Tribological Behaviour Of Nitrided Layer On 2Cr13 Steel Under Dry Sliding And Grease- Lubricated Conditions In Vacuum

2Cr13 martensite stainless steel, which is a common candidate for space friction couples such as gears of spacecraft, has wide future applications due to its erosion resistance, high mechanical properties and processability to nitriding. In order to meet the requirement of space applications for the nitrided layer on 2Cr13 steel, it is of important significance to explore systematically the tribological behavior and wear mechanisms of the nitrided layer under conditions of dry sliding and grease lubrication in vacuum. In this thesis, the tribological behavior and wear mechanisms of the gas- and plasma-nitrided 2Cr13 steel were explored in vacuum using a pin-on-disk tribo-meter machine, respectively. The effects of normal load, sliding velocity and grease lubricant on the tribological behavior and wear mechanisms of the nitrided layer were investigated. At the same time, the variation in microstructure, residual stress and micro-hardness of the nitrided layer were examined before and after wearing. Based on the above analyses, critical conditions of the transitions in tribological behavior and wear mechanisms were proposed. In addition, the degradation mechanism of the lubrication grease was clarified according to the characteristics of interaction between grease and plasma-nitrided layer during sliding friction in vacuum.Experimental results show that the friction force exhibits continuous fluctuation with sliding distance when the gas- and plasma-nitrided layers slide against themselves under lower sliding velocities (0.2m/s-0.4m/s) in vacuum. In contrast, the friction force exhibits discontinuous fluctuation with siding distance for the two nitrided layers under higher velocities and larger loads, and has lower fluctuation amplitude in the case of the plasma-nitrided layer. The average friction coefficients present a similar variation with normal load and sliding velocity for the two nitrided layers during the dry sliding in vacuum. In the case of lower sliding velocities or loads, the two nitrided layers exhibit smaller wear volumes. When the sliding velocity or load exceeds a certain critical value, the wear volumes of the two nitrided layers increase with increasing the sliding velocity or load. The plasma-nitrided layer has higher critical load for the sudden transition of wear volume. Excepting the high load and velocity condition of 90N and 1.6m/s, the wear volume is lower for the plasma-nitrided layer than for the gas-nitrided layer.Under dry sliding in vacuum, both the nitrided layers show a transition from mild wear to severe adhesive or even delamination wear, with increasing load and sliding velocity. The mild wear involves fine cutting and mild adhesion. The critical load and sliding velocity for the transition are higher for the plasma-nitrided layer than for gas-nitrided layer. Based on the wear volumes, the temperatures of worn surfaces and the morphological features of worn surfaces and debris, wear mechanism maps are established for the gas- and plasma-nitrided layers under dry sliding in vacuum.After sliding for 2000m under the high load and sliding velocity condition of 90N and 1.6m/s, cracks and plastic flow lines appear in the diffusion zone of the gas- and plasma-nitrided layers, and the plastically deformed area has a remarkable increase in hardness. During the friction and wear, formation of amorphous microstructure is observed in the transition layer that is closely near the compound layer in the two nitrided layers. The analysis also indicates that dynamic restoration/ recrystallization could take place in the compound layer of the gas-nitrided sample, during the dry sliding friction, forming new fine grains ofγ′-Fe₄N phase with stacking fault substructure and a different contrast from the matrix.Under the friction and wear condition of grease lubrication in vacuum, the effect of load is more obviousier than that of sliding velocity on the friction and wear behavior of the nitrided layer on 2Cr13 steel. The friction force is basically stable under lower loads (≤30N or 10N), and shows fluctuation at higher loads (>30N or 10N) with its amplitude being increased as increasing the load. Under the condition of sliding velocities higher than 1.2m/s and higher loads, the fluctuation of friction force is featured by alternating peaks and plateaus. The average friction coefficient increases obviously with increasing the load, but decreases with increasing the sliding velocity (except for the case of lower load). Also, the decreasing amplitude of the average friction coefficient increases with increasing the load. The plasma-nitrided layer shows mild wear under grease-lubrication and all the tested conditions. Also, with increasing load and sliding velocity, a transition takes place from mild wear involving fine cutting and mild adhesion to mixed mild wear characterized by fine cutting, mild adhesion, mild fatigue pitting and local mild delamination. Wear mechanism map was established for the plasma-nitrided layer on 2Cr13 steel under grease lubrication in vacuum.It is revealed that the plasma-nitrided layer is chemically reacted with PFPE grease during the friction and wear in vacuum, forming FeF₃. The produced FeF₃ can be served as a catalyst to promote the degradation of PFPE grease, forming acid fluorides. Increasing the load and sliding velocity would accelerate the reaction between grease and nitrided layer, leading to formation of fatigue pits on worn surfaces under higher loads and larger sliding velocities.