Abstract
Plasma-sprayed, molten molybdenum particles (~50 µm diameter) were photographed during impact (with velocity ~135 m/s) on a glass surface that was maintained at either room temperature or 400°C. A droplet approaching the surface was sensed using a photodetector and after a known delay, a laser was triggered to illuminate the spreading splat and photograph it with a CCD camera. A rapid two-color pyrometer was used to collect the thermal radiation from the impacting particles to follow the evolution of their temperature and size after impact. Molten molybdenum particles impacting on a surface at room temperature splashed and broke up after impact leaving only a small portion adhering to the substrate. On a surface held at 400°C, there was no splashing and a circular splat remained on the surface. Splats on a glass surface held at room temperature had a large maximum spread diameter, approximately 2.7 times that on a hot surface. The cooling rate on a cold surface was an order-of-magnitude lower than that on a hot surface, suggesting that thermal contact resistance was much greater.
