Simulation and Gas Temperature Selection
The simulation results from the ©KSS software at a fixed gas pressure of 2 MPa are shown in Fig. 2. The calculated particle temperatures at corresponding gas temperatures are also listed in Table 2. The particle temperatures and velocities presented are those for the mean particle size at 40 mm from the nozzle exit (when reaching the substrate in experiments). It can be seen from Fig. 2 that both the particle temperature and velocity increase generally linearly with the gas temperature. The particle temperature increases more significantly than particle velocity, e.g., Tp is five times higher at 550 °C than 250 °C, whereas the particle velocity only increases by 21%.
The gas temperatures for each substrate were selected based on the simulation results, and the rationale is as follows. It was proposed in our previous paper that cold spray onto the polymeric substrate is a two-step process, each step has its own window and the overall window for a non-monolayer coating is where the two windows overlap (Ref 20). However, it is relatively difficult to practically determine the window for developing the first layer. One main reason is that the properties of polymers change largely with temperature. For thermoplastics, their properties show a relatively sudden change at Tg. Thus, it is worthwhile comparing the impact behavior at particle temperatures above and below Tg. Based on the simulation results, gas temperatures of 550 and 425 °C were chosen for PEI, 425 and 350 °C for PEEK, and 400 and 250 °C for ABS (The Tgs of the three thermoplastics are presented in Fig. 2 by the dashed lines). The particle temperature margins with respect to Tg of each thermoplastic were not intentionally kept the same in this qualitative study (There are various uncertainties, e.g., Tp varies with particle size, glass transition occurs within a range of temperatures instead of one temperature). For CFRP, which has a thermosetting matrix, 425 and 250 °C were used, which result in a 108 °C difference in particle temperature according to the simulation. Theoretically, the particle velocity should be identical when comparing the effect of particle temperature (i.e., decrease gas pressure at higher temperatures to compensate the velocity increase); the velocity increase was considered negligible in this work for simplification purpose (velocity within 8% for a 100 °C interval).
With the calculated particle velocity and temperature, the software may further calculate a deposition efficiency (DE) by comparing the particle velocity to the conventional critical velocity, vcrit, at that temperature. The DE calculation assumes that the particle and substrate materials are identical (Ref 30), so the results will be different on polymeric substrates. Nevertheless, the calculated DE results (not shown, ~ 50% at 250 °C, ~ 90% at 350 °C, then increases slightly) suggest that the process conditions used in this work are all within the conventional cold spray window [i.e., the buildup window according to (Ref 20)]. This means if the first layer can be developed, it is then theoretically possible to build up a thick coating under those conditions.
Figure 3 shows the SEM image of a copper particle cold sprayed onto the steel substrate at 425 °C. The splat formation is well captured. It can be seen that the particle successfully adhered to the substrate, showing obvious flattening and a significant amount of plastic deformation, especially at its rim. This indicates that the adiabatic shear instability was activated and is in accordance with the simulation results (i.e., particle velocity higher than critical velocity). A trial cold spray of copper at such a condition led to successful coating deposition. However, such plastic deformation and particle flattening were not observed on the polymeric substrates, even when spraying under the same condition.
For all the polymeric substrates, the copper particles found at the top surfaces do not show significant amounts of plastic deformation. Figure 4 presents the copper particles cold sprayed at 550 and 425 °C on PEI substrates. The former led to a particle temperature of ~ 235 °C and the latter ~ 155 °C. It can be seen from Fig. 4(a) and (b) that both particles penetrated the substrates and remained adhered, without significant particle deformation (e.g., flattening). There are ‘interaction’ zones in the substrate around the particles and clear signs of substrate deformation in both samples. The interaction zone is larger at higher temperature (Fig. 4a). The substrate deformation should be caused by a combination of thermal softening (e.g., melting) and deformation under force (e.g., plastic deformation). There is no significant difference between the two samples in Fig. 4(a) and (b), even though one has a particle temperature above Tg and the other below. On the other hand, the two substrates exhibit differences where particle bouncing occurred, as shown in Fig. 4(c) and (d). At 550 °C, it can be seen that the bouncing copper particle was ‘smeared’ or ‘drawn’ over the polymer in Fig. 4(c). Melting of PEI at the impact site had occurred, and adhesion had been achieved. Whereas at 425 °C, it shows two craters in Fig. 4(d) with no copper particles being held. From the center of the craters, some stringy/fibrous structures are observed. This is similar to the ‘Spinnbarkeit’ phenomenon (a rheological term refers to the spinnability of viscoelastic fluids) and is likely formed by attaching/melting, bouncing/drawing and detaching/solidifying. Thus, it implies that localized melting has occurred despite the fact that the calculated particle temperature immediately before impact is lower than the Tg of PEI. It should also be pointed out that the copper particles in Fig. 4 (and other figures in this section) look different from those in Fig. 1, and this may be caused by thermal etching [(i.e., formation of facets and grain boundary grooves at metal surfaces when subjected to high temperature and suitable environments, (Ref 31)], or it may also be attributed to that the SEM images of the polymeric substrates were in BSE mode (SE for feedstock powder).
Cold spray of copper at 425 °C on PEI can result in successful coating, but delamination of the coating was observed in previous work (Ref 20). The delamination may be related to inadequate substrate melting or particle penetration, according to the single impact observation in this work. Hence, it is inferred that increase either gas temperature or pressure may help to improve bonding between copper and PEI.
For PEEK cold sprayed at 425 and 350 °C, the results are shown in Fig. 5. The copper particles found in both samples basically remain spherical. It can be seen from Fig. 5(a) that a copper particle is anchored in the substrate surface at 425 °C, with an obvious substrate interaction zone around the particle. Cold spray of copper at this temperature led to successful thick coatings (Ref 20). At a lower temperature of 350 °C, the interaction zone around the adhered particle can barely be observed in the substrate, as shown in Fig. 5(b). It is likely that the particle did not penetrate the substrate at 350 °C but attached superficially instead, which signifies inadequate anchorage. Indeed, significantly fewer particles remain adhered at the lower temperature, leaving more craters in the sample. A closer look at the craters after the particles bounced off is shown in Fig. 5(c). The crater is similar to that in Fig. 4(d), as some cavities and the ‘Spinnbarkeit’ phenomenon can be observed. This suggests that some localized substrate melting and bonding has been achieved at the impact sites, but is inadequate to hold the particles. It should be mentioned that during the SEM characterization of the 350 °C sample, some adhered copper particles were swept away from the substrate by the electron beam, signifying weak adhesion to the substrate. This is understandable considering that: (a) the particle temperature, 107 °C, is well below Tg, and (b) PEEK is a semicrystalline thermoplastic in which the crystalline part does not gradually soften with temperature increase (Ref 32). Based on these observations, it is inferred that higher particle velocity or temperature may be beneficial for stronger anchorage/interlocking than at 350 °C and 2 MPa. However, single particle impact is not practical, whereas successive impact is more realistic in practice, for which good bonding may still be achieved.
The SEM images showing the ABS substrates after single impact tests are presented in Fig. 6. After cold spraying at 400 °C, a copper particle was found penetrated deeply into the substrate. This may be attributed to the weak resistance to heat of ABS. It implies that cold spray of copper at temperatures higher than 400 °C can cause severe substrate erosion. Indeed, severe erosion was observed in the previous work when cold spraying copper onto ABS at 425 °C (Ref 20). At a lower temperature of 250 °C, particle adherence and the associated interaction zones can be observed. The particles that remain attached are mainly fine particles at 250 °C. Particle detachment also occurred at some locations, leaving a few craters at the substrate surface. The ‘Spinnbarkeit’ can also be seen at some craters (not shown), indicating that local melting had occurred upon impact, even though the calculated particle temperature is lower than of Tg. A trial spray of copper at 250 °C and 2 MPa failed to deposit continuous coating due to substrate erosion. Future work on cold spraying of metals onto ABS requires careful selection of the temperature/pressure combination, such that severe erosion can be avoid while good interlocking can be achieved.
Unlike the thermoplastics, the CFRP used in this work has a thermosetting matrix. The CFRP samples after single impact tests at 425 and 250 °C are shown in Fig. 7. It can be seen that at both temperatures, cavities have been generated adjacent to the copper particles in the substrates after impact. The cavity formation probably results from the brittleness of the CFRP substrate. Because of the cavity formation, the particles are not securely anchored, as can be seen from Fig. 7; it is reasonable to believe that if there were successive impacts, the particle would have been removed and enlargement of the cavity (i.e., erosion) would have occurred. The results signify that erosion of the substrate, instead of coating deposition, can occur during cold spraying of copper on CFRP at similar conditions. This is confirmed by the experimental results in (Ref 3, 20).
Discussion on the Deposition Behavior
In general, it can be seen that the copper particles did not experience significant amounts of plastic deformation on the polymeric substrates, even though the process conditions were selected within the conventional cold spray window (i.e., adiabatic shear instability should occur on metallic substrates). This confirms that the conventional adiabatic shear instability mechanism is inapplicable when developing the first metallic layer on polymeric substrates. For polymeric substrates, the particles penetrated the polymers and generally remained spherical in this work. Good particle/substrate interlocking may be achieved when the substrate shows good capability of thermal deformation (e.g., PEI and PEEK in this work). This may result from the local softening of the thermoplastics, which has been observed by various researchers (Ref 20, 21, 24). For thermosets or thermoset matrix composites (e.g., CFRP in this work), which lack the ability of thermal deformation or reflow, particle/substrate interlocking is relatively difficult due to the brittleness of the substrate. In addition, the polymer’s resistance to erosion at relatively high temperature is another important factor. An example of this is ABS in this work, on which coating attempts failed with particle temperatures both above and below its Tg, even though ABS is a thermoplastic and can be thermally softened. Therefore, other mechanical properties apart from thermal softening, such as substrate susceptibility to erosion, can also govern the deposition process. For future work, the correlation between the deposition behavior and various mechanical properties of polymers at different temperatures is recommended.
Without significant particle deformation, the bonding can only be achieved by mechanical interlocking/anchorage between the first metallic layer and the polymers. From another point of view, substrate damage is necessary, but must be to the right extent. Excessive damage can lead to severe substrate erosion (e.g., ABS at 400 °C), whereas insufficient damage may result in no/poor bonding (e.g., PEI at 425 °C). Similar to conventional cold spray, the two main parameters that influence bonding between metal and the polymer substrates are velocity and temperature. Chen et al. recently reported the effect of pressure/velocity on single particle deposition during cold spray of copper on PEEK (Ref 25). It was found that with increasing gas pressure/particle velocity but fixed gas temperature, the copper particles penetrated deeper into PEEK and more wrinkles and jets formed in the PEEK substrate around the copper particles. This trend may be attributed to both more mechanical penetration (higher kinetic energy) and more thermal softening (kinetic energy dissipation induced temperature increase). In this work, the gas temperature was varied and the pressure was kept unchanged. It is inevitable that increasing gas temperature will lead to increasing particle velocity at the same gas pressure, but the particle temperature increase is much more significant than particle velocity. It is reasonable to believe that the differences at different gas temperatures observed in this work are due to the temperature difference. Based on the results of this work, it can be seen that varying the particle temperature in the vicinity of Tg may effectively alter the bonding condition of metals on a thermoplastic. Nevertheless, it is also true that thermal softening and mechanical penetration occur simultaneously and are difficult to differentiate, although one may be dominant.
Cold spray of metal on polymer substrates is different from cold spray of hard metal on relatively softer metals. One major difference is due to the glass transition of thermoplastics, at temperatures above Tg the thermoplastics almost lose all their mechanical properties. This is similar to the melting of metals, but the melting points of metals are barely reached during cold spray, even in the adiabatic deformation area. Another major difference between polymers and soft metals is the largely different thermal conductivity (a few orders of magnitudes lower for polymers). Heat generated at the metal/polymer interface cannot be dissipated as fast as at the metal/metal interface. This can make the thermal effect even more significant on polymeric substrates. It is of interest to monitor/simulate the polymer substrate surface temperature near the particle interface during cold spray for further studies.