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Thermal stability and hardness of metastable Co–C composite alloy films
Materials Science and Engineering A312 (2001) 248– 252 www.elsevier.com/locate/msea
Thermal stability and hardness of metastable Co–C composite alloy films Yoichi Fukumiya, Yumiko Haga, Osamu Nittono * Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2 -12 -1 Oh-okayama, Meguro-ku, Tokyo 152 -8552, Japan Received 28 April 2000; received in revised form 25 July 2000
Abstract Co–C alloy films containing metastable cobalt carbides have been prepared by a new type of co-sputtering deposition technique and their thermal stability was examined and discussed in terms of the concept of the formation enthalpy. Dynamic hardness and magnetic properties, such as saturation magnetization, were also evaluated for the Co– C alloy films. Both hardness and magnetization of the films were found to decrease almost linearly with increasing fraction of carbon content in the film, suggesting that the measurement of hardness can serve as a monitor of film qualities. It is also shown that the hardness of metastable cobalt carbides is greater than that of the Co–C alloy film. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Composite film; Dynamic hardness; Magnetization; Cobalt carbide; Phase stability
1. Introduction Transition metal – carbon alloys such as Fe – C, Ni –C and Co –C alloys have attracted research interest so far because of their scientific and industrial importance. Hardness and some of the magnetic properties of such transition metal – carbon alloys are unique properties and important for their applications. Understanding both the formation ability and the thermal stability of transition metal carbides is also indispensable for reliable industrial applications. Among these transition metals, Ni and Co are known to form a simple eutectic binary phase diagram with carbon (or graphite), and metastable metal – metal carbide phase boundaries may be drawn on the equilibrium phase diagram (or double diagram) [1]. Nickel and cobalt carbides are thought to be difficult to form thermodynamically. However, such metastable carbides are reported to be formed by the cementation of metal powders (or films) [2,3], by co-sputtering [4,5], by mechanical alloying [6,7] and by ion implantation [8]. Recently, the authors developed a dual source deposition system, by which various types of as-deposited * Corresponding author. Fax: + 81-3-57342874. E-mail address: [email protected] (O. Nittono).
films, such as amorphous films, metal carbide films and composite films (or films composed of partially hydrogenated amorphous carbon, carbides and transition metal particles) are formed by adjusting deposition conditions, and succeeded in producing transition metal carbide films, such as Ni3C [9] and Co3C [10], at elevated temperatures above 473 K. It was found that such transition metal carbides show a ferromagnetic nature [9,10]. Furthermore, they investigated phase changes in such metastable transition metal carbides, and proved that the formation ability of the carbides was well explained by considering the difference in the formation enthalpy for the carbides: the order of the formation enthalpy is Ni –C\ Co –C \ Fe –C [7,11]. Our previous studies [9–11] suggested that Co –C composite alloy films (amorphous carbon coexisting with cobalt carbides and cobalt particles, hereafter referred to as composite alloy films) are formable by mainly adjusting methane gas flow rates even when glass substrates are kept at ambient temperature. In this study, therefore, we produced such composite alloy films with different carbon contents by adjusting a supply of carbon atoms, and investigated their thermal stability after annealing. Film qualities, such as dynamic hardness and magnetization, were also examined in terms of the carbon content in the film for reliable applications.
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 8 6 0 - 8
Y. Fukumiya et al. / Materials Science and Engineering A312 (2001) 248–252
2. Experimental The composite alloy films were deposited on glass substrates by a dual source deposition system, which has both an ECR (electron cyclotron resonance) microwave plasma source for supply of active carbon atoms and a Kaufman-type ion source for metal atoms in the same chamber Fig. 1. The base pressure of the chamber was less than 8.9× 10 − 5 Pa and sputtering was carried out under Ar pressure of 2.7× 10 − 2 Pa. Ion beam current and acceleration voltage were 20 mA and 2 kV, respectively. The methane (CH4) gas flow rate varied from 0.005 to 0.30 sccm with a constant hydrogen (H2) gas flow of 20 sccm by using two mass flow controllers. By tilting the cobalt target (99.99% purity) and by adjusting gas flow rates, we prepared the composite alloy films as well as pure Co films at ambient temperature (about 300 K). The total film thickness was 300– 700 nm, measured by a scanning stylus method. The carbon content was analyzed and evaluated by using Auger electron spectroscopy. In order to investigate the decomposition behavior during heating the films, some of the composite alloy films
Fig. 1. Schematic diagram of the dual-source deposition system.
249
were annealed at elevated temperatures above 573 K for 3 h in a vacuum of lower than 1× 10 − 4 Pa. X-ray identification of crystal structures was carried out by using the q–2q method with Cu Ka radiation for the as-deposited and annealed films at ambient temperature. Supplemental identification of phases in the films was done by electron diffraction. Microstructure and surface morphology of the films were observed by a transmission electron microscope (TEM) with an acceleration voltage of 300 kV and by a high resolution scanning electron microscope (FE-SEM), respectively. Magnetic properties such as saturation magnetization and coercive force were evaluated from the H –B hysteresis loops measured by a vibrating sample magnetometer (VSM) at ambient temperature. In this study the hardness of the film was measured by a dynamic ultra micro hardness tester (or a Berkovich indenter). The dynamic hardness was evaluated by both the maximum load (Pmax given in mN) and the maximum displacement of the indenter (hmax in mm) from a load–displacement curve measured by a depth-sensing indenter [12]. We employed the dynamic hardness defined by DHT115 = h Pmax/h 2max, where h is a constant depending on the shape of the diamond tip, having a tip angle of 115° (h= 3.86). All the measurements were carried out at ambient temperature under the following loading conditions: a constant loading rate of 0.142 mN s − 1, a maximum load of 0.5 mN, a holding time of 5 s followed by unloading. For the calibration of dynamic hardness measurements, a relationship between dynamic hardness and Vickers hardness (Hv) was firstly examined for two reference standard blocks, metal and ceramic, with Vickers hardness values of 700 and 1760, respectively. Fig. 2 shows the relationship between dynamic hardness (DHT115) and Vickers hardness (Hv) for both standard blocks, indicating that there seems to be a semi-quantitatively linear relationship between the two hardnesses. Thus, in this study we evaluated the dynamic hardness of the film with this measuring grade. It was also found that the dynamic hardness of the ceramic standard block showed less load dependence than the hardness of the metal block, being similar to the previously reported data [12].
3. Results and discussion
3.1. Structural characterization and thermal stability of composite alloy films
Fig. 2. Relationship between dynamic hardness (DHT115) and Vickers hardness (Hv) for two standard blocks, metal (Hv = 700) and ceramic (Hv =1760), evaluated with a Berkovich indenter [12]. The present hardness of the film was evaluated with this measuring grade.
Many deposition experiments have revealed that Co –C alloy films, such as amorphous films, carbide films and composite alloy films, are formed by adjusting mainly the CH4 gas flow rate. However, cobalt carbide films, which do not contain any amorphous carbon,
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Fig. 3. XRD profiles for cobalt –carbon alloy films with various carbon contents, produced by mainly adjusting the methane gas flow rate at ambient temperature.
Fig. 4. Phase changes after annealing at elevated temperatures in a composite alloy film containing 24 at.% C.
were very difficult to form, unlike in previous studies carried out at elevated temperatures [10]. Cobalt carbide particles were easily formed under low gas flow rates of less than 0.02 sccm, and when the gas flow rate was increased, cobalt carbide particles coexisted with cobalt (solid solution) particles in the amorphous carbon matrix. The present experiments revealed that Co3C (strictly speaking Co3 − zC) and Co2C carbides formed at the same time irrespective of the gas flow rate, but the former was always major and the latter minor. According to previous results [10], we regarded the carbides as thermodynamically metastable phases. In this study, Co3C carbides were identified as hexagonal [8], and Co2C carbides as orthorhombic (or a distorted close-packed hexagonal) [3]. It was also found that, comparing cobalt carbide (Co3C) with nickel carbide(Ni3C), the former was slightly easier to form with a wide range of carbon content than the latter, this being consistent with previously reported results [7,11]. Fig. 3 shows X-ray diffraction (XRD) profiles for as-deposited composite alloy films with various carbon
contents, deposited by changing the CH4 gas flow rates. For the films containing 20 at.% C and 24 at.% C, X-ray peaks, such as Co2C 002 (weak), Co3C 10·1 (strong), Co3C 00·2 (weak) and aCo (hcp Co, 00·2, 10·1), are seen in the same film, though some of them overlap with each other. This indicates that the films are composed of cobalt carbides (Co3C\ Co2C), cobalt particles (with a smaller quantity than carbide) and amorphous carbon, even when they are produced at ambient temperature (about 300 K). Observed broad peaks may be due to small grains of constituent phases of the film. In fact, TEM observations revealed that these as-deposited films consist of carbide particles and cobalt particles of about 10 nm in diameter, which are homogeneously dispersed in the amorphous carbon matrix. This is the same as the previous result [10]. No sharp peak is seen when the carbon content is more than 36 at.% C, indicating that the films are amorphous. Thus, in this study we did not produce any films containing carbon contents of more than 47 at.% C. A similar tendency was also seen for nickel–carbon composite alloy films [9]. Fig. 4 shows XRD profiles for a composite alloy film with a carbon content of 24 at.% C, annealed at elevated temperatures above 573 K. This film is also composed of major Co3C carbide and minor CO2C carbide. When the film was annealed at 673 K, a strong Co3C 101 peak showed a drastic decrease in intensity, and several detectable peaks ascribed to aCo (hcp Co), such as 10·0, 00·2 and 10·1, appeared instead. This indicates that a phase change (or phase decomposition) took place in the films during annealing at elevated temperatures. All the present results of phase changes in the composite alloy films are also explainable on the basis of the previous results [10]. The cobalt carbides decomposed into aCo, bCo (both solid solutions) and carbon (or graphite) at elevated temperatures above 773 K, according to the decomposition reactions: (1) Co3C a-Co (hcp solid solution slightly supersaturated with carbon)+ b-Co (fcc solid solution, with a much smaller quantity than a-Co) + C (carbon or graphite); and (2) Co2C a-Co +C. Furthermore, the thermal stability of the cobalt carbides was reasonably explained based on the concept that the formation enthalpy is strongly related to the thermal stability for the carbides, as already discussed in the previous study [11,13]. Observed broad peaks are also ascribed to small grains (or particles) of constituent phases in the films. In this connection, detailed TEM observations revealed that many stacking faults were introduced into individual carbide particles after annealing at 623 K, though such TEM images were not reproduced here. Therefore, these stacking faults may also be attributed to the peak broadening. Furthermore, detailed analyses of X-ray profiles revealed that the peaks due to the cobalt car-
Y. Fukumiya et al. / Materials Science and Engineering A312 (2001) 248–252
bides slightly shifted toward the higher angle after annealing, thus resulting in the shrinkage of the lattice spacing. This lattice shrinkage suggests a partial dissociation of carbon atoms from the carbides. Stacking faults observed in cobalt carbides may be associated with such dissociation of carbon atoms from the carbides. Similar lattice shrinkage was also seen for nickel carbide films [9].
3.2. Hardness of film and magnetic property We first examined the surface morphology of the composite alloy films by FE-SEM. FE-SEM observations proved that the surface smoothness was sufficient for sub-micron depth-sensing indentation measurements. The side length of a triangle produced by an indenter with a diamond tip was smaller than 500 nm,
Fig. 5. Relationship between dynamic hardness (DHT115) and carbon content (at.% C) for the as-deposited composite alloy films, together with a value evaluated after annealing at 623 K for a Co – C composite alloy film containing 24 at.% C.
Fig. 6. Relationship between dynamic hardness (DHT115) and fraction of cobalt content (at.% C) for the as-deposited Co –C composite alloy films that do not contain cobalt carbides.
251
and the maximum displacement of the indenter (hmax), or the depth of the pit produced by the indenter, was confirmed to be more shallow than one-third of the film thickness. Fig. 5 shows the relationship between dynamic hardness (DHT115) and carbon content (at.% C) for as-deposited composite alloy films, which are composed of cobalt particles and cobalt carbide particles that are dispersed in an amorphous carbon matrix. The hardness decreases almost linearly with increasing carbon content, though the hardness is slightly larger for films of 20 and 24 at.% C. This increased hardness may be ascribed to the constituent phases of the films. In fact, as additionally plotted in Fig. 5, the film of 24 at.% C showed a large decrease in hardness after annealing at 623 K. As already shown in Fig. 4, after annealing at 623 K the cobalt carbides (Co3C is major) showed a partial phase decomposition into Co3C, a-Co and carbon (or graphite). Therefore, this decrease in dynamic hardness is mainly ascribed to the phase decomposition of cobalt carbides. This result also means that the cobalt carbides have a slightly greater dynamic hardness than the cobalt particles. As already reported by the authors [10], rod-like graphite crystals formed around the carbides. Thus, such graphite particles may be attributed to a slightly higher dynamic hardness of the film. Fig. 6 also shows the relationship between dynamic hardness (DHT115) and the fraction of cobalt content (fCo) for the as-deposited composite alloy films. The hardness of the film increases almost linearly with increasing fraction of cobalt content. In this connection, magnetic properties such as saturation magnetization and coercive force were also evaluated from the hysteresis loops measured by VSM for various composite alloy films. All the hysteresis curves showed a rectangular shape characteristic of a soft magnetic nature, and the magnetization was nearly saturated at a magnetic field of 1 T for all the composite alloy films. This is consistent with the observed grain size of cobalt and carbide particles, about 10 nm in diameter. In this study, measured values of the magnetization were considered to be directly related to the summation of cobalt atoms in the cobalt carbide particles and cobalt particles. The coercive force was almost constant, though it was slightly smaller for the films with higher carbon content. The magnetization (Ms) also varied almost linearly with increasing fraction of cobalt content in the film. Observed characteristics are also explainable by considering the constituent phases of the films, such as cobalt carbide particles and cobalt particles, which are homogeneously dispersed in the amorphous carbon matrix. Based on these experimental data, including dynamic hardness and magnetic properties, Fig. 7 reproduces a relationship between magnetization (Ms) evaluated at 1 T and dynamic hardness
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composite alloy films were qualitatively explained in terms of constituent phases in the films. A simple hardness measurement would be a promising monitor, determining film qualities such as magnetization and concentration of magnetic metals, especially for composite alloy films that are composed of homogeneously dispersed magnetic alloy particles in an amorphous nonmagnetic matrix.
Acknowledgements
Fig. 7. Relationship between magnetization (Ms) and dynamic hardness (DHT115) for as-deposited Co –C composite films.
(DHT115) for the composite alloy films. There seems to be a linear relationship between saturation magnetization and dynamic hardness. The above experimental results suggest that the hardness measurement can serve as a monitor. This can evaluate the qualities of the film, such as the content and the magnetization, especially for the composite alloy films which are composed of fine magnetic particles dispersed homogeneously in an amorphous nonmagnetic matrix. For useful applications of the present monitoring using dynamic hardness measurements, further detailed study must be done for many types of composite alloy films which are composed of homogeneously dispersed alloy particles.
4. Concluding remarks The thermal stability of metastable cobalt carbides was examined and discussed in terms of the concept that the formation enthalpy is strongly related to the thermal stability. Film qualities such as dynamic hardness and magnetic properties observed for the Co– C
.
The authors are grateful to Dr Ji Shi, M. Azumi and M. Wakamori for help in the formation of the films. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of Phase Transformations (1997–99) from the Ministry of Education, Science, Sports and Culture, Japan.
References [1] T.B. Massalski, Binary Alloy Phase Diagram, ASM, Metals Park, 1986, p. 556. [2] S. Nagakura, J. Phys. Soc. Jpn. 13 (1958) 1005 (for Ni). [3] S. Nagakura, J. Phys. Soc. Jpn. 16 (1958) 1213 (for Co). [4] T. Konno, R. Sinclair, Acta Metall. Mater. 42 (1994) 1231. [5] T. Konno, R. Sinclair, Met. Sci. Eng. A179/180 (1994) 297. [6] T. Tanaka, S. Nasu, K.N. Ishihara, R.H. Shingu, J. Less. Common Met. 1 (1991) 872. [7] T. Tanaka, K.N. Ishihara, R.H. Shingu, Met. Trans. 23A (1992) 2431. [8] B.X. Liu, J. Wang, Z.Z. Fabg, J. Appl. Phys. 69 (1991) 7342. [9] J. Shi, O. Nittono, J. Mater. Sci. Lett. 11 (1992) 22. [10] M. Azumi, J. Shi, Y. Haga, O. Nittono, in: M.A. Imam, R. De Nale, S. Hanada, Z. Zhoiig, D.N. Lee (Eds.), Proceedings of PRICM’3, The Minerals, Metals & Materials Society, 1998, p. 953. [11] O. Nittono, M. Azurni, Y. Hashiba, M. Wakamori, in: M. Koiwa, K. Otsuka, T. Miyazaki (Eds.), Poceedings of Solid – Solid Phase Transformation (JIMIC-3), The Japan Institute of Metals, 1999, p. 1309. [12] W.C. Oliver, G.M. Pharr, J. Mater. Res. 7 (1992) 1564. [13] M.C. Cadeville, C. Lermer, J.M. Friedt, Physica B86 – 88 (1977) 432.
Thermal stability and hardness of metastable Co–C composite alloy films Yoichi Fukumiya, Yumiko Haga, Osamu Nittono * Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology, 2 -12 -1 Oh-okayama, Meguro-ku, Tokyo 152 -8552, Japan Received 28 April 2000; received in revised form 25 July 2000
Abstract Co–C alloy films containing metastable cobalt carbides have been prepared by a new type of co-sputtering deposition technique and their thermal stability was examined and discussed in terms of the concept of the formation enthalpy. Dynamic hardness and magnetic properties, such as saturation magnetization, were also evaluated for the Co– C alloy films. Both hardness and magnetization of the films were found to decrease almost linearly with increasing fraction of carbon content in the film, suggesting that the measurement of hardness can serve as a monitor of film qualities. It is also shown that the hardness of metastable cobalt carbides is greater than that of the Co–C alloy film. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Composite film; Dynamic hardness; Magnetization; Cobalt carbide; Phase stability
1. Introduction Transition metal – carbon alloys such as Fe – C, Ni –C and Co –C alloys have attracted research interest so far because of their scientific and industrial importance. Hardness and some of the magnetic properties of such transition metal – carbon alloys are unique properties and important for their applications. Understanding both the formation ability and the thermal stability of transition metal carbides is also indispensable for reliable industrial applications. Among these transition metals, Ni and Co are known to form a simple eutectic binary phase diagram with carbon (or graphite), and metastable metal – metal carbide phase boundaries may be drawn on the equilibrium phase diagram (or double diagram) [1]. Nickel and cobalt carbides are thought to be difficult to form thermodynamically. However, such metastable carbides are reported to be formed by the cementation of metal powders (or films) [2,3], by co-sputtering [4,5], by mechanical alloying [6,7] and by ion implantation [8]. Recently, the authors developed a dual source deposition system, by which various types of as-deposited * Corresponding author. Fax: + 81-3-57342874. E-mail address: [email protected] (O. Nittono).
films, such as amorphous films, metal carbide films and composite films (or films composed of partially hydrogenated amorphous carbon, carbides and transition metal particles) are formed by adjusting deposition conditions, and succeeded in producing transition metal carbide films, such as Ni3C [9] and Co3C [10], at elevated temperatures above 473 K. It was found that such transition metal carbides show a ferromagnetic nature [9,10]. Furthermore, they investigated phase changes in such metastable transition metal carbides, and proved that the formation ability of the carbides was well explained by considering the difference in the formation enthalpy for the carbides: the order of the formation enthalpy is Ni –C\ Co –C \ Fe –C [7,11]. Our previous studies [9–11] suggested that Co –C composite alloy films (amorphous carbon coexisting with cobalt carbides and cobalt particles, hereafter referred to as composite alloy films) are formable by mainly adjusting methane gas flow rates even when glass substrates are kept at ambient temperature. In this study, therefore, we produced such composite alloy films with different carbon contents by adjusting a supply of carbon atoms, and investigated their thermal stability after annealing. Film qualities, such as dynamic hardness and magnetization, were also examined in terms of the carbon content in the film for reliable applications.
0921-5093/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 0 9 3 ( 0 0 ) 0 1 8 6 0 - 8
Y. Fukumiya et al. / Materials Science and Engineering A312 (2001) 248–252
2. Experimental The composite alloy films were deposited on glass substrates by a dual source deposition system, which has both an ECR (electron cyclotron resonance) microwave plasma source for supply of active carbon atoms and a Kaufman-type ion source for metal atoms in the same chamber Fig. 1. The base pressure of the chamber was less than 8.9× 10 − 5 Pa and sputtering was carried out under Ar pressure of 2.7× 10 − 2 Pa. Ion beam current and acceleration voltage were 20 mA and 2 kV, respectively. The methane (CH4) gas flow rate varied from 0.005 to 0.30 sccm with a constant hydrogen (H2) gas flow of 20 sccm by using two mass flow controllers. By tilting the cobalt target (99.99% purity) and by adjusting gas flow rates, we prepared the composite alloy films as well as pure Co films at ambient temperature (about 300 K). The total film thickness was 300– 700 nm, measured by a scanning stylus method. The carbon content was analyzed and evaluated by using Auger electron spectroscopy. In order to investigate the decomposition behavior during heating the films, some of the composite alloy films
Fig. 1. Schematic diagram of the dual-source deposition system.
249
were annealed at elevated temperatures above 573 K for 3 h in a vacuum of lower than 1× 10 − 4 Pa. X-ray identification of crystal structures was carried out by using the q–2q method with Cu Ka radiation for the as-deposited and annealed films at ambient temperature. Supplemental identification of phases in the films was done by electron diffraction. Microstructure and surface morphology of the films were observed by a transmission electron microscope (TEM) with an acceleration voltage of 300 kV and by a high resolution scanning electron microscope (FE-SEM), respectively. Magnetic properties such as saturation magnetization and coercive force were evaluated from the H –B hysteresis loops measured by a vibrating sample magnetometer (VSM) at ambient temperature. In this study the hardness of the film was measured by a dynamic ultra micro hardness tester (or a Berkovich indenter). The dynamic hardness was evaluated by both the maximum load (Pmax given in mN) and the maximum displacement of the indenter (hmax in mm) from a load–displacement curve measured by a depth-sensing indenter [12]. We employed the dynamic hardness defined by DHT115 = h Pmax/h 2max, where h is a constant depending on the shape of the diamond tip, having a tip angle of 115° (h= 3.86). All the measurements were carried out at ambient temperature under the following loading conditions: a constant loading rate of 0.142 mN s − 1, a maximum load of 0.5 mN, a holding time of 5 s followed by unloading. For the calibration of dynamic hardness measurements, a relationship between dynamic hardness and Vickers hardness (Hv) was firstly examined for two reference standard blocks, metal and ceramic, with Vickers hardness values of 700 and 1760, respectively. Fig. 2 shows the relationship between dynamic hardness (DHT115) and Vickers hardness (Hv) for both standard blocks, indicating that there seems to be a semi-quantitatively linear relationship between the two hardnesses. Thus, in this study we evaluated the dynamic hardness of the film with this measuring grade. It was also found that the dynamic hardness of the ceramic standard block showed less load dependence than the hardness of the metal block, being similar to the previously reported data [12].
3. Results and discussion
3.1. Structural characterization and thermal stability of composite alloy films
Fig. 2. Relationship between dynamic hardness (DHT115) and Vickers hardness (Hv) for two standard blocks, metal (Hv = 700) and ceramic (Hv =1760), evaluated with a Berkovich indenter [12]. The present hardness of the film was evaluated with this measuring grade.
Many deposition experiments have revealed that Co –C alloy films, such as amorphous films, carbide films and composite alloy films, are formed by adjusting mainly the CH4 gas flow rate. However, cobalt carbide films, which do not contain any amorphous carbon,
250
Y. Fukumiya et al. / Materials Science and Engineering A312 (2001) 248–252
Fig. 3. XRD profiles for cobalt –carbon alloy films with various carbon contents, produced by mainly adjusting the methane gas flow rate at ambient temperature.
Fig. 4. Phase changes after annealing at elevated temperatures in a composite alloy film containing 24 at.% C.
were very difficult to form, unlike in previous studies carried out at elevated temperatures [10]. Cobalt carbide particles were easily formed under low gas flow rates of less than 0.02 sccm, and when the gas flow rate was increased, cobalt carbide particles coexisted with cobalt (solid solution) particles in the amorphous carbon matrix. The present experiments revealed that Co3C (strictly speaking Co3 − zC) and Co2C carbides formed at the same time irrespective of the gas flow rate, but the former was always major and the latter minor. According to previous results [10], we regarded the carbides as thermodynamically metastable phases. In this study, Co3C carbides were identified as hexagonal [8], and Co2C carbides as orthorhombic (or a distorted close-packed hexagonal) [3]. It was also found that, comparing cobalt carbide (Co3C) with nickel carbide(Ni3C), the former was slightly easier to form with a wide range of carbon content than the latter, this being consistent with previously reported results [7,11]. Fig. 3 shows X-ray diffraction (XRD) profiles for as-deposited composite alloy films with various carbon
contents, deposited by changing the CH4 gas flow rates. For the films containing 20 at.% C and 24 at.% C, X-ray peaks, such as Co2C 002 (weak), Co3C 10·1 (strong), Co3C 00·2 (weak) and aCo (hcp Co, 00·2, 10·1), are seen in the same film, though some of them overlap with each other. This indicates that the films are composed of cobalt carbides (Co3C\ Co2C), cobalt particles (with a smaller quantity than carbide) and amorphous carbon, even when they are produced at ambient temperature (about 300 K). Observed broad peaks may be due to small grains of constituent phases of the film. In fact, TEM observations revealed that these as-deposited films consist of carbide particles and cobalt particles of about 10 nm in diameter, which are homogeneously dispersed in the amorphous carbon matrix. This is the same as the previous result [10]. No sharp peak is seen when the carbon content is more than 36 at.% C, indicating that the films are amorphous. Thus, in this study we did not produce any films containing carbon contents of more than 47 at.% C. A similar tendency was also seen for nickel–carbon composite alloy films [9]. Fig. 4 shows XRD profiles for a composite alloy film with a carbon content of 24 at.% C, annealed at elevated temperatures above 573 K. This film is also composed of major Co3C carbide and minor CO2C carbide. When the film was annealed at 673 K, a strong Co3C 101 peak showed a drastic decrease in intensity, and several detectable peaks ascribed to aCo (hcp Co), such as 10·0, 00·2 and 10·1, appeared instead. This indicates that a phase change (or phase decomposition) took place in the films during annealing at elevated temperatures. All the present results of phase changes in the composite alloy films are also explainable on the basis of the previous results [10]. The cobalt carbides decomposed into aCo, bCo (both solid solutions) and carbon (or graphite) at elevated temperatures above 773 K, according to the decomposition reactions: (1) Co3C a-Co (hcp solid solution slightly supersaturated with carbon)+ b-Co (fcc solid solution, with a much smaller quantity than a-Co) + C (carbon or graphite); and (2) Co2C a-Co +C. Furthermore, the thermal stability of the cobalt carbides was reasonably explained based on the concept that the formation enthalpy is strongly related to the thermal stability for the carbides, as already discussed in the previous study [11,13]. Observed broad peaks are also ascribed to small grains (or particles) of constituent phases in the films. In this connection, detailed TEM observations revealed that many stacking faults were introduced into individual carbide particles after annealing at 623 K, though such TEM images were not reproduced here. Therefore, these stacking faults may also be attributed to the peak broadening. Furthermore, detailed analyses of X-ray profiles revealed that the peaks due to the cobalt car-
Y. Fukumiya et al. / Materials Science and Engineering A312 (2001) 248–252
bides slightly shifted toward the higher angle after annealing, thus resulting in the shrinkage of the lattice spacing. This lattice shrinkage suggests a partial dissociation of carbon atoms from the carbides. Stacking faults observed in cobalt carbides may be associated with such dissociation of carbon atoms from the carbides. Similar lattice shrinkage was also seen for nickel carbide films [9].
3.2. Hardness of film and magnetic property We first examined the surface morphology of the composite alloy films by FE-SEM. FE-SEM observations proved that the surface smoothness was sufficient for sub-micron depth-sensing indentation measurements. The side length of a triangle produced by an indenter with a diamond tip was smaller than 500 nm,
Fig. 5. Relationship between dynamic hardness (DHT115) and carbon content (at.% C) for the as-deposited composite alloy films, together with a value evaluated after annealing at 623 K for a Co – C composite alloy film containing 24 at.% C.
Fig. 6. Relationship between dynamic hardness (DHT115) and fraction of cobalt content (at.% C) for the as-deposited Co –C composite alloy films that do not contain cobalt carbides.
251
and the maximum displacement of the indenter (hmax), or the depth of the pit produced by the indenter, was confirmed to be more shallow than one-third of the film thickness. Fig. 5 shows the relationship between dynamic hardness (DHT115) and carbon content (at.% C) for as-deposited composite alloy films, which are composed of cobalt particles and cobalt carbide particles that are dispersed in an amorphous carbon matrix. The hardness decreases almost linearly with increasing carbon content, though the hardness is slightly larger for films of 20 and 24 at.% C. This increased hardness may be ascribed to the constituent phases of the films. In fact, as additionally plotted in Fig. 5, the film of 24 at.% C showed a large decrease in hardness after annealing at 623 K. As already shown in Fig. 4, after annealing at 623 K the cobalt carbides (Co3C is major) showed a partial phase decomposition into Co3C, a-Co and carbon (or graphite). Therefore, this decrease in dynamic hardness is mainly ascribed to the phase decomposition of cobalt carbides. This result also means that the cobalt carbides have a slightly greater dynamic hardness than the cobalt particles. As already reported by the authors [10], rod-like graphite crystals formed around the carbides. Thus, such graphite particles may be attributed to a slightly higher dynamic hardness of the film. Fig. 6 also shows the relationship between dynamic hardness (DHT115) and the fraction of cobalt content (fCo) for the as-deposited composite alloy films. The hardness of the film increases almost linearly with increasing fraction of cobalt content. In this connection, magnetic properties such as saturation magnetization and coercive force were also evaluated from the hysteresis loops measured by VSM for various composite alloy films. All the hysteresis curves showed a rectangular shape characteristic of a soft magnetic nature, and the magnetization was nearly saturated at a magnetic field of 1 T for all the composite alloy films. This is consistent with the observed grain size of cobalt and carbide particles, about 10 nm in diameter. In this study, measured values of the magnetization were considered to be directly related to the summation of cobalt atoms in the cobalt carbide particles and cobalt particles. The coercive force was almost constant, though it was slightly smaller for the films with higher carbon content. The magnetization (Ms) also varied almost linearly with increasing fraction of cobalt content in the film. Observed characteristics are also explainable by considering the constituent phases of the films, such as cobalt carbide particles and cobalt particles, which are homogeneously dispersed in the amorphous carbon matrix. Based on these experimental data, including dynamic hardness and magnetic properties, Fig. 7 reproduces a relationship between magnetization (Ms) evaluated at 1 T and dynamic hardness
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composite alloy films were qualitatively explained in terms of constituent phases in the films. A simple hardness measurement would be a promising monitor, determining film qualities such as magnetization and concentration of magnetic metals, especially for composite alloy films that are composed of homogeneously dispersed magnetic alloy particles in an amorphous nonmagnetic matrix.
Acknowledgements
Fig. 7. Relationship between magnetization (Ms) and dynamic hardness (DHT115) for as-deposited Co –C composite films.
(DHT115) for the composite alloy films. There seems to be a linear relationship between saturation magnetization and dynamic hardness. The above experimental results suggest that the hardness measurement can serve as a monitor. This can evaluate the qualities of the film, such as the content and the magnetization, especially for the composite alloy films which are composed of fine magnetic particles dispersed homogeneously in an amorphous nonmagnetic matrix. For useful applications of the present monitoring using dynamic hardness measurements, further detailed study must be done for many types of composite alloy films which are composed of homogeneously dispersed alloy particles.
4. Concluding remarks The thermal stability of metastable cobalt carbides was examined and discussed in terms of the concept that the formation enthalpy is strongly related to the thermal stability. Film qualities such as dynamic hardness and magnetic properties observed for the Co– C
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The authors are grateful to Dr Ji Shi, M. Azumi and M. Wakamori for help in the formation of the films. This work was partially supported by a Grant-in-Aid for Scientific Research on Priority Area of Phase Transformations (1997–99) from the Ministry of Education, Science, Sports and Culture, Japan.
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