Study On Surface Friction And Wear Characteristics Of Rotating Ultrasonic Grinding Of Si 3 N 4 Ceramics

Si₃N₄ ceramics, with its high working temperature, good mechanical strength,and excellent physical and chemical properties,has become the most ideal materials in developing aerospace dynamic system components.However,due to its high value of hardness and low value of fracture toughness, low efficiency and tool wearing seriously comes out when machining the Si₃N₄ ceramics materials with conventional grinding . The rotary ultrasonic grinding, with the advantages of low cutting force and high machining efficiency, is gradually applied to the machining of Si₃N₄ ceramic components. Si₃N₄ ceramic components used to work in special conditions, and generally there are relative movement between the components contacting with each other. The friction and wear behavior of the surface directly affects the reliability and service life of ceramic components. The friction and wear behavior is a series of physical or chemical processes occurring in the machining surfaces of the parts,and is closely related to the machining surface features. Therefore, rotary ultrasonic grinding surface of Si₃N₄ ceramic is taken as the object, the methods of the combining of theoretical analysis, mathematical modeling and experimental research is taken to study its surface friction and wear properties in typical conditions, and to reveal the friction and wear behavior of rotary ultrasonic grinding surface of Si₃N₄ ceramic and how it changes .This research is of great significance for improving the reliability and service life of Si₃N₄ ceramic Components in the aerospace dynamic system.The contact and mechanical relationship of the structural ceramics during the friction is firstly analyzed. Based on the contact characteristics and material properties and fractal theory,it is pointed that the total load of the contact surface should include the elastic deformation load, the plastic deformation load and the microscopic crack cracking load. The theoretical formula is analysed and deduced ,based on which the fractal model of the friction factor of structure ceramic is established. Tt is showed through the analysis that the friction coefficient of friction after the fractal dimension of the surface profile was similar to the normal distribution curve when the initial surface profile fractal dimension is 1.4,1.45,1.5 and 1.55.Meanwhile, three typical wear models, elastic-plastic wear, brittle fracture wear and abrasive wear, are established based on the elastic-plastic mechanics and indentation fracture mechanics. In the elastic-plastic wearing stage of Si₃N₄ ceramic materials ,the relationship between actual contact area and load of material machined surface depends on the surface profile curve and contact state; In the friction and wearing stage , internal stress field of materials including Boussinesq field, Sailudi Boussinesq field and residual stress field of component indentation , and the crack will produce and expand, which brittle fracture wear happens ,when stress field intensity of crack tip in crack system is above the critical stress intensity;In abrasive wearing stage, the friction and wear stress of material surface is related to the movement of the abrasive particles, and the wear effect of the wear particles differs from abrasive movement modes.The uniform experimental table of the temperature, the load and the frequency of the reciprocating frequency are designed, and friction and wear experiments are carried out by taking face contact of Si₃N₄ ceramic sample of rotary ultrasonic grinding as subjects. The results of the experiments showed the morphology of ceramic material surface after the friction and friction coefficient variation. And the influence of each factors on the friction coefficient is analyzed. It comes out that: the microstructure of the micro crack is the remarkable characteristic of the surface of the Si₃N₄ ceramics after friction; because of the existence of micro cracks, the fractal dimension of the surface topography after the friction is higher than that of the initial surface ,which is 2.52; friction factor changes with time, and it increase firstly then decreases and tends to be stable finally; compared with ordinary grinding,the coefficient of rotary ultrasonic grinding of Si₃N₄ ceramic is relatively larger; 160? is the inflection point of Si₃N₄ ceramic friction factor from the decline to rising; coefficient of friction get the maximum when the applied load is 360N and reciprocating frequency is 80Hz.By observation of The friction surface of the Si₃N₄ ceramic under certain temperature conditions and EDS, the form and mechanism of surface wearing in the rotary ultrasonic grinding surface of Si₃N₄ ceramics has been researched, and the Variation of the amplitude paramete of the wear surface under different friction conditions has been analyzed.It comes out that: The wear forms of the rotary ultrasonic grinding surface of the Si₃N₄ ceramic are mainly abrasive wear, spalling wear, fracture wear and oxidation wear; the main mechanism of the spalling wear and fracture wear is the appearance and the extension of the microscopic cracks; the main mechanism of abrasive wear is plastic deformation of material,and it has a great relationship with the fracture toughness of material surface; Oxidation wear is due to the chemical reaction of Si elements in ceramic materials with O in certain temperature,and a SiO2 oxide film with a lower binding strength is formed; with temperature and load increasing,the arithmetical mean deviation and root mean square deviation of the wear surface are declined; however, the arithmetic mean deviation of wear surface of Si₃N₄ ceramic trend increasing with the increasing of reciprocating frequency.