Effect of cooling methods on the tribological, oxidation and corrosion properties of CeO2 composite boronizing coatings on 4Cr5MoSiV1 steel

The cooling method has a very important effect on the microstructure and properties of the boride layer. In this study, a rare earth-Cr composite boronizing method to prepare boride ceramic coatings on 4Cr5MoSiV1 alloy steel was used. The samples were then cooled by four different methods: water cooling (WC, 450 °C /s), furnace cooling (FC, 5 °C /min), air cooling (AC, 20 °C /s), and FC with 500°C holding 2h (HFC, two-stage cooling, 5 °C /min). The effects and mechanisms of these cooling methods on the microstructure, hardness, wear, high temperature oxidation, and corrosion properties of boride layers were investigated. The results showed that the boride layer was composed of FeB, Fe2B, CrB and Cr2B phases. Sample WC with the faster cooling rate had the highest surface hardness of 1724.21 HV, but cracks penetrating the boride layer were observed. In the friction wear test, sample HFC has the lowest coefficient of friction and wear rate, with the surface wear mechanism being oxidative wear. In addition, sample HFC formed a closed and complete oxide film on the surface, showing excellent high temperature oxidation resistance. Sample FC shows good corrosion resistance in acid and alkali corrosion immersion tests.