To investigate the influence of disinfectant solution on inner properties of the die stone, two aldehyde disinfectant solutions, and four high-strength dental stones and one dental stone were examined. SEM observation and Knoop hardness of the fracture surface of the die stone after immersion in disinfectant solution were examined. Knoop hardness was much influenced and decreased up to 400 μm depth from the surface of the die stone. Because a similar finding was obtained after immersion in water, the influence of water on hardness was suggested to be considerably large. Immersion in disinfectant solution including water made the bond between die stone crystal looser with numerous microspaces. Some die stones showed disintegration of crystal grains.Especially dental stone, had the appearance of the fracture surface of a sand stone.

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The thermal expansion of the magnesia investment that result from the oxidizing expansion of Zr metal particles were measured on three kinds of apparatuses. The investment specimens were allowed to stand for 2 and 24 hrs from the start of mixing at room temperature prior to the test and were fired with a heating rate of 2℃, 6℃ and 10℃/min, respectively. Major differences were observed in the expansion rate derived from these three apparatuses. Moreover, the expansion rates were affected by both the heating rates and the storage periods of specimen. The results of the fitness test were not consistent with that obtained by the thermal expansion test. At each casting temperature, the compressive strength of the investment increased with increase in the time of storage. The fitness may be influenced by the compressive strength.

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Titanium castings were prepared in molds of a phosphate-bonded silica investment. The effects of degree of surface polishing on the electrochemical corrosion behavior of the castings with the surface layered structure were studied quantitatively in 1% NaCl, 5% HCl, and 0.11% NaF (buffered to pH 4) solutions. Four kinds of surfaces ranging from a sandblasted surface at the α-case layer to the innermost surface were evaluated. Furthermore, the surface of wrought titanium was examined as a control. All the surfaces similarly showed good corrosion resistance in the NaCl solution except for the sandblasted surface. Titanium was not considered to be corrosion resistant in the solution containing 500ppm F. The corrosion resistance in the HCl solution tended to become better as the surface polishing treatment removed more of the reacted surface layers.The corrosion resistance of the innermost surface of the castings was equal to or better than that of wrought titanium.

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The effects of finishing treatments on the surface of the innermost structure on the electrochemical corrosion behavior of titanium castings were studied quantitatively. Five kinds of finishing treatments ranging from sandblasting to buff-polishing were evaluated in the 5% HCl solution that was selected based on part 1 of this study. All of the measured characteristic values of electrochemical corrosion behavior were affected significantly by the finishing treatments. The corrosion resistance of titanium castings was judged to be the best when buff-polished and the worst when sandblasted. The characteristic values were compared to those obtained on the reacted surface layers with the same finishing treatments in part 1. The corrosion resistance of the reacted surface layers of titanium castings was inferior to that of the innermost structure.

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The conditions for the cast-on technique for pure titanium was examined. Different heating and mold temperatures was settled to measuring the separating strength between the primary and secondary castings. EPMA observation and X-ray diffractions was carried out to clarify the mechanism of cast-on technique. Separating strength tended to increase as the molding temperature was increased with 700℃ heating temperature. Molding temperature gave any positive influence except that heating temperature. Greater separating strength was measured with the mold temperarture over 300℃ at the 700℃ heating temperature. Cast-on technique in pure titanium with precise fit was able to execute on the condition with 700℃ of heating and 200℃ molding temperatures.

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The effects of fluoride concentration on corrosion characteristics of CP Ti, Ti-6Al-4V and Ti-Ni alloys were examined. Specimens were exposed to different concentrations of NaF from 0.025% to 2.0%.Potentiodynamic anodic polarization, corrosion potential and polarization resistance, and released Ti, were evaluated, and the corroded surfaces examined using SEM. The corrosion resistance of pure Ti was markedly decreased by exposure to 0.025 and 0.5% NaF and Ti-alloys by that to 0.1 and 0.5% NaF. CP titanium and titanium alloy did not show passivation in NaF of a concentration of 0.5% and above on anodic polarization behavior. Tarnish and surface deterioration were observed for CP titanium and titanium alloys following a 24-hour exposure to NaF at concentrations between 0.5 and 2.0%. Therefore, CP titanium and titanium alloys can not withstand the exposure to NaF solution exceeding a concentration of 0.5%.

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Surface texture and mechanical properties of commercial titanium and stainless steel miniplates and related screws for jaw fixation were examinied. Titanium plates had coarser surface texture than stainless steel plates with many polishing scratches and micro concaveconvexities. The plate with a golden color was treated by anodic oxidation to increase corrosion resistance. The hardness of tested plates differed with the product and the surface was much harder than the inside in all products. All plates fractured elastically after sufficient plastic deformation under tensile tests. Tensile properties of the titanium plates were inferior to those of stainless steel plates. SEM findings of the clinically fractured surface of the plates appeared different from the tensile findings test that indicated that plates might receive brittle fracture in vivo. Therefore, strengthening of titanium plates with material composition, design, and surface treatment may increase durability in vivo.

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Hybrid titanium miniplates for the fixation and reconstruction of bone chips in a maxillofacial field, which has two parts with different mechanical properties in one plate, were experimentally fabricated by joining the commercially pure (cp) titanium block with plasma activated sintering process to increase mechanical durability. The sintered part of the hybrid plate was harder than that of the cp titanium parts of the hybrid plate. The tensile test showed that the screw hole part of the plate, which was a weak point for the conventional plates, had been strengthened by the sintering. However, since the tensile strength of the hybrid plates was slightly lower than that of the cp titanium plates used as a control and the fracture was observed at the interface between the sintered part and cp titanium part of the hybrid plate, further experiments must be done to strengthen the joining surface of the plates.

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We examined the priming effect of N-methacryloyl aspartic acid (N-MAsp) and N-acryloyl aspartic acid (N-AAsp), which we synthesized, on the adhesion between composite resin and dentin. We measured the tensile bond strength between composite resin and dentin promoted by the aqueous solution with various concentrations of amino derivatives. The dentin surface treated with the derivatives and the dentin-resin interface were also observed by SEM. Maximum bond strength was obtained at the concentration of 5wt% in N-MAsp groups and at 20 wt% in N-AAsp groups, being 10.4 MPa and 16.0 MPa, respectively. The thickness of the hybrid layer formed in the dentin treated with 5 wt% N-MAsp and 20 wt% N-AAsp aqueous solution was 0.7 μm and 1.8 μm, respectively.

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The compact ram-type electric discharge machine for fabricating small holes was experimentally developed. Electric discharge machining properties of titanium (Ti), Co-Cr-Mo alloy (Co), Ag-Pd-Cu-Au alloy (Pd), and Ag-Sn alloy (Ag)were evaluated on this machine. Cast specimens with a thickness of 3, 5, and 10 mm were prepared. Hollow cylinder copper bars with a diameter of 0.7, 1.2, and 1.6 mm were used as an electrode. The effect of the rotation of electrode was examined. Higher machining speed was obtained in the specimens of Ag, Pd, Co, and Ti in decreasing order. On the contrary, higher electrode elimination ratio was obtained of the specimens of Ti, Co, Pd and Ag in decreasing order. Rotation of the electrode was useful to increase machining speed and to decrease electrode elimination ratio and clearance. Small holes ware prepared using electric discharge machining with a developed machine to apply to dental laboratory works.

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