Claudinei dos Santos

Evaluation of the reliability of Si3N4-Al2O3 -CTR2O3 ceramics through Weibull analysis

The objective of this work has been to compare the reliability of two Si3N4 ceramics, with Y2O3/Al2O3 or CTR2O3/Al2O3 mixtures as additives, in regard to their 4-point bending strength and to confirm the potential of the rare earth oxide mixture, CTR2O3, produced at FAENQUIL, as an alternative, low cost sinter additive for pure Y2O3 in the sintering of Si3N4 ceramics. The oxide mixture CTR2O3 is a solid solution formed mainly by Y2O3, Er2O3, Yb2O3 and Dy2O3 with other minor constituents and is obtained at a cost of only 20% of pure Y2O3. Samples were sintered by a gas pressure sintering process at 1900 °C under a nitrogen pressure of 1.5 MPa and an isothermal holding time of 2 h. The obtained materials were characterized by their relative density, phase composition and bending strength. The Weibull analysis was used to describe the reliability of these materials. Both materials produced presented relative densities higher than 99.5%t.d., b-Si3N4 and Y3Al5O12 (YAG) as cristalline phases and bending strengths higher than 650 MPa, thus demonstrating similar behaviors regarding their physical, chemical and mechanical characteristics. The statistical analysis of their strength also showed similar results for both materials, with Weibull moduli m of about 15 and characteristic stress values so of about 700 MPa. These results confirmed the possibility of using the rare earth oxide mixture, CTR2O3, as sinter additive for high performance Si3N4 ceramics, without prejudice of the mechanical properties when compared to Si3N4 ceramics sintered with pure Y2O3.

Effects of extreme cooling methods on mechanical properties and shear bond strength of bilayered porcelain/3Y-TZP specimens.

OBJECTIVES This study investigated the effect of extreme cooling methods on the flexural strength, reliability and shear bond strength of veneer porcelain for zirconia. METHODS Vita VM9 porcelain was sintered on zirconia bar specimens and cooled by one of the following methods: inside a switched-off furnace (slow), at room temperature (normal) or immediately by compressed air (fast). Three-point flexural strength tests (FS) were performed on specimens with porcelain under tension (PT, n=30) and zirconia under tension (ZT, n=30). Shear bond strength tests (SBS, n=15) were performed on cylindrical blocks of porcelain, which were applied on zirconia plates. Data were submitted to one-way ANOVA and Tukeys post hoc tests (p<0.05). Weibull analysis was performed on the PT and ZT configurations. RESULTS One-way ANOVA for the PT configuration was significant, and Tukeys test revealed that fast cooling leads to significantly higher values (p<0.01) than the other cooling methods. One-way ANOVA for the ZT configuration was not significant (p=0.06). Weibull analysis showed that normal cooling had slightly higher reliability for both the PT and ZT configurations. Statistical tests showed that slow cooling decreased the SBS value (p<0.01) and showed less adhesive fracture modes than the other cooling methods. CLINICAL SIGNIFICANCE Slow cooling seems to affect the veneer resistance and adhesion to the zirconia core; however, the reliability of fast cooling was slightly lower than that of the other methods.

A contribution of X ray diffraction analysis in the determination of creep of Si3N4 ceramics

The understanding of the creep behavior of silicon nitride (Si3N4) is extremely complex because of a large number of parameters influencing simultaneously the creep deformation of the materials. In general, the main creep mechanisms acting in these materials are grain boundary sliding or materials transport by solution-precipitation process. In this work, the creep behavior has been monitored by X ray diffraction analysis, determining the peak intensity ratio of the (101) and (210) planes of b-Si3N4. This characterization technical, allied the microstructural analysis can contribute to determination of creep mechanisms acting in material. The b-Si3N4 grains are highly elongated with aspect ratios ranging between 3 and 11. Therefore, the intensity of the peaks related to the basal plane (101) tends to be higher when compared to the lateral planes (210). During creep deformation occurs alignment of the elongated b-Si3N4 grains in the plane parallel to the direction of the applied load, reflecting in the peak intensity ratio. Crept samples presented variations in the (101)/(210) peak intensity ratio of b-Si3N4 indicating that grain rotation can to be contributing with creep deformation. In this way, the use of X ray diffractometry as a mean to characterize microstructural changes during creep has been shown successfully.

Effect of air-abrasion regimens and fine diamond bur grinding on flexural strength, Weibull modulus and phase transformation of zirconium dioxide

Purpose This study evaluated the effect of air abrasion and polishing regimens on the flexural strength of yttrium stabilized polycrystalline tetragonal zirconia (Y-TZP). Methods From Y-TZP blocks (InCeram 2000 YZ Cubes; Vita Zahnfabrik, Bad Säckingen, Germany) 120 bars (25 mm × 4 mm × 1.2 mm) were obtained according to ISO 6872:2008 and randomly divided into 4 groups: Group C: (control) without surface treatment (n = 30); Group APA: Air abrasion with aluminum oxide (44 µm) (n = 30); Group SC: Silica-coating (CoJet, 30 µm) (n = 30); Group FD: Fine diamond bur (n = 30). Subsequently, all specimens were subjected to 4-point bending test (in distilled water at 37°C) in a universal testing machine (EMIC DL 1000; São José dos Pinhais, Paraná, Brazil); cross-head speed: 0.5 mm/min). The characteristic strength (σ0) of each specimen was obtained from the flexural strength test and evaluated using Weibull analysis. X-ray diffraction analysis was utilized to quantity the monoclinic phase. The surface topography of specimens was analyzed using 3D optical profilometer and scanning electron microscopy (SEM) after surface conditioning methods. The flexural strength data (σ4p) were statistically analyzed by 1-way ANOVA, Tukey test (α = 0.05) and Weibull (m = modulus, σ0 = characteristic strength) were calculated. Results The mean ± standard deviations (MPa) of the groups were as follows: C: 1196.2 ± 284.2a; APA: 1369.7 ± 272.3a; SC: 1207.1 ± 229.7a and FD: 874.4 ± 365.4b. The values (m) and (σ0) were as follows: C: 4.5 and 1308.12; APA: 5.9 and 1477.88; SC: 6.0 and 1300.28; and FD: 2.6 and 985.901, respectively. Conclusions Air particle abrasion with neither silica nor alumina showed significant difference compared to the control group but grinding with fine diamond bur impaired the flexural strength of the zirconia tested.

Characterization of ceramic powders used in the inCeram systems to fixed dental Prosthesis

InCeram (Vita Zahnfabrik- Germany) is known as a high strength ceramic being used for core crowns and for fixed partial denture frameworks. InCeram system consists of slip-casting technique which is used for to build the framework, which is then pre-sintered obtaining an open-pore microstructure. The material gains its strength by infiltration of the lanthanum glass into the porous microstructure. In this work, commercial alumina (Al2O3), alumina-zirconia (Al2O3-ZrO2) and glasses lanthanum oxide-rich powders, used in InCeram system, were characterized, using x ray diffraction, dilatometry and scanning electron microscopy. The characteristics of these powders were related aiming to consider their substitution for new ceramic materials.

Effect of Al2O3 Addition on the Mechanical Properties of Biocompatible ZrO2-Al2O3 Composites

In this work, the effects of alumina additions on the properties of the ZrO2-Al2O3 ceramic composites were investigated. Samples of ZrO2 with Al2O3 additions varying between 0 and 30wt-% were prepared. The powder mixtures were milled, compacted by uniaxial cold pressing and sintered at 16000C, in air, for 2 hours. The sintered samples were characterized by their relative density, phase composition and microstructure. As mechanical properties at room temperature, their Vickers hardness and fracture toughness were determined: In all sintering conditions and Al2O3 amounts, the samples presented relative density higher that 99%. The Al2O3 addition produces a linear increase of the hardness, reaching values between 1350 and 1610 HV for the addition of 0 and 30% of alumina, respectively. The fracture toughness was near to 8 MPam1/2 in all conditions. The phase composition, microstructure and relative density were correlated in order to interpret the mechanical properties obtained.

Microstructural evidence of beryllium in commercial dental Ni-Cr alloys

The focus of this work was to determine microstructural features in commercial Ni-Cr alloys which could be used to identify indirectly the presence of beryllium. Thus, eight commercial alloys were characterized by chemical analysis, thermal analysis, X-ray diffraction (XRD), scanning electron microscopy - back-scattered electron images (SEM/BSE), energy-dispersive spectroscopy (EDS). The results indicate that the presence of beryllium can be inferred from microstructural analysis via XRD and SEM/BSE. The X-ray diffractograms of the beryllium-containing alloys showed clearly the existence of the NiBe intermetallic phase. SEM/BSE images of these alloys show a very characteristic eutectic microstructure which also indicates the presence of this element. These characteristics are not observed in the beryllium-free alloys.

Degradation of Y2O3-Stabilized ZrO2 Ceramics in Artificial Saliva: ICP Analysis of Dissolved Y3+ and Zr4+ Ions

ZrO2 ceramics are one of the most important materials used in dental prostheses production, due to their excellent mechanical resistance and chemical inertness in the mouth environment. Nevertheless, the combination of low pHs and fluoride presence is able to reduce the chemical stability of these ceramics. In this work, the resistance of commercial blocks of micrometric and nanometric sized Y2O3-stabilized ZrO2 ceramics (ProtMat Materiais Avançados® and Ivoclar®) was evaluated in Fusayama artificial saliva of different pHs with (and without) the presence of fluoride ions. The study was based on the analysis of the amount of Zr4+ and Y3+ ions dissolved in the artificial saliva after different exposure times using ICP OES technique. The XRD technique was also employed to investigate the phase transformations occurring during the degradation process in artificial saliva. The micrometric sized ZrO2 ceramics presented higher resistance in the artificial saliva than nanometric sized structures.

Properties of Nanostructured 3Y-TZP Blocks Used for CAD/CAM Dental Restoration

Several CAD/CAM systems are available to dental prosthesis laboratories which can be used to fabricate all-ceramic copings and frameworks. The use of these systems presents low demand, due principally the high blocks ceramics cost used for theses systems. Usually, these ceramic blocks are sintered at high temperatures, between 1450 and 15500C, resulting in micrometric ZrO2 microstructure. A considerable innovation in these ceramics systems used in CAD/CAM applications was introduced by the use of nanometric-tetragonal ZrO2 blocks, which are sintered at low sintering temperatures resulting in nanometric grains morphology and improved mechanical properties. The purpose of the present work is to characterize the mechanical properties of nanoparticled zirconium oxide blocks comparing with commercial micrometric ceramic parts. XRD patterns showed that the blocks have only the tetragonal-ZrO2 as crystalline phase. The tetragonal-monoclinic transformation phase was responsible for the excellent mechanical properties. Nanometric blocks presented hardness of 13GPa, fracture toughness of 11MPam1/2 , bending strength of 1020MPa and Weibull modulus, m=14, while micrometric ZrO2 blocks similar hardness, fracture toughness 8.5MPam1/2, bending strength of 850MPa and Weibull modulus of 10.

Sinterização e propriedades mecânicas do compósito Y-TZP/Al2O3

In this work, the influence of Al2O3 addition on the tetragonal ZrO2 ceramic properties was studied, aiming the use of this ceramic as dental materials. Samples containing 0, 10, 20 e 30 wt.% were cold pressed (80 MPa), and sintered in air, at 1500, 1550 e 1600 °C, for 120 min. The effects of the sintering conditions were analyzed by X-ray diffraction and Scanning Electron Microscopy. Hardness and fracture toughness were determined by Vickers indentation method. Furthermore, samples were submitted to the 4 point bending test. Dense ceramics were obtained, with hardness values varying around 13.4 and 15.8 GPa, depending on the Al2O3 content added in the ZrO2 matrix. On the other hand, no significant difference in the fracture toughness was detected by the Al2O3 addition, with KIC around 8 MPa.m1/2, in all conditions. The high hardness, fracture toughness, flexural strength (sf = 685 MPa) and reliability (m=11) presented by the ceramic composites with 20wt.% Al2O3, indicates the potential of these materials as bioceramics for dental materials.