8 STRUCTURE OF PYROCARBONS X.Bourrat 1 Introduction Pyrocarbon is the solid form of carbon deposited on a hot surface by cracking of gaseous or liquid hydrocarbons.Pyrocarbon is made up with small or extended graphene layers of sp2 hybridized carbons more or less saturated with hydrogen.But what makes his a unique char- acter is that these graphene layers can be piled in a high anisotropic way along the deposit surface.The anisotropy of the texture and the density are the key parameters characterizing this material with also the size distribution of the layers and the hydrogen content. Three main events have boosted researches in the second half of the twentieth century: the discovery of carbon/carbon composites at the end of the fifties and their application as strategic material (Lamicq,1984;Buckley,1993).Pyrocarbon is the major matrix material. Then pyrocarbons were developed to be used as coating in nuclear fuel industry in the sixties for which the fluidized bed processing was widely developed,that is the chemical vapor deposition(CVD)of high temperature pyrocarbons(Bokros,1969;Lefevre and Price, 1977).Finally,the strong development of carbon brakes in the last two decades of the twentieth century has focused the interest on infiltration of low temperature pyrocarbon (chemical vapor infiltration,CVD).Many other applications are existing.But the expansion of the carbon/carbon brakes,market the necessity to lower the prices (for fast train and truck markets)and the emergence of new comers has intensified the research all other the world at the turn of the century. Nowadays the main issue is a fine control of structure with more and more accuracy.The main interest is focused on low temperature pyrocarbon for CVI with the highest deposit rate at lowest price.Local probes are needed to measure the main properties as,e.g.the elas- tic properties,thermal conductivity and naturally nanostructure parameters,i.e.anisotropy, density,graphene structure or closed porosity,etc. First,this chapter will rationalize the very different pyrocarbon types following the two major transitions:low and high temperature transitions.Then in the next part,the recent efforts achieved to relate the structure (and texture)to the processing conditions in the case of low temperature pyrocarbons will be documented:carbon layer,cones and regenerative features.Finally,is a review of the growth mechanisms in relation with structure develop- ment and various approaches to quantify the structural parameters:density and anisotropy. 2 The various pyrocarbons Bokros (1965,1969)introduced a first comprehensive distinction among pyrocarbons by means of optical microscopy.At that time four structural groups were distinguished ©2003 Taylor&Francis
8 STRUCTURE OF PYROCARBONS X. Bourrat 1 Introduction Pyrocarbon is the solid form of carbon deposited on a hot surface by cracking of gaseous or liquid hydrocarbons. Pyrocarbon is made up with small or extended graphene layers of sp2 hybridized carbons more or less saturated with hydrogen. But what makes his a unique character is that these graphene layers can be piled in a high anisotropic way along the deposit surface. The anisotropy of the texture and the density are the key parameters characterizing this material with also the size distribution of the layers and the hydrogen content. Three main events have boosted researches in the second half of the twentieth century: the discovery of carbon/carbon composites at the end of the fifties and their application as strategic material (Lamicq, 1984; Buckley, 1993). Pyrocarbon is the major matrix material. Then pyrocarbons were developed to be used as coating in nuclear fuel industry in the sixties for which the fluidized bed processing was widely developed, that is the chemical vapor deposition (CVD) of high temperature pyrocarbons (Bokros, 1969; Lefevre and Price, 1977). Finally, the strong development of carbon brakes in the last two decades of the twentieth century has focused the interest on infiltration of low temperature pyrocarbon (chemical vapor infiltration, CVI). Many other applications are existing. But the expansion of the carbon/carbon brakes, market the necessity to lower the prices (for fast train and truck markets) and the emergence of new comers has intensified the research all other the world at the turn of the century. Nowadays the main issue is a fine control of structure with more and more accuracy. The main interest is focused on low temperature pyrocarbon for CVI with the highest deposit rate at lowest price. Local probes are needed to measure the main properties as, e.g. the elastic properties, thermal conductivity and naturally nanostructure parameters, i.e. anisotropy, density, graphene structure or closed porosity, etc. First, this chapter will rationalize the very different pyrocarbon types following the two major transitions: low and high temperature transitions. Then in the next part, the recent efforts achieved to relate the structure (and texture) to the processing conditions in the case of low temperature pyrocarbons will be documented: carbon layer, cones and regenerative features. Finally, is a review of the growth mechanisms in relation with structure development and various approaches to quantify the structural parameters: density and anisotropy. 2 The various pyrocarbons Bokros (1965, 1969) introduced a first comprehensive distinction among pyrocarbons by means of optical microscopy. At that time four structural groups were distinguished © 2003 Taylor & Francis
based on their appearance when observed under polarized light (on polished sections) The main concern was the deposition using the fluidized-bed CVD in a broad range of temperature. Then,Kotlensky et al.(1971),Granoff and Pierson (1973),and principally Lieberman and Pierson(1974,1975)documented the case of carbon composite infiltration at low tem- perature(T<1400C)and low partial pressure of hydrocarbon.They were the first to study the texture in relation with the processing conditions:matrix texture fall into three major types identified as rough laminar(RL),smooth laminar(SL),and isotropic pyrocarbons(D). They were the first to establish the important low temperature transition between rough and smooth laminar for infiltration process. 2.1 The low temperature transition:SL←→RL(8O0-l,400°C Nowadays,the transition between high and low density pyrocarbons is well established.It is under control of the gas phase species,itself controlled by the residence time (Dupel et al., 1995),the temperature or pressure.Transition is due to a change in the heterogeneous growth mechanisms in the range of 800-1,400C(Feron et al.,1999).These transitions could be called the CVI transitions because it is of major concern in 3D preform infiltration(Lavenac et al.,2000).A progressive passage from SL to a low-density I was also clearly established at very short residence time (and/or lower temperature and pressure)(Lavenac et al,2001).This passage occurs through the intermediate of dark laminar(DL)(Doux,1994)by a progressive increase of disclination defects in the hexagonal lattice,the pentagons (Bourrat et al.,2001): SL←→RL 2.1.I Smooth laminar pyrocarbon(SL) When observed by reflected light SL is characterized by a medium reflectance(see Fig.8.1). (Reflectance measures the ratio of light reflected by the polished surface.)Under cross- polars SL exhibits a large and well defined extinction cross known as the "Maltese-cross" (around fiber cross-sections).When rotating the stage,the rolling extinction parallel to the polars is smooth,thus this texture is called "smooth"laminar pyrocarbon.An example of this texture is provided on Fig.8.1a.When measured,the density is found to be intermedi- ate 1.8<d<1.95.The anisotropy is medium too:extinction angle,Ae,measured in cross- polarized light is 12<Ae<18 on a scale which goes up to 22 (see Section 5.3).The orientation angle,OA,measured by electron diffraction is 40<OA<70.OA measures the disorder along the anisotropy plane which decreases down to 15. 2.1.2 Rough laminar pyrocarbon (RL) This texture has a high reflectance.When observed with the polarizer alone and because of carbon dichroism,a gray branch parallel to the polarizer appears around fiber cross- sections:reflectance is higher parallel to the graphene planes.Under crossed polars a highly contrasted Maltese-cross appears around the fiber cross sections(Fig.8.1b).The extinction of the branches is irregular.For that reason it is called"rough"laminar.The roughness is provided by the prismatic texture due to the formation of fine cones generated on fiber ©2003 Taylor&Francis
based on their appearance when observed under polarized light (on polished sections). The main concern was the deposition using the fluidized-bed CVD in a broad range of temperature. Then, Kotlensky et al. (1971), Granoff and Pierson (1973), and principally Lieberman and Pierson (1974, 1975) documented the case of carbon composite infiltration at low temperature (T 1400 C) and low partial pressure of hydrocarbon. They were the first to study the texture in relation with the processing conditions: matrix texture fall into three major types identified as rough laminar (RL), smooth laminar (SL), and isotropic pyrocarbons (I). They were the first to establish the important low temperature transition between rough and smooth laminar for infiltration process. 2.1 The low temperature transition: SL ↔ RL (800–1,400 C) Nowadays, the transition between high and low density pyrocarbons is well established. It is under control of the gas phase species, itself controlled by the residence time (Dupel et al., 1995), the temperature or pressure. Transition is due to a change in the heterogeneous growth mechanisms in the range of 800–1,400 C (Féron et al., 1999). These transitions could be called the CVI transitions because it is of major concern in 3D preform infiltration (Lavenac et al., 2000). A progressive passage from SL to a low-density I was also clearly established at very short residence time (and/or lower temperature and pressure) (Lavenac et al., 2001). This passage occurs through the intermediate of dark laminar (DL) (Doux, 1994) by a progressive increase of disclination defects in the hexagonal lattice, the pentagons (Bourrat et al., 2001): SL ↔ RL 2.1.1 Smooth laminar pyrocarbon (SL) When observed by reflected light SL is characterized by a medium reflectance (see Fig. 8.1). (Reflectance measures the ratio of light reflected by the polished surface.) Under crosspolars SL exhibits a large and well defined extinction cross known as the “Maltese-cross” (around fiber cross-sections). When rotating the stage, the rolling extinction parallel to the polars is smooth, thus this texture is called “smooth” laminar pyrocarbon. An example of this texture is provided on Fig. 8.1a. When measured, the density is found to be intermediate 1.8 d 1.95. The anisotropy is medium too: extinction angle, Ae, measured in crosspolarized light is 12 Ae 18 on a scale which goes up to 22 (see Section 5.3). The orientation angle, OA, measured by electron diffraction is 40 OA 70. OA measures the disorder along the anisotropy plane which decreases down to 15. 2.1.2 Rough laminar pyrocarbon (RL) This texture has a high reflectance. When observed with the polarizer alone and because of carbon dichroism, a gray branch parallel to the polarizer appears around fiber crosssections: reflectance is higher parallel to the graphene planes. Under crossed polars a highly contrasted Maltese-cross appears around the fiber cross sections (Fig. 8.1b). The extinction of the branches is irregular. For that reason it is called “rough” laminar. The roughness is provided by the prismatic texture due to the formation of fine cones generated on fiber © 2003 Taylor & Francis
(a) Figure 8./Low temperature transition in CVI conditions:(a)SL:smooth laminar pyrocarbon and (b)RL:rough laminar pyrocarbon(Cross-polarized light,bar is 10um,after DouX.1994). surface asperity and transmitted across the whole deposit (see Section 3.2).RL density is high:2.018 up to the maximum (app.22)and OA the disorder is low <25(typically 15). 2.1.3 Isotropic pyrocarbon(1) Isotropic texture has a low reflectance and a very weak anisotropy (or not).Under cross- polars a faint Maltese-cross can hardly be seen.It shows very fine grains with a poor brightness.This texture is often mingled with"isotropic-sooty"pyrocarbons(Section 2.2). The later can have a high density whereas isotropic (ISO)of low density systematically ©2003 Taylor&Francis
surface asperity and transmitted across the whole deposit (see Section 3.2). RL density is high : 2.0 d 2.2. Anisotropy is high too: Ae 18 up to the maximum (app. 22) and OA the disorder is low 25 (typically 15). 2.1.3 Isotropic pyrocarbon (I) Isotropic texture has a low reflectance and a very weak anisotropy (or not). Under crosspolars a faint Maltese-cross can hardly be seen. It shows very fine grains with a poor brightness. This texture is often mingled with “isotropic-sooty” pyrocarbons (Section 2.2). The later can have a high density whereas isotropic (ISO) of low density systematically Figure 8.1 Low temperature transition in CVI conditions: (a) SL: smooth laminar pyrocarbon and (b) RL: rough laminar pyrocarbon (Cross-polarized light, bar is 10m, after Doux, 1994). (a) (b) © 2003 Taylor & Francis
exhibits a low density:d~1.6.The measure of the extinction angle gives values lower than4°. 2.1.4 Dark laminar pyrocarbon (DL) Dark laminar has no particular interest,it is only an intermediate in the passage between isotropic and smooth laminar pyrocarbons (SL).As isotropic,it does not seem to be result- ing from a different mechanism but the same heterogeneous mechanism as smooth laminar (Bourrat et al.,2001).It is defined by a faint anisotropy and intermediate density: 4°<Ae<12°and1.6<d<1.8. 2.2 The high temperature transition:L←)G←→IS(L,400-2,000CO In the CVD range of 1,400-2,000C,as processing temperature is increased,density decays and then restores again.This characterizes the high temperature transition reported by many authors in the case of surface deposition (see Fig.8.2).This second transition towards an "isotropic"grade grown at high temperature was first reported by Brown and Watt(1958). Diefendorf(1970)observed that what is responsible is the"soot"nucleated in gas phase and that it can be avoided by decreasing the hydrocarbon partial pressure(Diefendorf,1960)as shown in Figs 8.2 (bold line)and 8.3.All these experimental data have been confirmed by many authors in fluidized bed(Bokros,1965)or static one(Tombrel and Rappeneau,1965). Later on,Loll et al.(1977)have shown that this transition from laminar (L)to granular(G) and isotropic sooty(IS)pyrocarbon was also existing in the case of CVI(of felt)as shown in Fig.8.4 under the form of an existence diagram. 2.51 2.267 1.5 8001,0001,2001,4001,6001,8002,0002,2002,400 (C) Figure 8.2 High temperature pyrocarbon transition:density versus processing temperature.Full bold line:Diefendorf(1960)2.3 Pa CH4;fine dash and dot:ibid.,5.3 kPa CH4;full fine line:Brown et al.(1959)20kPa CH4 or C4H4;dashed fine line:Blackman et al.(1961)CHa and bold dot and dash:Mayasin and Tesner (1961):100 KPa H2and2 to 10%CH4 ©2003 Taylor&Francis
exhibits a low density: d ~ 1.6. The measure of the extinction angle gives values lower than 4. 2.1.4 Dark laminar pyrocarbon (DL) Dark laminar has no particular interest, it is only an intermediate in the passage between isotropic and smooth laminar pyrocarbons (SL). As isotropic, it does not seem to be resulting from a different mechanism but the same heterogeneous mechanism as smooth laminar (Bourrat et al., 2001). It is defined by a faint anisotropy and intermediate density: 4 Ae 12 and 1.6 d 1.8. 2.2 The high temperature transition: L ↔ G ↔ IS (1,400–2,000 C) In the CVD range of 1,400–2,000 C, as processing temperature is increased, density decays and then restores again. This characterizes the high temperature transition reported by many authors in the case of surface deposition (see Fig. 8.2). This second transition towards an “isotropic” grade grown at high temperature was first reported by Brown and Watt (1958). Diefendorf (1970) observed that what is responsible is the “soot” nucleated in gas phase and that it can be avoided by decreasing the hydrocarbon partial pressure (Diefendorf, 1960) as shown in Figs 8.2 (bold line) and 8.3. All these experimental data have been confirmed by many authors in fluidized bed (Bokros, 1965) or static one (Tombrel and Rappeneau, 1965). Later on, Loll et al. (1977) have shown that this transition from laminar (L) to granular (G) and isotropic sooty (IS) pyrocarbon was also existing in the case of CVI (of felt) as shown in Fig. 8.4 under the form of an existence diagram. Figure 8.2 High temperature pyrocarbon transition: density versus processing temperature. Full bold line: Diefendorf (1960) 2.3 Pa CH4; fine dash and dot: ibid., 5.3 kPa CH4; full fine line: Brown et al. (1959) 20 kPa CH4 or C4H4; dashed fine line: Blackman et al. (1961) CH4 and bold dot and dash: Mayasin and Tesner (1961): 100 KPa H2 and 2 to 10 % CH4. 2.5 2.267 d 2 1.5 1 800 1,000 1,200 1,400 1,600 (°C) 1,800 2,000 2,200 2,400 © 2003 Taylor & Francis
20 Laminar aromatic (uo) Isotropic 10 sooty Continuously nucleated Low Surface temperature nucleated 0 1,200 1.700 2,200 Temperature(C) Figure 8.3 High temperature transition after Diefendorf(1970) ◆ 30 8 20 No infiltration 6 Smooth laminar (CVD only or soot) 10 Rough laminar 0 Granular Isotropic 000 1,100 1,200 Temperature (C) Figure 8.4 Existence diagram of the low temperature transition demonstrated in infiltration of a felt.After Loll et al.(1977). The structural aspects of the growth mechanisms were studied by Kaae et al.(1972)and Kaae(1975,1985).With increasing temperature,laminar pyrocarbon is more and more regen- erated by gas-phase nucleated particles as shown in sketch of Fig.8.5.Transition occurs from regenerated laminar(Fig.8.6a)to granular(Fig.8.6b)and to isotropic sooty (Fig.8.6c,d). 2.2.1 Granular pyrocarbon It results from a mechanism where most of the carbon still grows directly onto the surface (molecular condensation)but gas phase-grown particles regenerate continuously ©2003 Taylor&Francis
The structural aspects of the growth mechanisms were studied by Kaae et al. (1972) and Kaae (1975, 1985). With increasing temperature, laminar pyrocarbon is more and more regenerated by gas-phase nucleated particles as shown in sketch of Fig. 8.5. Transition occurs from regenerated laminar (Fig. 8.6a) to granular (Fig. 8.6b) and to isotropic sooty (Fig. 8.6c, d). 2.2.1 Granular pyrocarbon It results from a mechanism where most of the carbon still grows directly onto the surface (molecular condensation) but gas phase-grown particles regenerate continuously Figure 8.3 High temperature transition after Diefendorf (1970). 20 Laminar aromatic Isotropic sooty Surface nucleated Low temperature 10 Pressure (torr) Continuously nucleated 0 1,200 1,700 Temperature (°C) 2,200 Figure 8.4 Existence diagram of the low temperature transition demonstrated in infiltration of a felt. After Loll et al. (1977). 1,000 1,100 1,200 0 10 20 30 Temperature (°C) Isotropic Granular Rough laminar No infiltration (CVD only or soot) Smooth laminar Volumic fraction of methane (%) © 2003 Taylor & Francis
(a) b 三 Small aliphatic @ Gas phase nucleated Figure 8.5 Model of the mechanism controlling the high temperature transition:(a)Low deposit- ing rate:regenerated-laminar and granular pyrocarbon;(b)High depositing rate: isotropic sooty(case of high density represented after Kaae,1985). small cones (Kaae,1985).Figure 8.5b gives a sketch to rationalize the mechanism (see Section 3.4). 2.2.2 Isotropic sooty At higher temperature a sort of"isotropic"deposit is observed(Fig.8.6c)which was named "isotropic sooty"(IS)by Diefendorf(1970).At the beginning,it has a high density (p=2.0). The lack of preferred orientation is provided by the size of the gas phase nucleated particles, these free-floating particles are much larger meanwhile too small to be resolved by optical microscopy:deposits look"isotropic"with optical microscopy.As temperature is increased, density progressively decays down to p=1.6(and even 1.5).Kaae shows that the change in density from high to low,is due to the molecular deposition and not to the particles structure which are still dense in most cases.If the molecular deposition is dense,then the density remains high:p=2.If this pyrocarbon is porous,the dense core is surrounded by a tangled structure close to that of glassy carbon:then the density drops down to a minimum (p=1.6). At higher temperature the density increases again because the tangled structure becomes coarser and then disappears.It is to note that concentration of hydrocarbon can be increased at a given bed temperature with the same effect.With a too low concentration of precursor this transition was not seen (Fig.8.2). Results obtained at General Atomic by Kaae were confirmed at CEA by Pelissier and Lombard(1975).As a matter of fact,the high temperature transition appears as a dramatic drop in density together with the occurrence of an isotropic structure.In this range textures are resulting from a different mechanism for which the particles grown in gas phase have a crucial role.Most of the authors agree with Kaae distinguishing L,G,or IS in-between 1,400 and2,000c: L←→G←→IS ©2003 Taylor&Francis
Figure 8.5 Model of the mechanism controlling the high temperature transition: (a) Low depositing rate: regenerated-laminar and granular pyrocarbon; (b) High depositing rate: isotropic sooty (case of high density represented after Kaae, 1985). Gas phase nucleated Small aliphatic (a) (b) small cones (Kaae, 1985). Figure 8.5b gives a sketch to rationalize the mechanism (see Section 3.4). 2.2.2 Isotropic sooty At higher temperature a sort of “isotropic” deposit is observed (Fig. 8.6c) which was named “isotropic sooty” (IS) by Diefendorf (1970). At the beginning, it has a high density ( 2.0). The lack of preferred orientation is provided by the size of the gas phase nucleated particles, these free-floating particles are much larger meanwhile too small to be resolved by optical microscopy: deposits look “isotropic” with optical microscopy. As temperature is increased, density progressively decays down to 1.6 (and even 1.5). Kaae shows that the change in density from high to low, is due to the molecular deposition and not to the particles structure which are still dense in most cases. If the molecular deposition is dense, then the density remains high: 2. If this pyrocarbon is porous, the dense core is surrounded by a tangled structure close to that of glassy carbon: then the density drops down to a minimum ( 1.6). At higher temperature the density increases again because the tangled structure becomes coarser and then disappears. It is to note that concentration of hydrocarbon can be increased at a given bed temperature with the same effect. With a too low concentration of precursor this transition was not seen (Fig. 8.2). Results obtained at General Atomic by Kaae were confirmed at CEA by Pelissier and Lombard (1975). As a matter of fact, the high temperature transition appears as a dramatic drop in density together with the occurrence of an isotropic structure. In this range textures are resulting from a different mechanism for which the particles grown in gas phase have a crucial role. Most of the authors agree with Kaae distinguishing L, G, or IS in-between 1,400 and 2,000 C: L ↔ G ↔ IS © 2003 Taylor & Francis
(a) (b) 0.5μm 0.25μm Figure 8.6 Structure evolution during the high temperature transition.(a)Pyrocarbon laminar (few small gas phase-grown particles);(b)Granular pyrocarbon (abundant gas phase-grown particles);(c)IS of high density(abundant dense particles co-deposited with homogeneous pyrocarbon);(d)IS of low density (abundant and dense particles co-deposited with glassy carbon-like pyrocarbon)(a and b:cross-polarized light,bar is 20 um,after Tombrel and Rappeneau(1965);(c)and(d):TEM after Kaae,1985). The high temperature transition has no more been studied till the seventies,in our knowledge. 2.3 Very high temperature pyrocarbons "Oriented"pyrocarbons used as conductive and gas-tight coating are deposited by CVD of methane at temperatures between 2,000 and 2,500C (e.g.2kPa of methane at 2,000C with a deposition rate of 100 um H-,Le Carbone Lorraine,1975).At higher temperature,works performed(Guentert,1962;Tombrel and Rappeneau,1965;Hirai and Yajima,1967;Bokros, 1969;and Goma et al.,1985)have shown by XRD and TEM that the deposit is highly oriented with a regenerated texture(Fig.8.7c).It was shown by XRD that they grow with ©2003 Taylor&Francis
The high temperature transition has no more been studied till the seventies, in our knowledge. 2.3 Very high temperature pyrocarbons “Oriented” pyrocarbons used as conductive and gas-tight coating are deposited by CVD of methane at temperatures between 2,000 and 2,500 C (e.g. 2 kPa of methane at 2,000 C with a deposition rate of 100mH1 , Le Carbone Lorraine, 1975). At higher temperature, works performed (Guentert, 1962; Tombrel and Rappeneau, 1965; Hirai andYajima, 1967; Bokros, 1969; and Goma et al., 1985) have shown by XRD and TEM that the deposit is highly oriented with a regenerated texture (Fig. 8.7c). It was shown by XRD that they grow with Figure 8.6 Structure evolution during the high temperature transition. (a) Pyrocarbon laminar (few small gas phase-grown particles); (b) Granular pyrocarbon (abundant gas phase-grown particles); (c) IS of high density (abundant dense particles co-deposited with homogeneous pyrocarbon); (d) IS of low density (abundant and dense particles co-deposited with glassy carbon-like pyrocarbon) (a and b: cross-polarized light, bar is 20m, after Tombrel and Rappeneau (1965); (c) and (d): TEM after Kaae, 1985). 0.5 µm 0.25 µm (a) (b) (c) (d) © 2003 Taylor & Francis
500um (b) 500um (c) 5nm Figure 8.7 Very high temperature pyrocarbon (processing temperature:2,100C):(a)Cross- polarized light on polished surface(PCH4=0.5 KPa);(b)Same in cross section (after Tombrel and Rappeneau,1965);(c)Same pyrocarbon in high resolution TEM(after Goma and Oberlin,1985). a turbostratic structure and a high degree of preferred orientation(Guenter,1962;Tombrel and Rappeneau,1965).In this case authors speak about direct deposition of carbon with a perfectly oriented turbostratic structure. 2.4 Pyrocarbons issued from new rapid densification processes The new processes as thermal gradient(Golecki et al.,1995)or pressure-pulse-CVI(Dupel et al.,1994)or film boiling (David et al.,1995;Bruneton et al.,1997),all provide classical textures or combinations of classical features known in CVD,except the possible mixed structures in the case of liquid immersion in the rapid densification process:mosaic pitch- based-and pyrolytic-type carbons(Rovilain,1999;Beaugrand,2000)as shown in Fig.8.8c. It is noteworthy that rough laminar is much easily produced by I-CVI than by any other process.In most cases,regenerative laminar(REL)(see Section 3.3)is obtained with the new rapid densification processes. ©2003 Taylor&Francis
a turbostratic structure and a high degree of preferred orientation (Guenter, 1962; Tombrel and Rappeneau, 1965). In this case authors speak about direct deposition of carbon with a perfectly oriented turbostratic structure. 2.4 Pyrocarbons issued from new rapid densification processes The new processes as thermal gradient (Golecki et al., 1995) or pressure-pulse-CVI (Dupel et al., 1994) or film boiling (David et al., 1995; Bruneton et al., 1997), all provide classical textures or combinations of classical features known in CVD, except the possible mixed structures in the case of liquid immersion in the rapid densification process: mosaic pitchbased- and pyrolytic-type carbons (Rovilain, 1999; Beaugrand, 2000) as shown in Fig. 8.8c. It is noteworthy that rough laminar is much easily produced by I-CVI than by any other process. In most cases, regenerative laminar (REL) (see Section 3.3) is obtained with the new rapid densification processes. Figure 8.7 Very high temperature pyrocarbon (processing temperature: 2,100 C): (a) Crosspolarized light on polished surface (PCH4 0.5 KPa); (b) Same in cross section (after Tombrel and Rappeneau, 1965); (c) Same pyrocarbon in high resolution TEM (after Goma and Oberlin, 1985). 5 nm (a) (c) (b) 500 µm 500 µm © 2003 Taylor & Francis
(a) (b) 5nm 5nm 得 (c) Figure 8.8 New rapid densification processing.(a)Comparison of laminar textures (LRE) obtained by pulse-CVI with a lateral growth of long defective layers with(b)rough laminar pyrocarbon obtained by I-CVI with a good stacking of small and straight lay- ers(Dupel et al.,1995);(c)Mosaic structure that can occur in the film boiling process aside classical laminar textures (after Beaugrand,2001,bar is 1 um). 2003 Taylor Francis
Figure 8.8 New rapid densification processing. (a) Comparison of laminar textures (LRE) obtained by pulse-CVI with a lateral growth of long defective layers with (b) rough laminar pyrocarbon obtained by I-CVI with a good stacking of small and straight layers (Dupel et al., 1995); (c) Mosaic structure that can occur in the film boiling process aside classical laminar textures (after Beaugrand, 2001, bar is 1m). c 5 nm 5 nm (a) (b) (c) © 2003 Taylor & Francis
3 Cones and regenerative features Among the distinctive growth-features of pyrocarbon is the cone generation.These features are important in considering the anisotropy of structure and thermo-mechanical properties, as well as in-service properties (e.g.tribology).Three main mechanisms and their mix have been recognized: substrate-generated cones or primary cones; secondary cones,self-generated within the deposit; secondary cones generated by gas-phase nucleated particles. Coffin (1964)has modeled the cone formation mechanism.He has definitely shown that they come from a simple roughness transmission effect due to the stacking,layer after layer. It is not the result of a nucleation/growth process. 3.1 Cones formation Flatness defects which can be transmitted come first from the support roughness.All lami- nar pyrocarbons possess primary cones generated onto the surface.Rough laminar pyrocar- bon alone keeps its primary cones exclusively all across the deposit. Let us suppose that the surface defect is a sphere lying on the support(Fig.8.9).Coffin (1964)has shown that the laminar growth propagates the defect layer after layer.At the beginning all asperities at the surface are transmitted exactly with a parabolic shape(2).On both sides of this "paraboloid"surface,the layer direction sharply changes by an angle a as for twinned crystals.This sharp bend when observed on a polished surface perpendicular to the deposit,appears as a parabolic curve with a drastic contrast variation related to the change of the layers direction.This stage lasts more or less depending on surface defects density.Then,the interference of adjacent growing cones leads to a honeycomb structure visible when looking down on the deposit surface.In cross section it shows a prismatic texture (3).The higher the surface roughness,the higher the a angle.This prismatic texture responsible for the rough extinction of the Maltese-cross branches was also called "columnar structure"by Bokros or fibrous structure by Tombrel and Rappeneau(1965)who have extensively studied the generation of cones as a function of temperature during the high temperature transition:laminar-granular transition. 3.2 Surface-generated cones Rough laminar appears to keep its primary cones across the full deposit.The more probable reason is because rough laminar does develop a highly oriented growth.The superposition of regenerative cones on the primary ones results in the progressive disappearance of them. So the pending question is:why rough laminar does not develop a regenerative growth as all laminars? Because Rough Laminar pyrocarbon is not regenerative,then primary cones survive providing its prismatic texture.Bourrat et al.(2002)have shown that the o angle in-between adjacent columns controls the future "grain boundaries"limiting the lateral graphitization of the crystallites.More importantly they point out that these boundaries control a unique transverse reinforcement in the weakest direction of the matrix (stacking).This is a very important property exclusively known in RL. ©2003 Taylor&Francis
3 Cones and regenerative features Among the distinctive growth-features of pyrocarbon is the cone generation. These features are important in considering the anisotropy of structure and thermo-mechanical properties, as well as in-service properties (e.g. tribology). Three main mechanisms and their mix have been recognized: ● substrate-generated cones or primary cones; ● secondary cones, self-generated within the deposit; ● secondary cones generated by gas-phase nucleated particles. Coffin (1964) has modeled the cone formation mechanism. He has definitely shown that they come from a simple roughness transmission effect due to the stacking, layer after layer. It is not the result of a nucleation/growth process. 3.1 Cones formation Flatness defects which can be transmitted come first from the support roughness. All laminar pyrocarbons possess primary cones generated onto the surface. Rough laminar pyrocarbon alone keeps its primary cones exclusively all across the deposit. Let us suppose that the surface defect is a sphere lying on the support (Fig. 8.9). Coffin (1964) has shown that the laminar growth propagates the defect layer after layer. At the beginning all asperities at the surface are transmitted exactly with a parabolic shape (2). On both sides of this “paraboloid” surface, the layer direction sharply changes by an angle as for twinned crystals. This sharp bend when observed on a polished surface perpendicular to the deposit, appears as a parabolic curve with a drastic contrast variation related to the change of the layers direction. This stage lasts more or less depending on surface defects density. Then, the interference of adjacent growing cones leads to a honeycomb structure visible when looking down on the deposit surface. In cross section it shows a prismatic texture (3). The higher the surface roughness, the higher the angle. This prismatic texture responsible for the rough extinction of the Maltese-cross branches was also called “columnar structure” by Bokros or fibrous structure by Tombrel and Rappeneau (1965) who have extensively studied the generation of cones as a function of temperature during the high temperature transition: laminar–granular transition. 3.2 Surface-generated cones Rough laminar appears to keep its primary cones across the full deposit. The more probable reason is because rough laminar does develop a highly oriented growth. The superposition of regenerative cones on the primary ones results in the progressive disappearance of them. So the pending question is: why rough laminar does not develop a regenerative growth as all laminars? Because Rough Laminar pyrocarbon is not regenerative, then primary cones survive providing its prismatic texture. Bourrat et al. (2002) have shown that the angle in-between adjacent columns controls the future “grain boundaries” limiting the lateral graphitization of the crystallites. More importantly they point out that these boundaries control a unique transverse reinforcement in the weakest direction of the matrix (stacking). This is a very important property exclusively known in RL. © 2003 Taylor & Francis