Table 6.Published characteristics of different chemical vapor infiltration methods used to densify C-C composite articles and described in this chapter CVI process Preform Process Reactor (Section no.) Multiple Thick/large Densification Pressure Precursor Cost of Special size time (w/r to adjustable efficiency capital fixturing (>2.5cm perform in all thickness) directions) Isothermal isobaric (2.2) Yes Yes Very long Yes Low High No Early thermal-gradient No Yes Long Yes Yes inductively-heated isobaric (2.3.1) Recent thermal-gradient Yes Yes Short Yes High Low No inductively-heated isobaric(2.3.2)】 Liquid-immersion No No Short No thermal-gradient inductively-heated isobaric atm. pressure (2.4) Forced-flow thermal- No No Short Very High Low Yes gradient (2.5) limited 3 用由 Hot w w 。。。象。。年 w H1EitiiGiittiiiititiiiii Cold Isothermal Thermal-gradient radiantly-heated radiantly-heated isobaric CVI(M) isobaric CVI(M) Liquid Cold Cold precursor 0 0 0 。 0 Cold 0 Cold Cold O Cold 0 0 Cold 0 0 Cold ● Liquid-immersion Thermal-gradient thermal-gradient inductively-heated inductively-heated isobaric CVI(M) isobaric CVI(S) Hot w 田田田 bbbpcccoapacmmceep进Y 444， Cold 是↑多↑3 Isothermal Thermal-gradient radiantly-heated radiantly-heated forced-flow CVI (S) forced-flow CVI(S】 Figure 6.3 Principles of the main chemical vapor infiltration methods (Golecki,1997); M=multiple preforms,S=single preform per run. ©2003 Taylor&Francis
Table 6.1 Published characteristics of different chemical vapor infiltration methods used to densify CøC composite articles and described in this chapter CVI process Preform Process Reactor (Section no.) Multiple Thick/large Densification Pressure Precursor Cost of Special size time (w/r to adjustable efficiency capital fixturing ( 2.5 cm perform in all thickness) directions) Isothermal isobaric (2.2) Yes Yes Very long Yes Low High No Early thermal-gradient No Yes Long Yes Yes inductively-heated isobaric (2.3.1) Recent thermal-gradient Yes Yes Short Yes High Low No inductively-heated isobaric (2.3.2) Liquid-immersion No No Short No thermal-gradient inductively-heated isobaric atm. pressure (2.4) Forced-flow thermal- No No Short Very High Low Yes gradient (2.5) limited Figure 6.3 Principles of the main chemical vapor infiltration methods (Golecki, 1997); M multiple preforms, S single preform per run. Isothermal radiantly-heated isobaric CVI (M) Thermal-gradient inductively-heated isobaric CVI (M) Liquid-immersion thermal-gradient inductively-heated isobaric CVI (S) Isothermal radiantly-heated forced-flow CVI (S) Thermal-gradient radiantly-heated forced-flow CVI (S) Thermal-gradient radiantly-heated isobaric CVI (M) Hot Hot Hot Cold Cold Cold Cold Liquid precursor Cold Hot Cold Cold Cold Cold Cold © 2003 Taylor & Francis
Different CVI methods are at different stages of technological and industrial maturity.Much data on infiltration of composites are unpublished or unavailable.Thus,the comparisons provided in this chapter are based on printed,public-domain studies,and patents.In the next sections,more detailed consideration is given to specific CVI methods used in fabrication of C-C articles. 3 Chemical vapor infiltration processes 3.1 Isothermal isobaric carbon CVI Isothermal isobaric CVI is in wide use since the 1960s for densification of C-Cs and other refractory composites(Kotlensky,1971;Savage,1993;Golecki,1997).In a common appli- cation,a large number of porous carbon disk brake-pad preforms,typically 15-55 cm in outer diameter (o.d.)by 2-3 cm in thickness,are placed in a hot-wall reactor,i.e.a furnace uniformly heated by radiation,at a temperature in the range 1,000-1,100C and exposed to a flow of reactant gas,e.g.CHa at 5-100 Torr=6.6 X 102-1.3 X 104 Pa (Thomas,1993); see Fig.6.4.In isothermal isobaric CVI,the densification kinetics of a thin (e.g.1-2mm thick)preform with initial density po follow an exponential approach with time to a"final" density value,pe(Loll et al.,1977;Marinkovic and Dimitrijevic,1987;Naslain et al.,1989): p(t)=po+(Pr-po)[1-exp(-t/T)] (6) where r is the time constant of the process;r decreases with increasing temperature accord- ing to an exponential (Arrhenius)relationship in the surface-reaction limited regime.For such a thin preform,Pr may equal the desired final density,but in a thicker preform the surface pores become clogged well before that density value is reached,requiring several interruptions of the infiltration to allow grinding the external surfaces in order to open the pores and enable further infiltration;see Fig.6.5.The total densification time is thus a strong supra-linear function of the thickness of the preform.For >2cm thick preforms, 600-2,000h may be required to achieve the desired density. Since from an economic viewpoint,it is desirable to minimize the densification time, higher temperature and precursor pressure would seem to be the direction to follow.However, Pump Preforms 区XX囚☒ X☒区XX☒ Heater 4一CH4 Figure 6.4 Simplified schematic diagram of a hot-wall,isothermal-isobaric chemical vapor infiltration reactor (not to scale). ©2003 Taylor&Francis
Different CVI methods are at different stages of technological and industrial maturity. Much data on infiltration of composites are unpublished or unavailable. Thus, the comparisons provided in this chapter are based on printed, public-domain studies, and patents. In the next sections, more detailed consideration is given to specific CVI methods used in fabrication of C–C articles. 3 Chemical vapor infiltration processes 3.1 Isothermal isobaric carbon CVI Isothermal isobaric CVI is in wide use since the 1960s for densification of C–Cs and other refractory composites (Kotlensky, 1971; Savage, 1993; Golecki, 1997). In a common application, a large number of porous carbon disk brake-pad preforms, typically 15–55 cm in outer diameter (o.d.) by 2–3 cm in thickness, are placed in a hot-wall reactor, i.e. a furnace uniformly heated by radiation, at a temperature in the range 1,000–1,100 C and exposed to a flow of reactant gas, e.g. CH4 at 5–100 Torr 6.6 102 1.3 104 Pa (Thomas, 1993); see Fig. 6.4. In isothermal isobaric CVI, the densification kinetics of a thin (e.g. 1–2mm thick) preform with initial density o follow an exponential approach with time to a “final” density value, f (Loll et al., 1977; Marinkovic and Dimitrijevic, 1987; Naslain et al., 1989): (t) o (f o)[1 exp( t/)] (6) where is the time constant of the process; decreases with increasing temperature according to an exponential (Arrhenius) relationship in the surface-reaction limited regime. For such a thin preform, f may equal the desired final density, but in a thicker preform the surface pores become clogged well before that density value is reached, requiring several interruptions of the infiltration to allow grinding the external surfaces in order to open the pores and enable further infiltration; see Fig. 6.5. The total densification time is thus a strong supra-linear function of the thickness of the preform. For 2 cm thick preforms, 600–2,000 h may be required to achieve the desired density. Since from an economic viewpoint, it is desirable to minimize the densification time, higher temperature and precursor pressure would seem to be the direction to follow. However, Figure 6.4 Simplified schematic diagram of a hot-wall, isothermal–isobaric chemical vapor infiltration reactor (not to scale). Heater CH4 Pump Preforms © 2003 Taylor & Francis
Time Figure 6.5 Schematic illustration of the densification kinetics of a thick preform in isothermal isobaric chemical vapor infiltration. 2.4 20 2.2 0.02 Laminar 2.0 15 aromatic 10 巴 1.8 Isotropic 10 sooty 1.6 40 1.4 1.2 Surface 150 Torr nucleatad 1.0 0 1,200 1,600 2,000 1,200 1.600 2000 Deposition temperature(C) Figure 6.6 Density and microstructure of infiltrated carbon versus temperature and pressure (Kotlensky,1971;reprinted by permission of the Society for the Advancement of Materials and Process Engineering). doing so may also lead to more premature closure of surface pores.Conversely,lower tem- peratures and pressures may reduce undesirable gas-phase nucleation and formation of tar and soot by-products.Lower pressure corresponds to higher gas-phase diffusivity,leading to more uniform distributions of density and microstructure within the composite.For exam- ple,the density minimum observed in carbon CVI(Kotlensky,1971)-see Fig.6.6-was interpreted as resulting from the incorporation of soot particles formed in the gas phase into the carbon deposited in the pores of the preform.If the pressure was reduced sufficiently, this density minimum could be eliminated.C-C composites having larger crystallite size, La,and improved thermal and electrical conductivities were produced at lower pressures (Stoller and Frye,1972).Rough-laminar(anisotropic)carbon,which is generally desired for many applications,was obtained at 35 Torr (4.7 X 103Pa),but not at 100-760 Torr (1.3 X 104-1.0 X 105 Pa).Rough-laminar carbon can be rendered more graphitic,i.e.more ordered microstructurally,by high-temperature treatment (graphitization)at =2,000C (Loll et al.,1977)and this may result in improved composite properties.Lower pressures also allow lower inlet precursor flow rates. The effect of different precursors on the carbon densification rate of 7.6 X 7.6 X 1.6cm carbon preforms was studied(Duan and Don,1995).Randomly oriented pitch-fiber tow preforms were impregnated with phenolic resin and densified at 1,000-1,150C and 15 Torr ©2003 Taylor&Francis
doing so may also lead to more premature closure of surface pores. Conversely, lower temperatures and pressures may reduce undesirable gas-phase nucleation and formation of tar and soot by-products. Lower pressure corresponds to higher gas-phase diffusivity, leading to more uniform distributions of density and microstructure within the composite. For example, the density minimum observed in carbon CVI (Kotlensky, 1971) – see Fig. 6.6 – was interpreted as resulting from the incorporation of soot particles formed in the gas phase into the carbon deposited in the pores of the preform. If the pressure was reduced sufficiently, this density minimum could be eliminated. C–C composites having larger crystallite size, La, and improved thermal and electrical conductivities were produced at lower pressures (Stoller and Frye, 1972). Rough-laminar (anisotropic) carbon, which is generally desired for many applications, was obtained at 35 Torr (4.7 103 Pa), but not at 100–760 Torr (1.3 104 1.0 105 Pa). Rough-laminar carbon can be rendered more graphitic, i.e. more ordered microstructurally, by high-temperature treatment (graphitization) at 2,000 C (Loll et al., 1977) and this may result in improved composite properties. Lower pressures also allow lower inlet precursor flow rates. The effect of different precursors on the carbon densification rate of 7.6 7.6 1.6 cm carbon preforms was studied (Duan and Don, 1995). Randomly oriented pitch-fiber tow preforms were impregnated with phenolic resin and densified at 1,000–1,150 C and 15 Torr Figure 6.5 Schematic illustration of the densification kinetics of a thick preform in isothermal isobaric chemical vapor infiltration. Time Density Figure 6.6 Density and microstructure of infiltrated carbon versus temperature and pressure (Kotlensky, 1971; reprinted by permission of the Society for the Advancement of Materials and Process Engineering). Deposition temperature (°C) 1,200 1,600 2,000 Density (g cm–3) 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 0.02 10 40 150 Torr 1,200 1,600 2,000 Pressure (Torr) 20 15 10 5 0 Laminar aromatic Isotropic sooty Surface nucleatad Continously nucleatad © 2003 Taylor & Francis
1,100C 830f1,150℃ 55 1,050°C 15 1.000C 10 0 sccm C3He CaHe=35sccm T=1,050C 0 0 40 80 1200 40 80 120 Time(h) Figure 6.7 Relative weight gain versus time in isothermal isobaric carbon CVI from CH4 and H2, showing the effects of temperature and C3H6 additions (Duan and Don,1995;reprinted with the authors'kind permission). (2.0X 103 Pa).The initial density and open porosity of the preforms were 1.25gcm-3 and 38 vol%,respectively.The preform weight was continuously recorded by means of an in-situ balance.CH4 at 400sccm was the primary precursor,with additions of 100sccm H2 and 0-75sccm propylene,C3H6.The hot zone of the furnace was 15.2cm in diameter by 30.5 cm long.The curves of fractional weight gain versus time had an exponential-type behavior and were fitted to the equation Am/m=a(pcvppo)In{1+az[exp(-a3t-a42)]-1), where the a,'s were constants.At 1,050C,the addition of increasing fractions of C3H6 resulted in increases of the initial deposition rate and of the final density,shown in Fig.6.7. The microstructure of the deposited carbon changed from isotropic in the absence of C3H6 to anisotropic when 35 sccm C3H6 was added.With 35 sccm C3H6,increasing the tempera- ture from 1,000 to 1,100C also increased the initial densification rate and the final density, whereas at 1,150C,premature surface pore closure and non-uniform deposition through the thickness were seen. Thus,the operating conditions of a hot-wall,isothermal isobaric CVI process involve a compromise which results in materials with the desired properties in the shortest possible time and with the minimum amount of labor,supplies,and energy. Advantages of isothermal isobaric CVI include: 。 The method is well-established and relatively well-understood. ● A large number of preforms can be densified simultaneously. The densification time per preform is relatively low for heavy loading of the reactor. Preforms of different and complex shapes and sizes can be readily densified in the same run,although the minimum dimension (usually the thickness)needs to be similar. The energy expenditure per part is relatively low. Disadvantages of isothermal isobaric CVI include: Premature surface crusting occurs before the desired bulk density is reached,necessi- tating several interruptions in the CVI process to grind surfaces. Very long hot processing time,typically 600-2,000h per batch(but the time does not depend on the number of parts being densified in a given reactor). The density within the article is generally highest at the surfaces and lowest in the inte- rior regions. ©2003 Taylor&Francis
(2.0 103 Pa). The initial density and open porosity of the preforms were 1.25 g cm3 and 38 vol%, respectively. The preform weight was continuously recorded by means of an in-situ balance. CH4 at 400 sccm was the primary precursor, with additions of 100 sccm H2 and 0–75 sccm propylene, C3H6. The hot zone of the furnace was 15.2 cm in diameter by 30.5 cm long. The curves of fractional weight gain versus time had an exponential-type behavior and were fitted to the equation m/ma1(CVD/o)ln{1a2[exp(a3ta4t 2 )]1}, where the aj ’s were constants. At 1,050 C, the addition of increasing fractions of C3H6 resulted in increases of the initial deposition rate and of the final density, shown in Fig. 6.7. The microstructure of the deposited carbon changed from isotropic in the absence of C3H6 to anisotropic when 35 sccm C3H6 was added. With 35 sccm C3H6, increasing the temperature from 1,000 to 1,100 C also increased the initial densification rate and the final density, whereas at 1,150 C, premature surface pore closure and non-uniform deposition through the thickness were seen. Thus, the operating conditions of a hot-wall, isothermal isobaric CVI process involve a compromise which results in materials with the desired properties in the shortest possible time and with the minimum amount of labor, supplies, and energy. Advantages of isothermal isobaric CVI include: ● The method is well-established and relatively well-understood. ● A large number of preforms can be densified simultaneously. ● The densification time per preform is relatively low for heavy loading of the reactor. ● Preforms of different and complex shapes and sizes can be readily densified in the same run, although the minimum dimension (usually the thickness) needs to be similar. ● The energy expenditure per part is relatively low. Disadvantages of isothermal isobaric CVI include: ● Premature surface crusting occurs before the desired bulk density is reached, necessitating several interruptions in the CVI process to grind surfaces. ● Very long hot processing time, typically 600–2,000 h per batch (but the time does not depend on the number of parts being densified in a given reactor). ● The density within the article is generally highest at the surfaces and lowest in the interior regions. Figure 6.7 Relative weight gain versus time in isothermal isobaric carbon CVI from CH4 and H2, showing the effects of temperature and C3H6 additions (Duan and Don, 1995; reprinted with the authors’ kind permission). Time (h) 0 40 80 120 0 40 80 120 Weight gain (%) 0 10 20 30 40 C3H6= 35 sccm 1,150 °C 1,100 °C 1,050 °C 1,000 °C 0 sccm C3H6 75 55 35 15 T = 1,050 °C © 2003 Taylor & Francis