Kinetics and mechanisms of high-temperature creep in polycrystalline aluminum nitride
- PDF / 1,476,682 Bytes
- 8 Pages / 612 x 828 pts Page_size
- 15 Downloads / 204 Views
K. L. More Oak Ridge National Laboratory, High Temperature Materials Laboratory, Oak Ridge, Tennessee 37831-6064
K. S. Ailey-Trent and R. F. Davis Department of Materials Science and Engineering, Box 7907, North Carolina State University, Raleigh, North Carolina 27695-7907 (Received 2 March 1992; accepted 30 December 1992)
The operative and controlling mechanisms of steady-state creep in hot-pressed A1N have been determined both from kinetic data within the temperature and constant compressive stress ranges of 1470 to 1670 K and 100 to 370 MPa, respectively, and from the microstructural results of TEM. No secondary phases were detected in the bulk or at the grain boundaries using Raman spectroscopy and HREM. The stress exponent was =«1.0 at all temperatures. The activation energies ranged between 558 and 611 kJ/mol. The most prominent microstructural features of the crept samples were elongated grains, strain whorls, and triple-point folds. Dislocations were generated only at the strain whorls in order to relieve the localized stress caused by intraboundary mechanical interaction among the grains. They contributed little to the observed deformation. The controlling mechanism for creep was diffusion-accommodated grain-boundary sliding. This mechanism was accompanied in parallel by relatively small amounts of unaccommodated grain-boundary sliding. Cavitation was not observed.
I. INTRODUCTION A1N crystallizes in the close-packed wurtzite structure (a = 0.311 nm, c = 0.498 nm). It possesses a slightly distorted, polar, noncentrosymmetric crystal structure consisting of two interpenetrating lattices of Al and N atoms which form metal and nonmetal sublattices that are displaced from each other by 0.385E0001].1'2 Of the 12 possible tetrahedrally coordinated sites in the N sublattice, Al can fill either the six upward- or the six downward-pointing sites. Only one type of tetrahedral site will be filled by the Al atoms in any given grain to minimize electrostatic repulsion. Hot-pressed A1N possesses excellent thermal conductivity (—200 W m ^ K " 1 ) , 3 a low dielectric constant (e' = 9.2 ± 0.05),4 and a coefficient of thermal expansion (43 X 10~ 7 /K) 5 close to that of Si within the temperature range 293-473 K.6 These properties, combined with good mechanical strength (250-500 MPa to 1773 K),7'8 make A1N a favored candidate as a substrate material for electronic packaging. The creep behavior of A1N at elevated temperatures is also of interest in terms of potential structural applications both alone and combined in solid solution with other ceramics such as SiC.9'10 J. Mater. Res., Vol. 8, No. 5, May 1993
http://journals.cambridge.org
Downloaded: 13 Mar 2015
All investigations concerned with the creep of A1N (including that reported herein) have used hot-pressed polycrystalline material. Spivak et al.11'12 were the first to report the creep behavior of this material (average grain size « 4 /im). The text of their paper implied that no additional phases were added to at least one of their materials. Four-point bending
Data Loading...
