Synthesis and post-treatment of hard coatings for cemented carbide cutting tools

Research output: ThesisDoctoral Thesis

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Abstract

The aim of the present thesis is to improve the efficiency of coating deposition and to optimize the hard coatings for more economic cutting processes. In order to make coating synthesis efficient, the sputter targets play a crucial role. Thus, the influence of two different sputter target types (i.e. powder metallurgically produced compound TiAl targets and mosaic TiAl targets prepared by casting) on the deposition process as well as on the structure and properties of the synthesized coatings was investigated. Using compound targets, a ~44 % higher deposition rate than for the mosaic targets could be obtained. Additionally, the Ti1-xAlxN coatings grown from the compound targets showed reduced friction and higher wear resistance compared to the mosaic targets. For further optimization of the Ti1-xAlxN coatings, a series of coatings with stresses ranging from tensile to compressive was successfully deposited using different bias voltages. The samples were vacuum annealed and the influence of the residual stresses and the grain size on the spinodal decomposition of the metastable coatings was illuminated. Tensile residual stresses promote the formation of more volume consuming fcc-TiN domains during decomposition, whereas compressive stresses foster the formation of smaller fcc-AlN domains. Smaller grain sizes cause earlier precipitation of w-AlN. In a next step, commercial TiAlTaN coatings were thermally treated in methane at 900 °C to combine the beneficial effects of both, age hardening and friction reduction due to the formation of a carbon deposit on top. As a result, friction coefficients comparable to values for DLC and MoS2, tested under similar conditions, could be obtained. A different possibility to enhance the coating performance is based on blasting treatments. The relaxation of the compressive stresses, introduced by dry-blasting of alpha- and kappa-Al2O3 CVD hard coatings using different blasting pressures and materials, at elevated temperatures was investigated as well as the effects of these dry-blasting treatments on the friction behavior. The stress relaxation strongly depends on the blasting medium. For alpha-Al2O3 dry-blasted using a globular medium, a complete stress relaxation could already be observed after annealing at 500 °C, while for alpha-Al2O3 dry-blasted using an edged medium as well as for kappa-Al2O3 an annealing temperature of 900 °C was necessary for stress relaxation. A drop of the friction coefficient at ~700 °C could be observed, which was due to softening of the blasting material transferred to the coating surface.
Translated title of the contributionHerstellung und Nachbehandlung von Hartstoffschichten für Hartmetall-Zerspannungswerkzeuge
Original languageEnglish
QualificationDr.mont.
Supervisors/Advisors
  • Mitterer, Christian, Assessor A (internal)
  • Keckes, Jozef, Assessor B (internal)
Publication statusPublished - 2013

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