A study on processing and hot corrosion behaviour of HVOF sprayed Inconel718-nano Al₂O₃ coatings

Highlights

• HVOF technique was used for depositing IN718-agglomerated nano-Al₂O₃ coatings on GCI.

• Coatings microstructure was investigated after mixing agglomerated nano-Al₂O₃ in various proportions in the IN718 matrix.

• Hot corrosion of the coatings was evaluated at 900 °C for 50 h duration.

• Coating with 30 wt% Al₂O₃ exhibited maximum hardness and hot corrosion resistance.

Abstract

In the present work, processing and hot corrosion behaviour of Inconel718-nano-Al₂O₃ based composite coatings with varying nano-Al₂O₃ contents (10, 20 and 30 wt. %) was studied. The nano-Al₂O₃ powders were synthesised by using manual granulation technique. The composite coatings were deposited on grey cast iron (GCI) substrate by using a high-velocity oxy-fuel process (HVOF). Hot corrosion behaviour of bare and coated specimens was determined at 900 °C in a high-temperature furnace for 50 h. The weight change data was used to establish the oxidation kinetics of bare and coated specimens. The coating with the highest proportion of nano-Al₂O₃ content exhibited the maximum hardness and corrosion resistance. The nano-sized alumina particles enhanced the hardness of the coatings by posing a higher constraint to the plastic deformation during the indentation. The presence of nano-Al₂O₃ and various protective oxides (Cr₂O₃, NiCr₂O₄, and TiO₂) resulted in the improved corrosion resistance of coatings in comparison to the GCI substrate.

Introduction

Grey cast iron (GCI) is a commonly used material for high-temperature applications such as turbocharger housings, turbocharger case, exhaust manifolds, furnace parts and machine tools [[1], [2], [3], [4], [5]]. These components must withstand the hostile environment of cyclic oxidation occurring at elevated temperatures. Failures of these components in engineering applications are generally initiated from the component’s surface [6]. The thermal degradation and mechanical action between the mating surfaces result in the rapid failure of the contacting surfaces and sub-surfaces. This process involves the development of pores and cracks on the top surface and the atmospheric oxygen enters through these paths to cause further oxidation in the sub-surface [[7], [8], [9], [10], [11], [12]]. To overcome the problem of thermal degradation of materials at high temperatures, various methods have been explored. These methods involve the application of surface coatings/cladding and development of high-temperature oxidation-resistant bulk materials. Among these techniques, surface coatings are mainly utilized as wear and corrosion-resistant coatings. The surface coatings appear to be the most viable options to counter the surface degradation of components at high temperatures [13]. The deposition of coatings can be performed by using various techniques such as weld overlays, diffusion coatings, and thermal spray coatings [14]. However, there are some problems related to the diffusion coatings as well as weld overlays such as metallurgical changes in the substrate. Excessive heat input, improper coating thickness, and several overlays pass results in the embrittlement of previous overlays. These problems limit the acceptability of diffusion coatings and weld overlay for general applications [15]. Recently, thermal spraying has attracted considerable attention due to its specific advantages, such as the wide range of material compositions that can be deposited on different substrates The production of high-quality coatings, fewer detrimental effects to base metals, on-site applications, cost-effectiveness, and environmental friendliness are some advantages in the application of coatings [16]. In thermal spraying, combustion takes place inside the chamber, and the products of combustion are generated; these products carry the feedstock powder particles at a very high velocity (200−1000 m/s). These particles (either in a molten, and semi-molten state) impact on the prepared surface of the substrate, where they are deposited in the form of splats to produce a coating of the desired thickness [17]. Hard and dense coatings with excellent wear and corrosion resistance can be produced by using the high-velocity oxy-fuel (HVOF) thermal spraying technique.

Generally, Ni-Cr based coatings are preferred to enhance the hot corrosion resistance of the mechanical components operated at high-temperature conditions. However, the utility of Ni-Cr based coatings is limited in providing the hot corrosion resistance. These coatings are not favoured to combat the wear due to their low hardness values. The wear and hot corrosion resistance of the Ni-Cr based coating can be further improved by introducing some hard phases in the soft Ni-Cr based matrix. It has been reported by many authors that the incorporation of an inert particles (reinforcement) in the metal-based matrix results in improved mechanical, tribological, and anti-corrosion properties of the coatings [[18], [19], [20]]. The NiCr-Cr₃C₂ based composite coating was deposited on the boiler steels with the help of a detonation gun spray (D-gun) process; the deposited coatings exhibited a lesser tendency of spallation under oxidation cycles and solid particle erosion at 700 °C [21]. The thermal cyclic oxidation resistance of the HVOF sprayed NiAl-40Al₂O₃, and Cr₃C₂-NiCr coatings on the 1018 low carbon steel substrate have been evaluated at 400 °C. The coating with 40 wt. % Al₂O₃ as reinforcement has shown the maximum thermal shock resistance [22]. The high-temperature oxidation resistance of Ni-Cr, NiCr-TiC, and NiCr-TiCRe based composite coatings deposited on the boiler steel by using the cold spray (CS) process was also investigated. It has been observed that the NiCr-TiCRe coating provides the maximum oxidation resistance at 700 °C [23]. In another study, the Inconel-718 (IN718) coating deposited on the GCI substrate by using the HVOF process exhibited a significant increase in the oxidation resistance as compared to the above mentioned coatings under similar cyclic oxidation condition of 900 °C and 50 cycles [24]. Moreover, it has been reported that the various mechanical properties viz, strength, hardness, and wear resistance of the composite coatings can be improved further by the addition of inert particles (reinforcement) to the nano-metric scale in the metal-based matrix [25]. However, the use of high-temperature thermal spray techniques results in the decomposition of nano-phase in the nanostructured composite coatings. This is mainly attributed to the higher surface area-to-volume ratio of nano-phase (reinforcement) in the coatings. The higher surface-area-to-volume ratio of nano-phase causes the nano-particles to interact largely with the high-temperature combustion products emerging out of the spray gun [26]. The HVOF process is generally preferred for the deposition of the nanostructured coatings due to its higher velocity and lower flame temperature amongst the other thermal spray processes, which results in the lesser decomposition of the useful nano-phase in the coatings.

In the present work, three different composite feedstock powders were prepared by mixing the Inconel-718 (IN718) and agglomerated nano-Al₂O₃ powders in different weight fractions viz. IN718 + 10 wt. % nano-Al₂O₃, IN718 + 20 wt. % nano-Al₂O₃, and IN718 + 30 wt. % nano-Al₂O₃, respectively. The HVOF coating process was employed for depositing the feedstock powders in the form of coatings on to a GCI substrate. The IN718 was selected as the matrix material because of its exceptional high yield strength and creep resistance at elevated- temperature. This alloy also exhibits excellent impact and tensile strength even at cryogenic temperatures [27]. The IN718 material is widely used in turbine blades, steam generators, jet engines, fusion reactor structures for high-temperature oxidation resistance [28]. Nano-alumina is a widely used material amongst the class of ceramics for its high thermal stability and hardness [[18], [19], [20]]. The HVOF spraying of as-received nano-alumina is not possible due to the lack of momentum gained by the powder particles (smaller sized) during the thermal spraying process. Therefore, the agglomeration of the nano-alumina was performed by manual mixing and mechanical sieving to get the powder particles in the range of 25−65 μm, as required for HVOF spraying process. The focus of the present research work is to analyse the effect of varying agglomerated nano-Al₂O₃ content (10, 20 & 30 wt. %) on the hot corrosion resistance and mechanical properties of the nanostructured composite coating in order to explore and establish its usefulness in high-temperature applications. The growth and spallation tendency of the oxide scales have also been analysed in the developed composite coatings subjected to the hot corrosion at 900 °C for 50 h duration. The outcome of this research shall be utilized for the typical engineering applications like rocketry parts, turbocharger housing and case, boilers, furnace parts, and engine manifolds subjected to high- temperature oxidation at elevated temperatures.