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Optimizing the sintering of alumina ceramics is the key to improve its density, mechanical properties and microstructure uniformity. The optimization suggestions are put forward from the aspects of ra
Advanced materials
1. raw material optimization
High purity raw material
Select high purity (> 99.5%) of alumina powder, reduce impurities (such as the SiO ₂, Na ₂ O) adverse impacts on the sintering. Particle size and distribution of sub-micron (0.1 ~ 1 microns) or nano-sized alumina powder, and optimized grading (coarse, medium and fine collocation), improve the packing density.
Additive selection
Sintering additives: add a small amount of MgO style (0.1 ~ 0.5 wt %) inhibition of abnormal grain grew up, promote the densification. The toughening phase: introduction of ZrO ₂ (3 ~ 5 wt %) improve fracture toughness by phase transformation toughening. Dispersant: improving slurry rheological property, prevent particles together.
2. The molding process optimization
Isostatic pressing forming
by cold isostatic pressing (CIP) or hot isostatic pressing (HIP) molding, to ensure uniform body density, reduce the sintering shrinkage difference. Stretch forming for chip ceramic, optimizing sizing agent (such as adhesive, plasticizer ratio), the control flow thickness and drying rate, avoid cracking. Ceramic injection molding of complex shape, optimizing feed formula and degreasing process, reduce the defects caused by residual organic matter.
3. Sintering system optimization
Temperature rise period, slow warming (1 ~ 5 ℃ / min) to 600 ℃, to ensure complete decomposition organic matter. Appropriate increase the rate of temperature within the range of 1200 ~ 1400 ℃ (5 ~ 10 ℃ / min), shorten sintering time.
Heat preservation phase sintering temperature is 1500 ~ 1700 ℃ normally, holding time 1 ~ 4 hours, depending on the powder size and additives. When adding MgO style can be appropriately reduce the sintering temperature (1450 ~ 1600 ℃). Cooling phase control cooling rate (< 5 ℃ / min), avoid cracking caused by thermal stress. Under 1000 ℃ can be appropriate to speed up the cooling rate.
4. Sintering atmosphere and environment
Air sintering for conventional alumina ceramic sintering atmosphere and environment, the cost is low, but volatile impurities should pay attention to the performance impact. Inert atmosphere sintering sintering in nitrogen or argon, avoid alumina and reaction atmosphere, suitable for high purity or doped alumina. Vacuum sintering promoting densification, reduce pores, but need to control the volatile loss.
5. Advanced sintering technology
hot-pressing sintering (HP) under the condition of pressure sintering (20 ~ 50 MPa), significantly increased density, suitable for high performance alumina ceramics. Discharge plasma sintering (SPS) using pulse current rapid heating (heating rate can reach 100 ~ 500 ℃ / min), in a short period of time (a few minutes to a few minutes), high density, fine crystal structure. Microwave sintering of microwave heating to realize rapid sintering uniformity, energy efficiency, suitable for thin wall or complex shape ceramic.
6. Post-processing and performance improvement
Annealing treatment, after sintering is annealed (below the sintering temperature of 50 ~ 100 ℃), eliminating residual stress and improve the mechanical properties. Surface treatment by polishing, chemical etching and coating to improve the surface quality, reduce the surface defects. Hot isostatic pressing (HIP) post-processing to HIP the sintering of ceramics (1200 ~ 1500 ℃, 100 ~ 200 MPa), further eliminate internal porosity, improve the density.
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