This review article on metal/ceramic joining is subdivided into the description of research activities in the fields of active metal brazing and diffusion bonding published in the last decade. Informations are given on active metal brazing of oxide, nitride, and carbide ceramics and on diffusion bonding of alumina, zirconia, magnesia, silicon nitride, aluminum nitride and silicon carbide ceramics to metals. Ultra high vacuum diffusion bonding and experiments using the model combination Nb/alumina are also regarded. Emphasis is laid on a concise reproduction of experimental data concerning the bonding conditions and the determintion of bond strength. The review demonstrates that much effort was devoted to studies on the formatin of interfacial reaction layers and on the efficiency of interlayers additonally introduced between the ceramic and the metal part to reduce internal stresses caused by thermal expansion misfit of the materials to be bonded.

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On joining of metals to ceramics, chemical reaction at the bonding interfaces is unavoidable in most of the combinations of ceramics and metals including brazes except for a few special cases. Thermodynamic possibility of the chemical reaction and the resulted interfacial structure are compared with the experimental results in the cases of silicon nitride and silicon carbide ceramics. The influence of the reaction products on the bond strength of the joints and the effect of impurities on the interfacial reaction are discussed.

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Direct metal/ceramic interface free of a reaction product layer and the atomic ledge structure aligned parallel to the trace of one of the slip planes of the metal was designed in niobium/alumina. Such interface should accommodate the thermal stress at high temperatures by nonconservative motion of the ledge involving the growth of ceramic side during cooling of the joined piece as solid solubilities of the ceramic components in metal decrease with the temperature. At lower temperatures where the nonconservative ledge motion is frozen, conservative motion of misfit dislocations results in stress concentration against the now immobile ledge and the stress may be relaxed by the emission of lattice dislocations into metal matrix.

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Joining technologies of ceramics to metals have been advanced in recent few years. There have a lot of difficult problems to be overcome in developing joining techniques. To control the chemistry of interfaces is, of course, one of the big key points. The active metal brazing has been established for solving this problem and gives us high quality of joints. To construct an appropriate interlayer for reducing thermal stress originating from thermal expansion mismatch between a ceramic and a metal is another critical problem. The soft metal interlayer and the soft metal/hard and low expansion metal laminate interlayer are proved to be two of the effective interlayers. The other bonding parameters such as surface roughness of bond face and bonding pressure are also important to get sound as well as strong ceramic/metal joints.

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Increasing technological applications of metal-ceramic systems have demanded a fundamental understanding of the properties of interfaces. In this paper, we describe our approach to the study of electronic structure of metal-ceramic interfaces. Electron spectroscopies have been used as primary techniques to investigate the interaction between metal overlayers and ceramic substrates under various experimental conditions. These data are further elucidated by theoretical calculations, from which the electronic structures of the interface have been deduced. A temperature dependence of the band structures of Al₂O₃ is first discussed, then the evolution of the electronic structure and bonding of Cu and Ni to Al₂O₃ is studied. The relationship between electronic structures and interfacial properties are also addressed.

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Y₂O₃-stabilized ZrO₂ or HfO₂ single crystals with (100), (110), or (111) surface are diffusion-bonded to polycrystalline niobium, and then joints are tensile tested. The bond strength is dependent on the surface orientation of ZrO₂ or HfO₂: higher bond strength is obtained in the (100) and (110) surfaces than in the (111) surface. Although any reaction layers are not observed by EPMA at the bonding interface, black dots are found on the torn surfaces of both the ceramic and the metal in the joints bonded at 1 773 K and above and then tensile tested, which may imply that some reaction has occurred at the bonding interface. Lattice matching is also considered for observed crystallographic orientation relationships.

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β-sialon-Ni bonded interface was investigated by SEM and TEM observations, using β-sialon/Ni/Mo bonded samples. Mechanical properties of the bonded samples were also evaluated. The bending strength of specimens bonded 1 373 K showed the maximum value (200 MPa). In the bonding at 1 273 and 1 373 K, no reaction phase was observed. In the high resolution lattice images of the interface for the specimen bonded at 1 373 K, a direct contact between Ni and β' phase (the main component of β-sialon) was revealed, and micro-twins and dislocations were observed in Ni side at the interface region. In the bonding at 1 473 K, Ni₅Si₂ was locally observed and void-like regions including Si_2-xAl_xN₂O particles were found to grow largely. The presence of silicide phase and large voids at the interface region possibly cause the poor mechanical strength of the specimens bonded at this temperature.

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The bonding of Ag with YBa₂Cu₃O_7-x (YBCO) superconductor was made by hot pressing and vacuum deposition. The electrical contact resistivity and adhesive force at the interface were measured. The following results have been obtained. The hot pressing above the temperature of 670 K for 18 ks gives a good bonding of Ag with YBCO. It aids the penetration of Ag into pores of YBCO. TEM observation revealed that the bonded interface has two kinds of morphology, that is with or without third phase. The electrical contact resistivity less than 10^-5 Ωcm² at 77 K is obtained in the hot pressed or annealed samples above 770 K. The adhesive force estimated from a peel test increased with the increase of the hot pressing temperatures. The fracture developes at YBCO grain boundaries as well as at the interface between Ag and YBCO. The optimum annealing temperature was determined as 770 K from the present experiments.

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X-ray collimation and diffraction conditions were optimized to evaluate the detailed distribution of residual stress quantitatively around the ceramic/metal interface, and the results were applied to the Si₃N₄/Cu/steel system. The residual stress value increased and finally saturated as the collimation and irradiation area decreased in the Si₃N₄ region close to the interface of the joint. The normal stress value across the interface saturated below 0.2 mm² to 240 MPa for Si₃N₄/t0.2 mm Cu/steel, and to 400 MPa for Si₃N₄/steel. It is recommended that φ0.1 mm collimation is suitable to evaluate its detailed distribution along the interface in which the irradiation area is 0.016-0.031 mm². Several characteristic features of distribution were successfully observed along the joined interface of Si₃N₄/t0.2 mm Cu/steel having a lapped surface; first large tensile normal stresses at both edges, secondly the minimum stress points 1-2 mm inside the edge which was originated by stress relief through a localized plastic deformation of the Cu buffer, and thirdly the maximum normal stress across the interface in the center region of the specimen which had never been predicted and could be caused by the three dimensional effect of the joined plate.

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Field assisted bonding of metal to zirconia utilizing the oxygen ionic conductivity of zirconia has been examined. A strengthening of sprayed zirconia to iron was attempted after spraying zirconia onto the iron substrate. And the effect of applying a DC voltage to the interface on the wettability of zirconia by liquid iron was also investigated because the liquid phase would be formed at the interface by applying a DC voltage to the sprayed zirconia-metal system. According to applying a DC voltage, the interfacial chemical reaction occurs at the interface. The joining strength can be improved even after spraying zirconia onto the metal substrate. And the wettability of zirconia by liquid iron was also improved. Results are summarized as follows:
(1) Application of a DC voltage to the interface can bring about the oxidation of iron and the deoxidation of zirconia to metallic zirconium at the interface.
(2) Two mechanisms can be considered for joining of zirconia to iron. They are the penetration of iron-oxide formed at the interface into the grain boundary of zirconia and the reaction between iron and metallic zirconium at the interface.
(3) This joining method is the new one to strengthen the sprayed zirconia after spraying it.
(4) This joining method has the advantage of not needing pretreatment of the substrate (e.g., sand blast or spraying metallic bond coat between substrate and zirconia overlayer, etc.).

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An electrical resistance heating system for joining ceramics was developed, which was based on input current control by thyristor switching. The joining of electroconductive ceramics was carried out in Ar gas by pinching adherends between two electrodes. If the current applied to the adherends was not controlled, cracks occurred in the ceramic by thermal shock, but it was clear that a high joint strength could be obtained by thyristor controlled current switching. Joined material contained no cracks and joining could be completed in a few minutes with use of low electric energy. The joint strength of ZrB₂/ZrB₂ bonded at 1 173 K was 200 MPa in 4 point bending mode, and that of electroconductive-SIALON bonded at 1 223 K was 400 MPa.

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The wettability of molten Cu on ti-precoating SiC was evaluated by a sessile drop technique. Titanium was coated by RF magnetron sputtering in argon gas. The contact angle of copper was measured by photography at 1 373 K in a vacuum. The equilibrium contact angle of copper on SiC was improved by increasing Ti precoating thickness. The Ti-precoating improves the wetting of copper on SiC. The formation of carbide and silicide such as TiC and Ti₅Si₃ at Cu/SiC interface accounts for the good wetting of copper on SiC.
The joining strength of copper with SiC was measured by fracture shear loading after joining copper to molybdenum. The joining strength of copper exhibits the maximum 10 μm of Ti thickness. The excess amounts of carbide and silicide cause the degradation of copper near the interface of Cu/SiC, though the formation of the compounds improve the wetting of copper. This is attributable to the maximum of joining strength of copper-metallized layer on SiC.

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The wetting characteristics, by molten aluminum, of three types of MgO, two sintered and one single crystal (100), were examined by means of the improved sessile drop method to make clear the effects of interfacial reactions and the difference of surface state on wetting.
The reaction zone products vary with the initial characteristics of the ceramics such as impurity content and grain boundaries. The contact angle in each system progresses through three phases (I, II, III). Similar contact angles were produced in phase II, regardless of the type of MgO, although the single crystal showed a slightly higher angle than the polycrystalline types. In phase III, however, contact angles become significantly different.
The phase III contact angle of the single crystal decreases more slowly than that of the sintered types since interfacial reactions progress relatively slowly. Different reaction products were formed at the interface depending on the type of MgO. These observations suggest that the chemical reactions at the interface must be clarified to understand the phase III behavior of wetting systems.

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It is well known that segregation of Ti and formation of TiN occurs when active brazing alloys such as Ag-Cu-Ti were used for Si₃N₄ joining. Si₃N₄-Si₃N₄ joining was carried out using several glass solders containing oxides of active metal, 1A group elements and 2A group elements without applying pressure. Si₃N₄-glass reaction and bonding behavior of these elements in Si₃N₄-Si₃N₄ joint were studied. The bonding strength obtained from Silicon nitride joints using glass solder in the system SiO₂-Al₂O₃-Li₂O was strong enough for practical application.

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In order to join Al₂O₃ with iron, an Fe-M₂O₃ composite (I) as an interlayer was applied. Here, M₂O₃ represents for Y₂O₃, Al₂O₃ and Y₂O₃·Al₂O₃. The Al₂O₃/I/Fe joint was prepared by hot-pressing. In the case that M₂O₃ is Y₂O₃, Al₂O₃/I/Fe joining can be completed through formation of a series of reaction sublayers: at the interface of the Al₂O₃/I, a chemical reaction sublayer, next to Al₂O₃, including iron, aluminium and oxygen is formed; and then next to it has a second sublayer of yttrium, aluminium and oxygen matrix mixed with dispersed iron as a different phase, the Fe-M₂O₃ interlayer and subsequently the I/Fe interface, where the inter-diffusion of iron has occurred. The thickness of second sublayer changes with the Y₂O₃ content. This layer plays a role in relieving thermal stress caused by a different thermal strain of Al₂O₃ and iron bulks, but the increase of its thickness makes the joint easier to break. With increase in Y₂O₃ content in Fe-Y₂O₃ interlayer, the layer tends to agglomerate, which results in the weakening of the joint. To bring the thickening of the reaction layer and the agglomeration under control, Fe-Y₂O₃·Al₂O₃ interlayer was found to be effective. For analysis of the boundary region, electron probe microanalysis (EPMA) was used. Tensile strength and thermal expansion for Fe/I/Fe, Al₂O₃/I/Al₂O₃ joints and Fe-Y₂O₃ composite sintered materials were measured.

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In order to obtain the joints of nickel and magnesia, which have no flat joining interface, a Ni-NiO composite having a stepwise controlled compositional gradient was used for joining fillers. This composite, which was made by the process of powder-metallurgy, was placed between a nickel block and a magnesia block and this set was held at 1 573 K under no pressure in air. In this manner, a good adhesive joint of nickel and magnesia was accomplished.
Nickel-concentration distribution on the longitudinal cross section of nickel/composite/magnesia joints smoothly decreased in order of nickel, composite, the solid-solution NiO-MgO, and magnesia. Such a joint has no flat joining interfaces which are weak points against fractures. Thus, bending strength of the joints increased than that of the direct joints of nickel and magnesia. The maximum value in the present work was 128 MPa. Fractures, whose planes became a dome shape, occurred almost in the solid-solution and magnesia in the vicinity of the composite filler. It seems that thermal stress in composite and solid-solution was released by the use of composite filler as compared with the direct joints of nickel and magnesia.

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Brazing of aluminum nitride to copper was performed using Ag-Cu-Ti, Ag-Cu-Ti-Co and Ag-Cu-Ti-Nb brazing filler metals in an argon atmosphere. The reaction layer formed at the interface between AlN and braze layer in AlN-Cu joints was found to increase by increasing brazing time for all brazing filler metals used. In the case of the fillen metal Ag-Cu-Ti-Nb, a thinner reaction layer was formed in comparison with the cases of using other filler metals. Form an EPMA analysis, it was found that not only Ti, but also Nb were concentrated in this thin reaction layer. From XRD analysis, it was found that TiN lattice was distorted by Nb addition. Shear strengths were measured for AlN-W joints at room temperature. The joint brazed at 1 173 K for 0.3 ks using Ag-Cu-Ti filler metal showed a strength of 118 MPa. In the case of using Ag-Cu-Ti-Co filler metal, almost the same shear strength, 116 MPa, was obtained after brazing at 1 173 K for 0.6 ks. The joints brazed at 1 173 K for 0.3 ks using Ag-Cu-Ti-Nb filler metal exhibited the maximum shear strength 147 MPa. The shear strength of the joints was discussed in relation to the thickness of the reaction layer.

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Bonding of nitride ceramics (Si₃N₄ and AlN) to metals was carried out in a vacuum of about 6 mPa using Cu-base active filler metals. Reaction layers existed at the interfaces between Si₃N₄ or AlN and insert layers, and Cu enriched layers were found in the bonding layers. Reaction layers in Si₃N₄ to W joints were composed of nitrides and silicides of active metals, and those in AlN-Cu joints were composed of nitrides of active metals. The thermodynamic calculation suggested that these products were formed by the reactions between nitride ceramics and the melted active filler metals. It was elucidated that the growth behavior of the reaction layer in Si₃N₄-W joints could be expressed by the Johnson-Mehl type equation with a time exponent 'n' of 1/2. It was deduced that the growth of reaction layers were controlled by the diffusion process of active metals in the reaction layers.

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