Martin Frieß

Discrete element simulation of transverse cracking during the pyrolysis of carbon fibre reinforced plastics to carbon/carbon composites

The fracture behavior of fiber-ceramics like C/C-SiC strongly depends on the initial damage arising during the production process. We study the transverse cracking of the 90{\deg} ply in [0/90]S cross-ply laminates due to the thermochemical degradation of the matrix material during the carbonization process by means of a discrete element method. The crack morphology strongly depends on the fiber-matrix interface properties, the transverse ply thickness as well as on the carbonization process itself. To model the 90{\deg} ply a two-dimensional triangular lattice of springs is constructed where nodes of the lattice represent fibers. Springs with random breaking thresholds model the disordered matrix material and interfaces. The spring-lattice is coupled by interface springs to two rigid bars which capture the two 0{\deg} plies or adjacent sublaminates in the model. Molecular dynamics simulation is used to follow the time evolution of the model system. It was found that under gradual heating of the specimen, after some distributed cracking, segmentation cracks occur in the 90{\deg} ply which then develop into a saturated state where the ply cannot support additional load. The dependence of the micro-structure of damage on the ply thickness and on the disorder in spring properties is also studied. Crack density and porosity of the system are monitored as a function of the temperature and compared to an analytic approach and experiments.

Investigation of Different Phenolic Resins and their Behavior during Pyrolysis to Form SiC/C-Composites

Specific phenolic resin samples have been developed as the carbon precursor for SiC/C composites. Liquid phenolic resins suitable for fiber-infiltration in the resin transfer moulding (RTM) process are synthesized by using versatile combination of the aromatic component (phenol, naphthalen-2-ol) with various formaldehyde equivalents such as methanal, 1,3,5,7tetraazatricyclo [3.3.1.13,7] decane (urotropine), and 1,3,5-trioxane, under different reaction conditions. Room temperature liquid resoles (RTLR) are obtained by using an excess of the formaldehyde component over phenol (≥2) under basic conditions. Upon heating RTLR can form a crosslinked network even without addition of a hardening reagent. In addition, novolacs are synthesized under acidic conditions using a phenol/formaldehyde ratio ≥1. Nitrogen-containing resins contain nitrogen due to reaction of phenol with urotropine. Novolacs and nitrogen-containing resins are solids at room temperature and not self-curing. To infiltrate these both resins into SiC fibers in the RTM process, they are dissolved in 2furanmethanol (furfuryl alcohol FA) and urotropine which is added as curing-agent. Both, the molecular weight and the amount of the dissolved phenolic resin have an influence on the viscosity and the carbon yield after pyrolysis which is important for this application. The aim was to create different phenolic resins for the fabrication in the RTM process and to characterize the carbon after pyrolysis with respect to the structure and porosity as these are key parameters to generate a stoichiometric SiC matrix by LSI.

Development of a SiNx-Based Barrier Coating for SiC Fibres

Uncoated SiC fibres in SiC/SiC composites manufactured by the liquid-silicon infiltration (LSI) process show a strong degradation as a result of silicon attack. The goal of this research is the development of a SiNx-based fibre coating, which acts as a barrier against the liquid silicon. The coating is applied by means of low-pressure chemical vapour deposition (LPCVD) utilising the gaseous precursors silane (SiH4) and ammonia (NH3) on a commercial SiC multifilament yarn. The result is an amorphous fibre coating with an increasing coating thickness and a variable chemical composition from the middle of the yarn to the edges. The coated fibres exhibit a reduced characteristic Weibull strength in comparison to the uncoated fibres. In order to examine the stability of the films, the coated fibres undergo a heat treatment at 1450 °C in different environments (vacuum, argon and nitrogen). In all environments, the amorphous SiNx coatings crystallise to the trigonal Si3N4. Depending on the coating thickness cracks and defects develop. However, the best results and the lowest amount of damaging occurs during the treatment in nitrogen.

Development and Characterisation of Phenolic Resin Based Liquid Silicon Infiltrated SiC/SiC Composites with SiNx Fibre Coating

SiC/SiC ceramics consist of silicon carbide fibres embedded in a silicon carbide matrix. As an alternative to classic CVI and PIP routes, Liquid Silicon Infiltration (LSI) was chosen as a technique with short process times to obtain composites with low porosity. Silicon carbide composites show good thermal shock resistance, a low coefficient of thermal expansion and excellent physical and chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in jet engines and in power engineering. To build up the matrix, different phenolic resin based carbon precursors were infiltrated in fibre preforms and thermally cured, pyrolysed and siliconized. The aim is to obtain a high carbon yield during pyrolysis and to control the pore morphology in a way that the following liquid silicon infiltration leads to a complete reaction of the carbon matrix with silicon to SiC. The siliconization behaviour and conversion into SiC in dependence of pore morphology and chosen precursor is analysed.At the same time a functional fibre coating has to be developed which protects the fibres from liquid silicon and simultaneously provides a weak fibre matrix bonding. A LPCVD-SiNx fibre coating has been chosen and is investigated in fibre composites especially in terms of protection and reactivity in different atmospheres during pyrolysis and siliconization.

Development of Damage-Tolerant Ceramic Matrix Composites (SiC/SiC) using Si-BN/SiC/pyC Fiber Coatings and LSI Processing

Silicon-carbide-fiber-reinforced silicon carbide matrix composites (SiC/SiC) exhibit good thermal shock resistance, a low coefficient of thermal expansion and excellent physical properties as well as chemical stability at elevated temperatures and are therefore regarded as promising candidates for various applications in heavily loaded turbine sections of jet engines. Liquid silicon infiltration was chosen as a technique characterized by short processing times to obtain composites with low porosity in a three-step process: infiltration of fiber preforms with a phenolic resin, pyrolysis and siliconization. Unfortunately, uncoated Tyranno SA3 fibers were cramped in the matrix, resulting in SiC/SiC with low strength and damage tolerance. In order to protect the fibers and to simultaneously provide a weak fiber matrix bonding, a CVD Si-BN/SiC/pyC fiber coating was chosen. The triple coating leads to a twofold higher bending strength of SiC/SiC as well as to more damage-tolerant fracture behavior compared to composites without fiber coating. Composites with various fiber volume contents are compared with regard to their mechanical properties. The microstructure of the composites was characterized by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and computed tomography (CT), especially with regard to the functionality of the fiber coating.

How to Tame the Aggressiveness of Liquid Silicon in the LSI Process

Liquid Silicon Infiltration (LSI) is a technique to manufacture non-oxide ceramic matrix composites such as C/C-SiC or SiC/SiC. In the beginning of this three-step process, fiber preforms are shaped and impregnated with phenolic resins. After curing, the preforms are pyrolyzed to convert the polymer matrix to a porous carbon matrix. This porosity is then used to infiltrate liquid silicon by capillary forces. Simultaneously, an exothermic reaction of silicon and carbon creates a silicon carbide matrix. Generally the liquid silicon reacts with any carbon and even with SiC present in the form of fibers, fiber coatings or matrix. Therefore, especially the fibers must be protected from Si attack effectively. The morphology of silicon carbide was observed to be heavily driven by Ostwald ripening. This can be suppressed by the addition of boron to the melt. The initially formed SiC crystals in C/C-SiC composites are hereby prevented from grain coarsening, resulting in almost completely preserved C/C blocks. For the manufacture of SiC/SiC composites, the silicon boron alloys allow an effective preservation of the nanocrystalline SiC-fibers. Thus, the use of Si based B containing alloys helps effectively to moderate and control the aggressive reaction during LSI process.