Surface engineering-controlled tribological behavior and adhesion strength of Ni-Cr coating sprayed onto carburized AISI 4340 steel substrate

Surface & Coatings Technology 370 (2019) 144–156

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Surface engineering-controlled tribological behavior and adhesion strength of Ni-Cr coating sprayed onto carburized AISI 4340 steel substrate

T

Auezhan Amanov Department of Mechanical Engineering, Sun Moon University, Asan 31460, Republic of Korea

A R T I C LE I N FO

A B S T R A C T

Keywords: Ni-Cr coating Surface roughness Wear Adhesion strength UNSM

In this study, a nickel‑chromium (Ni-Cr) coating was sprayed onto carburized AISI 4340 (UNS G43400) steel substrate by high-velocity oxygen fuel (HVOF) method. As-sprayed Ni-Cr coating was subjected to ultrasonic nanocrystal surface modification (UNSM) technology to control the tribological performance and adhesion strength. The aim of the current study is to improve the durability of Ni-Cr coating by the application of UNSM technology. The UNSM technology reduced the surface roughness and increased the surface hardness of the assprayed coating by about 64 and 25% that were associated with the elimination of high peaks and valleys, filling up micro-pores, respectively, where the unfilled open-pores and micro-cracks may be found as a stress concentration cite to suppress a crack initiation and crack propagation and a less number of open-pores may hinder the initiation and propagation of micro-cracks. The evaluation of the tribological performance of Ni-Cr coatings was evaluated using a wear tester in dry conditions, while the adhesion strength was obtained using a progressive scratch tester. As a result, the friction coefficient of the as-sprayed coating was reduced from about 1.1 to 0.75, and the adhesion strength was increased from about 3.3 to 6.1 N by the application of UNSM technology. The reduction in surface roughness of the UNSM-treated coating can be considered as the main parameter that influences on the frictional behavior, while increase in surface hardness determines the wear resistance and adhesion strength. It was found that the UNSM technology improved the tribological performance and adhesion strength of the coating.

1. Introduction Ultra-supercritical (USС) boilers are used in the power plant to produce an electric power [1]. Advanced surface engineering-controlled coatings for use in USС boiler applications are of interest to achieve a much higher efficiency and better performance of power plants operating at various conditions. Type of coatings and their durability are one of the key factors that maintain the reliability and accessibility of USС boilers in the power plant [2]. Coatings that are used in USС boilers of the power plant need to meet the standard regulations resisting against some important surface chemical-, heat- and mechanical- and induced properties such as corrosion, heat transfer, wear, creep, adhesion, oxidation, erosion, etc. [3,4]. Thermal spray coatings (TSСs) were employed to prevail the oxidation and corrosion resistances of USС boilers as TSСs protect the surface of USС boilers from environmental issues. Many efforts have been made to select the best suitable coatings in terms of durability and service life to maintain the reliability and accessibility of a power plant. For example, Abe et al. investigated the effect of TSС on the steam oxidation resistance of nickel‑chromium (Ni-Cr) coating for USС boilers [5]. They found that

the oxidation resistance was increased by the application of TSС coating. The same authors made an effort to improve the oxidation resistance of Cr steel by pre-oxidation process [6]. They reported the possibility of controlling the oxidation resistance of Cr steel suggesting that the boiler made of steel can be produced for power plants. However, the surface mechanical-induced properties, namely, friction and wear, and adhesion strength between top coating and substrate of TSСs for USС boilers are still one of the unsolved problems in USС boilers in the power plant. There are a few studies are still focusing on the improvement in wear resistance of the WС-Сo coating [7], but, recently, there is a growing interest in spraying of metal alloys, ceramic coatings and Ni-Cr [8,9], where the latter one is usually used in USС boilers in the power plant due to the restriction of substrate from the wear process. The main tribological property of the coatings is a wear-resistant that provides an improvement in service lifetime of the coated USС boilers. It is well established that the wear property of the coating can be contolled by changing the surface hardness, while the surface roughness determines the frictional behavior and the substrate surface roughness affects the adhesion bonding of the coatings [10,11]. However, some defects of

E-mail address: [email protected]. https://doi.org/10.1016/j.surfcoat.2019.04.087 Received 20 December 2018; Received in revised form 12 April 2019; Accepted 27 April 2019 Available online 30 April 2019 0257-8972/ © 2019 Elsevier B.V. All rights reserved.

Surface & Coatings Technology 370 (2019) 144–156

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(а)

(b)

Smаshed single powder

50 μm

50 μm Fig. 1. SEM images of feedstock powder of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

(UNSM) technology with Ni-Cr coating was carried out. It was proved earlier that the UNSM technology was able to control the frictional behavior of the TSСs [24]. The main mechanism was associated with the surface properties such as integrity, hardness, microstructure [25]. Thus, the aim of this investigation is to control the tribological performance and adhesion strength of Ni-Cr coating to improve the durability of the USС boilers in the power plant. In the present study, a UNSM technology was applied to a Ni-Cr coating sprayed onto carburized AISI 4340 steel by HVOF method to improve its tribological performance and adhesion strength. The frictional behavior and adhesion strength of the UNSM-treated Ni-Cr coating were evaluated and the obtained results were compared with that of the as-sprayed coating. Moreover, the frictional reduction, wear resistance enhancement and delamination mechanisms of the Ni-Cr coatings are systematically investigated and discussed based on the microscopic, chemical and Raman spectroscopy characterizations.

TSСs such as unmelted particles, micro-cracks, micro-pores, etc. are inevitable due to the spray processing, where a high surface roughness and the presence of those defects may deteriorate the mechanical properties (strength) of the coatings as well [12–14]. TSCs play a significant role in improving the efficiency and performance of power plants through surface-related properties, but a combination of coatings with surface engineering technologies can further improve the efficiency and performance of power plants by increasing the wear resistance and adhesion strength of USС boilers rather than only coating and substrate alone. For example, surface engineering technologies such as shot peening (SP), laser shock peening (LSP), ultrasonic peening (UP) can also increase the adhesion strength between the coating and substrate [15–18]. Moreover, one of the easiest and time consuming approach to improve the durability of the coatings is the application of surface engineering technologies that eliminate the surface and interface drawbacks of the coatings such as unmelted particles, microcracks, micro-pores generated during the spray processing. In this regard, a surface engineering method is in need to be applied for the coatings to control the frictional behavior and adhesion strength. For example, the effect of laser heat treatment (LHT) process on the wear resistance of tungsten carbide-cobalt carbide-nickel (WС-СrС-Ni) coating sprayed by high-velocity oxygen fuel (HVOF) method was investigated in the previous study [19]. The role of post-LHT process was found to be remarkable, where the reduced number of porosities increased the surface hardness. In turn, the wear resistance of WС-СrС-Ni coating was increased by post-LHT process. Another study on the influence of laser-based treatment on the micro-hardness of tungsten carbide-cobalt chromium (WС-СoСr) coating was reported earlier [20]. It was discovered that the laser-based treatment was capable of reducing the number of porosities and increasing the surface hardness. Furthermore, Senthilkumar et al. investigated the role of thermal cycle on the grain size and surface hardness of Ni-Cr coating, where the grain size was increased and surface hardness was reduced with increasing the temperature [21]. Furthermore, Sheibani Aghdam et al. compared the friction performance of Ni-Cr coatings with various Ni/Cr ratio, where the decrease in Ni increased the surface hardness and friction coefficient [22]. The surface hardness was increased by a decrease in Ni due to the heteregenous distribution of Ni particles and also Ni phase strengthening effect. Also, the increase in hardness by heat treatment can be understood in terms of lower precitipation of Ni [23]. As of now, most of the earlier investigations on TSCs are focused on playing with microstructure and surface hardness by optimizing the deposition or spraying parameters. However, there is a lack of empirical research on hybrid surface engineering, namely coating + surface engineering of the coatings, on the frictional behavior together with adhesion strength of Ni-Cr coatings in the literature. Therefore, a research on the combination of ultrasonic nanocrystal surface modification

2. Materials and method 2.1. Procedure In this study, a Ni-Cr alloyed powder was sprayed onto carburized AISI 4340 (UNS G43400) steel by HVOF method. An average powder size of 50 to 75 μm (see Fig. 1(a)) (≥99.5% purity, Hardfacing Сo., Ltd., Korea) was selected as a precursor material. The powders were mixed by a ball mill for 1 h and then dried for 20 min at 120 °С. As a substrate, the grit blasted carburized AISI 4340 steel with dimensions of 100 × 10 × 4 mm3 was manufactured. A Ni-Cr coating with an approximate thickness of 150 μm was sprayed using by HVOF system (HF1800) [26]. The pressure of propane, nitrogen and oxygen was about 0.25, 0.40 and 0.45 MPa, respectively. Fuel and gas rate was 30 l/h and 20 l/h, respectively. The coating was sprayed at a current of 800 A, a voltage of 40 V, and the distance between substrate and nozzle was 150 mm. An average surface roughness (Ra) of the coating was found to be about 7.5 μm. Following the coating spraying, a UNSM technology was applied to the Ni-Cr coating. The surface of the coatings was cleaned by highpressure air spray for the deliverance of impurities. A UNSM technology is a cold-forging process that allows to obtain a smooth surface along with high surface hardness because of the partially eliminated micropores from the surface of the coating. In this UNSM technology, a tungsten carbide (WС) ball smashed the coating (see Fig. 1(b)) with the parameters listed in Table 1 [24,27,28]. 2.2. Frictional behavior The friction coefficient and wear rate of the coatings were obtained 145

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Table 1 Optimized UNSM parameters. Frequency, kHz

Amplitude, μm

Static load, N

Linear speed mm/min

Interval, μm

Ball diameter, mm

Ball material

20

10

2

2000

70

2.38

WС

Table 2 Wear test conditions. Applied normal load, N

Reciprocating speed, mm/min

Sliding stroke, mm

Sliding cycles

Maximum Hertzian contact pressure, GPa

10

2.51

4

2500

0.68

obviously seen that the surface of the as-sprayed coating was altered by the application UNSM technology as shown in Fig. 2(b), a reduction in surface roughness because of elimination of high peaks, and also partially elimination of open-pores, which are filled up with coating powder debris that generated by continuous high-frequency smashing. High magnification SEM images of the as-sprayed and UNSM-treated coatings are shown in Fig. 2(a1 and b1). It can be seen that the microcracks and micro-pores of the as-sprayed coating was somehow disappeared after UNSM technology. Also, it is thought that the initial micro-cracks formed on the surface of the as-sprayed coating can be replenished by coating powder debris as well. It needs to be mentioned here that the partially eliminated high peaks and valleys, and micropores have a considerable influence on the surface roughness and hardness/strength of the coating, where the unfilled open-pores and micro-cracks may be found as stress concentration cites to suppress a crack initiation and crack propagation, and a less number of open-pores may hinder the initiation and propagation of micro-cracks [29]. SEM micrographs of the UNSM-treated coating indicated no signs of openpores and micro-cracks as shown in Fig. 2(b) as well. Fracture toughness is a vital property which indicates the absorption of energy deformation till fracture. It is believed that it may be increased by a partial reduction of open-pores and micro-cracks, thereby increasing the plasticity of the coating after UNSM technology. Shrivastava and Upadhyaya investigated the fracture toughness of Ni-Cr coating after heat treatment at various temperatures [30]. It was reported earlier that the fracture toughness of Ni-Cr coating increased by heat treatment temperature, but the hardness was reduced. The increase in toughness and reduction in hardness are related to the microstructure of the coating, where surface quality has a significant effect on the fracture toughness. As a result, the UNSM-treated coating was found to be somehow free from micro-cracks and open-pores in comparison with the as-sprayed one (see Fig. 2(a and b)). Moreover, a distribution of chemical elements over the surface is also of interest after UNSM technology. A distribution of Ni and Сr elements of the as-sprayed and UNSM-treated coatings were analysed by EDS as shown in Fig. 2(a2, a3, b2 and b3). It can be seen from elemental distribution mapping that no significant distinct in uniformity and chemical composition of Ni and Cr elements along with microstructure before and after UNSM technology. It was reported earlier that the UNSM technology does not change the chemical composition of the coatings [24,25]. However, the amount of С at the top surface was increased after UNSM technology, which is associated with the migration of С from the matrix towards top surface that can be explained by diffusion pumping mechanism as shown in Fig. 2(a4 and b4). Previously, it has been also demonstrated that the UNSM technology migrated С from the matrix towards top surface resulting in the increase in С amount at the top surface that has a great influence on the increase in mechanical properties as well [31]. Fig. 3 presents 3D LSM was employed in order to take a close look at the defects such as high-peaks and valleys, and micro-pores on the surface of the as-sprayed and UNSM-treated coatings. One can be observed that the as-sprayed coating demonstrated a very rough surface along with a number of micro-pores with a diameter in the range from

using a wear tester (Anton-Paar, Austria) slid against SAE52100 (UNS G52986) steel (7.14 mm in diameter). Both the as-sprayed and UNSMtreated coatings were employed to the test 3 times each under the identical conditions that are listed in Table 2. 2.3. Adhesion strength Adhesion strength between Ni-Cr coating and carburized AISI 4340 steel substrate was obtained using a single stroke scratch tester (Anton Paar, Austria). Adhesion strength of the as-sprayed and UNSM-treated coatings was repeated 2–3 times as well depending on the similarity of the results. Scratch test conditions are listed in Table 3. The adhesion strength of the coating to the substrate was determined by acoustic emission (AE) analysis. 2.4. Measurements and analysis The surface roughness profilometer (SJ-210, Mitutoyo, Japan) and micro-Vickers hardness tester (MVK E3, Mitutoyo, Japan) were used to measure the surface roughness (Ra) and surface hardness of the coatings. The distance of each measurement points of the surface roughness and surface hardness was 4 mm, and indentation load at 300 gf measured the surface hardness for 12 s. Both the measurements were repeated 3 times in order to achieve reliable data. The surface and wear track morphologies, and chemical alteration of the coatings were analyzed using a scanning electron microscope (SEM: JSM-6510, Jeol, Japan) and a three-dimensional laser scanning microscopy (3D LSM: VK-X100, Keyence, Japan) and also energy-dispersive X-ray spectroscopy (EDS: JED2300, Japan). The chemical composition and surface properties of the coatings before and after wear tests were studied by Raman spectroscopy (LabRam HR Evolution, Horiba, Japan) using a 532.2 nm line from a frequency-doubled Nd-YAG laser with a power of 8 mW as the excitation source in the Raman shift range of 300–2100 cm−1 with a spectral resolution of 1 cm−1. 3. Results and discussions 3.1. Surface and interface morphology SEM morphologies together with EDS mapping of the as-sprayed and UNSM-treated coatings are shown in Fig. 2. The micrographs of the as-sprayed coating showed typical characteristics of re-solidification molten particles and confirmed the presence of open micro-pores distributed over the surface as presented in Fig. 2(a). It is also evident that the as-sprayed coating had high peaks and valleys, and micro-cracks, which ensure a high surface roughness and a poor durability. It is Table 3 Scratch test conditions. Load, N

Speed, mm/min

Length, mm

Diamond tip radius, μm

20–160

20

14

200

146

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(а)

(b) Miсro-сrасks Flаttened/smаshed peаk

High-peаks

Miсro-pores

(а1)

(b1)

Miсro-pores Unmelted powder Miсro-pores High-peаks

Flаttened/smаshed peаk

Miсro-сrасks

(а2)

(b2)

(а3)

(b3)

(а4)

(b4)

Fig. 2. Surface morphology and Ni-Cr elements distribution of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings. 147

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(b)

(а)

Fig. 3. 3D LSM images of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

100 to 200 μm (see Fig. 3(a)). Fig. 3(b) presents the number of those micro-pores are somewhat reduced where the area of the flat surface was increased because of the reduction in high peaks and valleys. However, the UNSM technology did not eliminate the whole micropores on the surface depending on their size, where the small ones were filled up, while the big ones neither filled up nor disappeared from the surface as shown in Fig. 3(b) by arrows. Moreover, the effect of valleys is more important than the effect of peaks in crack initiation, where the UNSM technology did not change the valley, but it reduced the surface roughness thanks to the peak reduction (see Fig. 3). An adhesion property of the Ni-Cr coating is very essential as the coating needs to withstand various types of mechanical loads and impacts, and also thermal cycles when they are in use in USС boilers in the power plant. Fig. 4 presents the cross-sectional SEM images of the assprayed and UNSM-treated coatings. The quality of the as-sprayed coating in terms of adhesive was deliberately poor (see Fig. 4(a)), where the number of defects at the contact interface (indicated by red rectangle) were reduced after UNSM technology as shown in Fig. 4(b) due to the continious smashing down the coating from the top. Also, the thickness of the as-sprayed coating of about 144 μm was shrinkaged after UNSM technology down to about 132 μm ensuring a more dense coating. It is well known that the improvement in quality not only in terms of surface, but also in terms of adhesive strength is very important to improve the durability of the coated parts, where a detachment/delamination of the coating from the metallic substrate may result in catastrophic failure of the coated USС boilers in the power plant.

3.2. Surface roughness and hardness The wear performance of the coatings depends on their quality such as surface roughness, hardness, thickness, adhesion strength, etc. [32]. The coating can be characterized by surface roughness, which increases with the particle size and reduces with spraying velocity and temperature [33]. Fig. 5(a) presents the surface roughness of the as-sprayed and UNSM-treated coatings along with the comparison of average surface roughness (Ra) and five highest peaks and lowest valleys – mean roughness depth (Rz) values. It was found that the surface roughness values of Ra and Rz of the as-sprayed coating were reduced by about 39 and 43% after UNSM technology, respectively. This is due to the elimination of high-peaks and valleys (a reduction in Rz), filling up micro-pores by a smashed coating powder debris, and also the presence of flattened surface, where a hard tip continuously bombarded the surface of the coating at a high frequency. The surface roughness of the coating is the dominant surface integrity that plays a vital role in controlling the frictional behavior, where the application of UNSM technology may be beneficial to controlling the friction coefficient and wear resistance of the coatings. It needs to be mentioned that the application of UNSM technology on the ceramic TSCs was already demonstrated in our previous studies, where the surface roughness was significantly reduced in comparison with that of the as-sprayed one owing to the absence of high-peaks and valleys and the presence of flattened surface of the coating [24,25]. The surface hardness and depth profile of the coatings were measured and the obtained measurement results are presented in Fig. 5(b and c). Obviously, the surface hardness of the as-sprayed coating, which was about 47.2 HRС, was increased up to 58.5 HRС after UNSM

(а)

(b)

Fig. 4. Сross-sectional SEM images of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings. 148

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Fig. 5. Surface roughness (a), surface hardness (b) and hardness as a function of depth (c) of the as-sprayed and UNSM-treated Ni-Cr coatings.

increase of micro-hardness of Ni-Cr coating. Furthermore, the hardness increase is also related with work hardening, strain rate and surface integrity, where the partial elimination of high-peaks and valleys, and micro-pores may also influence [24]. In addition, a tensile test needs to be carried out on Ni-Cr coating in order to understand the influence of UNSM technology on the mechanical properties such as UTS, YS, elongation, etc.

technology as shown in Fig. 5(b). A comparison in hardness as a function of depth throughout the thickness of the as-sprayed and UNSM-treated coatings is presented in Fig. 5(c). The hardness measurement locations along the depth were selected randomly considering the distance between indentations. Both the as-sprayed and UNSMtreated coatings were gradually reduced with depth, where the effective depth of UNSM technology was found to be about 100 μm. This is first associated with the change in the microstructure of the coating, where the diameter of originally sprayed powders was reduced/smashed resulting in the refinement in particle size. It is well known that the particles size determines the hardness of the coatings based on HallPetch expression [33]. The hardness may be increased with refining the grain size. In addition, during UNSM treatment, the induced compressive stress is made a plasticly deformed zone with a large plastic strain. The large strain produced during the deformation leads to dislocation slides inside the grain and when the amount of dislocation is enough, a new grain boundary formed due to change the orientation of smaller part crystal inside of grain. This caused the grain is refined to smaller grain size that makes difficult for dislocation to cross between grain (different slip system), and travel and accumulate or in other words it means more obstacle to dislocation motion and higher strength because more stress is required for deformation. The accumulation of dislocation are precursors to crack initiation and failure. Grain refinement thereby delaying failure and increasing the hardness of the coating. As a result, the induced compressive residual stress may be attributed to the distortion of the lattice and to the generation of dislocation by severe plastic deformation (SPD). Skamat et al. reported the effect of the vibratory frequency treatment on the number of grains and micro-hardness [34]. They found that the increase in a number of grains led to

3.3. Frictional behavior Сomparison in the friction coefficient and wear resistance of the assprayed and UNSM-treated coatings is presented in Fig. 6. As for the assprayed coating, the friction coefficient was rapidly increased during the first 250 cycles from about 0.25 to 0.75 as presented in Fig. 6(a). With continuing the reciprocating sliding, the friction coefficient was further gradually increased reaching a friction coefficient value of > 1.0, and then it was reduced a bit up to a friction coefficient of 0.95 and finally it got stabilized to a value of about 0.90 at the end of reciprocating sliding. Hence, the time dependent frictional behavior of the as-sprayed coating can be divided into three stages: (1) deformation of asperity took place at the initial contact, where the friction coefficient dropped that affected the static friction; (2) then started slowly increasing till a certain value of friction coefficient, where the rising slope depended on the plastic deformation-induced wear particles; (3) finally the friction coefficient started gradually dropping that led to a reduction in friction force due to the lessening a plowing and an asperity deformation. Fig. 6(b) that the frictional behavior of the UNSMtreated coating differed from the frictional behavior of the as-sprayed one. In turn, the frictional behavior of the UNSM-treated coating can be 149

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(а)

(b)

(с)

Fig. 6. Friction coefficient (a and b) and wear rate (c) of the as-sprayed and UNSM-treated Ni-Cr coatings.

behavior [10,32]. It is well documented that the deformation of asperities at the contact interface determined the static friction coefficient and they also have a significant influence on the kinetic friction coefficient [35]. Lee and Polycarpou studied the role of roughness on the unlubricated frictional behavior, where they concluded that the initial surface roughness had a significant effect on the frictional behavior, especially static friction coefficient [36]. Hence, a reduction in surface roughness of the as-sprayed by UNSM technology is associated with the reduction in friction coefficient under the same unlubricated conditions in comparison with the as-sprayed coating. The wear rate that calculated based on the cross-sectional profiles of the as-sprayed and UNSM-treated coatings is presented in Fig. 6(c). Achieving a wear track of the as-sprayed coating was easy, but the wear track of the UNSM-treated coating was determined according to the oxide-rich regions since the surface of the UNSM-treated coating was already flattened. The wear of both the coatings was very mild, but the UNSM-treated coating exhibited a higher resistance to wear, which may be attributed to the surface hardness increase that can be considered as a dominant parameter [37]. Moreover, the oxidation level of the UNSM-treated coating was higher (11.84%) in comparison with the assprayed one (3.52%) as shown in Fig. 7. The formation of the oxidation layer is an important phenomenon that may lead to a better frictional behavior [38] as it may serve as a solid lubricant [39]. Furthermore, the as-sprayed coating had relatively more micro-pores and micro-cracks on the surface in comparison with that of the UNSM-treated coating (see Fig. 8(a and b)), which led to a decrease in wear resistance and is

divided into four regions: (1) static friction that occurred as a consequence of the reduction in surface roughness, where contact asperities were lesser in comparison with the as-sprayed one resulting in the increase in friction coefficient first and then reduced which is still higher than the kinetic friction; (2) kinetic friction occurred due to the higher hardness of counterface bearing steel ball, whose contact asperities get mirror polished resulting in reduction in friction coefficient; (3) and then a running-in stage started, where the friction coefficient gradually increased with increasing reciprocating sliding time (4) and finally reached the steady-state frictional behavior. Such a frictional behavior of the as-sprayed coating may be associated with the initial rough surface roughness that may lead to an increase in contact stress and a reduction in contact area due to the limited true contacts of asperities at the contact interface. A stabilization of the friction coefficient after 1800 cycles clarified in terms of change in the contact area, which is enlarged due to the wear occurrence resulting in a reduction in contact pressure. The friction coefficient of the UNSM-treated coating was about 0.5 at the initial stage, which slightly increased to 0.75 during dry sliding up to 610 cycles and then remained steady at a value of 0.70–0.75. UNSM technology demonstrated a reduction in the average friction coefficient of the as-sprayed coating by 33%, where a higher friction coefficient at the beginning of the test that was related to the initial low surface roughness as well. However, an increase rate of the as-sprayed coating was much higher in comparison with the UNSMtreated coating. The initial surface roughness of the coatings can be considered as the main parameter that influences on the frictional 150

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Fig. 7. SEM images of wear tracks generated on the surface of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

of the coatings strongly dependent on the residual stress of the coatings [40]. Therefore, it is very crucial to induce a high compressive residual stress. The UNSM technology is capable of inducing a compressive residual stress at the top surface up to 2 GPa and a deep compressive residual as a function of depth up to 1 mm depending on the property of materials and also UNSM technology parameters [27]. In case of UNSM treatment, induced residual stress can be considered to be the sum of a tensile stress component by thermal effect and a compressive one by plastic deformation due to ultrasonic impact. Tensile residual stress remains constant even if the UNSM treatment impact force increases, because the UNSM effect may be reset every impact, whereas the compressive residual stress increases with UNSM treatment impact, until the saturation of plastic deformation. Compressive residual stress

prone to produce debris at the contact surface. It is also seen from highmagnification LSM images in Fig. 8(a1 and b1) that the selected part of the wear track of the as-sprayed coating was obviously visible demostrating less flattened surface, while the wear track of the UNSM-treated coating was hardly observed due to the less amount of wear on the flattened surface. Thus, the friction coefficient of the as-sprayed was higher and unstable, while the UNSM-treated coating demonstrated a relatively lower and stable frictional behavior. Another reason for the better frictional behavior after UNSM technology is a hardened plastically deformed layer by large plastic strain. Moreover, as shown in Fig. 5(c), the gradual reduction in hardness as a function of depth may be found to be beneficial to improving the bearing capacity of the coating as well. In a previous study, it was reported that the durability

(а)

(b)

(а1)

(b1)

Flаttened surface

Fig. 8. Low and high magnification 3D LSM images of the wear track generated on the surface of the as-sprayed (a, a1) and UNSM-treated (b, b1) Ni-Cr coatings, respectively. 151

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partially eliminated surface defects such as micro-pores, micro-cracks, etc. As a result, the UNSM-treated coating reduced the friction coefficient by about 22% in comparison with the as-sprayed coating as shown in Fig. 9. AE signal of the coatings was also recorder during scratching to evaluate the adhesion strength between coating and steel substrate in Fig. 9. It was evident that a sharp dip was detected at a scratching distance of about 2 mm in AE plot along with an increasing progressive load as shown in Fig. 9(a). This sharp dip is an indication of cohesive failure that was detected owing to the presence of tensile stress of the as-sprayed coating. With applying a progressive load along with scratching distance, AE signal detected another high sharp dip at a scratching distance of about 3.3 mm. This sharp dip is an indication of adhesion failure that was the beginning of delamination of the assprayed coating. AE plot of the UNSM-treated coating along with an increasing progressive load is presented in Fig. 9(b). The UNSM-treated coating demonstrated a much better adhesion strength in comparison with the as-sprayed coating, where the adhesion failure was initiated at a scratching distance of about 6.3 mm, which implying that the UNSM technology was capable of increasing the adhesion strength between coating and steel substrate. As a result, it was gathered from microscratch tests at a progressive load that the as-sprayed and UNSM-treated coatings started to fail at a scratching distance of about 3.3 and 6.3 mm, corresponding to a critical load of 52.5 and 81.9 N, as shown in Fig. 10, respectively. Determination of adhesion energy, which is an interfacial tensions of the coatings, is an important phenomenon for evaluating the quality, durability and performance of the coatings. Adhesion energy of the coatings at the critical failure point was also measured using the following equations [42]:

(а)

(b)

0.5

σS = Fig. 9. Adhesion strength of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

0.15 ⎛ PHf ⎞ E f0.3 E 0.2 R ⎝ H ⎠ ⎜

⎟

W = K2 (σS + σR )2t can be induced due to the high contact stress of pin or ball when striking the surface with ultrasonic vibration. These strikes cause the surface is plastically deformed, which cause the plastic zone in the subsurface layers. As a result, the surface tensile residual stress changes into compression one with increasing the UNSM treatment impact. In addition, surface micro-pores may be eliminated or closed upon UNSM treatment. Hence, it is worth mentioning here that a highly induced compressive residual stress by the application of UNSM technology may be also responsible for the improvement in frictional behavior and wear resistance in comparison with the as-sprayed coating owing to the postponed micro-crack nucleation and propagation.

(1)

1 − vf2 Ef

(2)

where: σS – the scratch test stress; R – is the indenter radius; P – critical load; Hf – is the hardness of the coating; H – is the substrate hardness; Ef – is the modulus of coating; E – is the modulus of substrate; W – is the adhesion energy; K2 – is the constant for spallation; σ = σR + σS – is the total stress; σR – is the residual stress; t – is the thickness of the coating and νf – is the Poisson's ratio of the coating. The critical load of the coatings was obtained from micro-scratch test results, while the modulus and Poisson's ratio were provided by a coating supplier. It was found that the adhesion energy of the UNSMtreated coating was much higher of about 43% in comparison with the as-sprayed coatings, where the role of adhesion strength (critical load),

3.4. Adhesion strength Micro-scratch tests on the as-sprayed and UNSM-treated coatings were performed using a progressive load to assess the adhesion strength and also to emphasize the influence of UNSM technology on the adhesion energy. Fig. 9 presents the micro-scratch results representing the comparison in friction coefficient and adhesion strength with respect to scratching distance. The friction coefficient of the as-sprayed coating was reduced throughout the scratching distance at the same progressive load. In the case of as-sprayed coating, the friction coefficient was rapidly increased at the initial stage of the scratching and then a bit reduced reaching a stable friction coefficient of about 0.30, and at the end of scratching the friction coefficient was increased again up to a value of about 0.50. An increase in friction coefficient of the as-sprayed coating at the early stage may be associated with the initial surface roughness [41]. In the case of UNSM-treated coating, the friction coefficient was found to be stable throughout the scratching distance with an unremarkable increase in friction coefficient at the end of scratching. Such a stable frictional behavior of the UNSM-treated coating may be attributed to the smoother and harder coating with the

Fig. 10. Сritical load and adhesion energy of the as-sprayed and UNSM-treated Ni-Cr coatings. 152

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(а)

(b)

Miсro-сrасks

Miсro-pores

(а1)

(b1)

(с) (d)

(c)(с)

Flаttened surface

(с1)

(d1)

Fig. 11. SEM and 3D LSM images of scratch grooves formed on the surface of the as-sprayed (a and a1 – end point; b and b1 – start point) and UNSM-treated (c and c1 – end point; d and d1 – start point) Ni-Cr coatings.

critical to determine the durability, service life and also performance. Fig. 11 presents the start and end points of scratch grooves formed on the surface of the coatings at a progressive load range from 20 to 160 N. The width of scratch grooves of both the coatings was gradually widened with increasing the progressive load as can be observed through

surface hardness and residual stress was significant. In order to confirm the increase the adhesion strength and adhesion energy, and also to comprehend the deformation behavior of the coatings, the scratch grooves formed on the surface of the coatings was characterized by SEM and LSM as the examination of adhesion and failure of the coatings are

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3.5. Raman spectroscopy analysis

SEM images (see Fig. 11(a, b, c and d)). Moreover, 3D LSM images allowed to obtain the penetration depth, where it was gradually penetrated with increasing the progressive load as can be seen in Fig. 11(a1, b1, c1 and d1). Interestingly, the degree of penetration of a probe into the UNSM-treated coating was lower in comparison with the as-sprayed one (see Fig. 11(b and c)) at the beginning of scratching because of the increase in surface and subsurface hardness, reduction in surface roughness and the presence of flattened surface, also the partially eliminated micro-pores from the surface of the coating. The wear volume of the as-sprayed and UNSM-treated coatings was quantified based on the dimensions of scratch grooves obtained with the aim of 3D LSM images. It was found that the UNSM-treated coating exhibited a lower wear volume by about 17.8% in comparison with the as-sprayed one. It is also of great interest to draw attention to the interior appearance of scratch-induced grooves, where some micro-pores and micro-cracks were visible inside the scratch groove formed on the surface of the as-sprayed coating, while less micro-pores and microcracks were found inside the scratch-induced groove generated on the surface of the UNSM-treated coating as presented in Fig. 11(a and c). Moreover, high magnification SEM images of the as-sprayed and UNSM-treated coatings subjected to scratch is shown Fig. 12(a and b), respectively. It was also seen that some micro-cracks were observed in the scratch groove formed on the surface of the as-sprayed and UNSMtreated coatings, but the UNSM-treated one exhibited a higher resistance to scratch compared to that of the as-sprayed one due to the increase in surface hardness. Moreover, EDS analysis at peeling off the coating was conducted, but no difference in chemistry for the assprayed and UNSM-treated coatings was found. Hence, indeed it is believed that the UNSM technology was found to able to improve the surface microstructure and surface properties, but also it had a tremendous influence on the sub-surface microstructure and properties as well. As an example for sub-surface alteration the change in hardness as a function of depth up to about 100 μm can be cited. Meanwhile, it needs to be mentioned that both the coatings were not delaminate completely up to a progressive load of 120 N, but the failure of both the coatings was found in cracking, spallation and plastic deformation formats. A modified surface of the as-sprayed coating with a lower surface roughness and a higher surface hardness was beneficial for the improvement in adhesion strength between coating and steel substrate. Overall, as expected, the UNSM technology was able to increase the adhesion strength, adhesion energy and to enhance the wear resistance of the as-sprayed coating that has an extraordinary impact on improvement of the tribological performance and adhesion strength of NiCr coating in USС boiler in the power plant.

Raman spectroscopy is a powerful technique for the identification of the chemical composition, structure and uniformity of the coatings. Fig. 13 presents the typical Raman spectra of both the coatings. Typical two overlapping Dawsonite (D-band) and Gibbsite (G-band) peaks were detected on the surface of the as-sprayed coating at 1339 and 1584 cm−1, respectively. The intensity ratio of ID/IG peaks of the assprayed and UNSM-treated coatings was found to be 0.94 and 1.08, respectively. The higher intensity ratio of ID/IG peaks of the UNSMtreated coatings in comparison with the as-sprayed coating obviously depicted the defective nature of reduced inter-defect distance with partially eliminated surface defects such as porosity, cracks, etc. [43]. As for the as-sprayed coating, the D-band and G-band peaks was predominantly detected as presented in Fig. 13(a). As for the UNSMtreated coating, as presented in Fig. 13(b), some nickel oxide (NiO), chromium carbide (Сr3С2) together with oxide layer were found on the surface that were generated because of the temperature increase at the contact interface between tip made of WС and the as-sprayed coating during UNSM technology. The UNSM technology on the as-sprayed coating was carried out under dry conditions resulting a thermally-induced oxidation on the surface, while some amount of carbides on the UNSM-treated coating were also the results of material transfer due to the minor wear of the tip. The presence of carbides supports the idea that carbon-based materials and oxidation can serve as protective tribofilms that provide an improvement in frictional behavior [44]. These all led to the conclusion that the UNSM technology resulted in a process similar to tribo-oxidation effect within the UNSM-treated area of the assprayed coating. Raman spectra of the wear tracks formed on the surface of the coatings were also investigated as presented in Fig. 14. It was revealed that except for D-band and G-band a new peak at 675 and 500 cm−1 was detected on the worn our surface of both the as-sprayed and UNSM-treated coatings, respectively. The detected peaks are associated with the oxidation and material transfer from the countersurface during dry sliding. The appearance of Fe and O shows that an oxidation occurred during direct contact of the steel ball under dry sliding conditions [45]. The peak detected at 675 cm−1 within the wear track of the as-sprayed coating that rubbed against a steel ball resulted in the presence of Fe-oxides such as FeO, Fe2O3 and Fe3O4, while the peak detected at 500 cm−1 within the wear track of the UNSM-treated coating that rubbed against a steel ball corresponds to the occurrence of NiO and (Fe, Сr)O3 [46]. Hence, it can be concluded that the level of oxide layer and Fe-oxide wear debris on the worn out surface of the UNSM-treated coatings was higher than that of the as-sprayed ones. This is associated with the lower contact pressure of the asperities of the UNSM-treated coating because of the smoother surface roughness than that of the as-sprayed one, where a relative high contact pressure can

(а)

(b) Flаttened surface Miсro-cracks Miсro-cracks

Fig. 12. High magnification SEM images of scratch grooves formed on the surface of the as-sprayed and UNSM-treated Ni-Cr coatings. 154

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(b)

(а)

Fig. 13. Raman spectra and deconvoluted peaks of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

(b)

(а)

Fig. 14. Raman spectra and deconvoluted peaks of the wear track formed on the surface of the as-sprayed (a) and UNSM-treated (b) Ni-Cr coatings.

Acknowledgement

easily destroy the formed oxide layer at the contact interface and also the wear debris can be embedded within the contact interface during dry sliding conditions that led to the reduction in wear resistance.

This study was supported by the Start-Up Research Project through the Ministry of Science, IСT and Future Planning of Korea (NRF2017R1С1B5017434).

4. Conclusions

References

In the present research, the effect of UNSM technology on the tribological performance and adhesion strength of Ni-Cr coating sprayed onto carburized AISI 4340 steel by HVOF method was investigated. It was found based on the surface morphology characterizations that the UNSM technology played a vital role in preventing from high-peaks, micro-pores, cracks and other micro-defects of the as-sprayed coating. UNSM technology was found to be beneficial to reducing the surface roughness of the as-sprayed coating by about 64% and increasing the surface hardness from about 48 to 60 HRС, which may be attributed to the elimination of high-peaks and valleys, filling up micro-pores, and refining the particle size. The average friction coefficient and wear rate of the as-sprayed coating were reduced by about 33% and 35% after UNSM technology, respectively. Adhesion strength of the as-sprayed coating was increased from 52.5 and 81.9 N after UNSM technology in turn resulting in increasing the adhesion energy by about 43% as well. Consequently, the UNSM technology was remarkably able to control the tribological performance and adhesion strength of Ni-Cr coating, which may be applied to the USС boilers in the power plant with the aim of increasing their durability and service life span.

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