Effects of Thermal Treatment on Microstructure and Wear Properties of Ni60/CeO₂ Composite Coating 35CrMoV Steel by Laser Cladding

Abstract

Laser cladding Ni60/2.0 wt.%CeO₂ self-lubricating anti-wear composite coatings on 35CrMoV steel were thermally treated at 25 °C, 500 °C, 600 °C and 700 °C for 1 h, respectively. The macroscopic morphology of composite coatings was characterized by optical microscopy. The evolution of microstructure was studied by scanning electron microscopy. The precipitated phase of the coating was analyzed by X-ray diffractometer. The microhardness and wear properties of the composite coatings were measured by microhardness tester and friction and wear test. The effects of thermal treatment and no thermal treatment on the mechanical properties of the composite coatings were systematically studied by the above experiments. The experimental analysis shows that the main phase composition of Ni60/2.0 wt.%CeO₂ composite coating is γ-(Ni, Fe), Cr₇C₃, Cr₂₃C₆, CrB, CrFeB and Cr₂Ni₃. Meanwhile, in XRD, it can be seen that the diffraction peak has shifted to the left and the single diffraction peak is decomposed into double diffraction peaks with the thermal treatment at 700 °C, which indicates that the lattice in the coating phase has a certain distortion. Compared with the coatings without thermal treatment, Ni60 cladding layer has more uniform solute distribution and a denser and more uniform structure after thermal treatment at 700 °C. The laser cladding anti-wear composite coating demonstrates that the wear resistance performance shows the greatest improvement after thermal treatment at 700 °C.

1. Introduction

Because of its good hardenability, creep properties and high strength, 35CrMoV steel is mainly used to manufacture various small and medium-sized parts [1,2]. However, its poor tribological performance limits its application as an important tribological moving component [3,4].

To solve these problems, laser cladding Ni60 composite coating can significantly improve the wear resistance of the substrate material surface to meet the performance requirements of the material surface [5,6]. Nevertheless, high stress and cracks are generated in the cladding layer due to the rapid cooling and heating of laser cladding [7,8]. Due to the characteristics of rare earth grain refinement to inhibit cracking, it has been successfully applied in the field of laser cladding [9,10]. The previous study showed that the microstructure was not completely refined, and a small number of pores and cracks were found when the content of CeO₂ in the Ni60 cladding layer was 2.0% [11]. Thermal treatment can eliminate residual stress and improve hardness, wear resistance and strength as a classical method [12,13]. Lu et al. [14] studied Ni60/H-BN and a self-lubricating wear-resistant coating was prepared on 304 stainless-steel by laser cladding technology. The samples were subjected to thermal treatment at 600 °C for 60 min. The experimental results show that the maximum microhardness of the coating increases from 667.7 HV to 765.0 HV. The friction coefficient of the coating decreased significantly after thermal treatment. In the study by Guo et al. [15], the friction and wear properties of the iron-based alloy cladding layer after thermal treatment were investigated. The experiment showed that the grain size of the thermal treatment scheme is small, and the austenite and lath martensite grains are obvious, which is beneficial to improve the hardness. Sun et al. [16] studied the microstructure transformation and grain size change in different thermal treatment stages. In the meantime, the size, shape and distribution of secondary phases during thermal treatment were studied, and the mechanism of microalloying elements on the microstructure and grain evolution during thermal treatment was discussed. The structure and composition were found to be uniform after thermal treatment.

However, the synergistic effect of CeO₂ and thermal treatment on the microstructure and mechanical properties of the Ni-based laser cladding layer of 35CrMoV steel is rarely discussed. Thus, the effects of the thermal treatment process at different temperatures and CeO₂ on the macro morphology, microstructure, content of precipitated phase, element segregation and wear properties of Ni-based laser cladding layer of 35CrMoV steel were investigated systemically in the present study.

2. Materials and Experimental Procedures

2.1. Materials

The 35CrMoV structural alloy steel (size: 100 mm × 60 mm × 20 mm) was used as the substrate. The chemical composition (wt.%) of the 35CrMoV substrate was C 0.35, Si 0.27, Mn 0.55, Cr 0.95, Mo 0.2, and V 0.15. The preplaced powder was a combination of Ni60 (average particle size: around 100 μm) and CeO₂ (average particle size: around 0.62 μm, ≥99.99% purity). The chemical compositions of Ni60 alloy powders were C 0.8, Si 0.4, Cr 15.5, Mo 0.2, Fe 15.0, B 3.0 and Ni excess. Ni60/2.0%CeO₂ (wt.%) powders were mechanically mixed for about 2 h with a ball milling speed of 200 r/min. The mixed powder was placed in the oven at 100 °C to dry for 2 h.

2.2. Laser-Cladding Process

The substrate surfaces were grinded on the oxide layer with abrasive paper and cleaned with alcohol before the experiment of laser cladding. The mixed powder was placed by the preset powder feeding method, and the powder was spread on the surface of the substrate with a thickness of 1.5 mm. The model of the cladding experiment laser (Laser-300, Xi’an Beisheng Laser Technology Co, Xi’an, China) is BS-OF-3000-15-4L. The laser optimized process parameters are listed in

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
The wavelength	1080 nm

.

2.3. Thermal Treatment

The samples were cut by the wire cutting method (size: 15 mm × 10 mm × 10 mm) and divided into three groups after laser cladding coating treatment. Thermal treatment at 500 °C, 600 °C and 700 °C for 1 h was carried out in a resistance furnace (Yahua Co Company, Wuhan, China) with a heating rate of 10 °C/min. After reaching the corresponding experimental temperature and keeping it for 60 min, the sample was placed in the furnace to cool.

2.4. Coating Characterization

The surface microstructure, composition and phase composition of Ni60/CeO₂ composite coating on 35CrMoV steel substrate at different temperatures were investigated by scanning electron microscopy (SEM, JEOL/JSM-5610LV, Ishizuka Electronics Corporation Mitaka, Japan), energy dispersive spectrometer (EDS, Ishizuka Electronics Corporation, Mitaka, Japan) and X-ray diffraction (XRD, BRUKER-AXS-D8, BRUKER AXS GMBH, Berlin, Germany). Wear tests were performed using a high-speed reciprocating tester (MFT-R4000, Zhongke Kaihua Technology Co, Lanzhou, China) with the reciprocating frequency of 2 Hz, a normal load of 5 N, the wear time of 20 mins and the wear scar length of 10 mm. The microhardness tester for testing the hardness of the coating is (HMAS-D1000SZ, Hardness Precision Instruments Co, Shanghai, China), and the loading force is 9.8 N for 10 s.

3. Results and Discussion

3.1. Macroscopic Morphology of Cladding Layer

The macroscopic morphology of the cladding layer of non-thermal treated and thermal treated samples is shown in Figure 1 The microstructure under non-thermal treatment experimental conditions is composed of relatively sparse block crystals (Figure 1a). A large number of rough and nonuniform columnar grains (Figure 1b) were shown when the sample was subjected to thermal treatment at 500 °C. The morphology of grain evolved from columnar crystals into equiaxed crystals after thermal treatment at 600 °C (Figure 1c). The microstructure was expressed in the form of small uniform and ordered equiaxed crystals after thermal treatment at 700 °C (Figure 1d).

To sum up, thermal treatment has a great influence on the microstructure of Ni60/2.0 %CeO₂ composite coating. It is noteworthy that the matrix grains after thermal treatment at 700 °C became finer, which can effectively enhance the wear resistance of the composite coating.

3.2. Microstructures Morphology of Cladding Layer

Microstructure changes in the coating at different thermal treatment temperatures are shown in Figure 2. The figure shows that the dendritic eutectic (indicated by P1) of the coating under the condition of overheating treatment has coarse and sparsely distributed tissue (Figure 2a), while the eutectic structure of coatings after thermal treatment is compact and uniform (Figure 2b–d). Based on EDS analysis (

Table 2. EDS results of the coating.

		Content/wt.%
		C	B	Si	Cr	Fe	Ni	Ce	O
25 °C	Spec.1	9.73	2.85	1.34	24.35	33.86	27.67	0.16	0.1
	Spec.2	2.32	—	3.09	8.99	38.14	47.77	0.14	0.09.
500 °C	Spec.3	15.6	1.56	0.77	28.15	27.57	27.91	0.12	0.06
	Spec.4	6.58	—	1.63	18.82	41.59	37.65	0.1	0.05
600 °C	Spec.5	2.87	1.25	2.38	11.86	39.65	41.79	0.12	0.1
	Spec.6	2.07	—	3.12	9.08	40.66	47.23	0.17	0.04
700 °C	Spec.7	1.02	0.16	2.29	11.37	40.66	44.82	0.21	0.16
	Spec.8	3.67	—	2.86	7.04	39.58	51.69	0.27	0.12

), it can be inferred that Ni, Fe, Cr, C and B are important elements in the dendritic zone of P1, which is identified as γ + Cr₇C₃ + Cr₂₃C₆ + CrFeB + Cr₂Ni₃ solid solution. Ni, Fe and Cr are the main elements in the P2 zone, which constitute the γ + Cr₂N₃ solid solution. EDS analysis showed that the content of B in tissue area of P1 decreased gradually with the increase in the thermal treatment temperatures, perhaps caused by the segregation of the B component at low thermal treatment temperature.

Figure 3 presents SEM images of a Ni60/2.0% CeO₂ composite coating after binary treatment at different thermal treatment temperatures. In the meantime, the volume fraction of the P2 eutectic structure and γ phase was calculated by Image Pro-Plus Software (

Table 3. The volume fraction of eutectic structure at different temperatures.

	25 °C	500 °C	600 °C	700 °C
γ + Cr2Ni3	0.57	0.55	0.46	0.36

). The processed image can accurately reflect the distribution of the eutectic structure and γ phase. The volume fraction of γ + Cr₂Ni₃ are 0.57, 0.55, 0.46 and 0.36% at the thermal treatment condition of 25 °C, 500 °C, 600 °C, 700 °C, respectively. Based on the above analysis, it can be found that the dendrites are refined after thermal treatment at 700 °C, the dendrite agglomeration disappears, and the coating structure is more uniform. From this, we can draw the conclusion that thermal treatment and CeO₂ play an important role in grain refinement. In the meantime, thermal treatment for solid solution in the organization of CeO₂ provides energy to further optimize the structure. Therefore, a coating with fine grain and excellent performance can be obtained.

3.3. Solute Segregation Analysis

Figure 4 shows the element distribution of composite coatings under different thermal treatment temperatures. The scanning map shows that Si, Cr, Fe and Ni segregate at the interdendritic regions. The solute segregation occurs in the grain boundary and dendrite internal region without thermal treatment during rapid solidification (Figure 4a–d). Element enrichment decreases gradually and alloying elements distribution tends to be uniform after thermal treatment temperature at 500–700 °C (Figure 4a–d, a₂–d₂). Uniformity of element distribution is best when the thermal treatment temperature is 700 °C.

Based on these analyses, element distribution in coating microstructure is highly affected by the element segregation behavior during the process of thermal treatment and CeO₂. This is because CeO₂ can promote the high fluidity and uniform distribution of alloying elements in the molten pool and inhibit the occurrence of segregation. [17,18,19]. In the meantime, thermal treatment provides energy for the strengthening effect of CeO₂ on the microstructure, and further acts on the element distribution, so that the elements are evenly distributed in the coating. Appropriate thermal treatment temperature and amount of CeO₂ addition can reduce residual segregation.

Figure 4. Element distribution of the composite coatings with different thermal treatment temperatures: (a–d) 25 °C-2%CeO₂; (a₁–d₁) 500 °C-2%CeO₂; (a₂–d₂) 600 °C-2%CeO₂; (a₃–d₃) 700 °C-2%CeO₂.

Figure 4. Element distribution of the composite coatings with different thermal treatment temperatures: (a–d) 25 °C-2%CeO₂; (a₁–d₁) 500 °C-2%CeO₂; (a₂–d₂) 600 °C-2%CeO₂; (a₃–d₃) 700 °C-2%CeO₂.

3.4. Phase Structure of Cladding Layer

The XRD patterns of Ni60/2.0% CeO₂ composite coatings at different thermal treatment temperatures are shown in (Figure 5a). It is not difficult to see that γ- (Ni, Fe), Cr₇C₃, Cr₂₃C₆, CrB, CrFeB and Cr₂Ni₃ are the main phases of the four composite coatings. The results showed that the peak intensity of Ni60/CeO₂ coating decreases obviously with the increase in temperature. It is considered that the internal Cr phase of coating decomposes with the increase in thermal treatment temperatures from 500 °C to 700 °C. The diffraction peak shifts to the left, and the full width at half maximum of the diffraction peak is the smallest under the experimental conditions of 700 °C thermal treatment (Figure 5b). According to the Scherrer formula, the micro-structure consists mostly of dense and uniform grain.

In addition, melting of coarse grains and the diffraction peaks are sharpened with the increase in temperature. The double peaks with poor separation become clear, and the peak sharpening leads to the change in microstructure, which has a certain influence on the performance [20].

3.5. Microhardness

Hardness is often considered to be one of the important indicators of material wear resistance [21,22]. Therefore, the hardness changes of coatings at different heat treatment temperatures were analyzed in detail in this paper, as shown in Figure 6. The whole microhardness distribution region consists of a coating region, heat affected region and substrate region. The experimental results show that the microhardness of the cladding layer without thermal treatment is 527.1~780.8 HV₁. The microhardness of the coating ranges from 558.4 HV₁ to 647.9 HV₁, 502 to 603.4 HV₁, 568.3 to 604.9 HV₁ after thermal treatment at 500 °C, 600 °C and 700 °C, respectively. The average microhardness of the coating is 662.74 HV₁, 590.03 HV₁, 561.64 HV₁, 591.23 HV₁ after thermal treatment at 25 °C, 500 °C, 600 °C and 700 °C, respectively. It can be clearly seen that the fluctuation range of microhardness value of the coating is the smallest after thermal treatment at 700 °C. Combined with the above microstructure and phase analysis, the crystal grains are more uniform, CrB, CrFeB and other hard phases are reduced after heat treatment. These hard phases result in an improvement in the toughness of the coating, which provides better wear resistance and reduces the wear of the material.

3.6. Friction and Wear Properties

Figure 7 shows the three-dimensional morphology of the Ni60 cladding layer at different thermal treatment temperatures when the addition amount of CeO₂ is 2.0%. It can be observed from Figure 7a that there are grooves, tissue tears and debris accumulation on the worn surface of the coating without thermal treatment. Under these conditions, the main wear mechanisms are abrasive wear and adhesive wear. It can be observed from Figure 7b that wear marks, wear chips and grooves with different depths appeared on the wear surface of the coating after thermal treatment at 500 °C. Under these conditions, the main wear mechanisms are abrasive wear and delamination. It can be seen from Figure 7c that there are a few tissues spalling and debris accumulation on the worn surface after thermal treatment at 600 °C, which indicates that the wear degree of the coating is gradually decreased. Under these conditions, the main wear mechanism is abrasive wear. It can be observed from Figure 7d that the wear surface has only shallow wear marks after thermal treatment at 700 °C, and the wear surface is relatively smooth and not serious. In this case, plastic deformation is the main wear mechanism. According to the above analysis, when the content of CeO₂ in Ni60 cladding is 2.0%, the best wear resistance can be obtained at 700 °C.

In conclusion, the coating with CeO₂ has higher interfacial bonding strength and the best wear resistance under the same wear condition. In the meantime, the heat treatment of the cladding layer not only reduces the wear degree of the coating, but also precipitates the hard phase, so that the hardness of the coating is greatly improved. The thermal treatment promoted the refinement and nucleation of CeO₂, which further refined the microstructure [23]. However, improper thermal treatment does not enhance the nucleation process of CeO₂. The main reason is that relatively low temperature cannot provide enough energy for CeO₂ nucleation, and if the temperature is higher, it will destroy the nucleation mechanism. Proper thermal treatment provides sufficient energy for CeO₂ nucleation.

Figure 8 is the wear degree of the Ni60 cladding layer under different thermal treatment temperatures with 2.0% CeO₂ addition. The experimental analysis showed that the wear quantity, wear width and wear depth of non-thermal treated coatings are 38,312 μm³, 380 μm and 7 μm, respectively. The wear of the coating is 26,472 μm³, the wear scar width is about 280 μm, and the abrasion depth is about 6.5 μm at the thermal treatment of 500 °C. The wear of the coating is 24,596 μm³, the wear scar width is about 330 μm, and the abrasion depth is about 6 μm at the thermal treatment of 600 °C. The wear of the coating is 15,736 μm³, the wear scar width is about 290 μm, and the abrasion depth is about 5 μm at the thermal treatment of 700 °C. According to the above experimental conclusion, under the heat treatment condition of 700 °C, the coating wear degree is the least and the wear resistance is the best.

To obtain the best friction performance at different thermal treatment temperatures, the friction coefficient curves at different thermal treatment temperatures are presented in (Figure 9a). The experimental analysis shows that the average friction coefficient of the coating is 0.544, 0.526, 0.450 and 0.211, respectively, at the thermal treatment temperature of 25 °C, 500 °C, 600 °C and 700 °C. It can be expressly observed that the friction coefficient is minimum at the thermal treatment of 700 °C (Figure 9b).

The friction coefficient relates to the microstructure of the material [24,25]. The microstructure of the coating with non-thermal treatment is unevenly distributed and the friction coefficient curve fluctuates greatly. The hard phase distribution in the coating becomes uniform and when the thermal treatment temperature gradually increases the friction coefficient curve, the fluctuation amplitude becomes smaller. In addition, the rare earth Ce element enhances the interfacial strength, which is conducive to improve the abrasive resistance of the alloy [26]. In conclusion, under the influence of heat treatment at 700 °C, the friction coefficient is optimized, and the wear resistance is improved.

4. Conclusions

The effects of the macro morphology, microstructure, precipitated phase, microhardness, and wear properties of the Ni60/2.0%CeO₂ composite coatings were thermal treatment at 25 °C, 500 °C, 600 °C and 700 °C for 1 h, respectively. It can be summarized as follows:

The microstructure of the Ni60/2.0% CeO₂ composite coating after thermal treatment at 700 °C is mainly composed of equiaxed grains. In the meantime, coating grain refinement, uniform structure, Si, Cr, Fe, Ni elements are evenly distributed. The dendritic region of P1 is mainly composed of γ + Cr₇C₃ + Cr₂₃C₆ + CrFeB + Cr₂Ni₃ solid solution. The tissues areas of P2 mainly consist of γ + Cr₂N₃ solid solution.

The Cr enriched phase in the coating is gradually decomposed, resulting in a sharp decline in the peak strength of the Ni60/CeO₂ coating, accompanied by an increase in the thermal treatment temperature from 500 °C to 700 °C. After thermal treatment at 700 °C, the diffraction peak tends to shift to the left, and the half-height width of the diffraction peak reaches the maximum. In addition, the diffraction peaks are sharp, and the single diffraction peak is decomposed into double diffraction peaks with the thermal treatment at 700 °C.

The range of micro-hardness of the coating reached the smallest with thermal treatment at 700 °C and the hardness of the cladding layer was uniform from 568.3 to 604.9 HV₁.

The wear volume is 38,312 μm³, 26,472 μm³, 24,596 μm³ and 15,736 μm³; moreover the average friction coefficient is 0.544, 0.526, 0.450 and 0.211 of Ni60/CeO₂ coatings, with thermal treatment at 25 °C, 500 °C, 600 °C and 700 °C for 1 h, respectively. It can be concluded from the friction coefficient and mass loss experiments that the wear resistance of the coating after 700 °C thermal treatment is fully improved.