Initial Surface Damage Research Articles

Influence of Morphology on the Healing Mechanism of PCL/Epoxy Blends.

Polycaprolactone (PCL) is being researched as a self-healing agent blended with epoxy resins by several reasons: low melting point, differential expansive bleeding (DBE) of PCL, and reaction induced phase separation (RIPS) of PCL/epoxy blends. In this work, PCL/epoxy blends were prepared with different PCL ratios and two different epoxy networks, cured with aliphatic and aromatic amine hardeners. The curing kinetic affects to the blend morphology, varying its critical composition. The self-healing behavior is strongly affected by the blend morphology, reaching the maximum efficiency for co-continuous phases. Blends with dispersed PCL phase into epoxy matrix can also show high self-healing efficiency because of the low PCL domains that act as reservoir of self-healing agent. In this last case, it was confirmed that the most efficient self-healable blends are one whose area occupied by PCL phase is the largest. These blends remain the good thermal and mechanical behavior of epoxy matrix, in contrast to the worsened properties of blends with bicontinuous morphology. In this work, the self-healing mechanism of blends is studied in depth by scanning electron microscopy. Furthermore, the influence of the geometry of the initial surface damage is also evaluated, affecting to the measurement of self-healing efficiency.

Sapphire advanced mitigation process: wet etch to expose sub-surface damage and increase laser damage resistance and mechanical strength.

A novel, to the best of our knowledge, method of wet chemical etching of sapphire workpieces (such as optics, wafers, windows, and cones), called the sapphire advanced mitigation process (or sapphire AMP), has been developed that exposes sub-surface mechanical damage created during the optical fabrication process and significantly enhances the surface laser damage resistance ($ \gt {2{\times}}$>2×) and mechanical strength (up to $\sim{2.6{\times}}$∼2.6×). Sapphire AMP involves first treating the workpiece with a mixture of sulfuric and phosphoric acid $([{\rm H_{2}{\rm SO_{4}}}]:[{\rm H_{3}{\rm PO_{4}}}]=1:3)$([H2SO4]:[H3PO4]=1:3) at 220°C, followed with phosphoric acid at 160°C, then with sodium hydroxide base (NaOH) and surfactant at 40°C, and finally with a high-pressure deionized water spray rinse. Sapphire AMP has been demonstrated on both A- and C-plane sapphire workpieces. The mechanism of this etch process involves the reaction of the sapphire $({\rm Al_{2}}{\rm O_{3}})$(Al2O3) surface with sulfuric acid $({\rm H_{2}}{\rm SO_{4}})$(H2SO4) forming aluminum sulfate $[{{\rm Al}_2}{({{\rm SO}_4})_3}]$[Al2(SO4)3], which has low solubility. The high phosphoric acid content in the first and second steps of sapphire AMP results in the efficient conversion of ${{\rm Al}_2}{({{\rm SO}_4})_3}$Al2(SO4)3 to aluminum phosphate $({\rm AlPO_{4}})$(AlPO4), which is very soluble, greatly reducing reaction product redeposition on the workpiece surface. Sapphire AMP is shown to expose sub-surface mechanical damage on the sapphire surface created during the grinding and polishing processes, whose etched morphology has either isotropic or anisotropic evolution depending on the nature of the initial surface damage. Sapphire AMP was also designed to remove the key known surface, laser absorbing precursors (namely, foreign chemical impurities, the fracture surface layer of preexisting sub-surface damage, and reaction product or foreign species redeposition or precipitation). Static and sliding indention induced surface microfractures on sapphire are shown after sapphire AMP to have a significant decrease in the fast photoluminescence intensity (a known metric for measuring the degree of laser damaging absorbing precursors). In addition, the onset of laser damage (at 351 nm 3 ns) on sapphire AMP treated workpieces was shown to increase in fluence from $\sim{4}$∼4 to $ \gt {9}.{5}\;{{\rm J/cm}^2}$>9.5J/cm2. Finally, biaxial ball-on-ring mechanical tests on sapphire disks showed an increase in the failure stress from 340 MPa (with pre-existing 28 µm flaws) to $\sim{900}\;{\rm MPa}$∼900MPa after sapphire AMP, which is attributed to the blunting of the surface microfractures.

The influence of machining processes on strain distribution and progressive failure characteristics when producing holes in CFRP

Machining strategy can significantly influence surface integrity when producing holes in CFRP laminate, which has drastic impact on mechanical behavior and service life of related structures. Therefore, it is vital to understand the effect of defects caused by machining on cracking/splitting initiation, propagation and final failure modes. In this paper, the influence of four different machining processes used to produce 6.0 mm diameter holes in CFRP was investigated in relation to mechanical behaviour under tensile loading. The processes involved were high power fiber laser cutting, abrasive waterjet machining, conventional drilling and ultraviolet laser multiple-pass machining. The complex strain distributions around the holes were analyzed using digital image correlation (DIC) method, while crack propagation was studied using high speed imaging technique. The correlation between initial surface integrity and mechanical behavior was developed. Stress concentrations and machining defects around holes accelerated damage accumulation with final failure modes highly related to fiber orientations rather than initial surface damage.

Towards a deeper understanding of the formation of friction-induced hillocks on monocrystalline silicon

Friction-induced hillocks can be produced on monocrystalline silicon by scratching under given conditions. Results show that the height of these hillocks increases with the applied normal load or number of scratching cycles, but decreases with the sliding velocity. Transmission electron microscope (TEM) and energy dispersive x-ray (EDX) analysis show that the hillock contains a thin superficial oxidation layer and a thick disturbed (amorphous and deformed) layer in the subsurface. Although the formation of the silicon hillock is the combined results of mechanical interaction and tribochemical reaction, the mechanical interaction plays a more dominant role. Further analysis indicates that the formation of hillock is mostly induced by the amorphization of crystal silicon during scratching. Low sliding speed is found to facilitate the formation of a thick amorphization layer under the same loading condition. Since the friction-induced hillock is the initial surface damage on the nanoscale, the results will shed new light on understanding and controlling the nanowear process of silicon in micro/nanoelectromechanical systems.

Microscratching in an electrochemical cell. Effect of gallium on the surface deformation in aluminum

The method of microscratching is combined with the electrochemical reduction of an active component on the tested surface to study the initial surface damage to metals and effects caused by active media. This allows us to model the influence of the active liquid metal in the absence of the liquid-metal phase at room temperature. We observed the phenomenon of surface plasticizing of aluminum caused by the reduction of gallium ions and discussed it in terms of the microscopic (dislocation) and macroscopic (finite-elements) approaches.

Electron microscopy study of oxidation-induced defects at the silicon-silicon-dioxide interface

It is shown by transmission and electron scanning microscopies and energy-dispersive X-ray spectroscopy that the conditions of thermal annealing and the initial surface damage influence the microstructure of the silicon-silicon-dioxide interface and the silicon underlayers, especially the structure and chemical composition of defects. Annealing underr vacuum enhances the dislocation formation induced by diamond polishing. Oxidizing annealing of similar samples induces stacking faults, while mechanochemically polished samples do not show any stacking faults after the same thermal treatment. Copper precipitation in {110} planes is observed. The shape of the precipitates depends on the density of nucleation sites in the near-surface layers as well as on the amount of contaminating copper: some small copper precipitates are detected when the silicon has been mechanically polished and oxidized, while giant copper colonies are revealed when the silicon sample has been mechanochemically etched.

An Experimental Study of the Effects of Surface Lay Orientations on Initial Surface Damage in Point Contacts

An experimental investigation to determine the effects of surface roughness lay direction relative to the direction of motion on initial surface damage was conducted using a four-ball machine under both simple sliding and pure rolling point contact conditions. Surface topographies ranging from 0.11 μm (4.45 μm) to 0.73 μm (28.61 μm.) RMS roughness and 31.5 μm (1239 μin.) to 59.6 μm (2345 μin.) in wavelength were utilized in the experimental program. A lithium complex grease was used as the lubricant for all tests. The experimental results indicate that surfaces with transverse lay orientations cause more initial surface damage than surfaces with longitudinal lay orientations for relatively low ratios of film thickness-to-composite surface roughness, and low ratios of surf ace wavelength-to-contact radius. Presented at the 47th Annual Meeting in Philadelphia, Pennsylvania May 4–7, 1992

Influence of initial surface damage on the final texture of chemically etched quartz plates

Various AT-cut quartz plates, mechanically lapped or polished prior to etching are etched at a constant temperature. The variations in the dissolution rate with the time of etching are found to depend on the surface damage in the initial stages of the chemical attack, but with prolonged etching all the dissolution rates tend to a common limiting value. Lapped and polished quartz plates exhibit opposite changes in roughness parameters with the depth of etch for short etching times. Subsequent dissolution leads, whatever the original surface preparation of the quartz plate, to equilibrium values of the roughness parameters characterized by the direction of the profilometry traces. Scanning electron micrographs are in agreement with these observations and reveal that pits of the same shape develop on polished and lapped quartz plates. Thus the initial surface damage has no influence on the final surface texture of deeply etched quartz plates which is determined primarily by the crystal orientation.