The friction-induced microstructures changes of 18Cr-8Ni austenitic stainless steels with different grain sizes

The friction and wear performance, wear morphology and friction-induced subsurface microstructural characteristics of 18Cr-8Ni austenitic stainless steels with coarse-grained (CG), heterogeneous ultrafine-grained (HUFG), and nano/ultrafine-grained (NG/UFG) microstructures after dry sliding wear under room temperature were studied. The results reveal that HUFG steel with a good match between hardness and plasticity exhibits the best wear performance, followed by CG steel, while NG/UFG steel with the highest hardness exhibits the poorest wear performance. From the element distribution map, the contents of O and Si in the delamination and wear debris are relatively high. O is relatively evenly distributed on the whole wear surface of HUFG steel, and there is a continuous oxide layer on its wear surface. After the wear test, the hardness increment near the wear surface of the CG sample is the largest, and the depth affected by sliding is the largest, followed by the HUFG sample, and those of the NG/UFG steel are the smallest. The repeated frictional shear stress causes the formation of cracks between the mechanical mixed layer and the plastic deformation layer, and the continuous expansion of cracks can help oxygen elements diffuse deeply, causing the deformation layer materials to fall off.


Introduction
Nano/ultrafine-grained (NG/UFG) metallic materials with mixed nano-grained and ultrafine-grained structures usually exhibit excellent strength, hardness, fatigue resistance and wear resistance [1][2][3][4], which gives them good prospects in structural and functional applications.Researchers have conducted in-depth studies on the mechanism of deformation, the optimization of the microstructure and the matching of the strength and plasticity of NG/UFG materials [5][6][7][8].Recent studies have shown that the cross-scale construction of materials can coordinate the properties controlled by different characteristic sizes, which can maximize the performance of materials [9].A novel investigation has indicated that a heterogeneous structure prepared by conventional cold rolling and subsequent annealing treatment exhibit a combination of high strength and high ductility for austenitic stainless steels [10,11].The microstructures of heterogeneous materials are characterized by micron or submicron grains (low strength) embedded into their nano-grained matrices (high strength) [12].The good strength ductility synergy is attributed to the plastic deformation gradient generated by geometrically necessary dislocation stacking and by accumulation around the soft/hard interfaces [13].
Wear resistance is an important factor that determines the service lifespans of metals and plays a crucial role in technology and engineering applications.Austenitic stainless steels have widespread applications in human′s livelihood, industries and military fields, as a result of their excellent corrosion resistance, oxidation resistance, forming performance and welding properties [14,15].However, the usage of austenitic stainless steels is limited due to their relatively low yield stress and hardness values [16,17].In addition, austenitic stainless steel has low wear resistance, which shortens its service life, especially in the case of dry sliding wear [18,19].Extensive investigations have shown that, NG/UFG austenitic stainless steels exhibit enhanced friction and wear properties relative to their CG counterparts [20][21][22][23].The enhanced wear resistance of NG/UFG materials is mainly attributed to their high hardness, which can effectively resist friction pair microcutting.In addition, other studies have shown that the wear resistance of NG/UFG materials depends not only on the hardness, but also on the external friction conditions [24][25][26].However, the wear behaviors and the wear mechanisms of austenitic stainless steels with heterogeneous structures are still insufficient.
The chemical composition, microstructure, grain size and phase distribution characteristics of austenitic stainless steel influence the wear mechanisms [4,27,28].Sliding wear of metals is a complex process, and a series of structural evolutions take place under the wear surface.As wear progresses, fracture, chemical reaction, material transfer and mechanical mixing continue to occur, and plastic deformation is generated beneath the wear surface, forming a wear-induced layer [29].Many studies have been devoted to revealing the underlying wear mechanism of austenitic stainless steel and have confirmed the existence of a mechanical mixed layer and a plastic deformation layer [30][31][32].The microstructure in the plastic deformation layer experiences severe plastic deformation, resulting in a nanocrystalline structure and very high hardness.In addition, due to the effect of friction stress, strain-induced martensitic transformation occurs, causing changes from austenite to εmartensite and/or to α′-martensite.Understanding the relationship between the microstructures and underlying wear mechanisms of NG/UFG austenitic stainless steel during sliding is helpful for optimizing the friction and wear properties.
Previous studies have mainly focused on the wear properties and the change in wear morphology of NG/ UFG austenitic stainless steel, while the evolution process of the subsurface microstructure has rarely been reported.In the present study, the dry sliding friction and wear properties of NG/UFG and heterogeneous ultrafine-grained (HUFG) austenitic stainless steels are investigated.The microstructure characteristics and failure mechanisms of the wear subsurfaces of austenitic stainless steels with different grain sizes are analyzed.

Materials and experimental procedures
A commercial 18Cr-8Ni austenitic stainless steel sheet with a thickness of 4.5 mm was used for the studied materials.The nominal chemical composition (wt%) was as follows: C, 0.04; Si, 0.39; Mn, 1.09; Cr, 17.94; Ni, 7.92; Cu, 0.0379; N, 0.047; and Fe, balance.The average grain size of the as-received coarse-grained (CG) austenitic steel was 24 μm.To obtain austenitic stainless steels with different grain sizes, the as-received stainless steel plates were cold rolled at a low temperature (−50 °C) and annealed at appropriate temperatures.The cold rolling experiment was carried out on a Φ325 mm × 400 mm cold rolling mill, and the annealing experiment was carried out by an electrical resistance furnace.When the cold rolling deformation was 86% and the annealing conditions were a temperature of 650 °C and time of 50 min, NG/UFG steel was obtained.When the cold rolling deformation was 64% and the annealing conditions were a temperature of 750 °C and a time of 15 min, HUFG steel was obtained.
A FM-700 microhardness tester was adopted to determine the Vickers microhardness (load of 10 g and holding time of 10 s).The microhardness tests were repeated three times under the same conditions, and the average values were taken as the final results.Perpendicular to the rolling direction, the annealed austenitic stainless steel sheets were cut into dog-bone-shaped tensile samples with a gauge length of 10 mm and width of 3.8 mm.Then, uniaxial tensile tests at room temperature (crosshead speed of 1 mm min −1 ) were performed in a testing machine (GMT7000 SANS).
Prior to sliding wear tests, austenitic stainless steels with different grain sizes were cut into samples 12 mm in length and 10 mm in width using electrical discharge machining.All sample surfaces were mechanically polished by sandpaper down to 1500 grit to remove surface defects, and then mechanically polished to ensure good surface smoothness.The dry sliding wear tests were performed on a ball-on-plate wear tester (MFT 5000) under constant normal loads of 30 N at a sliding velocity of 5 mm s −1 with a reciprocating stroke of 2.5 mm.The experimental equipment and working principle are shown in figure 1.A Si 3 N 4 ceramic ball with a diameter of 4 mm was selected as the sliding ball, which was held by an elastic clamp and applied positive pressure to the surface of the sample.The wear experiments were repeated three times under the same conditions, and the average values were taken as the final wear volumes.
The microstructures of the austenitic stainless steels with different grain sizes were observed by EBSD using a ZEISS ULTRA 55 field-emission scanning electron microscope (SEM).For EBSD analysis, the samples were prepared by grinding and electrochemical polishing.The electrochemical polishing solution is C 2 H 5 OH: HClO 4 : H 2 O = 13:2:1 (volume ratio).The electrochemical polishing voltage is 22 V and the time is 20 s.The morphologies of the wear surface and cross-section were examined by SEM.Microstructural analysis was carried out for the sliding ball surface after sliding wear by using an optical microscope (Nikon Eclipse MA100).
A field emission electron probe microanalyzer (EPMA, JEOL JXA-8530F) was used to analyze the chemical element distribution of the wear surface and cross-section.

Results and discussion
3.1.Microstructures and mechanical properties of stainless steels with different grain sizes Figure 2 presents the microstructural characteristics of the austenitic stainless steels with different grain sizes.When the cold rolled sample is annealed at 650 °C for 50 min, the NG/UFG microstructure can be obtained, which includes equiaxed nano/submicron-sized grains and micron-sized grains.The grain size distribution is relatively uniform, with an average grain size of 400 nm, as shown in figure 2(a).The cold rolled specimen annealed at a high temperature of 750 °C for 15 min presents a typical heterogeneous character in which the micron-sized grains are surrounded by nano/submicron grains (figure 2(b)).The grain size of HUFG steel across multiple length scales has an average grain size of 890 nm.Nano/submicron grains are mainly derived from strain-induced martensitic reverse transformation, while large micron sized grains are mainly related to recrystallization of cold rolled deformed austenite grain [33].According to previous study, after 86% cold rolling deformation, the content of strain-induced martensite can reach up to 97% [6].The microstructure of NG/UFG steel is mainly obtained from strain-induced martensitic reverse transformation, and its grain size is finer and more uniform compared to HUFG steel.As presented in figure 2(c), the CG steel exhibits a homogeneous microstructure with an average grain size of 24 μm.Some annealing twins can be observed.Figure 2(d) displays the mechanical properties of NG/UFG, HUFG and CG steels.The yield strengths of NG/UFG, HUFG and CG steels are 1017 MPa, 684 MPa and 298 MPa, respectively.Accordingly, the ductility values of NG/UFG, HUFG and CG steels are 23%, 54% and 74%, respectively.Due to the significant refinement of grain size in HUFG and NG/UFG steels, their grain boundary area increases, and the hindrance of grain boundaries to dislocations increases, resulting in a significant increase in their yield strengths.The hardness changes in austenitic stainless steels with different grain sizes are consistent with those of the yield strength, and the hardness decreases with increasing grain size.The hardnesses of NG/UFG, HUFG and CG steels are 343 HV, 274 HV and 190 HV, respectively.

Friction and wear behaviors of stainless steels with different grain sizes
Figure 3 shows the 3D surface topographies and the corresponding cross-sectional profiles of the NG/UFG, HUFG and CG steels.From the 3D topographies, the wear scar depth and width values of the experimental steels are different, as shown by the color change from red (highest) to blue (deepest).The wear scars result from severe plastic deformation induced by the indentation of the Si 3 N 4 friction pair.Some deformed materials are pushed to both sides of the wear scar, forming ridges along both sides of the groove.A certain amount of fine wear debris is observed on the wear surface of the experimental steels.The pile-ups of deformed material are observed on both sides of the wear scar, and the pile-up material of CG steel is the most abundant.According to the calculation, the wear volumes of NG/UFG, HUFG and CG steels are 15.6×10 −3 mm 3 , 13.5×10 −3 mm 3 and 14.8×10 −3 mm 3 , respectively.This finding indicates that the wear resistance of NG/UFG steel is the poorest, HUFG steel is the best, and CG steel is in the middle.Figure 4 shows the morphologies of the wear scar edge of NG/UFG, HUFG and CG steels.Deformed material pile-ups and wear debris are observed at the edges of the wear scars of the three experimental steels.It is worth noting that the degree of deformation of the microstructure at the edge of the wear scar varies.The surface at the edge of the wear scar of NG/UFG steel is relatively flat, and no obvious plastic deformation is observed.The surface at the edge of the wear scar of HUFG steel becomes uneven, resulting in plastic deformation, but no shear band is observed.However, a large number of shear bands are observed on the surface of CG steel at the edge of the wear scar.In most metal materials, slip bands are produced by strain localization, and the dislocation source cannot exceed the grain size.When the grain size decreases, high stress is required to move dislocations [34].NG/UFG steel cannot accommodate high-density dislocations, which effectively inhibits the occurrence of strain localization.
Figure 5 shows the wear surface morphologies and element distributions on the wear surfaces of NG/UFG, HUFG and CG steels.The wear surfaces of austenitic stainless steels exhibit different characteristics as the grain size increases.Sliding grooves are observed on the wear surfaces of the NG/UFG, HUFG and CG steels, which are consistent with the abrasive wear characteristics of metals and related to plastic deformation work and fracture [35].In addition, the wear surface of NG/UFG steel is characterized by delamination, spalling, cracks  and wear debris.After repeated reciprocating sliding motion, large oxide patches and cracks appear on the rough wear surface, which is a typical characteristic of delamination, indicating the oxidation wear mechanism.As sliding progresses, the surface material peels off and forms many spalling pits [36].A large amount of wear debris is observed on the wear surface.Some of this debris may come from the hard particles removed from the surface during the sliding process [37], and the rest of the debris may be the fine wear particles formed by the delamination breaking off [38].
According to the element distribution maps, in addition to the original Fe, Cr, and Ni in the steel, O and Si are also found on the wear surface of the experimental steel.The presence of O occurs due to a large amount of heat generated between the sample surface and the friction pair during the sliding wear process, forming an oxide layer, which further confirms the existence of oxidation wear.Moreover, the oxide layer may delaminate with the reciprocating motion between the friction pair and the experimental steel, and then form spalling pits.The Si detected on the wear surface appears due to the migration of Si from the Si 3 N 4 ball to the wear surface during the sliding process, which is the result of a chemical reaction [39].The distribution of O on the wear surface is discontinuous, and the contents of O and Si in the delamination and wear debris are relatively high.The wear surface of NG/UFG steel has many broken delamination and spalling pits, exposing the fresh metal layer that has not been oxidized.The exposed fresh surface contains very low O content relative to the unbroken delamination.The oxide layer is brittle in nature, and it is damaged and peeled off under the reciprocating friction of the friction pair, resulting in the removal of a large number of materials on the surface.
In contrast, the wear surface of HUFG steel is relatively flat.Delamination and cracks appear on the wear surface, but large peeling pits and large amounts of fine wear debris do not appear.From the element distribution map of HUFG steel, O is relatively evenly distributed on the whole wear surface, and there is a continuous oxide layer on the wear surface.There are relatively few broken layers on the wear surface and few exposed fresh metal layers that are not oxidized.These oxide layers can play a good role in protection and lubrication, thus effectively preventing the friction pair from direct contact with the new metal surface and diminishing abrasive wear [40].The wear surface morphology of CG steel is different from that of NG/UFG and HUFG steels.On the wear surface of CG steel, some discontinuous delamination with high O content and some cracks can be observed.In addition, there are grooves and deformation areas caused by plowing.The O contents in these areas are very low, indicating that there is no obvious oxidation phenomenon.
Figure 6 shows the surface morphologies of Si 3 N 4 ball friction pairs of NG/UFG, HUFG and CG steels.After the wear test, elliptical plane wear tracks are formed on the Si 3 N 4 balls.As the hardness values of NG/UFG, HUFG and CG steels decrease, the wear degrees of Si 3 N 4 balls decrease gradually.The area of the Si 3 N 4 ball wear scar corresponding to NG/UFG steel is the largest, followed by HUFG steel, and CG steel is the smallest.When the hardness of austenitic stainless steel increases, its ability to resist the wear of friction pairs increases, and the Si 3 N 4 ball loss increases.In addition, large amounts of adhesive materials and wear debris can be observed on the wear surface of the Si 3 N 4 ball.By comparing the amounts of adhesive materials and wear debris on different friction pairs, it can be found that the number of Si 3 N 4 balls corresponding to NG/UFG steel is the largest, followed by CG steel, and HUFG steel is the smallest.This phenomenon is consistent with the abovementioned results of wear morphology; that is, a large amount of wear debris is generated on the wear surface of NG/UFG steel, and the wear debris on HUFG steel is the least.
Curves of micro-Vickers hardness as a function of distance from the wear surface are shown in figure 7. The microhardness test is conducted at an interval of 10 μm beneath the wear surface.The microhardness values of the steels with different grain sizes have similar trends, which gradually decrease from the wear surface to the interior of the samples.The variations in the microhardness values of the three steels are significantly different.After wear tests, the hardness near the wear surface of the CG sample is the largest; after a depth of 100 μm, the drop is dramatic from 391 to 200 HV.The wear surface hardness value of the HUFG steel reaches 352 HV, which is lower than that of the CG sample, and the depth affected by sliding is approximately 70 μm.However, the NG/ UFG sample has the highest matrix hardness (300 HV), and the smallest increases in hardness (344 HV) and affected depth (40 μm) after the wear test.The affected depths beneath the wear surfaces of the samples are related to the deformation layer, which occurs due to the strain hardening effect.The CG steel has the strongest hardening ability, which may be because the CG can accommodate further dislocations, and the dislocation density contributes greatly to the hardness.A similar microhardness variation has been reported by other researchers, and they considered that the hardness increase is mainly attributed to the sliding wear-induced plastic deformation and refined microstructure [41,42].For metastable austenitic stainless steel, strain-induced martensite is produced during sliding wear [43,44], which also contributes to the hardness increase.
Figures 8-10 show the SEM morphologies and elemental distributions of the wear scar cross sectional microstructure of NG/UFG, HUFG and CG steels.In the process of dry friction and wear, the material surface undergoes very complex changes, such as microstructure evolution, severe plastic deformation, surface oxidation and elemental diffusion on the contact surface.These changes lead to significant differences between the microstructures of the wear surfaces, subsurface materials and matrix materials.The friction-induced layers of the three experimental steels contain two different sublayers: the mechanical mixed layer and the deformation layer.The top layer near the surface is a thin mechanical mixed layer composed of fine-grained mixed materials, and its formation process is related to deformation, transfer, chemical reaction and mechanical mixing [45].Studies have shown that the structure of the mechanical mixed layer is considered to be highly nonequilibrium, characterized by large numbers of grain boundaries and high-density dislocations in the structure [46].Previous research has demonstrated that HUFG steel with the thickest mechanical mixed layer has the best wear performance, which is related to their good plasticity and work hardening ability [4].The following is the main analysis of how the mechanical mixing layer is formed.
It has been reported that the composition of this layer is different from that of the deformation layer and the undeformed material [47,48].EDS analyses indicate that the mechanical mixed layers of austenitic stainless steels with different grain sizes have considerable amounts of oxygen, indicating that they contain many oxides.The frictional shear force causes a large amount of accumulated strain under the wear surface, and the subsurface structure undergoes severe plastic deformation, resulting in refinement, bending, fracture, and fragmentation.From figures 9(a) and 10(a), microcracks are observed on the wear subsurface of the HUFG and CG steels.These cracks start at the interface between the mechanical mixed layer and the plastic deformation layer, continuously extending into the plastic deformation layer and parallel to the sliding direction.The formation of these cracks is caused by cyclic contact stresses under multiple sliding [49].mixed layer enter the crack, filling the inside of the crack with oxides.However, no cracks are observed on the wear subsurface of NG/UFG steel, but spalling pits are left after material fracture, as shown in figure 8(a).The spalling pit near the mechanical mixed layer contains a certain amount of O, while the part near the plastic deformation layer does not contain O (figure 8(b)).This finding indicates that after the material falls off, the exposed fresh surface undergoes oxidation again.It can be inferred that the formation of the mechanical mixing layer is closely related to the plasticity of austenitic stainless steel.Among the three experimental steels, NG/UFG steel has the worst plasticity, resulting in rapid crack propagation and material detachment in its deformation layer.CG steel has the best plasticity, and long cracks and a portion of stripped material can be observed on its deformation layer.
Based on the above analysis, it is believed that the crack formation process between the mechanical mixed layer and the plastic deformation layer during the wear process is as shown in figure 11.During the wear process, under the action of repeated frictional shear stress, the fatigue mechanism leads to the formation of cracks between the mechanical mixed layer and the plastic deformation layer [49].As the wear process progresses, cracks continue to propagate toward the interior of the plastic deformation layer, accompanied by the continuous diffusion of oxides in the mechanical mixed layer into the cracks, ultimately leading to the detachment of the plastic deformation layer material.As dry sliding wear continues, plastic deformation accumulates, generating heterogeneous deformation substructures, making near-surface materials prone to local shear instability [29].After the outermost layer of the mechanical mixed layer is layered and broken, the stripped material undergoes deformation, oxidation, and fragmentation under repeated frictional shear forces, forming a new mechanical mixed layer again.

Conclusions
The friction and wear experiments of NG/UFG, HUFG and CG austenitic stainless steels at room temperature were studied.The effects of grain size on the wear properties, wear morphologies, and friction-induced subsurface microstructures of austenitic stainless steels were analysed.The main results are as follows: (1) The friction and wear properties of NG/UFG, HUFG and CG austenitic stainless steels are not directly determined by hardness; instead, they are closely related to the microstructure.The hardness of NG/UFG is the highest, but its friction and wear properties are poor.The performance of HUFG is the best, followed by CG steel.
(2) Under the action of frictional heat, oxidation occurs on the wear surface.There are large amounts of delamination and debris with high oxygen contents on the wear surface of NG/UFG steel, which are products of oxidative wear.The distribution of oxygen elements on the wear surface of HUFG steel is uniform, forming a continuous oxide layer.
(3) The influences of friction on the microstructures of the subsurface layers of experimental steels with different grains are different.The subsurface structure of NG/UFG steel with the highest hardness is the shallowest when affected by friction, and the hardness increment is the smallest.The hardness increment near the wear surface of the CG steel is the largest, and the depth affected by friction is the largest, followed by the HUFG steel.
(4) Reciprocating friction causes cracks between the mechanical mixed layer and the plastic deformation layer.The continuous expansion of the cracks is accompanied by the diffusion of oxygen, which eventually causes the deformation layer to fall off.

Figure 1 .
Figure 1.Working principle schematic diagram of friction and wear experiment equipment.

Figure 2 .
Figure 2. EBSD micrographs and grain size distribution diagrams of (a) NG/UFG; (b) HUFG; (c) CG, and (d) mechanical properties of the austenitic steels with different grain sizes.
From the distribution map of Fe, O, and Si around the microcracks of HUFG steel (figure 9(b)), a large amount of O and a small amount of Si are distributed on the mechanical mixed layer, which is consistent with the detection results of O and Si on the wear surface.Positions 1, 2, and 3 in figure 9(b) represent the microstructure of the plastic deformation layer, the location of crack initiation, and the middle of the crack, respectively.No presence of O is found in position 1.Positions 2 and 3 contain certain amounts of O, and the content of O at position 2 is relatively high.A similar phenomenon is observed on the wear subsurface of CG steel (figure 10(b)).Cracks separate a portion of the material from the matrix.As the wear process progresses, oxides in the mechanical

Figure 7 .
Figure 7. Variation of hardness as a function of distance from the wear surface after wear tests.

Figure 8 .
Figure 8.(a) cross section morphologies and (b) EDS analyses of the friction-induced layer of the NG/UFG.

Figure 9 .
Figure 9. (a) cross section morphologies and (b) EDS analyses of the friction-induced layer of HUFG.

Figure 10 .
Figure 10.(a) cross section morphologies and (b) EDS analyses of the friction-induced layer of CG.