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Abstract

This doctoral thesis describes the synthesis and characterization of calcium phosphate (CaP) coatings on zirconia ceramics. The main application of coated zirconia is in medicine, particularly in the area of dental bone implants. Due to the favourable biological characteristics of CaP, it can be expected that the application of CaP coatings on zirconia bone implants will improve the quality of the bone-implant interface and increase the lifetime of the zirconia implants.
The first part of my research was dedicated to the development of a biomimetic synthesis procedure for the deposition of CaP coatings on zirconia and to the characterization of the coating’s composition and formation. The developed synthesis procedure involved two steps. In the first step, zirconia substrates were immersed in the supersaturated CaP solution with a pH = 7.4 and after 1 hour of soaking a thin (200 nm) calcium hydroxyapatite (HA) seeding layer formed on the zirconia substrates via three stages: i) precipitation of amorphous CaP; ii) precipitation of octacalcium phosphate (OCP); iii) transformation of OCP to HA. The subsequent immersion of these coated zirconia substrates in a fresh solution with a similar composition but a lower pH (pH = 7.0) led to the rapid growth of the biomimetic CaP coating with a lamellar topography (second step). After 11 hours of immersion in the second solution (pH=7.0) the thickness of the biomimetic CaP coating was 12 µm. The biomimetic CaP coating was composed of two compositionally distinct layers. The lower layer was composed of HA nanoparticles with an average size of 6 nm, while the upper layer was composed of lamellar octacalcium phosphate (OCP) monocrystals. The thickness of the biomimetic CaP coating increased with the immersion time in the second solution. The test of bioactivity with the simulated body fluid confirmed that the biomimetic CaP coatings are bioactive. Moreover, the results of different mechanical tests indicate, that the prepared biomimetic CaP coatings possess relatively low adhesion to the substrate, which could be a limiting factor for their application in clinics.
In the second part of my research I studied the influence of the thermal processing of biomimetic CaP coatings on their composition, dissolution rate and, in particular, on their
bonding strength to the zirconia substrate. Heating at 600 °C did not have any influence on the topography of coatings, but it promoted the phase transformation of the OCP crystals to HA and calcium pyrophosphate (CPP). The adhesion of the as-treated coating was only slightly higher compared to the biomimetic CaP coating. The dissolution rate of the coating heated at 600 °C was higher than the biomimetic CaP coatings. After heating the biomimetic CaP coatings at 800 °C and 900 °C they were composed of β-tricalcium phosphate (β-TCP) and β-CPP and after heating at 1100 °C and 1200 °C the β-TCP was the only phase present in the coating. When the coatings that were prepared by heating between 800 in 1200 °C were sonicated in an ultra-sound bath for a short period of time a majority of the coating was removed with only a thin β-TCP coating remaining on the zirconia substrate. As-prepared thin β-TCP coatings were the focus of my further research. The heating temperature influenced the topography and the thickness of thin β-TCP coatings, as well as their dissolution rate in a physiological solution. Moreover, various mechanical tests showed that thin β-TCP coatings possess excellent mechanical strengths and therefore the potential for applications in clinics.