Mykola Lugovy

Mechanical properties of ZrB2–SiC ceramic composites: room temperature instantaneous behaviour

Abstract Abstract The mechanical properties of ZrB2–30 wt-%SiC ultra high temperature ceramic composites have been studied. The composite was processed by hot pressing at 2100°C, 30 MPa and 45 min dwell time to achieve a good densification. Young’s modulus, single edge V notch beam fracture toughness, hardness, stress–strain deformation, four-point bending strength and Weibull parameters were measured. Fractography and microstructure analyses of ZrB2–30 wt-%SiC ceramic composite were also performed.

Inelastic deformation behavior of La0.6Sr0.4FeO3 perovskite

Inelastic deformation behavior of the rhombohedral La0.6Sr0.4FeO3 perovskite is reported. Such behavior is very similar to the ferroelastic deformation of LaCoO3 or lead zirconate titanate perovskites, except that no twin structure was found to exist in La0.6Sr0.4FeO3. The possible mechanisms responsible for such nonlinear deformation behavior are discussed.

In-situ neutron diffraction of LaCoO3 perovskite under uniaxial compression. I. Crystal structure analysis and texture development

The dynamics of texture formation, changes in crystal structure, and stress accommodation mechanisms have been studied in perovskite-type R3¯c rhombohedral LaCoO3 during uniaxial compression using in-situ neutron diffraction. The in-situ neutron diffraction revealed the complex crystallographic changes causing the texture formation and significant straining along certain crystallographic directions during compression, which are responsible for the appearance of hysteresis and non-linear ferroelastic deformation in the LaCoO3 perovskite. The irreversible strain after the first loading was connected with the appearance of non-recoverable changes in the intensity ratio of certain crystallographic peaks, causing non-reversible texture formation. However, in the second loading/unloading cycle, the hysteresis loop was closed and no further irrecoverable strain appeared after deformation. The significant texture formation is responsible for an increase in the Youngs modulus of LaCoO3 at high compressive stresse...

In-situ neutron diffraction of LaCoO3 perovskite under uniaxial compression. II. Elastic properties

Calculations of elastic constants and development of elastic anisotropy under uniaxial compression in originally isotropic polycrystalline LaCoO3 perovskite are reported. The lattice strains in individual (hkl) planes as well as average lattice strain were determined both for planes oriented perpendicular and parallel to the loading direction using in-situ neutron diffraction. Utilizing average lattice strains as well as lattice strains along the a and c crystallographic directions, an attempt was made to determine Poissons ratio of LaCoO3, which was then compared with that measured using an impulse excitation technique. The elastic constants were calculated and Youngs moduli of LaCoO3 single crystal in different crystallographic directions were estimated.

Boron Carbide/Boron Carbide-Carbon Nanofibers Laminates with Weak Interfaces

Three layered B4C/B4C-Cnanofibers laminates have been produced using a hot pressing technique. The laminates were designed with thick (~2.6 mm) outer layers of B4C and a thin (~90 µm) center layer of B4C-70 wt% Cnanofibers. It was found that low tensile thermal residual stress develops in the thick B4C outer layers. The compressive thermal residual stresses exist in the thin central layer. The magnitude of the tensile residual stress was estimated as 11.3 ± 2.5 MPa. The effect of weak interfaces on failure mechanism of the laminate was studied.

7 – Design of tough ceramic laminates by residual stresses control

Publisher Summary Ceramics have found their use in numerous crosscutting industrial applications because of excellent hardness, wear, corrosion resistance, and ability to withstand high temperatures. The best approach to increasing the fracture toughness that enables the structural application of ceramics is through the development of ceramic composites. However, since these materials have a very high density of weak interfaces, they are not very strong. Several publications on ceramics show that the use of layered materials is the most promising method for controlling cracks by deflection, bifurcation, microcracking or internal stresses. Layered structures clearly offer a key to greater reliability at a moderate cost and new applications may result as more complex structures are tailored to specific applications. The way to achieve the highest possible mechanical properties is to control the level of residual stresses in individual layers. One can increase the strength and apparent fracture toughness of ceramics by creating a layer with compressive stresses on the surface. In such a way, surface cracks will be arrested and, therefore, higher failure stresses are achieved. The sign and value of residual stresses can be established by theoretical prediction. There have also been a number of experimental studies of laminated ceramics that were conducted using the models, attempting to maximize the mechanical properties. In the case of symmetrical laminates, it can be done by choosing layer compositions such that the Coefficient of Thermal Expansion (CTE) of the odd layers is smaller than the CTE of the even ones.

Two-layered Cantilever Sensor: A Simplified Mechanical Analysis of Dimensional Limitations

A model that predicts minimal length and thickness of all ceramic two layer cantilever sensor for chemical and biological detection is proposed. The model allows the estimation of minimal length and thickness where the conditions for the safe cantilever operation are satisfied. Two materials have been chosen for the consideration of the piezoelectric and non-piezoelectric layers in the layered cantilever. A piezoelectric material is lead zirconate titanate and a non-piezoelectric material is boron carbide. Different conditions, such as von Mises criterion and Mohrs strength theory are considered to find safe stress level in the clamped cross-section of the cantilever.