Preparation of MCrAlY–Al2O3 Composite Coatings with Enhanced Oxidation Resistance through a Novel Powder Manufacturing Process
MCrAlY–Al2O3 composite coatings were prepared by high-velocity oxygen fuel thermal spraying with bespoke composite powder feedstock for high-temperature applications. Powder processing via a suspension route was employed to achieve a fine dispersion of α-Al2O3 submicron particles on the MCrAlY powder surface. This was, however, compromised by ~ 50% less flowability of the feedstock during spraying. Nevertheless, the novel powder manufacturing process introduced in this study has shown potential as an alternative route to prepare tailored composite powder feedstock for the production of metal matrix composites. In addition, the newly developed MCrAlY–Al2O3 composite coatings exhibited superior oxidation resistance, compared to conventional MCrAlY coatings, with the formation of nearly exclusively Al2O3 scale after isothermal oxidation at 900 °C for 10 h. The addition of α-Al2O3 particles in the MCrAlY coatings as a second phase was found to have promoted the formation of YAG oxides (YxAlyOz) during spraying and also accelerated the outwards diffusion of Al, which resulted in enhanced oxidation resistance.
Keywordsalumina bond coat HVOF MCrAlY oxidation suspension
Alumina-forming coatings, also known as bond coats, have been widely used on hot section of gas turbine engines since early 1960s (Ref 1-5). Among them, a bond coat system of MCrAlY (M = Ni/Co) alloy has been developed based on a balance among several aspects that include oxidation, corrosion and ductility (Ref 6-8). High-velocity oxygen fuel (HVOF) thermal spraying is one of the most widely used coating techniques for the deposition of MCrAlY coatings for high-temperature corrosion/oxidation protection (Ref 9-11). HVOF thermal-sprayed MCrAlY coatings are normally used within the temperature range of 950-1100 °C with the formation of a 1-10-μm-thick protective α-Al2O3 scale. This, however, creates a temperature gap for the use of MCrAlY bond coats in other high-temperature applications, e.g., ultra-supercritical (USC) steam boilers and turbines, and other turbomachinery for higher efficiencies and lower emissions, which are in operation at temperatures below 950 °C but in excess of 650 °C (Ref 12-14). No protective α-Al2O3 scale would form on the coating surfaces at these low temperatures as restricted by both kinetic and thermodynamic factors. Metastable θ- and γ-Al2O3 that are non-protective (voids) would form instead in the temperature range of 700-950 °C (Ref 15), which are also fast growing with oxidation rate constants (kp) being 2-3 orders of magnitude higher than that of α-Al2O3.
Efforts have been made to develop MCrAlY-Al2O3 composite coatings with dispersion of Al2O3 particles on the surface as nucleation sites to promote the early formation of a coherent α-Al2O3 scale (Ref 16-20). The challenge is, however, there is no established process to prepare composite powder feedstock with a homogenous distribution of the second phase particles with submicron or nano-sizes that are prone to agglomeration (Ref 6). A recent study by Bolelli et al. (Ref 21) produced NiCrAlY–Al2O3 composite coatings by a “hybrid” spraying technique with two injection nozzles feeding both dry NiCrAlY powder and a suspension of Al2O3 particles simultaneously into the gas stream. More recently, satelliting, as a manufacturing technique (Ref 22-25), was introduced to mix small quantities of hard reinforcing submicron or nanoparticles into larger volumes of soft metallic matrix powders. The “satellited” powders were obtained by mixing the two powders in a water solution containing 2.7 wt.% of polyvinyl alcohol (PVA) as a binder, followed by a drying process. This satelliting process has been patented (Ref 22), and it is now used for a wide range of composite manufacturing applications that involves powder feedstock, e.g., additive manufacturing and cold spraying (Ref 23-26). In this study, a similar powder mixing process via a suspension route was introduced with the aim to achieve a uniform distribution of Al2O3 submicron particles with MCrAlY powders, albeit without any binders. The composite feedstock powders were then sprayed onto stainless steels by conventional HVOF thermal spraying. The oxidation behavior of the composite coatings during isothermal heat treatment at 900 °C for 10 h was studied compared to the conventional MCrAlY coatings. The effect of the α-Al2O3 dispersion on the composite coatings’ oxidation was also discussed.
Powder and Substrate
A commercial MCrAlY powder for thermal spraying (Amdry9624, spheroidal, gas atomized, Oerlikon Metco, UK) was used as feedstock with a nominal composition of Ni Bal., Cr 21.0-23.0, Al 9.0-11.0, Y 0.8-1.2 wt.% and a nominal size distribution of − 37 + 11 μm, mixed with a commercial alumina powder (CR1, 100% alpha, Baikowski, France) with a D50 particle size of 1 µm. The feedstock powders were sprayed onto AISI 304 stainless steels substrates (nominal composition Fe-19.0Cr-9.3 Ni-2.0Mn-0.05C wt.%) with a dimension of 60 × 25 × 2 mm.
Preparation of Composite Feedstock
A suspension route was employed to prepare the composite feedstock without any additives. Ethanol denatured (industrial methylated spirit) with a concentration of ethanol over 99% in volume was used as the suspension medium. The mixing process was completed in less than 10 min on a hot plate at 150 °C with a magnetic stirring (500 rpm) until fully dried. An AS-300 Hall Flow Meter was used to test the flowability of the composite powder feedstock before spraying. The flowability of the composite powder feedstock was found to be poor, so equal weight of original MCrAlY powders was added to improve the flow rate of the composite feedstock and mixed in a rotary milling system (CAPCO Test Equipment 2VS, Suffolk, UK) at a speed of 250 rpm for less than 1 h—the conditions chosen did not change the particle size and shapes. The homogeneity of the final feedstock was checked under optical microscope. The final composite powder feedstock containing 5 wt.% of Al2O3 was then used for spraying.
HVOF Thermal Spray Process
Substrates were first grit blasted with brown alumina (F100, 0.125-0.149 mm) under 6 bar pressure and then cleaned by an ultrasonic acetone bath to remove any embedded alumina particles. The substrates were mounted onto a carousel rotating at 73 rpm with a vertical axis of rotation. Metjet IV, a liquid fuel-based HVOF thermal spray system (Metallisation Ltd., UK), was used for the deposition of the MCrAlY–Al2O3 composite and the conventional MCrAlY coatings with kerosene as the fuel. Both coatings were deposited with six passes under the same condition, and the samples were air-cooled during spraying. The length of the nozzle of the HVOF thermal spray gun was 100 mm, and a standoff distance of 356 mm was used during the spray runs. The flow rates of kerosene and oxygen were 476 mL/min and 900 L/min, respectively, giving a total flow rate of 28.2 g/s and 100% stoichiometry, while nitrogen was used a carrier gas for the powder. The as-sprayed samples were then cut into a dimension of 10 × 10 × 2 mm and placed in alumina crucibles. Isothermal oxidation treatment was carried out at 900 °C for 10 h in a box furnace with a heating/cooling rate of 10 °C/min.
Scanning electron microscope with JEOL in-lens Schottky field emissions source (FEG-SEM, JEOL 7100F, USA) equipped with an energy-dispersive x-ray (EDX) detector (silicon drift detector size: 150 mm2, Oxford Instruments, UK) was used to examine the feedstock powder, the coating surface and cross sections in secondary electron (SE) and back-scattered electron (BSE) modes. An x-ray diffractometer (XRD, D500 Siemens Germany) with Cu Kα radiation (1.5406 Å) was used to identify the phases presented in the powders and coatings in the 20° ≤ 2θ ≤ 90° range with a step size of 0.05° and dwell time of 2 s. For the oxidation analysis, the coating samples were mounted in epoxy resin filled with Ballotini glass beads for low shrinkage, and the cross sections were cut and polished down to a 1 μm finish.
Thermodynamic calculation of oxidation products as a function of effective partial pressures of O2 at 900 °C was carried out by Thermo-Calc® software (Version 2017b) with TCS Ni-based superalloys (TCNi8.1) database following CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) (Ref 27, 28). The use of Thermo-Calc® on alumina-forming and chromia-forming alloys systems has been validated by our previous work (Ref 29, 30) and others (Ref 31-38).
Composite Feedstock Powders
As-sprayed Composite Coatings
The MCrAlY-based composite coatings reinforced by alumina or other refractory oxides (e.g., Y2O3, ZrO2) have already been developed previously via different routes (Ref 41-45), and most of these studies were only focused on the wear performance of the composite coatings. This is because the addition of ceramic reinforcement hard phase into metallic coatings was primarily aimed at improving its mechanical strength against abrasion from room temperatures up to 800 °C, while the high-temperature oxidation behavior was only investigated by a few studies (Ref 46, 47). As a result, the beneficial effect of the alumina addition on the oxidation behavior of MCrAlY composite coatings has not been extensively investigated, especially at the early stages of oxidation. The experimental results in this study have demonstrated the improvement in the oxidation resistance with the formation of exclusive α-Al2O3 scale on the MCrAlY–Al2O3 composite coatings produced from the composite powder feedstock containing 5 wt.% of α-Al2O3 submicron particles. Although new Al2O3 also formed in between lamella and on the top surface during spraying due to the oxidation of MCrAlY powder, the original α-Al2O3 particles addition was still detectable by the EDX elemental analysis (Fig. 4 and 5) based on the different segregations of Y on the top surface and cross section of these two coatings. The EDX semiquantitative analysis gave the concentration of Y in the composite coatings (cross section: 1.6 wt.% and top surface: 1.2 wt.% based on three measurements) relatively higher than that of the MCrAlY coatings (cross section: 1.2 wt.% and top surface: 1.0 wt.%). Since the starting concentration of Y in the MCrAlY powder was only 0.8-1.2 wt.%, it means that the relative concentration of Y in the selected area for EDX analysis of the composite coatings was increased 1/3 to 1/2. This is possible because the high specific surface area of the submicron alumina particles would significantly increase the probability of reaction with Y and O during spraying, forming stable YAG compounds (YxAlyOz) as a mixture of Y2O3 and Al2O3 leading to a locally higher Y content near the alumina particles. The presence of YAG was also observed by Toma et al. (Ref 48) in the HVOF thermally sprayed MCrAlY coatings doped with Al and Y metallic powders in the feedstock. Furthermore, Cr is another important indicator that could reveal the presence of the original α-alumina. The coexistence of Al and Cr is commonly seen in oxides (e.g., corundum) after oxidation of MCrAlY powders (Ref 18). The absence of Cr in the Al2O3-rich oxides in the composite coatings (Fig. 5) indicates that these lamella oxides in the composite coatings were retained from the original alumina particles that contain no Cr at all.
The dispersion of α-Al2O3 particles, as a second phase, as well as other reactive element (RE) oxides, on the oxidation resistance of ODS (oxide dispersion strengthening) alloys have been discussed at length (Ref 49, 50). The mechanism involves with the dynamic-segregation theory (DST), in which these oxide dispersions mainly segregate at the oxide grain boundaries and oxide–metal interface (Ref 51). The segregations of α-Al2O3 and YAG oxide would hinder the outwards diffusion of undesirable outward cation transport (Ref 52). In particular, the diffusion coefficient of Al along the grain boundary of Al2O3 is several orders of magnitude higher than that of Ni and Cr [Al: 2.8 × 10−4 m2/s; Cr: 6.9 × 10−11 m2/s; Ni: 2.53 × 10−10 m2/s (Ref 53)]. As a result, the growth of these non-protective spinel and NiO oxides would be greatly inhibited by the dispersed Al2O3 particles. The EDX semiquantitative results in Fig. 7 already showed that the Al content on the composite coatings top surface after oxidation (26.1 wt.%) is nearly twice that on the MCrAlY coatings (13.6 wt.%), while the initial compositions of both coatings prior to oxidation are nearly the same with the Al content being 10-11 wt.% and the Ni content being 62-63 wt.%. The increase in the Al content after oxidation indicates that the outwards diffusion of Al in the composite coatings during oxidation outperformed the competitive outwards diffusion of Ni. The element of Cr, on the other hand, remains a similar level (16-17 wt.%) in both coatings after oxidation (Fig. 7), and also, appears to be granular precipitates with an isolated and dispersed distribution, and a size of 1-2 µm inside the two coatings as shown earlier in Fig. 9. It implies that the Cr was less involved in the competitive outwards diffusion with Ni and Al during oxidation. In addition, the dispersion of the α-Al2O3 and YAG would act as the nucleation sites for the newly formed α-Al2O3 phase (Ref 15) and also suppress the formation of other transient metastable Al2O3 (e.g., θ, γ-Al2O3) or shorten its phase transformation to stable α-Al2O3 (Ref 54, 55). It would also inhibit interfacial void growth, thus improving scale adhesion. To summarize, the improved oxidation resistance of the composite coatings with accelerated outwards diffusion of Al is believed to be caused by the dispersion of the α-alumina particles, in combination with the YAG formation during spraying of the composite powder.
A detailed process has been introduced in this study for the preparation of MCrAlY–Al2O3 composite coatings via a suspension route followed by HVOF thermal spraying. A good distribution of alumina submicron particles was observed with the MCrAlY powders, and this, however, led to poor flowability for thermal spraying. Moreover, the presence of α-Al2O3 particles promoted the oxidation of Y during spraying forming YAG inter-lamella oxide. The composite coatings exhibited also superior oxidation resistance with the formation of nearly exclusively Al2O3 scale after short-term oxidation tests at 900 °C for 10 h in comparison with the MCrAlY coatings. The presence of α-Al2O3 particles in the composite coatings is believed to have accelerated the outwards diffusion of Al cations and inhibited Ni and Cr along the grain boundaries during oxidation, which therefore promoted the formation of Al2O3 scale.
This work was supported by the Engineering and Physical Sciences Research Council (Grant Number EP/M01536X/1). The authors would like to acknowledge Mr. Rory Screaton for the experimental assistance in thermal spray. Dr. Mingwen Bai would also like to acknowledge many helpful discussions on power plant materials with Prof. Hao Liu and Prof. Wei Sun at the University of Nottingham during the USC-CFB-CMM project meetings.
- 5.R. Lima, D. Nagy, and B. Marple, Bond Coat Engineering Influence on the Evolution of the Microstructure, Bond Strength, and Failure of TBCs Subjected to Thermal Cycling, J. Therm. Spray Technol., 2015, 24(1-2), p 152-159Google Scholar
- 13.G.R. Holcomb, B.S. Covino Jr., S.J. Bullard, S.D. Cramer, M. Ziomek-Moroz, and D.E. Alman, Ultra Supercritical Turbines–Steam Oxidation, Albany Research Center (ARC), Albany, 2004Google Scholar
- 16.D. Maghet, G. Marginean, I. Mitelea, A. Davidescu, and W. Brandl, Comparison of oxidation behaviour of various thermally sprayed MCrAlY coatings, The European corrosion congress, Breisgau, 2007Google Scholar
- 22.A. Clare and A. Kennedy, Additive Manufacturing. Google Patents, 2016.Google Scholar
- 29.Z. Pala, M. Bai, F. Lukac, and T. Hussain, Laser Clad and HVOF-Sprayed Stellite 6 Coating in Chlorine-Rich Environment with KCl at 700 °C. Oxid. Met. 2017, p 1-23.Google Scholar
- 50.B. Pint, Progress in Understanding the Reactive Element Effect Since the Whittle and Stringer Literature Review, in Proceedings of John Stringer Symposium on High Temperature Corrosion. ASM International Materials Park, Ohio, 2003, p. 9-19Google Scholar
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.