METALLIC PERMEATION BARRIER FOR CRYOGENIC COMPOSITE PROPELLANT STORAGE AND TRANSFER APPLICATIONS

Transcription

1 METALLIC PERMEATION BARRIER FOR CRYOGENIC COMPOSITE PROPELLANT STORAGE AND TRANSFER APPLICATIONS O. Drescher (1), E. Njuhovic (2), S. Glöde (3), J. Persson (4), V. Altstädt (5) (1) MAGNA STEYR Fahrzeugtechnik AG & Co KG, Aerospace Division, Liebenauer Hauptstraße 317, 8041 Graz, Austria, (2) University of Bayreuth, Department of Polymer Engineering, FAN A / Universitätsstraße 30, Bayreuth, Germany, edin.njuhovic@uni-bayreuth.de (3) Lüberg Elektronik GmbH & Co. Rothfischer KG, Hans-Striegl-Str. 3, Weiden, Germany, stefan.gloede@lueberg.com (4) ESA/ESTEC, Mechanical Engineering Department, Keplerlaan 1,PO Box 299, 2200 AG Noordwijk ZH, The Netherlands, jan.persson@esa.int (5) University of Bayreuth, Department of Polymer Engineering, FAN A / Universitätsstraße 30, Bayreuth, Germany, altstaedt@uni-bayreuth.de ABSTRACT Mass savings for cryogenic transfer and storage applications using fibre-reinforced polymers (FRP) is a key approach at MAGNA STEYR s Aerospace division. The permeable nature of FRP materials requires an impermeable metallic barrier; its development forms the herein presented project. The described results are the outcome of a comprehensive two-dimensional (2-D) sample test programme defining the essential sub-processes of the metallisation before entering the three-dimensional (3- D) component test and furthermore the demonstrator verification phase. 1. INTRODUCTION For transportation and storage of cryogenic fluids like liquid- helium (LHe, T=4.2K), oxygen (LOx, T=90K) respectively hydrogen (LH2, T=20K), steel, titan and aluminium materials are commonly used for aerospace applications. For such components fibre-reinforced polymers (FRP) bear a potential for mass savings among other advantages. However, due to the permeable nature of those materials an impermeable barrier as it is given by metallic components is needed to prevent the loss of respective propellants at temperature conditions down to 4.2K. In the frame of various running projects at MAGNA STEYR s Aerospace division, permeations tests with different material combinations on selected composite test samples (uncoated and coated) have been performed. After thermal cycling of the uncoated FRP substrates (up to 50 cycles between 330K and 77K) no induced damages or failures (e.g. micro-cracks) have been found. Nevertheless, it can be concluded, that the relatively high measured leakage rates are related to the diffusion of helium due to the substrate s specific microstructure. It can be traced back by identified extremely small, so called nano-cracks (local disbondings between fibre and matrix <1µm), which is an inherent characteristic to the material s given manufacturing process. Therewith the measured leakage rates of uncoated FRP substrates are unacceptable for e.g. a double walled cryostat, as they would lead to a rapid breakdown of the insulating highvacuum and thus generating heavy evaporation rates of the respective storage fluid. This intolerable material performance can be improved by using a metallic surface coating, applied as a thin layer by galvanic deposition and still providing the advantage of the FRP material s inherent substantial mass savings. 2. DESIGN PHILOSOPHY From the project StorHy, led by MAGNA STEYR and funded by the European Union, where comprehensive investigations on different coating materials and processes were performed [1], galvanic deposited copper is the material s choice showing the best compromise for achieving functional as well as process requirements. Thereby the chosen double coated tank wall design philosophy (Fig. 1) fulfils multiple functions/ requirements: Reduction of helium or hydrogen permeation

2 through the inner tank wall Reduction of ambient air permeation through the outer tank wall Outgassing prevention of inner and outer FRP tank wall structures Prevention of leakage induced by micro-cracking of each mechanism and the availability of the pretreatment processes, the investigations within this project are limited to the following mechanical and chemical adhesion mechanisms: Mechanical Adhesion The principle theory of mechanical adhesion is based on interlocking effects of the adhesive with the substrate; it is also called the push button principle. The adhesive enters the porosities, slots and cavities of a rough structured adherent surface. Additionally, the increase in roughness leads to an increase of the surface area [3]. In order to increase the surface roughness, blasting and direct chemical etching are selected as most suitable treatment methods. Figure 1. Metallic Coating Design Philosophy on double walled FRP Cryostat Structure The pre-defined FRP tank laminate consist of two different sub laminates: The outer compensation layers (both sides of the tank wall) made from glass fibrereinforced polymers (GFRP) and the inner structural layers made from carbon fibre-reinforced polymers (CFRP). Hereby the outer GFRP compensation layers shall partly compensate the thermo-mechanical induced stresses as well as insulate the metallic coating from the CFRP, preventing corrosive effects. The inner CFRP core layers shall serve as the actual structural layers carrying most of the structural loads. From the above presented tank laminate concept, the metallic coating is applied on GFRP material only and thus most of the development activities are focused on this kind of substrate material. However, principle feasibility applying the envisaged coating process on CFRP substrate materials were investigated, but not part of the herein presented work. 3. METALLISATION For a better understanding of the presented development activities the main sub-processes of the complete metallisation procedure are described in the following sub-paragraphs. 3.1 Surface Preparation Regardless the selected coating process, one of the main difficulties is to create consistently high adhesive strength levels between the FRP substrate and the coating system. The substrate s surfaces are often treated in a way that the topology of the material is modified in order to achieve at least one or more adhesion mechanisms between the substrate and the coating. In literature, a lot of adhesion theories are discussed, however due to the different binding energy Selective chemical etching of a thermoplastic fleece, which is integrated during production as a top layer of the composite substrate, relates to the creation of interlockings and thus is selected to cover the so called push button principle. Chemical Adhesion The principle theory of chemical adhesion lays in the forming of interphase bonds on atomic or molecular level. The strongest joins are formed by swapped atoms (ionic bonding) or shared outer electrons (covalent bonding) of different materials, whereas the hydrogen bonding forms a weaker bonding [3]. In order to achieve chemical adhesion with the primer material, which is applied after the surface preparation, new polar-groups have to be introduced to the substrate s surface and thus activating it. For polymer materials wet-chemical treatments are preferred surface modifications due to low production costs compared to plasma and vacuum processes. Therefore a two step synthesis reaction is selected [2]. 3.2 Primer Application Once the substrate s surface preparation process has been performed, the palladium based primer needs to be applied, providing the starting conditions for the coating process on the organic substrate material. Due to certain advantages and disadvantages two different application principles have been assessed within the project: The application by spraying and by dipping. 3.3 Coating Application The actual coating or metallisation process can be divided into two steps: The electroless and the galvanic plating.

3 In order to provide a conductive starting layer for the galvanic plating a first layer of chemical copper (electroless plating) is applied. In the frame of a cost efficient industrial process the limiting maximum thickness of chemical copper applied is around ~5µm. Therefore, a galvanic metallisation shall be used in a second step for achieving the required permeation barrier thickness of ~50µm. Depending on the required coating layer ultimate strain level two different galvanic processes are available. 3.4 Surface Finishing In the final step of the complete metallisation process a protective surface finish layer has to be applied in order to avoid any oxidation of the galvanic copper in case of contact with ambient fluids. In principle the following surface finish materials and processes are applicable: be tested from the earliest stages of the project. Thereby the test s main purposes are: Prove of basic concept Parameter definition, e.g. minimum coating thickness Metallisation process parameter verification For measuring the permeation behaviour during the trade off phase, a 2-D sample test setup was used. The test setup mainly consists of a gaseous helium (GHe) supply, the test chambers assembly (Fig. 2) and a helium leak detector. Dipped 1,2,3-Benzotriazole Chemical silver Galvanic gold Galvanic Nickel & galvanic gold Galvanic Nickel & chemical gold From a requirements and also cost perspective chemical silver is selected as the most promising surface finishing material and process for further investigations. 4. TRADE OFF PHASE In the trade off phase the different surface preparation and primer application processes were applied on different 2-D sample plates. The herefrom resulted test samples were verified by numerous destructive and non destructive tests like: Permeation test Peel and scribe grid test Pull off test Ductility test Microscopic cross section test Eddy current test Universal surface test (UST) Atomic force microscopy (AFM) Surface energy and contact angle test Scanning electron microscopy (SEM) Tensile fatigue test Due to the vast amount of data gained from testing only the most important tests and their related results are described in the following sub-paragraphs. 4.1 Permeation Test The provision of an impermeable barrier is the top requirement for the copper coating within the herein presented work. Thus the permeation behaviour had to Figure 2. Assembly of Test Chambers with 2-D Test Sample - Permeation Test Setup The test chambers assembly is formed by two single chamber flanges made from stainless steel, which are geometrically closed by the respective 2-D test sample (Ø ~80mm and 2mm thick) placed in the middle. The assembly is joint by bolts and sealed by an indium seal on both sides. In order to minimise the test s complexity and thus being able to efficiently handle the huge amount of test samples the test time was kept to a short term period of six hours at room temperature (RT) conditions; only a few samples were tested over long term periods of 16-20h and 72h for verification reasons. Before permeation was measured all samples were 20-times thermal cycled between 330K and 77K using an hot air oven and liquid nitrogen (LN2), thus keeping the test s conditions more realistic. Fig. 3 shows the result of a 72h long term test for a coated 2-D sample. The blue graph represents the measured ambient test temperature, the red graph the helium test pressure (~10baro) applied after 3h of conditioning to one of the test chambers, the black graph the vacuum pressure (~ mbar) applied to the opposite chamber and the green graph the helium leak rates measured by the leak detector. Note, that all graphs given in Fig. 3 are dependent on the samples surface area, thickness and test pressure.

4 investigating the different applied parameters of the metallisation sub processes. The peel strength test was performed at RT conditions in accordance with ASTM B533; the test setup (Fig. 5) was directly derived from that standard. Figure 3. Long Term Permeation Result of selective chemical etched fleece-modified coated FRP Sample (COAT :SN003:PERM4) For the majority of coated samples tested during the trade off phase, no essential differences in permeation behaviour was observed. Thereby the level of measured leakage rates was at the lower measurable limit of the helium leak detector and thus cannot be exactly quantified. However, an attempt of presenting the achievements shall be given by Fig. 4, where permeation results of an uncoated FRP (black graph), a coated FRP (green graph) and a stainless steel reference sample (red graph) are compared to each other. Figure 5. Peel Strength Test Setup From each of the prepared 2-D test sample plates three peel samples were cut and prepare for the test. One side of each sample was peel tested before, whereas the other side was peel tested after a thermal cycling procedure. Fig. 6 shows the achieved peel strength results over the roughness measured after the respective surface preparation method. However, the level of peel strength for the mechanical blasted, the selective chemical etched as well as the wet-chemical activated substrates is considered as low to medium sized. Extraordinary poor performance was measured for the direct chemical etching. Figure 4. Long Term Permeation Results comparing uncoated FRP, coated FRP and Steel Samples Note, that all graphs given in Fig. 4 are independent on the sample surface area, thickness and test pressure. The direct comparison reveals that the uncoated FRP sample shows a leaking behaviour, whereas the coated FRP and the stainless steel plate show a leak tight behaviour. Furthermore, a difference in leak rates around three orders of magnitude is achieved by the metallisation of 2-D FRP samples. 4.2 Peel Strength Test Besides the permeation behaviour, the resulting adhesive strength of the copper coating is of fundamental importance. In addition, measuring the peel strength revealed to be a perfect test method for Figure 6. Achieved Peel Strength versus Roughness of non-thermal cycled Samples Although adhesion was low to medium, neither

5 delamination nor reduced peel strength levels were observed after thermal cycling, apart for all direct chemical etched substrates, where all samples failed by massive delamination of the copper coating. mean strain level of 0.5%, an amplitude of 0.1%, a frequency of 10Hz and cycles per sample. Finally, two combinations of surface preparation methods led to higher peel strength levels, whereas blasting combined with selective chemical etching of fleece-modified substrates showed extraordinary high adhesion properties. Due to the blasting, the fleece fibres get more exposed and thus the etching process runs more effective, creating the cavities needed for the push button principle (Fig.7). Figure 8. Tensile Fatigue Test Setup For all fatigue tested samples no delaminations or other failures of the copper coating were observed neither during nor after the performed cycles. Again, no detectable influence in peel strength was measured from the thermal cycling. However the tensile fatigue loading led to a decrease in peel strength of ~25%. Figure 7. Microscopic Picture (20x magnified) of selective chemical etched fleece-modified coated FRP Sample (COAT :SN005:PEEL1) The variation of primer application did not show any detectable effect on the coating s adhesive strength; however the decision of which method shall be applied is also strongly driven by the coated component s geometry. 4.3 Tensile Fatigue Test Testing the degradation of the coating s adhesion after an applied high-cycle fatigue (HCF) loading was another important test of the trade off activities. Due to the in-plane stress state of a cryogenic storage vessel a tensile fatigue test was selected and performed on samples prepared with the best surface preparation candidate. Test and test setup (Fig. 8) was in accordance with the ASTM D3479 standard. In order to make the degree of degradation quantifiable, the peel strength was measured before (on separate samples) and after the tensile fatigue test (on the fatigue tested samples). Keeping the complexity of the test to a minimum its performance was limited to RT conditions. Therefore a set of five samples was fatigue tested in a non-thermal cycled condition (NTC) and another set after a thermal cycling (TC) was applied. The sinusoidal HCF loading was derived from finite element method (FEM) analyses of a typical LHe tank space application. The test was strain controlled with a 4.4 Conclusions From the performed trade off activities the following essential conclusions have been drawn: Uncoated 2-D FRP samples show a leaking behaviour, whereas coated 2-D FRP and 2-D stainless steel samples show a leak tight behaviour. The leak rates of 2-D FRP samples can be decreased three orders of magnitude by the metallisation. Blasting combined with selective chemical etching of fleece-modified FRP substrates leads to extraordinary high adhesion properties and thus is the selected surface preparation method for future development activities. No influence by varying the primer application method was observed, either on the coating s adhesion or on its permeation behaviour. Due to the strong driver of the coated component s geometry a final decision has not been taken yet. For the majority of tested samples neither delamination nor failure of the copper coating was detected after thermal cycling between 330K and 77K; only direct chemical etched samples failed by massive delamination of the coating. Additionally no influence on the adhesion behaviour due to the performed thermal cycles was observed. For all fatigue tested samples no delaminations or other failures of the copper coating were observed. However the tensile fatigue loading led to a decrease in peel strength of ~25%.

6 5. VERIFICATION PHASE The overall aim of the verification phase is to detail the investigations on the selected sub-processes and to transfer them to representing 3-D structures given by respective running projects D Sample Tests By applying the selected sub-processes, further and detailed 2-D testing will be performed investigating the following topics: Process stability Surface finishing Metallisation of FRP to metal transitions, e.g. on inserts Influence of modified thermal cycling (e.g. change of temperature difference to 350K 4.2K, etc.) Influence of modified fatigue testing (e.g. type of loading, number of cycles, strain level, etc.) Influence of induced defects on the coating s permeation behaviour Hereby the most promising and efficient test out of the trade off phase will be selected and performed with an increased number of samples D Component Tests For testing the influences of transferring the selected sub-processes on 3-D level, a typical pressure vessel shaped component was analysed and designed (Fig. 9). and GFRP epoxy prepregs Helical wound cylinder and dome section FRP dome closure and flange manufactured by hand layup FRP dome closure can be easily replaced by metal insert Once the ongoing manufacturing will be finished the first metallisation test trials shall be started in order to make necessary adjustments of the related subprocesses. The verification programme of the 3-D components involves the following tests: Leakage tests Proof- and burst pressure tests Thermal cycling tests Pressure cycling tests 6. DEMONSTRATION PHASE In the demonstration phase the processes selected, the experience made, the results gained and the lessons learned from previous phases shall be applied to the LHe tank project (Fig. 10). Within this project, assigned by ESA/ESTEC, demonstrator hardware of a potential in flight operating cryostat shall be built and tested, aiming for a technology readiness level of TRL5. Figure 10. Rendering of LHe Tank Demonstrator 7. REFERENCES [1] Schultheiß, D. (2007). Permeation Barrier for Lightweight Liquid Hydrogen Tanks, Dissertation, Universität Augsburg. Figure 9. FEM Model of 3-D Component The component can be described by the following features: Diameter of ~400mm and height of ~450mm Full FRP design using a hybrid laminate of CFRP [2] Siau, S. & Vervaet, A. & Van Calster, A. & Demeter, U. & Schacht, E. (2006). Epoxy Polymer Surface Modification through wet-chemical organic Surface Synthesis for Adhesion Improvement in Microelectronics, Thin Solid Films 495, [3] Suchentrunk, R. et. al. (2007). Kunstoff- Metallisierung, Eugen Leuze Verlag, Bad Sauglau.