Jin Zhihao

The influence of brazing conditions on joint strength in Al2O3/Al2O3 bonding

The brazing of alumina ceramic to itself was performed using Ag57Cu38Ti5 filler alloy. The bonding was carried out in a vacuum of 7 × 10−3 Pa, and the joining conditions were at 1073, 1123, 1173, 1223, 1273 and 1323 K for 1.8ks under a pressure of 0.01 MPa, at 1123 K with a pressure of 0.01 MPa for 0, 0.3, 0.9, 1.8, 2.7 and 3.6 ks, and at 1123 K for 1.8 ks with pressures of 0, 0.01, 0.05, 0.10, 0.15, 0.20 and 0.30 MPa, to determine the effects of joining temperature, pressure and holding time on the joint strength. The joint strength was measured by shear tests. The interface microstructures and fractured surfaces after testing were observed by scanning electron microscopy (SEM). It was shown that the shear strength of Al2O3/Al2O3 joints was largely affected by the joining conditions; it first increased and then decreased with increasing joining temperature, pressure and holding time and depended mainly on the strength of interfacial reaction layer itself and the interface bonding strength between the reaction layer and the ceramic. The maximum joint strength was obtained when the reaction occurred under a suitable temperature, pressure and time, and the reaction layer thickness was about 2 μm. SEM observations revealed that there were four types of fracture and each kind corresponded to a different strength.

Interfacial reaction of alumina with Ag-Cu-Ti alloy

The interfacial reaction of Al2O3 and Ag-Cu-Ti alloy was investigated by scanning electron microscope (SEM) and X-ray diffraction (XRD), respectively. It was shown that Al2O3 ceramic reacted strongly with Ag-Cu-Ti alloy. With the increasing heating temperature and holding time, the reaction layer thickness increased and its growth was mainly controlled by the diffusion of titanium through the reaction layer. The reaction products were Cu2Ti4O and AlTi at or below 1123 K. However, there were two distinct layers at the interface at or above 1173 K, one layer in the vicinity of ceramic consisting mainly of Ti2O and TiO and the other layer near the alloy was CuTi2, a layer transition structures with a Al2O3/Ti2O+TiO/Ti2O +TiO+CuTi2/CuTi2/Ag-Cu formed at the interface according to the SEM and XRD analyses results. A lower or a higher joining temperature and a shorter or a longer holding time were disadvantageous for a stable and high reliable joined interface from the point of view of interfacial microstructures and morphologies.

An investigation of the corrosion–abrasion wear behavior of 6% chromium martensitic cast steel

Abstract The corrosion–abrasion wear resistance of 6% chromium martensitic cast steel containing 0.35–0.95% carbon was investigated in three kinds of sand slurries with pH values of 5, 7 and 10, respectively. The corrosion rate of the martensitic cast steel in the solutions with the same pH values was also measured and related with its corrosion–abrasion wear behavior. It is shown that the corrosion–abrasion wear resistance of the steel increased with increase in its carbon content. However, the corrosion process of the corrosive solution could influence the abrasion wear rate of this steel considerably.

Comparison between fatigue behavior of some ceramics: a new concept of intrinsic stress-corrosion exponent n0

Abstract The fatigue behavior was systematically investigated with three typical ceramic materials, including Y 2 O 3 –ZrO 2 , Si 3 N 4 and a machinable glass–ceramic. Cyclic, static and dynamic fatigue tests were conducted in three environments, i.e., moist air, distilled water and kerosene. The effects of environment and loading condition on fatigue behavior were analyzed. The difference between materials was discussed. Experimental results showed that, for all the three materials, fatigue life under cyclic load was the shortest and a low value of n (fatigue exponent) was obtained. Compared with the other materials, Si 3 N 4 ceramic had a very large n value under static load. Therefore the static fatigue for Si 3 N 4 ceramics may be ignored. Cyclic load decreases the fatigue life of transformation-toughening ceramics (Y–TZP) more seriously, while no remarkable difference was observed between cyclic and static fatigue for original glass of the glass–ceramic. A new idea about the physical meaning of n value and the concept of intrinsic stress-corrosion exponent n 0 were introduced. The exponent n can be separated into two terms, i.e., n = n 0 + n μ , where n 0 is a material constant and just related to the most basic properties, such as atom-bonds or crystalline structure, n μ reflects the contribution of microstructure toughening and is very sensitive to environments or loading conditions. This hypothesis can be used to describe and explain the experiment results successfully.

Investigation of the behaviour of rare earth element cerium in aluminium–lithium alloys by the method of internal friction

Abstract The behaviour of rare earth element Ce in 2090 Al–Li alloys has been studied by the method of low frequency internal friction. The results showed that trace solution Ce atoms can segregate at the grain boundary of Al–Li alloys, improve the grain boundary strength of the alloys, increase the activation energy of the grain boundary, and decrease the tendency of inter-granular fracture of the alloys. Rare earth element Ce can decrease the tendency of reduction of the elastic modulus of 2090 Al–Li alloys after heat cycle, which would benefit the high temperature mechanical properties.

The effect of molybdenum net interlayer on thermal shock resistance of Al2O3/Nb brazed joint

Abstract The effect of molybdenum net interlayers on the thermal shock behavior of Al2O3/Nb brazed joints was investigated. The net was able to increase the thermal shock resistance of the joint considerably. When brazed with Ni5lTi49 and Cu52Ni18Ti30 solders separately and quenched into water from 700°C, the thermal shock resistance of the joints brazed with the nets was raised by more than 180% and 130% respectively, compared to those of the joints without nets. The fracture photographs of the joints with the nets were different from those without nets. It was supposed that the existence of the net could reduce the residual stress markedly.

Decreasing the sintering temperature of diamond-bit matrix material by the addition of the element P

Abstract Matrix material of low melting point was prepared by the modification process of Cu–P pre-alloy. Experimental results showed that adding such trace elements as Si, Sb and Ce improved the toughness and weldability of this copper alloy with high P content. Using a matrix powder of low melting point had the advantage of decreasing the sintering temperature and manufacturing cost, increasing the drilling efficiency and prolonging the service life of the diamond bit.

A comparison of failure predictions by dynamic bending and cyclic bending fatigue techniques for Y2O3ZrO2 ceramics

Abstract Predictions of failure time under load were made for three Y 2 O 3 ZrO 2 ceramics using dynamic bending and cyclic bending fatigue techniques. Agreement of failure predictions by the two techniques was obtained for two of the materials. The implications of these results with regard to crack propagation mechanisms and transformation toughening effects are discussed.

The effect of the microstructure on the static fatigue behaviour of Si3N4

The effect of the microstructure on the static fatigue behaviour of two Si3N4 samples containing different sintering aids has been studied. The results show that the static fatigue behaviours of the two samples are consistent with each other in various media, i.e. the rate of crack growth of both materials is greatest in water, followed by air, and then kerosene. A distinctive microstructure endows one of the samples with a higher fracture toughness. In turn this results in a higher crack propagation resistance and a longer service life at the same stress level.

Mechanical and environmental factors in the cyclic and static fatigue of a machinable glass-ceramic

Abstract The effects of environment on cyclic and static fatigue behaviour were investigated with a machinable glass-ceramic and its original glass. Tests were conducted at ambient temperatures in three environments, i.e. moist air, distilled water and kerosene, and an indentation/bending technique was used. The results indicate that cyclic loading increases crack growth rates of glass-ceramic in all the three environments, but it just has a negligible effect on the original glass. The two materials were very sensitive to the amount of water in the test environment. For static fatigue of the glass-ceramic, both the slopes of d a d t-K eff curves and their position along the abscissa varied with the environments, but under other conditions, only the position varied.