Twenty years of experience with carbon/ceramic brakes: Status and perspectives

Carbon/ceramic brakes can be regarded as a successful spin-off from space technology to terrestrial applications. First attempts in the nineties of last century started to use carbon fiber reinforced silicon carbide (C/SiC) as heat sink materials in high performance brake systems. Originally, these materials were developed for thermal protection systems (TPS) of spacecraft. Meanwhile, LSI-derived C/C-SiC composites have proven their outstanding performance for frictional applications by demonstrating high and stable coefficients of friction and low wear rates. Today, these ceramic composite materials are used in series products for high performance brake systems in automotive and industrial applications (e.g. brake discs, pads for emergency brakes of elevators). High coefficients of friction which are constant over a wide range of sliding velocities and pressures have been achieved with appropriate counterpart materials. Specific modifications of the C/C-SiC microstructure in terms of matrix composition, fiber dimension and thermophysical properties were necessary and result in composite materials which differ widely from the original TPS-material. The development of an automotive brake system comprising C/C-SiC brake discs and organic based pads led to a lifetime brake which makes a brake disc change obsolete. The further success of these innovative materials, however, is strongly dependent on the reduction of the production costs and the development of light-weight ceramic brakes with life cycle costs (LCC) comparable to the current cast iron brakes. The presentation describes the development and evolution of carbon/ceramic brake discs and pads over the last twenty years, summarizes the state-of-the-art, and gives a perspective to future demands and challenges in process technology and material development

Development of yttrium and ytterbium silicates from their oxides and an oligosilazane precursor for coating applications to protect SI3N4 ceramics in hot gas environments

Environmental barrier coatings are required to protect Si3N4 against hot gas corrosion and enable its application in gas turbines. In comparison to other environmental barrier coatings, rare-earth silicate-based coatings stand out due to the very low corrosion rates in moist environments at high temperatures and the compatibility of thermal expansion coefficient to Si3N4 ceramics. Thus, the polymer-derived ceramic route was used to synthesize yttrium and ytterbium silicates in the temperature range of 1000-1500 °C for basic investigations regarding their intrinsic properties from a mixture of Y2O3 or Yb2O3 powders and the oligosilazane Durazane 1800. After pyrolysis above 1200 °C in air, the corresponding silicates are already the predominant phases. The corrosion behaviour of the resulting composites was assessed after exposure to flowing moist air at 1400 °C for 80 h. The material containing Yb2SiO5 and Yb2Si2O7 as main crystalline phases undergoes the lowest corrosion rate (-1.8 µg cm-2 h-1). In contrast, the corrosion rate of yttrium-based composites remained at least ten times higher. Lastly, the processing of Y2O3/Durazane 1800 as well-adherent, crack-free and thick (40 µm) coatings on Si3N4 was achieved after pyrolysis at 1400 °C in air. The resulting coating consisted of an Y2O3/Y2SiO5 top-layer and an Y2Si2O7 interlayer due to diffusion of silicon from the substrate and its interaction with the coating system

In Situ Generated Yb₂Si₂O₇ Environmental Barrier Coatings for Protection of Ceramic Components in the Next Generation of Gas Turbines

Abstract In face of an accelerating climate change, the reduction and substitution of fossil fuels is crucial to decarbonize energy production. Gas turbines can operate with versatile fuel sources like natural gas and future fuels such as hydrogen and ammonia. Furthermore, thermal efficiencies above 60% can be achieved using non‐oxide silicon‐based ceramic components. However, water vapor is one of the main combustion products leading to rapid corrosion because of volatilization of the protective SiO2 layer at 1200 °C. An in situ generated Yb2Si2O7 double layered environmental barrier coating system composed of silazanes and the active fillers Yb2O3 and Si processed at 1415 °C for 5 h in air protects a Si3N4 substrate very effectively from corrosion. It exhibits a dense microstructure with a total thickness of 68 µm, overcomes 15 thermal cycling tests between 1200 and 20 °C and shows almost no mass loss after very harsh hot gas corrosion at 1200 °C for 200 h (pH2O = 0.15 atm, v = 100 m s−1). The excellent adhesion strength (36.9 ± 6.2 MPa), hardness (6.9 ± 1.6 GPa) and scratch resistance (28 N) demonstrate that the coating system is very promising for application in the next generation of gas turbines