Kristina Roder

Development and Characterisation of Phenolic Resin Moulding Materials for the Production of New Short Fibre-Reinforced C/C-SiC Composites

This article focuses on the development of phenolic resin moulding materials for the production of new carbon fibre-reinforced ceramic composite materials based on C/C-SiC by utilising the LSI (liquid silicon infiltration) production method. The production of these moulding materials is being accomplished by combining phenolic resin and carbon fibres with the addition of a few selected parts of processing aids, during which the influence of the used lubricants on the processability of the moulding materials is examined. The starting materials, microstructures and mechanical properties of the materials were characterised at every step of the entire process (CFRP and C/C composites) as well as the end of the whole production (C/C-SiC composites). During this investigation a link between the portions of the lubricant used, the forming of the porosity and the impact on the mechanical properties was discovered. In regards to the optimisation of the process the involved parties were able to determine an optimal lubricant ratio.

An innovative production method for a C/C-SiC brake disc, suitable for a large-scale production

The industrial progress and the market-oriented management concepts result in increasing demands of the industry for improved performances while simultaneously improving on the energy efficiency. This results in the necessity for a progressive implementation of high-performance materials in areas such as the automotive industry, engineering, as well as applications in aerospace industry. Especially construction elements taking on large stresses call for excellent mechanical properties even at extremely high temperatures. In these high-temperature ranges only engineering ceramics, which unfortunately are very limited regarding the application due to the very brittle characteristics, showcase sufficient strength values.

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.

Influence of Hardener Content and Curing Parameters on the Microstructure and Mechanical Properties of Porous C/C Composites

Carbon fibre reinforced carbon composites (C/C) are characterised by their excellent thermal, chemical and mechanical properties. The intrinsic porosity and fibre reinforcement grant them an excellent damage tolerance. The production of complex structures is time consuming and very expensive. An innovative approach to this topic is the integration of simple geometric ceramic composite materials within complex polymer structures. The motivation of this contribution is to investigate the influence of hexamethylenetetramine as hardener (hardener content: 4, 8, 12 and 16 %) and curing parameters (tempered and non-tempered) on the microstructure and mechanical properties of the porous C/C composites. During the course of this contribution, selected carbon fibre reinforced polymer (CFRP) composites with different porosities were produced while adjusting the resin or hardening agent-ratio, as well as the processing parameters. Subsequent to the curing of the CFRP samples, porous C/C composites were produced by means of a pyrolysis process. The final part of the contribution is comprised of the microstructural analysis by light microscopy and the explanation of the flexural strengths, by utilising a “three-point-bending test”.

Großserientaugliche Formgebung durch Duroplast-Spritzgießen zur Herstellung von faserverstärkter Keramik

KurzfassungIn gemeinsamer Kooperation der Professuren Verbundwerkstoffe (Prof. G. Wagner), Strukturleichtbau und Kunststoffverarbeitung (Prof. L. Kroll) sowie Polymerchemie (Prof. S. Spange) der Technischen Universität Chemnitz wurde ein großserientaugliches Formgebungsverfahren zur Herstellung faserverstärkter C/C-SiC-Verbunde entwickelt. Das Verfahren wird die Produktionskosten erheblich senken.AbstractIn joint cooperation of the departments of composite materials (Prof. G. Wagner), lightweight structures and polymer technology (Prof. L. Kroll), and polymer chemistry (Prof. S. Spange) of Technische Universität Chemnitz a process suitable for large-scale production of fibre-reinforced C/C-SiC composites has been developed. This process will significantly reduce the production costs.

Investigation of Mechanical Properties and Failure Behaviour of CFRP, C/C and C/C-SiC Materials Fabricated by the Liquid-Silicon Infiltration Process in Dependence on the Matrix Chemistry

Fibre-reinforced ceramics have a higher fracture toughness compared to monolithic ceramics. Therefore, they are an attractive material for lightweight structural components in high-temperature applications. The liquid-silicon infiltration (LSI) process is a cost-efficient manufacturing route for fibre-reinforced ceramics consisting of three processing steps. A carbon-fibre-reinforced plastic (CFRP) composite is fabricated, which is converted in a porous C/C composite by pyrolysis. Liquid silicon is infiltrated to form a dense C/C-SiC composite. The performance of the composites strongly depends on the raw materials. The matrix chemistry in particular plays a key role in developing composites with tailored functional properties. The aim of this work is to investigate the mechanical properties and the failure behaviour of CFRP, C/C and C/C-SiC composites in dependence on the matrix chemistry. The composites are fabricated by the liquid-silicon infiltration process. Different phenolic resins as matrix polymers are used. The mechanical properties are characterized by bending tests and the failure behaviour is observed in-situ. Additionally, microstructural and fractrographic analyses are done. It can be shown that the formation of the microstructure and therefore the mechanical properties and the failure behaviour are strongly influenced by the matrix chemistry over all processing steps.