Stefan Flauder’s research while affiliated with University of Bayreuth and other places

This study shows the first successful fabrication of a carbon fiber‐reinforced silicon carbide (C/C‐SiC) via the liquid silicon infiltration process using two thermoplastic polyetherketoneketone (PEKK) powders as carbon precursors. Samples are analyzed after each processing step and compared to polyetheretherketone (PEEK)‐derived reference samples. The carbon yield was above 65% for PEKK and 54% for PEEK, possibly due to the greater crosslinking potential of PEKK. Rheological measurements showed a higher melt viscosity of PEKK 60/40 (which consists of 60% terephtalic and 40% isophtalic moieties) than PEEK. The higher viscosity of PEKK 60/40 affected the carbon fiber‐reinforced plastic (CFRP) microstructure because it resulted in more matrix pores and some partly saturated fiber tows in the PEKK 60/40‐derived CFRP. The incomplete infiltration of fiber tows by PEKK in the CFRP state led to the undesirable reaction of the single carbon fibers during final siliconization. However, the effect was of minor influence, and the microstructure and phase composition remained unaffected. Mechanical testing of PEEK‐ and PEKK‐derived C/C‐SiC showed excellently comparable properties with a mean flexural strength above 200 MPa and a strain to failure above 0.55%. Thus, PEKK‐derived C/C‐SiC is a suitable alternative thermoplastic polymer for C/C‐SiC fabrication.

Ceramic matrix composites like carbon fiber-reinforced silicon carbon (C/C–SiC) are brake materials for applications at high thermo-mechanical loads. This study investigates the influence of three C/C–SiC pad materials with different compositions on frictional performance. For this purpose, the ceramic pad materials were tested against a steel disk on a dynamometer at brake pressures from 20 to 60 MPa. A large effect due to the different Si and SiC contents of the three pad materials on the frictional behavior was expected. Although the wear rates differed from 40 to 140 mm3/MJ, only marginal differences were found for the coefficient of friction. Hence, additional tests to interrupt and sequence the brake procedure revealed the formation of an intermediate metallic transfer layer from the steel disk on the ceramic pads. The formation and disappearance of this third-body, which was not found in start-complete-stop testing, and the consequences for the frictional properties are discussed.

This work investigates the influence of the hexamethylenetetramine (hexa) content on the rheological features, pyrolysis reactions, mass-loss kinetics and char yield for two phenolic resins, each with different novolak-hexa mixtures from 0 wt.-% to 15 wt.-%. Curing and viscosity of resins was studied using a rheometer. Pyrolysis, evolved gases and char yield were investigated with TG-FTIR-GC/MS. The obtained results show that the curing of resins shifts to lower temperatures whereas the viscosity and the residual carbon amount both increase with increasing hexa-contents. The presented data suggests a hexa-induced microgel formation for phenolic resins. Models for viscosity and char yield were developed and present a valuable tool to tailor the resin-hardener mixture for the desired application, enable a more efficient processing technology and contribute to a deeper understanding of the pyrolysis of novolak resins.

The appropriate assessment of mechanical properties is essential to design ceramic matrix composites. The size effect of strength plays a key role for the material understanding and the transfer from lab-scale samples to components. In order to investigate the size effect for carbon fiber-reinforced silicon carbon (C/C-SiC) under tensile load, a tensile testing with a minimum of deviation from the pure tensile loading is necessary. Hence, a hybrid edge/face-loading test device for self-alignment and centering of C/C-SiC tensile samples was developed, evaluated and proved to ensure pure tensile load. The mechanical analysis of more than 190 samples with two different cross-sections fabricated from the same material population revealed no significant difference in tensile strength. Although the volume under load was increased from 129 to 154 mm³, the tensile strengths of 162 ± 7 and 164 ± 6 MPa did not change. These results are discussed regarding the weakest link and energetic size effect approaches.

This study analyzed the influence of the sample volume, number of tested specimen, and testing method on the flexural strength of fabric-reinforced ceramic matrix composites. For this purpose, seven different batches of C/C-SiC were prepared with four different sample thicknesses to determine the flexural strengths and Weibull moduli by three- and four-point flexural tests. The result showed that C/C-SiC exhibits a size effect of strength under bending load because a decrease of measured flexural strength with increased specimen size was observed. This size effect was discussed regarding the Weibull weakest link approach and the concept of quasi-brittle materials.The determined Weibull moduli were comparable for the same load condition but dissimilar for the identical material if the load condition were changed from three- to four-point bending. Hence, the Weibull modulus was found to be not an inherent material constant for C/C-SiC and the Weibull weakest link approach seems not appropriate.

Short fiber reinforced ceramic matrix composites (SF-CMCs) represent an attractive alternative to continuous fiber reinforced ceramic matrix composites, although their mechanical properties are considerably lower. Lightweight components in combustion environments (e.g., nozzles, burners), in aerospace (e.g., telescopes), heat treatment (e.g., charging racks) or in frictional applications (e.g., brake disks, pads, clutch linings) represent successful applications for this type of ceramic matrix composite (CMC). This article gives an overview of the current status and summarizes the international activities on short fiber CMCs.

The synthesis of C/C-SiC via liquid silicon infiltration (LSI) with thermoplastic carbon precursors can lead to the formation of macropores in the C/C state. The macro-pore pattern formation can be controlled and used as a new level in microstructure for ceramic matrix composites. The pore formation occurs in the CFRP state with the remelting of the thermoplastic matrix and can be fixed to the C/C state after liquid phase pyrolysis. The macro-pores are primarily induced by a non-linear elastic recovery of the fiber preform and not via gas formation and release during pyrolysis. The formation of the macroporosity is described and explained. These pores can be tailored by process control in size and shape. The pore shapes can vary from isolated spherical pores to an interconnected tube like macro-pore network. The relationship between starting setting variables of the fiber preform, the process conditions, and the resulting structure are discussed.

In order to improve the fracture toughness of ZrB2 ceramics, as-received and heat treated short carbon fiber reinforced ZrB2-based composites were fabricated by hot pressing. The toughening effects of the fibers were studied by investigating the relative density, phase composition, microstructure and mechanical properties of the composites. It was found that the densification behavior, microstructure and mechanical properties of the composites were influenced by the fibers’ surface condition. The heat treated fiber was more appropriate to toughen the ZrB2-based composites, due to the high graphitization degree, low surface activity and weak interfacial bonding. As a result, the fracture toughness of the composites with heat-treated fiber is 7.62 ± 0.12 MPa m1/2, which increased by 10% as compared to the composites with as-received fiber (6.89 ± 0.16 MPa m1/2).