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Improving the mechanical properties of cold spray metalized polymers
Cold sprayed metallic coatings on polymers provide the benefits of a non-traditional manufacturing solution. However, the process steps for formulating the coating layer often require trial and error to secure an optimal coating. Many common testing methods for interface adhesion and the coating's mechanical performances only give semi-quantitative measurements. As a result, it's challenging to build the connection between the process-structural relationships of the coated materials. This work established a process simulation framework and created an experimental material characterization method to quantify the mechanical strength of cold spray coatings onto polymer substrates.
Particle velocity, mass flow rate, and powder flowability were measured from a low-pressure cold spray system. Increases in mass flow rate are a consequence of good powder flowability. The developed tools and the measurement devices allow quantifying the powder flowability to deposition efficacy in cold spray coatings. Knowing the experimental parameters, this research utilized a three-network polymer model based on high strain-rate impact tests to simulate the nonlinear time-dependent response of polymer deformation during the cold spray impact with both rigid and deformable particles. The particle's material properties, velocity, and size were systematically studied to obtain various responses from the finite element analysis of the polymer deformation. The numerical results were mapped into diagrams and validated with the experimental results of cold spraying Cu and Al2O3 powders.
The cold spray process controls the adhesion strength between the coating and the substrate but does create a relatively wide distribution of film thickness and properties. Therefore, a mechanical test fixture was built to track electrical conductivity and coating fragmentation during tensile testing of metalized polymers. A modified Weibull model used the crack density, fragment length, and the measured specimens strength/strain to calculate the coated strength distribution at a fixed crack density and the mean strength as a function of fragment length. The coatings between 74 μm – 120 μm show an interfacial shear strength between 25 – 53 MPa and an energy release rate between 15 – 32 J/m2. The interfacial shear strength of thinner coatings between 23 μm -37 μm reaches as high as 250 MPa but eventually saturate, and the energy release rate range between 43 – 45 J/m2. In addition, results show that both interfacial shear strength and energy release rate increase as the coating thickness decreases.
Cold sprayed multi-layer (Sn/Cu) and electroless plated specimens were built successfully using the information from the process simulation model. Cold spray coating increases the engineering performance of the coated substrate. The studies have demonstrated selecting appropriate process parameters for multiple metal/polymer combinations to achieve a successful coating.