Preparation & characterization of SiO2 interface layer by dip coating technique on carbon fibre for Cf/SiC composites Kundan Kumar, C. Jariwala, R. Pillai, N. Chauhan, and P. M. Raole Citation: AIP Conference Proceedings 1675, 020046 (2015); doi: 10.1063/1.4929204 View online: http://dx.doi.org/10.1063/1.4929204 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1675?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Effect of surface preparation and interfacial layer on the quality of SiO2/GaN interfaces J. Appl. Phys. 109, 084511 (2011); 10.1063/1.3572236 Graded composition and valence states in self-forming barrier layers at Cu – Mn / SiO 2 interface Appl. Phys. Lett. 96, 012101 (2010); 10.1063/1.3269602 Preparation and optical characterization of Au / SiO 2 composite films with multilayer structure J. Appl. Phys. 93, 4485 (2003); 10.1063/1.1560569 HfO 2 – SiO 2 interface in PVD coatings J. Vac. Sci. Technol. A 19, 2267 (2001); 10.1116/1.1382879 Characterization of rapid thermally nitrided SiO2/Si interface by the conductance technique Appl. Phys. Lett. 57, 2217 (1990); 10.1063/1.104161
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Preparation & Characterization of SiO2 Interface Layer by Dip Coating Technique on Carbon fibre for Cf/SiC Composites Kundan Kumar1, 2, b), C. Jariwala1, a), R. Pillai1, N. Chauhan1,P.M Raole1 1
Fusion Reactor Material Development & Characterization Division, Institute for Plasma Research, GIDC Electronics Estate, Sector-25, Gandhinagar-382016, INDIA 2 Centre for Nanotechnology, Central University of Jharkhand, Ratu-Lohardaga Road, Brambe, Ranchi-835205 INDIA Corresponding author: a)
[email protected] b)
[email protected]
Abstract. Carbon fibres (Cf) are one of the most important reinforced materials for ceramic matrix composites such as Cf - SiC composites and they are generally sought for high temperature applications in as space application, nuclear reactor and automobile industries. But the major problem arise when Cf reinforced composites exposed to high temperature in an oxidizing environment, Cf react with oxygen and burnt away. In present work, we have studied the effect of silica (SiO2) coating as a protective coating on Cffor the Cf / SiC composites. The silica solution prepared by the sol-gel process and coating on Cf is done by dip coating technique with varying the withdrawing speed i.e. 2, 5, 8 mm/s with fixed dipping cycle (3 Nos.). The uniform silica coating on theCf is shown by the Scanning Electron Microscope (SEM) analysis. The tensile test shows the increase in tensile strength with respect to increase in withdrawing speed. The isothermal oxidation analysis confirmed enhancement of oxidation resistance of silica coated Cf as compared tothe uncoated Cf.
INTRODUCTION Carbon fibres (Cf) are one of the most important reinforcement used in the advanced composite material because of their excellent properties such as high strength to weight ratio, high specific modulus, low coefficient of thermal expansion [1-5], and high temperature stability under non oxidizing atmosphere. Because of these unique properties, they become important high-temperature structural materials widely used in space shuttle, rocket nozzles, hypersonic vehicles nuclear reactors and propulsion systems. Cf reinforced Silicon Carbide (SiC) Matrix composites are a type of ceramic matrix composites very well known for their properties such as high thermal and mechanical strength, low density, radiation tolerance, chemical stability and stiffness compared to the conventional materials like metals and alloys [6-10]. But the main limitation of the Cf are that it begins to oxidized in oxygen environment at temperature near to 400ºC, which restricts its application in the oxidizing environment at higher temperature. As they come in contact with the oxygen at or above 400 ºC they start degrading and as a result in the deterioration of the composite’s properties, simultaneously. To overcome this difficulty and to widen the applicability of Cf,there is need of protective coating on it which improves its oxidation resistance and minimize the diffusionof carbon outwards from the Cf [11]. So regardless of the Cf applications, an external protective coating is essential which remain intact throughout the operation and have the inherent ability to restrict the oxygen from reaching Cf. One such promising material is silica which has high melting point (1873 to 1998 K) and low oxygen permeability [12]. Therefore, has good oxidation resistance property and considered as a good protective coating materials. Reinforcing SiO2 coated Cfin a SiC matrix (i.e. Cf /SiC-composites) can be used for very high temperature applications for with extended life span. Interface coating can be done mainly two way by a dry process (CVD, PECVD, PVD etc.) and by a wet process (dip coating, spin coating, spray coating). Here we have followed simple dip coating technique. Dip coating is a simple and known from ages for depositing onto various substrates. It is an economical& simple process. It offers a relatively fair control over the thickness and uniform coating can be achievedby this process [13]. It includes low temperature operation. So in this paper, study of Advanced Materials and Radiation Physics (AMRP-2015) AIP Conf. Proc. 1675, 020046-1–020046-4; doi: 10.1063/1.4929204 © 2015 AIP Publishing LLC 978-0-7354-1322-1/$30.00
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effect of silica coating on the Cfby dip coating technique at different withdrawal speed such as 2, 5 and 8 mm/s with fixed dip cycle (3 Nos.) have been attempted for Cf / SiC composites
EXPERIMENTAL 1. Raw materials Tetraethyl orthosilicate (TEOS) (98%) used as silica precursor was purchased from Acros organics, (USA). Ethanol (99.99%) for solvent and HCl for catalyst purpose were purchased from Changshu Yangyuan (China) and Rankem chemicals (India), respectively.
2. Silica Solution Preparation Process Silica solution was prepared by hydrolysis-condensation (sol gel process) of silicon containing inorganicorganic compounds. TEOS was used as a precursor for the silica solution. A known volume of TEOS was added in ethanol and the solution was continuously stirred. In another beaker HCl was added in known volume of water and the pH of the solution was kept below 2 and added drop wise into the solution of TEOS and ethanol, then after the solution were continuously stirred for 3 hours.
3. Coating of Carbon Fibres Polyacrylonitrile (PAN) based carbon fibers TAIRYFIL TC – 33 (6K) (make: Formosa Plastics Corporation, Taiwan) with a diameter of about 6–7μm, used in this present study. The Cfwere de-sized and cleaned by the treatment of Cf in acetonefor 10 minutes in an ultrasonic bath.The de-sized and cleaned Cf tows were coated by Dip Coating system (supplied by M/s. Holmarc, India, Model no: HO-TH-02B). De-sized fibres were dipped into the silica solution by keeping fixed dipping cycle (3 Nos.) and changing the withdrawing speed such as 2 mm/s, 5 mm/s and 8 mm/s. All the Cf samples were coated at room temperature with controlled environment. After each dip coating Cf were dried up to 150 °C by Infrared(IR) heater integrated into the dip coating system and final heat treatment at 400 °C for 2 hr.
RESULTS AND DISCUSSION 1. Microstructural Analysis of Carbon Fibres The surface microstructure was studied by SEM(440i, LEO make, UK), images of coated & uncoated portion of Cf are shown in fig. 1 (a). All the Cf were dipped 3 times with varying withdrawing speed i.e. 2 mm/s, 5 mm/s and 8 mm/s shown in fig.1 (b), (c) & (d), respectively. Fig. 1 (a) shows the difference between coated and uncoated portion of Cf. In which uncoated Cf has parallel grooves and ridges along the fibre axis which is the characteristics of the Cf.
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Fig. 1.SEM Images of(a) coated& uncoated Cfportion & coated Cfwithwithdrawal speed (b) 2mm/s, (c) 5mm/s) (d) 8 mm/s.
Fig. 1 (b) shows non uniform & less coating on the Cffor withdrawing speed 2 mm/s, Fig. 1 (c) shows relatively uniform coating on Cf as compared to the Cf with withdrawal speed 2 mm/s. Whereas fig. 1 (d)shows the coated Cf with withdrawing speed 8 mm/s has better uniformity of the coating than the coated Cf having withdrawal speed 5 mm/s. Hence, increasing the withdrawing speed correspondingly there is increase inuniformity in the coating on the Cf surface.
2. Mechanical Property of Carbon Fibres The tensile strength of uncoated and coated carbon fibres were studied using InstronTensile-Tester. The tensile strengthof the uncoated Cf was 526.88 MPa.The relative tensile strength (C/Co) of coated and uncoated Cf shows that there is increase in the tensile strength of the Cf after coating and also increases with increase in withdrawal speed as shown in fig. 2 (a). Here, C is the tensile strength of coated & Cois the tensile strength of uncoated Cf. In case of 2 mm/s withdrawing speed the tensile strength obtained is similar to the uncoated C f. Whereas the tensile strength enhanced by 1.063 and 1.248 times thanthe uncoated Cf when the withdrawing speed was 5 mm/s and 8 mm/s, respectively. So, there is continuous increase in the tensile strength of the coated Cf with increase in the withdrawal speed. This shows that there is better adhesion of silica on Cfas well as uniform coating of silica on fibres andhence the enhancement of its strength.
3. Oxidation Property of Carbon Fibres The oxidation property of Cfwere investigated by isothermal oxidation of the Cfat 400oC and 600oC in electric-resistance furnace for 1 hr., with heating at the rate of 30 °C/ min. The weight loss of uncoated Cfwas 25 % whereas the weight loss for the coated Cf having withdrawing speed 2 mm/s, 5 mm/s and 8 mm/s was 20 %, 19 % and 15 % respectivelyat an oxidation temperature of 400 ºC.This shows that Cfcan oxidized even at 400 ºC. Whereas the weight loss at 600 °C for uncoated Cf increased to 44.45 % and for coated fibres having withdrawing speed 2mm/s, 5 mm/s and 8 mm/s it was increased to 40 %, 36.64 % and 30.77 % respectively as shown in fig. 2 (b). Here decreased in weight loss of coated Cf as there is increase in withdrawing speed which is due to increase in uniformity of silica coating on Cf. The enhancement of the oxidation resistance property of Cfis due to the formation of silica layer on fibres surface which act as barrier between Cf and oxygen.
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Fig. 2.(a) Relative tensile strength of Cfat different withdrawal speed(b) Weight loss of Cf at an oxidation temperatureof 400 °C and 600ºC at different withdrawal speed.
CONCLUSION Uniform coating of silica on Cfby dip coating methodis obtained by use of TEOS as silica precursor. Coating uniformity can be varied by varying the withdrawal speed as seen by SEM analysis and it was observed that uniformity increases with increase in withdrawal speed. In this study the withdrawing speed for coating was 2, 5 & 8 mm/s with fixed dipping cycle(3 Nos.).The tensile result shows that the increase in strength of coated Cfas compared to the uncoated fibres. The coated fibres retained more than 80 %& 60 % of its weight whereas the uncoated fibres retained 75 %& 55 % of its total weight when isothermally oxidized at 400 ºC and 600 ºC. Hence these silica coating of Cf by dip coating method confirms the enhancement of oxidation resistance of coated Cfas compared to the uncoated Cf.
ACKNOWLEDGEMENT The author (KK)would like to express sincere thanks to Institute for Plasma Research (IPR)& Board of Research in Fusion Science and Technology (BRFST) for providing financial support by National Fusion program(NFP) Internshipfor this project work The authors (KK &CJ)also thank to Ms. Seema Patel (ATIRA, Ahmedabad) for fibre tensile strength testing measurement.
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