Preparation and Properties of Core-Shell Structure/Ni60 Coating

Hennan Key Laboratory of Intelligent Manufacturing of Mechanical Equipment, Henan Engineering Research Center of New Energy Vehicle Light Weight Design and Manufacturing, Zhengzhou University of Light Industry, Zhengzhou 430002, China Zhengzhou University of Light Industry, Zhengzhou 430002, China Measurement and Physical and Chemical Analysis Center, Xi’an North Qinghua Electromechanical Co., Ltd., Xi’an City, China


Introduction
Supersonic plasma technology uses a plasma arc as the heat source to heat the working gas to form a hightemperature and high-speed plasma jet; the plasma spray coatings present better corrosion and wear resistance compared with the flame and arc spray coatings [1]. is is used to heat, accelerate, and form a molten particle flow that accelerates (or accelerate after atomization) to form high-speed droplets and impacts the substrate before being flattened into a coating. Rapid cooling solidification deposition is a material forming method that forms a coating on the surface of the substrate; however, this method only results in a mechanical bond between the substrate and the coating, resulting in limited bonding strength [2,3]. In the past, supersonic spraying was realized using a single component powder as the raw material, making it difficult to realize particular requirements [4,5]. erefore, studies on composite powders have become more relevant. Conventional composite powders are mixed mechanically; however, it is difficult to achieve uniform mixing and to avoid segregation of the components, which could significantly affect the performance of the coating [6]. e core-shell composite powder is a new type of composite material developed in recent years and is composed of the core particles and the coating layer. On the one hand, the different phases in the core-shell structure composite powder can achieve a uniform degree of mixing of the components at the particle level, which overcomes the disadvantages of conventional mixed composite powders, such as the composition segregation and uneven mixing. On the other hand, it has the favorable properties of both shell and core materials, the core-shell structure can prepare the coating with good bonding, no obvious cracks and pores, uniform distribution, and wear-resistant performance, and the bonding force between the coating and the substrate will improve the quality and service time of the coating, which can endow the powder with a favorable property profile [7][8][9][10]. In this article, Ni-based powder for supersonic plasma spraying is used as the main raw material, and the Co-composite process for metallurgical bonding in the transition layer is realized by adding core-shell structure materials [11]. e core-shell materials are Ni (Al), Ni (MoS2), and Ni (C). e comparative verification of the preparation process and performance of the coating is explored.

Raw Materials.
In this experiment, the raw material is self-soluble Ni60 powder. Because the powder has excellent comprehensive properties, especially corrosion resistance, wear resistance, and strong antioxidation ability, it is suitable for supersonic plasma spraying and thermal spraying. e chemical composition of the Ni60 powder (in wt%) is Cr 14.0-16.0, Si 3.5-5.0, B 2.5-3.5, C 0.6-1.0, Fe ≤ 5, and balanced Ni. e Ni (Al) core-shell composite powder has a "self-adhesive" effect, in which Ni : Al � 4 : 1 (wt%). In the process of thermal spraying, Ni and Al react to form intermetallic compounds and release a lot of heat. is reaction process can last until the powder collides with the surface of the substrate, which could improve the bonding strength between the coating and the substrate. At the same time, there is a "self-adhesive" effect between the coating and the Ni60 powder, which realizes the metallurgical bonding of the coating and enhances the wear resistance of the coating [12,13]. e results show that Ni (C) core-shell powder has good bonding, uniform distribution of C, and wear resistance, in which Ni : C � 4 : 1 (wt%). e Ni (MoS2) shell core composite powder, which has a composition of Ni : MoS2 � 4 : 1 (wt%), is often used to prepare self-lubricating coatings.
e MoS2 can wet and solidify the lubricating material, which improve the adhesion, heat resistance, and corrosion and wear resistance of the coating [14].
In the spraying process, the hard phase particles rebound less due to the coating effect of the bonding phase. High-quality coatings can be prepared easily on the substrate. e diameter of the sprayed powder purchased from Shanghai Haotian Metal Powder Co., Ltd is approximately 20-50 μm, and the purity is 99.5%. ese three kinds of core-shell structure powders were, respectively, added to Ni60 powder at a proportion of 15% (wt). e coating prepared using Ni60/Ni (Al) powder was designated as NNA: the one using Ni60/Ni (MoS2) powder was named NNM and the one using Ni60/Ni (C) powder was named NNC. Figure 1(a) shows the core-shell structure, and Figure 1(b) shows the micromorphology of the core-shell structure.

Preparation
Coating. HLD-8000 supersonic plasma spraying equipment produced by Zhengzhou Lijia ermal Spraying Machinery Co., Ltd. was used to prepare the composite coating. e matrix material was a round 45# steel plate (Φ20 mm × 10 mm). Before spraying, the preprocess steps included sandblasting, roughening, acetone cleaning, acidification, and alcohol cleaning. Before preparation, the powders were mixed in a ball mill having a milling rate of 150 r/min; the ball-material ratio was 3 : 1, and the ball milling time was 4H, and the material of the ball is GCr15 steel. e spraying process parameters after preheating the substrate are as follows: spraying voltage 110 V, spraying current 320 A, spray gun distance 120 mm, powder feeding voltage 11 V, Ar gas 3.5 m 3 /h, secondary gas H2 0.25 m 3 /h, and spraying layer thickness ∼0.5 mm. After spraying, the sand is slowly cooled to room temperature. e schematic of the coating preparation process is shown in Figure 2.

Characterization Methods.
e phase structure of the coating was analyzed using Shimadzu 7000 X-ray diffraction equipment (XRD, Rigaku Compact, Tokyo, Japan). e surface and cross section morphologies of the coating were observed by using a Merlin compact scanning electron microscope (SEM, uringia, Germany). e nanohardness of the coating surface was tested by using Hysitron Ti 950 nanoindenter. e pressure head was loaded with 5 N, and the indentation depth was 150 nm.
An MFT-R4000 reciprocating friction and wear testing machine was used to test the friction and wear of the coating (Zhongke Kaihua Technology Co., Ltd., Gansu); the coating of the wear test was the polishing coating of the s surface ample. e disc ball contact mode was adopted between the coating and the counter wear parts. e ball bearing steel (GCr13) with a hardness of 600Hv and diameter of 3 mm was selected for the reciprocating test (L � 8 mm). e test was carried out in air at room temperature for 30 min, and the selected loads were 10 N. e micro XAM-800 3D profilometer was used to characterize the worn surface, and the maximum wear depth and wear scar distance were measured. Figure 3 shows the micromorphology of the three kinds of coatings. Considering the surface, there are micropores on all three kinds of coatings, and the pores in Figure 3 (a) are fewer as calculated by the Gibbs free energy formula [15]: where ΔH is the enthalpy, ΔS is the entropy, and T is the temperature.

Microstructure Analysis of Coatings.
3Ni When the spraying heat source temperature is 660-680°C, an exothermic reaction occurs between the components, and the Ni3Al, NiAl, and Al2O3 intermetallic compounds are formed. e matrix and spray particles are heated sufficiently by the exothermic reaction, which facilitates the micrometallurgical bonding between the molten spray particles and the thin layer on the surface of the matrix, generates a strong bonding force, and produces a "self-adhesive" effect [16]. Porosity software ImageJ2x was used to test the porosity of the four coatings, respectively, (a) NNA(4.3%), (b) NNM(5.7%), (c) NNC(7.2%), and (d) Ni60(2.4%). e increase in porosity in Figure 3(b) is due to the existence of some pores in the gap between the core and shell of Ni (MoS2) with the core-shell structure. In Figure 3(c), the porosity of the coating surface increases, which is may be due to the metallurgical reaction of Ni (C) produced by the core-shell during the spraying process. A part of C exists in the coating as a free substance, and a part of C reacts, which causes the CO to escape from the coating in the form of a gas and increases the porosity. However, the above three coating structures form a metallurgical bond, and there is no layered structure with mechanical bonding in the Ni60 coating, as shown in Figure 3(d).

Analysis of Coating Phase Structure.
e XRD diagram in Figure 4, which depicts all three coating types, shows only the Ni diffraction peak, and there are no diffraction peaks  corresponding to the intermetallic compound. Hence, it can be deduced that the core-shell structure of powder in the supersonic plasma jet has not undergone a strong exothermic reaction. is may be because (1) the content of other elements in the Ni-based powder is very low, especially in the core-shell structure of the powder. As core-shell structure, Al, MoS2, and C are not firmly bonded to the Ni shell, and some of them have fallen off [17][18][19], as shown in Figure 1(b). When the mixed powder is sent to the supersonic plasma jet, these cored-shell structure particles are easily impacted by the rigid jet and cannot be melted uniformly. (2) e particle size and specific gravity of Al and C in the Ni (Al) and Ni(C) coreshell structure powders differ greatly from that of Ni, and the acceleration of Al and C powders after peeling is considerably high. It is difficult for the powders to melt synchronically and undergo exothermic reaction [20]. Figure 5 shows the indentation load displacement curves of the three coating materials. All three materials have experienced loading and unloading stages during the test process. With the increase in indentation depth, the loading curve changes smoothly, and there is no "step" curve, which also indicates that the internal structure of the coating is relatively uniform. When the indentation depth reaches 137 nm, the loading force of the three coating materials reaches the maximum value, and the nanohardness in Figure 5(a) reaches the maximum value of 6815 μN. is is also due to the "self adhesion" effect of Al particles in the Ni (Al) core-shell structure materials, which forms intermetallic compounds and encourages mutual diffusion between particles, and the bonding strength of the coating is enhanced, which is also consistent with the change of surface pores in the micromorphology in Figure 3. e nanohardness in Figure 5(b) is 5750 μN. is is because the addition of Ni (MoS2) core-shell structure materials does not form intermetallic compounds, and MoS2 particles influence dispersion strengthening and increase the bonding strength of the coating. Figure 5(c) shows a nanohardness of 2000 μN. Compared with the other two kinds of coatings, the nanohardness of Ni (C) core-shell structure material is greatly reduced, which is due to the chemical reaction between C particles and oxygen at high temperatures, resulting in the release of gases that increase the porosity of the coating and reduce the bonding strength of the coating, which is also consistent with the micropore morphology in Figure 3(c).

Analysis of Wear Performance of Coatings.
e friction and wear properties can reflect the bonding strength of the coating. Figure 6 shows the relationship between the friction coefficient and sliding time of the three coating materials. Using the reciprocating friction of the stainless steel ball at the coating interface, it can be seen from the curve in Figure 6 that in the initial stage (0-1 min), the friction coefficient of the three coating materials increased rapidly and then decreased. e reason is that the core-shell structure materials are added to the three coating materials. At the initial stage, a part of the Ni shell structure first participates in the wear. After the friction failure of the Ni shell surface, the cores of the three core-shell structures, namely, Al, MoS2, and C, begin to participate in the friction. ese three core-shell structure materials react with the Ni60 powder, and the friction coefficient also changes. It can be seen from Figure 6 that the friction coefficient of Figure 6(a) is the lowest at approximately 0.4.
is is because Al particles participate in the reaction during the spraying process, with the formation of Ni3Al, NiAl, and A2O3 intermetallic compounds, resulting in an increase in bonding strength and a decrease in friction coefficient. e friction coefficient of Figure 6(b) is approximately 0.7 as the MoS2 core structure has certain lubricating properties, but the additional quantity is limited, and there is no intermetallic compound or hard phase in the coating. e friction coefficient in Figure 6(c) is 1.1, which is the worst among the three coating materials. is is because the graphite is coated with nickel at the initial stage. Under the load, the nickel shell wears away, the wear surface becomes rough, and the friction coefficient increases rapidly. With the continuous appearance of C  particles, the lubrication effect of graphite stops the increase in the wear coefficient but gradually flattens and stabilizes at approximately 1.1. e micromorphology of the coating surface after friction was characterized. Figure 7 shows the worn morphology of the three coatings, and Figure 7(a) shows the worn morphology of NNA. e wear trace width is 0.75 mm, and the maximum wear depth is 32.6 μm. Figure 7(a)1 shows that the main wear mechanism is that the GCr15 ball applies load in the form of normal stress and adheres to Ni and Al on the surface of the matrix. When there is relative sliding, the surface of the matrix is damaged and peels off continuously, resulting in furrows and pits. When the adhesive point is in the pit, furrowing will also occur along the pit, and the wear mechanism is adhesive wear. With the increase in friction time, the abrasive particles produced on the surface undergo constant rubbing, which makes the wear surface of the substrate susceptible to a slight spalling phenomenon that is manifested as the abrasive wear mechanism. Figure 7(b) shows the morphology of NNM coating after wear. e wear width increases to 0.85 mm, and the maximum wear depth decreases to 30.4 μm. It can also be seen from Figure 7 (b1) that large areas of spalling appear on the surface of the coating. is is because nickel, as a hard phase, can bear a certain load after being stressed [21]. MoS2, as a dispersed phase in the coating, has a low bonding strength with Ni, resulting in particle spalling in the friction process, which is manifested as abrasive wear mechanism. Figure 7(c) shows the worn morphology of the NNC coating. e maximum width of wear trace is 0.95 mm, and the maximum wear depth is 37.8 μm. is is because C in the coating undergoes reactions, and pores also appear in the coating. However, due to the increase in compressive stress and shear stress, cracks begin to appear on the wear surface, and a small number of exfoliation pits appear, as shown in Figure 7(c1). is is because the hard phase Ni peeled out during wear is repeatedly rubbed and rolled under high stress, which results in the plastic deformation of the wear surface. With the repeated cutting action of abrasive particles, the furrow deepens, and Shock and Vibration 7 subsequently, cracks are generated.
e cracks continue to expand until slight spalling occurs. e wear mechanism of these two processes is mainly abrasive in nature.

Conclusion
(1) e nickel-based core-shell composite coating was prepared by the supersonic plasma method. e coating has no obvious defects such as cracks and pores, and the coating structure is metallurgically bonded.
(2) Al, MoS2, and C phases do not appear in the phase structure of NNA, NNM, and NNC coatings. e nanohardness of the three coatings is 6815 μN, 5750 μN, and 2000 μN. Meanwhile, the friction coefficients are 0.4, 0.7, and 1.1, respectively, and the wear resistance of the NNA coating is better. (3) Under a load of 10 N and reciprocating friction process, the NNA coating mainly shows adhesive wear and abrasive wear, while NNM and NNC coating mainly shows abrasive wear [22].

Data Availability
(1) e nickel-based core-shell composite coating was prepared by the supersonic plasma method. e coating has no obvious defects such as cracks and pores, and the coating structure is metallurgically bonded. (2) Al, MoS2, and C phases do not appear in the phase structure of NNA, NNM, and NNC coatings. e nanohardness of the three coatings is 6815 μN, 5750 μN, and 2000 μN. Meanwhile, the friction coefficients are 0.4, 0.7, and 1.1, respectively, and the wear resistance of the NNA coating is better. (3) Under a load of 10 N and reciprocating friction process, the NNA coating mainly shows adhesive wear and abrasive wear, while NNM and NNC coating mainly shows abrasive wear.

Conflicts of Interest
e authors declare that they have no conflicts of interest.