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Low-pressure gas-phase pack cementation coating of complex-shaped alloy surfaces
Materials Science and Engineering, A I2I (1989) 379 386
379
Low-pressure Gas-phase Pack Cementation Coating of Complex-shaped Alloy Surfaces* L. VANDENBULCKE, G. LEPRINCE and B. NCIRIt Centre de Recherches sur la Chimie de la Combustion et des Hautes Tempkratures, CNRS, 45071 Orlkans Cedex 2 (France)
(Received February 6, 1989)
Abstract A new pack cementation process is reported where the substrates are treated in the gas phase, over a pack, in a semi-open chamber maintained under reduced pressure. A stable halide is always present in the condensed state in the chamber and the process is controlled by the metal activities in the pack, the total pressure and the temperature o f the treatment. It is applied here to the aluminization of massive substrates and external and internal surfaces o f pure nickel and nickel alloy tubes, but it can be extended to various geometries such as complex-shaped substrates or porous materials. The conditions necessary to increase the gaseous transport rate are deduced from a gaseous diffusion model which takes into account the Knudsen diffusion contribution at the lower pressures, inside tubes or narrow cavities. The intrinsic diffusion rates in the gas phase are compared with the intrinsic solid state diffusion rates under various conditions of aluminization ranging from high to low aluminium activity, total pressure and temperature ranging ,from 0.3 to 760 Torr and from 670 to 950 °C respectively. It is shown how these parameters can be varied in conjunction to obtain a uniform coating on a given geometry. Finally, parts to be coated are classified into two groups depending on whether the Knudsen diffusion is negligible or not. This process extends to the transport o f other metals and some co-transport.
I. Introduction Various coatings and processes are available at the present to protect the superalloys in the corro-
*Invited paper. tPresent address: UniversityMohammed Ben Abdellah, Department of Chemistry, B.P.1796, Frs-Atlas, Morocco. 0921-5093/89/$3.50
sive and oxidative environments encountered at high temperature, ranging from thermochemical diffusion treatment by open flow or pack cementation processes, eventually combined with other prior deposition, to the modern overlay coatings of MCrAIY compositions applied by spray or physical vapour deposition processes. These modern processes are used to protect external surfaces, but their "throwing power" in all internal surfaces is negligible or zero, and only the chemical processes offer the possibility of coating these internal surfaces with uniform layers. The coating of small bores, for example in cooled blades, is also difficult to achieve with the classical pack cementation process, which is not always effective and can lead to the obstruction of the holes. Another technique which avoids most of the drawbacks encountered in the pack process was developed by PWA using a forced circulation of the gaseous phase but its industrial application is tedious. An intermediate process was also studied which utilizes the principles of the aluminization by the pack cementation process except that the substrates to be coated are out of contact with the powder mixture [1]. Good results were obtained with complex aluminium halides of an alkali or alkaline earth metal as the activator of the process, but the length-to-diameter ratio of the internal passage was lower than 10. A different process is reported here where the substrates are treated in the gas phase, over a pack, in a semi-open chamber maintained under reduced pressure and always containing a stable halide in the condensed state. It is applied here to aluminization but this process is general and extends to other elements and co-deposition. The conditions necessary to obtain a uniform coating thickness require the gaseous transport rate to be increased relative to the intrinsic solidstate diffusion rate. The possibility of increasing © ElsevierSequoia/Printed in The Netherlands
380 the intrinsic diffusion rate in the gas phase in a semi-closed chamber is deduced from a diffusion model. A low aluminium-activity process was first applied to the aluminization of steel tubes of various length-to-diameter ratios [2]. This process is applied here to massive nickel substrates embedded in the pack and also hung in the gas phase, and to pure nickel and nickel alloy tubes treated in the gas phase. It is extended to various conditions of temperature, pressure and aluminium activity, and it is shown how these parameters can be varied in conjunction to obtain the desired coating on a given geometry.
2. Experimental techniques The coating chamber consisted of a semi-open retort 5 cm in diameter by 12cm long. It contained the pack and the parts to be coated and was heated in a steel tube surrounded by a tubular furnace. The leak rate of the retort was low and the temperature at one end of the retort was slightly lower than that at the other; under these conditions, the activator, here A1F3, was always present in the condensed state in this zone during the treatment and the incorporation of solid A1F3 in the coating was avoided, even at the lower temperature of treatment (of the order of 700 °C). After evacuation at ambient temperature, a flow of hydrogen was maintained at fixed reduced pressure during the experiment in the steel tube. The pressures employed were in the range 0.1-20 Torr (13.3-2666 Pa); it was fixed, for each condition of temperature and metal activity in the pack, at a value 2-5 times greater than the sum of the partial pressures of the halides present inside the retort which were calculated at equilibrium with the pack. The packs consisted of powders of A I - M ( M - Fe, Cr, Ni) alloys, A1203 and AIF 3 as the activator. A1-M alloys were powder mixtures of the pure metals previously treated at 1000 °C for 24 h. Their compositions were preferably chosen to be within the two-phase domains of the binary phase diagrams to ensure that the aluminium activity had a fixed value during the treatment at a given temperature. Specimens to be coated consisted of massive cylinders of 99.5% pure nickel and tubes of pure nickel and Inconel 600. One cylinder of nickel was embedded in the pack and another hung over the pack. Tubes of two internal diameters (1 and l.Smm) and of the same length ( l l 0 m m ) were
hung over the pack. Weight gains of the samples were measured on the massive specimens and the coating thicknesses inside and outside the tubes were measured at various positions from the entrance of the tubes on polished cross-sections.
3. Theoretical study of the aluminium gaseous transport rate Various authors have studied the gaseous diffusion [3], the solid state diffusion [4, 5] or both [6-1 l] to model different thermochemical treatments of steels or superalloys as well as of pure iron and nickel. The equilibrium at the gas-pack interface can be calculated. It depends on the metal activities in the A1-M alloys and it assumes to consider all the possible gaseous, liquid and solid species formed with H, O, F, Al and M atoms. Detailed calculations of the equilibrium between the gaseous phase and various packs were reported previously by different authors [3, 12-14]. Under reduced pressure, the gaseous phase in equilibrium with a pack containing an AI-Ni or AI-Cr (Al > 20 at.%) alloy, AI203 and solid AIF3 is essentially composed of hydrogen, HF and fixed partial pressures of the aluminium fluorides as long as solid AIF 3 is present. All the other species can be neglected to study the aluminium transport in the gas phase. As shown previously at atmospheric pressure [9-11] the flux of each species can be written independently as the sum of a diffusion term and a flow term which results from the variation in the total number of gaseous moles produced by the chemical reactions: J,=
i-- ~--~ d----~
where Ji is the diffusion flux of species i in the x direction perpendicular to the surface to be coated, U is the flow velocity, D, is the diffusion coefficient and Pi the partial pressure of species i. Under reduced pressure, hydrogen is no longer a dominant species and the Dr. r~2 were replaced by the D,, m (diffusion coefficient of species i in the mixture) [2]. The effective binary diffusivities Di, m were calculated using the Wilke formula [15] for an average composition between the values at the gas-pack and gas-coating interfaces. The flux of one element k is related to the flux of the molecular species i by the relation:
381
Jk = Z •ikJi i=1
where 2ik is the stoichiometric coefficient of the element k in the species i. We showed previously that the fluxes of the gaseous species, the net flux of aluminium and the partial pressures at the gas-coating interface, p S, can be calculated by solving the system of equations which give the fluxes of the elements (aluminium, fluorine, hydrogen), which set the mass action relationships between the pes for a given value of the aluminium activity at the coating surface, and prescribe a constant value to the total pressure [9, 11]. It can be noted that the equilibrium at the gas-pack interface was calculated with the AIF3 pressure in equilibrium with solid AIF 3 at the temperature of the pack, which is slightly higher than the actual temperature of the solid A1F3 in the retort. In the diffusion calculations, it was supposed that no solid A1F3 could be deposited at the coating surface, as obtained experimentally. As the pressure of gaseous A1F3 increases from the pack to the coating surface, these approximations impose a supersaturation of gaseous A1F 3 at the coating surface with respect to the equilibrium pressure over the solid A1F3 at the pack and substrate temperature. These approximations cannot, however, change the conclusions derived from the calculation results. Figure 1 shows the net instantaneous flux JA~ of aluminium (for a diffusion distance of unity) as a function of the aluminium concentration at the coating surface on a nickel substrate for different values of the total pressure. The aluminium activity in the pack was fixed at 0.08, which corresponds to an A I - C r alloy in the two-phase domain fl + v2, for example at 55 at.% Ai. The net transport rate of aluminium decreases rapidly when the aluminium activity at the surface approaches the aluminium activity in the pack; this is the condition for the process to be principally controlled by solid state diffusion. Such a limitation leads to coatings which are uniform in thickness and composition and will be more easily reached when the aluminium transport in the gas phase is high, at low pressure in accordance with the influence of pressure on the diffusion coefficients. For a constant pressure of 10Torr (1333 Pa), the influence of the alloy composition in the pack on the net gaseous transport rate of aluminium is shown in Fig. 2. When the aluminium activity at the coating surface is very different from that in
E
3
20
10 - 6 100
10 - 7
760
10 - 8
10 - 9 20
i
i
i
L
30
40
50
60
NS I (at %)
Fig. 1. Net instantaneous flux of aluminium in the gas phase as a function of the aluminium concentration at the coating surface for different total pressures (T = 900 °C; 55 at.% A1 in AI-Cr, Ni substrate).
the pack, this influence is poor. However, a higher aluminium concentration could theoretically be reached at the coating surface when the aluminium activity increases if the solid state diffusion remains the rate-limiting step. As the rate of solid state diffusion is, for example at 1173 K, about 450 times higher when the surface concentration increases from 53 to 63 at.% Al in A1-Ni,
i
E
"x•l(c)
10-6
10 . 7
[
(d)
10-8
10 - 9 20
I
I
I
I
30
40
50
60
N/~I (at %)
Fig. 2. Net instantaneous flux of aluminium in the gas phase as a function of the aluminium concentration at the coating surface for different aluminium activities in the pack ( T = 900 °C; P = 10Torr; nickel substrate). (a) Pure aluminium; (b) about 75 at.% Al in AI-Cr; (c) about 65 at.% A1 in A1 -Cr; (d) about 55 at.% Al in AI-Cr.
382
the transport rate of aluminium will in fact be rate-limiting at high aluminium activity at this temperature of 1173 K, and the aluminium surface concentration will vary if the diffusion distance varies between the pack and the surface for parts treated in the gas phase even under reduced pressure. It is obvious that the solid state diffusion rate can also be controlled by the temperature of the treatment. Moreover, the temperature permits the aluminium transport rate in the gas phase to be controlled. Figure 3 shows, for a moderate aluminium activity in the pack, the order of 0.08, and at constant surface concentrations of 55 at.% A1 in A1-Ni, the influence of the total pressure on the net aluminium transport rate for temperatures in the range 973-1173 K. Each curve has been limited at low pressure when the sum of the partial pressures of the aluminium fluorides is half of the total pressure. This limit decreases as the temperature decreases and lower total pressures can be employed at lower temperatures. Therefore if higher transport rates are predicted at higher temperature, for a given total pressure, lower temperatures of treatment allow the aluminium transport rate to be increased by using lower pressures. Therefore, for
10-7 -
10 -9
~
10 -10
........ 0.1
r 1
, , , ..... I 10
K
~
T=
973K~
........
I 100
~
i
,,,,,,
P (Tort)
Fig. 3. Net instantaneous flux of aluminium in the gas phase as a function of the total pressure at different temperatures (nickel substrate; about 55 at.% A1 in A1-Cr; NAts = 55 at.% AI in A1-Ni).
a given aluminium activity in the pack, the solid state diffusion can be decreased at lower temperatures while increasing the gaseous transport rate, and uniform coatings can be obtained in the gas phase. This result is particularly true for outer surfaces treated in the gas phase. When articles with small tubes or narrow cavities are treated in the gas phase, the result can be somewhat different because the aluminium transport rate can be limited by Knudsen diffusion at the lower pressures, as the resistance due to molecule-wall collisions appears, which is not negligible compared with the resistance due to molecule-molecule collisions. In that case, the diffusion coefficients are given approximately by [ 16] +
1
-1
The Knudsen diffusion coefficient, DKi, in a tube can be calculated from the gas kinetic theory
d (8RT~ 1/2 D Ki = 3 ~,-~ii,] where d is the tube diameter and Mi the molecular weight of species i. As the diffusion model employed here is only unidimensional, the diffusion coefficient D; must be strictly employed to model the coating of a metallic surface which closes one end of an inert tube such as alumina, but the results can be qualitatively extended to metallic tubes. It was supposed in this calculation that the gaseous phase at the entrance of the tube (of length 1 cm) was in equilibrium with the pack because the diffusion rate outside the tube was supposed to be not rate-limiting. Figure 4 shows the influence of the total pressure on the net flux of aluminium on external surfaces, and inside tubes of 1 mm diameter where a limitation by Knudsen diffusion arises. These calculations were carried out for different coupled conditions of aluminium activity in the pack and temperature that may lead to different aluminium concentrations at the coating surface. All values were chosen in order that the different pairs of surface concentrations and temperature lead to the same solid state diffusion rate. This figure confirms the high aluminium transport rate in the gas phase at low pressure and shows that different combinations of aluminium activity, temperature and pressure can be employed, especially to coat external surfaces, but also internal ones eventually
383
f~0
u~
0
O
E
E
10 -7
10-7
10-8
10-8
I~1"~ aI
a 2 (55 at% AI) b 1 (59 at% AI)
10-9
10-9
b~,
(54 at%Ai)
b 2 (60 at% AI) 10-10
I
0.1
I
I IIIIii
I
1
I
I i1~
I,[
10
i
i
t i ....
[
100
t
i
i ilttt
P (Torr)
Fig. 4. Net instantaneous flux of aluminium in the gas phase as a function of the total pressure for three different coupled conditions (nickel substrate). (a) T = 900 °C; about 55 at.% A1 in A14Sr; NA~~ = 54.6 at.% AI in AI-Ni. (b) T = 800 °C; about 6 5 a t . % A1 in A142r; N A ~ = 5 9 . 5 a t . % AI in A1-Ni. (c) T = 700 °C; about 75 at.% A1 in AI-Cr; NA~~ = 60.5 at.% A1 in A1 Ni. The broken curves take into account the K n u d s e n diffusion inside tubes of 1 m m diameter.
with complex geometries if the Knudsen contribution is negligible. It is even clear that a high aluminium activity process at low temperature and very low pressure ( 0 . 2 T o r r or less) is a solution at least as reliable as the low aluminium activity process at higher temperature and pressure (about 10Torr) to transport aluminium to massive substrates. When the Knudsen diffusion becomes an important limiting factor, at pressures lower than 10 Torr for a diffusion path of 1 mm, it appears more interesting to employ a low-activity process at higher temperature and pressure to coat thin tubes or narrow cavities. As will be shown in Section 4, a limitation by the Knudsen diffusion can also occur for aluminium transport inside the pack, for the high aluminium activity-low temperature-low pressure process. The last factor which can arise in the control of the process is the sensitivity of the solid state diffusion rate and the aluminium gaseous transport rate to a small difference in the surface concentration. Various studies have shown that
10 -1°
........ 0.1
I
........
1
I
10
........
I
100
........
P (Torr)
Fig. 5. Net instantaneous flux of aluminium in the gas phase (with K n u d s e n diffusion) as a function of the total pressure for two different conditions (nickel substrate). (a) T = 900 °C; about 55 at.% A1 in A142r; NAlS= 54 and 55 at.% AI in AI-Ni. (b) T = 800 °C; about 65 at.% A1 in AI~Cr; NA~~ = 59 and 60 at.% AI in A1-Ni.
the solid state diffusion rate varies more rapidly at high aluminium surface concentrations. The difference between the gaseous transport rates also increases when the aluminium surface concentration increases as shown in Fig. 5, which takes into account the Knudsen diffusion. More uniform coating inside thin cavities should be obtained at low activity also as a function of this additional factor.
4. Experimental results When the surface composition of a coating is maintained at a fixed value, the mass of aluminium which diffuses perpendicularly to the surface of a semi-infinite solid follows the solid state diffusion law according to the square root of time: W~ 2 = k s t
where ks is a parabolic constant which is a function of the surface composition and the diffusion coefficient. Levine and Caves [3] and then various other authors [6-11] have shown that the net mass of aluminium transported inside the pack by the gas
384
phase also follows a parabolic law as the diffusion distance increases with a zone depleted in aluminium. Finally, even if the intrinsic gaseous transport rate is constant, it is possible to find an equivalent parabolic treatment which would give the same incorporation rate and surface composition at a given treatment time [11]. Therefore the experimental results on the deposition rate inside the outside the pack will be presented for simplicity and compared as a parabolic constant k. 4.1. Deposition on external surfaces in and out of the pack Massive substrates of pure nickel were treated in and out of the pack in various coupled conditions of aluminium activity, temperature and pressure which led to deposition rates of the same order, i.e. k in the range 10 - 6 to 10-5 g2cm-4 h -1. The experimental conditions and parabolic rate constants are given in Table 1 and the values of k are reported in Fig. 6. It appears clearly that a high transport rate can be obtained by this process for treatment in the gas phase. At low temperature, high activity and low pressure the incorporation rate is even higher in the gas phase than in the pack, as indicated previously from Fig. 4 because the gaseous aluminium transport rate inside the pack is limited by the Knudsen diffusion. At temperatures higher than 800 °C, moderate aluminium activity and higher pressure, the incorporation rate becomes higher in the pack except at 920 °C where they are equal. It can be seen that at 670-675 °C and 900 °C the difference decreases when the k values decrease, obviously because the choice of a lower activity at a given temperature leads to a more important influence of the solid state diffusion rate.
I ,,C
10-5
II t l 10 - 6
I
10-7
I
L
I
700
800
900
T (oc)
Fig. 6. Experimental parabolic aluminium deposition rate constants for treatment in the pack (full symbols) and in the gas phase (open symbols) corresponding to the conditions of Table 1.
Concerning the nature of the coating, classical results were obtained. They can be deduced from Fig. 7 which shows some experimental values of k together with the variations with temperature of the parabolic rate constant for solid state diffusion at different Ni-AI compositions. The Ni2A13 phase is obtained at high activity and low temperature, and the NiAI phase at low activity and higher temperature. It can also be seen for some values of the aluminium concentration in the pack that the experimental k
TABLE I Parabolic rate constants in and out of the pack for a l a a i n i m incorporation in massive nickel substrates as a function of the treatment conditions Run number
% A1 in A1-Cr (at.%) T (°C) P (Torr) t (h) k in the pack × 106 (g2 cm 4 h - l ) k out of the pack x 106 (g2 cm-4 h - I )
28
32
34
31
41
37
40
42
43
45
44
84 670 0.3 24 1.7 3.9
70 675 0.15 24 0.66 0.96
70 740 0.13 36 3.5 3.1
70 760 0.4 24 3.2 4.7
65 750 0.25 48 2.9 1.8
65 800 1 24 5.0 4.2
59 850 7 36 7.6 5.0
55 880 7.5 24 5.6 3.5
50 900 10 24 4.6 3.4
45 900 10 36 1.1 1.1
45 920 15 24 2.5 2.5
385 I
/
E o
// /
/ /
/ / / (-75)
/ /
/
/
Ni 2 AI 3 rain -59
at% AI
/
/
/
/
O)
0
/ /
/
//~ ///(65)~1// ~-// /
j,'
at% AI
-55
7/
/
/ 10-5
NiAI max
/ /
/
T h e r e f o r e u n i f o r m coatings can be expected by t r e a t m e n t in the gas p h a s e u n d e r r e d u c e d pressure, over all the activity range, on substrates where the K n u d s e n diffusion c o n t r o l is negligible, eventually inside c o m p l e x - s h a p e d internal surfaces w i t h o u t small holes; this is o b t a i n e d by p r o p e r c o n t r o l o f the value o f k over the whole range•
/ 61 at% AI
Ni2AI 3 max / - 63 at% AI /
I(55)
~ Z
.(50)
/
/
4.2. D e p o s i t i o n
,,,(,,is)
on
internal
surfaces
out
of
the
pack /,
10-6
/
/ / /
/
~
i
/
,,,'
/
,'
/
~"/
I
T u b e s o f inconel a n d pure nickel, 1 a n d 1.5 m m in d i a m e t e r a n d 110 m m in length were c o a t e d in the gas p h a s e in the same e x p e r i m e n t s as those r e p o r t e d for the massive substrates. F o u r typical results are given in T a b l e 2. T h e y confirm the p r e d i c t i o n o f Figs. 4 a n d 5. A t high a l u m i n i u m activity, low t e m p e r a t u r e a n d low pressure, the K n u d s e n diffusion limits the i n c o r p o r a t i o n rate inside the tubes a n d nonu n i f o r m coatings were o b t a i n e d . M o r e o v e r , no c o a t i n g was o b t a i n e d at the centre o f the tubes treated in run 28 ( T a b l e 1) at 0.3 T o r r . A fairly g o o d result was only reached at low a l u m i n i u m activity, higher t e m p e r a t u r e a n d a pressure o f 10 T o r r where the influence o f the K n u d s e n diffusion is low (Fig. 4). F r o m runs 43 a n d 45, it a p p e a r s that, at a given t e m p e r a t u r e , a better u n i f o r m i t y is o b t a i n e d at the lowest activity which leads to a lower k, b u t a c o m p r o m i s e is necessary with the d u r a t i o n o f t r e a t m e n t . H o w ever, the u n i f o r m i t y is i m p r o v e d for given conditions when the d u r a t i o n increases as the solid state diffusion rate decreases with time. F i g u r e 8 r e p o r t s the thickness v a r i a t i o n s inside the tubes treated in run 45.
/ / /
II
/
/
i
I /
/
i
I
5 3 at% AI /
10 -7
/ 5 0 at% AI
I
I
L
700
800
900
T (°C)
Fig. 7. Experimental parabolic aluminium deposition rate constants for treatment in the pack (full symbols) and in the gas phase (open symbols); values in parentheses indicate the aluminium concentration in the A1-Cr alloy (at.%). The broken curves give the variations of the theoretical parabolic solid state diffusion rate constant as a function of temperature for different surface concentration in at.% AI in AI-Ni (from refs. 6 8).
values follow, as same law as the for t r e a t m e n t in perature, a n d in temperature.
a function solid state the gas o r out o f
o f t e m p e r a t u r e , the diffusion, especially phase at low temthe p a c k at higher
TABLE 2 Process conditions and experimental values of the coating thickness inside Inconel tubes of length 110 mm. The values of k are experimental on massive substrates also treated in the gas phase Run number
% AI in AI Cr
T
P
t
k
('C)
(Torr)
(h)
(g2 c m
4 h- i)
(at.%)
31
70
760
0.4
24
4.7 × 10 -6
37
65
800
1
24
4.2
43
50
900
10
24
3.4 x 10 6
45
45
900
I0
36
× 10 - 6
1.1 x 10 6
Tube diameter
Coating thicknesses (/xm)
(mm)
At the entrance
At 10 mm
At 20 mm
At 30 mm
1
33 33 36 36 41 41 32 33
18 20 18 21 30 34 27 29
12 14 14 16 26 29 26 28
10
9
8
12 11 13 25 29 25 28
12 I0 12 25 28 25 28
11 10 12 24 28 25 27
1.5 1
1.5 1
1.5 1
1.5
At 40 mm
At 50 mm
386
process has been extended to porous materials such as felts [18].
A
E v ¢J
3O
References
(b)
20
10
I
20
I
I
40
I
I
60
I
I
80
I
I
L (mm)
Fig. 8. Thickness variations of an AI-Ni coating inside a tube l l 0 m m long (run 45): (a) diameter= 1.5mm; (b) diameter = I mm.
5. Conclusions The low-pressure gas-phase pack cementation process offers all the possibilities of the classical pack cementation process to coat external surfaces over the whole aluminium activity-temperature range. When the Knudsen diffusion contribution is negligible, uniform coatings can also be obtained eventually over the whole activity-temperature range inside complex-shaped internal surfaces but without small holes. When long holes with small diameters are present, a low aluminium activity, higher pressure and temperature are better employed. This process is general and has been extended to other metals such as chromium, hafnium, yttrium and some codeposition Cr-A1, A1-Hf [17]. This
1 R. S. Parzuchowski, Thin Solid Films, 45(1977) 349. 2 L. Vandenbulcke and B. Nciri, in J. O. Carlsson and J. Lindstrom (eds.), Proc. 5th Euro. Conf. on CVD, Uppsala, June 1985, Uppsala University, Sweden, 1985, p. 283. 3 S. R. Lcvine and R. M. Caves, J. Electrochem. Soc., 121 (8) (1974) 1051. 4 A. J. Hickl and R. W. Heekel, Metall. Trans. A, 6(1975) 431. 5 B. Nciri and L. Vandenbulcke, Surf. Technol., 24(1985) 365. 6 A. Sivakumar and L. L. Seigle, Metall. Trans. A, 7(1976) 1073. 7 B. K. Gupta, A. K. Sarkbel and L. L. Seigle, Thin Solid Films, 39 (1976) 313. 8 B. P. Gupta and L. L. Seigle, Thin Solid Films, 73(1980) 365. 9 B. Nciri and L. Vandenbulcke, J. Less-Common Met., 95 (1983) 191. 10 G. H. Marijnissen and J. A. Klostermann, in J. Bloem, G. Verspui and L. R. Wolff (eds.), Proc. 4th Euro. Conf. on CVD, Eindhoven, May-June 1983, Philips Congress Centre, Eindhoven, 1983, p. 363. 11 B. Nciri and L. Vandenbulcke, Thin Solid Films, 139 (1986) 311. 12 P. N. Walsh, in G. F. Wakefield and J. M. Blocher, Jr. (¢ds.), Proc. 4th Int. Conf. on Chemical Vapour Deposition, Boston, 1973, Electrochemical Society, Pennington, N J, 1973, p. 147. 13 B. Nciri and L. Vandenbulcke, J. Less-Common Met., 95 (1983) 55. 14 V.A. Ravi, P. Choquet and R. A. Rapp, Proc. of Mater. Res. Soc. Int. Meeting on Advanced Materials, Tokyo, 1988, Trans. Jpn. Inst. Met., in press. 15 C. R. Wilke, Chem. Eng. Prog., 46(1950) 95. 16 T. K. Sherwood, R. L. Pigford and C. R. Wiike, Mass Transfer, McGraw-Hill, New York, 1975. 17 G. Leprince, S. Alperine, L. Vandenbulcke and A. Walder, Mater. Sci. Eng., A120/121 (1989) 419.
379
Low-pressure Gas-phase Pack Cementation Coating of Complex-shaped Alloy Surfaces* L. VANDENBULCKE, G. LEPRINCE and B. NCIRIt Centre de Recherches sur la Chimie de la Combustion et des Hautes Tempkratures, CNRS, 45071 Orlkans Cedex 2 (France)
(Received February 6, 1989)
Abstract A new pack cementation process is reported where the substrates are treated in the gas phase, over a pack, in a semi-open chamber maintained under reduced pressure. A stable halide is always present in the condensed state in the chamber and the process is controlled by the metal activities in the pack, the total pressure and the temperature o f the treatment. It is applied here to the aluminization of massive substrates and external and internal surfaces o f pure nickel and nickel alloy tubes, but it can be extended to various geometries such as complex-shaped substrates or porous materials. The conditions necessary to increase the gaseous transport rate are deduced from a gaseous diffusion model which takes into account the Knudsen diffusion contribution at the lower pressures, inside tubes or narrow cavities. The intrinsic diffusion rates in the gas phase are compared with the intrinsic solid state diffusion rates under various conditions of aluminization ranging from high to low aluminium activity, total pressure and temperature ranging ,from 0.3 to 760 Torr and from 670 to 950 °C respectively. It is shown how these parameters can be varied in conjunction to obtain a uniform coating on a given geometry. Finally, parts to be coated are classified into two groups depending on whether the Knudsen diffusion is negligible or not. This process extends to the transport o f other metals and some co-transport.
I. Introduction Various coatings and processes are available at the present to protect the superalloys in the corro-
*Invited paper. tPresent address: UniversityMohammed Ben Abdellah, Department of Chemistry, B.P.1796, Frs-Atlas, Morocco. 0921-5093/89/$3.50
sive and oxidative environments encountered at high temperature, ranging from thermochemical diffusion treatment by open flow or pack cementation processes, eventually combined with other prior deposition, to the modern overlay coatings of MCrAIY compositions applied by spray or physical vapour deposition processes. These modern processes are used to protect external surfaces, but their "throwing power" in all internal surfaces is negligible or zero, and only the chemical processes offer the possibility of coating these internal surfaces with uniform layers. The coating of small bores, for example in cooled blades, is also difficult to achieve with the classical pack cementation process, which is not always effective and can lead to the obstruction of the holes. Another technique which avoids most of the drawbacks encountered in the pack process was developed by PWA using a forced circulation of the gaseous phase but its industrial application is tedious. An intermediate process was also studied which utilizes the principles of the aluminization by the pack cementation process except that the substrates to be coated are out of contact with the powder mixture [1]. Good results were obtained with complex aluminium halides of an alkali or alkaline earth metal as the activator of the process, but the length-to-diameter ratio of the internal passage was lower than 10. A different process is reported here where the substrates are treated in the gas phase, over a pack, in a semi-open chamber maintained under reduced pressure and always containing a stable halide in the condensed state. It is applied here to aluminization but this process is general and extends to other elements and co-deposition. The conditions necessary to obtain a uniform coating thickness require the gaseous transport rate to be increased relative to the intrinsic solidstate diffusion rate. The possibility of increasing © ElsevierSequoia/Printed in The Netherlands
380 the intrinsic diffusion rate in the gas phase in a semi-closed chamber is deduced from a diffusion model. A low aluminium-activity process was first applied to the aluminization of steel tubes of various length-to-diameter ratios [2]. This process is applied here to massive nickel substrates embedded in the pack and also hung in the gas phase, and to pure nickel and nickel alloy tubes treated in the gas phase. It is extended to various conditions of temperature, pressure and aluminium activity, and it is shown how these parameters can be varied in conjunction to obtain the desired coating on a given geometry.
2. Experimental techniques The coating chamber consisted of a semi-open retort 5 cm in diameter by 12cm long. It contained the pack and the parts to be coated and was heated in a steel tube surrounded by a tubular furnace. The leak rate of the retort was low and the temperature at one end of the retort was slightly lower than that at the other; under these conditions, the activator, here A1F3, was always present in the condensed state in this zone during the treatment and the incorporation of solid A1F3 in the coating was avoided, even at the lower temperature of treatment (of the order of 700 °C). After evacuation at ambient temperature, a flow of hydrogen was maintained at fixed reduced pressure during the experiment in the steel tube. The pressures employed were in the range 0.1-20 Torr (13.3-2666 Pa); it was fixed, for each condition of temperature and metal activity in the pack, at a value 2-5 times greater than the sum of the partial pressures of the halides present inside the retort which were calculated at equilibrium with the pack. The packs consisted of powders of A I - M ( M - Fe, Cr, Ni) alloys, A1203 and AIF 3 as the activator. A1-M alloys were powder mixtures of the pure metals previously treated at 1000 °C for 24 h. Their compositions were preferably chosen to be within the two-phase domains of the binary phase diagrams to ensure that the aluminium activity had a fixed value during the treatment at a given temperature. Specimens to be coated consisted of massive cylinders of 99.5% pure nickel and tubes of pure nickel and Inconel 600. One cylinder of nickel was embedded in the pack and another hung over the pack. Tubes of two internal diameters (1 and l.Smm) and of the same length ( l l 0 m m ) were
hung over the pack. Weight gains of the samples were measured on the massive specimens and the coating thicknesses inside and outside the tubes were measured at various positions from the entrance of the tubes on polished cross-sections.
3. Theoretical study of the aluminium gaseous transport rate Various authors have studied the gaseous diffusion [3], the solid state diffusion [4, 5] or both [6-1 l] to model different thermochemical treatments of steels or superalloys as well as of pure iron and nickel. The equilibrium at the gas-pack interface can be calculated. It depends on the metal activities in the A1-M alloys and it assumes to consider all the possible gaseous, liquid and solid species formed with H, O, F, Al and M atoms. Detailed calculations of the equilibrium between the gaseous phase and various packs were reported previously by different authors [3, 12-14]. Under reduced pressure, the gaseous phase in equilibrium with a pack containing an AI-Ni or AI-Cr (Al > 20 at.%) alloy, AI203 and solid AIF3 is essentially composed of hydrogen, HF and fixed partial pressures of the aluminium fluorides as long as solid AIF 3 is present. All the other species can be neglected to study the aluminium transport in the gas phase. As shown previously at atmospheric pressure [9-11] the flux of each species can be written independently as the sum of a diffusion term and a flow term which results from the variation in the total number of gaseous moles produced by the chemical reactions: J,=
i-- ~--~ d----~
where Ji is the diffusion flux of species i in the x direction perpendicular to the surface to be coated, U is the flow velocity, D, is the diffusion coefficient and Pi the partial pressure of species i. Under reduced pressure, hydrogen is no longer a dominant species and the Dr. r~2 were replaced by the D,, m (diffusion coefficient of species i in the mixture) [2]. The effective binary diffusivities Di, m were calculated using the Wilke formula [15] for an average composition between the values at the gas-pack and gas-coating interfaces. The flux of one element k is related to the flux of the molecular species i by the relation:
381
Jk = Z •ikJi i=1
where 2ik is the stoichiometric coefficient of the element k in the species i. We showed previously that the fluxes of the gaseous species, the net flux of aluminium and the partial pressures at the gas-coating interface, p S, can be calculated by solving the system of equations which give the fluxes of the elements (aluminium, fluorine, hydrogen), which set the mass action relationships between the pes for a given value of the aluminium activity at the coating surface, and prescribe a constant value to the total pressure [9, 11]. It can be noted that the equilibrium at the gas-pack interface was calculated with the AIF3 pressure in equilibrium with solid AIF 3 at the temperature of the pack, which is slightly higher than the actual temperature of the solid A1F3 in the retort. In the diffusion calculations, it was supposed that no solid A1F3 could be deposited at the coating surface, as obtained experimentally. As the pressure of gaseous A1F3 increases from the pack to the coating surface, these approximations impose a supersaturation of gaseous A1F 3 at the coating surface with respect to the equilibrium pressure over the solid A1F3 at the pack and substrate temperature. These approximations cannot, however, change the conclusions derived from the calculation results. Figure 1 shows the net instantaneous flux JA~ of aluminium (for a diffusion distance of unity) as a function of the aluminium concentration at the coating surface on a nickel substrate for different values of the total pressure. The aluminium activity in the pack was fixed at 0.08, which corresponds to an A I - C r alloy in the two-phase domain fl + v2, for example at 55 at.% Ai. The net transport rate of aluminium decreases rapidly when the aluminium activity at the surface approaches the aluminium activity in the pack; this is the condition for the process to be principally controlled by solid state diffusion. Such a limitation leads to coatings which are uniform in thickness and composition and will be more easily reached when the aluminium transport in the gas phase is high, at low pressure in accordance with the influence of pressure on the diffusion coefficients. For a constant pressure of 10Torr (1333 Pa), the influence of the alloy composition in the pack on the net gaseous transport rate of aluminium is shown in Fig. 2. When the aluminium activity at the coating surface is very different from that in
E
3
20
10 - 6 100
10 - 7
760
10 - 8
10 - 9 20
i
i
i
L
30
40
50
60
NS I (at %)
Fig. 1. Net instantaneous flux of aluminium in the gas phase as a function of the aluminium concentration at the coating surface for different total pressures (T = 900 °C; 55 at.% A1 in AI-Cr, Ni substrate).
the pack, this influence is poor. However, a higher aluminium concentration could theoretically be reached at the coating surface when the aluminium activity increases if the solid state diffusion remains the rate-limiting step. As the rate of solid state diffusion is, for example at 1173 K, about 450 times higher when the surface concentration increases from 53 to 63 at.% Al in A1-Ni,
i
E
"x•l(c)
10-6
10 . 7
[
(d)
10-8
10 - 9 20
I
I
I
I
30
40
50
60
N/~I (at %)
Fig. 2. Net instantaneous flux of aluminium in the gas phase as a function of the aluminium concentration at the coating surface for different aluminium activities in the pack ( T = 900 °C; P = 10Torr; nickel substrate). (a) Pure aluminium; (b) about 75 at.% Al in AI-Cr; (c) about 65 at.% A1 in A1 -Cr; (d) about 55 at.% Al in AI-Cr.
382
the transport rate of aluminium will in fact be rate-limiting at high aluminium activity at this temperature of 1173 K, and the aluminium surface concentration will vary if the diffusion distance varies between the pack and the surface for parts treated in the gas phase even under reduced pressure. It is obvious that the solid state diffusion rate can also be controlled by the temperature of the treatment. Moreover, the temperature permits the aluminium transport rate in the gas phase to be controlled. Figure 3 shows, for a moderate aluminium activity in the pack, the order of 0.08, and at constant surface concentrations of 55 at.% A1 in A1-Ni, the influence of the total pressure on the net aluminium transport rate for temperatures in the range 973-1173 K. Each curve has been limited at low pressure when the sum of the partial pressures of the aluminium fluorides is half of the total pressure. This limit decreases as the temperature decreases and lower total pressures can be employed at lower temperatures. Therefore if higher transport rates are predicted at higher temperature, for a given total pressure, lower temperatures of treatment allow the aluminium transport rate to be increased by using lower pressures. Therefore, for
10-7 -
10 -9
~
10 -10
........ 0.1
r 1
, , , ..... I 10
K
~
T=
973K~
........
I 100
~
i
,,,,,,
P (Tort)
Fig. 3. Net instantaneous flux of aluminium in the gas phase as a function of the total pressure at different temperatures (nickel substrate; about 55 at.% A1 in A1-Cr; NAts = 55 at.% AI in A1-Ni).
a given aluminium activity in the pack, the solid state diffusion can be decreased at lower temperatures while increasing the gaseous transport rate, and uniform coatings can be obtained in the gas phase. This result is particularly true for outer surfaces treated in the gas phase. When articles with small tubes or narrow cavities are treated in the gas phase, the result can be somewhat different because the aluminium transport rate can be limited by Knudsen diffusion at the lower pressures, as the resistance due to molecule-wall collisions appears, which is not negligible compared with the resistance due to molecule-molecule collisions. In that case, the diffusion coefficients are given approximately by [ 16] +
1
-1
The Knudsen diffusion coefficient, DKi, in a tube can be calculated from the gas kinetic theory
d (8RT~ 1/2 D Ki = 3 ~,-~ii,] where d is the tube diameter and Mi the molecular weight of species i. As the diffusion model employed here is only unidimensional, the diffusion coefficient D; must be strictly employed to model the coating of a metallic surface which closes one end of an inert tube such as alumina, but the results can be qualitatively extended to metallic tubes. It was supposed in this calculation that the gaseous phase at the entrance of the tube (of length 1 cm) was in equilibrium with the pack because the diffusion rate outside the tube was supposed to be not rate-limiting. Figure 4 shows the influence of the total pressure on the net flux of aluminium on external surfaces, and inside tubes of 1 mm diameter where a limitation by Knudsen diffusion arises. These calculations were carried out for different coupled conditions of aluminium activity in the pack and temperature that may lead to different aluminium concentrations at the coating surface. All values were chosen in order that the different pairs of surface concentrations and temperature lead to the same solid state diffusion rate. This figure confirms the high aluminium transport rate in the gas phase at low pressure and shows that different combinations of aluminium activity, temperature and pressure can be employed, especially to coat external surfaces, but also internal ones eventually
383
f~0
u~
0
O
E
E
10 -7
10-7
10-8
10-8
I~1"~ aI
a 2 (55 at% AI) b 1 (59 at% AI)
10-9
10-9
b~,
(54 at%Ai)
b 2 (60 at% AI) 10-10
I
0.1
I
I IIIIii
I
1
I
I i1~
I,[
10
i
i
t i ....
[
100
t
i
i ilttt
P (Torr)
Fig. 4. Net instantaneous flux of aluminium in the gas phase as a function of the total pressure for three different coupled conditions (nickel substrate). (a) T = 900 °C; about 55 at.% A1 in A14Sr; NA~~ = 54.6 at.% AI in AI-Ni. (b) T = 800 °C; about 6 5 a t . % A1 in A142r; N A ~ = 5 9 . 5 a t . % AI in A1-Ni. (c) T = 700 °C; about 75 at.% A1 in AI-Cr; NA~~ = 60.5 at.% A1 in A1 Ni. The broken curves take into account the K n u d s e n diffusion inside tubes of 1 m m diameter.
with complex geometries if the Knudsen contribution is negligible. It is even clear that a high aluminium activity process at low temperature and very low pressure ( 0 . 2 T o r r or less) is a solution at least as reliable as the low aluminium activity process at higher temperature and pressure (about 10Torr) to transport aluminium to massive substrates. When the Knudsen diffusion becomes an important limiting factor, at pressures lower than 10 Torr for a diffusion path of 1 mm, it appears more interesting to employ a low-activity process at higher temperature and pressure to coat thin tubes or narrow cavities. As will be shown in Section 4, a limitation by the Knudsen diffusion can also occur for aluminium transport inside the pack, for the high aluminium activity-low temperature-low pressure process. The last factor which can arise in the control of the process is the sensitivity of the solid state diffusion rate and the aluminium gaseous transport rate to a small difference in the surface concentration. Various studies have shown that
10 -1°
........ 0.1
I
........
1
I
10
........
I
100
........
P (Torr)
Fig. 5. Net instantaneous flux of aluminium in the gas phase (with K n u d s e n diffusion) as a function of the total pressure for two different conditions (nickel substrate). (a) T = 900 °C; about 55 at.% A1 in A142r; NAlS= 54 and 55 at.% AI in AI-Ni. (b) T = 800 °C; about 65 at.% A1 in AI~Cr; NA~~ = 59 and 60 at.% AI in A1-Ni.
the solid state diffusion rate varies more rapidly at high aluminium surface concentrations. The difference between the gaseous transport rates also increases when the aluminium surface concentration increases as shown in Fig. 5, which takes into account the Knudsen diffusion. More uniform coating inside thin cavities should be obtained at low activity also as a function of this additional factor.
4. Experimental results When the surface composition of a coating is maintained at a fixed value, the mass of aluminium which diffuses perpendicularly to the surface of a semi-infinite solid follows the solid state diffusion law according to the square root of time: W~ 2 = k s t
where ks is a parabolic constant which is a function of the surface composition and the diffusion coefficient. Levine and Caves [3] and then various other authors [6-11] have shown that the net mass of aluminium transported inside the pack by the gas
384
phase also follows a parabolic law as the diffusion distance increases with a zone depleted in aluminium. Finally, even if the intrinsic gaseous transport rate is constant, it is possible to find an equivalent parabolic treatment which would give the same incorporation rate and surface composition at a given treatment time [11]. Therefore the experimental results on the deposition rate inside the outside the pack will be presented for simplicity and compared as a parabolic constant k. 4.1. Deposition on external surfaces in and out of the pack Massive substrates of pure nickel were treated in and out of the pack in various coupled conditions of aluminium activity, temperature and pressure which led to deposition rates of the same order, i.e. k in the range 10 - 6 to 10-5 g2cm-4 h -1. The experimental conditions and parabolic rate constants are given in Table 1 and the values of k are reported in Fig. 6. It appears clearly that a high transport rate can be obtained by this process for treatment in the gas phase. At low temperature, high activity and low pressure the incorporation rate is even higher in the gas phase than in the pack, as indicated previously from Fig. 4 because the gaseous aluminium transport rate inside the pack is limited by the Knudsen diffusion. At temperatures higher than 800 °C, moderate aluminium activity and higher pressure, the incorporation rate becomes higher in the pack except at 920 °C where they are equal. It can be seen that at 670-675 °C and 900 °C the difference decreases when the k values decrease, obviously because the choice of a lower activity at a given temperature leads to a more important influence of the solid state diffusion rate.
I ,,C
10-5
II t l 10 - 6
I
10-7
I
L
I
700
800
900
T (oc)
Fig. 6. Experimental parabolic aluminium deposition rate constants for treatment in the pack (full symbols) and in the gas phase (open symbols) corresponding to the conditions of Table 1.
Concerning the nature of the coating, classical results were obtained. They can be deduced from Fig. 7 which shows some experimental values of k together with the variations with temperature of the parabolic rate constant for solid state diffusion at different Ni-AI compositions. The Ni2A13 phase is obtained at high activity and low temperature, and the NiAI phase at low activity and higher temperature. It can also be seen for some values of the aluminium concentration in the pack that the experimental k
TABLE I Parabolic rate constants in and out of the pack for a l a a i n i m incorporation in massive nickel substrates as a function of the treatment conditions Run number
% A1 in A1-Cr (at.%) T (°C) P (Torr) t (h) k in the pack × 106 (g2 cm 4 h - l ) k out of the pack x 106 (g2 cm-4 h - I )
28
32
34
31
41
37
40
42
43
45
44
84 670 0.3 24 1.7 3.9
70 675 0.15 24 0.66 0.96
70 740 0.13 36 3.5 3.1
70 760 0.4 24 3.2 4.7
65 750 0.25 48 2.9 1.8
65 800 1 24 5.0 4.2
59 850 7 36 7.6 5.0
55 880 7.5 24 5.6 3.5
50 900 10 24 4.6 3.4
45 900 10 36 1.1 1.1
45 920 15 24 2.5 2.5
385 I
/
E o
// /
/ /
/ / / (-75)
/ /
/
/
Ni 2 AI 3 rain -59
at% AI
/
/
/
/
O)
0
/ /
/
//~ ///(65)~1// ~-// /
j,'
at% AI
-55
7/
/
/ 10-5
NiAI max
/ /
/
T h e r e f o r e u n i f o r m coatings can be expected by t r e a t m e n t in the gas p h a s e u n d e r r e d u c e d pressure, over all the activity range, on substrates where the K n u d s e n diffusion c o n t r o l is negligible, eventually inside c o m p l e x - s h a p e d internal surfaces w i t h o u t small holes; this is o b t a i n e d by p r o p e r c o n t r o l o f the value o f k over the whole range•
/ 61 at% AI
Ni2AI 3 max / - 63 at% AI /
I(55)
~ Z
.(50)
/
/
4.2. D e p o s i t i o n
,,,(,,is)
on
internal
surfaces
out
of
the
pack /,
10-6
/
/ / /
/
~
i
/
,,,'
/
,'
/
~"/
I
T u b e s o f inconel a n d pure nickel, 1 a n d 1.5 m m in d i a m e t e r a n d 110 m m in length were c o a t e d in the gas p h a s e in the same e x p e r i m e n t s as those r e p o r t e d for the massive substrates. F o u r typical results are given in T a b l e 2. T h e y confirm the p r e d i c t i o n o f Figs. 4 a n d 5. A t high a l u m i n i u m activity, low t e m p e r a t u r e a n d low pressure, the K n u d s e n diffusion limits the i n c o r p o r a t i o n rate inside the tubes a n d nonu n i f o r m coatings were o b t a i n e d . M o r e o v e r , no c o a t i n g was o b t a i n e d at the centre o f the tubes treated in run 28 ( T a b l e 1) at 0.3 T o r r . A fairly g o o d result was only reached at low a l u m i n i u m activity, higher t e m p e r a t u r e a n d a pressure o f 10 T o r r where the influence o f the K n u d s e n diffusion is low (Fig. 4). F r o m runs 43 a n d 45, it a p p e a r s that, at a given t e m p e r a t u r e , a better u n i f o r m i t y is o b t a i n e d at the lowest activity which leads to a lower k, b u t a c o m p r o m i s e is necessary with the d u r a t i o n o f t r e a t m e n t . H o w ever, the u n i f o r m i t y is i m p r o v e d for given conditions when the d u r a t i o n increases as the solid state diffusion rate decreases with time. F i g u r e 8 r e p o r t s the thickness v a r i a t i o n s inside the tubes treated in run 45.
/ / /
II
/
/
i
I /
/
i
I
5 3 at% AI /
10 -7
/ 5 0 at% AI
I
I
L
700
800
900
T (°C)
Fig. 7. Experimental parabolic aluminium deposition rate constants for treatment in the pack (full symbols) and in the gas phase (open symbols); values in parentheses indicate the aluminium concentration in the A1-Cr alloy (at.%). The broken curves give the variations of the theoretical parabolic solid state diffusion rate constant as a function of temperature for different surface concentration in at.% AI in AI-Ni (from refs. 6 8).
values follow, as same law as the for t r e a t m e n t in perature, a n d in temperature.
a function solid state the gas o r out o f
o f t e m p e r a t u r e , the diffusion, especially phase at low temthe p a c k at higher
TABLE 2 Process conditions and experimental values of the coating thickness inside Inconel tubes of length 110 mm. The values of k are experimental on massive substrates also treated in the gas phase Run number
% AI in AI Cr
T
P
t
k
('C)
(Torr)
(h)
(g2 c m
4 h- i)
(at.%)
31
70
760
0.4
24
4.7 × 10 -6
37
65
800
1
24
4.2
43
50
900
10
24
3.4 x 10 6
45
45
900
I0
36
× 10 - 6
1.1 x 10 6
Tube diameter
Coating thicknesses (/xm)
(mm)
At the entrance
At 10 mm
At 20 mm
At 30 mm
1
33 33 36 36 41 41 32 33
18 20 18 21 30 34 27 29
12 14 14 16 26 29 26 28
10
9
8
12 11 13 25 29 25 28
12 I0 12 25 28 25 28
11 10 12 24 28 25 27
1.5 1
1.5 1
1.5 1
1.5
At 40 mm
At 50 mm
386
process has been extended to porous materials such as felts [18].
A
E v ¢J
3O
References
(b)
20
10
I
20
I
I
40
I
I
60
I
I
80
I
I
L (mm)
Fig. 8. Thickness variations of an AI-Ni coating inside a tube l l 0 m m long (run 45): (a) diameter= 1.5mm; (b) diameter = I mm.
5. Conclusions The low-pressure gas-phase pack cementation process offers all the possibilities of the classical pack cementation process to coat external surfaces over the whole aluminium activity-temperature range. When the Knudsen diffusion contribution is negligible, uniform coatings can also be obtained eventually over the whole activity-temperature range inside complex-shaped internal surfaces but without small holes. When long holes with small diameters are present, a low aluminium activity, higher pressure and temperature are better employed. This process is general and has been extended to other metals such as chromium, hafnium, yttrium and some codeposition Cr-A1, A1-Hf [17]. This
1 R. S. Parzuchowski, Thin Solid Films, 45(1977) 349. 2 L. Vandenbulcke and B. Nciri, in J. O. Carlsson and J. Lindstrom (eds.), Proc. 5th Euro. Conf. on CVD, Uppsala, June 1985, Uppsala University, Sweden, 1985, p. 283. 3 S. R. Lcvine and R. M. Caves, J. Electrochem. Soc., 121 (8) (1974) 1051. 4 A. J. Hickl and R. W. Heekel, Metall. Trans. A, 6(1975) 431. 5 B. Nciri and L. Vandenbulcke, Surf. Technol., 24(1985) 365. 6 A. Sivakumar and L. L. Seigle, Metall. Trans. A, 7(1976) 1073. 7 B. K. Gupta, A. K. Sarkbel and L. L. Seigle, Thin Solid Films, 39 (1976) 313. 8 B. P. Gupta and L. L. Seigle, Thin Solid Films, 73(1980) 365. 9 B. Nciri and L. Vandenbulcke, J. Less-Common Met., 95 (1983) 191. 10 G. H. Marijnissen and J. A. Klostermann, in J. Bloem, G. Verspui and L. R. Wolff (eds.), Proc. 4th Euro. Conf. on CVD, Eindhoven, May-June 1983, Philips Congress Centre, Eindhoven, 1983, p. 363. 11 B. Nciri and L. Vandenbulcke, Thin Solid Films, 139 (1986) 311. 12 P. N. Walsh, in G. F. Wakefield and J. M. Blocher, Jr. (¢ds.), Proc. 4th Int. Conf. on Chemical Vapour Deposition, Boston, 1973, Electrochemical Society, Pennington, N J, 1973, p. 147. 13 B. Nciri and L. Vandenbulcke, J. Less-Common Met., 95 (1983) 55. 14 V.A. Ravi, P. Choquet and R. A. Rapp, Proc. of Mater. Res. Soc. Int. Meeting on Advanced Materials, Tokyo, 1988, Trans. Jpn. Inst. Met., in press. 15 C. R. Wilke, Chem. Eng. Prog., 46(1950) 95. 16 T. K. Sherwood, R. L. Pigford and C. R. Wiike, Mass Transfer, McGraw-Hill, New York, 1975. 17 G. Leprince, S. Alperine, L. Vandenbulcke and A. Walder, Mater. Sci. Eng., A120/121 (1989) 419.