Scale effect of the testing furnaces on the oxidation of silicon carbide ceramics

Journal of Materials Science Letters -     

Intermediate temperature oxidation in silicon nitride yttria ceramics

Silicon nitride billets with both 4% and 8% Y₂O₃ additives have been subjected to oxidation treatments for up to 300 h, in air, in the temperature range 700 to 1000° C. Flexure strength and weight gain measurements together with both scanning and transmission electron microscopy and X-ray diffraction studies were conducted on these billets in an effort to understand the oxidation process. It appears that the degradation phenomena is associated with both the formation of phases outside the Si₃N₄-Si₂ON₂-Y₂Si₂O₇ compatibility triangle of the system Si₃N₄-SiO₂-Y₂O₃ and with the decomposition of W-containing phases at and near the grain boundaries.     

Effects of oxidation on intergranular phases in silicon nitride ceramics

The effects of oxidation on changes in the secondary phases of two Si₃N₄ ceramics were investigated by transmission electron microscopy. The Si₃N₄ materials were oxidized at 1400 °C for 168 h in laboratory air. One material, sintered with 5 vol% Yb₂O₃+0.5 vol% Al₂O₃, containing a Yb₂Si₂O₇ crystalline secondary phase, displayed no gross changes following oxidation. However, the thickness of the amorphous intergranular film was observed to have decreased by 20% from its initial thickness of 1.0 nm. The second Si₃N₄ material, sintered with 5 wt% Y₂O₃+1 wt% MgO, had a completely amorphous secondary phase. Devitrification of the secondary phase at multiple-grain junctions to -Y₂Si₂O₇ accompanied the outward diffusion of additive and impurity cations occurring in the residual amorphous intergranulàr films during oxidation. Substantial cavitation and intergranular phase depletion was observed at both multiple-grain junctions and two-grain boundaries. The equilibrium thickness of the amorphous intergranular film consequently decreased from 1.2 to 0.9 nm following oxidation. Purification of the amorphous intergranular films by diffusion of cations to the surface led to a reduction in impurity concentration, resulting in the observed thinning of grain-boundary films.     

Reduction of sodium-accelerated oxidation of silicon nitride ceramics by aluminium implantation

Norton NBD 200 silicon nitride ceramics were implanted with sodium to a dose of 7.0×10¹⁵cm^-2 at 72 keV (1 at% peak sodium content at 100 nm). The sodium-implanted samples were further implanted with aluminium to 7.3×10¹⁵cm^-2 at 87 keV (1 at% peak aluminium content at 100 nm). The implanted and unimplanted samples were oxidized in 1 atm dry oxygen at 1100 and 1300°C for 2–6 h. Profilometry and scanning electron microscopy measurements indicated that sodium implantation led to up to a two-fold increase in the oxidation rate of silicon nitride. The sodium effect was effectively neutralized when aluminium was co-implanted. The opposite effects of sodium and aluminium on the oxidation resistance of silicon nitride can be attributed to their different roles in modifying the structure and properties of the oxide formed. This revised version was published online in November 2006 with corrections to the Cover Date.     

氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响

以硅粉和氮化硅晶须为原料,通过添加30%(质量分数)成孔剂球形颗粒,以聚乙烯醇作粘结剂,采用干压成型工艺,反应烧结制备了多孔氮化硅陶瓷,分析对比了氮化硅晶须对反应烧结氮化硅多孔陶瓷介电性能的影响.实验结果表明,随着氮化硅晶须加入量的升高,氮化硅多孔陶瓷的介电常数和介电损耗都升高,介电性能恶化.     

Sodium-assisted oxidation of reaction-bonded silicon nitride

The oxidation of reaction-bonded silicon nitride in air, and with small amounts of sodium carbonate applied to the sample surface, has been studied. The action of the alkali is to cause short-term enhanced oxidation,which is terminated when specific compositions of the product sodium silicate glass are attained. These correspond closely to liquidus compositions in the Na₂O-SiO₂ system, and it is postulated that the retardation in the oxidation rate at this stage is due to the formation of a stable tridymite film at the silicon nitride-glass interface. The implications for the high temperature stability of reactionbonded silicon nitride components in alkali contaminated atmospheres are discussed.     

Corrosion-mechanical failure of silicon nitride ceramics in air

Translated from Fiziko-Khimicheskaya Mekhanika Materialov, No. 6, pp. 17–21, November–December, 1988.     

The compression creep behaviour of silicon nitride ceramics

A comparison has been made of the compression creep characteristics of samples of reaction-bonded and hot-pressed silicon nitride, a sialon and silicon carbide. In addition, the effects of factors such as oxide additions and fabrication variables on the creep resistance of reaction-bonded material and the influence of dispersions of SiC particles on the creep properties of hot-pressed silicon nitride have been considered. For the entire range of materials examined, the creep behaviour appears to be determined primarily by the rate at which the development of grain boundary microcracks allows relative movement of the crystals to take place. Now with the BNF Metals Technology Centre, Wantage.     

The microstructures of silicon nitride/alumina ceramics

The microstructures of materials formed by sintering or hot-pressing mixtures of silicon nitride and alumina have been studied by transmission electron microscopy. The probable mechanism of transformation of the reactants to form β′-silicon aluminium oxynitride (β′-sialon) via a liquid phase sintering process, which is analogous to a similar transformation in hot-pressed silicon nitride containing a magnesia additive, is proposed. The origin and crystal symmetry of an unknown second phase is discussed. The residual quantity of this phase, known as the X-phase, is controlled mainly by the silica impurity content of the initial silicon nitride powder.     

Joining of silicon nitride ceramics by hot pressing

The joining of hot-pressed silicon nitride ceramics, containing Al₂O₃ and Y₂O₃ as sintering aids, has been carried out in a nitrogen atmosphere. Uniaxial pressure was applied at high temperature during the joining process. Polyethylene was used as a joining agent. Joining strength was measured by four-point bending tests. The effects of joining conditions such as temperature (from 1400 to 1600°C), joining pressure (from 0.1 to 40 MPa), holding time (from 0.5 to 8 h) and surface roughness (R _max) of the joining couple (about 0.12, 0.22 and 1.2m) on the joining strength were examined. The joining strength was increased with increases in joining temperature, joining pressure and holding time. Larger surface roughness caused lower joining strength. The higher joining strength was attributed to a larger true contact area. The area was increased through plastic deformation of the joined couple at elevated temperatures. The highest joining strength attained was 567 MPa at room temperature, which was about half the value of the average flexural strength of the original body. The high temperature strength measured at 1200° C did not differ very much from the room-temperature value.     

Densification of silicon nitride ceramics under sinter-HIP conditions

The influence of several sinter-HIP variables on the densification behaviour of silicon nitride-based ceramics has been investigated. The processing conditions were studied for Si₃N₄ powder mixtures containing controlled amounts of Y₂O₃+Al₂O₃ or Y₂O₃+MgO. The specimens were subjected to sinter-HIP cycles under argon or nitrogen atmospheres at various temperatures and pressures. The final density of the powder compacts exhibited a strong dependence not only on the applied pressure, the composition and the processing temperature, but also on the pressurization rate and the initial pressurization time. The microstructural changes induced by the application of high pressure were followed by transmission electron microscopy. On examination by TEM, large concentrations of dislocations, generated inside some-Si₃N₄ grains, were observed. Characterization of these dislocations showed thatb=0001 is their most frequently found Burgers vector. Also, two relaxation mechanisms, tending to release the stored energy of deformation in the-Si₃N₄ grains, namely grain penetration (a form of strain-induced boundary migration) and grain fragmentation (the formation of subgrains due to rearrangement of dislocations into low energy configurations), have been identified. The intergranular phases formed were characterized by energy dispersive X-ray spectroscopic analysis, X-ray diffraction and electron diffraction. The influence of different sinter-HIP cycles on the transformation of silicon nitride without additives was also investigated by X-ray diffractometry.     

Corrosion of silicon nitride ceramics under hydrothermal conditions

Corrosion behaviour of Si₃N₄ ceramics containing Y₂O₃, Al₂O₃ and AIN as sintering aids was investigated under hydrothermal conditions at 200–300 C and saturated vapour pressures of water for 1–10 days. Hydrothermal corrosion resulted in the dissolution of the Si₃N₄ matrix and the formation of a product layer consisting of the original grain-boundary phases and hydrated silica. The dissolution rate of Si₃N₄ ceramics decreased with decreasing crystallinity of the grain-boundary phase. The dissolution rate could be adequately described by a parabolic plot in the initial stage of the reaction. The apparent activation energies were 83.5–108 kJ mol^–1, and the bending strength of the corroded samples decreased from 600 to 400 MPa in the initial stage of the reaction upto a weight loss of 0.004 g cm^–2, and then was almost constant up to a weight loss of 0.012 g cm^–2.     

Nitridation of whisker-reinforced reaction bonded silicon nitride ceramics

Ceramic matrix composites were fabricated from silicon carbide whisker-reinforced reaction bonded silicon nitride. Optimal dispersion of the SiC whiskers in the silicon powder slip was achieved by milling and pH control; a pH range of 4–5 giving the best results. Only a slight drop in green density was observed for a 30 wt% addition of SiC whiskers. The effects of the whisker additions on the nitridation kinetics of reaction bonding were investigated and the additions were found to increase the induction period before nitridation and to slightly decrease the nitridation rate but green density and temperature were still found to be the main factors controlling nitridation. Modulus of Rupture measurements for the composites showed a decrease in strength compared to the monolithic material.     

High-strength silicon nitride ceramics obtained by grain-boundary crystallization

A monolithic type of toughened silicon nitride ceramics has been developed from Si₃N₄-Y₂O₃ systems. However, because of the existence of a second phase, the fracture strength decreases at elevated temperatures. To improve the high-temperature strength of silicon nitride, some additional components were investigated. It was found that the addition of hafnia to the Si₃N₄-Y₂O₃-AIN system gave a greater high-temperature strength based on the promotion of grain-boundary phase crystallization: namely, 126 kg mm⁻² in 3-point bend strength at 1300 °C for the hot-pressed specimen, and 90 kg mm⁻² at 1300 °C for the pressureless sintered specimen. The role of the hafnia in crystallization is not yet clear, and is being characterized by electron microscopy and microanalysis.