Growth Kinetics and Tribological Properties of NbC Coating on Different Steel Substrates Prepared by Pack Cementation Method

Abstract:

The growth kinetics process and tribological properties of niobium carbide (NbC) coatings prepared by the pack cementation method on the 40Cr and 45 steel substrates were investigated with the main raw materials of iron niobium powder, ammonium chloride, and alumina under the conditions of different temperatures (1123~1273 K) and different treatment durations (1~4 h). Results show that the coating is compact and well bonded with the substrate interface, and it is mainly composed of NbC phase. The thickness of the coatings on 40Cr and 45 steel substrates is 1.703±0.285~8.457±0.240 and 1.987±0.355~9.247±0.275 μm, respectively. The growth kinetics study shows that the coating growth is controlled by the diffusion process, and the thickness has a parabolic relationship with the treatment duration. The activation energies of the NbC growth process on the 40Cr and 45 steel substrates are 113.80 and 102.76 kJ/mol, respectively. After treatment at 1273 K for 4 h, the hardness of NbC coating reaches more than 21 560 MPa, which is 5.49~8.06 times higher than that of the steel substrate. With GCr15 steel ball as the grinding material, the average coefficient of friction of NbC coating on the 40Cr and 45 steel substrates is 0.393 and 0.342, respectively; the average coefficient of friction of substrate is 1.3~1.6 times higher than that of the NbC coating. The volume wear rate of NbC coating is about 34.9%~37.5% of that of the steel matrix, indicating that the NbC coating has excellent wear resistance and wear reduction performance. The friction and wear mechanism of NbC coating is abrasive wear, adhesion wear, and oxidation wear.

Key words:pack cementation method;NbC coatings;different substrates;growth kinetics;friction