THERMOCHEMICAL TREATMENT

UDC 621.785.53

CIRCULATION METHOD FOR DEPOSITING DIFFUSION COATINGS

B. N. Arzamasov1 and V. N. Simonov1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 9, pp. 3 – 7, September, 2010.

The physicochemical fundamentals of directed mass transfer of coating elements with the help of heteroge-

neous chemical reactions occurring in a circulating gas flow successively washing the source of the coating

element and the surface of the saturated part at nonisothermal and isothermal states of the reaction space are

considered. Experiments and simulation are used for determining the effect of the process parameters on the

thickness and phase composition of coatings on internal and external surfaces of machine parts and on their life.

Key words: diffusion, coatings, chemical reaction, thermodynamics, kinetics.

INTRODUCTION

Processes of thermochemical treatment of parts should

be ecologically safe, wasteless, resource-saving, and econo-

mically efficient. In the 1960s specialists of the Bauman

Moscow State Technical University started working at a

novel circulation method for thermochemical treatment of

internal and external surfaces of machine parts under the

guidance of Professor D. A. Prokoshkin and Assistant Pro-

fessor B. N. Arzamasov. Prokoshkin and Arzamasov pre-

sented a report on “A circulation method for saturating molyb-

denum with some elements” at a scientific conference at the

Baikov Institute for Metallurgy and Materials Science [1].

The method was accepted at the level of the State in 1961

[2]. Today 11 Patents and Inventor’s Certificates on the topic

have been registered [3 – 12]. A detailed scientific substanti-

ation of the circulation method of diffusion saturation of me-

tals and alloys has been given in [13, 14].

The laws of equilibrium thermodynamics have been used

to analyze computationally the effect of temperature on the

direction of occurrence of chemical reactions. The study has

shown that circulation deposition of coatings can involve the

phenomenon of transfer of the diffusing element in a closed

working space of an installation (reactor) by reversible

chemical reactions due to creating different temperatures on

the source of the element of the coating and on the surface of

the saturated part. The driving force of the transfer is the gra-

dient of the partial pressure of the gas transferring the diffus-

ing element, which arises between the zones of location of

the initial material and the saturated surface of the part.

Four possible variants of the circulation method based on

the occurrence of reversible reactions of disproportioning,

dissociation, or reduction by hydrogen under the action of

the temperature drop between the initial material and the sa-

turated surface of the part have been developed.

The saturated parts and the material containing the diffu-

sion element are located separately in the working chamber

of the installation, and the atmosphere is moved reversibly

by a fan. Reversible motion of the atmosphere is required for

depositing uniform coatings on parts. The thickness of the

coating is affected by the speed of the motion of the atmo-

sphere, which can be controlled.

Nonisothermal processes of mass transfer and experi-

mental equipment for diffusion aluminizing, chromizing,

alumosiliconizing, and titanizing of steels and nickel alloys

have been developed by S. N. Fedoseeva, B. G. Kolmakov,

L. A. Pimenova, N. K. Bul’, and R. G. Mel’nikov with the

aim of raising the corrosion resistance.

Studies [15, 16] have shown that the thickness of the

coatings is the highest in a gas flow with a speed correspond-

ing to the transition of a laminar flow to a turbulent one. This

process in implemented in the circulation devices with the

help of a fan with variable rotation frequency.

The pilot facility for aluminizing of nickel refractory al-

loys was designed by B. G. Kolmakov and produced at

MMPP “Salyut.” The studies performed with the help of this

equipment sowed that the nonuniform temperature field in

Metal Science and Heat Treatment Vol. 52, Nos. 9 – 10, 2010

403

0026-0673/10/0910-0403 © 2010 Springer Science + Business Media, Inc.

1N. É. Bauman Moscow State Technical University, Moscow, Rus-

sia (e-mail: simonov vn@mail.ru).

the muffle resulted in high scattering of the thickness of coat-

ings on parts in a charge, and this stimulated the search for

new technological solutions.

The next stage in advancement of the circulation method

of deposition of coatings for raising the efficiency of the pro-

cess and the quality of the coatings obtained was substantia-

tion of mass transfer under isothermal conditions, when dif-

ferent activities of the diffusing element in the initial material

and on the saturated surface create a gradient of partial pres-

sure of the transferring gas in the working chamber of the fa-

cility. This variant of metallization is based on considerations

of equilibrium thermodynamics [17] and has been developed

in the studies of M. V. Unchikova, Yu. A. Puchkov, and other

researchers.

SIMULATION OF SATURATION

BY THE METHOD OF CIRCULATION

The multifactor nature and the interrelation between the

processes of formation of the atmosphere and the heteroge-

neous chemical reactions on the surfaces of the sources of

the coating components and of the treated parts required a

deeper analysis. It became necessary to develop a concept of

staged analysis of these processes by creating physical and

mathematical models based on the principles of nonequilib-

rium thermodynamics, gas mechanics, and diffusion laws.

Ternary phase diagrams with the gaseous component

(iodine, chlorine), the metal of the matrix, and the saturating

component were created for determining the phase composi-

tion of the surface of diffusion coatings depending on the

mass content of the reagents [18]. In order to determine the

phase composition of a coating with allowance for the fac-

tors of the process of saturation in aluminizing of nickel al-

loys a method for plotting ternary phase diagrams with a ga-

seous component has been developed. The equilibrium com-

position was computed using minimization of the Gibbs en-

ergy of the system, i.e.,

G (T, p, n ) � min; (1)

n ai ji

i

� �

�

�� = bj, j = 1, ..., m, (2)

where n is the vector of a composition where the components

are equilibrium concentrations of the substances of the sys-

tem in its phases (�) ni�

, bjis the content of the jth element

in the system, m is the number of the elements forming the

system, and aji

�is the number of atoms of the jth element in

the ith substance, which is present in phase �.

RESULTS AND DISCUSSION

The optimization problem represented by Eqs. (1) and (2)

was solved by the simplex method of linear programming.

Figure 1 presents an isothermal section of the diagram of the

Al – Ni – I system. We also designed such diagrams for the

Al – Ni – Cl system.

The phase diagram determines the kinds of equilibrium

arising between the gas atmosphere and the phases forming

the surface of the coating at the specified temperature. It fol-

lows from the diagram that the coating on the surface may

have a single-phase of two-phase state, which depends on the

mass proportion of the reagents (iodine, aluminum, and

nickel). The sequence of the phase states of the surface of

coatings reflected in Fig. 1 by the evolution trajectory Ni – a

indicates alternation of single-phase and two-phase states

such as �, (� + �� ), ��, (�� + �), �, etc. If the source of alumi-

num contains excess mass, the saturating capacity of the at-

mosphere evaluated in terms of the content of aluminum in-

creases progressively, and the coating is represented by a set

of layers of a solid solution of �, ��, �, and subsequent phases

of the Ni – Al diagram.

Theoretical analysis of the conditions of the phase state

was performed using the rule of Gibbs – Konovalov. Due to

the large radius, the atoms of halide � do not dissolve in the

phases of the coating, and the phase rule acquires a changed

form

C = K – F + 1. (7)

Analysis has shown that in the process of aluminizing of

nickel the surface of the diffusion coating should be sin-

gle-phase. A two-phase state exists only in the stage of nucle-

ation and growth on the interface of the earlier-formed

�-phase and the subsequent ��-phase, which is richer with

aluminum, etc. In order to check the adequacy of the predic-

tion of the phase condition with the help of ternary phase dia-

grams with a gaseous component, we saturated a nickel foil

with aluminum in an iodine atmosphere in a sealed quartz

404 B. N. Arzamasov and V. N. Simonov

I

I + NiI + AlI2 3

NiI

+A

lI

2

3+

Ni

��

3+

+A

lI

� 3+ AlI �+

AlI

�

�+

3

�

+A

lI

��

+�

+A

lI�

+

3

�A

lIL+

AlI

� + +AlI AlI3

�+

AlI

+L

Al Al, %

Ni,

%

I,%

Ni

a

c

Fig. 1. Isothermal section of the phase diagram of the Al – Ni – I

system at a temperature of 1000°C and a pressure of 0.1 MPa.

ampoule under conditions meeting the computed mass pro-

portion. The ampoule was charged with 25 g aluminum foil

20 m thick, 11.5 g aluminum, and 16.5 g crystalline iodine.

Then the air was evacuated to a residual pressure of 3 Pa.

Saturation was performed at a temperature of 1000°C for 5 h.

An x-ray diffraction analysis showed formation of a NiAl (�)

phase in the foil, which coincided with the computed data.

The kinetics of growth of the coating and its phase com-

position depend on many factors (the temperature, the gas

flow rate, the chemical activator, the sizes of the charged part

and of the source of the element of the coating, the geometri-

cal sizes of the used muffle, etc.).

A model of the kinetics of diffusion aluminizing has been

created as applied to specific equipment using the principles

of mosaic nonequilibrium thermodynamics [19].

The out parameters of the model are the main characte-

ristics of the diffusion layer (the thickness, the phase compo-

sition, and the content of aluminum in the coating), the va-

lues of which are specified by the design and process docu-

mentation for aluminide coatings on nickel alloys.

The model was created using the principle of singling

out elementary interrelated kinetic processes in a complex

physicochemical system and the principle of local equilib-

rium due to I. Prigogine, the laws of occurrence of chemical

reactions, and the laws of mass transfer, of gas mechanics,

and of diffusion. The approach does not contradict the princi-

ples of nonequilibrium thermodynamics and of the kinetics

of hierarchical processes.

The parameter determining the change in the state of the

reacting system is the capacity of the source Js , which in-

creases upon growth in the reaction surface of the source and

in the speed of the gas flow fed to the source. The factor con-

trolling the capacity of the source is the rate of diffusion

feeding of aluminum chlorides to the surface of the source.

In the startup stage the flow from the source Js is divided into

flow Ja of aluminum into the atmosphere, which raises its

saturating capacity, flow Jd of diffusion of aluminum into the

metal, which forms the coating, and flow Jm of aluminum

onto the cooled regions of the muffle and of the fixtures. In

the mode of steady operation of the reactor the saturating ca-

pacity of the atmosphere remains virtually unchanged, and

therefore the flow of aluminum into the gas phase decreases.

All the flows are interrelated, which creates conditions

for automatic control of the state of the system. The most im-

portant control parameters of the reacting system are the

temperature, the time, the reaction surfaces of the source, of

the charge of the parts and of the working chamber, and the

speed of the gas flow.

Physicochemical analysis has made it possible to single

out four subsystems in an operating reactor of a circulation

facility, which are responsible for the level of the flows of the

mass of aluminum. These are the source, the charge of parts,

the internal surface of the muffle, and the halide gas atmo-

sphere. The subsystems interact through 11 elementary ki-

netic processes, i.e.,

– diffusion of components of the atmosphere to the sur-

face of the source under the action of the gradient of their

partial pressures in the atmosphere and in the zone of local

equilibrium through a Nernst boundary layer;

– chemical reaction between AlCl3 and aluminum, which

yields AlCl2 and AlCl;

– redistribution of the products of the reaction in the gas

phase;

– motion of the products of the reaction from the source

to the charge in a forced gas flow;

– chemical reaction on the surface of the charge accom-

panied by segregation of aluminum;

– diffusion of the aluminum into the metal;

– redistribution of the products of the reaction in the at-

mosphere;

– motion of the atmosphere from the charge to the parts

of the muffle;

– diffusion motion of the components of the atmosphere

through the Nernst boundary layer to the surface;

– chemical reaction accompanied by segregation of alu-

minum;

– redistribution of the components of the atmosphere

and motion to the source.

Each kinetic process is represented by analytical or dif-

ferential equations. The systems of the equations are united

into three blocks and solved in an associated manner with the

help of the “Kinmod” software.

The behavior of the nonequilibrium system is divisible

into two stages with allowance for the flows of the mass of

aluminum from the source and on the sinks, which meet con-

dition

Js

= Ja

+ Jd

+ Jm

(4)

in the stage of startup and condition;

Js

= Jd

+ Jm

(5)

in the steady stage.

The expressions for flows Js, Jm , and Jd have a form re-

sembling the first equation of the Fick law.

It has been proved experimentally that the physico-

chemical model describes adequately the kinetics of diffu-

sion saturation of nickel with aluminum with a probability of

up to 90%.

Figures 2 and 3 present the results of computation of the

thickness and phase composition of a coating as a function of

the speed of the motion of the atmosphere.

Table 1 presents computed and experimental results of

an evaluation of the thickness of a coating as a function of

the time of saturation in aluminizing of nickel.

Processes of deposition of coatings onto internal surfaces

of void parts have been modeled and implemented. The ef-

fect of the gas dynamics of the saturating atmosphere on the

special features of the mass transfer and of the occurrence of

the chemical reactions in a complex-geometry gas conduit

Circulation Method for Depositing Diffusion Coatings 405

has been determined with the aim to ensure a uniform thick-

ness of the coating on the internal cavity [20]. A method has

been created for computing the service life of aluminide

coatings in an oxidizing atmosphere [21].

CONCLUSIONS

The theoretical and practical experience of deposition of

single- and multicomponent coatings accumulated at the de-

partment of materials science of the Bauman Moscow State

Technical University has been used for designing commer-

cial circulation devices. As a result of the joint work of the

department and of the “Salyut” aircraft enterprise designers

of the plant have created commercial installations DA-2M,

UMDP, and other devices for conducting aluminizing,

chromizing, and chromoaluminizing processes under indus-

trial conditions. The processes are used for commercial pro-

duction of aircraft engines, which has lowered the cost of de-

position of coatings by a factor of 5 as compared to the ear-

lier technology of chromoaluminizing from powders.

REFERENCES

1. B. N. Arzamasov and D. A. Prokoshkin, “Circulation method of

saturation of molybdenum with some elements” in: Reports on

the Theory of High-Temperature Strength, Coll. Works [in Rus-

sian], Akad. Nauk SSSR, Moscow (1961), pp. 208 – 213.

2. B. N. Arzamasov and D. A. Prokoshkin, “A method for saturat-

ing refractory metals with aluminum, silicon, and zirconium in

a gas atmosphere, USSR Inv. Certif. No. 148318,” Byull. Izobr.

Polezn. Modeli, No. 12 (1962).

3. V. N. Simonov, N. N. Zubkov, S. G. Vasil’ev, et al., “A method

for hardening the surface of parts, RF Patent No. 2015202,”

Byull. Izobr. Polezn. Modeli, No. 12 (1994).

4. V. N. Simonov, B. N. Arzamasov, Yu. S. Eliseev, et al., “A

method for multicomponent diffusion saturation of the surface

of a part, RF Patent No. 2186873, MKI C23C 10�14, C 23,

C12�02,” Byull. Izobr. Polezn. Modeli, No. 10 (2002).

5. V. N. Simonov, B. N. Arzamasov, Yu. S. Eliseev et al, “A

method for diffusion chromoaluminizing of the surface of a

part, RF Patent No. 2270880, MKI C23C 10�14,” Byull. Izobr.

Polezn. Modeli, No. 2 (2006)

6. V. N. Simonov, B. N. Arzamasov, Yu. P. Shkretov, and

G. V. Okhmatovskii, “A method for diffusion metallization of

steel articles in a circulating gas flow, RF Inv. Certif.

No. 1573891,” Byull. Izobr. Polezn. Modeli, No. 5 (1991).

7. V. N. Simonov, B. N. Arzamasov, Yu. P. Shkretov, et al., “A

method for aluminizing parts from metals and alloys, RF Inv.

Certif. No. 1299161 RF, MKI C 23 C 10�48,” Byull. Izobr.

Polezn. Modeli, No. 3 (1984).

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method for thermochemical treatment of parts from refractory

nickel alloys, RF Inf. Certif. No. 1088399,” Byull. Izobr. Polezn.

Modeli, (1984).

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method for detecting gas flow patterns, RF Inv. Certif.

No. 1148449,” Byull. Izobr. Polezn. Modeli, (1985).

406 B. N. Arzamasov and V. N. Simonov

TABLE 1. Computed and Experimental Values of the

Thickness of Coatings Deposited on Nickel Due to Alumi-

nizing for Different Times

alum

, h hcomp

, m hexp

, m

1 23.12�22.26 –

2 34.8�38.48 –

3 45.7�48.96 49

5 58.32�50.68 51

Notes. 1. In all the cases the process of aluminizing was

conducted at 1000°C.

2. The numerators present the thickness hcomp computed

with the help of the kinetic model; the numerators present

the corresponding values computed using regression anal-

ysis with confidence 99%.

44

43

42

41

40

39

38

37

36

35

0.1 0.3 0.5 0.7 0.9 1.1 1.3 1.5 1.7

vf , m sec�

hc , m

Ni

ZhS6U

Fig. 2. Thickness hc

of a coating on nickel and on alloy ZhS6U as a

function of the speed of the flow of aluminum halides vf

at a tempe-

rature of 1000°C and deposition time of 3 h.

0.38

0.32

0.24

0.16

0.08

�

��

�

5

1

2

3

4

Al, %

0 4 8 12 16 20 24 28 32 36 40 44 48

h, m

Fig. 3. Variation of the content of aluminum and of the phase com-

position over the thickness of coatings (h is the distance from the

surface) deposited at various speeds of the flow of aluminum

halides: 1 ) vf

= 1.5 m�sec; 2 ) vf

= 1.1 m�sec; 3 ) vf

= 0.5 m�sec;

4 ) vf

= 0.3 m�sec; 5 ) vf

= 0.1 m�sec.

10. V. N. Simonov, B. N. Arzamasov, Yu. P. Shkretov, et al., “A

method for diffusion metallization of articles from metals, Posi-

tive Decision on Appl. No. 4209875�02.”

11. V. N. Simonov, B. N. Arzamasov, A. V. Velishchanskii, et al., “A

device for depositing coatings from atmospheres, Positive Deci-

sion on Appl. No. 462928�31-02.”

12. V. N. Simonov, N. V. Abraimov, et al., “A method for diffusion

saturation of parts, RF Patent No. 2347847, MKI C23 C 10�14,

C23C 10�16,” Byull. Izobr. Polezn. Modeli, No. 6 (2009).

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method of deposition of coatings,” Izv. Vysh. Uchebn. Zaved.,

Mashinostroenie, No. 9, 116 – 119 (1966).

14. B. N. Arzamasov, Thermochemical Treatment of Metals in Acti-

vated Atmospheres [in Russian], Mashinostroenie, Moscow

(1979), 224 p.

15. B. N. Arzamasov, V. A. Rodionov, and B. G. Kolmakov, “Com-

putation of gas dynamic parameters of halide mixtures for cir-

culation devices,” Izv. Vysh. Uchebn. Zaved., Mashinostroenie,

No. 3, 136 – 139 (1968).

16. B. N. Arzamasov, V. A. Rodionov, B. G. Kolmakov, and

D. A. Prokoshkin, “Effect of the speed of gas flow on the pro-

cess of aluminizing by the circulation method,” Zashch. Pokr.

Met., No. 4, 45 – 47 (1971).

17. V. N. Simonov, B. N. Arzamasov, and A. D. Khaidorov, “Alu-

minizing of nickel in a medium of iodides by the circulation

method,” Izv. Vysh. Uchebn. Zaved., 113 – 116 (1975).

18. V. N. Simonov, M. V. Unchilova, and E. E. Yakubova, “Ther-

modynamic simulation of the process of aluminizing by the cir-

culation method and computation of the phase composition of

the coating,” Metalloved. Term. Obrab. Met., No. 5, 34 – 37

(1996).

19. V. N. Simonov, “A method for simulating the kinetics of alumi-

nizing of nickel and nickel-base alloys by the circulation

method,” Metalloved. Term. Obrab. Met., No. 6, 28 – 30 (2005).

20. V. N. Simonov, L. V. Yazonkin, and Yu. P. Shkretov, “Analysis

of the causes of nonuniform thickness of coatings deposited in

activated atmospheres,” Uprochn. Tekhnol. Pokr., No. 5, 45 – 48

(2008).

21. V. N. Simonov, Yu. P. Shkretov, and L. V. Yazonkin, “A method

for computing phase transformation in aluminide coatings on

nickel refractory alloys and their life under the action of high

temperatures and an oxidizing medium,” Metalloved. Term.

Obrab. Met., No. 10, 39 – 42 (2004).

Circulation Method for Depositing Diffusion Coatings 407