The Oxidation Stability of Boron Nitride Thin Films on MgO and TiO 2 Substrates
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THE OXIDATýON STABILITY OF BORON NITRIDE THIN F0UVMS ON MgO AND T102 SUBSTRATES XIAOMEI QIU*, ABHAYA K. DATYE*, ROBERT T. PAINE** and LAWRENCE. F. ALLARD*** Center for Microengineered Ceramics and Departments of *Chemical and Nuclear Engineering and **Chemistry, University of New Mexico, Albuquerque, NM 87131. **High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, TN 37831. ABSTRACT The stability of BN thin film coatings (2-5 nm thick) on MgO and TiO2 substrates was investigated using transmission electron microscopy (TEM). The samples were heated in air for at least 16 hours at temperatures ranging from 773 K - 1273 K. On MgO supports, the BN thin film coating was lost by 1073 K due to a solid state reaction with the substrate leading to formation of Mg2B2OS. No such reaction occurred with the 1102 substrate and the BN was stable even at 1273 K. However, the coating appeared to ball up and phase segregate into islands of near-graphitic BN and clumps of T1302 (futile). The oxidizing treatment appears to promote the transformation from turbostratic BN to graphitic BN. INTRODUCTION Boron nitride thin coatings have attracted a great deal of interest for modifying the interface in fiber-reinforced composites to improve fiber pullout and prevent interfacial reaction [1]. The high temperature oxidation stability of boron nitride may provide distinct advantages over graphite despite the higher cost. However, there is no definitive data in the literature on the oxidation resistance of thin films of BN. Bulk BN powders have been reported to oxidize at 1073 K following parabolic oxidation kinetics [2]. The extent of oxidation was monitored in this particular study by following changes in weight as the sample was progressively oxidized. Lavrenko et al. [3] reported that the oxidation resistance of BN powders was greatly influenced by sample pretreatment, for example the extent of annealing in N2. They found that oxidation of pyrolytic BN occurred at negligible rates at 1173 K. However, measurable weight losses were detected in TGA measurements at 1373 K, and quite rapid oxidation occurred at 1473 K. Significant oxidation of the graphitic BN did not occur at temperatures below 1223 K. Interestingly, they observed a weight loss with oxidation time for the pyrolytic BN but an increase in weight for graphitic (more ordered) BN. This they attributed to differences in porosity of the sample: pyrolytic powders, being more porous, led to loss of volatile boron oxide, which was retained in the graphitic samples that had a denser microstructure. Borek et al. [4] studied the effect of crystallinity and surface area of BN on its oxidation resistance. While they observed that the high surface area, poorly crystallized samples oxidized at rates that were an order of magnitude faster than the more crystalline samples, they concluded that if oxidation rates were normalized to surface area, no effect of crystallinity could be detected. All of these previous studies used TGA analysis to study oxidation behavior.
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