*3.4. Hardness*

Figure 9 presents the hardness depth profile in the transverse cross-section of the laser clad coating. The hardness was tested with a load of 1000 g. The variation of hardness reflects the coating microstructure and element changes, which are responsible for the hardness variation. Due to the interaction between the multi-principal elements that will promote the internal fine-grain strengthening, dispersion strengthening, and solid solution strengthening of the alloy during the solidification process, the hardness of the HEA coating is much higher than that of the Ti6Al4V substrate. The hardness of FeCoCrNi coating exceeds 850 Hv when the parameters of Case 1 and Case 2 are selected. Results indicate that a better FeCoCrNi coating is obtained than that from the literature [28]. Meanwhile, Figure 9b shows that the average hardness of HAZ is slightly higher than the substrate. This increase is likely to be attributed to the localized metallurgical changes that occur in the HAZ associated with the rapid heating and cooling above the <sup>α</sup>-β phase-transition temperature of this specific alloy but below its melting point. It can also be seen that the microhardness of HEA coating in Case 3 is significantly lower than that in Case 1 and Case 2, which is related to the element content in coatings, as shown in Figure 8. The content of Ti is much higher than the other element, hence the balance of high entropy is broken.

**Figure 9.** (**a**) Hardness depth profile in the transverse cross-section of the laser cladded coating, and (**b**) average microhardness on the cross-section of samples under different specific energy.
