A method for promoting densification sintering and increasing hardness of transition metal borides by adding trace amount of silicon dioxide

Abstract

The application belongs to the technical field of inorganic nonmetallic hard material, and discloses a method for promoting densification sintering of transition metal boride and improving hardness of the transition metal boride by adding trace silicon dioxide. The application synthesizes B2O3-SiO2 glass phase by adding a small amount of silicon dioxide in the process of preparing the transition metal boride, effectively refines the crystal grains, and prepares a submicron transition metal boride powder with good sintering performance, so that the transition metal boride ceramic with high density and high hardness can be prepared; the transition metal boride ceramic after sintering has a density of about 98%, and still has high hardness under high load.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of inorganic non-metallic hard material, and particularly relates to a method for promoting densification sintering of transition metal borides and improving hardness of the transition metal borides by adding trace silicon dioxide. BACKGROUND

[0002] Transition metal borides (TMBs), such as WB2 and ZrB2, are considered as ideal structural materials for extreme environment applications (such as high-temperature nuclear reactors, jet engines and catalytic materials) due to their high melting point (>3000℃), high hardness, excellent thermal shock resistance, corrosion resistance and electrocatalytic performance, and the ability to be synthesized at normal pressure. However, there are still great challenges in the densification sintering of transition metal borides, even under extreme conditions such as high temperature / high pressure or long time solid phase sintering.

[0003] Studies have shown that the grain surface of transition metal borides is usually covered with a layer of amorphous B2O3 with a thickness of nanometers, and the melting point of B2O3 is 450℃, and it will volatilize at about 1400℃. During high-temperature sintering, this phenomenon will on the one hand cause the existence of liquid phase for a long time, and thus cause grain coarsening and reduction of sintering activity; on the other hand, the volatilization of B2O3 at high temperature leads to the reduction of boron source, resulting in a large number of pores on the surface of the sintered ceramic sample. These factors may be the main reasons for the difficulty in densification sintering of transition metal borides and the low hardness.

[0004] Currently, the measures to solve this problem mainly include the following: (1) removing byproduct B2O3 through boron thermal reduction method combined with water washing process, and preparing ultrafine ZrB2 powder at 1000°C, and the density can be increased to about 98% under SPS-2000°C, but the process cannot completely remove B2O3 (oxygen content is 1.98wt%), and the process is relatively complex, which is not suitable for large-scale industrial production (Journal of the European Ceramic Society 40(12) (2020) 3844-3850. (2) Diluting HF etching oxide by adding methanol, under the best etching conditions, the oxygen content of ZrB2 can be reduced from 1.23wt% to 0.34wt%, and the density can reach 99.5%, but this process will use a large amount of chemicals, which does not meet the concept of green production (Journal of Alloys and Compounds 548 (2013) 173-179.). (3) Adding a second phase that can react with B2O3, such as Si3N4, AlN, ZrN, etc., by generating BN and metal oxides, but metal oxides will make it more difficult to densify the composite ceramic. For example, Li et al. added 5Vol% and 10Vol% AlN, and prepared ZrB2-15Vol%SiC-AlN ceramics by hot pressing at 1900°C, when the content of AlN is 5Vol%, the sample has higher density (96.4%), and when too much AlN is added, it will react with B2O3 to generate Al2O3, causing holes to appear on the surface of the sample (Acta Crystallographica Sinica 38(S1) (2009) 36-39.).

[0005] The above methods are based on reducing the oxygen content to achieve densification sintering, but the process is relatively complex, and the residual oxide cannot be completely removed, which affects the densification sintering of the ceramic. Therefore, it is urgent to find a method to solve the problem of B2O3 affecting the densification sintering. SUMMARY

[0006] In order to overcome the shortcomings and deficiencies in the prior art that the existence of amorphous oxide layer B2O3 on the surface of transition metal boride makes it difficult to densify and sinter, thereby affecting the mechanical properties, the primary purpose of the present application is to provide a method for promoting the densification sintering of transition metal boride and improving its hardness by adding a small amount of silicon dioxide; by adding a small amount of silicon dioxide (SiO2), it reacts with B2O3 to form a glass phase during sintering, thereby promoting densification sintering, and obtaining a transition metal boride bulk material with higher density and mechanical properties.

[0007] Another purpose of the present application is to provide a transition metal boride prepared by the above method.

[0008] Still another object of the present application is to provide an application of the above-mentioned transition metal boride.

[0009] The object of the present application is achieved by the following technical solutions:

[0010] A method for promoting the densification sintering of transition metal boride and improving the hardness thereof by adding trace amount of silicon dioxide, comprising the following steps:

[0011] S1, the following raw materials: silicon dioxide (SiO2) powder, transition metal oxide (TMO S ) powder, boron carbide (B4C) powder, carbon source powder are weighed and loaded into a jar under argon environment, grinding balls are added, and ball milling is carried out under argon protective atmosphere and room temperature conditions to obtain uniformly mixed powder;

[0012] S2, the uniformly mixed powder obtained in step S1 is dry pressed into a green body by a dry press machine, and the green body is heat treated at 1300-1500℃ under vacuum for 0.5-2h, and then cooled in the furnace, and the obtained loose bulk body is ground and sieved to obtain transition metal boride (TMBs) powder with added silicon dioxide;

[0013] S3, the transition metal boride (TMBs) powder with added silicon dioxide obtained in step S2 is placed into a graphite mold for pre-pressing, and solidification sintering is carried out by spark plasma sintering at 1500-1700℃ for 5-15min, and then the transition metal boride bulk material is obtained after cooling.

[0014] The ball milling time in step S1 is 0.5-3h; the grinding balls are tungsten carbide; the mass ratio of the raw materials to the grinding balls is 1:(3-5); the molar ratio of the transition metal oxide (TMO S ) powder, boron carbide (B4C) powder, and carbon source powder is 2:(1-1.5):(4.7-5); the added amount of the silicon dioxide (SiO2) powder is xVol.% of the uniformly mixed powder, and 0

[0015] The transition metal oxide (TMO S ) powder in step S1 is WO3, ZrO2, TiO2, or HfO2, and is preferably WO3.

[0016] The dry pressing pressure in step S2 is 3-30Mpa; the heat treatment time is 1-2h; the grinding is carried out in an agate mortar, and the sieving is carried out on a 200 mesh sieve.

[0017] The pre-pressing pressure in step S3 is 3-30Mpa; the spark plasma sintering is carried out under vacuum or argon protective atmosphere, and the sintering pressure is 10-30Mpa; and the heat treatment time is 10min.

[0018] The ball milling in step S1 is performed by using a high-energy ball mill, a vibration ball mill, a planetary ball mill, a field-assisted mill, a plasma-assisted high-energy ball mill or a stirred ball mill.

[0019] The sintering in step S3 is performed by using a hot-press sintering furnace or a pressureless sintering furnace.

[0020] A transition metal boride bulk material prepared by using the method has a density of 98.36%, and a hardness of 24.9±0.48 Gpa, which is superior to an optimal WB2 ceramic material prepared by using the prior art (a density of 95.8% and a hardness of 24.3±0.36 Gpa).

[0021] The transition metal boride bulk material is applied in the fields of high-performance coating, hydrogen catalysis, powder metallurgy, polishing, cutting tools, high-temperature-resistant materials and wear-resistant materials.

[0022] Principles of the present application:

[0023] B2O3 can form a eutectic glass phase B2O3-SiO2 with SiO2. According to a binary phase diagram of B2O3 and SiO2, the melting point of the B2O3-SiO2 glass phase can be adjusted by controlling the content of SiO2 in B2O3. The present application utilizes this principle, and adds SiO2 to the transition metal boride to generate B2O3-SiO2 with B2O3 on the surface of the boride. By controlling the amount of SiO2 added, liquid-phase sintering is promoted, and the densification effect of the transition metal boride ceramic is improved, and the mechanical properties are improved. For a specific transition metal boride powder, the control of the amount of added SiO2 is the key to achieving high density. The present application is suitable for all cases related to the sintering of transition metal boride ceramics.

[0024] The present application has the following advantages and beneficial effects relative to the prior art:

[0025] (1) The transition metal boride powder obtained by the present application has a high yield and fine grains at a low cost, and is suitable for industrial mass production.

[0026] (2) The present application can improve the sintering effect of the transition metal boride (TMB SA small amount of SiO2 is added in the process to synthesize B2O3-SiO2 glass phase, effectively refining the grain, preparing a sub-micron transition metal boride powder with good sintering performance, which can be used to prepare transition metal boride ceramics with high density and high hardness; the sintered transition metal boride ceramic has a density of ~ 98%, and still maintains high hardness under high load. The effect of a small amount of SiO2 on the grain size refinement of WB2 powder and the promotion of block material sintering is proved.

[0027] (3) The transition metal boride powder prepared by the present application can be applied to high-performance coating, hydrogen catalysis, powder metallurgy and related industrial fields such as polishing and grinding; the transition metal boride ceramic can be used in cutting tools, high-temperature-resistant materials and wear-resistant materials and related industrial fields. BRIEF DESCRIPTION OF DRAWINGS

[0028] Figure 1 XRD pattern of the tungsten diboride powder prepared in Examples 1-2.

[0029] Figure 2 SEM pattern of the tungsten diboride powder prepared in Examples 1-2.

[0030] Figure 3 Powder particle size distribution pattern of the tungsten diboride powder prepared in Examples 1-2.

[0031] Figure 4 XRD pattern of the tungsten diboride block prepared in Examples 3-4.

[0032] Figure 5 SEM pattern of the cross section of the tungsten diboride block prepared in Examples 3-4.

[0033] Figure 6 Hardness change pattern of the tungsten diboride block prepared in Examples 3-4. DETAILED DESCRIPTION

[0034] The content of the present application will be further illustrated below in combination with specific examples, but should not be understood as a limitation of the present application.

[0035] The ball mill used in the following examples is a high-energy ball mill (SPEX, USA, model 8000M), and the raw materials used in the following examples are tungsten trioxide (WO3, purity 99.9%, particle size 500 nm, Shanghai Xiantian Nanometer Material Co., Ltd., China), boron carbide (B4C, purity 99.9%, particle size 1 micron, Shanghai Xiantian Nanometer Material Co., Ltd.), carbon black (purity 99.9%, particle size 30 nm, Cabot), and silicon dioxide (SiO2, purity 99.9%, particle size 1 micron, Shanghai Xiantian Nanometer Material Co., Ltd.).

[0036] Example 1

[0037] 1. Weigh tungsten trioxide powder, boron carbide powder, and carbon black powder in an argon atmosphere at a molar ratio of 4:1.075:4.8 and place them in a tungsten carbide ball mill jar. Add six tungsten carbide grinding balls with a size of 11 μm. The mass ratio of grinding balls to mixed powder is 4:1. Ball mill in an argon atmosphere for one hour to obtain a uniformly mixed powder.

[0038] 2. The uniformly mixed powder obtained in step 1 is pre-pressed into a blank using a dry press. Then, the block is placed in an atmosphere furnace and kept at 1400℃ in a vacuum environment for one hour. After cooling with the furnace, the resulting loose block is broken up, ground in an agate mortar, and passed through a 200-mesh sieve to obtain tungsten diboride powder.

[0039] Example 2

[0040] 1. Weigh tungsten trioxide powder, boron carbide powder, and carbon black powder in an argon atmosphere at a molar ratio of 4:1.075:4.8, then add silica powder. Place all powders in a tungsten carbide ball mill jar, add six tungsten carbide grinding balls (11 μm in size), and maintain a grinding ball to powder mass ratio of 4:1. Ball mill in an argon atmosphere for one hour to obtain a uniformly mixed powder. The silica powder is added at 1 vol.%, 2 vol.%, and 3 vol.% of the uniformly mixed powder, respectively.

[0041] 2. The uniformly mixed powder obtained in step 1 is pre-pressed into a blank using a dry press, and then the block is placed in an atmosphere furnace and kept at 1400℃ in a vacuum environment for one hour. After cooling with the furnace, the resulting loose block is crushed, ground in an agate mortar, and passed through a 200-mesh sieve to obtain tungsten diboride powder with 1 Vol.%, 2 Vol.%, and 3 Vol.% SiO2 added.

[0042] Figure 1 These are XRD patterns of the fine-grained, lamellar tungsten diboride powders prepared in Examples 1 and 2. Figure 1 The phase composition of the synthesized powder can be observed. After heat treatment at 1400℃, the main phase of all samples is WB2 type WB2 (P63 / mmc, ICCD PDF#43-1386). However, a small amount of tungsten trioxide phase appears, which is due to insufficient reaction rate, resulting in incomplete oxide reaction and subsequent oxide residue. No silica phase was detected in the XRD pattern, because the amount added was very small, and the intensity of the detected diffraction peak was very weak compared to that of tungsten diboride.

[0043] Figure 2 and Figure 3 These are SEM images and particle size distribution diagrams of the fine-grained, lamellar tungsten diboride powders prepared in Examples 1 and 2.Figure 2 It can be obviously seen that the addition of SiO2 powder can effectively promote the decrease of the grain size of the tungsten diboride flaky grains. From the powder grain size distribution graph of Example 1, it can be seen that the grain size distribution is reduced to different degrees after the addition of SiO2 powder, especially when 1 Vol.% SiO2 is added, the powder grain size is reduced from 0.43 microns without addition to 0.29 microns. Figure 3

[0044] Example 3

[0045] The tungsten diboride powder obtained in Example 1 was placed in a graphite mold and pre-pressed at 10 MPa, and then sintered in a spark plasma sintering furnace at a sintering pressure of 30 MPa, with argon as the sintering atmosphere, at 1600°C for ten minutes, and after cooling, the tungsten diboride block was demolded.

[0046] Example 4

[0047] The tungsten diboride powder obtained in Example 2 with the addition of 1 Vol.%, 2 Vol.%, and 3 Vol.% SiO2 was respectively placed in a graphite mold and pre-pressed, and then sintered in a spark plasma sintering furnace at a pressure of 30 MPa, with argon as the sintering atmosphere, at 1600°C for ten minutes, and after cooling, the tungsten diboride block with the addition of 1 Vol.%, 2 Vol.%, and 3 Vol.% SiO2 was demolded.

[0048] Figure 4 The XRD pattern of the tungsten diboride block prepared in Examples 3-4, from which it can be seen that the main phase of the tungsten diboride block sintered at 1600°C is WB2 type WB2 (P63 / mmc, ICCD PDF #43-1386), and no diffraction peak of WO3 is detected in the XRD pattern, which is because the residual WO3 in the powder is fully reacted in the spark plasma sintering furnace, so no diffraction peak of WO3 is seen. Figure 4

[0049] The SEM graph of the tungsten diboride block prepared in Examples 3-4, from which it can be seen that when no SiO2 is added, a small amount of pores can be seen between the grains, and the density is 96.6%; when 1 Vol.% SiO2 is added, the pores are obviously reduced, the grains are uniformly distributed and tightly packed, and the density is 98.4%. When 2 Vol.% SiO2 or 3 Vol.% SiO2 is added, the pores between the grains are obviously increased, and the densities are 95.2% and 97.0%, respectively. Figure 5

[0050] Figure 6 ​​Hardness change plot of the tungsten diboride bulk prepared for Example 3-4 under 9.8 N load. As can be seen from the plot, the hardness of the tungsten diboride bulk increased from 23.1 ± 0.95 GPa to 24.9 ± 0.48 GPa with the addition of 1 Vol. % Si02, while the hardness of the tungsten diboride bulk was 23.7 ± 0.61 GPa and 22.5 ± 0.56 GPa with the addition of 2 Vol. % Si02or 3 Vol. % Si02.

[0051] The above examples are the preferred embodiments of the present application, but the embodiments of the present application are not limited to the above examples, and any changes, modifications, substitutions, combinations, simplifications made without departing from the spirit and principles of the present application should be equivalent replacement methods, and are included in the protection scope of the present application.

Claims

1. A method for promoting the densification and sintering of WB2 and improving its hardness by adding trace amounts of silica, characterized in that... The following steps are included: S1. Weigh and pack the following raw materials—silica powder, tungsten trioxide, boron carbide powder, and carbon source powder—in an argon atmosphere, add grinding balls, and ball mill under an argon protective atmosphere and at room temperature to obtain a uniformly mixed powder; the amount of silica powder added is 1 Vol.% of the uniformly mixed powder. S2. The powder obtained in step S1 is dry-pressed into a blank using a dry press, and kept at a vacuum of 1300℃~1500℃ for 0.5~2h. After cooling in the furnace, the resulting loose block is ground and sieved to obtain WB2 powder with added silica. S3. Place the WB2 powder with added silica obtained in step S2 into a graphite mold for pre-pressing, and then solidify and sinter it by spark plasma sintering at 1500~1700℃ for 5~15min. After cooling, WB2 bulk material is obtained.

2. The method for promoting the densification and sintering of WB2 and improving its hardness by adding trace amounts of silica according to claim 1, characterized in that: The ball milling time in step S1 is 0.5~3h; the grinding ball is tungsten carbide; the mass ratio of the raw material to the grinding ball is 1:(3~5); the molar ratio of tungsten trioxide, boron carbide powder and carbon source powder is 2:(1~1.5):(4.7~5).

3. The method for promoting the densification and sintering of WB2 and improving its hardness by adding trace amounts of silica according to claim 1, characterized in that: The pressure of the dry pressing in step S2 is 3~30 MPa; the heat preservation time is 1~2 hours; the grinding is carried out in an agate mortar; and the sieving is carried out in a 200-mesh sieve.

4. The method for promoting the densification and sintering of WB2 and improving its hardness by adding trace amounts of silica according to claim 1, characterized in that: The pre-pressing pressure in step S3 is 3~30 MPa; the discharge plasma sintering is carried out under a vacuum or argon protective atmosphere, and the sintering pressure is 10~30 MPa; the holding time is 10 min.

5. The method for promoting the densification and sintering of WB2 and improving its hardness by adding trace amounts of silica according to claim 1, characterized in that: The ball milling described in step S1 is a high-energy ball mill, a vibratory ball mill, a planetary ball mill, a field-assisted ball mill, or a stirred ball mill.

6. A WB2 bulk material prepared by the method according to any one of claims 1-5, characterized in that: The WB2 bulk material has a density of 98.36% and a hardness of 24.9 ± 0.48 GPa.

7. The application of the WB2 bulk material according to claim 6 in the fields of high-performance coatings, hydrogen catalysis, powder metallurgy, grinding and polishing, cutting tools, high-temperature resistant materials and wear-resistant materials.

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