3.1 Effect of load and frequency on lifetime
Fig. 4 shows the change of material wear life. Wear depth is relatively large at the beginning and the difference of wear life is only 40 h at 100 MPa compared to that at 110 MPa. It can be seen that the wear loss fluctuates greatly with time under two conditions. The main reason is that the dynamic change of tribological behavior and the coupling effect of mechanical impact lead to the lubrication state of contact surface being extremely unstable. Besides, the thickness of the fabric is only 0.38 mm, and thermal deformation of the friction pair has a great effect on the measurement results due to accumulated friction heat generated during the friction. So after each test, static load is maintained to eliminate the influence of thermal expansion of friction pair. At 100 MPa, wear depth has an apparent change until it reaches 70 h. It is analyzed that the factors such as friction temperature, equipment operation error and measurement error can be excluded, so it is probably caused by the change of tribological properties of the self-lubricating material. The fluctuation of wear depth in a small range can be attributed to the machine vibration and measurement accuracy (relative stable stage is more than 200 h). Whereas the fluctuation of wear depth in a large range is due to effect of self-lubrication behavior on the friction pair (mainly at the early stage), as shown in Fig. 5. In Fig. 6, because the oscillating frequency is unstable under high load, the data of two points at 1 Hz and 3Hz are recorded with the load is held at 110 MPa. It shows the change of wear rate with varying frequency. At 90 MPa and 100 MPa, wear rates rise abruptly, whereas the difference of wear rates under two frequencies is not obvious at 110 MPa.
Fig. 7 shows the initial surface topography of the Nomex/PTFE fiber reinforced composite. The actual contact area between the composite and its counterpart during friction depends on the fabric structure and material density. The actual contact area is far less than the apparent contact area between the fabric and its counterpart. Therefore, wear is firstly occurred on its outer layer of the fiber buckling peak protruding from the surface of the fabric, and then is gradually extended into interior. In fact, under such a high load (more than 90 MPa) the wave of fabric is not obvious, as shown in Fig. 8. The fibers tend to disperse uniformly under the action of load.
Because the fiber reinforced composite is mainly damaged by fatigue wear during long time friction, local high load of the friction surface has no direct effect on material wear under the normal lubrication. It is due to damage of the transfer film from contact surface, and then the reinforced fibers are exposed to be directly contact and abraded with the metal influencing tribological behavior of the material on this region. Figure 9 shows fatigue wear of the broken fiber. Normally due to self-healing mechanism of the transfer film, the exposed fibers are quickly supplemented by surrounding polytetrafluoroethylene fibers, maintaining the region in a better lubrication state. Only when the transfer film can not be formed in time or there is not enough fibers supplemented after serious wear, the fiber will be directly worn. Comparing to load, the change of frequency has a more obvious effect on the transfer film. Fatigue fracture will be occurred on the exposed fibers due to mechanical shear action under high frequency. Figure 10 shows the unworn fibers under normal condition. Figure 11 shows the fibers cut directly without friction test. The worn surface shown in Fig. 12 is obtained after 1200 oscillating friction cycles at 90 MPa and 7.5 Hz. The shear fracture of fibers can be seen clearly in the figure, which is similar to the fracture morphology in Fig. 11.
Fig. 13(a) shows the morphology of the worn surface of the composite after 1200 oscillating cycles at 90 MPa and 7.5 Hz, and the contour curve along the black mark is shown in Fig. 13(b). It is noted that there are many wear pits on the surface of the worn material, and the internal fibers are exposed, and some fatigue fracture have occurred. These are the parts where the lubrication layer are damaged during the friction. Compared with Fig. 7, it can be seen from Fig. 13 that the smoothness of the material surface with good lubrication state is better than that in the initial state. At 90 MPa, 100 MPa and 110 MPa, there are three wear modes in the Nomex fiber reinforced composite. Firstly, the resin and PTFE fiber coated on the surface of material undergo adhesive wear under local high temperature during dry friction. Secondly, fatigue fracture has been occurred because the PTFE and Nomex fiber suffered mechanical shear force, as shown in Fig. 12. Thirdly, the micro convex peaks of the counterpart undergo fatigue wear at the instant high temperature of the friction surface, and thus some metal particles are formed. These particles exist in the friction interface to cause abrasive wear, as shown in Fig. 14.
3.2 Effect of friction temperature on wear loss
Fig. 15 presents the change of wear loss when the environment temperature is 20 °C and 75 °C . Environment temperature has an obvious effect on the material life. The wear life reaches 180 h at 20 °C, while it is only 70 h at 75 °C. Figs. 16 and 17 show the friction coefficient and friction temperature at 20 °C and 75 °C, respectively. At 20 °C, the highest friction temperature is 129 °C and the friction coefficient is relatively stable. Whereas the maximum of friction temperature at the environment temperature of 75 °C reaches 210 °C, and the friction coefficient fluctuates sharply and is extremely unstable. It is indicated that the self-lubricating behavior of the material and its service life decay rapidly under high environment temperature.
The failure mode of fiber reinforced composite at high friction temperature can be explained as follows. Friction heat causes the lubrication layer and transfer film on the material surface to become unstable, easily be destroyed. In Fig. 18, local material is peeled off and some reinforced fibers are exposed. Under the high temperature of the friction surface, the strength and wear resistance of material have been reduced, and shear fracture and thermal fatigue wear are easily to occur when the composite directly contacts with its counterpart, as shown in Fig. 19. When wear increases, the self-lubricating effect further reduces, and the fluctuation of the friction coefficient is increased and the friction temperature further rises. High temperature reacts on the material and wear loss increases. In practice, this effect could lead to an excessive fit clearance of spherical plain bearing, the increased impact force and the local high load. These all lead to a decrease in the lubrication performance and aggravation of the wear.
Wear debris generated during friction are analyzed as follows. The transfer film are mainly consumed by being squeezed out from material. After that the region is supplemented by the matrix, with the transfer film being in a dynamic stability. When the specimen is worn to be with a certain thickness, the remaining material is not enough to supplement the consumption, and it will be worn out. Seen from Fig. 19, the high environment temperature leads to the high friction temperature, and thus breaks the dynamic balance of self-lubrication layers and aggravates the material damage.
Through the analysis of the whole life test, it is concluded that the friction heat plays a leading role in the process of wear out failure. High friction temperature greatly weakens the friction and wear performance of the material, resulting in a failure of the material in a short time. The residual thickness of specimen worn under high friction temperature is larger than that at low temperature. This is the local wear failure.