Titanium Matrix Composites Via Vacuum Plasma Spraying: Part 2: Plasma Instability and Mechanisms of Co-Spray Deposition

Abstract

High speed cinematography has shown that the plasma torch used in our vacuum plasma spraying experiments exhibits regular fluctuations in length. The frequency of plasma fluctuation (∼300 Hz) is insensitive to the feed gas constituents (that supply the plasma), although this factor does influence plasma dimensions. The effect of plasma fluctuations on ‘in flight’ melting of metal powders remains unclear. The mechanisms of co-spray deposition have been elucidated by the use of a modified ‘wipe test’. Embedding of the ceramic phase into the relatively ‘soft’ substrate surface, followed by coverage with molten titanium ‘splats’, appears to be the principal mechanism by which the ceramic particles become incorporated into the composite deposit. A moderate amount of disintegration and recoil of the ceramic particles accompanies this process. In flight metal-ceramic collisions play a minor role with regard to co-deposition. PM/0546