A preparation method of super wear-resistant MoCoB-based cermet

Abstract

The present invention discloses a method for preparing a super wear-resistant MoCoB-based cermet. The disclosed method comprises: ball milling, drying and pressing a mixed powder of Mo, Co and B in sequence to obtain a green body, and then sintering the green body. The sintering process comprises: ‑3 ~10 ‑1 Pa, and then (1)-(3) are performed in sequence: (1) the temperature is raised from room temperature to 1250-1350°C and kept for 10-60 min, with a heating rate of 5-15°C / min; (2) the temperature is lowered to 1150-1240°C and kept for 10-30 h, with a cooling rate of 1-15°C / min; (3) the furnace is then cooled to room temperature. The present invention uses fewer types of raw materials, does not require the addition of elements and reinforcing phases, is easy to operate, and is suitable for industrial application and promotion; a high-hardness MoCoB-based cermet with fine and uniformly distributed grain size is prepared by a new sintering process, which can be used as a super wear-resistant material.

Description

Technical Field

[0001] The invention relates to MoCoB-based cermet-related technologies, and in particular to a method for preparing super-wear-resistant MoCoB-based cermet. Background Art

[0002] Ternary boride-based cermets have the advantages of high hardness, low density, high bending strength, good wear resistance and strong corrosion resistance, and can be used as wear-resistant parts for injection molding machines, can-making tools and copper extrusion molds. In addition, the thermal expansion coefficient of ternary boride-based cermets is similar to that of steel, and it has a natural advantage as a coating material for steel substrates. Among them, MoCoB-based cermets are new ternary boride-based cermets prepared by the "reactive boriding sintering" method. It has the advantages of high hardness, strong wear resistance and strong corrosion resistance, and the raw materials do not contain the strategic scarce resource W element. Therefore, it is favored by many researchers.

[0003] MoCoB-based cermets are composed of MoCoB hard phase and Co binding phase, wherein the MoCoB hard phase is not directly added as a raw material, but is sintered by the intermediate product and the metal binding phase through the boriding reaction during the sintering process. MoCoB hard phase is the hardest ternary boride known so far by first principle calculation, and has great potential in the field of super wear-resistant materials. The hardness of MoCoB-based cermets prepared by existing conventional methods is about 88.0HRA, which has a high hardness and can meet the requirements of conventional service conditions as a wear-resistant material, but there is still a certain gap from the theoretical value, which limits its development prospects in the field of super wear-resistant materials. Therefore, it is of great theoretical significance and engineering value to study how to improve the hardness of MoCoB-based cermets so that it has super wear resistance. The prior art improves the hardness of MoCoB-based cermets by adding carbon fiber, but adding carbon fiber will increase the cost of raw materials and greatly increase the process cycle. For example, during ball milling, due to the winding effect of carbon fiber, it takes a long period for the mixed powder to reach a completely uniformly distributed state. In addition, during powder pressing, the powder is entangled by carbon fibers, which is not conducive to its flow, so that it cannot fully and tightly fill the cavity of the mold, causing uneven density distribution of the green body and segregation of the sintered body composition. Summary of the invention

[0004] In view of the defects or shortcomings of the prior art, the present invention provides a method for preparing a super wear-resistant MoCoB-based cermet.

[0005] To this end, the method provided by the present invention comprises: ball milling, drying and pressing the mixed powder of Mo, Co and B in sequence to obtain a green body, and then sintering the green body. The sintering process comprises: sintering the green body at a vacuum degree of 10 -3 ~10 -1Pa, perform (1)-(3) in sequence: (1) raise the temperature from room temperature to 1250-1350°C and keep it for 10-60 min at a heating rate of 5-15°C / min; (2) reduce the temperature to 1150-1240°C and keep it for 10-30 h at a cooling rate of 1-15°C / min; (3) then cool the furnace to room temperature.

[0006] Optionally, calculated by mass percentage: Mo is 38.0% to 54.0%, Co is 41.0% to 57.0%, and B is 3.0% to 7.0%.

[0007] Preferably, the sintering is carried out in a vacuum carbon tube furnace.

[0008] Furthermore, the hardness of the super wear-resistant MoCoB-based cermet is 88.7-91.1 HRA, and the mass wear rate is 9.10×10 -5 ~10.2×10 -5 g·m -1 .

[0009] Furthermore, the grain size of the MoCoB-based cermet is 3.9-6.7 μm.

[0010] Compared with the prior art, the present invention has the following advantages: 1. fewer raw materials are used, no elements or reinforcing phases need to be added, the operation is simple, and the invention is suitable for industrial application and promotion; 2. a new sintering process is adopted, which not only ensures that the particle rearrangement movement in the MoCoB-based metal ceramic is fully carried out, so that the MoCoB hard phase is evenly distributed, but also inhibits the dissolution-precipitation mechanism of the MoCoB hard phase grains, thereby preventing them from growing fully; finally, a high-hardness MoCoB-based metal ceramic with fine and evenly distributed grains is obtained, which can be used as a super wear-resistant material. BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is the XRD pattern of the MoCoB-based cermet in Example 1 of the present invention;

[0012] Figure 2 This is a fracture morphology photograph of the MoCoB-based cermet in Example 1 of the present invention;

[0013] Figure 3 This is a fracture morphology photograph of the MoCoB-based cermet in the comparative example of the present invention. DETAILED DESCRIPTION

[0014] Unless otherwise specified, the scientific and technical terms and methods herein are understood or implemented according to the knowledge of ordinary technicians in the relevant field using existing related methods. It should also be understood that the temperature and concentration involved in this article are approximate values ​​for illustration purposes. Although methods and materials similar to or equivalent to the methods and materials described herein can be used for the implementation of the present disclosure, some suitable methods and materials are described below. The publications, patent applications, patents and other references mentioned herein are partially incorporated herein by reference. If there is a conflict, this article shall prevail. In addition, the materials, methods, solution concentrations and embodiments are only exemplary and are not intended to be limited. In the specific scheme, the skilled person can use the conventional experimental period according to the disclosure of the present invention to optimize the material ratio, concentration, operating parameter value, and reactant addition order involved in the method to achieve the purpose of the present invention.

[0015] The wear resistance of metal ceramics is mainly determined by their hardness. The MoCoB hard phase occupies about 75% (volume fraction) in MoCoB-based metal ceramics. The hardness of metal ceramics is mainly provided by the MoCoB hard phase, and its content, size and distribution state will affect the hardness of metal ceramics. Among them, although increasing the content of the hard phase can improve its hardness, it will greatly reduce its toughness, resulting in a decrease in service safety. The hardness of metal ceramics can be improved by reducing the MoCoB grain size and improving its distribution state without changing other properties. Therefore, the hardness and wear resistance of MoCoB-based metal ceramics with fine and uniform MoCoB grain size can be improved. After in-depth research by the inventors, it was found that the MoCoB-based metal ceramics were prepared using the "reaction boronization sintering" principle. During sintering, as the temperature increases, MoCoB hard phase and eutectic liquid phase will be generated in sequence. The hard phase particles are gradually distributed evenly through particle rearrangement driven by capillary forces. At higher temperatures, MoCoB particles gradually grow in the liquid phase through a dissolution-precipitation mechanism. Therefore, the sintering process parameters can be improved according to this rule, and a new sintering process can be adopted: first, a lower temperature is selected above the eutectic liquid phase temperature for a certain period of time; second, the temperature is lowered, and a certain temperature is selected in the solid phase and liquid phase temperature range for insulation, and then the furnace is cooled. The advantages of this process are that the first insulation temperature is above the liquid phase, the particles are evenly distributed after rearrangement, and the two insulation temperatures are relatively low, and the dissolution-precipitation mechanism of the MoCoB hard phase is always suppressed. Therefore, the grains will not grow fully, and high-hardness, ultra-wear-resistant MoCoB-based cermets are obtained.

[0016] The purpose of the compression molding of the present invention is to make the powdered raw material preliminarily dense, and the shape and size of the obtained green body depends on the shape of the mold. Existing molds used in the ceramic field, such as molds with a volume size greater than 3mm*3mm, are suitable for the present invention.

[0017] The present invention is further described in detail by way of examples. The raw materials Mo, Co and B powder used in the examples of the present invention are all commercially available chemically pure powder reagents, which are ball-milled using a planetary ball mill (QM-3SP4), then dried using a rotary evaporator (QE-2000), and then bidirectionally pressed using a stainless steel mold to form a green body, which is then sintered in a vacuum carbon tube furnace (ZT-25).

[0018] In the present invention, the grain size of MoCoB-based cermet is tested by using image analysis software (Image Pro Plus, Version 6.0); the hardness of MoCoB-based cermet is measured by using HRS-150 Rockwell hardness tester; the dry sliding friction and wear test of cermet is carried out by using MMG-500 pin-disc friction and wear tester, the grinding pair is Si3N4 ceramic disc, and the spindle speed is 100 r·min -1 , the wear radius is 12.5mm, the test time is 120min, and the test load is 150N.

[0019] Embodiment 1:

[0020] Mo, Co and B powders with a purity of not less than 99.0% are selected as raw materials, and the weight percentage is: Mo: 38.0%, Co: 55.0%, B: 7.0%; the mixed powder is ball-milled, and then dried and pressed into a cylindrical (diameter 44mm, height 8-9mm) green body; then the green body is placed in a vacuum carbon tube furnace for sintering, and the sintering process parameters are: the temperature is raised from room temperature to 1250°C and kept for 10min, and the heating rate is 1°C / min; the temperature is reduced to 1150°C and kept for 10h, and the cooling rate is 1°C / min; then the furnace is cooled to room temperature; the vacuum degree during the sintering process is 10 -3 Pa.

[0021] The preparation method of the super wear-resistant MoCoB-based cermet in this embodiment is simple. The obtained cermet comprises a MoCoB hard phase and a Co binder phase (see XRD spectrum). Figure 1 ), microstructure (see Figure 2 ) and mechanical properties are as follows: grain size is 3.9 μm, hardness is 91.1 HRA, and mass wear rate is 9.10×10 -5 g·m -1 .

[0022] Embodiment 2:

[0023] Mo, Co and B powders with a purity of not less than 99.0% are selected as raw materials, and the weight percentage is: Mo: 46.0%, Co: 49.0%, B: 5.0%; the mixed powder is ball-milled, and then dried and pressed into a cylindrical (diameter 44mm, height 8-9mm) green body; then the green body is placed in a vacuum carbon tube furnace for sintering, and the sintering process parameters are: the temperature is raised from room temperature to 1300℃ and kept for 30min, and the heating rate is 10℃ / min; the temperature is reduced to 1200℃ and kept for 20h, and the cooling rate is 10℃ / min; then the furnace is cooled to room temperature; the vacuum degree during the sintering process is 10 -2 Pa.

[0024] The preparation method of the super wear-resistant MoCoB-based cermet in this embodiment is simple. The microstructure and mechanical properties of the obtained cermet are as follows: grain size is 5.1 μm, hardness is 89.8 HRA, and mass wear rate is 9.7×10 -5 g·m -1 .

[0025] Embodiment 3:

[0026] Mo, Co and B powders with a purity of not less than 99.0% are selected as raw materials, and the weight percentage is: Mo: 54.0%, Co: 43.0%, B: 3.0%; the mixed powder is ball-milled, and then dried and pressed into a cylindrical (diameter 44mm, height 8-9mm) green body; then the green body is placed in a vacuum carbon tube furnace for sintering, and the sintering process parameters are: the temperature is raised from room temperature to 1350℃ and kept for 60min, and the heating rate is 15℃ / min; the temperature is reduced to 1240℃ and kept for 30h, and the cooling rate is 15℃ / min; then the furnace is cooled to room temperature; the vacuum degree during the sintering process is 10 -1 Pa.

[0027] The preparation method of the super wear-resistant MoCoB-based cermet in this embodiment is simple. The microstructure and mechanical properties of the obtained cermet are as follows: grain size is 6.7 μm, hardness is 88.7 HRA, and mass wear rate is 10.2×10 -5 g·m -1 Comparative Example 1:

[0028] The control example uses the same raw material composition as Example 1, except that the sintering process is to keep the temperature at 1250°C for 10 hours and then cool the furnace. The product obtained has poor lethality and is extremely brittle.

[0029] Comparative Example 2:

[0030] The control example uses the same raw material composition as Example 1, except that the sintering process adopts the existing conventional sintering process, and the specific process parameters are: the temperature is raised from room temperature to a maximum temperature of 1400°C, and kept warm for 30 minutes, the heating rate is 15°C / min, and the furnace is directly cooled after the heat preservation.

[0031] The MoCoB-based cermet microstructure obtained in this comparative example (see Figure 3 ) and mechanical properties are as follows: grain size is 7.3 μm, hardness is 87.9 HRA, and mass wear rate is 10.8×10 -5 g·m -1 .

[0032] The test properties of the MoCoB-based cermets of the above embodiments and comparative examples are shown in Table 1.

[0033] Table 1 Microstructure and mechanical properties of MoCoB-based cermets

[0034]

[0035]

[0036] from Figure 2 , Figure 3 It can be seen from the test results in Table 1 that, compared with Comparative Example 2, the grain size of the MoCoB-based cermet prepared by the new sintering process is reduced, the hardness is increased, and the maximum hardness reaches above 91HRA, which is a major breakthrough; at the same time, the mass wear rate is reduced. Compared with the conventional sintering process, the wear weight loss can be reduced by up to 10.1%, and the wear resistance is greatly improved.

Claims

1. A method for preparing a super wear-resistant MoCoB-based cermet, characterized in that: The method comprises: ball milling, drying and pressing a mixed powder of Mo, Co and B in sequence to obtain a green body, and then sintering the green body. The sintering process comprises: -3 ~10 -1 Pa, perform (1)-(3) in sequence: (1) raise the temperature from room temperature to 1250-1350°C and keep it for 10-60 min at a heating rate of 5-15°C / min; (2) reduce the temperature to 1150-1240°C and keep it for 10-30 h at a cooling rate of 1-15°C / min; (3) then cool the furnace to room temperature; By mass percentage: Mo is 38.0% to 54.0%, Co is 41.0% to 57.0%, and B is 3.0% to 7.0%; The hardness of the super wear-resistant MoCoB-based cermet is 88.7-91.1 HRA, and the mass wear rate is 9.10×10 -5 ~10.2×10 -5 g·m -1 ; The grain size of the MoCoB-based cermet is 3.9-6.7 μm.

2. The method for preparing the super wear-resistant MoCoB-based cermet according to claim 1, characterized in that: The sintering is carried out in a vacuum carbon tube furnace.

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