| Engineering ceramics are the ceramic materials applied in various environments such as wear resistance, corrosion resistance, and high temperature resistance etc. It has been produced and machined to satisfy kinds of need by means of designing microstructure, and it emphasize particularly on mechanical properties. Because the strength and toughness of single-phase ceramics are lower, adding second phases in single-phase ceramics is an effective approach to strengthen and toughen the engineering ceramics. In this paper, ceramic composites have been prepared by hot pressing sintering with different second phases, including microsized particles, nanosized particles or glass phase, and the mechanical properties of the ceramic composites have been investigated. The experimental results are as follows:As for nanosized SiC strengthening Al2O3 ceramic composites,1～2wt% SiC addition enhanced the flexural strength of pure Al2O3 from 280 MPa to 516 MPa and the fracture toughness from 3.2 MPa·m1/2 to about 5 MPa·m1/2, which indicates that small amount nanosized SiC addition can lead to a great improvement of mechanical properties for Al2O3 ceramics. Nanosized SiC particles were located within Al2O3 grains and significantly refined the grain size of pure Al2O3 by inhibiting grain boundary migration, therefore decreased the size of microcrack in the sintered ceramics, and this was the main reason for strength improvement. In addition, the residual compressive stress on the Al2O3 grain boundary generated due to the mismatch of thermal expansion coefficient between SiC and Al2O3 matrix, which also resulted in the improvement of strength and toughness.As for microsized SiC strengthening Al2O3 ceramic composites, the sintering propery became poor with the increase of SiC addition and the sintering temperature was increased. The flexural strength of ceramic composites was enhanced with adding the SiC content, and the flexural strength reached 615 MPa for 20wt% SiC addition. The fracture toughness increased at first then decreased and the highest fracture toughness was 7.6 MPa·m1/2 for 5wt%SiC composite. Grain refinement and the residual radial compressive stress and tangential tensile stress in the matrix are the main reasons for strength improvement, and fracture toughness gradually decreased with the tendency of transgranular fracture. The observation of microstructure showed the good bonding of Al2O3/SiC interface. As for liquid phase sintered AlN ceramics, the addition of YAlO low melting point phase promoted the densification of AlN ceramics. The measurement of mechanical properties showed that the strength and toughness of pure AlN were increased with the addition of second phase due to the elimination of pures, and the flexural strength and fracture toughness were increased from 245 MPa and 2.88 MPa·m1/2 to 383 MPa and 3.1 MPa·m1/2 respectively. The content of YA10 phase increased and covered the whole AlN grain boundary with the increase of sintering aids, which however lowered the strength and toughness. The observation of microstructure indicated the poor wettability between AIN matrix and glass phase, therefore, the poor interfacial bonding. Moreover, the tensile stress between AIN and glass phase resulted from thermal mismatch between the two phases. This further weakened the interfacial bonding, made crack propagate more easily, and led to the decline of mechanical properties.As for B4C/SiC two phase ceramics, the addition of SiC basically had no influence on the densificaton of B4C ceramics. The influencing factors were mainly the size of starting powders, sintering temperature, and pressure. Pure B4C and B4C/SiC composites with full density were obtained using the ultrafine submicrosized B4C powder under the sintering temperature of 2000℃and pressure of 30 MPa. It was found the strong B4C/SiC interfacial bonding by the observation of microstructure, and the crack propagated through SiC grain transgranularly or along SiC grain boundary. The flexural strength of pure B4C increased from 500 MPa to the highest of 700 MPa with the addition of SiC for ceramic composites, however the fracture toughness decined slightly because of the tendency of transgranular fracture. |
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