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.     

Fracture toughness,strength and Vickers hardness of yttria-ceria-doped tetragonal zirconia/alumina composites fabricated by hot isostatic pressing

Yttria-ceria-doped tetragonal zirconia ((Y, Ce)-TZP)/alumina (Al₂O₃) composites were fabricated by hot isostatic pressing (HIP) at 1400–1600 °C and 147 MPa for 30 min in Ar gas using fine powders prepared by hydrolysis of ZrOCl₂ solution. The mechanical properties of these ceramic composites were evaluated. The fracture toughness and bending strength of the composites consisting of 25 wt% Al₂O₃ and tetragonal zirconia with compositions 4 mol% YO_1.5-4 mol% CeO₂-ZrO₂, 2.5 mol% YO_1.5-4 mol% CeO₂-ZrO₂ and 2.5 mol% YO_1.5-5.5 mol% CeO₂-ZrO₂ fabricated by HIP at 1400 °C were 6–7 MPa m^1/2 and 1700–1800 MPa. Fracture toughness, strength and hardness of (Y, Ce)-TZP/Al₂O₃ composites were strongly dependent on HIP temperature. The fracture strength and hardness were increased, and grain growth of zirconia grains and phase transformation from the tetragonal to the monoclinic structure of (Y, Ce)-TZP during HIP in Ar at high temperature (1600 °C) were suppressed by the dispersion of Al₂O₃ into (Y, Ce)-TZP.     

Destabilization of calcia stabilized zirconia

Calcia stabilized zirconia (CSZ) is destabilized by Al₂O₃, SiO₂ and TiO₂ at elevated temperatures. The product of destabilization, termed partially destabilized zirconia (PDZ) has been found to possess excellent resistance to thermal shock. The thermomechanical properties of PDZ depend, to a great extent, on the degree of destabilization which is the amount (in weight percent) of free ZrO₂ formed as a consequence of the destabilization reaction. The kinetics of the destabilization reaction has been studied by quantitative X-ray powder diffraction technique, and from the kinetic data, the optimum amount of the destabilizer required to get superior quality PDZ has been estimated.     

Vickers microindentation hardness studies of β-Sn single crystals

The Vickers microindentation hardness anisotropy profile and load dependence of apparent hardness of white tin (β-Sn) single crystals having different growth directions were investigated. Indentation experiments were carried out on the (001) crystallographic plane at indentation test loads ranging from 10 to 50 mN. Examinations reveal that the degree of the hardness anisotropy decreases with increasing indentation test load. Also, the materials examined exhibit significant peak load dependence (i.e., indentation size effect (ISE)). The traditional Meyer's law, proportional specimen resistance (PSR) model and modified PSR (MPSR) model, were used to analyze the load dependence of the hardness. While Meyer's law can not provide any useful information about the observed ISE, the load-independent hardness (i.e., H_PSR and H_MPSR) values can be estimated for different crystallographic directions, using the PSR and MPSR models. Briefly, for microindentation hardness determinations of β-Sn single crystals, the MPSR model is found to be more effective than the PSR model.     

Relationship between fatigue limit and Vickers hardness in steels

In this work, a method to predict the fatigue limit by using Vickers hardness measurements is proposed. Tests carried out in small regions of different annealed, quenched and quenched-tempered alloy steels allowed an improvement of the empirical Murakami-Endo's equation.In this method, the plastic deformation caused by the indentation is assumed to be the defect from which the process of initiation and propagation of cracks originate, analogously to small cracks.Fatigue limits for four kinds of steels in different metallurgical states (annealed, quenched and quenched-tempered) were estimated in two different ways, and the obtained values were compared to the experimental ones. A good correlation between Vickers hardness and the fatigue limits estimated by direct plastic deformation zone measurements using optical microscopy was envisaged.     

Strength characterization of MgO-partially stabilized zirconia

The flexural strength of MgO-partially stabilized zirconia was evaluated as a function of temperature (20–1300 °C in air environment), applied stress and time. The indentation-induced-flaw technique did not produce well-defined symmetrical cracks of controlled size, whose length (on the tensile surface) or depth (on the fracture face) can be measured unambiguously, and therefore it should not be used for measuring fracture toughness. The sudden decrease in fracture strength at moderately low temperatures (200–800 °C) is believed to be due to stability of the tetragonal phase and relative decrease in the extent of the stress-induced martensitic phase transformation of the tetragonal to monoclinic phase. Flexural stress rupture testing at 500–800 °C in air indicated the material's susceptibility to time-dependent failure, and outlines safe applied stress levels for a given temperature. Stress rupture testing at 1000 °C and above at low applied stress levels showed bending of specimens, indicating the onset of plasticity or viscous flow of the glassy phase and consequent degradation of material strength.     

Microwave synthesis of yttria stabilized zirconia

Yttria stabilised zirconia (YSZ) nanocrystals, with a mean size between 5 and 10 nm, were prepared by microwave flash synthesis. Flash synthesis was performed in alcoholic solutions of yttrium, zirconium chloride and sodium ethoxide (EtONa) using a microwave autoclave (RAMO system) specially designed by authors. Energy dispersive X-ray analysis (EDX), X-ray powder diffraction (XRD), BET adsorption technique, photon correlation spectroscopy (PCS) transmission and scanning electron microscopy (TEM and SEM) are used to characterized these nanoparticles. Compared with conventional synthesis, nanopowders can be produced in a short period (e.g. 10 s), both high purity and stoechiometric control are obtained. Nevertheless, this mean of production is more cheaper and much faster than the ones commonly used to produce yttria stabilized zirconia (YSZ) by conventional sol-gel techniques.     

Slip casting of partially stabilized zirconia

Casting behaviour and rheological properties are studied in order to define the appropriate conditions under which to prepare slips for the production of high-temperature ceramics. Various commercial powders have been used, which were characterized with respect to morphology, particle size distribution and specific surface area. Zirconia slips with 75 wt% solid content were prepared with distilled water and ethanol as dispersing agent, with and without deflocculant. Hydrochloric acid and tetramethylammonium hydroxide were used to control the pH. Investigations into rheology, i.e. the dependence of viscosity and shear stress on shear rate, were performed. The slip, green and sedimentation bulk densities were measured.     

Electrical conductivity of tetragonal stabilized zirconia

The electrical conductivity change on annealing for tetragonal stabilized zirconia (TZP) was studied with the help of a.c. impedance dispersion analysis techniques. The dependences of the conductivity on annealing time at 1000 ° C and on temperature cycling between room temperature and 1000 ° C were investigated. A decrease in conductivity of about 30% at 1000 ° C of TZP with 3 mol% Y₂O₃ was observed during the first 200 h of annealing at 1000 ° C, and no change was observed during further annealing. A similar result was observed for TZP with 2.9 mol% Sc₂O₃. For TZP with 3.0mol% Yb₂O₃, the conductivity decreased gradually during an annealing time of over 2000 h. The impedance dispersion analysis at lower temperature suggested that the decrease in electrical conductivity by annealing at 1000 ° C could be attributed to the increases of both grain boundary and intragrain resistance. No monoclinic phase was observed for the samples annealed at 1000 ° C for 2000 h. On the other hand, a trace of a monoclinic phase was found for TZP with 3mol% Y₂O₃ after the 50th temperature cycling, but no significant decrease in conductivity was observed with the cycling.     

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.     

Strength characterization of yttria-partially stabilized zirconia

The flexural strength of yttria-partially stabilized zirconia was evaluated as a function of temperature (20–1000 °C in air), applied stress and time. The material was susceptible to strength degradation at low temperatures (200 and 300 °C) due to the phase transformation of the tetragonal structure to monoclinic, possibly accompanied by microcracking. In this temperature range, the material was incapable of sustaining low applied stress levels of 276 MPa for any significant duration (> 100 h < 500 h). Stress rupture testing at 600 °C and above identified the onset of viscous flow of the glassy phase and consequent degradation of material strength.     

Fatigue strength prediction for inhomogeneous face-centered cubic metal based on Vickers hardness

To find the Vickers hardness (HV) value for predicting the fatigue strength of inhomogeneous face-centered cubic (FCC) metals, HV tests were performed on SUH660 stainless steel. The results indicate that the intrinsic hardness distribution can be obtained from the HV distribution in test zones according to the Vickers hardness definition. The soft zone greatly affects the fatigue strength of an inhomogeneous FCC metal. Therefore, for another inhomogeneous FCC metal in which fatigue cracks initiate and propagate easily in the softest zone, the fatigue limit can be predicted using the mean HV value of the softest zone.