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 °C) as well as on heated substrates (500 °C). 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.