*3.2. Influence of Rotational Speed*

To study the influence of revolution speed on the formation of Ti coatings, continuous Ti coatings on steel balls were also prepared at 400 rpm, with the results displayed in Figure 4. We can see that continuous Ti coatings have been formed after 4 h of the mechanical coating process (Figure 4b). The thickness of the Ti coatings was increased with the increase of duration from 4 h to 20 h. However, continuous Ti coatings began to separate from steel balls as the duration was further increased to 26 and 32 h. Therefore, we can say the evolution includes the following four stages: nucleation, growth of nuclei, formation of coatings, and exfoliation. The evolution is similar to that of Fe and Zn coatings [22,23].

**Figure 4.** SEM images for cross-section of Ti samples prepared by mechanical coating at 400 rpm after different durations: (**a**) 1 h; (**b**) 4 h; (**c**) 8 h; (**d**) 12 h; (**e**) 16 h; (**f**) 20 h; (**g**) 26 h; and (**h**) 32 h.

The weight increase of 50 steel balls during mechanical coating at different revolution speeds was recorded as illustrated in Figure 5. The weight increase means that more Ti powder particles coat the steel balls. We found that the weight of the steel balls increased with the increase in duration. However, the weight increase became greater with the increase of revolution speed from 200 to 400 rpm at the same mechanical coating process duration. This suggests that a higher revolution speed can accelerate the coating of Ti powder particles on the surface of steel balls. The average thickness change of continuous Ti coatings was also monitored as shown in Figure 6. We can note that the data at 200 rpm is absent because continuous Ti coatings were not even successfully prepared after 50 h. This hints that continuous Ti coatings may not be formed at revolution speeds of 200 rpm or lower. The average thickness evolution of continuous Ti coatings at 300 and 400 rpm is similar to the weight increase change in Figure 5. When rotational speed was 400 rpm, the weight began to decrease when the time came to 26 h, as the formed coatings began to peel off. If milling time is prolonged any further, the exfoliation of metallic coatings will continue. Therefore, we did not provide data after 26 h. According to the parameters named "collision strength" and "collision power" which we proposed in published work [23], the energy transferred to the metallic powder particles from the balls quickly increases with the increase of rotation speed of the ball mill. Greater collision power means larger transferred collision energy, which creates severe plastic deformation. The cold welding among metallic powder particles occurs only when plastic deformation is greater than a critical value [24].

**Figure 5.** Weight increase of 50 steel balls versus duration of mechanical coating at different revolution speeds.

**Figure 6.** Average thickness of Ti coatings versus duration of mechanical coating at different revolution speeds.

#### *3.3. Influence of Surface Roughness and Hardness*

The SEM images for the morphologies of the samples prepared by the mechanical coating process are shown in Figure 7. Figure 7a,b show the influence of surface roughness on the coatings' formation. No evident difference can be observed from the SEM images. From Figure 8, we can see that the initial coating rate for the polished balls was slightly greater than those which were unpolished. In other words, the decrease in surface roughness favors the adhesion of metallic powder particles to the surface of metallic balls. We believe that the surface roughness improvement can decrease the air volume reserved in the cavities in the surface of the balls. The contact area among the balls and the metallic powder particles was increased, which can increase the possibility of cold welding. Therefore, the surface roughness improvement accelerated the formation of metallic coatings. On the other hand, surface roughness improvement decreased the quantity of the cavities in the surface. Therefore, the interaction opportunity—specifically the mechanical inter-locking between the cavities and the metallic particles—was decreased. Finally, the surface roughness improvement hinders the formation of metallic coatings. According to the above results, we can conclude that the influence of the surface roughness on the formation of metallic coatings is rather complex; the coexistence of promoting and

obstructive factors made the influence negligible. As for the influence of the substrates' hardness, the formation situation of Ti coatings is given in Figure 7c,d. We can clearly see that more Ti powder particles were adhered to the annealed steel ball than to the steel ball. In other words, Ti powder particles more easily coat the softer steel balls. A slight difference in weight increase shown in Figure 8 also proved this. The influence of balls' surface hardness on the coating of metallic powder particles can also be attributed to the cold welding of metallic powder. As discussed above, the cold welding among balls and metallic powder particles happens only when a critical plastic strain is satisfied. After they were annealed, the balls became softer than that before annealing. During the collision among balls and metallic powder particles, the softer surface of the balls welds with the metallic particles more easily. After the surface of these balls is totally coated with metallic powders after 12 h of ball milling, the interaction among balls and metallic powder particles has been replaced by that among metallic powder particles. Therefore, the influence of surface parameters including roughness and hardness cannot be studied any more.

**Figure 7.** SEM images of morphologies of the samples: (**a**) TS300-4; (**b**) TSS300-4; (**c**) TS300-8; and (**d**) TSY300-8.

**Figure 8.** Weight increase of 50 steel balls during mechanical coating versus duration.
