Combustion Synthesis and Mechanical Properties of SiC Particulate Reinforced Molybdenum Disilicide

Abstract

Intermetallic composites of molybdenum disilicide reinforced with various amounts of silicon carbide particulate were produced by combustion synthesis from their elemental powders. Elemental powders were mixed stoichiometrically then ball-milled. The cold- pressed mixture was then chemically ignited at one end under vacuum at approximately 700°C. The combustion temperature of the process was approximately 1600°C which was lower than the melting point of molybdenum disilicide. This processing technique allowed the fabrication of the composites at 700°C within a few seconds, instead of sintering at temperatures greater than 1200°C for many hours. The end product was a porous composite, which was densified to >97% of the theoretical density by hot pressing. The grains of the matrix were 8-14 μm in size surrounded by SiC reinforcement of 1-5 μm. The morphology and structure of the products were studied by x-ray diffraction and scanning electron microscopy (SEM). Samples were prepared for hardness, fracture strength, and toughness testing at room temperature. There were improvements in the mechanical properties of the composites with increasing SiC reinforcement. The hardness of the materials increased from 10.1 ± 0.1 GPa (959 ± 13 kg/mm2) to 11.7 ± 0.6 GPa (1102 ± 52 kg/mm2) to 12.7 ± 0.4 GPa (1199 ± 36 kg/mm2) with the 10 vol% and 20 vol% SiC reinforcement, respectively. The strength increased from 195±39 MPa to 237±39 MPa with 10 vol% and to 299 ± 43.2 MPa with a 20 vol% SiC reinforcement. The fracture toughness increased from 2.79 ± 0.36 MPa.m1/2 to 3.31± 0.41 MPa.m1/2 with 10 vol% SiC and to 4.08± 0.30 MPa.m1/2 with 20 vol% SiC. The increase in hardness and flexural strength is due to the effective load transfer across the strong interface in the composites. The main toughening mechanism is crack deflection by the residual stress in the materials, induced by the differences in the thermal expansion coefficients and the elastic moduli of the matrix and reinforcement.