Polymeric coatings manufactured by thermal spray processes exhibit variable mechanical and adhesion properties that depend on their exact processing schedules. One important advantage of these coatings is that they can be readily repaired by re-spraying any delaminated or otherwise defective regions. In some instances the repaired region exhibits better mechanical attributes than the original coating. In this study the repairability of several classes of polymeric and polymer-ceramic composite coatings were investigated with a focus on the interfacial adhesion properties. The coatings include those of monolayer and bilayer ethylene methacrylic acid (EMAA), and CaCO 3 -EMAA composites. The coating thickness did not influence the interfacial adhesive strength between the coating and substrate; while a higher preheat temperature produced a greater interfacial cohesion for the monolayer coating on a metal substrate. The substrate preheat temperature played a dominant role concerning the peel strength of the coating. Greater peel strengths were achieved between polymers, at least twofold greater than that between the polymer and the steel substrate when the pre-heat temperature was greater than the melting point of the polymer. The peel strength of the composite coating decreased with filler content; both on the steel substrate and on a previously sprayed polymer coating. On the basis of these observations, the adhesion mechanism between polymers was explained with a model that relied on the formation of welding points.

Phase composition control is a prime concern for plasma sprayed hydroxyapatite [Ca10(PO4)6(OH)2; i.e., HA] coatings due to the complexity of both HA structure and plasma spray process. The present study aims to better understand the phase formation mechanism in the HA coating through compositional, structural and microstructural studies of HA coatings obtained from various spraying processes. A process model was established by considering both a single HA splat formation and coating buildup processes. Three HA recrystallization mechanisms were proposed on the basis of the temperature-time experiences of particles, their cooling rates, and the heat and hydroxyl accumulation during coating formation. The model explained very well the experimental results. It was concluded that crystallinity alone was not capable of reflecting the coating composition due to the existence of various portions of crystalline HA; i.e., unmelted, recrystallized and dehydroxylated HA, as well as the gradient compositional structure from the coating interface to the surface. Some newly formed nanocrystalline regions were also revealed in the coating microstructure.

Hydroxyapatite/polymer composite coatings of different volume ratios were produced using a Plastic Flame Spray (PFS) system. The intent of this processing is to obtain a coating with an optimal combination of biological and mechanical properties of these two materials for skeletal implants. The composite coatings were produced with a mechanical blend of EMMA and hydroxyapatite powder from a fluidized bed powder feeder. Characterization was conducted by scanning electron microscopy on the surface morphology, polished cross-sections and fracture surface morphology of the coatings. The bioactivity of the coatings was evaluated with a calcium ion meter, and the stress-strain behavior was investigated by tensile testing. The biological and mechanical properties were found to be related to the volume and the distribution of the hydroxyapatite in the polymer matrix.

Hydroxyapatite (HA) coatings are used to improve the adhesion of bone onto implanted devices. This approach increases the integrity and hence the lifetime of the implant. Several orthopaedic appliances (HA coated and macrotextured) were recovered from patients after revision surgery. The implants were cleaned and sterilised in ethanol or formaldehyde before being photographed and sectioned for analysis. X-ray diffraction indicated that the remaining coating was of high crystallinity. Micro textured areas such as ribbings and fenestrations subjected the coating to different modes of stress which has affected the coating. Adhesive failure was evident on implants attributed to dissolution of the amorphous phase at the interface. Observation of the microstructure with scanning electron microscopy showed that coating degradation begins at the surface where the coating is resorbed and continues along the substrate-coating interface thereby compromising interface strength. The microstructure and the dissolution of retrieved implants are discussed in relation to the general coating features in plasma sprayed HA coatings.

The thermal spray process melts powder at very high temperatures and propels the molten material to the substrate to produce a coherent deposit. This heating produces a certain amount of vaporization of the feedstock. Upon exiting the plasma plume the fast cooling conditions lead to condensation of the vapor. An electrical low pressure impactor was used to monitor the concentration of ultra-fine particles at various radial and axial distances. Metal, namely iron powder, showed very high concentration levels which increase with distance. Ultra-fine particles from ZrO 2 -8Y 2 O 3 reached a peak concentration at 6 cm. Use of an air barrier during spraying decreases the population of ultra-fine particles facilitating the production of a stronger coating.