Research On The Preparation, Microstructure And Properties Of Mosi₂Matrix Composites For Heating Elements

ABSTRACT:MoSi2intermetallics possess an unusual combination of the properties, such as low density, high melting temperature and good oxidation resistance at high temperatures. They have been widely applied to heating elements in high-temperature resistance furnace. The preparation of MoSi2matrix composites was proposed composited with Aluminum silicate and ZrO2. The microstructure and properties of composites were investigated by scanning electron microscope, X-ray diffraction instrument, microhardness tester and electronic universal testing machine. The main conclusions are as followed:(1) The results show that the crystallize size of homemade MoSi2heating element is about20μm. The thin and well-distributed homogeneously dark glass phase is its advantage. The density, bending strength, Vickers hardness and fracture toughness are95.13%,205MPa,8.22GPa and3.25MPa·m1/2, respectively. Which are more3.03%,36.67%,19.50%, and23.11%than that of the MoSi2heating element made by domestic manufacturers.(2) The surface of MoSi2heating elements appeared three-tier structure after the high-temperature oxidation. The outer layer is an oxide film composed mainly of SiO2, the second outer layer is MosSi3, the interior is MoSi2matrix. The growth law of oxidized surface layer obeys formula L=0.43+5.05×107exp (-28397.9/T) t1/2, and activation energy is236kJ/mol. Because the thickness of oxidized surface layer increased, the bending strength of MoSi2heating element can be improved. When MoSi2heating element oxidized for3h at1600℃, the thickness of oxidized surface layer increases22.3μm, the bending strength of oxidized MoSi2heating element reaches273MPa and increases51.67%than before oxidation.(3) With the increase of ZrO2content, hardness, flexural strength, fracture toughness of MoSi2matrix composites will be improved. The hardness, flexural strength, fracture toughness were enhanced to11.02GPa,322MPa,4.23MPa·M1/2, respectively in composites containing9%vol ZrO2particles from9.51GPa,158MPa,2.73MPa-M1/2 respectively in composites without ZrO2particles. Due to ZrO2can refine grain and improve density of MoSi2matrix, the mechanical properties of MoSi2matrix will be improved.(4) ZrO2can improve the electrical resistivity and thermal shock resistance of MoSi2matrix heating material.Compared to MoSi2ceramics, for composites containing9vol%ZrO2, the electrical resistivity more than18.93%, the thermal shock temperature improved100℃. Due to toughening effect of ZrO2and crack deflection mechanism, thermal shock resistance of MoSi2matrix is improved.