Structure, Mechanical Properties, and Oxidation Behavior of Nanolayered MoSi2/SiC Coatings
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ABSTRACTThe structure, hardness and elastic properties, and oxidation behavior of a nanolayered MoSi2/SiC structure were examined. DC magnetron and rf diode sputtering were used to deposit MoSi2 and SiC, respectively. The total number of sublayers of each kind was 90. The nominal thickness of the MoSi2 sublayers was 10 and that of the SiC 3 nm. The microstructure of the as-deposited samples was amorphous whereas annealing at 500 C for I h resulted in the transformation of MoSi2 into C40 type hexagonal structure as determined using cross-section transmission electron microscopy (TEM). However, silicon carbide still remained amorphous. Hardness and Young's modulus were determined by using a nanoindentation technique. Hardness of 11.6 and 20.8 GPa were obtained for the asdeposited and annealed structure, respectively. Corresponding Young's moduli were 220 and 290 GPa, which yield values of 232 and 332 GPa for Young's moduli of amorphous and crystalline MoSi2. Oxidation tests were carried out in wet oxidation conditions at 400 and 500 C. The degree of oxidation was determined by measuring oxygen concentration on the surface using a nuclear reaction 16O(d,p) 17O at a deuterium energy of 950 keV. The MoSi2/SiC coating gave good protection against oxidation for unalloyed steel. As compared to a single MoSi2 coating a slightly higher degree of oxidation and a significantly different mechanism of oxidation was observed. This was also supported by secondary ion mass spectroscopy (SIMS) measurements. Cracking of the coating caused by a thermal mismatch and transformation stresses was worse in the case of the single MoSi2 layer than in the case of the composite coating. Thus cracking of the coating can be significantly reduced by using a nanolayered structure.
