Conclusions

Thermal spray is a unique materials processing tool capable of processing the widest range of materials into useful forms: coatings of widely varying thickness, monolithic spray-formed materials, and spray-formed composite or graded structures. Thermal spray processing commonly uses powders as feedstock, and as described here both the form and characteristics of the starting powders play key roles in the development of the final thermally sprayed microstructure. The process heat source, combustion versus plasma, also has a significant influence on the microstructure of the coating or spray-formed deposit. The ultimate properties of sprayed materials, for example in coatings, are quantified in terms of the density/porosity, hardness, wear and/or corrosion resistance, thermal conductivity, and electrical resistivity. Key physical properties of spray-formed structures can also include coefficient of thermal expansion, tensile and creep strength, and fracture toughness. The structures of thermally sprayed materials therefore depend on the materials processing route used, including powder processing, heating rates and dwell times, particle velocity, and on the environmental exposure during processing.

Advances in thermal spray will continue to stimulate its growth as an important powder consolidation technique, especially for difficult-to-form materials. Important advances to date include:

• Emergence of the SHS route as an economical means of producing a wide range of improved composite feedstock materials for thermal spray forming

• Ever-growing knowledge and application bases for reactive plasma spray processing for synthesizing a wide range of advanced materials

• Growth of thermal spray as a method of producing a wide range of FGMs for thermal, mechanical, chemical, and electrical applications

Thermal spraying, developed just over a century ago, is now on the verge of many new and important forming applications. Thermal spray forming is now benefiting from the increased processing and materials knowledge that will allow increased use of its structures and characteristics. Further understanding of the potential for thermal spray consolidation of powders and the potential of this route for forming, however, necessitates that the influences of the processes on material microstructures be studied and understood. The last 25 years of research have been dedicated to this, and now new applications for the forming of materials for molds, dies, tooling, functionally graded structures, refractory metals, and composites are being established. The future challenges facing thermal spray are cost effectiveness, together with improved structure/property control, using highly reliable, repeatable processes. The potential is there, and many recent advances in materials and process control have ensured that industry is using thermal spray as both a coating and a forming process.

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