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19/06/2023 Dpt. Seminar and Presentation of Shrikant Joshi, University West at 10:00, 1.32

Topic: Exploiting novel thermal spray approaches to realize a new generation of functional coatings

Presentation of Shrikant Joshi, University West,

Seminar  of Functional Materials and Composites "FMC"   on Monday, June 19, 2023, at 10:00 a.m. in meeting room 1.32 Solid building


The recent emergence of high-velocity air-fuel (HVAF) spray systems and axial-feed capable atmospheric plasma spray (APS) torches have rendered new capabilities to thermal spray researchers and practitioners. In “high velocity” thermal spray techniques, the powder particles are accelerated to very high velocity and processed at temperatures much lower than in APS, leading to coatings that are extremely dense and display very limited thermal damage to the feedstock. The HVAF technique in particular, by virtue of the use of compressed air in place of pure oxygen, lowers the flame and the particle temperatures (~1500 ºC) beside enhancing the kinetic energy. Thus, it can harness combined benefits of cold spray and high-velocity oxy-fuel (HVOF) techniques. The use of  HVAF to realize high-performance tribological coatings, as well as its potential to deposit a host of metallic coatings with minimal oxygen uptake, will be discussed.

Given the traditional difficulties associated with feeding very fine powders, thermal spraying with liquid feedstock is being increasingly recognized to offer an exciting opportunity to obtain coatings with characteristics that are vastly different from those produced using conventional spray-grade powders. The two extensively investigated variants of this technique are Suspension Plasma Spraying (SPS), which utilizes a suspension of fine powders in an appropriate medium, and Solution Precursor Plasma Spraying (SPPS), which involves use of a suitable solution precursor that can form the desired particles in situ. The advent of axial feed capable high power APS systems in recent times has eliminated concerns regarding low deposition rates/efficiencies associated with liquid feedstock. The 10-100 µm size particles that constitute conventional spray powders lead to individual splats that are more than an order of magnitude larger compared to those resulting from the fine (approximately 100 nm - 2 µm in size) particles present in suspensions in SPS or formed in situ in SPPS. The distinct characteristics of the resulting coatings are directly attributable to the above very dissimilar ‘building blocks’ responsible for their formation. The talk will discuss the salient features associated with SPS and SPPS processing, highlight their versatility for depositing a vast range of functional coatings with diverse attributes and discuss their superior performance through some illustrative examples. A further extension of deploying solutions and suspensions that involves use of hybrid powder-liquid feedstock combinations for thermal spraying will also presented. This represents a unique pathway to deposit coatings with unusual microstructures. The talk will describe an elegant arrangement that permits simultaneous and/or sequential spraying of hybrid powder-suspension feedstock. The use of this new approach to create various function-dependent coating architectures with multi-scale features will be described. Convenient realization of layered, composite and graded coatings using this technique has been demonstrated through specific examples.  The utility of this method to develop a wide array of composite coatings will be discussed in particular detail, and illustrated through incorporation of oxides and carbides in a metallic or ceramic matrix. The potential applications for such hybrid processing can open new avenues for exploring novel material combinations and microstructures. Promising early results have encouraged extension of this approach to varied thermal spray systems as well as for facile incorporation into coatings of challenging constituents such as graphene. 

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