A new coating process called polymer thermal spraying (PTS) was recently developed to accommodate the deposition of heat sensitive polymeric materials over a broad range of substrates. The novel process uses an electro-resistive element to heat the main process gas, which could be air, any pure gas, or gas mixture. This paper describes the process and presents three case studies in which it is used to produce blast mitigation coatings for civil structures, super-hydrophobic coatings for corrosion protection, and flame resistant polyimide syntactic foams for thermal insulation.

The high velocity oxy-fuel (HVOF) combustion spray process has previously been shown to be a successful method for depositing pure polymer and polymer/ceramic composite coatings. Polymer and polymer-ceramic composite particles have high melt viscosities and require the high kinetic energy of HVOF in order to generate sufficient particle flow and deformation on impact. One of the goals of reinforcing polymer coatings with particulate ceramics is to improve their durability and wear performance. Composite coatings were produced by ball-milling 60 µm Nylon-11 together with nominal 10 vol.% of nano and multi-scale ceramic reinforcements and HVOF spraying these composite feedstocks onto steel substrates to produce semi-crystalline micron and nano-scale reinforced coatings of polymer matrix composites. The room temperature dry sliding wear performance of pure Nylon-11, Nylon-11 reinforced with 7 nm silica, and multi-scale Nylon-11/silica composite coatings incorporating 7 to 40 nm and 10 µm ceramic particles was determined and compared. Coatings were sprayed onto steel substrates, and their sliding wear performance determined using a pin-on-disk tribometer. Coefficient of friction was recorded and wear rate determined as a function of applied load and coating composition. Surface profilometry and scanning electron microscopy were used to characterize and analyze the coatings and wear scars.

Numerical predictions and experimental observations have been correlated to improve the qualitative understanding of the degree of thermal degradation occurring during the HVOF spray deposition of Nylon-11. Particle residence time (<1 ms) in the HVOF jet was insufficient for significant decomposition of the Nylon-11 but was sufficient for noticeable discoloration (yellowing) of the particles of a powder with a mean particle size of 30 µm. Experimental observations showed this to be the case even though numerical predictions indicated that the temperature of a 30 µm diameter particle should be considerably higher than the upper degradation limit of Nylon-11. Initial thermal oxidation of Nylon-11 promotes the formation of carbon-carbon double bonds that strongly absorb in the visible spectrum even at concentrations of parts per million, resulting in discoloration of the Nylon.

A three-dimensional model of particle impact and deformation on rough surfaces has been developed for HVOF sprayed polymer particles. Fluid flow and particle deformation was predicted by the Volume of Fluid (VoF) method using Flow-3D® software. The effect of roughness on the mechanics of splatting and final splat shapes was explored through the use of several prototypical rough surfaces, e.g. steps and grooves. In addition, a numerical representation of a more realistic rough surface, generated by optical interferometry of an actual grit blasted steel surface, was also incorporated into the model. Predicted splat shapes were compared with SEM images of Nylon 11 splats deposited onto grit blasted steel substrates. Rough substrates led to the generation of fingers and other asymmetric three-dimensional instabilities that are seldom observed in simulations of splatting on smooth substrates.

The high velocity oxy-fuel (HVOF) combustion spray process has been demonstrated to be a suitable technique for the deposition of nano-reinforced polymer matrix composite coatings by controlling both the particle dwell time and the substrate temperature. HVOF-sprayed polymer matrix composites incorporating reinforcements with size scales ranging from 7 nm to 100 µm are being studied to bridge between the nano and conventional scale regimes. Microstructural characterization has been used to characterize the dispersion and distribution of the ceramic reinforcements within the polymer matrix. The effect of particle size distribution on reinforcement dispersion and distribution has been studied, and the influence of substrate temperature on coating adhesion has also been investigated. Changes in crystallinity, as determined by Differential Scanning Calorimetry (DSC), are being correlated to coating microstructure, reinforcement loading and process parameter variations. Results of optical and scanning electron microscopy, scratch testing and DSC characterization of the feedstock materials and sprayed coatings are presented. Coatings of nominal 60 µm Nylon 11 with 10 vol. % of nano and micron size hydrophilic silica reinforcements exhibited a ~22 % improvements in scratch resistance compared to pure Nylon 11 coatings. An ~15 % improvement in scratch resistance was obtained for coatings containing 10 vol. % nano scale hydrophilic silica reinforcement.

Thermal spray has traditionally been used for depositing metallic, carbide and ceramic coatings, however, it has recently been found that the high kinetic energy of the High Velocity Oxy-Fuel (HVOF) thermal spray process also enables the solventless processing of high melt viscosity polymers, eliminating the need for harmful, volatile organic solvents. A primarily goal of this work was to develop a knowledge base and improved qualitative understanding of the impact behavior of polymeric particles sprayed by the HVOF combustion spray process. Numerical models of particle acceleration, heating and impact deformation during HVOF spraying of polymer particles have been developed. A Volume-of-Fluid (VoF) computational fluid mechanics package, Flow3D®, was used to model the fluid mechanics and heat transfer during particle impacts with a steel substrate. The radial temperature profiles predicted using particle acceleration and heat transfer models were used as initial conditions in Flow3D® together with a temperature-dependent viscosity model to simulate polymer particles with a low temperature, high viscosity core and high temperature, lower viscosity surface. This approach predicted deformed particles exhibiting a large, nearly hemispherical, core within a thin disk, and was consistent with experimental observations of thermally sprayed splats made using an optical microscope.

The use of polymer matrix composites [PMC's] in the gas flow path of advanced turbine engines offers significant benefits for aircraft engine performance, but their useful lifetime is limited by their poor erosion resistance. HVOF and flame sprayed polymer/cermet functionally graded coatings based on a polyimide matrix filled with varying volume fractions of WC-Co are being investigated to improve the erosion and oxidation resistance of polymer matrix composites. A study of the coating's effectiveness as erosion barriers was accomplished through a statistical analysis of the results of solid particle erosion testing of coated and uncoated PMC samples using a design of experiments [DoE] approach. Three coating systems and a control were evaluated in a randomized test matrix. The coatings were tested at room temperature and 250 °C, using an alumina erodent impacting the coatings at a speed of 100 m/s at angles of 20° and 90°. Erosion volume loss at 250 °C was approximately twice than at room temperature, but the maximum erosion volume loss did not exceed 0.30 mm 3 at the elevated temperature. In general, as the angle of incidence of the eroding material increased from 20 degrees to 90 degrees the volume loss increased.

An initial stage of the current work including a preliminary comparative numerical analysis of molten polymer and metal droplets upon impact on a cold steel substrate was presented. A commercially available Volume-of-Fluid [VoF] code was used to model particle deformation and cooling of molten nylon-11 and zinc droplets on impact with a steel substrate. Comparison between polymer and metal splatting was chosen in order to better understand how large fundamental differences between the materials affected their spreading behavior under similar thermal spray conditions. It was found that the inertia is more strongly balanced by the viscous flow resistance in molten polymers while high surface tension of molten metals may lead to particle breakup onto rivulets and satellite during later stage of particle deformation. Spreading ratios of nylon-11 and zinc droplets were 0.53 and 0.34, respectively, owing to the zinc droplets being almost twice the size of the nylon. Zinc splats less than 5 ìm in thickness spread fully and solidified in less than 1.5 µs. Over the same time interval, 17 µm thick nylon-11 splats were also fully developed, however, only a thin boundary layer [<2 µm] was solidified owing to a significantly lower thermal conductivity.

The use of polymer matrix composites (PMC's) in the gas flow path of advanced turbine engines offers significant benefits for aircraft engine performance but their useful lifetime is limited by their poor erosion resistance. High velocity oxy-fuel (HVOF) sprayed polymer/cermet functionally graded (FGM) coatings are being investigated as a method to address this technology gap by providing erosion and oxidation protection to polymer matrix composites. The FGM coating structures are based on a polyimide matrix filled with varying volume fractions of WC-Co. The graded coating architecture was produced using a combination of internal and external feedstock injection, via two computer-controlled powder feeders and controlled substrate preheating. Porosity, coating thickness and volume fraction of the WC-Co filler retained in the coatings were determined using standard metallographic techniques and computer image analysis. The pull-off strength (often refered to as the adhesive strength) of the coatings was evaluated according to the ASTM D 4541 standard test method, which measured the greatest normal tensile force that the coating could withstand. Adhesive/cohesive strengths were determined for three different types of coating structures and compared based on the maximum indicated load and the surface area loaded. The nature and locus of the fractures were characterized according to the percent of adhesive and/or cohesive failure, and the tested interfaces and layers involved were analyzed by Scanning Electron Microscopy.

High velocity oxy-fuel (HVOF) sprayed, functionally graded polyimide/WC-Co composite coatings on polymer matrix composites (PMC's) are being investigated for applications in turbine engine technologies. This requires that the polyimide, used as the matrix material, be fully crosslinked during deposition in order to maximize its engineering properties. The rapid heating and cooling nature of the HVOF spray process and the high heat flux through the coating into the substrate typically do not allow sufficient time at temperature for curing of the thermoset. It was hypothesized that external substrate preheating might enhance the deposition behavior and curing reaction during the thermal spraying of polyimide thermosets. An additional difficulty arises from the low thermal conductivity and low specific heat capacity of the PMC substrate, which prevent effective substrate preheating by the HVOF jet as in the case of metallic substrates. A simple analytical process model for the deposition of thermosetting polyimide onto polymer matrix composites by HVOF thermal spray technology has been developed. The model incorporates various heat transfer mechanisms and enables surface temperature profiles of the coating to be simulated, primarily as a function of substrate preheating temperature. Four cases were modeled: (i) no substrate preheating; (ii) substrates electrically preheated from the rear; (iii) substrates preheated by hot air from the front face; and (iv) substrates electrically preheated from the rear and by hot air from the front. Thermal properties of the polyimide needed for the simulations were determined by Differential Scanning Calorimetry (DSC) and Thermo-Gravimetric Analysis (TGA). Microstructural characterization of the coatings and the morphology of polyimide splats sprayed both with and without substrate preheating were analyzed using standard metallographic techniques. Coating temperature in cases (iii) and (iv) never dropped below the crosslinking temperature of the polyimide feedstock. This was the critical condition required for the curing reaction and successful deposition of thermosets by HVOF thermal spraying.

In this paper, WC-Co reinforced polymer matrix coatings are sprayed on preheated steel and carbon-fiber reinforced substrates, producing relatively dense, adherent coatings. The particle morphology of the feedstock materials and the microstructure of the HVOF sprayed coatings are characterized and the thermal properties of the polymer powder and coatings are compared. It was found that the deposition and build-up of the polymer coating was only successful when substrates were preheated to the curing temperature of the thermosetting polyimide powder used. Layered coatings of varying polyimide and WC-Co content have been successfully deposited, showing that it is possible to produce graded composite coatings consisting of pure polymer at the substrate and pure WC-Co on the surface. Paper includes a German-language abstract.