Microstructure and Properties of 3Y-TZP/LZAS Glass-ceramic Functionally Gradient Coatings

In order to improve toughness of glass-ceramic coatings, 3mol% yattria-tetragonal zirconia polycrystal/Li2O-ZnO-Al2O3-SiO2 (3Y-TZP/LZAS) glass-ceramic functionally gradient coatings on Q235 steel substrate were prepared by slurry method. Phase constitution and microstructure of samples were examined by XRD and SEM, respectively. Their micro-hardness and fracture toughness were measured by indentation method. And their bonding strength of the coatings was also investigated by pulling test. Resulting data indicate that the seamless interfaces among toughening coatings, LZAS transition layer and substrate are obtained. SiO2 in the LZAS glass reacts with iron oxides at interfacial region forming Fe2SiO4 and FeSiO3, which causes tight interface bind between gradient coatings and steel substrate. Micro-hardness and fracture toughness from inner substrate to top surface on the section of the coatings increase gradually. Residual compressive stress toughening, 3Y-TZP particles toughening and 3Y-TZP transformation toughening are the dominant factors for improving fracture toughness of the gradient coatings. Bonding strength of the coatings is up to 16.3 MPa. The gradient coatings withstand more than 30 thermal cycles at 300 degrees C, demonstrating their excellent thermal sock resistance.