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Carburising of steel AISI 1010 by using a cathode arc plasma process
Surface & Coatings Technology 187 (2004) 1 – 5 www.elsevier.com/locate/surfcoat
Carburising of steel AISI 1010 by using a cathode arc plasma process Chengming Li a,*, Qi He b, Weizhong Tang a, Fanxiu Lu a a
School of Materials Science and Engineering, University of Science and Technology Beijing, No.30 Xueyuan Road, Haidian Zone, Beijing 100083, PR China b Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China Received 18 January 2003; accepted in revised form 19 January 2004 Available online 6 May 2004
Abstract The properties and processes of cathode arc plasma carburizing were studied in this paper. Graphite with high purity and high strength was selected as the cathode arc source to provide carbon atoms. The carbon steel AISI 1010 was used as the substrate. A surface concentration of 1.05 wt.%C and a carburised layer thickness of 780 Am was obtained at 950 jC in 30 min. The diffusion coefficient of carbon atoms in carburizing layer was estimated by using the Matano-Plane method. The exponential relationship of diffusion coefficient and the distance to surface are determined. D 2004 Elsevier B.V. All rights reserved. Keywords: Cathode arc; Carburising; Plasma; Diffusion
1. Introduction Carburising is one of the most widely used heat treatment techniques for steel, mainly because of the superior combinations of mechanical properties it can offer. Among the various competing carburising processes (gas, liquid, solid and plasma carburising), plasma carburising is unique because it is performed with the help of DC or pulsed glow discharges. The introduction of a plasma into the carburising process brings with it many advantages [1 –3]: simple and precise control of carbon concentrations at the surface of the sample, favorable microstructures of carburised surface layers without the danger of internal oxidation, low emission rate of by-products and environmental pollutions, possibility to case-harden high alloy steel, suitability of selective carburising, and the last but not the least, rapid carburising rate and hence suitability of being integrated into production lines. Compared with glow discharges, arc discharges are normally operated at higher power levels and therefore can produce more intense plasmas. By using cathode arc sources made from metallic or semiconducting materials,
* Corresponding author. Tel.: +86-10-6233-2475; fax: +86-10-62332336. E-mail address: [email protected] (C. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.026
thin solid films and protective coatings with novel properties can be deposited at relatively higher deposition rates. In this paper, a new process of plasma carburising will be described. This process employs a graphite cathode arc source to supply the active carbon species. It will be shown that by combining such a cathode arc source with a suitable DC biasing, very high carburising rates of steel samples could be obtained. Examples will be given for carbon steel AISI 1010, for which a carbonized surface layer of thickness of 780 Am has been produced within half an hour.
2. Experimental Fig. 1 shows a schematic diagram of the equipment used in the cathode arc plasma carburising experiments. The base pressure of the equipment was 1 Pa and the argon pressure during the operation was 6 Pa. A cathode arc source made of high purity graphite was used to supply carbon vapors needed for the plasma carburising. The cathode arc was initialized by a shot-firing circuit and maintained by a DC arc power source. The diameter of the arc source was 65 mm and the current density was between 102 and 108 A/m2 [4 –7]. During the operation, a DC bias was applied between the graphite arc source and the samples made from carbon steel AISI 1010. With a DC bias voltage applied, a glow
2
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
Fig. 1. Schematic diagram of equipment used in cathode arc plasma carburising. Fig. 3. Dependence of carburising layer thickness on source – sample distance.
discharge was maintained between the arc source (as the anode) and the samples (as the cathode). Before the carburising, the surface of the samples was sputter cleaned by using argon plasmas. The specimen was heated to the treatment temperature using a closed auxiliary SiC heating elements with power of 0 – 10 kW. Treatment temperature was measured by a pyrometer and a thermocouple, which is located 1 cm above the specimen holder. Then carbon cathode arc was initialized and carbon vapors were produced. With the DC bias applied, fluxes of carbon containing ions, atoms and small particles were driven to the samples and carburised surface layers were formed. The range of bias variable was 500 –1500 V and bias current was 0 –5 A. Typical processing conditions were as follows: bias voltage 700 V, source –sample distance 130 mm, arc current 70 A and deposition time 30 min, unless otherwise specified. Microstructures of the samples and thickness of the carburised layers were analyzed by optical microscopy. Profiles of carbon concentration in the carbonized layers were determined by wavelength dispersion X-ray spectrometry. Maximum surface carbon content was firstly measured and then the profiles of carbon concentration were examined in the cross-section of the carbonized layers. Microhardness
Various processing parameters, including the biasing voltage, the distance between the arc source and the samples, the arc current as well as the time of the carburising process, all have influences on the rate of carburising. The dependence of carburising layer thickness on these four processing parameters for the carbon steel AISI 1010 are summarized in Figs. 2– 5. It can be seen from Fig. 2 that the carburising rate increases with the increase of biasing voltage. This is understandable since high biasing voltage would induce high ion fluxes bombarding the sample surfaces, with the result of high fluxes of carbon species depositing onto the samples. In addition, intense bombardment of ions would generate high densities of structural defects on the sample
Fig. 2. Dependence of carburised layer thickness on bias voltage.
Fig. 4. Dependence of carburising layer thickness on arc current.
of the carbonized surface layers was measured by using micro Vickers.
3. Results and discussions 3.1. Effects of processing parameters on carburising rate
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
3
species bombarding the sample surfaces and hence the rapid formation of thick carburised surface layers. Fig. 5 shows the time dependence of thickness of carburised surface layers. And just as any theory would predict, the longer the processing time, the thicker the carburising layer. An approximate parabolic interrelationship between the carbonized layer thickness and processing time would be expected [8]. 3.2. Microstructure, profiles of carbon concentration and microhardness of carburised layers
Fig. 5. Dependence of carburising layer thickness on processing time.
surface, and hence creating rapid diffusion channels for carbon atoms into the interior of the steel. In meantime, the diffusion of carbon also accelerates with the rise of temperature. From Fig. 3 we can see that as the distance between the arc source and the samples was increased, the thickness of the carburised layers decreased. This result could be explained by the fact that at long distances, low ion current would be maintained, and high probability of ion scattering and hence loss of the ion energy of the bombarding carbon ions would result. All these would result in a decrease in the carburising rate. Fig. 4 shows the dependence of thickness of carburised layers on the arc source current. The thickness of carburised layers is increased with increasing the arc source current. It infers that high arc currents would produce plasmas of high densities, which would result in large fluxes of active
Cross-section microstructures of carburised surface layers produced at 950 and 1050 jC are shown in Fig. 6. For the sample carburised at 950 jC (Fig. 6a), a structural change from eutectic pearlite at the surface to ferrite in the interior is discernable, with an intermediate layer in between. No carbide network was identified within the carburised layer. Generally carbide network are formed at higher temperature and/or high-carbon concentration in the carburized layer. As shown in Fig. 6b, coarse plate-like carbide networks could be identified in the carburised layer for the sample carburiszed at 1050 jC (Fig. 6b), indicating that this carburising temperature is a little bit higher than optimal. Figs. 7 and 8 show profiles of carbon concentration and microhardness of the carburised surface layer, for a sample carbonized at 950 jC and then quenched from 860 jC. Maximum surface carbon content was firstly measured and then the profiles of carbon concentration were examined in cross-section of the carbonized layers. The carbon concentration at the surface is 1.05 wt.% and decreases slowly from the surface to the interior. Accompanying this decrease in
Fig. 6. Cross-section optical micrographs for AISI 1010 steel, showing the carburised surface layer after carburising at 950 jC (a), and the plate-like carbide networks at 1050 jC (b). Etchant: 4 wt.% nitric acid in ethyl alcohol.
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C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
age of ions are in multiple charge state (+2, +3, etc.); (5) the average kinetic energy of the ions is high (10 – 100 eV). The ions from the cathode arc are accelerated by the substrate bias voltage. The atoms and ions of carbon are transferred to the surface of the sample by plasma processes. In plasma carburizing, the carbon flux ( j) is a function of the gas composition, the electric current density of the substrate (i), the gas flow (u), the gas pressure ( P) and the temperature (T), i.e.: j ¼ f ðtype of gas; i; u; P; T Þ:
Fig. 7. Profile of carbon concentration of carburised surface layer.
carbon concentration is a change in microhardness of the surface layer. At the sample surface, microhardness above HV800 has been obtained. It can be seen from our results that the cathode arc plasma carburizing is far more efficient than conventional processes. We have shown that a case depth of approximately 0.78 mm can be obtained within half an hour at 950 jC, with a surface carbon concentration of 1.05 wt.%. Besides, there is also no danger for internal oxidation and hydrogen embrittlement, because no hydrocarbons are used. 3.3. Carburizing mechanism It is reasonable to assume that the carburising process of steel is heterogeneous, just as in the case of glow discharge plasma carbonization. Firstly, fluxes of active carbon species with high dynamic energies would bombard the surface of the steel, and then penetration of carbon atoms into the samples would start at individual spots and then extend to all over the surfaces [9]. At last, diffusion of carbon atoms would take place homogeneously in the interior of the samples. However, detailed mechanism of this carbon transportation is not well understood. Rembges proposed that the carbonization process would initialize with the formation of cementite within the plasma, and then it would condense on the surface of the samples. Subsequent decomposition of the deposit would provide a source of carbon atoms [10]. Edenhofer insisted that the use of plasma would not influence the diffusion rate of carbon atoms in steel. However, a high rate of carbon species transportation in the plasma would result in a rapid increase in carbon concentration at the surface, and thus carbon diffusion would start earlier, leading to a reduction of processing time [11]. Cathode arcs have the following important physical characteristics [12 – 15]: (1) a plasma is generated by a small and intense arc spot that moves rapidly and randomly over the surface of the cathode; (2) plasma is formed from cathode material; (3) a high percentage (10 – 100%) of ions in the particles are eroded from cathode; (4) a high percent-
ð1Þ
For the given and controlled carburizing temperature, the control of the carbon flux depends only on the single parameter of electric current density, i.e.: j ¼ f ðiÞ
ð2Þ
here i stands for the electric current density of the substrate, and the gas composition, u, P, and T are all kept constant. However, in cathode arc carburizing, the control of the carbon flux depends on the cathode arc current I and the electric current density of the substrate i; i.e.: j ¼ f ðI; iÞ
ð3Þ
when argon gas is used, and the other parameters are constant. Therefore, the mechanism for generation and transportation of carbon species in cathode arc plasma carburizing process is not very much different from that of glow discharge plasma carburizing. While the excitation and ionization of carbon atoms or particles take place in the surface of the graphite by arc discharge in this method, however, the excitation and ionization of the hydrocarbon molecules only takes place in the cathode-fall region, i.e. in the vicinity of the sample in the glow discharge plasma carburizing method. Approximately 70% of carbon particles from the graphite cathode arc source can be ionized [16].
Fig. 8. Profile of hardness of the carbonized surface layer.
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
5
The ion energy is 30 –40 eV [17] depending on the bias voltage. The carbon ions or particles will be accelerated by the bias voltage towards the substrate (the sample), and the intense bombardment on sample surface would result in a large number of defects on its surface. It is inferable that large number of point defects bound to be generated in collision cascades, which will greatly enhance the diffusion of carbon atoms. The diffusion of carbon atoms consist of the isothermal diffusion and defects enhanced diffusion. In the thermal diffusion, the diffusion coefficient is a function of composition.
2. The cathode arc source plasma carburizing is less liable to internal oxidation and hydrogen embrittlement because the carbon atoms are provided through graphite rather than hydrocarbon. 3. Besides carburizing temperature and carburising time, case depth (carburizing layer thickness) is mainly depended on substrate bias and the source –sample distance, which increases with increasing bias voltage and decreases with increasing source—sample distance.
Dth ¼ f ðq; xÞ
References
ð4Þ
where q is the carbon concentration of the carburizied layer. x is the distance to the surface of carburizied layer. Another diffusion coefficient is the function of defects density in the carburizing layer, Dd ¼ f ðq; xÞ
ð5Þ
where q is the defect density introduced from ion bombardment. The diffusion coefficient Dn of carbon are estimated by Matano-plane method [18] and fitted by least square method according to Fig. 7: Dn ¼ Dth þ Dd ¼ 1:1921010 expð0:00206xÞm2 =s
ð6Þ
4. Conclusions 1. A case depth of 0.78 mm in the cathode arc plasma carburizing can be obtained within half an hour at 950 jC. Cathode arc source plasma carburising is far more efficient than conventional processes due to the bombardment of the intense energetic carbon ions generated by d.c. arc discharge and accelerated by the bias voltage onto the surface of the substrate (sample), and the enhanced carbon diffusion resulted by the high-density defects induced by the bombardment.
[1] B. Edenhofer, W. Grafen, J. Muller-Ziller, Surf. Coat. Technol. 142 – 144 (2001) 225 – 234. [2] A.J. Hick, Heat Treat. Met. 28 (2001) 77 – 81. [3] J.M. Baek, Y.R. Cho, D.J. Kim, K.H. Lee, Surf. Coat. Technol. 131 (2000) 568 – 573. [4] E. Hantzsche, B. Juettner, IEEE Trans. Plasma Sci. 13 (1984) 230 – 234. [5] G.A. Mesyats, S.A. Barengolts, IEEE Trans. Plasma Sci. 29 (2001) 704 – 707. [6] G.P. Smith, R. Dollinger, D.P. Malone, A.A. Gilmour, J. Appl. Phys. 51 (1980) 3657 – 3662. [7] B. Juettner, J. Phys. D 14 (1981) 1265 – 1275. [8] Chinese mechanical engineering society, Handbook of metals heat treatment, 2001, 306. [9] K.T. Rie, Surf. Coat. Technol. 112 (1999) 56 – 62. [10] K.T. Rembges, J. Luhr, Carburizing: Processing and Performance, ASM International, 1989, pp. 341 – 348. [11] B. Edenhofer, Heat Treat. Met. 18 (1991) 6 – 12. [12] H. Kelly, A. Marquez, F. Minotti, C.F. Fontan, J. Phys. D: Appl. Phys. 31 (1998) 1737 – 1741. [13] S. Thomas, A. Andre, IEEE Trans. Plasma Sci. 27 (1999) 911 – 944. [14] G.A. Mesyats, IEEE Trans. Electr. Insul. 20 (1984) 729 – 734. [15] J.E. Daalder, J. Phys. D: Appl. Phys. 9 (1976) 2379 – 2395. [16] D.Y. Fang, J. Phys. D: Appl. Phys. 15 (1982) 833 – 844. [17] J.M. Laeferty, Vacuum Arcs Theory and Application, 1980. [18] Z.F. Qie, Diffusion and Phase Transformation of Solid Metals, China machine press, 1998.
Carburising of steel AISI 1010 by using a cathode arc plasma process Chengming Li a,*, Qi He b, Weizhong Tang a, Fanxiu Lu a a
School of Materials Science and Engineering, University of Science and Technology Beijing, No.30 Xueyuan Road, Haidian Zone, Beijing 100083, PR China b Institute of Surface Engineering, Taiyuan University of Technology, Taiyuan 030024, Shanxi, PR China Received 18 January 2003; accepted in revised form 19 January 2004 Available online 6 May 2004
Abstract The properties and processes of cathode arc plasma carburizing were studied in this paper. Graphite with high purity and high strength was selected as the cathode arc source to provide carbon atoms. The carbon steel AISI 1010 was used as the substrate. A surface concentration of 1.05 wt.%C and a carburised layer thickness of 780 Am was obtained at 950 jC in 30 min. The diffusion coefficient of carbon atoms in carburizing layer was estimated by using the Matano-Plane method. The exponential relationship of diffusion coefficient and the distance to surface are determined. D 2004 Elsevier B.V. All rights reserved. Keywords: Cathode arc; Carburising; Plasma; Diffusion
1. Introduction Carburising is one of the most widely used heat treatment techniques for steel, mainly because of the superior combinations of mechanical properties it can offer. Among the various competing carburising processes (gas, liquid, solid and plasma carburising), plasma carburising is unique because it is performed with the help of DC or pulsed glow discharges. The introduction of a plasma into the carburising process brings with it many advantages [1 –3]: simple and precise control of carbon concentrations at the surface of the sample, favorable microstructures of carburised surface layers without the danger of internal oxidation, low emission rate of by-products and environmental pollutions, possibility to case-harden high alloy steel, suitability of selective carburising, and the last but not the least, rapid carburising rate and hence suitability of being integrated into production lines. Compared with glow discharges, arc discharges are normally operated at higher power levels and therefore can produce more intense plasmas. By using cathode arc sources made from metallic or semiconducting materials,
* Corresponding author. Tel.: +86-10-6233-2475; fax: +86-10-62332336. E-mail address: [email protected] (C. Li). 0257-8972/$ - see front matter D 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2004.01.026
thin solid films and protective coatings with novel properties can be deposited at relatively higher deposition rates. In this paper, a new process of plasma carburising will be described. This process employs a graphite cathode arc source to supply the active carbon species. It will be shown that by combining such a cathode arc source with a suitable DC biasing, very high carburising rates of steel samples could be obtained. Examples will be given for carbon steel AISI 1010, for which a carbonized surface layer of thickness of 780 Am has been produced within half an hour.
2. Experimental Fig. 1 shows a schematic diagram of the equipment used in the cathode arc plasma carburising experiments. The base pressure of the equipment was 1 Pa and the argon pressure during the operation was 6 Pa. A cathode arc source made of high purity graphite was used to supply carbon vapors needed for the plasma carburising. The cathode arc was initialized by a shot-firing circuit and maintained by a DC arc power source. The diameter of the arc source was 65 mm and the current density was between 102 and 108 A/m2 [4 –7]. During the operation, a DC bias was applied between the graphite arc source and the samples made from carbon steel AISI 1010. With a DC bias voltage applied, a glow
2
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
Fig. 1. Schematic diagram of equipment used in cathode arc plasma carburising. Fig. 3. Dependence of carburising layer thickness on source – sample distance.
discharge was maintained between the arc source (as the anode) and the samples (as the cathode). Before the carburising, the surface of the samples was sputter cleaned by using argon plasmas. The specimen was heated to the treatment temperature using a closed auxiliary SiC heating elements with power of 0 – 10 kW. Treatment temperature was measured by a pyrometer and a thermocouple, which is located 1 cm above the specimen holder. Then carbon cathode arc was initialized and carbon vapors were produced. With the DC bias applied, fluxes of carbon containing ions, atoms and small particles were driven to the samples and carburised surface layers were formed. The range of bias variable was 500 –1500 V and bias current was 0 –5 A. Typical processing conditions were as follows: bias voltage 700 V, source –sample distance 130 mm, arc current 70 A and deposition time 30 min, unless otherwise specified. Microstructures of the samples and thickness of the carburised layers were analyzed by optical microscopy. Profiles of carbon concentration in the carbonized layers were determined by wavelength dispersion X-ray spectrometry. Maximum surface carbon content was firstly measured and then the profiles of carbon concentration were examined in the cross-section of the carbonized layers. Microhardness
Various processing parameters, including the biasing voltage, the distance between the arc source and the samples, the arc current as well as the time of the carburising process, all have influences on the rate of carburising. The dependence of carburising layer thickness on these four processing parameters for the carbon steel AISI 1010 are summarized in Figs. 2– 5. It can be seen from Fig. 2 that the carburising rate increases with the increase of biasing voltage. This is understandable since high biasing voltage would induce high ion fluxes bombarding the sample surfaces, with the result of high fluxes of carbon species depositing onto the samples. In addition, intense bombardment of ions would generate high densities of structural defects on the sample
Fig. 2. Dependence of carburised layer thickness on bias voltage.
Fig. 4. Dependence of carburising layer thickness on arc current.
of the carbonized surface layers was measured by using micro Vickers.
3. Results and discussions 3.1. Effects of processing parameters on carburising rate
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
3
species bombarding the sample surfaces and hence the rapid formation of thick carburised surface layers. Fig. 5 shows the time dependence of thickness of carburised surface layers. And just as any theory would predict, the longer the processing time, the thicker the carburising layer. An approximate parabolic interrelationship between the carbonized layer thickness and processing time would be expected [8]. 3.2. Microstructure, profiles of carbon concentration and microhardness of carburised layers
Fig. 5. Dependence of carburising layer thickness on processing time.
surface, and hence creating rapid diffusion channels for carbon atoms into the interior of the steel. In meantime, the diffusion of carbon also accelerates with the rise of temperature. From Fig. 3 we can see that as the distance between the arc source and the samples was increased, the thickness of the carburised layers decreased. This result could be explained by the fact that at long distances, low ion current would be maintained, and high probability of ion scattering and hence loss of the ion energy of the bombarding carbon ions would result. All these would result in a decrease in the carburising rate. Fig. 4 shows the dependence of thickness of carburised layers on the arc source current. The thickness of carburised layers is increased with increasing the arc source current. It infers that high arc currents would produce plasmas of high densities, which would result in large fluxes of active
Cross-section microstructures of carburised surface layers produced at 950 and 1050 jC are shown in Fig. 6. For the sample carburised at 950 jC (Fig. 6a), a structural change from eutectic pearlite at the surface to ferrite in the interior is discernable, with an intermediate layer in between. No carbide network was identified within the carburised layer. Generally carbide network are formed at higher temperature and/or high-carbon concentration in the carburized layer. As shown in Fig. 6b, coarse plate-like carbide networks could be identified in the carburised layer for the sample carburiszed at 1050 jC (Fig. 6b), indicating that this carburising temperature is a little bit higher than optimal. Figs. 7 and 8 show profiles of carbon concentration and microhardness of the carburised surface layer, for a sample carbonized at 950 jC and then quenched from 860 jC. Maximum surface carbon content was firstly measured and then the profiles of carbon concentration were examined in cross-section of the carbonized layers. The carbon concentration at the surface is 1.05 wt.% and decreases slowly from the surface to the interior. Accompanying this decrease in
Fig. 6. Cross-section optical micrographs for AISI 1010 steel, showing the carburised surface layer after carburising at 950 jC (a), and the plate-like carbide networks at 1050 jC (b). Etchant: 4 wt.% nitric acid in ethyl alcohol.
4
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
age of ions are in multiple charge state (+2, +3, etc.); (5) the average kinetic energy of the ions is high (10 – 100 eV). The ions from the cathode arc are accelerated by the substrate bias voltage. The atoms and ions of carbon are transferred to the surface of the sample by plasma processes. In plasma carburizing, the carbon flux ( j) is a function of the gas composition, the electric current density of the substrate (i), the gas flow (u), the gas pressure ( P) and the temperature (T), i.e.: j ¼ f ðtype of gas; i; u; P; T Þ:
Fig. 7. Profile of carbon concentration of carburised surface layer.
carbon concentration is a change in microhardness of the surface layer. At the sample surface, microhardness above HV800 has been obtained. It can be seen from our results that the cathode arc plasma carburizing is far more efficient than conventional processes. We have shown that a case depth of approximately 0.78 mm can be obtained within half an hour at 950 jC, with a surface carbon concentration of 1.05 wt.%. Besides, there is also no danger for internal oxidation and hydrogen embrittlement, because no hydrocarbons are used. 3.3. Carburizing mechanism It is reasonable to assume that the carburising process of steel is heterogeneous, just as in the case of glow discharge plasma carbonization. Firstly, fluxes of active carbon species with high dynamic energies would bombard the surface of the steel, and then penetration of carbon atoms into the samples would start at individual spots and then extend to all over the surfaces [9]. At last, diffusion of carbon atoms would take place homogeneously in the interior of the samples. However, detailed mechanism of this carbon transportation is not well understood. Rembges proposed that the carbonization process would initialize with the formation of cementite within the plasma, and then it would condense on the surface of the samples. Subsequent decomposition of the deposit would provide a source of carbon atoms [10]. Edenhofer insisted that the use of plasma would not influence the diffusion rate of carbon atoms in steel. However, a high rate of carbon species transportation in the plasma would result in a rapid increase in carbon concentration at the surface, and thus carbon diffusion would start earlier, leading to a reduction of processing time [11]. Cathode arcs have the following important physical characteristics [12 – 15]: (1) a plasma is generated by a small and intense arc spot that moves rapidly and randomly over the surface of the cathode; (2) plasma is formed from cathode material; (3) a high percentage (10 – 100%) of ions in the particles are eroded from cathode; (4) a high percent-
ð1Þ
For the given and controlled carburizing temperature, the control of the carbon flux depends only on the single parameter of electric current density, i.e.: j ¼ f ðiÞ
ð2Þ
here i stands for the electric current density of the substrate, and the gas composition, u, P, and T are all kept constant. However, in cathode arc carburizing, the control of the carbon flux depends on the cathode arc current I and the electric current density of the substrate i; i.e.: j ¼ f ðI; iÞ
ð3Þ
when argon gas is used, and the other parameters are constant. Therefore, the mechanism for generation and transportation of carbon species in cathode arc plasma carburizing process is not very much different from that of glow discharge plasma carburizing. While the excitation and ionization of carbon atoms or particles take place in the surface of the graphite by arc discharge in this method, however, the excitation and ionization of the hydrocarbon molecules only takes place in the cathode-fall region, i.e. in the vicinity of the sample in the glow discharge plasma carburizing method. Approximately 70% of carbon particles from the graphite cathode arc source can be ionized [16].
Fig. 8. Profile of hardness of the carbonized surface layer.
C. Li et al. / Surface & Coatings Technology 187 (2004) 1–5
5
The ion energy is 30 –40 eV [17] depending on the bias voltage. The carbon ions or particles will be accelerated by the bias voltage towards the substrate (the sample), and the intense bombardment on sample surface would result in a large number of defects on its surface. It is inferable that large number of point defects bound to be generated in collision cascades, which will greatly enhance the diffusion of carbon atoms. The diffusion of carbon atoms consist of the isothermal diffusion and defects enhanced diffusion. In the thermal diffusion, the diffusion coefficient is a function of composition.
2. The cathode arc source plasma carburizing is less liable to internal oxidation and hydrogen embrittlement because the carbon atoms are provided through graphite rather than hydrocarbon. 3. Besides carburizing temperature and carburising time, case depth (carburizing layer thickness) is mainly depended on substrate bias and the source –sample distance, which increases with increasing bias voltage and decreases with increasing source—sample distance.
Dth ¼ f ðq; xÞ
References
ð4Þ
where q is the carbon concentration of the carburizied layer. x is the distance to the surface of carburizied layer. Another diffusion coefficient is the function of defects density in the carburizing layer, Dd ¼ f ðq; xÞ
ð5Þ
where q is the defect density introduced from ion bombardment. The diffusion coefficient Dn of carbon are estimated by Matano-plane method [18] and fitted by least square method according to Fig. 7: Dn ¼ Dth þ Dd ¼ 1:1921010 expð0:00206xÞm2 =s
ð6Þ
4. Conclusions 1. A case depth of 0.78 mm in the cathode arc plasma carburizing can be obtained within half an hour at 950 jC. Cathode arc source plasma carburising is far more efficient than conventional processes due to the bombardment of the intense energetic carbon ions generated by d.c. arc discharge and accelerated by the bias voltage onto the surface of the substrate (sample), and the enhanced carbon diffusion resulted by the high-density defects induced by the bombardment.
[1] B. Edenhofer, W. Grafen, J. Muller-Ziller, Surf. Coat. Technol. 142 – 144 (2001) 225 – 234. [2] A.J. Hick, Heat Treat. Met. 28 (2001) 77 – 81. [3] J.M. Baek, Y.R. Cho, D.J. Kim, K.H. Lee, Surf. Coat. Technol. 131 (2000) 568 – 573. [4] E. Hantzsche, B. Juettner, IEEE Trans. Plasma Sci. 13 (1984) 230 – 234. [5] G.A. Mesyats, S.A. Barengolts, IEEE Trans. Plasma Sci. 29 (2001) 704 – 707. [6] G.P. Smith, R. Dollinger, D.P. Malone, A.A. Gilmour, J. Appl. Phys. 51 (1980) 3657 – 3662. [7] B. Juettner, J. Phys. D 14 (1981) 1265 – 1275. [8] Chinese mechanical engineering society, Handbook of metals heat treatment, 2001, 306. [9] K.T. Rie, Surf. Coat. Technol. 112 (1999) 56 – 62. [10] K.T. Rembges, J. Luhr, Carburizing: Processing and Performance, ASM International, 1989, pp. 341 – 348. [11] B. Edenhofer, Heat Treat. Met. 18 (1991) 6 – 12. [12] H. Kelly, A. Marquez, F. Minotti, C.F. Fontan, J. Phys. D: Appl. Phys. 31 (1998) 1737 – 1741. [13] S. Thomas, A. Andre, IEEE Trans. Plasma Sci. 27 (1999) 911 – 944. [14] G.A. Mesyats, IEEE Trans. Electr. Insul. 20 (1984) 729 – 734. [15] J.E. Daalder, J. Phys. D: Appl. Phys. 9 (1976) 2379 – 2395. [16] D.Y. Fang, J. Phys. D: Appl. Phys. 15 (1982) 833 – 844. [17] J.M. Laeferty, Vacuum Arcs Theory and Application, 1980. [18] Z.F. Qie, Diffusion and Phase Transformation of Solid Metals, China machine press, 1998.