Internal friction and torsional creep behavior of chemically vapor deposited boron nitride
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Internal friction and torsional creep behavior of chemically vapor deposited boron nitride Giuseppe Pezzotti Department of Materials, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, 606 Kyoto, Japan
Hans-Joachim Kleebe Institut f¨ur Materialforschung, Universit¨at Bayreuth, D-95440 Bayreuth, Germany
Ken’ichi Ota Institute of Scientific and Industrial Research, Osaka University, Ibaraki-shi, Mihogaoka 8-1, 561 Osaka, Japan
Toshihiko Nishida Department of Materials, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, 606 Kyoto, Japan (Received 26 August 1997; accepted 25 February 1998)
Dense hexagonal BN processed via chemical vapor deposition (CVD) was tested with respect to damping, shear modulus, and torsional creep rate up to temperatures as high as ø2300 ±C. The microstructural characteristics of the material both before and after creep testing were studied by high-resolution electron microscopy (HREM). The CVD process yields a homogeneous nanosized microstructure with no other secondary phase detectable. Damping experiments revealed no plastic relaxation during testing up to ø2000 ±C, which is consistent with the fact that also no creep deformation could be detected below such a high temperature. Small porosity and an increased amorphization process were noted by HREM inspection after stress exposure at ø2300 ±C. These phenomena may be responsible for both the enhanced damping capacity and the creep rate of the material which, in the range of the present testing conditions, seems to follow the simple viscoelastic behavior of a Maxwell solid.
I. INTRODUCTION
Dense covalent nonoxide ceramics of relatively high purity (e.g., Si3 N4 , SiC, BN, etc.) can be obtained via sintering, eventually coupled with controlled atmospheres and/or high pressure techniques.1–3 However, in these materials, oxygen contamination unavoidably arises from simply exposing the starting powder to the ambient atmosphere. A glassy oxide phase can thus be found to segregate at grain boundaries in the polycrystalline body after densification, although no extraneous phase was intentionally added.4–6 Despite the rather low fraction of intergranular glass, which can arise from oxygen contamination, the inherent glass viscosity dominates the primary creep stage of nonoxide polycrystals.7–9 On the other hand, diffusion-controlled processes and secondary-phase crystallization, which may take place in the glass phase upon long exposure to high temperature, are the main factors dictating the medium- and long-range creep behavior.10–12 From the characterization of the high-temperature mechanical behavior of nonoxide ceramics, it is now understood that very creep resistant polycrystalline materials can be obtained despite the presence of glassenriched grain boundaries. For example, the presence of small atomic fractions of either N or C in the otherwise J. Mater. Res., Vol. 13, No. 12, Dec 1998
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pure SiO2 intergranular films of some Si3 N4
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