Effect of Machining on Damage and Strength of Green Ceramics
Castellana, Jesse Philip
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Powder ceramics are being used in new applications including car engine valves and dental implants. These ceramics are formed by mixing ceramic particles with polymer binders into a slurry and then either casting, extruding, or pressing them into a shape. They are then dried and fired. Various methods exist for shaping and post-processing the ceramics after firing. However, these techniques often suffer from high cost or low efficiency. An alternative is to machine the ceramics before firing, while they are in what is called the green state. This is much cheaper and has higher material removal rates than machining in the fired state. Machining of fired ceramics is known to induce damage that reduces the strength of the machined part. Green machining has also been shown to reduce strength. However, the mechanism for this reduction and how aspects of machining and the material composition impact the magnitude of the reduction is unclear. This thesis examines the relationship between machining parameters and material composition on resultant forces and surface roughness in green turning of aluminum oxide rods. The effect of force and roughness on the strength is also studied. The first set of experiments involved turning green alumina samples at various levels of the machining parameters and for samples made with different material compositions to understand how these factors impacted the resultant force and surface roughness. A design of experiment was used and the results showed that feed and rake angle were important for both forces and roughness, and that particle size was important for forces. Other factors and their second level interactions were also statistically significant, but the above factors had the largest effects. It is notable that higher feed led to lower resultant force. This may be because ploughing rather than cutting occurred at low feeds, resulting in higher forces. Additionally, cutting with a positive rake angle tool resulted in higher force than with a neutral rake angle tool. For the second round of experiments, longer alumina rods were turned on a lathe and then tested in four-point bending test for strength in the green state. A subset of the machining and material factors from the first round of experiments were varied during turning to achieve different resultant forces and surface roughness. The forces and roughness were then correlated with bending strength. The results showed that neither force nor roughness had a statistically significant effect on strength. This indicates that either the machining induced damage, if any occurred, is minor compared to the internal flaws from extrusion or that the reduction in strength is minor compared to the inherent variability of strength between green samples. Another possibility is that machining induced damage is less important in the more ductile green state than in the brittle fired state. Future work should focus on understanding how green machining impacts fired strength.