Boron nitride ceramic material

Boron nitride ceramics were first discovered in Berman‘s laboratory more than 100 years ago, and the material was developed on a larger scale in the late 1950s.

Boron nitride (BN) ceramic is a new type of ceramic material with excellent performance and great development potential, including five isomers, namely hexagonal boron nitride (h-BN) and wurtzite boron nitride (w -BN), trigonal boron nitride (r-BN), cubic boron nitride (c-BN) and orthorhombic boron nitride (o-BN).

Widely used in machinery, metallurgy, chemical, electronics, nuclear energy and aerospace.

Hexagonal form of boron nitride, zinc blende crystal form and wurtzite crystal form

1. Introduction to Boron Nitride Ceramics

The boron nitride ceramic is a crystal composed of a nitrogen atom and a boron atom, and has a molecular formula of BN and a molecular weight of 24.81. The chemical composition is 43.6% boron and 56.4% nitrogen with a theoretical density of 2.27 g/cm3.

Boron nitride ceramic powder has the characteristics of looseness, lubrication, light weight, easy moisture absorption, and white color. The boron nitride article is ivory white. At present, research on boron nitride is mainly focused on the study of its hexagonal phase (H-BN) and cubic phase (C-BN).

The performance of boron nitride can be mainly divided into the following aspects:

✔In terms of mechanical properties: it has the advantages of non-abrasive, low wear, dimensional safety, good lubricity, fire resistance and easy processing.

✔In terms of electrical characteristics: it has the advantages of good dielectric strength, low dielectric constant, low loss at high frequency, microwave penetration, and good electrical insulation.

✔In terms of thermal characteristics: it has the advantages of high heat conduction, high heat capacity, low thermal expansion, thermal shock resistance, high temperature lubrication and high temperature stability.

✔In terms of chemical properties: it has the advantages of non-toxicity, chemical stability, corrosion resistance, oxidation resistance, low humidity, bio-safety and non-stick properties.

2. Hexagonal boron nitride ceramics

Hexagonal boron nitride ceramics (h-BN) are the most commonly used forms of boron nitride. The structure of h-BN is similar to that of graphite, and has a hexagonal layer structure, lattice constant a=0.2504nm, c=0.6661nm, theoretical density 2.27g/cm3, melting point 3000°C, soft texture, strong processability, and color White, commonly known as "white graphite."

Graphite structure and hexagonal boron nitride ceramic structure

Hexagonal boron nitride (h-BN) has excellent electrical insulation, excellent chemical stability and excellent dielectric properties.

Thermal properties of hexagonal boron nitride: no obvious melting point, sublimation at 3000 ° C in 0.1 MPA nitrogen, 3000 ° C in inert gas, heat resistant to 2000 ° C in neutral reducing atmosphere, temperature in nitrogen and argon Up to 2800 ° C, the stability in oxygen atmosphere is poor, the use temperature is below 1000 ° C.

Hexagonal boron nitride is one of the most thermally conductive materials in ceramic materials. The thermal conductivity is ten times that of quartz and has a high thermal conductivity of 60 W/(m·K) perpendicular to the c-axis direction.

The low coefficient of thermal expansion, equivalent to quartz, is the smallest of ceramics, with a thermal expansion coefficient of 41*10-6 m/K in the c-axis direction and 2.3*10-6 m/K in the d-axis direction, so it is resistant to heat. The shock performance is very good.

Thermal properties of h-BN and other materials

Material maximum use temperature °C thermal conductivity

[W/( mK)]

Thermal expansion coefficient

10-6m/K

h-BN

900 (oxygen)

2800 (nitrogen)

25.1

0.7 (⊥)

7.5 (∥)

BeO

2000

255.4

7.8

Al2O3

1750

25.1

8.6

Talc porcelain

1100

2.51

8.7

ZrO2

2000

2.09

10.0

Quartz glass

130

1.67-4.19

6.5

Fluororesin

25

----Mechanical properties of hexagonal boron nitride ceramics: friction coefficient as low as 0.16, no increase at high temperature, higher temperature resistance than molybdenum disulfide and graphite, oxidizing atmosphere can be used up to 900 ° C, vacuum can be used to 2000 ° C. It has poor lubricating performance at normal temperature, so it is often used as a high-temperature lubricant in combination with graphite fluoride, graphite and molybdenum disulfide. Hexagonal boron nitride is a soft material with a Mohs hardness of only 2. It has good machinability, can be driven, milled, planed, drilled, ground, and cut, and has high processing precision. Therefore, it can be processed into high-precision parts and components by general machining methods.

Electrical properties of hexagonal boron nitride: Hexagonal boron nitride is a good conductor of heat and a typical electrical insulator. The normal temperature conductivity can reach 10^16~10^18Ω/cm. Even at 1000 °C, the resistivity is still 1014~106Ω/cm. The dielectric constant of h-BN is 3~5. The dielectric loss is (2~8)*10-4, and the breakdown strength is twice that of Al2O3, reaching 30~40kV/mm, so it is an ideal high-frequency insulation, high-voltage insulation and high-temperature insulation material.

Chemical properties of hexagonal boron nitride: HBN has excellent chemical stability. It does not react with general metals, rare earth metals, precious metals, semiconductor materials, glass, molten salts, inorganic acids, and alkalis. Most metal melts, such as steel, stainless steel, Al, Fe, Ge, Cu, Ni, Zn, etc., do not wet or act. Therefore, it can be used as a high temperature thermocouple cover, a metal crucible, a vessel for transporting liquid metal, a pump part, a mold for casting steel, and a high temperature electrical insulating material.

3. Cubic boron nitride

Cubic boron nitride (c-BN) was first synthesized by American General Electric (GE) under high temperature and high pressure conditions in the 1950s. Its hardness is second only to diamond and much higher than other materials, so it Together with diamonds are called superhard materials.

Superhard materials are widely used in sawing tools, grinding tools, drilling tools and cutting tools. Diamonds are easily oxidized at high temperatures, especially with iron-based elements, and are not suitable for ferrous metal processing of iron-based elements. Cubic boron nitride crystal structure is similar to diamond, hardness is slightly lower than diamond, and is often used as abrasive and tool materials.

In 1957, R.H. Wintov of the United States first developed boron nitride, but natural cubic boron nitride has not yet been discovered.

Cubic boron nitride cell

The performance of cubic boron nitride mainly includes high hardness and thermal stability, and the microhardness is second only to synthetic diamond.

热 Its thermal stability is better than synthetic diamond, it can maintain high enough mechanical properties and hardness at high temperature, and has good red hardness;

It has stable structure, high oxidation resistance, good chemical stability, and is especially good compared with diamond. It does not react with iron elements at temperatures up to 1100~1300 °C, so it is especially suitable for processing ferrous materials. ;

✔The thermal conductivity is smaller than that of diamond, but it is higher than cemented carbide and has good thermal conductivity;

✔ high flexural strength;

As a grinding material, ✔ has a long service life and good wear resistance.

However, the single crystal cubic boron nitride has a small crystal grain size and anisotropy, and has a cleavage surface which is easily cleaved, and has a large brittleness, and is highly susceptible to cleavage and breakage.

c-BN has high hardness, chemical inertness and thermal stability at high temperatures, so it is widely used as an abrasive c-BN grinding wheel in grinding.

Since c-BN has characteristics superior to other tool materials, people tried to apply it to cutting at the beginning, but the crystal of single crystal c-BN is small, it is difficult to make a tool, and c-BN is very sinter. Poor, it is difficult to make a large c-BN sintered body, until the 1970s, the former Soviet Union, China, the United States, the United Kingdom and other countries have successfully developed c-BN sintered body as a cutting tool - polycrystalline cubic boron nitride PCBN (Polycrystalline Cubic Boron Nitride). Since then, PCBN has been used in various fields of cutting with its superior cutting performance, especially in the cutting of high-hardness materials and difficult-to-machine materials.

After more than 30 years of development and application, PCBN tool materials for processing different materials have emerged.

PCBN tool

4. Preparation and application of boron nitride

4.1 Preparation and application of h-BN

1) Borax-urea (ammonium chloride) method

The borax-urea (ammonium chloride) method is a method in which anhydrous borax and urea are mixed and then heated in an ammonia gas stream to obtain boron nitride powder. The reaction equation is:

This method can realize continuous production and improve production efficiency. However, in the reaction process, the glass phase often appears to reduce the yield significantly, and the post-treatment is difficult. Therefore, it is necessary to further study the reaction mechanism and improve the synthesis process. The borax-urea method is a traditional method for preparing h-BN powder. It has low production cost, low investment, simple process and is suitable for industrial production. However, incomplete reaction of the raw materials or formation of by-products containing C during the reaction process may result in h- The content of BN is not high, and the purity of boron nitride obtained by synthesis is not high, and the uniformity of particle size is poor.

2) Water (solvent) thermal synthesis

The water (solvent) thermal synthesis method uses water (or organic solvent) as a reaction medium in an autoclave to create a high-temperature and high-pressure reaction environment by heating the autoclave, so that a substance which is usually insoluble or insoluble is dissolved and reacted. New crystals. Hydrothermal method is usually used to synthesize oxide or metal element ultrafine powder, and research on preparation of non-oxide superfine powder is still in its infancy.

The selection of suitable boron and nitrogen sources (such as ammonium borate, melamine) has an important effect on increasing h-BN content. Water is preferred as environmentally friendly solvent, but higher temperatures are required, while organic solvents can significantly lower the reaction temperature.

The process conditions of the hydrothermal method are relatively easy to control, the product particle size can reach nanometer scale, the uniformity and the sphericity are good, but the yield is generally low.

3) Chemical vapor deposition (CVD)

The h-BN powder prepared by CVD method generally adopts a hot wall reactor, and the gaseous raw materials containing B and N are introduced into a reaction chamber through a carrier gas, and a chemical reaction occurs between the gaseous raw materials at a high temperature to form a BN powder, wherein the boron source is common. Boron-containing compounds such as BF3, BCl3, BBr3 or B2H6 are used, and the nitrogen source is generally NH3 or N2.

The purity and sphericity of h-BN powder prepared by CVD method are high, but various factors need to be precisely controlled during the preparation process.

4) Application of h-BN

Hexagonal boron nitride is a material with high temperature resistance, corrosion resistance, high thermal conductivity, high insulation and excellent lubricating properties. It is widely used in petroleum, chemical, mechanical, electronics, electric power, textile, nuclear industry, aerospace and other sectors. .

 h-BN坩埚, insulating tube, evaporation boat and coating

4.2 Preparation and application of c-BN and PCBN

1) Preparation of c-BN

At present, the synthesis of c-BN single crystals is mainly synthesized by static high-pressure catalyst method, usually using hexagonal boron nitride (h-BN) and different catalysts as raw materials, at high temperature (1400~1800 °C) and high pressure (4~8GPa). Under the synthesis of c-BN single crystal powder, the color is mostly black or amber. The earliest synthetic c-BN in China used metallic magnesium as a catalyst, and later mainly used metal borides. The c-BN synthesized with nitrogen boride has a light color, mostly pale yellow, amber or colorless transparent crystal, and the crystal form is complete, the crystal surface is smooth, and the single particle compressive strength is high.

At present, the commonly used catalysts are mainly metal nitrides, borides and nitrogen borides to synthesize c-BN. Common nitrides are: Li3N, Mg3N2, Ca3N2; nitrogen borides are Li3BN2, Mg3B4N4, Cs3B2N4.

The crystalline state, B2O3 content, adsorbed water content, grain size and particle size of h-BN have significant effects on h-BN→c-BN conversion and c-BN crystal growth.

2) Preparation of PCBN

The c-BN single crystal powder of suitable particle size is sintered into polycrystalline cubic boron nitride at high temperature (1500~2000 °C) and high pressure (5~9 GPa) in the presence or absence of a binder.

In order to strengthen the bonding between c-BN grains, synthetic PCBN usually incorporates a certain proportion of binder. The choice of binder ultimately has an important impact on the structure and performance of the PCBN. The metal-based binder composed of metal and its alloy can play a good role in improving the toughness of PCBN, but the binder is easy to soften at high temperature, which is unfavorable to wear resistance; while ceramic as a binder can solve the softening problem at high temperature. However, PCBN has poor impact resistance and short life. Nowadays, a mixed type of ceramic and metal or metal alloy is used.

3) Application of c-BN and PCBN

A small particle c-BN single crystal can be used as the abrasive material. The c-BN abrasive tool bonds the c-BN abrasive particles into a product having a certain geometry as a superhard material by means of the action of the bonding agent. PCBN overcomes the shortcomings of c-BN single crystals such as easy cleavage and anisotropy, and is mainly used to make tool materials. PCBN tools are particularly suitable for high speed machining or high precision machining.