Simultaneously Improving Microstructures and Wear Properties of Ni60 Coating by Heat Treatment

Abstract

Ni60 self-lubricated anti-wear composite coatings were successfully precipitated on the 35CrMoV substrate by laser cladding technology. The effects of heat treatment on the macro-morphology, microstructure, precipitated phase, microhardness, and wear properties of the composite coatings with different heat treatment temperatures (25 °C, 500 °C, 600 °C, and 700 °C for 1 h) were investigated systemically. The macro-morphology, microstructure, precipitated phases, and elements distribution of laser cladding layers were detected by optical microscopy (OM), scanning electron microscopy (SEM), X-ray diffraction (XRD), and energy dispersive spectroscopy (EDS), respectively. The mechanical and tribological properties of the cladding layers were tested using a microscopic Vickers hardness tester and friction and wear tester, respectively. The results show that the main phases of Ni60 coatings are composed of γ-(Ni, Fe), Cr₇C₃, Cr₂₃C₆, CrB, CrFeB, and Cr₂Ni₃. In particular, the micro-structure and mechanical properties reach the best levels after heat treatment at 600 °C. The micro-hardness, average friction coefficient, and wear volume of the cladding layer are 771.4 to 915.8 HV₁ and 0.434 and 2.9546 × 10⁻⁵ mm³, respectively. In conclusion, the micro-structure and mechanical properties of the cladding layer are greatly improved by the proper heat treatment temperature.

1. Introduction

The wear properties of 35CrMoV steel are poor under heavy load and wet conditions, as the service life and working stability of the components are dramatically reduced. Further, 35CrMoV steel is applied to a load-bearing connector of the coal shearer, due to its excellent hardenability, creep, and high-strength properties [1,2]. Due to the excellent corrosion resistance and wear properties of Ni60 (Ni-Cr-B-Si) alloys, they have been widely used in metallurgy, mining, and other fields for the repair and protection of fragile parts [3]. Therefore, laser cladding Ni60-based coating on the 35CrMoV substrate is an effective way to reinforce the surface performance of components [4]. Nevertheless, the cracks and stress concentration are susceptible to generation in the laser cladding process [5]. There is an increasingly prominent solution in enhancing the surface properties of the coating by post-treatment methods, such as aging, quenching, heat treatment, and tempering [6,7,8].

Heat treatment is an effective method to improve surface properties. Heat treatment can eliminate residual stress, further improving the mechanical and tribological properties of the coating [9,10,11,12]. Lu et al. [13] reported that Ni60/h-BN self-lubricating anti-wear composite coating on 304 stainless steels were heat-treated at 600 °C for 1 h. The results indicated that the maximum microhardness of the coatings was first increased from 667.7 HV₁ to 765.0 HV₁. The friction coefficients of the coating after heat treatment were decreased, obviously. Guo et al. [14] investigated the friction and wear properties of laser cladding iron-base alloy by heat treatment. The results showed the crystalline grains of the coating are smaller after heat treatment for austenite and lath martensite grains, which are helpful in improving the hardness. Sun et al. [15] studied the microstructure conversion and grain size development in various stages of heat treatment. Meanwhile, magnitude, shape, and distribution in the secondary phase during heat treatment were investigated to explore the function mechanism of micro-alloyed elements on the evolution of microstructure and grain during heat treatment. Homogeneous microstructure and composition are obtained after heat treatment. Chai et al. [16] studied the effect of heat treatment on microstructures and properties of a 304 stainless-steel coating on 27SiMn steel by laser cladding. The results showed that the heat treatment process can refine the grain size of cladding layers, which makes microstructures denser. The heat treatment process can improve the plasticity and toughness and it enhances the tensile strength and reduces the microhardness of materials. Gong et al. [17] studied the influence of heat treatment on the microstructure and mechanical properties of the FeCrNi coating produced by laser cladding. The results showed that the heat treatment, at a temperature ranging from 1073 to 1273 K, refined the grains in the substrate materials and effectively removed the soft zone of hardness between the fused zone and base material, mainly due to a secondary quench of heat treatment. When the temperature of heat treatment was 1073 K, the maximum ultimate tensile strength values of the laser cladding component were obtained.

However, reports on reinforced laser cladding Ni-based alloys via heat treatment are scarce [18,19]. In this study, the composite coatings on 35CrMoV steel were heat-treated at different temperature (25 °C, 500 °C, 600 °C, and 700 °C for 1 h). The wear and hardness tests were analyzed. Further, the effects of heat treatment on the macro-morphology, microstructure, and element segregation of the Ni60 cladding layer were systematically investigated systemically.

2. Materials and Experimental Procedures

2.1. Materials

First, 35CrMoV steel (KENNAMETAL Co., Shanghai, China) samples with dimensions of 100 mm × 60 mm × 20 mm were used for the substrate. In order to eliminate the oxide film on the surface of 35CrMoV steel, the specific process is as follows: the surface of 35CrMoV steel is polished with fine sandpaper and then cleaned with acetone. The raw powder material is the Ni60 (ZHUJIN Co., Jingdezhen, China) with a composition (wt.%) of 0.8 C, 0.4 Si, 15.5 Cr, 0.2 Mo, 15.0 Fe, 3.0 B, and balance Ni and the particle size is 100 μm. Excellent fluidity and uniform Ni60 powder is obtained by the powder by ball milling for 3 h with a ball milling speed of 200 r/min. Additionally, it is dried in a desiccator at 375 K for 2 h.

2.2. Laser-Cladding Process

The 35CrMoV matrix was firstly surface-treated (burning–cleaning–drying) and then the powder was preset on the substrate with a thickness of 1.5 mm through a self-made mold. The coatings were then obtained by BS-OF-3000-15-4L industrial laser equipment (WISCO.HGLASER, Wuhan, China).

Table 1. Laser-cladding parameters.

Parameters	Value
Power	3000 w
Spot sizes	15 mm × 3 mm
Scanning speed	120 mm/min
Overlap ratio	0.4
Gas flow rate of argon	15 L/min
Wavelength	1080 nm

shows the optimized laser processing parameters.

2.3. Heat Treatment

After laser-cladding treatment, the coatings were cut by electro-sparking machining. Heat treatment was carried out by Muffle furnace (BYT, Nanjing, China) with a heating rate of 10 °C /min. Laser-clad coatings were heat treated at 500 °C, 600 °C, and 700 °C for 1 h, respectively, followed by furnace cooling.

2.4. Coating Characterization

The samples were cut into 10 × 10 × 10 mm samples by wire cutting and the surfaces of the coatings were cleaned by a film grinder. The phase composition of the coatings was detected by X-ray diffraction (BRUKER-AXS-D8, Germany). The SEM sample was polished to a mirror surface with a grinder (SYNTEM, Shangdong, China) and a polishing machine (ORTLAL, Tianjin, China) and then the sample was corroded with aqua regia (hydrochloric acid and nitric acid 3:1) for 25 s, scrubbed with 99% alcohol, and then dried. The microstructure of the coating was observed using a scanning electron microscope (SEM, JEOL/JSM-5610LV, JEOL, Japan) with energy-dispersive spectrometry (EDS, JEOL, Japan) for analyzing the distribution of elements. The tribological behavior of the cladding was detected by a high-speed reciprocating wear and friction tester (MFT-R4000, QUANLITEST, Jinan, China), and a Si₃N₄ ball with a diameter of 6 mm was used for grinding materials. The tests were carried out with a wear time of 20 min, a load of 5 N, a reciprocating frequency of 2 Hz, and a wear length of 10 mm. Surface topography 3D characteristics and wear volume of the coatings were analyzed by a MicroXAM-800 non-contact optical profiler (ADE Corporation, Westwood, MA, USA). A Vickers microhardness Tester (HMAS-D1000SZ, WHW, Shanghai, China was used to test the hardness of the cladding layer; the loading force was 9.8 N and lasted for 10 s.

3. Results and Discussion

3.1. Phase Structure of Cladding Layer

Figure 1 shows the XRD pattern of the Ni60 coatings with different heat treatment temperatures. The diffraction peaks of γ-(Ni, Fe), Cr₇C₃, Cr₂₃C₆, CrB, CrFeB, and Cr₂Ni₃ were detected in all specimens. It is obvious that the diffraction peak intensity of Cr₇C₃, Cr₂₃C₆, and Cr₂Ni₃ of the four composite coatings after heat treatment at 500 °C for 1 h is higher than that of other heat-treatment temperatures (Figure 1A₂). This can be attributed to the fact that laser cladding is a rapid non-equilibrium solidification process [20,21]. During the laser-cladding process, parts of boron atoms do not have enough time to combine with carbon or nickel atoms or the solute in the unstable supersaturated solid solution (Ni,Fe) in the molten pool [22]. In the process of heat treatment, atomic diffusion is active and boron atoms are prone to react with carbon and nickel atoms to form Cr₇C₃, Cr₂₃C₆, and Cr₂Ni₃ phases. Thus, the intensity of the diffraction peaks of Cr₇C₃, Cr₂₃C₆, and Cr₂Ni₃ increases (Figure 1A₂) [13]. After heat treatment at 600 °C, the characteristics of the diffraction peaks changed. It can be clearly seen in Figure 1A₁ that the diffraction peak angle shifted 0.11 degrees to the left relative to the diffraction peak angle of coating without heat treatment, and the half width of the diffraction peak becomes larger. According to the Debye–Scherrer Formula (1), the microstructure is refined with heat treatment at 600 °C. This result is consistent with SEM analysis. The phenomenon of diffraction peak shift can be explained as that after solid solution treatment, the atoms of precipitated phase elements, such as C, Cr, and Fe, are dissolved back into the matrix and the solid solution atoms are not precipitated but squeezed into the crystal cell to produce distortion. After heat treatment at 700 °C, the intensity of the diffraction peak increases and shifts to the right due to element segregation. This illustrates that improper heat treatment cannot refine the microstructure and make elements uniform.

$$D=\frac{\mathrm{k}\lambda }{\beta \cos \theta }$$

(1)

In Equation (1), D is the grain size, K is the Scherrer constant, λ is the wavelength of the X-ray, β is the half-height width of the diffraction peak, and θ is the Bragg diffraction angle.

The results indicate that strengthening phases of the non-heat-treatment cladding layer are composed of Cr₇C₃, Cr₂₃C₆, CrB, and CrFeB. In addition, CrB, CrFeB, and other boride ceramic phases can simultaneously improve the hardness and the brittleness of the cladding layer. However, severe thermal stress occurs from excessive brittle phases in rapid heating and rapid non-equilibrium solidification processes, which leads to cracks in the laser-clad coating [23,24]. After heat treatment at 600 °C, diffraction peak intensity decreases, brittle phases number decreases, and the coating exhibits a refined grain as well as excellent properties.

3.2. Macro-Morphology of Cladding Layer

The macro-morphologies (OMs) of the coatings after different heat-treatment temperatures are shown in Figure 2. The microstructure morphology of a non-heat-treatment specimen is a relatively coarse columnar grain (Figure 2a). A large number of columnar crystals separate out when the sample is heat treated at 500 °C (Figure 2b). Especially after heat treatment at 600 °C, the eutectic structure is refined and distributed uniformly (Figure 2c). However, the eutectic structure has a solid solution phenomenon and gradually develops into rod-shaped crystals with heat treatment at 700 °C (Figure 2d). It is worth noting that the matrix after heat treatment at 600 °C exhibits obvious grain refinement and a refined crystal structure, which can improve the wear resistance of composite coatings.

3.3. Microstructure

Figure 3 shows SEM image analyses of a Ni60 cladding layer on a 35CrMoV steel substrate with different heat treatments. It can be seen that heat treatments have great influence on the microstructure of the coating. The phases of the coating without heat treatment are composed of a reticular structure and coarse eutectic with large grain size and sparse structure distribution (Figure 3a). With the temperature of the heat treatment increasing, the microstructure of the coatings changes obviously. The coarse dendritic eutectic appears with a “decomposition” phenomenon and the dendritic eutectic of a larger size was refined after heat treatment at 500 °C (Figure 3b). In particular, after heat treatment at 600 °C, the reticular grains distribute uniformly and the massive solid solution is further refined (Figure 3c). Moreover, the microstructure becomes relatively sparse and coarse and the microstructure of the solid solution transfers to being rodlike after heat treatment at 700 °C (Figure 3d).

The results obtained by energy spectrum analysis (ESP) of points (P) are given in

Table 2. EDS results of the coating.

Spectrum of P	C/wt.%	B/wt.%	Si/wt.%	Cr/wt.%	Fe/wt.%	Ni/wt.%
Spec. 1	2.32	8.56	3.09	8.99	38.14	47.77
Spec. 2	3.47	5.55	2.80	21.05	21.82	45.31
Spec. 3	5.74	1.16	3.74	9.02	25.78	56.26
Spec. 4	2.33	7.05	0.43	42.67	31.56	15.97

. It can be deduced that the dendrite region of P (dendritic tissue marked by red triangle) is mainly composed of Ni, Fe, Si, Cr, C, and B, which is regarded as γ, Cr₇C₃, Cr₂₃C₆, Cr₂Ni₃, CrB, and CrFeB solid solution. In the microstructure, borides are typically brittle phases, such as CrB and CrFeB. The high content of B in the massive structure is more brittle, resulting in poor cladding-layer properties [25]. Carbides exhibit excellent high hardness, which can improve cladding-layer strength [26]. It can be obtained from Table 2 that, after the 600 °C heat treatment, the content of B decreased by 86.4% compared with that of the coating without heat treatment, but the content of C increased by 51%, indicating that the brittle-phase CrB and CrFeB decreased greatly and the enhanced-phase Cr₇C₃ and Cr₂₃C increased, which was conducive to the suppression of cracks, thus, improving the properties of the coating. This demonstrates that heat treatment at 600 °C promotes the redistribution of the elements, which is beneficial for improving the wear resistance of the coatings.

3.4. Solute Segregation Analysis

Figure 4 shows the distribution of Ni, Fe, Si, and Cr elements in the composite coating after 1 h of heat treatment at different heat−treatment temperatures. The redistribution behavior of solute atoms during laser−cladding solidification leads to the uneven distribution of alloying elements, and the element segregation caused by the above behavior seriously affects the microstructure uniformity of the cladding layer and reduces the mechanical properties in the coating. Therefore, heat treatment is often used to reduce or eliminate micro−segregation and improve the thermal deformation stability of the cladding layer. During the rapid solidification, solute segregation appears between the crystal boundaries and the inner area of the dendrites without heat treatment (Figure 4a–d). Homogenization was carried out at a 500 °C heat−treatment temperature to analyze the diffusion behavior of Ni, Fe, Si, and Cr elements. Ni and Cr elements were mainly distributed between the dendrites, while Fe and Si elements still had different degrees of segregation on the dendrites, indicating that the element segregation could not be eliminated at 500 °C (Figure 4a₁–d₁). Homogenization was carried out at a 600 °C heat−treatment temperature, element enrichment degree disappears, and element distribution is the best (Figure 4a₂–d₂). After homogenization under heat treatment at 700 °C, the enrichment degree of Ni, Fe, Si, and Cr gradually increases and the distribution uniformity degree of alloying elements becomes worse (Figure 4a₃–d₃). Due to element segregation, there is a concentration gradient between the central part of the dendrite and the surrounding part of the dendrite, and the growth of the dendrite is affected by solute diffusion. This indicates that heat treatment at 700 °C cannot refine the grain and reduce segregation to improve the properties of the alloy but can aggravate the element segregation. It can be concluded from the above investigation that heat treatment promotes element redistribution in the coating. The element segregation caused by laser cladding is reduced.

3.5. Microhardness

In order to evaluate the effect of different heat-treatment temperatures on the microhardness of coating, microhardness distribution along the depth direction was measured, as shown in Figure 5. The profiles can be divided into three zones: coating, heat affect zone, and substrate. It can be clearly seen that microhardness was closely related to the heat treatment. Non-heat-treated coatings exhibit the smallest micro-hardness (633.6 to 805.22 HV₁) and the average hardness is 702.68 HV₁. After heat treatment at 500 °C, the average hardness of the coating increases to 826.22 HV₁ and the coating microhardness ranges from 669.6 HV₁ to 947 HV₁. The increase in heat-treatment temperature improves coating microhardness. Obviously, the surface microhardness at 600 °C is comparatively higher (771.4 to 915.8 HV₁), with an average hardness of 843.1 HV₁. Nevertheless, the microhardness of the coating ranges from 660.2 to 857.6 HV₁ and average hardness decreases to 760.77 HV₁ after heat treatment at 700 °C. The microhardness results (Figure 5) indicate that the average value of coating microhardness with 600 °C is the highest and more stable. The microhardness enhancement is more significant at 600 °C.

3.6. Tribological Properties

In order to intrinsically reflect the tribological performance of coatings, wear experiments were carried out by a wear-testing machine (MFT-R4000). The wear time, load, reciprocating stroke, and frequency of experimental parameters are 20 min, 30 N, 5 mm, and 15 Hz, respectively. The Si₃N₄ ball is used as the grinding ball. The 3D wear morphology of wear scratch was carried out by the MicroXAM-800 non-contact optical profiler (ADE Corporation, USA). Figure 6 shows the 3D (three-dimensional) wear morphologies of coatings with different heat-treatment temperatures. The wide groove scratches, wear chips, tribo-film, and shallow plow appeared on the surface of the non-heat-treated coating (Figure 6a and Figure 7a). The formation of the tribo-film is possibly due to the combination of oxidative and adhesive wear phenomena. The occurrence of the adhesive wear can be attributed to the counter-material of inert Si₃N₄ ball, which results in initial material transferred from the coating onto the ball. Therefore, the formation of debris and peel-off areas is caused by the breaking-off of the tribo-film. Moreover, due to the hardness of the counter-material Si₃N₄ ball and the presence of debris in the contact, abrasive lines occurred simultaneously. Thus, the results indicated that the coating with non-heat treatment exists in fatigue wear, plastic deformation, adhesive, oxidative, and abrasive wear. Figure 6b and Figure 7b show that the wear mechanism is mainly abrasive wear, adhesive wear, and plastic deformation after heat treatment at 500 °C, which can be attributed to the uneven distribution of reinforcement phases that results in the rough wear surface. It is worth noting that the wear surface of the coating is relatively flat and smooth after heat treatment at 600 °C, and the wear mechanism exhibits mainly plastic deformation and adhesive wear (Figure 6c and Figure 7c). However, when the temperature increases to 700 °C, the deep grooves on the worn surface of the coating are caused by improper heat treatment (Figure 6d and Figure 7d). Therefore, the wear mechanism is mainly adhesive wear and plastic deformation. The SEM images of wear scratch morphology intrinsically reflect the tribological performance of coatings. The results justified the conclusion of the wear mechanism in the above investigations: Ni60 cladding exhibits the best wear resistance at 600 °C.

The wear volume and curves of width and depth of layers are shown in Figure 8. The wear volume reflects the wear resistance of materials. Detailed results are shown in

Table 3. The wear volume, width, and depth of coatings.

Sample	Wear Volume/10−5 mm3	Wear Width/μm	Wear Depth/μm
25 °C	4.0651 mm3	385 μm	7.5 μm
500 °C	3.7532 mm3	330 μm	6.5 μm
600 °C	2.9546 mm3	290 μm	5 μm
700 °C	3.3572 mm3	360 μm	6 μm

. It can be clearly observed that, with the heat treatment increasing to 500 °C, the wear volume of the coating decreases to 3.7532 × 10⁻⁵ mm³, the wear width is about 330 μm, and the wear depth is about 6.5 μm. In particular, after heat treatment at 600 °C, the wear volume of the coating is 2.9546 × 10⁻⁵ mm³, the wear width is about 290 μm, and the wear depth is about 5 μm. However, the wear of the coating increases to 3.3572 × 10⁻⁵ mm³ and the wear width and wear depth rise simultaneously after heat treatment at 700 °C. It can be concluded that the coating after the 600 °C heat treatment obtained the best wear resistance.

The temperatures of heat treatment are responsible for the friction coefficient. To sum up, under the same wear conditions, the wear properties in the untreated coating are poor. The proper heat-treatment temperature promotes reinforced phase production and the redistributed phases not only reduce the stress concentration, but also enhance the bonding with the matrix interface [27,28]. During the wear processing, the coating after 600 °C heat treatment’s wear mechanism is plastic deformation. Owing to the heat accumulated during continuous sliding friction, forming a hardened or oxide layer on the worn surface of the coating can be prevented by the higher combined force between the phase and the matrix, thus, exhibiting excellent wear resistance.

The curves of the friction coefficient with different heat-treatment temperatures are presented in Figure 9. The friction pair combination has a great influence on the friction and wear performance, which is directly related to whether the coating performance can meet the requirements of the working conditions on the friction coefficient. The Si₃N₄ was used to conduct friction and wear experiments on the grinding ball to explore the wear resistance of Ni60 coating. Comparing the friction performance of coatings at different heat-treatment temperatures, the results show that average friction coefficients of the coatings are 0.601, 0.561, 0.434, and 0.458 with different heat-treatment temperatures of 25 °C, 500 °C, 600 °C, and 700 °C, respectively. When the heat-treatment temperature increases to 600 °C, the wear mass loss obviously decreases. Owing to the reinforced phases of the coatings without heat treatment segregating, the curve swings largely throughout the whole stage of the friction experiments. The brittle phases distribute uniformly in the coating and the friction coefficient curves are prone to being stable with the increasing heat-treatment temperature. The uniform reinforced phases improve the metallurgical bonding strength in the matrix phases; thus, the heat treatment plays an important role in grain refinement and element redistribution, resulting in a reduction in the friction coefficient.

4. Conclusions

The Ni60 self-lubricated anti-wear composite coatings were successfully precipitated on a 35CrMoV substrate by laser-cladding technology. The effects of heat treatment on the macro-morphology, microstructure, precipitated phase, microhardness, and wear properties of the Ni60 coatings with different heat-treatment temperatures (25 °C, 500 °C, 600 °C, and 700 °C for 1 h, respectively) were investigated. The following conclusions can be drawn:

The micro-structure of the Ni60 composite coating is mainly composed of refined crystals at 600 °C heat treatment. The coating exhibits refined grain, a homogeneous microstructure, and uniform element distribution of Si, Cr, Fe, and Ni. The dendritic areas of P mainly consist of a γ + Cr₇C₃ + Cr₂₃C₆ + Cr₂Ni₃ + CrFeB solid solution.

The phases of enriched Cr of the coating gradually decompose with the increase in heat-treatment temperatures from 500 °C to 700 °C, resulting in the intensity of peak Ni60 coatings decreasing dramatically. The diffraction peak shifts to the left and the half-height width of the diffraction peak is the largest after heat treatment at 600 °C.

The mechanical properties of the coating are significantly improved by the 600 °C heat treatment. The micro-hardness of the cladding layer becomes more stable, ranging from 771.4 to 915.8 HV₁, the average friction coefficient is 0.434, with a small range of fluctuation, and the wear volume is reduced to 2.9546 × 10⁻⁵ mm³.