*3.3.1 Effect of CH4 flow rate ratio*

Braic et al. performed the initial studies on development of high-entropy carbide coatings for tribological and biomedical applications. Their research group deposited (TiAlCrNbY)C high-entropy carbide coatings from co-sputtering of elemental targets using DC-magnetron sputtering with different CH4 flow ratios (RC), and at constant substrate temperature and substrate bias of 400°C and −100 V, respectively [54], where CH4 flow ratio is given by: RC = CH4/(CH4 + Ar). The XRD data showed a change of structure from nanostructured broad FCC phase (0% RC) to a single FCC carbide phase (10 and 17% RC) and then to amorphous phase at higher carbon concentration (26 and 33% RC). The coating morphology changed from slightly higher surface roughness of 7 nm (0% RC) to fine grained surface roughness of 2 nm (33% RC) with increasing CH4 flow ratio. The hardness values increased from 8.2 GPa (0% RC) to 22.6 GPa (26% RC). Similarly, Braic et al. studied the effect of CH4 flow ratio on (TiZrNbHfTa)C high-entropy carbide coating with elemental target co-sputtering using DC-magnetron sputtering on Ti6Al4V alloy substrate [55]. However, in this work, formation of only FCC solid solution was observed at RC of 13 and 35% with hardness values of 22.4 and 32.1 GPa, respectively. Similar to earlier work, the surface roughness and crystallite size decreased, while the hardness increased with increasing CH4 flow ratio. In another work, Braic et al. deposited (CuSiTiYZr)C high-entropy carbide coating using elements with large atomic radii differences and reported the effect of different CH4 flow ratios on the structural and mechanical properties. In all the deposited high-entropy carbide coatings, the XRD showed the formation of amorphous phase irrespective of the amount of carbon. The higher lattice distortion in the high-entropy carbide coatings resulted in hardness values of 20.7 GPa (25% RC), 27.2 GPa (35% RC), and 29.5 GPa (50% RC). Thus, proving the work of Zhang et al. [56] and Guo et al. [57] in the case of high-entropy alloys, a solid solution is formed when the constituent elements have a close atomic radius. Similarly, Jhong et al. developed (CrNbSiTiZr)C highentropy carbide coatings and studied the effect of increasing CH4 flow ratio on the structural evolution and mechanical properties [58]. In this system, the structure of high-entropy carbide coating changed from FCC solid solution phase at lower RC of 3–10% to amorphous phase at higher RC of 15–20%. Such structural change from FCC to amorphous phase resulted in reducing the hardness from 32.8 to 22.3 GPa.
