The effect of anelasticity and phase transformation on crack growth in Y-TZP ceramics

Crack growth behavior has been investigated under monotonic and cyclic loadings for Y-TZP that produces remarkable anelastic strain. Monotonic loading testing was carried out under the condition of various stress rates (8 × 10², 8 × 10^–1 and 8 × 10^–4 MPa/s) and temperatures (RT and 373 K). Resistance of crack propagation was observed at the lowest stress rate at elevated temperature. Cyclic fatigue crack growth rate was examined under the condition of different frequencies and stress waveforms. Crack growth rate clearly depended on stress waveform, which was explicable by exhaustion and restoration of anelasticity at the crack tip region. Experimental results make it clear that anelasticity works as strong resistance against crack growth. In this study, the effect of environment-induced tetragonal to monoclinic phase transformation on fracture strength was also investigated for pre-cracked sample. Aged (transformed) samples have shown extreme crack closure and considerable improvement in strength.     

Microstructural development of the isothermal phase transformation during ageing at 250 °C in 2Y-TZP ceramics

The behaviour of isothermal phase transformation, during ageing at 250 °C, in 2Y-TZP and related microstructural development, were investigated. The mode of microstructural development due to isothermal phase transformation was greatly dependent on grain size and the density of the sintered body. Isothermal phase transformation and related microstructural changes began at the specimen surface, near the pores, where the change of strain free energy for a t-m transformation was small and water vapour was easily contacted. The transformed region showed porous microstructures, which appeared due to the formation of cracks upon isothermal transformation.     

Yttria migration in Y-TZP during high-temperature annealing

The microstructural and phase changes occurring during the high temperature (1300 to 1550° CO annealing of Y-TZP were studied using X-ray fluorescence, X-ray diffraction, and TEM. Two processes occurred simultaneously involving the diffusion of yttrium. The Y-TZP partitioned into yttria-rich and yttria-poor phases throughout the material, because the material lies in a two-phase two-phase field of the yttria-zirconia phase diagram. The other process involved the segregation of yttrium to the surface, the extent of which was shown to vary with the state of the surface (ground or polished), annealing temperature, and silica content. Migration of yttrium to the surface caused a significant surface composition change (i.e. from 4.7wt% Y₂O₃ at room temperature to 8.9 wt % Y₂O₃ at 1550°C for 3 h), resulting in a microstructure and phase composition different from the bulk.     

Grain-size dependence of toughness and transformability of 2mol% Y-TZP ceramics

Journal of Materials Science Letters -     

Activation energy and grain growth in nanocrystalline Y-TZP ceramics

Grain growth of nanocrystalline yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) was studied between 1250 and 1650 °C and compared to the Y-TZP data from the literature. All the results exhibited two distinctly different behaviour, where slow and fast grain growth regimes with activation energies 280 and 546 kJ mol⁻¹ prevail below and above 1400 °C, respectively. Analysis of the data with respect to the grain growth mechanisms and the diffusion kinetics was in agreement with limited Y³⁺ lattice diffusion within the cubic zirconia nano-grains below 1400 °C. This results in slow grain growth kinetics. Redistribution of Y³⁺ above 1400 °C together with the equilibrium phase assemblage lead to enhanced grain boundary diffusion of Y³⁺ and results in enhanced grain growth of the sub-micrometer tetragonal grains.     

Material strengthening by crack and cavity healing

The δ _c -model of a crack-containing elastoplastic body was used to specify parameters that influence the healing of crack-type defects by filling them with the other material. The relationships are derived, which can predict the extent of crack-containing body strength recovery with various fillings. The “characteristic distance” of crack (filled or free) growth is established, when the intensity of external loads reaches its limit. It is shown that this distance is the constant of the material for unfilled macrocracks and constitutes a certain portion of the plastic zone formed near the defect.     

Temperature dependence of anelastic behavior in 3Y-TZP ceramics

The unique anelastic behavior in 3Y-TZP has been observed under several temperatures in the range 77–353 K. By employing an elastic strain subtractor and in-situ XRD technique, we found that the anelastic deformation of 3Y-TZP occurred more easily at an elevated temperature, whereas this anelastic behavior disappears dramatically at 77 K, and that the anelastic strain was not induced by the t → m phase transformation.

In the cyclic loadings, the stress versus anelastic strain hysteresis loops were also measured at each temperature. From the shape change of these loops, the possible mechanisms of anelastic deformation of 3Y-TZP were explored. Furthermore, the cyclic softening was observed at every temperature although it was not strong at the lower temperature.

The rupture strengths of this material at different temperatures were investigated as well. It was found that the rupture strength of 3Y-TZP non-crack sample was obviously higher in the lower temperature range than in the higher temperature range. However, in pre-cracked samples, the strength reduction observed in the non-crack sample at higher temperatures could be remedied to some extent due to the mechanism related to anelastic behavior.     

Nanostructured and near defect-free ceramics by low-temperature pressureless sintering of nanosized Y-TZP powders

Nanosized (∼6 nm) Y-TZP (3 mol% Y₂O₃) powders have been produced by chemical co-precipitation (Y-inorganic + Zr-organic precursors) and thorough isopropanol-washing step, after calcining in air at 450 °C. The nanocrystalline Y-TZP powders consisted of spherical soft agglomerates (∼100 nm in size) which were easily broken down during compaction resulting in a very uniform green microstructure with a narrow pore size distribution (average pore size less than 6.5 nm) and no detectable compacting defects. In spite of the relatively low green density (43% theoretical), Y-TZP powder compacts sintered to near theoretical density in the very low-temperature range of 1000 °C for 80–100 h to 1070 °C for 2 h, maintaining a grain size in the nanoscale (< 100 nm) and the sintered bodies were nearly defect-free. Hardly any grain growth took place up to 1000 °C; it was very rapid above this temperature. This revised version was published online in November 2006 with corrections to the Cover Date.     

Eu~(2+)掺杂3Y-TZP陶瓷模拟天然牙的荧光性

将3Y-TZP预烧结瓷块浸泡在Eu(NO3)3溶液中经还原气氛烧结制备Eu2+掺杂的牙科3Y-TZP陶瓷。采用扫描电镜、X射线衍射仪和荧光光谱仪对其微观形貌、物相和发光特征进行表征;采用MTT实验评价其细胞毒性。结果表明,Eu2+掺杂3Y-TZP陶瓷烧结致密、晶粒大小均匀,尺寸约为450nm。主晶相为t-ZrO2,未见其它物相的衍射峰。在365nm激发下,发射波长为位于470nm的宽带蓝光发射,属于Eu2+的4f65d1→4f7(8S7/2)能级跃迁。该发射光谱与天然牙发射光谱接近,可模拟天然牙的荧光性能。Eu2+掺杂3Y-TZP陶瓷的细胞毒性为I级,具备口内应用的安全基础。     

Strength recovery of machined Al2O3/SiC composite ceramics by crack healing

Alumina is used in various fields as a machine component. However, it has a low fracture toughness, which is a weakness. Thus, countless cracks may be initiated randomly by machining, and these cracks decrease the component's mechanical properties and reliability. To overcome this problem, a crack‐healing ability could be a very useful technology. In this study, Al₂O₃/SiC composite was sintered. This alumina exhibits excellent crack‐healing ability. Small specimens for a bending test were made from the Al₂O₃/SiC. A semicircular groove was machined using a diamond ball‐drill. The machining reduced the local fracture stress from approximately 820–300 MPa. The machined specimens were crack‐healed under various conditions. The fracture stress of these specimens after crack healing was evaluated systematically from room temperature (RT) to 1573 K. It was found that the local fracture stress of the machined specimen recovered almost completely after crack healing. Therefore, it was concluded that crack healing could be an effective method for improving the structural integrity of machined alumina and reducing machining costs.     

Moisture assisted crack growth in ceramics

A method is described to study subcritical crack growth in ceramic materials. Large, macroscopic size cracks were used and quantitative crack velocity measurements were made on glass and sapphire as a function of applied force, temperature and environment. The measured crack velocity was a complex function of stress and water vapor concentration in the environment and portions of the data could be adequately explained by the stress corrosion theory of Charles and Hillig.

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Zusammenfassung Es wird über eine Methode zum Studium der unterkritischen Bruchfortpflanzung berichtet. Grosse, makroskopische Bruchspalten wurden beobachtet and zwar wurde die Bruchgeschwindigkeit in Glas and Saphir als Funktion von angewandter Kraft, Temperature and Atmosphäre gemessen. Die gefundene Bruchfortpflanzungsgeschwindigkeit stand in komplexer Beziehung zu Zugspannung und Wasserdampfkonzentration und ein Teil der Daten liess rich auf Grund der Spannungskorrosionstheorie von Charles und Hillig erklären.

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Résumé Une methode est decrite pour l'étude de la croissance des fentes sous-critiques dans des materiaux ceramiques. Des fentes longues de grandeur macroscopique ont été utilizées et les mesures quantitatives de la vitesse de formation des fentes ont été pris dans du verre et du saphir comme une fontion de la force appliquéé, de la temperature et de Fentourage. La vitesse mesurée de la formation des fentes est une fonction complexe de la force et de la concentration de vapeur dans l'entourage, et part des resultats pouvait être expliquée suffisamment bien avec la théorie de Charles et Hillig pour la corrosion par la contrainte.

     

Environmentally-controlled non-equilibrium crack propagation in ceramics

A general framework is developed for environmentally-controlled non-equilibrium crack propagation and applied to ceramic materials that exhibit microstructurally-controlled fracture resistance variations. Increasing fracture resistance with crack length, arising from frictional interlocking of predominantly intergranular fracture surfaces, is modelled by the influence of a localized line force behind the crack tip. An indentation fracture mechanics analysis incorporates the fracture resistance variation to describe the inert strength of ceramic materials as a function of dominant flaw size. Non-equilibrium fracture is modelled as the competition between thermally-activated bond-rupture and bond-healing processes, in which the activation barriers are modified by the net mechanical energy release rate acting on a crack. The resulting dependence of crack velocity on mechanical energy release rate is used to describe the strength of ceramic materials as a function of applied stressing rate in a reactive environment. The deconvoluted crack velocity behavior allows both the macroscopic reactive environment fracture resistance and the atomistic lattice traps for fracture to be determined. An implication is that fracture resistance variations are more important in determining observed fracture behavior in reactive environments than in inert environments.     

Tetragonal to monoclinic transformation in Y-TZP joined with metallic materials

Joints between Y-TZP (Yttria containing Tetragonal Zirconia Polycrystal) and metallic materials (type SUS304 stainless steel and Mo) were fabricated, using brazing alloy (Ag-Cu-Ti) sheet. Y-TZP disks having different Y₂O₃ contents, grain size were prepared for changing their transformability from tetragonal to monoclinic phase. Y-TZP disks with various thickness were joined with metal disks with constant thickness in order to change the thermal stress states. Transformation in Y-TZP was investigated by changing cooling rates from the joining temperature.Transformed fraction was larger under presence of tensile thermal stress (_rr). The transformed fraction decreased when cooled at a faster rate, which was related with time-dependent characteristics of the transformation in Y-TZP. A large fraction of transformation was detected in the coarse grained Y-TZP joined with Mo, although no transformation was detected in the unjoined state when cooled at the same rate. Transformation of Y-TZP joined with metallic materials was discussed, considering the effects of residual stress and the time dependent features of the transformation.     

Photochemical crack healing in cinnamate-based polymers

Four kinds of cinnamate-type monomers were synthesized as healing agents. Photoirradiation of the monomers gave cyclobutane-containing crosslinked polymers via [2+2] cycloaddition. Cyclobutane cleavage upon cracking of the crosslinked polymers and re-cycloaddition of the cracked polymers were investigated by FT-IR spectroscopy. Photochemical crack healing was demonstrated by measurement of flexural strength of crosslinked, cracked, and healed polymers. It was observed that microcracks with width of 200 nm to 2 microm were healed by photoirradiation.