G. A. Gogotsi

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.

Strain and Fracture of a Ceramic Based on Lanthanum Chromite

The bending behavior of a ceramic — a lanthanum chromite-based solid solution La1 – xCaxCrO3 (x = 0.025) doped with 5 – 40 wt.% Y2O3 — is studied in the temperature range of 20 – 1400°C. At 20°C the composition with 5 – 10 wt.% Y2O3 is strained inelastically and exhibits relatively superior mechanical properties in comparison to compositions containing 20 – 40 wt.% Y2O3. These compositions are prone to brittle fracture within the linear elastic strain. Based on the strain and fracture characteristics of lanthanum chromite at 20°C, its mechanical behavior at high temperatures is predicted.

A micro-Raman study of phase transformations of zirconia crystals upon introduction of a vickers indentor

A micro-Raman study of zirconia crystals and ceramics performed with the use of argon, He - Ne, and semiconductor lasers is described. The results of the study are shown to depend on the wavelength of the radiation source used. It is shown that argon radiation determines only the monoclinic phase formed upon indentation of low-strength crystals of partially stabilized zirconia, which is not observed in the case of tests of high-strength crystals. The indentation behavior of crystals of partially stabilized zirconia is shown to depend on the tetragonal-monoclinic phase transformation and the rotation of typical ferroelastic domains.

Comparative analysis of fracture toughness test methods for ceramics and crystals at room and lower temperatures

Flexural and indentation tests to estimate the fracture toughness of ceramics and crystals were compared. It was demonstrated that different methods can give reliable estimates in tests of homogeneous ceramics and quite ambiguous in tests of nonhomogeneous materials. Zirconia ceramics partially stabilized with magnesia were shown to exhibit a low-temperature inelastic-elastic transition in the temperature range of their fracture toughness variations.

Fracture toughness, strength, and other characteristics of yttria-stabilized zirconium ceramics

The mechanical behavior of zirconium ceramics with different porosities (456 and 1330 MPa) is studied with an accent on the fracture toughness (the study of other characteristics in a wide temperature range plays an auxiliary role) The tests are predominantly performed with the use of a Vickers pyramid (theIF andIS methods) and under flexure (theSEVNB andSENB methods) The results of the indentation and subsequent bending of the specimens are used to plot aR-curve that turns out to be plane It is shown that despite the substantial difference in the strength of the studied variants of ceramics their crack resistance differs inconsiderably The values of the critical coefficients of stress intensity determined by various methods for the same ceramics are shown to differ The results obtained are analyzed using the data of a micro-Raman analysis The tests for crack resistance by the method ofSEVNB (bending of a beam with a polishedV-notch) show that the specimens do not undergo a tetragonal-monoclinic phase transformation during the deposition of a stress concentrator

Crack resistance of ceramics and composites with ceramic matrix (SEVNB method)

The SEVNB method for studying crack resistance of ceramics and composites with a ceramic matrix is considered. Novel data on the crack resistance of various ceramics are presented. The special features of the SEVNB method which make it an optimum instrument for determining the crack resistance of ceramics and some composites with a ceramic matrix are described.

Raman spectroscopy and mechanical behavior of zirconia materials

A physicomechanical and Raman investigation of partially stabilized zirconia ceramics with an additive of magnesium monoxide is described. It is shown that for temperatures from 59 to −60°C the material undergoes a low-temperature elastic-inelastic transformation, whereas the Raman spectra exhibit only a gradual variation of the phase composition with reduction of the temperature. In crystals with additives of yttria and ceria no substantial phase transformation related to the introduction of an indentor was discovered. A comparative analysis of Raman spectra of cubic zirconia crystals and ceramics with yttrium and ferric oxides has been made, with the conclusion that the special features of the processes controlling the mechanical behavior of such materials should be studied in greater detail.

Strength and crack resistance of zirconium dioxide crystals containing yttrium and terbium oxides

ConclusionsWe studied the mechanical behavior of the partially stabilized (with yttrium oxide) zirconium dioxide crystals in which a small quantity of terbium oxide was introduced as an additive. The obtained data indicate the suitability of this material for practical applications. The effect of heat treatment on the physical and mechanical properties of such materials was revealed. It was established that on cooling up to −150°C, their strength and thermal shock resistance are improved.The lack of correspondence between the values of the crack resistance determined according to the methods of bend testing and indentation was established. The existence of a significant barrier to fracture was observed. The two possible types of surfaces of the crystals under consideration were revealed and the relationship between the structure of these surfaces (and, also, the near-surface layers adjoining them) and the resistance to fracture was established.

The Mechanical Behavior of Lanthanum Cobaltite-Based Perovskites with a Mixed Ion-Electron Conductivity at Different Temperatures

The mechanical behavior of a perovskite-type ceramic based on partially substituted lanthanum cobaltites is studied over a wide temperature range. The La0.8Ca0.2CoO3 composite is shown to be the most inelastically deformable, high-strength, and crack-resistant. In perovskites, the brittleness can be considered as a measure of both inelasticity and ferroelasticity. The perovskites are tested for deformability using bending and indentation techniques, and an analogy in the results obtained by the two techniques is noted. Specific features of the fracture of perovskites tested by the Vickers indentation method are discussed. The crack resistance and stress-strain diagrams of the La0.8Ca0.2CoO3 composite are studied as a function of temperature. Fractographic test data are used to analyze the results obtained.

PROBLEM OF EVALUATING THE CRACK RESISTANCE IN CERAMICS OF Si3N4 AND ZrO2

The mechanical behavior of silicon nitride and zirconium dioxide ceramics is investigated in a wide temperature range. Much attention is paid to crack resistance under the conditions of the ambient. Data obtained by different methods used for evaluating crack resistance are analyzed, and it is shown that the best results have been obtained in tests in which the concentrator was an indentation of a Vickers pyramid. The analysis was conducted using fractographic investigations of the materials and data of tests of a model material of zirconium dioxide.