In all thermal sprayed coatings, including those deposited by HVOF, the porosity inherently present as well as in situ thermal degradation of feedstock are recognized limitations for demanding wear applications. The supersonic HVAF spray technique provides highly adherent coatings with negligible porosity (<1 vol. %) due to higher particle velocities at impact and provides new prospects to impact the current state-of-the-art approaches, from both a technical and economic standpoint [23
]. The ability to further refine microstructures significantly employing different nozzle configurations (Figure 2
) is an added feature that can be gainfully utilized to deposit high-performance coatings.
2.1. Corrosion Protection Coatings
With a specific view to extend capabilities of the Thermal Spray Group beyond its extant expertise on TBCs to address other industrially relevant areas, preliminary investigations were first carried out to demonstrate the promise of HVAF to outperform HVOF coatings in both corrosion and wear prone environments. The results illustrated in Figure 3
, summarized from different studies carried out in the authors’ group [23
], provide an ideal foundation to embark on other projects focusing primarily on advantages that can be derived from HVAF deposition of coatings. Motivated by the ability of HVAF to mitigate decarburization of carbide feedstock and also yield nearly fully dense coatings, a majority of the HVAF-related efforts address wear and corrosion applications.
For reasons of sustainability and cost, there is growing interest in the use of biomass, waste, and industrial byproducts such as black liquor for power generation. For example, many power generation technologies are capable of using biomass as a fuel, and over 2900 active biomass power plants exist worldwide [29
]. Similarly, there are also important financial considerations that make use of black liquor from the kraft process attractive for power generation. Boilers utilizing wastes, prone to significant corrosion and degradation of components in different operating environments, are well-documented [30
]. Corrosion from alkali-chlorides is also significant in such boilers and is often further complicated if demolition wood from old buildings or railroad beams, with appreciable content of metals like lead and zinc, is used [31
]. Similarly, the black liquor recovery boilers (BLRBs) are also beset with problems of corrosion [33
]. Power plants also often face the combined attack of corrosion and erosion, with the combustion chamber, surfaces of superheaters and economizers, and ash removal systems being prone to such synergistic damage [34
Notwithstanding the above, and despite the considerable resources invested to develop means of combating corrosion, it continues to present a major challenge in power production from renewable fuels. Development of new alloys to address the problem has started to yield diminishing returns and, consequently, application of protective coatings is widely acknowledged to be the most promising option. Coatings deposited employing different techniques and utilizing varied material chemistries have been researched and deployed in boilers [35
] but a coating that can be maintenance free for an extended period as desired by the industries remains elusive.
In recognition of the above, a chosen focus area at University West deals with the development of advanced HVAF-sprayed coatings to combat corrosion in boilers. Specifically, nearly fully dense Ni-based coatings with the ability to form different protective scales (alumina, chromia, and alumina–chromia) have been studied in considerable detail and also subjected to both laboratory and field testing. The corrosion performance of candidate HVAF-sprayed coatings—Ni21
—has been evaluated through detailed laboratory studies in ambient air, moisture, and HCl-laden environments [36
]. All coatings were highly protective in all environments in the absence of KCl due to the formation of corresponding protective scales of alumina or chromia on the coating surface. When KCl was introduced, chromia-forming coatings degraded through a two-stage mechanism: (1) formation of K2
and Cl- followed by diffusion of Cl- through oxide grain boundaries, leading to the formation of Cl2
, metal chlorides, as well as a non-protective oxide, and (2) inward diffusion of the formed Cl2
through defects in the non-protective oxide, leading to metal chloride evaporation and breakaway oxidation.
Corrosion behavior of the chromia-forming Ni21
Cr coating was improved by the addition of alloying elements such as Al and Mo. It was also shown that adding dispersed SiO2
further increased the corrosion resistance of the coatings. The oxide scale formed in the presence of SiO2
effectively suppressed Cl- ingress and lowered the corrosion rate, since the formed oxide was continuous, adherent, and rich in Cr. The performance of the coatings in the complex Cl-containing environment was ranked as (from highest to lowest corrosion resistance): Ni21
AlY > Ni5
Al > Ni21
Mo > Ni21
Cr, confirming the enhanced corrosion protection of chromia-forming coatings in the presence of alloying elements and dispersed SiO2
. While a typical result is summarized in Figure 4
, further details regarding this work are available in the various publications mentioned previously [36
2.2. Wear Resistant Coatings
Wear failures have been known to have a great economic impact on the engineering industry globally [42
]. Decrease in the cost of operation by reducing overhaul and/or component replacement frequency is often intimately linked to wear of components. Continuous efforts to seek improved solutions to extend durability and enhance performance of wear-prone components have been responsible for the sustained interest in exploring various coatings for this purpose [43
]. Although recent developments have enhanced prospects of providing superior protection to wear-prone components, the life of present-day state-of-the-art coatings remains short of industry ambitions. The physical vapor deposited (PVD) or chemical vapor deposited (CVD) coatings are nearly fully dense, provide excellent properties, and are metallurgically bonded to the substrate unlike the mechanically anchored thermally sprayed coatings; however, they are typically thin (<5 μm) and often limited in their ability to address large components by virtue of being in-chamber processes.
Consequently, thermal spray processes have been the focus of a large number of research efforts, and have also been well-established for numerous industrial applications that demand thick coatings. Among the thermal spray methods, HVOF and plasma spray techniques have been particularly extensively investigated for wear applications. The HVOF sprayed coatings offer the possibility of depositing dense and homogenous coatings constitute the current state-of-the-art for many industrial wear applications. A wide range of coatings deposited by the HVOF route have been tested, including WC-Co based coatings and Cr3
]. However, although a few reports investigating wear behavior of HVAF-sprayed coatings have emerged during the past few years [47
], the evaluation of these coatings in industrial environments remains an uncharted territory. Nonetheless, the continuous demand for increasingly superior performance can potentially be accomplished through one or more of the following: higher density, improved adhesion and cohesion, refined microstructure, and minimal in situ damage to coating feedstock. The HVAF technique presents an ideal opportunity to accomplish the above.
Given the above benefits of the HVAF route, and the correspondingly vast potential for its adoption by the industry, wear resistant coatings based on traditional carbides as well as other promising alternative materials have been a subject of increasing interest at University West. The efforts have already led to some exciting results in terms of obtaining extremely dense coatings with minimal porosity, high adhesion strength, and impressive tribological properties [23
] and enhanced the knowledge base to serve as the foundation for further industry-relevant developments.