Effects of surface chemistry on splat formation during plasma spraying

Abstract

Plasma spray technology has been widely employed for deposition of engineering coatings. Coatings produced with customizable surface properties are used for a variety of industrial applications. The coatings are formed as molten coating particles impact and spread out to form overlapping splats. Thus, the performance of the coatings is closely linked with the way the individual splats are formed. The layered microstructure and splat-substrate bonding are profoundly affected by the spraying conditions, substrate surface, the particle conditions, and the relative distance between the torch and the substrate [1-3]. This thesis explains on the effects of substrate surface conditions on splat formation and morphology. NiCr single splats were plasma-sprayed onto aluminum substrates, which were subjected to thermal and hydrothermal pretreatments to grow specific types of oxide/ hydroxide layers on the surface and to alter the surface roughness. It was observed that splat formation and morphology were not directly correlated to the substrate surface roughness in both nano-scale and micro-scale (up to 2 μm), but were sensitive to surface chemistry, particularly the thickness and concentration of the hydroxide layer at the outermost surface. Upon impact by molten droplets at high temperature, the substrate surface was heated, resulting in the dehydration of surface hydroxide to oxide and releasing water vapor. The insulating layer of released water delayed the heat conduction from the splat to the substrate. The splat was in liquid state long enough to undertake internal ruptures, which led to splat splashing. Thermal treatment of the substrate prior to spraying can efficiently remove the water, thereby greatly improving the physical contact between the splat and the substrate. The improved contact enhanced the formation of favorable disk splats, increased the number of splats and significantly reduced splashing and the bubble formation at the splat underside. Cross sections of the splat-substrate interfaces were examined. Besides mechanical bonding, chemical bonding with inter-diffusion was also frequently observed. For wellbonded splats, there were occurrences of substrate melting and intermixing with the disk splats on the substrate which was heated at 350°C and kept at this temperature during ii spraying. In contrast, with the splats poorly adhered to the substrate, the oxide layer on the substrate surface was not removed or redistributed upon impact of the droplet. Three-dimensional computational models were developed using the commercial finite element modeling package Ansys CFX11 to simulate the impact and solidification of molten nickel splat on aluminum substrate. Different values of thermal contact resistance ranging from 5x10-9 W-1.m2.K to 1x10-5 W-1.m2.K were examined. Disk splats were formed with low values of thermal contact resistance, whereas splashing was correlated to high values of this parameter. These results agreed well with experimental observations. Based on the correlation, the mechanism of splashing was proposed. The effects of the oxide layer and gas release on the splat morphology were also examined subject of investigation. However, it was found that splat morphology was not influenced by the simple presence of the oxide layer on the substrate surface.