- Email: [email protected]
Study of the cavitation resistance of Sn-based coatings
Author’s Accepted Manuscript Study of the cavitation resistance of Sn-based coatings Ignacio Tudela, Rolandas Verbickas, Grazina Burmistroviene, Yi Zhang www.elsevier.com/locate/wear
PII: DOI: Reference:
S0043-1648(17)31711-8 https://doi.org/10.1016/j.wear.2018.07.002 WEA102450
To appear in: Wear Received date: 27 November 2017 Revised date: 30 June 2018 Accepted date: 3 July 2018 Cite this article as: Ignacio Tudela, Rolandas Verbickas, Grazina Burmistroviene and Yi Zhang, Study of the cavitation resistance of Sn-based coatings, Wear, https://doi.org/10.1016/j.wear.2018.07.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STUDY
OF THE CAVITATION RESISTANCE OF
SN-
BASED COATINGS I GNACIO T UDELA *, R OLANDAS V ERBICKAS , G RAZINA B URMISTROVIENE AND
Y I Z HANG †
Daido Metal Co., Ltd. European Technical Centre UK, Research& Development Department, Winterhay lane, Ilminster, TA19 9PH, United Kingdom.
ABSTRACT: In this study, the resistance to cavitation erosion of different Sn-based journal bearing overlay coatings were studied and compared. An ultrasonic horn set-up was used to generate cavitation over the surface of samples coated with the evaluated coatings, the volume loss during the tests was estimated and the surface morphology and structure of the overlay coatings was investigated after tests. The results show how the overlay coating structure has an effect on the cavitation resistance of the Sn-based coatings studied, as the multilayered Sn-based overlay coating exhibited a significantly enhanced resistance to cavitation erosion compared with monolayered Sn-based overlay coatings. KEYWORDS: Pb-free; Sn-based coatings; cavitation resistance; journal bearings
* †
Dr. Ignacio Tudela, Email: [email protected] Dr. Yi Zhang, Email: [email protected]
1
1. INTRODUCTION Pb-based overlay coatings have been widely used in trimetal journal bearings due to their good seizure resistance and lubrication properties [1,2]. However, Pb constitutes a serious hazard for the environment due to its highly poisonous nature and its particular harmfulness to children [3] and the legislative framework that removed Pb from the automotive market [4] is gradually extending into medium and high speed engines [5-7]. A suitable option to address this is the replacement of Pb-based overlay coatings with Pb-free Sn-based overlay coatings. State-of-art Sn-based overlay coatings can exhibit superior performance compared to conventional Pb-based overlay coatings [8]. Nevertheless, current and emerging design trends in the large engine industry [9-12] will lead to an even more demanding environment for engine components. Trends such as more extensive use of turbocharging [13,14] and increase in rotational speeds and combustion/inertial loads [15] could also result in cavitation playing a critical role in engine bearing damage in the future. Cavitation erosion in bearings, once a rare phenomenon which has become more important since the early 1970’s [14], generally occurs when the pressure of the lubricant locally falls below its vapour pressure due to i) flow fluctuations caused by the interaction between the design features of the journal bearing and the crankshaft (e.g. oil grooves and holes, drillings, etc.) and ii) transient oil film pressures (e.g. changes in crankshaft eccentricity) [13]. This means that cavitation erosion in an engine journal bearing can be the result of different phenomena which results in the occurrence of cavitation [16]: 1. Suction erosion when the crankshaft recedes rapidly from the surface of the journal bearing. 2. Discharge erosion due to a fluctuating discharge into the bearing groove caused by oil displacement of ahead of a crankshaft moving radially. 3. Flow erosion due to the flow of lubricant across discontinuities in the journal bearing (e.g. edges of oil grooves, supply oil holes, joint face reliefs, etc.). 4. Impact erosion caused by a drilling supplying oil from a main to a crankshaft big-end bearing passes from a grooved to an ungrooved portion of the surface of the crankshaft big-end bearing. 5. Dispersed erosion occurring along with rippling of the bearing overlay. In order to further enhance the structural strength of Sn-based overlay coatings, the authors have proposed a multilayered Sn-based overlay coating [17] where the addition of an SnNi intermetallic compound (IMC) layer splits the bulk Sn-based coating into two Sn-based sublayers, resulting in a further enhancement of the fatigue, wear and seizure performance of 2
Sn-based overlay coatings [18]. In this study, the cavitation performance of the multilayered Snbased overlay coating is evaluated and compared with a monolayered Sn-based overlay coating. The results show that the multilayered Sn-based overlay coating exhibits further enhanced resistance to cavitation as the multilayered structure with alternating Sn/IMC layers provides higher structural strength.
2. MATERIALS AND METHODS 2.1. BEARING OVERLAY MATERIALS Table 1 summarises the monolayered and the multilayered Sn-based overlay coatings [18]. Both overlay coating materials (thickness = 20 µm) were applied onto standard semi-circular bronze bearings with an external diameter of 56 mm. Prior to the application of the overlay coatings, the bearings were pre-treated to ensure proper coverage and adhesion. The bearings were firstly degreased in a commercial alkaline cleaner and then anodically etched in a concentrated HCl solution. A 2-3 µm Ni layer was electrodeposited over the bronze lining right before the application of the overlay coatings to act as a diffusion barrier layer between the overlay coating and the bronze lining. Both monolayered and multilayered Sn-based overlay coatings were electrodeposited by proprietary processes using different electrolytes [18]. In the case of the multilayered Sn-based overlay coating, an additional intermediate IMC layer was incorporated into the structure between the Sn sublayers aiming at the enhancement of various bearing properties. Figure 1 displays the overlay structure of both monolayered and multilayered Sn-based overlay coatings. Figure 1A shows the overlay structure of the monolayered Sn–based overlay coating where different features have been highlighted: A is the bronze substrate, B is the Ni diffusion barrier layer, C is the monolayered Sn–based overlay and D are SnCu intermetallic compound particles uniformly distributed within the matrix. The overlay structure of the multilayered Sn-based overlay coating is shown in Figure 1B: A is the bronze substrate, B is the Ni diffusion barrier layer and C are the thinner Sn–based sublayers which are separated by the intermediate IMC layer D (SnNi). SnCu phases D are also present in the multilayered Sn-based overlay, but in this case they are no longer dispersed within the Sn-based layers as the majority migrated into the areas between the thinner Snbased sublayers C and the IMC layer D after thermal annealing.
3
2.2. CAVITATION EXPERIMENTS The experimental set-up (Figure 2) used is analogous to that of previous studies on cavitation performance of bearing materials available in the literature [8,14,13,19]. Bearings were immersed in a water bath at room temperature. An ultrasonic horn closely placed on top of the bearings was used to generate cavitation. The horn had a cylindrical end to simulate shaft/bearing geometry. Experiments were repeated at least three times to ensure the reproducibility of the data. The parameters of the experiments are displayed in Table 2. Each bearing was carefully cleaned and weighed before and after the experiments to accurately account for the erosion loss caused by cavitation. Following Okada and Iwai’s methodology [19], erosion loss is expressed in this paper in terms of volume loss after estimating the overall density of the different layers in the overlays (approximated density of monolayered Sn-based overlay coating: 7.3 g/mm3, approximated density of multilayered Sn-based overlay coating with IMC layer: 7.5 g/mm3). Regarding the length of the experiments, each sample was tested for a total of 10 minutes [20] (with intermediate measurements after 5 minutes). <10-minute cavitation tests are a common practice in the industry [13,20] when evaluating the resistance to cavitation of bearing overlay coating materials as Pb-based overlay coatings, which are the most widely used in this field, are completely removed within the first few minutes of this type of experiments [8,20]. 10 minutes were also enough to completely remove the monolayered Sn-based overlay coating in the majority of the areas within the tested region, which implies that any longer experiment time would have resulted in erosion beyond the overlay coating. If the experiments were any longer, any wear loss recorded would not only be solely linked to the overlay removal, but also to the potential erosion of the Ni diffusion barrier layer underneath the Sn-based overlay and the bronze substrate under the Ni diffusion barrier layer. The surface morphology and overlay coating structure of tested and untested areas of the monolayered and multilayered Sn-based overlays were examined with a Hitachi TM3030Plus Tabletop Scanning Electron Microscope (SEM) operating in backscatter mode at an accelerating voltage of 15 keV to better understand the effect of the presence addition of the functional IMC layer on the cavitation resistance of the multilayered Sn-based overlay.
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3. RESULTS AND DISCUSSION 3.1. VOLUME LOSS Figure 3A displays the cumulative volume loss of both monolayered and multilayered Sn-based overlays. In both cases, it was observed that the cumulative volume loss increased with time and was very similar for both overlays after 5 minutes as both surfaces being eroded effectively consisted of very similar material (Figure 1) with nearly identical hardness of ≈15 Hv (Table 1) during the first half of the experiments. This changed during the second half of the experiments, as the cumulative volume loss for the monolayered overlay was 2.5 times that of the multilayered overlay after 10 minutes. This suggests that the progressive erosion of material from the top sublayer results in the eroded surface progressively moving closer to the IMC layer which, with a hardness of ≈560 Hv (Table 1), starts to exert significant influence on the resistance to cavitation of the multilayered overlay as a whole. In both cases, no ‘incubation’ stage (negligible erosion rate [21]) was observed during the experiments (Figure 3B). Nevertheless, whereas the erosion rate in the monolayered overlay clearly increased with time under the so-called ‘acceleration’ stage (erosion rate is variable and increasing with time [21]), the erosion rate did not change significantly for the multilayered Sn-based overlay, as it seemed to reach a low and relatively constant value during the second half of the experiment (cavitation erosion occurring under the so-called ‘maximum rate’ stage when a constant erosion rate is observed [21]).
3.2. SURFACE AND CROSS-SECTION ANALYSIS Figure 4 displays the SEM images of the monolayered overlay after tests. In the untested region, large Sn grains covered the surface of the overlay (Figures 4B and C) which clearly consisted of a Sn matrix. However, a variety of surface morphologies and structures were noticeable in the tested region (Figure 4A). In the centre, large protruding structures were observed between craters and undulations of the surface (Figure 4D), and some deep cracks were clearly observed within the overlay (Figure 4E). These protruding structures seemed heavily strained by the action of the bubbles, directly as a result of their non-spherical collapse near the surface resulting in the formation of high-speed liquid jets or indirectly by shockwaves generated during their oscillation and collapse, indicating that the overlay underwent plastic deformation. In this scenario of plastic deformation, fatigue would potentially become an important degradation mechanism, resulting in more pronounced wear due to the detachment of overlay material through micro-cracks coalescing near the overlay/diffusion barrier layer interface, which is what can be observed in certain areas surrounding the centre of the tested region (Figure 4F and G). Once most of the overlay had been removed, the remaining surface would 5
exhibit a smoother surface finish as the harder Ni barrier layer (hardness of ≈270 Hv [18]) is exposed. This is the case of the rest of the areas in the tested region, where a large amount of very small pits and small craters are observed (Figure 4H and I). The appearance of these areas is consistent with that of Ni-based self-fluxing alloys reported by Wu et al. [22]. In these areas, small pit damage would occur when the Ni diffusion layer is firstly exposed to cavitation. Cavitating bubbles would then nucleate and grow near these small pits, with some progressively growing wider and deeper [22]. Figure 5 displays the SEM images of the multilayered Sn-based overlay after test. Large Sn grains covered the surface (Figure 5B) of the overlay which consisted of a two Sn-based sublayers separated by the IMC layer (Figure 5C). Again, different surface morphologies and structures were noticed in the tested region of the bearing, as shown in Figure 5A. In the centre, nodule-like structures protruding from the surface of the overlay were also observed (Figure 5D). However, no deep cracks were noticed, as just some minor cracks propagating until the top Sn sublayer/IMC layer interface were observed (Figure 5E). Again, these nodule-like structures seemed to be strained by the action of cavitating bubbles as the overlay underwent plastic deformation during the ‘acceleration’ stage [21]. And again, wear through fatigue would eventually occur due to the detachment of overlay material through micro-cracks propagating through the top Sn sublayer. But the micro-cracks would now coalesce near the top Sn sublayer/IMC layer interface instead of the overlay/diffusion barrier layer interface, as shown in the vast majority of the tested areas besides the centre of the tested region (Figures 5F and G). Finally, overlay removal was noticed in a very few areas of the tested region of the multilayered overlay (Figures 5H and I). These areas were randomly localised in the periphery of the tested region. The reason for this is that, in any ultrasonic horn, the cavitation is outstandingly strong near the edges of the emitting surface [23,24]. Therefore, such strong activity of cavitating bubbles and the formation of streamers in those areas would cause localised removal of the overlay. As previously reported by the authors, the overall beneficial effect of the presence of the IMC layer and the resulting multilayered structure is [18]: 1. Higher shear strength due to thinner Sn-based sublayers [25,26]. 2. Plastic deformation in the top Sn-based sublayer has significantly lesser chance to accumulate due to shorter progression route in thinner Sn-based sublayers [18]. 3. Barrier effect of the hard-soft interfaces which would block dislocation glide resulting in dislocations in Sn-based sublayers not gliding across the Sn-based sublayer/IMC layer interface [27].
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The higher shear strength and lower accumulation of plastic deformation related to having thinner Sn-based sublayers, although can be clearly observed in the cross-section of the tested samples (protruding structures in Figures 4E and G for the monolayered Sn-based overlay coating are heavily strained and deformed, whereas significantly lesser plastic deformation is observed in Figures 5E and G for the multilayered Sn-based overlay coating) does not seem to have a significant effect on the resistance to cavitation (they do have an effect in the resistance to wear and fatigue [18]). Although the Sn-based multilayered overlay coating would present higher shear strength and lower chance to accumulate plastic deformation, the cumulative volume loss and erosion rate (Figure 3A and B, respectively) was very similar for both overlay coatings when the erosion was predominantly within the first few microns of the soft Sn-based material. It is after the initial erosion stage when presence of the hard-soft Sn-based sublayer/IMC layer interface can be clearly observed, not only in the cross-section of the tested samples (Figures 4E and G for the monolayered Sn-based overlay coating and Figures 5E and G for the multilayered Sn-based overlay coating), but also in the cumulative volume loss and erosion rate (Figure 3A and B, respectively). And this would be related to the barrier effect that blocks dislocation glide resulting in dislocations in Sn-based sublayers not gliding across the Snbased sublayer/IMC layer interface, which results in cracks not progressing through the IMC layer, preventing the complete delamination and removal of the overlay as shown in Figure 5G. Overall, the cumulative volume loss measured during the cavitation experiments, combined with the observations from the investigation of tested samples of the monolayered and multilayered Sn-based overlays, indicate the following cavitation wear mechanism for both materials:
Monolayered Sn-based overlay 1. Plastic deformation at the surface of the overlay causes the initiation of cracks from the surface. 2. Cracks progressively propagate towards the overlay/diffusion barrier layer interface. 3. Cracks coalesce near the overlay/diffusion barrier layer interface and larger portions of the overlay are detached. The cavitation erosion rate increases during the second half of the experiment.
Multilayered Sn-based overlay: 1. Plastic deformation at the surface of the overlay causes the initiation of cracks from the surface. 2. Cracks progressively propagate towards the top Sn sublayer/IMC layer interface.
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3. Cracks coalesce near the top Sn sublayer/IMC layer interface and small portions of the overlay are detached. The rest of the overlay structure remains with localised removal in areas with particularly strong cavitation activity. The overall cavitation erosion rate remains constant during the second half of the experiment. These results here presented must not mean that the resistance to cavitation of monolayered Sn-based overlays should be diminished, as its outstanding performance against cavitation erosion in comparison with conventional Pb-based overlay coating is well known as shown in Figure 6, which show that the nature of the material (Pb vs Sn with similar hardness) has an inherent effect on the resistance to cavitation. Moreover, these results highlight how the addition of the functional IMC layer and the resulting hard-soft interfaces further enhance the excellent cavitation resistance of monolayer Sn-based overlays, making the multilayer structure a suitable candidate for even harsher engine operating conditions.
4. CONCLUSIONS The overall resistance to cavitation of monolayered Sn-based overlay coatings can be further enhanced by adding an IMC layer splitting the bulk Sn-based coating into two Sn-based sublayers. Plastic deformation occurred at the surface of both monolayered and multilayered overlays, causing the initiation and progressive propagation of cracks from the surface. However, whereas these cracks coalesce near the overlay/diffusion barrier layer interface in the monolayered overlay, coalescence occurs near the top Sn sublayer/IMC layer interface in the multilayered overlays. This results in the delamination of relatively small portions of the top Snbased sublayer only in the case of the multilayered overlay, compared to the large portions of material detached from the monolayered material. The reason for this would be presence of the IMC layer, which acts as a ‘barrier’ against cavitation erosion preventing the significant increase in erosion rate expected during the ‘acceleration’ stage.
ACKNOWLEDGEMENTS The authors would like to acknowledge the European Commission for its funding through the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691503.
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TABLES Table 1. Monolayer and multilayer Sn-based overlays used in the present study. Monolayer 1st sublayer, top 2nd sublayer, middle 3rd sublayer, bottom
Material
Multilayer
Sn-based alloy (≈15 Hv) [18]
Sn-based alloy (≈15 Hv) [18]
20 µm
8 µm
Material
n/a
SnNi IMC layer (≈560 Hv) [18]
Thickness
n/a
4 µm
Material
n/a
Sn-based alloy (≈15 Hv) [18]
Thickness
n/a
8 µm
Thickness
Table 2. Experimental parameters defined for the cavitation experiments conducted in the present study. Cavitation test conditions Bearing dimension
Ø56 mm
Frequency
19 kHz
Power output
600 W
Oscillation amplitude
≈25 µm
Medium
water
Temperature
10-20 °C
Clearance
0.5 mm
Horn diameter
Ø12 mm
FIGURES CAPTIONS Figure 1. A) Monolayered Sn–based overlay: SEM backscattered electron image (left) and secondary electron image (right) micrographs of overlay structure. Adapted from B) Multilayered Sn–based overlay: SEM backscattered electron image (left) and secondary electron image (right) micrographs of overlay structure. Adapted from Ref. [18], with permission from Elsevier. Figure 2. Experimental set-up used in cavitation experiments. Figure 3. Figures of merit estimated from cavitation experiments for monolayered ( continuous line) and multilayered ( - dotted line) Sn-based overlay coatings: (A) cumulative
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volume loss, (B) estimated erosion rate. Scatter bars indicate maximum/minimum measured values. Figure 4. SEM images of a monolayered Sn-based overlay coating after cavitation experiment: (A) general overview of surface of tested region; (B) surface and (C) cross-section images of untested overlay coating area, and (D, F, H) surface and (E, G, I) cross-section images of different tested overlay coating areas. Figure 5. SEM images of a multilayered Sn-based overlay coating after cavitation experiment: (A) general overview of surface of tested region; (B) surface and (C) cross-section images of untested overlay coating area, and (D, F, H) surface and (E, G, I) cross-section images of different tested overlay coating areas. Figure 6. Comparison of the Sn-based overlays studied in this study with a conventional Pbbased overlay: Cumulative volume loss for monolayered Sn-based overlays ( - thick line), multilayered Sn-based overlays ( - dotted line) and Pb-based overlay [8] ( - dashed line).
REFERENCES [1]
F. P. Bowden, D. Tabor, The friction and lubrication of solids, Clarendon Press, Oxford, 1950.
[2]
I. Kerr, M. Priest, Y. Okamoto, M. Fujita, Friction and wear performance of newly developed automotive bearing materials under boundary and mixed lubrication regimes, P. I. Mech. Eng. J. – J. Eng. 221 (2007) 321-331.
[3]
World Health Organization - Regional Office for Europe, http://www.euro.who.int/en/health-topics/environment-andhealth/pages/news/news/2013/10/join-forces-on-lead-poisoning-prevention-week
[4]
European Union directive 2000/53/EC. Annex II – Amendment to Annex of the end of life vehicle directive, 2005.
[5]
European Union directive 2011/65/EU.
[6]
The Hong Kong International Convention for the Safe and Environmentally Sound Recycling of Ships, http://www.imo.org/about/conventions/listofconventions/pages/the-hong-konginternational-convention-for-the-safe-and-environmentally-sound-recycling-ofships.aspx
[7]
European Union regulation 1257/2013..
[8]
H. Tsuji, Y. Tomita, N. Kawakami, I. Kerr, J. Harrison, Development of a new tin based overlay for medium speed diesel engines, 24th CIMAC World Congress on Combustion Engine Technology, Kyoto, Japan, June 7-11, 2004. 10
[9]
C. Kroeger, Building Customer Value with Natural Gas Fuel Across the Markets for Large Engines, 6th AVL Large Engines Techdays, Graz, Austria, 6th-7th May, 2014.
[10] K. Portin, Technology Trends for Wärtsilä Gas Engines, 6th AVL Large Engines Techdays, Graz, Austria, 6th-7th May, 2014. [11] T. Gocmez, The Diesel Engine – Propulsion Technology for the Future, ICAT'14, Antalya, Turkey, 12th-15th August, 2014. [12] W. Müller, Current Development Activities for the MWM Engine Family TCG2032, 6th AVL Large Engines Techdays, Graz, Austria, 6th-7th May, 2014. [13] D.R. Garner, R.D. James, J.F. Warriner, Cavitation erosion damage in engine bearings: theory and practice, J. Eng. Power 102 (1980) 847-857. [14] R.D. James, Erosion damage in engine bearings, Tribology International 8 (1975) 161-170. [15] D. Dini et al., STLE Annual Meeting & Exhibition, Lake Buena Vista, USA, May 18-21, 2014 [16] D. Dowson, C.M. Taylor, Cavitation in bearings, Ann. Rev. Fluid Mech. 11 (1979) 35-66. [17] Y. Zhang, M. Pal, E. Banchelli, I. Kerr, Lead-free tin or tin-based overlay for a plain bearing, GB patent 2529382, 2016. [18] Y. Zhang, I. Tudela, M. Pal, I. Kerr, High strength tin-based overlay for medium and high speed diesel engine bearing tribological applications, Tribology International 93 (2016) 687-695. [19] T. Okada, Y. Iwai, Resistance to wear and cavitation erosion of bearing alloys, Wear 110 (1986) 331-343. [20] C. Evans, J.F. Warriner, Bearings and bearing metals, in: R. Baranescu, B. Challen (Eds.), Diesel engine reference book, 2nd ed., Butterworth-Heinemann Ltd, Oxford, 1999. [21] L.A. Espitia, A. Toro, Cavitation resistance, microstructure and surface topography of materials used for hydraulic components, Tribology International (2010) 2037-2045. [22] S.K. Wu, H.C. Lin, C.H. Yeh, A comparison of the cavitation erosion resistance of TiNi alloys, SUS304 stainless steel and Ni-based self-fluxing alloy, Wear 244 (2000) 85-93. [23] A. Moussatov, C. Granger, B. Dubus, Cone-like bubble formation in ultrasonic cavitation field, Ultrason. Sonochem. 10 (2003) 191-195. [24] O. Louisnard, A simple model of ultrasound propagation in a cavitating liquid. Part II: Primary Bjerknes force and bubble structures, Ultrason. Sonochem. 19 (2012) 66-76. [25] J.S. Koehler, Attempt to design a strong solid, Phys. Rev. B 2 (1970) 547-551. [26] S.L. Lehoczky, Strength enhancement in thin‐layered Al‐Cu laminates, J. Appl. Phys. 49 (1978) 5479-5485. [27] W. Wang, R.N. Singh, Influence of the microstructure on the mechanical properties of Ni/Sn multilayered composites, Mater. Sci. Eng. A 271 (1999) 306-314.
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HIGHLIGHTS:
Cavitation resistance of Pb-free Sn-based overlay coatings has been evaluated.
Coating structure has an effect on cavitation resistance of Sn-based overlay coatings.
Cavitation resistance further enhanced when adding an intermetallic middle layer.
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PII: DOI: Reference:
S0043-1648(17)31711-8 https://doi.org/10.1016/j.wear.2018.07.002 WEA102450
To appear in: Wear Received date: 27 November 2017 Revised date: 30 June 2018 Accepted date: 3 July 2018 Cite this article as: Ignacio Tudela, Rolandas Verbickas, Grazina Burmistroviene and Yi Zhang, Study of the cavitation resistance of Sn-based coatings, Wear, https://doi.org/10.1016/j.wear.2018.07.002 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
STUDY
OF THE CAVITATION RESISTANCE OF
SN-
BASED COATINGS I GNACIO T UDELA *, R OLANDAS V ERBICKAS , G RAZINA B URMISTROVIENE AND
Y I Z HANG †
Daido Metal Co., Ltd. European Technical Centre UK, Research& Development Department, Winterhay lane, Ilminster, TA19 9PH, United Kingdom.
ABSTRACT: In this study, the resistance to cavitation erosion of different Sn-based journal bearing overlay coatings were studied and compared. An ultrasonic horn set-up was used to generate cavitation over the surface of samples coated with the evaluated coatings, the volume loss during the tests was estimated and the surface morphology and structure of the overlay coatings was investigated after tests. The results show how the overlay coating structure has an effect on the cavitation resistance of the Sn-based coatings studied, as the multilayered Sn-based overlay coating exhibited a significantly enhanced resistance to cavitation erosion compared with monolayered Sn-based overlay coatings. KEYWORDS: Pb-free; Sn-based coatings; cavitation resistance; journal bearings
* †
Dr. Ignacio Tudela, Email: [email protected] Dr. Yi Zhang, Email: [email protected]
1
1. INTRODUCTION Pb-based overlay coatings have been widely used in trimetal journal bearings due to their good seizure resistance and lubrication properties [1,2]. However, Pb constitutes a serious hazard for the environment due to its highly poisonous nature and its particular harmfulness to children [3] and the legislative framework that removed Pb from the automotive market [4] is gradually extending into medium and high speed engines [5-7]. A suitable option to address this is the replacement of Pb-based overlay coatings with Pb-free Sn-based overlay coatings. State-of-art Sn-based overlay coatings can exhibit superior performance compared to conventional Pb-based overlay coatings [8]. Nevertheless, current and emerging design trends in the large engine industry [9-12] will lead to an even more demanding environment for engine components. Trends such as more extensive use of turbocharging [13,14] and increase in rotational speeds and combustion/inertial loads [15] could also result in cavitation playing a critical role in engine bearing damage in the future. Cavitation erosion in bearings, once a rare phenomenon which has become more important since the early 1970’s [14], generally occurs when the pressure of the lubricant locally falls below its vapour pressure due to i) flow fluctuations caused by the interaction between the design features of the journal bearing and the crankshaft (e.g. oil grooves and holes, drillings, etc.) and ii) transient oil film pressures (e.g. changes in crankshaft eccentricity) [13]. This means that cavitation erosion in an engine journal bearing can be the result of different phenomena which results in the occurrence of cavitation [16]: 1. Suction erosion when the crankshaft recedes rapidly from the surface of the journal bearing. 2. Discharge erosion due to a fluctuating discharge into the bearing groove caused by oil displacement of ahead of a crankshaft moving radially. 3. Flow erosion due to the flow of lubricant across discontinuities in the journal bearing (e.g. edges of oil grooves, supply oil holes, joint face reliefs, etc.). 4. Impact erosion caused by a drilling supplying oil from a main to a crankshaft big-end bearing passes from a grooved to an ungrooved portion of the surface of the crankshaft big-end bearing. 5. Dispersed erosion occurring along with rippling of the bearing overlay. In order to further enhance the structural strength of Sn-based overlay coatings, the authors have proposed a multilayered Sn-based overlay coating [17] where the addition of an SnNi intermetallic compound (IMC) layer splits the bulk Sn-based coating into two Sn-based sublayers, resulting in a further enhancement of the fatigue, wear and seizure performance of 2
Sn-based overlay coatings [18]. In this study, the cavitation performance of the multilayered Snbased overlay coating is evaluated and compared with a monolayered Sn-based overlay coating. The results show that the multilayered Sn-based overlay coating exhibits further enhanced resistance to cavitation as the multilayered structure with alternating Sn/IMC layers provides higher structural strength.
2. MATERIALS AND METHODS 2.1. BEARING OVERLAY MATERIALS Table 1 summarises the monolayered and the multilayered Sn-based overlay coatings [18]. Both overlay coating materials (thickness = 20 µm) were applied onto standard semi-circular bronze bearings with an external diameter of 56 mm. Prior to the application of the overlay coatings, the bearings were pre-treated to ensure proper coverage and adhesion. The bearings were firstly degreased in a commercial alkaline cleaner and then anodically etched in a concentrated HCl solution. A 2-3 µm Ni layer was electrodeposited over the bronze lining right before the application of the overlay coatings to act as a diffusion barrier layer between the overlay coating and the bronze lining. Both monolayered and multilayered Sn-based overlay coatings were electrodeposited by proprietary processes using different electrolytes [18]. In the case of the multilayered Sn-based overlay coating, an additional intermediate IMC layer was incorporated into the structure between the Sn sublayers aiming at the enhancement of various bearing properties. Figure 1 displays the overlay structure of both monolayered and multilayered Sn-based overlay coatings. Figure 1A shows the overlay structure of the monolayered Sn–based overlay coating where different features have been highlighted: A is the bronze substrate, B is the Ni diffusion barrier layer, C is the monolayered Sn–based overlay and D are SnCu intermetallic compound particles uniformly distributed within the matrix. The overlay structure of the multilayered Sn-based overlay coating is shown in Figure 1B: A is the bronze substrate, B is the Ni diffusion barrier layer and C are the thinner Sn–based sublayers which are separated by the intermediate IMC layer D (SnNi). SnCu phases D are also present in the multilayered Sn-based overlay, but in this case they are no longer dispersed within the Sn-based layers as the majority migrated into the areas between the thinner Snbased sublayers C and the IMC layer D after thermal annealing.
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2.2. CAVITATION EXPERIMENTS The experimental set-up (Figure 2) used is analogous to that of previous studies on cavitation performance of bearing materials available in the literature [8,14,13,19]. Bearings were immersed in a water bath at room temperature. An ultrasonic horn closely placed on top of the bearings was used to generate cavitation. The horn had a cylindrical end to simulate shaft/bearing geometry. Experiments were repeated at least three times to ensure the reproducibility of the data. The parameters of the experiments are displayed in Table 2. Each bearing was carefully cleaned and weighed before and after the experiments to accurately account for the erosion loss caused by cavitation. Following Okada and Iwai’s methodology [19], erosion loss is expressed in this paper in terms of volume loss after estimating the overall density of the different layers in the overlays (approximated density of monolayered Sn-based overlay coating: 7.3 g/mm3, approximated density of multilayered Sn-based overlay coating with IMC layer: 7.5 g/mm3). Regarding the length of the experiments, each sample was tested for a total of 10 minutes [20] (with intermediate measurements after 5 minutes). <10-minute cavitation tests are a common practice in the industry [13,20] when evaluating the resistance to cavitation of bearing overlay coating materials as Pb-based overlay coatings, which are the most widely used in this field, are completely removed within the first few minutes of this type of experiments [8,20]. 10 minutes were also enough to completely remove the monolayered Sn-based overlay coating in the majority of the areas within the tested region, which implies that any longer experiment time would have resulted in erosion beyond the overlay coating. If the experiments were any longer, any wear loss recorded would not only be solely linked to the overlay removal, but also to the potential erosion of the Ni diffusion barrier layer underneath the Sn-based overlay and the bronze substrate under the Ni diffusion barrier layer. The surface morphology and overlay coating structure of tested and untested areas of the monolayered and multilayered Sn-based overlays were examined with a Hitachi TM3030Plus Tabletop Scanning Electron Microscope (SEM) operating in backscatter mode at an accelerating voltage of 15 keV to better understand the effect of the presence addition of the functional IMC layer on the cavitation resistance of the multilayered Sn-based overlay.
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3. RESULTS AND DISCUSSION 3.1. VOLUME LOSS Figure 3A displays the cumulative volume loss of both monolayered and multilayered Sn-based overlays. In both cases, it was observed that the cumulative volume loss increased with time and was very similar for both overlays after 5 minutes as both surfaces being eroded effectively consisted of very similar material (Figure 1) with nearly identical hardness of ≈15 Hv (Table 1) during the first half of the experiments. This changed during the second half of the experiments, as the cumulative volume loss for the monolayered overlay was 2.5 times that of the multilayered overlay after 10 minutes. This suggests that the progressive erosion of material from the top sublayer results in the eroded surface progressively moving closer to the IMC layer which, with a hardness of ≈560 Hv (Table 1), starts to exert significant influence on the resistance to cavitation of the multilayered overlay as a whole. In both cases, no ‘incubation’ stage (negligible erosion rate [21]) was observed during the experiments (Figure 3B). Nevertheless, whereas the erosion rate in the monolayered overlay clearly increased with time under the so-called ‘acceleration’ stage (erosion rate is variable and increasing with time [21]), the erosion rate did not change significantly for the multilayered Sn-based overlay, as it seemed to reach a low and relatively constant value during the second half of the experiment (cavitation erosion occurring under the so-called ‘maximum rate’ stage when a constant erosion rate is observed [21]).
3.2. SURFACE AND CROSS-SECTION ANALYSIS Figure 4 displays the SEM images of the monolayered overlay after tests. In the untested region, large Sn grains covered the surface of the overlay (Figures 4B and C) which clearly consisted of a Sn matrix. However, a variety of surface morphologies and structures were noticeable in the tested region (Figure 4A). In the centre, large protruding structures were observed between craters and undulations of the surface (Figure 4D), and some deep cracks were clearly observed within the overlay (Figure 4E). These protruding structures seemed heavily strained by the action of the bubbles, directly as a result of their non-spherical collapse near the surface resulting in the formation of high-speed liquid jets or indirectly by shockwaves generated during their oscillation and collapse, indicating that the overlay underwent plastic deformation. In this scenario of plastic deformation, fatigue would potentially become an important degradation mechanism, resulting in more pronounced wear due to the detachment of overlay material through micro-cracks coalescing near the overlay/diffusion barrier layer interface, which is what can be observed in certain areas surrounding the centre of the tested region (Figure 4F and G). Once most of the overlay had been removed, the remaining surface would 5
exhibit a smoother surface finish as the harder Ni barrier layer (hardness of ≈270 Hv [18]) is exposed. This is the case of the rest of the areas in the tested region, where a large amount of very small pits and small craters are observed (Figure 4H and I). The appearance of these areas is consistent with that of Ni-based self-fluxing alloys reported by Wu et al. [22]. In these areas, small pit damage would occur when the Ni diffusion layer is firstly exposed to cavitation. Cavitating bubbles would then nucleate and grow near these small pits, with some progressively growing wider and deeper [22]. Figure 5 displays the SEM images of the multilayered Sn-based overlay after test. Large Sn grains covered the surface (Figure 5B) of the overlay which consisted of a two Sn-based sublayers separated by the IMC layer (Figure 5C). Again, different surface morphologies and structures were noticed in the tested region of the bearing, as shown in Figure 5A. In the centre, nodule-like structures protruding from the surface of the overlay were also observed (Figure 5D). However, no deep cracks were noticed, as just some minor cracks propagating until the top Sn sublayer/IMC layer interface were observed (Figure 5E). Again, these nodule-like structures seemed to be strained by the action of cavitating bubbles as the overlay underwent plastic deformation during the ‘acceleration’ stage [21]. And again, wear through fatigue would eventually occur due to the detachment of overlay material through micro-cracks propagating through the top Sn sublayer. But the micro-cracks would now coalesce near the top Sn sublayer/IMC layer interface instead of the overlay/diffusion barrier layer interface, as shown in the vast majority of the tested areas besides the centre of the tested region (Figures 5F and G). Finally, overlay removal was noticed in a very few areas of the tested region of the multilayered overlay (Figures 5H and I). These areas were randomly localised in the periphery of the tested region. The reason for this is that, in any ultrasonic horn, the cavitation is outstandingly strong near the edges of the emitting surface [23,24]. Therefore, such strong activity of cavitating bubbles and the formation of streamers in those areas would cause localised removal of the overlay. As previously reported by the authors, the overall beneficial effect of the presence of the IMC layer and the resulting multilayered structure is [18]: 1. Higher shear strength due to thinner Sn-based sublayers [25,26]. 2. Plastic deformation in the top Sn-based sublayer has significantly lesser chance to accumulate due to shorter progression route in thinner Sn-based sublayers [18]. 3. Barrier effect of the hard-soft interfaces which would block dislocation glide resulting in dislocations in Sn-based sublayers not gliding across the Sn-based sublayer/IMC layer interface [27].
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The higher shear strength and lower accumulation of plastic deformation related to having thinner Sn-based sublayers, although can be clearly observed in the cross-section of the tested samples (protruding structures in Figures 4E and G for the monolayered Sn-based overlay coating are heavily strained and deformed, whereas significantly lesser plastic deformation is observed in Figures 5E and G for the multilayered Sn-based overlay coating) does not seem to have a significant effect on the resistance to cavitation (they do have an effect in the resistance to wear and fatigue [18]). Although the Sn-based multilayered overlay coating would present higher shear strength and lower chance to accumulate plastic deformation, the cumulative volume loss and erosion rate (Figure 3A and B, respectively) was very similar for both overlay coatings when the erosion was predominantly within the first few microns of the soft Sn-based material. It is after the initial erosion stage when presence of the hard-soft Sn-based sublayer/IMC layer interface can be clearly observed, not only in the cross-section of the tested samples (Figures 4E and G for the monolayered Sn-based overlay coating and Figures 5E and G for the multilayered Sn-based overlay coating), but also in the cumulative volume loss and erosion rate (Figure 3A and B, respectively). And this would be related to the barrier effect that blocks dislocation glide resulting in dislocations in Sn-based sublayers not gliding across the Snbased sublayer/IMC layer interface, which results in cracks not progressing through the IMC layer, preventing the complete delamination and removal of the overlay as shown in Figure 5G. Overall, the cumulative volume loss measured during the cavitation experiments, combined with the observations from the investigation of tested samples of the monolayered and multilayered Sn-based overlays, indicate the following cavitation wear mechanism for both materials:
Monolayered Sn-based overlay 1. Plastic deformation at the surface of the overlay causes the initiation of cracks from the surface. 2. Cracks progressively propagate towards the overlay/diffusion barrier layer interface. 3. Cracks coalesce near the overlay/diffusion barrier layer interface and larger portions of the overlay are detached. The cavitation erosion rate increases during the second half of the experiment.
Multilayered Sn-based overlay: 1. Plastic deformation at the surface of the overlay causes the initiation of cracks from the surface. 2. Cracks progressively propagate towards the top Sn sublayer/IMC layer interface.
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3. Cracks coalesce near the top Sn sublayer/IMC layer interface and small portions of the overlay are detached. The rest of the overlay structure remains with localised removal in areas with particularly strong cavitation activity. The overall cavitation erosion rate remains constant during the second half of the experiment. These results here presented must not mean that the resistance to cavitation of monolayered Sn-based overlays should be diminished, as its outstanding performance against cavitation erosion in comparison with conventional Pb-based overlay coating is well known as shown in Figure 6, which show that the nature of the material (Pb vs Sn with similar hardness) has an inherent effect on the resistance to cavitation. Moreover, these results highlight how the addition of the functional IMC layer and the resulting hard-soft interfaces further enhance the excellent cavitation resistance of monolayer Sn-based overlays, making the multilayer structure a suitable candidate for even harsher engine operating conditions.
4. CONCLUSIONS The overall resistance to cavitation of monolayered Sn-based overlay coatings can be further enhanced by adding an IMC layer splitting the bulk Sn-based coating into two Sn-based sublayers. Plastic deformation occurred at the surface of both monolayered and multilayered overlays, causing the initiation and progressive propagation of cracks from the surface. However, whereas these cracks coalesce near the overlay/diffusion barrier layer interface in the monolayered overlay, coalescence occurs near the top Sn sublayer/IMC layer interface in the multilayered overlays. This results in the delamination of relatively small portions of the top Snbased sublayer only in the case of the multilayered overlay, compared to the large portions of material detached from the monolayered material. The reason for this would be presence of the IMC layer, which acts as a ‘barrier’ against cavitation erosion preventing the significant increase in erosion rate expected during the ‘acceleration’ stage.
ACKNOWLEDGEMENTS The authors would like to acknowledge the European Commission for its funding through the European Union’s Horizon 2020 research and innovation programme under grant agreement No 691503.
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TABLES Table 1. Monolayer and multilayer Sn-based overlays used in the present study. Monolayer 1st sublayer, top 2nd sublayer, middle 3rd sublayer, bottom
Material
Multilayer
Sn-based alloy (≈15 Hv) [18]
Sn-based alloy (≈15 Hv) [18]
20 µm
8 µm
Material
n/a
SnNi IMC layer (≈560 Hv) [18]
Thickness
n/a
4 µm
Material
n/a
Sn-based alloy (≈15 Hv) [18]
Thickness
n/a
8 µm
Thickness
Table 2. Experimental parameters defined for the cavitation experiments conducted in the present study. Cavitation test conditions Bearing dimension
Ø56 mm
Frequency
19 kHz
Power output
600 W
Oscillation amplitude
≈25 µm
Medium
water
Temperature
10-20 °C
Clearance
0.5 mm
Horn diameter
Ø12 mm
FIGURES CAPTIONS Figure 1. A) Monolayered Sn–based overlay: SEM backscattered electron image (left) and secondary electron image (right) micrographs of overlay structure. Adapted from B) Multilayered Sn–based overlay: SEM backscattered electron image (left) and secondary electron image (right) micrographs of overlay structure. Adapted from Ref. [18], with permission from Elsevier. Figure 2. Experimental set-up used in cavitation experiments. Figure 3. Figures of merit estimated from cavitation experiments for monolayered ( continuous line) and multilayered ( - dotted line) Sn-based overlay coatings: (A) cumulative
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volume loss, (B) estimated erosion rate. Scatter bars indicate maximum/minimum measured values. Figure 4. SEM images of a monolayered Sn-based overlay coating after cavitation experiment: (A) general overview of surface of tested region; (B) surface and (C) cross-section images of untested overlay coating area, and (D, F, H) surface and (E, G, I) cross-section images of different tested overlay coating areas. Figure 5. SEM images of a multilayered Sn-based overlay coating after cavitation experiment: (A) general overview of surface of tested region; (B) surface and (C) cross-section images of untested overlay coating area, and (D, F, H) surface and (E, G, I) cross-section images of different tested overlay coating areas. Figure 6. Comparison of the Sn-based overlays studied in this study with a conventional Pbbased overlay: Cumulative volume loss for monolayered Sn-based overlays ( - thick line), multilayered Sn-based overlays ( - dotted line) and Pb-based overlay [8] ( - dashed line).
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HIGHLIGHTS:
Cavitation resistance of Pb-free Sn-based overlay coatings has been evaluated.
Coating structure has an effect on cavitation resistance of Sn-based overlay coatings.
Cavitation resistance further enhanced when adding an intermetallic middle layer.
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