Abstract
The aim of this study is to develop a novel nitride material as a protective coating, which enhances the surface hardness, wear resistance, thermal stability and oxidation resistance of the machining tools. Multi-component (AlCrTaTiZr)-Six-N coatings were deposited on silicon wafers, cemented carbide substrates and cutting inserts by reactive RF magnetron co-sputtering of an equimolar AlCrTaTiZr alloy target and a pure silicon target. The effect of silicon content on the microstructures, mechanical properties, tribological behavior and oxidation resistance was studied. Nitride films with low silicon content remained a simple FCC (face-centered cubic) structure. As the silicon content reached 7.9 at%, thermodynamically driven phase separation occurred, leading to a nanocomposite structure consisting of an FCC solid-solution nitride and an amorphous SiNx phase. These nitride films exhibited high hardness of 34 GPa and remained a constant level up to 7.9 at% Si. The reduced hardness at silicon content of 10.2 at% was attributed to the appreciable amounts of softer amorphous segregation. These nitride films showed a mild polishing wear behavior with higher friction coefficient from 0.79 to 0.83 and a higher wear rate from 6.0×10-6 mm3/N•m to 9.8×10-6 mm3/N•m with increasing silicon content, while the film with the silicon content of 10.2 at% showed a tribochemical wear behavior having the lowest friction coefficient of 0.74 and an almost undetectable wear rate due to the oxidized products and transferred materials on the worn surface. Silicon incorporation significantly improved the oxidation resistance of the (AlCrTaTiZr)N films. The nitride film with the silicon content of 7.9 at% annealed at 1000 °C for 2 h in air only had a 330 nm-thick oxide layer. This improvement was attributed to the amorphous SiO2 boundary around the crystallites, and the nanocomposite structure (for high silicon content) with the presence of amorphous SiNx phase. The presence of protective skin surface consisting SiO2 and Al2O3 and the subsequent sublayer consisting of Cr2O3 and Al2O3 also accounted for the further improved oxidation resistance at such a high temperature. Comparing with the present state-of-the-art protective hard coatings, the inserts with multi-component nitride coatings exhibited lower tool wear rates when machining stainless steel. The silicon-containing nitride coating had better performance when machining SKD11 tool steel due to the much improved oxidation resistance. In this study, the optimum silicon content of the multi-component nitride coatings is 7.9 at% since it gives the best combination result of hardness and oxidation resistance and consequently the best cutting performance. These coatings are indeed applicable to the manufacturing and processing industry.