AbstractAdvanced ceramic materials are assuming increasing industrial importance because of their special properties. However, the extreme hardness and brittleness of these materials make their machining efficiency low. The difficulty arises from shaping ceramics whilst conserving their surface integrity, strength and high quality surface finish. Manufacturing processes such as grinding with diamond bonded wheels present one solution. The use of a diamond grinding wheel on the surface of ceramics, creates a complex system of elastic/plastic deformation, and subsequent cracking. For ceramic materials such as silicon nitride, two types of damage can occur; firstly surface damage in the form of machining marks which cause stress concentrations and surface cracks and secondly sub-surface cracking. These problems caused a major delay in the utilisation of these materials in the aeronautics industry. This research was aimed at providing a better understanding of the grinding process and parameters involved in machining ceramic component.
An extensive preliminary study was carried out to understand the behaviour of advanced ceramic material during grinding. The effect of very high infeed and grinding wheel parameters were studied as an initial investigation. A system for on line detection of grinding wheel surface condition during grinding was developed during this research. Acoustic emission AE technique to monitor the wheel condition was also investigated during the experimental work.
The grinding machine used is a commercially available machine with a stiffness coefficient of 18.5 N/µm. The machine was modified to accommodate a higher feed rate. The purpose of using this machine was to investigate how a commercial machine would grind hard material such as advanced ceramics and what the effect would be on the process of crack initiation and propagation during grinding. The effect of different grinding wheel and machine tool parameters on crack initiation and propagation was investigated and represent the main concern of this research. Experimental design using modified L27 orthogonal array was used. The response of the surface roughness, fracture strength, AE spectral amplitude, grinding forces, grinding energy, crack size, sub-surface cracks distance from the ground surface, depth of surface damage due to different grinding wheel and machine parameters used in the fractional factorial array were all investigated.
Three basic types of cracks that were found to occur after the passage of an abrasive grain on the surface of ceramics materials were identified. These are radial, median and lateral cracks. Only radial cracks and machine marks are visible on the surface, median and lateral cracks (parallel to the surface) are formed below the affected zone and thus not visible. The load of an individual grain and its shape on the grinding wheel surface were found to have an influence on the crack initiation and propagation. Higher damage depth on the ground surface was measured when the grain size increased or was extremely sharp. Median cracks were found to be at a deeper location from the ground surface (2-15 µm) compared with lateral cracks. The dimensions and depth of median cracks beneath a ground surface was found to increase when the levels of grinding wheel grit size, wheel bond hardness, depth of cut and table feed were increased. However, locations of lateral cracks were found not to be affected when theseparameters increased and found to occur at 2-4 µm below the surface. The median cracks were located close to the ground surface at low table feed rates compared with deeper location when the table feed rate increased.
A correlation between AE, SEM and 3D data and the ground surface condition was carried out. The condition of the ground surface whether smooth or rough was identified and linked with AE signals. The link between these observations show that an increase in percentage of surface fracture is accompanied by low-amplitude long duration events which can be assumed as the characteristic of brittle mode grinding. This observation was confirmed by the SEM and the 3D observations. Higher AE spectral amplitude was found to be associated with smooth ground surfaces. However, sub surface median cracks were found for this condition. This correlation was the first reported attempt to link different experimental measurements and parameters.
Different artificial cracks were initiated on ceramic specimen surfaces in both longitudinal and transverse directions using v-shaped discs. This method of initiating cracks is more realistic than that produced by a single diamond scratch. The very random nature of diamond grit shape, size and location on the surface of the wheel would alter the characteristics of a crack initiated in a real grinding process. This technique used for creating an artificial crack is the first reported attempt hitherto. The groove generation mechanism observed using SEM was almost brittle when the depth of scratch increased. At larger groove depths, a deeper locations of sub-surface median cracks were found. Grinding processes were then carried out on these cracks to investigate their propagation. The AE spectral amplitude was found to be higher for specimens ground with artificial cracks compared with specimens ground without artificial cracks. The sub-surface cracks initiated due to grinding specimens with artificial cracks were found to be at deeper locations than to those for ceramic specimens ground without artificial cracks. The effect of remaining damage depths on the ground surface on the fracture strength were also studied. Increasing the diamond grinding wheel grit sizes, wheel bond hardness and table feed were found to be the most influential factors that increase the propagation distance of the artificial cracks during grinding.
A computer model based on finite element analysis package was used to study the behaviour of the ceramics in grinding. This package was used to establish a theoretical model which was validated using experimental results. Normal and tangential grinding forces from experimental work were used as input to the FE model. These forces were chosen to indicate different wheel and machine parameters. The model simulates the process of grinding by applying these forces on the surface of the ceramic specimen to study the crack initiation, propagation and their distance from the ground surface. A complete simulation for the grinding process was made including removal of material during the process.
|Date of Award
|Talal Maksoud (Supervisor)