Analysis of gallium phosphide nanoindentation and cutting mechanism: a molecular dynamics approach

Gallium phosphide (GaP) is industrially important for light-emitting diodes (LEDs) and optics; however, manufacturing limitations and alternative materials restrict its use. To expand GaP applications and take advantage of its optical properties, a deeper understanding of its deformation behaviour is essential. In this context, this dissertation investigates GaP deformation through nanoindentation and ultraprecision machining using experiments and molecular dynamics simulations, focusing on its brittle-ductile transition. The experiments consisted in nanoindentations and plunge tests. Nanoindentations with 5 mN and 10 mN loads at rates of 0.1 mN/s, 0.5 mN/s, and 1.0 mN/s were performed on a GaP monocrystalline sample. Load-displacement curves and atomic force microscopy measurements showed GaP hardness consistent with literature. Pop-ins appeared in two forms: long pop-ins (~1.9 mN) ending the elastic phase and short pop-ins characterizing the plastic phase. The initial and final contact pressures of the long pop-ins decreased with loading rate increase. Plunge tests were performed also on a Gap monocrystalline sample using rake angles -20° and -30°, plunge angle of 0.005°, and a single crystal diamond tool. The plunge tests showed the cut started as ductile and changed to brittle in depths of cut larger than ~100 nm. A molecular dynamics simulation of nanoindentation was performed using a 3.5 nm repulsive sphere as indenter tip moving at 0.4 Å/ps (40 m/s) and a 14 nm x 11 nm x 14 nm GaP workpiece. Results showed pile-up formation and a brittle-ductile transition beneath the indenter tip. Using the load-displacement curve, an estimation of GaP’s hardness value was obtained as close to the experimental value. Plastic deformation started at the first dislocation when the load-displacement curve deviated from the Hertzian curve. In addition, temperature and the number of atoms with coordination number 6 increased during plastic deformation phase. Due the agreement of these results with experiments and literature, the force field for GaP was considered valid for the analysis of deformation behaviour in nanoindentation and a valid candidate for machining simulations. Machining simulations with rake angles of -10°, -20°, and -30° and cutting speed of 1200 m/min in 87 nm x 33 nm x 5 nm and 87 nm x 33 nm x 10 nm GaP workpieces were performed. Noticeably, more negative rake angles produced more stacking faults along the direction of the chip flow, and ductile phases appeared in the tertiary shear zone for -10° and -20° and in the secondary shear zone for -30° as a result from shear and hydrostatic loads. A stagnation zone occurred where the normal cutting force was 45° with cutting direction, aligning with a GaP slip plane. Stacking faults presence intensified due to periodic boundaries, as confirmed by models with larger cutting thickness. Finally, brittle-ductile transition, driven by shear and hydrostatic stress, contributed less to chip formation than stacking faults, contradicting literature that suggests a significant ductile phase formation in front of the tool cutting edge. To the best of the author's knowledge, the transition of GaP from zincblende to β-tin during nanoindentation and machining processes was simulated for the first time. Additionally, stacking faults were observed as a deformation mechanism. During cutting, the variation in normal cutting force along the cutting edge can explain the occurrence of both mechanisms, as well as the formation of the stagnation zone. These findings led to the conclusion that, in addition to the rake angle, the geometry of the tool cutting edge at the nanometric scale is of extreme significance for the occurrence of these cutting mechanisms in GaP ultraprecision machining.