Wear, friction, and microhardness of a thermal sprayed PET: poly (ethylene terephthalate) coating

Abstract

The effects of combustion thermal spraying parameters namely combustion pressure, feeding rate, and carrier gas on the wear resistance, friction coefficient, and Knoop hardness of poly (ethylene terephthalate) (PET) films were investigated. The PET coatings were characterized by measuring the wear coefficients by calowear-type testing, the friction coefficients by a pin-on-disk test, and Knoop hardness. The abrasive wear and friction coefficients of the coatings were compared with the values of a post consumer PET bottle chip reference sample. The structural characteristics of the coatings were investigated by X ray diffraction. Statistical analysis of the results allowed for the systematic characterization of the influence of the process variables mentioned above on the coating wear, friction, and microhardness values. Specifically, this study shows that the process parameters affect the wear coefficient and Knoop hardness significantly, but not the friction coefficient. The degree of crystallinity of the PET coatings varied from 20 to 26%. Keywords: thermal spray, PET, tribology*e-mail: rogerioxavier@yahoo.com.br, jose.branco@cetec.br, vilmaccosta@hotmail.com, calado@eq.ufrj.br 122 Nunes et al. Materials Researchparameters were 10 rpm, 14 mm trail diameter, 10-N load, 50 turns, and a traveled distance of 2,200 mm. Each sample was measured in triplicate. The samples were weighed before and after testing for calculation of mass loss. Wear testAbrasive wear was evaluated by calowear testing, which involves the interaction of a rotating steel sphere with the sample surface with alumina as an abrasive. A semi-spherical crater was formed on the contact surface of the sample. The wear rate was calculated from the crater dimension using Equations 2, 3, and 4:where D is the crater diameter, n the number of turns, R the sphere radius, V the lost volume, S the sliding distance, and Q the wear rate 12 .The wear coefficient (K) was calculated with the Archad equation 10 , where W is the applied load and H the hardness.The sliding distances of the sphere were 100, 200, 300, 500, and 1000 turns and the applied load was 13.7 N. Each sample was measured in triplicate. The crater diameter was measured by 3D profilometry. Results and Discussion Coating structureUsing an optical microscope, the presence of bubbles or pores at the substrate/coating interface and in the PET coating medium was not observed. However, they were observed on the coating-free surface. The formation of bubbles is explained in the following way. The final coating temperature was around 410 °C, below the degradation temperature of PET (420 °C). Apparently, the temperature reached by the deposited layers was sufficient for their complete coalescence between the substrate and the coating medium. Particle degradation was insignificant. This will be further discussed with the results of the infrared spectroscopy. Due to the heterogeneous size of the PET particles, the smallest ones probably melted, while the larger ones were semi-melted. During the natural cool...

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