Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Thermal-spraying of polymers can be traced back to the 1940s, when polyethylene (PE) was first produced by E.I. du Pont de Nemours & Company. Early work with flame-spraying guns was unsuccessful because the equipment, designed for spraying metals, produced a flame that was both too hot and too short to melt the PE without degradation. The technology has advanced considerably in the past 20 years. Flame-spray equipment and hardware accessories have been designed by numerous companies (PFS Thermoplastics, Alamo, Eutectic + Castolin, etc.) specifically for polymeric materials. The new equipment provides either a lower flame temperature or uses special cooling shrouds to cool the center of the flame, providing a longer residence time at a lower temperature. Many plastics can now be completely melted in-flight and allow heat-sensitive components to be coated with plastic.

A range of physical and chemical properties of flame‐sprayed ethylenemethacrylic acid copolymer (EMAA) were assessed, following different processing conditions. Coatings were produced at a range of specific temperatures by varying the propane flow rate and gun traverse rate. The flame spraying process oxidizes the EMAA copolymer during processing, the extent of oxidation increasing with greater deposition temperatures. Coatings were scanned using dielectric relaxation spectroscopy at a frequency range from 102 to 106 Hz over a temperature interval of ‐20 to 85°C. The glass transition temperature (usually denoted as the β′ relaxation in this system), is attributed to the microBrownian motion of long chain segments in the amorphous phase and is found to decrease with increasing deposit temperature. The oxidation process appears to reduce the position of the β relaxation due to chain scission. The molecular weight for the EMAA powder was reduced from 22,693 g/mol to 9302 g/mol when deposited at 271°C as shown by gel permeation chromatography. Despite the decrease in chain length, the activation energies for β′ relaxation increased with increasing coating temperatures. This is attributed to the increased polarity of the oxidized coatings resulting in greater intermolecular association, which outweighs the decreased chain length.

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MesoScribe Technologies has developed a process for producing embedded, conformal, thick film sensors based on Direct Write technology. Thermocouple and heat flux sensors can be fabricated directly onto engineering components and embedded into functional coatings. This provides for a variety of vital advantages: reliability, robustness and survivability in extremely harsh environments, cost effective implementation, and fabrication onto surfaces that are large, conformal (non-flat) and flexible.Embedded thermocouples and heat flux sensors were deposited onto superalloy substrates and subjected to a number of high temperature tests including isothermal furnace heating, cyclic burner rig testing, and continuous flame impingement.Initial testing yields Seebeck coefficients within 3% of commercial thermocouples. Results also demonstrate that embedded Type K thermocouples survive over 200 thirty minute burner rig cycles with surface temperatures exceeding 1150°C. Embedding thermocouples at different depths within the TBC allows for simultaneous temperature measurements within the temperature gradient. In addition, over 20 hours of continuous flame impingement have been recorded with stable output. Embedded thermocouples were also tested at NASA GRC using a 3.5 kW CO 2 high heat flux laser which also allows extraction of thermal conductivity. The test comprised of 75 thirty minute cycles with a surface temperature of 1150°C and metal interface temperature of 930°C for a total duration of 41 hours. This very first test showed the capability of the embedded TC in terms of performance and durability. This paper will summarize the harsh environment test results as well as provide an overview of the capabilities of Direct Write technology to instrument propulsion and space structures. 1,2 1 0-7803-9546-8

Polymeric composites, with either metallic or ceramic fillers, have been manufactured by thermal spray, and their mechanical properties have been measured. The advantage of this technology is that it allows on-site manufacture and is a repairable composite system, with virtually no cure time and no release of volatile organic compounds. Fracture mechanisms have been studied to examine mechanical modeling of the composite system.

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