Manufacturing of silicon carbide (SiC) ceramics
Silicon carbide is a very important technical ceramic. It is usually a sintered, hard, black solid that could withstand a very high working temperature. Due to the high physical strength, SiC is also used to make structural parts in a lot of high-temperature machinery.
1. Reaction sintering ceramics
1.1 preparation of ceramic compacts
The ceramic powder (80-90%, composed of silicon carbide powder and boron carbide powder) and carbon source powder (3-15%, composed of carbon black or phenolic resin) and molding agent (5-15%, phenolic resin, polyethylene glycol, hydroxyethylcellulose, or paraffin) are mixed evenly with the ball mill, and the mixed powder is obtained by spray-drying and granulating. Then the prepared material is pressed into the mold. The green state ceramic compacts with various shapes are obtained.
1.2 preparation of sintered infiltrating agent compacts
60-80% silicon powder, 3-10% silicon carbide powder, and 37-40% boron nitride powder were mixed evenly and pressed in the die to obtain the sintered infiltrating agent compacts.
1.3 preparation of reaction sintered ceramic products
The ceramic compacts and the sintered infiltrating agent compacts are stacked together and heated to 1450-1750 ℃ in a vacuum furnace with a vacuum degree of not less than 5 × 10-1 PA, and then sintered for 1-3 h to obtain the reaction sintered ceramic products.
After tapping the surface of the sintered ceramics, the infiltrating agent residue was removed, and the compact ceramic sheet was obtained, and the original shape of the compact was maintained.
1.4 preparation of silicon carbide ceramics
The reactive sintering process is adopted, that is: liquid silicon or silicon alloy with reactive activity at high temperature infiltrates into porous green state ceramic containing carbon under the action of capillary force, and then reacts with carbon to form silicon carbide. The volume expansion of silicon carbide will occur and the remaining pores will be filled with silicon. Porous ceramics can be pure carbon or silicon carbide/carbon matrix composites. In the former, organic resin, pore-forming agent, and solvent are mixed and then catalytic solidified and pyrolyzed to obtain porous carbon. The latter is to pyrolyze SiC particles/resin matrix composites to obtain SiC/carbon matrix composites or α - SiC and carbon powder are used as starting materials, the composite was obtained by pressing or grouting.
2 preparation method of solid-phase sintered SiC ceramics
2.1 mixing of raw materials
According to the weight percentage, 70-90% of submicron α - SiC (200-500nm), 0.1-5% boron carbide, 4-20% resin, and 5-20% organic binder are put into the mixer and mixed with pure water. After 6-48 hours, the mixed slurry will pass through 60-120 mesh sieve.
2.2 spray granulation
The screened slurry is sprayed and granulated by a spray tower. The inlet temperature of the spray granulation tower is 180-260 degrees, and the outlet temperature is 60-120 degrees. The bulk density of the material after the granulation is 0.85-0.92g/cm, the fluidity is 8-11s/30g, and the material after granulation is screened over 60-120 mesh for backup.
2.3 molding
Select the mold according to the shape of the required products, put the above-granulated materials into the mold cavity, mold at 50-200 MPa pressure at room temperature to obtain the green blank; or put the molded blank into the isostatic pressing equipment, and conduct isostatic pressing at 200-300 MPa to obtain the green blank after secondary pressing.
2.4 vacuum sintering
After the green billets prepared by the above steps are put into the vacuum sintering furnace for sintering, the qualified ones are the finished products of silicon carbide ceramics. In the above-mentioned sintering process, the sintering furnace is vacuumed first. When the vacuum degree reaches 3-5 × 10-2Pa, inert gas is pumped into the sintering furnace to atmospheric pressure and then heated. The relationship between heating temperature and time is room temperature to 800 ℃, 5-8 hours, Heat preservation for 0.5-1 h, from 800 ℃ to 2000-2300 ℃, 6-9 h, 1-2 h, and then cooled with the furnace to room temperature.