In this work, ceramic coatings were formed on Ti6Al4V titanium alloy using a technique of plasma electrolytic oxidation. Plasma electrolytic oxidation was carried out in electrolytes with different chemical compositions and the effect of the electrolyte on the macro-and microstructure, pore size, phase composition and wear resistance of coatings was estimated. Three types of electrolytes based on sodium compounds were used, including phosphate, hydroxide, and silicate. The composition of the electrolyte affects the intensity and size of microcharges and the volume of gas release of various electrolytes. The plasma electrolytic oxidation processes were carried out at a fixed voltage (270 V) for 5 minutes. The results showed that the coating was mainly composed of rutile- and anatase TiO2 , but a homogeneous structure with lower porosity and a large number of crystalline anatase phases was obtained in the coating prepared in the silicate-based electrolyte. The diffractogram electrolytes did not reveal the peaks of the crystalline phases associated with the PO4 3— and SiO3 2— anions. This means that these anions included only oxygen in the coatings. The morphology and phase composition of the samples were studied using a scanning electron microscope and an X-ray diffractometer, respectively. Wear resistance was evaluated by the “ball-disc” method on the TRB3 tribometer. The wear resistance of various coatings formed on Ti6Al4V titanium alloys showed completely different wear resistance. The lowest coefficient of friction (µ = 0.3) was demonstrated by the coating obtained based on phosphate. This may be due to a large number of crystal phases of rutile. The sample prepared in a hydroxide-based electrolyte showed a high wear coefficient (µ=0.52). This effect can be obtained by eliminating surface defects (microcracks and micropores).
Functional-gradient titanium/hydroxyapatite (TiHA) coatings were obtained using detonation spraying technology to improve the structure and mechanical properties. To obtain functional-gradient coatings, pulsed energy sources are best suited, namely, detonation spraying, in which the energy of the explosion of gas mixtures is used as a source of pulsed action. By controlling the modes of detonation spraying, it is possible to vary the temperature and rate of coating deposition; accordingly, it is possible to obtain a certain structuralphase structure of the coatings. The structural-phase state and tribological properties of TiHA detonation coatings were investigated by modern materials science methods: X-ray phase analysis (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDX-mapping), profilometry and ball-disk wear-resistance test. The results showed that the coatings had a continuously gradient elemental composition across the cross-section of the coatings with no boundary between the elemental layers of the coatings. The amount of Ti gradually decreased and the amount of hydroxyapatite gradually increased in the direction from the substrate to the surface of the coatings, which allows to expand the possibilities of using TiHA-coatings for bone implants. Since the surface layer is composed of HA, the resulting functional-gradient coating demonstrates excellent biocompatibility and the ability to create new bone tissue. The excellent mechanical strength of the functionally graded coatings is ensured by the Ti phase.
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