Enhancing the toughness of nano-composite coating for light alloys by the plastic phase transformation of zirconia

Abstract

Toughness of ceramic coatings is essential for engineering deploys of light alloys, especially for the scenarios tackling with corrosion and wear. This work reports a toughened ZrO2/MgO coating on Mg alloy enabled by stress-induced martensite transformation from tetragonal zirconia (t-ZrO2) to monoclinic zirconia (m-ZrO2), which is achieved by the plasma electrolytic oxidation (PEO) process. Observations reveal that the interfaces between ZrO2 and MgO grains are essential in inducing phase transition during the toughening process. Interestingly, semi-coherent interfaces were found between t-ZrO2 and MgO, and the mismatched atoms at the interface cause local lattice distortions. These lattice distortions provide an appropriate dislocation density (2.2 × 105 /m2–4.1 × 105 /m2), which not only ensures the stability of the interface but also provides a stress transfer channel to induce martensite transformation of ZrO2. The plastic phase transformation is accompanied with volume expansion of ZrO2 grains, thus a compressive stress field is generated to resist the crack propagation with an enhancement of the plasticity of the composite ceramics. As a result, the toughness of the enhanced coating is about 1.15 times higher than that of the traditional PEO coating. This work provides an alternative strategy for designing toughened ceramic coatings for light alloys.