Masanobu Yoda

Mechanical properties and microstructures of cast Ti–Cu alloys

OBJECTIVES This study evaluated the mechanical properties of cast Ti-Cu alloys with the hope of developing an alloy for dental casting with better mechanical properties than unalloyed titanium. METHODS Ti-Cu alloys with five concentrations of copper (0.5, 1.0, 2.0, 5.0 and 10.0 mass%) were made in an argon-arc melting furnace. The alloys were cast into magnesia-based molds using a centrifugal casting machine. The microstructure, microhardness profile of the specimen cross section, tensile strength, yield strength, and elongation were determined for the castings. Scanning electron microscope fractography was undertaken for the fractured surfaces after tensile testing. XRD was performed on the polished specimens. Results were analyzed using one-way ANOVA and the Student-Newman-Keuls tests. RESULTS The mean tensile strengths of all the cast Ti-Cu alloys were significantly (p<0.05) higher than for cast commercially pure titanium (CP Ti). Of the Ti-Cu alloys tested, the 5 and 10% Cu alloys had significantly higher strength than the rest. The 10% Cu alloy exhibited the lowest mean elongation. CP Ti and the 0.5 and 1% Cu alloys showed higher ductility. The bulk hardness of all the cast Ti-Cu alloys, except for the 10% Cu alloy, and CP Ti was approximately the same. SIGNIFICANCE By alloying with copper, the cast titanium became stronger. Increases in the tensile strength (30%) and yield strength (40%) over CP Ti were obtained for the 5% Cu alloy. Elongation was approximately 3%, which was similar to cast Ti-6Al-4V. Ti-Cu alloys, such as the 5% Cu alloy, could be used for prosthetic dental applications if other properties necessary for dental castings are obtained.

Grindability of cast Ti -Cu alloys

OBJECTIVE The purpose of the present study was to evaluate the grindability of a series of cast Ti-Cu alloys in order to develop a titanium alloy with better grindability than commercially pure titanium (CP Ti), which is considered to be one of the most difficult metals to machine. METHODS Experimental Ti-Cu alloys (0.5, 1.0, 2.0, 5.0, and 10.0 mass% Cu) were made in an argon-arc melting furnace. Each alloy was cast into a magnesia mold using a centrifugal casting machine. Cast alloy slabs (3.5 mm x 8.5 mm x 30.5 mm), from which the hardened surface layer (250 microm) was removed, were ground using a SiC abrasive wheel on an electric handpiece at four circumferential speeds (500, 750, 1000, or 1250 m/min) at 0.98 N (100 gf). Grindability was evaluated by measuring the amount of metal volume removed after grinding for 1min. Data were compared to those for CP Ti and Ti-6Al-4V. RESULTS For all speeds, Ti-10% Cu alloy exhibited the highest grindability. For the Ti-Cu alloys with a Cu content of 2% or less, the highest grindability corresponded to an intermediate speed. It was observed that the grindability increased with an increase in the Cu concentration compared to CP Ti, particularly for the 5 or 10% Cu alloys at a circumferential speed of 1000 m/min or above. SIGNIFICANCE By alloying with copper, the cast titanium exhibited better grindability at high speed. The continuous precipitation of Ti(2)Cu among the alpha-matrix grains made this material less ductile and facilitated more effective grinding because small broken segments more readily formed.

Bond strength of binary titanium alloys to porcelain

The purpose of this study was to investigate the bond strength between porcelain and experimental cast titanium alloys. Eleven binary titanium alloys were examined: Ti-Cr (15, 20, 25 wt%), Ti-Pd (15, 20, 25 wt%), Ti-Ag (10, 15, 20 wt%), and Ti-Cu (5, 10 wt%). As controls, the bond strengths for commercially pure titanium (KS-50, Kobelco, Japan) and a high noble gold alloy (KIK, Ishifuku, Japan) were also examined. Castings were made using a centrifugal casting unit (Ticast Super R, Selec Co., Japan). Commercial porcelain for titanium (TITAN, Noritake, Japan) was applied to cast specimens. The bond strengths were evaluated using a three-point bend test according to ISO 9693. Since the elastic modulus value is needed to evaluate the bond strength, the modulus was measured for each alloy using a three-point bend test. Results were analyzed using one-way ANOVA/S-N-K test (alpha = 0.05). Although the elastic moduli of the Ti-Pd alloys were significantly lower than those of other alloys (p = 0.0001), there was a significant difference in bond strength only between the Ti-25Pd and Ti-15Ag alloys (p = 0.009). The strengths determined for all the experimental alloys ranged from 29.4 to 37.2MPa, which are above the minimum value required by the ISO specification (25 MPa).

Clinical Chipping of Zirconia All-Ceramic Restorations

Advancements in CAD/CAM systems employed in dentistry have made possible the application of yttria tetragonal zirconia polycrystal (Y-TZP) in zirconia-based all-ceramic restorations. Y-TZP has excellent flexural strength and fracture toughness and is used in molar crowns as well as frameworks of fixed partial dentures (FPDs). The use of Y-TZP in clinics has increased over the past several years, and it is now used in implant abutments and denture frameworks. While the demand for Y-TZP is increasing, chipping of porcelain used in the zirconia framework has been noted as a problem in zirconia-based all-ceramic restorations from a clinical point of view. We have previously used Cercon® smart ceramics with Y-TZP frames in clinics but have noticed the chipping of porcelain in a large number of cases over time. This review article focuses on the chipping of zirconia all-ceramic restorations by taking into account the following aspects: (1) clinical performance of zirconia all-ceramic restorations, (2) influence of frame thickness and porcelain firing schedules, and (3) reduction in porcelain chipping.

Strength of porcelain fused to Ti-20% Ag alloy made by CAD/CAM

Titanium and titanium alloys are difficult to machine. This problem arises when milling, using dental CAD/CAM systems. In a previous study, an experimental binary titanium alloy with 20 mass% Ag showed good grindability. In this study, the fracture strength of porcelain fused to a Ti-20 mass% Ag alloy crown made using a CAD/CAM (GN-1, GC, Japan) system is investigated. As controls, similar pure titanium (JIS grade II) samples made using cast and using the CAD/CAM system were also examined. The crowns were made assuming a maxillary left central tooth. The fracture strengths were statistically analyzed using one-way ANOVA followed by Tukey pairwise tests. There was no significant difference in the fracture strength of porcelain fused to metal crowns between the Ti-20 mass% Ag alloy frame crowns and the pure titanium frame crowns.

Strength of porcelain fused to pure titanium made by CAD/CAM

This study is an investigation of the bond strength of porcelain to pure titanium, cast and machined by CAD/CAM processes, and a determination of the fracture strength of porcelain fused to pure titanium crown by both methods. The bond strength was evaluated according to the surface treatment conditions. The mean bond strengths were ranged from 36.1 to 49.4 MPa. The surface treatment conditions had a significant effect on the bond strength. The mean fracture strength for the cast frame crowns was 1667 N, and that for the machined frame crowns was 1554 N. There was no significant difference between the two methods. Acceptable bond strength and fracture strength were achieved by both methods.

Effect of Zirconia Frame Thickness on Fracture Toughness of Veneer Porcelain

This review article focused on the influence of the zirconia thickness on the strength of the veneer porcelain, which was evaluated with a fracture toughness test according to JIS R 1607. The veneer porcelains for the zirconia all-ceramic crown were fired on two different thicknesses (0.4 mm and 0.8 mm) of zirconia frames (ZAC). As the controls, veneer porcelains fired on metal frames for porcelain fused to metal crown (PFM) were used. The fracture toughness value evaluated by measuring the extent of cracking associated with a Vickers indentation on the veneer porcelain part was significantly larger at the points of 1.5 mm in 0.4 mm thickness frame of ZAC samples compared with in the 0.8 mm frame.

Educational effect on tooth preparation of visual feedback using computer graphics

This review article describes the educational effect on tooth preparation of visual feedback using computer graphics. The result is that in the feedback group, the average occlusal reductions showed significant differences at the three cusps, and the axial wall taper showed significant differences in the three regions. A series of studies indicates that visual feedback using computer graphics was effective for tooth preparation practices.

Preparation of TiO2 coating on dental metal materials by plasma CVD

In this study, TiO2 films were prepared by Plasma-enhanced Chemical Vapor Deposition (PECVD) using Ti(O-i-Pr)2(dpm)2 precursors. The effects of the deposition conditions on the crystalline phases, microstructures, and color of the deposited films were investigated. At a substrate pre-heating temperature T pre = 623 K, the crystalline phase of the deposited film changed from amorphous to anatase to rutile with increasing microwave power PM from 0 to 3.0 kW, and the microstructure changed from dense to granular. The deposited film was gray or black but changed to white at a higher T pre (923 K).

Fitness of Zirconia all-ceramic crowns with different cervical margin forms

The purpose of this study was to evaluate the marginal and internal fitness of zirconia all-ceramic crowns. Three brass dies were prepared with the following cervical margin forms: a shoulder and two types of rounded (curvature radius: 0.2 and 0.5 mm) shoulder preparations. Five crowns for each type of die were fabricated using the CAD/CAM system. The fitness was evaluated with a replica technique using silicon impression materials. The mean gap dimensions and standard deviations at the margins were 36 ± 32 µm for the shoulder, 27 ± 29 µm for the 0.2 mm rounded shoulder, and 41 ± 33 µm for the 0.5 mm rounded shoulder. There were no significant differences among the three groups regarding marginal fitness. The zirconia all-ceramic crowns made using the CAD/CAM system showed clinically acceptable marginal fitness.