Thick Low-Friction nc-MeC/a-C Nanocomposite Coatings on Ti-6Al-4V Alloy: Microstructure and Tribological Properties in Sliding Contact with a Ball

Abstract

In this paper, we show that duplex surface treatment, combining oxygen diffusion hardening with the subsequent deposition of thick, low-friction nanocomposite nc-MeC/a-C coatings to improve the tribological properties of the Ti-6Al-4V alloy. We have synthesized, in a magnetron sputtering process, the nanocomposite nc-MeC/a-C coatings (where Me denotes W or Ti transition metal) consisting of two dissimilar materials (nanocrystallites of transition metal carbides MeC and an amorphous carbon matrix a-C). The nano and microstructure of the substrate material and coatings were examined with the use of scanning and transmission electron microscopy as well as by X-ray diffractometry. It was found that different carbide nanocrystals of the same transition metal were embedded in an amorphous carbon matrix of both coatings. The HRTEM analysis indicated that the volume fraction of tungsten carbides in the nc-WC/a-C coating was equal to 13 pct, whereas in the nc-TiC/a-C one the volume fraction of the titanium carbides was equal to just 3 pct. The tribological properties, hardness, and scratch resistance of the coatings were investigated as well. The coefficient of friction (COF) of the coatings during dry sliding against 6 mm diameter alumina ball reached very low value, 0.05, in comparison with an oxygen-hardened alloy, whose COF was equal to 0.8. This lowfriction effect of the coatings has been attributed to the formation of a self-lubricating film in sliding contact. The coatings exhibited similar failure morphology in the scratch tests. Even though the hardness was rather low, the coatings exhibited a very good wear resistance during sliding friction. The wear rate of the nc-WC/a-C coating was equal to 0.08 9 10 À6 mm 3 N À1 m À1 and for the nc-TiC/a-C one it was 0.28 9 10 À6 mm 3 N À1 m À1 .

Nanoparticle Size Effects on Phase Stability for Molybdenum and Tungsten Carbides

“…205,206 The β-W 2 C phase is only reported at >20 nm particle sizes and only from a single reference. 207 To the best of our knowledge, ε-W 2 C nanoparticles are reported only in the intermediate range of 6−9 nm, 208,209 consistent with Figure 10a. We were unable to find any reports for γ-W 2 C particle sizes.…”

“…When tungsten oxide is used as a precursor for synthesis, oxycarbides are formed at temperatures <900 K, and the temperature is increased to obtain tungsten carbide phases. , The β-W 2 C phase is only reported at >20 nm particle sizes and only from a single reference . To the best of our knowledge, ε-W 2 C nanoparticles are reported only in the intermediate range of 6–9 nm, , consistent with Figure a. We were unable to find any reports for γ-W 2 C particle sizes.…”