*3.5. Wear Resistance*

Figure 10 shows the wear morphology of the sample surface after the above-mentioned wear resistance test. It can be seen from the figure that the surface wear of the TC4 substrate is very serious, and there are pits and adhesive materials, which is attributed to the that under the action of plastic deformation and large pressure, the surface material of the substrate will adhere to the surface of the friction pair and transfer during the sliding process. Therefore, pits and adhesive materials are formed. In addition, deep furrows were also found. This is because metal particles caused by wear will adhere to the surface of the friction pair, thereby forming grooves on the surface of the substrate. Meanwhile, the main component of the cladding layer of Case 3 is the Ti element, so there is not much difference in the wear morphology. Compared with the TC4 matrix, it shows slight adhesion wear and abrasive wear. A large amount of Laves phase is distributed in the cladding layer, which increases the hardness of the cladding layer, so abrasive wear is effectively suppressed. XRD results show that a HEA solid solution phase is generated in the cladding layer, which improves the plasticity of the material, and the adhesion wear is also suppressed.

**Figure 10.** The wear morphology of TC4 and coatings. (**a**) TC4, (**b**) Case 1, (**c**) Case 2, (**d**) Case 3.

Figure 10b,c are the wear morphology of Case 1 and Case 2, respectively. A nearly straight crack with a smooth fracture appearance can be found on the wear morphology of Figure 10b, so brittle fracture will likely occur under the action of larger pressure, which will seriously a ffect the practical application of Case 1. Therefore, su fficient specific energy is essential during laser cladding of FeCoCrNi high entropy alloy on the Ti substrate. Case 2 only su ffered slight scratches during the wear process. Besides, as can be seen from the figure, the wear resistance of Case 2 is significantly higher than that of the TC4 matrix. This is because Case 2 has an equal proportion of elements, solid solution strengthening, and second phase strengthening, which results in a significant improvement in the wear resistance of the cladding layer.

Due to the obvious brittle fracture of Case 1, it is di fficult to define the size of the wear region. Therefore, only the wear size statistics of the TC4 substrate and Case 2 and Case 3 are performed. The results are shown in Table 5. It can be seen that the wearing depth and width of the HEA cladding layer is significantly lower than that of the TC4, and the wear cross-sectional area is calculated using the probe of the friction-wear machine, and its wear rate was further calculated, as shown in Table 5. The results show that the HEA cladding layer plays a significant role in improving the wear resistance of the TC4 surface, which can e ffectively protect the matrix material. Results indicate that the wear resistance of Case 2 is improved by 5 times relative to the TC4 matrix, while the wear resistance of Case 3 is improved by 2.84 times. The reason is that as the hardness increases, the wear resistance of the material will also increase. Therefore, the feasibility and extent of improve wear resistance of titanium alloy by laser cladding of HEA is revealed. It is worth noting that the appropriate process parameters are limited to a small range during laser cladding of FeCoCrNi high entropy alloy on the Ti substrate.


**Table 5.** The size of the wear region of the cladding layer and substrate under di fferent specific energy.
