Composite Coatings Employing a Novel Hybrid Powder and Solution-Based Plasma Spray Technique for Tribological Applications

Composite Coatings Employing a Novel Hybrid Powder and Solution-Based Plasma Spray Technique for Tribological Applications

G. Sivakumar (International Advanced Research Center for Powder Metallurgy and New Materials, India) and S. V. Joshi (International Advanced Research Center for Powder Metallurgy and New Materials, India)
Copyright: © 2015 |Pages: 27
DOI: 10.4018/978-1-4666-7489-9.ch003
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Solution Precursor Plasma Spray (SPPS) technique is an emergent technique which offers significant performance advantages with various functional coatings like YSZ, ZnFe2O4, LSM, metal-doped ZnO, etc. Apart from the above capabilities, efforts towards development of an innovative hybrid approach through simultaneous deposition of powder and solution resulted in coatings with unique microstructures. Some illustrative examples of the large variety of coatings that can be realized through the hybrid route combining the conventional plasma spray and SPPS techniques are discussed. The attractive prospects offered by hybrid technique for spraying nanocomposite coatings are specifically highlighted through a case study. Successful development of hybrid coatings using a Mo-alloy powder and a suitable oxide-forming solution precursor has been shown to exhibit improved sliding wear performance. The relationship between the splats, the ensuing coating microstructure at varied processing conditions, and its tribological behaviour of the coatings is comprehensively discussed.
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Among various coating methods, the thermal spray based techniques have gained the most widespread industrial acceptance till date. Ever since the first application of thermal spraying was demonstrated in 1909 by Dr. M.U. Schoop (Schoop, 1909), the technique has been widely used for applying coatings on various components to protect them particularly from corrosive attack or wear. The availability of numerous variants of the thermal spray technique, the diversity of spray materials and continuing advances in spray-control systems have together created new opportunities for the thermal spray industry. Above all, the versatility of the thermal spray process is unmatched by any other surface modification technology (Holmberg and Matthews, 1994, Pawlowski, 1995, Schneider et al., 2006, Fauchais et al. 2001).

Thermal spraying is a generic term which relates to processes in which the desired coating material is deposited in semi-molten or molten or solid state on a substrate to form a layer. The feedstock material used conventionally has been in the form of powder, rod or wire with use of powder based feedstock being most popular. Numerous spray variants, such as flame spray, arc spray, plasma spray, HVOF, detonation spray, etc. (Holmberg and Matthews, 1994, Pawlowski, 1995, Schneider et al., 2006)are available today, with each process employing a unique method of generating a high enthalpy stream to heat and accelerate the particles. It may be noted that the spray variants are distinct, particularly as far as the gas temperature and gas velocity profiles in the high enthalpy zone are concerned. These most crucially determine the physical state of particles at the time of deposition (molten, semi-molten or solid) and the velocity to which the injected particles are accelerated prior to impact with the substrate to be coated.Typical deposition efficiencies of most thermal spray variants range between 30-60%. This is a major concern from a commercial standpoint, due to the use of typically expensive powder feedstock, which alone accounts for nearly 60% of the overall coating costs, and the low deposition efficiencies make thermal spray a relatively expensive deposition technique.

In recent times, there is also a growing interest in spraying nanostructured or nano-sized powders, mainly due to the characteristic advantages that are expected to accrue compared to micron-sized powders. Thermal spraying is a rapid consolidation process with a potential to realize nanostructured features in a single step (Fauchais et al. 2008). Accordingly, it has been presumed that use of nano-sized feedstock can improve the characteristics and tribological performance of various coatings (Stewart et al., 1999, Lima and Marple, 2007, Qiao et al. 2003). However, the use of nanosized particles for spraying presents numerous challenges, with some prominent ones being,(i) high production costs associated with manufacture of nanosized feedstock, (ii) difficulty in injecting nanosized particles into the core of the spray plume/flame, and (iii) lack of inertia of fine-sized particles after melting to impact the substrate.

One method that has been attempted to overcome the problem of feeding is to agglomerate the nano/sub-micron sized particles prior to feeding (Lima and Marple, 2007). However, this usually results in an inhomogeneous melting behavior, yielding a combination of nano-sized and micron-sized (Qiao et al. 2003). An alternative approach that has been adopted by various researchers involves use of a liquid instead of gas as a carrier medium and therefore, introduce the nanoparticles in the form of a suspension to eliminate feeding related problems (Fauchais et al. 2008, Bouyer et al., 1996, Fauchais et al., 2011, Bolelli et al., 2010).

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