A method for preparing a high toughness and strong wear-resistant WC-Co coating

Abstract

The application relates to a preparation method of a high-toughness and strong-wear-resistant WC-Co coating and belongs to the technical field of thermal spraying cermet coating. Firstly, slurry mixed with WC powder, Co powder and an organic binder solution is sprayed and granulated to prepare spherical powder, then low-temperature heat treatment is carried out to obtain spherical WC-Co powder with certain cohesive strength, the spherical WC-Co powder is mechanically mixed with fine-particle boron nitride powder, high-temperature heat treatment is carried out above the eutectic point of WC-Co, WC-Co powder is separated through airflow classification, the powder after high-temperature heat treatment is deposited on the surface of a metal base through supersonic flame spraying, and a strong-wear-resistant WC-Co coating with nanocrystalline Co can be obtained. The application technology converts amorphous Co in a traditional coating into nanocrystals, improves the toughness of the coating and does not reduce the hardness of the coating.

Description

Technical Field

[0001] This invention belongs to the field of thermal spray metal-ceramic coating technology, specifically relating to a process method for preparing a WC-Co coating containing nanocrystalline Co with high toughness and strong wear resistance using ultrasonic flame spraying. Background Technology

[0002] Thermally sprayed WC-Co coatings, due to their relatively high hardness and wear resistance, are widely used for surface wear protection of various metal parts, significantly extending the service life of engineering equipment. However, with the increasing complexity of working environments, WC-Co coatings prepared by traditional thermal spraying methods cannot meet the stringent requirements for coating toughness and wear resistance in some applications involving high-stress impact and high-speed friction. Therefore, developing WC-Co coatings with stronger wear resistance is of great significance for expanding their applications.

[0003] The performance of thermally sprayed WC-Co coatings is influenced by various factors, among which the content and structure of Co, the binder phase, are crucial. Increasing the Co content in the coating can improve its ductility and toughness, but its hardness and wear resistance will decrease accordingly. Furthermore, in WC-Co coatings prepared by existing methods, most of the Co is in an amorphous state. This not only significantly reduces the overall plastic deformation capacity of the coating, but also, due to the disordered atomic arrangement at the WC / Co phase interface, hinders strain coordination at the WC / Co phase interface. Consequently, dislocations in WC cannot stimulate the generation and movement of phase dislocations in Co through stress transfer at the interface. This leads to stress concentration near the interface, causing crack propagation along grain boundaries. This is the main mechanism by which traditional WC-Co coatings experience rapid wear failure under external stress.

[0004] To address the aforementioned issues, this invention proposes a novel method for preparing thermally sprayed WC-Co coatings. This method involves increasing the Co content in the coating to transform the amorphous Co in traditional coatings into nanocrystalline structures, thereby improving the coating's toughness without reducing its hardness, thus obtaining a WC-Co coating with strong wear resistance. Summary of the Invention

[0005] The process flow and principle of the preparation method provided by this invention are as follows: First, a slurry containing WC powder, Co powder and organic binder solution is spray-granulated to prepare spherical powder. Then, low-temperature heat treatment is performed to obtain spherical WC-Co powder with a certain cohesive strength. The obtained spherical WC-Co powder is mechanically mixed with fine boron nitride powder and subjected to high-temperature heat treatment at a temperature above the WC-Co eutectic point. The WC-Co powder is then separated by airflow classification. The powder after high-temperature heat treatment is deposited on the surface of a metal substrate by supersonic flame spraying to obtain a WC-Co coating with nanocrystalline Co that is highly wear-resistant.

[0006] This invention provides a method for preparing a highly wear-resistant WC-Co coating, characterized by comprising the following steps:

[0007] (1) Using anhydrous ethanol as a medium, WC powder and Co powder with an average particle size of less than 0.8 μm were wet-milled and mixed. The mass fraction of Co in the mixed powder was 17%-25%. The dried WC-Co powder was mixed with polyethylene glycol and deionized water to form a slurry. The spherical powder was agglomerated and granulated using a centrifugal spray dryer. The obtained spherical powder was subjected to low-temperature heat treatment and air classification in sequence to prepare WC-Co spherical powder with a particle size distribution of 10-30 μm and a certain cohesive strength. The low-temperature heat treatment temperature was 1100-1180℃ and the holding time was 1-2h. The first-stage frequency of the air classifier was 17Hz and the second-stage frequency was 23Hz.

[0008] (2) After the spherical WC-Co powder that has undergone low-temperature heat treatment is mixed evenly with boron nitride powder with an average particle size of less than 5 μm, a second high-temperature heat treatment is carried out. The heat treatment temperature is 1300-1350℃ and the temperature is held for 1-2 hours. After cooling, the two are separated by airflow classification to obtain pre-alloyed spherical WC-Co powder. The classification frequency is 25Hz.

[0009] (3) A coating is prepared by spraying pre-alloyed spherical WC-Co powder onto the surface of a steel substrate using a supersonic flame spraying process. The coating thickness is 200-300μm. The spraying process parameters are: kerosene flow rate 22-23L / h, oxygen flow rate 930-950L / min, spraying distance 340-360mm, powder feeding rate 50-70g / min, and horizontal moving speed of the spray gun 0.4-0.6m / s. After cooling, a WC-Co coating with nanocrystalline Co and strong wear resistance is obtained.

[0010] The main technical features and advantages of the method of this invention are: (1) This invention increases the toughness of the WC-Co coating by increasing the Co content, and at the same time uses the pre-alloying of powder to form nanocrystalline Co in the coating to increase the hardness. Based on the synergistic effect of the two factors, the prepared coating has strong wear resistance; (2) The Co content in the initial sprayed powder and the coating in this invention is significantly increased compared with the traditional coating (the Co mass fraction is generally 12%-17%). Due to better plasticity and toughness, the powder can be fully deformed when deposited on the substrate, reducing residual stress. In addition, more Co melts during thermal spraying and can fully fill the pores. Therefore, the density of the prepared coating can be significantly increased, and it is easier to obtain a non-porous WC-Co coating; (3) This invention uses a high melting point (about 3000℃), good chemical stability (does not react chemically with WC and Co), and low density (2.3g / cm³). 3 This is far lower than the 13.1-14.4 g / cm³ of WC-Co powder. 3(3) Fine-particle (less than 5 μm) boron nitride powder separates the spherical WC-Co powder after low-temperature heat treatment, avoiding the adhesion of WC-Co particles during high-temperature heat treatment. At the same time, the fine-particle boron nitride can be easily separated by subsequent airflow classification, thereby avoiding the introduction of impurity elements into the WC-Co powder. (4) The combination scheme of the spray powder composition structure design and thermal spraying process proposed in this invention solves the technical problem that it is difficult to improve the hardness and toughness of WC-Co metal ceramic coatings in a coordinated manner. The method is simple and easy to implement. The application of the technology of this invention can make the coating obtain super wear resistance. Attached Figure Description

[0011] Figure 1 The scanning electron microscope (SEM) morphologies of WC-Co coatings with different Co contents prepared by the present invention and conventional processes are shown below; wherein, (a) is the SEM morphology of the WC-17Co coating prepared in Example 1, (b) is the SEM morphology of the WC-25Co coating prepared in Example 2, and (c) is the SEM morphology of the WC-25Co coating prepared in the comparative example.

[0012] Figure 2 The transmission electron microscope (TEM) microstructure of the WC-17Co coating prepared in Example 1 of the present invention is shown below; wherein, (a) is a bright-field TEM image of the WC-17Co coating prepared in Example 1, and (b) is the microstructure of the nanocrystalline Co phase in the WC-17Co coating prepared in Example 1.

[0013] Figure 3 The images show the transmission electron microscopy (TEM) microstructure of the WC-25Co coating prepared by the comparative example of the present invention; wherein, (a) is a bright-field TEM image of the WC-25Co coating prepared by the comparative example, and (b) is a selected area electron diffraction (SID) spectrum of the Co phase in the WC-25Co coating prepared by the comparative example. Detailed Implementation

[0014] The present invention will be further described below with reference to the embodiments, but the present invention is not limited to the following embodiments.

[0015] Example 1

[0016] Using anhydrous ethanol as the medium, WC powder and Co powder with an average particle size of less than 0.8 μm were wet-milled and mixed. The mass fraction of Co in the mixed powder was 17%. The dried WC-Co powder was mixed with polyethylene glycol and deionized water to form a slurry, which was then agglomerated and granulated using a centrifugal spray dryer. The obtained spherical powder was then subjected to low-temperature heat treatment and air classification to prepare WC-Co spherical powder with a particle size distribution of 10-30 μm and a certain cohesive strength. The heat treatment temperature was 1180℃ and the holding time was 1 h. The first stage frequency of the air classifier was 17 Hz and the second stage frequency was 23 Hz. The spherical WC-Co powder after low-temperature heat treatment was then mixed with polyethylene glycol and deionized water to form a slurry. The spherical WC-Co powder with an average particle size of less than 0.8 μm was then mixed with anhydrous ethanol and deionized water to form a slurry. After homogenizing 5μm boron nitride powder, a second high-temperature heat treatment was performed at 1350℃ for 1 hour. Upon cooling, the two powders were separated by airflow classification to obtain pre-alloyed spherical WC-Co powder at a classification frequency of 25Hz. A coating of 200μm was prepared by spraying the pre-alloyed spherical WC-Co powder onto the steel substrate using a supersonic flame spraying process. The spraying parameters were: kerosene flow rate 23L / h, oxygen flow rate 950L / min, spraying distance 360mm, powder feed rate 50g / min, and horizontal movement speed of the spray gun 0.6m / s. After cooling, a WC-17Co coating with nanocrystalline structure and strong wear resistance was obtained. The scanning electron microscope microstructure of the prepared coating is shown below. Figure 1 As shown in (a), the microstructure under transmission electron microscopy is as follows: Figure 2 As shown in (a) and (b), Figure 2 Figure (b) shows the nanocrystalline Co in the coating. The microhardness (test load 300 g), fracture toughness (test load 5 kg), and wear rate (test load 80 N, test time 30 min, wear distance 150 m) of the coating are listed in Table 1.

[0017] Example 2

[0018] Using anhydrous ethanol as the medium, WC powder and Co powder with an average particle size of less than 0.8 μm were wet-milled and mixed. The mass fraction of Co in the mixed powder was 25%. The dried WC-Co powder was mixed with polyethylene glycol and deionized water to form a slurry, which was then agglomerated and granulated using a centrifugal spray dryer. The obtained spherical powder was subjected to low-temperature heat treatment and air classification to prepare WC-Co spherical powder with a particle size distribution of 10-30 μm and a certain cohesive strength. The heat treatment temperature was 1100℃ and the holding time was 2h. The first stage frequency of the air classifier was 17Hz and the second stage frequency was 23Hz. The spherical WC-Co powder after low-temperature heat treatment was then mixed with polyethylene glycol and deionized water to form a slurry. The spherical WC-Co powder with an average particle size of less than 0.8 μm was then mixed with anhydrous ethanol and deionized water to form a slurry. After homogenizing 5μm boron nitride powder, a second high-temperature heat treatment was performed at 1300℃ for 2 hours. Upon cooling, the two powders were separated by airflow classification to obtain pre-alloyed spherical WC-Co powder at a classification frequency of 25Hz. A coating of 300μm was prepared by spraying the pre-alloyed spherical WC-Co powder onto the steel substrate surface using a supersonic flame spraying process. The spraying parameters were: kerosene flow rate 22L / h, oxygen flow rate 930L / min, spraying distance 340mm, powder feed rate 70g / min, and horizontal movement speed of the spray gun 0.4m / s. After cooling, a WC-25Co coating with nanocrystalline structure and strong wear resistance was obtained. The scanning electron microscope microstructure of the prepared coating is shown below. Figure 1 As shown in (b), the microhardness (test load 300g), fracture toughness (test load 5kg), and wear rate (test load 80N, test time 30min, wear distance 150m) of the coating are listed in Table 1.

[0019] In a comparative example, anhydrous ethanol was used as the medium to wet-mill and mix WC powder and Co powder with an average particle size of less than 0.8 μm. The mass fraction of Co in the mixed powder was 25%. The dried WC-Co powder was mixed with polyethylene glycol and deionized water to form a slurry, which was then agglomerated and granulated using a centrifugal spray dryer. The obtained spherical powder was then subjected to low-temperature heat treatment and air classification to prepare WC-Co spherical powder with a particle size distribution of 10-30 μm and a certain cohesive strength. The heat treatment temperature was 1100℃. The substrate was kept at a constant temperature for 2 hours. The first-stage frequency of the air classifier was 17 Hz, and the second-stage frequency was 23 Hz. A coating of pre-alloyed spherical WC-Co powder was prepared by spraying it onto the steel substrate using a supersonic flame spraying process. The coating thickness was 300 μm. The spraying parameters were: kerosene flow rate 22 L / h, oxygen flow rate 930 L / min, spraying distance 340 mm, powder feed rate 70 g / min, and horizontal movement speed of the spray gun 0.4 m / s. After cooling, a WC-12Co coating with amorphous Co microstructure was obtained. The scanning electron microscope microstructure of the prepared coating is shown below. Figure 1 As shown in (c), the diffraction patterns of the transmission electron microscopy microstructure and the Co phase are as follows: Figure 3 As shown in (a) and (b), Figure 3 Figure (b) shows that Co in the coating is in an amorphous state. The microhardness (test load 300g), fracture toughness (test load 5kg), and wear rate (test load 80N, test time 30min, wear distance 150m) of the coating are listed in Table 1.

[0020] Table 1 shows the microhardness, fracture toughness, and wear rate of WC-Co coatings with different Co contents prepared by the present invention and conventional processes.

[0021]

Claims

1. A method for preparing a high-toughness, high-wear-resistant WC-Co coating, characterized in that, Includes the following steps: (1) Using anhydrous ethanol as a medium, WC powder and Co powder with an average particle size of less than 0.8 μm were wet-milled and mixed. The mass fraction of Co in the mixed powder was 17%-25%. The dried WC-Co powder was mixed with polyethylene glycol and deionized water to form a slurry. The spherical powder was agglomerated and granulated using a centrifugal spray dryer. The obtained spherical powder was subjected to low-temperature heat treatment and airflow classification in sequence to prepare WC-Co spherical powder with a particle size distribution of 10-30 μm. The low-temperature heat treatment temperature was 1100-1180℃ and the holding time was 1-2 h. (2) After the spherical WC-Co powder that has undergone low-temperature heat treatment is mixed evenly with boron nitride powder with an average particle size of less than 5 μm, a second high-temperature heat treatment is carried out. The heat treatment temperature is 1300-1350 ℃ and the temperature is held for 1-2 h. After cooling, the two are separated by airflow classification to obtain pre-alloyed spherical WC-Co powder. (3) A pre-alloyed spherical WC-Co powder is sprayed onto the surface of a steel substrate using a supersonic flame spraying process to obtain the WC-Co coating with nanocrystalline Co and a coating thickness of 200-300 μm.

2. The method according to claim 1, characterized in that, Step (1) The first-stage frequency of the air classifier is 17 Hz and the second-stage frequency is 23 Hz.

3. The method according to claim 1, characterized in that, Step (2) The grading frequency is 25 Hz.

4. The method according to claim 1, characterized in that, The spraying process parameters for step (3) are: kerosene flow rate 22-23 L / h, oxygen flow rate 930-950 L / min, spraying distance 340-360 mm, powder feeding rate 50-70 g / min, and horizontal movement speed of the spray gun 0.4-0.6 m / s.

5. The high-toughness, high-wear-resistant WC-Co coating prepared by the method according to any one of claims 1-4.

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