Effects of AISI H11 surface integrity on the residual stresses and adhesion of TiAlN/substrate compounds

Abstract

The material of the substrate prior to the deposition of PVD coatings as well as its surface roughness and microstructure, hardness, and Young's modulus both on the surface as well as in the material matrix, and their residual stresses on the surface and in depth, have a high influence on the adhesion of the coating to the substrate. During metal forming processes, tools are exposed to high loads, which promote plastic deformation in the material of the tool and thus reduce its lifespan. In this regard, different substrate pre-treatments, allow modifying the hardness and toughness of the substrate, positively affecting the adhesion of the coating/substrate compound as well as the overall performance of the PVD coated tool.In this investigation, four different pre-treatments were performed on the hot work tool steel AISI HI I (X37CrMoV5-1). Plasma nitriding, two different heat treatments, and polishing sequences (austenitizing, quenching and double tempering) as well as a combination of these two processes (heat treatment prior plasma nitriding). Subsequently, TiAlN coatings with a 3 μm thickness were deposited onto the four differently treated substrates by means of Direct Current Magnetron Sputtering. Residual stresses of the substrate surface, before and after coating deposition were determined by means of X-ray diffraction. Additionally, residual stresses depth profiles of the steel after each pre-treatment prior to the deposition of the coatings were measured utilizing selective electropolishing of the substrate surface. To evaluate the adhesion of the TiAlN monolayers to the coatings, scratch tests and Rockwell C adhesion tests were performed on the coating/substrate compounds. The adhesion of the coatings, deposited on steel with lower residual stresses prior to the deposition and flatter residual stress gradients, was improved.