Fig. 1. Surface morphologies of gradient coating with different contents of KF-6 and TiB2. (a) Sample K1; (b) sample K2; (c) sample K3

Fig. 2. Cross-sectional morphology of gradient coating. (a) Bonding zone; (b) transition layer

Fig. 3. XRD patterns of strengthened layer for different contents of KF-6 and TiB2

Fig. 4. Microstructure of strengthened layer for sample K1. (a) Top; (b) middle; (c) high magnification

Fig. 5. Microstructure of strengthened layer for sample K2. (a) Top; (b) middle; (c) bottom; (d)(e) high magnification

Fig. 6. EDS maps of element distribution in strengthened layer for sample K2

Fig. 7. Microstructure of strengthened layer for sample K3. (a)(b)(c) Top; (d)(e) middle

Fig. 8. Longitudinal distributions of microhardness of gradient coating

Fig. 9. (a) Friction coefficient versus time; (b) comparison chart of mass losses