Coating Amorphous Metals

Featuring Mixed Oxide Finish

 

Coating amorphous metal parts is similar to coating most other metal components. Some processes work better than others, but overall, conventional methods work well.

Surface treatment of crystalline metal parts

When considering a surface treatment, engineers must find what they want and need from their component regarding performance and cosmetics in its final state.

Many surface finishes yield purely cosmetic results, and others produce performance-based results. Performance-based results can include corrosion resistance, hardness, the coefficient of friction, resistance to fingerprinting or reflectivity. Cosmetic-based results mostly include color and surface finish.

Examples
Image
Doorknob
3 surface treatment steps
Needs: Cosmetic appearance, low cost.

After forging the net-shape component, doorknobs require coining, milling, drilling, and tapping processes leading to a coating. This coating is commonly electroplating or physical vapor deposition (PVD).

Image
Silverware
2 surface treatment steps
Needs: Corrosion resistance, cosmetic appearance, moderate performance/durability, food safe.

Typically, after blanking, rolling, annealing, and cutting, a fork will undergo a buff polish process, silver plating, and possibly some form of polishing.

Image
Revolver
1 surface treatment step
Needs: Performance, corrosion resistance, durability.

Commonly known as blueing, a protective coating is added to most firearms. Blueing entails creating a chemical reaction to produce a durable barrier. Plating, parkerizing, and anodizing are variants to the blueing process.

Surface treatment of amorphous metals

When considering surface treatment with AMM parts, two things should be noted:

  1.  Amorphous alloys eject from the mold after experiencing almost no shrinkage (0.25%). This lack of shrinkage means the parts mimic the mold cavity nearly exactly, giving the part designer freedom to design an array of surface textures into the mold cavity. Amorphous metal parts eject from the mold easier when it has a polished finish, meaning by default most parts have a surface finish between 0.05µm and 0.1µm. Similarly, a mold that is brushed, blasted, machined, or given some other texture will produce AMM parts that mimic that surface. Many of the surface features that can be molded in with AMM require additional or secondary operations if another metal forming process was used, which adds cost to the final part.
  2.  Amorphous metals have a random atomic structure that leads to a unique set of premium material properties. These material properties serve as a significant advantage to part designers, but it should be known that the properties cannot change after molding. Processes typically used to add hardness or strength are not suggested because they rely on either forming crystal structures in the material, or stressing already existing structures (which amorphous metals do not have).

With these “rules” in mind, a variety of post-processed finishes have been applied to AMM parts, including tumbling, sand-blasting, polishing, PVD, e-coating, and painting. One surface treatment that Liquidmetal R&D has recently had success with is a mixed oxide finish.

Mixed oxide finish is intriguing to the Liquidmetal team because it provides a coloring option that is both cosmetic and functional. The process creates a very thin layer of the native metal alloy atoms in their oxidized states. The approximate thickness of this layer is about 2 µm, but extremely hard (1400 Vickers), making the material, even more, scratch and wear resistant than usual (530 Vickers).

You can read about the details of mixed oxide finishes with Liquidmetal amorphous alloys here.

Summary

Treating the surface of any metal part requires an understanding of what the desired outcome should be; functional, cosmetic, or both.

Liquidmetal amorphous alloy material properties are set at the moment of solidification in the molding process, but engineers have identified that the native mixed oxide finish can add visually appealing colors and practical ceramic-like hardness to the surface of the part which improves the general wear properties.

Surface treatment for Liquidmetal parts does not end there; e-coating, PVD, media blasting, painting, and tumbling are common processes used to add color or surface texture. These processes do not change material properties and are generally considered cosmetic.

In the end, untreated Liquidmetal amorphous metal parts naturally achieve the goals of many costly post-processing steps that crystalline metals require (especially after machining). This process capability reduces cost and variability significantly in the final product. If a functional or cosmetic change is required that cannot be molded; the Liquidmetal team has an array of options to ensure you do not make a single trade-off.