The effect of particle impact temperature on the deposit efficiency and cutting wear rate of 88/12 WC/Ni coatings sprayed with the top gun HVOF thermal spray system
Date
1997
Authors
Lambrecht, Hendrik Oliver.
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Abstract
A relatively new thermal spray system, the TOP GUN High Velocity Oxy Fuel (HVOF) system, has been developed which is able to deposit coatings of high quality. The supplier's parameters, however, do not always obtain the quality that the system is supposed to achieve. This has led into an investigation of the effect of spray parameters on the quality and properties of the coating.
The investigation ass. es that the coating quality and properties are influenced by the coating particle impact temperature. The gun parameters which affect the particle impact temperature are spray distance, volume flow rate, oxygen fuel ratio and the combustion chamber size. Samples sets with varying spray distances were sprayed with 88/12 tungsten carbide / nickel (WC/Ni} powder onto 15 mm diameter copper tubing. In each set one of the gun parameters which affect the particle impact temperature was changed: The samples were then analysed with respect to deposit efficiency, wear resistance, hardness and microstructure.
The results show that the deposit efficiency is directly proportional to the wear resistance. The hardness 25 well as the porosity level of the coating are not related to the wear resistance of the coating.
It appears that the deposit efficiency and wear resistance of a coating is related to the bonding of the coating particles to one another. If the coating particle remains bonded during the wear process the material loss is limited to the wear of the WC in the coating particles. If the coating particle is disbonded during the wear process the material loss is directly proportional to the bond strength of the particles.
The mechanisms which affect the wear resistance of a coating are not the same as those which affect the hardness. The hardness trends are thus not related to the wear resistance trends. Since the coating consists of a composite of WC and Ni it is the concentration of the we which determines-the hardness of the coating. If the Ni content in a coating particle becomes liquid due to overheating, it will flatten severely on impact allowing some of the we to be repelled and be lost to the coating. This loss in WC results in a reduction of hardness
Description
An 18 point research project submitted to the faculty of Engineering, University of the
Witwatersrand, Johannesburg, in the partial fulfillment of the requirement for the degree
of Masters of Science in Engineering
Keywords
Coatings., Protective coatings., SPRAYING EQUIPMENT.