This paper briefly summarizes the principle of laser cladding technology, and systematically introduces the research status and existing problems of laser cladding pure ceramic coating, metal-ceramic composite coating, bioceramic coating, nano-ceramic coating, and precursor conversion ceramic coating. The effects of laser cladding process parameters and auxiliary treatment on the internal structure, macroscopic morphology and performance of ceramic coatings are summarized, and improvement measures and prospects for laser cladding metal-based ceramic coatings are proposed.

With the development of science and technology, in industrial construction and actual production, facing complex use environments such as high temperature, high pressure, over-acid and over-alkali, the surface of metal parts will be seriously damaged, which will greatly reduce the service performance of parts or even make them scrapped, and also increase production costs. In this regard, surface modification technologies such as plasma spraying, physical vapor deposition, chemical vapor deposition, carburizing and nitriding, and submerged arc welding have been proposed to improve the friction and wear, high temperature resistance, and corrosion resistance of metal parts. Laser cladding is a new type of surface treatment technology that uses a high-energy density laser beam to melt the cladding material and the substrate at the same time, so that the cladding material and the substrate form a metallurgical bond, and the heat-affected zone formed with the substrate after cladding is small.

The performance of laser cladding coating is related to the cladding material and process parameters. The cladding material plays a decisive role in the structure and performance of the cladding coating. Ceramics have the characteristics of wear resistance, high temperature resistance, corrosion resistance, and oxidation resistance, and have attracted widespread attention. They have great potential in the field of laser cladding metal surface modification. Laser cladding technology can improve the wear resistance, corrosion resistance, hardness and other properties of ceramic materials. Oxide ceramics have high temperature resistance and oxidation resistance, while nitride ceramics have self-lubrication and wear resistance. The dense ceramic coating obtained by laser cladding technology is beneficial to extend the service life of industrial production parts and enhance the use effect, save resources and reduce costs. This study summarizes the research status of laser cladding ceramic coatings from the perspective of laser cladding technology principles, systematic classification of laser cladding ceramic coating materials and process parameters, and the influence of auxiliary treatment on the structure and performance of ceramic coatings, and looks forward to the direction of related laser cladding ceramic coatings.

1 Principles of laser cladding ceramic coating technology

Laser cladding is a multidisciplinary technology that integrates laser technology, computer-aided technology and control technology. Laser cladding uses a high-energy-density laser beam as a heat source to melt the substrate and the cladding material simultaneously to form a cladding layer. The cladding layer can be divided into four parts: the cladding zone, the interface zone, the heat-affected zone, and the substrate. According to the powder feeding method, it is divided into synchronous powder feeding and pre-setting powder. The principle diagram is shown in Figures 1 and 2. Compared with the pre-setting powder method, synchronous powder feeding is more complicated. The cladding material is transported to the laser beam during the laser cladding process through the powder feeding nozzle, and a cladding layer is formed on the surface of the substrate. Compared with pre-setting powder, this method can reduce the dilution rate of the cladding layer and the impact on the substrate. Laser cladding technology has the following advantages: ① After laser cladding, the coating material forms a metallurgical bond with the substrate, and the bonding performance is good; ② The processing time is short and the cooling speed is fast, which is easy to form a fine microstructure and improve work efficiency; ③ The dilution rate is low, the deformation of the heat-affected zone is small, and the impact on the substrate is small; ④ The coating composition can be controlled, the operation is simple, and it is easy to realize automation; ⑤ Environmental protection and material saving. Therefore, laser cladding technology has been one of the research hotspots in recent years.

2 Types of laser-prepared ceramic coatings

After decades of development, laser cladding ceramic coatings have achieved certain results. At present, according to the classification of laser cladding ceramic coating materials, ceramic coatings mainly include pure ceramic coatings, metal-ceramic composite coatings, nano-ceramic coatings, bioceramic coatings, laser cracking ceramic precursor conversion ceramic coatings, etc.

2.1 Pure ceramic coatings

Laser cladding pure ceramic coatings are formed by metallurgical bonding of ceramic powders such as oxides, carbides and silicides with metal substrates through laser cladding technology to form a dense ceramic coating. The oxides are mainly Al2O3, SiO2 and ZrO2, and WC and TiC are widely used in carbides. Sun Ronglu et al. clad TiN ceramic coatings on the surface of TC4 alloys through laser cladding technology, and obtained dense and non-porous cladding layers through SEM observation. The wear resistance of the cladding layer is significantly improved compared with that of TC4 alloy. The mutual solubility and affinity between multiphase ceramic components are excellent. LI Z L et al. used laser cladding to prepare a multiphase Al2O3-TiB2-TiC ceramic coating with high microhardness and enhanced wear resistance on the surface of a carbon steel substrate. Compared with the carbon steel substrate, the wear resistance and microhardness are improved. The microstructure phase components were determined by XRD to be Al2O3, TiB2, TiC and α-Fe phase. Figure 3 is the SEM morphology of the cladding layer with different Al2O3 contents. It can be seen that TiC and TiB2 are white granular, and Al2O3 is black strips or blocks, which are evenly distributed between the cladding layers. Observing the cladding layers with different Al2O3 components, when the addition amount of Al2O3 alone increases to 30%, the ceramic coating has good surface quality, the coating structure is dense, and the cladding layer has the best performance. Analysis of the wear resistance of the cladding layer found that the wear resistance is best when the Al2O3 content is 30%.

When the laser cladding ceramic coating contains more hard reinforcement phases, it plays a positive role in improving the friction performance and hardness of the coating. At the same time, the multi-ceramic phase design can effectively reduce the size of the phase in the surface coating and refine the grains. At this time, the cladding layer is not easy to crack. Since the bonding strength between the coating of the ceramic material after laser cladding and the metal substrate is not enough, cracks and holes or even coating peeling may occur under high-intensity service conditions. The main reason is that the thermal expansion coefficient, compatibility, lubricity, elastic modulus and other parameters of the ceramic material and the metal are different; secondly, from the perspective of chemical properties, the bonding mode and crystal structure of the two materials are different. Ceramics are generally covalent bonds and metals are generally metallic bonds. The compatibility between the two is poor. In addition, the laser cladding process is a rapid cooling and solidification process. The internal stress of the material in the cladding layer cannot be released, which will also cause the coating to crack, which puts higher requirements on the material and preparation process of the coating.

2.2 Metal-ceramic composite coatings

Metal-ceramic composite coatings are generally formed by mixing ceramic powder as a reinforcing phase with metal self-fluxing alloy powder. In most cases, a ball mill is used to mix the ceramic powder and metal powder to make the mixed particles more uniform. When designing and selecting materials, factors such as the thermal expansion physical properties and wettability of the material should be considered. The ceramic materials of metal-ceramic composite coatings mainly refer to carbides, nitrides, oxides, borides and silicides. The metal self-fluxing alloy powder is generally nickel-based, cobalt-based and iron-based. Metal-ceramic composite coatings combine the excellent properties of metal such as toughness and ductility with the wear resistance and corrosion resistance of ceramics to achieve the effect of 1+1>2.

(1) Co-based ceramic composite coatings Co-based alloy powders have good heat resistance, corrosion resistance, wear resistance and other properties. The performance of Co-based alloy coatings is enhanced by adding ceramic phases or generating ceramic phases in situ. Studies have found that carbides are widely used as reinforcing phases. Xiao Jupeng et al. prepared WC/Co06 composite coatings with different mass fractions on the surface of 42CrMo steel by laser cladding technology, in which the diameter of Co06 powder was 100~150 μm and the diameter of WC was 10~15 μm. The results show that the coating surface is smooth, without obvious pores and cracks. Compared with the substrate, the hardness of the coating increases with the increase of WC content. Liang Weiyin et al. clad WC/TiC/Co-based metal composite coatings on the surface of YG8 alloy. The surface morphology and interface after cladding are shown in Figure 4. It was found that there were pores on the surface of fine particles. This is because there are unmelted WC particles in the cladding layer, and the surface and both sides of the powder absorb less laser energy during the reaction process, so that the residual gas in the powder cannot be discharged in time. The phase composition in the cladding layer is mainly WC, W2C, and (Ti, W) C1-x, Co3C, Co4W2C, and Co3W3C. The results show that the wear resistance and hardness of the cladding layer are better than those of the substrate.

(2) Fe-based ceramic composite coating. Fe-based alloy powder has the characteristics of high hardness, good wear resistance and low price. The performance of Fe-based coating is enhanced by introducing ceramic particles into Fe-based alloy. WEI A et al. studied the effect of different WC contents on the microstructure and properties of laser cladding iron-based coating. The results show that the cladding layer is dense and has no obvious cracks and pores, as shown in Figure 5. It can be seen that the microstructure above the cladding layer gradually changes from planar crystals to columnar crystals and cellular crystals, while the middle and upper parts of the cladding layer are mainly equiaxed crystals. With the increase of WC content, the microhardness and wear resistance of the coating increase. When the WC addition is 16%, the coating hardness (HV) is the highest, which is 826.2. The wear mechanism of the coating is mainly abrasive wear, accompanied by slight adhesive wear and oxidative wear. Dai Xiaoguang et al. used laser cladding technology to prepare Fe-based SiC metal ceramic composite coatings and studied the structure and properties of the coatings. It was found that the coating surface was smooth and crack-free, and the phases in the coating were mainly composed of α-Fe, M7C (3 M=Fe, Cr and Ni) and Fe2Si, indicating that SiC in the molten pool reacted chemically with the Fe matrix to generate M7C and Fe2Si in situ. It also has a precipitation strengthening effect on the matrix. The average microhardness (HV0.2) of the generated metal ceramic composite cladding layer is 572, which is 1.5 times that of the matrix.

(3) Ni-based ceramic composite coating Nickel-based self-fluxing alloy powder has high tensile strength, good fatigue resistance, high temperature corrosion resistance, good toughness and wear resistance, and is widely used in actual industrial production. At present, there are many types of nickel-based alloy powders, such as Ni20, Ni25, Ni60, Inco-nel625, etc. Due to the insufficient wear resistance of nickel-based alloys, the performance of nickel-based alloys is generally enhanced by adding ceramic phases. GAO Z T et al. studied the effect of CeO2 on laser cladding Ni60 coating and found that the addition of CeO2 can significantly improve the microstructure. When the CeO2 content increases from 0 to 4.0%, the width of the eutectic structure decreases from 1.74 μm to 237.5 nm, as shown in Figure 6. It can be found that when the CeO2 addition reaches 4.0%, the minimum number of cracks is obtained. In addition, from the perspective of friction coefficient and mass loss, the coating with 4.0% CeO2 addition exhibits excellent wear resistance.

KOTARSKA A et al. used laser cladding technology to prepare Inconel 625 high-temperature alloy powder mixtures with TiC particle volume fractions of 10%, 20% and 40%, respectively. The main components are Ni, Cr, Fe, and the matrix microstructure is austenite dendrites, as shown in Figure 7. The average microhardness (HV) of the composite coating increases with the increase of TiC content, and is higher than that of the metal Inconel 625 coating, which is 258~342. With the addition of TiC reinforced particles, the erosion rate at 30° and 90° impact angles decreased by 46% and 61%, respectively. Since TiC is a brittle phase, as the TiC particle content in the composite coating increases, the erosion rate of the coating decreases when the impact angle is 30°, while the erosion rate of the coating increases slightly when the impact angle is 90°, which is also consistent with the erosion wear mechanism.

Since the ceramic powder is directly in contact with the outside world after mixing with the alloy powder, the ceramic powder will be burned at high temperature, lose carbon, oxidize and other problems during the cladding process, which will affect the microstructure and properties of the cladding layer. Therefore, a coated powder is designed. WANG Z et al. used 316L stainless steel as a substrate and prepared core-shell structure 420 stainless steel powder and nano WC-Co powder as cladding materials by high-energy ball milling. The nano WC-Co powder was evenly coated on the surface of the 420 powder. The metal ceramic composite coating without holes and cracks can be obtained by laser cladding technology. The nano WC-Co powder is protected during the laser cladding process, and the nano WC-Co particles are evenly distributed in the cladding layer, and the microhardness of the cladding layer is significantly improved. Chen Shengzuan et al. designed different compositions of Ni-coated nano-alumina to be pre-placed on the surface of stainless steel, and prepared the surface of the nano-alumina coating uniformly distributed by laser cladding. The test found that the wear resistance of the coating was significantly improved after Ni-coated Al2O3.

The above studies show that ceramic powder is added to the metal self-fluxing alloy powder as a reinforcing phase, combining the wear resistance, corrosion resistance, and oxidation resistance of ceramics with the toughness, high elastic modulus, and high strength of metals, and the organization and properties of the coating can be affected by controlling the amount of added ceramic phase. In general, the composite coating will not be incompatible with the substrate, and there will be no peeling or large cracks. If the added ceramic powder particles are irregular, the ceramic coating will crack or even fall off due to stress concentration. The coating technology is used to prevent the ceramic powder from directly contacting the outside world during laser cladding, which can avoid oxidation and burning at high temperatures. At the same time, the ceramic phase is more evenly distributed in the coating to obtain a denser coating.

2.3 Bioceramic coatings

Bioceramic materials have good bioactivity, osteoinductivity and osteoconductivity, and have good compatibility with the human body. However, they are brittle, low in strength and poor in toughness, and are generally not suitable for bone sites. Therefore, using laser cladding technology to combine bioceramic coatings with metals (such as titanium alloys) can improve the bioactivity, corrosion resistance, and bone bonding ability of metal implants. At present, bioceramic coatings mainly include bioinert ceramics and bioactive ceramic materials. Bioinert materials generally include oxides of Al, Zr, and Mg, and non-oxides are mainly borides, nitrides, carbides, etc. Both bioactive ceramic coatings and inert ceramic coatings have good biocompatibility, but bioinert ceramics have better mechanical properties than active ceramics. Bioactive ceramics are mainly prepared with hydroxyapatite (HAP) as raw material. However, the direct preparation of ceramic coatings with HAP as raw material has high cost and HAP powder is easy to decompose, and the obtained ceramic coating is unstable. Therefore, some researchers mixed CaHPO4·2H2O and CaCO3 powders in a certain proportion as precursors to synthesize HAP bioceramic coatings in situ, which prevented the decomposition of bioceramics. Jia Ru [50] used the pre-setting method to synthesize hydroxyapatite (HAP) coatings in situ using mixed powders of CaHPO4·2H2O and CaCO3 as precursors, and studied the influence of single-factor process parameters on the surface component HA of the coating. The results showed that when the laser power was 300 W, the scanning speed was 9 mm/s, and the focal length was 25 mm, the surface morphology of the coating was the best, and no cracks or holes appeared. As the distance from the coating surface increases, the hardness of the coating increases first and then decreases, and the coating exhibits good biocompatibility under various process parameters. Xu X et al. prepared silicon nitride (Si3N4) and calcium phosphate composite bioceramic coatings on the surface of Ti-6Al-4V (TC4) alloy using Nd∶YAG pulsed laser, CO2 continuous laser and semiconductor continuous laser. It was found that under appropriate laser parameters, Si3N4 and Si3N4/TCP (tricalcium phosphate) composite coatings can form dense metallurgical ceramic composites with the substrate. After testing, the surface morphology of both coatings is good and has good mechanical properties. Compared with the substrate, the microhardness of the composite ceramic coating on the surface is significantly improved. In addition, the Si3N4 and calcium phosphate composite coating can obtain good biological activity.

Rare earth elements, nanomaterials, etc. are added to HAP to form a composite bioceramic coating to improve the bonding strength and life of the substrate and coating. Shi Jiaxin used laser cladding technology to add rare earth lanthanum oxide (La2O3) to the bioceramic coating and found that when the amount of La2O3 added was 0.8%, the hardness was the highest, about 2.3 times that of the substrate, and the corrosion current density was reduced, the corrosion rate decreased, and the corrosion resistance of the cladding layer was improved. The immersion test found that when 0.2%~0.4% La2O3 was added to the cladding layer, the coating showed good bioactivity. FAN G et al. successfully prepared bioactive Ca-P and Al2O3 and ZrO2 eutectic ceramic coatings using laser cladding technology, as shown in Figure 8. It was found that laser power plays a dominant role in determining the phase composition and surface morphology of the biocompatibility and bioactivity of the Ca-P coating. At 50 W, a rough surface containing a large number of flower-like agglomerates can be observed (see Figure 8a), but at 60 W and 70 W, the surface of the laser-clad Ca-P coating formed a smooth surface, as shown in Figures 8b and 8c. The analysis shows that the laser power of 50 W significantly improves the bioactivity of the coating compared with other powers. A dense ceramic coating with good biocompatibility and metallurgical bonding was prepared by laser cladding. Compared with the substrate, the mechanical properties of the bioceramic coating are also significantly improved.

2.4 Nanocomposite ceramic coating

The nanoceramic coating has extremely high hardness and good friction and corrosion resistance. The coating and the substrate are metallurgically bonded, with good bonding performance, and it is not easy to peel off under high-intensity service conditions. The fine nano-coating grains can effectively inhibit the growth of grains and hinder the movement of dislocations, making the nano-coating generally dense, crack-free and with good performance. Commonly used nano-ceramic coatings include Al2O3, ZrO2, SiO2, TiO2, SiC and WC. WANG X et al. used pre-prepared nano-TiC powder and 12CrNi2 powder to prepare laser cladding gradient coatings on the surface of 40Cr gear steel. The results show that as the amount of prepared nano-TiC powder increases, the nano-ceramic particles are transformed into a large-volume hard phase structure, and the microhardness (HV) of the coating is significantly improved, from 612 at the bottom of the coating to 1 088 at the top of the coating. With the increase of microhardness and the transformation of TiC hard phase, the friction coefficient of the gradient coating is reduced by 50%, the grinding loss is reduced by 40%, and the wear resistance under heavy load and lack of lubricant is significantly improved. ZHANG S H et al. mixed micron-scale Ni-based alloy with 1.5% micron CeO2 and 1.0%~3.0% nano CeO2 powder, respectively, and prepared composite coatings on low-carbon steel Q235 using laser cladding technology. It was found that the 1.5% micron CeO2 composite coating grew oriented dendrites and equiaxed dendrites from the interface to the central area, and the nano CeO2 composite coating grew multiphase dendrites and fine equiaxed dendrites. The addition of CeO2 powder greatly improves the microhardness and wear resistance of the coating. Nano CeO2 improves the hardness and wear resistance of the micron-level coating, and compared with the addition of 1.0% and 3.0%, the hardness and wear resistance of the coating with 1.5% nano CeO2 are the best. WANG HJ et al. studied the effect of nano CeO2 on Ti-based coatings. TiCN+SiO2 composite ceramic coatings were successfully prepared by laser cladding process without adding or adding 3% CeO2 nanoparticle additives. The coating is mainly composed of TiCN and TiN phases, as shown in Figure 9. It can be seen that the grain morphology of the upper, middle and lower three regions of the coating without adding CeO2 is dendrite, and the dendrites in the upper and middle regions are larger. After adding CeO2 nanoparticles, the grain morphology of the upper and middle regions of the coating is uniform and dense granular crystals. This is mainly because CeO2, as a surfactant material, can reduce the critical nucleation radius, inhibit the growth of grains, and is beneficial to improve the microhardness and wear resistance of the coating. The average microhardness (HV0.2) of the coating without CeO2 and the coating containing CeO2 nanoparticles is 1 050 and 1 230 respectively. Wear test was carried out under SBF (simulated body fluid lubrication) conditions. The addition of CeO2 nanoparticles significantly improved the wear resistance of the coating. The wear volume loss of the coating is about 2.2% of the substrate. The wear mechanism of the coating without CeO2 includes abrasive wear, adhesive wear and fatigue wear, while the wear mechanism of the coating with CeO2 nanoparticles does not include fatigue wear. The addition of CeO2 coating structure refines the grains and improves the fatigue wear resistance of the coating.

Compared with general powders, ultrafine nanoparticle powders have high surface activity, good chemical activity, low melting point, and unique and excellent electromagnetic and optical properties, which have attracted extensive research at home and abroad. Nanoceramics have better advantages in strength, wear resistance, and oxidation resistance than general ceramic coatings. The crystal structure of nano-scale ceramics is fine equiaxed crystals, and the coating is denser. Research on laser cladding nano-ceramic coatings has not yet fully explored the excellent properties of ceramic materials, and further exploration is needed.

2.5 Laser pyrolysis of ceramic precursors to convert ceramic coatings

Organic polymer precursors to inorganic ceramics (PDC) synthesis is a feasible method for preparing ceramic coatings on the surface of metal substrates, etc. Research on laser pyrolysis of precursors to prepare ceramic coatings has achieved certain results. LIU J et al. prepared SiC ceramic coatings on the surface of 45 steel by laser pyrolysis of polysilazane (PSZ). The microstructure and morphology of the ceramics were analyzed by XRD, SEM and TEM, and it was found that the laser pyrolysis product was mainly β-SiC, with a face-centered cubic structure and a polycrystalline structure, which enhanced the oxidation resistance and creep resistance of the substrate surface. QIAO Y L et al. used γ-glycidyloxypropyltrimethoxysilane and tetrabutyl titanate to prepare titanium-type organic silicon precursor on the metal surface by chemical reaction, and used continuous different laser scanning to prepare SiTiOC ceramic coating, analyzed the composition and structure of SiTiOC ceramic coating, and studied its tribological properties, as shown in Figure 10. It can be seen that at 350 W and 500 W, the surface morphology of SiTiOC ceramic coating is flat and dense, and there is no obvious change compared with 0 W. At 800 W, uniform particles appear. When the laser power is large, the sintering is more complete, and the organic polymer is ceramicized and pyrolyzed, thereby increasing the particle size of the ceramic.

Wang Sijie et al. prepared Ti-Si organic ceramic film by laser pyrolysis of tetrabutyl titanate and silane coupling agent (KH560). The results show that the coating surface morphology is best when the laser power is 600 W. As the laser power increases, the organic functional groups gradually decrease, and the degree of their conversion to inorganic groups is higher. The wear resistance of the new ceramic phases of SiO2 and TiO2 generated at 600 W is better.

Compared with the traditional heating and cracking of ceramic precursors, laser cracking reduces the thermal effect of high-temperature heating on the substrate, and the experimental process is simple and easy to achieve industrial application. The ceramic precursor is cracked into an inorganic ceramic coating by organic polymer laser. The coating exhibits excellent wear resistance and corrosion resistance, and the particles are fine and evenly distributed, and it is well bonded to the metal substrate. However, the coating has a large porosity, which is due to the overflow of small molecular gases during the pyrolysis of the ceramic precursor. Therefore, when designing the coating material system, some fillers are added to fill the pores in time during laser cracking, so that the ceramic coating is denser. Huang Kening et al. added different mass fractions of ferrocene to polysiloxane to prepare SiOC (Fe) ceramic coatings. They found that when the mass fraction of ferrocene was 20%, there were basically no pores on the surface of the ceramic coating by SEM observation, and the ceramic coating became dense and smooth. Through the test, it was found that ferrocene and polysiloxane under the action of laser underwent non-equilibrium free radical chemical reaction to generate Fe2O3, Fe3O4, and Fe3C phases, which had a filling effect on the coating, and as the content increased, the coating surface became denser. Zhao Jixin et al. mixed Ti powder with different mass fractions of polydimethylsiloxane by laser pyrolysis and ultrasonically dispersed it. It was found that SiC, TiC, and TiO2 were formed by laser pyrolysis, and as the mass fraction of Ti powder increased, the surface morphology of the coating became denser and more uniform. The volume of TiC and TiO2 generated by the reaction is larger than that of free carbon, which plays a role in filling pores. However, the research in this area is not yet mature, and further research is still needed on more material filling effects, pyrolysis precursor process control, and ceramic coating performance quality standard control.

3. Influence of process parameters on ceramic coating

Laser process parameters such as scanning speed, spot diameter, and laser focal length have an important influence on the geometry, dilution rate, coating thickness, organizational morphology, and surface characteristics of the coating. Therefore, in order to obtain high-quality ceramic coatings, the process parameters must be adjusted and optimized. If the laser rate is too low or the scanning speed is too fast, the cladding layer cannot be completely melted, and the cladding coating effect cannot be achieved. At the same time, the cladding layer will solidify rapidly, resulting in a large number of structural defects in the coating, such as pores and cracks. However, if the laser power is too high or the scanning speed is too slow, the molten pool will exist for too long after melting, and the coating material will be overburned for too long, resulting in excessive sintering of the coating material, resulting in loss of surface alloy components and the formation of craters. Under appropriate laser power, the slower the scanning speed, the longer the reaction time of the molten pool, so that the cladding materials such as ceramics are fully melted. The faster the scanning speed, the lower the dilution rate. The spot diameter determines the width of a single cladding. When the spot is larger, the cladding efficiency will be improved, but it requires more energy than a smaller spot, that is, the laser power needs to be increased. AGHILI S E et al. studied the effects of process parameters such as laser power, powder feeding speed and scanning speed on the geometric properties of a single-pass cladding layer. The results show that when the laser power is 300 W, a good dense cladding layer is formed. When the laser power is too high, part of the cladding layer falls off. When the laser power is constant at 400 W, by reducing the scanning speed and increasing the powder feeding speed, some poor bonding phenomena can be observed, resulting in coating detachment. Under a certain power, by increasing the scanning speed, the interaction time between the powder particles and the laser beam can be shortened, so that the heat input to the center of the molten pool is reduced, and the impact on the metal matrix is ​​smaller. The faster the scanning speed, the smaller the width of the cladding layer.

FAN P F et al. designed a single factor experiment to study the effects of laser power, scanning speed and powder feeding speed on the geometric dimensions, dilution rate and hardness of the WC-Co50 cladding layer. The results show that as the laser power increases, the cladding layer width, molten pool depth and dilution rate all increase, while the effect on the cladding layer height is small, and the average hardness gradually decreases. As the scanning speed increases, the height of the cladding layer and the depth of the molten pool decrease significantly, while the width of the cladding layer decreases slightly. At the same time, the average hardness gradually decreases. As the powder feeding speed increases, the cladding layer width, molten pool depth and dilution rate gradually decrease, and the cladding layer height and average hardness gradually increase, and the influence of the three on the quality of a single cladding layer is powder feeding rate> laser power> scanning speed. LI Y X et al. predicted the correlation between the main process parameters and the geometric characteristics of the coating through linear regression analysis. TiBCN ceramics were used as a reinforcing phase to enhance the performance of Ti-based coatings. The results show that the cladding height, cladding width, cladding depth and dilution rate are mainly affected by laser power, powder feeding speed and scanning speed; the cladding height is related to the powder feeding speed and conforms to the linear regression equation, and has little to do with the laser power. The cladding layer width increases with the increase of laser power and the decrease of scanning speed. When the laser power and scanning speed are the same and the powder feeding speed is different, the cladding layer width is basically equal. This shows that the powder feeding speed has little effect on the cladding layer width.

By optimizing the process parameters, the geometric properties of the ceramic coating can be significantly improved, and defects such as cracks and holes can be reduced, which is of positive significance for obtaining dense and excellent ceramic coatings. It is difficult to obtain the best microstructure and properties of the cladding layer with many process parameters. Experiments can be carried out with the help of orthogonal experiments, single factor experiments, linear regression equations, and mathematical model establishment to save time and cost.

4　Influence of auxiliary treatment on ceramic coating

Optimizing laser cladding process parameters can improve the quality and performance of ceramic cladding layer, but there are still some defects that cannot be avoided, such as dendrite segregation and uneven internal composition. In order to better solve these problems, auxiliary treatment methods have attracted attention, including heat treatment assistance, magnetic field assistance, ultrasonic assistance, etc. Studies have found that heat treatment assistance reduces the cracks of the coating and refines the grain size, while improving the friction and wear properties of the ceramic coating. Studies have shown that magnetic field treatment can refine grains and increase microhardness during metal solidification.

4.1　Ultrasonic assistance

LI M Y et al. used ultrasonic vibration-assisted technology to prepare high-hardness laser-clad nickel-WC CaF2 coatings on the surface of medium carbon steel, and studied the microstructure, element distribution, phase composition, microhardness and wear resistance of the cladding layer. The results show that ultrasonic vibration during laser cladding can reduce the aggregation of WC particles and generate new phases. The cladding layer without ultrasonic vibration is composed of (Fe, Ni) and WC particles, and the ultrasonic vibration cladding coating is composed of (Fe, Ni), Cr2C6, W2C and WC particles. Through ultrasonic vibration, the two phases of Cr2C6 and W2C are increased, and the grains are refined to make the cladding layer more uniform. When the ultrasonic vibration power is 800 W, due to the combined effect of grain refinement, hard WC phase dispersion strengthening and solid solution strengthening, the average microhardness of the cladding layer is improved relative to the hardness of the cladding layer. In addition, the wear mass loss and friction factor of the coating after ultrasonic vibration are lower than those of the coating without ultrasonic vibration. Finally, the friction and wear analysis found that the wear mass loss with ultrasonic vibration is 0.2 mg and the friction factor is 0.38, which are 80% and 17% lower than those without ultrasonic vibration, respectively. LI M Y et al. used ultrasonic-assisted laser cladding to form a 60% Ni, 0.8% WC and La2O3 metal-ceramic composite coating. It was found that under ultrasonic vibration, the dendrite structure of WC ceramic particles in the cladding layer was destroyed, so that the grain structure was refined and evenly distributed. When the ultrasonic power was 600, 700 and 900 W, the WC particles were concentrated at the bottom of the cladding layer, and the hardness of the cladding layer was also affected to a certain extent with the change of ultrasonic vibration. When the ultrasonic power was 800 W, the hardness and wear resistance of the coating were improved, which was mainly due to the strengthening of carbides, the refinement of the structure and the uniform distribution of WC particles in the coating. For the laser cladding layer without ultrasonic vibration, after the ring-on-disc (ROD) wear test, typical abrasive wear and fatigue wear characteristics can be found on the wear surface, and spalling pits appear along the sliding direction. For the composite coatings prepared with different ultrasonic vibration powers, in the ROD wear test at 600, 700 and 900 W, typical abrasive wear appeared on the coating surface, and shallow and mild wear appeared on the coating surface at 800 W. Ultrasonic assistance refines the internal grains of the coating, enhances the hardness and wear resistance of the ceramic coating, and new phases may appear through ultrasonic assistance.

4.2 Electromagnetic field assistance

The main technical principle of electromagnetic field assisted treatment is that the microscopic electrons of the ceramic coating material interact with the electromagnetic field, affecting the chemical reaction process, affecting the flow state of the molten pool during the cladding process, and then affecting the microstructure and element distribution. Figure 11 is a schematic diagram of the electromagnetic field assisted treatment principle. It can be seen that under the action of the magnetic field, the convection and stirring of the molten pool are enhanced, the temperature gradient and metal viscosity of the molten pool are reduced, and the grains can be refined, thereby affecting the organization and mechanical properties of the ceramic coating.

LIANG G et al. used magnetic field assisted laser cladding technology to synthesize directional array TiN reinforced AlCoCrFeNiTi HEA coating. The results show that the lattice constant of the BCC phase increases from 0.316 nm to 0.319 nm after the addition of a magnetic field. Since the magnetic field changes the solidification shape of TiN, TiN
has a larger contact area with BCC, and the micro-stress will increase the lattice constant of the BCC phase. After the addition of a magnetic field, the number of dendrites in the cladding layer decreases, and the number of equiaxed crystals and cellular dendrites gradually increases. The uniformity of element distribution in the cladding layer is improved. After the electromagnetic assistance is applied, the material flow in the molten pool is accelerated, the elements of the cladding layer respond to the periodic changes of the electromagnetic force, and the electromagnetic field affects the crystallization, which refines the grains in the cladding layer and improves the uniformity of grain distribution. In addition, this method can reduce the dilution rate of the cladding layer and improve the overall hardness of the cladding layer. HUO K et al. prepared Inconel718-WC composite coating by electromagnetic field-assisted laser cladding. Under the action of the composite electromagnetic field, a downward Ampere force is generated, and the enhanced Marangoni convection makes the WC particles evenly distributed in the composite coating. The enhanced Marangoni convection destroys the long columnar eutectic carbides and refines the microstructure of the coating. The study found that the WC particles decompose in the molten pool to form submicron eutectic carbides, feathery eutectic carbides and spherical eutectic carbides, as shown in Figure 12. The decomposition of WC particles is closely related to the spatial position of WC particles and the distance between WC particles. The higher the spatial position of WC particles relative to the molten pool, the greater the distance between WC particles, and the more serious the decomposition. Under the action of electromagnetic field, the nucleation rate of eutectic carbides is accelerated, and the reinforcing phases Fe3W3C and Ni17W3 increase and are evenly distributed. The microhardness (HV0.2) of the coating at 20 mT and 100 A is 530, which is significantly higher than that of the coating without auxiliary laser cladding (400).

4.3 Heat treatment assistance

The use of heat treatment assisted laser cladding can reduce or even avoid excessive residual stress caused by uneven temperature distribution between the coating and the substrate, which affects the microstructure and properties of the coating. CHEN T et al. studied the effect of heat treatment assistance on the performance of TiC/TiB bio-inert ceramic coatings. The coatings were annealed at 400, 600 and 800 °C, and the effects of heat treatment on the microstructure, residual stress, microhardness, fracture toughness and wear resistance of the coating were investigated. It was found that with the increase of annealing temperature, the residual stress, fracture toughness and wear resistance of the cladding layer were improved. TAO Y F et al. first heated the substrate and cladding powder to 400, 600 and 800 ℃ to prepare TiC and TiB2 reinforced TiNi/Ti2Ni based composite coatings, and then stored them at three temperatures for 3 h. The results showed that with the increase of temperature, the residual stress on the coating surface decreased and the wear volume increased. The reason was that the strain hardening effect weakened, the density of the reinforcement body decreased, and the cracking sensitivity of the laser cladding layer decreased. When the ambient temperature changed from room temperature to high temperature, the wear mechanism of the coating changed from a single brittle debonding to a combination of micro-cutting and brittle debonding, and the high wear resistance was maintained under the premise of effectively eliminating residual stress. In summary, heat treatment has a positive effect on the microstructure and properties of laser cladding coatings.

5 Conclusion and Prospect

The current research status and characteristics of different types of laser cladding ceramic coatings are summarized, and the existing problems and development trends are discussed. For a series of defects such as holes, cracks, and stress concentration caused by factors such as different thermal expansion coefficients of materials in laser cladding ceramic coatings, the main solution is currently through optimizing process parameters, material systems, and external field auxiliary treatment. Although the research on laser cladding ceramic coatings has made some progress, there are still some problems that need to be improved.

(1) Cladding materials. In the face of increasingly harsh working environments, it is necessary to design ceramic materials that are more adaptable to the working environment. The design should ensure that the metal substrate and the ceramic coating have similar physical properties, and ensure the compatibility and wettability of the two. Improving wettability includes increasing the wettability temperature of the ceramic material and metal interface and increasing the surface energy of ceramic powder particles, or adding appropriate alloying elements. When designing and controlling the composition of the added ceramic hard phase coating, the negative impact on the substrate should be reduced. In laser cladding, there is a temperature difference between the molten pool and the substrate, resulting in the actual chemical composition not matching the designed composition. Therefore, it is necessary to design a reasonable material composition based on the characteristics of the laser cladding process. The properties of ceramic metal materials can be predicted and optimized by using first-principles calculations, high-throughput, and multi-scale calculations. Design a special material system for laser cladding ceramic coatings. Finally, the development and design of ceramic coatings should consider material costs and certain performance improvements.

(2) Process parameter optimization For the optimization of laser cladding ceramic coating process, it is necessary to analyze and optimize according to the characteristics of different types of ceramic coatings such as metal-ceramic composite coatings, bioceramic coatings, and nano-ceramic coatings. Laser power, spot size, powder feeding speed, scanning speed, and other process parameters will affect the internal structure, macroscopic morphology, and mechanical properties of the ceramic coating. A suitable process can reduce problems such as cracks, holes, and dilution rate of the ceramic coating. This process requires continuous experimental testing and analysis, but the best combination of process parameters is obtained through various optimization algorithms and empirical formulas. At the same time, with the help of computer technology, the temperature field and stress field of the molten pool in the laser cladding process can be simulated and analyzed, and the influence of fluid flow and temperature changes in the molten pool on the microstructure of the cladding layer can be quantitatively studied.

(3) Auxiliary processing Combining laser cladding technology with auxiliary processing technology can reduce cracks and holes in the ceramic coating, refine grains, generate new phases, and improve the internal structure and macroscopic surface quality and mechanical properties of the coating. In order to reduce the roughness of the coating surface and obtain ceramic coatings with better performance, in the future we should continue to conduct in-depth research on the coordination effect of auxiliary processing methods and laser cladding ceramic coating materials, and select appropriate single or multiple auxiliary processing methods according to different cladding conditions. At the same time, we can research and develop more auxiliary processing methods to contribute to the further improvement of the performance of laser cladding ceramic coatings, and standardize and industrialize them.