Metallization of Al2O3 ceramic by magnetron sputtering Ti/Mo bilayer thin films for robust brazing to Kovar alloy
Introduction
Alumina (Al2O3) ceramic has recently attracted considerable interests in a wide variety of applications including electronics, aerospace, nuclear power, automobile, cutting tools and biomaterials due to their superior physical, chemical and mechanical properties [1], [2]. The primary prerequisite for practical utilization of Al2O3 ceramic is to join them to metallic parts strongly [3], [4]. However, a large difference in physical and mechanical properties between ceramics and metals, including coefficient of thermal expansion (CTE), chemical composition and Modulus of Elasticity (MOE), makes it quite difficult to obtain ceramic/metal joints with adequately mechanical integrity [5]. Currently, one of effective solutions is metallization of Al2O3 ceramic by coating metallic layer on surface, followed by sintering to improve adherent strength and brazing property [1], [6], [7], [8]. Typically, in 1970s, Molybdenum–manganese (Mo–Mn) method was developed for metallizing Al2O3 ceramic owing to its effectiveness and reliability, which is closely related to the formation of dense metal/glass composite layers through glass phase migration from Al2O3 into metallizing layers [6], [7]. Nevertheless, in order to ensure glass phase to migrate favorably, Mo–Mn metallizing process has to be subjected to an extremely high sintering temperature up to 1500 °C in hydrogen [6]. Obviously, both the higher demands for equipments and larger costs for manufactures limit its wider application to a large degree. Therefore, feasible approaches with advantages of simplicity and economy to achieve the metallization of Al2O3 ceramic urgently need to be investigated.
It has been reported that titanium (Ti) can act as a kind of active element to wet Al2O3 ceramic effectively by means of changing the chemistry of Al2O3 ceramic surfaces on a basis of in situ chemical reaction between Ti and Al2O3 [9], [10], [11], [12]. However, Ti is prone to be oxidized to TiO2 at elevated temperatures, thereby evidently reducing the wettability of Al2O3 ceramic even though being carried out under a relatively high vacuum of 5×10−3 Pa (equilibrium oxygen partial pressure at 1200 °C only is 10−25 Pa [13]), which could be fatally harmful to subsequent brazing for practical applications. Kang et al. also displayed the evidence of TiO2 formation on the surface of Ti-coated silicon nitride through heat treatment at 980 °C for 10 min under vacuum [14]. One common method to overcome this issue is to utilize one refractory metal layer to protect the pure Ti film from being oxidized. Nevertheless, Ti can readily react with most of refractory metals to form intermetallics with the exception of Mo, which just has a very limited solubility in Ti. As a result, Mo could serve as a potential protector of Ti coating on the surface of Al2O3 to prevent from oxidization during metallization; meanwhile, it also could serve as a potential isolator of Ti to inhibit the occurrence of side reactions between Ti and metal elements in filler alloys on the other side of Mo during brazing. Moreover, note that Mo has a very close CTE (5.2×10−6/K at 293 K) to that of Al2O3 substrate (7.8×10−6/K at 293 K), beneficial to relieve residual stress during cooling of sintering. Therefore, a reasonable hypothesis on designing Ti/Mo bilayer thin film configuration to metallize Al2O3 ceramic could be proposed effectively.
To test the feasibility of design philosophy, Ti/Mo bilayer thin films were deposited onto Al2O3 ceramic by magnetron sputtering with a subsequent high temperature sintering to ensure robust brazing of Al2O3 ceramic to Kovar alloy, which just is one of candidates with close CTE to Al2O3. The interface reaction process between Ti film and Al2O3 ceramic as well as the joining strength between metallized Al2O3 ceramic and Kovar alloy were investigated systematically. The experimental results show that the Ti film can react with Al2O3 ceramic to form Ti3Al and TiO during sintering, in which the amount, size and morphology of TiO crucially depend on sintering temperature. As the temperature reaches 1200 °C, a plenty of spherical TiO particles with ~150 nm in diameter can be created across the Ti/Al2O3 interfaces, resulting in the strong adhesion between Ti film and Al2O3 substrate and the optimal brazing joining strength between metallized Al2O3 ceramic and Kovar alloy. The interface reaction mechanism of metallization and the probable reasons for improvement of joining strength are discussed in detail.
Section snippets
Experimental material and methods
The base materials are Al2O3 ceramic with purity of 95% and 4J33-Kovar alloy (Fe–33 wt% Ni–17 wt% Co). The filler alloy is commercial Ag–28 wt% Cu eutectic alloy with a thickness of 50 μm (Shanghai Mick Welding Material co., LTD). Kovar alloy in the form of ring possesses an outer diameter of 16 mm, inner diameter of 9.8 mm and thickness of 1 mm. The detailed dimensions of Al2O3 ceramic are shown in Fig. 1.
Ti and Mo bilayer thin films were deposited onto Al2O3 ceramic substrates successively by
Microstructure and phase of metallized bilayer
Fig. 2 shows the surface and cross-sectional SEM micrographs of interface deposition bilayer of metallized Al2O3 ceramic after sintering at different temperatures, respectively. It can be seen clearly from Fig. 2a and b that the surface of Al2O3 ceramic is covered by a continuous and dense metallized layer after metallization sintering. The cross-sectional SEM micrographs further exhibit the metallized layer consists of double thin films, in which one film bordering the Al2O3 substrate can be
Conclusion
A simple and effective approach to metallize Al2O3 ceramic has been developed by magnetron sputtering Ti/Mo bilayer thin films with a subsequent high temperature sintering, which can produce strong adhesion between Ti film and Al2O3 ceramic by in situ chemical reaction. The active Ti can react with Al2O3 to form Ti3Al and TiO during the high-temperature sintering, in which the amount of TiO in the metallized layer is scarce as the temperature is below 1057 °C, while over which the amount can be
Acknowledgments
We give thanks to financial support by the National Natural Science Foundation of China (11076109), the “HongKong Scholars Programme” Funded Project (XJ2014045, G-YZ67), the China Postdoctoral Science Foundation Funded Project (2015M570784), the Scientific Research Fund of Sichuan Provincial Department of Education (16ZB0002), and the Talent Introduction Program of Sichuan University (YJ201410). Also, the authors thank Prof. S.L. Wang for her help in preparation of cross-sectional TEM samples.
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These authors contributed equally.