Sprayed Bond Coat Research Articles

Improvement of thermal plasma sprayed coating adhesion strength by laser microtexturing of aluminum

Grit blasting and laser processing are two common methods used for surface microtexturing to enhance coating adhesion strength. This paper reports a thermomechanical method of laser microtexturing of Al 7075 alloy using a nanosecond pulsed laser to generate a large increase in surface area. The entire surface area of the substrate was microtextured using laser melting, ablation, and resolidification. The laser microtextured surface morphology consisted of tightly packed pillar-like surface features. The morphology of the microtexture was varied by controlling the laser processing parameters. The laser microtextured surface was coated with metallic particles using a thermal spray process. The tensile adhesive strength of the thermally sprayed metallic coating was higher by over 17% for a 5-μm deep microtexture, compared to that of grit-blasted samples. Also, this is the highest reported adhesion strength for laser-based microtexturing methods for thermally sprayed bond coatings.

Evaluation of New High Entropy Alloy as Thermal Sprayed Bondcoat in Thermal Barrier Coatings

Evaluation of New High Entropy Alloy as Thermal Sprayed Bondcoat in Thermal Barrier Coatings

Corrosion Properties of Thermally Sprayed Bond Coatings Under Plasma-Sprayed Chromia Coating in Sulfuric Acid Solutions

Plasma-sprayed chromia coatings are known to have excellent corrosion and wear properties in highly acidic conditions at ambient and elevated temperatures, but are not watertight due to their intrinsic porosity. Therefore, in applications involving aggressive environments, the whole component is usually made of a corrosion-resistant alloy, to which the Cr2O3 coating imparts the necessary wear resistance. However, in such aggressive environments, the survival of thermal spray metallic bond layers becomes an issue. The present study deals with the performance in sulfuric acid solutions of coated systems consisting of a Hastelloy C-276 substrate and a plasma-sprayed Cr2O3 top coating with four different intermediate bond coatings. The bond coatings were HVOF-sprayed Ni-20Cr, Hastelloy C-276 and Ultimet alloys and plasma-sprayed tantalum. Open-circuit measurement, electrochemical polarization and electrochemical impedance spectroscopy tests were carried out at room temperature (RT) in solutions with various concentrations. Also, static immersion tests were performed at RT and 60 °C. The results revealed that the HVOF-sprayed Ni-20Cr and Ultimet alloy coatings were significantly attacked by the sulfuric acid electrolyte, especially at 60 °C, whereas the HVOF-sprayed Hastelloy C-276 and plasma-sprayed Ta coatings performed significantly better.

Comparative study between high-velocity oxygen fuel and flame spraying using MCrAlY coats on a 304 stainless steel substrate

High-velocity oxygen fuel (HVOF) and flame spraying (FS) are alternative methods to thermal spraying processes to produce dense high-quality coatings. The aim of this study is to compare the microstructure and mechanical properties for HVOF and FS using a thermally sprayed bond coat NiCoCrAlY and CoNiCrAlY powders on an AISI 304 stainless steel substrate. The microstructure and composition of coatings were characterized by an X-ray Diffraction (XRD) analysis and Electron Microscopy (SEM) coupled to an Energy Dispersive Spectroscopy (EDS) detector. HVOF showed higher quality coatings compared to FS in terms of porosity and the presence of unfused particles for both employed powders. The mechanical properties and results indicated that the yield strength of the NiCoCrAlY (HVOF) coating was 1.4-folds FS, but the flexural bending modulus was almost the same. For the CoNiCrAlY powder, HVOF gave a higher yield strength and a higher flexural modulus than FS as the oxygen affinity of the CoNiCrAlY powder was lower than NiCoCrAlY given the high Co content in the former versus the latter. The results also indicated that hardness increased by about 83%, and by 58% for the NiCoCrAlY HVOF and FS coating alloys, and by 42% and 20% more for the CoNiCrAlY HVOF and FS coatings than the stainless steel substrate hardness, respectively.

Air Plasma Sprayed Bond Coat Oxidation Behavior and its Resistance to Isothermal and Thermal Shock Loading

An experimental investigation was conducted to find the effect of spraying method of coating of Thermal Barrier Coatings (TBCs) on their oxidation behaviour and resistance to various thermal loading. Isothermal and thermal shock tests were performed in order to study oxidation behaviour of air plasma sprayed Bond Coat (BC) and assess its effect on TBCs lifetime under the stated loadings. Specimens, after loading were investigated using Field Emission Scanning Electron Microscopy (FE-SEM). Results showed in spite of forming various oxide layers within the BC its oxidation in Bond Coat/Top Coat interface behave the same as samples coated by other methods and no failure was observed at substrate/BC interface or within the BC.

High-Temperature Thermal Properties of Ba(Ni1/3Ta2/3)O3 Ceramic and Characteristics of Plasma-Sprayed Coatings

To evaluate its applicability as a ceramic top coat in thermal barrier coatings, Ba(Ni1/3Ta2/3)O3 (BNT) was synthesized by a solid-state reaction method and its phase stability, coefficient of thermal expansion (CTE), and thermal conductivity were measured. Coatings with a structure of plasma-sprayed BNT/yttria-partially stabilized zirconia (YSZ) double ceramic coat and a high-velocity oxygen fuel (HVOF)-sprayed bond coat were fabricated. The thermal shock behavior of the coatings was investigated, and the phase composition and microstructure evolution of the BNT/YSZ coatings were characterized. The results showed that the BNT powder had single perovskite structure that remained unchanged after sintering at 1500 °C for 100 h. The average CTE of the BNT ceramic at 25-1400 °C was found to be 10.2 × 10−6 K−1, comparable to that of YSZ. The thermal conductivity at 1200 °C was found to be 2.56 W m−1 K−1. During thermal shock, the BNT/YSZ coatings spalled layer by layer, which can be attributed to the compositional deviation combined with the large temperature gradient in the BNT coating.

Highly oxidation resistant and cost effective MCrAlY bond coats prepared by controlled atmosphere heat treatment

MCrAlY bond coats with high oxidation resistance are essentially important for improving the life-time of thermal barrier coatings (TBCs). In this study, highly oxidation resistant MCrAlY bond coats were prepared by a cost-effective approach involving air plasma spraying (APS) followed by a controlled atmosphere heat treatment (a diffusion treatment performed in a furnace filled with Ar). The results confirmed that a pure α-Al2O3 thermally grown oxide (TGO) was successfully formed on the surface of the heat-treated APS bond coats during isothermal oxidation. The oxidation resistance of the resulting APS bond coats was as high as that of low-pressure plasma sprayed bond coats, which are very expensive. The high oxidation resistance of the APS bond coats fabricated in this study can be attributed to the structural changes of the interface between the splats inside the coating. During the controlled atmosphere heat treatment, the lamellar oxides present between the splats within the as-sprayed APS bond coats converted into isolated oxide particles, and the metal elements could diffuse freely within the coating after the healing of the interface between the lamellar splats. The effect of the changes in the interfacial microstructure of the bond coats on the growth of the TGO was discussed.

Surface Microstructure and High Temperature Oxidation Resistance of Thermal Sprayed NiCoCrAlY Bond-Coat Modified by Cathode Plasma Electrolysis

The surface properties of the air-plasma sprayed bond-coat have been modified by cathode plasma electrolysis (CPE). After modification, a re-melted layer without obvious pores and oxide stringers is formed, the gain size of re-melted layer is approximately 80–120 nm. It is shown, from cyclic oxidation at 1100 °C, that a thin oxide scale mainly composed of α-Al2O3 has been formed on the modified bond-coat and the oxidation resistance of the modified bond-coat has been significantly improved. Such beneficial result can be attributed to following effects: during CPE process, the plasma discharges with high temperature take place on the bond-coat surface. With plasma discharge treatment, the surface is melted and quickly re-solidified, the grain size decreases, and the pores and oxide stringers disappear. During cyclic oxidation, owing to the above modification of surface properties, the critical content of Al for selective oxidation is significantly decreased. Therefore, a continuous Al2O3 scale is formed.

The application of photoluminescence piezospectroscopy for residual stresses measurement in thermally sprayed TBCs

Photoluminescence piezospectroscopy (PLPS) was used as a non-destructive technique for the measurement of residual stresses within the thermally grown oxide (TGO) layer beneath plasma-spray thermal barrier coatings (TBC). The technique has proved to be very effective for such measurements in YSZ thermal barrier coatings applied by EB-PVD but its application to thermal sprayed coatings has been hindered by optical scattering in plasma sprayed coatings of usual thicknesses. PLPS experiments were performed on TBCs with cold sprayed bond coatings and several different ceramic layer thicknesses after thermal cycling. The results are discussed as a function of the coating characteristics like bond coat spraying process, thickness of both bond and top coat, microstructural features, and damage accumulation.

Performance Testing of Suspension Plasma Sprayed Thermal Barrier Coatings Produced with Varied Suspension Parameters

Suspension plasma spraying has become an emerging technology for the production of thermal barrier coatings for the gas turbine industry. Presently, though commercial systems for coating production are available, coatings remain in the development stage. Suitable suspension parameters for coating production remain an outstanding question and the influence of suspension properties on the final coatings is not well known. For this study, a number of suspensions were produced with varied solid loadings, powder size distributions and solvents. Suspensions were sprayed onto superalloy substrates coated with high velocity air fuel (HVAF) -sprayed bond coats. Plasma spray parameters were selected to generate columnar structures based on previous experiments and were maintained at constant to discover the influence of the suspension behavior on coating microstructures. Testing of the produced thermal barrier coating (TBC) systems has included thermal cyclic fatigue testing and thermal conductivity analysis. Pore size distribution has been characterized by mercury infiltration porosimetry. Results show a strong influence of suspension viscosity and surface tension on the microstructure of the produced coatings.

Adhesion Strength of Thermal Barrier Coatings with Thermal-Sprayed Bondcoat Treated by Compound Method of High-Current Pulsed Electron Beam and Grit Blasting

Microstructural characteristics and adhesion properties of air plasma spraying, thermal barrier coatings (TBCs) having thermally sprayed bond coats treated by high-current pulsed electron beam (HCPEB) irradiation, and a combination treatment of HCPEB and grit blasting were investigated. Microstructure observations revealed that the original coarse surface of thermally sprayed bond coat was significantly changed to interconnected bulged nodules with a compact appearance after HCPEB irradiation. With further grit blasting treatment, these series of bulged nodules appeared with successive and uniform jagged edges. Surface roughness of the bond coat after compound treatment became higher due to these bulged nodules with jagged edges, which was conducive to ceramic deposition. The results of the tensile test revealed that the adhesion strength of thermally sprayed TBCs with the bond coats conducted by a compound treatment was obviously increased. The combination treatment of HCPEB and grit blasting improved the interfacial bonding strength as well as the interfacial homogeneity.

コールドスプレーおよび減圧プラズマ溶射によりボンドコートを施工した遮熱コーティングの熱成長酸化物成長挙動とそのインピーダンス特性

コールドスプレーおよび減圧プラズマ溶射によりボンドコートを施工した遮熱コーティングの熱成長酸化物成長挙動とそのインピーダンス特性

Investigation of the bond coats for thermal barrier coatings on Mg alloy

Bond coats play a significant role in manipulating the stability of 8YSZ thermal barrier coatings (TBCs) deposited on Mg alloy. In this study, different bond coats from the Al@Ni, Ni@Al and MCrAlY (M=Co, Ni) powders were prepared on the Mg alloy by atmospheric plasma spraying. The spraying behavior of the powders, the microstructure, oxidation and corrosion resistance of the produced bond coats were investigated to find the positive bond coat for TBCs on Mg alloy. Results indicate that the composition and structure of original powders affect the structure and properties of the sprayed bond coats. For the candidate bond coats, the MCrAlY bond coat is demonstrated as the most appropriate one for 8YSZ TBCs deposited on Mg alloy substrate, which is mainly attributed to its moderate thermal expansion match and excellent corrosion and oxidation resistance under the simulated corrosion and relatively high temperature conditions.

Thermal cyclic oxidation performance of plasma sprayed zirconia thermal barrier coatings with modified high velocity oxygen fuel sprayed bond coatings

In this study, the Hastelloy-X superalloy samples were overlaid by a CoNiCrAlY bond coating using a high pressure high velocity oxygen fuel (HVOF) spray process. A thin platinum film around 7.5μm thick was further applied to the surface of CoNiCrAlY coating by the magnetron sputtering deposition. Then the superalloy coupons with bare bond coat and with Pt deposited bond coat were pack aluminized at 850°C for 4h to produce (Ni,Co)Al and PtAl2 phases on surfaces, respectively. After that, all samples were overlaid with the yttria-stabilized zirconia (YSZ) top coats by air plasma spraying (APS). Then specimens were subjected to a thermal cycling test at 1100°C. Thermal cycling tests were 1h at 1100°C followed by 10min of forced-air cooling to ambient temperature outside the furnace. The weights of all the specimens were measured every 2cycles. Then effects of aluminizing and Pt-aluminizing on the cyclic oxidation performance and microstructure evolutions of the coatings were evaluated. Scanning electron microscopy (SEM), X-ray diffractometry (XRD) and electron probe microanalyzer (EPMA) were used to identify crystalline phases and microstructures of each coating. The results proved that the roughness of the CoNiCrAlY coating was not changed after the aluminizing or the Pt-aluminizing process. The specific weight gain of the thermal barrier coatings (TBCs) with aluminizing or with Pt-aluminizing bond coating was lower than that of TBCs with only HVOF sprayed bond coat. The cyclic oxidation life of sprayed MCrAlY/ZrO2–Y2O3 thermal barrier coating can be improved effectively by either aluminizing or Pt-aluminizing treatment.

Study of the high temperature oxidation performance of Thermal Barrier Coatings with HVOF sprayed bond coat and incorporating a PVD ceramic interlayer

The performance of an intermediate Cr3C2 ceramic layer applied by PVD between the bond coat and the ceramic top coat in a TBC system was evaluated. The thickness of the transitional layer was kept around 1–2μm. Two substrate materials and two distinct bond coats were combined in the tests. High Velocity Oxygen Fuel (HVOF) and Atmospheric Plasma Spraying (APS) were used respectively for bond coat and top coat deposition. Isothermal oxidation tests were performed at 1000°C in static air atmosphere. Thermal grown oxide (TGO) was measured and correlated to the exposition times. Results are discussed in terms of the TGO growth rate and changes in residual stresses. The results suggest an improvement in the oxidation resistance of the bond coat because of the presence of the intermediate layer.

S044025 コールドスプレーボンドコートを有する遮熱コーティングの熱成長酸化物生成・成長挙動評価

S044025 コールドスプレーボンドコートを有する遮熱コーティングの熱成長酸化物生成・成長挙動評価

Variation of splat shape with processing conditions in plasma sprayed alumina coatings

This paper is on various splat shapes obtained using three alumina based powders sprayed on various substrates. The parameters considered were substrate preheating temperature, nozzle diameter, and secondary and primary gas flow rates. The splat shape was found to be strongly dependent on spraying conditions. The substrate preheating temperature determined the degree of substrate wetting by the splat. A change in either nozzle diameter or primary gas flow rate brought about a change in the particle momentum and subsequently, a change in splat shape. The splat shape differed widely on an as – sprayed bond coat as compared to a polished one, owing to splat confinement by surface asperities. Sub-microscale surface roughness of polished substrate surfaces showed an increase with the preheating temperature and this in turn, resulted in better substrate wetting by the splats.

The role that bond coat depletion of aluminum has on the lifetime of APS‐TBC under oxidizing conditions

AbstractBond coat oxidation as well as bond coat depletion of Al are still believed to be a major degradation mechanism with respect to the lifetime of thermal barrier coating (TBC) systems. In this study the top coat lifetime is described as being limited by both bond coat depletion of Al and mechanical failure of the top coat. The empirical results are introduced by considering three spallation cases, namely, Al depletion failure, thermal fatigue failure, and thermal aging failure. Al depletion failure occurs when the Al content within the bond coat reaches a critical value. In this paper bond coat depletion of Al is modeled by considering the diffusion of Al into both the thermally grown oxide (TGO) and substrate. The diffusion model results are compared to Al concentration profiles measured with an electron beam microprobe. These measured results are from oxidized air plasma sprayed TBC systems (APS‐TBC) with vacuum plasma sprayed (VPS) bond coats for exposures up to 5000 h in the temperature range of 950–1100 °C. This paper focuses on the Al depletion failure and how it relates to top coat spallation.

A physics-based life prediction methodology for thermal barrier coating systems

A novel mechanistic approach is proposed for the prediction of the life of thermal barrier coating (TBC) systems. The life prediction methodology is based on a criterion linked directly to the dominant failure mechanism. It relies on a statistical treatment of the TBC’s morphological characteristics, non-destructive stress measurements and on a continuum mechanics framework to quantify the stresses that promote the nucleation and growth of microcracks within the TBC. The last of these accounts for the effects of TBC constituents’ elasto-visco-plastic properties, the stiffening of the ceramic due to sintering and the oxidation at the interface between the thermally insulating yttria stabilized zirconia (YSZ) layer and the metallic bond coat. The mechanistic approach is used to investigate the effects on TBC life of the properties and morphology of the top YSZ coating, metallic low-pressure plasma sprayed bond coat and the thermally grown oxide. Its calibration is based on TBC damage inferred from non-destructive fluorescence measurements using piezo-spectroscopy and on the numerically predicted local TBC stresses responsible for the initiation of such damage. The potential applicability of the methodology to other types of TBC coatings and thermal loading conditions is also discussed.

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.