The influence of milling parameters on the properties of the milled powders and the resultant coatings

The high energy ball milling technique is an effective method for producing powders for metal matrix composite (MMC) coatings. The properties of MMC coatings strongly depend on the amount, size and distribution of hard phases in the metal matrix, which are influenced by properties and morphology of starting powders and milling parameters. In this study, Al2O3 dispersion strengthened NiCr spray powders were produced using the high energy ball milling process with different milling parameters. Two of the milled powders and the unstrengthened matrix material were sprayed using a high velocity oxy-fuel spraying process. Both the milled powders and the coatings were studied in terms of their microstructure and microhardness. The abrasive wear resistance of the coatings was evaluated using pin-on-disk tests, during which a disk was covered by a 400 SiC grinding paper. It was found that better milling results were achieved using a longer milling time, a higher ball to powder ratio and bigger milling balls. Pre-milling of the ceramic powder led to more homogeneous dispersion structure with finer Al2O3 particles. The harder the milled powder, the harder and the more wear resistant the coating.