Papers by Author: Richard Dobedoe

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: 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.