November 8-12, 2015
Ceramic materials exhibit superior mechanical properties at high temperatures. But, their use as structural components is severely limited because of their brittleness. Fiber-reinforced ceramic matrix composites (FRCMC), by incorporating fibers in ceramic matrices, not only exploit their attractive high-temperature strength but also enhance their toughness thereby rendering these attractive for many applications. Some of these applications require an understanding of the interrelationship among processing, fiber-matrix interface and mechanical properties both at room and elevated temperatures.
Processing of FRCMC is typically done by chemical vapor infiltration (CVI), filament winding and hot-pressing, and polymer infiltration and pyrolysis (PIP) approaches. Most of these approaches do not lead to full-density unless external pressure is applied during processing. A novel approach of melt-infiltration (MI) is a promising technique for fabricating fully dense and net-shape SiC fiber-reinforced SiC composites. The processing of FRCMC by MI was pioneered, invented and developed for making fully dense, net- and complex-shape silicon carbide (SiC) fiber-reinforced ceramic matrix composites . The melt infiltration process technology exhibits processing-simplicity in which a porous preform consisting of carbon, SiC particulates and reinforcing SiC fibers is infiltrated with molten silicon (Si) for in-situ formation of SiC. This innovative processing is unique in that it is complete in a few minutes and produces net-shape and fully-dense composites inexpensively compared to other ceramic composite processes requiring applied pressure at very high temperatures for densification leading to shape change upon consolidation.
The high temperature mechanical properties of MI composites may be limited by the properties of the fiber and Si-SiC matrix. In particular, the matrix properties depend on the amount of Si in the Si-SiC matrix phase. Therefore, Si-SiC composites containing 10-45 vol% Si were fabricated using melt-infiltration process and their microstructure and mechanical properties were measured between 1250-1550°C. Elastic modulus, strength, and creep behaviors were studied including the influence of the Si content on these properties. Elastic modulus and strength were shown to decrease with increases in both Si content and temperature. The composites containing large amounts of the continuous Si phase exhibited extremely low strength near the melting point of Si, while the composites containing small amounts of Si with a continuous SiC network retained 120 to 200 MPa strength even above the melting point of Si. The composites containing 10 and 20 vol% Si exhibited excellent creep resistance up to 1550C, well above the melting point of silicon. Creep rates as low as 10-10 to 10-9 were obtained at stresses of 40 to 75% of the strengths. Microstructural examination of the crept composites with 36 and 45 vol% Si showed a correlation between the creep behavior and nucleation and linkage of cavity in the tensile side leading to tertiary creep and creep failure. These results will be discussed and presented.
Raj Singh, "Processing, properties and prospects for melt infiltrated (MI) SiCf-ceramic composites" in "Composites at Lake Louise (CALL 2015)", Dr. Jim Smay, Oklahoma State University, USA Eds, ECI Symposium Series, (2016). http://dc.engconfintl.org/composites_all/69