Tanguy Rouxel

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.

Tape-cast alumina-zirconia laminates: Processing and mechanical properties

Abstract Alumina-zirconia laminar ceramics made from layers of different compositions with different stacking sequences were fabricated by tape casting and compared. A phosphate ester dispersant was optimized in MEK/EtOH and an optimum formulation of organic components for tape casting was defined. The fracture resistance, toughness and elastic properties were characterized. A significant improvement of both the fracture resistance and the toughness, from 380 to 560 MPa and from 3.7 to 8 MPa√m respectively, was gained between the pressed alumina monolith and the tape-cast Al2O3-ZrO2 composites. The improvement was tentatively related to the presence of residual stresses at both the microscopic scale (phase transformation toughening) and at the macroscopic scale (interface effects).

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.

SiC particle reinforced oxynitride glass: Processing and mechanical properties

Abstract After optimization of the processing route, the mechanical properties of the composites were evaluated with varying particle sizes and volume fractions of reinforcement. The best dispersion of the particles in the composite was obtained by using attrition milling followed by spray-drying; nevertheless, ball-milling led to satisfactory results for particle sizes higher than 3 μm. Elastic moduli, hardness and fracture toughness increase with the volume fraction of SiC. Fracture strength increases with both decreasing particle size and increasing volume fraction to reach 400 MPa for a glass matrix composite containing 47 vol% of SiC with 1 μm average panicle size. A further improvement is achieved by crystallizing the matrix.

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 .

Free silicon and crystallization in silicon nitride based ceramics and in oxynitride glasses

Changes in microstructure induced by heat treatments between 1000 and 1500 °C in Si3N4 based glasses and ceramics were characterized using x‐ray diffraction and Raman scattering. The presence of free silicon appears to be a common feature of these families of materials. Si precipitates were observed by transmission electron microscopy in a bulk glass and in a polycrystalline ceramic. It is shown that heat treatments affect the free silicon content and that its decrease upon annealing is closely related to the crystallization of secondary phases. The incidence of the amount of free silicon on the material color strongly suggests that silicon microcrystals are responsible for most of the color fluctuations reported in silicon nitride glasses and ceramics. The higher the free Si content, the darker they are.

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.

Viscoplastic forming of Si3N4-based ceramic and glass-matrix particulate composites

Abstract Two different routes were explored to make ceramic parts by plastic deformation at high temperature. The first route consists of hot-forming at a temperature close to the sintering one. About 100% elongation was achieved above 1773 K in an α-phase rich silicon nitride, densified with Al 2 O 3 and Y 2 O 3 , with an average grain size of 0.6 μm. The second route uses glass-rich materials (paniculate composites) and offers the advantage of much lower forming temperatures, typically around 1273 K. In this latter case, post-forming strengthening can be achieved through a crystallization treatment of the glass. SiC particle reinforced oxynitride glass composites with up to 40 vol% SiC were successfully shaped and further strengthened this way.

Ductility and creep resistance of a silicon nitride ceramic

Metals are ductile materials in their annealed state and can usually be readily net-shaped by cold-working. A consequence of cold-working is work-hardening that provides at the end of the forming step a material with improved mechanical resistance. Silicon nitride ceramics have been developed for use at temperatures higher than those allowed by metallic superalloys. However, a drawback of these structural ceramics was the difficulty of making parts with complex geometry and the dimensional tolerances required for mechanical applications. Since the demonstration by Wakai et al. of the superplastic behavior of a covalent crystal composite, Si{sub 3}N{sub 4}/SiC, the availability of commercial nanosized Si{sub 3}N{sub 4} powders has allowed to tailor microstructures that could be deformed by more than 80% in tension. The counterpart of this high ductility is a decrease in creep resistance compared with recent advanced silicon nitride. However, a creep resistance microstructure can be developed by a heat treatment that could be included after the working sage as the final step in the processing schedule.