Dense Coatings Research Articles

Tribological Study of HVOF Sprayed Tungsten Carbide Coated Stainless Steel

Corrosion and Wear, or a combination of both, are the main causes of degradation of metals used in the various industrial sectors. Degradation of the metals can be slowed down by adding a layer of resistant coating on the metal surface. This coating separates the base metal from a corrosive environment, reduces wear, and improves the appearance of the metal. The workpiece after coating becomes a composite that has properties far better than the substrate alone. There are various coating techniques, HVOF is one of the most important and widely used processes to protect the metals from wear, corrosion by providing hard and dense coatings. WC coating done by the high-velocity oxy-fuel (HVOF) spray method is the widely used method for providing a layer of corrosive resistance to a wide range of materials that are used in many different industries. In this study, Tungsten carbide (WC-12CO) Coating, HVOF Spray method was studied in great detail. Tungsten Carbide coating was done on a SUS400 Stainless steel substrate using HVOF Spray Process. An, Experiment was done to analyze the various effect of different parameters namely, oxygen rate, propane (fuel) rate, powder rate, spray distance on hardness, and surface roughness of a SUS 400 Stainless Steel substrate. Process optimization was done by using Taguchi and ANOVA methods. It was found that achieving maximum hardness was greatly dependent on propane (fuel) rate followed by powder rate, spray distance, and oxygen rate. The hardness decreases with the increasing fuel rate. And, achieving minimum surface roughness was greatly dependent on spray distance followed by oxygen rate, propane (fuel) rate, powder rate. Surface Roughness increases with increasing spray distance.

Corrosion behavior of cold sprayed 7075Al composite coating reinforced with TiB2 nanoparticles

Cold spray (CS), as a novel surface treatment technique, has been widely used for repairing damaged components and as protecting coatings, especially for the applications of aircraft and marine components. To further improve the mechanical properties, while maintaining the corrosion resistance of pure Al and Al alloy coatings, ceramic particles are commonly added into the Al matrix to produce particle-strengthened Al matrix composite (P-AMC) coatings. In this study, dense 7075Al composite coatings reinforced with uniformly distributed in-situ TiB2 nanoparticles (hereafter named TiB2/7075Al) were successfully produced by CS using a gas-atomized composite powder and the propulsive gases of air and helium. Following this, the corrosion behavior of the cold sprayed (CSed) TiB2/7075Al composites was investigated using Tafel polarization, electrochemical impedance, spectroscopy, and immersion tests in 0.1 M and 0.6 M NaCl solutions, and were compared to those of the CSed pure 7075Al coating and bulk 7075Al-T6 material. Electrochemical tests revealed that the composite coating exhibits a higher corrosion rate than the pure 7075Al coating, primarily caused by a galvanic coupling between TiB2 nanoparticles and the more active Al matrix. Compared to the air-processed coatings, the He-processed ones exhibit higher densities of defects, like dislocations, and precipitates due to larger plastic deformation of the sprayed particles, which result in more active sites for corrosion and thus lower corrosion resistance. Low-temperature annealing treatment (230 °C/6 h) reduces these defects to increase the corrosion resistance of the CSed coatings. Comparatively, high-temperature annealing treatment (412 °C/4 h) results in the coarsening of precipitates and grain growth to increase the corrosion rate of the coatings. Corrosion mechanisms of the CSed and annealed composite coatings related to the microstructure evolution were investigated in detail.

Dense Niobium Oxide Coating on Carbon Black as a Support to Platinum Electrocatalyst for Oxygen Reduction

AbstractCarbon black (CB) is commonly used to support Pt as an electrocatalyst in fuel cells. However, it is easily corroded in electrochemical reactions, such as in the oxygen reduction reaction (ORR), leading to catalyst degradation. In this paper, we report results of protecting the CB using an ultrathin 5 nm film of niobium oxide conformally coated on the CB using a new coating technique. Electrochemical test in ORR shows only a 1.7 % activity loss after 5000 cycles, demonstrating an excellent durability of the electrocatalyst. Compared to the electrocatalyst without niobium oxide coating, it shows a 25 mV improvement in half‐wave potentials, indicative of a better kinetics. A positive shift in binding energy was found in Pt 4 f, implying electron delocalization has occurred when Pt is interfaced with the niobium oxide support. The activity enhancement is attributed to the electronic structure change in the electrocatalyst as a result of metal‐support interactions.

Microstructure and wear performance of alumina/graphene coating on textured Al2O3/TiC substrate composites

The incorporation of graphene-based nanodispersoids in ceramic coatings is known to enhance the wear behavior of these materials. In this study, for the first time, alumina/graphene nanoplatelets (GNPs) coatings were deposited through Suspension High Velocity Oxy-Fuel (SHVOF) thermal spray on Al2O3/TiC substrate composites, which were laser microtextured with grid and line surface patterns. Dense coatings with around 8−10 μm thickness were obtained by spraying alumina suspension containing 1 wt. % GNPs. The tribological performance of the alumina/GNPs coatings on the Al2O3/TiC substrate composites was evaluated using ball-on-disc sliding wear tests. The alumina/GNPs coated Al2O3/TiC substrate composites textured with grid pattern showed the best tribological performance reaching a steady coefficient of friction (CoF) at lower sliding distances and presenting the lowest wear rate of the Al2O3/TiC substrate composites and the counterbody.

Low thermal conductivity in high-entropy rare-earth pyrosilicate solid-solutions for thermal environmental barrier coatings

Dense thermal environmental barrier coatings (TEBCs) with low thermal conductivities are needed for the protection and insulation of SiC-based ceramic matrix composites (CMCs) in future gas-turbine engines. Here we demonstrate experimentally that the high-temperature thermal conductivity of dense β-Yb2Si2O7 pyrosilicate ceramic can be reduced progressively in β-(Y0.1Yb0.9)2Si2O7 and β-(Y0.5Yb0.5)2Si2O7 solid-solutions for TEBC application. We also demonstrate that by going to a dense five-component equi-atomic single-phase β-(Y0.2Yb0.2Sc0.2Gd0.2Lu0.2)2Si2O7 pyrosilicate high-entropy ceramic (HEC), the thermal conductivity can be reduced further to 1.52 W.m.K−1 (at 1000 °C). This is attributed to enhanced phonon scattering due to the increased mass-disorder with the incorporation of the additional RE3+ cations in the HEC. Although the high-entropy aspect and bonding-disorder do not appear to contribute significantly to the phase stability or the low thermal conductivity in such HECs, they are strong candidates for future TEBCs for CMCs.

Gamma-phase Zn-Ni alloy deposition by pulse-electroplating from a modified deep eutectic solution

This article describes the electroplating of ZnNi alloy from a modified deep eutectic solvent (DES), a mixture of choline chloride (ChCl) and ethylene glycol (EG) commonly known by its commercial name Ethaline. In this study the Ethaline was modified with propylene carbonate (PC) to decrease the solution viscosity. Boric acid was also used as an additive to improve surface finish and adhesion. The modifications were shown to increase the reduction rate of metal ions through improved mass transport and also to improve the quality of finish (morphology and interfacial adhesion) of the coating. We demonstrate that it is possible to produce dense, thick and adherent coatings of a γ-phase ZnNi alloy with 81–85% Zn on mild steel substrates using either potentiostatic deposition or controlled current pulse-plating techniques. Mild steel is a typical substrate for a sacrificial anti-corrosion coating used in many applications where the alloy serves to protect the steel from corrosion in harsh environments.

Comparisons of plasma-sprayed and sputtering Al0.5CoCrFeNi2 high-entropy alloy coatings

In this study, the Al0.5CoCrFeNi2 high entropy alloy (HEA) powders fabricated via vacuum gas-atomization were applied to deposit two different coatings by plasma-spraying and sputtering processes. The as-atomized powders possessed an initial status of single face-centered cubic (FCC) phase, uniform element distribution and spherical shape. Further, the powders were plasma-sprayed onto stainless steels with various spraying powers. The results showed that a higher current could induce the powders fully melted, resulting in a dense coating layer and fewer defects such as voids and unmelted powders. While the current raised from 500 to 750 A, the element distribution kept uniform throughout the coating layer and the phase constitution stayed at the initial condition of pure FCC phase. The hardness of dense spraying coatings exhibited an increasing trend from HV206 to HV271 with spraying currents. In addition, the atomized Al0.5CoCrFeNi2 powders were sintered to get a homogeneous sputtering target for thin-film deposition process. The analyses of sputtering coatings still revealed that the phase constitution of pure FCC was corresponding to the raw powders. The density and deposition rate were affected by the working pressure and substrate bias. High pressure would cause energy loss, resulting in the lower density. Moreover, the substrate bias could increase the diffusion rate of metal ions to achieve the thin films with high hardness. The coating prepared by these two methods possessed the same phase constitution, which indicated that Al0.5CoCrFeNi2 high entropy alloy could exhibit excellent stability in high-energy occasions.

Coat color and morphological hair traits influence on the mechanisms related to the heat tolerance in hair sheep.

The principal focus of this study was to describe how the coat characteristics could affect the heat exchanges in animals managed in a hot environment. The Morada Nova ewes were monitored once a month, during 10 consecutive months, in three commercial flocks. Initially, an analysis was performed to measure the differences regarding the coat color in the thermoregulation mechanisms. The animals were grouped into 4 different groups according to coat tonality, as follows: dark red animals (group 1, N = 23), intermediate red color (group 2, N = 27), light red animals (group 3, N = 30), and white-coated animals (group 4, N = 30). The data were collected from 1100 to 1400 h, after the animals were exposed to 30 min of direct sunlight. The cluster analysis was performed considering the hair structural characteristics such as coat thickness (CT, cm), hair length (HL, mm), hair diameter (HD, m), and number of hairs (NH, hairs per unit area), after that these clusters were compared in relation to thermoregulatory mechanisms that include rectal temperature (RT, °C), respiratory rate (RR, breaths min-1), cutaneous evaporation (CE, °C), and respiratory evaporation (RE, W m-2). The groups were characterized and compared using mean and standard deviation, and the differences between the clusters were compared using the Tukey test with a 5% probability of error. In relation to coat color, no differences were found in groups 1, 2, and 3 regarding the activation of the thermoregulation mechanisms. The most different was observed in the totally white coat that presented different thermoregulatory responses as the highest sweating rate. White-coated animals showed a non-pigmented epidermis, and the hair structure is responsible to promote skin protection as necessary, such as a dense coat (1242.7 hair cm-2), long hair (14.2 mm), and thicker coat (7.38 mm). In red-coated animals, the hair structure favored heat loss to the environment, such as short hairs, less thick coat, and less hairs per square centimeter. All evaluated animals showed the rectal temperature within the reference limits for the ovine species, regardless of the coat color. In the analysis of clusters related to the physical structure of hair, it was possible to observe that the animals with thick hair, short hair, and less dense coat tended to have a higher capacity to eliminate heat through their respiratory rate and showed less intense heat loss by cutaneous evaporation. We verified that coat color presents a direct influence on the hair structure and the activation of mechanisms related to thermoregulation.

Design of a PLD–grown Y2O3 protective barrier for fusion relevant applications

Advanced and innovative solutions are currently under investigations in order to solve the long-standing issues of materials constituting the breeding blanket of a future fusion reactor. Tritium permeation through steels, corrosion perpetrated by the breeding media, magnetohydrodynamic (MHD) effects and intense neutron irradiation hinder the realization of this technology. Recently, the employment of protective nanoceramic coatings has been considered as one of the few viable solutions. In particular, the pulsed laser deposition (PLD) technique allows to produce dense, compact and uniform coatings, which are the fundamental characteristics when designing a protective barrier. Yttrium oxide (Y2O3) is one of the most promising candidates given its superior stability when facing lithium-based heavy liquid metals and also its low nuclear activation yield, thus it was selected as subject of study and characterzation. PLD-grown Y2O3 coatings were then tested in order to assess their feasability as protective barriers at blanket relevant conditions. Basic characterization of the as deposited and annealed (450 °C) films was perfomed (scanning electron microscopy, atomic force microscope, grazing incident angle x-rays diffraction, nanoindenation). A coated sample was also exposed to static Pb-16Li at.% for 1000 h, showing optimal corrosion resistance and thermal stability at high temperature (550 °C). Finally, the electrical resistivity of this material was measured (up to 200 °C) in order to evaluate its ability to mitigate the MHD detrimental effects. To conclude, a preliminary characterization of the PLD-grown Y2O3 coatings proved the possibility to employ them as multifunctional protective barrier to enable the accomplishment of the breeding blanket technology.

A suspension high velocity oxy-fuel thermal spray manufacturing route for silicon carbide – YAG composite coatings

Silicon carbide (SiC) coatings obtained through thermal spraying represent a major engineering challenge due to the high tendency of SiC to decompose during the spraying process. In order to avoid this decomposition, suspension high velocity oxy-fuel (SHVOF) thermal spray was used in combination with a novel SiC powder modified by addition of yttrium aluminium garnet (YAG) as the feedstock. The colloidal stability of the feedstock in water was optimised in order to obtain stable and well-dispersed suspensions suitable for SHVOF spraying. Dense SiC-YAG coatings with ~80 μm thickness were obtained and the coatings showed promising results in terms of porosity, microstructure, phase distribution and mechanical properties. Energy dispersive X-ray spectroscopy and X-ray diffraction showed a lack of degradation of the SiC phase in the coatings, thanks to the formation of a molten YAG phase during the spray process, which prevents SiC decomposition.

Study on the formation of Mn-P coatings with significant corrosion resistance on Q235 carbon steels by adjusting the ratio of phosphorus to manganese

The Mn-P coating prepared by changing the phosphorus-manganese ratio significantly improves the corrosion resistance of Q235 carbon steel. Surface morphology, composition of the coating was analyzed by scanning electron microscopy (SEM) equipped with energy dispersive spectroscopy (EDS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The corrosion resistance of the coatings was evaluated by potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and salt spray test (SST). Experiments prove that thin and dense coatings can be obtained when Mn2+:PO43− =1:7, the salt spray can withstand more than 30h and there is no obvious corrosion. Finally, the growth mechanism is explained.

Advances in the control of electrophoretic process parameters to tune the ytterbium disilicate coatings microstructure

AbstractSuspensions of ytterbium disilicate in isopropanol were prepared using iodine dispersant. Their zeta potential, electrical conductivity, and pH dependence with iodine concentration is detailed. Electrophoretic deposition was performed on silicon substrates at various voltages (100‐200 V) and times (until 10 minutes) and the growth dynamic was investigated. It was observed that the deposited mass reaches a maximum value for [I2] = 0.2 g/L, and the coating microstructure becomes porous at higher iodine concentrations. Current density and voltage measurements allowed to correlate this behavior to the increase of free protons concentration in the suspension. In these conditions, it was proved that porosity increases with the increase in applied voltage, and a compaction occurs as the deposition time increases. This has been related to the coating resistance increase and subsequent decrease in effective voltage in the suspension. The denser coatings (20% of porosity) were obtained in the case of suspension without iodine, at the minimum applied voltage and for the longest deposition times.

Very low-pressure plasma-sprayed dense yttria-stabilized zirconia coatings using an axial bi-cathode plasma torch

In order to increase the plasma power and enhance the heating of sprayed powder, a bi-cathode plasma torch with axial powder injection was developed for the very low-pressure plasma spraying (VLPPS). By using this torch, uniform and dense yttria-stabilized zirconia (YSZ) coatings were prepared at different spraying distances. The plasma characteristics, microstructures and mechanical properties of YSZ coatings were investigated in detail. Results show that the new plasma torch had the ability to vaporize part of the YSZ powder. Meanwhile, the YSZ powder underwent a distinct phase change, at a maximum of 50% monoclinic zirconia phase transformed to the tetragonal-prime zirconia phase. The sprayed coatings had hybrid microstructures, including unmelted particles, molten droplets, and vapor clusters. With increasing spraying distance, the melting state of YSZ coating got better, while the microstructure became looser, leading to the porosity increased from 2.7% to 7.4%. The best mechanical properties were found in the densest coating, whose microhardness and Young's modulus was 11.7 ± 0.6 GPa, 167.8 ± 4.0 GPa, respectively, indicating that the mechanical performance of the YSZ coating was more dependent on the density, rather than the melting state. The dense VLPPS ceramic coatings may be applied for corrosion and heat resistance against oxidation, as well as the solid oxide fuel cells.

Effect of YSZ sol-gel coating on interaction of Crofer22 APU with sealing glass for solid oxide fuel/electrolysis cell

Interactions between the metal interconnect and glass sealant in the solid oxide cells stack are critical to retaining the system integrity. To address this issue, in the current study, 8YSZ coating is applied on the Crofer22 APU steel via sol-gel and dip-coating method. Dense and continuous 8YSZ coatings are obtained on Crofer22 APU substrates. X-ray diffraction and Raman spectroscopy results show that both cubic and tetragonal structures of 8YSZ are formed. A glass-based sealing paste is applied on bare and 8YSZ coated Crofer22 APU steel. The interfacial compatibility and chemical stability of steel/glass and steel/8YSZ/glass joint couples are investigated using SEM and EDS line scan analysis after the sealing process as well as 1000 h (10 thermal cycles) aging at 800 °C in air atmospheres. Very good compatibility is found between the coated steel and the glass. No significant interactions, cracks formation, or failure are observed at both steel/8YSZ and 8YSZ/glass interfaces, suggesting that the YSZ protective layer can effectively act as a barrier to interdiffusion of metal and glass elements. Finite element analysis shows that the generated residual stresses in the presence of the coating layer are not significant to be detrimental for the examined interfaces.

Control of the Mg-Treated Iron Casting Skin Formation by S-Diffusion Blocking at the Metal–Mould Interface

Having established that sulphur presence in the mould materials appears to have an important contribution in graphite degeneration at least in the casting surface layer, a research program is undertaken to explore the possible beneficial effect of sulphur diffusion blocking at the metal–mould interface. Test samples, with and without a thin steel sheet (up to 3 mm thickness) application on the inner surface of the mould cavity, before iron melt pouring, are considered for structure analysis. A higher nodulizing potential (0.048% Mgres, 0.015% Ceres, and 0.006% Lares) decreases the occurrence of surface graphite degeneration in castings obtained in rigid chemically bonded resin sand moulds, using P-toluol sulfonic acid (PTSA) hardener (S-including), but it is not enough to avoid this phenomenon (200–400 μm skin in present experimental conditions). The casting skin appears to have different values, depending on the evaluation technique (un- and Nital-etching direct measurement, or graphite parameters variation on the casting section). In the presence of a thin steel sheet at the metal–mould interface, the casting skin thickness decreases or is just excluded. It is supposed that it acts as a barrier, blocking S-diffusion from the mould media into the iron melt. Without this S-diffusion, the graphite degeneration in the casting surface layer could be avoided, or at least diminished. For industrial application, the increasing of residual content of nodulizing elements is a limited solution, and it is recommended to use barriers to block S transfer on the mould/metal surface, such as dense coatings or coatings with desulphurization capacity.

Contribution in optimization of Zn Cold-sprayed coating dedicated to corrosion applications

Cold spray coatings elaboration requires optimization of numerous parameters in order to reach the best coatings' performances. A systematic approach can be avoided by using an experimental design. A Doehlert uniform shell design was applied in this work to Zn cold sprayed coating. Coatings were produced on steel substrates to provide high corrosion resistant layers. N2 temperature and gas pressures varied from 200 °C to 320 °C and from 2.0 MPa to 3.0 MPa, respectively. Performance maps were plotted for two descriptors: (i) the interfacial porosity volume (substrate coating interface defects), (ii) coating thickness (i.e., coating durability), and validated with the polarization resistance (i.e., corrosion resistance). Results reported three set of optimized temperature and gas pressure values according to the descriptor.Two optimized protection modes can be obtained: a sacrificial mode from heterogeneities in the coating (290 °C and 3.0 MPa), and a barrier protection mode from homogeneous and dense coatings (320 °C and 2.5 MPa). Mechanism of corrosion has been investigated.

Effect of thermal energy on the deposition behaviour, wear and corrosion resistance of cold sprayed Ni-WC cermet coatings

Cold spraying is emerging as one of the versatile coating techniques applicable in vast variety of applications ranging from overlay coatings to structural repairs. Though variety of metals and their alloys have been successfully deposited using this technique, deposition of cermet with high volume percentage of carbide particulates is still challenging. Existing reports reveal that the deposition of cermet coatings was obtained from blend of metal (binder) and ceramic (particulates) powders. Understanding the deposition behaviour of commercially available cemented carbide in cold spraying has technical significance since these cermets have huge industrial potential to protect components from wear and corrosion. Deposition of cermet using cold spray has a challenge since increase in particle velocity not only increases the plastic deformability of binder phase but also reduces the particulate phase in coatings since the supersonic velocity leads to fracturing and shattering of particulates. State of binder and particulate in the feedstock plays a crucial role in successful deposition. In this work, two different Ni-WC cemented carbides were deposited and comprehensively characterized for inter-splat bonding state and powder state and the same is correlated with the mechanical and corrosion performances of the coatings. The role of thermal energy in the coating formation is studied and discussed in detail. Along with a proper feedstock, engineered nozzle design to enhance the thermal energy input to the feedstock is required to obtain dense cemented carbide coatings with high particulate retainment.

Heat-Treatment of Aluminium-Nickel Composite Cold Sprayed Coating

Intermetallic compounds, especially aluminides, show good high-temperature strength, oxidation resistance, high melting points, and thus have received considerable attention as potential substitutes for superalloys in high-temperature applications. Aluminides are especially interesting because they are stable up to the critical temperature of ordering, which is close to the melting temperature. In the Al-Ni system, the most studied intermetallics are Ni3Al, NiAl and NiAl3. In the presented study, Al and Ni powders were mixed together with Al2O3 in various proportions to produce dense coatings by low-temperature cold spraying. Two types of post-deposition treatments were applied to produce aluminides, namely furnace heating and resistance spot welding. The former caused a long time diffusion while the latter a self-propagating high temperature synthesis. Both heating methods enabled formations of intermetallic phases. However, the furnace heating provides high porosity. The microstructure of the samples was analyzed by SEM (scanning electron microscope), EDS (energy dispersive X-ray spectroscopy) and XRD (X-ray diffraction) together with microhardness measurements.

Microstructure, surface stress and surface temperature response of ZrB2–SiC based coatings

SiC/ZrB2–SiC, SiC/ZrB2–SiC/SiC, SiC/ZrB2–SiC/ZrB2 and SiC/ZrB2–SiC/ZrB2/SiC coatings were prepared on graphite by pack cementation and chemical vapor deposition (CVD). The microstructure, surface stress, ablation resistance and surface temperature response during ablation of these coatings were systematically studied. The results indicated that pack cemented SiC/ZrB2–SiC coating can effectively relax mismatch between graphite substrate and outer dense coatings. The CVD ZrB2 layer with loose structure can further increase the compressive stress of outer CVD SiC layer, and avoid the formation of cracks in SiC layer. The surface temperature response during ablation is mainly affected by coating microstructure evolution and emissivity of different components.

MICROSTRUCTURE AND MICROHARDNESS EVALUATION FOR NiCrAlY MATERIALS MANUFACTURED BY SPARK PLASMA SINTERING AND PLASMA SPRAYING

NiCrAlY deposited by different thermal spraying methods is commonly used as the bond coat material in thermal barrier coatings (TBCs). In the present study, two experimental coatings were deposited by hybrid water stabilized plasma (WSP-H) and radio frequency inductively coupled plasma (RF-ICP) using the same feedstock powder. Spark plasma sintering (SPS) was used to manufacture a compact NiCrAlY from the same feedstock powder as a reference material. Microstructure, internal oxidation, phase characterization and quantification of the mechanical behaviour in terms of microhardness were studied. The investigations clearly showed microstructural and mechanical differences between the NiCrAlY samples manufactured by different plasma technologies. The results confirmed that SPS and RF-ICP provide dense structures with no oxides due to the fabrication under protective atmosphere and similar mechanical properties. Thus, RF-ICP may be used for deposition of very dense coatings with microstructure and hardness comparable to compacted materials prepared by SPS.