CoNiCrAlY Coatings Research Articles

Utilisation of electrospark deposition to restore local oxidation resistance properties in damaged NiCoCrAlY and CoNiCrAlY coatings

The electrospark deposition (ESD) technique has been studied as a potential method to locally repair damaged MCrAlY coatings, permitting restoration of the oxidation properties. The results show that for both the NiCoCrAlY and CoNiCrAlY systems, the ESD process produced an Al supersaturated solid solution, which upon heating, decomposed into the equilibrium γ and β phases. The oxidation products developed at the surface of the ESD were similar in evolution and nature to the scale produced on the substrates, but the thickening was found to be approximately double that of the substrate. This increased oxidation response was associated with a more homogeneous distribution of Al near the surface, and a shorter, faster diffusion path for diffusion towards the surface created by the small grain size of the ESD deposit.On a étudié la technique de dépôt par électroétincelle (ESD) comme méthode potentielle de réparation locale des revêtements endommagés de MCrAlY, permettant le rétablissement des propriétés d’oxydation. Les résultats montrent que dans les deux systèmes de NiCoCrAlY et de CoNiCrAlY, le procédé d’ESD produisait une solution solide sursaturée d’Al, laquelle, lors du chauffage, se décomposait en phases d’équilibre γ et β é. Les produits d’oxydation développés à la surface du ESD étaient similaires en évolution et en nature à l’écaille produite sur les substrats, mais on a trouvé que l’épaississement était approximativement le double de celui du substrat. Cette augmentation de la réponse d’oxydation était associée à une distribution plus homogène de l’Al près de la surface et à un chemin plus court et plus rapide de diffusion vers la surface, créé par la petite taille de grain du dépôt par ESD.

Cyclic oxidation behaviour of different treated CoNiCrAlY coatings

High velocity oxygen fuel (HVOF) spraying method was used in order to obtain very dense and good adhesive CoNiCrAlY-coatings deposited onto nickel-based alloy. The coatings were differently treated (preoxidized, vacuum treated or electron beam irradiated) before their exposure to cyclic oxidation tests in air at 1000°C for periods up to 5h. Changes of the coatings morphology and structure were analysed by scanning electron microscopy (SEM) and X-ray diffraction technique (XRD). The surface temperature of the samples was measured during cooling, between the oxidation cycles, and finally was associated with the thickness of the grown protective oxide scale on the CoNiCrAlY-surface. The experimental results demonstrated that depending on the thickness respectively on the different structures of the grown oxide scale, the cooling rate of the sample surface will be different as well.

Improvement in the microstructure and property of plasma sprayed metallic, alloy and ceramic coatings by pre-/during-treatment of dry-ice blasting

Dry-ice blasting was introduced into plasma spray as substrate's under-cooling system to improve the microstructure and property of coatings. In this study, steel, CoNiCrAlY and Al2O3 coatings were deposited by plasma spray coupled with the pre-/during-treatment of dry-ice blasting. Moreover, the effect of dry-ice blasting as a pre-/during-treatment and only the during-treatment was examined to discover a better way to use dry-ice blasting during thermal spraying. It is revealed that a denser steel or CoNiCrAlY alloy coating with a lower content of oxide can be achieved with the application of dry-ice blasting during the plasma spraying. Such decrease of the oxide content is mainly attributed to the better cooling effect of dry-ice pellets. Interestingly, it is also found that the pre-treatment of dry-ice blasting on the substrate can help to get metallic, alloy and even ceramic coatings with improved adhesions. The adhesive strengths of steel, CoNiCrAlY and Al2O3 coatings were increased by 14%, 5% and 30%, respectively, compared with those of the coatings deposited with conventional air cooling. The cleaning effect of dry-ice blasting is responsible for the increase of the adhesive strength. Based on the present study, dry-ice blasting can be proposed as a pretreatment and cooling system during plasma spray.

On the Early Stage Isothermal Oxidation of APS CoNiCrAlY Coatings

The aim of this study is to analyze the evolution of microstructural and room temperature mechanical properties of air plasma sprayed (APS) CoNiCrAlY coatings before and after early stage high-temperature oxidation. To this purpose, selected samples were isothermally heat treated at 1110 °C for different durations. Phase analysis and oxide scale characterization were performed using x-ray diffraction. Morphological and microstructural features of as-sprayed and oxidized CoNiCrAlY coatings were analyzed by scanning electron microscopy and energy dispersive x-ray spectroscopy. After heat treatment, a duplex oxide scale, composed of an inner α-Al2O3 layer and an outer spinel-type oxide layer, was observed on coating top-surface. The nanoindentation technique was employed to study the evolution of the mechanical properties. An increase in Young’s modulus and hardness with increasing the aging time was observed, this effect was mainly addressed to the partial densification of coating microstructure.

A Comparative Study of Wear Effect on the Microstructures Behavior of CoNiCrAlY Coatings fabricated by APS, HVOF and C GDS Coatings

This work focuses on the micro-abrasion wear and microstructural properties of CoNiCrAlY coatings fabricated on nickel based super alloy substrates by using the atmospheric plasma spraying (APS), high-velocity oxygen fuel (HVOF), cold gas dynamic spraying (CGDS) methods. Tribological tests were performed on the samples in order to understand the wear mechanism of thermally sprayed coatings and influence of the coating microstructure on wear mechanism. The microstructures of as sprayed and weared coatings were investigated by scanning electron microscopy. Initial surface topography was examined by surface profilometer. Coating hardness measurements were performed with a micro-hardness tester. The micro-abrasion tests were carried out with some different durations. The lateral fracture was observed as wear mechanism on the specimens. The wear surface resistance have changed with coating process and surface features of specimens depending on the coating process.

Influence of Surface Finishing on the Oxidation Behaviour of VPS MCrAlY Coatings

CoNiCrAlY coatings were produced by means of the vacuum plasma spraying (VPS) process onto CMSX-4 single crystal nickel superalloy disk substrates. As-sprayed samples were annealed at high temperatures in low vacuum. Three kinds of finishing processes were carried out, producing three types of samples: as-sprayed, mechanically smoothed by grinding, ground and PVD coated by using aluminum targets in an oxygen atmosphere. Samples were tested under isothermal conditions, in air, at 1000 °C, and up to 5000 h. Morphological, microstructural and compositional analyses were performed on the coated samples in order to assess the high temperature oxidation behavior provided by the three different surface finishing processes. Several differences were observed: grinding operations decrease the oxidation resistance, whereas the PVD process can increase the performances over longer time with respect of the as-sprayed samples.

Thermal Cycle Properties of Plasma Sprayed Oxidation-Resistant Metallic Coatings

Thermal cycle resistance of Ni-20Cr, Ni-50Cr and CoNiCrAlY coatings produced by air plasma spraying was investigated according to Japanese Industrial Standard Testing method for thermal cycle resistance of oxidation resistant metallic coatings (JIS H 8452: 2008). The specimens were exposed to a cyclic heating and cooling regimen comprised of up to 100 cycles of 10 hours heating to 1000 °C or 1093 °C in air followed by cooling. The thermal cycle resistance of oxidation-resistant metallic coatings was found to depend strongly on testing temperature and on the chemical composition of the coating materials. In thermal cycle testing at 1000 °C, no remarkable failure was observed in any specimen. However, in thermal cycle testing at 1093 °C, spalling was observed over the entire surface of the Ni-20Cr coating, although the Ni-50Cr and the CoNiCrAlY coatings exhibited excellent thermal cycle resistance even upon exposure to 100 thermal cycles. The CoNiCrAlY coating showed mass gain with increasing number of thermal cycles due to preferential oxidation between thermal spray particle splats. Furthermore, the failure behavior of specimens was investigated in detail by SEM, XRD, EPMA, etc.

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

In this study, high velocity oxy-fuel (HVOF) and vacuum plasma spraying (VPS) coatings were sprayed using a Praxair (CO-210-24) CoNiCrAlY powder. Free-standing coatings underwent vacuum annealing at different temperatures for times of up to 840 h. Feedstock powder, and as-sprayed and annealed coatings, were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The hardness and Young’s modulus of the as-sprayed and the annealed HVOF and VPS coatings were measured, including the determination of Young’s moduli of the individual phases via nanoindentation and measurements of Young’s moduli of coatings at temperatures up to 500 °C. The Eshelby inclusion model was employed to investigate the effect of microstructure on the coatings’ mechanical properties. The sensitivity of the mechanical properties to microstructural details was confirmed. Young’s modulus was constant up to ~200 °C, and then decreased with increasing measurement temperature. The annealing process increased Young’s modulus because of a combination of decreased porosity and β volume fraction. Oxide stringers in the HVOF coating maintained its higher hardness than the VPS coating, even after annealing.

Depth Sensing Indentation of Oxidized Plasma Sprayed CoNiCrAlY Coatings

The aim of the present work is to analyze the evolution of microstructural and mechanical properties of Air Plasma Sprayed (APS) CoNiCrAlY coatings after early stage high-temperature oxidation. Phase analysis and oxide scale characterization were performed by using X-Ray Diffraction (XRD). The microstructural features of CoNiCrAlY coat- ings were analyzed by Scanning Electron Microscopy (SEM), while Nanoindentation (NI) technique was employed to study the evolution of the mechanical properties.

Oxidation behaviour of CoNiCrAlY bond coats produced by plasma, HVOF and cold gas dynamic spraying

Abstract This paper examines and compares the microstructure and oxidation behaviour of CoNiCrAlY coatings manufactured by the APS, HVOF and CGDS deposition techniques. The coatings microstructural features were characterized by means of SEM and XRD analyses. Coating samples were then subjected to isothermal heat treatments at 1000 °C. Oxide growth rates were obtained from a series of mass gain measurements while oxide scale compositions were determined from SEM, XRD and EDS analyses. Results obtained in this study show that the as-sprayed CGDS and HVOF coatings exhibit similar microstructures, whereas the APS coating features high levels of visible defects and oxide content. Oxidation experiments revealed low oxide growth rates for both the CGDS and HVOF coatings as a result of low porosity and oxide content. The oxide scale on the CGDS and HVOF coatings after 100 h of oxidation were composed mainly of alumina without the presence of detrimental fast-growing mixed oxides. The presence of Cr 2 O 3 and dispersed NiO was however also observed for the HVOF coating. As expected, the APS coating featured the onset of mixed oxides in the early stages of oxidation. From these results, it appears that potential improvements to the bond coat oxidation behaviour can be achieved using low-temperature processing methods such as CGDS.

Degradation of Thermal Barrier Coatings by Fuel Impurities and CMAS: Thermochemical Interactions and Mitigation Approaches

Degradation of free-standing yttria-stabilized zirconia (YSZ) and CoNiCrAlY coatings (300 μm) due to V2O5 and a laboratory-synthesized CMAS was investigated at temperatures up to 1400 °C. Reactions, phase transformations, and microstructural development in coatings were examined by using x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The molten deposits destabilized the YSZ and reacted with the thermally grown oxide with various phase transformations and reaction product formation. A dense, continuous environmental barrier overlay, based on oxides, applied by electrophoretic deposition was effective in mitigating the molten deposit attack. Enriching CMAS composition with Al promoted the crystallization of anorthite platelets and MgAl2O4 spinel, and mitigated CMAS ingression. EPD MgO overlay was also effective in protection against V2O5 melt by formation of magnesium vanadates. EPD alumina overlay deposited on thermal barrier coatings with APS 8YSZ and bond-coated IN939 superalloy retained its adhesion and structural integrity after prolonged furnace thermal cycle test at 1100 °C.

Effect of Thermal Treatment on High-Temperature Mechanical Properties Enhancement in LPPS, HVOF, and APS CoNiCrAlY Coatings

The mechanical properties of a MCrAlY coating significantly influence the initiation of cracks in the superalloy substrate under thermomechanical-fatigue conditions. Previous studies have developed a convenient method for evaluating the mechanical properties of sprayed coatings by lateral compression of a circular tube coating. This method does not need chucking, and manufacturing the free-standing coating is quite straightforward. In this study, the mechanical properties of the free-standing CoNiCrAlY coatings prepared using low-pressure plasma spraying (LPPS), high-velocity oxyfuel (HVOF) spraying, and atmospheric plasma spraying (APS) were systematically measured with the lateral compression method at room temperature through to 920 °C. The effect of postspray thermal treatments, in vacuum and in air, on the mechanical properties was investigated in the 400 to 1100 °C temperature range. It was found that high-temperature thermal treatment in air was effective in increasing the bending strength and Young’s modulus. It was especially effective on the APS coatings, which were produced using powders with average size 60 μm, and on HVOF coating, whose bending strengths increased by approximately three times. On the contrary, the enhancement in the LPPS and APS coatings produced with powders 21 μm in size was found to be approximately 1.6 times.

High temperature oxidation of plasma and HVOF thermal sprayed CoNiCrAlY coatings in simulated gas turbine and furnace environments

CoNiCrAlY powders were thermal sprayed using plasma (atmospheric plasma spraying) and high velocity oxyfuel procedures onto AISI 310 austenitic stainless steel specimens in order to study the cyclic oxidation behaviour of these coatings in air and in a simulated gas turbine environment (10% oxygen) at 800 and 1000°C. The oxidation behaviour and hardness evolution of both coatings were significantly different, and the observed weight gain for the plasma spray coating was always much greater due to the presence of substantial open interconnected porosity. In both cases, protective alumina (Al2O3) and spinel CoAl2O4 layers were created.

Cyclic oxidation behavior and microstructure evolution of aluminized, Pt-aluminized high velocity oxygen fuel sprayed CoNiCrAlY coatings

In this study, the Hastelloy-X superalloy samples were firstly overlaid by a CoNiCrAlY bond coating utilizing a high pressure, high velocity oxygen fuel (HVOF) spray process. Then platinum thin film approx. 7.5 µm thick was introduced to selected test samples of CoNiCrAlY coatings by a magnetron sputtering deposition process. Then the HVOF sprayed superalloy coupons, with and without Pt coating were pack aluminized for 4 h at 850 °C to produce (Co,Ni)Al and PtAl 2 aluminide phases on their surfaces, respectively. All specimens were subjected to a thermal cycling test at 1100 °C. Then the aluminizing and Pt-aluminizing effects relative to cyclical oxidation behavior 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. Results clearly indicated that the surface roughness of the HVOF sprayed CoNiCrAlY coatings were unchanged after aluminizing or the Pt-aluminizing process. The oxide scales spalled after 50 h and 100 h cyclic oxidation for the HVOF sprayed sample and aluminized sample respectively, while the oxide scale attached successfully to the substrate for the Pt-aluminized sample after testing for 150 h. It is obvious that the Pt-aluminizing process significantly improves the oxidation resistance of HVOF sprayed coatings, while the isolated aluminizing process demonstrated negligible effect.

The Effect of Heat Treatment on the Oxidation Behavior of HVOF and VPS CoNiCrAlY Coatings

Free-standing VPS and HVOF CoNiCrAlY coatings were produced. The as-sprayed HVOF coating retained the γ/β microstructure of the feedstock powder, and the VPS coating consisted of a single (γ) phase. A 3-h, 1100 °C heat treatment in vacuum converted the single-phase VPS coating to a two-phase γ/β microstructure and coarsened the γ/β microstructure of the HVOF coating. Oxidation of free-standing as-sprayed and heat-treated coatings of each type was carried out in air at 1100 °C for a duration of 100 h. Parabolic rate constant(s), Kp, were determined for free-standing, as-sprayed VPS and HVOF coatings as well as for free-standing coatings that were heat treated prior to oxidation. The observed increase in Kp following heat treatment is attributed to a sintering effect eliminating porosity from the coating during heat treatment. The lower Kp values determined for both HVOF coatings compared to the VPS coatings is attributed to the presence of oxides in the HVOF coatings, which act as the barrier to diffusion. Oxidation of the as-sprayed coatings produced a dual-layer oxide consisting of an inner α-Al2O3 layer and outer spinel layer. Oxidation of the heat-treated samples resulted in a single-layer oxide, α-Al2O3. The formation of a thin α-Al2O3 layer during heat treatment appeared to prevent nucleation and growth of spinel oxides during subsequent oxidation.

Temperature Estimation Based on Microstructural Change of Coatings for a Gas Turbine — Influence of Deposition Process on Microstructural Change and Estimated Temperature —

CoNiCrAlY coatings are deposited by the air plasma spraying (APS), the high velocity oxygen fuel spraying (HVOF) and the low pressure plasma spraying (LPPS). Oxides and pores are formed in the coatings during the deposition process, and their content in APS coating is the largest among the three types of coatings while that in LPPS coating is the smallest. In order to estimate the service temperature of a gas turbine component by means of the microstructural change of the coatings, these three types of specimens are exposed to high temperature in air. A diffusion layer grows at the boundary between the coating and the substrate due to the interdiffusion during the exposure. The layer thickness increases in proportion to the square root of the test time. The temperature estimation based on the relation is applicable to HVOF coating as well as to LPPS coating. However, it is difficult to apply the method to APS coating since its layer thickness is much smaller than those in the other coatings. The exposure test is also conducted in air by means of thermal barrier coatings with CoNiCrAlY bond coats prepared by HVOF and LPPS. HVOF bond coat contains larger amount of the oxides and pores than LPPS one. The bond coat is oxidized during the test, and the Al content decreases in the layer at the vicinity of the surface. The layer thickness increases in proportion to the square root of the test time. The applicability of the temperature estimation based on the relation is clarified for HVOF bond coat as well as for LPPS one.

Stress Histories of Yttria-Stabilized Zirconia and CoNiCrAlY Coatings during Thermal Spraying

Thermal barrier coating (TBC) systems which are used for insulating the substrates of gas turbine blades from high temperature can be made by thermal spraying. The TBC system has residual stresses because of high temperature deposition and the thermal expansion mismatch in the system. In this study, how the residual stress occurs in TBC system was examined by both experimental measurement and FEM analysis. The Yttria-stabilized zirconia (YSZ) top coating was deposited by atmospheric plasma spraying (APS). CoNiCrAlY bond coatings were deposited by high velocity oxygen-fuel (HVOF) spraying and APS. The temperatures of YSZ, CoNiCrAlYs and substrates were measured during thermal spraying. The temperature of YSZ was the highest and that of CoNiCrAlY(HVOF) was the lowest among the three types of spray processes. The residual stresses were elastically calculated by FEM based on the measured temperature histories. The residual stress of YSZ and CoNiCrAlYs on two types of substrates were also measured by X-ray diffraction method. It was confirmed from FEM analysis that residual stress consisted of primary quench stress and secondary thermal mismatch stress. The quench stress was caused by the quenching of coating particles during deposition which occurs due to the huge thermal capacity of the substrate. The thermal mismatch stress was caused by the difference in linear expansion coefficients between coating and substrate. It was found that not only these two mechanisms but also microcrack formation caused by quench played an important role in the residual stress. The temperatures at which residual stresses might begin to occur in the coatings were shown based on the stress relaxation by microcrack formation. It was also found that peening effect played an important role in the residual stress of HVOF sprayed coating.

Indentation of Metallic and Cermet Thermal Spray Coatings

Indentation methods are presented by which the elastic and inelastic stress-strain characteristics of metallic thermal spray (TS) coatings on substrates may be extracted. The methods are based on existing techniques for brittle solids, and adapted for the finite geometry associated with coatings. Basic assumptions and derivations are given, along with guidelines for experimental measurement. Using these, indentation inelastic stress-strain curves are generated for NiCrAlY and Ni-Al bondcoats, as well as WC-Co cermet coatings. Elastic moduli are extracted for CoNiCrAlY coatings. Results are briefly discussed in the context of the effect of feedstock material, process and post-process heat treatment on the intrinsic properties of splats as well as their in-coating cohesion. The methods presented are attractive, particularly for the TS industry, due to the minimal specimen preparation and lack of intricate equipment required for measurement.

CoNiCrAlY microstructural changes induced during Cold Gas Dynamic Spraying

The present study is part of an ongoing research project that aims to develop high performance bond coats by means of Cold Gas Dynamic Spraying (CGDS) for the manufacturing of thermal barrier coatings (TBC). The objective of this work is to investigate the microstructure of a CGDS coating and compare it to that of the original feedstock powder in order to determine whether any microstructural changes have occurred during the deposition process. CoNiCrAlY coatings were deposited using the CGDS system developed at the University of Ottawa Cold Spray Laboratory. Scanning electron microscopy, transmission electron microscopy and X-ray diffraction techniques were used to assess the phases and microstructure of the original feedstock powder and coatings produced. Contrarily to the generally accepted theory that the CGDS process does not lead to changes in the deposited material's microstructure and phase, results from the analysis performed in this study demonstrate the occurrence of important microstructural and phase changes. Evidence of grain refinement of the γ-phase matrix down to the nanometre scale as well as partial dissolution of β-phase precipitates was observed. It is believed that these changes are attributed to the severe plastic deformation encountered by the deposited particles.

Degradation of free-standing air plasma sprayed CoNiCrAlY coatings by vanadium and phosphorus pentoxides

Use of alternative and/or low-cost fuels such as syngas, petcoke and coal/petcoke blend in gas turbine engines requires a thorough understanding in high temperature degradation of protective coatings. Deleterious combustion by-products can deposit, adhere, melt and degrade the protective coatings and underlying structural substrates. In this investigation, degradation of air plasma sprayed (APS) free-standing CoNiCrAlY coatings in contact with two different corrosive oxide contaminants, namely V 2O 5 and P 2O 5 were examined at high temperature. Different degradation mechanisms were observed from V 2O 5 melt interaction with CoNiCrAlY at 700 °C and 900 °C. At 700 °C, formation of chromium–aluminum orthovanadate (Cr,Al)VO 4 was observed with no evidence of severe degradation. However, at 900 °C, extensive dissolution of CoNiCrAlY constituents by V 2O 5 was found with reaction products such as nickel–cobalt orthovanadate (Ni,Co) 3(VO 4) 2 and (Ni,Co)(Al,Cr) 2O 4 spinel. Interaction of P 2O 5 melt with CoNiCrAlY at 350 °C for 2 h revealed the extensive consumption of constituents through formation of polyphosphate compounds such as (Ni,Co)(PO 3) 2 and (Cr,Al)(PO 3) 3.