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Ion-plated titanium carbide coatings
Thin Solid Films, 22 (1974) 111-120 © Elsevier Sequoia S.A., Lausanne---Printed in Switzerland
I 11
ION-PLATED TITANIUM CARBIDE COATINGS
WILLIAM R. STOWELL Battelle, Columbus Laboratories, Columbus, Ohio 43201 (U.S.A.) (Received December 10, 1973; accepted January 22, 1974)
A new development in the field of coatings is the reactive ion-plating process. In this process, metal evaporated from a source reacts with an atmosphere of mixed gases. The reaction can take place in the gas phase before the material deposits, or it can take place on the substrate. The stoichiometry of the reaction can be controlled by adjusting at least one of several process parameters. This technique allows the formation of carbides, nitrides, oxides and other materials and provides a method of controlling the stoichiometry of the deposited material. The ability of the process to provide graded stoichiometry through a coating layer has made possible the application of adherent coatings to difficult-to-coat substrates. This prevents, for instance, a sharp boundary zone between materials which have greatly differing thermal coefficients of expansion. The dependence of the coating composition upon the deposition parameters of gas pressure, substrate voltage and evaporation rates from the source is discussed. The application of the technique in the coating of titanium and mild-steel substrates with titanium carbide is discussed. Photomicrographs and hardness data for the deposited films are presented.
INTRODUCTION
This paper describes a new development in the field of coating depositionthe reactive ion-plating process. In this process, metal evaporated from a source reacts with an atmosphere of mixed gases which have been ionized by a glow discharge. The reaction can take place in the gas phase before the material deposits, or it can take place on the substrate. The stoichiometry of the reaction can be controlled by adjusting one or more of several control parameters which regulate the process. This technique can be used for the formation of carbides, nitrides, oxides and other materials. It also provides a straightforward method for controlling the stoichiometry of the deposited material. Adherent coatings have been applied to difficult-to-coat substrate materials by taking advantage of the ability of the process controls to provide graded stoichiometry through a coating layer, and of the inherent tenacity 1-* of films produced by ion plating. The gradual transition from the properties of the substrate material to those of a
112
w.R. STOWELL
metallic compound eliminates sharp boundaries between two quite different materials. Bunshah 5, 6 of UCLA has recently published the results of his work on what he terms "activated reactive evaporation ". His technique is similar to this work, but it has some differences. To cause reaction between evaporating metal atoms and a gas species, he applies a positive potential to form a discharge around a probe situated between the metal vapor source and the substrate. In the process discussed in this paper, activation of the reacting species occurs in the vicinity of the substrate, which is maintained at a high negative potential. In the region of the substrate, a cathodic glow discharge is created by applying a negative 2-5 kV to the substrate which ionizes and decomposes the reactive gas species and allows interaction between the evaporated metal atoms as they approach the substrate and the activated gaseous species which surround the substrate. The reactive ion-plating process is physically analogous to reactive sputtering processes which have been described by many authors (too numerous to list). In the reactive sputtering technique, metal atoms are sputtered from a target surface onto a substrate and can react to form compounds if a reactive gas is present. Under proper conditions of the glow discharge, active gas concentration and sputtering rate, many compounds have been formed. However, typical sputtering rates are one or two orders of magnitude lower than rates for electronbeam evaporation. Consequently, in applications where film thicknesses of several hundred microns are required, the economics of such a low rate process tend to make sputtering unattractive. EXPERIMENTAL EQUIPMENT
Figure 1 is a schematic diagram of the basic ion-plating equipment used in this work. The electron-beam evaporation source is a standard l in. rod fed source. Electrons generated at the gun filament are magnetically deflected through an angle of 270 ° while they are electrically accelerated through a potential of 10 kV to provide energy to heat the source for evaporation. To prevent rapid degradation of the electron-emitting filament, it is separated from the deposition portion of the coating unit by a flow-limiting baffle. This baffle arrangement makes it possible to maintain pressures of less than 1 Ixm in the vicinity of the filament, while pressures are as high as 60 gm in the deposition section of the coater. The substrate holder for flat pieces is located directly above the evaporation source at an adjustable distance from it, and is connected to the coating chamber by a high voltage vacuum feed-through. Special substrate-holding arrangements were required to reactively ionplate material onto cylindrical specimens. Figure 2 is a schematic diagram of the fixture used to apply a uniform coating to a cylindrical specimen. The mechanical drive had to be electrically isolated from the substrate. To do this it was necessary to design a dark-space shielding arrangement so that an insulating block would not become coated during the deposition process. It was also necessary to provide a voltage contact which would allow rotation of the substrate.
113
ION-PLATED TITANIUM CARBIDE COATINGS
104 TORR~
S~Sl"|M
Fig. 1. Experimental apparatus for ion plating by the hot-filament electron-beam technique.
CONTROL PARAMETERS
Pressure measurement is difficult when reactive gas species are present in the coating system because of reactions with the hot filament of the ionization gauge. In the experiments described, the life of the vacuum gauge was shortened to a maximum of several hours. In these experiments, carbon soot deposited from the gas species onto the interior of the vacuum ionization gauge until the gauge components were shorted out. To provide a backup measurement of the pressure, a thermocouple gauge was installed along with a Schulz-Phelps high pressure ionization gauge. The thermocouple gauge was not satisfactory because of its slow response. However,
114
W. R. STOWELL Work piece
High voltage lead
Fig. 2. Rotating fixture for a cylindrical substrate.
it did provide reassurance when static measurements were made. Judging from the correlation of measurements made with the ionization gauge and with the thermocouple gauge, the ionization gauge did not degenerate in accuracy of measurement but simply ceased to function when the carbon deposit on the interior of the tube became sufficiently conducting. The power supply used to provide for voltage as a control parameter in this set of experiments had a regulated voltage output with a current output of from 0 to 500 mA. Voltage regulation was desirable because the current to the substrate tended to vary with the sweep of the electron-beam gun across the evaporation source. The associated variation in ionization would be expected to cause a variation in the deposition rate of carbon; however, the effect was not noticed in the deposited film. This result is understandable since the electronbeam sweep rate of about 1 cycle/s was too high to allow resulting fluctuations in structure to be observed metallographically. EXPERIMENTS AND RESULTS
In one experiment gas pressure was varied to control the composition of a titanium carbide coating. The substrate, a specially prepared fiat panel, was located 14 cm above the titanium evaporation source. Material was deposited at an average rate of 15 ~tm/min (measured by dividing the final thickness of the deposit by the time). The substrate voltage was maintained at 3 kV, but the pressure of ethylene gas (the reactive species) in the coating chamber was varied during the deposition. At a gas pressure of 12 ~tm, the deposition of carbon into the film was slow. At a pressure of 26 Ixm, heavy depositions of carbon occurred. Figure 3 shows the
ION-PLATED TITANIUM CARBIDE COATINGS
115
Fig. 3. Deposited film prepared from the evaporation of titanium into 12-26 Inn of ethylene ( × 120).
deposited film as it appeared after deposition. For most of the deposition cycle the gas pressure was maintained at 12 Ixm. Before completion of the deposition, however, it was adjusted to 26 pm for approximately 1 min and then to 20 ~tm for approximately 3 min with evaporation rate and voltage constant. The dark and gray zones visible in the photomicrograph corresponded to the periods of 26 and 20 Ima pressure, respectively. This deposited film was stripped from its specially prepared substrate, and was somewhat ductile as produced. Since TiC is a brittle compound, the ductility (measured by bending the film around a a in. radius) of the film along with metallography indicated that it was not chemically reacted but that a fine mixture of titanium and carbon had been co-deposited. To cause compound formation, several heat treatments were performed. Figure 4 shows the stripped film following a 1 h vacuum heat treatment at 1040°C. Apparent precipitation of particles occurred within the film and a continuous hard layer began to form along the outer edge. Hardness readings were 240 K H N (10 g load) in the light zone and 450 K H N in the particle-containing zone.
Fig. 4. The same film as in Fig. 3 after vacuum heat treatment (1040 °C, 1 h) ( x 120).
116
W. R. STOWELL
The polished surface of the same film is shown in Fig. 5. Here one sees a dispersion of particles which formed during the heat treatment. Hardness readings were 320 K H N in the matrix material but the particles gave hardness readings of 780 K H N , which is consistent with their being small unsupported TiC particles.
Fig. 5. Surface view of the same film as in Figs. 3 and 4, polished ( × 300). Figure 6 shows the same film after an additional heat treatment of 1 h at 1200 °C. The material along the outer edge gave hardness readings of 2000 K H N , while that below the outer edge measured 450 K H N . A portion of the film from the same experiment was vacuum heat-treated for 5 h at 955 °C (see Fig. 7). The outer edge of the film measured 2960 K H N . The hardness dropped to 300 K H N in the m a j o r portion of the film.
Fig. 6. Outer edge of the same filmas in Figs. 3-5 after vacuum heat treatment (1200 °C, 1 h) ( x 600). Figure 8 shows a rod of titanium 1 in. in diameter and 2 in. long which was coated by means of the reactive ion-plating process. A set o f experiments using a rod of titanium for the substrate was performed to determine the effect o f changing the substrate voltage while maintaining the evaporation rate and gas
117
ION-PLATED TITANIUM CARBIDE COATINGS
Fig. 7. Outer edge of the film after vacuum heat treatment (955 °C, 5 h) ( x 600).
pressure (25 ~tm) effectively constant. Material was deposited on the cylindrical substrate at an average rate of 5.1 ~tm/min; the voltage was held at 3.5 kV for the first 5 min (one-half the total deposition time) and was gradually changed to 4.5 kV during the last 5 min of deposition with a corresponding increase in substrate current. The as-produced specimen is shown in Fig. 9. There appear to be heavier carbon concentrations in the outer portion of the film corresponding to increased deposition with the higher substrate voltage.
Fig. 8. Cylindrical specimen as coated.
Fig. 9. Coating on a titanium rod produced under conditions of varying voltage ( x 600).
Figure 10 shows the same specimen after a heat treatment of 5 h at 900 °C, revealing the resulting particle formation. From hardness profiles before and after heat treatment (see Fig. 11) it is apparent that the film developed a significant gradation in hardness during this heat treatment.
118
w.R. STOWELL
Fig. 10. The same coating as in Fig. 9 after vacuum heat treatment (900 °C, 5 h) ( x 600).
/
Fig. 12. The same coating as in Figs. 9 and 10 after additional vacuum heat treatment (955 °C, 3 h) ( × 600).
AS PRODUCED
HEAT T R E A T E D
\ KNH 1. 2. 3. 4.
399 473 495 443
10-Gram Load
900 C
KNH
iTf
1. 2. 3. 4.
5 Hours
10-Gram Load
880 682 599 557
(a)
(b) Fig. 11. Hardness profiles in the coating deposited on a titanium rod with voltage variation.
Figure 12 shows the same coating following an additional heat treatment of 3 h at 955 °C. Two distinct zones are evident. The hardness in the outer zone was 1000 K H N . The hardness in the intermediate zone decreased from 800 K H N just below the particle-containing zone to 500 K H N at the interface between the coating and the substrate. Another deposition experiment was performed on a titanium substrate during which the gas pressure was maintained at 25 ~trn and the substrate voltage was held at 2 kV. The coating was deposited on the cylindrical substrate at an average rate of 5 ~un/min. The as-deposited coating is shown in Fig. 13, and it appears that the coating is fairly uniform. The same specimen after vacuum heat treatment at 950 °C for 5 h is shown in Fig. 14. The average hardness through the coating was 340_ 20 K H N (25 g load). A final experiment was made to explore the effects of using high gas pressure, high voltage and high titanium evaporation rate. During this deposition, the gas
ION-PLATED TITANIUM CARBIDE COATINGS
Fig. 13. Coating as deposited on a titanium rod with constant control parameters ( x 600).
119
Fig. 14. The same coating as in Fig. 13 vacuum heat treated (950 °C, 5 h) ( × 600).
pressure was changed from 60 to 65 ~ n and back, the mild-steel substrate voltage was held constant at 5 kV, and the average deposition rate was 11 I~m/min--more than twice the average deposition rate on the other cylindrical substrates. The as-deposited coating is shown in Fig. 15. Several significant points can be made.
Fig. 15. Coating as deposited on a mild-steel substrate under the highest pressure, voltage and evaporation rate conditions studied ( x 600).
(1) Cracks are present that cannot be attributed to differential thermal contraction (the coefficient of thermal expansion of mild steel is higher than that of titanium or TIC). These cracks are similar to those that have been attributed in the past to gas entrapped during deposition. (2) The pressure change from 60 to 65 pm and back to 60 pxn produced a
120
W. R. STOWELL
corresponding darkening of the coating, presumably because of the increased carbon deposition. (3) Such a coating would obviously not provide acceptable erosion resistance. CONCLUSIONS
With ethylene gas in the coating chamber during the process of reactive ion plating using evaporated titanium, co-deposition of titanium and carbon results. As judged solely from metallographic information, film hardness after heat treatment corresponds in magnitude to the hardness reported by Bunshah for films deposited onto heated substrates; the concentration of carbon in the deposited film at a fixed deposition rate can be varied by varying either the gas pressure or the substrate voltage. Following deposition, heat treatment of the film causes the formation of hard phases, presumably of titanium carbide. Dense adherent films can be produced at rates of condensation much greater than those attainable by sputtering. Upper limits to the range of gas pressure, voltage and condensation rate exist beyond which film quality deteriorates. ACKNOWLEDGEMENTS
The author gratefully acknowledges the technical assistance of Ray Coleman and Gene Sigler, and the support of the Materials Application and Coatings Section of Battelle's Columbus Laboratories. REFERENCES 1 2 3 4
D . M . Mattox, Fundamentals of ion plating, J. Vac. Sci. Technol., 10 (1) (1973) 47. D.L. Chambers and D. C. Carmichael, Res. Develop., 22 (5) (1971) 32. C.T. Wan, D. L. Chambers and D. C. Carmichael, J. Vac. Sci. Technol., 8 (1971) 99. H . R . Harker and R. J. Hill, The deposition of multicomponent phases by ion plating, J. Vac. Sci. Technol., 9 (6) (1972) 1395. 5 A . C . Raghuram and R. F. Bunshah, The effect of substrate temperature on the structure of titanium carbide deposited by activated reactive evaporation, J. Vac. Sci. Technol., 9 (6) (1972) 1389. 6 R.F. Bunshah and A. C. Raghuram, Activated reactive evaporation process for high rate deposition of compounds, J. Vac. Sci. Technol., 9 (6) (1972) 1385.
I 11
ION-PLATED TITANIUM CARBIDE COATINGS
WILLIAM R. STOWELL Battelle, Columbus Laboratories, Columbus, Ohio 43201 (U.S.A.) (Received December 10, 1973; accepted January 22, 1974)
A new development in the field of coatings is the reactive ion-plating process. In this process, metal evaporated from a source reacts with an atmosphere of mixed gases. The reaction can take place in the gas phase before the material deposits, or it can take place on the substrate. The stoichiometry of the reaction can be controlled by adjusting at least one of several process parameters. This technique allows the formation of carbides, nitrides, oxides and other materials and provides a method of controlling the stoichiometry of the deposited material. The ability of the process to provide graded stoichiometry through a coating layer has made possible the application of adherent coatings to difficult-to-coat substrates. This prevents, for instance, a sharp boundary zone between materials which have greatly differing thermal coefficients of expansion. The dependence of the coating composition upon the deposition parameters of gas pressure, substrate voltage and evaporation rates from the source is discussed. The application of the technique in the coating of titanium and mild-steel substrates with titanium carbide is discussed. Photomicrographs and hardness data for the deposited films are presented.
INTRODUCTION
This paper describes a new development in the field of coating depositionthe reactive ion-plating process. In this process, metal evaporated from a source reacts with an atmosphere of mixed gases which have been ionized by a glow discharge. The reaction can take place in the gas phase before the material deposits, or it can take place on the substrate. The stoichiometry of the reaction can be controlled by adjusting one or more of several control parameters which regulate the process. This technique can be used for the formation of carbides, nitrides, oxides and other materials. It also provides a straightforward method for controlling the stoichiometry of the deposited material. Adherent coatings have been applied to difficult-to-coat substrate materials by taking advantage of the ability of the process controls to provide graded stoichiometry through a coating layer, and of the inherent tenacity 1-* of films produced by ion plating. The gradual transition from the properties of the substrate material to those of a
112
w.R. STOWELL
metallic compound eliminates sharp boundaries between two quite different materials. Bunshah 5, 6 of UCLA has recently published the results of his work on what he terms "activated reactive evaporation ". His technique is similar to this work, but it has some differences. To cause reaction between evaporating metal atoms and a gas species, he applies a positive potential to form a discharge around a probe situated between the metal vapor source and the substrate. In the process discussed in this paper, activation of the reacting species occurs in the vicinity of the substrate, which is maintained at a high negative potential. In the region of the substrate, a cathodic glow discharge is created by applying a negative 2-5 kV to the substrate which ionizes and decomposes the reactive gas species and allows interaction between the evaporated metal atoms as they approach the substrate and the activated gaseous species which surround the substrate. The reactive ion-plating process is physically analogous to reactive sputtering processes which have been described by many authors (too numerous to list). In the reactive sputtering technique, metal atoms are sputtered from a target surface onto a substrate and can react to form compounds if a reactive gas is present. Under proper conditions of the glow discharge, active gas concentration and sputtering rate, many compounds have been formed. However, typical sputtering rates are one or two orders of magnitude lower than rates for electronbeam evaporation. Consequently, in applications where film thicknesses of several hundred microns are required, the economics of such a low rate process tend to make sputtering unattractive. EXPERIMENTAL EQUIPMENT
Figure 1 is a schematic diagram of the basic ion-plating equipment used in this work. The electron-beam evaporation source is a standard l in. rod fed source. Electrons generated at the gun filament are magnetically deflected through an angle of 270 ° while they are electrically accelerated through a potential of 10 kV to provide energy to heat the source for evaporation. To prevent rapid degradation of the electron-emitting filament, it is separated from the deposition portion of the coating unit by a flow-limiting baffle. This baffle arrangement makes it possible to maintain pressures of less than 1 Ixm in the vicinity of the filament, while pressures are as high as 60 gm in the deposition section of the coater. The substrate holder for flat pieces is located directly above the evaporation source at an adjustable distance from it, and is connected to the coating chamber by a high voltage vacuum feed-through. Special substrate-holding arrangements were required to reactively ionplate material onto cylindrical specimens. Figure 2 is a schematic diagram of the fixture used to apply a uniform coating to a cylindrical specimen. The mechanical drive had to be electrically isolated from the substrate. To do this it was necessary to design a dark-space shielding arrangement so that an insulating block would not become coated during the deposition process. It was also necessary to provide a voltage contact which would allow rotation of the substrate.
113
ION-PLATED TITANIUM CARBIDE COATINGS
104 TORR~
S~Sl"|M
Fig. 1. Experimental apparatus for ion plating by the hot-filament electron-beam technique.
CONTROL PARAMETERS
Pressure measurement is difficult when reactive gas species are present in the coating system because of reactions with the hot filament of the ionization gauge. In the experiments described, the life of the vacuum gauge was shortened to a maximum of several hours. In these experiments, carbon soot deposited from the gas species onto the interior of the vacuum ionization gauge until the gauge components were shorted out. To provide a backup measurement of the pressure, a thermocouple gauge was installed along with a Schulz-Phelps high pressure ionization gauge. The thermocouple gauge was not satisfactory because of its slow response. However,
114
W. R. STOWELL Work piece
High voltage lead
Fig. 2. Rotating fixture for a cylindrical substrate.
it did provide reassurance when static measurements were made. Judging from the correlation of measurements made with the ionization gauge and with the thermocouple gauge, the ionization gauge did not degenerate in accuracy of measurement but simply ceased to function when the carbon deposit on the interior of the tube became sufficiently conducting. The power supply used to provide for voltage as a control parameter in this set of experiments had a regulated voltage output with a current output of from 0 to 500 mA. Voltage regulation was desirable because the current to the substrate tended to vary with the sweep of the electron-beam gun across the evaporation source. The associated variation in ionization would be expected to cause a variation in the deposition rate of carbon; however, the effect was not noticed in the deposited film. This result is understandable since the electronbeam sweep rate of about 1 cycle/s was too high to allow resulting fluctuations in structure to be observed metallographically. EXPERIMENTS AND RESULTS
In one experiment gas pressure was varied to control the composition of a titanium carbide coating. The substrate, a specially prepared fiat panel, was located 14 cm above the titanium evaporation source. Material was deposited at an average rate of 15 ~tm/min (measured by dividing the final thickness of the deposit by the time). The substrate voltage was maintained at 3 kV, but the pressure of ethylene gas (the reactive species) in the coating chamber was varied during the deposition. At a gas pressure of 12 ~tm, the deposition of carbon into the film was slow. At a pressure of 26 Ixm, heavy depositions of carbon occurred. Figure 3 shows the
ION-PLATED TITANIUM CARBIDE COATINGS
115
Fig. 3. Deposited film prepared from the evaporation of titanium into 12-26 Inn of ethylene ( × 120).
deposited film as it appeared after deposition. For most of the deposition cycle the gas pressure was maintained at 12 Ixm. Before completion of the deposition, however, it was adjusted to 26 pm for approximately 1 min and then to 20 ~tm for approximately 3 min with evaporation rate and voltage constant. The dark and gray zones visible in the photomicrograph corresponded to the periods of 26 and 20 Ima pressure, respectively. This deposited film was stripped from its specially prepared substrate, and was somewhat ductile as produced. Since TiC is a brittle compound, the ductility (measured by bending the film around a a in. radius) of the film along with metallography indicated that it was not chemically reacted but that a fine mixture of titanium and carbon had been co-deposited. To cause compound formation, several heat treatments were performed. Figure 4 shows the stripped film following a 1 h vacuum heat treatment at 1040°C. Apparent precipitation of particles occurred within the film and a continuous hard layer began to form along the outer edge. Hardness readings were 240 K H N (10 g load) in the light zone and 450 K H N in the particle-containing zone.
Fig. 4. The same film as in Fig. 3 after vacuum heat treatment (1040 °C, 1 h) ( x 120).
116
W. R. STOWELL
The polished surface of the same film is shown in Fig. 5. Here one sees a dispersion of particles which formed during the heat treatment. Hardness readings were 320 K H N in the matrix material but the particles gave hardness readings of 780 K H N , which is consistent with their being small unsupported TiC particles.
Fig. 5. Surface view of the same film as in Figs. 3 and 4, polished ( × 300). Figure 6 shows the same film after an additional heat treatment of 1 h at 1200 °C. The material along the outer edge gave hardness readings of 2000 K H N , while that below the outer edge measured 450 K H N . A portion of the film from the same experiment was vacuum heat-treated for 5 h at 955 °C (see Fig. 7). The outer edge of the film measured 2960 K H N . The hardness dropped to 300 K H N in the m a j o r portion of the film.
Fig. 6. Outer edge of the same filmas in Figs. 3-5 after vacuum heat treatment (1200 °C, 1 h) ( x 600). Figure 8 shows a rod of titanium 1 in. in diameter and 2 in. long which was coated by means of the reactive ion-plating process. A set o f experiments using a rod of titanium for the substrate was performed to determine the effect o f changing the substrate voltage while maintaining the evaporation rate and gas
117
ION-PLATED TITANIUM CARBIDE COATINGS
Fig. 7. Outer edge of the film after vacuum heat treatment (955 °C, 5 h) ( x 600).
pressure (25 ~tm) effectively constant. Material was deposited on the cylindrical substrate at an average rate of 5.1 ~tm/min; the voltage was held at 3.5 kV for the first 5 min (one-half the total deposition time) and was gradually changed to 4.5 kV during the last 5 min of deposition with a corresponding increase in substrate current. The as-produced specimen is shown in Fig. 9. There appear to be heavier carbon concentrations in the outer portion of the film corresponding to increased deposition with the higher substrate voltage.
Fig. 8. Cylindrical specimen as coated.
Fig. 9. Coating on a titanium rod produced under conditions of varying voltage ( x 600).
Figure 10 shows the same specimen after a heat treatment of 5 h at 900 °C, revealing the resulting particle formation. From hardness profiles before and after heat treatment (see Fig. 11) it is apparent that the film developed a significant gradation in hardness during this heat treatment.
118
w.R. STOWELL
Fig. 10. The same coating as in Fig. 9 after vacuum heat treatment (900 °C, 5 h) ( x 600).
/
Fig. 12. The same coating as in Figs. 9 and 10 after additional vacuum heat treatment (955 °C, 3 h) ( × 600).
AS PRODUCED
HEAT T R E A T E D
\ KNH 1. 2. 3. 4.
399 473 495 443
10-Gram Load
900 C
KNH
iTf
1. 2. 3. 4.
5 Hours
10-Gram Load
880 682 599 557
(a)
(b) Fig. 11. Hardness profiles in the coating deposited on a titanium rod with voltage variation.
Figure 12 shows the same coating following an additional heat treatment of 3 h at 955 °C. Two distinct zones are evident. The hardness in the outer zone was 1000 K H N . The hardness in the intermediate zone decreased from 800 K H N just below the particle-containing zone to 500 K H N at the interface between the coating and the substrate. Another deposition experiment was performed on a titanium substrate during which the gas pressure was maintained at 25 ~trn and the substrate voltage was held at 2 kV. The coating was deposited on the cylindrical substrate at an average rate of 5 ~un/min. The as-deposited coating is shown in Fig. 13, and it appears that the coating is fairly uniform. The same specimen after vacuum heat treatment at 950 °C for 5 h is shown in Fig. 14. The average hardness through the coating was 340_ 20 K H N (25 g load). A final experiment was made to explore the effects of using high gas pressure, high voltage and high titanium evaporation rate. During this deposition, the gas
ION-PLATED TITANIUM CARBIDE COATINGS
Fig. 13. Coating as deposited on a titanium rod with constant control parameters ( x 600).
119
Fig. 14. The same coating as in Fig. 13 vacuum heat treated (950 °C, 5 h) ( × 600).
pressure was changed from 60 to 65 ~ n and back, the mild-steel substrate voltage was held constant at 5 kV, and the average deposition rate was 11 I~m/min--more than twice the average deposition rate on the other cylindrical substrates. The as-deposited coating is shown in Fig. 15. Several significant points can be made.
Fig. 15. Coating as deposited on a mild-steel substrate under the highest pressure, voltage and evaporation rate conditions studied ( x 600).
(1) Cracks are present that cannot be attributed to differential thermal contraction (the coefficient of thermal expansion of mild steel is higher than that of titanium or TIC). These cracks are similar to those that have been attributed in the past to gas entrapped during deposition. (2) The pressure change from 60 to 65 pm and back to 60 pxn produced a
120
W. R. STOWELL
corresponding darkening of the coating, presumably because of the increased carbon deposition. (3) Such a coating would obviously not provide acceptable erosion resistance. CONCLUSIONS
With ethylene gas in the coating chamber during the process of reactive ion plating using evaporated titanium, co-deposition of titanium and carbon results. As judged solely from metallographic information, film hardness after heat treatment corresponds in magnitude to the hardness reported by Bunshah for films deposited onto heated substrates; the concentration of carbon in the deposited film at a fixed deposition rate can be varied by varying either the gas pressure or the substrate voltage. Following deposition, heat treatment of the film causes the formation of hard phases, presumably of titanium carbide. Dense adherent films can be produced at rates of condensation much greater than those attainable by sputtering. Upper limits to the range of gas pressure, voltage and condensation rate exist beyond which film quality deteriorates. ACKNOWLEDGEMENTS
The author gratefully acknowledges the technical assistance of Ray Coleman and Gene Sigler, and the support of the Materials Application and Coatings Section of Battelle's Columbus Laboratories. REFERENCES 1 2 3 4
D . M . Mattox, Fundamentals of ion plating, J. Vac. Sci. Technol., 10 (1) (1973) 47. D.L. Chambers and D. C. Carmichael, Res. Develop., 22 (5) (1971) 32. C.T. Wan, D. L. Chambers and D. C. Carmichael, J. Vac. Sci. Technol., 8 (1971) 99. H . R . Harker and R. J. Hill, The deposition of multicomponent phases by ion plating, J. Vac. Sci. Technol., 9 (6) (1972) 1395. 5 A . C . Raghuram and R. F. Bunshah, The effect of substrate temperature on the structure of titanium carbide deposited by activated reactive evaporation, J. Vac. Sci. Technol., 9 (6) (1972) 1389. 6 R.F. Bunshah and A. C. Raghuram, Activated reactive evaporation process for high rate deposition of compounds, J. Vac. Sci. Technol., 9 (6) (1972) 1385.