Spray Drying Of Aqueous Slurry & Preparation Of SiC-AlN Multiphase Ceramics

Silicon carbide (SiC) ceramics is an advanced structural material with excellect properties such as resistance to extreme temperature, corrosion and abrasion, low thermal expansion and high thermal conductivity, and it is widely used in many industries. However, it is difficult to manufacture and its inherent brittleness and low toughness values are significant deterrents to its use in other potential applications. AlN can react with SiC and form an extensive solid solution at temperatures between 1800°C and 2100 °C to improve the sintering behaviour of SiC and obtain SiC-AlN multiphase ceramic. SiC-AlN multiphase ceramic has excellent oxidation resistance and mechanical behaviour at high temperature, and is widely focused. The researches on the SiC-AlN concentrates on the high percent of AlN system and the optimization of sintering program, but few researches on high quality SiC-AlN composite powders and manufacturing processes of SiC-AlN multiphase ceramic.On the basis of reviewing the present research on SiC, its composite ceramics and spray drying technique, the compositions of SiC-AlN multiphase ceramic was designed, AlN powder was surface modified against hydrolysis was studied by oleic acid, and low viscosity aqueous suspensions of SiC-AlN was obtained with dispersants in existence. Water-based slurry was spray dried for producing a granulated, flowable and high compaction properties of SiC-AlN granule pellets. SiC-AlN multiphase ceramic with high density, strength and toughness was prepared by pressureless sintering with liquid phase. The machine seals of SiC-AlN multiphase ceramics were industrial manufactured. These reseaches are favorable to obtain high quality SiC-AlN powder and high property multiphase ceramics, and consequently extend the application field of SiC ceramics.The resistance to hydrolysis of AlN powder was improved by coating oleic acid and PEG on the surface of AlN particles. The pH value of aqueous suspensions with 4wt% AlN is below 10 still after stirred for 48 hour, and only few AlN is hydrolytic. The absolute value zeta potential increases from 27mV to 46mV when dispersant (0.2wt% TMAH & 1wt%PEG) was introduced to SiC-AlN water-based slurry, which tends to enhance the dispersability of powder. The rheological behaviour of SiC-AlN water-based slurry is non-newtonian flow behaviour which exhibited slight shear thinning rheologies, and the viscosity of aqueous suspensions is 60 mPa·s when solid content is 50wt% with additive (0.2wt%TMAH + 2wt%PVA+1 wt%PEG).Aqueous slurry was spray pelletization by rotary atomizing, and flowability and compaction mechanism of granulated powder were investigated. When solid content is50wt%, temperature of dry air and exhaust air are 230℃ & 87℃, rotate speed of atomizer is 12000 r/min, and the granulated powder is produced. Compared with raw SiC powder, the packing density of granulated powder increases from 0.53 g/cm³ to 0.91 g/cm³, and angle of repose reduces from 42° to 26°. Relationship of pressure and green body density showes that the granulated powder pellets begin to be destroyed from 80～160MPa, and when the forming pressure exceeded 160MPa, the granulated powder pellets in green body could be completely destroyed. The related density of green body is 58% with a forming pressure of 160MPa.SiC-AlN multiphase ceramics was fabricated by pressureless sintering with liquid phase, and the relationship of density and sintering temperature, holding time and A1N content was discussed. The weight loss, contractiveness, grains size and aspect ratio will increase with the enhancing of sintering temperature and holding tim the best sintering temperature and holding time are different with varied content of A1N. The SiC-AlN multiphase ceramics with 10wt% A1N sintered at 2050℃ for 4 hour had related density of 98.8%, flexural strength of 521MPa, hardness (Hv) of 26.8 GPa, and fracture toughness of 6.25 MPa·m^1/2. SiC-AlN multiphase ceramics is consist of 2H solid solution and α-SiC, and the content of A1N was different between core and rim of 2H-grains. The toughening of SiC-AlN multiphase ceramics is attributed to crack deflecting, crack bridging, grains pull-out and tearing-open. SiC-AlN multiphase ceramics was densified across liquid phase appeared, wich is Y-Al-Si-O-N system. The vaporizing A1N was deposited on the surface of SiC powder, finally SiC and AlN forms SiC-AlN solid solution layer on the surface of SiC grains, subsequently, homogenous and platelike grains are carried out.On the basis of researches on spray pelletization of aqueous slurry and SiC-AlN multiphase ceramics, machine seals of SiC-AlN multiphase ceramics were industrial fabricated. The multiphase ceramics had related density of 98-99%, flexural strength of 495MPa, hardness (Hv) of 22.8±0.4 GPa, and fracture toughness of 4.5±0.3 MPa·m^1/2, compared with the ware of S-SiC and RBSC, the flexural strength and fracture toughness of SiC-AlN multiphase ceramics increase evidently.