Adhesive properties of the Mo(001)//MoSi2 (001) heterophase interface with and without C, O, B, S, and Nb impurities are calculated using a first principles local density functional approach. The adhesive energy and interfacial strength of the impurity-free interface are 10% to 15% smaller than the respective values for cleavage along the (001) planes of Mo and MoSi2. All of the impurities were found to decrease the Mo//MoSi2 adhesive energy. The substitutional impurities S and Nb decrease the interfacial strength, while the interstitial impurities C, O, and B increase it. All of the impurities increase the interfacial spacing in proportion to their covalent radii. The impurity effects on adhesion may be described in terms of competing bonding and strain effects.

It is shown that for metal/oxide systems with weak or moderate reactivity, wetting depends mainly on parameters related with the detailed chemistry and structure of interfacial reaction products rather than on parameters related with the intensity of interfacial reactions. Consequently, wetting can be promoted by alloying a metal matrix M with a reactive solute B capable of modifying in a favourable sense the metal/oxide interface. This can be achieved via two mechanisms depending on the value of the Wagner's interaction parameter εBo. When εBo<0 (moderate interactions between solute B and dissolved oxygen), the solute B can modify the liquid-side of the interface by adsorption, an effect that can be strongly enhanced by oxygen coming from dissolution of the oxide substrate. When εBo<<0 (strong O-B interactions in M), the solute B can lead to the formation, at the solid-side of the interface, of a new phase. When this new phase features metallic bonding wetting can be strongly improved.

This paper reviews the effects of chemical bonding, reaction, interfacial structure, fabrication, specimen geometry and testing conditions on the strength and fracture behavior of metal/ceramic interfaces. It is shown that a number of important properties of metal/ceramic interfaces such as the wetting behavior and work of adhesion can be qualitatively predicted from simple bonding models based on the elements in the metal and ceramic. In addition, the interfacial structure can often be predicted from principles of equilibrium thermodynamics and minimization of interfacial energy for relatively thick metal/ceramic layers. More quantitative description of interfacial structure employing atomistic calculations has been performed for simple interfaces and this area is progressing. The fracture behavior of metal/ceramic interfaces is a complicated process which depends on many factors such as the specimen geometry and loading conditions, strength of the interfacial bond, thermal, elastic and fracture properties of the metal and ceramic, thickness of the metal layer and testing environment. Advances in this area include the development of favorable specimen geometries for measuring interface properties and an understanding of the relationship among the phase angle of loading, crack trajectory and interface fracture energy for these geometries. Conversely, little is known about the stress corrosion and fatigue behavior of metal/ceramic interfaces although data on these time dependent failure modes are beginning to appear in the literature. Much progress has been made but considerably more work isneeded to understand the properties of metal/ceramic interfaces.

Advanced aluminium and titanium alloys, metal matrix composites (MMC's) and intermetallic compounds are of considerable interest to the aerospace industry. These materials offer significant mechanical improvements over many conventional materials. Appropriate joining technologies are being developed to utilise the advantages these materials offer in aerospace applications. Diffusion bonding offers considerable potential as a joining process. This keynote paper will review diffusion bonding with reference to these advanced metallic systems.

This paper reviews the processing factors in joining ceramics to metals concerning the reliability. Thermal expansion mismatch has a great influence not only on the absolute value of strength but also on the reliability of joints. Large thermal stress increases the scatter of joint strength because of the presence of defects induced during joining process. One should insert an appropriate interlayer to relax the stress between a ceramic and a metal. Surface roughness also has some influence on the reliability. A roughly ground bond face leads large scatter in strength. Scratches must be removed before joining. Unjoined area reduces joint strength especially in solid-state joining. In brazing, the homogeneity in the braze layer should be also controlled carefully. A slight applied pressure during brazing can preserve the integrity of joints.

We have designed, constructed, and are operating a unique capability for the production of highly controlled homophase and heterophase interfaces: an ultrahigh vacuum diffusion bonding machine. This machine is based on a previous design which is operating at the Max Planck Institut für Metallforschung, Institut für Werkstoffwissenschaft, Stuttgart, FRG. In this method, flat-polished single or polycrystals of materials with controlled surface topography can be heat treated up to 1500°C in ultrahigh vacuum. Surfaces of annealed samples can be sputter cleaned and characterized prior to bonding. Samples can then be precisely aligned crystallographically to obtain desired grain boundary misorientations. Material couples can then be bonded at temperatures up to 1500°C and pressures up to 10 MPa. Results are presented from our initial work on Mo grain boundaries and Cu/Al2O3 interfaces.

The joining of ceramics using active metal braze alloys is an attractive technique and has been used for a variety of material systems. The braze alloys usually contain an active element such as Ti which can reduce the ceramic and form a strong bond across the interface. Microstructural characterizations of brazed interfaces have been widely reported, though reasons for the formation of reaction products and their sequence is not clear due to the lack of thermodynamic information on the phases formed.

The Ag-Cu-Ti/Al2O3 system contains an interface where Ti, the active element in the braze alloy, reduces Al2O3 to form layers of reaction products which include (Ti,Cu,Al)6O, an M6X type compound. In this study we have looked at the Ti-Cu-O system as a first step towards understanding the Ag-Cu-Ti/Al2O3 system. A section of the Ti-Cu-O ternary was investigated at 945°C. The system contains two M6X type compounds, Ti4Cu2O and Ti3Cu3O, which have independent single phase fields. A Ti activity diagram was generated from the available thermodynamic information and the knowledge of the ternary section. The two reaction sequences reported for such interfaces were analyzed based on the activity diagram.

Interfacial reactions between the melts of several borate glasses and titanium have been investigated by analytical scanning electron microscopy (ASEM) and by x-ray photoelectron spectroscopy (XPS). A thin titanium boride interfacial layer is detected by XPS after short (30 minutes) thermal treatments. ASEM analyses after longer thermal treatments (8–120 hours) reveal boron-rich interfacial layers and boride precipitates in the Ti side of the interface.

Multi layer alumina coatings consisting of layers of either pure α-A12 O3 or κ-Al2O3, were deposited in a hot wall CVD reactor. In order to nucleate and grow a desired alumina polymorph (α/κ) in each layer, thin α- and κ-modification layers were deposited between the alumina layers. This investigation examines the interfacial structure of the κ-modification and the alumina layers. The materials were examined using a combination of XRD, SEM, TEM and EDX.

The κ-modification layers exhibited an FCC structure and were composed of (Ti,AI)(C,O). Orientation relationships were frequently found at the (i) α-A12O3/κ-modification/κ-A12O3 layer interfaces and (ii) κ- A12O3/κ-modification/κ- A12O3 layer interfaces:

Interfaces between Al and amorphous C or amorphous Sio2 were prepared by sputter deposition of the ceramic phases onto the sputter cleaned surfaces of large grain Al. In situ TEM was used to study the behavior of the extrinsic dislocations (slip traces) at the metal/ceramic interface formed by the motion of Al lattice dislocations which intersect the interphase boundary plane. The proximity of the extrinsic lattice dislocation to the metal/ceramic interface places this interphase boundary under a highly localized stress. Relaxation behavior of extrinsic defects at the interface region is a function of the defect standoff distance from the interface, image stress and the applied stress. An experimental technique is proposed to estimate the minimum shear stress the various interfacial regions are able to withstand. Research supported by the Department of Energy.

SEM, x-ray microprobe and EDS analysis was performed on a uniquely prepared joint of graphite soldered to copper with 60Sn-40Pb. The presented data shows a segregation of chromium out from the metallization paste to the graphite/solder interface. Dot maps of the other elemental species were also generated.

The mechanical properties of interfaces in Ti/Al2O3 composites were characterized by four point bending and fiber pushout tests. To determine the bi-material fracture toughness with four point bending tests, planar interfaces were evaluated as sandwich composites. By changing the processing temperature from 700°C to 1000°C, the interfacial fracture energy was found to increase ug to 950°C. It then decreases when the processing temperature is further increased to 1000°C. This is because of the formation of the intermetallic compound (Ti3Al). Interfacial shear strength and interfacial frictional stress of 323MPa and 312 MPa were obtained, respectively, by performing pushout tests of the Al2O3 fiber reinforced Ti matrix composites. These values are smaller than the shear yielding strength of the Ti matrix which is 525 MPa.

It is known that the fracture resistance of glass-copper interfaces depends strongly on the water content in ambient gaseous environments. In the present study, subcritical crack growth stimulated by water and other environmental species is investigated for such interfaces. Tests were conducted in various liquids, namely water, N-methylformamide, and n-butanol. All were found to accelerate fracture with the greatest effects from liquid water. Results are considered in the context of current models for stress-corrosion crack growth.

In the past several years there has been an explosive growth in the use of microwave energy for the processing of a host of materials. Microwave energy provides rapid internal heating which results in an overall reduction in the processing time. The important features of microwave processing are described, as well as several applications.

Microwave energy has been used by a few groups for the joining of alumina, mullite, silicon nitride and silicon carbide. The work performed by these groups will be reviewed. Typically, a single mode microwave applicator has been used to join ceramics at temperatures ranging between 1250°C - 1800°C. Microwave joining of ceramics was achieved in a matter of minutes, in contrast to hours reported by conventional methods. The strength of the joints was equal to or greater than the as-received materials. Joining of specimens of sintered silicon carbide (Hexoloy ™ ) using interlayers, and direct joining of reaction bonded silicon carbide (RBSC) to itself and Hexoloy™ has been accomplished recently. Both single mode and multimode microwave applicators were used and larger specimens of RBSC having complex shapes were joined using hybrid heating. The paper describes microwave joining apparatus, techniques and results.

Electron probe microanalysis (EPMA) and transmission electron microscopy (TEM) have been used to examine the SiC/Al interfaces in microwave joined Si-SiC/Al/Si-SiC and α-SiC/Al/α-SiC. Both the SiC/Al interfaces display intimate contact between the ceramic and metal and are free of porosity. EPMA of the α-SiC/Al/α-SiC joints reveals that no Al has diffused into the bulk α-SiC, unlike the reported diffusion of Al in Si-SiC/Al/Si-SiC. The TEM investigations show that while the Si-SiC/Al/Si-SiC interface is reaction-free, the α-SiC/Al/α-SiC joint contains Si at the interface. The TEM findings are correlated to the strength data available on these joints and the possible reasons for the presence of Si in the absence of Al4C3 in the α-SiC/Al/α-SiC joint are discussed.

Due to their superior mechanical properties and high strength to weight ratio, metal matrix composite materials are increasingly being used in aerospace, automotive and defense industries. The use of a fusion related conventional joining technique, such as arc welding, leads to the formation of undesirable non-equilibrium phases in the weld zone, where as diffusion and adhesive joining techniques are rather slow. Non contact multi-kilowatt laser joining, though fully developed for high speed autogenous joining of different alloys, is not yet adequate for joining of metal matrix composites. Efforts have been made by several researchers to control the composition and microstructure of laser joined aluminum metal matrix composites by controlling the energy input and by the use of filler wires. In view of the current industry trend to use more light weight structural composite materials, and greater use of high energy density beams for a variety of metalworking applications, the authors have reviewed the current status of high speed laser joining of metal matrix composites. Emphasis is placed on problems and various techniques which can be used to achieve a better control on composition and microstructure during high energy laser joining of aluminum metal matrix composites.

A rapid infrared joining (RIJ) technique has been developed at the University of Cincinnati for high temperature materials. This technique takes only a few seconds to a few minutes to join parts up to 1.8 cm in thickness. The advantages of the RIJ technique are quick, simple, inexpensive, no vacuum or pressure needed, no effect on the microstructure of the base material, flexible and feasible for portable operation. For titanium joining, both Ti-6AI-4V and β21S/SCS-6 composite have been successfully joined with infrared at about 1000 °C for 30 seconds in argon with a TiNiCu brazing alloy. The maximum joint shear strength is up to 554 MPa, which is higher than that of any bond joined with conventional brazing techniques. Our study has shown that the longer the joining time, the wider the joining affected zone. Prolonged joining cycles allow for attack of the titanium alloy by the molten brazing alloy. Results from the titanium matrix composite (TMC) joining show that the joining strength for the infrared bonded parts is superior to those processed with the conventional techniques.

Capacitor discharge welding (CDW) is a rapid solidification joining process where high cooling rates (106 K/s) are obtained as a result of the large weld surface area to small weld volume. The objective of this study, directed by the U.S. Bureau of Mines and the Oregon Graduate Institute of Science and Technology, was to use ultra-high speed photography to quantify transient arc behavior during the CDW cycle. The simple cylindrical geometries of the CD welds have been used to formulate analytical models which are compared to the high speed photographs of the welding process. The high speed photographs were analyzed with respect to welding time and process weld variables and compared to predicted values from the analytical model. The detailed photographic analyses revealed that material is continuously ejected as a plasma from the weld area due to induced magnetic forces, rather than having the liquid metal squeezed out of the weld upon contact. It was found that welding time was controlled by tip length and drop height. Results from high speed photographs found the arc travel speed around tube welds to be 109m/s. Finally, the high speed photographs revealed that the velocity of arc propagation during ignition was fast enough to allow the CDW process to be modelled as onedimensional heat flow.

Discontinuously reinforced Al alloys (DRAs) are difficult to join using conventional fusion welding techniques due to inhomogeneous redistribution of the reinforcements and reaction between molten Al and SiC. Two specialized welding techniques, capacitor discharge welding and flash welding, were assessed as possible ways to avoid these problems. Using these techniques, welds were made with two DRAs: 8009/SiC/llp and 2009/SiC/15w. Capacitor discharge welding resulted in the formation of Al-carbides in the weld zones, but flash welding produced satisfactory joints in both DRAs.

The U.S. Bureau of Mines has studied the capacitive discharge weld interface between Al and Fe using optical and transmission electron microscopy (TEM). Capacitive discharge welding (CDW) is a rapid solidification process in which the amount of molten metal at the interface is small compared to the sample size. As a result, high cooling rates (106 K/s) can be achieved, providing a weldment made up of small grains. Large magnetic fields produced by the process tend to mix the molten metals at the interface to form a complex alloy, the nature of which depends upon the starting electrode materials. This effect is characterized by a marblecake pattern in optical micrographs. The amount of mixing is related to the melting temperature of the cathode with respect to the anode. Increased mixing occurs when the melting temperature of the cathode is higher than the melting temperature of the anode. TEM has revealed that when aluminum is the anode material, the iron grains of the cathode are surrounded by a layer of aluminum. When the iron is the anode, a thin layer of iron surrounds the aluminum grains.