Study on particle impact angle in powder jet machining (Dental treatment by hydroxyapatite film coating)

Abstract

Powder jet machining is one of blasting processes conducted under room temperature and atmospheric pressure. This process brings both deposition and removal process, and in this study, it refers to powder jet deposition (PJD) and abrasive jet machining (AJM). As an application of PJD, the authors have proposed an innovative dental treatment method with the hydroxyapatite (HA) fine particle. By this method, thick HA coating can be fabricated directly in the human oral cavity. In this study, the effect of the particle impact angle was investigated as a parameter that affects the machining phenomenon. The experiments showed that the machining phenomenon transited depending on the blasting angle. In the vertical blasting condition, PJD process was just observed. On the other hand, in the more acute blasting angle such as 45 deg. or 60 deg. both coating and removal process appeared at the same time and in the most acute angle of 30 deg. only removal process was detected. The TEM observations showed that the impact surface of the HA substrate deformed and the deformation depth increased as the blasting angle get more acute. The smoothed particle hydrodynamics (SPH) method was utilized for the analysis for the fracture behavior of the HA substrate. The result indicated that the decrease of the impact angle induced the increase of the strain and the temperature of the interface between the particle and the substrate. Thus it is concluded that the shear stress, which refers to the impact angle, induces the destruction of the substrate by the deformation and the brittle fracture due to the thermal stress.