Thermal Spray Techniques for Fabrication of Meso-Electronics and Sensors

Using optical emission spectroscopy in atmospheric conditions to track the inflight reduction of plasma sprayed TiO2-x feedstock for thermoelectric applications

Thermal spray deposition (specifically Atmospheric Plasma Spraying, APS) is a well-established surface coating technology with a broad scope of applications (i.e., insulative coatings, tribological coatings, anti-corrosion coatings, etc.). In addition, there is a constant drive to introduce the APS process into new and emerging fields. One such niche application for APS would be sub-stoichiometric TiO2-x coatings with enhanced thermoelectric performance (compared to the bulk material). The APS process in this context has a unique ability-given the use of hydrogen as a plasma gas-to reduce TiO2-x material during processing. However, to this point, there is neither a reliable nor self-consistent method to assess (nor control by parametric optimization) the inflight reduction of molten oxide particles during processing. This study shows that using Optical Emission Spectroscopy (OES), it can be possible-even in atmospheric conditions-to identify characteristic emission peaks associated with the inflight reduction of TiO2 during APS. Using this OES data, the input spray processing parameters and their influence on coating microstructure and the degree of inflight reduction of the material will be shown. Results suggest under equilibrium conditions only a minimal amount of hydrogen gas is needed in the plasma to fulfill the TiO2 reduction.

Thermally Sprayed Functional Coatings and Multilayers: A Selection of Historical Applications and Potential Pathways for Future Innovation

Thermal spray coatings are material systems with unique structures and properties that have enabled the growth and evolution of key modern technologies (i.e., gas turbines, structurally integrated components, etc.). The inherent nature of these sprayed coatings, such as their distinctive thermal and mechanical properties, has been a driving force for maintaining industrial interest. Despite these benefits and proven success in several fields, the adoption of thermal spray technology in new applications (i.e., clean energy conversion, semiconductor thermally sprayed materials, biomedical applications, etc.) at times, however, has been hindered. One possible cause could be the difficulty in concurrently maintaining coating design considerations while overcoming the complexities of the coatings and their fabrication. For instance, a coating designer must consider inherent property anisotropy, in-flight decomposition of molten material (i.e., loss of stoichiometry), and occasionally the formation of amorphous materials during deposition. It is surmisable for these challenges to increase the risk of adoption of thermal spray technology in new fields. Nevertheless, industries other than those already mentioned have benefited from taking on the risk of implementing thermal spray coatings in their infrastructure. Benefits can be quantified, for example, based on reduced manufacturing cost or enhanced component performance. In this overview paper, a historical presentation of the technological development of thermal spray coatings in several of these industries is presented. Additionally, emerging industries that have not yet attained this level of thermal spray maturation will also be discussed. Finally, where applicable, the utility and benefits of multilayer functional thermal spray coating designs will be demonstrated.