Metallization of polymers or polymer metallization is a technique in which a metallic layer is formed on the surface of polymeric parts, diminishes the inherent deficiencies of polymer surface while extending the applicability of polymers.
Metallization of polymers realizing through the very popular cold spray additive manufacturing technique is an emerging thermal spray approach for coating of thick metallic layers on the surface and the sub-surface regions of polymers, and the fiber-reinforced composites, which in result guarantees high productivity, eco-friendliness, and industrial scalability of the coating process.
In the cold spray technique, a supersonic stream of preheated gas accelerates the solid metallic powder particles toward a polymer substrate, where metallized of polymer layers are constructed layer-by-layer via impact-induced heating and particle deformation.
Figure curtesy: Schematic diagrams metallization of PEEK by pure aluminium coating and aluminum oxide coating on PEEK substrate. Surface and Coatings Technology, 446, 128752
Adding an appropriate amount of aluminum and aluminum oxide can effectively improve electrical conductivity of polymer substrate due to the elimination of pores on the polymer surface, while enhancing the bonding between Al-Al splashes. The highest electrical conductivity has been observed for the (67 % of bulk Al) embedded on the PEEK substrate.
Environmentally conductive technique for the metallization of polymers today uses magnetron sputtering., particularly high-power impulse magnetron sputtering (HiPIMS). One of the main benefits of HiPIMS is the low thermal loading of the substrates. Apart from this, HiPIMS also offers excellent adhesion of coated layers on the substrates.
The exciting things are:
HiPIMS facilitates direct metallization of polymers
The interface of Polymer substrate and metallized polymer layer can be tuned or modified by HiPIMS current. With a present positive reversal of HiPIMS voltage, much pronounced quality of metallized polymer can be produced
The adhesion can be directly controlled and optimized by choosing peak current in unipolar HiPIMS; the adhesion is further improved with positive reversal of HiPIMS voltage with Bipolar HiPIMS.
Importantly, no pre-treatments of polymers are necessary before metallization.
The metallization of polymers through many other methods requires pretreatments, lacquers or interface coatings usually must be applied prior to metallization to improve adhesion. Therefore, the main challenge is a reliable and an improved film adhesion, which in turn can be directly optimized by HiPIMS current.
As we discussed, HiPIMS offers excellent adhesion without any requirements for the chemical pretreatment.
Al coated PMMA substrates after tape test (left of the substrate) and combined cross cut and tape test (right of the substrate)
Coated with Direct Current Magnetron Sputtering (DCMS)
HiPIMS50A peak current
HiPIMS100A peak current
HiPIMS150A peak current
Figure Curtesy: Surface and Coatings Technology Volume 290 Pages 77-81
Increasing peak current leads to an increased ionization of energetic deposits in the plasma that subsequently enhances the adhesion to the level where no failure on the surface of the polymer is detected when examined the samples after a genetic and standard crosscut tape test. Improved adhesion can be attributed to the nature of the HIPIMS discharge, where 80% of the sputtered target atoms from the cathode are ionized before they condensed to the substrate.
However, the discharge current in HiPIMS should be kept at optimum level for better adhesion, whereas an excessive increase in discharge current leads to cohesive failure of coating layers (while adhesive failure occurs in poor ionized plasma associated with DCMS coatings)