*3.4. Influence of Substrate Material*

We also studied the influence of substrate material on the formation of continuous Ti coatings. When stainless steel (SUS-304) balls were chosen as the substrate (Figure 9), the formation of continuous Ti coatings took about 10 h, which was identical to that using steel (SUJ-2) balls as the substrate. This means that the required time to form continuous Ti coatings on stainless steel and steel balls was identical. However, the formation of continuous Ti coatings on Al2O3 balls in the same condition took 20 h [25]. Therefore, we can conclude that the formation of Ti coatings on steel balls is easier and quicker than on ceramic balls. In other words, the substrate plays a key role in the formation of Ti coatings. From the above results, we can state that the formation of Ti coatings on steel balls is much easier and quicker than on ceramic balls. The influence of substrate material on the formation of metallic coatings can be explained as follows.

**Figure 9.** SEM images of surface morphologies of Ti coatings on stainless steel (SUS-304) balls prepared by mechanical coating at 300 rpm after different duration: (**a**) 0 h; (**b**) 4 h; (**c**) 10 h; (**d**) 16 h and (**e**) 60 h.

Firstly, it is well-known that the hardness of Al2O3 is about 2300 HV, which is far greater than those of steel and stainless steel. When substrate hardness decreased from 2300 HV to 809 HV, the required time to form continuous Ti coatings decreased from 20 h to 10 h. With a further decrease of substrate hardness from 809 HV to 187 HV, the required time hardly decreased, indicating that decreasing the substrates' hardness within a certain range can shorten the formation of Ti coatings. The influence of substrate hardness has been discussed above. Secondly, the material transfer from metal to ceramics is difficult than cold welding between metal materials [26]. Some works [24,26] have proved that the interaction between metallic particles and ceramic balls belongs to mechanical self-locking due to the plastic deformation of metallic particles. However, the cold welding occurred among the fresh surface of the balls and metallic powder particles when balls became metallic ones. The strength of self-locking is lower than that of cold welding.
