In Japan, thermal spraying is now becoming one of the most effective surface modification methods for industrial use. This method is widely used for a variety of purposes, such as cathodic protection of steel structures such as bridges and offshore structures, and wear and corrosion resistance of industrial machinery parts. We also develop functional coatings for advanced technologies.
To achieve this, it becomes increasingly important to maintain the performance and properties of spray coatings composed of metallic, ceramic or cermet materials. Research and development for industrial applications is primarily driven by collaborations between end users and coating manufacturers.
In this article, a thorough review of developments in industrial applications, and the fundamental research for thermal spraying in Japan are summarized.
1. Coating Application
1.1 Steel and Nonferrous Industries
Thermal spraying has applied for some major industrial sectors, such as:- Steel rolling and finishing
- Aluminum rolling and finishing
- Pulp and paper
- Aeronautics
- Energy
- Automotive
- Others
The steel industry is a huge market. Different types of coils are used in steel rolling and processing plants. Typical applications are continuous annealing lines (CAL), continuous annealing and processing lines (CAPL) and continuous galvanizing lines (CGL). Figure 1 shows a general scheme of a continuous annealing line. Many thermal spray hearth rolls are used in heat treatment furnaces today.
An important function of the coating is to increase the resistance of the rolling surface to the steel plate at high temperatures for assembly and removal. In Japan, the surface quality and mechanical properties of cold-rolled or galvanized sheets are highly developed in response to the strict specifications of customers such as automobile manufacturers. Therefore, the surface quality of the rollers is very important to maintain high productivity. For this reason, plasma-blasted cobalt or nickel superalloys, oxide/metal composites and chromium carbide cermets are used.
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| Figure 1 General scheme of a continuous annealing line |
1.2 Aeronautical Industry
A major application of spray coatings in the aerospace industry is in various parts of commercial aircraft turbine engines. Atmospheric plasma thermally sprayed zirconium ceramic coatings are widely used as thermal insulation coatings. Copper-nickel-indium plasma coatings are often used to reduce frictional wear. However, these specifications are complemented by the performance of major US and European engine manufacturers. MCrAlY vacuum plasma coating for vanes and motor vanes is also used for the same application.
1.3 Paper Industry
In the paper industry, WC Cermet mirror-smooth coatings are applied to rolls in the coating department of a paperboard mill. This application is designed to replace the hard chrome finish. WC cermet coating, which is superior to chrome in terms of wear resistance, has always allowed wiper blades to be installed on the roller surface. Dust-free facial care has been greatly improved. Keeping roll surfaces clean is still an important issue in paper mills. Therefore, hybrid coatings are developed with wear-resistant and coagulation-limiting materials.
1.4 Automotive Applications
Thermal spray coatings are widely used in automotive engine parts, and automobile manufacturers have developed their own applications. A typical component is an oxygen sensor composed of a solid zirconium mass electrolyte for sensing the oxygen concentration of exhaust gases. A schematic diagram of the cell and a diagram of the operating system are shown in Figure 2. The platinum electrode of the U81 cell is combined with zirconium spinel or plasma-atomized magnesium aluminate to prevent gas flow erosion.
2. New Process Technology
2.1 Development of Functional Materials
National laboratories, research institutes and large corporations studied the use of warriors as national projects. Some components of solid oxide fuel cells (SOFCs) used in power generation are formed by thermal spraying. Investigate the performance of spray molded components such as gas permeability and polarization behaviour.
2.2 Reactive Low-Pressure Plasma Spraying
The synthesis of composite materials is achieved by low-pressure reactive plasma atomization in which nitrides are formed by the plasma reaction. Examples include titanium nitride/titanium composite coatings with improved strength and high temperature abrasion resistance, aluminum-based hybrid materials reinforced with aluminium nitride and silicon carbide fabrics, and inter-metallic composite (IMC) coatings composed of a inter-metallic and nitride composite matrix.
Development. Intermetallic coating materials are studied as materials resistant to oxidation at high temperatures. For example, titanium-palladium alloy coatings are developed using a vacuum plasma spray process in an argon atmosphere on bulk titanium materials. Excellent wet corrosion resistance has been reported to be comparable to bulk titanium materials.
2.3 Radio-Frequency Plasma Spray
Radio frequency (RF) plasmas have large hot regions that are several centimetres in diameter. The process is carried out in an ambient atmosphere and plasma flow rates can be up to several meters per second. When used in plasma spraying, sufficient melting of the sprayed particles is achieved, comparable to conventional DC plasmas. In addition, the combination of RF technology with conventional permanent plasma minimizes the formation of porosity in the coating. For example, this process explores how to form TiAl3, MoSi2 as functional gradient materials and yttria-stabilized zirconia as solid electrolyte by sputtering them, and to synthesize LaCoO3 and NiO as electrodes for solid oxide fuel cells.
2.4 Laser Spray
Modification of sprayed coatings with laser sources is one of the important topics in research and development organizations in Japan. One is the deposition of reactive metals such as titanium. The other is to modify the deposited layer by re-dissolution or diffusion with the substrate. An example of precision thermal spraying is the use of a laser spraying process and a fine powder feeder to deposit thin films of metal and/or ceramic onto printed circuit boards to manufacture electronic circuits.
2.5 Multi-Electrode Plasma Spray
The development of central powder plasma guns is a major innovation and these major innovations are needed to take plasma spray coatings to a new level. Research on the three-electrode plasma torch and gas tunnel plasma injection is ongoing.
2.6 Miscellaneous
2.6.1 Spraying Phenomenon
The analysis of spray phenomena is important for the optimization of spray conditions. One of the main research topics is the measurement of plasma flow and particle scattering velocity. It also studies the flame temperature distribution and the impact force of scattering particles.
We quantified the relationship between the plasma jet temperature and its length, the effect of particle melting conditions, and the effect of particle velocity on adhesion strength.
2.6.2 Testing and Characterization
There are many unknown factors in the properties of thermal spray coatings. Determining bond strength between atomized particles, substrate adhesion strength, and coating residual stress is a common research objective. New testing and characterization methods have been explored in this area. The evaluation of adhesive strength using a centrifugal device combined with an adhesive-free tensile test procedure and an acoustic emission test method was proposed. Residual stress analysis is performed using the X-ray diffraction method. Physical properties such as thermal conductivity and anisotropy are active research topics. This value is very useful for designing and evaluating gradient materials.
TOCALO's motto begins with the words "Technologies and Ideas". Our main priority is to develop unique and creative skills. The Thermal Spray Technology Laboratory plays a central role in the development of our technology.
As the #1 and only company in the thermal spray industry, Labs continues to expand thermal spray applications into new technologies through its continued commitment to superior service and technology.
The R&D laboratory with Thermal Spray technology performs a comprehensive evaluation and analysis of prototype coatings. We develop coatings that meet our customers' performance specifications, test their physical properties, and perform analytical evaluations to ensure that the coatings meet customer requirements.
Our analytical evaluation covers the categories listed in the table, but can also be extended to simulation testing and quality control analysis using real machines.
We are also developing new test methods and test equipment to better evaluate the physical properties of coatings to improve coating performance during processing, as well as existing equipment to better evaluate their compatibility with existing equipment.
