In Situ Preparation of Nano-Cu/Microalloyed Gradient Coating with Improved Antifriction Properties

Abstract

In order to solve the failure problem between the crankshaft and the connecting rod friction pair, nano-Cu/microalloyed gradient coating was prepared on the surface of the crankshaft by in situ gaseous nitriding process. Electron probe analysis confirmed the change of delamination after the in situ nitriding process, and the formed coating included three layers: the upper layer is a nano-copper layer, the middle is a microalloyed layer (containing Cu, N, and Fe elements), and the bottom is a nitriding diffusion layer. The change of profile hardness curve was analyzed by microhardness test. The bonding force between the coating and the substrate was analyzed by the scratch test. The friction and wear test showed that the nano-Cu/microalloyed coating could achieve the effect of antifriction. Moreover, self-lubricating and antifriction mechanisms of nano-copper coating were proposed. These results indicated that the in situ gaseous nitriding process could provide a new surface modification technique for the precision friction pairs.

1. Introduction

As the “heart” of the refrigerator, the compressor plays an important role in product quality, performance, and energy consumption level [1]. As the core transmission component of the compressor, the crankshaft connecting rod has a great impact on the performance of the compressor. In the working process, the eccentric shaft part of the crankshaft drives the connecting rod to make circular eccentric movement, which is complex in force and easy to cause wear on the surface of parts and lead to failure, as shown in Figure 1. At the same time, the repeated pressure load in the compressor may also produce excessive adhesive wear on the crankshaft surface. Relevant studies have shown that oil lubrication problems and design defects of lubrication grooves on the surface of crankshaft are important reasons for crankshaft wear [2,3]. There are studies that improve the lubrication system by changing the geometry of the helical channel to maximize the oil mass flow at the crankshaft outlet [4]. Xing et al. used fullerene C60 nano oil for lubrication, the cop of the test compressor increases by 5.45% on average [5].

Copper and copper alloy materials have excellent lubricating property, which is often added in friction materials [6,7]. Generally, the tribological properties of nano-metals are better than those of corresponding coarse-grained metals [8]. Therefore, as a self-lubricating material [9,10], nano-Cu can improve the wear resistance of the material, and has anti-friction effect [11]. Y et al. [12] studied the abrasive wear behavior of SiCp/Al composites (MMCs) prepared by liquid metallurgy to understand the effect of applied load and weight fraction on the pin disk structure. The results show that the wear rate of the composites decreases slightly with the increase of SiCp content, and increases with the increase of load. Liu et al. [13] prepared Ni-Cu-P coating on the surface of ZK61M magnesium alloy by electroless plating. With the increase of copper content, the Ni-Cu-P coating became denser and the crystallinity gradually increased. Copper has a higher point potential than nickel, which can accelerate the selective dissolution of nickel, lead to the enrichment of phosphorus and copper on the coating surface, contribute to the formation of passive film, and improve the corrosion resistance. N.W. et al. [14] proposed a new type of cellulose nanocrystalline (CNC) nanoparticles as a green additive to improve the tribological properties of lubricants. The mixing of CNC nanoparticles in engine oil significantly reduces the friction and wear rate, thus improving the lubricating performance of engine oil.

In order to reduce the wear of crankshaft connecting rod friction pair, nano copper gradient-modified layer was prepared on the surface of crankshaft by in situ nitriding method to improve its lubrication performance, so as to provide guidance for improving the reliability of the compressor.

2. Experimental Procedures

2.1. Materials

CuO nanoparticles were synthesized by complex precipitation method [14] with copper sulfate as raw material and sodium hydroxide as reducing agent. Then the CuO nanoparticles were filtered by a filter device (SHZ-D(Ⅲ), Bangxi Instrument Technology Co., Ltd., Shanghai, China) and put into a vacuum thermostat (TZF-6030U, Shanghai Zhetu Scientific Instrument Co., LTD, Shanghai, China) for drying treatment at 50 °C. 42CrMo steel is used as the base material, and the chemical composition is shown in

Table 1. The chemical composition of 42CrMo Steel (wt. %).

Materials	C	Si	Mn	S	P	Mg	Cr	Mo	Fe
42CrMo	0.44	0.30	0.72	0.03	0.03	-	1.12	0.22	Balance

.

2.2. In Situ Preparation of Nano-Cu/Microalloyed Gradient Coating

In order to explore the effect of substrate surface roughness on coating performance, the samples were divided into three groups: 400 grit SiC abrasive paper rough grinding surface (1#), 2000 grit SiC abrasive paper fine grinding surface (2#), and polished surface (3#). After pretreatment, the samples were treated by ultrasonic cleaning for 15 min. Then the dried CuO nanoparticles were mixed evenly with the binder (Mixed reagent of 90% terpineol and 10% ethyl cellulose) and coated on the surface of the sample by manual brushing. Finally, the samples were nitrided in Tubular nitriding furnace (The related nitriding parameters are shown in

Table 2. The parameters of in situ gaseous nitriding process (Holding time 3 h).

Code	Gas Flow/mL	Temperature/°C	Furnace Pressure Difference/Pa	Surface Finish	Decomposition Rate
0#	55–60	530	550	control	30–35%
1#				400 grit	30–35%
2#				2000 grit	30–35%
3#				Polished	30–35%

). Meanwhile, the samples without coating were nitrided under the same conditions for comparison (0#). In the nitriding process, ammonia gas was used as the gas medium, and the positive pressure of 550 Pa was kept in the furnace by U-tube manometer. The ammonia decomposition rate was measured with an ammonia decomposition meter. The test process is shown in Figure 2.

2.3. Characterization of Nano-Cu/Microalloyed Gradient Coating

The surface topography and cross-section microstructure of the coating on 42CrMo Steel were examined using scanning electron microscopy (Nova Nano SEM450, FEI company, Hillsboro, OR, USA). Surface analysis and linear scanning were carried out using EPMA (JXA-8230, Nippon Electronics Co (JEOL), Shojima City, Tokyo, Japan) at an acceleration voltage of 15 kV and a current intensity of 10 mA. The phase composition of the sample surface after nitriding was determined by X-ray diffraction (Rigaku D/max 2200 X-ray diffractometer, Nippon Electronics Co (JEOL), Shojima City, Tokyo, Japan) performed using Cu-Kα radiation (λ = 10.54 nm).

The microhardness of the coating was obtained using a Vickers microhardness tester (TIME6610 M, FUTURE-TECH company, Beijing Times Peak Technology Co., LTD, Beijing, China) with a load of 50 g. Scratch tests of the coating were evaluated using a CETR/UMT-3 tribometer; the mating ball was GCr15 steel (Elastic modulus E is 206 GPa, Poisson’s ratio ν₁ is 0.3) with a radius of 6.35 mm and hardness of 53 HRC. A standard diamond rockwell indenter with a 120° angle and a tip radius of 200 μm was used. Scratch tests were carried out by linear loading from 0 to 200 N at a loading rate of 0.31 N/s.

The critical load of the contact specimen is calculated according to the Hertzian contact theory, at room temperature, the specimens were placed on a multi-functional friction and wear testing machine (CETR-UMT-3MO, Brock (Beijing) Technology Co., LTD, Beijing, China). The friction pair was GCr15 steel and the load of 2 N was applied to carry out the friction and wear test. The wear speed was 10 mm/s and the length of wear mark was 5 mm. Through the reciprocating linear dry sliding friction and wear experiment, the change curve of friction coefficient with time was obtained. In addition, the wear volume of the sample was measured by three-dimensional topography instrument to explore the wear rate.

In order to better simulate the actual working environment, the wear resistance of the coating under oil lubrication was tested by four-ball friction tester (MMW-1, Shandong Zhongyi Instrument Co. LTD, Jinan, Shandong, China). The test is divided into three groups. The friction pair consists of four equal diameter GCr15 steel balls (radius r = 6.35 mm), the upper sample is untreated steel ball, and the lower sample is the untreated steel ball, the nitrided steel ball, and the in situ nitrided steel ball. An hour-long friction and wear test were carried out under the condition of 10 N load and 1200 rpm acceleration test. The change curve of friction coefficient with time was obtained, and the wear spot diameter was measured after accelerated test to explore the wear amount.

3. Results and Discussion

3.1. Nano CuO Nanoparticles

The SEM characterization results of nano CuO are shown in Figure 3a,b. The micro morphology of nano CuO is flake, and these nanoparticles are stacked together. Figure 3c shows the X-ray diffraction pattern of CuO powder, corresponding to (PDF-#895898) standard card, the diffraction planes corresponding to the angle of CuO were marked in the figure, respectively. CuO nanoparticles are monoclinic, and only CuO phase, indicating the purity of CuO nanoparticles.

3.2. Nano-Cu/Microalloyed Gradient Coating

Before the gas nitriding, CuO nanoparticles were coated on the sample surfaces. In the process of gas nitriding, ammonia molecules were decomposed into hydrogen and active nitrogen atoms. Under the nitriding atmosphere, the reaction temperature of CuO with hydrogen is around 258.9 °C [15]. Therefore, the reaction of CuO nanoparticles with hydrogen was reduced to form the nano-Cu layer. The reaction mechanism is shown in Figure 4.

After the in situ gaseous nitriding process, the surface morphology of the as-prepared samples on 42CrMo surface are shown in Figure 5. Different types of sandpaper are used for pre-surface treatment to improve the bonding force between copper layer and surface. The surface roughness of the treated copper layer after factory brushing treatment is 0.20 µm, which meets the installation precision requirements of the crankshaft. On the upper surface of 1# and 2# specimens, a small amount of agglomeration was shown, and the surface of 3# sample is loose. From the chemical composition in

Table 3. Chemical compositions of nano-Cu/microalloyed gradient coating on 42CrMo surface after in situ nitriding process (wt. %).

Elements	C	N	O	Cr	Fe	Cu
1#	3.14	1.65	2.8	0.3	21.95	70.17
2#	2.74	3.39	3.5	0.24	31.76	58.37
3#	1.72	1.95	2.67	0.2	21.2	72.26

, compared with the rest of the elements, higher amount of copper was detected on the surface.

Figure 5d shows the X-ray diffraction patterns of four samples after nitriding. The characteristic peaks of metallic copper were well resolved, indicating their good crystallinity. Due to the nitriding effect, γ’-Fe₄N phase was detected on the 0# sample, corresponding to (PDF #06-0627) standard card. Compared with the X-ray diffraction pattern without copper layer (0#), there are obvious copper diffraction peaks in 1#, 2#, and 3# samples, which are consistent with the EDS results, and there are low intensity ε-Fe_2-3N diffraction peaks, corresponding to (PDF #50-0985, PDF #49-1663) standard card, indicating that a new phase is formed after nitriding. No oxide peak was found in the four samples, indicating that no obvious oxidation occurred during nitriding. According to the Jade software, the average grain sizes of nano-Cu on the surface of 1#, 2#, and 3# samples were 46 nm, 67 nm, and 64 nm, respectively. The refinement of grain size can contribute to the increase of wear resistance [16].

Figure 6 presents the cross-section microstructures of the nano-Cu/microalloyed gradient coating and the corresponding linear scanning profiles. From the cross-section, it can be seen that the copper layer presents a loose structure consistent with the surface morphology. Indicated by the linear scanning profiles in Figure 6b, the nano-Cu/microalloyed gradient coating contained three layers: the upper nano-Cu layer made up of nano-Cu particles with thickness of 10 μm, the middle part is composed of Cu, N, Fe microalloyed layer, and the bottom nitriding diffusion layer. It was found that the copper element diffused into the compound layer, indicating that there was interface diffusion phenomenon in the nitriding process, forming a microalloyed layer. No oxygen was found in EDs scanning of the coating, indicating that no oxidation occurred in Cu particles during nitriding.

Indicated by the XRD analysis, the average grain size of nano-Cu was between 40 and 70 nm. Due to the nanocrystalline grains size affect [17,18], the melting point of nano-Cu was lower than that of bulk copper, the melting point of nano-copper (the particle size 13 nm and 104 nm, respectively) was 224.4 °C and 413.5 °C, respectively, so the melting point decreased with the decrease of nano-grain size [19]. Therefore, during the nitriding treatment (530 °C), the nano-Cu could melt and diffuse (Figure 6b). Due the small size of active nitrogen atoms compared with Cu, they could diffuse further into the substrate to form diffusion layer with lots of Fe-N compounds (ε-Fe_2-3N and γ’-Fe₄N).

3.3. Microhardness Analysis and Binding Force of Nano-Cu/Microalloyed Gradient Coating

Figure 7 shows the profile microhardness distribution of the as-prepared specimens. It can be seen that the surface hardness of samples 1#, 2# and 3# is about 450 HV₀.₀₅, which is lower than that of sample 0#. This is due to the soft surface copper layer. However, the microhardness of the middle microalloy layer is about 570 HV₀.₀₅, which improves the surface strength. Because the copper is nano scale, the hardness increases with the decrease of nano grain size, which provides strength support for the contact stress of the friction pair surface.

Figure 8 presents the scratch morphologies of the three samples with nano-Cu/microalloyed gradient coating after scratch test under different loads. The scratch morphologies under 70 N load were shown in Figure 8a,c,e, which revealed that the nano-Cu/microalloyed gradient coatings exhibited excellent plasticity and good bonding strength. Figure 8b,d,f showed the scratch morphologies of the nano-Cu/microalloyed gradient coating under high loads, and lots of blocky stripping pits could be observed at the edge of the scratches.

Due to the tension and friction stress at the rear edge of the scratch needle, lots of conformal cracks were formed [20]. Figure 8b,d,f show the micro morphology of the nano copper layer after peeling off. At the edge of the scratch, there are massive peeling pits left after peeling off. The bonding strength of the 1#, 2#, and 3# samples are 109 N, 107 N, and 150 N, respectively, indicating that the surface roughness affects the formation of the surface coating. The conformal crack size of 3# sample at 150 N was less than that of 1# and 2# samples, indicating that the plastic yield strength of 3# sample at scratches was larger. Laugier suggested that the energy required for coating peeling must be balanced by releasing stored elastic energy [21]. The energy released consists of three parts, around the indentation, the internal stress of the coating, and the stress generated by friction.

Compared with the crankshaft based on QT500-7, after in situ treatment, the bonding strength between the copper coating and the microalloy layer reaches 34 N [22]. In this experiment, 42CrMo alloy steel is used as the base material of crankshaft, and the bonding strength of copper layer shows more excellent performance. Surface roughness has a certain effect on the adhesion of the coating [23].

3.4. Dry Friction Test on Wear Resistance of Nano-Cu/Microalloyed Gradient Coating

Figure 9 shows the total wear volume loss of 42CrMo steel along depth profiles under 2 N load. It can be found that under the same load, with the decrease of surface roughness, the wear volume gradually decreases, and 3# sample shows the smallest wear volume. Compared with uncoated samples, the wear volume of 1# sample was greater than that of 0# sample, and the wear of 2# sample was equal to that of 0# sample. On the one hand, the bonding force of nano-copper layer was lower than that of 3# sample, which was exfoliated prematurely in the wear process and could not play a good role in antifriction effect [24]. On the other hand, the microhardness of the sub-surface of 1# sample was smaller than that of 0# sample, which led to a decrease in attrition resistance and an increase in wear volume [25,26].

Figure 10 shows the micro-morphology after wear test under 2 N load. It can be seen from the figure that there were grooves and a large number of exfoliation pits on the surface of 0# sample without the nano-copper layer. The wear mechanism was the combination of abrasive wear and delamination wear. In comparison, there were a few spalling pits and grooves on the surface of 1# and 2# samples, respectively, and the wear surface of 3# sample was relatively flat. Results showed that copper was used as lubricating material at the initial stage of wear tests. Hard particles were pressed into the surface of wear scratch to make plastic deformation in the wear tests process, and grooves were formed during sliding friction. Due to the existence of nano-copper layer, it could act as a lubricant and avoided the direct contact of tribo-pairs in the process of wear tests, which reduced the amount of wear on the surface.

Figure 11 shows the friction coefficient of 42CrMo steel during wear tests after gas nitriding. The upward trend of friction coefficient curve can be divided into two stages. In the initial stage representing the break-in period, the curve of friction coefficient of 0# sample rose rapidly, and then maintained a relatively stable trend where the friction coefficient was stabilized. By comparison, it was found that the friction coefficient of 1#, 2#, and 3# samples first rose to stage and became stable, then rose rapidly and finally entered a stable state. Results show that the nano-copper layer on the surface plays a better lubrication role, resulting in a lower friction force in the early stage of friction. With the development of the friction test, the amount of debris increased and adhered to the surface of the worn surface, as shown in the dashed line marked area in Figure 10. Due to the good plasticity of copper, the plastic deformation of debris under the applied load led to an increase in friction force and friction coefficient [27].

3.5. Wet Friction Test of Wear Resistance of Copper Coating

In order to better simulate the actual working environment and in combination with the dry friction test results, the four-ball friction and wear tester was used to conduct the friction test on the polished specimen under the condition of mineral oil lubrication and the results are shown in Figure 12. Figure 12a shows the time-dependent curves of the friction coefficients of untreated, conventional, and in situ gas nitriding samples, respectively. It can be seen that the friction coefficients of the two groups with treatment are lower than that of the untreated group, revealing that the effect of wear reduction could be achieved after surface treatment. The friction coefficient of the three groups increased rapidly in the initial stage, and the friction coefficient of the untreated group and the in situ treatment group tended to be stable in the operation stage, while the friction coefficient of the nitriding treatment group decreased first and then became stable. Compared with the in situ treatment group, the friction coefficient of nitriding treatment group is higher in the transient start-up stage of the test, which may be caused by the loose surface layer, leading to a higher friction coefficient in the early running in stage [28]. The friction coefficient of the in situ treatment group is relatively stable. This is because the surface copper layer has good self-lubricating properties [29], which shows a relatively good anti-wear effect, which effectively avoids the wear problem caused by the large friction force of the friction pair at the instant start-up stage.

After the four-ball friction tester, the wear scar morphologies were observed, which are shown in Figure 12b. The wear mark sizes corresponding to the steel balls with three different surface treatments are 54 μm, 54 μm, and 51 μm, respectively. The results show that there is no significant difference in the wear scar diameter between the untreated sample and the conventional gas nitriding treatment sample. The wear scar diameter of in situ gas nitriding treatment is the smallest, indicating the best anti-wear effect. It can be seen from Figure 12a that the anti-wear and anti-wear effect of steel ball treated by in situ gas nitriding process is the best.

The antifriction mechanism of nano-Cu/microalloyed gradient coating can be explained in Figure 13. In the friction process, due to the existence of nano copper layer, it can effectively prevent the direct contact between the grinding couple. With the interaction between the friction pairs, the nano copper layer containing self-lubricating materials form a composite transfer layer on the pair, which transforms into mutual friction between the coating materials and greatly reduce the friction coefficient [22].

4. Conclusions

In this paper, nano-Cu/microalloyed gradient coating on 42CrMo steel surface was prepared by in situ gaseous nitriding treatment after the activation of CuO nanomaterials. The surface roughness of 42CrMo affects the bonding force of copper layer/microalloy layer and the microstructure of copper layer. When polishing is adopted, the surface of the sample is loose, but the bonding strength of the copper layer can reach 150 N. Nano-Cu layer affects the adsorption concentration and diffusion of active nitrogen atoms on the surface, thus reducing the brittleness of nitride compound layer and enhancing its binding strength, which forms a metallurgical bond. Abrasive wear and adhesive wear are the main failure mechanisms of crankshaft connecting rod. When the crankshaft is modified by nano copper/micro alloy, the copper layer on the surface of the crankshaft forms a copper transfer film on the surface of the wear couple connecting rod, which reduces the adhesive wear. At the same time, the particle size of the abrasive particles is small and is covered by the copper layer, that is, embedded in the copper layer, reducing the wear of the abrasive particles. It provides a new way for the preparation of copper coating with good anti friction performance.