Void formation and spatial distribution in plasma sprayed Nd-Fe-B coatings

Formation of voids is inevitable in plasma sprayed coatings and the role of voids on coating properties has long been established. In fact, the void content within coatings is adjusted by optimizing the process parameters to obtain coatings with desirable performance. Quantification of voids via image analysis allows determination of not only the void content within a coating, but also the spatial distribution of the voids. Void content in plasma sprayed neodymium iron boron (Nd-Fe-B) coatings was adjusted by changing the standoff distance, and was found to vary from 1.8 to 8.2%. Spatial distribution parameters, which include nearest neighbor distance (d min), mean near neighbor distance (d mean), and nearest neighbor angle (θn), were determined via the Dirichlet tessellation method. Coefficient of variation (COV) values of d min and d mean allow determination of inhomogeneity and degree of clustering of the voids within a coating. The θn values reveal the anisotropic behavior of voids within plasma sprayed coatings. The influence of void content and its spatial distribution within the coatings on the microhardness and elastic modulus of coatings was determined.