Stability of Boron Nitride Coatings on Ceramic Substrates
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STABILITY OF BORON NITRIDE COATINGS ON CERAMIC SUBSTRATES ABHAYA K. DATYE*, XIAOMEI QUI*, THEODORE T. BOREK*, ROBERT T. PAINE* AND LAWRENCE F. ALLARD# *Center for Microengineered Ceramics, University of New Mexico, Albuquerque, NM 87131. #High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831.
ABSTRACT When BN is synthesized via polymeric precursors and applied to ceramic substrates, tough adherent coatings of hexagonal-BN (h-BN) are obtained after annealing at 1200'C in N2 . The study of these coatings is facilitated by using nonporous oxide powders containing single crystal particles of submicron size (e.g. These oxide powders permit high cubes of MgO) as model ceramic substrates. resolution TEM examination of the BN coatings with no further sample preparation. In this study, san~ples of BN/MgO cubes containing 50 wt% BN were heated in air at elevated temperatures for 16 hours to study the oxidation resistance of BN coatings. The BN coating was found to be stable at 600 0 C, but the 700*C-treated sample showed evidence for partial amorphization of the coating and reaction with MgO. A significant fraction of the MgO in the 8001C-treated sample had transformed to Mg2B2O5. The reaction of MgO with the BN coating under oxidizing ambients leads to loss of the cubic morphology in the precursor powder.
INTRODUCTION Ceramic coatings represent an important means to modify interfaces in composite materials. Boron nitride coatings can be used to prevent reaction at the fiber-matrix interface and to improve fiber pullout, which is important for attaining high toughness in these composites. Fiber coatings can also protect the fibers from mechanical damage and chemically isolate the fiber from the matrix during processing. Little is known about the high temperature oxidation resistance of ceramics such as BN. The oxidation of bulk BN powders [1] and acrogels [2] at elevated temperatures has been reported in the literature. However, thin films of BN on ceramic substrates may behave differently and should be investigated before these coatings can be incorporated in composite materials. This study is part of a larger program aimed at examining the applicability of BN coatings to oxide and nonoxide ceramic substrates. Since the coatings of interest are only applied as thin films, it is necessary to use transmission electron microscopy (TEM) to observe changes caused by oxidation. Cross-section TEM of thin coatings is fraught with the difficulties in preparing thin sections and associated artifacts due to sample preparation. Hence, we have used nonporous oxide powders as model ceramic substrates to facilitate study of these materials [3-5]. The sample was similar to that reported earlier [4-5] and was prepared by physically mixing equal weights of MgO smoke cubes and a preceramic BN polymer, and heating under N2 at 1200 'C for 12 hours. The TEM samples were prepared by simply dipping a holey carbon TEM grid into the powder and dusting off the excess. Details of the BN polymer synthesis have been
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