Air Plasma Spray Process Research Articles

In-Situ and Ex-Situ Measurement of Stresses In Environmental Barrier Coatings

Abstract Ceramic Matrix Composites (CMCs) offer a step improvement in temperature capability over the current best high temperature super alloys for gas turbine applications. Silicon Carbide (SiC) based CMCs react with water vapor at temperature and therefore need an Environmental Barrier Coating (EBC) for protection against the water vapor attack. EBC is typically deposited on a CMC component through Air Plasma Spray (APS) process, which involves melting and fast quenching of molten ceramic powders (such as ytterbium disilicate and ytterbium monosilicate). The fast quenching and subsequent cool down to room temperature introduce residual stress in the EBC. In addition, EBC coated CMC components often go through a heat treatment process afterwards. For Ybsilicates, this both stabilizes/crystalizes the EBC and increases hermiticity by healing cracks and pores. This heat treatment at elevated temperature changes the coating residual stress state. It is important to be able to measure the coating stresses during deposition and heat treatment so that APS process improvement and EBC coating life assessment can be made. This paper describes novel methods to measure coating stresses continuously during APS and during heat treatment up to 1400°C, using an optical deformation and curvature-based approach.

Investigation of corrosion behavior of Al–TiO2 nanocomposite produced using air plasma spray and accumulative roll bonding method

This work is an attempt to study the Al–TiO2 nanocomposite properties applying accumulative roll bonding process. For this purpose, nano Titania powder was first coated on aluminum foil by air plasma spraying process. The nanocomposite was then fabricated in one, two, three, five, and seven cycles by ARB. To investigate the effect of ARB process cycles on an Al-5% wt. TiO2 nanocomposite, characterization tests including XRD, quantitative XRD analysis by MAUD software, FESEM, EDS, electrochemical impedance, and TAFEL polarization tests were performed on the samples. The results indicate that the crystallite size reduced from 156 to 31 nm and the dislocation density quadrupled in the composite. The results of XRD analysis showed an increase in the dislocation density in the composite when the number of cycles increases. The reduction of crystallite size and increase of the grain boundaries have caused a sudden increase in the dislocation density. As the ARB cycle increases, the local porosity decreases and the local cracks due to the separation of the titanium sprayed droplets disappear in the matrix phase, and the reinforcing phase has more diffusion in the matrix. With the increase of ARB cycles, the percentage of titanium and oxygen elements on the surface was decreasing, but they remained until the last cycle. Among the non-rolled and rolled samples in different cycles, ARBed samples after 1 cycle and 5 cycles had not only the lowest and highest corrosion current densities but also the highest and lowest total resistance (Rt), respectively. This indicates max corrosion resistance due to the decrease in defect and porosity resulting from plasma spray and min corrosion resistance as a result of formation of galvanic cell between Al-matrix and Ti-reinforcing particles, respectively.

Effects of Top Ceramic Layers with an Ultrathin Dense Layer on the Thermal–Physical Properties of Thermal Barrier Coatings

Thermal barrier coatings have been used to protect superalloys under extremely harsh conditions. The durability of TBCs with a NiCoCrAlY bond layer prepared via the air plasma spray process and an ultrathin dense layer prepared via magnetron sputtering was investigated under different corrosion conditions. This paper discusses the corrosion resistance improvement mechanism of TBCs with a dense layer produced by magnetron sputtering under corrosion conditions with environmental contaminants such as calcium–magnesium–aluminum–silicon oxide systems (CMAS) at 1300 °C and NaCl-Na2SO4-V2O5 (NV) at 900 °C. The corrosion results show that CMAS will react with the stabilizers of zirconium oxide, which will change the rate of the phase transition, as determined via X-ray diffraction tests. A thermal ablation test verified that TBCs with a dense layer have a better corrosion resistance and better thermal insulation properties. All results show that preparing TBCs with a dense layer via the magnetron sputtering method will be an efficient method to improve TBCs’ properties at high temperatures in the future.

Plasma-sprayed Al2O3-TiB2-SiC ternary composite coatings and its wear behaviour based on SiC content

ABSTRACT This paper aims to study the effect of adding different SiC content on the wear performance of Al2O3-TiB2-SiC ternary composite coatings produced by the air plasma spraying process. The study used SHS powders as primary materials, consisting of H3BO3, Al, and TiO2, and 5, 10, and 15 Vol.% SiC. The microstructure and wear specifications of the coatings were characterised using FESEM, microhardness, and pin-on-disk methods. The results showed that the addition of SiC led to higher hardness and lower wear track width and rate compared to Al2O3-TiB2 composite coatings. The best wear behaviour was observed in Al2O3-TiB2-10%SiC and 15 wt% SiC composite coatings. The main wear mechanisms were found to be brittle fracture, delamination and adhesive for all samples.

Effect of plasma sprayed flyash based composite coatings on corrosion resistance

With the aim of exploring the possibility of achieving a low-cost thermal spray coating to prevent wear, erosion and corrosion. In the current study, flyash-Al2O3 and flyash-SiC composite coatings were effectively created using the air plasma spray process on substrates of Al6061 alloy. NiCr material is used as the bond coat to improve the bond strength between the coat and the substrate. Taguchi's DoE method is applied to for spray process parameters optimization. In addition, the developed coating is subjected to microstructure analysis and long-term immersion corrosion testing (1 year) in an aqueous environment to assess corrosion properties.The results revealed that the over a certain test period, the developed flyash-SiC coating has greater corrosion resistance than the uncoated and flyash-Al2O3 coated Al6061. It is noticed that the corrosion resistance of the flyash-Al2O3 coating shifts to a negative value with respect to the uncoated substrate. The uncoated sample is extensively pitted and locally corroded, as shown by the SEM image of the corroded surface. Flyash-corroded Al2O3's surface exhibits extensive degradation in the form of peeling, breaking, and cracking of the splats. With flyash-SiC composite coating a very minor corrosion splat deterioration is seen.

Stability and plasma etching behavior of yttrium-based coatings by air plasma spray process

In recent days, many studies have focused on fluorine-saturated yttrium-based coatings for better material performance in semiconductor processing chambers. In this investigation, yttrium oxide (Y2O3) and yttrium oxyfluoride (YOF) powders were synthesized using a simple solid-state wet chemical process. Then, the Y2O3 and YOF powder feedstocks were coated on an aluminum (Al) substrate using the atmospheric plasma spraying (APS) technique at different plasma powers of 25.6, and 25.2, 33.2 kW, respectively. Microstructure, macroscopic property, and chemical stability of dense Y2O3 and YOF-coated samples were analyzed. The Vickers hardness values of Y2O3 and YOF coated samples were 369.2 ± 32, and 267.1 ± 25, 241.9 ± 31 at plasma powers of 25.6, and 25.2, 33.2 kW, respectively. Y2O3 sample was showed higher hardness value due to less porosity. The etching behavior of the Y2O3 and YOF coatings was analyzed using PECVD- NF3 plasma. X-ray photoelectron spectrum analysis (XPS) has confirmed the fluorination on the etched surfaces. The relative intensity ratios of the YF to YO peaks on the Y2O3 and YOF coatings after exposure to NF3 plasma were 1.30 and 1.48, respectively. This indicated that the YOF coating showed a strong fluorination layer and superior resistance for fluorine-plasma exposure compared to Y2O3, which is more appropriate for use as a protective material for the inner wall reactors. Therefore, YOF coated samples have good stability with high erosion resistance, which is highly suggested for applications in the semiconductor industry.

Influence of air plasma spraying process parameters on the thermal barrier coating deposited with micro- and nanopowders

Abstract This study investigates the optimal conditions to deposit a thermal barrier coating using micro- and nanopowders in the air plasma spraying (APS) process. The influence of the APS process parameters on the thickness, porosity and hardness of the yttria-stabilized zirconia (YSZ; ZrO2 × 8Y2O3) coatings deposited with a single-electrode plasma gun was determined. The temperature and velocity of melted particles were determined by the DPV diagnostic system to decrease the number of experimental processes. The current and H2 flow rate were changed in this research. Metco-6700 YSZ micropowder has already been used in plasma spray physical vapor deposition. The results of this study suggest the possibility of using it for APS. The particles of this powder are characterized by high temperature (2,700°C–2,900°C) and high speed (>380 m/s). The highest thickness of the coating was obtained with 6 NLPM (normal liter per minute) H2 flow and 800 A current. Difficulties were observed with the feeding of the powder particles at higher H2 flow. The results showed that using APS, deposition of Metco-6609, a nanopowder normally used in suspension plasma spraying, is possible. In this research, this powder was fed using a carrier gas. The coatings were around 40 μm thick and had high porosity. The lowest porosity of the coating was obtained at a current of 600 A and H2 flow rate of 12 NLPM. In the coatings, unmelted spherical particles were also visible.

Optimization of industrial coatings tribological parameters by studying its application on mechanical components using Taguchi coupled WASPAS method

Heat insulated industrial ceramic coatings are gaining popularity and becoming more appealing for some application domains due to their high toughness, thermal shielding characteristics at elevated temperatures, protection from chemical degradation, wear avoidance, and environmental corrosion protection when coated on the surface of mechanical systems and their segments. To describe the tribological properties of Industrial Coatings, a superior comprehension of their performance and an exhaustive examination of their exhibition are required. In this experimental evaluation, Taguchi-based Weighted Aggregated Sum Product Assessment (WASPAS) has been employed to streamline the process parameters of different tribological attributes of industrial coatings applied through an air plasma spray process. Utilizing Taguchi L16 factorial blended level exploratory plan, tribological attributes, for example, wear loss and the coefficient of friction has also been assessed. In view of ANOVA results, it was reported that pressure applied over the coated surface played the significant role in assisting with limiting wear loss and the coefficient of friction and to amplify industrial coating life.

Influence of CNTs in protecting Air Plasma Sprayed Al2O3-3wt%TiO2 Coated Surface

The Al2O3-3wt%TiO2 powders reinforced by Carbon nanotubes (CNTs) were used to prepare composite coatings by Air Plasma Spraying (APS) process. The coatings derived from APS process were varied in the percentage proportion of CNTs such as 2wt%, 4wt%, and 6wt% respectively. The coated surfaces were evaluated by means of scratch testing. The SEM micrographs of scratched samples showed the intensity of penetration during scratching gets reduced due to the presence of CNTs film over the surface. Also, the CNT reinforced coated samples showed a significant improvement in mechanical and microstructural properties such as microhardness, porosity as well as surface roughness.

Characteristics of Thermally Sprayed Alumina-Titania Ceramic Coatings obtained from Conventional and Nanostructured Powders - A Review

ABSTRACT Alumina-titania thick coatings with finely layered microstructure are usually deposited using Air Plasma Spraying (APS) process. In the APS process, nitrogen or argon is used as the primary plasma gas. Microstructural characteristics of alumina-titania coatings are typically affected by the percentage of Titania (TiO2) present in the coatings. On the other hand, coatings obtained from nanostructured agglomerated alumina-titania powders have shown superior wear resistance over the corresponding conventional coatings. The properties of conventional Alumina-Titania coatings can also be modified by reinforcing them with some fillers like graphene, Carbon Nanotubes (CNTs), etc. From the past decade, significant research work has been in progress both industrial and academia side and still there is a lot of scope for future research on this process. This detailed review gives an overall view of Alumina-Titania coatings including their significance, microstructural, mechanical, wear, corrosion behavior and the effect of various reinforcements on their characteristics.

Failure Mechanisms of APS-YSZ-CoNiCrAlY Thermal Barrier Coating Under Isothermal Oxidation and Solid Particle Erosion

The high-temperature oxidation and solid particle erosion of thermal barrier coating (TBC) system which consists of a 8 wt.% yttria-partially stabilized zirconia (YSZ) top coat and CoNiCrAlY bond coat deposited on Inconel 718 substrate via air plasma spraying (APS) process are studied experimentally. Isothermal oxidation tests of the APS-TBCs are conducted at 1050, 1100 and 1150 °C in air for up to 1970 h. Solid particle erosion tests are also performed on both as-deposited and heat-treated APS-TBC specimens at selected particle impingement angles and velocities in room temperature. The scanning electron microscopy (SEM) analyses of the cross sections of the APS-TBC specimens after the oxidation tests show the formation of thermally grown oxide (TGO) scale due to the oxidation of CoNiCrAlY bond coat, and the oxidation kinetics of TGO growth is described by the parabolic rate equation. The failure of the APS-TBC system under isothermal oxidation is associated with the spallation of the top coat through propagation and coalescence of cracks along the coating interface, which is affected by the TGO growth. The solid particle erosion rate of the top coat is found to increase with impingement angle and reach the maximum erosion rate at normal impingement of particles. The erosion resistance of the APS-TBC is observed to increase after the APS-TBC specimen has been exposed at 1100 and 1150 °C for 72 h, probably due to the sintering effect on the top coat, which results in reduction of top coat porosity.

Experimental and Numerical Investigation of Residual Stress in Coatings on Steel

Abstract In the present work, an alumina–titania (13 %) coating has been deposited by the air plasma spray process on a mild steel substrate by preheating the substrate to different temperatures. Coating thickness was measured by a scanning electron microscope, the hardness of coatings was measured, and morphology was also checked. The crack compliance technique has been used to determine the residual stress, as it is a very simple method, and also, it is compatible with conventional machining equipment and wire electric discharge machining equipment. Specimens were cut by the diamond cutter up to certain depth in microns, and the strain relieved was recorded. For finding out the residual stress, these recorded values of strain have been used by doing a finite element analysis (FEA) using the ANSYS 15.0 (Ansys, Inc., Canonsburg, PA) software. For validation of the model, uniaxial tension was applied on a sample by using the INSTRON universal testing machine (Instron Inc., Norwood, MA), and strain values at both the coating side and the substrate side were recorded and matched with the model under the same value of the load. After verifying the model, an equation has been developed using the Minitab software (Minitab Inc, Bengaluru, India), and the strains previously recorded were used as an input to the equation obtained, and the value of residual stress was calculated. The maximum value of residual stress was found at the coating–substrate interface, and the values of residual stress for sample preheated at 250°C, 200°C, and 150°C were found to be compressive, with the magnitude of −176.28 MPa, −243.54 MPa, and −255.42 MPa, respectively.

Preparation of dense ScSZ thin films by plasma spraying with densified ScSZ powders

Low-pressure plasma spray (LPPS) technology was applied to make scania-stabilized zirconia (ScSZ) thin films for solid oxide fuel cell (SOFC). Agglomerated ScSZ powders and densified ScSZ powders sintered at high temperature were synthesized and sprayed to make dense ScSZ thin films in the LPPS processes. SEM results indicate that the densified ScSZ powders can be easier molten in plasma stream to form more uniform and denser thin films. The influence of spraying parameters on the microstructure of the films was investigated. It is found that longer spraying distance was beneficial to the melting of the flying powders to make high-quality thin films. The ionic conductivity of the thin film (about 20 μm thick) was about 0.025 S/cm measured at 700 °C. A half-cell sample was made via air plasma spray process by using the densified ScSZ powder. The result indicated that a denser and thinner ScSZ should be made for higher electrochemical property.

Process and properties of dense and porous vertically-cracked yttria stabilized zirconia thermal barrier coatings

Highly durable thermal barrier coatings (TBCs) are desired for various applications in hot sections of gas turbine engines. In this work, dense vertically-cracked (DVC) and porous vertically-cracked (PVC) yttria stabilized zirconia (YSZ) TBCs were produced using the air plasma spray process with a specially engineered powder. The effects of plasma enthalpy on the in-flight particle behavior and coating microstructures were investigated. Thermal conductivity, thermal cycling behavior, as well as, failure mechanisms of both DVC TBCs and PVC TBCs were evaluated. The thermal conductivity of PVC TBCs was found to be approximately 40%–50% lower than that of DVC TBCs due to their higher porosity. In addition, the thermal cycling life of PVC TBCs was demonstrated to be more than 1.8 times that of DVC TBCs with the same substrates and bond coats. PVC TBCs also showed excellent sintering resistance after 743 one-hour thermal cycles at 1125 °C. This work demonstrates that highly durable 8YSZ PVC TBCs with simultaneously high strain tolerance, low thermal conductivity, and high sintering resistance can be produced by the application of an engineered powder feedstock.

A Critical Review on Nano structured Coatings for Alumina-Titania (Al2O3-TiO2) Deposited by Air Plasma Spraying Process (APS)

Abstract The thick coatings obtained from nanostructured powders with fine layered microstructure are usually deposited using Air Plasma Spraying process (APS). In APS, nitrogen or argon gas is used as primarily. Microstructural characteristics of coatings for nanostructure powders are typically affected by the proportion of undissolved or partly melted zone (PM regions) present in coatings microstructure. It is observed that volume portion of partly melted regions is seen to be very much smaller in coatings obtained from nanostructurerd Al2O3-TiO2 powders deposited with nitrogen as compared to argon as the essential plasma gas. Coatings acquired from agglomerated powders of nanostructured alumina-titania shown to qualify superior resistance to wear over coatings by conventional powders. The Bi-modal representation of microstructures in nanostructured coatings with homogeneous dispersion of partially melted areas in fully melted region are considered to be responsible for improved properties of nanostructured coatings. This paper attempts to critically review the recent works done on atmospheric plasma sprayed nanostructured Al2O3-TiO2 coatings deposited by either argon or nitrogen as primary gas

High-temperature wear and frictional behavior of partially oxidized Al with NiCr composite coating

The influence of composite coating in improving wear and frictional behavior from room temperature to 600 °C was investigated. Partially oxidized Al powder was prepared with a flame spray process by spraying pure Al powder into distilled water. The composite powder is the mixture of 30 weight percent of partially oxidized Al and 70 weight percent of NiCr alloy powder. The composite powder was subsequently coated on MDN321 steel by air plasma spray process. The composite coatings are characterized with respect to adhesion strength, porosity, micro-hardness, and density. Wear and frictional behavior of coatings are evaluated under disc speed of 1 and 2 m s−1, loads of 10, 20 and 30 N and 3000 m sliding distance. The test results indicated that at room temperature, frictional heat generated due to applied load produce three-body abrasion at the interface caused to increase the wear and friction in the coating. The oxide film formed at high temperature due to plastic deformation avoids surface degradation at the interface and reduce the wear and friction. The worn surfaces at 600 °C consist phases of α-Al2O3, NiO, and Cr3O. These phases are contributing to improving the wear resistance of the coating more than 4-times compared to uncoated steels under varying load and sliding velocities. The coefficient of friction reduced with increase in temperature due to generated oxides act as lubricants at the interface.

Measurement on the thermal barrier ability of ceramic coatings in a flame thermal shock tester

Ceramic materials are widely used at high temperatures due to their low thermal conductivity. Thermal barrier coatings (TBCs) have been applied in hot sections in industrial gas turbines and air engines. The measurement of the thermal barrier ability of the TBCs is very important for coating property evaluation, that can be done in hot gases with temperature gradient through the coatings. In this study, a flame thermal shock tester was used for such investigation. Thick TBC, containing a MCrAlY bond coat and a YSZ top coat, was sprayed onto a superalloy substrate by air plasma spray (APS) process. In the test, hot flame impacted on the top coat surface with the temperature controlled by an infrared thermometer. A thermocouple was attached onto the backside of the substrate to measure the backside temperature. Therefore, the temperature gradient in the TBC can be evaluated. The thermal barrier ability of the TBCs in different back cooling rate was studied.

Effect of reinforcement of carbon nanotubes on air plasma sprayed conventional Al2O3-3%TiO2 ceramic coatings

Abstract The characterization of carbon nanotubes (CNT) doped plasma sprayed ceramic coatings were carried out by varying the percentage of CNT as 2 wt%, 4 wt%, and 6 wt%. The coatings were deposited using conventional Alumina-3 wt% Titania powders mixed with CNT using Air Plasma Spraying (APS) process. The adhesion strength of this ceramic coating is increased due to the addition of CNTs. This phenomenon is due to CNT bonding across the powder splats on the coatings which results in the refinement of coating features. On the other hand distinct factors impacting adhesion of coatings were found to be absence of CNT in bond coat and uneven distribution of CNT in top coat. The surface roughness also plays a vital role in characterization of coatings. It was also found that the surface roughness of the coatings increased with increasing percentage of CNT due to the pores available in conventional powders were filled with carbon nanotubes. The overall effect of adding CNTs to the Alumina-Titania coating system improves both mechanical and microstructural properties.

Experimental investigation of the relationship between thermal barrier coating structured porosity and homogeneous charge compression ignition engine combustion

Heat transfer has a profound influence on homogeneous charge compression ignition combustion. When a thermal barrier coating is applied to the combustion chamber, the insulating effect magnifies the wall temperature swing, decreasing heat transfer during combustion. This enables improvements in both thermal and combustion efficiency without the detrimental impacts of intake charge heating. Increasing the temperature swing requires coatings with lower thermal conductivity and heat capacity. A promising avenue for simultaneously decreasing both thermal conductivity and capacity is to increase the porosity fraction. A proprietary solution precursor plasma spray process enables discrete organization of the porosity structure, called inter-pass boundaries, which in turn produces a step-reduction in thermal conductivity for a given porosity level. In this investigation, yttria-stabilized zirconia is used to create four different thermal barrier coatings to study the potential of structured porosity as means of improving the “temperature swing” behavior in a homogeneous charge compression ignition engine. The baseline coating is “dense YSZ,” applied using a standard air-plasma spray process. Next, significant reductions of the thermal conductivity are achieved by utilizing the solution precursor plasma spray process to create inter-pass boundaries with a moderate overall porosity. Performance, efficiency, and emissions are compared against both a baseline configuration with a metal piston and an air-plasma spray “dense YSZ” coating. Experiments are carried out in a single-cylinder gasoline homogeneous charge compression ignition engine with exhaust re-induction. Experiments indicate that incorporating structured porosity into thermal barrier coatings produces tangible gains in combustion and thermal efficiencies. However, there is an upper limit to porosity levels acceptable for homogeneous charge compression ignition engine application because an elevated porosity fraction leads to excessive surface roughness and undesirable fuel interactions. Comparison of the coatings showed the best results with coating thickness of up to 150 µm. Thicker coatings led to slower surface temperature response and attenuated swing temperature magnitude.

Laser Thermal Gradient Testing and Fracture Mechanics Study of a Thermal Barrier Coating

It is critical for thermal barrier coating (TBC) development that a testing method be used to understand the potential and limitation of a coating’s durability and integrity under gas turbine engine operating conditions. To this end, a TBC-coated button is tested using a laser high-heat flux facility. The ceramic coating is ZrO2-8 wt.% Y2O3 applied via the air plasma spraying process on top of a NiCoCrAlY bond coating and an Inconel alloy 617 substrate button of 25.4 mm diameter. The coated button is subject to precisely controlled laser heating on the top side (1150 °C) and a temperature gradient of 63.9 °C/mm through the button overall thickness. The coated button lasts 160.9 h or 570 cycles of laser heating. The void fraction change before and after the test, the thermal conductivity change during the laser test and the failure assessment are presented. After the test, significant horizontal cracks exist in the top coating close to the thermally grown oxide (TGO) layer and near the button center. Based on the cracks and the TGO layer geometry, the stress intensity factor and strain energy release rate are computed. The combined experimental and computational approach can lead to a TBC lifetime model.