D2: Coating of carbon fibre textile preforms to improve the infiltration properties and stability in aluminum alloys
Engineering Design, Machine Elements, Product Development
Final Report Abstract
Carbon fibre based 3D-textile preforms were coated with TiN and SiC by chemical vapor deposition (CVD) and with Al2O3 by atomic layer deposition (ALD). TiN deposition was realized with a gas mixture of TiCl4, N2 and H2. A homogeneous coating of carbon fibres in the 3D- textile preform is achieved at a temperature between 800°C and 850°C, a pressure of 3 kPa and an optimized value of the TiCl4 concentration. For the TiN deposition activation energies EA between 116 - 163 kJ/mol are determined from the Arrhenius plot between 800 - 920°C for different process pressures. In this temperature range the TiN deposition is controlled by surface reaction kinetics which is a favourable range for a homogeneous coating. The examination of the oxidation behavior of coated carbon fibres shows that TiN with a thickness of 25 nm improves the oxidation resistance of the carbon fibres only slightly. The oxidation resistance is improved significantly by increasing TiN thickness however there is an opposite effect regarding to the tensile strength. Mean tensile strength about 2.2 GPa and 2.1 GPa are measured for carbon fibres coated at 800°C and 850°C with thin TiN layers of thickness 30 - 40 nm compared to 3.9 GPa for an uncoated carbon fibre. For a higher deposition temperature of 920°C the mean tensile strength will be nearly zero due to the strong brittleness of the fibres. The decrease of tensile strength is caused by the higher thermal load, a coarser-grained TiN structure and a stronger formation of a hard carbonitride interface between TiN and carbon fibre. For avoiding strong brittleness of the fibres the structure of coated TiN should be less crystalline. TiN shows a good wettability behaviour and it works well as protective layer if the contact time with the aluminium melt is short. For long contact times it reacts with Al melt to AlN and the protective effect is lost. SiC was investigated as a second CVD coating. Two processes with different gas mixtures of SiCl4/C2H4/H2 and Si2Cl6/C2H4/H2/Ar were applied. Both processes gave coatings with strong thickness variations of more than 50% between fibres of the face side and inner fibres of a fibre bundle. Furthermore SiC prepared by the SiCl4 process at 950°C contains free silicon. In contrast SiC coatings of the Si2Cl6 process are stoichiometric and have a high crystallinity despite the deposition was performed at lower temperature. These coatings are more homogeneous, conformal and adhere better than coatings prepared with SiCl4 precursor. The oxidation resistance is improved significantly by a SiC coating prepared with the Si2Cl6 process. The higher oxidation resistance compared to the SiC of the SiCl4 process is favoured by the stoichiometric composition without free silicon, the better morphology and adherence. A directly deposited SiC coating with a thickness of 100 nm leads to a decrease of the fibre strength from 3.9 GPa to approx. 2 GPa. A less reduction of the tensile strength can be achieved by adjusting the SiC thickness below 50 nm. The aluminium alloy wets the SiC coated fibre with a contact angle higher than 140° that means it shows a poor wetting. Infiltration experiments were not performed. As third layer type thin Al2O3 coatings were examined. These layers were deposited by ALD using the precursors trimethylaluminium (TMA) and ozone. A homogeneous alumina coating on 3D-textile preforms was achieved with optimized ALD cycle parameters at a temperature of 220°C. ALD allows an excellent conformal coating of complex shaped 3D-textile preforms. Al2O3 coatings prepared at 220°C show a sufficient adhesion on the fibres. The oxidation resistance of the carbon fibres was improved significantly even for a low Al2O3 thickness of 20 nm. An ALD-Al2O3 coating with a thickness less than 50 nm leads only to a minor reduction of the tensile strength of the carbon fibre. The tensile strength decreases from 3.9 GPa for an uncoated fibre to 3.2 GPa for the alumina-coated fibre. This is a remarkable advantage compared to CVD-TiN and CVD-SiC coatings both leading to a much stronger reduction of tensile strength. ALD alumina shows an excellent wettability for aluminium and Al alloy melts and it works very well as protective layer also for long contact times with the melt. It is a very promising protective layer for the application on fibre materials.
Publications
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„Al2O3 protective coatings on carbon fiber-based 3D-textile preforms prepared by ALD for application in metallic composite materials”, 13th AUTEX World Textile Conference, 22.05.- 24.05.2013, Dresden
M. Krug, A.Z. Abidin, M. Höhn, I. Endler, N. Sobczak and A. Michaelis
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„Surface modification of 3D-carbon textile preforms by means of CVD for application in MMC – Effect of the layer composition and structure on the infiltration properties of aluminum alloys“, Tagungsband 19. Symposium Verbundwerkstoffe und Werkstoffverbunde (Herausgeber:A. Wanner, K.A. Weidenmann), Karlsruhe, 03.07.- 05.07.2013, Seite 674-682
A.Z. Abidin, M. Höhn, I. Endler, I. Uhlig, N. Sobczak, R. Kozera, F. Kachold, A. Michaelis
