Takai Kenichi

Hydrogen desorption behaviors of pure iron with a body-centered-cubic (bcc) lattice and Inconel 625 with a face-centered-cubic (fcc) lattice were examined during tensile deformation using a quadrupole mass spectrometer in a vacuum chamber integrated with a tensile testing machine. Hydrogen and water desorption was continuously detected simultaneously under the application of a tensile load and strain to the specimens. Hydrogen desorption promoted by tensile deformation can be found by deducting both fragment hydrogen dissociated from H₂O and H₂ desorbed under unloading from the total hydrogen desorption out of hydrogen-charged specimens during tensile deformation. Hydrogen desorption from hydrogen-charged specimens was detected under various strain rates of 4.2×10⁻⁵/s, 4.2×10⁻⁴/s and 4.2×10⁻³/s.<br>Hydrogen desorption did not increase under elastic deformation. In contrast, it increased rapidly at the proof stress when plastic deformation began, and reached its maximum, then decreased gradually for both pure iron and Inconel 625. This desorption behavior is considerable related to hydrogen dragging by dislocation mobility. The desorbed hydrogen contents promoted by tensile deformation were measured using thermal desorption analysis (TDA). The TDA results showed that the desorbed hydrogen content differed at each strain rate. The largest desorbed hydrogen content promoted by tensile deformation was 16% of the initial hydrogen content in pure iron with high hydrogen diffusion rate when it was deformed at a strain rate of 4.2×10⁻⁴/s. In contrast, that of Inconel 625 with low hydrogen diffusion rate was 9% of the initial hydrogen content when it was deformed at a strain rate of 4.2×10⁻⁶/s. This deference of the desorbed hydrogen content transported by dislocations depended on the balance between the hydrogen diffusion rate and moving dislocation velocity.

A calcium phosphate compound (Hydroxyapatite) has similar composition and crystal structure to an organism bone. Except an absorbent calcium phosphate compound, the composition that resembled apatite and a deposit having configuration generate it on the surface of apatite ceramic in vivo. In other words apatite ceramics does an organism bone and direct bonding through an apatite deposit without causing negativism. Generally this function is named bioactivity. These functions can inhibit ionic elution, roosting, wear and fretting occurring in metal biomaterial, and be extremely important from a point of view to use in vivo. However, ceramics material is extremely inferior in mechanical properties in comparison with metal material. Therefore, an application to locus accompanied by high load is difficult. It is used as bone filling material such as shank or body of vertebra under the present conditions. In other words low load is applied to locus to be accompanied by. Therefore, static load than cyclic load is important when longterm use was considered. However, bioactivity ability of apatite ceramic material and relation of mechanical properties were not clarified. A fatigue characteristic in consideration of organism environment is particularly unclear. Furthermore, it is necessary to evaluate a fatigue characteristic and crack propagation behavior when micro structure changes by apatite and chemical reaction with body fluid. This study, a static fatigue characteristic of apatite ceramics in simulated body fluid environment was examined.

Biocompatible materials, such as bioactive ceramics and bioactive glasses can be effective in the repair of bone defects during orthopaedics surgery. These materials have been found by observation to exhibit varying degrees of osteoconductive behavior. The hydroxyapatite ceramics and bioactive glass ceramics was known as a highly bioactive ceramics, and replacements of lost bone. However, it is to be inferior to a fracture characteristic in a weak point of apatite ceramics. In the present study, surface structural changes of apatite ceramics with the bioactive function and mechanical property in simulated body fluid were investigated. Sub-micrometer hydroxyapatite ceramics powder was used as starting materials for making hydroxyapatite ceramics. Pressure less sintering was preformed at 1300℃ in O_2 atmosphere using the pre-sintered bodies. Fracture resistance (K_Q) was evaluated by ASTM E399-90 method. And also, fracture resistance tests were performed using compact tension specimens. It has been confirmed by SEM observation, thin-film X-ray diffraction matter and FT-IR reflection spectroscopy that the apatite layer can be reproduced on the surface of the hydroxyapatite ceramic even in a cellular simulated body fluid with ion concentrations nearly equal to those of human blood plasma. As a result, corrosion degradation of hydroxyapatite ceramics were preferentially recognized on hydroxyapatite particle after short time immersion into simulated body fluid. The general tendency of drastic decrease in fracture resistance was recognized in these materials. The fracture resistance of the specimen was found to decrease with increasing corrosion degradation, especially after Sweeks immersion in simulated body fluid. This remarkable degradation in fracture resistance is considered to be caused by crack propagation through corroded pit. However, the specimens after 4weeks immersion into simulated body fluid showed improved fracture resistance compared with those of corroded hydroxyapatite ceramics. There improvements in fracture resistance may be brought about through the mechanics was shown to induce the apatite layer formation on it's surface in some areas between 4 and Sweeks by simulated body fluid.

Metals are by far the oldest materials used in surgical procedures. Titanium alloys are hoped to be used much more for applications as implant materials in the orthopedic and dental medical fields because of their mechanical properties, such as biocompatibility, corrosion resistance and specific strength compared with other metallic implant materials. The performance of any biomedical material is controlled by two characteristics, biofunctionality and biocompatibility. Biofunctionality defines the ability of the device to perform the required function, whereas biocompatibility determines the compatibility of the material with the body. This biocompatibility is improved by coating the surface in contact with living tissues with calcium phosphates, specially hydroxyapatite. Some of the new implants utilize titanium alloys substructure coated with a thin layer of calcium phosphates ceramics, hydroxyapatite, or the plasma spray technique. Hydroxyapatite coating are designed to produce a bioactive surface promoting bone growth and inducing a direct bond between the implant and the hard tissues. The titanium metal also forms the bone like apatite layer on its surface in simulated body fluid, when it has been previously treated with NaOH aqueous solution to form a sodium titanium hydro gel layer on its surface. In the present study, surface structural changes of Ti-6Al-4V alloys with the alkali treatments and mechanical property in simulated body fluid were investigated. Thus it is expected that alkali treated titanium alloys could also form the bone like apatite layer on its surface in the living body and bond to living bone through the apatite layer. Studies have demonstrated that the bone bonding ability of titanium alloys could be evaluated by testing the titanium alloys in a simulated body fluid. In test results of with apatite coating specimens extremely higher fracture strength, compared with monolithic Ti-6Al-4V alloys whose fracture strength was 60MPa・m^<1/2>. For apatite coating Ti-6Al-4V alloys the general tendencies in the fracture strength depending upon apatite coating were understood as follows. With apatite coating Ti-6Al-4V alloys it is recognized that speculated that this tight bond might be attributed to a graded interface structure between the apatite layer and the substrates.

The mechanism for fatigue failure in extremely high cycle fatigue in the regime of N > 10^7 is studied on a bearing steel, JIS SUJ2. Special focus was given to the fracture morphology in the vicinity of fracture origin (subsurface non-metallic inclusion) of a heat treated bearing steel (Specimen QT). The particular morphology looks dark during optical microscopic observation. Specimens with short fatigue life of the order of N_f=10^5 did not have such a dark area, ODA (optically dark area). To investigate the influence of the hydrogen trapped by nonmetallic inclusions on fatigue properties, specimens heat treated in a vacuum followed by quenching and tempering (Specimen VQ) were prepared. Specimens VQ contained 0.07ppm hydrogen as compared to 0.80ppm hydrogen for conventional Specimens QT. Specimens VQ had a slightly smaller ODA than Specimens QT. Hydrogen was detected by a Secondary Ion Mass Spectrometer around the inclusion at fracture origin of Specimens QT and Specimens VQ. Thus, it can be concluded that the formation of ODA is closely related to hydrogen trapped by nonmetallic inclusions. Estimations of fatigue limit by the √areaparameter model based on the original size of inclusions for fatigue limit defined for 10^7 cycles are ,10% unconservative. Considering the size of ODA into fatigue limit estimation, the √area parameter model can predict the mechanical fatigue threshold for small cracks without influence of hydrogen. The mechanism of duplex S-N curve is also discussed.