UDC 621.793.6

SIMULATION OF THE PROCESS OF CHROMIZING AND SILICONIZING

UNDER THE CONDITIONS OF SELF-PROPAGATING

HIGH-TEMPERATURE SYNTHESIS

B. P. Sereda1 and I. V. Kruglyak1

Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 29 – 33, April, 2002.

INTRODUCTION

Coatings deposited under the conditions of self-propa-gating high-temperature synthesis (SHS processes) accompa-nied by vapor-transport reactions have many peculiar proper-ties. They consist of a film of the deposited product (as a re-sult of vapor-phase precipitation) and a wide transition diffu-sion (gradient) zone (produced by diffusion saturation) [1].The method for depositing vapor-transport SHS coatings hasbeen suggested by É. A. Shtessel’ and E. P. Kostogorov [2]and studied in detail by the authors of [3 – 7]. The method isbased on the occurrence of exothermic reactions in a mode ofpropagating combustion waves. In contrast to the known dif-fusion methods of chromizing and siliconizing [8] that areenergy-intensive and long lasting, the SHS method can pro-vide diffusion layers in a mode of combustion or thermal ig-nition in a short time (from several minutes to 1 – 2 h). Theformed layer is from several microns to 1 – 2 mm thick. Thethickness of the protective layer may be controlled by thetime parameters of the process and the thermophysical char-acteristics of the SHS mixtures. In the present work we mod-eled the processes of chromizing and siliconizing under theconditions of SHS.

METHODS OF STUDY

The processes of SHS chromizing and siliconizing wereconducted in open-type reactors with argon blowing. Theprocess of formation of coatings in a mode of spontaneousthermal ignition consisted of five stages, i.e., inert heating,spontaneous thermal ignition, heating, isothermal hold, andcooling. The formation of coatings in a combustion mode oc-curred in three stages, namely, heating of the parts to the be-ginning of formation of saturating halides, their decomposi-tion, and chemical transport reactions.

The chemicothermal treatment under the conditions ofSHS was conducted at 1000 – 1200°C. The maximum timeof the isothermal hold was 60 min. The saturating mediumwas a mixture of aluminum powders, aluminum oxide, chro-mium oxide, metallic iodine, silicon, ammonium chloride,and ammonium fluoride.

The saturation process in the combustion mode was initi-ated with the help of an exothermal tablet or an electric spi-ral. The combustion wave moved at a speed of 1 mm�sec.

The thickness of the layers was determined under aNeophot-2 microscope under a magnification of up to � 500.The microstructure was determined by etching in a 3% solu-tion of nitric acid in alcohol.

RESULTS AND DISCUSSION

The rates of the chemical processes in SHS are deter-mined by kinetic rules that depend on the temperature and onthe diffusion factors. Assuming that in the stage of initialheating the deceleration of the diffusion processes in the va-por phase is not high, and the rate of temperature variation islow compared to that of vapor-phase chemical reactions, wemay assume that the equilibrium composition of the productsat each temperature is specified. Then, we may compute theequilibrium composition of the reaction products at somespecific temperatures and obtain a qualitative picture of thechemical development of the process.

We computed the equilibrium composition of the prod-ucts for an initial mixture consisting of M substances con-taining l chemical elements. At fixed values of the volume ofthe reactor V and the temperature T these elements can enterchemical reactions and form mk-substances in k = 0, 1, ..., g

differing phases. The set of substances includes l atomic and(m – l ) molecular compositions, the reactions of which arerepresentable in the form of equations of dissociation. Themathematical formulation of the problem is reduced tominimization of the thermodynamic potential.

The computation was performed with the use of the“ASTRA” software [9].

Metal Science and Heat Treatment Vol. 44, Nos. 3 – 4, 2002

1630026-0673/02/0304-0163$27.00 © 2002 Plenum Publishing Corporation

1 Moscow Automobile and Road Institute (State Technical Univer-sity), Moscow, Russia; Zaporozhye State Engineering Academy,Zaporozhye, Ukraine.

A study of the equilibrium composition of the products ata temperature of combustion of powdered dichromotrioxide,aluminum, and iodine has shown that the combustion of thepowder mixture with a vapor-transport agent (VTA) mayyield a gas phase bearing iodine compounds that include thefollowing elements: I, I2, AlI2, CrI, CrI2, and CrI3.

The increase in the content of VTA in the mixture leadsto a certain decrease in the adiabatic temperature of the com-bustion and a simultaneous increase in the amount of gas-eous iodides. This confirms that the elements can be trans-ported in order to form a coating.

The existence of limiting temperatures the heating towhich is accompanied by propagation of the combustionfront (1000 – 1200 K) somewhat limits the range of applica-tion of the combustion mode in the production process. Onthe contrary, the ignition mode is free of these constraints. Ifwe add an inert substance (up to 80 – 85%)2 to the initialmixture, the maximum temperature of the process can be re-duced to appropriate values. Figure 1 presents computedequilibrium compositions of gaseous and condensed pro-ducts of a system bearing Cr2O3 + Al + Al2O3 + I2 for a widerange of the initial reagents depending on the temperature.With increase in the content of the inert diluent the chro-mium yield decreases. The primary compounds in the gasphase at 1200 – 1800 K are chromium iodides CrI, CrI2, CrI3and atomic and molecular iodine. The mass fraction of eachof the iodides has a maximum at various temperatures. Withincrease in the amount of iodine in the mixture the positionof the maxima shifts to higher temperatures. The content ofgaseous iodides is proportional to the amount of the VTA inthe mixture and virtually does not depend on the amount ofthe inert additive.

For a content of I2 ranging from 3 to 10% the amount ofthe VTA is always in deficit relative to the reduced chro-mium and therefore reacts fully. The presence of extremumpoints on the curves describing the dependence of the equi-librium content of gaseous iodides on the temperature maybe explained by their decomposition due to the reactions ofdisproportionation and thermal decomposition. The contentof gaseous aluminum iodides in the composition of equilib-rium products is inconsiderable and ranges between 0.01 and0.02% for the considered temperatures.

It is known that the transfer of the components to the sa-turated surface through the vapor phase is of principal impor-tance for diffusion saturation in powder systems. However,these processes are often conducted under isothermal condi-tions. In the present work we studied the possibility of depo-siting chromized coatings under nonisothermal conditions. Inthis case the chemical transport reactions, the realization ofwhich due to gas carriers in the power mixtures is connectedwith the presence of temperature gradients, may be of greatimportance.

The realization of chemical transport should intensify thesupply of the gaseous components to the surface of the parts,which will affect the rate of formation of the coating.

The possibility of chemical transport in a combustionwave is based on the fact that the combustion process occurswith consecutive changes in the temperature regimes, and thetemperature at each point of the mixture assumes a series ofcontinues values (from the initial temperature to the combus-tion one).

Thus, a coating can be deposited in the process of com-bustion of a powder mixture combined with chemical trans-port reactions (CTR). This is physically possible becausehigh temperature gradients that appear at rather short dis-tances exist directly at the combustion front of the system.These drops may amount to 1000 K at a distance of fractionsof a millimeter. At such short diffusion paths the intensity ofthe diffusion transfer of the gaseous components participat-ing in chemical transport reactions is very high.

It is known that the use of CTR is based on realization ofa reversible reaction of the type [10]

M +m

nHn � MHm , (1)

where M is the deposited element, H is a halogen, and MHm

is a volatile halide.For the transfer and precipitation of element M the equi-

librium of this reaction should shift to the requisite side. Theelement and the part should have different temperatures.

The principal solid-phase reaction between elements M

and N is the driving force of the propagation of the combus-tion front over the powder mixture. If the mixture bears ahalogen, reactions of type (1) may occur in the zone of heat-ing of the combustion wave.

At a low temperature the reaction yields gaseous com-pounds. At a high temperature they decompose, precipitating

164 B. P. Sereda and I. V. Kruglyak

M , %c M , %g

1000 1200 1400 1600 1800T, K

80

60

40

20

0

1.2

0.8

0.4

0

5 6

7

1 2

9

3 4

8

Fig. 1. Dependence of the equilibrium composition of reactionproducts on the temperature for the Cr2O3 + Al + Al2O3 + I2 sys-tem. The composition of the powder reaction mixture (in mass frac-tions) is 0.199Cr2O3 + 0.071Al + 0.70Al2O3 + 0.03I2. Mg is themass fraction of gaseous products (the solid lines) and Mc is themass fraction of condensed products (the dashed lines): 1 ) Cr;2 ) CrI2; 3 ) I2; 4 ) CrI2; 5 ) Al2O3; 6 ) CrI; 7 ) I; 8 ) CrI3; 9 ) AlI3.

2 Here and below the content of the elements and compounds isgiven in mass fractions.

on the surface of the part placed in the power medium. Thelength of the diffusion path is comparable with the width ofthe heated zone in the combustion wave. For this reason, thetime of transportation is short, and the precipitation rate isquite high.

In contrast to the traditional processes of diffusion satu-ration, the coating in the considered process primarily growson the surface of the part. However, small transition layersmay form at the stage of cooling of the mixture and the partafter the passage of the combustion wave.

Basing ourselves on the considered features of combus-tion and propagation of heat waves we may evaluate thequalitative characteristics of the process.

The thermal pattern of the process is determined by threecharacteristic times, namely,

1. The combustion time �c in which the reaction frontpasses a distance equal to the heated zone,3 i.e.,

�c =a

V

m

2, (2)

where am is the thermal diffusivity of the mixture and V is thespeed of propagation of the combustion wave.

2. The time of heating of the specimen �h . We assumethat the heat exchange between the surface of the specimenand the surrounding mixture is described by boundary condi-tions of the third kind. Then it follows from the solution tothe transient problem of thermal conduction that the heatingtime of the specimen is determinable from the equation

�h =l

a

p

p pBi

2

124 � ( )

, (3)

where lp is the diameter of the part, ap is the thermaldiffusivity of the part, �1 (Bi)p is determined for cylindricalgeometry from the characteristic equation

J0 (�)�J1 (�) = 1�Bi � �, (4)

where J0 (�), J1 (�) are first-kind Bessel functions of zeroand first orders, respectively; Bi is the Biot factor (for ourcase Bi � 0 � �1 � 2Bi.

Substituting the requisite values into Eq. (4) we obtainthe following expression for �h :

�h =l Cp2

8

p p

m

�

�,

where lp is the diameter of the part, Cp and �p are heat capa-city and density of the material of the part respectively, and�m is the thermal conductivity of the mixture.

3. The cooling time of the mixture �cool after the end ofthe combustion determined as

�cool = � �

��

�

��R

l Q A

a

p h

m rBi2

21

12

ln( )

( )

/

�, (5)

where R is the radius of the reactor, lp is the diameter of thepart, Qh is the characteristic temperature, A1 is a parameterallowing for the geometry of the processes, and am is thethermal diffusivity of the mixture.

The model of the deposition of chromized coatings in thecombustion regime involves three stages.

In stage I of heating of the part due to ignition of the re-action mixture the reaction front passes a distance X0 equal tothe heated zone. In this time the coating does not form. Iodi-ne vapors diffuse from the mixture into the layer X0 . Theiodine vapors react in this layer with chromium reduced fromthe oxide and form CrI2. X0 is equal to 0.1 – 0.2 mm.

Stage II consists of the decomposition of chromiumdiiodide in the layer. Chromium diiodide precipitates on thesurface and decomposes in subsequent heating to t > 1000°Cwith deposition of chromium on the surface. In addition tothe precipitated chromium the process yields a small diffu-sion zone. At this moment the concentration of active chro-mium atoms on the surface is high. The high chemical poten-tial predetermined by the concentration gradient dc�dx pro-motes the beginning of formation of a diffusion layer consist-ing of a (Cr, Fe)7C3 phase. Computations show that in thisstage the formed layer has a thickness of about 10 �m.

Stage III consists in the occurrence of chemical transportreaction during the cooling of the reactor. The rate of coolingis a very important factor characterized by criterion Bi. Thecomputed thicknesses h � 80 – 120 �m. In addition to theformation of active chromium atoms the reaction produces adiffusion zone. Under unsteady temperature conditions achromized layer of a much greater thickness than in the caseof traditional chromizing under isothermal conditions formsduring the cooling time. After the cooling, a transition zoneconsisting of � + (Cr, Fe)7C3 is observed in the micro-structure of the specimen in addition to the phases mentionedabove.

For the process with dynamic heating to occur under theconditions of spontaneous ignition, the proportion of theheating rate provided by the external heat source to the heat-ing rate of the reactor with the specimens (parts) should bepositive. Under extremely high rates of external heating thespontaneous ignition may transform into firing. Under veryslow external heating spontaneous ignition does not occurbecause the mixture burns off substantially. However, as wehave mentioned above, ignition starts at some critical heatingrates.

Thus, the mode of spontaneous ignition is realized in thecase where the time of external heating is less than that ofheating of the reactor as a whole.

Simulation of the Process of Chromizing and Siliconizing 165

3 In the formulas throughout the paper the subscript “m” is used forthe characteristics of the reaction mixture, the subscript “p” isused for the coated part, and the subscript “r” is used for the reac-tor in which the reaction occurs.

In the mode of spontaneous ignition chromized coatingsform in five stages.

In stage I (of inert heating) the base exothermic reactionis absent. The reaction mixture heats to the temperature ofspontaneous ignition.

In stage II (of spontaneous thermal ignition) the tempera-ture grows to the maximum value tmax at a rate of 400 –600 K�sec. The reaction occurring in this stage yields ele-mentary chromium and its compounds with the carrier (I2 ),yielding volatile halides. If the activation energy of the inter-action of the mixture elements with the carrier is lower thanthe activation energy of the primary alumothermic process,the reaction of formation of volatile halides occurs in aquasi-stationary mode in parallel with the primary reaction.If the temperature of spontaneous ignition is lower than thatof the beginning of intense formation of the volatile CrI2 ha-lide, the formation of halides occurs only in the stage ofnonstationary temperature growth.

When the maximum temperature tmax exceeds the tem-perature of the pyrolysis of chromium diiodide, the latter de-composes rapidly. For thin parts the thickness of the coatingobtained in this stage is extremely small and depends on theamount of volatile halides formed during the induction of thethermal explosion.

In stage III (of heating of the parts) the physical pro-cesses are similar to those occurring in the combustion pro-cess. The thickness of the coatings obtained in this stage is ofthe same order. In addition to the precipitated chromium asmall diffusion zone begins to form in this stage.

The combustion processes differ only in the fact that thetemperature on the surface of the part at the initial moment isequal to the temperature of the medium before the momentof ignition.

In contrast to the combustion processes, where the tem-perature on the surface of the part is practically uncontrolla-ble and chiefly depends on the thermophysical characteris-tics of the part and of the mixture, the temperature on the sur-face of the part in this process may be a controllable quantity.This makes it possible to change the characteristics of thecoating in this stage.

In stage IV (of isothermal hold) active chromium atomsform continuously, and the precipitated chromium diffusesinto the material of the part. The layer consists of pure chro-mium and a zone of chemical compounds represented by the(Cr, Fe)23C6 carbide and a transition diffusion zone. In thisstage the temperature in the reactor does not change, and pro-cesses typical for stationary diffusion saturation occur in theabsence of temperature gradients. It should be noted that thesaturation occurs in the powder medium of the just reducedactive metal. In addition, the austenite formed upon theabrupt increase in the temperature is characterized by a highdislocation density, fine grains, and fine blocks. In this stagein such media the processes of diffusion saturation occurmore actively.

In stage V (of cooling) the processes are similar to thosethat occur in combustion. After the mixture is cooled to thetemperature of the ambient, the chromized coating consistsof chromium, carbide phases, and a transition diffusion zone.

The formation of siliconized coatings also occurs in fivestages.

In stage I of inert heating the reaction mixture is heatedto the ignition temperature. The diffusion layer is not formed.

In stage II of spontaneous thermal ignition silicon halidesbegin to form during the occurrence of the primary exother-mic reaction.

In stage III of heating of the part the temperature falls to

that of siliconizing. An �-phase starts to form.In stage IV of isothermal hold a siliconized diffusion

zone appears together with a zone of chemical compoundsrepresented by Fe3Si.

In stage V of cooling the formation of the layer is less in-tense. Carbon is squeezed into the depth of the specimen,forming a carbon-enriched transition zone.

The formation of siliconized layers in the combustionmode occurs as in chromizing in three stages.

In stage I of passage of the combustion wave the coatingis not formed. A vapor phase enriched with the saturatingcomponent (silicon) emerges.

In stage II of heating of the part the active atoms begin todiffuse into the surface of the specimen due to the highchemical potential predetermined by the concentration gradi-ent dc�dx.

166 B. P. Sereda and I. V. Kruglyak

à b

dc

1 1

1 1

2 2

2

2

3 3

33

Fig. 2. Microstructure of protective coatings deposited under theconditions of SHS (1 is the protective layer, 2 is the transition zone,3 is the substrate; � 500): a, b ) chromized [1 ) (Cr, Fe)7C3;2 ) � + (Cr, Fe)7C3 ] and siliconized [1 ) �-solid solution of siliconin iron; 2 ) � + �� (Fe3Si)] layers deposited in the combustion mode(onto a substrate of steel 45 at t = 1050°C); c) chromized layer[1 ) Cr + (Cr, Fe)7C3; 2 ) � + (Cr, Fe)7C3 ] deposited in the mode ofspontaneous thermal ignition (onto a substrate of steel 45 att = 1000°C, � = 20 min, Bi = 0.8); d ) siliconized layer [1 ) ��-phase(Fe3Si); 2 ) � + ��] deposited in the mode of spontaneous thermal ig-nition (onto a substrate of steel 20 at t = 1000°C, � = 45 min,Bi = 0.8).

In stage III of cooling the layer is formed less intenselydue to the decrease in the diffusivity of the saturating compo-nent because of the lower temperature.

Figure 2 presents the microstructure of chromized andsiliconized coatings obtained in the combustion and sponta-neous thermal ignition modes.

CONCLUSIONS

It is shown that the “ASTRA” software may be used forsuccessful simulation of SHS processes of chromizing andsiliconizing and efficient determination of the phase compo-sition of various zones of the diffusion layer.

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Simulation of the Process of Chromizing and Siliconizing 167