CC BY
18ÏSHS2019
Moscow, Russia
CONSOLIDATION OF SHS DERIVED SiC BASED COMPOSITES Z. P^dzich*", K. Wojteczko", A. Wojteczko", A. Gubernat", and M. M. Bucko"
aAGH University of Science and Technology, Krakow, Poland
*e-mail: pedzich@agh.edu.pl
DOI: 10.24411/9999-0014A-2019-10119
HS synthesis could be a useful method of homogeneous incorporation of minority phases into ceramic matrix. The aim of presented studies was to check the possibility of incorporation phases containing yttrium or chromium into silicon carbide polycrystalline material. The scheme of experimental procedure was as follows: in the first step SHS synthesis of silicon carbide was conducted starting from elemental silicon and graphite powders mixed with suitable oxide additive (Y2O3 or &2O3) in the amount of 5% in each case. Substrates were compacted and heated up to 1550°C when SHS reaction started. The phase composition of porous products achieved during synthesis were examined by XRD method. Results confirmed proper stoichiometry of starting mixture. Whole silicon and carbon reacted and there was no free elemental silicon or carbon detected in products (see Figs. 1a, 1b).
Fig. 1. The phase composition of powders after SHS synthesis: (a) powder containing yttria in the starting mixture; (b) powder containing chromia in the starting mixture.
Yttria included into reacting mixture transformed into yttriasilicate phase (Y2SÏ2O7) (Fig. 1a). What is worth to noticed silicon carbide produced in SHS reaction was composed of two moissanite phases. The typical coarse grains 3C type phase and the nanometric one. Rietveld procedure applied for calculations distinctly indicated the presence of the majority of this nanometric 3C type phase. Powder achieved in the presence of chromia (Fig. 1b) also was composed of two SiC 3C type crystallites differ in size. The amount of nanometric one was slightly less than in powder contained yttria additive. The whole oxygen was removed from the system and chromium was present only in the Cr5Si3 phase. Powders achieved in SHS procedure were consequently milled and consolidated by hot-pressing. Conditions of HP procedure was as follows: the graphite stamps, argon atmosphere, 2150°C maximum temperature with 45 min of soaking time. Heating rate was 15°C/min and applied pressure was 25 MPa. After consolidation materials were examined for their phase composition and microstructural observation were conducted by means of SEM technique.
Figures 2a and 2b show phase composition of sintered materials. In both cases HP process caused significant changes in SiC phase composition. In both cases nano SiC phase disappeared what is obvious because of the highest diffusional activity of nano grains during sintering. Surprisingly, the structure of 3C type was detected as a dominant phase not 6H structure type usually typical for sintered SiC bodies. Such phase composition of sintered bodies could be
XV International Symposium on Self-Propagating High-Temperature Synthesis
caused by the presence of external pressure during sintering and by the presence of nanograins of phases contain yttrium or chromium in the starting powder. The most important statements were that finally both, yttrium and chromium were present in sintered bodies as carbide phases and that reduction conditions during HP process led to free carbon appearance.
Fig. 2. The phase composition of HP bodies: (a) the sample containing yttria in the starting mixture; (b) the sample containing chromia in the starting mixture.
Figures 3 a and 3b show morphology of carbide inclusions in sintered bodies. In both cases observed carbide grains were not so fine, rather in micro- than nanometric size. The inclusions size is bigger than yttria and chromia starting powders. It fact suggests that SHS process non preserved the system against agglomeration of phases dispersed in SiC matrix.
..... (a)' ...... (b)
Fig. 3. Carbide inclusions in sintered bodies: (a) yttrium carbide YC2 and (b) chromium carbide Cr3C2.
