Microstructural, mechanical and tribological properties of finely grained Al2O3 coatings obtained by SPS and S-HVOF methods

In this work suspension-high velocity oxygen fuel spraying (S-HVOF) and suspension plasma spraying (SPS) were applied to produce Al2O3 coatings using home-made and commercial available Al2O3 water-based liquid feedstocks containing 40 wt% of submicrometer-sized raw powders. The aim of this study was to investigate the differences in the microstructure, mechanical properties and tribological performance of the suspension sprayed coatings with these two techniques. The microstructure of SPS coatings was slightly porous with fine particles of sintered, non-melted powders entrapped between the well melted lamellas. On the contrary, the S-HVOF coatings were very homogeneous in terms of morphology and of very low porosity in range of 1 to 4 vol%. With initial alpha-Al2O3 powders, it was possible to spray alpha-phase rich coatings, both in the case of SPS and S-HVOF, as determined by X-ray diffraction, with up to 47 vol% of alpha-Al2O3 in the S-HVOF coating and about 62 vol% in SPS one. The microhardness, adhesion strength and dry sliding wear resistance testing showed a good performance of coatings sprayed by using liquid feedstocks, especially the S-HVOF sprayed ones. Finally, the influence of feedstock characteristics and spray process conditions on the mechanical and tribological properties was analyzed.