The addition of a granular material on the surface of the polymer substrate is an alternative method that has been used to increase the roughness of the substrate surface, in lieu of grit blasting. Gonzalez et al. roughened the surface of glass fiber-reinforced epoxy tubes by the addition a layer of garnet sand during the curing process prior to the flame spray deposition of A1-12Si coatings. In contrast to results reported by Hounnic et al., where the grit blasting process was employed, this technique did not alter the strength of the FRPC tubes and the internal pressure required to cause fracture and damage of the coated FRPC specimens remained unchanged from that of the uncoated FRPC specimens. The addition of a ceramic granular material, with its low thermal conductivity, also provides a barrier to heat transfer from the high-temperature flame and particles, protecting the polymer substrate from high-temperature degradation during the thermal spray process. Aiso, Robitaille et al.suggested that the granular metal layer can be added to the substrate surface during the curing process of the polymer substrate to promote thermal spray deposition. In this study,a layer of Cu particles was added onto the unidirectional/weave carbon fiber-reinforced epoxy substrate during the curing process to enable the deposition of cold-sprayed Zn coatings without inducing any damage to the substrate fibers. Deposition on the substrate with no surface pretreatment led to erosion of epoxy and exposure and damage of the carbon fibers.The idea of incorporating granular material on the surface of the polymer substrates during the curing process has also been employed for deposition of conductive coatings onto elastomers such as PU.
Boyer et al.roughened the castable PU surface by the addition of Cu particles during the curing process. It was found that flame-sprayed Al-12Si particles interlocked with the Cu particles to form acontinuous electrically conductive coating. In anotherstudy conducted by Ashrafizadeh et al. the addition of the same metal powder as that of the spraying powder was suggested to roughen the thermo-setting PU substrate during the curing process, which ensured consistency between the roughening agent and the deposited particles. In another study by LoperaValle and McDonald, it was shown that the layer of granular sand material can also be added to the thermo-setting substrate(glass fiber-reinforced epoxy composite)surface by applying a layer of adhesive after the curing process, which allowed the deposition of coatings consisting of 80 wt.% nickel and 20 wt.% chromium(Ni-20Cr). Figure 1 shows a cross section image of the deposited Ni-20Cr coating and the sand layer added onto the FRPC substrate. As evidenced in Fig 1, the metal coating and the granular particles used for roughening the polymer-based substrate are bound by mechanical interlocking.
Fig. 1 Backscattered scanning electron microscope (SEM) image of the cross section of a flame-sprayed Ni-20Cr coating deposited onto a fiber-reinforced polymer composite (FRPC) substrate
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