Microstructural characterisation of porous TiO ₂ ceramic coatings fabricated by plasma electrolytic oxidation of Ti

This study aims at providing a thorough examination of compositional and microstructural heterogeneities in the porous oxide ceramic surface layers formed on Ti by plasma electrolytic oxidation (PEO). The PEO-titania layers of up to 5μm thick were produced using 0.02...0.04M NaH ₂ PO ₄ electrolyte solutions in the voltage range of 450...500V DC. Advanced methods of X-ray diffractometry (XRD), scanning electron microscopy (SEM) and field-emission transmission electron microscopy (FE-TEM) were employed to observe the surface layer morphology and characterise its crystal structure across the whole coating thickness. As revealed by XRD analysis, the surface layers consist of both anatase and rutile evenly distributed across the layer thickness. Detailed TEM studies showed that a continuous amorphous layer exists at the top of the PEO coating, the layer comprises mainly TiO ₂ , with some phosphorus incorporated from the electrolyte. Underneath, there is a porous crystalline layer with uniformly distributed nano-scale pores (<50nm), anatase nanocrystallites (<100nm) and submicrometer-scale (0.1 to 1μm) rutile crystallites. Large micrometre size pores (about 1...3μm in diameter) surrounded by nanocrystalline anatase are found to exist at the bottom of the porous crystalline layer, adjacent to the thin interfacial barrier layer. Correlations between thermal-physical properties of the coating material, heat dissipation conditions and microstructural evolution in the surface layer during coating formation are discussed.