Role of condensates and adsorbates on substrate surface on fragmentation of impinging molten droplets during thermal spray

We propose that the presence of condensates/adsorbates on low temperature substrate surfaces may be a significant factor responsible for splat fragmentation of impacting molten droplets. Vaporization and rapid expansion of condensates/adsorbates upon molten droplet impact cause instability of the spreading droplet. Plasma spraying experiments, using radio frequency induction processing of ZrO2, were designed to test this hypothesis. In order to obtain different levels of condensates/adsorbates, steel substrates were heated in a vacuum chamber (at 250 torr) and allowed to cool under vacuum for different periods of time, ranging from 1 to 62 h before splat deposition. For comparison, splats were also produced on ambient (25 degreesC) as well as on heated substrates (500 degreesC). It was found that splat morphology changed from highly fragmented to a contiguous, disk-like shape with a decreased level of surface condensates/adsorbates, although the substrate temperature was maintained at ambient temperature. (C) 2001 Published by Elsevier Science B.V. All rights reserved.