High Energy Ball Milling - a Promising Route for Production of Tailored Thermal Spray Consumables

The high energy ball milling technique permits production of composite powders suitable for application in thermal spray processes. Different milling systems are compared concerning their potential for production of composite powder feedstock for spraying processes. Hard phase materials like carbides or oxides are incorporated into various metallic matrices including light weight alloys based on titanium and aluminum. For optimized choice of matrix alloy and processing parameters a homogeneous distribution of hard phases with a size mostly smaller than 100 nm and perfect embedding in the metal matrix are achieved. Examples of oxide reinforced aluminum, carbide and boride reinforced self fluxing nickel based alloys and boride reinforced titanium alloy powders are presented. For optimized milling process roughly 90 wt.-% of the applied powder constituents form composite powder particles with suitable size for application in thermal spray processes. The use of HVOF spraying permits to maintain the extremely fine structure of the composite powders in the produced coatings. Characterization of the composite powders and the sprayed coatings is carried out by optical microscopy, SEM and XRD. Only negligible increase of the hard phase crystallite size but significant reduction of lattice distortion is observed after HVOF spraying. The composite coatings are characterized concerning their performance under severe wear stress. In addition to the Taber Abraser wear test an oscillating and a dry erosion wear test at elevated temperature are applied. The high potential of HVOF coatings produced from high energy ball milled composite powders is proved although the performance of conventional HVOF WC-Co 88-12 applying spray dried powder feedstock is not fully reached.