Molecular dynamics study on mechanical cleavage mechanisms of GaAs and experimental verification

Laser bars are fast becoming a key device in a range of specific industrial applications. Mechanical cleavage technology is a new and efficient method for fabricating laser bars. However, there is little detailed analysis of the relationship between scratching step and cleavage step. To close this research gap, a molecular dynamics study on cleavage mechanisms of GaAs are reported investigating surface and subsurface damage, and molecular dynamics method could accurately describe the nano-scale processes in semiconductors at atomic scale. Simulation results show that the scratching depth has a significant effect on the scratch quality as compared with other parameters during the scratching process. Then, a series of cleavage experiments were carried out to verify the simulation results and further explore the influence of key scratching parameters on cleavage plane morphology. Experimental results correlate well with the simulations. Consequently, the achieved optimal combination of parameters have been found to be a scratching load of 10 g, scratching speed of 20 mm/s and scratching length of 0.6 mm, which provides direct guidance for the cleavage processing of GaAs materials. Under optimal conditions, the length and surface roughness of the undamaged area can reach 11.77 mm and 0.43 nm respectively.     

Research on material removal mechanism and radial cracks during scribing single crystal gallium nitride

A diamond conical indenter with the cone angle of 60° and a tip arc radius of 5 μm was used to perform the gradual loading scribing experiments on gallium nitride (0001) crystal plane along the (Li et al., 2019; Wang et al., 2018; Yao et al., 2020; Fang and Zhang, 2013; Feng et al., 2009; Zhang et al., 2007; Jing et al., 2007; Yonenaga et al., 2018; Nowak et al., 1999; Shimada et al., 1998) [11-20] crystal orientation, and the material removal mechanism and machining properties of gallium nitride are studied. Single crystal gallium nitride will have the surface and sub-surface damage during machining, and the study of the stress field during scribing can effectively analyze and control the surface and sub-surface damage. Therefore, the stress field for scribing gallium nitride is acquired by superposing the boussinesq field, cerruti field and sliding blister field which have been led-in anisotropy parameters. Then on the basis of the distribution of the maximum tensile stress on the scribing surface, the nucleation and initial spread angle of radial cracks under our experimental conditions are analyzed, and for gallium nitride, the more severe the brittle spalling accompanying radial crack spread is, the greater the initial spread angle of radial cracks is.     

Synthesis and photoluminescent property of AlN nanobelt array

An AlN nanobelt array has been synthesized on Si substrate by an oxide-assisted vapor transport and condensation method at 900 °C. The nanobelts are 1–3 μm in length, 20–150 nm in width, and the ratio of width to thickness is in the range of 2–5. The nanobelts are single-crystalline hexagonal wurtzite AlN with [001] growth direction. The growth mechanism and photoluminescent property of the AlN nanobelt array were discussed.     

Research of laser remelting on the thermal-mechanical behaviors and heat treatment of yttria-stabilized zirconia coatings

In this study, the thermal and mechanical behaviors were investigated by simulating laser remelting of atmospheric plasma-sprayed yttria-stabilized zirconia coatings, and the molten depth and regions of stress concentration were compared between simulation and experiment. The heat  treatment process of the remelted coating was also simulated. The crack formation mechanism in the YSZ coating remelted by laser and the heat-treatment effect on residual stress were investigated. Results showed that the simulated results were consistent with the experimental measurements, and the residual thermal stress was the main cause of cracks formation. The coating remelted by a laser power of 1500 W and a scanning rate of 9 mm/s possessed less residual concentrated stress and segmented cracks. Heat treatment released concentrated stress, which was still accurate for the ceramic coating. If the coatings were slowly heated to demonstrate heat treatment after laser remelting, the cracks in the remelted layer decreased correspondingly.     

Effect of crack opening on the local diffusion of chloride in cracked mortar samples

The paper presents an experimental study on chloride penetration in cracked mortar specimens. A mechanical expansive core was used to generate cracks of constant width across the thickness of the sample. Sixteen specimens with crack openings ranging from 6 to 325 µm were subjected to a test designed to allow chloride diffusion along the crack path for a period of 14 days. Chloride penetration tests were carried out on mortars at 28 days and 2 years. Relationships between crack opening and chloride–ion diffusion along a crack are presented and discussed. The results show that crack opening significantly affects chloride–ion diffusion along a crack. Overall, chloride diffusion along a crack decreases with crack opening. On the other hand, no chloride diffusion occurs in cracks with an opening of 30 µm or less. This crack-opening threshold agrees with the critical crack opening obtained from a stress-displacement curve of a mortar sample subjected to uniaxial tension. At crack openings greater than the threshold value, chloride diffusion along the crack path depends on mortar age. This result suggests that self-healing could reduce chloride diffusion in cracks.     

Experimental study on the mechanism of strain rate on grinding damage of zirconia ceramics

To study the mechanism of strain rate on the grinding damage of zirconia ceramics (ZrO₂), a ZrO₂ surface grinding experiment was conducted to analyse the effect of strain rate on the grinding force, surface morphology and subsurface cracks. As the strain rate increased, the grinding force decreased significantly; the grinding scratches gradually became continuous and clear, and the grinding surface gradually became smooth. Increasing the strain rate can greatly reduce the length of subsurface cracks; however, the length of subsurface cracks drops slowly and tends to stabilise when the strain rate reaches a certain value. The direction of subsurface cracks is related to the internal defects of the material, and there is a trend of approaching the surrounding cracks and defects. When the internal defects were stimulated by the impact stress wave, the cracks started to grow and expand from the tip, and the expanding direction remained at approximately 125° in the direction of the defect length. The results show that an increase in the grinding strain rate can effectively improve the grinding quality of zirconia ceramics, which has guiding significance for practical production.     

Experimental study of adhesively bonded CFRP joints subjected to tensile loads

The tensile performance of adhesively bonded CFRP joints has been investigated experimentally. In this study, overlap length, adherend thickness, adherend width and scarf angle were chosen as design parameters. All load–displacement curves are linear, except that the thicker single-lap joints behave slightly nonlinearity due to the bending effect caused by eccentric loading. The lap shear strength is not directly proportional to overlap length, adherend thickness, adherend width and scarf angle for the brittle adhesive studied in the paper. The major failure mode includes adhesive shear failure and adherend delamination failure, sometimes accompanying with some fiber pull-out. Finally, the lap shear strength of three different lap types with similar bonding area (W=25 mm, L=10 mm, θ=5.71°) and adherend thickness (0.96 mm) was analyzed. It is found that the double-lap joint has the highest ultimate failure load. However, when considering the lap region weight, the scarf-lap joint is the most efficient.     

Thermal barrier coatings with interface modified by 3D mesh patterns: Failure analysis and design optimization

Surface patterning of the bond coat using a three‐dimensional mesh offers a promising approach to improve the durability of the thermal barrier coatings (TBCs), in which the geometry parameters of the mesh play a vital role. The objective of this work is to investigate the failure behavior of the air‐plasma sprayed TBCs with mesh, and to identify the optimal mesh design. The study revealed that the failure sequence of the TBCs with mesh patterns consisted of (I) initiation of the interfacial and ridge cracks (around the top of the mesh); (II) cracks propagation and buckling of the YSZ layer; (III) interfacial cracks deflection and coalescence with ridge cracks, leading to final spallation. The critical parameters governing each step of the failure sequence were discussed and proposed. For a typical TBCs with YSZ thickness about 200 μm, the critical mesh height h and spacing length L is about 110 μm and 7 mm, respectively, when the mesh width w is fixed at about 480 μm.     

The microstructure of highly-oriented graphite tape prepared from mesophase pitch by melt-blowing

A highly-oriented continuous carbon/graphite tape was prepared from a coal tar-based mesophase pitch by melt-blowing using a slit type nozzle. The average thickness and width of tape were ca. 20 μm and 7.2 mm, respectively. The molten precursor pitch was continuously extruded through the slit nozzle to produce a pitch tape along the drawing direction. The tape was oxidized, carbonized and graphitized. The micro area X-ray diffraction revealed that the carbon layer planes were oriented parallel to the tape surface indicating a strong anisotropy. The orientation was greater in the tape drawing direction than in the tape width direction, and also in the center area than in the edge area. The mechanical properties were higher than those of conducting polymers, but lower than those of carbon fiber with lower elastic modulus grade.     

Effect of plasma sprayed and laser re-melted Al2O3 coatings on hardness and wear properties of stainless steel

Commercially available austenic stainless steel substrate was coated with commercially available, raw Al₂O₃ powder applied by means of plasma spraying method and then re-melted with CO₂ laser beam of various parameters. Tribological and mechanical properties of the 120 J/mm and 160 J/mm laser re-melted coatings were compared with the tribological and mechanical properties of the “as-sprayed” coating. The influence of the laser beam of various parameters on the microstructure, phase constituents, and mechanical and tribological properties of the ceramic coating was investigated by means of scanning electron microscopy, light microscopy, computer tomography, X-ray diffraction technique and nanoindentation tests. The micro sliding wear performance of the coatings was tested using a nanoindenter. The study showed an improvement of the mechanical and tribological properties caused by the laser treatment. The best results were achieved for coating re-melted with 120 J/mm laser beam.     

Interaction between corrosion crack width and steel loss in RC beams corroded under load

This paper presents results and discussions on an experimental study conducted to relate the rate of widening of corrosion cracks with the pattern of corrosion cracks as well as the level of steel corrosion for RC beams (153 × 254 × 3000 mm) that were corroded whilst subjected to varying levels of sustained loads. Steel corrosion was limited to the tensile reinforcement and to a length of 700 mm at the centre of the beams. The rate of widening of corrosion cracks as well as strains on uncracked faces of RC beams was constantly monitored during the corrosion process, along the corrosion region and along other potential cracking faces of beams using a demec gauge. The distribution of the gravimetric mass loss of steel along the corrosion region was measured at the end of the corrosion process. The results obtained showed that: the rate of widening of each corrosion crack is dependent on the overall pattern of the cracks whilst the rate of corrosion is independent of the pattern of corrosion cracks. A mass loss of steel of 1% was found to induce a corrosion crack width of about 0.04 mm.     

Process parameters analysis of direct laser sintering and post treatment of porcelain components using Taguchi's method

The effects of laser sintering parameters (laser power, scan speed and hatching space) and post sintering process (heating rate, sintering temperature and holding time) on the physical and mechanical properties of porcelain components have been investigated. The study has been carried out using the Taguchi's method for the experimental design. In the laser sintering process, lower laser energy density and higher hatching space will increase the final mechanical properties of the porcelain components. A stress relief principle has been put forward to explain the different influence of the factors. The appropriate laser sintering parameters are attained in this paper: laser power 50 W; scan speed 85 mm/s; and hatching space 0.6 mm. Sintering temperature has been determined to be the most important factor in the post sintering process. Appropriate sintering temperature for the laser sintered porcelain bodies is in the range of 1425–1475 °C regarding the mechanical properties of the porcelain components. The maximum bending strength, 34.0 ± 4.9 MPa, is reached.     

Defect detection in refractories using the radiation method

Conclusions Radiation methods of control are useful for detecting hidden volume defects in refractories (large pores and cavities) with minimum dimensions of 2.5–3% of the size of the article in the direction of radiation. Overpressing and thermal cracks are detected with openings of 0.15 mm or more, and a depth of spread of not less than 10–15 mm. Such sensitivity is close to actual demands ordinarily placed on refractories testing.The main parameters of radiation methods for checking refractories are the supply voltage to the x-ray tube, the distance from the source of radiation to the test object, the length of the control section, etc., and these may be selected in accordance with the existing recommendations for radiographie control procedures [5, 7].Translated from Ogneupory, No. 8, pp. 6–10, August, 1987.     

Centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates using a femtosecond laser with square-shaped Flat-Top focus spots

Femtosecond (fs) lasers have been proved to be reliable tools for high-precision and high-quality micromachining of ceramic materials. Nevertheless, fs laser processing using a single-mode beam with a Gaussian intensity distribution is difficult to obtain large-area flat and uniform processed surfaces. In this study, we utilize a customized diffractive optical element (DOE) to redistribute the laser pulse energy from Gaussian to square-shaped Flat-Top profile to realize centimeter-scale low-damage micromachining on single-crystal 4H–SiC substrates. We systematically investigated the effects of processing parameters on the changes in surface morphology and composition, and an optimal processing strategy was provided. Mechanisms of the formation of surface nanoparticles and the removal of surface micro-burrs were discussed. We also examined the distribution of subsurface defects caused by fs laser processing by removing a thin surface layer with a certain depth through chemical mechanical polishing (CMP). Our results show that laser-induced periodic surface structures (LIPSSs) covered by fine SiO₂ nanoparticles form on the fs laser-processed areas. Under optimal parameters, the redeposition of SiO₂ nanoparticles can be minimized, and the surface roughness Sa of processed areas reaches 120 ± 8 nm after the removal of a 10 μm thick surface layer. After the laser processing, micro-burrs on original surfaces are effectively removed, and thus the average profile roughness Rz of 2 mm long surface profiles decreases from 920 ± 120 nm to 286 ± 90 nm. No visible micro-pits can be found after removing ~1 μm thick surface layer from the laser-processed substrates.     

Thermal stability of anisotropic bonded magnets prepared by additive manufacturing

In this research, anisotropic NdFeB + SmFeN hybrid and NdFeB bonded magnets are additively printed in a polyphenylene sulfide (PPS) polymer binder. Printed NdFeB + SmFeN PPS bonded magnets displayed excellent magnetic properties (B_r [remanence] = 6.9 kG [0.69 T], H_cj [coercivity] = 8.3 kOe [660 kA/m], and BH_max [energy product] = 9.9 MGOe [79 kJ/m³]) with superior corrosion resistance and thermal stability. The anisotropic NdFeB bonded magnet shows a high coercivity of 14.6 kOe (1162 kA/m) with a BH_max of 8.7 MGOe (69 kJ/m³). The coercivity and remanence temperature coefficients for NdFeB + SmFeN hybrid bonded magnets are −0.10%/K and −0.46%/K, and for NdFeB bonded magnets are −0.14%/K and −0.53%/K in the range of 300–400 K, indicating that the hybrid bonded magnets are thermally stable. The average flux aging loss for hybrid magnets was also determined to be very stable over 2000 h at 448 K (175°C) in air with 2.04% compared to that of NdFeB magnets with 3.62%.     

Frequency-doubled short nanosecond fiber lasers for glass drilling and grinding

We developed a fiber laser based 515 nm single-mode green laser with pulse width of 5 ns, peak power of greater than 20 kW, and repetition rates from 100 kHz to 500 kHz, which is suitable for glass drilling and grinding. The lithium triborate (LBO) crystal is used for second harmonic generation of 515 nm laser with efficiency of 68%. Glass drilling speed of 50 mm/s @ 0.5 mm thick glass was achieved for industrial environment continuous operation, which is equivalent to 1 s of time to drill a 4 mm diameter hole on a 2 mm thick glass. The typical chip sizes for the rear and front sides are 80 μm and 120 μm, respectively. Seven thousand holes are drilled in 24 hours in glass product line, and the success rate is larger than 99.7%. This laser glass drilling approach is rapidly accepted by glass industrial. Glass surface grinding was demonstrated by using 1 ns pulse width 515 nm lasers.     

Study on the subsurface damage mechanism of optical quartz glass during single grain scratching

The single grain scratching SPH simulation model was established to study the subsurface damage of optical quartz glass. Based on the analysis of the stress, strain and scratching force during scratching, the generation and propagation of subsurface cracks were studied by combining with the scratch elastic stress field model. The simulation results show that the cracks generate firstly at the elastic-plastic deformation boundary in front of the grain (φ = 28°) due to the influence of the maximum principal tensile stress. During the scratching process, the median crack closes to form the subsurface damage by extending downward, the lateral crack promotes the brittle removal of the material by extending upward to the free surface, and microcracks remain in the elastic-plastic boundary at the bottom of the scratch after scratching. The depth of subsurface crack and plastic deformation increases with rising scratching depth. The increase of scratching speed leads to the greater dynamic fracture toughness, accompanied by a significant decrease of the maximum depth of subsurface crack and the number of subsurface cracks. The subsurface residual stress is concentrated at the bottom of the scratch, and the residual stress on both sides of the scratch surface would generate and propogate the Hertz crack. When the scratching depth is less than 1.5 μm or the scratching speed is greater than 75 m/s, the residual stress value and the depth of residual stress are relatively small. Finally, the scratching experiment was carried out. The simulation analysis is verified to be correct, as the generation and propagation of the cracks in the scratching experiment are consistent with the simulation analysis and the experimental scratching force indicates the same variation tendency with the simulation scratching force. The research results in this paper could help to restrain the subsurface damage in grinding process.     

The effects of (di-,tri-valent)-cation partitioning and intercalant anion-type on the solubility of hydrotalcites

Synthetic hydrotalcites were produced by a co-precipitation method. The hydrotalcites are represented by the general formula [M^II_(1-x)M^III_(x)(OH)₂][Aⁿ⁻]_x/n·zH₂O, where M^II is a divalent cation (eg, Mg²⁺or Ca²⁺), M^III is a trivalent cation (eg, Al³⁺) and Aⁿ⁻ is the interlayer anion. Herein, M^II = Mg, and M^III = Al such that [Mg/Al] = [2, 3] (atomic units) and Aⁿ⁻, represents intercalant species including: OH⁻, SO₄²⁻ and CO₃²⁻ anions. The thermochemical data of each compound including their solubility constants (K_so), density and molar volume were quantified at T = 25 ± 0.5°C, and P = 1 bar. The solubilities of the synthetic hydrotalcites, irrespective of their divalent-trivalent cation partitioning ratio, scaled as CO₃²⁻ < SO₄²⁻ < OH⁻; in order of decreasing solubility. The type of anion, very slightly, affected the solubility with less than ±1 log unit of variation for [Mg/Al] = 2, and ±2 log units of variation for [Mg/Al] = 3. The solubilities of these phases were strongly correlated with that of gibbsite (Al(OH)₃); such that activity of the [AlO₂⁻] species was solubility determining with increasing pH. The tabulated thermodynamic data were used to construct solid-solution models for phases encompassing both cation distribution ratios and to calculate stable phase equilibria relevant to alkali-activated slag (AAS) systems for diverse activator compositions.     

Effect of the shape of hydrogel samples on their stretch-fracture properties

Hydrogels have broad application prospects, but the measurement of their mechanical properties often lacks stability. This study investigates the mechanical properties of hydrogels, with a specific focus on the influence of sample geometry on the tensile-fracture testing results. In the process of stretching the hydrogel along its length, increasing the width and thickness will result in uneven stress distribution. When the width of PAM hydrogel is three times that of initial sample (5 mm of width), the elastic modulus, maximum stress, and maximum strain of PAM hydrogel are reduced by about 16.8%, 69.2%, and 26.5%, respectively. Similarly, compared to the initial sample (1 mm of thickness), the elastic modulus of the triple thickness sample was reduced by about 6.5%, the maximum stress was reduced by 31%, and the maximum strain was reduced by 18.3%. In contrast, increasing the length of the hydrogel can improve the tensile properties of the hydrogel. Finite element calculations support these findings that the size increase in the loading direction improves the stress dispersion uniformity. These results indicate that the shape (length, width and thickness) of the hydrogel sample affects the tensile properties of the hydrogel and should be paid attention in related studies.     

Autogenous healing of sea-water exposed mortar: Quantification through a simple and rapid permeability test

Concrete has an autogenous ability to heal cracks potentially contributing to its functional water tightness and durability. Here, we quantify the crack-healing capacity of sea-water submerged mortar specimens through a simple and rapid permeability test. Defined crack width geometries were created in blast furnace slag cement specimens allowing healed specimens to be quantified against unhealed specimens. Specimens with 0.2 mm wide cracks were not permeable after 28 days submersion. Specimens with 0.4 mm cracks had decreases in permeability of 66% after 28 days submersion, and 50–53% after 56 days submersion. Precipitation of aragonite and brucite in the cracks was the main cause of crack healing. Healing potential was dependent on the initial crack width, thermodynamic considerations and the amount of ions available in the crack. To our knowledge, this is the first study to quantify the functional autogenous healing capacity of cracked sea-water exposed cementitious specimens.