Abstract
The surface modification engineering is a useful technique to achieve desired properties of mechanical strength, thermal stability, anti-corrosion and wear-resistance onto the surface and thus plays an indispensable role in industry, especially in machining process. However, some inevitable problems restrict the continuous development of hard coatings in this field, such as high friction, thermal degradation and strong adhesive interaction at contact surface during the process. To overcome these drawbacks, the development of a novel self-lubricating hard coating with superior anti-abrasion at elevated temperature is compulsory. In this study, a new CrAlN/VN multilayer coatings are fabricated by RF reactive magnetron sputtering. The bilayer periods are altered from 10 to 40 nm, and the individual layer thickness ratio of CrAlN to VN is kept at 1. Characterizations by TEM, dark-field images, and SEM reveal dense and coherent columnar in coatings. Owing to the interfacial strengthening with plenty of interfaces, CrAlN/VN multilayer coatings with appropriate bilayer period exhibit superior plastic deformation resistance, H3/E*2. With an appropriate bilayer period of 16 nm, the H3/E*2 boosts to a maximum around 0.36. Furthermore, the tribological properties are examined by a ball-on-disc wear test at room temperature and 700oC. At room temperature, the wear rates of CrAlN/VN multilayer coatings are lower than that of CrAlN due to structural strengthening. Particularly, the multilayer coating with a bilayer period of 16 nm reveals the lowest wear rate of 1.0×10−6 mm3N-1m-1. After wear test at 700 oC, it can be observed that the coefficient of friction (COF) values for CrAlN/VN multilayer coatings significantly reduce with increasing temperature, which is attributed to the formation of sufficient solid and liquid self-lubricating vanadium oxides at elevated temperature. Moreover, mechanical strengthening in multilayer coatings with numbers of interfaces is beneficial for lowering the wear rate. Especially, the value can be down to 1.6×10−5 mm3N-1m-1 for the one with bilayer period of 16 nm. In addition, the worn morphology after wear test at 700oC is much favorable with less spallation and debris on the wear track, as compared to CrAlN monolayer. The X-ray photoelectron spectroscopy (XPS), focused ion beam (FIB) and transmission electron microscopy (TEM) techniques are further used to examine the worn surface characteristics after wear test at elevated temperature to further probe the hybrid anti-wear mechanisms. A hybrid mechanism, including structural strengthening and elemental contribution, is proposed to highlight the favorable anti-wear property in CrAlN/VN multilayer coatings at elevated temperature.