THE S IL ICONIZtNG OF REFRACTORY METALS
UNDER NONEQUIL IBR IUM CONDIT IONS
E. P. Nechtporenko , V. M. Kr ivoruchko , and
Physicotechnical Institute, Academy of Sciences, UkrSSR Translated from Poroshkovaya Metallurgiya, No. 10 (34), pp. 67-70, October, 1965 Original article submitted November 22, 1964
A. S. Mf t ro fanov
The kinetics of formation and growth of various chemical compounds, including silicides of refractory metals, constitute a complex physicochemical process. Chemical reaction is preceded by chemisorptton, which is difficult to separate directly from the former. This is followed by the growth of a layer of chemical reaction products, pro- vided the latter are nonvolatile. The chemfsorption and initial layer growth period is characterized by an absence of equilibrium at the phase boundaries, so that the concentrations of the reacting materials are variable. This cir- cumstance, of course, influences the layer growth kinetics.
In the case of sufficiently thick layers, the kinetics of layer growth during the nonequtlibrium period may obey a linear law with respect to time. In the course of time, as a result of the increased layer thickness, the dif- fusion of atoms of one of the components, rather than the boundary process, begins to determine the rate in the case of dense layers. Consequently, the linear law of layer growth becomes transformed into a parabolic one. Strictly speaking, the parabolic law becomes effective at the instant when a constant concentration of a component, present in the form of a chemical compound, is established at the phase boundaries.
This picture of the growth of chemical compound layers may become much more involved when a slight amount of a third element participates in the reaction or when the system of the resultant layers proves to be a multiphase one [ 1- 5].
K inet i cs of Vacuum S i l i con iz ing of Mo lybdenum in the Presence of a Smal l Amount of A luminum
Silicon, as well as molybdenum, tungsten, and tantalum, almost invariably contains impurities which exert a certain influence on the vacuum siliconizfng process. In particular, aluminum may constitute such an impurity. Figure 1 shows the kinetics of growth of a molybdenum disflicide layer in the presence of 1.0-1.2 wt. % aluminum dispersed in silicon (curve 1), as well as siliconizing in chemically-pure silicon (curve 2). It can be seen from a comparison of these curves that, for sufficiently thick layers, the growth law is linear in the former case and para- bolic in the latter. This indicates that, in the former case, varying conditions prevail at the layer interface as regards the concentrations of the reacting components.
At a given temperature, aluminum has a much higher vapor pressure than silicon and, consequently, inter- feres with the supply of the latter to the reacting surface. On the other hand, as a result of the removal of alumi- num impurities under vacuum siliconizing conditions, its vapor concentration above the reacting surface constantly decreases. Thus, during siliconizing in a silicon powder slightly contaminated with aluminum, the aluminum con- centration on the reacting surface decreases in the course of time, while the silicon concentration increases. Under these conditions, the thickness of the layer within a certain thickness range exerts no influence on the casing-growth kinetics, since, compared with the parabolic law, the layer grows in this case with a certain acceleration. In the case of sufficiently thick layers, this leads to a linear siliconizing law.
In the course of time, much of the aluminum is el iminated from the powder, and the linear law changes into a parabolic one, which indicates that constant concentrations of free and, consequently, combined silicon have be- come established at the phase boundaries.
It is characteristic that aluminum impurities exert a significant influence on the silfconizing process not only with regard to its kinetics, by decreasing the rate of growth of the casing and changing the growth law frorri parabolic
f~ . - - 2 ..-"i
f . t -
I - " 71
0 tO 20 a0 40 so ~0 70 Time, h
Effect of t ime on MoSi2 layer thickness at sil icontzing temperature of 1250~C: 1) alu- minum content 1-1.2 wt.~ 2) sil iconizing in chemical ly pure silicon.
to linear, but also with regard to the structure of the resulting silicides. Molybdenum disi l icide usually has a tetragonal struc- ture. The sil iconizing of molybdenum in the presence of alu- minum results in the formation of a MoSt 2 phase which has a hexagonal structure. Figure 2 shows a photomicrograph of a molybdenum specimen coated with hexagonal MoSt 2.
Thus, slight amounts of aluminum present in the form of vapors above the reaction surface markedly affect the kinetics of si l iconizing. In addition, by reducing the supply of silicon and entering into the molybdenum disiltcide lattice, they de- crease the absolute rate of growth of the latter.
The l inear law of growth of the si l icide layer can readily be interpreted mathemati.cally by assummg the absence of equi- l ibrium on the phase boundary, In the case of the parabolic law,
~he rate of growth of the layers depends in the first approximation on two factors, Ac and D, which are incorporated in the rate constant k of the parabolic law y2 = kt. The absence of equil ibrium on the phase boundaries in the case of isothermal process conditions (when D = const) indicates a change of the boundary concentrations and, eonse- que~!tly, Ac, Thus, under nonequilibrium conditions, k in fact constitutes a variable parameter, which linearly de- pends on &c,
The sil icon concentration at the si l icide layer boundary adjoining molybdenum may in our case be regarded as constant, Then, Ac will be determined by the sil icon concentration at the inner boundary layer, Cboun d.
At a low-percentage aluminum content of silicon, the change of aluminum concentration with t ime at con- stant temperature may he taken to be linear. This means that Cboun d and, consequently, k will also be linear func- tions of t ime, i ,e,, k = k't. Hence,
te"t y~ = k t = k ' t ~, y= 9
and the law of growth of the layer thickness is expressed by a straight line.
Naturally, this simple mathematical interpretation of the complex phenomenon of siltcide layer growth under nonequil ibtium conditions does not aspire to provide a full explanation of the kinetic curves of stlicontztng,
Fig, 2. Photomicrograph of molybdenum disi l icide layer with hexagonal structure on molybdenum. From top to bottom: MoSi 2, MosSi 3, Mo. X840.
,of ~1,6 -/ g'...2
0,8 -~ "~
300 400 0 100 2'00
Fig. 3. Initial stage of siliconizing of molyb- denum at temperatures of (deg C): 1) 1200; 2) 1250; 3) 1300.
,5/ 3 l J 2
_ . - -<
10o 200 300 Time, rain
Fig. 4. Initial stage of siliconiztng of tanta- lum at temperatures of (deg C): 1) 1200; 2) 1250; 3) 1300.
Kinet i cs of Growth of Mu l t iphase S i l t c ide Layers
A linear law of growth may apply not only in the case of dense, sufficiently thick single-phase layers, as was demonstrated above, but ,also in the case of multiphase layers. Once again, this is due to the absence of equilibrium on the phase boundaries. Figures 3 and 4 show the kinetics of growth of silicide phases in the systems Mo-Si and Ta-S i during the initial period of siliconizing. As can be seen from these figures, the rate of growth of the whole layer increases in the course of time, which may be attributed to the increase of the parameters Ac and D on tran- sition from the phase MoaSi to the phase M%Si a, etc. In fact, each of fl~ese kinetic curves represents a set of para- bolas characterizing the kinetics of growth of each phase separately.
It is always possible to select the siltconizing temperature in such a manner that, within a certain range of layer thicknesses, the rate of growth of the whole layer will obey a linear Iaw with respect to time, constituting the envelope of a family of several parabolas:
g2 _ kt -t- A,
where A is a variable parameter characterizing the nucleation time of a phase.
Naturally, with increasing thickness of each phase and of the whole layer in the course of time, equilibrium is established on the phase boundaries, and the layer growth law changes from linear to parabolic, since diffusion becomes the sole rate-determining factor.
It is shown that, during the vacuum siliconiztng of refractory metals in the absence of an equilibrium silicon concentration at the phase boundaries, the growth of the silicide layer as a function of time may obey. a linear law. This is observed during the initial period of siliconizing, when new phases are formed, as well as in the presence of impurities whose vapor pressure is much higher than that of silicon.
L ITERATURE C ITED
K. Hauffe, Reactions in Solid Bodies and on Their Surfaces[Russian translation], Moscow, IL (1963), Vol. 2. V. E. Ivanov, E. P. Nechiporenko, V. M. Krivoruchko, and A. S. Mttrofanov, Fiz. Metal. i Metalloved., 17, 6 (1964). K. Hauffe and H. Pfeiffer, Zs. Metallkunde, Bd.44, H. 1 (1953). A. R. Cox and R. Brown, J. Less-Common Metals, 6, No. 1 (t964). The Use of Vacuum in Metallurgy [in Russian], Moscow, Acad. Sci. USSR Press (1963), p. 168.