Development of copper-enriched porous coatings on ternary Ti-Nb-Zr alloy by plasma electrolytic oxidation

In this paper, a preparation method and characteristics of porous coatings enriched in copper distributed in the whole volume on a ternary Ti-Nb-Zr alloy biomaterial obtained by plasma electrolytic oxidation (PEO) in an electrolyte containing H3PO4 within Cu(NO3)2 at potentials of 180 and 450 V are presented. It has been shown that the PEO potential has impact on the thickness of the coatings, i.e., the higher the potential used, the thicker the coating obtained. Using XPS study, it was shown that copper inside the coating appears as Cu+ and Cu2+ ions, while titanium, niobium, and zirconium appear as Ti4+, Nb5+, and Zrx+ (x ≤ 2), respectively. It was also found that the roughness of PEO coating formed at 450 V is higher than the one obtained at 180 V, and it is well correlated with bigger pores after the PEO treatment. Additionally, in this paper two PEO coating models composed of three sub-layers are presented. The thickness of the outer top porous sub-layer obtained after PEO oxidation at both 180 and 450 V equals to about 2 μm, while the semi-porous as well as transition sub-layers are thicker after PEO processing at 450 V (5 μm) than those obtained at 180 V (4 μm thick). The creation of the top porous and transition compact sub-layer of PEO coating may be explained by switch-on and switch-off of the PEO potential, while the middle and semi-porous sub-layers are most likely formed during the stable voltage conditions of PEO treatment.

A very important point in antibacterial coatings has to be improved for optimized biocompatibility. Some previous papers described the porous and bactericidal coatings on alloys, such as Ti6Al4V [38] and Ti-Nb-Zr-Sn [44] obtained by PEO known also as microarc oxidation (MAO). In those articles, the focal point of analysis was dedicated to the presence of elements displaying a detrimental action in the human body, such as vanadium, aluminum [38, 56,57], and tin [38,58].

Material
Titanium-niobium-zirconium alloy (Ti-20Nb-6Zr, composition in wt% and at%) samples submitted to PEO (microarc Fig. 1 SEM picture of coating formed on Ti-Nb-Zr after mechanical pretreatment by abrasive paper 120 at ×6000 magnification Fig. 2 EDS result of coating formed on Ti-Nb-Zr after mechanical pretreatment by abrasive paper 120 at ×6000 magnification oxidation) served for the study. Samples were prepared by mechanical abrasive treatment (Fig. 1) in the form of rectangular specimens of dimensions 35 × 15 mm cut off from a metal sheet 1 mm thick with chemical composition shown in Fig. 2 and Table 1.

Setup and parameters
The PEO was performed at voltages of 180 ± 10 and 450 ± 10 V, consecutively, during 3 min of processing. A setup used for the studies was presented in our earlier paper [72]. The studies were carried out in the electrolyte of initial temperature of 20 ± 2°C. For the studies, the electrolyte, i.e., 3.2 mol/L of Cu(NO 3 ) 2 in orthophosphoric acid H 3 PO 4 , was used. For each run, an electrolytic cell made of glass was used, containing up to 500 mL of the electrolyte.

SEM and EDS studies
The scanning electron microscope is a Quanta 250 FEI in low vacuum and ESEM mode and a field emission cathode. The energy-dispersive (EDX) system is a NORAN System SIX with a nitrogen-free silicon drift detector. A ×6000 magnification for SEM images and EDS analyses was used.

XPS studies
The X-ray photoelectron spectroscopy (XPS) measurements on titanium samples were performed on a SCIENCE SES 2002 instrument using a monochromatic X-ray source Al K(alpha) (hν = 1486.6 eV) (18.7 mA, 13.02 kV) (Gammadata Scienta). Scan analyses were carried out with an analysis area of 1 × 3 mm and a pass energy of 500 eV with an energy step 0.2 eV and step time of 200 ms. The binding energy of the spectrometer has been calibrated by the position of the Fermi level on a clean metallic sample. The power supplies were stable and of high accuracy. The experiments were carried out in an ultra-high-vacuum system with a base pressure of about 6 × 10 −8 Pa. The XPS spectra were recorded in normal emission. For the XPS analyses, the CasaXPS 2.3.14 software (Shirley background type) [73] with the help of XPS tables [73][74][75][76][77][78][79] was used. All the binding energy values presented in this paper were charge-corrected to C 1s at 284.8 eV.

2D roughness measurements
A computerized Hommel Tester T800 system of Hommelwerke GmbH was used for measuring the surface roughness, and the GDOES for measuring crater depth. It was equipped with a sliding measuring head Waveline 60 Basic/51808 and a sensor TKL100/17 MO435005. The measuring needle beam was equal to 3.5 μm with an angle of 87°. The tracing, evaluation, and single measuring lengths were equal to 4.8, 4.0, and 0.8 mm, respectively. Due to the porous surface, non-contact methods for surface roughness [81] were not possible to be used, inter alia, because of the uncertainties in measurement results [82].
According to the EN ISO 4287:1999 [83] and DIN 4768 [84] standards, the following roughness parameters have been measured: arithmetic mean of the sum of roughness profile values (Ra), mean peak-to-valley height (Rz DIN ), root mean square deviation of the roughness profile (Rq), total height of the roughness profile (Rt), the ratio of the developed profile length to the evaluation length (L 0 ), and profile peak density (D). All the results were processed by using the software STATISTICA ver. 12 [85].

Results and discussion
In Figs. 3 and 4, the SEM pictures of coating formed on one of the superelastic alloys [86,87], i.e. the Ti-Nb-Zr alloy surface after PEO treatment at voltages of 180 and 450 V, respectively,   Table 2. However, the reader must be aware that the EDS signal of phosphorus, niobium, and zirconium are very close to one another. Therefore, the data presented in Table 2 must be treated as orientation ones only. Hence, for comparing the two surfaces, the copper-to-phosphorus (Cu/P) ratio was used. The higher Cu/ P ratio was found for oxidation at 180 V, equaling to 0.55. After treatment at 450 V, the ratio was lower (Cu/P = 0.39). The pores are mostly opened in the PEO coating surface after the treatment at 450 V and mostly closed for oxidation at 180 V. The porous coating obtained after PEO treatment at 450 V has a lot of sharp and open pores (Fig. 4), which are also one within another. The coating grown at 180 V is not so porous as that one formed at 450 V. It is most compact with some craters (Fig. 3). It can suggest that for oxidation at 180 V the pores are mostly closed and developed inside the coating, whereas the PEO treatment at 450 V results in the formation of open pores.
In Figs. 5, 6, 7, 8, 9, 10, 11, and 12, the GDOES depth profiles of copper (325 nm), phosphorus (178 nm), oxygen       180 V (about 4 μm thick) with a similar amount of copper inside. The signals of phosphorus (Fig. 6) and oxygen (Fig. 7) are higher after PEO at 180 V than at 450 V, which may be interpreted literally as a higher amount of phosphorus as well as the effect of layer porosity with the depth of analysis. The third inner layer (ca. 4 μm for PEO at 180 V and ca. 5 μm at 450 V) is the interface between the matrix and coating obtained by PEO. The analyses of titanium (Fig. 10), niobium (Fig. 11), and zirconium ( Fig. 12) signals confirm the above stated reasoningthicker coating obtained at a higher voltage (450 V) and existence of the transition layer found in the coating. Additionally, in that layer a peak of hydrogen (Fig. 8) was detected, which may be interpreted as "frozen" in the PEO structure phosphoric acid molecules and/or hydrogen phosphate ions. The spectrum of nitrogen (Fig. 9) is similar after two PEO treatments, and the highest signal was recorded for coating after PEO at 180 V, which  confirms the phenomena described in [44]. The detected peaks related to the top surface (top 10 nm) may be explained as surface organic contaminations.
To understand the chemical composition of the top layer (top 10 nm) of PEO coatings, XPS measurements were performed. They are presented in Fig. 13. With XPS, there is no problem separating the phosphorus, niobium, and zirconium signals, as it was the case with EDS. The survey XPS analysis of the PEO coatings obtained at 180 and 450 V indicates that they are composed of phosphorus, titanium, niobium, zirconium, and copper with organic carbon-oxygen contaminations.
To know the oxidation stages of titanium, niobium, zirconium, and copper, high-resolution XPS measurements were performed, and the results are displayed in Figs   Ra arithmetic mean of the sum of roughness profile values, Rz DIN mean peak-to-valley height, Rq root mean square deviation of the roughness profile, Rt total height of the roughness profile, L 0 the ratio of the developed profile length to the evaluation length, D profile peak density  [76,77]. It must be pointed out that the XPS results can be related only to the top 10 nm, therefore they can be treated mainly as a suggestion of oxidation stages, especially in the presence of organic contaminations which provide signals of carbon and nitrogen; they are presented in Tables 3 and 4. The chemical compositions of the studied top nano-layer of the PEO coatings show that for TNZ alloy a higher voltage resulted in formation of a coating containing more oxides than phosphates of titanium and/or niobium and/or copper; it appears that the detected amount of zirconium in this coating is lower than the embedded copper. The amount of copper is higher after the PEO treatment at 180 V; however, because of a high amount of contamination signals (C 1s, N 1s, partly O 1s), it is very hard to state clearly what the chemical composition of this surface layer is, with distinguishing between the contamination layer and the PEO coating.  Tables 5  and 6 as well as in Fig. 17, the roughness parameters of the PEO coating formed on TNZ alloy at 180 and 450 V, respectively, are presented. Based on these results, it should be concluded that the surface roughness after PEO treatment at 180 V is lower than that after oxidation at 450 V, which was also noted in the case of the PEO of Ti-Nb-Zr-Sn alloy in the same electrolyte [53]. Based on the obtained results, conclusions related to the porosity of the PEO coatings may be drawn, i.e., the higher the roughness parameter, the bigger pores are observed, which was confirmed by SEM and roughness studies (Ra). The significance tests were performed, and that way, the null hypothesis H 0 was formulated. It is assumed that the means of studied populations of roughness parameters are equal for surfaces obtained at two voltages, i.e., 180 and 450 V. Based on the obtained roughness parameters, the probability "p" for all parameters was found. The low probability p from the level of significance (α) indicates that the hypothesis of equality of expected values must be rejected. Proposed 2D roughness parameters such as Ra (p = 4.21 × 10 −4 ), Rz (p = 8.28 × 10 −4 ), Rq (p = 1.88 × 10 −3 ), Rt (p = 4.1 × 10 −2 ), and L (p = 2.25 × 10 −5 ) are significantly different for two surfaces obtained at two voltages (180 and 450 V), and all of them can be used for the evaluation of these surfaces. Only the use of a parameter D (p = 1.15 × 10 −1 ) is not possible in the case of differentiated surfaces after PEO at 180 and 450 V. Roughness parameters such as Ra, Rz, Rq, and L are the best for characterization obtained surfaces, which confirms the probability p and the box and whisker plots in Fig. 17. Figure 18 shows the models of the coatings formed on Ti-Nb-Zr (TNZ) alloy after 3 min of PEO treatment at voltages of 180 and 450 V in the electrolyte containing H 3 PO 4 within Cu(NO 3 ) 2 . The two models were created on the basis of GDOES and XPS results presented in this paper. Three sublayers of PEO coatings are found, i.e., top porous layer with thickness of 2 μm, middle semi-porous layer with thickness of 4-5 μm, and transition compact layer 4-5 μm thick. The model is similar to that one presented in [38]; however, the thicknesses of sub-layers are different. It is also noted that in the transition layer of the present model, one may assume with a high probability that there are molecules of phosphoric acid as the peak of hydrogen in GDOES suggests. Most importantly is the fact that the structure of the obtained models presented in this work is very similar to that given in [38]. This indicates that the obtained results are reproducible and fully reliable.

Conclusions
The following conclusions may be formulated after the PEO treatment of tertiary Ti-Nb-Zr alloy studied: 1. Porous coatings on titanium-niobium-zirconium alloy surface, enriched in copper distributed in the whole volume, were obtained. 2. The PEO potential has impact on the thickness of the coatings: the higher the potential used, the thicker coating is obtained. 3. Copper inside the coating appears as Cu + and Cu 2+ ions, while titanium, niobium, and zirconium appear as Ti 4+ , Nb 5+ , and Zr x+ (x ≤ 2), respectively. 4. The roughness of PEO coating formed at 450 V is higher than that obtained at 180 V, and it is well correlated with bigger pores obtained after the PEO treatment. 5. Three sub-layers, i.e., outer (porous), inner (semi-porous), and transition (adjacent to the matrix), may be separated in the PEO coating formed on Ti-Nb-Zr alloy. 6. The thickness of the outer top porous sub-layer obtained after PEO oxidation at both 180 and 450 Vequals to about 2 μm, while the semi-porous as well as transition sublayers are thicker after PEO processing at 450 V (5 μm) than those obtained at 180 V (4 μm). 7. It is most likely that the transition sub-layer, adjacent to the matrix, is formed at the very beginning of PEO treatment (switch-on of the PEO potential) and the outer, porous sub-layer, is formed at the end of PEO treatment (switch-off of the PEO potential), while the second, semi-porous sub-layer, is formed during the stable voltage conditions of PEO treatment.