Ž . Thin Solid Films 385 2001 132141 Role of condensates and adsorbates on substrate surface on fragmentation of impinging molten droplets during thermal spray Xiangyang Jiang  , Yuepeng Wan, Herbert Herman, Sanjay Sampath Center for Thermal Spray Research, Department of Materials Science and Engineering, Uni  ersity of New York at Stony Brook, Stony Brook, NY 11794-2275, USA Received 16 January 2000; received in revised form 30 November 2000; accepted 14 December 2000 Abstract 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 ZrO , were designed to test this hypothesis. In order to obtain different levels of condensatesad- 2 Ž . sorbates, steel substrates were heated in a vacuum chamber at 250 torr and allowed to cool under vacuum for different periods Ž . of time, ranging from 1 to 62 h before splat deposition. For comparison, splats were also produced on ambient 25°C as well as Ž . 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.  2001 Published by Elsevier Science B.V. All rights reserved. Keywords: Condensates; Adsorbates; Molten drops; Thermal spray; Fragmentation 1. Introduction Fragmentation of molten droplets is a commonly observed phenomenon in many metallurgical processes, which involve the impact of molten droplets on a substrate or pre-deposited materials, such as splat quenching, thermal spray, spray forming, microcasting,   etc. 1,2 . A fundamental understanding of fragmenta- tion mechanisms is not only of scientific interest, but has technical consequences as well. It is commonly believed that splat fragmentation is generally detrimen-   tal to coating quality 3  6 , affecting not only the splat  solidification rate 3 , but also the phase selection, deposit microstructure development and deposition efficiency. It is also found that cracksvoids form at  Corresponding author. Tel.: 1-309-578-6624; fax: 1-309-578- 2953. Ž . E-mail address: jiang xiangyang@cat.com X. Jiang .  ZrO splat and substrate interfaces in the presence of 2  fragmented splats 4 . It is known that improved me- chanical and physical properties of deposits are achieved when splat fragmentation is avoided in Mo   and ZrO systems 5,6 . A large number of studies have 2 shown that contiguous morphology of splats is benefi-   cial to the overall quality of coatings 3  5. 1.1. Sur  ey of the fragmentation mechanism for molten droplets A suitable mechanism to explain the fragmentation of impacting droplets has not been developed. The fragmentation of non-thermal spray particles, such as rain or fuel droplets which do not experience a phase change during deposition, has received considerable attention in the field of fluid dynamics, where fragmen- tation is generally attributed to fluid flow instability,   such as Rayleigh  Taylor or Weber instabilities 7  9. 0040-609001$ - see front matter  2001 Published by Elsevier Science B.V. All rights reserved. Ž . PII: S 0 0 4 0 - 6 0 9 0 01 00769-6