In this study, WC-CoCr coatings were deposited on an aluminum substrate by HVOF spraying. Layer thicknesses between 50 and 150 μm were achieved by stepwise increase of the number of torch scans. The stepwise method is shown to make the coatings not only thicker, but also denser due to peening effects and changes in the splat formation mechanism. It also explains the incremental improvement in coating hardness and corrosion and wear resistance observed over the first few torch scans, the largest of which occurs between the second and third scans. The coatings are also compared to anodized films and were found to have superior wear and impact resistance but less resistance to corrosion.

Finishing of thermally sprayed metallic, ceramic and cermet coatings is required in order to meet tolerances and requirements on surface roughness in most industrial applications. Conventional machining is a costly and time-consuming process, difficult to automate. Therefore, this study investigates and develops a new technique highly amenable for automation: Fast Regime - Fluidized Bed Machining (FR-FBM). Atmospheric Plasma Sprayed (APS) TiO 2 , Cr 2 O 3 and HVOF-sprayed WC-17%Co and Tribaloy-800 coatings, deposited on AISI 1040 steel substrates, were subjected to FR-FBM treatment. The effects of the leading operational parameters, namely, abrasive size, jet pressure and processing time, were evaluated on all coatings by using a two/three-levels full factorial Design Of Experiments (DOE). The FR-FBM treated surfaces were observed by FE-SEM and their surface finishing was evaluated by contact profilometry. Significant improvements in surface finishing of all the machined thermally sprayed coatings can always be detected, with FR-FBM being able to guarantee the precision and the respect of the closest geometrical tolerances.

Residual stresses in plasma-sprayed Al 2 O 3 and Cr 2 O 3 coatings, deposited using commercial powders, and in HVOF-sprayed ceramic coatings, deposited using conventional Al 2 O 3 and Cr 2 O 3 feedstock and nanostructured- Al 2 O 3 feedstock, were studied by combining X-ray diffraction, substrate chemical removal technique and analytical modelling. The in-situ curvature technique was also employed for HVOF-sprayed Al 2 O 3 coatings, for further verification. Both HVOF-sprayed Al 2 O 3 -based coatings display similar, tensile residual stresses (≈120 MPa) near the top surface and possess moderate through-thickness stress gradients (≈10 - 20 MPa). Plasma-sprayed Al 2 O 3 possesses a smaller through-thickness stress gradient and a larger near-surface stress (≈220 MPa): this latter result seems to be due to higher quenching stresses in APS Al 2 O 3 , as determined by analytical computation The analytical model is validated by its fairly good agreement to the experimental results obtained both by substrate chemical removal and by in-situ curvature. Cr 2 O 3 -based coatings possess a lower near-surface residual stress (≈20 MPa); the HVOF one also exhibits a very large stress gradient (≈80 MPa). Machining and sliding processes (like polishing and dry sliding tribological testing) change their surface residual stresses to compressive ones.