Interface interaction of additively manufactured composite systems coated by PVD - Correlation of surface hardening, residual stresses, roughness, and fatigue strength

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Additive manufacturing processes such as laser beam melting (LBM) have become a common manufacturing process in the field of prototyping and open up new possibilities for the user in terms of design freedom of metallic components for the mechanical and automotive engineering with high-cost efficiency at the same time. Such components are very often subject to dynamic loads. To protect tribologically stressed functional surfaces from premature wear, the application of nitride PVD hard coatings has become an established approach in surface technology. However, the fatigue behavior of LBM steel substrate PVD coating composites under cyclic loading were not investigated. First preliminary work shows that coating 316L substrates manufactured by LBM with CrAlN PVD hard coatings lead to an increase in low-cycle fatigue strength. The coating growth as well as the resulting adhesion strength are influenced by the microstructure and the residual stress state of the LBM substrate surface as a result of the building process as well as mechanical pretreatment procedures, whereby the direct correlations however are unsettled. Furthermore, their influence on fatigue strength is unclear. The aim of this research project is to investigate the influence of different interface modifications of steel surfaces processed employing LBM, i.e. an adjustment of the mechanical properties (interface hardening, residual stress state) and crystalline microstructure by means of mechanical, thermal and thermochemical substrate pretreatments, on the nucleation and growth phase of PVD hard coatings as well as their effect on the fatigue strength of LBM substrate — PVD coating composites. For this purpose, the austenitic steel 1.4404 (X2CrNiMo17-12-2 respectively 316L) and the tempered steel 1.6773 (36NiCrMo16) manufactured by LBM are subjected to different processes like polishing, plasma nitriding and stress relief annealing to provide different surfaces (-interfaces) properties. The resulting microstructural and mechanical properties are analyzed using radiographic examinations as well as mechanical technological test methods. Depending on the differently pretreated LBM substrates, the adhesion strength and coating growth of different PVD Cr1-xAlXN stoichiometries are investigated. The long-term behavior of the LBM substrate PVD coating composites is evaluated by means of fatigue tests (high cycle fatigue). The knowledge gained will be correlated with the results of microstructure analysis to determine the material mechanical processes due to mechanical, thermal, and thermochemical processing procedures of (coated) LBM components to increase fatigue strength.