*3.1. Preparation of Ti Coatings on Steel Balls*

The XRD patterns of the Ti-coated steel balls are presented in Figure 1. We could see the diffraction peaks of Ti in addition to those of Fe from the XRD patterns when the duration of mechanical coating processing was increased to 4 and 8 h. This means that some Ti powder particles adhered to the surface of the steel balls. When processing time reached 10 h, the diffraction peaks of Fe could no longer be observed, indicating that continuous Ti coatings had formed on the steel balls.

**Figure 1.** XRD patterns of the Ti coatings prepared by mechanical coating on steel balls at 300 rpm.

The surface morphologies of Ti-coated steel balls were recorded by SEM and are displayed in Figure 2. When the duration of the mechanical coating process was 4 and 8 h, Ti powder particles discontinuously coated the surfaces of steel balls (Figure 2a,b). With the increase of process duration to 10 h, continuous Ti coatings formed (Figure 2c). The surface of the Ti coatings became rugged, and humps were formed with further increase of duration to 50 h. The results from the SEM images

are consistent with that reflected from the XRD patterns in Figure 1. Figure 3 shows the SEM images of the cross-section of the samples prepared by mechanical coating. Although the coating of Ti powder particles on steel balls was not clearly observed (Figure 3a,b) when the duration was 4 or 8 h, the formation of continuous Ti coatings was confirmed after 8 h of mechanical coating (Figure 3c). The coatings' evolution in Figure 3 agreed with that in Figures 1 and 2. Therefore, we can say that continuous Ti coatings on steel balls were prepared at 300 rpm after 10 h of mechanical coating process from the above results.

**Figure 2.** SEM images for the surface morphologies of the samples prepared by mechanical coating at 300 rpm after different duration: (**a**) 4 h; (**b**) 8 h; (**c**) 10 h; (**d**) 12 h; (**e**) 16 h; (**f**) 20 h; (**g**) 26 h; (**h**) 32 h; and (**k**) 50 h.

**Figure 3.** SEM images for the cross-section of the samples prepared by mechanical coating at 300 rpm after different durations: (**a**) 4 h; (**b**) 8 h; (**c**) 10 h, (**d**) 12 h; (**e**) 16 h; (**f**) 20 h; (**g**) 26 h; (**h**) 32 h; and (**k**) 50 h.
