Fabrication method and characteristics Ceramic Fibers: Alumina fiber SiC fiber Aim of 4 th week class

6. Ceramic Fibers ▶ ceramic fiber is high strength and elastic modulus with high-temperature capability and a general freedom from environmental attack fabrication- CVD, polymer Pyrolysis, sol-gel silicon carbide fibers- CVD deposited on a W (sometimes carbon) substrate heated to 1300 ℃

Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6. Ceramic Fibers This fiber gives a buffer layer at the surface that allow fiber strength to be maintainted Even during the high-temperature incorporation into a metal matrix. Sol-Gel Process of making Oxide fibers  Formulate sol  2. Concentrate to form a viscous gel  3. Spin the precursor fiber  4. Calcine to obtain the oxide fiber

Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Nonoxide Silicon : Silicon Carbide(fiber type is commercially available) & Silicon Nitride  Silicon Fibers Manufacturing: CVD(conventional) Controlled pyrolysis of polymeric precursors  SiC whiskers

 Reactive gas mixtures : Alkyl Silanes(30%) Hydrogen(70%)  Mercury Seals are used at both ends  To grow 20 μm monofilament takes 20 Sec.  Exhaust gas: 95% of Original mixture + HCl(g): CVD process of making SiC fiber is very similar to that of B fiber manufacture  Atm: Excess. H 2 excessive Si will form at the both ends Insufficient H 2 Chlorosilanes will not be reduced Free C will be present  Final monofilament( μm): A sheath of β-SiC with some of α-SiC on W core: {111} in SiC deposit// fiber axis  CVD Silicon Carbide Fibers Substrate : W or Carbon, Heated to 1300C, Direct current (250mA) Very High Frequency(VHF~60MHz)

CH 3 SiCl 3 (g) H2 SiC(s) + 3HCl(g) Fig.2.39 CVD process for SiC monofilament fabrication Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Textron Specialty Materials : developed surface modified SiC fibers: SCS fibers e.g. SCS-6  SCS-6 : Thicker fiber(142 μm) by CVD of Si & C containing compounds onto pyrolytic Graphite coated carbon fiber core(37 μm)

Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Non-oxide fiber process via controlled pyrolysis of a polymeric precursor 1. Polymer characterizaton (yield, molecular weight, purity, etc.) 2. Melt spin polymer into a precursor fiber 3. Cure the precursor fiber to crosslink the molecular chains, making it infusible during the subsequent pyrolysis Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Nonoxide fibers via polymers: Controlled pyrolrisis of a polymer precursor because SiC Fibers from CVD is thick not flexible

In Ar gas Curing in Air at ~1000C followed by ~550C cross linking of polymerchains, H 2 & Methyl group decompose Conversion of SiC done above 850C Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Commercial variety of Nicalon fiber is armorphous:density is 2.6g/cm3, lower than β -SiC because of high impurities: SiO2 & free C Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

2.8 Whiskers dia: a few micron length: a few mm aspects ratio: 50`10,000 Rice Hull : cellulose, silica, organic and inorganic materials  Coking at 700C in inert or reducing atm. to drive out volatile compounds result in equal amount of SiO2 and free C  Heating coked rice hull between C for 1Hr to form SiC: 3C + SiO2 ⇒ SiC + 2CO  The residue is heated up to 800C to remove any free C: mixture of whiskers and particles

Fig.2.45a Schematic of SiC whisker production process starting from rice hulls Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Fig. 2.45b SEM micrograph of SiC whiskers obtained from rice hulls Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

SiO 2 (g)+ C(s)→SiO(g)+CO(g) Supersaturated sol. of C and Si in liquid catal. precipitates out SiC whiskers Tensile strength: 1.7~23.7GPa from 40 tests Length ~10mm dia:~5.9μm Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

6. Ceramic Fibers This fiber gives a buffer layer at the surface that allow fiber strength to be maintained even during the high- temperature incorporation into a metal matrix. Silicon carbide based ceramic fibers(SiC, Nicalon fibers) Silicon nitrideSi 3 N 4 ): reactive CVD using volatile silicon compounds mSiCl 4 + nNH 3→ k Si 3 N 4 Boron carbide(BC) Boron nitride(BN)

7. Comparison of fibers Advanced fibers exhibit Low density: Young ’ s modulus Tensile strength Direction of applied stress should be considered due to the anisotropy of fiber. Brittleness of fiber overcome by the combination with matrix materials

Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

7. Comparison of fibers Direction of applied stress should be considered due to the anisotropy of fiber. Brittleness of fiber overcome by the combination with matrix materials Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Applications of carbon fibers Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Applications of carbon fibers Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)

Applications of carbon fibers Krishan K. Chawla, Composite materials science and engineering, Springer-Verlag, (1998)