Effect of two-step sintering on the hydrothermal ageing resistance of tetragonal zirconia polycrystals

The effects of two-step sintering (TSS) on the mechanical properties and hydrothermal ageing resistance of yttria-stabilized tetragonal zirconia polycrystals (Y-TZP) were investigated. In TSS, the first step involved heating the samples up to 1400 °C at a heating rate of 10 °C/min and holding the samples at this temperature for 1 min. The second step involved sintering by cooling the samples down to 1200 °C and holding the samples at this temperature for various holding times (t) ranging from 0 to 30 h before cooling to room temperature. Moreover, TSS promoted densification with increasing holding time without sacrificing the mechanical properties of the sintered body and causing abnormal grain growth. The average grain size was found not to be affected by the long holding times, and the final microstructure composed of a uniformly distributed tetragonal grain having sizes ranging from 0.24 to 0.26 µm. The beneficial effect of TSS in suppressing the hydrothermal ageing of Y-TZP has been revealed in the present work. In particular, samples sintered at t=20 and 30 h exhibited excellent resistance to low-temperature degradation when exposed to superheated steam at 180 °C, attributed mainly to the enhance densification of the sintered bodies.     

High temperature creep behaviour of 4 mol% yttria tetragonal zirconia polycrystals (4-YTZP) with grain sizes between 0.38 and 1.15 μm

The high temperature mechanical behaviour of 4% mol yttria tetragonal zirconia polycrystals (4-YTZP) with different grain sizes (0.38 < d < 1.15 μm) has been analyzed by means of compression creep tests. The working temperature was 1350 °C and the strain rates ranged between 5 × 10⁻⁷ and 2 × 10⁻⁴ s⁻¹. Experimental results have been fitted to the conventionally accepted creep law for superplastic ceramics. Thus, stress exponents and activation energies have been measured as a function of the grain size. The dependence of strain rate on grain size has also been determined. The experimental data are discussed with respect to the existing theoretical models for these materials.     

Isothermal sintering kinetic of 3YTZ and 8YSZ: Cation diffusion

We have studied the sintering kinetic of 3 and 8 mol% of yttria stabilized zirconia under isothermal conditions. Sintering was performed in the temperature range between 1200 and 1450 °C. The sintering kinetic process was followed by measuring density as a function of sintering time. A model was applied to the first stage of densification. Sintering obeys to the grain boundary diffusion mechanism for both materials. It was possible to calculate the activation energy as well as the diffusion coefficients. 887 and 731 kJ mol^?1 were the activation energies for the initial stage of sintering for 3YTZ and 8YSZ respectively.Finally the diffusion activation energy was estimated for both materials. The diffusion coefficients were also estimated at 1400 °C in 4.05×10^?14 and 6.00×10^?11 cm² s^?1 for tetragonal and cubic zirconia respectively. The obtained results support the observations of a faster densification for 8YSZ.     

Effect of cation doping on lattice and grain boundary diffusion in superplastic yttria-stabilized tetragonal zirconia

Lattice diffusion coefficients D_l and grain boundary diffusion D_gb coefficients of hafnium were studied for 0.5 and 1 mol% cation-doped yttria-stabilized tetragonal zirconia at the temperature range from 1283 to 1510 °C. The diffusion profiles were determined by two experimental techniques: secondary ion mass spectroscopy and electron microprobe analysis. Additionally the first principle calculations of the electronic states of Zr⁴⁺, dopant cations and O^2? anions and elastic properties in 3Y-TZP were performed. Superplastic strain rate versus stress and inverse temperature was also measured. For 1 mol% doped samples the significant increase of the grain boundary diffusion and superplastic strain rate was observed. Correlations between the calculated ionic net charges and D_gb indicate that enhancement of D_gb was caused by the reduction of ionic bonding strength between metal cation and oxygen anion in zirconia. The new constitutive equation for superplastic flow of yttria-stabilized tetragonal zirconia ceramics was obtained.     

Effects of sintering on the mechanical and ionic properties of ceria-doped scandia stabilized zirconia ceramic

The effect of conventional sintering from 1300 to 1550 °C on the properties of 1 mol% ceria-doped scandia stabilized zirconia was investigated. In addition, the influence of rapid sintering via microwave technique at low temperature regimes of 1300 °C and 1350 °C for 15 min on the properties of this zirconia was evaluated. It was found that both sintering methods yielded highly dense samples with minimum relative density of 97.5%. Phase analysis by X-ray diffraction revealed the presences of only cubic phase in all sintered samples. All sintered pellets possessed high Vickers hardness (13–14.6 GPa) and fracture toughness (~3 MPam^1/2). Microstructural examination by using the scanning electron microscope revealed that the grain size varied from 2.9 to 9.8 µm for the conventional-sintered samples. In comparison, the grain size of the microwave-sintered zirconia was maintained below 2 µm. Electrochemical Impedance Spectroscopy study showed that both the bulk and grain boundary resistivity of the zirconia decreases with increasing test temperature regardless of sintering methods. However, the grain boundary resistivity of the microwave-sintered samples was higher than the conventional-sintered ceramic at 600 °C and reduced significantly at 800 °C thus resulting in the enhancement of electrical conduction.     

Surface microstructural changes of spark plasma sintered zirconia after grinding and annealing

Spark plasma sintered zirconia (3Y-TZP) specimens have been produced of 140 nm 372 nm and 753 nm grain sizes by sintering at 1250 °C, 1450 °C and 1600 °C, respectively. The sintered zirconia specimens were grinded using a diamond grinding disc with an average diamond particle size of about 60 µm, under a pressure of 0.9 MPa. The influence of grinding and annealing on the grain size has been analysed. It was shown that thermal etching after a ruff grinding of specimens at 1100 °C for one hour induced an irregular surface layer of about a few hundred nanometres in thickness of recrystallized nano-grains, independently of the initial grain size. However, if the ground specimens were exposed to higher temperature, e.g. annealing at 1575 °C for one hour, the nano-grain layer was not observed. The resulted grain size was similar to that achieved by the same heat treatments on carefully polished specimens. Therefore, by appropriate grinding and thermal etching treatments, nanograined surface layer can be obtained which increases the resistance to low temperature degradation.     

Sintering of zirconia ceramics by intense high-energy electron beam

A comparative analysis of the efficiency of zirconia ceramics sintering by thermal method and high-energy electron beam sintering was performed for compacts prepared from commercial TZ-3Y-E grade powder. The electron energy was 1.4 MeV. The samples were sintered in the temperature range of 1200–1400 °C. Sintering of zirconia ceramics by high-energy accelerated electron beam is shown to reduce the firing temperature by about 200 °C compared to that in conventional heating technique. Ceramics sintered by accelerated electron beam at 1200 °C is of high density, microhardness and smaller grain size compared to that produced by thermal firing at 1400 °C. Electron beam sintering at higher temperature causes deterioration of ceramics properties due to radiation-induced acceleration of high-temperature recrystallization at higher temperatures.     

96Zr diffusion in polycrystalline scandia stabilized zirconia

Diffusion of the stable tracer isotope ⁹⁶Zr in 12 mol% polycrystalline scandia stabilized zirconia was studied in air in the temperature range from 1200 to 1600 °C. Secondary ion mass spectroscopy (SIMS) was used to record the tracer diffusion profiles. The activation enthalpies for bulk and grain boundary diffusion were determined to be (5.0 ± 0.4) eV and (3.9 ± 0.5) eV, respectively, with the latter being six to seven orders in magnitude faster than the first. Using XRD, it was proved that the diffusion occurs only in the cubic phase.     

Effect of dopants and sintering temperature on microstructure and low temperature degradation of dental Y-TZP-zirconia

Accelerated ageing of dental TZP were investigated at 134 °C for 2 h under 2.3 bar water vapor pressure. The TZP blanks were sintered in the range from 1350 to 1580 °C. The average grain size of 1350 and 1400 °C sintered materials were <0.3 μm whereas higher sintering temperatures led to larger grain sizes. The grain size and dopants influence the stability of tetragonal phase of zirconia under LTD conditions. The Y-TZP with average grain sizes <0.3 μm did not reveal the martensitic tetragonal-monoclinic phase transformation after ageing, whereas zirconia with grain sizes larger 0.3 μm showed fractions of monoclinic phase. Alumina and Ceria stabilized grain size and Y-TZP against LTD. Y-TZP with low amounts of Fe₂O₃ (<0.15%) used for coloring did not show any detrimental effects under LTD conditions. As the Y-TZP ceramics with grain size larger than 0.3 μm are not stable under LTD conditions they are not recommended for long term use in moist environment.     

Tailoring the grain size of zirconia polycrystalline fibers by cetyltrimethyl ammonium bromide

In the study, cetyltrimethyl ammonium bromide (CTAB) has been found to have a significant effect on the grain size of zirconia (ZrO₂) polycrystalline fibers from a precursor of polyacetylacetonatozirconium (PAZ). A relation between the variation in the grain size of the fibers sintered at 1000 °C for 1 h and CTAB addition has been established by scanning electron microscopy (SEM) and matching the results by a Gaussian Function. The grain size increased firstly and then decreased at an elevated weight percent of CTAB with respect to PAZ. ZrO₂ nucleation at 500 °C and grain growth during thermal treatment between 500 and 1000 °C were investigated by thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), high resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD). The results indicate that the addition of CTAB accelerated the decomposition of PAZ. The enhanced decomposition promoted ZrO₂ nucleation and the removal of residual carbon. While excess CTAB resulted in the increase of residual carbon on grain surface. The general kinetics of the grain growth was evaluated by Arrhenius equation and the results did matched well with the variation in grain size.     

Low temperature degradation resistant nanostructured yttria-stabilized zirconia for dental applications

When used in prosthetic dentistry, zirconia encounters severe durability issues due to low temperature degradation: exposure to humidity results in a transition from tetragonal to monoclinic phase, associated to disruptive integrity loss. Recently it has been shown that size-induced stabilization helps maintaining zirconia in tetragonal form, when the grain size is reduced to the nano-range. Objective of this work is to demonstrate the applicability of High Pressure Field Assisted Sintering (HP-FAST) to the preparation of dense, nanostructured samples of tetragonal yttria stabilized zirconia, with yttria content between 0.5 and 3 mol% and showing resistance to low temperature degradation. The yttria stabilized zirconia nanopowders were prepared by a hydrothermal method. Sintering by HP-FAST was performed at 900 °C in 5 min, under a pressure of 620 MPa. Resistance to low temperature degradation was tested at 134 °C, under vapor pressure, for up to 40 h. Both pristine and aged samples were characterized by X-ray diffraction, high-resolution scanning electron microscopy and nanoindentation tests in continuous stiffness measurement mode. The sintered samples presented a grain size between 20 and 30 nm and low or null monoclinic content. Both parameters resulted unaffected by ageing. The best results in terms of phase composition and mechanical properties have been obtained with the material containing 1.5 mol% of yttria. These results induce to reconsider the use of yttria stabilized zirconia as material for dental prosthetic systems requiring long-term durability.     

Densification of nanocrystalline Y2O3 ceramic powder by spark plasma sintering

Nanocrystalline Y₂O₃ powders with 18 nm crystallite size were sintered using spark plasma sintering (SPS) at different conditions between 1100 and 1600 °C. Dense specimens were fabricated at 100 MPa and 1400 °C for 5 min duration. A maximum in density was observed at 1400 °C. The grain size continuously increased with the SPS temperature into the micrometer size range. The maximum in density arises from competition between densification and grain growth. Retarded densification above 1400 °C is associated with enhanced grain growth that resulted in residual pores within the grains. Analysis of the grain growth kinetics resulted in activation energy of 150 kJ mol^?1 and associated diffusion coefficients higher by 10³ than expected for Y³⁺ grain boundary diffusion. The enhanced diffusion may be explained by combined surface diffusion and particle coarsening during the heating up with grain boundary diffusion at the SPS temperature.     

Phase transformation and morphological evolution of electrospun zirconia nanofibers during thermal annealing

Pure zirconia nanofibers were fabricated by electrospinning zirconia-polymer precursor and subsequent annealing. Fiber properties such as polymer decomposition, crystallization formation, phase transformation, surface morphologies, etc., were investigated by various techniques, including thermogravimetric analysis (TGA) and differential thermal analysis (DTA), high temperature differential scanning calorimeter (HTDSC), powder X-ray diffractometer (XRD), field emission scanning electron microscopy (FESEM), etc. It was found that the crystallization of as-spun fibers started at 450 °C and the initial crystallized zirconia phase was tetragonal (t), which began transforming to monoclinic (m) phase at 650 °C as evidenced by XRD; HTDSC showed at different thermal circles, the m-to-t transformation temperatures remained virtually unchanged while the reverse t-to-m temperatures systematically shifted from 924.9 to 978.6 °C as the progress of thermal circles; FESEM examinations revealed that fibers calcined to 1000 °C went through thermal grooving due to surface diffusion during heat treatment; fibers heated to 1370 °C formed the so-called “bamboo wires”, where volume diffusion was the dominant driving force.     

Micromechanics modeling of creep fracture of zirconium diboride–silicon carbide composites at 1400–1700 °C

The creep deformation of the ultra-high temperature ceramic composite ZrB₂–20%SiC at temperatures from 1400 to 1700 °C was studied by a micromechanical mode in which the real microstructure was adopted in finite element simulations. Based on the experiment results of the change of activation energy with respect to the temperature, a mechanism shift from diffusional creep-control for temperatures below 1500 °C to grain boundary sliding-control for temperatures above 1500 °C was concluded from simulations. Also, the simulation results revealed the accommodation of grain rotation and grain boundary sliding by grain boundary cavitation for creep at temperatures above 1500 °C which was in agreement with experimental observations.     

Manipulating microstructure and mechanical properties of CuO doped 3Y-TZP nano-ceramics using spark-plasma sintering

Nano-powder composites of 3Y-TZP doped with 8 mol% CuO were processed by spark-plasma sintering (SPS). A 96% dense composite ceramic with an average grain size of 70 nm was obtained by applying the SPS process at 1100 °C and 100 MPa for 1 min. In contrast to normal, pressureless, sintering during SPS reactions between CuO and 3Y-TZP were suppressed, the CuO phase was reduced to metallic Cu, while the 3Y-TZP phase remained almost purely tetragonal. Annealing after SPS results in grain growth and tetragonal to monoclinic zirconia phase transformation. The grain size and monoclinic zirconia phase content are strongly dependent on the annealing temperature. By combining the processing techniques studied in this work, including traditional pressureless sintering, properties of the composite ceramic can be tuned via manipulation of microstructure. Tuning the mechanical properties of dense 8 mol% CuO doped 3Y-TZP composite ceramic by utilising different processing techniques is given as an example.     

Densification behavior and microstructure evolution of hot-pressed SiC–SiBCN ceramics

Densification behavior and microstructure evolution of hot-pressed SiC–SiBCN ceramics were studied between 1660 °C and 1830 °C. Polyborosilazane was chosen as the SiBCN precursor and pyrolyzed at 1000 °C in inert atmosphere before use. Samples with SiBCN contents of 10% and 20% in weight were prepared. During the sintering, at temperatures <1660 °C, the density of all the samples showed a minor increase because of solid state particles rearrangement. Above 1660 °C, the density increased rapidly because of the grain boundary sliding with a non-Newtonian viscous boundary phase. After grain boundary sliding, grain-boundary diffusion enhanced by B and C elements from the SiBCN material was responsible for the further densification. The microstructure of the samples hot pressed at 1660 °C appeared particle packing state. The two samples can achieve almost full density when they were hot pressed at 1830 °C/40 MPa for 90 min.     

Plastic deformation of dense nanocrystalline yttrium oxide at elevated temperatures

Nanocrystalline yttrium oxide, Y₂O₃ with 110 nm average grain size was plastically deformed between 800 °C and 1100 °C by compression at different strain rates and by creep at different stresses. The onset temperature for plasticity was at 1000 °C. Yield stress was strongly temperature dependent and the strain hardening disappeared at 1100 °C. The polyhedral and equiaxed grain morphology were preserved in the deformed specimens. The experimentally measured and theoretically calculated stress exponent n = 2 was consistent with the plastic deformation by grain boundary sliding. Decrease in the grain size was consistent with decrease in the brittle to ductile transition temperature.     

Densification and grain growth during sintering of porous Ce0.9Gd0.1O1.95 tape cast layers: A comprehensive study on heuristic methods

The sintering behavior of porous Ce_0.9Gd_0.1O_1.95 (CGO10) tape cast layers was systematically investigated to establish fundamental kinetic parameters associated to densification and grain growth. Densification and grain growth were characterized by a set of different methods to determine the dominant sintering mechanisms and kinetics, both in isothermal and at constant heating rate (iso-rate) conditions. Densification of porous CGO10 tape is thermally activated with typical activation energy which was estimated around 440–470 kJ mol^?1. Grain growth showed similar thermal activation energy of ～427 ± 22 kJ mol^?1 in the temperature range of 1100–1250 °C. Grain-boundary diffusion was identified to be the dominant mechanism in porous CGO10 tapes. Grain growth and densification mechanism were found strictly related in the investigated temperature range. Porosity acts as a grain growth inhibitor and grain boundary mobility in the porous body was estimated around 10^?18–10^?16 m³ N^?1 s^?1 at the investigated temperature range.     

Complex impedance studies of low temperature synthesized fine grain PZT/CeO2 nanocomposites

Fine grain nanocomposites of (100 ? x) PbZr_0.52Ti_0.48O₃ ? (x) CeO₂ with x = 0.5, 1 and 2 wt%, were prepared and characterized for structural and microstructural changes. Addition of ceria nanoparticles resulted into a fine grain microstructure with average grain size ranging from 600 nm to 440 nm and a significant decrease in sintering temperature (～200 °C). Size distribution profile, as analyzed by lognormal distribution function suggests a very narrow size distribution. X-ray diffraction analyses of sintered samples reveal that fine grain PZT/CeO₂ nanocomposite could retain distorted tetragonal structure even with grain size as low as 440 nm. Further, complex impedance spectroscopy studies were performed to illustrate the electrical properties of bulk and grain boundary phases in fine grain ceramics. Two electrical processes in the impedance spectra at temperatures above 350 °C were attributed to bulk and grain boundary phase. Magnitude of grain boundary capacitance and corresponding transition was found to be strongly dependent on grain size of the system. Both bulk and grain boundary relaxation processes follows Arrhenius law.     

Sintering behaviour and microstructure of 3Y-TZP + 8 mol% CuO nano-powder composite

Nanocrystalline 3Y-TZP and copper-oxide powders were prepared by co-precipitation of metal chlorides and copper oxalate complexation–precipitation, respectively. A significant enhancement in sintering activity of 3Y-TZP nano-powders, without presence of liquid phase, was achieved by addition of 8 mol% CuO nano-powder, resulting in an extremely fast densification between 750 and 900 °C. This enhancement in sintering activity was explained by an increase in grain-boundary mobility as caused by dissolution of CuO in the 3Y-TZP matrix. The nano-powder composite was densified to 96% by pressureless sintering at 1130 °C for 1 h. Considerable tetragonal to monoclinic phase transformation of the zirconia phase was observed by high temperature XRD analysis. This zirconia phase transformation is discussed in terms of reactions between CuO and yttria as segregated to the 3Y-TZP grain boundaries.