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Kinetics of chemical vapor deposition of tungsten carbide
Thin Solid Films, 219 (1992) 103-108
103
Kinetics of chemical vapor deposition of tungsten carbide C. Michael Kelly Villanova University, Department of Chemical Engineering, Villanova, PA 19085 (USA)
Diwakar Garg and Paul N. Dyer Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195 (USA)
(Received November 29, 1991; revised March 3, 1992; accepted June 1, 1992)
Abstract
Hard tungsten carbide based coatings were deposited by chemical vapor deposition (CVD) at low temperatures onto metal substrates by hydrogen reduction of tungsten hexafluoride (WF6) in the presence of dimethyl ether (DME). These coatings contain a mixture of tungsten and tungsten carbide in the form of W3C, W2C, or a combination of W3C and W2C depending on reaction conditions. They exhibit layered morphology, which is primarily responsible for their excellent erosion and wear-resistance properties. The deposition of these coatings with layered morphology is believed to involve two simultaneous reactions, leading to the deposition of tungsten and tungsten carbide phases with crystallite size varying between 50 and 150/~. Since the deposition of these coatings in a large-scale commercial reactor requires knowledge about the rate and order of simultaneous reactions, a pilot-scale CVD reactor was designed and operated to investigate reaction kinetics. This paper describes an experimental approach to determine the apparent reaction order and kinetic rate constant for depositing the coating with optimal properties. It also discusses the significance of these kinetic parameters on scaling up reactor design.
1. Introduction
Hard tungsten carbide based coatings with excellent erosion and wear resistance have been successfully deposited onto various substrates by hydrogen reduction of WF 6 in the presence of dimethyl ether (DME) in a chemical vapor deposition (CVD) reactor [1-4]. These coatings contain two distinct layers deposited over the nickel interlayer, as shown in Fig. 1. The function of the nickel interlayer is to protect the substrate from attack by hot and corrosive hydrofluoric acid (HF) produced during the deposition reactions. The inner layer of the coating consists of pure columnar tungsten, deposited by hydrogen reduction of tungsten hex-
lO ~.m
l
TUNGSTEN/ TUNGSTEN CARBIDE TUNGSTEN .NICKEL A M - 3 5 0 STAINLESS STEEL SUBSTRATE
Fig. 1. Cross-sectional photomicrograph of erosion and wear resistant coating with an outer layer containing a mixture of tungsten and tungsten carbide (W2C) with layered morphology.
0040-6090/92/$5.00
afluoride (WF6) in the absence of DME. Above this is an outer layer of tungsten carbide with a fine-grained, non-columnar morphology. The outer tungsten carbide layer of these coatings, when deposited within a relatively narrow range of temperature and partial pressures of reactant gases, exhibits a distinct lamellar morphology (Fig. 1) and contains a mixture of a pure tungsten phase and a carbon-containing phase in the form of W2C, W3C, or a mixture of W2C and W3C. The individual sublayers in the outer tungsten carbide layer contain intermingled phases of tungsten and tungsten carbide with their proportions varying from one layer to the next [4]. The deposition of these tungsten carbide coatings is postulated to involve two simultaneous reactions, leading to the deposition of pure tungsten (reaction A) and tungsten carbide in the form of W2C, W3C, or a mixture of W2C and W3C (reaction B). Models have also been postulated to explain the deposition of layered tungsten carbide coatings by natural oscillation of reactions between these two reactions with the same periodicity as observed in the coatings [5]. Similar naturally occurring oscillations in CVD reactions have been reported by Bartsch et al. [6] The effective deposition of these tungsten carbide coatings in a scaled-up commercial reactor requires knowledge of the reaction rate order and rate constant for the two reactions or the combined reactions at
© 1992-- Elsevier Sequoia. All rights reserved
104
C. M. Kelly et al. / CVD of tungswn carbide
preferred temperature and reagent partial pressures. The deposition of pure tungsten by hydrogen reduction of W F 6 has been extensively studied and is reported to be of very low order in the partial pressure of W F 6 [7-9]. However, nothing has been reported on either the reaction mechanism or the reaction order of the postulated second reaction. Since it is not possible experimentally to study the postulated second reaction (reaction B) in the absence of the first (reaction A), the reaction order of the second reaction cannot be determined independently. Therefore, it is necessary to develop a kinetic model to determine the reaction rate order for the combined reactions. Deposition studies carried out in a pilot-scale CVD reactor have shown that tungsten carbide coatings with desired composition, morphology, and erosion and wear-resistance properties can only be deposited in a narrow range of temperature and partial pressures of reactants [1 4]. Any departure from this range of operating parameters, especially the partial pressures of the reactants, has been found to result in deposition of tungsten carbide coatings with columnar morphology and poor erosion and wear properties. The scope of the present investigation has been limited to determining deposition kinetics when operating at conditions that produce coatings with the desired lamellar morphology and optimal properties. No attempt has been made to extend the kinetics to other operating regimes. Studies have also shown that the thickness of these coatings needs to be fairly uniform (within a range of 10%) to obtain optimum erosion and wear-resistance properties. It is well known that reactant gases flow through a collection of objects placed in a CVD reactor without any back mixing. This changes the concentration of reactants along the length of the reactor owing to their depletion. These changes may result in deposition of tungsten carbide coatings with variable thicknesses and properties on the parts. Therefore, when scaling up the reactor design, it is necessary for the design engineers to be able to compute changes in the concentration of reactants, which affect composition, morphology, and erosion and wear-resistance properties of these coatings. This requires knowledge of the reaction order and the rate constant. To determine the reaction rate order of the combined reactions and to investigate the effect of depletion of reactants on coating composition, morphology, and deposition rate, a multi-section, pilot-scale reactor was designed and operated under reaction conditions known to deposit tungsten carbide coatings with desired composition, morphology, and erosion and wearresistance properties. The details of reactor design and test results are discussed below.
2. Experimental details A pilot-scale inductively heated, hot-wall cylindrical graphite reactor, shown in Fig. 2(a), was initially used to develop layered tungsten carbide coatings. The cylindrical reactor was 14 cm in diameter and had a reaction zone approximately 20.4 cm long. Reactant gases, premixed at ambient temperature, were introduced from the top in this reactor. In order to create a uniform flow in the reaction zone, a perforated distributor plate was interposed just downstream of the injection port [10]. The perforated plate also served to pre-heat reactant gases to the desired temperature before they entered the reaction zone. Use of the distribution system was necessary to deposit coatings with uniform composition and thickness. However, it resulted in secondary deposition of the coating on the hot surfaces above the substrates being coated, depleting reactants and changing their concentrations by an unknown amount before reaching the substrates. This deposition made investigation of the rate and reaction kinetics in this reactor very difficult. To overcome the problem of the inability to measure W F 6 partial pressure in the reaction zone, an 8.9 cm in diameter inner graphite reactor chamber, which could fit within the original pilot-scale reactor, was designed and assembled. Both the inner and the outer reactor chambers were sectionalized, so that they could be disassembled and weighed accurately before and after each deposition experiment. All the test coupons for the kinetic study were placed in the inner chamber. The sections of the inner reactor chamber are shown in Fig. 2(b), and the assembled reactor is illustrated in Fig. 2(c). The five sections of this inner cylindrical reactor chamber were separated by perforated distributor plates similar to the plate used in the original reactor. Seven small test coupons made of graphite were supported on each distributor plate to study the deposition rate, reaction kinetics, and coating composition and morphology. As the gaseous reactants flowed through the reactor, they deposited a tungsten carbide coating onto the distributor plates, cylindrical walls, and test coupons. Weight changes of the various reactor sections were then used to calculate the depletion of reactants and to estimate their partial pressures at the location of the test coupons. The tungsten carbide deposition experiments were carried out in the presence of a large excess of hydrogen at a total reactor pressure of 5.33 kPa (40 Torr) and a nominal temperature of 500 °C. The inlet gas comprised W F 6 , DME, hydrogen, and an inert diluent gas such as argon; the composition was accurately controlled by mass flow controllers. The partial pressures of hydrogen and diluent in the inlet gas were kept constant in all the experiments. The flow rates of reactants WF~, and DME
105
C. M. Kelly et al. / CVD of tungsten carbide
,~FEEDGASES
FEEDGASES REACTOR CHIMNETr~ REACTOR COVERam_ . . . . . . ~ PERFORATED ~_. _~ PLATE 7 ........ NORMAL_ _ i s Ri
......... ~'
~
R"~ION-- ~ ZONE /
~
~U.~,~)RT ~b .
J~
.
.
.
.
.
.
.
age) ZONE ~ICS
SECTION
~ .
3.5"DIAMETER CYLINDRICAL GRAPHITE REACTOR INNER
9 PERFORATED PLATE
~". . . . . d .
SUPPORT PLATE
GASESOUT
GASESOUT a. OUTER REACTORASSEMBLY
b. INNERREACTORASSEMBLY
c. ASSEMBLEDREACTOR (test couponsillustrated)
Fig. 2. Pilot scale CVD reactor design.
were selected to match the range of W F 6 and D M E partial pressures found previously to produce the tungsten carbide coating (a mixture of tungsten and tungsten carbide in the form of W2C) with the desired lamellar morphology and the best erosion and wear-resistance properties [ 1-4]. The partial pressure of W F 6 in the feed gas (at the inlet to the reactor) was varied from approximately 1.0 to 5.0 Torr while keeping the ratio of W F 6 to D M E constant at approximately 10 in all the experiments. The depletion of W F 6 reactant was determined from the weight gain by the individual sections of the reactor and test coupons, and the partial pressure of W F 6 at the inlet of each tier (or reactor section) was calculated by material balance. The coating rate was determined from the weight gain by the coupons coated in each tier. Additionally, in terms of thickness, the coating rate was determined by sectioning and measuring the coating thickness on the coupons coated in each tier. Because water and other sources of oxygen had previously been found to affect the nature of both tungsten and tungsten carbide coatings, extreme care was taken to eliminate even trace amounts of water and oxygen both from gaseous feed lines and the reactor. All reactant gases were analyzed before use and found to be free of moisture. At the beginning of each deposition experiment, all feed lines that might have been exposed to atmosphere (e.g. during disassembly steps after the previous experiment) were evacuated and baked out to remove adsorbed oxygen and moisture. Additionally, the reactor was carefully leak checked to minimize chances of contamination by ambient air during the deposition experiment. Finally, no evidence of moisture
or oxygen contamination was noted on any of the test coupons.
3. Resultsanddiscussion 3.1. R e a c t a n t concentrations
The computed partial pressure of reactants in each tier in the reactor depends on the reaction stoichiometry and on the ratio of tungsten and W2C in the coating. Since neither of these could be determined dffectly and accurately, reasonable assumptions were made about them for material balancing purposes. The deposition of tungsten carbide coating is postulated to involve the following two simultaneous reactions A and B: Reaction A
WF 6
+ 3H2 ~ W + 6HF
Reaction B
3WF 6 + D M E + 6H 2 W2C q- WOF 4 d- CH 4 d- 14HF
The hydrogen reduction of W F 6 for depositing tungsten in the absence of D M E has been extensively studied [7-9], and the stoichiometry of reaction A has been well established. However, studies of tungsten carbide deposition by hydrogen reduction of W F 6 in the presence of D M E have not been reported in the literature, thus the stoichiometry of this reaction is unknown. The major by-products observed in the reactor exhaust gas by an in-line mass spectrometer are HF, WOF4 and CH 4. Therefore, reaction B is believed to represent the deposition of W2C by hydrogen reduction of W F 6 in the presence of DME.
106
c. M. Kelly et al. / CVD of tungsten carbide
Because W F 6 is consumed in both reactions, material balances involving the conversion of W F 6 would be expected to depend on the ratio of W F 6 participating in the tungsten deposition reaction (reaction A) to that participating in W2C deposition reaction (reaction B). Since the ratio of tungsten and W2C in the coating could not be determined accurately, the ratio of moles of W F 6 participating in reaction B to those in reaction A was taken as an adjustable parameter to compute reactant concentrations in each section of the reactor. The partial pressure of W F 6 w a s computed on each section of the reactor assuming (1) that equal amounts of W F 6 participated in the two reactions and (2) ten times as much W F 6 participated in reaction A as in reaction B. The results of these computations for a representative deposition experiment are shown in Fig. 3. Although the partial pressure of WF 6 on each section varies depending on the assumed W F 6 participation ratio, the slope of the overall curve or the apparent reaction order is essentially the same, regardless of the assumption. Therefore, the assumption about the proportions of tungsten and WaC in the coating or moles of W F 6 participating in reactions A and B has a minor effect on the computed WF 6 partial pressure and apparent reaction kinetics. Because the weight percent of carbon incorporated in the tungsten carbide coating was small, it was difficult to calculate accurately the concentration of D M E in each section of the reactor by material balance. It is, however, known that the ratio of the partial pressures of WF6 and D M E must be within a narrow range to deposit tungsten carbide coatings with the desired composition and morphology. Since all the test coupons placed in the upper tiers of the sectionalized reactor were coated with the
uJ
0.01
3.2. A p p a r e n t reaction rate order
The partial pressure of W F 6 in each section of the reactor was calculated for all the tungsten carbide deposition experiments, assuming the ratio of moles of W F 6 participating in reaction B to those in reaction A to be 0.5. The partial pressure of WF6 was then plotted against the observed coating rate (based upon weight gain) in Fig. 4. The coating rate appeared to show first-order dependence, over a wide range of W F 6 partial pressure (partial pressure of W F 6 varied by over a factor of three). A similar plot, assuming equal moles of
NOTE: BAR INDICATES RANGE OF WF6 PARTIAL PRESSURE COMPUTED BY MATERIAL BALANCE AS FUNCTION OF RATIO OF REACTION B TO REACTION A
w
B
I--
< rr
f
n
m
m
ZA
<'~
desired composition and morphology, and since the coating on the test coupons placed in the bottom tier showed only a slight departure from the desired composition and morphology, it was believed that the ratio of the partial pressures of W F 6 and DME did not change considerably from the top to the bottom of the reactor. Therefore, the depletion of WF 6 was considered to be an adequate surrogate for the depletion of both reactants within the range of WF6 partial pressure studied and reported here. However, it would be unwise to extrapolate the results to a more complete conversion, as this would probably take the ratio of W F 6 and D M E partial pressures out of the acceptable range and change the overall reaction mechanism. Figure 3 shows that the coating rate declined as the gases moved down in the reactor. This decline in coating rate was believed to be related to a decrease in the partial pressure of the reactants. Therefore, it would be desirable to select carefully the length, diameter, and packing density in the scaled-up commercial reactor to minimize variation in coating thickness between the parts coated at the top and the bottom of the reactor.
0.004
m
Oca Z'-"
I REACTION B
O
o.
REACTION A
I =0.1
REACTION B REACTION A
=
1.0
O O
I
0.001 0.4
COMPUTED
I IIII
I 1.0
WF 6 PARTIAL
2.0 PRESSURE,
torr
Fig. 3. The dependence of computed WF6 partial pressure on assumed proportions of competing deposition reactions (result of a single deposition experiment illustrated).
C. M. Kelly et al. / CVD of tungsten carbide
107
3.4. Coating composition KI
~,<
0.01
-
oO OO
rr
z.-:. t-
c
0.005
-
<~ o~
-
0 0"~ Z ~
_
o
o
0
0.001 0.4
I
I
I
0.5
1.0
2.0
W F s P A R T I A L P R E S S U R E , torr
Fig. 4. Effectof reactant depletion on coating rate assuming the ratio of moles of W F 6 participating in reaction B to those in reaction A to be 0.5.
participating in the two reactions, showed that the dependence of coating rate o n W F 6 partial pressure was 0.9 power, which is still essentially first order. The coating rate in terms of coating thickness also showed close to first-order dependence on WF6 partial pressure and is best represented by the following expression: WF 6
rate = 0.17 × (partial pressure of
WF6) n
where the rate is in micrometers per minute, the partial pressure is in torr, and the exponent n varies between 0.9 and 1.0 depending on the assumed ratio of moles of W F 6 participating in reaction B to those in reaction A, under the experimental conditions studied. The first-order dependence of the tungsten carbide deposition rate o n W F 6 partial pressure was in contrast to a low-order dependence (reported to be as low as 1/6th power) on W F 6 partial pressure for the deposition of pure tungsten [7-9]. This information, once again, showed the importance of carefully selecting the length, diameter, and packing density in the scaled-up commercial reactor to minimize the variation in coating thicknesses between the parts coated at the top and the bottom of the reactor.
X-ray diffraction and transmission electron microscopy analyses of the coatings deposited in various experiments revealed the presence of distinct phases of tungsten and tungsten carbide in the form of W2C [3, 4]. Optical microscopy indicated that the majority of the coatings had the desired layered (lamellar) morphology similar to the outer layer in Fig. 1. Interestingly, however, tungsten carbide coatings deposited at the lowest partial pressures of W F 6 began to show slight departures from the desired layered morphology. These observations suggest that it would be desirable to select carefully the range of W F 6 partial pressure while depositing tungsten carbide coatings in a scaled-up commercial reactor.
4. C o n c l u s i o n s
A multi-section pilot-scale CVD reactor was designed and operated to determine the reaction kinetics and critical parameters required to scale up reactor design for depositing tungsten carbide coatings with layered morphology. The deposition of these coatings was postulated to involve two simultaneous reactions, one for depositing the tungsten phase and the other for the tungsten carbide phase. The experimental results revealed the overall tungsten carbide coating deposition by the postulated simultaneous reactions to be much slower than that of pure tungsten deposition by hydrogen reduction of WF6. The reaction rate order for the overall tungsten carbide coating deposition was found to have first-order dependence of W F 6 partial pressure, which contrasted with 1/6th order dependence o n W F 6 partial pressure for the tungsten deposition reaction. The experimental results also showed the importance of carefully selecting the length, diameter, packing density, and range of W F 6 partial pressure for depositing tungsten carbide coatings uniformly and with the desired composition and layered morphology in a scaled-up commercial reactor.
3.3. Reaction m e c h a n i s m
Interestingly, the tungsten carbide deposition reaction, which has been postulated to be a combination of two simultaneous reactions, was found to proceed much more slowly than the deposition of pure tungsten a l o n e - - t h e deposition rate of tungsten carbide was approximately one-half that of pure tungsten under identical conditions of temperature and W F 6 partial pressure. The carbon-producing reaction involving D M E is presumed to tie up active surface sites, thereby slowing the overall deposition rate.
References
1 D. Garg, D. Dimos, P. N. Dyer and R. E. Stevens, Erosion-resistant coatings containing tungsten carbide by low-temperature CVD, paper presented at the 15th Int. Conf. on Metallurgical Coatings, San Diego, CA, April 1988.
2 P. N. Dyer, D. Garg, S. Sunder, H. E. Hintermann and M. Maillat, Wear-resistant coatings containing tungsten carbide deposited by low-temperatureCVD, paper presented at the 15th Int. Conf. on Metallurgical Coatings, San Diego, CA, April 1988.
3 D. Garg, P. N. Dyer, D. Dimos, S. Sunder, H. E. Hintermann and M. Maillat, Low-temperatureCVD tungsten carbide coatings for
108
4
5 6 7
C. M. Kelly et al. / CVD of tungsten carbide
wear/erosion resistance, Proc. 12th Annu. Conf. on Composites and Advanced Ceramic Materials, Vol. 2, American Ceramic Society, Westerville, FL, 1988, pp. 1215-1222. D. Garg, P. N. Dyer, D. Dimos, S. Sunder, H. E. Hintermann and M. Maillat, Low-temperature chemical vapor deposition tungsten carbide coatings for wear/erosion resistance, J. Am. Ceram. Sot., 75 (4) (1992) 1008-1011. D. W. Skafand C. M. Kelly, Sustained oscillation in simple models for chemical vapor deposition, Thin Solid Films, 207(1992) 57-64. K. Bartsch, A. Leonhart and E. Wolf, J. Phys. IV, Coll. C2, l (1991) 563 570. W. A. Bryant and G. H. Meier, Kinetics of the chemical vapor
deposition of tungsten, J. Electrochem. Soc., 120 (4) (1973) 559 565. 8 W. A. Bryant, J. Electrochem. Soc., 125 (9) (1978) 1534 1543. 9 P. Van der Putte, The reaction kinetics of the H 2 reduction of WF,, in the chemical vapor deposition of tungsten films. In E. K. Broadbent (ed.), Tungsten and Other ReJ?actory Materials .l~r ULSI Applications H, Materials Research Society, Pittsburgh, PA, 1987, p. 77. 10 D. Garg and C. M. Kelly, Reactor design for chemical vapor deposition of tungsten carbide coatings, paper presented at the Annu. Meet. Americim Institute of Chemical Engineers, Los Angeles, CA~ November 1991.
103
Kinetics of chemical vapor deposition of tungsten carbide C. Michael Kelly Villanova University, Department of Chemical Engineering, Villanova, PA 19085 (USA)
Diwakar Garg and Paul N. Dyer Air Products and Chemicals, Inc., 7201 Hamilton Blvd., Allentown, PA 18195 (USA)
(Received November 29, 1991; revised March 3, 1992; accepted June 1, 1992)
Abstract
Hard tungsten carbide based coatings were deposited by chemical vapor deposition (CVD) at low temperatures onto metal substrates by hydrogen reduction of tungsten hexafluoride (WF6) in the presence of dimethyl ether (DME). These coatings contain a mixture of tungsten and tungsten carbide in the form of W3C, W2C, or a combination of W3C and W2C depending on reaction conditions. They exhibit layered morphology, which is primarily responsible for their excellent erosion and wear-resistance properties. The deposition of these coatings with layered morphology is believed to involve two simultaneous reactions, leading to the deposition of tungsten and tungsten carbide phases with crystallite size varying between 50 and 150/~. Since the deposition of these coatings in a large-scale commercial reactor requires knowledge about the rate and order of simultaneous reactions, a pilot-scale CVD reactor was designed and operated to investigate reaction kinetics. This paper describes an experimental approach to determine the apparent reaction order and kinetic rate constant for depositing the coating with optimal properties. It also discusses the significance of these kinetic parameters on scaling up reactor design.
1. Introduction
Hard tungsten carbide based coatings with excellent erosion and wear resistance have been successfully deposited onto various substrates by hydrogen reduction of WF 6 in the presence of dimethyl ether (DME) in a chemical vapor deposition (CVD) reactor [1-4]. These coatings contain two distinct layers deposited over the nickel interlayer, as shown in Fig. 1. The function of the nickel interlayer is to protect the substrate from attack by hot and corrosive hydrofluoric acid (HF) produced during the deposition reactions. The inner layer of the coating consists of pure columnar tungsten, deposited by hydrogen reduction of tungsten hex-
lO ~.m
l
TUNGSTEN/ TUNGSTEN CARBIDE TUNGSTEN .NICKEL A M - 3 5 0 STAINLESS STEEL SUBSTRATE
Fig. 1. Cross-sectional photomicrograph of erosion and wear resistant coating with an outer layer containing a mixture of tungsten and tungsten carbide (W2C) with layered morphology.
0040-6090/92/$5.00
afluoride (WF6) in the absence of DME. Above this is an outer layer of tungsten carbide with a fine-grained, non-columnar morphology. The outer tungsten carbide layer of these coatings, when deposited within a relatively narrow range of temperature and partial pressures of reactant gases, exhibits a distinct lamellar morphology (Fig. 1) and contains a mixture of a pure tungsten phase and a carbon-containing phase in the form of W2C, W3C, or a mixture of W2C and W3C. The individual sublayers in the outer tungsten carbide layer contain intermingled phases of tungsten and tungsten carbide with their proportions varying from one layer to the next [4]. The deposition of these tungsten carbide coatings is postulated to involve two simultaneous reactions, leading to the deposition of pure tungsten (reaction A) and tungsten carbide in the form of W2C, W3C, or a mixture of W2C and W3C (reaction B). Models have also been postulated to explain the deposition of layered tungsten carbide coatings by natural oscillation of reactions between these two reactions with the same periodicity as observed in the coatings [5]. Similar naturally occurring oscillations in CVD reactions have been reported by Bartsch et al. [6] The effective deposition of these tungsten carbide coatings in a scaled-up commercial reactor requires knowledge of the reaction rate order and rate constant for the two reactions or the combined reactions at
© 1992-- Elsevier Sequoia. All rights reserved
104
C. M. Kelly et al. / CVD of tungswn carbide
preferred temperature and reagent partial pressures. The deposition of pure tungsten by hydrogen reduction of W F 6 has been extensively studied and is reported to be of very low order in the partial pressure of W F 6 [7-9]. However, nothing has been reported on either the reaction mechanism or the reaction order of the postulated second reaction. Since it is not possible experimentally to study the postulated second reaction (reaction B) in the absence of the first (reaction A), the reaction order of the second reaction cannot be determined independently. Therefore, it is necessary to develop a kinetic model to determine the reaction rate order for the combined reactions. Deposition studies carried out in a pilot-scale CVD reactor have shown that tungsten carbide coatings with desired composition, morphology, and erosion and wear-resistance properties can only be deposited in a narrow range of temperature and partial pressures of reactants [1 4]. Any departure from this range of operating parameters, especially the partial pressures of the reactants, has been found to result in deposition of tungsten carbide coatings with columnar morphology and poor erosion and wear properties. The scope of the present investigation has been limited to determining deposition kinetics when operating at conditions that produce coatings with the desired lamellar morphology and optimal properties. No attempt has been made to extend the kinetics to other operating regimes. Studies have also shown that the thickness of these coatings needs to be fairly uniform (within a range of 10%) to obtain optimum erosion and wear-resistance properties. It is well known that reactant gases flow through a collection of objects placed in a CVD reactor without any back mixing. This changes the concentration of reactants along the length of the reactor owing to their depletion. These changes may result in deposition of tungsten carbide coatings with variable thicknesses and properties on the parts. Therefore, when scaling up the reactor design, it is necessary for the design engineers to be able to compute changes in the concentration of reactants, which affect composition, morphology, and erosion and wear-resistance properties of these coatings. This requires knowledge of the reaction order and the rate constant. To determine the reaction rate order of the combined reactions and to investigate the effect of depletion of reactants on coating composition, morphology, and deposition rate, a multi-section, pilot-scale reactor was designed and operated under reaction conditions known to deposit tungsten carbide coatings with desired composition, morphology, and erosion and wearresistance properties. The details of reactor design and test results are discussed below.
2. Experimental details A pilot-scale inductively heated, hot-wall cylindrical graphite reactor, shown in Fig. 2(a), was initially used to develop layered tungsten carbide coatings. The cylindrical reactor was 14 cm in diameter and had a reaction zone approximately 20.4 cm long. Reactant gases, premixed at ambient temperature, were introduced from the top in this reactor. In order to create a uniform flow in the reaction zone, a perforated distributor plate was interposed just downstream of the injection port [10]. The perforated plate also served to pre-heat reactant gases to the desired temperature before they entered the reaction zone. Use of the distribution system was necessary to deposit coatings with uniform composition and thickness. However, it resulted in secondary deposition of the coating on the hot surfaces above the substrates being coated, depleting reactants and changing their concentrations by an unknown amount before reaching the substrates. This deposition made investigation of the rate and reaction kinetics in this reactor very difficult. To overcome the problem of the inability to measure W F 6 partial pressure in the reaction zone, an 8.9 cm in diameter inner graphite reactor chamber, which could fit within the original pilot-scale reactor, was designed and assembled. Both the inner and the outer reactor chambers were sectionalized, so that they could be disassembled and weighed accurately before and after each deposition experiment. All the test coupons for the kinetic study were placed in the inner chamber. The sections of the inner reactor chamber are shown in Fig. 2(b), and the assembled reactor is illustrated in Fig. 2(c). The five sections of this inner cylindrical reactor chamber were separated by perforated distributor plates similar to the plate used in the original reactor. Seven small test coupons made of graphite were supported on each distributor plate to study the deposition rate, reaction kinetics, and coating composition and morphology. As the gaseous reactants flowed through the reactor, they deposited a tungsten carbide coating onto the distributor plates, cylindrical walls, and test coupons. Weight changes of the various reactor sections were then used to calculate the depletion of reactants and to estimate their partial pressures at the location of the test coupons. The tungsten carbide deposition experiments were carried out in the presence of a large excess of hydrogen at a total reactor pressure of 5.33 kPa (40 Torr) and a nominal temperature of 500 °C. The inlet gas comprised W F 6 , DME, hydrogen, and an inert diluent gas such as argon; the composition was accurately controlled by mass flow controllers. The partial pressures of hydrogen and diluent in the inlet gas were kept constant in all the experiments. The flow rates of reactants WF~, and DME
105
C. M. Kelly et al. / CVD of tungsten carbide
,~FEEDGASES
FEEDGASES REACTOR CHIMNETr~ REACTOR COVERam_ . . . . . . ~ PERFORATED ~_. _~ PLATE 7 ........ NORMAL_ _ i s Ri
......... ~'
~
R"~ION-- ~ ZONE /
~
~U.~,~)RT ~b .
J~
.
.
.
.
.
.
.
age) ZONE ~ICS
SECTION
~ .
3.5"DIAMETER CYLINDRICAL GRAPHITE REACTOR INNER
9 PERFORATED PLATE
~". . . . . d .
SUPPORT PLATE
GASESOUT
GASESOUT a. OUTER REACTORASSEMBLY
b. INNERREACTORASSEMBLY
c. ASSEMBLEDREACTOR (test couponsillustrated)
Fig. 2. Pilot scale CVD reactor design.
were selected to match the range of W F 6 and D M E partial pressures found previously to produce the tungsten carbide coating (a mixture of tungsten and tungsten carbide in the form of W2C) with the desired lamellar morphology and the best erosion and wear-resistance properties [ 1-4]. The partial pressure of W F 6 in the feed gas (at the inlet to the reactor) was varied from approximately 1.0 to 5.0 Torr while keeping the ratio of W F 6 to D M E constant at approximately 10 in all the experiments. The depletion of W F 6 reactant was determined from the weight gain by the individual sections of the reactor and test coupons, and the partial pressure of W F 6 at the inlet of each tier (or reactor section) was calculated by material balance. The coating rate was determined from the weight gain by the coupons coated in each tier. Additionally, in terms of thickness, the coating rate was determined by sectioning and measuring the coating thickness on the coupons coated in each tier. Because water and other sources of oxygen had previously been found to affect the nature of both tungsten and tungsten carbide coatings, extreme care was taken to eliminate even trace amounts of water and oxygen both from gaseous feed lines and the reactor. All reactant gases were analyzed before use and found to be free of moisture. At the beginning of each deposition experiment, all feed lines that might have been exposed to atmosphere (e.g. during disassembly steps after the previous experiment) were evacuated and baked out to remove adsorbed oxygen and moisture. Additionally, the reactor was carefully leak checked to minimize chances of contamination by ambient air during the deposition experiment. Finally, no evidence of moisture
or oxygen contamination was noted on any of the test coupons.
3. Resultsanddiscussion 3.1. R e a c t a n t concentrations
The computed partial pressure of reactants in each tier in the reactor depends on the reaction stoichiometry and on the ratio of tungsten and W2C in the coating. Since neither of these could be determined dffectly and accurately, reasonable assumptions were made about them for material balancing purposes. The deposition of tungsten carbide coating is postulated to involve the following two simultaneous reactions A and B: Reaction A
WF 6
+ 3H2 ~ W + 6HF
Reaction B
3WF 6 + D M E + 6H 2 W2C q- WOF 4 d- CH 4 d- 14HF
The hydrogen reduction of W F 6 for depositing tungsten in the absence of D M E has been extensively studied [7-9], and the stoichiometry of reaction A has been well established. However, studies of tungsten carbide deposition by hydrogen reduction of W F 6 in the presence of D M E have not been reported in the literature, thus the stoichiometry of this reaction is unknown. The major by-products observed in the reactor exhaust gas by an in-line mass spectrometer are HF, WOF4 and CH 4. Therefore, reaction B is believed to represent the deposition of W2C by hydrogen reduction of W F 6 in the presence of DME.
106
c. M. Kelly et al. / CVD of tungsten carbide
Because W F 6 is consumed in both reactions, material balances involving the conversion of W F 6 would be expected to depend on the ratio of W F 6 participating in the tungsten deposition reaction (reaction A) to that participating in W2C deposition reaction (reaction B). Since the ratio of tungsten and W2C in the coating could not be determined accurately, the ratio of moles of W F 6 participating in reaction B to those in reaction A was taken as an adjustable parameter to compute reactant concentrations in each section of the reactor. The partial pressure of W F 6 w a s computed on each section of the reactor assuming (1) that equal amounts of W F 6 participated in the two reactions and (2) ten times as much W F 6 participated in reaction A as in reaction B. The results of these computations for a representative deposition experiment are shown in Fig. 3. Although the partial pressure of WF 6 on each section varies depending on the assumed W F 6 participation ratio, the slope of the overall curve or the apparent reaction order is essentially the same, regardless of the assumption. Therefore, the assumption about the proportions of tungsten and WaC in the coating or moles of W F 6 participating in reactions A and B has a minor effect on the computed WF 6 partial pressure and apparent reaction kinetics. Because the weight percent of carbon incorporated in the tungsten carbide coating was small, it was difficult to calculate accurately the concentration of D M E in each section of the reactor by material balance. It is, however, known that the ratio of the partial pressures of WF6 and D M E must be within a narrow range to deposit tungsten carbide coatings with the desired composition and morphology. Since all the test coupons placed in the upper tiers of the sectionalized reactor were coated with the
uJ
0.01
3.2. A p p a r e n t reaction rate order
The partial pressure of W F 6 in each section of the reactor was calculated for all the tungsten carbide deposition experiments, assuming the ratio of moles of W F 6 participating in reaction B to those in reaction A to be 0.5. The partial pressure of WF6 was then plotted against the observed coating rate (based upon weight gain) in Fig. 4. The coating rate appeared to show first-order dependence, over a wide range of W F 6 partial pressure (partial pressure of W F 6 varied by over a factor of three). A similar plot, assuming equal moles of
NOTE: BAR INDICATES RANGE OF WF6 PARTIAL PRESSURE COMPUTED BY MATERIAL BALANCE AS FUNCTION OF RATIO OF REACTION B TO REACTION A
w
B
I--
< rr
f
n
m
m
ZA
<'~
desired composition and morphology, and since the coating on the test coupons placed in the bottom tier showed only a slight departure from the desired composition and morphology, it was believed that the ratio of the partial pressures of W F 6 and DME did not change considerably from the top to the bottom of the reactor. Therefore, the depletion of WF 6 was considered to be an adequate surrogate for the depletion of both reactants within the range of WF6 partial pressure studied and reported here. However, it would be unwise to extrapolate the results to a more complete conversion, as this would probably take the ratio of W F 6 and D M E partial pressures out of the acceptable range and change the overall reaction mechanism. Figure 3 shows that the coating rate declined as the gases moved down in the reactor. This decline in coating rate was believed to be related to a decrease in the partial pressure of the reactants. Therefore, it would be desirable to select carefully the length, diameter, and packing density in the scaled-up commercial reactor to minimize variation in coating thickness between the parts coated at the top and the bottom of the reactor.
0.004
m
Oca Z'-"
I REACTION B
O
o.
REACTION A
I =0.1
REACTION B REACTION A
=
1.0
O O
I
0.001 0.4
COMPUTED
I IIII
I 1.0
WF 6 PARTIAL
2.0 PRESSURE,
torr
Fig. 3. The dependence of computed WF6 partial pressure on assumed proportions of competing deposition reactions (result of a single deposition experiment illustrated).
C. M. Kelly et al. / CVD of tungsten carbide
107
3.4. Coating composition KI
~,<
0.01
-
oO OO
rr
z.-:. t-
c
0.005
-
<~ o~
-
0 0"~ Z ~
_
o
o
0
0.001 0.4
I
I
I
0.5
1.0
2.0
W F s P A R T I A L P R E S S U R E , torr
Fig. 4. Effectof reactant depletion on coating rate assuming the ratio of moles of W F 6 participating in reaction B to those in reaction A to be 0.5.
participating in the two reactions, showed that the dependence of coating rate o n W F 6 partial pressure was 0.9 power, which is still essentially first order. The coating rate in terms of coating thickness also showed close to first-order dependence on WF6 partial pressure and is best represented by the following expression: WF 6
rate = 0.17 × (partial pressure of
WF6) n
where the rate is in micrometers per minute, the partial pressure is in torr, and the exponent n varies between 0.9 and 1.0 depending on the assumed ratio of moles of W F 6 participating in reaction B to those in reaction A, under the experimental conditions studied. The first-order dependence of the tungsten carbide deposition rate o n W F 6 partial pressure was in contrast to a low-order dependence (reported to be as low as 1/6th power) on W F 6 partial pressure for the deposition of pure tungsten [7-9]. This information, once again, showed the importance of carefully selecting the length, diameter, and packing density in the scaled-up commercial reactor to minimize the variation in coating thicknesses between the parts coated at the top and the bottom of the reactor.
X-ray diffraction and transmission electron microscopy analyses of the coatings deposited in various experiments revealed the presence of distinct phases of tungsten and tungsten carbide in the form of W2C [3, 4]. Optical microscopy indicated that the majority of the coatings had the desired layered (lamellar) morphology similar to the outer layer in Fig. 1. Interestingly, however, tungsten carbide coatings deposited at the lowest partial pressures of W F 6 began to show slight departures from the desired layered morphology. These observations suggest that it would be desirable to select carefully the range of W F 6 partial pressure while depositing tungsten carbide coatings in a scaled-up commercial reactor.
4. C o n c l u s i o n s
A multi-section pilot-scale CVD reactor was designed and operated to determine the reaction kinetics and critical parameters required to scale up reactor design for depositing tungsten carbide coatings with layered morphology. The deposition of these coatings was postulated to involve two simultaneous reactions, one for depositing the tungsten phase and the other for the tungsten carbide phase. The experimental results revealed the overall tungsten carbide coating deposition by the postulated simultaneous reactions to be much slower than that of pure tungsten deposition by hydrogen reduction of WF6. The reaction rate order for the overall tungsten carbide coating deposition was found to have first-order dependence of W F 6 partial pressure, which contrasted with 1/6th order dependence o n W F 6 partial pressure for the tungsten deposition reaction. The experimental results also showed the importance of carefully selecting the length, diameter, packing density, and range of W F 6 partial pressure for depositing tungsten carbide coatings uniformly and with the desired composition and layered morphology in a scaled-up commercial reactor.
3.3. Reaction m e c h a n i s m
Interestingly, the tungsten carbide deposition reaction, which has been postulated to be a combination of two simultaneous reactions, was found to proceed much more slowly than the deposition of pure tungsten a l o n e - - t h e deposition rate of tungsten carbide was approximately one-half that of pure tungsten under identical conditions of temperature and W F 6 partial pressure. The carbon-producing reaction involving D M E is presumed to tie up active surface sites, thereby slowing the overall deposition rate.
References
1 D. Garg, D. Dimos, P. N. Dyer and R. E. Stevens, Erosion-resistant coatings containing tungsten carbide by low-temperature CVD, paper presented at the 15th Int. Conf. on Metallurgical Coatings, San Diego, CA, April 1988.
2 P. N. Dyer, D. Garg, S. Sunder, H. E. Hintermann and M. Maillat, Wear-resistant coatings containing tungsten carbide deposited by low-temperatureCVD, paper presented at the 15th Int. Conf. on Metallurgical Coatings, San Diego, CA, April 1988.
3 D. Garg, P. N. Dyer, D. Dimos, S. Sunder, H. E. Hintermann and M. Maillat, Low-temperatureCVD tungsten carbide coatings for
108
4
5 6 7
C. M. Kelly et al. / CVD of tungsten carbide
wear/erosion resistance, Proc. 12th Annu. Conf. on Composites and Advanced Ceramic Materials, Vol. 2, American Ceramic Society, Westerville, FL, 1988, pp. 1215-1222. D. Garg, P. N. Dyer, D. Dimos, S. Sunder, H. E. Hintermann and M. Maillat, Low-temperature chemical vapor deposition tungsten carbide coatings for wear/erosion resistance, J. Am. Ceram. Sot., 75 (4) (1992) 1008-1011. D. W. Skafand C. M. Kelly, Sustained oscillation in simple models for chemical vapor deposition, Thin Solid Films, 207(1992) 57-64. K. Bartsch, A. Leonhart and E. Wolf, J. Phys. IV, Coll. C2, l (1991) 563 570. W. A. Bryant and G. H. Meier, Kinetics of the chemical vapor
deposition of tungsten, J. Electrochem. Soc., 120 (4) (1973) 559 565. 8 W. A. Bryant, J. Electrochem. Soc., 125 (9) (1978) 1534 1543. 9 P. Van der Putte, The reaction kinetics of the H 2 reduction of WF,, in the chemical vapor deposition of tungsten films. In E. K. Broadbent (ed.), Tungsten and Other ReJ?actory Materials .l~r ULSI Applications H, Materials Research Society, Pittsburgh, PA, 1987, p. 77. 10 D. Garg and C. M. Kelly, Reactor design for chemical vapor deposition of tungsten carbide coatings, paper presented at the Annu. Meet. Americim Institute of Chemical Engineers, Los Angeles, CA~ November 1991.