Improvement of fracture toughness in dense ATZ composites prepared from zirconia powders with different yttria content
Marek Grabowy, Yurii Delikhovskyi, Agnieszka Wilk, Zbigniew Pędzich
Quarterly No. 4, 2021 pages 161-168
DOI:
keywords: ATZ composites, fracture toughness, toughening mechanisms, crack propagation
abstract Alumina toughened zirconia (ATZ) composites with 2.3 vol.% Al2O3(ATZ-B) and 12.3 vol.% Al2O3(ATZ-10) were fabricated. The used starting zirconia powders were prepared as a mixture of powders with different yttria content. The alumina additive was commercially available Al2O3 powder. The specific preparation method and optimized sintering conditions allowed us to achieve ATZ products with exceptional properties. These properties were compared with 3Y-TZP sintered samples prepared from commercial powder (Tosoh). The structural and mechanical properties of the investigated ATZ composites were systematically studied. The microstructures were observed by scanning electron microscopy (SEM) on polished and thermally etched surfaces, then the micrographs were binarized and subjected to stereological analysis. Dense (> 99% of relative density), uniform and pore-free microstructures with homogeneously distributed Al2O3 inclusions without any visible agglomerates were obtained. The Vickers hardness and Young’s modulus were enhanced according to the rule of mixtures for the composites. The mechanical behaviour was especially oriented towards increasing the fracture toughness. The K1C parameter reached the extraordinary value of 12.7 MPam1/2 for ATZ-B and 9.8 MPam1/2 for ATZ-10. Comparatively, K1C of the 3Y-TZP reference material was 5.1 MPam1/2. The mechanisms contributing to the increase in K1C were identified to explain the reason for such a large improvement in the fracture toughness. The investigations were particularly focused on crack propagation analysis. The identified mechanisms include crack path deviation and mixed transgranular-intragranular crack migration (crack bridging), crack propagation through theAl2O3 grains and frequent changes in the fracture propagation directions of a high angle (close to even 90°). Nevertheless, the occurrence of tm (tetragonal to monoclinic) transformation of the ZrO2 phase was considered to be the main toughening factor. Due to the specific method of preparation, leading to an intensification of yttrium diffusion during sintering, the final microstructure revealed very small grains of a tetragonal zirconia phase. These grains exhibited high transformability, which was the main reason for the distinctin crease in fracture toughness.