Surface Crater Formation Research Articles

A reexamination of the relationship between β to α crystal transition and the surface crater formation in biaxially oriented polypropylene film

The formation mechanism of the crater-like surface of biaxially oriented polypropylene (BOPP) film remains controversial. To clarify this issue, in this work, three isotactic polypropylene (iPP) samples with different tacticities are selected to prepare BOPP films. It is shown that the relative β crystal content (kβ) in the cast sheet increases with tacticity. The β crystal transforms to the α crystal when stretched at 145 °C along the machine direction. For all the samples, rough surfaces are obtained even if the kβ value of the cast sheet is very low (kβ < 0.1). The arithmetic mean roughness (Ra) of the BOPP films increases with the increase of the kβ in the cast sheet. While higher kβ in the iPP cast sheet contributes to a higher Ra value, the β crystallites hinder the enlargement of roughening rings, and cause surface defects (cavitation), which are detrimental to applications. Our results indicate that the β crystal in the cast sheet is not essential for the crater formation and a higher β crystal content is not always beneficial for BOPP capacitor films.

Nanosecond laser irradiation of Yttria stabilized Zirconia: Pulsed laser ablation and surface treatment

Nanosecond pulsed laser irradiation resulting in material ablation and surface micro structuring of Yttria Stabilized Zirconia (YSZ) targets, has been investigated. Our study aims to evaluate suitable ablation conditions enabling either, creation of desired YSZ surfaces having micro porosity for directed use, or generation of YSZ vapors for deposition of homogenous, dense and particle free YSZ films via conventional Pulsed Laser Deposition (PLD) technique. The work being reported here involves a thermal model based theoretical simulation describing heat transport, melting and vaporization of a laser irradiated YSZ target. Our simulation has been validated against our experimental data on nanosecond laser ablation from a sample target of ceramic YSZ. Surface porosity generated on laser irradiated YSZ targets for low laser fluence levels ∼ 1 ​J/cm2 has been explained in terms of pore formation via entrapment of gas bubbles in the solidifying interface on laser melting and re-solidification thereafter, of the YSZ target. To understand surface crater formation on laser irradiated YSZ targets at high laser fluence levels ∼10 ​J/cm2 maximum temperature reached by the laser irradiated YSZ target has been compared with the thermodynamic critical temperature of YSZ. Our study confirms that judicious choice of laser fluence enables either, creation of surfaces having micro porosity of desired nature on YSZ target, or controlled generation of YSZ vapors for deposition of good quality YSZ films and coatings using laser deposition technique.

Hole formation effect on surface morphological response of plasma-facing tungsten

We report simulation results on the effect of helium (He) bubble bursting-mediated surface hole formation on the surface morphological response of tungsten plasma-facing components (PFCs) in nuclear fusion devices. Our analysis is based on an atomistically informed, continuum-scale model, which is capable of accessing the spatiotemporal scales relevant to the fuzz nanostructure formation process on the surface of PFC tungsten. Our simulations account, in an empirical fashion, for two types of subsurface bubble dynamical phenomena in the nanobubble region of PFC tungsten during He plasma irradiation, involving bubble bursting and surface crater formation. We demonstrate that the hole formation effect on the PFC tungsten surface accelerates the growth rate of nanotendrils and the onset of fuzz formation. As a result, the predicted incubation time for surface nanotendril growth is shortened, in agreement with experimental data of incubation fluence at comparable plasma exposure conditions. We also explore systematically the dependence of the PFC surface morphological response on the areal density of holes introduced at regular time intervals onto the He-implanted tungsten surface, a parameter in our analysis that serves as a proxy for the rate of He bubble bursting. More importantly, our simulations capture fine surface features in the PFC tungsten surface morphology and predict that the average spacing between nanotendrils is on the order of 100 nm, consistent with the experimental findings.

Studying a Silica Film Implanted with Zn and Irradiated with Swift Xe Ions

The effect irradiation with swift heavy Xe ions at an energy of 167 MeV has on the structure and properties of a Zn-implanted SiO2 film is studied. The implantation of Zn ions is found to result in the formation of amorphous zinc nanoparticles around 10 nm in size at a depth near the projective range of zinc ions (Rp ≈ 40 nm) in the SiO2 film. Xe irradiation of the film dampens the exciton recombination–induced peak in the photoluminescence spectrum at a wavelength of 370 nm. It also raises the peak at 430 nm, which is associated with radiation defects. Bombarding the surface of the SiO2 film with Хе ions results in the formation of surface craters surrounded by hillocks, and the emergence of Zn-containing nanoparticles.

High vapour pressure nanofuel droplet combustion and heat transfer: Insights into droplet burning time scale, secondary atomisation and coupling of droplet deformations and heat release

Combustion characteristics of ethanol-water (EW) droplets laden with ceria nanoparticles are investigated. The present experimental study focuses on three facets of droplet combustion (i) burning time scale of droplets with and without NPs, (ii) pathways of secondary atomisation due to interface deformations and (iii) coupling of droplet shape deformations and flame heat release. A theoretical vaporisation timescale is advocated which considers natural convection-based evaporation, mass loss due to daughter droplet ejections, and flow through porous media. Droplets seeded with ceria nanoparticles, exhibit arrested surface undulations although internal ebullition is discernibly enhanced as compared to EW droplets without NPs. Deformations and formation of surface craters in EW droplets are traced to the imbalance between local vapour recoil (due to rapid ethanol vaporisation) and surface tension. Such craters collapse and form high-speed ligaments which eventually break at the tip through Rayleigh–Plateau mechanism. This pathway of secondary atomisation of EW droplets has been elucidated using a modified local weber number. On the contrary, for nanofuels, bubble rupture is the mechanism behind the surface crater formation. Proper orthogonal decomposition (POD) technique is utilised for investigating the droplet shape and flame heat release coupling. EW droplet shape and HR are found to be a synced system with a phase lag arising from the flame response timescale. However, a weak coupling is detected for nanofuel droplets.

On the formation of surface craters by rapid cluster ions

A theoretical approach is developed to describe the dominant features of process by which craters form on the copper, gold and solid argon surfaces under self impacts of high velocity cluster ions. It is showed that, in the shock wave conditions generated at the impact of cluster on the surface, the dynamical process begins when the shock reaches the target free surface inducing the formation of a rarefaction wave(release wave) which propagates inward into the compressed region. The rarefaction causes the shocked material to move toward the free surface and expand into a vacuum forming the crater. It is found that the size of cluster has a significant influence on the mechanism of crater formation. In addition to metals (Cu and Au) the proposed approach has been successfully extended to impact of argon clusters on solid argon substrate. The approach has been validated using molecular dynamics simulated data for copper, gold and argon available in the literature. All simulated crater volumes have been analytically reproduced with a good agreement on the basis of a hemispherical shape.

Modelling of crater formation on anode surface by high-current vacuum arcs

Anode melting and crater formation significantly affect interruption of high-current vacuum arcs. The primary objective of this paper is to theoretically investigate the mechanism of anode surface crater formation, caused by the combined effect of surface heating during the vacuum arc and pressure exerted on the molten surface by ions and electrons from the arc plasma. A model of fluid flow and heat transfer in the arc anode is developed and combined with a magnetohydrodynamics model of the vacuum arc plasma. Crater formation is observed in simulation for a peak arcing current higher than 15 kA on 40 mm diam. Cu electrodes spaced 10 mm apart. The flow of liquid metal starts after 4 or 5 ms of arcing, and the maximum velocities are 0.95 m/s and 1.39 m/s for 20 kA and 25 kA arcs, respectively. This flow redistributes thermal energy, and the maximum temperature of the anode surface does not remain in the center. Moreover, the condition for the liquid droplet formation on the anode surfaces is developed. The solidification process after current zero is also analyzed. The solidification time has been found to be more than 3 ms after 25 kA arcing. The long solidification time and sharp features on crater rims induce Taylor cone formation.

Influence of copper alloy microheterogeneities on the formation of surface relief under high power ion beam irradiation

Changes in the morphology and surface composition of copper alloys (L63 brass, VB23NTs bronze, and a Cu-Al alloy) containing components with different degrees of volatility, under the action of a high power ion beam of nanosecond duration, are investigated. It is demonstrated that various alloy components affect the formation of surface craters during ion beam irradiation. The probable mechanisms of the observed phenomena are discussed.

Pulsed electron beam surface melting of CoCrMo alloy for biomedical applications

The use of CoCrMo alloys in biomedical applications has come under scrutiny recently due to unacceptable revision rates of certain hip resurfacing and total hip arthroplasty designs. Failure analysis has demonstrated that solid and soluble wear debris and corrosion products, released from the joints have resulted in adverse local tissue reactions (ALTR), pseudo-tumour formation and ultimately implant retrieval and replacement. In order to improve the surface properties of a wrought CoCrMo alloy, a low energy high current pulsed electron beam surface treatment process was investigated. Samples were irradiated at two cathode voltages of 15 and 35kV at pulse numbers of 1, 15 and 25. At low beam energies a polishing effect was observed as a result of surface melting. At higher beam energies a higher Ra value was the result of the formation of surface craters. Nano-indentation and scratch testing of the treated surface were carried out using a nano-indenter. Depth profiling nano-indentation was performed using a Berkovich tip in load control. Loading was performed in 8mN increments up to 160mN at a rate of 3.5mN/s, with a 60s dwell period and 40% unloading. The results demonstrated that the surface treatment process reduced both the modulus and the hardness of the surface in comparison to the control. Scratching was performed with a 20μm radius spherical diamond and loading rate of 2mN/s up to a maximum of 100mN, over a 1mm scratch length. Similar scratch depths for both control and treated surfaces were observed. However, an improvement in the dynamic friction coefficient was observed at certain beam energies. These results are discussed in the light of XRD evidence that suggested rapid cooling of the surface induced preferential formation of an ε-martensite HCP phase which may be beneficial for biomedical applications.

Dry Sliding Friction and Wear Behaviour of an Electron Beam Melted Hypereutectic Al–Si Alloy

The economic and environmental benefits of using light-weighting technology in automotive applications continue to attract attention for feasible commercial solutions. This study investigates the use of pulsed electron beam melting of a hypereutectic Al–Si alloy as a possible modification procedure for cylinder crankcase bore facing surfaces. Machined surfaces of an A390 alloy were subjected to five pulsed electron doses with an applied cathode potential between 16.5 and 36 kV. It was found that increasing beam accelerating voltages led to an initial decrease (1.4 μm R a) but subsequent increase (4.0 μm R a) in average surface roughness values associated with surface crater formation due to sub-surface melting and eruption. Surfaces were tested under dry sliding tribological conditions against 52100 bearing steel in a reciprocating geometry. Average dynamic friction coefficients were higher (0.9) compared to the untreated alloy surface (0.6) as a result of a greater degree of adhesion to the counterface. However, FIB cross sections of worn surfaces indicated that this activated an oxidative type wear process which ultimately led to the formation of a beneficial surface tribo-film on the EBM-treated surfaces, improving the specific wear rates by up to 66%.

Crater formation via homoepitaxy of adatoms dislodged from reducing oxide islands on metal surfaces

The reduction of ${\text{Cu}}_{2}\text{O}$ islands on Cu(100) surfaces under vacuum annealing leads to the formation of surface craters on the substrate surface surrounding the reducing oxide islands. These craters exhibit growth instability characterized by increased slope of both the inner and outer facets of the crater rims. We suggest that the crater formation and the growth instability are related to the homoepitaxial growth of Cu adatoms dislodged from the reducing ${\text{Cu}}_{2}\text{O}$ islands. Incorporation of such atomic processes in kinetic Monte Carlo simulations reproduces the morphological features of the experimentally observed craters.

Pulsed laser treatment of plasma-sprayed thermal barrier coatings: effect of pulse duration and energy input

Pulsed laser treatments of plasma-sprayed thermal barrier coatings can provide good corrosion resistance of protected components without impairing thermal fatigue resistance of the ceramic layers. Laser treatments are performed over a wide range of pulse durations and energy inputs, and their effects on microstructure, crystalline grain size and chemical composition of the remelted thin upper layer are investigated. Particular attention is given to macro and microcracking originating on the surface, gas bubble motion inside the melted layer and consequent surface crater formation. Density, shape, dimension and distribution of craters in the laser-irradiated zone are correlated with pulse duration and energy input of the laser beam. A numerical simulation of temperature distributions and heat phenomena originating in the ceramic coating during laser irradiation is presented, in order to explain the influence of laser characteristics on the quality of the coating surface.

Cathode spot energy transfer simulated by a focused laser beam

Minimum conditions for the formation of surface craters by laser irradiation were studied experimentally and theoretically for various metals. The critical power density for crater formation within 20 ns was about 10/sup 11/ W/m/sup 2/. It is therefore concluded that crater formation by ion bombardment requires an ion current density on the order of 10/sup 10/ A/m/sup 2/. >

Direct observation of spike effects in heavy-ion sputtering

Abstract Transmission electron microscopy has been used to study the formation of surface craters by Bi+ and Bi2 + bombardment of Au in the energy range 10–500 keV. Craters ∼5 nm in diameter were observed in individual displacement cascades of energy 50 keV. The observed energy and energy—density dependence of crater yields, sizes and fine structure indicate that the craters were formed in extra-high-energy—density surface spikes which occur with a low (∼10−2) but finite probability due to fluctuations in energy deposition in individual cascades. In light of the present experimental evidence, we propose a modified ‘thermal-spike’ model for the explanation of the quite large deviations from linear cascade theory that have been observed by conventional self-ion and molecular-ion sputtering-yield measurements in heavy metals.

Surface craters induced by displacement cascades in heavy metals

Past investigations of the erosion of surfaces under bombardment by energetic particles have mainly focused on the measurement of integral sputtering yields and the examination of surface structural changes by scanning electron microscopy. Both types of observations generally require the use of rather high particle doses (≳1015cm-2) and provide only indirect information on the primary sputtering mechanisms. For more direct information, transmission electron microscopy (TEM) was applied to study the interaction of individual displacement cascades with surfaces in thin Au films. First results from low dose (1010 - 1012cm-2) Bismuth ion irradiation revealed the existence of small craters where cascades intersect a surface. In the present paper it will be shown that this surface crater formation strongly depends on cascade energy and cascade energy density. Also, details of the observed crater contrast will be discussed in view of its implications in regard to crater formation mechanisms.Thin Au films, prepared by vacuum evaporation, were bombarded with Bismuth ions.