Takayuki Yoneyama

Wear properties of Ti and Ti-6Al-7Nb castings for dental prostheses.

Titanium has been increasingly applied to dental prostheses because of its biocompatibility. However, application remains limited, due to the low strength and poor wear resistance of unalloyed titanium. The purpose of this study is to evaluate the wear resistance of high-strength Ti-6Al-7Nb alloy castings for dental application. Test specimens were cast from commercially pure titanium (CP-Ti grades 2 and 3) and Ti-6Al-7Nb alloy ingots, and subjected to a wear test simulating the occlusal loading pattern. Wear resistance was evaluated by the weight loss during the test. Ti-6Al-7Nb alloy was found to exhibit lower weight loss than CP-Ti. Moreover, scanning electron microscopic (SEM) observation after the test revealed that the worn surface of Ti-6Al-7Nb alloy is much smoother than that of CP-Ti. These results indicate that Ti-6Al-7Nb alloy castings can be used to produce dental prostheses of improved wear resistance and mechanical strength.

Mechanical properties and corrosion resistance of Ti-6Al-7Nb alloy dental castings.

With the aim of applying a novel titanium alloy, Ti–6Al–7Nb, to a dental casting material, a comprehensive research work was carried out on its characteristics, such as castability, mechanical properties and corrosion resistance in the present study. As a result, Ti–6Al–7Nb alloy exhibited sufficient castability by a dental casting method for titanium alloys and enough mechanical properties for dental application. It is also showed excellent corrosion resistance through an immersion test in 1.0% lactic acid and an anodic polarization test in 0.9% NaCl solution. From these results, it is concluded that this Ti–6Al–7Nb alloy is applicable as a dental material in place of Ti–6Al–4V alloy, which includes cytotoxic vanadium.

Influence of aging heat treatment on mechanical properties of biomedical Ti–Zr based ternary alloys containing niobium

Titanium–zirconium based alloys containing a small amount of niobium were investigated in order to evaluate their possible use as biomedical materials. Zirconium, which belongs to the IVa group, is known to have good corrosion resistance and biocompatibility similar to titanium. As the titanium–zirconium system shows a complete solid solution, a wide variation of alloy design is available and large quantities of solid-solution hardening must be possible. Niobium, having a β-phase stabilizing effect, was chosen as a ternary element in order to control desirably the microstructure. There have been no reports which suggest its harm to a living body. The alloys containing 2% or 3% niobium showed the highest hardness value after aging heat treatment at 773 K. In contrast to this, no alteration of hardness was seen in specimens aged at 1073 K. Through conventional X-ray diffractometry and in situ X-ray analysis using a hot stage, β-phase precipitation in the A matrix was identified. From the above results, it is concluded that alloys containing 2%–3% niobium are hopeful candidates for new kinds of biomedical alloys, when they are heat treated under suitable conditions.

Dental Casting of Titanium and Ni-Ti Alloys by a New Casting Machine

The purpose of this study was to develop a new casting machine for titanium and Ni-Ti alloys. Properties of cast pure titanium and Ni-Ti alloys were studied by means of the tensile and casting tests. As gas in the mold was removed by the mold being heated under a high vacuum, the reaction between the molten metal and the mold decreased. The new control system and the two types of crucibles developed proved very useful for prevention of internal macro-defects in castings and for improvement of castability. Mechanical properties and castability of pure titanium were improved. Ni- Ti alloys could be cast without loss of their shape-memory effect or super-elasticity characteristics. The new casting machine is thought to be promising for the casting of titanium and Ni-Ti alloys. Furthermore, we can cast these alloys using conventional techniques and investments.

Effect of heat treatment on transformation temperatures and bending properties of nickel–titanium endodontic instruments

AIM To investigate the effect of heat treatment on the bending properties of nickel-titanium endodontic instruments in relation to their transformation behaviour. METHODOLOGY Nickel-titanium super-elastic alloy wire (1.00 mm Ø) was processed into a conical shape with a 0.30 mm diameter tip and 0.06 taper. The heat treatment temperature was set at 440 or 500 degrees C for a period of 10 or 30 min. Nonheat-treated specimens were used as controls. The phase transformation behaviour was examined using differential scanning calorimetry. A cantilever-bending test was used to evaluate the bending properties of the specimens. Data were analyzed by ANOVA and the Tukey-Kramer test (P = 0.05). RESULTS The transformation temperature was higher for each heat treatment condition compared with the control. Two clear thermal peaks were observed for the heat treatment at 440 degrees C. The specimen heated at 440 degrees C for 30 min exhibited the highest temperatures for M(s) and A(f), with subsequently lower temperatures observed for specimens heated at 440 degrees C for 10 min, 500 degrees C for 30 min, 500 degrees C for 10 min, and control specimens. The sample heated at 440 degrees C for 30 min had the lowest bending load values (P < 0.05), both in the elastic range (0.5 mm deflection) and in the super-elastic range (2.0 mm deflection). The influence of heat treatment time was less than that of heat treatment temperature. CONCLUSIONS Change in the transformation behaviour by heat treatment may be effective in increasing the flexibility of nickel-titanium endodontic instruments.

Characterization of the spatial immobilization manner of poly(ethylene glycol) to a titanium surface with immersion and electrodeposition and its effects on platelet adhesion

Both terminals of a poly(ethylene glycol) (PEG) molecule were terminated with amines. The PEG was electrodeposited onto titanium (Ti) to give it the biofunctions such as blood compatibility. The effects of the amine of PEG terminals and the pH at PEG solution on the adsorption amount of PEG molecules and the thickness of PEG layer immobilized on the Ti surface were evaluated using quartz crystal microbalance technique and X-ray photoelectron spectroscopy. The interfacial reactivity between PEG terminals and the Ti surface was the largest at pH 11, according to the interaction between the charge of terminal amines of PEG and the point of zero charge of Ti oxide. The orientations of PEG molecules immobilized on the Ti surface with immersion or electrodeposition at pH 11 were determined by Fourier transform infrared reflection absorption spectroscopy. Consequently, the terminal amines of PEG were oriented perpendicularly to the surface in electrodeposition rather than in immersion. The charged PEG randomly immobilized on the Ti surface with immersion led to platelet aggregation, whereas U-shaped PEG molecule immobilized with electrodeposition inhibited platelet adhesion and aggregation. The immobilization manners of PEG on the Ti surface were strongly associated with a biofunction such as platelet adhesion.

Castability of Ti-6Al-7Nb alloy for dental casting

Castability of Ti-6Al-7Nb alloy, CP Ti, and Co-Cr alloy was examined for mesh type and plate type specimens. The casting was carried out with a pressure type casting machine and commercial molding material. The castability of the mesh type specimen was evaluated in terms of the number of cast segments (castability index), and that of the plate type was evaluated by the area of the specimen (casting rate). X-ray images processed by a digital imaging technique were used to identify the casting porosity. The casting rate of the specimens increased with increasing thickness of the specimens. It was concluded that the castability index and the casting rate of Ti-6Al-7Nb alloy was slightly lower than that of CP Ti, and higher than that of Co-Cr alloy, were as Ti-6Al-7Nb alloy showed fewer casting porosities than CP Ti and smaller ones than Co-Cr alloy, which was advantageous for increasing the reliability of the casting properties.

Effect of heat treatment with the mould on the super-elastic property of Ti–Ni alloy castings for dental application

Tensile property of Ti–50.85Ni (mol %) alloy castings was investigated quantitatively in relation to the thermal behavior accompanied with phase transformation to evaluate the effect of heat treatment after casting with the mould in air. The heat treatment temperature was 713 or 773 K, and the period was 0.9, 1.8, or 3.6 ks. Apparent proof stress of the castings decreased with increasing period of heat treatment, and the decrease was larger with the treatment at 773 K. Residual strain also decreased by the heat treatment, however, it was low with the treatment for relatively short period, i.e. 713 K-0.9 and 1.8 ks, and 773 K-0.9 ks treatments. From the thermal behavior measured by differential scanning calorimetry (DSC), the ascent in the transformation temperatures and the increase in the thermal peak height appeared to influence the changes in the tensile property. These changes by heat treatment were believed to be effective to utilize more flexibility, less stress and less permanent deformation in dental castings.

Effects of triazine dithione and hydrophobic phosphate monomers on bonding to Ag-Pd-Cu-Au alloy and titanium with a methacrylic resin-based luting agent.

PURPOSE The purpose of the current study was to evaluate the effects of thione and phosphate functional monomers on bond strength and durability of a methacrylic resin luting agent joined to silver-palladium based alloy and titanium. MATERIALS AND METHODS Disk specimens of two different sizes (10 mm and 8 mm in diameter by 2.5 mm thickness) were prepared from silver-palladium-copper-gold (Ag-Pd-Cu-Au) alloy and titanium. The specimens were ground with abrasive paper, and divided into four groups based on the following conditions: 1) unprimed control; 2) priming with Alloy Primer (thione and phosphate); 3) priming with Estenia Opaque Primer (phosphate); and 4) priming with VPrimer (thione). The disks were bonded with tri-n-butylborane (TBB)-initiated resin, and shear bond strengths were determined both before and after thermocycling. RESULTS The Alloy Primer and V-Primer agents containing a triazine dithione monomer (VTD) bonded the Ag-Pd-Cu-Au alloy, whereas the Alloy Primer and Estenia Opaque Primer agents containing a hydrophobic phosphate monomer (MDP) bonded titanium. The reduction in bond strength after 20,000 thermocycles was not remarkable for the two groups (Alloy Primer and Ag-Pd-Cu-Au alloy; V-Primer and Ag-Pd-Cu-Au alloy). CONCLUSION The thione monomer was effective in bonding the Ag-Pd-Cu-Au alloy, whereas the phosphate monomer was useful for bonding titanium. The thione did not interfere with the bonding of MDP to titanium, and MDP did not negatively affect bonding of VTD to the Ag-Pd-Cu-Au alloy. The combinations of two thione primers and the Ag-Pd-Cu- Au alloy exhibited the best adhesive performance within the limitations of the current experiment.

Non-linear finite element stress analysis of plastic deformation in Co-Cr wrought-wire clasps

OBJECTIVES The purpose was to assess the influence of plastic deformation by bending on stress, flexibility and permanent deformation in wrought-wire clasps. METHODS A three-dimensional finite element model of a straight wire (120 mm in length and 1.0mm in diameter) was created. The non-linear stress-strain relationship of a commercial Co-Cr alloy straight wrought-wire, measured by means of tensile test (n = 5) was put into the program. Bending to an angle of 90 degrees or 120 degrees and subsequent unloading processes with spring back phenomenon, were simulated in the clasp shoulder and arm. The stress distributions were analyzed at loading and unloading. Thereafter, the clasp models were deflected outwardly 0.25, 0.50, and 0.75 mm at the clasp tip, to simulate the removal and insertion of a denture. RESULTS Under the bending force, the maximum tensile stress was recorded at the outside surface of the bending corner; while after unloading, the maximum tensile stress appeared at the inside of the bending angle. By deflection of the clasp tip, this stress increased up to 203% of that before deflection. The change of stress by deflection was larger at the shoulder than at the arm. The load required for deflection was approximately 43% larger in the models with the arm angle of 120 degrees than those with an angle of 90 degrees . SIGNIFICANCE The results suggest that the permanent deformation of wrought-wire clasps is likely to initiate at the clasp shoulder, while clasp flexibility is dependent on the bending angle of the clasp arm.