Front view and (b) elevation of cold sprayed samples set in holder prior to salt fog chamber testing.

Cold Sprayed Coatings with Dual Nanoparticle Reinforcements for Wear and Corrosion Protection

As a form of additive manufacturing, the use of cold spray has made significant strides since different-sized particles were explored in the process. There is proof that micron-sized particles down to nanoparticles enhance the mechanical material properties of a substrate in wear resistance.

Ships and the components onboard ships experience high degrees of stress from sea motion, operational loads, and frequent operational changes. As such, excessive component wear, protective coating deterioration, and complete mechanical failure is not uncommon.

When this occurs, repair is based on a timeline that accounts for the degree of mission impact, availability of parts, and operational requirements. Often repair or replacement is limited by what personnel can perform locally. This can result in delaying repair or replacement of deficient components (sometimes for years) until operational requirements can support a shipyard maintenance period.

Having degraded or broken components fosters an environment where personnel live with deficiencies and contributes to overall ship degradation. Therefore, providing the ship or unit with more tools to enable the repair of components on station is vital to supporting the overall Department of Defense mission.

Frequently sailors use paint onboard ships to place a nonreactive barrier between bare metal and a corrosive at-sea environment. This is very helpful for reducing general corrosion, but paint can chip or delaminate, exposing a metal surface and resulting in localized corrosion if the exposed metal is not repainted. In addition, paint has minimal use for wear or impact applications. In the event of high wear against a painted surface, the topcoat can easily be worn away, often requiring a reapplication of the paint coat.

Physical Vapor Deposition (PVD) and Chemical Vapor Deposition (CVD) are two methods of depositing coatings uniformly around a substrate within a closed environment. It is possible to obtain a high degree of controllability of thickness, including nanometric dimensions. It is understood that PVD and CVD have lower application rates when compared to other surface modification technologies and generally do not provide coatings as thick as some other treatments, like thermal spraying a large area with coatings over 100 μm. While both coating methods can be easily performed in a lab environment with the right equipment, they are not suitable for performing in-situ on a ship. Fully enclosing an installed part is usually not realistic and accessing tight spaces make either PVD or CVD less than an ideal choice for coating or repairing a component.

Plasma spray coatings use molten metal particles to impact a substrate, which then cool and solidify rapidly. High temperatures result in microstructure phase changes and subsequent residual stresses once the coatings cool down. These “quenching stresses” can result in tens of MPa residual stresses for ceramic reinforcements that can result in extensive splat cracking. High residual stresses may affect the margin to yield stress that some other coating methods may not inherently produce because they do not reach the high temperatures resulting from plasma spray. In addition, plasma spray processes require temperatures above 2000K for the particles in-flight. On a ship where there is limited electrical power, this can unduly burden the turbine generators from the high-energy draw.

Cold spray is a low energy method that could be employed on ships to repair failed or degraded components and minimize the number of material deficiencies onboard the ship. When lead times for replacement parts are excessive (>12 months), a cold sprayed coating of the same or similar material could be applied to reduce downtime and repair costs. Additionally, cold spray can also be used to provide protective coatings to prevent material deterioration from corrosion or mechanical wear. To support the use of this technology aboard ships, further research must be conducted to determine the mechanical properties and corrosion resistance of various application-compatible cold spray coatings.

Cold spray can be performed under a range of pressures, with high pressure normally resulting in a coating with better overall adhesion and a harder coating. Additionally, multiple reinforcements may have a synergistic effect, enhancing the properties of the base coating more than any one reinforcement alone. Starting with a high-pressure process would be a proof of concept for dual particle reinforcement, and future testing could closer simulate ship limitations and capabilities as a feasible possibility to repairing and manufacturing components.

This work was done by Travis Norrell for the Naval Postgraduate School. For more information, download the Technical Support Package (free white paper) below. NPS-0021

This Brief includes a Technical Support Package (TSP).
Cold Sprayed Coatings with Dual Nanoparticle Reinforcements for Wear and Corrosion Protection

(reference NPS-0021) is currently available for download from the TSP library.

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