Jean-Louis Besson

Laminar ceramic composites

Abstract The processing of laminar ceramic composites from stacking of layers obtained by tape casting is described. In the case of composites for structural applications, the reinforcement mechanisms are briefly reviewed. It is shown that both strength and toughness can be improved. The evaluation of residual stresses allows a strategy for tailoring the mechanical properties of such composites to be developed. As an example, results are given in the case of laminar composites with layers made of alumina with various zirconia contents.

Yttrium oxynitride glasses: Properties and potential for crystallisation to glass-ceramics

Abstract Silicon nitride-based ceramics contain oxynitride glass phases at the grain boundaries which can impair subsequent high temperature properties. Studies of bulk glasses in the Y-Si-Al-O-N system have been carried out and it has been shown that up to 10 atomic % N can be incorporated into these oxynitride glasses. Nitrogen increases the viscosity, hardness and glass transition temperature of the glasses. Heat treatments of Y-Si-Al-O-N glasses have been carried out and the crystalline phases formed are reported. Further improvements are possible if glass-ceramic processes using two-stage heat treatments are introduced. This paper reviews the development of oxynitride glasses, the effects of nitrogen on properties and reports on the glassceramic heat treatments.

Yttrium SiAlON glasses: structure and mechanical properties — elasticity and viscosity

Abstract Two series of oxynitride glasses in the YSiAlON system, with varying ratios of Al Si and Al Y , were studied from both structural and mechanical point of view. A Raman scattering study showed that by changing the Al; content, at constant Y cationic equivalent concentration (e/o), aluminum substitutes for silicon without affecting the degree of polymerisation (cross-linking) of the glass network, whereas at constant Si e/o, replacement of yttrium by aluminum results in a higher polymerisation degree. The effect of glass composition on the hardness, Youngs modulus, the thermal expansion coefficient and the glass transition temperature range are discussed in light of the corresponding structural changes.

Raman spectra of SiYAlON glasses and ceramics

Abstract Silicon nitrides are ceramics that exhibit excellent properties for application at high temperatures in advanced heat engines as well as in oxidizing environments. Their densification with Y2O3 and Al2O3 as sintering aids leads to a two-phase material, called β′-Sialon, containing β′-Si3N4 grains where Si and N are partially substituted by Al abd O, and a mainly glassy intergranular phase which controls the high temperature behaviour of these ceramics. Nitrogen glasses, with compositions close to that of the grain boundary phase, have been synthesized to study the intrinsic properties of this secondary phase. Raman spectra obtained from both SiYAlON ceramics and glasses are reported and compared. Raman scattering spectroscopy appears to be an appropriate technique to investigate crystallization of nitrogen glasses. Further, this technique is very sensitive to the occurrence of free silicon in silicon nitride based materials. Residual silicon, resulting from incomplete nitridation or from decomposition during processing, leads to a narrow and strong Raman band located at about 520 cm−1.

Temperature dependence of Young's modulus in Si3N4-based ceramics: roles of sintering additives and of SiC-particle content

Abstract The temperature dependence of Youngs modulus has been investigated by ultrasonic-echography in the 20–1400 °C temperature range for different oxynitride glasses, silicon nitride ceramics, and SiC/Si 3 N 4 particulate composites. Glasses exhibit a transition between a slow softening (elastic) regime and a rapid softening one which almost coincides with the glass transition range, and is located between 840 and 920 °C. The transition observed in ceramics is located between 1080 and 1150 °C and accounts for the behavior of the intergranular amorphous films. The higher the Y/Al ratio or the SiC content, the higher the transition temperature, and the smaller the softening rate above the transition range. Following the theories of thermally activated flow phenomena and of hierarchically constrained dynamics for glass relaxation, an expression for the correlation factor ( b ) was derived from the softening rate above the transition temperature. The estimated values for b range between 0.5 and 0.7 and are consistent with previously reported values, as obtained by stress relaxation or by mechanical spectroscopy techniques.

Superplastic Forming of an α-Phase Rich Silicon Nitride

The deformation behavior of fine-grained α-phase rich silicon nitride materials has been studied between 1550°C and 1615°C, both in compression and in tension. First, it is shown that higher the α-phase content, better the superplastic forming ability. A large tension-compression flow asymmetry was evidenced. For instance, shear-thickening flow shows up in compression whereas shear-thinning is observed in tension. Furthermore, much higher flow stresses and hardening rates are reported in compression than in tension. Elongation of more than 80% were achieved for strain rates between 2.5 and 5×10−5S−1. In the light of our results and of the abundant literature dealing with the high temperature deformation in silicon nitride, a sketch of the different deformation stages is proposed, which emphasizes the tension-compression flow asymmetry. Starting from the promising results obtained at the laboratory scale, the feasibility for net-shaping of a real part was demonstrated by hot-forging of a parabolic shell.

Yttrium sialon glasses: Nucleation and crystallization of Y35Si45Al20O83N17

Abstract The crystallization of a glass with a composition Y 35 Si 45 Al 20 O 83 N 17 expressed in equivalent %, which corresponds to a normalized composition Y 0.28 Si 0.27 Al 0.16 ON 0.13 , has been studied using an ultrasonic technique. The results were compared with those obtained by more classical techniques such as high temperature X-ray diffraction or differential thermal analysis. The analysis of the data was made within the frame of the general theory of transformation kinetics classically used for the interpretation of differential thermal analysis and differential scanning calorimetry experiments. The structure was studied by transmission electron microscopy at different stages of the glass to glass-ceramic transformation. The devitrification was shown to develop by homogeneous nucleation in the bulk and crystal growth was controlled by diffusion. The optimum nucleation temperature was determined to be 960°C and an activation energy for crystal growth rate found equal to 1025 ± 20 kJ mol −1 .

Microstructure and mechanical behaviour of self-reinforced Si3N4 and Si3N4-SiC whisker composites

Abstract Monolithic Si3N4 and Si3N4-SiC whisker composites were fabricated by hot pressing or hot isostatic pressing. They were sintered in the 1600–1800°C temperature range with 6 wt% Y2O3 and 3 wt% Al2O3 as additives. Morphological aspects of whiskers were statistically determined by image analysis and different matrix microstructures were observed after chemical etching. Then, a correlation was established with mechanical properties. When the same sintering conditions are used, the composite rupture stress increases or decreases with respect to that of the corresponding monolithic Si3N4 matrices. The increase is attributed to an effective load transfer mechanism which involves stress concentration at fibre-matrix interfaces. These interfaces can become the new critical defects in the microstructure when the whiskers are too large. The resistance to short cracks was determined by indentation. The single edge precracked beam (SEPB) method allowed characterization of the resistance to long crack propagation. The toughness increases both with the aspect ratio of whiskers and/or elongated β-Si3N4 grains and with the precrack length (R-curve effect). The improvement is mainly due to the bridging of crack borders by acicular shapes or ligaments of unbroken matter. The R-curve corresponds to the enlargement of the active clamping zone as the crack extends. At a given precrack length the fracture toughness is strongly dependent upon the potential diameter of bridges, whereas the R-curve steepness rises with the density of clamping sites.

Large tensile ductility of high purity polycrystalline yttria

Abstract It is shown that high purity polycrystalline yttria (Y 2 O 3 ) exhibits a high ductility, with elongation over 40%, for temperatures between 1400 and 1550°C, with strain rates from 10 −5 to 10 −4 s −1 . Deformation is accompanied by grain elongation and alignment toward the tensile axis and by a dynamic grain growth. The presence of a threshold flow stress was independently deduced from several sets of experiments, including relaxation, creep and constant strain rate tests. Accounting for the threshold stress results in an activation energy of 350 kJ/mol, a stress sensitivity exponent between 2 and 2.5 and a grain size exponent between 1.2 and 2.3.

Changes in elasticity and viscosity of a SiYA1ON glass during structural relaxation in the transformation range

Abstract Ultrasonic echography and a three-point bending creep test have been used to study the change in Youngs modulus, E, and viscosity, η, during the structural relaxation of a SiYA1ON glass at 870°C, i.e., 40°C below the glass transition temperature. The increase in viscosity has been discussed and modelled in the frame of the fictive temperature theory and a relatively good fitting has been obtained between calculation and experimental results. In addition to the evolution of E and η, Raman scattering measurement are presented, which brings to light a microstructural change during the relaxation process.