Where is the difficulty of mechanical processing of alumina ceramics reflected

99 alumina ceramic refers to engineering ceramics with an alumina content higher than 99%. According to the standard, 99 alumina ceramic material has high hardness, strength, low expansion coefficient, insulation, wear resistance, and corrosion resistance. It has a wide range of applications in mechanical manufacturing, aerospace, precision instruments, petrochemicals, and other fields.
Aluminum oxide ceramic parts are usually formed by hot pressing sintering. Due to the deformation and shrinkage caused by sintering, further precision machining is generally required to ensure the dimensional and shape accuracy of the parts. However, alumina ceramic materials generally have a relatively high elastic modulus, high hardness, high brittleness, and strong crack sensitivity. Therefore, their mechanical processing difficulty mainly manifests in processing hardness and brittleness.
AL203 mainly has three crystal forms: alpha, beta, and gamma. Among them, the alpha AL203 crystal form is stable, and at 1300 ℃, I3 and gamma crystals almost completely transform into alpha crystals. In the crystalline form of α - AL203, the atomic bonds formed between aluminum ions and oxygen ions are mostly covalent, ionic, or mixed bonds. Therefore, the binding energy between atoms is high and has strong directionality, which is manifested as high material brittleness, small plastic deformation, and easy cracking.
Its hardness is equivalent to that of carbide hard alloys, several times higher than that of steel. Typically, high-purity alumina ceramics have a density of 3980 (Kg-m4), a tensile strength of 260 (MPa), an elastic modulus between 350-400 (GPa), a compressive strength of 2930 (MPa), and a hardness of 99HRA. The strength and hardness of 99 alumina ceramics have decreased, and according to our measurements of the experimental samples, their hardness at room temperature has also reached 70HRA.
Under normal circumstances, the microstructure of alumina ceramics is equiaxed grains, which are polycrystalline structures composed of ionic or covalent bonds. Therefore, the fracture toughness is low. Under external loads, stress can cause fine cracks on the ceramic surface, which can rapidly propagate and lead to brittle fracture. Therefore, during the cutting process of alumina ceramics, the phenomenon of collapse often occurs, that is, small cracks appear on the ceramic surface.