C. Michotte

On the effect of Ta on improved oxidation resistance of Ti–Al–Ta–N coatings

Formation of protective oxide scales is the main reason for the high oxidation resistance of TiAlN based coatings. Here the authors report on further improvement in the oxidation resistance of TiAlN by Ta alloying. An industrial-scale cathodic arc evaporation facility was used to deposit Ti–Al–Ta–N coatings from powder metallurgically produced Ti38Al57Ta5 targets. After oxidation in ambient air, a significantly reduced oxide layer thickness in comparison to unalloyed TiAlN reference material was observed. Energy-dispersive x-ray spectroscopy line scans and secondary ion mass spectroscopy depth profiling showed that the oxide scale consists of an Al-rich top layer without detectable amount of Ta and a Ti–Ta-rich sublayer. Transmission electron microscopy investigations revealed α-Al2O3, rutile-type TiO2, and anatase-type TiO2 as the scale forming oxides. Furthermore, the Ti–Ta-rich sublayer consists of a porous layer at the oxide-nitride interface but appears dense toward the Al-rich top layer. The improve...

1.16 – Coating Applications for Cutting Tools

Typically, hard coatings are produced via condensation from the vapor phase. In chemical vapor deposition (CVD) gaseous precursor vapors at elevated temperatures react and form the desired chemical compound of the coating. Since high temperatures are necessary to deposit most of these hard coatings, substrate materials are mainly limited to cemented carbides. CVD processes are suitable for coating parts with complex geometries, can produce relatively thick coatings, and characteristically yield crystalline oxide coatings. Physical vapor deposition (PVD) processes generate the vapor from a solid source by physical methods and are generally conducted at lower temperatures, enabling the use of metallic substrate materials. Ionization of the reactant species and low deposition temperatures permits the deposition of metastable coatings, far from the thermodynamic equilibrium, a major reason why PVD is a flexible process that can produce a broad variety of coatings. This chapter addresses the history of CVD and PVD coatings, their technological aspects, coating architectures, and finally their range of application and why in some regions one technology dominates and in others both are competitive.

Temperature-dependent wear mechanisms for magnetron sputtered AlTiTaN hard coatings

AlTiTaN coatings have been demonstrated to have high thermal stability at temperatures up to 900 °C. It has been speculated that the high oxidation resistance promotes an improved wear resistance, specifically for dry machining applications. This work reports on the influence of temperature up to 900 °C on the wear mechanisms of AlTiTaN hard coatings. DC magnetron-sputtered coatings were obtained from an Al(46)Ti(42)Ta(12) target, keeping the substrate bias at -100 V and the substrate temperature at 265 °C. The coatings exhibited a single-phase face-centered cubic AlTiTaN structure. The dry sliding tests revealed predominant abrasion and tribo-oxidation as wear mechanisms, depending on the wear debris formed. At room temperature, abrasion leading to surface polishing was observed. At 700 and 800 °C, slow tribo-oxidation and an amorphous oxide formed reduced the wear rate of the coating compared to room temperature. Further, an increase in temperature to 900 °C increased the wear rate significantly due to fast tribo-oxidation accompanied by grooving. The friction coefficient was found to decrease with temperature increasing from 700 to 900 °C due to the formation of oxide scales, which reduce adhesion of asperity contacts. A relationship between the oxidation and wear mechanisms was established using X-ray diffraction, Raman spectroscopy, scanning electron microscopy, surface profilometry, confocal microscopy, and dynamic secondary ion mass spectrometry.