JOURNAL OF MATERIALS SCIENCE 36 (2 0 0 1 ) 5767 – 5772 High temperature deformation behavior of crystallized precursor-derived Si-B-C-N ceramics MARTIN CHRIST, ANDR ´ E ZIMMERMANN, ACHIM ZERN, MARKUS WEINMANN, FRITZ ALDINGER Max-Planck-Institut f ¨ ur Metallforschung and Institut f ¨ ur Nichtmetallische Anorganische Materialien, Universit ¨ at Stuttgart, Pulvermetallurgisches Laboratorium, Heisenbergstrasse 5, D-70569 Stuttgart, Germany E-mail: zimmermann@mf.mpg.de Annealing of amorphous precursor-derived Si-B-C-N ceramics for 3 h at 1800 ◦ C in 15 bar nitrogen yielded a material consisting of SiC with a mean grain size of about 100 nm, embedded in a turbostratic B-C-N phase. The deformation behavior of this material was investigated by compression tests at 1400 ◦ C in air. Dependent on the applied stress the strain rate decreased with time. The deformation rate consists of a stress dependent component that is proportional to the applied stress, which indicates that this part of the deformation mechanism is based on viscous flow. Furthermore, the influence of oxidation on the deformation behavior is investigated. C  2001 Kluwer Academic Publishers 1. Introduction Precursor-derived covalent ceramics based on sili- con, boron, carbon and nitrogen (Si-B-C-N) proved to posess high thermal [1–4], chemical [5] and mechani- cal [6] stability due to the lack of oxidic grain boundary phases. Detailed studies on the mechanical properties of as-thermolysed Si-C-N [7, 8] and Si-B-C-N [9–11] materials revealed an outstanding mechanical stability of the amorphous state at rather high temperatures. It has been shown, that the plastic deformation consists of a stress-dependent and a stress-independent compo- nent [7, 11]. The first component is proportional to the applied stress [7, 8, 11, 12] and is assumed to be caused by plastic flow, while the second one can be ascribed to shrinkage [11]. The plastic behavior of the amorphous ceramics could be explained by a deformation model, which has been developed for metallic glasses. Ac- cording to this model [13, 14], materials flow is based on structural defects consisting of deviations from the “ideal” amorphous structure, i. e. fluctuations in the free volume distribution. During post-annealing of as-thermolysed materials, the free volume heals out partially leading to a decrease of the specific volume and a decrease of the flow defect concentration and thus to an increase of the viscosity of the amorphous material. To our knowledge, no detailed investigations of the mechanical high-temperature properties of crystalline precursor-derived ceramics are published until now. This paper reports on crystallization experiments of Si-B-C-N ceramics and their mechanical behavior at high temperature under compression. 2. Experimental procedure The ceramics investigated here were derived from a boron-modified polyvinylsilazane (MW33) with the idealized molecular structure {B[C 2 H 4 -Si(H)NH] 3 } n [5]. Polymer synthesis yielded a brittle and unmeltable product. For further processing, the polymer was ground using a vibrating-disk mill and sieved to particle sizes smaller than 32 μm. For compaction the polymer powder was heated in a graphite die up to a temperature of about 350 ◦ C, where the polymer softens, and sub- sequently densified by an uniaxial pressure of 48 MPa. The green bodies obtained were converted into amor- phous ceramic monoliths via thermolysis in an argon atmosphere by heating with a heating rate of 1 ◦ C/min and subsequent annealing for 2 h at 1400 ◦ C. For crystallization, the as-thermolysed ceramics were further annealed in nitrogen atmosphere (15 bar) at 1800 ◦ C for 3 h in boron nitride crucibles. The applied heating and cooling rates were 10 ◦ C/min. To monitor the density change of the pore-free ceramics during the crystallization process, amorphous MW33-derived ce- ramic powder with a particle size smaller than 32 μm was annealed in the same run. The density ρ p of the powder was measured by helium gas pycnometry and the bulk density ρ bulk of the ceramics by submersion of the specimen in mercury using the Archimedes princi- ple. The crystallization was investigated by XRD and TEM. Electron spectroscopic imaging (ESI) was used to detect the lateral distribution of the elements. Further- more, the chemical composition of the specimen was determined before and after the crystallization by ele- mental analysis. The concentration of nitrogen and oxy- gen was determined by carrier gas hot extraction. The concentration of silicon and boron was determined by Fourier-transformation infrared spectrometry (Fluorine Volatilization-FTIRS) and optical emission spectrome- try with an inductively coupled plasma (ICP-OES) [15]. From the crystallized samples, specimen with a height of 3 mm and a cross sectional area of 0022–2461 C  2001 Kluwer Academic Publishers 5767