Iron-based alloy coatings with different solid lubricants of WS2 (mass fraction 0-8.0%) and CaF2 (mass fraction 5.0%) and different solid lubricants of h-BN (mass fraction 0-2.0%) and CaF2 (mass fraction 0-2.0%) were prepared on the surface of CL60 wheel steel by laser cladding technology. The microstructure, dry sliding friction and wear behavior and wear mechanism of the iron-based alloy coatings with different solid lubricants were compared. The results show that all coatings are mainly composed of dendrites and eutectic structures, and the surface hardness reaches about 800HV, which is about twice that of CL60 steel. With the increase of WS2 content, the friction coefficient of WS2+CaF2/iron-based alloy coating decreases, and the wear mass loss first decreases and then basically stabilizes. When the WS2 mass fraction is 6.0%, the wear mass loss is the lowest, which is 26.7% lower than that of the iron-based alloy coating without solid lubricant. At this time, the pores are the least, the wear surface damage is slight, and the wear mechanism is abrasive wear. With the increase of h-BN content and the decrease of CaF2 content, the friction coefficient and wear mass loss of h-BN+CaF2/Fe-based alloy coating first decrease and then increase. When the mass fraction of CaF2 and h-BN is 1.0%, the stable friction coefficient and wear mass loss are the smallest, which are reduced by 32.7% and 33.3% respectively compared with the Fe-based alloy coating without solid lubricant. At this time, the grains are the finest, the structure is the densest, the wear surface is mainly fine wear marks, and the wear mechanism is slight abrasive wear.

The increase of vehicle speed and load makes the wear and damage of railway vehicle wheel-rail system increasingly serious. In particular, when railway vehicles run on small radius curve sections, there are problems of flange-track angular contact and large slip, which leads to severe sliding friction, thereby greatly reducing the service life of the wheel [1-2]. Therefore, it is urgent to carry out research on improving the strength and friction and wear properties of wheel materials. Laser cladding technology is an advanced surface modification technology [3]. This technology strengthens the substrate surface by cladding a high-performance alloy coating on the substrate surface to improve the wear resistance and fatigue resistance of the substrate surface [4-7]. In recent years, a large number of researchers have introduced solid additives into the cladding material to regulate the performance of the surface laser cladding layer [8-9]. Wang et al. [10] clad an iron-based alloy coating on the surface of wheel-rail materials and added an appropriate amount of La2O3 during the laser cladding process to make the coating grains finer, and further improve the wear resistance and rolling contact fatigue resistance. Ding Haohao et al. [11] prepared alloy coatings on the surface of train wheel materials by using mixed cladding materials composed of hexagonal boron nitride (h-BN), CaF2 and iron-based alloy powders in different proportions. The rolling friction and wear test results showed that the addition of h-BN+CaF2 had a positive effect on reducing the friction coefficient of the cladding layer. When the mass ratio of iron-based alloy powder, h-BN powder and CaF2 powder was 98:1:1, the wear resistance and fatigue resistance of the coating were the best. Ding et al. [12] prepared iron-based alloy powder laser cladding layers with different WS2 powder contents on wheel materials. The rolling test results of double wheels showed that rolling contact fatigue cracks preferentially initiated and extended along the boundary between dendrites and eutectic phases. When the mass fraction of WS2 was 6%, the wear rate of the cladding layer was the lowest and the rolling contact fatigue crack was the shortest. From the above studies, it can be seen that the addition of WS2 powder, CaF2 powder and h-BN powder particles can reduce the rolling friction coefficient and wear rate of the cladding layer. The WS2 structure belongs to the hexagonal crystal system and has a layered structure. The layers are affected by the van der Waals force, so the shear strength is low. Under the action of friction, it is easy to form a lubricating transfer film on the contact surface, thereby reducing the friction coefficient of the friction pair and reducing the wear amount [13]. Adding CaF2 to the laser cladding material can effectively improve the wettability of the molten pool liquid, increase the fluidity of the liquid, and refine the grains [14]. h-BN has a layered crystal structure similar to graphite and molybdenum disulfide and can be used as a solid lubricating additive [15]. Therefore, WS2, CaF2 and h-BN have been used as solid additives in the surface strengthening treatment of wheel materials. During the wheel-rail operation process, the tread and the rail are mainly in rolling contact, while the wheel flange and the rail side are mainly in sliding friction. However, current research mainly focuses on the effect of solid lubricant additives on the rolling contact fatigue performance of laser cladding layer on wheel surface, and there is no research on the effect of solid lubricant additives on the dry sliding friction and wear performance of laser cladding layer.

In order to study the effect of different solid lubricant contents on the wear resistance of iron-based alloy coating on wheel steel surface, the authors used laser cladding technology to prepare iron-based alloy coatings with different WS2 + CaF2 and h-BN + CaF2 contents on the surface of CL60 wheel steel, and used a pin-disc wear tester to conduct sliding friction and wear tests at room temperature in a dry state to simulate the wear of the wheel flange and rail side of the train wheel on the curved section of the railway line. The friction factor, wear mass loss, surface damage and wear mechanism of WS2 + CaF2 / iron-based alloy coating and h-BN + CaF2 / iron-based alloy coating were studied; the research results can provide certain theoretical guidance for the application of laser cladding technology in the field of train wheel surface strengthening.

1 Sample preparation and test method

The base material was taken from scrapped railway CL60 steel wheels. The disc sample with a size of φ55mm×10mm was cut from the rim 2~3mm below the wheel tread by wire cutting. The laser cladding material was iron-based alloy powder. The solid lubricant additives were WS2 powder, h-BN powder and CaF2 powder, all of which had a purity greater than 99% and were commercially available. Among them: the iron-based alloy powder was spherical and rod-shaped, with an average particle size of about 80μm, and the chemical composition was shown in Table 1; WS2, h-BN and CaF2 powders were all in the form of fine flakes, with a particle size distribution in the range of 0.2~0.8μm[11-12].

According to the composition in Table 2, different mass fractions of WS2 and CaF2 and different mass fractions of h-BN and CaF2 powders were added to the iron-based alloy powder to prepare mixed powders. After mixing and drying, a TR-3000 multi-mode cross-flow CO2 laser was used to prepare the cladding coating on the surface of the substrate disk sample by synchronous powder feeding. The laser power was 1900W, the rectangular spot size was 7mm×1mm, the scanning speed was 180mm·min-1, and the powder feeding rate was 15g·min’-1. After laser cladding, the surface was ground and polished to make the surface roughness of the coating reach 0.05~0.1μm.

Metallographic samples were cut from the laser cladding samples. After grinding, polishing and etching with aqua regia, the cross-sectional microstructure of the cladding layer was observed using an OLYMPUSBX60M optical microscope (OM) and a PhenomPro scanning electron microscope (SEM). The energy dispersive spectrometer (EDS) attached to the SEM was used for micro-area composition analysis. The hardness of the sample was tested using an MVK-H21 Vickers hardness tester with a load of 4.9N and a holding time of 10s. Dry sliding friction and wear tests were carried out using a self-developed pin-disc wear tester. The disc sample was a laser cladding sample. The dual cylindrical pins were taken from the near surface of the top of the U71Mn rail and had a size of φ6mm×25mm. The contact surface between the pin and the disc was a circle with a diameter of 4mm. The vertical force during the test was set to 300N, corresponding to a contact stress of about 18.5MPa. The rotation speed of the disk sample was 100r·min-1, and the total rotation was 3.6×104 cycles. Each test condition was repeated 3 times. The sample was placed in ethanol for ultrasonic cleaning. After drying, the mass before and after the friction and wear test was weighed using an electronic balance with an accuracy of 0.1mg, and the wear mass loss was calculated. The worn surface and cross-sectional morphology were observed using OM and SEM.

2 Experimental results and discussion

2.1 Microstructure and microhardness

The author’s previous study [5] found that the interface between the laser cladding iron-based alloy coating and the substrate was smooth and well metallurgically bonded. The coating was composed of dendrites and cellular crystals, and the heat-affected zone was mainly acicular martensite. As shown in Figure 1, there are no pores and cracks on the surface of the 2# coating. Its structure is similar to that of the iron-based alloy coating, both of which are composed of dendrites and eutectic structures. Compared with the iron-based alloy coating, the surface structure of the 2# coating is denser and finer, and the number of cellular crystals produced is larger. This is because the added CaF2 has a lower density and poor interface compatibility with the metal matrix. It is easy to move upward in the laser molten pool, resulting in greater fluidity of the molten pool, which promotes uniform distribution of the structure and uniform grain size [16]. With the addition of WS2, pores with a diameter of 3~5μm appear in the coating. The 3# coating (WS2 mass fraction is 2%) has the most pores. With the increase of WS2 content, the pores decrease; the 6# coating has a broken structure and cracks at the grain boundary. Adding too much WS2 will reduce the ductility and toughness of the metal material. At the same time, under the action of high temperature during laser cladding, the coating material will produce hot brittleness, all of which lead to the appearance of defects such as broken structure and cracks. It can be seen that when the mass fractions of WS2 and CaF2 are 6.0% and 5.0% respectively, the WS2+CaF2/iron-based alloy coating (5# coating) has the least pores and the best forming quality. As shown in Figure 2, the 3# coating is mainly composed of iron, chromium, silicon, sulfur, manganese and other elements, among which the sulfur element mainly comes from the solid lubricating material WS2[17]. As shown in Figure 3: the iron and chromium elements in the 3# coating are concentrated, among which the iron element exists in the eutectic structure and the chromium element is mainly enriched in the dendrite structure; the sulfur element exists in the form of sulfide and is enriched, and is unevenly distributed in the coating. The presence of sulfide is beneficial to improving the friction reduction performance of the coating. The authors’ previous study [11] found that h-BN+CaF2/Fe-based alloy coatings prepared with different proportions of h-BW, CaF2 and Fe-based alloy powders are composed of dendrites and eutectic structures. The addition of h-BN and CaF2 refines the coating structure, and when the mass fractions of h-BN powder and CaF2 powder are both 1.0%, the coating structure is the densest and the grains are the finest.

According to previous studies [5, 11-12], the iron-based alloy coating with a mass fraction of 5% CaF2 by laser cladding is mainly composed of (Fe, Ni) solid solution, Cr7C3 hard carbide and Ni-Cr-Fe phase [5, 11-12]. After adding WS2 with a mass fraction of 2.0% to 8.0%, a new CrS phase appears in the coating. This is because the WS2 in the coating decomposes in large quantities during the laser cladding process (the decomposition temperature is about 539℃), and sulfur reacts with chromium to form a new phase CrS. CrS has lubricating properties and is beneficial to enhancing the wear resistance of the coating. When laser cladding h-BN+CaF2/iron-based alloy coating, dendrite structure precipitates first, and then the intergranular liquid phase rapidly solidifies to form a lamellar eutectic structure. The coating is composed of Cr7C3 hard carbide and Ni-Cr-Fe, CrB, Cr2N, h-BN and CaF2 phases [11]. Under the direct irradiation of high-energy laser beam, part of h-BN in the molten pool will float up quickly and decompose into boron and nitrogen elements, and react with chromium elements in the molten pool to generate new phases such as CrB and Cr2N in situ; the presence of undecomposed h-BN in the cladding layer helps to improve the self-lubricating performance of the coating [18].

As can be seen from Figure 4, the content of WS2+CaF2 and h-BN+CaF2 has no obvious effect on the surface hardness of the iron-based alloy coating. The surface hardness is about 800HV, which is about twice that of the substrate CL60 steel (about 341.57HV). The (Fe, Ni) solid solution and strengthening phases such as Cr7C3 of the WS2+CaF2/iron-based alloy coating are evenly distributed in the cladding layer, thus producing a dispersion strengthening effect. The hard phases such as Cr7C3, CrB, and Cr2N of the h-BN+CaF2/iron-based alloy coating are distributed horizontally or staggered in the coating, playing the role of a skeleton and improving the hardness of the coating [19].

2.2 Friction coefficient and wear amount

As shown in Figure 5, the friction coefficient curve of the coating can be divided into two stages: the initial wear stage and the stable wear stage. The friction coefficient in the initial wear stage increases rapidly, and the friction coefficient in the stable wear stage is in a dynamic equilibrium state. After about 2000 rotations of the 1# coating (iron-based alloy coating), its friction coefficient tends to be stable, and the 2#~4# coatings are basically stable after about 5000 rotations. The friction coefficient of 5# coating and 6# coating suddenly decreased during the sliding friction wear process. The main reason is that the plasticity of WS2 and CrS lubricating phases increases under the action of friction heat. Under the action of friction force, the lubricating phase spreads on the wear surface, which transforms the direct high-stress contact between the mating part and the coating into indirect contact between the mating part and the lubricating film and between the lubricating film and the coating, resulting in a decrease in the friction coefficient [20]. After about 18,000 rotations, the friction coefficient of 5# coating and 6# coating enters the stable wear stage. The stable friction coefficient of 1# coating is the largest, about 0.52, and the wear mass loss is also the largest, about 0.048g. Compared with 1# coating, the friction coefficient and wear mass loss of the iron-based alloy coating with WS2+CaF2 are lower, and with the increase of WS2+CaF2 content, the friction coefficient and wear mass loss both decrease. The stable friction coefficient of the 6# coating is the smallest, about 0.32, which is 38.5% lower than that of the 1# coating; the wear mass loss of the 5# coating is the smallest, about 0.035g, which is 26.7% lower than that of the 1# coating. The wear resistance of the coating is greatly improved. There is an in-situ synthesized CrS phase in the coating, which plays a lubricating role in the coating. With the increase of WS2 content, the content of CrS lubricating phase in the coating increases, which further improves the wear resistance of the coating [21-22]. However, when the mass fraction of WS2 increases from 6% to 8%, the higher content of sulfides in the coating reduces its strength, thereby limiting the further improvement of wear resistance [23].

As can be seen from Figure 6, the friction coefficients of different h-BN+CaF2/iron-based alloy coatings all show a trend of increasing first and then being in dynamic equilibrium. When the total mass fraction of h-BN+CaF2 is added to 2.0%, with the increase of h-BN content and the decrease of CaF2 content, the friction coefficient and wear mass loss of the coating both show a trend of decreasing first and then increasing. When the mass fractions of CaF2 and h-BN are both 1.0%, the stable friction coefficient and wear mass loss of the coating are the smallest, about 0.35 and 0.033g, respectively, which are reduced by 32.7% and 33.3% compared with the iron-based alloy coating. When only CaF2 or h-BN is added to the laser cladding iron-based alloy coating, its friction coefficient and wear mass loss are higher than those of the coating with both h-BN and CaF2 added, indicating that CaF2 and h-BN have a synergistic effect, better lubrication effect, and can further improve the wear resistance of the coating. In summary, after adding WS2+CaF2 and h-BN +CaF2 to the iron-based alloy powder, a self-lubricating coating can be formed, which is beneficial to improving the wear resistance of the coating. WS2+CaF2 can significantly reduce the friction coefficient of the coating, while h-BW+CaF2 can significantly reduce the wear mass loss. Among them, in the WS2+CaF2/iron-based alloy coating, the coating with a mass fraction of 6.0% WS2 and a mass fraction of 5.0% CaF2 has the best wear resistance, while in the h-BN+CaF2/iron-based alloy coating, when the mass fractions of CaF2 and h-BN are both 1.0%, the coating has the best wear resistance.

2.3 Wear morphology and wear mechanism

As shown in Figure 7, after the sliding friction wear test, the surface of the 1# coating produced partial delamination damage [24] and obvious plowing due to severe plastic deformation, and the wear mechanism was severe adhesive wear and abrasive wear [25]. Compared with the 1# coating, the number of peeling pits on the surface of the 2# coating with only CaF2 added was significantly reduced, while the number of plowing was slightly reduced. Its wear mechanism was mainly abrasive wear, accompanied by slight adhesive wear. Compared with the 1# coating and the 2# coating, the wear surface of the coating with WS2 and CaF2 (3#~6# coating) only has shallow furrow-like wear marks, and the peeling and plastic deformation are not obvious, indicating that the wear resistance of the coating is good. This is because the addition of WS2 particles can play a lubricating role and reduce wear. At this time, the wear mechanism is abrasive wear. As the mass fraction of WS2 increases to 8%, the flaky peeling pits on the coating surface appear again, indicating that the wear resistance of the coating is reduced to a certain extent at this time. The main reason is that the excessive WS2 particles reduce the strength of the coating; the wear mechanism of the coating is abrasive wear accompanied by a certain degree of peeling.

As can be seen from Figure 8, the surface of the 1# coating against the rail sample is distributed with wide and deep furrows along the friction direction, and severe plastic deformation occurs at the same time. The wear mechanism is severe adhesive wear and abrasive wear. The surface of the 2# coated rail sample showed plastic deformation and adhesion marks, as well as flaking. The wear mechanism was adhesive wear and abrasive wear. After adding WS2, the plastic deformation of the surface of the rail sample was significantly reduced. That is, the simultaneous addition of WS2 and CaF2 can not only improve the wear resistance of the coating, but also reduce the damage to the grinding pair, thus protecting the grinding pair[26]. The surface of the 5# coated rail sample (WS2 mass fraction of 6%) was basically smooth with slight scratches. Its wear mechanism was slight abrasive wear. As the WS2 mass fraction increased to 8%, wear debris appeared on the surface of the rail sample and there were local flaking pits. The wear mechanism was abrasive wear with a certain degree of flaking.

As shown in Figure 9, the wear marks on the surfaces of 7# and 8# coatings are relatively rough, with clear grooves and flaking, showing severe abrasive wear; the surface of 9# coating is relatively smooth, with relatively small scratches, only accompanied by a small number of pits, and no large amount of flaking, and the wear mechanism is slight abrasive wear; there are slight furrows on the surfaces of 10# and 11# coatings, accompanied by flaking pits, and the wear mechanism is abrasive wear and adhesive wear. In summary, after adding h-BN +CaF2 at the same time, the degree of damage to the coating surface is relatively light, and when the mass fractions of h-BN and CaF2 are both 1.0%, the coating surface damage is the lightest and the wear resistance is the best. This is basically consistent with the results of the friction factor curve and the wear mass loss.

As shown in Figure 10, the surfaces of the rail specimens worn with 7# and 8# coatings have serious plastic deformation, block peeling and agglomerated wear debris, and their wear mechanisms are mainly abrasive wear and adhesive wear; the surfaces of the rail specimens worn with 9# coatings have slight wear marks and no large amount of wear debris accumulation, and their wear mechanism is mainly abrasive wear; the surfaces of the rail specimens worn with 10# and 11# coatings have serious wear, and there are more peeling pits and wear debris accumulation on the furrows, and their wear mechanism is mainly adhesive wear accompanied by abrasive wear. In summary, after adding mixed WS2 and CaF2 as well as h-BN and CaF2 to the iron-based alloy powder, the wear resistance of the coating is improved; among them, in the WS2+CaF2/iron-based alloy coating, the iron-based alloy coating containing 6.0% WS2 and 5.0% CaF2 by mass and the wear surface of the rail sample are slightly damaged, and the wear mechanism is abrasive wear, while in the h-BN+ CaF2/iron-based alloy coating, the alloy coating surface containing 1.0% CaF2 and h-BN by mass and the wear surface of the rail sample are the least damaged, and the wear mechanism is slight abrasive wear.

As shown in Figure 11, the 1# coating is seriously damaged, and there are large spalling pits with a length of about 25 μm in the cross section [27]. Compared with the 1# coating, when 5% CaF2 and 0~6% WS2 are added (2#~5# coatings), the spalling pit size of the coating cross section is greatly reduced, and when the WS2 mass fraction is 6%, the coating damage is the slightest, the spalling pit size is the smallest, and the surface quality is excellent; the 6# coating (WS2 mass fraction 8%) has a small spalling pit group in the cross section, mainly because the excessive WS2 particles reduce the strength of the coating. Under the alternating stress of the friction pair, microcracks are formed and expanded, resulting in material shedding [28]. Compared with the iron-based alloy coating and WS2+CaF2/iron-based alloy coating, the spalling pit size of the h-BN+CaF2/iron-based alloy coating cross section is significantly smaller. When only 2% CaF2 was added (7# coating), a group of small spalling pits appeared on the cross section of the iron-based alloy coating, and grooves of a certain depth were formed; after adding h-BN at the same time, the spalling area became smaller, the degree of wear was reduced, and when the mass fractions of h-BN and CaF2 were both 1.0%, the size of the spalling pits on the cross section of the coating was significantly reduced, indicating that the reasonable matching of the content of h-BN and CaF2 is conducive to improving the wear resistance of the coating during dry sliding friction and wear.

As can be seen from Figure 12, when the WS2+CaF2/iron-based alloy coating and the grinding sample moved relative to each other and friction occurred, the self-lubricating coating was squeezed and deformed under the dual action of continuous load and friction, and the WS2 and CrS sulfide particles inside the coating were squeezed out of the surface, gradually forming a uniform and continuous sulfide lubricating film [29-30], thereby protecting the substrate. For h-BN+CaF2/Fe-based alloy coating, h-BN has a layered crystal structure, the bonding between molecules in each layer is covalent bond, and the bonding between adjacent layers is weak van der Waals force. This structural characteristic makes it easy to slide relative to each other under the action of external force, so h-BN exhibits good lubrication performance [31]. Adding a reasonable proportion of CaF2 and h-BN to the Fe-based alloy coating will reduce the grain boundary tension and grain boundary energy, inhibit grain growth, and thus refine the grains [32]. During the dry sliding friction and wear process, the h-BN phase in the coating is gradually exposed to the coating surface due to the extrusion force, forming a lubricating film on the wear surface, thereby reducing the wear degree of the coating [32]. As shown in Figure 13, under the dual effects of load and friction, plastic slip occurs on the coating surface, and the eutectic and dendrite structures gradually form plastic flow lines. The hard phases such as Cr7C3 and CrB in the eutectic structure are compressed and deformed, and the strength decreases. Cracks initiate in the hard phases and extend along the eutectic structure into the coating. When the cracks extend to the grains, the cracks will bend to the surface and extend due to the large amount of energy required for transgranular fracture, thus forming spalling damage.

3 Conclusions

(1) The WS2+CaF2/Fe-based alloy coating and h-BN+CaF2/Fe-based alloy coating prepared on the surface of CL60 wheel steel by laser cladding method are mainly composed of dendrites and eutectic structures. As the WS2 content in the WS2+CaF2/iron-based alloy coating increases, the pores decrease, but when the WS2 mass fraction is 8.0%, the coating shows microstructure fragmentation and cracking; when the WS2 and CaF2 mass fractions are 6.0% and 5.0% respectively, the pores are minimal and the coating quality is good. When the mass fractions of CaF2 and h-BN are both 1.0%, the h-BN+ CaF2/iron-based alloy coating has the finest grains and the densest structure. The WS2+CaF2 and h-BN+CaF2 contents have no significant effect on the surface hardness of the iron-based alloy coating, and the surface hardness is about 800HV, which is about twice that of the substrate CL60 steel.

(2) Compared with the iron-based alloy coating, the friction coefficient and wear mass loss of the iron-based alloy coating with WS2+CaF2 added are lower; with the increase of WS2+CaF2 content, the friction coefficient decreases, and the wear mass loss first decreases and then basically stabilizes. When the mass fractions of WS2 and CaF2 are 8.0% and 5.0%, respectively, the stable friction coefficient is the smallest, which is 38.5% lower than that of the iron-based alloy coating. When the mass fractions of WS2 and CaF2 are 6.0% and 5.0%, respectively, the wear mass loss is the smallest, which is reduced by about 26.7%. With the increase of h-BN content and the decrease of CaF2 content, the friction coefficient and wear mass loss of h-BN+CaF2/Fe-based alloy coating first decrease and then increase. When the mass fractions of CaF2 and h-BN are both 1.0%, the stable friction coefficient and wear mass loss are the smallest, which are reduced by 32.7% and 33.3% respectively compared with the Fe-based alloy coating. The addition of WS2+CaF2 can more significantly reduce the friction coefficient of the coating, while the addition of h-BN+CaF2 can significantly reduce the wear mass loss.

(3) When the mass fractions of WS2 and CaF2 are 6.0% and 5.0% respectively, the wear surface of WS2+CaF2/Fe-based alloy coating is slightly damaged. This is because in the process of sliding friction wear, the sulfide particles are broken and a uniform sulfide lubricating film is formed on the coating surface. Its wear mechanism is abrasive wear. When the mass fractions of CaF2 and h-BN are both 1.0%, the wear surface of the h-BN+CaF2/Fe-based alloy coating is mainly characterized by fine wear marks, and the damage is lighter. This is because during the sliding friction wear process, the h-BN phase in the coating is gradually exposed on the coating surface due to the extrusion force, forming a lubricating film on the wear surface, and the wear mechanism is slight abrasive wear.