In order to improve the wear resistance of copper materials, four kinds of coatings including pure Ni, Ni-20Cu, Ni-20Cu-10Mo and Ni-20Cu-15Mo (molar fraction, %) were laser clad on copper substrate. The microstructure of Ni-20Cu-10Mo coating was analyzed, and the friction and wear behavior of copper substrate and coating were studied. The effects of Ni, Cu and Mo content on the microstructure and wear resistance of coating prepared by laser cladding were investigated. The results show that the coatings are mainly composed of Ni-based solid solution, presenting a non-equilibrium solidification morphology of plane crystal-cellular dendrite-equiaxed crystal; the microhardness of pure Ni, Ni-20Cu, Ni-20Cu-10Mo and Ni-20Cu-15Mo coatings are 137.0HV, 141.4HV, 151.3HV and 143.7HV, respectively, and the average friction coefficients are 0.64, 0.54, 0.16 and 0.42, respectively; compared with the copper substrate, the microhardness of the four coatings are increased by 22.65%, 26.59%, 35.45% and 28.65%, respectively, and the average friction coefficients are increased by 29.67%, 40.66%, 82.42% and 53.85%; among them, the average friction coefficient of the Ni-20Cu-10Mo coating is the lowest (0.16), which is 17.58% of that of the copper matrix, which changes the friction and wear mechanism of the copper matrix from adhesive wear mechanism to abrasive wear mechanism.

Copper alloy materials and their components support the development of new generation information technology, advanced rail transportation equipment, aerospace equipment and other technical fields, and play an important role in my country’s major projects, economic construction and national security [1]. However, the poor surface wear resistance of copper materials is one of the main factors restricting their long-term application in the fields of crystallizer components. A feasible way to improve the wear resistance of copper materials is to add alloying elements to the copper matrix [2]. At present, the wear-resistant copper alloy systems that have entered the mass production stage in China mainly include aluminum bronze, tin bronze, silicon bronze, manganese bronze, aluminum brass, silicon brass and lead brass [3]. However, the alloying strengthening method often changes its comprehensive mechanical properties and physical and chemical properties such as electrical conductivity. Preparation of surface coating is another way to improve the wear resistance of copper substrate materials. Through a series of coating surface modification technologies such as coating-plating-plating [4-11], the surface wear resistance of copper materials can be significantly improved without affecting the original properties of the copper substrate [12].

Laser cladding technology uses a high-power laser beam to quickly melt the coating powder and the surface of the metal substrate to form a metallurgical bonding coating with high bonding strength. It is particularly suitable for surface strengthening of copper alloys in copper crystallizers used under harsh conditions such as high temperature and wear [13]. Both Ni and Cu have face-centered cubic structures. The two can achieve infinite solid solution by replacing atoms in the grains. In addition, Ni and Cu have good wettability and similar thermophysical parameters. Ni-based alloy powder can form a coating with good interface bonding performance with the copper substrate [14]. For example, by laser cladding, a Ni coating with high bonding strength, no pores, and no cracks can be obtained on the surface of pure copper. Its friction coefficient is 2.0, which is only 57% of that of pure copper[15]. However, the intrinsic hardness of the Ni coating is low and the wear resistance is limited[16]. Therefore, domestic and foreign researchers often consider adding elements such as Cr, Zr, Nb, Si or strengthening phases such as WC on the basis of pure Ni coating, in order to further improve the hardness and wear resistance of the coating[17−20].

In past studies, there are relatively few reports on the use of Mo as a strengthening element for Ni-based coatings for laser cladding of copper substrates. This may be due to two constraints: 1) the thermal properties of Cu and Mo do not match; 2) the mutual solubility of Mo and Cu is low. This brings challenges to the research and development of Mo-Cu system coatings. This study attempts to introduce three elements, Ni, Cu, and Mo, as the main components of the laser cladding layer. Four types of cladding layers, namely pure Ni, Ni-20Cu, Ni-20Cu-10Mo, and Ni-20Cu-15Mo, are prepared. Ni-Cu-Mo cladding layers with good metallurgical bonding with the copper substrate are obtained. The microstructures of the four cladding layers and their influence mechanisms on the surface hardness and friction and wear properties of the copper substrate are studied.

1 Experiment

1.1 Experimental materials and methods

The cold-rolled copper alloy substrate was cut into small cubes of 40 mm×40 mm×10 mm by wire-cutting electrospark cutting. Before laser cladding, the copper substrate was sandblasted to remove surface oil and oxides, and then ultrasonically cleaned in 75% ethanol solution (mass fraction). The Cu powder, Ni powder, and Mo powder used for laser cladding are spherical metal powders with a purity of not less than 99% (mass fraction), and the average particle size of the powder is 15~45 μm. In order to make the coating composition uniform, the metal powder needs to be mixed in a three-dimensional motion mixer for 12 h in advance to fully mix the powder at a speed of 10 r/min. Since copper has a high reflectivity to laser, and the reflectivity of the pre-set powder layer on the copper substrate to laser is lower than that of the exposed copper surface to laser, this experiment uses the pre-set powder method to pre-form the uniformly mixed alloy powder on the surface of the copper substrate by air pressure spraying, and the thickness of the pre-set layer is 300~400 μm. The experimental process is shown in Figure 1, and the composition of the coating powder is shown in Table 1.

1.2 Experimental device and analytical instrument

A 500 W fiber laser is used to prepare the Ni-Cu-Mo coating, and the laser spot diameter is 70~200 μm. Before laser cladding, the air in the operating chamber of the laser cladding equipment was extracted in advance until the oxygen content in the operating chamber was as low as 0.04% (volume fraction), and then Ar with a purity of not less than 99% (volume fraction) was filled in the operating chamber of the laser cladding equipment as a protective gas during the laser cladding process. The specific process parameters of laser cladding are as follows: laser power 250 W, scanning speed 100 mm/s, spot diameter 130 μm, overlap rate 40%. The friction coefficient of the copper substrate and the coating was tested on a dry sliding friction and wear tester. The friction pair was a GCr15 steel ring, the friction speed was 400 r/min, and the loading pressure was 10 N. The coating phase was characterized by a copper target X-ray diffractometer (XRD), the microstructure of the copper substrate and the coating was observed by a metallographic microscope (OM) and a field emission electron microscope (SEM), and the element distribution was analyzed by an energy dispersive spectrometer (EDS).

2 Results and discussion

2.1 Coating microstructure analysis

Figure 2 shows the coating macroscopic morphology and cross-sectional metallographic photographs. As shown in Figure 2, the coating surface is flat, there are a few holes visible to the naked eye on the surface of the pure Ni coating, there are no obvious holes and cracks on the surface of the Ni-20Cu coating, Ni-20Cu-
10Mo coating, and Ni-20Cu-15Mo coating, but there is coating peeling on the surface of the Ni-20Cu-10Mo coating and Ni-20Cu-15Mo coating. The coating thickness is in the range of 200~300 μm, the structure is dense, there are many holes and cracks on the cross section of the pure Ni coating, and there are no obvious cracks on the cross section of the Ni-20Cu coating, Ni-20Cu-10Mo coating, and Ni-20Cu-15Mo coating. Because the laser energy distribution is close to the Gaussian distribution, the interface between the coating and the copper substrate is not straight, but presents overlapping concave contours. There are no defects such as inclusions, holes, cracks, etc. at the interface between the cladding and the copper substrate, and the interface is densely bonded, showing metallurgical bonding characteristics.

During the laser cladding process, the molten pool temperature is high and the cooling rate is fast. The microstructure of the cladding is affected by the direction of the maximum heat flow loss and the ratio of the solid-liquid interface temperature gradient G to the solidification rate R[21]. As shown in Figure 3, the bottom of the molten pool is mainly plane crystals, and the reasons for their formation may be based on the following three points: 1) The heat of the molten pool has not been effectively conducted at the moment of its formation, and there is a maximum temperature gradient G between the molten pool and the copper substrate; 2) The molten pool has not yet begun to nucleate and grow, and the solidification rate R in the molten pool is approximately 0; 3) The interface between the bottom of the molten pool and the copper substrate can be used as a nucleation point in the early stage of solidification. According to the principle of metal solidification[22], when the molten pool begins to nucleate under the above conditions, it mainly grows into the molten pool in the form of plane crystals.

Since the copper matrix has good thermal conductivity, it can quickly conduct the heat in the molten pool to all directions of the copper substrate, resulting in a decrease in the temperature of the molten pool, an increase in the temperature of the copper matrix, and a decrease in the temperature gradient G between the molten pool and the copper matrix. Therefore, the planar crystal growth mode at the front of the solid-liquid interface is quickly broken; in addition, the melting points of the three elements Ni, Cu, and Mo are different. Ni and Cu with lower melting points solidify first, and Mo with higher melting points is enriched at the front of the solid-liquid interface, resulting in supercooling of the components at the front of the solid-liquid interface, and finally the cellular growth characteristics appear. As shown in Figure 4, the cellular growth of the molten pool leads to the main cellular columnar crystals and cellular dendrites in the middle of the molten pool. As the solidification continues, the heat of the molten pool is continuously transferred to the copper matrix, which further reduces the temperature gradient G between the molten pool and the copper matrix. The reduction in the temperature of the molten pool is also conducive to the increase of the solidification rate R. The ratio of G to R continues to decrease, which eventually leads to a small difference in the growth rate in all directions inside the molten pool, and equiaxed crystals appear in the coating, as shown in Figure 5.

2.2 Coating phase and element analysis

Figure 6 shows the XRD spectra of the four coatings. During the analysis, it was found that the diffraction peaks of the four coatings were similar to the angle distribution of the diffraction peaks of Ni, but slightly shifted to the left. Considering that Cu and Ni can form an infinite solid solution, and Mo and Ni can also form a substitutional solid solution, it is preliminarily inferred that the main phase of the four coatings is a Ni-based solid solution. When the Cu atoms with an atomic radius of 1.28 Å and the Mo atoms with an atomic radius of 1.39 Å replace the Ni atoms with an atomic radius of 1.24 Å in the unit cell, the interplanar spacing increases. According to the Bragg equation [23], the wavelength of the diffraction beam remains unchanged. When the interplanar spacing increases, the diffraction angle decreases, which is manifested as the diffraction peak on the diffraction spectrum shifting to the left. Figure 7 shows the surface scanning analysis results of Ni, Cu, and Mo elements in the Ni-20Cu-10Mo coating. As shown in Figure 7, the content of Cu element in the interface area between the coating and the substrate is slightly higher than that in the coating. The reason may be that the high energy input of laser cladding quickly melts the surface layer of the copper substrate, making the interface between the coating and the substrate higher in copper content than the overall coating. However, due to the short laser cladding time, according to Fick’s law, the copper atoms have not had time to fully diffuse to the coating surface and have begun to solidify, so there is a phenomenon of high concentration of copper atoms at the interface between the coating and the substrate and low concentration of copper atoms at the coating surface. Compared with the Cu element, the distribution of Ni and Mo elements in the coating is more uniform and dispersed, and there is no obvious element segregation phenomenon.

2.3 Analysis of coating wear resistance

Figure 8 shows the microhardness of the copper substrate and four coatings. The microhardness of the four coatings of pure Ni, Ni-20Cu, Ni-20Cu-10Mo, and Ni-20Cu-15Mo are 137.0HV, 141.4HV, 151.3HV, and 143.7HV, respectively. Compared with the hardness of the copper matrix of 111.7HV, they have all increased by 22.65%, 26.59%, 35.45%, and 28.65%, respectively. Combined with the results of XRD analysis, the main phases of the coatings of the four components are all Ni-based substitutional solid solutions. When Cu and Mo atoms with larger atomic radius replace Ni atoms in the unit cell, it is bound to cause a certain degree of lattice distortion [24], thereby increasing the movement resistance of dislocations during deformation, increasing the ability of the coating to resist plastic deformation, and improving the microhardness of the coating. Figure 9 shows the friction coefficient curves of the four coatings and the copper substrate. The average friction coefficients of the coatings and the substrate are shown in Table 2. It can be seen from Table 2 that the average friction coefficient of the coating with only Ni and Cu elements added is lower than that of the copper substrate, but the friction coefficient fluctuates greatly; after adding 10% (molar fraction) Mo element, the average friction coefficient of the coating is further reduced, and the fluctuation is relatively gentle; but as the Mo content continues to increase to 15%, the average friction coefficient of the coating increases. The friction coefficient curve is relatively stable in the early stage. As the friction time increases, the friction coefficient gradually increases and fluctuates greatly [25].

The reasons for the change in friction coefficient may include the following three aspects: 1) Laser cladding has the characteristics of rapid heating and rapid cooling, which leads to grain refinement. When dislocations move in a polycrystal, the slip resistance increases due to the different orientations of the grains on both sides of the grain boundary. Therefore, the slip band in the grain on one side cannot directly enter the second grain, but accumulates at the grain boundary, causing the strength to increase. The finer the grains, the more grain boundaries there are, the more places where dislocations are blocked, and the higher the strength of the coating [25-26]. 2) The formation of Ni-based solid solution in the coating has the effect of solid solution strengthening. Therefore, even if the Mo element is not added, the hardness of the two Ni-based coatings will increase, resulting in a decrease in the friction coefficient of the coating [27]. 3) After adding the Mo element, it can be seen from Figure 10(a) and (b) that the Mo particles are uniformly dispersed nanoparticles in the Ni-20Cu-10Mo coating. Since the Mo element has a high hardness, it can be used as a hard phase when it is uniformly dispersed in the coating. The Mo atoms interact with the dislocations, hindering the movement of the dislocations and increasing the deformation resistance of the coating, thereby further increasing the hardness of the coating and making it difficult for the friction pair to invade the coating under the action of normal stress, reducing plastic deformation. In addition, the Mo particles are uniformly distributed in the coating and play a pinning role, thereby reducing the friction coefficient [27−28]. However, when the Mo content is 15%, the average friction coefficient of the coating increases. This may be because when the Mo content is too high, it is not enough to be completely melted during the laser cladding process, and the Mo element forms a discontinuous network structure at the grain boundary (see Figure 10 (c) and (d)), resulting in a decrease in coating performance and an increase in the friction coefficient. At the same time, the particle size of the Mo element distributed in the coating is relatively large, causing the hard phase to coarsen. When the particles of the hard phase are small, the dislocation moves in a way of cutting the particles. As the particle radius increases, the dislocation line no longer cuts the particles, but moves by bypassing the particles. Since the critical shear stress required for the dislocation to bypass the particles is lower than that required to cut the particles, the strengthening effect weakens as the size of the hard phase increases, resulting in an increase in the friction coefficient and a decrease in wear resistance.

Figure 11 shows the surface morphology scanning electron microscope photos of the four coatings and the copper substrate after the friction and wear test. During the dry sliding friction and wear process, the sample will be subjected to the positive stress of the micro-convex body of the friction pair. Due to its low hardness, the copper substrate is prone to plastic deformation, resulting in continuous tearing and peeling on the surface. As shown in Figure 11(a), after the friction and wear test, there are a large number of peeling pits and plastic tears on the surface of the copper substrate, accompanied by debris. As the friction and wear test continues, these debris will cause abrasive wear, resulting in the appearance of plowing grooves on the surface of the copper substrate. Therefore, the friction and wear mechanism of the copper substrate is a mixed mechanism with adhesive wear as the main mechanism and abrasive wear as the auxiliary mechanism [29]. Unlike the copper substrate, the friction and wear mechanism of the Ni-20Cu-10Mo coating is mainly abrasive wear. As shown in Figure 11(b), there is almost no plastic tearing and spalling pits on its surface, and only some narrow and shallow furrows exist. This may be due to the presence of dispersed Mo atoms that hinder the movement of dislocations and improve the deformation resistance of the coating. During the continuous friction and wear process of the high-hardness GCr15 steel friction pair, only some narrow furrows are produced on the coating surface [16].

Comparing the microscopic morphologies of the copper substrate, pure Ni coating, and Ni-20Cu-10Mo coating after the dry sliding friction and wear test shown in Figure 11, it can be found that from the copper substrate to the pure Ni coating and then to the Ni-20Cu-10Mo coating, the plastic tearing and spalling pits on the surface gradually decrease, the material’s ability to resist plastic deformation gradually increases, and the wear resistance gradually increases. In addition, by comparing the morphologies of the Ni-20Cu-10Mo coating and the Ni-20Cu-15Mo coating after the dry sliding friction and wear test, it can be found that the surface plastic tearing of the coating containing 10% Mo content is less than that of the coating containing 15% Mo content. This is because when the Mo content is too high, due to the limitation of the laser power, the number of Mo particles that cannot be completely melted increases, causing the dislocation mechanism to change from a cutting mechanism to a bypass mechanism. The strength and hardness of the coating are reduced, and it is more prone to plastic deformation, resulting in more plastic tearing on the surface.

3 Conclusions

1) The four coatings prepared by laser cladding have metallurgical bonding characteristics with the copper substrate. Affected by the maximum heat flow loss direction and the ratio of the temperature gradient G at the solid-liquid interface to the solidification rate R, the bottom of the coating is mainly planar crystals, the middle is mainly cellular dendrites, and the upper part is equiaxed crystals.

2) The main phases of the four coatings of pure Ni, Ni-20Cu, Ni-20Cu-10Mo and Ni-20Cu-15Mo (molar fraction, %) are all Ni-based solid solutions, and the Ni, Cu, and Mo elements are evenly dispersed in the coating.

3) The hardness of the four coatings is higher than that of the copper substrate, the average friction performance is lower than that of the copper substrate, and the wear resistance is improved. Among them, the Ni-20Cu-10Mo coating has the best performance, with a microhardness of 151.3HV and an average friction and wear coefficient of 0.16, which is 17.58% of that of the pure copper substrate. After the preparation of this coating, the friction and wear mechanism changes from the adhesive wear mechanism of the copper substrate to the abrasive wear mechanism.