Showing papers in "Surface & Coatings Technology in 2018"

Enhanced corrosion and wear resistance properties of carbon fiber reinforced Ni-based composite coating by laser cladding

Abstract: To enhance the wear resistance and corrosion resistance of Ni-based coatings, carbon fibers reinforced nickel-based composite coatings (CFs/Ni) were fabricated on the surface of 1Cr13 stainless steel by laser cladding (LC). The microstructure characteristics, microhardness, wear and corrosion performances of the composite coatings were investigated. The results show that CFs can effectively improve the corrosion and wear resistances of Ni-based coatings. With increasing laser scanning speed, the morphology of CFs in composite coatings is more integral and the corrosion and wear resistances of the composite coatings are improved. Especially, when laser scanning speed is increased to 8 mm/s, the average microhardness of the composite coating reaches up to 405 HV0.2, which is about 1.3 times higher than that of Ni-based coating. Moreover, the corrosion current density and the wear rate of the composite coating are only 7% and 55% of those of the Ni-based coating, respectively, which is attributed to the good properties and homogeneous distribution of CFs and finer microstructure of composite coating.

Laser shock peening of laser additive manufactured Ti6Al4V titanium alloy

Abstract: Laser shock peening is combined with laser additive manufacturing to modify the surface microstructures and mechanical properties of as manufactured Ti6Al4V titanium alloy. Microstructural evolution, microhardness distribution, residual stress distribution and mechanical properties are examined before and after LSP. After peening, the interplanar spacing of lattices of both α and β phases decreases without any new phase formation. Grain refinement is achieved with average grain size of α phase decreasing from 33.6 to 24.3 μm. High density of dislocation lines, tangles, and multi-directional mechanical twins are observed. Residual stress is turned from tensile to compressive state with an affected depth of around 700 μm. The hardening layer reveled by microhardness is around 900 μm in depth. Grain refinement accounts for the yield strength, ultimate tensile strength, and elongation enhancements after peening.

Sol-gel deposition of hydroxyapatite coatings on porous titanium for biomedical applications

Abstract: The stress shielding and the poor osseointegration in titanium implants are still problems to be resolved. In this context, this work proposes a balanced solution. Titanium samples were fabricated, with a porosity of 100–200 μm of pore size employing space-holder technique (50 vol% NH4HCO3, 800 MPa at 1250 °C during 2 h under high vacuum conditions), obtaining a good equilibrium between stiffness and mechanical resistance. The porous titanium substrates were coated with hydroxyapatite, obtained by sol-gel technique: immersion, dried at 80 °C and heat treatment at 450 °C during 5 h under vacuum conditions. Phases, surface morphology and interfacial microstructure of the transverse section were analyzed by Micro-Computed Tomography, SEM and confocal laser, as well as the infiltration capability of the coating into the metallic substrate pores. The FTIR and XRD showed the crystallinity of the phases and the chemical composition homogeneity of the coating. The size and interconnected pores obtained allow the infiltration of hydroxyapatite (HA), possible bone ingrowth and osseointegration. The scratch resistance of the coating corroborated a good adherence to the porous metallic substrate. The coated titanium samples have a biomechanical and biofunctional equilibrium, as well as a potential use in biomedical applications (partial substitution of bone tissue).

Self-degradation of micro-arc oxidation/chitosan composite coating on Mg-4Li-1Ca alloy

Abstract: Regulating degradation rate and moderate pH micro-environment for biodegradable magnesium alloys face huge challenge. The chemical and morphological characteristics of micro-arc oxidation (MAO) and chitosan (CS) composite coatings, fabricated on Mg-4Li-1Ca alloy, are analyzed through field-emission scanning electronic microcopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform infrared spectroscopy. Corrosion resistance of the samples is evaluated via hydrogen evolution, potentiodynamic polarization and electrochemical impedance spectroscopy in Hank's solution. Results indicated that the MAO and CS coating enhances the corrosion resistance and antibacterial growth activity. With increasing immersion time, the degradation of the MAO/CS coatings gives rise to a decrease in pH value and leads to a rapid increase in hydrogen evolution rate after an immersion in Hank's solution after 100 h. The MAO/CS coatings retain the solution pH at a moderate level (less than or equal to 8.25). A novel self-degradation mechanism of the MAO/CS coating on Mg-Li-Ca alloy is proposed due to the fact that MAO/CS coating is cathodic relative to the substrate.

Synthesis, characterization, corrosion and bioactivity investigation of nano-HA coating deposited on biodegradable Mg-Zn-Mn alloy

Abstract: Electric discharge machining (EDM) is widely used to cut and shape the biomedical device substrates, following often a separate coating process to deposit another barrier layer to improve the corrosion performance. This paper presents an innovative single in-situ surface modification technique to deposit nano-hydroxyapatite (nHA) coating on biodegradable Mg-Zn-Mn alloy while shaping the device substrate via electric discharge machining (EDM). The key benefit of the technique is to carry out the whole process in a single setup, hence saving time and cost. The aim of the coating is to control implant's degradation rate and to improve in-vitro bioactivity with human cells. Morphology, elemental, and chemical composition of the nHA coated Mg-Zn-Mn surface were characterized by FE-SEM, EDS, and XRD, respectively, while microhardness is measured by a Vickers hardness tester. Corrosion tests were performed via potentiodynamic polarization measurements in a SBF (simulated body fluid) to evaluate the degradation kinetics. In-vitro cell culture study was carried out to evaluate biocompatibility and cell attachment onto the modified surface. Surface characterization results revealed that a biomimetic nHA containing interconnected nano-porosities of size 5-10 μm had been yielded on the substrate surface, which is beneficial for the apatite growth and osseo-integration. The deposited coating layer has comprised of Mg, Zn, Mn, O, Ca and P elements and formed intermetallic oxide phases, such as CaMg, Mg-Zn, Mn-CaO, Mn-P, and Ca-Mn-O, which improved the in-vitro corrosion performance. The degradation rate of Mg-Zn-Mn alloy was reduced by 90.85% from 0.82 mm/year to 0.07 mm/year by the deposition of nHA-coating layer. The microhardness of the modified surface was measured as 234 HV, which was 1.5 fold higher than the untreated surface. The corroded surface analysis showed that the dense intermetallic phases of nHA coating acted as a stable barrier layer, thus prohibiting the surface from degradation, and hence, improving the corrosion resistance. The in-vitro bioactivity analysis revealed that the nHA containing layer exhibited the superior bioactivity and promotes adhesion, growth, and proliferation of human osteoblastic MG-63 cells.

In vitro degradation behavior, antibacterial activity and cytotoxicity of TiO2-MAO/ZnHA composite coating on Mg alloy for orthopedic implants

Abstract: Magnesium alloys as biodegradable materials have received great attention for orthopedic application as a result of their good biocompatibility, bioactivity, and mechanical properties. However, the clinical use of Mg alloys is restricted by high degradation rate. In order to reduce the degradation rate, TiO2 incorporated micro-arc oxidation (TM) coatings were prepared on Mg- Ca alloy using micro-arc oxidation (MAO). Subsequently, zinc-doped hydroxyapatite (ZH) coating was deposited by electrophoretic deposition (EPD) on the MAO coating. The electrochemical test results demonstrated that the deposition of ZH composite coatings on Mg alloy significantly reduces its corrosion rate and improves its charge transfer resistance. Antibacterial activity of the coating against Escherichia coli (E. coli) was studied using disk-diffusion and spread plate methods. The results revealed that the inhibition zone amplified after deposition of TM and ZH coatings on Mg alloy, whereas more inhibition zone was found around ZH coating. In addition, the number of E. coli colonies reduces to 92% after ZH coating implying its good antibacterial properties. The cytotoxicity test indicated that cell viability of MG63 osteoblast cells cultured with ZH extracts was higher compared to the TM coating and bare Mg alloy. These results confirm that Mg alloy coated by TM/ZH exhibits high corrosion resistance, antibacterial activity and favorable bioactivity and cytocompatibility, indicating their substantial potentials for biomedical applications.

Effects of graphene nano-platelets (GNPs) on the microstructural characteristics and textural development of an Al-Mg alloy during friction-stir processing

Abstract: The aim of this research is to characterize the unique microstructural features of Al-matrix nanocomposites reinforced by graphene nano-platelets (GNPs), fabricated by multi-pass friction-stir processing (FSP). During this process, secondary phase GNPs were dispersed within the stir zone (SZ) of an AA5052 alloy matrix, with a homogenous distribution achieved after five cumulative passes. The microstructural characteristics and crystallographic textures of different regions in the FSPed nanocomposite, i.e., base metal (BM), heat affected zone (HAZ), thermo-mechanical affected zone (TMAZ), and SZ, were evaluated using electron back scattering diffraction (EBSD) and transmission electron microscopy (TEM) analyses. The annealed BM consisted of a nearly random crystal orientation distribution with an average grain size of 10.7 μm. The SZ exhibited equiaxed recrystallized grains with a mean size of 2 μm and a high fraction of high-angle grain boundaries (HAGBs) caused by a discontinuous dynamic recrystallization (DDRX) enhanced by pinning of grain boundaries by GNPs. The sub-grains and grain structure modification within the HAZ and TMAZ regions are governed by dislocation annihilation and reorganization in the grain interiors/within grains which convert low-angle to high-angle grain boundaries via dynamic recovery (DRV). The FSP process and incorporation of GNPs produced a pre-dominantly {100} cube texture component in the SZ induced by the stirring action of the rotating tool and hindering effect of nano-platelets. Although, a very strong {112} simple shear texture was found in the HAZ and TMAZ regions governed by additional heating and deformation imposed by the tool shoulder. These grain structure and texture features lead to a hardness and tensile strength increases of about 55% and 220%, respectively.

Microstructure and properties of in-situ TiN reinforced laser cladding CoCr2FeNiTix high-entropy alloy composite coatings

Abstract: To prolong the service life of agitator blades in Phosphoric acid reactors, the in-situ TiN particles reinforced CoCr2FeNiTix (x = 0, 0.5, 1) high-entropy alloy (HEA) coatings have been successfully fabricated by laser cladding on 904L stainless steels. The microstructures and phase compositions were analyzed by metallurgical microscope (OM), scanning electron microscope (SEM), X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The microhardness, wear resistance, and corrosion resistance were measured by microhardness tester, wear and friction machine as well as electrochemical workstation, respectively. The experimental results indicate the phase structures of the coatings are composed of FCC plus TiN and a few Laves phases. The microstructure of the coating is columnar crystal while no Ti elements adding. With the addition of Ti elements, the coating consists of irregular dendritic and granular TiN ceramics as well as a few Laves phases. In the terms of wear resistance, the hardness of the coating containing Ti elements increase significantly. As x = 1, hardness of the coating is more than 3 times higher than substrate, and its wear mass is approximately 1/3 of the substrate. According to the electrochemical polarization curve, the corrosion resistance of the HEA coatings is lower than that of the substrate.

Effects of conventional, severe, over, and re-shot peening processes on the fatigue behavior of mild carbon steel

Abstract: The present study investigates experimentally the effects of different shot peening treatments, including conventional, severe, over, and re-shot peening on microstructure, mechanical properties, and fatigue behavior of AISI 1050 mild carbon steel. Different shot peening treatments were performed using various effective parameters by considering the influences of increasing Almen intensity and coverage. Optical microscopy and field emission scanning electron microscopy observations and X-Ray diffraction measurements were carried out to analyze grains refinement in each shot peening treatment. Microhardness and residual stress measurements were taken from shot peened surfaces to the core material to investigate the mechanical properties. The fatigue behaviors of the specimens were examined by using the axial fatigue test. The results indicated that post-grinding, re-shot peening, and severe shot peening processes have significant effects on fatigue life improvement.

Preparation, structure, and properties of an AlCrMoNbZr high-entropy alloy coating for accident-tolerant fuel cladding

Abstract: A nearly equal molar ratio AlCrMoNbZr high-entropy alloy (HEA) coating was deposited on N36 zirconium alloy substrates using magnetron co-sputtering technology to enhance the corrosion resistance of light water reactor (LWR) fuel cladding. The microstructure, mechanical properties, surface wettability and corrosion resistance of the AlCrMoNbZr coating were systematically investigated. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) analyses showed that the AlCrMoNbZr HEA coating contained a composite of amorphous and bcc-structured nanocrystals. A nanoindentation test revealed that the coating had a high hardness of 11.8 GPa, and a scratch test indicated that the coating well adhered to the N36 substrate. The contact angle test showed that the static contact angle was 109°, which indicated the good hydrophobic property of the coating. The corrosion measurement showed that the AlCrMoNbZr coating remained effective after it was immersed in static pure water at 360 °C and 18.7 MPa for 30 days, and no N36 substrate oxides formed, which indicated a superior corrosion resistance. Furthermore, the corrosion behaviour of the coating was discussed. The weight gain measurement (8.8 mg/dm2 weight gain) proved that the coating experienced an oxidation process, and during the process, protective Nb2Zr6O17, ZrO2 and Cr2O3 formed on the surface of the coating. Hence, the AlCrMoNbZr HEA coating is a potential candidate material for accident-tolerant fuel (ATF) coatings.

Superhydrophobic carbon nanotubes/epoxy nanocomposite coating by facile one-step spraying

Abstract: A new superhydrophobic nanocomposite coating was prepared by combining the bisphenol A diglycidyl ether (BADGE) epoxy resin with unmodified multi-walled carbon nanotubes (MWCNTs). The mixture solutions were coated on the Q235 carbon steel surface through a facile and effective one-step spraying method. The highest water contact angle reached ~154° when 30% MWCNTs were added into the coating matrix. From the scanning electron microscopy (SEM) images, the superhydrophobic hierarchical microstructures were established via the agglomerated MWCNTs cured with epoxy resin. Furthermore, the as-prepared superhydrophobic nanocomposite coating with 30% MWCNTs showed excellent anti-icing property in the water-dripping and static freezing tests at the temperature of −10 °C. The superhydrophobic nanocomposite coating was endowed with an extraordinary mechanical durability, with the superhydrophobic state being retained even after 100 cycles in the tape-peeling test. From the electrochemical impedance spectroscopy (EIS) results, the low-frequency modulus first decreased due to the increasing conductivity of the coating surface, and then increased for the coating with 30% MWCNTs due to the air trapped in the superhydrophobic microstructure.

Improvement of mechanical properties and anticorrosion performance of epoxy coatings by the introduction of polyaniline/graphene composite

Abstract: We reported the preparation of epoxy coatings incorporated with a composite filler of poly(styrenesulfonate)-polyaniline/reduced graphene oxide (PSS-PANI/rGO) and their performance on the mechanical and anticorrosion properties. The rGO platelets were first dispersed in the PSS solution, followed by in situ oxidative polymerization of aniline. The resulting PSS-PANI/rGO composite was then blended with a bisphenol-A type epoxy at different loadings by tri-roller mill, and was subsequently cured with a curing agent. The ultimate tensile strength and tensile toughness of the epoxy composite at a loading of only 0.5 wt% PSS-PANI/rGO were improved by 39% and 127%, respectively, when compared to the respective values of the pristine epoxy. This was ascribed to their strong interfacial bonding upon curing by the reaction between the PANI and epoxy. Furthermore, the potentiodynamic polarization of the carbon steels coated with the epoxy/PSS-PANI/rGO composite revealed that the anticorrosion performance was greatly improved when compared to those with the neat epoxy and epoxy/rGO coatings. The superior anticorrosion effect was attributed to its larger tortuosity of diffusion pathways, improved interfacial strength between the epoxy and filler, and the passivation layer formed by the presence of polyaniline which was confirmed by X-ray photoelectron spectroscopy. The improved anticorrosion and toughness would allow the coatings to withstand externally mechanical impact and prevent corrosion effectively.

Development of high-efficient synthetic electric heating coating for anti-icing/de-icing

Abstract: One novel multi-layered anti-icing/de-icing coating has been developed by combination of the fronted electric heating coating and top super-hydrophobic coating. Compared with the conventional electric heating method where the elements are embedded in fiber reinforced plastic, this multi-layered coating can reduce the energy consumption by up to 58% for anti-icing. The anti-icing experiments demonstrate that the multi-layered coating not only realizes anti-icing with low-temperature (the anti-icing temperature was about 7 °C), but also possesses high-efficient electric heating and anti-icing performances. From the viewpoint of micro-scale, the experiment of ice drop detaching from substrate was conducted to demonstrate that the thin water layer greatly reduces the ice adhesive force. The electric heating coating has the potential for diverse industrial applications due to its convenient fabrication, damage resistance and high-efficient anti-icing/de-icing properties.

Pulsed laser deposition of copper and zinc doped hydroxyapatite coatings for biomedical applications

Abstract: Research on biomaterials which promote osseointegration and reduce bacterial colonization is of great interest because of their potential to enhance metal prostheses for the repair or regeneration of critically damaged or fractured bone. Hydroxyapatite (HA) coatings doped with copper (Cu) and zinc (Zn) emerge as good candidates for this purpose since both elements are parts of the biological metabolism and exhibit antibacterial properties. Two series of Cu-HA and Zn-HA coatings with different concentrations were fabricated by pulsed laser deposition (PLD). Their physicochemical characterization, performed by fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), x-ray diffraction (XRD) and x-ray photoelectron spectroscopy (XPS), revealed typical apatite growth in globular aggregates and demonstrated the efficient incorporation of Cu and Zn into the HA coatings. Using the MC3T3-E1 pre-osteoblast cell line, the evaluation of the biological response showed proper cell proliferation and osteogenic activity, confirmed respectively by the MTT test and the alkaline phosphatase quantification. Cu and Zn doping adds antibacterial properties to the coating, resulting in significant reductions of Escherichia coli and Staphylococcus aureus biofilms. In conclusion, this study advances that either Cu-HA or Zn-HA materials offer promising alternatives to be used in metallic prostheses for the repair or regeneration of critically damaged or fractured bone tissue.

Mechanical, wear, corrosion and biological properties of arc deposited titanium nitride coatings

Abstract: Cathodic arc deposition of ceramic coatings is gaining importance due to their good adhesion and minimum residual stresses. In this work we have arc deposited TiN coatings on Ti6Al4V alloy and evaluated their mechanical, corrosion, in vitro tribological and biological properties for implant applications. Coatings exhibited high hardness of 33.4 ± 10 GPa and Young's modulus of 458.4 ± 79 GPa. Scratch tests revealed that the coatings fail adhesively at 2.9 ± 0.3 GPa. Hydrophilic nature of the coatings enabled protein adsorption leading to surface passivation and significant increase in the in vitro corrosion resistance. The inherent in vitro wear rate of the coatings was 6.8 ± 1.7 × 10−7 mm3/N·m against Al2O3 ball. The wear rate of ultrahigh molecular weight polyethylene tested against TiN coatings was 1.9 ± 0.7 × 10−5 mm3/N·m, which appears to be better than against CoCrMo alloy. In vitro biocompatibility studies performed using mouse embryonic fibroblast cell line (NIH3T3) demonstrated that the coatings are non-toxic and exhibit excellent cell-materials interactions. Further improvement in the performance of present cathodic arc deposited TiN coatings can be achieved by minimizing/eliminating coating defects such as pits and macro/micro particles, which accelerated the localized damage during articulation.

Fabrication of superhydrophobic coating on magnesium alloy with improved corrosion resistance by combining micro-arc oxidation and cyclic assembly

Abstract: A superhydrophobic composite coating was fabricated on AZ91 magnesium alloy by combining micro-arc oxidation (MAO) and cyclic assembly in phytic acid and Ce(NO3)3 solution. The influences of assembling cycles on surface morphology and wettability were characterized by scanning electron microscopy and atomic force microscopy. It showed that a micro-nano hierarchical structure was constructed after three cycles of assembly, resulting in a superhydrophobic surface with a contact angle of 159°. Compared with the direct cyclic assembly on bare Mg alloy, the prior MAO treatment could notably reduce the cracks caused by hydrogen evolution during assembling process, and the following assembling process could in return repair the porous defects of MAO layer. Electrochemical tests indicated that the superhydrophobic composite coating increased the corrosion resistance of Mg alloy by three orders of magnitude in 3.5 wt% NaCl solution. Furthermore, 72 h of electrochemical tests revealed that the composite coating could provide long-term corrosion protection for Mg alloy. It indicates that the combination of prior MAO and latter cyclic assembly will be a promising strategy to construct corrosion-resistant composite coating on Mg alloy substrate.

Microstructural design of self-lubricating laser claddings for use in high temperature sliding applications

Abstract: Nickel-based self-lubricating claddings with the addition of Ag and MoS2 were prepared by means of laser cladding on stainless steel substrates, aiming at their implementation in metal forming appl ...

PTMS/OH-MWCNT sol-gel nanocomposite for corrosion protection of magnesium alloy

Abstract: In this work, hydroxylated Multi-Walled Carbon Nanotubes (OH-MWCNTs) were embedded in phenyl-trimethoxysilane (PTMS) sol-gel coating at different concentrations to improve the corrosion protection capacity. The obtained nanocomposites (PTMS/OH-MWCNT) were applied on AM60B magnesium alloy for effective corrosion protection. Chemical interaction between OH-MWCNTs and PTMS silane matrix was confirmed by FTIR. The SEM images were revealed that a uniform but cracked sol-gel coating was obtained without incorporation of OH-MWCNTs. Also, uniform sol-gel nanocomposites without any micro-cracks were achieved when various concentrations of OH-MWCNT (200, 500, and 1000 ppm) were embedded in the silica film. Both the pure and nanocomposite coatings showed suitable adhesion to the substrate and thickness of the pure coating (1.4–1.5 μm) was not changed significantly by incorporation of OH-MWCNTs. The average roughness of the pure PTMS film was about 0.2 nm and was increased to 0.5 nm by incorporation of OH-MWCNTs at 200 ppm concentration. However, the surface roughness was decreased by further increasing the carbon nanotube content in the sol-gel coating probably due to formation of more compact silica film. Corrosion resistance of the magnesium alloy in Harrison's corrosive electrolyte was significantly increased after application of PTMS coating which was ascribed to barrier properties of the film along with hydrophobic nature of the phenyl group. Also, the corrosion resistance of PTMS sol-gel film was considerably promoted after incorporation of OH-MWCNTs at 500 ppm concentration which was attributed to the chemical interaction between incorporated OH-MWCNTs and silane matrix, filling of the defects of coating by the carbon nanotubes, and also formation of longer tortuous pathways for the corrosive species. The water contact angle on the pure sol-gel coating was increased from about 86.95 to 94.65° by incorporating 500 ppm of OH-MWCNTs indicating considerable improvement in the hydrophobic property.

Improved hardness and wear resistance of plasma sprayed nanostructured NiCrBSi coating via short-time heat treatment

Abstract: NiCrBSi alloy coatings deposited by atmospheric plasma spray technology were heated at 440 °C, 460 °C, and 480 °C for a short-time respectively to achieve a better understanding of the relationship between their microstructures and properties. Optical microscopy, scanning electron microscopy, X-ray diffraction, transmission electron microscopy and tribological tests were conducted to evaluate the microstructures and wear performance of the coatings. Both the hardness and wear resistance of the coatings increase at the initial stage of short-time heat treatment followed by decreasing. The heat-treated coatings show 15.4% higher maximum hardness and 60% lower minimum wear volume than the as-sprayed one. The as-sprayed NiCrBSi coating possesses a mixed microstructure mainly composed of the crystalline phases and the amorphous phase (about 10 vol%). After 3-min heat treatment at 480 °C, the volume fraction of the amorphous phase decreases to 1.6 vol%, while the fractions of compounds (such as Ni3B and Cr3B4) and γ-Ni increase correspondingly. The precipitates of Ni3B and Cr3B4 are responsible for the hardening of the coatings. As heat treatment proceeds, the γ-Ni grains coarsen, leading to a deceasing tendency in the hardness and wear resistance of the coatings. As the abrasive modes of the coatings are not varied after short-time heat treatment, the hardness of the coatings plays a dominant role in the wear resistance of the coatings.

Influence of coating thickness on residual stress and adhesion-strength of cold-sprayed Inconel 718 coatings

Abstract: In the cold spray process, deposition of particles takes place through intensive plastic deformation upon impact in a solid state at the temperatures well below their melting point. The high particle impact velocity causes high local stresses which lead to deforming the particles and the substrate plastically in the proximity of the particle–substrate interface. As a result, high residual stresses are introduced in cold spray coatings due to the peening effect of the particles collisions with the substrate. In this study, a powder based on the chemical composition of IN 718 was cold-sprayed on IN 718 substrates by using nitrogen gas for an application as a repair tool for aero engine components. The magnitude of the residual stress and its distribution through the thickness were measured by using the hole-drilling and the bending methods. Residual stress was also estimated by using an approach based on the physical process parameters. Mainly compressive residual stresses were observed in cold-sprayed IN 718 coatings. Accumulation of residual stresses in the coatings is highly affected by peening during deposition and it decreases with increase in thickness. It has been observed that the adhesion-strengths of cold-sprayed IN 718 coatings are highly influenced by coating thickness and residual stress states of the coating/substrate system. In the presence of residual stresses in the coatings, adhesion-strength decreases with increasing coating thickness. The energy-release-rate criterion has been used to predict adhesion-strength with increasing coating thickness. Predicted bond-strength values are close to the measured adhesion-strength values and decrease with increase in coating thickness.

The potential of glycerol and phenol towards H2 production using steam reforming reaction: A review

Abstract: Hydrogen production via the steam reforming of glycerol, the main by-product of biodiesel production, and the pyrolysis/gasification route of biomass are two processes that have drawn considerable attention by the scientific community due to their potential in reducing our dependence on fossil based sources and in mitigating the effects of greenhouses gases on our planet. However, the commercial exploitation of the processes depends on the development of cheap, active and stable catalysts. In the present review, the key literature on the glycerol and phenol steam reforming reactions is presented and discussed. The impact of crucial variables for both the reactions at hand are discussed, such as active metal nature, metal loading, support, reaction temperature, method of preparation, poisoning resistance and coking tolerance. For catalytic systems design purposes the aspect of how the catalyst physicochemical characteristics affect the catalytic performance is addressed. Particular attention is given at the issue of coke resistance of the catalysts due to its detrimental effect for the reactions at hand. Natural materials, such as calcites, dolomites and olivines, utilized for the phenol steam reforming reaction are discussed.

Evaluation of protective coatings for offshore applications. Corrosion and tribocorrosion behavior in synthetic seawater

Abstract: Coatings have been widely used in the corrosion protection of metallic materials in marine environments. In this work, the corrosion and tribocorrosion behavior of three potential coatings employed in offshore applications has been evaluated. The coatings studied were a Thermally Sprayed Carbide coating with an organic sealant (C1), a Thermally Sprayed Aluminum with an organic sealant (C2), and an epoxydic organic coating reinforced with ceramic platelets (C3). Electrochemical Impedance Spectroscopy and Potentiodynamic Polarization techniques have been employed to assess the corrosion performance of the coatings in synthetic seawater. Furthermore, unidirectional ball-on-disc tribocorrosion tests were performed to study the response of the coatings subjected to simultaneous action of wear and corrosion. The coatings were found to provide to the steel substrate with enhanced corrosion resistance, both in absence and during wear process, and to improve in the tribological properties with lower coefficients of friction in seawater. The coating less affected by sliding in terms of corrosion resistance was C2 coating, which also showed the lowest coefficient of friction.

The impact of AA7075 cold spray coating on the fatigue life of AZ31B cast alloy

Abstract: To improve the fatigue resistance of magnesium alloys, AA7075 powder, which has high fatigue strength and high adhesion strength, was successfully coated on as-cast AZ31B substrate. The coating proved to be dense, with a porosity of

Influence of ball-burnishing on roughness, hardness and corrosion resistance of AISI 1045 steel

Abstract: The ball-burnishing process involves a surface plastic deformation that improves the physical-mechanical properties of manufactured parts. In this work, was used to study the influence of the main parameters of the ball-burnishing process on features such as mean surface roughness and hardness of cylindrical AISI 1045 steel samples. The experimental stage was based on a 33 factorial design and the Response Surface Methodology. An equation was proposed to model the mean roughness in the studied experimental region. Moreover, the polarization technique (Tafel) and electrochemical impedance spectroscopy (EIS) were used to evaluate the burnishing process effect on corrosion resistance of a sample machined with the optimal burnishing parameters obtained herein. Finally, burnishing process effect on the phase change was also evaluated by using X-ray diffraction. As a result, the factor that showed higher influence was the burnishing force for both features, roughness and hardness. The data shows that it is possible to reduce the surface roughness from 3.51 μm to 0.61 μm and to increase the hardness from 202 HB to 236 HB using ball-burnishing process, and to improve the corrosion resistance in AISI-1045 steel.

Behavior of plasma sprayed Cr coatings and FeCrAl coatings on Zr fuel cladding under loss-of-coolant accident conditions

Abstract: To fabricate the oxidation resistance accident tolerance fuel cladding, Cr layer and FeCrAl alloys were deposited on Zircaloy-4 using atmospheric plasma spraying technology. The specimens were exposed under a simulated loss-of-coolant accident condition. XRD, SEM and EDS techniques were carried out to explore their high temperature behavior. According to the analysis results, both Cr coating and FeCrAl coating were successfully prepared by atmospheric plasma spraying system. Cr coating had superior oxidation resistance. Compact Cr2O3 scale forming on Cr coating acted as an oxygen diffusion barrier, whereas FeCrAl coating degraded due to inter-diffusion, leading to its poor protection for the cladding. Difference in standard Gibbs free energy of formation of Fe and Zr caused the prior oxidation of Zr. Accordingly, Zr-rich and Zr-depleted zones occurred in FeCrAl-coated specimens.

Facile fabrication of durable superhydrophobic silica/epoxy resin coatings with compatible transparency and stability

Abstract: It is a practical route for taking advantage of the high binding force of epoxy resin to fabricate mechanically robust superhydrophobic surfaces. However, this kind of superhydrophobic surfaces is hard to be endowed with high transparency. Here, we developed transparent and durable superhydrophobic bilayer coatings containing hydrophobic silica particles on top and an epoxy resin bonding layer at the base via a simple approach with common apparatus and reagents. The transmittance of the coated glass at 550 nm is as high as 90.3%, close to the transmittance of 91.8% for the bare glass. The stabilities of these coatings were tested by sandpaper abrasion tests, simulated seawater immersion and heat treatment experiments. The results demonstrate the as-prepared superhydrophobic coatings have excellent self-cleaning ability, good mechanical strength, high chemical and thermal stabilities. Additionally, the method presented here can effectively reach a proper balance between high transparency and excellent durability. The self-cleaning superhydrophobic coatings are independent of the nature of the substrates and have great application potential in industry and real-life.

Increasing thickness and protective properties of PEO-coatings on aluminum alloy

Abstract: The paper presents results of the study aimed at assessing the effect of duty cycle (D) during plasma electrolytic oxidation (PEO) on morphology, composition, and protective properties of the coatings produced on 5754 aluminum alloy in a mixed electrolyte. It is shown that increasing the duty cycle of a microsecond current pulses leads to a decrease of porosity and an increase of thickness of the PEO-layers, which are composed of γ-Al2O3, β-Al2O3, AlPO4, and Al2Mo3C. This improved the barrier properties and microhardness of the coating. The Young's modulus increased with an increase of the quantity of electricity due to the changes of morphological and chemical structure of the coatings. The PEO-coatings produced at a higher duty cycle and longer oxidation time are more wearproof as compared to ones formed at a shorter oxidation time and lower D values. The obtained data allowed confirming the hypothesis on phase formation mechanism.

Process parameter optimisation of laser clad iron based alloy: Predictive models of deposition efficiency, porosity and dilution

Abstract: As a candidate coating material for heat-exchanger surfaces in commercial power generation boiler, an amorphous/glass forming Fe-Cr-B alloy NanoSteel SHS 7170 was deposited by a 2 kW fibre laser onto a boiler grade steel substrate (15Mo3). A comprehensive trial with 28 single track optimisation runs was carried out to develop models of the influence of three processing parameters, laser power, laser traverse speed and powder feed rate, on powder deposition efficiency, dilution and porosity. It was found that deposition efficiency is dependent on laser power and powder feed rate, increasing with increasing power and decreasing powder feed rate when tested within the parameter window of laser power ranging from 0.4 to 2 kW; traverse speed varying from 150 to 1200 mm min‑1; and powder feed rate varying from 4 to 10 g min‑1. Similarly, it was found that dilution is also dependent on laser power and powder feed rate. Dilution increases with increasing power and decreases with increasing powder feed rate within the same parameter window discussed above. This means that through processing parameter selection, these properties can be adjusted to suit their application. Porosity was found to be independent of processing parameters and instead mostly dependent on the feedstock material. A model was produced for predicting porosity within a powder feedstock, found to be 8.5%. These models were used to successfully produce an optimised coating.

Microstructure investigation of Inconel 625 coating obtained by laser cladding and TIG cladding methods

Abstract: The purpose of this study was to investigate the microstructure of the Ni base Inconel 625 coatings that were fabricated on the Inconel 738 substrate with two processes of laser cladding and TIG cladding For this purpose, single-pass samples were precipitated to obtain optimal parameters In the laser cladding method, laser power, laser scanning rate and powder feed rate were considered as variables, and current and its type were considered as variables for the TIG method Based on the results, using the parameters of optimum single-pass samples that were free of porosity, crack and had minor geometric dilution, coatings were applied in both methods with 50% overlap In order to microstructural, elemental and phasic characterization, field emission scanning electron microscopy (FESEM) equipped with energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) were used Based on the results, the microstructure of coatings from surface to interface of coating/substrate consists of coaxial, columnar and cellular structure dendrites, respectively, and the laser coating has a finer microstructure due to the higher cooling rate In addition, the austenite, carbide and Laves phases were observed in both coatings

Highly oxidation resistant and cost effective MCrAlY bond coats prepared by controlled atmosphere heat treatment

Abstract: MCrAlY bond coats with high oxidation resistance are essentially important for improving the life-time of thermal barrier coatings (TBCs). In this study, highly oxidation resistant MCrAlY bond coats were prepared by a cost-effective approach involving air plasma spraying (APS) followed by a controlled atmosphere heat treatment (a diffusion treatment performed in a furnace filled with Ar). The results confirmed that a pure α-Al2O3 thermally grown oxide (TGO) was successfully formed on the surface of the heat-treated APS bond coats during isothermal oxidation. The oxidation resistance of the resulting APS bond coats was as high as that of low-pressure plasma sprayed bond coats, which are very expensive. The high oxidation resistance of the APS bond coats fabricated in this study can be attributed to the structural changes of the interface between the splats inside the coating. During the controlled atmosphere heat treatment, the lamellar oxides present between the splats within the as-sprayed APS bond coats converted into isolated oxide particles, and the metal elements could diffuse freely within the coating after the healing of the interface between the lamellar splats. The effect of the changes in the interfacial microstructure of the bond coats on the growth of the TGO was discussed.