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Deposition of multicomponent chromium boride based coatings by pulsed magnetron sputtering of powder targets
Surface & Coatings Technology 200 (2005) 1616 – 1623 www.elsevier.com/locate/surfcoat
Deposition of multicomponent chromium boride based coatings 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, University of Sheffield, 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 3 October 2005
Abstract It is well known that composite transition metal boride (and also nitride or carbide) based materials show great promise for a wide range of tribological applications. The wear and friction properties of plasma assisted PVD hard films can, in certain cases, be enhanced significantly by introducing some amount of an additional material (such as a solid lubricant) into the coating. However such measures can, for example, introduce further issues with regard to reduced corrosion resistance (and even increased friction in humid environments). An aim with complex multi-element composite materials is often, therefore, to combine the advantages of several constituent phases to enable the coating to survive under varying wear conditions by exhibiting ‘‘adaptive’’ properties, without harming other coating characteristics. Sputtering of multi-component coatings from blended powder targets offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence of the deposited film. It also solves potential cracking problems associated with overheating during the sputtering of brittle ceramic targets with relatively low thermal (and sometimes electrical) conductivity. Furthermore, pulsed magnetron sputtering can have a significant beneficial impact on PVD coating structure and properties. In this study, dense multicomponent chromium boride-based coatings with MoSx , Ti and C alloying additions were deposited by pulsed magnetron sputtering of blended powder targets. Auger Electron Spectroscopy revealed that the films had extremely uniform throughthickness compositions and that contamination by oxygen was below 3 at.%. Scratch testing of coatings with increasing amount of MoSx showed that improvements in adhesion are obtained only up to a certain point, beyond which properties deteriorate. Ti and C addition to the CrB-MoSx films dramatically improves their behaviour on stainless steel in corrosive environments, exceeding that of bare stainless steel. It also enhances the hardness and adhesion of the coatings. This paper presents the results of investigations of the structure, composition, corrosion resistance and mechanical and tribological properties of these coatings. D 2005 Elsevier B.V. All rights reserved. Keywords: Powder targets; Pulsed magnetron sputtering; Multicomponent coatings; Chromium diboride; Molybdenum disulphide; Corrosion resistance
1. Introduction The functionality of modern tribological coatings can be improved by combining materials of different properties into composites [1 –4]. Composite transition metal boride, nitride or carbide based materials with specific properties show great promise for a wide range of tribological applications. For example, the wear and friction properties of PVD hard films can in some cases be enhanced significantly by * 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.2005.08.089
introducing some amount of solid lubricant material into the coating. One advantage of such dispersion-enhanced coatings is based on a continuous presence of the lubricating compound throughout the working period [5]. However there may be issues with regard to reduced corrosion resistance and increased friction in humid environments [6,7]. More complex, nanostructured multi-element composite materials are able to combine the advantages of several constituent phases to survive under varying wear conditions by exhibiting ‘‘adaptive’’ properties [4]. The technique of depositing multicomponent coatings by pulsed magnetron sputtering (PMS) of loosely-packed
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
blended powder targets, developed by Kelly et al. [8], offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence optimizing the deposited film by allowing quick exploration of novel materials and their properties. It also solves some problems associated with overheating during sputtering of brittle ceramic targets, which are commonly sintered, or hotpressed. Furthermore, the PMS process can have a significant beneficial impact on PVD coating structure and properties. For example, it has been shown that smooth, fully dense and highly crystalline CrB2 coatings can be deposited by PMS at substrate temperatures below 150 -C [9]. This indicates that PMS can facilitate the deposition of very high quality films onto temperature sensitive substrate materials such as low alloy steels, the light alloys or metallic and polymeric composites. The advantages of PMS have been published previously and were recently reviewed by Arnell et al. [10]. In this study, multicomponent chromium boride based coatings, with MoSx , Ti and C alloying additions, were deposited by PMS of loosely-packed blended powder targets. This paper presents results of the characterization of these coatings in terms of their structure, composition, corrosion resistance and mechanical and tribological properties. The results demonstrate the potential of the PMS deposition technique as a means of developing novel singleor multi-phase/component coating materials with enhanced combinations of mechanical, tribological and corrosionresistant properties.
2. Experimental details Coatings were produced by magnetron sputter deposition using equipment described in detail elsewhere [8,9]. The target to substrate distance was 100 mm. To ensure a constant substrate temperature during deposition and to solve some adhesion issues encountered in earlier studies, [9] a water-cooled substrate holder was used. During all runs the substrate temperature never exceeded 150 -C. All other configuration parameters can be found in Ref. [9]. For coating deposition four different target material compositions were used, consisting of different amounts of blended Cr, B, MoS2, Ti and C powders; the target compositions are given in Table 1. Prior to deposition, the substrates were cleaned ultrasonically in isopropanol. After attaining a base vacuum
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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 applied 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 increased) to give the required deposition settings. For the first 30 min of each run, the substrates were masked by means of a shutter. This was necessary for target conditioning to occur and for stable deposition parameters to be obtained [9]. Films were deposited onto polished Al pin stubs, silicon wafers, stainless steel (AISI 304) and tool steel (AISI M2) coupons by pulsed DC magnetron sputtering. A dual channel, 10 kW Advanced Energy Industries Pinnacle Plus magnetron driver was used. The power supply was operated in power regulation mode at an input power of 500 W. The deposition time was fixed at 240 min. All runs were carried out at 100 kHz target pulsing frequency and 70% duty cycle. Two runs were carried out for each target with substrate bias voltages preset to 0 V and 30 V. The deposited coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), operating in glancing angle mode (beam angle 8-) using Cu Ka radiation, and by auger electron spectroscopy (AES). AES depth profiling was performed by Ar ion milling. The ion beam energy was 3 keV. Mechanical properties were evaluated by nanoindentation of films deposited on Si wafers using a Nanotest 500 (Micromaterials Ltd., UK). A Berkovich pyramid diamond indenter was used to make twenty indentations with a depth of about 145 nm in each sample, and the results presented here represent the averages for each group. The linear loading/unloading rate was 0.13 mN s 1 and the dwell time was 10 s for all indentations. Coatings were also analyzed via scratch adhesion testing [11– 13], employing a Teer Coatings ST3001 scratch tester with a 1.5-mm radius chrome steel ball indenter. Progressively increasing loads from 1 to 140 N were used for each scratch. The steel ball was replaced after each test. Scratch tests were performed on the coated tool steel specimens with a constant velocity of 10 mm min 1 and a linearly increasing loading rate of 100 N min- 1 (at 30 –40% humidity). Coating failure point was determined by optical microscopy. The corrosion resistance of the coatings deposited on stainless steel substrates was evaluated by potentiody-
Table 1 Target compositions Target no.
T1 T2 T3 T4
Atomic percent of elements in the target, %
In text and pictures referred as:
Cr
B
Mo
S
Ti
C
24.9 23.2 21.7 15.1
62.3 57.9 54.2 37.7
4.3 6.3 8.0 5.7
8.5 12.6 16.0 11.3
– – – 15.1
– – – 15.1
(CrB2.5)8.54(MoS2)1.46 (CrB2.5)7.86(MoS2)2.14 (CrB2.5)7.3(MoS2)2.7 (CrB2.5)4.21(MoS2)1.58(TiC)4.21
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namic polarization tests in a synthetic environment of 3.5% NaCl aqueous solution using a Solartron 1287 Electrochemical Interface Potentiostat. A conventional three-electrode 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 Oring 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 1.
3. Results and discussion 3.1. Magnetron sputtering of loosely-packed powder targets Sputtering of loosely-packed powder (LPP) targets is not a widely used technique, and it has yet to be characterized in full detail. The deposition of Cr – B coatings from such targets [9] showed that when a fresh mixture of powder was sputtered for the first time, the target underwent a Fconditioning_ period, during which the target voltage dropped in magnitude from 537 to 340 V, with a corresponding increase in the target current from 0.37 to 0.59 A, over the course of a 5 hour period (at a target power setting of 200 W [9]). This fall in target voltage and increase in current (while maintaining the same target power) may be due to both increased amorphous boron electrical conductivity (arising from the increase in target temperature during operation) and the formation of a more conductive chromium boride crust [9]. However it is noted that these target surface compositional changes may also affect the plasma conditions associated with the target (e.g. secondary electron yield) and this may also contribute to the changes in voltage/current characteristics observed. Similar target behaviour was also observed in the present work during the sputtering of Cr– B targets containing some amount of MoS2, Ti and C. Although the differences in time of Ftarget conditioning_ were not investigated, a trend of longer required conditioning times with increased MoS2 content was observed. In common with the behaviour seen for porous, hotpressed compact CrB2 targets [14], at the beginning of the deposition process, outgassing from loosely-packed powder targets tends to cause contamination of the deposited films—mainly by oxygen. Oxygen contamination is recognized to appear from two main sources. The first of these is the oxidized layer at the target surface; this source is more important for loosely-packed or pressed powder targets than for cast targets because of their high surface-to-volume ratio. The second is diffusion from the bulk target material; again, this is obviously especially significant for targets made of pressed or loosely-packed
powder. However, the main disadvantage for pressed powder targets in the latter respect is that, during target conditioning, only the region near the surface of the target is degassed. Films deposited after this can be relatively free of oxide; but as sputtering progresses, the surface of the target gradually recedes into the (heavily contaminated) bulk, resulting in deterioration of the film properties with deposition time. No such effect was observed during the sputtering of LPP targets. Because of the manner in which LPP targets are made [8,9], the diffusion of contaminants from the bulk takes place at the beginning of coating deposition and ceases quite quickly. The outgassing rate is strongly dependent on target temperature, i.e. the more power applied to the target, the more rapid is the outgassing, which can in turn cause target damage. Therefore appropriate target power ramping times should be selected. During this period the substrates can be masked by means of a shutter. Despite attempting to avoid oxygen contamination of the substrate surface during the main outgassing period (which is monitored by the changes of pressure in the deposition chamber) by means of shutter, a small (¨ 1 –3 at.%) amount of oxygen was always detected in the coating, with the source presumably being the oxidized surface layer. The presence of oxygen was shown by the simulation of the RBS spectra initially [9], and proved by auger electron spectroscopy (AES) in the current study (Fig. 1). (Note: the higher amount of oxygen seen in Fig. 1 during initial film growth is avoidable by using longer ‘‘shutter closed’’ times). Fig. 1 shows AES depth profiles of coatings deposited at 30 V substrate bias voltage from all four targets. It can be seen that the films are very uniform in composition throughout the thickness and that the film composition is generally matched closely to the target composition. However some compositional deviations, the reasons for which we can not explain in this paper, are also evident. As Fig. 1a shows, the Cr:B atomic ratio in the film deposited from the (CrB2.5)8.54(MoS2)1.46 target (Table 1) is 1:1.8, which was an expected result, based on our earlier studies. The Cr:B ratio in the films deposited from the remaining targets dropped to 1:1.3, 1:0.8 and 1:1 for targets (CrB2.5)7.86(MoS2)2.14, (CrB2.5)7.3(MoS2)2.7 and (CrB2.5)4.21 (MoS2)1.58(TiC)4.21, respectively. It is not clear at this stage why there is such a drop in the Cr:B ratio and how it relates to the increased levels of the other elements in the target. One other interesting observation in the deviations in film uniformity and elemental composition is the different atomic percentages of Ti and C, compared to that in the target. For example, the atomic percentage of Ti and C in the coating deposited from target composition no. 4 is considerably lower than that in the target. However, again, the reasons are not clear. It may, of course, be an artefact of the analysis process involving sputter-etch depth profiling of the film, which has been shown to affect the accuracy of this technique [15].
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
Fig. 1. AES depth profiles of the films deposited from targets nos. 1 – 4 at
3.2. Structure and properties of deposited films SEM studies showed that the Cr –B based MoS2 and Ti– C containing coatings exhibit fully dense and featureless structure with very smooth surfaces. All coatings were of the order of 2 Am thick. These morphologies are very similar to those of CrB2 coatings deposited from similar targets and under similar conditions [9]. Such properties are clearly advantageous for anti-corrosion and anti-wear performance. By way of example, Fig. 2 shows the SEM micrograph of the fracture section of a coating deposited from target no. 4 onto a polished Al substrate. The dense, non-columnar morphology of these coatings is largely attributable to the design of the deposition system and the use of the PMS process in preference to continuous DC processing [8– 10].
Fig. 2. SEM micrograph of CrBTiC-MoS coating.
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30 V DC bias.
Fig. 3 shows glancing angle XRD spectra of the coatings deposited from targets no. 1, no. 2 and no. 3. Differences in film phase composition are apparent from these spectra. CrB2 (001), (100) and (101) reflections are more distinct in the films having a lower MoS2 content. It is also noticeable that the application of a higher substrate bias voltage promotes crystallization (e.g. increases in MoS2 (002) and (118) peaks) of these phases. Films with higher MoS2 content exhibit far less details in their XRD spectra. This may be related to the fact that these coatings also show a lower Cr:B ratio (Fig. 1) as chromium boride (CrB) has a tendency to form X-ray amorphous structures [9,16]. Results of nanoindentation measurements are summarized in Table 2 and shown in Fig. 4. The hardness of the coatings was found to be strongly dependent on the MoS2 content of the target. Coatings deposited from target no. 1 exhibited hardness values of ¨ 15 GPa. A further increase in MoS2 content resulted in a decrease in hardness, as expected. Ti and C alloyed films, deposited from the (CrB2.5)4.21(MoS2)1.58(TiC)4.21 target, even with a significantly higher amount of MoSx in them (Fig. 1a and d; Table 2), are as hard as films deposited from the (CrB2.5)8.54(MoS2)1.46 target (Fig 4b). This improvement in hardness may perhaps be attributable to an enhanced nano- (or amorphous) structure, but deeper structural studies are required to understand such effects. 3.2.1. Scratch testing Strong adhesion is one of the most important property requirements of a tribological coating [11 – 13]. The
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Fig. 3. GAXRD of the coatings deposited from targets nos. 1 – 3.
Coatings deposited from target no. 1 (C1, C2), despite having the highest hardness, exhibited the lowest L c. No indications of chipping or cracking was present prior to failure (Fig. 5a). The removal of the coating was immediate and complete, pointing to the brittleness of these films. This kind of failure is typical for thin ceramic coatings and occurs in response to the compressive stresses generated ahead of the moving indenter. Grooves seen in the track are a result of an abrasive wear mechanism. Softer films with slightly higher MoSx contents (C3 and C4, Table 2) performed best during scratch testing (Fig. 5b). No coating failure (in terms of flaking) was observed, even at the maximum 140 N load, in spite of the fact that traditionally it is accepted that harder coatings give higher values of L c [12,13]. No statistically significant variations in
generally accepted method of evaluating the adhesion of a surface coating is the scratch test, in which an indenter tip is drawn over the coated surface to produce a scratch. The load on the indenter is progressively increased and the value of the load at which adhesive failure is observed is known as the critical load (L c). In these experiments, the discussion of the results is made with respect to coating failure mode criteria published elsewhere [12,13]. Fig. 5a –d shows sections of the scratch testing tracks of sample nos. 2, 4, 6 and 8 (Table 2), either at the failure point or at the end point of the scratch if no failure was detected. These optical micrographs are typical of the responses of the coatings from each target composition. The critical loads are summarized in Table 2.
Table 2 Compositions (as obtained from AES) and some properties of coatings Coating no./ Target no.
Substr. Bias
B
Mo
S
Ti
C
O
Friction coeff.
Critical load L c, N
H, GPa
Cr
E v, GPa
C1/1 C2/1 C3/2 C4/2 C5/3 C6/3 C7/4 C8/4
0V 30 V 0V 30 V 0V 30 V 0V 30 V
– 29.9 – 30.6 – 33.0 – 24.6
– 53.1 – 40.5 – 26.7 – 24.9
– 7.9 – 12.1 – 17.1 – 12.5
– 6.5 – 13.8 – 20.5 – 12.3
– – – – – – – 9.5
– – – – – – – 14
– 2.6 – 3 – 2.8 – 2.3
0.15 0.14 0.19 0.15 0.18 0.15 0.17 0.17
53 – 72 84 >140 >140 82 80 90 65 – 80
14.7 14.3 10.4 12.0 9.6 10.9 13.2 14.3
152 158 129 131 117 122 148 158
Averaged atomic percent of elements in the coatings, %
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
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is a biproduct of the same reactions but occurs primarily due to the formation of MoO3 [7]. Comparisons of potentiodynamic polarization results (Fig. 6a– b) from coated samples and uncoated steel show that, in terms of corrosion resistance, stainless steel specimens coated with chromium boride coatings containing different amounts of MoSx additions, behave poorly. The corrosion potential of specimens is below or well below that of bare stainless steel and there is either no passivation, or passivation takes place at a higher corrosion current.
Fig. 4. (a) Hardness dependence on the content of MoS2 in the target and (b) enhancement of hardness of the films by incorporation of Ti and C.
the friction coefficient were observed, with all values being of the order of 0.15. A further increase in MoSx content in the coatings (C5 and C6) resulted in a decrease in L c to values close to those of C1 and C2. Results of scratch testing of samples C1 to C6 suggest that additions of MoSx to hard coatings can improve the adhesion properties up to a certain point. These improvements are achieved due to the chemical composition and structural changes in the films. Fig. 5d shows the occurrence of failure during the scratch testing of CrB – TiC – MoSx coatings. Brittle tensile cracking behind the indenter is observed. As the load is increased, chipping of the film at the edges of the track also occurs. However, despite such cracking and spallation, the films were still present at the highest load (140 N). Thus, the presence of additional Ti and C in the films may result in increased brittleness, but improved adhesion. 3.2.2. Potentiodynamic polarization testing Coated components frequently need good corrosion resistance, because they rarely operate in completely dry environments. It has been reported that, in humid environments, MoS2 can accelerate corrosion processes through oxidation reactions that form H2SO4 [7]. Increased friction
Fig. 5. Different failure modes of coatings C2 (a), C4 (b), C6 (c) and C8 (d) during scratch testing (white arrow indicates direction of the scratch).
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Fig. 6. Potentiodynamic polarization tests results of coatings.
However, it is also noticeable that an increasing amount of MoSx in the films does tend to improve their behaviour (e.g. coating C6 ( 30 Vi (CrB2.5)7.3(MoS2)2.7, in Fig. 6a)). This is unexpected, since a mixture of different chemical compositions and phase structures in adjacent grains might be expected to promote galvanic interactions within the coating itself (and at the coating/substrate interface), but may be attributable to the lower crystallinity (Fig. 3) and/or very small grain sizes of these coatings, which may suppress the conventional galvanic behaviour seen for polycrystalline multiphase materials. Additions of Ti and C into CrB – MoS coatings can dramatically improve their behavior in corrosive environments and even significantly exceed that of stainless steel by shifting up E corr to less negative values and extending the passivation zone (Fig. 6b). Despite the excellent enhancement in the corrosion resistance provided by the CrB – TiC– MoSx films, these samples exhibit relatively poor results during scratch testing. Further investigation of target and deposited film composition may allow the combination of superior corrosion resistance and improved tribological behaviour to be found.
4. Conclusions Fully dense multi-elemental CrB –MoSx and CrB – TiC – MoSx coatings were deposited by pulsed magnetron sputtering from loosely-packed blended powder targets at low substrate temperatures. Sputtering of LPP targets offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence of the deposited film and is a powerful technique to allow quick
exploration of novel materials. LPP targets also offer an effective alternative to the problematic sputtering of solid boride and pressed powder composite targets. AES revealed that films deposited from LPP targets had extremely uniform through-thickness compositions and that contamination by oxygen was minimal. Scratch testing of coatings with increasing amounts of MoSx showed that improvements in adhesion are obtained only up to a certain point, beyond which properties deteriorate. Improvements appear to be achieved due primarily to the chemical composition and structural changes in the films. Ti and C alloying of the CrB-MoSx films dramatically improves their behaviour on stainless steel in corrosive environments, exceeding that of bare stainless steel. It also enhances the hardness and adhesion of the coatings. The friction coefficients of all the coatings were found to be ¨ 0.15 against a chrome steel ball in 30– 40% humidity. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
C. Donnet, A. Erdemir, Surf. Coat. Technol. 180 – 181 (2004) 76. S. Hogmark, S. Jacobson, M. Larsson, Wear 246 (2000) 20. H.A. Jehn, Surf. Coat. Technol. 131 (2000) 433. A.A. Voevodin, J.S. Zabinski, Compos. Sci. Technol. 65 (2005) 741. Y.W. Bae, W.Y. Lee, T.M. Besmann, C.S. Yust, P.J. Blau, Mater. Sci. Eng., A 209 (1996) 372. W.O. Winer, Wear 10 (1967) 422. D.Y. Yu, J.A. Wang, Yang, Thin Solid Films 293 (1997) 1. P.J. Kelly, Y. Zhou, A. Postill, Thin Solid Films 426 (2003) 111. M. Audronis, P.J. Kelly, R.D. Arnell, A.V. Valiulis, Pulsed magnetron sputtering of chromium boride films from loose powder targets, Surf. Coat. Technol. (in press) Corrected, doi:10.1016/j.surfcoat.2004.11. 007. R.D. Arnell, P.J. Kelly, J.W. Bradley, Surf. Coat. Technol. 188 – 189 (2004) 158.
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623 [11] J. Valli, J. Vac. Sci. Technol., A, Vac. Surf. Films 4 (1986) 3007. [12] V. Bellido-Gonzalez, N. Stefanopoulos, F. Deguilhen, Surf. Coat. Technol. 74 – 75 (1995) 884. [13] P.J. Burnett, D.S. Rickerby, Thin Solid Films 154 (1987) 403. [14] D.R. Wheeler, W.A. Brainard, J. Vac. Sci. Technol. (1978) 24.
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[15] M.A. Baker, R. Gilmore, C. Lenardi, W. Gissler, Appl. Surf. Sci. 150 (1999) 255. [16] M. Zhou, Y. Makino, K. Nogi, M. Nose, Thin Solid Films 343 – 344 (1999) 234.
Deposition of multicomponent chromium boride based coatings 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, University of Sheffield, 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 3 October 2005
Abstract It is well known that composite transition metal boride (and also nitride or carbide) based materials show great promise for a wide range of tribological applications. The wear and friction properties of plasma assisted PVD hard films can, in certain cases, be enhanced significantly by introducing some amount of an additional material (such as a solid lubricant) into the coating. However such measures can, for example, introduce further issues with regard to reduced corrosion resistance (and even increased friction in humid environments). An aim with complex multi-element composite materials is often, therefore, to combine the advantages of several constituent phases to enable the coating to survive under varying wear conditions by exhibiting ‘‘adaptive’’ properties, without harming other coating characteristics. Sputtering of multi-component coatings from blended powder targets offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence of the deposited film. It also solves potential cracking problems associated with overheating during the sputtering of brittle ceramic targets with relatively low thermal (and sometimes electrical) conductivity. Furthermore, pulsed magnetron sputtering can have a significant beneficial impact on PVD coating structure and properties. In this study, dense multicomponent chromium boride-based coatings with MoSx , Ti and C alloying additions were deposited by pulsed magnetron sputtering of blended powder targets. Auger Electron Spectroscopy revealed that the films had extremely uniform throughthickness compositions and that contamination by oxygen was below 3 at.%. Scratch testing of coatings with increasing amount of MoSx showed that improvements in adhesion are obtained only up to a certain point, beyond which properties deteriorate. Ti and C addition to the CrB-MoSx films dramatically improves their behaviour on stainless steel in corrosive environments, exceeding that of bare stainless steel. It also enhances the hardness and adhesion of the coatings. This paper presents the results of investigations of the structure, composition, corrosion resistance and mechanical and tribological properties of these coatings. D 2005 Elsevier B.V. All rights reserved. Keywords: Powder targets; Pulsed magnetron sputtering; Multicomponent coatings; Chromium diboride; Molybdenum disulphide; Corrosion resistance
1. Introduction The functionality of modern tribological coatings can be improved by combining materials of different properties into composites [1 –4]. Composite transition metal boride, nitride or carbide based materials with specific properties show great promise for a wide range of tribological applications. For example, the wear and friction properties of PVD hard films can in some cases be enhanced significantly by * 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.2005.08.089
introducing some amount of solid lubricant material into the coating. One advantage of such dispersion-enhanced coatings is based on a continuous presence of the lubricating compound throughout the working period [5]. However there may be issues with regard to reduced corrosion resistance and increased friction in humid environments [6,7]. More complex, nanostructured multi-element composite materials are able to combine the advantages of several constituent phases to survive under varying wear conditions by exhibiting ‘‘adaptive’’ properties [4]. The technique of depositing multicomponent coatings by pulsed magnetron sputtering (PMS) of loosely-packed
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
blended powder targets, developed by Kelly et al. [8], offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence optimizing the deposited film by allowing quick exploration of novel materials and their properties. It also solves some problems associated with overheating during sputtering of brittle ceramic targets, which are commonly sintered, or hotpressed. Furthermore, the PMS process can have a significant beneficial impact on PVD coating structure and properties. For example, it has been shown that smooth, fully dense and highly crystalline CrB2 coatings can be deposited by PMS at substrate temperatures below 150 -C [9]. This indicates that PMS can facilitate the deposition of very high quality films onto temperature sensitive substrate materials such as low alloy steels, the light alloys or metallic and polymeric composites. The advantages of PMS have been published previously and were recently reviewed by Arnell et al. [10]. In this study, multicomponent chromium boride based coatings, with MoSx , Ti and C alloying additions, were deposited by PMS of loosely-packed blended powder targets. This paper presents results of the characterization of these coatings in terms of their structure, composition, corrosion resistance and mechanical and tribological properties. The results demonstrate the potential of the PMS deposition technique as a means of developing novel singleor multi-phase/component coating materials with enhanced combinations of mechanical, tribological and corrosionresistant properties.
2. Experimental details Coatings were produced by magnetron sputter deposition using equipment described in detail elsewhere [8,9]. The target to substrate distance was 100 mm. To ensure a constant substrate temperature during deposition and to solve some adhesion issues encountered in earlier studies, [9] a water-cooled substrate holder was used. During all runs the substrate temperature never exceeded 150 -C. All other configuration parameters can be found in Ref. [9]. For coating deposition four different target material compositions were used, consisting of different amounts of blended Cr, B, MoS2, Ti and C powders; the target compositions are given in Table 1. Prior to deposition, the substrates were cleaned ultrasonically in isopropanol. After attaining a base vacuum
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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 applied 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 increased) to give the required deposition settings. For the first 30 min of each run, the substrates were masked by means of a shutter. This was necessary for target conditioning to occur and for stable deposition parameters to be obtained [9]. Films were deposited onto polished Al pin stubs, silicon wafers, stainless steel (AISI 304) and tool steel (AISI M2) coupons by pulsed DC magnetron sputtering. A dual channel, 10 kW Advanced Energy Industries Pinnacle Plus magnetron driver was used. The power supply was operated in power regulation mode at an input power of 500 W. The deposition time was fixed at 240 min. All runs were carried out at 100 kHz target pulsing frequency and 70% duty cycle. Two runs were carried out for each target with substrate bias voltages preset to 0 V and 30 V. The deposited coatings were examined by scanning electron microscopy (SEM) and X-ray diffraction (XRD), operating in glancing angle mode (beam angle 8-) using Cu Ka radiation, and by auger electron spectroscopy (AES). AES depth profiling was performed by Ar ion milling. The ion beam energy was 3 keV. Mechanical properties were evaluated by nanoindentation of films deposited on Si wafers using a Nanotest 500 (Micromaterials Ltd., UK). A Berkovich pyramid diamond indenter was used to make twenty indentations with a depth of about 145 nm in each sample, and the results presented here represent the averages for each group. The linear loading/unloading rate was 0.13 mN s 1 and the dwell time was 10 s for all indentations. Coatings were also analyzed via scratch adhesion testing [11– 13], employing a Teer Coatings ST3001 scratch tester with a 1.5-mm radius chrome steel ball indenter. Progressively increasing loads from 1 to 140 N were used for each scratch. The steel ball was replaced after each test. Scratch tests were performed on the coated tool steel specimens with a constant velocity of 10 mm min 1 and a linearly increasing loading rate of 100 N min- 1 (at 30 –40% humidity). Coating failure point was determined by optical microscopy. The corrosion resistance of the coatings deposited on stainless steel substrates was evaluated by potentiody-
Table 1 Target compositions Target no.
T1 T2 T3 T4
Atomic percent of elements in the target, %
In text and pictures referred as:
Cr
B
Mo
S
Ti
C
24.9 23.2 21.7 15.1
62.3 57.9 54.2 37.7
4.3 6.3 8.0 5.7
8.5 12.6 16.0 11.3
– – – 15.1
– – – 15.1
(CrB2.5)8.54(MoS2)1.46 (CrB2.5)7.86(MoS2)2.14 (CrB2.5)7.3(MoS2)2.7 (CrB2.5)4.21(MoS2)1.58(TiC)4.21
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namic polarization tests in a synthetic environment of 3.5% NaCl aqueous solution using a Solartron 1287 Electrochemical Interface Potentiostat. A conventional three-electrode 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 Oring 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 1.
3. Results and discussion 3.1. Magnetron sputtering of loosely-packed powder targets Sputtering of loosely-packed powder (LPP) targets is not a widely used technique, and it has yet to be characterized in full detail. The deposition of Cr – B coatings from such targets [9] showed that when a fresh mixture of powder was sputtered for the first time, the target underwent a Fconditioning_ period, during which the target voltage dropped in magnitude from 537 to 340 V, with a corresponding increase in the target current from 0.37 to 0.59 A, over the course of a 5 hour period (at a target power setting of 200 W [9]). This fall in target voltage and increase in current (while maintaining the same target power) may be due to both increased amorphous boron electrical conductivity (arising from the increase in target temperature during operation) and the formation of a more conductive chromium boride crust [9]. However it is noted that these target surface compositional changes may also affect the plasma conditions associated with the target (e.g. secondary electron yield) and this may also contribute to the changes in voltage/current characteristics observed. Similar target behaviour was also observed in the present work during the sputtering of Cr– B targets containing some amount of MoS2, Ti and C. Although the differences in time of Ftarget conditioning_ were not investigated, a trend of longer required conditioning times with increased MoS2 content was observed. In common with the behaviour seen for porous, hotpressed compact CrB2 targets [14], at the beginning of the deposition process, outgassing from loosely-packed powder targets tends to cause contamination of the deposited films—mainly by oxygen. Oxygen contamination is recognized to appear from two main sources. The first of these is the oxidized layer at the target surface; this source is more important for loosely-packed or pressed powder targets than for cast targets because of their high surface-to-volume ratio. The second is diffusion from the bulk target material; again, this is obviously especially significant for targets made of pressed or loosely-packed
powder. However, the main disadvantage for pressed powder targets in the latter respect is that, during target conditioning, only the region near the surface of the target is degassed. Films deposited after this can be relatively free of oxide; but as sputtering progresses, the surface of the target gradually recedes into the (heavily contaminated) bulk, resulting in deterioration of the film properties with deposition time. No such effect was observed during the sputtering of LPP targets. Because of the manner in which LPP targets are made [8,9], the diffusion of contaminants from the bulk takes place at the beginning of coating deposition and ceases quite quickly. The outgassing rate is strongly dependent on target temperature, i.e. the more power applied to the target, the more rapid is the outgassing, which can in turn cause target damage. Therefore appropriate target power ramping times should be selected. During this period the substrates can be masked by means of a shutter. Despite attempting to avoid oxygen contamination of the substrate surface during the main outgassing period (which is monitored by the changes of pressure in the deposition chamber) by means of shutter, a small (¨ 1 –3 at.%) amount of oxygen was always detected in the coating, with the source presumably being the oxidized surface layer. The presence of oxygen was shown by the simulation of the RBS spectra initially [9], and proved by auger electron spectroscopy (AES) in the current study (Fig. 1). (Note: the higher amount of oxygen seen in Fig. 1 during initial film growth is avoidable by using longer ‘‘shutter closed’’ times). Fig. 1 shows AES depth profiles of coatings deposited at 30 V substrate bias voltage from all four targets. It can be seen that the films are very uniform in composition throughout the thickness and that the film composition is generally matched closely to the target composition. However some compositional deviations, the reasons for which we can not explain in this paper, are also evident. As Fig. 1a shows, the Cr:B atomic ratio in the film deposited from the (CrB2.5)8.54(MoS2)1.46 target (Table 1) is 1:1.8, which was an expected result, based on our earlier studies. The Cr:B ratio in the films deposited from the remaining targets dropped to 1:1.3, 1:0.8 and 1:1 for targets (CrB2.5)7.86(MoS2)2.14, (CrB2.5)7.3(MoS2)2.7 and (CrB2.5)4.21 (MoS2)1.58(TiC)4.21, respectively. It is not clear at this stage why there is such a drop in the Cr:B ratio and how it relates to the increased levels of the other elements in the target. One other interesting observation in the deviations in film uniformity and elemental composition is the different atomic percentages of Ti and C, compared to that in the target. For example, the atomic percentage of Ti and C in the coating deposited from target composition no. 4 is considerably lower than that in the target. However, again, the reasons are not clear. It may, of course, be an artefact of the analysis process involving sputter-etch depth profiling of the film, which has been shown to affect the accuracy of this technique [15].
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
Fig. 1. AES depth profiles of the films deposited from targets nos. 1 – 4 at
3.2. Structure and properties of deposited films SEM studies showed that the Cr –B based MoS2 and Ti– C containing coatings exhibit fully dense and featureless structure with very smooth surfaces. All coatings were of the order of 2 Am thick. These morphologies are very similar to those of CrB2 coatings deposited from similar targets and under similar conditions [9]. Such properties are clearly advantageous for anti-corrosion and anti-wear performance. By way of example, Fig. 2 shows the SEM micrograph of the fracture section of a coating deposited from target no. 4 onto a polished Al substrate. The dense, non-columnar morphology of these coatings is largely attributable to the design of the deposition system and the use of the PMS process in preference to continuous DC processing [8– 10].
Fig. 2. SEM micrograph of CrBTiC-MoS coating.
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30 V DC bias.
Fig. 3 shows glancing angle XRD spectra of the coatings deposited from targets no. 1, no. 2 and no. 3. Differences in film phase composition are apparent from these spectra. CrB2 (001), (100) and (101) reflections are more distinct in the films having a lower MoS2 content. It is also noticeable that the application of a higher substrate bias voltage promotes crystallization (e.g. increases in MoS2 (002) and (118) peaks) of these phases. Films with higher MoS2 content exhibit far less details in their XRD spectra. This may be related to the fact that these coatings also show a lower Cr:B ratio (Fig. 1) as chromium boride (CrB) has a tendency to form X-ray amorphous structures [9,16]. Results of nanoindentation measurements are summarized in Table 2 and shown in Fig. 4. The hardness of the coatings was found to be strongly dependent on the MoS2 content of the target. Coatings deposited from target no. 1 exhibited hardness values of ¨ 15 GPa. A further increase in MoS2 content resulted in a decrease in hardness, as expected. Ti and C alloyed films, deposited from the (CrB2.5)4.21(MoS2)1.58(TiC)4.21 target, even with a significantly higher amount of MoSx in them (Fig. 1a and d; Table 2), are as hard as films deposited from the (CrB2.5)8.54(MoS2)1.46 target (Fig 4b). This improvement in hardness may perhaps be attributable to an enhanced nano- (or amorphous) structure, but deeper structural studies are required to understand such effects. 3.2.1. Scratch testing Strong adhesion is one of the most important property requirements of a tribological coating [11 – 13]. The
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Fig. 3. GAXRD of the coatings deposited from targets nos. 1 – 3.
Coatings deposited from target no. 1 (C1, C2), despite having the highest hardness, exhibited the lowest L c. No indications of chipping or cracking was present prior to failure (Fig. 5a). The removal of the coating was immediate and complete, pointing to the brittleness of these films. This kind of failure is typical for thin ceramic coatings and occurs in response to the compressive stresses generated ahead of the moving indenter. Grooves seen in the track are a result of an abrasive wear mechanism. Softer films with slightly higher MoSx contents (C3 and C4, Table 2) performed best during scratch testing (Fig. 5b). No coating failure (in terms of flaking) was observed, even at the maximum 140 N load, in spite of the fact that traditionally it is accepted that harder coatings give higher values of L c [12,13]. No statistically significant variations in
generally accepted method of evaluating the adhesion of a surface coating is the scratch test, in which an indenter tip is drawn over the coated surface to produce a scratch. The load on the indenter is progressively increased and the value of the load at which adhesive failure is observed is known as the critical load (L c). In these experiments, the discussion of the results is made with respect to coating failure mode criteria published elsewhere [12,13]. Fig. 5a –d shows sections of the scratch testing tracks of sample nos. 2, 4, 6 and 8 (Table 2), either at the failure point or at the end point of the scratch if no failure was detected. These optical micrographs are typical of the responses of the coatings from each target composition. The critical loads are summarized in Table 2.
Table 2 Compositions (as obtained from AES) and some properties of coatings Coating no./ Target no.
Substr. Bias
B
Mo
S
Ti
C
O
Friction coeff.
Critical load L c, N
H, GPa
Cr
E v, GPa
C1/1 C2/1 C3/2 C4/2 C5/3 C6/3 C7/4 C8/4
0V 30 V 0V 30 V 0V 30 V 0V 30 V
– 29.9 – 30.6 – 33.0 – 24.6
– 53.1 – 40.5 – 26.7 – 24.9
– 7.9 – 12.1 – 17.1 – 12.5
– 6.5 – 13.8 – 20.5 – 12.3
– – – – – – – 9.5
– – – – – – – 14
– 2.6 – 3 – 2.8 – 2.3
0.15 0.14 0.19 0.15 0.18 0.15 0.17 0.17
53 – 72 84 >140 >140 82 80 90 65 – 80
14.7 14.3 10.4 12.0 9.6 10.9 13.2 14.3
152 158 129 131 117 122 148 158
Averaged atomic percent of elements in the coatings, %
M. Audronis et al. / Surface & Coatings Technology 200 (2005) 1616 – 1623
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is a biproduct of the same reactions but occurs primarily due to the formation of MoO3 [7]. Comparisons of potentiodynamic polarization results (Fig. 6a– b) from coated samples and uncoated steel show that, in terms of corrosion resistance, stainless steel specimens coated with chromium boride coatings containing different amounts of MoSx additions, behave poorly. The corrosion potential of specimens is below or well below that of bare stainless steel and there is either no passivation, or passivation takes place at a higher corrosion current.
Fig. 4. (a) Hardness dependence on the content of MoS2 in the target and (b) enhancement of hardness of the films by incorporation of Ti and C.
the friction coefficient were observed, with all values being of the order of 0.15. A further increase in MoSx content in the coatings (C5 and C6) resulted in a decrease in L c to values close to those of C1 and C2. Results of scratch testing of samples C1 to C6 suggest that additions of MoSx to hard coatings can improve the adhesion properties up to a certain point. These improvements are achieved due to the chemical composition and structural changes in the films. Fig. 5d shows the occurrence of failure during the scratch testing of CrB – TiC – MoSx coatings. Brittle tensile cracking behind the indenter is observed. As the load is increased, chipping of the film at the edges of the track also occurs. However, despite such cracking and spallation, the films were still present at the highest load (140 N). Thus, the presence of additional Ti and C in the films may result in increased brittleness, but improved adhesion. 3.2.2. Potentiodynamic polarization testing Coated components frequently need good corrosion resistance, because they rarely operate in completely dry environments. It has been reported that, in humid environments, MoS2 can accelerate corrosion processes through oxidation reactions that form H2SO4 [7]. Increased friction
Fig. 5. Different failure modes of coatings C2 (a), C4 (b), C6 (c) and C8 (d) during scratch testing (white arrow indicates direction of the scratch).
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Fig. 6. Potentiodynamic polarization tests results of coatings.
However, it is also noticeable that an increasing amount of MoSx in the films does tend to improve their behaviour (e.g. coating C6 ( 30 Vi (CrB2.5)7.3(MoS2)2.7, in Fig. 6a)). This is unexpected, since a mixture of different chemical compositions and phase structures in adjacent grains might be expected to promote galvanic interactions within the coating itself (and at the coating/substrate interface), but may be attributable to the lower crystallinity (Fig. 3) and/or very small grain sizes of these coatings, which may suppress the conventional galvanic behaviour seen for polycrystalline multiphase materials. Additions of Ti and C into CrB – MoS coatings can dramatically improve their behavior in corrosive environments and even significantly exceed that of stainless steel by shifting up E corr to less negative values and extending the passivation zone (Fig. 6b). Despite the excellent enhancement in the corrosion resistance provided by the CrB – TiC– MoSx films, these samples exhibit relatively poor results during scratch testing. Further investigation of target and deposited film composition may allow the combination of superior corrosion resistance and improved tribological behaviour to be found.
4. Conclusions Fully dense multi-elemental CrB –MoSx and CrB – TiC – MoSx coatings were deposited by pulsed magnetron sputtering from loosely-packed blended powder targets at low substrate temperatures. Sputtering of LPP targets offers a straightforward, rapid and cost-effective way of varying the elemental composition of the target and hence of the deposited film and is a powerful technique to allow quick
exploration of novel materials. LPP targets also offer an effective alternative to the problematic sputtering of solid boride and pressed powder composite targets. AES revealed that films deposited from LPP targets had extremely uniform through-thickness compositions and that contamination by oxygen was minimal. Scratch testing of coatings with increasing amounts of MoSx showed that improvements in adhesion are obtained only up to a certain point, beyond which properties deteriorate. Improvements appear to be achieved due primarily to the chemical composition and structural changes in the films. Ti and C alloying of the CrB-MoSx films dramatically improves their behaviour on stainless steel in corrosive environments, exceeding that of bare stainless steel. It also enhances the hardness and adhesion of the coatings. The friction coefficients of all the coatings were found to be ¨ 0.15 against a chrome steel ball in 30– 40% humidity. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
[10]
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[15] M.A. Baker, R. Gilmore, C. Lenardi, W. Gissler, Appl. Surf. Sci. 150 (1999) 255. [16] M. Zhou, Y. Makino, K. Nogi, M. Nose, Thin Solid Films 343 – 344 (1999) 234.