Using laser cladding technology to prepare surface strengthening coatings is one of the important ways to improve the service performance of materials. That is, according to the service environment requirements, the coating composition and organizational structure can be adjusted by changing the cladding process, thereby improving its performance and extending the service life of parts. In recent years, doping rare earth elements in coatings has been one of the research hotspots in the field of laser cladding. Rare earth elements have a special electronic structure that can purify alloy solutions and refine grains; appropriate addition of rare earth elements can effectively reduce the dilution rate, refine the grains of the cladding layer, reduce porosity, and relieve residual thermal stress, thereby reducing the crack sensitivity of the coating. Therefore, coatings with added rare earth elements have better strength, toughness, corrosion resistance and wear resistance. Designing metal-based coatings containing different types (components) of rare earth elements and combining them with appropriate laser cladding parameters is expected to become an ideal means of surface strengthening of mechanical equipment and key components in extreme environments. Starting from summarizing the effects of three commonly used rare earth oxides (lanthanum oxide, cerium oxide and yttrium oxide), this paper reviews the research on the influence of rare earth elements on the microstructure and properties of metal-based coatings, and prospects for the problems to be solved in the field of surface engineering and future development directions.
Wear and corrosion are two main forms of material failure. One-third of the world’s energy loss and 70% to 80% of the failures of electromechanical equipment are caused by various forms of wear failure, which causes huge economic losses. Unlike general mechanical properties and physical properties, wear resistance is not an inherent property of the material, but is affected by multiple factors such as contact conditions, working conditions, environment and medium. It is a system property. The wear failure of the material starts from the surface, so the surface performance is the key to determining the wear resistance of the material. Corrosion failure not only leads to a huge waste of energy and resources (according to statistics, the economic losses caused by corrosion each year account for 3% of the global GDP), but also easily causes environmental pollution and accident hazards, seriously affecting people’s lives and even threatening life safety. Similar to wear failure, all corrosion failures start from the surface damage of the material.
Therefore, to improve the wear resistance and corrosion resistance of the material surface, it is very important to choose a reasonable surface modification method. Surface modification technology refers to a technology that uses a certain process to make the material surface obtain a different organizational structure and performance from its base material. According to different process characteristics, surface modification technology can be divided into three categories: surface tissue transformation technology, surface alloying technology and surface coating technology.
1) Surface tissue transformation technology improves material properties by changing the surface tissue structure characteristics or stress state, such as laser surface quenching and annealing technology, as well as surface processing hardening technologies such as shot peening and rolling.
2) Surface alloying technology mainly uses foreign materials to compound with the substrate to form a surface alloying layer that is different from both the substrate and the added material, such as thermal diffusion technology, ion implantation technology and laser surface alloying technology.
3) Surface coating technology mainly optimizes the surface performance of the substrate by adding an external coating or plating. The substrate does not participate or rarely participates in the reaction of the coating and contributes little to the coating composition. General surface coating technologies include vapor deposition, chemical solution deposition, laser cladding, thermal spraying and spray (stack) welding technology. Since surface coating technology can select or design the surface coating composition and control the surface performance according to the purpose of mechanical equipment, its application range is very wide.
Compared with surface coating technologies such as magnetron sputtering and spraying, laser cladding technology has the following significant advantages.
1) High metallurgical bonding strength. The high-energy laser beam instantly and completely melts the pre-set (or synchronously transported with the laser beam) raw material powder, and forms a metallurgically bonded dense cladding layer with the substrate surface that solidifies rapidly after micro-melting, quickly achieving the performance improvement of the substrate surface.
2) Low heat input and greatly reduced thermal deformation. Compared with arc cladding, the heat input of laser cladding is greatly reduced, and the thermal deformation of the substrate is significantly reduced. Therefore, thin-walled parts that cannot be clad or welded by arc can be clad by laser cladding.
3) Low coating dilution rate. By adjusting the process parameters and combining the characteristics of low heat input, a cladding layer with a low dilution rate can be obtained, further improving the metallurgical bonding strength, wear resistance and corrosion resistance.
4) Fast cooling rate. Because of the fast solidification rate, it is easy to obtain fine-grained structure or phase structure that cannot be obtained in equilibrium state (such as amorphous state, etc.).
5) There is almost no restriction on the selection of powders, especially when preparing high melting point alloy coatings on the surface of low melting point metals or adding strengthening phases to the cladding layer, the wear resistance, corrosion resistance and fatigue resistance of the substrate surface can be improved to varying degrees to meet the use requirements under various complex working conditions.
Although the advantages of laser cladding technology are obvious, due to its fast cooling speed, it is easy to cause component segregation in the cladding layer. In addition, the mismatch of thermal expansion coefficients between the substrate and the cladding layer material can easily produce large residual thermal stress, induce cracks in the cladding layer and cause damage to parts. The above factors have become a technical bottleneck restricting the large-scale application of laser cladding. Rare earth elements have a special electronic structure that can refine grains, purify alloy melts, and improve alloy strength and corrosion resistance. Studies have shown that the appropriate addition of rare earth elements to powder can effectively promote the homogenization of the cladding layer, relieve residual thermal stress, and thus reduce the crack sensitivity of the coating. Therefore, this paper focuses on the effect of adding three rare earth oxides, lanthanum oxide, cerium oxide and yttrium oxide, to the laser cladding layer on the coating structure and performance, in order to prepare a coating material that performs well under the interaction of corrosion and wear in extreme environments.
2 Effect of rare earth elements on coating microstructure and forming quality
The modification effect of rare earth elements on coating structure is mainly manifested in grain refinement, organization purification, and reduction of dilution rate, thereby effectively improving the coating forming quality and achieving the purpose of improving the service performance of the coating.
2.1 Grain refinement
Grain refinement can increase the grain boundary area and dislocation density, thereby achieving the purpose of improving the strength and hardness of the material. The effect of rare earth elements in refining grains is mainly manifested in the following aspects.
1) Rare earth elements have strong chemical activity and element affinity. Under the action of the laser beam, some rare earth oxides decompose, and the rare earth elements produced are easy to react with other elements to form stable compounds during the crystallization process, thereby increasing the nucleation points of the cladding layer and improving the nucleation rate. ZHANG et al. found that the yttrium-containing intermetallic compounds formed by the decomposition of Y2O3 in the molten pool can significantly increase the nucleation rate and thus refine the grains (see Figure 1a). Undecomposed rare earth oxides can serve as non-spontaneous crystallization cores to increase the nucleation rate and promote grain refinement. CHEN et al. found that nano-La2O3 particles can serve as non-spontaneous nucleation cores to refine the grains, as shown in Figure 1b.
2) Rare earth elements are segregated at the grain boundaries, which will reduce the driving force for grain growth in the coating and limit grain growth. As shown in Figure 1c, after adding 1% Y2O3+Ce2O3 to the coating, the purpose of refining the coating microstructure is achieved through the segregation of rare earth elements at the grain boundaries.
2.2 Purification of Organization
The purification effect of rare earth elements is mainly manifested in the following aspects.
1) Improve the fluidity of the molten pool, accelerate the escape of the gas generated by the reaction, and reduce defects such as loose coating organization and pores. As shown in Figure 2a, the addition of CeO2 can not only improve the surface smoothness of the coating, but also effectively reduce the porosity of the coating. GAO et al. added an appropriate amount of rare earth La2O3 (mass fraction 1.6%) to the nickel-based coating, which promoted the fluidity of the molten pool, increased the gas escape rate and organizational uniformity in the molten pool, and greatly reduced the content of impurity elements in the coating.
2) Rare earth elements can generate high-melting-point expellable compounds with impurity elements such as S, Si, and N, reducing harmful inclusions in the coating. As WANG et al. found, the impurities of the coating with rare earth addition were significantly less than those of the coating with rare earth addition (see Figure 2b).
3) Rare earth elements can improve the shape and distribution of inclusions in the coating structure and reduce the harm of inclusions to the coating performance. As shown in Figure 2c, after the addition of La2O3, the inclusions were dispersed, the particle size was reduced, and the shape changed from the original polygon to a circle or ellipse.
2.3 Reduce the dilution rate
According to the research of LUO et al., the dilution rate η can be expressed by formula (1), that is, η=A2/(A2+A1) (1)
Where A1 is the cross-sectional area of the cladding layer (m㎡); A2 is the cross-sectional area of the melted substrate (m㎡).
During the laser cladding process, in order to make the chemical composition of the coating and the original cladding powder the same to the greatest extent, give full play to the original protective function of the cladding powder, and reduce the influence of the substrate on the coating performance, it is usually required to reduce the dilution rate as much as possible under the premise of ensuring a good metallurgical bond between the coating and the substrate. Rare earth elements can increase the melting latent heat of the coating material, shorten the solidification time, and weaken the diffusion and movement of elements, reduce the dilution effect of the substrate on the coating, and maintain the composition and performance of the coating material.
CUI et al. added 0-4% (mass fraction) CeO2 to the cladding powder to explore the effect of the addition of rare earth elements on the dilution rate. As shown in Figure 3, an appropriate amount of rare earth elements (see Figure 3d, 3% CeO2) can minimize the coating dilution rate. The research of LI et al. also shows that during the laser cladding process, CeO2 particles can absorb a large amount of energy from the laser beam, shorten the existence time of the molten pool, and increase the degree of supercooling, thereby inhibiting the diffusion of particles in the molten pool to a certain extent and reducing the dilution rate of the substrate to the coating components.
2.4 Improving the quality of coating formation
Defects such as high porosity and cracks will greatly affect the quality and mechanical properties of laser cladding coatings. If the coating powder is damp, oxidized, or undergoes oxidation reaction at high temperature before cladding, it is very easy to produce gas and cause pore defects; the substrate and the cladding layer material have different physical properties such as different melting points, and the rapid heating and quenching of high-density lasers are easy to produce large residual thermal stresses, inducing cracks in the cladding layer and causing damage to parts. The addition of rare earth elements can improve the heat exchange process in the molten pool. It is easy to react with harmful elements such as O, C, and Si, which can reduce pores and impurities in the coating and relieve stress concentration, thereby improving the forming quality of the coating.
SHU et al. believe that the rare earth element Ce has sufficient wettability in the molten pool, and its extremely active characteristics make it very easy to migrate in the molten pool. In addition, as a typical surfactant, Ce can greatly reduce the surface tension between the components of the molten pool compared with W and Ni elements in the molten pool, thereby reducing the contact angle of each component and improving the wettability of the solid-liquid interface. Therefore, an appropriate amount of rare earth elements can improve the surface quality of the cladding layer. CUI et al. found that Ce tends to form low-melting point compounds with elements such as O, Si, and S, and decomposes into slag under laser heating. The slag floats up in the molten pool and takes away the gas, which plays a role in purifying the grain boundary and relieving stress concentration. In addition, as mentioned above, rare earth elements can refine the grains, and the smaller the grain size in the coating, the larger the grain boundary area, and the more conducive to the regulation of grain boundary dislocation and slip. In this case, the residual stress in the coating can be relieved by adjusting the grain boundary. Figure 4 shows the mechanism by which CeO2 reduces the residual stress of the coating by refining the grains.
WANG et al. used 0-10% rare earth La 2O3, CeO2, and Y2O3 as additives to study the effect of rare earth elements on the macroscopic quality of the coating surface. Since the composite coatings containing three different rare earth elements have the same rules, only Y2O3 is used as an example for summary and discussion. As shown in Figure 5a, the Ni60 cladding layer without Y2O3 has poor fluidity in the molten pool, slow gas escape, and a large number of pores and gullies on the coating surface along the cladding direction. When the Y2O3 content is 0.5% and 2%, the macroscopic morphology of the cladding layer is significantly improved, but large-scale shedding still occurs. This is because the rare earth content in the cladding layer is low, which leads to uneven flow of the molten pool during laser processing, affecting the discharge of elements such as B and Si in the cladding layer. When the rare earth element content increases to 3% to 10%, the forming quality of the cladding layer is further improved. The reason is that the appropriate amount of rare earth elements improves the convection of the molten pool, promotes the uniform distribution of cladding elements in the molten pool, is conducive to the removal of impurities and gases, and the coating structure is more uniform.
ZHANG et al. found that, first, CeO2 can improve the absorption rate of laser irradiation energy, reduce the thermal expansion difference between the coating and the substrate, and reduce the tendency of coating cracking. Secondly, the addition of CeO2
improves the absorption rate of laser irradiation energy (equivalent to increasing heat input), effectively reduces the cooling rate of the cladding layer, and relieves thermal stress; and CeO2 increases the melting and solidification latent heat of the cladding material, and narrows the solidification temperature range of the molten pool. Finally, CeO2 can purify the microstructure, reduce the inclusion content, and further reduce the tendency of crack formation. Combining the above factors, the addition of CeO2 effectively inhibits the generation of coating cracks, as shown in Figure 5b.
3 Effect of rare earth elements on mechanical properties of coatings
The main effects of rare earth elements are: ①Promote the occurrence of solid solution strengthening and dispersion strengthening. ②Promote the precipitation of hard phases and improve the microhardness of the coating. ③ Reduce the average friction force borne by microscopic particles and the friction factor of the coating, and improve wear resistance. ④ Promote the formation of a passivation film in the coating during corrosion, reduce the occurrence of pitting and local corrosion, and improve the corrosion resistance of the coating.
3.1 Solid solution strengthening and dispersion strengthening
Solid solution strengthening refers to the lattice distortion of the solute atoms dissolved in the solid solution, which increases the resistance to dislocation movement and makes it difficult to slip, thereby increasing the strength and hardness of the alloy solid solution. Dispersion strengthening refers to the method of improving performance by adding hard particles to uniform materials. Rare earth elements are usually concentrated in dislocations, grain boundaries and phase boundaries in the coating structure, producing many distortion zones, attracting a large number of C, B, Si and other atoms to fill in the lattice gaps or enrich to form atomic clusters, producing solid solution strengthening and dispersion strengthening.
WANG et al. believe that the addition of the second phase La2O3 promotes the dispersion of particles in the coating. Under the action of solid solution strengthening and dispersion strengthening, the microhardness of the coating is significantly improved (see Figure 6a). LIANG et al. prepared Ni60+Y2O3 coating on Al substrate, and the corresponding TEM is shown in Figure 6b. Y2O3 promotes the formation of solid solution, such as Al4Ni3, a solid solution of Cr, Fe, Si, C, and Y at the c2 position; and AlCr2, a solid solution of Ni, Fe, C, and Y at the c3 position. Al4Ni3 and AlCr2 have high hardness and are dispersed and embedded in Al with good plasticity, which improves the comprehensive mechanical properties and plays a good role in protecting the substrate.
3.2 Promote the precipitation of hard phases such as carbides and borides
Rare earth elements can not only promote the precipitation of hard phases such as carbides and borides, increase the microhardness of the cladding layer, and reduce plowing and adhesion; but also most rare earth elements exist at the grain boundaries, which can increase the resistance to crack propagation and reduce coating wear.
WAN et al. used Y2O3 to modify laser cladding Al-12Si coating. From the XRD diffraction pattern (see Figure 7a), it can be seen that α-Mg and Mg2Si phases are mainly formed in the unmodified coating, accompanied by a certain amount of
Al12Mg17 and Al3Mg2 secondary phases. In the Y2O3 modified coating, in addition to the Mg2Si phase and Al12Mg17 phase, a new hard phase Al4MgY is also formed, which improves the wear resistance of the coating and reduces the wear rate. In addition, no other secondary phases are observed in the coating, which is due to the purification effect of rare earth elements on the molten pool during the cladding process.
SHI et al. studied the phase composition, element distribution and friction and wear characteristics of the coating by adding different amounts of nano-La2O3 to Ni60A/SiC composite powder. As shown in Figure 7b, hard second phases such as Cr7C3 and CrC were formed in the coating with La2O3 added, which promoted the improvement of the comprehensive performance of the coating.
3.3 Improving the friction performance of the coating
As mentioned above, the addition of rare earth elements can not only promote the refinement of the coating structure, reduce defects such as porosity, internal stress and cracks, and improve the forming quality of the coating, but also strengthen it through solid solution strengthening or dispersion strengthening and inducing hard phase precipitation. Therefore, the wear resistance and corrosion resistance of the coating can be significantly improved.
LIU et al. significantly improved the microhardness of the SMA/La2O3 composite coating by adding different amounts of La2O3 to the SMA (shape memory alloy) coating. When the addition amount was 0.9%, the microhardness reached 450HV0.2, which was 7% higher than that of the substrate. According to the Hall-Petch formula, the smaller the grain size per unit volume, the higher the grain interface energy. Due to the random arrangement of atoms, the grain boundary has a higher dislocation density, resulting in dislocation entanglement and a significant increase in motion resistance. As can be seen from the previous article, the addition of rare earth elements has a significant effect of refining grains, which promotes the improvement of the microhardness of the composite coating.
XU et al. prepared Ni-WC coatings with different mass fractions (3%, 6%, 9%) of La2O3 added on the surface of S136 steel substrate. The study found that the wear resistance of 6% La2O3 was the best, not only the COF was the lowest, but also the wear rate and wear volume were the smallest (see Figure 8a-c). On the contrary, the wear resistance of 9% La2O3 was relatively poor. This is because when La2O3 is added in an appropriate amount, it can play a good role in refining grains, purifying tissues, and improving mechanical properties. However, excessive La2O3 will lead to La2O3 agglomeration at grain boundaries, hindering dislocation movement, increasing grain boundary brittleness, and easily extending brittle fracture along the surface microcrack direction during wear, and reducing wear resistance.
SHU et al. prepared CoFeCrNiSiB high entropy alloy coatings with different CeO2 contents (0-4%). As shown in Figure 8d, the microhardness of each coating shows a similar law: the microhardness gradually decreases from the surface to the substrate, decreases sharply at the junction, and then tends to stabilize. When the CeO2 content is 2%, the increase in the microhardness of the coating is the largest. However, as the CeO2 content increases further, the microhardness of the coating decreases instead. This is because after CeO2 is added in excess, due to its ability to easily absorb impurities, the size of the inclusions increases and cannot float up in time to become residues and accumulate, and CeO2 will agglomerate, causing the coating performance to decrease.
3.4 Improving the corrosion resistance of coatings
The effects of rare earth elements on the characteristics of coating structure are as follows.
1) It can make the coating surface structure dense, and it is easy to form a passivation film during the corrosion process, which effectively slows down the corrosion rate. LI et al. found that after adding rare earth element Y, the grains are refined as a whole, and the ceramic particles also have a certain hindering effect on the current. Therefore, the corrosion potential of the S316 coating with Y added is positive relative to the substrate, forming a stable passivation film area, the self-corrosion point increases, and the self-corrosion current density decreases significantly (see Figure 9a polarization curve).
2) Generate intermetallic compounds and modified inclusions, reduce the point difference with the substrate, and avoid pitting corrosion. REN et al. found that adding CeO2 can significantly increase the capacitance arc diameter of the coating (see Figure 9b), indicating that the charge transfer at the interface between the electrolyte and the electrode is more difficult and the pitting resistance is stronger.
3) Rare earth segregates at the interface, reduces the interface energy, inhibits the dendrite spacing, and avoids local corrosion. MOHAMMED et al. [62] found that Ce is easy to segregate at the grain boundary, which effectively refines the coating structure, promotes the formation of local passivation film, strengthens the coating’s ability to resist local corrosion, and significantly reduces the self-corrosion current density compared to the substrate (see Figure 9c polarization curve), making the coating better resist the erosion damage of Cl- ions.
4 Conclusion
In summary, rare earth elements have the functions of refining grains, purifying structure, reducing dilution rate, promoting hard phase precipitation, and producing solid solution strengthening. Therefore, they play a great auxiliary role in improving the forming quality of laser cladding coatings (such as relieving internal stress, eliminating cracks and pores), and improving hardness, corrosion resistance and wear resistance.
At present, the application of rare earth elements in laser cladding coatings has achieved some results in terms of action mechanism, but there are still some shortcomings. Future research and development can focus on the following aspects.
1) Rare earth elements are often added through oxides, which will inevitably introduce a large amount of O elements, which will easily lead to the formation of pores. Therefore, it is possible to try to add rare earth elements to the cladding layer in the form of other compounds or single substances, which can not only avoid excessive O elements, but also give full play to the role of rare earth elements.
2) Current research mainly focuses on La, Ce and Y elements. In the future, other rare earth elements can be tried to play a role in laser cladding coatings to further expand the application range of rare earth elements.
3) At present, there are few studies on the creep resistance, fatigue, fracture, impact and other aspects of rare earth enhanced laser cladding coatings. In the future, research in these fields should be strengthened to meet the needs of complex working conditions.
4) Rare earth elements are generally easy to exist in the middle and upper parts of the coating, so the auxiliary effect of rare earth elements is weaker at the bottom. In the future, external magnetic fields, vibration fields and other means can be used to make the distribution of rare earth elements uniform.
