High-Temperature Mechanical Properties of Al2O3-SiC Composites

Abstract

The Al2O3-SiC composite series were investigated to better understand the effect of second-phase dispersion on the mechanical properties, in special, at high temperatures. SiC dispersions with average particle sizes of 2 and 8 ym were used to form 2 composite series, containing 1.25 to 30 vol% of the dispersed phase. These composites were fabricated by hot-pressing. Hardness, fracture toughness and strength were measured from room temperature to 1400°C. At room temperature, the maximum fracture toughness was observed at about 5 vol% for both composite series. The fracture strength of 2-ym composites increased clearly up to the volume fraction corresponding to the fracture toughness maximum, while the 8-ym composites showed no improved strength, more or less retaining their strength with increasing volume fraction, These results may be interpreted using the concept of toughening mechanism by microcracking. The striking finding in the present system is that the fracture strength of Al2O3 was improved significantly at high temperatures by incorporating SiC into Al2O3, The fracture strength of the composites containing 5 vol% of 2-µm SiC remained essentially constant up to 1100°C ~500 MPa at room temperature and ~485 MPa at 1100°C), while the fracture strength of Al2O3 alone degraded rapidly above 800°C ~230 MPa at 1100°C). Considerable improvement of hardness was also observed at high temperatures for this composite systems. Based on the fracture surface observstions and the deformation/fracture behavior around indentations, the degradation or retention in strength and hardness at high temperatures was correlated with slow crack growth caused by nucleation, growth and/or coalescence of cavities. The hardness and strength correlation was also dicussed at high temperatures.