|M.Sc Student||Levinson Tomer|
|Subject||Microstructure and Properties of Green-Machined Sintered|
|Department||Department of Materials Science and Engineering||Supervisor||Professor Wayne D. Kaplan|
|Full Thesis text|
In the processing and production of ceramic parts, grinding is the primary method used to reach micron-level precision of dimensions. Although grinding techniques are well known, they are costly and subjected to geometrical limitations. By machining the compact prior to sintering, (i.e. green machining) to a near-net-shape it is possible to produce parts economically and with reduced geometrical limitations. Green machining requires mechanical properties that can be attained by injection molding, gel-casting, or hydrolysis-assisted solidification (HAS).
Injection molding which uses thermoplastic binders requires a pyrolysis stage to remove the binder and has a relatively large shrinkage after sintering (~20%), therefore injection molding usually requires an additional grinding phase. Gel-casting uses a concentrated slurry of ceramic powder in a solution of organic monomers which is polymerized in situ to form a green body in the shape of the mold cavities. This method suffers from cracks, distortions, or uncontrolled shrinkage unless a meticulous drying processing is used. Additionally the used monomers are mostly toxic making gel-casting implementation limited in the industry. Hydrolysis-assisted solidification (HAS) is a relevantly new method for net-shaping ceramic green parts from aqueous suspensions in nonporous molds. The process exploits the thermally activated hydrolysis of aluminum nitride (AlN) powder within the ceramic suspension. Similar to gel-casting, HAS is very sensitive to drying and can suffer from cracks and distortion.
The current study investigated the microstructure and mechanical properties of sintered alumina as a function of AlN concentration (for HAS), which were machined in the green state prior to sintering. The results were compared with samples machined after sintering. Green bodies of alumina were produced by slip casting and pressure filtration. The green ceramic preforms were then fired to promote different degrees of necking between the ceramic particles, and machined to a near-net shape prior to sintering. The measured fracture strength (in bending) showed a good correlation to the density of the samples prepared with different percentages of added AlN. The large amount of porosity detected in the HAS samples suggest unwanted flocculation prior to green body formation. Analysis of fracture surfaces revealed that for sintered alumina, pores were the dominant flaw lowering the fracture strength. Machining of fired alumina samples also weakened the flexural strength and suggests enlargement of the flaw size during machining.