JOURNAL OF MATERIALS SCIENCE 32 (1997) 653 — 659 Production, bonding strength and electrochemical behaviour of commercially pure Ti / Al 2 O 3 brazed joints O. C. PAIVA * , M. A. BARBOSA ‡ INEB-Instituto de Engenharia Biome & dica, Prac ,a do Coronel Pacheco 1, 4050 Porto, *also ISEP-Polytechnic Engineering Institute of Porto, Dept. of Mechanical Engineering, Rua de S. Tome & , 4200 Porto, and ‡ also Dept. of Metallurgical Engineering, FEUP, Rua dos Bragas, 4099 Porto Codex, Portugal The brazing of commercially pure titanium to Al 2 O 3 has been studied. Two different brazing alloys within the Ag —Cu —Ti system and pure silver were selected as bonding agents. Titanium hydride(TiH 2 ) additions were also tested, with the aim of improving the wetting of the ceramic surface by the melted brazing alloy. The mechanical and electrochemical behaviour of the produced joints was assessed, and related to chemical and morphological features resulting from an analysis by scanning electron microscopy and energy dispersive spectroscopy.It waspossible to producejointspresenting high integrity,good strength and high resistance to corrosion. The best results were obtained when using an Ag —26Cu —3Ti brazing alloy. The addition of TiH 2 increased the mechanical properties, leading to a maximum bonding strength of 808 MPa, as determined in three-point bending tests. In most of the cases, for a maximum deflection of 5 mm, there was only a partial detachment of the ceramic/metal joints. The lowest values for the corrosion rates (i corr 1.38 A cm !2 ) determined in potentiodynamic experiments also correspond to the use of the Ag —26Cu —3Ti brazing alloy. The bonding strength and electrochemical results could be explained in terms of the different chemical compositions of the interfaces. The use of TiH 2 additions proved to be quite effective, allowing for the replacement of the usual metallizing and plating pre-treatments needed for the brazing of ceramics to metals. 1. Introduction The unique properties of structural ceramics, such as alumina, make them well suited for a range of applica- tions. This type of material is becoming increasingly important in engineering, especially in both structural and insulating applications [1, 2]. However, ceramics present poor machinability and their brittle nature limits a more extensive use. In order to avoid the processing limitations, some form of joining is usually employed to form the final component. The joint may be of either a ceramic/ceramic or a ceramic/metal type. Brazing is the most effective and widely used tech- nique for joining structural ceramics (such as alumina) to ceramics and to metals [3—6]. The joining techniques currently used require me- tallization of the ceramic [3]. The ceramic is initially metallized by applying a Mn—Mo paste which is then burned in. To improve brazing properties, a nickel coating is then applied on top. The brazing process can then be carried out with conventional brazing alloys. While the process is complex, it can be con- siderably simplified by using active brazing alloys, without separate pretreatment [3, 4]. For a successful braze, the ceramics must be wetted by metals and alloys to produce strong bonds. How- ever, many technologically important ceramics are unwetted by conventional brazes based on copper and silver [3, 4, 7]. To achieve the necessary wetting, the chemistry of the metal/ceramic interface must be changed and hence some components of the braze must be active enough to alter the composition and the chemistry of the ceramic surface [3, 7—9]. It is now well established that the wetting properties of various metals and alloys can be dramatically improved by small additions of titanium, aluminium, silicon and other interfacially active elements [3, 4, 7]. An active element may be defined as one which can interact with the ceramic and form a strong chemical bond at the interface [6, 10]. The improvements in the wetting behaviour produced by titanium additions are very frequently accompanied by improvements in the strength of brazed joints [7]. Bonding of materials, such as ceramics and metals, with distinct thermal expansion coefficients can be a problem, especially when heat is applied [8]. In addition, thermal shock, loss of hermetic sealing 0022—2461  1997 Chapman & Hall 653