In high velocity thermal spray processes, loss of carbon through reaction with oxygen is predicted to be limited during WC particle impact and rapid solidification. This is due to rapid cooling of the entire splat that causes difficulties in carbon diffusion through the solidified splat matrix, and the unfavourable reaction ther-modynamics based on Gibbs free energy minimisation, which fa-vours the metallic Co oxidation over that of carbon at low temperatures.
Instead, coating decarburisation at impact occurs predomi-nantly by rebounding of whole carbide particles, as first postulated by Li in the HVOF spraying of Cr3C2-NiCr coatings. During impact of WC-Co composite particles, the metallic binder phase is well molten and the constituent WC particles are forced to flow with the liquid binder during splat formation, Fig.1. If the carbidegrains are smaller than the splat thickness, they will tend to move with the flowing liquid and remain encapsulated during solidifi-cation. However, when the constituent WC particle size is greater than the resulting splat thickness, the dynamic impact force of the WC particles on the surface exceeds the drag on the particle by theliquid binder, leading to WC particles rebounding off the surface.The efficiency of the rebounding mechanism depends on the car-bide size, the degree of melting of the binder and the impact ve-locity. As such, decarburisation by rebounding is expected to occur to a greater extent during HVOF spraying compared to plasma spraying, where the lower plasma plume velocity reduces the particle impact velocity and the extent of splat spreading. Prefer-ential rebounding of large carbide grains leads to a loss of not only carbon, but reduction in the overall carbide content.
Fig.1. Schematics describing the mechanisms of carbon loss by WC rebounding, based on the description of Li for Cr3C2-NiCr composite coatings.
本文由桑尧热喷涂网收集整理。本站文章未经允许不得转载；如欲转载请注明出处，北京桑尧科技开发有限公司网址：http://www.sunspraying.com/
|
