Wetting and interfacial behavior of molten Cu on Co–Si(–Mo) coated SiC

Abstract

The wetting and spreading of molten Cu on SiC substrates with or without Co–Si(–Mo) coatings at 1120 °C were investigated by the sessile drop technique. The Co–Si(–Mo) coatings on the SiC substrate were prepared by liquid phase sintering process under a vacuum. The interfacial behaviors of the coating/substrate systems and the Cu/coated SiC wetting couples were analyzed. The experimental results indicated that the final contact angle of the Cu/SiC system at 1120 °C, for a holding time of 10 min, decreased from ~142° without coating to 12°, 15–27° and 7° with the corresponding Co–Si, Co–Si–10Mo and Co–Si–20Mo coatings. This result was closely related with the interactions between the Cu drop and the coatings. No reaction layer was observed at any of the coating/SiC interfaces before the wetting tests. However, a thin Mo–Co–Cu–Si layer and a graphitization layer with different thicknesses formed at the Cu/Co–Si–10Mo coated SiC and Cu/Co–Si–20Mo coated SiC interfaces. Moreover, the graphitization layer disappeared at the Cu/Co–Si–10Mo coated SiC interface when the thickness of the Co–Si–10Mo coating increased to ~60 μm.

Introduction

The wetting and interfacial behavior of liquid metal/solid ceramic systems plays a key role in many manufacturing processes, such as in the fabrication of metal–ceramic composites [1], brazing of ceramic components [2], and electronic packaging of ceramic substrates [3]. Good wettability and interfacial bonding are often desirable for these applications. The main problems are the poor wettability of the SiC ceramic by most pure metals and/or the strong reactivity between transition metals and the ceramic [4]. Actually, the poor wettability makes the preparation of metal–SiC composites or joining of SiC ceramic difficult, and the strong reactivity could reduce the interface bonding strength [5], [6], [7].

Indeed, many studies have been conducted to improve the wettability of pure metals or alloys on SiC ceramic substrate, as reported in Ref. [4]. At present, two main technological methods are used to improve wettability. One is to add active alloying elements, such as Ti [8], [9], [10] and Cr [11], [12] into the brazing filler, or to add Si into some pure metals, such as Co [13], Cu [14], [15], [16], [17], Ni [18], [19], Sn [20], Fe [21], Ag [22] and Au [23]. The additives have quite a different influence on the interfacial behavior of the metal/SiC system. The addition of active alloying elements can commonly result in serious chemical reactions at the interface; however, the addition of Si can reduce the interfacial reactions between the metals and SiC. The other method is to produce a metallic coating, such as Ti [24], Ni [25], [26], Cr [27] or Mo [19], on the ceramic substrate by electroless plating, plasma pulses or ion implantation techniques.

Although the above two methods can improve the wettability of metals/SiC systems, there are still some problems that remain to be solved. For instance, it is possible to promote the interfacial reaction of the metal/SiC system by adding active elements into the filler metals, but the formation of interfacial reaction products can be harmful for brazing or fabricating of metal–ceramic composites. The addition of Si into metals can reduce the interfacial reactions, but here again, the possible formation of metal silicides can destroy the usefulness of the brazing filler metals due to their brittleness. In order to improve the wettability and control the reactivity of metals/SiC system, the surface metallization on the SiC ceramic with metallic coatings seems to be a good opportunity. However, till now, the coatings of SiC reported in the literatures are mainly elementary metal, such as Ti, Ni and Cr; however, in these cases, it is difficult to control the interfacial reactions because theses metals are highly reactive with SiC at high temperature. In the present study, in order to improve the wettability and control the interfacial reactions simultaneously, the SiC surface was modified by Co–Si–(Mo) coatings which do not react with SiC even at a temperature as high as 1350 °C, thanks to their high Si content [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. The influence of the Co–Si(–Mo) coatings on wetting and interfacial behavior of molten Cu/SiC system was investigated by sessile drop tests and by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS) analyses.