M. Mohedano

Metal release from ceramic coatings for dental implants.

OBJECTIVES Two types of ceramic coatings on commercially pure titanium for dental implant applications with different Ca/P ratios in the range from 1.5 to 4.0, and two different thicknesses (∼5 and ∼15μm) were examined with the aim of underpinning the effect of coating composition, thickness and microstructure on the corrosion behavior and hydroxyapatite forming ability in SBF. METHODS Bioactive coatings were formed on Ti by plasma electrolytic oxidation (PEO). The composition, structure, and morphology of the materials were characterized before and after the immersion in simulated body fluid solution (SBF) at 37°C for up to 4 weeks. All the materials were screened with respect to metal ion release into SBF. RESULTS Only thick PEO coating with overstoichiometric Ca/P ratio of 4.0 exhibited capacity to induce the precipitation of hydroxyapatite over the short period of 1 week. Long term Ti(4+) ion release from all PEO-coated materials was 2-3 times lower than from the uncoated Ti. Metal ion release is attributed mostly to chemical dissolution of the coating at initial stages of immersion. SIGNIFICANCE The long term stability was greater for thin PEO coating with overstoichiometric Ca/P ratio of 2.0, which exhibited ∼95ngcm(-2) of Ti(4+) ions release over 4 weeks. Thin PEO coatings present economically more viable option.

Bioactive plasma electrolytic oxidation coatings—The role of the composition, microstructure, and electrochemical stability

A Plasma electrolytic oxidation (PEO) process was used to produce bioactive coatings on Ti. PEO coatings with Ca/P atomic ratio of 1.7 and 4.0 were fabricated and characterized with respect to their morphology, composition, and microstructure. AC and DC electrochemical tests were used to evaluate the effect of (i) organic additives (amino acids, proteins, vitamins, and antibiotics) in alpha-minimum essential medium (α-MEM) on electrochemical stability of noncoated and PEO-coated Ti and (ii) coating composition, microstructure, and corrosion behavior on the cell response in α-MEM. PEO-coated Ti showed higher corrosion resistance than the noncoated Ti in MEM with and without organic additives by an order of magnitude. The corrosion resistance in α-MEM decreased with time for nonmodified Ti and increased for PEO-coated Ti; the latter was because of the adsorption of the proteins in the coating pores which increased the diffusion resistance. The presence of Ca and P in titanium oxide coating at the Ca/P ratio exceeding that of any stoichiometric Ca-P-O and Ca-P-O-H compounds facilitates faster osteoblast cell adhesion.

Influence of Gd on the Corrosion Behavior of AM50 and AZ91D Magnesium Alloys

The corrosion behavior of AM50 and AZ91D Mg alloys containing 0.2 wt% to 1.0 wt% Gd was evaluated in 3.5 wt% sodium chloride (NaCl) solution using electrochemical, gravimetric, and hydrogen evoluti...

Salt spray corrosion behaviour of new Mg–Al alloys containing Nd or Gd

Abstract The influence of chloride concentration on the salt spray corrosion behaviour of new AZ91D and AM50 alloys containing rare earth (RE) elements was evaluated. The corrosion rate of both materials increased with increasing chloride concentration, particularly for NaCl concentrations above 2 wt-%. The addition of Nd or Gd reduced the amount of β-Mg17Al12 phase and resulted in the formation of RE containing intermetallics that were less noble than the Al–Mn inclusions but more noble than the β-Mg17Al12 phase. The latter modifications decreased the corrosion rate of the AM50 alloy by up to 90% but did not give a clear benefit to the corrosion resistance of the AZ91D alloy.

Recent advances in energy efficient PEO processing of aluminium alloys

Abstract Plasma electrolytic oxidation (PEO) coatings developed under voltage-controlled mode on various commercial wrought, gravity cast and rheocast aluminium alloys were discussed with respect to enhancement of their tribological and corrosion performance and minimization of the PEO energy consumption. It is demonstrated that use of conventional porous anodic film precursors reduces the PEO energy consumption by up to 50%. The wear of 6082 alloy with PEO coatings with added α-Al2O3 particles is two times lower compared with electrolytic hard chrome. The long-term corrosion resistance of the PEO-coated A356 rheocast alloy is enhanced via use of a precursor and hydrophobic post-treatment.

Hierarchically organized Li–Al-LDH nano-flakes: a low-temperature approach to seal porous anodic oxide on aluminum alloys

This work suggests a low-temperature sealing approach for tartaric–sulfuric acid (TSA) anodized AA2024 based on hierarchically organized Li–Al-layered double hydroxide (LDH) structures. The new proposed sealing is expected to be directly competitive to the standard hot water sealing (HWS) approaches because of its reduced treatment temperature and high protection efficiency. A hierarchical organization of in situ formed LDH nano-flakes across the depth length of the TSA pores, from the macrodown to the nano-size range, was observed with transmission electron microscopy (TEM). Electrochemical impedance spectroscopy (EIS) studies showed that the densely packed LDH arrangement at the porous oxide layer is directly related to the drastically improved barrier properties of TSA. Moreover, LDH flake-like structures worked as “smart” reservoirs for corrosion inhibiting vanadium species (VOx) that are released on demand upon the onset of corrosion. This was confirmed using a scanning vibrating electrode technique (SVET), giving relevant insights into the time-resolved release activity of VOx and the formation of the passivation layer on cathodic intermetallics, corroborated with EDX and analytical Raman spectroscopy. Passive and active corrosion protection was imparted to the anodic layer via new Li–Al-LDH structures with long-term protection exceeding that of standard HWS procedures.

Electrochemical corrosion behaviour of Mg-Al alloys with thermal spray Al/SiCp composite coatings

The corrosion protection of Mg–Al alloys by flame thermal spraying of Al/SiC particles (SiCp) composite coatings was evaluated by electrochemical impedance spectroscopy in 3.5 wt.% NaCl solution. The volume fraction of SiCp varied between 5 and 30%. The as-sprayed Al/SiCp composite coatings revealed a high number of microchannels, largely in the vicinity of the SiCp, that facilitated the penetration of the electrolyte and the subsequent galvanic corrosion of the magnesium substrates. The application of a cold-pressing post-treatment reduced the degree of porosity of the coatings and improved the bonding at the coating/substrate and Al/SiC interfaces. This resulted in improved corrosion resistance of the coated specimens. The effectiveness of the coatings slightly decreased with the addition of 5–30 vol.% SiCp compared with the unreinforced thermal spray aluminium coatings.

Powder Metallurgical Synthesis of Biodegradable Mg‐Hydroxyapatite Composites for Biomedical Applications

Biodegradable Mg alloys are a new class of temporary implant materials for musculo-skeletal surgery. Recent studies show that Mg-based alloys can be biocompatible and there is a high demand to design Mg alloys with adjustable corrosion rates and suitable mechanical properties. An approach to solving this challenge might be the use of Mg metal matrix composites (Mg-MMC). In this study, a Mg-MMC composed of ZK60 was investigated as the base material and hydroxyapatite (HA) particles were added for tailoring its properties. The composite was produced by high-energy ball milling followed by hot extrusion. This processing route was chosen, as HA in contact with molten Mg releases a toxic gas (phosphine – PH3). The HA particles were homogeneously distributed in the ZK60 matrix after ball milling and the composite was consolidated by hot extrusion. This work presents the influence of different amounts of HA on corrosion behavior and mechanical properties of the composite. Corrosion properties were evaluated by immersion and electrochemical measurements in physiological media at 37 °C. A slight improvement in the corrosion resistance was observed for Mg-MMC due to the presence of more stable corrosion products. Compression tests were used to measure the mechanical properties. Under compression, samples showed a slight increase in the compressive yield strength with the addition of HA, while the ultimate strength did not change significantly.