Lapping and polishing of additively manufactured 316L substrates and their effects on the microstructural evolution and adhesion of PVD CrAlN coatings

Abstract

Laser powder bed fusion (LPBF) enables the manufacturing of three-dimensional parts from metallic powder with a high degree of design-freedom. Due to the low hardness in order to protect tribologically stressed surfaces against severe wear, the surface of LPBF parts made of 316L stainless steel needs to be modified by coating processes such as physical vapor deposition (PVD) using hard coatings. Although 316L is among the most investigated material for LPBF, the deposition of PVD hard coatings such as ternary nitride coatings on additively manufactured 316L substrates has not been studied yet.This study aims at evaluating the effects of lapping and polishing of 316L stainless steel substrates processed by LPBF on the resulting surface integrity and its influence on the microstructural evolution and adhesion of PVD CrAlN coatings. A 316L substrate bulk material served as reference throughout the investigations. The 316L substrate CrAlN coating composites were examined with respect to the microstructural characteristics and residual stresses using nanoindentation and two-dimensional X-ray diffraction. To assess the adhesion of the CrAlN coating on the differently pre-treated 316L substrates, scratch and Rockwell C adhesion tests were conducted. The resulting scratch tracks and Rockwell C indents were metallographically examined in order to determine the failure modes. The results demonstrate that the deposited PVD CrAlN coatings differ in their texture and mechanical properties, such as hardness and residual stresses, depending on the 316L substrate surface conditions. The CrAlN coatings generally exhibit good adhesion to both the 316L LPBF substrate and 316L bulk material. However, the findings indicate improved adhesion of CrAlN coatings on lapped 316L substrates. The residual stress gradient at coating-substrate interface and imperfections at the surface of the 316L LPBF substrate are found to primarily influence the performance and failure mode of the CrAlN coating.