Showing papers in "Surface & Coatings Technology in 2012"

Hard nanocomposite coatings: Thermal stability, oxidation resistance and toughness

Abstract: The article reports on the enhanced hardness of nanocomposite coatings, their thermal stability, protection of the substrate against oxidation at temperatures above 1000 °C, X-ray amorphous coatings thermally stable above 1000 °C and new advanced hard nanocomposite coatings with enhanced toughness which exhibit (i) low values of the effective Young's modulus E ⁎ satisfying the condition H/E ⁎ > 0.1, (ii) high elastic recovery W e ≥ 60%, (iii) strongly improved tribological properties, and (iv) enhanced resistance to cracking; here E ⁎ = E(1−ν 2 ), E is the Young's modulus and ν is the Poison's ratio. At the end trends of next development of hard nanocomposite coatings are briefly outlined.

A review of plasma-assisted methods for calcium phosphate-based coatings fabrication

Abstract: The review is focused on the latest achievements in the field of plasma-assisted fabrication of biocompatible CaP-based coatings for medical implants with the emphasis on the coatings composition, structure, mechanical and biological performance. The discussed properties of biocompatible CaP coatings have been recently prepared using the most frequently applied plasma-assisted techniques such as plasma spraying (PS), radio-frequency (RF) magnetron sputtering, pulsed laser deposition (PLD), and ion beam-assisted deposition (IBAD). The review shows that plasma-assisted fabrication allows us to prepare dense, homogeneous, pore-free and high adherent biocompatible coatings able to prevent the leaching of toxic ions from metal to the surrounding tissues or rough and porous coatings capable of stimulating osteogenesis of a new bone. The main advantages and limitations of the described techniques of CaP-based coatings fabrication are presented as well as the most important challenges and critical issues are highlighted.

Surface improvement of shafts by the deep ball-burnishing technique

Abstract: In this article, deep ball-burnishing as a mechanical surface treatment for improving productivity and quality of rotating shafts is presented. When this technique is combined and applied after conventional turning, the resulting process is rapid, simple and cost-effective, directly applicable in lathes and turning centers of production lines. This process provides good surface finish, high compressive residual stresses, and hardness increment of the surface layer. These characteristics are the key for the fatigue life improvement of the component, and for wear resistance due to the higher hardness. This work presents a complete analysis of the principal beneficial aspects produced by the application of ball-burnishing. To determinate the influence of each process parameter, several tests were carried out. Once the optimum parameters were established, a complete analysis of the surface characteristics was performed. Surface topographies, sub-surface micro-hardness and residual stresses were measured. Complementary, a finite element model of ball-burnishing was used to understand and predict residual stress values and their variety with the process parameters. Results show that burnishing is an economical and feasible mechanical treatment for the quality improvement of rotating components, not only in surface roughness but in compressive residual stresses as well.

Thermal stability and oxidation resistance of Ti–Al–N coatings

Abstract: Ti1 − xAlxN coatings are widely used for wear resistant applications due to their excellent mechanical and thermal properties, which depend to a great extent on the Al content. Here, we concentrate on a comparative study of the effect of Al content on crystal structure, thermal stability and oxidation resistance of Ti1 − xAlxN coatings. In agreement to earlier studies, thermal annealing of the individual cubic (c) and wurtzite (w) structured metastable Ti1 − xAlxN coatings induces decomposition into their stable phases c-TiN and w-AlN. The decomposition process for c-Ti1 − xAlxN involves an intermediate formation of cubic Al-rich and Ti-rich domains which results in a hardness increase to 34.7 and 34.4 GPa for x = 0.52 and 0.62 when annealed at 950 and 900 °C, respectively. In general, coatings with an Al content closer to the solubility limit, exhibit an earlier decomposition process, and hence an earlier peak-hardness. During exposure of the Ti1 − xAlxN coatings to ambient air at elevated temperatures Al2O3, TiO2 and Al2TiO5 are formed. The oxidation resistance of as-deposited single-phase Ti1 − xAlxN coatings, cubic or wurtzite structured, increases with increasing Al content. However, coatings containing Al contents at the metastable solubility limit, which result in a mixed cubic–wurtzite structure, have the worst oxidation resistance of the Al-containing coatings investigated. The single phase wurtzite structured coating w-Ti0.25Al0.75N shows the best oxidation resistance, with only ~0.7 μm oxide scale thickness, after thermal exposure for 20 h at 850 °C in ambient air.

Nanostructured multi-element (TiZrNbHfTa)N and (TiZrNbHfTa)C hard coatings

Abstract: Multi-element (TiZrNbHfTa)N and (TiZrNbHfTa)C coatings were deposited on C45 and M2 steel substrates by co-sputtering of pure metallic Ti, Zr, Nb, Hf and Ta targets in Ar + N2 and Ar + CH4 reactive atmospheres, respectively TiN, TiC and TiZrNbHfTa metallic coatings were also prepared as reference All the films were comparatively investigated for elemental and phase composition, texture, microstructure, hardness, friction and wear behavior X-ray structural analysis revealed the formation of single-solution phases ((111) texture) As compared to the reference binary coatings (TiN and TiC), the multi-component films exhibited significantly higher hardnesses (a difference in hardness of about 10 GPa in the case of nitrides) and also superior wear performance The best friction and wear behavior was found for the (TiZrNbHfTa)C film, for which the friction coefficient and wear rate were of 015 and 08 × 10− 6 mm3 N− 1 m− 1, respectively

Fretting wear and friction reduction of CP titanium and Ti–6Al–4V alloy by ultrasonic nanocrystalline surface modification

Abstract: Application of surface modification techniques is expected to be a viable solution to mitigate fretting damage and to reduce friction. In this paper, the aim was to improve the fretting wear and friction characteristics of commercially pure titanium (CP Ti) and Ti–6Al–4V alloy by using an ultrasonic nanocrystalline surface modification (UNSM) technique. Lubricated fretting wear and friction tests were conducted with a ball-on-flat configuration on untreated and UNSM-treated specimens using silicon nitride (Si 3 N 4 ) balls. The results showed that the fretting wear and friction coefficient characteristics of the UNSM-treated specimens were improved compared to those of the untreated specimens. Moreover, it was found that the fretting wear scar diameter and depth of the UNSM-treated specimens were smaller and shallower compared to those of the untreated specimens. Surface analysis was performed using a scanning electron microscope (SEM).

Effects of WC–Ni content on microstructure and wear resistance of laser cladding Ni-based alloys coating

Abstract: Different WC–Ni contents of NiCrBSi/WC–Ni composite coatings were produced on stainless steel by laser cladding. The effect of WC–Ni doping on the microstructure and tribological properties of the conventional NiCrBSi coating were systematically investigated. It has been found that the microhardness and wear resistance of the Ni-based alloy coatings are greatly increased after adding the WC–Ni particles, due to the formation of hard WC phase and a partial dissolution of WC particles on the Ni matrix after laser cladding. The laser cladding NiCrBSi/WC–Ni composite coatings, involving only mild abrasive and adhesive wear when sliding against the AISI-52100 counterpart (under ball-on-disk and ring-on-block wear test conditions). Thus, it is concluded that the incorporation of WC phase is an effective and attainable way to improve the tribological properties of conventional Ni-based alloys coatings.

Effective solution for the tribological problems of Ti-6Al-4V: Combination of laser surface texturing and solid lubricant film

Abstract: Titanium alloys are characterized by poor tribological properties, and the traditional use of titanium alloys has been restricted to nontribological applications. Surface texturing has been recognized as an effective means of surface engineering to improve tribological properties of sliding surfaces. In this study, the patterns of micro-dimple with different dimples density were fabricated on the surface of Ti-6Al-4V by using laser. The effect of dimples density on the friction behavior of the titanium alloy was investigated under dry friction and coated MoS 2 . The results showed that the textured surface with higher dimples density had lower friction coefficients only at low load and speed under dry friction. When combining the solid lubricant with dimples, some textured samples showed excellent tribological performance for all applied loads. The optimum surface pattern was found, and the mechanisms for friction reduction and anti-wear were discussed.

Effect of nitrogen content and substrate bias on mechanical and corrosion properties of high-entropy films (AlCrSiTiZr)100 −xNx

Abstract: High-entropy alloy and nitride films of (AlCrSiTiZr)100 − xNx containing large Zr atoms and small Si atoms were deposited on 6061 aluminum alloy and mild steel substrates by DC reactive magnetron sputtering at various nitrogen flow ratio (RN). The composition, crystalline structure, and film morphology were analyzed by electron probe X-ray microanalyzer (EPMA), X-ray diffractometer (XRD), transmission electron microscope (TEM), and scanning electron microscope (SEM), respectively. Also, their hardness and elastic modulus were studied by nanoindentation. The corrosion behavior was studied by anodic polarization analysis in 0.1 M H2SO4 aqueous solution at room temperature. The properties of films deposited under substrate bias application were also studied. The tendency of the present composition to form amorphous or low-crystallinity structure is high because of its large difference in atomic size. Films demonstrate pure amorphous structures even as the nitrogen content reaches as high as 22.4 at.%. All coatings can provide better corrosion resistance on both 6061 aluminum alloy and mild steel substrates. Under condition of no applied substrate bias, films of (AlCrSiTiZr)100 − xNx deposited at RN = 30% give the best corrosion resistance. Substrate bias of − 100 V effectively improves the corrosion resistance of the amorphous film of (AlCrSiTiZr)100 − xNx (RN = 5%). The factors that might influence the corrosion resistance were also discussed.

Evolution of microstructure in laser deposited Al-11.28%Si alloy

Abstract: Laser melting of Al–Si alloys has been investigated extensively, however, little work on the microstructural evolution of laser deposited Al–Si alloys has been reported to date. This paper presents a detailed microstructural investigation of laser deposited Al–11.28Si alloy. Laser aided direct metal deposition (DMD) process has been used to build up solid thin wall samples using Al 4047 prealloyed powder. The evolution of macro- and microstructures of laser deposited Al–Si samples was investigated using X-ray diffraction, optical microscopy, scanning electron microscopy and electron backscattered diffraction techniques. Microstructural observation revealed that the morphology and the length scale of the microstructures are different at different locations of the sample. A periodic transition of microstructural morphology from columnar dendrite to microcellular structure was observed in each layer. The observed difference in the microstructure was correlated with the thermal history of the deposit.

Fabrication and characterization of graphene-reinforced waterborne polyurethane nanocomposite coatings by the sol–gel method

Abstract: Functionalized graphene-reinforced polyurethane nanocomposite coatings were prepared using the sol–gel method. This method not only provides a “green” strategy for fabricating the graphene-based nanocomposites, but also realizes the covalent functionalization of graphene nanosheets with polymer matrix. The functionalization of graphene with conjugated organosilanes is favorable for improving mechanical and thermal properties of the nanocomposite coatings, which is mainly attributed to the homogeneous dispersion of functionalized graphene in the polymer matrix and strong interfacial interactions between the two components. A 71% increase in tensile strength and a 86% improvement of Young's modulus are observed by the addition of 2.0 wt.% of functionalized graphene. The experimentally determined Young's modulus corresponds well with the theoretical simulation under the hypothesis that the graphene sheets are randomly dispersed in the polymer matrix.

Electrodeposition of Ni–Co alloys from a deep eutectic solvent

Abstract: Pure Ni and three Ni–Co alloys films, i.e. Ni–4 wt.%Co, Ni–18 wt.%Co, and Ni–40 wt.%Co, are electrodeposited at room temperature from the choline chloride/ethylene glycol deep eutectic solvent dissolved by nickel or/and cobalt chlorides. Electrodeposition mechanism, microstructure, and corrosion properties of the films are investigated. Surface morphology and chemical composite of the films are significantly dependent on the Ni 2+ and Co 2+ concentrations in the electrolytes. Interestingly, it is found that the amount of cobalt in the Ni–Co alloy films is significantly lower than that present in the electrolytes, which indicates an absence of anomalous codeposition process for the non-aqueous electrolytes. However, anomalous codeposition of Ni–Co deposits is frequently observed for the aqueous electrolytes. The Ni–Co alloy films possess face-centered cubic structures and refined grains revealed by X-ray diffractometer and scanning electron microscope. Potentiodynamic polarization measurements show that the Ni film exhibits the noblest corrosion potential and the lowest corrosion current compared with the Ni–Co alloys films. Moreover, the more Co content the Ni–Co films have, the more negative corrosion potential and the higher corrosion current the films exhibit.

Fabrication of Superhydrophobic and Oleophobic sol–gel Nanocomposite coating

Abstract: Superhydrophobic sol–gel nanocomposite coatings with improved oleophobic property were obtained by incorporation of a perfluoroalkylmethacrylic copolymer in a hybrid sol–gel matrix containing fumed silica nanoparticles. The coatings exhibited a water contact angle (WCA) of 158° and contact angle of 146° for ethylene glycol and 113° for lubricant oil. Though the sol–gel nanocomposite coatings in the absence of fluoropolymer were superhydrophobic with WCA of 155°, they were oleophilic with oil contact angle of

Plasma sprayed gadolinium zirconate thermal barrier coatings that are resistant to damage by molten Ca–Mg–Al–silicate glass

Abstract: Air plasma sprayed (APS) Gd 2 Zr 2 O 7 thermal barrier coatings (TBCs) are found to be highly effective in resisting high-temperature (1200 °C) penetration of molten Ca–Mg–Al–silicate (CMAS) glass deposit for prolonged durations (up to 1 week). In contrast, conventional APS 7YSZ TBCs are found to be fully penetrated by the molten CMAS glass under the same testing conditions. This resistance is attributed to the formation of a sealing layer made of crystalline Ca–apatite phase (based on Ca 2 Gd 8 (SiO 4 ) 6 O 2 ) as a result of the high-temperature chemical interactions between the APS Gd 2 Zr 2 O 7 TBC and the CMAS glass. The resistance to penetration of molten silicate deposits offered by Gd 2 Zr 2 O 7 composition TBCs is relatively insensitive to both the type of molten silicate deposits (CMAS sand, volcanic ash, coal fly ash) and the TBC microstructure (APS, EB-PVD).

Preparation of epoxy microcapsule based self-healing coatings and their behavior

Abstract: In this paper, epoxy microcapsules were prepared by interfacial polymerization of epoxy resin droplets with ethylenediamine (EDA), and the capsules were characterized by scanning electron microscope (SEM). Then, the coatings containing epoxy microcapsules were applied on carbon steels, and their behavior and self-healing effect were investigated by electrochemical impedance spectroscopy (EIS) technique and SEM observation. The experimental results demonstrate that the artificial scratches were successfully healed in about 4-h after made. Furthermore, coating prepared from 20 wt.% epoxy microcapsules shows the best performance among all the prepared coatings.

Role of Tin+ and Aln+ ion irradiation (n = 1, 2) during Ti1-xAlxN alloy film growth in a hybrid HIPIMS/magnetron mode

Abstract: Metastable Ti1-xAlxN (0.4 less than= x less than= 0.76) films are grown using a hybrid approach in which high-power pulsed magnetron sputtering (HIPIMS) is combined with dc magnetron sputtering (DC ...

Characterization of trivalent chromium process coating on AA2024-T3

Abstract: Trivalent Chromium Process (TCP) coatings, which have been used recently as an alternative to chromate conversion coatings (CCCs), have been characterized. The TCP coating was found to be a dense layer consisting of particles, hundreds of nm in size on the coating surface, which is similar to CCCs. The TCP coating has a two layered structure, with zirconium-chromium mixed oxide in the outer layer and aluminum oxide or oxyfluoride at the metal/coating interface. No Cr(VI) was found, which supports its use as an environmentally friendly replacement for CCC. The TCP thickness was in the range of 40–120 nm, considerably thicker than the zirconium based coating without chromium species. The high vacuum condition in the SEM dehydrates the coating quickly resulting in the formation of mud-crack artifacts. The TCP coating provides corrosion protection to the AA2024-T3 through suppressing the oxygen reduction reaction on aluminum alloy surfaces by acting as a protective barrier layer. Two pretreatments were investigated but found to result in little difference in the resulting TCP formed on AA2024-T3.

A comparative study on the direct and pulsed current electrodeposition of hydroxyapatite coatings on surgical grade stainless steel

Abstract: Hydroxyapatite [Ca 10 (PO 4 ) 6 (OH) 2 , (HAP)] coatings were developed on 316L stainless steel substrate from the electrolyte containing hydrogen peroxide (H 2 O 2 ) with the concentration ranging from 600 to 3000 ppm by both the direct and pulsed current electrodeposition methods. The effects of direct current density upon the addition of H 2 O 2 into the electrolyte on the phase purity and morphology of the as-deposited coatings were reported. The influence of pulsed parameters such as peak current density and pulse on and off time on the deposit compositions was also examined and compared with direct continuous current deposition in relation to the crystallinity, microstructure and the corresponding phases. X-ray diffraction (XRD) and Fourier transform infrared spectroscopic (FT-IR) techniques were performed in order to assure the purity, phase compositions of the coating and the morphology of the coating were characterized by scanning electron microscopic (SEM) technique. The results showed that the coating consists of mixed phases of calcium phosphate (Ca–P) in the absence of H 2 O 2 in the electrolytic bath. Whereas the addition of H 2 O 2 lowers the deposition current with the formation of smooth and uniform layer comprised solely of HAP. It is highly beneficial to increase the peroxide concentration from 600 to 2000 ppm for the deposition of pure HAP. While increasing the peroxide concentration to 3000 ppm, the coating morphology is not uniform as evidenced from the SEM result. Moreover, the increased adhesion and crystallinity of the HAP coating were achieved by pulsed current electrodeposition method at lower current density with longer pulse off time. The results of pulsed electrodeposition show that the relaxation time of the pulse is beneficial for the growth of HAP because it allows the diffusion of ions from bulk solution to the surface of electrode and thus lowers the concentration polarization in the next pulse on time. The combination of pulsed electrodeposition and addition of H 2 O 2 into the electrolyte promisingly improve the physico-chemical properties of HAP coating.

Effect of particle size and co-deposition technique on hardness and corrosion properties of Ni–Co/SiC composite coatings

Abstract: Ni–Co/SiC alloy matrix composite coatings were electrodeposited in a modified Watt's bath containing micro and nano sized SiC particles by using conventional electro-co-deposition (CECD) and sediment co-deposition (SCD) techniques. The deposits were characterized using SEM, EDX and XRD analyses, and microhardness and potentiodynamic polarization measurements. The maximum incorporation of the SiC micro- and nano-particles was obtained using the SCD technique at deposition current densities of 2 and 3 A/dm2, respectively. It was found that in the composite coatings, incorporation of SiC particles improves the microhardness of unalloyed Ni and Ni–Co alloy matrices. The nanocomposite coatings exhibit higher microhardness values than microcomposite ones. The potentiodynamic polarization measurements in 3.5% NaCl solution revealed that the corrosion resistance of the Ni–Co/SiC nanocomposite coatings is much higher than the Ni–Co alloy and Ni–Co/SiC microcomposite coatings. Moreover, corrosion resistance of Ni–Co/SiC nanocomposite coatings deposited by SCD technique is higher than the ones deposited by CECD technique. Corrosion resistance of the studied Ni–Co/SiC composite coatings was considerably affected by Co content, SiC particle size and content. Hardness enhancement was related to the structural features, and corrosion behavior was discussed based on the formation of corrosion micro cells, diminishing the effective metallic area, and increasing and hindering the corrosion paths.

Bonding behavior studies of cold sprayed copper coating on the PVC polymer substrate

Abstract: Metallization of the polymer surface by cold spray technique is an attractive way of improving their electrical conductivity and erosive resistance properties. However, it is still challenging to achieve a dense and thick high-strength metal coating, such as copper, on the polymer surface by cold spray system, due to excessive surface erosion during the processing. In order to overcome these difficulties, in the present work, two different types of interlayers are proposed. The polyvinyl chloride (PVC) polymer substrate was initially coated with a spherical copper powder and a tin powder separately and then a thick copper coating was fabricated using a dendritic copper powder. The properties of the coatings, such as shear adhesion strength, Vickers hardness and electrical resistivity, were measured and the influence of the interlayer on the above properties and bonding behavior was studied using scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDX) and focused ion beam dissection (FIB) techniques. The results show that the deposition efficiency is highly sensitive to the glass transition temperature of the substrate, irrespective of the process gas pressure. The coating with spherical copper interlayer shows poor shear adhesion strength due to the dislodged polymer debris at the spherical/dendritic copper coating interface. The electrical conductivity of the coating is fairly low as compared to the bulk copper due to the pores and inter-particle defects.

Increased thermal stability of Ti–Al–N thin films by Ta alloying

Abstract: To increase the thermal stability of protective coatings has always been a major topic for application oriented materials development. Here, we study the impact of Ta on the thermal stability of magnetron sputtered Ti 1− x Al x N thin films using a combined ab initio and experimental approach. With increasing Ta content in Ti 1− x − y Al x Ta y N, a decreasing energy of formation, as well as an increasing interaction of the transition metal d -states go along with an increase in cohesive energy, which point toward a higher stability of the quaternary system. The ab initio predicted increasing bonding strength of Ti 1− x − y Al x Ta y N with increasing Ta content was corroborated by structural and mechanical investigations of Ti 1− x − y Al x Ta y N thin films with y = 0, 0.03, 0.05, and 0.1, respectively. With increasing Ta content, a hardness increase of 25%, from ~ 30 GPa to ~ 40 GPa, can be observed after deposition. Annealing experiments in vacuum show that the decomposition process of the supersaturated solid solution towards their stable constituents c-Ti 1- z Ta z N (with z = y /(1 − x )) and hexagonal (wurtzite structure, w) w-AlN, is effectively retarded from ~ 900 to ~ 1200 °C with increasing Ta content. The involved decomposition into c-Ti–Ta-rich and c-Al-rich domains results in hardness maxima of ~ 38 to ~ 42 GPa for y = 0 and 0.1, respectively. Oxidation experiments for 20 h at 850 and 950 °C yielded fully oxidized Ta-free coatings, whereas the addition of only 3 at.% Ta to the metal sublattice results in the form of a layered oxide scale and remaining unoxidized nitride layer thicknesses of ~ 95 and ~ 87% (with respect to the as deposited films), respectively.

Residual stresses in cold spray Al coatings: The effect of alloying and of process parameters

Abstract: Al and Al alloy cold spray coatings were deposited on Mg substrates using two different cold spray systems - a Kinetic Metallization system (convergent barrel, sonic nozzle) and a CGT system (convergent-divergent barrel, supersonic nozzle). The residual stress profiles in the coatings were measured using neutron diffraction with high spatial resolution. In the first part of the study, the residual stress profile in pure Al coatings was compared when sprayed using the Kinetic Metallization system (using both helium and nitrogen as the driving gas), and with the CGT system using nitrogen gas. In this way the effect of impact velocity and process temperature was studied. In the second part of the study, the residual stress profile was compared in coatings of pure Al, 7075 Al and 6061 Al sprayed using the same process conditions in the CGT system. The residual stress profiles depend more on the alloy content, i.e. intrinsic resistance to plastic deformation, than on the processing conditions, and this is interpreted using a simple model that incorporates the effect of peening stresses in the cold spray process.

Assessment of duplex coating combining plasma electrolytic oxidation and polymer layer on AZ31 magnesium alloy

Abstract: Mechanical and corrosion tests were performed on a polymer-coated AZ31 magnesium alloy pre-treated by plasma electrolytic oxidation (PEO). Results were compared with a fluorotitanate–zirconate conversion coating pre-treatment. Mechanical performance was assessed by standardized adhesion (ISO 2409:2007), impact (ISO 6272‐1:2004) and impact + adhesion (ISO 6272/ASTMD2794) pass/fail tests. Corrosion behavior was monitored using electrochemical impedance spectroscopy (EIS), ac/dc/ac measurements and continuous exposure to salt fog per ASTM B117 and cyclic exposure per VDA 621‐415 (VDA). The PEO + polymer coating revealed lower impact resistance but better corrosion resistance than the Ti/Zr + polymer coating. The ac/dc/ac procedure demonstrated to be faster than EIS measurements for evaluation of the corrosion performance of studied coatings.

Microstructure and wear properties of laser-deposited functionally graded Inconel 690 reinforced with TiC

Abstract: Functionally gradient material (FGM) can be tailored to the structural requirements of the final product. In this study, titanium carbide (TiC) reinforcement particles were embedded in Inconel 690 with laser direct deposition to build functionally gradient metal matrix composites (FGMMCs). The microstructures of the MMC and distribution of TiC particles were characterized with an optical microscope, SEM and X-ray diffraction. There was a near absence of internal voids in the deposited TiC-Inconel 690 MMC. With the volume percentage of TiC particles in the depositions varied from 0 to 49%, a drastic evolution in the microstructure was observed and the presence of TiC particles over 30% yielded a refinement of the matrix microstructure and introduction of a finely dispersed crystalline phase. High-temperature dissolution of TiC was not detected under the conditions used. Micro-hardness and wear resistance tests showed a significant improvement with increased TiC content.

Calcium–magnesium–alumino-silicate (CMAS) degradation of EB-PVD thermal barrier coatings: Characterization of CMAS damage on ex-service high pressure blade TBCs

Abstract: The use of thermal barrier coatings (TBCs) on turbine blades and vanes located in the hot sections of gas turbine engines has allowed higher engine operating temperatures leading to temperatures of the order of 1200 °C at the surface of the ceramic coating. At such temperatures, under service conditions, thermal barrier coatings are susceptible to corrosion by molten calcium–magnesium–alumino-silicates (CMAS) resulting from the ingestion of siliceous mineral debris (dust, sand, ash) by the engine. This study consists in a microstructural analysis of CMAS induced degradation of standard 8YPSZ EB-PVD thermal barriers observed on high pressure turbine blades of military engines removed from service. The CMAS/TBC interactions are mainly observed in the hottest zones of the blade pressure side. CMAS infiltration in the TBC porous microstructure (inter-columnar gaps, pores, cracks) down to the thermally grown oxide TGO interfacial layer is observed as well as dissolution of the 8YPSZ into the CMAS melt, TBC transformation from tetragonal 8YPSZ to monoclinic Y-depleted zirconia and formation of a Zr-bearing phase at the interface between CMAS and TBC. CMAS not only turns out to be mainly constituted of CaO, MgO, Al 2 O 3 , and SiO 2 but also contains a large amount of iron oxide Fe 2 O 3 . Comparison with previous published data shows that CMAS composition depends on the flight conditions to a large extent. A part from the loss of column integrity and the modification of the porous morphology resulting from the CMAS chemical attack, large vertical separations between highly sintered columns are observed in the CMAS infiltrated TBC as well as delamination cracks in the upper part of the top coat possibly leading to progressive TBC spallation. These results are discussed in the light of similar studies on CMAS/TBC interaction and of existing sintering and delamination mechanisms.

High temperature hot corrosion behaviour of NiCr and Cr3C2–NiCr coatings on T91 boiler steel in an aggressive environment at 750 °C

Abstract: 80Ni–20Cr and 75Cr 3 C 2 –25(Ni–20Cr) coatings were deposited on T91 boiler tube steel by high velocity oxy-fuel (HVOF) process to enhance high-temperature corrosion resistance. Hot corrosion studies were conducted on bare and HVOF-coated steel specimens after exposure to a molten salt (Na 2 SO 4 –60%V 2 O 5 ) environment at 750 °C under cyclic conditions. Each cycle consisted 1 h of heating in the silicon carbide tube furnace followed by 20 min of cooling in air. The weight change measurements were performed after each cycle to establish the kinetics of corrosion using thermogravimetric technique. X-ray diffraction (XRD), scanning electron microscopy/energy dispersive X-ray analysis (SEM/EDAX) and X-ray mapping techniques were used to analyse the corrosion products. The bare steel experienced higher weight gain, which may be attributed to the formation of unprotective Fe 2 O 3 dominated oxide scales. The 80Ni–20Cr coating was found to be more protective than the 75Cr 3 C 2 –25(Ni–20Cr) coating. The phases revealed in the oxide scale of the coated specimens were mainly oxides and spinels of nickel and chromium, which are reported to be protective against the hot corrosion.

Encapsulation of triethanolamine as organic corrosion inhibitor into nanoparticles and its active corrosion protection for steel sheets

Abstract: Triethanolamine (TEA), a corrosion inhibitor for zinc and steel, was introduced into nano-sized particles as nanoreservoirs to increase longevity of inhibitive property and prevent degradation caused by direct addition of corrosion inhibitor into coating layer. TEA-incorporated nanoparticles with average particle size around 400–450 nm were successfully synthesized by sequential emulsion polymerization, occupying around 5% of total solid weight of particles during a neutralization process. Encapsulated TEA was released from the capsule inside when the pH level of environment became acidic or alkaline due to an acid–base interaction or ionization of seed material in specific conditions. In the corrosion tests, the encapsulated TEA decreased the corrosion rate of steel substrate owing to its adsorption on steel surface and the resistance of coating layer against corrosive environment was much higher and remained its resistance as immersion time increased when TEA was incorporated in coating layer in the encapsulated form. Based on the scanning vibrating electrode technique (SVET) result, anticorrosive ability of the encapsulated TEA seemed to improve due to the spontaneous passivation of exposed metal on the defected region of coated steel.

A comparative study of the effect of mechanical and ultrasound agitation on the properties of electrodeposited Ni/Al2O3 nanocomposite coatings

Abstract: In our study, Ni/Al 2 O 3 nanocomposite coatings were obtained by electrochemical deposition of alumina nanoparticles from nickel Watts baths with different concentrations of Al 2 O 3 particles in solution. A comparison of the effects of mechanical (300 rpm) and ultrasonic (24 kHz, 38 W cm − 2 ) stirring on the properties of electrodeposited Ni/Al 2 O 3 composite coatings was performed, observing an enhancement of the Al 2 O 3 incorporation in the composite coating and an improvement of the uniformity of particles distribution in the nickel metal matrix when ultrasonic energy was applied to the process. Moreover, the characterization of the coatings also showed that for both types of agitation, the incorporation of Al 2 O 3 nanoparticles led to changes in the morphology and structure of the nickel matrix leading to an improvement of its tribological properties. These properties were further enhanced when ultrasonic stirring was used during the composite coating electrodeposition.

Adaptive NbN/Ag coatings for high temperature tribological applications

Abstract: article i nfo Nanocomposite films that consist of niobium nitride with silver nanoinclusions were created using unbal- anced magnetron sputtering to investigate their potential as adaptive, friction reducing coatings. The coatings were tribotested against a Si3N4 counterface in the 25 to 1000 °C temperature range. The coatings displayed coefficients of friction in the 0.15 to 0.30 range at T>700 °C. Post-wear testing structural and chemical char- acterization revealed that, in the low to mid-temperature range, silver migrated to the surface to reduce friction. At higher temperatures, oxygen, silver and the transition metal reacted to form lubricious binary metal oxide phases (silver niobate) in addition to pure silver. In situ Raman spectroscopy measurements were taken during heating and wear testing at 750 °C to identify the evolution of phases in the coatings surface and in the wear track. The analysis of the in situ Raman spectroscopy data revealed the various stages of formation of these binary metal oxides. The coatings were subsequently doped with MoS2 to investigate the effect of the introduction of a low temperature lubricant. The addition of MoS2 did not appreciably reduce the room temperature coefficient of friction, likely due to the miscibility of this compound with the transition metal nitride. However, the coefficient of friction was significantly reduced at high temperatures because of the synergistic lubricious effect of silver niobates and molybdates.

Anti-corrosion performance of a new silane coating for corrosion protection of AZ31 magnesium alloy in Hank's solution

Abstract: Mg alloys can be used as bioresorsable metallic implants. However, the high corrosion rate of magnesium alloys has limited their biomedical applications. Although Mg ions are essential to the human body, an excess may cause undesirable health effects. Therefore, surface treatments are required to enhance the corrosion resistance of magnesium parts, decreasing its rate to biocompatible levels and allowing its safe application as bioresorbable metallic implants. The application of biocompatible silane coatings is envisaged as a suitable strategy for retarding the corrosion process of magnesium alloys. In the current work, a new glycidoxypropyltrimethoxysilane (GPTMS) based coating was tested on AZ31 magnesium substrates subjected to different surface conditioning procedures before coating deposition. The surface conditioning included a short etching with hydrofluoric acid (HF) or a dc polarisation in alkaline electrolyte. The silane coated samples were immersed in Hank's solution and the protective performance of the coating was studied through electrochemical impedance spectroscopy (EIS). The EIS data was treated by new equivalent circuit models and the results revealed that the surface conditioning process plays a key role in the effectiveness of the silane coating. The HF treated samples led to the highest impedance values and delayed the coating degradation, compared to the mechanically polished samples or to those submitted to dc polarisation.