Tribological Properties of Hard Metal Coatings Sprayed by High-Velocity Air Fuel Process

Abstract

Lowering the thermal energy and increasing the kinetic energyof hard metal particles sprayed by the newly developed HVAF systems can significantly reduce their decarburization, and increases the sliding wear and corrosion resistance of the resulting coatings, making the HVAF technique attractive, both economically and environmentally, over its HVOF predecessors. Two agglomerated and sintered feedstock powder chemistries, WC-Co (88/12) and WC-CoCr (86/10/4), respectively, with increasing primary carbides grain size from 0.2 to 4.0 microns, have been deposited by the latest HVAF-M3 process onto carbon steel substrates. Their dry sliding wear behaviors and friction coefficients were evaluated at room temperature via Ball-on-disk (ASTM G99-90) wear tests against Al2O3 counterparts, and via Pin-on-disk (ASTM G77-05) wear tests against modified martensitic steel counterparts in both dry and lubricated conditions. Sliding wear mechanisms, with the formation of wavy surface morphology and brittle cracking, are discussed regarding the distribution and size of primary carbides. Corrosion behaviors were evaluated via standard Neutral Salt Spray, Acetic Acid Salt Spray, accelerated corrosion test, and electrochemical polarization test at room temperature. The optimization of the tribological properties of the coatings is discussed, focusing on the suitable selection of primary carbide size for different working load applications

Advanced Coatings by Thermal Spray Processes

“…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 [23,24]. 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.…”

“…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][24][25][26][27][28], 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.…”

“…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][24][25][26][27][28] and enhanced the knowledge base to serve as the foundation for further industry-relevant developments.…”

Microstructure and Tribological Properties of Plasma-sprayed WC-17 Co Coatings with Different Carbide Grain Size Distribution

“…The dependence of the coefficient of friction on the below mechanisms such as matrix material elastic deformation, adhesion, hard protrusions, asperities, ploughing proposed in the several literatures 12,13,24) . At the beginning stage of the test, two surfaces are brought into sliding contact, the soft ductile cobalt matrix between WC particles suffers severe deformation, then forms a thin layer at the sliding interface (called for tribofilm) 16) , which reduces the coefficient of friction by acting as a lubricant.…”

KCl-Induced High-Temperature Corrosion Behavior of HVAF-Sprayed Ni-Based Coatings in Ambient Air

“…High-velocity air-fuel (HVAF), a promising method to produce dense and adherent coating, works based on high-velocity impact of molten or semi-molten particles which could be used to apply a wide range of coating materials including Ni-based powders (Ref [23][24][25][26]. Recent advances in design and development of new HVAF guns have reduced the cost and enhanced the quality of coatings, resulting in booming markets for HVAF technology (Ref [27][28][29]. The coatings produced by HVAF, characterized by lower porosity, less in situ formed oxide content and more interlamellar cohesion, are capable of reducing the diffusion of aggressive agents such as chlorine toward substrate/coating interface (Ref 30,31).…”