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Design, structure and properties of organomorphic composites as new materials
Author’s Accepted Manuscript Design, structure and properties of organomorphic composites as new materials E.A. Bogachev
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PII: DOI: Reference:
S0272-8842(18)32789-5 https://doi.org/10.1016/j.ceramint.2018.09.310 CERI19714
To appear in: Ceramics International Received date: 29 July 2018 Revised date: 30 September 2018 Accepted date: 30 September 2018 Cite this article as: E.A. Bogachev, Design, structure and properties of organomorphic composites as new materials, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.310 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, structure and properties of organomorphic composites as new materials E.A.Bogachev JSC Kompozit, 4 Pionerskaya, Korolev, Moscow region 141070, Russia E-mail address: [email protected]
Abstract Reinforcing basis of high-temperature composite materials conventionally consists of derivatives of multifilament fibers: fabrics, rods, plaited yarns, etc. Another example of an appropriate preform is a biomorphic preform (BP) obtained by pyrolysis of wood. On the one hand, organomorphic preforms (OP) are prepared based on filaments of organic precursors of carbon, silicon carbide, and carbonitride fibers - polyacrylonitrile, polycarbosilane, and polysilazane, respectively. On the other hand, nonoxidizing pyrolysis of densified bundles of such fibers is used as in the case of BP. Autohesion interaction occurring upon contact of compressed polymer filaments during the initial stage of pyrolysis inherits at compression kept up to OP transition into inorganic state. As a result, uniform dense (0.4-0.5 of ρfiber) strong preforms are formed with narrow and very small equivalent diameter of pores: from few to 3040 mm. Reinforcement in preforms can be uni-, bi- or three-directional. Subsequent densification of OP by vapor or liquid phase methods allows obtaining organomorphic composites (OC) with improved properties, some of which (gas permeability, surface roughness, capability of making articles with thickness of no more than 0.3 mm) are unattainable for existing high-temperature structural composites.
Keywords: precursors; organic fibers; diffusion; pyrolysis; organomorphic composites
1
1.Introduction
Reinforcement of high-temperature structural composites like C-C, C-SiC, SiCSiC traditionally consists of complex inorganic thread being made as bundles of several thousand continuous filaments. Fabrics, rods, and woven preforms with different reinforcement structures are formed based on such bundles of filaments. This approach makes it possible to reach thread strength sufficient for machine processing when creating a reinforcement structure. About twenty years ago, it was found that wood (a natural polymer) could serve as a precursor of a preform for high-temperature composites [1]. It was shown that biomorphic porous carbon preforms could possess rather high density (Table 1).
Table 1 Density of various wood species after pyrolysis at 1000 oC [2]. Wood species
Pine
Oak
Maple
Beech
Ebony
Density after pyrolysis,
0.31
0.50
0.51
0.55
0.87
3
g/cm
Widespread occurrence of biomorphic materials, especially for producing so called ecoceramics [2, 3], results from unique channel microstructure of wood as well as from presence in its chemical composition of components assuring coherency and strength of a preform after pyrolysis.
2
Itis fair to assume that along with complex yarn based preforms and biomorphic preforms, composite reinforcing systems combining the above-mentioned preforms in some principal issues could exist.
2.Materials and methods Polyacrylonitrile (PAN) fibers (-CH2-CH(CN)-)n Pyron®, nonwoven fabrics OKSIPAN® based on these fibers, synthesized polycarbosilane (PCS) fibers (-R1R2Si(C)nR3R4-)m, where R1-R4=Alk, Ar, H, and polysilazane (PS) fibers (-R1R2Si-NR3-)n, whereR1-R4=Alk, Ar, H, manufactured by JSC GNIIChTEOS (Russia) were used as raw materials. All fibers were previously thermally stabilized by oxidation at a temperature of 100-240oC (depending on fiber composition) which cross-linked macromolecules of polymer and converted them into infusible state. Structure of polymeric fibers after thermal stabilization was analyzed by SAXS and WAXS methods as well as by microtomography. Studies were conducted by means of Bruker D8 DISCOVER high-resolution diffractometer using CuKα radiation (at a wavelength of 1.54056 Å) in line-focus parallel beam mode as well as Skyscan-2011 nanotomograph under the following scanning parameters: U=50 kV, I=200 mА, rotation angle=0.3o, averaging over 3 frames. Spatial resolution was 0.25-0.36 mm/pixel. Pyrolysis of compressed bundles of filaments was performed at an applied load up to 0.1 MPa in a resistant furnace under inert atmosphere or under vacuum of 0.1 Pa at a temperature up to 1000-1250oC [4, 5]. To assure different reinforcement structures, fibers were put into a graphite container in mutually perpendicular directions and unidirectionally (1D) (Fig.1 a, b). To obtain carbon flat 2D- and cylindrical 3D-
3
preforms, non-woven fabrics were used too. The stacked layers of fabric were needle punched. In order to study strength of separate carbon filaments, they were withdrawn from unidirectional organomorphic PAN-based preform after pyrolysis. Filament strength were determined by means of Zwick/Roell 5 kN ProLine instrument in accordance with GOST 32667-2014 using 40 mm gage length at loading speed of 2 mm/min. Densification of preforms obtained after pyrolysis were performed by CVI- and LI-techniques. Densification with carbon was made from coal pitch and natural gas by thermal gradient method, while densification with silicon carbide was made using polycarbosilane manufactured by JSC GNIIChTEOS and monomethylsilane CH3SiH3(Perm Chemical Company, Ltd., Russia, monomethylsilane content is 99.93% vol., no chlorinated impurities) [6]. Porotech 3.2 instrument was used to study porosity of samples by the contact reference porometry method [7]. Roughness of polished surfaces was determined by means of profilometer-profilograph Portable Surface Roughness Tester-TR200. OC microstructure was investigated by optical and scanning electron microscopy using Altami MET 3C metallographic digital microscope and JEOL JSM-7001F и FEI Quanta 600 FEG scanning electron microscopes. Gas permeability of organomorphic C/C composites was determined by air using Darcy law form for laminar flow for cylindrical samples (3 mm in width, 50 mm in diameter) at original facility JSC Kompozit.
3.Theory
4
As far as is known, organic precursors of filaments used in high-temperature composites are polymeric fibers: polyacrylonitrile - for carbon filaments, polycarbosilane - for silicon carbide filaments, polysilazane -for carbonitride filaments. As a rule, they are pyrolyzed in a complex yarn assuring conditions required forpyrolysis (overheating control, occurrence of tension, etc.). At the same time, filaments by themselves can be classified with good reason as dispersed bodies. Even though their length is by many orders of magnitude larger than diameter, the latter makes only 20-40 mm. It is commonly believed that stability of dispersed objects in a limited volume is subject to the Coulomb-Mohr rule: = + ∙
н
,
where C- autohesion of polymers; is a tangent of the internal friction angle; !
is the normal pressure.
According to the autohesion theory developed by the soviet scientist S.S.Voyutsky more than half century ago, polymers of the same composition show selfdiffusion ability upon contact in thin films [8]. At that, lateral fragments of polymer macromolecules diffuse into each other. It was shown [9] that if two samples of the same amorphous polymer would be engaged to contact at a temperature over glass transition point, the contact surface would gradually become in distinguishable from the bulk polymer. Herewith, the number of joining bridges depends both on contacting time and molecular weight of polymer chains. The most important factors influencing the autohesion bond strength are [8]: ·
temperature (it grows with increase in temperature);
5
·
pressure (it grows with increase in pressure);
·
contact time (it grows with increase in the contact time);
·
surface imperfection (promotes autohesion growth).
Thus, during the initial stage of pyrolysis the precompressed fibers meet a considerable thermomechanical impact at places of contact which promotes autohesion.
4.Results and discussion Investigation of PAN-, PCS- and PS-fibers using wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques points to mainly amorphous structure of all fibers under study (Fig.2 a, b). This structure was formed after thermostabilyzing treatment due to destruction of the initial paracrystalline fiber's structure, formation of cyclic fragments and intermolecular cross-links [10]. Only PAN-fiber exhibits a wide and weak maximum at 2θ of near 0.8o (see Fig.2 b) corresponding to the average typical scale of nonuniformities along the fiber of near 10 nm. These conclusions concerning structure of fibers are proved by microtomography results (Fig.3 a, b, c). Whereas PAN-filaments possess to some extent ordered structure with compacted surface layer and more in compact volume, PCS-and PS-filaments are completely amorphous with respect to supermolecular structure. Such structure is favorable for diffusion because amorphous bodies due to non-equilibrium structural state possess larger specific volume, entropy and intrinsic energy than paracrystals. Meanwhile, it is not clear how to determine quantitatively an autohesion level in preforms consisting of polymer fibers pressed to each other. This is a difficult task due
6
to the small size of organic fibers and uncertainty of total surface area of a contact leading to autohesion. However, its occurrence in OP was confirmed experimentally. This is evidenced by a considerable strength obtained without using any binder (Fig.3). Approximate mechanism of formation and retention of diffusion cross-linking of preforms during pyrolysis consists in the following (Fig.4). When exposed to pressure and holding for several hours, the diffusion contact starts forming as early as from 180-200oC. Diffusion factor value required for crosslinking was estimated to be ca. 10-10 cm2/s. This value is pretty accessible in polymers. Due to constant suppression, contacts between fibers remain over the whole period of pyrolysis despite a considerable shrinkage of fibers with respect to both diameter (for PAN-fibers - from 20 to 10 mm, for PCS-fibers - from 40 to 30 mm) and length (for PAN-preforms - by 8-9%, for PCS-preforms - by 25-30%). The microstructural analysis also points to the presence of diffusion cross-linking between filaments in a carbonized pressed PAN-preform (Fig.5). It is observed the very large scatter in the measured strength data of carbon fibers removed from unidirectional preforms of pyrolyzed PAN: 1.9±1.2 GPa. It looks like an indirect proof of formation and retention of autohesion contact during pyrolysis.
7
It may be assumed that strength of fibers correlates in some way with their involvement into autohesion contact. It is highly likely that breakage of autohesion contact during extraction of fibers from a preform introduces defectivity into their structure and leads to decrease their strength. However, along with extremely low values, strength of some filaments was found to be rather high and close to the level for commercially available fibers. Pyrolysis behavior of two kinds of Pyron®-fibers (curled nonwoven staple and straight fibers put unidirectionally into the mold) was compared at the same conditions (Fig. 6). It was found that density values of the preforms were determined to be 0.2 g/cm3 (from staple) and 0.8 g/cm3 (from straight fibers) at the same applied pressure of 0.1 MPa. This can be attributed to the fact that flexural deformation of curled nonwoven fibers demands much higher forcing to reach the required density and, hence, there quired number of autohesion contacts. During densification of a preform from straight fibers deformation by flexible mechanism practically does not occur. Comparison of properties of BP and OP (weight loss during pyrolysis, density, shrinkage, porosity, average pore diameter – see Table 2) points to considerable advantages of the organomorphic preforms: - open porosity of BP is accompanied by very large fraction of closed porosity, whereas in organomorphic carbon preforms even at higher density all the porosity is open and its value is much higher; - pyrolysis of BP is accompanied by tremendous shrinkage (above 20%). Shrinkage of OP is usually much lower;
8
- chemical composition of OP may be much more multivarious comparing to the same of BP from wood.
Table 2 Comparison of properties of BP [2] and OP. Characteristics of
Oak
Maple
Beech
Ebony
biomorphic or
PAN-
PCS-
fibers
fibers
organomorphic reinforcing basis Weight loss during
70
75
74
65
45
20
26
30
31
23
12
25
0.50
0.51
0.55
0.87
0.6
1.0
30/40
43/22
42/21
23/20
64/0
53/0
pyrolysis, % Shrinkage during pyrolysis, % Density, g/cm
3
Porosity, % (open/closed)
Schematic diagram for preparation of organomorphic composites based on OP is depicted in Fig.7. The research results of microstructure (Fig.8) and porosity (Fig.9) for PANbased OP in the form of 2D-plate indicate a high structural uniformity: whereas open porosity of the preform is equal to 64%, its density amounts to 0.59 g/cm3 (ca. 0.4 of the
9
carbon fiber density).The equivalent diameter of the main fraction of pores (above 4/5) falls within the range 4-35 mm. Small and uniform pore allows efficient liquid-phase densifing organomorphic preforms and minimizing the number of impregnation cycles when conducting LIprocesses. The melt pitch wetting the surface is retained in preform pores by excessive bubble pressure: ∆# =
$% &
, where
Δp - is additional pressure under the warped surface of liquid; σ - is the surface tension; r- pore radius. After both gas-phase and liquid-phase densification, distances between fibers remain the same or even decrease due to shrinkage of the material of densifying matrix (Fig.10). High values of open porosity allow applying successfully both CVI- and LImethods for OP densification. OC-samples show outstanding capabilities to ensure low and uniform roughness of surface (Fig.11). The strength-related properties of OC considerably exceed characteristics of isographite even though these materials possess high anisotropy in reinforcement directions (Table 3).
Table 3 Properties of OC with different chemical composition and reinforcement structure.
10
Material properties
Isographite SED-70
Density, g/cm
3
Darcy coefficient
C/C
SiC/SiC
3D. Matrix:
2D. Matrix:
1 D. Matrix:
pyrocarbon
pitch coke
CVI-SiC
1.85
1.75-1.80
1.90
2.20
N/A
-
N/A
150 (R)
-19
8.5·10
N/A*
2
(p.d. of 5 bar), m
Tensile strength, MPa
>70
N/A*
280
> 150
N/A*
Compression strength, 189
180-240 (R)
180-230
N/A*
MPa (Z)
330-390 (Z)
15-20(Z) Bending strength,
78
MPa
Grain size, mm
20
20-30
20-30
30-40
Thermal expansion
5.6
0-1.1, from
(-0.5-2.0),
N/A*
50° to 400°С
from 20° to
-6
-1
coefficient ×10 , K
1900°С *Data are in working As may be seen from the presented data, material of a bushing made from OC of C/C (see Fig.11) shows tensile strength of 150 MPa in radial direction. This value is by
11
many times higher than the same of isographite. Herewith, gas tightness of the material is close to permeability of metals - 8.10-19 m2 (Fig.12). Application range of organomorphic composites seems to be unlimited: from the thinnest (0.3 mm and higher) electrodes of ion engines with surface roughness at least Ra 0.8 to optical mirrors of space telescopes; from hot section elements of aircraft engines to hot pressing OC-molds instead of isographite ones.
5.Conclusions First results on preparation and investigation of OC are indicative of important distinctions of these composites from existing composite materials for high-temperature applications. Reinforcing basis with bonding assured not by mechanical braiding of complex yarns or pyrolyzed coke but by diffusion cross-linking (autоhesion) of fibers at polymer stage assumes extremely small distance between filaments. This peculiarity leads to unusual properties of OC such as low and uniform surface roughness as well as gas permeability close to the metallic level. Thermostabilized filaments of polyacrylonitrile (carbon fiber precursor), polycarbosilane (silicon carbide fiber precursor), polysilazane (silicon carbonitride fiber precursor) can be used as raw materials for preparation of OC-reinforcing systems. Consolidation of filaments into a reinforcing preform takes place at the initial stage of pyrolysis (at heating to 200-300oC). At these temperatures autohesion interaction of polymeric fibers occurs accompanied by interpenetration of macromolecule segments into micropores at contact points. This diffusion cross-linking inherits when keeping compression conditions over the whole period of pyrolysis. It seems that there are no
12
limitations to use some other polymers with high coking residue (e.g. rayon yarn) as raw materials for organomorphic preforms. Pressing of nonwoven raw material consisting of stapled fibers with curled structure requires pressing force which is much larger than that of straight fibers. It may be explained by flexible mechanism of fiber deformation under pressing: for unidirectional filaments compressed perpendicular to layout flexural deformation is close to zero; on the contrary, for curled nonwoven fibers it is very high. Studies of microstructure and porosity of organomorphic preforms point to very small (30-40 mm in diameter) pores between fibers with uniform pore volume distribution. This dictates high and uniform roughness of OC (Ra 0.8 and better) as well as Darcy coefficient at a level of permeability of metals. Comparison of properties of BP and OP (weight loss during pyrolysis, density, shrinkage, porosity, average pore diameter, open porosity) points to considerable advantages of OP over BP, thus opening favorable application prospects for OC as compared to BC. Another advantage of organomorphic composites is wide variation of their composition. Due to very small and uniform structural cell, properties of carbon-carbon OC are at a level of the best grades of isographite and many commercial carbon-carbon composites. Radial reinforced cylindrical drum shells can be a substitute for isographite in hot pressing molds because their tensile strength exceeds the strength of isographite by several times.
6.Acknowledgment
13
The author extends his great appreciation to the colleagues for their contribution into the research, development and preparation of OC: the post graduate student of MRTU A.B.Elakov, the principal researcher of the Institute of chemical physics of RAS professor R.A.Turusov, CEO of Niagara Ltd. A.P.Beloglazov, the director of the SaintPetersburg state university Centre for X-ray diffraction studies O.S.Grunsky.
7.References [1] C. E. Byrne, D. C. Naglee, Carbonization of wood for advanced materials applications, Carbon 35 (1997) 259-266. [2] J. Ramirez-Rico, J. Martinez-Fernandez, M. Singh, Biomorphic ceramics from wood-derived precursors, International Materials Reviews 62 (2017) 465-485. [3] M. Singh, J. Martinez-Fernandez, A.R. de Arellano-Lopez, Environmentally conscious ceramics (ecoceramics) from natural wood precursors, Current Opinion in Solid State and Materials Science 7 (2003) 247-254. [4] E.A. Bogachev, Method of manufacturing of porous preform - basis of a composite material, Patent application RF №2018103144 dated 29.01.2018. [5] E.A. Bogachev, A.B. Yelakov, A.P. Beloglazov, Yu.A. Denisov, A.N. Timofeev. Method of manufacturing of porous preform - basis of a composite material, Patent RF №2620810 dated 29.05.2017. [6] E.A. Bogachev, A.V. Lakhin, A.N. Timofeev, MMS-technology: first results and prospects, Ceramic transactions 248 (2014) 243-253. [7] E.V. Kogan, Yu.M. Volfkovich, V.V. Kulakov, A.M. Kenigfest, V.V. Avdeev, V.E. Sosenkin, N.F. Nikol’skaya, Porous structure of carbon–carbon friction composites
14
studiedby gas adsorption and standard contact porosimetry techniques, Inorganic Materials 48 (2012) 676–679. [8] S.S. Voyutskii, Autohesion and adhesion of high polymers, WileyInterscience, New York, 1963. [9] S. Prager, M. Tirrell,The healing process at polymer–polymer interfaces, The Journal of Chemical Physics 75 (1981) 5194. [10] I.P. Dobrovol'skaya, Pyrolysis of oriented polymers. Structure and properties
of carbon fibers, Dissertation (in Russian). - Saint-Petersburg (2007) 309.
Fig.1. Stages for formation of silicon carbide OP: a – placement of unidirectional PCSfilaments in a graphite container; b – SiC-preform after pyrolysis Fig.2 WAXS (a) and SAXS (b) results for PAN-, PCS- and PS-fibers Fig.3. 3D-microtomography of PAN- (a), PCS- (b) and PS- (c) fibers Fig.4. Mechanism of OP formation Fig.5. Examples of diffusion cross-linking (autohesion) of filaments in an organomorphic PAN- preform observed after its pyrolysis Fig.6. Appearance after pyrolysis of curled nonwoven staple (top) and straight fibers put unidirectionally into the mold (bottom) Fig.7. Schematic diagram for preparation of OC based on OP Fig.8 Microstructure of organomorphic carbon plate preform Fig.9. Integral (a) and differential (b) pore volume distribution vs. logarithm of pore equivalent radius into the organomorphic carbon preform Fig.10. Microstructure of organomorphic composites with different chemical composition and reinforcement structure: a – 2D C/C composite; b – 2D C/SiC composite; c – 1D SiC/SiC composite 15
Fig. 11. Roughness Ra of 3D C/C organomorphic composite Fig.12. Air pressure drop through 2D OC C/C sample vs. time at various initial outside air pressure values for every sample
16
www.elsevier.com/locate/ceri
PII: DOI: Reference:
S0272-8842(18)32789-5 https://doi.org/10.1016/j.ceramint.2018.09.310 CERI19714
To appear in: Ceramics International Received date: 29 July 2018 Revised date: 30 September 2018 Accepted date: 30 September 2018 Cite this article as: E.A. Bogachev, Design, structure and properties of organomorphic composites as new materials, Ceramics International, https://doi.org/10.1016/j.ceramint.2018.09.310 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Design, structure and properties of organomorphic composites as new materials E.A.Bogachev JSC Kompozit, 4 Pionerskaya, Korolev, Moscow region 141070, Russia E-mail address: [email protected]
Abstract Reinforcing basis of high-temperature composite materials conventionally consists of derivatives of multifilament fibers: fabrics, rods, plaited yarns, etc. Another example of an appropriate preform is a biomorphic preform (BP) obtained by pyrolysis of wood. On the one hand, organomorphic preforms (OP) are prepared based on filaments of organic precursors of carbon, silicon carbide, and carbonitride fibers - polyacrylonitrile, polycarbosilane, and polysilazane, respectively. On the other hand, nonoxidizing pyrolysis of densified bundles of such fibers is used as in the case of BP. Autohesion interaction occurring upon contact of compressed polymer filaments during the initial stage of pyrolysis inherits at compression kept up to OP transition into inorganic state. As a result, uniform dense (0.4-0.5 of ρfiber) strong preforms are formed with narrow and very small equivalent diameter of pores: from few to 3040 mm. Reinforcement in preforms can be uni-, bi- or three-directional. Subsequent densification of OP by vapor or liquid phase methods allows obtaining organomorphic composites (OC) with improved properties, some of which (gas permeability, surface roughness, capability of making articles with thickness of no more than 0.3 mm) are unattainable for existing high-temperature structural composites.
Keywords: precursors; organic fibers; diffusion; pyrolysis; organomorphic composites
1
1.Introduction
Reinforcement of high-temperature structural composites like C-C, C-SiC, SiCSiC traditionally consists of complex inorganic thread being made as bundles of several thousand continuous filaments. Fabrics, rods, and woven preforms with different reinforcement structures are formed based on such bundles of filaments. This approach makes it possible to reach thread strength sufficient for machine processing when creating a reinforcement structure. About twenty years ago, it was found that wood (a natural polymer) could serve as a precursor of a preform for high-temperature composites [1]. It was shown that biomorphic porous carbon preforms could possess rather high density (Table 1).
Table 1 Density of various wood species after pyrolysis at 1000 oC [2]. Wood species
Pine
Oak
Maple
Beech
Ebony
Density after pyrolysis,
0.31
0.50
0.51
0.55
0.87
3
g/cm
Widespread occurrence of biomorphic materials, especially for producing so called ecoceramics [2, 3], results from unique channel microstructure of wood as well as from presence in its chemical composition of components assuring coherency and strength of a preform after pyrolysis.
2
Itis fair to assume that along with complex yarn based preforms and biomorphic preforms, composite reinforcing systems combining the above-mentioned preforms in some principal issues could exist.
2.Materials and methods Polyacrylonitrile (PAN) fibers (-CH2-CH(CN)-)n Pyron®, nonwoven fabrics OKSIPAN® based on these fibers, synthesized polycarbosilane (PCS) fibers (-R1R2Si(C)nR3R4-)m, where R1-R4=Alk, Ar, H, and polysilazane (PS) fibers (-R1R2Si-NR3-)n, whereR1-R4=Alk, Ar, H, manufactured by JSC GNIIChTEOS (Russia) were used as raw materials. All fibers were previously thermally stabilized by oxidation at a temperature of 100-240oC (depending on fiber composition) which cross-linked macromolecules of polymer and converted them into infusible state. Structure of polymeric fibers after thermal stabilization was analyzed by SAXS and WAXS methods as well as by microtomography. Studies were conducted by means of Bruker D8 DISCOVER high-resolution diffractometer using CuKα radiation (at a wavelength of 1.54056 Å) in line-focus parallel beam mode as well as Skyscan-2011 nanotomograph under the following scanning parameters: U=50 kV, I=200 mА, rotation angle=0.3o, averaging over 3 frames. Spatial resolution was 0.25-0.36 mm/pixel. Pyrolysis of compressed bundles of filaments was performed at an applied load up to 0.1 MPa in a resistant furnace under inert atmosphere or under vacuum of 0.1 Pa at a temperature up to 1000-1250oC [4, 5]. To assure different reinforcement structures, fibers were put into a graphite container in mutually perpendicular directions and unidirectionally (1D) (Fig.1 a, b). To obtain carbon flat 2D- and cylindrical 3D-
3
preforms, non-woven fabrics were used too. The stacked layers of fabric were needle punched. In order to study strength of separate carbon filaments, they were withdrawn from unidirectional organomorphic PAN-based preform after pyrolysis. Filament strength were determined by means of Zwick/Roell 5 kN ProLine instrument in accordance with GOST 32667-2014 using 40 mm gage length at loading speed of 2 mm/min. Densification of preforms obtained after pyrolysis were performed by CVI- and LI-techniques. Densification with carbon was made from coal pitch and natural gas by thermal gradient method, while densification with silicon carbide was made using polycarbosilane manufactured by JSC GNIIChTEOS and monomethylsilane CH3SiH3(Perm Chemical Company, Ltd., Russia, monomethylsilane content is 99.93% vol., no chlorinated impurities) [6]. Porotech 3.2 instrument was used to study porosity of samples by the contact reference porometry method [7]. Roughness of polished surfaces was determined by means of profilometer-profilograph Portable Surface Roughness Tester-TR200. OC microstructure was investigated by optical and scanning electron microscopy using Altami MET 3C metallographic digital microscope and JEOL JSM-7001F и FEI Quanta 600 FEG scanning electron microscopes. Gas permeability of organomorphic C/C composites was determined by air using Darcy law form for laminar flow for cylindrical samples (3 mm in width, 50 mm in diameter) at original facility JSC Kompozit.
3.Theory
4
As far as is known, organic precursors of filaments used in high-temperature composites are polymeric fibers: polyacrylonitrile - for carbon filaments, polycarbosilane - for silicon carbide filaments, polysilazane -for carbonitride filaments. As a rule, they are pyrolyzed in a complex yarn assuring conditions required forpyrolysis (overheating control, occurrence of tension, etc.). At the same time, filaments by themselves can be classified with good reason as dispersed bodies. Even though their length is by many orders of magnitude larger than diameter, the latter makes only 20-40 mm. It is commonly believed that stability of dispersed objects in a limited volume is subject to the Coulomb-Mohr rule: = + ∙
н
,
where C- autohesion of polymers; is a tangent of the internal friction angle; !
is the normal pressure.
According to the autohesion theory developed by the soviet scientist S.S.Voyutsky more than half century ago, polymers of the same composition show selfdiffusion ability upon contact in thin films [8]. At that, lateral fragments of polymer macromolecules diffuse into each other. It was shown [9] that if two samples of the same amorphous polymer would be engaged to contact at a temperature over glass transition point, the contact surface would gradually become in distinguishable from the bulk polymer. Herewith, the number of joining bridges depends both on contacting time and molecular weight of polymer chains. The most important factors influencing the autohesion bond strength are [8]: ·
temperature (it grows with increase in temperature);
5
·
pressure (it grows with increase in pressure);
·
contact time (it grows with increase in the contact time);
·
surface imperfection (promotes autohesion growth).
Thus, during the initial stage of pyrolysis the precompressed fibers meet a considerable thermomechanical impact at places of contact which promotes autohesion.
4.Results and discussion Investigation of PAN-, PCS- and PS-fibers using wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering (SAXS) techniques points to mainly amorphous structure of all fibers under study (Fig.2 a, b). This structure was formed after thermostabilyzing treatment due to destruction of the initial paracrystalline fiber's structure, formation of cyclic fragments and intermolecular cross-links [10]. Only PAN-fiber exhibits a wide and weak maximum at 2θ of near 0.8o (see Fig.2 b) corresponding to the average typical scale of nonuniformities along the fiber of near 10 nm. These conclusions concerning structure of fibers are proved by microtomography results (Fig.3 a, b, c). Whereas PAN-filaments possess to some extent ordered structure with compacted surface layer and more in compact volume, PCS-and PS-filaments are completely amorphous with respect to supermolecular structure. Such structure is favorable for diffusion because amorphous bodies due to non-equilibrium structural state possess larger specific volume, entropy and intrinsic energy than paracrystals. Meanwhile, it is not clear how to determine quantitatively an autohesion level in preforms consisting of polymer fibers pressed to each other. This is a difficult task due
6
to the small size of organic fibers and uncertainty of total surface area of a contact leading to autohesion. However, its occurrence in OP was confirmed experimentally. This is evidenced by a considerable strength obtained without using any binder (Fig.3). Approximate mechanism of formation and retention of diffusion cross-linking of preforms during pyrolysis consists in the following (Fig.4). When exposed to pressure and holding for several hours, the diffusion contact starts forming as early as from 180-200oC. Diffusion factor value required for crosslinking was estimated to be ca. 10-10 cm2/s. This value is pretty accessible in polymers. Due to constant suppression, contacts between fibers remain over the whole period of pyrolysis despite a considerable shrinkage of fibers with respect to both diameter (for PAN-fibers - from 20 to 10 mm, for PCS-fibers - from 40 to 30 mm) and length (for PAN-preforms - by 8-9%, for PCS-preforms - by 25-30%). The microstructural analysis also points to the presence of diffusion cross-linking between filaments in a carbonized pressed PAN-preform (Fig.5). It is observed the very large scatter in the measured strength data of carbon fibers removed from unidirectional preforms of pyrolyzed PAN: 1.9±1.2 GPa. It looks like an indirect proof of formation and retention of autohesion contact during pyrolysis.
7
It may be assumed that strength of fibers correlates in some way with their involvement into autohesion contact. It is highly likely that breakage of autohesion contact during extraction of fibers from a preform introduces defectivity into their structure and leads to decrease their strength. However, along with extremely low values, strength of some filaments was found to be rather high and close to the level for commercially available fibers. Pyrolysis behavior of two kinds of Pyron®-fibers (curled nonwoven staple and straight fibers put unidirectionally into the mold) was compared at the same conditions (Fig. 6). It was found that density values of the preforms were determined to be 0.2 g/cm3 (from staple) and 0.8 g/cm3 (from straight fibers) at the same applied pressure of 0.1 MPa. This can be attributed to the fact that flexural deformation of curled nonwoven fibers demands much higher forcing to reach the required density and, hence, there quired number of autohesion contacts. During densification of a preform from straight fibers deformation by flexible mechanism practically does not occur. Comparison of properties of BP and OP (weight loss during pyrolysis, density, shrinkage, porosity, average pore diameter – see Table 2) points to considerable advantages of the organomorphic preforms: - open porosity of BP is accompanied by very large fraction of closed porosity, whereas in organomorphic carbon preforms even at higher density all the porosity is open and its value is much higher; - pyrolysis of BP is accompanied by tremendous shrinkage (above 20%). Shrinkage of OP is usually much lower;
8
- chemical composition of OP may be much more multivarious comparing to the same of BP from wood.
Table 2 Comparison of properties of BP [2] and OP. Characteristics of
Oak
Maple
Beech
Ebony
biomorphic or
PAN-
PCS-
fibers
fibers
organomorphic reinforcing basis Weight loss during
70
75
74
65
45
20
26
30
31
23
12
25
0.50
0.51
0.55
0.87
0.6
1.0
30/40
43/22
42/21
23/20
64/0
53/0
pyrolysis, % Shrinkage during pyrolysis, % Density, g/cm
3
Porosity, % (open/closed)
Schematic diagram for preparation of organomorphic composites based on OP is depicted in Fig.7. The research results of microstructure (Fig.8) and porosity (Fig.9) for PANbased OP in the form of 2D-plate indicate a high structural uniformity: whereas open porosity of the preform is equal to 64%, its density amounts to 0.59 g/cm3 (ca. 0.4 of the
9
carbon fiber density).The equivalent diameter of the main fraction of pores (above 4/5) falls within the range 4-35 mm. Small and uniform pore allows efficient liquid-phase densifing organomorphic preforms and minimizing the number of impregnation cycles when conducting LIprocesses. The melt pitch wetting the surface is retained in preform pores by excessive bubble pressure: ∆# =
$% &
, where
Δp - is additional pressure under the warped surface of liquid; σ - is the surface tension; r- pore radius. After both gas-phase and liquid-phase densification, distances between fibers remain the same or even decrease due to shrinkage of the material of densifying matrix (Fig.10). High values of open porosity allow applying successfully both CVI- and LImethods for OP densification. OC-samples show outstanding capabilities to ensure low and uniform roughness of surface (Fig.11). The strength-related properties of OC considerably exceed characteristics of isographite even though these materials possess high anisotropy in reinforcement directions (Table 3).
Table 3 Properties of OC with different chemical composition and reinforcement structure.
10
Material properties
Isographite SED-70
Density, g/cm
3
Darcy coefficient
C/C
SiC/SiC
3D. Matrix:
2D. Matrix:
1 D. Matrix:
pyrocarbon
pitch coke
CVI-SiC
1.85
1.75-1.80
1.90
2.20
N/A
-
N/A
150 (R)
-19
8.5·10
N/A*
2
(p.d. of 5 bar), m
Tensile strength, MPa
>70
N/A*
280
> 150
N/A*
Compression strength, 189
180-240 (R)
180-230
N/A*
MPa (Z)
330-390 (Z)
15-20(Z) Bending strength,
78
MPa
Grain size, mm
20
20-30
20-30
30-40
Thermal expansion
5.6
0-1.1, from
(-0.5-2.0),
N/A*
50° to 400°С
from 20° to
-6
-1
coefficient ×10 , K
1900°С *Data are in working As may be seen from the presented data, material of a bushing made from OC of C/C (see Fig.11) shows tensile strength of 150 MPa in radial direction. This value is by
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many times higher than the same of isographite. Herewith, gas tightness of the material is close to permeability of metals - 8.10-19 m2 (Fig.12). Application range of organomorphic composites seems to be unlimited: from the thinnest (0.3 mm and higher) electrodes of ion engines with surface roughness at least Ra 0.8 to optical mirrors of space telescopes; from hot section elements of aircraft engines to hot pressing OC-molds instead of isographite ones.
5.Conclusions First results on preparation and investigation of OC are indicative of important distinctions of these composites from existing composite materials for high-temperature applications. Reinforcing basis with bonding assured not by mechanical braiding of complex yarns or pyrolyzed coke but by diffusion cross-linking (autоhesion) of fibers at polymer stage assumes extremely small distance between filaments. This peculiarity leads to unusual properties of OC such as low and uniform surface roughness as well as gas permeability close to the metallic level. Thermostabilized filaments of polyacrylonitrile (carbon fiber precursor), polycarbosilane (silicon carbide fiber precursor), polysilazane (silicon carbonitride fiber precursor) can be used as raw materials for preparation of OC-reinforcing systems. Consolidation of filaments into a reinforcing preform takes place at the initial stage of pyrolysis (at heating to 200-300oC). At these temperatures autohesion interaction of polymeric fibers occurs accompanied by interpenetration of macromolecule segments into micropores at contact points. This diffusion cross-linking inherits when keeping compression conditions over the whole period of pyrolysis. It seems that there are no
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limitations to use some other polymers with high coking residue (e.g. rayon yarn) as raw materials for organomorphic preforms. Pressing of nonwoven raw material consisting of stapled fibers with curled structure requires pressing force which is much larger than that of straight fibers. It may be explained by flexible mechanism of fiber deformation under pressing: for unidirectional filaments compressed perpendicular to layout flexural deformation is close to zero; on the contrary, for curled nonwoven fibers it is very high. Studies of microstructure and porosity of organomorphic preforms point to very small (30-40 mm in diameter) pores between fibers with uniform pore volume distribution. This dictates high and uniform roughness of OC (Ra 0.8 and better) as well as Darcy coefficient at a level of permeability of metals. Comparison of properties of BP and OP (weight loss during pyrolysis, density, shrinkage, porosity, average pore diameter, open porosity) points to considerable advantages of OP over BP, thus opening favorable application prospects for OC as compared to BC. Another advantage of organomorphic composites is wide variation of their composition. Due to very small and uniform structural cell, properties of carbon-carbon OC are at a level of the best grades of isographite and many commercial carbon-carbon composites. Radial reinforced cylindrical drum shells can be a substitute for isographite in hot pressing molds because their tensile strength exceeds the strength of isographite by several times.
6.Acknowledgment
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The author extends his great appreciation to the colleagues for their contribution into the research, development and preparation of OC: the post graduate student of MRTU A.B.Elakov, the principal researcher of the Institute of chemical physics of RAS professor R.A.Turusov, CEO of Niagara Ltd. A.P.Beloglazov, the director of the SaintPetersburg state university Centre for X-ray diffraction studies O.S.Grunsky.
7.References [1] C. E. Byrne, D. C. Naglee, Carbonization of wood for advanced materials applications, Carbon 35 (1997) 259-266. [2] J. Ramirez-Rico, J. Martinez-Fernandez, M. Singh, Biomorphic ceramics from wood-derived precursors, International Materials Reviews 62 (2017) 465-485. [3] M. Singh, J. Martinez-Fernandez, A.R. de Arellano-Lopez, Environmentally conscious ceramics (ecoceramics) from natural wood precursors, Current Opinion in Solid State and Materials Science 7 (2003) 247-254. [4] E.A. Bogachev, Method of manufacturing of porous preform - basis of a composite material, Patent application RF №2018103144 dated 29.01.2018. [5] E.A. Bogachev, A.B. Yelakov, A.P. Beloglazov, Yu.A. Denisov, A.N. Timofeev. Method of manufacturing of porous preform - basis of a composite material, Patent RF №2620810 dated 29.05.2017. [6] E.A. Bogachev, A.V. Lakhin, A.N. Timofeev, MMS-technology: first results and prospects, Ceramic transactions 248 (2014) 243-253. [7] E.V. Kogan, Yu.M. Volfkovich, V.V. Kulakov, A.M. Kenigfest, V.V. Avdeev, V.E. Sosenkin, N.F. Nikol’skaya, Porous structure of carbon–carbon friction composites
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studiedby gas adsorption and standard contact porosimetry techniques, Inorganic Materials 48 (2012) 676–679. [8] S.S. Voyutskii, Autohesion and adhesion of high polymers, WileyInterscience, New York, 1963. [9] S. Prager, M. Tirrell,The healing process at polymer–polymer interfaces, The Journal of Chemical Physics 75 (1981) 5194. [10] I.P. Dobrovol'skaya, Pyrolysis of oriented polymers. Structure and properties
of carbon fibers, Dissertation (in Russian). - Saint-Petersburg (2007) 309.
Fig.1. Stages for formation of silicon carbide OP: a – placement of unidirectional PCSfilaments in a graphite container; b – SiC-preform after pyrolysis Fig.2 WAXS (a) and SAXS (b) results for PAN-, PCS- and PS-fibers Fig.3. 3D-microtomography of PAN- (a), PCS- (b) and PS- (c) fibers Fig.4. Mechanism of OP formation Fig.5. Examples of diffusion cross-linking (autohesion) of filaments in an organomorphic PAN- preform observed after its pyrolysis Fig.6. Appearance after pyrolysis of curled nonwoven staple (top) and straight fibers put unidirectionally into the mold (bottom) Fig.7. Schematic diagram for preparation of OC based on OP Fig.8 Microstructure of organomorphic carbon plate preform Fig.9. Integral (a) and differential (b) pore volume distribution vs. logarithm of pore equivalent radius into the organomorphic carbon preform Fig.10. Microstructure of organomorphic composites with different chemical composition and reinforcement structure: a – 2D C/C composite; b – 2D C/SiC composite; c – 1D SiC/SiC composite 15
Fig. 11. Roughness Ra of 3D C/C organomorphic composite Fig.12. Air pressure drop through 2D OC C/C sample vs. time at various initial outside air pressure values for every sample
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