Key Engineering Materials Vols. 368-372

Abstract: According to an actual design for the joining of ceramics and metal, four joining design principles, heat expansion matching principle, homogeneous transition principle, cushion principle and thermal protection principle, are proposed in this paper. The weakness and stress concentration point of the joining are analyzed through finite element analysis. Following the joining design principles, the design scheme and finite element analysis are improved on. Finally the satisfactory residual intensity coefficient is given.

Abstract: Joining of SiC ceramic to graphite is important from both technical and economical points of view. High temperature brazing of recrystallized SiC ceramic to high strength graphite has been realized using Ni-51Cr (consisting of Ni + 51wt% Cr powders) powders as filler. The obtained maximum three-point bending strength of joints is 32.3MPa, which is equal to 80.8% of the strength of the graphite. Microstructure and phase analysis reveals that interdiffusions and chemical reactions take place in the weld zone. A reaction layer and an interlayer form in the interfacial area. The reaction layer, of which the thickness is about 60-100μm, is contacted with the SiC ceramic. The interlayer with the thickness of about 200μm exists between the graphite and the reaction layer. The reaction layer is mainly composed of Ni2Si, while the interlayer is mainly composed of Cr23C6 and Ni2Si.

Abstract: In order to make a good ceramic metal halide lamps, to form the solid hermetic sealing supplied by the ceramic discharge and metal electrode is very important. It is different with the quartz tube, after heating softly, ceramics tube can’t stick with metal electrode lead together through squeezing. For the sealing, it is necessary to adopt the appropriate sealing materials. In our research work, studied the thermal expansion coefficient of the Al2O3-Dy2O3-SiO2 sealing material was a key for ceramic metal halide lamp.

Abstract: WC steel bond hard alloy powders were compacted by explosive compaction on the surface of carbon steel, then the composite layer was prepared by liquid-phase sintering in vacuum. The phases and microstructures of the interface of composite layer were observed by SEM, EDS, TEM. Result showed that the thickness of the interface was about 30μm, and the elements of W, Cr, C were detected in the interface. The structures of the interface were pearlites which were composed of the layered tablets of M23C6 and ferrite. The layered tablets were perpendicular to the interface. The structures were the pearlites and ferrites in the side of the carbon steel near the interface but globular carbides, whose phase is W3Fe3C, and pearlites in side of steel bond hard alloy. During sintering at 1623K, all elements of metal-powder and C, V, which were decomposed by hard-phase, interdiffused in the interface. The multiple carbides were found in the interface and steel bond hard alloy. As a result, the powders formed into compact alloy by itself, and at the same time there was a good metallurgical bonding between the carbon steel and the hard alloy.

Abstract: Pulse current heating technology is used to join TiB2-Ni cermets with Ti6Al4V at different temperatures (1023K, 1073K and 1123K) with 7MPa pressure and Cu foil and Ni foil are used as joining interlayer. Joint points increase with welding temperature. The microstructure of joints is observed through SEM images and micrographs. The diffusion coefficient (D0) of Cu at the Cu/TiB2 interface is calculated by the EPMA patterns. The diffusion principle of pulse current heating and influences of electric field and current on atom diffusion are studied.

Abstract: A novel technique to join ceramics was described using microwave heating. SiC was added in the interlayer as a microwave-absorbing material, joining of 95 Al2O3 was realized by raising temperature of SiC itself and then inducing microwave-absorbing of oxides. The strength of joining, microstructure of interlayer and mechanism of joining were investigated.

Abstract: Microstructure in anchoring site of TiC cermets/steel explosive cladding plate was analyzed by transmission electron microscope (TEM), scanning electron microscope (SEM) and energy-dispersive spectrometer (EDS). An intermittent micro-crystal layer, with thickness of about 10μm, was observed in anchoring area. The inner crystal size was in range of some nanometer to hundreds of nanometer. The phases of the layer are mainly austenite, ferrite, and some titanium carbide. A flow layer was observed in the area near the steel side. The diffusion of Ti is the most apparent and the diffusion length is about 15μm. The observation of the microstructure in anchoring area revealed the nature of explosive compound in metallurgical anchoring.

Abstract: Low-fusing dental porcelain was fused on titanium surface. The adhesion between the titanium and porcelain was evaluated by three-point flexure test. It was shown that the failure of the titaniumporcelain predominantly occurred at the alloy-oxide interface. Rutile phase was present on the interface debonding from porcelain. The influence of corrosion on bonding strength was also investigated. The results suggested that, after being immersed in artificial saliva with pH of 2.7, 5.4 and 7.0, no decreasing of the bonding strength of Ti-porcelain occurred.

Abstract: Carbon fiber reinforced CMCs (ceramic matrix composites) are promising high temperature structural materials for aeronautical and astronautical industries. But the engineering application of the materials strongly depends on the joining technique. As a fundamental research, high strength graphite can be used to simulate carbon fiber. The study of wettability of graphite by liquid metals contributes to the development of joining technique of carbon fiber reinforced CMCs. The wettability of graphite/Ni plus Cr system was investigated using the sessile drop method. The contact angle was determined by high temperature photography. The microstructure and composition of the interfacial zone were analyzed by SEM, EDX and XRD. The contact angle of graphite/Ni plus Cr system decreases with increasing the Cr content. It becomes zero, when the Cr content reaches 50wt%. The wetting of graphite/Ni plus Cr system is chemical wetting. The contact angle of the system decreases with increasing the wetting temperature or prolonging the holding time. The higher the Cr content, the more remarkable the effects of the temperature and the holding time on the contact angle are. Microstructure and phase analysis reveals that inter-diffusions and interfacial reactions take place in the wetting process. The reaction products include Cr3C2. The formation of Cr3C2 contributes to the interfacial bonding and results in the accumulation of Cr in the interfacial zone.

Abstract: The interfacial arc-microcracks have large influence on the behaviors of cermets. The emerging cause of arc-microcracks is interpreted and the interaction between ceramic particles and arc-microcracks is accounted for determining the equivalent eigen strains by using the random two phase element method. Then the effective volumetric modulus and shear modulus of the cermets is estimated. The results show the effective volumetric modulus and shear modulus of the cermets is associated with the stiffness of the matrix and ceramic particles, the volume fractions of ceramic particles and arc-microcracks, the subtending angle and thickness of arc-microcrack..