Contact angles and spreading kinetics of Al and Al–Cu alloys on sintered AlN

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Abstract

Wetting (both final contact angles and spreading kinetics) and bonding (Wa) of pure Al and Al–Cu alloys on sintered AlN are studied by sessile drop experiments under high vacuum at 1100°C. At this temperature, the process which controls the spreading rate is shown to be the deoxidation of AlN surface. The mechanical behaviour of solidified droplets on cooling can be explained satisfactorily taking into account two parameters: the work of adhesion of the alloys on AlN and the yield stress of the alloys.

Introduction

Aluminium nitride is a hard synthetic ceramic material; its high thermal conductivity and low dielectric constant make it an attractive material for electronic applications, including heat sinks, hybrid substrates, semiconductor packages, high-frequency acoustic wave devices, and dielectric optical enhancement layers. Its main structural application is for ceramic lightweight armour, aircraft and vehicular, given its low density (3.26 g/cm3) [1], [2]. In some applications, interfaces with liquid metals are involved, and it is thus important to investigate AlN wettability by liquid metals and alloys. The purpose of the present work was to study wetting (final contact angle θF and spreading kinetics) of Al on AlN, and the effects on these properties of copper additions to aluminium.

A recent review [3] has shown that low or moderate melting point pure metals, such as Sn, In, Ga, Pb, Ge, Ag, Au, Cu, do not wet AlN (θF≫90°). Particularly interesting are molten Si and Al, which seem to wet AlN without any significant reactivity. However, in the case of Al, the results given in the literature [4], [5], [6], [7], [8], [9], [10] are not altogether in agreement. With pure AlN, sessile drops experiments performed in a static He-3% H2 atmosphere at 1100°C [4] gave a value of θF∼128°. With AlN containing a small amount of Y2O3 particles, added as sintering agents, θF decreased to about 90°; this improvement in wetting has been attributed to the formation of Al2O3 at the Al/AlN interface, by reaction between aluminium and Y2O3. All the other determinations, carried out under high vacuum, gave wetting final contact angles (θ<90°), above a temperature of 850–900°C, depending on the study (Table 1). However spreading kinetics differ strongly.

Rhee [5] reports that the contact angle of aluminium on various non-oxide ceramics, including sintered AlN, assumed a constant value after 30 min at any temperature in the range from 700 to 900°C. Tomsia et al. [6] measured an initial contact angle of 150° at 1000°C, decreasing to a steady contact angle of 53° after an isothermal holding of 30 h. Nicholas et al. [7] measured transient contact angles during a continuous rise in temperature (5 K min−1) towards 1150°C. They observed a transition from non-wetting (θ>90°) to wetting (θ<90°) at about 950°C. Their results at 1000°C and 1100°C indicate much faster spreading kinetics than in [6]. No information on spreading kinetics at 1000°C is given by Naidich and Taranets in reference [8], but in a more recent work [9] the same authors measured a steady contact angle of 39° after 30 min at 1100°C. Finally, Ho et al. [10], working on sintered AlN containing Y2O3 as a sintering aid observed that at 850°C the contact angle decreased from 112 to 75° in 200 min, without reaching a steady value.

This work is divided into two parts: in the first the wetting of Al on AlN is studied, focusing on spreading kinetics; in the second the effects of copper additions to aluminium on the final contact angles and on spreading kinetics are considered. The mechanical behaviour of solidified drops under the stress produced during cooling is also discussed and correlated with interfacial features.

Section snippets

Experimental procedure

The experimental apparatus used for the sessile drop experiments consist of an alumina tube heated externally by a silicon carbide resistor. The tube is connected to a gas supply and to a vacuum system consisting of a rotary vane pump and an oil diffusion pump, each equipped with a liquid nitrogen trap. The system provides a vacuum of 10−4 to 5×10−5 Pa.

The sample is placed in the middle of the alumina tube, just above the thermocouple. Windows are fitted at each end of the tube, allowing for

Results

In the wetting of aluminium on ceramics, the effect of oxygen is a well-known problem, usually causing Al drops to be covered with an oxide layer, which inhibits wetting [14], [15]. Therefore, before studying the Al/AlN system, a limited number of sessile drop experiments were performed for the Al/Al2O3 system, chosen as a reference because of its well-known behaviour [15], [16], to check that deoxidation of aluminium drops could be achieved with the installation and experimental conditions

Characterisation

For Al/AlN samples, cooling does not cause any rupture, and SEM micrographs of cross-sections (Fig. 9) show strong interfaces, with no cracks along the interface or inside the ceramic or the solidified drop. The same figure shows that there are no more secondary phases at the interface, while they can be clearly seen (arrow 1) far from the interface. They have been dissolved by the liquid metal, and the resulting holes are filled with aluminium (arrow 2 in Fig. 9). This is confirmed by

Discussion

The experimental results are listed in Table 3. The values of the work of adhesion Wa are also given, calculated from data on surface energies σLV of Al–Cu alloys at 1100°C [20] and values of θF, according to the Young-Dupré equation:cosθY=WaσLV−1where θY is the intrinsic contact angle of the system (θYθF is assumed in these calculations).

After drop deoxidation, the contact angle of Al on AlN at 1000°C lies between 70–75°, while the same angle on Al2O3 is 86±3° (Table 4). The difference is

Adhesion of solidified drops

It is widely recognized that the main contribution to the fracture energy, Γi, of an interface formed between a metallic layer and a ceramic substrate is due to the energy dissipated by plastic deformation of the metal. This contribution depends mainly on two parameters: the yield stress of the metal, σY, and the work of adhesion of the metal on the substrate, Wa. Although the precise expression of ΓI as a function of Wa and σY is not well-known, it is well established that Γi increases with

Conclusions

The rate of spreading kinetics of Al droplets on sintered AlN under high vacuum is controlled by two deoxidation processes:

(i) deoxidation of Al at low temperature (T<900–950°C), as in wetting experiments for pure Al on alumina or graphite substrates; at the end of this stage the contact angle of deoxidised Al on sintered AlN is close to 75°(ii) deoxidation of AlN surface, occurring at higher temperature (T>1000°C); this process, which corresponds to in situ cleaning of the AlN surface, is caused

Acknowledgements

The authors would like to thank Professor M. Dupeux (LTPCM) for helpful discussions on the adhesion of solidified droplets. Also the society Biophy Research for the ‘Time of Flight’—Secondary Ion Mass Spectroscopy analyses.

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