Abstract
Surface modification engineering is the technology to deposit a foreign material onto the surface of interest to improve specific desired properties. This study aimed to develop a new multi-component material as a protective coating, which would enhance the surface strength, thermal stability, and corrosion resistance of the tool steels. The multi-component (TiAlCrSiV)xNy coatings were fabricated by utilizing a high entropy alloy (HEA) target in RF magnetron sputtering. Through hardness measurements, wear tests, heat treatments, and corrosion tests, effect of each additional element in harsh environments was evaluated. The multi-component (TiAlCrSiV)xNy nitrides (denoted as MCN in the following text) exhibited a f.c.c. crystallined structure and an enhanced hardness as high as 36.4 GPa. According to the XRD patterns and TEM analyses, the improved mechanical properties were attributed to their nano-crystalline structures. After a heat treatment at 600 oC in air, a mixed amorphous oxide layer formed on the surface of MCN coating and protected the coating from further oxygen attack. Subsequently, the formations of CrVO4 and TiO2 were verified after annealing the coating at 700 oC in air. The outward diffusion of Ti, revealed by EDS quantitative data, implied that these crystalline oxides no longer acted as a protection and serious oxidation occurred from this temperature. Due to the absence of self-lubricating V2O5 oxide, the reduced friction coefficient was only found in the as-deposited MCN coatings. Although addition of Ti and V was disfavored in consideration of oxidation, the multi-component TiAlCrSiV coating (denoted as MC in the following text) revealed its amorphous nature as well as good corrosion resistance against NaCl solution. Being a metallic interlayer between coating and steel substrates, it also greatly improved the polarization resistance of the steel/MC/MCN sample. Hence, the coated tools could reveal both enhanced surface hardness and improved corrosion resistance. Concerning about both oxidation and corrosion resistance, Ti and V were removed in the parallel study, and the amorphization was achieved by adjusting the Si content of the modified coating. The nanocomposite structure of the modified CrAlSiN coating was investigated by TEM techniques, and the grain refinement by adding Si was verified. It was found that the columnar structure of CrAlN coating could be altered to a compact one after Si addition, and the polarization resistance of the coated sample was thus improved. Therefore, the CrAlSiN coating with high hardness, good thermal stability and corrosion resistance was suggested as a potential candidate coating for tool steels.