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Growth of diamond films on stainless steel
Thin Solid Films, 212 (1992) 169-172
169
Growth of d i a m o n d films on stainless steel H. Chen, M. L. Nielsen, C. J. Gold and R. O. Dillon Department of Electrical Engineering and Center for Microelectronic and Optical Materials Research University of Nebraska, Lincoln, NE 68588-0511 (USA)
J. DiGregorio and T. Furtak Physics Department, Colorado School of Mines, Golden, CO 80401 (USA) (Received August 22, 1991)
Abstract Continuous diamond films have been deposited on 304 stainless steel with a silicon intermediate layer by means of hot-filament chemical vapor deposition. The most serious problems for growing diamond films on ferrous metals are a long nucleation period, the catalytic effect of iron, and thermal expansion mismatch. In order to reduce these unwanted effects, three approaches have been adopted. First, an intermediate layer was used to block carbon diffusion, enhance adhesion and suppress sp2 carbon formation. Second, oxygen-assisted low temperature deposition was used to minimize the thermal expansion effect and also avoid the phase transition of iron alloys. Third, initial and secondary nucleation was enhanced to yield small grain sizes in the continuous film. The morphology and quality of the deposited films were characterized by scanning electron microscopy and Raman spectroscopy respectively. Pull-off adhesion tests showed that the intermediate layer was strongly bonded to both the steel substrate and diamond film.
1. Introduction Ferrous metals are one of the most popular materialso in today's industry. If diamond films could be grown on them, there would be many applications using just the mechanical properties and chemical inertness of diamond. However, there are at least three major obstacles for growing diamond on ferrous metals [1- 3]. First, carbon can diffuse into these metals with a relatively high diffusion rate at high temperature. This means the diamond nucleation time can be long and the mechanical properties of the substrate are changed since it has been carburized. Second, iron has a catalytic effect on growth of sp 2 dominated amorphous and nanocrystalline carbon [3, 4], which is commonly called black carbon. Therefore, in many cases diamond films are actually grown on a layer of soft black carbon instead of carburized iron. Third, the thermal expansion coefficients between diamond and ferrous metals are not compatible and this mismatch usually causes poor adhesion and high residual stress. Initially we tried t o grow diamond film directly on ferrous metal substrates and concentrated on increasing the diamond nucleation site density and reducing the black carbon formation. After fully exploring deposition parameters, continuous diamond films without a black carbon layer were grown on 304 stainless steel, but the problem of poor adhesion remained. Thus an
0040-6090/92/$5.00
intermediate layer serving as a carbon diffusion barrier and adhesion enhancer became necessary. There are several materials that meet these requirements and are under investigation. However, in this work we concentration our study on silicon to serve these purposes. The effect of thermal expansion coefficients will be reduced by smaller grain sizes if the material between grains allows more movement of the diamond crystallites than would be possible in equivalent regions of single crystal diamond. Lower deposition temperatures directly reduce the effect of thermal expansion. Since low temperature deposition tends to reduce grain size, the lower temperature techniques could be doubly important. The goal of this work is to grow a continuous polycrystalline diamond film with submicron grains which is strongly bonded to the ferrous substrate.
2. Experimental details Diamond films were synthesized in a hot-filament-assisted chemical vapor deposition ( H F C V D ) apparatus. The substrates were on a stainless steel heater, which was powered by passing current through an enclosed tungsten wire. A 0.25 mm tungsten wire was used as the hot filament. The deposition temperature was monitored by a D type thermocouple attached to the back side of the heater. During deposition, the substrate
~ 1992 - - Elsevier Sequoia. All rights reserved
170
H. Chen et al. / Growth of diamond films on stainless steel
TABLE 1. Experimental parameters Tungsten filament temperature (°C) Heater temperature (°C) Estimated substrate surface temperature (°C) Distance between substrate and filament (ram)
200o 450-550 550-700 15
Gas flow rate (sccm)
02 0.25- I CH4 0.5 4 H2 95-99.2
Total gas flow rate (sccm) Total gas pressure (Torr)
100 20 (26.7 mbar)
Deposition time (h) Growth rate 0tm h -~)
3-24 h <0.1 0.4
surface temperature was a function of heater temperature, hot filament temperature and size, distance from the hot filament, pressure and gas flow rate. It could be 100-200 °C higher than the thermocouple reading. In order to avoid a phase transition of stainless steel [5], which occurs around 720 °C, our heater temperatures were 450 and 550 °C. For the purpose of reducing the temperature gradient to the substrate, the distance between the substrate and hot filament was 15 mm, which is larger than the commonly used distance of 5 - 1 0 mm. The filament temperature was 2000 °C, which is lower than that used by m a n y other workers. According to the study by Sch~ifer and co-workers [6] on the atomic hydrogen concentration profile in an H F C V D system, almost all of our parameters have been set at the lowest limit for growing good quality diamond at a reasonable rate. The total gas flow rate and pressure were maintained at 100 sccm and 20 T o r r respectively. A small amount of oxygen was used for reducing non-diamond growth at low temperatures. There is an upper limit for both the concentrations of oxygen and methane in an H F C V D system. The resistance of the filament and its lifetime were dramatically reduced when the oxygen concentration was higher than 1%. When the methane concentration was over 4% and with a small amount of oxygen, most of the carbon condensed on the filament instead of the cooler substrate. A typical gas ratio was 0 . 2 5 - 1 - 9 8 . 7 for O 2 - C H n - H 2. The summary of deposition parameters and growth rate is shown in T a b l e 1. The surface of the 304 stainless steel substrate was either mirror polished by 0.3 ~tm 0t-A120 3 grit or roughly scratched by 3M Scotch Brite, and ultrasonically cleaned by Alconox and acetone. Then, a silicon intermediate layer was deposited either by magnetron sputtering or e-beam evaporation. We used 100 nm intermediate layers on most of the substrates. For each deposition, the steel substrate was accompanied by a small piece of silicon acting as a control sample. The surface morphologies of the silicon coated
stainless steel and the silicon were very different. Usually the film on silicon had larger and better faceted diamond particles.
3. Results and discussion 3.1. Direct diamond growth on f e r r o u s metals
Nickel, iron, 304 stainless steel and 321 stainless steel were used as substrates for the direct growth of diamond without using an intermediate layer. During the deposition, a layer of black carbon rapidly formed when the substrate temperature was either too high or too low. A narrow temperature window around 950 °C allowed the growth of diamond on 304 stainless steel without forming any visible black carbon. However, even under this promising condition, the adhesion between the substrates and the diamond particles was very weak. Usually the coating could easily be wiped off. Figure 1 shows the surface morphology of a seeded 304 stainless steel substrate after 40 h growth without using any intermediate layer.
Fig. 1. SEM micrograph of diamond particles directly grown on a submicron diamond powder seeded 304 stainless steel substrate. 3.2. Nucleation site density and growth rate
Diamond was grown on the silicon coated stainless steel without any pre-deposition treatment, but the nucleation site density ( N S D ) was low, less than 1 × l 0 4 mlT1-2, and the growth rate was slow. After 24 h the average particle size was smaller than 2 lam. With a 40 nm hydrogenated amorphous carbon coating on top of the intermediate layer, the N S D was enhanced by an order of magnitude, (see Fig. 2), but it did not form a continuous film. Increased oxygen and methane concentrations have been used by m a n y groups to improve the diamond
H. Chen et al. / Growth o f diamond filrns on stainless steel
Fig. 2. SEM micrograph of the particle morphology for hydrogenated carbon enhanced nulceation.
171
Fig. 4. SEM micrograph of the diamond film with the submicron grain size grown on a diamond powder scratched intermediate silicon layer at 700 °C surface temperature after 6 h. the initial nucleation sites. This phenomenon has been carefullly studied by Iijima, Aikawa and Baba [9] and their result shows that the N S D can be as high as l 0 9 m m -2. A continuous diamond film of submicron sized grains was formed after 6 h. Figure 5 shows the surface morphology with an average grain size of 3 pm after 24 h of growth, indicating that the grain size is a function of deposition time. The film thickness increased from 2 ~tm after 6 h to about 10 I-tm after 24 h; however, the 10 ~tm film cracked during cooling. The continuous 2 ~tm thick film on coated stainless steel was reproduced in another 6 h deposition. Much thicker films would be hard to form because the remaining problem is the difference of thermal expansion coefficients. Films thicker than 4 lain usually cracked
Fig. 3. The result of high carbon deposition, 0.25'¼,02 and 2% CH 4 in H2 at 700 "C. quality and growth rate [7, 8] at a substrate temperature lower than 800 °C. Our results clearly indicate that the growth rate increased with a higher carbon concentration but the diamond structure was degraded. An example of this is given by the morphology in Fig. 3 which was made at 0 . 2 5 - 2 - 9 7 . 7 for 0 2 - C H 4 - H 2 , The ball-shaped carbon, without any visible faceted diamond, was also found with various precursor gas combinations such as 0 . 5 - 2 - 9 7 . 5 , 0 . 5 - 3 - 9 6 . 5 and 1 - 4 - 9 5 . 3.3. Gra& size and film thickness The most promising result is shown in Fig. 4. The intermediate coating was lightly scratched by submicron diamond powder and then ultrasonically cleaned. The diamond powder abrading pretreatment planted diam o n d debris onto the substrate surface, thus increasing
Fig. 5. The same deposition conditions as the sample in Fig. 4 after 24 h,
172
H. Chen et al. / Growth o f diamond films on stainless steel
'E
--,
epoxy cement was used to attach each side of the substrate to the ends of two cylindrical rods. A force normal to the substrate surface was applied to each rod in an attempt to detach the film. The two samples tested showed that the bonding between the film and the substrate was stronger than the epoxy. The epoxy pulled off at about 7.2 kg mm 2.
(o) _
<,~
"~
f/
~ /(grating ghost) ....
g E
4. Summary
c~
1 O0
'
13'00
'
15'00
'
700
Wavenumber Shift (cm--1) Fig. 6. Raman spectra (a), (b), and (c) of the deposits shown in Figs. 2, 3, and 4, respectively; (d) is the reference from a synthetic diamond.
during the cooling phase. It might be possible to obtain a thicker film with submicron grains if the microcrystalline diamond film had larger thermal expansion coefficient and/or a smaller Young's modulus than single crystal diamond. This can be attempted by enhancing secondary nucleation with pulses of a higher carbon concentration. 3.4. Film quality The desired small particle size and low deposition temperature produced some sp 2 bonding in the films. The film quality was determined by studying the morphology in SEM pictures and Raman spectroscopy. Figure 6 shows the corresponding Raman spectra of the samples in Figs. 2, 3, and 4. The spectra (a) in Fig. 6 have a strong diamond peak at about 1332 cm-i and very broad peaks at high wavenumbers from sp 2 carbon formation. Similar non-diamond peaks can be seen in the spectra (b), where the diamond peak becomes much smaller due to a higher growth rate. The spectra (c), from the 6 h deposition, have a relatively flat background indicating a better quality of the film. 3.5. Adhesion test The preliminary studies on the mechanical properties of these films have been conducted with a scratch test and a pull-offadhesion test. Scratch testing was performed by scratching the diamond coated samples using a steel needle. The radius of the needle tip was larger than the average diamond particle size. The sample passed the test if metal debris was left on the diamond surface without any damage to the coating. Most of our samples with detectable diamond on the surface passed this test. Adhesion testing was also performed by a direct pull-off method using the Sebastian Five apparatus. An
With the help of a silicon intermediate layer and lower deposition temperature, growth of a well-adhered diamond film on 304 stainless steel has been demonstrated. We successfully used the thin intermediate layer for reducing carbon diffusion, blocking the iron catalytic effect and improving adhesion. Oxygen-assisted low temperature deposition was required to reduce the effect of thermal expansion mismatch. The results have been reproduced by careful control of the major parameters, such as temperature, temperature gradient, and gas composition.
Acknowledgments This work has been supported by the Electric Power Research Institute and the Nebraska Energy Office under the contracts RP2426-31 and UNR-06 respectively. The authors wish to thank Dr. Seraiya Naris and Jeff Lewis for assistance.
References 1 N. Ohtake, H. Tokura, Y. Kuriyama, Y. Mashimo and M. Yoshikawa, in J. P. Dismukes, A. J. Purdes, B. S. Meyerson, T. D. Moustakas, K. E. Spear, K. V. Ravi and M. Yoder (Eds.), Proc. 1st Int. Syrup. on Diamond and Diamond-Like Films, The Electrochemical Society, Pennington, NJ, 1989. 2 B. Lux and R. Haubner, in R. E. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Proc. o f Nato A S I (Advanced Study Institute), on Diamond and Diamond-like Films and Coatings, N A T O - A S I Series B: Physics, Vol. 266, Plenum, New York, 1991. 3 T. P. Ong and R. P. H. Chang, Appl. Phys. Lett., 58 (4) (1991) 358. 4 D.N. Belton and S. J. Schmieg, J. Appl. Phys., 66(9) (1989) 4223. 5 R. A. Lula, Stainless Steel, American Society for Metals, Metals Park, OH, Rev. edn., 1986, pp. 29-32. 6 L. Sch/ifer, C. P. Klages, U. Meier and K. Kohse-H6inghaus, Appl. Phys. Lett., 58 (6) (1991) 571. 7 S. J. Harris and A. M. Weiner, Appl. Phys. Lett., 55(21) (1989) 2179. 8 F. Jansen, M. A. Machonkin and D. E. Kuhman, J. Vac. Sci. Teehnol. A, 8 (5) (1990) 3785. 9 Sumio Iijima, Yumi Aikawa and Kazuhiro Baba, J. Mater. Res., 6 (7) (1991) 1491.
169
Growth of d i a m o n d films on stainless steel H. Chen, M. L. Nielsen, C. J. Gold and R. O. Dillon Department of Electrical Engineering and Center for Microelectronic and Optical Materials Research University of Nebraska, Lincoln, NE 68588-0511 (USA)
J. DiGregorio and T. Furtak Physics Department, Colorado School of Mines, Golden, CO 80401 (USA) (Received August 22, 1991)
Abstract Continuous diamond films have been deposited on 304 stainless steel with a silicon intermediate layer by means of hot-filament chemical vapor deposition. The most serious problems for growing diamond films on ferrous metals are a long nucleation period, the catalytic effect of iron, and thermal expansion mismatch. In order to reduce these unwanted effects, three approaches have been adopted. First, an intermediate layer was used to block carbon diffusion, enhance adhesion and suppress sp2 carbon formation. Second, oxygen-assisted low temperature deposition was used to minimize the thermal expansion effect and also avoid the phase transition of iron alloys. Third, initial and secondary nucleation was enhanced to yield small grain sizes in the continuous film. The morphology and quality of the deposited films were characterized by scanning electron microscopy and Raman spectroscopy respectively. Pull-off adhesion tests showed that the intermediate layer was strongly bonded to both the steel substrate and diamond film.
1. Introduction Ferrous metals are one of the most popular materialso in today's industry. If diamond films could be grown on them, there would be many applications using just the mechanical properties and chemical inertness of diamond. However, there are at least three major obstacles for growing diamond on ferrous metals [1- 3]. First, carbon can diffuse into these metals with a relatively high diffusion rate at high temperature. This means the diamond nucleation time can be long and the mechanical properties of the substrate are changed since it has been carburized. Second, iron has a catalytic effect on growth of sp 2 dominated amorphous and nanocrystalline carbon [3, 4], which is commonly called black carbon. Therefore, in many cases diamond films are actually grown on a layer of soft black carbon instead of carburized iron. Third, the thermal expansion coefficients between diamond and ferrous metals are not compatible and this mismatch usually causes poor adhesion and high residual stress. Initially we tried t o grow diamond film directly on ferrous metal substrates and concentrated on increasing the diamond nucleation site density and reducing the black carbon formation. After fully exploring deposition parameters, continuous diamond films without a black carbon layer were grown on 304 stainless steel, but the problem of poor adhesion remained. Thus an
0040-6090/92/$5.00
intermediate layer serving as a carbon diffusion barrier and adhesion enhancer became necessary. There are several materials that meet these requirements and are under investigation. However, in this work we concentration our study on silicon to serve these purposes. The effect of thermal expansion coefficients will be reduced by smaller grain sizes if the material between grains allows more movement of the diamond crystallites than would be possible in equivalent regions of single crystal diamond. Lower deposition temperatures directly reduce the effect of thermal expansion. Since low temperature deposition tends to reduce grain size, the lower temperature techniques could be doubly important. The goal of this work is to grow a continuous polycrystalline diamond film with submicron grains which is strongly bonded to the ferrous substrate.
2. Experimental details Diamond films were synthesized in a hot-filament-assisted chemical vapor deposition ( H F C V D ) apparatus. The substrates were on a stainless steel heater, which was powered by passing current through an enclosed tungsten wire. A 0.25 mm tungsten wire was used as the hot filament. The deposition temperature was monitored by a D type thermocouple attached to the back side of the heater. During deposition, the substrate
~ 1992 - - Elsevier Sequoia. All rights reserved
170
H. Chen et al. / Growth of diamond films on stainless steel
TABLE 1. Experimental parameters Tungsten filament temperature (°C) Heater temperature (°C) Estimated substrate surface temperature (°C) Distance between substrate and filament (ram)
200o 450-550 550-700 15
Gas flow rate (sccm)
02 0.25- I CH4 0.5 4 H2 95-99.2
Total gas flow rate (sccm) Total gas pressure (Torr)
100 20 (26.7 mbar)
Deposition time (h) Growth rate 0tm h -~)
3-24 h <0.1 0.4
surface temperature was a function of heater temperature, hot filament temperature and size, distance from the hot filament, pressure and gas flow rate. It could be 100-200 °C higher than the thermocouple reading. In order to avoid a phase transition of stainless steel [5], which occurs around 720 °C, our heater temperatures were 450 and 550 °C. For the purpose of reducing the temperature gradient to the substrate, the distance between the substrate and hot filament was 15 mm, which is larger than the commonly used distance of 5 - 1 0 mm. The filament temperature was 2000 °C, which is lower than that used by m a n y other workers. According to the study by Sch~ifer and co-workers [6] on the atomic hydrogen concentration profile in an H F C V D system, almost all of our parameters have been set at the lowest limit for growing good quality diamond at a reasonable rate. The total gas flow rate and pressure were maintained at 100 sccm and 20 T o r r respectively. A small amount of oxygen was used for reducing non-diamond growth at low temperatures. There is an upper limit for both the concentrations of oxygen and methane in an H F C V D system. The resistance of the filament and its lifetime were dramatically reduced when the oxygen concentration was higher than 1%. When the methane concentration was over 4% and with a small amount of oxygen, most of the carbon condensed on the filament instead of the cooler substrate. A typical gas ratio was 0 . 2 5 - 1 - 9 8 . 7 for O 2 - C H n - H 2. The summary of deposition parameters and growth rate is shown in T a b l e 1. The surface of the 304 stainless steel substrate was either mirror polished by 0.3 ~tm 0t-A120 3 grit or roughly scratched by 3M Scotch Brite, and ultrasonically cleaned by Alconox and acetone. Then, a silicon intermediate layer was deposited either by magnetron sputtering or e-beam evaporation. We used 100 nm intermediate layers on most of the substrates. For each deposition, the steel substrate was accompanied by a small piece of silicon acting as a control sample. The surface morphologies of the silicon coated
stainless steel and the silicon were very different. Usually the film on silicon had larger and better faceted diamond particles.
3. Results and discussion 3.1. Direct diamond growth on f e r r o u s metals
Nickel, iron, 304 stainless steel and 321 stainless steel were used as substrates for the direct growth of diamond without using an intermediate layer. During the deposition, a layer of black carbon rapidly formed when the substrate temperature was either too high or too low. A narrow temperature window around 950 °C allowed the growth of diamond on 304 stainless steel without forming any visible black carbon. However, even under this promising condition, the adhesion between the substrates and the diamond particles was very weak. Usually the coating could easily be wiped off. Figure 1 shows the surface morphology of a seeded 304 stainless steel substrate after 40 h growth without using any intermediate layer.
Fig. 1. SEM micrograph of diamond particles directly grown on a submicron diamond powder seeded 304 stainless steel substrate. 3.2. Nucleation site density and growth rate
Diamond was grown on the silicon coated stainless steel without any pre-deposition treatment, but the nucleation site density ( N S D ) was low, less than 1 × l 0 4 mlT1-2, and the growth rate was slow. After 24 h the average particle size was smaller than 2 lam. With a 40 nm hydrogenated amorphous carbon coating on top of the intermediate layer, the N S D was enhanced by an order of magnitude, (see Fig. 2), but it did not form a continuous film. Increased oxygen and methane concentrations have been used by m a n y groups to improve the diamond
H. Chen et al. / Growth o f diamond filrns on stainless steel
Fig. 2. SEM micrograph of the particle morphology for hydrogenated carbon enhanced nulceation.
171
Fig. 4. SEM micrograph of the diamond film with the submicron grain size grown on a diamond powder scratched intermediate silicon layer at 700 °C surface temperature after 6 h. the initial nucleation sites. This phenomenon has been carefullly studied by Iijima, Aikawa and Baba [9] and their result shows that the N S D can be as high as l 0 9 m m -2. A continuous diamond film of submicron sized grains was formed after 6 h. Figure 5 shows the surface morphology with an average grain size of 3 pm after 24 h of growth, indicating that the grain size is a function of deposition time. The film thickness increased from 2 ~tm after 6 h to about 10 I-tm after 24 h; however, the 10 ~tm film cracked during cooling. The continuous 2 ~tm thick film on coated stainless steel was reproduced in another 6 h deposition. Much thicker films would be hard to form because the remaining problem is the difference of thermal expansion coefficients. Films thicker than 4 lain usually cracked
Fig. 3. The result of high carbon deposition, 0.25'¼,02 and 2% CH 4 in H2 at 700 "C. quality and growth rate [7, 8] at a substrate temperature lower than 800 °C. Our results clearly indicate that the growth rate increased with a higher carbon concentration but the diamond structure was degraded. An example of this is given by the morphology in Fig. 3 which was made at 0 . 2 5 - 2 - 9 7 . 7 for 0 2 - C H 4 - H 2 , The ball-shaped carbon, without any visible faceted diamond, was also found with various precursor gas combinations such as 0 . 5 - 2 - 9 7 . 5 , 0 . 5 - 3 - 9 6 . 5 and 1 - 4 - 9 5 . 3.3. Gra& size and film thickness The most promising result is shown in Fig. 4. The intermediate coating was lightly scratched by submicron diamond powder and then ultrasonically cleaned. The diamond powder abrading pretreatment planted diam o n d debris onto the substrate surface, thus increasing
Fig. 5. The same deposition conditions as the sample in Fig. 4 after 24 h,
172
H. Chen et al. / Growth o f diamond films on stainless steel
'E
--,
epoxy cement was used to attach each side of the substrate to the ends of two cylindrical rods. A force normal to the substrate surface was applied to each rod in an attempt to detach the film. The two samples tested showed that the bonding between the film and the substrate was stronger than the epoxy. The epoxy pulled off at about 7.2 kg mm 2.
(o) _
<,~
"~
f/
~ /(grating ghost) ....
g E
4. Summary
c~
1 O0
'
13'00
'
15'00
'
700
Wavenumber Shift (cm--1) Fig. 6. Raman spectra (a), (b), and (c) of the deposits shown in Figs. 2, 3, and 4, respectively; (d) is the reference from a synthetic diamond.
during the cooling phase. It might be possible to obtain a thicker film with submicron grains if the microcrystalline diamond film had larger thermal expansion coefficient and/or a smaller Young's modulus than single crystal diamond. This can be attempted by enhancing secondary nucleation with pulses of a higher carbon concentration. 3.4. Film quality The desired small particle size and low deposition temperature produced some sp 2 bonding in the films. The film quality was determined by studying the morphology in SEM pictures and Raman spectroscopy. Figure 6 shows the corresponding Raman spectra of the samples in Figs. 2, 3, and 4. The spectra (a) in Fig. 6 have a strong diamond peak at about 1332 cm-i and very broad peaks at high wavenumbers from sp 2 carbon formation. Similar non-diamond peaks can be seen in the spectra (b), where the diamond peak becomes much smaller due to a higher growth rate. The spectra (c), from the 6 h deposition, have a relatively flat background indicating a better quality of the film. 3.5. Adhesion test The preliminary studies on the mechanical properties of these films have been conducted with a scratch test and a pull-offadhesion test. Scratch testing was performed by scratching the diamond coated samples using a steel needle. The radius of the needle tip was larger than the average diamond particle size. The sample passed the test if metal debris was left on the diamond surface without any damage to the coating. Most of our samples with detectable diamond on the surface passed this test. Adhesion testing was also performed by a direct pull-off method using the Sebastian Five apparatus. An
With the help of a silicon intermediate layer and lower deposition temperature, growth of a well-adhered diamond film on 304 stainless steel has been demonstrated. We successfully used the thin intermediate layer for reducing carbon diffusion, blocking the iron catalytic effect and improving adhesion. Oxygen-assisted low temperature deposition was required to reduce the effect of thermal expansion mismatch. The results have been reproduced by careful control of the major parameters, such as temperature, temperature gradient, and gas composition.
Acknowledgments This work has been supported by the Electric Power Research Institute and the Nebraska Energy Office under the contracts RP2426-31 and UNR-06 respectively. The authors wish to thank Dr. Seraiya Naris and Jeff Lewis for assistance.
References 1 N. Ohtake, H. Tokura, Y. Kuriyama, Y. Mashimo and M. Yoshikawa, in J. P. Dismukes, A. J. Purdes, B. S. Meyerson, T. D. Moustakas, K. E. Spear, K. V. Ravi and M. Yoder (Eds.), Proc. 1st Int. Syrup. on Diamond and Diamond-Like Films, The Electrochemical Society, Pennington, NJ, 1989. 2 B. Lux and R. Haubner, in R. E. Clausing, L. L. Horton, J. C. Angus and P. Koidl (eds.), Proc. o f Nato A S I (Advanced Study Institute), on Diamond and Diamond-like Films and Coatings, N A T O - A S I Series B: Physics, Vol. 266, Plenum, New York, 1991. 3 T. P. Ong and R. P. H. Chang, Appl. Phys. Lett., 58 (4) (1991) 358. 4 D.N. Belton and S. J. Schmieg, J. Appl. Phys., 66(9) (1989) 4223. 5 R. A. Lula, Stainless Steel, American Society for Metals, Metals Park, OH, Rev. edn., 1986, pp. 29-32. 6 L. Sch/ifer, C. P. Klages, U. Meier and K. Kohse-H6inghaus, Appl. Phys. Lett., 58 (6) (1991) 571. 7 S. J. Harris and A. M. Weiner, Appl. Phys. Lett., 55(21) (1989) 2179. 8 F. Jansen, M. A. Machonkin and D. E. Kuhman, J. Vac. Sci. Teehnol. A, 8 (5) (1990) 3785. 9 Sumio Iijima, Yumi Aikawa and Kazuhiro Baba, J. Mater. Res., 6 (7) (1991) 1491.