▶ In all cases, microstructural examination of crept samples showed that existing phases at grain boundaries were associated to the deformation processes. This highlights the importance of the ...presence and the amount of grain boundary glass. ▶ Crystallization of the remnant phase during heat treatment in nitrogen atmosphere gives rise to further crystallization of the new phases in the Nd–Si–O–N system such as Nd
4Si
3O
12 and Nd
2Si
3O
3N
4. A consequence of this crystallization is a significant reduction in stress exponents and creep rates for the heat treated samples. ▶ Diffusional creep may prevail for lower temperatures, low glass content and stresses. Cavitation would start to operate and become increasingly prevalent with increase in stress, temperature and decrease in crystallinity of the grain boundary phase.
At the present work, samples of silicon nitride with 12
wt% of yttrium/neodymium oxides mixture were formed by gas-pressure sintering. Pos sintering heat treatments in nitrogen with a stepwise temperature variation were performed in some samples. The short term compressive creep tests were undertaken in an argon atmosphere, over a stress range of 50–300
MPa and temperature range of 1200–1400
°C. Values of stress exponents near unity for (i) low temperature testing in all materials and (ii) all temperatures for heat treated samples suggest diffusion accommodation processes, involving ambipolar diffusion of ionic species in the grain boundary phases. Crystallization of the remnant phase during heat treatment in a nitrogen atmosphere gives rise to further formation of new phases in the Nd–Si–O–N system such as Nd
4Si
3O
12 and Nd
2Si
3O
3N
4. A consequence of this crystallization is a significant reduction in stress exponents and creep rates for the heat treated samples. The wedge crack observed after creep testing at specimens in its as-sintered condition may be related to the increased probability of cavitation in the second phase glassy layers at triple point junction.
The objective of this work was the development and characterization of a ceramic composite based on (Ce,Y)-TZP/Al2O3 aiming an application on dental implants, comparing it with conventional ...monolithic 3Y-TZP ceramics, currently used for the same type of application.
Ceramic samples, 3Y-TZP (n = 40) and (Ce,Y)-TZP/Al2O3 (n = 40), were sintered at 1500 °C - 2h and characterized by relative density, X-Ray diffraction (XRD) and microstructure. Then, the samples of both materials were divided into two groups: 1) samples with original (as sintered) surfaces; 2) samples with conditioned, polished, surfaces. All samples were submitted to hydrothermal degradation tests, on an autoclave (134 °C - 2 bar), for 10 h in artificial saliva. The degraded samples were characterized by XRD and the polished group were also characterized by their elastic moduli, Vickers hardness and fracture toughness (Vickers indentation method). Both groups were also submitted to a flexural strength test, 3B–P testing, for which the data were interpreted using Weibull statistics.
All sintered specimens presented nearly full densification. After the hydrothermal degradation tests, 3Y-TZP samples presented 16.4% of monoclinic (m)-ZrO2 phase while the composite samples withheld 100% of tetragonal (t)-ZrO2 phase. Both materials presented equiaxial ZrO2 grains with an average size of 0.48 ± 0.17 μm and 0.75 ± 0.22 μm, respectively, for the monolithic and composite ceramics. In the composites, is observed the presence of well distributed Al2O3 grains on the ZrO2 matrix, in two distinct morphologies: equiaxial grains and platelets. The composites (Ce,Y)-TZP/Al2O3 presented average values of elastic moduli, Vickers hardness and fracture toughness of 228.3 ± 6.5 GPa, 1427 ± 46 HV e 11.3 ± 0.4 MPa m1/2, respectively. An inversely proportional relationship is observed between the roughness and the bending strength, since the 3Y-TZP samples presented a average strength of 860.7 ± 81 MPa (as-sintered) and 965.4 ± 93 MPa (polished) while the (Ce,Y)-TZP/Al2O3 composites presented average strength of 810.6 ± 147 MPa (as-sintered) and 952.6 ± 88 MPa (polished).
The composites (Ce,Y)-TZP/Al2O3 showed high resistance to degradation in saliva and adequate properties for use as dental implants. Values of flexural strength (>950 MPa) and Weibull modulus (m > 10) were similar to the conventional 3Y-TZP ceramics. Moreover, its hardness, elastic modulus and fracture toughness were higher than those obtained for 3Y-TZP. The expressive values of KIC obtained for (Ce,Y)-TZP/Al2O3 composites are results of association of different toughening mechanisms acting simultaneously in the material.
•Stress corrosion cracking identified as cause of insulator pin failure in tower 639.•Failure occurred at the highest towers of the Mojú river crossing.•Several analyses were conducted to determine ...the main cause of the pin failure.•Atmospheric circulation traps pollutants approaches have been proposed.•Extractive activities created a microclimate around the top of the highest towers.
This work investigates the causes of the failure of an insulator pin at tower 639 of the 500 kV transmission power line (TL) Tucuruí-Vila do Conde in the State of Pará, Brazil. Such a tower is of one the highest in this TL and is located at one bank of the large Moju river, in the Amazon Forest region. The failure was caused by stress corrosion cracking of the galvanized 1045 steel pin. Detailed investigation of the failed pin, of the design of the insulators and of the maintenance procedure did not reveal errors that could indicate incompetence, recklessness, or technical negligence. After several analysis (chemical, microscopic and environmental), it was concluded that the root cause of the failure proved to be associated with a novel phenomenon. Illegal extractive activities and wildfires generated pollution, and the lack of natural filters/obstacles allowed the generation of a polluted microclimate at the top of the higher towers mounted at the two riverbanks. This accelerated the stress corrosion of the pin.
This study aimed to identify the potential use of the ceramic composite ZrO
(CeO
)-Al
O
as a dental implant due to its intrinsic geometry and different masticatory loads based on finite element ...simulations. Ceramic samples were sintered at 1500 °C-2h, and characterized: The mechanical properties of the ceramic composite (hardness, fracture toughness, flexural strength, Young's Modulus, and Poisson ratio) were determined, in addition to the relative density and its structural characteristics. Commercial dental implant designs (incisal and third-molar) on CAD models were used in this study as an initial implant geometry applied in a typical simulated mandible anatomy. Finite element models were generated for implant geometries using CAD and CAE techniques. Loading cases were considered based on different intensities (100-500 N) and orientation angles to the implant axis (0° and 45°) to reproduce human masticatory conditions. For comparison purposes, the numerical predictions were compared with finite element simulations of gold-standard titanium implants. Ce-TZP/Al
O
sintered ceramics showed flexural strength of 952.6 ± 88 MPa, hardness and fracture toughness of 1427 ± 46 HV and 11.3 ± 0.4 MPa m
, respectively, beside Young's modulus of 228.3 ± 65 GPa and Poisson ratio of 0.28. For both Ce-TZP/Al
O
dental implant geometries, the implant prototypes showed adequate mechanical behavior regardless of the masticatory load value or the orientation angle applied in the simulations: All finite element predictions are lower than the values established by Mohr Coulomb's failure criterion, allowing the feasibility, preliminarily, of the proposed ceramics for dental implant applications without fracture risk.
Silicon nitride compressive creep behavior in argon atmosphere da Silva, Cosme Roberto Moreira; Neto, Flaminio Levy; Araújo, José Alexander ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
06/2008, Letnik:
485, Številka:
1
Journal Article
Recenzirano
Gas pressure sintering, using pure neodymium oxide and a mixture of neodymium oxide and yttrium oxide as sintering aids, was used to process silicon nitride samples. The short-term compressive creep ...behavior in argon was evaluated over a stress range of 50–300
MPa, and temperature range of 1250–1400
°C. Microstructural analysis by X-ray diffractometry and transmission electron microscopy showed that secondary crystalline phases, which form from the remnant glass, are dependent upon composition and percentage of additives. Stress exponent values near to unity were obtained for lower stress and temperature testing and materials with low glass content, suggesting grain boundary diffusion accommodation processes. Cavitation will thereby become prevalent with increase in stress, temperature and amount of the grain boundary phase, and decrease in the degree of crystallization of the intergranular phase.
The objective of this work was the development and characterization of a ceramic composite based on (Ce,Y)-TZP/Al
O
aiming an application on dental implants, comparing it with conventional monolithic ...3Y-TZP ceramics, currently used for the same type of application.
Ceramic samples, 3Y-TZP (n = 40) and (Ce,Y)-TZP/Al
O
(n = 40), were sintered at 1500 °C - 2h and characterized by relative density, X-Ray diffraction (XRD) and microstructure. Then, the samples of both materials were divided into two groups: 1) samples with original (as sintered) surfaces; 2) samples with conditioned, polished, surfaces. All samples were submitted to hydrothermal degradation tests, on an autoclave (134 °C - 2 bar), for 10 h in artificial saliva. The degraded samples were characterized by XRD and the polished group were also characterized by their elastic moduli, Vickers hardness and fracture toughness (Vickers indentation method). Both groups were also submitted to a flexural strength test, 3B-P testing, for which the data were interpreted using Weibull statistics.
All sintered specimens presented nearly full densification. After the hydrothermal degradation tests, 3Y-TZP samples presented 16.4% of monoclinic (m)-ZrO
phase while the composite samples withheld 100% of tetragonal (t)-ZrO
phase. Both materials presented equiaxial ZrO
grains with an average size of 0.48 ± 0.17 μm and 0.75 ± 0.22 μm, respectively, for the monolithic and composite ceramics. In the composites, is observed the presence of well distributed Al
O
grains on the ZrO
matrix, in two distinct morphologies: equiaxial grains and platelets. The composites (Ce,Y)-TZP/Al
O
presented average values of elastic moduli, Vickers hardness and fracture toughness of 228.3 ± 6.5 GPa, 1427 ± 46 HV e 11.3 ± 0.4 MPa m
, respectively. An inversely proportional relationship is observed between the roughness and the bending strength, since the 3Y-TZP samples presented a average strength of 860.7 ± 81 MPa (as-sintered) and 965.4 ± 93 MPa (polished) while the (Ce,Y)-TZP/Al
O
composites presented average strength of 810.6 ± 147 MPa (as-sintered) and 952.6 ± 88 MPa (polished).
The composites (Ce,Y)-TZP/Al
O
showed high resistance to degradation in saliva and adequate properties for use as dental implants. Values of flexural strength (>950 MPa) and Weibull modulus (m > 10) were similar to the conventional 3Y-TZP ceramics. Moreover, its hardness, elastic modulus and fracture toughness were higher than those obtained for 3Y-TZP. The expressive values of K
obtained for (Ce,Y)-TZP/Al
O
composites are results of association of different toughening mechanisms acting simultaneously in the material.
The creep behaviour of Si3N4 was monitored by XRD, determining the peak intensity ratio of the (101) and (210) planes of beta-Si3N4. This characterisation, allied to the microstructural analysis, can ...contribute to determination of creep mechanisms acting in the material. The beta-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, alignment of the elongated beta-Si3N4 grains occurs in the plane parallel to the direction of the applied load, reflecting the peak intensity ratio. Crept samples presented variations in the (101) /(210) peak intensity ratio of beta-Si3N4 indicating that grain rotation can contribute to creep deformation. In this way, the use of XRD to characterise microstructural changes during creep was demonstrated. 17 refs.
In this work was obtained solid electrolytes of fully stabilized zircônia with doped of 10 and 12 mol% of Re2O3 (mixed oxides rare earth), for use in oxygen sensors and or fuel cells. The specimens ...were prepared by uniaxial pressing and sintered using two heating schedules, S1 and S2. Impedance diagrams show that the crystalline phases and the grain size change the electrical behavior of the ceramics. The sample with the best electrical performance was obtained with 10 mol% doped and was sintered with the curve S1. The value of the total conductivity of this sample was 2,85x10-3W-1.cm-1 (taken at 600 ° C). When making a comparison between this values of conductivity with the reported in the literature is identified similarity with or traditional system zirconia-yttria the most widely used commercially as electrolyte oxygen sensors and fuel cells, confirming the potential use of the oxides mixed rare earth for these specific applications.
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 s o 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.
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