(70az) Production and Characterization of High Aspect Ratio Metal Flakes

The production of high aspect ratio metal flakes through milling is an important part of the paint and coating industry. The United States Army also uses high aspect ratio metal flakes of specific dimensions in obscurant clouds that scatter and attenuate infrared radiation. Ductile metal particles are initially flattened in a milling process increasing the aspect ratio. As milling continues, cold welding of metal flakes can take place increasing the particle size and decreasing the aspect ratio. At extended milling times particles will also fracture leading to a further decrease in particle size and aspect ratio. This study is focused on understanding the mechanism(s) of particle deformation during a milling process leading to the production of metal flakes of specific thickness and diameter. To achieve this result milling parameters such as media loading, rotational rate, media size, and media density were investigated. Light scatting data shows an evolution in particle size with time where metal particles are initially flatten as seen by the increase in particle size, followed by fracture, which results in a decrease in particle size. This evolution can also be seen in images taken with a scanning electron microscope.

In order to understand the mechanism(s) that result in the flattening and breakage of metal flakes it is important to characterize any changes in the microstructure of the metal particles during the milling process. Nanoindentation experiments were performed to measure changes in the elasticity and hardness of the metal flakes. X-ray diffraction was used to determine changes in the crystallinity of the particles. By identifying changes in the materials properties it is possible to take steps that prevent particle breakage during processing. These measures may include annealing to reduce the strain hardening and manipulation of the milling parameters.