Key Takeaways & Executive Findings
- •• Optimized wet ball milling at 60 r/min produces mullite/nano-fly ash feedstock powders with a uniform particle size of 14 µm, enhancing plasma spray deposition efficiency to 35%. • Wet-milled coatings exhibit significantly lower porosity (7.9%) and superior mechanical properties, including hardness of HV1 647 and fracture toughness of 1.41 MPa·m0.5. • Residual stress analysis reveals higher tensile stresses (up to 165.95 MPa) in wet-milled coatings due to finer particles and rapid thermal cycling, which may influence coating durability. • The study demonstrates a sustainable approach by repurposing fly ash, aligning with circular economy principles and reducing raw material consumption in coating applications.
Abstract
This study examines how ball milling parameters, specifically rotational speeds (20, 40, and 60 r/min) in dry and wet conditions, affect the development of mullite/5wt% nano-fly ash coatings on AISI 410 steel, focusing on their impact on feedstock powders and plasma-sprayed coatings. Optimized milling parameters at 60 r/min under wet conditions yielded high-quality feedstock powders with a particle size of 14 µm and limited size distribution. Coatings produced from wet-milled powders demonstrated a higher deposition efficiency (35%) due to their smaller, uniformly distributed particles, which enhanced melting during the spraying process. These coatings also exhibited significantly lower porosity (7.9%), resulting in denser structures with superior mechanical properties, including a hardness of HV1 647, fracture toughness of 1.41 MPa·m0.5, and a smoother surface finish with a roughness (Ra) of 6.1 µm. Residual stress analysis showed that wet-milled coatings had higher residual stresses, reaching up to 165.95 MPa, compared to dry-milled coatings. This increase is attributed to finer particle sizes and rapid thermal cycling during deposition, which intensified tensile stresses within the coating. These results highlight the importance of optimizing milling parameters to enhance coating performance and process efficiency.
1. Introduction
Recently, green or sustainable coatings were primarily distinguished by their minimal or zero emissions throughout production and application. Nevertheless, the concept has evolved to encompass a wider range of sustainability considerations, highlighting factors such as health, climate impact, and adherence to circular economy principles. Ensuring sustainability within the coating industry entails diverse initiatives, including conserving energy and resources, minimizing waste generation, enhancing process efficiency, and incorporating renewable materials [1]. To attain these sustainability objectives, embracing innovative methodologies is imperative, and one promising avenue entails integrating fly ash as an additive in plasma-sprayed coatings. Fly ash, a residue generated from coal, oil, and biofuel combustion in thermal power plants, offers a sustainable solution by curbing raw material consumption and providing cost-effectiveness through repurposing an industrial byproduct [2].
Plasma-sprayed coatings find wide applications across diverse industries such as aerospace, industrial gas turbines, automotive, oil and gas, and medical components. These coatings are instrumental in elevating component performance and sustainability. They provide insulation against extreme temperatures, extend material lifespan, decrease corrosion and wear rates, and mitigate the need for premature replacements. The adoption of these coatings resonates with global endeavors to embrace environmentally friendly materials and practices [3–6]. Mullite coatings, highly regarded for their ability to protect against extreme temperatures, wear, and corrosion, have attracted significant interest across various industries. Plasma spraying, recognized as a versatile thermal spray technique, is extensively employed for applying mullite coatings because of its capability to produce dense and firmly adhered coatings, surpassing alternatives like zirconia [7].
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Peerawatt Nunthavarawong, Torsak Boonthai, Masaki Fuchiwaki (2025). Mechanical properties of mullite/5wt% nano-fly ash feedstock powders produced using mechanical alloying methods for plasma spraying: Towards sustainable coating solutions. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-025-3137-3
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Frequently Asked Questions
What is the main objective of this study?
The study aims to investigate the effect of ball milling parameters (rotational speed and wet/dry conditions) on the development of mullite/5wt% nano-fly ash feedstock powders and their resulting plasma-sprayed coatings on AISI 410 steel, focusing on mechanical properties and process efficiency.
How does wet milling improve coating properties?
Wet milling at 60 r/min produces finer and more uniformly distributed particles (14 µm), which enhance melting during plasma spraying, leading to higher deposition efficiency (35%), lower porosity (7.9%), and improved mechanical properties such as hardness (HV1 647) and fracture toughness (1.41 MPa·m0.5).
What is the significance of using fly ash in this research?
Fly ash is an industrial byproduct that is repurposed as a sustainable additive, reducing raw material consumption and waste, aligning with circular economy principles and promoting environmentally friendly coating solutions.
What are the key findings regarding residual stress?
Wet-milled coatings exhibit higher residual stresses (up to 165.95 MPa) compared to dry-milled coatings, attributed to finer particle sizes and rapid thermal cycling during deposition, which intensify tensile stresses within the coating.
What is the potential application of these coatings?
The mullite/nano-fly ash coatings are suitable for protecting components in extreme environments, such as aerospace, industrial gas turbines, automotive, and oil and gas industries, where resistance to high temperatures, wear, and corrosion is critical.
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