Thermal spraying is a well-known coating technology with many variations in spraying techniques, feedstock materials and substrate materials. These unique variations increased its industrial applicability in different fields, including aerospace, automotive, chemical process, corrosion protection, and medical applications. However, one of the main limitations of thermal spray is the difficulty of depositing several nitride ceramics directly using conventional techniques. This is due to the decomposition of nitride particles under high temperature without a stable melting phase. This chapter presents reactive plasma spraying (RPS) technology as a promising solution for the in situ fabrication of several nitride ceramic coatings. The main attractive prospects of RPS for fabricating nitride coatings are specifically highlighted. Successful development of various high-temperature nitride coatings, such as AlN, Fe4N and Si3N4, are presented. Process optimization, the relationship between reaction and process parameters and the influence on coatings formation are comprehensively discussed.
TopIntroduction
Surface coating is a suitable solution to improve the performance of industrial parts by improving the surface function ability, enhancing the component lifetime and reducing the manufacturing cost. The coating techniques are divided into thin film and thick film deposition techniques. Thin film deposition techniques are mainly based on chemical/physical vapor phase deposition phenomena. Thick films can be deposited using several techniques, including wet chemical or electrochemical process, overlay welding, brazing, and thermal spraying. Each technique has certain advantages and certain limitations. Among the thick film deposition techniques, thermal spray deposition is well known industrially due to the broad variations of the following:
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Materials (metallic or nonmetallic materials)
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Material forms (powder, ceramic rod, wire, molten materials, suspension, solution-precursor)
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Spraying systems (high velocity oxy-fuel, flame, wire arc, detonation, plasma, laser-plasma, atmospheric vacuum, and high frequency)
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Substrate materials, forms, shapes and dimensions (length, width and thickness)
These unique variations in thermal spray increase its industrial applicability in different fields, including aerospace, automotive, power, chemical process, corrosion protection, and medical applications. In general, thermal spray technology is based on molten or semi-molten phase deposition to form the coatings. Thus, the solid feedstock (in the form of a powder, wire, rod, suspension, or solution precursor) is introduced into a hot environment (gas, flame, or plasma jet) to form liquid droplets, which are accelerated and propelled onto a substrate material. The molten or semi-molten particles impact on the substrate, rapidly solidify and coalesce to form the thermal-sprayed coating. However, one of the limitations of thermal spray technology is the difficulty of depositing several nitride ceramics directly using conventional thermal spray techniques. This is attributed to the decomposition of nitride particles under high temperature without a stable melting/semi-melting phase.
One suitable solution to fabricate nitride ceramics is to use the reactivity of plasma and to encourage the plasma nitriding reaction of the molten feedstock materials. Reactive plasma spraying (RPS) is a promising solution for in situ fabrication of nitride ceramic coatings and other compositions, which are difficult to deposit using conventional thermal spray techniques. The process is based on the chemical reactions between the feedstock materials (elemental metal or non-metal powder) and the surrounding active species in the plasma (such as atoms, ions, and radicals) to develop the required materials. Several nitride ceramics, including titanium nitride (TiN), aluminum nitride (AlN), iron nitride (Fe4N) and silicon nitride (Si3N4) have been successfully fabricated using the RPS process. In the last decade, development of the reactive plasma spray technology to fabricate thick nitride ceramic coatings has received significant attention in the field of thermal spraying.
This chapter presents the main concepts of reactive plasma spraying technology and the applicability of reactive spraying in different spraying systems and feedstock materials. Furthermore, it summarizes the state of current knowledge about plasma nitriding as well as the main aspects and challenges for implementing RPS to fabricate different nitride ceramic coatings using the RPS process.