Mechanical properties of a transformation-toughened glass-ceramic

The fracture toughness of a transformation-toughened glass-ceramic containing 12 vol% t-ZrO₂ was measured. Heat-treatment conditions were selected to produce from the same glass composition a glass-ceramic which contained 12 vol% m-ZrO₂ in a nearly identical matrix. The transformation-toughened material was found to have a fracture toughness 65% greater than the m-ZrO₂-containing material at room temperature. However, at 77 K both materials were found to have approximately the same fracture toughness. Additionally, the susceptibility of the two glass-ceramics to fatigue was determined. No improvement in fatigue behaviour of the transformation-toughened glass-ceramic over the other glass-ceramic was observed.     

Microstructure and mechanical properties of hot pressed ZrB2-SiCp-ZrO2 composites

The present paper investigated the microstructure and mechanical properties of ZrB₂-10 vol.%SiC_p-10 vol.%ZrO₂ composites hot pressed at three temperatures. Phase transformability from t-ZrO₂ to m-ZrO₂ during fracture was analyzed through calculating the volume fractions of m-ZrO₂ and t-ZrO₂ on polished and fracture surfaces. The densification temperature was found to have a significant effect on the microstructure, phase transformation and the properties of the composites. When the composite was hot pressed at 1950 °C, the average grain size was 9.5 µm, and the fracture toughness was 4.5 MPa·m^1/2. Comparatively, when the composite was hot pressed at 1750 °C, the average grain size was 3.4 µm, and the fracture toughness increased by ~ 50% to 6.8 MPa·m^1/2.     

Effect of MgF<Subscript>2</Subscript> on hot pressed hydroxylapatite and monoclinic zirconia composites

Hydroxylapatite (HA) has been widely used in biomedical applications because of its excellent biocompatibility in the human body. A total of 25 wt% monoclinic (m) zirconia–HA composites (with and without 5 wt% MgF₂) were synthesized to investigate their mechanical properties and phase stability. In HA–m-ZrO₂ composites, HA and m-ZrO₂ reacted to form CaZrO₃ when there was no F⁻ present in the composite and m-ZrO₂ partially transformed to tetragonal ZrO₂. When MgF₂ was added into the system, it improved the thermal stability of the phases, densification, hardness, and fracture toughness of the composites and it caused the m-ZrO₂ to transform completely to t-ZrO₂ by incorporating the Mg²⁺ ions present in MgF₂ in the ZrO₂. Moreover, the stability of HA was improved by incorporating the F⁻ ions from MgF₂ in place of OH⁻ ions in HA. Substitution of OH⁻ by F⁻ ions was verified by the change in HA’s hexagonal lattice parameters. A fracture toughness of 2.0 MPa√m was calculated for the composite containing MgF₂.     

Effects of heteroflocculation of powders on mechanical properties of zirconia alumina composites

Zirconia-toughened alumina (ZTA) composites colloidally processed from dense aqueous suspensions (>50 vol% solids) had ZrO₂ content varying from 5 to 30 vol%. Tetragonal zirconia (TZ) was used in the unstabilized, transformable form (0Y-TZ), in the partially transformable form, partially stabilized with 2 mol% yttria (2Y-TZ), and in the non-transformable form stabilized with 3 mol% yttria (3Y-TZ). After sintering in air to 99% theoretical density, the elastic properties, flexure strength and fracture toughness were examined at room temperature. Dynamic moduli of elasticity of fully deagglomerated compositions did not show the effects of microcrack formation during sintering, even for materials with unstabilized zirconia. In all compositions made from submicron powders and with low content of dispersed phase (less than 10 to 20 vol %), the strength increased with increasing ZrO₂ content to a maximum of 1 GPa, irrespective of the degree of stabilization of t-ZrO₂. With increasing content of the dispersed phase (> 20 vol%), heteroflocculation of powder mixtures during wet-processing led to the formation of ZrO₂ grain clusters of increasing size. Residual tensile stresses built within cluster/matrix interfaces upon cooling not only facilitated the t-m ZrO₂ phase transformation in final composites with transformable t-ZrO₂, but also led to lateral microcracking of ZrO₂/Al₂O₃ interfaces. This enhanced fracture toughness, but at larger ZrO₂ contents the flexure strength always decreased due to intensive microcracking, both radial and lateral. The important microstructural aspects of strengthening and toughening mechanisms in ZTA composites are related in discussion to the effects of heteroflocculation of powder mixtures during wet-processing.     

Fabrication,characterization and mechanical properties of SiC-whisker-reinforced Y-TZP composites

The microstructure and mechanical properties of hot-pressed yttria-stablized tetragonal zirconia polycrystals (Y-TZP) reinforced with up to 30 vol % SiC whiskers were investigated. The homogeneously dispersed and fully dense SiC whisker/Y-TZP composites were fabricated by wet-mixing the constitutents and uniaxially hot-pressing the resulting powder. The grain size of the matrix depended on the whisker volume fraction and the hot-pressing temperature. The significant increase of fracture toughness of about MPa m^1/2 at 10 Vol % SiC and a small increase in strength were achieved by uniformly dispersing the whiskers in the Y-TZP matrix. Fracture surfaces revealed evidence of toughening by the mechanisms of crack deflection, pullout, and crack bridging by the whiskers and also a phase transformation of ZrO₂. The observed increase in the fracture toughness of Y-TZP due to the addition of SiC whiskers was correlated with existing models of toughening mechanisms. Good agreement was achieved between the theoretical predictions and the experimental toughness values, obtained from the Y-TZP/SiC_w composites.     

Improvement of room temperature fracture toughness in Cr2Nb-based alloys by heat treatment

The microstructure and room temperature fracture toughness of binary Cr/Cr₂Nb alloys annealed at 1653 K for 30 h were investigated at both the hypo- and hypereutectic compositions. The experimental results indicate that the high temperature heat treatment has a beneficial effect on the room temperature fracture toughness of the Cr₂Nb/Cr alloys. After the heat treatment, the room temperature fracture toughness of the hypo- and hypereutectic alloys are increased by about 212 and 203%, which are 15 and 8 times higher than that of as-cast Cr₂Nb Laves phase (1·2 MPa m^1/2). The fractographic analysis indicates that in the annealed condition, the strengthen of lamellar eutectic cohesive strength can provide significant toughening of the matrix by crack deflection, crack blunting and crack bridging mechanisms.     

Thermal shock resistance of Mg-PSZ

The thermal shock resistance (TSR) of Mg-PSZ (a partially stabilized ZrO₂) containing either tetragonal t- or monoclinic m-ZrO₂ precipitates was studied. The samples containing t-ZrO₂ had superior TSR, particularly in having a higher retained strength after thermal fracture had been initiated. The t-ZrO₂ in the surface regions of shocked samples transformed to a new polymorph with orthorhombic (o) symmetry; the stress-induced t o transformation may be responsible in part for the improved TSR in samples containing t-ZrO₂ compared to those in which all the particles have m symmetry.     

Microstructure and mechanical properties of yttria-stabilized tetragonal zirconia polycrystals containing dispersed TiC particles

The microstructure and mechanical properties of hot-pressed yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) ceramics containing up to 30 vol % TiC particles were studied. Adding TiC particles to Y-TZP improved the bending strength and fracture toughness. With 20 vol% TiC particles the maximum bending strength and fracture toughness reached 1073±30.4 MPa and 14.56±0.25 MPa m^1/2, respectively. The residual tensile stress induced by the thermal expansion difference between ZrO₂ and TiC must have inhibited the tetragonal-monoclinic transformation. The stress-induced phase transformation was therefore not the dominant toughening mechanism. High-densities of dislocations within TiC particles and microcracking were detected by TEM. The improved toughness of the materials is considered to be the result of crack deflection, crack bowing of TiC particles and microcracking toughening of ZrO₂.     

Effect of inclusion size on mechanical properties of alumina toughened cubic zirconia

The relationship between alumina inclusion size and mechanical properties of particulate cubic zirconia-alumina composites was studied. The composites of the diverse size and content of alumina inclusions and of the nearly constant size of zirconia grains were used. Physical mixtures of the 8 mol% Y₂O₃-ZrO₂ nano-powder and the γ-Al₂O₃ or α -Al₂O₃ micro-powder were cold isostatically pressed and then pressurelessly sintered for 2 h at 1300^∘C in air. The γ -Al₂O₃ and α -Al₂O₃ powder was composed of the particles of 0.17 and 0.36 μ m in size, respectively. Crystallites of the zirconia powder had the size of 6 nm. Microstructural features of the composites have been characterised quantitatively. Hardness, critical stress intensity factor and bending strength of the composites was measured and correlated with the microstructural features. Depending on the size and content, the alumina inclusions influenced strength of the composites by influencing their fracture toughness and the presence of flaws of critical size. An increase in size of the alumina inclusions was accompanied by the increase of fracture toughness due to the additional contribution of large alumina inclusions to the crack deflection mechanism. It was found that decreasing the alumina inclusion size significantly below the cubic zirconia matrix grain size (more than 3 times) did not lead to the increased values of fracture toughness of the composites. The highest increase in fracture toughness (up to 3.9 MPa⋅ m^0.5) has been found when the inclusion size was comparable to the matrix grain size.     

Microstructure characterization of fibrous HAp-(20 vol.% t-ZrO2)/Al2O3-(25 vol.% m-ZrO2) composites by multi-pass extrusion process

Core-shell structured HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites were fabricated using a multi extrusion process. The shell of Al₂O₃-(m-ZrO₂) phases was selected due to their excellent biocompatibility and mechanical properties and the core was designed with t-ZrO₂ dispersed in the HAp matrix. The t-ZrO₂ and m-ZrO₂ particles (< 400 nm) were homogeneously dispersed in the HAp and Al₂O₃ phases, respectively. In the HAp-(t-ZrO₂) core region, a heavy strain field contrast was observed due to the mismatch of their thermal expansion coefficients. The values of relative density, bending strength and Vickers hardness of the third pass fibrous HAp-(t-ZrO₂)/Al₂O₃-(m-ZrO₂) composites, which were sintered at 1400 °C, were about 93%, 169 MPa, and 792 Hv, respectively.     

Effect of test method and crack size on the fracture toughness of a chain-silicate glass-ceramic

The fracture toughness of a canasite glass-ceramic with a highly acicular, interlocked grain structure was measured by a number of different methods. The values at room temperature obtained by the chevron-notch, short-bar and notched-beam methods ranged from 4 to 5 M Pa m^–1/2, well above literature values for other glass-ceramics. Similar values of toughness were obtained by the fracture of bars with indentation cracks introduced with loads ranging from 1.96 to 400 N, but only for crack sizes >200 m, with lower values for cracks of smaller size. The toughness values obtained by the direct measurement of the size of the indentation cracks were appreciably lower than the values obtained by all other methods over the total range of indentation loads and corresponding crack size. SEM fractography showed that the surface within the indentation cracks was appreciably smoother than the surrounding fracture surface. The high values of fracture toughness were attributed to the combined mechanisms of crack-deflection and microcrack-toughening due to the stress-enhanced creation of microcracks caused by the residual stresses which arise from the thermal expansion anisotropy of the canasite monoclonic crystal structure. The strong negative temperature dependence of the fracture toughness suggests that at room temperature microcrack toughening represents the primary contributing mechanism to the fracture toughness. The combined effects of crack-deflection and microcrack-toughening can lead to the development of glass-ceramics with greatly improved resistance to crack propagation.     

Nb2O5对ZTM-Al2O3性能及ZrO2增韧机制的影响

探讨了Ｎｂ２Ｏ５对ＺＴＭ－Ａｌ２Ｏ３的性能和ＺｒＯ２在瓷体中增韧机制的影响。发现Ｎｂ２Ｏ５的引入可明显提高瓷体中ｍ－ＺｒＯ２含量而降低ｔ－ＺｒＯ２含量,材料的机械性能也随Ｎｂ２Ｏ５添加量的增大出现了显著的改善,并且有韧性的平方正比于ｍ－ＺｒＯ２含量的关系,ｍ－ＺｒＯ２含量的增加强化了微裂纹增韧是材料性能改善的原因。     

Fracture toughness and reliability in high-temperature structural ceramics and composites: Prospects and challenges for the 21st century

The importance of high fracture toughness and reliability in Si₃N₄, and SiC-based structural ceramics and ceramic matrix composites is reviewed. The potential of these ceramics and ceramic matrix composites for high temperature applications in defence and aerospace applications such as gas turbine engines, radomes, and other energy conversion hardware have been well recognized. Numerous investigations were pursued to improve fracture toughness and reliability by incorporating various reinforcements such as particulate-, whisker-, and continuous fibre into Si₃N₄ and SiC matrices. All toughening mechanisms, e.g. crack deflection, crack branching, crack bridging, etc essentially redistribute stresses at the crack tip and increase the energy needed to propagate a crack through the composite material, thereby resulting in improved fracture toughness and reliability. Because of flaw insensitivity, continuous fibre reinforced ceramic composite (CFCC) was found to have the highest potential for higher operating temperature and longer service conditions. However, the ceramic fibres should display sufficient high temperature strength and creep resistance at service temperatures above 1000°C. The greatest challenge to date is the development of high quality ceramic fibres with associate coatings able to maintain their high strength in oxidizing environment at high temperature. In the area of processing, critical issues are preparation of optimum matrix precursors, precursor infiltration into fibre array, and matrix densification at a temperature, where grain crystallization and fibre degradation do not occur. A broad scope of effort is required for improved processing and properties with a better understanding of all candidate composite systems.     

Zirconia toughened cordierite

Zirconia-cordierite ceramic composites were fabricated by the hot pressing of cordierite and zirconia powders at 1400 ° C/20 MPa for 10 min. Both the fracture toughness and fracture strength have been enhanced by the addition of zirconia. Such improvements are due to stress-induced transformation toughening and to the generation of residual stresses. Micro-crack toughening has been found not to occur in the composite as a consequence of the large thermal expansion mismatch. The transformability of zirconia inclusions in the cordierite matrix is higher than in the case of an alumina matrix, although the inclusions have a rounded morphology, resulting from the presence of a liquid phase at the hot-pressing temperature.     

Strength improvement in beta″ alumina by incorporation of zirconia

Beta alumina ceramic electrolytes for use in Na/S batteries are inherently weaker than most engineering ceramics due to the presence of weakly-bonded conduction planes in the crystal structure and to difficulties in controlling grain growth during firing. Substantial improvement in microstructural control is obtained by incorporation of monoclinic zirconia (m-ZrO₂) or partially stabilized zirconia (PSZ) resulting in increases in strength and fracture toughness to around 350 MPa and 4 MPam^1/2, respectively. PSZ may adversely influence the electrical resistivity of the ceramic owing to the presence of impurities. With most zirconia powders a high level of retention of tetragonal zirconia (t-ZrO₂) is obtained at levels of addition up to 15% by weight ZrO₂. At these levels ZrO₂/-Al₂O₃ ceramics show low resistivity and stable resistance in Na/S cells.     

Preparation of Si3N4-SiC composites by microwave route

Si₃N₄-SiC composites have been microwave sintered using β-Si₃N₄ and β-SiC as starting materials. Si₃N₄ rich compositions (95 and 90 vol.% Si₃N₄) have been sintered above 96% of theoretical density without using any sintering additives in 40 min. A monotonic decrease in relative density is observed with increase in SiC proportion in the composite. Decrease in relative density has manifested in the reduction of fracture toughness and microhardness values of the composite with increase in SiC content although the good sintering of matrix Si₃N₄ limits the decrease of fracture toughness. Highest value of fracture toughness of 6.1 MPa m^1/2 is observed in 10 vol.% SiC composite. Crack propagation appears to be transgranular in the Si₃N₄ matrix and the toughening of the composites is through crack deflection around hard SiC particles in addition to its debonding from the matrix.     

The alumina fibre/tetragonal zirconia polycrystal composite system

Preliminary data on the mechanical properties of the tetragonal zirconia polycrystal (TZP) matrix as affected by the incorporation of Al₂O₃ fibres are presented. Measurements show that an addition of 10% by weight of Al₂O₃ fibres to TZP resulted in a twofold increase in fracture toughness over the monolithic TZP. The toughening mechanisms in the Al₂O₃ fibre/TZP system appear to be crack branching and deflection, in addition to martensitic phase transformation.     

3Y-TZP/LZAS微晶玻璃功能梯度涂层的微观结构与性能

为了提高钢基体微晶玻璃涂层的韧性, 在Q235钢基体上采用涂搪法制备了钇稳定四方相氧化锆/Li₂O- ZnO-Al₂O₃-SiO₂ (3Y-TZP/LZAS) 微晶玻璃功能梯度涂层。采用XRD、SEM分析了梯度涂层的物相组成和微观结构, 采用压痕法测试并计算了涂层的显微硬度和断裂韧性, 通过粘接-拉伸法测试了涂层的结合强度。结果表明, 3Y-TZP/LZAS微晶玻璃功能梯度涂层各层之间的界面结合紧密; 涂层与钢基体依靠玻璃中的SiO₂与铁的氧化物发生界面反应形成牢固的结合, 反应产物为Fe₂SiO₄和FeSiO₃; 涂层的显微硬度和断裂韧性沿涂层厚度方向逐渐增大, 涂层韧性提高是表面残余压应力增韧、3Y-TZP相变及3Y-TZP的颗粒增韧共同作用的结果; 梯度涂层与Q235的结合强度达16.3 MPa。热震实验表明, 梯度涂层在300℃下经历30余次热循环, 表现出较好的抗热震性能。     

Processing and mechanical properties of short fibers/Si3N4p reinforced BaO·Al2O3·2SiO2 ceramic matrix composites

Dense short silicon carbide (SiC_sf) and carbon fibers (C_sf) reinforced BaAl₂Si₂O₈ (BAS) glass-ceramic composites with silicon nitride were fabricated by hot-pressing technique. The phase characterization, microstructure, mechanical properties and fracture behavior of the composites were investigated by X-ray diffraction, scanning electron microscopy, transmission electron microscopy and three-point bending tests. The results showed that short silicon carbide and carbon fibers disperse homogeneously in BAS matrix, and had good chemical compatibility with the glass-ceramic matrix. The addition of Si₃N₄ could successfully eliminate the microcracks in the BAS matrix induced by the thermal mismatch between the fiber and matrix. Both the added short fibers could effectively reinforce the BAS glass-ceramic by the associated toughening mechanisms such as crack deflection, fiber bridging and pullout effects.     

The interface,microstructure and mechanical properties of Cf/LAS glass-ceramic composites

Carbon fibre (C_f)-reinforced lithium aluminium silicate (LAS) glass-ceramic matrix composites were prepared by using LAS ultrafine powders and LAS sol as starting materials and binder, respectively. The effects of fibre content, hot-pressing temperature and pressure on the mechanical properties of the composites were studied. By means of SEM and theoretical calculation, the effects of thermal mismatching between fibre and matrix, and the microstructure on the mechanical properties of the composites were analysed and discussed. The flexural strength and fracture toughness of C_f/LAS glass-ceramic matrix composite prepared were 740 MPa and 19.5 MPa m^1/2, respectively. The wettability of carbon fibre with matrix was also investigated.