November/December AMP_Digital

iTSSe TSS A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 8 3 9 iTSSe TSS PEEK COATING ON METALLIC SUBSTRATES Impact Innovations produces a PEEK coating on steel and Al substrates using its impact spray system (ISS) 5/8 with optimized process parameters including gas temperature and pressure, powder feed rate, and spray gun travel speed. Figure 1a shows the deposition of PEEK coating on the as-re- ceived steel substrate. While PEEK coatings can be achieved on steel substrates, coating bond strength is poor (Figs. 1b andc). Itwaspossible tobuildup thePEEKcoatingbecause the powder particles heated up and softened during deposition. However, when the spray nozzle moves away after depositing the first layer, depositedparticles shrink due to cooling andbe- come brittle. When the nozzle returns to deposit the next layer, delamination of the previous layer occurs. Bond strength and coating performance were improved by optimizing process parameters andmodifying the spray systemhardware compo- nents. It was also observed that substrate preparation prior to deposition was essential to enhance bond strength. Figure 1d shows the PEEK coating on the steel substratewithout delami- nation. Bond strength appears to be good, although it was not measured quantitatively. An Al cylinder was coated using the optimized parame- ters and substrate preparation to demonstrate the feasibility of producing a thick ( ∼ 3 mm) PEEK coating. Bond strength be- tween the PEEK coating and Al substrate was assessed via the machinability of the coating. Figure 2a shows the PEEK coating after machining to a thickness of ∼ 2 mm, indicating the good machinability of the cold spray PEEK coating. Cross sections of the coating on the substrate (Figs. 2b and c) show the high interface quality between coating and substrate as well as be- tween consecutive coating layers. The coating is free of cracks. Quantitative evaluation of bond strength is difficult because the coating forms a tight ring around the Al cylinder. However, macroscopic examination and the machining test show good adhesion. This work potentially opens up new applications for cold gas spray technology. METALLIC COATINGS ON CFRP COMPOSITE SUBSTRATES Particle impact energy (particle velocity andmass) is one of the most important factors influencing metal particles to adhere to a polymer substrate. Lupoi andO’Neill [14] report that the impact energy required to bond a singlemetal powder par- ticle and polymer substrate should be less than 0.002 mJ. It is possible to achieve lower impact energy by decreasing particle velocity, but particle deformation is essential during impact on the substrate in order to form the coating. Therefore, softer materials (e.g., lead and tin) theoretically have a good possi- bility of adhering to polymer substrates. Deposition of elec- trically conductive coatings, such as Cu, Ag, and Al, on CFRP composite substrates is challenging due to the high impact energy of powder particles, which results in substrate erosion. By optimizing process parameters, Impact Innovations developed a procedure to deposit Cu and Ag coatings on CFRP composites without using a bonding layer. Figure 3a shows spraying an Ag coating on a composite substrate. Figures 3b and c show a uniform coating of Ag and Cu, respectively, on composite substrates. Coating thickness up to several milli- meters is possible for both materials. Bond strength was as- sessed via a machinability test. Ag and Cu coatings on CFRP composites were machined successfully without any delami- nation (Figs. 4a and b). Side views of the spray profiles of Ag FEATURE 5 Fig. 3 — (a) Direct deposition of Ag and Cu coatings on CFRP composite using cold gas spray; (b) uniform coating of Ag on composite; (c) uniform coating of Cu on composite. Fig. 4 — Qualitative assessment of bond-strength of (a) Ag and (b) Cu coatings on CFRP composite via machinability test and via visual inspection of the side view of the spray profile of (c) Ag and (d) Cu.