.alpha.'-Sialon ceramics: a review

Abstract

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A review of the processing, composition, and temperature-dependent mechanical and thermal properties of dielectric technical ceramics

“…Sialon (or 'SiAlON') is a solid solution based on silicon nitride, with Al 3+ and O 2− replacing Si 4+ and N 3− in the crystal lattice [66][67][68][69][70], resulting in a material with high strength, wear resistance, thermal shock resistance and oxidation ressitance [71]. The phases of sialon of interest for engineering applications are α [72,73] …”

“…β -sialon is the most widely used sialon, with higher fracture toughness (≈ 7-8 MPa·m 1 2 for β -sialon, ≈ 3-4 MPa·m 1 2 for α -sialon [74]) and room temperature strength, but lower hardness (≈ 1500-1700 kg·mm −2 H v for β -sialon, ≈ 1900-2100 kg·mm −2 H v for α -sialon) and thermal shock resistance than α -sialon [71,75,76]. A good combination of strength, hardness, and fracture toughness, intermediate to those of monolithic α and β -sialons, can be obtained from duplex sialons [75], with hardness and thermal shock resistance increasing and toughness decreasing with increasing α -phase content [71,77]. The higher lattice asymmetry and increased defects from greater Al-O and Si-N substitution, and presence of interstitial cations, results in lower thermal conductivity in α -sialon due to increased phonon scattering [78].…”

“…The properties of the oxynitride glass and the volume fraction present control the bulk properties of the sialon (or silicon nitride), having lower elastic moduli and higher thermal expansion coefficients than the sialon (or silicon nitride) grains [79]. The oxynitride glassy grain boundary phase typically present in β -sialon ceramics leads to a deterioration in chemical and mechanical properties at elevated temperatures [71], reducing high temperature strength and creep resistance due to glassy phase softening. This limits the practical maximum operating temperature to approximately 1000°C.…”

“…The cations of metal oxide sintering aids used to stabalise the α -phase in the production of α -sialon may be interstitially incorporated into the solid solution, and this results in reduced volume of intergrannular glassy phase [71]. α -sialons consequentially retain their mechanical properties better at elevated temperatures and have a higher maximum operating temperature (up to 1400°C).…”

Relations between composition and microstructure of sialons

“…As a consequence, for example, composite ~' +/3'-sialons were obtained in the surface layer of the sintered sample C, although monolithic :(-sialon was expected according to the phase diagrams.V, 1 6.1 7 In a similar way, the oxides also can enter the lattice of :(-sialons by reacting with yttria and nitrides: Si3N 4 4-AIN + Y2034-SiO 2 4-A1203 -+ 7'-sialons (3) Therefore, an overall composition shift of sialons composition towards the oxygen-rich side is expected. Additionally, the oxygen impurities present in the nitrides were also found to increase the amount of the amorphous grain boundary phase.…”