3.2.2. Wear

The SEM images (Figure 9) show the wear track of the 300 nm DLC coated HNBR after a dry ball on disc test. It is evident that in the majority of the run area, the surface go<sup>t</sup> smoother. The DLC coating was slightly pressed down due to the normal load and the microstructures were plastically deformed because of the tangential traction, which was generated by the sliding motion. Some piles of small crystal-like fragments can be found on the run track (Figure 10g). DLC is a very hard material and the thickness of the coating is just 300 nm. That means that when the counter body slid over the surface, both the elastomeric substrate and the coating experienced a deformation. The difference is that the substrate deformed viscoelastically and the coating showed a plastic deformation. Meanwhile, the cracks of the coating can also be ascribed to the enormous difference in hardness between the two materials.

Two positions of the wear track of 300 nm Hybrid\_A coated HNBR were shown in Figure 9c,e. Particles can be observed in the troughs, which were located between every two peaks. White particles (Figure 3a,f) can be MoO3, the oxidation product of MoS2, which has a negative effect on the performance [15,36]. As shown in Table 4, in hybrid coatings, MoO3 possesses larger portion than MoS2. According to [67], when less than 30% of the MoS2 converted to MoO3, wear performance is still good. However, when it is greater than 50%, the wear behavior gets poor. As can be seen from Figure 3a,g, a part of the particles were generated during the coating process. Particles were also generated through dynamic motion in crack area. All of these particles were collected during the test in the trough. As can be observed in Figure 9d, some of the particles were pressed on the surface when the ball slid over.

From the same coating and substrate, sheet-like wear particles are visible in Figure 9e,f. This phenomenon can be attributed to surface fatigue [68]. Due to the repeated plastic deformation, sheet-like particles were gradually generated and separated from the coating.

**Figure 9.** SEM micrographs: wear track of 300 nm DLC coated HNBR (**a**) and 300 nm Hybrid\_A coated HNBR (**<sup>c</sup>**,**<sup>e</sup>**). Related areas are marked and shown with high magnification (**b**,**d**,**f**).

Compared to the 300 nm Hybrid\_A coating on FKM before (Figure 3i) and after (Figure 10a) the test, a grea<sup>t</sup> number of cracks was generated during the test. This can be related to the dense particle-like microstructures of uncoated FKM. When the porous and loose coating was pressed by the counter body, it deformed more heavily and easily than other coatings. Besides, due to its lowest hardness among the four elastomers, the deformation of FKM is the largest. These two reasons could explain this phenomenon.

**Figure 10.** *Cont.*

**Figure 10.** Scanning electron microscope (SEM) micrographs: wear track of 300 nm Hybrid\_A coated FKM (**a**), 300 nm MoS2 coated FKM (**c**), 300 nm DLC coated TPU (**<sup>e</sup>**,**g**), and 300 nm MoS2 coated TPU (**h**). Related areas are marked and shown with high magnification (**b**,**d**,**f**).

Not like the 300 nm Hybrid\_A coating, no obvious alteration could be found on the 300 nm MoS2 coating after 10,000 cycles. Only the contact area was pressed and subsequently crushed into small pieces (Figure 10c). This can be attributed to the S–Mo–S sandwich structure of MoS2, which facilitates the sliding motion on its surface [15].

The wear track of 300 nm DLC coated TPU (Figure 10e,f) presented very similar microstructures as MoS2 coated FKM. That means only the DLC coating in the contact area was pressed into small pieces. However, plenty of wear particles, which are around 1 μm, were found close to the edge of the run track (Figure 10g). In some areas, they were piled up together. At the beginning of the test, the DLC coating was pressed into small pieces. However, some of the small particles that were detached from the substrate, rolled down from the sides to the middle of the groove. More and more particles were gathered on the lane with more cycles. At this moment, the particles were pushed out of the lane when the counter body slid over. Still quite a number of particles were found on the track after the test. Apparently, the dynamic movement of these small particles has influenced the tribological behavior to some extent. This can explain why DLC is the best coating for TPU under lubricated conditions but presented worse tribological properties than MoS2 in dry tests. There is a strong possibility that under lubricated conditions the wear particles can be carried out of the track by grease. This is also one of the main functions of a lubricant [69].

Because of its low hardness and good shear characteristics no obvious particles were found on the MoS2 coated TPU. Slight abrasive wear can be observed on the surface (Figure 10h). This is also one of the major wear processes on polymers [70]. Due to its special properties and good adherence on TPU, 300 nm MoS2 shows the best tribological properties in dry tests.
