Walter Krenkel

C/C-SiC composites for advanced friction systems

Ceramic Matrix Composites (CMC), based on reinforcements of carbon fibres and matrices of silicon carbide, show superior tribological properties in comparison to grey cast iron or carbon/carbon. In combination with their low density, high thermal shock resistance and good abrasive resistance, these Si-infiltrated carbon/carbon materials, called C/SiC or C/C-SiC composites, are promising candidates for advanced friction systems. Generally, the carbon fibres lead to an improved damage tolerance in comparison to monolithic SiC, whereas the silicon carbide matrix improves the wear resistance compared to carbon/carbon. In combination with new design approaches cost-efficient manufacturing processes have been developed and have lead to successfully tested prototypes of brake pads and disks, especially for passenger cars and emergency brake systems.

The morphology of silicon carbide in C/C-SiC composites

Abstract In the present investigations C/C–SiC has been studied by means of SEM, X-ray diffraction (XRD) and TEM to reveal the morphology of the silicon carbide areas. It was found that there exist two different areas of SiC, a fine grained β-SiC layer with a high amount of stacking faults at the C–SiC interface, and a zone of coarser β-SiC at the SiC–Si interface. From these observations, reaction mechanisms governing the siliconization of porous C/C preforms are proposed. After an initial reaction of carbon with silicon vapour, liquid silicon has to diffuse through the already formed SiC. A violent reaction far away from equilibrium conditions and a high number of nucleation sites leads to the observed formation of a fine grained SiC with a high density of stacking faults. Thermodynamically this is an instable configuration so that the coarser grained zone emerges by solution and precipitation.

Carbon Fibre Reinforced Silicon Carbide Composites (C/SiC, C/C-SiC)

Ceramic matrix composites (CMC), based on reinforcements of carbon fibres and matrices of silicon carbide (called C/SiC or C/C-SiC composites) represent a relatively new class of structural materials. In the last few years new manufacturing processes and materials have been developed. Short fibre reinforcements, cheap polymer precursors and liquid phase processes reduced the costs by almost one order of magnitude in comparison to first generation C/SiC composites which were originally developed for space and military applications. Besides high mass specific properties and high thermal stability, functional properties like low thermal expansion and good tribological behaviour play an increasing importance for new commercial applications like brake disks and pads, clutches, calibration plates or furnace charging devices.

Selective cross-linking of oligosilazanes to tailored meltable polysilazanes for the processing of ceramic SiCN fibres

An interesting alternative for the processing of non-oxide ceramic fibres at lower costs than that for the current commercially available fibre types was developed by modifying different commercially available liquid oligosilazanes (ML33 and HTT1800) into polysilazanes by selective cross-linking via the N–H and Si–H groups with tetra-n-butylammoniumfluoride (TBAF) as a catalyst. Termination of the reaction with calcium borohydride allows the processing of meltable solid polysilazanes (ML33S and HTTS) with tailored chemical and thermal properties, to fulfil the requirements for the melt spinning of mechanically stable and homogeneous polymeric fibres. The chemical and thermal stability of the polysilazanes ML33S and HTTS were investigated by using GPC, DSC and rheological measurements. These techniques indicate the dependency of the molecular weight and glass temperature on the catalytical cross-linking conditions. Polymers with up to ∼10 000 g mol−1 show glass–liquid transition (Tg) between 65 and 81 °C and viscoelasticity, which are essential properties for the melt-spinning process. The thermal stability of ML33S is ensured up to 220 °C. In contrast the thermal stability of HTTS is limited to 170 °C due to the presence of vinyl-groups. The viscoelastic behaviour of the polymer melts, measured by oscillatory rheometry, and the sufficient thermal stability allowed the continuous processing of stable green fibres by melt spinning in the temperature range of 110 to 130 °C. After fast electron beam irradiation curing of the green fibres and pyrolysis of continuous amorphous ceramic SiCN fibres from both ML33S and HTTS polysilazanes were successfully synthesized, while a defined Tg point influences the shape and the smoothness positively.

cBN particle filled SiCN precursor coatings

Abstract A polymer derived ceramic composite coating consisting of the polycarbosilazane ABSE (matrix) and cBN (filler) is developed. The final coating thickness is varied by adjusting the withdrawal speed, the viscosity of the solution and the pyrolysis temperature. The coatings exhibit excellent adhesion to the substrate confirmed by cross-cut and bending tests. The environmental barrier properties of the coatings were investigated with static, isothermal oxidation experiments testing coated and uncoated steel substrates. It was revealed that the weight gain during oxidation was strongly reduced by coating the substrates and hence the coatings offer the possibility to enhance the lifetime and performance of steel substrates under rough conditions.

Fiber ceramic structures based on liquid impregnation technique

Abstract For future reentry vehicles ceramic matrix composites (CMC) are very favorable materials for hot structures and thermal protection systems. This paper describes the fabrication and evaluation of CMC-components manufactured by the liquid impregnation technique. This process offers the potential for a low-cost CMC production for all components which can be fabricated by resin transfer molding (RTM) or other techniques for reinforcing polymers. The two main fabrication steps, the component shaping and the matrix conversion to ceramics, are described in detail. Experimental data on infiltration parameters as well as mechanical properties and oxidation resistance of the produced fiber ceramics were determined. Emphasis was given on manufacturing real components like panels with integrated stringers to demonstrate the feasibility of the process.

Processing and characterization of large diameter ceramic SiCN monofilaments from commercial oligosilazanes

This work reports the processing of large diameter ceramic SiCN monofilaments via the precursor route using two chemically different polysilazanes ML33S and HTTS self-synthesized from respective commercially available oligosilazanes. The melt-spinning of continuous polymer fibers with controllable diameters from 35 to 150 μm and their pyrolysis to ceramic SiCN fibers is not influenced by differences in the chemical structure of the polysilazanes. In contrast, the necessary e-beam curing dose is reduced by Si-vinyl groups from 600 kGy for ML33S (vinyl free) to 200 kGy for HTTS derived polymer fibers. The curing step leads to an enhanced handleability important for further pyrolysis at 1100 °C in nitrogen and to an increase in ceramic yield. The resulting ceramic SiCN fibers from both systems have similar mechanical and thermal behavior, indicating quite a low influence of the polysilazane type on these properties. For the first time a comprehensive investigation of the effect of fiber diameter on the tensile strength is reported for SiCN fibers. The average strength increases from ∼800 MPa for 90 μm diameter fibers to ∼1600 MPa for the 30 μm diameter fibers. Bend Stress Relaxation (BSR) tests demonstrated that no stress relaxation occurs up to 1000 °C for SiCN monofilaments and the creep resistance is equal to or better than commercially available SiC monofilaments produced by chemical vapour deposition (CVD). The oxidation resistance is also comparable to commercially available oxygen free CVD SiC fibers (SCS-6). The ceramic fibers in this study were pyrolyzed at low temperature (1100 °C) and have high oxygen content (13 to 29 wt%). The high temperature creep resistance and oxidation resistance is expected to improve if the oxygen content is reduced and the pyrolysis temperature increased.

From Polymer to Ceramics: Low Cost Manufacturing of Ceramic Matrix Composite Materials

Abstract A cost efficient manufacturing route for ceramic matrix composites (CMC) has been developed by DLR. Starting with a carbon fibre reinforced plastic composite and converting the matrix from carbon to silicon carbide via the reaction with liquid silicon, CMC materials of high mass specific properties and of excellent thermal shock resistance can be achieved. Taking advantage of the simplicity of the process and the low raw material costs, C/C-SiC composites offer a wide range of possible applications. Very complex C/C-SiC structures have already been realized and demonstrate the transferability of the materials characteristics from samples to real components. Within industrial cooperation, C/C-SiC materials are being developed as aerospace hot structures and as friction materials for weight saving brake construction. Further prospective products like jet vanes or heat exchangers show new applications where C/C-SiC composites are attractive alternatives to conventional materials.

Influence of carbon fiber’s surface state on interlaminar shear properties of CFRP laminate

In order to investigate the influence of carbon fiber’s surface state on the interlaminar shear properties of carbon fiber-reinforced plastic (CFRP) laminate, the carbon fiber’s surface state was modified by thermal treatment at elevated temperatures. The interlaminar shear strength (ILSS) of CFRP laminates reinforced with treated fibers was measured by means of short-beam shear test, and the surface state of fiber was characterized by Electron Spectroscopy for Chemical Analysis (ESCA) analysis to reveal the dominate factor for controlling the ILSS. Combining the ILSS measurement with the ESCA analysis, the results indicated that: (1) the ILSS is strongly dependent on the oxygen-containing functional groups on the surface of carbon fiber; (2) the fiber treated at 600 °C has the highest oxygen-containing functional groups that lead to the highest ILSS of CFRP; and (3) at temperatures beyond 600 °C, the oxygen-containing functional groups decrease with increasing the heat treatment temperature, resulting in a low ILSS of CFRP laminates. Furthermore, from the microstructure observation, it was found that the CFRP mainly failed in the mode of multi-interlaminar shear. The multi-interlaminar shear failure in the CFRP laminates with low ILSS is more severe due to a weak fiber-matrix interface.

Fabrication of fiber composites with a MAX phase matrix by reactive melt infiltration

Due to the inherent brittleness of ceramics it is very desirable to increase the damage tolerance of ceramics. The ternary MAX phases are a promising group of materials with high fracture toughness. The topic of this study is the development of ceramic matrix composites (CMCs) with a matrix containing MAX phases, to achieve a damage tolerant structural composite material. For this purpose carbon fiber reinforced preforms with a carbon-titanium carbide matrix (C/C-TiC) were developed and infiltrated with silicon by a pressureless reactive melt infiltration. Finally liquid silicon caused the formation of SiC, TiSi2 and Ti3SiC2 in the matrix of the composite.