Roles of preoxidation, Cu 2 O particles, and interface pores on the strength of eutectically bonded Cu/a-Al 2 O 3 H. Ghasemi a, * , A.H. Kokabi a , M.A. Faghihi Sani a , Z. Riazi b a Department of Materials Science and Engineering, Sharif University of Technology, Tehran, Iran b Bonab Research Center, Bonab, Iran article info Article history: Received 27 February 2007 Accepted 17 June 2008 Available online 24 June 2008 Keywords: Ceramic–metal (A) Bonding (D) Mechanical strength (E) abstract The influences of CuO layer thickness, Cu 2 O particles, and pores on mechanical properties and micro- structure of alumina–copper eutectic bond have been investigated. The furnace atmosphere in the first stage was argon gas with 2  10 6 atm oxygen partial pressure. In the second stage, the furnace atmo- sphere was same as the first stage except for the cooling interval between 900 and 1000 °C, the hydrogen gas was injected into furnace atmosphere. Finally, in the last stage a vacuum furnace with 5  10 8 atm pressure was chosen for bonding procedure. Peel strength of first stage specimens shows that CuO layer with 320 ± 25 nm thick generates the maximum peel strength (13.1 ± 0.3 kg/cm) in joint interface. In the second stage, by using the hydrogen gas, a bond interface free of any Cu 2 O oxide particle was formed. In this case, the joint strength has increased to 17.1 ± 0.2 kg/cm. Finally, the bonding process in vacuum fur- nace indicates that the furnace gas does not have considerable effect on joint interface pores. Further- more, bonding process in vacuum furnace reduces the peel strength of joint due to formation of more pores. Thorough study of pores formation is presented. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Gas–metal eutectic bonding of alumina–copper was first devel- oped by Burgess and Neugebauer [1]. So far, the mechanical prop- erties of eutectic bonding and corresponding interactions within the bonding area have been studied extensively by several authors [2–8]. The bonding of copper to alumina is carried out at 1075 ± 2 °C in a properly controlled oxygen-containing atmo- sphere. It has been demonstrated that oxygen promotes wetting of liquid copper to alumina and hence it is necessary to achieve good spreading of the liquid along the bond interface [2,9]. Yoshino [2] observed that the peel strength of alumina–copper bond is pre- dominantly affected by dissolved-oxygen concentration. He re- ported that the Cu 2 O particles and voids have, also, contributions to true bond strength. Thus, oxygen seems to affect the peel strength of alumina/copper bond principally in two ways: through change in cohesive bond energy and through formation of Cu 2 O particles. The measured bond strength is the combination of these effects. Yoshino predicted that by removing the oxide particles from the interface, the peel strength of the joint can be raised up to 15 kg/cm [2]. Moreover, Trumble et al. [7] have reported that the Cu 2 O particles can be reduced after bonding without any loss in bond strength. Kim and Kim [5] studies on sandwich type Cu– Al 2 O 3 couple shows that by creating CuAlO 2 phase at the interface, the bond strength increases up to an optimum CuAlO 2 layer thick- ness which after that brittleness of CuAlO 2 lowers the bond strength. Likewise, Seager et al. [6] have investigated more pre- cisely the effect of CuAlO 2 layer thickness and have confirmed that thick CuAlO 2 layer is detrimental to bond strength. Following the other researcher investigations, Ning et al. [8] have stated that good bonding strength can be obtained by a certain thickness of CuO layer which is a function of alumina substrate profile. On source of pores formation and their effect on peel strength, Yoshino [2] reported that the pores within the bonding interface can be formed due to release of oxygen gas in the liquid copper; by increasing the CuO layer thickness, their percentage and size will be larger. However, Seager et al. [6] have found that smaller pores (1–3 lm) may be pullouts of the Cu 2 O particles observed on alu- mina fracture surface, while the large pores observed by Yoshino are the result of argon entrapment in liquid copper during process- ing. Concerning the influence of pores on mechanical strength of eutectic bond, Reimanis [10] have observed that crack-front per- turbation occurs when the crack tip is in contact with a pore; the crack front is drawn into the pore and causes debonding of the re- gions immediately surrounding the pore. Despite several investiga- tions, the accurate contribution of preoxidation and Cu 2 O particles on peel strength of mentioned joint is unclear. There is, further- more, uncertainness about the sources of pores and their effect on peel strength of the bond. The goal of current research is to shed 0261-3069/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.matdes.2008.06.034 * Corresponding author. Tel.: +1 416 543 9757. E-mail address: hadighasemi@alum.sharif.edu (H. Ghasemi). Materials and Design 30 (2009) 1098–1102 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/matdes