The Investigation Of Mechanism And Thermal Phenomena In High Speed Deep Grinding Of Advanced Ceramics

Advanced engineering ceramics have excellent mechanical properties, such as high strength, high hardness and great resistance to friction. However, these excellent properties also render their manufacturing difficult and easily cause damage in the machined surface and subsurface. As a result, the machining efficiency is low and its associated cost is high. This has thus limited the widespread applications of advanced engineering ceramics. This thesis project thus aimed at developing a high speed deep grinding (HSDG) technology for efficiency machining of advanced ceramics and investigating the associated removal mechanisms.Three advanced engineering ceramics, including alumina, nitride silicon and yttria partially stabilized tetragonal zirconia (PSZ), were ground under various HSDG conditions. Effects of microstructures, mechanical properties of the ceramics and grinding conditions on grinding surface/subsurface characteristics, grinding force, grinding energy and grinding temperature were systematically investigated. A study was also performed to understand the generation of grinding heat and the mechanism responsible for the rapid rise of temperature under some extreme HSDG conditions. Based on the results from the investigations, we have successfully applied the HSDG for grinding the ceramics using a wheel velocity up to 160m/s. This enabled to achieve a depth of cut of 6 mm, producing a removal rate of 120 mm~3/(mm·s), while the depth of subsurface micro crack was not greater than 10μm which can be removed readily by the following grinding process.The investigations indicated that the removal modes of ceramics were determined by their microstructures and mechanical properties. The alumina used had relatively large grains size, high hardness and low fracture toughness, so the critical depth of ductile and brittle transition was relatively low, having tendency towards fracturing during grinding. The ground surfaces of alumina were generated by brittle fracture and the cracking along grain boundaries were observed frequently in its subsurface. The PSZ and silicon nitride had relatively small grains, low hardness and high fracture toughness, so the surfaces of PSZ and nitride silicon were generated by the combined removal modes of brittle and ductile, and the lateral cracking were generated occasionally in the subsurface of PSZ. Under the same grinding conditions, the value of surface roughness of the ground alumina (R_a about 0.9μm) was the highest among the three ceramics, and the surface roughness values of PSZ and silicon nitride were very close (R_a about 0.7μm).In order to understand the removal mechanism of engineering ceramics under HSDG, an analytical grinding force model of the HSDG ceramic was developed. The model results were in good agreement with the experimental results. Experiment and numerical results indicated that the material mechanical properties, removal modes and grinding conditions significantly influenced the grinding force of the ceramics. The increase in microhardness of ceramics resulted in a greater grinding force when the ductile removal was dominant. When the brittle fracture was dominant, the increase in fracture toughness of ceramics and the decrease in micro hardness of ceramics resulted in greater grinding forces. The experiment also showed that the special energy of PSZ or silicon nitride was higher than that of alumina.The measurement of grinding temperature involved in the HSDG processes indicated that 93 percent of the grinding energy was expended on the attrition and ploughing between the wheel and the workpiece, which was then generated into grinding heat. A small portion of the grinding heat was transferred into the workpiece, but the majority of the heat was removed from the grinding zone by the coolant and the removal of chips, and was transferred into the grinding wheel. Under the normal HSDG conditions, the temperature was in the range of 100～300℃. There existed quite linear relationships between the heat flux and temperature of grinding zone for the three ceramics. Under some extreme conditions, the temperature could rapidly rise to about 600 to 1100℃, close to the temperature of dry grinding. The experiment and theoretical investigations indicated that the film boiling of coolant in grinding zone might be responsible for this rapid temperature rise.