Air Plasma Spray Technique Research Articles

Microstructural evaluation and thermal oxidation behaviors of YSZ/NiCoCrAlYTa coatings deposited by different thermal techniques

In this paper, two coating techniques, the high velocity oxy-fuel (HVOF) and air plasma spray (APS) techniques, were used to deposit a bond coat of NiCoCrAlYTa on the Inconel 625 substrate, followed by applying a topcoat of yttria-stabilized zirconia (YSZ). The samples were preoxidized in an argon-controlled furnace at a temperature of 1000 °C for 12 and 24 h to characterize the microstructure of a thermally grown oxide (TGO) using the two coating techniques. The most suitable preoxidized samples were further tested for isothermal oxidation at 1000 °C for up to 120 h, and a hot corrosion test was performed at 1000 °C for up to 52 h or until spalling occurred. As-sprayed and oxidized samples prepared with different coating techniques were evaluated in terms of their microstructure using different characterization methods, such as field emission scanning electron microscopy (FESEM), variable pressure scanning electron microscopy (VPSEM), energy dispersive X-ray spectroscopy (EDS) equipped with energy dispersive X-ray and X-ray diffraction (XRD) analyses. In addition, the mechanical properties of these samples were evaluated using adhesion tests. The results show that the YSZ/NiCoCrAlYTa coating applied with the HVOF technique forms a more thin and continuous layer of TGO than that obtained when applying a YSZ/NiCoCrAlYTa coating using the APS technique, indicating that a severe brittle oxidation interface exists between the two layers. The results also indicate that the mechanical strength obtained from the adhesion test of the coated samples is observably affected by the oxidation behaviors obtained with the different deposition techniques chosen.

Formation of vertical cracks in air plasma sprayed YSZ coatings using unpyrolyzed powder

Thick thermal barrier coatings (TTBCs) with vertical cracks deposited by air plasma spray (APS) and solution precursor plasma spray (SPPS) techniques have been widely investigated to achieve good thermal insulation along with reasonable service life. In this study, synthesized unpyrolyzed YSZ powder was air plasma sprayed in order to produce segmentation crack TTBCs. The microstructure and hardness of the deposits were then compared with those of the conventional TTBCs and dense vertically cracked (DVC)TTBCs. In this regard, spraying parameters were optimized to achieve deposits with the appropriate amount of unpyrolyzed particles in them to assist inducing vertical cracks in the deposited layers. The effect of the unpyrolyzed particles on microstructure, porosity, and microhardness of plasma sprayed coatings were also evaluated and compared. The new fabricated coating showed a bimodal structure combining non-molten sub-micron size particles and conventional splats along with segmentation cracks with higher amount of porosity and lower hardness compared to those of the DVC coatings. The results implied that, depositing unpyrolyzed powder by APS, as a new approach for achieving segmentation crack TTBCs, is very promising.

Hot Corrosion Behaviour of CNT Reinforced Zirconium Yttrium Coatings in Molten Salt Environment

The present work investigates the hot corrosion behaviour of CNT-reinforced zirconium yttrium coatings on T-91 boiler tube steel in a molten salt (Na2SO4–60 wt%V2O5) environment at 600 °C under cyclic conditions. Air plasma spray technique was used for the development of coatings. The un-coated coated and all the as-coated specimens were subjected to hot corrosion exposure in a silicon tube furnace at 600 °C for 50 cycles. The corrosion behaviour was examined through mass gain measurements after the completion of each cycle. The corroded product was examined by XRD and SEM with EDAX analysis. After critical examination it was concluded that the oxide scale of Fe2O3 was formed on un-coated specimen as a result of it suffered from thermal spallation. All the as-coated specimens show the lower mass gains with good adhesiveness of oxide scale with the substrate steel. The CNT-reinforced zirconium yttrium coatings was found to provide better corrosion resistance in the hot corrosion environment due to uniform dispersion of CNT in the coatings matrix and the formation of protective zirconium oxide in the scale.

Erosion studies of plasma sprayed WC-12%Co, Cr3C2-25%NiCr, 80%Ni-20%Cr, 87%Al2O3-13%TiO2 coatings on ASTM A36 steel

WC-12%Co, Cr3C2-25%NiCr, 80%Ni-20%Cr, 87%Al2O3-13%TiO2 coatings were deposited by Air Plasma Spray (APS) technique on ASTM A36 steel. Erosion behaviour of sprayed coated samples were investigated at three different impact angles (45°, 60° and 90°). The mechanism of erosion was investigated on uncoated and coated samples. The rate of erosion of the substrate first increased with increasing angle of impact and then gradually decreased. But the erosion rate of the coated samples increased steadily. Substrate shows the higher metal removal whereas coating shows the higher erosion resistance. WC-12%Co coated samples performed well in erosion testing and offered best erosion resistance as compared to the substrate and all other coatings. Ductile fracture takes place on substrate material at higher impact angle. Overall, all the coated samples have shown less erosion propensity when compare to the bare one.

Effect of Critical Plasma Spray Parameter on Characteristics of Nanostructured Alumina-Titania Coatings

Micro and nanostructured alumina-13wt% titania coatings were sprayed via air plasma spraying (APS) technique using nitrogen as primary plasma gas. Microstructural characteristics and mechanical properties of nanostructured coatings are regularly influenced by percentage of unmelted/partially melted regions. Hence, the coatings were obtained as a function of critical plasma spray parameter (CPSP) defined as the ratio of arc power to the primary plasma gas flow rate. It was watched that for nanostructured coatings properties like porosity, micro hardness and percentage of partially melted/ unmelted regions were seen to be monotonic functions of CPSP. However, influence of CPSP on volume fraction of partially melted regions apparently is much smaller in nanostructured Al 2 O 3 -13wt%TiO 2 coatings deposited using nitrogen as compared to argon as the primary plasma gas

Recent developments in the designing of deposition of thermal barrier coatings – A review

The thermal barrier coatings (TBCs) provide insulation to the components of gas turbine by providing higher amount of thermal energy and also reduce the cooling requirement of the top ceramic surface. The top ceramic coat plays a significant role in surging the efficiency of the TBCs. The 8 wt% Yttria partially stabilized zirconia (YSZ) is the top coat material is widely used because of its ability to withstand aggressive conditions of high temperature and pressure. Air plasma Spraying (APS) technique frequently used in gas turbine industry to deposit 8 wt%YSZ TBCs. There are some important spray parameters which play very important role in the deposition of TBCs such as powder feed rate (g/min), current (A), voltage (V), Power (KW) and standoff/spray distance (mm), and these parameters help to regulate the phase composition, isothermal thermal stability, improved microstructure, corrosion resistance and mechanical properties like residual stresses and bonding strength. Recently new surface modification techniques have been developed such as plasma-enhanced chemical vapour deposition (PECVD), electrostatic spray assisted vapour deposition (ESAVD), and solution precursor plasma spray (SPPS), sol-gel and suspension plasma spraying (SPS). These processes enhance the features of developed coatings which include the formation of vertical cracks by utilizing segmentation process and so on. Furthermore, the effect of varying the deposition process parameters like substrate temperature low plasma gun speed, high passage thickness, contact area between splats have been discussed. This review gives a good understanding about the TBCs and inspires researchers to find new ideas for the improvement in this field.

Fluffy Polyfluoroalkoxy Layer Produced by Air Plasma Spraying Based on “Grapeshot” Effect

Fluoropolymers with low surface energies are widely used in friction reduction, anti-corrosion, and anti-ice coatings. The fabrication of hierarchical micro- and nanostructures with fluoropolymers is beneficial for industrial application. In this study, a superhydrophobic composite coating was prepared by an improved air plasma spraying technique using feedstocks of Cr and polyfluoroalkoxy (PFA). A fluffy structure was found on the surface of the coating. A similar structure could not be found on the surface when the Fe and PFA powders were used as the feedstocks under the same condition. The characterization results suggest that the fluffy layer was derived from PFA. The formation of such a fluffy structure was attributed to the evaporation of Cr during spraying and their interaction with PFA droplets. This spraying method provides a new route for preparing hierarchical polymers, which have potential applicability in functional coatings.

Sensing Capability of Air Plasma-Sprayed SnO2 Coating in the Presence of Hydrogen and Carbon Monoxide

This report deals with the sensing characteristics of hydrogen (H2) and carbon monoxide (CO) gases using a tin oxide (SnO2) functional coating deposited on an alumina plate using air plasma spraying technique. This coating exhibits a porous morphology that contains both mesoporous and macroporous regions conducive to superior gas sensing. Initially, gas sensing measurements were performed by varying the operating temperature at a fixed gas concentration using a dynamic sensing setup. The coating showed maximum response % at 275 °C for H2 and CO gases. However, higher response % was obtained in the presence of H2 over CO. Sensing performance was further investigated by varying the target gas concentration at 275 °C. The coating exhibited a higher response for H2 compared to that of other plasma-sprayed SnO2 coating reported in the literature. The SnO2 coating under investigation demonstrated good sensor response and repeatability, moderate operating temperature and quick response time.

Effect of iron content on the wear behavior and adhesion strength of TiC–Fe nanocomposite coatings on low carbon steel produced by air plasma spray

In this study, the effect of Fe content on the abrasion behavior of TiC–Fe nanocomposite coatings applied on the CK45 steel substrate by air plasma spray method was investigated. For this purpose, milled TiC powder was prepared at 1, 2, 3 and 4 h milled TiC powder for 4 h was selected as the suitable sample. In the next step, a suitable sample mixture with different iron powder concentrations of 5, 10, 15, 20 and 25% was prepared by mechanical milling. The granulated mixture was applied to the substrate using air plasma spray technique. Microstructural and phase analyzes were performed using X-ray diffraction (XRD) and Scanning Electron Microscopy (SEM). According to the results of Williamson-Hall calculations, the TiC crystallites' size decreased by 49 nm–29 nm, and network strain reached 0.16% by increasing milling time from 1 h to 4 h. Studies have shown that the coatings contain titanium carbide, iron oxide, and titanium oxide, with the number of phases formed depending on the amount of iron in the chemical composition. Investigation of the tribological properties of the coating layer showed that with increased iron content in the coating, the wear resistance of the samples is reduced. Hardness tests on coatings indicate that adding iron to nanocomposite from 5 to 25% reduces hardness from 1025 to 699 Hv. It can be argued that a slight increase in the adhesion strength of the coating to the substrate is due to increased wettability because of the formation of molten iron in the coating.

Tribological behavior of plasma-sprayed Cr2O3 coatings on C/C composites sliding against different counterparts

In this study, Cr2O3 coating was fabricated on the surface of SiC-coated C/C composites by air plasma spray (APS) technique. Tribological behaviors of the Cr2O3 coatings sliding against stainless steel, WC-Co, Si3N4 and PTFE under low and high loads were investigated and compared. The results showed that the Cr2O3 coating significantly improved the wear resistance of C/C composite through reducing the wear rates. The wear rates rankings of the four friction pairs were Cr2O3/Si3N4 > Cr2O3/WC-Co > Cr2O3/stainless steel > Cr2O3/PTFE at all tested loads. While the friction coefficients were influenced by the applied loads. The Cr2O3/stainless steel pair displayed higher friction coefficients and lower wear rates compared with those of the Cr2O3/WC-Co and Cr2O3/Si3N4 pairs. Although the hardness of WC-Co was similar to that of Si3N4, the wear and friction behaviors of the Cr2O3/WC-Co and Cr2O3/Si3N4 pairs were quite different. The Cr2O3/PTFE pair displayed the lowest friction coefficient among the four kinds of friction pairs. The relevant wear mechanisms of different friction pairs were analyzed in detail.

Mechanical and wear behaviour of nanostructure TiO2–Ag coating on cobalt chromium alloys by air plasma spray and high velocity oxy-fuel

Cobalt chromium alloys constitutes the base of one important group of biometallic with excellent mechanical properties. However, due to its non-bioactive and non-antimicrobial surface, cobalt chromium alloys are vulnerable to wear corrosive attack and bacterial infection. Hence, coating cobalt chromium alloys surface with superior biomaterials is the best proven techniques. In this work, nanostructure silver adopted titanium dioxide coating was deposited on cobalt chromium alloys using air plasma spray (APS) and high velocity oxy-fuel (HVOF) techniques. Nanostructure TiO2–Ag coating demonstrated superior hardness, wear resistance and lower coefficient of friction when compared to bare CoCr alloys. Reduced plastic deformation detected by microstructural analysis indicate that the interference of nano-structure coating in metallic matrix help lower the wear rate of the sample. Meanwhile, low porosity of the coating produced by HVOF technique shown a better wear resistance result than APS technique.

Influence of Critical Plasma Spray Parameter on Microstructural and Tribological Characteristics of Nanostructured Tungsten Carbide-Cobalt Coatings

Nanostructured tungsten carbide-cobalt coatings were deposited by air plasma spraying (APS) technique, using nitrogen as primary plasma gas. The characteristics of nanostructured coatings are typically affected by the percentage of unmelted/partially-melted regions present in the coatings. Hence, the coatings were obtained as a function of critical plasma spray parameter (CPSP), which is defined as the ratio of arc power to the primary plasma gas flow rate. It was observed that tungsten (W) rather than tungsten carbide (WC) was the dominant phase in nanostructured coatings due to decarburization. CPSP was found to have a significant effect on wear performance of APS nano-structured coatings. Wear resistance was highest for coatings deposited at lowest CPSP. Flaking was more prevalent in nanostructured coatings deposited at the highest CPSP.

Thermal conductivity of air plasma sprayed yttrium heavily-doped lanthanum zirconate thermal barrier coatings

The yttrium heavily doped La2Zr2O7 solid solutions coatings, with a Y to La molar ratio of 1:1, have been successfully prepared by air plasma spraying technique. The evolution of phase composition, phase structure and thermal conductivity of such coatings with annealing at 1300 °C has been investigated. The results show that, a single pyrochlore structure can be retained for coating after annealing up to 48 h, beyond which the fluorite phase begins to precipitate out. By comparing thermal conductivities to those undoped counterparts at a similar porosity level, we find a considerably flat thermal conductivity versus temperature (k-T) curve, suggesting the existence of a strong phonon scattering source, which is inferred as rattlers. In addition, after the segmentation of the fluorite phase, the thermal conductivity of corresponding coatings rises considerably, indicating that the fluorite phase has a higher thermal conductivity than that of pyrochlore phase. Moreover, while the as-sprayed coatings show a clear indication of radiative thermal conduction beyond 1000 °C, the thermal conductivity of annealed coatings do not show such an uprising trend after 1000 °C, suggesting that the radiative thermal conduction has been greatly suppressed. The reason is proposed as the formation of local dipoles due to local enrichment of certain elements influences the propagation of electromagnetic waves and thus suppresses the radiative thermal conduction.

Thermal Shock Resistance of Yb<sub>2</sub>SiO<sub>5</sub>/Si and Yb<sub>2</sub>Si<sub>2</sub>O<sub>7</sub>/Si Coatings Deposited on C/SiC Composites

Rare-earth silicates, especially ytterbium silicate (Yb2SiO5and Yb2Si2O7), have been developed for promising environmental barrier coatings (EBCs) for SiC-matrix composites. In this study, double-layer Yb2SiO5/Si and Yb2Si2O7/Si EBC systems were deposited on C/SiC composites by air plasma spray (APS) technique, respectively. Both systems were subjected to thermal shock tests at 1400 °C. The evolution of phase composition and microstructure of those samples before and after thermal shock test were characterized. Results showed that there were penetrating microcracks in the top Yb2SiO5layer and horizontal microcracks at the Yb2SiO5-Si interface after thermal shock test. While extremely few microcracks and no horizontal microcracks were presented in the Yb2Si2O7/Si sample. The EDS analysis also showed that the Si bond layer of the Yb2SiO5/Si sample was oxidized more serious than that of the Yb2Si2O7/Si sample. The different thermal shock behaviors of both systems were clarified based on the thermal expansion behavior, phase composition and microstructure analysis.

Mechanical Properties of Layered La2Zr2O7 Thermal Barrier Coatings

Lanthanum zirconate (La2Zr2O7) has been proposed as a promising thermal barrier coating (TBC) material due to its low thermal conductivity and high stability at high temperatures. In this work, both single and double-ceramic-layer (DCL) TBC systems of La2Zr2O7 and 8 wt.% yttria-stabilized zirconia (8YSZ) were prepared using air plasma spray (APS) technique. The thermomechanical properties and microstructure were investigated. Thermal gradient mechanical fatigue (TGMF) tests were applied to investigate the thermal cycling performance. The results showed that DCL La2Zr2O7 + 8YSZ TBC samples lasted fewer cycles compared with single-layered 8YSZ TBC samples in TGMF tests. This is because DCL La2Zr2O7 TBC samples had higher residual stress during the thermal cycling process, and their fracture toughness was lower than that of 8YSZ. Bond strength test results showed that 8YSZ TBC samples had higher bond strength compared with La2Zr2O7. The erosion rate of La2Zr2O7 TBC samples was higher than that of 8YSZ samples, due to the lower critical erodent velocity and fracture toughness of La2Zr2O7. DCL porous 8YSZ + La2Zr2O7 had a lower erosion rate than other SCL and DCL La2Zr2O7 coatings, suggesting that porous 8YSZ serves as a stress-relief buffer layer.

Mechanical property evaluation of carbon nanotubes reinforced plasma sprayed YSZ-alumina composite coating

Yttria (8%) stabilized zirconia (8YSZ)-alumina-carbon nanotube (CNT) composite coatings were deposited using air plasma spray (APS) technique. Three zirconia alumina (ZA) composite coatings, without CNT reinforcement, and with 1%CNT, 3%CNT reinforcement were deposited using identical process parameters (500A and 66V). Indentation method was used to characterize mechanical behavior of coatings. The CNT reinforced composite coating showed superior mechanical properties as compared to the conventional 8YSZ coating. The addition of alumina and 1%CNTs increased the Young's modulus by around 25% and 40%, respectively. However, the increase in CNT content decreased the Young's modulus as a result of CNT agglomeration. The hardness, fracture toughness increased with addition of alumina and CNT. The fracture toughness increased from 0.55 ± 0.26MPam1/2 for 8YSZ to 1.76 ± 0.65MPam1/2 for 17%alumina-3%CNT reinforcement due to various toughening mechanisms like crack deflection, bridging etc. The maximum displacement of indenter was found to decrease from ~ 176nm for conventional YSZ coating to ~ 120nm for 3% CNT reinforced coating. The surface roughness (Ra) was found to be decrease with alumina addition and CNT reinforced coating. The CNT reinforced coating showed surface roughness of around 6µm. Scanning electron microscopy (SEM) with energy dispersive spectroscopy (EDS) was used to characterize the coating microstructure and elemental composition respectively.

High‐Temperature Low‐Cycle Fatigue Behaviour of MAR‐M247 Coated with Newly Developed Thermal and Environmental Barrier Coating

This study investigates the strain‐controlled low‐cycle fatigue (LCF) behaviour of an untreated and surface‐treated MAR‐M247 superalloy in a symmetrical push‐pull cycle with a constant strain rate at 900°C in laboratory air. A newly developed experimental thermal and environmental barrier coating (TEBC) system, consisting of a 170 μm thick CoNiCrAlY bond coat (BC) and a bilayer ceramic top coat (TC), with an interlayer and an upper layer, was deposited using air plasma spray techniques. The ceramic interlayer with an average thickness of 77 μm was formed from agglomerated and sintered yttria‐stabilized zirconia. An experimental mixture of mullite (Al6Si2O13) and hexacelsian (BaAl2Si2O8) at a ratio of 70/30 vol.% was sprayed as the upper layer. The average thickness of the TC was 244 μm. The specimen sections were investigated using a TESCAN Lyra3 XMU scanning electron microscope (SEM) to characterise the microstructure of both the TEBC and the substrate material. The fatigue damage mechanisms in the TEBC‐coated superalloy were studied. The fatigue life curves in the representation of the total strain amplitude versus the number of cycles to failure of the TEBC‐coated and uncoated superalloy were assessed. TEBC was found to have a slight, positive effect on the fatigue life of MAR‐M247.

Influence of microstructure on the wear and corrosion behavior of detonation sprayed Cr2O3-Al2O3 and plasma sprayed Cr2O3 coatings

Cr2O3-20wt% Al2O3 cermet composite and Cr2O3 coatings were deposited by the detonation spray coating (DSC) and air plasma spray techniques on Ti-6Al-4V substrates. The microstructure and phase formation in the coatings were analysed. Micro hardness and fracture toughness of coatings were measured using Vickers diamond pyramid indenter. Owing to the well-defined layered structure with individually richer Cr2O3 and Al2O3 phases together with oxide-based solid solution at the splat boundaries, the DSC Cr2O3-Al2O3 composite coating exhibits dense microstructure and better indentation fracture toughness than the well established plasma sprayed Cr2O3 coating. Although the plasma sprayed Cr2O3 coating also exhibits layered structure; the relatively poor indentation fracture toughness is attributed to the significantly higher porosity and weaker inter-splat bonding. Towards understanding the relative tribological behavior, the sliding and three-body abrasive wear tests were conducted. The material removal mechanism and associated degradation behavior of detonation sprayed Cr2O3-Al2O3 coating has been correlated with coating microstructure comparatively against the plasma sprayed Cr2O3 coating. Electro-chemical corrosion tests of coatings in salt solution (artificial sea water) reveals an improved corrosion protection by Cr2O3-Al2O3 composite coating as compared to Cr2O3 coating. Enhanced tribological performance and corrosion resistance was attributed to the dense and crack free structure of Cr2O3-Al2O3 composite coating than its counterpart.

Effect of powder particle size on vibration damping behaviour of plasma sprayed alumina (Al2O3) coating on AISI 304 stainless steel substrate

The damping capacity of plasma sprayed alumina (Al2O3) coatings on AISI 304 stainless steel was investigated in this study as a function of particle size of the starting alumina powder. The coatings were prepared from different sizes alumina powder using commercial air plasma spraying (APS) technique. The damping properties of coated samples were characterized by damping capacity (Q−1) measured experimentally using dynamic mechanical analyzer (DMA). The surface morphology of the coatings was studied using scanning electron microscope (SEM). The results revealed that the coating was porous and was able to improve the damping capacity of bare substrate. It was also observed that the powder particle size had a significant effect on the damping characteristics of the coatings. The damping values were found to be increased with the increase in particle size in the measured strain range. This behaviour was correlated with the microstructure investigated by SEM.

Environmental effects of thermal barrier coating with waste cooking palm oil methyl ester blends in a diesel engine

ABSTRACTWaste cooking oil was selected as the source material for the development of an alternative fuel. The conversion of waste cooking palm oil (WCO) into waste cooking palm oil methyl ester (WCOME) was carried out through a transesterification process. The chemical components and composition of the prepared neat methyl ester was investigated using Fourier Transform Infrared spectroscopy (FTIR) and Gas Chromatography Mass Spectrometry (GC-MS) analyses. Physio-chemical properties were determined based on ASTM standards. A thermal barrier coating was applied to the engine combustion chamber using an air plasma spray technique to reduce exhaust emission. A Kirloskar TV1 model single-cylinder direct injection diesel engine with high precision eddy current dynamometer was used for experimentation. Low brake thermal efficiency and high brake specific fuel consumption was observed for 20WCOME in the coated engine. Low CO, HC and smoke emissions were observed for 10WCOME and 20WCOME with an increase in NOx emission.