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A study on the microstructure and tribological behavior of cold-sprayed metal matrix composites reinforced by particulate quasicrystal
A study on the microstructure and tribological behavior of cold-sprayed metal matrix composites reinforced by particulate quasicrystal Xueping Guo, Jingfeng Chen, Hongliang Yu, Hanlin Liao, Christian Coddet PII: DOI: Reference:
S0257-8972(14)00489-7 doi: 10.1016/j.surfcoat.2014.05.062 SCT 19449
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
16 January 2014 2 May 2014 18 May 2014
Please cite this article as: Xueping Guo, Jingfeng Chen, Hongliang Yu, Hanlin Liao, Christian Coddet, A study on the microstructure and tribological behavior of coldsprayed metal matrix composites reinforced by particulate quasicrystal, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.05.062
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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT A study on the microstructure and tribological behavior of coldsprayed metal matrix composites reinforced by particulate quasicrystal Xueping GUO 1, 2 *, Jingfeng CHEN 1, 2, Hongliang YU 1, 2, Hanlin LIAO 3, Christian CODDET 3 1 Marine Engineering College, Jimei University, Xiamen 361021, P.R. China 2 Fujian Provincial Key Laboratory of Ship and Ocean Engineering, Xiamen 361021, P.R. China 3 IRTES -LERMPS ( Laboratoire d’Etudes et de Recherches sur les Matériaux, les Procédés et les Surfaces)- UTBM (Université de
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Technologie de Belfort-Montbéliard), Site de Sévenans, 90010 Belfort Cedex, France
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Keywords: Cold spray; Quasicrystal; MMC; Tribology
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In the present study, mechanical blends of AlCuFeB quasicrystal and tin bronze powders were deposited by cold spray process to obtain metal matrix composites (MMCs) reinforced with quasicrystalline particulates. The influences of the incorporation of quasicrystal particles on the particle deposition behavior, microstructure and microhardness of the composite coatings were investigated. In order to evaluate the influence of reinforcing quasicrystal phase on the tribological behavior of the coatings, ball-on-disc sliding tribological tests were conducted in an ambient condition. The results showed that the incorporation of quasicrystal particles reduced the porosity and increased the microhardness of the composite coatings. At the same time, a reduction of the friction coefficient and an increase of wear rate were found. Wear mechanism were discussed and correlated to the microstructure and microhardness of coatings.
1. Introduction
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Quasicrystal materials have unique atomic structures and consequently well-determined physical, chemical and mechanical properties, such as low surface energy, high hardness, low coefficient of friction (COF), good wear- and corrosion-resistance [1]. It has been known that the mechanical properties of quasicrystal materials are similar to those of brittle ceramics or intermetallic compounds. This limits the potential applications of quasicrystals as structural materials. However, quasicrystals can be used as a particulate-reinforcing phase for polymer- or metal-matrix composites or to form thin films/coatings for surface modifications. Among various coating processes, thermal spray techniques have predominant advantages for fabricating quasicrystalline (or composite) coatings due to its flexibility and operability of the process. Numerous studies concerning the characterization of microstructures and properties of quasicrystalline coatings prepared by thermal spray have been conducted [2-4]. However, using these conventional thermal spray processes, the occurrence of unexpected phase transformation of quasicrystals is usually inevitable due to the high temperature of the process [5, 6]. Cold spray is a low temperature deposition process in which particles are accelerated through a De-Laval type nozzle, and the coating in cold spray process is obtained solely as a result of the accumulation of plastic deformation of solid particles impinging upon a substrate. As a competitive candidate for preparing high quality coatings, cold spray technique has attracted much interest in fabrication of metallic coatings since its emergence. Cold spray can also be used to prepare MMC coatings which usually exhibit improved physical or mechanical properties. This provides a new means for the preparation of desired MMCs, and the properties of cold-sprayed MMCs may be conveniently tailored by a proper addition of reinforcing particles. In this study, mechanically blended mixtures of AlCuFeB quasicrystal and bronze powders were deposited by cold spray process, aiming at tailoring quasicrystal-reinforced MMC coatings. Powder mixtures with different AlCuFeB contents were used to prepare composite coatings for evaluating the influence of quasicrystal fraction on the characteristics of deposited coatings. The structure-property relationships of the composite coatings were investigated. As one of the major objectives of this study, the tribological behavior of the quasicrystal particles reinforced coatings were investigated and correlated to their microstructures and mechanical properties.
2. Experimental procedure 2.1. Powders Inert gas atomized AlCuFeB quasicrystal (referred as QC hereinafter) and Cu-8wt.%Sn (CuSn8) powders were used as the feedstocks in this work. The as-atomized QC powder is in a nominal atomic composition of Al59.1Cu25.6Fe12.1B3.2. Both QC and CuSn8 particles have a spherical morphology. Particle size distribution were measured by Mastersizer 2000 (Malvern, UK). The size of the both particles follows a typical Gaussian distribution feature. For the CuSn8 powder, the d (10), d (50) and d (90) were 8.4 µm, 17.1 µm, and 33.8 µm, respectively. As to the QC powder, the d (10), d (50) and d (90) were 7.9 µm, 17.1 µm, and 32.2 µm, respectively. It is clear that the sizes of CuSn8 and QC powders were in a similar distribution range. Both of the powders were in-house made and more technical details regarding manufacture of these powders are available in our previous publication [7]. Mixtures of
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ACCEPTED MANUSCRIPT CuSn8 with three different QC fractions (19.5 %, 36.8 % and 57.6% in volume) were mechanically blended and used as initial powder feedstocks for coating deposition, and the obtained CuSn8 and composite coatings will hereinafter be referred as CuSn8, QC19, QC36 and QC57, respectively.
2.2. Cold spray conditions
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A commercially available Kinetic 3000 cold spray system (CGT GmbH, Germany) coupled with a self-designed nozzle was used for coating deposition. Compressed air and high pressure argon were respectively used as the main gas and the powder carrier gas. Pressure and temperature of the main gas were maintained at about 27 bars and 540 °C, respectively. The pressure of the carrier argon gas was 30 bars. The standoff distance between the nozzle exit and the sample surface was kept constant at 30 mm for all cold spray processes. The moving speed of spray gun manipulated by an ABB robot was maintained at 50 mm/s. Four passes of the spray gun were made for each sample, and the thickness of obtained coatings was about 350 µm. In this work, mild steel disks with a dimension of Ø43 x 4 mm were chosen as substrates, and the disks were degreased and grit blasted following usual procedures prior to coating deposition.
2.3 Coating characterization
2.4 Tribological tests
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The cross-sectional microstructure of the prepared coatings was examined by optical microscope (Nikon, ECLIPSE ME600) and scanning electron microscope (JEOL, JSM-5800LV, Japan). The porosity level of coatings was evaluated based on polished cross-sections by using an image processing method. Ten optical images were used to calculate the average porosity. It should be noted here that only the deeper zone was taken into account for analyzing the porosity level, because the top layer was relatively porous than the deeper zone in the coatings. Energy Disperse Xray analysis have been conducted for at least three times over different areas of each coating to determine the fraction of QC within the coatings. Microhardness of coatings was measured by a Vickers hardness tester (Leica VMHT30A, Germany) at 2.94 N load with 15 s dwelling time, and the corresponding sizes of the indentation marks were larger than a single splat during the hardness measurement. The microhardness given in this study is an average of at least 10 measurements randomly taken from the cross-sections of coatings.
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Tribological tests were conducted in a dry sliding condition through a re-constructed ball-on-disc tribometer (CSM, Switzerland) to evaluate the tribological performances of the prepared coatings. All the coatings were ground using diamond sandpapers (following 300 mesh, 600 mesh, 800 mesh, 1200 mesh and finally 4000 mesh), then they were polished using diamond slurries down to an average surface roughness of about 0.04 µm. All the coatings were treated in the same procedure to assure that they could have a same initial surface topography. WC-Co balls with a 6 mm-diameter and a mirror finished surface were used as the counterpart. The polished coatings slide against the WC-Co ball in a linear oscillation mode at a mean sliding velocity of 70 mm/s. The length of each stroke was 11.6 mm and the frequency of the oscillation movement was kept at 7 Hz. The normal load was in the range of 2 N and the total sliding distance was 280 mm. The positive peaks of the COF curves were extracted and averaged as the mean COF for each tribotest. Morphologies of the wear traces were examined by optical microscope and scanning electron microscope. In this work, the width of at least four locations along the wear trace were measured by microscope and averaged to compare coatings’ wear rates. The wear rate in this work is defined as the width of wear trace divided by the normal load (N) and total sliding distance (m).
3. Results and discussion 3.1 Coating structures Fig. 1 shows the cross-sectional morphologies of CuSn8, QC19, QC36 and QC57 coatings, respectively. Compared to the original spherical morphology of CuSn8 particles, the deposited CuSn8 particles were well flattened, especially for the smaller ones, as indicated by single arrows in Fig. 1 (a) and (b). It seems that both the pure CuSn8 and the composite coatings present compact structures with few defects on the interface of splats. Compared to pure copper, the deformation ability of bronze particles is usually insufficient for the closure of the interfacial gaps between deposited particles due to a relatively lower deformability. For the composite coatings, QC particles are dispersed uniformly in the bronze matrix. It seems that most of the embedded QC particles have a dimension less than 10 µm, and only the small QC particles (approximately below 5 µm) have maintained their original spherical morphology, as shown in Fig. 1 (c). Statistically, the average size of QC particles deposited into the coating is obviously smaller than that in the starting powder. Presumably, part of the larger QC particles in the starting powder has rebounded off during the impacting process and consequently were not deposited into the CuSn8 matrix. Fragmentation of QC particles during the deposition process may also be responsible for this decrease, i.e., part of the larger QC particles have fragmented into smaller ones. SEM observation shown in Fig. 1 (d) clearly indicates that some of deposited QC particles have
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broken into smaller pieces during the deposition process. The small fragments presented in Fig. 1 (b), as marked by circles, are considered to be residuals of the crushed QC particles, while the rest of the broken particles could have been rebounded off during the impacting process. The fragmentation of brittle AlN particles has been also observed in the cold-sprayed Al/AlN composite [8]. Due to the high impacting momentum, large QC particles could be more susceptible to break into small pieces during the impacting process. Irissou et al. declared that the more brittle particles are included in the starting powder, the higher is the probability that a brittle particle hits another one, and the increase of the concentration of brittle particles in the starting powder may increase the amount of fragmented particles [9]. It seems here that the particle momentum plays a dominant role in the impacting fragmentation of QC particles.
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Fig. 2 (a) shows the porosity level of the prepared coatings. Overall, the porosities of composite coatings are much lower than that of the pure CuSn8 coating, and a slight decrease of porosity level with the increase of the QC fraction in starting powder is observed for the three composite coatings. This indicates an important influence of the incorporation of QC particles in the starting powder on the densification of coating structures. Similar results can be found in the literature, e.g., Irissou et al. declared that an incorporation of 7 wt.% Al2O3 ceramic particles in the Al powder can effectively densify the Al/Al2O3 composite coatings [9]. An interpretation concerning this densification can be attributed to the strong blasting effect of the non-ductile hard particles [7]. The quantitative analysis reported in Fig. 2 (b) shows that the volume fractions of QC phase are 7.5%, 11.5%, and 20.5% for the QC19, QC36 and QC57 coatings respectively. These values are lower than the starting mixed volume fractions. This behavior has been also observed in previous work, when the starting volume fraction (75 wt.%) of Al2O3 particles was reduced to 26 wt.% after coldspraying of MMCs [9]. In the literature [8-10], the amount of reinforcing particles entrapped in the cold-sprayed MMCs is lower than that in the starting powder mixture when a blended powder is used for coating deposition. The deposition of hard and brittle reinforcing particles (such as QC, Al2O3 and SiC etc.) in cold spray process can only be considered as a result of the “embedding” effect. The discrepancy of the QC contents between the final coating and the starting powder can generally be attributed to the rebounding effect of QC particles.
3.2 Coating hardness
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The microhardness of the coatings is illustrated in Fig. 3. The incorporation of QC phase significantly increases the hardness of composite coatings. The coating microhardness increases with the increment of the QC fraction. The increase of coating hardness can mainly be attributed to both the increased flattening of CuSn8 matrix (due to strong blasting effect of QC particles) and the reinforcing effect of hard QC particles which significantly limit the plastic deformation of the CuSn8 matrix.
3.3 Tribological behavior
Fig. 4 shows the COFs and the wear rates of the CuSn8 and the composite coatings tested in a dry sliding condition. The COFs of the coatings are in the same range, i.e. from 0.67-0.72, and only a slight decrease in the COF with the increasing QC fraction is observed. The slight reduction in COF can be attributed to the decreased adhesion between the sliding pairs with enhancing QC particle fraction. The wear rates of CuSn8 and QC19 coatings are similar, but they are lower than that of QC36 and QC57 coatings. A previous study [7] has demonstrated that an incorporation of about 8.5 vol.% QC particles can effectively improve the tribological performances of CuSn8 based MMC. An even better tribological performance was accordingly expected for the MMCs containing a higher content of QC particles. However, the results here seem not to be in accordance with this expectation. The non-linear evolution of wear rates presented in Fig. 4 (b) indicates that different wear behaviors have taken place for the coatings. Fig. 5 (a) shows that a lot of grooves and craters were generated in the wear trace of the CuSn8 coating, as indicated by circles, and the wear of the CuSn8 coating is mainly characterized by both micro-ploughing and particle delamination. The fatigue wear of the CuSn8 coating is mainly attributed to the delamination and rupture of coating layer as a result of the initiation and propagation of micro cracks. For the QC19 coating, it seems that the microploughing is no longer the most important factor contributing to the wear. However, more and even tinier craters were generated in the worn surface, as indicated by single arrows in Fig. 5 (b). The formation of these tiny craters was assumed to be resulted from the delamination of the surface layer. Due to the reinforcing effect of QC particles, a lower in-depth deformation and thus a thinner strain layer involved in the frictional process were expected for the QC19 than those of the CuSn8 coating. As a result, the micro-ploughing of CuSn8 coating can be alleviated due to the load-bearing effect of QC particles, as demonstrated by the presence of many scratches in Fig. 5 (b). Moreover, the initiation and propagation of micro-cracks can only occur within a relatively thinner sub-surface layer. This is assumed to be the main reason for the generation of the tinier craters. The QC36 and QC57 coatings have a higher wear rate than the QC19 coating. It is observed that numerous deep worn areas were generated in the wear trace and most of them were filled with small wear particles, as indicated by the single arrows in Fig. 5 (c). For particulate reinforced MMCs, the interfaces between reinforcing particles and the matrix are weakly bonded regions [9, 11, 12]. The highest pressure in this case was as high as 0.67 Gpa when estimated using Hertizian contact theory (taking E-modulus and Poisson’s ratio values of bulk CuSn8 material). Accordingly, the extremely high contact pressure can trigger the debonding of QC particles. When filled with high amount of QC particles, numerous debonded QC particles could be fragmented when
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ACCEPTED MANUSCRIPT they were entrapped into the sliding bodies and led to severe abrasion and thereby fast wear of the coating. Thus, when the content of QC particles was high, the abrasion exerted by debonded particles could compensate its beneficial aspect (load-bearing effect). Moreover, the reduced coating ductility and fracture toughness when incorporating high fraction of hard particles can also generate a high wear rate.
4. Conclusions
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1. Composite coatings present much denser microstructures than the pure CuSn8 coating, and the porosity level of composite coatings decreases with increasing the content of incorporated QC phase. It is believed that the hammering effect on the coating exerted by the QC particles enhances the coating porosity.
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2. The ratio of the deposited QC particles to its fraction in the initial powders is about 30%-40%. The low ratio is ascribed to the rebounding of QC particles upon impacting and thereby the low deposition efficiency.
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3. The coating hardness increased with the increase of the incorporation of QC phase. The reinforcing effect of QC particles is evident.
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3. The COF decreases slightly with the increase of the QC content in coatings. Incorporation of low-loading QC particles improves the wear resistance of CuSn8 coating. The improved wear resistance can be mainly attributed to the reinforcing effect of the QC particles which reduce ploughing on the coating and also reduce the depth of frictional layer. However, a higher concentration of QC phase adversely aggravated the wear resistance of QC36 and QC57 coatings possibly due to the fact that debonded QC particles can cause severe abrasion.
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Acknowledgement The authors gratefully appreciate the financial support of the Application and Basic Research Project of Ministry of Transport of PR. China (Grant No. 2013329815280), the Natural Science Foundation of Fujian Province (Grant No. 2013D033) and the Project of Technology Foundation for Selected Overseas Chinese Scholar (Ministry of Personnel of China).
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E. Huttunen-Saarivirta, Microstructure, fabrication and properties of quasicrystalline Al-Cu-Fe alloys: a review, J. Alloy. Compd. 363 (2004) 150-174. C. G. Zhou, F. Cai, H. B. Xu, and S. K. Gong, Cyclic oxidation behavior of Al-Cu-Fe-Cr quasicrystalline coating on titanium alloy, Mat. Sci. Eng. a-Struct. 386 ( 2004) 362-366. W. D. Yuan, T. M. Shao, E. Fleury, D. Se, and D. R. Chen, Microstructure and tribological properties of plasma sprayed Al-Cu-Fe quasicrystalline coatings after laser post-treatment processing, Surf. Coat. Tech. 185 (2004) 99-105. L. P. Feng, T. M. Shao, Y. J. Jin, E. Fleury, D. H. Kim, and D. R. Chen, Temperature dependence of the tribological properties of laser re-melted Al-Cu-Fe quasicrystalline plasma sprayed coatings, J. Non-Cryst. Solids. 351 (2005) 280-287. C. G. Zhou, F. Cai, J. Kong, S. K. Gong, and H. B. Xu, A study on the tribological properties of lowpressure plasma-sprayed Al-Cu-Fe-Cr quasicrystalline coating on titanium alloy, Surf. Coat. Tech. 187 (2004) 225-229. D. J. Sordelet, M. F. Besser, and J. L. Logsdon, Abrasive wear behavior of Al-Cu-Fe quasicrystalline composite coatings, Mater. Sci. Eng. A 255 (1998) 54-65. X. P. Guo, G. Zhang, W. Y. Li, Y. Gao, H. L. Liao, and C. Coddet, Investigation of the microstructure and tribological behavior of cold-sprayed tin-bronze-based composite coatings, Appl. Surf. Sci. 255 (2009) 3822-3828. D. T. Morelli, A. A. Elmoursi, T. H. VanSteenkiste, D. W. Gorkiewicz and B. Gillispie, Kinetic Spray of Aluminum Metal Matrix Composites For Thermal Management Applications, presented at the Thermal Spray 2003: Advancing the Science & Applying the Technology, Materials Park, Ohio, USA, 2003. E. Irissou, J. G. Legoux, B. Arsenault, and C. Moreau, Investigation of Al-Al2O3 cold spray coating formation and properties, J. Therm. Spray. Techn. 16 (2007) 661-668. A. Shkodkin, A. Kashirin, O. Klyuev and T. Buzdygar, Metal particle deposition stimulation by surface abrasive treatment in gas dynamic spraying, J. Therm. Spray. Techn. 15 (2006) 382-386. W. Z. Wang, F. Sun, X. Guo, H. L. Liao, O. Elkedim, and J. C. Liang, Effect of substrate surface temperature on the microstructure and ionic conductivity of lanthanum silicate coatings deposited by plasma spraying, Surf. Coat. Tech., 205 (2011) 3665-3670.
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B. N. Mordyuk, M. O. Iefimov, G. I. Prokopenko, T. V. Golub, and M. I. Danylenko, Structure, microhardness and damping characteristics of Al matrix composite reinforced with AlCuFe or Ti using ultrasonic impact peening, Surf. Coat. Tech., 204 (2010) 1590-1598.
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Figure captions:
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Fig. 1 SEM observations of the cross-sectional morphologies of (a) CuSn8, (b) QC19, (c) QC36 and (d) QC57 coatings; the single arrows present the well flattened small particles, and the circles show small fragmented QC particles
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Fig. 2 (a) Porosity levels and (b) volume fractions of QC of cold-sprayed CuSn8 and composite coatings
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Fig. 4 (a) Coefficients of friction and (b) wear rates of the cold-sprayed coatings tested in a dry sliding condition
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Fig. 5 Morphologies of the wear traces of (a) CuSn8, (b) QC19, (c) QC36 and (d) QC57 coatings tested in a dry sliding condition; the circles present the grooves and craters generated in the wear trace of the CuSn8 coating, and the single arrows indicate the craters generated in the wear trace of the composite coatings
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ACCEPTED MANUSCRIPT Highlights
Particulate QC reinforced bronze-based MMCs were prepared by cold spraying.
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Composite coatings present a dense microstructure and a high hardness. The ratio of deposited QC particles to its fraction in initial powders is 30%-40%.
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A moderate incorporation of QC phase improves the wear resistance of coating.
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S0257-8972(14)00489-7 doi: 10.1016/j.surfcoat.2014.05.062 SCT 19449
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
16 January 2014 2 May 2014 18 May 2014
Please cite this article as: Xueping Guo, Jingfeng Chen, Hongliang Yu, Hanlin Liao, Christian Coddet, A study on the microstructure and tribological behavior of coldsprayed metal matrix composites reinforced by particulate quasicrystal, Surface & Coatings Technology (2014), doi: 10.1016/j.surfcoat.2014.05.062
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 proof before it is published in its final 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.
ACCEPTED MANUSCRIPT A study on the microstructure and tribological behavior of coldsprayed metal matrix composites reinforced by particulate quasicrystal Xueping GUO 1, 2 *, Jingfeng CHEN 1, 2, Hongliang YU 1, 2, Hanlin LIAO 3, Christian CODDET 3 1 Marine Engineering College, Jimei University, Xiamen 361021, P.R. China 2 Fujian Provincial Key Laboratory of Ship and Ocean Engineering, Xiamen 361021, P.R. China 3 IRTES -LERMPS ( Laboratoire d’Etudes et de Recherches sur les Matériaux, les Procédés et les Surfaces)- UTBM (Université de
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Technologie de Belfort-Montbéliard), Site de Sévenans, 90010 Belfort Cedex, France
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Keywords: Cold spray; Quasicrystal; MMC; Tribology
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In the present study, mechanical blends of AlCuFeB quasicrystal and tin bronze powders were deposited by cold spray process to obtain metal matrix composites (MMCs) reinforced with quasicrystalline particulates. The influences of the incorporation of quasicrystal particles on the particle deposition behavior, microstructure and microhardness of the composite coatings were investigated. In order to evaluate the influence of reinforcing quasicrystal phase on the tribological behavior of the coatings, ball-on-disc sliding tribological tests were conducted in an ambient condition. The results showed that the incorporation of quasicrystal particles reduced the porosity and increased the microhardness of the composite coatings. At the same time, a reduction of the friction coefficient and an increase of wear rate were found. Wear mechanism were discussed and correlated to the microstructure and microhardness of coatings.
1. Introduction
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Quasicrystal materials have unique atomic structures and consequently well-determined physical, chemical and mechanical properties, such as low surface energy, high hardness, low coefficient of friction (COF), good wear- and corrosion-resistance [1]. It has been known that the mechanical properties of quasicrystal materials are similar to those of brittle ceramics or intermetallic compounds. This limits the potential applications of quasicrystals as structural materials. However, quasicrystals can be used as a particulate-reinforcing phase for polymer- or metal-matrix composites or to form thin films/coatings for surface modifications. Among various coating processes, thermal spray techniques have predominant advantages for fabricating quasicrystalline (or composite) coatings due to its flexibility and operability of the process. Numerous studies concerning the characterization of microstructures and properties of quasicrystalline coatings prepared by thermal spray have been conducted [2-4]. However, using these conventional thermal spray processes, the occurrence of unexpected phase transformation of quasicrystals is usually inevitable due to the high temperature of the process [5, 6]. Cold spray is a low temperature deposition process in which particles are accelerated through a De-Laval type nozzle, and the coating in cold spray process is obtained solely as a result of the accumulation of plastic deformation of solid particles impinging upon a substrate. As a competitive candidate for preparing high quality coatings, cold spray technique has attracted much interest in fabrication of metallic coatings since its emergence. Cold spray can also be used to prepare MMC coatings which usually exhibit improved physical or mechanical properties. This provides a new means for the preparation of desired MMCs, and the properties of cold-sprayed MMCs may be conveniently tailored by a proper addition of reinforcing particles. In this study, mechanically blended mixtures of AlCuFeB quasicrystal and bronze powders were deposited by cold spray process, aiming at tailoring quasicrystal-reinforced MMC coatings. Powder mixtures with different AlCuFeB contents were used to prepare composite coatings for evaluating the influence of quasicrystal fraction on the characteristics of deposited coatings. The structure-property relationships of the composite coatings were investigated. As one of the major objectives of this study, the tribological behavior of the quasicrystal particles reinforced coatings were investigated and correlated to their microstructures and mechanical properties.
2. Experimental procedure 2.1. Powders Inert gas atomized AlCuFeB quasicrystal (referred as QC hereinafter) and Cu-8wt.%Sn (CuSn8) powders were used as the feedstocks in this work. The as-atomized QC powder is in a nominal atomic composition of Al59.1Cu25.6Fe12.1B3.2. Both QC and CuSn8 particles have a spherical morphology. Particle size distribution were measured by Mastersizer 2000 (Malvern, UK). The size of the both particles follows a typical Gaussian distribution feature. For the CuSn8 powder, the d (10), d (50) and d (90) were 8.4 µm, 17.1 µm, and 33.8 µm, respectively. As to the QC powder, the d (10), d (50) and d (90) were 7.9 µm, 17.1 µm, and 32.2 µm, respectively. It is clear that the sizes of CuSn8 and QC powders were in a similar distribution range. Both of the powders were in-house made and more technical details regarding manufacture of these powders are available in our previous publication [7]. Mixtures of
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ACCEPTED MANUSCRIPT CuSn8 with three different QC fractions (19.5 %, 36.8 % and 57.6% in volume) were mechanically blended and used as initial powder feedstocks for coating deposition, and the obtained CuSn8 and composite coatings will hereinafter be referred as CuSn8, QC19, QC36 and QC57, respectively.
2.2. Cold spray conditions
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A commercially available Kinetic 3000 cold spray system (CGT GmbH, Germany) coupled with a self-designed nozzle was used for coating deposition. Compressed air and high pressure argon were respectively used as the main gas and the powder carrier gas. Pressure and temperature of the main gas were maintained at about 27 bars and 540 °C, respectively. The pressure of the carrier argon gas was 30 bars. The standoff distance between the nozzle exit and the sample surface was kept constant at 30 mm for all cold spray processes. The moving speed of spray gun manipulated by an ABB robot was maintained at 50 mm/s. Four passes of the spray gun were made for each sample, and the thickness of obtained coatings was about 350 µm. In this work, mild steel disks with a dimension of Ø43 x 4 mm were chosen as substrates, and the disks were degreased and grit blasted following usual procedures prior to coating deposition.
2.3 Coating characterization
2.4 Tribological tests
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The cross-sectional microstructure of the prepared coatings was examined by optical microscope (Nikon, ECLIPSE ME600) and scanning electron microscope (JEOL, JSM-5800LV, Japan). The porosity level of coatings was evaluated based on polished cross-sections by using an image processing method. Ten optical images were used to calculate the average porosity. It should be noted here that only the deeper zone was taken into account for analyzing the porosity level, because the top layer was relatively porous than the deeper zone in the coatings. Energy Disperse Xray analysis have been conducted for at least three times over different areas of each coating to determine the fraction of QC within the coatings. Microhardness of coatings was measured by a Vickers hardness tester (Leica VMHT30A, Germany) at 2.94 N load with 15 s dwelling time, and the corresponding sizes of the indentation marks were larger than a single splat during the hardness measurement. The microhardness given in this study is an average of at least 10 measurements randomly taken from the cross-sections of coatings.
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Tribological tests were conducted in a dry sliding condition through a re-constructed ball-on-disc tribometer (CSM, Switzerland) to evaluate the tribological performances of the prepared coatings. All the coatings were ground using diamond sandpapers (following 300 mesh, 600 mesh, 800 mesh, 1200 mesh and finally 4000 mesh), then they were polished using diamond slurries down to an average surface roughness of about 0.04 µm. All the coatings were treated in the same procedure to assure that they could have a same initial surface topography. WC-Co balls with a 6 mm-diameter and a mirror finished surface were used as the counterpart. The polished coatings slide against the WC-Co ball in a linear oscillation mode at a mean sliding velocity of 70 mm/s. The length of each stroke was 11.6 mm and the frequency of the oscillation movement was kept at 7 Hz. The normal load was in the range of 2 N and the total sliding distance was 280 mm. The positive peaks of the COF curves were extracted and averaged as the mean COF for each tribotest. Morphologies of the wear traces were examined by optical microscope and scanning electron microscope. In this work, the width of at least four locations along the wear trace were measured by microscope and averaged to compare coatings’ wear rates. The wear rate in this work is defined as the width of wear trace divided by the normal load (N) and total sliding distance (m).
3. Results and discussion 3.1 Coating structures Fig. 1 shows the cross-sectional morphologies of CuSn8, QC19, QC36 and QC57 coatings, respectively. Compared to the original spherical morphology of CuSn8 particles, the deposited CuSn8 particles were well flattened, especially for the smaller ones, as indicated by single arrows in Fig. 1 (a) and (b). It seems that both the pure CuSn8 and the composite coatings present compact structures with few defects on the interface of splats. Compared to pure copper, the deformation ability of bronze particles is usually insufficient for the closure of the interfacial gaps between deposited particles due to a relatively lower deformability. For the composite coatings, QC particles are dispersed uniformly in the bronze matrix. It seems that most of the embedded QC particles have a dimension less than 10 µm, and only the small QC particles (approximately below 5 µm) have maintained their original spherical morphology, as shown in Fig. 1 (c). Statistically, the average size of QC particles deposited into the coating is obviously smaller than that in the starting powder. Presumably, part of the larger QC particles in the starting powder has rebounded off during the impacting process and consequently were not deposited into the CuSn8 matrix. Fragmentation of QC particles during the deposition process may also be responsible for this decrease, i.e., part of the larger QC particles have fragmented into smaller ones. SEM observation shown in Fig. 1 (d) clearly indicates that some of deposited QC particles have
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broken into smaller pieces during the deposition process. The small fragments presented in Fig. 1 (b), as marked by circles, are considered to be residuals of the crushed QC particles, while the rest of the broken particles could have been rebounded off during the impacting process. The fragmentation of brittle AlN particles has been also observed in the cold-sprayed Al/AlN composite [8]. Due to the high impacting momentum, large QC particles could be more susceptible to break into small pieces during the impacting process. Irissou et al. declared that the more brittle particles are included in the starting powder, the higher is the probability that a brittle particle hits another one, and the increase of the concentration of brittle particles in the starting powder may increase the amount of fragmented particles [9]. It seems here that the particle momentum plays a dominant role in the impacting fragmentation of QC particles.
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Fig. 2 (a) shows the porosity level of the prepared coatings. Overall, the porosities of composite coatings are much lower than that of the pure CuSn8 coating, and a slight decrease of porosity level with the increase of the QC fraction in starting powder is observed for the three composite coatings. This indicates an important influence of the incorporation of QC particles in the starting powder on the densification of coating structures. Similar results can be found in the literature, e.g., Irissou et al. declared that an incorporation of 7 wt.% Al2O3 ceramic particles in the Al powder can effectively densify the Al/Al2O3 composite coatings [9]. An interpretation concerning this densification can be attributed to the strong blasting effect of the non-ductile hard particles [7]. The quantitative analysis reported in Fig. 2 (b) shows that the volume fractions of QC phase are 7.5%, 11.5%, and 20.5% for the QC19, QC36 and QC57 coatings respectively. These values are lower than the starting mixed volume fractions. This behavior has been also observed in previous work, when the starting volume fraction (75 wt.%) of Al2O3 particles was reduced to 26 wt.% after coldspraying of MMCs [9]. In the literature [8-10], the amount of reinforcing particles entrapped in the cold-sprayed MMCs is lower than that in the starting powder mixture when a blended powder is used for coating deposition. The deposition of hard and brittle reinforcing particles (such as QC, Al2O3 and SiC etc.) in cold spray process can only be considered as a result of the “embedding” effect. The discrepancy of the QC contents between the final coating and the starting powder can generally be attributed to the rebounding effect of QC particles.
3.2 Coating hardness
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The microhardness of the coatings is illustrated in Fig. 3. The incorporation of QC phase significantly increases the hardness of composite coatings. The coating microhardness increases with the increment of the QC fraction. The increase of coating hardness can mainly be attributed to both the increased flattening of CuSn8 matrix (due to strong blasting effect of QC particles) and the reinforcing effect of hard QC particles which significantly limit the plastic deformation of the CuSn8 matrix.
3.3 Tribological behavior
Fig. 4 shows the COFs and the wear rates of the CuSn8 and the composite coatings tested in a dry sliding condition. The COFs of the coatings are in the same range, i.e. from 0.67-0.72, and only a slight decrease in the COF with the increasing QC fraction is observed. The slight reduction in COF can be attributed to the decreased adhesion between the sliding pairs with enhancing QC particle fraction. The wear rates of CuSn8 and QC19 coatings are similar, but they are lower than that of QC36 and QC57 coatings. A previous study [7] has demonstrated that an incorporation of about 8.5 vol.% QC particles can effectively improve the tribological performances of CuSn8 based MMC. An even better tribological performance was accordingly expected for the MMCs containing a higher content of QC particles. However, the results here seem not to be in accordance with this expectation. The non-linear evolution of wear rates presented in Fig. 4 (b) indicates that different wear behaviors have taken place for the coatings. Fig. 5 (a) shows that a lot of grooves and craters were generated in the wear trace of the CuSn8 coating, as indicated by circles, and the wear of the CuSn8 coating is mainly characterized by both micro-ploughing and particle delamination. The fatigue wear of the CuSn8 coating is mainly attributed to the delamination and rupture of coating layer as a result of the initiation and propagation of micro cracks. For the QC19 coating, it seems that the microploughing is no longer the most important factor contributing to the wear. However, more and even tinier craters were generated in the worn surface, as indicated by single arrows in Fig. 5 (b). The formation of these tiny craters was assumed to be resulted from the delamination of the surface layer. Due to the reinforcing effect of QC particles, a lower in-depth deformation and thus a thinner strain layer involved in the frictional process were expected for the QC19 than those of the CuSn8 coating. As a result, the micro-ploughing of CuSn8 coating can be alleviated due to the load-bearing effect of QC particles, as demonstrated by the presence of many scratches in Fig. 5 (b). Moreover, the initiation and propagation of micro-cracks can only occur within a relatively thinner sub-surface layer. This is assumed to be the main reason for the generation of the tinier craters. The QC36 and QC57 coatings have a higher wear rate than the QC19 coating. It is observed that numerous deep worn areas were generated in the wear trace and most of them were filled with small wear particles, as indicated by the single arrows in Fig. 5 (c). For particulate reinforced MMCs, the interfaces between reinforcing particles and the matrix are weakly bonded regions [9, 11, 12]. The highest pressure in this case was as high as 0.67 Gpa when estimated using Hertizian contact theory (taking E-modulus and Poisson’s ratio values of bulk CuSn8 material). Accordingly, the extremely high contact pressure can trigger the debonding of QC particles. When filled with high amount of QC particles, numerous debonded QC particles could be fragmented when
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4. Conclusions
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1. Composite coatings present much denser microstructures than the pure CuSn8 coating, and the porosity level of composite coatings decreases with increasing the content of incorporated QC phase. It is believed that the hammering effect on the coating exerted by the QC particles enhances the coating porosity.
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2. The ratio of the deposited QC particles to its fraction in the initial powders is about 30%-40%. The low ratio is ascribed to the rebounding of QC particles upon impacting and thereby the low deposition efficiency.
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3. The coating hardness increased with the increase of the incorporation of QC phase. The reinforcing effect of QC particles is evident.
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3. The COF decreases slightly with the increase of the QC content in coatings. Incorporation of low-loading QC particles improves the wear resistance of CuSn8 coating. The improved wear resistance can be mainly attributed to the reinforcing effect of the QC particles which reduce ploughing on the coating and also reduce the depth of frictional layer. However, a higher concentration of QC phase adversely aggravated the wear resistance of QC36 and QC57 coatings possibly due to the fact that debonded QC particles can cause severe abrasion.
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Acknowledgement The authors gratefully appreciate the financial support of the Application and Basic Research Project of Ministry of Transport of PR. China (Grant No. 2013329815280), the Natural Science Foundation of Fujian Province (Grant No. 2013D033) and the Project of Technology Foundation for Selected Overseas Chinese Scholar (Ministry of Personnel of China).
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B. N. Mordyuk, M. O. Iefimov, G. I. Prokopenko, T. V. Golub, and M. I. Danylenko, Structure, microhardness and damping characteristics of Al matrix composite reinforced with AlCuFe or Ti using ultrasonic impact peening, Surf. Coat. Tech., 204 (2010) 1590-1598.
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Figure captions:
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Fig. 1 SEM observations of the cross-sectional morphologies of (a) CuSn8, (b) QC19, (c) QC36 and (d) QC57 coatings; the single arrows present the well flattened small particles, and the circles show small fragmented QC particles
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Fig. 4 (a) Coefficients of friction and (b) wear rates of the cold-sprayed coatings tested in a dry sliding condition
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Fig. 5 Morphologies of the wear traces of (a) CuSn8, (b) QC19, (c) QC36 and (d) QC57 coatings tested in a dry sliding condition; the circles present the grooves and craters generated in the wear trace of the CuSn8 coating, and the single arrows indicate the craters generated in the wear trace of the composite coatings
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ACCEPTED MANUSCRIPT Highlights
Particulate QC reinforced bronze-based MMCs were prepared by cold spraying.
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Composite coatings present a dense microstructure and a high hardness. The ratio of deposited QC particles to its fraction in initial powders is 30%-40%.
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A moderate incorporation of QC phase improves the wear resistance of coating.
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