Preparation of preformed yarn to synthesize carbon-carbon composites

Abstract

The carbon fibre-reinforced carbon composites, popularly known as the carboncarbon (C/C) composites, are of interest because of their ability to retain strength and structural integrity till 30000C either in vacuum or in inert environment. C/C composites also possess outstanding specific strength and stiffness. Hence, the C/C composites are suitable for application in thermal protection systems used in nose cones and leading edges of hypersonic and re-entry type vehicles, which are exposed to elevated temperatures. Due to impressive high temperature properties, the applications of C/C composites in the aerospace industry have increased significantly in recent decades. Significant research has been carried out on the conventional C/C composites manufacturing methods and their properties. The conventional C/C composites synthesis requires many steps and consumes more time which leads to comparatively more cost. This research is mainly focused on a new method of C/C composite-synthesis by giving importance on preform preparation which includes reinforcement and matrix materials. In order to simplify the manufacturing of C/C composites, it was decided to develop a new production method, using preformed yarn, which contains the carbon fibre filaments as well as the matrix materials (coke and pitch binder).This method is named as preformed yarn (PY) method. This PY enables us easily to fabricate primary workpieces such as unidirectional (UD) sheets, cloth sheets, tapes and chopped yarns. Further, it is possible to prepare pipe and tube shaped standby preforms. From these workpieces and preforms, the C/C composites can easily be produced by hot press molding method. This investigation presents the construction of preformed yarn machine, and to produce preformed yarn bundles. In the beginning, it was decided to prepare proto-type PY model and three carbon fibre weight percentages of PY were prepared. Using this PY, unidirectional C/C composites were synthesized and various properties were analyzed. The C/C composites were prepared by varying the weight percentage of carbon fibre. The PY was prepared with carbon fibre (reinforcement) filaments surrounded byii coke and pitch (matrix materials), which were enclosed in nylon-6 as sleeving material for easy handling and processing. Three types of PY samples were prepared with carbon fibre fractions of 30wt%, 40wt% and 50wt% respectively and accordingly the composites were synthesized. In each case, the PY was chopped and filled unidirectional into a die of required shape and hot pressed at 600°C. Subsequently, impregnation of the carbon fibre preforms by pitch was carried at 2500C under a pressure of 0.1 MPa for 2-3 hrs. Then the pitch impregnated carbon fibre preforms was heated to 800◦C and hot isostatic-pressed at 90–100 MPa pressure for 24 hrs. This was followed by carbonization at 9000C for 48 hrs and graphitization at 25000C for 24 hrs, each under 1 atm. pressure. The process cycle from pitch impregnation to graphitization was repeated 2 to 3 times, until a density of 1.7 g/cc could be obtained in the investigated C/C composites. The characteristics such as microstructure (SEM images), XRD analysis, hardness, compressive strength, impact energy, creep, wear property, thermal degradation, have been studied. It is observed that, as the carbon fibre weight percentage increased the properties have also improved. The C/C composite made by the proposed PY technique is superior to any other conventional method. Among the three C/C composites containing different weight percentages of the fibre content, the one with 50wt% carbon fibre showed better properties. After developing a prototype model, it was further decided to design automatic (using PLC) machine for producing 50wt% carbon fibre-PY. The design is presented with relevant drawings. The sleeve weight percentage was reduced considerably in automatic PY technique as sleeve is not much important in composites except for holding and handling purposes. Due to this the weight percentage of matrix is proportionately increased against the reduction of weight percentage of the sleeve and weight percentage of carbon fibres would remain same.