REACTION BONDING AND MECHANICAL-PROPERTIES OF MULLITE SILICON CARBIDECOMPOSITES/

Based on the RBAO technology, low-shrinkage mullite/SiC/Al2O3/ZrO2 com posites were fabricated. A powder mixture of 40 vol% Al, 30 vol% Al2O3 , and 30 vol% SiC was attrition milled in acetone with TZP balls which introduced a substantial ZrO2 wear debris into the mixture. The precu rsor powder was isopressed at 300-900 MPa and heat-treated in air by t wo different cycles resulting in various phase ratios in the final pro ducts. During heating, Al oxidizes to Al2O3 completely, while SIC oxid izes to SiO2 only on its surface. Fast densification (at >1300 degrees C) and mullite formation (at 1400 degrees C) prevent further oxidatio n of the SiC particles. Because of the volume expansion associated wit h the oxidation of Al(28%), SiC(108%), and the mullitization (4.2%), s intering shrinkage is effectively compensated. The reaction-bonded com posites exhibit low linear shrinkages and high strengths: shrinkages o f 7.2%, 4.8%, and 3%, and strengths of 610, 580, and 390 MPa, correspo nding to compaction pressure of 300, 600, and 900 MPa, respectively, w ere achieved in samples containing 49-55 vol% mullite. HIPing improved significantly the mechanical properties: a fracture strength of 490 M Pa and a toughness of 4.1 MPa.m(1/2) increased to 890 MPa and 6 MPa.m( 1/2), respectively.