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A general model for vacuum condensates and vacuum diffusive coatings
Vacuum/volume 46/number11/pages1337to 1346/1995
~
Pergamon 0042-207X (95)00020-7
Copyright© 1995 ElsevierScienceLtd Printed in Great Britain.All rightsreserved 0042-207X/95 $9.50+.00
A general model for vacuum condensates and vacuum diffusive coatings R I Shishkov and E M Lisichkova, Technical University, Rousse, Bulgaria
received 10January 1995
The paper suggests a general mode/for the classification of metal coatings obtained in vacuum. The model gives the thickness, the bond character and the bond to the coated substrate, depending on: temperature, forming time and condensation rate. The coatings have been classified in two basic groups: vacuum condensates and vacuum diffusive coatings, separated by a layer zone having an intermediate character. The model is presented graphicafly and a prognosis is made on the change in thickness of the coating of each group, changing the technological parameters. The model suitability is tested for coatings obtained in a vacuum furnace by magnetron sputtering of a ferro-chromium target in a neutral medium by the method of plasma vacuum diffusive metallizing in element sputtering conditions (PVDM-E) on steel substrates with a composition (12%Cr; 1.5%C; 0.5%Mo), in the temperature range 850-1000°C and a process duration of 60120 rain for two condensation rates. The thickness, the structure and the phase composition of the coatings are presented by the three-coordinate figures. A classification has been made, according to the suggested general model.
Introduction
A number of well known investigations exist which aim to determine the structure and the topography of vacuum deposited coatings (condensates) depending on the basic technological parameters: temperature of the substrate, pressure of the working inert gas and the state of the surface 1-5.These investigations cover both thin and thick coatings, obtained either by evaporation or by sputtering of a number of pure elements and compounds in vacuum. Zone models have been developed, where the relation TJTm (T~---substrate temperature; Tin--melting temperature for the coating material) in certain ranges is the basic factor for obtaining one or another kind of layer structure during deposition. The above quoted authors agree about the kind of structural zones established and their temperature ranges, independent of different compositions of the investigated coatings and substrates, of the methods used (evaporation or sputtering) and of the change in wide range deposition rates. In the first structural zone, at Ts/Tm ~ 0.3, the layer has been structured by dispersed grains with an unclearly expressed growth grain direction. In the second zone, 0.3 ~< Ts/Tr, <<.0.5, the structure is characterized as being constant in width, having clearly distinguished column-like crystals for the whole layer thickness. Between both zones, the 'T' zone is the intermediate one, having a bi-modal structure of large and small crystals. Using relation TJTm >>-0.5, the obtained structures belong to a third zone and include widely columned, equi-axial or polyhedric large crystals. The surface or the bulk diffusion, acting as shading effects, are the determining factors
for the structure-formation of the layer at different temperatures. The presence of a smooth transition (two kinds of structure) between the second and third structural zones is explained by the unfinished process of recrystallization during the layer's growth. According to Grovenor 5, the initial layer structure always consists of fine, equi-axial grains, that are very unstable at high temperatures, since the better mobility and lower activation energy, necessary for a boundary migration, cause their spontaneous transition to the corresponding zone structure. Considerable experience has been gained in the field of thermochemical treatment in obtaining and investigating diffusive vacuum coatings, resistive to corrosion, wear and tear, thermal changes and the like. The authors, Ponomarenko 6 and Bakaliuk 7, studying diffusive interaction in vacuum metallizing, have worked at high temperatures (900-1300°C) for long periods (618 h). The results presented on the structure, phase composition, properties and on the processes that determine them, have been generally interpreted for the corresponding pair's substratedeposit, thus ensuring their natural relationship. Investigations in this field are related to studying and prognosticating the diffusive interaction as a result of the bulk heterodiffusion through the boundary surface coating-substrate. A number of possible intermediate kinds of coating exist between the two extremes of vacuum condensate and the classical diffusive coating. These intermediates were, until now, unable to be realised in practice. A review of publications connected with investigations of vacuum deposited coatings by evaporation and sputtering, leads to the following conclusions.
1337
R I Shishkov and E M Lisichkova: Vacuum eondensates and vacuum diffusive coatings
(1) The obtained structures, presented as structural and zone models in relation to the reduced temperature (TJTm), are related to thin and thick layers of a number of elements, obtained at low temperatures or at high deposition rates at a duration where the diffusive exchange between the substrate and the coating is either very slow, or is completely absent. (2) Most of the substrates used are neutral (glass, A1203) , i.e. they do not interact diffusively with the coating. Even in cases where the substrate-coating system in one way or another allows a diffusive interaction, these processes are not commented onl'2't The thick diffusive intermediate zone, observed by Thornton, when depositing Ti on Ta substrates in the temperature range T = 900-1200°C, due to the fact that fl-Ti and Ta form a series of various solutions at high temperatures, has not found a general expression in his model. (3) The metallurgical layer phase composition has been discussed only from the point of view of the coating composition and the conditions for its correspondence to the equilibrium phase diagram of state 1'2'5.The physical processes and the growth mechanism of the newly obtained phases, resulting from the diffusive interaction with the substrate chemical composition, are not commented on. (4) Deviations from the expected structure in the structural and zone models (SZM) are to be expected only from the point of view of thin layers, soiled by the substrate and also by the deposition of alloys, influencing the fluctuation of the grain boundaries. No comments have been made on the structural growth in the layer through a rising diffusion from the substrate and the recrystallization kinetics under conditions of concentration stimulated phase recrystallization. (5) References 1-5 report that the deposition rate is constant for all substrate temperatures (Ts). By the simultaneous deposition on substrates at a given temperature gradient, the re-evaporation rate, i.e. the chemical desorption of the falling particles, is dependent on the surface temperature. There are no data available on the actual deposition rate and on the layer thickness by varying the deposition temperature (condensation), which is of a considerable importance. (6) In the field of thermochemical treatment, no generalized modelling has been made on the influence of the basic technological parameters (substrate temperature---T,; condensation rate--/L; time for metallizing--t) on the structures obtained and on the phase composition of the coating, regardless of the equilibrium diagram of state, to which the system belongs. (7) The published results and models up until now have made it impossible to formulate a general idea about the changes in the structure, phase composition and character of the vacuum coatings after changing the basic technological parameters in wide ranges, beginning with the complete absence of diffusive interaction between the substrate and the coating and ending with cases, where the latter is the determinant.
A general model for vacuum condensates and vacuum diffusive coatings
The basic technological parameters for obtaining coatings in vacuum, that determine the process of interaction between the coating and the substrate, are the substrate temperature and the time of metallization. If the chemical composition of the coating and the substrate include elements whose nature allows diffusive interaction, the following considerations should be taken into 1338
account. Independent of the duration of metallization, the diffusive interaction between the deposited condensate and the substrate is possibly above a given temperature, determined by the physical nature of the participating elements and their chemical interaction. At the same time, in order to develop a diffusive interaction, a factor having a particular influence on the coating character, sufficient time is necessary, which will decrease with increase in Ts. It follows from the above that for a given condensate-substrate system, at certain boundary depositing conditions, regarding the substrate temperature (Ts) and the time of metallization (t), the obtained coating will be a condensate having an adhesive bond to the substrate. In fact, the known zone models from the literature discuss the case, where even at Ts/Tm ~ O, they do not examine the possible zones of amorphous-, amorphous and crystal- and ultradispersed state. Following the mechanism of heterodiffusion across the boundary surface, depending on the deposition conditions, the changes in the character of interaction are presented generally and ideally by Figure 1. The initial moment for starting the diffusion exchange between the coating and the substrate, changes with Ts and time t and is expressed by line 'a'. Such an exchange at high temperatures takes considerably less time. The coatings in zone 'A' have an adhesive bond and belong to the group of the vacuum condensates (VC). At high temperatures and great lengths of time, the coatings are completely diffusive and include those cases above the 'b' line. They are obtained using thermochemical treatment and belong to the 'B' zone. The coatings, positioned between the 'A' and the 'B' zones (i.e. above the 'a' line and under the 'b' one) contain an intermediate diffusive zone but show a different character, depending on its thickness. When the reciprocal diffusive exchange covers microvolumes around the boundary surface substrate--coating and a microdiffusive transition has taken place, the coatings show typical features and properties of vacuum condensates (VCt). They are obtained under sputtering conditions and are positioned between lines 'a' and 'c'. In the other case, the coatings, obtained under time-temperature conditions between the 'b' and 'd' lines, are of a considerable thickness of the intermediate diffusive layer. Nevertheless there remains some quality of residual condensate on their surface, that has not been affected by the heterodiffusion on their surface, nor by the heterodiffusion processes. These coatings are very similar to vacuum-diffusive coatings (VDCs) in their properties and behavior. Neither the characteristics of vacuum condensates, nor those of vacuum diffusive coatings, are expressed predominantly in the zone, positioned between the 'c' and the 'd' lines. This is the zone in which coatings are of an intermediate character. The known literature sources interpret the 'A' and the 'B' zones as being independent and the intermediate cases are not covered by them. The present general model aims to develop an idea concerning the influence of the basic technological parameters (substrate temperature--Ts, metallization time--t, condensation rate--/L) on the thickness, coating character and its bond to the coated substrate at constant sputtering rate (Rs) and a condensatesubstrate system, including elements interacting diffusively. Figure 2 illustrates graphically in a qualitative way the 'G' zone from Figure 1, in coordinates thickness of the coating--6, Ts, t, at sputtering capacity--Ps = const. This is the graphical representation of the nature of the general model, accounting for : the kind of coating, the coating thickness, the type of coating-
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings o ~3o
VM ~
: ~ ~ :-----~VDM
Zone "G"
I
',, I',, ~
~
(VDCs without RCt)
'~"
%
"..
""'..
~
__..----- VDC s with Re t Intermediate coatings a
VCt with a microdiffusive bond
C7
Zone "A" (VDCs with an adhesive bond) 0
Ts/T m
Figure 1. A general zone plan in T~-t coordinates, studied from the SZM (structural and zone models) and thermochemical treatment (diffusive metallizing in vacuum).
Ps = const.
J 6(p~m)
/ / /L/_/
// /*"
/
//
/ //
/
To
//
ii i
/"
///
/
11
Surface Ts,*C -...,
vet
VC t
Adh. bond
:
Diffus. bond
VM S
u
b
VDCs i
::With RCt
VDCs
t
VDM r
a
t
\
Without RCt
A: s
\
e
\ N'\.~l
~1
/
/
./'
'//
Figure 2. Coating thickness (3) and character in relationship to the substrate temperature and the time of metallizing in real coordinates.
substrate bond and the possibility of obtaining one or more intermediate diffusive layers, depending on the diagram of state between the components from the substrate and the deposited material. The model deals with possibility of converting the coating from a typical vacuum condensate to a typically diffusive coating during the same vacuum metallizing process.
According to the basic technological parameters (Ts, t) and the chemical composition of the system substrate-coating (condensate) and by following the diagram of state, the number and phase composition of the possible intermediate diffusive layers can be predicted. The coatings in the model (Figure 2) are divided into two basic 1339
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
groups. The first one includes vacuum coatings (VC~), having an adhesive bond and a microdiffusive transition towards the substrate, i.e. vacuum condensates (VC0. The second group includes diffusive coatings (VCD,), i.e. coatings, obtained completely or in their greater part in a diffusive way, characteristic for the classical thermochemical treatment (CTT). The diffusive coatings are also divided into two subgroups: VDCs with a residual condensate and VDC~ without a residual condensate (RCt). The regions of the two groups are separated by an intermediate zone 'A' (Figure 2), in which the coatings are of an intermediate character. In this zone, the thickness of the diffusive transition between VCt and the substrate grows, reaching dimensions near those of the coating and leading to qualitative changes in its behavior. It starts as a typical condensate and ends as a diffusive one, obtained by the methods of thermochemical treatment. The boundaries of this zone, Tl and T2 (Figure 2) are relative and their position is determined by the composition of the condensate and the substrate, by the sputtering (condensation) rate, by the time needed to obtain the coating and by the determined (observed) properties for the specific case. In order to classify the obtained coating, in correspondence with the model groups, the value 'relative thickness of the diffusive part in the coating' has been introduced and is called coefficient of the coating diffusiveness (Koc). It is determined by the relation of the thickness of the diffusive part (60) to the general (total) thickness of the whole coating (6c) and characterizes the extent of the diffusive interaction for a given system substrate-coating at the corresponding deposition conditions. Koc is of practical significance to characterize the coating and its relation to the corresponding group. KDC = (6D/6C) X 100%
(1)
For coatings, belonging to the intermediate zone 'A' from the model, KDC satisfies the inequality :
a <~ Koc <~ b
(2)
In general, for most properties, giving the coating character, one could accept boundary values for a ~ 0.1 and b ~ 0.3, where the thickness of the intermediate diffusive layer at zone 'A' will constitute 10-30% of the total coating thickness. For KDc lower than 0.1 (10%), the coating can be defined as VC, with a microdiffusive transition to the substrate, while for values higher than 0.3 (30%), it could be classified in the group of VDC~ with RCt. When the diffusive zone has covered the whole coating thickness KDC = 1 (100%), the coating belongs to the VDC~ without RCt. Since the diffusive layer also grows towards the substrate during the process of interaction and the determination of its real disposition towards the initial boundary surface is difficult, in practice the coatings can be presented in two ways : toward the initial substrate surface and toward the metallographically observed boundary on the substrate side after saturation. Figure 2 represents graphically the model in real coordinates, where, prior to deposition, the abscissa is the initial (saturated) surface. Depending on their character, the coatings are disposed towards the abscissa. Figure 3 presents the model in conditional coordinates, since the metallographically determined boundary between the coating and the substrate is disposed on the abscissa (Figure 2--line m). In practice, during the process of metallization, only the real coordinates make it possible to determine the changes in the dimensions of the parts. 1340
According to the model, changing the technological parameters in wide ranges changes the coating thickness, following a different rule; depending on its group affiliations and on the dominating processes. In the zone of the first group (VDt) (there are a number of papers and models on it), raising the substrate temperature due to the increase of the re-evaporation, the condensation rate (Re) should fall, decreasing also the thickness of the VCt (6c). At the time of deposition, the coating thickness is determined by Re, which is a function of Ts (at Ps = const).
6o = Rot
(3)
After reaching the temperature To (Figures 2 and 3) where significant diffusive processes start, independent of the decrease of the condensation rate (Ro), due to the presence and to the intensive growth of the diffusive transition, one should expect the growth of the whole coating. At the same time, there also occurs a change in its character: from VC,, via a transitive coating, VDC~ with RCt, towards a VDC~ without RCt. At a given temperature (Tcr), due to a balance in the intensity of the processes of condensation and diffusive interaction with the substrate, the coating character becomes completely diffusive, without RCt. Raising the temperature in the zone of the VDCs with RCt, the change of the general coating thickness is determined by the simultaneous influence of the re-evaporation and the diffusive interaction, which in general is a complex function. The coating thickness in the VDs without RCt changes according to the temperature, following the laws of exponential growth of the diffusive transition layer 7 10.
6c = 2x/-Dtt = 2 x / A e - e m r , t
(4)
D--coefficient of diffusion;A--pre-exponential factor, not depending on the temperature ; Q--heat of diffusion (activation energy) ; R--gas constant ; T--absolute temperature. The disposition of the intermediate (transition) zone 'A' and the critical temperature (Tcr) will be determined by the kind of sputtered metal, by the substrate composition and by the sputtering power (Ps). Decreasing Ps, the intermediate zone 'A' and the critical temperature (T~) will move towards lower temperatures. The influence of the sputtered metal and the substrate composition in zone 'A' and Tcr is in fact determined by their influence on the rate of the diffusive processes. The model (Figures 2 and 3) also includes the influence of time on coating formation. The beginning of its coordinate axis in this specific case assumes values, sufficient for a significant diffusive interaction, nevertheless the diffusion rate in vacuum condensates exceeds that of a thick material. The coating thickness in the VM zone grows linearly with time, since the condensation rate is a constant value for the same Ts. 6 c = k I t,
(5)
kl is a constant, characterizing P~ for a given Ts at Ps = const. In the VDM zone, where VDC~ without RCt are formed, the changing of t will influence the thickness as by classical thermochemical treatment. 3~ = k2x//t
(6)
kz---coefficient of proportionality, depending on a great number of factors for the corresponding Ts8'9. The rules for structuring diffusive layers in each specific case depend on the studied system and on the technological
R I Shishkov and E M Lisichkova: Vacuum condensates and v a c u u m diffusive coatings
Ps = const. I
-
J
I
8Clam)
/ ~J
I
I// /1
i
'
/ / To
T'I/T2
i :
RC,
/
II T f f
t,
It
% \
Surface
: ' ~. n I>'....
VCt
VC t
VDCs
X..._ '\ \
m DIZ in the
Ts'*C
substrate
VDCs ".,.
i
f
Adh. bond
Diffus. bond
: With RCt
:
.,,..
VM
iA It:
S
u
b
.I"
Without R ~ . \
s
VDM •9
t
It
r
a
t
e
Figure 3. Coating thickness (3) and character in relationship to the substrate temperature and the time of metallizing in conditional coordinates.
conditions. The mathematical models for their determination come to solving the equations of diffusion at various initial and boundary conditions, about which models exist9. The difficulties are related to the multilateral and to the interrelated character of the diffusive processes at heterodiffusion between the substrate and the coating, including: boundary-, bulk reaction- and reciprocal diffusions taking place in the principally metastable condensate. These processes sometimes run parallel to recrystallization, phase recrystallization, etc. The above mentioned processes are very much influenced by the structure, by the kinds of the boundaries, by the nature of the diffusate elements, by the stress etc. The problem becomes additionally complicated by multi-layered diffusive coatings when the sub-layers, heterogeneous by structure and phase composition, influence each other. Due to the diverse and dynamic character of the diffusive processes, the analytical modelling methods in this specific case are involve some approximations. The experimental methods require either the drawing of concentration curves at a fixed time, or the drawing of kinetic curves at fixed coordinates. The change in the coating thickness has a mixed character in the transitional zone 'A' and in the zone where VDCs with R C t are formed. In this case, raising the temperature increases the non-linearity.
~c = klt--~, + k 2 ~ ,
(7)
The mixed character of the change of the general coating thickness is due to the linear relationship of the condensate growth (5) on the one hand and on the non-linear growth of the transition diffusive layer thickness (6) on the other. The value 31 in the equation (7) is that part of the condensate that interacts diffusively with the substrate.
By increasing the time necessary for coating formation, the critical temperature (T,) should fall to some extent at first, which is due to two factors. On the one hand, the correlation between the time, necessary for the initial formation of a sufficient dense coating prior to the beginning of the diffusive interaction with the substrate, to the total metallization time, decreases. On the other hand, the diffusive transition, obtained from the interaction of the 6i part of the condensate, grows more intensively. Through further extension of the process beyond the limits, shown on Figures 2 and 3, one should expect a further increase in T=, due to the fading of the diffusive interaction across the formed diffusive layer. After the appearance of a diffusive bond, by raising the substrate temperature and by increasing the duration of the process, a number of intermediate layers can be formed (Figure 2 and Figure 3--1-4). They are moving relatively towards the condensate surface. The number, the phase composition and the order of the appearance of the intermediate layers are determined by the diagram of the state of the components of the condensatesubstrate system. In order to characterize such complex multilayered coatings, one can introduce the parameter 'a sub-layer structure' (Si). As a result of the diffusive interaction, it illustrates the multiplicity of the obtained structural and phase modifications in the coating. For example, a diffusive layer, formed in the coating, is a kind of a structure and phase composition. It is indicated as a first sub-layer structure (S0. The appearance in the coating of a subsequent diffusive sub-layer with another structure and phase composition is indicated as a second sub-layer structure ($2). If there exists a third sub-layer structure, it will be indicated by $3, etc. for the whole row of possible intermediate layers in the system. Since the type and succession of the phase sub-layers for the specific system condensate-substrate are 1341
R I Shishkov a n d E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
known from the diagram of state, it is expedient that the number of Si should follow them. Nevertheless, transition sub-layers are missing in the coating. For example, according to the diagram of state, at a low Rc and by intensive diffusion, the diffusive layer might be structured only of a second-in-succession sublayer. In such cases, nevertheless this is a single layer, it will be a second sub-layer structure and it will give a precise identification of the formed coating. If Si gives the relative thickness of the :h intermediate diffusive layer to the whole diffusive layer, then it can define the coating also in a qualitative way. Si = (SD~/~D) x 100%
(8)
Following the succession of formation, the sub-layer numbered the 1st, in the model on Figures 2 and 3, can be expressed by : S, = (5D,/aD) X 100%
(9)
If the whole diffusive part of the coating contains only the sublayer then $1 = 1 or 100%. If there are more sub-layers, then: for the second sub-layer from Figures 2 and 3 S2 = (6DJSD) X 100%
(10)
$3 = (6D3/fD) X 100%
(11)
for the third one
etc. If the specific coating is defined in the following way: KDC = 0 . 5 ($1 = 0 . 6 ; $2 = 0 . 3 ; $3 --- 0.1), it means that it is a VDC~ with RCt and that 50% of it is diffusive ; 60% of this is the first sub-layer, 30% is the second one and 10% makes the intermediate layer from the third structural and phase kind. Presented in this way, the VDCs obtain a suitable form for modelling the processes of their formation. This is also suitable by a process of investigation to characterize them precisely. The diffusive interaction between the coating and the substrate also leads to the formation of diffusive influence zones (DIZ), correspondingly in the condensate (Figure 2 and Figure 3--line q) and in the substrate (Figure 2 and Figure 3--line n). The diffusive changes of the chemical composition in them are insignificant and insufficient for a considerable change in the phase composition. Based on the general idea of vacuum metallization (VM) and vacuum diffusive metallization (VDM), the present paper has examined the adequacy of the proposed model in the field of transitional and diffusive coatings. The investigations cover coatings with a high chromium concentration, obtained under conditions of low temperature plasma, from a magnetron source at
various substrate temperatures and at a different process duration by the method of plasma-vacuum diffusive metallization at element sputtering conditions (PVDM-E) ~1.
Experimental The coatings were obtained by magnetron sputtering in Argon in a one-chamber vacuum furnace for plasma diffusive surface saturation ~2.An alloy ferro--chromium target was sputtered (65 % Cr; 0.1% C; 1.5% Si; 33.4% Fe), having a sputtering zone diameter of 90 mm, 30 mm width and an indirect cooling. The sputtering was carried out at an operating pressure of 6.10 -3 mbar, at an initial pressure of 6.10 - 4 mbar. The sputtering power was maintained constant--2.4 kW. The substrates were made of steel (1.5% C; 12% Cr; 0.5% Mo), with dimensions of diameter 15 mm and thickness 8 mm. They were positioned at a distance from the target Lt_s = 80 mm and 130 mm and R = 45 mm. They were heated to different temperatures in the range 850-1000°C by a radiation heater and by duration of the process 60-120 min. The condensation rate was changed in the ranges 0.16-0.12 /~m/min for L,_, = 80 and from 0.08 to 0.06 #m/rain for Lt_~ = 130 mm at T~ varying between 850 and 1000°C. The condensation rate was determined by the weighting method by deposition on A1203 ceramic substrates simultaneously with the steel ones. The temperature was measured using thermocouple Pt-PtRh. The structure was studied in cross-section using a metallographic microscope type Epitype-2 and Neophot-21, while the phase composition was investigated by the X-ray diffractometer U R D G with an Fe anode.
Results and discussion Microstructural analysis shows that the coatings obtained are diffusive in character, that their type and structure correspond to the intermediate zone 'A' (Figure 3) and to the VDM zone for both distances between the substrate and the target (L,_s = 80 and 130 mm). The values of Koo given in Table 1, are obtained using relation (2). They make it possible to classify the coatings according to the proposed model. For Lt_s = 80 mm (Figure 4) up to 900°C the coatings are intermediate (0.1 ~< KDc ~< 0.3) The coatings obtained under the same conditions but at L, s = 130 mm (Figure 5), due to the lower condensation rate, have a smaller coating thickness which is basically compensated for by the thickness of the residual condensate. In this case the boundary of the intermediate zone T2 falls and the coatings have an intermediate character up to about 850°C. At a temperature of 1000°C for both levels the coatings belong to the group of VDCs without RCt. In the other cases they are VDCs with Rft. The critical temperature for Rc ~ 0.12 #m/min
Table 1. Lt-~ mm 80 130
1342
KDc T(°C) t(min)
850
900
950
1000
60 90 120 60 90 120
0.159 0.189 0.196 0.192 0.23 0.25
0.2 0.26 0.28 0.333 0.357 0.435
0.45 0.476 0.5 0.555 0.667 0.71
1 1 1 1 1 1
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
(a)
(a)
~
R
C
t
I
B(p,m)
~(l~m)
I 20 I0 850
850
900
950
900
1000
950
1000
~oc
Ts.°C
(b) (b)
B(~m) 20
120
2ot--/ ~ ~ - - - ; q " - -e." - I y/ / .
I0
850 850
~,,.~
900
950
900
1000
,..
120
Dd-~..'6o
950
1000
~°c
Ts,°C
(c)
850 850
900
6(l~m) (d)
~
10F
L-" ~,o
950 T~,°C
~ 900
B(~m)(~)
201-/~,~ - - - 7
6
0 ,~o
90
,ooo
T¢°C Figure 4. Thickness (6) and character in conditional coordinates of high chromium conccntrated coatings onto steel(12%Cr; 1.5%C ; 0.5%Mo) at Lt_s = 80 mm :
(a) (b) (c) (d)
950
1000
~oc
2
.~~-~'~2-
900
1000
total thickness ; residual condensate--c~-rigid sol. (Cr, Fe) + [or+ (Cr, Fe)23C6]; first diffusive layer--(Cr, Fe)23C6-{-:(-rigid sol (Cr, Fe) ; second diffusive layer--(Cr, Fe)7C 3 q- (Cr, Fe)23C6 q- :(-rigid sol. (Cr, Fe).
is about 1000°C. In order to measure it more accurately, a larger number of experiments should be carried out in a temperature range of Ts = 950-1050°C. For Rc ~ 0.07 #m/min To, changes from about 980°C at 60 min, to 960°C at 120 min. The change in Rc at a constant sputtering power, respectively from 0.16 #m/min and 0.08 /~m/min at 850°C to 0.12 #m/min and 0.06/~m/min at 1000°C, explains the decrease in the total coating thickness (Figures 4 and 5). The amount of RC t is larger at temperatures 850-900°C and
,oF L-" 850
~
2
~ - - / ~ . ~ , 2 0 . ~ f _ ~ - - - :/;" --- ~ - - ~ ' - ' - - - - D ~ 9 ° ~ - / " i f6° 900
950
1000
Ts,°C Figure 5. Thickness (6) and character in conditional coordinates of high chromium concentrated coatings onto steel(12% Cr ; 1.5% C ;0.5%Mo) atLts= 130mm: (a) totalthickness ; (b) residualcondensate--s-rigid sol. (Cr, Fe) + [a+ (Cr, Fe)23C6] ; (c) firstdiffusivelayer--(Cr, Fe)23C6+ s-rigidsol (Cr, Fe) ; (d) second diffusive layer--(Cr, Fe)TC3+(Cr, Fe)23C6+~-rigid sol. (Cr, Fe).
because of this, the influence of re-evaporation is considerable. At temperatures 950-1000°C the amount and the significance of the diffusive layer grow relatively and the total coating thickness does not change significantly in the investigated ranges. The experimental data verify the theoretical model pertaining to influence of the process duration on the critical temperature. The idea given by the model (Figure 3) is also proof of the process kinetics in the transitional zone and in the zone where VDCs with RCt and without RCt are formed. It has been established that the coatings deposited on steel and more precisely RCt, are thicker 1343
R I Shishkov and E M Lisichkova: Vacuum condensates and v a c u u m diffusive coatings
than the thickness obtained for ceramic substrates. This can be explained by the influence of the following two factors : (1) Influence of the state- and structure of the surface on the formation of a dense initial condensate layer. (2) A temperature difference between the steel- and ceramic substrates. The first factor is obvious since both kinds of substrate have a different structure. The second factor is also important due to the following considerations. The temperature of the samples has been measured indirectly by a thermo-element, introduced into the heating chamber space 12. By means of the plasma, extra energy has been fed from the magnetron to the substrates. The quantity of this energy will depend on the distance to the target and will influence the real temperature. It therefore follows that decreasing L, s will cause the real substrate temperature to be increased to some extent. At the same time, the ceramic substrates, which are of a smaller mass and thickness, will have a higher temperature at e q u a l Lt-s. This is because of the greater ratio between the surface and the volume and the poorer heat conductivity. This will lead to an increase in the rate of re-evaporation and will decrease the Re. Increasing the time of metallization, the influence of the first factor should fade. This is because the relative part of time, necessary to obtain the initial condensate, in relation to the total process time, will decrease. This has been proved by the results obtained. Increasing the target-substrate distance should cause the influence of the second factor to fade. This effect has also been proven more clearly by increasing the time of metallization, when the influence of the first factor slows down. The diffusive layers obtained at distance Lt_s = 130 mm always have a smaller thickness in comparison to those obtained at Lt_s = 80 mm (Figures 4 and 5). This is obviously due to the differences in temperature on the one hand and the difference in the condensation rate on the other. The latter is one of the basic factors determining the concentration surface gradient, motivating the opposite reaction diffusion. From the microstructural point of view, some other features also prove the general theoretical model. By increasing the time and a temperature, the diffusive layer stratifies and the different sub-layers move towards the coating surface. Two types of intermediate diffusive layers (sub-layers) have been observed, which differ in structure and phase composition. The initially formed sub-layer for a period of 60 min has a column-like structure and phase composition, shown on Figures 4 and 5. It is the first sublayer structure. At 1000°C, for 60 min and for both distances L,_s = 80 mm and L,_~ = 130 mm this layer spreads all over the diffusive part, i.e. S~ = (5n,/5o) = 1 (100%). The coatings can be signified in the following way: KDC = 1 (S1 = 1). Increasing the time to 120 min, at all temperatures a sub-layer occurs, having equi-axial grains and a phase composition corresponding to the following highly carbonized part from the diagram of state. At 1000°C, for 120 min and at L,s = 130 mm, the parameter S 2 = (Sn216n) = 1 (100%) which means that the whole coating has been structured by the second sub-layer structure and that it can be signified in the following way: KDc = 1 ($2 = 1). At the same time, at the same T, and t, for L ~ = 80 mm, 48% of the coating is the first sub-layer structure and 52% of it is the second one. The coating character in this case can be signified in the following way: KDC = 1 (S~ = 0.48; $2 = 0.52). Studying the microstructure of cross-sections of some coatings deposited on 1344
steel and having a composition as the one quoted above, obtained by the method of PVDM-E, the same structures are observed that have been found in the structural and zone models-SZM j'2'5.The essential point is that they do not correspond to the specific temperature ranges, whereas some combine themselves in one coating. In fact, the structure in the RCt is dependent mainly on SZM, while the diffusive layer follows the rules of thermochemical treatment. The deposition temperature Ts has been changed by the experiments in the ranges 850-1000°C. The reduced temperature T,/Tm for the given compositions of ferro-chromium (Tm = 1650°C) has been changed in the range 0.52-0.61. It is to be expected that, in neutral substrates and through the absence of a diffusive interaction, the structure of the whole condensate should correspond to a third zone of the Movchan-Demchishin model and that it should have wide column-like or equi-axial big crystals. In fact the RCt on steel substrates has a dispersed structure; it has unclearly expressed boundaries, possesses considerable defects and porosity, more typical for the first zone. Grovenor 5 shows that a single change in the coating composition and pollution from the residual atmosphere influence the mobility of the grain boundaries, initially obtained by deposition. The energy, activating growth in such cases, is considerable and the transformation of the structure to a recrystallized one (more balanced) and corresponding to the temperature (Ts/Tm) is difficult. This will cause diversions from the SZM to more dispersed structures, characteristic for lower temperatures. In this case it is probable that independently of the high temperature Ts, the complex coating composition (ferro-chromium) and the presence of a residual atmosphere are the reason why the structure of the RCt corresponds to the first zone 1. By increasing the temperature and the time in the investigated ranges, one can only observe growth of the RCt connected with processes of coalescence. According to Thornton 4, first zone structures are characterized by an inability to undergo changes by heating to Ts/T,~ ~ 0.7. Some recrystallization is possible only in the interior of the grains, as the presence of a large number of open boundaries hinders obtaining of column-like grains. In support of the idea about the influence of condensate chemical composition on the temperature ranges of the different structural zones is the fact that the temperature ranges of similar compositions with amorphous and crystal states has been moved to 300 and 350°C, i.e. to Ts/Tm, respectively 0.3 a n d 0.3213 .
At the same time, at t = 60 min, for the coatings deposited on steel, one observes column-like grains, grown from the base, i.e. a structure, corresponding to the second SZM zone. The columnlike layer is of varying thickness, it grows with temperature and time and has a character of change, corresponding to the regularities of diffusive layer growth. The coating composition includes the strong carbide-former Cr, which stimulates the codiffusion of the C from the substrate and that of the Cr from the coating. According to ref 7, the reaction diffusion with metalloids flows according to a simple internode mechanism, since the activation energy for the heterodiffusion and for the migration between the nodes of the crystal lattice is the same. On the other hand, the dispersed condensate has a large boundary surface, a defectiveness and a small thickness which makes the heterodiffusion easier. There is a gradient of the chemical potential on the coating-substrate boundary, which determines the direction of the migration of the atoms. The increased concentration of Cr and C atoms increases the thickness of the layer in the
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
boundary zone and provides a change in their bulk concentrations. After the concentrations of the elements reach those concentrations characteristic for the new phase, i.e. the chemical compound (Cr, Fe)23C6, there exists an energetic prerequisite for its creation. The re-structuring of the crystal lattice in a complex wall-centered lattice of the carbide is accompanied by the additional procurement of more balanced structure, reducing the layer's stresses. The new structure has the direction of the diffusive flow, as the migration of the grains away from each other is limited by the meeting of other column-like grains. The height reached by the column-like grains, is determined by the diffusion front reached with stoichiometric concentrations for the new phase. The thickness of this intermediate layer grows when increasing the temperature and the time and that is due to the increase in the diffusion rate, resulting from increasing the temperature and the time of its influence. This has been observed for both distances Lt_s till the formation of the second diffusive sublayer. The diffusive exchange in the RC,, which is just above the column-like grains, probably influences the mobility of only some of the boundaries. The neighbouring grains, whose borders have a reduced defectiveness, will combine and that will promote the obtaining of a bi-modal structure, characteristic of the 'T' zone from the SZM. The observed structure in the DIZ in the condensate corresponds to that zone. At a higher Re ~ 0.14/tm/min the DIZ has a greater thickness due to a decrease in the diffusion processes in comparison to the condensation ones. In the obtained coatings a second intermediate diffusive layer has been formed as a part of the substrate through increasing the time of metallization. The substrates are made of highly carbonized steel (1.5% C). The continuing diffusive flow of the carbon from the substrate to the coating provides a concentration prerequisite for the formation of the next more highly carbonized (Cr, Fe)7C3 from the diagram of state. However, its growing conditions differ a lot from those of the previous one. The carbide (Cr, Fe)23C 6 was formed as a result of the phase recrystallization, running at conditions of reduced heterodiffusion in the strongly expressed and small thickness condensate, while the upgoing C diffusion, taking place across the initially formed diffusive layer of (Cr, Fe)23C6, r u n s with difficulty. Because of this, the phase recrystallization to a complex hexagonal crystal lattice of the carbide (Cr, Fe)7C 3 runs under conditions of saturation with C and a difficult chromium exchange between the substrate and the carbide (Cr, Fe)23C6. These are prerequisites for the formation of a dispersed and a more balanced (equi-axial) structure in the second diffusive sub-layer. It has the characteristic features of the third zone of SZM. By studying some ferro-chromium coatings, deposited on this or on other kinds of steel, one has the impression that the obtained structures in the different layers correspond to the given ones in the SZM 1'2.Of special interest is the fact that they succeed each other not by changing T,/Tm but by the thickness of a single coating, obtained at the same T~, as a result of the diffusive interaction with the substrate. For example, for a duration of 120 min up to a temperature of about 960°C the coating consists of: RC, which has a structure, analogous to the first zone of Movchan-Demshichin's model; DIZ--corresponding to the zone T; the first intermediate diffusive layer with a column-like structure of the second zone and the second intermediate sublayer with equiaxial grains--corresponding to the third zone. The presence or absence of some layers in the coating at the same Rc depends on the technological parameters T, and t, which
influence the heterodiffusion in this case. A duration of 60 min is insufficient to obtain the second diffusive sub-layer in the ranges of the studied temperatures. Since the rate of heterodiffusion exceeds Re, there are no RC, and DIZ above Tcr and at L,_s = 130 mm and 120 min the first diffusive sub-layer is missing. However, in all cases, DIZ is observed in the substrate under the coating. These results show that the basic technological parameters Re, Ts and t influence the character of the coatings, their thickness, structure and their phase composition. The investigations of some of the parameters separately are an exception at constant or boundary values of the rest ones. The experiments, presented in this manuscript, are limited to changing 7", and t to certain values for two distances Lt s, providing in this way two different average R e•
In general, Re is of considerable importance for the final result of the surface treatment. The change in Rc has a multiple effect, connected with: a change in energy of the particles, an extra heat effect, a layer thickness and defectiveness, a concentration diffusion gradient etc. Rc considerably influences the initial moment (Ts, t) for a diffusive interaction between the coating and the substrate in two cases. Firstly, when it has very small values and when the time, necessary to obtain a thick layer before the heterodiffusion starts, increases very much. Secondly, when it has very high values, strongly decreasing the time, necessary for metallization. When Re is limited to values, at which it has no considerable influence on the beginning of the heterodiffusion (line 'a' on Figure 1 and temperature To from Figures 2 and 3), its change mainly influences the coating thickness, obtained for a given time. In this way, the relative thickness of the diffusive layer (KDc) will keep changing, changing by this the coating group and the characteristic temperatures--T1, T2, T , (Figures 2 and 3). In order to obtain transitional coatings, VDCs with or without RCt, decreasing Ro, lower temperatures and shorter durations will be needed. The opposite, an increase in Re will cause the obtaining of the corresponding coating groups at higher Ts and t. As a result of heterodiffusion in the studied coatings, the comments on the type of the structure- and phase forming has been confirmed by the measured microhardness and by the X-ray structural and phase analysis. The phase analysis of RCt and that of the different diffusive layers (Figures 4 and 5) has been determined by a comparison of the results of the X-ray structural and durometric analyses to the type of the triple diagram ' C r - F e - C '~4. The presence of a aphase has been confirmed by the electrochemical testings on the photographing of potentiondynamic curves. The presence of DIZ in the condensate has been determined by the poorer appearance of its microstructure and it has been confirmed by the measured higher hardness of the DIZ, compared to that of the condensate. The hardness of the first diffusive layer at a metallization temperature of 1000°C and Lt_s ----80 mm changes from about M H W ° 1000 for 60 min up to MHV 2° 1400 for 120 min. At the same temperature for Lt s = 130 mm the hardness of the first layer is about M H W ° 1100 at t = 60 min. The hardness of the second diffusive layer at 1000°C, 90 min and 120 min and Zt-s = 130 mm is correspondingly about MHV 2° 1300 and about 1700, while for L, ~ = 80 mm for 120 min at 1000°C MHV 2° is about 1700. The results obtained for hardness are easy to explain, taking into consideration the fact that increasing Ts and t due to the stronger diffusive interaction in the transition diffusive layer and its saturation with C from the steel, moves its composition to the right in the triple diagram of ' C r - F e - C ' . The phase composition 1345
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
of the coating, directed to the substrate, changes by decreasing the quantity of the s-rigid solution (a). At the same time, the quantity of (Cr, Fe)23C6 increases and the carbide (Cr, Fe)7C3 is obtained. The increasing of Lt_s also leads to increasing the hardness, due to the relative delay of the condensation processes, compared to the diffusive ones, i.e. considerably more carbide phases are formed. The poorer hardnesses obtained, compared to those from the tables, for Cr23C 6 and Cr7C 3 and to those determined in 15, are a result of the presence of the s-rigid solution and of the mixed composition of the carbides, due to ferrochromium sputtering. It is known that Fe dissolves in the Cr23C6 carbide up to 35-40% and in Cr7C3 up to 50%, which decreases the hardness. Conclusions The investigated coatings with high chromium concentration prove the suitability of the presented general model for vacuum condensates and vacuum diffusive coatings. The coatings obtained by magnetron sputtering of a ferret--chromium target (65%Cr; 0.1%C; 33%Fe), for 60-120 min on substrates made of steel with a composition of 12%Cr; 1.5%C; 0.5%Mo, are intermediate up to about 900°C at a condensation rate 0.14 gm/min and up to about 850°C at a condensation rate 0.07 #m/min. The critical temperature (Tcr) for a target-substrate distance o f L t ~ = 80 mm (at Rc = 0.12/~m/min) is about 1000°C and for Zt-s = 130 mm (at Rc = 0.06 #m/min) it is about 970°C. When the process duration increases, Tcr decreases, thus at L~_, = 130 mm the change of To, is from about 980°C at 60 min up to about 960°C at 120 min. The two diffusive intermediate layers obtained are bi- and three-phasal, in accordance with the triple diagram of state " C r - F e - C " . The first layer consists of (Cr, Fe)23C6 and or-rigid solution and the second one consists of (Cr, Fe)TC3; (Cr, Fe)23C6 and s-rigid solution. The practical
1346
application of the plasma vacuum diffusive metallizing (chromium plating), when the magnetron sputtering source works in elemental conditions (PVDM-E), is suitable for open plane surfaces because of its more complicated configuration, due to the higher differences in Lt_,, the character, the kind and the properties of the coating will differ very significantly. References 1B A Movchan and A V Demchishin, Investiyatin# the Structure and the Properties of Thick Vacuum Condensates Made of Nickel, Titanium, Aluminium Oxides and Oxides of the Zirconium, Metal Physics and Metal Science. 2j A Thornton, J Vacuum Science and Technology, 11, 4, (1974). 3j A Thornton, J Vacuum Science and Technology, 12, 4, (1975). 4j A Thornton, Ann Rev Mater Sci, pp 239-260 (1977). 5C R M Grovenor, H T G Hentzel and D A Smith, Acta Metall, 12, 5, 773-781 (1984). 6E P Ponomarenko, Steel and alloy metallization in vacuum. Technika, Kiev, 1974. 7Ia H Bakaliuk and E V Proskurkin, Production of tubes with metal coatings. Metallurgy, Moscow, 1973. 8S Z Bockstein, Diffusion and metal structure. Metallur#y, Moscow, 1973. 9L N Lirikov and V I Issaichev, Diffusion in metals and alloys. Scientific Word, Kiev, 1987. I°M Kanev, R Shishkov et al., Vacuum chemical and thermochemical treatment. Technika, Sofia, 1984. I~R Shishkov and E Lisichkova, Method for Vacuum Plasma Diffusive Metallizing, Patent No 48518 (30.11.1988). ~2R Shishkov, E Lisichkova, Laboratory vacuum furnace for plasma diffusive surface saturation. Proc of the Nat Con#ress on Metal Science and Thermochemical Treatment. Varna, 3-5 Oct. (1991) 373. ~3R Shishkov and I Dermendjiev, Structure and phasal composition of magnetron deposited chromium-metalloid coatings. Proc of the Nat Congress on Metal Science and Thermochemical Treatment. Varna, 3-5 October (1991) 324. 14B G Livshitz, Metallography. Metallurgy, Moscow, 2nd, 1971, pp 323326. ~SR Shishkov, Vacuum chromium plating of carbon steels. Ph.D. degree thesis, Technical University of Rousse, 1980.
~
Pergamon 0042-207X (95)00020-7
Copyright© 1995 ElsevierScienceLtd Printed in Great Britain.All rightsreserved 0042-207X/95 $9.50+.00
A general model for vacuum condensates and vacuum diffusive coatings R I Shishkov and E M Lisichkova, Technical University, Rousse, Bulgaria
received 10January 1995
The paper suggests a general mode/for the classification of metal coatings obtained in vacuum. The model gives the thickness, the bond character and the bond to the coated substrate, depending on: temperature, forming time and condensation rate. The coatings have been classified in two basic groups: vacuum condensates and vacuum diffusive coatings, separated by a layer zone having an intermediate character. The model is presented graphicafly and a prognosis is made on the change in thickness of the coating of each group, changing the technological parameters. The model suitability is tested for coatings obtained in a vacuum furnace by magnetron sputtering of a ferro-chromium target in a neutral medium by the method of plasma vacuum diffusive metallizing in element sputtering conditions (PVDM-E) on steel substrates with a composition (12%Cr; 1.5%C; 0.5%Mo), in the temperature range 850-1000°C and a process duration of 60120 rain for two condensation rates. The thickness, the structure and the phase composition of the coatings are presented by the three-coordinate figures. A classification has been made, according to the suggested general model.
Introduction
A number of well known investigations exist which aim to determine the structure and the topography of vacuum deposited coatings (condensates) depending on the basic technological parameters: temperature of the substrate, pressure of the working inert gas and the state of the surface 1-5.These investigations cover both thin and thick coatings, obtained either by evaporation or by sputtering of a number of pure elements and compounds in vacuum. Zone models have been developed, where the relation TJTm (T~---substrate temperature; Tin--melting temperature for the coating material) in certain ranges is the basic factor for obtaining one or another kind of layer structure during deposition. The above quoted authors agree about the kind of structural zones established and their temperature ranges, independent of different compositions of the investigated coatings and substrates, of the methods used (evaporation or sputtering) and of the change in wide range deposition rates. In the first structural zone, at Ts/Tm ~ 0.3, the layer has been structured by dispersed grains with an unclearly expressed growth grain direction. In the second zone, 0.3 ~< Ts/Tr, <<.0.5, the structure is characterized as being constant in width, having clearly distinguished column-like crystals for the whole layer thickness. Between both zones, the 'T' zone is the intermediate one, having a bi-modal structure of large and small crystals. Using relation TJTm >>-0.5, the obtained structures belong to a third zone and include widely columned, equi-axial or polyhedric large crystals. The surface or the bulk diffusion, acting as shading effects, are the determining factors
for the structure-formation of the layer at different temperatures. The presence of a smooth transition (two kinds of structure) between the second and third structural zones is explained by the unfinished process of recrystallization during the layer's growth. According to Grovenor 5, the initial layer structure always consists of fine, equi-axial grains, that are very unstable at high temperatures, since the better mobility and lower activation energy, necessary for a boundary migration, cause their spontaneous transition to the corresponding zone structure. Considerable experience has been gained in the field of thermochemical treatment in obtaining and investigating diffusive vacuum coatings, resistive to corrosion, wear and tear, thermal changes and the like. The authors, Ponomarenko 6 and Bakaliuk 7, studying diffusive interaction in vacuum metallizing, have worked at high temperatures (900-1300°C) for long periods (618 h). The results presented on the structure, phase composition, properties and on the processes that determine them, have been generally interpreted for the corresponding pair's substratedeposit, thus ensuring their natural relationship. Investigations in this field are related to studying and prognosticating the diffusive interaction as a result of the bulk heterodiffusion through the boundary surface coating-substrate. A number of possible intermediate kinds of coating exist between the two extremes of vacuum condensate and the classical diffusive coating. These intermediates were, until now, unable to be realised in practice. A review of publications connected with investigations of vacuum deposited coatings by evaporation and sputtering, leads to the following conclusions.
1337
R I Shishkov and E M Lisichkova: Vacuum eondensates and vacuum diffusive coatings
(1) The obtained structures, presented as structural and zone models in relation to the reduced temperature (TJTm), are related to thin and thick layers of a number of elements, obtained at low temperatures or at high deposition rates at a duration where the diffusive exchange between the substrate and the coating is either very slow, or is completely absent. (2) Most of the substrates used are neutral (glass, A1203) , i.e. they do not interact diffusively with the coating. Even in cases where the substrate-coating system in one way or another allows a diffusive interaction, these processes are not commented onl'2't The thick diffusive intermediate zone, observed by Thornton, when depositing Ti on Ta substrates in the temperature range T = 900-1200°C, due to the fact that fl-Ti and Ta form a series of various solutions at high temperatures, has not found a general expression in his model. (3) The metallurgical layer phase composition has been discussed only from the point of view of the coating composition and the conditions for its correspondence to the equilibrium phase diagram of state 1'2'5.The physical processes and the growth mechanism of the newly obtained phases, resulting from the diffusive interaction with the substrate chemical composition, are not commented on. (4) Deviations from the expected structure in the structural and zone models (SZM) are to be expected only from the point of view of thin layers, soiled by the substrate and also by the deposition of alloys, influencing the fluctuation of the grain boundaries. No comments have been made on the structural growth in the layer through a rising diffusion from the substrate and the recrystallization kinetics under conditions of concentration stimulated phase recrystallization. (5) References 1-5 report that the deposition rate is constant for all substrate temperatures (Ts). By the simultaneous deposition on substrates at a given temperature gradient, the re-evaporation rate, i.e. the chemical desorption of the falling particles, is dependent on the surface temperature. There are no data available on the actual deposition rate and on the layer thickness by varying the deposition temperature (condensation), which is of a considerable importance. (6) In the field of thermochemical treatment, no generalized modelling has been made on the influence of the basic technological parameters (substrate temperature---T,; condensation rate--/L; time for metallizing--t) on the structures obtained and on the phase composition of the coating, regardless of the equilibrium diagram of state, to which the system belongs. (7) The published results and models up until now have made it impossible to formulate a general idea about the changes in the structure, phase composition and character of the vacuum coatings after changing the basic technological parameters in wide ranges, beginning with the complete absence of diffusive interaction between the substrate and the coating and ending with cases, where the latter is the determinant.
A general model for vacuum condensates and vacuum diffusive coatings
The basic technological parameters for obtaining coatings in vacuum, that determine the process of interaction between the coating and the substrate, are the substrate temperature and the time of metallization. If the chemical composition of the coating and the substrate include elements whose nature allows diffusive interaction, the following considerations should be taken into 1338
account. Independent of the duration of metallization, the diffusive interaction between the deposited condensate and the substrate is possibly above a given temperature, determined by the physical nature of the participating elements and their chemical interaction. At the same time, in order to develop a diffusive interaction, a factor having a particular influence on the coating character, sufficient time is necessary, which will decrease with increase in Ts. It follows from the above that for a given condensate-substrate system, at certain boundary depositing conditions, regarding the substrate temperature (Ts) and the time of metallization (t), the obtained coating will be a condensate having an adhesive bond to the substrate. In fact, the known zone models from the literature discuss the case, where even at Ts/Tm ~ O, they do not examine the possible zones of amorphous-, amorphous and crystal- and ultradispersed state. Following the mechanism of heterodiffusion across the boundary surface, depending on the deposition conditions, the changes in the character of interaction are presented generally and ideally by Figure 1. The initial moment for starting the diffusion exchange between the coating and the substrate, changes with Ts and time t and is expressed by line 'a'. Such an exchange at high temperatures takes considerably less time. The coatings in zone 'A' have an adhesive bond and belong to the group of the vacuum condensates (VC). At high temperatures and great lengths of time, the coatings are completely diffusive and include those cases above the 'b' line. They are obtained using thermochemical treatment and belong to the 'B' zone. The coatings, positioned between the 'A' and the 'B' zones (i.e. above the 'a' line and under the 'b' one) contain an intermediate diffusive zone but show a different character, depending on its thickness. When the reciprocal diffusive exchange covers microvolumes around the boundary surface substrate--coating and a microdiffusive transition has taken place, the coatings show typical features and properties of vacuum condensates (VCt). They are obtained under sputtering conditions and are positioned between lines 'a' and 'c'. In the other case, the coatings, obtained under time-temperature conditions between the 'b' and 'd' lines, are of a considerable thickness of the intermediate diffusive layer. Nevertheless there remains some quality of residual condensate on their surface, that has not been affected by the heterodiffusion on their surface, nor by the heterodiffusion processes. These coatings are very similar to vacuum-diffusive coatings (VDCs) in their properties and behavior. Neither the characteristics of vacuum condensates, nor those of vacuum diffusive coatings, are expressed predominantly in the zone, positioned between the 'c' and the 'd' lines. This is the zone in which coatings are of an intermediate character. The known literature sources interpret the 'A' and the 'B' zones as being independent and the intermediate cases are not covered by them. The present general model aims to develop an idea concerning the influence of the basic technological parameters (substrate temperature--Ts, metallization time--t, condensation rate--/L) on the thickness, coating character and its bond to the coated substrate at constant sputtering rate (Rs) and a condensatesubstrate system, including elements interacting diffusively. Figure 2 illustrates graphically in a qualitative way the 'G' zone from Figure 1, in coordinates thickness of the coating--6, Ts, t, at sputtering capacity--Ps = const. This is the graphical representation of the nature of the general model, accounting for : the kind of coating, the coating thickness, the type of coating-
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings o ~3o
VM ~
: ~ ~ :-----~VDM
Zone "G"
I
',, I',, ~
~
(VDCs without RCt)
'~"
%
"..
""'..
~
__..----- VDC s with Re t Intermediate coatings a
VCt with a microdiffusive bond
C7
Zone "A" (VDCs with an adhesive bond) 0
Ts/T m
Figure 1. A general zone plan in T~-t coordinates, studied from the SZM (structural and zone models) and thermochemical treatment (diffusive metallizing in vacuum).
Ps = const.
J 6(p~m)
/ / /L/_/
// /*"
/
//
/ //
/
To
//
ii i
/"
///
/
11
Surface Ts,*C -...,
vet
VC t
Adh. bond
:
Diffus. bond
VM S
u
b
VDCs i
::With RCt
VDCs
t
VDM r
a
t
\
Without RCt
A: s
\
e
\ N'\.~l
~1
/
/
./'
'//
Figure 2. Coating thickness (3) and character in relationship to the substrate temperature and the time of metallizing in real coordinates.
substrate bond and the possibility of obtaining one or more intermediate diffusive layers, depending on the diagram of state between the components from the substrate and the deposited material. The model deals with possibility of converting the coating from a typical vacuum condensate to a typically diffusive coating during the same vacuum metallizing process.
According to the basic technological parameters (Ts, t) and the chemical composition of the system substrate-coating (condensate) and by following the diagram of state, the number and phase composition of the possible intermediate diffusive layers can be predicted. The coatings in the model (Figure 2) are divided into two basic 1339
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
groups. The first one includes vacuum coatings (VC~), having an adhesive bond and a microdiffusive transition towards the substrate, i.e. vacuum condensates (VC0. The second group includes diffusive coatings (VCD,), i.e. coatings, obtained completely or in their greater part in a diffusive way, characteristic for the classical thermochemical treatment (CTT). The diffusive coatings are also divided into two subgroups: VDCs with a residual condensate and VDC~ without a residual condensate (RCt). The regions of the two groups are separated by an intermediate zone 'A' (Figure 2), in which the coatings are of an intermediate character. In this zone, the thickness of the diffusive transition between VCt and the substrate grows, reaching dimensions near those of the coating and leading to qualitative changes in its behavior. It starts as a typical condensate and ends as a diffusive one, obtained by the methods of thermochemical treatment. The boundaries of this zone, Tl and T2 (Figure 2) are relative and their position is determined by the composition of the condensate and the substrate, by the sputtering (condensation) rate, by the time needed to obtain the coating and by the determined (observed) properties for the specific case. In order to classify the obtained coating, in correspondence with the model groups, the value 'relative thickness of the diffusive part in the coating' has been introduced and is called coefficient of the coating diffusiveness (Koc). It is determined by the relation of the thickness of the diffusive part (60) to the general (total) thickness of the whole coating (6c) and characterizes the extent of the diffusive interaction for a given system substrate-coating at the corresponding deposition conditions. Koc is of practical significance to characterize the coating and its relation to the corresponding group. KDC = (6D/6C) X 100%
(1)
For coatings, belonging to the intermediate zone 'A' from the model, KDC satisfies the inequality :
a <~ Koc <~ b
(2)
In general, for most properties, giving the coating character, one could accept boundary values for a ~ 0.1 and b ~ 0.3, where the thickness of the intermediate diffusive layer at zone 'A' will constitute 10-30% of the total coating thickness. For KDc lower than 0.1 (10%), the coating can be defined as VC, with a microdiffusive transition to the substrate, while for values higher than 0.3 (30%), it could be classified in the group of VDC~ with RCt. When the diffusive zone has covered the whole coating thickness KDC = 1 (100%), the coating belongs to the VDC~ without RCt. Since the diffusive layer also grows towards the substrate during the process of interaction and the determination of its real disposition towards the initial boundary surface is difficult, in practice the coatings can be presented in two ways : toward the initial substrate surface and toward the metallographically observed boundary on the substrate side after saturation. Figure 2 represents graphically the model in real coordinates, where, prior to deposition, the abscissa is the initial (saturated) surface. Depending on their character, the coatings are disposed towards the abscissa. Figure 3 presents the model in conditional coordinates, since the metallographically determined boundary between the coating and the substrate is disposed on the abscissa (Figure 2--line m). In practice, during the process of metallization, only the real coordinates make it possible to determine the changes in the dimensions of the parts. 1340
According to the model, changing the technological parameters in wide ranges changes the coating thickness, following a different rule; depending on its group affiliations and on the dominating processes. In the zone of the first group (VDt) (there are a number of papers and models on it), raising the substrate temperature due to the increase of the re-evaporation, the condensation rate (Re) should fall, decreasing also the thickness of the VCt (6c). At the time of deposition, the coating thickness is determined by Re, which is a function of Ts (at Ps = const).
6o = Rot
(3)
After reaching the temperature To (Figures 2 and 3) where significant diffusive processes start, independent of the decrease of the condensation rate (Ro), due to the presence and to the intensive growth of the diffusive transition, one should expect the growth of the whole coating. At the same time, there also occurs a change in its character: from VC,, via a transitive coating, VDC~ with RCt, towards a VDC~ without RCt. At a given temperature (Tcr), due to a balance in the intensity of the processes of condensation and diffusive interaction with the substrate, the coating character becomes completely diffusive, without RCt. Raising the temperature in the zone of the VDCs with RCt, the change of the general coating thickness is determined by the simultaneous influence of the re-evaporation and the diffusive interaction, which in general is a complex function. The coating thickness in the VDs without RCt changes according to the temperature, following the laws of exponential growth of the diffusive transition layer 7 10.
6c = 2x/-Dtt = 2 x / A e - e m r , t
(4)
D--coefficient of diffusion;A--pre-exponential factor, not depending on the temperature ; Q--heat of diffusion (activation energy) ; R--gas constant ; T--absolute temperature. The disposition of the intermediate (transition) zone 'A' and the critical temperature (Tcr) will be determined by the kind of sputtered metal, by the substrate composition and by the sputtering power (Ps). Decreasing Ps, the intermediate zone 'A' and the critical temperature (T~) will move towards lower temperatures. The influence of the sputtered metal and the substrate composition in zone 'A' and Tcr is in fact determined by their influence on the rate of the diffusive processes. The model (Figures 2 and 3) also includes the influence of time on coating formation. The beginning of its coordinate axis in this specific case assumes values, sufficient for a significant diffusive interaction, nevertheless the diffusion rate in vacuum condensates exceeds that of a thick material. The coating thickness in the VM zone grows linearly with time, since the condensation rate is a constant value for the same Ts. 6 c = k I t,
(5)
kl is a constant, characterizing P~ for a given Ts at Ps = const. In the VDM zone, where VDC~ without RCt are formed, the changing of t will influence the thickness as by classical thermochemical treatment. 3~ = k2x//t
(6)
kz---coefficient of proportionality, depending on a great number of factors for the corresponding Ts8'9. The rules for structuring diffusive layers in each specific case depend on the studied system and on the technological
R I Shishkov and E M Lisichkova: Vacuum condensates and v a c u u m diffusive coatings
Ps = const. I
-
J
I
8Clam)
/ ~J
I
I// /1
i
'
/ / To
T'I/T2
i :
RC,
/
II T f f
t,
It
% \
Surface
: ' ~. n I>'....
VCt
VC t
VDCs
X..._ '\ \
m DIZ in the
Ts'*C
substrate
VDCs ".,.
i
f
Adh. bond
Diffus. bond
: With RCt
:
.,,..
VM
iA It:
S
u
b
.I"
Without R ~ . \
s
VDM •9
t
It
r
a
t
e
Figure 3. Coating thickness (3) and character in relationship to the substrate temperature and the time of metallizing in conditional coordinates.
conditions. The mathematical models for their determination come to solving the equations of diffusion at various initial and boundary conditions, about which models exist9. The difficulties are related to the multilateral and to the interrelated character of the diffusive processes at heterodiffusion between the substrate and the coating, including: boundary-, bulk reaction- and reciprocal diffusions taking place in the principally metastable condensate. These processes sometimes run parallel to recrystallization, phase recrystallization, etc. The above mentioned processes are very much influenced by the structure, by the kinds of the boundaries, by the nature of the diffusate elements, by the stress etc. The problem becomes additionally complicated by multi-layered diffusive coatings when the sub-layers, heterogeneous by structure and phase composition, influence each other. Due to the diverse and dynamic character of the diffusive processes, the analytical modelling methods in this specific case are involve some approximations. The experimental methods require either the drawing of concentration curves at a fixed time, or the drawing of kinetic curves at fixed coordinates. The change in the coating thickness has a mixed character in the transitional zone 'A' and in the zone where VDCs with R C t are formed. In this case, raising the temperature increases the non-linearity.
~c = klt--~, + k 2 ~ ,
(7)
The mixed character of the change of the general coating thickness is due to the linear relationship of the condensate growth (5) on the one hand and on the non-linear growth of the transition diffusive layer thickness (6) on the other. The value 31 in the equation (7) is that part of the condensate that interacts diffusively with the substrate.
By increasing the time necessary for coating formation, the critical temperature (T,) should fall to some extent at first, which is due to two factors. On the one hand, the correlation between the time, necessary for the initial formation of a sufficient dense coating prior to the beginning of the diffusive interaction with the substrate, to the total metallization time, decreases. On the other hand, the diffusive transition, obtained from the interaction of the 6i part of the condensate, grows more intensively. Through further extension of the process beyond the limits, shown on Figures 2 and 3, one should expect a further increase in T=, due to the fading of the diffusive interaction across the formed diffusive layer. After the appearance of a diffusive bond, by raising the substrate temperature and by increasing the duration of the process, a number of intermediate layers can be formed (Figure 2 and Figure 3--1-4). They are moving relatively towards the condensate surface. The number, the phase composition and the order of the appearance of the intermediate layers are determined by the diagram of the state of the components of the condensatesubstrate system. In order to characterize such complex multilayered coatings, one can introduce the parameter 'a sub-layer structure' (Si). As a result of the diffusive interaction, it illustrates the multiplicity of the obtained structural and phase modifications in the coating. For example, a diffusive layer, formed in the coating, is a kind of a structure and phase composition. It is indicated as a first sub-layer structure (S0. The appearance in the coating of a subsequent diffusive sub-layer with another structure and phase composition is indicated as a second sub-layer structure ($2). If there exists a third sub-layer structure, it will be indicated by $3, etc. for the whole row of possible intermediate layers in the system. Since the type and succession of the phase sub-layers for the specific system condensate-substrate are 1341
R I Shishkov a n d E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
known from the diagram of state, it is expedient that the number of Si should follow them. Nevertheless, transition sub-layers are missing in the coating. For example, according to the diagram of state, at a low Rc and by intensive diffusion, the diffusive layer might be structured only of a second-in-succession sublayer. In such cases, nevertheless this is a single layer, it will be a second sub-layer structure and it will give a precise identification of the formed coating. If Si gives the relative thickness of the :h intermediate diffusive layer to the whole diffusive layer, then it can define the coating also in a qualitative way. Si = (SD~/~D) x 100%
(8)
Following the succession of formation, the sub-layer numbered the 1st, in the model on Figures 2 and 3, can be expressed by : S, = (5D,/aD) X 100%
(9)
If the whole diffusive part of the coating contains only the sublayer then $1 = 1 or 100%. If there are more sub-layers, then: for the second sub-layer from Figures 2 and 3 S2 = (6DJSD) X 100%
(10)
$3 = (6D3/fD) X 100%
(11)
for the third one
etc. If the specific coating is defined in the following way: KDC = 0 . 5 ($1 = 0 . 6 ; $2 = 0 . 3 ; $3 --- 0.1), it means that it is a VDC~ with RCt and that 50% of it is diffusive ; 60% of this is the first sub-layer, 30% is the second one and 10% makes the intermediate layer from the third structural and phase kind. Presented in this way, the VDCs obtain a suitable form for modelling the processes of their formation. This is also suitable by a process of investigation to characterize them precisely. The diffusive interaction between the coating and the substrate also leads to the formation of diffusive influence zones (DIZ), correspondingly in the condensate (Figure 2 and Figure 3--line q) and in the substrate (Figure 2 and Figure 3--line n). The diffusive changes of the chemical composition in them are insignificant and insufficient for a considerable change in the phase composition. Based on the general idea of vacuum metallization (VM) and vacuum diffusive metallization (VDM), the present paper has examined the adequacy of the proposed model in the field of transitional and diffusive coatings. The investigations cover coatings with a high chromium concentration, obtained under conditions of low temperature plasma, from a magnetron source at
various substrate temperatures and at a different process duration by the method of plasma-vacuum diffusive metallization at element sputtering conditions (PVDM-E) ~1.
Experimental The coatings were obtained by magnetron sputtering in Argon in a one-chamber vacuum furnace for plasma diffusive surface saturation ~2.An alloy ferro--chromium target was sputtered (65 % Cr; 0.1% C; 1.5% Si; 33.4% Fe), having a sputtering zone diameter of 90 mm, 30 mm width and an indirect cooling. The sputtering was carried out at an operating pressure of 6.10 -3 mbar, at an initial pressure of 6.10 - 4 mbar. The sputtering power was maintained constant--2.4 kW. The substrates were made of steel (1.5% C; 12% Cr; 0.5% Mo), with dimensions of diameter 15 mm and thickness 8 mm. They were positioned at a distance from the target Lt_s = 80 mm and 130 mm and R = 45 mm. They were heated to different temperatures in the range 850-1000°C by a radiation heater and by duration of the process 60-120 min. The condensation rate was changed in the ranges 0.16-0.12 /~m/min for L,_, = 80 and from 0.08 to 0.06 #m/rain for Lt_~ = 130 mm at T~ varying between 850 and 1000°C. The condensation rate was determined by the weighting method by deposition on A1203 ceramic substrates simultaneously with the steel ones. The temperature was measured using thermocouple Pt-PtRh. The structure was studied in cross-section using a metallographic microscope type Epitype-2 and Neophot-21, while the phase composition was investigated by the X-ray diffractometer U R D G with an Fe anode.
Results and discussion Microstructural analysis shows that the coatings obtained are diffusive in character, that their type and structure correspond to the intermediate zone 'A' (Figure 3) and to the VDM zone for both distances between the substrate and the target (L,_s = 80 and 130 mm). The values of Koo given in Table 1, are obtained using relation (2). They make it possible to classify the coatings according to the proposed model. For Lt_s = 80 mm (Figure 4) up to 900°C the coatings are intermediate (0.1 ~< KDc ~< 0.3) The coatings obtained under the same conditions but at L, s = 130 mm (Figure 5), due to the lower condensation rate, have a smaller coating thickness which is basically compensated for by the thickness of the residual condensate. In this case the boundary of the intermediate zone T2 falls and the coatings have an intermediate character up to about 850°C. At a temperature of 1000°C for both levels the coatings belong to the group of VDCs without RCt. In the other cases they are VDCs with Rft. The critical temperature for Rc ~ 0.12 #m/min
Table 1. Lt-~ mm 80 130
1342
KDc T(°C) t(min)
850
900
950
1000
60 90 120 60 90 120
0.159 0.189 0.196 0.192 0.23 0.25
0.2 0.26 0.28 0.333 0.357 0.435
0.45 0.476 0.5 0.555 0.667 0.71
1 1 1 1 1 1
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
(a)
(a)
~
R
C
t
I
B(p,m)
~(l~m)
I 20 I0 850
850
900
950
900
1000
950
1000
~oc
Ts.°C
(b) (b)
B(~m) 20
120
2ot--/ ~ ~ - - - ; q " - -e." - I y/ / .
I0
850 850
~,,.~
900
950
900
1000
,..
120
Dd-~..'6o
950
1000
~°c
Ts,°C
(c)
850 850
900
6(l~m) (d)
~
10F
L-" ~,o
950 T~,°C
~ 900
B(~m)(~)
201-/~,~ - - - 7
6
0 ,~o
90
,ooo
T¢°C Figure 4. Thickness (6) and character in conditional coordinates of high chromium conccntrated coatings onto steel(12%Cr; 1.5%C ; 0.5%Mo) at Lt_s = 80 mm :
(a) (b) (c) (d)
950
1000
~oc
2
.~~-~'~2-
900
1000
total thickness ; residual condensate--c~-rigid sol. (Cr, Fe) + [or+ (Cr, Fe)23C6]; first diffusive layer--(Cr, Fe)23C6-{-:(-rigid sol (Cr, Fe) ; second diffusive layer--(Cr, Fe)7C 3 q- (Cr, Fe)23C6 q- :(-rigid sol. (Cr, Fe).
is about 1000°C. In order to measure it more accurately, a larger number of experiments should be carried out in a temperature range of Ts = 950-1050°C. For Rc ~ 0.07 #m/min To, changes from about 980°C at 60 min, to 960°C at 120 min. The change in Rc at a constant sputtering power, respectively from 0.16 #m/min and 0.08 /~m/min at 850°C to 0.12 #m/min and 0.06/~m/min at 1000°C, explains the decrease in the total coating thickness (Figures 4 and 5). The amount of RC t is larger at temperatures 850-900°C and
,oF L-" 850
~
2
~ - - / ~ . ~ , 2 0 . ~ f _ ~ - - - :/;" --- ~ - - ~ ' - ' - - - - D ~ 9 ° ~ - / " i f6° 900
950
1000
Ts,°C Figure 5. Thickness (6) and character in conditional coordinates of high chromium concentrated coatings onto steel(12% Cr ; 1.5% C ;0.5%Mo) atLts= 130mm: (a) totalthickness ; (b) residualcondensate--s-rigid sol. (Cr, Fe) + [a+ (Cr, Fe)23C6] ; (c) firstdiffusivelayer--(Cr, Fe)23C6+ s-rigidsol (Cr, Fe) ; (d) second diffusive layer--(Cr, Fe)TC3+(Cr, Fe)23C6+~-rigid sol. (Cr, Fe).
because of this, the influence of re-evaporation is considerable. At temperatures 950-1000°C the amount and the significance of the diffusive layer grow relatively and the total coating thickness does not change significantly in the investigated ranges. The experimental data verify the theoretical model pertaining to influence of the process duration on the critical temperature. The idea given by the model (Figure 3) is also proof of the process kinetics in the transitional zone and in the zone where VDCs with RCt and without RCt are formed. It has been established that the coatings deposited on steel and more precisely RCt, are thicker 1343
R I Shishkov and E M Lisichkova: Vacuum condensates and v a c u u m diffusive coatings
than the thickness obtained for ceramic substrates. This can be explained by the influence of the following two factors : (1) Influence of the state- and structure of the surface on the formation of a dense initial condensate layer. (2) A temperature difference between the steel- and ceramic substrates. The first factor is obvious since both kinds of substrate have a different structure. The second factor is also important due to the following considerations. The temperature of the samples has been measured indirectly by a thermo-element, introduced into the heating chamber space 12. By means of the plasma, extra energy has been fed from the magnetron to the substrates. The quantity of this energy will depend on the distance to the target and will influence the real temperature. It therefore follows that decreasing L, s will cause the real substrate temperature to be increased to some extent. At the same time, the ceramic substrates, which are of a smaller mass and thickness, will have a higher temperature at e q u a l Lt-s. This is because of the greater ratio between the surface and the volume and the poorer heat conductivity. This will lead to an increase in the rate of re-evaporation and will decrease the Re. Increasing the time of metallization, the influence of the first factor should fade. This is because the relative part of time, necessary to obtain the initial condensate, in relation to the total process time, will decrease. This has been proved by the results obtained. Increasing the target-substrate distance should cause the influence of the second factor to fade. This effect has also been proven more clearly by increasing the time of metallization, when the influence of the first factor slows down. The diffusive layers obtained at distance Lt_s = 130 mm always have a smaller thickness in comparison to those obtained at Lt_s = 80 mm (Figures 4 and 5). This is obviously due to the differences in temperature on the one hand and the difference in the condensation rate on the other. The latter is one of the basic factors determining the concentration surface gradient, motivating the opposite reaction diffusion. From the microstructural point of view, some other features also prove the general theoretical model. By increasing the time and a temperature, the diffusive layer stratifies and the different sub-layers move towards the coating surface. Two types of intermediate diffusive layers (sub-layers) have been observed, which differ in structure and phase composition. The initially formed sub-layer for a period of 60 min has a column-like structure and phase composition, shown on Figures 4 and 5. It is the first sublayer structure. At 1000°C, for 60 min and for both distances L,_s = 80 mm and L,_~ = 130 mm this layer spreads all over the diffusive part, i.e. S~ = (5n,/5o) = 1 (100%). The coatings can be signified in the following way: KDC = 1 (S1 = 1). Increasing the time to 120 min, at all temperatures a sub-layer occurs, having equi-axial grains and a phase composition corresponding to the following highly carbonized part from the diagram of state. At 1000°C, for 120 min and at L,s = 130 mm, the parameter S 2 = (Sn216n) = 1 (100%) which means that the whole coating has been structured by the second sub-layer structure and that it can be signified in the following way: KDc = 1 ($2 = 1). At the same time, at the same T, and t, for L ~ = 80 mm, 48% of the coating is the first sub-layer structure and 52% of it is the second one. The coating character in this case can be signified in the following way: KDC = 1 (S~ = 0.48; $2 = 0.52). Studying the microstructure of cross-sections of some coatings deposited on 1344
steel and having a composition as the one quoted above, obtained by the method of PVDM-E, the same structures are observed that have been found in the structural and zone models-SZM j'2'5.The essential point is that they do not correspond to the specific temperature ranges, whereas some combine themselves in one coating. In fact, the structure in the RCt is dependent mainly on SZM, while the diffusive layer follows the rules of thermochemical treatment. The deposition temperature Ts has been changed by the experiments in the ranges 850-1000°C. The reduced temperature T,/Tm for the given compositions of ferro-chromium (Tm = 1650°C) has been changed in the range 0.52-0.61. It is to be expected that, in neutral substrates and through the absence of a diffusive interaction, the structure of the whole condensate should correspond to a third zone of the Movchan-Demchishin model and that it should have wide column-like or equi-axial big crystals. In fact the RCt on steel substrates has a dispersed structure; it has unclearly expressed boundaries, possesses considerable defects and porosity, more typical for the first zone. Grovenor 5 shows that a single change in the coating composition and pollution from the residual atmosphere influence the mobility of the grain boundaries, initially obtained by deposition. The energy, activating growth in such cases, is considerable and the transformation of the structure to a recrystallized one (more balanced) and corresponding to the temperature (Ts/Tm) is difficult. This will cause diversions from the SZM to more dispersed structures, characteristic for lower temperatures. In this case it is probable that independently of the high temperature Ts, the complex coating composition (ferro-chromium) and the presence of a residual atmosphere are the reason why the structure of the RCt corresponds to the first zone 1. By increasing the temperature and the time in the investigated ranges, one can only observe growth of the RCt connected with processes of coalescence. According to Thornton 4, first zone structures are characterized by an inability to undergo changes by heating to Ts/T,~ ~ 0.7. Some recrystallization is possible only in the interior of the grains, as the presence of a large number of open boundaries hinders obtaining of column-like grains. In support of the idea about the influence of condensate chemical composition on the temperature ranges of the different structural zones is the fact that the temperature ranges of similar compositions with amorphous and crystal states has been moved to 300 and 350°C, i.e. to Ts/Tm, respectively 0.3 a n d 0.3213 .
At the same time, at t = 60 min, for the coatings deposited on steel, one observes column-like grains, grown from the base, i.e. a structure, corresponding to the second SZM zone. The columnlike layer is of varying thickness, it grows with temperature and time and has a character of change, corresponding to the regularities of diffusive layer growth. The coating composition includes the strong carbide-former Cr, which stimulates the codiffusion of the C from the substrate and that of the Cr from the coating. According to ref 7, the reaction diffusion with metalloids flows according to a simple internode mechanism, since the activation energy for the heterodiffusion and for the migration between the nodes of the crystal lattice is the same. On the other hand, the dispersed condensate has a large boundary surface, a defectiveness and a small thickness which makes the heterodiffusion easier. There is a gradient of the chemical potential on the coating-substrate boundary, which determines the direction of the migration of the atoms. The increased concentration of Cr and C atoms increases the thickness of the layer in the
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
boundary zone and provides a change in their bulk concentrations. After the concentrations of the elements reach those concentrations characteristic for the new phase, i.e. the chemical compound (Cr, Fe)23C6, there exists an energetic prerequisite for its creation. The re-structuring of the crystal lattice in a complex wall-centered lattice of the carbide is accompanied by the additional procurement of more balanced structure, reducing the layer's stresses. The new structure has the direction of the diffusive flow, as the migration of the grains away from each other is limited by the meeting of other column-like grains. The height reached by the column-like grains, is determined by the diffusion front reached with stoichiometric concentrations for the new phase. The thickness of this intermediate layer grows when increasing the temperature and the time and that is due to the increase in the diffusion rate, resulting from increasing the temperature and the time of its influence. This has been observed for both distances Lt_s till the formation of the second diffusive sublayer. The diffusive exchange in the RC,, which is just above the column-like grains, probably influences the mobility of only some of the boundaries. The neighbouring grains, whose borders have a reduced defectiveness, will combine and that will promote the obtaining of a bi-modal structure, characteristic of the 'T' zone from the SZM. The observed structure in the DIZ in the condensate corresponds to that zone. At a higher Re ~ 0.14/tm/min the DIZ has a greater thickness due to a decrease in the diffusion processes in comparison to the condensation ones. In the obtained coatings a second intermediate diffusive layer has been formed as a part of the substrate through increasing the time of metallization. The substrates are made of highly carbonized steel (1.5% C). The continuing diffusive flow of the carbon from the substrate to the coating provides a concentration prerequisite for the formation of the next more highly carbonized (Cr, Fe)7C3 from the diagram of state. However, its growing conditions differ a lot from those of the previous one. The carbide (Cr, Fe)23C 6 was formed as a result of the phase recrystallization, running at conditions of reduced heterodiffusion in the strongly expressed and small thickness condensate, while the upgoing C diffusion, taking place across the initially formed diffusive layer of (Cr, Fe)23C6, r u n s with difficulty. Because of this, the phase recrystallization to a complex hexagonal crystal lattice of the carbide (Cr, Fe)7C 3 runs under conditions of saturation with C and a difficult chromium exchange between the substrate and the carbide (Cr, Fe)23C6. These are prerequisites for the formation of a dispersed and a more balanced (equi-axial) structure in the second diffusive sub-layer. It has the characteristic features of the third zone of SZM. By studying some ferro-chromium coatings, deposited on this or on other kinds of steel, one has the impression that the obtained structures in the different layers correspond to the given ones in the SZM 1'2.Of special interest is the fact that they succeed each other not by changing T,/Tm but by the thickness of a single coating, obtained at the same T~, as a result of the diffusive interaction with the substrate. For example, for a duration of 120 min up to a temperature of about 960°C the coating consists of: RC, which has a structure, analogous to the first zone of Movchan-Demshichin's model; DIZ--corresponding to the zone T; the first intermediate diffusive layer with a column-like structure of the second zone and the second intermediate sublayer with equiaxial grains--corresponding to the third zone. The presence or absence of some layers in the coating at the same Rc depends on the technological parameters T, and t, which
influence the heterodiffusion in this case. A duration of 60 min is insufficient to obtain the second diffusive sub-layer in the ranges of the studied temperatures. Since the rate of heterodiffusion exceeds Re, there are no RC, and DIZ above Tcr and at L,_s = 130 mm and 120 min the first diffusive sub-layer is missing. However, in all cases, DIZ is observed in the substrate under the coating. These results show that the basic technological parameters Re, Ts and t influence the character of the coatings, their thickness, structure and their phase composition. The investigations of some of the parameters separately are an exception at constant or boundary values of the rest ones. The experiments, presented in this manuscript, are limited to changing 7", and t to certain values for two distances Lt s, providing in this way two different average R e•
In general, Re is of considerable importance for the final result of the surface treatment. The change in Rc has a multiple effect, connected with: a change in energy of the particles, an extra heat effect, a layer thickness and defectiveness, a concentration diffusion gradient etc. Rc considerably influences the initial moment (Ts, t) for a diffusive interaction between the coating and the substrate in two cases. Firstly, when it has very small values and when the time, necessary to obtain a thick layer before the heterodiffusion starts, increases very much. Secondly, when it has very high values, strongly decreasing the time, necessary for metallization. When Re is limited to values, at which it has no considerable influence on the beginning of the heterodiffusion (line 'a' on Figure 1 and temperature To from Figures 2 and 3), its change mainly influences the coating thickness, obtained for a given time. In this way, the relative thickness of the diffusive layer (KDc) will keep changing, changing by this the coating group and the characteristic temperatures--T1, T2, T , (Figures 2 and 3). In order to obtain transitional coatings, VDCs with or without RCt, decreasing Ro, lower temperatures and shorter durations will be needed. The opposite, an increase in Re will cause the obtaining of the corresponding coating groups at higher Ts and t. As a result of heterodiffusion in the studied coatings, the comments on the type of the structure- and phase forming has been confirmed by the measured microhardness and by the X-ray structural and phase analysis. The phase analysis of RCt and that of the different diffusive layers (Figures 4 and 5) has been determined by a comparison of the results of the X-ray structural and durometric analyses to the type of the triple diagram ' C r - F e - C '~4. The presence of a aphase has been confirmed by the electrochemical testings on the photographing of potentiondynamic curves. The presence of DIZ in the condensate has been determined by the poorer appearance of its microstructure and it has been confirmed by the measured higher hardness of the DIZ, compared to that of the condensate. The hardness of the first diffusive layer at a metallization temperature of 1000°C and Lt_s ----80 mm changes from about M H W ° 1000 for 60 min up to MHV 2° 1400 for 120 min. At the same temperature for Lt s = 130 mm the hardness of the first layer is about M H W ° 1100 at t = 60 min. The hardness of the second diffusive layer at 1000°C, 90 min and 120 min and Zt-s = 130 mm is correspondingly about MHV 2° 1300 and about 1700, while for L, ~ = 80 mm for 120 min at 1000°C MHV 2° is about 1700. The results obtained for hardness are easy to explain, taking into consideration the fact that increasing Ts and t due to the stronger diffusive interaction in the transition diffusive layer and its saturation with C from the steel, moves its composition to the right in the triple diagram of ' C r - F e - C ' . The phase composition 1345
R I Shishkov and E M Lisichkova: Vacuum condensates and vacuum diffusive coatings
of the coating, directed to the substrate, changes by decreasing the quantity of the s-rigid solution (a). At the same time, the quantity of (Cr, Fe)23C6 increases and the carbide (Cr, Fe)7C3 is obtained. The increasing of Lt_s also leads to increasing the hardness, due to the relative delay of the condensation processes, compared to the diffusive ones, i.e. considerably more carbide phases are formed. The poorer hardnesses obtained, compared to those from the tables, for Cr23C 6 and Cr7C 3 and to those determined in 15, are a result of the presence of the s-rigid solution and of the mixed composition of the carbides, due to ferrochromium sputtering. It is known that Fe dissolves in the Cr23C6 carbide up to 35-40% and in Cr7C3 up to 50%, which decreases the hardness. Conclusions The investigated coatings with high chromium concentration prove the suitability of the presented general model for vacuum condensates and vacuum diffusive coatings. The coatings obtained by magnetron sputtering of a ferret--chromium target (65%Cr; 0.1%C; 33%Fe), for 60-120 min on substrates made of steel with a composition of 12%Cr; 1.5%C; 0.5%Mo, are intermediate up to about 900°C at a condensation rate 0.14 gm/min and up to about 850°C at a condensation rate 0.07 #m/min. The critical temperature (Tcr) for a target-substrate distance o f L t ~ = 80 mm (at Rc = 0.12/~m/min) is about 1000°C and for Zt-s = 130 mm (at Rc = 0.06 #m/min) it is about 970°C. When the process duration increases, Tcr decreases, thus at L~_, = 130 mm the change of To, is from about 980°C at 60 min up to about 960°C at 120 min. The two diffusive intermediate layers obtained are bi- and three-phasal, in accordance with the triple diagram of state " C r - F e - C " . The first layer consists of (Cr, Fe)23C6 and or-rigid solution and the second one consists of (Cr, Fe)TC3; (Cr, Fe)23C6 and s-rigid solution. The practical
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application of the plasma vacuum diffusive metallizing (chromium plating), when the magnetron sputtering source works in elemental conditions (PVDM-E), is suitable for open plane surfaces because of its more complicated configuration, due to the higher differences in Lt_,, the character, the kind and the properties of the coating will differ very significantly. References 1B A Movchan and A V Demchishin, Investiyatin# the Structure and the Properties of Thick Vacuum Condensates Made of Nickel, Titanium, Aluminium Oxides and Oxides of the Zirconium, Metal Physics and Metal Science. 2j A Thornton, J Vacuum Science and Technology, 11, 4, (1974). 3j A Thornton, J Vacuum Science and Technology, 12, 4, (1975). 4j A Thornton, Ann Rev Mater Sci, pp 239-260 (1977). 5C R M Grovenor, H T G Hentzel and D A Smith, Acta Metall, 12, 5, 773-781 (1984). 6E P Ponomarenko, Steel and alloy metallization in vacuum. Technika, Kiev, 1974. 7Ia H Bakaliuk and E V Proskurkin, Production of tubes with metal coatings. Metallurgy, Moscow, 1973. 8S Z Bockstein, Diffusion and metal structure. Metallur#y, Moscow, 1973. 9L N Lirikov and V I Issaichev, Diffusion in metals and alloys. Scientific Word, Kiev, 1987. I°M Kanev, R Shishkov et al., Vacuum chemical and thermochemical treatment. Technika, Sofia, 1984. I~R Shishkov and E Lisichkova, Method for Vacuum Plasma Diffusive Metallizing, Patent No 48518 (30.11.1988). ~2R Shishkov, E Lisichkova, Laboratory vacuum furnace for plasma diffusive surface saturation. Proc of the Nat Con#ress on Metal Science and Thermochemical Treatment. Varna, 3-5 Oct. (1991) 373. ~3R Shishkov and I Dermendjiev, Structure and phasal composition of magnetron deposited chromium-metalloid coatings. Proc of the Nat Congress on Metal Science and Thermochemical Treatment. Varna, 3-5 October (1991) 324. 14B G Livshitz, Metallography. Metallurgy, Moscow, 2nd, 1971, pp 323326. ~SR Shishkov, Vacuum chromium plating of carbon steels. Ph.D. degree thesis, Technical University of Rousse, 1980.