Study On The Preparation And Corrosion Resistance Of Silicon Nitride Ceramics

Silicon nitride ceramic is a kind of structural ceramic with high strength and toughness, which has a broad development prospect.Meanwhile, it has many advantages,such as high thermal stability, wear resistance and corrosion resistance.The purpose of this paper is to find a suitable sintering additive system and process conditions to prepare the silicon nitride ceramics with excellent properties.The ?-Si3N4 powder is taken as the raw material and the pressureless sintering technology is used for preparing the silicon nitride ceramic in this paper. The rare earth oxides and rare earth fluorides are studied as sintering additives.The influence of composition and preparation process on the structure and mechanical properties of silicon nitride ceramics was studied.The corrosion resistance of silicon nitride ceramics was also studied.The main research work and results of the paper are summarized as follows:(1) The composition and preparation process of rare earth oxides doping silicon nitride ceramic were studied. The results indicated that when the content of Si3N4, MgO, Y2O3, La2O3 was 90%, 4%, 3%, 3% respectively, the hardness HRA of the sample reached the maximum value of 90 at 1750 ?. The bending strength of the sample was 640 MPa.When the content of Si3N4, MgO, Y2O3, CeO2, La2O3 was 90%, 4%, 3%, 1%, 2% respectively, the bending strength of the sample reached the maximum value of 722 MPa and the hardness of HRA was 86.5 at 1750 ?.(2) The effect of process parameters on the properties of silicon nitride ceramics was studied.The content of Si3N4,MgO,Y2O3,CeO2, La2O3 of silicon nitride ceramics was 90%, 4%, 3%, 1%, 2% respectively. Under the condition of sintering temperature at 1750 ? for 2 hour and cold isostatic pressure at 200 MPa, the comprehensive mechanical properties of the samples were best. The hardness HRA was 86.5 and the bending strength was 722 MPa at this condition.(3)The composition and preparation process of rare earth fluorides doping silicon nitride ceramic were studied. The results indicated that when the content of Si3N4, MgO, CeF3 was 92%, 4%, 4% respectively, the hardness HRA of the sample reached the maximum value of 89.5 at 1750 ?. The bending strength of the sample was 760.7 MPa. Comparing with the rare earth oxides doping samples, there is no obvious difference on hardness between two kinds of samples. But the bending strength of the rare earth fluorides doping silicon nitride ceramics is higher than the other.(4)The corrosion resistance of rare earth oxides and rare earth fluorides doping silicon nitride ceramics against hydrochloric acid, sodium hydroxide, hydrofluoric acid and sodium carbonate, cryolite molten salt was studied. It is indicated that the two kinds of silicon nitride ceramic samples can not be corroded by hydrochloric acid. Both the two kinds of silicon nitride ceramic samples can be corroded by high concentration sodium hydroxide at 50 ?. After corroded by the high concentration sodium hydroxide, the samples weight added obviously and the surface morphology changed.The two kinds of silicon nitride ceramic samples can be corroded by the hydrofluoric acid of different concentrations. Meanwhile, the corrosion of the rare earth fluorides doping sample was more serious. The two kinds of silicon nitride ceramic samples can be corroded by the sodium carbonate and cryolite molten salt. The weight and surface morphology variation was more obvious in sodium carbonate molten salt and the corrosion leval in sodium carbonate molten salt was more serious.