Key Engineering Materials Vols. 336-338

Abstract: Joining of ceramics is of importance from both technical and economical points of view. Joining of recrystallized SiC ceramic to itself has been realized by hot pressing reaction joining process using Ni-51Cr powders (consisting of Ni+51wt%Cr powders) as filler. The optimized technological parameters have been obtained by orthogonal experiment, which are joining temperature of 1150°C, holding time of 20min, joining pressure of 10MPa and cooling rate of 0.25°C/s. The most effective factor to influence the weld strength is joining pressure within the range of testing. Under the optimized conditions, the maximum relative bending strength of joint, 70.7%, is achieved. The microstructure and phase composition of the weld zone were examined by SEM, EDX and XRD. The results show that interdiffusions and chemical reactions take place in the joining process. A reaction layer, which is mainly composed of Ni2Si and Ni3C with a little Cr, exists between the welding base material SiC ceramic and the filler reaction product layer called as interlayer, which is mainly composed of Ni2Si and Cr.

Abstract: Based on the fractal theories, using image processing software (IPS) to handle the scan electron microscope photos of particles, this paper gives a simple and applied method to pick up the boundary line’s fractal dimension (FD) of particles’ projective numerical images. Take example for Al(OH)3 material particles, adopting respectively relative Box-counting (BC) method and relative C-S method, we have calculated the boundary FD values of the research objects and compared FD with other quantitative token parameters, which shows FD is a sort of more refined quantitative token parameter. Finally, utilizing the method of Generator, we get some fractal figures which have nearly same FD values with Al(OH)3 material particles, utilizing that we can simulate and analyze the complexity of the boundary and surface micro-topography of Al(OH)3 material particles.

Abstract: A relative method, defined as indirect approach to evaluate the material properties via the relationship between unknown properties and a known property, is proposed to estimate some properties that could not be measured by the traditional methods for ceramics. Experiments and theoretic analysis based on the relative method were carried out in this study to estimate the properties in following aspects: determining the temperature dependence of elastic modulus of some machineable ceramics by comparing the deflections; obtaining the modulus and strength of ceramic coatings supported by substrates, from the variation in properties of the rectangular beam samples before and after coating; estimating the residual stresses in tempered glass by comparing the change in the surface strength after strengthening.

Abstract: The compressive strengths of three kinds of ceramic microbeads, ZrO2 toughened mullite, zirconium silicate and industrial zirconia, were studied in this paper. The densities of these ceramic microbeads were 2.9, 3.5 and 5.3g/cm3. The weighted average of compressive strength of these ceramic microbeads was 446.9, 500.8 and 917.3 N/mm2. It was concluded that the compressive strength of ceramic microbeads was proportional to the toughness and Young's modulus of the materials, and a larger diameter of the ceramic microbeads led to a higher compressive strength.

Abstract: A glass layer with high hardness and fatigue resistance is coated on the aluminum titanate (Al2TiO5) – mullite (3Al2O3⋅2SiO2) composites to improve wear and fatigue resistances. Mullite is added to aluminum titanate to constraint the decomposition of aluminum titanate and exhibit high mechanical strength as well as high elastic modulus. The aluminum titanate composites with various quasi-ductilities through controlling the contents of mullite in the substrate layers are prepared in this study. The results of the load-displacement curves and the resulting contact damages on the surface of glass coating layer by spherical indentation indicate that mullite addition and the glass coating is effective.

Abstract: Yttria-stabilized cubic zirconia is well known the material that possesses high oxygen ionic conductivity and chemical stability over wide ranges of temperature and oxygen partial pressure and thus it is widely used as an oxygen sensor, thermal barrier and solid oxide fuel cell (SOFC) electrolyte. In the present study, 8 mol% yttria-stabilized cubic zirconia with SiO2 addition up to 10 wt% was studied with respect to the microstructure, fracture toughness and hardness. XRD results showed that SiO2 had very limited solubility of 0.3 wt% in cubic zirconia. This suggests that only small part of SiO2 dissolved in cubic zirconia and the rest of SiO2 segregated at grain boundaries and multiple junctions. This glassy phase also wetted the zirconia grains and prevented the grain growth and the formation of facetted grains. Both hardness and fracture toughness were measured using a Vickers indenter. It is observed that the introduction of SiO2 decreased the hardness and increased the fracture toughness of cubic zirconia. The hardness and fracture toughness also showed the same trend with increasing SiO2 content.

Abstract: The nanoindentation unloading behavior of some brittle ceramics with modified surfaces was analyzed. It was found that the unloading data may be described well with a quadratic polynomial. The physical meaning of the quadratic polynomial in describing the nanoindentation unloading behavior was then discussed by considering the effect of residual contact stress on the force-displacement relationship. It was suggested that the quadratic polynomial may be considered as a modified form of the basic forcedisplacement relationship for the contact of an isotropic elastic half-space by a rigid conical punch.

Abstract: Fatigue behaviors of conventional alumina ceramics in cyclic loading with hemisphere and cylinder punches are investigated with modified small punch (MSP) tests. Residual strengths are used to quantify the degree of fatigue damage. The dependences of the residual MSP strength on loading mode, cyclic loading magnitude, frequency, and puncher shape have been discussed in detail. Fatigue behavior studies could be very convenient and effective because MSP tests could perform the cyclic loading and then evaluate the residual strength consistently and effectively.

Abstract: Dynamic fatigue behavior of two kinds of Y-TZP ceramics was investigated at room temperature. The results showed that dynamic fatigue and slow crack growth behaviors exist in zirconias. Crack growth exponents of original and pre-cracked samples were obtained. Crack growth exponents of the two original zirconias are 15.60 and 21.00, respectively. Crack growth exponents of annealed pre-cracked samples are close to the original samples, indicating that the pre-crack experimental method is reasonable and effective. Because of the influence of residual stress indcued by indentation, crack growth exponents of unannealed samples are different from that of original samples. So residual stress should be removed.

Abstract: Recent experiment showed that nano-fibers composite ceramics, fabricated through SHS process, could acquire high toughening and strengthening. Composite ceramics are mainly composed of fiber eutectic with random orientation, in which nanometer sized zirconia fibers are dispersed within the alumina matrix. First, it can be visualized that tensile force is transmitted from the matrix to the nano-fiber by means of shear stress that develop along the fiber- matrix interface. Then the shear stress on the surface of the fiber eutectic is obtained. It related to the volume fraction and slenderness ratio of the nano-fibers, and external strain of the fiber eutectic. The maximum shear stress is at the ends of the fiber eutectic perpendicular to applied tensile stress. As maximum shear stress on the surface of the fiber eutectic is equal to the critical shear stress, the composite ceramics would crack. So the applied tensile stress on composite ceramics arousing the surface crack of the fiber eutectic is gotten.