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Theoretical and Experimental Studies on the Chemical Vapor Deposition of Boron Carbide Lionel G. Vandenbulcke Centre de Recherches sur la Chimie de la Combustion et des Hautes Temp&atures, C.N.R.S., 45045 Orlhns Cedex, France
Coatings of boron carbides are deposited from BCI,-CH,-H, mixtures by using a stagnation flow technique in a cold-wall reactor. The deposition rates and the solid compositions are compared with a mass transfer-equilibrium model, and the actual degree of approach to equilibrium is shown a s a function of the experimental parameters. In many conditions, the surface kinetics resistance is an important factor that limits the deposition rates and determines the composition of the deposit. In addition to the composition, the structure and the morphology of the 9-C deposits are studied a s a function of the temperature and the amount of departure from equilibrium. A distinction is made between the stable phases of the binary 6-C phase diagram and several kinetically favored phases. The ease with which layers of uniform structure, composition, and thickness are obtained is studied as a function of the deposition conditions at the gas-solid interface.
Introduction The chemical vapor deposition of boron carbide has already been studied by several authors, either to obtain continuous homogeneous coatings (Cochran and Donaldson, 1970; Donaldson et al., 1968,1973; Fitzer, 1969; Moore and Volk, 1969) or to determine the crystalline structure of the boron-carbon phases (Amberger et al., 1971; Mierzejewska and Niemyski, 1965; Ploog, 1974; Ploog and Amberger, 1971; Ploog et al., 1972). They have generally pointed out the difficulty in depositing a single phase, especially on a large surface. The solid deposited often contains carbide phases other than the rhombohedral carbide phase and also free boron or free carbon. To explain these results Ducarroir and coauthors (Ducarroir and Bernard, 1975, 1976; Nadal and Ducarroir, 1980) carried out a thermodynamic study on the B-C-H-C1 system in a large range of temperature and composition of the inlet gas mixture including H2, BCl,, and CH4. Studies reported here are part of a program that tries to establish the relationships between the mechanisms of deposition and the structure/composition of the solid deposited by taking into account the actual closeness of the state of the system to equilibrium or the amount of departure from equilibrium at the gas-solid interface in an open-flow reactor. In previous papers, the deposition mechanisms of an element like pure boron were studied, and the solid structures were connected to the principal parameters, the substrate temperature and the supersaturation of the gaseous phase at the gas-solid interface (Vandenbulcke and Vuillard, 1977, 1979). The deposition mechanisms of a binary solid, boroncarbon, are deduced here from the comparison between the experimental data and a model that combines the mass transfer in the gas phase and a dynamic equilibrium at the gas-solid interface. The influence of the surface kinetics constraint is shown, and the deposit structure, morphology, and uniformity are studied as a function of the deposition conditions a t the gas-solid interface. Experimental Techniques The deposition equipment and experimental procedure have been described in detail elsewhere (Vandenbulcke et al., 1981). A stagnation flow technique was employed where a stream of reactants H2-BC1,-CH4 was introduced in a cylindrical nozzle and blown perpendicularly onto a flat disk of graphite heated by a high-frequency generator. In such a jet impinging on a flat plate, the boundary layer
thickness and the mass transfer rates are nearly constant in the stagnation zone when the substrate is set within the core of the jet, and the deposition conditions are welldefined. The deposit thickness in this zone was measured microscopically on transverse cross sections, and the deposition rate, in pm/min, was deduced from experiments at five deposition times. Boron concentration in the deposit was determined by prompt y-ray spectrometry with cy particles of 3.5 meV from a Van de Graff accelerator. In all experiments the total pressure was 1 atm, the substrate temperature (7') was varied in the range 1400-2000 K, and the reactant mass flow rates (F)were 0.0016, 0.04, or 0.08 g cm-' s-l. Various inlet gas compositions were employed. Theoretical Analysis The boron carbide deposition process concerns the formation of dense adhering deposits, and the heterogeneous reactions are generally favored. The extreme case of homogeneous reactions in the gas phase far from the substrate is generally avoided, as it usually results in soot formation in the gas phase and porous or nonadherent solid being deposited. So the gas phase is supposed "chemically frozen", as no chemical reactions are taken into account out of the interface. A t the gas-solid interface some homogeneous parallel reactions must be, however, considered, which lead to intermediate gaseous compounds in the deposition process or gaseous byproducts. Thermodynamic analyses of the B-H-C1 and B-C-H-C1 systems have shown that HBC12 is such an intermediate and/or byproduct, which is formed in appreciable quantity at equilibrium; the other species can be neglected (Carlton et al., 1970; Ducarroir and Bernard, 1975,1976; Thebault et al., 1978; Vandenbulcke, 1979; Vandenbulcke and Vuillard, 1977). Nonequilibrium analyses of the gaseous byproducts in boron filament production have also indicated that HBC12 is a product of the boron deposition process (Talley, 1965). The chemical vapor deposition of boron carbide is therefore assumed to proceed in sequence as follows: (1)transport of reactants to the deposition surface (H2,BC13, CH,); (2) reactions at the gas-solid interface that are of two kinds-heterogeneous reactions which yield the B-C deposit and the gas product (HCl) and homogeneous reactions which give one intermediate but also byproduct of the reaction (HBCl,); (3) transport of products away from the interface (HC1 and HBC1,). We had shown previously that a deposition process entirely limited by the surface kinetics (step 2) is only a
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with the substrate, and the deposition rates are a function theoretical boundary case as the influence of the vapor of the mass transfer in the gas phase. The integration of transport can never be completely eliminated except at the Stefan-Maxwell equations allows us to take into acvery low pressures, which are not employed here (Vancount the differences in the diffusion rates of the various denbulcke, 1979). In most cases the local concentrations, gaseous species and to deduce the dynamic equilibrium especially the partial pressures a t the gas-solid interface, are different from the inlet (p?)or bulk-stream values bo), at the interface and a quantitative deposition rate of the solid. and the deposition rate and solid composition depend in In this calculation, the bulk-stream compositions of the part, a t least, on the transport processes; they must be gas phase are obtained from the arithmetic average of inlet calculated from diffusional considerations, taking into account the actual geometry of the reactor. and outlet compositions. On the other hand, the individual fluxes are not all independent; they are generally related Numerical solutions for unidimensional interdependent to each other by reaction stoichiometries except when diffusion of all the important gaseous species in a nondilute several parallel reactions proceed concurrently. This is the gas system can be obtained by solving the Stefan-Maxwell case when one intermediate in the conversion of the inlet equations. For an n component mixture there are n - 1 species into solid is also a nonnegligible byproduct of the equations that relate the concentration gradient of one deposition. Here the hydrogen reduction of boron trispecies to the molar fluxes and the mole fractions of all chloride leads either to the formation of solid boron carcomponents, their binary diffusivities, and the pressure bide, eventually via HBC12as an intermediate compound and temperature of the system (Bird et al., 1960). There is one more relationship between the n molar fraction cjXj in the reduction, or to the formation of HBCl,, which diffuses away from the gas-solid interface: = 1. Integration of these equations is carried out for boundary layer thicknesses extracted from a mass trans4BC1, CH4 4H2 B4C 12HC1 (2) port relationship for an axisymmetric jet impinging on a flat plate: BCl, + H2+ HBC12 + HC1 (3) N~h/Ns,l/~ 0.80N~:~~ (1) Only the compound B4C is considered in this calculation, as the thermodynamic properties of nonstoichiometric where NSh is the Sherwood number, Nsc the Schmidt boron carbide are not actually determined. It can be denumber, and NRe the Reynolds number. posited as a single phase or codeposited either with pure This general procedure was shown previously (Vandenboron or pure carbon as a function of the deposition conbulcke, 1979; Vandenbulcke and Vuillard, 1977). The ditions. When only boron carbide is deposited, the dedetailed calculations depend on the deposition conditions position rate of boron is directly determined by the dea t the gas-solid interface. Several situations can arise: position rate of carbon. The unknowns are the flux of (1)When the surface kinetics is sufficiently fast to allow HBClz that diffuses away from the surface and the dethe maximum conversion of reactants into products at the position rate of boron carbide. When two phases, B4C-B gas-solid interface, the interfacial partial pressures nearly or B,C-C, are deposited, the calculation becomes more reach the equilibrium values and a dynamic equilibrium tedious as the deposition rates of boron and carbon are not is maintained. The process is controlled by the mass directly related, and one more unknown is introduced in transfer combined with this interfacial dynamic equilibthe problem, which is compensated by one more equilibrium. From the exponential temperature dependence of rium constant for the deposition of pure boron or pure the kinetic constants, it is obvious that this situation is carbon. In all cases, a trial and error method was employed characteristic not only of high-temperature processes but that permits integration of the Stefan-Maxwell equations also of very low mass transfers. by the RungeKutta-Gill procedure. At any value of the (2) If the equilibrium is very favorable, the process is boundary layer thickness 6, the interfacial mixture was purely mass transfer controlled. tested for thermodynamical resistance. A numerical so(3) When the surface kinetics is not sufficiently fast to lution was obtained when the mass transfer correlation and allow the maximum conversion of reactants into products the thermodyamical constraints were satisfied. The calat the gas-solid interface, the interfacial partial pressures culation depends on the values of the physical parameters. depart from the equilibrium values and are a function of Some of them, the binary diffusion coefficients, are indethe surface kinetics; they also depend on the mass transfer pendent of composition. Physical film properties, the characteristics of the system as the interfacial values are average diffusivity and the mixture viscosity, were calcudifferent from the inlet or bulk-stream ones. The deposlated from the arithmetic average composition and temition process is controlled by a combination of the mass perature of the bulk stream and the interface, as reported transfer and the surface kinetics. previously (Vandenbulcke and Vuillard, 1977). The case of full diffusion control, case 2, does not occur A. Results of the Mass Transfer-Equilibrium in the B-C-H-Cl system where the conversion of reactants Calculations. Figure 1 presents in continuous lines the into products is never complete. One model, corresponding variations of the boundaries between the deposition doto case 1,will be developed and compared to the experimains of pure B4C,B4C-B, and B4C-C as functions of the mental results, and the influence of the kinetics restraint, initial partial pressures of boron trichloride (PoBc1,)and case 3, will be shown. methane (pocH,);hydrogen makes up the difference to 1 Mass Transfer-Equilibrium Model atm. Calculations were carried out at T = 1400 K and F = 0.04 g cm-2 s-l. Fundamental to the investigation of any chemical vapor In the conditions of deposition currently employed exdeposition process now is a thermodynamic analysis. The perimentally, the deposition range of pure B4C is limited B-C-H-C1 system has been studied by several authors by the two-phase domain B4C-B for low input partial (Ducarroir and Bernard, 1975,1976;Vandenbulcke, 1979). pressures of methane and B4C-C for higher p0cH,. But it is of course an approximation to consider that a These results are compared with the boundaries obthermodynamic study can predict the deposition conditained from a classical thermodynamic calculation, in tions a t the gas-solid interface in an open flow system. dashed lines. One can note that the deposition range of Even when a dynamic equilibrium is maintained, only a pure boron carbide is narrower and shifts to higher values portion of the gas stream has an opportunity to equilibrate
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0.012
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Figure 2. Variations with p°CH, of the partial pressure ratios (,and the deposition rates of the different phases; T = 1400 K, F = 0.04 g cm-2 SKI, poBCl, = 0.2 atm.
0
0.01
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pgH4(atm)
Figure 1. Deposition domains of the solid phases calculated: (a) with the mass transfer-equilibrium model (continuous lines); (b) with a classical thermodynamic calculation (dashed lines), T = 1400 K, F = 0.04 g cm-2 s-l,
c_-
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of poBcl, and lower values of P'CH, when the diffusion in the boundary layer is considered: this results from the different diffusivities of H,, BCl,, and CH4. The heavier component, BCl,, is retarded with respect to the lighter ones, H2 and CHI, and higher input values of poBcl, and lower values of p°CH, are necessary to compensate for this phenomenon. Results on the composition of the gaseous phase in dynamic equilibrium a t the interface are expressed in terms of partial pressure ratios [j
P'j = 7100 Pi
(4)
where p', is the partial pressure of component j a t the interface (poj= poBcI, for EmCIP). Figure 2 gives the results of the calculation carried out in the conditions of Figure 1 but with a constant inlet partial pressure of boron trichloride (PoBc1,= 0.2 atm), together with the deposition rates of the solid phases expressed in mole (or atomegram) cm+ s-l. The variations of the deposition rates are directly related to the variations of Ej, but the absolute values are essentially a function of the conditions of forced convection determined principally by the mass flow rate of reactants
F. The results obtained with a constant inlet partial pressure of methane (POCK = 0.02 atm) are shown in Figure 3. The interfacial partial pressure of boron trichloride is increases. Accordingly, lower relative to poBCl when poH2 the deposition rate of $oron carbide increases significantly. The deposition rate of free carbon increases rapidly for high values of p o H l and very more slowly at high values of poBcls; this result is explained by high diffusion rates of methane in hydrogen-rich mixtures. Besides, the interfacial partial pressure of methane increases normally when the inlet partial pressure of hydrogen increases. B. Comparison with the Experimental Results. In the model of mass transfer through a "chemically frozen" boundary layer combined with a dynamic equilibrium at
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30
-
20
-
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0 -
0 8
2
-
b 0.2
0.4
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~ $ ~ ~ ( a t m ~
Figure 3. Variations with p o ~ c ,of, the partial pressure ratios [ j and the deposition rates of the different phases; T = 1400 K, F = 0.04 g cm-2 s-l, p°CH,= 0.02 atm.
the gas-solid interface, the conversion of reactants into products at the interface and the deposition rate are maximum for the deposition conditions studied. It is the rate that would be achieved in the absence of surface resistances. These calculations can provide evidence for significant surface resistance to the deposition when the actual experimental deposition rate is very much lower than the calculated one and the solid composition very different from the equilibrium expectations. From the deposition rate (in ymlmin) and the composition of the solid, the experimental deposition rates of boron and carbon were deduced. We remember that the homogeneity range of the rhombohedral carbide phase is about 10-20 atom % C, say from B9C to B4C (Bouchacourt et al., 1978,1979;Elliot, 1965; Kieffer et al., 1971); the limit B& is obtained in nonequilibrium deposition conditions (Vandenbulcke and Vuillard, 1981). So, only the experimental limit of the deposition of free carbon can be quantitatively compared to the calculated one, and the homogeneity range of the carbide phase should be more extended for high boron contents than predicted by the calculation. Figure 4a gives, as a function of p°CHr, the theoretically predicted and actually obtained deposition rates a t T = 1400 K for a constant inlet partial pressure of boron tri-
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-8,C
boron Carbide
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boron Catbide -BgC 0
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p t H 4 (atm)
Figure 4. Comparison between the calculated (with the mass transfer-equilibrium model) and the experimental deposition rates of boron = 0.2 atm, and carbon: (- -1 boron (calculated); (- - 0 -) boron (experimental); (-1 carbon (calculated); (-X-) carbon (experimental);poBCl3 F = 0.04 g cm-2 s-l; (a) T = 1400 K; (b) T = 1600 K; (c) T = 1800 K; (d) T = 1900 K.
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chloride (poBcl, = 0.2 atm). The deposition rates of boron have been divided by a factor of 4 to get a better understanding of the results; they are represented by dashed lines. The carbon deposition rates are given by the continuous lines. The deposition rate of pure boron is slightly limited by some surface kinetic constraint. When methane is added to the reactants, the deposition rate of boron falls far from the rate predicted when a dynamic equilibrium is maintained a t the interface. Accordingly, the deposition rates of carbon are very slow. Both show that the process is severely hindered by the surface kinetics, and this is confirmed by the results on solid composition. The pure boron carbide phase is deposited when pocb is greater than 0.05 atm and lower than 0.17 atm as compared to the predicted values of 0.0117 and 0.0195 atm for stoichiometric B4C at equilibrium. At T = 1600 K (Figure 4b) pure boron is deposited in conditions of dynamic equilibrium at the interface, but the deposition of boron carbide is still limited by some surface kinetic constraint. The boron carbide phase, without free carbon, is accordingly deposited for values of inlet partial pressures of methane up to 0.09 atm. At T = 1800 K (Figure 4c) the boron deposition rates approximate better the predicted values. We can note particularly an increase of the deposition rate in the range 0.015-0.025 p°CH, that is characteristic of conditions which approach the equilibrium ones. The variations of the deposition rate of carbon show that the surface kinetic
limitations are still extant, in agreement with the advent of free carbon for p°CH,2 0.045 atm compared to the theoretical value of 0.0214 atm. At T = 1900 K (Figure 4d) the carbon deposition rates approach the calculated values. We can also note an important increase of the boron deposition rate for p°CH4 in the range 0.025-0.035 atm that can be ascribed to some homogeneous reactions; this is confirmed by experiments carried out at temperatures of 2000 K where homogeneous reactions clearly extended, leading to nonadherent crystals of boron carbide. High temperatures favor near-equilibrium conditions as the temperature dependence of the chemical kinetics is much greater than the temperature dependence of the gaseous diffusion. The combined influence of the temperature and the inlet partial pressure of methane on the amount of departure from equilibrium can be observed in Figure 5, which compares the experimental boundary between the deposition domains of pure boron carbide and boron carbide plus free carbon (B/C = 4 in first approximation) to the theoretical ones calculated by a classical thermodynamic study and the model of mass transferequilibrium. In spite of some possible sources of error in estimation of the diffusion coefficients, exactness of the thermodynamic data, and experimental deposit compositions, the comparison of the three boundaries summarizes clearly the capital importance of the amount of departure from equilibrium. As the temperature decreases, higher
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Figure 5. Variations in the experimental and calculated boundaries (where the atomic ratio B/C = 4) as functions of p0cHland T; poBc13 = 0.2 atm, F = 0.04 g cm-2 s-l.
supersaturations in CH4 are necessary to deposit solids with high carbon content. Figure 5 allows us to make some remarks upon the nature of the boundary layer. The mass transfer-dynamic equilibrium curve is indeed never reached, even at high temperatures. As this model supposes a pure diffusion in the gas phase, the results obtained in the temperature range 1800-1900 K confirm that some homogeneous intermediate reactions occur in the gas phase. As indicated previously, this phenomenon increases with the temperature, as confirmed by the formation of "soot" a t temperatures greater than 1900 K; the curve derived from the thermodynamic study is then approached. In cold-wall reactors, the gas phase can therefore be supposed "chemically frozen" when the process is severely hindered by the surface kinetics or when the chemical reactions are not too favored thermodynamically. The other extreme assumption is to consider the chemical processes preponderant to the diffusion process and the gas mixture in chemical equilibrium a t any location of the boundary layer. The actual intermediate situation of boron carbide deposition in cold-wall reactors, but a t high temperature, shows the complex nature of the process when the surface reactions nearly reach the equilibrium conditions. Experimental parameters other than the temperature and the inlet partial pressure of methane determine the actual closeness of the state of the system to equilibrium; these parameters are the inlet partial pressure of boron trichloride and the total mass flow rate of reactants. The influence of p o B c l , can be observed in Figure 6, which compares the theroretical and experimental deposition rates of boron and carbon in the following conditions: T = 1600 K, p°CH, = 0.025 atm, and F = 0.04 g cm-2 s-l. The amount of departure from equilibrium decreases when poBcl, increases because the conversion of reactants into products a t equilibrium decreases with posa, more rapidly than the surface kinetics does. The influence of the total mass flow rate of reactants is shown in Figure 7, which gives the deposit composition as a function of p°CHl and two different mass flow rates. Higher supersaturation in methane is necessary to deposit solids with high carbon content when the total mass flow rate increases. The amount of departure from equilibrium increases when p°CHl and the mass flow rate increase and when poBc13
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Figure 6. Influence of the inlet partial pressure of BC13 on the deposition rates of boron and carbon: (- - -) boron (mass transferequilibrium calculation); (- - 0 - -) boron (experimental); (-) carbon (calculation); (-X-) carbon (experimental);p°CHa= 0.025 atm, F = 0.04 g cm-2 s-l, T = 1600 K.
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Figure 7. Solid composition as a function of the inlet partial pressure of CH4: T = 1400 K; p0Bcl3= 0.2 atm;).( F = 0.0016 g cm-2 SI; (m) F = 0.08 g cm-2 s-l.
and the temperature decrease. It depends also on the kind of reactants employed, especially the carbon-containing species as shown in the titanium carbide deposition process (Vandenbulcke, 1981). With many experimental conditions, the conversion of reactants into products a t the interface, the deposition rate, and the solid composition depart from the equilibrium values, and the influence of both chemical kinetics and mass transfer must be considered.
Influence of the Surface Kinetics Studies on processes controlled by a combination of mass transfer and surface kinetics concerned essentially the deposition rate of an element. In the case of a binary solid like boron carbide the solid composition is in addition determined by the process, and different surface mechanisms can occur when the solid composition varies within the homogeneity range of the single phase; even more may occur when different phases are deposited. A mass transfer-kinetics model was developed previously (Vandenbulcke, 1981) that correlates reasonably well the deposition rates and solid compositions over the boron
Ind. Eng. Cham. Prod. Res.
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24, NO. 4, 1985 573
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Figure 8. Influence of temnerature on the ratio n of the exoerimental deposition rate to the theoretical one; poBcb = 0.2 atm, F = 0.04 g em-’ 8-l. I
carbide homogeneity range, a t least in the range of experimental conditions employed. The mechanism of deposition confirms the prominent effect of CH, in limiting the conversion of both boron- and carbon-containing reactants into solid products, especially a t the lower temperatures. This can also be summarized here in Figure 8, which presents the influence of temperature on the ratio q between the experimental deposition rate and the rate calculated by the mass transfer-equilibrium model at the same composition, B/C = 4. (Values slightly higher than unity are obtained at high temperature because some homogeneous reactions arise in the process.) The mass transfer-kinetics model showed also that the decomposition kinetics of CH,, either homogeneous or after adsorption, was low, and high supersaturations in CH, were necessary to deposit carbon-rich boron carbides a t the lower temperatures employed. This is clearly shown here in Figure 5. F d y the amount of the kinetic constraints determines very different deposition conditions, even if boron carbide of all compositions can be deposited a t each temperature. These deposition conditions determine the structure and the morphology of the deposits. A. Influence on t h e Deposit Structure. Different structures were characterized by metallographic techniques and X-ray diffraction. In the single-phase domain, the well-known rhombohedral boron carbide phase appeared. It is the phase obtained by solidification from the melt over a large homogeneity range, from 10-11 to about 21.7 wt % C (Bouchacourt et al., 1978,1979;Elliot, 1965; Kieffer et al., 1971). Another boron carbide appeared in the single-phase domain. Its powder diffraction data correspond to data reported previously by Amberger et al. (1971) and identified as (B13C2)C. The phase width appears to be much larger than the value given by them (20.1-20.4 wt % C, but it will be called “B13C3”in the following. In the boron-rich two-phase domain, ,%rhombohedral boron and tetragonal boron carbide (BdB2C2)were also identified. On the carbon-rich side, free carbon was codeposited with rhombehedral boron carbide or B13C3,but amorphous
m
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“0“s
mron
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mron
f r P D mrmn
Figure 9. Depition domains of the various structures BS a function of T and pocH,;poBCl3 = 0.2 atm, F = 0.04 g c d 8’.
B-C solids were also obtained. Figure 9 presents the deposition domains of the different structures as a function of the temperature and the inlet partial pressure of methane. In the single-phase domain, the rhombohedral boron carbide is replaced by B13C3as pocH,increases. The deposition range of B13C3expands as the temperature decreases and the amount of departure from equilibrium increases; it appears as a kinetically favored phase. At 1900 K, when near-equilibrium conditions are reached, it disappears completely in favor of the rhombohedral boron carbide phase, which is the equilibrium phase encountered in the binary phase diagram. In the same way, rhombohedral boron solid solution is codeposited in the boron-rich two-phase domain at high temperatures, and B,C2 is codeposited a t a temperature lower than 1700 K. It becomes clear that BSoC2is a kinetically favored phase. In the carbon-rich region, free carbon is codeposited with rhombohedral boron carbide under near-equilibrium conditions and with B13C3when the amount of departure from equilibrium becomes appreciable. Under high supersaturation conditions and for T < 1500 K, amorphous B-C solids are deposited. B. Influence on the Deposit Morphology. Two other influences of the interfacial conditions of deposition can be observed, the degree of crystallization or grain size and the uniformity in thickness and composition of the deposits. It is well-known that the deposits are better crystallized and the crystal grain size increases when the temperature increases and the supersaturation decreases. Figure 10 shows the surface morphology of several deposits of nearly the same composition located in the single-phase domain
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Figure IO. Surface morphology of typical deposits obtained at three temperatures in the following conditions, p°K13= 0.2 atm. F = 0.04 g c N 2 s-‘: (top) 1400 K. pacH,= 0.14 atm; (middle) 1800 K. paCRI = 0.035 atm; (bottom) 1900 K, pnCH. = 0.03 atm.
*
(17.5 0.5 wt % C) hut deposited in various conditions of temperature and inlet partial pressure of methane. A t 1400 K, the surface morphology is characterized by badly crystallized or amorphous layers. As the temperature increases and the amount of departure from equilihrium decreases, the deposits are better crystallized and the grain size increases up to 20-30 wn a t 1900 K. As a function of the deposition conditions, the deposits can be more or less uniform in thickness and composition as well as in structure and morphology. These variations of the characteristics of the deposits arise from the dual influence of the depletion of reactants and variation in mass transfer rate along the surface. It has been shown on pure boron that uniformity of the thickness and the structure is improved by conditions that increase the surface kinetics control of the deposition process a t low temperature, low pressure, and sufficiently high gas flow rate (Vandenhulcke, 1983). It was concluded that it should be possible to obtain uniform amorphous boron layers into extended surfaces without particular attention to the gas flow dynamics, hut a careful control of the gas flow pattem appeared to be neceSSary to deposit uniform coatings under
0
.
B
12
111
0
.
8
I*
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Fiiure 11. Variations of the solid commition as a function of
near-equilibrium conditions, which results in well-crvstallized layers. These conclusions can be extended to the hinarv B-C system, but another factor plays an important dart in determining the uniformity of the composition; it is the sensitivity of the solid composition to small variations in the gaseous composition. Figure 11 shows the variations of the composition measured as a function of the temperature and the inlet partial pressure in methane. It is clear that, with near-equilibrium conditions of deposition, a t high temperature a small variation of the gaseous composition produces a rapid variation of the solid composition, especially in the composition range 18-25 w t % C. On the contrary, at lower temperatures, the solid composition is not very sensitive to the variations of the gaseous composition. It will he therefore difficult to obtain well-crystallized coatings of uniform thickness and composition, especially in the composition range 1&25 wt % C. In contrast, uniform fine-grained or quasi-amorphous layers of each composition can he obtained easily under kinetically limited conditions (Vandenhulcke et al., 1981).
Conclusions The chemical vapor deposition in a multicomponent gas-solid system is a complex process in which many factors can affect the deposition mechanism. They are not generally independent, and information about their relative importance can he inferred from the comparison of the experimental results with various theoretical model. A mass transfer-equilihrium model gives more information than a classical thermodynamic study as the predicted deposition rates and solid compositions are quantitatively compared to the experimental ones and a possible departure from equilibrium is clearly deduced. Closeness to equilibrium is a complex function of temperature, inlet gas composition, total pressure. and gas flow conditions. As expected from the influence of temperature on the surface kinetics, near-equilibrium conditions are especially
Ind. Eng. Chem. Prod. Res. Dev. 1985, 2 4 , 575-578
approached a t high temperatures where the deposition rates and the solid compositions are fairly well evaluated by the model and a classical thermodynamic study. It can be noted that some homogeneous reactions occur at once, and a model of pure diffusion in the gas phase can no longer be valid. A t low temperatures, the deposition of boron carbide is limited by the surface kinetics. Accordingly, the deposition rates and the solid compositions far depart from the thermodynamic predictions. Boron-carbon solids can therefore be deposited with very different deposition conditions a t the gas-solid interface. These interfacial conditions of deposition, temperature, and amount of departure from equilibrium determine the structure, the composition, and the morphology of the binary B-C deposits. The phases in the B-C phase diagram are produced under near-equilibrium conditions. They are the @-rhombohedral boron solid solution, the rhombohedral boron carbide, and the free carbon. Other forms are often obtained by chemical vapor deposition under kinetically favored conditions of deposition, here BMB2C2, (BI3C2)C,and amorphous B-C solids. Under conditions of high departure from equilibrium and low temperature, boron carbide deposits of each composition can be deposited. The deposits are badly crystallized or amorphous. As the kinetics is the limiting step of the process, the structure, the composition of the solid, and the deposition rate are not very sensitive to some differences in the gas flow dynamics or the gaseous composition. Layers of uniform thickness, composition, and structure can easily be obtained. With near-equilibrium conditions, at high temperature the deposits are polycrystalline. The thickness and the composition of the deposits are very sensitive to the gas flow pattern and the gaseous composition, especially for boron carbide of a composition that approaches B4C. These conditions of deposition are not favorable to obtain continuous uniform layers, particularly on extended surfaces or complex-shaped substrates.
Acknowledgment
575
Registry NO.BCls, 10294-34-5; CHI, 74-82-8; boron carbide, 60063-34-5.
Literature Cited Amberger, E.; Druminskl, M.; Ploog, K. J. Less-Common Met. 1971, 23, 43. Bird, R. 6.; Stewart, W. E.; Lightfoot, E. N. "Transport Phenomena"; Wiley: New York, 1960; p 570. Bouchacourt, M.; Ruste, J.; Thevenot. F. J. Less-Common Met. 1978, 59, 139. Bouchacourt, M.; Ruste, J.; Thevenot, F. J. Microsc. Spectrosc. Nectron 1979, 4 , 143. Carlton, H. E.; Oxley, J. H.; Hall, L. H.; Blocher, J. M., Jr. "Proceedings, 2nd International Conference on Chemical Vapor Deposition"; Blocher. J. M., Jr.; Withers, J. C., Eds.; The Electrochemical Society: Pennlngton, NJ, 1970; p 209. Cochran, A. A.; Donaldson. J. B. Metall. Trans. 1970, 7 , 2875. Donaldson, J. G.; Stephenson, J. B.; Cochran, A. A.; Rep. Invest. U S . Bur. Mines 1988. 7750, 1. Donaidson, J. G.; Stephenson, J. 6.; Cochran, A. A. Nectrodeposition Surf, Treat. 1973174, 2 , 149. Ducarroir, M.; Bernard, C. "Proceedings, 5th International Conference on Chemical Vapor Deposltion"; Blocher, J. M., Jr.; Hintermann, H. E.; Hall, L. H., Eds.; The Electrochemical Society: Pennington, NJ, 1975; p 72. Ducarroir, M.; Bernard, C. J. Nectrochem. SOC. 1976, 723, 136. Eiilot, R. P. "Constitution of Binary Alloys", 1st Suppl.; McGraw-Hill: New York, 1965; p 110. Fitzer. E. Chem. Eng. Tech. 1989, 4 7 , 331. Kleffer, R.; Gugel, E.; Leimer, G.; Ettmayer, P. Ber. Dtsch. Keram. Ges. 1971, 4 8 , 3 8 5 . Mierzejewska, S.; Niemyskl, T. J. Less-Common Met. 1985, 8 , 368. Moore, A. W.; Volk, H. F.; U . S . C . F . S . T . I . AD Rep. (1969). 1989, 693882, 1. Nadal, M.; Ducarrolr, M. presented at the 9th International Symposium on the reactivity of SolMs, Cracovie, Poland, Sept 1-6, 1980. Ploog, K. J. Less-Common Met. 1974, 35, 115, 131. Ploog, K. J. Cryst. Growth 1974, 2 4 / 2 5 , 197. Ploog, K.; Amberger, E. J. Less-Common Met. 1971, 23, 33. Ploog, K.; Schmidt, H.; Amberger, E.; WIII, G.; Kossobutzi, K. H. J. LessCommon Met. 1972. 29, 161. Talky, C. P. "High modulus high strength reinforcements for structural composites", Texaco experlment, Inc., Technical Report ML-TDR-64-88, Part 111, 1965. p 174. Thebault, J.; Naslain, R.; Nagenmuller, P.; Bernard, C. J. Less-Common Met. 1978, 58, 7. Vandenbulcke, L. "Proceedings, 7th International Conference on Chemical Vapor Deposltion"; Sedgwick, T. 0.; Lydtin, H., Eds; The Electrochemical Society: Pennington, NJ, 1979; p 315. Vandenbulcke. L. "Proceedings. 8th International Conference on Chemlcal Vapor Deposition"; Blocher J. M., Jr.; Vuillard, G.; Wahl, G., Eds; The Electrochemical Society: Pennington, NJ, 1981; pp 32, 95. Vandenbulcke, L. Thin Solid Films 1983, 702, 149. Vandenbulcke, L.; Vuillard, G. J. Electrochem. SOC.1977, 724, 1931, 1937. Vandenbulcke. L.; Vuiliard, G. J. Less-Common Met. 1979, 67, 65. Vandenbulcke, L.; Vuillard, 0. J. Less-Common Met. 1981-2, 8 2 , 49. Vandenbulcke, L.; Herbin, R.; Basutpu, M.; Barrandon, J. N. J. Less-Common Met. 1981, 8 0 , 7.
I gratefully recognize R. Herbin for her assistance with the experimental work and Dr. M. B a s u t p and J. N. Barrandon who carried out the analyses.
Received for review D e c e m b e r 7, 1984 Revised manuscript received March 21, 1985 Accepted April 24, 1985
Composite Electroplating Using the Brush Technique Josef Ehrhardt Battelle-Institut e.V., 6000 Frankfurt a m Main GO, West Germany
On the basis of the galvanic dispersion deposition technique and the brush electroplating technique, a method has been developed that permits dispersion coatings resistant to wear, corrosion, and mechanical stresses to be deposited in hollow parts made from a variety of materials.
Introduction According to the galvanic brush technique, a brush, a tissue, or a spongy material like a tampon permeated by electrolyte has to contact the workpiece (cathode) that is to be coated. This technique, which originally had been used only for repair purposes, has been further developed. Thus, special high-speed electrolytes (elektrolytes developed for use with high electrical current density) and 0198-432"
1224-0575$01.50/0
water-cooled anodes are available today, which permit current densities to be applied that are more than 100 times higher than those used in conventional electrolysis. This, together with the high plating rate, makes the brush or tampon technique suitable not only for repair work and for coating workpieces that cannot be electroplated by conventional methods because of their large size but also for direct integration into the mechanical production of @ 1985 Amerlcan Chemical Society
