Frank L. Riley

Oxygen mobility in silicon dioxide and silicate glasses: a review

Abstract Silicon dioxide and silicate glass films are formed on silicon nitride and silicon carbide ceramics during exposure to high-temperature oxidising atmospheres, and oxygen transport through the film is potentially a rate-controlling step. Recent published literature concerning oxygen permeation and diffusion through amorphous and crystalline silicon dioxide, and silicate glasses, is reviewed. Data for diffusion coefficients are collected to facilitate the assessment of probable dominant oxygen transport mechanisms, and associated rates, under given sets of oxidation conditions.

The α/β silicon nitride phase transformation

The literature on the α/β silicon nitride transformation is reviewed briefly. Data are presented on the kinetics of the tranformation of 1600° C on low and high purity silicon nitride powders. The addition of magnesia increased the rate of transformation while the addition of yttria had no effect. Scanning electron photomicrographs show clearly the morphology changes that accompany the transformation. It is concluded that the transformation occurs via a solution-precipitation mechanism and that α and β are probably low and high temperature forms of silicon nitride.

Silicon nitride crystal structure and observations of lattice defects

In view of the considerable progress that has been made over the last 40 years on the microstructural design of silicon nitride and related materials of tailored properties for specific applications, a clear review of the current understanding of the crystal structure and crystal chemistry of silicon nitride is timely. The crystal structures, crystal chemistry, and lattice defect nature of silicon nitride are critically reviewed and discussed, with emphasis placed firstly on the structural nature of α-silicon nitride (whether it is a pure silicon nitride, or should better be regarded as an oxynitride); and secondly on the space group of β-silicon nitride (whether it is P63/m or P63). In conjunction with recent observations of vacancy clusters in α-silicon nitride, a comprehensive view compatible with all the experimental facts with respect to the structural nature of α-silicon nitride is tentatively presented.

The properties of aqueous phase suspensions of barium titanate

Abstract Recent literature is reviewed concerning the influence of suspension pH and ion concentrations on the zeta-potential of near-stoichiometric BaTiO3 powders. New work is reported concerning the surface chemistry and rheology of BaTiO3 powder in aqueous phase suspensions prepared without the use of surfactants or polyelectrolytes.

Effects of silicon carbide nano-phase on the wet erosive wear of polycrystalline alumina

Abstract Wet erosive wear rates for alumina-silicon carbide nanocomposites have been measured and compared with those for pure polycrystalline aluminas of similar grain size. In pure materials for mean grain sizes >2 μm the dominant wear mechanism appears to be grain-boundary microfracture, leading to grain pullout; for finer grain sizes the worn surfaces are smooth. Within both grain size ranges significant reductions in wear rate are found for the nanocomposite materials, thus extending the span of established benefits obtained through the incorporation of silicon carbide nanoparticles into ceramic materials.

Mechanical properties of magnesia-spinel composites

Abstract A set of dense magnesia-magnesium aluminate spinel composites has been prepared by hot-pressing magnesia powder using 0 to 30 wt.% of spinel powder of mean particle size 3, 11 and 22 μm. Bend stength, modulus, and fracture toughness, have been measured. Strength and modulus decrease with increasing spinel content, and for a given loading, spinel particle size, as a result of microcracking caused by thermal expansion mismatch between the magnesia matrix grains and the spinel particles, the effects of which may be intensified by recrystallization of spinel. Fracture is predominantly transgranular for the pure magnesia, and intergranular for the composite materials. Possible reasons for the differences in behaviour between this system and the SiC–Al 2 O 3 system with a similar thermal expansion mismatch are examined.

Alumina-coating of silicon nitride powder

Abstract α-Silicon nitride powder has been coated with aluminium oxide generated by the in-situ hydrolysis of aluminium isopropoxide in isopropanol solution. TEM examinations show clearly the coating structure and the homogeneity of dispersion of the aluminium oxide over the surfaces of the silicon nitride particles. Benefits of this method of addition of densification additives are shown in a faster sintering densification rate of coated particles compared with conventionally mixed powders, and in the homogeneity of the final microstructure.

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.

Grain size effects on the wet erosive wear of high-purity polycrystalline alumina

Abstract The wet erosive wear of a set of 100% polycrystalline alumina materials of tailored grain sizes in the range 1–12 μm has been studied using a slurry of coarse alumina grit in water. Detailed observations of worn surfaces identify two wear mechanisms: microfracture and grain detachment, and tribochemical wear. Measured wear rates are a function of (grain size)− 1 2 . and a linear relationship between wear rate and four-point bend strength is also seen. Grain detachment is the dominant wear mechanism for grain sizes > 2 μm.

Grain-size dependence of the wear of alumina

A simple model is presented which, although empirical in nature, is consistent with experimental observations on the wear and abrasion rates of high purity alumina of tailored grain size. It is postulated that the critical wear process, reduced to its simplest terms, involves the nucleation and propagation of grain boundary microcracks. A microcrack is nucleated and travels at a characteristic velocity along a low energy grain face until a multi-boundary junction is encountered, when development of the crack is temporarily inhibited. The crack thus progresses with alternating periods of steady growth and delay, and ultimately causes the local loss of material through grain detachment. It is shown that this model leads to a wear rate grain-size dependence of the form seen for a variety of conditions such as erosion, sawing, grinding and sliding.