Pressureless sintered self-reinforced yttrium-silicon aluminum oxygen nitride ceramics.

摘要

This thesis is concerned with the study of the self-reinforcement behaviour and mechanical properties (hardness, fracture toughness and flexural strength) of Yttrium stablized SiAlON ceramics, which are the solid solutions of Si 3N4, Al2O3 and AlN. The overall objective of this work is to develop composition and microstructure of Si3N 4 ceramics that will possess a fracture toughness approaching that of beta-Si 3N4 while maintaining Vicker's hardness close to that of alpha-SiAlON.;In this study, Y doped SiAlON was selected where the level of Al and oxygen was varied. Systematic studies of the effect of chemical composition, morphology of grains and sintering temperature on the densification and mechanical properties of pressureless sintered Y-SiAlON ceramics were performed. It was found that Si3N4 could be pressureless sintered in excess of 98% theoretical density to form SiAlON ceramics by using alumina (Al 2O3), aluminium nitride (AIN) and yttria (Y2O 3) as the sintering additives at sintering temperatures ranging from 1750 to 1920 °C. The typical microstructure is elongated alpha- and/or beta-SiAlON grains uniformly distributed in a matrix of equiaxed or slightly elongated grains. The highest fracture toughness of 6.3 MPa·m1/2, fracture strength of 520 MPa and hardness of 17.5 GPa was found for Y 0.667Si8.5Al3.5O1.5N14.5 (single phase (alpha-SiAlON) sintered at 1850 °C for 120 mins. For this composition, the maximum aspect ratio of elongated grains is ∼7 (maximum grain length ∼14 mum and diameter ∼2 mum).;The effect of CeO2 addition on sintering temperature, microstructure and mechanical properties has also been studied. Significant change in grain morphology of Y-SiAlON was observed after CeO2 was added. Samples containing ceria additive had a much larger volume fraction of the elongated grains compared to samples without ceria. The highest fracture toughness of 10.5 MPa·m1/2 was measured in samples with composition Y0.167Si10Al2O1.5N 14.5 (mixed alpha+beta-SiAlON) and ceria content of 6.5 wt. % while the samples retained the hardness at 16 GPa. The average aspect ratio of the elongated grains is 9 (maximum grain length ∼9 mum and diameter ∼1 mum) for this composition. The XRD and EDS analyses revealed that most of CeO 2 precipitated at the grain boundaries and a small amount of CeO 2 dissolved into a phase. Crack bridging and crack deflection are identified as the major toughening mechanisms in the investigated Y and/or Ce doped SiAlON system.