Life and Tribological Properties of PTFE/Kevlar Braided Liner under Different Accelerating Stresses

In order to solve the problems of long-life cycle and low efficiency evaluation of self-lubricating braided liner and to determine the influence of accelerated stress selection on liner reliability in accelerated life tests, the accelerated life test of the self-lubricating braided liner was carried out by using a heavy-duty reciprocating friction and wear testing device. The experimental material was PTFE (Polytetrafluoroethylene)/Kevlar braided liner and GCr15 bearing steel. The load and frequency were used as the accelerated stress respectively. The wear debris and surface of the liner in different stages were characterized by scanning electron microscopy and optical microscope to study the friction and wear properties of the PTFE/Kevlar braided liner. The accelerated life data of the liner under 20~50 MPa and 5~10 Hz were processed by the best linear unbiased estimation method and the least square method to establish the Weibull-Inverse power-law accelerated life model. The research results showed that the wear process of PTFE/Kevlar braided liner was mainly divided into three stages: slight wear stage, steady wear stage and severe wear stage. The main wear stages of PTFE/Kevlar braided liner were slight wear and steady wear stage. The severe wear stage was the stage where failure characteristics appear. In slight wear stage, the surface of the liner was mainly covered by a transfer film formed from PTFE. The friction coefficient was small but the wear rate was fast. The wear debris of the liner was mainly in powder form and the main failure mode was the adhesive wear of PTFE. In steady wear stage, the surface of the liner was mainly composed of a small amount of PTFE and Kevlar fiber. The friction coefficient increased and the anti-wear performance was the best during this stage. The wear debris was mainly in slender flake form and the main failure modes of the liner were the adhesive wear of PTFE and the abrasive wear of Kevlar fiber. In severe wear stage, the surface of the liner was mainly composed of a small amount of PTFE and resin that bonded to the substrate. The friction coefficient and the wear amount of the liner increased sharply during this stage. The wear debris of the liner was mainly in strip form and the failure mode was mainly the shear fracture of Kevlar fibers. The load mainly accelerated the wear of the PTFE/Kevlar braided liner by increasing cracks on the surface. As the load increased, the friction coefficient of the PTFE/Kevlar braided liner decreased, the adhesive wear and abrasive wear intensified. Frequency weakened properties of fiber and resin by generating frictional heat on the wear surface, the friction coefficient of the liner fluctuated greatly in different wear stages with the increase of frequency due to the changed properties of the liner. Compared with the load, the effect of frequency on the tribological behavior of the PTFE/Kevlar braided liner at different stages was more complicated. As the frequency increased, the failure mode of the liner changed from wear failure to fiber shear fracture. The actual reliability of the accelerated life model with the load as the accelerating stress was 98%, while the actual reliability of the accelerated life model with the frequency as the accelerated stress was only 78%. The different accuracy of the mode indicated that the failure mode of the liner had changed with frequency as its accelerating stress. Under the accelerated load of 20~50 MPa, the time required for the life test of the gasket was effectively shortened, and the life prediction and reliability estimation of the PTFE/Kevlar braided liner were realized.