Abstract
Surface modification engineering, including coating deposition and surface treatment techniques, is the technology to deposit a foreign material onto the surface of interest to improve specific desired properties. In this study, nanostructured CrN/AlN multilayer coatings with different modulation periods were fabricated by RF magnetron sputtering technique. The hardness of as-deposited CrN/AlN coating with of 4 nm was 28.2 GPa, which was 60% higher than that estimated by rule of mixture. The hardness enhancement was caused by the specific coherent interfaces between cubic CrN and metastable cubic AlN. The enhanced hardness for CrN/AlN multilayer coatings annealed at 850oC in vacuum could prevail, similar to the as-deposited state, and the nano-layered structure still existed. The hardness degradation ratio of CrN/AlN coating with modulation period of 4 nm was only 8.1% at 700oC, which was superior to that of CrN coating. Furthermore, the microstructure of CrN/AlN coatings exhibited a dense columnar structure and the surface roughness of multilayer coating retained below 5 nm after annealing at elevated temperatures. After heat treatment at 800oC for 1hr, only one oxide layer smaller than 50 nm in thickness was found in the annealed CrN/AlN coating with 4 nm. This amorphous oxide layer identified by EDS was a metal deficient oxide, in which Al2O3 and Cr2O3 were mixed to form solid solution. It is worthy to note that this Al2O3-Cr2O3 solid solution was still existed even after heat treatment at 950oC for 1 hr. In comparison, a thick oxide layer around 260 nm was formed on the surface of TiN/AlN coating with 4 nm. The oxide layer formed on the coating was composed of three distinct regimes, including Al-riched oxide with excess oxygen on the top surface, a crystalline Al-depleted TiO2 layer, 30-80 nm thick above the nitride coating and in between, was mixed with nano-crystalline Al2O3 and TiO2 films. After heat treatment at 950oC, the bilayer structure of TiN/AlN coating disappeared instead of the thick oxide layer with cracks found on the surface. As a result, the CrN/AlN coating exhibited superior stability compared to the TiN/AlN coating at elevated temperatures. In addition, for CrN/AlN multilayer coating with 12.3 nm modulation followed by heat treatment at 800oC for 1hr, three kinds of oxide layer around 60 nm formed on the surface was observed, including the Al-rich layer covered at the topmost surface, the mixed nano-crystalline Al2O3 and Cr2O3 film and the spherical Cr-rich grains embedded in between. After heat treatment at 900oC and 950oC for 1 hr, a large crystalline grains were formed on the surface due to the grain growths of both Al-rich and Cr-rich oxides, which was much different in the CrN/AlN coating with 4 nm. This implied that the interface in the multilayer coating played an important role in oxidation resistance at elevated temperatures. The oxidation behaviors and mechanisms of CrN/AlN with different modulation periods and TiN/AlN multilayer coatings were discussed and proposed. It was concluded that mechanical properties and thermal stability of CrN/AlN multilayer coating with 4 nm were much superior to that of CrN, AlN, CrN/AlN with 12.3 nm and TiN/AlN coatings. A promising nanostructured hard coating candidate was then developed.