Abstract
The processes which take place during solidification of plasma-sprayed molybdenum were studied with the aid of optical and electron microscopical techniques. After leaving the plasma gun, the spray particles travelled freely over a certain distance, thereby absorbing a certain amount of oxygen from the plasma gas (argon), which was being polluted by the surrounding air. The particles were then slowed down in a countercurrent of argon, and hence cooled down relatively slowly. As a result MoO2 was formed, which was segregated during solidification along the grain boundaries of the crystals. On the other hand, when the particles were sprayed directly on to a substrate, the metal cooled down very quickly; hence the segregation of MoO2 was for the most part suppressed. Depending on the cooling rate a finely dispersed phase of MoO2 may arise. The extent to which these processes take place determines the properties, including the microhardness of the deposited metallic layer. A formula is derived for the cooling rate in the solidified material near the solidification temperature, based upon the Neumann–Schwartz formula. Influences of the temperature dependence of the thermophysical quantities are considered and evaluated by numerical methods. Correlations are found between microhardness and cooling rate under different conditions.
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