JOURNAL OF MATERIALS SCIENCE LETTERS 19, 2 0 0 0, 1529 – 1531 High-temperature fracture toughness of SiC-Mo 5 Si 3 C composite Q. ZHU, K. SHOBU Kyushu National Industrial Research Institute, 841-0052, Japan E-mail: qszhu@hotmail.com Silicon carbide-based composites are candidate materi- als for high-temperature structural applications in heat engines and heat recovery systems [1, 2]. For these ap- plications, the ceramic components are often needed to be manufactured by processes with near-net-shape ca- pability to minimize the requirements to perform very expensive and difficult post-machining operations [3]. Among the various fabrication processes, the melt infil- tration process, which has the advantages of near-net- shape capability and short processing time, is proposed to be one of the potential processes for such appli- cations [4]. However, SiC-based composites made by conventional melt infiltration processes showed poor high-temperature mechanical properties, which are pri- marily limited by the low melting-point residual phases as exhibited by the reaction-bonded SiC [5–7]. Much effort has thus been made to reduce or eliminate the low-melting-point residual phases in the composites. One way to reduce the residual phases is through al- loy infiltration [8–10]. Better approaches have been demonstrated by Lim et al. [11] and Shobu et al. [12], where high-melting-point MoSi 2 and Mo(Si,Al) 2 were directly infiltrated into the SiC performs. Both the in- filtrated SiC-MoSi 2 and SiC-Mo(Si,Al) 2 composites exhibited significant increases (more than a 40% in- crease at 1400 ◦ C) in fracture strength between 1200 and 1400 ◦ C. In a previous report [13], a novel SiC- Mo 5 Si 3 C composite was fabricated by directly infil- trating high-melting-point Mo 5 Si 3 C into the SiC pre- forms. The composite also showed significant increases in fracture strength between 1200 and 1500 ◦ C, which was attributed to the toughening effects provided by the infiltrated Mo 5 Si 3 C phases at elevated tempera- tures. To clarify this point, the fracture toughness of the composite is studied in the present report. The fracture toughness for ceramics can be characterized by vari- ous techniques, such as the double torsion (DT) tech- nique [14], the double cantilever beam (DCB) tech- nique [15], the indentation fracture (IF) technique [16], the indentation strength (IS) technique [17], the sin- gle edge precracked beam (SEPB) technique [18], the chevron notched beam (CNB) technique [19], etc. The IS method was adopted in the present study because it has proved to be efficient and economical [20]. The preparation procedure for the composite as well as the infiltration characteristics have been reported elsewhere [13, 21], so they are only briefly introduced here. Namely, a SiC preform of dimensions 40 × 16 × 6 (mm) was prepared from the raw SiC powder (α, circa 3.2 μm, purity 99%, Showa Denko, Japan) through a cold isostatic pressing (CIP) process. An infiltrant of the Mo 45 Si 30 C 5 composition was prepared from the raw powders of MoSi 2 (circa 2.93 μm, Japan New Met- als), SiC (the same as above) and Mo (circa 1.3 μm, purity 99.94%, Japan New Metals). The infiltration was performed at 2100 ◦ C in an induction furnace in 1 atm argon. Typically, the temperature was ramped to 2100 ◦ C in 5 min from 1000 ◦ C, held for 20 min, followed by a 5-min cool down to 1000 ◦ C, where the power was cut off. The powder mixture was found to melt above 2000 ◦ C, and spontaneous infiltration could be achieved above 2050 ◦ C. Although 20 min for in- filtration was employed in the present study, several minutes were found to be sufficient for a full infiltra- tion. The infiltrated composites are relatively dense, but the relative density could only reach 95% of the theoretical value at best. The remaining porosity is at- tributed to the uninfiltrated areas up to 20 μm and closed pores in the SiC matrix that could not be reached during the infiltration process, as exhibited by the typical mi- crostructure of the composite in Fig. 1. Also, the volu- metric shrinkage during solidification would contribute to the porosity. Further improvement in product density is very difficult. The XRD and SEM/EDS investigations confirmed that the composite is only composed of SiC and Mo 5 Si 3 C. Beams for the IS testing were cut from the infiltrated samples using a diamond saw, and ground to the nom- inal dimensions of 3 × 1.5 × 12 (mm). The tensile sur- face was polished with 1 μm diamond paste to remove residual stress due to machining and to produce a fin- ish for optical microscopic observations. The edges of the prospective tensile face were slightly chamfered. Figure 1 Microstructure of SiC-Mo 5 Si 3 C composite infiltrated at 2100 ◦ C for 20 min under 1 atm argon atmosphere. Dark phases are SiC and bright phases are Mo 5 Si 3 C. 0261–8028 C  2000 Kluwer Academic Publishers 1529