Regulation of sintering temperature of zirconia-based ceramics

The results on regulation of the sintering temperature of ceramics by treating the surface of powder particles with organic liquids are presented. It is established that in some cases the sintering temperature is reduced by 350°C.Translated from Steklo i Keramika, No. 10, pp. 25 – 29, October, 1995.     

Mechanical properties of quartz ceramics at their service temperature

Translated from Steklo i Keramika, No. 10, pp. 21–22, October, 1989.     

Evaluation of the properties of some kinds of zirconia-based structural ceramics

Zirconia-based ceramic articles produced by the Klein and Syntek Keramik firms (Germany) are investigated by the methods of x-ray, luminescent, and ultrasound analyses. The density, the porosity, ultimate bending strength at room temperature, Weibull's modulus, crack resistance, and Young's modulus are investigated for specimens cut from the articles and are compared with the properties of domestic specimens based on zirconia partially stabilized by Y₂O₃. The structure and the composition are investigated by the methods of scanning electron microscopy, petrography, and x-ray and chemical analyses. The high-temperature strength is measured and the structure of fractures caused by these tests is investigated by the method of scanning electron microscopy. Translated from Ogneupory, No. 3, pp. 16 – 19, March, 1996.     

Mechanical properties of low temperature synthesized dense and fine-grained Cr2AlC ceramics

Mechanically activated hot-pressing technology was used to synthesize a fine-crystalline Cr₂AlC ceramic at relatively low temperatures. A mixture of Cr, Al and C powders with a molar ratio of 2:1.2:1 was mechanically alloyed for 3 h, and then subjected to hot pressing at 30 MPa and different temperatures for 1 h in Ar atmosphere. The results show that a dense Cr₂AlC ceramic with a grain size of about 2 μm can be synthesized at a relatively low temperature of 1100 °C. The synthesized fine-grained Cr₂AlC has a high density of 99%, which is higher than the 95% density for the coarse-grained Cr₂AlC (grain size of about 35 μm) as synthesized by hot pressing unmilled Cr, Al and C. The flexural strength, fracture toughness and Vickers hardness of the fine-grained Cr₂AlC were determined and compared with the values for the coarse-grained Cr₂AlC.     

Mechanical properties of zirconia composite ceramics

Composite materials based on 8 wt% yttria partially stabilized zirconia, with additions of gadolinium zirconate, lanthanum lithium hexaaluminate, yttrium aluminum garnet and strontium zirconate were characterized. Samples were fabricated by hot-press sintering at 1550 °C. The effect of the secondary phase content on the mechanical properties of the composites was evaluated. Hardness, elastic modulus and fracture toughness of the fabricated composites were determined by means of depth-sensitive indentation testing. The fracture toughness of the samples as determined by the indentation method was found to increase with increasing YSZ content, reaching 3 MPa·m^0.5 for samples with 80 wt% YSZ. The fracture toughness appeared to be affected by thermal expansion coefficient mismatch, crack bridging and crack deflection.     

Mechanical properties of LaFeO3 ceramics

The fracture strength, fracture toughness and apparent Young’s modulus of LaFeO₃ ceramics in the temperature region 25–800 °C are reported. The fracture strength of the material was observed to increase from 202 ± 18 MPa at room temperature to 235 ± 38 MPa at 800 °C. The room temperature fracture toughness was 2.5 ± 0.1 MPa m^1/2. The fracture toughness decreased to 2.1 ± 0.1 MPa m^1/2 at 600 °C, followed by an increase to 3.1 ± 0.3 MPa m^1/2 at 800 °C. The temperature dependence of the fracture toughness correlates well with the crystallographic strain, |(a−c)|/(a+c), and ferroelastic toughening of LaFeO₃ materials is inferred. Non-elastic stress–strain behaviour of the LaFeO₃ materials due to ferroelasticity was confirmed by cyclic compression experiments, and residual strain was observed in the material after unloading.     

Mechanical and electrical properties of superplastically foamed titania-based ceramics

Ceramic monofoams based on titania were fabricated using superplastic deformation driven by the evolution of gas from a foam agent. Titania-based polyfoam with porosities of up to 25% can be fabricated by dispersing only 1 mol% of foam agent (silicon carbide). When the mechanical strength of superplastically foamed titania was compared with that of fully densified titania and conventionally fabricated porous titania, the superplastically foamed ceramics retained 70% of the mechanical strength of the dense ceramics, while that of the conventional porous ceramic decreased to 40%. Niobium-doped semiconducting titania polyfoam was also fabricated. The electrical resistivity and affect of ambient humidity were similar to those of the dense ceramics.     

Mechanical properties of structural ceramics based on AI203

The results of an investigation of the mechanical properties of structural ceramics based on AI₂O₃ and ZrO₂ are presented. It is shown that the properties of the ceramics change markedly with the sintering temperature and the composition. The optimum composition in the form of a solid solution of oxides is determined, which gives a ceramic with elevated mechanical characteristics that sinters at 1600°C.Translated from Steklo i Keramika, No. 8, pp. 20 -22, August, 1996.     

Mechanical properties of ZrB2- and HfB2-based ultra-high temperature ceramics fabricated by spark plasma sintering

Flexural strengths at room temperature, at 1400 °C in air and at room temperature after 1 h oxidation at 1400 °C were determined for ZrB₂- and HfB₂-based ultra-high temperature ceramics (UHTCs). Defects caused by electrical discharge machining (EDM) lowered measured strengths significantly and were used to calculate fracture toughness via a fracture mechanics approach. ZrB₂ with 20 vol.% SiC had room temperature strength of 700 ± 90 MPa, fracture toughness of 6.4 ± 0.6 MPa, Vickers hardness at 9.8 N load of 21.1 ± 0.6 GPa, 1400 °C strength of 400 ± 30 MPa and room temperature strength after 1 h oxidation at 1400 °C of 678 ± 15 MPa with an oxide layer thickness of 45 ± 5 μm. HfB₂ with 20 vol.% SiC showed room temperature strength of 620 ± 50 MPa, fracture toughness of 5.0 ± 0.4 MPa, Vickers hardness at 9.8 N load of 27.0 ± 0.6 GPa, 1400 °C strength of 590 ± 150 MPa and room temperature strength after 1 h oxidation at 1400 °C of 660 ± 25 MPa with an oxide layer thickness of 12 ± 1 μm. 2 wt.% La₂O₃ addition to UHTCs slightly reduced mechanical performance while increasing tolerance to property degradation after oxidation and effectively aided internal stress relaxation during spark plasma sintering (SPS) cooling, as quantified by X-ray diffraction (XRD). Slow crack growth was suggested as the failure mechanism at high temperatures as a consequence of sharp cracks formation during oxidation.     

The bending stress distribution in bilayered and graded zirconia-based dental ceramics

The purpose of this study was to evaluate the biaxial flexural stresses in classic bilayered and in graded zirconia-feldspathic porcelain composites. A finite element method and an analytical model were used to simulate the piston-on-ring test and to predict the biaxial stress distributions across the thickness of the bilayer and graded zirconia-feldspathic porcelain discs. An axisymmetric model and a flexure formula of Hsueh et al. were used in the FEM and analytical analysis, respectively. Four porcelain thicknesses were tested in the bilayered discs. In graded discs, continuous and stepwise transitions from the bottom zirconia layer to the top porcelain layer were studied. The resulting stresses across the thickness, measured along the central axis of the disc, for the bilayered and graded discs were compared. In bilayered discs, the maximum tensile stress decreased while the stress mismatch (at the interface) increased with the porcelain layer thickness. The optimised balance between both variables is achieved for a porcelain thickness ratio in the range of 0.30–0.35. In graded discs, the highest tensile stresses were registered for porcelain rich interlayers (p=0.25) whereas the zirconia rich ones (p=8) yield the lowest tensile stresses. In addition, the maximum stresses in a graded structure can be tailored by altering compositional gradients. A decrease in maximum stresses with increasing values of p (a scaling exponent in the power law function) was observed. Our findings showed a good agreement between the analytical and simulated models, particularly in the tensile region of the disc. Graded zirconia-feldspathic porcelain composites exhibited a more favourable stress distribution relative to conventional bilayered systems. This fact can significantly impact the clinical performance of zirconia-feldspathic porcelain prostheses, namely reducing the fracture incidence of zirconia and the chipping and delamination of porcelain.     

Mechanical properties of ZrB2 ceramics determined by two laboratories

The mechanical properties for zirconium diboride (ZrB₂) were measured at two laboratories and compared. Two billets of ZrB₂ were prepared by hot-pressing commercial powder. The relative densities of the billets were >99% and with an average grain size of 5.9 ± 4.5 µm. Both laboratories prepared American Society for Testing and Materials (ASTM) C1161 B-bars for strength and ASTM C1421 bars with notch configuration A for fracture toughness. Specimens were machined by diamond grinding at the Army Research Laboratory (ARL) and electrical discharge machining (EDM) at Missouri S&T. Strength bars tested at Missouri S&T were polished to a .25 μm finish while the bars were tested as-ground at ARL. Strengths were 473 ± 79 MPa for the Missouri S&T bars and 438 ± 68 for the ARL bars while the fracture toughness values were 3.9 ± .7 MPa•m^1/2 for the Missouri S&T bars and 4.4 ± .6 MPa•m^1/2 for the ARL bars. Vickers hardness was measured by both laboratories over a range of indentation loads. The resulting hardness values were on the low end of previously reported values and were quite different from each other especially at indentation loads ≤20N. The study demonstrated that the properties of materials tested to ASTM standards at different laboratories can be compared directly. In addition, strength and fracture toughness were nearly identical for bars prepared by conventional diamond grinding or EDM.     

Mechanical properties of ceria-calcia stabilized zirconia ceramics with alumina additions

Ceria-stabilized zirconia-based composites have been developed aiming to obtain ceramic materials with enhanced hardness, strength, fracture toughness, and resistance to low temperature degradation. These composites are based on ceria-calcia stabilized zirconia (10 mol% CeO₂-1 mol% CaO TZP) and α-alumina (0?15 wt%) as a second phase. Raw materials in the form of powders were dispersed through ball milling, dried by slip casting, and subsequently grounded before being pressed and conventionally sintered at 1450 °C. Compared to the strength and hardness of 10Ce-TZP ceramics (typically 500 MPa and 6 GPa), an increase was observed for all compositions, especially for 10Ce-1CaO-5Al₂O₃ (739 MPa and 10.2 GPa). Single Edge V-Notched Beam fracture toughness values ranged from 5.1 to 6.6 MPa?√m, indicating a loss of transformability for all compositions. As in 10Ce-1CaO-TZP co-doped ceramics, the aging resistance of all alumina-containing composites was also excellent.     

Mechanical properties of sintered ZrB2-SiC ceramics

ZrB₂ ceramics containing 10-30 vol% SiC were pressurelessly sintered to near full density (relative density >97%). The effects of carbon content, SiC volume fraction and SiC starting particle size on the mechanical properties were evaluated. Microstructure analysis indicated that higher levels of carbon additions (10 wt% based on SiC content) resulted in excess carbon at the grain boundaries, which decreased flexure strength. Elastic modulus, hardness, flexure strength and fracture toughness values all increased with increasing SiC content for compositions with 5 wt% carbon. Reducing the size of the starting SiC particles decreased the ZrB₂ grain size and changed the morphology of the final SiC grains from equiaxed to whisker-like, also affecting the flexure strength. The ceramics prepared from middle starting powder with an equiaxed SiC grain morphology had the highest flexure strength (600 MPa) compared with ceramics prepared from finer or coarser SiC powders.     

Uniaxial plastic deformation in the zirconia-based nanocrystalline ceramics containing a silicate glass

Nanocrystalline yttria-stabilized tetragonal zirconia polycrystal (nc-Y-TZP) powders coated with silicate based glasses were cold isostatically pressed and sintered near to the full density (98–99%). Two glasses with different compositions were used: 93 SiO₂–1 Na₂O–6 SrO (mol%) (designated as SNS glass) and 58 SiO₂–29 Al₂O₃–13 SrO (designated as SAS glass). Uniaxial compression tests of the pure (glass-free) nc-Y-TZP samples yielded strain rates as high as 2·10⁻⁴ s⁻¹ under 60 MPa at 1300 °C. Comparable strain rates were measured in the SNS glass-containing samples, with the maximum of 3·10⁻⁴ s⁻¹ at 1300 °C under a stress of 80 MPa (5 vol.% SNS glass content). Compression tests under 100 MPa exhibited relatively high strain rates of 5·10⁻⁴ and 10⁻⁴ at 1300 °C and 1200 °C, respectively, in the 15 vol.% SAS glass samples. The strain rates measured in the SAS glass-containing samples were achieved at temperatures lower by 100 °C compared to the similar strain rates in the glass-free and SNS glass-containing samples. The microstructure of the deformed samples was similar to that of samples before deformation, within which the ultrafine and equiaxed character of the grains was preserved. Clear evidence for cooperative grain boundary sliding was observed in the SAS glass-containing samples.     

Mechanical properties and grain orientation evolution of zirconium diboride-zirconium carbide ceramics

The effect of ZrC on the mechanical response of ZrB₂ ceramics has been evaluated from room temperature to 2000 °C. Zirconium diboride ceramics containing 10 vol% ZrC had higher strengths at all temperatures compared to previous reports for nominally pure ZrB₂. The addition of ZrC also increased fracture toughness from $～3\text{.5}\text{MPa}\sqrt{\text{m}}$ for nominally pure ZrB₂ to $～4\text{.3}\text{MPa}\sqrt{\text{m}}$ due to residual thermal stresses. The toughness was comparable with ZrB₂ up to 1600 °C, but increased to $4\text{.6}\text{MPa}\sqrt{\text{m}}$ at 1800 °C and 2000 °C. The increased toughness above 1600 °C was attributed to plasticity in the ZrC at elevated temperatures. Electron back-scattered diffraction analysis showed strong orientation of the ZrC grains along the [001] direction in the tensile region of specimens tested at 2000 °C, a phenomenon that has not been observed previously for fast fracture (crosshead displacement rate = 4.0 mm min^?1) in four point bending. It is believed that microstructural changes and plasticity at elevated temperature were the mechanisms behind the ultrafast reorientation of ZrC.     

Mechanical properties of textured ceramics from muscovite–kaolinite alternate layers

An organized network of mullite anisotropic crystals embedded in a silico-aluminate matrix material is obtained at interfaces of sintered alternate layers of muscovite and kaolinite minerals. The nucleation and growth of mullite anisotropic crystals occur preferentially along the muscovite basal planes through topotactic reaction with the high temperature form of muscovite.Simultaneously to structural transformations, dehydroxylation of muscovite induces an exfoliation process, which is temperature and time dependent. The kinetics of this process was controlled using an appropriate thermal cycle and uniaxial load. During sintering, the control of mullite size mainly depends in temperature and the addition of a small quantity of low-temperature liquid phase also favors the growth of mullite. But liquid induces the weakening of the organization degree of the mullite network. Quantitative texture analysis (QTA) and SEM were used to characterize microstructural characteristics.Flexural strength, Young modulus and fracture toughness are closely related to size and organization degree of the mullite network. In general, mullite length favors the increase of strength and fracture toughness. But a high organization degree of the mullite network favors the occurrence of interconnected crystals and increases mechanical properties.     

Mechanical and microstructural properties of cameroonian bauxite ceramics for ballistic applications

Thermal reactions of bauxite, kaolin, and talc compound were investigated at 1550°C to obtain high rigidity ceramic for ballistic applications. The compressed strength and density of the bauxite ceramics with kaolin and talc substitutions varied from 195 to 455 MPa and density from 2.85 to 4.05 g/cm³. The Young’s modulus varied from 107 GPa to 222 GPa with water absorption varying from 1.4 wt% to 5.9 wt% for 0 wt% and 15 wt% substitution of kaolin. The substitution of bauxite-kaolin by talc up to 7.5 wt% contributes to the resorption of microporosity and increase the Young's modulus from 107 to 195 GPa. The XRD of bauxite ceramic with kaolin substitution showed the presence of corundum and mullite; whereas the XRD of bauxite ceramic with kaolin and talc substitution showed the presence of corundum, mullite, and spinel. The ballistic simulation with abaqus dassult SIMULA using the JH-2 model predict that an impact with velocity of 525 to 810 m/s on the 10 mm thick bauxite ceramic does not erode or damage for a projectile consisting of tungsten alloy with dimensions: 12 mm in diameter, 61.5 mm length, and 72 g of mass. The bauxite ceramics can be used for ballistic applications.     

Mechanical behavior and ultrasonic non-destructive characterization of elastic properties of cordierite-based ceramics

The structural and morphological evolutions of cordierite-based ceramics produced from stevensite/andalusite mixture sintered from 1150 to 1350 °C were studied using X-ray diffraction (XRD) and scanning electron microscopy (SEM). The mechanical behavior was investigated by three-point bending and Brazilian tests. The elastic properties were evaluated using ultrasonic non-destructive testing (NDT). XRD results revealed that the main crystalline phase formed at 1300 and 1350 °C was cordierite with traces of mullite. A linear-elastic behavior followed by brittle fracture was observed in three-point bending test with the presence of multiple discontinuities. Flexural and diametral compression strength values of cordierite sintered at 1300 °C were 39.4±4 and 21.8±2 MPa, respectively. The elastic properties such as Young's modulus (38.7–45.1 GPa), shear modulus (17.90–19.42 GPa) and Poisson ratio (0.08–1.6) of cordierite-based ceramics produced at 1350 and 1300 °C were also determined.     

Mechanical properties and recrystallization of alumina ceramics reinforced by silicon carbide whisker

The mechanical properties and recrystallization of a material consisting of Al₂O₃ and SiC whisker are considered for different hot pressing regimes. The best properties are obtained at an SiC whisker content of 20% in hot pressing in an argon medium at 1600°C (the ultimate bending strength is 600 MPa, the critical coefficient of stress intensityK _Ic=5.5 MPa · m^1/2). The bending strength measured at 1000°C is at least 400 MPa for the material pressed at 1700°C in air. It is established that the presence of SiC whisker on grain boundaries of Al₂O₃ noticeably impedes the growth of alumina grains in recrystallization for all chosen regimes of hot pressing (1650 – 1900°C, 15 – 45 min). The dominant grain size is 1 –5 µm. A mathematical model is suggested for evaluating the maximum grain size in uniform grain growth, which isD*=0.571 [1++(1+16.151/f)^1/2]d, whered, f are the diameter and the volume fraction of SiC whisker. The material can be recommended for use under conditions of thermomechanical stress and abrasive wear.Translated from Ogneupory, No. 5, pp. 8 – 12, May, 1995.     

Elevated temperature thermal properties of ZrB2-B4C ceramics

The elevated temperature thermal properties of zirconium diboride ceramics containing boron carbide additions of up to 15 vol% were investigated using a combined experimental and modeling approach. The addition of B₄C led to a decrease in the ZrB₂ grain size from 22 µm for nominally pure ZrB₂ to 5.4 µm for ZrB₂ containing 15 vol% B₄C. The measured room temperature thermal conductivity decreased from 93 W/m·K for nominally pure ZrB₂ to 80 W/m·K for ZrB₂ containing 15 vol% B₄C. The thermal conductivity also decreased as temperature increased. For nominally pure ZrB₂, the thermal conductivity was 67 W/m·K at 2000 °C compared to 55 W/m·K for ZrB₂ containing 15 vol% B₄C. A model was developed to describe the effects of grain size and the second phase additions on thermal conductivity from room temperature to 2000 °C. Differences between model predictions and measured values were less than 2 W/m·K at 25 °C for nominally pure ZrB₂ and less than 6 W/m·K when 15 vol% B₄C was added.