Numerical simulation of the ball impact process

To optimize the operation conditions of the ball impact process, a novel dry surface treatment process developed by the authorsʹ research group, a simulation model was developed on the basis of the discrete element method (DEM). From the calculation results, it was found that the balls impact the top or bottom inner surface as a solid group. When the filling fraction and ball diameter were 19% and 5.0 mm, respectively, the kinetic energy at collision for 1 s was 94 J, and the contact stress on the top inner surface attained an average of 4300 MPa and a maximum of 14,000 MPa. When the ball diameter was above 7 mm, the average contact stress was constant at approximately 6000 MPa, and when the diameter was less than 7 mm, the contact stress had a wide distribution and was notably high for balls with high relative velocity. Therefore, to achieve a significant ball impact, it is reasonable to use a few balls with small diameter so as to obtain high relative velocity and prevent collisions between the balls. In addition, it was shown that there is a strong relationship between the calculated contact stress and the adhesion of actually fabricated hydroxyapatite coatings on Ti substrate.