Unstabilized Zirconia Research Articles

Ultrafine ZrO2 powder by Laser Evaporation: Preparation and properties

AbstractUltrafine oxide powders were produced by CO2 laser evaporation of coarse ZrO2 powder or compact stabilized ZrO2 materialThe 10.6μm radiation in the power range 1–4kW was generated by a transversal flow Co2 laser which can oscillate in cw and pw operationThe 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.

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.

Microcrack coalescence in alumina-zirconia composites

Zirconia dispersed alumina ceramics exhibit better thermal and mechanical properties, such as thermal shock resistance, fracture toughness and fracture strength, than conventional alumina ceramics [1]. The toughening mechanisms operating in an alumina-zirconia two-phase ceramic material are determined by such compositional and microstructural parameters as the particle size, size distribution and volume fraction of zirconia inclusions, the type and amount of stabilizing agent in the zirconia inclusions, and the size and size distribution of the alumina matrix [2]. Stress-induced transformation toughening in an alumina matrix containing metastable tetragonal zirconia grains is the principal toughening and strengthening mechanisms responsible for the improved mechanical properties. In contrast, the volume expansion and shear strain associated with the spontaneous ZrO2 (t) to ZrO2 (m) transformation in an alumina matrix containing unstabilized zirconia grains may result in the formation of mierocracks on cooling from the sintering temperature [3, 4]. Similarly, microcracks develop on cooling from the sintering temperature in many two-phase ceramic materials due to the differential thermal contractions between the inclusion phase and the matrix phase [5]. The fracture toughness of a ceramic material may be significantly improved by the occurrence of a well-defined microcrack network, although the fracture strength is degraded as a result of the increase in critical flaw sizes. Claussen and co-workers [3, 4] observed that the fracture toughness of an alumina matrix containing unstabilized zirconia inclusions at the grain boundaries and grain junctions peaked at a specific zirconia volume fraction, depending on the particle size of the zirconia inclusions. At the peaktoughness volume fraction, the material demonstrates a fall in fracture strength. Li [6] observed a similar maximum in fracture toughness and fall in fracture strength at 5.0 vol% Hf0.75Zr0.2502 for one of the hafnia-zirconia dispersed alumina ceramics fabricated by conventional pressureless sintering. It was concluded that the fall in fracture strength was related to the coalescence of microcracks at a high enough zirconia volume fraction in the alumina matrix. Attempts have been made of investigating the effects of Various types of microcrack on the fracture toughness and fracture strength of zirconia dispersed ceramic matrices [7-9]. Discrete microcracks, by their ability to extend in the stress field of a propagating crack or to deflect the propagating crack, contribute to toughening [3, 4]. Over-extended microcracks and the subsequent coalescence affect

Toughening of Layered Ceramic Composites with Residual Surface Compression

Effects of macroscopic residual stresses on fracture toughness of multilayered ceramic laminates were studied analytically and experimentally. Stress intensities for edge cracks in three‐layer, single‐edge‐notch‐bend (SENB) specimens with stepwise varying residual stresses in the absence of the crack and superimposed bending were calculated as a function of the crack length by the method of weight function. The selected weight function and the method of calculation were validated by calculating stress intensities for edge cracks in SENB specimens without the residual stresses and obtaining agreement with the stress‐intensity equation recommended in ASTM Standard E‐399. The stress‐intensity calculations for the three‐layer laminates with the macroscopic residual stresses were used to define an apparent fracture toughness. The theoretical predictions of the apparent fracture toughness were verified by experiments on three‐layer SENB specimens of polycrystalline alumina with 15 vol% of unstabilized zirconia dispersed in the outer layers and 15 vol% of fully stabilized zirconia dispersed in the inner layer. A residual compression of ∼400 MPa developed in the outer layers by the constrained transformation of the unstabilized zirconia from the tetragonal to the monoclinic phase enhanced the apparent fracture toughness to values of 30 MPa.m1/2 in a system where the intrinsic fracture toughness was only 5 to 7 MPa.m1/2.

Densification studies of SRBSN with unstabilized zirconia by means of dilatometry and electron microscopy

A correlation of densification behaviour and microstructural development of ZrO2-fluxed sintered reaction-bonded Si3N4 (SRBSN) is reported in the light of dilatometry and both high-resolution electron microscopy (HREM) and analytical electron microscopy (AEM). Two distinct dilatometer maxima were observed using a modified dilatometer with improved sensitivity. The relatively small first dilatometer maximum, at approximately 1750‡C, is due to the formation of a highly viscous silica-rich liquid phase at the initial stage of sintering. The second maximum, showing a more pronounced densification rate, is related to a radical change in secondary-phase chemistry at approximately 1900‡C. In the SRBSN system, ZrO2 acts as an effective sintering aid because of the increased sintering temperatures and high N2 overpressure during the second dilatometer event, which promotes active participation of ZrO2 in the liquid-phase formation process. Sintering cycles were interrupted at the temperatures corresponding to the two dilatometer maxima. From these specimens, thin TEM foils were prepared, which correspond to the microstructures of the two densification events. Conventional TEM and HREM observations revealed significant microstructural differences, which could be related to the densification behaviour of the SRBSN-ZrO2 system. The good high-temperature performance of the densified SRBSN material is due to (i) the formation ofin situ large-grown Β-Si3N4 grains, (ii) a completely crystallized secondary phase (m-ZrO2), and (iii) very thin amorphous phase- and grain-boundary films of approximately 1.0 and 0.5 nm, respectively.

Tetragonal-cubic inversion in unstabilized zirconia dispersed in alumina matrix

Tetragonal-cubic inversion in unstabilized zirconia dispersed in alumina matrix

Stress–Strain Behavior of Duplex Ceramics: I, Observations

The stress–strain behavior of a variety of duplex ceramics was measured with the aid of strain gauges attached to the tensile surface of beams in four‐point flexure. The duplex materials consisted of a matrix of alumina and, alternatively, of yttria–tetragonal‐zirconia composites without (2Y‐TZP) and with alumina (3Y‐TZP20A). Up to 20 vol% pressure zones of different sizes containing unstabilized zirconia were dispersed within the matrices. Different types of stress–strain behavior were distinguished. The results were compared with microcrack and shear band observations of indented and R‐curve specimens.

Etching behaviour and associated transformation of stabilized zirconia

Ceramography of stabilized zirconia is a difficult task, often requiring hot etching [1]. In previous work we developed a technique of mechanically weakening the grain boundaries by ultrasonic machining and then etching at room temperature [2]. Reports on pure (unstabilized) zirconia show that microstructures could be attained even without etching [3]. The major difference between the stabilized and unstabilized forms of zirconia is the presence of the tetragonal (t) phase in the former and its absence in the latter. The fact that unstabilized zirconia does not need etching means that the monoclinic (m) phase of zirconia does not need etching. Similarly, the fact that stabilized zirconia is difficult to etch simply means that t-phase is not easily etched. However, etching is based totally on differences in the chemical composition of the surface [4]. Accordingly, an etchant that is capable of etching zirconia should be able to do so irrespective of the physical structure of the zirconia, i.e. irrespective of whether it is in m-phase or t-phase. Therefore, the difficulty of etching zirconia samples containing t-phase particles needs explanation. In order to assess the etching behaviour of the different phases of zirconia, two samples were chosen: sample A with 33.9% t-phase (the remainder being m-phase) and sample B with 100% t-phase. Both samples were prepared from 12 tool % ceria-stabilized zirconia (TOSOH, TZ-12CE), by dry pressing with a suitable binder and sintering. Sample A was sintered at 1600 °C for 2 h, furnacecooled and subsequently heat-treated at 400 °C for 2 h. Sample B was sintered at 1350 °C for 2 h and air-quenched (i.e. removed from the furnace soon after the soaking time was over and allowed to cool in still air). These sintering schedules were selected from previous experiences [5] of tailoring the phase structure in 12 mol % ceria-stabilized zirconia. Normally, surfaces are polished before etching. However, in zirconia there are numerous reports on the t-to-m phase transformation arising due to various stress inducing environments such as machine grinding [6-8], electron-beam heating during transmission electron microscopy observation [9] and even indentation [10]. In particular, 12 mol % ceria-stabilized zirconia is highly transformable and t-to-m phase transformation in this material has been reported even due to polishing [11]. Since polishing would induce t-to-m phase transformation,

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 inKlc 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.

The Temperature Dependence of Yield Stress and Fracture Toughness in Unstabilized Zirconia Crystals

ABSTRACTThe flexural strength and the single edge notch beam fracture toughness of undoped ZrO2 crystals, grown by the skull melting technique, were examined from room temperature to 1400°C. On heating the toughness increased with test temperature to a maximum of 4.0 MPajm at 1225°C then gradually decreased to 2.6 MPa/m. Upon cooling after a 20 minute hold at 1250°C an increase in toughness to 5 MPa/m was observed at 1200°C; upon cooling to lower temperatures Kic gradually diminished. The loaddeflection curves for the flexural strength tests showed marked nonlinearity before failure for samples tested on cooling. The temperature dependence of the apparent yield stress suggests that initial yielding occurs by slip above 1200°C but that from 1200°C to 1050°C the observed yielding is due to stress induced tetragonal to monoclinic transformation.

The thermal diffusivity and conductivity of transformation-toughened solid solutions of alumina and chromia

The thermal diffusivity of a series of solid solutions of alumina and chromia transformation toughened with a dispersed phase of unstabilized zirconia was measured by means of the laser-flash method from room temperature to 1400° C. It was found, in general, that the thermal diffusivity could be decreased significantly by the combined effects of solid solution alloying, microcracking and by the presence of the low conductivity dispersed phase of zirconia. The decrease in thermal diffusivity by microcracking was found to be present in the solid solution with low chromia content which underwent extensive grain growth. The effectiveness of solid solution formation and microcracking on thermal diffusivity was found to be greatest at the lower and intermediate ranges of temperature. The decrease in the thermal diffusivity due to the zirconia inclusions was found to be effective over the total temperature range. A numerical example is presented for the thermal conductivity calculated from the thermal diffusivity multiplied by the volumetric heat capacity.