Key Engineering Materials Vols. 280-283

Abstract: We established a new expression to describe the nanoindentation unloading data by assuming that the Berkovich indenter behaves as a conical punch, rather than a paraboloid punch, and properly considering the effect of residual contact stress on the unloading load-displacement relation. The validity of this new approach was confirmed by analyzing the experimental data obtained for a series of brittle materials. It was shown that, compared with the generally adopted power law, this new expression has much clearer physical significance.

Abstract: By assuming that the test material has a load-independent nanohardness number, a linear relationship was predicted to exist between the reciprocal of the initial unloading stiffness, 1/SM, and the inverse square root of the peak load, (1/Pmax)0.5, and the load-independent hardness can be obtained directly from the slope of the 1/SM−(1/Pmax)0.5 straight line. This prediction was then verified by analyzing the experimental data obtained on soda-lime glass and a tetragonal ZrO2 polycrystalline. The indenter area function established based on the resultant load-independent hardness was found to deviate from the perfect Berkovich indenter and such a deviation can be attributed to the indenter deformation occurring during indentation as well as the indenter tip rounding.

Abstract: Polypropylene fiber is a new measure to prevent plastic cracks of concrete. Effects of the parameters, such as dosage and types of fibers, on the plastic cracks were studied systematically. The properties of cracking resistance of mortar, ordinary concrete and high performance concrete were investigated by using samples of two types in shape. The results show that: (1) polypropylene fibers may increase the cracking resistance of concrete further; (2) as smaller quantity of cement and higher quantity of aggregate as possible should be used to prevent concrete form cracking; (3) the main reason why polypropylene fibers increase cracking resistance of concrete is that they increase strain capacity of concrete at early age, decrease shrinkage strain, improve plastic tensile strength and decrease tensile stress of the capillary.

Abstract: Based on the SEM observation on cracks in lined ceramics of the composite pipes, the stress fields in lined ceramics were analyzed. The results indicated that there are the compressive field and heat stress field in lined ceramics, resulting in the existence of radial cracks and net cracks; besides being influenced by wall thickness of ceramic layer, the strength of compressive stress field is also dependent on bonding strength between metal and ceramics, and the strength of heat stress field is dependent on temperature gradient in lined ceramics during cooling of the ceramics.

Abstract: Ni base alloy ceramic grain composite coating is used mostly in high temperature condition, so thermal fatigue failure will be easy. If the thermal expansion coefficients and elastic modulus of Ni base alloy and ceramic grain are different, there will be thermal stresses between grain and matrix in thermosyphon. The thermal stress will arouse the initiation and growth of thermal fatigue crack. We use Eshebly-Mori-Tanaka method to study the thermal stress field in matrix and grain. It can be shown that the more difference of thermal expansion coefficients between Ni base alloy and ceramic grain is, the bigger the thermal stress is. The thermal stress relates to volume fractions and elastic constants of Ni base alloy and ceramic grain. Based on low-cycle fatigue crack growth rate formation, the thermal fatigue life was computed. The bigger thermal stress is, the smaller thermal fatigue life is. Thermal life is an exponential function of crack initiation length and critical length.

Abstract: By the use of comprehensive transformation criterion and the method of weight function, the fracture enhancement of mixed-mode II-III crack in transformation toughened ceramics is predicated. The theoretical expressions of the toughening effect for both the stationary and steady-state growing crack are given respectively. The results show there is no toughening effect for the stationary crack and the toughening effect for the steady-state growing crack is associated with the modulus of elasticity, the width of transformation and its volume fraction.

Abstract: An optimum model for the composition design of the advanced ceramic material is built based on the wear resistance with the combination of both theoretical and experimental methods, with KIC 3/4×H1/2 working as the coefficient of wear resistance when the abrasive wear dominates. Results show that the tested SiC/(W,Ti)C/Al2O3 ceramic material can be expected to achieve the highest wear resistance when the volume fraction of SiC and (W,Ti)C is about 15% and 17%, respectively. The optimum composite is then fabricated with the hot pressing technique. Its wear resistance is approximately 43% higher than that of the pure alumina ceramic when used as the tool material in the machining of the hardened tool steel. The increment of the wear resistance of the developed ceramic material coincides well with that predicted from the optimum model. It proves that the method proposed in the present study is feasible for the ceramic materials mainly with wear mechanisms of abrasive wear.

Abstract: Based on retrieval for domestic and international literatures about the grinding fluids special for ceramic machining use, this paper put forward a method to enhance the engineering ceramic grinding machining efficiency using the organic grinding fluid added with the long chain alcohol The experiment, performed on the constant force cutter, greatly increased the ceramic grinding efficiency by 2.4 times through the comparative experiment for several normal grinding fluids as well as by adding additives. In addition, this paper conducted the matter structural analysis for the liquid-solid interface using the Raman Spectrum technology and preliminarily discussed the mechanism of this grinding fluid.

Abstract: The machining of ceramic balls is one of the main difficulties of the manufacture process of ceramic ball bearings. This paper introduces a new grinding technique for green ceramic balls, the twowheels grinding technique. The sphering mechanism for the green ceramic balls ground with the technique was analyzed in theory, and the influencing factors were studied by experiment. The experimental research presents that the spherical deviation of the ground green ceramic balls can be improved to 0.02mm.

Abstract: Wettability of ceramics by liquid metals is of importance for joining of ceramics to metals and manufacturing of cermets. Wettability of reaction bonded SiC /Ag plus Ti system was studied. The contact angle was determined by high temperature photography. The microstructure and composition of the interfacial region were analyzed. For SiC/pure Ag system, the contact angle at 998°C in vacuum is 128.4º. The wettability of the system is remarkably improved by adding Ti into the metallic filler. Also, the contact angle of the system is strongly affected by the wetting temperature and the dwelling time. The results indicate that interfacial reactions take place during the test, and the reactions facilitate the wetting. Microstructural study reveals that inter-diffusion takes place and a reaction layer forms at the interface.