Relationships between microstructure, mechanical properties of plasma-sprayed Ni-50Cr coatings and spray parameters

The Ni-50Cr coatings are prepared under different plasma spray conditions which are arranged according to the orthogonal experimental design method using three typical parameters of plasma arc current, Ar flow and spray distance in order to obtain the quantitative regression correlations between spray parameter and coating mechanical properties, and examine the relationship between the mechanical properties and microstructure features. The mechanical properties of sprayed Ni-Cr coatings are characterized by hardness, abrasive wear weight loss and erosion rates. The microstructure of the coatings is examined by optical and electron scanning microscopes. The porosity in the coating is quantitatively estimated from by the increase in Cr content of the coating after the impregnation of Cr₂O₃ into pores of the coating. The effects of spray parameters on the mechanical properties of the coating are discussed according to regression formulas systematically. The results evidently show that three typical parameters have significant inter-effects on the microstructure and properties of the Ni-Cr coatings. There are evident correlations among the hardness and abrasive wear resistance, and porosity for plasma sprayed Ni-Cr coatings. With the decrease in the porosity of the coating, the hardness and abrasive wear resistance of the Ni-Cr coating are increased. On the other hand, there are no correlations between erosion resistance and hardness, and porosity as well. It has been found that the abrasive wear occurs mainly by micro-cutting of abrasives while the erosion occurs mainly by the successive separation of flattened particles exposed to the surface which are subjected to the direct impact of erosive particles. Therefore, the abrasive wear resistance is primarily dependent on the density and hardness of the coating, while the erosion wear is mainly dependent on the condition of the cohesion between flattened particles. The results also revealed that the deposition of partially melted particles into the coating deteriorates the erosion resistance of the coating, especially at low impact angle.