Carbon-fiber-reinforced CMCs for high temperatures: processing flexibility and tailored microstructures
VALLE M. 1, CAVALLI L. 1, GIACOMETTI F. 1, AKRAM Y. 1, BOIOCCHI M. 1, CANT┘ M. 1
1 Petroceramics, Stezzano, Italy
Ceramic-matrix-composites (CMCs) overcome some of the major issues of monolithic ceramics, as they combine remarkable stability under mechanical stresses with lightness as well as resistance to high temperatures, wear and oxidation. The capability to withstand extreme conditions makes CMCs suitable for the most demanding applications in sectors including the automotive and aerospace industries (e.g. components of rocket engines and heat shields for space vehicles, parts of aircraft exhaust systems, brake rotors for high performance vehicles).
In the great family of CMCs, carbon fiber reinforced composites densified by means of CVI (Chemical Vapor Infiltration) - Cf-SiC - stand out as the unique solution, especially in the aerospace industry, as they are in compliance with the most demanding technical requirements. Although CMCs are replacing other materials in applications where the higher costs are offset by improvements in performance, the reduction of both costs and processing times is among the unsolved challenges which still limit the take off of these composites for certain uses.
Liquid Silicon Infiltration (LSI) is a favorable solution for the densification of Cf-SiC composites, as it offers several advantages over other CMCs manufacturing technologies. LSI densification happens in few hours (instead of days/weeks) and therefore it is cost-effective and time-saving when compare to CVI. LSI does not entail hazardous gases emissions, which reduces risks for human-health and the environment. In addition to all of that, LSI can be tuned and made suitable for different typologies of Cf-performs regardless of internal structure (e.g. 2D vs 2.5D vs 3D), shape and dimension. Furthermore, elements from both alike and different Cf-performs can be joined thorough the LSI process, which guarantees a strong coupling of the parts.
In this frame, recently developed CMCs named ISiComp® and OxyComp® have shown outstanding resistance to mechanical stress even under extreme thermo-oxidative conditions.
Manufacturing of complex-shaped components made of ISiComp® and OxyComp® is possible thanks to the accurate selection of materials and processing parameters. An innovative shaping process was developed specifically for these composites; pyrolysis and LSI were designed and tuned in order to control shrinkage and deformation during critical steps. This advances allow to overcome some of the major limitations of LSI in the development of Cf-CMCs.
The good control on the evolution of the fiber-matrix interface throughout the manufacturing process lies behind the tailored microstructures of ISiComp® and Oxycomp®, which define their distinct properties. The two trademarks have in fact different and peculiar characteristics in terms of thermo-mechanical behavior in use and can therefore can be selected according to the final application requirements.
ISiComp® (bending strength ~250 MPa, tensile strength ~170 MPa) was designed to withstand extreme thermo-mechanical stresses up to 1650°C in inert or low fO2 atmosphere. Oxycomp® was developed for more oxidative environments, in which it can endure long term exposure up to 600°-800°C preserving it mechanical properties (bending strength ~170 MPa, tensile strength ~120 MPa).