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.

The High Velocity Oxy-Fuel (HVOF) combustion spray process has been used successfully for spraying polymers and polymer-matrix composite coatings. Spraying of polymer ceramic composite powders produced by ball-milling nominal 60 ..m Nylon-11 with different size scale (7 nm to 15 µm) ceramic reinforcements is an effective method of producing semi-crystalline micron and nano-scale reinforced composite coatings. Polymer matrix composite coatings with nominal 10 vol. % of different size scale silica and alumina reinforcements have been produced. The levels of filler loading in both the feedstock powders and HVOF-sprayed coatings were determined by thermo-gravimetric analysis (TGA) and compared using ashing. Particle size analysis, microstructural characterization and the elemental compositions of the feedstock powders and as-sprayed coatings were determined by optical and scanning electron microscopy and energy dispersive spectroscopy. The influence of dispersion, distribution and size of the reinforcing phase was studied and correlated to coating microstructure and process parameter variations. The scratch resistance of the coatings was measured as a function of reinforcement size and compared with those of the pure HVOF-sprayed Nylon-11 coatings.

The high velocity oxy-fuel (HVOF) combustion spraying of ball-milled Nylon-11/ceramic composite powders is an effective, economical and environmentally sound method for producing semi-crystalline nano- and micron-scale reinforced polymer composite coatings. Polymer matrix composite coatings reinforced with multiple scales of ceramic particulate materials are expected to exhibit improved load transfer between the reinforcing phase and the matrix due to interactions between large and small ceramic particles. An important step in developing multi-scale polymer matrix composite coatings and associated load transfer theory is determining the effect of reinforcement size on the distribution of the reinforcement and the properties of the composite coating. Composite feedstock powders were produced by dry ball milling Nylon-11 with fumed silica particles of 7, 20 and 40 nm, with fumed alumina particles of 50 and 150 nm, and with white calcined alumina 350 nm, 1, 2, 5, 10, 20, 25 and 50 µm at 10 % by volume overall ceramic phase loadings. The effectiveness of the ball-milling process as a function of reinforcement size was evaluated by SEM, EDS microanalysis and by characterizing the behavior of the powders during HVOF spraying. The microstructures of the as-sprayed coatings were characterized by optical microscopy, SEM, EDS and XRD. The reinforcement particles were found to be concentrated at the splat boundaries within the coatings, forming a series of interconnected lamellar sheets with good 3-dimensional distribution. The scratch resistance of the coatings improved consistently and logarithmically as a function of decreasing reinforcement size and compared to those of HVOF sprayed pure Nylon-11.

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.

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.