Session: 16-01-01: NSF-funded Research (Grad & Undergrad)

Paper Number: 76804

Start Time: Wednesday, 02:25 PM

76804 - Collision-Angle-Dependent Extreme Mechanical and Tribological Responses of Block-Copolymer Microparticles for Solid-State Additive Manufacturing 

   Applications of polymers in a solid-state additive manufacturing method, known as cold spray (CS), have extensively been increased. The high-strain-rate (HSR) tribological characteristics of individual microparticles (μPs) at the high-velocity impact interface are critical to develop and optimize the advanced additive manufacturing processes, but have been not understood in depth. Specifically, multi-phase polymers such as phase-separated block copolymers (BCP) are promising for CS since HSR material properties of μPs can be systematically tailored by adjusting the volume fractions of polymer blocks. To precisely observe the HSR responses of BCP μPs consisting of multi-domains, we have introduced an angled-collision experiment with semi-empirical models of mechanical and tribological characteristics.

   The model materials to investigate the HSR behaviors of μPs were polystyrene (PS) and polystyrene-block-polydimethylsiloxane (PS-b-PDMS) BCPs. Two groups of PS-b-PDMS BCPs had distinctive cylindrical and lamellar nanostructures depending on their volume fractions of PDMS. The μPs were produced by spray drying of a 9 wt% solution dissolved in toluene in a vertical chamber, and the atomization conditions were optimized to generate μPs with diameters between 10 and 30 µm. Compared to the amorphous PS-μPs, the μPs of BCPs went through microphase separation into PS (glassy) and PDMS (rubbery) domains during the spray drying, and their nanostructures had limited short-range ordering due to the fast evaporation of solvent.

   The HSR dynamic behaviors of PS and BCP μPs were investigated by using the laser induced projectile impact test (LIPIT) technique with different collision angles, i.e., 90 degrees (90°-LIPIT) and 45 degrees (45°-LIPIT). Moreover, the effects from the nanostructures of BCP μPs and thermal conditions at the collision interface were demonstrated by conducting the two LIPITs. The 90°-LIPIT was conducted to determine the bonding windows of each group of BCP μPs colliding onto rigid substrates (glass or silicon) under perpendicular collisions. Conversely, the 45°-LIPIT enabled a further understanding of the HSR tribological nonlinearity at the impact interface with fused silica and sapphire substrates by decomposing the collision components into the normal (compression-dominant) and the tangential (shear-dominant) directions to the impact surface. The impact velocities of a single μP during both LIPITs were accurately controlled up to 700 mžs^-1, and the entire collision process of a single μP was recorded using femtosecond stroboscopic imaging.

   The time-averaged linear momentum changes of a single μP during collision was measured in terms of coefficients of restitution (CoRs) from 90°- and 45°-LIPIT, respectively. The bonding windows of μPs were defined based on the results of the 90°-LIPIT, and the HSR collision responses of BCP μPs during the perpendicular impact were quantified with a CoR model consisting of sigmoid functions representing the elastic and adhesive effects of μPs. The effective coefficient of friction between μP and the substrate was defined using the normal and tangential CoRs and impact velocities, and the tribological responses were quantified with a semi-empirical model of the effective coefficient of friction consisting of adhesion and viscosity terms. The results from the two LIPITs and the in-depth analysis showed that the effects of nanostructures and thermal conditions at the impact interface determined the bonding windows and the tribological nonlinearity during HSR collisions. Morphologies of μPs before and after collisions, including plastic deformations, were investigated by using scanning electron microscopy. Furthermore, the impact-induced morphological changes of nanostructures were explored by imaging focused ion beam milled surfaces of μPs.

   This research enabled a further understanding of the HSR behaviors of BCPs, such as the bonding windows and the tribological nonlinearity at the interface determined by the material properties and the near-adiabatic thermal interactions between μP and the substrate. The methodology and the results from this study are expected to aid advanced performances of polymers with multi-phases in additive manufacturing industries.

 

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

Presenting Author: Ara Kim UMASS Amherst

Authors:

Ara Kim UMASS Amherst
Jae-Hwang Lee UMASS Amherst