Development of Machining-Assisted Plasma Spraying System

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Plasma spraying is one of the most economical ceramic-coating technologies. Its industrial use is increasing more and more : thermal barriers for turbine blades and wear resistant coatings for guideways and moving parts, sensors, etc. However, there remain some problems to overcome. Reliability of the coating and increase in the adhesion strength are the issues which arise from poor understanding of pre- and post-treatments to be incorporated in the spraying process and adhesion mechanism of sprayd coatings.The present study has developed the Machining-Assisted Plasma Spraying (MAPS) , not only to raise the adhesion strength of ceramic coatings over metal substrates but also to combine spraying with pre- and post-treatments. It is of essence to conduct spraying over a clean and chemically active substrate surface that has just been obtained by machining and then grind to the coating just after being sprayd, if necessary. These processes are integrated into a conventional lathe on which a spray gun and a grinding unit are mounted. The major results obtained are as follows :(1) For an alumina coating sprayd over a steel bar, a scratch test has been performed on the MAPS apparatus. MAPS was found to be superior to conventional spraying.(2) The effect of bond coats on thermal shock resistance has been investigated. Some bond coatings prevent the alumina from adhesion failure in a thermal shock test. In particular, Ni-Cr alloy is most favorable, over which alumina is sprayd together with the machining of the bond coating.(3) A plausible mechanism for adhesion in the MAPS process has been discussed on the basis of the Dupre equation. The primary reason for the increase in adhesion strength is due to chemical activity of the substrate which arises from machining.