As a new type of surface engineering technology, laser cladding has the advantages of high efficiency, low dilution rate, good bonding between coating and substrate, dense coating structure and low pollution. It is widely used to prepare harder, more wear-resistant and more corrosion-resistant composite coatings. Carbon nanotubes (CNTs) have unique nanotube structure and excellent mechanical properties. They are ideal reinforcement materials for preparing laser cladding coatings. CNTs-enhanced coatings have become a research hotspot in the field of laser cladding in recent years. This paper discusses the strengthening mechanism of CNTs in laser cladding coatings from four aspects: second phase strengthening, fine grain strengthening, dislocation strengthening and load transfer. CNTs can improve the performance of cladding coatings. The research status of CNTs-enhanced laser cladding coatings is summarized and analyzed from the aspects of the addition amount and dispersion mode of CNTs and the influence of CNTs on the hardness, wear resistance, strength, plasticity, toughness and corrosion resistance of the coating. Finally, the problems existing in the current research of CNTs-enhanced laser cladding coatings and the future development direction are prospected.

In traditional factories and mines, mechanical equipment is often exposed to harsh environments such as high temperature and high pressure for a long time, and the surface of key parts often suffers from wear, damage, local surface peeling and other failure forms, which sometimes seriously affect the normal operation of the entire equipment. In recent years, the number and degree of damage of high value-added parts have increased dramatically [1-3]. Preparing a coating with excellent performance on the surface of parts is an important way to extend the service life of parts and reduce economic losses. The coating prepared by laser cladding is dense and has fine grains, which greatly improves the wear resistance of parts. A well-bonded surface fusion zone can also be formed between the coating and the substrate, which can make the coating closely connected to the substrate and not easy to fall off. In addition, the advantages of laser cladding technology such as small heat input, small deformation of the workpiece, and small thermal impact on the substrate make it the preferred technical means of the remanufacturing industry [4-5].

Carbon nanotubes (CNTs) are a new type of carbon material with a unique tubular structure and nanometer diameter. For a long time, carbon nanotubes have been considered as ideal reinforcement materials for manufacturing ceramic and metal matrix composite coatings due to their high strength, light weight, good geometric properties, excellent mechanical properties and chemical stability [6-8]. Laser cladding composite coatings reinforced with carbon nanotubes have excellent properties, including high mechanical strength and excellent tribological properties, and have potential application prospects.

1 Mechanism of strengthening effect of CNTs on coatings

Like nanodiamonds, fullerenes, graphene, etc., CNTs are also a kind of nanocarbon material [9]. Carbon nanotubes are made of rolled-up graphene sheets with a layered structure. The outer diameter is generally 2 to 20 nm, and the inner diameter is even smaller, some are only about 1 nm. For multi-walled carbon nanotubes (MWCNTs), the distance between layers is fixed (~0.34 nm). CNTs are considered to be a typical one-dimensional nanomaterial because the length of the tube is generally in the micrometer level and has a very large aspect ratio. The introduction of CNTs in laser cladding can significantly improve the performance of the coating. Its strengthening mechanism is mainly reflected in the second phase strengthening, grain refinement, dislocation strengthening and load transfer mechanism.

(1) After being irradiated by a high-energy laser beam, the aspect ratio of CNTs is greatly reduced and they will partially dissolve in the molten pool [10-11]. The escaped C atoms react in situ with other metal elements in the molten pool to generate carbides such as WC, W2C and TiC. The generated carbides are dispersed and evenly distributed in the cladding layer as reinforcement phases, thereby increasing the hardness and wear resistance of the coating.

(2) During the cladding process, CNTs can act as heterogeneous nucleation centers, increase the nucleation rate, and thus refine the grains and improve the coating performance [12]. In the study of CNTs-reinforced Inconel 625 composite coating by Zhao et al. [12], CNTs acted as heterogeneous nuclei to refine grains. The middle part of the cladding layer showed finer equiaxed grains with an average size of about 3 μm. As shown in Figure 1, the grains of CNTs/Inconel 625 cladding layer were finer than those without CNTs.

(3) The carbon allotropes transformed from CNTs and damaged CNTs are mainly distributed at the grain boundaries, hindering the displacement of grains through the pinning effect. When the material matrix undergoes plastic deformation, it can only form dislocations to bypass CNTs, thereby increasing the regional lattice distortion energy and improving the mechanical properties of the coating [13].

(4) CNTs or their allotropes are evenly dispersed in the cladding layer to form an interconnected grid structure. During the fracture process, they bear the main load and play a bridging role in the pores and cracks.

2. Addition amount and dispersion method of CNTs

Whether carbon nanotubes can be evenly dispersed has always been the main challenge of carbon nanotube reinforced composites, whether in polymer, ceramic or metal matrix. This is because carbon nanotubes have a huge specific surface area of ​​up to 200 m2/g. Due to the action of van der Waals force, they are easy to form agglomerates [14]. The agglomeration of CNTs will cause the performance of the composite coating to drop sharply. The elastic modulus, strength and performance of the composite coating are closely related to the distribution of the added CNTs [15]. Wu Xiwang et al. [16] also pointed out that in order to give full play to the excellent properties of CNTs and improve the performance of composite materials, the first thing to solve is the dispersion problem of CNTs. Therefore, it is very important to pre-treat CNTs before cladding. The uniform dispersion of CNTs is the prerequisite for obtaining a cladding layer with excellent performance.

2.1. The amount of CNTs added

The amount of CNTs added has a significant effect on the performance of the coating. Some researchers have found that the mechanical and tribological properties (tensile strength, hardness, and wear) of the composite materials prepared by adding excessive CNTs will deteriorate sharply [17-18]. The reason for the deterioration is that the excessive CNTs cannot be evenly dispersed in the composite material. This problem occurred when the author’s research group ball-milled the composite powder of CNTs. Due to the addition of excessive CNTs, after the ball milling, agglomerated CNTs large particles can be clearly observed on the surface of the composite powder, as shown in Figure 2. This will have a serious impact on the forming quality of the cladding layer, and more pore defects will appear in the cladding layer, as shown in Figure 3.

Liu et al. [19] studied the effect of CNTs content on the performance of WC/Ni cladding layer. When the content of carbon nanotubes increased from 0% to 5% (mass fraction, the same below), the microhardness and wear resistance of the coating increased first and then decreased. When the content of CNTs increased from 3% to 5%, cracks and pore defects began to appear in the cladding layer, as shown in Figure 4. In the friction and wear test, the surface of the coating containing 3% carbon nanotubes was smooth with only some slight grooves, while the friction surface of the sample containing 5% CNTs was rough and a lot of debris was observed. This is because the agglomeration of CNTs increased the crack sensitivity of the coating and produced adhesive wear.

Zhao et al. [20] prepared a Cu-Fe alloy cladding layer reinforced with CNTs. The addition amounts of CNTs were 0%, 1%, 2.6% and 4% (mass fraction, the same below), respectively. The wear resistance and hardness of the cladding layer also showed a trend of increasing first and then decreasing. When the addition amount of CNTs was 2.6%, the cladding effect was the best; when the addition amount of CNTs increased to 4%, the excessive CNTs content caused agglomeration, which hindered the heat transfer of the molten pool and slowed down the cooling rate of the molten pool. At the same time, the agglomerated CNTs could not effectively prevent the collision and aggregation of the iron-rich phase, resulting in grain coarsening. Agglomerated CNTs also increased the porosity of the cladding layer and increased the defects of the cladding layer. The aggregated iron-rich phase is easy to split during dry sliding friction. After splitting, it is integrated into the wear surface as an abrasive and forms many grooves through micro-cutting wear, which reduces the wear resistance of the coating.

2.2 Dispersion of CNTs

At present, the main methods used to improve the dispersion of carbon nanotubes in laser cladding technology include mechanical alloying, chemical nickel plating, ultrasonic treatment and multiple composite methods [21]. Zhao Longzhi et al. [22] used the mechanical alloying method to put the composite powder into a planetary ball mill for ball milling, with a ball-to-material ratio of 3:1, a rotation speed of 200 r/min, and a ball milling time of 10 h. After the ball milling was completed, it was placed in a drying oven for complete drying for 2 h, and then cladding was performed to obtain a cladding layer with uniform dispersion of CNTs. Zhang Nan et al. [23] found that nickel plating can not only modify the surface of CNTs, but also play a certain role in improving the dispersion of CNTs. Nickel plating can effectively prevent CNTs from being damaged under laser irradiation. At the same time, the nickel layer has good material compatibility with the iron-based amorphous coating, which can achieve a good combination of CNTs and iron-based amorphous coating. Wang Tiancong et al. [24] mixed 1% nickel-plated CNTs and iron-based amorphous powders in a ball mill for 30 min at a ball milling speed of 150 r/min, and obtained a composite powder with good tubular structure and uniform dispersion. It was also found that too long ball milling time would damage the CNTs structure and affect its toughening effect. Zhang Jiacheng et al. [25] first added CNTs to anhydrous ethanol and ultrasonically dispersed them for 1 h, then added the remaining powder to the CNTs dispersion, stirred it magnetically at room temperature for 30 min at a speed of 500 r/min, and finally poured the mixed powder liquid into a ball mill and milled it for 1 h at a speed of 275 r/min. Zhao et al. [26] prepared CNTs/ Ti-6Al-4V composite powders by using electrostatic self-assembly. In order to break the agglomeration of CNTs and make them hydrophilic, the surface of CNTs was first chemically modified, and then the CNTs were purified by HNO3 for 12 h. The outer layer of CNTs was functionalized by a mixture of HNO3 and H2SO4 (volume ratio 1:3) under ultrasonic action, and mechanically mixed at 323 K for 4 to 6 h. Then, the acid-treated CNTs were dispersed in ethanol by ultrasound, and the Ti-6Al-4V powder was also dispersed in ethanol. Subsequently, the dispersed CNTs were added dropwise to the Ti-6Al-4V suspension and mixed. 0.3 h, and finally, the obtained suspension was completely dried in a drying furnace at 343 K for 24 h to obtain CNTs/ Ti-6Al-4V composite powder with uniform dispersion of CNTs.

In summary, whether CNTs can be uniformly dispersed in the composite material has a significant impact on the molding quality of the cladding layer. By reasonably pretreating CNTs, a CNTs-reinforced coating with good quality and excellent performance can be obtained.

3 Effect of CNTs on the performance of composite coatings

3.1 Effect of CNTs on the hardness and wear resistance of composite coatings

Generally speaking, the hardness of a material is proportional to its wear resistance. The increase in the hardness of the cladding layer also represents the improvement in wear resistance. Savalani et al. [27] prepared CNTs/Ti composite coatings with different CNTs contents on pure titanium substrates. The hardness of the coating increased with the increase in CNTs content. When the CNTs content reached 20% (mass fraction), the microhardness of the cladding layer increased by 600% compared with pure titanium, reaching 1125HV0.5. With the increase in CNTs content, the wear depth of the coating decreased rapidly. The improvement of the hardness of the composite coating is attributed to the formation of hard carbides. TiC is generated in situ by titanium powder and CNTs during the cladding process. The TiC particles are evenly dispersed in the cladding layer to resist dislocation movement and play a role of dispersion strengthening. The fine TiC dendrites in the cladding layer are evenly dispersed in the titanium matrix to form a network structure and strengthen the grain boundaries. The dispersion strengthening and grain boundary strengthening of the cladding layer work together to improve the hardness and wear resistance of the cladding layer. Li et al. [28] clad a mixed powder of CNTs and titanium powder on a titanium substrate and measured the high temperature wear characteristics of the cladding layer at 500 °C. The results showed that the CNTs-reinforced coating has higher high temperature wear resistance than the titanium matrix, and it increases with the increase of CNTs content, as shown in Figure 5. The average value of the friction coefficient of the reinforced coating is lower and more stable. The main wear mechanisms of titanium substrate at high temperature are abrasive wear, adhesive wear, severe plastic deformation and oxidation, while the main wear mechanisms of reinforced coating are adhesive wear and oxidation.

Pascua et al. [29] prepared CNTs reinforced nickel-based composite coating. Compared with the sample without CNTs, the addition of CNTs can effectively refine the microstructure and promote the formation of hard phase Cr7C3. The microhardness and wear resistance of the coating increased by 25% and 5.5%, respectively. Zhou Xiaowei et al. [30] prepared CNTs/Ni60 composite coating on the surface of 45 steel. When the addition amount of CNTs was 0.4% (mass fraction), the best performance cladding layer was obtained. The average hardness of CNTs/Ni60 cladding layer can reach 900HV0.3, which is 12.5% ​​higher than that of Ni60 cladding layer, the wear amount is reduced by 50%, and the friction coefficient is smaller and more stable. The wear surface of Ni60 cladding layer has plowing marks and peeling pits, while the surface of CNTs/Ni60 cladding layer is very smooth with only slight wear marks. CNTs increase the density of the cladding layer. The escaped C atoms react with the metal elements in the alloy to form carbide (Cr2C) reinforcement phase, which is evenly dispersed in the cladding layer and plays a role of dispersion strengthening. Due to the self-lubricating effect of CNTs, a dense protective film will be formed on the surface of the cladding layer during the friction process, blocking the direct contact between the friction pairs [31], thereby improving the wear resistance of the coating. Zhao Longzhi et al. [22] prepared CNTs/SiC/Ni60A composite coating on the surface of 45 steel. The hardness and wear resistance of the coating first increased and then decreased with the change of CNTs content. When the CNTs addition amount was 3% (mass fraction), it reached the extreme value, with the highest hardness reaching 1 058.3HV0.2, the lowest friction coefficient reaching 0.181, and the lowest wear amount reaching 0.001 1 g.

Zhang Peng et al. [32] prepared TiO2-CNTs/FeNi36 cladding layer, in which CNTs were coated by TiO2, and the addition amount was 0.5%, 1%, 1.5%, and 2% (mass fraction, the same below). The hardness of the composite coating increased with the increase of CNTs content, and finally tended to be stable (~315HV), which is 2.5 times the hardness of the unreinforced coating. The specific strength and specific stiffness of CNTs are extremely high. The addition of CNTs can effectively improve the ability of the coating to resist deformation. Moreover, CNTs are coated by TiO2 and are not easily damaged by laser, which can retain more CNTs, thereby improving the hardness of the cladding layer. Pei et al. [33] prepared a functional gradient composite coating of CNTs/hydroxyapatite (HA). CNTs were evenly dispersed in the composite coating to form an interconnected network. The microhardness of the coating increased with the increase of CNTs content. When the addition of CNTs reached 5%, the hardness of the coating could be increased by 46.8%. Yang Lijun et al. [34] pre-prepared a uniform and dense CNTs pre-coating on the surface of TC4 and prepared a CNTs/Ti laser cladding layer. The surface hardness of the CNTs cladding layer was 35% higher than that of the substrate surface, and the friction coefficient was reduced to 0.4 at the lowest. CNTs promoted grain refinement during the solidification of the molten pool, thereby improving the hardness of the coating. He et al. [35] prepared a 5% CNTs+TiC cladding layer on the surface of TC4. The microhardness value of the top of the cladding layer reached a maximum of 2800HV0.5. The closer to the bottom of the cladding layer, the fewer TiC dendrites there were, and the hardness gradually decreased to 500HV0.5. The friction coefficient of the cladding layer was 0.467, which was 27% lower than the friction coefficient of the TC4 substrate (0.637). The wear morphology of the substrate and the cladding layer is shown in Figures 6 and 7. The wear mechanism of the TC4 substrate is mainly plastic deformation and plowing, and the wear mechanism of the cladding layer is mainly abrasive wear.

In summary, the addition of CNTs promotes the generation of carbide reinforcement phase in the coating, refines the grain structure, effectively improves the hardness and wear resistance of the coating, reduces the friction coefficient of the coating and improves the wear form of the coating.

3.2 Effect of CNTs on coating strength, plasticity and toughness

CNTs have extremely high mechanical strength, and their elastic modulus and tensile strength can reach 1 TPa and 200 GPa respectively [36]. Adding CNTs to the cladding layer can improve the strength, plasticity and toughness of the coating to a certain extent. Chen et al. [37] prepared IN718/NiPCNTs composite coating. The typical morphology of liquefaction cracks was produced in IN718/10NiPCNTs coating, as shown in Figure 8. Although the cracks still exist, their length is greatly shortened. As shown in Figure 9, the bridging of the added CNTs with the Laves particles and the dendrite bonding area improves the stress transfer in the interdendritic region, which can effectively reduce the crack sensitivity of the coating. Compared with IN718 coating, after adding 5% and 10% CNTs, the yield stress of the coating increased by 3.6% and 26.3%, respectively, and the ultimate tensile strength increased by 2.5% and 16.7%, respectively, but the elongation decreased from 22.7% to 18.1% and 14.7%, respectively. The addition of CNTs led to the formation of eutectic Laves phase, which improved the mechanical strength of the material but also reduced the ductility of the material. Yu et al. [38] prepared CNTs reinforced Al-Si10Mg composites by selective laser melting. The average yield strength and elongation of the unreinforced material were 329 MPa and 9%, respectively, while those of the reinforced material were 380 MPa and 7%, respectively. Liu et al. [39] prepared CNTs/AlSi10Mg composites by selective laser melting. Compared with AlSi10Mg coatings, the average tensile strength, yield strength and elongation of CNTs-reinforced coatings were improved. Wang Tiancong et al. [24] prepared nickel-plated CNTs (contents of 0%, 0.25%, 0.5%, and 1%) toughened iron-based amorphous coatings. With the increase of CNTs content, the crack length obtained by indentation method continued to decrease, and the fracture toughness of the coating continued to increase, reaching a maximum of 7.67 MPa·m1/2. The CNTs uniformly dispersed in the coating fully exerted their fine grain strengthening effect and load transfer mechanism. The bridging effect of CNTs effectively inhibited the generation of cracks, and the strength, plasticity and toughness of the coating were enhanced to a certain extent.

3.3 Effect of CNTs on the corrosion resistance of composite coatings

Adding CNTs also improves the corrosion resistance of the cladding layer to a certain extent. The CNTs preserved during the cladding process are evenly dispersed in the cladding layer, forming a uniformly distributed interconnected network, which can bridge the pores and cracks in the coating, thereby improving its corrosion resistance.

Yuan et al. [40] used high-speed laser cladding technology to prepare nickel-coated CNTs-reinforced iron-based amorphous composite coatings on the surface of 45 steel. The addition amounts of CNTs were 0%, 0.25%, 0.5%, and 1% (mass fraction), respectively. With the increase of CNTs addition, the self-corrosion potential of the coating gradually increased, the corrosion current density gradually decreased, and the corrosion resistance of the coating was improved. The corrosion mechanism of the coating is shown in Figure 10. A part of CNTs gathers in the pores, bridges the pore defects in the coating, reduces the possibility of the formation of galvanic cells around the pores, and slows down the corrosion rate; another part of CNTs will exist in the cracks to bridge the cracks, the crack extension is restricted, and the corrosion path is reduced; the purified CNTs will remove static catalyst particles, inhibit the generation of corrosion centers, and thus improve the corrosion resistance of the coating.

Zhou Xiaowei et al. [41] prepared CNTs enhanced Ni60 laser cladding layer on the surface of A3 steel. The relationship between the corrosion rate and time of the cladding layer with different CNTs contents is shown in Table 1. When the CNTs content is 0.3%, the coating has the best corrosion resistance. Compared with the pure Ni60 cladding layer, the corrosion resistance is more than doubled, and only a small number of shallow pitting pits appear on the corroded surface; while obvious pitting pits appear on the Ni60 coating and the corrosion is uneven. On the one hand, the retained CNTs make the coating denser, isolate the corrosive medium, and accelerate the passivation of the nickel-based alloy; on the other hand, the reinforcement phase formed during the cladding process is evenly dispersed in the cladding layer, thereby preventing the growth of corrosion pits and improving the corrosion resistance of the coating.

Wu et al. [42] clad TiC-Cr7C3-CNTS metal ceramic coating on the surface of 304 stainless steel. The cladding layer has higher corrosion resistance than the substrate. In terms of corrosion morphology, many corrosion pits will form on the surface of the substrate, but no pitting is found on the coating. The TiC, Cr7C3 and CNTs reinforcements in the coating are evenly distributed in the cladding layer, effectively preventing the occurrence of pitting. The above studies show that the uniform dispersion of CNTs and various reinforcement phases in the coating effectively isolates the corrosive medium, reduces the corrosion path, prevents the growth of corrosion pits, and effectively improves the corrosion resistance of the coating.

4 Conclusion and Outlook

With the development and application of laser cladding technology, the research on using CNTs as a reinforcing phase to improve the performance of laser cladding coatings has made some progress, but the coating performance is still a certain distance away from the expected results, and there is still a lot of room for development. This paper summarizes and analyzes the research on laser cladding CNTs reinforced composite coatings from the aspects of the strengthening mechanism of CNTs on the coating, the addition amount and dispersion method of CNTs, and the influence of CNTs on the mechanical properties, tribological properties and corrosion resistance of the composite coating.

(1) The strengthening mechanism of CNTs on laser cladding coatings can be divided into four aspects: second phase strengthening, fine grain strengthening, dislocation strengthening and load transfer mechanism. The strengthening of CNTs on coating performance is the result of the combined effect of the above four mechanisms.
(2) The amount and dispersibility of CNTs play a vital role in reducing defects in the cladding layer and improving the quality of the cladding layer. Excessive CNTs are prone to agglomeration and are difficult to disperse, resulting in defects such as pores and cracks in the laser cladding layer.
(3) Adding an appropriate amount of CNTs can effectively improve the hardness, wear resistance, strength, plasticity and toughness of the laser cladding coating, and improve the corrosion resistance of the coating.

CNTs are very easy to agglomerate, making it extremely difficult to disperse them evenly in the coating. Therefore, their addition amount has always been very low, and the dispersibility of CNTs has become a major problem restricting its development. In addition, the interface reaction between CNTs and the substrate is also a problem. The intensity of the interface reaction has a great influence on the bridging effect and load transfer mechanism of the interface. In view of this, further research can be considered in the following aspects in the future:
(1) Find new CNTs pretreatment methods to reduce the agglomeration tendency of CNTs and improve the dispersion ability of CNTs in composite coatings.
(2) Develop a universal quantitative scheme for evaluating the dispersibility of CNTs in order to compare the ability of various processes to disperse CNTs in microstructures.
(3) Conduct interface research between CNTs and the matrix. Study the interface phase composition between CNTs and the matrix, the bonding strength between CNTs and the reinforcement phase, and its enhancement mechanism.