Stéphane Jouannigot

Evidence of a green luminescence band related to surface flaws in high purity silica glass

Using luminescence confocal microscopy under 325 nm laser excitation, we explore the populations of defects existing in or at the vicinity of macroscopic surface flaws in fused silica. We report our luminescence results on two types of surface flaws: laser damage and indentation on fused silica polished surfaces. Luminescence cartographies are made to show the spatial distribution of each kind of defect. Three bands, centered at 1.89 eV, 2.75 eV and 2.25 eV are evidenced on laser damage and indentations. The band centered at 2.25 eV was not previously reported in photo luminescence experiments on indentations and pristine silica, for excitation wavelengths of 325 nm or larger. The luminescent objects, expected to be trapped in sub-surface micro-cracks, are possibly involved in the first step of the laser damage mechanism when fused silica is enlightened at 351 nm laser in nanosecond regime.

Green luminescence in silica glass: A possible indicator of subsurface fracture

We investigate the nature of defects triggering laser damage in fused silica in subsurface fractures in nanosecond near ultraviolet regime. Mechanical, laser induced surface flaws as well as pristine silica surface were characterized by optical microscopy and luminescence confocal microscopy before and after acid etching. In all cases, photoluminescence decreases with etching time assessing the existence of defects close to the surface. Spectral analysis of the evolution of these signals during etching allows new interpretations of the nature of precursors inducing damage. Green luminescence around 2.25 eV is seen as a potential subsurface fracture indicator leading to laser damage.

Temperature dependence of luminescence for different surface flaws in high purity silica glass

In situ temperature dependence of the Photoluminescence under 325nm irradiation is used to investigate defect populations existing in different surface flaws in high purity fused silica. Five photoluminescence bands peaking at 1.9, 2.1, 2.3, 2.63 and 3.11 eV have been detected in the spectral area ranging from 1.6 up to 3.6 eV. The Gaussian deconvolution of spectra allows dividing the five luminescence bands in two categories. The former corresponds to bands showing a significant intensity enhancement while temperature decreases; the latter corresponds to bands remaining insensitive to the temperature evolution. Such a behavior brings new information on defects involved in laser damage mechanism at 351 nm in nanosecond regime.

Non-linear behavior of polymeric resin matrix composites : Modeling of laminae and laminates

This paper presents an analytical method to model the non-linear behavior of polymeric resin matrix composites. Complementary energy density is used to derive a stress-strain relation which is non-linear in uniaxial loadings in the longitudinal, transverse directions and in shear. The laminae behaviors are modeled by an n-order terms equation, with n going from two to five according to the non-linearity behavior. This paper presents results for [mat/O well-balanced woven fabric/mat] E-glass/polyester laminates with high matrix volume fraction and shows a good agreement with experimental data.

Nanoindentation, Microscratch, Friction and Wear Studies of Carbon-Fibre Reinforced SiC-Si Matrix Composite

This paper describes some tribological properties of a carbon / silicon carbide – silicon composite (C/SiC-Si). Friction and wear tests were conducted at room temperature under various relative humidity rates using a pin-on-disc test apparatus. Further study aimed at assessing the extent of the damage induced by indentation / scratch through the different parts of the composite. The mechanisms involved were discussed after examining the morphology of the worn surfaces and debris by scanning electron microscopy. For the different friction conditions, rather polished surfaces were generated but the morphology of the debris varied according to the relative humidity of the atmosphere. The indentation / scratch study revealed that damage occurred mostly as fragmentation of the carbon fibres, cracking of the silicon carbide matrix, ductile deformation and to a less extent cracking of the silicon matrix. Introduction Ceramic matrix composites made of carbon fibre and silicon carbide matrix offer a great potential as structural materials for tribological applications. Carbon fibres generally improve the toughness of silicon carbide, whereas silicon carbide shows high hardness, thermal stability and low chemical reactivity, leading to superior friction properties. Several processes have been developed for fabrication of such composite materials: slurry infiltration and hot pressing, polymer impregnation pyrolysis (PIP), liquid silicon infiltration (LSI) and chemical vapour infiltration (CVI). The present study deals with the dry friction and wear properties of a carbon fibre / silicon carbide (CVI) and silicon (LSI) composite sliding against itself at room temperature under various relative humidity rates (HR = 0%, 30% and 72%). We also examined the following micromechanical characteristics: hardness, elastic modulus, scratch resistance. Experimental Techniques Material. A 3D PAN-based carbon fibre preform with a pyrocarbon coating was used as reinforcement. The silicon carbide matrix has been deposited via chemical vapour infiltration (CVI) followed by molten silicon capillary impregnation, to obtain a high-density material. Friction and wear tests. The tribological study was performed using a pin-on-disc tribometer. Tests were conducted at room temperature (20°C) under various relative humidity rates (0%, 30% and 72%). The sliding velocity was 0.3 ms -1 and the normal pressure 3.4 MPa. The wear rate W (g m -1 Pa -1 ) is determined according to the following relationship:

Investigation of the Behavior of Carbon/Carbon Composites under Contact Loading Using Scratch Tests

The behavior of a 3D carbon/carbon (C/C) composite under contact loading is investigated using the scratch test technique. Single scratch tests and multi-pass scratch tests were performed. Quantitative results in terms of penetration depth profiles showed that the C/C composite exhibits a significant elastic recovery under low loads. Correlations between scratch profiles and observations (using optical microscopy and scanning electron microscopy) allowed identification of the damage mechanisms in composite constituents. Introduction Indentation-based techniques are an excellent method for measuring local mechanical properties (Young’s modulus E, hardness H, fracture toughness KIC) of surfaces of all types of materials [1]. Indentation tests were recently applied to various varieties of carbon [2,3] and C/C composites [3]. Scratch tests allow the behavior of materials under normal and transverse contact loads to be investigated [4]. The aim of this paper is to identify the basic phenomena that dictate the response of a C/C composite under contact loading using scratch tests. Experimental Material. The 3D C/C composite examined in this paper was manufactured by SNECMA (Le Haillan, France). The fiber preform contained PAN-based carbon fibers (8-10 μm diameter). The pyrocarbon matrix was deposited via chemical vapor infiltration. It consisted of a highly anisotropic rough laminar type carbon. Experiments. The scratch tests were performed using a MTS NanoIndenter. A Berkovitch indentor was used face-first, the displacement rate was 1 μm/s. Two types of scratch tests were performed. Single scratch test under various loads (50, 100, 150 and 200 mN), and multi-pass scratch tests which consisting in successive scratches in the same groove, under 150 mN. Profiles of the initial surface and the subsequent grooves were determined using scratch tests under a very low load of 20 μN. Five tests were performed under each load. The tests were conducted at room temperature and in ambient air. Results and Discussion Previous studies using nanoindentation tests showed that the carbon matrix and carbon fibers in a C/C composite exhibit a purely elastic behavior under low loads and damage without plastic deformation under higher loads [3]. Fig. 1 shows penetration depth profiles obtained after a single pass test on fibers that are parallel and perpendicular to the surface. It can be noticed that the penetration depth is much smaller after unloading when comparing with that one measured under Key Engineering Materials Online: 2004-05-15 ISSN: 1662-9795, Vols. 264-268, pp 2271-2274 doi:10.4028/www.scientific.net/KEM.264-268.2271

Analysis of the Local Mechanical Behaviour of Ceramics and Composites through Nanoindentation/Scratching and Raman Microspectroscopy

Abstract. Nanoindentation/scratch tests were carried out on Si and SiC wafers and C/C-SiC-Si composites. The specimens were submitted to morphological and Raman microspectroscopy analyses to investigate the damage process and the local structural alteration and/or stress redistribution induced by the contact loading. Silicon displays a moderate stiffness and important local plastic deformations. The plastic strain at the indentor print produces compressive residual stresses, partially released by cracking. Silicon carbide is about twice as stiff as silicon. Indentation and scratching also induce local cracking and structural disordering, but to a much lesser extent. Carbon is a soft material and it exhibits a pure elastic behaviour, with no significant damage. The appropriate combination in a composite of such different phases is expected to give rise to high tribological properties.

Non-linear behavior of polymeric resin matrix composites : Modeling of laminates

This paper presents an analytical method to model the non-linear behavior of polymeric resin matrix composites. Complementary energy density is used to derive a stress-strain relation which is non-linear in uniaxial loadings in the longitudinal, transverse directions and in shear. The laminae behaviors are modeled by an n-order terms equation, with n going from two to five according to the non-linearity behavior. This paper presents results for [mat/0 well-balanced woven/mat] E-glass/polyester laminates with high matrix ratio and shows good agreement with experimental data.