Thermal expansion responses of pressure infiltrated SiC/Al metal-matrix composites

Aluminium-matrix composites containing thermally oxidized and unoxidized SiC particles featuring four average particle diameters ranging from 3 to 40 mu m were produced by vacuum assisted high pressure infiltration. Their thermal expansion coefficient (CTE) was measured between 25 and 500 degrees C. Oxidation of the SiC particles in air produces the formation at their surface of silicon oxide in quantities sufficient to bond the particles together, and confer strength to preforms. After infiltration with pure aluminium, the composites produced showed no sign of significant interfacial reaction. The CTE of the composite reinforced with unoxidized SiC particles featured an abrupt upward deviation 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 with oxidized SIC particles, these two transitions were removed, their CTE varying smoothly and gradually from the lower elastic bound to the upper elastic bound as temperature increases. With both composite types, the CTE decreased as the average particle size decreased. This work illustrates the benefits of three-dimensional reinforcement continuity for the production of low-CTE metal matrix composites, and shows a simple method for producing such composites.