Paper SE-TuP7
High-temperature Tribological Investigations of CrAlN and CrAlSiN in Ambient and Inert Atmospheres

Tuesday, October 20, 2015, 6:30 pm, Room Hall 3

CrAlN-based hard coatings are nowadays widely used in industrial cutting applications to protect the base material of the tools against wear and oxidation. Due to alloying of the binary CrN with Al, enhanced mechanical properties of CrAlN could be achieved as a result of solid solution hardening, if the face-centered cubic structure of CrN is retained, i.e. Al atoms substitute Cr in the crystal lattice. An improved oxidation resistance is based on the formation of thin oxide layers on the coating’s surface hindering further oxidation. With the addition of Si, a nanocomposite of crystalline CrAl(Si)N grains and an amorphous SiNx grain boundary phase can be formed resulting in a further enhancement of the mechanical properties. The oxidation resistance could also be further improved due to the formation of an additional SiOx phase on the coating surface. The tribological properties of these coatings at high temperature have already been studied in detail [1, 2]. However, these experiments were performed in ambient atmosphere impeding a clear identification of the main wear mechanism like abrasive or oxidative wear. Even though CrAlN and CrAlSiN are oxidation resistant at the test temperatures up to 700 °C, oxidation in the tribological contact zone might still play an important role if the protective oxide layer on the surface is removed and freshly exposed coating material can rapidly oxidize.

Therefore, CrAlN and CrAlSiN coatings were analyzed in ball-on-disk tests in ambient and inert Ar (+ N₂) atmosphere at temperatures ranging from room temperature up to 700 °C. The coatings were synthesized by DC magnetron sputter deposition in an industrial-scale system using composite Cr₅₀Al₅₀ and Cr₄₅Al₄₅Si₁₀ targets. The tribological tests against alumina counterparts revealed a coefficient of friction independent of the used atmosphere. The wear coefficient as determined by 3D optical profilometry, however, showed a non-uniform behavior. At medium temperature of about 400 °C higher wear in inert than oxidative atmosphere was noticed, whereas the opposite trend was observed at high temperatures of up to 700 °C. A subsequent analysis of the wear scars and the wear debris by scanning electron microscopy and Raman spectroscopy revealed further details about the wear mechanisms.

[1] A.E. Reiter, C. Mitterer, M. Rebelo de Figueiredo, R. Franz, Tribol. Lett. 37 (2010) 605–611.

[2] T. Polcar, A. Cavaleiro, Surf. Coatings Technol. 206 (2011) 1244–1251.