T. Yuhta

Phase composition of sputtered films from a hydroxyapatite target

Abstract Hydroxyapatite (HA) was coated onto a cellulose filter acting as a substrate from a crystalline HA powder target using radio-frequency magnetron sputtering at Ar pressure of 0.5–5.0 Pa and discharge power of 50–150 W. After coating, the films were heated to 700 °C to remove the substrate and the crystalline phases were identified using XRD. The Ca/P ratio of the films was analyzed using EDS. The films were composed of HA, β-tricalcium phosphate (TCP), β-calcium pyrophosphate (PYR) and CaO. The weight ratio of HA and the Ca/P ratio of the films decreased with increasing Ar pressure and were largest at 100 W of discharge power. After coating, the surface of the HA target was decomposed into α-TCP, β-TCP and CaO.

A functionally graded titanium/hydroxyapatite film obtained by sputtering.

A functionally graded film of titanium/hydroxyapatite (HA) was prepared on a titanium substrate using a radio frequency magnetron sputtering. The ratio of titanium to HA was controlled by moving the target shutter. The film was composed of five layers, with overall film thickness of 1 μm. The HA was concentrated close to the surface, while the titanium concentration increased with proximity to the substrate. The bonding strength between the film and the substrate was 15.2 MPa in a pull-out test and the critical load from a scratch test was 58.85 mN. The corresponding values of a pure HA sputtered film were 8.0 MPa and 38.47 mN, respectively. The bonding strength of a pure HA plasma spray coating was 10.4 MPa in the pull-out test. The graded film and the pure HA film were sputter-coated to a thickness of 1 μm on titanium columns (10 mm in length and 4 mm in diameter). These columns were implanted in diaphyses of the femora of six adult dogs and a push-out test was carried out after 2, 4, and 12 weeks. After 12 weeks, the push-out strengths of the graded film, the pure HA film and the non-coated columns were 3.7, 3.5, and 1.0 MPa.

Examination of cavitation-induced surface erosion pitting of a mechanical heart valve using closing velocities

Abstract Recently, cavitation on the surface of mechanical heart valves has been studied as a cause of fractures that occur in implanted mechanical heart valves. Several factors, including peak dp/dt of the ventricular pressure, maximum closing velocity of the leaflet, and squeeze flow, have been studied as indices of the cavitation threshold. In the present study, cavitation erosion on the surface of a mechanical valve was examined by focusing on squeeze flow and the water-hammer phenomenon during the closing period of the valve. A simple solenoid-actuated test device that can directly control the valve closing velocity was developed, and opening–closing tests of 3000 and 40 000 cycles were performed at various closing velocities. The results showed that there was a closing velocity threshold above which erosion pitting was induced and that the threshold was about 0.4 m/s in the valves used in this study. Cavitation-induced erosion pits were observed only in regions where squeeze flow occurred immediately before valve closure. On the other hand, the number of the pits was found to be closely related to the area of the water hammer-induced pressure were below the critical pressure defined by water vapor pressure. Therefore, it was concluded that cavitation is initiated and augmented by the two pressure drops due to squeeze flow and the water-hammer phenomenon, respectively.

Synthesis of Calcium Phosphates by Gas-Solid Reaction of CaO-P 2 O 5

Calcium phosphates are useful materials in fertilizer, dentifrice, biomaterials, etc., and have been popularly synthesized by a wet method, a hydrothermal method, a flux method, and a solid-state reaction method. However, there has been no report on synthesis by chemical vapor deposition (CVD) method or gas-phase reaction. In the present study, calcium phosphates were synthesized by gas-solid reaction of CaO solid and P 2 O 5 gas. The CaO/P 2 O 5 ratio of starting materials was changed from 5.06 to 0.64, and the reaction was carried out at 400-1100;C. These calcium phosphates were investigated using X-ray diffractometry (XRD) and scanning electron microscope (SEM). By gas-solid reaction of CaO-P 2 O 5 , g -MET only was synthesized at 400-500;C, g -MET, HA, and g -PYR at 600-700;C, and g -MET, HA, g -PYR, and g -TCP above 800;C. Synthesized calcium phosphates were independent on the CaO/P 2 O 5 ratio of starting materials but dependent on the reaction temperature.