Active Metal Brazing for Reliable Ceramic Joints Using TiN Coating
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Tae Woo Kim School of Mechanical Engineering, Kookmin University, Seoul, 136-702, Korea
Osung Seo and Shinhoo Kang School of Materials Science and Engineering, Seoul National University, Seoul 151-742, Korea (Received 3 November 2000; accepted 1 November 2001)
To improve the strength of brazed joints, a TiN-coated layer was introduced as an interlayer and reaction barrier for ceramic-ceramic joints. Al2O3, Si3N4, and ZrO2 ceramics were brazed with a Ag–Cu–In–Ti alloy after a TiN coating was applied on the ceramic substrates. The strength of as-brazed and thermally treated Al2O3 specimens with a TiN coating was found to be equivalent to that of bulk Al2O3. However, the Si3N4 and ZrO2 systems showed a significant reduction in joint strength. The difference in the strength can be explained on the basis of the bond strength of the coating, the reaction products, and residual stress developed at the joint interfaces.
I. INTRODUCTION
Active metal brazing procedures are widely used for the joining of high-performance ceramic materials commonly used in high-temperature, corrosive or wear resistance applications.1–5 The addition of active metal elements such as Ti, Hf, or Zr serves to enhance the wettability and reactivity of the brazing alloy to ceramics.5–11 However, it is well known that excessive reaction between ceramic materials and the active metal, which occurs during the brazing process or an extended exposure to elevated temperature, can result in brittle reaction products.12–15 This represents a major factor along with thermal expansion mismatch in the deterioration of the joint strength of ceramic materials. To improve joint strength, the formation or effect of reaction products and residual stress developed at the joining interfaces needs to be minimized. The cause of the residual stress at the joints is the mismatch in the coefficients of thermal expansion (CTE) among the constituent materials. Difficulties and complexity in stress analysis reside in the determination of final dimensions and physical and mechanical properties of each of the constituent layers and phases formed in the joints. Therefore, the analyses often require realistic and reasonable assumptions for a simple and ideal interface structure to estimate the stress. However, the finite element analysis (FEA) performed for an ideal interface structure may show limited success in providing reasons for joint strength variation, particularly when the joint involves substantial interfacial reactions. The effect of 246
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J. Mater. Res., Vol. 17, No. 1, Jan 2002 Downloaded: 18 Mar 2015
microstructural changes near the interface, such as the nature of reaction products, needs to be addressed along with FEA results. The present study describes an approach wherein a TiN-coated layer is introduced between the brazing alloy and the parent ceramic materials as a reaction barrier and an interlayer for thermal expansion mismatch. A TiN interlayer was selected, since it is an extremely stable compound with strong metallic char
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