Processing and Thermal Shock Resistance of a Polymer-Derived MoSi 2 / SiCO Ceramic Composite Luca Bergero, Vincenzo M. Sglavo,* and Gian Domenico Soraru* ,w Dipartimento di Ingegneria dei Materiali e Tecnologie Industriali, Universita` di Trento, I-38050 Trento, Italy In this paper, we report a study on the thermal shock resistance (TSR) of MoSi 2 /SiCO ceramic composites obtained through controlled pyrolysis of a gel-derived precursor. MoSi 2 -filled gel is prepared by casting a sol obtained from MoSi 2 powder dis- persed in methyltriethoxysilane. The pyrolysis product can be described as a porous ceramic composite formed by a SiCO matrix with a dispersion of MoSi 2 particles. Mechanical char- acterization is performed on bar samples by four-point bending. The TSR is investigated either by evaluating the R parameter (associated with strength, elastic modulus, and thermal expan- sion coefficient), or with the conventional water quenching tech- nique. In both cases, the results suggest that the studied ceramic material displays a good TSR, which makes it a candidate for high-temperature application. I. Introduction T HE polymer route to advanced ceramics containing Si, C, N, B, and O is a chemistry-based process in which the ceramic material is obtained through a pyrolysis process starting from suitable pre-ceramic precursors. 1 Covalent ceramics obtained with this method are usually amorphous or nanocrystalline and display unusual properties such as high thermal stability and creep resistance, excellent oxidation, and corrosion resistance, even in harsh environments. 2–4 Recently, they have also shown interesting optical properties. 5 The polymer-to-ceramic transformation process involves a complex sequence of chemical and physical changes with evo- lution of gaseous by-products, resulting in mass loss and volu- metric shrinkage. These factors favor the fabrication of small components such as fibers, films, and MEMS. 3,6 However, pro- duction of bulk samples is also possible. Indeed, Greil 7 devel- oped the Active Filler Controlled Pyrolysis (AFCOP) method in which a filler (active or inert) is used to reduce the shrinkage during pyrolysis, while Riedel et al. 8 demonstrated the feasibility of producing centimeter-sized Si-based ceramics through pyro- lysis of a green body obtained by pressing polymeric pre-ceramic powders. Colombo and Modesti 9 used the direct foaming and pyrolysis of pre-ceramic polymer/polyurethane solutions to pro- duce large-scale ceramic foams. The AFCOP method has mainly been applied to synthesize SiCO-based ceramics using commercial polysiloxane as precur- sors. Accordingly, most of the common ceramic powders such as Al 2 O 3 , SiC, SiO 2 (quartz and amorphous), ZrO 2 , and MoSi 2 have been used as fillers to fabricate various SiCO-based ceramic com- posites. 10 These materials are usually developed for high-temper- ature application, and studies have been accordingly reported on their oxidation and/or high-temperature stability. 11,12 Neverthe- less, an important feature such as the thermal shock resistance (TSR) of such materials has not yet been reported. The only data available in the literature refer to polymer-derived SiCO foams. 13 In this paper, we present a study on the TSR of an MoSi 2 / SiCO ceramic composite prepared through pyrolysis of a gel- derived precursor. The interest in this material lies in the good combination of high-temperature stability and good electrical conductivity, which makes it an excellent candidate for the pro- duction of high-temperature ceramic resistors. 14 II. Experimental Procedure (1) Synthesis of Filled Gels MoSi 2 -filled gel precursors were prepared by casting a sol ob- tained from methyltriethoxysilane, MTES (ABCR, Karlsruhe, Germany) with a dispersion of MoSi 2 powders (Aldrich, Stein- heim, Germany). The process involves the formation of a sus- pension of the MoSi 2 powders (d max o2 mm) in the MTES, the addition of water to promote hydrolysis of the alkoxide, and a second step in which a basic solution is added to promote the condensation and the formation of the MoSi 2 -filled gel. The amounts of MTES and MoSi 2 have been chosen in order to have a 100V MoSi2 =ðV MoSi2 þ V gel Þ ratio of 40%. It has to be noticed that the volume fraction of filler estimated as above refers only to the skeleton of the filled gels without considering the open porosity that is also present in the samples. In a typical preparation, 17.67 g of MoSi 2 is added to 14.96 mL of MTES. The suspension is stirred for 40 min at 401C and cooled to room temperature. Then, 3.04 mL of distilled water is added. After 10 min, the condensation step is boosted by adding 1.5 mL of an ammonia solution (30% wt). The total amount of water added to the MTES in the two steps is chosen to have a molar ration of H 2 O/MTES 5 3. Soon after, the sol is cast in Plexiglas vertical molds (5 mm  5 mm  100 mm) and gel for- mation occurs almost instantly. The wet gel is then aged in this liquid for 1 day at room temperature. In this stage, the wet gels are subjected to a load acting over the 5 mm  5 mm section. Different loads from 50 to 300 g (which correspond to com- pressive stresses in the range from 20 to 118 kPa) were tested. We found that the application of the load during the aging step dramatically decreases the probability of development of cracks in the gel, and facilitates the production of samples well suited for mechanical characterization. On the other hand, all the measured properties of the final ceramic components were found to be independent of the load applied during the aging step. After 1 day of aging in liquid, the samples were quickly trans- ferred to an empty test tube, which was sealed and placed in an oven for 3-day aging at 601C. The samples were then removed from the oven and allowed to dry slowly (by creating a few holes in the test tube) for 3 days at room temperature and for 3 days at 601C. From this procedure, a 5 mm  5 mm  80 mm gel sample was obtained. (2) Pyrolysis of the Filled Gels For the polymer-to-ceramic transformation, the filled gel sam- ples are placed in a foamed alumina sample holder, which is J ournal J. Am. Ceram. Soc., 88 [11] 3222–3225 (2005) DOI: 10.1111/j.1551-2916.2005.00550.x r 2005 The American Ceramic Society 3222 J. R. Hellman—contributing editor *Member, American Ceramic Society. w Author to whom correspondence should be addressed. e-mail: soraru@ing.unitn.it Manuscript No. 20219. Received September 23, 2004; approved February 25, 2005.