Carbon - Ceramic composites

By : Charu Lakshmi T R ( 04 )

Rinkal Vaghasia ( 13 ) Sem : 4 Guided by : Ms R.H. Patel Milan M. vyas Materials Science Department

Date : 16/04/15

Outline• Introduction Carbon to carbon ceramic composites Applications • Material Requirements • Method followed• Characterization• References

Carbon - An Introduction

• Carbon materials can be used in various fields like aerospace , defense ,sports , medicine.

• They can withstand high temperatures.• They have good thermal shock resistance,

good strength, good thermal & electrical conductivity , corrosion resistance, low density, low friction & wear properties.

Limitation : It is used less as it degrades in oxidizing atmosphere even at low temperatures.

• Coating of oxidation resistant refractory

oxide and carbide on carbon materials but there is a possibility of formation of cracks due to difference in coefficients of thermal expansion Or Doping them with boron & phosphorous compounds which block active sites for oxidation of carbon. This doesn’t work at higher temps.

• Carbon ceramic composites are used. Ceramic protects carbon from oxidation , mechanical strength improves (hardness & flexural) , open porosity reduces.

Carbon ceramic composites

• Carbon-ceramic composites refer to a special class of carbon based materials which cover the main drawbacks of carbon, particularly its proneness to air oxidation, while essentially retaining its outstanding properties.

• Carbon-ceramic composites, are found to possess a unique self-healing property , which protects the material from air oxidation in the event of any accidental or other damage.

• Carbon-matrix composites are important for lightweight high-temperature structures. Due to its superior oxidation resistance, silicon carbide is used in combination with carbon, whether it is a part of the matrix or a part of the reinforcement.

• For extended life applications the matrix of these composites need to be modified with Si, Zr, Hf or by multilayer deposition of SiC ,Silica etc. to avoid oxidation.

Conventional methods

These composites can be made by• Using thermosetting resin as matrix• Using pitch has matrix.• Using CVD method where hydrocarbon gases are used.• Using at least one sinterable component.

Composites with resin as matrix have lower densities, lower thermal conductivity and well distributed microporosity then the composites obtained with pitch as matrix (mesopores) and by CVD method (closed pores).

Resin based composites have high flexural strength, low toughness, low thermal conductivity.

Applications of CCC• Carbon and silicon based composites can be used as

nuclear reactor materials due to their high temperature stability and low susceptibility to irradiation and non brittle fracture.

• They can be also used as high temperature components in chemical reactors, Stirling engines, gas turbines, etc.

• They can be used in braking systems for the automotive sector and rapid train systems. Carbon-Carbon composites are used in aircraft brake technology.

• The ideal material for use in aerospace applications, such as rocket nozzles, exhaust cones, heat shields. e.g.: Nosetip of Agni missile and break pads of LCA

• Carbon-carbon composites have excellent gas non-permeability. They can be used in heat exchangers, parts of fuel batteries.

Experimental requirements• Phenol formaldehyde Resin (Novolac)

to get high carbon content.• SiC particles (85% purity)• Graphite Flakes• Porcelain container + ceramic balls• Hot press• Furnace• Tribometer

PF Resin• Phenolic resins are among the most commonly used binders

in friction materials. It can be of resol type, a novolac or even a modified version. It is the first industrially produced synthetic resin (1909,Baekland,Erkner/ germany)

• They are usually readily mixed with other constituents of friction material composites and therefore, may be used in relatively high content. They offer resistance to temperatures to the order of 250°C, and they char to high carbon content. They have superior fire resistance, long- term durability, and resistance to hydrocarbon and chlorinated solvents.

• Phenolics are also inherently brittle – limitation. So tougheners such as epoxy resin, Rubber particles are added to increase flexibility, Cashew nut shell liquid to minimize cracking of composite.

In all the 4 composites made,this is used as matrix.And this matrix is subjected to carbonization.

SiC particles.• Silicon Carbide is a leading material for components and

devices operating at high temperature, high power and under harsh environments.

• Micro-sized SiC particles and whiskers are commonly used as reinforcement materials for ceramics, metals and alloys for various structural and tribological applications.

• Silicon carbide (SiC) is a material with outstanding physical and mechanical properties.

• It has high mechanical strength, high hardness, low density, high thermal conductivity, low thermal expansion coefficient, large band gap and excellent oxidation and corrosion resistances.

• Silicon carbide (SiC) is highly resistant to heat. The material is stable up to temperatures near 1,600 °C in air and its decomposition temperature is 2,545°C.

• Grains of silicon carbide can be bonded together by sintering to form very hard ceramics which are widely used in applications requiring high endurance, such as car brakes, car clutches and ceramic plates.

Graphite flakes• Graphite are used in frictional materials for heavier

(nonautomotive) vehicles.• Graphite increases the efficiency of the frictional

materials in terms of lifetime, electrical and thermal conductivity, lubricity and hardness, mechanical strength and resistance to corrosion and reduction of noise.

• Natural graphite has high electrical conductivity but poor wettability with the liquid resin. Synthetic graphite is a product made by high-temperature treatment of amorphous carbon materials, usually calcined petroleum coke and coal tar pitch.

Composites in the making

• 4 composites were made, 2 of graphite flakes and 2 of SiC particles.

Steps followed : -• Pf-Graphite flakes were taken in 80-20% ratio

and 20-80% ratio respectively, (48-12 gm & 12-48 gm)

• SiC-Pf were taken in 60-40% and 40-60% ratio.(36-24gm & 24-36 gm).

• They were properly mixed and size was reduced considerably by ball milling.

Ball milling

• Ball milling for graphite flakes and Pf resin was carried out for 10-12 hrs at 750-900 rpm.• Ball milling for SiC and Pf resin was carried out for 18-20 hrs at 1000-1200 rpm.• Excess material stuck on the balls were removed after every three hours , to ensure proper milling of the particles by the balls.• After milling, container and balls were cleaned for the next batch. Particle size obtained : ( 0.063 mm )

Mould for particulate composite

• The Rectangular Mould has to be cleaned with sand paper and then with acetone to remove previously applied wax or lubrication layer surface impurities, dust, rust etc.• Aluminum foil is applied on the connecting parts of the mould.• Fresh layer of wax of negligible thickness is applied on the foil.• The particle mixture is filled and pressure is applied with a spatula. P<100 PSI.• Then finally pressure above 300 PSI is applied using hot press.

Hot pressing

• The mould kept on the hot press has to be properly insulated with bricks and glass wool.

• For graphite flake and Pf resin composite pressure of 50 kg/cm^2 and temperature of 93-95° C was applied using the hot press.

• Temperature was applied using joule heating technique. Voltage of 120 and 60V was applied to control heating by upper and lower heater.

• Temperature was allowed to raise till 82°C and current was switched off. The green composite’s temperature raises to 93°C due to mould’s hotness. And it was held at that temperature for one hour. Temperature is measured using thermocouple.

• For SiC and Pf resin composite pressure of 100 kg/cm^2 and 140°C was provided for proper sintering. And one hour standing time at that temperature is given.

Green composite

Carbonization of Pf Resin Matrix• Carbonization is heating in inert atmosphere. In our

green composite, Pf is the matrix, as it chars it yields high carbon content, we get carbon ceramic composites.

• For carbonization, green parts were heated to 1000° C in furnace filled with Nitrogen gas. Heating schedule was from room temperature to 200°C with a heating rate of 60° C/h, followed by a slower rate 30°C/h to 600°C, and then a 50°C/h rate to 1000°C, holding for 1h and then cooling has to be done by reducing the temperature by 40°C/h to room temperatures.

• The heating rate has to be such that resin shrinkage is controlled within limits , porosity and dimensional stability of the samples is maintained. sudden change in temperature might lead to crack formation in the composite .

• After carbonization, the phenolic resin molecules form an amorphous (glassy) carbon material that bonds the SiC particles together.(4)

Characterization• Surface texture, morphology can be analyzed with

the help of SEM.• The phase composition of the elementary powders ,

carbonized composites can be determined by XRD analysis.

• The precursor material SiC was 85% pure, to study the sample characterization and composition XRD will be used.

• The precise measurement of the lattice constant gives the exact composition of the composite.

• Surface roughness and the variation of coefficient of friction with composition of composite can be analyzed by studying the wear rate obtained from tribometer.

References• 1) Synthesis of Phenol formaldehyde and friction materials

reinforced brake liner material. (By Sridhar B S et al 2014 )• 2) Effects of phenolic resin and fly ash on coefficient of

friction of brake shoe composite ( By E. Surojo et al 2014 )• 3) Synthesis of Silicon Carbide Based Composite Material

for Industrial Applications (By Sridhar B S et al 2014 )• 4) Cf/SiC/C Composites for tribological application.( By Rak

Z.S et al 2011)• 5) Effect of Compositional Parameters on the

Characteristics of C-SiC-B4C Composites (By G.Bhatia et al 2004)

• 6) Effect of Different Graphite Materials on the Properties of Bipolar Plates Fabricated by Selective Laser Sintering.( By Nannan G et al 2011)