Tribological Properties of MoCoB Cermet Coupled with Different Counterparts Materials

Cermets have many potential applications in the field of wear-resistant structural materials, such as aerospace, equipment manufacturing and automotive, etc. M^Ⅰ_xM^ⅡB_x cermet (M^Ⅰ and M^Ⅱ represent different transition metal atoms, x=1,2) is a type of ternary transition metal boride, which consists of transition metal atomic layers and boron atomic layers alternately, forming a three-dimensional network structure connected by strong covalent bonds. This crystal structure gives the materials many excellent properties, such as high melting point, high toughness, high hardness and good thermal conductivity. In comparison, MoCoB cermet has lower sintering temperature and excellent mechanical properties, and can be successfully synthesized by vacuum liquid-phase sintering or SPS sintering. However, the investigation on the tribological properties when coupling with counterpart materials still needs to be further explored. In this study, dense MoCoB cermet material was successfully prepared by vacuum hot-press sintering method, in which the molar ratio of three elements powder was 1:1.25:1. The specimen was sintered at 1250 ℃ with a pressure of 30 MPa for 40 min, and finally cooled to room temperature in the furnace to obtain MoCoB cermet block material. The microstructure, mechanical properties of MoCoB cermet, and its tribological properties coupling with four different counterpart materials were investigated. The applied load, rotation radius and rotation speed during the friction test were 20 N, 4 mm and 300 r/min, respectively, and the test time was 30 min. The friction and wear mechanisms were systematically analyzed and discussed. It was concluded that the MoCoB cermet prepared by this sintering process was pure phase. The measured density and relative density of as prepared MoCoB specimen are about 8.7 g/cm³and 97.8%, respectively, indicating that the MoCoB specimen had a good densification. Its Vickers hardness was as high as 16.4±0.2 GPa, and the etched MoCoB cermet grains was lath-shaped with an average length of less than 10 μm. In addition, it was also confirmed that the tribological properties of MoCoB cermet were sensitive to the counterpart materials. The friction coefficient decreased with the increase of the hardness value of the counterpart materials, while the wear rate was not directly related to the hardness value of the counterpart materials. When MoCoB cermet was coupled with ZrO₂, which had a lower hardness, the MoCoB cermet showed the highest friction coefficient (0.70) and wear rate 2.3×10^–5 mm³/(N·m). The worn surface of MoCoB cermet was covered with a large of wear debris, and the wear degree was quite severe. Abrasive wear and adhesive wear were the main wear mechanisms. As MoCoB cermet slid against with Al₂O₃ ball, some abrasive debris could be detected on the MoCoB worn surface, and the tribo-film with fatigue cracks could be found as well. The main wear mechanisms were conceived to be adhesive wear. For MoCoB/Si₃N₄ tribo-pair, the MoCoB cermet could exhibit excellent wear-resistant property, of which the wear rate was as low as 1.6×10^–6 mm³/(N·m), and the wear mechanism was dominated by slight abrasive wear. It was found that a layer of SiO₂ oxide layer could be generated on the surface of the Si₃N₄ ball, which could play a good anti-friction and wear-resistant effect. When MoCoB cermet was coupled with a high hardness value of SiC, the tribo-oxidation products generated, which was caused by the tribo-oxidation effect on the MoCoB worn surface could reduce the friction effect. The friction coefficient was as low as 0.48. The main wear mechanisms were oxidative wear and fatigue wear.