Vitrified CBN grinding wheel is a kind of superhard grinding wheel which is sintered by CBN abrasive and vitrified bond under high temperature and high-pressure condition.
Introduction of CBN
CBN ( cubic boron nitride) is a kind of synthetic superhard material. With high hardness, good chemical stable, good thermal conductivity and not react with Fe group elements at high temperature, CBN is widely used for grinding steel, cast iron materials with high speed, high efficiency, and high precision.
Boron nitride has 4 different structures: hexagonal close-packed layered structure(hBN), hexagonal wurtzite structure(wBN), rhombohedral structure(rBN) and cubic zinc blende structure(cBN).
cBN belongs to the cubic system, along the C-axis direction, and the atomic layers are lined in the mode of ABCABC……, as shown in Fig.1.1(d). Two different atoms composite the atoms of two face-centered cubic lattices in the crystal structure of cBN, containing weak ionic bond and covalent bond. And the boron atoms and nitrogen atoms form 109°28” with the bond length of 0.157 nm.
Each layer in a CBN crystal is composed of nitrogen atom or boron atom, and the monolayer composed of the nitrogen atom and the monolayer composed of boron atom alternates with each other and is composed according to the principle of close packing of spheres. Both CBN and diamond are zinc blende sp3 hybrid cubic structure, both lattice constants are very similar, and the bonding bonds are basically the same. So, the similarity of the structures determines their similar performance.
Compared with diamond, CBN has lower thermal conductivity and hardness. It is because the nitrogen-boron bond in CBN has about 22%iconicity, which also cause higher heat resistance and larger impact thermal resistance of CBN than that of the diamond. CBN has six weak bonds along (011) face, so it has better self-sharpness than diamond.
Performance requirements of vitrified CBN grinding wheel
1.Lower refractoriness of the bond
The refractoriness affects the firing temperature of the abrasive. Usually, the abrasive is fired at the temperature higher than 50℃ of the bond. CBN is oxidized easily above 900℃, and CBN forms boron oxide under the action of oxygen or air.
Therefore, it is usually required that the refractoriness of the vitrified bond should not higher than 900 °C.
2.The higher strength of the bond
The strength of vitrified CBN grinding wheel is determined by the CBN strength, bond strength, and the holding force of the bond to the abrasive. In general, CBN abrasive strength is high, so the bond strength and interfacial bonding strength between CBN abrasive and the bond determine the mechanical strength of the CBN grinding wheel.
3. The thermal expansion coefficient of the vitrified bond matches the CBN abrasive
It is usually required they are equal or similar. When the temperature changes, both are uniformly stretched, and the interface is not cracked.
4.Good high-temperature wettability and fluidity
Vitrified bond plays a crucial role in the wettability and fluidity of the CBN abrasive and the bonding state between the abrasive grains and the bond at high temperature. Good wettability and fluidity can make vitrified bond evenly distributed around the CBN abrasive grains during the firing process, and more completely wrap the abrasive grains. It does not cause the defects such as pores at the interface, making bonding firm between the bond and abrasive, improving the holding force for CBN abrasive.
5.Good thermal conductivity and chemical inertness
Vitrified CBN grinding wheel produces a large amount of heat due to the high speed grinding effect. If the thermal conductivity of the vitrified bond is very low, it is not good for the diffusion of grinding heat, burns the workpiece and reduces the processing quality of the workpiece. In addition, the vitrified bond has good chemical inertness, and it does not deteriorate and not react with the workpiece under working conditions.
6.Good technology properties
Good technology properties of the bond include good formability, high strength of dry and wet body, small shrinkage, good high-temperature stability, wide sintering range. And the finished products are not cracked, not deformated, forming.
7.No obvious chemical reaction with CBN abrasive
In the preparation of vitrified bond CBN grinding wheel, in addition to the high-temperature effect, CBN abrasive should contact with vitrified bond inevitably. Some elements and compounds react with CBN at high temperature. The alkali metal oxide（K2O Na2O, Li2O）easily reacts with CBN at high temperature, and it causes corrosion to the abrasive, destroysCBN structure and organization, reduces the mechanical properties of CBN, thereby causing the overall grinding performance of the grinding wheel severely declined.
Preparation of vitrified CBN grinding wheel
The preparation of the vitrified bond CBN wheel requires the formula and preparation process. It includes the type and grain size of CBN abrasive, auxiliary abrasive, the bond amount, pore, molding density, and sintering system.