Effect of Mo on microstructure and sliding wear performance of CoCrFeNiMox high entropy alloy coatings by laser cladding

The CoCrFeNiMox (x = 0, 0.5, 1, 1.5 in molar ratio) high-entropy alloys (HEAs) coating were prepared on AISI1045 steel by laser cladding. The microstructure, hardness and tribological properties were investigated through x-ray diffraction, scanning electron microscopy (SEM), microhardness tester and reciprocating tribometer. The result shows that the coating is metallurgically bonded to the substrate and exhibits columnar and equiaxed grains microstructure consisted of main face-centered cubic (FCC) solid solution and few body-centered cubic (BCC) struture when x≤ 1.0. The (Fe, Cr, Mo)-riched σ phase shows up in the CoCrFeNiMo1.5 FCC matrix due to excess Mo addition. CoCrFeNiMox HEA coatings shows an obvious increase of surface hardness from 261 HV0.2 to 793 HV0.2 by increasing the Mo content to 1.5 in the molar ratio, and exhibited the most excellent wear resistance among all the compositions designed in this work. The wear resistance of the CoCrFeNiMo1.5 alloy coating is about 2.2 times higher than that of the coating without Mo. The wear mechanism changes with increasing Mo content, though abrasive wear is a common mechanism, more adhesive wear occurred at low Mo content but few oxidation and fatigue wear occured at high Mo content.


Introduction
High entropy alloys (HEAs), namely multi-component alloys proposed by Cantor et al [1] and Yeh et al [2], are new alloy systems different from conventional alloys such as Aluminum alloy, Steel and Nickel alloy, etc They contain at least five principal elements with atomic concentration between 5% and 35% resulting in a high configuration entropy of mixing ΔS conf, and a low Gibbs free energy ΔG, which make it easy to form a solid solution with simple structures such as body-centered cubic (BCC) and face-centered cubic (FCC) instead of intermetallics or complex phases during solidification [3][4][5].Several special intrinsic characteristics including high mixing entropy, lattice distortion [6,7], sluggish diffusion [8], and cocktail effect [9] make HEAs exhibit a series of unusual properties such as high hardness [10,11], excellent wear resistance [12,13], superior oxidation resistance and distinguished corrosion resistance [14,15].
Wear is a severe headache in the industries due to the inevitable material loss leads to the components failure and high economy cost.Except for the wear-resistant steels, most structural materials exhibit inferior wear performance, therefore, it is a very popular topic to overcome most engineering failures such as wear, fatigue and corrosion occurred on surfaces or modifying material surfaces.HEAs could be used as a wear or corrosion resistant material or coating materials due to its super properties.Laser cladding is an attractive surface modification technique to prepare hard coatings or films which enhance wear resistance of the components' surfaces [16,17].Laser cladding uses the heat radiated by a laser beam with high-energy to melt the the cladding layer and the shallow substrate, and the melted two parts formed a good metallurgical bond after solidification [18][19][20][21].The advantages of the technique are a high temperature gradient, a high cooling rate (10 3 −10 6 K s −1 ), and the ease of obtaining fine crystalline structures with light component segregations and improving solubility limitation and developing amorphous and substable phases that cannot be produced in equilibrium.Through the appropriate selection of the alloying elements and structural design, HEAs show competitive wear resistance.For the AlxCo 1.5 CrFeNi 1.5 Tiy high-entropy alloys investigated by Chuang et al [22], Co 1.5 CrFeNi 1.5 Ti and Al 0.2 Co 1.5 CrFeNi 1.5 Ti alloys are found to have better wear resistance at least two times than that of conventional wear-resistant steels SUJ2 and SKH51with similar hardness.HEAs coatings prepared by laser cladding characterized as excellent wear resistance and bonding strength have got considerable attention in recent years [23,24].Juan et al [25].fabricated FeCoNiCrAlMo x (x = 0.25, 0.75, 1.0, 1.25, 1.5) coating on 45 carbon steel by laser cladding and FeCoNiCrAlMo was found to have a maximum hardness of 706 HV, best resistance and lowest coefficient of friction in the HEA coatings.
HEAs such as CoCrFeNi system alloys can be considered as a promising engineering structural material candidate.Gludovatz et al [26] investigated CrMnFeCoNi alloy and found that it has a tensile strength above 1 GPa and fracture toughness value exceeding 200 MPa•m 1/2 , which is better than most pure metals and alloys.The alloy is comparable to structural ceramics and even approaches the strength of some bulk metallic glasses in this respect.The single-faced CoCrFeNi alloy with an FCC structure has a tensile strength of 472 MPa and a low yield strength of only 155 MPa [27].The Mo addition into the CoCrFeNi FCC solid solution can obtain a combination of high strength and superior ductility.Due to the large atomic size of Mo, the formation of higher lattice distortion not only produces solid solution strengthening, but also promotes hard and brittle (Cr, Mo)rich σ and (Mo, Cr)-rich μ phases in the FCC solid solution, forming precipitation hardening.Mechanical properties and tribological behaviour of FeCrNi-based high-entropy alloys are strongly influenced by precipitated topological close-packed (TCP) phases (σ and μ) with hardnesses up to 15 Gpa [27,28].Qiu et al [29] prepared the CoCrFeNiMo coating by laser cladding and the result showed it had a high surface hardness of 741 HV and the excellent wear resistance was acquired.It was suggested fine-grained strengthening, BCC structure and Laves phase resulting in higher hardness and wear resistance.However, the effect of Mo content on the tribological properties of CoCrFeNiMo coating is not clear.The medium carbon steel has some good properties but not ideal hardness and wear resistance.In order to improve the surface properties of the AISI1045 steel, CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5 in molar ratio) HEA coatings were prepared by laser cladding on the AISI1045 steel substrate.In this paper, the effect of Mo alloying on wear properties of the CoCrFeNiMo x coatings with rapidly solidified microstructure was investigated.

Experimental procedure
The AISI1045 steel with the main element contents (wt %), C% = 0.44, Si% = 0.31, Mn% = 0.62, S% = 0.02, P% = 0.02, the other Fe and the structure (pearlite + ferrite) was cut into the dimensions of 50 mm × 35 mm × 15 mm and used as the substrate materials.The mixed elemental powders of Co, Cr, Fe, Ni and Mo were used as cladding materials.The nominal chemical composition of the powder mixture with the mole ratio of CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) was obtained by mechanically mixing commercially pure element powders of Co, Cr, Fe, Ni and Mo with a purity of 99.0%-99.5% and mesh size of 200-300.In the experiment, the AISI1045 steel surface was treated by grinding machine, and cleaned by acetone to remove the dirt and oil.The Co, Cr, Fe, Ni and Mo powders were blended in a ball milling manner with a mass ratio of ball to power 5:1, rotate speed of 200 r min −1 and milling time of 5 h.Before the laser cladding, the mixed powder was mixed thoroughly with an organic solvent, pre-coated uniformly on the AISI1045 steel surface, the layer thickness is about 0.4 mm, dried at 80 °C and preheated before laser cladding.The laser cladding (figure 1(a)) was carried out on a 700W pulsed pulse Nd:YAG solid laser equipment (JHM-1GXY-700B) and the laser processing parameters of 330 A current, 2.5 ms pulse width, 20 Hz frequency, 100 mm min −1 scan rate, 1mm beam diameter, 50% overlapping rate.Argon was used as a shielding gas to avoid or decrease oxidation during laser melting.Finally, the specimens were put into an electric furnace heated at 200 °C for 1 h to reduce the internal stress and crack caused by rapid cooling.
The phase composition of CoCrFeNiMo x alloy coatings were identified by x-ray diffractometer (XRD, UItima IV, Japan) with Kα1 radiation at 40 kV/40 mA, scanning from 20 to 100°in 2θ at a scanning speed of 10°/min.The microstructure was investigated by a scanning electron microscope (SEM, Phenom XL, Netherlands) operated at 15 kV with an energy dispersive spectrometer (EDS).The cross-section hardness distribution was tested by a microhardness tester (HXD-1000TMSC/LCD) with a loaded of 200 g and a duration time of 20 s.The friction and wear properties of the coatings were evaluated using a reciprocating tribometer (Rtec Co.Ltd, USA) with a configuration of ball-on-plate (figure 1(b)).The surfaces of fabricated samples were ground and polished before the testing.The YG6 ceramic ball with a diameter of 5 mm and chemical composition of 94%WC and 6% Co was used as the counter sliding surface.The test conditions were set as the applied load of 10 N, duration of 30 min, a sliding stroke of 5 mm, reciprocating frequency of 5 HZ, which slides a total distance of 90 m.To ensure repeatability, each test was repeated at least three times.After tribological tests, the 3D surface profiles of the wear tracks on plates were measured by the 3D profiler (Rtec, USA).

Results and analysis
3.1.Phase analysis Figure 2(a) shows the x-ray diffraction (XRD) pattern of the CoCrFeNiMo x (x = 0, 0.5,1.0,1.5)HEA cladding layers.The results show that CoCrFeNi, CoCrFeNi 0.5 , CoCrFeNi 1.0 alloy coatings exhibits main FCC solidsolution phase and few BCC or σ phase.CoCrFeNi 1.5 alloy coating consist of FCC and σ phase two phases obviously.Figure 2(b) shows the diffraction peak of (111) crystal plane near the 43°shifts to the left with an increase in Mo addition, indicating the lattice distortion increases with the solid solubility of Mo increases in the FCC matrix according to the Bragg equation (2dsin θ = nλ).The corresponding lattice parameter of FCC( 111) is calculated to be 3.6046, 3.6313, 3.6552, 3.6473 Å for CoCrFeNi, CoCrFeNi 0.5 , CoCrFeNi and CoCrFeNi 1.5 coatings, respectively.Since the atomic radius of Mo (1.362Å) is larger than that of Co (1.251 Å), Fe (1.241 Å), Cr (1.249 Å) and Ni (1.246 Å), the alloy strengthening is realized by the addition of Mo and the phase composition become different.Some Mo is distributed in the multi-element supersaturated FCC solid solution and increase the distortion of the crystal lattice.When Mo content exceeds its solubility limit, the precipitation of σ phase would appear in the FCC matrix.
The phase formation and structural stability in HEAs coatings can be predicted by the thermodynamic parameter ΔH mix (mixing enthalpy), δ (atomic size difference), ΔS mix (mixing entropy), Ω (solid solutionforming ability) [30].The core criteria proposed by Zhang et al [31]is to satisfy the conditions, δ 6.6% and Ω1.1.Moreover, Guo et al [32] proposed that the FCC phases could develop in HEAs when VEC > 8, BCC phases can exists when VEC < 6.87, and both BCC and FCC phases exist when 6.87 VEC 8.0.The calculated  Moreover, the likelihood for the sigma phase tends to develop in Cr-containing alloys can be determined using the equivalent chromium content formula below [33]: ECC %Cr 0.31% Mn 1.76% Mo 0.97% W 2.02% V 1.58% Si 2.44% Ti 1.7% Nb 1.22% Ta 0.266% Ni 0.177% Co.
If the equivalent Cr content (ECC) exceeds 17∼18 wt%, the steel is very likely to develop the σ phase.In this case, the ECC of CoCrFeNiMo x (x = 0, 0.5, 1, 1.5) alloys is 11.5, 40.3, 60.6 and 75.5 wt%, respectively.Therefore, Mo can promote the formation of the sigma phase, and the excess addition of Mo into FeCrNiMnMo x alloys could increase the likelihood of developing the σ phase.

Microstructure and EDS analysis
A cross-sectional morphology of the CoCrFeNiMo x coating is demonstrated in figure 3(a).It suggests that the coating with a thickness of about 350 μm is free of cracks and pores and formed a good metallurgical bond with the substrate.The cross-section microstructure of the coatings is depicted in figures 3(b)-(e) .The CoCrFeNi coating's microstructure is columnar grain-shaped, yet with the addition of Mo, the microstructure shifts to equiaxed polygonal grains and the grain size diminishes.The grain size of CoCrFeNiMo coating is less than 3 μm.The columnar or equiaxed grains boundaries could spreads over a wider area.As the grains and interdendritic areas are marked as zone A and zone B, EDS analysis of the two regions is listed in table 2. A mass of Co, Cr, Fe, Ni and Mo are dissolved in the FCC solid solution grains and a slight tendency of Mo, Cr or Fe segregation can be found in the grains boundaries.The excessive content of Fe should caused by some dilution effect of the coating during the laser cladding process, which would promote the developing of FCC+BCC duplex.Excess Mo, Cr or Fe segregation in grains boundaries could form the hard (Fe, Cr, Mo)-riched σ phase, and even the tiny FeCrMo-type sigma phase precipitated in the CoCrFeNiMo 1.5 coating.The rapid cooling rate   by laser cladding can effectively relieve the component segregation in the HEAs coatings and bring about remarkable grain refinement.
As can be seen, the Mo free CoCrFeNi coating has a relatively low average microhardness of 261 HV 0.2 , which is higher than the microhardness of equiatomic CoCrFeNi bulk HEA (161 HV 0.2 ) prepared by vacuum arc melting [34] due to the rapid solidification of laser cladding processing.The average microhardness of the CoCrFeNiMo 0.5 , CoCrFeNiMo, and CoCrFeNiMo 1.5 coatings are 427 HV 0.2 , 508 HV 0.2 , and 793 HV 0.2 , respectively.It is worth noting that the hardness curve of the CoCrFeNiMo 1.5 coating is relatively stable, which is three times higher than the iron substrate with the hardness of about 210 HV 0.2 .The hardness of the coating is improved by Mo addition.The increase of Mo content improved the microhardness of the coatings mainly due to the combined effects of solid-solution strengthening, grain refinement and precipitate strengthening of the FeCrMo-type σ phase.the solid solution of Mo with a large atomic radius (1.36 Å) leads to lattice distortion, which improves the slip resistance ability of dislocation and causes an increase in microhardness.Secondly, the hardness enhancement could be attributed to the refined microstructure.In the third, the rise in microhardness of CoCrFeNiMo 1.5 coatings becomes sharper could be attributed to the second-phase strengthening of the precipitated fine (Fe, Cr, Mo)-riched σ phase.According the studies of Liu at al [34], the σ phase in the CoCrFeNiMo bulk HEAs has hardness of 21 GPa, which is much higher than that ( 0.9 GPa) of the Mo-saturated FCC solid solution.

Wear behavior
The tribological properties of the CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) coatings are evaluated by reciprocating sliding wear tests.The test data is fitted by software to obtain the variation of the friction coefficient of the alloy coating with the sliding time, which is shown in figure 5(a), and the data in (a) is sorted and processed to draw the average friction coefficient (b).It can be observed that the friction coefficients have a sharply increase at the  beginning and instantly decrease and that of CoCrFeNi and CoCrFeNiMo 0.5 coating fluctuates more obviously in the initial friction stage within 3-8 min and then become some stable, but that of CoCrFeNiMo and CoCrFeNiMo 1.5 can quickly reach the steady state.The average friction coefficient values for CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) coatings were about 0.60, 0.59, 0.54 and 0.50, respectively, which indicated that the addition of Mo decreased the coefficient.Figure 6 shows the schematic diagram of wear process, which visualizes first increase then decrease of the friction coefficient.The severe extrusion appear in the two contact surfaces under positive pressure in the early stages of friction process and bring into plastic deformation due to the absence of lubrication.As a result, large shear strains accumulate near local contact points on the material surface, leading to plow lines, local adhesions, or micro-crack formation on the surface and even causing localized spalling.The curves varied with time could be connected with the scratching and the formation and peeling-off of wear debris during reciprocating friction.The large fluctuation of the friction coefficient should be caused by the periodic localized fracture of surface layer or the periodic accumulation and elimination of debris on the worn surface.The coefficient of friction increases as the large-sized debris accumulates on the worn surface while it decreases as the debris depart from the worn surface.The contact surface is worn flat gradually after the first short running-in period, the friction coefficient tends to be stable and enters the stable wear stage.The friction coefficient variation is linked to the composition and original microstructure of the material.The HEA coatings with high Mo content have the characteristics of high hardness and high strength, The Mo addition in the HEA alloys can reinforce the substrate,and the formation of some oxide debris such as FeO, Cr2O3 and MoO is beneficial for decreasing the friction coefficient.
Figures 7(a) ∼ (d) shows the typical three-dimensional wear scar morphology of the CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) coatings.It can be seen that the wear scar of CoCrFeNi coating is wide and deep, the width and depth were decreased with more Mo addition and CoCrFeNiMo 1.5 coating has the smallest depth and width of  wear scar.As seen from the cross-section curves of wear scars of coatings in figure 7(e).The measured wear scar depth is 29.4μm, 20.3μm, 8.2μm and 4.7μm, and the corresponding wear scar width is 792.2 μm, 594.8 μm, 500.4μm and 171.1 μm, respectively.Figure 7(f) shows the CoCrFeNiMox coatings' average wear volume and specific wear rate.The wear rate (W) of the coatings was determined by Archard's formula [35].
where V is the measured wear volume loss (mm 3 ), N is the normal load (N), and d is the total friction and wear reciprocating distance (m).It is shown that the average wear volumes are 0.319, 0.248, 0.173 and 0.104 mm 3 , and the calculated wear rate is 3.54 × 10 −4 , 2.43 × 10 −4 , 1.91 × 10 −4 , 1.11 × 10 −4 mm 3 /N•m for CoCrFeNi, CoCrFeNiMo 0.5 , CoCrFeNiMo and CoCrFeNiMo 1.5 alloys respectively.The decrease in volume and rate of wear, linking the lesser depth and breadth of the wear scar, means the better wear resistance.realized with the increase of the Mox content.The Mo addition greatly enhanced the wear resistance of coatings due to fine microstructure, solution strengthening and second-phase strengthening,which lead to the superior microhardness.

Worn surface
Figure 6 presents the SEM images of the worn surface morphologies of the CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) coatings after tribological tests.There are many obvious grooves parallel to the sliding direction, indicating the occurrence of abrasive wear in all CoCrFeNiMo x coatings.Meanwhile, more or less dark layer zone appeared, which contain rich in O besides Fe, Ni,Cr,Co or Mo, according EDS analysis (figure 8 or table 3 ) indicating the occurrence of oxidation wear occurs on the worn surface of coating.It can be observed in figures 8(a), (b) that there are a large area of delamination and patches besides grooved, significant ductile deformation along the sliding direction on the worn surface of CoCrFeNi, CoCrFeNiMo 0.5 coatings.This indicates the worn surface undergoes periodic delamination fracture and the adhesive wear mechanism is predominantly of delamination wear.This indicates the worn surface undergoes periodic delamination fracture and the adhesive wear mechanism is predominantly of delamination wear.So the wear mechanisms of CoCrFeNiMo x (x = 0. 0.5) coatings are a combination of adhesion and abrasion wear.Figures 8(c), (d) shows the CoCrFeNiMo, CoCrFeNiMo 1.5 coatings exist shallower continuous grooves and the decreased surface roughness and less deformation, indicating that the wear mechanism turn to be the typical abrasive wear.In the mean time, there exist thinner and more continuous dark oxidation film in the coating,which implies the oxidation reactions have occured during sliding and the oxidation layer exist stably.EDS spot analyses indicate that the dark zones contain more molybdenum oxide in the coating with increasing Mo content.The MoO 2 has relatively high melting point (2600 °C), high hardness (HV1130) and strong covalent bond.The strengthened oxidation layer with more Molybdenum oxide content adhering to the coating surface would shield the coating from microcutting, lead to the smoothening of the surfaces and further effectively reduces the wear loss of the coating.The higher hardness of coating can provide more structural support for the oxidation layer and the oxide have higher hardness, making for improving the wear resistance.In the other hand, few pitting was observed in CoCrFeNiMo 1.5 coating pointing to localized flaking a result of surface fatigue.The wear mechanisms of CoCrFeNiMo x (x = 1.0, 1.5) coatings turn to be primary abrasive wear and few oxidation wear and fatigue wear.
It is well known that the wear resistance of metals is highly dependent on their microstructure, phase composition and phase distribution.The YG6 possesses a superior hardness of 20.1 GPa and relatively higher than the CoCrFeNiMo x coatings.During the friction process, hard YG6 asperities can be embedded into relatively soft surface of CoCrFeNi and CoCrFeNiMo 0.5 coatings under the load.The subsurface layer of coating is subjected to severe extrusion, ploughing and deformation, and produces work-hardening, brittleness, and the shed hard phase resulting in abrasive wear.Due to the big shear stress and leading rapid adhesion and delamination of the surface, and the harder phase fell off to form hard abrasive debris on the coating and produced many plowing grooves.For the CoCrFeNiMo and CoCrFeNiMo 1.5 coatings with high Mo content, solid-solution strengthening, grain refinement and especially tiny precipitate strengthening of the FeCrMo-type σ phase will enhance the resistance to hinder the penetration of the friction pair, inhibits the appearance of spalling layer, lessen microcutting, exhibiting good wear resistance.In the other hand, the brittle debonding due to fatigue may become more frequently seen as hardness increases and strength and ductility decreases, making it more likely that the problem will become more common.According to study in [34], CoCrFeNiMo alloy  present better wear resistance and the high-volume fraction of μ precipitates in CoCrFeNiMo1.5 alloy causes micro-fatigue wear.It is not the case in this study, the rapid solidification by laser cladding make CoCrFeNiMo 1.5 coating a greatly refined microstructure and hinder the precipitation and grow up of the second phase and lessen the fatigue wear.

Conclusion
In this experiment, the microstructure, hardness, friction and wear properties of the CoCrFeNiMo x (x = 0, 0.5, 1.0, 1.5) coatings fabricated by laser cladding were discussed.The following conclusions can be drawn as follows: (1) The cladding coating is metallurgically bonded to the substrate and exhibits main columnar and equiaxed grains microstructure consisted of main face-centered cubic (FCC) solid solution and few body-centered cubic (BCC) struture when x 1.0.The (Fe, Cr, Mo)-riched σ phase shows up in the CoCrFeNiMo 1.5 FCC matrix due to excess Mo addition.The Mo addition and laser cladding make the grain refinement effectively.
(2) With the increase of the Mox molar ratio from 0 to 1.5, the hardness of CoCrFeNiMo x coating was improved from 261 HV 0.2 to 793 HV 0.2 , which could be attributed to the combined effects of Mo on solidsolution strengthening, grain refinement and precipitate strengthening.
(3) With increasing Mo content in CoCrFeNiMo x coatings, the wear coefficient and wear volume loss decreases and the wear resistance improved effectively, and the wear mechanism change from a mixed mechanism of adhesive wear, abrasive wear to the primary abrasive wear, few oxidative wear and fatigue wear.CoCrFeNiMo 1.5 alloy coating has a wear resistance that is about 2.2 times higher than that of the coating without Mo.

Figure 2 .
Figure 2. (a) XRD patterns of cladding layers ; (b) Effect of Mo on the diffraction angle.

Figure 4 .
Figure 4.The cross-sectional microhardness distribution curves of the coatings.

Figure 5 .
Figure 5. (a) real-time coefficient of friction (b)Average coefficient of friction.

Figure 6 .
Figure 6.The schematic diagram of wear process.

Figure 7 .
Figure 7. (a)-(d) 3D morphology of the wear scar;(e) Cross-section curves of wear scars ; (f) Average wear volume and average specific wear rate for CoCrFeNiMo x coatings.

Table 1 .
Parameters of FeCrNiMnMo x high entropy alloy HEAs.

Table 2 .
The EDS results of the CoCrFeNiMox coatings (at%).

Table 3 .
Chemical compositions (at.%) of oxidation layer in worn surfaces of coatings in figure7.