Showing papers in "Surface & Coatings Technology in 2020"

Post-processing of the Inconel 718 alloy parts fabricated by selective laser melting: Effects of mechanical surface treatments on surface topography, porosity, hardness and residual stress

Abstract: The turbine blade test parts were manufactured by the selective laser melting (SLM) process using a nickel-based pre-alloyed Inconel (IN) 718 powder. Various mechanical post-processing techniques, such as barrel finishing (BF), shot peening (SP), ultrasonic shot peening (USP), and ultrasonic impact treatment (UIT), were applied to improve the surface layer properties of the SLM-built specimens. Effects of mechanical surface treatments on surface topography, porosity, hardness, and residual stress were studied. In comparison with the SLM-built state the surface roughness (Sa = 5.27 μm) of the post-processed specimens were respectively decreased by 20.6%, 26.2%, and 57.4% after the BF, USP, and UIT processes except for the SP-treated ones. The Sz parameter was reduced in all treated SLM-built specimens except for the SP-treated ones. The surface microhardness of the SLM-built specimen (~390 HV0.025) was increased after the BF (by 14.2%), USP (by 23.8%), UIT (by 50%), and SP (by 66.5%) processes. The deepest hardened layers were formed after the UIT and SP processes. Residual porosity of the SLM-built specimen was decreased by 23.1%, 40.6%, 55%, and 84% after the BF, SP, USP and UIT processes, respectively. The UIT process formed a densified subsurface layer of significantly reduced porosity (0.118%). All mechanical surface treatments successfully transformed the tensile residual stresses generated in SLM-built specimen into the compressive residual stresses (−201.4...510.7 MPa). The thickness of hardened, densified and compressed near-surface layers ranges from ~80 μm after BF to ~140 μm after USP, and ~180 μm after SP and UIT processes, which correlates to the accumulated energy and deformation extent of the treated surface. The effect of the accumulated energy on the outcomes of the applied surface treatments is also addressed.

Microstructure and wear behavior of FeCoNiCrMn high entropy alloy coating deposited by plasma spraying

Abstract: FeCoNiCrMn high entropy alloy coatings were deposited on steel substrate by atmospheric plasma spraying at different H2 flow rates and subsequently annealed. The microstructure and the phase composition of the prepared coatings were investigated, and the hardness of the coating was determined. The wear behavior of the as-sprayed and annealed FeCoNiCrMn coatings against WC-Co ball was evaluated under dry sliding condition. The results show that the fluffy structures characterized on the surfaces of as-sprayed coatings were formed due to the volatilization and oxidation of Mn element. The coatings were single FCC phase along with some oxides. The wear rate of the as-sprayed coatings was reduced by half as the H2 flow rate increasing from 3 to 6 L/min. Annealing can greatly improve the wear resistance and scratch resistance of the as-sprayed coatings. The characterization of wear tracks demonstrates that splat spalling was the dominating wear mechanism for the as-sprayed coatings. The improvement of wear resistance by increasing H2 flow rate and annealing was attributed to the enhancement of cohesive strength among splats.

Effect of shot peening coverage on hardness, residual stress and surface morphology of carburized rollers

Abstract: Shot peening is currently becoming a widely used surface strengthening technique that can refine the material grain, increase the hardness and introduce a certain depth of residual compressive stress layer. At the same times it changes the surface topography which may have a deleterious effect on the contact fatigue life. A fully understanding on the mechanism of shot peening for high-strength steels is to be explored. In this paper, the effect of shot peening coverage on residual stress, surface roughness, microhardness and microstructure of rollers are experimentally investigated. Experimental results show that the shot peening leads to a slight increase of surface and near-surface hardness from 690 HV to 740 HV, and an appreciably increasing of the subsurface maximum compressive residual stress. Moreover, some retained austenite in the the near-surface layer are transformed to martensite after shot peening, and wherein the grain refinement occurs.

Biocompatible, hydrophobic and resilience graphene/chitosan composite aerogel for efficient oil−water separation

Abstract: Three-dimensional aerogels with hydrophobic and lipophilic properties have attracted wide attention in the effective cleaning of oil-spills. However, cost-effectiveness, biodegradability, and recycling are still challenging in the application of aerogels for oil-water separation. In this paper, we have prepared high biocompatibility, low cost, and hydrophobic composite aerogels (WCA = 148°) through directional freezing-drying technology by using chitosan (CS) as the skeleton substrate, reduced graphene oxide nanosheets (rGO) as enhancements and hydrophobic silicon particles/polydimethylsioxane (H-SiO2/PDMS) as the hydrophobic modifier. The composite aerogel prepared has good adsorption capacity (18–45 g/g), good chemical and thermal stability in a harsh environment. More importantly, the adsorbed organic solvents and oils adsorbed can be easily extruded from aerogels due to its excellent compressive properties. In addition, adsorption rate for pump oil and silicone oil is accelerated with the increase of temperature. The adsorption kinetics of aerogel on silicone oil and pump oil can be well fitted by pseudo-first order kinetic equation.

Electrodeposited Ni-Co alloy-particle composite coatings: A comprehensive review

Abstract: Electrodeposited composite and nanocomposite layers have attracted considerable attention as engineering coatings, due to their favourable tribological properties and corrosion resistance. Ni Co alloy matrix composite coatings are particularly significant due to their extensive and developing industrial applications. Ceramic nanoparticles are commonly incorporated into the Ni Co alloy matrix to further improve their properties, opening new windows for industrial applications. This review aims to comprehensively assess the influence of operating parameters (including the type of current control, plating bath conditions, particle loading and size) on the deposit properties. Developments in this field are summarized. Fundamental and technological aspects requiring further R & D are identified.

Study of the stability of the structural properties of CeO2 microparticles to helium irradiation

Abstract: The work is devoted to the study of the radiation resistance of CeO2 microparticles to irradiation with low-energy Не2+ ions with an energy of 40 keV, the irradiation fluence was 1015–1016 ion/cm2. The study found that, in contrast to the initial spherical microparticles, irradiated microparticles are a mixture of sintered sphere-like agglomerates, with porous inclusions. According to energy dispersive analysis, an increase in the irradiation fluence leads to an increase in the oxygen content and its uneven distribution over the structure, which leads to a violation of the stoichiometry of the microparticles under study. It was found that the displacement of atoms from the lattice sites as a result of irradiation, as well as their subsequent migration, leads to a change in interplanar spacings and the appearance of distortions and deformations of the crystal lattice. According to X-ray diffraction data, changes in the crystal structure of the microparticles under study are associated with deformation and distortion of the crystal structure due to migration of defects and an increase in the vacancy density of oxygen defects, as well as partial swelling of the crystal structure.

Effect of WC content on microstructures and mechanical properties of FeCoCrNi high-entropy alloy/WC composite coatings by plasma cladding

Abstract: FeCoCrNi high-entropy alloy/WC composite coatings were fabricated via plasma cladding on steels adding different mass fraction of WC. Effects of WC content on the microstructure and mechanical properties of the coatings were studied. The results showed that WC content significantly affected microstructure and the wear resistance of the coatings. With the increase of WC content, the microstructures of the coatings became complex. When WC content was more than 60%, the coatings consisted of WC, FCC phase of HEA matrix as metal bond, Fe3W3C carbide phase and Cr-rich secondary solid solution phase. The Fe3W3C carbides improved the hardness and wear resistance of the coating. When WC content was at a high proportion of 60%, the HEA/WC coating had the best wear resistance with the minimum volume wear rate of 3.27 × 10−7 mm3/N·m and the high hardness of 59.6 HRC, which was better than commercial Ni60/WC coating with the same WC content.

Study of high temperature friction and wear performance of (CoCrFeMnNi)85Ti15 high-entropy alloy coating prepared by plasma cladding

Abstract: (CoCrFeMnNi)85Ti15 high-entropy alloy (HEA) coating prepared on Q235 steel substrate via plasma cladding was investigated for its microstructure, hardness, and high temperature tribological performance. The microstructure of (CoCrFeMnNi)85Ti15 HEA consists of FCC and BCC solid solutions as well as an intermetallic sigma phase. The FCC solid solution is characterized by a lattice parameter of 3.669 A and is enriched with Co-Cr, the BCC solid solution has a lattice parameter of 2.998 A and is enriched with Fe-Mn, and the intermetallic sigma phase is enriched with Ni-Ti. The micro-hardness of the Q235 steel substrate, the CoCrFeMnNi HEA coating, and the (CoCrFeMnNi)85Ti15 HEA coating is 123.6 ± 6.2, 150.1 ± 7.4, and 910.5 ± 26.6 HV, respectively. As the hardness of the (CoCrFeMnNi)85Ti15 HEA coating is enhanced six times more than that of the CoCrFeMnNi HEA coating, it exhibits a lower wear rate and a better wear resistance at high temperatures. The best wear resistance, which corresponds to a wear rate of 4.08 × 10−6 mm3·N−1·m−1, is obtained at 400 °C. The wear mechanisms of the (CoCrFeMnNi)85Ti15 HEA coating are predominantly oxidation wear and contact fatigue, although there is evidence of oxidation wear and adhesive wear at 800 °C.

Oxidation performance of Si-HfO2 environmental barrier coating bond coats deposited via plasma spray-physical vapor deposition

Abstract: Current state of the art (SOA) environmental barrier coating (EBCs) systems necessary for SiC/SiC ceramic matrix composites (CMCs) rely upon a metallic silicon bond coat. While this layer provides durability and adhesion, the upper use temperature of these systems is limited by the melting point of silicon (1414 °C). Turbine engine temperatures already exceed this melting point and in order to reduce or eliminate cooling for advanced components, new bond coat materials are required for EBC systems. One potential EBC bond coat that has been proposed is a composite layer of silicon and hafnium oxide (Si-HfO2). This coating concept intends to provide a well-bonded interface between silicon and the substrate, similar to the ‘pure’ metallic silicon bond coat, but with a higher temperature capability owed to the addition of HfO2. Two-layer systems of Si-HfO2 bond coat and a Yb2Si2O7 EBC top coat were deposited using Plasma Spray- Physical Vapor Deposition (PS-PVD), which is a hybrid coating method technique capable of vapor or liquid deposition. Coatings were deposited on bulk α-SiC and tested for oxidation performance in both laboratory air and in 90%/10% H2O/O2. A thermally grown oxide (TGO) of SiO2 was formed at the substrate/bond coat interface and the growth of this layer was measured and compared to literature values to determine TGO growth rates.

Corrosion and wear behavior of an electroless Ni-P/nano-SiC coating on AZ31 Mg alloy obtained through environmentally-friendly conversion coating

Abstract: In this paper, an environmentally-friendly pretreatment has been used for electroless Ni P on AZ31 magnesium alloy. Saturated aqueous NaHCO3 solution was used to form carbonate compounds on the surface of the substrate prior to Ni-P/nano-SiC electroless plating. Various amounts of nano-SiC particles were used to enhance the hardness, corrosion and wear resistance of the coatings. The coatings were characterized by using scanning electron microscopy (SEM) equipped with energy dispersion spectroscopy (EDS), and via X-ray diffraction (XRD). Polarization tests were carried out for investigating the corrosion resistance of the coatings. Pin on disk tests were used to study wear behavior of the coatings. The effect of heat treatment temperatures on the coatings structure, hardness and corrosion behavior were studied. However, the hardness of the substrate decreased due to the growth of Mg grains at high temperatures. Moreover, heat treated Ni P coatings at 300 °C exhibited better corrosion resistance compared to other heat treatment temperatures. As a result, the temperature of 300 °C was chosen as the optimum temperature. Results also indicated that 1 g L−1 nano-SiC in plating bath provided a uniform composite Ni P electroless coating with high hardness (795 HV) and corrosion resistance. Adding more nano-particles to the bath resulted in agglomeration of the particles and did not have positive effect on properties. The wear behavior of coatings were investigated using steel pin (AISI 52100) as the counterpart in pin-on-disk wear test. The optimum as-plated composite coating (Ni-P/1SiC) produced the best wear resistance at a rate of 4.2 × 10−5 mm3 N−1 m−1 wear rate and the highest corrosion resistance belonged to Ni-P/1SiC after annealing at 300 °C.

Coarse TiC particles reinforced H13 steel matrix composites produced by laser cladding

Abstract: Coarse TiC particles (more than 50 μm) reinforced H13 steel composites produced by laser cladding with various ceramic volume fraction were investigated. The effects of the ceramic volume fraction on the microstructure and hardness of the produced TiC/H13 composite were obtained. The results indicated that preheating conbined with slowing laser beam scanning speed were relatively useful for crack suppression and formation of composites with high TiC volume fraction. The microstructure of the TiC/H13 composite consisted of various TiC particles, including coarse TiC, initial fine TiC, primary TiC, lamellar-type eutectic TiC, austenite and martensite. By increasing the concentration of TiC in the composite, the volume fraction of residual austensute and the carbon content of martensite as well as the composite hardness had a significant increase. Nanoindentation hardness results showed that the hardness of the initial TiC particle was the highest, followed by the primary TiC, among the various shaped TiC within the deposit. The highest average hardness of the TiC/H13 composite was 1365 HV, which was more than twice as much as the H13 substrate hardness.

Effects of heat treatment combined with laser shock peening on wire and arc additive manufactured Ti17 titanium alloy: Microstructures, residual stress and mechanical properties

Abstract: Wire and arc additive manufactured (WAAM) metal parts usually contain large columnar grains and detrimental tensile residual stress, affecting their mechanical performance. In this work, a post-treatment method combining heat treatment (HT) and laser shock peening (LSP) was employed to alter the microstructure and mechanical properties of WAAM Ti17 titanium alloy. The results show severe plastic deformation was induced in the surface layer, which, in turn, led to a high-level surface compressive residual stress (~−763 MPa) by combination treatment of HT and LSP. Meanwhile, high-density dislocations and mechanical twins were observed in coarse α phases after treatment by laser shock wave, and gradually evolved into refined α phases. The elongation of samples was significantly improved by 15% while ensuring original ultimate tensile strength (UTS, 1153 ± 13 MPa) after HT and LSP treatment. This combined HT and LSP method helps enhance the mechanical performance of WAAM parts through changing their microstructure and residual stress distribution.

Investigation of the influence of the thickness of nanolayers in wear-resistant layers of Ti-TiN-(Ti,Cr,Al)N coating on destruction in the cutting and wear of carbide cutting tools

Abstract: The paper presents the results of the investigation into the formation of the nanolayer structure of the Ti-TiN-(Ti,Cr,Al)N coating and its influence on the thickness of coatings, their resistance to fracture in scratch testing, and the wear resistance of coated tools in turning 1045 steel. The structure of the coatings with the nanolayer thicknesses of 302, 160, 70, 53, 38, 24, 16, and 10 nm was studied using scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution (HR) TEM. It is shown that the grain sizes in the nanolayers decrease to certain values with an increase in the thickness of the nanolayers, and then, with a further decrease in the nanolayer thickness, the grain sizes of the nanolayer grow as the interlayer interfaces cease to produce a restraining effect on the growth of the grains. The study found that the nanolayer thickness influenced the wear of carbide cutting tools and the pattern of fracture for the Ti-TiN-(Ti,Cr,Al)N coatings.

High-temperature oxidation behavior of CuAlNiCrFe high-entropy alloy bond coats deposited using high-speed laser cladding process

Abstract: A promising new bond coat using CuAlNiCrFe high-entropy alloy was proposed in this paper The difference in pre-oxidation conditions between the traditional thermal-sprayed MCrAlY bond coat and CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding was investigated The result confirmed that the CuAlNiCrFe high-entropy alloy bond coat deposited using high-speed laser cladding can complete the pre-oxidation faster and form a continuous α-Al2O3 thermally grown oxide (TGO), which shortens the initial oxidation stage, thereby avoiding the formation of other oxides and spinel structures In the subsequent isothermal oxidation process, the block-like structure of the high-speed laser cladding layer and the sluggish diffusion effect of the high-entropy alloy work together to ensure the slow and continuous supply of aluminum element to TGO layer and obtain a low growth rate Also, the diffusion between the bonding layer and the substrate is controlled at a low level, and the new CuAlNiCrFe bond coat exhibits excellent oxidation and diffusion resistance With the consumption of aluminum, the phase structure of the high-entropy alloy bond coat changed from BCC to FCC, but it still maintained a stable simple solid solution structure

Effect of heat treatment on phase stability and wear behavior of laser clad AlCoCrFeNiTi0.8 high-entropy alloy coatings

Abstract: To explore the effect of phase stability on the properties of high entropy alloy, the AlCoCrFeNiTi0.8 coatings are prepared by laser cladding, and then the microstructure evolution and wear resistance of the coatings after heat treatment at various temperatures are studied comprehensively. The coating is composed of BCC-FeCr solid solution matrix, B2-AlNi precipitated phase and in-situ TiC particle phase. The microstructure is stable below 700 °C. The coarsening behavior of AlNi precipitates can be observed after heat treatment at 900 °C, and the precipitates present an Ostwald ripening phenomenon after heat treatment at 1200 °C. There is a close relationship between the wear resistance and the microstructure evolution of the coating, that is, the coarsening and ripening of the precipitates could lead to the decrease of wear resistance. The coating shows a significant decrease in wear resistance after heat treatment at 1200 °C. The volume wear rate of the as-deposited coating is 1.36 × 10−6 mm3·N−1·m−1, while that of the coatings after heat treatment at 700 °C, 900 °C, and 1200 °C is 1.64 × 10−6 mm3·N−1·m−1, 1.96 × 10−6 mm3·N−1·m−1, and 6.96 × 10−6 mm3·N−1·m−1, respectively.

Facile surface coating of metal-tannin complex onto PVDF membrane with underwater Superoleophobicity for oil-water emulsion separation

Abstract: In this study, inspired by self-cleaning property of fish scale in water, a novel superhydrophilic and underwater superoleophobic tannin-metal complex@polyvinylidene fluoride (TA-Fe@PVDF) nanocomposite membrane was prepared by a facile and one-pot surface coating technology at room temperature using the ferric nitrate and tannic acid as the building blocks. The as-fabricated TA-Fe@PVDF nanocomposite membrane was characterized by scanning electron microscopy (SEM), attenuated total reflection fourier transform infrared spectroscopy (ATR-IR), atomic force microscope (AFM) and contact angle measuring instrument. It was found that the TA-Fe complexes improved the surface wetting, separation efficiency and repeatability. The optimal TA-Fe@PVDF-1 composite membrane was obtained at the concentration ratio of TA and Fe3+ of 1:3, which show underwater oil contact angle of 155°, and low oil adhesion. The separation efficiencies of TA-Fe@PVDF-1 composite membrane for various oil-water mixtures and oil-in-water emulsions are higher than 99.5%. The membrane flux can reach 2551 L m−2 h−1 bar−1 for surfactant-stabilized petroleum ether-in-water emulsion separation, and 4505 L m−2 h−1 bar−1 for petroleum ether/water mixture. This nanocomposite membrane exhibits excellent repeatability and good chemical stability (salt/acid/alkali resistance), to be potentially applied for the treatment of oily wastewater.

Deposition of HA-TiO2 by plasma spray on β-phase Ti-35Nb-7Ta-5Zr alloy for hip stem: Characterization, mechanical properties, corrosion, and in-vitro bioactivity

Abstract: This research work presents the deposition of titanium oxide (TiO2) reinforced hydroxyapatite (HA) on Ti-35Nb-7Ta-5Zr alloy by plasma spray deposition technique for Hip-stem application. The effect of TiO2 reinforcement (15 and 30 wt%) in HA has been studied on coatings morphology, microstructure, mechanical properties, corrosion resistance, and in-vitro bioactivity was investigated. The microstructure, elemental composition, and phase composition were characterized by FE-SEM, EDS, and XRD. The morphology analysis showed that of HA-coating contains micro-cracks and splats like structure with large globules. The reinforcement of TiO2 in HA improved the microstructure, prevents the formation of micro-cracks, and formed a dense structure. The cross-section morphology shows that HA-15%TiO2 (HA-Tx) and HA-30%TiO2 (HA-Ty) coatings (thickness 185–200 μm) were mechanically bonded with the substrate as compared HA-coatings, that helps in enhancing implant stability. Results also show that microhardness and adhesion strength of HA/TiO2 coatings improved with the reinforcement of TiO2 and HA-30%TiO2 possessed high micro-hardness and adhesion strength (1.35 GPa and 32.5 MPa) in comparison with HA-coating (0.63 GPa and 18.5 MPa). The electron dispersive spectroscopy (EDS) analysis and X-ray diffraction analysis (XRD) results show that HA-TiO2 coatings contain HA and TiO2 phases that prevent the formation of amorphous contents. In contrast, HA-coating contains amorphous content (CaO, β-TCP, and TTCP). The HA-TiO2 coatings exhibit excellent corrosion resistance as compared to HA-coating because of the presence of TiO2 lamellar structure in the HA matrix that acted as a barrier to corrosion and improves the corrosion resistance. The TiO2 reinforcement in HA/TiO2 coating not only increase the corrosion resistance but also help in promoting chemical integration of MG-63 osteoblastic cellular structure through the formation of apatite on the surface of the implant. Furthermore, the in-vitro bioactivity analysis results showed that Mg-63 osteoblastic cells have excellent adhesion and growth on the HA-30%TiO2 coated substrates.

Improvement of high entropy alloy nitride coatings (AlCrNbSiTiMo)N on mechanical and high temperature tribological properties by tuning substrate bias

Abstract: High entropy alloy nitride coatings (HEAN), showing superior mechanical strength, high oxidation resistance, and thermal stability, have often been used in protective hard coatings field. However, the high temperature tribological field of novel HEA nitride films has not yet been well studied. The correlations among tribological properties, compressive residual stress, mechanical properties, and self-lubricating oxide are still limited. In this research, the novel (AlCrNbSiTiMo)N coatings were fabricated on both Inconel-718 and Si (100) substrate by radio frequency (RF) magnetron sputtering via tuning both substrate bias at 300 °C deposition temperature. The (AlCrNbSiTiMo)N coatings with a specific substrate bias exhibit an outstanding hardness of 34.5±0.8 GPa. In the wear test at 700 °C, the films deposited at −100 V revealed the lowest wear rate around 1.2 × 10−6 mm3N−1 m−1. With Molybdenum doping, the MoO3 Magneli phase was observed on the surface at elevated temperature wearing process, and the friction coefficient at high temperature decreased significantly due to a lubricating surface. The coating exhibited the average friction coefficient value of 0.48 in the wear test of 700 °C. The high temperature tribological performance was addressed and related to the mechanical properties, the plastic deformation resistance H3/E2, and the excess residual stress of the films. This study provides a new design for hard coatings applied to severe wearing conditions. By tuning substrate bias, the (AlCrNbSiTiMo)N coatings exhibit outstanding mechanical and tribological characteristics, which will be a promising candidate for high temperature tribological protective film.

Versatile bioactive and antibacterial coating system based on silica, gentamicin, and chitosan: Improving early stage performance of titanium implants

Abstract: The aim of this work is to develop and characterize a multifunctional and dual surface coating system for titanium orthopedic implants by applying two different cost-effective, scalable, and non-complex coating technologies (spray and electrophoretic deposition). The first deposit is formed by a sprayed hybrid sol-gel layer combined with bioactive glass particles (45S5, BG), and the outer part of the dual coating consists of a chitosan-gelatin/silica (Si) - antibiotic (gentamicin, Ge) composite layer applied by electrophoretic deposition. The application of sol-gel enclosed BG drops onto the surface was done to enhance the bioactivity of the double-layered surface coating system. After the BG is dissolved, thus generating a calcium‑silicon rich medium, the re-deposition of hydroxyl‑carbonate apatite occurs. Regarding the antibacterial inhibition properties, antibacterial activity to both strains used (S. aureus and E. coli) was obtained for the chitosan/gelatin/Si Ge nanoparticle coatings on titanium substrates, showing a large inhibition area around the samples. Both the bare Ti samples and the coatings with chitosan/gelatin matrix did not successfully inhibit bacterial growth. As expected, the presence of silica-based glasses and coatings based on amorphous silica enhanced cell viability. The deposition of BG was done with the aim of extending the bioactive effect of the system, considering the presence of a porous degradable organic layer deposited on top, which was shown to be partially degraded after 7 days. The sol gel sprayed BG layer combined with chitosan/gelatin biopolymers filled with Si Ge nanoparticles presents a suitable technology to generate bioactive and antibacterial surfaces to enhance Ti implant performance.

Ammonium-grafted graphene oxide for enhanced corrosion resistance of waterborne epoxy coatings

Abstract: This study reports an eco-friendly waterborne epoxy (WBE) coating with excellent corrosion resistance by adding ammonium grafted graphene oxide (AGO) as co-dispersant. AGO, as a co-dispersant, addresses the problem of a lot of micro-defects caused by the intrinsic mismatch between epoxy resin and amine curing agents during the formation of aqueous epoxy resin. Also, the compatibility of GO in the epoxy resin matrix is also improved by the ammonium modification. The ammonium modification process was systematically studied using Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) spectra, UV–vis spectra, Raman spectra and X-ray photoelectron spectra (XPS). Scanning electron microscope (SEM) images show that AGO sheets has successfully inhibited the formation of micro-pores and micro-defects, forming a more compact WBE/AGO composite coating. As a result, the corrosion resistance of the coating has been greatly improved as investigated by the potentiodynamic polarization test. The corrosion rate of WBE/AGO is decreased by three orders of magnitude compared to pure WBE coating. The corrosion protection mechanism of the WBE/AGO coating is studied via Electrochemical impedance spectroscopy (EIS) test, and equivalent electrical circuits are applied to fitting the EIS results and reveal the effect of AGO in the composite coating.

Tribomechanical properties of hard Cr-doped DLC coatings deposited by low-frequency HiPIMS

Abstract: Cr-doped diamond-like carbon (Cr-DLC) films with Cr contents ranging from 3 up to 20 at. % were synthesised in a codeposition process with HiPIMS (Cr deposition) and DC-pulsed technology (C deposition). The application of HiPIMS at low frequencies was observed to significantly enhance the energy density during the Cr plasma discharge due to the interaction of Cr–C species. The higher energy bombardment at low HiPIMS frequencies allowed doping with Cr the DLC structure avoiding the graphitization of the carbon structure. EELS spectroscopy was used to evaluate sp3 content and Raman was used for sp2 structural characterization of the films. Enhanced mechanical properties (hardness up to 30 GPa) were observed with nanoindentation for Cr-doped DLC at low frequencies. High temperature nanoindentation tests were also performed from room temperature to 425 °C in order to evaluate the evolution of hardness and Young Modulus with temperature. The results showed that the mechanical properties at high temperature mainly depend on the initial sp3-sp2 structure. Tribological tests were carried out in air from room temperature to 250 °C. Cr-doped DLC coatings deposited by low-frequency HiPIMS showed lower friction and wear compared to undoped DLC.

Antibacterial coating of Ti-6Al-4V surfaces using silver nano-powder mixed electrical discharge machining

Abstract: Previous studies have revealed the potential of powder mixed electrical discharge machining (PMEDM) with regards to concurrently machining part geometry and coating an antibacterial layer on medical devices. This study is aimed at further demonstrating this potential. In order to do so, the PMEDM process was varied by adding different concentrations of silver nano-particles into the dielectric fluid and used to machine Ti-6Al-4V. Afterwards, the resulting machined and coated surfaces were characterized with regards to surface integrity, the coating layer's thickness, microhardness and chemical elements as well as antibacterial property. Material removal rate, tool wear and pulse signals were also analysed in order to give an insight on process feasibility. From both qualitative and quantitative results, it could be established that the surfaces machined and coated by PMEDM method have demonstrated a significant reduction of not only the amount of S. aureus bacteria, but also the number of bacterial clusters on the coating layer's surface. Moreover, the coating layer's silver content, which depends on the powder concentration suspended in the dielectric fluid, plays a vital role in the antibacterial property. As compared to surfaces without silver, surfaces containing approximately 3.78% silver content showed a significant decrease in both bacterial numbers and clusters, whereas a further increase in silver content did not result in a considerable bacterial number and cluster reduction. Regarding the machining performance, as compared to EDM without powder, machining time is remarkably decreased by using the PMEDM method.

Mechanical properties and thermal stability of reactively sputtered multi-principal-metal Hf-Ta-Ti-V-Zr nitrides

Abstract: Crystalline (Hf,Ta,Ti,V,Zr)N nitride thin films, with a high-entropy metal-sublattice, were synthesized at 440 °C by reactive magnetron sputtering using an equimolar Hf-Ta-Ti-V-Zr-compound target. The coatings are single-phase fcc structured mono-nitrides for N2/(Ar + N2) flow-rate-ratios (fN2) between 30 and 45%. For higher fN2 a small fraction of a second phase (next to the fcc matrix) can be detected by X-ray diffraction (XRD) and selected area electron diffraction (SAED). All coatings studied (prepared with fN2 between 30 and 60%) show similar chemical compositions and hardness (H) values between 30.0 and 34.0 GPa with indentation moduli of ~460 GPa. Atom probe tomography (APT) indicates a homogenous distribution of all elements within our fcc-(Hf,Ta,Ti,V,Zr)N even after vacuum-annealing at 1300 °C. While H decreased from 32.5 to 28.1 GPa by this annealing treatment, the coating is still single-phase fcc structured with a defect density (expressed by XRD and SAED features, transmission electron microscopy contrast, and grain sizes) comparable to the as-deposited state. Only after vacuum-annealing at 1500 °C, XRD and APT reveal the formation of hexagonal structured (Ta,V)2N. The onset of nitrogen-loss – detected by thermogravimetric analysis – is ~1350 °C. Based on our results we can conclude that the sluggish diffusion within our fcc-(Hf,Ta,Ti,V,Zr)N warrants the single-phase fcc structure up to 1300 °C, although ab initio based calculations would suggest the lower-entropy products [fcc-(Hf,Zr)N, fcc-(Ta,V)N, and fcc-TiN] and [fcc-(Hf,Zr)N and fcc-(Ta,Ti,V)N] to be energetically more stable up to 1302 K.

Enhanced corrosion resistance and biocompatibility of biodegradable magnesium alloy modified by calcium phosphate/collagen coating

Abstract: Magnesium (Mg) and its alloys have been regarded as one of the most promising biodegradable implant materials, whereas the rapid degradation rate and potential cytotoxicity hinder their clinic applications. In order to improve the biocompatibility and biocorrosion resistance of Mg alloy, a composite coating which composed of calcium phosphate (CaP) and collagen (Col) was successfully fabricated on the surface by chemical conversion and dip-coating methods. The chemical compositions, surface morphologies and corrosion resistance of the CaP/Col coating were investigated using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), electrochemical and immersion test. Results revealed that the collagen coating efficiently sealed the cracks and pores of CaP coating, and significantly enhanced the corrosion resistance. Furthermore, the composition of the CaP/Col coating was similar to that of bone, which can more effectively promote the adhesion, proliferation and differentiation of osteoblasts, showing excellent biocompatibility and biosafety. These observations indicated that the CaP/Col coating could protect Mg alloy from fast degradation, greatly enhance biocompatibility and osteoinductivity, and thus could be promising for orthopedic implant applications.

Effect of local treatment temperature of ultrasonic nanocrystalline surface modification on tribological behavior and corrosion resistance of stainless steel 316L produced by selective laser melting

Abstract: In this study, the effects of local treatment temperature of ultrasonic nanocrystal surface modification (UNSM) on tribological behavior and corrosion resistance of stainless steel 316L produced by selective laser melting (SLM) were investigated The as-printed samples were treated by UNSM at room and high temperatures (RT and HTs) and their strength, tribological behavior and corrosion resistance were compared and discussed Dry friction coefficient of the UNSM-treated at RT and HT samples was reduced from 073 to 056 and 067 to 052, while the dry sliding wear resistance at RT and HT was enhanced from 725 × 10−8 to 58 × 10−8 and 701 × 10−8 to 52 × 10−8 mm3/Nmm, respectively Increase in local treatment temperature of UNSM resulted a lower friction coefficient and higher wear rate compared to that of the UNSM at RT

Investigation on the corrosion behavior and biocompatibility of Ti-6Al-4V implant coated with HA/TiN dual layer for medical applications

Abstract: Ti-6Al-4V alloy has wide applications in the medical industry due to its unique mechanical properties and biocompatibility. However, in the long-term use of these alloys, the release of aluminum and vanadium can result in serious illnesses. For solving these problems, the implant surface modification can be done to improve the corrosion and biocompatibility properties. In this study, TiN coating was applied by Plasma-Assisted Chemical Vapour Deposition (PACVD) method along with hydroxyapatite coating (HA) by sol-gel method on Ti-6Al-4V substrate surface. Afterwards, X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FE-SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Atomic Force Microscopy (AFM) and potentiodynamic polarization and electrochemical impedance tests were used for evaluating the phase, morphology, chemical composition, surface roughness, and corrosion behavior in the simulated body fluid (SBF), respectively. Results showed that the surface roughness of HA/TiN, HA, TiN and substrate specimens were the highest to minimum values with roughness of 30.86±0.5, 25.1±0.42, 20.43±0.14 and 15.1±0.02 nm, respectively, and the HA-TiN composite coating had the lowest corrosion current density, highest polarization resistance and corrosion potential. Results of cell viability and proliferation demonstrated that the HA/TiN nanocomposite coating is a good choice for dental and orthopedic implants due to its corrosion resistance and biocompatibility.

Corrosion properties of laser cladded CrCoNi medium entropy alloy coating

Abstract: CrCoNi medium entropy alloy is an intriguing new material showing excellent mechanical properties but little is known about its corrosion properties. In this study, CrCoNi coating is fabricated by laser cladding, and its corrosion resistance and passive film properties in 3.5 wt% NaCl solution and 0.5 M H2SO4 solution have been investigated and compared with SS304. Optical microscope (OM) and Scanning electron microscope (SEM) shows that CrCoNi coating is near-fully dense without evident defects. In 3.5 wt% NaCl solution, CrCoNi coating is more easily corroded than SS304 while its passive film stability is better. The passive film formed on CrCoNi coating is a n-type semiconductor which consists of a single Cr oxide layer and the X-ray photoelectron spectroscopy (XPS) results reveal the presence of Cl− ion. In 0.5 M H2SO4 solution, CrCoNi coating exhibits better corrosion resistance than SS304. Electrochemical impedance spectroscopy (EIS) measurements reveal a slight pitting behavior during immersion and XPS results reveal that the Cr oxide and its hydroxides are the primary constituents of the passive film. Energy dispersive spectrometer (EDS) mapping shows Cr plays an important role in preventing pitting corrosion in both test solutions.

Pure copper components fabricated by cold spray (CS) and selective laser melting (SLM) technology

Abstract: To study effect of the different metal additive manufacturing techniques on microstructural evolution, phase constitution and the relationship between the microstructure and tensile behavior of pure Cu parts, components were manufactured by selective laser melting (SLM) technology and cold spraying (CS) technology, respectively. The microstructure of Cu parts was detected using an optical microscope (OM) and scanning electron microscopy (SEM). The XRD spectrum revealed that only Cu phase is formed in the SLM Cu and CS Cu samples. The microstructure of the SLM Cu samples is constituted by the polycrystalline grains with substructures including the columnar dendrites and the equiaxed structures. As for the CS Cu samples, only the equiaxed grains were detected. In terms of the main physical properties, the averaged electrical conductivity of SLM Cu sample is 41% IACS, while that of CS Cu sample is 73% IACS. For the microhardness, the mean microhardness value of the CS Cu and the SLM Cu samples is 144.2 ± 4.3 HV0.05 and 83.6 ± 5.2 HV0.05, respectively. With regard to the statistic mechanical properties, the yield strength (YS) and ultimate tensile strength (UTS) of the SLM Cu part is approximately 185.8 ± 6.1 MPa and 242.2 ± 8.2 MPa, respectively.

Robust nickel-reduced graphene oxide-myristic acid superhydrophobic coating on carbon steel using electrochemical codeposition and its corrosion resistance

Abstract: We report a facile approach to fabricate superhydrophobic (SHP) coating of nickel (Ni)–reduced graphene oxide (rGO)-myristic acid on carbon steel (CS) surface with superior corrosion resistance and self-cleaning ability, using electro-codeposition. The electro-deposition current density and electrolyte bath temperature are optimized to obtain the desired hierarchical surface textures with very high water contact angle (WCA). The microstructure, chemical compositions, surface roughness, and self-cleaning properties were examined using FESEM, XRD, XPS, EDX, AFM, water bouncing etc. A WCA of 174 ± 1.5°, with a sliding angle (SA) of ~1°, was obtained at 60 mA·cm−2 and 45 °C. The FESEM surface morphology showed a hierarchical pinecone structured Ni–rGO composite film. The good adhesion and mechanical stability of SHP coating is achieved by phosphating the surface prior to deposition. The SHP surface showed a two order reduction in the corrosion current density and ~3 orders higher impedance, as compared to uncoated surface. This study demonstrates a new approach for fabricating mechanically and chemically durable SHP coating on CS specimens.

Surface modification of titanium alloy using hBN powder mixed dielectric through micro-electric discharge machining

Abstract: In the present study, hard, wear, and corrosion resistance coating is deposited on titanium alloy with hexagonal boron nitride powder (avg. size 70 nm) suspended in deionized water by micro-electro discharge coating process. The influence of input process parameters on surface integrity, wear properties, and corrosion resistance is studied. XRD analysis of coating surface shows the presence of phases such as BN, Al2O3, TiN, TiAlN, TiO, and CuO. Micro-hardness of the coating surface increases by five times as compared to titanium alloy (parent material). At the parameter settings of 60 V, 0.7 duty factor, and 12 g/l, the maximum deposition rate of 5.65 ± 0.1 × 10−4 g/min and recast layer thickness of 13.1 μm is obtained. Pin on disk wear test results reveal the reduction of wear rate of the coated surface to one-fourth of that of the base material. The average coefficient of friction (COF) for the deposited BN surfaces reduces to 0.26 from 0.4 (substrate material). Corrosion resistance of the coated surface is found to be 1.24 μm/year in flowing water conditions and 1.07 μm/year in stagnant water whereas for the parent material it is 5.92 μm/year in flowing water condition and 4.89 μm/year for stagnant water condition.