Processing of C/SiC composites by different

methods

Submitted By:Lakshya NarulaSophomoreDepartment of Ceramic Engineering,I.I.T (B.H.U), VaranasiU.P. 221005

Under the guidance of:Dr. Abhijit Ghosh

Scientific Officer (G)Glass and Advanced Materials Division,

Bhabha Atomic Research Centre

Supervised By:Dr. Madangopal Krishnan

HEAD, Glass and Advanced Materials DivisionBhabha Atomic Research Centre

CANDIDATE’S DECLARATION

This is to certify that the work which is being hereby presented by us in this project titled “Processing of C/SiC composites by different approaches” during summer intern at Glass and Advance Materials Division, Bhabha Atomic Research Centre, is a genuine account of my work carried out during the period from May 2015 to July 2015 under the guidance of Dr. Abhijit Ghosh (S/O,G) , Glass and Advance Materials Division , BARC.

The matter embodied in the project report, to the best of my knowledge, has not been submitted before elsewhere.

Dated: July 02, 2015

1. I N T R O D U C T I O N CMCs are constituted by the coupling of long fibers reinforcement (usually carbon fibers) and a refractory ceramic matrix (i.e. carbon, or silicon carbide) and represent a class of ceramic materials characterized by good mechanical properties, thermo-mechanical stability and fracture toughness: their fracture behavior sets them apart from conventional monolithic ceramics, allowing for a variety of uses in which damage tolerance is the main requirement. [1]

The extremely good high temperature fracture toughness of CMCs is provided by the crack bridging effect of the carbon fibers: stress concentrations, e.g. notches or holes, are reduced by stress redistribution and inelastic deformation. In case of overloading, monolithic ceramics break immediately, while CMC materials are still able to carry load even if the elastic mechanical load range is exceeded. Such a damage tolerant behavior constitutes an important point for the safety issues in particular for space re-entry vehicles. [1]

Carbon Fiber reinforced composites can be used up to 3000K in inert atmosphere or vacuum. Due to its oxidation above 873K, its use is limited and has become a problem to be used in aerospace sector in high temperature applications. [2] SiC is the most suitable material due to its high oxidation resistance, superior temperature and thermal shock ability and high creep resistance. [3] Replacement of Carbon Matrix by SiC is the most important breakthrough and carbon fiber reinforced silicon carbide (C/SiC) composites provide excellent thermo-mechanical properties at temperatures up to 2000oC (Zhong et al., 1998; He et al., 2001; Mentz and Muller, 2006).

The intrinsic features of SiC make C fiber reinforced SiC matrix composites (C/SiC composites) attractive structural materials for aerospace applications, since C/SiC composites can overcome the inherently brittle fracture behavior of monolithic SiC ceramics. C/SiC composites usually exhibit higher fracture toughness and less scatter of mechanical properties than SiC ceramics. At the same time, the excellent intrinsic features of SiC ceramics are retained. [4]

C/SiC composites fabrication processes such as chemical vapor infiltration (CVI), [5-7] reaction sintering, [8]

liquid phase sintering, [9] polymer impregnation and pyrolysis, [10] chemical vapor reaction [11], reaction bonded silicon [12] and their combined processes [13, 14] are being developed.

The physical and mechanical properties of continuous fiber-reinforced ceramic matrix composites (CMCs) depend on the properties of their various constituents, their geometry and concentration (e.g., volume fraction of fibers, fiber/matrix interphase structure, and matrix properties). [5]

2. L I T E R A T U R E R E V I E W

2.1 Sintering

Because of the high melting point of the raw materials, the fabrication of ceramics usually includes a heat treatment step in which a powder already formed into a required shape is converted into a dense solid. This step is called sintering. In other words, sintering can be defined as a process of compacting and forming a solid mass of material by heat and/or pressure without melting it to the point of liquefaction. [15]

Solid State Sintering

A type of sintering in which powder does not melt; instead, the joining together of the particles and the reduction in the porosity (i.e., densification) of the body, as required in the fabrication process, occurs by atomic diffusion in the solid state. This type of sintering is usually referred to as solid state sintering. While solid state sintering is the simplest case of sintering, the processes occurring and their interaction can be fairly complex. [15]

The shaped green body is heated to a temperature that is 0.5 to 0.9 of the melting point of the material of the green body. The powder does not melt, instead, the joining together of the particles and the reduction in the porosity of the body, as required in the fabrication process, occurs by atomic diffusion in the solid state. [15]

The driving force for sintering is the reduction in surface free energy of the consolidated mass of particles. This reduction in energy can be accomplished by atom diffusion processes that lead to either densification of the body (by transport matter from inside the grains into the pores) or coarsening of the microstructure (by rearrangement of matter between different parts of the pore surfaces without actually leading to a decrease in the pore volume). Controlling factors are sintering conditions like sintering temperature, rate of heating, and sintering atmosphere. [15]

Liquid Phase SinteringA common difficulty in solid-state sintering is that coarsening may dominate the densification process, with the result that high densities are difficult to achieve. This difficulty is especially common in highly covalent ceramics (e.g., Si3N4 and SiC). One solution is the use of an additive that forms a small amount of liquid-phase between the grains at the sintering temperature. This method is referred to as liquid-phase sintering. The liquid phase provides a high diffusivity path for transport of matter into the pores to produce densification but is insufficient, by itself, to fill up the porosity. A classic example of liquid-phase sintering in ceramics is the addition of 5–10wt% of Magnesium Oxide (MgO) to Si3N4. The presence of the liquid phase adds a further complexity to the sintering process, but the benefits can be significant, as demonstrated by the widespread use of liquid phase sintering in industry. [15]

Reaction bonded SinteringThe term reaction bonding (or reaction forming) is commonly used to describe fabrication routes where a porous solid preform reacts with a gas (or a liquid) to produce the desired chemical compound and bonding between the grains. Commonly, the process is accompanied by little or no shrinkage of the preform so that very close dimensional tolerances can be achieved for the finished component. Reaction bonding is used on a large scale as one of the fabrication routes for Si3N4 and SiC. [15]

Reaction bonded silicon carbide (RBSC) represents the most important example of the fabrication route based on the reaction between a solid and a liquid. Commonly, a mixture of SiC particles (5–10µm), carbon, and a polymeric binder is formed into a green body by pressing, extrusion or injection molding. In some variations, silicon carbide particles and a carbon-forming resin are used as the starting mixture. The binder or resin is burned off or converted to microporous carbon by pyrolysis, after which the porous preform is infiltrated with liquid Si at temperatures somewhat above the melting point of Si (1410oC). Reaction between the carbon and Si occurs according to: [15]

Si (l) +C (s) SiC (s)

The reaction product crystallizes on the original SiC grains and bonds them together. The infiltration and reaction processes occur simultaneously. Capillary pressure provides the driving force for infiltration, and good wetting of the surfaces by liquid Si is a key requirement. The kinetics of the infiltration are complex. [15]

Milling OperationsParticle size reduction, or comminution is an important step in many technological operations. The process itself is defined as the mechanical breakdown of solids into smaller particles without changing their state of aggregation. It may be used to create particles of a certain size and shape (including nano-size), to increase the surface area and induce defects in solids which is needed for subsequent operations such as chemical reactions, sorption etc. Milling not only increases the surface area of solids, it is likely to increase the proportion of regions of high activity in the surface. However, the size reduction of solids is an energy intensive and highly inefficient process. 2-4% of all electricity generated is used in size reduction.

Operations of following mills were used in the project-

1. Planetary MillPlanetary ball mill owes its name to the planet-like movement of its vial (s). Since the vials and the supporting disc rotate in opposite directions, the centrifugal forces alternatively act in like and opposite directions. This causes the milling balls to run down the inside wall of the vial – the friction effect, followed by the material being milled and milling balls lifting by of and travelling freely through the inner chamber of the vial and colliding against the opposite inside wall. Planetary mills exploit the principle of centrifugal acceleration instead of gravitational acceleration. The enhancement of the forces acting on the balls in relation to conventional ball mill is achieved by the combined action of two centrifugal fields. The charge inside vials performs two relative motions: a rotatory motion around the mill axis and a planetary motion around the vial axis. These mills produce high mechanical activation after a relatively short milling time. In principle, it is possible to obtain the gravitational accelerations in values 50-100 g. The energy density in these mills is 100-1000 times higher than the energy density used earlier in conventional milling equipment.

3. E X P E R I M E N T A L P R O C E D U R E S

Two types of mixtures were made to examine the characteristics of the fibre reinforced matrix composites.

3.1 Sample Preparation

Sample 1:

Laboratory made (BARC, Mumbai) carbon fibre was used as a reinforcement and pure and fine powder (1-2µm) SiC was used as a matrix for the composite. 10gms of carbon fibre was washed with acetone and then, was cleaned with acetone by performing ultrasonification for about 30 minutes. Carbon fibre was then cut into about 30mm length and kept for drying for about 2 hours. Fibre was then again cut into about 0.5-1 mm length and then mixed with 90gm Silicon carbide, 10gm of Yittrium-Aluminium-Garnet as sintering agent and 4gms of Poly Vinyl Butryl (PVB) as binder in a pot. A paste of this mixture was made using L.R Alcohol. About 800gms of Titanium Carbide (TiC) balls were added for milling operation and the whole mixture was then milled in a pot mill overnight (45rpm). The final mixture obtained was then dried till it was turned to powder. Eight pellets were made using 13mm dye by uniaxial pressing under a pressure of 100MPa.

Sample 2:

Laboratory made (BARC,Mumbai) carbon fibre was used as a reinforcement and a mixture of SiC (75µm) and SiC (22µm) was used as matrix for the composite. 10gms of carbon fibre was washed with acetone and then, was cleaned with acetone by performing ultrasonification for about 30 minutes. Carbon fibre

was then cut into about 30mm length and kept for drying for about 2 hours. Fibre was then again cut into about 0.5-1 mm length and then mixed with 45gms of each type of SiC and 4gms of PVB + 1gm of PVV as binder. A paste of this mixture was made using L.R Alcohol. About 200gms of Titanium Carbide (TiC) balls were added for milling operation and the whole mixture was then milled in a pot mill overnight (52rpm). The final mixture obtained was then dried till it was turned to powder. 14 pellets from this powder were made using 13mm dye by varying pressure from 66.67MPa to 266.67MPa. The full details are given in Table 1. Three bars of this powder were also made by pressing in a uniaxial press and then in Cold Iso-static Press (Engineered Pressure Systems Int., Belgium) under 200MPa pressure.

Sample 3:

100gms of Silicon (Si) powder was taken and was mixed with 2gms of PVB. A paste of this mixture was made using L.R Alcohol. About 145gms of Titanium Carbide (TiC) balls were added for milling operation and the whole mixture was then milled in a TiC pot in planetary mill for 3 hrs. (120rpm). Final mixture obtained was then dried till it was turned to powder. 6 pellets of about 10mm thickness were made from this powder using 13mm dye at a pressure of about 233.33MPa.

3.2 Operations

1. Four pellets from sample 1 were sintered in protective atmosphere furnace (Super-Kanthal insulation, BARC Mumbai) at 1600oC in atmosphere of nitrogen with uneven rate of heating till 1200oC and then 60oC/hr. till 1600oC.

2. Theoretical density of the sample 1 pellets were found out by the formula,

1dm

= M 1d 1

+ M 2d 2

+ M 3d 3

+ … + Mndn

, where; ............................................ (1)

dm = theoretical density of the mixture;d1, d2, d3 and dn are the densities of 1st, 2nd, 3rd and nth components respectively, andM1, M2, M3 and Mn are the mass fractions of 1st, 2nd, 3rd and nth components respectively in the mixture.

3. True Density of the pellets from sample 1 was found by boiling the pellets in distilled water for about 2 hours and then measuring various weights. The density was found by the formula,

Density (ƍ) =(W dry )

(W soaked−W suspended) , where; ...................... (2)

Wdry = Weight of pellet in air,Wsoaked = weight of pellet just after taking out after boiling, andWsuspended = weight of pellet when suspended in water.

4. Geometrical density of the pellets from sample 2 was found out by measuring their diameter, weight and thickness and then putting all things in the formula,

Dg = MassVolume=

(W dry )tπ r2

, where; ........................................................... (3)

Dg = Geometrical density of the pelletWdry = weight of pellet in air,t = thickness of the pellet, andr = radius of the pellet.

5. Porosity of the pellets was evaluated by the formula,

%age porosity =( (W dry−Wsoaked )(W soaked−W suspended ) )∗100, where;.…… (4)

Wdry = Weight of pellet in air,Wsoaked = weight of pellet just after taking out after boiling, andWsuspended = weight of pellet when suspended in water.

4. R E S U L T S

4.1 Density Measurements

Theoretical density of the sample was found to be 3.07 from formula (1), very close to the actual density i.e. 3.2 and true density of the 1600oC product was found to be 1.64 from formula (2), i.e. 53.42% of the actual density or in other words this much percent of green body started to dense at 1600oC and 46.35% porosity was evaluated from formula (4). This showed that only first stage of sintering is just initiated at 1600oC.

The graph of Density vs. Pressure was plot for the geometrical density found from formula (3) of pellets from sample 2. The full data is given in table 1.

Pressure (MPa) Density (gm/cc)66.67 1.81100 1.95133.33 2.03166.67 2.09200 2.12233.33 2.17266.67 2.17

Table 1.Data showing the variation of pressure vs density.

Graph of geometrical density of pellets of sample 2 is shown in Fig. 1. It was found that 233.33MPa pressure is the optimum pressure as density becomes after this pressure.

Fig. 1 Graph showing variation of density with applied pressure in uniaxial pressing.4.2 Microstructure Analysis

Green Microstructure of the powder of sample 1 is shown in Fig. 2. The C/SiC composite showed a good dispersion of fibres in powder, with fibres oriented in random directions, which meant that mixture was well milled. Theoretical composite density obtained by formula (1) was found to be 3.07.

Green microstructure of the fractured surface of sample 1 pellets is shown in Fig. 3. The big holes in the figure show the fibre pull-out. Also, powder distribution is shown in Fig. 4 and attachment of powder to the fibre is shown in Fig. 5. We can see that powder is distributed in a good way and powder size is a few hundred nanometres after milling.

Fig. 2 Microstructure of green powder of C/SiC – YAG

Fig. 3 Green body microstructure of fractured surface of C/SiC – YAG.

Fig. 4 Green microstructure of C/SiC – YAG powder.

50 100 150 200 250 3001.7

2

2.3

Density(gm/cc)

Density(gm/cc)

Fig. 5 Green Microstructure of C/SiC – YAG composite. Here we can see the attachment of fibre and

powder in a better way.

SEM analysis of the green microstructure of the powder of sample 2 is shown in Fig. 6. The C/SiC composite showed a good dispersion of fibres in powder, with fibres oriented in random directions, which meant that mixture was well milled. Fig. 7 shows the effect of choosing big sized particles of SiC. After milling, sharp edges of powder are formed, which cut the fibres into micrometre size during milling.

Fig. 6 Green microstructure of C/SiC powder.

Fig. 7 SEM analysis showing fibre cutting effect after choosing big sized particles of SiC.

ACKNOWLEDGEMENTS

I would like to express my sincere gratitude and deepest regards for myproject Guide, Dr. Abhijit Ghosh, for his constant support and motivation throughout the Project work. It would have been impossible without his valuable guidance, relentless support, discerning thoughts and loads of inspiration that led me forward to delve deeper into the issue.

I am highly thankful to Dr. Madangopal Krishnan, Head, Glass and Advance Materials Division, Bhabha Atomic Research Centre, Trombay, Mumbai for supervising and giving this project and providing a summer internship opportunity in the best research institution of the country.

I am thankful to the office staff Shri. Shoeb Kaleem Hashmi and Shri. Sachin Zhinge for helping us with the experimental setups required in the course of our Project work, to access the lab equipment and helping us in using the same. I also thank the office of Glass and Advanced Material Division for helping us through the paperwork involved in the various aspects of our work. They also provided valuable guidance and thoroughly monitored and encouraged us during our work.

5. R E F E R E N C E S

1. Davide Alfano, Properties and applications of Silicon Carbide, Ch-10, “Carbon fiber reinforced silicon carbide composites of aerospace applications”, Italian Space Research Centre, Italy.

2. N. Padmavathi, J. Subrahmanyam, K.K. Ray, R. Mohanrao, P. Ghosal, Sweety Kumari, “Carbon fiber reinforced silicon carbide mini-composites-solution approach”, Journal of Mat. Processing Tech. 204 (2008) 434–439

3. Handbook of ceramic composites, Narottam P. Bansal, 2005 4. TATSUYA HINOKI*, EDGAR LARA-CURZIO AND LANCE L. SNEAD, “MECHANICAL PROPERTIES OF HIGH PURITY SiC FIBER-REINFORCED CVI-SiC MATRIX COMPOSITES”, Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN37831

5. T. HINOKI, L.L. SNEAD, T. TAGUCHI, N. IGAWA, W. YANG, T. NOZAWA, Y. KATOH and A. KOHYAMA, Ceramic Transactions, 144, 55, The American Ceramic Society, Westerville (2002).

6. T.M. BESMANN, J.C. McLAUGHLIN, HUA-TAY LIN, “Fabrication of Ceramic Composites: Forced CVI,” J. Nucl. Mater., 219, 31 (1995).

7. S. BERTRAND, P. FORIO, R. PAILLER and J. LAMON, “Hi-Nicalon/SiC Minicomposites with (Pyrocarbon/SiC) n Nanoscale Multilayered Interphases,” J. Am. Ceram. Soc., 82, 9, 2465 (1999).

8. S.P. LEE, Y. KATOH, J.S. PARK, S. DONG, A. KOHYAMA, S. SUYAMA and H.K. YOON, “Microstructure and Mechanical Characteristics of SiC/SiC Composites with Modified-RS Process,” J. Nucl. Mater., 289, 30 (2001).

9. Y. KATOH, S.M. DONG AND A. KOHYAMA, “Thermo-Mechanical Properties and Microstructure of Silicon Carbide Composites Fabricated by Nano-Infiltrated Transient Eutectoid Process,” Fusion Engineering and Design, 61-62, 723 (2002).

10. A. KOHYAMA, M. KOTANI, Y. KATOH, T. NAKAYASU, M. SATO, T. YAMAMURA AND K. OKAMURA, “High-Performance SiC/SiC Composites by Improved PIP Processing with New Precursor Polymers,” J. Nucl. Mater., 283-287, 565 (2000).

11. W. KOWBEL, J.C. WITHERS, R.O. LOUTFY, C. BRUCE AND C. KYRIACOU, “Silicon Carbide Fibers and Composites from Graphite Precursors for Fusion Energy Application,” J. Nucl. Mater., 219, 15 (1995).

13. M. KOTANI, A. KOHYAMA AND Y. KATOH, “Development of SiC/SiC Composites by PIP in Combination with RS,” J. Nucl. Mater., 289, 37 (2001).

14. A. ORTONA, A. DONATO, G. FILACCHIONI, U. DE ANGELIS, A. LA BARBERA, C.A. NANNETTI, B. RICCARDI AND J. YEATMAN, “SiC–SiCf CMC Manufacturing by Hybrid CVI–PIP Techniques: Process Optimisation,” Fusion Engineering and Design, 51-52, 159 (2000).

15. M.N REHEMAN, Ceramic Processing and Sintering.