Enhancement of the Tribological Properties of Plasma Sprayed Alumina

Sammanfattning: Thermal spraying is the name of a large group of coating deposition techniques used to deposit thick layers for a variety of applications. The principle is to melt the material, and rapidly propel the droplets towards a substrate where they flatten and solidify. When properly used, the substrate is not significantly heated. Spraying enables deposition of practically any material with a stable molten phase, on any solid material. Sprayed ceramics are used to reduce wear by sliding and by hard particles. However, due to the defect-filled microstructure resulting from spraying, the coatings typically have poor mechanical and tribological properties compared to dense, sintered materials.By varying the spraying parameters, the microstructure of the coatings was influenced, and also the wear rate and cohesion, which is difficult to quantify. Improved tribological properties resulted from spraying with axial particle injection equipment and using narrowly size distributed, spherically shaped powder particles, compared to conventional equipment and powder particles.A new method to seal the open pores of sprayed ceramic coatings by electrolysis is proposed and evaluated. It was found that almost complete sealing could be obtained, in some cases giving a drastically improved wear behaviour. The studied electrolytically deposited sealants were Pb, Sn, Cu and Ni. The latter was found to give the best performance. It also proved possible to apply PVD coatings to the sprayed ceramics, and it was shown that sprayed and Ni-sealed alumina can be superior to ball bearing steel to support thin, low friction PVD coatings.