- Email: [email protected]
Nitriding at low temperature
Surface and Coatings Technology 131 Ž2000. 284᎐290
Nitriding at low temperature 夽 M.P. Fewell a,U , J.M. Priest a , M.J. Baldwina , G.A. Collins b, K.T. Short b a Physics and Electronics Engineering, Uni¨ ersity of New England, Armidale, NSW 2351, Australia Australian Nuclear Science and Technology Organisation, Pri¨ ate Mail Bag 1, Menai, NSW 2234, Australia
b
Abstract This paper reports advances in the use of low-pressure rf plasmas for nitriding with an emphasis on treatments at temperatures of 250᎐450⬚C; i.e. well below those used by more conventional methods. The treatment of austenitic stainless steel AISI-316 was chosen to represent the efficacy of such plasmas for nitriding over a wide temperature range, producing thicker nitrogen-rich layers at low temperature than more conventional methods in the same process time. This is due to a lower activation energy. Application of high-voltage pulses to the workpiece Žplasma-immersion ion implantation, PI 3 . increases the thickness of the nitrogen-rich layer but does not significantly alter the activation energy. Other aspects of the process investigated include the role of hydrogen, various regimes of plasma-based cleaning, process gas purity and the variation of workpiece bias, from zero up to the 10s of kV characteristic of PI 3. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Nitriding; Low temperature; Austenitic stainless steel; Low-pressure rf plasma; PI 3
1. Introduction A major theme in the development of new plasma᎐surface-engineering treatments is the lowering of the process temperature w1x. Lower temperature means lower cost, less distortion and less surface roughening. Perhaps more importantly, certain alloys suffer degradation of their properties at the temperatures normally associated with conventional processes. For example, austenitic stainless steel can lose its corrosion resistance when nitrided at temperatures above 480⬚C, due to migration of chromium w2x. Lowering the process temperature widens the range of alloys and components that can be treated. Plasma nitriding is conventionally performed with a glow discharge at pressures of 1᎐10 mbar Ž100᎐1000
夽
An invited paper presented at the 2nd Asian᎐European International Conference on Plasma Surface Engineering, Beijing, 15᎐19 September 1999. U Corresponding author. Tel.: q61-2-6773-2388; fax: q61-2-67733413. E-mail address: [email protected] ŽM.P. Fewell..
Pa.. A negative bias voltage of between 300 V and 1000 V maintains the discharge, which is confined to a few centimetres around the workpiece. Although this bias voltage is usually pulsed to allow better temperature control and suppress arcing, there is a lower limit on the process temperature owing to the energy deposition by the ion bombardment that accompanies the discharge. A number of workers have investigated the use of low-pressure plasmas as a nitriding environment w3x. At pressures of approximately 10y3 mbar Ž0.1 Pa., a plasma is generated by a source separate from the workpiece, such as rf excitation, microwaves or energetic electrons from heated filaments, and diffuses throughout the treatment chamber. Because of the reduced collisionality at the lower pressure, the plasma contains a large number of active species, which increases the nitriding efficiency. It is not necessary to increase the power of the plasma generator as the surface area of the workpiece is increased. Also, the discharge is inherently stable and has no tendency to transform into an arc, another important advantage over conventional plasma nitriding.
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 3 - 3
M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
Under these conditions, an arbitrary bias can be applied to the workpiece to tailor the energetic ion bombardment. The ion and neutral fluxes are generally lower than those at higher pressure, so the temperature can be kept low. Ion bombardment is useful for surface activation, sputtering and controlling the structure of the nitrided surface. Even high negative biases can be used Žtypically 20᎐50 kV., so that nitrogen mass transfer is not only by thermochemical absorption but also by implantation to depths of 0.1 m. This process, which is generally called plasma immersion ion implantation ŽPIII or PI 3 TM ., allows ‘nitriding’ temperatures to be brought down further, even below 200⬚C, where the albeit shallow case is still capable of producing improvements in wear performance of some components w4x. Since high concentrations of nitrogen can be produced in the surface of a metal independently of thermochemical absorption processes, PI 3 has been applied to a range of alloys at temperatures between 200⬚C and 500⬚C w5x. In one sense, PI 3 can be considered as a duplex process. Since it uses ion energies of several 10s of kV, the duty cycle of the high voltage bias must be kept low Žtypically - 5%.. Consequently, for most of the time, the workpiece is subject only to the background flux of ions and excited neutrals from the rf plasma. This in itself can result in effective nitriding w6x. The aim of simplifying the treatment process as much as possible is a strong incentive toward reducing the bias voltage on the workpiece. This has led us to research the nitriding efficacy of the rf plasma alone, so as to understand more clearly the actual role of high-energy ion bombardment. This paper reports recent developments in the application of low-pressure rf plasmas, both with and without workpiece bias, as effective media for low temperature nitriding. The particular issues addressed are: 䢇
䢇 䢇
䢇
䢇
Is the plasma ‘activity’ sufficient to nitride over a wide temperature range? Does hydrogen enhance the process efficiency? What is the effect of various regimes of plasmabased cleaning? Do impurities in the process gas have a negative influence? What is the role of ion bombardment at energies from a few 10s of eV up to the 10s of keV characteristic of PI 3 ?
2. Experimental details The latest development of the PI 3 research equipment towards industrial application involves a hot-wall vacuum furnace w7x in which a uniform temperature is
285
maintained over a volume 500 mm in diameter and 600 mm long. A base pressure of 1᎐2 = 10y6 mbar is achieved after 1 h of pumping. A low-pressure rf glow discharge, 10 ᎐ 3 ᎐10 ᎐ 2 mbar Ž0.1᎐1 Pa., is maintained in the chamber by 200᎐300 W of radio frequency power at 13.56 MHz applied to an antenna at one end of the chamber. Negative bias can be applied to the components to give an independently controlled flux of ions that bombards all exposed surfaces. This bias can either be dc Žup to 1 kV. or pulsed Žup to 50 kV in amplitude and typically 100 s duration.. Typically, the workpieces are heated to the treatment temperature in a mixture of argon and hydrogen. Since radiative heating is less efficient at low temperatures, ion bombardment is used to assist in heating the components; this also assists in cleaning and preparation of the surfaces. The process gas is generally highpurity nitrogen but hydrogen, methane or argon can be added to assist with the treatment of particular alloys. A typical treatment lasts 3᎐5 h. Workpiece temperature is monitored by a two-colour infrared pyrometer which views the working table through a quartz window. At low values of workpiece bias, this temperature is identical to the furnace temperature. With the highenergy ion bombardment typical of PI 3 , however, there is a slight difference between the workpiece temperature and that of the furnace itself. This difference is not as great as might be imagined since secondary electrons from the workpiece also heat the furnace walls. To assist in our research, we have also developed a small UHV reactor in which samples can be nitrided under carefully controlled conditions of process-gas composition. The internal dimensions of this reactor are 400 mm in diameter and 300 mm high, and the plasma is maintained by up to 100 W of rf power at 13.56 MHz. Samples are loaded and unloaded through a vacuum load lock and are heated by a radiative element in the support stage. In this paper, we concentrate on the treatment of austenitic stainless steel AISI-316, since there are good commercial incentives to develop an efficient nitriding method for this material at relatively low temperatures Žapprox. 400⬚C. where a hard, wear-resistant surface can be produced while maintaining corrosion resistance. Typically, we use layer thickness measurements, X-ray diffraction and instrumented indentation to assess the nitriding efficiency w8x. The high chromium content of this alloy means that the treated layer is relatively thin, but easily identifiable with the full range of microscopies Žoptical, SEM, TEM and scanning probe microscopy.. The nitrogen-rich phase produced at temperatures below 480⬚C has a characteristic X-ray diffraction pattern of peaks corresponding to an expansion Žwith a slight triclinic distortion. of the fcc austenite lattice w9x. Its high hardness is dramatically revealed
M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
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by microhardness measurements with 0.05-N indents, whereas measurements at 1 N load are more sensitive to the layer thickness. For thin layers, nitrogen concentration profiles can be measured by SIMS w10x, which we calibrate by proton back-scattering w6x. Further details of the various diagnostic techniques are given in refs. w6᎐10x.
3. Results 3.1. The nitriding efficiency as a function of temperature Samples of AISI-316 were treated for a process time t of 3 h in the hot-wall rf nitriding furnace at ANSTO w7x at temperatures T between 300⬚C and 550⬚C with a pure nitrogen plasma at a pressure of 4 = 10y3 mbar Ž0.4 Pa. and a dc bias of y250 V. The samples were sectioned to measure the thickness d of the treated layer. The results are shown as filled circles in Fig. 1, which is an Arrhenius plot Ž d 2rt as a function of 1rT .. The layers produced at temperatures below 500⬚C consisted of the nitrogen-rich expanded austenite phase w9x, which is transformed to CrN and ␣-Fe at higher temperatures. However, Fig. 1 shows that the diffusion process controlling the layer thickness is similar over the whole temperature range, and can be described by E d2 s D⬁exp y a , t kT
ž
/
where D⬁ is the effective diffusion coefficient at infinite temperature and Ea is the activation energy.
Fig. 1. Arrhenius plot for data obtained from PI 3 nitriding Žfilled triangles ., rf-plasma nitriding in the hot-wall furnace Žfilled circles. and in the UHV chamber Žopen squares.. The behaviour reported for pulsed dc glow nitriding ŽPPN. and high flux-ion implantation ŽBII. is also indicated.
Fig. 2. Ža. Expansion and Žb. distortion of the austenite lattice using data from rf-nitriding in the UHV chamber Žopen squares. and from PI 3 experiments Žfilled triangles..
The lower end of the temperature range was explored further in the UHV chamber at UNE with a pure nitrogen plasma at same pressure of 0.4 Pa and the samples grounded to the chamber walls. SIMS w10x was used to measure the thin layers on these very low temperature samples. These data are also shown in Fig. 1 Žopen squares. and indicate that the nitriding process is similar in both chambers and extends down to 250⬚C. If the 300⬚C sample is excluded from the data obtained on the hot wall furnace, an activation energy of 70 " 3 kJ mol ᎐ 1 fits both sets of data. The expansion and triclinic distortion w9x of the austenite lattice are shown as a function of temperature as open squares in Fig. 2. These data were obtained by XRD from a set of samples treated in the UHV chamber at UNE. The expansion and distortion are greatest at the lowest temperatures, indicating that, although the layer is thin, there is a relatively high concentration of nitrogen. We have also investigated the plasma parameters as a function of temperature in these devices using Langmuir probes and optical emission spectroscopy w11x. Fig. 3 shows the plasma potential and electron temperature as a function of temperature in the hot-wall furnace. There is a steady increase with temperature in both these quantities to a maximum of approximately 400⬚C,
M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
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hydrogen prior to nitriding. These samples indicate improved nitriding efficiency over the no-hydrogen situation, but neither is as effective as the addition of hydrogen to the process gas. Increased cleaning time results in an increase in surface concentration of nitrogen in the treated layer but no change in the depth of the layer, suggesting that the effect of the hydrogen cleaning is confined to the near-surface region. In contrast, addition of hydrogen to the process gas increases both the surface concentration and the depth of nitrogen, and gives the greatest lattice expansion ŽFig. 4b.. 3.3. The effect of impurities in the process gas Fig. 3. Plasma potential Žtriangles. and electron temperature Žcircles. as a function of temperature in the hot-wall furnace.
indicating that the population of high-energy electrons Žabove 10 eV. increases in density. Interestingly, this correlates with results from our fundamental investigations on the surface interactions of excited nitrogen species, which showed enhanced metastable-particle induced secondary electron production from stainless steel with temperature w7x.
One of the questions that must be asked in operating at low pressures is: How clean must the process chamber and the process gas be? Early experiments in the hot-wall furnace showed that nitriding efficiency was severely impaired by the presence of small amounts of oxygen w7x; workpiece biases of a few hundred volts were necessary during the nitriding process to inhibit the formation of oxide layers and obtain reproducible
3.2. The role of hydrogen Since it is common to use hydrogen in conventional dc glow discharge nitriding, we have investigated the effect of adding hydrogen to the process gas. The results, which are presented in detail elsewhere w8x, show that, provided the partial pressure of nitrogen is held constant, the addition of hydrogen at concentration in the range of 5᎐50% results in thicker layers and enhanced surface hardness compared with treatments in pure nitrogen. Excessive amounts of hydrogen Ž; 75%. retard the nitriding process. Optical-emission spectroscopy indicates that the addition of hydrogen does not increase the concentration of active nitriding species, indeed the reverse is true. Hence the beneficial effect of hydrogen must be due to the action of hydrogen atoms and molecules at the workpiece surface. Fig. 4 shows nitrogen concentration profiles obtained from SIMS w10x calibrated by proton back-scattering for samples treated in various gas mixtures in the UHV chamber at UNE ŽT s 400⬚C, t s 3 h, workpiece grounded.. Also shown in Fig. 4 are the XRD patterns around the Ž111. austenite peak which clearly show the expansion of the austenite lattice in the nitrogen-rich surface layer. There is a substantial improvement over the pure nitrogen treatment by the inclusion of 25% hydrogen in the process gas. Also shown in Fig. 4 are results for samples treated in pure nitrogen with 1 h and 3 h of treatment in pure
Fig. 4. Ža. SIMS nitrogen profiles and Žb. X-ray diffraction patterns around the Ž200. austenite peak ŽCu K ␣ radiation. for various gas-composition and cleaning regimes in the UHV chamber.
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M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
Fig. 5. Layer thickness for various gas-composition and cleaning regimes in the UHV chamber using both Matheson and industrial grades of nitrogen.
results. However, the results from the UHV chamber shown in Figs. 1, 2 and 4 were obtained with a grounded workpiece, i.e. only a few 10s of eV of ion bombardment resulting from the plasma potential ŽFig. 3.. This indicates that, in clean conditions, oxide formation is not a problem. To investigate this point further, the experiments in Section 3.2 were repeated with industrial-grade nitrogen in place of the Matheson-grade nitrogen Ž- 5 ppm oxygen. used above. Fig. 5 shows the layer thickness obtained with both grades of gas with no hydrogen cleaning, with hydrogen pre-clean and with hydrogen added to the process gas. The results are essentially identical to those shown in Fig. 4; there is no significant difference in using the industrial- grade gas. This may be because the actual impurity content of this gas is low Žspecifications state that the oxygen content is - 0.2%, but the actual assay of the gas supplied is not known., but does indicate that the requirements on the gas quality are not particularly stringent.
voltage for AISI-3136 treated in the hot-wall chamber at 400⬚C for 5 h at a pressure of 0.01 mbar Ž1 Pa.. These data are extracted from the Taguchi array but agree with specific data points also shown in Fig. 6a. There is a slight increase in surface hardness, most likely due to ion bombardment creating a high surface concentration of nitrogen, but there is a decrease in the 1 N microhardness, indicating a detrimental effect on the nitriding efficiency and a decrease in the layer thickness. Turning to higher bias voltages, which must be pulsed, PI 3 experiments have shown that a substantial increase in nitriding efficiency is possible w4x. Data from a series of 5 h PI 3 treatments at a negative bias voltage of 35 kV and average nitrogen dose rate of 2.25= 10 14 cmy2 sy1 w13x are shown in Figs. 1 and 2 Žfilled triangles.. There is very little change in the activation energy, as seen in Fig. 1 Ž88 " 6 kJ moly1 for the PI 3 data., but there is an increase in the thickness of the nitrogen-rich layer. This indicates a higher rate of nitrogen influx in PI 3 , created by the high energy ion bombardment. The point is dramatically highlighted by the XRD results presented in Fig. 2 for this set of samples. The expansion of the austenite lattice is high Žup to 10%. over the whole temperature range and the consequent tri-
3.4. The role of ion bombardment The effects of pressure and bias voltage were investigated using a Taguchi L9 parametric array w12x in the hot-wall furnace with temperature and time being the dominating variables. It appears that pressure has very little effect on the nitriding efficiency, a result confirmed by a scan of pressures up to 0.1 mbar Ž10 Pa.. This result is surprising since we know that the plasma becomes confined to the antenna at the top of the furnace when the pressure increases much above 0.01 mbar Ž1 Pa.. This implies that the ions are not necessary for effective nitriding of austenitic stainless steel. A scan of bias voltages up to 400 V obtained from the Taguchi array supports this. Fig. 6a shows 50 mN and 1 N microhardness measurements as a function of bias
Fig. 6. Microhardness for AISI-316 treated in the hot-wall furnace for 5 h at 400⬚C as a function of Ža. dc bias to workpiece and Žb. PI 3 voltage. The data points Žcircles. are extracted from Taguchi L9 experimental arrays. The grey squares indicate specific experimental data.
M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
clinic distortion is very large, particularly at the higher temperatures. This indicates a very high concentration of nitrogen in the expanded austenite. Another Taguchi L9 array, using temperature, time, PI 3 voltage and nitrogen ion dose rate as variables, has systematically investigated the exact role that high energy ion bombardment plays. Only preliminary results are available so far; some of these are shown in Fig. 6b for the application of HV biases between 5 and 25 kV for a 5-h treatment at 400⬚C with 0.01 mbar Ž1 Pa. nitrogen and an average nitrogen dose rate of 2.25= 10 14 cmy2 sy1. The results from rf nitriding Žy250 V dc bias. are also shown in Fig. 6b. There is a slight increase in the 50 mN microhardness but a more substantial increase in the 1 N result, reflecting the thicker layers created by PI 3 ŽFig. 1.. All the data analysed to date indicate that there is an optimum bias voltage for the treatment of AISI-316 of approximately 15 kV.
4. Discussion There are several distinctive characteristics of nitriding using low-pressure rf plasmas. Most importantly, treatment temperatures can be extended well below those usually associated with nitriding: a ‘nitrided’ layer is produced at temperatures as low as 250⬚C. This is due to the relatively low value of activation energy, as shown in Fig. 1. For comparison, we also indicate in Fig. 1 results obtained from pulsed dc-glow discharge nitriding Žactivation energy of 110 kJ moly1 . w14x and high flux, low-energy ion beam implantation Žactivation energy of 190 kJ moly1 . w15x. Even though both of these produce thicker modified layers at higher temperatures, comparable to those obtained by PI 3, the higher activation energy means that they are less effective at low temperature than low-pressure rf nitriding. Secondly, operating pressures are very low, typically three orders of magnitude lower than those used for conventional plasma nitriding and six orders of magnitude lower than those used for gaseous nitriding. Pure nitrogen can be used, although small admixtures of hydrogen increase the nitriding efficiency. The low pressure means that the plasmas are inherently susceptible to contamination by outgassing from the reactor walls, but the results presented here indicate that this is not a significant effect. Cleaning in hydrogen or argonrhydrogen mixtures also improves the nitriding efficiency. Thirdly, the separation of workpiece bias from plasma generation reduces the likelihood of arcing and allows better control of the process, and also means that almost any substrate bias can be used, as may best enhance the properties of the nitrided layer. In particular, high voltage biases ŽPI 3 . can be used to create very
289
high concentrations of nitrogen in the first 100 nm, independent of the treatment temperature. The subsequent fate of the nitrogen is, however, strongly dependent on the temperature, the composition of the alloy and its structure. There are some important points to be considered for the industrial application of the process. In particular, the cost of high-vacuum conditions, and high-voltage pulsing in the case of PI 3 , must be shown to provide advantages commensurate with the investment. Another way of phrasing this is to ask: How large are the potential markets for the new applications created by the availability of a low-temperature nitriding process?
5. Conclusions Low-pressure rf plasmas are effective environments for nitriding over a very wide temperature range Ž200᎐500⬚C.. This treatment method produces thicker nitrogen-rich layers at low temperature than more conventional methods for a given process time. This is due to a lower activation energy, indicating that different aspects of the nitriding process are rate limiting with low-pressure rf plasmas than with other treatment methods. Application of high-voltage pulses to the workpiece Žplasma-immersion ion implantation. increases the thickness of the nitrogen-rich layer over that obtained with the workpiece grounded to the chamber walls, but does not significantly increase the activation energy for the process.
Acknowledgements This research is supported by the Australian Research Council, the Australian Institute of Nuclear Science and Engineering and by Australian Postgraduate Research Awards. References w1x K.T. Rie, E. Menthe, A. Matthews, K. Legg, J. Chin, MRS Bull. 21 Ž1996. 46. w2x Z.L. Zhang, T. Bell, Surf. Eng. 1 Ž1985. 131. w3x T. Czerwiec, H. Michel, E. Bergmann, Surf. Coat. Technol. 108r109 Ž1998. 182. w4x G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Surf. Coat. Technol. 103r104 Ž1998. 212. w5x G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Heat Treatment Metals 4 Ž1995. 91. w6x M.J. Baldwin, G.A. Collins, M.P. Fewell, S.C. Haydon, S. Kumar, K.T. Short, J. Tendys, Jpn. J. Appl. Phys. 36 Ž1997. 4941. w7x J.M. Priest, M.J. Baldwin, M.P. Fewell, S.C. Haydon, G.A. Collins, K.T. Short, J. Tendys, Thin Solid Films 345 Ž1999. 113. w8x M.J. Baldwin, M.P. Fewell, S.C. Haydon, S. Kumar, G.A. Collins, K.T. Short, J. Tendys, Surf. Coat. Technol. 123 Ž2000. 29.
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w9x M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, Surf. Coat. Technol. 131 Ž2000. 300. w10x M.J. Baldwin, S. Kumar, J.M. Priest, M.P. Fewell, K.E. Prince, K.T. Short, Thin Solid Films 345 Ž1999. 108. w11x J.M. Priest, G.A. Collins, K.T. Short, M.P. Fewell, M.J. Baldwin, in: Proceedings of the 22nd AINSE Plasma Science and Technology Conference, Canberra, February 1999, ISSN 13246313, p. 45. w12x G. Taguchi, System of Experimental Design: Engineering Methods to Optimize Quality and Minimize Costs, UNIPUBrKraus Int. Publications, White Plains, NY, 1987.
w13x G.A. Collins, D.R. Mitchell, K.T. Short, J. Tendys, E. Menthe, K.-T. Rie, Formation and transformation of nitrogen-rich phases in austenitic stainless steel, in: Proceedings of the 6th International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, September 1998, Surf. Coat. Technol. Žsubmitted.. w14x E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116r119 Ž1999. 199. ¨ ¨ R. Wei, P.J. Wilbur, Surf. Coat. w15x D.L. Williamson, O. Ozturk, Technol. 65 Ž1994. 15.
Nitriding at low temperature 夽 M.P. Fewell a,U , J.M. Priest a , M.J. Baldwina , G.A. Collins b, K.T. Short b a Physics and Electronics Engineering, Uni¨ ersity of New England, Armidale, NSW 2351, Australia Australian Nuclear Science and Technology Organisation, Pri¨ ate Mail Bag 1, Menai, NSW 2234, Australia
b
Abstract This paper reports advances in the use of low-pressure rf plasmas for nitriding with an emphasis on treatments at temperatures of 250᎐450⬚C; i.e. well below those used by more conventional methods. The treatment of austenitic stainless steel AISI-316 was chosen to represent the efficacy of such plasmas for nitriding over a wide temperature range, producing thicker nitrogen-rich layers at low temperature than more conventional methods in the same process time. This is due to a lower activation energy. Application of high-voltage pulses to the workpiece Žplasma-immersion ion implantation, PI 3 . increases the thickness of the nitrogen-rich layer but does not significantly alter the activation energy. Other aspects of the process investigated include the role of hydrogen, various regimes of plasma-based cleaning, process gas purity and the variation of workpiece bias, from zero up to the 10s of kV characteristic of PI 3. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Nitriding; Low temperature; Austenitic stainless steel; Low-pressure rf plasma; PI 3
1. Introduction A major theme in the development of new plasma᎐surface-engineering treatments is the lowering of the process temperature w1x. Lower temperature means lower cost, less distortion and less surface roughening. Perhaps more importantly, certain alloys suffer degradation of their properties at the temperatures normally associated with conventional processes. For example, austenitic stainless steel can lose its corrosion resistance when nitrided at temperatures above 480⬚C, due to migration of chromium w2x. Lowering the process temperature widens the range of alloys and components that can be treated. Plasma nitriding is conventionally performed with a glow discharge at pressures of 1᎐10 mbar Ž100᎐1000
夽
An invited paper presented at the 2nd Asian᎐European International Conference on Plasma Surface Engineering, Beijing, 15᎐19 September 1999. U Corresponding author. Tel.: q61-2-6773-2388; fax: q61-2-67733413. E-mail address: [email protected] ŽM.P. Fewell..
Pa.. A negative bias voltage of between 300 V and 1000 V maintains the discharge, which is confined to a few centimetres around the workpiece. Although this bias voltage is usually pulsed to allow better temperature control and suppress arcing, there is a lower limit on the process temperature owing to the energy deposition by the ion bombardment that accompanies the discharge. A number of workers have investigated the use of low-pressure plasmas as a nitriding environment w3x. At pressures of approximately 10y3 mbar Ž0.1 Pa., a plasma is generated by a source separate from the workpiece, such as rf excitation, microwaves or energetic electrons from heated filaments, and diffuses throughout the treatment chamber. Because of the reduced collisionality at the lower pressure, the plasma contains a large number of active species, which increases the nitriding efficiency. It is not necessary to increase the power of the plasma generator as the surface area of the workpiece is increased. Also, the discharge is inherently stable and has no tendency to transform into an arc, another important advantage over conventional plasma nitriding.
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 9 3 - 3
M.P. Fewell et al. r Surface and Coatings Technology 131 (2000) 284᎐290
Under these conditions, an arbitrary bias can be applied to the workpiece to tailor the energetic ion bombardment. The ion and neutral fluxes are generally lower than those at higher pressure, so the temperature can be kept low. Ion bombardment is useful for surface activation, sputtering and controlling the structure of the nitrided surface. Even high negative biases can be used Žtypically 20᎐50 kV., so that nitrogen mass transfer is not only by thermochemical absorption but also by implantation to depths of 0.1 m. This process, which is generally called plasma immersion ion implantation ŽPIII or PI 3 TM ., allows ‘nitriding’ temperatures to be brought down further, even below 200⬚C, where the albeit shallow case is still capable of producing improvements in wear performance of some components w4x. Since high concentrations of nitrogen can be produced in the surface of a metal independently of thermochemical absorption processes, PI 3 has been applied to a range of alloys at temperatures between 200⬚C and 500⬚C w5x. In one sense, PI 3 can be considered as a duplex process. Since it uses ion energies of several 10s of kV, the duty cycle of the high voltage bias must be kept low Žtypically - 5%.. Consequently, for most of the time, the workpiece is subject only to the background flux of ions and excited neutrals from the rf plasma. This in itself can result in effective nitriding w6x. The aim of simplifying the treatment process as much as possible is a strong incentive toward reducing the bias voltage on the workpiece. This has led us to research the nitriding efficacy of the rf plasma alone, so as to understand more clearly the actual role of high-energy ion bombardment. This paper reports recent developments in the application of low-pressure rf plasmas, both with and without workpiece bias, as effective media for low temperature nitriding. The particular issues addressed are: 䢇
䢇 䢇
䢇
䢇
Is the plasma ‘activity’ sufficient to nitride over a wide temperature range? Does hydrogen enhance the process efficiency? What is the effect of various regimes of plasmabased cleaning? Do impurities in the process gas have a negative influence? What is the role of ion bombardment at energies from a few 10s of eV up to the 10s of keV characteristic of PI 3 ?
2. Experimental details The latest development of the PI 3 research equipment towards industrial application involves a hot-wall vacuum furnace w7x in which a uniform temperature is
285
maintained over a volume 500 mm in diameter and 600 mm long. A base pressure of 1᎐2 = 10y6 mbar is achieved after 1 h of pumping. A low-pressure rf glow discharge, 10 ᎐ 3 ᎐10 ᎐ 2 mbar Ž0.1᎐1 Pa., is maintained in the chamber by 200᎐300 W of radio frequency power at 13.56 MHz applied to an antenna at one end of the chamber. Negative bias can be applied to the components to give an independently controlled flux of ions that bombards all exposed surfaces. This bias can either be dc Žup to 1 kV. or pulsed Žup to 50 kV in amplitude and typically 100 s duration.. Typically, the workpieces are heated to the treatment temperature in a mixture of argon and hydrogen. Since radiative heating is less efficient at low temperatures, ion bombardment is used to assist in heating the components; this also assists in cleaning and preparation of the surfaces. The process gas is generally highpurity nitrogen but hydrogen, methane or argon can be added to assist with the treatment of particular alloys. A typical treatment lasts 3᎐5 h. Workpiece temperature is monitored by a two-colour infrared pyrometer which views the working table through a quartz window. At low values of workpiece bias, this temperature is identical to the furnace temperature. With the highenergy ion bombardment typical of PI 3 , however, there is a slight difference between the workpiece temperature and that of the furnace itself. This difference is not as great as might be imagined since secondary electrons from the workpiece also heat the furnace walls. To assist in our research, we have also developed a small UHV reactor in which samples can be nitrided under carefully controlled conditions of process-gas composition. The internal dimensions of this reactor are 400 mm in diameter and 300 mm high, and the plasma is maintained by up to 100 W of rf power at 13.56 MHz. Samples are loaded and unloaded through a vacuum load lock and are heated by a radiative element in the support stage. In this paper, we concentrate on the treatment of austenitic stainless steel AISI-316, since there are good commercial incentives to develop an efficient nitriding method for this material at relatively low temperatures Žapprox. 400⬚C. where a hard, wear-resistant surface can be produced while maintaining corrosion resistance. Typically, we use layer thickness measurements, X-ray diffraction and instrumented indentation to assess the nitriding efficiency w8x. The high chromium content of this alloy means that the treated layer is relatively thin, but easily identifiable with the full range of microscopies Žoptical, SEM, TEM and scanning probe microscopy.. The nitrogen-rich phase produced at temperatures below 480⬚C has a characteristic X-ray diffraction pattern of peaks corresponding to an expansion Žwith a slight triclinic distortion. of the fcc austenite lattice w9x. Its high hardness is dramatically revealed
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by microhardness measurements with 0.05-N indents, whereas measurements at 1 N load are more sensitive to the layer thickness. For thin layers, nitrogen concentration profiles can be measured by SIMS w10x, which we calibrate by proton back-scattering w6x. Further details of the various diagnostic techniques are given in refs. w6᎐10x.
3. Results 3.1. The nitriding efficiency as a function of temperature Samples of AISI-316 were treated for a process time t of 3 h in the hot-wall rf nitriding furnace at ANSTO w7x at temperatures T between 300⬚C and 550⬚C with a pure nitrogen plasma at a pressure of 4 = 10y3 mbar Ž0.4 Pa. and a dc bias of y250 V. The samples were sectioned to measure the thickness d of the treated layer. The results are shown as filled circles in Fig. 1, which is an Arrhenius plot Ž d 2rt as a function of 1rT .. The layers produced at temperatures below 500⬚C consisted of the nitrogen-rich expanded austenite phase w9x, which is transformed to CrN and ␣-Fe at higher temperatures. However, Fig. 1 shows that the diffusion process controlling the layer thickness is similar over the whole temperature range, and can be described by E d2 s D⬁exp y a , t kT
ž
/
where D⬁ is the effective diffusion coefficient at infinite temperature and Ea is the activation energy.
Fig. 1. Arrhenius plot for data obtained from PI 3 nitriding Žfilled triangles ., rf-plasma nitriding in the hot-wall furnace Žfilled circles. and in the UHV chamber Žopen squares.. The behaviour reported for pulsed dc glow nitriding ŽPPN. and high flux-ion implantation ŽBII. is also indicated.
Fig. 2. Ža. Expansion and Žb. distortion of the austenite lattice using data from rf-nitriding in the UHV chamber Žopen squares. and from PI 3 experiments Žfilled triangles..
The lower end of the temperature range was explored further in the UHV chamber at UNE with a pure nitrogen plasma at same pressure of 0.4 Pa and the samples grounded to the chamber walls. SIMS w10x was used to measure the thin layers on these very low temperature samples. These data are also shown in Fig. 1 Žopen squares. and indicate that the nitriding process is similar in both chambers and extends down to 250⬚C. If the 300⬚C sample is excluded from the data obtained on the hot wall furnace, an activation energy of 70 " 3 kJ mol ᎐ 1 fits both sets of data. The expansion and triclinic distortion w9x of the austenite lattice are shown as a function of temperature as open squares in Fig. 2. These data were obtained by XRD from a set of samples treated in the UHV chamber at UNE. The expansion and distortion are greatest at the lowest temperatures, indicating that, although the layer is thin, there is a relatively high concentration of nitrogen. We have also investigated the plasma parameters as a function of temperature in these devices using Langmuir probes and optical emission spectroscopy w11x. Fig. 3 shows the plasma potential and electron temperature as a function of temperature in the hot-wall furnace. There is a steady increase with temperature in both these quantities to a maximum of approximately 400⬚C,
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hydrogen prior to nitriding. These samples indicate improved nitriding efficiency over the no-hydrogen situation, but neither is as effective as the addition of hydrogen to the process gas. Increased cleaning time results in an increase in surface concentration of nitrogen in the treated layer but no change in the depth of the layer, suggesting that the effect of the hydrogen cleaning is confined to the near-surface region. In contrast, addition of hydrogen to the process gas increases both the surface concentration and the depth of nitrogen, and gives the greatest lattice expansion ŽFig. 4b.. 3.3. The effect of impurities in the process gas Fig. 3. Plasma potential Žtriangles. and electron temperature Žcircles. as a function of temperature in the hot-wall furnace.
indicating that the population of high-energy electrons Žabove 10 eV. increases in density. Interestingly, this correlates with results from our fundamental investigations on the surface interactions of excited nitrogen species, which showed enhanced metastable-particle induced secondary electron production from stainless steel with temperature w7x.
One of the questions that must be asked in operating at low pressures is: How clean must the process chamber and the process gas be? Early experiments in the hot-wall furnace showed that nitriding efficiency was severely impaired by the presence of small amounts of oxygen w7x; workpiece biases of a few hundred volts were necessary during the nitriding process to inhibit the formation of oxide layers and obtain reproducible
3.2. The role of hydrogen Since it is common to use hydrogen in conventional dc glow discharge nitriding, we have investigated the effect of adding hydrogen to the process gas. The results, which are presented in detail elsewhere w8x, show that, provided the partial pressure of nitrogen is held constant, the addition of hydrogen at concentration in the range of 5᎐50% results in thicker layers and enhanced surface hardness compared with treatments in pure nitrogen. Excessive amounts of hydrogen Ž; 75%. retard the nitriding process. Optical-emission spectroscopy indicates that the addition of hydrogen does not increase the concentration of active nitriding species, indeed the reverse is true. Hence the beneficial effect of hydrogen must be due to the action of hydrogen atoms and molecules at the workpiece surface. Fig. 4 shows nitrogen concentration profiles obtained from SIMS w10x calibrated by proton back-scattering for samples treated in various gas mixtures in the UHV chamber at UNE ŽT s 400⬚C, t s 3 h, workpiece grounded.. Also shown in Fig. 4 are the XRD patterns around the Ž111. austenite peak which clearly show the expansion of the austenite lattice in the nitrogen-rich surface layer. There is a substantial improvement over the pure nitrogen treatment by the inclusion of 25% hydrogen in the process gas. Also shown in Fig. 4 are results for samples treated in pure nitrogen with 1 h and 3 h of treatment in pure
Fig. 4. Ža. SIMS nitrogen profiles and Žb. X-ray diffraction patterns around the Ž200. austenite peak ŽCu K ␣ radiation. for various gas-composition and cleaning regimes in the UHV chamber.
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Fig. 5. Layer thickness for various gas-composition and cleaning regimes in the UHV chamber using both Matheson and industrial grades of nitrogen.
results. However, the results from the UHV chamber shown in Figs. 1, 2 and 4 were obtained with a grounded workpiece, i.e. only a few 10s of eV of ion bombardment resulting from the plasma potential ŽFig. 3.. This indicates that, in clean conditions, oxide formation is not a problem. To investigate this point further, the experiments in Section 3.2 were repeated with industrial-grade nitrogen in place of the Matheson-grade nitrogen Ž- 5 ppm oxygen. used above. Fig. 5 shows the layer thickness obtained with both grades of gas with no hydrogen cleaning, with hydrogen pre-clean and with hydrogen added to the process gas. The results are essentially identical to those shown in Fig. 4; there is no significant difference in using the industrial- grade gas. This may be because the actual impurity content of this gas is low Žspecifications state that the oxygen content is - 0.2%, but the actual assay of the gas supplied is not known., but does indicate that the requirements on the gas quality are not particularly stringent.
voltage for AISI-3136 treated in the hot-wall chamber at 400⬚C for 5 h at a pressure of 0.01 mbar Ž1 Pa.. These data are extracted from the Taguchi array but agree with specific data points also shown in Fig. 6a. There is a slight increase in surface hardness, most likely due to ion bombardment creating a high surface concentration of nitrogen, but there is a decrease in the 1 N microhardness, indicating a detrimental effect on the nitriding efficiency and a decrease in the layer thickness. Turning to higher bias voltages, which must be pulsed, PI 3 experiments have shown that a substantial increase in nitriding efficiency is possible w4x. Data from a series of 5 h PI 3 treatments at a negative bias voltage of 35 kV and average nitrogen dose rate of 2.25= 10 14 cmy2 sy1 w13x are shown in Figs. 1 and 2 Žfilled triangles.. There is very little change in the activation energy, as seen in Fig. 1 Ž88 " 6 kJ moly1 for the PI 3 data., but there is an increase in the thickness of the nitrogen-rich layer. This indicates a higher rate of nitrogen influx in PI 3 , created by the high energy ion bombardment. The point is dramatically highlighted by the XRD results presented in Fig. 2 for this set of samples. The expansion of the austenite lattice is high Žup to 10%. over the whole temperature range and the consequent tri-
3.4. The role of ion bombardment The effects of pressure and bias voltage were investigated using a Taguchi L9 parametric array w12x in the hot-wall furnace with temperature and time being the dominating variables. It appears that pressure has very little effect on the nitriding efficiency, a result confirmed by a scan of pressures up to 0.1 mbar Ž10 Pa.. This result is surprising since we know that the plasma becomes confined to the antenna at the top of the furnace when the pressure increases much above 0.01 mbar Ž1 Pa.. This implies that the ions are not necessary for effective nitriding of austenitic stainless steel. A scan of bias voltages up to 400 V obtained from the Taguchi array supports this. Fig. 6a shows 50 mN and 1 N microhardness measurements as a function of bias
Fig. 6. Microhardness for AISI-316 treated in the hot-wall furnace for 5 h at 400⬚C as a function of Ža. dc bias to workpiece and Žb. PI 3 voltage. The data points Žcircles. are extracted from Taguchi L9 experimental arrays. The grey squares indicate specific experimental data.
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clinic distortion is very large, particularly at the higher temperatures. This indicates a very high concentration of nitrogen in the expanded austenite. Another Taguchi L9 array, using temperature, time, PI 3 voltage and nitrogen ion dose rate as variables, has systematically investigated the exact role that high energy ion bombardment plays. Only preliminary results are available so far; some of these are shown in Fig. 6b for the application of HV biases between 5 and 25 kV for a 5-h treatment at 400⬚C with 0.01 mbar Ž1 Pa. nitrogen and an average nitrogen dose rate of 2.25= 10 14 cmy2 sy1. The results from rf nitriding Žy250 V dc bias. are also shown in Fig. 6b. There is a slight increase in the 50 mN microhardness but a more substantial increase in the 1 N result, reflecting the thicker layers created by PI 3 ŽFig. 1.. All the data analysed to date indicate that there is an optimum bias voltage for the treatment of AISI-316 of approximately 15 kV.
4. Discussion There are several distinctive characteristics of nitriding using low-pressure rf plasmas. Most importantly, treatment temperatures can be extended well below those usually associated with nitriding: a ‘nitrided’ layer is produced at temperatures as low as 250⬚C. This is due to the relatively low value of activation energy, as shown in Fig. 1. For comparison, we also indicate in Fig. 1 results obtained from pulsed dc-glow discharge nitriding Žactivation energy of 110 kJ moly1 . w14x and high flux, low-energy ion beam implantation Žactivation energy of 190 kJ moly1 . w15x. Even though both of these produce thicker modified layers at higher temperatures, comparable to those obtained by PI 3, the higher activation energy means that they are less effective at low temperature than low-pressure rf nitriding. Secondly, operating pressures are very low, typically three orders of magnitude lower than those used for conventional plasma nitriding and six orders of magnitude lower than those used for gaseous nitriding. Pure nitrogen can be used, although small admixtures of hydrogen increase the nitriding efficiency. The low pressure means that the plasmas are inherently susceptible to contamination by outgassing from the reactor walls, but the results presented here indicate that this is not a significant effect. Cleaning in hydrogen or argonrhydrogen mixtures also improves the nitriding efficiency. Thirdly, the separation of workpiece bias from plasma generation reduces the likelihood of arcing and allows better control of the process, and also means that almost any substrate bias can be used, as may best enhance the properties of the nitrided layer. In particular, high voltage biases ŽPI 3 . can be used to create very
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high concentrations of nitrogen in the first 100 nm, independent of the treatment temperature. The subsequent fate of the nitrogen is, however, strongly dependent on the temperature, the composition of the alloy and its structure. There are some important points to be considered for the industrial application of the process. In particular, the cost of high-vacuum conditions, and high-voltage pulsing in the case of PI 3 , must be shown to provide advantages commensurate with the investment. Another way of phrasing this is to ask: How large are the potential markets for the new applications created by the availability of a low-temperature nitriding process?
5. Conclusions Low-pressure rf plasmas are effective environments for nitriding over a very wide temperature range Ž200᎐500⬚C.. This treatment method produces thicker nitrogen-rich layers at low temperature than more conventional methods for a given process time. This is due to a lower activation energy, indicating that different aspects of the nitriding process are rate limiting with low-pressure rf plasmas than with other treatment methods. Application of high-voltage pulses to the workpiece Žplasma-immersion ion implantation. increases the thickness of the nitrogen-rich layer over that obtained with the workpiece grounded to the chamber walls, but does not significantly increase the activation energy for the process.
Acknowledgements This research is supported by the Australian Research Council, the Australian Institute of Nuclear Science and Engineering and by Australian Postgraduate Research Awards. References w1x K.T. Rie, E. Menthe, A. Matthews, K. Legg, J. Chin, MRS Bull. 21 Ž1996. 46. w2x Z.L. Zhang, T. Bell, Surf. Eng. 1 Ž1985. 131. w3x T. Czerwiec, H. Michel, E. Bergmann, Surf. Coat. Technol. 108r109 Ž1998. 182. w4x G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Surf. Coat. Technol. 103r104 Ž1998. 212. w5x G.A. Collins, R. Hutchings, K.T. Short, J. Tendys, Heat Treatment Metals 4 Ž1995. 91. w6x M.J. Baldwin, G.A. Collins, M.P. Fewell, S.C. Haydon, S. Kumar, K.T. Short, J. Tendys, Jpn. J. Appl. Phys. 36 Ž1997. 4941. w7x J.M. Priest, M.J. Baldwin, M.P. Fewell, S.C. Haydon, G.A. Collins, K.T. Short, J. Tendys, Thin Solid Films 345 Ž1999. 113. w8x M.J. Baldwin, M.P. Fewell, S.C. Haydon, S. Kumar, G.A. Collins, K.T. Short, J. Tendys, Surf. Coat. Technol. 123 Ž2000. 29.
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w9x M.P. Fewell, D.R.G. Mitchell, J.M. Priest, K.T. Short, G.A. Collins, Surf. Coat. Technol. 131 Ž2000. 300. w10x M.J. Baldwin, S. Kumar, J.M. Priest, M.P. Fewell, K.E. Prince, K.T. Short, Thin Solid Films 345 Ž1999. 108. w11x J.M. Priest, G.A. Collins, K.T. Short, M.P. Fewell, M.J. Baldwin, in: Proceedings of the 22nd AINSE Plasma Science and Technology Conference, Canberra, February 1999, ISSN 13246313, p. 45. w12x G. Taguchi, System of Experimental Design: Engineering Methods to Optimize Quality and Minimize Costs, UNIPUBrKraus Int. Publications, White Plains, NY, 1987.
w13x G.A. Collins, D.R. Mitchell, K.T. Short, J. Tendys, E. Menthe, K.-T. Rie, Formation and transformation of nitrogen-rich phases in austenitic stainless steel, in: Proceedings of the 6th International Conference on Plasma Surface Engineering, Garmisch-Partenkirchen, September 1998, Surf. Coat. Technol. Žsubmitted.. w14x E. Menthe, K.-T. Rie, Surf. Coat. Technol. 116r119 Ž1999. 199. ¨ ¨ R. Wei, P.J. Wilbur, Surf. Coat. w15x D.L. Williamson, O. Ozturk, Technol. 65 Ž1994. 15.