Influence of the finish-machining by turning and diamond smoothing on the tribological properties of Fe17Cr2Ni0.2C thermally sprayed coatings

Abstract

In the presented work, cylindrical specimens of aluminium EN AW-5754 are pre-machined by turning to provide fine-structured surfaces before the subsequent coating with Fe17Cr2Ni0.2C by Atmospheric Plasma Spraying (APS). The pre-machining is done with specific MCD-tipped tools that generate helical dovetail structure elements in μm-range and offer high adhesive tensile strength of the applied coating. In this way, finish-machining of the iron based coating is possible. It is carried out at varied tool feed (0.05 mm – 0.15 mm) by turning with CBN-tipped indexable inserts. Depth of cut (0.05 mm) and cutting speed (250 m/min) are kept constant. Specimens are then machined by diamond smoothing with different radial forces (50 N – 200 N), constant tool feed (0.05 mm), and constant machining speed (100 m/min). Properties and geometric features of the machined surfaces are characterised by tactile measurement, 3D laser scanning microscopy, SEM, and tribological analyses. The analyses include the determination of opened pores and pulled-out coating material, that are brought together in the combined surface parameter of the valley void volume. Machining by diamond smoothing after turning generally leads to decreased surface roughness. In addition, the smoothing forces lead to increased absolute values of surface-near compressive residual stresses.The results of this work contribute to the substitution of common honing processes as finish machining steps of thermal spray coatings, especially focussing on the non-homogeneous character of these coatings. In this work, for the specific application and when comparing similar surface finish qualities, it is proven that at diamond smoothing results in less tool wear than finish-machining by turning. As a main result, a material and energy efficient finish-machining process for thermal spray coatings is positively evaluated.