Pylin Sarobol

Room Temperature Deformation Mechanisms of Alumina Particles Observed from In Situ Micro-compression and Atomistic Simulations

Aerosol deposition (AD) is a solid-state deposition technology that has been developed to fabricate ceramic coatings nominally at room temperature. Sub-micron ceramic particles accelerated by pressurized gas impact, deform, and consolidate on substrates under vacuum. Ceramic particle consolidation in AD coatings is highly dependent on particle deformation and bonding; these behaviors are not well understood. In this work, atomistic simulations and in situ micro-compressions in the scanning electron microscope, and the transmission electron microscope (TEM) were utilized to investigate fundamental mechanisms responsible for plastic deformation/fracture of particles under applied compression. Results showed that highly defective micron-sized alumina particles, initially containing numerous dislocations or a grain boundary, exhibited no observable shape change before fracture/fragmentation. Simulations and experimental results indicated that particles containing a grain boundary only accommodate low strain energy per unit volume before crack nucleation and propagation. In contrast, nearly defect-free, sub-micron, single crystal alumina particles exhibited plastic deformation and fracture without fragmentation. Dislocation nucleation/motion, significant plastic deformation, and shape change were observed. Simulation and TEM in situ micro-compression results indicated that nearly defect-free particles accommodate high strain energy per unit volume associated with dislocation plasticity before fracture. The identified deformation mechanisms provide insight into feedstock design for AD.

Deformation Behaviors of Sub-micron and Micron Sized Alumina Particles in Compression.

The ability to integrate ceramics with other materials has been limited due to high temperature (>800°C) ceramic processing. Recently, researchers demonstrated a novel process, aerosol deposition (AD), to fabricate ceramic films at room temperature (RT). In this process, sub-micron sized ceramic particles are accelerated by pressurized gas, impacted on the substrate, plastically deformed, and form a dense film under vacuum. This AD process eliminates high temperature processing thereby enabling new coatings and device integration, in which ceramics can be deposited on metals, plastics, and glass. However, knowledge in fundamental mechanisms for ceramic particles to deform and form a dense ceramic film is still needed and is essential in advancing this novel RT technology. In this work, a combination of experimentation and atomistic simulation was used to determine the deformation behavior of sub-micron sized ceramic particles; this is the first fundamental step needed to explain coating formation in the AD process. High purity, single crystal, alpha alumina particles with nominal sizes of 0.3 μm and 3.0 μm were examined. Particle characterization, using transmission electron microscopy (TEM), showed that the 0.3 μm particles were relatively defect-free single crystals whereas 3.0 μm particles were highly defective single crystals or particles contained low angle grain boundaries. Sub-micron sized Al2O3 particles exhibited ductile failure in compression. In situ compression experiments showed 0.3μm particles deformed plastically, fractured, and became polycrystalline. Moreover, dislocation activity was observed within these particles during compression. These sub-micron sized Al2O3 particles exhibited large accumulated strain (2-3 times those of micron-sized particles) before first fracture. In agreement with the findings from experimentation, atomistic simulations of nano-Al2O3 particles showed dislocation slip and significant plastic deformation during compression. On the other hand, the micron sized Al2O3 particles exhibited brittle fracture in compression. In situ compression experiments showed 3μm Al2O3 particles fractured into pieces without observable plastic deformation in compression. Particle deformation behaviors will be used to inform Al2O3 coating deposition parameters and particle-particle bonding in the consolidated Al2O3 coatings.