W. KrenkeL, F Berndt Materials Science and Engineering A 412(2005)177-181 1600C rapidly fills the cracks within some minutes. The dense 3. C/C-SiC composites for advanced friction systems carbon matrix inside the fibre bundles shields off the highl reactive silicon. A layer of silicon carbide is formed around the 3. 1. Mechanical and thermo-physical properties segments and only a small amount of lead-bearing carbon fibres are converted and damaged First investigations with respect to brake applications started A thermal treatment of the carbon fibres prior to their imple- in 1994 with C/C-SiC composites using a 2D fibre architec- mentation into the polymer matrix de-activates the fibre surfaces ture with a fibre orientation parallel to the friction surface[12] and reduces the FMB in the polymer stage. Exposing the fibres The disks were tested against pads of the same ceramic mate in inert atmosphere at temperatures beyond 600C removes the rial as no conventionally available pad material sustained the functional surface groups of the fibres to a certain degree, result- high temperatures. This tribological system resulted in surface ing in reduced bonding forces in the fibre-matrix interphase. temperatures of more than 1000C, high coefficients of friction By increasing the pre-treatment temperature beyond 900 C the(CoFand considerably low wear rates. Nevertheless, these first FMB decreases further and the ILSS for the CFRP composite generation C/C-SiC composites were not suitable for an indus- is typically below 20 MPa. During pyrolysis these weak inter- trial use in brake disks and pads, as they were too expensive phases lead to a shrinkage away from the matrix to the fibres, to produce and their Cof was not stable enough. Consequently, resulting in carbon fibre tows without any segmentation. During C/C-SiC composites and adapted processes have to be devel siliconization much more silicon in comparison to composites oped to improve these CMC materials for their applicability in with untreated fibres is absorbed, resulting in higher densities. high-performance brake systems As there is no coating of the fibres used, the liquid silicon ca Different C/C-SiC composites have been developed by react with most of the filaments. This reduces the load-bearing diverse manufacturers, all characterised by low porosities(lower capability of the fibres considerably and the fracture behaviour than 6%), high fibre volume contents(higher than 30%)and of the composite becomes more similar to that of monolithic moderate mechanical properties. Table 1 summarizes the cu ceramics[9,101 rent status of the mechanical and thermo-physical properties of short fibre reinforced C/C-SiC composites, available in the open 2. 2. Aerospace applications literature, in comparison with grey cast iron and carbon/carbon materials [13-20]. Due to the different ceramic and fibre frac The LSI process was established 1990 at the german tions of the composites, the properties vary in a wide range. If Aerospace Center in Stuttgart (DLR)to develop C/C-SiC com- higher mechanical properties are required, e.g. in brake linings posites for space applications. First tests concentrated on the of elevators and cranes, a two-dimensional fibre reinforcement n-fight verification of material samples, which were integrated is preferred. Table 2 gives a complemental overview of the prop- into ablative heat shields. The nose cap of X-38, which was erties of C/C-SiC brake materials with a 2D fibre reinforcement planned to serve as a technology carrier for a new Crew Return As most of this r&d work is conducted under confidentiality. Vehicle of the International Space Station, is regarded as the there is a lack of comparative properties and statistical data. current stage of the materials development for thermal protec- Nevertheless, the variety of published properties demonstrates tion systems(TPS)of spacecraft [11]. It is made of C/C-Sic the high flexibility of the lsi process opposites derived from fabric prepregs using the autoclave technique. The thin-walled shell(wall thickness approximately 3.2. Tribological properties 6 mm)was manufactured in a net-shape technology, realised y the non-destructive evaluation of the microstructure and an The main requirements on new high performance brake mate accurate control of the shrinkage behaviour during all thermal rials are processing steps Much shorter operational times of only some seconds are stable dynamic and static coefficients of friction demanded for jet vanes which are used to divert the direction high wear stability for lifetime brake systems; of thrust in solid fuel rockets of missiles. However, they are low weight to reduce the unsprung mass of the transportation loaded by considerably higher thermo-mechanical stresses than TPS-structures of spacecraft. Advanced jet vanes are made of high degree of freedom in the structural design(e.g for inter- C/C-SiC composites and coated with a ceramic surface pro- nal cooling ducts, attachments) tection(e.g. CVD-SiC) in order to withstand the immense low life cycle costs blast of solid particles (e.g. Al2O3). The tailoring of the microstructure of C/C-Sic composites for jet vanes conse- th transverse heat fluxes from the outer region(friction quently requires a compromise between high fracture toughness surface)to the centre of the composite are necessary to avoid (high carbon fibre contents) and high resistance to abrasion an overheating of the friction surface, i.e. high transverse con- (high SiC contents). Both applications, hot structures for space- ductivities must be adjusted. Standard C/C-SiC composites with craft as well as jet vanes for missiles were designed as lim- bi-directional reinforcements used in hot structures of spacecraft ited life structures, whereas quite longer lifetimes are required have been modified in their composition and microstructure and for terrestrial applications like new brake disks in passenger led to essential improvements. Particularly, by increasing the silicon carbide content of the composite the transverse thermal178 W. Krenkel, F. Berndt / Materials Science and Engineering A 412 (2005) 177–181 1600 ◦C rapidly fills the cracks within some minutes. The dense carbon matrix inside the fibre bundles shields off the highly reactive silicon. A layer of silicon carbide is formed around the segments and only a small amount of lead-bearing carbon fibres are converted and damaged. A thermal treatment of the carbon fibres prior to their imple￾mentation into the polymer matrix de-activates the fibre surfaces and reduces the FMB in the polymer stage. Exposing the fibres in inert atmosphere at temperatures beyond 600 ◦C removes the functional surface groups of the fibres to a certain degree, result￾ing in reduced bonding forces in the fibre–matrix interphase. By increasing the pre-treatment temperature beyond 900 ◦C the FMB decreases further and the ILSS for the CFRP composite is typically below 20 MPa. During pyrolysis these weak inter￾phases lead to a shrinkage away from the matrix to the fibres, resulting in carbon fibre tows without any segmentation. During siliconization much more silicon in comparison to composites with untreated fibres is absorbed, resulting in higher densities. As there is no coating of the fibres used, the liquid silicon can react with most of the filaments. This reduces the load-bearing capability of the fibres considerably and the fracture behaviour of the composite becomes more similar to that of monolithic ceramics [9,10]. 2.2. Aerospace applications The LSI process was established 1990 at the German Aerospace Center in Stuttgart (DLR) to develop C/C–SiC com￾posites for space applications. First tests concentrated on the in-flight verification of material samples, which were integrated into ablative heat shields. The nose cap of X-38, which was planned to serve as a technology carrier for a new Crew Return Vehicle of the International Space Station, is regarded as the current stage of the material’s development for thermal protec￾tion systems (TPS) of spacecraft [11]. It is made of C/C–SiC composites derived from fabric prepregs using the autoclave technique. The thin-walled shell (wall thickness approximately 6 mm) was manufactured in a net-shape technology, realised by the non-destructive evaluation of the microstructure and an accurate control of the shrinkage behaviour during all thermal processing steps. Much shorter operational times of only some seconds are demanded for jet vanes which are used to divert the direction of thrust in solid fuel rockets of missiles. However, they are loaded by considerably higher thermo-mechanical stresses than TPS-structures of spacecraft. Advanced jet vanes are made of C/C–SiC composites and coated with a ceramic surface pro￾tection (e.g. CVD–SiC) in order to withstand the immense blast of solid particles (e.g. Al2O3). The tailoring of the microstructure of C/C–SiC composites for jet vanes conse￾quently requires a compromise between high fracture toughness (high carbon fibre contents) and high resistance to abrasion (high SiC contents). Both applications, hot structures for space￾craft as well as jet vanes for missiles were designed as lim￾ited life structures, whereas quite longer lifetimes are required for terrestrial applications like new brake disks in passenger cars. 3. C/C–SiC composites for advanced friction systems 3.1. Mechanical and thermo-physical properties First investigations with respect to brake applications started in 1994 with C/C–SiC composites using a 2D fibre architec￾ture with a fibre orientation parallel to the friction surface [12]. The disks were tested against pads of the same ceramic mate￾rial as no conventionally available pad material sustained the high temperatures. This tribological system resulted in surface temperatures of more than 1000 ◦C, high coefficients of friction (CoF) and considerably low wear rates. Nevertheless, these first generation C/C–SiC composites were not suitable for an indus￾trial use in brake disks and pads, as they were too expensive to produce and their CoF was not stable enough. Consequently, C/C–SiC composites and adapted processes have to be devel￾oped to improve these CMC materials for their applicability in high-performance brake systems. Different C/C–SiC composites have been developed by diverse manufacturers, all characterised by low porosities (lower than 6%), high fibre volume contents (higher than 30%) and moderate mechanical properties. Table 1 summarizes the cur￾rent status of the mechanical and thermo-physical properties of short fibre reinforced C/C–SiC composites, available in the open literature, in comparison with grey cast iron and carbon/carbon materials [13–20]. Due to the different ceramic and fibre frac￾tions of the composites, the properties vary in a wide range. If higher mechanical properties are required, e.g. in brake linings of elevators and cranes, a two-dimensional fibre reinforcement is preferred. Table 2 gives a complemental overview of the prop￾erties of C/C–SiC brake materials with a 2D fibre reinforcement. As most of this R&D work is conducted under confidentiality, there is a lack of comparative properties and statistical data. Nevertheless, the variety of published properties demonstrates the high flexibility of the LSI process. 3.2. Tribological properties The main requirements on new high performance brake mate￾rials are • stable dynamic and static coefficients of friction; • high wear stability for lifetime brake systems; • low weight to reduce the unsprung mass of the transportation system; • high degree of freedom in the structural design (e.g. for inter￾nal cooling ducts, attachments); • low life cycle costs. High transverse heat fluxes from the outer region (friction surface) to the centre of the composite are necessary to avoid an overheating of the friction surface, i.e. high transverse con￾ductivities must be adjusted. Standard C/C–SiC composites with bi-directional reinforcements used in hot structures of spacecraft have been modified in their composition and microstructure and led to essential improvements. Particularly, by increasing the silicon carbide content of the composite the transverse thermal