Dense Coatings Research Articles

Microstructural Evolution of High-Entropy Intermetallic Compounds during Detonation Spraying

This study aims at investigating the feasibility of depositing quality coatings from various high-entropy intermetallic compounds (HEICs) using detonation spraying (DS). Four different HEIC coatings, namely (NbTaVCrTi)Al3, (NbTaVNiFe)Al3, (NbTaVZrHf)Al3, and (FeNiCoCrMn)(MoCr), were prepared by DS on low alloy steel substrates. The HEIC powders were first prepared by arc melting followed by ball milling and then used as reinforcement particles to deposit HEIC coatings. Elemental segregation was observed for all the as-cast samples. Powders with average particle sizes of about ~25 µm for (NbTaVCrTi)Al3, ~22 µm for (NbTaVNiFe)Al3, ~34 µm for (NbTaVZrHf)Al3, and ~18 µm for (FeNiCoCrMn)(MoCr) were obtained. (NbTaVCrTi)Al3, (NbTaVNiFe)Al3, and (NbTaVZrHf)Al3 HEICs exhibited a nearly single D022 (TaAl3 type) structure, while (FeNiCoCrMn)(MoCr) exhibited a single D8b (FeCr type) structure. Dense coatings consisted of a lamellar microstructure and sound bonding with the substrate, and low porosity was obtained for all the samples. Crystal structures of the HEIC samples were highly retained during DS, whereas all the samples underwent some degree of oxidation. Microhardness values of 745 HV for (NbTaVCrTi)Al3, 753 HV for (NbTaVNiFe)Al3, and 862 HV for (NbTaVZrHf)Al3 were obtained, which are significantly higher than the microhardness of the substrate (~140 HV). Among all the samples, (FeNiCoCrMn)(MoCr) exhibited the highest microhardness values of about 1047 HV.

Crack-free wear resistance coating on carbon steel obtained by directed energy deposition of Fe-base partial amorphous powders

Laser Directed Energy Deposition of an Fe-base partial amorphous powder was carried out to study the feasibility of manufacturing partial amorphous coatings without macroscopic defects and high hardness values onto carbon steel substrates for wear resistance applications. Cold cracking can be avoided by preheating the substrate beyond 400 °C. Process parameters optimization can also guarantee fully dense coatings with hardness values above 800 HV. Results from x-ray diffraction correlate the high hardness to the crystallite size of the matrix phase and the presence of hardness enhancer phases, such as borides/carbo-borides and amorphous microstructure. Finally, wear resistance analyses on the coatings revealed an excellent response and confirmed the potential of combining this technology with Fe-base amorphous materials to increase wear resistance.

Surface Characteristics, Properties and Wear Resistance OF TIB2 BASED Hard-Alloy Coatings Obtained by Electrospark Deposition at Negative Polarity on Ti6Al4V Alloy

In the present work, the possibilities of improving the surface properties of titanium alloy Ti6Al4V by electrospark deposition (ESD) at negative polarity were investigated. The coatings were deposited with a TiB2-TiAl-based hard-alloy electrode with nano-sized additives of NbC and ZrO2. The effect of polarity in ESD at low pulse energy (0.01-0.07 J) on the surface roughness, thickness, composition and structure of coatings was studied by profilometric, metallographic, XRD, SEM and EDS methods. In both polarities, dense and uniform coatings were obtained with roughness and thickness, which could be varied by changing the ESD modes within the range Ra=1.5÷4.5 µm, d= 6÷20 µm and microhardness from 9 to 12 GPa, respectively. It was found that the negative polarity coatings obtained are denser and uniform with higher thickness, lower roughness, finer structure, and lower coefficient of friction, and can be successfully used to reduce the surface roughness and defects of 3D printed titanium alloy. Abrasion and erosion wear tests showed that the wear resistance of coated titanium surfaces at both polarities was 2.2 to 3.5 times higher than that of the substrate. Coatings deposited in positive polarity demonstrated higher resistance to abrasive wear, while those in negative polarity are more resistant to erosive wear.

Strategy to Generate Oxide-Free Molten Metal Droplets for Fully Dense Corrosion-Resistant Coatings by Atmospheric Plasma Spraying

Thermal spray techniologies are very efficient to deposit metal coatings, which have been applied to different industrial fields for protection of metals from wear and corrosion. However, severe oxidation during inflight of spray particles introduces large amount of oxide inclusions in the coating which limits lamellar bonding formation and thus full utilization of coating material performance potential. In this project, the spray powders containing the deoxidizers such as carbon and boron are designed to develop in-situ deoxidizing effect to generate oxide-free molten metal droplets by air plasma spraying (APS) in ambient atmosphere for depositing dense coatings with sufficiently bonded splats. The thermodynamic and kinetic conditions for continuous deoxidization during whole in-flight molten droplets are presented. The experiments were conducted for NiCr, NiCrCu, CuNi coatings with boron as deoxidizer and for NiAl and FeAl coatings with dispersed diamond as deoxidizer. Results show that through powder design and spray condition optimization different spray particles can be heated to temperatures from 2100°C to 2500°C. It was found that the oxide contents in all coatings decreases with increasing spray distance, which indicates that deoxidizing effect of deoxidizer is maintained during whole spray particle in-flight. The examination shows that all APS coatings present dense microstructure. The electrochemical test reveals that the corrosion only occurs to coating surface and no trace of corrosive solution penetration into coating. As a result, the corrosion-resistant metal coatings can be realized by APS in ambient atmosphere through developing in-situ deoxidizing effect and subsequently oxide-free molten droplets with using deoxidizer-containing spray powders.

Development of oxidation-resistant and electrically conductive coating of Ti–Al–C system for the lightweight interconnects of solid oxide fuel cells

The paper studies oxidation resistance and electrical conductivity of dense coatings produced by vacuum-arc deposition technique on α-titanium thin (0.1 mm) substrate using a hot pressed Ti2AlC–TiC target. The coatings were deposited at low (7 mA/cm2) and high (15 mA/cm2) current densities on the substrate and marked LCD and HCD, respectively. This provided different local chemical and phase compositions of the coatings. It was found that phase compositions of the coatings differ from that of the target. The HCD coating has high oxidation resistance evaluated in terms of the specific weight gain (Δm/S = 0.06 mg/cm2) as well as high surface electrical conductivity (σ = 1.23·106 S/m) after long-term (1000 h) holding at 600 °C in the air due to the formation of an over thin (450 nm) Ti–Al-(C, O, N) near-surface layer. The thin titanium substrate with such Ti–Al–C coating is recommended as a lightweight interconnect of an intermediate-temperature solid oxide fuel cell.

Plasma-sprayed coatings of 62W bioactive glass: Mechanical properties and in vitro behavior

Bioactive glasses, due to their chemical and biological properties, are promising materials for being used in orthopedic applications. In this study, coatings of the novel 62W glass, with a composition in the system SiO2-CaO-P2O5-MgO, were analyzed. The 62W feedstock powder was prepared from a mixture of reagents by the melt-quenching route and deposited onto titanium alloy substrates by atmospheric plasma spray (APS) technique. The cross-section and the phase of the coatings were analyzed by scanning electron microscopy and X-ray diffraction. Mechanical and in vitro tests were conducted on the obtained coatings to evaluate their bioactive ability. The results of the cross-sections show dense coatings with the absence of the characteristic pores that appear when spraying the usual bioactive glasses. The obtained coatings present a bond strength of 32 ± 3 MPa with the substrate without crystalline phases on the microstructure. The presence of magnesium oxide in the composition favours the glass's processing and increases its crystallization temperature and, thus, the proper stacking of the deposited particles. The bioactive capacity of the 62W coatings was demonstrated at short periods after in vitro evaluation in SBF and TRIS/HCl solutions. The study reveals the potential use of 62W glass coatings for bone implant applications.

Highly-Efficient and Robust Zn Anodes Enabled by Sub-1-µm Zincophilic CrN Coatings.

For exploring advanced Zn-ion batteries (ZIBs) with long lifespan and high Coulombic efficiency (CE), the critically important point is to limit the undesired Zn dendrite and parasitic reactions. Among the coating for electrode is a promising strategy, relying on the trade-off between its thickness and stability to achieve the ultra-stable Zn anodes in ZIBs. Herein, a submicron-thick (≈0.4µm) zincophilic CrN coatings are fabricated by a facile and industry-compatible magnetron sputtering approach. It is exhilarating that the ultrathin and dense CrN coatings with strong adsorption ability for Zn2+ exhibit an impressive lifespan up to 3700h with ≈100% CE at 1mAcm-2 . Along with the experiments and theoretical calculations, it is verified that the introduced CrN coatings cannot only effectively suppress the dendrite growth and notorious parasitic reactions, but also allow the uniform Zn deposition due to the reduced nucleation energy. Moreover, the as-assembled Zn@CrN‖MnO2 full cell delivers a high specific capacity of 171.1mAhg-1 after 1000 cycles at 1Ag-1 , much better than that of Zn‖MnO2 analog (97.8mAhg-1 ). This work provides a facile strategy for scalable fabrication of ultrathin zincophilic coating to push forward the practical applications of ZIBs.

Effect of Si contents on microstructure and mechanical characteristics of TiAlSiN thin film deposited by HiPIMS using different Ti[sbnd]Si target compositions

High Power Impulse Magnetron Sputtering (HiPIMS) PVD deposition technology offers smoother coating enhanced mechanical properties and improved substrate-coating adhesion and has become a superior alternative to direct current magnetron sputtering (DCMS). The technology utilizes high peak power with a small pulse duration, leading to a much denser plasma in the order of 1018 m−3. This results in dense, defect-free, and high-quality smoother coatings. In this study, TiAlSiN coatings were deposited on WC-10Co (wt%) cermet using HiPIMS technology with two different TiSi contents. A pair of TiSi targets, either of compositions Ti77Si23 or of Ti66Si34, were used during the depositions along with another pair of Ti40Al60, in successive cathode-stages. The impact of Ti and Si atomic percentages on the microstructure and mechanical properties of the coatings was investigated using various techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), 3D-surface profiling, nano-indentation, Rockwell indentation-adhesion, and scratch test. The coating's thickness and deposition rate were estimated from the cross-section morphology. The results suggested that the TiAlN phase likely existed in an amorphous phase of Si3N4 in TiAlSiN coatings. As the Ti atomic percentage in the layer increased, the hardness improved, reaching up to 42 GPa (max). Additionally, the coating surface roughness decreased from 14.3 nm to 10.9 nm, and the grain size decreased from 10 nm to 9 nm. The adhesion strength observed under the Rockwell-indentation test was rated as HF1 in both cases. Furthermore, the indentation test revealed similar crack patterns, but an increase in Ti content significantly improved the coating-substrate adhesion, as confirmed by the Scratch test. The scratch test demonstrated a maximum load of 173 N during the complete delamination (Lc3) of TiAlSiN from the carbide substrate.

Fabrication of Cr2AlB2 and Cr4AlB4 MAB Phase Coatings by Magnetron Sputtering and Post-Annealing

Cr2AlB2 and Cr4AlB4 are members of the MAB phases that exhibit unique properties of both metals and ceramics. However, despite these unique characteristics, Cr2AlB2 and Cr4AlB4 phase coatings have not been widely investigated. In this study, Cr2AlB2 and Cr4AlB4 MAB phase coatings were fabricated by magnetron sputtering at room temperature and post-annealing. A composite target, consisting of a phase-pure disc-shaped CrB target overlapped by uniformly dispersed fan-shaped Al slices, was placed parallel to the substrates. The Al content of the coatings was adjusted by altering the areal proportion of the Al slices. MAB phases have crystallized upon post-annealing the as-deposited coatings on Al2O3(0001) substrates in Ar. The phase compositions and morphologies of the crystalline coatings were found to be dependent on the Al content and the annealing temperature. As-deposited coatings with a Cr:Al:B ratio close to 2:1:2 could crystallize as pure and dense Cr2AlB2 phases within the temperature range of 650–800 °C; higher annealing temperatures resulted in the decomposition of Cr2AlB2, while crystallization at lower temperatures was not evident from X-ray diffraction. As-deposited coatings with a Cr:Al:B ratio close to 3:1:3, despite containing a relatively higher Al content than required by the stoichiometry of Cr4AlB4, exhibited insufficient crystallization of Cr4AlB4 with unknown phases below 840 °C. Higher annealing temperatures resulted in the coexistence of Cr4AlB4 and CrB, indicating that achieving phase-pure and well-crystallized Cr4AlB4 coatings proved challenging, possibly due to the inevitable loss of Al during annealing. The configuration of the composite target and the substrates provides a promising strategy for fabricating phase-pure and dense Cr2AlB2 coatings.

Microstructural evolution and properties of He-charged a-Si coatings prepared by magnetron sputtering

The introduction of porosity in nanomaterials via magnetron sputtering in helium atmospheres is an interesting strategy for the design of functional materials. Amorphous silicon coatings, with high concentration of He incorporated in the form of overpressurized nano-bubbles, present modified optical properties in comparison with dense coatings of similar composition.In this work, annealing process in He atmosphere from 300 °C up to 1200 °C is performed to evaluate stability after the release of He from the a-Si:He coatings (thickness ∼1700 nm) and to favor matrix crystallization and defect evolution. The work is focused on these effects on the silicon film and its properties. The composition and microstructural evolution of the coatings annealed to temperatures as high as 1200° were investigated by electron microscopy and X-Ray diffraction. Annealing effects on the optical properties and bandgap of the silicon coatings were evaluated by ellipsometry and UV–Vis spectrometry. Helium release from the coating, densification, defects evolution and ordering due to crystallization have a small effect on the refractive index, but cause a significant change in the material band gap from 1.37 eV to 1.80 eV.

Controlling Superhydrophobicity on Complex Substrates Based on a Vapor-Phase Sublimation and Deposition Polymerization.

The superhydrophobic properties of material surfaces have attracted significant research and practical development in a wide range of applications. In the present study, a superhydrophobic coating was fabricated using a vapor-phase sublimation and deposition process. This process offers several advantages, including a controllable and tunable superhydrophobic property, a dry and solvent-free process that uses well-defined water/ice templates during fabrication, and a coating technology that is applicable to various substrates, regardless of their dimensions or complex geometric configurations. The fabrication process exploits time-dependent condensation to produce ice templates with a controlled surface morphology and roughness. The templates are sacrificed via vapor sublimation, which results in mass transfer of water vapor out of the system. A second vapor source of a polymer precursor is then introduced to the system, and deposition occurs upon polymerization on the iced templates, replicating the same topologies from the iced templates. The continuation of the co-current sublimation and deposition processes finally renders permanent hierarchical structures of the polymer coatings that combine the native hydrophobic property of the polymer and the structured property by the sacrificed ice templates, achieving a level of superhydrophobicity that is tunable from 90° to 164°. The experiments demonstrated the use of [2,2]paracyclophanes as the starting materials for forming the superhydrophobic coatings of poly(p-xylylenes) on substrate surfaces. In comparison to conventional vapor deposition of poly(p-xylylenes), which resulted in dense thin-film coatings with only a moderate water contact angle of approximately 90°, the reported superhydrophobic coatings and fabrication process can achieve a high water contact angle of 164°. Demonstrations furthermore revealed that the proposed coatings are durable while maintaining superhydrophobicity on various substrates, including an intraocular lens and a cardiovascular stent, even against harsh treatment conditions and varied solution compositions used on the substrates.

The Potential of High-Velocity Air-Fuel Spraying (HVAF) to Manufacture Bond Coats for Thermal Barrier Coating Systems

Driven by the search for an optimum combination of particle velocity and process temperature to achieve dense hard metal coatings at high deposition efficiencies and powder feed rates, the high-velocity air-fuel spraying process (HVAF) was developed. In terms of achievable particle velocities and temperatures, this process can be classified between high-velocity oxy-fuel spraying (HVOF) and cold gas spraying (CGS). The particular advantages of HVAF regarding moderate process temperatures, high particle velocities as well as high productivity and efficiency suggest that the application of HVAF should be also investigated for the manufacture of MCrAlY (M = Co and/or Ni) bond coats (BCs) in thermal barrier coating (TBC) systems. In this work, corresponding HVAF spray parameters were developed based on detailed process analyses. Different diagnostics were carried out to characterize the working gas jet and the particles in flight. The coatings were investigated with respect to their microstructure, surface roughness and oxygen content. The spray process was assessed for its effectiveness. Process diagnostics as well as calculations of the gas flow in the jet and the particle acceleration and heating were applied to explain the governing mechanisms on the coating characteristics. The results show that HVAF is a promising alternative manufacturing process.

Microstructural and Tribological Characteristics of Composites Obtained by Detonation Spraying of Iron-Based Alloy-Carbide Powder Mixtures.

iron-based coatings have exhibited good mechanical properties, such as high hardness and good wear resistance, which are desirable properties in applications such as automobile brake rotors. iron-based coatings are also good replacements for Co- and Ni-based coatings, which are costly and could have health and environmental concerns due to their toxicity. In this research, three different iron-based coatings were deposited using the Detonation Gun Spraying (DGS) technology onto aluminum substrates, including the steel powders alone (unreinforced), and steel powders mixed with Fe3C and SiC particles, respectively. The microstructural characteristics of these coatings and mechanical properties, such as hardness and wear resistance, were examined. The morphology and structure of the feedstock powders were affected by the exposure to high temperature during the spraying process and rapid solidification of steel powders that resulted in the formation of an amorphous structure. While it was expected that steel particles reinforced with hard ceramic particles would result in increased hardness, instead, the unreinforced steel coating had the highest hardness, possibly due to a higher degree of amorphization in the coating than the other two. The microstructural observation confirmed the formation of dense coatings with good adhesion between layers. All samples were subjected to ball-on-disk wear tests at room temperature (23 °C) and at 200 °C. Similar wear resistances of the three samples were obtained at room temperature. At 200 °C, however, both ceramic reinforced composite samples exhibited higher wear rates in line with the reduction in their hardness values. This work explains, from the microstructural point of view, why adding hard particles to steel powers may not always lead to coatings with higher hardness and better wear resistance.

Fabrication of fuel electrode supported proton conducting SOFC via EPD of La2Ce2O7 electrolyte and its performance evaluation

The proton conducting La2Ce2O7 electrolyte has already been shown to be chemically more stable to H2O and CO2 than BaCeO3 based electrolyte materials. It has become widely popular as an alternative electrolyte for low to intermediate temperature operation in solid oxide fuel cells. Few initial studies have reported the fabrication of LCO-based fuel cells using the co-pressing process. Here, we present an alternative, simple, cost-effective and efficient method for the deposition of uniform and dense La2Ce2O7 electrolyte coatings, which is a key requirement for any fuel cell fabrication process. In this work, La2Ce2O7 is synthesized by a conventional solid state reaction route and we have established the process of fuel cell fabrication followed by its investigation for SOFC application as electrolyte material. Different alcohols (e.g. butanol, ethanol, acetone, acetyl acetone and isopropanol) are used as dispersion medium, the suspension chemistry and subsequent coating of LCO powders by electrophoretic deposition are investigated. A thin and good quality coating of about 11 μm thickness is obtained in an ethanol-based suspension under deposition conditions at 50 V for 2 min. Fuel electrode supported half cells with LCO electrolyte fabricated by electrophoretic deposition and performance test of fabricated cell is evaluated in moist H2 (∼3% H2O) as reducing medium and static air as oxidizing medium.

Diffusion behaviour and corrosion rate of rare earth monosilicate-based EBCs under CMAS exposure

The thermochemical corrosion behaviour of different rare earth (RE)-based (Yb, Lu and Er) environmental barrier coatings (EBCs) between 1 and 48 h in low calcium–magnesium–aluminosilicate (CMAS) load environments at 1300 °C was studied. The EBCs were coated on reaction-bonded SiC–B4C substrates using the dip-coating method. Each coating is composed of a dense RE2SiO5-based bottom coat and a porous RE2O3 topcoat. The microstructures and compositions of the corroded samples were examined using scanning electron microscopy and energy dispersive spectroscopy. CMAS infiltrated the topcoat through pores and cracks, settled in these defects, and dissolved in the neighbouring coating areas to precipitate as a Ca–Mg–Si–RE garnet. The diffusion coefficients of Ca and Si in the CMAS–EBC corroded specimens were calculated. Yb and Lu silicates have the lowest diffusion coefficient values, whereas Er silicate has the highest values. The formed garnet filled the pores and inhibited further diffusion of corrosive species in the coat. The corrosion rates were calculated based on the physical properties of the coat and found to be lower than those of coatings produced using atmospheric plasma spraying. The diffusion coefficient and corrosion rate were successfully used to evaluate various corrosion-resistant coatings. The results show that single-layer dip-coated Yb silicate is a cost-effective CMAS-resistant RE-based EBC.

The impact of binding energies on the necessary conditions in aerosol deposition

AbstractAerosol deposition (AD) enables the formation of dense ceramic coatings by high velocity impact of submicron‐sized particles. However, up to now, it is still not clear how the material properties of the ceramic powder particles influence their impact behavior and possible success in layer build‐up in AD. Therefore, in order to provide a broader understanding, this study utilizes molecular dynamic (MD) simulations to investigate the impacts of single‐crystalline particles while manipulating binding energies, particle sizes, and impact velocities, addressing a rather wide range of different materials and process conditions. The findings reveal that increasing binding energies from 0.22 to 0.96 eV necessitates up to three times higher velocities to reach thresholds for bonding and fragmentation, which are linked to potential layer formation. For conditions above the velocity thresholds given by individual binding energies, similarities in the deformation and fragmentation patterns are derived. Consequently, rough estimations regarding the required particle impact velocities for AD of different materials can be inferred.

Preparation of highly dense Ti-based coatings with enhanced corrosion protection performance via cold spray

In the current study, relatively large TC4 powder (D50: 35.1 μm) were blended with Ti powder (D50: 24.2 μm) in different mixing fractions (0, 10, 30, 50, 100 vol%) to prepare corrosion protection Ti-based coatings on 1045 steel substrates via cold spray under moderate ‘intensity’ deposition condition. The idea is to utilize the strong hammering effect of hard TC4 particles to significantly densify the originally loose Ti layer, while the inclusion of TC4 particles after deposition will not cause severe galvanic problem with Ti due to similar free potential in corrosive medium, which avoids the deterioration of the corrosion resistance of Ti-based coating. Results show that cold sprayed Ti-based coating with high density (99.8 %) was achieved when the mixed powder contains 30 vol% TC4 particles. Moreover, only a slight decrease trend in deposition efficiency of the mixed powder is shown with the increase of TC4 content. Electrochemical tests and cross-sectional microstructural observations indicate that this coating is not permeable during the entire 24 h immersion in 10 wt% HCl solution, thus provided excellent corrosion protection for the substrate. The present method of cold spraying Ti and TC4 mixed powders exhibits great potential for economical and effective preparations of Ti-based corrosion protection coatings.

Effect of Reaction Temperature on the Molten Salt Electrodeposition of Silicide Coating on Molybdenum Substrate and its Oxidation Behavior

In this study, silicide coatings were successfully prepared on Mo substrates using NaCl-KCl-NaF molten salt as the reaction medium and K2SiF6 as the silicon source. The electrochemical reduction mechanism of Si(IV) was studied by cyclic voltammetry and chronoamperometry technology. The effect of reaction temperature on the phase composition, microstructure, and growth kinetics of the electrodeposited coatings were investigated in detail. The long-term oxidation behaviors of Mo and MoSi2-coated Mo samples at 873 K were also compared. The results revealed that the electrodeposition of silicide coatings was closely related to the reaction temperature. Dense and coherent single-layer MoSi2 coatings could be obtained at 1023 K and 1073 K, while double-layer coatings with inner MoSi2 layer and outer Si layer could be obtained at 1123 K. Under the same electrodeposition time, the overall thickness of the silicide coatings increased with the reaction temperature. Moreover, long-term oxidation tests at 873 K proved that MoSi2 coatings could effectively prevent the Mo substrate from oxidation. The mass gain of the MoSi2-coated Mo sample was only 0.46 wt% after 500 h of oxidation, which could be attributed to the formation of the continuous SiO2 protective layer.

Effect of process parameters on the porosity in laser-directed energy deposition of Al2O3 reinforced Inconel-based composite coating

Metal matrix composites (MMC) possess an excellent combination of mechanical and metallurgical properties but are difficult to process. Laser-directed energy deposition (DED) technique can be effectively employed to fabricate MMC coatings, producing dense coatings with sound metallurgical bonding. In this investigation, Inconel 718/ alumina (Al2O3) MMC coating is manufactured using powder-fed laser DED process. At first, different wt. % of alumina particles (i.e., 1, 3, 5, and 7 wt%) were mixed with Inconel 718 powder, and after that, four different types of MMC coatings were fabricated. By analyzing the mechanical properties and microstructure of those MMC coatings, the optimum amount of Al2O3 in MMC coating is determined. It is observed that IN718 −5 wt% Al2O3 composite coating exhibits better mechanical properties (i.e., hardness = 625 ± 6 HV), so further experiments were conducted with this coating. In the second phase of this investigation, the effect of laser power, scanning speed, and powder feed rate on porosity is analyzed. Beyond a laser power of 1700 W, porosity increases significantly due to the formation of gas porosity. On the other hand, beyond a scanning speed of 500 mm/min, the energy input is insufficient to melt the powder, which leads to the enhancement of lack-of-fusion porosity. With an increase in the powder feed rate, the lack of fusion porosity grows due to a higher laser power attenuation. The mid-range of laser power and scanning speed and lower range of powder feed rate are suitable for creating MMC coatings with better mechanical properties.

A Study on Long-Term Oxidation and Thermal Shock Performance of Nanostructured YSZ/NiCrAlY TBC with a Less Dense Bond Coat.

This paper focused on studying the performance of a nanostructured thermal barrier coating (TBC) system deposited by APS, which had a bond coat with inter-lamellar porosities that resulted during the manufacturing process. The higher porosity level of the bond coat was studied as a possible way to keep the thickness of the TGO under control, as it is distributed on a higher surface, thereby reducing the chance of top-coat (TC) spallation during long-term oxidation and high-temperature thermal shock. The TBC system consisted of nanostructured yttria partially stabilized zirconia (YSZ) as a top coat and a conventional NiCrAlY bond coat. Inter-lamellar porosities ensured the development of a TGO distributed on a higher surface without affecting the overall coating performance. Based on long-term isothermal oxidation tests performed at 1150 °C, the inter-lamellar pores do not affect the high resistance of nanostructured TBCs in case of long-term iso-thermal oxidation at 1150 °C. The ceramic layer withstands the high-temperature exposure for 800 h of maintaining without showing major exfoliation. Fine cracks were discovered in the ceramic coating after 400 h of isothermal oxidation, and larger cracks were found after 800 h of exposure. An increase in both ceramic and bond-coat compaction was observed after prolonged high-temperature exposure, and this was sustained by the higher adhesion strength. Moreover, in extreme conditions, under high-temperature thermal shock cycles, the TBC withstands for 1242 cycles at 1200 °C and 555 cycles at 1250 °C.