A novel plasma-sprayed Ti4O7/carbon nanotubes/Al2O3 coating with bifunctional microwave application

High-performance microwave absorption coatings are critically required in the stealth defense system of military platforms. Regrettably, just optimizing the property but neglecting the application feasibility seriously inhibits its practical application in the field of microwave absorption. To face this challenge, the Ti₄O₇/carbon nanotubes (CNTs)/Al₂O₃ coatings were successfully fabricated by a plasma-sprayed method. For the different oxygen vacancy-induced Ti₄O₇ coatings, the enhanced ε' and ε'' values in the frequency of X-band is due to the synergistic manipulation of conductive path, defects and interfacial polarization. The optimal reflection loss of Ti₄O₇/CNTs/Al₂O₃ sample (0 wt% CNTs) is -55.7 dB (8.9 GHz of 2.41 mm), while the electromagnetic interference shielding effectiveness of Ti₄O₇/CNTs/Al₂O₃ sample (5 wt% CNTs) increases to 20.5 dB as the enhanced electrical conductivity. In special, the flexural strength of Ti₄O₇/CNTs/Al₂O₃ coatings first increases from 48.59 MPa (0 wt% CNTs) to 67.13 MPa (2.5 wt% CNTs) and then decreases to 38.31 MPa (5 wt% CNTs), demonstrating that an appropriate amount of CNTs evenly dispersed in the Ti₄O₇/Al₂O₃ ceramic matrix can effectively play the role of CNTs as the strengthening phase of the coatings. This research will provide a strategy by tailoring synergistic effect of dielectric loss and conduction loss for oxygen vacancy-mediated Ti₄O₇ material to broaden the application of absorbing or shielding ceramic coatings.

Plasma sprayed fluoride and zinc doped hydroxyapatite coated titanium for load-bearing implants

Titanium (Ti) alloys show excellent fatigue and corrosion resistance, high strength to weight ratio, and no toxicity; however, poor osseointegration ability of Ti may lead to implant loosening in vivo. Plasma spraying of hydroxyapatite [HA, Ca₁₀ (PO₄)₆ (OH)₂] coating on Ti surfaces is commercially used to enhance osseointegration and the long-term stability of these implants. The biological properties of HA can be improved with the addition of both cationic and anionic dopants, such as zinc ions (Zn²⁺) and fluoride (F^-). However, the hygroscopic nature of fluoride restricts its utilization in the radiofrequency (RF) plasma spray process. In addition, the amount of doping needs to be optimized to ensure cytocompatibility. We have fabricated zinc and fluoride doped HA-coated Ti6Al4V (Ti64) to mitigate these challenges using compositional and parametric optimizations. The RF induction plasma spraying method is utilized to prepare the coatings. Multiple parametric optimizations with amplitude and frequency during the processing result in coating thicknesses between 80 and 145 μm. No adverse effects on the adhesion properties of the coating are noticed because of doping. The antibacterial efficacy of each composition is tested against S. aureus for 24, 48, and 72 h, and showed that the addition of zinc oxide and calcium fluoride to HA leads to nearly 70 % higher antibacterial efficacy than pure HA-coated samples. The addition of osteogenic Zn²⁺and F^- leads to 1.5 times higher osteoblast viability for the doped samples than pure HA-coated samples after 7-days of cell culture. Zn²⁺ and F^- doped HA-coated Ti64 with simultaneous improvements in anti-bacterial efficacy and in vitro biocompatibility can find application in load-bearing implants, particularly in revision surgeries and immune-compromised patients.

Properties of silver contained coatings on CoCr alloys prepared by vacuum plasma spraying

The silver contained coatings on cast Cobalt Chrome (CoCr) alloys were prepared by vacuum plasma spraying technique. The Scanning Electron Microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray Diffraction (XRD), energy dispersive spectrometry (EDS), properties of corrosion resistance, wear resistance and effect of vitro antibacterial on the surface of silver contained coating were investigated. The cytotoxicity of the coatings was performed with L-929 fibroblasts by MTT assay. SEM showed that the surfaces of the coatings were dense, smooth, no obvious cracks except only a few pores. XRD analysis indicated that the contents of the surface were mainly Ag and Cr except a small amount of Ag₂O, Cr₂O₃. EDS analysis indicated that the distributions of Cr and Ag were uniform without any large-scale clustering. The wear resistance of silver coatings is similar to that of CoCr alloys, and the corrosion resistance is slightly better than that of CoCr alloys. The Ag coating had no significant effect on the proliferation of L929 cells. The antibacterial results indicated that the number of S. mutans and C. albicans were significantly reduced on the surface of silver contained coating than that of CoCr alloys. All the results indicated that the silver contained coatings can be achieved by vacuum plasma spraying technique with good surface characteristic and antibacterial properties and have promising applications in biomedical area.

Corrosion resistance and hydrophilic properties of plasma sprayed Ni+5%Al coatings

In this research, the AZ31 Mg alloy was coated with Ni5Al powder, using a plasma spray method. Effects of nuzzle distance and number of passes on corrosion behavior, hydrophilic properties and phase structure of the coated layer were studied. Samples in different distance of nozzle (150 and 300 mm) and different number of passes (2, 4 and 6) were examined. Corrosion behavior characterization was carried out using electrochemical impedance spectroscopy and potentiodynamic polarization methods. Hydrophilic properties of the coated layer were also investigated by the contact angle method. Results showed that by increasing the number of passes, the corrosion resistance and the contact angle were increased. On the other side, by increasing the nuzzle distance, the corrosion resistance and the contact angle were decreased. The Coated sample with 6 pass coating and nozzle distance of 150 mm had the best corrosion behavior and hydrophilic properties.

Experimental and numerical investigation of cavitation-induced erosion in thermal sprayed single splats

Hydraulic components are coated by thermal spraying to protect them against cavitation erosion. These coatings are built up by successive deposition of single splats. The behavior of a single splat under mechanical loading is still very vaguely understood. Yttria-stabilized zirconia (YSZ) and stainless-steel splats were obtained by plasma spraying onto stainless steel substrates. The velocity and temperature of particles upon impact were measured and the samples were subsequently exposed to cavitation erosion tests. An acoustic cavitation simulation estimated the water jet velocity and hammer stresses exerted by bubble collapse on the surface of the specimen. Although the results suggested that high stress levels resulted from cavitation loading, it was clear that weak adhesion interfaces played a crucial role in the accelerated cavitation-induced degradation.

Alumina-fluorapatite composite coating deposited by atmospheric plasma spraying: An agent of cohesion between bone and prostheses

In order to remedy the poor biological and tribological properties of 316L stainless steel (SS), plasma sprayed bio-ceramic coatings have been widely investigated. In the present study, a small amount of fluorapatite (Fap) was introduced into alumina in order to enhance its bioactivity. The powder feedstock was sprayed on 316L substrate by Atmospheric Plasma Spraying (APS) technology. The roughness profiles and average roughness values were determined using 3D profilometry. The cross sectional morphologies of the coatings were examined by scanning electron microscopy (SEM). Adhesive strength, micro-hardness and tribological properties were also examined. Experimental results revealed that Al₂O₃/Fap coating showed a good microhardness property revealing that the calcium aluminates were quite effective in improving the Fap mechanical behavior. The tribological characteristics of both alumina and alumina-Fap coating were also compared to those of classical hydroxyapatite (Hap) coatings as reported in the literature. The main finding of this work was that Fap coating can contribute to the cohesion between bone and prostheses and thus ensure a more durable and reliable prostheses.