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Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings
Accepted Manuscript Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings
Fengyuan Shu, Binglin Zhang, Tao Liu, Shaohua Sui, Yuxin Liu, Peng He, Bin Liu, Binshi Xu PII: DOI: Reference:
S0257-8972(18)31193-9 https://doi.org/10.1016/j.surfcoat.2018.10.086 SCT 23949
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
4 June 2018 20 September 2018 28 October 2018
Please cite this article as: Fengyuan Shu, Binglin Zhang, Tao Liu, Shaohua Sui, Yuxin Liu, Peng He, Bin Liu, Binshi Xu , Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.10.086
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ACCEPTED MANUSCRIPT Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings
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Fengyuan Shua, c, Binglin Zhanga, Tao Liua, Shaohua Suia, Yuxin Liub, Peng Hec, Bin Liud, Binshi Xub a Harbin Institute of Technology at Wehai, Weihai, 264209, China b National Key Laboratory for Remanufacturing, Beijing, 100072, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China d School of Materials Science and Engineering, Jiangsu University of Scienece and Technology, Zhenjiang, 212003, China * Corresponding author. Tel. : +86 15266123725; Fax: +86 0631-5687305; Email: [email protected] Abstract
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The CoCrBFeNiSi high-entropy alloy (HEA) coatings were fabricated on H13 steel using different laser powers. The microstructure, mechanical and chemical properties of the coating and the internal relations between them were investigated, in which 3D wear trace morphology was adopted to analyze the wear resistance of different coatings. The coatings could be divided into three layers with different microstructure including the bottom dendritic layer, the upper amorphous layer and the transition layer. The amorphous content in the coatings was dependent upon laser power which influenced dilution rate and actual cooling rate through changing heat input. Increased amorphous content led to increased microhardness of the coatings which could approach five times of that of the substrate. When the amorphous content decreased, cladded coatings exhibited deeper furrows, more serious adhesive wear and oxidation wear, which gave birth to wider cross section area of the worn track, higher wear weight loss and thus deteriorated wear resistance of the coatings. Moreover, higher amorphous content in the coatings had led to more excellent corrosion resistance to HCl and NaCl solution. Key words: Laser power, High-entropy alloy, Amorphous coatings, Wear, Corrosion 1. Introduction
As concluded by the existing research, materials with higher entropy of mixing should possess better glass forming ability (GFA) [1, 2]. Critical cooling rate [3] and the dimensionless quantity λn [4] were proposed as criteria of evaluating GFA, considering the solute atom radius, the covalent radius, Pauling electronegativity, valence number, molten point and mole percent of every component element. With regard to HEAs, the stacking density of ordered atomic clusters in the liquid alloys should be enhanced. Moreover, the high entropy contribution lowering the Gibbs free energy of the solid solution phase and the confusion principle [2, 5] simultaneously 1
ACCEPTED MANUSCRIPT favored the formation of the amorphous phase. As compared with the traditional thermal spraying techniques, laser cladding was characterized by rapid cooling rate (104-106 K/s) [6, 7] which was an important factor for GFA [8, 9]. So, HEAs were very suitable for fabricating amorphous coatings with appealing combined properties, especially when laser cladding technology was utilized. Therefore, advantages of laser cladding, amorphous alloys and high-entropy alloys could be combined, after which amazing performances of the coatings were expected [10, 11]. The 6FeNiCoSiCrAlTi HEA coatings had been successfully laser cladded on a
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low carbon steel substrate and had exhibited high microhardness and excellent soft magnetic properties [12]. The FeCoNiCrCu HEA coating was also prepared by laser
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cladding and possessed high corrosion resistance, microhardness and softening
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resistance properties [13]. High hardness, striking elastic modulus, desirable fracture toughness and high-temperature softening resistance were combined in the laser cladded FeCoNiCrCuTiMoAlSiB0.5 HEA coatings containing lath-like martensite
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phase [14]. Amorphous-matrixed coatings with the amorphous phase accounting for 49 vol. % were obtained with FeCrCoNiSiB HEA powder [15]. The wear resistance
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and corrosion resistance of the amorphous layer prepared with HEA powders were obviously beyond that of the crystalline layer and the substrate [16]. The outstanding wear resistance and microhardness of the amorphous coatings should be attributed to
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their high amorphous content [17]. Wang [18] prepared Fe-based alloy coatings by
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pulsed laser beam and analyzed the differences in mechanical performances caused by different laser power. Microhardness of the coatings was found to gradually decreased as the laser power increased, which could be attributed to the increase of dilution ratio. High dilution ratio caused by increased laser power gave birth to
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coarsened dentrite which should be closely related to the deteriorated corrosion resistance [19]. The microstructure evolution was dependent upon the contribution of
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dilution ratio by the substrate which resulted in the redistribution of alloying elements in the coating [20]. Different laser cladding power in the range of 700-1000 W were used to obtain various dilution ratios in the Ni-Fe-B-Si-Nb coatings [10]. The difference in laser power input changed the dilution ratio, phase, composition and microstructure of the laser cladded coatings, while lower dilution ratio resulted in higher amorphous content in the coatings which exhibited better mechanical performances. The amorphous-matrixed composite coatings had been successfully fabricated by laser cladding CoCrBFeNiSi HEA powder on low carbon steel substrate and had been proved to possess excellent mechanical properties [21]. However, further investigations were desperately expected upon the influences of process parameters 2
ACCEPTED MANUSCRIPT on the microstructure and the service behaviors against mechanical and corrosion damages. Therefore, based on the previous studies, CoCrBFeNiSi HEA powder was utilized to prepare amorphous coatings with different laser power ranging from 233W to 700W, after which the effect of laser power on the microstructure, wear resistance and corrosion resistance of the coatings was analyzed. 2. Experimental procedure
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H13 low carbon steel with dimension of 200 mm ×100 mm ×20 mm was used as substrate material. The Co34Cr29B14Fe8Ni8Si7 (at. %) HEA powder was pre-placed on H13 steel plates of which the chemical composition was presented in Table 1. The gas flow rate of pure argon as protection gas was 20 L/min during cladding with SL-80 Nd: YAG laser machine. The laser cladding processing parameters were as follows: scanning speed of 10 mm/min, defocusing distance of 20 mm, light spot diameter of 2.2-2.5 mm, overlapping ratio of 50% between adjacent tracks and preset powder thickness of 200 μm. The coatings cladded with different laser power were successfully prepared under the condition of 233 W, 476 W, 583 W and 700 W. The microstructure of the coatings was detected with optical digital microscope (DSX510, JAPAN), field emission scanning electron microscope (FESEM, ZEISS), energy-dispersive spectroscope (EDS) and X-ray diffraction (XRD, D/max 2500). The microstructure was also detected by transmission electron microscope (TEM, JEOL-2100) and selected area electron diffraction (SAED). Microhardness distribution in the cross section of the coatings was obtained using TH701 microhardness tester with impression load of 0.2 Kg. Wear resistance of the coatings was measured using high-temperature ball-on-disc type friction and wear testing machine (HT-1000). Zirconia ceramic balls (diameter of 5 mm) were selected as the counterpart material. The tested surface got previously ground by 400 grit SiC papers and cut into samples with dimension of 15 mm × 15 mm × 5mm. The experimental parameters were: loaded mass of 1150 g, frictional radium of 3 mm, rotational speed of 1120 r/min, temperature of 500 ºC and wearing time of 60 min. Each micro hardness and wear rate value was the everage value of three samples under the same test condition. Wear mechanism of the coatings was determined according to the fractographic examination the wear surface by FESEM and EDS. The three-division (3D) height contour of wear trace was obtained with the optical digital microscope. Corrosion resistance of the upper layer to 3.5 wt. % NaCl solution and 1 mol/L HCl solution was analyzed at electrochemical workstation (IM6E, Germany) which included working electrode, a Saturated Calomel Electrode (SCE) as the reference electrode and a platinum electrode as the auxiliary electrode to form a three-electrode system. All electrochemical experiments were performed at under room temperature according to the Chinese national technique standard GB/T 24196-2009. The corroded area of the working electrode exposed to the acid solutions was 10 mm by 10 mm. As 3
ACCEPTED MANUSCRIPT the preparation for the electrochemical experiments, the tested samples as working electrodes were welded with copper wire, mounted in epoxy resin, cleaned with alcohol and dried with cool air. Potentiodynamic polarization curves tests were carried out with the sweep rate of 0.5 mV/s. Electrochemical impedance spectroscopy (EIS) plots were obtained under the condition of 10 mV amplitude and within 10 mHz to 10 kHz frequencies. Table 1 The chemical composition of H13 steel (wt. %)
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C 0.32-0. 45
Si 0.80-1. 20
Mn 0.20-0. 50
Cr 4.75-5 .50
Mo 1.10-1. 75
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3. Results and discussion
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3.1 Microstructure of the coatings Fig. 1 showed the SEM image of the upper layer in the coatings fabricated by different laser power. The upper layer of the coatings was composed of amorphous phase and equiaxed dendrites of which the content was closely related with laser powder. Less content of equiaxed dendrites was obtained with lower laser power. The equiaxed dendrites content and size increased simultaneously with the increase of laser power. Fig. 2 exhibited the XRD diagram of the upper layer and proved the existence of crystalline phases including FeNi3 phase, β(Co) phase and Co2B phase. As compared with the amorphous matrix, the FeNi3 phase marked as F has covered a much smaller area while the Co2B compound marked as P was inserted in the amorphous matrix as shown in the insert in Fig. 1(a). Widened peak was also found near 44 degrees, which indicated the presence of amorphous phase in the coatings and corresponded well with Fig. 1.
Fig. 1 The microstructure of the upper layer in the coatings obtained with different laser power, (a) 233 W, (b) 476 W, (c) 583 W, (d) 700 W.
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Fig.2 XRD test result of coatings cladded with different laser power, (a) laser power of 476 W, (b) the assembled diagram.
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Fig. 3(a) and (b) presented the microstructure and SAED test result of the upper layer by TEM, respectively. Long-range disordered arrangement were exhibited by atoms in the amorphous phase, as a result, diffuse scattering ring with a bright spot in the center was formed as the identification of amorphous phases as shown in Fig. 3(b).
Fig. 3 TEM test result, (a) the bright field image of microstructure of the upper layer, (b) SAED of area as marked in (a).
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3.2 Mechanical and chemical properties of the coatings The microhardness of the coatings was obviously higher than that of the substrate and decreased with the increase of the distance from the upper surface as shown in Fig. 4(a). The content of amorphous phase was evaluated in our previous research by the area ratio of amorphous phase [15]. By fitting the Pseudo-Voigt function with the assistance of MDI Jade software, the proportion of the amorphous phase in the upper layers was obtained and shown in Fig. 4(b). When the laser power was 476 W, the microhardness of the coatings was between 700 and 1192.5 HV0.2 which was about 2.9 to 5 times of that of the substrate. The difference in microhardness between the coatings should be attributed to the different amorphous content. Fig. 5 presented the EIS plot and polarization curves of the substrate and coatings under the condition of 3.5 wt. % NaCl solution and 1 mol/L HCl solution. The radius of the impedance spectrum corresponding to the laser power of 476 W was obviously larger than that of the other coatings and the substrate, as indicated by Fig. 5(a) and Fig. 5(c). The corrosion potential of the coatings obtained with laser power of 476W 5
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was no lower than that of the the others in both solutions, though the passivation point was shown higher than that of the other coatings, as indicated by Fig. 5(b) and Fig. 5(d). The reduction of amorphous content helped elevate the corrosion potential of the coatings and led to obvious decrease in the radius of the impedance spectrum. Thus, the amorphous content was the main factor that determined the corrosion resistance of the coatings. In addition, the corrosion resistance of the coatings was much better than that of the substrate. The tested samples had experienced serious corrosion damage during the accelerated corrosion test process. The substrate got uncovered by the coatings at the later stage of the test process, as a result, the passivation potential was influenced and thus not suggested as the reference standard of corrosion resistance.
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Fig. 4 Microhardness distribution and amorphous content in the upper layer of the coatings obtained with different laser power, (a) microhardness distribution, (b) the amorphous content obtained from XRD patterns.
Fig. 5 Electrochemistry test result of the substrate and coatings, (a) EIS diagram in 3.5 wt. % NaCl solution, (c) polarization curves in 3.5 wt. % NaCl solution, (c) EIS diagram in 1 mol/L HCl solution, (d) polarization curves in 1mol/LHCl solution. 6
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Coatings obtained with lower amorphous phase had exhibited higher friction coefficient as shown in Fig. 6(a). The relationship between the amorphous content and the friction and wear weight loss was shown in Fig. 6(b). According to Fig. 4(b), the amorphous content decreased as the laser power increased. So, the wear resistance of the coatings with higher amorphous content was better than that of the coating with lower amorphous content.
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Fig. 6 Results of friction and wear test, (a) variation process of friction coefficient along time, (b) wear weight loss of different tested surfaces
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3.3 Discussions In order to determine the relationship between the wear resistance and the amorphous content, further research was carried out upon the wear surface of the coatings by SEM. The surface morphology of the coatings was characterized with furrows as a result of abrasive wear and a small amount of adhesive wear. The coatings with higher amorphous content had shown shallower furrows, less adhesive wear and oxidation wear. Therefore, coatings with higher amorphous content possessed better wear resistance, which was in consistence with the wear test result in Fig. 6. Then 3D surface topography of the worn surface was adopted to analyze the wear resistance in different coatings as shown from Fig. 9 to Fig. 13. The shape of cross section edge of every worn track was approximately parabolic as shown in Fig. 8(b) and Fig. 9(b). The cross section area or the area surrounded by the parabola and abscissa axes became smaller as the amorphous content got decreased in the coatings, which could be confirmed by both qualitative analysis and quantitative small-square-area method. In addition, the cross-sectional area of the wearing tracks in the substrate was obviously larger than that in the cladded coatings. Therefore, it was visually proved that the cladded coatings significantly elevated the wear resistance of the working surface. Moreover, the increment in the content of amorphous phase led to improvement of wear resistance.
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Fig. 7 The SEM morphology of worn surfaces in the coatings cladded with different laser power, (a) 233 W, (b) 476 W, (c) 583 W, (d) 700 W.
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Fig. 8 The 3D surface topography of the worn surface in the coating cladded with 233 W, (a) 3D wear trace morphology, (b) cross section.
Fig. 9 The 3D surface topography of the worn surface in the coating cladded with 476 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 10 The 3D surface topography of the worn surface in the coating cladded with 583 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 11 The 3D surface topography of the worn surface in the coating cladded with 700 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 12 The 3D surface topography of the worn surface in the substrate, (a) 3D wear trace morphology, (b) cross section.
The middle layers of coatings were composed of amorphous and equiaxed dendrites as shown in Fig. 13. The content of equiaxed dendrites was defined in the same way as that of amorphous phase and referred to as the area ratio of equiaxed dendrites. As compared with the vast amorphous area, the black islands was identified and marked as equiaxed dendrites as shown in Fig. 13. Obviously, elevated laser power were beneficial to increasing the content of the equiaxed dendrites. Fig. 14 presented the SEM micromorphology of the bottom layers in the coatings obtained under the condition of different laser power. Metallurgical bonding had been obtained between the base metal and the coatings of which the bottom layer was mainly 9
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composed of primary dendrites and secondary dendrites. The content of amorphous phase and crystalline phases gradually decreased and increased from the upper layer to the middle layer of the coatings, respectively. Therefore, the distribution of amorphous and crystalline phases throughout the coatings could be represented by Fig. 15 in which the bottom layer of the coatings were composed of dendrites instead of amorphous phases.
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Fig. 13 The microstructure of the middle layer in the coatings cladded with different laser power, (a)233 W, (b) 476 W, (c) 583 W, (d) 700 W.
Fig. 14 The microstructure of the bottom layer in the coatings cladded with different laser power, (a)233 W, (b) 476 W, (c) 583 W, (d) 700 W. 10
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EDS analysis was then carried out along the path SE from the upper layers to the substrate as marked in Fig. 16(a). Little fluctuation in the relative concentration curves of Cr, Co, Fe, Si and Ni elements was found in the upper stage, or in the upper and middle layers in the coatings. However, the content of Fe and the other elements increased and decreased sharply, respectively, in the bottom layers of the coatings, especially in the dendrite zone.
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Fig. 15 Schematic diagram of microstructure distribution in the cross section of the coatings.
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Fig. 16 The macro morphology and element distribution in the cross section of the coating, (a) the coatings cross-section, (b) element distribution along path SE as marked in (a).
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As the content of Fe elements in the substrate was much higher than that of the coatings, obvious dilution effect by the substrate upon the cladded coatings led to the increase of Fe elements and the formation of dendrites at the bottom of the coatings as shown in Fig. 16. Fig. 17(a) was the schematic diagram of the cross section of the coating in which A1 and A2 was that of the melted base metal and the coatings beyond the base metal, respectively. The dilution ratio (η) could be obtained by η=A1/(A1 + A2) [22] and exhibited in Fig. 17(b).
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Fig. 17 The schematic diagram of dilution rate and the test result, (a) the cross section of the coatings, (b) the dilution rate in coatings cladded with different laser power.
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Increasing power led to the increase in dilution ratio in Fig. 17(b). The determinant factors of amorphous microstructure for a certain metallic alloy system included cooling rate and composition of metallic alloy system [18]. The increase of laser power led to the increase of heat input on coatings and substrate. On the one hand, more melted substrate metal was melted to give birth to higher dilution rate and thus more obvious change in the chemical composition of the HEA coatings. So the critical cooling rate of the coatings was changed as a result. On the other hand, for metallic glass alloys, the actual cooling rate was the most important factor to inhibit the nucleation and growth of the competing crystalline phases [23]. With regard to the thermal cycling process, the increase of heat input led to the decrease of actual cooling rate and thus reduced the amorphous content as shown in Fig. 4(b). On the one hand, comparing with the substrate, a large number of Co, Cr, Ni as passivating elements improved the corrosion resistance of the coatings. On the other hand, amorphous coatings were less prone to galvanic corrosion due to the absence of microstructure defects and non-uniform chemical composition [24, 25]. Therefore, the corrosion resistance of coatings with the highest amorphous content was more excellent than that of the other coatings. The equiaxed dendrites content and the columnar dendrites size increased simultaneously with the increase of laser power. Microhardness of the upper layer of the coatings also varied along the laser power and amorphous content. As well known, the amorphous alloy exhibited no long-range ordered microstructure, then no crystalline defects such as grain boundary, dislocation and precipitation would be obtained [26]. The wear resistance of the coatings was then related to the microstructure and main phases [27]. As a result, the amorphous content in the coatings had played a crucial role in the tribological properties of the coatings. Conclusions (1) Different amorphous content in laser cladded Co34Cr29B14Fe8Ni8Si7 (at. %) high entropy alloy coatings were successfully obtained with different laser power and the H13 steel as the substrate. Metallurgical bonding was obtained between the 12
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coatings and the substrate. (2) The coatings was composed of three layers according to the microstructure including the dendritic layer at the bottom, the amorphous layer in the top and the transition area. The coverage area (or the size) of the dendrites at the bottom increased obviously as the laser power got elevated. The amorphous content in different coatings decreased from 81.15% to 33.79% as the laser power increased from 233 W to 700 W, which was attributed to the dilution effect and dependent upon the heat input. (3) Higher microhardness, better wear and corrosion resistance was attributed to higher amorphous content in the coatings. Lower amorphous content in the coatings had lead to deeper furrows, more serious adhesive wear and oxidation wear, which gave birth to wider cross section area of worn track, higher wear weight loss and thus deteriorated wear resistance.
Acknowledgement
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This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2016EEQ03) and the Natural Scientific Research Innovation Foundation in HIT (No. HIT.NSRIF.201703). Special appreciation shall be sent to Dr. Jia Liu who has been giving wholehearted supports to the first author. The authors wish to thank Mr. Dingli Zhao from WARD Australia Pty. Ltd. for fruitful discussions about electrochemical investigations.
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Highlights 1 The Co34Cr29B14Fe8Ni8Si7 (at. %) high-entropy alloy amorphous coatings cladded with different laser power were fabricated on H13 steel substrate. 2 Laser power influenced the amorphous content through changing dilution rate and actual cooling rate. 3 3D wear trace morphology was adopted to analyze the wear resistance of coatings. 4 Higher amorphous content in the coatings led to higher microhardness, better wear and corrosion resistance.
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Fengyuan Shu, Binglin Zhang, Tao Liu, Shaohua Sui, Yuxin Liu, Peng He, Bin Liu, Binshi Xu PII: DOI: Reference:
S0257-8972(18)31193-9 https://doi.org/10.1016/j.surfcoat.2018.10.086 SCT 23949
To appear in:
Surface & Coatings Technology
Received date: Revised date: Accepted date:
4 June 2018 20 September 2018 28 October 2018
Please cite this article as: Fengyuan Shu, Binglin Zhang, Tao Liu, Shaohua Sui, Yuxin Liu, Peng He, Bin Liu, Binshi Xu , Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings. Sct (2018), https://doi.org/10.1016/j.surfcoat.2018.10.086
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ACCEPTED MANUSCRIPT Effects of laser power on microstructure and properties of laser cladded CoCrBFeNiSi high-entropy alloy amorphous coatings
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Fengyuan Shua, c, Binglin Zhanga, Tao Liua, Shaohua Suia, Yuxin Liub, Peng Hec, Bin Liud, Binshi Xub a Harbin Institute of Technology at Wehai, Weihai, 264209, China b National Key Laboratory for Remanufacturing, Beijing, 100072, China c State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, 150001, China d School of Materials Science and Engineering, Jiangsu University of Scienece and Technology, Zhenjiang, 212003, China * Corresponding author. Tel. : +86 15266123725; Fax: +86 0631-5687305; Email: [email protected] Abstract
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The CoCrBFeNiSi high-entropy alloy (HEA) coatings were fabricated on H13 steel using different laser powers. The microstructure, mechanical and chemical properties of the coating and the internal relations between them were investigated, in which 3D wear trace morphology was adopted to analyze the wear resistance of different coatings. The coatings could be divided into three layers with different microstructure including the bottom dendritic layer, the upper amorphous layer and the transition layer. The amorphous content in the coatings was dependent upon laser power which influenced dilution rate and actual cooling rate through changing heat input. Increased amorphous content led to increased microhardness of the coatings which could approach five times of that of the substrate. When the amorphous content decreased, cladded coatings exhibited deeper furrows, more serious adhesive wear and oxidation wear, which gave birth to wider cross section area of the worn track, higher wear weight loss and thus deteriorated wear resistance of the coatings. Moreover, higher amorphous content in the coatings had led to more excellent corrosion resistance to HCl and NaCl solution. Key words: Laser power, High-entropy alloy, Amorphous coatings, Wear, Corrosion 1. Introduction
As concluded by the existing research, materials with higher entropy of mixing should possess better glass forming ability (GFA) [1, 2]. Critical cooling rate [3] and the dimensionless quantity λn [4] were proposed as criteria of evaluating GFA, considering the solute atom radius, the covalent radius, Pauling electronegativity, valence number, molten point and mole percent of every component element. With regard to HEAs, the stacking density of ordered atomic clusters in the liquid alloys should be enhanced. Moreover, the high entropy contribution lowering the Gibbs free energy of the solid solution phase and the confusion principle [2, 5] simultaneously 1
ACCEPTED MANUSCRIPT favored the formation of the amorphous phase. As compared with the traditional thermal spraying techniques, laser cladding was characterized by rapid cooling rate (104-106 K/s) [6, 7] which was an important factor for GFA [8, 9]. So, HEAs were very suitable for fabricating amorphous coatings with appealing combined properties, especially when laser cladding technology was utilized. Therefore, advantages of laser cladding, amorphous alloys and high-entropy alloys could be combined, after which amazing performances of the coatings were expected [10, 11]. The 6FeNiCoSiCrAlTi HEA coatings had been successfully laser cladded on a
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low carbon steel substrate and had exhibited high microhardness and excellent soft magnetic properties [12]. The FeCoNiCrCu HEA coating was also prepared by laser
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cladding and possessed high corrosion resistance, microhardness and softening
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resistance properties [13]. High hardness, striking elastic modulus, desirable fracture toughness and high-temperature softening resistance were combined in the laser cladded FeCoNiCrCuTiMoAlSiB0.5 HEA coatings containing lath-like martensite
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phase [14]. Amorphous-matrixed coatings with the amorphous phase accounting for 49 vol. % were obtained with FeCrCoNiSiB HEA powder [15]. The wear resistance
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and corrosion resistance of the amorphous layer prepared with HEA powders were obviously beyond that of the crystalline layer and the substrate [16]. The outstanding wear resistance and microhardness of the amorphous coatings should be attributed to
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their high amorphous content [17]. Wang [18] prepared Fe-based alloy coatings by
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pulsed laser beam and analyzed the differences in mechanical performances caused by different laser power. Microhardness of the coatings was found to gradually decreased as the laser power increased, which could be attributed to the increase of dilution ratio. High dilution ratio caused by increased laser power gave birth to
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coarsened dentrite which should be closely related to the deteriorated corrosion resistance [19]. The microstructure evolution was dependent upon the contribution of
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dilution ratio by the substrate which resulted in the redistribution of alloying elements in the coating [20]. Different laser cladding power in the range of 700-1000 W were used to obtain various dilution ratios in the Ni-Fe-B-Si-Nb coatings [10]. The difference in laser power input changed the dilution ratio, phase, composition and microstructure of the laser cladded coatings, while lower dilution ratio resulted in higher amorphous content in the coatings which exhibited better mechanical performances. The amorphous-matrixed composite coatings had been successfully fabricated by laser cladding CoCrBFeNiSi HEA powder on low carbon steel substrate and had been proved to possess excellent mechanical properties [21]. However, further investigations were desperately expected upon the influences of process parameters 2
ACCEPTED MANUSCRIPT on the microstructure and the service behaviors against mechanical and corrosion damages. Therefore, based on the previous studies, CoCrBFeNiSi HEA powder was utilized to prepare amorphous coatings with different laser power ranging from 233W to 700W, after which the effect of laser power on the microstructure, wear resistance and corrosion resistance of the coatings was analyzed. 2. Experimental procedure
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H13 low carbon steel with dimension of 200 mm ×100 mm ×20 mm was used as substrate material. The Co34Cr29B14Fe8Ni8Si7 (at. %) HEA powder was pre-placed on H13 steel plates of which the chemical composition was presented in Table 1. The gas flow rate of pure argon as protection gas was 20 L/min during cladding with SL-80 Nd: YAG laser machine. The laser cladding processing parameters were as follows: scanning speed of 10 mm/min, defocusing distance of 20 mm, light spot diameter of 2.2-2.5 mm, overlapping ratio of 50% between adjacent tracks and preset powder thickness of 200 μm. The coatings cladded with different laser power were successfully prepared under the condition of 233 W, 476 W, 583 W and 700 W. The microstructure of the coatings was detected with optical digital microscope (DSX510, JAPAN), field emission scanning electron microscope (FESEM, ZEISS), energy-dispersive spectroscope (EDS) and X-ray diffraction (XRD, D/max 2500). The microstructure was also detected by transmission electron microscope (TEM, JEOL-2100) and selected area electron diffraction (SAED). Microhardness distribution in the cross section of the coatings was obtained using TH701 microhardness tester with impression load of 0.2 Kg. Wear resistance of the coatings was measured using high-temperature ball-on-disc type friction and wear testing machine (HT-1000). Zirconia ceramic balls (diameter of 5 mm) were selected as the counterpart material. The tested surface got previously ground by 400 grit SiC papers and cut into samples with dimension of 15 mm × 15 mm × 5mm. The experimental parameters were: loaded mass of 1150 g, frictional radium of 3 mm, rotational speed of 1120 r/min, temperature of 500 ºC and wearing time of 60 min. Each micro hardness and wear rate value was the everage value of three samples under the same test condition. Wear mechanism of the coatings was determined according to the fractographic examination the wear surface by FESEM and EDS. The three-division (3D) height contour of wear trace was obtained with the optical digital microscope. Corrosion resistance of the upper layer to 3.5 wt. % NaCl solution and 1 mol/L HCl solution was analyzed at electrochemical workstation (IM6E, Germany) which included working electrode, a Saturated Calomel Electrode (SCE) as the reference electrode and a platinum electrode as the auxiliary electrode to form a three-electrode system. All electrochemical experiments were performed at under room temperature according to the Chinese national technique standard GB/T 24196-2009. The corroded area of the working electrode exposed to the acid solutions was 10 mm by 10 mm. As 3
ACCEPTED MANUSCRIPT the preparation for the electrochemical experiments, the tested samples as working electrodes were welded with copper wire, mounted in epoxy resin, cleaned with alcohol and dried with cool air. Potentiodynamic polarization curves tests were carried out with the sweep rate of 0.5 mV/s. Electrochemical impedance spectroscopy (EIS) plots were obtained under the condition of 10 mV amplitude and within 10 mHz to 10 kHz frequencies. Table 1 The chemical composition of H13 steel (wt. %)
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C 0.32-0. 45
Si 0.80-1. 20
Mn 0.20-0. 50
Cr 4.75-5 .50
Mo 1.10-1. 75
S 0.03
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3. Results and discussion
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3.1 Microstructure of the coatings Fig. 1 showed the SEM image of the upper layer in the coatings fabricated by different laser power. The upper layer of the coatings was composed of amorphous phase and equiaxed dendrites of which the content was closely related with laser powder. Less content of equiaxed dendrites was obtained with lower laser power. The equiaxed dendrites content and size increased simultaneously with the increase of laser power. Fig. 2 exhibited the XRD diagram of the upper layer and proved the existence of crystalline phases including FeNi3 phase, β(Co) phase and Co2B phase. As compared with the amorphous matrix, the FeNi3 phase marked as F has covered a much smaller area while the Co2B compound marked as P was inserted in the amorphous matrix as shown in the insert in Fig. 1(a). Widened peak was also found near 44 degrees, which indicated the presence of amorphous phase in the coatings and corresponded well with Fig. 1.
Fig. 1 The microstructure of the upper layer in the coatings obtained with different laser power, (a) 233 W, (b) 476 W, (c) 583 W, (d) 700 W.
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Fig.2 XRD test result of coatings cladded with different laser power, (a) laser power of 476 W, (b) the assembled diagram.
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Fig. 3(a) and (b) presented the microstructure and SAED test result of the upper layer by TEM, respectively. Long-range disordered arrangement were exhibited by atoms in the amorphous phase, as a result, diffuse scattering ring with a bright spot in the center was formed as the identification of amorphous phases as shown in Fig. 3(b).
Fig. 3 TEM test result, (a) the bright field image of microstructure of the upper layer, (b) SAED of area as marked in (a).
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3.2 Mechanical and chemical properties of the coatings The microhardness of the coatings was obviously higher than that of the substrate and decreased with the increase of the distance from the upper surface as shown in Fig. 4(a). The content of amorphous phase was evaluated in our previous research by the area ratio of amorphous phase [15]. By fitting the Pseudo-Voigt function with the assistance of MDI Jade software, the proportion of the amorphous phase in the upper layers was obtained and shown in Fig. 4(b). When the laser power was 476 W, the microhardness of the coatings was between 700 and 1192.5 HV0.2 which was about 2.9 to 5 times of that of the substrate. The difference in microhardness between the coatings should be attributed to the different amorphous content. Fig. 5 presented the EIS plot and polarization curves of the substrate and coatings under the condition of 3.5 wt. % NaCl solution and 1 mol/L HCl solution. The radius of the impedance spectrum corresponding to the laser power of 476 W was obviously larger than that of the other coatings and the substrate, as indicated by Fig. 5(a) and Fig. 5(c). The corrosion potential of the coatings obtained with laser power of 476W 5
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was no lower than that of the the others in both solutions, though the passivation point was shown higher than that of the other coatings, as indicated by Fig. 5(b) and Fig. 5(d). The reduction of amorphous content helped elevate the corrosion potential of the coatings and led to obvious decrease in the radius of the impedance spectrum. Thus, the amorphous content was the main factor that determined the corrosion resistance of the coatings. In addition, the corrosion resistance of the coatings was much better than that of the substrate. The tested samples had experienced serious corrosion damage during the accelerated corrosion test process. The substrate got uncovered by the coatings at the later stage of the test process, as a result, the passivation potential was influenced and thus not suggested as the reference standard of corrosion resistance.
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Fig. 4 Microhardness distribution and amorphous content in the upper layer of the coatings obtained with different laser power, (a) microhardness distribution, (b) the amorphous content obtained from XRD patterns.
Fig. 5 Electrochemistry test result of the substrate and coatings, (a) EIS diagram in 3.5 wt. % NaCl solution, (c) polarization curves in 3.5 wt. % NaCl solution, (c) EIS diagram in 1 mol/L HCl solution, (d) polarization curves in 1mol/LHCl solution. 6
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Coatings obtained with lower amorphous phase had exhibited higher friction coefficient as shown in Fig. 6(a). The relationship between the amorphous content and the friction and wear weight loss was shown in Fig. 6(b). According to Fig. 4(b), the amorphous content decreased as the laser power increased. So, the wear resistance of the coatings with higher amorphous content was better than that of the coating with lower amorphous content.
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Fig. 6 Results of friction and wear test, (a) variation process of friction coefficient along time, (b) wear weight loss of different tested surfaces
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3.3 Discussions In order to determine the relationship between the wear resistance and the amorphous content, further research was carried out upon the wear surface of the coatings by SEM. The surface morphology of the coatings was characterized with furrows as a result of abrasive wear and a small amount of adhesive wear. The coatings with higher amorphous content had shown shallower furrows, less adhesive wear and oxidation wear. Therefore, coatings with higher amorphous content possessed better wear resistance, which was in consistence with the wear test result in Fig. 6. Then 3D surface topography of the worn surface was adopted to analyze the wear resistance in different coatings as shown from Fig. 9 to Fig. 13. The shape of cross section edge of every worn track was approximately parabolic as shown in Fig. 8(b) and Fig. 9(b). The cross section area or the area surrounded by the parabola and abscissa axes became smaller as the amorphous content got decreased in the coatings, which could be confirmed by both qualitative analysis and quantitative small-square-area method. In addition, the cross-sectional area of the wearing tracks in the substrate was obviously larger than that in the cladded coatings. Therefore, it was visually proved that the cladded coatings significantly elevated the wear resistance of the working surface. Moreover, the increment in the content of amorphous phase led to improvement of wear resistance.
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Fig. 7 The SEM morphology of worn surfaces in the coatings cladded with different laser power, (a) 233 W, (b) 476 W, (c) 583 W, (d) 700 W.
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Fig. 8 The 3D surface topography of the worn surface in the coating cladded with 233 W, (a) 3D wear trace morphology, (b) cross section.
Fig. 9 The 3D surface topography of the worn surface in the coating cladded with 476 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 10 The 3D surface topography of the worn surface in the coating cladded with 583 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 11 The 3D surface topography of the worn surface in the coating cladded with 700 W, (a) 3D wear trace morphology, (b) cross section.
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Fig. 12 The 3D surface topography of the worn surface in the substrate, (a) 3D wear trace morphology, (b) cross section.
The middle layers of coatings were composed of amorphous and equiaxed dendrites as shown in Fig. 13. The content of equiaxed dendrites was defined in the same way as that of amorphous phase and referred to as the area ratio of equiaxed dendrites. As compared with the vast amorphous area, the black islands was identified and marked as equiaxed dendrites as shown in Fig. 13. Obviously, elevated laser power were beneficial to increasing the content of the equiaxed dendrites. Fig. 14 presented the SEM micromorphology of the bottom layers in the coatings obtained under the condition of different laser power. Metallurgical bonding had been obtained between the base metal and the coatings of which the bottom layer was mainly 9
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composed of primary dendrites and secondary dendrites. The content of amorphous phase and crystalline phases gradually decreased and increased from the upper layer to the middle layer of the coatings, respectively. Therefore, the distribution of amorphous and crystalline phases throughout the coatings could be represented by Fig. 15 in which the bottom layer of the coatings were composed of dendrites instead of amorphous phases.
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Fig. 13 The microstructure of the middle layer in the coatings cladded with different laser power, (a)233 W, (b) 476 W, (c) 583 W, (d) 700 W.
Fig. 14 The microstructure of the bottom layer in the coatings cladded with different laser power, (a)233 W, (b) 476 W, (c) 583 W, (d) 700 W. 10
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EDS analysis was then carried out along the path SE from the upper layers to the substrate as marked in Fig. 16(a). Little fluctuation in the relative concentration curves of Cr, Co, Fe, Si and Ni elements was found in the upper stage, or in the upper and middle layers in the coatings. However, the content of Fe and the other elements increased and decreased sharply, respectively, in the bottom layers of the coatings, especially in the dendrite zone.
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Fig. 15 Schematic diagram of microstructure distribution in the cross section of the coatings.
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Fig. 16 The macro morphology and element distribution in the cross section of the coating, (a) the coatings cross-section, (b) element distribution along path SE as marked in (a).
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As the content of Fe elements in the substrate was much higher than that of the coatings, obvious dilution effect by the substrate upon the cladded coatings led to the increase of Fe elements and the formation of dendrites at the bottom of the coatings as shown in Fig. 16. Fig. 17(a) was the schematic diagram of the cross section of the coating in which A1 and A2 was that of the melted base metal and the coatings beyond the base metal, respectively. The dilution ratio (η) could be obtained by η=A1/(A1 + A2) [22] and exhibited in Fig. 17(b).
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Fig. 17 The schematic diagram of dilution rate and the test result, (a) the cross section of the coatings, (b) the dilution rate in coatings cladded with different laser power.
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Increasing power led to the increase in dilution ratio in Fig. 17(b). The determinant factors of amorphous microstructure for a certain metallic alloy system included cooling rate and composition of metallic alloy system [18]. The increase of laser power led to the increase of heat input on coatings and substrate. On the one hand, more melted substrate metal was melted to give birth to higher dilution rate and thus more obvious change in the chemical composition of the HEA coatings. So the critical cooling rate of the coatings was changed as a result. On the other hand, for metallic glass alloys, the actual cooling rate was the most important factor to inhibit the nucleation and growth of the competing crystalline phases [23]. With regard to the thermal cycling process, the increase of heat input led to the decrease of actual cooling rate and thus reduced the amorphous content as shown in Fig. 4(b). On the one hand, comparing with the substrate, a large number of Co, Cr, Ni as passivating elements improved the corrosion resistance of the coatings. On the other hand, amorphous coatings were less prone to galvanic corrosion due to the absence of microstructure defects and non-uniform chemical composition [24, 25]. Therefore, the corrosion resistance of coatings with the highest amorphous content was more excellent than that of the other coatings. The equiaxed dendrites content and the columnar dendrites size increased simultaneously with the increase of laser power. Microhardness of the upper layer of the coatings also varied along the laser power and amorphous content. As well known, the amorphous alloy exhibited no long-range ordered microstructure, then no crystalline defects such as grain boundary, dislocation and precipitation would be obtained [26]. The wear resistance of the coatings was then related to the microstructure and main phases [27]. As a result, the amorphous content in the coatings had played a crucial role in the tribological properties of the coatings. Conclusions (1) Different amorphous content in laser cladded Co34Cr29B14Fe8Ni8Si7 (at. %) high entropy alloy coatings were successfully obtained with different laser power and the H13 steel as the substrate. Metallurgical bonding was obtained between the 12
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coatings and the substrate. (2) The coatings was composed of three layers according to the microstructure including the dendritic layer at the bottom, the amorphous layer in the top and the transition area. The coverage area (or the size) of the dendrites at the bottom increased obviously as the laser power got elevated. The amorphous content in different coatings decreased from 81.15% to 33.79% as the laser power increased from 233 W to 700 W, which was attributed to the dilution effect and dependent upon the heat input. (3) Higher microhardness, better wear and corrosion resistance was attributed to higher amorphous content in the coatings. Lower amorphous content in the coatings had lead to deeper furrows, more serious adhesive wear and oxidation wear, which gave birth to wider cross section area of worn track, higher wear weight loss and thus deteriorated wear resistance.
Acknowledgement
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This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2016EEQ03) and the Natural Scientific Research Innovation Foundation in HIT (No. HIT.NSRIF.201703). Special appreciation shall be sent to Dr. Jia Liu who has been giving wholehearted supports to the first author. The authors wish to thank Mr. Dingli Zhao from WARD Australia Pty. Ltd. for fruitful discussions about electrochemical investigations.
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Highlights 1 The Co34Cr29B14Fe8Ni8Si7 (at. %) high-entropy alloy amorphous coatings cladded with different laser power were fabricated on H13 steel substrate. 2 Laser power influenced the amorphous content through changing dilution rate and actual cooling rate. 3 3D wear trace morphology was adopted to analyze the wear resistance of coatings. 4 Higher amorphous content in the coatings led to higher microhardness, better wear and corrosion resistance.
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