What is the working hardness and brittleness of 99 alumina?

Ceramics have one thing in common, that is, hardness is great, but it is very brittle, so it is very difficult to process. So what is the processing hardness and brittleness of alumina ceramics with purity of 99? The following Kezhong Ceramics Factory will give you an analysis.

99 alumina ceramics refers to engineering ceramics with alumina content higher than 99%. According to the national standard GB/T5593-1999, 99 alumina ceramics have high hardness, high strength, low expansion coefficient, insulation, wear resistance and corrosion resistance, and are widely used in machinery manufacturing, aerospace, precision instruments, petrochemical industry and other fields. Alumina ceramic parts are usually formed by hot pressing sintering. Because sintering often leads to deformation and shrinkage, further finishing is generally needed to ensure the dimensional accuracy and shape accuracy of the parts. However, alumina ceramic materials generally have quite large elastic modulus, high hardness, high brittleness and strong crack sensitivity. Therefore, its machining difficulty is mainly manifested in machining hardness and machining brittleness.

The processing hardness of alumina ceramics: AL203 mainly includes α 、β 、γ Three crystal forms, of which α -AL203 is the most stable crystalline form, with I3 and &gamma at 1300℃; Crystallization is almost completely transformed into α Crystallization At α The atomic bonds formed by aluminum ions and oxygen ions in the crystalline form of -AL203 are mostly covalent bonds, ionic bonds or their mixed bonds, so the bonding energy between atoms is high and has strong directionality, which is manifested in the material's brittleness, small plastic deformation and easy cracking. Its hardness is equivalent to that of carbide cemented carbide, which is several times higher than that of steel. Usually, the density of high-purity alumina ceramics can reach 3980(Kg-m4), the tensile strength can reach 260(MPa), the elastic modulus can range from 350 to 400 (GPA), and the compressive strength can reach 2930(MPa), especially its hardness can reach 99HRA. The strength and hardness of 99 alumina ceramics have decreased, and according to our determination of experimental samples, its hardness also reaches 70HRA at room temperature.

Processing brittleness of alumina ceramics: Usually, the microstructure of alumina ceramics is equiaxed grain, which is a polycrystalline structure composed of ionic bonds or covalent bonds, so the fracture toughness is low. Under the action of external load, the stress will cause fine cracks on the ceramic surface, and the cracks will rapidly expand and brittle fracture will occur. Therefore, in the cutting process of alumina ceramics, there will often be a phenomenon of collapse, that is, a small gap will appear on the ceramic surface. The reasons for the collapse phenomenon are:

(1) The final separation between the removed part of the material and the machined surface is caused by tensile failure, which is not the result of normal cutting.

(2) Cracks caused by chipping cutting deformation generally crack all the way down along the surface of the workpiece. At this time, the cutting and bonded workpiece matrix are peeled off together due to the cutting tensile stress, resulting in chipping. It should be noted that the greater the tensile stress, the more serious the collapse phenomenon, which may lead to the waste of the whole workpiece.

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