Fabrication of hydroxyapatite coatings by the ball impact process

The authors applied the ball impact process, developed by the authorsʹ research group, to fabricate hydroxyapatite (HAp) particle coatings on biocompatible metals such as Ti and its alloys. During this process, under ambient temperature, normal pressure, and in a standard air atmosphere, metal or ceramic balls are accelerated and impacted repeatedly on a metal surface by means of mechanical vibrations; simultaneously, powder particles are sandwiched between the balls and the surface. Consequently, the metal surface is rapidly coated with the powder particles. Using a ZrO2 vibration chamber and ZrO2 balls, the authors have successfully fabricated high-density HAp particle coatings with a cover ratio greater than 95% and a thickness of 50 μm, after several tens of minutes of treatment. Experiments were conducted by varying the parameters such as ball diameter, treatment time, and ball filling fraction. The results revealed that the ball diameter should be greater than 5.0 mm and the treatment time longer than 10 min for the fabrication of coatings with high cover ratio and sufficient thickness. This is because, under these conditions, mechanical mixing occurs between the Ti substrate and HAp particle coatings at their interface, and this phenomenon is assumed to enhance adhesion. To decrease the surface roughness of the coatings, repeated treatments using small balls are required.