THERMAL-EXPANSION RESPONSES OF PRESSURE INFILTRATED SIC AL METAL-MATRIX COMPOSITES/

Aluminium-matrix composites containing thermally oxidized and unoxidiz ed SiC particles featuring four average particle diameters ranging fro m 3 to 40 mu m were produced by vacuum assisted high pressure infiltra tion. Their thermal expansion coefficient (CTE) was measured between 2 5 and 500 degrees C. Oxidation of the SiC particles in air produces th e formation at their surface of silicon oxide in quantities sufficient to bond the particles together, and confer strength to preforms. Afte r infiltration with pure aluminium, the composites produced showed no sign of significant interfacial reaction. The CTE of the composite rei nforced with unoxidized SiC particles featured an abrupt upward deviat ion upon heat-up near 200 degrees C, and a second abrupt decrease near 400 degrees C. The first transition is attributed to an inversion of stress across particle contact points. When composites are produced wi th oxidized SIC particles, these two transitions were removed, their C TE varying smoothly and gradually from the lower elastic bound to the upper elastic bound as temperature increases. With both composite type s, the CTE decreased as the average particle size decreased. This work illustrates the benefits of three-dimensional reinforcement continuit y for the production of low-CTE metal matrix composites, and shows a s imple method for producing such composites.