Papers by Author: Gurdial Blugan

Abstract: The effect of different sintering additives on the high temperature oxidation and corrosion behaviour of Si3N4 ceramics is investigated. A corrosion furnace was set up which allows testing of ceramic discs at test temperatures upto 1500°C. It was possible to conduct tests under ambient oxygen conditions, or with increased water vapour as well as testing with corrosive gases such as HCl or SO2. Si3N4 was prepared with MgO, Al2O3, Y2O3 and Al2O3+Y2O3 sintering additives. These discs were subjected to different oxidation and corrosion test conditions at temperatures of 1200°C and 1500°C for upto 128 h. The effects that the different corrosion enviroments have on the corrosion resistance of the Si3N4 materials are presented.

Abstract: Two different designs of high fracture toughness micro-laminate ceramics were produced containing 50 μm thick Si3N4 layers and 100 μm thick Si3N4 + TiN layers. The first design with external tensile layers had a predicted maximum apparent fracture toughness of 10.5 MPa m1/2. The second design with external compressive layers had a predicted maximum apparent fracture toughness of 18.0 MPa m1/2. The fracture toughness of these micro-laminates was tested by the SEVNB method. A stiff testing machine was used to measure the R-curve behavior by observing crack growth in single notched specimens. A soft testing machine was used to measure the R-curve behavior using several specimens with notches at different depths.

Abstract: Recent developments have shown that producing multi-layer ceramic laminates with alternative layers under compressive and tensile stress can lead to significant improvements in toughness at a low cost. However, in many cases the improvements in fracture toughness is associated with the presence of surface “edge cracks” in the compressive layers or the use of porous interfaces between the layers. At the same time such effects can limit the performance of ceramics when used in harsh environments. This review covers the development of silicon nitride based laminate structures and characterisation of these multi-layer structures. The work presents the results of macro-layered laminates with layers greater than 150 μm thickness. The apparent fracture toughness of different designs is measured and the conditions for failure tolerant effects, including crack deflection, bifurcation and edge cracking, are shown and discussed. The structural and processing limitations of the macro-layered laminates are also presented. The development of a weight function analysis as an effective design tool for developing micro-layered laminates with layers of approximately 50 μm thickness is discussed along with the apparent fracture toughness results from these micro-laminates. The failure tolerant behaviour as well as the ease of producing micro-layered laminates with a toughness of 2-3 times higher than that of silicon nitride is shown.

Abstract: Si3N4-TiN based multi-layer ceramics laminates have been produced. With external compressive layers, laminates with a three-fold increase in KIc over the monolithic ceramics have been realised. When external tensile layers are used in conjunction with thin internal compressive layers, energy absorbing crack deflection and bifurcation processes are observed.

Abstract: Multi-layer laminates were produced using alternating layers of Si3N4 and Si3N4+TiN. The differences in the coefficient of thermal expansions between the alternating layers lead to residual stresses after cooling. These are compressive in the Si3N4 layers and tensile in the Si3N4+TiN layers. The existence of these stresses in the laminates effect the crack propagation behaviour during failure. Different designs of laminates were produced with external layers under compression and tension exhibiting different failure mechanisms. Facture toughness was measured by SEVNB method. In systems with external layers under compression the measured fracture toughness was up to three times that of Si3N4, i.e. up to 17 MPa m1/2. In systems with external layers under tension during failure the energy absorbing effects of crack deflection and crack bifurcation were obtained. High temperature tests were performed to determine the onset temperature for residual stresses in these laminates. Micro-laminates with compressive layers of only 30 µm thickness with high strength and fracture toughness and were manufactured.

Abstract: Modern waste burners are equipped with catalysts to reduce NOX. During operation the catalysts and their performance are influenced by, for example rapid temperature changes, facility vibrations, cleaning procedures and unwashed exhaust gas (when the catalyst is installed before the washer). The catalyst discussed in this paper comprised over 2’500 extruded elements of 150 x 150 x 770 mm3. During commissioning the catalyst elements exhibited a tendency to spall off pieces, thus preventing acceptance of the plant. For the failure analysis, one element from the catalyst was removed and two spare elements were selected. The investigation comprised a visual check, fractography, measurement of the most important physical, mechanical and chemical properties, and a microstructural analysis. It could be shown that: − Processing defects, such as extrusion defects, were the source of cracks which led to pieces spalling off; − Hot steam, used for periodic cleaning, reduced the structural strength by half; − The elements were exposed to higher mechanical loads than expected during transport; − The cleaning process, in combination with particles transported by the exhaust gas, has an erosive effect on the catalyst material; − Blockages, caused by catalyst material among other things, could be found at different depths; − The chemical composition did not significantly differ between used and unused elements. In summary, the failure analysis led to an understanding of the failure mechanism and to a set of recommendations for improvements whose implementation ultimately led to the plant being cleared for operation.