Humberto Naoyuki Yoshimura

Flexural strength and failure modes of layered ceramic structures

OBJECTIVE To evaluate the effect of the specimen design on the flexural strength (σ(f)) and failure mode of ceramic structures, testing the hypothesis that the ceramic material under tension controls the mechanical performance of the structure. METHODS Three ceramics used as framework materials for fixed partial dentures (YZ--Vita In-Ceram YZ; IZ--Vita In-Ceram Zirconia; AL--Vita In-Ceram AL) and two veneering porcelains (VM7 and VM9) were studied. Bar-shaped specimens were produced in three different designs (n=10): monolithic, two layers (porcelain-framework) and three layers (TRI) (porcelain-framework-porcelain). Specimens were tested for three-point flexural strength at 1MPa/s in 37°C artificial saliva. For bi-layered design, the specimens were tested in both conditions: with porcelain (PT) or framework ceramic (FT) layer under tension. Fracture surfaces were analyzed using stereomicroscope and scanning electron microscopy (SEM). Youngs modulus (E) and Poissons ratio (ν) were determined using ultrasonic pulse-echo method. Results were statistically analyzed by Kruskal-Wallis and Student-Newman-Keuls tests. RESULTS Except for VM7 and VM9, significant differences were observed for E values among the materials. YZ showed the highest ν value followed by IZ and AL. YZ presented the highest σ(f). There was no statistical difference in the σ(f) value between IZ and IZ-FT and between AL and AL-FT. σ(f) values for YZ-PT, IZ-PT, IZ-TRI, AL-PT, AL-TRI were similar to the results obtained for VM7 and VM9. Two types of fracture mode were identified: total and partial failure. SIGNIFICANCE The mechanical performance of the specimens was determined by the material under tension during testing, confirming the study hypothesis.

Two-body abrasive wear of Al–SiC composites

Abstract The two-body abrasive wear behaviour of aluminium matrix composites reinforced with silicon carbide particles has been investigated. The metal matrix composites were fabricated by a powder metallurgy route involving a final hot extrusion step, with Al 1100 matrix and α-SiCp reinforcement with mean sizes of 10, 27 and 43 μm, in the proportions of 5, 10 and 20 vol.%. Using a pin-on-disc apparatus, two-body abrasion tests were carried out against silicon carbide and alumina abrasives with four different grit sizes. The microstructural characterisations were performed using light microscopy. The dominant wear mechanisms were evaluated using scanning electron microscopy. It was shown that SiCp particles reinforcement improved the abrasion resistance against all the abrasives used. This improvement generally was higher against alumina than against silicon carbide. The abrasion resistance increased with an increase in the volume fraction and size of SiCp particles reinforcement. The results also showed that the abrasion resistance decreased with increasing the relative abrasive penetration depth, until a critical value; above this limit, the abrasion resistance was generally independent of the penetration depth.

Effect of the microstructure on the lifetime of dental ceramics

OBJECTIVES To evaluate the effect of the microstructure on the Weibull and slow crack growth (SCG) parameters and on the lifetime of three ceramics used as framework materials for fixed partial dentures (FPDs) (YZ - Vita In-Ceram YZ; IZ - Vita In-Ceram Zirconia; AL - Vita In-Ceram AL) and of two veneering porcelains (VM7 and VM9). METHODS Bar-shaped specimens were fabricated according to the manufacturers instructions. Specimens were tested in three-point flexure in 37°C artificial saliva. Weibull analysis (n=30) and a constant stress-rate test (n=10) were used to determine the Weibull modulus (m) and SCG coefficient (n), respectively. Microstructural and fractographic analyzes were performed using SEM. ANOVA and Tukeys test (α=0.05) were used to statistically analyze data obtained with both microstructural and fractographic analyzes. RESULTS YZ and AL presented high crystalline content and low porosity (0.1-0.2%). YZ had the highest characteristic strength (σ(0)) value (911MPa) followed by AL (488MPa) and IZ (423MPa). Lower σ(0) values were observed for the porcelains (68-75MPa). Except for IZ and VM7, m values were similar among the ceramic materials. Higher n values were found for YZ (76) and AL (72), followed by IZ (54) and the veneering materials (36-44). Lifetime predictions showed that YZ was the material with the best mechanical performance. The size of the critical flaw was similar among the framework materials (34-48μm) and among the porcelains (75-86μm). SIGNIFICANCE The microstructure influenced the mechanical and SCG behavior of the studied materials and, consequently, the lifetime predictions.

Porosity dependence of elastic constants in aluminum nitride ceramics

Aluminum nitride is a relatively new engineering ceramic and its mechanical properties have not been extensively studied. In this work, the effects of porosity (~0 to 38%) on sonic velocities, Poissons ratio, and elastic moduli of partially sintered A1N, both pure and with additives, were investigated. The elastic constants, determined by the ultrasonic pulse-echo method, were significantly influenced by the porosity, but not by the different types of additives used (2 wt. (%) of CaCO3, CaO, and Y2O3). All elastic constants evaluated decreased with increasing porosity, but the rates of decrease were higher for elastic moduli compared do sonic velocities and Poissons ratio. The results were analyzed in the light of stress concentration and loading bearing area models proposed in the literature.

Effect of processing induced particle alignment on the fracture toughness and fracture behavior of multiphase dental ceramics

OBJECTIVE To investigate the processing induced particle alignment on fracture behavior of four multiphase dental ceramics (one porcelain, two glass-ceramics and a glass-infiltrated-alumina composite). METHODS Disks (Ø12 mm x 1.1mm-thick) and bars (3 mm x 4 mm x 20 mm) of each material were processed according to manufacturer instructions, machined and polished. Fracture toughness (K(Ic)) was determined by the indentation strength method using 3-point bending and biaxial flexure fixtures for the fracture of bars and disks, respectively. Microstructural and fractographic analyses were performed with scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. RESULTS The isotropic microstructure of the porcelain and the leucite-based glass-ceramic resulted in similar fracture toughness values regardless of the specimen geometry. On the other hand, materials containing second-phase particles with high aspect ratio (lithium disilicate glass-ceramic and glass-infiltrated-alumina composite) showed lower fracture toughness for disk specimens compared to bars. For the lithium disilicate glass-ceramic disks, it was demonstrated that the occurrence of particle alignment during the heat-pressing procedure resulted in an unfavorable pattern that created weak microstructural paths during the biaxial test. For the glass-infiltrated-alumina composite, the microstructural analysis showed that the large alumina platelets tended to align their large surfaces perpendicularly to the direction of particle deposition during slip casting of green preforms. SIGNIFICANCE The fracture toughness of dental ceramics with anisotropic microstructure should be determined by means of biaxial testing, since it results in lower values.

Mechanical properties and porosity of dental glass-ceramics hot-pressed at different temperatures

The objective of this work was to evaluate biaxial-flexural-strength (σf), Vickers hardness (HV), fracture toughness (KIc), Youngs modulus (E), Poissons ratio (ν) and porosity (P) of two commercial glass-ceramics, Empress (E1) and Empress 2 (E2), as a function of the hot-pressing temperature. Ten disks were hot-pressed at 1065, 1070, 1075 and 1080 °C for E1; and at 910, 915, 920 and 925 °C for E2. The porosity was measured by an image analyzer software and sf was determined using the piston-on-three-balls method. KIc and HV were determined by an indentation method. Elastic constants were determined by the pulse-echo method. For E1 samples treated at different temperatures, there were no statistical differences among the values of all evaluated properties. For E2 samples treated at different temperatures, there were no statistical differences among the values of σf, E, and ν, however HV and KIc were significantly higher for 910 and 915 °C, respectively. Regarding P, the mean value obtained for E2 for 925 °C was significantly higher compared to other temperatures.

Effect of ion exchange on strength and slow crack growth of a dental porcelain

OBJECTIVES To determine the effect of ion exchange on slow crack growth (SCG) parameters (n, stress corrosion susceptibility coefficient, and sigma(f0), scaling parameter) and Weibull parameters (m, Weibull modulus, and sigma(0), characteristic strength) of a dental porcelain. METHODS 160 porcelain discs were fabricated according to manufacturers instructions, polished through 1 microm and divided into two groups: GC (control) and GI (submitted to an ion exchange procedure using a KNO3 paste at 470 degrees C for 15 min). SCG parameters were determined by biaxial flexural strength test in artificial saliva at 37 degrees C using five constant stress rates (n=10). 20 specimens of each group were tested at 1 MPa/s to determine Weibull parameters. The SPT diagram was constructed using the least-squares fit of the strength data versus probability of failure. RESULTS Mean values of m and sigma(0) (95% confidence interval), n and sigma(f0) (standard deviation) were, respectively: 13.8 (10.1-18.8) and 60.4 (58.5-62.2), 24.1 (2.5) and 58.1 (0.01) for GC and 7.4 (5.3-10.0) and 136.8 (129.1-144.7), 36.7 (7.3) and 127.9 (0.01) for GI. Fracture stresses (MPa) calculated using the SPT diagram for lifetimes of 1 day, 1 year and 10 years (at a 5% failure probability) were, respectively, 31.8, 24.9 and 22.7 for GC and 71.2, 60.6 and 56.9 for GI. SIGNIFICANCE For the porcelain tested, the ion exchange process improved strength and resistance to SCG, however, the materials reliability decreased. The predicted fracture stress at 5% failure probability for a lifetime of 10 years was also higher for the ion treated group.

Al2O3/GdAlO3 fiber for dental porcelain reinforcement

The aim of this study was to test the hypothesis that the addition of continuous or milled GdAlO3/Al2O3 fibers to a dental porcelain increases its mechanical properties. Porcelain bars without reinforcement (control) were compared to those reinforced with long fibers (30 vol%). Also, disk specimens reinforced with milled fibers were produced by adding 0 (control), 5 or 10 vol% of particles. The reinforcement with continuous fibers resulted in significant increase in the uniaxial flexural strength from 91.5 to 217.4 MPa. The addition of varied amounts of milled fibers to the porcelain did not significantly affect its biaxial flexural strength compared to the control group. SEM analysis showed that the interface between the continuous fiber and the porcelain was free of defects. On the other hand, it was possible to note the presence of cracks surrounding the milled fiber/porcelain interface. In conclusion, the reinforcement of the porcelain with continuous fibers resulted in an efficient mechanism to increase its mechanical properties; however the addition of milled fibers had no significant effect on the material because the porcelain was not able to wet the ceramic particles during the firing cycle.

Sintering of 6H(α)-SiC and 3C(β)-SiC powders with B4C and C additives

The sintering behavior of three fine industrial SiC powders (two 6H(α)-type and one 3C(β)-type) has been comparatively investigated. The powders were pressureless sintered with B4C and C additives between 1950°C and 2250°C in a high temperature dilatometer with flowing Ar atmosphere. The densification and shrinkage rate curves, polytype content, and grain growth were correlated with physical and chemical characteristics of starting powders. One of 6H(α)-type powders presented good sinterability only after extensive milling, even though it presented small average particle size, narrow particle size distribution and high specific surface area. The main difference in densification behavior among powders was the narrower shrinkage rate curve of β-SiC powder, with its maximum shifted to higher temperature. Grain growth and phase transformation simultaneously occurred. In α-SiC, 6H polytype partially transformed to 4H. This transformation was favored by aluminum impurity and resulted in a microstructure with more elongated grains. In β-SiC, 3C transformed mainly to 6H, 15R and 4H, introducing many stacking faults which resulted in elongated SiC grains.

Mechanical Properties and Microstructure of Zinc Oxide Varistor Ceramics

ZnO varistors are nonlinear resistors used as surge arresters in power transmission and distribution for the protection of electronic devices. Electrical characteristics of these materials have been extensively studied, but their mechanical behavior is not completely understood. It has been suggested that the breakdown of ZnO varistors is related to microstructure heterogeneities and processing defects. These defects are the same that usually control the mechanical strength of ceramic materials. In this work, mechanical properties (flexural strength, fracture toughness, elastic constants, and hardness) of five commercial blocks of ZnO varistors (class I) from different producers were measured and correlated to their microstructure. Pore fraction and size significantly affected the flexural strength.