RF-sputtering법에 의해 (Ti, Zr)N 복합 코팅된 금형강의 표면특성 연구

Abstract

STD 61 steel has been widely used for tools, metallic mold and die for press working because of its favorable mechanical properties such as high toughness, and creep strength as well as excellent oxidation resistance. The STD 61 tool steel coated with TiN and ZrN by sputtering results in improvement of wear and corrosion resistance, strength at high temperature and pitting corrosion resistance, especially in Cl- contained environment. In this study, the effects of sputtering conditions i.e substrate temperature, sputtering power, sputtering time and buffer layer on the surface characteristics of the STD 61 tool steel coated with TiN, ZrN and (Ti,Zr)N by RF-sputtering method have been studied by using OM, FESEM, AFM, EDS, XRD, XRR, scratch and nano-indentation tests. Corrosion behavior of the sputtered specimen was also investigated by potentiostat in a 3.5 %NaCl solution. The results obtained are as follows; (1) Microstructures of quenchied and tempered STD 61 tool steel showed martensite structure which spherical MC, M3C and M23C6 were uniformly distributed in the matrix but those carbides were coarsen with increasing tempering temperature. (2) From the results of surface characteristics of coated specimen, the ZrN coated surface showed finer granular than that of TiN coated surface. The coated layer structures of ZrN and TiN were grown to (111) and (200) preferred orientation. (3) From the results of XRR test for surface roughness, density and growth rate of coating film, surface roughness and growth rate of ZrN coated film and (Ti,Zr)N composite coated film revealed lower values those of TiN coated film, while, density of ZrN and (Ti,Zr)N composite coated film showed higher values than that of TiN coated film. (4) From the nanohardness and elastic modulus test, nanohardness value and elastic modulus of ZrN coated film became higher than those of TiN coated film. In case of (Ti,Zr)N composite coated film, nanohardness value and elastic modulus showed the hybrid character between TiN and ZrN films. (5) From adhesion test of coated film, TiN coated film had better adhesion for Ti buffer layer compared with ZrN film. In case of (Ti,Zr)N composite coated film, the coated layer did not easily breakdown and had low friction force, friction coefficient and good adhesion. (6) From the corrosion test of coated film, in (Ti, Zr)N composite coated film, corrosion current density was lower and pitting corrosion resistance was higher than those of none-coated, TiN coated and ZrN coated films..