Reaction Sintered SiC Ceramic Materials Preparation, Structure And Properties

In recent years, advanced structural ceramics, represented by reaction sinteredSiC ceramics, are widely used in the mechanical seal industry. In the petroleum,chemical industries, pumps, blenders and others usually work in the toughcondition of high-temperature and high-pressure, corrosion and wear. Therefore,higher properties of the seal materials are required. In order to improve theproperties of these ceramic materials, the preparation process of them should beoptimized, and relationship between microstructure and properties should beclarified. Because wear resistance of the seal ceramics has great effect on thequality and servicing life of them, study on the wear resistance of the seal ceramicsis very important.In this paper, fine SiC powder of7μm in diameter was chosen as rawmaterials. Dense SiC ceramic materials were prepared through a reaction sinteringtechnique. Phases, microstructures, mechanical properties and fractographies of thereaction sintered SiC ceramics were studied, accordingly. Effects of main technicalparameters on the microstructures and mechanical properties of the reactionsintered SiC ceramics were also investigated. In terms of the ring block typefriction and wear experiment, the friction and wear properties of the reactionsintered SiC ceramics on the dry or oil lubrication conditions were studied,respectively. In this experiment, one kind of imported pressureless sintering SiCceramic ring was used as the friction ring, and the reaction sintered SiC ceramicsdeveloped by the author’s group were used as the friction blocks.When the forming pressure of117MPa is applied, the SiC performs areexcellent in which the SiC particles and the graphite nano-particles distributeuniformly with3-dimensional connecting pores formed to ensure a greatself-infiltration ability of the molten Si. The reaction sintered SiC ceramics have ahigh SiC content and a low free Si content. After reaction sintering at1550℃, thedensity, Rockwell hardness (HRA) and bending strength of the ceramics canachieve3.01g·cm-3,95and410MPa, respectively. The mechanical properties ofthe reaction sintered SiC ceramics are high enough to completely meet thetechnological requirements of the ceramic mechanical seals. The reaction sintered SiC ceramics are composed of α-SiC, β-SiC and free Si. During sintering, the Celement in the SiC performs gradually dissolves into the liquid Si to form asupersaturated solution, leading to the formation of the β-SiC particles on thesurfaces of the α-SiC particles. It was concluded that the reaction sintering of SiCfollowed the dissolution-precipitation mechanism.Wear of the reaction sintered SiC ceramics under the dry friction conditionincludes three stages, i.e. running-in, stable wear and severe wear. The wearmechanism of the reaction sintered SiC ceramics is a mixture of the abrasive wearand the oxidative wear. In this situation, the friction factor and wear volume arehigher. Under the condition of oil lubrication, the friction and wear of the reactionsintered SiC ceramics only includes two stages of running-in and stable wear withsmaller friction factor and wear volume. The oil film between the friction block andring play role in reducing the contact of the friction ring and block. It results in thedecrease of the wear and fatigue of the grinding surface. The wear surface is noteasily deprived; meanwhile, the abrasive particles can be brought away from thewear surface by the flowing lubricant oil. The extents of the depriving wear and theabrasive wear are decreased. The wear volume of the wear pair is accordinglyreduced. As the loading increased, the wear volume and friction factor of thereaction sintered SiC ceramics increase; however, that of the reaction sintered SiCceramics are bigger under the dry condition. In general, the reaction sintered SiCceramics have an excellent wear resistant property.