Anodic oxidation of zirconium in silicate solutions

Antibacterial Properties and Biocompatibility of Multicomponent Titanium Oxides: A Review

The simple oxides like titania, zirconia, and ZnO are famous with their antibacterial (or even antimicrobial) properties as well as their biocompatibility. They are broadly used for air and water filtering, in food packaging, in medicine (for implants, prostheses, and scaffolds), etc. However, these application fields can be broadened by switching to the composite multicomponent compounds (for example, titanates) containing in their unit cell, together with oxygen, several different metallic ions. This review begins with a description of the synthesis methods, starting from wet chemical conversion through the manufacturing of oxide (nano)powders toward mechanosynthesis methods. The morphology of these multicomponent oxides can also be very different (like thin films, complicated multilayers, or porous scaffolds). Further, we discuss in vitro tests. The antimicrobial properties are investigated with Gram-positive or Gram-negative bacteria (like Escherichia coli or Staphylococcus aureus) or fungi. The cytotoxicity can be studied, for example, using mouse mesenchymal stem cells, MSCs (C3H10T1/2), or human osteoblast-like cells (MG63). Other human osteoblast-like cells (SaOS-2) can be used to characterize the cell adhesion, proliferation, and differentiation in vitro. The in vitro tests with individual microbial or cell cultures are rather far away from the real conditions in the human or animal body. Therefore, they have to be followed by in vivo tests, which permit the estimation of the real applicability of novel materials. Further, we discuss the physical, chemical, and biological mechanisms determining the antimicrobial properties and biocompatibility. The possible directions of future developments and novel application areas are described in the concluding section of the review.

Effect of (NH4)2ZrF6, Voltage and Treating Time on Corrosion Resistance of Micro-Arc Oxidation Coatings Applied on ZK61M Magnesium Alloys

The effects of (NH₄)₂ZrF₆ concentration, voltage and treating time on the corrosion resistance of ZK61M magnesium alloy micro-arc oxidation coatings were studied by orthogonal experiments. The SEM result shows that the surface roughness and porosity of MAO coatings increased with (NH₄)₂ZrF₆ concentration, voltage and treating time as a whole, except the porosity decreased with treating time. EDS, XRD and XPS analysis show that (NH₄)₂ZrF₆ was successfully incorporated into coatings by reactive incorporation, coatings are dominantly composed of ZrO₂, MgO, MgF₂ and amorphous phase Mg phosphate. Potentiodynamic polarization was used to evaluate the corrosion property of coatings. When the concentration of (NH₄)₂ZrF₆ is 6 g/L, the voltage is 450 V, and the treating time is 15 min, the coating exhibits the best corrosion resistance which corrosion current density is four magnitudes lower than substrate attributed to the incorporation of ZrO₂ and the deposition of MgF₂ in the micropores.

Fabrication and Characterization of New Functional Graded Material Based on Ti, Ta, and Zr by Powder Metallurgy Method

In view of the aging population and various diseases worldwide, the demand for implants has been rapidly increasing. Despite the efforts of doctors, engineers, and medical companies, the fabrication of and procedures associated with implants have not yet been perfected. Therefore, a high percentage of premature implantations has been observed. The main problem with metal implants is the mechanical mismatch between human bone and the implant material. Zirconium/titanium-based materials with graded porosity and composition were prepared by powder metallurgy. The whole samples are comprised of three zones, with a radial gradient in the phase composition, microstructure, and pore structure. The samples were prepared by a three-step powder metallurgy method. The microstructure and properties were observed to change gradually with the distance from the center of the sample. The x-ray diffraction analysis and microstructure observation confirmed the formation of diffusive connections between the particular areas. Additionally, the mechanical properties of the obtained materials were checked, with respect to the distance from the center of the sample. An analysis of the corrosion properties of the obtained materials was also carried out.

Micro + Nano: Conserving the Gold Standard Microroughness to Nanoengineer Zirconium Dental Implants

Zirconium has achieved popularity as a biomaterial for dental and orthopedic implants; however, its bioinertness can compromise implant-tissue integration, especially in compromised patient conditions. More recently, various nanoengineering strategies have been explored to enhance the bioactivity of Ti-based implants; however, nanoengineering of Zr-based implants has not been adequately explored. In this pioneering attempt, we report on the optimized fabrication of various nanostructures on microrough Zr surfaces and explore the influence of the underlying surface topography. In-depth optimization of electrochemical anodization (EA) is performed by tuning various parameters, including substrate topography, voltage/current and time, onto microrough (micromachined) and extremely rough Zr substrates, which represent clinically relevant implant surfaces. Variations of EA factors yielded various nanotopographies, including nanotubes, nanograss and nanotemplates, offering different topographical and chemical combinations. EA optimization and precise current-voltage recording was performed to arrive at clinically translatable and reproducible nanostructures on Zr surfaces. This study will pave the way toward the fabrication of the next generation of nanoengineered Zr-based orthopedic and dental implants.

Plasma Electrolytic Oxidation Ceramic Coatings on Zirconium (Zr) and ZrAlloys: Part I—Growth Mechanisms, Microstructure, and Chemical Composition

Recently, a significant number of research projects have been directed towards designing and developing ceramic coatings for zirconium-based substrates due to their outstanding surface properties and utilization in modern technologies. The plasma electrolytic oxidation (PEO) coating is an environmentally friendly wet coating method that can be performed in a wide range of electrolytes. The surface characteristics of PEO coatings can be tailored by changing electrochemical parameters, electrolyte composition, and substrate alloying elements to adopt a conformal and adhesive PEO ceramic coating for the final demanding applications in chemical, electronics, and energy technologies. This review focuses on deriving a deeper fundamental understanding of the PEO growth mechanisms and the effect of process parameters on transient discharge behavior at breakdown, initiation, and growth of the oxide layer and incorporating species from the electrolyte. It highlights the fundamental microstructural properties associated with structural defects, phase transformation, and the role of additives.

Novel Porous Phosphorus⁻Calcium⁻Magnesium Coatings on Titanium with Copper or Zinc Obtained by DC Plasma Electrolytic Oxidation: Fabrication and Characterization

In this paper, the characteristics of new porous coatings fabricated at three voltages in electrolytes based on H₃PO₄ with calcium nitrate tetrahydrate, magnesium nitrate hexahydrate, and copper(II) nitrate trihydrate are presented. The SEM, energy dispersive spectroscopy (EDS), glow discharge optical emission spectroscopy (GDOES), X-ray photoelectron spectroscopy (XPS), and XRD techniques for coating identification were used. It was found that the higher the plasma electrolytic oxidation (PEO) (micro arc oxidation (MAO)) voltage, the thicker the porous coating with higher amounts of built-in elements coming from the electrolyte and more amorphous phase with signals from crystalline Ca(H₂PO₄)₂∙H₂O and/or Ti(HPO₄)₂∙H₂O. Additionally, the external parts of the obtained porous coatings formed on titanium consisted mainly of Ti4+, Ca2+, Mg2+ and PO₄3-, HPO₄2-, H₂PO₄-, P₂O₇4- as well as Zn2+ or copper Cu⁺/Cu2+. The surface should be characterized by high biocompatibility, due to the presence of structures based on calcium and phosphates, and have bactericidal properties, due to the presence of zinc and copper ions. Furthermore, the addition of magnesium ions should accelerate the healing of postoperative wounds, which could lead to faster patient recovery.

Effect of DC Plasma Electrolytic Oxidation on Surface Characteristics and Corrosion Resistance of Zirconium

Zr is a valve metal, the biocompatibility of which is at least on par with Ti. Recently, numerous attempts of the formation of bioactive coatings on Zr by plasma electrolytic oxidation (PEO) in solutions that were based on calcium acetate and calcium β-glycerophosphate were made. In this study, the direct current (DC) PEO of commercially pure zirconium in the solutions that contained Ca(H₂PO₂)₂, Ca(HCOO)₂, and Mg(CH₃COO)₂ was investigated. The treatment was conducted at 75 mA/cm² up to 200, 300, or 400 V. Five process stages were discerned. The treatment at higher voltages resulted in the formation of oxide layers that had Ca/P or (Mg+Ca)/P ratios that were close to that of hydroxyapatite (Ca/P = 1.67), determined by SEM/EDX. The corrosion resistance studies were performed using electrochemical impedance spectroscopy (EIS) and DC polarization methods. R(Q[R(QR)]) circuit model was used to fit the EIS data. In general, the coatings that were obtained at 200 V were the most corrosion resistant, however, they lacked the porous structure, which is typical for PEO coatings, and is sought after in the biomedical applications. The treatment at 400 V resulted in the formation of the coatings that were more corrosion resistant than those that were obtained at 300 V. This was determined mainly by the prevailing plasma regime at the given process voltage. The pitting resistance of Zr was also improved by the treatment, regardless of the applied process conditions.

Comparative investigations of structure and properties of micro-arc wollastonite-calcium phosphate coatings on titanium and zirconium-niobium alloy

Investigation results of micro-arc wollastonite–calcium phosphate (W–CaP) biocoatings on the pure titanium (Ti) and Zr–1wt.%Nb (Zr–1Nb) alloy were presented. The voltages of 150–300 V generate the micro-arc oxidation (MAO) process with the initial amplitude current of 150–550 A and 100–350 A for Ti and Zr–1Nb substrates, respectively. The identical dependencies of changes of the coating thickness, surface roughness and adhesion strength on the process voltage were revealed for the both substrates. The W–CaP coatings with the thickness of 10–11 μm were formed on Ti and Zr–1Nb under the low process voltage of 130–150 V. Elongated wollastonite particles with the size in the range of 40–100 μm were observed in such coatings. The structure of the coatings on Ti was presented by the X–ray amorphous and crystalline phases. The X–ray reflexes relating to the crystalline phases of Ti and wollastonite were observed only in XRD patterns of the coatings deposited under 130–200 V on Ti. While, the crystalline structure with phases of CaZr₄(PO₄)₆, β–ZrP₂O₇, ZrO₂, and Zr was detected in the coatings on Zr–1Nb. FT–IRS, XRD, SEM, and TEM data confirmed that the increase of the process voltage to 300 V leads to the dissociation of the wollastonite. No toxic effect of specimens on a viability, morphology and motility of human adipose–derived multipotent mesenchymal stem cells was revealed in vitro.

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Investigations of microarc wollastonite–calcium phosphate biocoatings on the titanium and Zr–1wt.%Nb alloy were presented.

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The thin coatings 10–11 μm with wollastonite particles formed under the low process voltage of 130–150 V.

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The increase of the process voltage to 300 V leads to the dissociation of the wollastonite.

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Coatings on both substrates have not toxic effect on the morphofunctional status of AMMSCs culture in vitro.

SEM, EDS and XPS Analysis of the Coatings Obtained on Titanium after Plasma Electrolytic Oxidation in Electrolytes Containing Copper Nitrate

In the paper, the Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Photoelectron Spectroscopy (XPS) results of the surface layer formed on pure titanium after plasma electrolytic oxidation (micro arc oxidation) at the voltage of 450 V are shown. As an electrolyte, the mixture of copper nitrate Cu(NO₃)₂ (10–600 g/L) in concentrated phosphoric acid H₃PO₄ (98 g/mol) was used. The thickness of the obtained porous surface layer equals about 10 μm, and it consists mainly of titanium phosphates and oxygen with embedded copper ions as a bactericidal agent. The maximum percent of copper in the PEO surface layer was equal to 12.2 ± 0.7 wt % (7.6 ± 0.5 at %), which is the best result that the authors obtained. The top surface layer of all obtained plasma electrolytic oxidation (PEO) coatings consisted most likely mainly of Ti₃(PO₄)₄∙nH₃PO₄ and Cu₃(PO₄)₂∙nH₃PO₄ with a small addition of CuP₂, CuO and Cu₂O.

Characteristics of multi-layer coating formed on commercially pure titanium for biomedical applications

An innovative multi-layer coating comprising a bioactive compound layer (consisting of hydroxyapatite and calcium titanate) with an underlying titanium oxide layer (in the form of anatase and rutile) has been developed on Grade 4 quality commercially pure titanium via a single step micro-arc oxidation process. Deposition of a multi-layer coating on titanium enhanced the bioactivity, while providing antibacterial characteristics as compared its untreated state. Furthermore, introduction of silver (4.6wt.%) into the multi-layer coating during micro-arc oxidation process imposed superior antibacterial efficiency without sacrificing the bioactivity.