Crack growth in transforming ceramics under cyclic tensile loads

The mechanisms of stable growth of short fatigue cracks (crack length up to 1 mm) at room temperature in magnesia-partially stabilized zirconia subjected to cyclic tensile loads were investigated. Single edge-notched specimens were fractured in the four-point bend configuration under cyclic and quasi-static tensile loads. At a load ratio of 0.1, the threshold stress intensity factor range, K, for fracture initiation in cyclic tension is as low as 3.4 M Pam^1/2, and catastrophic failure occurs at K=6.6 M Pam^1/2. For crack length less than 1 mm and for plane strain conditions, growth rates are highly discontinuous, and periodic crack arrest is observed after growth over distances of the order of tens of micrometres. Crack advance could only be resumed with an increase in the far-field stress intensity range. The mechanisms of short crack advance in cyclic tension are similar to those observed under quasi-static loads, and the tensile fatigue effect appears to be a manifestation of static failure modes. A model is presented to provide an overall framework for the tensile fatigue crack growth characteristics of partially stabilized zirconia. Experimental results are also described to demonstrate the possibility of stable room temperature crack growth under cyclic tension in fine-grained tetragonal zirconia polycrystals, partially stabilized with Y₂O₃. The growth of cracks in transformation-toughened ceramics is found to be strongly influenced by the crack size and shape, stress state and specimen geometry.     

Microstructural and chemical stability of Y-ZrO2 reinforced β″-alumina in molten sodium sulfide and sulfur

The stability of -alumina reinforced with 10 vol% of tetragonal partially stabilized 3 mol% Y₂O₃-ZrO₂ (3Y-ZrO₂) and with 10 vol% of cubic 8 mol% Y₂O₃-ZrO₂ (8Y-ZrO₂) in molten sulfur or molten Na₂S₄ has been examined using scanning electron microscopy (SEM) X-ray diffraction (XRD) and electron probe microanalysis (EPMA) both before and after immersion at 350 °C. Tetragonal partially stabilized 3 mol % Y₂O₃-ZrO₂ was destabilized when reinforced into -alumina and immersed in molten Na₂S₄. Destabilization without incorporation into -alumina or using molten S as the immersion medium was minor. EPMA analyses indicated that the presence of -alumina enhanced zirconia destabilization in that -alumina can react with the molten corrodants to form corrosion products which are known corrosion agents for the leaching of Y₂O₃ from partially stabilized 3Y-ZrO₂. From XRD analyses, changing from partially stabilized 3Y-ZrO₂ to cubic 8Y-ZrO₂ in the composite increased resistance against phase destabilization. EPMA analyses revealed that the depletion was almost halted for cubic 8Y-ZrO₂ suggesting that the change in the zirconia phase used had reduced the chemical reactivity between Y₂O₃ and the corrodants. In order to avoid depletion destabilization of zirconia in -alumina, corrosion resistance can be increased by reducing chemical reactivity by using fully stabilizing zirconia. In addition, partially stabilized tetragonal zirconia may still be considered for use if a less reactive stabilizer such as CeO₂ is used.     

Nanocrystalline zirconia-yttria system–a Raman study

Nano sized zirconia (ZrO₂) powders doped with different amount of yttria (Y₂O₃) (3, 5 and 8 mol%) were prepared through coprecipitation method. The crystallite size estimated from the X-ray peak broadening is around 10 nm. Phase identification was carried out using XRD and Raman spectroscopy. Raman spectroscopic study of the synthesized materials show clear evidence of the presence of single phase cubic structure in the case of 8 mol% Y₂O₃ doped fully stabilized zirconia (8Y-FSZ); tetragonal phase in the case of zirconia doped with 3 mol% Y₂O₃ (3Y-TZP-tetragonal zirconia polycrystal) and a mixture of cubic and tetragonal phases for 5 mol% Y₂O₃ doped partially stabilized zirconia (5Y-PSZ). Raman technique is therefore an effective tool to distinguish the phases present in the calcined nano sized powders of zirconia.     

Short-rod elastic-plastic fracture toughness test using miniature specimens

The standard ASTM-E399 plane-strain fracture toughness (K _IC) test requires (1) the test specimen dimensions to be greater than a minimum size and, (2) fatigue precracking of the specimen. These criteria render many materials impractical to test. The short-rod elastic-plastic plane-strain fracture toughness test proposed by Barker offers a method of testing not requiring fatigue precracking and furthermore, it appears that test specimens smaller than that stipulated by ASTM can be used to obtain validK _IC values. In this study, the use of a modified miniature short-rod fracture toughness test specimen was investigated. Our miniature short-rod specimen is approximately 7 mm long and 4 mm diameter. These mini specimens are well suited for the purpose of testing biomaterials. The value of the minimum stress intensity factor coefficient (Y _m ^* ) for the mini short-rod specimens was determined experimentally using specimens machined from extruded acrylic rod stock. An elastic-plastic fracture toughness analysis using the mini specimens gave values ofK _IC for extruded acrylic (nominally PMMA) equal to 0.67 ± 0.06 MPa m^1/2. The problem of testing non-flat crack growth resistance curve materials (such as PMMA) using the short-rod fracture toughness test method is discussed. A modification to the test procedure involving the use of aY ^* value corresponding to a short crack length is suggested as a method of overcoming this difficulty.Nomenclature a crack length - a ₀ initial crack length - a ₁ length of the chevron notch on the mini short-rod specimen - a _m critical crack length — crack length atY _m ^* - C specimen compliance - C dimensionless specimen compliance = CED - D mini short-rod specimen diameter - E Young's modulus - K ₁ stress intensity factor - K _1C plane-strain fracture toughness - K _max fracture toughness calculated usingP _max - P load applied to the test specimen during a short-rod fracture toughness test - P _c load applied to the test specimen atY _m ^* - P _max maximum load applied to the specimen during a short-rod fracture toughness test - p plasticity factor - W mini short-rod specimen width - Y ^* stress intensity factor coefficient - Y _m ^* minimum of the stress intensity factor coefficient - dimensionless crack length =a/W - ₀ dimensionless initial crack length = ₀/W - ₁ dimensionless chevron notch length =a ₁/W - _m dimensionless critical crack length =a _m/W     

Microstructure of Y-PSZ after ageing at high temperature

Partially stabilized zirconia (PSZ) materials containing 2.5 and 5.0 mol % Y₂O₃ were prepared by pressureless sintering and aged at 1200° C for 1000 IS, and their microstructures were analysed by transmission electron microscopy and electron diffraction methods. Tetragonal zirconia polycrystal (TZP) containing 2.5 mol % Y₂O₃ before ageing showed nearly 100% tetragonal microstructure and 0.5 m grain size, but after ageing the microstructure changed greatly, exhibiting no simple grain structure over wide areas. Repeated twin structures within the grains were observed. Y-PSZ material containing 5.0M01% Y₂O₃ before ageing showed a tetragonal (I structure within a cubic (c) stabilized ZrO₂ matrix, After ageing, structures of fine strip crystals crossed each other orthogonally within the cubic matrix and typical diffuse scattering in the diffraction pattern was observed. Repeated twins were found on the plane of (100)_m, and the orientational relationship between tetragonal (t) and monoclinic (m) crystal was determined to be (100)_m [(100)_t, [010]_m \tT [001b]_t.     

Subcritical crack growth in partially stabilized zirconia(PSZ)

Studies on the subcritical crack growth behaviour of partially stabilized zirconia (ZrO₂-I, 5 to 10 vol% tetragonal phase; ZrO₂-II, 35%) were carried out using the double-torsion technique and data from the dynamic fatigue of unnotched bend specimens. The results of subcritical crack growth support the model of stress induced transformation from the tetragonal to monoclinic modification. Differences in the crack growth parameter n (as-received condition) using the double-torsion technique or bend specimens may be explained by the special nature of subcritical crack extension at stressed surfaces for these different specimen types. The log v-log K _i plot of ZrO₂-I using the double torsion technique shows a plateau of constant velocity, which has to be attributed to a tetragonal-monoclinic transformation. After annealing (1500° C, 5 h) the plateau has vanished and the n value is comparable to bend test in an as-received condition.     

Thermal-stress resistance and fracture toughness of two tool ceramics

The thermal-stress resistance and fracture strength behaviour of two oxide ceramics (a hot-pressed pure Al₂O₃ and a composite ceramic NTK-HC2) subjected to severe thermal shocks have been investigated. The damage resistance parameter (K _IC/ _f)² for both ceramics is also determined for a wide range of temperatures (25° to 800° C) and cross-head rates (10^–2 cm min^–1 2.5 m sec^–1). Fracture strength behaviour of these two oxide ceramics is shown to follow Hasselman's model where the instantaneous strength loss at the critical quenching temperature may be calculated using appropriate (K _IC/ _f)² values to give good agreement with experimental results. Repeated shocks show some further degradation in the retained strength for both ceramics so that these materials are susceptible to thermal fatigue. It is found that both materials possess similar resistance to crack initiation (i.e. similar T _c and retained strength after shocking through T _c) but the pure oxide ceramic has higher resistance to crack propagation.     

Vickers Indentation Fracture (VIF) modeling to analyze multi-cracking toughness of titania, alumina and zirconia plasma sprayed coatings

For massive brittle materials, the fracture toughness in mode I, K_IC, can be determined using various reliable techniques. Besides, Vickers Indentation Fracture (VIF) technique has been developed to locally determine fracture toughness. However, since the indentation test generates a complex three-dimensional crack system around the indent, fracture toughness, K_C, is calculated instead of K_IC. Consequently some authors rightly reject the VIF technique to determine standard fracture toughness by arguing that the literature counts numerous VIF crack equations thus revealing discrepancies of this technique. Nevertheless in some cases (e.g. brittle ceramic coatings) inclusive material techniques are not applicable since presence of the substrate and/or multi-crack network can modify the crack propagation into the coating.In this work, we employed VIF technique to study multi-cracking behavior of titania, alumina and zirconia ceramic oxide coatings obtained by plasma spraying. To calculate VIF toughness, we propose (i) to select two crack equations for radial-median and Palmqvist cracking modes respectively, (ii) to adjust the crack equation of Miranzo and Moya for intermediate cracking mode, (iii) to develop a mathematical approach to determine the cracking mode, (iv) to take into account the multi-crack network by defining an equivalent four-crack system and (v) to propose a universal crack equation applicable independently of the cracking mode.     

Fracture toughness of Si3N4 measured with short bar chevron-notched specimens

The short bar chevron-notched specimen was used to measure the plane strain fracture toughness of hot-pressed Si₃N₄. Specimen proportions and chevron-notch angle were varied, thereby varying the amount of crack extension to maximum load (upon which K_ic was based). The measured toughness (4.68 ± 0.19 MN m^3/2) was independent of these variations, inferring that the material has a flat crack growth resistance curve.Nomenclature a crack length - a _A crack length at arrest of unstable crack advance - a ₁ length of chevron notch at specimen surface (distance from line of load application to point of chevron emergence at specimen surface) - a ₀ initial crack length (distance from line of load application to tip of chevron) - a _R crack length at ending of stable crack extension (conversely, crack length at onset of abrupt, unstable crack advance) - B specimen thickness - H specimen half-height - K _1A stress intensity factor at arrest of unstable crack advance - K _IR stress intensity factor at end of stable crack extension (crack growth resistance) - K _IC plane strain fracture toughness - P _max maximum applied load in fracture toughness test - W specimen width - Y ^* dimensionless stress intensity factor coefficient for chevron-notched specimen - Y ^* _m minimum value ofY ^* as a function of - a/W - ₀ a ₀/W - ₁ a ₁/W     

Internal friction,crack length of fracture origin and fracture surface energy in alumina-zirconia composites

Two series of alumina-zirconia composites, i.e. alumina-unstabilized zirconia and alumina-partially stabilized zirconia with 3 mol % Y₂O₃, with different zirconia content were slip casted and fired at 1550°C for 3 h. Elastic constant, bending strength and fracture toughness were measured. Internal friction was determined to follow the formation of cracks, nondestructively, which could be one of the fracture origins. The crack length of the fracture origin and the fracture surface energy were calculated by applying Griffith's fracture theory. Microstructures of the fracture surfaces were observed using a scanning electron microscope. For the unstabilized zirconia system, the increase in the internal friction of the order from 10⁻⁴ to 10⁻³ was a guide to find the formation of cracks which lead to the fracture. The increase in the cracks becoming a fracture origin lead to the increase inK _lc and also to the apparent increase in the fracture surface energy. For the partially stabilized zirconia system, the increase in the fracture surface energy with an increase in zirconia content, keeping low internal frictions of the order of 10⁻⁴, indicates the intrinsic strengthening of the grain boundaries in comparison to the unstabilized zirconia system. Internal friction is the most suitable nondestructive physical quantity to find the microcracks which leads to the fracture.     

Control of the tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia in hot water

The tetragonal-to-monoclinic phase transformation of yttria partially stabilized zirconia caused by annealing in hot water was investigated in the temperature range 80 to 200° C using sintered bodies in zirconia containing 2, 3 and 4 mol % Y₂O₃. Three approaches, alloying ZrO₂(Y₂O₃) with 0 to 20wt% CeO₂, dispersing 0 to 40 wt % Al₂O₃ into ZrO₂(Y₂O₃) ceramics and decreasing the grain size of zirconia, were examined to inhibit the tetragonal-to-monoclinic phase transformation. The amount of monoclinic phase formed decreased with increasing concentrations of CeO₂ alloyed and Al₂O₃ dispersed, and with decreasing grain size of zirconia.     

Mössbauer Study of Tetragonal ZrO2–Y2O3–Fe2O3 Solid Solutions

Tetragonal Zr_0.886Y_0.057Fe_0.057O_{2 –} solid solutions prepared by calcining coprecipitated and successively precipitated hydroxide mixtures were studied by Mössbauer spectroscopy immediately after calcination and after long-term storage. The results indicate that the solid solutions prepared via coprecipitation and successive precipitation contain Fe³⁺ in two (octahedral coordination) and three (octahedral, fivefold, and tetrahedral coordinations) inequivalent sites, respectively. Partial Fe³⁺ substitution for Y³⁺ is shown to prevent or substantially slow down the low-temperature structural degradation of stabilized zirconia.     

A fractographic criterion for subcritical crack growth boundaries at internal fracture origins in hot pressed silicon nitride

Using the elliptic integral method, stress intensity factors (K _I) were estimated at boundaries defined by fracture features observed at various distances from internal fracture origins in H.P. silicon nitride. The fracture origins are surrounded by regions of transgranular fracture. At the outer boundaries of these regionsK _I is less thanK _IC showing that these are regions of subcritical crack growth. Regions of hummocks and depressions were observed surrounding the regions of transgranular fracture.K _I was calculated at the elliptical boundary determined by the outer edge of the nearest of these features to the fracture origin. At this boundary,K _I K _IC. Therefore, these features can be used to locate the subcritical crack growth boundary.     

Effect of zirconia (3 mol% yttria) additive on mechanical properties and structure of alumina ceramics

The effects of ZrO₂-3 mol% Y₂O₃ additives containing 7.3, 15, 23.3 and 32 vol% of ZrO₂ on _f, K _IC, H _v and the microstructure of hot-pressed alumina-based ceramics were investigated. The presence of the m-, t- and t-ZrO₂ phases was discovered by using X-ray diffraction and transmission electron microscopy. An inhomogeneous distribution of Y₂O₃ in the ZrO₂ grains was observed. The variation of the mechanical properties of the ceramics is explained by the influence of different toughening mechanisms and by a change in the structure of the material.     

Y(E)-doped tetragonal zirconia polycrystalline solid electrolyte

Polycrystalline dense zirconia containing 100% metastable tetragonal phase was obtained by alloying zirconia with 2 to 3 mol % Y₂O₃ (Er₂O₃). A critical temperature, approximately 1400°C, for full densification (100% theoretical density) was found, and above that a sudden density decrease in all sintered bodies took place. The fracture toughness (K _IC) was found to be strongly dependent on the grain size, and a critical grain size (0.29μm in Y-TZP and 0.38μm in Er-TZP) existed beyond which a steepK _IC decrease was produced. Phase composition and microstructural development seem to influence such mechanical behaviour.     

Fracture resistance of 8 mol% yttria stabilized zirconia

Anin situ technique for the assessment of fracture resistance employing double cantilever beam (DCB) specimens was developed in the present study. The side-grooved DCB specimens were loaded with pure bending moments in a specially designed and fabricated test fixture which went inside the specimen chamber of a scanning electron microscope. The study as conducted on a 8 mol% fully stabilized cubic phase yttria (Y₂O₃) stabilized zirconia (YSZ) ceramic. The powder processed sheets were sintered at 1600°C for 2 h in a zirconia tube furnace. The mode I applied energy release rate, G_I was determined for both pure YSZ and treated YSZ. Two sets of experiments were conducted for the complete characterization of the ceramics. Three fracture toughness values were determined for the pure and treated ceramics, viz. (i) at the onset of the crack initiation,G _ic, (ii) at the arrest of a subcritical crack, G_ia and (iii) at the onset of the fast fracture,G _if. Two analyses of the experimental data were carried out, viz. method of extrapolation and statistical analysis. In case of the pure YSZ, a transgranular mode of the stable crack growth was identified to be predominant. The porous coating treatment appeared to have positive effects as the crack initiation resistance increased due to electrode layers. The stable crack growth behaviours of the ceramics were investigated by monitoring the crack growth velocity as a function of appliedG values. The results obtained were of direct significance in designing and fabrication of SOFC stacks.     

Formation Kinetics and Mechanisms of BaFe9.5Al2.5O19 and SrFe9.5Al2.5O19 Solid Solutions

The kinetics and mechanisms of solid-state reactions in BaCO₃ + 4.75Fe₂O₃ + 1.25Al₂O₃ and SrCO₃ + 4.75Fe₂O₃ + 1.25Al₂O₃ powder mixtures, leading to the formation of BaFe_9.5Al_2.5O₁₉ and SrFe_9.5Al_2.5O₁₉ magnetoplumbite solid solutions, were studied in the range 1140–1270 K by x-ray diffraction and magnetization measurements. The results demonstrate that the reaction intermediates are BaFe₂O₄, SrFeO_{3 – x} , SrAl₂O₄, BaFe_{12 – x} Al _x O₁₉, SrFe_{12 – x} Al _x O₁₉ and BaAl_{12 – x} Fe _x O₁₉, SrAl_{12 – x} Fe _x O₁₉ solid solutions containing less aluminum compared to the final product, and BaAl₁₂O₁₉ and SrAl₁₂O₁₉ solid solutions based on BaFe_9.5Al_2.5O₁₉ and SrFe_9.5Al_2.5O₁₉. The constant K _Y in the Yander equation (1 – = K _Y is shown to exhibit Arrhenius behavior: K _Y = 5.08 × 10¹⁴exp(–390 × 10³/RT) for BaFe_9.5Al_2.5O₁₉ and K _Y = 3.22 × 10⁴exp(–155.1 × 10³/RT) for SrFe_9.5Al_2.5O₁₉.     

Crack resistance curves of alumina and zirconia at room temperature

Ceramic three-point bend specimens were pre-cracked in a displacement-controlled test in air at room temperature to form sharp cracks of different lengths. Critical stress intensity factors (K _IC were then measured as a function of sharp crack length in a fast-fracture, load-controlled test. Crack resistance curves (K _IC against crack length) were determined for three commercially pure aluminas of different grain size, a debased alumina containing a glassy phase, and a partially stabilized zirconia (PSZ) material. The crack resistance curves proved to be flat for the finer-grained and the debased alumina. A steeply rising crack resistance curve was, however, observed for a pure coarse-grained alumina material which is explained by friction effects of the cracked microstructure behind the measured crack front. The effect is influenced by the test procedure itself. Though crack branching takes place the crack resistance curve of PSZ is completely flat, which is attributed to fast fracture testing where only the most dangerous flaw is activated.     

Sintering of hydroxylapatite-zirconia composite materials

Sintering of hydroxylapatite-zirconia (doped with 3 mol% Y₂O₃) composite powder compacts was studied. Hydroxylapatite powder was prepared from Ca(OH)₂ and H₃PO₄, and zirconia powder was prepared from ZrOCl₂ · 8H₂O and YCl₃. The sinterability of hydroxylapatite-zirconia composite powder compacts depends strongly on differential shrinkage between the powder components of the composite. Smaller differential shrinkage results in better sinterability. By increasing the calcination temperature of zirconia powder and/or decreasing that of hydroxylapatite powder improves the sinterability of the composite powder compacts. The phase distribution and total amounts of crystal phases depend on the sintered density of compacts. Hydroxylapatite and cubic zirconia are the major phases of compacts with high sintered densities, whereas - and -tricalcium phosphate and CaZrO₃ are the major phases of compacts with low sintered densities.     

Fracture toughness of CaO-P2O5-B2O3 glasses and glass-ceramics determined by indentation

The fracture toughness, (K _IC) of CaO-P₂O₅-B₂O₃ glasses and glass-ceramics was investigated using both Vickers indentation and the notched beam technique (NBT). Five representative equations were applied and it was found that for the variation of K _IC with B₂O₃ content, the Lawn and Fuller equation showed the best correspondence with the NBT. The values of fracture toughness obtained from the Lawn and Fuller equation showed the same trend with B₂O₃ content as that determined by NBT, although the values from indentation were on average 33% lower. The determination of absolute fracture toughness by indentation requires a correction factor which can be obtained by calibration using NBT. A significant increase in K _IC occurred after a 37CaO-37P₂O₅-20B₂O₃-6Al₂O₃ (mol%) glass was converted to a glass-ceramic. The much higher K _IC for the glass-ceramic measured by NBT (1.32 MN m^–3/2) compared with that from indentation (0.89 MN m^–3/2) is attributed to internal stresses due to thermal expansion differences between the crystalline and residual glass phases leading to additional microcrack toughening.