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Shigeo Hasegawa

Bio: Shigeo Hasegawa is an academic researcher from Meirin College. The author has contributed to research in topics: Flexural strength & Grinding. The author has an hindex of 1, co-authored 1 publications receiving 10 citations.

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TL;DR: The flexural strength values of Crys-Cera and Empress decreased with increasing amounts of grinding, while that of OCC increased with increased grinding, and the surfaces of all specimens were smoother when the amount of grinding was deeper.
Abstract: The aim of this study was to determine a suitable amount of grinding of a flexural test specimen for clinical evaluation of dental ceramics. Three dental ceramics (Crys-Cera, OCC and IPS Empress Cosmo) were selected. Five types of bar-shaped patterns were prepared for a three-point bending test, and four types of disk-shaped patterns for a biaxial flexural test. Ceramic specimens were fabricated using these patterns in accordance with the manufacturers' instructions. All specimens were ground with diamond polishing pads to yield either bar or disk specimens with the same final geometry. The surface roughness and X-ray diffraction (XRD) patterns of the specimens were examined. The flexural strength values of Crys-Cera and Empress decreased with increasing amounts of grinding, while that of OCC increased with increased grinding. The surfaces of all specimens were smoother when the amount of grinding was deeper. The XRD patterns of Crys-Cera and Empress changed with increased grinding; however, that of OCC did not change regardless of the extent of grinding. The flexural strengths of three dental ceramics differed with the amount of grinding probably because of changes of surface roughness and crystalline component composition. Unground specimens provided important insight into clinical failure modes.

10 citations


Cited by
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Journal ArticleDOI
TL;DR: There was no difference in flexural strength of uncolored and colored Y-TZP ceramic and the fatigue limit of un colored, FS4 and FS7 zirconia may be defined as lying between 60 and 65% of the stress to failure.

166 citations

Journal ArticleDOI
TL;DR: Surface roughness may not be the only feature that determines strength of pressable materials, and other issues such as porosity, microstructural residual stresses, surface and bulk defects may also be pertinent.

137 citations

Journal ArticleDOI
TL;DR: In this article, the authors explore selective laser melting of yttria-stabilized zirconia (YSZ) ceramic by a 1 μm wavelength fiber laser; investigate the influence of different laser powers and different scanning velocities on the microstructure, the relative density, the deformation of ceramic sample and the micro-hardness; and analyze the crystal structure transformation during the fabrication.
Abstract: Purpose – The purpose of this paper is to explore selective laser melting of yttria-stabilized zirconia (YSZ) ceramic by a 1 μm wavelength fibre laser; investigate the influence of different laser powers and different scanning velocities on the microstructure, the relative density, the deformation of ceramic sample and the micro-hardness; and analyze the crystal structure transformation during the fabrication. Design/methodology/approach – During the fabrication, the 5 mm × 5 mm × 5 mm YSZ ceramic samples are fabricated by rapid prototyping (RP) machine MCP Realizer SLM 250; density and microscopic photographs show the ceramic melting situation. The density of cubic sample with different laser powers and different scanning velocities is measured by Archimedes method. The microstructure of samples and powder is observed by SEM. The micro-hardness is measured by the Vickers micro-hardness equipment, and the crystal structure transition is studied by XRD. Findings – It is possible to melt YSZ powder complete...

41 citations

Journal ArticleDOI
TL;DR: The purpose of this study was to evaluate the effects of controlled polishing on the flexural strength of dental ceramics by using a custom-made machine that applied standardized loads and speeds that coincided with the mean Loads and speeds used by experienced prosthodontists.
Abstract: Statement of problem Many studies on the strengthening effects of grinding and polishing, as well as heat treatment on ceramics, are not well standardized or use commercially available industrial polishing systems. The reported effectiveness of these strengthening mechanisms on ceramics may not be applicable to clinical dentistry. Purpose The purpose of this study was to evaluate the effects of controlled polishing on the flexural strength of dental ceramics by using a custom-made machine that applied standardized loads and speeds that coincided with the mean loads and speeds used by experienced prosthodontists. Material and methods A total of 140 aluminous dental ceramic bar-shaped specimens (Vitadur Alpha Enamel) measuring 1.5 × 2.0 × 25 mm were fabricated and divided into 12 groups (for most groups, n=10). Specimens were untreated, polished with different polishing systems, polished at different speeds, ground and autoglazed, polished and autoglazed, autoglazed and polished, polished with loose (paste) and bonded abrasives, or overglazed. Simulated clinical polishing was performed on the ceramic specimens by using a customized polishing apparatus that allowed independent control over the relevant polishing parameters (abrasive hardness, applied load, linear speed, rotational velocity, and wheel stiffness). Flexural strength (MPa) was measured with a 4-point bending test, and subjective surface roughness was assessed with scanning electron microscopy. Autoglazing was performed at various stages of the polishing sequence to determine the effects of polishing on surface stresses. Mean values, standard deviations, independent-sample t tests, 1-way and 2-way analyses of variance, Dunnett t tests and Kruskal-Wallis tests were applied to the data (α=.05). Results Under a clinical load of 0.6 N for a coarse polishing wheel, 1.0 N for a medium polishing wheel, and 1.3 N for a fine polishing wheel, a linear speed of 499 mm/min, and a rotational velocity of 10,000 rpm, the use of clinical polishing instruments did not affect the flexural strength of the aluminous ceramics studied ( P =.274). At higher rotational velocity (20,000 rpm), specimens polished with the diamond polishing system produced statistically weaker specimens compared with those that had been polished at 10,000 rpm ( P =.019). Autoglazing treatment of the diamond-polished specimens did not reverse the strength degradation ( P =.125). Conversely, diamond polishing of the autoglazed specimens resulted in significant flexural strength reduction ( P =.029). Fine-diamond-bonded abrasive significantly reduced flexural strength ( P =.025). Conclusions Simulated clinical polishing at 10,000 rpm did not appear to substantially strengthen or weaken the ceramic specimens. Polishing at 20,000 rpm reduced flexural strength of the ceramic bars.

39 citations

Journal ArticleDOI
TL;DR: The Weibull modulus of the glazed group was the largest (16.3), and the surface roughness was found to influence the WeibULL modulus.
Abstract: The purpose of the present study was to test the hypothesis that glazing will affect the Weibull modulus or the mean flexural strength of dental porcelain. Four groups (n = 30) of specimens were prepared from feldspathic body porcelain (Vita VMK 68). The specimens were tested in groups: original glazed condition (control), and after grinding separately with 1,000-grit, 600-grit, and 100-grit silicon carbide abrasive. A three-point flexure test was undertaken for each specimen. The flexural strength of porcelain and the Weibull modulus were then obtained using a computer program and load-to-failure data. Flexural strength decreased as surface roughness increased, and the differences were significant (p < 0.05), as determined by Tukey’s multiple range test, except between the 600-grit group and the 100-grit group. The Weibull modulus of the glazed group was the largest (16.3), and the surface roughness was found to influence the Weibull modulus.

38 citations