E. E. Lomonova

Vickers and Knoop Indentation Behaviour of Cubic and Partially Stabilized Zirconia Crystals

Abstract Hardness and fracture toughness of cubic zirconia crystals (CZC) doped with yttria and partially stabilized zirconia crystals (PSZC) doped with yttria, terbia and ceria were studied at indentation loads of 0.1–300 N. MOR and K IC of these materials were measured under bending conditions. A maximum hardness value of H K = 21.2 GPa was obtained for CZC at a 0.1 N indentation load when a longer diagonal of the Knoop indenter being directed along 〈110〉. Starting from a 2 N load, PSZC exhibited lateral cracks and phase transition zones. Radial cracks in Vickers indentation tests of Y-Tb-PSZC and Y-Ce-PSZC appeared above 40 N and 50 N respectively. Such cracks in Knoop indentation tests were observed above 300 N. Orientation anisotropy of lateral cracks was noticed in all crystals, radial ones were revealed only in CZC. After grinding off the specimen surfaces, lateral cracks were found to be present under the impressions. The calculations of fracture toughness K IC v . using the equation proposed by the authors gave a value corresponding to those materials determined on bending.

Strength and fracture toughness of zirconia crystals

Abstract Single crystals up to 40 mm wide and 120 mm long made of zirconia with yttria addition were investigated over the temperature range of −140 to 1400°C by the methods conventionally used for advanced ceramics. The stress-strain curves were analysed. Up to 1000°C the curves of all the crystals were linear (the ‘brittleness measure’, χ = 1), and on heating up to 1400°C a minimum value of the ‘brittleness measure’ for partially stabilized crystals was 0·73. At 1400°C the MOR of cubic single crystals is almost 2 times higher than at 20°C and is greatly dependent on a loading rate. The strength of PSZ single crystals decreases with an increase in temperature. The decrease of test temperature from ambient to −140°C results in a 20% increase in strength of these crystals, the latter being practically independent of a loading rate. The relation between the elastic moduli and the fracture toughness of single crystals was investigated. A maximum value of K Ic = 16 M Pa m 1 2 at Ed = 301 G Pa was measured. It is shown that the R-curves are flat and that a fracture barrier effect exists. The mechanical behaviour of crystals was also studied by Vickers indentation. Partial substitution of yttria with ytterbia increases the fracture toughness of these materials. The results of fractographic investigations were used to analyse the test data. Information sufficient to compare single crystal materials with advanced ceramics and necessary to estimate their potential applications in ambient- and high-temperature devices has been obtained.

Synthesis of Refractory Materials by Skull Melting Technique

This chapter discusses methods of growing refractory oxide single crystals and synthesis of refractory glasses by skull melting technique in a cold crucible. It shows the advantages of radiofrequency (RF) heating of dielectric materials in a cold crucible and points out some specific problems regarding the process of growing crystals by directional crystallization from the melt and by pulling on a seed from the melt. The distinctive features of the method of directional crystallization from the melt are discussed in detail on the example of technology of materials based on zirconia, i.e., cubic single crystals and partly stabilized single crystals. It is shown that the size and quality of crystals are functions of the process conditions, such as thermal conditions under crystallization, growth rate, and chemical composition. We provide an overview of research on the structure, phase composition, and physicochemical properties of crystals based on zirconia. The optical, mechanical, and electric properties of these crystals make them suitable for a number of technical and industrial applications in optics, electronics, materials processing, and medicine. In this chapter, we also consider some problems regarding the synthesis of refractory glasses by skull melting technique. The physicochemical and optical properties of glasses are given and their practical applications in technology are discussed. We note that one of the better developed and most promising applications of skull melting technique is the immobilization of liquid and solid waste (also radioactive waste) into solid-state materials by vitrification.

Structure and mechanical properties of crystals of partially stabilized zirconia after thermal treatment

The phase composition and morphology of the twin structure of the Y2O3-stabilized zirconia crystals (from 2.8 to 4.0 mol %) after the thermal treatment at 1600°C have been investigated by X-ray diffractometry and transmission electron microscopy. It is shown that as the concentration of the stabilizing Y2O3 impurity increases, the character of the twin structure changes, and the amount of the untransformed phase t′ increases. The dependence of the hardness and crack resistance of the crystals of partially stabilized zirconia on the Y2O3 concentration and the indenter orientation is investigated using the microindentation method. The sample with the lowest concentration of the stabilizing Y2O3 impurity turned out the most crack resistant. This can be explained by a high content of tetragonal phase t in it, which provides the transformation strengthening mechanism of the material, and by a more multilevel character of twinning.

Structure and phase composition studies of partially stabilized zirconia

Zirconia single crystals doped with 2.8, 3.2, 3.7, and 4.0 mol % of Y2O3 have been studied. The phase composition and structure have been studied by X-ray diffraction analysis and transmission electron microscopy. It has been established that all investigated samples has two ZrO2 tetragonal phases with tetragonality c/a = 1.006–1.007 and c/a = 1.014–1.015 independent of the stabilizing impurity content. The former phase is not transformed, whereas the latter is transformed into a monoclinic one. In all cases the samples have a developed twin structure. Twinning hierarchy is observed: there are first-, second-, third-order, etc., twins, each of them containing the next-order twins inside them. Elastic stress relaxation occurs by twinning rather than by generation of dislocations. The stabilizing impurity content affects the structure nonmonotonically; the minimum dimensions of the twin lamellas refer to the Y2O3 concentration of 3.2 mol %.

Preparation and properties of Y2O3 partially stabilized ZrO2 crystals

We have studied the effect of composition and growth conditions on the structure and properties of 2.5–5 mol % Y2O3 partially stabilized ZrO2 crystals grown by directional solidification in a cold-wall crucible. The phase composition and density of the crystals have been determined. The crystals are shown to be uniform in composition, with local changes in Y2O3 content within ±0.5 mol %. The dimensions and quality of the single crystals are influenced by the growth conditions.

Zirconia crystals with yttrium and cerium oxides

Results obtained for a new material, namely, zirconia crystals stabilized by Y2O3 and CeO2 and containing a technological addition of neodymium oxide, tested at room temperature are described. Special features of the surfaces of the crystal blocks grown and the distribution of the additions introduced into the charge over their height are investigated. The dependences of the amounts of the stabilizing components and the fracture properties of the investigated material on its strength are determined. Certification results for the mechanical characteristics of the material are presented (mean four-point bending strength 1250 MPa, crack resistance in bending of a notched bar 11.43 MPa·m1/2 at an elasticity modulus of 366 GPa for the crystallographic direction 〈100〉). The crystals are investigated in the crystallographic plane {100} by applying Vickers and Knoop indenters, and their hardness at different loads (beginning with 0.1 N) is determined. It is established that zones of phase transformations are formed near the indentations in addition to radial and lateral cracks. Problems of change in the specific features of the mechanical behavior of the crystals with change in the valence of cerium (most experiments were conducted for Ce2O3), with the heat treatment, etc. are considered. The results are analyzed using data of fractographic investigations and the new data obtained in tests of similar crystals partially stabilized by yttrium and terbium oxides.

Mechanical properties of partially stabilized zirconia crystals studied by kinetic microindentation

Using kinetic microindentation, we have studied the strength and elastic characteristics of partially stabilized zirconia (PSZ) crystals. The results demonstrate that the mechanical properties of the crystals depend on yttria stabilizer concentration. Additional doping with ceria leads to an increase in the kinetic hardness, kinetic Young’s modulus, and plasticity of the crystals, thus improving their mechanical characteristics. The PSZ crystals have high plasticity for nonmetallic materials.

Thermal conductivity of single-crystal ZrO2-Y2O3 solid solutions in the temperature range 50–300 K

The thermal conductivity of ZrO2−xY2O3 single crystals (x = 0.5, 1.5, 2.0, 2.5, 3.0, 8.0 mol %) has been studied experimentally in the temperature range 50–300 K. The influence of high-temperature annealings on the thermal conductivity has been analyzed.

Spectroscopy of optical centers of Eu3+ ions in partially stabilized and stabilized zirconium crystals

The structure of the optical centers of Eu3+ ions in tetragonal (ZrO2)1–x–y(Y2O3)x(Eu2O3)y (х = 2.7–3.6; y = 0.1) and cubic (ZrO2)1–x–y(Y2O3)x(Eu2O3)y (х = 8–38; y = 0.1–0.5) crystals of solid solutions on the basis of zirconium dioxide is studied using the methods of optical and Raman-scattering spectroscopy. Characteristic optical centers of Eu3+ ions with different crystalline environments are revealed in the above compounds.