The deformation and fracture mechanism of titanium cast into a phosphate-bonded silica alumina investment mold was examined by means of 3-point bending test, acoustic emission(AE)analysis, and metallographic examination.The α case formed on the casting increased the elastic limit and the maximum load, and so the removal of the surface by pickling decreased them.For the as-cast, sharp cracks propagated through the Si, P-rich layer into the acicular structure in the elastic deformation stage and then opened in the plastic deformation stage.As with the room-temperature mold casting, low-amplitude AEs were generated at a constant rate throughout the entire deformation process, in which crack formation, propagation, and opening occurred in the tension-side surface ranging from the center to both the end supports.As with the 800℃ mold casting, low-amplitude but high-energy AEs of a continuous type were generated during the elastic deformation, and they suggested simultaneous propagation of the primary crack.In the latter half stage of plastic deformation, the load and the AE rate were remarkably reduced.This is because a few main cracks near the center propagated continuously within the internal structure which possesses little resistance to the crack propagation.As with the pickled casting, AEs were generated not in the elastic deformation stage but in the plastic deformation stage, thus releasing high energy.The cause of such high-amplitude AEs is obscure.

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A stone cast in a mold of an alginate impression material can have a rough surface.The surface roughness of a dental stone cast against the alginate impression material, was decreased by mixing with a surfactant solution.Among the surfactants tested the Quartamin 24 P cationic surfactant, particularly in a 1% solution, was the most effective for decreasing surface roughness in all four combinations of alginate impression materials with dental stones.The gelating reaction of the alginate impression material was depressed by the 24 P solution to a small extent, the elastic strain was scarcely affected.Loss of water from the gelled impression mateiral by evaporation was hardly affected by the 24 P solution.Another action of the 24 P solution was to retard the setting rate of the dental stone.Statistically significant difference was observed between surface roughness and wettability of the contact surface with dental stone.

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Thermal diffusivity, specific thermal capacity and thermal conductivity were measured by the xenon flash method to evaluate the thermal properties on 5 commercial kinds of temporary sealing cements.Thermal diffusivity of polycarboxylate temporary sealing cements were 0.2918 to 0.4444×10^-2cm²s^-1, and that of eugenol or nogenol temporary sealing cements were 0.1454 to 0.2574×10^-2cm²s^-1.The specific thermal capacity of polycarboxylate temporary sealing cements was 1.1558 to 1.5938Jg^-1K^-1, and that of eugenol or nogenol temporary sealing cements 0.7500 to 0.9004Jg^-1K^-1.The thermal conductivity of polycarboxylate temporary sealing cements was 1.0293 to 1.4616Wm^-1K^-1, and that of eugenol or nogenol temporary sealing cements 0.2497 to 0.3779Wm^-1K^-1.

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A significant correlation has been found between static creep of amalgam and clinical marginal fracture characteristics.To reveal mechanical details, stress, time and temperature dependencies of creep rate of amalgam have been studied.However, the relationship between static creep properties and creep behavior of the amalgam in the cavity under occlusal force has only been done from groundless discussion.The creep of amalgam in Class I cavity was then simulated by an axisymmetric elastic creep finite element method.In this procedure, the stress and time dependence of creep strain was assumed as follows:ε^c=kσ^mtⁿ Here, exponents m and n of stress and time dependencies were postulated as 2.0 and 1.0, respectively.The creep values of 0.1, 1.0 and 8.0%(tested by American Dental Association Specification no.1)were selected for the creep calculation of amalgam.Occlusal force was assumed as 40 N on the caps of enamel and vertical direction to the axis of the tooth.Simulated results show that the creep strain of amalgam in the cavity did not increase linearly with time because the amalgam deforms to become stress-free and creep rate is a function of the stress.It was concluded that elastic deformation of the crown which has no restorative material is an important factor to determine the creep strain and the gap between the cavity wall and restorative material.

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The objective of this study is to examine the relationship between contact angle and tensile bond strength of dental luting cements.Contact angles of four different luting cements(zinc phosphate, polycarboxylate, glass ionomer and resin cements)on four different casting alloys(Au-Ag-Cu, Ag-Pd, Ni-Cr, and Ti)or dentin were examined.A volume of 0.2ml of each cement mixture was placed on each material polished with #600 SiC and a polyvinylsiloxane impression was made after the setting of the cement.After a replica was fabricated from the impression, the cross-section of the replica was prepared and photographed to measure the contact angle of each cement on each substrate.The tensile bond strength between each cement and each metal was determined according to the method described in the previous study.The contact angle of the resin cement on any substrate were the lowest values among the cements examined;however, the zinc phosphate cement had the highest values of contact angles on any substrate(p<0.05).The resin cement showed the greatest tensile bond strength to any materials among the cements examined(p<0.05).A significant correlation between the contact angle and tensile bond strength of the cements was found(correlation coefficient=-0.895, p<0.05).The results suggested that the wettability of luting cements is one of the factors predicting the adhesive properties of the cements.

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A basic study was carried out on the reaction products on the surface of titanium castings obtained with molds lined with high purity oxide pellets(e.g., SiO₂, Al₂O₃, MgO, and YSZ).In a preliminary study, high purity titanium plates(99.99%)were heat treated to examine the relationship between color changes and the reaction products on their surface.The following results were obtained using a TF-PSPC(Thin Film Position Sensitive Proportional Counter)at an angle of the two-degree incidence and EPMA(Electron Probe X-ray Micro Analyzer)of the surface of these materials.When high purity titanium plates were treated by heat in the atmosphere, rutile was generated as the primary product on their surface, and the rutile production increased rapidly above 900℃.The products responsible for coloring by casting at less than 600℃ could not be identified by TF-PSPC under the conditions of this study.In the surface layer of the titanium castings produced by the actual casting process, the depth of penetration of major elements of various high purity oxide pellets with which the molds were lined decreased in the order of quartz>alumina>magnesia and zirconia, and that of TITAVEST-PS was the greatest next only to quartz.The depth of penetration of Si into the surface of titanium castings depended on the silicon concentration on the cast surface at a casting temperature of 900℃, but it was dependent on the casting temperature at a fixed silicon concentration.The structure of the hardened layer on the surface of titanium castings into which Si had penetrated differed with the casting temperature.

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Deterioration of relining resins by exposure to thermal cycling was evaluated.The test materials consisted of self-curing, light-curing denture base relining resins and heat-curing denture base resin.After polymerization, the materials were exposed 10, 000 times to thermal cycling at a temperature ranging from 4℃ to 60℃.The same kinds of resins immersed in distilled water at 37℃ for 14 days were also examined.The surface roughness, gloss, color tone, knoop hardness, contact angle, abrasion resistance and bacterial adherence were examined.The materials were also observed by scanning electron microscopy The surface roughness, gloss, color tone, abrasion resistance, bacterial adherence of the resins exposed to thermal cycling were more affected than those of resins immersed in distilled water, particularly, those of self-curing resins.

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To understand the interface phenomena between metals and 4-META/MMA-TBBO resin, the relationship between the amount of hydroxyl groups on the surface of the metal and that of the resin adsorbed by the metal, the relative orientation of functional groups in the resin to the surface of the metal and the bonding states between the metal and the resin were examined using Fourier transform infrared reflection absorption spectroscopy(FT-IR-RAS)and X-ray photoelectron spectroscopy(XPS).The amount of the resin absorbed by the metal increased with the increase of hydroxyl groups.In addition, in the case of non-precious metals, C=O and C-O axes in polar groups in the resin relatively oriented normal to the surface of the metals and the metal-complexes were formed at the interfaces.On the other hand, in the case of the precious metals, such phenomena did not occur.That is, 4-META/MMA-TBBO resin adheres to the non-precious metals more strongly than the precious metals.

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The working time and setting time, viscosity, elastic recovery, elastic modulus, hardness(H_DA)and dimensional change of 10 commercial denture base soft lining materials(5 acrylates, 3 silicones, 1 fluoro polymer and 1 olefin polymer)were measured.The working time for the six materials examined ranged from 2.5 to 17.0 min, the setting time ranged from 3.8 to 14.3 min.In the case of two silicones, the viscosity was markedly increased after the start of measurement.Six materials reached above 90% of elastic recovery at 2〜9 min after the start of mixing.The elastic recovery for two silicones at 30 min was nearly 100%.The elastic modulus ranged from 1.51×10⁶ to 2.74×10⁶dyne/cm².These values were lower than those obtained for the oral soft tissue(0.7〜4.4×10⁷dyne/cm²).Olefin polymer and one acrylate(heat cured)had a higher hardness value than the other materials.For acrylates, the values of elastic modulus and hardness increased with time(30 day).In dimensional change, four acrylates showed expansion for 2〜5 hr after the start of mixing.

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The durability of six commercially available composite resins for inlays was examined by storage for various periods in boiling water and by thermal cycling between 4 and 60℃.Bending strength and elastic modulus of these materials decreased by storage of the specimens in boiling water, and did not greatly change with the increase in storage period between 5 and 30 days in boiling water.On the other hand, Knoop hardness also decreased by storage in boiling water and Knoop hardness of the specimens stored in boiling water for 30 days was about 20 and the values were equal to that of unfilled resins.The variation in Knoop hardness on depth from the surface of these specimens stored in boiling water for 30 days was determined.The specimens were affected up to 0.4-0.8mm in depth from the surface by storage in boiling water.Bending strength and elastic modulus decreased after 10, 000 thermal cycles, but did not change with increasing number of thermal cycles.In conclusion, the durability of composite resins for inlays could be evaluated by the bending test of specimens stored in boiling water for a short period such as 5 days and the degradation of composite resins for inlays could be evaluated by the determination of the variation in Knoop hardness on depth from the surface.

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The indentation microfracture(IM)method was applied to estimate the fracture toughness and residual stress in the surface of dental porcelains which were slowly cooled and tempered by two different treatments.The shape parameter, m, in Weibull plots of the K_c`s which were measured by IM method, was calculated to be 7.7 to 9.1 for the slow cooled specimen.Tempering had no significant effect on Weibull parameter.Factor of safety for the porcelain used for prosthesis was estimated to be 3.4 to 4.9 for the failure probability of 10^-5.Surface residual stress was estimated to be 26-28 and 64-88MPa(in compression)for the porcelain tempered in ambient air and forced air, respectively, from the median crack size after indentation.Estimated residual stress was in good agreement with the results of computer simulation by Asaoka et al.However, the residual stress could not be calculated for the porcelain with crystallization in its glass transition range during cooling because the reference sample was difficult to prepare.In concludsion, the fracture toughness and residual stress in micro-area(100〜400μm)of the dental baked-on porcelain prosthesis can be measured by the IM method.

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The interface reaction between molten titanium and quartz glass in the casting process was investigated from the standpoint of the flow nature in the mold cavity.The quartz glass by contact with the molten titanium was decomposed into oxygen and silicon.The former together with titanium formed the outermost layer known as α-case.The latter is preferentially dissolved into the molten titanium to form the contaminated layer.The molten titanium including the silicon can cause the inner contamination region by the mechanisms of(1)surface concave defects, (2)collision of some streams, and(3)"ejection"in a turbulent flow.Furthermore, the viscosity of molten titanium necessarily increases by contamination and then resistance to flow increases, by which the castability can be affected.

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Dental casting techniques of titanium have made marked progress in recent years.However, as titanium is very active in the melting condition, layers reacted with the mold are formed on the surface of the castings.The corrosion characteritics on the reacted surface layer of titanium castings were studied in saline solution by using electro-chemical technique and observation by SEM.Three types of surface condition were tested-A:cleaned with ultrasonic cleaning equipment;B:sand-blasted;C:buffed.Al though the surface condition A showed a lower corrosion resistance than the two other conditions, its corrosion resistance was much better than those of dental alloys such as Ni-Cr, Co-Cr and silver alloys.Voltamogram of the surface condition B was similar to that of the surface condition C.SEM observations before and after the voltammetry did not show marked changes on the surface condition B.As a result, the reacted surface layer on the titanium castings seems to be reliable with respect to the corrosion resistance in a static condition.

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Self-etching dentin primer(M-MEMA)containing N-methylolmethacrylamide(MEMA)and maleic acid was prepared to improve the bond strength of light-cured composite resin systems to dentin.The bond strength of Silux with Scotchbond_2(SB2)and Scotchbond(SB), and Photo Clearfil A with Clearfil Photo Bond(CP)to dentin treated with M-MEMA was significantly higher than that to dentin treated with Scotchprep(SP).The widened dentin tubules on dentin surfaces treated with SP and M-MEMA were visible with SEM.M-MEMA widened dentin tubules more greatly than SP.Also, for M-MEMA, the intertubular dentin was rougher than that for SP.The thickness of the treated layers was about 1.5μm.The bonding agents created acid-proof dentin layers with different thickness on the treated dentin surfaces.The layer thicknesses were about 2μm for CP and less than 1μm for the others, regardless of the type of self-etching dentin primers.The SEM of the specimens after shear bond tests revealed resin-dentin interface failure or acid-proof dentin layer cohesive fracture.

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Accuracy and the shape of laminate veneers are significantly affected by the complicate technical procedures and the ability of the operator.A CAD/CAM production system for laminate veneer restoration was developed.Initially, a system which was composed of a laser displacement meter and a computer-controlled scanning machine was assembled to measure the three dimensional shape of stone teeth models automatically.To obtain a higher accuracy of measurement, several modifications were made, for instance, color of stone model was examined and gray was concluded to be the most suitable color, and further, the tracking measurement method to keep the distance between a displacement meter and the stone model constant was effective to measure a model with rugged surface accurately.By the present system developed, three dimensional shape measurement of a maxillar model was carried out, and high accuracy data were obtained.

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The apparatus was designed to measure the linear shrinkage of composite resin during hardening, and the linear shrinkage of composite resin against the direction of the length was investigated as a function of filler content in the composite resin using the apparatus.The linear shrinkage of composite resin in low filler content was very small, compared with the linear shrinkage value calculated from the density of monomer, polymer and silica filler, because the viscosity of resin paste was very low and the flow of the resin past during hardening occurred.However, when the filler content become higher, the linear shrinkage was close to the calculated linear shrinkage because the flow of resin past was small, the shrinkage was equally done against the three direction.

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It is very important to investigate the mechanism for shrinkage in composite resin during hardening, because the developing force caused by the shrinking of composite resin in the cavity applies stress to human tooth tissue.In this study, the apparatus was prepared to measure the developing force in composite resin during hardening.The shrinking stress in a different curing type of composite resin, light cured and chemically cured type, was observed, and then the mechanism of shrinkage in composite resin was discussed.Initially, the shriking stress of composite resin increased dramatically.Later, the shrinking stress suddenly decreased, which indicates the detachment at the interface between the composite resin and the cavity wall.The shrinking stress obtained with the open type cavity was smaller than that obtained with the closed type cavity, and the shrinking stress of the chemically cured resin appeared faster than that of the light cured resin, but the maximum value of the shrinking stress was the same.

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