Multilayered ceramic composites are considered as very promising materials for different engineering applications. However, the relationship between residual stresses and the mechanical behavior of laminates has not yet been adequately studied. We propose to experimentally study residual stresses and their effect on the mechanical performance of silicon nitride and boron carbide based multilayered ceramics using a micro-Raman spectroscopy and indentation methods, as well as standard flexure tests. Our previous work demonstrated a significant increase in fracture toughness due to a design, which created a high compressive stress in the thin (100 micro m) silicon nitride layers and a low tensile stress in the thick (500-700 micro m) silicon nitride – titanium nitride layers. The residual stresses were controlled by the amount of TiN in layers with residual tensile stresses and the layers thickness. The fracture toughness of pure Si3N4 ceramics was measured to be ~5 MPa m1/2, while the apparent fracture toughness of Si3N4/Si3N4-TiN laminates was in the range of 16-19 MPa m1/2 depending on the composition and thickness of the layers. The same approach used for design of three layered boron carbide based laminates has led to the composites with apparent KIc exceeding of 9 MPa m1/2. The significant increase in fracture toughness was achieved by the control over the level of residual stresses in separate layers, which allows maximizing mechanical properties of laminates. However the relationship between residual stresses and the mechanical behavior of ceramic laminates has not yet been optimized and the measurement of the residual stresses in the separate layers of the laminates is a real challenge.
The proposed modeling-experimental program will demonstrate unequivocally that the concept of controlled residual stresses can be employed to develop high performance ceramic laminates. This project addresses the problem by using micro-Raman and sharp indentation for residual stress measurements; employing analytical modeling for calculation of the residual stresses; as well as studying their effect on the mechanical properties of ceramic microstructures composed of periodic, ordered arrays of different silicon nitride and boron carbide based materials. The resulting local variations of thermal expansion and Young’s modulus produce oscillating internal stresses, which inhibit brittle failure. This research produces a fundamental knowledge about the relationship between residual stresses and the mechanical behavior of multilayered ceramic composites, improve our understanding of laminates’ strength and fracture toughness at room and high temperatures, and lead to the development of damage-tolerant ceramics with high mechanical properties significantly exceeding that of current non-oxide ceramics.
The main goal is to determine the exact values of the residual stresses in silicon nitride and boron carbide based laminates and incorporate them in composite design. The research results in a clear identification of the microstructural parameters that control residual stresses in laminates. Further understanding of the toughening mechanisms (bifurcation, R-curve behavior, deflection of a moving crack) on mechanical characteristics will be achieved.
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