| Al2O3 ceramic has excellent properties, such as high strength, good high-temperature resistance and excellent electric insulation, etc., and has been widely used in the fabrication of vacuum electric devices. However, with the enhancement of products performance, the stainless steel that is joined to Al2O3 does not suffice because the local high temperature in extreme conditions, which will affect the product reliability seriously. Ni Al alloy has low density, high strength and good high-temperature resistance, and is thought to be a good substitute for stainless steel to improve product performances. General ceramic/metal brazing joints have poor high-temperature properties and very large residual stress. To improve the joint performance, whiskers were epitaxially grown on the surfaces of Al2O3 ceramic and Ni Al alloy. The direct bonding of Al2O3 by growth of whiskers were used to form joints with good high-temperature properties. In addition, the whisker-coated Al2O3（Al2O3（W） for short） was brazed to itself and to the whisker-coated Ni Al alloy（Ni Al（W） for short）. By control of the reaction of the whiskers, composite regions can be formed in the seam, and further strengthen the brazing joints.Whiskers were grown on the surfaces of Al2O3 ceramic and Ni Al alloy using B2O3, H3BO3 and（B2O3 + co-solvant materials）, respectively, as the added raw materials. The morphology of the as-produced whiskers were investigated by SEM. The effects of raw material composition and growth parameters on the whisker morphology were studied. The crystallographic phase and growth habit of the whiskers were analyzed by XRD and TEM. Moreover, the growth mechanism was discussed. The results showed that uniformly distributed whiskers with good crystallinity could be obtained when sintered at 950℃ for 4 h by adding B2O3 as the raw material. Good one-dimensional morphology of the whiskers would be destroyed when sintered at a over-high temperature or holded for too long a time. The morphology of the whiskers produced by adding H3BO3 as the raw material was similar to that of the whiskers obtained by adding B2O3 as the raw material. Whiskers with large size and obvious faults at the roots obtained by adding Na Cl as the co-solvant material generally had small angles between the growth direction and the ceramic surface. When adding K2SO4 as the co-solvant material, whiskers with relatively good crystallinity could only be obtained at 1100℃ for 2 h. While, those whiskers also had small angles between the growth direction and the substrate surface. Only relatively small whiskers were obtained when using B2O3 + Mn O2 as the raw material. The whiskers fabricated at 950 ℃ were orthorhombic phase Al4B2O9, and the whiskers fabricated at temperatures above 1050 ℃ were orthorhombic phase Al18B4O33. The as-produced whiskers were epitaxially grown on the substrate surfaces, and the mismatch between the（101—0） planes of Al2O3 and the（220） planes of Al4B2O9 is determined to be 0.03%. The whisker growth process could be explained by S-L-S self-catalysis mechanism. Based on the studies on the growth of whiskers on Al2O3, metal borate whiskers were fabricated on the surface of Ni Al alloy using B2O3 as the added raw material. Results revealed that uniformly distributed Al4B2O9 whiskers with smooth surface could be obtained at 950℃ for 4 h. In addition to Al4B2O9 phase, Ni3（BO3）2 phase was also oberved in the products fabricated at 1050℃. Subsequently, the crystallographic phase and growth habit of Ni3（BO3）2 whiskers grown on pure Ni substrates were analyzed in det ail.Al2O3 ceramics were directly bonded by epitaxial growth of whiskers. The effects of interlayer constituent and bonding parameters on the morphology of the whiskers and the high-temperature strength of the joints were investigated. The bonding mechanism was also discussed. When bonding at relatively low temperatures using pure B2O3 interlayer, small whiskers were observed in the joint, as well as the residual B2O3. As the bonding temperature or the holding time increased, more whiskers could grow across the joint and then realize the direct joining, resulting in the increase of the joint strength. The 800℃ bend strength of the joints reached the highest value（27 MPa） when bonded at 1100℃ for 4 h. The fracture occurred within the whiskers. Too high bonding temperature or too long holding time would be harmful to the growth and the maintenance of aluminum borate in one-dimensional form. When adding K2SO4 or Mn O2 into B2O3 interlayer, the number and the size of the whiskers in the joints decreased obviously, as well as joints strengh decreased dramatically.The wettability of Al4B2O9-whisker-coated Al2O3 was analyzed. Results showed that the reactive Ag Cu Ti alloy had good wettabiligy on the whisker-coated ceramic. The wetting process of liquid Ag Cu Ti alloy on the coarse surfaces were reactive wetting process, and the capillary force between whiskers could promote the spreading of the liquid alloy on the whisekr-coated surface. Subsequently, Al2O3（W） was vacuum brazed by Ag Cu Ti interlayer. The effects of brazing parameters on the microstructure and the mechanical properties of the joints were studied. The whisker/interlayer interface consisted of a continous（Cu,Al） 3Ti3 O layer outside and a complex transition zone inside, which consisted of Ag nanoparticles, titanium oxides, Ti B2 and（Cu,Al）3Ti3O nanoparticles. As the bonding parameters increased, the reaction between the whiskers and the liquid alloy was enhanced, finally leading to the transformation of whiskers into nanoparticles dispersively distributed in the joints. When the whiskers retained in the brazing seam, the bend strength of the joints was obviously higher than the smooth Al 2O3 joints. The bend strength of the joint brazed at 820℃ for 10 min reached 313 MPa. Pulling out and break of the whiskers were obviously observed on the fractured surfaces. Growth of whiskers on Al2O3 ceramics can not only help increase the joint strength, but also help decrease the bonding temperature. In addition, based on the microstructural observation, the mircrostructure evolution of the joint was discussed.Al2O3（W） and Ni Al（W） were vacuum brazed by Ag Cu Ti interlayer. The effects of brazing parameters on the microstructure and the mechanical properties of the joints were contrastively analyzed. The results revealed that the growth of whiskers on the surface of Ni Al alloy prevented the excessive consumption of the active elemnets in the liquid brazing alloy. In additon, the whiskers retained in the joint resulted in the increase of the joint strength. The joint brazed at 820℃ for 10 min had the maximum shear strength（63 MPa）. In order to avoid the violent reaction between whiskers and the brazing alloy, reactive air brazing was selected to join Al2O3（W） to Ni Al（W） using Ag Cu powder as the interlayer. The results showed that the whiskers on the substrate surfaces reacted with Cu O in the liquid alloy. As the content of Cu in the brazing alloy decreased, the extent of reaction of the whiskers reduced, and accordingly the shear strength of the joints increased. |
