As one of the most important raw materials for the production of cemented carbide, the particle morphology, size, particle size distribution and impurity content of wolfram carbide (WC) powder will directly affect the quality and application of cemented carbide. WC powder can be divided into ultra coarse tungsten carbide, micron tungsten carbide, sub micron tungsten carbide, sub nano tungsten carbide and nano tungsten carbide according to the particle size. In terms of application, submicron tungsten carbide powder is mainly used to produce cemented carbide, superhard cutting tools, jet engine parts and kiln structural parts.
Properties of submicron tungsten carbide powder
From the definition, tungsten carbide is a compound composed of transition metal tungsten and non-metallic carbon. Its chemical formula is WC and its molecular weight is 195.85.
From the physical and chemical properties, WC looks like a black granular powder with a melting point of about 2870 ℃ and a boiling point of about 6000 ℃. It is insoluble in water, hydrochloric acid and sulfuric acid, but easily soluble in the mixed acid of nitric acid and hydrofluoric acid. It has the characteristics of similar hardness to diamond, good conductivity and thermal conductivity, low coefficient of thermal expansion, high modulus of elasticity and compressive strength.
It is worth mentioning that the particle size of submicron WC powder is between micron and submicron, i.e. 100nm to 1.0 μ M, so it is not as easy to agglomerate as sub nano WC in a certain environment, that is, it has better dispersion performance. At the same time, it does not need to have a long milling time like micro WC, which is more conducive to the preparation of sub microcrystalline cemented carbide. However, it is not suitable for 3D printing technology because the particles are too large and the products produced are relatively rough.
Preparation of submicron tungsten carbide powder
The smaller the particle size of WC powder, the shorter the sintering time required in the material preparation process, and the lower the temperature required for densification. For example, nano WC powder begins to densify at 500 ° C, while sub micron WC powder begins to densify at 1200 ° C. Therefore, the preparation of WC powder with particle size less than 100nm can lay a good foundation for its subsequent sintering process.
In recent years, the main methods for preparing ultrafine or nano WC powders are: mechanical alloying, direct reduction carbonization, sol gel method, vapor phase carbonization, fixed bed chemical vapor phase method, plasma method, etc.
1 mechanical alloying method
Liu Lin et al. Adopted the mechanical alloying method, first mixed the W powder and C powder according to the atomic ratio of 1:1, put them into the steel pipe and introduced argon, then selected the 12mm diameter WC grinding ball, adopted the ball material ratio of 18:1, and finally carried out high-energy ball milling on the planetary ball mill. Through this method, WC powder with an average grain size of 7.2nm was obtained. Maxueming et al. Used mechanical alloying technology to reduce the particle size to about 75 μ M of W powder and C powder were mixed according to the atomic ratio of 1:1, and the selected ball material ratio was 30:1. The WC powder with an average grain size of 11.3nm was obtained by ball milling on qm-1f planetary high energy ball mill for 100h.
2 direct reduction carbonization
The reduction carbonization methods for preparing ultrafine WC powder can be divided into two categories: (1) two-step reduction carbonization: the first step is to decompose and reduce the precursor containing w to prepare W powder; The second step is to mix W powder with the substance containing C and heat it to high temperature, then carbonize it through chemical reaction to produce WC powder. In this method, W powder and C powder were mixed and reacted at high temperature (1400-1600 ℃) to form WC powder. (2) The one-step reduction carbonization method is the direct reduction carbonization method: the precursor containing W is mixed with the substance containing C, and then directly reduced and carbonized at high temperature to form WC powder. This method can not only improve the production efficiency of WC powder, but also obtain more uniform distribution of WC powder and finer grains.
Some experts obtained WC powder with grain size of 15-30nm by direct reduction carbonization. The preparation method is as follows: with WO3 and C as raw materials, the mixture of WO3 and C is first wet ground, in which the atomic ratio of C to W is greater than 1, then the wet ground slurry is spray dried, and then the intermediate product of WC powder and excess C is prepared by reducing carbonization at high temperature (1000-1100 ℃) with N2 as the protective gas, and finally the carbon content is adjusted to (6.13 ± 0.05)%.
Nano tungsten carbide powder was obtained by embedding direct reduction carbonization method. Reduction and combination reactions are carried out in A12O3 embedding device, which can provide reducing atmosphere at high temperature, so as to avoid oxidation of WC. The raw materials WO3 and C were pretreated by high-energy ball milling, and then the synthesis reaction was carried out at 1300 ℃ for 3 hours. Finally, the reaction products were pretreated by high-energy ball milling, and WC powder with grain size of 26nm was prepared after ball milling for 40H.
3 sol gel method
Nano tungsten carbide powder is prepared by sol gel / in-situ carbonization method. The preparation steps are as follows: first, hydrogen peroxide (mass fraction of H202 is 30%) is added to W powder (200 mesh), and glacial acetic acid and absolute ethanol are used as stabilizers to prepare yellow tungsten sol. The excess water is removed by evaporation, and then absolute ethanol dissolved in phenolic resin is added. After ultrasonic mixing, the sol containing w source and C source is obtained, After aging, gel was obtained. Finally, WC powder with grain size of 10.2nm was prepared by carbonization at 900 ℃ with H2 and Ar as shielding gas.
4 gas phase carbonization
The nano tungsten carbide powder was obtained by the gas phase carbonization method in Japan. He used WCl6 as the w source and CH4 as the gas phase C source to prepare the WC powder with the grain size of 20-30nm by chemical reaction at high temperature (1300-1400 ℃), and discussed in detail the grain size relationship between the reactant product system and the reaction temperature. The Tokyo tungsten company of Japan has applied for a patent for the preparation of ultra-fine WC powder by direct carbonization with WO3 as the w source and CO as the carbonization gas. The particle size and C content of the prepared WC powder can be controlled.
5 fixed bed chemical vapor method
WC powder of about 15nm was successfully prepared by fixed bed chemical vapor phase method. With nano WO3 as w source and acetylene as C source, the preparation steps are as follows: put the nano WO3 into a quartz reaction boat, and then put the boat into a high-temperature stainless steel tubular reactor; After vacuumizing, H2 is introduced. After holding at 660 ℃ for 1.5h, the nano WO3 powder is completely reduced to nano á -w powder. Then, the flow of H2 is reduced and acetylene is introduced. After holding at 800 ℃ for 4h, the nano á -w powder is transformed into WC powder.
6 plasma method
There is another common method to prepare ultra-fine / nano WC powder: plasma method, which uses the plasma as the heat source, and its temperature can reach 4000 ~ 5000 ℃. Under such high temperature, the powder raw materials will decompose and react to produce the required products. This method generally uses WO3, WC or w as the w source and CH4 as the C source. After the reaction, it mainly produces β- The research of WC or W2C, Japan’s Kuriyama, etc. shows that when the molar ratio of CH4 to WC is greater than 15, β- The mass fraction of WC is 90%-95% and the particle size of the powder is about 10nm. TEM observation β- The grain size of WC is 5-20nm, which has good fractional property.