Improvement in Contact Strength of Si3N4/SiC Composite by Crack Healing

Ceramics have been used as bearing and cutting tool components, which are subjected to contact loading during their operation. The presence of surface cracks on these components decreases their contact strength. Thus, the reliability of ceramic components can be increased by improving their contact strength through crack healing. In the present study, the effects of crack healing on the contact strength of a silicon carbide-(SiC-) reinforced silicon nitride (Si3N4) composite subjected to various machining processes were investigated. The contact strength of this composite was evaluated using a sphere indentation test in which acoustic emission was used. The results showed that the contact strength of the composite improved when it was subjected to crack healing in combination with rapping; this was true even when the composite had cracks due to a heavy machining process. 1. Introduction Structural ceramics have excellent mechanical properties, corrosion resistance, and wear resistance. However, their fracture toughness is lower than that of metals. As a result, surface cracks may develop on the surface of ceramic components during machining or operation. These cracks can considerably decrease the reliability and strength of the components. A common method for fabricating ceramic components involves the use of diamond abrasives for machining them. This process usually results in countless cracks on the surface layers of the components. Thus, the machining of ceramics is very costly, and not all minute surface cracks can be removed. To overcome this problem, some researchers have attempted to recover the strength of ceramics by means of crack healing. Chu et al. [1] showed that a 100？μm long surface indentation crack in mullite reinforced using 20 vol% SiC particles can be completely healed by subjecting it to heat treatment at 1300°C for 1？h in air. Ando et al. [2] reported that crack healing can be achieved by the oxidation of SiC and Si3N4 in a Si3N4/SiC composite. Nakao et al. [3] stated that the oxidation of SiC generates exothermic heat and causes a volume expansion of approximately 80% in the condensing phase. As a result, the spaces between crack walls are filled with the healing material and the walls are bonded strongly. Crack-healing behavior in machined ceramics has also been studied by several researchers. Wu et al. [4] reported that an Al2O3/SiC composite showed a substantial increase in strength after grinding and heat treatment in air at 1250°C. Liu et al. [5] reported that the heating of ground ceramic components made from alumina (Al2O3)