Nucleation and Abnormal Grain Growth of Alpha-Al 2 O 3 in Gamma-Alumina Matrix
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NUCLEATION AND ABNORMAL GRAIN GROWTH OF ALPHA-A12 0 3 IN GAMMA-ALUMINA MATRIX T. C. Chou and T. G. Nieh Lockheed Missiles and Space Company, Research and Development Division, 0/93-10, B/204, Palo Alto, CA 94304 ABSTRACT The microstructures of reactive sputter-deposited alumina films have been studied by transmission electron microscopy. The as-deposited films contained y-A12 0 3 phase in an amorphous alumina matrix. Annealing of the films at 1200 0 C for 2 h resulted in nucleation and concurrent anomalous grain growth of a-A12 0 3 in a polycrystaUine y-Al 2 0 3 matrix which exhibited a layered microstructure and was strongly textured along [001]. The grain sizes of a-A12 0 3 varied from 3 to 20 pm, while the average grain size of y-A12 0 3 was only about 50 nm. It appears that the nucleation kinetics of a-A12 0 3 was slow. As a result, the abnormal grain growth of a-A12 0 3 proceeded by consuming surrounding y-Al 20 3 grains. An atomic model is presented to explain the origin of layered structure in y-A12 0 3 . The nucleation mechanism of a-A120 3 in V-alumina matrix is suggested. Orientation relationships between y- and a-A12 0 3 are reported. The anomalous grain growth of aA12 0 3 is discussed in terms of yla interface boundary migration. INTRODUCTION Aluminum oxide, in either bulk or thin film coating forms, finds many important applications as a component for structural, tribological, and electrical usages. For those applications, the thermodynamically-stable phase a-A12 0 3 (commonly known as corundum) is desired because of its high melting point (due to strong bonding), high temperature stability, extreme hardness, and a high dielectric constant. Aluminum oxide is known to exist in various metastable polymorphs in addition to a-A12 0 3 [1]. The polymorphs include: gamma (y), delta (6), theta (9), eta (q), kappa (K), chi (X), beta (P6),and iota ( i ). It has been noted that the starting phase of A12 0 3 polymorphs may vary, depending upon the processing techniques and conditions, and phase transformation sequence can take place by various routes during post-processing treatments [1-9]. By the physical vapor deposition technique, a commonly reported transformation route is: amorphous -. y -) 0 + 6 -0 a, in which the transformation products at each stage may be a mixture of several phases. Recently, thin alumina films (50 nm to 1.2 pm in thickness) were produced by r.f. reactive sputtering deposition [10], and their microstructures were studied after annealing at temperatures ranging from 800 to 1200 0 C. A few striking results were observed in the films annealed at 1200OC: nucleation and concurrent explosive grain growth of a-A12 0 3 took place in a textured, polycrystalline y-A12 0 3 matrix; y- and a-A12 0 3 phases exhibited special orientation relationships. In this paper, we summarize the experimental results and offer explanations for the concurrence of nucleation/abnormal grain growth of a-A12 0 3 . EXPERIMENTAL PROCEDURES Alumina films were deposited from a hot pressed A12 0 3 target (99.99%) by r.f. mag
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