99 alumina ceramics refers to engineering ceramics with an alumina content higher than 99%. According to the national standard GB/T5593-1999, 99 alumina ceramic materials have high hardness and strength. It has high coefficient of expansion, low expansion coefficient, insulation, wear resistance and corrosion resistance. It is widely used in machinery manufacturing, aerospace, precision instrumentation, petrochemical industry and other fields. Alumina ceramic parts are usually formed by hot pressing and sintering. Since sintering often causes deformation and shrinkage, further finishing is generally required to ensure the dimensional accuracy and shape accuracy of the parts. However, alumina ceramic materials generally have a large elastic modulus, high hardness, high brittleness, and strong crack sensitivity. Therefore, their machining difficulty is mainly reflected in the processing hardness and processing brittleness.
Analysis of difficulties in processing alumina ceramics
Processing hardness of alumina ceramics: AL203 mainly has three crystal forms: α, β, and γ. Among them, α-AL203 is the most stable crystal form. I3 and γ crystallize at 1300°C. Almost completely transformed into α crystal. In the crystal form of α-AL203, the atomic bonds formed by aluminum ions and oxygen ions are mostly covalent bonds, ionic bonds or their mixed bonds. Therefore, the binding energy between atoms is very high and has strong directionality. Its specific performance The material is highly brittle, has small plastic deformation, and is prone to cracks; its hardness is equivalent to the hardness of carbide cemented carbide, several times higher than steel. Usually the density of high-purity alumina ceramics can reach 3980 (Kg-m4), and the tensile strength Up to 260 (MPa), the elastic modulus is between 350-400 (GPa), the compressive strength is 2930 (MPa), especially its hardness can reach 99HRA. The strength and hardness of 99 alumina ceramics have been reduced. According to our measurement of experimental samples, its hardness also reaches 70HRA at room temperature.
Processing brittleness of alumina ceramics: Usually, the microstructure of alumina ceramics is equiaxed grains, which is a polycrystalline structure composed of ionic bonds or covalent bonds. , so the fracture toughness is low, under external loadUnder the action, the stress will cause fine cracks on the ceramic surface, and the cracks will expand rapidly and cause brittle fracture. Therefore, during the cutting process of alumina ceramics, chipping phenomenon often occurs, that is, small cracks appear on the ceramic surface. . The reasons for the collapse phenomenon are:
(1) The final separation of the removed part of the material and the machined surface is caused by tensile damage, which is not the result of normal cutting.
(2) Cracks caused by chipping and cutting deformation generally crack along the surface of the workpiece. At this time, the cutting and phase bonding due to cutting tensile stress The workpiece matrix peels off together to form a chipping phenomenon. It should be noted that the greater the tensile stress, the more serious the cracking phenomenon will be, which may lead to waste of the entire workpiece.
