Visualization of Transformation Toughening for Newly Developed Ce-TZP/Alumina Nanocomposite

Objectives: The aim of this study was to perform a microscopic fracture mechanics assessment for newly developed Ce-TZP/Alumina nanocomposite and to provide information on transformation zone size and generated residual stress fields around a crack path according to in situ Raman spectroscopy, in comparison to a conventional 3 mol% Y-TZP.

Methods: The investigated Ce-TZP/Alumina nanocomposite achieved three times the toughness and equivalent or even higher strength of Y-TZP. This material was composed of 10 mol% Ce-TZP as a matrix phase and 30 vol% of alumina as a secondary phase. It partly possessed an intragranular microstructure, in which nanometer sized alumina particles were trapped within the sub-micron sized Ce-TZP grains. In order to conduct fracture mechanics assessments, cracks were produced from the corners of a Vickers indentation by pressing an indenter onto a polished surface of the specimen. Raman spectra were collected with microscopic spatial resolution and analyzed by using a triple monochromator spectrometer (T-64000, ISA Jovin-Ivon/Horiba Group, Japan) equipped with a charge-coupled detector (high-resolution CCD camera). In Raman mapping, a suitable excitation frequency was a monochromatic blue line emitted by an Ar-ion laser at a wavelength of 488 nm.

Results: For Ce-TZP/Alumina nanocomposite, both significant amount of tetragonal-to-monoclinic (t-m) transformation and rather high compressive residual stresses were visualized behind and ahead of the crack path in relatively large areas of the order of tens of micron meters. On the other hand, in 3Y-TZP, rather small amounts of t-m transformation and less compressive residual stresses could be observed. In contrast, only in 3Y-TZP, a pronounced tensile residual stress was observed in the zone immediately ahead of the crack tip.

Conclusion: According to the present quantitative characterization based on in situ Raman microprobe spectroscopy, the actual microscopic mechanism behind the unique toughening and strengthening behaviors of Ce-TZP/Alumina nanocomposite was clarified.