Low temperature preparation of stabilized zirconia

The preparation of stabilized zirconia by thermal decomposition of metal alkoxides is reported. Formation of stabilized zirconia takes place at 400° C. The a.c. conductivity of the samples has been measured from 400 to 1000°C. The best conductivity is found in ZrO₂doped with 15 per cent CaO, which at 400° C is 2.37×10⁻⁶ Ω⁻¹ cm⁻¹ and at 1000°C is 1.26×10⁻² Ω⁻¹ cm⁻¹, with an activation energy of 1.16eV. Transport number measurements show that stabilized zirconia prepared by this method is purely an oxygen ion conductor.     

Ultrafine ZrO2 powder by Laser Evaporation: Preparation and properties

Ultrafine oxide powders were produced by CO₂ laser evaporation of coarse ZrO₂ powder or compact stabilized ZrO₂ material The 10.6μm radiation in the power range 1–4kW was generated by a transversal flow Co₂ laser which can oscillate in cw and pw operation The vaporization rate depends on the relative position of the focal plane to the surface of the ZrO₂ powder, the laser intensity and the supplied energy input. At a laser intensity of 4.2 · 10⁵ Wcm⁻² the optimum vaporization rate is 130 g · h⁻¹ (cw-operation of the laser). The produced powders consist of spherical particles; their diameters vary in the range from 5 to 200 nm can be controlled by the process conditions. The surface area (BET) is adjustable from 10 to 30 m² · g⁻¹. The powders of unstabilized zirconia show an unusual high content of the tetragonal phase. In case of chemically stabilized zirconia the composition can change during the process of evaporation and recondensation.     

Low temperature and strain rate dependence of fracture stress and fracture toughness on thin Fe40Ni40B20 amorphous ribbon

The fracture stress and the critical stress intensity factor of the Fe₄₀Ni₄₀B₂₀ amorphous metallic ribbons 20 μm thick were measured in the temperature range 4.2–300 K and at deformation rates from 3.3×10⁻⁶ to 1.25×10⁻³ m⁻¹ with the aim to obtain more information on the condition for the onset and development of the inhomogeneous plastic deformation and fracture.     

Effect of ZrO2 inclusions on fracture properties of MgCr204

The effects of unstabilized ZrO₂ inclusions on the strength, fracture surface energy and thermal-shock resistance of MgCr₂O₄ have been evaluated. The fracture surface energy for MgCr₂O₄-ZrO₂ composites was observed to depend on the agglomerate particle size distribution, and volume fraction of the ZrO₂ inclusions. Large, nonuniformly distributed ZrO₂ inclusions tended to produce a relatively small increase in the fracture surface energy of MgCr₂O₄. The fracture surface energy increased with increasing ZrO₂ content to a maximum value of 24.5J m^–2 at 16.5 vol % ZrO₂, and decreased as the ZrO₂ content increased further. It is proposed that this four-fold increase in fracture surface energy results from the absorption of energy due to microcrack formation in the MgCr₂O₄ matrix, which results primarily from the tensile stresses due to the tetragonal monoclinic phase transformation of ZrO₂ and the associated volume expansion. The improvement in mechanical properties, specifically the four-fold increase in fracture surface energy, resulted in a substantial increase in thermal-shock resistance of MgCr₂O₄-ZrO₂ composites as indicated by the results of thermal-shock experiments.     

The fracture energy of some epoxy resin materials

The double cantilever beam technique has been used to measure the fracture energy of a series of cured epoxy resins. A range of materials was produced by varying the proportion of resin to hardener agent (Araldite CT200 and hardener HT901 — phthalic anhydride, CIBA Ltd) and by adding silica flour or aluminium powder as a filler. The fracture energy for crack initiation in the cured epoxy resins ranged from approximately 10³ to 10⁵ erg cm⁻², and in the “filled resins” from 10⁴ to 5×10⁵ erg cm⁻², as the proportion of hardener increased. The fracture energy for crack arrest was somewhat lower at the higher hardener contents and particularly for the Al-filled material.     

Quasi-static and dynamic compressive behaviour of poly(methyl methacrylate) and polystyrene at temperatures from 293 K to 363 K

Flow stress, Young’s Modulus, energy and strain of fracture of poly(methyl methacrylate) (PMMA) and polystyrene (PS) were studied under compressive loading at strain rates of 10⁻⁴–10 s⁻¹ and temperatures from 293 K to temperatures ∼20 K below T _g. It was found that the energy of fracture shows an increase in the quasi-static strain rate (10⁻⁴–10⁻³ s⁻¹) region and becomes constant in the low strain rate (10⁻²–10 s⁻¹) region, while the strain of fracture shows a slow decrease with rate over the strain rate range tested. The activation energies and volumes of PMMA and PS at yield stress, 20% and 30% strain were evaluated using Eyring’s theory of viscous flow. ΔG was found to be constant for all strain rates and strains for both PMMA and PS. The activation volume for both materials increased as a function of strain.     

Thermal shock fracture behaviour of ZrO2 based ceramics

Thermal shock fracture behaviour of alumina, mullite, silicon carbide, silicon nitride and various kinds of zirconia based ceramics, such as magnesia partially stabilized zirconia (Mg-PSZ), yttria and ceria doped tetragonal zirconia polycrystals (Y-TZP and Ce-TZP), Y-TZP/Al₂O₃ composites and yttria doped cubic stabilized zirconia (Y-CSZ), was evaluated by the quenching method using water, methyl alcohol and glycerin as quenching media. Thermal shock fracture of all materials seemed to proceed by the thermal stress due to convective heat transfer accompanied by boiling of the solvents under the present experimental conditions. Thermal shock resistance of zirconia based ceramics increased with increasing the fracture strength, but that of Y-TZP and Y-TZP/Al₂O₃ composites was anormalously lower than the predicted value.     

Vickers microhardness indentation and fracture mechanics of chalcogenide arsenic-selenium glasses

Measurements of Vickers microhardness have been carried out on As _x Se_1−x glass (0.28 <x < 0.60). The diamond pyramid hardness number as a function of composition revealed a maximum at 40% As, the stoichiometric composition, indicating that this composition is the most ordered and strongest of the alloys. Deviation from stoichiometry was found to increase the disorder and introduce weaker bonds. An attempt was made to use the indentation approach to determine the fracture toughness of the investigated glasses. Therefore, the extent of surface traces of well-developed penny-like (conchoidal) cracks extending from the corners of Vickers indents were measured and found to obey Lawn's relationP/C ^3/2 = constant (whereP is the indenter load andC is the characteristic crack dimension). An approximate value of the fracture toughness was inferred from these measurements.     

Critical evaluation of indentation fracture toughness measurements with Vickers indenter on yttria‐stabilized zirconia dental ceramics

The purpose of this study was to investigate and analyze fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia (Y‐TZP) dental ceramics by the Vickers indentation fracture test. In order to determine fracture toughness, the Vickers indenter was used under the load of 294.20 N (HV30). The cracks, which occur from the corners of a Vickers indentation, were measured and used for fracture toughness determination, through five mathematical models according to (I) Anstis, (II) Evans and Charles, (III) Tanaka, (IV) Niihara, Morena and Hasselman and (V) Lankford. Morphology of indentation cracking was determined by scanning electron microscope. The most adequate model for determination of fracture toughness (K_Ic) of yttria stabilized tetragonal zirconia dental ceramics by the Vickers indentation fracture test is Lankford model.     

Role of the medium in the character of delayed fracture of high-nitrogen steels

Halide ions efficiently accelerate corrosion processes under stresses due to adsorption of Hal-ions on high-nitrogen chromium-manganese steels and increase in the degree of filling of the surface (in the process of the change Cl⁻→Br⁻) with hydroxyhalide intermediates (FeHalOH)²⁻, (FeHalOH)⁻, and Cu₂(OH)₃Cl. Fracture of specimens made of 18Mn-18Cr steel in water at room temperature in the process of tests for long-term strength occurs according to a toughness mechanism due to the initiation, growth, and coalescence of micropores. In saturated NaCl and KBr solutions, fracture has a slight effect on the mechanism of initiation and coalescence of micropores. In a CuCl₂ saturated solution, a certain part of the surface is covered by products of corrosion, while the other part is covered by large pores. At the bottom of the pores, chaotically oriented micropores and inclusions are observed. Karpenko Physicomechanical Institute. Ukrainian Academy of Sciences, L'viv. Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 36, No. 1, pp. 102–104, January–February, 2000.     

Calcia-doped yttria-stabilized zirconia for thermal barrier coatings: synthesis and characterization

Doping with other oxides has been a stabilization method of ZrO₂ for thermal barrier coating applications. Such a stabilized system is 7–8 mol% YO_1.5-doped zirconia (7YSZ), which has been in use for around 20 years. In this study, calcia (CaO) and yttria (Y₂O₃) have been used for doping ZrO₂ to produce a stable single-phase cubic calcia-doped yttria-stabilized zirconia (CaYSZ). This has been synthesized using wet chemical synthesis as well as by solid-state synthesis. Unlike partially stabilized zirconia where 5 mol% CaO is doped into ZrO₂, CaYSZ has been found to be stable up to 1600 °C. Detailed CaYSZ synthesis steps and phase characterization are presented. Wet chemical synthesis resulted in a stable single-phase CaYSZ just after 4 h treatment at 1400 °C, whereas a 36 h annealing at 1600 °C is required for CaYSZ synthesis during solid-state processing. The CaYSZ has been found stable even for 600 h at 1250 °C. Coefficient of thermal expansion and sintering temperature of CaYSZ was found to be 11 × 10⁻⁶ K⁻¹ and 1220 °C, respectively, which are comparable to 7YSZ. An increase in sintering rate with increasing dopant concentration has also been observed.     

The fracture of an epoxy polymer containing elastomeric modifiers

The fracture energies of elastomer-modified epoxy polymers have been determined over a range of strain rates from 10⁻² to 10³ sec⁻¹. The modifiers included a liquid carboxyterminated butadiene acrylonitrile and a solid rubber. They were used alone and also in combination. In all cases, the modifiers increased the toughness of the base resin by orders of magnitude and one combination of liquid and solid rubber increased toughness by 60 times. There was a general decrease in fracture energy with increasing strain rate but even during impact testing the modified epoxys were 10 to 20 times tougher than the base polymer. Scanning electron microscopy revealed that, when combined with the liquid rubber, the solid rubber induced a localized shear yielding.     

Mixed-mode interfacial fracture toughness for thermal barrier coating

A new interfacial fracture test method was developed for measuring the mixed-mode interfacial fracture toughness of thermal barrier coated material over a wide range of loading phase angles. The principle of this developed method is based on peeling the coating from the substrate due to compressive loading to the coating edge, as forming a shear loading to the interface, and slinging loading such as beam bending, as normal loading to the interface. The complete closed form of the energy release rate and associated complex stress intensity factor for our testing method is shown. An yttria stabilized zirconia (YSZ) coating, which was sprayed thermally on Ni-based superalloy, was tested using the testing device developed here.The results showed that the energy release rate for the coating-interfacial crack increased with loading phase angle, which is defined by tan⁻¹ for a ratio of stress intensity factor K₂ to K₁. It was noticed that the interfacial energy release rate increasing with mode II loading could be mainly associated with the contact shielding effect due to crack surface roughness rubbing together.     

Crack-size dependence of fracture toughness in transformation-toughened ceramics

Fracture toughness (K _IC) has been determined for Y₂O₃-partially stabilized zirconia, Y₂O₃-partially stabilized hafnia, CaO-partially stabilized zirconia and Al₂O₃+ZrO₂ composites. It is shown thatK _IC determined using the identation technique may not yield a unique number but may depend upon the crack size (C) (on the indent load). The slope ofK _IC againstC ^1/2 yields the magnitude of the surface stress created by the tetragonal monoclinic transition on the surface induced by grinding.K _IC determined using the double cantilever beam (DCB) technique, on the other hand, is shown to be independent of crack length.     

Influence of temperature and strain rate on cohesive properties of a structural epoxy adhesive

Effects of temperature and strain rate on the cohesive relation for an engineering epoxy adhesive are studied experimentally. Two parameters of the cohesive laws are given special attention: the fracture energy and the peak stress. Temperature experiments are performed in peel mode using the double cantilever beam specimen. The temperature varies from −40 to + 80°C. The temperature experiments show monotonically decreasing peak stress with increasing temperature from about 50 MPa at −40°C to about 10 MPa at + 80°C. The fracture energy is shown to be relatively insensitive to the variation in temperature. Strain rate experiments are performed in peel mode using the double cantilever beam specimen and in shear mode, using the end notch flexure specimen. The strain rates vary; for peel loading from about 10⁻⁴ to 10 s⁻¹ and for shear loading from 10⁻³ to 1 s⁻¹. In the peel mode, the fracture energy increases slightly with increasing strain rate; in shear mode, the fracture energy decreases. The peak stresses in the peel and shear mode both increase with increasing strain rate. In peel mode, only minor effects of plasticity are expected while in shear mode, the adhesive experiences large dissipation through plasticity. Rate dependent plasticity, may explain the differences in influence of strain rate on fracture energy between the peel mode and the shear mode.     

Interfacial fracture characteristic and crack propagation of thermal barrier coatings under tensile conditions at elevated temperatures

Thermal barrier coatings (TBCs) have been extensively used in aircraft engines for improved durability and performance for more than fifteen years. In this paper, thermal barrier coating system with plasma sprayed zirconia bonded by a MCrAlY layer to SUS304 stainless steel substrate was performed under tensile tests at 1000°C. The crack nucleation, propagation behavior of the ceramic coatings in as received and oxidized conditions were observed by high-performance camera and discussed in detail. The relationship of the transverse crack numbers in the ceramic coating and tensile strain was recorded and used to describe crack propagation mechanism of thermal barrier coatings. It was found that the fracture/spallation locations of air plasma sprayed (APS) thermal barrier coating system mainly located within the ceramic coating close to the bond coat interface by scanning electron microscope (SEM) and energy dispersive X-Ray (EDX). The energy release rate and interface fracture toughness of APS TBCs system were evaluated by the aid of Suo–Hutchinson model. The calculations revealed that the energy release rate and fracture toughness ranged, respectively, from 22.15 J m⁻² to 37.8 J m⁻² and from 0.9 MPa m^1/2 to 1.5 MPa m^1/2. The results agree well with other experimental results.     

Hertzian fracture of Pyrex glass under quasi-static loading conditions

Hertzian fracture tests were conducted using an Instron on Pyrex glass specimens with various surface conditions, including lubricants, employing steel, Al₂O₃, WC and Pyrex glass indentors of 0.79 to 12.7 mm radius under ambient air and high vacuum environments at cross-head speeds of 8.5×10^–6 to 2.1×10^–4m sec^–1. The results were not in strict accord with Auerbach's law, nor any of the existing energy-balance Hertzian fracture theories. Rather, they indicated that surface roughness and friction modified the Hertz stress field so that the maximum tensile stress at the surface occurred outside the contact circle. Further, they indicated that Hertzian fracture occurred by the direct, unstable growth into a cone crack of a pre-existing flaw at the displaced site of the maximum tensile stress, the flaw size responsible for the fracture decreasing with decrease in ball size (contact radius). Once a cone crack occurred, its length and growth were described reasonably well by Roesler's theory; however, his constant appears to be too high by a factor of about 5. A surface energy of @ 4 J m^–2 was derived from bend tests on specimens similar to those used in the Hertzian fracture tests. Using this value, the crack sizes which lead to fracture were estimated to range between 0.6 and 3.5 m for the conditions investigated here. The increase in the critical load for Hertzian fracture with indentation velocity was concluded to be due to kinetic effects of water vapour acting at the tip of the crack.     

Orientation dependence of fracture toughness measured by indentation methods and its relation to surface energy in single crystal silicon

Fracture toughness of silicon crystals has been investigated using indentation methods, and their surface energies have been calculated by molecular dynamics (MD). In order to determine the most preferential fracture plane at room temperature among the crystallographic planes containing the 〈001〉, 〈110〉 and 〈111〉 directions, a conical indenter was forced into (001), (110) and (111) silicon wafers at room temperature. Dominant {110}, {111} and {110} cracks were introduced from the indents on (001), (011) and (111) wafers, respectively. Fracture occurs most easily along {110}, {111} and {110} planes among the crystallographic planes containing the 〈001〉, 〈011〉 and 〈111〉 directions, respectively. A series of surface energies of those planes were calculated by MD to confirm the orientation dependence of fracture toughness. The surface energy of the {110} plane is the minimum of 1.50 Jm⁻² among planes containing the 〈001〉 and 〈111〉 directions, respectively, and that of the {111} plane is the minimum of 1.19 Jm⁻² among the planes containing the 〈011〉 direction. Fracture toughness of those planes was also derived from the calculated surface energies. It was shown that the K _IC value of the {110} crack plane was the minimum among those for the planes containing the 〈001〉 and 〈111〉 directions, respectively, and that K _IC value of the {111} crack plane was the minimum among those for the planes containing the 〈011〉 direction. These results are in good agreement with that obtained conical indentation.     

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.     

Subcritical crack growth,surface energy,fracture toughness,stick-slip and embrittlement

It is proposed that the difficulties encountered with the meaning of subcritical crack growth arose from a misunderstanding of the Griffith equation. This equation is G=2γ for an equilibrium crack (stable or unstable) where γ is the intrinsic surface energy. When G>2γ the crack has a velocity v depending on the crack extension force G−2γ, even in a vacuum, and the following equation, well verified for adherence of elastomers, G−2γ=2γφ _T(v) where φ _T(v) is related to viscoelastic losses or internal friction at the crack tip, is generalized to other materials. At a critical speed v _c, dφ/dv becomes negative; as a negative branch cannot be observed the velocity jumps to high values on a second positive branch, so that G=G _c is a criterion for crack speed discontinuity, not the Griffith criterion. The multiplicative factor 2γ on the right-hand side accounts for the shift of the v-K curves with environment. No stress corrosion is needed to explain subcritical crack growth. Subcritical crack growth in glasses and ceramics and velocity jump in brittle polymers are shown to agree with this proposal. This model can also explain stick-slip motion when a mean velocity is imposed in the negative branch. Occurrence of velocity jump or stick-slip depends on the geometry tested and the stiffness of the apparatus. A second kind of stick-slip associated with cavitation in liquid-filled cracks is discussed. When the surrounding medium can reach the crack tip and reduce the surface energy, even at the critical speed v _c, the critical strain energy release rate G _c is reduced in the same proportion as γ, and a loading which would have given subcritical growth will give a catastrophic failure. Reduction of surface energy in the Rehbinder effect and in embrittlement by segregation is discussed. Finally, the evolution of ideas concerning the Irwin-Orowan formula and fracture toughness is examined.