Structurally Integrated, Damage-Tolerant, Thermal Spray Coatings

Abstract

Thermal spray coatings are used extensively for the protection and life extension of engineering components exposed to harsh wear and/or corrosion during service in aerospace, energy, and heavy machinery sectors. Cermet coatings applied via high-velocity thermal spray are used in aggressive wear situations almost always coupled with corrosive environments. In several instances (e.g., landing gear), coatings are considered as part of the structure requiring system-level considerations. Despite their widespread use, the technology has lacked generalized scientific principles for robust coating design, manufacturing, and performance analysis. Advances in process and in situ diagnostics have provided significant insights into the process-structure-property-performance correlations providing a framework-enhanced design. In this overview, critical aspects of materials, process, parametrics, and performance are discussed through exemplary studies on relevant compositions. The underlying connective theme is understanding and controlling residual stresses generation, which not only addresses process dynamics but also provides linkage for process-property relationship for both the system (e.g., fatigue) and the surface (wear and corrosion). The anisotropic microstructure also invokes the need for damage-tolerant material design to meet future goals.

Electrochemical Behavior, Microstructure, and Surface Chemistry of Thermal-Sprayed Stainless-Steel Coatings

“…In thermal spraying, the successive impingements of hot gas-accelerated molten and semi-molten particles onto a substrate lead to the formation of a coating. The development of this industry-scale technique has allowed the replacement of erosion-corrosion resistant hard-chrome coatings in different applications including valves, hydraulic pistons, and aircraft landing gears [1,2]. When compared with other standard coating deposition methods such as weld-overlay, thermal spray has a reduced effect on the substrate's chemistry and geometry, making it an interesting process for the repair of parts used, for example, in the paper and steel industries [3] as well as in hydroelectric turbines [4].…”

Nature inspired, multi-functional, damage tolerant thermal spray coatings

“…The majority of thermal spray coating applications for surface protection involve some combination of impact, erosion and contact wear situations where the coating requires a combination of hardness and a degree of damage tolerance. Industrial developments have largely focused on solving these problems through the use of hard, dense and in some cases multi-phase cermet coatings, however it has become clear in recent times that even metallic spray coatings have low in-plane fracture toughness, and as such, invoke damage tolerant design consideration [17][18][19][20][21]. Furthermore, much of the work addressing toughness of spray deposited layers has indicated substantially reduced numbers toughness values as compared to their bulk counter parts, in a large part due to the weak inter-splat bonding and porosities in these splat layered coatings.…”

Effect of Deposition Rate and Deposition Temperature on Residual Stress of HVOF-Sprayed Coatings

“…Residual stresses play a determinant role in fatigue life, as well. Enhanced fatigue life in HVOF coatings deposited in compressive residual stress has been demonstrated by various studies (Ref [15][16][17][18][19]. Residual stresses are inherently present in TS coatings as in any other manufacturing process where thermal input, expansion mismatch of materials, and temperature gradients are present.…”