The powder jet deposition (PJD) process is for creation of a hydroxyapatite (HA) layer on human teeth. To develop the PJD device, the layer-forming properties have improved. Created HA layers with a new handpiece-type PJD device demonstrate excellent material properties in vitro. Titanium dioxide (TiO 2 ) is known to cause whitening because of the selective reflection of the light. Therefore, we assessed the possibility of using the creation of TiO 2 -HA layers with the new PJD device as a new treatment for discolored teeth. In this study, the microstructural and mechanical properties of TiO 2 -HA layers were evaluated under the same conditions of the previous study. These properties were evaluated before and after 500 cycles of thermal cycling (5-55 °C). Furthermore, the CIE L * a * b * color system was used for color measurement and ⊿E * values for color differences were calculated. The maximum thickness of the TiO 2 -HA layers was about 60 μm. There were no significant differences in thickness, hardness, or bonding strength before and after thermal cycling. The layers showed an increased L * parameter and a decreased b * parameter, and the color difference ⊿E * was approximately 6.7 units. Creation of TiO 2 -HA layers by PJD might be a valuable new treatment for discolored teeth.

Powder jet machining can be used for both material removal (abrasive jet machining) and material deposition processes (powder jet deposition) by changing the powder spray conditions. The transition from the removal to the deposition is investigated. The transition between these two phenomena, removal and deposition, is studied through simulating the impact phenomena of two alumina particles with a glass substrate using the Smoothed Particle Hydrodynamics (SPH) method in this study. Crystalline structures of an alumina film created were observed with a Transmission Electron Microscope (TEM). Observation indicates that a thick multicrystalline film is created when the blasted particles are monocrystalline alumina. It is also concluded that fractures within the particle material are necessary for the deposition process, and that fragments less than 100 nm in size contribute to the deposition of the thick films.

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.

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.

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