Volume 110, Issue 4

The microstructural evolution of AISI 316L austenitic stainless steel subjected to interrupted uniaxial tensile testing is investigated in relation to the strain hardening behavior. The microstructural evolution is characterized by means of scanning electron microscopy equipped with electron backscattered diffraction, optical profilometry and atomic force microscopy. The experimental results show that the increase in level of deformation causes increased surface roughness, extrusion height, plastic strain value, fraction of low-angle grain boundaries and geometrically necessary dislocations. The tensile work hardening analysis reveals a three-stage behavior, where the deformation mechanism is dominated by localization of plastic strain in slip bands due to dislocation–dislocation interactions in stage II and by rotation of austenite grains and dynamic recovery which is due to dislocation annihilations in stage III.

Ni–Co/WC composite coating was fabricated on ZG45 steel substrate through vacuum sintering cladding technology. Its microstructure, composition, and three-point bending behavior were analyzed through scanning electron microscopy, electron probe microanalysis, X-ray diffraction, and mechanical testing. Results show that the distribution of WC particles presents a 3D net-like shape. A metallurgical fusion area is observed at the interface between the composite coating and the substrate. The entire coating consists of a composite coating area, a transition area, and a diffusion fusion area. The main phases of the coating are WC, Cr 7 C 3 , Cr 23 C 6 , Ni 3 Si, FeNi 3 , and Ni–Co solid solution. Three-point bending results reveal that the softness coefficient of the whole sample when the composite coating was in a compressive stress state was larger than that when the composite coating was in a tensile stress state. The bending strength of the sample is 520 MPa when the composite coating is in a compressive stress state. This value is 66 % higher than that when the composite coating is located at the bottom. The cracks simultaneously extend to the substrate and interface when the composite coating was in a compressive stress state and to the substrate when the composite coating was in a tensile stress state. The fracture of the composite area is brittle, and the substrate fracture is ductile.

This work examined the effects of a TiN coating on the high-cycle-fatigue (HCF) and very-high-cycle-fatigue (VHCF) properties of the Ti-6Al-4V alloy using a three-point bending ultrasonic fatigue testing machine. Three different TiN coatings with total thicknesses of 5, 15 and 25 μm were deposited on a Ti-6Al-4V alloy substrate. Scanning electron microscopy was used to examine the fracture surface. Fatigue results indicate that there is no traditional fatigue limit, and S–N curves show a continuously decreasing trend for uncoated and coated specimens. Fractography analysis shows a change from the surface to the subsurface initiations for the uncoated specimens and nucleation at the defects of the coatings for the coated specimens. A TiN coating can decrease the fatigue strength of the substrate, and the fatigue life decreases with increasing total thickness. Furthermore, delamination can be observed at the interface in the HCF regime.

The solid state phase equilibria in the Dy–Nb–Si ternary system at 1 273 K were experimentally determined by means of X-ray powder diffraction, differential scanning calorimetry and scanning electron microscopy equipped with energy dispersive X-ray analysis in the whole compositional range. The isothermal section of the Dy–Nb–Si ternary phase diagram contains 12 single-phase regions, 22 two-phase regions and 11 three-phase regions. At 1 273 K, the highest solubilities of Nb in β-DySi 2 , β-DySi 1.67 , Dy 3 Si 4 , β-DySi, Dy 5 Si 4 and Dy 5 Si 3 are extremely low (all less than 1 at.%). Whereas the maximum solubilities of Dy in NbSi 2 and Nb 5 Si 3 are approximately confirmed to be 1.12 at.% and 1.33 at.% Dy, respectively. Combining differential scanning calorimetry results with X-ray powder diffraction analysis, it is proved that the transformation temperatures of β-DySi 1.67 →α-DySi 1.67 and Dy 3 Si 4 →β-DySi 1.67 + β-DySi are 980 K and 1 480 K, respectively.

SnO/SnO 2 composites were synthesised through a simple solid-phase grinding-assisted ultrasonic reaction by using SnCl 2 and NH 4 HCO 3 as raw materials. Results showed that SnCl 2 reacted with NH 4 HCO 3 to form Sn 6 O 4 (OH) 4 and NH 4 Cl through solid-phase grinding. Sn 6 O 4 (OH) 4 was subsequently hydrolysed to SnO nanosheets. A small amount of SnO grains was transformed to SnO 2 . Different treatment processes played important roles in the composition, microstructural morphology and photocatalytic activity of SnO/SnO 2 composites. The direct ultrasonication of ground products yielded microspheres comprising numerous SnO nanosheets and SnO 2 nanoparticles with excellent visible photocatalytic properties, which can degrade 93.5% of methyl orange within 75 min.

Nowadays bioactive glasses represent one of the most successful bioceramics used for bone tissue restorations. In this work, three types of silica sands (White, Yellow and Gray Sands) and calcite from Cuban natural deposits were employed to synthesize glasses from the system SiO 2 –CaO–Na 2 O. The ions released from glasses were evaluated through in vitro tests in Tris-HCl and in simulated body fluids. All sands had purity around 99.2% of SiO 2 and contained traces (ppm) of Zr, Cr, Ba, Ce and Sr ions, while calcite raw material had traces of Sr, Cr, Zr, Ce and Zn. All glasses induced a pH change in Tris-HCl from 7.4 to 9 after 24 h; they had similar ion-release behavior in the in vitro solutions tested and showed a significant bioactive performance after 5 h. This work illustrates the potentialities of the use of natural resources to develop medical products when recognized trademark materials are not available.

Scaffolds were developed from mixtures of calcium aluminate (CA) and a bioactive glass (BG) and the effect of glass content on the mechanical strength and bioactivity of scaffolds was studied. Three different mixtures were tested: 95CA-5BG, 92.5CA-7.5BG and 90AC-10BG wt.%. Also, for comparison purposes, pure CA was used. In order to obtain the calcium aluminate cements (CAC), mixtures were hydrated using a water/mixture ratio by weight (w/c) of 0.4 and, with the resulting pastes, scaffolds were made by the lost-spheres method using PMMA beads. These materials showed interconnected pores (215 μm average diameter). Obtained scaffolds were loaded with antibiotic (gentamicin sulfate) and its release kinetics was studied. The evaluation of the in-vitro bioactivity was carried out by immersing scaffolds in a simulated body fluid (SBF) for 1, 7, 14 or 21 days at 36.5 °C. Compressive strength was evaluated before and after each immersion period. In all cases the formation of a Ca,P-rich compound on the surface of the scaffolds was detected after immersion in SBF. The amount of the bioactive compound formed as well as compressive strength increased as the amount of bioglass was increased. A controlled antibiotic release in SBF, with diffusion-controlled kinetics, was observed. In addition, scaffolds were not hemolytic. According to the results obtained, these materials are promising candidates for biomedical applications as drug delivery systems.

Titanium and its alloys are considered interesting materials for endosseous implants. However, they still present drawbacks related to their in-vivo behavior that can be overcome by coatings, such as apatite. This work focuses on the deposition of apatite coatings on commercially pure titanium (grade II) substrates previously pre-calcified. The influence of the temperature used in the thermal treatment on the microstructure and tribo-mechanical surface properties was analyzed. The coatings were structurally and chemically characterized and their tribo-mechanical behavior was evaluated. The nano-apatite coatings were only formed on surfaces with successive treatments in NaOH and CaCl 2 solutions. In addition, scratch tests showed that after the heat treatment the nano-apatite coatings had high bond strength to the substrate.

Calcium silicate–poly(n-butyl-2-cyanoacrylate) nanocomposite has been studied as a promising biomaterial to immobilize small bone fragments in order to achieve adequate fracture healing. The aim of this work was to investigate the in-vitro and in-vivo biocompatibility of the synthetic nanocomposite produced. The samples were characterized by means of infrared spectroscopy, thermogravimetric analysis, scanning and transmission electron microscopy. Bioactivity, degradability, cytotoxicity and implantation tests were performed. Experimental results showed the degradability of the nanocomposite, no cytotoxicity and a significant apatite nucleation and growth on the material surfaces after seven days of immersion in simulated body fluid. The new bone formation observed on the surface of the nanocomposite implanted in vivo is an indicator of its biocompatibility.

Calcium phosphate cements are known for their biocompatibility and bioactivity, which has led to a significant increase in their use. They have the advantage of being prepared as a malleable paste that sets in a few minutes and can be easily applied in bone defects. In this research work, 6 brushite cement formulations were prepared and the effect of collagen addition and the release of tetracycline were evaluated. All cements gave a malleable paste that set in 2 or 3 min, resulting in a material with pores of 5–20 μm. The cements showed compression resistance values between 0.8 and 1.7 MPa, and the addition of collagen was the most influential factor in the increase of this property. Up to 40 % of the drug was released after 7 days and the effectiveness of its antimicrobial activity was confirmed.

To improve the properties of compact Ti alloys for biomedical applications, porous Ti-16Nb-10Zr-(0–15)Ta (wt.%) alloys were produced by the space-holding method using an ammonium hydrogen carbonate space holder. The pore size distribution, porosity ratio and mechanical properties of the obtained porous alloys were investigated. Also, the effect of Ta addition on the microstructure and mechanical properties was investigated. It has been determined that the sintered porous Ti–Nb–Zr–Ta alloys have suitable mechanical properties (elastic modulus: 36.32–38 GPa, transverse rupture strength: 154–281 MPa) for hard tissue implants.

In this study, the effect of Zr content on the corrosion resistance of Ti-16Nb alloy produced by powder injection molding was investigated. Electrochemical corrosion tests were carried out in simulated body fluid using electrochemical impedance spectroscopy and polarization curve analysis. It is observed that, with the addition of 10 wt.% Zr to the base alloy, the corrosion current density and the corrosion rate decreased from 2.23 to 1.55 μA cm −2 and 961.7 × 10 −3 mpy to 703 × 10 −3 mpy, respectively. As a result, the addition of Zr has been effective in increasing the corrosion resistance of the Ti16Nb alloy.