Study of cracking mechanism and wear resistance in laser cladding coating of Ni-based alloy

: Ni-based coatings with the addition of a plastic phase- an austenitic stainless net were prepared using laser cladding technology, and the CeO 2 was added in cladding layers. The cracking mechanism, microhardness, microstructure, phase composition, and wear properties were investigated. The relationship between thermal stress and the elastic and plastic fracture had been developed from the standpoint of fracture mechanics and thermal elastic fracture mechanics. The Fracture criterion of the nickel-based coating was obtained, and the study has shown that the crack sensitivity could be reduced by decreasing the thermal expansion coefficient Δα . Then a new method was proposed, in which the substrate was prefabricated the stainless steel net. It was found that the number of cracks reduced significantly with the addition of stainless steel net. When the stainless steel net with 14 mesh was added in Ni-based coatings, the average microhardness of nickel composite coating was 565 HV 0.2 , which was 2.6 times higher than that of the 45 steel substrate. Although the rare earth oxide 4 wt.% CeO 2 and stainless steel net were added in the Ni-based coating reducing the microhardness (the average microhardness is 425 HV 0.2 ), the wear resistance of it improved substantially. The wear volume of Ni-Based composite coating was 0.56×10 5 mm 3 ·N -1 ·m -1 , which was 85.1% lower than that of 45 steel and 61.9% lower than that of Ni-based coating without CeO 2 and stainless steel net. The experiment results have shown that the Nickel-based composite coating is equipped with low crack sensitivity and high abrasive resistance with austenitic stainless net and the rare earth oxide 4 wt.% CeO 2 .


Methodology on crack suppression 2.1 Crack formation mechanism
During laser processing, the mismatch of expansion and shrinkage between the coating and the substrate is generally related to the difference in elastic modulus and thermal expansion coefficients. Based on thermal thermoelasticity theory, the thermal stress σ generated after solidification of the molten pool can be calculated by Eq (1) [30]: Where ƒ( x, L) is a function of the analysis location and the initial length of the specimen, E is the modulus of elasticity, Δα is the thermal expansion coefficient of deposited material, ΔT is the temperature difference between the analysis position and the restricted area. In addition, a large temperature gradient along the deposition direction is generated in the molten pool, during which the temperature of the substrate is lower than that of the coatings owing to the good thermal conductivity of the substrate. Therefore, the cladding layer's shrinkage will be restrained by the substrate when the molten pool cools and solidifies, and the tensile stress is generated in the coating, which might bring a great tendency of cracking [30].

Elastic and plastic fracture in Ni-based alloy coating
By observing the cracks, the tensile stress leading to the fracture of the cladding layer is perpendicular to the direction of crack cracking, which belongs to the crack opening type. According to the COD criterion of elasticplastic fracture mechanics, the critical opening displacement obtained by the D-M model is taken as the physical parameters of the crack-tip field.
The AB segment of the crack surface is extended forward and intersects with the vertical line of the tip D at point E. The Crack opening displacement δ is measured by 2ED. The crack tip opening displacement is illustrated in Fig 1(a).
It is assumed that there are infinite microcracks of equal length in the direction perpendicular to the scanning direction, The crack length is 2a, The interval between adjacent cracks is 2h, the uniform tensile stress in the scanning direction is σ, the length of microcrack and its simplified plastic zone is 2c=2a+2R, a schematic illustration of the microcracks of the coating as shown in Fig 1(b). Cracking occurs when the crack opening displacement reaches the critical value δc. At this time, the critical tensile stress can be calculated by Eq (2

Fracture criterion of nickel-based coating
Thermal stress of coating is expressed in two forms, Δα<0 when the thermal expansion coefficient of the substrate is less than that of the cladding layer, and the coating is subjected to tensile stress. On the contrary, Δα>0 when the thermal expansion coefficient of the substrate is high than that of the cladding layer and the coating is subjected to compressive stress. In order to protect Ni-based coating from cracking, it should be ensured that the maximum tensile stress σxmax in the nickel-based coating is less than the critical tensile stress σsc when there exist crack tendency in cladding layers. According to Eqs. (1) and (2), the micro-crack propagating condition can be described as follows:  [32]. The authors conclude that reduce the temperature gradient by preheating significantly decreased the residual stress after laser cladding. However, there is little information about the thermal expansion coefficient on cracks sensibility. Hence, this paper focuses mainly on the thermophysical parameters Δα [33]. Cracks could be suppressed by reducing the absolute difference of thermal expansion coefficient between the Ni-based coating and the substrate.

Initial materials
A medium carbon steel plate (0.45 wt.% C) was selected as the substrate material, and its dimension was 55 mm × 37 mm × 10 mm. 304 stainless steel net with eight different mesh (8 mesh, 10 mesh, 12 mesh, 14 mesh, 16 mesh, 20 mesh, 24 mesh, and 30 mesh, respectively) were selected for an experiment. The chemical composition of the 45 steel and the 304 stainless steel net was shown in Table 1. Ni60 powders with a particle size of 40-80μm were used as the raw powder. The chemical composition of the powder is listed in Table 2. The CeO2 particles with the contents of 4% were adsorbed on the Ni60 powders by mechanical ball grinding, and the morphology of two kinds of powders was showed in

Experiment apparatus and process parameters
Ni-based composite coatings were performed on the LDM-800 laser additive manufacturing system built by Shenyang aerospace university, as schematically shown in Fig 3. It consists of a 6 kw IPG fiber laser, a three-axis motion execution system, a three-bin coaxial powder feeder (RC-PGF-D3), a coaxial powder feeding system with a Precitec laser cladding head (YC52), and an oxygen analyzer and control system. Pure argon with 99.99% purity was used to deliver the powders and shield the molten pool from oxidation, which ensured the oxygen level below 50 ppm. The reciprocating scanning strategy was adopted, and the single-track cladding layer length was 35 mm. The process parameters of laser cladding were shown in Table 3. Before the experiment, the powder was dried in a vacuum oven at 80 °C for 8 h in order to increase fluidity. The substrate was ground and polished with sandpaper to remove the oxide layers, then was cleaned with ethanol. The 304 stainless steel net was washed with acetone to remove oil and then placed on the substrate.

Characterization of the coating
After the experiments, cross-sections of all coatings were obtained by using the wire cutting machine, then The specimens were polished with 400 # to 1500 # grit sandpapers, and the microstructure was observed by an etchant ( H2O and HNO3 and HF with a ratio of 7: 3: 1). The microstructure of coatings was studied and analyzed by metallographic microscope (MR5000). Zeiss sigma500 scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS) was employed to further confirm the microstructure and identify the and chemical compositions of the coatings. The phase constitution of the cladding layer was performed using D/max-2500/PC X-ray diffraction (XRD) with a copper target, and the range of diffraction angle (2θ) was selected as 10° 90°, X-ray tube high voltage: 40KV, X-ray light tube current: 100mA. The XRD was implemented with a voltage of 40 kV, a current of 30 mA, a continuous scanning speed of 8°/min, and scanning from 10° to 90°. The microhardness of cladding layers from the surface to the bottom was tested using an HVS-1000 Vickers microhardness tester with a loading force of about 0.98N for 10 s. One point was measured vertically every 0.1 mm at the cladding layer, each point was measured three times, and the final microhardness was the average value of measurements. The friction and wear tests were carried out on the MS-G5000 wear experimental machine, and the Si3N4 balls were employed as friction pairs with a diameter of 5mm. The tests were conducted with a rotation speed of 200 rpm/min and a radius of 5.5 mm, and 45 min friction tests with a 3 N normal load. The wear tracks after the 310 m sliding test were observed with a Keyence Digital Microscope VHX-500F. The wear volumes were assessed by Alpha-Step D-100 profilometer.

Macroscopic morphology analysis of nickel-based composite coating
When the cladding layers was detected by dye penetrant test, there are transverse and longitudinal cracks in the Ni-based coating without net, as illustrated in Fig 4. This is attributed to the thermal stresses that develop during solidification acting on grain boundaries, which is the main cause for cracking tendency. It is believed that cracks are occurred when the stress exceed the yield limit of materials. It is well known that Austenitic stainless steel offers a comprehensive property of lower yield strength, better antioxidant properties, strong toughness, and weldability. The stainless steel net are prefabricated on the substrate before the laser cladding experiment to obtain the coating Ni-based composite coating which was achieved after solidification. Experimental results showed that the 304 stainless steel net (7×10-6 ~ 21×10-6/K) on the substrate with different mesh leading to the cracks in the cladding layer decreased. This is because the thermal expansion coefficient of 304 stainless steel net is higher than that of the 45 steel (11.9×10-6~13.1×10-6/K). The stainless steel net which was prefabricated on the surface of 45 steel substrate can be regarded as a part of the substrate. At this time, the overall thermal expansion coefficient αo = (α45 + α304) is greater than 45 steel. The absolute difference between the thermal expansion coefficient of the substrate after pre-fabricated the 304 stainless steel net and the Ni-based coating (13.9×10 -6~2 0×10 -6 /K) decrease. From the above Eq (2), the thermal stress of cladding layer decreased, which led to reduce the crack sensitivity. However, there still existed cracks in Ni-based composite coating with the addition of stainless steel nets of 16 mesh and 24 mesh. It demonstrated that the mesh number of the net is not the larger the better, nor the smaller the better, there exist a critical range to inhibit both the initiation and growth of crack.

Analysis of cross-section morphology of nickel-based composite coating
From Fig 5, some large pores exist in Ni-based composite coating with 8 mesh stainless steel net prefabricated on the substrate. Because there is a gap between the stainless steel net and the substrate, when the melting pool rapidly solidified, the gas in the gap can not be eliminated in time, forming pores. Compared with the cross-section morphology of Ni-based coating without net, there was incomplete melting stainless steel wire in the Ni-based composite coating which the substrate was prefabricated stainless steel net with 10 mesh, and the presence of microcracks in the Ni-based composite coating which the stainless steel net with 12 mesh was prefabricated on the substrate. However, there was no obvious cracks or pores appeared in the composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate. It was found that there were small cellular pores in nickel-based composite coating which the stainless steel net with 24 mesh and 30 mesh were prefabricated on the substrate through an optical microscope. When the specimen subject to external forces, the existence of porosity has a positive effect on crack propagation, improving the crack sensitivity and decreasing the mechanical property of specimens. The above studies suggested that the precipitating gas hole existed in nickel-based composite coating, and the gas content in a molten solute increased during the solidification of molten metal, and the gas that precipitated near the bottom of grain boundaries. Similarly, it has large driving force to precipitate gas. The solute of lower solubility are enriched at the intergranular phase, which provide geometric stabilization for bubble micronuclei. Shrinkage cavity which is associated with solidification shrinkage, and it is in a vacuum state at the initial stage, which creates favorable conditions for bubble precipitation.

Analysis of microhardness of nickel-based composite coating
The microhardness profile along the depth direction ofNi60A coating are showed in Fig 6. When the the stainless steel net was prefabricated on the substrate, the microhardness of the composite coating decreased. Due to the stainless steel net was melted at high temperatures, the Fe content increased. As a result, the precipitation of the hard phase was suppressed in the melt pool, which caused a reduction in the microhardness [34]. However, it was found that the microhardness of the coating was improved as the number of meshes of stainless steel net increases. When the stainless steel net with 14 mesh was prefabricated on the substrate, the highest average microhardness of the cladding layer reaches 565HV0.2, which was approximately 2.6 times higher than that of the substrate. This is due to the fact that as the number of mesh of the stainless steel net increase, the diameter of wire become increasingly slender. Besides, the content of Fe element decrease with the reduction of the mass of stainless steel net per unit area prefabricating on the substrate, which contribute to the precipition of the hard phase increases. When 4wt.% CeO2 was added in Ni-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate, the average hardness of the Ni-based composite coating was found to be 425 HV0.2 which showed a 31% reduction compared Ni-based composite coating without rare earth oxides, but it was still about 2 times as much as that of the 45 steel substrate. Due to the addition of CeO2, the laser energy absorbed by molten pool increases, increasing the convection and inducing a large number of Fe element from the substrate come into the molten pool [35]. When the surface has undergone a relatively short running-in stage, the contact area between the surface and Si3N4 ball is smaller and had large contact stress. The microconvex bodies on the surfaces were abraded vigorously and improving the friction coefficient. When the friction coefficient leveled off, it was demonstrated that the wear process gradually enters a stable stage. It can be seen in Figure 6 that the 45 steel shows highest friction coefficient, and the average friction coefficient is 0.76. This is because the difference between the hardness of 45 steel and Si3N4 ball is more significant, which the steel surface is highly susceptible to damage. In the process of friction and wear, the production of wear debris increased frictional resistance, the friction coefficient increased. As it can be seen, the Ni60 coating exhibited an average friction coefficient of 0.50. This is due to the precipitation of borides and carbides can significantly improving the hardness of the cladding coating, offering the advantage of resistance to abrasive wear. When the substrate was prefabricated stainless steel net with 8 mesh, 10 mesh, 12 mesh, and 14mesh respectively, the friction coefficient increased as the mesh number decreased. The average friction coefficient of Ni-based composite coating with different stainless steel net was 0.64, 0.59, 0.57and 0.46, respectively. The friction coefficient of Ni-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate decreased by about 70% than that of the 45 steel and decreased by about 8% than that of the Ni60 coating. It was demonstrated that the Ni-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate had good antiwear and friction-reduction behaviour. The relative investigations reveal that the pores and cracks of the Ni-based Alloy Coating with addition of 4 wt.% CeO2 is increased comparing with that of the coating without CeO2 [36]. Therefore, the average coefficient of friction of Ni-based composite coating in Ni-based composite coating that the stainless steel net with 14 mesh was prefabricated on the substrate was 0.49, after the 4 wt.% CeO2 was added to it. The Ni-based composite coating increased by 6.5% compared to coating without rare earth oxides and decreased by 36% compared to 45 steel.

Analysis of wear scar morphology and wear amount of nickel-based composite coating
The surface morphologies of the wear scars can be revealed by Fig 8, the depth and width of the wear scar of the modified nickel-based composite coating were less than 45 steel, and it was demonstrated that the the modified coatings are apparently much excellent wear resistant than 45 steel. Under cyclic contact stresses, the surface of 45 steel was presented severe wear, and the contact-welding was generated in a local high temperature environment. Adhesion occurred due to the bonding of the two contact surfaces atoms. The adhesive spots were sheared off and spalled during the subsequent sliding friction. Because of the lower hardness of 45 steel, the microcracks is generated and propagated freely. Then A large number fatigue spalling pits were found on the worn surface. The bulk of wear debris exhibited large plastic deformation after repeated rolling. As the time progresses, part of the undischarged wear debris adhered to the worn surface and wear continued. According to the above analysis, the wear mechanisms of the 45 steel were fatigue wear and adhesive wear. When stainless steel net was prefabricated on the the substrate, as the mesh number increased, the width of the wear scars narrower. It was demonstrated that the antifriction properties of Ni-based composite coating improves as the mesh number increased. It can be consistent with variation of the friction coefficient of Ni based alloy composite coating. The Ni-based Alloy Coating that the stainless steel net with 10 mesh was prefabricated on the substrate after the addition of the rare earth oxide 4 wt.% CeO2, the width of the wear scars slightly increased, and the fatigue spalling pits were not found, which indicated that There are no large pieces of wear debris on the worn surface. In summary, when the content of CeO2 was 4 wt.%, the nickel-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate shows better wear resistance. To further investigate the wear properties of nickel-based composite coating by laser cladding, the Wear volumes of the specimens were quantified separately using a surface profiler, as observed in Fig 9. In fact, in a radial section profile of erasion trace, which the outside edges are significantly higher than that of the inner ones. Due to the plastic deformation of the wear debris produced by the surface spalling after repeated rolling. Through comparative analysis, it can be found that the 45steel had the widest section profile of erasion trace, and the maximum depth was 7um, and the wear volume of that was 3.75×10 -5 mm 3 ·N -1 ·m -1 . The width of the erasion trace of Ni60 coating decreased significantly, and the wear volume of that was 1.47×10 -5 mm 3 ·N -1 ·m -1 , which was 60.8% lower than that of 45 steel. The width of the erasion trace of the Ni-based composite coating that the stainless steel net with 14 mesh was prefabricated on the substrate decreased significantly, and the wear volume of that was 2.89×10 -5 mm 3 ·N -1 ·m -1 , which is 22.9% lower than that of 45 steel. When the rare earth oxide 4 wt.% CeO2 was added to it, the depth of the erasion trace of Ni-based composite coating decreased significantly, and the wear volume of that was 0.56×10 -5 mm 3 ·N -1 ·m -1 , which was 80.6% lower than that of Ni-based coating without rare earth oxide, and 85.1% lower than that of 45 steel substrate. Although the rare earth oxide 4 wt.% CeO2 added in the Ni-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate reducing the microhardness, the abrasion wear resistance of it improved substantially. This is consistent with previous studies [37,38] indicating that there is no correlation between wear resistance and hardness in multiphase materials. Hardness describes only the hard body's penetration resistance, but it does not include mechanisms of formation and propagation of cracks which finally lead to the separation of the material [39]. The wear resistance of multiphase materials is controlled not only by the hard phase of materials, but also the strength and toughness of the matrix phase.

XRD phase analysis of nickel-based composite coating
To further investigate the mechanism of reduction of hardness and increase of wear resistance of Ni-based composited coating with the addition of 4 wt. % CeO2, Fig 10 exhibits the phase composition of materials using the XRD technique. Ni60 coating is mainly composed of the matrix γ-(Ni, Fe) solid solution and the reinforcement phases such as M23C6, M7C3 and hard phase CrB, CeCr2B. Due to the high contents of Fe, Ni and Cr of 304 stainless steel , peaks of nickel-based composite coating that the stainless steel net with 14 mesh was prefabricated on the substrate intensified at 2θ = 44.1°. But there is no difference in phase composition compared to Ni60 coating. The appearance of peak broadening at the diffraction angle of about 51.0°and 75.3°was obtained through the addition of the rare earth oxide 4 wt.% CeO2 into Ni-based composite coating, and there are mainly hard phase CrB and eutectic phase M23C7, at the same time, a new phase of CeCr2B appeared.  Table 4 reveal the micro-morphologies and the constituent phases of the nickel-based composite coatings. Fig 11(a) shows that nickel-based composite coating mainly consists of bulk phase with irregular shape (A ), interdendritic eutectics with the rod-shape structure (B ) cellular dendrite phase(C ). Due to B and C belong to low atomic number elements, can not be determined by EDS analysis. the estimation of the constituent phases can be done as follows. Through XRD analysis and reference to the relevant literature, the following can be inferred. The bulk eutectic contains a high concentration of Ni, Cr, and Fe, slight amount of C elements ,and B poor. It could be inferred that this phase is probably borides CrB. The interdendritic eutectics with the rod-shape structure are identified as γ-(Fe,Ni) + M7C3 (M=Fe, Ni, Cr) with fewer quantities of C. As can be seen from Table 3, the relatively high content of Ni is overwhelmingly in cellular dendrite phase in which relatively low contents of C and Cr are dissolved. It can be confirmed that the cellular dendrite is the primary γ-(Fe,Ni) solid solution. Fig 11(b) shows that nickel-based composite coating with the addition of 4 wt.% CeO2 mainly consists of blocky phase (D), flake-like eutectic phase (E), cellular dendrite phase(F). Compared to coating without rare earth oxide, the Fe element content increased and the Cr element content decreased. On the one hand, the laser energy absorbed by the molten pool increases due to the addition of CeO2, increasing the convection and inducing the excessive melting of the substrate [35]. It contributes to an amount of Fe coming into the molten pool. On the other hand, Fe and Cr belong to the elements of the third period, and the radius of the Fe atom is close to that of Cr atom. When the Fe concentration is higher, the Cr atoms are replaced with Fe to form solid solution phase. A new phase CeCr2B is formed after the addition of CeO2, due to the addition of CeO2 are comparatively less in quantity and the finite dimensions of the probe, the element of Ce failes to detect. Combined with XRD analysis and relevant references, the blocky phase (marked with D in Fig 11(b)) may be identified as CrB with significant quantities of Fe. The flake-like eutectic phase (marked with E in Fig 10(b)) may be identified as γ-(Fe,Ni) + M23C7 (M=Fe, Ni, Cr) eutectics with small quantities of Si. The cellular dendrite phase may be identified as γ-(Fe,Ni) solid solution with small quantities of C, Cr and Si.  Through comparative analysis, when 4wt.% CeO2 is added in Ni-based composite coating which the stainless steel net with 14 mesh was prefabricated on the substrate, the size of the cellular dendrons increase and the resistance of the crystal boundary reduce. When an external force is applied, the plasticity distortion is very east to go through the crystal boundary from one crystal grain to another [40]. Besides, hardness describes only the hard body's resistance to penetration [39]. The hardness of coating reduced. When the cladding layer is subjected to thermal stress, part of the internal stress will be removed by plastic deformation, which would reduce the cracking tendency. It can be seen from Fig 11(b) that the size of the bulk phase decrease. When the melt pool temperatures higher than 2050 ℃, CeO2 could be decomposes to Ce ion. In addition, Ce enriches as inner adsorption element mainly at grain boundary, which decreases the interfacial energy and Gibbs free energy of cladding layer, and reduces the driving force for crystal growth, thereby hindering the growth of crystals [41]. From this, hard phases (CeCr2B) are distributed at grain boundaries of bulk phases. The ductile phase γ-(Fe,Ni) is precipitated after the primary hard phase during laser cladding, and the ductile phase is prone to inhibition, so the amount of the hard phase decrease contributing to the number of ductile phases increased. When the surface layer is subjected to external force, it can reduce the probability of damage to the surface layer in an elastic deformation manner within a certain range. The softer matrix phase is worn, and the hard phase which plays a certain supporting and bearing role is not easy to be peeled off duo to the small size of it, thus, reducing the viscous force. In addition, the softer matrix phase act as inlaying wear debris and hard particle, therefore the wear resistance is improved greatly.

Conclution
(1) The fracture criterion of nickel-based coating was presented basing on thermoelasticity theory and fracture mechanics. Factors that influence the Microscopic cracks included material thermophysical parameters(E, Δα, ΔT ), specimen length and critical crack opening displacement when cracking occurs. (2) The difference in thermal expansion coefficient Δα was reduced by prefabricating an austenitic stainless net on the surface of the substrate, and a significant number of the transverse and longitudinal cracks were dispeared in the Ni-based coating. However, a few macroscopic cracks were found in Ni-based coating with the addition of stainless steel nets of 16 mesh and 24 mesh.
(3) As the number of meshes of stainless steel net increased, the average microhardness of the nickel-based composite coating improved and the friction coefficient decreased. When the stainless steel net with 14 mesh was prefabricated on the substrate, the highest average microhardness of nickel composite coating is 565HV which is 2.6 times higher than that of the substrate and the average friction coefficient is 0.46 which is decreased by about 70% than that of the 45 steel substrate and decreased by about 8% than that of the Nibased coating without nets. (4) After the 4 wt.% CeO2 was added to Ni-based composite coating that the substrate was prefabricated stainless steel net with 14 mesh, the average friction coefficient is 0.49, and the average hardness was found to be 386 HV. Comparing with coating without rare earth oxides, the average hardness showed a 36% reduction and the average coefficient of friction increased by 6.5%. But the wear resistance of coating with the addition of 4 wt.% CeO2 and stainless steel net with 14 mesh improved substantially. The wear volume of that was 0.56×10 -5 mm 3 ·N -1 ·m -1 , which was 80.6% lower than that of Ni-based coating without rare earth oxide, and 85.1% lower than that of 45 steel substrate.

Availability of data and materials
The datasets supporting the conclusions of this article are included within the article.