Communication Novel Ballistic Processing of Sn-0.7Cu Thick Films D. CAVERO, K. STEWART, and K. MORSI The present paper discusses a novel process (Ballistic Processing) for the ultra-rapid processing of textured and un-textured thick and potentially thin films. The effect of processing velocity (14.6 to 36.1 m/s) on the developed external structure and internal microstructure of Sn-0.7Cu thick film is discussed. Film thicknesses ranging from 6.08 to 12.79 lm were produced and characterized by two-dimensional hypoeutectic microstructures. Both film thickness and dendrite arm spacing decreased with an increase in processing velocity. DOI: 10.1007/s11661-016-3838-3 Ó The Minerals, Metals & Materials Society and ASM International 2016 The behavior and structure of materials subjected to ultra-rapid (extreme) processing techniques are expected to be different than during conventional processing. Processes such as explosive compaction [1] and rapid solidification [2] have provided options for the processing of materials with unique characteristics and features. Thick films have been defined as those having thick- nesses between 2 and 100 lm, whereas below 2 lm, they are considered thin films. [3] Processing approaches for films include splat quenching, [4,5] thermal spray [6] plasma physical vapor deposition (PVD), [7] flash evap- oration, [8] sputtering, [9] electroplating [10] and sol-gel coating. [11] These are complex processes involving high vacuum, chemicals or multiple processing steps. For thick films, screen printing is fundamentally a cost-effective pro- cessing technique, [11] involving the use of bonding materials like glass or metallic oxides that usually reduce the conductivity of the film, or an active metal (that can increases the processing cost [12] ). Amorphous films have also received a great deal of attention because of their promising applications in micro-electro-me- chanical (MEMS) devices and biomedicine. [13] Within the area of melt processing, planar-flow melt spinning (PFMS) is a common and effective process for manufacturing long ribbons of metallic glasses. [14] The process consists of flowing molten metal through a nozzle onto a rotating chilled (with water or other fluids) wheel, to increase the temperature gradients at the casting surface, which allows for undercooling of the material into an amorphous state. Wheel speed has strong effects on both cooling rate and ribbon thickness. Typical speeds investigated in the planar-flow melt spinning process for metals can range from 5 to 80 ms 1 with typical thicknesses between 8 lm and a few hundred micrometers depending on material and process conditions. [15,16] The present paper discusses for the first time ballistic processing (BP), a novel ultra- rapid processing technique with possibilities in thin, thick, and amorphous film processing, in addition to additive manufacturing. The process can be viewed as an extension of splat quenching [17] with the novelty being melt quenching on a rapidly moving substrate. A tin-copper (Sn-0.7Cu) alloy was used as a model material due to its low melting point‘; however, obvi- ously, results are applicable to many other material systems. Although the present paper investigates pro- cessing velocities up to 36.1 m/s, higher ballistic veloc- ities even above the speed of sound are possible with Ballistic Processing, which is the subject of a separate study. The present paper provides an initial insight into the capabilities of BP and the developed microstructures in the lower velocity regime. The process is capable of producing films thinner than that produced with con- ventional PFMS and allows higher geometric freedom than PFMS and splat cooling. The experimental equipment for this investigation was all custom built at San Diego State University’s Advanced Materials Processing Lab (AMPL). The casting material used was a low melting point Kester K100LD lead-free solder alloy (Sn-0.7Cu). A schematic of the BP process and setup is shown in Figure 1. An aluminum carrier (projectile) holds a flat wax substrate. The wax is machinable and purchased from MachinableWax.com, Inc., The wax was initially melted, and cast into the carriers using a glass micro- scope slide at the bottom to ensure a flat casting surface with very good surface finish. The projectile/substrate was designed having an inclined surface of 15 deg. For each experiment, the solid Sn-0.7Cu solder alloy was melted in a stainless steel crucible heated by a tape heater to a temperature of 648 K (375 °C) prior to casting. A plunger valve at the bottom of the crucible is opened after the desired temperature is reached. The molten solder was then allowed to flow through a rectangular slit 0.9 mm in thickness and 31.75 mm in width and a curtain of molten alloy was formed as seen in Figure 1. The carrier/substrate was then accelerated pneumatically inside the barrel, using air pressures of 200, 700, 1000, and 1500 psi. The end of the barrel was D. CAVERO, Former MS Student, K. STEWART, MS Student, and K. MORSI, Professor, are with the Department of Mechanical Engineering, San Diego State University 5500 Campanile Dr. San Diego, CA 92182. Contact e-mail: kmorsi@mail.sdsu.edu Manuscript submitted March 31 2016. Article published online November 1, 2016 46—VOLUME 48A, JANUARY 2017 METALLURGICAL AND MATERIALS TRANSACTIONS A