Dry sliding up to 7.5 m/s and 800 °C of thermally sprayed coatings of the TiO 2–Cr 2O 3 system and (Ti,Mo)(C,N)–Ni(Co)

Abstract

Since the term ‘lubricious oxides’ was introduced to tribology in order to describe expected low coefficients of friction (COF) and high wear resistances in unlubricated (dry) sliding conditions numerous studies have been carried out on bulk materials and thermal spray coatings of the Ti–O system. Recently, studies were performed on the binary TiO 2–Cr 2O 3 system, as well as with (Ti,Mo)(C,N)–NiMo cermets or TiC-based thermally sprayed hardmetal coatings, which might form beneficial titanium suboxides as a result of tribo-oxidative and/or high-temperature reactions. In this paper, we shortly summarize the characteristics and dry sliding wear resistance of thermally sprayed oxide coatings of the Ti–O and TiO 2–Cr 2O 3 systems, as well as of (Ti,Mo)(C,N)–Ni(Co) hardmetal coatings. The prediction of solid lubrication properties of titanium suboxides was inspired from the planar oxygen defects present in the Magnéli-phases of titania, e.g., so-called crystallographic shear planes. While such structures with some limitations can be realised in coatings, the significant disadvantage is that TiO x tends to re-oxidize in air above ∼380 °C. The iso-structural phases in the TiO 2–Cr 2O 3 phase diagram can be prepared in air and have stable oxygen content. However, as can be concluded from the phase diagram, during thermal spraying phase transformations occur as well. In this paper, the wear resistance of alumina dry sliding against APS-sprayed coatings of three compositions in the TiO 2–Cr 2O 3 systems (compositions 75Cr 2O 3/25TiO 2, 27Cr 2O 3/73TiO 2 and 23.5Cr 2O 3/76.5TiO 2) is studied from room temperature up to 800 °C and sliding speeds in the range of 0.1–7.5 m/s. The results are compared with those of a chromia and a titania suboxide coatings, as well as with (Ti,Mo)(C,N)–Ni(Co) coatings and bulk (Ti,Mo)(C,N)–NiMo cermets as self-mated couples.