Rajaa A Mirghani
email: rmirghani@kfmc.med.sa

Ishraga Elagib

Gehad Elghazali

Urban Hellgren

Lars L Gustafsson

Effects of Plasmodium falciparum infection on the pharmacokinetics of quinine and its metabolites in pregnant and non pregnant Sudanese women

Keywords: malaria, quinine, metabolites, pharmacokinetics, pregnancy

Purpose

The study aimed to investigate the effects of plasmodium falciparum infection on the pharmacokinetics of quinine and its metabolites in pregnant and non pregnant Sudanese women.

Methods

In a case-control study, nine pregnant & eight non-pregnant Sudanese women infected with P. falciparum were treated with intramuscular artemether. Before given artemether they received a single dose of quinine hydrochloride as intravenous infusion. Blood samples were collected

frequently and analysed for quinine and its metabolites (phase I). One week later (after clearance of parasitaemia) the quinine part of the protocol was repeated (phase II).

Results

During phase I, the mean ± SD AUC of quinine and its major metaboplite, 3hydroxyquinine, in pregnant women was 428.2±132.4 and 27.8±14.1 µmol.l - 1 .h, respectively. In non pregnant women the mean ± SD AUC of quinine and 3-hydroxyquinine, was 517.8±100.0 and 32.3±15.3 µmol.l -1 .h. In pregnant women the mean (90% confidence intervals) AUC ratio of phase I/phase II of quinine and 3-hydroxyquinine was 1.6 (0.61, 4.22) and 1.01 (0.18, 5.60). In non pregnant the mean (90% confidence intervals) AUC ratio of phase I/phase II of quinine and 3-hydroxyquinine, was 1.93 (1.74, 2.15) and 1.19 (0.95, 1.47).

Conclusions

Plasmodium falciparum infection significantly increases plasma concentration of quinine in non pregnant women and showed the same trend in pregnant women. Pregnant women and children under the age of five are in particular vulnerable to the disease. In endemic areas, pregnant women are highly susceptible to malaria with both higher frequency and severity than other groups. Many complications are associated with Plasmodium falciparum malaria infection during pregnancy such as low birth weight, stillbirth, maternal anaemia and mortality [START_REF] Shulman | Importance and prevention of malaria in pregnancy[END_REF] Quinine is an important drug for the treatment of severe or multi-drug-resistant P. falciparum malaria [START_REF] White | The treatment of malaria[END_REF]. Quinine is eliminated mainly by metabolism with no more than 13% of the absorbed drug excreted unchanged in urine [START_REF] Mirghani | Metabolism and elimination of quinine in healthy volunteers[END_REF]. The pharmacokinetics of quinine are reported to be altered during malaria and this alteration is in proportion to the severity of the infection. A marked increase in the area under the plasma concentration versus time curve (AUC) and volume of distribution was reported earlier. Pukrittayakamee et al have reported the reduction of total quinine clearance from 3.4 mg. kg -1 . min -1 during convalescence to 1.4 mg. kg -1 . min -1 during infection [START_REF] White | Quinine pharmacokinetics and toxicity in cerebral and uncomplicated falciparum malaria[END_REF][START_REF] Pukrittayakamee | A study of the factors affecting the metabolic clearance of quinine in malaria[END_REF][START_REF] White | Assessment of the pharmacodynamic properties of antimalarial drugs in vivo[END_REF].

Therapeutic treatment doses with quinine are safe during pregnancy [START_REF] Looareesuwan | Quinine and severe falciparum malaria in late pregnancy[END_REF][START_REF] Looareesuwan | Quinine and severe falciparum malaria in late pregnancy[END_REF][START_REF] Mcgready | Quinine and mefloquine in the treatment of multidrug-resistant Plasmodium falciparum malaria in pregnancy[END_REF][10].

Quinine efficacy in pregnant women was reported to be poor [START_REF] Mcgready | A randomized comparison of artesunate-atovaquoneproguanil versus quinine in treatment for uncomplicated falciparum malaria during pregnancy[END_REF]. This may be caused by the physiological changes which are associated with pregnancy.

The combination of the malaria infection with pregnancy may affect the metabolism and pharmacokinetics of drugs. However, there is a lack of detailed studies that compare the pharmacokinetics of quinine and its metabolites during infection to that at recovery in pregnant and non pregnant women.

1. Ethics

An informed consent was obtained from women who participated in the study after being given adequate information on the objectives and benefits of the project. The study was monitored and supervised by qualified medical personnel whose first responsibility was the welfare of the patients enrolled in the study. At all times, proper patient management took priority over continuation of the study. The study was carried out following WHO Guidelines for good clinical practice. The ethical committee of the University of Khartoum´s Faculty of Medicine and the Human Ethics committee at the Federal Ministry of Health, Khartoum, Sudan approved the study protocol.

Study design and subjects

It was a case-control study in which nine pregnant and eight non-pregnant Sudanese women in their second or third trimester infected with P. falciparum were asked to participate. The patients received a single dose of (10mg/kg body weight) quinine hydrochloride (lab. Renaudin, Saint Cloud, France) as intravenous infusion during 2 hours. Twelve hours later they received the first dose of artemether (methyl ether of dihydroartemisinin, Kunming Pharmaceutical Corp. Qigongli, West Suburb, Kunming, China) 1.6mg/kg body weight, intramuscularly. The second dose of artemether was given eight hours after the first dose. Thereafter they were given one dose daily for four days. All drug administrations were supervised by our staff.

Blood samples were collected before quinine administration and at 0.5, 1, 2, 4, 12, 24, 48 and 72 hours thereafter (phase I). One week later after the initial dose administration at phase I, the quinine part of the protocol was repeated Duplicate samples of plasma were analysed for quinine and its metabolites 3hydroxyquinine (3-OHQ), (10R)-10,11-dihydroxyquinine ((10R)-DOHQ) and (10S)-10,11-dihydroxyquinine ((10S)-DOHQ) using high performance liquid chromatography (HPLC) as described [START_REF] Mirghani | Simultaneous determination of quinine and four metabolites in plasma and urine by highperformance liquid chromatography[END_REF]. The lower limit of quantitation of quinine and its metabolites was 5 nM. Three different concentration ranges of the calibration curves were used. For each calibration curve a blank plasma sample and four to five calibration points were used. The within and between assay coefficient of variations of the method were less than 15% for all the analytes.

.

Parasitological and clinical data:

For diagnosis of malaria infection and during follow up after treatment, blood was obtained by a finger prick.

Thick and thin blood films were prepared and stained with Giemsa (Ph 7.0, diluted with phosphate buffered saline). The thick blood films were used to detect the malaria parasite and the thin films for defining the parasite species. All women enrolled in this study were those who presented with mild and uncomplicated P. falciparum malaria, those who presented with mixed infections or with severe malaria as defined by the WHO criteria for severe disease [START_REF] Health Organization | Severe falciparum malaria[END_REF] were excluded.

All the subjects were followed very closely. Body temperature was recorded every 6 hr until it decreased to 37.5 C and remained stable for the next 48 hr, then daily. Blood parasite counts were estimated every 6 hr until cleared, then daily. Radical cure was defined as the absence of asexual parasites during the 28-day follow-up. Parasite clearance time (PCT) was the time from the initiation of treatment until the first time a slide was negative for the asexual parasite forms, and remained negative for the next 24 hr. Fever clearance time (FCT) was defined as the time from the start of treatment until the body temperature subsided to 37.5 ºC and remained at or below this temperature for the next 48 hr.

3. Pharmacokinetic calculations

The pharmacokinetic parameters were determined using non-compartmental methods. The total area under the plasma concentration time curve (AUC) of quinine and its metabolites was determined as AUC

4. Statistical analysis

We used log e -transformed data and determined geometric means of the pharmacokinetic parameters for quinine, 3-hydroxyquinine, (10R)-10,11dihydroxyquinine and (10S)-10,11dihydroxyquinine in phase1 and phase II, respectively with 95% confidence intervals (CIs) from the untransformed data separate for pregnant and non-pregnant (control group) women.

Intraindividual ratios of geometric means of phase I to phase II for the pharmacokinetic parameters with corresponding 90% CIs were calculated.

Mann-Whitney U test was used to check for differences in t 1/2 and t max between phase I and phase II for pregnant and non pregnant women.

Results

Quinine was detected in the sample of one of the pregnant patients before drug administration and thus this subject was excluded from the study.

All the patients completed the study except for two of the controls that quitted the second part of the study. The treatment was generally well tolerated and only tinnitus was reported by a minor number of patients. All participants recovered uneventfully from their P.

falciparum infection.

The ages and weights of the pregnant women were (mean±SD) 26.6± 8.5 years and 55.7±14.9 kg, respectively. The ages and weights of the control group were (mean±SD) 25.2±7.9 years and 52.3±12.7 kg, respectively. 1 and2.

In pregnant women, there were no statistically significant changes in the pharmacokinetic parameters of quinine and its metabolites between phase I and II of the study as presented by the ratios of geometric means and 90% confidence intervals shown in Table I In this study we compared the pharmacokinetics of quinine and three of its metabolites during infection to their pharmacokinetics at recovery. This was done in pregnant and nonpregnant (control) women. Quinine pharmacokinetics have been reported to be changed by a number of factors such as co-administration of inhibitors [START_REF] Mirghani | The roles of cytochrome P450 3A4 and 1A2 in the 3-hydroxylation of quinine in vivo[END_REF] or inducers [START_REF] Wanwimolruk | Marked enhancement by rifampicin and lack of effect of isoniazid on the elimination of quinine in man[END_REF] of CYP3A4, cigarette smoking [START_REF] Pukrittayakamee | Effects of cigarette smoking on quinine pharmacokinetics in malaria[END_REF] and Plasmodium falciparum infection [START_REF] Babalola | Pharmacokinetics of quinine in African patients with acute falciparum malaria 1998[END_REF].

The pharmacokinetics of medications taken during pregnancy can be modified due to physiological changes such as alterations in gastrointestinal transit time, increased glomerular filtration rate [START_REF] Hladunewich | The dynamics of glomerular filtration in the puerperium[END_REF] and increased total body water and fat. Furthermore, levels of sex hormones change significantly during pregnancy [START_REF] Xu B Pekkanen | Maternal sex hormones in early pregnancy and asthma among offspring: a case-control study[END_REF] and that in turn may affect the activity of the different cytochrome P450 enzymes [START_REF] Laine | No sex-related differences but significant inhibition by oral contraceptives of CYP2C19 activity as measured by the probe drugs mephenytoin and omeprazole in healthy Swedish white subjects[END_REF][START_REF] Laine | A screening study on the liability of eight different female sex steroids to inhibit CYP2C9, 2C19 and 3A4 activities in human liver microsomes[END_REF][START_REF] Palovaara | Inhibition of cytochrome P450 2B6 activity by hormone replacement therapy and oral contraceptive as measured by bupropion hydroxylation[END_REF][START_REF] Palovaara | The effect of ethinyloestradiol and levonorgestrel on the CYP2C19-mediated metabolism of omeprazole in healthy female subjects[END_REF], which are involved in the metabolism of endogenous compounds and drugs. In general, changes in pharmacokinetics of drugs during pregnancy are well documented [24]. Specifically, these changes may alter exposure to quinine, which could compromise its efficacy or increase concentration dependent side effects.

Cytochrome P450 3A4 is the major drug metabolising enzyme [START_REF] Shimada | Interindividual variations in human liver cytochrome P-450 enzymes involved in the oxidation of drugs, carcinogens and toxic chemicals: studies with liver microsomes of 30 Japanese and 30 Caucasians[END_REF], which catalyses the biotransformation of quinine to 3-hydroxyquinine. A known marker for induction of hepatic cytochrome P450 3A4 is 6β-hydroxycortisol formation [START_REF] Kovacs | Urinary excretion of 6 betahydroxycortisol as an in vivo marker for CYP3A induction: applications and recommendations[END_REF][START_REF] Totsuka | Increase in urinary excretion of 6beta-hydroxycortisol in common marmosets as a marker of hepatic CYP3A induction[END_REF][START_REF] Eeckhoudt | Midazolam and cortisol metabolism before and after CYP3A induction in humans[END_REF][START_REF] Galteau | Urinary 6beta-hydroxycortisol: a validated test for evaluating drug induction or drug inhibition mediated through CYP3A in humans and in animals[END_REF]. Using this marker studies performed in pregnant women have shown a significant increase in the formation of this metabolite during pregnancy in comparison to postpartum [START_REF] Ohkita | Increased 6-hydroxycortisol excretion in pregnant women: implication of drug-metabolizing enzyme induction[END_REF] indicating higher activity of CYP3A.

The statistically significant increase in the AUC and C max of quinine during infection that we found in the control group is consistent with what was reported in literature [31].

Quinine is known to bind to α-acid glycoprotein which is an acute phase reactant that significantly increases during malaria infection [32]. This explains the increased levels of quinine during infection. Even though the elevation of quinine AUC and C max in pregnant women was not statistically significant, it was in agreement with the results found in the control group. This may be explained by the greater inter-individual variability in the pregnant women group. The lack of significant changes in the AUC and C max of the metabolites indicates that the malaria infection has no pronounced effect on the drug metabolizing enzymes which are involved in the formation of quinine metabolites.

Furthermore, the degree of protein binding and excretion of the metabolites may be different than that of the parent drug. The small sample size and wide intra-individual variability may as well be possible explanations of the lack of significant differences in the pharmacokinetics of the metabolites.

In conclusion, our results indicate that plasmodium falciparum infection has more profound effect on the plasma concentration of quinine than on the plasma concentration of quinine metabolites when a single dose of quinine is administered. However, steady state pharmacokinetic parameters of the drug and its metabolites have to be studied in order to determine whether quinine dose adjustment is needed during infection with p. falciparum.

F

  Malaria is a major global health problem and especially in sub-Saharan Africa.

4

 4 In this study, we investigated the pharmacokinetics of quinine and its metabolites in pregnant and non-pregnant Sudanese control group of women during acute P. falciparum malaria infection and after treatment.

6 (

 6 phase II) in order to compare the pharmacokinetics of quinine and its metabolites during infection and after treatment. Blood samples were centrifuged at 2500 rpm, plasma was separated and kept at -20°C until analysis.

For

  parasite detection, 100 fields were examined irrespective of the parasites encountered. Parasitological data were cross-checked by another technician, blinded to the results and verified by the senior technologist if there was any controversy. The number of asexual parasites per 200 leukocytes was counted and parasite density (asexual parasites/µl) was calculated assuming a normal value of 6000 leukocytes/µl.

  The mean parasite clearance time (PCT) was 51.2 +/-23.2 hours while the fever clearance time (FCT) was 38.0 +/-31.3 hours. All P. falciparium infected women cleared parasites and were free of fever by the end of day 3 except for one pregnant women (primigravidae) who had a parasite clearance time by day 4 following initiation of treatment. None of the study subjects had parasites in blood or fever before the start of phase II of the study or on day 14, 21 and 28. The plasma concentration profiles of quinine, 3-hydroxyquinine, (10R)-10,11dihydroxyquinine and (10S)-10,11-dihydroxyquinine for pregnant and non pregnant women during phase I and II are shown in figures 1A-1D, respectively. Pharmacokinetic parameters of quinine, 3-hydroxyquinine, (10R)-10,11-dihydroxyquinine and (10S)-10,11-dihydroxyquinine in pregnant and non pregnant women during phase I and phase II are shown in Table

  Pharmacokinetic parameters of quinine, 3-hydroxyquinine, (10R)-10,11 dihydroxyquinine and (10S)-10,11dihydroxyquinine when a single 10mg/kg dose of quinine hydrochloride was given to 8 non pregnant women (controls) during phase I and 6 non pregnant women during phase II of the study. Results are shown as Geometric Means (95% CI) and Ratios (phase I / phase II) of Geometric Means with (90% CI

Fig 1 :

 1 Fig.1A

F

  Only slight mean changes were observed in the AUC and C max of the other two metabolites, and no remarkable differences were found in the t 1/2 or t max of quinine and its metabolites.

	during phase I the geometric mean AUC and C max of quinine were higher by 60 and 55 %, 4. Discussion:
	respectively.
	As shown in Table II for the control group, there was a significant decrease in the geometric
	mean AUC and C max of quinine which is reflected by the geometric mean ratio of phase I to
	phase II (1.93, 90 % CI 1.74, 2.15) and (1.35, 90 % CI 1.21, 1.50), respectively. However,
	no significant changes were observed in t 1/2 (p=0.89) and t max (p=0.78) of quinine.
	F o r Furthermore, we observed no significant changes in the pharmacokinetic parameters (AUC, C max , t 1/2 and t max ) for the three metabolites of quinine. F o r
	P P
	e e
	e r e r
	R R
	e e
	v i e v i e
	w w
	. However,
	10 11

Table I .

 I Pharmacokinetic parameters of quinine, 3-hydroxyquinine, (10R)-10,11dihydroxyquinine and (10S)-10,11dihydroxyquinine when a single 10mg/kg dose of quinine hydrochloride was given to 9 pregnant women during phase I and II of the study.Results are shown as Geometric Means (95% CI) and Ratios (phase I / phase II) of

	Geometric Means with (90% CI). T max and t 1\2 are presented as median (range)
		C max (µmol/l)		t max (h)	t 1\2 (h)	AUC (µmol.l -1 .h)
	Quinine
	Geometric Mean in phase I	30.92		2.0	12.5	455.94
	(95% CI)	(19.38, 49.32)	(1.0 -4.0)	(6.2 -15.7)	(368.22, 564.54)
	Geometric Mean in phase II (95% CI) Ratios of Geometric Means o 19.99 (16.59, 24.08) r 1.54	2 (2.0 -4.0)	12.8 (8.6 -16.6)	284.90 (222.00, 365.61) 1.6
	(90% CI)	P (0.35, 6.79)				(0.61, 4.22)
	3-hydroxyquinine	e
	(95% CI) Geometric Mean in phase I	( 0.53, 1.38) r e 0.86	12.0
			R
			e
				v i e
				w
		13

  Silamut, K.,White, N.J., et al., 1985. Binding of quinine to plasma proteins in falciparum malaria." Am. J. Trop. Med. Hyg. 34, 681-686. 32. Wanwimolruk, S., Denton, J.R., 1992. Plasma protein binding of quinine: binding to human serum albumin, alpha 1-acid glycoprotein and plasma from patients with malaria. J. Pharm. Pharmacol. 44, 806-811.

	31.
	o r F o r F o r F o r F o r F o r
	P P P P P P
	e e e e e e
	e r e r e r e r e r e r
	R R R R R R
	e e e e e e
	v i e v i e v i e v i e v i e v i e
	w w w w w w
	21 26

European Journal of Clinical Pharmacology

Acknowledgments

The authors are indebted to the patients who agreed to participate in this study. The local health authorities in Kassala and the staff of New Halfa Hospital especially Dr. Isag Adam are acknowledged for their support and contribution. The study received partial financial support from the Swedish Agency for Research Development with Developing Countries (SIDA, SAREC).

Conflicts of interest statement

The authors have no conflict of interest concerning the work reported in this paper.


Adriana W Blom
email: a.w.blom@tudelft.nl

Mostafa M Abdalla

Zafer Gürdal

Optimization of Course Locations in Fiber-Placed Panels for General Fiber Angle Distributions

Keywords: A. Laminate, A. Structural composite, A. Fibers, E. Fiber placement, Optimization

Fiber-reinforced composites are usually designed using constant fiber orientation in each ply. In certain cases, however, a varying fiber angle might be favorable for structural performance. This possibility can be fully utilized using tow placement technology. Because of the fiber angle variation, tow-placed courses may overlap and ply thickness will build up on the surface. This thickness buildup affects manufacturing time, structural response, and surface quality of the finished product. This paper will present a method for designing composite plies with varying fiber angles with composite plates or panels. The thickness build-up within a ply is predicted as function of ply angle variation using a streamline analogy. It is found that the thickness build-up is not unique and depends on the chosen start locations of fiber courses. Optimal fiber courses are formulated in terms of minimizing the maximum ply thickness, maximizing surface smoothness or combining these objectives with and without periodic boundary conditions.

Introduction

In industry fiber-reinforced composites are usually designed using a constant fiber orientation in each ply. The fiber angles in these laminates are typically 0, 90, and ±45 degrees. Traditionally the choice of these lay-ups was motivated by manufacturability, while nowadays lay-ups with changing or even non-conventional fiber angles are avoided because of the lack of allowables. However, research on composites with a varying in-plane fiber orientation has shown that variable stiffness can be beneficial for structural performance [START_REF] Hyer | The use of curvilinear fiber format in composite structure design[END_REF][START_REF] Hyer | The use of curvilinear fiber format to improve buckling resistance of composite plates with central circular holes[END_REF][START_REF] Nagendra | Optimization of tow fiber paths for composite design[END_REF][START_REF] Waldhart | Analysis of tow placed, parallel fiber, variable stiffness laminates[END_REF][START_REF] Parnas | Optimum design of composite structures with curved fiber courses[END_REF][START_REF] Hale | Integrated design and analysis tools for reduced weight, affordable fiber steered composites[END_REF][START_REF] Setoodeh | Design of variable stiffness laminates using lamination parameters[END_REF][START_REF] Abdalla | Design of variable stiffness composite panels for maximum fundamental frequency using lamination parameters[END_REF][START_REF] Tatting | Design and manufacture of elastically tailored tow placed plates[END_REF][START_REF] Tatting | Automated finite element analysis of elastically-tailored plates[END_REF][START_REF] Jegley | Tow-steered panels with holes subjected to compression or shear loading[END_REF][START_REF] Huang | Optimization of fiber orientations near a hole for increased loadcarrying capacity of composite laminates[END_REF][START_REF] Blom | Design of variable-stiffness conical shells for maximum fundamental eigenfrequency[END_REF][START_REF] Sun | Use of material tailoring to improve buckling capacity of elliptical composite shells[END_REF][START_REF] Blom | Optimization of a composite cylinder under bending by tailoring stiffness properties in circumferential direction[END_REF][START_REF] Alhajahmad | Design tailoring for pressure pillowing using towplaced steered fibers[END_REF][START_REF] Alhajahmad | Optimal design of a pressurized fuselage panel with a cutout using tow-placed steered fibers[END_REF], because variable-stiffness laminates are able to redistribute the loading more efficiently than constant-stiffness laminates. In most cases curvilinear fiber paths manufactured by tow placement are used to construct the variable-stiffness laminates [4, 5, 9-11, 15, 18-20]. Jegley, Tatting and Gürdal [START_REF] Tatting | Design and manufacture of elastically tailored tow placed plates[END_REF][START_REF] Tatting | Automated finite element analysis of elastically-tailored plates[END_REF][START_REF] Jegley | Tow-steered panels with holes subjected to compression or shear loading[END_REF] designed variable-stiffness flat plates with holes and demonstrated their effectiveness by building and testing several specimens. Due to fiber angle variation, a tow-placed shell typically exhibits gaps and/or overlaps between adjacent courses and ply thickness will change along the surface [START_REF] Tatting | Design and manufacture of elastically tailored tow placed plates[END_REF][START_REF] Tatting | Automated finite element analysis of elastically-tailored plates[END_REF][START_REF] Jegley | Tow-steered panels with holes subjected to compression or shear loading[END_REF][START_REF] Blom | Path definitions for elastically tailored conical shells[END_REF]. The amount of gap/overlap affects structural response, manufacturing time, and surface quality of the finished product. This paper presents a method for designing composite plies which have spatially varying fiber angles. Since fiber-reinforced laminates usually consist of multiple plies, optimizations for specific loading conditions result in multiple plies with different fiber angle distributions. The fiber angle distribution per ply can be used as a direct input for the optimization, as is done by most researchers so far [START_REF] Hyer | The use of curvilinear fiber format in composite structure design[END_REF][START_REF] Hyer | The use of curvilinear fiber format to improve buckling resistance of composite plates with central circular holes[END_REF][START_REF] Nagendra | Optimization of tow fiber paths for composite design[END_REF][START_REF] Waldhart | Analysis of tow placed, parallel fiber, variable stiffness laminates[END_REF][START_REF] Parnas | Optimum design of composite structures with curved fiber courses[END_REF][START_REF] Hale | Integrated design and analysis tools for reduced weight, affordable fiber steered composites[END_REF][START_REF] Setoodeh | Design of variable stiffness laminates using lamination parameters[END_REF][START_REF] Abdalla | Design of variable stiffness composite panels for maximum fundamental frequency using lamination parameters[END_REF][START_REF] Tatting | Design and manufacture of elastically tailored tow placed plates[END_REF][START_REF] Tatting | Automated finite element analysis of elastically-tailored plates[END_REF][START_REF] Jegley | Tow-steered panels with holes subjected to compression or shear loading[END_REF][START_REF] Huang | Optimization of fiber orientations near a hole for increased loadcarrying capacity of composite laminates[END_REF][START_REF] Blom | Design of variable-stiffness conical shells for maximum fundamental eigenfrequency[END_REF][START_REF] Sun | Use of material tailoring to improve buckling capacity of elliptical composite shells[END_REF][START_REF] Blom | Optimization of a composite cylinder under bending by tailoring stiffness properties in circumferential direction[END_REF][START_REF] Alhajahmad | Design tailoring for pressure pillowing using towplaced steered fibers[END_REF][START_REF] Alhajahmad | Optimal design of a pressurized fuselage panel with a cutout using tow-placed steered fibers[END_REF][START_REF] Blom | Path definitions for elastically tailored conical shells[END_REF][START_REF] Wu | Structural response of compression-loaded, tow-placed, variable stiffness panels[END_REF][START_REF] Wu | Design and analysis of tow-steered composite shells using fiber placement[END_REF], or it can be obtained in a post processing step, where an optimum laminate stiffness distribution is approximated as closely as possible, as described by Setoodeh [START_REF] Setoodeh | Generating curvilinear fiber paths from lamination parameters distribution[END_REF]. In these optimizations the ply thickness is usually assumed to be constant, without taking into consideration manufacturing issues. In the current paper the fiber angle distribution per ply is assumed given, being one of the plies within an optimized laminate. The thickness build-up is predicted as function of ply angle variation using a streamline analogy. It is found that the thickness build-up is not unique and depends on the chosen start locations of fiber courses. Optimal distributions of fiber courses are formulated in terms of minimizing the maximum ply thickness or maximizing surface smoothness, either with or without periodic boundary conditions. Subsequently the discrete thickness build-up resulting from the tow-placement process can be determined based on the streamline distribution. Results will be compared to the smeared thickness approximation. An overview of the analysis sequence is given in Fig. 1. Finally, a number of applications for the developed methods and suggestions for future research are given.

Streamline Analogy

For the construction of discrete fiber paths a streamline analogy is being used. For this application each streamline represents the centerline of a course, or if the course width is made infinitely small each streamline will represent a single fiber. Mathematically a streamline is represented by a stream function

Ψ(x, y) = C (1)
which connects all the points with a constant value C. For a given fiber angle distribution θ(x,y), the streamlines can be found by solving the following partial differential equation:

dΨ ds = ∂Ψ ∂x dx ds + ∂Ψ ∂y dy ds = Ψ ,x cos θ + Ψ ,y sin θ = 0 (2)
A unique solution for the stream function (and thus the location of the stream lines) depends on the boundary conditions. Before seeking a solution to the stream function, additional considerations relevant to the physical representation of the fiber paths are in order. As stated earlier, the streamlines represent the central path of a finite width course. Unless the streamlines are parallel, the successive courses will always overlap each other when no gaps are allowed between them (or alternatively, the gaps will form between the passes if two successive finite width passes are not allowed to overlap). The amount of overlap depends on the distance between the course centerlines. If the distance is decreased, then the overlap area is increased. Although in reality these overlaps are discrete, a first approximation to the amount of overlap could be made by smearing out this discrete overlap to form a continuous thickness distribution. In this case, the smeared thickness, t, will be inversely proportional to the distance between adjacent courses, which can be explained as follows. If a number of N courses with a given width, w, and thickness has a fixed volume V , and if these successive courses are placed closer than the width of the courses, then the total width covered is less then N • w, and the thickness has to be increased in order to maintain the same material volume V . When the distance between two streamlines is |dn|, then t ∝ 1/|dn| (as explained above). Since Ψ, n = dΨ/dn and dΨ between two streamlines is constant according to Eq. ( 1) the thickness t will be proportional to Ψ ,n as follows:

t ∝ 1 |dn| = 1 dΨ/Ψ ,n = Ψ ,n dΨ ∝ Ψ ,n (3)
If dΨ is assumed to be a unity, then t = Ψ ,n , which can be used to derive a direct correlation between the thickness distribution and the fiber angle variation (see Appendix A):

-s∇

(ln t) = n∇θ (4)
in which s and n represent the tangent and normal vectors to a streamline, respectively, as shown in Fig. 2. The physical explanation of Eq. ( 4) is that the change in thickness along a streamline depends on the change of the fiber orientation perpendicular to that streamline. Since both vectors s and n depend on the given fiber angle distribution θ(x, y), the only unknown in Eq. ( 4) is the thickness. Hence, the thickness can now be determined by solving this equation, but since it is a differential equation boundary conditions are needed in order to obtain a unique solution. In accordance with streamline theory, boundary conditions are only needed at the inflow boundary, where the inflow boundary is arbitrarily defined by:

s • N ≤ 0 ( 5
)
where s is the vector tangent to the streamline and N is the outward normal vector to the boundary, as shown in Fig. 2. By changing the thickness at the inflow boundary, the thickness distribution inside the domain and at the outflow boundaries will change.

. Streamline definitions 3 Determining Boundary Conditions

There exist an infinite number of possible boundary conditions for which the thickness distribution associated with the streamlines can be found, but the most difficult part is to find the ones that are physically sensible for the problem in hand. In this paper the boundary conditions are established such that they fulfill a certain optimality condition. The optimality conditions to be demonstrated in this paper are minimization of maximum thickness, maximization of smoothness, and a combination of these two. In addition, constraints such as periodicity of the boundary conditions can be enforced as well.

General Solution

By using the following change of variables: τ = ln t, Eq. ( 4) becomes:

-s∇τ = n∇θ (6)
The above equation is solved numerically by discretizing the derivatives, so that it is written as:

[M ]τ = B (7)
where [M ] is the matrix that represents the left hand side of Eq. ( 6), τ is the vector that represents τ at every grid point and B is the vector that represents the right hand side of Eq. ( 6), as well as the boundary conditions. If the thickness at the inflow boundaries is assumed to be equal to one everywhere (τ = ln t = ln 1 = 0), a nominal solution can be found for τ , which will be referred to as τ 0 . A general solution of Eq. ( 7) can be expressed as:

τ = τ 0 + [T ]τ in (8)
where each column j in matrix [T ] represents the influence of boundary grid point j on the thickness distribution in the complete domain, while satisfying Eq. [START_REF] Setoodeh | Design of variable stiffness laminates using lamination parameters[END_REF]. Since these columns are independent of each other and since Eq. ( 7) is a linear equation, any linear combination of these columns also represents a solution, as given by Eq. ( 8). The entries in τ in all render the thickness at a single point on the inflow boundary. By substituting Eq. ( 8) in Eq. ( 7), the thickness can be optimized for one of the criteria mentioned earlier by using τ in as design variables.

Often it is desired to have at least one layer of material everywhere so that no gaps exist. Therefore it is required that the thickness over the entire domain is at least one, such that τ ≥ 0 in all optimizations described below. Optimization is performed by using the optimization toolbox in Matlab [START_REF][END_REF]

Minimized Maximum Thickness

Minimizing the maximum thickness of the plate is the first optimality criterion that will be elaborated on in this paper. This criterion is relevant for judging how practical the resulting thickness distribution would be in a real life structure, as well as for determining the manufacturability of a plate with constant thickness for a given fiber angle distribution. If the thickness build-up is too severe (i.e. if one point is 100 or 1000 times thicker than the average plate thickness) it will not be practical for realistic structures.

In order to solve the min-max problem, the bound formulation as introduced by Olhoff [START_REF] Olhoff | Multicriterion structural optimization via bound formulation and mathematical programming[END_REF] is used. This formulation introduces a new variable α, which represents the maximum thickness and which also serves as the new objective function for the minimization. Additionally, a constraint on the thickness at each grid point is being introduced so that the thickness never exceeds the minimized maximum thickness, α. Mathematically:

F min = α s.t. τ i ≤ α i = 1, 2, ..., N g (9)
where N g is the number of grid points. The design variables that result from the optimization are substituted in Eq. ( 8) and then the thickness distribution is found by changing variables again:

t i = e τi .

Maximized Smoothness

Another possible optimization objective is to maximize the smoothness of the thickness distribution of the composite panel. Although in reality the change in thickness will always be discrete due to the discrete nature of tow courses, it would still be desirable for ply drops/overlaps to be distributed throughout the panel rather than to be concentrated at particular regions. In order to achieve this objective, smoothness is defined as the norm of the rate of change of thickness. Smoothness is maximized by minimizing the H 1 -norm of the thickness:

min 1 2 τ T [K]τ (10)
where [K] is the matrix that discretizes the Laplacian. Substituting the expression for τ (Eq. ( 8)) into the argument of Eq. ( 10) gives:

1 2 τ T [K]τ = 1 2 τ T 0 [K]τ 0 + τ T 0 [K][T ]τ i n + 1 2 τ T in [T ] T [K][T ]τ in (11)
The first term on the right hand side of Eq. ( 11) is constant, so that the objective function to be minimized is:

F min = 1 2 τ T in [K r ]τ in -f r τ in (12)
with

[K r ] = [T ] T [K][T ] f r = -τ T 0 [K][T ] (13)
The minimum of Eq. ( 12) can be found by differentiating it and equating it to zero, so that:

[K r ]τ in = f r (14)
This is a linear system that can be solved for τ in . However, the [K r ]-matrix is one time singular and therefore one entry of τ in is given an assumed value so that the system can be solved. After the solution is substituted in Eq. ( 8), a constant can be added to τ such that the condition of τ ≥ 0 is being met (this will not change the H 1 -norm, but will change the absolute value of thickness).

Combined Objective Function

Since both minimizing the maximum thickness and maximizing the smoothness are valid optimization criteria, designers might consider combining the two in order to obtain a better design. Depending on the design requirements, different weighting factors can be assigned to the individual criterion. The objective functions of Eqs. ( 9) and ( 12) can then be combined to form a new objective function:

F min = (1 -w) α α * + w 1 2 τ T in [K r ]τ in -f r τ in 1 2 τ T in * [K r ]τ in * -f r τ in * (15)
In this equation w is the weighting factor that indicates the importance of the smoothness in the optimization. Furthermore the two objective functions are normalized by α * and 1 2 τ T in * [K r ]τ in *f r τ in * , respectively, where α * is the smallest maximum thickness obtained from Eq. ( 9) and τ in * are the design variables for maximum smoothness as obtained by Eq. ( 12).

Periodic Boundary conditions

The present formulation would also be valid for cylindrical shells since points on a cylindrical surface are in one to one correspondence to points on a rectangular panel. Nevertheless, an important difference exists; in the case of a cylindrical shell the solution must be periodic. When the ply angle variation is periodic, continuity in thickness is obtained by including the thickness periodicity constraints in the optimization routines described above. For periodicity in y-direction, this takes the form:

τ (x i , 0) = τ (x i , b) 0 ≤ x i ≤ l (16)
where l is the length and b is the width of the panel.

Discrete Fiber Courses

Once the smeared thickness distribution is obtained through one of the optimizations described above, the corresponding stream function can be obtained by integrating Ψ ,n over dn:

Ψ(x, y) = Ψ ,n dn = dΨ dx dx dn dn + dΨ dy dy dn dn = Ψ ,x dx + Ψ ,y dy (17)
The derivatives of Ψ with respect to x and y can be expressed as functions of Ψ ,s and Ψ ,n as follows:

Ψ ,x = Ψ ,s cos θ -Ψ ,n sin θ Ψ ,n = Ψ ,s sin θ + Ψ ,n cos θ (18)
Since Ψ ,s = 0 and Ψ ,n = t, the combination of Eqs. ( 17) and (18) will give:

Ψ(x, y) = - x 0 t(x * , y * ) sin θ(x * , y * )dx * + y 0 t(x * , y * ) cos θ(x * , y * )dy * (19)
Both t(x, y) and θ(x, y) are known functions, so that Ψ(x, y) can be solved. By plotting the contour lines of Ψ at fixed increments the streamlines are found that could represent the centerlines of the actual fiber courses. The constant of integration will determine the exact location of the fiber courses, which can be used for staggering in case of multiple plies with the same fiber angle distribution.

Once the course centerlines are known, discrete courses can be constructed by calculating the course edges. If a point on the path centerline is defined by {x c , y c } the course edges are found by:

x e = x c ∓ p sin θ c y e = y c ± p cos θ c ( 20
)
where p is half the total course width and θ c is the fiber orientation angle at {x c , y c }, as shown in Fig. 3. When the centerline reaches the domain boundary one edge is still inside the domain and therefore the centerline is extrapolated until both edges are outside the domain boundary (e.g. the blue lines in Fig. 3). After determining the location of the course boundaries the number of layers at each point in the laminate can be found. First the results for a panel with a one-dimensional fiber angle distribution are presented. The differences between the optimization criteria are shown and the influence of periodic boundary conditions on the thickness distribution are demonstrated. For these criteria the corresponding thickness distributions resulting from discrete courses are also given. Second, the thickness distributions for two panels with a polynomial angle distribution in two directions are compared.

Unidirectional Fiber Angle Variation

To illustrate the differences between the various optimality criteria described in section 3, an example panel is analyzed which has the following linear angle variation in x-direction:

θ(x, y) = -30 -30 x l (21)
such that the fiber orientation is -30 degrees at the left side of the panel (x = 0) and -60 degrees at the right side of the panel (x = l). The length to width ratio of the panel is 3. This fiber angle distribution is only used for illustration of the presented theory and is not part of an optimized laminate definition.

Smeared Thickness Distribution

The thickness distribution of a panel for which the maximum thickness is minimized is shown in Fig. 4(a). The resulting thickness along the left and top edge is unity, indicating that there are no overlaps on these sides. The maximum thickness occurs in the lower right corner of the panel.

The thickness distribution of a panel with maximized smoothness is presented in Fig. 4(b). When compared to the first panel the maximum thickness is increased by approximately 20 percent, while smoothness is improved by 40 percent. The smeared thickness for the combined objective with w = 0.5 is plotted in Fig. 4(c). Since both maximum thickness and smoothness are included in the objective function, the increase in maximum thickness is only 7 percent, while the improvement in smoothness is 30 percent when compared to the first panel. Finally a panel with periodic boundary conditions is shown in Fig. 4(d). The maximum thickness of this panel is more than 40 percent larger than the smallest maximum thickness, and also smoothness is decreased.

Two Dimensional Fiber Angle Distributions

In Fig. 6 the smeared thickness for two different fiber angle distributions are presented. The graphs on the left correspond to a two-dimensional 2 nd order Lobatto polynomial and the graphs on the right correspond to a two-dimensional 3 rd order Lobatto polynomial, as described by Setoodeh [START_REF] Setoodeh | Generating curvilinear fiber paths from lamination parameters distribution[END_REF].

The angle distributions shown represent one ply in a laminate that resulted from an optimization to match a given optimal lamination parameter distribution as closely as possible [START_REF] Setoodeh | Generating curvilinear fiber paths from lamination parameters distribution[END_REF]. Due to the smaller number of design variables of the 2 nd order polynomial when compared to the 3 rd order polynomial (4 versus 9 variables), the latter better matched the given stiffness distribution. For more details on the laminate optimization the reader is referred to reference [START_REF] Setoodeh | Generating curvilinear fiber paths from lamination parameters distribution[END_REF].

In Figs. 6(c) and 6(d) the thickness due to a uniform unit inflow is plotted. In both figures regions with values smaller than one are present, which indicates that in these areas there is no complete coverage of the surface. In other areas the thickness is larger than one, implying that overlaps are present. For the 2 nd order polynomial (Fig. 6(c)) the largest thickness is about five times larger than the thinnest region, while for the 3 rd order polynomial the thickest region is more than a million times thicker than the thinnest region. If no gaps are allowed and all thicknesses were scaled accordingly, this would result in a non-acceptable amount of overlap for the last variation.

Optimizing the thickness distribution in order to eliminate gaps and to minimize maximum thickness results in the smeared thicknesses shown in Figs. 6(e) and 6(f). The maximum thickness of the 2 nd order polynomial distribution is reduced by approximately 35 percent, while the the amount of overlap for the 3 rd order polynomial distribution is reduced from one million to only 186 times maximum overlap. Finally smoothness is maximized in Figs. 6(g) and 6(h) at the cost of increased thickness. For the second order polynomial the increase is thirty percent, while for the third order polynomial the increase is 145 percent.

Based on these results, the plate with the second order angle distribution would be preferred to the third order distribution, because the amount of overlap resulting from the latter is not practical from a manufacturability point of view. Due to the streamline analogy presented in this paper this observation can be made without having to fit discrete courses to the given fiber angle distribution. Once a configuration has been selected, the discrete courses matching one of the boundary conditions can be generated. In Fig. 7(a) the discrete courses for a uniform unit inflow are shown. The white areas indicate gaps, as was already predicted by the smaller the values smaller than one in the continuous thickness distribution of Fig. 6(c). Figs. 7(b) and 7(c) display the discrete thickness distributions for respectively the smallest maximum thickness and maximum smoothness conditions. These plots indicate that the maximum number of layers for the maximum smoothness distribution is indeed larger, while the difference in smoothness is not significant due to the small number of (h) Maximized smoothness for 3x3 Lobatto Fig. 6. Fiber angle and thickness distributions for 2 nd and 3 rd order Lobatto polynomials layers in this ply. The ply thickness optimization can be used to judge if it possible to obtain a constant-thickness ply by cutting tows on the outside of fiber courses. In the case of the 3x3 Lobatto variation the amount of overlap as predicted by the streamline analogy already indicates that too many overlaps would occur and that cutting tows is inefficient. The 2x2 Lobatto variation on the other hand has a maximum overlap of 6 layers, for which a local reduction of the course width by a factor 6 can result in a constant-thickness ply. Using fiber placement this is feasible, since the course width could be reduced from 32 to 5 or 6 tows locally in order to eliminate the overlaps. In other cases the thickness distribution might be fed back to the laminate optimization routine in order to include the effect of thickness buildup in the optimization, instead of assuming a constant thickness per ply.

Conclusions and Outlook

In this paper it was demonstrated that a stream line analogy can be successfully used to predict and influence the thickness distribution in a ply with variable fiber angles. Different objectives for optimization were considered, and in the future others, such as minimum volume, might be Rearrangement gives:

-cos θ t ,x t + sin θ t ,y t = -sin θ • θ ,x + cos θ • θ ,y

Using the following definitions: Eq. ( 25) can be written as:

-s∇(ln t) = n∇θ (27)

Fig. 1 .

 1 Fig. 1. Analysis sequence

Fig. 2

 2 Fig. 2. Streamline definitions

Fig. 3 .

 3 Fig. 3. Finite width course and path extrapolation

Fig. 4 .

 4 Fig. 4. Thickness distribution for various optimization criteria

Fig. 5 .

 5 Fig. 5. Discrete thickness build-up (black = 1 layer, white = 2 layers)

(c) Discrete thickness build-up for 2x2 Lobatto with maximum smoothness Fig. 7. Discrete thickness build-up explored as well. The thickness information can be used to judge if a constant-thickness ply can be obtained, as constant thickness is often assumed in the optimization phase of variable-stiffness laminate design. Alternatively, the thickness optimization result can be included in the laminate optimization in order to take thickness buildup into account in the laminate design.

In addition to one-ply designs, the developed methods could be used to design complete laminates with both varying fiber angles and varying thickness. One approach would be to design the laminate using lamination parameters and thickness [START_REF] Setoodeh | Design of variable stiffness laminates using lamination parameters[END_REF][START_REF] Abdalla | Design of variable stiffness composite panels for maximum fundamental frequency using lamination parameters[END_REF][START_REF] Setoodeh | Generating curvilinear fiber paths from lamination parameters distribution[END_REF] as spatially varying design variables, and then as a post-processing step, multiple plies with varying fiber angles and their corresponding thickness distributions could be fit in order to match both the desired lamination parameters and thickness distribution as close as possible.

Finally, a similar method can be developed for curved surfaces in order to expand the applicability of the method.

Appendix A

The derivatives of the stream function are:

The second derivatives of the stream function are:

Continuity of the second derivatives of the stream function implies that Ψ ,xy = Ψ ,yx so that:

Ψ ,xy -Ψ ,yx = t(cos θ • θ ,y -sin θ • θ ,x ) -(sin θ • t ,y + cos θ • t ,x ) = 0 (24)


Maurizio Natali

Josè Kenny

Luigi Torre

Phenolic matrix nanocomposites based on commercial grade resols: synthesis and characterization

Polymer layered silicate nanocomposites based on a commercial grade resol were produced using a simple, low labor cost, mechanical approach which allowed to avoid the process of intercalative polymerization of phenol and formaldehyde. Commercial compatibilized montmorillonite was selected as the main nanoreinforcement, while the matrix was a resol diluted in methanol. The aim of this work was to optimize the production technique of the above mentioned nanocomposites. Therefore intercalation of the resin was promoted by high speed mixing, and the processing parameters were varied in order to find the optimum dispersion. The produced nanocomposites were characterized and compared by means of X-ray diffraction, SEM and thermogravimetric analysis. The results of the characterization tests indicated that it was possible to obtain a good degree of dispersion as well as and uniform distribution of the nanoclay platelets. However, TGA measurements showed that the introduction of well dispersed nanoclays did not result in a consistent improvement of thermal stability respect that of the neat resol.

Introduction

Polymeric ablative composites are key materials in the aerospace sector. Their main function is to protect aerodynamic surfaces and propulsion structures from extreme environments. The walls of a rocket combustion chamber, for example, can experience temperatures as high as 3,000°C and must withstand pressures up to 100 atm. Moreover, ablative materials are also used to protect the space vehicles during the re-entry flight. This protective function is accomplished by a self-regulating heat and mass transfer process typically known as ablation [START_REF] Torre | Ablative Plastics[END_REF]. Polymeric ablative composites are composed by a continuous phase, the matrix and, a discontinuous phase, which is constituted by the reinforcements and functional fillers, both phases can actively participate to the ablation process. Most polymeric ablators are based on thermosetting resins and, in this group of polymers, phenolic resins are undoubtedly the most employed and important matrices. [START_REF] Knop | Chemistry and Application of Phenolic Resins[END_REF] In fact, beside their exceptional thermal stability, they possess excellent features such as, high dimensional stability and thermal insulation. These properties can be directly related to their high crosslink density and their chemical structure. [START_REF] Stephen | [END_REF] Furthermore, the high crosslinking density of phenolic matrices improves the mechanical performance of the char, leading a better protection of the inner layers of the ablator. [4] During last forty years, in order to improve the quality of composites, fillers such as mineral powders or fibers, have been combined to polymers. However, in last twenty years, researchers focused their attention on nanocomposites, i.e. on the use of nanofllers such as nanoclays, nanosilica or, nanofibers. In this class of composites, Polymer Layered Silicate Nanocomposites (PLSN) based on montmorillonite (MMT) have attracted great attention because the introduction of these kind of nanofllers can lead to improved mechanical and barrier properties, [5]- [7] as well as of the thermal features, [8]- [9], without a relevant increase of the specific weight of the neat polymer.

Although phenolic resins are widely used as ablative matrices, few attempts to use these resins as a matrix for layered silicate nanocomposites are present in literature. This fact can be related to difficulty to obtain the exfoliation or even the intercalation of the nanoclay because of the rigid three-dimensional structure of the phenolic resin molecules, particularly, with resols. [10] To solve this problem, Usuki et al. [11] attempted to synthesize phenolic resin/layered silicate nanocomposites by an intercalative polymerization approach, employing phenol and formaldehyde and a modified montmorillonite. Lee et al., [10] as well as, Choi et al. [12]- [13] synthesized phenolic resin/layered silicate nanocomposites by melt intercalation of a linear novolac. Currently, only fairly limited researches on resol type phenolic resin layered silicate nanocompositesvia in situ polymerization of monomers -have been published. [14]- [16] In fact, it was showed that it is more difficult to obtain exfoliated structures employing resol type phenolic resins than using novolac type phenolic resins. According to literature, this situation can be explained considering that, even before the cure, resols generally possess a more crosslinked structure than novalacs. [17] However, up now, the mainstream approach to process phenolic resin layered silicate nanocomposites, is based on the in situ polymerization of monomers i.e., phenol and formaldehyde molecules need to be absorbed into the interlayer spaces of the nanoclay and then, polymerized in situ. [18]- [19] In other words, with this technique -based on a high labor cost chemical approach -it is necessary to employ both phenol and formaldehyde as starting materials.

In this research, PLSN were obtained using a commercial grade resol diluted in methyl alcohol. The employed processing technique was based on a simple mechanical approach (i.e., using a mixing tool) procedure used quite successfully in the preparation of nanocomposites based on other matrices, such as polyester resins. Literature surveys have shown that this is not an usual way to process resol type phenolic matrix nanocomposites. In fact, according to Kaynak and Tasan [20] it seems that up to now, only in their work, the mixing of the clay with the liquid resin was employed as a method to produce PLSN. To our knowledge, no others studies seem to involve the production of resol based nanocomposites. In our research, three processing techniques were evaluated and, in one case, even the effects of an ultrasonic source were considered. The nanocomposites obtained using these three mixing methods were examined by XRD, TGA and SEM analysis.

Experimental Section

Materials

The matrix chosen for the production of nanocomposite specimens was a single stage phenolic resin -a resol -diluted in methyl alcohol (kindly provided by SHA). The nanoclay was Cloisite® 30B, an organic treated MMT with methyl tallow bis-2-hydroxylethyl quaternary ammonium (Southern Clay Products).

Determination of the curing cycle of the neat resol

Even if phenolic resins are the oldest thermosetting polymers, their processing is rather difficult. [START_REF] Stephen | [END_REF] In fact, since during the crosslinking process of phenolic matrices the by-product of the polycondensation reaction is mostly water, voids and fractures can be formed during the processing. Moreover, also the solvent in which the resol is diluted may contribute to complicate this problem. Generally, because of their features, resols can be cured both applying a source of heat or also, employing a curing agent. In this work, in order to simplify the production of the nanocomposite samples, phenolic resin was processed by the application of heat. Since during the reaction there is no release of additional gas due to the curing agent, this approach allowed to minimize the creation of voids and fractures. Another advantage of this curing approach is that, excluding the use of an hardener, very short gel times, which may inhibit the release of gas, are avoided.

The first experimental step of this research was to study the curing cycle of the pristine matrix. Preliminary process parameters were acquired performing a series of Differential Scanning Calorimetry (DSC) analyses on the resol. Measurements were performed on Mettler-Toledo 822e Model. However, in order to minimize the presence of voids and fractures in the cured matrix, the effects of the applied temperature and pressure profiles during the curing cycle, were studied.

First, the resol was polymerized at atmospheric pressure. The following temperature profile was used: 24h @ 60°C and then heating up to 180°C at an heating rate of 10°C/30 min. The obtained specimens are shown Fig. 1(a) it is clearly visible that these samples are characterized by the presence of a few macro fractures. In order to obtain a lower degree of voids, it was decided to process the resol under vacuum. The same curing cycle was applied. As a result, the obtained samples were characterize by a great amount of micro bubbles and cracks, which, as is possible to observe in Fig. 1(b), were uniformly distributed throughout the bulk of the sample. According to the obtained results, the use of vacuum during the curing cycle, simply introduced another kind of problem. After many different trials, we came to the conclusion that the use of vacuum was not necessary, but conversely, the key approach to obtain a fractures and voids free sample was more related to the temperature profile. Particularly, it was observed that, in order to obtain a good cured material, the most important thing to do was simply to enlarge the first step of the curing cycle -i.e, to delay the gelling of the resin -keeping the resol at a temperature of 60°C for a longer time. In such a way, the production and the trapping of gas was minimized. The main drawback of such curing cycle was that, in order to obtain a sample with no void, very large processing time was necessary. The complete cure of the material required, in fact at least 5 days. Such processing time is not usual for standard series production, but, for the purpose of the study, and in general, for the production of extreme temperature resistant materials, it can be surely considered more than acceptable.

A photograph of a neat resol specimen cured with the optimized cycle, is showed in Fig. 2. From this pictures it is possible to observe that the samples produced with the optimized curing cycle were fractures and voids free. In order to better evaluate the effectiveness of such cure cycle, Scanning Electron Microscopy (SEM) was used, a Zeiss Supra 25 SEM was employed, and SEM pictures of a fracture surface of the obtained sample are shown in Fig. 3 and Fig. 4. It is possible to observe that the SEM analyses on the fracture surface related to the well cured neat resol specimen, confirmed that no bubbles or in general defects were generated during the cure. This is a remarkable result especially considering that SEM was not able to detect any voids.

In order to evaluate the degree of cure obtained by the employed curing cycle, DSC tests were performed on the neat resol. In particular, dynamic scans from -30°up to 350°C at an heating rate of 10°C /min. were performed. In order to obtain a readable signal it has been necessary to utilized pressurized pans. Two Figure 4: SEM detail of a fracture surface referred to a neat resol sample prepared using a good curing cycle. Even, at higher magnification no defects could be found. dynamical scan on the liquid and on the cured neat resol, are shown in (Fig. 5). From a numerical point of view, the obtained degree of cure related to the specimens treated with the selected temperature profile was 94.3%.

Preparation of the nanocomposites

In order to produce the nanocomposites, three processing techniques were considered. In the first one a mixture of resol plus the nanoclay was processed using a mechanical mixer set up at a speed of 2,000 rpm. We will refer to this blend as sample 1. In the second process the blend was prepared using the same mixer, at the same speed, but in combination with an ultrasonic source provided by an ultrasonic bath (sample 2). For these two samples, the same mixing impeller (Heidolph RZR 2041) was used. The third blend was processed employing a higher speed mixer i.e., 30,000 rpm (sample 3) which was equipped with a smaller diameter stirring impeller. More in details the ratio between the diameters of the tools used in the first two blends over the one used in the third blend was about 1:3.

In all the cases, the blends were prepared according to the following procedure: first the resol was weighted and then, the selected amount of nanofiller was added. Before its use, the nanoclay was spread on an aluminum plate and then, dried for 48 hours at 60°C. The nanoclay percentage was always 5% in weight (resol:nanofiller, 95:5) and, the mixing time was 3 hours. All stirring operations were performed at room temperature. In order to remove the air introduced in the blends during the mixing process, all mixtures were degassed for 40 min at a pressure of 0.1 atm. Then, the produced blends were polymerized employing the same curing cycle used for the neat resol.

Results and discussion

According to the optical analysis, all the produced specimens showed no presence of voids or fractures. Furthermore, no phase separation between the matrix and the nanoclay could be observed. Pictures related to the three produced blends are showed in figure (sample 1, Fig. 6 ). It should be underlined that, the cure cycle adopted, was characterized by a very long residence time under the gel point, such situation, promoting the phase separation between the nanofiller and the matrix, can seriously compromise the homogeneity of the nanocomposites. [20] Therefore having obtained homogeneous and good quality samples can be considered as a very remarkable result.

SEM characterization

In order to confirm the above statements, the morphology and the dispersion of the clay sheets in the he produced nanocomposites were studied by means of Scanning Electron Microscopy. For each sample studied, two kind of SEM images were produced. The first one, taken at a resolution of 20µm, was used to observe the overall dispersion of the clay in the matrix. Then, in order to evaluate the degree of disassembly of the stacked layers, other SEM images were acquired at a deepest magnification i.e., 2µm. The SEM pictures obtained for the nanocomposites produced with the three different techniques, are shown in figures 9 through 14. It is possible to observe from the pictures taken at lower magnification, that the homogeneity of the dispersion was of good quality (blend 1, Fig. 9, blend 2, Fig. 10 and blend 3, Fig. 11). Higher resolution pictures (sample 1, Fig. 12, sample 2, Fig. 13 and sample 3, Fig. 14) confirmed that the platelets have lost their original ordered stacked structure to assume a disordered intercalated structure. All the produced nanocomposites did not show any presence of voids as in the case of the neat resol, confirming that, the curing cycle used to polymerize neat matrix yielded good quality samples also in the case of the nanocomposites.

XRD characterization

Figure 15 shows the X-ray diffraction patterns of the produced blends. In order to evaluate the de degree of disaggregation of nanoclay tactoids, the XRD profile of neat nanoclay was used as reference. According to experimental results, the basal spacing of the Cloisite® 30B is about 1.79 nm. Concerning the evaluated blends, in all of them, the interlamellar spacing between the nanoclay sheets was increased when the nanofiller was introduced and mixed into the phenolic matrix. Although an increase of the interlayer distance, or the absence of a peak in the X-Ray diffraction pattern, can be considerer a necessary, but not sufficient condition for the exfoliation of nanoclay [START_REF] Koo | Polymer Nanocomposites: Processing, Characterization, and Applications[END_REF], the increase of the d-spacing observed in the figure coupled with the indication of the SEM give an encouraging insight that the resin molecules have penetrated into the layers of the nanoclay i.e, that a certain degree of intercalation or even exfoliation was obtained. Regarding the employed processing techniques, XRD measurements confirmed the effectiveness of all three mixing methods. The contribution of low power ultrasounds bath (second technique i.e., sample 2) did not result very relevant. Finally, XRD plot related to nanocomposite obtained with the highest speed mixing (sample 3) showed no remarkable differences with the other two. Concluding, according to XRD analysis, it resulted that the simplest technique -using the 2,000 rpm mixer -could be considered the most appropriate. In fact, this approach showed the minimum ratio between complexity of the processing technique and obtained results.

Thermogravimetric analysis

In order to evaluate the thermal behavior of the produced materials as a function of temperature, a series of thermogravimetric analyses was performed. In order to get a good repeatability, nanocomposite specimens were milled up to get a narrow particle diameter distribution of about 200µm. Then, samples of about (10±2)mg were characterized. The measurements were performed in a not oxidative atmosphere (i.e., in nitrogen), applying a linearly growing temperature ramp, at an heating rate of 20°C/min. Measurements were performed on Seiko Exstar Fig. 17 shows the corresponding DTA traces. According to the obtained results, all produced nanocomposites showed a very similar behavior until about 350°C. In this range of temperatures, the degradation rate of all nanocomposites was generally lower than the neat resol. Over this temperature, sample 1 and sample 2 started to exhibit a worse performance than the neat resol i.e., a greater degradation velocity. Furthermore, after this temperature, sample 3 showed a degradation rate very close to neat resol. However, once a temperature of about 425°C, was reached, the third nanocomposite began to loose its mass at a velocity lower than each other material. At a temperature of about 475°C, even sample 1 and sample 2, began to show a lower degradation rate than the pristine resol. Then, starting from this temperature up to 1,000°C, all nanocomposites always shown a lower degradation velocity than the neat matrix.

A similar degradation behavior, was found by other researchers, such as by Choi et al., [START_REF] Choi | [END_REF]. At date, the decrease in thermal stability of neat resol loaded with silicate is not clear.

However, because of its thermal features, sample 3 can be considered the most performing material. In fact, the thermal stability of this nanocomposite was superior than the other studied systems and finally, even its residual mass at 1,000°C was slightly greater than any other material.

If we define the oxidation temperature T o as the value at which the derivate of the weight W with respect to T is maximum i.e., referring to T o as the temperature of the maximum rate of oxidation, it is possible to affirm that the effect of the nanoclays on the thermal stability of the system is appreciable only on the degradation rate and not on the temperature T o.

Conclusions

In this research three different kind of nanocomposites based on a liquid, commercial grade resol were prepared. They were produced using a simple mechanical approach i.e., using a stirring tool. The main advantage of such process lays in the fact that it was not necessary to employ the intercalative polymerization of phenol and formaldehyde. Because of the nature of phenolic resins, the first step of the research was related to the optimization of a suitable curing cycle, which could avoid the production of fractures and voids in the specimens. An effective temperature profile for the cure of the neat matrix was found and, the same method, was successfully employed for the nanocomposites. All the produced neat resols and nanocomposite samples showed no voids, even at a micrometric scale. Furthermore, even if this curing method was based on the extension of the gel time, in no case, this condition led to phase separation between the nanoclay and matrix. SEM results showed that all three mixing techniques were effective i.e., they were able to promote good dispersion as well as distribution of the clay platelets. The intercalation of the nanoclay sheets was confirmed by X-ray diffraction analyses. In comparison with pristine resol, TGA measurements showed a lower degradation rate of all nanocomposites until about 350°C and over about 475°C. However, only nanocomposite produced with the higher speed mixing showed a residual mass slightly greater than neat resol.

Figure 1 :Figure 2 :

 12 Figure 1: Cured specimens of the neat resol prepared employing a unfavorable processing technique. Fig.1(a) refers to a typical sample obtained with a temperature profile leading to a too short gel time. Fig.1(b) shows a representative neat resol specimen prepared applying both an unfavorable curing cycle and pressure.

Figure 3 :

 3 Figure 3: SEM scan of a fracture surface related to a well cured neat resol specimen. No fractures or voids could be detected.

Figure 5 :

 5 Figure 5: Results of DSC analysis referred to neat resol.

  (a) and Fig.6(b)), figure (sample2, Fig.7(a) and Fig.7(b)) and figure (sample 3, Fig.8(a) and Fig.8(b)

Figure 6 :

 6 Figure 6: Characteristic appearance of a specimen prepared with the blend produced with first technique which employs a 2,000 rpm mixer. Fig.6(b) fracture surface of the sample. No phase separation between matrix and the filler could be observed.

Figure 7 :Figure 8 :

 78 Figure 7: Typical specimen prepared with the blend produced using a 2,000 rpm mixer combined with an ultrasonic source. Fig.7(b) fracture surface of the sample. No phase separation between matrix and the filler could be observed.

Figure 9 :

 9 Figure 9: SEM picture of nanocomposite 1. Nanoclay platelets are homogeneously distributed. No clay agglomerations could be detected.

Figure 10 :

 10 Figure 10: SEM picture of nanocomposite prepared with second technique. Nanoclay sheets are both finely dispersed as well as distributed in the matrix. No clay agglomerations could be detected.

Figure 11 :

 11 Figure 11: SEM picture of nanocomposite prepared with the higher speed mixer. Even in this situation, nanoclay platelets are uniformly dispersed.

Figure 12 :

 12 Figure 12: Detail of some nanoclay platelets dispersed in the first blend. The scale is the same of Figure 4.

Figure 13 :

 13 Figure 13: Detail of some nanoclay platelets of the second sample.

Figure 14 :

 14 Figure 14: Detail of some nanoclay platelets of the third blend.

Figure 15 :

 15 Figure 15: X-Ray patterns of the produced nanocomposites.

Figure 16 :

 16 Figure 16: TGA traces of the produced nanocomposites.

Figure 17 :

 17 Figure 17: Degradation rate trends of the produced nanocomposites. Sample 3 shown the best resistance to thermal degradation.

Preprint submitted to Composites Science and TechnologyDecember 28, 2009


A Delcamp

L Maillé

S Saint

R Pailler

A Guette

Al-O-N AND Al-O-B-N THIN FILMS APPLIED ON Si-O-C FIBERS

Keywords: A. Coating, A. Ceramic-matrix composites, B. Mechanical properties, Oxidation

Protective coatings (Al-O-N and Al-O-B-N) on Si-O-C fibers (Tyranno ZMI) were applied in order to enhance oxidation resistance under severe thermo-mechanical conditions in the 400-600°C temperature range. The coating process consisted in three steps: (i) the transformation of the Si-O-C fiber surface into microporous carbon; (ii) the impregnation of these carbon microporous layers by an aluminium trichloride (AlCl 3 ) solution and then, (iii) a final heat-treatment under ammonia. Processing parameters were studied in order to select the best conditions. Using these conditions, obtained results have shown that coatings were present around each fiber, with a controlled thickness, and that the mechanical properties of the fibers were preserved. Although, these coatings did not entirely stop the oxygen ingress, it has been shown that they strongly reduced the oxidation of the fiber.

Introduction

New environmental rules as well as economical competition have constrained the aeronautic industry to develop new products with low structural weight and ceramized engines in order to reduce NO x and CO 2 emissions. This is the result of a better combustion and enables also a reduction of aircraft noise.

In order to reach these objectives ceramic matrix composites (CMC) have been selected since they display excellent thermo-mechanical properties as well as low densities [START_REF] Naslain | Introduction aux matériaux composites, 2-Matrices métalliques et céramiques[END_REF].

The aim of the present study was focused on developing low cost raw materials and processes in order to be competitive on the civil aeronautic market. Low cost ceramic fibers such as Tyranno ZMI fibers were therefore selected [START_REF] Shibuya | 9th Cimtec World Forum on New Materials -Symposium V -Advanced structural fiber composites -Florence[END_REF]. When used in an oxidizing environment at elevated temperature, it has been shown that the use of a self healing matrix usually provides an accurate protection of the CMCs against oxidation [START_REF] Naslain | [END_REF]. However, when moderate temperatures are considered, self healing matrices have proved to be not so efficient. Hence, when CMCs exhibit matrix cracks, these cracks are not entirely healed because of the low rate of formation of the oxide. Further oxygen diffuses into the fibers, leading therefore to a catastrophic failure of the CMC [START_REF] Filipuzzi | Oxydation des composites SiC/SiC et de leurs constituents : approche expérimentale, modélisation et influence sur le comportement mécanique[END_REF]. In order to prevent any propagation of the oxygen up to the fibers, and therefore to enhance the lifetime of these materials, it has been decided to set up a protective oxidation barrier on the surface of the Si-O-C fibers. This layer should posses all the following distinct features: to provide an individual and uniform coating around the fibers, while avoiding bonding between them; to prevent any oxygen diffusion to the fibers; to be stable during the processing of the matrix and of the interphase; to present a good adhesion with both the fibers and the interphase; and to preserve the mechanical properties of the fibers. Different materials may be considered regarding these expected properties: refractory oxides such as ZrO 2 or Al 2 O 3 [START_REF] Baklanova | [END_REF][6]; silicon based ceramics such as Si-B-C-N, Si-C-N [7][START_REF] Kern | 27th International Cocoa Beach Conference on Advanced Ceramics Composites, COCOA BEACH -FL, Book Series[END_REF] or Si-Al-C-N [9][10][11], monazite [12][13][14][15], BN [16][17][18][19][20][21] and LaAlO x [22][23][24][25][26].

Two new materials were selected for the present work: Al-O-N and Al-O-B-N [27][START_REF] Gogotsi | US Patent[END_REF][29][30]. As a matter of fact, the aluminium as an element exhibits a low density as well as a stable oxide (Al 2 O 3 ). Besides, most nitrides have attractive refractory properties. Boron was also selected as an adequate element because of an important property of all boron containing species which is to form fluid oxide phases (B 2 O 3 ) over a broad temperature range (600-1200°C) in an oxidizing atmosphere. The Al-O-N-B material could thus be used to design self-healing composites with improved lifetimes. It bases on the in-situ formation of the fluid oxide, which fills the matrix cracks and slows down the in-depth diffusion of oxygen [3;31].

Experimental procedure

Thin coatings deposition

As already mentioned, Tyranno ZMI Si-O-C fibers (from UBE Industry, Japan), were used for this study. The microstructure of this first generation of Si-O-C fibers is composed of β-SiC crystallites (2nm), free carbon and a Si-O-C phase resulting from the precursor reticulation step [START_REF] Coustemer | AMAC-CODEMAC Bordeaux-France[END_REF]. The chemical composition of these fibers (Si 37.7 at.%, C 53.4 at.%, O 8.8 at.% and Zr 0.1 at.%) has been determined using an electron probe microanalyser (SX 100 from CAMECA). The average fibre diameter is 13µm.

In order to promote an adequate adhesion between the Al-O-N or Al-O-N-B thin coatings and the fiber surface, microporous carbon layers with a controlled thickness were first produced, using a thermochemical treatment around 600°C with chlorine gas [START_REF] Mcnallan | [END_REF]. This specific treatment leads to an extraction of Si from SiC by chlorine at the fiber surface, according to the following reaction 1 [34]: SiC (s) +2 Cl 2(g) = SiCl 4(g) +C (s) . The microporous carbon thus obtained was found to possess a specific surface area, calculated using the Brunauer-Emmet-Teller equation, of 994m 2 /g. Most of the pores size (> 91%) are smaller than 2nm and the pore volume measures 0.45cm 3 /g (these values were calculated using the semi-empirical Horvath and Kawazoe approach). After this heat treatment, fibers were impregnated with an anhydrous aluminium trichloride (AlCl 3 ) solution.

Only one impregnation was performed during 15 minutes, at ambient temperature, using a batch process. The samples were then heated up to about 120°C in order to evaporate the solvent, and a thermal treatment in an ammonia atmosphere during 60 minutes, at a constant flow rate of 20 sccm and at various temperatures (900, 950 and 1000°C), was finally performed.

In the case of the Al-O-B-N thin coatings, the fiber preforms were impregnated with boron acid prior to the impregnation with an aluminium trichloride solution. A heat treatment at 1150°C during one hour was then performed under an ammonia flow.

Three different types of composite materials were produced and tested: composites C1 only made with a pyrocarbon interface and a SiC matrix, considered as the reference material; composites C2 made with Al-O-N thin coatings and a SiC matrix; and composites C3 made with a pyrocarbon interface, an Al-O-N thin coating and a SiC matrix.

Thin coatings characterization

Both structural, chemical and mechanical characterizations were performed for each kind of coating.

The cross-section morphology was investigated with the aid of a high resolution Scanning Electron Microscopy (SEM, Hitachi S4500). Coating thicknesses as well as the quality of their adhesion to the fibers were studied by the observation of the fracture surfaces.

Microstructural characterizations of the various coatings were performed with a Philips CM30ST (LaB 6 , 300kV) Transmission Electron Microscope (TEM). The chemical composition and thicknesses of the coatings were evaluated using an Auger Electron Spectroscopy (VG Microlab 310 F) coupled with an argon sputter device. The analysis of the atomic content of each element was determined in the depth profile mode.

Room temperature fiber strength was determined by tensile tests performed on single fibers according to a procedure describe in [START_REF] Delcamp | Protection de fibres base SiC pour composites à matrice cérammique[END_REF]. These tests were performed on twenty fibers for each sample, with a gauge length of twenty five millimetres. Each fiber extracted from a tow was fixed on paper frame holders using a hard acrylic resin. Ultimate tensile strength (σ r , in GPa), strain to rupture (ε r , in %) and Young's modulus (E, in GPa) of the Si-O-C fibers were derived from these tests. Mechanical tests on single ply (2D fiber weaves) with a SiC matrix were also performed.

Static oxidation tests were performed at 600 °C in air during 7000 hours. Silica thicknesses developed at the surface of the fibers were determined by SEM after a few weeks.

Results and discussion

Various processing parameters related to the impregnation and the nitridation have been studied.

The impregnation process

As mentioned before, aluminium compounds were deposited at the fiber surface through the impregnation by an aluminium trichloride solution (AlCl 3 ) of microporous carbon first created at the ZMI fiber surface by chlorination [27-28;36-37]. In this respect, in two fluids were tried to solve the AlCl 3 powder, namely water and ether.

Although water proved to be a good solvent for hydrated AlCl 3 powder, this solution yet caused an important introduction of oxygen into the coating. Conversely, ether also proved to be a good solvent when using anhydrous AlCl 3 powder, and consequently forms a coating with a lower oxygen content. Ether was therefore preferred to impregnate the aluminium trichloride solution.

In a first attempt to impregnate a maximum of aluminium compounds into the microporous carbon, a saturated aluminium trichloride solution was realised. Yet, the high viscosity of this solution did not easily allow the impregnation of the fiber preforms by the aluminium compounds, as illustrated by the high resolution SEM image exhibited in figure 1. This which shows that thin coatings at the fiber surface were not homogeneous, with an excess of matter deposited at the fiber surface. It was therefore then decided to use only thin coatings requiring a lower concentration of aluminium trichloride solution. As expected, a homogeneous microstructure could thus be obtained (figure 2 a) in which no bridge between Si-O-C fibers could be evidenced (figure 2 b). These amorphous coatings, which formed an individual and uniform coverage around each fiber, could thus be expected to efficiently protect the fiber surfaces.

The nitridation process

Nitridations were performed on samples previously impregnated with AlCl 3 solutions at 900°C following the method described by L. Chen et al. [27;36-37], and then extended at 950°C and 1000°C.

The samples thus obtained were also analysed by Auger electron spectroscopy. Results, shown in figure 3, confirm that the impregnation of the aluminium compounds into the microporous carbon network was quite successful. However, the chemical compositions presented in tables 1 and 2 reveal that although all silicon compounds should have been consumed (after equation 1), it was yet possible to observe the presence of this element but in low amounts (figure 3).

Variations in the concentration of the chemical elements (Al, N, O, B and C) present in the coatings were observed according to the nitridation temperature. Besides, these Auger Electron Spectroscopy analyses have also shown that the thickness of the nitrated coatings tend to decrease as the temperature of the nitridation process was increased (figure 3). This last phenomenon can be explained by the reaction occuring between ammonia and carbon which leads to the formation of HCN and to a carbon depletion [START_REF] Van Dijen | [END_REF]. A densification of Al-O-N-B and Al-O-N coatings and therefore a decrease of their thickness, were thus observed after using higher nitridation temperatures. In addition to their densification and concomitant carbon consumption, it has been evidenced that, in the case of Al-O-N thin films, the aluminium and oxygen content increased with the temperature of nitridation, as presented in table 1. These last observations may be explained by a de-mixing process in gradually taking place in the coating as the nitridation temperature was increased, leading to the formation of Al 2 O 3 while liberating nitrogen. Conversely, in the case of the Al-O-N-B coatings, such phenomena were not observed (table 2). Finally, high resolution TEM showed that both thin films were homogeneous, amorphous (no ring patterns of selectedarea electron-diffraction were observed) and adhere to the fibers.

Mechanical properties and oxidation behaviour

Some mechanical properties (Young's modulus, stress and strain to rupture) of single fibers coated with Al-O-N and Al-O-N-B thin films were studied. Results are reported in tables 1 and 2.

Whereas heat treatments performed up to 950°C did not degrade the mechanical properties of the various coated ZMI fibers, both stress and strain to rupture of the fibers coated by Al-O-N and nitrated at 1000°C were found to decrease. This phenomenon can be explained by a reaction between the ammonia and the free carbon contents of the fibre as well as by the aforementioned a de-mixing process occuring in Al-O-N when the temperature of the nitridation process is increased.

On the other hand, stress and strain to rupture of Al-O-N-B coated fibers, nitrated at 1000°C, proved to be stable. Indeed, at this temperature the ammonia reacts preferentially with boron oxide to form boron nitride and water. This formation of boron nitride, which acts as an interphase (confirmed by TEM in figure 4), permits to preserve the mechanical properties of the fibers. Indeed, TEM analyses showed that these Al-O-N-B thin coatings were amorphous, and mainly composed of aluminium, boron, nitrogen and boron compounds with few nano-crystals of BN and Al 4 B 2 O 9 .

Results of the mechanical tests performed on different single ply composites made of treated ZMI fiber in a CVI-SiC matrix are presented in figure 5.

Composites C2 did not present acceptable mechanical properties. Indeed, the observed very limited non linear domains tend to evidence an inherent brittleness (with strains to rupture less than 0.4%). These results can be attributed to a noticeable degradation of the fiber as compared to the reference material C1. Conversely, samples C3 with both an Al-O-N coating and a pyrocarbon interface did not reveal any degradation of the mechanical properties, as compared to the reference material. It may thus be concluded that the pyrocarbon interphase is still necessary to obtain an overall non linear/non brittle behaviour.

Results of static oxidation tests are presented in figure 6

Conclusion

Two different protective coatings, Al-O-N and Al-O-N-B respectively, have been applied

on Si-O-C fibers in order to enhance their oxidation resistance under severe thermomechanical conditions. Aluminium and nitrogen were introduced into a microporous carbon surface by impregnation of an aluminium trichloride solution, followed by a heat treatment and by a nitridation of these thin films (consisting of a thermal treatment in an ammonia atmosphere). Auger spectroscopy and SEM associated to an EDS detector have confirmed the effectiveness of the presence of Al-O-N and Al-O-N-B thin coatings at all the fiber surfaces. These coatings were sufficiently homogeneously distributed to allow the absence of bridging between the fibers. The mechanical compatibility of these applied coatings have been assessed with the help of tensile tests performed on single fibre as well as on composites produced from a 2D fabric containing the fibre with the different coatings mentioned above after impregnation with SiC matrix.

Obtained results have shown that, in both cases, the coatings preserve the mechanical integrity of the fibers, if an additional pyrocarbon interphase is present. Oxidation experiments at 600°C showed that the different coatings considerably decrease oxide scale formation on the fibers which will lead to an increase of the lifetime of the ceramic matrix composites.

List of figure and table captions

  . The usual passive oxidation of SiC based fibers leads to the formation of a silica layer. The presence of Al-O-N or Al-O-N-B coatings has resulted in an important reduction of the silica scale. However, neither Al-O-N nor Al-O-N-B coatings were able to totally prevent the oxygen ingress towards the fiber surface.

Figure 1 :

 1 Figure 1: High resolution SEM image of thin coatings at the fiber surface with a saturation

Figure 2 :

 2 Figure 2: High resolution SEM images of thin coatings with a concentration of aluminium

Figure 3 :

 3 Figure 3: Auger spectroscopy analyses of thin coatings after nitridation at a) 900°C, b)

Figure 4 :

 4 Figure 4: TEM images of Al-O-N-B thin coatings: a) Bright field image; b) High

Figure 5 :

 5 Figure 5: Tensile stress-strain curve of a simple 2-D weave Si-O-C fiber reinforced SiC as

Figure 6 :

 6 Figure 6: Silica thicknesses (x) created at 600 °C in air during forty weeks, on Tyranno

d 2 :

 2 Thickness of Al-O-N or C coatings, φ: diameter of fibre; r : stress at rupture; E: Young's modulus; r : strain to rupture

Figure 1 27

 1 Figure 1

Table 1 :

 1 Thicknesses and mechanical properties of fiber with Al-O-N thin coatings after different nitridation temperatures.

	d 1 :Thickness of Al-O-N or C coatings; φ: diameter of fibre; r : stress at rupture; E:
	Young's modulus; r : strain to rupture

Table 2 :

 2 Thicknesses and mechanical properties of fiber with Al-O-N-B thin coatings after different nitridation temperatures.

Table 1

 1

	Fiber	d 1	φ φ φ φ	Chemical composition (at.%)	r	E	r
		(nm) (µm) C	O	Al N	Si (GPa) (GPa) (%)
	without any treatment	-	13.0						3.2 ± 0.7	210 ± 8	1.5 ± 0.3
	with the carbon coating	560 ± 46	13.0						3.1 ± 0.3	162 ± 6	1.9 ± 0.3
	With Al-O-N
	coating after 900°C nitridation	246 ± 13	12.3 63 15 15	5	2	3.4 ± 0.4	178 ± 22	1.9 ± 0.3
	treatment
	With Al-O-N
	coating after 950°C nitridation	129 ± 12	12.1 11 38 29 16	6	3.2 ± 0.4	± 8	1.7 ± 0.2
	treatment
	With Al-O-N
	coating after 1000°C nitridation	39 ± 15	11.9 10 47 30	9	4	2.5 ± 0.8	198 ± 16	1.3 ± 0.4
	treatment

Table 2

 2

	Fiber	d 2	φ φ φ φ	Chemical composition (at.%)	r	E	r
		(nm) (µm) C	O	Al N	B (GPa) (GPa) (%)
	without any treatment	-	13.0					3.2 ± 0.7	210 ± 8	1.5 ± 0.3
	with the carbon	560						3.1 ±	162	1.9
	coating	± 46 12.3					0.4	± 6	± 0.3
	with Al-O-N-B
	coating after 1000°C nitridation	180 ± 13	12.2 15 45 17 18	5	3.0 ± 0.4	205 ± 22	1.5 ± 0.2
	treatment
	with Al-O-N-B
	coating after 1050°C nitridation	140 ± 16	12.1 15 25 17 25 18	3.0 ± 0.4	200 ± 21	1.5 ± 0.2
	treatment
	with Al-O-N-B
	coating after 1100°C nitridation	70 ± 10	12.0 15 15 10 30 30	2.8 ± 0.5	185 ± 7	1.5 ± 0.3
	treatment

Acknowledgments

The authors acknowledge the financial support of ADEME for this work and M. Lahaye and P. Weisbecker for Auger electron spectroscopy and Cross Polishing respectively.