M. Ivosevic

Adhesive/cohesive properties of thermally sprayed functionally graded coatings for polymer matrix composites

High-velocity oxyfuel (HVOF) sprayed polyimide/WC-Co functionally graded (FGM) coatings with flame-sprayed WC-Co topcoats have been investigated as solutions to improve the solid-particle erosion and oxidation resistance of polymer matrix composites (PMCs) in the gas flow path of advanced turbine engines. Porosity, coating thickness, and volume fraction of the WC-Co phase retained in the graded coating architecture were determined using standard metallographic techniques and computer image analysis. The adhesive bond strength of three different types of coatings was evaluated according to ASTM D 4541. Adhesive/cohesive strengths of the FGM coating were measured and compared with those of pure polyimide and polyimide/WC-Co composite coatings and also related to the tensile strength of the uncoated PMC substrate perpendicular to the thickness. The FGM coatings exhibited lower adhesive bond strengths (∼6.2 MPa) than pure polyimide coatings (∼8.4 MPa), and in all cases these values were lower than the tensile strength (∼17.6 MPa) of the reference uncoated PMC substrate. The nature and locus of the failures were characterized according to the percent adhesive and/or cohesive failure, and the interfaces tested and layers involved were analyzed by scanning electron microscopy.

Effect of substrate roughness on splatting behavior of HVOF sprayed polymer particles: Modeling and experiments

A three-dimensional model of particle splatting on rough surfaces has been developed for high-velocity oxyfuel (HVOF) sprayed polymer particles and related to experimentally observed polymer splats. Fluid flow and particle deformation were predicted using a volume of fluid (VoF) method using Flow-3D software. Splatting behavior and final splat shapes were simulated on a realistic rough surface, generated by optical interferometry of an actual grit-blasted steel surface. Predicted splat shapes were compared with scanning electron microscopy images of nylon 11 splats deposited onto grit-blasted steel substrates. Rough substrates led to the formation of fingers and other asymmetric three-dimensional instabilities that are seldom observed in simulations of polymer splatting on smooth substrates.

Erosion/Oxidation Resistant Coatings for High Temperature Polymer Composites

Thermally sprayed coatings are being studied and developed as methods of enabling lightweight composites to be used more extensively as structural components in propulsion applications in order to reduce costs and improve efficiency through weight reductions. The primary goal of this work is the development of functionally graded material (FGM) polymer/metal matrix composite coatings to provide improved erosion/oxidation resistance to polyimide-based polymer matrix composite (PMC) substrates. The goal is to grade the coating composition from pure polyimide, similar to the PMC substrate matrix on one side, to 100% WC-Co on the other. Both step-wise and continuous gradation of the WC-Co loading in these coatings are being investigated. Details of the processing parameter development are presented, specifically the high velocity oxy-fuel (HVOF) combustion spraying of pure PMR-II matrix material and layers of various composition PMR-II/WC-Co blends onto steel and PMR-15 composite substrates. Results of the HVOF process optimization, microstructural characterization, and analysis will be presented. The sprayed coatings were evaluated using standard metallographic techniques - optical and scanning electron microscopy (SEM). An SEM + electron dispersive spectroscopy (EDS) technique has also been used to confirm retention of the PMR-II component.