Zirconia Grains Research Articles

Evaluation of the homogeneity in Pb(Zr,Ti)O3–zirconia composites prepared by the hetero-agglomeration of precursors using the Voronoi-diagram approach

Hetero-agglomeration of precursor particles was employed to achieve a homogeneous distribution of tetragonal zirconia (TZ) grains within a lead zirconate titanate (PZT) ceramic matrix. The surface charge of the zirconia particles in the aqueous suspension was modified by the addition of citric acid. At pH 5, the citric-acid-modified TZ particles were negatively charged, while the PZT particles were positively charged, which led to the agglomeration of the two types of particles. The homogeneity of the TZ distribution in the PZT–TZ ceramic composites prepared from the hetero-agglomerated particles was evaluated using Voronoi-diagram analyses. The results showed that the homogeneity of the composites prepared using the citric-acid-modified TZ particles was higher than in the case where the TZ particles were not modified. The curves for the crack-growth resistance were also determined in order to investigate the impact of particle homogeneity on the fracture behavior.

Lu2O3-SiO2-ZrO2 Coatings for Environmental Barrier Application by Solution Precursor Plasma Spraying and Influence of Precursor Chemistry

As environmental barrier coatings are subjected to thermal stress in gas turbine engines, the introduction of a secondary phase as zircon (ZrSiO4) is likely to increase the stress resistance of Lu2Si2O7 coatings generated by induction plasma spraying using liquid precursors. In a first step, precursor chemistry effect is investigated by the synthesis of ZrO2-SiO2 nanopowders by induction plasma nanopowder synthesis technique. Tetraethyl orthosilicate (TEOS) as silicon precursor and zirconium oxynitrate and zirconium ethoxide as zirconium precursors are mixed in ethanol and produce a mixture of tetragonal zirconia and amorphous silica nanoparticles. The use of zirconium ethoxide precursor results in zirconia particles with diameter below 50 nm because of exothermic thermal decomposition of the ethoxide and its high boiling point with respect to solvent, while larger particles are formed when zirconium oxynitrate is employed. The formation temperature of zircon from zirconia and silica oxides is found at 1425 °C. Second, coatings are synthesized in Lu2O3-ZrO2-SiO2 system. After heat treatment, the doping effect of lutetium on zirconia grains totally inhibits the zircon formation. Dense coatings are obtained with the use of zirconium ethoxide because denser particles with a homogeneous diameter distribution constitute the coating.

Microstructure of veneered zirconia after surface treatments: A TEM study

ObjectiveClinical studies reveal that veneer chipping is one major problem associated with zirconia based dental restorations, the underlying mechanisms being still investigated. We semi-quantitatively analyzed the effects of different surface treatments (thermal etching, 35/105μm sandblasting and coarse bur drilling (150μm)) on the microstructure of a zirconia veneered dental ceramic. MethodsThe relative monoclinic content on zirconia surfaces was determined using X-ray diffraction (XRD). The microstructure at the zirconia–veneer interface has thereafter been investigated using transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was used to qualitatively assess the depth of the stress-induced phase transformation. ResultsSandblasting or bur drilling significantly roughened the zirconia surface. A reverse transformation of already transformed monoclinic zirconia grains back into the tetragonal polymorph has been observed after thermal veneering treatment. In TEM, the mechanically treated samples revealed a highly damaged area of 1–3μm from the interface. The presence of monoclinic phase in veneered zirconia samples has been observed in SAED up to depths of 4μm (35μm sandblasted), 11μm (105μm sandblasted) and 9μm (150μm diamond drilled) below the interface. SignificanceRegardless of the treatment protocol and produced roughness, the veneering ceramic perfectly sealed the zirconia surface. XRD showed an increased amount of monoclinic phase on the surface treated zirconia. However after thermal treatment, the monoclinic phase was re-transformed into the tetragonal polymorph. TEM/SAED analysis has found indication for a greater extend of the monoclinic transformation into the bulk zirconia compared to the treatment related defective zone depth.

Effects of Integrated Carbon as a Light Absorber on the Coloration of Photonic Crystal-Based Pigments

Three-dimensionally ordered macroporous (3DOM) materials prepared by colloidal crystal templating are examples of photonic crystals that can exhibit structural color. The color intensity can vary widely, from a pale, nearly white opalescence to vivid, brilliantly metallic colors. Such variations are observed even for 3DOM materials of a single nominal composition that exhibit virtually identical structural order in scanning electron micrographs and are prepared from the same colloidal crystal templates. In this study we investigate the cause of the variations in color intensity for 3DOM ZrO2 systems, considering both the role of zirconia grains in the skeleton of the photonic crystal and the presence or absence of carbonaceous components in the material. Such components act as broad spectral light absorbers and are introduced either directly in the synthesis through the precursor and the polymeric template or by postsynthesis addition and carbonization of sucrose solutions. We conclude that grain-size eff...

Influence of crystal structure on debonding failure of zirconia veneered restorations

ObjectiveDelamination of porcelain from intact zirconia framework was recently reported as the most common failure mode of these restorations. The aim of this study was to investigate the influence of different laboratory surface treatments on crystals structure and fracture strength of zirconia veneered restorations. MethodsZirconia discs received airborne particle abrasion with either 50 or 120μm alumina particles then half of the specimens were annealed to remove surface pre-stresses, while assintered discs served as a control. Crystal structure of each group was evaluated using X-ray diffraction analysis (XRD). The discs were then veneered with porcelain and biaxially loaded to fracture with the veneer surface in tension (α=0.05). ResultsRegarding debonding failure, 50μm particle abrasion significantly increased biaxial flexure strength compared to as-sintered specimens. On the contrary 120μm particle abrasion resulted in significant reduction in flexure strength and was associated with higher percent of monoclinic phase (7%). However for both types of particle sizes, annealing reduced initial failure load as it led to complete reverse transformation of the monoclinic phase which was associated with zirconia grain pull-out at the critical crack location. SignificancePaying attention to the surface treatment of zirconia before veneering could reduce chances of delamination and significantly improve the strength of the bilayered restorations.

Development and mechanical characterization of novel ceramic foams fabricated by gel-casting

Porous ceramic materials are of considerable interest for a variety of chemical and industrial applications in extremely harsh conditions, particularly at very high temperatures for long time periods. A combined gel-casting-fugitive phase process employing agar as a natural gelling agent and polyethylene spheres as pore formers was exploited to produce porous ceramic bodies. Alumina and alumina–zirconia powders were used to prepare samples having a porosity of about 65–70–75 vol%. The composite powder was produced by a surface modification route, i.e. by coating a well-dispersed alpha-alumina powder with a zirconium chloride aqueous solution. On thermal treatment, ultra-fine tetragonal zirconia grains were formed on the surface of the alumina particles. SEM observations and image analysis were used to characterize the microstructure of porous samples and uniaxial compressive tests were carried out to measure their mechanical behavior.

Synthesis of spherical (30 nm) and rod-like (200 nm) zirconia co-reinforced mullite nanocomposites

The objective of this research is to study microstructure and phase evolution of zirconia–mullite (ZM) nanocomposites which were synthesized by the direct transformation from amorphous precursor monoliths. The monolithic precursors were heat treated at 950–1250 °C to obtain ZM composites. Scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were employed to investigate the microstructure of the composites. A unique nano- and submicron-zirconia co-reinforced mullite composite microstructure was obtained after a controlled crystallization. The nano tetragonal zirconia (t-ZrO2) grains (<50 nm) were spherical and embedded in the mullite lattice. The unconfined zirconia grains showed a preferred growth direction along 〈1 0 0〉 direction of t-ZrO2, and formed submicron grains (<650 nm) with bar-like shape. The precursor derived ZM composites also demonstrated enhanced mechanical properties comparing with the conventional powder processed ZM composites.

Crack simulation of nano-bioceramic composite microstructures with cohesive failure law: Effects of sintering, loads and time

Crack behaviour of zirconia toughened alumina (ZTA) microstructures are simulated with a two-dimensional finite element simulation. Finite element models are developed using actual microstructure images of zirconia toughened alumina and a bilinear cohesive zone law. Simulation conditions are similar to those found at frictional contact between a femoral head and an acetabular cup of hip prosthesis. Effects of microstructures and contact stresses are investigated in terms of crack generation. Moreover, fatigue behaviour of a microstructure is determined by performing simulations under cyclic loading conditions. It is identified that total crack length observed in a microstructure increases with increasing the magnitude of applied contact stress. Cyclic simulation results show that progressive crack growth occurs with respect to number of fatigue cycles. In addition, it is demonstrated that zirconia grains resist crack growth in microstructures.

Plasma Dissociated Zircon (PDZ) Processing; Influence of the Zr:Si Ratio in the Composition, Microstructure and Thermal Re-Crystallization

Plasma dissociated zircon (PDZ) has attractive applications in ceramic colouring, as well as the preparation of fine ZrO2 crystals, coatings and refractory materials. Generally the PDZ is obtained from different natural or synthetic zircon powders, if the cooling rate is high enough a complete dissociation is achieved, with rounded (dendritic) zirconia grains imbibed in a fused silica (non crystalline) matrix. The principal objective of this work is to establish the influence of the incorporation of both products of the zircon dissociation (fused silica and crystalline zirconia) to the initial powders mixture utilized for the processing of PDZ in an indigenous thermal plasma argon furnace. Both structural and microstructural characterization was carried out. A thermal annealing at 1500¼C with different soaking time (1-15hours) was completed in order to establish the thermal stability to the thermodynamically favoured recombination, showing that after this time a recombination was achieved, and this behaviour was not influenced by the Zr:Si ratio. The addition of fused silica resulted in an important decrease of the zirconia grains size, while the zirconia addition had the opposite result, showing that within this range (1-10μm) the final zirconia grains size can be selected and controlled with this processing variable. The present processing route showed an alternative way for obtaining different size of monoclinic zirconia powder.

Zirconia Reinforcement of Cement Free Alumina Refractory Castables by Two Routes

The cement free castables (CFC) and the ultra low cement castables (ULCC) differ from the conventional castables not only in the fact that alumina cement content and that the water required for elaboration is considerable lower, but also that they exhibit outstanding physical-chemical properties and mechanical properties. In this work we compared two routes of introducing zirconia to cement free alumina castables with a zirconia free castables, one by adding electrofused zirconia grains and the second by incorporating reused grains from electrofused AZS block formerly used as floor of glass melting furnaces. Comparable microstructures and textural properties were achieved, this fact permitted to evaluate the zirconia influence in the mechanical and fracture properties of the studied materials. There was a direct correlation with the zirconia content of both the mechanical properties at room temperature and at high temperatures (1350¡C). Finally, while at room temperature the incorporation of AZS grains proved to be more efficient to reinforce electrofused zirconia proved to be the most effective additive for reinforcement of castables studied if they were to be used at elevated temperatures.

Residual stress delaying phase transformation in Y-TZP bio-restorations

Engineering favorable residual stress for the complex geometry of bi-layer porcelain-zirconia crowns potentially prevents crack initiation and improves the mechanical performance and lifetime of the dental restoration. In addition to external load, the stress field depends on initial residual stress before loading. Residual stress is the result of factors such as the thermal expansion mismatch of layers and compliance anisotropy of zirconia grains in the process of sintering and cooling. Stress induced phase transformation in zirconia extensively relaxes the residual stress and changes the stress state. The objective of this study is to investigate the coupling between tetragonal to monoclinic phase transformations and residual stress. Residual stress, on the surface of the sectioned single load to failure crown, at 23 points starting from the pure tetragonal and ending at a fully monoclinic region were measured using the micro X-ray diffraction sin2 ψ method. An important observation is the significant range in measured residual stress from a compressive stress of −400 MPa up to tensile stress of 400 MPa and up to 100% tetragonal to monoclinic phase transformation.

In situ growth of carbon nanotubes in alumina–zirconia nanocomposite matrix prepared by solution combustion method

In this work successful synthesis of multiwall carbon nanotube (CNT) using solution combustion and chemical vapor deposition (CVD) methods are reported. Ceramic nanocomposite samples of (Al2−xFexO3)–(y)ZrO2 formula with x=0.017, 0.034 and 0.17 and y=0.15 were initially prepared. These were then subjected to CVD process during which the in situ reduction of iron oxide to metallic iron (Fe/Fe3C) phase/s provided the necessary catalyst for the CNT formation. The formation of long flexible filaments with a smooth and regular surface bridging between alumina–zirconia (AZ) grains could be detected. The diameters of the formed filaments were in the range of ∼70 to ∼320nm and length of the order of some tens of micrometers. However, transmission electron microscope (TEM) examinations also revealed the existence of small amounts of Bamboo-like carbon along with more or less straight CNTs. This could be related to the lack of strong interactions between the metallic iron phase/s and the nanocomposite support.

Influence of Zirconia Content and Grain Size on Physical and Mechanical Properties of Zirconia Toughened Alumina Ceramics

Physical and mechanical properties as well as microstructure of zirconia toughened alumina ceramics with different zirconia content were tested. The results show that the best physical and mechanical properties are obtained in sub-micron composite ceramic when zirconia content is 25%; while the physical and mechanical properties of nano-zirconia toughened alumina ceramics are far better than those of sub-micron zirconia toughened alumina ceramics. Therefore physical and mechanical properties of ceramics can be improved significantly by adding proper amount of zirconia into alumina matrix and refining zirconia particles.

Effects of Ca-, Mg- and Si-doping on microstructures of alumina–zirconia composites

The aim of this work was to obtain zirconia toughened alumina composites with different microstructures, using a simple process (powder mixing and natural sintering). Adjusting the amount of zirconia directly controls the size and localization of zirconia grains and the size of the alumina grains. Doping the composite with CaO, MgO and SiO2 allows further control of the microstructures. The influence of the thermal treatments is also investigated. The composites exhibit different structures (nano/nano-, micro/nano- and micro-composites) with zirconia and alumina grains as small as 100 and 200 nm, respectively, and with the proportion of intragranular zirconia grains varying between 0% and 90%. Zirconia plays a major role on grain size distributions as compared to CaO and MgO, whose role is almost negligible. The use of SiO2 leads to micro/nano composites with intragranular zirconia particles. The influence of these different additions is related to adjustments of the grain boundaries mobility.

Zircon–zirconia (ZrSiO4–ZrO2) dense ceramic composites by spark plasma sintering

Abstract Dense reinforced zircon (ZrSiO4)–20 vol.% zirconia (ZrO2) ceramic composites were obtained by spark plasma sintering (SPS) starting from high energy milled commercially available powders. The sintering temperature and holding time resulted in two different microstructural configurations. In the first configuration, the nano sized zirconia nanoparticles (100 nm) act as a bonding phase continuously dispersed along the zircon micronic (1–4 μm) grains. In the second one, a continuous zircon phase with well dispersed zirconia grains was achieved. Both configurations led to improvement in fracture toughness and Vickers hardness if compared to pure zircon material processed under the same sintering condition. By comparing the developed microstructure configurations, the second one exhibits higher fracture toughness (almost 4.0 MPa m1/2) due to the more effective zirconia reinforcement effects.

Influence of the zirconia transformation on the thermal behavior of zircon–zirconia composites

During a heating–cooling cycle, zirconia (ZrO2) undergoes a martensitic transformation from monoclinic to tetragonal structure phases, which presents special hysteresis loop in the dilatometry curve at temperatures between 800 and 1100 °C. Monoclinic zirconia (m-ZrO2) particles reinforced ceramic matrix composites not always present this behavior. In order to elucidate this fact a series of zircon–zirconia (ZrSiO4–ZrO2) ceramic composites have been obtained by slip casting and characterized. The final properties were also correlated with the zirconia content (0–30 vol.%). The influence of the martensitic transformation (m–t) in well-dispersed zirconia grains ceramic composite on the thermal behavior was analyzed. Thermal behavior evaluation was carried out; the correlation between the thermal expansion coefficients with the zirconia content showed a deviation from the mixing rule applied. A hysteresis loop was observed in the reversible dilatometric curve of composites with enough zirconia grains (≥10 vol.%). Over this threshold the zirconia content is correlated with the loop area. The transformation temperatures were evaluated and correlated with the zirconia addition. When detected the m–t temperature transformation is slightly influenced by the zirconia content (due to the previously evaluated decrease in the material stiffness) and similar to the temperature reported in literature. The reverse (cooling) transformation temperature is strongly decreased by the ceramic matrix. The DTA results are consistent with the dilatometric analysis, but this technique showed more reliable results. Particularly the endothermic m–t transformation temperature showed to be easily detected even when the only m-ZrO2 present was the product of the slight thermal dissociation of the zircon during the processing of the pure zircon material.

Effects of Monoclinic ZrO&lt;sub&gt;2&lt;/sub&gt; with Different Particle Size on Properties of Zirconia Refractories

The zirconia refractories were prepared using partially stabilized zirconia grain and monoclinic zirconia powder as the matrix, partially stabilized zirconia particles as the aggregate, and phenolic resin as the binder, and four sizes of monoclinic zirconia powder (D50=1 μm, 3 μm, 6 μm, 10 μm) were used as additives. Besides, an improved specimen was prepared using a specific monoclinic zirconia powder as additive. Properties of specimens with different sizes of m-ZrO2powders were researched, including apparent porosity, bulk density, cold modulus of rupture, pore size distribution. Moreover, phase composition and microstructure were analyzed. The results show that, particle size of monoclinic zirconia powders greatly affect the sintering of materials. The specimens with finer m-ZrO2powder have lower porosity, higher bulk density and cold modulus of rupture, and the grain boundaries of zirconia particles were fuzzy, showing that the matrix of the specimens were almost sintered completely. While in specimens using coarser m-ZrO2powder, the grain boundaries of zirconia particles were clear, showing that the matrix of the specimens were not sintered completely.. Though finer monoclinic zirconia powder promoted the sintering of materials in the test, it had negative impact on the microporus structure, for large pores were observed in the specimen with fine m-ZrO2powder. Finally, the improved specimen with promoted sintering and optimized microstructure as well as phase composition was obtained by using specific monoclinic zirconia powder as additive.

Microstructure of Interface between Zirconia and Veneer Porcelain

To study whether the veneering technique will have an impact on zirconia grain and the bonding type and relationship on interface between zirconia and veneer porcelain. Materials and methods: After sintered, zirconia was annealed for 15 minute to finish the phase transition from m to t.4 types of veneer porcelains were sintered and observed with SEM (scanning electron microscopy) and TEM (transmission electron microscopy). Results: With etching time extending, it appeared that many materials loosed and corrosional pit deepened, enlarged in the veneer porcelain, which made crystallize structure move into veneering surface. Composition of interface mainly was amorphous glass matrix and zirconia. Energy spectrum analysis showed that there was no remnant glass composition in the zirconia side departing from interface. SEM showed that crystal in veneering side did not participate interface bonding. Conclusion: The interface between 4 types of veneer porcelains and zirconia bonded well. Veneering sintering technique didn’t change lattice structure of zirconia, which still was tetragonal structure. The specific bonding property of interface still remained to be analyzed further to determine.

Microstructure of oxide layers formed on zirconium alloy by air oxidation, uniform corrosion and fresh-green surface modification

Cladding materials with superior corrosion resistance and anti-hydrogen pickup have been developed for high burnup nuclear fuel. We have suggested a surface modification of the cladding materials for this purpose and invented a new surface modification method “Fresh-Green”. The Fresh-Green treatment oxidizes and carbonizes a material surface in the same process. Zircaloy-2 with the Fresh-Green treatment showed the improvement of corrosion resistance in autoclave tests. In order to investigate the effect of surface modifications on the corrosion resistance, a synchrotron radiation experiment and a TEM observation were performed on different oxide layers formed on Zircaloy-2. The oxide layers were formed by air-oxidation, an autoclave test and the Fresh-Green treatment. Crystal structures of all the samples were transformed as Zr > Zr 3O > tetragonal ZrO 2 > monoclinic ZrO 2 from the matrix to the surface. Columnar grains of monoclinic zirconia were arranged unidirectionally in the Fresh-Green oxide layer treated at a low temperature. Diffusing capacity for oxygen influenced the crystal structure of the oxide layers.

Microstructure of oxides in thermal barrier coatings grown under dry/humid atmosphere

The microstructure of thermally grown oxide (TGO) in thermal barrier coatings (TBCs) oxidized under dry/humid atmosphere at 1100 °C has been characterized by transmission electron microscopy. A thin and continuous oxide layer is formed in the as-deposited TBCs produced by electron beam physical vapor deposition. The TGO formed in dry atmosphere consists of an outer layer of fine α-alumina, zirconia grains and an inner layer of columnar α-alumina grains. However, a small amount of spinel is observed in the TGO under humid atmosphere. The presence of water vapour promotes the formation of spinel.