Patrice Berthod
email: patrice.berthod@centraliens-lille.org

Jean Lamour

High temperature properties of several chromium-containing Co-based alloys reinforced by different types of MC carbides (M=Ta, Nb, Hf and/or Zr)

Keywords: High temperature alloys (A), Microstructure (C), Oxidation (C), Thermal analysis (D), Scanning electron microscopy (D)

Five cast cobalt alloys based on Co-8Ni-30Cr-0.4/0.45C and containing Ta, Nb, Hf and/or Zr were studied by metallography in the as-cast condition and after treatments at 1300°C. The obtained MC carbides were all interdendritic with a eutectic script-like morphology. For similar carbon contents, the HfC carbides are the most developed in the as-cast microstructure and the most stable at 1300°C. As-cast, the TaC carbides are less developed than the former and they tend to become more fractioned and less present in microstructure at 1300°C. The NbC carbides, which have initially the same morphology and the same fraction as TaC, rapidly dissolve at 1300°C. The cobalt alloys containing HfC or TaC are chromia-forming at 1300°C. The NbC-containing alloy catastrophically oxides after only few hours at 1300°C. The average hardness is the highest for the HfC-containing alloy and the lowest for the NbC-containing alloy.

Introduction

The strengthening of some of the alloys or superalloys designed for high temperature applications is due to carbides, and especially to MC carbides which are of the most stable at high temperature (around 1200°C). Among them, the most used in cast alloys is TaC carbide [START_REF] Sims | The Superalloys[END_REF], notably in cobalt-based superalloys [START_REF] Beltran; In | Superalloy II -High temperature materials for aerospace and industrial power[END_REF][START_REF] Michon | [END_REF]. Tantalum is a carbide-former element which is more efficient than chromium. Therefore TaC carbides can be easily obtained in Cr-rich cobalt alloys instead of chromium carbides [4]. It is also true in chromium-rich ferritic or austenitic alloys based on (Fe,Cr) or (Fe,Ni,Cr) [5,6] but TaC are more difficult to obtain in (Ni,Cr)-based alloys [7].

Among the other MC carbides which can be used for the strengthening of alloys at high temperatures, one can cite firstly NbC. The niobium carbides, which can induce an increase in both hardness and wear resistance of steels [8] or cermets [9], are able to improve the mechanical properties at high temperature of refractory alloys or intermetallics [10]. The HfC carbide is among the most stable ones at very high temperatures. Hafnium carbides are notably used for the mechanical reinforcement of very refractory alloys, based on molybdenum, tungsten and/or rhenium [11][12][13]. The zirconium carbide (ZrC) is another MC carbide which is also used in alloys for applications at very high temperatures, for example in Mo-based or Nb-Mo based alloys [14][15][16].

TaC is the most used MC carbide for strengthening the cast cobalt-based superalloys, while NbC, HfC or ZrC seem being more rarely considered as strengthening particles in such alloys. The purpose of this work is to better know the ascast microstructures of Co-based alloys containing one or several of these MC-former elements, their refractoriness and their behavior when exposed at very high temperatures.

Experimental details of the study 2.1. The studied alloys

Five cobalt-based alloys, all containing about 8 wt.%Ni, 30 wt.%Cr, 0.4 wt.%C and different contents in M elements (M = Ta, Nb, Hf and Zr) were elaborated by high frequency (300kHz) induction melting under an inert atmosphere (0.3 bar of pure argon). The fusion of the charge (total mass ≅ 50g) and solidification of the obtained liquid alloy were realized in the water-cooled copper crucible of the furnace. These alloys are denominated as indicated in Table 1 which also gives the obtained chemical compositions of these alloys, determined by Energy Dispersion Spectrometry for all elements except carbon. The contents in the latter element were assumed being equal to the targeted values, on the one hand since the same elaboration procedure always leads to a very good respect of the carbon content as earlier verified several times by spark spectrometry analysis (technique efficient for this light element even in low contents), and on the other hand since this good respect of the targeted C contents is confirmed by the obtained carbides densities in the alloys studied here.

The obtained ingots were cut in order to obtain compact samples (less than 1 cm 3 ) for the as-cast microstructures examinations and for the high temperature treatments, as well as smaller parts (about 2 × 2 × 8 mm 3 ) to measure the intervals of fusion of the alloys.

Thermal analysis

Differential Thermal Analysis (DTA) experiments were performed on the five alloys, in order to measure their temperatures of fusion's start and of fusion's end. This was done using a Setaram TGA 92-16.18 apparatus, following a thermal cycle composed by a heating rate of 20 K min -1 up to 1,200 °C, a slower heating at 5 K min -1 up to 1,500 °C for a better accuracy in this zone of temperatures. The cooling was done first at 5 K min -1 down to 1,200°C, followed by a second one at 20 K min -1 down to room temperature. For the assessment of the solidus and liquidus temperatures of the alloys it was preferred here to consider the temperatures of fusion's start and fusion's end determined on the heating part of the DTA curve. Indeed, taking the values at solidification, or even the average values of start and end of fusion and solidification, may lead to minimized temperatures because of nucleation delay and undercooling during solidification.

High temperature exposures and metallographic characterization

The samples corresponding to the alloys which presented a melting temperature high enough (higher than 1300°C: "CoTa", "CoNb" and "CoHf"), were polished all around with 1200-grit paper. They were heated in a resistive tubular furnace up to 1,300°C, at approximately 20 K min -1 . A first sample was taken out the furnace at the end of heating, a second one after 5 hours spent at 1,300°C and the third one after 20 hours spent at 1,300°C. All of them were then air-cooled down to room temperature. After cutting in two equal parts (using a Buehler Isomet 5000 precision saw), and embedding in a cold resin (Escil CY230 + HY956), the mounted samples were polished, firstly by using SiC paper from 120 to 1,200 grit under water, and secondly with a 1µm-diamond paste.

A Scanning Electron Microscope (SEM, model: XL30 Philips) was used for metallographic observations, essentially in the Back Scattered Electrons mode under an acceleration voltage of 20kV. The region of interest was either the middle of the sample's bulk (characterization of the carbides evolution without influence of oxidation), or the zone affected by oxidation (for characterizing the surface degradation by oxidation). The chemical composition of each alloy was analyzed in the bulk of the as-cast alloy, by using the Energy Dispersion Spectrometry device of the SEM (except for carbon which is a too light element, and which was assumed being equal to the targeted values as already explained above).

The surface fractions of carbides were measured on four BSE micrographs randomly taken in the bulk of each sample (as-cast and for the three durations at 1,300°C). These micrographs were taken with the SEM in BSE mode and a magnification × 250 (areas of almost 0.1mm²). The BSE detector led to different levels of gray, depending on the average atomic number of the phases. After a careful rating of the brightness/contrast SEM parameters, the matrix (gray) and the MC carbides (whiter than matrix) were then well separated, what allowed the Photoshop CS software of Adobe measuring the surface fractions of carbides, after a rating of gray level avoiding to take into account also carbides close to the sample surface but not really emerging. Each final surface fraction, average of three measures, was supposed to be close to the volume fractions.

Hardness measurements

In order to know the hardness evolution with the 1300°C-treatment duration, macro-indentation was performed on the three alloys selected for the heat treatments, by using a Testwell Wolpert apparatus. Four Vickers indentations with a load of 30kg were performed in the bulk of each sample, far from the zone affected by oxidation.

Characterization of surface deterioration by oxidation

The resistance of the alloys against high temperature oxidation was characterized by observing the surface of cross sections of the samples exposed at 1300°C with the SEM in BSE mode, by measuring the depths or thickness of the zone affected by oxidation and by characterizing the oxide scales (thickness, nature and porosity). Electron Probe Micro Analysis (EPMA) was also performed, with a Cameca SX100 apparatus working by Wavelength Dispersion Spectrometry. A short profile was performed per sample through the entire external oxide scale while a longer profile was performed through the entire zone of alloy affected by oxidation. This allowed identifying the natures of the different oxides present.

Results

As-cast microstructures of the studied alloys

The as-cast microstructures of the five alloys are illustrated by SEM micrographs in Fig. 1. The "CoTa" alloy is composed of a dendritic matrix and TaC carbides forming a eutectic with matrix in the interdendritic spaces. The same features are met in the "CoNb" microstructure, with NbC carbides instead of TaC, and in the "CoHf" alloy with HfC carbides. The latter seem to be denser than the interdendritic TaC or NbC carbides of the two former alloys. In addition, one can notice the presence of coarser white particles, especially rich in Hf (and even probably being pure Hf). Indeed their WDS analyses with microprobe showed that they contain more than 95wt.% of Hf, while the other detected elements seemingly belonged to the subjacent matrix which often participates to the interaction with the electrons beam when the analyzed particles are too small. The "CoTH" alloy, which contains both tantalum and hafnium, is also composed of a dendritic matrix and interdendritic carbides, but the latter are less dense and less script-like than in the three previous alloys. The last alloy, "CTHZ" which contains Ta, Hf and Zr together, is especially rich in interdendritic carbides and intermetallic particles which cannot be analyzed because of their too fine structures.

Results of DTA experiments

Differential Thermal Analysis was carried out for the five alloys. Generally, two endothermic peaks were found in the heating part of the obtained curves while the cooling part of the curves displayed two exothermic peaks, as illustrated in Fig. 2.

The temperatures of fusion's start et fusion's end, which were taken as estimations of the solidus and liquidus temperatures, were measured at the beginning and end of heat absorption. Their values are given in Table 2. Except for the "CTHZ" alloy, all temperatures of fusion's start are equal to 1300°C or higher. In the increasing order, and by considering only the alloys containing one M element, one meets first the TaCcontaining alloy, then the NbC-containing alloy and finally the HfC-containing alloy. The alloy containing both Ta and Hf starts melting at almost the same temperature as the "CoTa" alloy. The "CTHZ" alloy, which is obviously too rich in M elements, is the less refractory of the five alloys since it starts melting near 1225°C. Only the alloys displaying both a sufficiently high temperature of fusion's start (equal or higher than 1300°C) and a simple structure of carbides (with only one M element in MC carbides) were considered for the exposures at 1300°C.

Behaviors of the bulk microstructures during a dwell at 1300°C

Three samples of each considered alloy ("CoTa", "CoNb" and "CoHf") were exposed at 1300°C for three different durations. The less exposed was air-cooled when it reached 1300°C (condition called "0h"), the second one remained at 1300°C during five hours ("5h") and the most exposed one staid at 1300°C during twenty hours ("20h"). The microstructure bulk behaviors of the three alloys were quite different from each other. The TaC network of the "CoTa" alloy, seemed already a little affected when reaching the targeted temperature. The fragmentation and rarefaction of the tantalum carbides were more marked after 5 hours, and, after 20 hours the script-like carbides were totally replaced by aligned blocky TaC carbides (Fig. 3). The same phenomena took place for the "CoNb" alloy, but faster since carbides were already fragmented when reaching 1300°C and they became especially rare after 20 hours (Fig. 4). In contrast the "CoHf" alloy was particularly less affected by time spent at 1300°C (Fig. 5). Indeed, the interdendritic carbide network still existed after 20 hours, even if the fragmentation of the HfC carbides was obvious (but less marked than in the "CoTa" alloy, and of course than in the "CoNb" alloy).

Three pinpoint WDS analysis were performed by EPMA in matrix in the centre of each sample, with calculation of the average value and the standard deviation taken as uncertainty (Table 3). It seems that the chromium content and the nickel content did not change with the exposure duration. In contrast, the Ta content in the "CoTa" matrix and the Hf content in the "CoHf" matrix both tend to decrease (with almost total disappearance of Hf in matrix), while the Nb content seems increasing in the "CoNb" matrix.

Behaviors of the alloys in oxidation at 1300°C

During the exposures at high temperature the alloys underwent a more or less severe oxidation. The metallographic characterization of the oxidized alloys led to see that the behaviors can be totally different between the three alloys (Fig. 6 and Table 4). The external oxide scale growing on the "CoTa" surface is rather thin at all times and contains an increasing thickness of chromia (Cr 2 O 3 ). Since this stoechiometric oxide is particularly protective, the oxidation of the alloy remains limited, even if a spinel oxide (CoCr 2 O 4 ) appeared between 0h and 5h and grew thereafter on the external oxide. Such spinel usually results from the reaction between an inner Cr 2 O 3 scale and a (Co,Ni)O outer scale, the latter formed at the first stage of oxidation (but which cannot be seen here probably because it was lost during cooling; in contrast this external divalent oxide is still present in the oxide scales of the two other alloys). Another oxide (internal CrTaO 4 ) grew with time inward the sub-surface: it is the unique penetration of oxidation which can be noted in this "CoTa" alloy. From the extreme surface of the "CoTa" alloy, a depletion in chromium developed inward the centre of the alloy, with a gradient of Cr content which decreases with time. The chromium content on the extreme surface of the alloy decreased from 16 wt.%Cr ("0h") down to 12 wt.%Cr ("20h"). The latter is a very low value which let think that the chromia-forming behavior of the alloy might be lost after only few supplementary hours, and therefore that catastrophic oxidation is imminent. Concerning the microstructure changes induced by oxidation in the alloy sub-surface, it can be noted that a carbide-free zone developed from the external surface, because of diffusion of tantalum toward the oxidation front (where it is oxidized into CrTaO 4 ), leaving carbides which were dissolving over an increasing depth in the alloy. One can notice that the first part of the chromium profile for "20h" (0.039 wt.%Cr × µm -1 ) seemingly corresponds to the carbide-free zone depth, while the inner gradient (three times lower than the first gradient) concerns an inner zone where carbides are still present.

The behavior of the "CoNb" alloy is totally different. Chromia existed only when temperature reached 1300°C, and the very low Cr content in extreme surface (7.2 wt.%Cr) explains why the chromia-forming behaviour was thereafter lost and replaced by a general oxidation which penetrated inward the alloy. This catastrophic behavior involved simultaneous oxidation of Cr, Co, Ni and Nb in more or less complex and more or less porous oxide. Chromia did not continue to diffuse and the Cr content remained still close to 30 wt.% at the moving frontier between the growing oxide and the part of alloy which was not yet oxidized.

The "CoHf" alloy displays a behavior which can be intermediate by comparison with the two previous ones. Chromia seems to be almost always present on surface, covered by the CoCr 2 O 4 spinel formed by reaction with the outer early (Co,Ni)O oxide (which was sometimes lost during cooling). The Cr gradient exists in the sub-surface of all samples, which proves that Cr continued diffusing toward the oxidation front. The chromium content in the extreme surface decreased down to 8.2 wt.% after 20h at 1300°C, which let think to a loss of the chromia-forming behavior few hours later. There is seemingly never a carbide-free zone in the sub-surface of the three "CoHf" samples, and then no correspondence between Cr gradient and carbide-free zone depth can be done.

Hardness of the alloys for the different treated states

Vickers indentation was performed four times per sample, with calculations of the average value and of the standard deviation (taken as uncertainty). Results are presented in the graphs displayed in Fig. 8. For an easier interpretation values (average ± standard deviation) of surface fractions measured by image analysis (on three BSE micrographs per sample) are added in these graphs. In the case of the "CoHf" alloy, two curves of surface fraction are plotted: one for the HfC carbides together with the Hf-rich precipitates, and one for only the HfC carbides (nevertheless the two curves are close to one another since the Hf-rich precipitates do not lead to important additional surface fractions). The hardness of the "CoTa" alloy follows the same tendency as TaC surface fractions, i.e. a continuous decrease from 310 down to 295 Hv 30kg , for TaC surface fraction decreasing from 7.7 to 4.3 surf.%. The same behavior, but drastically accelerated in time, can be seen for the "CoNb" alloy, with fast decreases in hardness (337 down to 254 Hv 30kg ) and in NbC surface fraction (4.2 down to 1.1 surf.%). For the "CoHf" alloy, the HfC surface fraction begins increasing from about 8 (as-cast) to about 10 surf.% ("5h") then decreases to about 8 ("20h"). This is the same evolution as the one of hardness that increases from 304 to 331 Hv 30kg , then decreases to 301 Hv 30kg ("20h").

Discussion

Thus, tantalum, niobium and hafnium lead to the same type of as-cast microstructure in a chromium-rich carbon-containing cobalt-based alloy: a dendritic matrix with script-like carbides forming an interdendritic eutectic with matrix. Although the carbon contents of these studied alloys are similar, different densities of interdendritic carbides were obtained. Obviously the carbide network obtained with Hf in the "HfC" alloy is more developed than the TaC network of the "CoTa" alloy, which is itself more developed than the NbC network in the "CoNb" alloy, although the atomic contents of M element (M=Hf, Ta, Nb) were the same. Surprisingly, the combination of Ta and Hf in the "CoTH" alloy, for a same total atomic content of carbide-former elements as the three previous alloys, led to less carbide (in the as-cast condition). The "CTHZ" alloy which contains three times the atomic content of M element in the "CoTa", "CoNb" and "CoHf" alloys, was especially rich in interdendritic carbides, and also in intermetallic compounds (mixed with the carbides) since the carbon content was the same as for all the other alloys. In the latter alloy the interdendritic compounds could be considered as matrix, instead the cobalt solid solution, because of its higher volume fraction.

The alloys with equal atomic contents for carbon and for the M elements present are the most refractory, with temperatures of fusion's start of at least 1300°C. When the atomic content of M elements exceeds too much the carbon atomic content, this temperature becomes significantly lower, as it is the case for other alloys in which M elements were less present than carbon and in which chromium carbides appeared in microstructures in addition to MC carbides [4].

When exposed at 1300°C, both morphology and surface fraction of carbides are changing. The decrease in surface fraction was fast for the "CoNb" alloy (with consequently an increase in Nb content in matrix), and obviously slower for the "CoTa" alloy (inversely the carbide fraction increased in the "CoHf" alloy). The fragmentation of NbC carbides was also very much faster for NbC than for the two other alloys. The NbC carbides are then not useful for mechanical properties while the other MC carbides seem being potentially able to maintain a significant level of mechanical resistance at high temperature.

The "CoTa" alloy is probably the best one for the resistance against oxidation at high temperature. Indeed oxidation remains essentially on surface, except a growing internal oxidation of both chromium and tantalum into CrTaO 4 , which is frequent in cobalt alloys reinforced by TaC carbides [17][18][19][20]. Even after 20 hours there was no generalized oxidation and the existence of a gradient of chromium content shows that Cr continued to diffuse to maintain the protective chromia scale. The latter unfortunately reacted with the first oxide formed, CoO (more precisely (Co,Ni)O ), particularly strongly because of the especially high temperature (1300°C). Thus, the real thickness of protective oxide was lower than what can be expected regarding the chromium quantity which diffused toward the oxidation front. After 20 hours at 1300°C, the chromium content was significantly lowered and catastrophic oxidation appeared to be imminent.

The behavior of the "CoHf" alloy in oxidation is similar to the previous alloy's ones, but it can be thought that the catastrophic oxidation was also imminent, as suggested by the critically low chromium content in extreme surface. The main difference with the "CoTa" alloy is the impossibility of Hf atoms to diffuse toward the oxidation front, i.e. differently to Ta atoms which leaved dissolving TaC carbides and diffused outward to be oxidized into CrTaO 4 just under the external surface of the "CoTa" alloy, allowing the development of a carbide-free zone. HfC carbides did not really dissolve, and on the contrary seemed coarsening since the Hf content in matrix, which was already very low in the as-cast microstructure (which also led to Hf-rich precipitates in addition to carbides), continued to decrease during the 1300°C exposures. This suggests that the exposure at 1300°C allowed finishing the formation of the (very stable) HfC carbides from Hf and C atoms initially trapped in matrix by solidification. In addition to its tendency to do not stay in a cobalt-base matrix, even in very small quantity, it seems that Hf atoms cannot diffuse through matrix and also along the interdendritic boundaries, as suggested by the absence of any carbide-free zone. Together with the high stability of the HfC carbides, this can also explain why HfC were much less affected by fragmentation than the other MC carbides. The oxidation of HfC occurred but only in extreme surface, seemingly into a complex oxide involving notably Hf and Cr rather than into HfO 2 to which oxidation of HfC alone can lead [21].

The Vickers hardness at room temperature is clearly related to the surface fraction of carbides, a relationship which can be affected by the degree of fragmentation of the carbides. The latter probably enhances the decrease in hardness when the carbide fraction decreases. The levels of hardness between the three alloys are sensibly different; for all heat treatments together the following order was observed: Hv ("CoNb", except as-cast) < Hv ("CoTa") < Hv ("CoHf"). This can be related to the carbide volume fractions (Fig. 9), but also to the differences of hardness between the different carbides (TaC: 1800Hv 50g , NbC: 2400 Hv 50g and HfC: 2900 Hv 50g [START_REF]Handbook of High-Temperature Materials -Materials Index[END_REF]). One might also take into account the differences between the matrixes themselves since their level of strengthening by heavy atoms (atomic weights: Nb < Hf ≈ Ta), which are moreover present in different quantities in solid solution (weight contents in matrix: Ta ≈ Nb >> Hf).

Conclusion

Chromium-containing cobalt-based alloys containing MC carbides and elaborated by casting are able to display simultaneously a high refractoriness (if not too rich in M elements) and a network of strengthening carbides which can be sufficiently developed in the interdendritic areas for 0.4-0.5 wt.% of carbon only. The stability of two of the studied carbides at a very high temperature (1300°C), and then the efficiency of strengthening they may bring, can be good (TaC) or very good (HfC), although the fragmentation phenomenon, faster for TaC than for HfC, probably reduces high temperature strength as it lowers hardness at room temperature. In addition, the resistance of the corresponding alloys against high temperature oxidation can be maintained at a high level for several tens hours (chromia-forming behavior). In contrast, NbC in a cobalt alloy cannot be exposed at high temperature without quick dissolution and early catastrophic oxidation (after only few hours at 1300°C). Among the three MC carbides tested here in a cast cobalt-based alloy, the one containing HfC seems the most able for applications at very high temperatures (even around 1300°C). Surface fractions of MC-carbides and Vickers hardness (30kg) in the alloys for the four states (as cast and aged for "0h", 5h and 20h at 1300°C

TABLES

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .Fig. 6 . 3 Fig. 7a .

 12345637a FIGURES

Table 1

 1

			Chemical compositions of the studied alloys
			(SEM/EDS, ± 1wt.% for all elements except C*)
	Alloys	Co	Ni	Cr	C*	Ta	Nb	Hf	Zr
	CoTa	Bal.	8.1	29.4	0.39*	6.5	/	/	/
	CoNb	Bal.	8.1	29.7	0.46*	/	3.3	/	/
	CoHf	Bal.	7.7	30.0	0.43*	/	/	6.6	/
	CoTH	Bal.	8.3	31.0	0.41*	3.2	/	3.0	/
	CTHZ	Bal.	8.3	29.8	0.40*	6.4	/	6.0	3.1
	*: targeted contents, supposed well respected (as usually verified for such alloys and
	this elaboration procedure)
						Table 2
	Values of the solidus temperatures and liquidus temperatures determined on the heating
					part of the DTA curves
	Alloys	"CoTa"	"CoNb"	"CoHf"		"CoTH"	"CTHZ"
	Temperatures
	Tliq (°C)		1450		1402		1421		1429	1311
	Tsol (°C)		1300		1310		1322		1300	1226
						Table 3
	Chemical composition of the matrix in the centre of all samples exposed at 1300°C
	Alloy		wt.%Cr		wt.%Ni		wt.%Ta
	"CoTa"	average	± std dev.	average	± std dev.	average	± std dev.
	"0h"	30.48	± 0.21	8.43	± 0.07	2.10	± 0.11
	"5h"	30.85	± 0.22	8.48	± 0.31	1.90	± 0.03
	"20h"	29.87	± 0.39	8.54	± 0.38	1.87	± 0.26
	Alloy		wt.%Cr		wt.%Ni		wt.%Nb
	"CoNb"	average	± std dev.	average	± std dev.	average	± std dev.
	"0h"	31.21	± 0.14	8.61	± 0.07	1.26	± 0.19
	"5h"	30.60	± 0.12	8.66	± 0.12	2.11	± 0.17
	"20h"	30.94	± 0.02	8.55	± 0.16	2.29	± 0.24
	Alloy		wt.%Cr		wt.%Ni		wt.%Hf
	"CoHf"	average	± std dev.	average	± std dev.	average	± std dev.
	"0h"	31.82	± 0.25	8.33	± 0.01	0.29	± 0.20
	"5h"	31.52	± 0.59	8.12	± 0.35	0.08	± 0.07
	"20h"	28.85	± 0.76	8.18	± 0.24	0.02	± 0.02

Table 4

 4 Natures and thicknesses of oxides scales, chromium contents in the most external zone of alloy and chromium gradients in sub-surface, after oxidation at 1300°C

	zone		oxide		substrate / bulk
	sample	3 rd scale	2 nd scale	1 st scale or internal	Cr mini d cfz	wt.% Cr × µm -1 1 st zone 2 nd zone
				oxidation
			Cr 2 O 3	CrTaO 4	16.0wt.%	0.130
	CoTa 0h	/				over	/
			6µm	(int. ox.)	25 µm	48 µm
		CoCr 2 O 4	Cr 2 O 3	CrTaO 4	15.6wt.%	0.146
	CoTa 5h					over	/
		4µm	15µm	(int. ox.)	72µm	130 µm
		CoCr 2 O 4	Cr 2 O 3	CrTaO 4	12.0 wt.%	0.039	0.013
	CoTa 20h					over	over
		18µm	40µm	(int. ox.)	338µm	320 µm	460 µm
			Ox. of Cr-		7.2 wt.%	0.855	0.298
	CoNb 0h	/	Co-Ni-Nb	Cr 2 O 3		over	over
			15µm	7µm	no c.f.z.	20µm	20µm
		(Co,Ni)O	Oxide of Cr-Co-Ni-Nb 29.4 wt.%	0.024
	CoNb 5h		compact	porous		over	/
		50µm	50µm	240µm	no c.f.z.	40µm
		(Co,Ni)O	Oxide of Cr-Co-Ni-Nb 23.3 wt.%	0.175
	CoNb20h		compact	porous		over	/
		120µm	180µm	760µm	no c.f.z.	40µm
		(Co,Ni)O	Ox. of Cr-		13.4 wt.%	0.498	0.241
	CoHf 0h		Co-Ni-Hf	Cr 2 O 3		over	over
		19µm	9µm	8µm	no c.f.z.	20µm	30µm
			(Co,Ni)O	CoCr 2 O 4	16.9 wt.%	0.127
	CoHf 5h					over	/
			16µm	15µm	no c.f.z.	120µm
			CoCr 2 O 4	Cr 2 O 3	8.2 wt.%	0.171	0.050
	CoHf 20h					over	over
			8µm	6µm	no c.f.z.	100µm	100µm

Acknowledgements

The author thanks the Common Service of Microanalysis of the Faculty of Science and Techniques of the University Henri Poincaré Nancy 1 for the EPMA measurements.


Manon Froger
email: manon.froger@iterop.com

Frederick Bénaben

Sébastien Truptil

Nicolas Boissel-Dallier

A non-linear business process management maturity framework to apprehend future challenges

Keywords:

Maturity evaluation A B S T R A C T

In a context where enterprises and organizations aim to optimise their behaviour, obtain certifications and labels, and benefit from the smart use of information systems and technology, two considerations drive this research: (1) the weak maturity level of worldwide Business Process Management (BPM), which exposes the need to reconcile academic theories with industrial contexts, and (2) the need for upcoming software functionalities that prioritize removing the barriers frequently encountered by industrialists when trying to implement the method. To reach such goals, this research work has developed a conceptual framework to represent the BPM implementation state. It is built along three axes: the BPM Cycle (Design, Enact, Maintain), the Field (Culture, Business, IT) and the Abstraction Level (Data, Jobs, Behaviour). An organization's overall BPM maturity can thus be evaluated by positioning its capabilities along the framework's axes. It is also suggested that the framework be used to track the implementation of new procedures in an organisation. The framework is presented and detailed before being applied to a complete case study.

Introduction

Businesses structure their operations to satisfy objectives and goals. These particular ways of working constitute their own business processes [START_REF] Vernadat | Techniques de modélisation en entreprise, applications aux processus opérationnels[END_REF]. Since the 1980s, myriads of methodologies (van der Aalst, 2013) have emerged to manage and improve business processes. The term Business Process Management (BPM) was first used in 2002 and suggests a lifecycle method to continuously improve and manage processes (vom Brocke & Rosemann, 2015). According to ter [START_REF] Ter Hofstede | Business process management: A survey[END_REF], and as shown in Fig. 1, BPM is a set of methods, techniques and software to (i) design and configure, (ii) execute, (iii) control and (iv) diagnose processes. In other words, BPM is made of multiple cycles which consist in (i) formalizing the way operations are conducted in the business (Design and Configuration), (ii) playing out processes as they were designed (Execution), (iii) collecting data and monitoring KPIs (Control) and (iv) deducing process weaknesses, which are then compensated for in the next cycle of the approach (Diagnosis).

BPM methodologies have been conceived by academics for industrialists. Since the 1980s, methodologies to manage business processes have been taught in universities as strategies to improve many aspects (van der Aalst, 2013) such as agility, productivity, or risk & compliance management. The method has shown satisfying results for large organizations like Ford Motor Co. and Wal-Mart (Al-Mashari & Zairi, 1999). But still, the weak worldwide level of BPM maturity in 2016 [START_REF] Harmon | The state of business process management 2014Retrieved from[END_REF] is evidence that BPM still does not constitute a suitable strategy for the majority of companies that are of smaller size. [START_REF] Imanipour | Obstacles in business process management (BPM) implementation and adoption in SMEs[END_REF] revealed that between 60% and 80% of attempts to implement BPM failed in 2012, revealing the need to reconcile theoretical approaches with industrial implementation methods [START_REF] Indulska | Business process modeling: Current issues and future challenges[END_REF]. New academic strategies should be based on industrial capabilities and should consider the reality of enterprise ecosystems.

BPM methods have existed since the end of the last century, a time when Information Systems (IS) were neither equipped nor thought to support the orchestration of processes in organizations (IS were mainly dedicated to managing databases and, at best, to connecting a heterogeneous set of software tools). In this context, modelling approaches were not designed to integrate computer systems, data sources and software tools that nowadays could (and maybe should) be involved in the facilitation and fluidity of the organization's behaviour. Approaches to modelling business processes and, more generally, the behaviour of organizations must incorporate this shift from a mode where the processes were modelled to be operationally managed by the human actors of the company to a mode where they are supported by modern IS. BPM methodologies should integrate this new aspect to prevent the expected decline in enthusiasm for them [START_REF] Harmon | The state of the BPM market -2018[END_REF].

BPM methodologies need to be more successful more often, and it is becoming urgent to conduct a thorough investigation of practical BPM implementations so as to identify the main difficulties faced by industrialists and to help them in bridging these gaps. On a societal level, this research generally aims to provide guidance in the design of new BPM implementation methods to ensure industrial success (the Social Challenge).

Maturity is defined as "the state of being complete, perfect, or ready" and the "fullness or perfection of growth or development" (Oxford University Press, 2012). In terms of evaluating BPM Maturity, two different aspects of an organization's maturity are important: Enterprise Maturity (a company's BPM capabilities) and Process Maturity (the condition of processes in general or distinct process types). For both aspects, BPM Maturity models are conceptual models provided to evaluate maturity through several desired or logical stages -the maturity levels -from an initial state to a more mature state [START_REF] Gottschalk | Maturity levels for interoperability in digital government[END_REF]. To conceive better implementation methods (the Social Challenge), we should first be able to confront existing methods with frequent industrial failures and classify them. To reach such a goal, we should be able to characterize an enterprise's BPM maturity depending on what has already been implemented by the organization, meaning that we first need to identify what criteria can characterize BPM maturity (Scientific Question 1). Such a framework should be able to characterize an organization's overall maturity in terms of BPM implementation and should indicate the achievements required to reach a higher maturity level (Scientific Question 2).

The next section builds a literature review that primarily describes the research supporting and answering the questions raised and described above (the Social Challenge and Scientific Questions 1 and 2). Section 3 gives the big picture of the framework and is mostly based on the results of the literature review. Section 4 details each position in the framework in term of achievements. Section 5 presents a case study to demonstrate how the framework can be used in an industrial context where the goal is to obtain ISO certification. Section 6 gives evaluations and discusses the limitations of the framework while Section 7 raises perspectives for its usability.

Literature review

This literature review is divided into four subsections. Subsection 2.1 gives some more detailed arguments supporting the need to reconcile academics and industrialists regarding the topic of BPM implementation. Subsection 2.2 defines BPM maturity and describes a sample of BPM maturity frameworks. Subsection 2.3 identifies important Business domains that our BPM Maturity framework should consider. Finally, subsection 2.4 explains why we chose ISO 9001:2015 (ISO, 2015) as a reference in terms of BPM maturity for our case study.

A gap between academic theories and industrial practices

An organization's success depends on its ability to remain competitive. Business Process Management (BPM) were quickly recognised as a common practice by both academics and industrials to increase one's Return On Investment (ROI) by optimising its internal operations [START_REF] Frolick | Business performance management: One truth[END_REF]. The BPM lifecycle approach aims at managing business processes from their design to their analysis and improvement. With the growing number of companies having a full BPM strategy since the 1990′s, process designing naturally became a core research subject for academics. Industrial applications evolved accordingly with the emergence of Business Process Management Suites (BPMS). BPMS are software systems driven by process designs to enact and manage operational business processes. BPMS developed rapidly during the 2000′s to meet the demands of industrialists looking for a way to computerize the method [START_REF] Ko | Business process management (BPM) standards: A survey[END_REF]. Among others, these suites include Appian, AuraPortal, Bizagi, Bonitasoft, Flokzu, Iterop, Kissflow, Process Street, ProcessMaker, Signavio and WorkflowGen. More and more organizations are adopting these BPMS, also known as Process-Aware Information Systems (PAIS), to manage and execute their processes [START_REF] Yongsiriwit | Modeling and mining business process variants in cloud environments[END_REF].

Progress made in Information technologies (IT) during the past few years has led to some new concepts with positive impacts on BPMS [START_REF] Ahmad | Business process reengineering: Critical success factors in higher education[END_REF][START_REF] Almajali | Applying the triangulation approach in IT -Business strategic alignment and sustainable competitive advantage[END_REF][START_REF] Benaben | Conception de systèmes d'information de médiation pour la prise en charge de l'interopérabilité dans les collaborations d'organisations[END_REF], such as process deduction [START_REF] Bidoux | Preference-based planning using the Multi-Attribute Utility Theory along with a Choquet integral: Application to organizations' interoperability in crisis management[END_REF][START_REF] Montarnal | Deduction of inter-organizational collaborative business processes within an enterprise social network[END_REF] or cognitive BPM [START_REF] Hull | Rethinking BPM in a cognitive world: Transforming how we learn and perform business processes[END_REF], also known as Machine Learning and Artificial Intelligence applied to BPM. But, 15 years later, some theorical concepts are still not integrated to BPMS. For instance, [START_REF] Meidan | A survey on business processes management suites[END_REF]) Key Performance Indicators (KPI) still cannot be integrated into process models, neither can they be linked to services, and BPMS still do not commonly provide process documentation generation features, despite this being a requirement to obtain certifications [START_REF] Sei | Capability maturity model® integration (CMMISM)[END_REF]. Academical theories are progressively outpacing industrial implementation.

According to [START_REF] Klun | BPM explained through cognitive dissonance and absorptive capacity[END_REF], the lack of success of BPM programmes is not due to insufficient human skill or deficiencies in methodologies, but rather to the lack of a theoretical underpinning of each step of the BPM implementation cycle, from achievements to rewards. Moreover, according to [START_REF] Benaben | Conception de systèmes d'information de médiation pour la prise en charge de l'interopérabilité dans les collaborations d'organisations[END_REF], the lack of actual assimilation of BPMS by enterprises is mainly due to the time shift between the emergence of BPM (during the 1990′s) and the emergence of BPMS (during the 2010′s): deploying an entire business process cartography was just too complicated and too intricate to succeed without the support of BPMS. Enterprises learned that it was not possible; and that belief was deeply rooted by the time BPMS started to reach the maturity level that would make them the solution. Enterprises need to move beyond this belief and BPMS must demonstrate their value. Therefore, companies desperately need progress in bridging the gap separating operations from concepts [START_REF] Møller | What is business process management: A two stage literature review of an emerging field[END_REF].

BPM maturity

More than a hundred maturity models have been developed in various domains. They can be classed as (i) descriptive i.e. for the assessment of an organization's current state, (ii) prescriptive, providing suggestions for improvement actions or (iii) comparative, to confront other organizations' level of maturity [START_REF] De Bruin | Understanding the main phases of developing a maturity assessment model[END_REF]. Very few maturity models are prescriptive, whereas descriptive and comparative models are much more commonly investigated [START_REF] Pöppelbuß | What makes a useful maturity model? A framework of general design, principles for maturity models and its demonstration in business process management[END_REF]. For the last 15 years, numerous researchers have conducted surveys to identify success and failure factors for BPM implementation [START_REF] Ahadi | An examination of the role of organizational enablers in business process reengineering and the impact of information technology[END_REF][START_REF] Buh | Critical success factors for different stages of business process management adoption -A case study[END_REF][START_REF] Chong | Business process management for SMEs: An exploratory study of implementation factors for the Australian wine industry[END_REF][START_REF] Škrinjar | Increasing process orientation with business process management: Critical practices[END_REF], thus highlighting the numerous domains involved in the BPM method such as Management, Strategic alignment, Employees, Communication, IT, Culture or Governance. van Steenbergen, Bos, Brinkkemper, van de Weerd, and Bekkers (2013) explain that most existing maturity models regrettably focus on only one dimension, even though business process performance not only relies on individual process characteristics but also on enterprise capabilities such as culture and expertise.

This paragraph aims to study the state of the art of existing BPM Maturity frameworks, with the results summarized in Table 1. To obtain a homogeneous and usable benchmark, we decided to focus on frameworks (1) that are not dedicated only to specific business domains (supply chain management, software development, etc.) and (2) taking multiple domains into account (business, knowledge, and information technology management). These frameworks are then compared to the proposed framework in Section 6. Identified frameworks are presented along with the domains they consider as relevant to assess BPM Maturity.

These frameworks all claim to evaluate the same characteristic: how advanced a company is in terms of BPM. Yet some frameworks base their evaluation on unique concepts that are not shared by other frameworks, such as Accountability, Performers or Governance for instance. To create our BPM Maturity framework, we need to identify the domains we consider as critical for BPM maturity measurement. In the next paragraph we select the relevant domains to build our own BPM Maturity assessment framework.

Critical domains involved in the evaluation of BPM Maturity

Despite their differences, the frameworks presented in Table 1 all share a common idea: the BPM Maturity of any company should at least be evaluated in the context of the step in the BPM Cycle that has been reached. This research already shares this idea by considering the BPM cycle as the first dimension of the framework, and the following paragraphs aim to define other critical domains.

2.3.1. Fields (Culture, Business, IT) [START_REF] Buh | Critical success factors for different stages of business process management adoption -A case study[END_REF] highlight the need to consider the different stages of BPM adoption independently of each other. BPM adoption is defined as the implementation in a structure of any BPM concepts, and it appears necessary to differentiate five states of BPM adoption, going from the initial concepts to full integration in the business: Awareness and understanding of BPM, Desire to adopt BPM, BPM projects, BPM program, and Productization of BPM. Previously cited authors (Table 1) have also concluded that BPM adoption covers at least two Fields: the organization's Culture and its Business. [START_REF] Alonso | Description of the structure of the IT demand management process framework[END_REF] further insist on the need to bridge the gap that exists between these two fields.

Grau and Moormann (2014) point out that any process initiative is tightly bound up with the company's culture and that this psychological aspect has been widely neglected. With the development of technology, each business concept now has a digital counterpart, leading [START_REF] Rahimi | Business process management and IT management: The missing integration[END_REF] to define a Business Process as the link between IT capabilities and business strategy. IT constitutes a critical Field, just as much as Culture (see Table 1 andCutura , 2013;[START_REF] Sherwin | Defining the critical success factors for business process management[END_REF] and Business. [START_REF] Kruger | Information management as an enabler of knowledge management maturity: A South African perspective[END_REF] underline the necessity to distinguish communication technology from both information management and knowledge management, but also to define the border existing between these three fields.

Over the years, a multitude of scientists [START_REF] Bai | A grey-based DEMATEL model for evaluating business process management critical success factors[END_REF][START_REF] Buh | Critical success factors for different stages of business process management adoption -A case study[END_REF][START_REF] Grau | Investigating the relationship between process management and organizational culture: Literature review and research agenda[END_REF][START_REF] Santos | Strategies for managing critical success factors of BPM initiatives in Brazilian public organizations: A qualitative empirical study[END_REF][START_REF] Trkman | The critical success factors of business process management[END_REF] and industrialists [START_REF] Burlton | BPM critical success factors: Lessons learned from successful BPM organizations[END_REF][START_REF] Cutura | 5 reasons for BPM project failure[END_REF][START_REF] Dyer | Scaling BPM adoption: From project to program with IBM business process manager[END_REF]PRIME BPM, 2017;[START_REF] Sherwin | Defining the critical success factors for business process management[END_REF] have conducted surveys to build lists of success and failure factors for BPM implementation in industrial contexts. Table 2 summarizes them and associates them to previously identified Fields.

Abstraction level (data, jobs, behaviour)

On another level, [START_REF] Benaben | Information System agility to support collaborative organisations[END_REF] define an organization as a set of functions providing capabilities, fed by information as inputs and organized according to processes describing the behaviour of the organization. These authors also discuss the counterparts of these concepts for the IT Level; Functions may be offered by software Applications, Information may be any piece of computerized Data used by the organization and Computerized Processes are called Workflows. They conceptualize three Abstraction Levels describing two Fields (Business and IT) that interlock with each other. The three interlocking Abstraction Levels and the concepts resulting from their intersection with Business and IT levels are summarized in Fig. 2. [START_REF] Hu | Mediating effects of business process for international trade industry on the relationship between information capital and company performance[END_REF] explain how Information is the foundation of the Business field. This is the idea depicted in Fig. 2, since each circle represents the core of a bigger circle: Information is at the core of Functions, and Functions are at the core of Processes, and together they describe the Business. In line with this representation, the IT field is depicted as a transversal field across the organization and is built using the same structure, so as to facilitate diffusion of Information through the Business Layer. Data, Applications and Workflows respectively are the IT equivalents of Information, Functions and Processes [START_REF] Bénaben | Semantic issues in model-driven management of information system interoperability[END_REF]. These three abstraction levels of the IT field can be interwoven following the same logic: Data is the foundation of Applications, and Applications are the foundation of Workflows.

A reference base to assess reaching a higher maturity level

In order to validate our framework, we need to compare it with existing references in terms of BPM Maturity. As a reference we chose ISO 9001:2015(ISO, 2015), which is a non-specific standard defining the foundations of a quality management system. Created during the 90′s, the requirements are recognised worldwide as enabling certified organizations to be more efficient and improve customer satisfaction. To obtain the certification, an organization must receive a third party into its physical structure to audit its practices against the requirements of the norm. ISO 9001:2015 is based on the principles of continuous improvement. Since it obliges organizations (1) to control internal development and growth, (2) to maintain processes in continuous accordance with client requirements, and (3) to preserve customer and employee satisfaction constantly, the ISO 9001:2015 certification is a guarantee of BPM maturity.

Our BPM maturity framework

The structure of the framework presented in this section is based on the conclusions of Paragraph 2. This section aims to set the fundamental definitions for the chosen axes and their labels. Fig. 3 depicts the resulting framework. As stated in the previous section, our framework is built along three axes of development; [START_REF] Sherwin | Defining the critical success factors for business process management[END_REF] the BPM Cycle, (2) the Fields and (3) the Abstraction Level.

If the (1) cycle of the BPM approach is considered as one dimension (see Fig. 1), it is possible to differentiate businesses progressing along the BPM lifecycle. By doing so, the framework would result in different maturity levels for businesses that have only created models compared to businesses that have designed and are also using models. However, it has been decided to group together the two steps of Control and Diagnosis under the name of Maintain, in order to keep similar granularity levels between steps. This axis, the BPM level, then consists of three labels: Design (building models), Enact (using models) and Maintain (diagnosing indicator results).

The consideration of (2) different Fields enables our model to reveal different maturity levels for organizations that have integrated concepts in their culture, compared to those that also apply them in their work, or those who have implemented them in their information system. The Business Field axis is made up of three labels: Culture (psychology and knowledge), Business (projects and programme within the business structure) and IT (computerized concepts). As stated in Section 2.1, we think that the emergence of new technologies, such as Artificial intelligence or process mining, could change the way BPM is implemented in organizations. Therefore, we particularly emphasise the need for our framework to represent the Business Field axis.

The last axis, (3) Abstraction Level allows the differentiation of organizations conducting improvements in their processes from those also improving their activities, and again from those doing this on the data they use. This last Abstraction Level axis is composed of three levels: Data (material used within Jobs), Jobs (Indivisible activity usually expected to produce outputs from inputs) and Behaviour (sequencing of Jobs). Our project likes to define an organization by how its data and skills interact with each other. This vision is fully in line with the way the Abstraction Level axis breaks down the Culture, Business and IT Fields. Therefore, we decided to use this axis in our framework, despite its originality and its novelty among the frameworks cited in Section 2.

In the next figures we will represent the framework as a cube composed of 27 positions. With a cube representation, we can see that each position has several neighbouring positions and that transitions from one position to another might represent a challenge. Using three axes to evaluate BPM Maturity prevents the framework from being linear, meaning that the positions are not ordered. However, it does not mean that reaching a particular position will not have any impact on the future positions obtainable. But before talking about difficulties and facilitators, we need to define the meaning of each position in the framework.

Detailed description of the BPM maturity framework

This section aims to define the 27 positions resulting from the framework presented in Section 3. Each position is the result of the intersection of three layers. To articulate the presentation, we have first chosen to detail, in Section 4.1, the intersection between Field Layers and Abstraction Levels by building a plane between these two axes. We then explain, in the three following paragraphs, the meaning of the 9 positions resulting from the intersection of each BPM Cycle layer with the plan previously detailed. The articulation of the paragraphs is illustrated in Fig. 4. We have chosen to present the framework positions along the BPM Cycle that has already been explained in Section 1, but the 27 positions are also presented along the two other axes in Appendix A. and Appendix B in Supplementary materials.

Intersection of abstraction levels and fields

In line with the results described in paragraph 2.3, concepts resulting from the intersection of the three Abstraction Levels with the Fields Business and IT have already been defined. As a reminder, a "Business is set of functions providing capabilities, fed by information as inputs and organized according to processes describing the Behaviour of the organization". The implemented (IT) counterparts of these three concepts are respectively Applications, Computerized Data and Workflows. For the Culture level it makes sense to use the word Knowledge to represent Data and to use Skill for Jobs. The last concept (Intersection of Culture with Behaviour) represents the awareness of the existence of sequences between Jobs and the acceptance of the need to monitor them. For this concept we have chosen the word Monitor. A recap of concepts resulting from the intersection of Abstraction Levels and Fields is presented in Table 3.

The following three paragraphs define the concepts resulting from the intersection between the plane presented in Table 3 and each level of the BPM Cycle axis.

Intersection of abstraction levels with fields within the design layer

The Design Layer corresponds to the implementation (for IT), the formalization (for Business) and the identification (for Culture) of models within the organization. In the Abstraction Level, Data gathers every material used within the organization, Jobs are equivalent to functions and Behaviour respectively refers to a sequence of goals and resources (for Culture), to a process (for Business) and to a Workflow (for IT). Table 4 sums up the different intersections of Fields and Abstraction Levels within the Design Level of the BPM Cycle axis.

Intersection of abstraction levels with fields within the enact layer

The Enact Layer corresponds to the act of using business concepts in an organization. Here, Culture stands for the comprehension of concepts, Business is for the main work of the organization and IT for the use of previous implementation. Following that definition, we can easily identify positions for expertise and experience; the knowledge (and so the culture) of enactment would be the expertise, baptised here "knowhow"; the business application of enactment would be the experience, here represented by "do". Table 5 summarizes the different intersections of Fields with Abstraction Levels within the Enact Level of the BPM Cycle axis.

Intersection of abstraction levels with fields within the maintain layer

The Maintain Layer is the combination of Control and Diagnosis. This Layer is mainly about Key Performance Indicators (KPIs), defined as measurable values (e.g. gathered using embedded sensors) representing how effectively a company is achieving key business objectives. They may reveal potential dysfunctions in the models and contribute to the appropriate management of organizations. In an industrial environment, Control is made possible thanks to the measurement of these KPIs and their comparison with target figures. Diagnosis corresponds to the analysis of malfunctions or of improvements to integrate into future models. KPIs can be behavioural (for Behaviour) or functional (for Jobs) and can also be set to evaluate the consistency, the lack of redundancy and the security of the Dataset. Table 6 sums up the different intersections of Fields and Abstraction Levels within the Maintain Level of the BPM Cycle axis.

Representation of the framework with its 27 achievements

Fig. 5 summarizes the 27 positions in our framework layer by layer, in terms of achievements. The next section develops the idea of constraints existing in the framework, and this concept is then further developed in the Perspectives section.

Structuring constraints existing in the framework

Now that resulting concepts have been precisely defined, we can validate the interlocking of the three Abstraction levels described in paragraph 2.3. It indeed does not make any sense to implement a workflow without having implemented applications or a database. We can therefore conclude that some prerequisite constraints might exist along the Abstraction Level axis, meaning that no positions of the Behaviour layer can be reached if the position directly below (Jobs Abstraction Level, same Field, same BPM Cycle level) has not been reached yet.

We could argue that the same kind of prerequisite constraints exists along the BPM Cycle axis because it does not make any sense to maintain a concept we never enacted. But since it is possible to enact a concept that has not previously been designed, we can only conclude that the maintain layer cannot exist without the enact layer, meaning that no positions of the Maintain layer can be reached if the position right behind it (Enact BPM Cycle Level, same Field, same Abstraction Level) has not been reached yet. As regards the Fields, since all levels can exist independently, there cannot be any prerequisite constraints along that last axis. The prerequisite constraints are represented in Fig. 6.

As a conclusion to this paragraph we would like to focus on the nonrelated levels, meaning those with no prerequisite constraints. Since the order to reach the non-related levels is not imposed by any structuring constraint, multiple paths can lead to higher maturity levels, and some paths might be more effective than others. This idea is more deeply developed in part 7 -Conclusion and Perspectives, where it is suggested that the framework may be improved through concepts of difficult and easy transitions from one position to another.

Defined in terms of achievements, our framework could be used to (i) assess overallorganization maturity but also to (ii) track the implementation of new processes, jobs and data in the company. The next section is dedicated to demonstrating these two different uses.

Case study

Iterop is an innovative French start-up in software editing. Its product, IteropSuite, is software for process modelling, enactment and monitoring. The company also uses its own product for its daily work. In 2015, Iterop undertook the process of obtaining ISO 9001:2015 (ISO, 2015) certification as a guarantee of quality for its customers, but also to demonstrate that process-oriented development is a fast and efficient method to achieve strategic goals. In 6 months, Iterop received the certification with more than 20 key points identified (Iterop, 2017).

As explained at the end of Section 4, this paper claims that the current framework could be used to assess the overall maturity of an organization, but also to track the implementation of new processes, jobs and data in the company. The two use-cases presented respectively in sub-sections 5.1 and 5.2 are dedicated to covering both these potential uses of the framework.

Iterop BPM Maturity before and after obtaining the ISO 9001:2015 certification

Since Iterop uses the workflow tool the company have developed on a daily basis, all behaviour, jobs and data is computerized. For this reason, every maturity level reached within the Business Field is also reached within the IT Field. The consequences in terms of maturity representation are that every position reached within the Business layer comes with the position at its right side (IT).

For the Model layer (Table 7), Iterop already had its Data and Functions identified, formalized and implemented before the certification. The company also had its processes formalized and implemented, whereas the identification of goals and resources was not systematic yet.

As concerns the Enact Layer (Table 8), Iterop is driven by the processes it follows. Thus, everything done by the company follows an implemented process (Workflow). Within the Culture Field, since all the formalization has been conceived by its principal users (and not by an external designer), we can consider that they know and comprehend the data they use, the jobs they do and the sequence of jobs they follow. However, we keep in mind that this knowledge can be lost with time and especially if dictated by a BPM software over a long period of time.

Within the Maintain Layer (Table 9), KPI for jobs and data were already identified and implemented but their measures had not been systematically analysed yet.

This paragraph describes the positions reached (and in what order) by Iterop thanks to the ISO 9001:2015 certification. ISO 9001:2015 (ISO, 2015) requires two main points: (i) the formalization of all exchanges with employees and clients and (ii) the analysis and improvement of processes. As regards the 1 st point, Iterop had to identify content for their missing model: Goals and resources [START_REF] Sherwin | Defining the critical success factors for business process management[END_REF]. Regarding the 2 nd point, Iterop had to progressively identify and implement behavioural KPI for each of their processes (2) and started analysing KPIs that were already implemented but not used enough (3) (Table 10).

Iterop's BPM Maturities before (left) and after (right) are pictured in Fig. 7.

Evolution of maturity during the implementation of new procedures required by the norm

The ISO 9001:2015 certification created the need for a new process for Iterop: that of auditing their internal processes and bringing corrections and improvements to models. The company had never conducted such audits before, and they chose to implement them using a process-oriented approach. The steps they followed are described in this paragraph and shown in Fig. 8.

The audit process, and the functions and data involved, were first formalized (1), as detailed in the norm. A new document for the new process contains a formalization of the process using a BPMN 2.0 diagram, where the functions involved are described and the data (needed and produced) is all detailed (types and description). The company quickly implemented the new dataset, set up mails and connections (Applications) to the calendar and resulting workflow (2) in their BPM software, Iterop, to (3) run them. The orchestration of workflows forced the employees to (4) follow the process as it was first formalized. Descriptions helped the actors (and not only the designer) to correctly do the jobs they were expected to do, and data was automatically used by the workflow. After several iterations of using the process, the actors [START_REF] Gottschalk | Maturity levels for interoperability in digital government[END_REF] acquired essential knowledge about the data they used, the jobs themselves and their sequencing. It was only after having acquired such knowledge of enactment that they could (6) identify data, functions, goals and resources that were involved in the process, and complete the documentation. This new expertise enabled them to (7) identify KPIs for processes, functions and data that were (8) implemented, allowing processes to be frequently diagnosed (9).

Framework evaluation and limitations

Contributions to theory

Other frameworks from the literature review show that Abstraction Levels are hardly ever investigated by existing maturity frameworks. In contrast, our framework recognizes that conducting continuous improvement for data or jobs is indeed a progression towards achieving high BPM maturity levels, whereas other frameworks only recognize

Table 7

Positions in the Model Layer reached by Iterop before obtaining the certification.

Table 8

Positions in the Enact Layer reached by Iterop before obtaining the certification.

Table 9

Positions in the Maintain Layer reached by Iterop before obtaining the certification.

processes. Even if this rare axis will cause our framework to be less comparable with others, our position is that this vision could be useful for maturity models aiming to guide BPM implementation, since it imposes a BPM approach not only on behaviour but also on functions and data.

As far as we know, this is the first time that the notion of prerequisite constraints in maturity models has been expressed. A prerequisite constraint can be seen as an axis along which a position cannot be reached without the previous one. For maturity models that are only based on one linear dimension, this notion already implicitly exists, but the absence of prerequisite constraint identification that helps guide the user in implementing their BPM approach is a shortcoming of other frameworks.

Implications for practice

An assessment of the framework is available in Appendix C in Supplementary materials. It follows the guidelines of [START_REF] Van Looy | Evaluating business process maturity models[END_REF]. This first evaluation reveals that the framework provides more in-depth descriptions but may be overly-dependent on the assessors' skills. There is specifically a limitation in the culture layer achievements of the proposed framework, where the evaluation sometimes relies on subjective techniques. However, we think it is important to measure the BPM culture of an organization to assess its overall BPM maturity level and so to keep this layer in the framework. [START_REF] Tarhan | Business process maturity models: A systematic literature review[END_REF] demonstrated that an increased maturity level, with respect to a maturity model, leads to an improved business performance, which is a goal coveted by every organization. Yet, since most BPMMs display descriptive rather than prescriptive characteristics (as raised in 2.2 -BPM Maturity), organization lacks guidelines to improve their maturity levels. The proposed framework describes positions in terms of achievements, thus helping organizations to see what they need to accomplish to reach new maturity levels.

The non-linear structure of the framework allows every organization to place their advancement in the structure, revealing that different strategies can exist to reach a high BPM maturity level. We plan on using that theory to investigate the role of IT in enabling the acquirement of new business capabilities. Nowadays, technologies have so much more capabilities than in the 80 s (Artificial Intelligence, Big Data, Cloud Computing) that it has become a common intuition to think that technology would shortly enable businesses to reach higher maturity levels in less time than they need today. An Adequate Alignment of IT Infrastructure and BPR Strategy has always been identified [START_REF] Al-Mashari | BPR implementation process: An analysis of key success and failure factors[END_REF][START_REF] Indulska | Business process modeling: Current issues and future challenges[END_REF][START_REF] Recker | How collaborative technology supports cognitive processes in collaborative process modeling: A capabilities-gains-outcome model[END_REF] as a key success factor for implementing a BPM strategy. But despite the natural partnership that has always existed between BPM and information technology [START_REF] Davenport | The new industrial engineering: Information technology and business process redesign[END_REF], industrial engineers have still not fully exploited the idea of taking better advantage of IT to implement a BPM strategy [START_REF] Yongsiriwit | Modeling and mining business process variants in cloud environments[END_REF]. We think that the framework could be used to support that intuition. [START_REF] Harmon | The state of the BPM market -2018[END_REF] points out that nowadays organizations do not implement BPM in the entire business, but rather focus on specific processes and "lurch from one process improvement effort to the next". To our knowledge, this is the first time a maturity model has been designed for use both in the evaluation of an organization's overallBPM maturity level and for the evaluation of a single procedure in the organization. Most maturity frameworks also contain achievements such as "processes need to be defined" or "data needs to be identified". We decided to consider the same achievements for single procedures. Thus, the framework can be used to follow the implementation of this new procedure until it reaches the same level as other existing procedures in the company. The difference between these two case studies resides in the positions that have been reached when starting the guidance: when implementing new procedures, no position has yet been reached, whereas guiding an organization in its entire BPM implementation requires determining what positions have already been reached. Due to its non-linearity, we consider this initial need to position a company within the framework as the main limitation of our work.

Limitations and avenues for future research

A first version of the framework did not consider the two steps, Control and Diagnosis, of the BPM approach as being merged, but the Diagnose layer appeared to be empty (composed only of achievements such as: identify flaws). We decided to keep the framework with only three levels for the BPM Cycle axis to widen the perspective for the future use of the framework. However, although some organizations might have their own KPIs identified, implemented and measured, they may not possess the required reengineering knowledge or skills in KPI results interpretation to effectively upgrade their processes. This step of the BPM approach is rarely reached by organizations and our framework does not dedicate an entire layer to it.

Compared to other frameworks, our proposal does not include concepts such as Strategic Alignment, Methodology, Leadership and Communication. These missing concepts may help a new achievement, or indeed prevent it from completing, meaning that our framework could still be enriched by identifying frontiers between layers or positions. We suggest that this framework could be enriched by characterizing frontiers between two positions. We would like future research to investigate those frontiers more fully. A frontier can have prerequisite constraints (like the ones identified in Section 4.5) or perhaps only simple barriers (making it difficult, but not impossible, for organizations to reach a new position from a previous one). On the other hand, the opposite concept may apply: frontiers may facilitate flows (meaning it would be easy for organizations to reach a new position from a position already obtained). We would like our future research to investigate these concepts of barriers and flows so that the framework in its 2.0 version could then be used to demonstrate where future research should be focusing on (barriers that need to be raised or flows that can be taken advantage of) to better reconcile theories and industrial implementation.

In line with this idea of improving the structure with barriers and flows, the framework could be seen as a relief map where barriers would be mountains and flows would be rivers. Thus, targeting a precise maturity level, organizations would rather take the most lucrative path, meaning the shortest one, or the one with the fewest mountains and the most rivers. This perspective might finally prove that IT is a real enabler for the acquisition of new business capabilities, which constitutes our main perspective for the framework.

Conclusion

Our goal was to focus on how to improve industrial BPM implementation and we strongly believe that, despite the numerous features existing to partially guide companies, industries lack guidance to implement the overall method. In order to support organizations in implementing their entire BPM approach, our research attempts to conceive a new, comprehensive framework for BPM maturity assessment. With regard to our first Scientific Question (What criteria can characterize BPM maturity?), we identified three domains defining an organization's overall BPM maturity: BPM Cycle (Model, Enact, Maintain), Field (Culture, Business, IT) and Abstraction Level (Data, Jobs, Behaviour). We described the 27 positions resulting from the intersections between these domains in terms of prescriptive achievements. The proposed framework can be used to (i) assess an organization's BPM maturity or (ii) help implement new procedures in the structure by indicating those achievements that need to be completed in order to reach a higher maturity level. This second use of the framework answers our second Scientific Question (What should an organization achieve to reach a higher maturity level?).

Depending on the organization, the order in which positions are reached can differ, meaning that our framework could reveal different strategies to reach a high maturity levels. Following this idea, we think that our framework could be seen has a travelling map in which one should avoid difficulties and prefer following a path with facilitating flows. Hopefully, numerous theories also complementing the BPM domain and could thus constitute these flows: data and process mining features, flow optimization tools, optimization of warehouse organizations, simulation, (flow, workshop, production, product), Lean management, business activity monitoring tools... We wish to improve our framework with these existing features to reveal easiest strategies to reach a perfect maturity level. Moreover, we think it could suggest how IT could evolve to raise remaining barriers for BPM implementation and thus constitute a guide for future research subjects.

Fig. 1 .

 1 Fig. 1. Business Process Management (BPM) according to van der Aalst, Netjes, and Reijers (2007).

Fig. 2 .

 2 Fig. 2. Abstraction Levels across Business and IT Levels according to Benaben and Vernadat (2017).

Fig. 3 .

 3 Fig. 3. Our BPM maturity framework structure: axis and levels.

Fig. 4 .

 4 Fig. 4. Articulation of paragraphs in Section 4.

Fig. 5 .

 5 Fig. 5. Our BPM maturity Framework: meaning of positions in terms of achievements.

Fig. 6 .

 6 Fig. 6. Prerequisite constraints existing in the framework.

Fig. 7 .

 7 Fig. 7. Iterop's BPM Maturity according to our framework before (left) and after (right) obtaining the ISO 9001:2015.

Fig. 8 .

 8 Fig. 8. 9 steps of the process-oriented implementation of Process Auditing.

Table 1

 1 State of the Art of existing BPM Maturity Frameworks.

	Framework Name	Reference	Criteria to measure advancement along BPM cycle	Other criteria to measure BPM Maturity
	BPM Maturity Model	Rosemann, De Bruin, and	Perspectives: Align, Design, Execute, Control, Improve	Factors: IT/IS, Culture, Accountability, Methodology,
	(BPMMM)	Power (2006), Rosemann		Performance
		and De Bruin (2005)
	Process Performance Index	Fisher (2004)	States of Maturity: Siloed, Tactically Integrated,	Five levers of change: Strategy, Controls, Process,
	(PPI)		Process Driven, Optimized Enterprise, Intelligent	People, IT
			Operating Network
	Process and Enterprise	Power (2007)	Enablers of Maturity: Design (purpose, context, and	Enterprise capabilities: Leadership (Awareness,
	Maturity Model (PEMM)		documentation), Performers (knowledge, skills, and	Alignment, Behaviour, Style), Culture (Teamwork,
			behaviour of people doing the work of the process),	Customer focus, Responsibility, Attitude toward
			Owner (identity, activities, and authority),	change), Expertise (People, Methodology), Governance
			Infrastructure (information systems and human	(Process Model, Accountability, Integration).
			resources), Metrics (definition and uses).
	Process Management	Rohloff (2009)	Program management: Set goals, Analyse, Define,	Categories: Process Portfolio & Target Setting System,
	Maturity Assessment		Realize, Review	Process Documentation, Process Performance
				Controlling, Process Optimization, Methods & Tools,
				Process Management Organization, Program
				Management, Qualification, Communication, Data
				Management, IT-Architecture
	Process Management	Cronemyr and Danielsson	Maturity Level: Awareness, Established, Improved,	Categories: Management of the Organisation,
	Maturity Model	(2013)	Adapted	Documentation, Way of Working, Layout of the process,
				Management of the Process, Users of the Process,
				Measurements, Improvements
	Model for Business Process	Moradi-Moghadam, Safari,	Index: Initial, managed, defined, quantitatively	Criteria: Design (Purpose, content, Documentation),
	Maturity Assessment	and Maleki (2013)	managed, optimising	Performers (knowledge, Skills, Behaviour), Owner
				(Identity, Activities, Authority) Infrastructure (IS, HR),
				Measurement (Def, use)

Table 2

 2 Critical Factors identified when implementing a BPM strategy.

	Field	Critical Factors for BPM
		implementation
	Social & Cultural	Communication
		Culture for organizational change
		Empowerment
		Organizational resistance
		People involvement
		Information withholding
		Rewards and motivation systems
		Leadership
		Championship and sponsorship
		Management of risks & Quality
		management system
	Business Organization, Structure, Project	Adequate job integration
	planning & Management	BPR effectiveness
		Job definitions and responsibilities
		allocation
		Teamwork and quality culture
		Aligning BPR strategy with
		corporate strategy
		Adequate financial support
		Goals and measures
		Unrealistic expectations
		Consulting
		Previous experiences
	IT Infrastructure	Aligning IT infrastructure with
		corporate strategy
		SOA architecture, interoperability
		IT investment and sourcing
		decisions
		Modelling tools, data mining, BAM,
		detecting tools
		IS integration
		Lack of BPM knowledge

Table 3

 3 Concepts resulting from the intersection of Axes Fields and Abstraction Level.

Table 4 9

 4 Positions in the Model Layer.

Table 5 9

 5 Positions in the BPM Cycle Level: Enact.

Table 6 9

 6 Positions in the BPM Cycle Level: Maintain.

Table 10

 10 Positions in the Design and Maintain Layers reached by Iterop after obtaining the certification.

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.ijinfomgt.2019.05. 013.


Viviana Fonti

Catherine Joulian
email: c.joulian@brgm.fr.

Anne-Gwenaelle Guezennec

Christopher G Bryan

Bioprospecting and the Microbial Ecology of a Coal Production Waste Dump

Keywords: Coal Waste, Microbial Ecology, Leptospirillum, Inhibition

In order to develop an optimised coal production waste bioleaching unit, a sampling campaign was carried out on the existing waste dump. Eleven samples from various surface locations of the heap were used for enrichment cultures at three temperatures (30°C, 42°C and 48°C).

Interestingly, none of the 11 cultures at 42°C grew well (low cell numbers and poor pyrite dissolution). Bioreactors inoculated with pooled enrichments showed the same results, indicating that the waste heap harboured organisms capable of growth and mineral oxidation at 30°C and 48°C but not 42°C. Analysis of the 48°C culture used for subsequent reactor tests using a variety of molecular microbiology methods (CE-SSCP, T-RFLP and high-throughput sequencing) revealed the culture was dominated by Sulfobacillus thermosulfidooxidans, Acidithiobacillus caldus and an unknown member of the Acidimicrobiaceae. Further tests at 42°C using a well-characterised bioleaching consortium, BRGM-KCC, performed well in initial reactors. However, during each batch the relative abundance of Leptospirillum ferriphilum decreased, and its numbers declined gradually with each subsequent subculture indicating some form of accrued inhibition. Coupled with the absence of a 42°C culture from the dump it was theorised that the waste was inhibitory to Leptospirillum spp. In order to further explore the possible reasons for the absence of Leptospirillum in the enrichments and bioreactors, a detailed analysis of the waste heap was undertaken using a range of geochemical and molecular biology tools. Results suggest the role of an as-yet unidentified organic carbon compound from the coal itself.

Introduction

Acid mine drainage from sulfidic coal production wastes is a significant challenge to the European Coal industry. This drainage can be treated, but most wastes will continue to produce AMD for decades. At the same time, the generation of electronic waste is growing year by year, nonetheless, recovering valuable raw materials from this waste by existing processes it is not always possible.

The EU RFCS-funded CEReS project aims at developing a co-processing strategy where i) the acidgenerating potential of coal mine waste is removed through accelerated bioleaching and (ii) base metals from electronic waste are recovered, while concentrating precious and critical elements into enriched substrates. The selection and adaption of an effective and efficient bioleaching consortium is key stage for achieving these two goals [START_REF] Bryan | The use of pyrite as a source of lixiviant in the bioleaching of electronic waste[END_REF].

It was hypothesised that microbes isolated from coal itself may be more efficient than allochthonous ones but the literature reports conflicting results [START_REF] Ye | Biodesulfurization of high sulfur fat coal with indigenous and exotic microorganisms[END_REF]. Cultures enriched from a hard coal mine in Poland and a well-characterised consortium, KCC, were tested for their ability in suiting both goals of the CEReS project. In order to design an optimal consortium, the microbial ecology of the sampling site and of experimental reactors was investigated in relation to their geochemistry.

Materials and methods

2-1. Bioleaching experiments

A sampling campaign was carried out on a waste dump in Janina mine (Libiąż, Poland). Samples, collected from various surface locations, were used for enriching bioleaching cultures at three temperatures, here named TW-30, TW-42 and TW-48, according to their enrichment temperature (30°C, 42°C and 48°C). Additional samples have been collected to characterise the microbial community of the heap.

2-2. Bioleaching experiments and process scale-up

Bioleaching experiments were performed in batch reactors using unweathered tailings from a spiral separator in Janina mine ((i.e. JST, Janina spiral tails)). Reactors were purged with CO2-enriched air (2% v/v) operating at 0.5 L/min. Experiments were carried out with increasing solid load, (5%, 10% and 15%) and at increasing working volumes (0.5L, 2L). Bioreactors were inoculated at 10% (v/v) with consortia described above, each at its respective temperature. Further experiments were performed inoculating with the allochthonous, moderately thermophilic consortium KCC. This consortium is well characterized (the main strains are Leptospirillum ferriphilum, Sulfobacillus benefaciens, Acidithiobacillus caldus and Ferroplasma acidiphilum) and has been already applied with high performances in several bioleaching applications [START_REF] Bryan | The efficiency of indigenous and designed consortia in bioleaching stirred tank reactors[END_REF][START_REF] Hubau | Continuous production of a biogenic ferric iron lixiviant for the bioleaching of printed circuit boards (PCBs)[END_REF]. Consortia performances were assessed monitoring the following variables: redox potential, pH, planktonic cell count, Fe concentration and speciation. At the end of each bioleaching run, samples of the slurry were centrifuged to remove the supernatant and stored at -80°C until subsequent extraction of genomic DNA (Power Soil DNA Isolation kit, Qiagen).

Samples collected in Janina mine were used for characterization of the microbial community by sequencing V4-V5 region of 16S rDNA (Illumina MiSeq, universal primer set 515WF/928WR). Obtained sequences were analysed using FROGS bioinformatics pipeline in the Galaxy platform [START_REF] Escudié | FROGS: Find, Rapidly, OTUs with Galaxy Solution[END_REF]. Variations in the community structure during bioreactor experiments and in enrichment cultures were monitored by CE-SSCP [START_REF] Bryan | The efficiency of indigenous and designed consortia in bioleaching stirred tank reactors[END_REF]. In addition, TW-48 consortium was sequenced by Oxford Nanopore MinIONS (universal primer set 515F/1492R), which allows to resolve lower taxonomic levels than Illumina MiSeq.

Results and Discussion

3-1. Microbial ecology of the site and enrichments of Fe/S oxidizing consortia Despite a certain variability among sampling points, the microbial community of the waste dump was largely dominated by strains typically found in metal rich, low pH environments (e.g. Acidithiobacillaceae). Several strains commonly found in saline environemnts were also present in large amount and ubiquituous in the dump (e.g. Acidihalobacter spp., Maritimimonas spp.).

Of the three microbial cultures enriched from this initial community, TW-42 showed poor pyrite dissolution and very low cell numbers (data not shown). On the contrary, TW-48 showed the best performances. Results obtained by combining CE-SSCP profiles and sequencing by MinIONS revealed that TW-48 was largely dominated by an unidentified strain affiliated to Acidimicrobiaceae family, followed by Sulfobacillus thermosulfidooxidans and At. caldus in lower relative abundances.

Due to its instability, it was no possible to obtain enough DNA for characterising TW-42.

3-2. Microbial communities in batch reactor experiments

Best pyrite dissolution performances were obtained in bioreactors inoculated with the consortia TW-48 and KCC (data not shown). The mesophilic consortium TW-30 showed significantly lower Fig. 1 CE-SSCP profiles of BRGM-KCC (panels "a", "b", "c" and "d") and TW48 (panels "e", "f", "g", "h") in 2L reactor experiments, increasing solid load.

pyrite dissolution yields and slower kinetics. As observed in shake flasks, bioreactors inoculated with TW-42 showed poor results, indicating that the waste heap harboured organisms capable of growth and mineral biooxidation at 30°C and 48°C but not 42°C. For these reasons, TW-48 and KCC were selected for deeper investigations. During scale-up both consortia became dominated by S. thermosulfidooxidans (Fig. 1). In particular, the relative abundance of L. ferriphilum in KCC inoculating reactors declined gradually. Coupled with the absence of a 42°C culture from the dump, it was theorised that the waste was inhibitory to Leptospirillum spp. Due to the presence of Cl (3% w/w) in JST we hypothesized an inhibition due to salt dissolution from coal. However, the highest concentration of chloride observed in the reactors (at 10% w/v solids) was found to be less than 15 mM, well below the range normally tolerated by this organism.

Conclusions

The waste dump sampled in this study harboured several strains known for their bioleaching potential, including Leptospirillum spp.. However, the scale-up of the bioleaching process seemed to drive consortia to a convergent dominance by Sulfobacillus spp.. In order to explore the possible reasons for the absence of Leptospirillum in the enrichment cultures and in reactors, a detailed analysis of the waste heap was undertaken using a range of geochemical and molecular biology tools.

The effect of the solid load, as well as the salt content of the waste, were excluded as possible causes of such alterations. Results seems to suggest the role of an as-yet unidentified organic carbon compound liberated from the coal itself in bioreactor conditions.

Acknowledgement:

This work funded by the European Commission under the Research Fund for Coal and Steel (CEReS project, grant No 709868).