Particulate-Reinforced Precursor-Derived Si–C–N Ceramics: Optimization of Pyrolysis Atmosphere and Schedules Sea-Hoon Lee, w Markus Weinmann, and Fritz Aldinger* Max-Planck-Institut fu¨ r Metallforschung and Institut fu¨ r Nichtmetallische Anorganische Materialien, Universita¨ t Stuttgart, Pulvermetallurgisches Laboratorium, 70569 Stuttgart, Germany Surface oxidation occurred during pyrolysis of SiC particulate- reinforced composites (PRCs) with a precursor-derived Si–C–N matrix. In contrast, such an oxidation was not observed in pure Si–C–N ceramics. The present investigation discusses the pos- sible reasons for this, and reports on the influence of such an oxidation on the microstructure and the mechanical and thermal properties of PRCs by the precursor-impregnation and pyrolysis method. The high-temperature mass stability of the PRCs in Ar deteriorated owing to the decomposition of SiO 2 formed by ox- idation. The effects of the pyrolysis schedule on the processing and mechanical properties of PRCs are also investigated. I. Introduction T HE precursor-impregnation and pyrolysis (PIP) technique has been intensively investigated during the last decades for the fabrication of structural composites for high temperature application. 1,2 The general procedure is to impregnate woven fiber fabrics with a liquid precursor, to cross-link the polymeric precursor by heating or applying ultraviolet irradiation, and finally to pyr- olyze the assembly at temperatures in the range of 10001–14001C in a controlled atmosphere. Weight loss and density increase (B1.0-B2.3 g/cm 3 ) during pyrolysis lead to a shrinkage of the precursor-derived ceramics. 3 Because of the volume shrinkage and weight loss during the polymer-to-ceramic conversion, 4 the pores of the composites cannot be completely removed after pyrolysis. Therefore, prep- aration of dense fiber-reinforced composites (FRCs) by the PIP technique requires additional efforts. One of the methods of overcoming this problem is to repeat the impregnation/pyrolysis process several times, 5 which, however, is time consuming and expensive. 6 Accordingly, filler materials such as SiC are added to the precursor in order to reduce the number of impregnation/ pyrolysis cycles. 7 Besides improving the processing of such ma- terials, the filler particle may also reinforce the matrix phase in between the fibers of the composites. This may play a beneficial role in the processing, as heating rates in the PIP process are commonly small (r11C/min) to prevent flaw formation by the gas generated during pyrolysis. 6 Precursor-derived Si–C–N ceramics have been investigated mainly because of their excellent high-temperature stability and oxidation resistance. Butchereit and Nickel 8 reported that the oxidation of such ceramics started only at 11001C, and that the weight gain as a result of passive oxidation was less than 1% up to 15001C in air. In accordance with these results, oxidation was not observed after pyrolysis of compacted precursor powders in the present study. In contrast, Si–C–N particulate-reinforced composites (PRCs) with a SiC filler always possessed an oxi- dized surface after pyrolysis in spite of the high oxidation re- sistance of SiC particles, 9 which may affect the mechanical properties and thermal stability of the PRC. In the present investigation, we report on the processing con- ditions and properties of particulate-filled precursor-derived Si– C–N composites. The focus is on the influence of the pyrolysis atmosphere and schedule on the oxidation of PRCs. Subse- quently, the pyrolysis behavior of PRCs using different heating rates is presented, and the mechanical and thermal properties of PRCs obtained using different pyrolysis schedules are analyzed. II. Experimental Procedure (1) Processing of PRC Fabrication of SiC filler /Si–C–N matrix PRC was performed by im- pregnating a SiC pellet (A-10, H. C. Starck, Goslar, Germany, d 50 : 0.51 mm, oxygen content: 0.9 wt%) with a commercially available liquid Si–C–N precursor (VL20, polysilazane, KION, Huntingdon Valley, PA). SiC powder was milled intensively in isopropanol by plane- tary milling for 5 h using SiC balls. After milling, the slurry was dried at 701C for 24 h while stirring. The powder thus obtained was then compacted at 600 MPa by cold isostatic pressing into pellets of +1.4 and 4 mm thickness. The pellets were intensively dried in a graphite furnace at 17501C for 2 h in Ar (1.5 atm) to remove SiO 2 and humidity from the surface of the SiC fillers. For impregnation into the pellets, the polymer was dissolved in dehydrated tetrahydrofuran (1:1 by volume) in order to de- crease its viscosity. After impregnating by vacuum infiltration, the solvent was removed under reduced pressure at 401C for 6 h. The mold was then capped and placed in a sealed metal con- tainer, and the polymer was cross-linked at 4301C for 6 h. Subsequently, the samples were pyrolyzed at 13501C for 2 h in Ar (argon 4.8>99.998 vol%, Messer, Sulzbach, Germany). For controlling the purity of the atmosphere during pyrolysis, a gas- providing device was used consisting of polyvinyl chloride pipes and screwed joints (termed N-Ar, Fig. 1(a)). To minimize humidity and oxygen impurities in the atmosphere and to prevent oxidation of the PRC samples during pyrolysis as much as possible, the sys- tem was modified by using Cu pipes, high-vacuum sealing, and a gas purification system (oxisorb s , Messer, guaranteed final puri- ties: oxygen o5 ppb, moisture o20 ppb, termed P-Ar, Fig. 1(b)). The oxygen content of the respective Ar gases at 251C was meas- ured at the outlet of the system using an oxygen sensor (Rapid- ox2000, Cambridge Sensotec, Cambridge, U.K.). As the pyrolysis schedule can also affect surface oxidation, different heating rates (0.51,11, and 51C/min) were used for the ceramization process. The impregnation and pyrolysis cycle was repeated up to six times to decrease the residual porosity of the as-thermolyzed PRC. The number of PIP cycles is listed in parentheses, e.g. PRCs with six PIP cycles are termed PRC(6). During the fabrication process, the samples were handled in Ar to prevent contact with air. (2) Characterization The true density and chemical compositions of the precursor- derived Si–C–N ceramics used in the present investigation J ournal J. Am. Ceram. Soc., 88 [11] 3024–3031 (2005) DOI: 10.1111/j.1551-2916.2005.00587.x r 2005 The American Ceramic Society 3024 R. Riedel—contributing editor *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: lee@mf.mpg.de Manuscript No. 20239. Received March 1, 2005; approved May 12, 2005.