HVOF Spraying of Mechanically Alloyed WC-Co Powder and Evaluation of its Coating Property

Abstract

Mechanical alloying (MA) method was introduced to produce a nano-grain WC powder as a source material and HVOF spraying of mechanical alloyed WC-Co powder was tried to fabricate a coating with nano-structure. Mechanical properties of the coatings obtained were evaluated and compared with the coating made from the commercially available conventional powder. The grain size of WC powder had a decreasing tendency up to nano meter order with increase of the alloying period. It was found that the grain size reached finally about 5nm after 100h MA treatment estimated by Scherrer’s equation. TEM observation result for the alloyed powder substantiated the estimation result of Scherrer’s equation.
HVOF coating of the mechanically alloyed powder had a dense and lamellar structure, and the fine carbide particles distributed homogeneously in the coating. The grain size of the coating was estimated about 9nm and it was found that the grain size is not affected so much by the spraying. Hardness test was performed both on the surface and the cross section of the coating. The coatings made from the powders for 25h, 50h and 100h MA showed higher value in the cross section micro hardness by 20.2%, 9.2% and —18.2% compared to that made from the conventional powder. On the other hand, the hardness for the coating surface revealed lower value in the coating made from MA powder. The improvement of toughness value by 21.5% was performed in the coating made from 100h MA powder compared to that made from the conventional powder. From the observation of the crack pass in the coating, it was found that the higher toughness value of the coating made from MA powder was attributed to the more complex crack pass in this coating.