Emilien Buet
email: emilien.buet@cea.fr

James Braun
email: james.braun@cea.fr

Cédric Sauder
email: cedric.sauder@cea.fr

Influence of Texture and Thickness of Pyrocarbon Coatings as Interphase on the Mechanical Behavior of Specific 2.5D SiC/SiC Composites Reinforced with Hi-Nicalon S Fibers

Keywords: interphase, pyrocarbon, ceramic matrix composites, texture, SiC, fibers

In the framework of SiC/SiC composite development for nuclear applications, the influence of pyrocarbon interphase texture and thickness on the mechanical behavior both on as-processed materials and on irradiated materials is a major concern. Thus, the PyC interphase influence has to be clearly addressed to define its optimal chemical vapor infiltration processing parameters. For this purpose, specific 2.5D SiC/SiC composites reinforced with Hi-Nicalon S fibers and with two kinds of PyC texture and thickness were produced. Transmission electronic microscopy allowed PyC thickness and microstructure/texture characterizations, whereas push-out and tensile tests were employed as experimental mechanical procedures. The original result is that PyC nature directly influences the interfacial shear stress and failure mode of the weakest interface, regardless of the PyC thickness within the studied range. Adhesive failure or cohesive failure are highlighted depending on the PyC CVI deposition mechanisms. Similar post-irradiation characterizations will be required to assess the role of irradiation on the PyC microstructure/texture evolution and mechanical behavior of these materials.

Introduction

SiC composites are widely studied for high-temperature severe environment applications. They are being introduced in aero-engines or land-based gas turbines in hot section components to replace superalloys to increase the performance while reducing the weight [START_REF] Spriet | CMC Applications to Gas Turbines[END_REF][START_REF] Dicarlo | Advances in SiC/SiC Composites for Aero-Propulsion[END_REF][START_REF] Lewis | Application of Uncooled Ceramic Matrix Composite for Turbine Blades for Performance Improvement of Advanced Turboshaft Engines[END_REF]. SiC excellent behavior in a neutron environment at high temperatures [START_REF] Katoh | Radiation Effects in SiC for Nuclear Structural Applications[END_REF][START_REF] Katoh | Radiation Effects[END_REF] is of particular interest for nuclear power plant core applications [START_REF] Sauder | Nuclear Applications[END_REF], where SiC/SiC could improve the performance and safety of light water reactors (LWR) [START_REF] Terrani | Accident Tolerant Fuel Cladding Development: Promise, Status, and Challenges[END_REF]. These materials are also one of the best options for the development of generation IV high-temperature nuclear reactor core materials [START_REF] Sauder | Nuclear Applications[END_REF].

For these applications, 3rd generation SiC fibers (Hi-Nicalon S, Tyranno SA3 or SA4 and Sylramic) reinforced SiC/SiC composites are considered because of their excellent corrosion resistance. Up to now, Hi-Nicalon S (HNS) fibers seem to be more relevant for nuclear applications [START_REF] Sauder | Nuclear Applications[END_REF][START_REF] Katoh | Continuous SiC Fiber, CVI SiC Matrix Composites for Nuclear Applications: Properties and Irradiation Effects[END_REF], even if recent studies tend to demonstrate that neutron irradiation leads to fiber degradation at a high dose and/or low irradiation temperature [START_REF] Koyanagi | Design and Strategy for Next-Generation Silicon Carbide Composites for Nuclear Energy[END_REF][START_REF] Katoh | Silicon Carbide and Its Composites for Nuclear Applications-Historical Overview[END_REF][START_REF] Koyanagi | Mechanical Property Degradation of High Crystalline SiC Fiber-Reinforced SiC Matrix Composite Neutron Irradiated to ~100 Displacements per Atom[END_REF].

In these materials, the deposition of an interphase (small coating) between the fiber and the matrix is required to ensure a specific fiber/matrix bonding. This fiber/matrix bonding must be strong enough to guarantee load transfer from the matrix to the fibers but not too strong to deflect matrix cracks at the interface (through interfacial debonding) [START_REF] Kagawa | Two Approaches for Interface Design of Continuous Fiber Ceramic Matrix Composites[END_REF]. Currently, for nuclear applications, thin pyrocarbon (PyC) deposits are used as an interphase material due to their chemical compatibility with SiC up to high temperatures and their ability to promote interfacial debonding [START_REF] Lamon | Microcomposite Test Procedure for Evaluating the Interface Properties of Ceramic Matrix Composites[END_REF][START_REF] Naslain | Fibre-Matrix Interphases and Interfaces in Ceramic Matrix Composites Processed by CVI[END_REF].

The pyrocarbon interphase microstructure processed by chemical vapor infiltration (CVI) can vary from isotropic to graphitic. Most of these low-temperature CVI pyrocarbons consist of rough laminar (RL) anisotropic pyrocarbon [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF]. Many pyrocarbon microstructures with different textural organizations can be produced by changing the CVI parameters. This textural organization is commonly used for PyC classification [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF][START_REF] Lavenac | Microstructure of the Pyrocarbon Matrix in Carbon/Carbon Composites[END_REF]. The influence of the nature of a PyC interphase (thickness and/or texture) on the mechanical behaviors of C/SiC was already studied [START_REF] Fellah | Impact of Ex-PAN Carbon Fibers Thermal Treatment on the Mechanical Behavior of C/SiC Composites and on the Fiber/Matrix Coupling[END_REF][START_REF] Zhao | Effect of Pyrolytic Carbon Interphase on Mechanical Properties of Mini T800-C/SiC Composites[END_REF][START_REF] Duan | Effect of Pyrocarbon Interphase Texture and Thickness on Tensile Damage and Fracture in T-700TM Carbon Fiber-Reinforced Silicon Carbide Minicomposites[END_REF], but these studies were only dedicated to carbon fiber reinforcement. Although the influence of pyrocarbon interphase thickness is well known in SiC/SiC composites [START_REF] Yang | Hi-Nicalon Fiber-Reinforced CVI-SiC Matrix Composites: I Effects of PyC and PyC-SiC Multilayers on the Fracture Behaviors and Flexural Properties[END_REF][START_REF] Sauder | Influence of Interface Characteristics on the Mechanical Properties of Hi-Nicalon Type-S or Tyranno-SA3 Fiber-Reinforced SiC/SiC Minicomposites: Influence of Interface Characteristics on the Mechanical Properties[END_REF][START_REF] Kabel | Micro-Mechanical Evaluation of SiC-SiC Composite Interphase Properties and Debond Mechanisms[END_REF][START_REF] Karakoc | Fiber/Matrix Debonding Evaluation of SiCf/SiC Composites Using Micropillar Compression Technique[END_REF][START_REF] Nakazato | Appropriate Thickness of Pyrolytic Carbon Coating on SiC Fiber Reinforcement to Secure Reasonable Quasi-Ductility on NITE SiC/SiC Composites[END_REF][START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF], its textural organization influence has only been studied recently on unidirectional model composites (minicomposites) [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF]. Moreover, the irradiation behavior of a carbon phase depends on its microstructure/texture [START_REF] Snead | Swelling of Nuclear Graphite and High Quality Carbon Fiber Composite under Very High Irradiation Temperature[END_REF], but the consequence on fiber/matrix bonding in SiC/SiC composites is still unknown and needs to be addressed.

In the present work, the influence of both pyrocarbon interphase thickness and texture on mechanical behavior was investigated on HNS reinforced 2.5D SiC/SiC composites. Specific small SiC/SiC specimens were processed for further post-irradiation characterizations. Two pyrocarbon thicknesses and textures were investigated using push-out and mechanical tensile tests to determine the fiber/matrix bonding strength. Pyrocarbon thickness and texture characterizations were assessed by transmission electron microscopy (TEM). Finally, all mechanical characterizations are correlated to PyC microstructure/texture.

Materials and Methods

SiC/SiC Specimen Processing

Hi-Nicalon S (NGS Advanced Fibers Co, Toyama, Japan) fibers were used as reinforcement. The fiber's properties, provided by the supplier, are summarized in Table 1. Previous studies were dedicated to specific characterizations of this fiber [START_REF] Dong | Characterization of Nearly Stoichiometric SiC Ceramic Fibres[END_REF][START_REF] Sauder | Tensile Creep Behavior of SiC-Based Fibers with a Low Oxygen Content[END_REF][START_REF] Buet | Influence of Chemical and Physical Properties of the Last Generation of Silicon Carbide Fibres on the Mechanical Behaviour of SiC/SiC Composite[END_REF] and concluded that the extreme fiber surface is composed of a thin carbon layer.

Characteristic HNS Fiber

Lot number/Type 418206 Young Modulus E (GPa) 381 Strength (GPa) 2.7 Density (g/cm 3 ) 3.0 Diameter (µm) 13 Sizing content (%) 0.5 Tex (g/km) 202

The final goal was the production of SiC/SiC specimens compatible with an experimental sodium fast reactor irradiation (BOR-60 in Dimitrovgrad, Russia). To be irradiated, samples need to be small enough to be enclosed in an 80 mm length and 30 mm diameter capsule. In this aim, 2.5D thin woven architecture was produced. This texture improves through-thickness strength, conductivity and delamination resistance in comparison with 2D architecture [START_REF] Bhatt | Influence of Fiber Architecture on Impact Resistance of Uncoated SiC/SiC Composites[END_REF]. The fibrous textile produced is composed of 3 warp and 4 weft layers with 10 yarn/cm density in both directions. PyC interphase and SiC matrix depositions were processed by CVI at a temperature below 1100 • C with propane (C 3 H 8 ) and methyltrichlorosilane (MTS) + H 2 as PyC and SiC precursors, respectively. Two PyC thicknesses and textures samples were produced to evaluate their influence. PyC thicknesses (30 nm and 150 nm) were processed to respect nuclear applications recommendations of SiC/SiC composites (e PyC ≤ 150 nm) [START_REF] Nozawa | The Effects of Neutron Irradiation on Shear Properties of Monolayered PyC and Multilayered PyC/SiC Interfaces of SiC/SiC Composites[END_REF][START_REF] Katoh | Effect of Neutron Irradiation on Tensile Properties of Unidirectional Silicon Carbide Composites[END_REF][START_REF] Nozawa | Intermediate-Temperature Neutron Irradiation Effects on Silicon Carbide Composites with Varied Fiber/Matrix Interfaces[END_REF]. As previous studies did not highlight a large difference in mechanical behavior within this thickness range, only two different thicknesses were processed. Highly anisotropic (HA) and lowly anisotropic (LA) PyC textures were targeted, as referenced in [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF]. After complete CVI SiC matrix densification, plates were cut into small specimens (length = 60 mm and width = 8 mm) for mechanical characterizations. Typical SiC/SiC specimens are presented in Figure 1a; details of specimen orientations for tensile mechanical tests are presented Figure 1b.

composites (𝑒

150 nm) [START_REF] Nozawa | The Effects of Neutron Irradiation on Shear Properties of Monolayered PyC and Multilayered PyC/SiC Interfaces of SiC/SiC Composites[END_REF][START_REF] Katoh | Effect of Neutron Irradiation on Tensile Properties of Unidirectional Silicon Carbide Composites[END_REF][START_REF] Nozawa | Intermediate-Temperature Neutron Irradiation Effects on Silicon Carbide Composites with Varied Fiber/Matrix Interfaces[END_REF]. As previous studies did not highlight a large difference in mechanical behavior within this thickness range, only two different thicknesses were processed. Highly anisotropic (HA) and lowly anisotropic (LA) PyC textures were targeted, as referenced in [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF]. After complete CVI SiC matrix densification, plates were cut into small specimens (length = 60 mm and width = 8 mm) for mechanical characterizations. Typical SiC/SiC specimens are presented in Figure 1a; details of specimen orientations for tensile mechanical tests are presented Figure 1b. As sample roughness is important in comparison with specimen thickness, porosity and section measurements were assessed by image analysis of several optical microscopy sections (Figure 2) rather than by dimensional measurements of samples thickness and width. The resulting characteristics of the manufactured SiC/SiC composites are presented in Table 2 for PyC thicknesses (no effect on PyC texture).

Thermal induced Residual Stresses

Thermal residual stresses are generated consecutively to the CVI process final cooling. Detailed thermal-induced residual stresses calculation in each phase (i.e., fiber, interphase and matrix) is detailed elsewhere [START_REF] Michaux | Young's Modulus, Thermal Expansion Coefficient and Fracture Behavior of Selected Si-B-C Based Carbides in the 20-1200 • C Temperature Range as Derived from the Behavior of Carbon Fiber Reinforced Microcomposites[END_REF]. Even if the model is based on a single fiber and concentric cylinders of interphase and matrix, good correlations for minicomposites [START_REF] Duan | Effect of Pyrocarbon Interphase Texture and Thickness on Tensile Damage and Fracture in T-700TM Carbon Fiber-Reinforced Silicon Carbide Minicomposites[END_REF] and 2D tubular composites [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF] have already been reported. The thermomechanical properties as a function of temperature were fitted with Equation (1) and the constants used for the calculations are presented in Table 3. As sample roughness is important in comparison with specimen thickness, porosity and section measurements were assessed by image analysis of several optical microscopy sections (Figure 2) rather than by dimensional measurements of samples thickness and width. The resulting characteristics of the manufactured SiC/SiC composites are presented in Table 2 for PyC thicknesses (no effect on PyC texture).

Coatings 2022, 12, x FOR PEER REVIEW 3 of 16 composites (𝑒 150 nm) [START_REF] Nozawa | The Effects of Neutron Irradiation on Shear Properties of Monolayered PyC and Multilayered PyC/SiC Interfaces of SiC/SiC Composites[END_REF][START_REF] Katoh | Effect of Neutron Irradiation on Tensile Properties of Unidirectional Silicon Carbide Composites[END_REF][START_REF] Nozawa | Intermediate-Temperature Neutron Irradiation Effects on Silicon Carbide Composites with Varied Fiber/Matrix Interfaces[END_REF]. As previous studies did not highlight a large difference in mechanical behavior within this thickness range, only two different thicknesses were processed. Highly anisotropic (HA) and lowly anisotropic (LA) PyC textures were targeted, as referenced in [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF]. After complete CVI SiC matrix densification, plates were cut into small specimens (length = 60 mm and width = 8 mm) for mechanical characterizations. Typical SiC/SiC specimens are presented in Figure 1a; details of specimen orientations for tensile mechanical tests are presented Figure 1b. As sample roughness is important in comparison with specimen thickness, porosity and section measurements were assessed by image analysis of several optical microscopy sections (Figure 2) rather than by dimensional measurements of samples thickness and width. The resulting characteristics of the manufactured SiC/SiC composites are presented in Table 2 for PyC thicknesses (no effect on PyC texture).

Thermal induced Residual Stresses

Thermal residual stresses are generated consecutively to the CVI process final cooling. Detailed thermal-induced residual stresses calculation in each phase (i.e., fiber, interphase and matrix) is detailed elsewhere [START_REF] Michaux | Young's Modulus, Thermal Expansion Coefficient and Fracture Behavior of Selected Si-B-C Based Carbides in the 20-1200 • C Temperature Range as Derived from the Behavior of Carbon Fiber Reinforced Microcomposites[END_REF]. Even if the model is based on a single fiber and concentric cylinders of interphase and matrix, good correlations for minicomposites [START_REF] Duan | Effect of Pyrocarbon Interphase Texture and Thickness on Tensile Damage and Fracture in T-700TM Carbon Fiber-Reinforced Silicon Carbide Minicomposites[END_REF] and 2D tubular composites [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF] have already been reported. The thermomechanical properties as a function of temperature were fitted with Equation (1) and the constants used for the calculations are presented in Table 3.

Thermal Induced Residual Stresses

Thermal residual stresses are generated consecutively to the CVI process final cooling. Detailed thermal-induced residual stresses calculation in each phase (i.e., fiber, interphase and matrix) is detailed elsewhere [START_REF] Michaux | Young's Modulus, Thermal Expansion Coefficient and Fracture Behavior of Selected Si-B-C Based Carbides in the 20-1200 • C Temperature Range as Derived from the Behavior of Carbon Fiber Reinforced Microcomposites[END_REF]. Even if the model is based on a single fiber and concentric cylinders of interphase and matrix, good correlations for minicomposites [START_REF] Duan | Effect of Pyrocarbon Interphase Texture and Thickness on Tensile Damage and Fracture in T-700TM Carbon Fiber-Reinforced Silicon Carbide Minicomposites[END_REF] and 2D tubular composites [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF] have already been reported. The thermomechanical properties as a function of temperature were fitted with Equation (1) and the constants used for the calculations are presented in Table 3.

Y = AT 3 + BT 2 + CT + D (1)
α t ( • C -1 ) =α l =α l C = 2 × 10 -9 C = 2 × 10 -9 D = 5 × 10 -6 D = 10 × 10 -6
A, B, C and D are constants, T the temperature ( • C) and Y the thermomechanical property.

The calculated residual thermal stresses of all SiC/SiC samples are presented in Figure 3. For all the composites, the level of residual thermal stresses in the fiber and matrix are similar and sufficiently low to ensure that fibers and matrix are undamaged after CVI processing (neither matrix cracks nor fiber failure). No effect of PyC thickness and texture is observed on fiber and matrix residual thermal stresses. The interphase is under tension in radial and circumferential directions and under compression in the longitudinal direction for all SiC/SiC samples. Interphase stresses in longitudinal and circumferential directions are quite high for the HA interphase.

Y = AT 3 + BT 2 + CT + D (1)
A, B, C and D are constants, T the temperature (°C) and Y the thermomechanical property.

Table 3. Coefficients used in the analytical model for Young's modulus, Poisson's ratio and coefficient of thermal expansion (l for longitudinal direction and t for transverse direction). The calculated residual thermal stresses of all SiC/SiC samples are presented in Figure 3. For all the composites, the level of residual thermal stresses in the fiber and matrix are similar and sufficiently low to ensure that fibers and matrix are undamaged after CVI processing (neither matrix cracks nor fiber failure). No effect of PyC thickness and texture is observed on fiber and matrix residual thermal stresses. The interphase is under tension in radial and circumferential directions and under compression in the longitudinal direction for all SiC/SiC samples. Interphase stresses in longitudinal and circumferential directions are quite high for the HA interphase. 2.

Properties

Characterization of PyC Texture

Due to the low thickness of processed interphases, PyC texture characterization is assessed by transmission electron microscopy (TEM, JEOL, Tokyo, Japan). A JEOL 2011 microscope equipped with a LaB6 gun, operating at 200 kV with a 0.144 nm resolution lattice fringe mode, was used. Sample cross-sections were cut and polished down to 100 2.

Coatings 2022, 12, 573 5 of 15

Characterization of PyC Texture

Due to the low thickness of processed interphases, PyC texture characterization is assessed by transmission electron microscopy (TEM, JEOL, Tokyo, Japan). A JEOL 2011 microscope equipped with a LaB 6 gun, operating at 200 kV with a 0.144 nm resolution lattice fringe mode, was used. Sample cross-sections were cut and polished down to 100 µm thickness before final thinning by "ion slicer" apparatus (JEOL). Samples were then deposited on a standard TEM copper grid. The (002) Lattice Fringe (002LF) highresolution mode was used to obtain the PyC structure and texture. As these pyrocarbon interphases are very thin, it is not possible to employ Raman microspectroscopy (RMS) and/or optical microscopy in the cross-polarization configuration characterizations to use the PyC classification (ReL, RL and SL) used in [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF]. That is why only the orientation angle (OA) [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF][START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF][START_REF] Kabel | Direct Observation of the Elasticity-Texture Relationship in Pyrolytic Carbon via in Situ Micropillar Compression and Digital Image Correlation[END_REF] was used to distinguish the PyC nature. The higher the OA, the more isotropic the PyC texture is. For OA in the 20-40 • range, PyC is highly anisotropic (HA); in the 60-70 • range, PyC is lowly anisotropic (LA). Fast Fourier transform (FFT) of 002 lattice fringe was applied on several areas of 30 × 30 nm to determine OA; therefore, this area includes sufficient basic structural units (BSU) for representative OA measurements [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF].

Mechanical Tests 2.4.1. Push-Out Tests

A nano-indentation tester (Nanotest 550, Micro Materials, Wrexham, United Kingdom) was used for single-fiber push-out tests with a maximal load of 500 mN. The same procedure as the one in [START_REF] Buet | Influence of Surface Fibre Properties and Textural Organization of a Pyrocarbon Interphase on the Interfacial Shear Stress of SiC/SiC Minicomposites Reinforced with Hi-Nicalon S and Tyranno SA3 Fibres[END_REF] was employed. The analysis of the load-displacement push-out test curves allows the measurement of the decohesion normal stress (σ d ) and the interfacial shear stress (τ). σ d is the stress needed to break the F/M bond and to initiate the fiber/matrix exfoliation and τ is the stress above which the F/M bonding is completely lost (followed by a free sliding of fiber). σd and τ measurements by push-out tests are the only way to obtain a quantitative evaluation of F/M bonding strength in composites.

To ensure reliable measurements, about 20 fibers were pushed out for each sample. Post mortem SEM observations were conducted to see if the flat punch indenter was centered on the fiber during the push-out test. Otherwise, results were not taken into account.

Tensile Mechanical Tests

Uniaxial tensile tests were performed on an Instron 2404 device (10 kN load cell, Instron, Norwood, United States) using the procedure described in the ISO 20,323 standard [START_REF]of Tubes: International Organization for Standardization[END_REF] on a minimum of 3 samples for each type of material. Samples are glued on end collars on the mechanical testing device with 3M scotch-Weld TM 9323 B/A epoxide structural glue. The crosshead displacement rate was 0.05 mm/min. The strain was measured with a 25 mm gauge length Instron 2620-603 extensometer (±1 mm course) calibrated with a high-resolution numeric calibrator (0.1 µm). Uniaxial tensile tests were performed along the warp direction as described in § 2.1 and Figure 1. An acoustic emission system fixed on the upper-end collar was used for matrix multicracking detection. The measured signal accounts for events over 50 dB (10 dB over background noise) and is presented as a cumulated signal corresponding to events.

To gain information on damage evolution (matrix multicracking) within the composite during tensile mechanical loading, up to 6 unloading-reloading sequences were conducted. Details about this testing procedure are available in [START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF]. E 0 is the initial Young's modulus (before matrix cracking) and is measured by the slope of the initial linear part of the stressstrain curve. Unloading-reloading cycles lead to the determination of reduced modulus (E/E 0 ), area of the cycles (∆W) and anelastic residual strains after unloading (ε r ) [START_REF] Camus | Development of Damage in a 2D Woven C/SiC Composite under Mechanical Loading: I. Mechanical Characterization[END_REF][START_REF] Baste | Inelastic Behaviour of Ceramic-Matrix Composites[END_REF]. The longitudinal elastic modulus (E) is determined by measuring the slope of the line passing through the summit of cycle (σ max , e max ) and the median point of the cycle for a σ max/2 stress [START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF][START_REF] Domergue | Relationships between Hysteresis Measurements and the Constituent Properties of Ceramic Matrix Composites: II, Experimental Studies on Unidirectional Materials[END_REF]. Reduced modulus decreases as a function of applied stress due to matrix multicracking occurring with raising stress. At high stress, if complete matrix multicracking occurs, the reduced modulus reaches an asymptotic behavior related to the fiber fraction in the mechanical solicitation direction and Young's modulus as expressed in Equation ( 2). E f and V f are the tow Young's modulus and the fiber volume fraction in composite, respectively. X is the volume fraction of fiber oriented in the tensile mechanical load direction (i.e., warp direction). In this study, X = 0.429 for the specific 2.5D woven architecture described in § 2.1.

lim ε→ε max E E 0 = X E f V f E 0 (2)

Results

Pyrocarbon Texture Characterization

The pyrocarbon microstructures were observed by HRTEM analysis of the (002) lattice fringes (Figure 4) on 150 nm interphase thickness samples. The OA values measured are presented in Table 4. Results confirmed that there is a strong difference in textural organization between the two kinds of processed pyrocarbon. As expected, LA (OA = 61 • ) and HA (OA = 44 • ) textures are confirmed. Even if only OA is not sufficient to thoroughly classify pyrocarbon [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF], the detailed analysis of the HRTEM (002) lattice fringes micrograph of the two pyrocarbon tends to demonstrate that PyC LA is probably a smooth laminar (SL) pyrocarbon and PyC HA a rough laminar (RL) pyrocarbon [START_REF] Vallerot | Quantitative Structural and Textural Assessment of Laminar Pyrocarbons through Raman Spectroscopy, Electron Diffraction and Few Other Techniques[END_REF]. As RMS and/or polarized optical microscopy analysis of such thin pyrocarbon is not possible to confirm this standard classification of processed pyrocarbon, the LA and HA denomination is retained here.

Push-Out Tests

At least 20 push-out tests were performed on each type of material to assess the effect of pyrocarbon thickness and texture on the fiber/matrix bonding in HNS/PyC/SiC composites. Figure 5 shows typical push-out test curves for each PyC interphase specimen. The consecutive mean values of σd and τ for all materials are reported in Table 5. σd is three times higher for PyCHA than for PyCLA for a given thickness and decreases when the PyC thickness increases. τ follows the same trends but differences are less pronounced. These results highlight the influence of PyC texture and PyC thickness on F/M bonding in HNS/PyC/SiC composites. PyC thickness reduces the F/M bonding, as previously demonstrated [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Igawa | Optimizing the Fabrication Process for Superior Mechanical Properties in the FCVI SiC Matrix/Stoichiometric SiC Fiber Composite System[END_REF][START_REF] Yang | Mechanical Properties of Several Advanced Tyranno-SA Fiber-Reinforced CVI-SiC Matrix Composites[END_REF], but to a greater extent for PyCLA interphase than for PyCHA interphase. Finally, the original result is that PyCLA leads to lower F/M bonding strength compared to PyCHA for a similar thickness.

Push-Out Tests

At least 20 push-out tests were performed on each type of material to assess the effect of pyrocarbon thickness and texture on the fiber/matrix bonding in HNS/PyC/SiC composites. Figure 5 shows typical push-out test curves for each PyC interphase specimen. The consecutive mean values of σ d and τ for all materials are reported in Table 5. σ d is three times higher for PyC HA than for PyC LA for a given thickness and decreases when the PyC thickness increases. τ follows the same trends but differences are less pronounced. These results highlight the influence of PyC texture and PyC thickness on F/M bonding in HNS/PyC/SiC composites. PyC thickness reduces the F/M bonding, as previously demonstrated [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Igawa | Optimizing the Fabrication Process for Superior Mechanical Properties in the FCVI SiC Matrix/Stoichiometric SiC Fiber Composite System[END_REF][START_REF] Yang | Mechanical Properties of Several Advanced Tyranno-SA Fiber-Reinforced CVI-SiC Matrix Composites[END_REF], but to a greater extent for PyC LA interphase than for PyC HA Coatings 2022, 12, 573 7 of 15 interphase. Finally, the original result is that PyC LA leads to lower F/M bonding strength compared to PyCHA for a similar thickness.

sites. Figure 5 shows typical push-out test curves for each PyC interphase specimen. The consecutive mean values of σd and τ for all materials are reported in Table 5. σd is three times higher for PyCHA than for PyCLA for a given thickness and decreases when the PyC thickness increases. τ follows the same trends but differences are less pronounced. These results highlight the influence of PyC texture and PyC thickness on F/M bonding in HNS/PyC/SiC composites. PyC thickness reduces the F/M bonding, as previously demonstrated [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Igawa | Optimizing the Fabrication Process for Superior Mechanical Properties in the FCVI SiC Matrix/Stoichiometric SiC Fiber Composite System[END_REF][START_REF] Yang | Mechanical Properties of Several Advanced Tyranno-SA Fiber-Reinforced CVI-SiC Matrix Composites[END_REF], but to a greater extent for PyCLA interphase than for PyCHA interphase. Finally, the original result is that PyCLA leads to lower F/M bonding strength compared to PyCHA for a similar thickness. 6 for each type of SiC/SiC specimen and summarized in Figure 7 for easier comparison. Mechanical properties are given in Table 6. Mechanical behaviors of all specimens are similar and correspond to intermediate/high interfacial shear stress (ISS) mechanical behavior [START_REF] Evans | The Physics and Mechanics of Fibre-Reinforced Brittle Matrix Composites[END_REF][START_REF] Lissart | Damage and Failure in Ceramic Matrix Minicomposites: Experimental Study and Model[END_REF]. Composites made of PyC LA have lower moduli and higher failure strain than PyC HA composites. No influence of interphase thickness is highlighted. For the same PyC interphase texture (LA or HA), modulus and failure characteristics are similar for 50 and 150 nm PyC thicknesses.

The debonding lengths (fiber pull-out) observed on SEM fracture surface micrographs (Figure 8) tend to confirm push-out results. Debonding lengths seem shorter for PyC HA composites than for PyC LA -based, which could confirm the higher F/M bonding strength for PyC HA interphase. Moreover, the influence of PyC interphase thickness is not clearly visible on PyC HA . This effect seems more visible for PyC LA , with longer debonding lengths for the thickest PyC. At this scale (above the tow scale) and for processed materials, differences between samples are small and only qualitative analysis of fracture surfaces is possible. graphs (Figure 8) tend to confirm push-out results. Debonding lengths seem shorter for PyCHA composites than for PyCLA-based, which could confirm the higher F/M bonding strength for PyCHA interphase. Moreover, the influence of PyC interphase thickness is not clearly visible on PyCHA. This effect seems more visible for PyCLA, with longer debonding lengths for the thickest PyC. At this scale (above the tow scale) and for processed materials, differences between samples are small and only qualitative analysis of fracture surfaces is possible.

Unloading-Reloading Cycles

stress-strain curves (with unloading-reloading are presented in Figure 9 for of SiC/SiC specimen. The evolution of reduced modulus (E/E0), area of cycles (∆W) and residual strain (εr) as a function of the maximum stress before unloading sequences is presented 10 for all composites.

The evolution of reduced modulus (Figure 10a) is similar for all materials, with a decrease above 50 MPa consecutive to the beginning of the matrix multicracking phenomenon. The saturation value of matrix cracking (Equation ( 2)) is reached in the 200-250 MPa range, before the ultimate failure, as observed in previous studies on HNS-based composites [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF]. This tends to demonstrate that ISS is not strong, but intermediate for these materials. No significant influence of interphase texture and/or thickness is observed on reduced modulus. The analyses of the area of cycles (Figure 10b) and residual strains (Figure 10c) evolution led to the same conclusions; therefore, PyC texture and/or thickness (within the studied range) have no significant influence on the composite damage resistance (matrix cracking and/or opening of cracks).

Unloading-Reloading Cycles Analysis

Typical stress-strain curves (with unloading-reloading sequences) are presented in Figure 9 for each type of SiC/SiC specimen. The evolution of reduced modulus (E/E 0 ), area of cycles (∆W) and residual strain (ε r ) as a function of the maximum stress applied before unloading sequences is presented in Figure 10 for all composites.

The evolution of reduced modulus (Figure 10a) is similar for all materials, with a rapid decrease above 50 MPa consecutive to the beginning of the matrix multicracking phenomenon. The saturation value of matrix cracking (Equation ( 2)) is reached in the 200-250 MPa range, before the ultimate failure, as observed in previous studies on HNSbased composites [START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Braun | Influence of an Original Manufacturing Process on the Properties and Microstructure of SiC/SiC Tubular Composites[END_REF]. This tends to demonstrate that ISS is not strong, but intermediate for these materials. No significant influence of interphase texture and/or thickness is observed on reduced modulus. The analyses of the area of cycles (Figure 10b) and residual strains (Figure 10c) evolution led to the same conclusions; therefore, PyC texture and/or thickness (within the studied range) have no significant influence on the composite damage resistance (matrix cracking and/or opening of existing cracks).

Discussion

Tensile mechanical behaviors do not clearly put into evidence any effect of PyC texture and thickness. For each PyC texture, PyC thickness (in the 30-150 nm range) has no significant influence on the tensile mechanical behavior, previously observed [START_REF] Yang | Hi-Nicalon Fiber-Reinforced CVI-SiC Matrix Composites: I Effects of PyC and PyC-SiC Multilayers on the Fracture Behaviors and Flexural Properties[END_REF][START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Igawa | Optimizing the Fabrication Process for Superior Mechanical Properties in the FCVI SiC Matrix/Stoichiometric SiC Fiber Composite System[END_REF]. It is possible that the PyC thickness range explored in this study is not sufficient. PyC texture only influences the failure strain, which is higher for a more isotropic PyC texture (PyCLA). Fracture surface analysis (Figure 8) after tensile tests is more interesting to highlight the qualitative effect of PyC texture and thickness. Fiber pull-out is higher for PyCLA than for PyCHA. Moreover, for each kind of texture, thicker PyC interphase leads to longer fiber pull-out. As longer fiber pull-out is related to weaker ISS, it can be deduced that F/M bonding strength is higher for PyCHA and that thicker PyC reduces the ISS.

If mechanical tensile tests and consecutive fracture surfaces only allow assessing the qualitative effect of PyC texture or thickness, push-out tests allow quantitative evaluation of F/M bonding strength by the determination of debonding shear stress (σd) and interfacial shear stress (τ). It is clear that PyCHA induces F/M bonding than PyCLA in HNSreinforced σd being three higher for PyCHA (Table 5). The influence of PyC thickness is also highlighted for both types of PyC. PyC tends to decrease F/M bonding, as already observed in previous studies It is demonstrated in this study that the influence of PyC thickness is more pronounced for PyCLA. This confirms qualitative observations based on fracture surfaces analysis. To understand the rupture mode of F/M bonding in all materials, high magnification SEM micrographs of typical F/M debonding zone for all specimens after the tensile test are presented in Figure 11.

On all specimens, PyC always remains bonded to the SiC matrix, revealing that the PyC/Matrix (PyC/M) interface is the strongest one, where debonding is never observed. The original result is that PyCLA and PyCHA lead to different failure modes. For PyCLA, interfacial failure occurs between the PyC interphase and the fiber (Figure 11c,d), leading to adhesive failure as observed in other studies [START_REF] Karakoc | Fiber/Matrix Debonding Evaluation of SiCf/SiC Composites Using Micropillar Compression Technique[END_REF][START_REF] Fellah | Influence of the Carbon Interface on the Mechanical Behavior of SiC/SiC Composites[END_REF]. The surface of HNS fiber is known to be composed of amorphous carbon resulting from the decomposition of the polymer sizing that is employed to improve weaving and handling capability. Indeed, the amorphous carbon layer/PyC interface is believed to be the weakest region of HNS-reinforced CVI composites, as observed in this case for PyCLA; however, PyCHA leads to a cohesive failure, with part of the PyC interphase remaining on both the matrix and fiber surfaces after fracture (Figure 11a,b), whatever the PyC thickness. This clearly demonstrates that the Fiber/PyCHA (F/PyC) interface is stronger than

Discussion

Tensile mechanical behaviors do not clearly put into evidence any effect of PyC texture and thickness. For each PyC texture, PyC thickness (in the 30-150 nm range) has no significant influence on the tensile mechanical behavior, unlike previously observed [START_REF] Yang | Hi-Nicalon Fiber-Reinforced CVI-SiC Matrix Composites: I Effects of PyC and PyC-SiC Multilayers on the Fracture Behaviors and Flexural Properties[END_REF][START_REF] Braun | Mechanical Behavior of SiC/SiC Composites Reinforced with New Tyranno SA4 Fibers: Effect of Interphase Thickness and Comparison with Tyranno SA3 and Hi-Nicalon S Reinforced Composites[END_REF][START_REF] Igawa | Optimizing the Fabrication Process for Superior Mechanical Properties in the FCVI SiC Matrix/Stoichiometric SiC Fiber Composite System[END_REF]. It is possible that the PyC thickness range explored in this study is not sufficient. PyC texture only influences the failure strain, which is higher for a more isotropic PyC texture (PyC LA ). Fracture surface analysis (Figure 8) after tensile tests is more interesting to highlight the qualitative effect of PyC texture and thickness. Fiber pull-out is higher for PyC LA than for PyC HA . Moreover, for each kind of texture, thicker PyC interphase leads to longer fiber pull-out. As longer fiber pull-out is related to weaker ISS, it can be deduced that F/M bonding strength is higher for PyC HA and that thicker PyC reduces the ISS.

If mechanical tensile tests and consecutive fracture surfaces only allow assessing the qualitative effect of PyC texture or thickness, push-out tests allow quantitative evaluation of F/M bonding strength by the determination of debonding shear stress (σ d ) and interfacial shear stress (τ). It is clear that PyC HA induces higher F/M bonding than PyC LA in HNSreinforced composites, σ d being three times higher for PyC HA (Table 5). The influence of PyC thickness is also highlighted for both types of PyC. PyC tends to decrease F/M bonding, as already observed in previous studies [START_REF] Sauder | Tensile Creep Behavior of SiC-Based Fibers with a Low Oxygen Content[END_REF][START_REF] Buet | Influence of Chemical and Physical Properties of the Last Generation of Silicon Carbide Fibres on the Mechanical Behaviour of SiC/SiC Composite[END_REF]. It is demonstrated in this study that the influence of PyC thickness is more pronounced for PyC LA . This confirms qualitative observations based on fracture surfaces analysis. To understand the rupture mode of F/M bonding in all materials, high magnification SEM micrographs of typical F/M debonding zone for all specimens after the tensile test are presented in Figure 11.

On all specimens, PyC always remains bonded to the SiC matrix, revealing that the PyC/Matrix (PyC/M) interface is the strongest one, where debonding is never observed. The original result is that PyC LA and PyC HA lead to different failure modes. For PyC LA , interfacial failure occurs between the PyC interphase and the fiber (Figure 11c,d), leading to an adhesive failure as observed in other studies on HNS-reinforced composites [START_REF] Karakoc | Fiber/Matrix Debonding Evaluation of SiCf/SiC Composites Using Micropillar Compression Technique[END_REF][START_REF] Fellah | Influence of the Carbon Interface on the Mechanical Behavior of SiC/SiC Composites[END_REF]. The surface of HNS fiber is known to be composed of amorphous carbon resulting from the decomposition of the polymer sizing that is employed to improve weaving and handling capability. Indeed, the amorphous carbon layer/PyC interface is believed to be the weakest region of HNS-reinforced CVI composites, as observed in this case for PyC LA ; however, PyC HA leads to a cohesive failure, with part of the PyC interphase remaining on both the matrix and fiber surfaces after fracture (Figure 11a,b), whatever the PyC thickness. This clearly demonstrates that the Fiber/PyC HA (F/PyC) interface is stronger than F/PyC LA one in HNS-reinforced composites. This is the consequence of the stronger adhesion of PyC HA on the fiber surface amorphous carbon. Moreover, PyC HA has the weakest bonds between its basal planes (more graphite-like structure) than PyC LA ; therefore, crack deflection between basal planes is more likely to occur in PyC HA . The main difference between PyC HA and PyC LA are CVI processing parameters that pilot the gas phase composition during deposition. As explained in previous studies for RL and SL PyC [START_REF] Lavenac | Microstructure of the Pyrocarbon Matrix in Carbon/Carbon Composites[END_REF][START_REF] Bouchard | Pyrocarbon Deposits on a Graphite Surface Observed by STM[END_REF], the mechanism of deposition is quite different between PyC HA (i.e., RL PyC) and PyC LA (i.e., SL PyC). On the basis of the contact angle determined in [START_REF] Bouchard | Pyrocarbon Deposits on a Graphite Surface Observed by STM[END_REF], PyC HA leads to an almost 0 • contact angle with the substrate. For this type of PyC, a total wetting is believed to occur, leading to a pyrocarbon adhesion energy on the substrate at least equal to its cohesive energy. The contact angle for PyC LA was wider; as a consequence, adhesion energy on the substrate lower than the cohesive energy within the pyrocarbon. In this previous study, the substrate was pyrographite (HOPG), which is a pure carbon material; similar results can be hinted for the adhesion of PyC on the amorphous carbon on the HNS fiber surface in this study. This analysis tends to confirm experimental results observed by push-out tests and fracture analysis after tensile mechanical tests of processed materials. F/PyC interface bonding is stronger for PyC HA (cohesive failure) than for PyC LA (adhesive failure). This is mainly the consequence of the different deposition mechanisms for both PyCs. F/PyCLA one in HNS-reinforced composites. This is the consequence of the stronger adhesion of PyCHA on the fiber surface amorphous carbon. Moreover, PyCHA has the weakest bonds between its basal planes (more graphite-like structure) than PyCLA; therefore, crack deflection between basal planes is more likely to occur in PyCHA. The main difference between PyCHA and PyCLA are CVI processing parameters that pilot the gas phase composition during deposition. As explained in previous studies for RL and SL PyC [START_REF] Lavenac | Microstructure of the Pyrocarbon Matrix in Carbon/Carbon Composites[END_REF][START_REF] Bouchard | Pyrocarbon Deposits on a Graphite Surface Observed by STM[END_REF], the mechanism of deposition is quite different between PyCHA (i.e., RL PyC) and PyCLA (i.e., SL PyC). On the basis of the contact angle determined in [START_REF] Bouchard | Pyrocarbon Deposits on a Graphite Surface Observed by STM[END_REF], PyCHA leads to an almost 0° contact angle with the substrate. For this type of PyC, a total wetting is believed to occur, leading to a pyrocarbon adhesion energy on the substrate at least equal to its cohesive energy. The contact angle for PyCLA was wider; as a consequence, adhesion energy on the substrate is lower than the cohesive energy within the pyrocarbon. In this previous study, the substrate was pyrographite (HOPG), which is a pure carbon material; similar results can be hinted for the adhesion of PyC on the amorphous carbon on the HNS fiber surface in this study. This analysis tends to confirm experimental results observed by push-out tests and fracture analysis after tensile mechanical tests of processed materials. F/PyC interface bonding is stronger for PyCHA (cohesive failure) than for PyCLA (adhesive failure). This is mainly the consequence of the different deposition mechanisms for both PyCs.

Conclusions

The influence of PyC interphase thickness (in the 50-150 nm range) and PyC nature in SiC/SiC composites was studied in detail by tensile mechanical tests and push-out tests. The influence of PyC thickness and PyC nature was minor on tensile behaviors and properties. Only the tensile strain was slightly higher for PyCLA. Fracture surfaces analysis after tensile tests allows a qualitative evaluation of the F/M bonding in composites and brings us to the

Conclusions

The influence of PyC interphase thickness (in the 50-150 nm range) and PyC nature in SiC/SiC composites was studied in detail by tensile mechanical tests and push-out tests. The influence of PyC thickness and PyC nature was minor on tensile behaviors and properties. Only the tensile strain was slightly higher for PyC LA . Fracture surfaces analysis after tensile tests allows a qualitative evaluation of the F/M bonding in composites and brings us to the same conclusion made after the push tests (quantitative measurements of F/M bonding strength). Thicker PyC reduces the F/M bonding strength. This trend is more pronounced for PyC LA than for PyC HA . In all materials, the weakest interface of the PyC interphase domain is the F/PyC interface. PyC HA leads to a cohesive failure with high ISS, whereas PyC LA leads to an adhesive failure with lower ISS. This is the consequence of the CVI deposition mechanisms between these two kinds of pyrocarbon, leading to different adhesion energy.

It has clearly been highlighted that the PyC microstructure/texture is the best processing parameter to control the nature of F/M bonding in SiC/SiC composites reinforced with HNS fibers. This is especially true when a thin PyC interphase is required, such as in nuclear applications, where PyC thickness cannot be significantly modified.

These new results are interesting for the nuclear application of HNS reinforced SiC/SiC composites, for which ISS is known to decrease following irradiation [START_REF] Nozawa | The Effects of Neutron Irradiation on Shear Properties of Monolayered PyC and Multilayered PyC/SiC Interfaces of SiC/SiC Composites[END_REF][START_REF] Katoh | Effect of Neutron Irradiation on Tensile Properties of Unidirectional Silicon Carbide Composites[END_REF][START_REF] Nozawa | The Effect of Neutron Irradiation on the Fiber/Matrix Interphase of Silicon Carbide Composites[END_REF]. The use of PyC HA as an interphase in these composites could be a way to reduce the degradation of F/M bonding following neutron irradiation. The next step will be the characterizations of these composites after a high dose of neutron irradiation to clearly identify the role of PyC interphase nature and thickness on their post-irradiation mechanical behavior. As post-irradiation experiments are quite difficult (experiments need to be conducted in glove boxes or in hot cells), SEM analysis after tensile mechanical tests could be the easiest way to understand the qualitative effect of irradiation on F/M bonding.

Figure 1 .

 1 Figure 1. SiC/SiC composites specimens as processed: (a) global picture and (b) specimen orientations for tensile mechanical tests.

Figure 2 .

 2 Figure 2. SiC/SiC sample section capture by optical microscopy.

Figure 1 .

 1 Figure 1. SiC/SiC composites specimens as processed: (a) global picture and (b) specimen orientations for tensile mechanical tests.

Figure 1 .

 1 Figure 1. SiC/SiC composites specimens as processed: (a) global picture and (b) specimen orientations for tensile mechanical tests.

Figure 2 .

 2 Figure 2. SiC/SiC sample section capture by optical microscopy.

Figure 2 .

 2 Figure 2. SiC/SiC sample section capture by optical microscopy.

Coatings 2022 , 16 Figure 3 .

 2022163 Figure 3. Radial, circumferential and longitudinal residual thermal stresses calculated in microcomposite with fiber and matrix volume fractions as given in Table2.

Figure 3 .

 3 Figure 3. Radial, circumferential and longitudinal residual thermal stresses calculated in microcomposite with fiber and matrix volume fractions as given in Table2.

Figure 4 .

 4 Figure 4. HRTEM (002) lattice fringes micrographs of the as-processed pyrocarbon: (a) PyCLA and (b) PyCHA.

Figure 4 .

 4 Figure 4. HRTEM (002) lattice fringes micrographs of the as-processed pyrocarbon: (a) PyC LA and (b) PyC HA .

Figure 5 .Figure 5 .

 55 Figure 5. Typical push-out test curve of HNS/PyCHA30nm/SiC, HNS/PyCHA150nm/SiC, HNS/PyCLA30nm/SiC and HNS/PyCLA150nm/SiC.

Figure 6 .

 6 Figure 6. Typical tensile stress-strain curves.

Figure 7 .

 7 Figure 7. Influence of PyC texture and thickness on the tensile stress-strain behavior.

Figure 6 . 16 Figure 6 .

 6166 Figure 6. Typical tensile stress-strain curves.

Figure 7 .

 7 Figure 7. Influence of PyC texture and thickness on the tensile stress-strain behavior.

StressFigure 7 .

 7 Figure 7. Influence of PyC texture and thickness on the tensile stress-strain behavior.

Figure 8 .

 8 Figure 8. Fracture surface after failure for each type of composites specimen: (a) HNS/PyCHA30nm/SiC, (b) HNS/PyCHA150nm/SiC, (c) HNS/PyCLA30nm/SiC and (d) HNS/PyCLA150nm/SiC.

Figure 8 .

 8 Figure 8. Fracture surface after failure for each type of composites specimen: (a) HNS/PyC HA30nm /SiC, (b) HNS/PyC HA150nm /SiC, (c) HNS/PyC LA30nm /SiC and (d) HNS/PyC LA150nm /SiC.

Figure 9 .Figure 9 .Figure 9 .Figure 10 .

 99910 Figure 9. Typical tensile stress-strain behaviors with unloading-reloading sequences with evolution of cumulated AE signal for (a) HNS/PyCHA30nm/SiC, (b) HNS/PyCHA150nm/SiC, (c) HNS/PyCLA30nm/SiC and (d) HNS/PyCLA150nm/SiC.

Figure 10 .

 10 Figure 10. Evolution of (a) reduced modulus (E/E 0 ), (b) the area of cycles (∆W) and (c) the residual strain (ε r ) as a function of the maximum applied stress before unloading for all composites.

Figure 11 .

 11 Figure 11. High magnification SEM micrographs of typical F/M debonding zone after failure for each type of composites specimen: (a) HNS/PyCHA30nm/SiC, (b) HNS/PyCHA150nm/SiC, (c) HNS/PyCLA30nm/SiC and (d) HNS/PyCLA150nm/SiC.

Figure 11 .

 11 Figure 11. High magnification SEM micrographs of typical F/M debonding zone after failure for each type of composites specimen: (a) HNS/PyC HA30nm /SiC, (b) HNS/PyC HA150nm /SiC, (c) HNS/PyC LA30nm /SiC and (d) HNS/PyC LA150nm /SiC.

Table 1 .

 1 Fiber's main characteristics provided by supplier.

Table 2 .

 2 SiC/SiC specimen characteristics. () standard deviation.

	Samples

Vf (%) Vm (%) Vi (%) Vp (%) Section (mm 2 ) Density (g/cm 3 )

	HNS/PyC30nm/SiC HNS/PyC150nm/SiC (GPa)	48.5 47.3	36.6 35.7	0.5 2.6	14.4 14.4	7.77 (0.28)	2.85 (0.09)

Table 2 . SiC/SiC specimen characteristics. () standard deviation. Samples Vf (%) Vm (%) Vi (%) Vp (%) Section (mm 2 ) Density (g/cm 3 )

 2

	HNS/PyC30nm/SiC HNS/PyC150nm/SiC (GPa)	48.5 47.3	36.6 35.7	0.5 2.6	14.4 14.4	7.77 (0.28)	2.85 (0.09)

Table 2 . SiC/SiC specimen characteristics. () standard deviation. Samples V f (%) V m (%) V i (%) V p (%) Section (mm 2 ) Density (g/cm 3 )

 2

	HNS/PyC 30nm /SiC HNS/PyC 150nm /SiC (GPa)	48.5 47.3	36.6 35.7	0.5 2.6	14.4 14.4	7.77 (0.28)	2.85 (0.09)

Table 3 .

 3 Coefficients used in the analytical model for Young's modulus, Poisson's ratio and coefficient of thermal expansion (l for longitudinal direction and t for transverse direction).

	Properties	HNS Fiber	CVI SiC Matrix	PyC LA	PyC HA
		A = -8.88 × 10 -5	A = -9.85 × 10 -5
	E l (MPa)	B = 1.23 × 10 -1 C = -6.41 × 10 1	B = 1.36 × 10 -1 C = -7.11 × 10 1	D = 55 × 10 3	D = 115 × 10 3
		D = 3.76 × 10 5	D = 4.18 × 10 5
	E t (MPa)	=E l	=E l	D = 30 × 10 3	D = 30 × 10 3
	ν 12 = ν 13	0.2 B = -2.25 × 10 -12	0.2 B = -4.51 × 10 -12	0.3 B = -1.22 × 10 -12	0.2 B = -3.25 × 10 -12
	α l ( • C -1 )	C = 4.75 × 10 -9 D = 2.91 × 10 -6	C = 9.36 × 10 -9 D = 1.30 × 10 -6	C = 3.97 × 10 -9 D = 1.58 × 10 -6	C = 5.71 × 10 -9 D = 0.765 × 10 -6

Table 4 .

 4 OA of processed pyrocarbon. () standard deviation.

	Samples	PyC LA	PyC HA
	OA	61 (5)	44 (4)

Table 5 .

 5 Results of push-out tests on HNS reinforced composites. () standard deviation.

	Samples	HNS/PyC HA30nm /SiC	HNS/PyC HA150nm /SiC	HNS/PyC LA30nm /SiC	HNS/PyC LA150nm /SiC
	σ d (MPa)	1276 (295)	989 (185)	470 (130)	270 (80)
	τ (MPa)	63 (16)	55 (8)	50 (14)	7 (2)
		3.3. Tensile Tests
		3.3.1. Tensile Mechanical Behavior
		Typical stress-strain curves with cumulated AE signals are presented in Figure

Table 6 .

 6 Mechanical

	Samples	HNS/PyCHA30nm/ SiC	HNS/PyCHA150nm/ SiC	HNS/PyCLA30nm/ SiC	HNS/PyCLA150nm/ SiC
	E (GPa)	365 (14)	355 (6)	291 (3)	302 (21)

characteristics mean values of all SiC/SiC specimen types. () standard deviation.

Table 6 .

 6 Mechanical

	Samples	HNS/PyCHA30nm/ SiC	HNS/PyCHA150nm/ SiC	HNS/PyCLA30nm/ SiC	HNS/PyCLA150nm/ SiC
	E (GPa)	365 (14)	355 (6)	291 (3)	302 (21)

characteristics mean values of all SiC/SiC specimen types. () standard deviation.

Table 6 .

 6 Mechanical characteristics mean values of all SiC/SiC specimen types. () standard deviation.

	Samples	HNS/

PyC HA30nm /SiC HNS/PyC HA150nm /SiC HNS/PyC LA30nm /SiC HNS/PyC LA150nm /SiC

	E (GPa)	365 (14)	355 (6)	291 (3)	302 (21)
	σ r (MPa)	319 (6)	297 (7)	337 (3)	325 (7)
	ε r (%)	0.497 (0.01)	0.465 (0.051)	0.582 (0.018)	0.641 (0.016)

Coatings 2022, 12, x FOR PEER REVIEW 10 of 16

Acknowledgments:

The authors acknowledge the support of Jean-Noël Rouzaud for TEM image captures and discussion about pyrocarbon microstructure.

Funding: This research received no external funding.

Institutional Review Board Statement: Not applicable.

Informed Consent Statement: Not applicable.

Data Availability Statement:

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest:

The authors declare no conflict of interest.


Patrice Ballester
email: patrice.ballester@gmail.com

Patrice Universty

O F Toulouse-Le Mirail

NEW DELHI, INDIA BALLESTER

est destinée au dépôt   Copyright Esposizione Universale di Roma EUR. Roma -world's fair 1936 -2006. The community of memories rediscover?

Such are the remarks of the philosopher Paul Ricoeur concerning the town of Rome, but these remarks can coincide perfectly with the object of our studies and the search of identity of a territory which then leads us to reflect on what could represent a district of Rome in the eyes of the world, Europe and Italians but also as a matrix of a painful but salutary rediscovery of architecture and a precursory urban installation and model with many regards: E42/ EUR.

In 1936, Mussolini decides to build a great new district, a new city -Third Rome -in the south of the eternal capital, Roma, in the direction of the Mediterranean Sea. The dictator projects to celebrate the twentieth birthday of "walk on Rome" with a named World's Fair "Olympiads of Civilization". He asks at the famous architect of the regime Marcello Piacentini to plan/design and to build the new district who takes the name of "Esposizione Universale di Roma 1942" so, E.42. The realization of this district is intended to become the heart of a modern-rationalist agglomeration and spearhead of the fascist urban policy. Work starts in 1938 but because of the Second World War, the exhibition did not take place. The new district is equipped by Mussolini with great administrative and juridical autonomy which it preserved well after 1945. The paradox lies in the fact that it was completed after the war and was arranged/developed on several occasions in 70 years of existence.

Currently, it forms a harmony at the same time administrative (great Italian ministries), residential (private housing), cultural (the presence of many museums), commercial (fair of Rome and new centre of Congress for 2007registered office of many companies) and sports equipment (sports complex built for the Olympic Games of 1960). The objective of this article is to answer the following question: (principal question) 70 years after creation and planning by a fascist system of a modern district, how did the community of memories (Romans) find itself within the context of a reinterpretation of the functions of the district through for a long time? How did the Italians reconcile themselves with this district, its plan, its architecture and its functions? Which were the stages of this re appropriation of the district by the policies, architects and town planners?

We will be based for that on archivist research in Rome with the headquarter of the E.U.R., a bibliography reasoned and new on the subject, of the questionnaires addressed to the current persons in charge for the district. The cover of the study will be to take into account three dimensions of the preliminary project and to compare the evolution of the actions of the Italian Republic with the heritage of the fascist system. ( 1 The result will show that in spite of the bad representation of this district, the Romans took again possession of this new area, reinterpreted this one and rediscovered the positive aspects of this planning and the architecture. The ephemeral becoming durable and even interpretable by the new Italian political community, the contemporary economists, architects and town planners. For this seventieth birthday of his creation, one will quote the remarks of Rossana Bossaglia, commissioner of an exhibition rehabilitating the architecture of the E.U.R in 2004. "Until little, one could not say that fascist architecture was beautiful, under penalty of being shown to find beautiful Fascism as a whole" [START_REF] Millon | The Role of History of Architecture in Fascist Italy[END_REF] . Today, the experts agree to underline the coherent action of a government in this planning action coming to a continuity between ancient Rome and fascist urban sprawling, more an interesting mixture of classicism and rationalist modernity (premise of the International Style) and also to put the question of the continuity of the city's planning in extension of urban network in the direction of the sea.

"EUR Urbanistica", a planning style for heritage?

The inscription of Eur42 in the Roman metropolis

In fact, this universal exhibition is to be integrated in an ambitious plan of urban reform with a metropolitan dimension concerning the town of Rome and its suburbs. From the very start of work, one speaks in the Italian press and the newspapers about architectures specialized, of "construction of a new district of Rome" or integration in a "Urbanistica Piano". It is indeed an administrative, sporting and cultural complex thought on this genesis as a structure becoming durable after an ephemeral exhibition. The occasion of an international exhibition is taken and one project the construction of equipment and buildings permanent in what one will then call "third Rome" especially by the members of the regime, the creation ex nihilo of a new city. We should not either forget the symbolic character system and the will by Mussolini to leave a heritage, a trace after the exhibition testifying in time and space to the record of the fascistic capacity through an environment architectural rationalist, organisational, arbitrary and monumental.

One of the most important points concerning the realization of this World's Fair is that the town planners of the time took note which this event would be for them the means of partly solving the problems of the control of the urban growth of the metropolis of Rome. It is necessary via EUR.42 to control the urban development of the thousand-year-old city towards the sea. The creation of the new district and the network of infrastructure are projected then with an aim of urban network to shorten/link the fabric Romans with the peripheral surfaces which developed in a spontaneous way to see anarchistic for some. The objective is well to restore the historical and ideological bond of the city with the installation with its peripheries (and that along the Tiber river) and the ancient harbour city of Ostie. It is included/understood whereas the choice of the district of the exhibition, it was a primary importance with many debates in the reviews of town planning and architecture and of the Italian press. But on September 15, 1936 following a visit of Benito Mussolini, the dictator announced the final choice of the space of the Three fountains for the realization of the district of the exhibition. 1.2 An impeded plan but ever forgotten: "Third Rome"

It is in 1936 that east creates the organization of management so called later the World's fair of Rome of 1942, E.42. The administration of management of the World'sFair E.42 was instituted with like objective first directing and bringing to its conclusion the realization of an administrative complex of a park of the exhibition, the whole for the year 1942. Nevertheless, the Second World War and Italian engagement at the side of the troops of the Axis led the Italian authorities to cancel the World's fair in spite of largely advanced work of a great part of the buildings. One decided as of the first times of the project to appoint the Senator Vittorio Cini, police chief general of the world's Fair and President of the administrative organization.

The organization of this world's fair implied all the intelligentsia of the fascistic regime. This team intervened and proposed ex nihilo solutions for the construction of a model district. The objective is that one remembers this world's fair as well as the great Parisian, London or American exhibitions. The objective was to manage and to link technological and functional choices emanating from an architectural language which clearly made reference to town planning and classical architecture Roman while perennializing the ephemeral infrastructure by a countable management of finances and the will to make last the urban developments once the finished world's fair. Concerning the itself project, we can be pressed on a patrimonial document concerning the presentation of the project; it is the report of presentation of work of studies for a World's fair in Rome [START_REF] Libera | Studio preliminare per l'espansione edilizia di Roma al mare e per la scelta delle aree da destinare all'esposizione mondiale del 1941 xx E.F[END_REF] . Thanks to this document, we know the intention and initial waitings that the whole of the professionals want to lean with this project:

-A large axis "Rome Sea"connecting in direction south the great central place of the exhibition to the pine forest of Fusano Manor house, towards north by joining the surface of the exhibition and the S.Giovanni place. Moreover, the connection with the city was ensured by four large arteries, the avenue Ostiense, a new layout corresponding to the current boulevard G.Marconi and a new connection to the west with the Magliana way which envisaged in more construction of a bridge on the Tevere river but which was never carried out.

-The zoning of the area was already well established with direct broad outline which remained constants for the later plans. A great central place, a central axis, an artificial lake in central position, a place towards the south, a monumental building of quadrangular form dominating all the zone of exhibition and offering a panoramic sight on the valley of Trevere.

-A monumental scenography pointing out the Roman classicism in architecture. The idea of force, order, arbitrary and authoritarian. An arbitrary recombining passed of the town of Rome by the choice of an ancient set of themes of architectural intervention.

-A great event pretexts with the unlimited budget but nevertheless depend on the international events.

-Will expresses to have an architecture allowing a simple and clear identification: Italia/Fascism/Rome/E.42.

January 5, 1937 the project was officially entrusted at a Commission on the Regulating Plan made up of the largest architects of the country, Payent, Piacentini, Piccinato, Rossi and Vietti. It is Piacentini which strong of the preceding experiment of the University Town of Rome which was shown ready to propose median solutions and supplementing the first preliminary draft. It had according to these friends of the very clear ideas about the urban organization and of the architectural language to employ for the exhibition. It wanted adequacy between the mode, architecture, the place and its functions. He becomes the general coordinator, the chief, of the group of architects "the largest buildings of the world's fair which then will become durable will constitute all together immense Forum. I imagined in fact a displacement in a Roman Forum, between places, columns, landscapes, arch and to see at the bottom of the left Coliseum and the bottom of the right Campidoglio. A traditional but modern vision analogical, very modern" 5 .

One sees well by this quotation that already all the elements of an urban scenography are created and in a coherent way compared to the ideas of the regime (monumental surface, recognizable architecture, Force and Modernism). The Place of the Forum is the central scenographic element. With the junction of the cardo and decumanus to the Roman, thanks to her structure and topography it makes it possible to the visitor to create a sequence of panoramas and visions monumental reinforcing the layout and fascinating of accounts this zone undulating. The axis Rome Sea, "the penetrating one", of north in the south is the complementary element of the place; it divides the district into two parts of equal surfaces. Along this central axis which does not take Via Imperial today Via C. Colombo, the Place of entry (the Door symbolic system of the Exhibition), the Imperial Place (today place G Marconi) is prolonged by a landscape prospect with the Central Park which in its turn dominates the artificial Lake. That the large imperial artery divides in addition.

For via Imperial, one proposed at the end of the district and at the end of the way the creation of an Arc of the Empire which one finds in the poster official of the exhibition E 42 of 1939 but today another prospect is proposed at the end of this monumental boulevard, it acts of its cylindrical form and Sport hall dominating the hill with a fall spectacular of water in the lake proposed and carried out by Vico in 1939 general consultant of the parks and gardens. In December 1937, Piacentini was named Director of the service of the architecture of the regulating organization of EUR. It acquires a total autonomy which enabled him to bring a series of modifications to the urban drawing of the district. The influence of the English cities' gardens and the too sinuous roads to its taste were reorganized. Another outline this time final of the plan was carried out in March 1938. It is a question of accentuating the monumental impression of the complex.

The road network was redrawn in a system of axis taking the Roman tradition like example with Cardo and Decumanus, a rigorous grid is drawn in the two parts of the district principal of the zone of exhibition, the landscape sights and prospects are accentuated and the side openings are reduced to their minimum. The buildings, of ticketing, restaurants, the Imperial door, are re-adjusted. The same applies to the principal transverse roads (Via Europe and Civilization of work) which connects the two parts of the district creating a visual prospect for fluidity and passage on the left and on the right of the Palate of the Receptions and Congresses (A.Libera) and Palate of Italian Civilization with in more one sight on the church of SS. Pietro and Paolo. The artificial lake is also redrawn, of irregular forms in the preceding studies; it takes a rigorously geometrical form with the Italian shape of three basins intersected with two bridges connecting the two parts of the district.

Normally, with the two extremes of the longitudinal axis of the lake two architectural elements owed closed the prospects for the two with dimensions ones, the theatre into full surface with G Michelucci and the building for the exhibition of the Agriculture of P.Marconi, G Samonà, G Viola. However, none of both were finished and in their place during the following decade one respectively arranged the new seat of the Bank of Rome and the building of the ENI (the petrochemical cartel of the Italian government founded in 1953). As for the place of entry, one it thought of the Roman, one decided to create a kind of anteroom with large columns making of the exhibition a closed zone. The place was closed in the pressure of the two columns. Finally, as for any exhibition one decided to create a park of entertainment in the south-western zone of the lake. In 1951, the extraordinary police chief then in load of EUR Virgilio Testa asked the government to resume not yet finished work and/or works. He is at the base of the rebirth of the district. He starts with a new team to study the possibilities of construction of building of new buildings for the zones of the Exhibition not yet built. One then notices a continuity in the change, a very large part of the buildings projected for the world's fair was supplemented and transformed to allow the installation of important administrative complexes, Ministers, Agencies Governmental, was various museums and sporting complexes.

The Northern area was devoted to the construction of buildings for the directional activities while the peripheral zones and the southern part were allocated functions of residential dwellings, services town and green zones already intended for this use within Piacentini. The project of Tested is quite simply to create a modern district including the whole of the organizations of direction of country as well as the establishments of companies to tertiary predominance while preserving monumental scenography, the green surfaces of the Plan of Piacentini as well as a residential zone [START_REF] Testa | L'EUR centro direzionale[END_REF] .

In 1960, the Olympic Games marked durably also the aspect of the district, like the Olympic Cycle-racing track, the swimming pool of the Pinks, the Sport hall or Piacentini proposed its solutions with the assistance of Nervi. A modern functional dimension of center decisional is then ratified by the Christian Democracy and the whole of controlling Italian which have followed one another for fifty years.

CHRONOLOGY

Regulative Agency

The great planning decisions Great questions.

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The style EUR, an architectural rediscovery?

2.1 Birth of the "style E 42"

A kind of style "Piacentini" = a system of place, monumental door, scenographic sight, pastiche of Roman art is then set up as soon as the architect takes the control of work and the choices of the adopted projects. It is itself which decided surfaces to put at the contest and consolidated its vision of the district with the monumental complexes the surfaces of access, the borders external interior of the district. "A through the mechanism of section of the projects winner, with the contest, it created for itself moreover a precis of architectural style which under the aegis of Piacentini and the control of the Office of the Technicians of the agency, in fact, they become a kind of collective language in which, separate like exception the Palace of the Congresses, it is difficult to recognize the individuality of the projects of authors. Actually, there was a regulating faculty of the Regulator Agency which made it possible to add convenient variables to the projects retained what heavily conditioned the choices of the projects of the participants, that is to say the practice of the prices ex-aequo concerning different culturally strong projects between them, that made so that so-called "the style E 42. Characterizing each realization of the Agency took an identity in a certain direction unreal and anonymity even if perfectly localisable. " 7 .

Between 1938 and 1941, the area of exhibition was a principally a meet of building site where the primordial infrastructures were realised. The layout of the Imperial voice, the emblematic construction of the Palaces and the monumental buildings were largely advanced, the Palate of the offices of the Agency of construction of E.42 was the completed first but also the Palaces of Civilization, of the Restaurant, the imperial Place and its buildings, of both Esedre, the INA, of INPS, the Palace of the Congresses, the Church of Saint Pietro and Paolo, the Basin of the Lake and the theatre with the lime pit air.

Inevitably, the history of the district is marked by its genesis by a will of standardization and arbitrary realization of a modernistic fascistic ideal city. It is noted that on official Internet site of EUR, one can read with the part history and urban project that this one "represents in its complex the synthesis, still today perfectly readable between the urban operation of last and a future fact of specific interventions but by respecting the original structure, and that it cannot miss intriguing at the time of a visit of the city by having an open and cultivated vision of its most recent history…" 8 .

The rediscovery of the style E 42 by the Romans

This rediscovery was done in several times:

-In a first the lives of the district and civil servant who live there and work have made it possible to maintain a link between the city and the district.

-In the second, the district accommodates since the Fifties the Fair of Italy and various less important fairs like that of Agriculture.

-In the third time, the cinema played a part in the fact that more and more the realizers take as decoration EUR and its scenographic value to turn of the scenes and to recall to the Romans their memory or sometimes dark history:

"Sometimes, thanks to certain adults, I went to discovered from another city: Rome always, but elsewhere in Rome. One day, my maternal uncle, the photographer Elio Luxardo, took us along by car, my cousin and me. It had taken the direction of the sea, and, from the improvist, it took a small road which I did not know. At a certain time, one stopped, and one continued with foot. We arrived in the district of EUR, built in the south of the city by the fascists for the world's fair of Rome, which was to be held in 1942. To this end, since 1939, Mussolini had made build, in the direction of Ostie, an ultramodern district on more than 400 hectares. In my childhood, post-war period, the district was almost given up, it did not have there even more road to reach it. At the beginning of the Fifties, EUR was regarded as a heritage of Fascism, and this district was cursed. There were large marble buildings, a church, a museum, an obelisk... It was an immense and deserted place. In this phantom city, the birds fluttered in the buildings, the grass had pushed back a little everywhere. This vision, this day, was one of the architectural emotions most important of my life, it was impressive. For this new film, I thus chose with attention the places of turning, of spaces either very known in the center of Rome, or ignored: large buildings with It Quadraro, prospects with Centocelle, working cities with Tor Vergata. The police officers of film, to carry out their survey, move in various points of the city. Certain people whom they meet live in the center of Rome, others in the suburbs, others still in the residential districts..." [START_REF] Argento | Il Cartaio[END_REF] .

Lastly, an ambitious project of rediscovery of the style E. 42 is undertaken by the tourist route which is borrowed by the visitors of the district by means of an extremely complete layer of presentation making it possible to connect the history of this district to the inhabitants of the city: "Exploring EUR" 10 allows the Romans and tourists "to Re-examines the entire spectrum of architectural Roman and national culture [...] At any misses the conflict between modernity and tradition had been has movement feature of the architectural ideology of the Italian fascist already since the early 1920 S " 11 . At the time of the conference or in the event of publication of the acts, I would present film extracts or the tourist route, but it should be emphasized that until the year 2000, the efforts of the municipality of Rome to provide quality tour guides on this symbolic space were very weak, as evidenced by the article in the New York Times, July 11, 1999, recalling NYT Walking Mussolini's Fascist Utopia by Michael Z. Wize who write : "A new brochure about the planned revitalization of EUR notes that the architecture's ideological message prompted calls for the entire quarter to be razed after the war. In the meantime, the brochure speaks of ''any disputes about its monumental role having long been resolved'' as a result of a ''more objective and rational'' evaluation. Nearly 60 years after the demise of the dictatorship that erected EUR, there is a remarkable lack of information at the site itself about its history. Aside from a small guide (available in Italian only) on sale at the Museum of Roman Civilization, I found little to document the origins of the district. Nonetheless, the aborted world's fair lives on as an unmistakable reminder of a vainglorious tyranny". The historians of art, architecture and architecture also spoke in the debates around EUR, various publications very few at the beginning then much more important as from the Seventies brought has to requalify the image of the district. It is especially that the occasion of the fiftieth birthday which the publications pointed out the role and the importance of the EUR choice by the fascists and especially that the idea to build such a district in direction of the south, sea gone up before the fascistic era Mussolinien. It is at the time of the fiftieth birthday of the creation of EUR that one finds a will of the policies and institutional Roman to point out the importance of third Rome for Italy. It should be noted that there were works reporting the importance and the rediscovery of Eur but as from the fiftieth anniversary 12 .

But four works refer concerning EUR now and brings more than one comment since. They are the works of Italo Insolate, Di Majo, Mariani, De Vico Fallani. In Casabella, G. Ciucci titrates besides: una storia will ancora incompleta speaking about last the publication on EUR [START_REF] Ciucci | una storia ancora incompleta[END_REF] .

2.3

The inscription of an authoritarian rationalist modernistic style in the city, the memories and the European culture ? Do the Italians and Romans they had shame of their urban past with EUR? Initially, yes, because to speak about architecture during the fascistic period and to find qualities with this one were to put itself except play of the media class politico. On the other hand, it is true that the historians, geographers and architects took this way for a long time. On the other hand, a last event can sweep the whole of the fears, fears and unvoiced comments on this district. It is into 2004 that it is held in the Palace of the Congresses of Libera the IGC. The European Intergovernmental Conference proceeded in the decorum of EUR district for precisely discussing the European Constitution. "Until little, one could not say that fascistic architecture was beautiful, under penalty of being shown to find beautiful Fascism as a whole" explains Rossana Bossaglia 14 . One finds the same order of idea with the remarks of Giorgio Muratore "Until the Sixties, it was only one dark page in the history of Italian architecture. From now on, after more than fifty years, we can have retreat. An architecture of quality -even resulting one historically contestable period -can be interesting all the same" 15 .

E42/EUR heritage for Rome, Italy and the world?

3.1 The great paradox: the world's fair ever carried out but most important with the plan of the territorial and functional stakes It is the largest of the paradoxes possible and conceivable if one is considered the history and the geography of the world's fairs in the world. The exhibition which was not carried out until the end -Eur.42 -left the most trace in the memories and the urban screen of a large European metropolis!

3.2 Eur42 a model for the mega events of end the XX 2nd century ?

We will be posed like a question: which place this atypical exhibition and was it took used as model for other achievements of great projects?

E 42/EUR Recall memories

In the case of a publication, I would present this third part, for the moment, I propose an explanatory diagram allowing showing via the succession of various temporalities the community of the memories found within the framework of the identification and urban reinterpretation of a district to living, to imagine, to discover.

In conclusion, EUR is the example of Italian Rationalism or functionalism combining classical architecture and modern. Nevertheless, EUR is also a perfect example of transfer of identity and of return on its past of people by the intermediary of an authoritative town planning to become a participative and regulating town planning. This third Rome always intrigues and remains for the urban history of the Italian capital an interrogative place, when we see the cinema director Fellini talking about this district, it should be said that this universal exhibition has developed a form of creativity that has escaped simple architecture fascinates to become a place of meeting and fair. (Fellini e l'EUR (1972) Federico Fellini -Il quartiere EUR di Roma (1972) https://www.dailymotion.com/video/x581wl)

  ) The initial urban network respected? -(2) The functions of the district perennialized? (3) Which types of evolution concerning the representations of this district? The whole between (a) 1936 -1954, (b) 1954 -1960, (c) 1960-1990, (d) 1990 -2006.

Figure 1 :

 1 Figure 1: The Government action, visits in 1940 (?) of Benito Mussolini at the E.42. Présentation at the Agency of the model of the Exhibition. Italian Central Archivesphotographic file.

Figure 2 :

 2 Figure 2: The inscription in 2005 of the EUR districts in the metropolitan plan (1), the south of Rome (2) and the scale of the district (3). Google Earth.

Figure 3 :

 3 Figure 3: In 1944, the south of the Roman agglomeration offered vast agricultural fields in waste lands for some and then allowed the realization of great installations like a world's fair. Here one guesses only one part of the work of the exhibition (1) the final plan of 1938 with installations of Piacentini. Italian Central Archives, EUR.

Figure 4 :

 4 Figure 4: During the conference, I would comment on this chart and attempt at schematization of Eur42/Eur in 2006. Heritage continued, functionality, palimpsest.

Figure 4 :

 4 Figure 4: Old the and new Congresses Hall on the site of EUR. Studio Fuksas, Massimiliano Fuksas projects, New Roma, EUR Convention Center and Hotel.

Figure 5 :

 5 Figure 5: The Eur.42 style, in these various dimensions. Italian Central Archive EUR 1939.

Figure 6 :

 6 Figure 6: Continuation, the style Eur. 42, Central Place, obelisk and colonnade.

Recall Memories of EUR.

Two temporalities and stakeholders and representation actions

1936/1939/1942

The Government action

1942/1950 ?

Uncertainty ?

1952/1954/1960

Des mega events JO Museum and a man Testa.

Change of toponimy and Functionality

1970/1990

A new Image A new logo A reconciliation Prestigious architecture Fellini visions

2000/2010

Bibliography:

-AA.VV. ( 1987) , E42. un progetto per l'Ordine Nuovo, Comunità, Milano.

-AA. VV. (1987), E42, utopia e scenario del regime, Catalogi Marsilio Venezia, Venizia.

-AA. VV.(1995), Eur.guida degli istituti culturali, Comunità, Milano.

-"Architecttura", april 1937.

-"Casabella", june 1937.

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