Extreme Tribological Characteristics of Block-Copolymer Microparticles Under Angled Supersonic Collisions

                  Mechanical characteristics of materials under high-strain rate (HSR) play an important role for advanced additive manufacturing processes. Cold Spray is one of the most efficient additive manufacturing methods utilizing high-velocity collisions in a range between 100 to 1,500 m/s of feedstock microparticles onto a substrate. Metals have been the principle materials for the cold spray method, however the application has been expanded to non-metallic materials and composites. We envision that the tailoring of material properties of cold-spray-able microparticles such as microstructure can achieve advanced performance of the end-product, in this perspective, multi-phased block copolymers (BCP) are interesting materials for the cold spray additive manufacturing as distinctive phase co-exist at the nanoscale and the diverse nanostructures offer a variety of applications. Furthermore, precisely defined HSR mechanical collision conditions demonstrated the HSR dynamic behaviors of the BCP, and the results will be priceless data for expanded applications in defense, biology, and manufacturing.

                  Precisely controlled single particle impact tests were conducted to investigate HSR behaviors of polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCP microparticles, which have distinctive phases consisting of the glassy (PS) and rubbery (PDMS). An ultrasonic atomization technique was used to produce BCP microparticles by spraying a 2.5 wt% PS-b-PDMS solution dissolved in toluene. In this project, microparticles having different microstructures such as cylinders and lamellae with different volume fractions of PS and PDMS were investigated.

                  Effects from the nanostructures of the BCP microparticles were demonstrated by conducting the single-particle impact experiments with controlled impact velocities up to 700 m/s by using the laser induced projectile impact test (LIPIT) technique, and the diameters of the microparticles ranged between 10 to 30 µm for the LIPIT. Moreover, substrate effects at the contact surface during the impact were also investigated by using diverse substrate materials such as fused silica, sapphire and an uncured PDMS layer with different thermal diffusivities. The bonding windows of each group of BCP microparticles were demonstrated through the normal impact test with a perpendicular angle between the propagating direction of the microparticle and the substrate surface. The dynamic behaviors of microparticles were investigated in depth by decoupling the components along the normal and tangential directions through an angled impact test with an inclined surface or the substrate. The entire collision motion before and after impact was recorded with an ultrafast imaging system using femtosecond illumination pulses. Particle morphologies before impact, plastic deformation and microstructures were explored by using scanning electron microscopy (SEM) and focused ion beam.

                  The results of this project demonstrate the effects of volume fraction and morphological scale, and the in-depth analysis of the angled LIPIT identified that the HSR dynamic behaviors of BCPs are decided by the contact condition such as a friction coefficient and thermal diffusivities. Pre-impact inelastic deformations and post-impact plastic deformations indicated that correlation between kinetic energy generated by acceleration force and material characteristics.

This project is funded by NSF MOMS Program with Siddiq Qidwai, CMMI-1760924, and Principal Investigator Jae-Hwang Lee