Diffusion bonding of ceramics: mullite,ZrO2-toughened mullite

Diffusion bonding of fine-grained mullite and ZrO₂-toughened mullite was performed in the temperature range from 1500 to 1550 °C in air. Uniaxial pressure was applied at high temperature during the bonding process. The surface roughness to be bonded (R _max) was about 3 m. Bonding strength was measured by four-point bending tests and the strength of the base material was measured by three-point bending tests. The effects on the bonding strength of bonding conditions such as temperature and applied strain were examined. Bonding strength increased with increasing bonding temperature and applied strain. The bonding strength of mullite and ZrO₂-toughened mullite was about 80% of the strength of the base material before bonding. The bonding strength of mullite was maintained up to 1000 °C.     

Diffusion bonding systems

Worldwide there is a trend to use diffusion bonding for more and more applications, for example for optical components, micro devices and heat transfer elements. Despite that the market for diffusion bonded product is still small today, more than 30 companies offer diffusion bonding systems worldwide. These systems are different, starting from basic concept and type of machine, the principle of heat input and force application. In the presentation an overview of commercially available diffusion bonding system concepts and their differences is given.     

Diffusion bonding of beryllium-copper alloys

A process has been developed for diffusion-bonding identical beryllium-copper alloy, 1.8 to 2.0 wt% Be, which has produced bond strengths comparable to that of the bulk. Bonding resulted from self-diffusion between two Be-Cu samples, brought into intimate contact and heated in a high vacuum. Metallurgical analyses of the diffusion bonds revealed the bond interface to be a continuous high-angle planar boundary. The analyses also revealed that oxides were present at the interface after bonding. The agreement between the experimental results and a theoretical model for diffusion-bonding of pure copper, derived by Hill and Wallach, was good. Operated for the US Department of Energy by Allied-Signal Inc., Kansas City Division under Contract No. DE-AC04-76-DP00613.     

Diffusion bonding of pd-ag rolled membranes

Cold-rolled and annealed Pd-Ag 25% at. foils of thickness 50 m have been joined by diffusion bonding in order to obtain permeator tubes used in laboratory applications for hydrogen separation and purification. The tubes produced present a sound and effective joint along the generatrix line: particularly, this procedure has shown better performance than arc welding, responsible for formation of thermally stressed zones and embrittlement of permeators.Based on the high diffusion of the Ag atoms, the joining procedure of the tubes consists of a thermal treatment at 1100°C for 1 h under controlled atmosphere (Ar-H 5%), where the overlapped limbs of the metal foil have been compressed in a special device.This joint obtained by diffusion bonding has shown absence of defects and good chemical and physical stability after thermal and hydrogenation cycling of the permeator tubes under their operating conditions. Characterization tests carried out in the temperature range 300–400°C on these membrane tubes have given hydrogen permeability values according to the literature.     

Thermo-compression bonding of alumina ceramics to metal

Alumina ceramics and Kovar with aluminum interlayer are pressed together under vacuum at temperatures around 600°C for joining. This process produces mechanically strong ceramic to metal bonds in one step in an economic manner. In order to arrive at the optimum conditions for solid-state bonding, effects of bonding temperature, pressure and time on the bond strength have been studied. Bonding kinetics is also elucidated. Irradiation of 99% Al₂O₃ ceramics by 4–5 MV X-rays has been found to increase the bond-strength sharply from 33 to 60 MPa with a dose of 15 k Rads for bonding temperatures around 540°C. The apparent activation energy for the bonding process (Q _B) depends strongly on the type of alumina ceramics. Irradiation of alumina ceramics (99%), prior to joining with Kovar, accelerates the solid-state bonding by reducing (Q _B) from 209 to 76 kJ/mole.     

Oxidation bonding of porous silicon carbide ceramics

A oxidation-bonding technique was successfully developed to fabricate porous SiC ceramics using the powder mixtures of SiC, Al₂O₃ and C. The oxidation-bonding behavior, mechanical strength, open porosity and pore-size distribution were investigated as a function of Al₂O₃ content as well as graphite particle size and volume fraction. The pore size and porosity were observed to be strongly dependent on graphite particle size and volume fraction. In contrast, the degree of SiC oxidation was not significantly affected by graphite particle size and volume fraction. In addition, it was found that the fracture strength of oxidation-bonded SiC ceramics at a given porosity decreases with the pore size but increases with the neck size. Due to the enhancement of neck growth by the additions of Al₂O₃, a high strength of 39.6 MPa was achieved at a porosity of 36.4%. Moreover, such a porous ceramic exhibited an excellent oxidation resistance and a high Weibull modulus.     

Transient liquid phase bonding of HfC-based ceramics

Transient liquid phase (TLP) bonding enables joining at lower temperatures than traditional bonding techniques and preserves the potential for high-temperature applications, making it particularly attractive for joining ultra-high-temperature ceramics (UHTCs) such as carbides and borides. The feasibility of a TLP joint between “pure” carbides has been recently demonstrated. The present study examines the interactions that occur between undoped HfC or MoSi₂-doped HfC and a Ni/Nb/Ni multilayer interlayer during TLP bonding. Bonding is performed at 1400 °C for 30 min in a high-vacuum furnace. SEM–EDS characterization shows that the reaction layer formed at the interlayer/ceramic interface contains mixed carbides and depending upon the ceramic, Ni–Nb–Hf, or Ni–Nb–Hf–Si, or Ni–Nb–Si alloys. Nanoindentation tests traversing the reaction layer between the bulk ceramic and Nb foil midplane also show a clear transition zone across which the indentation modulus and hardness vary. Crack-free joints have been obtained with undoped HfC. The addition of 5 vol% MoSi₂ introduces small (<5 μm long) isolated cracks within the reaction layer, whereas with 15 vol% MoSi₂ added, cracking was pervasive within the reaction layer. When the reaction layer exceeds a critical thickness, as in the case of the bond obtained with HfC doped with 15 vol% MoSi₂, residual stresses become sufficiently large to cause extensive cracking and bond failure. The results suggest a need to characterize and balance the positive role of additives on sintering with the potentially deleterious role they may have on joining.     

MoSi2发热元件冷热端扩散接合工艺的研究

为了研究扩散接合工艺参数对二硅化钼发热元件冷热端的接合强度的影响,对接合端面状态、接合温度、保温时间、接合压力和接合气氛等工艺条件进行了对比分析,并根据接合部位的断裂强度和微观结构的研究结果,建立了在本实验条件下最佳发热元件冷热端扩散接合工艺条件:接合端面粗糙度为1.5 μm,接合温度1570～1600℃,保温时间30～60s,接合载荷15kg,接合气氛为氩气.     

Diffusion bonding of AZ91 using a silver interlayer

Diffusion bonding of AZ91 alloy with a silver interlayer was carried out at 480 °C for different times under 1 MPa in a vacuum of 2 × 10⁻³ Pa. Shear test was applied to measure the shear strengths of the joints in the room temperature. The shear strength values of all bonded samples were found around 65–70 MPa. SEM–EDS studies indicated that the melting occurred along the interface of bonded samples as a result of transfer of atoms between the interlayer and the matrix during bonding. XRD results confirmed that the interlayer dissolved in the interface of joints. Investigations of the fracture surfaces showed that a good bonding was obtained by plastic deformation.     

Diffusion bonding of TiAl using Ni/Al multilayers

With the stimulus of temperature and pressure Ni and Al can quickly react and produce the intermetallic compound NiAl. This reaction is highly exothermic and high temperatures can be attained in the surroundings. These characteristics make Ni/Al multilayers very attractive to technological applications as localised heat sources. In this study, Ni/Al multilayer thin films are used to promote bonding between TiAl intermetallic alloys. Ni and Al alternated nanolayers were deposited by d.c. magnetron sputtering onto TiAl samples, with periods of 5, 14 and 30 nm. Joining experiments were performed at 900 °C for 60 or 30 min, in a vertical furnace with a vacuum level better than 10⁻² Pa. Applied pressures of 5 MPa were tested. The microstructure of the cross-sections of the bond interface was analysed by energy dispersive X-ray spectroscopy and characterised by scanning electron microscopy. The observation of the microstructure for 14 and 30 nm period multilayers revealed sound bonding, while for 5 nm period porosity and cracks within the interlayer thin film were observed. The interface is divided into three distinct zones: one with columnar grains, another with very small equiaxed grains and the third with larger equiaxed grains. The joining process appears to depend on the diffusion of Ni and Ti across the interface and is assisted by the nucleation of nanometric grains at the interface. The mechanical strength of the joints was evaluated by shear tests. The bonds produced at 900 °C/5 MPa/60 min/14 nm exhibited the highest shear strength of 314 MPa.     

Some observations on the wetting and bonding of nitride ceramics

Several series of experiments have been conducted to gain information about the wettability of AlN, BN, Si₃N₄ and two sialon ceramics by potential braze materials. It was possible to achieve wetting of all five ceramics using aluminium, copper-titanium alloys, and a Ag-28Cu-2Ti alloy. Wetting by aluminium and the Ag-28Cu-2Ti alloy was usually good. Both wetting and non-wetting alloys containing titanium reacted to form TiN and it is argued that the achievement of wettability is associated with a certain degree of hypostoichiometry. While aluminium should also have reacted, no clear evidence was obtained. In supplementary experiments it was found that bonds formed by brazing with aluminium at 1000 °C could have shear strengths as great as 60MPa. Although the experimental work was preliminary in nature, it suggested that good brazing systems could be developed.     

Diffusion bonding stainless steel to alumina using aluminium interlayers

A study has been conducted to identify the effects of fabrication temperatures pressures, times and other variables on the strengths of diffusion-bonded joints between alumina and BS321 stainless steel produced using aluminium foil interlayers. The strengths of the alumina-aluminium and steel-aluminium interfaces were found to be influenced differently by some fabrication parameters, thus increasing the fabrication temperature promoted alumina-aluminium bonding but also accelerated the growth of ultimately weakening intermetallic layers at steel-aluminium interfaces. It was concluded that the optimum conditions for bonding BS321 stainless steel to alumina could be achieved by using a 0.5 mm aluminium foil, applying a 50 MPa pressure for 30 min in an evacuated chamber at 625° C. In discussing the results of this study, attention is paid to the problems or advantages of using foils and metal components other than aluminium or BS321 steel and particular note is taken of thermal expansion mismatch effects.     

Diffusion bonding and testing of Al-alloy lap shear test pieces

A quantitative measure of the effect of processing variables on the shear strength of solid state diffusion bonds is often difficult to obtain because of the scatter in test data. This may be reduced by improving the bonding and testing techniques. Two jigs for bonding and testing small Al-alloy lap shear test pieces are described. These jigs enabled the precise measurement of shear stress-strain curves for lap joints and led to reproducible shear strength values. Results obtained for diffusion bonded lap joints between clad Al-Zn-Mg (7010) alloy are described.     

Diffusion bonding of zirconia to silicon nitride using nickel interlayers

The possibilities of diffusion bonding of zirconia to silicon nitride using a nickel interlayer were studied by carrying out bonding experiments under various processing conditions. The process parameters considered were temperature, bonding pressure and interlayer thickness. The optimal process conditions were determined by evaluating the mechanical strength using shear strength testing. It was found that the bonding is optimal in the temperature range 1000–1100°C. The bond strength appears to be independent of the bonding pressure and interlayer thickness if threshold values are exceeded (bonding pressure >14 MPa, interlayer thickness >0.2 mm). At the Si3N4 Ni interface, Si3N4 decomposes, forming a solid solution of silicon in nickel. At the ZrO2–Ni interface, no reaction was observed. © 1998 Kluwer Academic Publishers     

Diffusion bonding of alumina to steel using soft copper interlayer

The diffusion bonding of a low steel to alumina has been studied in the present work. Thin foils of a soft metal (copper) were used to reduce the effects of the residual stresses produced in the joint by thermal expansion mismatch. The strength of the joint was found to be influenced by the bonding parameters, but principally by the oxygen content both on the surface and in the copper matrix. The diffusion bonds have been mechanically tested using a three-point bending test. Maximum bending strengths of 100 MPa were achieved by using a 0.1 mm copper foil, and bonding in a oxidizing atmosphere (P _{O 2}=10₄Pa). SEM and EDS investigations have shown the presence of reaction products in the copper-alumina interface which controls the mechanical properties of the joint.     

Diffusion bonding of titanium alloy to stainless steel wire mesh

Abstract

The feasibility and appropriate processing parameters of diffusion bonding of titanium alloy to stainless steel wire mesh directly and with a nickel interlayer have been investigated. The microstructures of the diffusion bonded joints were observed by optical microscopy, scanning electron microscopy, X-ray diffraction, and electron probe microanalysis and the main factors affecting diffusion bonding were analysed. The maximum shear strengths of the joints were 72 and 148 MPa for direct bonding and indirect bonding using a nickel interlayer respectively. Atomic diffusion and migration between titanium and iron are effectively prevented by adding pure nickel as the interlayer metal, and a firm joint is obtained.