Influence of the processing conditions on the characteristics of the clad layers produced with laminar plasma technology

Laminar plasma technology was used to produce ceramic hardened layers of Al₂O₃-40% mass Ni composite powders on stainless steel substrates. In order to investigate the influences of processing conditions on the morphologies of the surface modified layers, two different powder-feeding methods were tested, one with carrier gas called the powder injection method, and the other without carrier gas called powder transfers method. The microscopic investigations demonstrate that the cross-section of the clad layers consists of two distinct microstructural regions, in which the Al₂O₃ phases exhibit different growth mechanisms. When the powder transfers method is adopted, the number density and volume fraction of the Al₂O₃ particles increase considerably and their distributions exhibit zonal periodical characteristics. When the powder-feeding rate increases, the microstructure of the Al₂O₃ phases changes from a small globular to a long needle shape. Finite element simulations show that the transient thermo-physical features of the pool substances, such as solidification rate and cooling rate, influence strongly the mechanisms of the nucleation and the directional growth of the Al₂O₃ phases in the thermal processing.