Tribological Properties of PTFE/Kevlar Fabric Composite in Rainwater Environment

PTFE/Kevlar fabric composite is widely used in joint bearings in some key fields, such as aerospace, rail transit and military equipment, because of its characteristics of self-lubrication, impact resistance, high strength and wear resistance. As a key part of joint bearings, the PTFE/Kevlar bearing liner replaces the outer ring, and directly contacts and wears with the inner ring. Therefore, the friction and wear performance of PTFE/Kevlar bearing liner in complex service environment is very important to the service safety of joint bearings. Experiencing rainy weather is very common for the joint bearings of main rotor during the service of helicopter. In order to explore the influence of environmental conditions on the frictional properties of PTFE/Kevlar bearing liners used in aircraft spherical plain bearing, the reciprocating wear tests were carried out considering the practical rainfall environment in the western Sichuan plain. The tests were performed on MXW-01 reciprocating friction and wear rig using ball-plane contact mode. The ball sample and plane sample were GCr15 bearing steel ball and PTFE/Kevlar woven liner, respectively. During the test, the plane sample surface was continuously sprayed with uniform rain water to maintain a completely wet environment. After the tests, the evolution of damage morphology of wear scar and chemical composition on wear surface were analyzed by using optical microscopy, field emission scanning electron microscopy, three-dimensional optical profiler and energy dispersing spectroscopy. Consequently, the tribological properties of PTFE/Kevlar fabric composite in rainwater environment were investigated. The results showed that the damage behaviors of PTFE/Kevlar liner material were manifested as deformation, breakage and transfer of PTFE fibers, and deformation and fatigue fracture of Kevlar fibers. The wear process of PTFE/Kevlar fabric in rainwater environment was divided into three stages: the early stage (N<1000), the middle stage (1000≤N<5000) and the late stage (N≥5000). Specifically, the early stage of wear represented the transfer stage of PTFE, in which the friction coefficient was gradually stable at 0.38, and the damage of PTFE/Kevlar composite was relatively slight. The middle stage of wear indicated a formation stage of PTFE transfer film. In this stage, the friction coefficient decreased gradually from the maximum value of 0.42 to the minimum value of 0.36. Meanwhile, the damage of the liner material increased slightly. The late wear stage suggested a destruction stage of transfer film, in which the friction coefficient fluctuated between 0.36 and 0.41. In this stage, the surface of PTFE/Kevlar fabric material was seriously damaged, and Kevlar fibers were exposed and deformed. It was found that from the early stage to the middle stage, with the increase in the number of cycles, the friction coefficient decreased, the maximum width and depth of wear scar increased, and the content of F element in the wear scar increased while the contents of C and O element decreased. On the other hand, from the middle stage to the late stage, the friction coefficient increased and fluctuated with the elevating number of cycles, the maximum width and depth of wear scar continued to increase, however, the F element content decreased and the C and O element content increased. Comparing with the dry condition, it can be included that the rainwater environment could promote the peeling and destruction of the transfer film. Therefore, the spherical plain bearing with self-lubricating PTFE/Kevlar lining should be avoided as far as possible in rainy weather for a long time.