Austenitic stainless steel is limited in its application due to its low hardness and poor wear resistance. Therefore, improving the surface properties of stainless steel is of great engineering significance for promoting its application. Composite coatings of Co-based alloy reinforced with MoSi2 of different mass fractions (0%, 20%, 40%) were prepared by laser cladding technology. The effects of MoSi2 addition on the microstructure, phase composition, hardness and friction and wear properties of the composite coatings were studied by scanning electron microscopy (SEM), X-ray diffractometer (XRD), electron probe microanalyzer (EPMA) and other methods. The results show that the addition of MoSi2 causes the microstructure of the composite coating to transform from columnar crystals to equiaxed crystals and planar dendrites, and has the effect of refining the structure. With the increase of MoSi2 content, the microhardness and wear resistance of the Co-based composite coating also increase. When the MoSi2 content is 0%, 20%, 40%, 40%, 0%, 2 …40%, 0%, 20%, 40%, 40%, 0%, 20%, 40%, 40%, 0%, 20%, 40%, 40%, 40%, 0%, 20 When the MoSi2 content is 40%, the microhardness of the MoSi2/Co-based composite coating is as high as 1 455 HV0.2, and the wear rate is 6.9×10-5 mm3/(N􀅰m); the hard phases (Cr5 Si3, MoSi2, Mo5 Si3 and Co2Mo3) and the new solid solution (Fe, Cr, Co)Si2 formed during the solidification process significantly improve the wear resistance of the composite coating; the wear mechanism of the MoSi2-reinforced Co-based alloy coating changes with the increase of MoSi2 content, that is, the synergistic effect of abrasive wear, adhesive wear and plastic deformation changes to adhesive wear, brittle microfracture and oxidative wear.

Austenitic stainless steel is widely used in aviation, metallurgy, machinery, petrochemical, construction, medical and other industries due to its excellent mechanical properties and corrosion resistance. However, low hardness and poor wear resistance limit its application in special environments and reduce the service life of the product. Therefore, while maintaining its overall toughness and strength, preparing a composite coating with higher and specific properties (such as wear resistance and oxidation resistance) on the surface of austenitic stainless steel is of great engineering significance for promoting its application. Laser cladding technology is a rapidly developing surface modification method that uses a high-energy laser beam to prepare a coating with special physical, chemical or mechanical properties on the metal surface, and can accurately control and remotely operate the repair of local and key components. Compared with other surface modification processes (thermal spraying and plasma electrolytic oxidation, etc.), laser cladding technology also has the advantages of good metallurgical bonding, small heat-affected zone, fast solidification rate (10’6 ~ 10’7 K/s), and low dilution rate [1-3]. Therefore, laser cladding technology has a good application prospect in industry.

Cobalt-based, iron-based and nickel-based alloy powders are commonly used cladding materials. Among them, cobalt-based alloy powders have good fluidity and wettability, and a low tendency to crack. According to the reported literature, the main ways to obtain ideal microstructure and high-performance coatings are: (1) directly adding ceramic reinforcement phase or generating ceramic reinforcement phase “in situ” in the matrix, such as WC[4], TiC[5], SiC[6], h-BN[7], etc. Xiao et al.[8] studied the wear mechanism of Fe-WC composite coatings with a mass fraction of 0-60% prepared by laser cladding technology. The results showed that the microhardness of the WC-Fe composite coating increased with the increase of WC particle content, and the wear resistance of the composite coating was significantly improved. (2) By adding alloying elements, intermetallic compounds (such as FeAl and Cr5 Si3 [9, 10]) are formed in the coating. The improvement of hardness and wear resistance is mainly attributed to the special crystal structure of the intermetallic compounds. (3) A supersaturated solid solution is formed in the coating. Due to the interaction between solute atoms and dislocations, the movement of dislocations is hindered, thereby improving the performance of the coating. MoSi2 is often used to prepare layered composite materials and various high-temperature resistant protective coatings due to its excellent performance and special crystal structure (tetragonal crystal structure, C11b type). However, there are few studies on the structure and wear properties of MoSi2/Co-based composite coatings with MoSi2 particles as the reinforcement phase. In this work, MoSi2-reinforced Co-based composite coatings with different mass fractions were prepared by laser cladding technology. The influence of MoSi2 particle content on the microstructure evolution and wear performance of the coating was investigated, and the wear mechanism of MoSi2 / Co composite coating was studied, in order to supplement the cobalt-based coating system for laser cladding, and also provide a relevant theoretical basis for the design of functional gradient composite materials.

1 Experiment

1.1 Test material and cladding

304 stainless steel was selected as the substrate, which was polished with sandpaper and cleaned with acetone before the experiment. Co-based alloy powder with a particle size of 50 ~ 110 μm (mass fraction Cr 27. 59%, W 4. 42%, Ni 2. 20%, Fe 1.64%, C 1.28%, Si 1.15%, MoSi2/Co composite coating) was used. 0.44%) as the matrix, and MoSi2 powder with mass fractions of 0, 20%, and 40% were added respectively. The cladding test selected GS ￣ＴＦＬ ￣ 10 kW CO2 laser, pre-set powder and single-layer single-pass scanning. By optimizing the process parameters and determining the final parameters: laser power 2.2 kW, scanning speed 8 mm/s, beam diameter 5 mm; pre-set powder thickness 4~5 mm, argon protection during cladding.

1.2 Microstructure and phase composition

The cladding block was cut perpendicular to the laser scanning direction to prepare the metallographic specimen, and the specimen was etched with a mixed solution (HCl, HNO3 volume ratio 3:1). The MeF3 optical microscope and The microstructure and wear morphology were observed by JEOL JSM-6700F scanning electron microscope (SEM); the coatings were analyzed by EPMA-1600 electron probe microanalyzer and Rigaku D/max-2400 X-ray diffractometer (XRD), where the diffraction conditions of XRD were Cu target Kα line, scanning speed 4 (°)/min, and step length 0.02°.

1.3 Microhardness and wear performance test

The microhardness test of all coating cross sections was carried out by HX-1000TM Vickers microhardness tester, with a load of 2 N, loading time 10 s. The cross-section coating was tested equidistantly and in parallel, and the average of 5 readings of each sample was taken as the microhardness of the corresponding sample.

At room temperature, a dry friction test was carried out using a Tribo ￣Ｓ￣Ｄ￣００００ Trirbometer testing machine, with a loading load of 5 N, a sliding frequency of 10 Hz, a maximum sliding speed of 6.28 cm/s, and a tungsten steel ball with a diameter of 6 mm. The wear rate KC can be used to understand the effect of the addition of MoSi2 particles on the wear resistance of the composite coating, and is calculated according to the following formula [11]:
Kc = πdA/ FS (1)
Where: d is the diameter of the wear track; A is the cross-sectional area of ​​the wear track; =area; F is the applied load; S is the total sliding distance.

2 Results and discussion

2.1 Microstructure and phase composition

Figure 1 shows the cross section and SEM morphology of the middle area of ​​the laser cladding MoSi2/Co-based alloy composite coating. During the laser cladding process, the molten pool temperature can reach about 2973 K, and the cooling rate is close to 10’6 ~ 10’7 K/s, which is a typical non-equilibrium solidification process. The microstructure morphology is affected by the direction of the maximum heat flow and is controlled by the ratio of the solid-liquid interface temperature gradient (G) to the solidification rate (R) [12]. The pure Co-based alloy coating has a good metallurgical bond with the substrate, as shown in Figure 1a, and is composed of columnar particles perpendicular to the bonding surface. The eutectic structure of the composite coating is composed of crystals and dendrites. Due to the high cooling rate in the local area of ​​the molten pool, finer secondary dendrite arms are formed, as shown in Figure 1b. In terms of micromechanical properties, columnar grains have no large-angle grain boundaries and can effectively transfer longitudinal loads, thereby improving the plasticity and creep resistance of the coating. When 20% MoSi2 is added, the solidification characteristics of the composite coating remain basically unchanged, but the morphology of the microstructure is affected, and some columnar crystals are transformed into equiaxed crystals, as shown in Figure 1c. When the dendrites are subjected to external pressure, the toughness of the dendrites will release a certain amount of energy, thereby hindering the propagation of cracks [8]. When the MoSi2 content reaches 40%, the organizational morphology of the composite coating is completely different from the former. The grains in the coating grow in the form of small planar dendrites with different morphologies (such as butterfly-shaped, bamboo-leaf-shaped, and petal-shaped), as shown in Figure 1d. As shown in Figure 1, the planar dendrite structure can not only transmit longitudinal stress well, but also effectively bear lateral load, thereby improving the comprehensive performance of the coating [8]. In addition, when MoSi2 particles are added, the effect of regional grain refinement is shown. Because when the melt near the unmelted MoSi2 solidifies, the heat will flow to the low-temperature MoSi2 particles remaining in the molten metal, forming micro-region directional growth [13], which further refines the structure. At the same time, the addition of MoSi2 particles will disrupt the directional growth characteristics of the cladding layer structure and reduce the anisotropy of the coating. With the increase of MoSi2 content, the above phenomenon becomes more obvious, and more synergistic grain refinement areas will be obtained, so that the cladding layer will produce a fine grain strengthening effect as a whole, as shown in Figure 1c and 1d.

Figure 2 is the XRD spectrum of the MoSi2/Co-based composite coating. It can be seen that the pure Co-based alloy coating is mainly composed of enriched γ-Co, Cr23C6 and Co3W3C phases, as shown in Figure 2a. According to the analysis results, the columnar dendrites are the matrix phase Co-based solid solution, surrounded by the eutectic structure composed of Co-based solid solution and carbides (Cr23C6 and Co3W3C). When MoSi2 particles are added, the main matrix phase remains unchanged, while the composition of the eutectic structure changes, and a small amount of Cr5Si3 and Co2Mo3 are formed, as shown in Figure 2b. With the increase of MoSi2 content, the type and intensity of silicide diffraction peaks increase. A small amount of incompletely melted MoSi2 diffuses with the Co-based alloy liquid. The planar dendrites are mainly composed of MoSi2, Mo5 Si3 and (Fe, Cr, Co) Si2 type solid solution, as shown in Figure 2c. These silicide hard phases help to improve the microhardness and wear resistance of the coating.

2.2 Microhardness

The hardness test results show that the addition of MoSi2 particles can significantly improve the microhardness of the Co-based alloy coating. The average microhardness of the pure Co-based alloy coating is 595 HV0.2, while the average microhardness of the composite coating with 20% and 40% MoSi particles is 695 HV0.2 and 1455 HV0.2, respectively. HV0.2. Compared with the pure Co-based coating, the microhardness of the MoSi2/Co-based composite coating increased by 1.2 and 2.4 times, respectively. With the increase of MoSi2 content, the formation of hard phase (Cr5Si3, MoSi2, Mo5Si3 and Co2Mo3) is the main reason for the significant increase in the microhardness of the composite coating; the formation of (Fe, Cr, Co)Si2 type new solid solution is also beneficial to improve the microhardness of the composite coating, playing a role in solid solution strengthening. In addition, the addition of MoSi2 particles refines the microstructure of the cladding layer, hinders the movement of dislocations and the migration of grain boundaries, and produces a fine grain strengthening effect, which makes the composite coating more durable. The microhardness of the composite coating is improved[8].

2.3 Friction and wear properties

Figure 3 is the friction coefficient curve of MoSi2/Co-based alloy composite coating. Under the same wear test conditions, all coatings experience two stages: running-in and stable wear, as shown in Figure 3. The friction coefficient of the pure Co-based coating is 0.225, and the friction coefficients of the composite coating containing MoSi2 particles are 0.241 and 0.301, respectively. With the increase of MoSi2 content, the friction coefficient of MoSi2/Co-based composite coating also increases. According to formula (1), the wear rates of MoSi2/Co-based composite coatings are 3.8×10′-4, 3.3×10′-4, 6.9×10′-5 ｍｍ’３ / (Ｎ􀅰ｍ). It can be seen that the wear resistance of MoSi2 / Co-based composite coatings increases with the increase of MoSi2 content, and the wear resistance of 40% MoSi2 composite coatings is the best.

The friction coefficient in the stable stage is the comprehensive effect of the interaction and deformation of micro-convex bodies, the “grooving” formed by wear particles, and the interatomic force. When the friction pair is made of the same type of material, the contribution of wear particles (wear chips, etc.) to the friction coefficient is greater than the hardness of the sliding surface. A lot of wear chips are generated during the sliding process, and the wear chips are mechanically interlocked with the hard surface [14]. When MoSi2 is 40%, the surface hardness is high and very little wear chips are generated, so its friction coefficient is relatively slightly larger. In addition, due to the high MoSi2 content, Mo5 Si3 , Co2Mo3 and other hard phases. The hard particles are hard and not easy to wear. At the same time, they can change the movement direction of the abrasive, forming a “shadow” protection effect, so that the composite coating presents a lower wear amount and exhibits excellent wear resistance.

Figure 4 is the SEM morphology of the surface wear morphology of the MoSi2/Co-based composite coating. The pure Co-based coating surface wear is more serious, and a wide and deep “plow” morphology parallel to the sliding direction appears. There is no obvious peeling, and there is wear debris accumulation on both sides of the groove and at the front of the sliding stop movement, as shown in Figure 4a. During the wear process, a high shear stress is generated at the contact point between the coating surface and the surface of the counter-abrasive body. Under the cutting action, the contact surface undergoes plastic deformation to form a side flash. The flash cracks and falls off during reciprocating rolling to form debris. It can be seen that pure Co The wear mechanism of the base coating is dominated by abrasive wear, accompanied by a certain amount of plastic deformation. When 20% MoSi2 is added, the “grooves” on the surface of the sample become narrower and shallower, and the amount of wear debris decreases, as shown in Figure 4b. The hard phases Cr5 Si3 and Co2Mo3 formed by the addition of MoSi2 play an effective bearing role in the friction process, inhibiting plastic deformation, hindering wear and shedding, and thus improving the wear resistance of the coating. In addition, a more obvious spalling phenomenon also occurred during wear. From a microscopic point of view, due to the small actual contact area and the large local stress, the contact points are bonded or welded, and spalling occurs due to shear or fatigue fracture during relative tangential motion. The spalling phenomenon of the composite coating indicates that adhesive wear has occurred [15]. The wear mechanism of the 20% MoSi2/Co-based composite coating has changed, which is a joint mechanism of abrasive wear, adhesive wear and plastic deformation.

With the increase of MoSi2 particle content (40%), the wear surface is very different from the previous one. The wear surface is dark, and there is no obvious “plowing groove”, accumulated wear debris and peeling phenomenon. When observing the wear debris morphology under high magnification, it is found that the wear debris is in the shape of fine corrugations that have not completely fallen off, as shown in Figure 4c, indicating that the wear resistance of the 40% MoSi2/Co composite coating is better. Because more hard reinforcement phases (MoSi2, Mo5 Si3 and Co2Mo3) are formed during the solidification process, the hard reinforcement phases inhibit the plastic deformation of the composite coating surface and prevent the expansion of microcracks. Although brittle fracture is aggravated during friction, the distribution of hard particles as the main carrier is more dense, which can effectively protect the coating and greatly reduce the surface contact area between the coating and the dual steel ball. In addition, the solid solution generated after the addition of MoSi2 has a solid solution strengthening effect on the coating, which improves the wear resistance of the composite coating. At the same time, the wear mechanism also changes again, turning into adhesive wear, brittle microfracture and oxidation wear. Navas et al. [16] found that under low contact load (3 N), the surface oxidation mechanism dominated the wear of Tribaloy T-800 layer (cobalt-based alloy family). Li et al. [17] studied the mechanism of MoSi2 strengthening nickel-based alloys and found that when the addition amount of MoSi2 increased to 30%, the wear of the composite material was significantly reduced. The surface exhibits typical brittle microfracture and oxidation wear characteristics.

3 Conclusion

(1) MoSi2/Co-based alloy composite coatings with different mass fractions were prepared by laser cladding technology. The addition of MoSi2 particles caused a significant change in the microstructure morphology, that is, the columnar crystals transformed into equiaxed crystals and planar dendrites, and had the effect of refining the microstructure; the types and quantities of phase components also increased with the increase of MoSi2 content.

(2) Compared with the microhardness of pure Co-based coating (595 HV0.2), the microhardness of MoSi2/Co-based composite coatings were 695 HV0.2 and 1 455 HV0.2, respectively, which were increased by 1.2 times and 2.4 times, respectively. With the increase of MoSi2 content, the microhardness of MoSi2/Co-based composite coatings increased by 1.2 times and 2.4 times. The wear resistance of Co-based composite coatings also improves, mainly due to fine grain strengthening, solid solution strengthening and second phase strengthening.

(3) The MoSi2-reinforced Co-based coating is mainly a combination of abrasive wear, adhesive wear and plastic deformation. When the MoSi2 content reaches 40%, the wear mechanism changes to adhesive wear, brittle microfracture and oxidation wear.