Overview of Thermal Spray Technology

This document is intended to educate members of government, industry, and academia as to the benefits of thermal spray technology. Further research in applied science/engineering and in fundamental science as well as collaboration among academia, government, and universities are needed to continue advance aspects of thermal spray technology from theory to practice. It is anticipated that greater understanding and appreciation of the value of thermal spray for improved sustainability will lead to industry and government funding for additional growth.

Appropriate application of thermal spray technology helps to improve and sustain our way of life by reducing energy consumption, providing environmental benefits, supporting human comfort, and reducing material waste.

Thermal spray is an established industrial method for the surfacing and resurfacing of engineered components.^{[1, 2]} Metals, alloys, metal oxides, metal/ceramic blends, carbides, wires, rods, and various composite materials can be deposited on a variety of substrate materials to form unique coating microstructures or near-net-shape components. Thermal spray coatings provide a functional surface to protect or modify the behavior of a substrate material and/or component. A substantial number of the world’s industries utilize thermal spray for many critical applications.^[2] Key application functions include:

• Restoration and repair

• Protection against corrosion

• Protection against forms of wear, such as abrasion, erosion, and scuffing

• Heat insulation or conduction

• Prevention of oxidation and hot corrosion

• Electrical conduction or insulation

• Near-net-shape manufacturing

• Sealing

• Engineered emissivity

• Abradable coatings

• Decoration

Thermal spray processes are easy to use, cost relatively little to operate, and have attributes that are beneficial to applications in almost all industries. The benefits of this environmentally friendly process are typically reduced cost, improved engineering performance, and/or increased component life (Fig. 1).

Figures 2(a) and 2(b) show the requirements for thermal spray: a heat/energy source and consumable materials.^[2] Gases, along with air in some cases, are needed to inject materials into the thermal spray gun/torch and to generate the necessary heat for melting. The high gas velocities associated with these processes cause the material to be propelled as fine molten droplets that impinge on the part, solidify, and adhere. The mechanism of bonding is mostly mechanical, but in some cases is also metallurgical. The result is that each layer bonds tenaciously to the previous layer, forming a lamellar “pancake-like” splat structure. The coating properties are directly dependent on the combination of kinetic and thermal energy.

Figure 3 shows the categories of thermal spray processes in the marketplace today.

Five basic thermal spray processes are available commercially:

Of the five processes, HVOF and detonation spraying are two that result in high bond strength with extremely dense microstructures. Plasma coatings are also known to have high bond strength with relatively dense oxide-free microstructures when sprayed in either low pressure plasma spray (LPPS) or vacuum plasma spray (VPS) systems.

Cold spray is a relatively new process that relies more on high velocity and kinetic energy and less on thermal energy. It is a solid state process; that is, the particles are not molten during spraying. Particle temperatures in cold spray are lower than in HVOF, but velocities are higher, enabling coating structures that resemble bulk wrought materials.

A summary of typical temperatures and velocities of the sprayed particles/droplets for the various processes and materials are shown in Figure 4(a) and 4(b). Additional details about each process are provided in the article “Thermal Spray Processes” in this Volume.

Figure 4(b) shows the relative particle temperatures versus velocities for various thermal spray materials. This fundamental understanding is critical for spray parameter optimization of process and material. For example, metal oxide ceramics with high melting points and low thermal conductivity typically require high enthalpy and particle temperatures with slow flame velocities for optimum performance and application cost. Carbide cermets require the opposite conditions: low particle temperature and high particle velocities. For cermets, these parameters result in optimized wear resistance with minimum decarburization and brittle phases being formed in the coating. As can be seen in Figure 4(b), other material types fall somewhere in between metal oxides and carbide cermets.

The market size for thermal spray is an estimated $6.5 billion per year.^[3] Key market segments are in aerospace and industrial gas turbine industries; it is estimated that 60% of thermal spray industry revenue is generated in these markets.^[4] Equipment and material suppliers constitute approximately 20% of the market, with the balance attributed to coating applicators and finishers. Historically, the most active regions for thermal spray technology have been in North America and Europe. However, developments over the last few years have shown higher market growth in Asia and South America. China, for example, has seen tremendous growth due to the high degree of professional engineers graduating yearly and its overall economic growth. Traditional markets such as aerospace, are expected to remain strong, but growth is expected to come from alternative energy, semiconductor/electronic devices, steel, and paper and pulp.^[5]

A few examples where thermal spray technology has helped to sustain our way of life by reducing energy consumption, providing environmental benefits, supporting human comfort, and reducing material waste are seen below. More examples of application success stories can be found in the ASM Handbook, Volume 5A, Thermal Spray Technology.^[6] The handbook captures the value and benefits thermal spray provides to society with examples from the following industries: turbine, marine and atmospheric corrosion, renewable energy, oil sands, automotive, electronic and semiconductor, biomedical, landing gears (replacement of hard chrome with thermal spray), primary metals/steel, paper, printing, nuclear, petrochemical, and textile. Addendum 2 also summarizes many of the applications highlighted in this handbook. A few applications critical to thermal spray and market growth are briefly discussed below.

Aerospace and industrial gas turbines will continue to expand as thermal spray technology and reliability improve. Important areas of growth are in advanced heat insulation materials (thermal barrier), wear resistant coatings, clearance control coatings, and oxidation/hot corrosion resistant alloys. The value of these coatings is higher operation temperatures leading to more efficient engines, reduced energy cost, reduced CO2 emissions resulting in less greenhouse effects to the environment and less material waste due to reduced maintenance times.

With a world population of more than 7 billion people and growing at the time of this writing, emerging countries, as well as technically advanced countries, will continue to need new and improved energy solutions.

Hydroelectric, wind, fuel cells, bio-fuels, biomass, and solar are just a few examples of where thermal spray can play a role in renewable energy solutions in addition to the more traditional fossil fuel areas such as coal, gas, and oil. Turbine manufactures and energy producers are aware of these sources of energy and have started to synergistically integrate multiple technologies when cost appropriate.

As medical technology helps to extend human lifespans, medical implants will become more common and thermal spray applications will play a bigger role. One key area where thermal spray technology is playing a role is as an inert surface for bone and tissue growth. Coatings have been used to support the advancement of knee and hip implants.

New environmental regulations imposed by governments on car mileage have forced manufacturers to look at alternative solutions to reduce car weight and improve engine efficiency. One area of growth has been the use of cylinder bore coatings on aluminum blocks. Aluminum blocks help reduce the weight of a vehicle while the porous nature of the coating helps to reduce friction between the cylinder and piston ring.

In addition to improvements in traditional processes, the industry continues to see growth in cold spray. This will result in the growth of salvage and repair applications for critical and expensive components. The U.S. military is developing and implementing specifications for critical components. Examples of key materials being studied include aluminum, zinc-aluminum, aluminum-magnesium, copper, titanium, and Ni-Fe-Cr alloys. Potential areas for growth in cold spray are in materials development and in material manufacturing processes.

Thermal spray has been shown to be a an environmentally friendly and sustainable technology relative to alternative technologies.^[6] At present, HVOF sprayed carbide coatings are being used to replace chromium electroplating for aircraft landing gears and hydraulic rods in industrial applications. Such examples of environment friendly sustainable manufacturing are desirable for gaining support for market growth.

Several professional organizations, trade associations, and research groups are actively working to further develop thermal spray technology and to provide opportunities for networking, education, and professional advancement for thermal spray technologists and researchers.

ASM International’s Thermal Spray Society (TSS), Germany’s Welding Society (DVS), Japan Thermal Spray Society (JTSS), European Thermal Spray Society (ETSA), Thermal Spray Committee of China Surface Engineering Association (TSCC), Japan Thermal Spray Association (JTSA), Korean Thermal Spray Association (KTSA), International Thermal Spray Association (ITSA), German Thermal Spray Association (GTS) as well as many other associations and societies are dedicated to expanding thermal spray applications, improving the technology, and promoting the industry. Toward this end, annual international and regional conferences are sponsored by leading professional societies such as the International Thermal Spray Conference (ITSC) sponsored by the ASM Thermal Spray Society, along with its partner the German Welding Society. This conference rotates every year between Asia, Europe, and North America. In addition, TSS publishes the Journal of Thermal Spray Technology and ASM Handbook, Volume 5A: Thermal Spray Technology.^[6] These publications, along with international conferences such as ITSC and smaller topical events, help the growth and promotion of emerging technology and applications.

Organizations in Asia promote conferences such as the Asian Thermal Spray Conference (ATSC), which provides a venue for thermal spray researchers in countries such as Japan, China, Korea, India, and Singapore to share advances. Additional organizations and conferences are listed in the article “Guide to General Information Sources” in ASM Handbook, Volume 5A.^[6]

A key objective for the thermal spray community is to bridge the gap between research and commercialization, with the goal of accelerating technology transfer to the market. A second goal is to demonstrate technological solutions to real problems. A third goal is education and training of students in industrial environments. Many industry organizations and trade groups exist that are working in support of these goals.

Many government research organizations and university-based institutes are performing research to support innovation and growth of thermal spray technology. A few key institutes from around the world are listed below to highlight that thermal spray is indeed recognized globally as a critical surface engineering science.

Many of these international research organizations and universities are exploring new ways to improve human life through new applications of thermal spray technology. Key research areas include, but are not limited to, improving the reliability and robustness of these processes, expanding materials and equipment technology, and advancing applications in energy and transportation sectors. Development of solution precursor and suspension thermal spray, cold spray, diagnostics, and modeling are examples of such innovative research areas. The article “Key Research Challenges in Thermal Spray Science and Technology” in this volume lists areas where research and development advances are needed to further expand the growth and applications of thermal spray processes.