The fretting wear properties and fretting wear mechanism of the MoNbTaVW refractory high entropy alloy coating prepared by laser cladding technology were studied under different loads (10 N, 20 N, 30 N), different fretting wear amplitudes (50 μm, 150 μm, 250 μm), and different cycle times (5 000, 10 000, 15 000). The results show that the prepared MoNbTaVW refractory high entropy alloy coating is composed of Fe7Ta3 type HCP solid solution phase, FCC solid solution phase and (Fe, Ni) matrix phase, in which the FCC phase is unmelted high entropy alloy powder. According to the orthogonal experimental range analysis, the fretting wear amplitude has the greatest impact on the wear volume, the fretting wear load has the second greatest impact on the wear volume, and the fretting wear cycle number has the smallest impact on the wear volume. Among them, the wear volume of the MonbTaVW refractory high entropy alloy coating reaches the maximum value under the fretting wear conditions of 15,000 times, 20 N, and 250 μm; the fretting wear load has the greatest impact on the friction coefficient, the fretting wear amplitude has the second greatest impact on the friction coefficient, and the fretting wear cycle number has the smallest impact on the friction coefficient. Among them, the friction coefficient of the MonbTaVW refractory high entropy alloy coating reaches the maximum value under the fretting wear conditions of 10,000 times, 30 N, and 150 μm. The fretting wear mechanism of the MonbTaVW refractory high entropy alloy coating is mainly oxidative wear and adhesive wear, and the wear debris produced by wear is mainly Ta and W oxides.
As a new and popular material, high entropy alloy has high hardness, strong oxidation resistance, outstanding mechanical properties, excellent wear resistance and corrosion resistance due to its unique design concept, high entropy, lattice distortion, slow diffusion and cocktail effect. Refractory high entropy alloy is a new type of multi-principal alloy developed on the basis of high entropy alloy, mainly composed of refractory metal elements (melting point higher than 1650℃, such as Nb, Ta, Mo, W, V and other elements). Refractory high entropy alloy has excellent strength, specific strength, high temperature performance and mechanical properties, and has broad application prospects. Refractory high entropy alloy represented by quaternary WTaNbMo and quinary MoNbTaVW is the hot spot in the research field of refractory high entropy alloy.
Due to its high hardness, high strength, good friction resistance and corrosion resistance, high entropy alloy coating soon became the focus of coating research. High entropy alloy coatings are usually prepared by plasma spraying, magnetron sputtering, electrochemical deposition, laser cladding and other means. Compared with other coating preparation technologies, laser cladding technology can melt most of the metal elements, and due to the fast cooling rate, the coating prepared by it has fine structure, dense structure and low dilution rate, and the substrate is little affected, which can ensure the excellent performance of the coating material; at the same time, the coating and the substrate are metallurgically bonded, with high bonding strength, and it is not easy to fall off under complex working conditions. Therefore, laser cladding technology has gradually become the first choice for the preparation of high entropy alloy coatings, especially refractory high entropy alloy coatings.
At present, the research on laser cladding refractory high entropy alloy coatings mainly focuses on corrosion resistance, hardness, wear resistance, microstructure and other aspects. Zhao et al. prepared WT a N b Mo refractory high entropy alloy by laser cladding, and believed that the high entropy alloy prepared with magnetic field assistance is more dense. Guan et al. prepared NbTiZr and NbTaTiZr refractory high entropy alloy coatings by laser cladding, and proved that the hardness of both coatings was higher than that of the substrate. Lou et al. prepared Al0.2CrNbTiV refractory high entropy alloy coatings by high-speed laser cladding. The study showed that the hardness and grain boundary hardening effect of the coating were higher than that of the substrate, and the ability to resist plastic deformation was also improved. Huang et al. prepared TiNbZrMo refractory high entropy alloy coatings by laser cladding, and studied its hardness, wear resistance and corrosion resistance. The results showed that the various properties of the coatings were better than those of the 316L substrate. However, based on the literature survey, it can be found that there are relatively few studies on the fretting wear tribological properties of high entropy alloy coatings prepared by laser cladding. Fretting wear is a special wear phenomenon. When the relative movement of two contacting objects produces wear damage at the micron level, the failure of parts caused by this type of wear is usually difficult to detect and evaluate, and the harm caused is extremely great. The factors affecting fretting wear are mainly related to the performance of the material, microstructure, and applied load. Refractory high entropy alloy coatings have excellent performance. Studying the fretting wear of refractory high entropy alloy coatings has far-reaching significance for reducing the harm caused by fretting wear. Based on orthogonal experimental design, this work conducts fretting wear experiments on the MoNbTaVW refractory high entropy alloy coating prepared by laser cladding with three factors (fretting wear load, fretting wear cycle number, fretting wear amplitude) and three levels (load is 10 N, 20 N, 30 N; number of cycles is 5 000, 10 000, 15 000; fretting wear amplitude is 50 μm, 150 μm, 250 μm) to study its fretting wear performance and fretting wear mechanism under different loads, different number of cycles and different amplitudes.
1 Experiment
1.1 Preparation of refractory high entropy alloy coating
The substrate material is 316L stainless steel, and the substrate size is 15 mm×15 mm×10 mm. Before laser cladding, the oxide scale on the surface of the substrate was removed by manual grinding, and then it was placed in anhydrous ethanol or acetone solution for ultrasonic cleaning to remove surface dirt before use. The MonbTaVW refractory high entropy alloy coating was prepared by TruDisk6006 laser cladding equipment. The laser cladding powder was spherical MonbTaVW refractory high entropy alloy powder produced by the Institute of New Materials of Guangdong Academy of Sciences. The powder diameter was 50-100 μm (as shown in Figure 1), and the atomic fraction of each element is shown in Table 1. The laser cladding parameters were laser power 3200 W, laser cladding speed 10 mm/s, laser spot diameter 4 mm, defocus 17 mm, overlap rate 50%, and synchronous powder feeding amount 15.9 g/min. During the cladding process, helium was used as the powder feeding and shielding gas source, and the gas flow rate was maintained at 2. 5 L/min.
1. 2 Characterization of the microstructure of refractory high entropy alloy coating
The phase composition of the MonbTaVW high entropy alloy coating was characterized by a Rigaku Smartlab SE X-ray diffractometer (the Kα target was a Cu target), with a scanning angle of 20-90° and a scanning speed of 5 (°)/min. The micromorphology and elemental composition of the high entropy alloy coating surface and the fretting wear surface were obtained by an Apreo S Hivac field emission scanning electron microscope.
1. 3 Fretting wear experiment of refractory high entropy alloy coating
The fretting wear experiment was carried out using a homemade fretting wear test machine, and a Si3N4 ceramic ball with a diameter of 6 mm was selected as the grinding ball. The fretting wear parameters were set as fretting wear cycle number 5000, 10000, 15000, fretting wear load 10N, 20N, 30N, fretting wear amplitude 50μm, 150μm, 250μm. After the fretting wear experiment, the wear morphology of the wear scar surface was obtained using a Bruker Control GT-K white light interferometer, and the wear volume was calculated by the two-dimensional cross-sectional profile area and the wear scar length.
In order to study the effects of different fretting wear cycle numbers (factor A), different fretting wear loads (factor B) and different fretting wear amplitudes (factor C) on the fretting wear performance of refractory high entropy alloy coatings, a three-factor three-level orthogonal experiment was developed, and the experimental factors and levels are shown in Table 2. According to the fretting wear orthogonal experiment, an orthogonal table was designed to reduce the number of experiments from 27 to 9, as shown in Table 3.
2 Results and discussion
2.1 Phase composition structure of MonbTaVW refractory high entropy alloy coating
Figure 2 shows the XRD diagram of MonbTaVW refractory high entropy alloy coating and powder. From the XRD diagram of high entropy alloy powder, it can be found that it is a single phase of face-centered cubic crystal structure (FCC); the refractory high entropy alloy coating is composed of HCP solid solution phase (reference standard XRD card PDF 12-0604 of Fe7Ta3), FCC solid solution phase (reference standard XRD card PDF 39-1178 of TaV2) and (Fe, Ni) matrix phase. Figure 3 shows the SEM and EDS diagrams of MonbTaVW refractory high entropy alloy coating prepared by laser cladding. From the SEM image of the surface morphology, it can be found that the coating is composed of bright white round particles, typical solidified dendrites and gray matrix, showing a typical solid solution structure of laser cladding high entropy alloy coating. The bright white round particles show that their elemental composition is W, Ta, Mo, Nb, and V in the EDS image. By comparing the XRD image, it can be judged that the bright white area is the high entropy alloy powder that is not completely melted during the laser cladding process. At the same time, according to the EDS image, it can be found that a large amount of Fe elements are distributed inside the coating. This is because the laser energy density is too high during the laser cladding process to prepare the high entropy alloy coating, resulting in a large amount of Fe elements in the matrix entering the coating, forming a Fe-rich gray matrix phase inside the coating (i.e., the (Fe, Ni) phase in Figure 2); the Fe element combines with the W, Ta, Mo, Nb, and V elements in the high entropy alloy to form a HCP phase crystal structure with Fe7Ta3 as the core. The HCP solid solution phase, FCC solid solution phase and (Fe, Ni) matrix phase are indicated by the arrows in Figure 3, respectively.
2.2 Friction coefficient and wear volume of MonbTaVW refractory high entropy alloy coating
The amplitude-tangential force (F-D) curve is an important parameter to characterize fretting wear, which can intuitively reflect the wear state of refractory high entropy alloy coating during fretting wear. As shown in Figure 4, under different fretting wear loads, different fretting wear amplitudes, and different fretting wear cycle times, the shape of the amplitude-tangential force curve is a parallelogram, which indicates that under different experimental conditions, the fretting wear on the surface of refractory high entropy alloy coating is complete slip (Gross slip).
The friction coefficient curve of fretting wear of MonbTaVW refractory high entropy alloy coating is shown in Figure 5. As can be seen from Figure 5, under different fretting wear conditions, before 1000 to 1500 fretting wear cycles, it is a friction running-in period, and the friction coefficient is unstable and shows an upward trend. After the running-in period, when the load is 10 N, the friction coefficient of the high entropy alloy coating with different fretting wear cycles and different fretting wear amplitudes is 0.21±0.03; when the load is 20 N, the friction coefficient of the high entropy alloy coating with different fretting wear cycles and different fretting wear amplitudes is 0.45±0.03; when the load is 30 N, the friction coefficient of the high entropy alloy coating with different fretting wear cycles and different fretting wear amplitudes is 0.63±0.02. The friction coefficient after stabilization is selected, and the range analysis is performed using orthogonal experiments, as shown in Table 4. Among them, T is the sum of the experimental results of the factors, t is the mean of the sum of the experimental results of the factors, and the range R is the reduction value of the large value in the t value. The larger the R, the greater the influence of the factor on the experimental results. As can be seen from Table 4, the influence of fretting wear load on the friction coefficient of high entropy alloy coating is the greatest, the influence of fretting wear cycle amplitude on the friction coefficient of high entropy alloy coating is second, and the influence of fretting wear amplitude on the friction coefficient of coating is the smallest, which is consistent with the results obtained from the friction coefficient curve.
Figure 6 is a three-dimensional morphology of the wear surface of MonbTaVW refractory high entropy alloy coating. As can be seen from Figure 6, when the fretting wear amplitude is 50 μm, the wear degree of the three coating surfaces is small; when the fretting wear amplitude is 250 μm, the wear degree of the three coating surfaces is large. Table 5 is the wear volume of the coating fretting wear calculated based on the three-dimensional morphology of the wear surface. The wear volume of the high entropy alloy coating surface obtained was subjected to orthogonal experimental range analysis, and the results are shown in Table 6. Among them, T is the sum of the experimental results of the factors, t is the mean of the sum of the experimental results of the factors, and the range R is the reduction value of the large value in the t value. The larger the R, the greater the influence of the factor on the experimental results. From the results of the range analysis, it can be seen that the fretting wear amplitude has the smallest effect on the fretting wear volume of the refractory high entropy alloy coating, the fretting wear load has the second largest effect on the fretting wear volume of the refractory high entropy alloy coating, and the fretting wear cycle number has the largest effect on the fretting wear volume of the coating.
2.3 Fretting wear mechanism of MonbTaVW refractory high entropy alloy coating
Figure 7 is a SEM image of the fretting wear morphology of MonbTaVW refractory high entropy alloy coating. As can be seen from Figure 7, the fretting wear surface of the coating is divided into three regions, namely the white circular area, the dark gray area at the wear mark, and the light gray area. The dark gray area is distributed in the friction area in a flake shape. Compared with Figure 6, it can be seen that the dark gray area appears as a lighter green in the three-dimensional morphology of the wear surface. From the EDS element analysis in Figure 8, it can be seen that this area is a metal oxide formed by the combination of W, Ta elements and O elements in the coating. Combining the EDS, SEM and three-dimensional morphology of the coating, it can be confirmed that the dark gray area is the wear debris generated on the surface of the high entropy alloy coating during fretting wear. These wear debris are not discharged in time during the fretting wear process and are compacted on the surface to form a friction layer. During the wear process, these friction layers will be destroyed first, so the friction layer can protect the coating from further wear, which is shown as a small wear volume in Figure 6. From the enlarged view of Figure 7, it can be seen that cracks are distributed on the friction layer (as shown by the arrows in Figure 7), which proves that the friction layer is continuously destroyed during the wear process, protecting the coating.
According to the EDS diagram of the light gray area (see Figure 8), except for the O element, other elements are evenly distributed in the light gray area, which indicates that the light gray area is the coating surface. In the SEM diagram (see Figure 7), there are cracks along the friction direction in the light gray area inside the wear trace, while the dark gray area is distributed along the friction direction. This is because the friction layer formed by the wear debris is destroyed and the coating below is exposed. The bright white circular area is the unmelted high entropy alloy powder when the high entropy alloy coating is prepared by laser cladding, which can be concluded from Figure 3. The bright white circular area and the light gray area do not aggregate with the O element, indicating that the MonbTaVW refractory high entropy alloy powder and the high entropy alloy coating have good oxidation resistance. The Fe element does not aggregate in the light gray area and the dark gray area, indicating that the wear debris does not contain the Fe element, and the Fe element on the coating surface does not react with the O element, which means that the HCP phase formed by the dilution of the Fe element during the laser cladding process has better wear resistance.
Based on the above experimental results, it can be considered that: in the early stage of wear, the surface of the MonbTaVW refractory high entropy alloy coating continuously produces wear fragments composed of metal oxides, and the wear fragments will be discharged along the direction of fretting wear, but some wear fragments are not discharged in time and are continuously compacted to form a friction layer on the coating surface. At this time, the coating surface shows adhesive wear. However, the thickness of the friction layer formed at the beginning is not enough. As the friction progresses, the friction layer continues to break, which is represented as the early running-in period on the friction coefficient curve. As the wear progresses, the friction layer continues to form and break, but more and more wear fragments will continue to be generated, the compacted friction layer becomes thicker and thicker, no longer breaks, and the friction coefficient gradually approaches stability. Therefore, the fretting wear mechanism of the MonbTaVW refractory high entropy alloy coating under different fretting wear loads, different fretting wear cycles and different fretting wear amplitudes is oxidation wear and adhesive wear.
3 Conclusions
(1) The prepared MonNbTaVW refractory high entropy alloy coating is composed of HCP solid solution phase, FCC solid solution phase and (Fe, Ni) matrix phase. Among them, the FCC solid solution phase is the partially unmelted MonNbTaVW refractory high entropy alloy powder, and the HCP solid solution phase is the HCP phase with Fe7Ta3 as the core formed by the Fe element in the matrix entering the coating in large quantities during the laser cladding process and combining with the W, Ta, Mo, Nb, and V elements in the high entropy alloy.
(2) According to the orthogonal experimental range analysis, the fretting wear load has the greatest influence on the friction coefficient of the MonbTaVW refractory high entropy alloy coating, followed by the fretting wear amplitude, and the number of fretting wear cycles is the smallest; the fretting wear amplitude has the greatest influence on the friction volume of the MonbTaVW refractory high entropy alloy coating, followed by the fretting wear load, and the number of fretting wear cycles is the smallest.
(3) The main wear forms of fretting wear of the MonbTaVW refractory high entropy alloy coating are oxidation wear and adhesive wear. The wear debris generated during fretting wear is mainly composed of oxides of Ta and W elements. The affinity of other elements of the high entropy alloy to oxygen is not high, indicating that the high entropy alloy coating has good oxidation resistance.
