Micron-sized Pits Research Articles

Investigation on the Machinability of Polycrystalline ZnS by Micro-Laser-Assisted Diamond Cutting.

Polycrystalline ZnS is a typical infrared optical material. It is widely used in advanced optical systems due to its excellent optical properties. The machining accuracy of polycrystalline ZnS optical elements must satisfy the requirements of high-performance system development. However, the soft and brittle nature of the material poses a challenge for high-quality and efficient machining. In recent years, in situ laser-assisted diamond cutting has been proven to be an effective method for ultra-precision cutting of brittle materials. In this study, the mechanism of in situ laser-assisted cutting on ultra-precision cutting machinability enhancement of ZnS was investigated. Firstly, the physical properties of ZnS were characterized by high-temperature nanoindentation experiments. The result revealed an increase in ductile machinability of ZnS due to plastic deformation and a decrease in microhardness and Young's modulus at high temperatures. It provided a fundamental theory for the ductile-brittle transition of ZnS. Subsequently, a series of diamond-cutting experiments were carried out to study the removal mechanism of ZnS during in situ laser-assisted cutting. It was found that the mass damage initiation depth groove generated by in situ laser-assisted cutting increased by 57.99% compared to the groove generated by ordinary cutting. It was found that micron-sized pits were suppressed under in situ laser-assisted cutting. The main damage form of HIP-ZnS was changed from flake spalling and pits to radial cleavage cracks. Additionally, the laser can suppress the removal mode difference of different grain crystallographic and ensure the ductile region processing. Finally, planning cutting experiments were carried out to verify that a smooth and uniform surface with Sa of 3.607 nm was achieved at a laser power of 20 W, which was 73.58% better than normal cutting. The main components of roughness were grain boundary steps and submicron pit. This study provides a promising method for ultra-precision cutting of ZnS.

The influence of various grease compositions and silver nanoparticle additives on electrically induced rolling-element bearing damage

Leakage currents accelerate surface degradation of metal contacts via small scale arcing across lubricating films, but recent observations suggest that metallic nanoparticle additives in lubricants may be useful to improve contact performance. These findings prompted a study that examined electrically induced surface pitting of steel contacts in the presence of several lubricating greases including some containing nanometer-sized colloidal silver (Ag) particles. Reciprocating rolling sphere-on-disk experiments were conducted under electro-tribological loads employing polyurea greases derived from mineral and synthetic base oils with and without additives. Friction forces and electrical resistance were monitored continuously during the tests; surface changes were characterized by means of optical spectroscopy, stylus profilometry, and scanning electron microscopy (SEM) including compositional analysis using energy dispersive spectroscopy (EDS). The observations demonstrate that surface pitting induced by arcing occurs mainly at the points were the rolling motion changes direction and that eroded metal is deposited along the wear grove. Micron-sized pits are formed which contain carbon and oxygen indicating that arcing causes decomposition of the hydrocarbon lubricants. Numerous findings indicate a significant inhibition of pitting is induced by the Ag nanoparticles; some greases containing other additives exhibit a similar, although less pronounced, effect.

Role of oxygen in surface kinetics of SiO2 growth on single crystal SiC at elevated temperatures

Understanding surface kinetics of SiO2 growth on single crystal SiC at elevated temperatures is crucial to fabricate high-performance SiC-based devices. However, the role of oxygen in the evolution mechanism of SiC surface at atomic scale has not been comprehensively elaborated. Here, we reveal the manipulation effect of oxygen on the competitive growth of thermal oxidation SiO2 (TO-SiO2) and thermal chemical vapor deposition SiO2 (TCVD-SiO2) on the 4H-SiC substrate at 1500 °C. TO-SiO2 is formed by the thermal oxidation of SiC, in which the substrate undergoes layer-by-layer oxidation, resulting in an atomically flat SiC/TO-SiO2 interface. TCVD-SiO2 growth includes the sublimation of Si atoms, the reaction between sublimated Si atoms and reactive oxygen, and the adsorption of gaseous SixOy species. A relatively high sublimation rate of Si atoms at SiC atomic steps causes the transverse evolution of the nucleation sites, leading to the formation of nonuniform micron-sized pits at the SiC/TCVD-SiO2 interface. The low oxygen concentration favors TCVD-SiO2 growth, whose crystal quality is much better than that of TO-SiO2 due to the high surface mobility in the thermal CVD process. We further achieve the epitaxial growth of graphene on 4H-SiC in an almost oxygen-free reaction atmosphere. Additionally, ReaxFF reactive molecular dynamic simulation results illustrate that the decrease in oxygen concentration can promote the growth kinetics of SiO2 on single crystal SiC from being dominated by thermal oxidation to being dominated by thermal CVD.

Topographic scale-range synergy at the functional bone/implant interface

We sought to explore the biological mechanisms by which endosseous implant surface topography contributes to bone anchorage. To address this experimentally, we implanted five groups of custom-made commercially pure titanium implants of varying surface topographical complexity in rat femora for 9 days; subjected them to mechanical testing; and then examined the interfacial bone matrix by electron microscopy. The five implant surfaces were prepared by combinations of dual acid etching and grit blasting the titanium substrates and, in some cases, modifying the created surfaces with the deposition of nanocrystals of calcium phosphate, which resulted in 10 samples per group. In parallel, we cultured rat bone marrow cells on surrogate implants constructed from polymer resin coated with the same calcium phosphate nanocrystals, and monitored the deposition of bone sialoprotein by transmission electron immunohisto-micrography. We found that implant samples modified with sub-micron scale crystals were bone-bonding, as described by the interdigitation of a mineralized cement line matrix with the underlying implant surface. The in vitro assay showed that bone sialoprotein could be deposited in the interstices between, and undercuts below, the nanocrystals. In addition, when mineralized, the cement line matrix globules occupied micron-sized pits in the implant surfaces, and in part obliterated them, creating an additional form of anchorage. Our results also showed that collagen, elaborated by the osteogenic cells, wrapped around the coarse-micron features, and became mineralized in the normal course of bone formation. This provided a mechanism by which coarse-micron implant features contributed to a functional interface, which we have previously described, that is capable of resisting the mechanical loading that increases as peri-implant bone matures. Thus, our findings provide mechanistic explanations for the biologically-relevant criteria that can be employed to assess the importance of implant surface topography at different scale-ranges.

Microcharacterization of the surface oxidation of Py/Cu multilayers by scanning X-ray absorption spectromicroscopy

The local surface oxidation of the permalloy surface layer in Py/Cu GMR multilayers on a micron lateral scale has been analyzed by means of a microspot-X-ray absorption spectromicroscope utilizing synchrotron radiation from the Advanced Light Source bending magnet beamline 6.3.2. Additionally, the GMR multilayer samples prepared by dc magnetron sputtering have been analyzed by cross-sectional transmission electron microscopy, hard X-ray reflection and magnetoresistance measurements. The formation of a passivating iron-oxide layer on the sample surface was identified by X-ray absorption near edge structure spectroscopy (XANES) near the Fe-2p edge while no indication for nickel-oxide formation could be found. Small micron-size pits of reduced iron-oxide concentration could be identified by XANES microscopy while the corresponding nickel distribution appeared to be homogeneous. The results are explained in terms of a local breakdown of the passivating oxide layer.

Statistical tools for the rapid development and evaluation of high-reliability products : William Q. Meeker and Michael Hamada. IEEE Transactions on Reliability, 44 (2), 187 (June 1995).

The electrostatic discharge (ESD) mechanism in dual-in-line package (DIP) and small outline J-lead (SOJ) semiconductor devices was investigated by examining the discharge waveforms generated in fixed gap discharge (FGD) tests as well as in approaching discharge (AD) tests. The discharge probe, after the FGD and AD tests, was examined by a Scanning Electron Microscope for ESD damage. The results of our study can be summarized as follows: (1) the breakdown voltage for the packaged semiconductor devices agrees with the Townsend model for air breakdown; (2) the general features of the discharge waveforms for the FGD tests are similar to those of the AD tests; (3) only nanosecond impulse (non-oscillatory) discharge waveforms are detected for all voltages (from 50 volts to 3000 volts); and (4) the micron-size pits, similar to those previously reported in the study of low voltage (less than 300 volts) discharges between close electrodes in air, are observed on the surface of our discharge probe. Based on these results, we conclude that the electrostatic discharges to the packaged semiconductor devices must occur through an air gap for all voltages and not through physical contact as is often suggested in discussions of Charged Device Model (CDM).

An experimental investigation of the electrostatic discharge (ESD) mechanism in packaged semiconductor devices

The electrostatic discharge (ESD) mechanism in dual-in-line package (DIP) and small outline J-lead (SOJ) semiconductor devices was investigated by examining the discharge waveforms generated in fixed gap discharge (FGD) tests as well as in approaching discharge (AD) tests. The discharge probe, after the FGD and AD tests, was examined by a Scanning Electron Microscope for ESD damage. The results of our study can be summarized as follows: (1) the breakdown voltage for the packaged semiconductor devices agrees with the Townsend model for air breakdown; (2) the general features of the discharge waveforms for the FGD tests are similar to those of the AD tests; (3) only nanosecond impulse (non-oscillatory) discharge waveforms are detected for all voltages (from 50 volts to 3000 volts); and (4) the micron-size pits, similar to those previously reported in the study of low voltage (less than 300 volts) discharges between close electrodes in air, are observed on the surface of our discharge probe. Based on these results, we conclude that the electrostatic discharges to the packaged semiconductor devices must occur through an air gap for all voltages and not through physical contact as is often suggested in discussions of Charged Device Model (CDM).

Loss of precious metal from Pt-Rh alloy under refractory oxide environments

Loss of precious metals from a Pt-10 wt% Rh alloy was studied at 1300° C in refractory oxides and fused quartz environments. After 60 and 150 h annealings, samples exposed to gaseous environments showed significant weight loss. The surface was attacked and characterized by micron-sized pits, as well as river-like striations. The pits were found to be enriched with silicon and/or aluminium depending upon the heating environment. Laser Raman microprobe identified that the major compound formed in the pits is α-cristobalite.

Laser‐induced fluid motion on a dye/polymer layer for optical data storage

The interaction of highly focused light with active dye/poly- mer layers has received considerable attention in these days because it creates micron-size pits useful as optical data storage sites. Pit formation during laser marking takes place in a very short period of time, usually between 50 and 500 nanoseconds (ns). It consists of the following steps: the material heats, then melts, and finally flows in the lateral direction. In other words, the highly focused laser pulse results in an uneven temperature profile on the surface of an active dye/polymer layer; the varia- tion in surface tension from one point to another creates tangen- tial stresses on the free surface and a capillary fluid motion is therefore induced. As a result, the free surface of the pit center depresses and a bulge is formed around the pit. To be a good recording medium, an active dye/polymer layer should have high light absorption and reflectivity at the wavelength of the emitting laser beam. The greater the absorption, the faster the marking. The encoded data can be read back based on the dif- ference in the surface reflectivity between the surroundings and the pit. The detailed mechanism of pit formation is not fully under- stood yet, but it is a typical transport phenomenon problem for a chemical engineer. Surprisingly, however, almost no attention has been given to this issue by chemical engineers; most of the work has been done by optical physicists and published in physi- cal journals. For example, Novotny and Alexandru (1 979) cal- culated the temperature profile during the light-induced mark- ing process and described the mechanism of structure change in dye/polymer systems. Wrobel et al. (1982) found that the sur- face tension forces were unimportant in rapid pit formation in thin polymer films. Broer and Vriens (1983) presented models to describe the hole-opening process, and concluded that surface tension is one dominant force during the process. Suh and coworkers investigated the writing mechanism in thin-textured inorganic films, and believed the surface tension gradient force to be one of the most effective driving forces to overcome the energy barriers for forming a pit (Suh and Craighead, 1985; Suh et al., 1985). Chung (1986) derived the governing equations for the transitional laser-induced fluid motion in extremely thin dye/polymer layers. The temperature rise in an optical layer