(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