Microstructural evolution and wear properties of chromium carbide coating formed by thermo-reactive diffusion (TRD) process on a cold-work tool steel

Abstract

In the current study, the possibility of forming a chromium carbide layer on the surface of an AISI W1 cold work tool steel was investigated. The coatings were fabricated through a thermo-reactive diffusion (TRD) treatment by the molten salt bath at 800, 900, and 1000 °C for 4, 5, and 6 h using chromium oxide powder. The structural and mechanical properties of the coatings formed at the different conditions (temperature and immersion time) were compared together. For this purpose, the coating layers were characterized by field emission scanning electron microscopy (FE-SEM) equipped with energy dispersive X-ray spectrometer (EDS) as well as X-ray diffraction (XRD) method. Moreover, the hardness and friction coefficient (COF) of the coated samples were examined through micro-indentation and ball-on disk tests, respectively. It was determined that at 800 °C, just chromium carbide nuclei are formed, and the temperature is insufficient to produce a complete chromium carbide layer on the substrate. XRD results of the coating produced at this temperature showed that the main phase is Fe–Cr solid solution. However, compact, adherent, uniform, and crack-free chromium carbide coatings with thickness ranging from 5.14 to 18.06μm were successfully obtained on the substrate at 900 and 1000 °C, mainly consisting of Cr7C3, Cr23C6, and Cr3C2. The XRD results showed that the possibility of forming chromium carbide phases increases with increasing temperature and immersion time. The hardness of the coatings was in the range 771 to 1600 HV, depending on the process condition. The results demonstrated that increasing the temperature and immersion time of the TRD process increases the thickness and hardness of coatings, resulting in low friction coefficient values and improvement in the wear resistance up to 6 times compared to the untreated AISI W1 sample.