- Abstract
- Introduction and Background
- Recent and Emerging Spray Processes
- Cold Spray
- Suspension and Solution Precursor Thermal Spray
- Very Low Pressure Plasma Spray
- Thermal Spray Markets and Applications
- Electronics Industry, Semiconductor Manufacturing
- Automobile Industry
- Land-Based and Aerospace Gas Turbines
- Energy Conversion and Harvesting
- Wear Resistant and Corrosion Resistant Coatings
- Process Robustness, Reliability, and Economics
- References
Key Research Challenges in Thermal Spray Science and Technology
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Published:2022
Abstract
This article summarizes the results of work completed by the ASM Thermal Spray Society Advisory Committee to identify key research challenges and opportunities in the thermal spray field. It describes and prioritizes research priorities related to emerging process methods, thermal spray markets and applications, and process robustness, reliability, and economics.
Introduction and Background
Thermal spray is regarded as a key surface engineering technology that underpins the competitiveness of critical manufacturing and engineering industries. It is being developed continuously to meet market evolution and new requirements. This evolution raises challenges that must be addressed for the advancement of thermal spray processes and coatings, and to direct academic research into promising industrial applications that offer opportunities for R&D investment.
Experts in academia and industry were surveyed by the Thermal Spray Advisory Committee to identify the scientific and technology issues facing recent and emerging spray processes as well as the future development of established processes. The outcome is a list of priority items that will enable industry and academia to develop improved products in terms of quality and performance at a lower life cycle cost.
Many of these topics and additional research opportunities are addressed in an extensive thermal spray roadmap article published in the Journal of Thermal Spray Technology in 2016.[1]
Recent and Emerging Spray Processes
Cold Spray
Cold spray is a solid-state coating process that uses a high-speed gas jet to accelerate powder particles toward a substrate where particles plastically deform and consolidate upon impact. This technique is suitable for the deposition of oxygen- and temperature-sensitive materials and can produce thick coatings exhibiting wrought-like microstructures with near theoretical density. Cold spray is being used in the military and in aerospace and energy industry applications.
The main research priorities identified are:
| Powders |
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| Cold spray equipment |
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| Spraying onto real parts |
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| Process control to increase process reliability |
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| Effect on substrate |
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| Bow shock |
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| Opportunities to embrace |
|
Suspension and Solution Precursor Thermal Spray
The demand for fine-structured coatings has led to the development of thermal spray processes that are based on plasma and HVOF systems. Thus, the use of liquid feedstock in the form of suspensions or solutions enables the production of coatings with unique microstructures such as columnar and segmented structures and ones with highly porous/high specific surface area, and dense features. Potential applications include thermal barrier coatings with stress relieving structures and with lower thermal conductivity for aerospace and gas turbines. Other applications include fuel cells, photocatalytic and anti-microbial coatings, and biomaterials or biotechnology uses.
These technologies are not sufficiently mature to meet industrial standards and customer needs. However, collaboration among researchers will provide understanding of the scientific principles and behavior of such processes. In this fashion, equipment manufacturers will develop processes that are reliable, robust, and easy to adopt within an industrial environment.
The research priorities identified to ensure a rapid increase in technology readiness include:
| Feedstock |
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| Spray process |
|
| Process control to increase process reliability |
|
| Opportunities |
|
Very Low Pressure Plasma Spray
Very low pressure plasma spray (VLPPS) differs from traditional thermal spray processes in that coatings are deposited at chamber pressures typically less than ~1000 Pa. Deposition may be in the form of fine molten droplets, vapor phase deposition, or a mixture of vapor and droplet deposition. The coatings exhibit similar quality to those produced by physical vapor deposition (PVD) or chemical vapor deposition (CVD); however, the VLPPS deposition rates can be an order of magnitude higher. The VLPPS process can be used to produce dense, high quality coatings in the 1 to 100 micrometer thickness range with lamellar, columnar, and mixed microstructures.
Specific questions to be answered to further industrial acceptance of this technology include:
| Plasma spray process |
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| Opportunities |
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Thermal Spray Markets and Applications
Thermal spray is used by a large spectrum of manufacturing industries. It is vital to improve the performance of components and industrial products to maximize their life cycle. However, some sectors, such as electronics applications, are still relatively unexplored. Also, traditional sectors are facing the global pressures on prices, regulatory demands, and global shortage of critical raw materials.
Electronics Industry, Semiconductor Manufacturing
Thermal spray technology can be further explored for electronic materials and electronics applications, keeping in mind that these sectors employ existing, competitive coating technologies. A wide range of electronics applications are promising, including sensors, wiring, antennas, radio frequency devices, and systems for energy storage and energy harvesting. The high throughput, material versatility, and processing at ambient conditions attributes of thermal spray are attractive features for these applications.
| Electronics industry required capabilities |
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| Opportunities |
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Automobile Industry
Advanced mechanical and automotive engineering is crucial for most national economies in order to compete in the global markets and, also, in value adding and creating jobs for people with higher academic education. Automotive engineering and industrial manufacturing are basic to mass production of thousands of parts per day. Manufacturing engineering in these industries requires a complex and deep knowledge base with a distinct interdisciplinary approach. High quality and productivity can only be achieved by process automation with highly sophisticated control systems.
To a certain extent, the thermal spray business and technology is still dominated by traditional small and medium size enterprises and spray shops. Their focus is on materials engineering and coating process technology that will add value to the products of their customers.
| Automotive industry application challenges |
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| Opportunities |
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Land-Based and Aerospace Gas Turbines
Thermal spraying is a well-established surface engineering technology in the gas turbine sector, especially in producing thermal barrier coatings (TBCs) that can withstand the high fuel gas temperature needed for efficient engine operation and low pollutant emissions.
TBCs for land-based and aerospace gas turbines are one of the current key market segments of thermal spraying. However, such applications are now facing new challenges such as increased firing temperatures, longer service life requirements, use of non-conventional fuels (such as syngas and low-BTU gases), global shortages of certain materials (e.g., Y, Yd, He), and larger and more complex parts.
| Gas turbine application research areas |
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| Opportunities |
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Energy Conversion and Harvesting
Both population growth and economic development are accompanied by an ever-increasing consumption of energy. However, the use of fossil fuels to meet this increasing demand has resulted in the rapid increase of carbon dioxide in the atmosphere, which in turn translates into climate changes and extreme weather events. It is therefore necessary to develop renewable energy sources and also to increase the thermal efficiency of fossil fuels.
Thermal spray technology has the potential to contribute to energy conversion and harvesting, in particular in the following applications:
| Thermoelectric Energy Converters (TEC) | Thermoelectric devices can convert thermal energy to electricity in a variety of applications (e.g., waste heat from power stations and vehicle exhaust pipes). Many materials exhibit high thermoelectric effects but generally within a narrow temperature range. Current developments involve the combination of several thermoelectric elements efficient at different temperature levels to broaden the useful temperature range and increase the output voltage. They are typically produced by powder metallurgical methods including sintering processes, which involve relatively long processing times at high temperatures, limiting hence the spectrum of applicable materials. In principle, thermal spraying can produce near-net shape multilayered structures with constant or functionally graded composition of material and porosity. Therefore, multi-layered TECs present an interesting challenge for plasma technology with potential for a large field of R&D and new applications. |
| Alkaline Water Electrolysers (AWE) | Hydrogen is becoming increasingly important as energy carrier and energy storage medium in connection with renewable energy sources. Up to now most hydrogen is produced from reforming of hydrocarbons (e.g., natural gas). An alternative is the production by water electrolysers using electricity from wind, solar or nuclear energy. Alkaline water electrolysers with standard pure metal electrodes have a long history. However, to make the hydrogen production more efficient new electrodes with activated surfaces should be developed, suited for intermittent operation as required by renewable energy sources. In small size such electrodes have already reached the desired quality, but electrodes in technical size (larger than 1 m2) represent still a significant challenge for thermal spray in an economical and reliable way. For this purpose, several issues must be addressed including optimization of (i) spray material with respect to composition and powder particle size and of (ii) electrode layer structures; (iii) preparation/activation of substrate surface for stable bonding of coating, (iv) means and methods for preventing deformation of large metal substrates during the spray process, and (v) industrialization of the production process, which also means improvement of spray equipment. |
| Solid Oxide Fuel Cells and High Temperature Electrolysers of Solid Oxide Electrolyser Cell-Type (SOEC) | Fuel cells directly convert chemical energy into electricity with an efficiency much higher than that of other energy conversion processes. Different fuel cell types exist in the whole temperature range between ambient temperature and about 1000 °C. The lower temperature types need mostly hydrogen as fuel gas, which must be produced first by reforming of hydrocarbon fuels, whereas the high temperature types, like SOFCs, have intrinsic reforming qualities and can use hydrocarbon fuels directly. Therefore, the latter are of great interest for combined electricity and heat supply in stationary application with outputs from 100 W to 2 MW. They can also be used as electricity source in vehicles as “Auxiliary Power Units” (APU). Hundreds of cells are then arranged electrically to form SOFC stacks and units. Thermal spray techniques are of potential interest to produce cells with limited or no post treatment. For planar “Metal Supported Cells” for car APUs, the multi-layered cell structure is sprayed directly on the surface of the metal base sheet This concept involves cells with an overall thickness of about one millimetre. However, the production of thin and fully dense electrolytes is one of the issues faced by thermal spraying to produce SOFCs. High temperature solid oxide electrolyser cells (SOEC) which need less specific electric power due to involved thermal energy have significant potential for hydrogen production and energy harvesting when they are coupled to a renewable or nuclear heat source. Their operation principle is the inverse of that of SOFCs with operating conditions even more demanding than those of SOFCs. The materials and processing methods are similar and therefore, thermal spray techniques may also be of interest for their production. |
Wear Resistant and Corrosion Resistant Coatings
The cost of wear and corrosion are estimated to be a significant fraction (up to 3-5%) of the gross domestic product in developed economies. These phenomena result in the degradation and eventual failure of components and systems in the processing and manufacturing industries and reductions in the service life of components.
Thermal sprayed coatings prevent, or limit, the effects of corrosion and wear by providing protection against chemical and physical interactions of a material with its environment. However, there is still room for the development of cost-effective coatings that can resist corrosion and wear under specific conditions. In addition, the search for reliable coatings that improve the performance and extend the operating range of applications of structural materials has accelerated because of worldwide concerns related to reducing energy consumption, conserving resources, and minimizing the emission of the products of corrosion to the environment.
Thermal spray coatings exhibit microstructures that are different from those in coatings produced by other coating technologies. Thermal spray microstructures have highly oriented features with lamellae of varied sizes and shapes parallel to the substrate surface; variable quality of contact and bonding between lamellae interfaces; and some porosity, cracks, and possible inclusions of partially melted particles. This morphology has important implications with regard to the performance of coatings used against corrosion and wear.
| Coatings for corrosion and wear resistance research areas |
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| Opportunities |
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Process Robustness, Reliability, and Economics
As is true for every coating technology, reliability and reproducibility have been essential for the early adoption of thermal spray technologies in industrial manufacturing. Process productivity, quality assurance, and customer confidence have been improved by the development of (i) thermal spray control systems that accurately control the gas, electrical, and cooling water requirements and (ii) diagnostic tools for the plume and/or the substrate.
Diagnostic tools are now well accepted in the thermal spray community. Over the last 20 years, they have been quite helpful, especially for spray parameter optimization, booth-to-booth parameter transfer, process troubleshooting, powder lot validation, go no-go monitoring, and similar analysis. However, continued development is needed before these tools are systematically used for daily production.
| Process robustness, reliability, and economics research areas |
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| Thermal spray hardware research opportunities |
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| Process control research opportunities |
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| Environmental impact research opportunities |
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Acknowledgments
This document was created in 2014. The following experts contributed to identify the scientific and technological key issues in thermal spraying:
Armelle Vardelle (France)
Christian Moreau (Canada)
Christopher C. Berndt (Australia)
Dianying Chen (USA)
Pierre Fauchais (France)
Rainer Gadow (Germany)
Aaron Hall (USA)
Rudolf Henne (Germany)
Friedrich Herold (USA)
Éric Irissou (Canada)
Eric Jordan (USA)
Yuk-Chiu Lau (USA)
Klaus Landes (Germany)
Chang-Jiu Li (China)
Jon Longtin (USA)
André G. McDonald (Canada)
Luc Pouliot (Canada)
Mark F. Smith (USA)
Robert Vassen (Germany)
Jan Wigren (Sweden)
Motohiro Yamada (Japan)
References
ASM Thermal Spray Society, Key Research Challenges in Thermal Spray Science and Technology, Thermal Spray Technology: Accepted Practices, By ASM Thermal Spray Society, ASM International, 2022, p 20–29, https://doi.org/10.31399/asm.tb.tstap.t56040020
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