Effect of Epoxy Resin Modification on the Cavitation Erosion Resistance of Polyamide-imide Coatings

Cavitation is a phenomenon of material damage under extreme conditions of localized high pressure and heat. It commonly occurs in pumps and other flow-through components and can severely limit the service life of these parts. Polyamideimide (PAI) coatings were originally developed to prevent cavitation erosion damage in steel components. However, because of their lightweight requirements in aerospace, they are now being used as light alloys that can withstand low temperatures. Notably, PAI coatings have high curing temperatures that exceed the withstanding temperatures of most lightweight alloys. Although the addition of epoxy resin (EP) is expected to significantly reduce the curing temperature of PAI, it may also alter its overall properties. The corresponding effect on cavitation erosion performance is currently unknown. To address this issue, we prepared pure PAI coatings (P-280) and EP-modified PAI coatings (P-200 and P-170) with varying PAI contents. Using an ultrasonic vibration-accelerated cavitation erosion test, we then compared the cavitation erosion performances of the samples. Through characterization using X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), and nanoindentation, we also analyzed the mechanical and thermal properties of the samples and their force / heat response behaviors under the effects of cavitation load and cavitation heat. This study investigated the mechanical and thermal properties of the samples and their force- and heat-response behaviors using three-dimensional optical shaping. The results indicated that the addition of EP could significantly reduce the curing temperature of PAI by 80-110 degrees C . However, this reduction led to the destruction of the mechanical properties of the material, including its toughness, which decreased to 8.21, 5.50, and 3.18 mJ<middle dot>m-3 in P-280, P-200, and P-170, respectively. This occurred because of the reduction in rigid molecular chains, such as the imide and benzene rings. In P-280, P-200, P-170, the tensile strength decreased gradually from 114.11 to 75.52 and 70.74 MPa. This reduction in strength led to a decrease in the bearing capacity of the coating and increased fatigue cracking under cavitation load, resulting in the formation of a greater number of larger spalling pits. However, the addition of EP significantly degraded the thermal stability of PAI, making it susceptible to melting and decomposition under cavitation heat. The reductions in temperature corresponding to a 5% weight loss of the P-170, P-200, and P-280 samples after 30 min of cavitation erosion were 15.24%, 14.82%, and 9.05%, respectively. This further accelerated the degradation of the mechanical properties of the coating surface and the damage caused by cavitation erosion. In addition, the heat generated by cavitation erosion promoted pyrolysis and hydrolysis of the molecular chains. XPS results indicated a reduction in the oxygen content after 30 min of cavitation erosion. Specifically, P-280, P-200, and P-170 decreased by 0.67, 1.9, and as much as 3.33at.%, respectively. The breakage of the molecular chains further deteriorated the overall performance of the coatings. The SEM morphology of the P-170 flaking debris showed melting under the heat of cavitation and the subsequent condensation of water into spherical debris particles. After 30 min of accelerated cavitation erosion, the mass losses of P-200 and P-170 were 1.7 and 3.6 mg, respectively. These values were 2.1 and 4.5 times higher than that of P-280, respectively. Considering the curing temperature, overall performance, and cavitation resistance of the coating, P-200 was deemed more suitable for application on the surface of light alloy parts. This study provides guidelines for the research and development of PAI coatings based on its investigation of the relationship between the overall and cavitation performances of PAI coatings under different EP contents.