Phosphate Coatings Enriched with Copper on Titanium Substrate Fabricated Via DC-PEO Process

The present paper covers the possible ways to fabricate advanced porous coatings that are enriched in copper on a titanium substrate through Direct Current Plasma Electrolytic Oxidation (DC-PEO) with voltage control, in electrolytes made of concentrated orthophosphoric acid with the addition of copper(II) nitrate(V) trihydrate. In these studies, solutions containing from 0 to 650 g salt per 1 dm3 of acid and anodic voltages from 450 V up to 650 V were used. The obtained coatings featuring variable porosity could be best defined by the three-dimensional (3D) parameter Sz, which lies in the range 9.72 to 45.18 μm. The use of copper(II) nitrate(V) trihydrate in the electrolyte, resulted, for all cases, in the incorporation of the two oxidation forms, i.e., Cu+ and Cu2+ into the coatings. Detailed X-Ray Photoelectron Spectroscopy (XPS) studies layers allowed for stating that the percentage of copper in the surface layer of the obtained coatings was in the range of 0.24 at% to 2.59 at%. The X-Ray Diffraction (XRD) studies showed the presence of copper (α-Cu2P2O7, and Cu3(PO4)2) and titanium (TiO2-anatase, TiO3, TiP2O7, and Ti0.73O0.91) compounds in coatings. From Energy-Dispersive X-Ray Spectroscopy (EDS) and XPS studies, it was found that the Cu/P ratio increases with the increase of voltage and the amount of salt in the electrolyte. The depth profile analysis by Glow-Discharge Optical Emission Spectroscopy (GDOES) method showed that a three-layer model consisting of a top porous layer, a semi-porous layer, and a transient/barrier layer might describe the fabricated coatings.


Characterization Methods of PEO Coatings
Images of the obtained coatings were completed by means of the scanning electron microscope FEI Quanta 250 that was equipped with a field emission electron gun. Each time the SEM imaging was carried out under a high vacuum, employing Everhart-Thornley detector under the secondary electron imaging mode.
Analysis of the atomic composition of the obtained coatings was performed by means of the energy-dispersive X-ray spectroscopy (EDS), while using the ultra-dry EDS detector of Thermo Scientific Co., co-operating with a scanning electron microscope FEI Quanta 250. For a quantitative determination, the atomic contents of Ti (Kα), P (Kα), and Cu (Kα) were taken into account. The studies were carried out on 10 samples, at the magnification of 500×, while using the accelerating voltage of 20 kV.
The XPS studies were performed by means of the SCIENCE SES 2002 apparatus, taking advantage of a monochromatic source of the X-ray radiation Al Kα (hν = 1486.6 eV), which was operating at the current of 18.7 mA and the voltage of 13.02 kV. Each time the analyzed surface area of the sample was 3 mm 2 , the pass energy of 500 eV with the leap of 0.2 eV and the leap time equaling 200 ms. The device power supply was stabilized to increase the energy accuracy that was supplied to the source of radiation. Measurements were performed in a vacuum chamber with an ultra-high vacuum equaling 6 × 10 -8 Pa.
For each of the studied samples, the XPS analysis for the binding energies of titanium (Ti 2p), phosphorus (P 2p), copper (Cu 2p3/2), oxygen (O 1s), carbon (C 1s), and nitrogen (N 1s), were done. All of the presented herewith XPS spectra have been calibrated against the binding energies of C-C, C-H for carbon (C 1s), equaling 284.8 eV, and the analyses of the obtained data were carried out while using the softwares of Casa XPS 2.3.14 (Casa Software Ltd., Teignmouth, Devon, UK).
The XRD studies were carried out in the porous PEO coatings. The diffractograms that were presented in this work were obtained by means of the D8 Advance measuring system of Bruker's Co.
(BRUKER Corporation, Billerica, MA, USA), equipped with a focusing circle/circumference//district of Bragg-Brentano type (2Θ/Θ), the source of X-ray radiation Cu Kα (40 kV, 40 mA), and the 192-channel detector LynxEye with a nickel aperture/diaphragm/cover. The studies of the surface topography were carried out by using a confocal laser microscope LEXT OLS 4000 by Olympus Co., which was equipped with the object lens/objective LEXT OLS 4000 by Olympus Co. (aperture 0.95, operating distance 0.35 mm). In the laser mode, a semiconductive source of radiation for the wave of 405 nm, operating on a permanent basis of a maximum power of 6 mW, and the divergency in the range from 0.12 up to 0.41 rad, was used. The white light LED was used in the optical mode. A photomultiplier was responsible for the detection system in a laser mode and a single panel CCD (2 × 10 8 pixels) -for a laser mode.
The profile analysis of the depth distribution of the elements of the obtained PEO coatings was performed by means of the GDOES apparatus. For this purpose, a Horiba Scientific Profiler 2 was used by taking advantage of the asynchronous frequency pulse generator of plasma. The studies were carried out in a low pressure (700 Pa) chamber, the operational frequency of the plasma generator was equal 3000 Hz, under the discontinuous mode equaling 0.25 cycle of the power of 40 W in the argon environment by using the anode with the diameter of 4 mm. Figure S1. Juxtaposition of exemplary SEM images (a-l) and three-dimensional (3D) maps obtained by using CLSM technique (m-o) on the PEO coatings fabricated with an electrolyte of concentrated orthophosphoric acid at voltages: 450 V (a, d, g, j, m), 550 V (b, e, h, k, n), and 650 V (c, f, i, l). Table S1. Results of surface topography parameters using CLSM method, on a titanium sample after PEO processing in a pure concentrated orthophosphoric acid electrolyte at different voltages 450 V, 550 V, and 650 V.              Figure S8. XPS spectra obtained for the PEO coating fabricated in the electrolyte consisting of orthophosphoric acid and copper(II) nitrate(V) trihydrate at concentration 50 g/dm 3 with a voltage of 550 V.