Abstract
Structural features of thermal spray coatings, e.g., porosity, can be beneficial as oil retention volumes in tribological systems in order to improve emergency running properties. While thermal spray coatings can already have a considerable degree of porosity depending on the coating conditions, the finish machining, e.g., by turning, has a significant influence on the final surface properties. Effects like near-surface deformation and subsequent closing of pores during the machining process should be prevented. In the present study, the influence of thermochemical surface hardening on the surface topography of wire arc sprayed 17Cr steel layers after finish turning was investigated. Successful surface hardening by gas nitriding was shown by light microscopic and phase analyses. The surface properties after the various treatment steps were characterized by the surface roughness parameters Ra and Rz, the valley void volume Vvv, and the Abbott curves. A rise of the valley void volume can be beneficial in tribological applications in which a suitable oil retention volume is required. Accordingly, a thermochemical treatment combined with an appropriate subsequent finishing process is suitable to significantly influence the surface properties of thermal spray steel coatings.
Highlights
Thermal spray coatings are characterized by a heterogeneous microstructure comprised of oxides, pores, and cracks due to the deposition process and the chosen thermal and kinetic parameters of the applied thermal spray process
While open porosity is a disadvantage in corrosion prevention applications, in tribological systems, it can improve the emergency running properties by acting as an oil reservoir [1,2,3]
The as-sprayed coating presents the typical structure of a wire arc sprayed sprayed deposit deposit comprising comprising of of steel steel and and oxide oxide lamellas lamellas with with embedded embedded pores pores (Figure (Figure1a,b)
Summary
Thermal spray coatings are characterized by a heterogeneous microstructure comprised of oxides, pores, and cracks due to the deposition process and the chosen thermal and kinetic parameters of the applied thermal spray process. Finishing of thermal spray coatings using tools with geometrically defined cutting edges is often limited due to the constitution of the deposited layer. Hard abrasive particles such as oxides and carbides, as well as the coating porosity resulting from the spraying process, cause rapid cutting tool wear [4,5,6,7,8,9,10]. Another problem is the comparably low coating adherence to the substrate. The selection of the cutting material and the adjustment of the machining parameters to the coating microstructure are essential to achieve the targeted surface properties
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