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Nanocrystalline transition alumina tape casts were used as interlayers to join conventional alumina ceramic pellets. The joining experiments were performed by hot pressing at 1200–1300 °C under uniaxial pressures of 55 and 80 MPa for 1- and 5-h durations, with and without a nanocrystalline interlayer. Successful joints were enabled only above 1250 °C in the presence of the interlayer. Generally, the joint 4-point bending strength increased with the increase in joining temperature, pressure, and duration. The average bending strength of the interface joined at 1250 °C was 245 ± 65 MPa compared to the pellet strength of 268 MPa. Postjoining heat treatments at 1400 °C for 3 h caused reduction in the joint strength. The interlayer at the joint exhibited homogeneous and crack-free microstructure. The changes in the joint strength were discussed with respect to the densification and grain growth behavior of the nanocrystalline interlayer. I. INTRODUCTION

Joining of ceramic parts becomes of prime importance due to their increased application as components in complex systems. Alumina is the most useful engineering oxide, used as a structural as well as a functional ceramic. However, sintering, densification, and diffusion bonding of alumina often necessitate heating to temperatures above 1600 °C. Joining by microwave heating1–3 was found to be effective only for impure aluminas containing a few percent of the glassy phases. This arises from the low dielectric loss of pure alumina, hence its ineffective heating. Superplastically deformable yttriastabilized tetragonal zirconia polycrystal (Y-TZP) inserts were also used for diffusion bonding of alumina4,5 and zirconia.6 However, these joints also necessitated heating to temperatures above 1375 °C. Recently, nanocrystalline transition ␥–alumina powder compacts were used for successful joining of conventional alumina disks at 1200 °C in vacuum.7,8 These powders were formed by laser ablation and had a median grain size of 14 nm. The present paper describes the joining of conventional alumina pellets by hot pressing in ambient atmosphere, using nanocrystalline alumina tape casts as interlayers. II. EXPERIMENTAL PROCEDURE

Commercial alumina powder (Ceralox, 0.5 ␮m mean grain size) was cold isostatically pressed into cylinders at 280 MPa and pressureless sintered at 1600 °C for 10 h. The resultant 97% dense cylinders of 20 mm in length and 15 mm in diameter were used for the joining experiments. The joining surfaces, perpendicular to the cylin1724

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J. Mater. Res., Vol. 15, No. 8, Aug 2000 Downloaded: 13 Mar 2015

der axis, were polished using 600-grit SiC abrasive paper. Finally, the cylinders were cleaned in an ultrasonic bath using ethanol and then acetone. Commercial nanocrystalline powder consisting of ␪, ␦, and amorphous alumina (Neomat, 10 nm median particle size) was used to form the tape casts. Flexible and deformable tape casts were formed using the following (nonaqueous) formulation (in grams): 100 alumina, 100 xylene, and 100 ethyl alcohol