Selective laser melting (SLM) to fabricate Al_0.5CoCrFeNi high entropy alloys with pre-mixed powders was studied in this paper. The influences of process parameters including laser power, scanning speed, and hatch spacing on the relative density of high-entropy alloys (HEAs) were investigated. A relative density of 99.92% can be achieved by optimizing the SLM process parameters with laser power 320 W, scanning speed 800 mm/s, and hatch spacing of 60 μm, respectively. Moreover, the microstructure of the HEAs was also studied using scanning electron microscopy (SEM) and x-ray diffraction (XRD). It was found that the microstructure of the HEAs was only composed of face-centered cubic and body-centered cubic phases, without complex intermetallic compounds. The mechanical properties of the HEAs were also characterized. At ambient temperature, the alloys had a high yield strength of about 609 MPa, tensile strength about 878 MPa, and hardness about 270 HV. Through a comparison with the corresponding alloys fabricated by vacuum induction melting, it can be concluded that the high entropy alloys fabricated by SLM had fine microstructures and improved mechanical properties.

A fundamental understanding of the mechanical properties and deformation behaviors of surface modified silicon during chemical mechanical polishing (CMP) processes is difficult to obtain at the nanometer scale. In this research, MD simulations of monocrystalline silicon covered with an amorphous SiO₂ film with different thickness are implemented by nanoindentation, and it is found that both the indentation modulus and hardness increase with the growing indentation depth owning to the strongly silicon substrate effect. At the same indentation depth, the indentation modulus decreases shapely with the increase of film thickness because of less substrate influence, while the hardness agrees well with the trend of modulus at shallow depth but mismatches at larger indentation depth. The observed SiO₂ film deformation consists of densification and thinning along indentation direction and extension in the deformed area due to the rotation and deformation of massive SiO₄ tetrahedra. The SiO₂ film plays an important role in the onset and development of silicon phase transformation. The thinner the SiO₂ film is, the earlier the silicon phase transformation takes place. So the numbers of phase transformation atoms increase with the decrease of SiO₂ film thickness at the same indentation depth. It is suggested that the thicker film should be better during CMP process for higher material removal rate and less defects within silicon substrate.

The surface planarity and asperity removal behavior on atomic scale in an ultrathin water environment were studied for a nanoscale process by molecular dynamics simulation. Monolayer atomic removal is achieved under both noncontact and monoatomic layer contact conditions with different water film thicknesses. The newly formed surface is relatively smooth without deformed layers, and no plastic defects are present in the subsurface. The nanoscale processing is governed by the interatomic adhering action during which the water film transmits the loading forces to the Cu surface and thereby results in the migration and removal of the surface atoms. When the scratching depth ≥ 0.5 nm, the abrasive particle squeezes out the water film from the scratching region and scratches the Cu surface directly. This leads to the formation of trenches and ridges, accumulation of chips ahead of the particles, and generation of dislocations within the Cu substrate. This process is mainly governed by the plowing action, leading to the deterioration of the surface quality. This study makes the ”0 nm planarity, 0 residual defects, and 0 polishing pressure” in a nanoscale process more achievable and is helpful in understanding the nanoscale removal of materials for developing an ultra-precision manufacture technology.

The well-known “size effect” is not only related to the diameter but also to the length of the small volume materials. It is unfortunate that the length effect on the mechanical behavior of nanowires is rarely explored in contrast to the intensive studies of the diameter effect. The present paper pays attention to the length-dependent mechanical properties of 〈111〉-oriented single crystal gold nanowires employing the large-scale molecular dynamics simulation. It is discovered that the ultrashort Au nanowires exhibit a new deformation and failure regime-high elongation and high strength. The constrained dislocation nucleation and transient dislocation slipping are observed as the dominant mechanism for such unique combination of high strength and high elongation. A mechanical model based on image force theory is developed to provide an insight to dislocation nucleation and capture the yield strength and nucleation site of first partial dislocation indicated by simulation results. Increasing the length of the nanowires, the ductile-to-brittle transition is confirmed. And the new explanation is suggested in the predict model of this transition. Inspired by the superior properties, a new approach to strengthen and toughen nanowires-hard/soft/hard sandwich structured nanowires is suggested. A preliminary evidence from the molecular dynamics simulation corroborates the present opinion.

18Ni(300) maraging steel, which has exceptional strength and toughness, is used in the field of aviation and aerospace. In this paper, using a high-speed tribo-tester, tribological behaviors of 18Ni(300) maraging steel were investigated under high-speed dry sliding conditions. Morphology of the worn surfaces and the debris was analyzed by scanning electron microscope, and the oxides of worn surfaces caused by friction heat were detected by X-ray diffraction. The experiment results reveal that the friction coefficient of frictional pairs declines with increasing load and speed. With the speed and load increasing, oxides of the worn surfaces of 18Ni(300) maraging steel change from FeO to Fe₃O₄ and the wear mechanism converts from adhesive wear into severe oxidative or extrusion wear.

Currently, monocrystalline silicon has been widely applied in micro-electro-mechanical systems (MEMSs). It is of importance to reveal the wear behavior of the MEMS and evaluate the planarization of silicon surface in chemical mechanical polishing (CMP). In this study, molecular dynamics simulation was used to investigate the nano three-body abrasion of monocrystalline silicon with a diamond ellipsoidal particle sandwiched between two silicon specimens. The normal load acting on the ellipsoidal particle was varied from 80 nN to 240 nN. Results indicate that the movement pattern of the particle changes from rolling to sliding when the normal load becomes greater than 160 nN. Using the criterion of particle movement pattern by comparing the value of e/h and coefficient of friction, the particle movement pattern can be accurately predicted. The evolution of force in the abrasion process depicts both friction force and normal load fluctuations in sinusoid-like curve for the rolling ellipsoidal particles, whereas the front cutting of particle results in an increase in the friction force, making it greater than the normal force for sliding particles under high velocity. The plastic deformation of monocrystalline silicon is attributed to the phase transformation, which is clearly impacted by the movement pattern of the particle. The rolling of the particle causes substrate deformation with periodical inhomogeneous characteristics, while sliding helps produce a high-quality surface and improves efficiency in the CMP process.