Hard coated high-speed steels are often used for tooling applications and consist of three different microstructural constituents: a martensitic matrix, M 6C and MC carbides. The behavior of these three during the various tool manufacturing steps has a detrimental effect on the final tool performance. The focus of the current work is the surface topography change due to variation of the substrate bias voltage applied during the Ar/Ti arc plasma sputter cleaning process which changed the sputtering behavior of the high-speed steel's microstructural constituents and the appearance of droplets on the sputtered surface. The surface topography was characterized by laser confocal and scanning electron microscopy. Furthermore, microstructural analysis was carried out on cross-section lamellae prepared by a focus ion beam technique and analyzed by transmission electron microscopy. At each of the three investigated substrate bias voltages (U b) of −600, −800 and −1000 V during the Ar/Ti arc plasma sputter cleaning process, the mentioned microstructural constituents behaved differently. At an U b threshold of −600 V, sputter erosion starts at M 6C carbides in an inhomogeneous material erosion pattern. At −800 V, a homogeneous sputter behavior was established for the entire sample. M 6C carbides showed the highest sputter erosion rate, on MC carbides Ti deposition occurred instead of sputter erosion, and the martensitic matrix was smoothened, as peaks and exposed features were sputtered preferentially. As of U b = −1000 V, the sputter erosion behavior of the martensitic matrix changed and exposed features were preserved. The reason for this change was identified to be Ti + ion implantation indicated by Monte Carlo simulations and the associated increase of the matrix' cohesive strength, calculated using Density Functional Theory. The published results in this study will help to further utilize Ar/Ti arc plasma sputter cleaning processes for intentional surface topography design.
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The authors thank Harald Leitner and Ingrid Schemmel (voestalpine Böhler Edelstahl GmbH & Co KG, Kapfenberg, Austria) for fruitful discussions about high-speed steels and Georg Erlacher (Materials Center Leoben Forschung GmbH, Leoben, Austria) for carbide size distribution analysis. Furthermore, we gratefully acknowledge the financial support under the scope of the COMET program within the K2 Center “Integrated Computational Material, Process and Product Engineering (IC-MPPE)” (Project No 886385 ). This program is supported by the Austrian Federal Ministries for Climate Action, Environment, Energy, Mobility, Innovation and Technology (BMK) and for Labour and Economy (BMAW), represented by the Austrian research funding association (FFG), and the federal states of Styria, Upper Austria and Tyrol.
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