Hybrid Processing with Powders and Solutions: A Novel Approach to Deposit Composite Coatings
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JTTEE5 24:1166–1186 DOI: 10.1007/s11666-015-0262-y 1059-9630/$19.00 Ó ASM International
Hybrid Processing with Powders and Solutions: A Novel Approach to Deposit Composite Coatings S. V. Joshi and G. Sivakumar (Submitted January 29, 2015; in revised form June 9, 2015) Thermal spraying employing either solution precursor or suspension feedstock offers an exciting opportunity to obtain coatings with microstructures and characteristics that differ vastly compared to those conventionally produced using spray-grade powders. Both solution precursor plasma spray and suspension plasma spray techniques have been explored extensively in recent years for depositing wide ranging ceramic coatings for various functional applications. Encouraged by the properties achieved using the above solution-based feedstocks, a hybrid approach involving dual injection of solutions and powders, either simultaneously or sequentially, has been proposed and demonstrated to yield novel coating architectures. Although prior work on such hybrid processing is very limited, this overview seeks to present the concept, outline associated challenges, and demonstrate its efficacy to realize coatings with exciting and unusual properties using some illustrative examples.
Keywords
applications, hybrid coating, in situ formed powder, splats, solution precursor plasma spray
1. Introduction The use of liquid-based feedstock, either in the form an appropriate precursor solution or a suspension of nanosized particles in a suitable medium, has been extensively investigated in the past few years. Although only few representative relevant research efforts are being cited throughout this paper for the sake of brevity, the above approach has widely been shown to be attractive to achieve unique coating microstructures with promising properties (Ref 1, 2). Early work by Karthikeyan et al. (Ref 3, 4) using atomized liquid droplets and suspensions to deposit nanostructured ceramic coatings provided the initial encouragement by demonstrating the possibilities of liquid-based spraying. Subsequently, the interest in exploring liquid-based spraying further has expanded significantly and is now being pursued actively by various research groups for diverse applications (Ref 5-16). The two most widely studied forms of liquid-based thermal spraying are the solution precursor plasma spray (SPPS) and the suspension plasma spray (SPS) techniques. Injection of a nanoparticle suspension rather than a spray-grade powder through a dedicated device into a thermal spray torch assembly to deposit coatings has been the basis of suspension spraying. The process requires a
S.V. Joshi and G. Sivakumar, International Advanced Research Center for Powder Metallurgy and New Materials (ARCI), Balapur P.O., Hyderabad, India. Contact e-mail: [email protected].
1166—Volume 24(7) October 2015
stable suspension of the desired powder to be coated, formulated using an aqueous or organic solvent, and typically uses fine starting powders directly without any additional agglomeration step (Ref 1, 2
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