Fabrication of concave micro lens array using laser patterning and isotropic etching

Micro lens arrays are widely used in optical communication and laser-fiber coupling applications. In this paper, a technique to fabricate concave micro lens arrays on glass substrate using a third harmonic Nd:YAG laser direct patterning and followed by chemical wet isotropic etching is presented. The patterning process was done on gold film, which was coated on a glass substrate by using a NC controlled laser ablation tool paths. The glass substrate is then etched by using hydrofluoric acid (HF) solutions whereby the exposed area will be dissolved away by chemical reaction with HF. The type of etching process is an isotropic etching which the etching rate is equal at all direction thus produce hemispherical concave profile on glass. The optimum laser patterning parameters is obtained and the effect of different types of HF solutions on etching efficiency is studied. The surface morphology, 2D and 3D profiles are also measured. Various micro lens diameters are fabricated with different values of lens sag.     

Fabrication of hybrid micro/nano-textured surfaces using rotary ultrasonic machining with one-point diamond tool

A novel rotary ultrasonic texturing (RUT) technique is proposed to fabricate hybrid periodic micro/nano-textures on flat surfaces. Different from conventional rotary ultrasonic machining, a tailored one-point diamond tool was manufactured and employed for RUT on surfaces of electroless nickel–phosphorus (Ni–P) plating. A one-dimensional longitudinal-vibration mode is used. The combined effect of ultrasonic vibration, rotation and feed motion leads to high-frequency periodic change of cutting edge׳s motion, which is the basic principle for the RUT process. Therefore, to accurately predict and control the texturing process, the cutting locus is firstly mathematically calculated. Hybrid periodic micro/nano-textures comprising linear grooves at the micrometer scale and sinusoidal grooves at the micrometer or nanometer scale were successfully fabricated on machined surfaces, which are in compliance with the results of the mathematical calculations. Different types of surface textures were generated by changing machining conditions. The surface generation mechanism of RUT is illustrated and discussed by analyzing the surface textural features, the cutting locus and the tool tip׳s geometry, including various tool faces, cutting edges, and the cutting corner. The requirements for RUT technique are concluded.     

Fabrication and characterization of micro dent arrays produced by laser shock peening on titanium Ti-6Al-4V surfaces

Surface micro dents may act as lubricant reservoirs to reduce friction and wear in sliding and rolling contact applications. Surface patterning has become a valuable technique for fabricating micro dents. Alternative methods such as micromachining present obvious limitations in comparison with laser shock peening (LSP). In this paper, the use of micro LSP along with an automatic XY table proves to be an attractive and reliable method for producing micro dent arrays with enhanced surface integrity. Surface topography, residual stress, and microhardness of the fabricated micro dent arrays on polished Ti-6Al-4V surfaces have been characterized. It has been shown that LSP is capable of efficiently fabricating mass micro dent arrays with controllable size via adjusting laser power. The center area of the peened dents has the highest hardness. In addition, high compressive residual stress can also be produced.     

Fabrication of highly transparent and superhydrophobic silica-based surface by TEOS/PPG hybrid with adjustment of the pH value

Preparation of superhydrophobic silica-based thin film with high transmittance (T%) in the visible light region by adjusting different pH value of mixing solution has been developed. The hybrid films were coated by the mixing solution which included precursor solution (sol–gel process) and polypropylene glycol (PPG) polymer solution. Rough surfaces were obtained by removing the organic polymer at high temperature and then the hydrophobic groups bonded onto the films were obtained by the reaction with hexamethyldisilazane (HMDS). Characteristic properties of the as-prepared surface of the films were analyzed by contact angle measurement, scanning electron microscopy (SEM), atom force microscopy (AFM), UV–VIS scanning spectrophotometer and Fourier transform infrared (FT-IR) spectrophotometer. The experimental parameters were mainly varied by the pH value (0, 1, 2.3, 6 and 8) of the mixing solution. The result showed that the contact angle of the film was greater than 150° and the transmittance of the film was greater than 90% simultaneously when the pH value of mixing solution was adjusted to 1. In addition, the highly transparent superhydrophobic surfaces was obtained by adding 20 μL acid solution into the mixing solution, which the contact angle of the film was 156.3° as well as the transmittance of the film at the 600 nm visible light was 97.9%.     

Fabrication of 3D photonic structure on glass materials by femtosecond laser modification with HF etching process

In this study, a novel method of an array of square columns with high aspect ratio (about 10) on quartz and a pattern of complex multi-layered woodpiles on slide glass are firstly fabricated by femtosecond laser inner modification with hydrofluoric acid (HF) etching. The three-dimension (3D) patterns composted with embedded gratings are scribed by femtosecond laser focused inside the bulk materials with the central wavelength of 517 nm, the repetition rate of 100 kHz, the pulse width of 350 fs and the power of 100 mW. For the quartz glass, a high aspect ratio (about 10) structure of squared column array with the width of 10 μm and the height of 100 μm is formed from a grid pattern etched by 15 wt% HF for 15 min and 5 wt% HF for 90 min. For the slide glass, a 3D piled-octahedron structure with the feature size of 10 μm is developed from a multi-layer woodpile pattern etched by 5 wt% HF for 2 h. Moreover, the optical band structures of both 3D structures are calculated by the plane wave expansion method. And then, the defect effect of light propagation with sample paths on square and piled-octahedron structures are designed to verify the optical characteristics by finite difference time domain (FDTD) simulation in this study.     

Structure and hardness of titanium surfaces carburized by pulsed laser melting with graphite addition

Titanium foils coated with graphite films 20 μm thick were irradiated by means of a pulsed Nd-YAG in order to harden this metal by surface melting and alloying. The relationships between irradiation parameters, microstructure and hardness of the synthesized composite coatings were determined. Four relevant parameters were defined as governing the irradiation processes. The parametric working field of the laser source was investigated next to its periphery. The influences of the relevant irradiation parameters on the microstructure and hardness of the melted zone were deduced from the metallographic analyses and Vickers micro-indentation tests of the cross sections of this composite zone. Such a zone was constituted always with hard titanium carbide and ductile metallic titanium, some times with the presence of lubricating graphite inclusions. The main advantage of such a surface treatment of titanium is to synthesize, under clearly defined irradiation conditions, a self-lubricating composite coating that resists abrasive or adhesive wear.     

Fabrication of A356 composite reinforced with micro and nano Al2O3 particles by a developed compocasting method and study of its properties

Aluminum/alumina composites are used in automotive and aerospace industries due to their low density and good mechanical strength. In this study, compocasting was used to fabricate aluminum-matrix composite reinforced with micro and nano-alumina particles. Different weight fractions of micro (3, 5 and 7.5 wt.%) and nano (1, 2, 3 and 4 wt.%) alumina particles were injected by argon gas into the semi-solid state A356 aluminum alloy and stirred by a mechanical stirrer with different speeds of 200, 300 and 450 rpm. The microstructure of the composite samples was investigated by Optical and Scanning Electron Microscopy. Also, density and hardness variation of micro and nano composites were measured. The microstructure study results revealed that application of compocasting process led to a transformation of a dendritic to a nondendritic structure of the matrix alloy. The SEM micrographs revealed that Al₂O₃ nano particles were surrounded by silicon eutectic and inclined to move toward inter-dendritic regions. They were dispersed uniformly in the matrix when 1, 2 and 3 wt.% nano Al₂O₃ or 3 and 5 wt.% micro Al₂O₃ was added, while, further increase in Al₂O₃ (4 wt.% nano Al₂O₃ and 7.5 wt.% micro Al₂O₃) led to agglomeration. The density measurements showed that the amount of porosity in the composites increased with increasing weight fraction and speed of stirring and decreasing particle size. The hardness results indicated that the hardness of the composites increased with decreasing size and increasing weight fraction of particles.