Mechanical Properties and the Microstructure of Al2O3/Al/Al2O3 Joints with the Surface Modification of Alumina by a Thin Layer of Ti + Nb

Al2O3/Al/Al2O3 joints were formed by liquid-state bonding of alumina substrates covered with a thin Ti + Nb coating of 900 nm thickness with the use of an Al interlayer of 30 μm at 973 K under a vacuum of 0.2 mPa for 5 min. The bond strength of the joints was examined by a four-point bending test at 295, 373, and 473 K. Optical, scanning, and transmission electron microscopies were applied for detailed characterization of the interface structure and failure characteristics of fractured joint surfaces. The analysis of the results has shown that (i) bonding occurred due to the formation of a reactive interface on the metal side of the joint in the presence of Al3Nb(Ti) precipitates and (ii) modification of Al2O3 by a thin layer of Ti + Nb increases the hardness at the interface and makes it possible to achieve reliable joints working at elevated temperatures.


Introduction
The metal/ceramic joint showed a series of unique properties that do not occur in any other material in such a combination. The applicability and occupied time of the final metal/ceramic joints depend on the dissipation of the stress concentration, which increases the ceramicÕs cracking strength in the formation of such interatomic bonds that will ensure obtaining a durable and reliable joint. The reliability of this joint depends on the interfacial properties between the metal and ceramic, e.g., adhesion properties and thermal stability. However, the differences in physical and chemical characteristics between the metal and ceramics cause severe changes in the region of the interface (a discontinuous change in the elastic, plastic, and thermal characteristics at the interface), and the difference between the values of linear coefficients of the thermal expansion of the metal and ceramic leads to the formation of residual stress at the interface. That is why in order to identify the optimum mechanical integrity of the metal/ ceramic joint, one shall consider a number of factors: the process of the bond formation, chemical composition, morphology and the structure of the metal/ceramic joint, thermal area and plastic incompatibility, the limit value of the metal strength and its thickness (it determines the distribution of residual stress and the stress occurring currently at the joint load), the number of defects in a ceramic material and at the interface, and also the resistance of the interfacial region to cracking (it determines the course of the cracking path) (Ref 1-4).
The phenomena occurring at the contact of liquid and solid phase have a decisive influence on the process of joining these materials. The most important ones include wetting of the ceramic material surface by liquid metal and mutual chemical interaction between the liquid metal and ceramic, related with diffusion processes. The reactivity and wetting behavior in a metal/ceramic system can in turn be improved, e.g., by applying thin metallic layers to the ceramic material surface ( Ref 5,6). Such layers can be reactive against the bonded materials so that a fixed bond on the phase boundaries is formed, creating diffusion, reactive intermediate microlayers in the most favorable case; the layers have a physical and chemical compatibility (thermodynamic and kinetic), which guarantees good mechanical characteristics of the joint. A number of studies focus on research of the influence between pure titanium and Al 2 O 3 together with the definition of the morphology of the reaction area which is created at the interface of Ti/Al 2 O 3 . Kelkar et al. in their study (Ref 7) on the basis of thermodynamic calculations present a concept that the influence between liquid Al and titanium covering Al 2 O 3 in a vacuum, even at low partial pressures of oxygen, leads to changes in the chemical composition of the interface at the side of the substrate by forming the following reaction layer: Al/Al 2 O 3 /TixOy/Ti/TixOy/Al 2 O 3 .
In the previous work (Ref 8), experimental results revealed that the addition of Ti and Nb improves the wetting; titanium influences the contact angle more than niobium (probably because it has a higher chemical affinity to oxygen); and at a thin layer less than 1 lm, the minimal contact angle was reached (40°for titanium and 108°for niobium at 1223 K).
The aim of this work was to study the effect of the Ti + Nb thin film on alumina on the interface structure, bond strength properties at room and elevated temperatures, and the mechanism of destruction of Al 2 O 3 /Al/Al 2 O 3 joints.

Experimental Procedure and Materials
The Al 2 O 3 /Al/Al 2 O 3 joints were produced by liquid-state bonding of 98% pure polycrystalline Al 2 O 3 blocks (14 9 14 9 20 mm) and 99.99% pure aluminum of 30 lm thick layer.
Foils of the metal (14 9 14 mm dimensions) were placed between the alumina blocks, and bonding was performed at 973 K with a vacuum of 0.2 mPa for 5 min. The heating and cooling rates used were 30 and 10 min, respectively. The alumina blocks of 3.8 g/cm 3 (almost 0% porosity) were produced by sintering at the temperature of 1923 K for 2 h; powders have the following starting composition: 99.9085% a-Al 2 O 3 , 0.009% CaO, 0.053% SiO 2 , 0.0029% MgO, 0.023% Fe 2 O 3 , and 0.0036% Na 2 O. The polycrystalline alumina contained 2% of Mg(SiO 4 ) and MgAl 2 O 4 spinels. The Al foil was mechanically and ultrasonically cleaned in acetone, rinsed in alcohol for 10 min, and then dried with warm air directly before bonding. The Al foils were positioned between two Al 2 O 3 blocks, and this assembly was placed in a vacuum chamber under a load applied by placing one of the Al 2 O 3 blocks on top of the assembly to maintain structural integrity during bonding.
The surfaces to be joined were lapped flat to a 1-lm finish, ultrasonically cleaned in acetone, and finally a 900-nm-thin Ti + Nb layer was deposited on the surface by physical vapor deposition (PVD). The film was deposited by rf sputtering of a mosaic Ti + Nb target in argon gas atmosphere onto different substrates such as Corning glasses and alumina, depending on the requirements imposed by further applications, i.e., for the determination of the film thickness and composition. The film thickness was measured mechanically by means of a Tally Step profilometer. Research with the use of a scanning electron microscope revealed that the thin layer is smooth and nonporous and that it contains both Ti and Nb. The thin film composition was determined from X-ray diffraction (XRD). The weight ratio Nb/(Nb + Ti) was found to be equal to 0.625 (62.5 wt.%).
After bonding, the joints were tested at 295, 373, and 473 K, using the four-point bending test with an inner span of 12.5 and outer span of 25 mm. Specimens of dimensions 3 9 3 9 36 mm 3 for the four-point bend testing were cut from one bonded sample. The specimens were tested using an INSTRON 1115 machine with the automatic recording of the applied load versus the corresponding displacement up to failure under a constant load displacement rate of 1 mm/min. The reported results are averages of between three and five tests that were run for each condition. The impact of temperature was analyzed within the range of 295-473 K on the course of jointÕs bending characteristics, due to the fact that in practice deformation at a temperature (above 0.25 of melting point) is used to minimize the deformation resistance and increase the plasticity of brittle materials.
The structure was examined on sandwich specimens after joining, using optical microscopy (LM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). LM/ SEM studies were performed on fractured joint specimens, cross sectioned in a plane inclined at about 25°to the joint surface (to enable the enlargement of the joint area available for examinations; it is noted that this technique lowers somewhat the spatial resolution of EDS composition line scans). Moreover, SEM/EDS studies were performed to study failure characteristics of fractured joint surfaces.
The specimens for TEM examination (Model PHILIPS CM20 TWIN operated at 200 KV) were prepared using a method for the preparation of coated material by sandwiching described in detail in (Ref 9) (comprising specimen prepreparation, bonding together by epoxy glue, mounting, mechanical pre-thinning, and thinning by ion-beam milling until perforation).

Results and Discussion
The four-point bending test results of the ceramic-metalceramic joints bonded at 973 K are summarized in Fig. 1  The fracture modes of the Al 2 O 3 /Ti + Nb/Al/Ti + Nb/Al 2 O 3 joints also differ from that of Al 2 O 3 /Al/Al 2 O 3 . Figure 2 presents images of the fracture surfaces resulting from the bending test at 295 K. As illustrated in Fig. 2, two types of fracture modes exist in the ceramic-to metal joints without and with Ti + Nb coating. In type I, the crack exists in the ceramic during initiating and propagating periods. In joints Al 2 O 3 /Al/Al 2 O 3 , brittle fractures occur along the cleavage planes of the Al 2 O 3 phase. This indicates that the bonding strength of Al/Al 2 O 3 interface is higher than that of the Al 2 O 3 . The region of Al/Al 2 O 3 interface is ''clean'' without segregants and reaction products; they tend to be detrimental as it can form brittle compounds. In type II (for the Al 2 O 3 /Ti + Nb/Al/Ti + Nb/Al 2 O 3 joints) cracking occurs both in the metal near the interface and along the interface, which suggests that this region represents the combination of the solid solution and precipitation strengthening (associated with the presence of intermetallics and of Al-rich solid solution, resulting from dissolution of coatings). For the explanation of the mechanism controlling the bonding process in Al 2 O 3 /Ti + Nb/Al/Ti + Nb/Al 2 O 3 joints, the structural examinations were carried out in the region of Al/ Ti + Nb/Al 2 O 3 bond by the method of optical microscopy, combined with SEM and TEM, shown in Fig. 3 and 4.
The bonding between Al 2 O 3 and Al layer was achieved by growing the Al 2 O 3 phase at the interface. The presence of isolated separate particles of Al 2 O 3 was confirmed experimentally by SEM/TEM studies in the region of the interface in Al 2 O 3 /Al/Al 2 O 3 joint (Fig. 3). SEM analysis of the fracture surface of this joint (Fig. 2a) also confirms the presence of Al 2 O 3 phases at the interface. It is obvious that oxygen plays an important role in improving the bond strength between Al and Al 2 O 3 , probably because of the formation of metal clusters (Ref 10) and chemical interaction between liquid Al and additives in ceramic (Ref 11). At a temperature below 1373 K, through a reduction reaction of silico-aluminate spinels by liquid Al, it is possible to form Al 2 O 3 precipitates at the interface, which in turn is conducive to reinforcement of the interface. Therefore, the microstructure of the joint shows a strong bonding at the interfaces. Bend testing at 295 K showed that fractures always occurred within the ceramic material.
Observations of the Al/Ti + Nb/Al 2 O 3 system interface under a magnification of 5009 revealed the presence of interfacial precipitates, along the interface (Fig. 4). SEM observations ( Fig. 4b) have proved the formation of new phases, i.e., a phase containing mainly Nb and Ti. The local analysis of the chemical composition of the above-mentioned precipitates has indicated that these are the precipitates of intermetallic phases rich in Nb and Ti (e.g., items 1, 2, and 3 in Fig. 4b, respectively). The chemical composition in the interface region also shows maps of the distribution of the elements like aluminum, titanium, and niobium (Fig. 4b).
Moreover, the last statement was confirmed experimentally by TEM studies of the Al/Ti + Nb/Al 2 O 3 system interface and the presence of particles of Al 3 Nb(Ti) in the region of interface (Fig. 4c). The structural analysis of the Al/Ti + Nb/Al 2 O 3 bond interfaces produced in the joining process at 973 K indicates that the joining effect has been accompanied by intense chemical interaction between the Ti + Nb film-coated substrate and liquid aluminum. The mechanism of this interaction is probably solutionprecipitation based and is related to the dissolution of Ti + Nb coating in liquid aluminum and the formation of a solid solution Al(Nb,Ti) in the contact region during the process of bonding. However, the precipitates of phases rich in Nb and Ti nucleate at the distance of approx. 5 lm from the interface from the supersaturated Al(Nb,Ti) solution, during the cooling of the joint to the ambient temperature ( Ref 12). Figure 5 illustrates the fracture surface of Al 2 O 3 /Ti + Nb/ Al/Ti + Nb/Al 2 O 3 joints after bending test at elevated temperatures. According to this figure, the fracture preceded in the metal layer near the metal-ceramic interface by ductile rupture mechanism. In the fracture surface, the conventional ductile fracture process generated the regions with dimples. The dimples are believed to initiate at the precipitates of intermetallic phases rich in Nb and Ti (e.g., items 1 and 2 in Fig. 5a). Finally, interfacial precipitates of intermetallic phases can both embrittle the region of the Al 2 O 3 /Ti + Nb/Al interface and plastically blunt and eliminate high stress concentrations. In the fracture surface obtained after bending test at 473 K, more dimples which are shallower and bigger than the dimples in the fracture surface after bending test at 373 K are observed. These fracture types observed in the Al 2 O 3 /Al/Al 2 O 3 and Al 2 O 3 / Ti + Nb/Al/Ti + Nb/Al 2 O 3 joints correspond with the bending strength results shown in Fig. 1

Summary and Conclusion
The effect of a thin film Ti + Nb interlayer on the microstructure and strength of Al 2 O 3 /Al/Al 2 O 3 joints at elevated temperatures is determined by the four-point bend testing, SEM, and TEM analyses.
The following conclusions were drawn: -Better quality of Al 2 O 3 /Ti + Nb/Al/T + Nb/Al 2 O 3 joints at elevated temperatures relative to Al 2 O 3 /Al/Al 2 O 3 joints is due to specific interfacial microstructure (resulting in a solution and precipitation of rich Nb and Ti intermetallic phases) combined with plastic deformation on the metal side during fracture and leads to the production of reliable metal/ceramic joints -Fractography of Al 2 O 3 /Al/Al 2 O 3 and Al 2 O 3 /Ti + Nb/Al/ Ti + Nb/Al 2 O 3 joints at elevated temperatures showed that