Studies On Corrosion And Wear Resistances Of Liquid-phase Sintered Silicon Carbide Ceramics

SiC ceramics have been widely applied in many industrial areas such as mechanical seals and wear/corrosion-resistant parts, due to their excellent properties e.g., high hardness and fracture strength, high abrasive and corrosion resistances as well as low coefficient of thermal expansion. Corrosion behavior of the liquid-phase sintered SiC (LPS-SiC) by adding BaO-Al2O3-Y2O3 sintering additives was studied by dipping in acidic and alkaline aqueous solutions at room temperature and 70℃, respectively. As a comparison, research on corrosion of reaction-bonded SiC ceramics (RB-SiC) was also carried out. Friction and wear properties of the LPS-SiC under the dry and oil lubrication conditions were investigated.Corrosion of the LPS-SiC specimens is varied with the acid solution. That is the rapidest in the HNO3 solution, the lowest in the H2SO4 solution and the moderate in the HCl solution. The bamboo-leaf-like corrosion pits are formed after the LPS-SiC specimens corroded in the acidic solutions, while corrosion of the regions beside the pits is not obviously. Corrosion of the LPS-SiC specimens results from the corrosion of the secondary phases by the acid solution. BAS has a higher corrosion rate than Y2Si2O7, so corrosion of the LPS-SiC specimens in the acidic solution is selective. It results in the dramatic reduction of strength of the LPS-SiC specimens. As the SiC specimens etched in the 3.53 mol/L HNO3 solution at room temperature for 75 days, about 100 μm thickness corrosion layer forms. The weight loss of the etched SiC specimens is 2.63 mg/cm2 and the bending strength is 255 MPa. Corrosion of the SiC specimens is accelerated in the 70℃ HNO3 solution at a corrosion rate about five times bigger than that in the same corrosion medium at room temperature.Corrosion of the LPS-SiC specimens is of uniform chemical corrosion of the secondary-phase oxides and reaction-controlled both at room temperature and 70℃ in 6.12 mol/L NaOH aqueous solution. Accompanying with peel-off of the SiC particles, corrosion of the LPS-SiC specimens in the NaOH solution leads to reduction of bending strength and increase of weight loss and surface roughness of the SiC specimens. Though corrosion mechanism is not changed, corrosion of the LPS-SiC is accelerated when etched in the 70℃ alkaline solution resulting in apparent increase of weight loss and strength reduction. Bending strength of the LPS-SiC specimens is reduced to 467 MPa after etched for 75 days at room temperature. However, after dipped in the 70℃ NaOH solution for 9 days, residual strength of the LPS-SiC specimens is 408 MPa. No matter in the room temperature or in 70 ℃ alkaline solution, corrosion resistance of the LPS-SiC is obviously higher than that of the RB-SiC.Under the dry friction condition, the wear of the LPS-SiC specimens includes three stages of running-in, stable wear and severe wear with wear mechanism of a mixture mode of abrasive wear and brittle fracture. It has a large wear volume loss. Under the oil lubrication condition, the wear of the LPS-SiC specimens includes two stages of running-in and stable wear with a smaller friction factor and wear volume. The wear volume of the LPS-SiC specimens increases with the increase of the the loading and the decrease of the oil droping rate.