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Thick boride coatings by chemical vapor deposition
Thin Solid Films, 95 (1982) 99-104
99
PREPARATION AND CHARACTERIZATION
T H I C K B O R I D E C O A T I N G S BY C H E M I C A L VAPOR D E P O S I T I O N * H. O. PIERSON AND A. W. MULLENDORE Sandia National Laboratories, Albuquerque, N M 87185 (U.S.A.) (Received March 25, 1982; accepted April 5, 1982)
In this paper we describe an experimental study of the chemical vapor deposition (CVD) of TiB2 by the hydrogen reduction of TiCI~ and BC13 with the purpose of obtaining very thick (more than 100 lain) and uniform coatings. The optimum deposition conditions were as follows: temperature, 900-950 °C; source gas ratios, [Ti]/[B] = 1/2 and [H]/[CI] = 6/1. With these conditions, deposition rates greater than 25 ~tm h - 1 were obtained. The composition was very uniform with an excess of boron over stoichiometry (probably in the form of free boron). A small amount of chlorine remained incorporated in the deposit probably as TiCI2 (0.05 wt.~o at 950 °C). The coatings were very hard (about 3700 kgf mm-2) and hardness was uniform through the thickness. With careful control of the deposition parameters, the CVD of uniform coatings with thickness of 1 mm or more appears feasible.
1. INTRODUCTION Boron forms very stable interstitial diborides with the refractory transition metals, particularly those of groups IVa (titanium, zirconium, hafnium) and Va (niobium, tantalum). Their stability may be related to their crystal structure which is dominated by the two-dimensional network boron configuration; this structure favors properties such as high melting point (HfB 2, 3250 °C), great hardness (TiB2, 3500 kgf m m - 2) and high electrical conductivity (ZrB2, 7 Ixf] cm) 1. The refractory borides are very hard but also brittle. This limitation may be partially circumvented if they are used as coatings over less brittle metallic or refractory substrates. A well-known process for applying boride coatings is chemical vapor deposition (CVD); this process has several advantages such as high purity of the deposit, relatively high rate of deposition, close chemical composition control and a technology which is already well developed 2. It also has limitations, particularly in the deposition of thick coatings (more than 100 ktm) due to an
* Paper presented at the International Conferenceon Metallurgical Coatings and ProcessTechnology, San Diego, CA, U.S.A., April 5-8, 1982. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
100
H . O . PIERSON~ A. W. MULLENDORE
inherent tendency to form columnar structures and the difficulty of controlling the deposition parameters over a long period of time. This may result in poor uniformity, leading to internal stresses, spalling and delamination. This study is an experimental investigation of the process variables and their effects on the structure and properties of the deposit with the purpose of obtaining thick coatings of greater uniformity and improved properties. The experimental work was essentially limited to TiB 2 but the results should also apply to other borides of the metals of group IVa and Va such as zirconium, hafnium, niobium and tantalum. 2.
THEORETICAL CONSIDERATIONS
The general CVD reaction considered in this study is the hydrogen coreduction of the chlorides, as follows: TIC14 + 2BC13 +
5H 2 ~
TiB/+ 10HC1
(1)
The thermodynamic equilibrium phase diagram of the quaternary system TiB-C1-H shows that generally this is the only feasible deposition reaction with the possible exception of the deposition of a mixed solid phase of TiB/and boron when the ratio of [Ti]/[B] is very low. It also shows that increasing the [-H]/[CI] ratio above a certain value does not increase the theoretical yield3. However, thermodynamic equilibrium in a flowing system is never reached and reaction kinetics, mass transport and surface interaction are major factors in controlling the reaction 4. To determine the relative importance of each of these factors could be a very complex task. Of more practical interest is the design of an experimental envelope with the goal of optimizing the deposition yield and the properties of the solid phase. To accomplish this the CVD conditions were varied as follows: temperature from 750 to 1000 °C, [Ti]/[B] ratio from 1 to 1/4 and [H]/[-C1] ratio from 2/1 to 15/1. 3.
EXPERIMENTAL DETAILS
Figure 1 is a schematic diagram of the cVD apparatus used in these experiments. A special graphite Poco AMX-5Q (Poco Graphite Inc.) was chosen for the substrate because of its close thermal expansion match with TiB2 (7.7 x 10 - 6 °C 1 and 7.9x 1 0 - 6 ° C - 1 respectively). In addition, several depositions were carried out on a tungsten carbide cermet (Kennametal 701 with 8.5% Co and 4.5% Cr binder). These substrates were cleaned ultrasonically using acetone and alcohol and heated in a vacuum at 300 °C immediately prior to deposition. Analysis techniques included metallography, X-ray diffraction, electron microprobe and Vickers microhardness (25 gf load) measurements. 4.
RESULTS AND DISCUSSION
4.1. Deposition rate
Figure 2 illustrates the deposition rate as functions of the CVD conditions of temperature, [Ti]/I-B] ratio and [H]/[C1] ratio. The rate increases sharply with temperature. It is negligible below 700 °C. Above 1000 °C the deposition tends to be powdery, probably the result of vapor phase nucleation.
101
BORIDE C O A T I N G S BY C V D
MASS FLOWMETER
TiCI4 RESERVOIR
10
BCIz
°
.~
(a)
POLYSTALTIC PUMP ARGON
OPTICAL % PYROMETER VAPORIZER
o o o
O ~ RF COIL C ~ (450 kHz)
I
i
800
900
1000
DEPOSITIONTEMPERATURE('C) 30
E ,'~
10
~
o 0
i 1/4
I
i
i
1/2
314
1
"riB RATIO
~~ (b) 30
o QUARTZ
SUSCEPTOR (POCOGRAPHr
THERMOCOUPLE (IN ALUMINA SLEEVE) TO RECORDERAND CONTROLLER
700
20 I0
EXHAUST
0
BUBBLER
(c)
i
0
5/1
'
'
10/1 t5/1 H/CI RATIO
20/1
Fig. 1. Schematic diagram ofa CVD reactor. Fig. 2. Effect of CVD variables on the deposition rate: (a) [-Ti]/[B] = 1/2, [H]/[C1] = 7/1; (b) To = 900 °C, [H]/[C1] = 4/1;(c) TD = 900 °C, [Ti]/[B] = 1/2. The deposition rate varies also with the a m o u n t of h y d r o g e n available ([H]/[C1] ratio). The o p t i m u m ratio is approximately 6-8, showing that a large excess of h y d r o g e n over stoichiometry is necessary. The effect of the [Ti]/[-B] ratio is less pronounced. However, a ratio of 1/2 (stoichiometry) seems optimum. The o p t i m u m C V D conditions for the highest deposition rate are thus TD [H]/[C1] [Ti]/[B]
900-950 °C 6/1-8/1 1/2
With these conditions, the deposition rate is 25 ~tm h - 1 or more.
4.2. Structure The crystal structure of TiB z is hexagonal (C-32, AIB2) and the lattice dimensions are generally reported as a 0 = 3.028 ~ and Co = 3.228/~ 1. s. Table I summarizes the results of the present study. The values of ao are consistently higher and those of Co lower than the above values. This indicates that stoichiometry is not obtained and that the presence of a second phase is a possibility. As expected, the crystallite size increases with increasing deposition temperature. It also reaches a m a x i m u m with EH]/[C1] = 4/1.
4.3. Composition Table II shows the compositional changes as functions of the deposition parameters. The value of x (in TiBx) appears to be independent of deposition
102
H.O. PIERSON, A. W. MULLENDORE
TABLE 1 X-RAY DIFFRACTION RESULTS AS FUNCTIONS OF CHEMICAL VAPOR DEPOSITION PARAMETERS
C VD parameters T(°C) [Ti]/[B]
X-ray diffraction results [H]/[CI] a o(A)
c o(/~)
c/a
Preferred orientation
Crystallite size (~)
900
1/2
2/1 4/1 7/1 15/1
3.0404(11) 3.0405 (16) 3.0424 (45) 3.0420(2)
3.1609(34) 3.1537 (49) 3.1576 (142) 3.1835(6)
1.040 1.037 1.038 1.047
100, 101 100 211,100 100, 101
750 1200 1000 550
800 850 900 950 1000
1/2
7/l
3.03040(7) 3.0456(1) 3.0431 (20) 3.0337(21) 3.0294 (5)
3.2227(23) 3.1664(2) 3.1570(61) 3.2220(72) 3.2289 (17 t
1.062 1.040 1.038 1.062 1.066
101 100 100, 211 101,100 100
480 490 1000 1050 1100
900
1/4 1/3 1/2 1/1
4/1
3.0425(15) 3.0422(23) 3.0405 (16) 3.0307 (2)
3.1590(47) 3.1561 (71) 3.1537 (49) 3.2250 (8)
1.038 1.037 1.037 1.064
100 100 100 100, 101
1500 700 1200 N.A. a
a Not applicable. TABLE II COMPOSITIONa AS A FUNCTION OF CHEMICAL VAPOR DEPOSITION PARAMETERS
C VD parameters
Composition
VHN5o (kgf mm 2)
[Ti]/I-B]
[HI/lUll
xb
Cl2 (wt.~o)
900
1/2
2/1 4/1 7/1 15/1
2.53 2.60 2.38 2.37
0.15 0.16 0.10 0.12
N.A. 3861 3830 4498
800 850 900 950 1000
1/2
7/1
2.36 2.31 2.34 2.35 2.36
0.22 0.17 0.08 0.05 0.03
3607 3830 3402 3300
900
1/4 1/3 1/2 1/1
2.45 2.60 2.60 2.35
0.09 0.l 1 0.16 0.19
N.A. 3300 3300 N.A.
TD C C)
4/1
a By electron microprobe. bin TiBx; obtained by difference. temperature but varies slightly with the variations in source gas ratios ([Ti]/[B] and [ H ] / [ C 1 ] ) . I n all c a s e s x is h i g h e r t h a n s t o i c h i o m e t r y (x = 2). A thermodynamic a n a l y s i s o f r e a c t i o n (1) s h o w s t h a t u n d e r r e s t r i c t e d d e p o s i t i o n c o n d i t i o n s it is p o s s i b l e t o o b t a i n T i B a s a s e c o n d p h a s e 3. H o w e v e r , n o n e
BORIDE COATINGS BY CVD
103
was detected either by X-ray diffraction or by electron microprobe. Free boron as a second phase is also thermodynamically possible. X-ray diffraction indeed showed some evidence of boron peaks and of a second phase tentatively identified by X-ray diffraction as non-stoichiometric TiB and indexed as an orthorhombic unit cell with the following lattice parameters: ao = 5.94 ~; b o = 3.025/~; Co = 4.556 A. The only extraneous element detected by the electron microprobe is chlorine (Table II). The amount of chlorine present is very dependent on the temperature of deposition and reaches its lowest amount at the highest temperature. It also increases with increasing [Ti]/[B] ratio but is not appreciably affected by the [H]/[C1] ratio. The presence of chlorine is probably due to the formation of TIC12 which is a relatively stable compound with a melting point of 1035 °C 6. TiC12 disproportionates as follows: 2TiC12(g) >475 °C TiC14 + Ti(s)
(2)
The rate of reaction (2) increases sharply with increasing temperature which would explain the rapid decrease in C12 content at the higher deposition temperatures. Since only a small amount of chlorine compound is present, a positive identification of TiC12 by X-ray diffraction was not possible. 4.4. Hardness Table II shows the Vickers microhardness (50 gfload) values taken on the cross section as functions of the deposition parameters. These values appear to be essentially independent of the deposition parameters and are in good agreement with other reported values 2. Measurements were also taken of the microhardness at various locations across the thickness of the deposit. Again, the results show little variation in hardness. (It should be noted that an exceptionally high value of 4498 kgf m m - 2 was obtained on the deposit with [H]/[CI] = 15/1; the reason for this is not clear at this stage.)
Fig. 3. Metallographiccross sectionofa TiB2deposit on graphite.
104
H . O . PIERSON, A. W. MULLENDORE
4.5. Uniformity Figure 3 shows a typical cross section of a thick deposit obtained with the optimum deposition parameters mentioned above (900 °C, [H]/[C1] = 6, [Ti]/[B] = 1/2). The typical columnar structure is evident throughout the deposit which appears uniform. As a corollary to the study, several thick TiB 2 coatings were deposited onto WC. (WC is a common control valve material in coal conversion reactors.) An electron microprobe examination through the thickness (more than 100 ~tm) showed an x value (in TiBx) of 2.271 _+_0.006 indicating a very uniform elemental distribution even in thick deposits and confirming the optical uniformity and the uniform hardness. 5. CONCLUSION
Thick coatings of TiB 2 (more than 100 ~tm) can be obtained by CVD on substrates such as graphite and hot-pressed carbide. These coatings have very uniform structure and composition and uniformly high hardness. These characteristics are essentially independent of the thickness and there seems to be no reason why considerably thicker and uniform coatings (i.e. more than 1 mm) could not be made in this manner. There should be a large number of applications for such coatings such as chemically resistant and erosion-resistant components for chemical processes (coal conversion, shale oil etc.), components for fusion reactors and anodes for aluminum reduction cells. ACKNOWLEDGMENTS
The authors would like to acknowledge the contributions of Paul Hlava (electron microprobe), Ed Graeber (X-ray diffraction) and Richard Curlee (application study). This work was sponsored by the U.S. Department of Energy under Contract DE-AC04-76-DP00789. REFERENCES l 2 3 4 5 6
V.I. Matkovich (ed.), Boron andRefraetory Borides, Springer, New York, 1977. H . O . Pierson and A. W. Mullendore, Thin SolidFihns, 72 (1980) 511-516. E. Randich and T. M. Gerlach, Thin Solid Films, 75 (1981) 271 291. C . F . Powell, J. H. Oxley and J. M. Blocher, Jr., Vapor Deposition, Wiley, New York, 1966. H.J. Goldschmidt, Interstitial Alloys, Plenum, New York, 1967. R.J. Clark, The Chemistry of Titanium and Vanadium, Elsevier, New York, 1968.
99
PREPARATION AND CHARACTERIZATION
T H I C K B O R I D E C O A T I N G S BY C H E M I C A L VAPOR D E P O S I T I O N * H. O. PIERSON AND A. W. MULLENDORE Sandia National Laboratories, Albuquerque, N M 87185 (U.S.A.) (Received March 25, 1982; accepted April 5, 1982)
In this paper we describe an experimental study of the chemical vapor deposition (CVD) of TiB2 by the hydrogen reduction of TiCI~ and BC13 with the purpose of obtaining very thick (more than 100 lain) and uniform coatings. The optimum deposition conditions were as follows: temperature, 900-950 °C; source gas ratios, [Ti]/[B] = 1/2 and [H]/[CI] = 6/1. With these conditions, deposition rates greater than 25 ~tm h - 1 were obtained. The composition was very uniform with an excess of boron over stoichiometry (probably in the form of free boron). A small amount of chlorine remained incorporated in the deposit probably as TiCI2 (0.05 wt.~o at 950 °C). The coatings were very hard (about 3700 kgf mm-2) and hardness was uniform through the thickness. With careful control of the deposition parameters, the CVD of uniform coatings with thickness of 1 mm or more appears feasible.
1. INTRODUCTION Boron forms very stable interstitial diborides with the refractory transition metals, particularly those of groups IVa (titanium, zirconium, hafnium) and Va (niobium, tantalum). Their stability may be related to their crystal structure which is dominated by the two-dimensional network boron configuration; this structure favors properties such as high melting point (HfB 2, 3250 °C), great hardness (TiB2, 3500 kgf m m - 2) and high electrical conductivity (ZrB2, 7 Ixf] cm) 1. The refractory borides are very hard but also brittle. This limitation may be partially circumvented if they are used as coatings over less brittle metallic or refractory substrates. A well-known process for applying boride coatings is chemical vapor deposition (CVD); this process has several advantages such as high purity of the deposit, relatively high rate of deposition, close chemical composition control and a technology which is already well developed 2. It also has limitations, particularly in the deposition of thick coatings (more than 100 ktm) due to an
* Paper presented at the International Conferenceon Metallurgical Coatings and ProcessTechnology, San Diego, CA, U.S.A., April 5-8, 1982. 0040-6090/82/0000-0000/$02.75
© ElsevierSequoia/Printedin The Netherlands
100
H . O . PIERSON~ A. W. MULLENDORE
inherent tendency to form columnar structures and the difficulty of controlling the deposition parameters over a long period of time. This may result in poor uniformity, leading to internal stresses, spalling and delamination. This study is an experimental investigation of the process variables and their effects on the structure and properties of the deposit with the purpose of obtaining thick coatings of greater uniformity and improved properties. The experimental work was essentially limited to TiB 2 but the results should also apply to other borides of the metals of group IVa and Va such as zirconium, hafnium, niobium and tantalum. 2.
THEORETICAL CONSIDERATIONS
The general CVD reaction considered in this study is the hydrogen coreduction of the chlorides, as follows: TIC14 + 2BC13 +
5H 2 ~
TiB/+ 10HC1
(1)
The thermodynamic equilibrium phase diagram of the quaternary system TiB-C1-H shows that generally this is the only feasible deposition reaction with the possible exception of the deposition of a mixed solid phase of TiB/and boron when the ratio of [Ti]/[B] is very low. It also shows that increasing the [-H]/[CI] ratio above a certain value does not increase the theoretical yield3. However, thermodynamic equilibrium in a flowing system is never reached and reaction kinetics, mass transport and surface interaction are major factors in controlling the reaction 4. To determine the relative importance of each of these factors could be a very complex task. Of more practical interest is the design of an experimental envelope with the goal of optimizing the deposition yield and the properties of the solid phase. To accomplish this the CVD conditions were varied as follows: temperature from 750 to 1000 °C, [Ti]/[B] ratio from 1 to 1/4 and [H]/[-C1] ratio from 2/1 to 15/1. 3.
EXPERIMENTAL DETAILS
Figure 1 is a schematic diagram of the cVD apparatus used in these experiments. A special graphite Poco AMX-5Q (Poco Graphite Inc.) was chosen for the substrate because of its close thermal expansion match with TiB2 (7.7 x 10 - 6 °C 1 and 7.9x 1 0 - 6 ° C - 1 respectively). In addition, several depositions were carried out on a tungsten carbide cermet (Kennametal 701 with 8.5% Co and 4.5% Cr binder). These substrates were cleaned ultrasonically using acetone and alcohol and heated in a vacuum at 300 °C immediately prior to deposition. Analysis techniques included metallography, X-ray diffraction, electron microprobe and Vickers microhardness (25 gf load) measurements. 4.
RESULTS AND DISCUSSION
4.1. Deposition rate
Figure 2 illustrates the deposition rate as functions of the CVD conditions of temperature, [Ti]/I-B] ratio and [H]/[C1] ratio. The rate increases sharply with temperature. It is negligible below 700 °C. Above 1000 °C the deposition tends to be powdery, probably the result of vapor phase nucleation.
101
BORIDE C O A T I N G S BY C V D
MASS FLOWMETER
TiCI4 RESERVOIR
10
BCIz
°
.~
(a)
POLYSTALTIC PUMP ARGON
OPTICAL % PYROMETER VAPORIZER
o o o
O ~ RF COIL C ~ (450 kHz)
I
i
800
900
1000
DEPOSITIONTEMPERATURE('C) 30
E ,'~
10
~
o 0
i 1/4
I
i
i
1/2
314
1
"riB RATIO
~~ (b) 30
o QUARTZ
SUSCEPTOR (POCOGRAPHr
THERMOCOUPLE (IN ALUMINA SLEEVE) TO RECORDERAND CONTROLLER
700
20 I0
EXHAUST
0
BUBBLER
(c)
i
0
5/1
'
'
10/1 t5/1 H/CI RATIO
20/1
Fig. 1. Schematic diagram ofa CVD reactor. Fig. 2. Effect of CVD variables on the deposition rate: (a) [-Ti]/[B] = 1/2, [H]/[C1] = 7/1; (b) To = 900 °C, [H]/[C1] = 4/1;(c) TD = 900 °C, [Ti]/[B] = 1/2. The deposition rate varies also with the a m o u n t of h y d r o g e n available ([H]/[C1] ratio). The o p t i m u m ratio is approximately 6-8, showing that a large excess of h y d r o g e n over stoichiometry is necessary. The effect of the [Ti]/[-B] ratio is less pronounced. However, a ratio of 1/2 (stoichiometry) seems optimum. The o p t i m u m C V D conditions for the highest deposition rate are thus TD [H]/[C1] [Ti]/[B]
900-950 °C 6/1-8/1 1/2
With these conditions, the deposition rate is 25 ~tm h - 1 or more.
4.2. Structure The crystal structure of TiB z is hexagonal (C-32, AIB2) and the lattice dimensions are generally reported as a 0 = 3.028 ~ and Co = 3.228/~ 1. s. Table I summarizes the results of the present study. The values of ao are consistently higher and those of Co lower than the above values. This indicates that stoichiometry is not obtained and that the presence of a second phase is a possibility. As expected, the crystallite size increases with increasing deposition temperature. It also reaches a m a x i m u m with EH]/[C1] = 4/1.
4.3. Composition Table II shows the compositional changes as functions of the deposition parameters. The value of x (in TiBx) appears to be independent of deposition
102
H.O. PIERSON, A. W. MULLENDORE
TABLE 1 X-RAY DIFFRACTION RESULTS AS FUNCTIONS OF CHEMICAL VAPOR DEPOSITION PARAMETERS
C VD parameters T(°C) [Ti]/[B]
X-ray diffraction results [H]/[CI] a o(A)
c o(/~)
c/a
Preferred orientation
Crystallite size (~)
900
1/2
2/1 4/1 7/1 15/1
3.0404(11) 3.0405 (16) 3.0424 (45) 3.0420(2)
3.1609(34) 3.1537 (49) 3.1576 (142) 3.1835(6)
1.040 1.037 1.038 1.047
100, 101 100 211,100 100, 101
750 1200 1000 550
800 850 900 950 1000
1/2
7/l
3.03040(7) 3.0456(1) 3.0431 (20) 3.0337(21) 3.0294 (5)
3.2227(23) 3.1664(2) 3.1570(61) 3.2220(72) 3.2289 (17 t
1.062 1.040 1.038 1.062 1.066
101 100 100, 211 101,100 100
480 490 1000 1050 1100
900
1/4 1/3 1/2 1/1
4/1
3.0425(15) 3.0422(23) 3.0405 (16) 3.0307 (2)
3.1590(47) 3.1561 (71) 3.1537 (49) 3.2250 (8)
1.038 1.037 1.037 1.064
100 100 100 100, 101
1500 700 1200 N.A. a
a Not applicable. TABLE II COMPOSITIONa AS A FUNCTION OF CHEMICAL VAPOR DEPOSITION PARAMETERS
C VD parameters
Composition
VHN5o (kgf mm 2)
[Ti]/I-B]
[HI/lUll
xb
Cl2 (wt.~o)
900
1/2
2/1 4/1 7/1 15/1
2.53 2.60 2.38 2.37
0.15 0.16 0.10 0.12
N.A. 3861 3830 4498
800 850 900 950 1000
1/2
7/1
2.36 2.31 2.34 2.35 2.36
0.22 0.17 0.08 0.05 0.03
3607 3830 3402 3300
900
1/4 1/3 1/2 1/1
2.45 2.60 2.60 2.35
0.09 0.l 1 0.16 0.19
N.A. 3300 3300 N.A.
TD C C)
4/1
a By electron microprobe. bin TiBx; obtained by difference. temperature but varies slightly with the variations in source gas ratios ([Ti]/[B] and [ H ] / [ C 1 ] ) . I n all c a s e s x is h i g h e r t h a n s t o i c h i o m e t r y (x = 2). A thermodynamic a n a l y s i s o f r e a c t i o n (1) s h o w s t h a t u n d e r r e s t r i c t e d d e p o s i t i o n c o n d i t i o n s it is p o s s i b l e t o o b t a i n T i B a s a s e c o n d p h a s e 3. H o w e v e r , n o n e
BORIDE COATINGS BY CVD
103
was detected either by X-ray diffraction or by electron microprobe. Free boron as a second phase is also thermodynamically possible. X-ray diffraction indeed showed some evidence of boron peaks and of a second phase tentatively identified by X-ray diffraction as non-stoichiometric TiB and indexed as an orthorhombic unit cell with the following lattice parameters: ao = 5.94 ~; b o = 3.025/~; Co = 4.556 A. The only extraneous element detected by the electron microprobe is chlorine (Table II). The amount of chlorine present is very dependent on the temperature of deposition and reaches its lowest amount at the highest temperature. It also increases with increasing [Ti]/[B] ratio but is not appreciably affected by the [H]/[C1] ratio. The presence of chlorine is probably due to the formation of TIC12 which is a relatively stable compound with a melting point of 1035 °C 6. TiC12 disproportionates as follows: 2TiC12(g) >475 °C TiC14 + Ti(s)
(2)
The rate of reaction (2) increases sharply with increasing temperature which would explain the rapid decrease in C12 content at the higher deposition temperatures. Since only a small amount of chlorine compound is present, a positive identification of TiC12 by X-ray diffraction was not possible. 4.4. Hardness Table II shows the Vickers microhardness (50 gfload) values taken on the cross section as functions of the deposition parameters. These values appear to be essentially independent of the deposition parameters and are in good agreement with other reported values 2. Measurements were also taken of the microhardness at various locations across the thickness of the deposit. Again, the results show little variation in hardness. (It should be noted that an exceptionally high value of 4498 kgf m m - 2 was obtained on the deposit with [H]/[CI] = 15/1; the reason for this is not clear at this stage.)
Fig. 3. Metallographiccross sectionofa TiB2deposit on graphite.
104
H . O . PIERSON, A. W. MULLENDORE
4.5. Uniformity Figure 3 shows a typical cross section of a thick deposit obtained with the optimum deposition parameters mentioned above (900 °C, [H]/[C1] = 6, [Ti]/[B] = 1/2). The typical columnar structure is evident throughout the deposit which appears uniform. As a corollary to the study, several thick TiB 2 coatings were deposited onto WC. (WC is a common control valve material in coal conversion reactors.) An electron microprobe examination through the thickness (more than 100 ~tm) showed an x value (in TiBx) of 2.271 _+_0.006 indicating a very uniform elemental distribution even in thick deposits and confirming the optical uniformity and the uniform hardness. 5. CONCLUSION
Thick coatings of TiB 2 (more than 100 ~tm) can be obtained by CVD on substrates such as graphite and hot-pressed carbide. These coatings have very uniform structure and composition and uniformly high hardness. These characteristics are essentially independent of the thickness and there seems to be no reason why considerably thicker and uniform coatings (i.e. more than 1 mm) could not be made in this manner. There should be a large number of applications for such coatings such as chemically resistant and erosion-resistant components for chemical processes (coal conversion, shale oil etc.), components for fusion reactors and anodes for aluminum reduction cells. ACKNOWLEDGMENTS
The authors would like to acknowledge the contributions of Paul Hlava (electron microprobe), Ed Graeber (X-ray diffraction) and Richard Curlee (application study). This work was sponsored by the U.S. Department of Energy under Contract DE-AC04-76-DP00789. REFERENCES l 2 3 4 5 6
V.I. Matkovich (ed.), Boron andRefraetory Borides, Springer, New York, 1977. H . O . Pierson and A. W. Mullendore, Thin SolidFihns, 72 (1980) 511-516. E. Randich and T. M. Gerlach, Thin Solid Films, 75 (1981) 271 291. C . F . Powell, J. H. Oxley and J. M. Blocher, Jr., Vapor Deposition, Wiley, New York, 1966. H.J. Goldschmidt, Interstitial Alloys, Plenum, New York, 1967. R.J. Clark, The Chemistry of Titanium and Vanadium, Elsevier, New York, 1968.