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The structure and properties of chromium diboride coatings deposited by pulsed magnetron sputtering of powder targets
Surface & Coatings Technology 200 (2005) 1366 – 1371 www.elsevier.com/locate/surfcoat
The structure and properties of chromium diboride coatings deposited by pulsed magnetron sputtering of powder targets M. Audronis a,*, P.J. Kelly b, R.D. Arnell c, A. Leyland a, A. Matthews a a
Department of Engineering Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK b Surface Engineering Group, Manchester Metropolitan University, Manchester M1 5GD, UK c Institute for Materials Research, University of Salford, Salford M5 4WT, UK Available online 13 September 2005
Abstract Metal boride coatings are attracting increased interest, due to the fact that they combine high hardness with good wear and corrosion resistance. These coatings are often deposited by the sputtering of hot-pressed or sintered ceramic targets. However, targets of this nature can be prone to cracking and, since the user is limited to a single composition per target, do not offer the opportunity to readily vary film stoichiometry. Titanium and zirconium borides have been widely studied; however, chromium diboride is recognised as offering superior performance in corrosive-wear environments. In this study, therefore, CrB2 coatings were deposited by the pulsed magnetron sputtering of loosely packed blended powder targets. This work was carried out in a deposition system specifically designed for the use of powder targets, and the effectiveness of this approach has been previously demonstrated through the production of a number of different multi-component coating materials. The CrB2 coatings deposited exhibit extremely dense, defect-free, crystalline structures with high hardness (> 30 GPa) and good corrosion resistant properties. This paper discusses the dependence of structure, mechanical properties and corrosion behaviour of such coatings on the DC pulsed plasma treatment conditions chosen. D 2005 Elsevier B.V. All rights reserved. Keywords: Chromium diboride; Pulsed magnetron sputtering; Pulsed biasing; Powder targets
1. Introduction Transition metal diboride coatings are attracting increased interest, due to the fact that they combine high hardness with good wear and corrosion resistance. Indeed, chromium diboride has been considered particularly suitable for applications in corrosive-wear environments [1,2]. However, to date, there are very few reports in the scientific literature concerning investigations of coatings of this material [1– 8]. Boride coatings are often deposited by sputtering hotpressed or sintered ceramic targets. However, targets of this nature can be prone to cracking and, in extreme cases, localised overheating can even cause the source material to explode. Also, solid targets do not offer the opportunity to
* Corresponding author. Tel.: +44 114 2225934; fax: +44 114 2225943. E-mail address: [email protected] (M. Audronis). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.022
readily vary film stoichiometry since the user is limited to a single composition per target. This may be a major drawback, since it has been reported that magnetron sputtering of boride coatings can produce films with significant stoichiometric deviations from the target composition [4,8,9]. The use of loosely packed blended powder targets offers a solution to the problems of target cracking. The starting materials are relatively cheap and the targets require no additional processing, such as hot-pressing, or bonding to a backing plate. This approach was recently exploited by Kelly et al. [10] in a deposition system specifically designed for powder target use. This system also incorporated a closed field unbalanced magnetron configuration [11] and the target was sputtered in the pulsed DC mode [12]. As reported earlier [8], the ability to conveniently vary the target composition allows the production of stoichiometric CrB2 coatings. Furthermore, these additional system design features produce deposition
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1366 – 1371
conditions that also prompt the formation of extremely dense, crystalline Fsuperhard_ coatings. This work was again carried out in the aforementioned deposition system [8,10], but the current objective was to study the achievable structures and properties of CrB2 films deposited under different plasma pulsing parameters and also under pulsed substrate bias conditions, both of which can significantly affect the energetic state of coating species arriving at the substrate surface, and thus influence strongly the properties of the growing film [13 – 18]. Besides the sputtering pulse frequency, which is often assumed to be a significant factor in inducing changes in the properties and structure of deposited films, we have found that changes in duty factor can also have an important influence. In this paper, the relationships between pulsed magnetron sputtering (PMS), pulsed biasing parameters and the resulting film properties are investigated and discussed. Our discussions are based on the target and substrate voltage waveform characteristics, SEM, XRD, nanoindentation analysis and potentiodynamic polarization test results in an aqueous chloride solution.
2. Experimental details Coatings were deposited in a magnetron sputter deposition rig, described in detail elsewhere [8,10], which contained a 180 mm diameter unbalanced magnetron, installed in the Fsputter-up_ configuration. The substrate holder, mounted 100 mm above the target, was watercooled to ensure repeatable substrate temperature profiles during each run. The substrate temperature remained in the range of 110– 150 -C during all coating deposition runs. For the deposition of CrB2 coatings, the target consisted of Cr and B powders (99.99% purity) blended in an atomic ratio of 1:2. For both powders, the average particle size was ¨ 5 Am, and the blend was mixed in a rotating drum for several hours. No further compaction was performed after mixing the blend. Additional pieces of solid boron
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(approximately 1 cm3 each) were also placed at regular intervals around the Fracetrack_ region to achieve the required film stoichiometry. Polished silicon wafer, stainless steel (AISI 304) and tool steel (AISI M2) coupons were used as substrates. Prior to deposition, the substrates were cleaned ultrasonically in isopropanol. After attaining a base vacuum pressure below 2.67 10 4 Pa (2 10 6 Torr), the chamber was backfilled with argon to 0.25 Pa and the substrates were DC sputter cleaned at 650 V for 20 min. During the sputter cleaning procedure, a low magnetron power of 20 W was retained to maintain the glow discharge. After cleaning, the argon pressure was adjusted to the working pressure of the deposition process, which was 0.2 Pa during all runs. The substrate bias voltage was then decreased (and the magnetron power was increased) to the chosen values shown in the experimental array of Table 1. For the first 30 min of the run, the substrates were masked by means of a shutter. This was necessary for target conditioning and stabilization of deposition parameters [8]. A dual channel, 10 kW Advanced Energy Industries FPinnacle Plus_ magnetron driver was used as a sputtering power source. In pulsed DC mode, this unit can deliver an asymmetric bipolar output at frequencies of up to 350 kHz. In this study, all deposition runs were carried out at a target pulse frequency of 100 kHz. The target power supply was operated in power regulation mode at 500 W. The deposition time was fixed at 240 min (excluding conditioning step). The treatment variables were duty cycle (80%, 70% and 60%) and substrate voltage bias conditions (floating, DC and pulsed DC). Two DC bias voltages of 30 V and 85 V were chosen. Substrate pulsed biasing was performed at 250 kHz and 350 kHz with a 50% duty factor (the target was pulsed at 100 kHz, 70% duty factor during these runs). At each substrate pulse frequency, two bias voltages were applied ( 50 V and 85 V); these values represent the mean negative bias voltage attained by the substrate during the pulse-on cycle. In all instances, the coatings deposited at 350 kHz and 85 V bias peeled off completely and, consequently, will not be discussed further. The pulsed substrate bias coatings produced at 250 kHz/
Table 1 Deposition parameters and some physical and structural properties of chromium diboride coatings Sample no.
Duty factor, %
Substrate bias
1 2 3 4 5 6 7 8 9 10 11 12
80 70 60 80 70 60 80 70 60 70 70 70
Floating Floating Floating 30V DC 30V DC 30V DC 85V DC 85V DC 85V DC P(1) P(2) P(3)
E corr, V 0.21 0.10 0.33 0.22 0.47 0.54 – 0.14 0.66 – 0.08 0.00
Passivation region, V
I (101), %
I (100), %
H, GPa
E red, GPa
0.21 1.01 0.95 1.02 1.1 0.59 – No passivation 0.57 – 0.94 0.86
23.4 43.5 40.7 0.2 1.0 19.0 6.3 34.9 11.4 19.9 4.6 0.4
5.7 11.3 14.2 0.0 0.3 7.0 1.5 13.0 4.6 7.8 2.5 0.2
23 25 21 38 34 31 35 – – 35 – –
217 218 205 258 243 244 244 – – 245 – –
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50 V, 250 kHz/ 85 V and 350 kHz/ 50 V will, therefore, be referred to as P(1), P(2) and P(3), respectively. During each run, mean current and voltage levels at the target and substrate, as indicated by the power supply outputs, were recorded. Voltage waveforms at the target and substrate were also stored for each run using a Tektronix 3014 digital oscilloscope. The deposited coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), operating in h – 2h mode using Cu Ka radiation. Mechanical properties were evaluated by nanoindentation of the films deposited on Si wafers using a Nanotest 500 (Micromaterials Ltd., UK). A Berkovitch pyramid diamond indenter was used to make twenty indentations with a depth of 145 nm in each sample. The linear loading/unloading rate was 0.13 mN s 1 and the dwell time was 10 s for all indentations. The results presented here represent the averages for each group. Standard deviations have been worked out from these results and are presented in the form of error bars in Fig. 5. The corrosion resistance of the coatings deposited on stainless steel substrates was evaluated by potentiodynamic polarization tests in a synthetic environment of 3.5% NaCl solution using a Solartron 1287 Electrochemical Interface Potentiostat. A conventional threeelectrode cell was used with a platinum counter electrode and a saturated calomel electrode (SCE) as a reference. The cell used was designed with a circular O-ring sealed orifice in the base. The flat specimen (working electrode) was pressed against the O-ring seal, to allow the surface area inside the circumference of the O-ring to be accessed by the solution. The area exposed to the electrolyte was thus 0.28 cm2. Polarization scans were performed at a fixed voltage scan rate of 100 mV/min.
The application of a pulsed voltage waveform at the magnetron target (Fig. 1a) is known to result in an increase of the energy and flux of bombarding ions at the growing film surface [12]. It has been shown that the energy and flux of the charged bombarding species depend on the nature of the plasma sheath at the substrate. Thus, it is the plasma conditions that largely determine the coating structure and properties. In turn, the plasma conditions can themselves be varied by changing parameters such as pulse frequency and duty factor. During PMS, the plasma potential remains the most positive potential in the system, even during the positive voltage overshoot phase at the beginning of the pulse off cycle (region 1 in Fig. 1a) [20,21]. This results in the complex temporal evolution of conditions at a floating substrate during the pulse cycle as the floating potential attempts to follow changes in the plasma potential but is restrained by factors such as the impedance of the plasma and of the substrate holder itself. To follow accurately the time-dependent changes in floating potential requires the use of Langmuir (or emissive) probes. However, some indication of the biasing conditions can be obtained through the use of an oscilloscope. Fig. 1b shows substrate voltage waveforms obtained in this manner during pulsed sputtering at target conditions of 100 kHz pulse frequency and 70% duty cycle (similar characteristics were obtained under the other biasing parameters investigated). The waveforms
3. Results and discussion 3.1. Deposition conditions: pulsed target and substrate The low-temperature plasma processing of solid surfaces is mainly affected by (i) energy per incident particle, which is related to energy transfer, (ii) particle flux density incident at the substrate, which is related to momentum transfer, (iii) energy flux density, representing a key parameter for the energetic conditions at the surface, and (iv) temperature of the substrate surface, which results from the parameters mentioned above. The surface temperature (which can also be influenced by externally applied heating or cooling) affects elementary processes such as adsorption, desorption and diffusion, as well as chemical reactions [19]. In the case of thin film deposition, the structure and morphology, as well as the stoichiometry of the film, depend strongly on the energetic conditions at the surface, with bombardment of the growing film by low-energy ions resulting in modifications of its properties [8,12,19].
Fig. 1. (a) Target and (b) substrate voltage waveforms during 100 kHz, 70% duty cycle pulsed magnetron sputtering.
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1366 – 1371
were obtained under conditions of floating bias and applied DC bias voltages of 30 and 85 V. The strong influence of the driving voltage waveform on conditions at the substrate is apparent from these traces. During the pulse-off phase at the target, there is a considerable degree of Fringing_ in the substrate waveforms, making interpretation impossible. The situation is far more stable during the pulse-on phase, where the target bias voltage varies in a more predictable manner. It should be noted that, during the pulse-on phase, the DC bias voltages reach significantly more negative values than the time-averaged values indicated by the power supply, which will increase the energy of ions incident at the substrate at this point. Overall, these waveforms indicate clearly that, during pulsed sputtering, the application of a DC bias voltage (or the selection of floating bias conditions) at the substrate does not lead to constant voltage conditions at the substrate. Rather, it seems likely that the bias conditions at the substrate will be influenced strongly by the driving voltage waveform at the target and that this will impact on the species incident at the substrate (detailed investigations of such effects will be the subject of further work). Recent studies have shown that pulsed biasing provides an additional means of controlling the plasma parameters and can be used to modify coating structures and properties [13 –18], including composition [16]. Fig. 2 shows the substrate voltage waveforms obtained under the operating conditions described earlier. As can be seen, depending on the pulse parameters, the substrate negative bias can reach instantaneous values very much in excess of the apparent applied values (up to 400 V during the pulse-on phase). Furthermore, during the pulse-off phase, very significant positive overshoot voltages can also be recorded (up to + 300 V).
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Fig. 3. SEM micrograph of the fracture section of CrB2 coating deposited at 100 kHz pulsing frequency, 80% duty cycle and floating substrate bias.
ogies. A typical example is shown in Fig. 3 (run conditions are summarised in the figure heading). XRD, however, revealed that there were certain differences in the crystalline structures (Fig. 4 and Table 1) of these films. Although all coatings had strong (001) peaks, the (101) and (100) peaks were also observed. The relative intensities of the (101) and (100) peaks to the (001) peak at 100% (I (101) and I (100), respectively) are shown in Table 1 for each deposition run performed. It can be seen from the graph and data that the intensity of these peaks relative to the (001) preferred orientation varies considerably with bias voltage. Lower intensities of the (101) and (100) peaks were registered for coatings biased negatively ( 30 and 85 V) during deposition. The application of a pulsed substrate bias (Fig. 2) also resulted in reduced intensities of these peaks or even eliminated them, thus allowing highly textured films to be obtained. It was also observed that the (001) peaks shifted to lower angles of 2h with decreasing duty factor, implying
3.2. Structure and properties of deposited films SEM examination of the CrB2 coatings revealed that all coatings exhibit extremely dense and featureless morphol-
Fig. 2. Substrate voltage waveforms during pulsed biasing conditions of 250 kHz/ 50 V [P(1)], 250 kHz/ 85 V [P(2)], and 350 kHz/ 50 V [P(3)] obtained during pulsed sputtering at 100 kHz, 70% duty.
Fig. 4. XRD spectra of chromium diboride coatings deposited under various plasma pulsing and substrate biasing conditions.
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M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1366 – 1371
Fig. 5. (a) Hardness dependence of chromium diboride coatings on (a) the pulsing and substrate biasing conditions and (b) the presence and fraction of (101) phase texture.
increased film stresses. This may perhaps arise from the higher energy ion bombardment that would be expected under these conditions, as discussed in Refs. [20 –23].
Nanoindentation studies showed that the film hardness varied strongly with substrate biasing conditions (Fig. 5a). Coatings deposited at a floating substrate potential exhibited the lowest hardness values (21 – 25 GPa), whilst those coatings deposited at a substrate bias voltage of 30 V DC exhibited much higher hardness values in the range 31– 38 GPa. Such a significant difference in hardness at these different bias conditions may well be attributable, in part, to changes in film composition (as was observed previously [8]) and to differences in structure, as revealed by XRD data. Duty factor, by its influence on the texture of the films, appears to have a significant effect on coating hardness. As can be seen in Fig. 5a, decreasing the duty factor tends to decrease the hardness of the films. Comparing the XRD data with the hardness data (Fig. 5b) reveals that strongly (001) textured films are hardest, and the presence (and/or increased fraction) of grains with other orientations decreases the hardness significantly. Of the pulsed bias films, only P(1) could be measured, due to spalling off of the other two films. Comparison of potentiodynamic polarization results (Fig. 6a– c) of coated samples to uncoated steel shows that coatings deposited under certain deposition parameters can improve stainless steel behaviour in an aqueous chloride by raising the corrosion potential E corr to less negative values and extending the passivation zone. Comparison of results between the coatings demonstrates similar trends to those discussed above, i.e., (001) textured coatings exhibit improved corrosion behaviour (samples no. 4, 5, 11 and 12) in comparison to coatings with proportionally larger (101) and (100) peaks. In the latter case, the coatings show poorer behaviour in terms of lower corrosion potential and a narrower passivation region. This is particularly apparent for the coatings deposited with pulsed biasing, which show
Fig. 6. Results of potentiodynamic polarization tests of chromium diboride coatings (sample numbers relate to the deposition parameters shown in Table 1).
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1366 – 1371
E corr (vs. SCE) values of ¨0 V and a passivation region width of ¨ 0.9 V (Fig. 6c) Coatings 2 and 3 are exceptions to this trend, in that they include large values for I (101) and I (100), but also have low E corr values and very large (compared to the other coatings) passivation regions. This suggests that for coatings with this type of structure (say I (101) > 40%), the corrosion mechanism changes and, in terms of corrosion behaviour, coatings with a large fraction of (101) can perform as well as (001) textured coatings. However, we can only speculate at this point about the reasons for these effects. (For example it may be that a combination of differently oriented grains promotes galvanic interactions between adjacent grains or maybe these differently oriented grains also exhibit slightly different chemical compositions, which might also influence corrosion interactions between grains or between the coating and the substrate.)
4. Conclusions Fully dense, crystalline, hard (> 30 GPa) and corrosion resistant CrB2 coatings can be deposited by PMS from loosely packed blended powder targets at low substrate temperatures. The use of this method to produce boride coatings provides an effective alternative to the problematic sputtering of solid boride targets, which are brittle and vulnerable to thermal shock. The DC substrate negative bias voltage is strongly modulated during PMS and its waveform is dependent on the duty factor and the time-averaged voltage preset on the power supply. Besides pulsing the target, substrate pulsed biasing appears also to be an effective measure to control the film structure, and thus the properties obtained. It is difficult to conclude unambiguously, without more detailed structural and compositional information (which unfortunately was beyond the scope of this paper, but will be the subject of a further study) about the interrelationships between deposition parameters, plasma conditions and film properties, such as hardness, texture and corrosion resistance. Despite this, it is clear that the structure and composition of the films can, at least in part, be controlled through the plasma pulsing and substrate biasing parameters. For example, parameters which promoted the growth of strongly (001) textured coatings also resulted in films of higher hardness and superior corrosion resistance.
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Acknowledgements The authors would like to thank Geoff France for producing the SEM micrographs and Dr. Aleksey Yerokhin for the help with corrosion testing.
References [1] P.A. Dearnley, N. Dortmann, K.L. Dahm, H. Weiss, Surf. Eng. 13 (1997) 139. [2] K.L. Dahm, L.R. Jordan, J. Haase, P.A. Dearnley, Surf. Coat. Technol. 108 – 109 (1998) 413. [3] L.R. Jordan, A.J. Betts, K.L. Dahm, P.A. Dearnley, G.A. Wright, Corrosion Science 47 (2005) 1085. [4] M. Zhou, Y. Makino, K. Nogi, M. Nose, Thin Solid Films 343 – 344 (1999) 234. [5] M. Zhou, M. Nose, Y. Makino, K. Nogi, Thin Solid Films 359 (2000) 165. [6] M. Mori, T. Shibayanagi, M. Maeda, M. Naka, Scr. Mater. 44 (2001) 2035. [7] Y. Sakamaoto, M. Nose, T. Mae, E. Honbo, M. Zhou, K. Nogi, Surf. Coat. Technol. 174 – 175 (2003) 444. [8] M. Audronis, P.J. Kelly, R.D. Arnell and A.V. Valiulis, Surf. Coat. Technol., in press. [9] F. Kunc, J. Musil, P.H. Mayrhofer, C. Mitterer, Surf. Coat. Technol. 174 – 175 (2003) 744. [10] P.J. Kelly, Y. Zhou, A. Postill, Thin Solid Films 426 (2003) 111. [11] W.D. Sproul, P.J. Rudnik, M.E. Graham, S.L. Rohde, Surf. Coat. Technol. 43 – 44 (1990) 270. [12] R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188 – 189 (2004) 158. [13] P.J. Kelly, R. Hall, J. O’Brien, J.W. Bradley, P. Henderson, G. Roche, R.D. Arnell, J. Vac. Sci. Technol., A, Vac. Surf. Films (19) (2001) 2856. [14] B.K. Gan, M.M.M. Bilek, D.R. McKenzie, M.B. Taylor, D.G. McCulloch, J. Appl. Phys. 95 (2004) 2130. [15] E.V. Barnat, T.M. Lu, J. Appl. Phys. 92 (2002) 2984. [16] S. Mukherjee, H. Reuther, F. Prokert, E. Richter, W. Moeller, Surf. Coat. Technol. 160 (2002) 93. [17] S.-B. Wang, A.E. Wendt, J. Appl. Phys. 88 (2000) 643. [18] W. Otano-Rivera, L.J. Pilione, J.A. Zapien, R. Messier, Proc. 44th Int’l AVS Symp. 16 (1998) 1331. [19] H. Kersten, H. Deutsch, H. Steffen, G.M.W. Kroesen, R. Hippler, Vacuum 63 (2001) 385. [20] S.K. Karkari, A. Vetushka, J.W. Bradley, J. Vac. Sci. Technol., A, Vac. Surf. Films (21) (2003). [21] C. Muratore, J.J. Moore, J.A. Rees, Surf. Coat. Technol. 163 – 164 (2003) 12. [22] H. Backer, P.S. Henderson, J.W. Bradley, P.J. Kelly, Surf. Coat. Technol. 174 – 175 (2003) 909. [23] J.W. Bradley, S.K. Karkari, A. Vetushka, Plasma Sources Sci. Technol. 13 (2004) 189.
The structure and properties of chromium diboride coatings deposited by pulsed magnetron sputtering of powder targets M. Audronis a,*, P.J. Kelly b, R.D. Arnell c, A. Leyland a, A. Matthews a a
Department of Engineering Materials, The University of Sheffield, Sir Robert Hadfield Building, Mappin Street, Sheffield, S1 3JD, UK b Surface Engineering Group, Manchester Metropolitan University, Manchester M1 5GD, UK c Institute for Materials Research, University of Salford, Salford M5 4WT, UK Available online 13 September 2005
Abstract Metal boride coatings are attracting increased interest, due to the fact that they combine high hardness with good wear and corrosion resistance. These coatings are often deposited by the sputtering of hot-pressed or sintered ceramic targets. However, targets of this nature can be prone to cracking and, since the user is limited to a single composition per target, do not offer the opportunity to readily vary film stoichiometry. Titanium and zirconium borides have been widely studied; however, chromium diboride is recognised as offering superior performance in corrosive-wear environments. In this study, therefore, CrB2 coatings were deposited by the pulsed magnetron sputtering of loosely packed blended powder targets. This work was carried out in a deposition system specifically designed for the use of powder targets, and the effectiveness of this approach has been previously demonstrated through the production of a number of different multi-component coating materials. The CrB2 coatings deposited exhibit extremely dense, defect-free, crystalline structures with high hardness (> 30 GPa) and good corrosion resistant properties. This paper discusses the dependence of structure, mechanical properties and corrosion behaviour of such coatings on the DC pulsed plasma treatment conditions chosen. D 2005 Elsevier B.V. All rights reserved. Keywords: Chromium diboride; Pulsed magnetron sputtering; Pulsed biasing; Powder targets
1. Introduction Transition metal diboride coatings are attracting increased interest, due to the fact that they combine high hardness with good wear and corrosion resistance. Indeed, chromium diboride has been considered particularly suitable for applications in corrosive-wear environments [1,2]. However, to date, there are very few reports in the scientific literature concerning investigations of coatings of this material [1– 8]. Boride coatings are often deposited by sputtering hotpressed or sintered ceramic targets. However, targets of this nature can be prone to cracking and, in extreme cases, localised overheating can even cause the source material to explode. Also, solid targets do not offer the opportunity to
* Corresponding author. Tel.: +44 114 2225934; fax: +44 114 2225943. E-mail address: [email protected] (M. Audronis). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.08.022
readily vary film stoichiometry since the user is limited to a single composition per target. This may be a major drawback, since it has been reported that magnetron sputtering of boride coatings can produce films with significant stoichiometric deviations from the target composition [4,8,9]. The use of loosely packed blended powder targets offers a solution to the problems of target cracking. The starting materials are relatively cheap and the targets require no additional processing, such as hot-pressing, or bonding to a backing plate. This approach was recently exploited by Kelly et al. [10] in a deposition system specifically designed for powder target use. This system also incorporated a closed field unbalanced magnetron configuration [11] and the target was sputtered in the pulsed DC mode [12]. As reported earlier [8], the ability to conveniently vary the target composition allows the production of stoichiometric CrB2 coatings. Furthermore, these additional system design features produce deposition
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1366 – 1371
conditions that also prompt the formation of extremely dense, crystalline Fsuperhard_ coatings. This work was again carried out in the aforementioned deposition system [8,10], but the current objective was to study the achievable structures and properties of CrB2 films deposited under different plasma pulsing parameters and also under pulsed substrate bias conditions, both of which can significantly affect the energetic state of coating species arriving at the substrate surface, and thus influence strongly the properties of the growing film [13 – 18]. Besides the sputtering pulse frequency, which is often assumed to be a significant factor in inducing changes in the properties and structure of deposited films, we have found that changes in duty factor can also have an important influence. In this paper, the relationships between pulsed magnetron sputtering (PMS), pulsed biasing parameters and the resulting film properties are investigated and discussed. Our discussions are based on the target and substrate voltage waveform characteristics, SEM, XRD, nanoindentation analysis and potentiodynamic polarization test results in an aqueous chloride solution.
2. Experimental details Coatings were deposited in a magnetron sputter deposition rig, described in detail elsewhere [8,10], which contained a 180 mm diameter unbalanced magnetron, installed in the Fsputter-up_ configuration. The substrate holder, mounted 100 mm above the target, was watercooled to ensure repeatable substrate temperature profiles during each run. The substrate temperature remained in the range of 110– 150 -C during all coating deposition runs. For the deposition of CrB2 coatings, the target consisted of Cr and B powders (99.99% purity) blended in an atomic ratio of 1:2. For both powders, the average particle size was ¨ 5 Am, and the blend was mixed in a rotating drum for several hours. No further compaction was performed after mixing the blend. Additional pieces of solid boron
1367
(approximately 1 cm3 each) were also placed at regular intervals around the Fracetrack_ region to achieve the required film stoichiometry. Polished silicon wafer, stainless steel (AISI 304) and tool steel (AISI M2) coupons were used as substrates. Prior to deposition, the substrates were cleaned ultrasonically in isopropanol. After attaining a base vacuum pressure below 2.67 10 4 Pa (2 10 6 Torr), the chamber was backfilled with argon to 0.25 Pa and the substrates were DC sputter cleaned at 650 V for 20 min. During the sputter cleaning procedure, a low magnetron power of 20 W was retained to maintain the glow discharge. After cleaning, the argon pressure was adjusted to the working pressure of the deposition process, which was 0.2 Pa during all runs. The substrate bias voltage was then decreased (and the magnetron power was increased) to the chosen values shown in the experimental array of Table 1. For the first 30 min of the run, the substrates were masked by means of a shutter. This was necessary for target conditioning and stabilization of deposition parameters [8]. A dual channel, 10 kW Advanced Energy Industries FPinnacle Plus_ magnetron driver was used as a sputtering power source. In pulsed DC mode, this unit can deliver an asymmetric bipolar output at frequencies of up to 350 kHz. In this study, all deposition runs were carried out at a target pulse frequency of 100 kHz. The target power supply was operated in power regulation mode at 500 W. The deposition time was fixed at 240 min (excluding conditioning step). The treatment variables were duty cycle (80%, 70% and 60%) and substrate voltage bias conditions (floating, DC and pulsed DC). Two DC bias voltages of 30 V and 85 V were chosen. Substrate pulsed biasing was performed at 250 kHz and 350 kHz with a 50% duty factor (the target was pulsed at 100 kHz, 70% duty factor during these runs). At each substrate pulse frequency, two bias voltages were applied ( 50 V and 85 V); these values represent the mean negative bias voltage attained by the substrate during the pulse-on cycle. In all instances, the coatings deposited at 350 kHz and 85 V bias peeled off completely and, consequently, will not be discussed further. The pulsed substrate bias coatings produced at 250 kHz/
Table 1 Deposition parameters and some physical and structural properties of chromium diboride coatings Sample no.
Duty factor, %
Substrate bias
1 2 3 4 5 6 7 8 9 10 11 12
80 70 60 80 70 60 80 70 60 70 70 70
Floating Floating Floating 30V DC 30V DC 30V DC 85V DC 85V DC 85V DC P(1) P(2) P(3)
E corr, V 0.21 0.10 0.33 0.22 0.47 0.54 – 0.14 0.66 – 0.08 0.00
Passivation region, V
I (101), %
I (100), %
H, GPa
E red, GPa
0.21 1.01 0.95 1.02 1.1 0.59 – No passivation 0.57 – 0.94 0.86
23.4 43.5 40.7 0.2 1.0 19.0 6.3 34.9 11.4 19.9 4.6 0.4
5.7 11.3 14.2 0.0 0.3 7.0 1.5 13.0 4.6 7.8 2.5 0.2
23 25 21 38 34 31 35 – – 35 – –
217 218 205 258 243 244 244 – – 245 – –
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50 V, 250 kHz/ 85 V and 350 kHz/ 50 V will, therefore, be referred to as P(1), P(2) and P(3), respectively. During each run, mean current and voltage levels at the target and substrate, as indicated by the power supply outputs, were recorded. Voltage waveforms at the target and substrate were also stored for each run using a Tektronix 3014 digital oscilloscope. The deposited coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), operating in h – 2h mode using Cu Ka radiation. Mechanical properties were evaluated by nanoindentation of the films deposited on Si wafers using a Nanotest 500 (Micromaterials Ltd., UK). A Berkovitch pyramid diamond indenter was used to make twenty indentations with a depth of 145 nm in each sample. The linear loading/unloading rate was 0.13 mN s 1 and the dwell time was 10 s for all indentations. The results presented here represent the averages for each group. Standard deviations have been worked out from these results and are presented in the form of error bars in Fig. 5. The corrosion resistance of the coatings deposited on stainless steel substrates was evaluated by potentiodynamic polarization tests in a synthetic environment of 3.5% NaCl solution using a Solartron 1287 Electrochemical Interface Potentiostat. A conventional threeelectrode cell was used with a platinum counter electrode and a saturated calomel electrode (SCE) as a reference. The cell used was designed with a circular O-ring sealed orifice in the base. The flat specimen (working electrode) was pressed against the O-ring seal, to allow the surface area inside the circumference of the O-ring to be accessed by the solution. The area exposed to the electrolyte was thus 0.28 cm2. Polarization scans were performed at a fixed voltage scan rate of 100 mV/min.
The application of a pulsed voltage waveform at the magnetron target (Fig. 1a) is known to result in an increase of the energy and flux of bombarding ions at the growing film surface [12]. It has been shown that the energy and flux of the charged bombarding species depend on the nature of the plasma sheath at the substrate. Thus, it is the plasma conditions that largely determine the coating structure and properties. In turn, the plasma conditions can themselves be varied by changing parameters such as pulse frequency and duty factor. During PMS, the plasma potential remains the most positive potential in the system, even during the positive voltage overshoot phase at the beginning of the pulse off cycle (region 1 in Fig. 1a) [20,21]. This results in the complex temporal evolution of conditions at a floating substrate during the pulse cycle as the floating potential attempts to follow changes in the plasma potential but is restrained by factors such as the impedance of the plasma and of the substrate holder itself. To follow accurately the time-dependent changes in floating potential requires the use of Langmuir (or emissive) probes. However, some indication of the biasing conditions can be obtained through the use of an oscilloscope. Fig. 1b shows substrate voltage waveforms obtained in this manner during pulsed sputtering at target conditions of 100 kHz pulse frequency and 70% duty cycle (similar characteristics were obtained under the other biasing parameters investigated). The waveforms
3. Results and discussion 3.1. Deposition conditions: pulsed target and substrate The low-temperature plasma processing of solid surfaces is mainly affected by (i) energy per incident particle, which is related to energy transfer, (ii) particle flux density incident at the substrate, which is related to momentum transfer, (iii) energy flux density, representing a key parameter for the energetic conditions at the surface, and (iv) temperature of the substrate surface, which results from the parameters mentioned above. The surface temperature (which can also be influenced by externally applied heating or cooling) affects elementary processes such as adsorption, desorption and diffusion, as well as chemical reactions [19]. In the case of thin film deposition, the structure and morphology, as well as the stoichiometry of the film, depend strongly on the energetic conditions at the surface, with bombardment of the growing film by low-energy ions resulting in modifications of its properties [8,12,19].
Fig. 1. (a) Target and (b) substrate voltage waveforms during 100 kHz, 70% duty cycle pulsed magnetron sputtering.
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were obtained under conditions of floating bias and applied DC bias voltages of 30 and 85 V. The strong influence of the driving voltage waveform on conditions at the substrate is apparent from these traces. During the pulse-off phase at the target, there is a considerable degree of Fringing_ in the substrate waveforms, making interpretation impossible. The situation is far more stable during the pulse-on phase, where the target bias voltage varies in a more predictable manner. It should be noted that, during the pulse-on phase, the DC bias voltages reach significantly more negative values than the time-averaged values indicated by the power supply, which will increase the energy of ions incident at the substrate at this point. Overall, these waveforms indicate clearly that, during pulsed sputtering, the application of a DC bias voltage (or the selection of floating bias conditions) at the substrate does not lead to constant voltage conditions at the substrate. Rather, it seems likely that the bias conditions at the substrate will be influenced strongly by the driving voltage waveform at the target and that this will impact on the species incident at the substrate (detailed investigations of such effects will be the subject of further work). Recent studies have shown that pulsed biasing provides an additional means of controlling the plasma parameters and can be used to modify coating structures and properties [13 –18], including composition [16]. Fig. 2 shows the substrate voltage waveforms obtained under the operating conditions described earlier. As can be seen, depending on the pulse parameters, the substrate negative bias can reach instantaneous values very much in excess of the apparent applied values (up to 400 V during the pulse-on phase). Furthermore, during the pulse-off phase, very significant positive overshoot voltages can also be recorded (up to + 300 V).
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Fig. 3. SEM micrograph of the fracture section of CrB2 coating deposited at 100 kHz pulsing frequency, 80% duty cycle and floating substrate bias.
ogies. A typical example is shown in Fig. 3 (run conditions are summarised in the figure heading). XRD, however, revealed that there were certain differences in the crystalline structures (Fig. 4 and Table 1) of these films. Although all coatings had strong (001) peaks, the (101) and (100) peaks were also observed. The relative intensities of the (101) and (100) peaks to the (001) peak at 100% (I (101) and I (100), respectively) are shown in Table 1 for each deposition run performed. It can be seen from the graph and data that the intensity of these peaks relative to the (001) preferred orientation varies considerably with bias voltage. Lower intensities of the (101) and (100) peaks were registered for coatings biased negatively ( 30 and 85 V) during deposition. The application of a pulsed substrate bias (Fig. 2) also resulted in reduced intensities of these peaks or even eliminated them, thus allowing highly textured films to be obtained. It was also observed that the (001) peaks shifted to lower angles of 2h with decreasing duty factor, implying
3.2. Structure and properties of deposited films SEM examination of the CrB2 coatings revealed that all coatings exhibit extremely dense and featureless morphol-
Fig. 2. Substrate voltage waveforms during pulsed biasing conditions of 250 kHz/ 50 V [P(1)], 250 kHz/ 85 V [P(2)], and 350 kHz/ 50 V [P(3)] obtained during pulsed sputtering at 100 kHz, 70% duty.
Fig. 4. XRD spectra of chromium diboride coatings deposited under various plasma pulsing and substrate biasing conditions.
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Fig. 5. (a) Hardness dependence of chromium diboride coatings on (a) the pulsing and substrate biasing conditions and (b) the presence and fraction of (101) phase texture.
increased film stresses. This may perhaps arise from the higher energy ion bombardment that would be expected under these conditions, as discussed in Refs. [20 –23].
Nanoindentation studies showed that the film hardness varied strongly with substrate biasing conditions (Fig. 5a). Coatings deposited at a floating substrate potential exhibited the lowest hardness values (21 – 25 GPa), whilst those coatings deposited at a substrate bias voltage of 30 V DC exhibited much higher hardness values in the range 31– 38 GPa. Such a significant difference in hardness at these different bias conditions may well be attributable, in part, to changes in film composition (as was observed previously [8]) and to differences in structure, as revealed by XRD data. Duty factor, by its influence on the texture of the films, appears to have a significant effect on coating hardness. As can be seen in Fig. 5a, decreasing the duty factor tends to decrease the hardness of the films. Comparing the XRD data with the hardness data (Fig. 5b) reveals that strongly (001) textured films are hardest, and the presence (and/or increased fraction) of grains with other orientations decreases the hardness significantly. Of the pulsed bias films, only P(1) could be measured, due to spalling off of the other two films. Comparison of potentiodynamic polarization results (Fig. 6a– c) of coated samples to uncoated steel shows that coatings deposited under certain deposition parameters can improve stainless steel behaviour in an aqueous chloride by raising the corrosion potential E corr to less negative values and extending the passivation zone. Comparison of results between the coatings demonstrates similar trends to those discussed above, i.e., (001) textured coatings exhibit improved corrosion behaviour (samples no. 4, 5, 11 and 12) in comparison to coatings with proportionally larger (101) and (100) peaks. In the latter case, the coatings show poorer behaviour in terms of lower corrosion potential and a narrower passivation region. This is particularly apparent for the coatings deposited with pulsed biasing, which show
Fig. 6. Results of potentiodynamic polarization tests of chromium diboride coatings (sample numbers relate to the deposition parameters shown in Table 1).
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E corr (vs. SCE) values of ¨0 V and a passivation region width of ¨ 0.9 V (Fig. 6c) Coatings 2 and 3 are exceptions to this trend, in that they include large values for I (101) and I (100), but also have low E corr values and very large (compared to the other coatings) passivation regions. This suggests that for coatings with this type of structure (say I (101) > 40%), the corrosion mechanism changes and, in terms of corrosion behaviour, coatings with a large fraction of (101) can perform as well as (001) textured coatings. However, we can only speculate at this point about the reasons for these effects. (For example it may be that a combination of differently oriented grains promotes galvanic interactions between adjacent grains or maybe these differently oriented grains also exhibit slightly different chemical compositions, which might also influence corrosion interactions between grains or between the coating and the substrate.)
4. Conclusions Fully dense, crystalline, hard (> 30 GPa) and corrosion resistant CrB2 coatings can be deposited by PMS from loosely packed blended powder targets at low substrate temperatures. The use of this method to produce boride coatings provides an effective alternative to the problematic sputtering of solid boride targets, which are brittle and vulnerable to thermal shock. The DC substrate negative bias voltage is strongly modulated during PMS and its waveform is dependent on the duty factor and the time-averaged voltage preset on the power supply. Besides pulsing the target, substrate pulsed biasing appears also to be an effective measure to control the film structure, and thus the properties obtained. It is difficult to conclude unambiguously, without more detailed structural and compositional information (which unfortunately was beyond the scope of this paper, but will be the subject of a further study) about the interrelationships between deposition parameters, plasma conditions and film properties, such as hardness, texture and corrosion resistance. Despite this, it is clear that the structure and composition of the films can, at least in part, be controlled through the plasma pulsing and substrate biasing parameters. For example, parameters which promoted the growth of strongly (001) textured coatings also resulted in films of higher hardness and superior corrosion resistance.
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Acknowledgements The authors would like to thank Geoff France for producing the SEM micrographs and Dr. Aleksey Yerokhin for the help with corrosion testing.
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