Synthesis and Characterization of Nitrogen-rich Calcium α-Sialon Ceramics

Abstract: In this thesis, a synthesis concept has been developed, which uses nitrogen-rich liquid phases for sintering of Ca-α-sialon ceramics. First, keeping the Si/Al ratios constant, the effects of N/O ratio on the properties and microstructure were investigated through a liquid phase sintering process. Second, nitrogen-rich Ca-α-sialon ceramics, with nominal compositions: CaxSi12-2xAl2xN16, x < 2.0, was synthesized and characterized. Third, mechanical and thermal properties of nitrogen-rich Ca-α-sialons were investigated in terms of high temperature deformation resistance,reaction mechanism, phase stability and oxidation resistance, and further correlated to their phase assemblage and microstructure observation.It has been found that increasing the N/O and Ca/Al ratio simultaneously in the materials could result in development of a microstructure with well shaped, high-aspect-ratio Ca-α-sialon grains, and an improvement in both toughness and hardness.For the nitrogen-rich Ca-α-sialon, mono-phasic α-sialon ceramics were obtained for 0.51 ≤ x ≤ 1.32. The obtained Ca-α-sialon ceramics with elongated-grain microstructures show a combination of high hardness and high fracture toughness. Compared with the oxygen-rich Ca-α-sialons, the nitrogen-rich Ca-α-sialons exhibited approximately 150 oC higher deformation onset temperatures and decent properties even after the deformation.The α-sialon phase was first observed at 1400 oC, however the phase pure Ca-α-sialon ceramics couldn’t be obtained until 1800 oC. The nitrogen-rich Ca-α-sialons were thermal stable, no phase transformation observed in the temperatures range1400-1600 oC. In general, mixed α/β-sialon showed better oxidation resistance than pure α-sialon in the low temperature range (1250-1325 oC), while α-sialons with compositions located at α/β-sialon border-line showed significant weight gains over the entire temperature range (1250-1400 oC).