EPIC – Enhancing Performance via Innovative Coatings of AM Parts
Introduction
This project was focused on the development of an electroless plating approach for the deposition of tribological coatings on parts produced using Fused Deposition Modeling and Stereolithography. Electroless plating would allow for uniform coating throughout the part regardless of the part’s complexity. Unique coating formulation would provide enhanced wear and friction resistance, hardness, high-temperature protection, and improved mechanical performance. Ultimately, developed plating methods, and formulation would greatly enhance the manufacturing of unique, high value, low volume components for military applications.
Additive Manufacturing of plastic parts is well developed and several methods exist to fabricate components such as Filament Extrusion, Vat Photopolymerization, and Binder Jetting. While plastic AM is mature, the barriers for adoption of the technology, particularly, for military applications are significant. Parts created by plastic AM methods lack functionality due to the limited selection of materials and poor performance.
Metal coating of plastic AM parts has the potential to enhance the component’s mechanical performance, increase lifespan, and decrease the mean time before failure.
Metallization of Plastic Parts
Common approaches for coating plastic parts are vacuum metallization, thermal spray, and cold spray.
Vacuum metallization (e.g. physical vapor deposition) – evaporation of metal by a physical process such as heating or sputtering and then transporting of the metal vapor to the part and its deposition by condensation.
Thermal spray (e.g. plasma spraying and flame spraying) – feedstock metal is melted by means of electric current or combustion flame and then propelled towards substrate where molten droplets flatten and rapidly solidify to form a coating.
Cold spraying – metal is deposited onto the substrate surface by accelerating metal particles to very high speeds using a carrier gas. Upon impact, particles with sufficient kinetic energy deform plastically and form a mechanical bond with the substrate. These techniques are unfit for coating of AM parts due to the following limitations:
- These methods are line-of-sight processes and cannot coat complex geometries required for manufacturing of parts such as casting molds, press molds, and fixtures
- Line-of-sight trajectories result in variable film thickness
- High capital costs make these processes unfavorable for coating small batches of AM parts
Plating on plastics is an alternative method to apply metallic coating on plastics. Plating can be achieved through the application of the electric current, which drives the reduction of metal ions to the conductive substrate immersed into the plating bath (electroplating) or through a set of chemical reactions in an aqueous solution, which occur without the use of external power (electroless plating). To electroplate on plastics, pre-plating is required to render the plastic conductive.
Methods for the plating on plastics with metals such as copper, nickel, tin, gold, and chrome are well developed. The average plating thickness varies between 1 µm to 100 µm depending on the plating technique and conditions. Electroplating is inexpensive, fast, and applicable to a wide variety of sizes and shapes. However, electroplating is a line-of-sight process that often results in an uneven coating when applied to surfaces with complex shapes. Generally, surfaces that are facing the anode will receive a thicker coating compared to surfaces not facing the anode during electroplating. Moreover, thicker deposits will form on the corners and thinner deposits in the recessed areas due to differences in current densities. Electroless plating is slower but provides uniform thickness, especially in deep recesses, bores, and blind holes.
Thus, electroless plating is the most favorable approach to apply uniform metal coating onto additively manufactured parts with complex geometry.
Project Goal: to develop an electroless plating approach to deposit tribological coatings onto additively manufactured plastic parts
Benefits: enhanced strength, durability, surface hardness, and heat resistance of plastic AM components
Outcomes: increased lifespan, decreased mean time to failure, expanded application envelope
Results: Preliminary experiments on the metallization of plastic AM parts are shown in the photo below. FFF (second from left and far right) and Vat Photopolymerization (first from left and second from right) parts coated with standard Ni and Cu plating solutions. Parts display visible differences in appearance. Components show uniform coating inside the recessed feature (eye). Note: no pre-processing (cleaning & conditioning) was performed prior to plating thus coating is not uniform.
