The friction coefficients of the Teflon coating under the four different conditions are measured in 10 minutes. The friction coefficients of the samples in the DR and DH conditions are relatively higher and more stable, as shown in Fig. 2(a). The friction coefficients of the samples with oil lubrication are much lower, and it is attributed to the hydrodynamic lubrication generated in the oil film. Moreover, the friction coefficients change with the testing time. For example, in OH condition, the friction coefficient drops rapidly after 50 seconds, then increases with time, and finally decreases gradually. It indicates that the state of surface of the coating changes during the friction test.
The changes happened on the surfaces of the samples during the test are examined by the interference microscope at the time of 40 s, 4 min and 10 min, as shown in Fig. 2(b-e). The corresponding wear rates are statistically calculated based on the morphology of the worn surfaces, as shown in Fig. 2(f). According to Fig. 2(f), two main results are found:
(1) Under room temperature, the dry friction causes higher wear rate than the oil lubrication does;
(2) While under high temperature, on the contrary, the oil lubrication causes a much higher wear rate.
The first result can be explained since the dry friction displays a higher friction coefficient at the room temperature, while the oil film provides a good lubrication environment to reduce the wear of the coatings. However, the second result is counterintuitive. The friction under oil lubrication has a lower friction coefficient but a higher wear rate. Actually, the wear rate of the coatings under OH condition is the highest, and it is much higher than the wear rate in other 3 conditions after the experiment is running for minutes. It indicates that there is a different wear mechanism of the coating with hot oil lubrication.
To reveal the wear mechanism of the coating under OH condition, the surfaces of the worn coatings under OH and DH conditions after 1-minute friction tests are examined using SEM, and the corresponding micrographs of worn surfaces are shown in Fig. 3. Under DH condition, the coating materials are compacted to the substrate of the sample, and the surface of the grinding crack is more smooth compared with the original porous surface, as shown in Fig. 3(a). The composition of the worn surface is examined using the Energy Dispersive Spectrometer (EDS) of SEM, as shown in Fig. 3(a1-a4). According to the evenly distribution of the elements C and F on the surface, the coating is considered to remain on the substrate and the coating layer is not broken.
While under OH condition, the coating in the wear track is found broken and detached from the aluminum substrate, as shown in Fig. 3(b). Under this condition, large debris of the coating is generated, which explains the notable raise of the wear rate, as shown in Fig. 2(f). According to the distribution of the elements on the surface as shown in Fig. 3(b1-b4), the coating is broken and the substrate is exposed. But unlike the scarce element O in DH, local oxidation under the OH is observed, which implies much more severe tribochemical oxidation reaction, and thus wear rate. Therefore, the hot oil accelerates the wear of the coating, and the wear mechanism of the coating with hot oil lubrication is different from that in DH or OR.
The Teflon coating has a porous structure, and the porosity is usually inevitable since its fibrils nature , verified by SEM shown in Fig. 3(a)(b). In hot oil lubrication, oil film will spread out on surface at the beginning, and flow into the holes in the Teflon coatings, as shown in Fig. 3(c1). The oil then permeates through the porous structure, and the high temperature accelerates the diffusion of the oil in the coating. Cracks form, and the oil enters the cracks under the friction, and the hydraulic pressure of the oil helps the expansion of the cracks. When the cracks approach the substrate, the coating material detaches and the wear debris is generated.
The schematics of the wear process with hot oil lubrication is shown in Fig. 3(c2). This explanation is consistent with the result found in the experiment. In Fig. 3(b), the coating is bulged in the grinding track that is contrary to the compacted surface in dry friction, which is attributed to the expansion of the oil in the cracks that peels the coating off the substrate. The wear of coating with the hydraulic pressure has also been found in the Huang’s experiment , where the hydraulic pressure is used to break the rocks in the quarrying. This effect has also been found in the cavitation erosion on metal surfaces [12, 13], where the hydraulic pressure accelerates the growth of the cracks and causes loss of the materials.
Another question is why this accelerated wear happens at high temperature rather than at room temperature. It is owing to the different diffusion rate and difficulty of the oil into the coating materials under different temperatures. A water washable HG-Z99S2 type dye penetrant flaw detection agent is used to simulate the oil flow in the coating under OH and OR in a visual way. Figure 3(d1) shows the optical images of as-bought pan’s cross-section morphology, and Fig. 3(d2-d3) show the different results of the agent in the coating under the high temperature and room temperature. At high temperature, the agent infuses into the coating materials, while the agent only stays at the surface of the coating at the room temperature. It illustrates that the temperature also plays an important role in the hot oil lubrication, and it also validates the effect of the hydraulic pressure in the enhanced wear of the coating.