Optical and mechanical properties of amorphous and crystalline yttria-stabilized zirconia thin films prepared by pulsed laser deposition

Abstract

Sub-micron thick yttria-stabilized zirconia (YSZ) layers ( t = 400–700 nm) containing 3 (3YSZ) or 8 mol.% (8YSZ) Y 2O 3 with microstructures ranging from isotropic amorphous to columnar polycrystalline and a variable porosity can be grown by pulsed laser deposition (PLD) varying the substrate temperature and oxygen background pressure. The dependence of the mechanical and optical properties on the film microstructure and chemical composition was investigated by nanoindentation experiments and transmission spectrophotometry. Dense amorphous YSZ layers exhibit a higher optical transmissivity, 0.2 eV lower band gap energy (5.5 vs. 5.7 eV), and up to 25% lower hardness (11.9 vs. 16.0 GPa) and reduced elastic moduli (231 vs. 278 GPa) compared with crystalline films, irrespective of the dopant level. High refractive indices, n 600 nm, in the range 2.18–2.23, i.e. close to single crystal reference data, are obtained for the low pressure PLD regime. Within these boundaries the index decreases with increasing Y 2O 3 content and is consistently slightly smaller for amorphous layers compared with crystalline films of the same composition, due to a lower atomic packing density. The gradual decrease in the refractive index for YSZ layers grown at background pressures above 1.0 Pa marks the development of pores in the form of inter-columnar voids and can be used for sensitive quantification of the film porosity. The lattice order affects the fracture behaviour, as amorphous coatings show plastic deformation mediated by shear bands, while the crystalline layers yield hoop and surface cracks upon indentation. The crystalline 3YSZ films exhibit an enhanced fracture toughness compared with 8YSZ, which is related to the stress-induced transformation to the monoclinic phase in partially stabilized zirconia.