Chemical vapor deposition of hard coatings : Development of W(C,N) coatings for cemented carbide and TiN deposition on a CoCrFeNi substrate
Abstract: There is a constant need for cutting tool material development to be able to machine new materials and improve the metal cutting efficiency. Inserts of indexable cutting tools usually consist of WC-Co cemented carbide (cc) with μm thick layers of ceramic coatings. Chemical vapor deposition (CVD) is a commonly used method for the synthesis of such coatings and this technique was also used in this thesis.The first part of the thesis focuses on a CoCrFeNi substrate, a multi-component alloy previously suggested as an alternative binder phase instead of Co in cc. Titanium nitride (TiN) is typically the first CVD coating layer on cc and therefore the chemical stability of CoCrFeNi in CVD of TiN using TiCl4, N2 and H2 precursors at 850–950 °C was studied. CoCrFeNi was stable in the CVD processes although small amounts of Cr were detected in the grain boundaries and on the top surface of the TiN coating deposited at 950 °C. Thermodynamic calculations were used to explain the experimental observations.In the second part of the thesis W(C,N) coatings were developed by CVD. The motivations behind the CVD of W(C,N) coatings were an expected compressive stress due to a favorable match between the thermal behavior of the coating and that of the cc substrate, and a high coating toughness due to the expected strong metallic character of the coating.A CVD process window was determined for the deposition of columnar hexagonal W(C,N) coatings at a low total pressure as a function of the WF6, CH3CN and H2 precursor partial pressures and the deposition temperature. Electron microscopy, diffraction techniques, spectroscopic methods and ion beam analyses were used to characterize the coatings. Correlations between the microstructure, texture and process parameters were found.The mechanical and tribological performances of the W(C,N) coatings were also investigated. The coatings had a high nanohardness of about 40 GPa and showed indications of a high toughness. The abrasive wear mechanism was primarily micro cutting or plowing. The coatings had good adhesion to the cc substrate.
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