The effect of temperature, matrix alloying and substrate coatings on wettability and shear strength of Al/Al 2 O 3 coupl
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1/30/04
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The Effect of Temperature, Matrix Alloying and Substrate Coatings on Wettability and Shear Strength of Al/Al2O3 Couples N. SOBCZAK, R. ASTHANA, M. KSIAZEK, W. RADZIWILL, and B. MIKULOWSKI A fresh approach has been advanced to examine in the Al/Al2O3 system the effects of temperature, alloying of Al with Ti or Sn, and Ti and Sn coatings on the substrate, on contact angles measured using a sessile-drop test, and on interface strength measured using a modified push-off test that allows shearing of solidified droplets with less than 90 deg contact angle. In the modified test, the solidified sessile-drop samples are bisected perpendicular to the drop/Al2O3 interface at the midplane of the contact circle to obtain samples that permit bond strength measurement by stress application to the flat surface of the bisected couple. The test results show that interface strength is strongly influenced by the wetting properties; low contact angles correspond to high interface strength, which also exhibits a strong temperature dependence. An increase in the wettability test temperature led to an increase in the interface strength in the low-temperature range where contact angles were large and wettability was poor. The room-temperature shear tests conducted on thermally cycled sessile-drop test specimens revealed the effect of chemically formed interfacial oxides; a weakening of the thermally cycled Al/Al2O3 interface was caused under the following conditions: (1) slow contact heating and short contact times in the wettability test, and (2) fast contact heating and longer contact times. The addition of 6 wt pct Ti or 7 wt pct Sn to Al only marginally influenced the contact angle and interfacial shear strength. However, Al2O3 substrates having thin (1 m) Ti coatings yielded relatively low contact angles and high bond strength, which appears to be related to the dissolution of the coating in Al and formation of a favorable interface structure.
I. INTRODUCTION
ALUMINA ceramics are widely used in contact with metals in applications such as joining, sealing, and metalmatrix composites. High metal/alumina bond strength is usually required in such applications; this may be achieved by promoting the metal/alumina wettability. The study of high-temperature wettability, bond strength, and their interrelationship is, therefore, of considerable technological importance. Much work has been done on the wettability in metal/Al2O3[1–35] and other[36–48] systems. Prior work has shown that alumina is not wetted by Al at its melting point; the literature data on the nonwetting-to-wetting transition show a wide dispersion (1083 to 1233 K) in the value of the transition temperature, Tw, at which a transition from the nonwetting to wetting behavior is achieved. The temperature, Tw, depends upon the test technique, atmosphere, substrate roughness, crystal orientation, and chemical purity of the substrate and the metal. In particular, the wetting properties of Al/Al2O3 are extremely sensitive to the ubiquitous atmospheric im
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