Vacuum arc plasma deposition of wear resistant coatings on polymer substrates

Thin wear resistant bi-layer ceramic/metal coatings were deposited onto polysulfone S2010 substrates using a triple-cathode vacuum arc plasma source connected to a cylindrical plasma duct, in which an axial magnetic field was imposed to guide a metal vapor plasma flow from the cathodes to the substrate. In some experiments, additional magnetic fields were applied by two beam steering coils oriented normal to the duct axis to increase the plasma flux to the substrate. Two types of metal intermediate layers were investigated: (1) refractory metals – Ti, Zr or Nb. and (2) highly conductive metals – Ag, Cu or Al. The ceramic layers were TiN or ZrN. The structure and composition of the coatings were studied using x-ray diffraction, Auger electron spectroscopy and scanning electron microscopy. Scratch and reciprocating wear tests were used to evaluate the adhesion strength and to study the friction and wear rates, respectively. It was shown that the bi-layer coatings with a refractory metal intermediate layer had either an amorphous or a mixed amorphous/nanocrystalline structure in both the metal and nitride layers. In some cases, carbide formed between the intermediate metal layer and the polymers. In coatings where carbide at the interface was not formed, wide area cracks were observed around the scratch and wear tracks, and their wear rate was higher than that for coatings where a carbide was formed. The conditions required for carbide formation to take place are not clear. Bi-layer coatings with a Nb intermediate layer detached during scratch and wear tests and had a high wear rate. This was possibly due to the substrate damage by the Nb plasma flux, in which the ion energy was much higher than that of Ti and Zr. The electrical surface resistivity of TiN/Ti and ZrN/Ti coated samples was high (3–8 Ω/□). High conductivity metals Cu, Ag and Al had superior adhesion to the polymer substrate. However, the deposition of a TiN layer on Ag layer caused immediate coating delamination from the substrate. In the case of ZrN/Ag coatings, which initially possessed good adhesion to the substrate, cracking and delamination of ZrN at the ZrN/Ag interface was observed after 1–7 days, while the Ag layer remained adherent to the substrate. In the cases of TiN/Cu and ZrN/Cu coatings, the nitride layers had poor adhesion to the Cu layer. The best results were obtained for ZrN/Al coatings, which had low surface resistivity (0.08–0.11 Ω/□) with good adhesion of the Al layer to the substrate and of the nitride layer to the Al. The addition of a ZrN layer on the Al decreased the reciprocating wear rate by a factor of 8–10.