|Engineering ceramics indentation methods crack-resistance Rayleigh waves acoustic microscopy machining parameters green ceramic bodies ultrasonic contact impedance acoustic impedance residual stress|
Indentation methods were examined at various engineering ceramics (Al2O3, ATC, SSiC, PSZ) to measure fracture toughness (Indentation - Crack Length, Indentation – Strength in Bending) and Young’s modulus (Ultrasonic Contact Impedance...UCI). A lithographic method was developed in order to measure the crack length depending on the indentation load. The forecast of the Lawn Model was confirmed that the diagonal cracks of a Vickers indent in the ceramics increasing during the unloading phase. In consideration of real crack profiles and of the 3D-FEM calculations of Marx, refinements of the Lawn model were discussed and a correction term for the short-crack toughness was formulated. The UCI-Method was also used to measure gradients of density on the surface of green ceramic bodies.
The surfaces of SSiC, SiSiC and SSN ground with various parameters were investigated with Rayleigh waves. A focused transducer at 35 MHz was used to find defects ³ 50 µm. The V(z)-curves at 1 GHz measured perpendicular to the ground surface points at microcracks up to 80 µm depth. The dispersion of Rayleigh waves at 20 MHz - 200 MHz measured on the ground surface with a laser-acoustic equipment produced comparable curves of Rayleigh-wave-velocities in depth direction. Combined with the results of the residual stress measurements and 4-point bending tests it was possible to evaluate effective guidelines for optimized machining parameters.