Thermal spray deposition of aluminum and zinc coatings on thermoplastics

Coatings of aluminum and zinc were applied on two thermoplastic materials, ultra-high molecular weight polyethylene (PE) and polytetrafluoroethylene (PTFE), using twin wire-arc spray spraying. Samples were either smooth (Ra = 0.2 μm) or roughened by grit blasting (Ra = 1.6 μm). Before the start of spraying samples were kept at either room temperature or preheated to 55 °C for PE and 95 °C for PTFE, so that the peak temperature during spraying reached the glass transition temperature of the polymer. Surface temperature was monitored during spraying using thermocouples. Coating adhesion strength was measured using pull tests. Single splats of metal on samples were examined using SEM images. Zinc coatings, about 260 ± 20 μm thick, formed on both polymer samples irrespective of their surface roughness. Adhesion strength was significantly higher on rough surfaces than smooth surfaces. Increasing the initial sample temperature also enhanced adhesion strength. Aluminum adhered only to the PTFE but not the PE. Aluminum droplets have a higher melting point than zinc and melted the sample upon impact, inhibiting mechanical bonding. These results suggest that the metal adhesion to thermoplastics can be significantly improved by pre-heating polymer samples prior to thermal spraying.