Kazuhiko Sakaki

Inorganic antimicrobial coating for titanium alloy and its effect on bacteria

BackgroundFor orthopedic implants, infection is a serious problem. Therefore, we considered an implant with antimicrobial ability can prevent infection. We tried to coat a titanium alloy surface with Novaron, a commercially available inorganic antimicrobial. The purpose of this study was to analyze the differences among the surfaces of materials coated using different processing pressures of the working gas and analysis of the antimicrobial activity.MethodsOne of the inorganic antimicrobials Novaron (grade VZ 600) was applied to titanium alloy (Ti6Al4V) plates. This antimicrobial has limited heat resistance, so we used cold spray technology to coat the titanium alloy with it. The principle of cold spray technology is spraying a powder in a high-velocity gas jet, accelerated by adiabatic expansion, against a substrate. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry (EDS) were used to analyze the differences among the surfaces of materials coated using different processing pressures of the working gas. The Japanese Industrial Standard (JIS) method (JIS Z2801: 2000) was used to analyze the antimicrobial activity against Staphylococcus aureus, Pseudomonas aeruginosa, and Klebsiella pneumoniae. Antimicrobial activity was analyzed only for the sample coated at 3.0 MPa.ResultsThe SEM and EDS results indicated that when the pressure of the working gas was increased, the antimicrobial coated the titanium adequately. This material showed good effects against S. aureus and P. aeruginosa and some effect for K. pneumoniae.ConclusionsAntimicrobial implants represent a preventive method against infection. There is a possibility of using them not only for clean operations but also for operations with suspected bacterial contamination, such as fixation of slight compound fractures.

The influence of nozzle design in the cold spray process

Publisher Summary One of the major characteristics of the cold spray process is the high-speed gas jet, which is governed by gas dynamics. In gas dynamics, supersonic flows are obtained with Convergent-Divergent [“CD”] nozzles (or de Laval nozzle), which are used for rocket motors. While the principal purpose of the design of a rocket motor nozzle is to maximize the thrust, in thermal spraying, the main purpose is to obtain better coating quality. The studies on thermal spraying show that the coating properties are principally determined by the thermal and kinetic energy states of particles upon impact with the substrate. To have a balance between these two states, various changes in the design of the High-Velocity Oxygen Fuel [“HVOF”] gun nozzle have been attempted. However, works concerning the influence of nozzle geometry on the thermal spray process are sparse. The effects of throat diameter and exit divergence of the gun nozzle on the HVOF process have been considered. The combustion gas flow (such as pressure, velocity, temperature and expansion state of gas jet from the nozzle exit), the particle behavior and, therefore, the nature of coatings were found to be significantly influenced by these nozzle parameters. In addition, the effect of the expansion state of the combustion gas jet on the HVOF process was investigated using a diverging nozzle exit.

Mechanical Properties of WC/Co Coatings Prepared by Cold Spraying

Microstructure, hardness and wear resistance of WC/Co coatings prepared by cold spraying and influence of powder properties, such as powder size, WC particle size and cobalt content, on those properties have been investigated in this study. Both smaller WC particle and low cobalt content are found to be effective to produce dense, hard and highly wear resistant coating. Smaller powder size is also favorable to make a coating with high mechanical properties. The cold sprayed coating using WC (0.2μm) /12% Co with powder size of -20+0μm has best mechanical properties within this study. This coating has high density compared to conventional HVOF sprayed coatings. Its abrasive wear resistance has been almost comparable to HVOF coating. Strong correlation is also seen between hardness and wear resistance.

Effect of gun nozzle geometry, increase in the entrance convergent section length and powder injection position on cold sprayed titanium coatings

Nozzle geometry influences gas dynamics, such as gas density, velocity and temperature, making sprayed particle behavior one of the most important parameters in cold spray process. Gas flow at the entrance convergent section of the nozzle takes place at relatively high temperature and are subsonic. Thus, this region is a very suitable environment for heating spray particle. In this study, numerical simulation and experiments were conducted to investigate the effect of nozzle contour (convergent -divergent and convergent-divergent-barrel), entrance geometry of convergent-divergent nozzle and powder injection position at nozzle on the cold spray process. The process changes inside the nozzle were observed through numerical simulation studies and the results were used to find a correlation with coating properties. A titanium powder was usedin the experiments. Working gas (is nitrogen) pressure and temperature at nozzle-intake were 3MPa and 626K, respectively. In addition, the change in the nozzle contour and the change in the convergent section length of the gun nozzle were found to have a slight effect on the coating microstructure. Powder injection position was also found to influence deposition efficiency and coating porosity with titanium powder. Deposition efficiency using axial injection was found to be higher than that using radial injection.