플라즈마 이온 질화에 따른 스테인레스강 고속화염용사 코팅의 미세조직 및 기계적 특성

Abstract

본 연구는 열변형 최소화 및 적층 두께 계량화/제어를 위해 스테인레스강 분말을 프레스 금형에 고속화염용사법 (High Velocity Oxygen-Fuel spraying)을 이용하여 적층시켜 공정 최적화를 수행하였다. 코팅층의 전처리 공정으로 Sand shot-blasting을 실시하여 일정 표면 거칠기 (Ra) 이상에서 코팅과 모재간 치밀한 계면과 높은 접합강도를 갖는 코팅층을 확보하였다. 고속화염용사 (HVOF spraying) 코팅 공정 중 산소와 연료가스의 증감을 통해 공정변수를 제어하였으며, 미세조직과 기계적 특성을 평가하여 비교하였으며, 변형경화 및 결정립 미세화에 의한 결정립계 강화를 구현하여 코팅 강도를 향상시켰다. 또한 코팅층의 내구성 향상을 위하여 플라즈마 질화처리를 실시하여 코팅층에 화합물층과 확산층으로 이루어진 질화층이 형성되었으며 플라즈마 이온 질화 공정은 N2와 H2의 혼합가스와 함께 아세틸렌 (C2H2)을 첨가하여 아세틸렌 첨가에 따른 질화층의 두께 증가를 확인하였다. 또한 미소경도 시험과 마모시험을 통해 질화처리된 코팅층의 기계적 특성(미소경도, 하중지지능력, 내마모성)을 평가하였다. 후 질화처리를 통해 γN, CrN, Fe3N, Fe4N의 석출과 코팅 내부의 질소 확산에 의한 고용강화 효과로 매우 높은 표면 경도를 가지는 것을 확인하였으며 질화층의 두께와 비례하여 높은 하중 지지능력(load bearing capacity)을 보였다. 또한 마모 중 마찰열에 의해 형성된 표면 산화물층의 영향으로 매우 뛰어난 내마모성을 가지는 것을 확인하였다. |High Velocity Oxygen-Fuel (HVOF) Spray coating and post-plasma ion nitriding to minimize thermal deformation and controlling of coating thickness have been studied. Prior to HVOF spraying, sand shot-blasting of the surface of substrate was perfomed in order to obtain bond strength between the coating and substrate. Stainless steel alloy feedstocks were deposited on the spheroidal graphite cast iron (FCD550) with various coating parameters such as oxygen and fuel flow. Microstructure evolution in the coatings and deposition efficiencies for various coating materials and HVOF coatings parameters were optimized. Mechanical properties (microhardness and bond strength) were also evaluated. In this work, to improve durability of coatings, plasma nitriding and nitrocarburizing on the HVOF sprayed stainless steel coatings were also investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steel coatings were plasma-nitrided and nitrocarburized using a N2 + H2 gas mixture and a gas mixture containing C2H2, respectively, at 550℃. The results showed that the plasma nitriding and nitrocarburizing produced thick nitriding layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C2H2 (plasma nitrocarburizing process) was added. The plasma nitriding and nitrocarburizing resulted in not only an increase of the surface hardness but also an improvement of the load bearing capacity of the HVOF sprayed stainless steel coatings because of the formation of CrN, Fe3N, and Fe4N phases. Also, the plasma nitrocarburized HVOF sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr23C6 on the surface. All of nitrided and nitrocarburized coatings showed excellent wear resistance under the severe wear conditions. Fe oxides formed by the frictional heating in fretting contacts during the pin on disc test acted as a lubricant further improved the wear properties of plasma nitrided and nitrocarburized coatings.; High Velocity Oxygen-Fuel (HVOF) Spray coating and post-plasma ion nitriding to minimize thermal deformation and controlling of coating thickness have been studied. Prior to HVOF spraying, sand shot-blasting of the surface of substrate was perfomed in order to obtain bond strength between the coating and substrate. Stainless steel alloy feedstocks were deposited on the spheroidal graphite cast iron (FCD550) with various coating parameters such as oxygen and fuel flow. Microstructure evolution in the coatings and deposition efficiencies for various coating materials and HVOF coatings parameters were optimized. Mechanical properties (microhardness and bond strength) were also evaluated. In this work, to improve durability of coatings, plasma nitriding and nitrocarburizing on the HVOF sprayed stainless steel coatings were also investigated. 316 (austenitic), 17-4PH (precipitation hardening), and 410 (martensitic) stainless steel coatings were plasma-nitrided and nitrocarburized using a N2 + H2 gas mixture and a gas mixture containing C2H2, respectively, at 550℃. The results showed that the plasma nitriding and nitrocarburizing produced thick nitriding layer consisting of a compound layer and an adjacent nitrogen diffusion layer depending on the crystal structures of the HVOF sprayed stainless steel coatings. Also, the diffusion depth of nitrogen increased when a small amount of C2H2 (plasma nitrocarburizing process) was added. The plasma nitriding and nitrocarburizing resulted in not only an increase of the surface hardness but also an improvement of the load bearing capacity of the HVOF sprayed stainless steel coatings because of the formation of CrN, Fe3N, and Fe4N phases. Also, the plasma nitrocarburized HVOF sprayed 410 stainless steel had a superior surface microhardness and load bearing capacity due to the formation of Cr23C6 on the surface. All of nitrided and nitrocarburized coatings showed excellent wear resistance under the severe wear conditions. Fe oxides formed by the frictional heating in fretting contacts during the pin on disc test acted as a lubricant further improved the wear properties of plasma nitrided and nitrocarburized coatings.