3.1 Coefficient of friction and micrograph analysis
Figure 2 illustrates the micro-motion operating state of the K417/DD6 friction pair under different cycle times and normal loads of 84 N, 334 N, and 522 N. The Ft-D curve of the K417/DD6 friction pair under different cycle times is presented in Fig. 2(a). Figure 2(a) shows that the Ft-D curves are all of the flat quadrilateral type under different cycle times, indicating complete slip between the contact surfaces of the friction pair when Fn=84 N. The friction fluctuation with displacement amplitude was significant under this parameter condition, which was consistent with the friction coefficient results. The abrasive debris in the contact area undergoes drastic evolution during micro-motion operation. Figure 2(b) shows that during the early stage of micromotion wear, the micromotion operation state graph still displays a parallelogram shape, indicating that the micromotion operation state is still within the complete slip zone when Fn=334 N. As the cycle period increases (N ≥ 4×105), the Ft-D curve gradually transforms into an elliptical shape. As the cycle period (N ≥ 4×105) increases further, the Ft-D curve transforms gradually to the elliptic type. This indicates that the micro-motion operation state in the contact area is transitioning from the complete slip zone to the mixed zone. From Fig. 2(c), it is evident that increasing the normal load to 522 N results in a complete slip state during the initial stage of micromotion operation. Furthermore, when the cyclic cycle is increased beyond N ≥ 104, the Ft-D curves become flattened elliptic type. Due to the gradual increase in load, contact stress also increases, causing a decrease in relative slip between the friction pair, and leading to the gradual dominance of elastic deformation in the contact region.
Figure 3 shows the trend of the friction coefficient under different normal load conditions at a high temperature of 750°C. It can be observed that the friction coefficient fluctuates to varying degrees under different normal loads. The change process of the friction coefficient can be divided into three stages: rising, falling, and stabilization, as the number of cycles increases. During micro-motion operation, hard micro-convex bodies on the surface of K417 and DD6 alloys cause roughness differences. Simultaneously, the oxide film generated on the surface of the two contacting bodies in a high-temperature environment results in greater friction under load, leading to increased material wear. This causes fluctuations in the coefficient of friction curve, known as the break-in stage. It is important to note that this stage is characterized by a greater degree of wear of the material. During the micro-motion experiment, the micro-convex body and the oxide film layer rupture, leading to the gradual exclusion and compaction of abrasive debris. This results in the wear entering a stable stage, causing a decrease in the magnitude of the fluctuation of the coefficient of friction. According to Fig. 3(b), the highest friction coefficient and overall fluctuation occur when the normal load is 84 N. It is important to note that the friction coefficient gradually decreases and stabilizes as the normal load increases.
3.3 Micro-motion wear mechanism analysis
Figure 6 shows the SEM micro-morphology of the abrasion marks on the surface of K417 nickel-based alloy after high-temperature micro-manipulation wear tests under different loading conditions. The surface of the wear marks exhibits delamination and furrow-like morphology features, with fine lumpy abrasive bodies observed at the delamination. The wear that occurs under these parametric conditions is primarily dominated by abrasive wear, adhesive wear, and oxidative wear. Compared to Fig. 6(a), Fig. 6(b) shows an enhancement of the adhesion effect with an increase in normal load, resulting in large-scale delamination in the wear region. A large number of fine and loose abrasive chips are scattered at the delamination, leading to the formation of groove-like morphological features on the surface of the three-body layer under the action of secondary micro-motion [27]. Under this parameter condition, the micro-motion operation state is in the complete slip region. Increasing the normal load results in an increase in the contact area and an enhancement of the adhesion effect, leading to large-scale delamination on the surface of the three-body layer. Under conditions of Fn=334 N, the wear forms mainly consist of abrasive wear, oxidative wear, adhesive wear, and fatigue delamination. Figure 6(c) shows that the three-body layer structure has gradient characteristics, and its main formation mechanism is the continuous evolution of third-body abrasive debris in the contact region. As the load increases, the adhesive effect in the contact region is enhanced, leading to adhesive wear. This causes changes to the morphology of the crater on the surface of the three-body layer and results in a large amount of abrasive chips overflowing from the sub-surface region of the three-body layer. The abrasive chips were continually refined, milled, and oxidized under the action of tangential force. Part of the abrasive chips are discharged out of the contact interface, while the other part was coated on the surface of the material. This results in the plow furrow-like morphology characteristic of the surface of the abrasive marks in the process of repeated micro-movement. The wear at this stage is mainly abrasive, adhesive, and oxidative.
Figure 7 displays the BSE morphology of the abrasion cross-section and the corresponding EDS line swept area under different normal load conditions. It can be observed from Fig. 7(a) that the third-body layer's structure is dense and smooth under the normal load condition of Fn=84 N. The overall gradient structure is formed by grey oxides and white oxides, and the third-body layer has a high degree of bonding with the substrate material. The third-body abrasive chips are fully melted and coated on the material's surface due to the high-temperature environment and heat generated in the friction process. The element O content shows oxygen-rich and oxygen-poor zones. In the oxygen-rich zone, the content of element Ni is relatively low, and the content of element Al is relatively high. In the oxygen-poor zone, the content of element Al and element Ni are relatively high. This phenomenon is analyzed to be possible. The surface of the abrasion marks on Al2NiO4 spinel oxide is composed of grey oxides in a three-body layer, as shown in Fig. 7(b) for a normal load of Fn=334 N. Increasing the normal load leads to higher contact stress in the contact area, causing noticeable cracks in the three-body layer. The O element content in the three-body layer is higher than that of other elements, and there is no apparent enrichment phenomenon. This is consistent with its structural state. At a normal load of Fn=522 N, the micromotion operation state is in the mixing zone, which causes the rupture of the three-body layer due to the further increase of the normal load, resulting in increased stress in the contact center region. Furthermore, the three-body layer comprises of grey oxide at the upper end and light grey oxide at the bottom. The overall structure is relatively loose and poorly bonded to the matrix material. The O element content is higher than that of other elements in the surface area of the three-body layer, with a certain enrichment of Cr and Co elements in this region. The adhesion effect is enhanced in the contact area due to the large normal load, resulting in more elements being involved in the oxidative wear process. The content of the O element sharply decreases in the middle region of the three-body layer, while an enrichment area of the O element exists in the bottom region of the three-body layer. The micro-movement process of the three-body layer involves a continuous cycle of repair and destruction. Under high load conditions, the combination of delamination caused by adhesive wear and oxidative wear results in the three-body layer structure exhibiting varying oxygen content in high-temperature environments.
Figure 8 displays the metallographic profile of K417 cross-section wear under different normal loads. The image reveals that a dense three-body layer formed in the high-temperature environment, which provides excellent protection and a good combination with the matrix. The overall structure of the three-body layer in the contact center area remains undamaged, and the γ'-strengthened phase only undergoes slight deformation during the continuous micro-motion process of the matrix surface layer. However, in the edge region of the three-body layer, cracks form at different angles due to the continuous stress effect. Additionally, the protective effect of the three-body layer is reduced in the region where its thickness crosses the thinning, resulting in a smaller deformation region. Figure 8(b) shows that an increase in normal load leads to structural rupture and micro cracking in the top region of the three-body layer due to high contact stress. The bonding between the three-body layer and the base material in the contact center remains strong. However, in the edge zone, the contact between the three-body layer and the base material starts to detach, leading to a decline in the protective effect on the base material. The deformation of the γ'-strengthened phase in the edge zone is greater, resulting in the formation of a deformation zone with a certain thickness. When the normal load Fn was increased to 522 N, the stress in the contact region increased, causing the three-body layer to begin separating from the base material. As a result, the protective effect of the three-body layer disappeared, as shown in Fig. 8(c). This indicates a separation trend between the two materials, which was not observed under the first two parameters during the cross-section metallography. Due to the high stress in the contact area under large load conditions, the adhesion effect becomes stronger, resulting in a decrease in the bonding degree between the three-body layer and the K417 high-temperature alloy, causing them to separate from each other. As micro-movement continues, the top region of the three-body layer develops micro-cracks, leading to crushing and eventually forming a crater.
Figure 9 displays the three-dimensional morphology of the white light of the surface abrasions of K417 alloy under different loading conditions and the two-dimensional contour curves of the corresponding positions. The three-dimensional morphology reveals a significant accumulation of abrasive chips due to the adhesion effect under different parameter conditions. This is primarily due to the material softening to varying degrees in the high-temperature environment. Upon comparing the profile curves of the wear marks, it is evident that an increase in normal load from 84 N to 334 N results in an increase in the width of the wear marks, the height of the bulge in the wear area, which reaches 32.18 µm, and the depth of the wear marks, which increases to 16.56 µm. Combined with the analysis in Fig. 2, the contact area increases as the load increases due to both parameter conditions being in a state of complete slip. The width of the wear marks also increases under the action of the repetitive tangential force. Additionally, the wear mark width increases. Under repeated tangential force, the width of the abrasion marks increases. Simultaneously, an increase in normal load enhances the adhesion effect in the contact area, increasing the height of abrasion mark accumulation. When the load was increased to 522 N, the height of the abrasive chip accumulation decreased, and the depth of the abrasive mark was reduced. This occurred because the micromotion running condition was in the mixed zone, causing the center contact area to be in the adhesion state due to the larger load, while the edge area was in the slip state. As a result, the depth of the abrasion mark edge was larger compared to the center area. It should be noted that comparing the damage results under the three-parameter conditions reveals that increasing the load condition causes the micromotion operation to transition from the complete slip zone to the mixed zone, resulting in a reduction in the degree of material damage.
The wear volume and material wear rate of K417 nickel-based high-temperature alloy were characterized under different loading conditions using a white light interferometer. The results are presented in Fig. 10. Figure 10(a) illustrates that the wear volume initially increases and then decreases with an increase in normal load, reaching a maximum value of 0.016 mm3 at a normal load of 334 N. Additionally, the wear rate exhibits an inverse relationship with the normal load. The increase in load enhances the adhesion effect between the contact areas, causing the micromovement running area to gradually transition to the mixing area, thereby reducing the loss of material. Please refer to Fig. 10(b) for a visual representation.
The work done by the friction force, which is also known as the material dissipation energy, is used to characterize the material damage in micromotor wear. This is obtained by integrating the closed-loop curve in the friction-displacement curve [28]. The dissipated energy for a single cycle can be calculated using the following equation:
\(\:{E}_{d}=\int\:fd\delta\:\) (3 − 1)
In Eq. (3 − 1), Ed represents the dissipated energy in a single cycle, f represents the friction force, and ∆δ represents the change in displacement amplitude.
The Ft-D curve in Fig. 1 is integrated using this equation to obtain the energy dissipation value for the corresponding number of cycles. The results of this calculation are shown in Fig. 11. It is evident from the figure that the dissipation energy of the material increases significantly with an increase in normal load. During the initial phase of micro-movement, the damage caused is more severe as the substantial contact object is the oxide film on the surface of the upper and lower specimens. When the contact state is dominated by three-body contact, the three-body layer can alleviate wear, resulting in a smaller energy dissipation value during the stabilization phase. Under small normal loads, the material undergoes elastic-plastic deformation, which helps to alleviate material damage and results in lower friction dissipation energy values. At a load of 334 N, the micromotor remains in a state of complete slip. As the load increases, the contact area also increases, accelerating the loss of abrasive chips during the wear process. This leads to greater damage to the material and an increase in dissipated energy. These findings are consistent with the wear depth and volume results. At a normal load of 522 N, the micromotion operation area is in the mixing zone. The increased load condition reduces the likelihood of relative slip in the central contact area, which in turn reduces the rate of abrasive chip spillage and minimizes material damage.
3.4 Wear zone chemical composition analysis
The results of EPMA electron microprobe surface scanning of K417 nickel-based high-temperature alloy wear scar surfaces under normal load conditions of Fn=84 N, Fn=334 N, and Fn=522 N are presented in Fig. 12. The analysis of the O element distribution reveals that the O content in the wear zone is higher than that in the unworn zone under all three parameter conditions. This suggests that the wear zone is no longer solely dependent on friction for chemical action in the contact area, especially in high-temperature environments. Additionally, the high-temperature environment facilitates the oxidation of O with the metal abrasive chips in the contact area, which ultimately results in the formation of a tristimulus layer on the material surface due to the tangential force. The wear region was found to contain the DD6 characteristic element Ta under various normal load conditions, indicating material transfer during micro-movement. Figure 12(a) shows a relatively homogeneous distribution of Ni, Cr, and Co elements in the wear region under a normal load of Fn = 84 N, with an enrichment of O element distribution. The analysis indicates the presence of spinel oxide NiCr2O4 in the enriched area of O, Ni, and Cr elements located at the top of the abrasion mark. When the load is increased to 334 N, the distribution of oxygen elements becomes more uniform compared to the distribution under small load conditions. At this point, the micromovement running state is in the complete slip zone, and the increased load improves the contact area. This results in the third body chips being more uniformly coated on the surface of the wear marks, as shown in Fig. 12 (b). The wear area shows a significant enrichment phenomenon in the distribution of Cr and Co elements. This is attributed to the transfer of materials caused by the high concentration of Cr and Co elements in DD6. It is important to note that this statement is based on objective observations and not subjective evaluations. Figure 12(c) shows that as the normal load increases to 522 N, the wear region experiences varying degrees of enrichment in O, Ni, Cr, and Co elements. The micromanipulation operation state is in the mixing at this point, and the increase in load enhances the adhesion effect in the contact region.