Fabrication of hydroxyapatite film by powder jet deposition

Abstract

The authors have proposed a new dental treatment method by using powder jet deposition (PJD) technique to make coatings on human teeth. The method utilizes hydroxyapatite (HA), which is the main element of human hard tissue, as a restoration material. The coating process is achieved by high speed impacts of the particles on a tooth, and it can be carried out under room temperature and the atmospheric pressure. The thick PJD film is obtained through the accumulation of fractured parts of particles. This study focuses on the microstructures of the HA particles and the deposited film. To investigate the effect of particle crystal structures, two types of HA particles were produced: one (Type A particle) is covered with an amorphous surface, whose thickness is approximately tens of micrometers; the other one (Type B particle) is a type of complete-crystal-growth particles, which are produced by heating the particle A at 230°C for 12 hours. The film fabrication experiments were performed with these particles and machined areas were observed with a scanning electron microscope. It was found that the fabricated film with Type A particles was spalled during blasting. In contrast, the fabricated HA film with Type B particles is well deposited without film spalling. To further investigate the microstructures of the HA film, observations with a transmission electron microscope were conducted. The dark field image of the film with a Type A particle implies that the created HA layer consists of HA crystals of one to tens of nanometers. The bright field image indicates that cracks of approximately 300 nm generated in the film. Thus the mechanical strength of the film fabricated using the Type A particle is not strong enough against the subsequent particle impacts and the pressure of blasted air. In contrast, cracks were not observed in the film created with the Type B particle.