Crater Depth Research Articles

Physical mechanism of pulsed laser interaction with fused silica optics during CO2 laser mitigation process

The intrinsic physical mechanism of CO2 laser rapid ablation mitigation micro-defects on fused silica optics surface are studied through experiments and simulations. Based on the multi-physics coupling mathematical model involving the stages of phase transition, melting flow and evaporative ablation, the calculated depth of Gaussian ablation crater and protrusion height at the edge formed by single laser pulse are 5.05 μm and 25 nm respectively, which agrees well with the experiment. Then, the temperature distribution, heat affected zone, solid-liquid transition process and surface morphology evolution under the action of moving pulsed laser are analyzed. Through studies of the relationship between laser parameters and processed surface quality, it can be concluded that the periodic structural features at the bottom of surface morphology are formed by the superposition of residual height of linear laser processing. Laser with short pulse width is more likely to cause material redeposition on the surface of fused silica optics.

Model tests of the influence of ground water level on dynamic compaction

The effect of groundwater level on dynamic compaction is significant but remains poorly understood. Model tests of dynamic compaction on sand with different groundwater levels were conducted to investigate the effect of groundwater depth in dynamic compaction. The crater depth, the dynamic stresses, and the pore water pressures induced by dynamic compaction were recorded and analyzed. The soil movements caused by the impacts were monitored using a high-speed camera, and the corresponding shear strain fields were generated employing the digital photography technology. For cases where the groundwater level is below the surface, the crater depth, the dynamic stresses, the normalized peak porewater pressures, and the shear strain response all increase as the water level becomes lower. Dewatering, as expected, is beneficial for ground improvement. Besides, the shear strain distributions indicate that the unsaturated sand above the water table experiences compression-induced compaction, previously observed in dynamic compaction on dry sand. Whether a compaction zone and the associated shear bands can be formed or not depends on the thickness of unsaturated soils above groundwater. By comparison, the saturated sand immersed in water displays the liquefaction-induced compaction. The densification of saturated sand in dynamic compaction is less efficient than that in dry or unsaturated sand in terms of energy utilization.

Depths of Copernican Craters on Lunar Maria and Highlands

We present a study on the relationship between the ratio of the depth of a crater to its diameter and the diameter for lunar craters both on the maria and on the highlands. We consider craters younger than 1.1 billion years in age, i.e. of Copernican period. The aim of this work is to improve our understanding of such relationships based on our new estimates of the craters's depth and diameter. Previous studies considered similar relationships for much older craters (up to 3.2 billion years). We calculated the depths of craters with diameters from 10 to 100 km based on the altitude profiles derived from data obtained by the Lunar Orbiter Laser Altimeter (LOLA) onboard the Lunar Reconnaissance Orbiter (LRO). The ratio h/D of the depth h of a crater to its diameter D can diverge by up to a factor of two for craters with almost the same diameters. The linear and power approximations (regressions) of the dependence of h/D on D were made for simple and complex Copernican craters selected from the data from Mazrouei et al. (2019) and Losiak et al. (2015). For the separation of highland craters into two groups based only on their dependences of h/D on D, at D<18 km these are mostly simple craters, although some complex craters can have diameters D>16 km. Depths of mare craters with D<14 km are greater than 0.15D. Following Pike's (1981) classification, we group mare craters of D<15 km as simple craters. Mare craters with 15<D<18 km fit both approximation curves for simple and complex craters. Depths of mare craters with D>18 km are in a better agreement with the approximation curve of h/D vs. D for complex craters than for simple craters. At the same diameter, mare craters are deeper than highland craters at a diameter smaller than 30-40 km. For greater diameters, highland craters are deeper.

Numerical modeling of fracture in ceramic micro-particles and insights on ceramic retention during composite cold spray process

The retention and fragmentation of the polycrystalline ceramic particles have a significant influence on the properties of the cold sprayed coatings. In this computational study, polycrystalline ceramic models were developed which demonstrated that ceramic fragmentation and retention were influenced by particle grain sizes and their impact velocities. The underlying mechanisms of crack initiation and propagation were outlined. Larger grain sizes showed higher crater depths and lower rebounding velocities, while smaller grain sizes exhibited significantly lower fragmentation. Based on the results, an optimum grain size range was proposed for lower fragmentation and higher retention. Moreover, using multiple particles and first-layer crater models, the beneficial role of surrounding metal particles on reducing fragmentation has been clarified and compared to the experimental observations. The present work provides theoretical insights on the damage mechanism and retention of ceramics during metal-ceramic composite cold spraying.

Multi-shot damage on Mo/Si multilayer induced by nanosecond EUV radiation

As a vital optical element working in the extreme ultraviolet (EUV) region, the damage mechanism on a Mo/Si multilayer has caught great attention. In this paper, using a nanosecond table-top EUV source, an S-on-1 EUV damage experiment was performed on a Mo/Si multilayer mirror with shot numbers S = 1, 2, 5, and 10. It was demonstrated that the multi-shot damage thresholds follow an inverse-power-law as a function of the pulse number. The incubation effect is explained based on the measured damage morphologies by an atomic force microscope. When the Mo/Si multilayer mirror was exposed to the EUV fluence above the single damage threshold, compaction of the layer structure and melting and resolidification processes are dominating the damage mechanism. It was also found that the crater depth and FWHM at high fluence are independent of the shot numbers, which is explained by the formation of the MoSi2 spacing layer.

Effects of workpiece polarity and flushing pressure on the arc plasma and the crater of single-pulsed arc discharges

Electro-arc machining (EAM) is a novel type of electrical discharge machining (EDM) that adopts high-energy arc discharges to erode workpiece materials. The workpiece polarity and the dielectric medium flushing have complex effects on the machining process and the EAM results. To fully understand these effects is beneficial to reveal the mechanism of EAM. Two single-pulsed arc discharge experiments with factors, namely, workpiece polarity and flushing pressure, were conducted. As the experimental results, the craters were measured using the microscopy images and surface profiles. The experiments found clear evidence of the polarity effect. The arc root diameter of positive workpiece polarity was smaller than that of negative workpiece polarity, while the crater diameter of positive workpiece polarity was bigger than that of negative workpiece polarity. Additionally, the effects of flushing are obvious. The arc plasma was moved along the flushing direction, which turned the common round crater into a crater with a tail. The crater depth was unaffected by workpiece polarity in the experiments and grew bigger when flushing was adopted, which means that flushing was beneficial to the material removal in EAM. It was deduced that the material removal appeared in the head of the crater mostly while the tail was composed of neglectable pits. The bulge height was increased slowly with an increase in flushing pressure, which was against good gap state and high material removal. Based on the results of the experiments, it is suggested that the EAM processes should adopt both arc-breaking methods to get better machining results.

Effect of arc extinction on formation of interlayer faulty fusions during consumable electrode arc welding of pipelines

Virtual study is performed using deterministic mathematical model of the welding process to assess the effect of arc interruptions on the formation of the weld pool and seam. It is found that interruptions in arc burning lead to the appearance of craters, the depth of which can reach the thickness of the filling layer. It is shown that the effect of the duration of the interruptions in arc burning on the crater depth depends weakly on the welding speed, but noticeably depends on the spatial position of the weld pool. It is necessary to take into account the criteria for the formation of defects from disturbances in the thermal circuit of the arc when analyzing of the welding monitoring data.

Fatigue behaviour of Plasma Sprayed Titania and Chromia Coatings

Cr2O3 and TiO2 powders were deposited by atmospheric plasma spray (APS) on steel substrates. Microstructural analysis of the coatings showed typical lamellar structure with good coating quality. Fatigue strength was studied by using cyclic testing (measuring with an inhouse-built apparatus the strength of the coated systems under a wide range of impact cycles) and static loading tests (Vickers tests standards with 600N and 1500N) measuring the adhesion properties of the coatings. In low cycles (1x103) Titania coatings exhibited better strength, while at intermediate (4.5x105) and high (1x106) number of impact cycles, both Chromia and Titania coatings exhibited quite similar strength characteristics. At low impact force the thickness of the coatings plays critical role with better performance obtained by Chromia coatings. During static loading both coatings exhibited similar characteristics at the crater diameter but with larger crater depth for Titania. Chromia coatings exhibited higher strength resistance than Titania coatings with better mechanical properties and coating structure.

Prediction of crater depth, surface roughness and erosion rate during abrasive jet machining of glass

Understanding the mechanism for brittle material removal via solid particle impact is important for various areas such as energy and aerospace industries. The existing analytical formulation of erosion damage, consistent with indentation fracture theory, overestimates actual erosion by over 200%. This study complements an existing analysis by prescribing a realistic shape and mechanical properties to the impinging particle. We quantitatively show that a lateral crack in glass does not nucleate at the indenter's tip or the theoretical plastic depth, but at an intermediate depth. This depth can be found by implementing the actual particle penetration depth into Hill's ratio. Several new geometries of the particle's impacting tip are proposed as ways to achieve adequate particle penetration depth instead of an expanding hemispherical cavity. The crater depth, surface roughness, and erosion efficiency of borosilicate glass are predicted with 10%, 24%, and 23% error, respectively, using a simple analytical routine.

Study on the Influence of Sand Erosion Process on the Wear and Damage of Heat-Treated U75V Rail Steel

Abstract Usually, rail materials are exactly affected by the erosion of windblown sand in the desert environment. For this reason, the influence of impact angle, particle velocity, and particle size on the erosion wear behavior of the U75V heat-treated rail steel, a material frequently employed in Chinese railways, were studied in this work. The results showed that, with increasing impact angle, the erosion rate increased between 15 deg and 45 deg, decreased between 45 deg and 75 deg, and then increased again between 75 deg and 90 deg. The highest erosion rate occurred at about 45 deg. When the particle velocity increased, the erosion rate increased approximately in a quadratic way. As the sand particle size increased, the erosion rate presented a decreasing trend. During the initial stage of erosion, shear craters, indentation craters, and ploughing craters were the main surface damage features. The shear craters predominated at the impact angle of 45 deg whereas the indentation craters predominated at 90 deg. During the steady-state of erosion, the rail damage was mainly composed of craters, platelets, and cracks. Both the length and depth of craters increased almost linearly with increasing particle velocity, whereas the increased rate of length was significantly higher than that of depth. The length and depth of craters increased with increasing particle size at 90 deg, whereas only the length increased with increasing particle size at 45 deg. The microstructure evolution and the formation mechanism of platelet at low impact angles were different from those at high impact angles. Platelet formation was the main erosion wear mechanism.

Characterization of microcraters fabricated on the silicon surface by single and multi-pulse laser ablation at various laser intensities

Microcraters on the silicon surface were fabricated with the help of a nanosecond pulsed laser. The features of microcraters fabricated with single laser pulse and pulse trains of various laser intensities were analyzed. In case of single pulse, the diameter and depth of micro-crater rise with the laser fluence and achieve saturation at higher laser intensities. For the range of fluence 62–122 J/cm2, the mass of ablated material was in the range of 0.27 × 10−9–2.69 × 10−9 g/pulse. Numerically simulated temperature trends were compared with the measured crater depths. The influence of laser intensity on the target temperature and onset time to melt/boil were also investigated. In case of pulse trains, the reduction in material removal efficiency is attributed to the plume shielding and accumulation of ablated material on the crater walls. We demonstrated that the laser intensity and number of laser pulses can be used to control the morphology of a microcrater.

Experimental study concerning the oblique impact of low- and high-density projectiles on sedimentary rocks

We conducted four sets of impact experiments using sedimentary rock targets and three different kinds of projectiles at a variety of impact angles in order to examine how the density of a projectile affects the dimensions of a crater as the angle of impact decreases, the threshold angle for the formation of elliptical craters, and the threshold angle for the formation of pits. The crater profiles, crater volume, equivalent diameter, length, width, depth, and ellipticity of each set were carefully measured to be used in comparison with small craters that formed on the weak rocky surfaces of planetary bodies. The results indicate that the crater volume, equivalent diameter, width, and depth decrease with the impact angle, while the length of the crater within a set does not decrease monotonically with impact angle. This trend in crater length is consistent with the results of previous studies. Although craters formed at higher impact angles have a central pit, the pit becomes unclear and eventually disappears as the impact angle decreases. A larger threshold angle is required for the formation of pits at slower impact velocity than at higher impact velocity. Our results suggest that the presence of a central pit is indicative of impacts at higher angles and/or higher velocity. The ratio of the volume of craters resulting from oblique impacts to that of craters formed by normal impacts was proportional to the power of the sine of the impact angle. The power index was found to range between 1.46 and 2.20, with an average of 1.57. Comparison of the averaged power index to the power index of the π-group crater scaling rules, it is experimentally suggested that the hypothesis indicating that the vertical velocity component controls crater formation is plausible on a brittle target. The threshold angles for the formation of elliptical craters for three different kind of projectiles were almost consistent with those obtained in previous studies. Our results strongly suggested that the threshold angle for the formation of elliptical craters for high-density impactor, such as iron meteorites, are higher than for rocky impactors. We then obtained a relationship between the threshold angle for the formation of pits and the cratering efficiency. It is revealed that the threshold angle for the formation of pits is greater than the threshold angle for the formation of elliptical craters, when the cratering efficiency is in the range 7–30. A well-developed pit-spall structure in the crater may be used to indicate both, the impact angle and the vertical component of the impact velocity.

Energy deposition parameters revealed in the transition from 3D to 1D femtosecond laser ablation of fluorite at high-NA focusing

Ultrashort-pulse laser surface and bulk nano- and micromachining of dielectrics have multiple promising applications in micro-optics, microfluidics, and memory storage. The fundamental principles relate intrinsic inter-band multi-photon (MPA) and laser-induced intra-band free-carrier absorption (FCA) to particular ablation mechanisms and features. These principles are yet to be quantified into a complete set of basic experimental laser-matter interaction parameters, describing photoexcitation, relaxation, and final ablation. In this study, we considered the characteristic double-crater structure of single-shot ablation spots on dielectric surfaces and single-shot transmission spectra to extract crucial information about the underlying basic processes of ultrafast photoexcitation and laser energy deposition. Specifically, energy-dependent crater profiles and accompanying prompt self-phase modulation (SPM) spectral broadening were studied in single-shot surface ablation experiments on fluorite (CaF2) surface photo-excited by tightly focused 515- or 1030-nm, 300-fs laser pulses. Crater size dependence demonstrated two slopes, scaling proportionally to the squared focal 1/e-radius at higher energies (intensities) for larger ablated spots, and a much smaller squared 1/e-radius at lower energies (intensities) for (sub) micron-wide ablated spots, indicating a transition from 1D to 3D-ablation. As a result, these slopes were related to lower-intensity wavelength-dependent multi-photon inter-band transitions and wavelength-independent higher-intensity linear absorption in the emerging near-critical electron-hole plasma (EHP), respectively. Crater depth dependences on the local laser intensity fitted in the corresponding ranges by multi- and one-photon absorption provided the corresponding absorption coefficients. Spectral broadening measurements indicated even values for the red and blue shoulders of the laser pulse spectrum, representing the SPM effect in the weakly excited fluorite at the leading pulse front and providing the corresponding Kerr coefficient. In the second regime, the blue-shoulder broadening value saturated, indicating the appearance of near-critical plasma screening at the trailing pulse front, which is consistent with our calculations. These complementary experiments and related analysis provided an important set of key basic parameters, characterizing not only surface ablation, but also propagation of high-intensity ultrashort laser pulses in bulk fluorite, and enabling precise forecasting of optimal energy deposition for high-efficiency ultrashort-laser micro-structuring of this dielectric material.

Characterization of multilayer coatings of aluminium, tungsten and molybdenum on steel substrate using laser-induced breakdown spectroscopy

Laser induced breakdown spectroscopy (LIBS) has been applied for the depth profiling of multilayer metallic coating structure which is simulated to the ITER-like wall materials in fusion devices such as EAST tokamak. The coatings were produced by magnetron sputtering and characterized by LIBS using a Nd:YAG laser at 1064 nm with pulse duration of 5 ns. The light from the resulting plasma was analyzed as a function of both number of laser pulse (N) and elemental spectral line intensity. The depth profile of the multilayer coating was determined by three different spectral signal processing methods; intensity background subtraction, intensity normalization and linear Pearson correlation. The intensity normalization method provides the best outcomes/solutions for thickness measurement and analysis of interfaces of multilayer coatings among other methods. The evolution of spectral intensity representing different materials was properly examined for 90 consecutive laser shots at the same spot. The ablation behaviour of the coating materials was analyzed in terms of ablation rate under the laser pulse of various fluences. In a much deep crater, some factors originating from laser ablation, such as material re-deposition and plasma shielding effect were analyzed. As a comparison, the depth, shape and diameter of craters were also measured using SEM and a profilometer. The influence of laser power density on the ablation rate and crater profile was investigated. The elemental composition, inside the crater, was verified by the EDX technique. The results indicate that LIBS has a potential power for in-situ depth profiling of multilayer coatings on the wall of the EAST.

Numerical Modeling of Lateral Dynamic Compaction on the Slope in Dry Sand

Dynamic compaction is a low-cost soil improvement method that is most commonly applied in sandy soils. Free fall compaction is not applicable to compact the soil in the slope, but it is possible to perform lateral dynamic compaction in the slope. In this method, the angle of impact of the tamper with the inclined surface is vertical. In this dynamic compaction method, the tamper is forced to move in a circular path by a cable with specific radius. In this study, ABACUS 6.14 software was used to simulate three-dimensional model of lateral dynamic compaction in the slope. The critical state model that used in numerical simulation is cap plasticity criteria. In this study, the effects of impact position, impact velocity and tamper weight on soil improvement for three different slope were investigated. The analysis focuses on crater depth and improvement region which are compared to the state of flat ground. It was found that on steep slopes to increase the tamper energy, the effect of increasing the impact velocity was greater than the effect of increasing the tamper mass to soil improvement. The crater depth due to lateral dynamic compaction is lower than the horizontal layer as well as the depth of soil improvement.

Defining the role of ‘zero wear volume’ in percussive impact

This work defines the previously undetermined contribution of the ‘zero wear’ volume, that is geometry change due to material compression that occurs before other mechanisms that cause change through actual material loss are initiated during to repetitive impact. Five metal alloys widely used in engineering applications, each with a different bulk hardness, were the subjects of the experiments. Using a reciprocating hammer type impact wear test apparatus, flat coupon type specimens were subjected to repetitive impact from a chrome steel ball acting normal to the surface. 36,000 impacts were applied at a nominal rate of 10 impacts per second, each with an impact energy of 0.23J and an impact force of 3.5 kN. The impact wear crater on selected worn specimens was examined using a 3D non-contact profilometer. Scanning electron microscopy techniques were used to further examine the damage on the specimens. The main damage mechanism was plastic deformation and surface fatigue due to spalling. Microcracks and adhered wear debris were noted on the specimens, but with no evidence of delamination, while subsurface examination showed no possible microcracks under the impacted surface and only surface pitting could be observed from subsurface examination. Analysis of the wear scars suggests that zero wear volume is the main contributor to the total volume ‘loss’ for all materials, and, for specific materials, plastic flow volume and bulk hardness could be a significant parameter in characterising zero-wear volume and crater depth.

Bennu's near-Earth lifetime of 1.75 million years inferred from craters on its boulders.

An asteroid's history is determined in large part by its strength against collisions with other objects1,2 (impact strength). Laboratory experiments on centimetre-scale meteorites3 have been extrapolated and buttressed with numerical simulations to derive the impact strength at the asteroid scale4,5. In situ evidence of impacts on boulders on airless planetary bodies has come from Apollo lunar samples6 and images of the asteroid (25143) Itokawa7. It has not yet been possible, however, to assess directly the impact strength, and thus the absolute surface age, of the boulders that constitute the building blocks of a rubble-pile asteroid. Here we report an analysis of the size and depth of craters observed on boulders on the asteroid (101955) Bennu. We show that the impact strength of metre-sized boulders is 0.44 to 1.7 megapascals, which is low compared to that of solid terrestrial materials. We infer that Bennu's metre-sized boulders record its history of impact by millimetre- to centimetre-scale objects in near-Earth space. We conclude that this population of near-Earth impactors has a size frequency distribution similar to that of metre-scale bolides and originates from the asteroidal population. Our results indicate that Bennu has been dynamically decoupled from the main asteroid belt for 1.75±0.75 million years.

Picosecond laser ablation of metals and semiconductors with low-transverse order Gaussian beams

Picosecond pulse duration laser–material interactions are extremely complex and much less studied than the physics of femtosecond and nanosecond ablation. Additionally, multimode laser beam structure can inhibit robust analysis and comparisons of effects at any pulse duration. To address these gaps, single-pulse laser ablation of Al, Si, Ti, Ge, and InSb in air and Ge in vacuum was studied using low-transverse order Gaussian beams at a 1064 nm wavelength and 28 ps pulse duration. Crater depths of 0.4 to 6.3 μm and volumes of up to 4000 μm3 were measured using a laser confocal microscope. Crater depths plateau with increasing fluence and are slightly higher for Ge in vacuum than in air. Crater volume increases linearly with fluence for all materials in air. In vacuum, the volume of material above the surface was less than in air and increased at a lower rate with increasing fluence. The ratio of volume above the surface (due to melt flow and redeposition) to volume below the surface plateaus for all materials to ∼0.7 in air and 0.4 for Ge in vacuum. The ablation efficiency, defined as atoms removed per incident photon, was higher at low fluences and decreased to ∼0.004 for all materials at higher fluences. Simulations using the directed energy illumination visualization tool showed that bulk melt flow out of the crater caused by the evaporation recoil pressure dominated at higher fluences. Plateauing of crater depth with fluence was caused by melt reflow into the crater, which effects smaller crater widths more than larger ones, as evidenced by comparing multimode results with TEM00 simulations. Recondensation of evaporated material was identified as the main difference between craters formed in air versus vacuum, and the Knudsen layer jump conditions in DEIVI were modified to account for an estimated ≈20 % recondensation rate. The simulations showed a resulting reduction in evaporation, which created less recoil pressure, driving less melt out of the crater. The results of this study help elucidate mass removal mechanisms in the picosecond pulse duration regime and their dependence on laser and environmental characteristics.

Sub-Picosecond Micromachining of Monocrystalline Silicon for Solar Cell Manufacturing

In this study a prototype sub-picosecond laser was investigated for cutting and scribing of silicon wafers. The Yb:KYW laser used for this investigation, unlike ultrashort systems used previously, generates pulses of 650 fs, i.e., between the pico and femtosecond range. The laser was placed in a micromachining setup, involving a galvo scanner and a telecentric lens. A study of the influence of the processing parameters on the crater width, depth, and quality of machining was carried out. The optimal parameters were found to be 343 nm, 200 kHz, 7 mm/s, and 15 pattern repetitions. The experiments were performed using samples of a silicon wafer of 210-µm thickness. The experimental results show that the sub-picosecond laser can be a promising and competitive tool for solar cell micromachining. In comparison to the commercially available ultrashort pulse laser systems, we find the sub-picosecond laser to be a more cost efficient and reliable source, than a femtosecond one. In addition, the prototype Yb:KYW design offers some unique parameters, such as repetition rate in the range of 100–400 kHz, UV wavelength or obtainable laser fluence close to the silicon ablation thresholds.

Considering Impact and Corrosion Resistance in the Performance of Heavy Wear Resistant Coatings

Laser clad coatings consisting of a Ni-based matrix and tungsten carbide (WC) hard phases are used for heavy wear protection of components in many industries. Matrix composition and WC type can be chosen to give these composites resistance to erosive/abrasive wear, as well as to the impacts of particles and corrosion. Maintaining ductility in the matrix is important, so that it can absorb impact energy and minimize cracking, thus avoiding penetration of corrosive media through cracks and subsequent corrosion in the substrate-cladding interface. Claddings were tested for their impact resistance by means of a single impact with a hard, spherical indenter. The depth of impact craters was measured, and the cracking behavior was analyzed. Corrosion resistance of Ni/WC composites, a HVOF WC/CoCr coating and a hard chrome reference coating were tested by electrochemical corrosion in a 3.5 wt.% NaCl solution and by exposure for up to 1000 h in a salt spray test.