Enhance the wear resistance of 42CrMo steel and improve its severe wear failure. Diamond/WC particle reinforced cobalt-based composite cladding layer was prepared on the surface of 42CrMo steel by synchronous powder feeding using laser cladding technology. The macroscopic morphology and microstructure, phase composition, microhardness and wear resistance of the cladding layer were studied with the help of SEM, EDS, XRD, microhardness tester and multi-functional comprehensive performance tester. Pretreatment of diamond with Ti/TiC powder can improve its ablation and graphitization; appropriate amount of ZrH2 improves the width-to-thickness ratio of the cladding layer and promotes the convection mass transfer of the molten pool. At the same time, the active element Zr improves the wetting properties of diamond particles and increases the holding force of the bonding relative to diamond. The multi-pass overlap transition of the cladding layer is uniform, and its microstructure is mainly composed of fine dendrites and dense network carbide eutectics. The cladding layer reacts with the substrate to generate a planar crystal structure in the bonding area, thereby improving the bonding strength of the cladding layer. The thermal characteristics of laser cladding make W2C, ZrC, γ-(Co, Fe), M6W6C, CoZr2, (Ti, Zr)O2, TiCx, Co3Ti and other phases exist in the cladding layer. The fine grain strengthening and dispersion strengthening make the average microhardness of the cladding layer (1 002HV0.2) three times that of the substrate. The average wear of the cladding layer is 1/2 of the average wear of the substrate, and the average friction coefficient of the cladding layer is also significantly lower than that of the substrate, indicating that the wear resistance of the cladding layer is improved. Its wear mechanism is mainly abrasive wear. The diamond in the cladding layer is passivated due to the friction load, but the diamond in the wear mark is intact and has not fallen off. The wear resistance of the diamond/WC particle reinforced cobalt-based composite cladding layer is significantly improved, and it can be used for 42CrMo steel surface strengthening.

42CrMo steel is an alloy structural steel and is generally used to manufacture important parts that work under high loads. 42CrMo steel is often used as the material for components such as drill bits, gears, and picks. Wear is the main failure mode of 42CrMo steel[1], which seriously restricts the development of high-efficiency and intelligent mechanical equipment. The use of reasonable processes to strengthen the surface of 42CrMo steel can significantly improve its structure and enhance its overall performance. Flame thermal spraying, plasma cladding, surfacing, brazing, laser cladding and other technologies can effectively enhance the wear resistance of the coating and form a good metallurgical bond with the substrate[2]. When thermal spraying technology is used to prepare coatings, the substrate surface pretreatment process is cumbersome[3]; when plasma cladding technology is used to strengthen the surface of 42CrMo steel, the high-energy plasma beam has a greater impact on the substrate structure and performance and has a high dilution rate[4]; research on the application of surfacing technology in 42CrMo steel shows that overburning will change the substrate structure, resulting in low production efficiency and poor working conditions, and is not suitable for mass production[5]. Compared with other surface strengthening processes, laser cladding technology is simple in substrate treatment process; the laser produces a small heat affected zone and low dilution rate, which has little effect on the original performance of the 42CrMo steel substrate and can ensure the metallurgical bonding between the cladding layer and the substrate [6-8]; it has high production efficiency and low cost, and can realize automated mass production, which is an effective process for 42CrMo steel surface strengthening [9-10].

WC, SiC and other particle-reinforced composite materials have the characteristics of good toughness and impact resistance of metal materials and high hardness and high wear resistance of reinforcement materials. Their preparation process is mature and conducive to industrial production. When diamond is used as a reinforcement to strengthen the surface of 42CrMo steel, its wear resistance can be significantly improved [11]. However, diamond is easily ablated and graphitized at high temperature, and its extremely low thermal expansion coefficient leads to low bonding strength with the metal substrate [12]. Therefore, diamond particles are limited to a certain extent in the application of metal surface strengthening. Diamond composite coatings were prepared by thermal spraying, brazing and other technologies. The interface between the metal matrix and diamond and the microstructure of diamond particles in the coating were studied. The results showed that the diamond exposure was high, and the high welding temperature caused severe ablation and graphitization of the diamond [13-14]. There were macro cracks and many pores in the coating. Diamonds that had not been pretreated were used to obtain diamond/metal matrix composite cladding layers by pre-setting powder feeding in laser cladding technology. The wear resistance of the cladding layer was significantly improved, but the bonding strength between diamond and the bonding phase was low and the degree of graphitization was high [15-16]. High-speed laser cladding of FeCoCrNiMo high entropy alloy/diamond composite coatings was studied, and the effects of different laser cladding process parameters on the microstructure of diamond particles and the wear resistance of the coating were analyzed. The results showed that appropriate laser cladding process parameters can reduce diamond particle ablation and graphitization, improve the bonding between diamond and the bonding phase, and improve the wear resistance of the coating [17]. At the same time, when preparing a coating containing diamond particles, doping an appropriate amount of Ti and Zr elements or pre-treating the surface of diamond can also improve the wettability of diamond, reduce the degree of graphitization, and thus obtain a highly wear-resistant and corrosion-resistant composite coating [18-19].

Based on the above analysis, this study used Ti/TiC powder to pre-treat diamond particles to improve the ablation and graphitization of diamond under the action of laser. At the same time, ZrH2 was added to improve the fluidity of the molten pool, improve the wettability of diamond, and thus improve the bonding force relative to diamond. The laser cladding technology was used to simultaneously feed powder, optimize process parameters, and prepare a highly wear-resistant composite cladding layer suitable for 42CrMo steel. The condition of diamond particles in the cladding layer was further detected, and the microstructure and performance of the cladding layer were analyzed. It provides a certain reference for effectively enhancing the wear resistance of 42CrMo steel and delaying the failure of 42CrMo steel parts.

1 Experiment

1.1 Materials and sample preparation

The matrix material is 42CrMo steel, and its chemical composition is shown in Table 1. The size of the matrix sample is 30 mm×30 mm×8 mm. After polishing, it is ultrasonically cleaned with anhydrous ethanol and dried for use.

The pretreatment process of diamond is to load the configured material powder into the ball mill for mechanical mixing. Among them, the selected materials are A-grade single crystal regular diamond (average particle size 100 μm), TiC powder (average particle size 0.5 μm), and Ti powder (average particle size 0.5 μm). The proportion is 70% diamond, 20% TiC powder, and 10% Ti powder (mass fraction). The speed of the planetary ball mill is set to 220 r/min. After mixing for 48 hours, it is taken out and stored in a vacuum to protect the diamond from being damaged by the laser beam. The reinforcing phase of the cladding material is block WC powder (average particle size 60 µm), diamond and TiC powder (average particle size 0.5 µm) after TiC/Ti pretreatment; the bonding phase is pure cobalt powder (average particle size 60 µm); the added phase is ZrH2 powder (average particle size 0.5 µm), which improves the structure and performance of the cladding layer. The purity of each powder is above 99.5%. According to the composition ratio in Table 2, each cladding material is placed in a planetary ball mill for mechanical mixing. The ball milling time is 1 h and the speed is 230 r/min. After the ball milling, it is taken out and stored in a vacuum. The morphology of the cladding material is shown in Figure 1a. The surface of the diamond after pretreatment is enriched with Ti and Zr elements and the flatness is deteriorated (Figure 1b), which increases the contact area between the metal bonding phase and the diamond surface, indicating that the quality of diamond pretreatment is high.

2a is a schematic diagram of laser cladding on the surface of 42CrMo steel. The experiment used a 1 kW HWL-RAW1000 laser cladding system, with high-purity argon as the gas source, and a three-hole coaxial carrier gas powder feeder to ensure continuous and stable powder feeding. The laser cladding process parameters are: laser power 700 W, powder feeding rate 21 g/min, scanning speed 180 mm/min, carrier gas flow rate 4 L/min, distance between substrate and laser cladding head 13 mm, overlap rate 30%. The overlap transition of multiple cladding layers prepared on the surface of 42CrMo steel sample is uniform. Because the density of diamond particles in the cladding material is relatively small, a large number of diamond particles are attached to the surface of the cladding layer, as shown in Figure 2b.

1.2 Microstructure characterization and performance testing

The metallographic samples were prepared by etching and polishing the cross-section with 10% (volume fraction) HNO3 alcohol solution. The macroscopic morphology and microstructure of the cladding layer were observed using VEGA 3 SBH SEM, and the distribution of tissue elements was detected by EDS. The cladding layer phase was analyzed with the help of TD-3600 XRD, with the scanning speed set to 2 (°)/min and the diffraction angle set to 20°~80°. The HMV-G21ST microhardness tester applied a load of 1.96 N (HV0.2) and maintained it for 15 s to measure the Vickers microhardness distribution from the cladding layer to the substrate (more than 3 points were tested at different positions in the horizontal direction at each height and the average value was taken). The friction and wear test was carried out using a CFT-I multifunctional comprehensive performance tester, with a reciprocating stroke of 5 mm, a reciprocating motion of 1,000 times/min, a load of 80 N, a duration of 30 min, and a grinding ball material of Al2O3 (diameter 5 mm). The mass of the sample before and after wear was measured using a FA224C electronic analytical balance with an accuracy of ±0.1 mg. The wear scar profile was measured using a Focus SM-1000 confocal three-dimensional profiler, and the wear scar micromorphology was analyzed using a scanning electron microscope.

2 Results and discussion

2.1 Macromorphology of cladding layer

Under the premise of keeping other cladding process parameters unchanged, 0.5%, 1%, and 1.5% ZrH2 powder were added to the cladding material respectively. The results showed that there were macro cracks inside the cladding layer with a ZrH2 mass fraction of 0% (Figure 3a). With the increase of ZrH2 powder content, the width and thickness (H+h) of the cladding layer gradually increased, and the width-to-thickness ratio of the single-pass cladding layer showed an upward trend (Figure 3b). Therefore, the addition of ZrH2 enhanced the convection of the molten pool, reduced the generation of pores and cracks in the cladding layer[20], and improved its average microhardness. When the ZrH2 content is added in large amounts, a large number of inclusions will be formed, which will easily generate microcracks at the interface of the reinforcement phase, resulting in a decrease in the hardness of the cladding layer[21]. The microhardness of the cladding layer is an important parameter for judging its wear resistance[22]. Therefore, when the ZrH2 addition amount is 1% when preparing the diamond/WC particle reinforced cobalt-based composite cladding layer, its wear resistance can be improved.

Comparing the morphology of diamonds in the cladding layers with ZrH2 mass fractions of 0%, 0.5%, 1%, and 1.5%, it is found that the pretreated diamonds have no ablation or graphitization, and the particles are intact. The wettability of diamond particles in the cladding layer prepared without adding ZrH2 powder is poor, as shown in the box in Figure 4a, and the diamond and the binder are not tightly bonded. In the cladding layer with a ZrH2 mass fraction of 0.5%, the binder phase and the diamond are well bonded locally, as shown in the yellow box in Figure 4b, indicating that its wettability is improved, but there are still micro-gaps at the bonding interface, as shown in the red box in Figure 4b. When the ZrH2 mass fraction is 1%, there are a small amount of pores in the cladding layer. The active component Zr element is beneficial to improving the wettability of diamond. The (Ti, Zr)O2, ZrC, and TiCx generated by the reaction are bonded to the diamond surface (Figure 4c), which expands the contact area with the metal binder phase and improves the holding force of the cladding layer on the diamond particles[23], thereby reducing shedding. When the ZrH2 mass fraction is 1.5%, the diamond particles are not easily detached. After the powder is decomposed by heat, a large amount of gas will be generated. The rapid condensation of the molten pool makes it impossible for the gas in the cladding layer to escape quickly, resulting in pores. If the amount of powder added is too much, inclusions will be generated during the reaction process [21], resulting in microcracks around the diamond particles (Figure 4d), reducing the holding force of the bond relative to the diamond and reducing the overall performance of the cladding layer.

The g area in Figure 4c is enlarged and analyzed in detail, showing that the diamond and the bond are tightly combined, and there is no microcrack at the bonding interface, as shown in Figure 4e. The EDS line scan spectrum of the elements shows that the C content increases first and then decreases, and then changes in a gradient and the content increases significantly. The diamond particles react to a certain extent under the action of the laser, which is conducive to the formation of new phases at the bonding interface; and at 3~4 µm from the starting point of the line scan, the Zr and Ti elements increase suddenly (Figure 4f). Combined with XRD analysis, it can be seen that new phases such as ZrC, CoZr2, and (Ti,
Zr)O2 are generated by the reaction at this location, indicating that when the quality of the diamond particles pretreatment is high, the unmelted TiC bonded to the surface and the new phases generated by the reaction can isolate and protect the diamond, reduce its ablation and graphitization under high temperature of the laser, and the good element transition phenomenon also indicates that the metal bonding phase and the diamond particles are chemically metallurgically bonded, which further improves the holding force of the bonding phase on the diamond. Therefore, adding 1% ZrH2 powder can improve the wettability of diamond in the cladding layer, and effectively increase the average microhardness of the cladding layer, thereby enhancing its wear resistance. On this basis, cladding was carried out according to the material composition ratio in Table 2, and the cross-sectional morphology, structure and properties of the cladding layer were studied.

The cross section of the sample has obvious regional boundaries (Fig. 5a), which can be divided into cladding layer, transition zone, heat-affected zone and substrate. The white blocks are unmelted WC particles. During the laser cladding process, WC partially melts to produce C elements, and the CO, CO2 and other gases generated by the reaction with O2 do not have time to escape [24]. At the same time, ZrH2 decomposes and releases H2 when heated, but the active component Zr can enhance the convection and mass transfer of the molten pool. Therefore, there are a few pores in the cladding layer, no macro cracks, and the structure of each area is dense. The rapid scanning of the high-energy laser beam causes the surface of the substrate to melt slightly, and the molten pool is quickly formed. At the interface where the cladding layer transitions to the substrate (120 µm from the starting point of the line scan), due to the different chemical compositions of the cladding layer material and the substrate, the content of elements such as Co, W, and Ti gradually decreases, while the content of Fe increases sharply (Fig. 5d). Different elements diffuse in a gradient manner. Fe has a strong affinity with C and Co[25]. During laser cladding, the convection mass transfer of the molten pool can promote the flow of elements. Some Fe elements in the matrix will diffuse into the cladding layer and participate in the reaction. The content of W, Ti and other elements in the transition zone is relatively low, and Co and Fe elements are enriched, which makes the toughness higher. The good element diffusion phenomenon indicates that the cladding layer and the matrix are metallurgically bonded (Figure 5b). The heat accumulation leads to a thicker heat-affected zone, and the microstructure of the area close to the bonding interface is coarse, but the matrix mainly forms a lath-shaped martensite structure under the action of the high-temperature laser beam (Figure 5c). The high-density dislocation inside it has a certain contribution to the structural strengthening, and the lattice distortion caused by supersaturated carbon also plays a role in solid solution strengthening[26]. However, the supersaturation of carbon in martensite is low, and its plasticity and toughness are good[27]. Therefore, the comprehensive performance of the heat-affected zone is good, and the impact on the safety of the matrix is ​​small. The gradient decrease of the microhardness from the cladding layer to the matrix is ​​achieved, and the thickness of the wear-resistant layer is indirectly increased.

2.2 Phase analysis

After rough grinding to remove the unmelted particles on the surface, the new phases in the XRD spectrum of the cladding layer are composed of W2C, TiCx, Co3Ti, CoZr2, (Ti, Zr)O2, ZrC, composite carbides (Co, Fe, W) and γ-(Co, Fe) solid solution (Figure 6).

The dilution of the matrix allows the Fe element with a strong affinity for the C element to dissolve into the cladding layer, forming new phases such as Fe6W6C and (γ-Fe). The (γ-Fe) and (γ-Co) generated at high temperature are both face-centered cubic structures[28], and the atomic radius of the Fe and Co elements is relatively small, which makes it easy to form a metastable γ-(Co, Fe) solid solution. The density of cast block WC particles is greater than that of diamond, and they tend to sink to the bottom of the molten pool. Therefore, the diffraction intensity of the new carbide phase in the cladding layer far from the bonding interface is low. The presence of ZrC, W2C, and Co6W6C indicates that WC particles and diamonds have reacted to varying degrees[29]. ZrH2 powder reacts at high temperatures to generate new phases with relatively stable chemical properties, such as (Ti, Zr)O2, CoZr2, and ZrC. Undissolved WC, TiC, and diamond particles can strengthen the cladding layer; the face-centered cubic structure of γ-(Co, Fe) solid solution has more slip planes and has a strong ability to hinder crystal plane slip[30], which plays a role in solid solution strengthening of the cladding layer; the intermetallic compound Co3Ti and many new carbide phases are evenly distributed in the cladding layer, which can disperse and strengthen the cladding layer.

2.3 Microstructure

The grain morphology at the bottom of the cladding layer is different and the stratification phenomenon is obvious. The bonding interface is a plane crystal, and the upper part is coarse dendrites and cellular dendrites. Large pieces of reticular eutectic structure are precipitated between the grains (Figure 7a). The rapid melting characteristics make the chemical composition at the bonding interface uneven and element segregation occurs, which in turn forms a nucleus-free equilibrium transition zone, which is conducive to the growth of plane crystals. When the high-energy laser beam begins to irradiate the substrate, within a certain range, the liquid phase temperature is higher and the supercooling is lower at a distance from the bonding interface. The positive temperature gradient is extremely large and the crystallization rate is the smallest. The latent heat of crystallization here is dissipated through the matrix wall, and plane crystals are easily generated. As the cladding process progresses, the bottom is farther away from the bonding interface, and the heat accumulation makes the supercooling of the liquid phase component larger. The negative temperature gradient also gradually decreases and has a specific direction, and the solidification rate increases. However, due to the dilution of the matrix, the Fe element content is relatively high, and fine nuclei are not obtained. Coarse dendrites are quickly formed along the descending direction of the temperature gradient. During the cladding process, the negative temperature gradient of the liquid phase in the middle of the cladding layer (Fig. 7b) continued to decrease, and the supercooling degree increased further. At the same time, it was less affected by the dilution rate, and there was a white particle phase, which caused the nucleation rate to increase sharply, forming a large number of non-oriented cellular dendrites. At the top of the cladding layer (Fig. 7c), the latent heat of crystallization dissipated from the surface, the temperature gradient reached the minimum, the solidification rate increased sharply, and the high-melting-point white particle phase (i.e., carbides of elements such as W, Zr, and Ti) was dispersed in the grains and grain boundaries, and served as the core of grain growth, reducing the nucleation supercooling around it, increasing the heterogeneous nucleation rate [31], and obtaining a large number of non-oriented growth and fine grain equiaxed dendrites and network eutectics formed by the aggregation of tiny solute atoms. The top structure was uniform and dense, and the comprehensive performance of the cladding layer was guaranteed.

By using EDS to detect different microstructures in Figure 7c, in the white particle phase 1 (Figure 8a), the mass fraction of W element is as high as 39.8%, and the content of Co and Fe elements is low. Compared with other positions, the content of C, Zr and Ti elements is relatively large. Combined with XRD, it is speculated that it is a carbide of W, Zr, Ti and other elements. In the equiaxed dendrite 2 (Figure 8b), the content of Co and Fe elements is greatly increased, the content of C and W elements accounts for a small proportion, and the content of Zr and Ti elements is extremely low. It is speculated that the equiaxed dendrite is a γ-(Co, Fe) solid solution of W and C elements. The WC particles melt to generate W elements with a larger atomic radius and dissolve into the Co-Fe solid solution, resulting in lattice distortion, which can effectively prevent crystal plane slip and enhance the ability of the cladding layer to resist plastic deformation [28]. In the eutectic 3 (Fig. 8c), the contents of W, Co, and Fe are relatively high, the contents of Zr and Ti are basically the same as those in the equiaxed dendrite 2, and the content of C increases slightly. Combined with the phase analysis, the organization includes the interstitial solid solution of γ-(Co, Fe) of the solid carbide and the composite carbides such as M6W6C. The eutectic organization is dense, the intergranular bonding is strong, and the cladding layer can be strengthened.

2.4 Microhardness distribution

The microhardness distribution of each area in the cross section of the cladding layer is measured from the top of the cladding layer (Fig. 9). Avoiding hard phase particles such as WC, the microhardness distribution of the cladding layer is uniform, with an average value of 1002HV0.2, and the hardness values ​​at the top and bottom are slightly higher. The drastic temperature gradient under the action of laser can rapidly generate multiple phases of alloy elements in the molten pool, increase the number of non-spontaneous nuclei, improve the nucleation rate [31], and gradually refine the grains from the bottom to the top of the cladding layer (Figure 7), which has the effect of fine grain strengthening [32]. The addition of ZrH2 powder enhances the fluidity of the molten pool, making the unmelted WC and TiC particles and new carbide phases such as W2C, Co3Ti, ZrC, and Co6W6C evenly distributed, which can disperse and strengthen the cladding layer. The presence of the eutectic network increases the total length of the grain boundary, which can effectively prevent the deformation of the cladding layer caused by dislocation slip movement and improve the microhardness of the cladding layer. The transition zone between the cladding layer and the substrate has a strong dilution effect, the content of the precipitated carbide hard phase decreases, and its structure is mainly coarse dendrites and planar crystals. The average microhardness value at this location drops to 795HV0.2. In the heat-affected zone where the quenching phase transformation occurs due to the continuous laser action, the microstructure is martensite[33], and the average microhardness value is 628HV0.2, which is higher than the average microhardness of the substrate (329HV0.2).

2.5 Friction and wear properties

The average friction coefficient of the substrate is 0.426 (Fig. 10a), which is 2.2 times the average friction coefficient of the cladding layer (0.192). During the running-in wear period, the friction curve of the cladding layer is in a downward state as a whole. The grinding ball and the cladding layer are in point contact at the beginning, and the cladding layer has micro-protrusions that are tightly bonded and have high hardness and have not been removed, which causes fluctuations in its friction curve. Under the action of the grinding ball, the flatness of the cladding layer gradually improves, and the friction resistance between the grinding ball and the cladding layer decreases, so its friction coefficient continues to decrease; the friction curve of the substrate is generally first decreasing and then increasing. Before the test, the surface of the substrate is polished to be smooth. The grinding ball and the substrate are in point contact with a small contact area, and the friction factor decreases. As the friction test progresses, the grinding ball will sink into the substrate, turning the point contact sliding friction into surface contact sliding friction. The friction area gradually increases, increasing the friction resistance, and the friction factor increases accordingly. After entering the stable wear period, the friction curve between the substrate and the cladding layer remains basically stable, the microhardness of the cladding layer is evenly distributed, and there is an equal hardness area at the bottom of the wear mark; however, there is a local fluctuation in the friction curve between the substrate and the cladding layer. 42CrMo steel undergoes plastic deformation during the friction and wear process, releasing its own residual stress, affecting the contact state between the substrate and the grinding ball, causing local fluctuations in the friction factor curve; the surface of the cladding layer has a strong ability to resist plastic deformation, and its microstructure is unevenly distributed. During the friction and wear process, WC particles and diamonds are broken and peeled off, changing the contact state between the cladding layer and the grinding ball, resulting in local fluctuations in the friction factor curve. The substrate wear amount is 3.3 mg (Figure 10b), which is twice the wear amount of the cladding layer (1.6 mg), and the wear resistance of the cladding layer is significantly improved. There is obvious material accumulation at the edge and end of the substrate wear mark (Figure 11a), no accumulation at the edge of the cladding layer wear mark (Figure 11b), and a small amount of material accumulation at the end. The wear mark depth of the substrate is 137.72 µm, which is twice the wear mark depth of the cladding layer (67.11 µm), so the wear rate of the cladding layer is low. The width-to-depth ratio of the wear mark is m=L/H, and the width-to-depth ratio of the cladding layer is 18.48, which is significantly greater than the width-to-depth ratio of the substrate wear mark (11.62), indicating that the cladding layer has a higher hardness and a stronger ability to resist plastic deformation. The bottom of the substrate wear mark is relatively smooth, while the bottom of the cladding layer wear mark has obvious protrusions, indicating that there are extremely hard tissues and particle phases there. The wear mark comparison analysis shows that the cladding layer has good wear resistance.

There is no hard phase in the matrix wear scar, and a large area of ​​local material peeling and transfer accumulation occurs at the bottom (Figure 12b), so adhesive wear is the main wear mechanism of the 42CrMo steel matrix. The relatively concentrated periodic stress during the friction process will produce a large cyclic strain, causing the matrix material to gradually develop fine cracks due to fatigue and continue to expand, and finally peel off in the form of flakes and layers (Figure 12a); the high-frequency point contact friction and wear test method causes small scratches in the matrix wear scar. The peeled material migrates under the action of continuous reciprocating friction load, so a more serious material bonding occurs at the end of the matrix wear scar (Figure 12c), which is the most typical feature of the adhesive wear mechanism.

There are diamond, WC particles and detached fine hard particles in the wear scar of the cladding layer, and some flake peeling occurs at the bottom, and the rest of the wear is small, accompanied by a large number of micro-grooves, and there is no obvious material accumulation (Figure 13b). The diamond in the wear scar of the cladding layer is relatively complete and firmly bonded to the bond without falling off; during the wear process, after the metal phase with lower hardness around the diamond is peeled off, the diamond particles protrude and will bear part of the friction load, so its edges and corners are gradually blunted; the periodic load causes the high-hardness grinding ball to continuously collide with the diamond, resulting in tiny fatigue brittle peeling of the diamond (Figure 13a). Due to the high microhardness of the cladding layer, the fine hard particles that fall off during the friction and wear process cannot be embedded in the cladding layer, and reciprocating scraping is performed in the wear scar. The three-body wear causes a large area of ​​micro-ploughing in the wear scar of the cladding layer. ZrH2 improves the fluidity of the molten pool, and then makes the carbide reinforcement phases such as W2C, Co6W6C, CO3Ti, and ZrC dispersed in the cladding layer, hindering micro-cutting, so the ploughing depth is shallow and the continuity is poor (Figure 13c). The content of WC particles in the cladding material is relatively high, which is the key substance for the entire cladding layer to bear the friction load. The bonding phase around the blocky WC peels off, exposing it on the surface of the cladding layer and resisting friction. When the wear intensifies, the WC particles produce microcracks due to fatigue stress and continue to expand, causing them to be crushed and slightly brittle to peel off (Figure 13d). The EDS line scan at the interface between the WC particles and the bonding shows that different elements change in a gradient, and the WC particles are well bonded to the cladding layer without falling off. At 12 µm from the starting point of the line scan, the W element increases suddenly in the cladding layer (Figure 13e). The WC particles partially melt under the action of the laser, which is conducive to the formation of new phases such as W2C, Co6W6C, and TiCx, and improves the wear resistance of the cladding layer. The friction and wear test results of the cladding layer show that its main wear mechanism is abrasive wear, and there is also some adhesive wear, which has better wear resistance than the substrate.

3 Conclusions

1) Diamond pretreated with Ti/TiC powder can improve the ablation and graphitization under high temperature of laser beam. Appropriate amount of ZrH2 powder can expand the width-to-thickness ratio of cladding layer and enhance the fluidity of molten pool; and the formation of new phases such as ZrC and (Ti, Zr)O2 improves the wettability of diamond, so that chemical metallurgical bonding is formed between diamond and bonding phase. During friction and wear, diamond gradually becomes passivated due to the friction load, and does not fall off, and the diamond holding force is relatively high.

2) The multi-pass overlap transition of diamond/WC particle reinforced cobalt-based composite cladding layer is uniform, there is no macro crack inside, and metallurgical bonding occurs with the substrate. The thermal characteristics of laser cladding make W2C, CoZr2, Co6W6C, Fe6W6C and CO3Ti dispersed in the cladding layer, enhancing its overall performance. Large uniform and dense eutectics are precipitated between the grains in the cladding layer, and its structure mainly includes γ-(Co, Fe) interstitial solid solution of solid carbide, M6W6C and other phases. The bottom of the cladding layer reacts to generate a planar crystal structure with good dislocation sliding ability, which improves the toughness of the joint; there are a large number of fine equiaxed crystals on the top, which improves the microhardness of the cladding layer.

3) The average microhardness of the cladding layer is 1 002HV0.2, which is 3 times the microhardness of the substrate. Its average friction coefficient is reduced by 0.234 compared with the substrate, the average wear of the cladding layer is 1/2 of the average wear of the substrate, and the cladding layer is mainly abrasive wear mechanism, and the wear resistance is significantly improved.