As-sprayed Coatings Research Articles

Mechanical properties of vacuum plasma sprayed reinforced hydroxyapatite coatings on Ti-6Al-4V alloy

The aim of this present work was to deposit reinforced hydroxyapatite coatings with a bond coat of zirconia on Ti-6Al-4V substrate using the vacuum plasma spray technique in order to improve the surface properties. To achieve that, HA + 10 wt% Al2O3 and HA + 10 wt% ZrO2 coatings with an intermediate layer of zirconia were deposited on Ti-6Al-4V implant substrate. Further, these coatings were subjected to post-coating heat treatment at 700 °C for 1 h. The morphology and phase analysis of bio-ceramic powders, as-sprayed and heat-treated coatings, have been characterized using a scanning electron microscope and an X-ray diffractrometer. Crystallinity, surface roughness, porosity, hardness and shear strength were also investigated. The wear behaviour of the coatings was evaluated using a Taber abrasion tester as per ASTM F1978 standard. The results indicate that heat-treated coatings exhibit lower wear resistance than as-sprayed coatings and show lower porosity and higher crystallinity and hardness after heat treatment. Slight improvement in shear strength was observed after heat treatment.

Wear and Corrosion Properties of 316L-SiC Composite Coating Deposited by Cold Spray on Magnesium Alloy

In order to improve the wear and corrosion resistance of commonly used magnesium alloys, 316L stainless steel coating and 316L-SiC composite coating have been deposited directly on commercial AZ80 magnesium alloy using cold spraying technology (CS). The microstructure, hardness and bonding strength of as-sprayed coatings were studied. Their tribological properties sliding against Si3N4 and GCr15 steel under unlubricated conditions were evaluated by a ball-on-disk tribometer. Corrosion behaviors of coated samples were also evaluated and compared to that of uncoated magnesium alloy substrate in 3.5 wt.% NaCl solution by electrochemical measurements. Scanning electron microscopy was used to characterize the corresponding wear tracks and corroded surfaces to determine wear and corrosion mechanisms. The results showed that the as-sprayed coatings possessed higher microhardness and more excellent wear resistance than magnesium alloy substrate. Meanwhile, 316L and 316L-SiC coating also reduced the corrosion current density of magnesium alloy and the galvanic corrosion of the substrates was not observed after 200-h neutral salt spray exposure, which demonstrated that corrosion resistance of a magnesium alloy substrate could be greatly improved by cold-sprayed stainless steel-based coatings.

Studies on corrosion resistance and bio-activity of plasma spray deposited hydroxylapatite (HA) based TiO2 and ZrO2 dispersed composite coatings on titanium alloy (Ti-6Al-4V) and the same after post spray heat treatment

In the present study, the effect of plasma spray deposited hydroxylapatite (HA) based TiO2 dispersed (HA+50wt.% TiO2), coating and post spray heat treatment to be referred as HA-TiO2 (heat treated at 650°C for 2h) and ZrO2 dispersed (HA+10wt.% ZrO2), to be referred as HA-ZrO2 coating (heat treated at 750°C for 2h) on corrosion resistance and bioactivity of Ti-6Al-4V substrate has been undertaken. There is partial decomposition of HA to tri-calcium-phosphate (Ca3(PO4)2) and formation of CaTiO3 phase in HA-TiO2 coating and CaZrO3 phase in the HA-ZrO2 coating. Corrosion study in Hank’s solution shows that there is shifting of corrosion potential (Ecorr) towards active potential (−1.1V(SCE) for as-sprayed and post spray heat treated HA-TiO2 coating, −1.1 V(SCE) for as-sprayed HA-ZrO2 coating and −1V(SCE) for HA-ZO2 coating after post spray heat treatment), and deterioration in pitting corrosion (Epit) resistance in as-sprayed coatings and the same after heat treatment (−0.7V(SCE) for both HA-TiO2 and HA-ZrO2 coating as compared to as received substrate (−0.3V(SCE)). The corrosion rate was increased for both the coatings with a maximum increase in HA-ZrO2 coating. Bioactivity test shows a higher degree of apatite deposition in as-sprayed coating and the same after heat treatment as compared to as received Ti-6Al-4V though the as-sprayed one showed a superior behavior.

Supersonic Plasma Spray Deposition of CoNiCrAlY Coatings on Ti-6Al-4V Alloy

Plasma spray is a versatile technology used for production of environmental and thermal barrier coatings, mainly in the aerospace, gas turbine, and automotive industries, with potential application in the renewable energy industry. New plasma spray technologies have been developed recently to produce high-quality coatings as an alternative to the costly low-pressure plasma-spray process. In this work, we studied the properties of as-sprayed CoNiCrAlY coatings deposited on Ti-6Al-4V substrate with smooth surface (R a = 0.8 μm) by means of a plasma torch operating in supersonic regime at atmospheric pressure. The CoNiCrAlY coatings were evaluated in terms of their surface roughness, microstructure, instrumented indentation, and phase content. Static and dynamic depositions were investigated to examine their effect on coating characteristics. Results show that the substrate surface velocity has a major influence on the coating properties. The sprayed CoNiCrAlY coatings exhibit low roughness (R a of 5.7 μm), low porosity (0.8%), excellent mechanical properties (H it = 6.1 GPa, E it = 155 GPa), and elevated interface toughness (2.4 MPa m1/2).

Corrosion behavior of low pressure cold sprayed Zn-Ni composite coatings

In this study, Zn-Ni composite coatings with different Ni contents were deposited on low-carbon steel using Low pressure cold spray (LPCS) method. The morphology, element distribution and phase compositions of powders and as-sprayed coatings were analyzed by using scanning electron microscopy, energy dispersive spectrometer and X-ray diffraction respectively. The corrosion protection mechanism of these composite coatings were investigated using electrochemical technique and their corrosion morphologies. The results show that Zn-Ni composite coatings exhibited a better sacrificial anodic protection and physical shielding effect, where the corrosion resistance of the coating with 10 wt% Ni in the feedstock was best. The corrosion products were mainly composed of Zinc hydroxide Zn(OH)2, basic zinc chloride hydrate ZnCl2·4Zn(OH)2·H2O, basic zinc carbonate hydrate Zn4·CO3(OH)6·H2O and the corrosion products is not mixed with alumina. The corrosion process mainly includes active dissolution of zinc, zinc cathodic protection, physical shielding caused by corrosion products.

Ultrasonic cavitation erosion of as-sprayed and laser-remelted yttria stabilized zirconia coatings

Cavitation erosion resistance of 8wt.% yttria stabilized zirconia has been investigated in specimens prepared by atmospheric plasma spraying and laser remelting post treatment. The results indicate that as-sprayed coatings involve defects such as primary cavities and initial micro cracks inside a particle and among the interfaces of particles. When the specimens are subjected to cavitation erosion, the micro cracks initiate and coalesce along with chip removals. Laser remelting produces a dense glazed layer with some cracks though the coatings. With the increasing of erosion time, large pieces are delaminated from coating-substrate interface leading to a significant mass loss. However, the resistance of laser remelted coatings to cavitation erosion is significantly improved when they are impregnated with epoxy by vacuum castable mounting. The relationship between cracks formed inside the laser remelted YSZ coatings and their damage mechanism under cavitation is discussed.

Characterization of Vacuum Plasma Sprayed Reinforced Hydroxyapatite Coatings on Ti–6Al–4V alloy

Two reinforced hydroxyapatite (HA) coatings with an intermediate layer of zirconia were deposited on Ti–6Al–4V by vacuum plasma spray (VPS) technique. In first coating, HA was reinforced with 10 wt % Al2O3 whereas in second coating, HA was reinforced with 10 wt % ZrO2. The objective of this study was to investigate the microstructure, phase formation and mechanical properties like hardness and bond strength of as-sprayed coatings and the coatings after post coating heat treatment at 700 °C for 1 h. The characterization of the coatings was performed by using SEM/EDAX, XRD, porosity, crystallinity and roughness measurement. The coatings were also evaluated for mechanical properties like hardness and tensile bond strength. It was observed that after post coating heat treatment, crystallinity increased and porosity decreased which indicated recrystallization of amorphous phases of as-sprayed coatings. Heat treatment resulted into improvement in cross-sectional hardness, however sharp decrease in bond strength was observed.

Microstructure and electric properties of low-pressure plasma sprayed β-FeSi2 based coatings

Thermoelectric material β-FeSi2 based coating was fabricated by the technique low-pressure plasma spray (LPPS) on the Al2O3 substrate from different alloy powders. During the process LPPS, the phase transformation had occurred through the peritectoid, eutectoid reaction and their inverse reaction. The grain size of the as-sprayed β-FeSi2 doped Co coatings was reduced comparing with the original feedstock powders, which implied the thermal conductivity could effectively decreased by the LPPS process. The room temperature electrical conductivity showed metal and semiconductor properties on the as-sprayed and annealed coatings. This method and the results could solve the problems of practical application especially on special-shaped surfaces such as the automobile exhaust pipe or other complex surface shape of the device.

Failure analysis of fine-lamellar structured YSZ based thermal barrier coatings with submicro/nano-grains

The catastrophic phase transformation of yttria partially stabilized zirconia (YSZ) is considered to be the main reason that leads to the failure of YSZ based thermal barrier coatings (TBCs) when their service temperatures are higher than 1200°C, which greatly limits the application of YSZ in the new-generation aero engines. In the present work, a fine-lamellar structured YSZ coating that consisted of submicro-/nano-grains showed good phase stability during a burner-rig test at 1270±20°C. The results from X-ray diffraction and Raman spectroscopy confirmed that the phase compositions of as-sprayed or failed coatings fully consisted of the t′ phase. The catastrophic tetragonal-monoclinic (t→m) phase transformation did not occur, indicating it was not the main reason for the spallation of YSZ coating. However, the analysis of residual stress within YSZ coating suggested that the failure of coating was associated with the severe sintering of YSZ and thermal expansion mismatch at high temperatures.

Isothermal oxidation behavior of HVAF-sprayed Ni and NiCr coatings in H2-H2O environment

The formation of a protective chromia scale on stainless steels is known to be suppressed by the presence of water vapor in reducing conditions. Thermal spray coatings present a promising approach to improve the durability of steels by transferring the first line of oxidation attack from the bulk steel to the coating. In the present work, isothermal oxidation behavior of Ni and NiCr coatings deposited by High-Velocity Air Fuel (HVAF) process on 304L stainless steel was investigated at 600°C for 168h. Ar-10%H2-20%H2O was selected as the oxidation environment to study the oxidation behavior of the coatings in a low pO2 environment containing H2 and H2O. BIB/SEM, EDS, and XRD techniques were used to characterize the as-sprayed coatings and to investigate the oxidation mechanisms in the coated samples. Results showed that both Ni and NiCr coatings imparted oxidation protection to the 304L substrate. The chromia-forming 304L steel presented a duplex but non-protective oxide scale comprising of an outer Fe3O4 layer on an inner (Fe, Cr)3O4-spinel oxide. In contrast, the NiCr coating presented superior oxidation behavior due to the formation of a continuous, thin, and slow-growing Cr2O3 scale. The Ni coating, too, protected the substrate owing to limited nucleation and growth of the deleterious NiO scale in the low-oxygen-activity environment.

Comparative analysis of tribological behavior of plasma- and high-velocity oxygen fuel-sprayed WC-10Co-4Cr coatings

PurposeThe purpose of this study is to prepare WC-10Co-4Cr coatings using two processes of plasma spraying and high-velocity oxygen fuel (HVOF) spraying. The decarburization behaviors of the different processes are analyzed individually. The microstructural characteristics of the as-sprayed coatings are presented and the wear mechanisms of the different WC–10Co–4Cr coatings are discussed in detail.Design/methodology/approachThe WC–10Co–4Cr coatings were formed on the surface of Q235 steel by plasma and HVOF spraying.FindingsPlasma spraying causes more decarburizing decomposition of the WC phase than HVOF spraying. In the plasma spraying process, η(Cr25Co25W8C2) phase appears and the C content decreases from the top surface of the coating to the substrate.Practical implicationsIn this study, two WC–10Co–4Cr coatings on Q235 steel prepared by plasma and HVOF spraying were compared with respect to the sliding wear behavior.Originality/valueThe wear mechanisms of the plasma- and HVOF-sprayed coatings were abrasive and oxidation, respectively.

Microstructure and oxidation resistant behavior of Er 2 Si 2 O 7 and Er 2 Si 2 O 7 /LaMgAl 11 O 19 coatings deposited on C f /SiC composites by APS at 1723 K

Abstract Er 2 Si 2 O 7 and Er 2 Si 2 O 7 /LaMgAl 11 O 19 coatings were fabricated on C f /SiC composites by atmospheric plasma spraying (APS) in order to improve the oxidation resistance of the substrate. Microstructure and oxidation resistant behavior of the coatings were investigated at 1723 K in the thermal shock furnace. The results showed that the as-sprayed coatings were uniformly distributed on the substrate and bonded well with the substrate. The bared substrate has the maximum weight loss of 43.8% after 8 thermal shock cycles with the total heating time of 120 min, while for the samples coated with the Er 2 Si 2 O 7 and Er 2 Si 2 O 7 /LaMgAl 11 O 19 coatings, the weight losses were only up to 3.73% and 1.88%, respectively. Due to the difference of coefficients of thermal expansion between coatings and substrate, the sintering of Er 2 Si 2 O 7 layer, the surface oxidation of substrate and the interfacial reactions between the ceramic layers, the through cracks and horizontal cracks appeared in the coatings with the thermal shock cycle increasing, which significantly led to the increasing weight loss for the coated samples. The oxidation protection of Er 2 Si 2 O 7 and Er 2 Si 2 O 7 /LaMgAl 11 O 19 coatings for C f /SiC composites at 1723 K is much attractive for the short-term application such as several thermal shock cycles with the total heating time of tens of minutes.

Isothermal oxidation of HVAF-sprayed Ni-based chromia, alumina and mixed-oxide scale forming coatings in ambient air

The power generation industry has been progressively shifting towards higher operating steam temperatures and pressures to increase efficiency and reduce CO2 emissions. However, higher operating temperatures lead to more aggressive oxidation of the boiler components. A promising route to improve the durability of degradation-prone components is through deployment of high-performance coatings. In the present work, four Ni-based coatings - Ni21Cr, Ni5Al, Ni21Cr9Mo, and Ni21Cr7Al1Y - thermally sprayed by the high-velocity air fuel (HVAF) technique on boiler steel (16Mo3) substrates were investigated. The isothermal oxidation behavior of the coatings was studied in ambient air environment at 600°C for different time intervals i.e. 1, 5, 10, 24, 48, 96, and 168h. The oxidation behavior of the as-sprayed and polished coatings was compared. The protective α-Al2O3 was not detected on the exposed alumina-forming NiAl coating. On the other hand, Cr2O3 along with a small amount of NiO were the main oxidation products on the surface of the NiCr and NiCrMo coatings, and were found to be relatively less protective. The mixed-oxide scale forming NiCrAlY coatings showed the best oxidation resistance due to the formation of a thin and slow-growing Al2O3 scale along with Ni(Al,Cr)2O4 and Cr2O3. The polished coatings were found to significantly reduce the oxidation rate in each case as the protective scale-forming elements were more uniformly supplied to the surface oxide scale by removing the surface asperities.

Microstructural evolution of cold-sprayed Inconel 625 superalloy coatings on low alloy steel substrate

This study illustrates microstructural evolution of INCONEL 625 superalloy coatings cold-sprayed on a 4130 chrome alloy steel with medium carbon content. INCONEL 625 powder (5–25 μm) were successfully cold sprayed without any oxidation. The comprehensive microstructure analysis of the as-sprayed coatings and of the substrate-coating interface was carried out using EBSD, TEM, and XRD. The coating microstructure at the substrate-coating interface was markedly different from the microstructure away from the interface. The coating microstructure at steel-coating interface consisted of a fine layer of small grains. The microstructure beyond this fine layer can be divided into splats, inter splat and intra splat boundaries. Both splat and splat boundaries exhibited deformation induced dislocations. Dynamic recovery of dislocations-ridden regions inside the splat was responsible for the development of sub grain structure inside a splat with both low and high angle grain boundaries. Splat-splat (inter splat) boundary consisted of a relatively high density of dislocations and shear bands as a result of adiabatic shear flow localisation. This flow instability is believed to enhance the microstructural integrity by eliminating porosity at splat-splat boundaries. Based on the microstructural analysis using electron microscopy, a plausible mechanism for the development of microstructure has been proposed in this work. Cold spray technique can thus be deployed to develop high quality coatings of commercial importance.

Formation and subsequent phase evolution of metastable Ti-Al alloy coatings by kinetic spraying of gas atomized powders

High temperature application of modern Ti-alloys is often limited by their insufficient oxidation resistance at temperatures exceeding 650°C. One way to overcome this obstacle is application of an overlay coating made of TiAl-based alloys. In this study kinetically sprayed coatings of Ti-46Al-8.5Nb-1Ta (at%) on a commercial alloy IMI-834 were investigated. The feedstock powder fabricated by gas atomization consisted of non-equilibrium α-Ti and β-Ti phases. Heat treatment of the powder at 750°C triggered transformation to the equilibrium ordered γ-TiAl and α2-Ti3Al phases. Mechanical properties of both the as-atomized and the heat treated powders were tested, showing that the as-sprayed one possesses lower hardness and higher ductility. Since such properties of the as-atomized powder are more suited for kinetic spraying, the as-atomized powder was used for coating fabrication. The as-sprayed coatings inherited the non-equilibrium phases of the atomized powder with significant deformation. The coatings were annealed at 750°C and immediate transformation to a refined microstructure containing the γ-TiAl and α2-Ti3Al phases was observed in <20min. Longer annealing time resulted in significant grain growth and formation of defect-free grains.

Role of Oxides and Porosity on High-Temperature Oxidation of Liquid-Fueled HVOF Thermal-Sprayed Ni50Cr Coatings

High chromium content in Ni50Cr thermally sprayed coatings can generate a dense and protective scale at the surface of coating. Thus, the Ni50Cr coating is widely used in high-temperature oxidation and corrosion applications. A commercially available gas atomized Ni50Cr powder was sprayed onto a power plant steel (ASME P92) using a liquid-fueled high velocity oxy-fuel thermal spray with three processing parameters in this study. Microstructure of as-sprayed coatings was examined using oxygen content analysis, mercury intrusion porosimetry, scanning electron microscope (SEM), energy-dispersive x-ray spectroscopy (EDX) and x-ray diffraction (XRD). Short-term air oxidation tests (4 h) of freestanding coatings (without boiler steel substrate) in a thermogravimetric analyzer at 700 °C were performed to obtain the kinetics of oxidation of the as-sprayed coating. Long-term air oxidation tests (100 h) of the coated substrates were performed at same temperature to obtain the oxidation products for further characterization in detail using SEM/EDX and XRD. In all samples, oxides of various morphologies developed on top of the Ni50Cr coatings. Cr2O3 was the main oxidation product on the surface of all three coatings. The coating with medium porosity and medium oxygen content has the best high-temperature oxidation performance in this study.

Mechanical and tribological properties of nano/micro composite alumina coatings fabricated by atmospheric plasma spraying

Adding nano particles can significantly improve the mechanical properties and wear resistance of thermal sprayed Al2O3 coating. However, it still remains a challenge to uniformly incorporate nano particles into traditional coatings due to their bad dispersibility. In the present work, nanometer Al2O3 (n-Al2O3) powders modified by KH-560 silane coupling agent were introduced into micrometer Al2O3 (m-Al2O3) powders by ultrasonic dispersion to afford nano/micro composite feedstock, and then four resultant coatings (weight fraction of n-Al2O3: 0%, 3%, 5% and 10%) were fabricated by atmospheric plasma spraying. The features and constitutes of feedstock and as-sprayed coatings, as well as their porosity, bonding strength, microhardness and frictional behaviors were investigated in detail. Results show that the nano/micro composite feedstock with uniform microstructure can be better melted in the spraying process, thereby obtaining coatings with denser microstructure, higher hardness and bonding strength. Added n-Al2O3 has no obvious effect on the friction coefficient of composite coatings, whereas can improve their wear-resistant and reduce the worn degree of counterpart. The wear mechanism of traditional coating is brittle fracture and lamellar peeling, while that of composite coating with weight fraction of n-Al2O3 of 10% is adhesive wear.

Laser Post Annealing of Cold-sprayed Al–Ni Composite Coatings for Green Energy Tasks

This paper considerers the principal feasibility of using cold gas dynamic spraying with laser post treatment technique to create Al-Ni composite coatings. The prepared mixture of two powders was coated on an aluminum substrate and laser annealed. As a result the AlxNy intermetallic phases were observed in the coating. Obtained laser-annealed coatings were studied by means of optical and electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffractometry and compared with as-sprayed coatings. Effects of laser processing parameters, such as scanning speed, power and beam shape on microstructural evolution of the composite coatings were discussed and optimal regimes for laser assisted CS were determined.

Microstructure and mechanical properties of plasma sprayed Al2O3– 13%TiO2Ceramic Coating

This paper focused on the effect of deposition conditions on the microstructural and mechanical properties of the ceramic coating. In this study, Al 2 O 3 – 13%TiO 2 coated mild steel were prepared by using atmospheric plasma spray technology with different plasma power ranging from 25 kW to 40 kW. The as-sprayed coatings consist of γ-Al 2 O 3 phase as the major phase and small amount of the titania phase existed in the coating structure. High degree of fully melted region was observed in the surface morphology for the coating sprayed with high plasma power, which lead to the high hardness and low percentage of porosity. In this study, nanoindentation test was carried out to investigate mechanical properties of the coating and the results showed that the coatings possess high elastic behaviour, which beneficial in engineering practice.

Effects of Spray Parameters and Post-spray Heat Treatment on Microstructure and Mechanical Properties of Warm-Sprayed Ti-6Al-4V Coatings

Warm spray is a novel thermal spray technique that allows the formation of dense and relatively pure Ti-6Al-4V coatings due to its capability to control the temperature of the propellant gas by diluting the combustion flame with an inert gas such as nitrogen. Recently, its combustion pressure has been increased from 1 to 4 MPa aiming to further increase particle velocity to over 1000 m/s. Two series of coatings with combustion pressure of 1 and 4 MPa and various nitrogen flow rates were prepared in this study. Effects of combustion pressure and nitrogen flow rate on the microstructure and mechanical properties of the Ti-6Al-4V coatings were systematically studied. Miniature tensile specimens with a total length of about 9 mm were used for static tensile tests. It was found that the spray parameters affect both the porosity and oxygen content of the coatings significantly and had remarkable effects on their mechanical properties. High level of porosity in the Ti-6Al-4V coatings reduced the effective cross-sectional area of the mini-specimens and caused a drop in their tensile strength and Young’s modulus. Subsequent heat treatments were found effective in significantly recovering the mechanical properties of the as-sprayed coatings.