Fe₅₄Cr₂₅Mo₁₇C₂B₂ alloy amorphous powders were produced by high pressure atomized Ar gas. In order to evaluate the wear behavior of different levels of Al₂O₃, we construct five models: pure Fe-based amorphous model; 5 wt% Al₂O₃/Fe amorphous composite model; 10 wt% Al₂O₃/Fe amorphous composite model; 15 wt% Al₂O₃/Fe amorphous composite model; and 20 wt% Al₂O₃/Fe amorphous composite model.

As shown in Figures 1A,B pure Fe-based amorphous coating model and an amorphous composite coating model (example of 15 wt% Al₂O₃/Fe amorphous composite amorphous model) are constructed in this study. In the simulation process, the size of the MD system is 230Å*41.613Å*39.349Å. The models are mainly composed of a diamond grinding ball and amorphous coating substrate. The upper part of the model is the indenter, which is a sphere with a radius of 10Å. The center of the sphere is located at x = -25Å, y = 20.8015Å, and z = 54.349Å. Detailed parameters of the MD simulation are shown in Table 1. The substrate is composed of three kinds of atoms: fixed atoms, thermostat atoms, and Newtonian atoms (Liu et al., 2007; Si et al., 2012). As depicted in Figure 1C, the free boundary condition is applied in the z direction and the periodic boundary in the x and y directions. The atoms at the bottom of the substrate with a thickness of 5Å are fixed atoms, which prevent the simulation models from shifting. Thermostat atoms with a thickness of 5Å are adjacent to the fixed atoms in order to dissipate heat. The remainder are Newtonian atoms.

The MD simulation with large-scale atomic/molecular massively parallel simulator (LAMMPS) is adopted to simulate the atomic-scale friction behaviors of contact between a diamond surface and an amorphous surface. There are six different atomic interactions in the simulation, concerning Fe-Fe, Fe-Al, Al-Al, Fe-O, Al-O, and O-O atoms in the substance. In this study, we adopt embedded atom potential (EAM), Morse potential, and Comb3 potential to describe the interaction of the atoms. After constructing the models, a periodic boundary is applied in three directions. In order to stabilize the energy, we adopt combined gradient (CG) to minimize the system temperature constant (300 K).

MD was performed using microcanonical ensemble (also known as NVT ensemble) for a time period of 80 ps with a timestep of 1 fs.

After relaxation progresses, indentation begins, and the intender slides along the surface of the substrate. The simulation process includes indentation and scratching. In the indentation stage, the indenter is placed on the top of 5Å of the substance. Within the NVE ensemble, the indenter above the left side moves downward with a speed of 1.0 Å/ps until the depression depth reaches 7Å. In the scratching stage, the intender moves along the positive side of the x-axis with a speed of 1.0 Å/ps. After the whole models reaches a balanced state, the sliding process starts. The total sliding time of friction is 400 ps. After the MD simulation, the atomic information is visualized by the Open Visualization Tool (OVITO).

A ball-on-disc friction-wear tester (SFT-2M, Zhongkekaihua, Technology development Co., Ltd., China) is adopted to evaluate the tribological properties of Fe-based metallic glassy coating and the sliding of composite coatings against commercially obtained Si₃N₄ balls (5 mm in diameter) in air. Each test was carried out using an applied load of 30 N. The sliding speed was selected at a constant value of 500 r/min, and the duration of sliding was 15 min.

In order to investigate the wear process, wear morphologies were studied. Figure 2 shows the scratched surfaces of the five simulation models at 400 ps. Usually, the wear proportion can be evaluated by the number of worn atoms in the pile-up. Compared to Figure 2A, the pile-up significantly increases in Figure 2B. It can be observed that the worn atomic number gradually increases and then decreases.

In order to further explain the abrasive resistance of the five simulation models, a cross-section is selected at a time of 400 ps. As shown in Figure 3, the compression depth of the models firstly increases and then gradually decreases. In addition, the data in the diagram are based on the change in the location of different atoms (shown in Figure 4). From this, we can confirm that the compression depth is the lowest when the Al₂O₃ content is 15 wt%. Above all, the effect of friction coefficient, wear rate and compression depth, and the abrasive resistance of the 15 wt% Al₂O₃ composite model are at optimum levels. This is consistent with results from the previous experiment.

In order to verify the reliability of the simulation data, they were compared to experimental data. The sliding wear performance of the coatings can be characterized by measuring the surface profile of the coating wear scar. Figure 5 shows the wear depth of the coatings with various Al₂O₃ content. The wear depth of the Fe-based amorphous coating is the largest, reaching 44 μm, which indicates that the coating has incurred serious wear during the grinding process with the Si₃N₄ friction pair. With an increase of Al₂O₃ content in the second phase of the composite coating, the wear depth decreases gradually, and when Al₂O₃ content increases to 15 wt%, the wear depth is the lowest. When Al₂O₃ content is further increased to 20 wt% in the second phase, the wear depth no longer decreases, but it is still higher than that of the Fe-based amorphous coating. This conclusion is consistent with the results of simulation.

Since it is rather difficult to directly observe progression of friction on materials by in situ experiments with atomic-scale resolution, MD simulation can show the motion of atoms during the sliding process. It is beneficial to understand the wear mechanism, especially for metallic glassy composite materials. Figure 6 shows the surface morphology evolution of five models with different Al₂O₃ content at 50 ps, 100 ps, and 300 ps during the sliding process. The atoms are colored according to height. A groove can be seen that has formed in the surface, and both sides of the groove have obvious material accumulation. Meanwhile, with the sliding process, the wear depth of both Fe-based amorphous coatings and composite coatings increased. However, compared with the composite coating, the wear depth of Fe-based amorphous coating is deeper, and the wear scar width is larger. At a time of 50 ps in the early stage of wear, the visible surface height clearly increases at the interface between the Al₂O₃ ceramic phase and the amorphous phase (as circled in Figure 6 (c) at 50 ps). This is due to the different elastic modulus of the Al₂O₃ ceramic phase and amorphous phase. The deformation degree of the two phases is different at load, resulting in stress concentration at the interface. The high stress induces the development of shear bands in the amorphous phase, which promotes the large plastic deformation of the amorphous coating (Chu et al., 2019b). In composite coatings, a plastic deformation zone around the indentation is observed. With regards to the wear resistance of the material, besides high levels of strength and hardness, a good plastic deformation ability is also an important factor affecting wear resistance. In other words, a material that maintains high strength while also having good plasticity will achieve the purpose of resisting crack propagation. In Figure 6 (c) and (d), plastic deformation zones are observed in the 10% and 15% Al₂O₃ composite coatings. Although Al₂O₃ was added during the brittle phase, the material stills shows fatigue wear due to the sliding process. The high stress concentration between the amorphous phase and ceramic phase results in viscosity flow in the amorphous phase, and the increase in height can be observed in the composite coatings. However, when the addition of Al₂O₃ is more than 20%, the fatigue wear of the ceramic phase increases.

In this study, three compression depths of 5Å, 7Å, and 9Å are chosen in order to explain the effect of compression depth on the friction and wear behavior of Fe-based metallic glassy coating and 15 wt% composite models. In MD simulation, the friction speed is 1.0 Å/ps.

Figure 7 shows scratched surfaces showing pile-up atoms at the time of 400 ps with a speed of 1.0 Å/ps. It is clear that the wear degree of the substance surface gradually increases as the compression depth increases. This is because the number of atoms that push on both sides and from above increase relatively with an increase in compression depth. In other words, the friction volume accumulated in front of the indenter is relatively larger, which leads to a compression depth proportional to the wear rate. At the same time, the wear degree of the 15 wt% Al₂O₃ composite model is proportionally lower than that of the Fe-based metallic glassy coating model. This indicates that the abrasive resistance of the amorphous composite coating is higher than the Fe-based amorphous coating. As shown in Figure 8, the wear rate gradually increases with an increase of compression depth. It is worth noting that the wear rate of the composite coating is higher than the Fe-based amorphous coating at a compression depth of 5Å. Following an increase of compression depth, the wear rate of the composite coating is lower than pure Fe-based amorphous coating. This is because with an increased compression depth, the distance between the atoms that are situated between the indenter and the substances decreases, resulting in the increased resistance of the atoms. The compressed atoms on the surface of the material display compressional deformation. During friction, the number of atoms accumulated continually increases, and the level of plastic deformation becomes deeper. Above all, an increase in compression depth is positive for the wear of materials, and the wear resistance of composite coating is superior to Fe-based amorphous coating.

According to the classification of sliding speeds for friction (Fang et al., 2014; Li et al., 2014), a general frictional speed is less than 0.45 Å/ps, a high frictional speed is 0.45–1.5 Å/ps, and an ultra-high frictional speed is more than 1.5 Å/ps. To further analyze the effects of sliding speed on materials, we chose five speeds: 0.1 Å/ps, 0.4 Å/ps, 1.0 Å/ps, 1.7 Å/ps, and 2.7 Å/ps. During MD simulation, the compression depth is maintained at 7Å. Figure 9A shows the evolution of the worn particles of the 15 wt% composite model as a result of different timesteps. It was found that the number of wear particles gradually increases with the timestep. When the time reaches 1,500 ps, the rate of increase of the number of worn particles begins to slows down; the faster the sliding speed, the earlier the time of wear. As illustrated in Figure 9B, sliding speed has a great impact on wear in the initial stage. The wear rate increases with the sliding speed and then maintains a linear growth. In our study, the indenter was set to measure specific parameters, and we have ignored the effects of bond wear on materials. According to the analysis, the relative sliding speed and surface area increase relatively with an increase in sliding speed, eventually leading to an increase in the degree of wear.

Figure 9C shows the changes between sliding distance and friction coefficient at different sliding speeds. The friction coefficient generally decreases as the sliding distance increases. On the other hand, fluctuation of the friction coefficient mainly occurs at the early stage, after which it remains stable. As shown in Figure 9D, the average friction coefficient shows a trend of increasing, and then decreasing and remaining stable. The results show that changes in speed have great impact on the degree of wear of a material under general and high frictional speed. However, at an ultra-high frictional speed, there is no obvious influence on the degree of wear of the material.