Degree Of Subsurface Damage Research Articles

Molecular dynamics study of crystal orientation effect on surface generation mechanism of single-crystal silicon during the nano-grinding process

Crystal orientation of the crystalline materials surfaces significantly affects the surface generation and subsurface damage of the materials during the ultra-precision machining. This work is based on the molecular dynamics (MD) simulation method to study the nano-grinding process of the single-crystal silicon workpiece with three different types of surface crystal orientations. And the MD simulation results are compared with the experimental results. The results show that the machined surface quality of workpieces with different surface crystal orientations is similar, but the degree of subsurface damage, processing force, phase transition, and internal residual stress is quite different. By comparing all the three types of the workpiece, the surface and subsurface quality of the single-crystal silicon workpiece with {110} surface crystal orientation has a better processing effect. This work reveals the evolution of the crystal structure of single-crystal silicon with different surface crystal orientations and the mechanism of subsurface damage from the atomic point of view.

Nondestructive Evaluation of Functionally Graded Subsurface Damage on Cylinders in Nuclear Installations Based on Circumferential SH Waves

Subsurface damage could affect the service life of structures. In nuclear engineering, nondestructive evaluation and detection of the evaluation of the subsurface damage region are of great importance to ensure the safety of nuclear installations. In this paper, we propose the use of circumferential horizontal shear (SH) waves to detect mechanical properties of subsurface regions of damage on cylindrical structures. The regions of surface damage are considered to be functionally graded material (FGM) and the cylinder is considered to be a layered structure. The Bessel functions and the power series technique are employed to solve the governing equations. By analyzing the SH waves in the 12Cr-ODS ferritic steel cylinder, which is frequently applied in the nuclear installations, we discuss the relationship between the phase velocities of SH waves in the cylinder with subsurface layers of damage and the mechanical properties of the subsurface damaged regions. The results show that the subsurface damage could lead to decrease of the SH waves’ phase velocity. The gradient parameters, which represent the degree of subsurface damage, can be evaluated by the variation of the SH waves’ phase velocity. Research results of this study can provide theoretical guidance in nondestructive evaluation for use in the analysis of the reliability and durability of nuclear installations.

The Quantitative Effect of Diamond Grit Size on the Subsurface Damage Induced in Dental Adjustment of Porcelain Surfaces

Diamond burs with different grit sizes are often applied to adjust ceramic prostheses in restorative dentistry. However, the quantitative influence of diamond grit size on subsurface damage in adjusting ceramic prostheses is unknown. The aim of this study was to investigate and visualize the quantitative effect of diamond bur grit size on subsurface damage in dental adjusting of a feldspar prosthetic porcelain. Diamond burs with coarse (106-125 rm), medium (53-60 microm), and fine (10-20 microm) grit sizes were selected. Dental adjusting-induced subsurface damage was quantitatively investigated with the aid of finite element analysis (FEA) and scanning electron microscopy (SEM). Significant differences in subsurface damage depth were found among the coarse, medium, and fine diamond burs (ANOVA, p < 0.05). Coarse diamond burs induced approximately 6-8 times deeper subsurface damage than fine burs. Diamond grit size is confirmed to be a controlling factor in determining the degree of subsurface damage. Subsurface damage depths also significantly increased with removal rate (ANOVA, p< 0.05). The correlation of the SEM-measured subsurface damage depths and the diamond grit sizes supports the FEA predictions. From a practical standpoint, dental porcelains should be adjusted using smaller diamond grit sizes with lower removal rates to minimize subsurface damage.

Fabrication and evaluation for extremely thin Si wafer

Grinding process on the Si wafer develops subsurface damage, which remarkably degrades deflective strength of the wafer and constitutes a barrier against producing a thin wafer for low-profile packaging. In this paper, the authors propose a new index for evaluation of the Degree of Subsurface Damage (DSD). Requiring no costly instrument, the new index is easily calculated via the external geometry of the ground wafer. With the new index, it is able to quantitatively evaluate the subsurface damage introduced by different processes (or wheel) and to estimate the minimally achievable thickness of the wafer by each process. Also, a novel fixed abrasive process of Chemo-Mechanical Grinding (CMG) has been proposed for stress relief. All results indicate that the subsurface damage after CMG is nearly zero.

A novel process for the generation of pristine sapphire surfaces

Sapphire is widely used as a substrate material for blue light emitting diode and laser diode devices, as well as for visible/infrared window and radome applications. The ensuing component performance is highly dependent on the quality of the surface finish, and degree of subsurface damage. A novel process has been developed which has the potential to replace the final stage chemical and mechanical polishing steps currently necessary to achieve the requisite surface finish. The process entails deposition of a thin film of Al on the (0 0 0 1) sapphire substrate, followed by an oxidation anneal. The final step is grain growth of the underlying substrate, which leads to complete single crystal conversion. The influence of heat treatment conditions on the final surface finish was investigated. Microstructural and topographical development of the surface layer was studied using scanning electron microscopy, orientation image microscopy, and atomic force microscopy.

Ductile Mode Machining of Commercial PZT Ceramics

A wide range of products rely on the high-precision fabrication of piece-parts using PZT ceramics. These include ultrasonic medical imaging transducers, ink jet printing heads, etc. Not only is precision important, but also the degree of sub-surface damage needs to be minimised because this is known to compromise performance by engendering depoling. It also leads to in-service problems due to ageing caused by the movement of damage-induced domain walls and cracks. A fundamental machinability investigation was undertaken, commencing with quasi-static and ruling tests, prior to performing diamond turning and grinding experiments. This research has helped in the understanding of the PZT ceramic mechanical and electrical characteristics when machined and also is being used to build up an optimised machining process model.

Indentation induced crack nucleation and propagation in single crystal MgO

A study of indentation induced cracks in single crystal Magnesium Oxide ([001] orientation) was conducted to understand the micromechanisms controlling crack nucleation (and subsequent propagation) in semibrittle solids. Contrary to the expectations based on generally accepted crack nucleation models, no evidence of dislocation pile-ups was observed. However, SEM and TEM study of the deformation structure within the indent impression did reveal the presence of shear fault induced microcracking. This phenomenon was confirmed by examination of cross-sections through the indents using SEM. When the indentation temperature was varied from ambient to 800°C, a direct correlation was observed between the degree of subsurface damage, and the absence or presence of surface radial cracks. Based on the afore-mentioned results, an alternate crack nucleation mechanism is proposed, whereby shear fault induced microcracks act as nuclei for subsequent radial cracking. Further, microdiffraction analysis of the misorientations induced outside the indent suggest that crack propagation parallel to {110} is the result of severe plastic incompatibility across this plane.

The effect of sliding time and speed on the wear of composite materials

The friction and wear properties of an Al 201 alloy and a unidirectionally oriented graphite fiber-aluminum matrix composite (T50-Al 201) were investigated. The experiments were conducted on a pin-on-disc type friction machine. The diameter of the pin was 0.22 cm and the load 4.46 N. The sliding velocity varied between 0.17 and 0.43 m s −1. The disc counterface was of commercially pure iron. It has been found that the friction coefficient μ and the wear rate W L of the composite material decrease as the sliding time is increased until a steady state value is reached. The steady state wear rate is proportional to the reciprocal of the sliding speed in accord with a recently proposed model. Scanning electron microscopy and Auger electron spectroscopy observations indicate that the high initial values of μ and W L are due to a high degree of matrix adhesion to the counterface accompanied by fiber breaking and transfer. The low steady state values of μ and W L are due to the formation of a film that impedes adhesion and confers some degree of self-lubrication. It is suggested that the observed variation of W L with sliding speed is related to changes in the degree of subsurface damage as the velocity is varied.