Structural and microstructural studies on SiC-SiO2 ceramic composites

In this study, SiC ceramic substrates were prepared with 99.5% silicon carbide powders which were came from waste silicon mud during the manufacturing process of solar cells. It is found that a SiO2 layer with 50 nm in thickness is present on the SiC surface by TEM analysis. The relative density of the SiC samples is too low after sintering to 1550 degrees C, which is only 60% of the relative density. It will cause a poor mechanical strength at lower relative density of samples. To improve the density of SiC samples, an amorphous SiO2 was introduced as a sintering aid. Different amounts of amorphous SiO2 from 1 wt% to 5 wt% were then added into SIC powders and made into discs. We call this modified-SiC. These samples were sintered at high temperatures from 1350 degrees C to 1500 degrees C. The SiO2 addition and sintering temperature effects on the density, phase evolution, microstructure and mechanical properties of modified-SiC were investigated. A Y-ZrO2 layer was then coated onto the SiC disc. Sintering temperature can be efficiently decreased and the mismatch between ZrO2 coatings and SiC disc can be reduced when SiO2 is added into SiC. It is found that SiC powders with 3 wt% SiO2 addition shows the most optimum outcome with higher density and stronger mechanical properties.

High Temperature Behavior of Nano-Structured Ceramic Composites Studied by Mechanical Spectroscopy

Mehdi Mazaheri

Silicon nitride based ceramics (SiAlONs); tetragonal polycrystalline zirconia (3Y‐TZP); alumina and their composites reinforced with different amount of multi‐ walled carbon‐nanotubes (CNTs) have been processed by Spark Plasma Sintering (SPS). High temperature mechanical spectroscopy measurements were performed in each material between room temperature and 1600 K. In each of these materials, anelastic and viscoplastic relaxation phenomena were investigated and responsible mechanisms were explained. In particular, grain boundary (GB) sliding, which is responsible for high temperature plasticity in fine grained ceramics, gives rise to a peak or an exponential increase in the high temperature mechanical loss. Peak and exponential background depend on two forces, which control the GB sliding: a friction force due to the GB viscosity and a restoring force due to the elasticity of the surrounding grains. In the present thesis, ceramics and composites have been processed, which allow one to study the role played by these forces on the thermo‐ mechanical behavior of these refractory materials. SiAlON ceramics were chosen for studying the viscous force due to the inter‐granular glassy phase. In zirconia and alumina, it is shown how the CNT reinforcements may improve the restoring force and consequently the creep resistance. Different grades of SiAlON ceramics, processed with different sintering aids (Ca2+, Y3+, Yb3+), with oxygen rich (CaO, Y2O3 and Yb2O3) and nitrogen rich (CaH2, YN and YbN) compounds, have been studied. Mechanical spectroscopy has been used to analyze the behavior of the residual glassy phase, present after sintering, either as grain‐boundary glassy (GB) films or glass pockets located in GB triple junctions. The mechanical loss spectra show a relaxation peak, which is due to a relaxation phenomenon (the so called “α‐relaxation”) associated with the glass transition in the amorphous phase. The peak position depends on the glass viscosity and the peak height is mainly affected by the glassy phase amount and the SiAlON bimodal microstructure, namely equiaxed versus elongated grains. Moreover the peak height depends on the restoring force provided by the neighboring grains, which limit the GB sliding process. As a matter of fact, a good correlation between the mechanical loss peak and plastic deformation in a compression test has been observed. It is concluded that the elongated grains in the bimodal microstructure provide a higher restoring force, which limits the GB sliding of smaller equiaxed grains. In the case of equiaxed fine grained oxide ceramics, such as zirconia and alumina, the restoring force has been increased by CNT additions, which can reinforce the GBs. 3Y‐TZPcomposites with a homogenous distributions of CNTs, ranging within 0.5 – 5 wt%, were processed by SPS. A significant improvement in room temperature fracture toughness and shear modulus as well as creep performance at high temperature were obtained, and interpreted as due to the GB reinforcing role of the CNTs. To support this interpretation, high‐resolution electron microscopy and Raman spectroscopy have been carried out. Moreover, a remarkable enhancement of the electrical conductivity up to ten orders of magnitude due to CNT additions has been obtained with respect to the pure ceramics. The isothermal spectrum of the 3Y‐TZP composites (measured at 1600 K) is composed of a mechanical loss peak at a frequency of about 0.1 Hz, which is superimposed on an exponential increase at lower frequency. This is interpreted as primarily due to GB sliding. The absence of a well‐marked peak in monolithic 3Y‐ TZP is justified by considering that the restoring force decreases at low frequencies or high temperatures. Therefore, GB sliding is no more restricted and the mechanical loss increases exponentially, which is correlated to macroscopic creep. With CNT additions the mechanical loss decreases and the relaxation peak is better resolved with respect to the background. This is interpreted by the pinning effect of CNTs on GBs, providing addition source of restoring force, which can hinder GB sliding at high temperature, resulting in a creep resistance improvement. Similarly to 3Y‐TZP based composites, alumina specimens (3 different types of alumina powders) reinforced with different amount of CNTs were sintered by SPS. It is shown how the initial particle size of the powders may affect the dispersion of CNTs. Measurement of different properties, such as hardness, fracture toughness and mechanical loss at high temperature, have evidenced the crucial role of the CNT dispersion in the obtained mechanical properties.

EPFL2013

Dynamic wetting in pressure-infiltration of ceramic particle preforms by molten metal

Gionata Schneider

This thesis investigates the interaction between mechanically induced flow of a liquid metal into a porous solid, and kinetic effects at the triple line. The approach is experimental and focuses on metal-ceramic systems for which the literature gives values of wetting and local kinetic parameters (Simga, SimgaLV, and interfacial phenomena near the triple line) at high temperature, gleaned by means of the sessile drop experiment. We investigate the infiltration with Al of Al2O3 preforms at 1000°C, 1050°C and 1100°C, and the infiltration with Cu-x (x=Si, Ti or Cr) of porous CGr between 1050°C and 1200°C. Those three alloys are known to form carbides better wetted by the liquid than is graphite. If measurements are taken at various values of applied metal overpressure, in the presence of thermally activated processes at the triple line, time intervenes in capillarity and the front does not stabilise. We observe that the metal continues to invade slowly the porous medium, gradually ¿creeping¿ into its pores under the action of local phenomena that take place at the triple line. We characterise such dynamic infiltration by measuring rates of steady isobaric infiltration, measuring also the variation of infiltration rates at Pth and Pth+0.05MPa. Saturation rates are then computed by means of a linear regression over 100s of the measured saturation for each pressure step. Data are interpreted using an analogy with high-temperature plasticity. This leads to assume that the applied pressure contributes a proportional reduction in the activation energy of the process that governs the rate of preform infiltration, the constant of proportionality defining an activation volume that characterises the thermally activated process. Cu-46at.pct.Si infiltrating carbon is a mildly reactive system that forms SiC at the interface. Isobaric saturation rates of infiltration and their dependence on variations in the applied pressure were conducted between 1050°C and 1200°C. It is found that an activation volume on the order of ~200nm3 makes the data compatible with the Arrhenius law, with an estimated activation energy value of ~400kJ/mol, which is realistic for a process limited by the rate of SiC growth along the interface. Aluminium infiltrating three different Al2O3 preforms shows similar behaviour as does the mildly reactive Cu-Si/C system if particles in the preform are highly angular, regardless of the presence or absence of Na impurities. Isobaric saturation rates measured between 1000°C and 1100°C give a similar activation volume as the Cu-Si/C system, on the order of ~200nm3, and an activation energy ~300kJ/mol, suggesting that the kinetics are likely limited by solid phase diffusion. It is also shown that sodium impurities in alumina serve to facilitate its infiltration by aluminium, likely because sodium alters the oxide skin that initially covers the melt. Finally, in the infiltration of Cu-1at.pct.Cr or Cu-10at.pct.Ti into CGr, one finds that interfacial reaction does not aid pressure infiltration. In the former, it is because initial reaction between melt and preform depletes the alloy in reactive Cr, leading to similar infiltration as with pure Cu. In the latter, initial carbide formation is so extensive that it blocks ingress of the metal into the preform. For alloying elements forming a better wetted interface to aid pressure infiltration, reaction must thus be sufficiently slow compared to the rate of metal flow.

EPFL2019

High Temperature Behavior of Nano-Structured Ceramic Composites Studied by Mechanical Spectroscopy

Mehdi Mazaheri

EPFL2013

Dynamic wetting in pressure-infiltration of ceramic particle preforms by molten metal

Gionata Schneider

EPFL2019