How to avoid unmelted particles during plasma spraying?
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Answer:
Plasma spray coating of polymers and polymer composites is not a commonly used process, mainly because of the low thermal stability of polymers compared to other metallic or ceramic substrates. For polymer coating, the literature mostly describes the use of flame spray or high-velocity oxy-fuel techniques, because these processes involve lower temperatures, thus minimizing thermal degradation. These processes are, for example, used to resurface polymer matrix composites in aerospace applications with erosion-resistant coatings [79–81].
PEEK is a high-temperature-resistant engineered thermoplastic showing a relatively high glass transition temperature (about 140 °C) and melting point (about 340 °C), also allowing short-time exposures at temperature above 200 °C. These features enable the use of plasma spray processes for PEEK-based components, provided that care is taken with accurate regulation of those parameters that most affect the cooling of the substrate: cooling system, position and distance of the plasma torch from the substrate, and relative movements during the coating process. An important aspect to be considered in the case of PEEK is its chemical degradation when subjected to UV irradiation. This kind of radiation is emitted by the plasma, and even in the absence of oxygen such as in an SPS or VPS process, polymer chain scissions at the aromatic ether bond may occur as a result of direct UV absorption, with consequent creation of hydroxyl groups (–OH) and ester groups (O–CO) (see also Fig. 9.10). The result is a loss in mechanical properties and discoloration [78,82]. Therefore, the coating process must also be regulated in order to minimize the exposure of the PEEK-based substrate to UV radiation.