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Thermal effect is inevitable during laser processing and is easy to induce cracks and damage on the hard and brittle materials, especially. The crack generation mechanism during laser ablation of single hole and groups of holes on alumina ceramic has been investigated. A heat conduction model of the nanosecond laser processing of a group of holes has been developed. The temperature field over a AL2O3 sample was modeled and simulated using COMSOL multi-physics. The temperature distribution on the AL2O3 sample was experimentally verified using an infrared thermometer. The present research provides guidance for the high-quality laser machining of group holes over large areas.

Control over the growth location and orientation of organic semiconductor single crystals (OSSCs) is of key importance to enable high-performance organic circuits. However, it is difficult to create well-patterned and aligned OSSCs simultaneously using previous approaches. In this work, we demonstrate a patterned microchannel dip-coating (PMDC) method for the patterning of OSSC arrays with high crystallinity and purely crystallographic orientation. The patterned microchannels provide regionally different wettability, permitting to define solution wetting locations. Moreover, the dip-coating process enables unidirectional dewetting of the solution, inducing a long-range ordered crystallization. The PMDC approach shows a good universality, and can be extended to a variety of soluble acenes, e.g. 2,8-difluoro-5,11-bis(triethylsilylethynyl)anthradithiophene (Dif-TES-ADT) and 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene). 50 discrete organic field-effect transistors (OFETs) made from the patterned Dif-TES-ADT single crystal arrays exhibit a high average mobility up to 1.5 cm2 V−1 s−1, surpassing that of the commonly used amorphous silicon transistors. This patterning approach constitutes a promising step that might ultimately allow us to use high-mobility OSSCs for integrated devices.

Aluminum nitride (AlN) is essential material to electronics industry. Compared to traditional machining methods, laser trepanning has become one of the most popular options for hole machining on AlN. Due to involvement of multiple parameters, such as scanning number, laser beam jump direction, scanning mode, and filling circle interval, the optimization of hole quality is complex. To tackle this problem, this paper systematically studied hole trepanning on AlN using nanosecond pulse laser in order to examine the relationship among trepanning parameters and hole dimensions, quality, and machining efficiency. Thereby, the optimized combination of parameters could be obtained to improve the quality and efficiency of hole machining. Moreover, possible effect mechanisms like effect of laser fluence on hole diameters, effects of jump directions and scanning modes on hole machining quality were discussed. Most importantly, it is the first time that the relationship among trepanning parameters, hole quality, and machining efficiency is presented.

The comparative study is presented of grooves ablation for the single- and multi-scan processing technology in Ni alloy using a 10 ps pulse laser with the wavelength of 532 nm and the repetition rate of 1 kHz. Results show, when the N (the effective ablation pulse number per unit area), for the single-scan processing of grooves, have been divided into multiple groups and combined with multiple times ablation, namely the multi-scan processing, the groove depth and morphology quality could be enhanced effectively. In addition, the temporary saturation of the groove depth and the bending on the deep groove bottom are reported. Especially for the bending phenomenon which hinders the further deepening of the groove, the related reasons and the solutions are discussed.

Ni-based superalloy with ceramic thermal barrier coatings (TBCs) is a composite material, which can be used in special environments with high temperature and high pressure such as aeroengine blade. In order to improve the cooling effect of the aeroengine, it is necessary to perform multi-size and large-area holes processing on the surface of blades. As a non-contact processing method with fast processing speed, good processing quality and almost no deformation, laser processing has been one of the important processing methods for film cooling hole processing of aeroengine blades. Percussion drilling is presented using picosecond ultrashort pulse laser in order to explore processing of deep holes in Ni-based superalloy, ceramic TBCs, and ceramic TBCs/substrate multilayer material. The effects of pulses, threshold and wavelength on hole diameter have been discussed, and the experiment on the deep hole ablation with 1064 nm wavelength has been performed. By analyzing the hole size and morphological characteristics of multiple processing parameters, the variation of hole cylindricity is obtained. A high-quality hole, without spatters around the periphery of hole entrance and without recast layer on the side-wall surface, in Ni-based superalloy coated with ceramic TBCs has been drilled. This research has potential applications to blade film cooling holes.

Comparing with the trepanning technology, cooling hole could be processed based on the percussion drilling with higher processing efficiency. However, it is widely believed that the ablating precision of hole is lower for percussion drilling than for trepanning, wherein, the melting spatter materials around the hole surface and the recast layer inside the hole are the two main issues for reducing the ablating precision of hole, especially for the recast layer, it can't be eliminated completely even through the trepanning technology. In this paper, the self-cleaning effect which is a particular property just for percussion ablating of holes has been presented in detail. In addition, the reasons inducing the self-cleaning effect have been discussed. At last, based on the self-cleaning effect of percussion drilling, high quality cooling hole without the melting spatter materials around the hole surface and recast layer inside the hole could be ablated in nickel-based superalloy by picosecond ultra-short pulse laser.

A theoretical study, based on the split as well as experiments, was conducted to investigate the formation of high-spatial-frequency laser-induced periodic surface structures (HSFLs) on rough indium-tin-oxide (ITO) films under 10-ps 532-nm-wavelength laser irradiation. At a peak laser fluence of 0.472 J/cm2, the theoretical periods of HSFLs (130–190 nm) matched the experimental values (128–200 nm). Both the theoretical and experimental results demonstrated that the transformation mechanism of laser-induced periodic surface structures (LIPSSs) from low-spatial-frequency LIPSSs (LSFLs) to HSFLs was attributed to split and the irregular period difference of HSFLs and LSFLs was attributed to the surface roughness. Deeper ablation occurred for LIPSSs with a larger period, and the difference at the ablated depth increased with increasing spot number. Therefore, the LIPSSs with the larger period were clearer demarcated and the initial pits in the convex portion of LIPSSs disappeared due to the laser-induced melting. Consequently, sub-100-nm-perioded HSFLs were invisible in spite of the theoretical minimum period of ~88.5 nm. Then, for pits of different depths, the difference of the ablated depth induced by a subsequent pulse can be narrowed by reducing the laser fluence. On this method, 83-nm-perioded HSFLs were obtained by reducing the peak laser fluence to 0.432 J/cm2. Keywords: Ripples, Split, Indium-tin-oxide film, Picosecond laser, Surface roughness, Drude-Lorentz model

Comparative study on micro-deep hole drilling by picosecond ultra-short pulse laser in four metals, including copper, aluminum alloy, titanium alloy, and nickel alloy, was presented. Destructive testing was performed to measure the depth, shape, and side-wall morphology of micro-deep holes in this study. Diameters and depths of micro-deep holes in four metals ablated using the same processing parameters were compared. The multiple thresholds of metals had been calculated. Relationships between hole dimensions and multiple thresholds (the average ablation threshold, the ablation threshold and the thermal threshold) and physical properties (for example, the heat capacity, the melting temperature, the optical penetration depth and the absorbance, etc.) of the four metals are examined. Furthermore, the surface and side-wall morphologies of the micro-deep holes in four metals were demonstrated. The influence mechanisms, especially the side-wall morphology, were discussed, such as the thresholds, conical emission, self-cleaning effect, physical properties of metals, the energy transmission, the plasma effect, thermochemical reaction, and so on.

Since rotor run out and bearing vibration are not considered simultaneously and are analyzed independently, it is common for one vibration amplitudes to reduce and the other to rise during the balancing process. In order to suppress the unbalance vibration of rotor and bearing at the same time, the dynamic model of an unbalanced rotor system is developed and verified to analyze its dynamic characteristics, based on this, the optimal sensor placements of the rotor bearing system are given. Then, an accurate description of the unbalanced state is presented, and the uniform balancing method is proposed based on the optimization objective function of the unbalance vectors. Finally, the experimental results reflect the effectiveness of the proposed method.

An experimental study is presented of the effect of wavelength on the shape and morphology evolution of micro holes ablated on stainless steel surface by a 10 ps Q-switched Nd:VAN pulsed laser. Two routes of hole development are associated with the visible (532 nm) and near-infrared (1064 nm) laser beams, respectively. The evolution of various geometric shapes and morphological characteristics of the micro holes ablated with the two different wavelengths is comparatively studied for other given processing conditions such as a laser power levels and the number of pulses applied. Plausible explanations, based on the light-materials interaction associated with laser micromachining, are also provided for the discernable paths of geometric and morphological development of holes under laser ablation.

In this study, a two-dimensional thermo-chemical reaction model with temperature-dependent thermo-physical parameters on Si3N4 with 10 ns laser was developed to investigate the ablated size, volume and surface morphology after single pulse. For model parameters, thermal conductivity and heat capacity of β-Si3N4 were obtained from first-principles calculations. Thermal-chemical reaction rate was fitted by collision theory, and then, reaction element length was deduced using the relationship between reaction rate and temperature distribution. Furthermore, plasma absorption related to energy loss was approximated as a function of electron concentration in Si3N4. It turned out that theoretical ablated volume and radius increased and then remained constant with increasing laser energy, and the maximum ablated depth was not in the center of the ablated zone. Moreover, the surface maximum temperature of Si3N4 was verified to be above 3000 K within pulse duration, and it was much higher than its thermal decomposition temperature of 1800 K, which indicated that Si3N4 was not ablated directly above the thermal decomposition temperature. Meanwhile, the single pulse ablation of Si3N4 was performed at different powers using a TEM00 10 ns pulse Nd:YAG laser to validate the model. The model showed a satisfactory consistence between the experimental data and numerical predictions, presenting a new modeling technology that may significantly increase the accuracy of the predicated results for laser ablation of materials undergoing thermo-chemical reactions.

Laser fusion welding technique was used for lap joining of Q235 steel to AA5052 aluminum alloy using a flux-cored Zn-22Al filler wire. The influence of laser power on the microstructure and mechanical properties of laser Al/steel joints was investigated. A fusion welded region and a brazed region were formed at the fusion zone/steel interface: Fe2Al5−xZnx and FeZn10 were formed at the both regions, while a new FeAl IMCs with much lower hardness and brittleness was observed in the welded region. The phase constitutions at the both regions were unvaried with the change of laser power, while the phase morphologies were slightly changed. The tensile-shear testing results showed that the joint fracture load first increased then decreased with the rising laser power and the maximum value reached 1225 N at laser power of 2800 W. The joint fractured at the fusion zone/steel interface when the laser power was less than 2800 W and it changed into fusion zone when the laser power reached 3000 W. The change of joint fracture load and fracture behavior was mainly attributed to the change of morphology of IMC and interfacial bonding length.

High density and high quality interconnects are necessary for the preparation of miniaturized and lightweight electronic products. Therefore, small-diameter and high-density through-holes in FPCs (Flexible Printed Circuits) are required. However, the current processing methods cannot further decrease the diameters and improve the quality of through-holes. Comparatively, ultrashort pulse laser is a good choice. In this paper, the processing technology for the microdrilling of through-holes in FPCs using a 10 ps pulse laser was systematically studied. The effects of laser parameters, including the wavelength, energy, pulses and polarization, on the drilling of through-holes were investigated. The various processing parameters were optimized and the plausible reasons were discussed. Finally, the desired small-diameter and high-density through-holes in FPCs were obtained. The experimental results showed that, through-holes with diameters of less than 10 µm and inlet interconnection pitches of 0 ~ 2 µm could be successfully drilled in FPCs using ultrashort pulse laser.

In this work, the effects of electroplated Cu (EP Cu) and Cu addition (0.7%) in Sn solder on the intermetallic compounds (IMCs) growth and void formation were clarified by comparison with solder joints comprising of high purity Cu (HP Cu) substrate and pure Sn solder. After aging processes, a new IMC, Cu3Sn, was formed at the interface, in addition to Cu6Sn5 formed in the as-soldered joints. The EP Cu and Cu addition (0.7%) both had limited effects on the total IMCs thickness. However, the effects varied on the growth behaviors of different IMCs. Comparing to the void-free interface between Sn and HP Cu, a large number of voids were observed at the Cu3Sn/Cu interface in Sn/EP Cu joints. The formation of these voids may be induced by the impurities and fine grain, which were introduced during the electroplating process. The addition of Cu suppressed the inter-diffusion of Cu and Sn at the interface. Consequently, the growth of the Cu3Sn layer and formation of voids were suppressed.

An experimental study is presented of micro-hole drilling on the surface of stainless steel using a 10-ps Q-switched Nd:VAN pulsed laser at two wavelengths, 532 and 1064 nm, with multiple powers and different number of pulses. Results show that two primary ablation mechanisms for ultrashort laser machining, i.e., vaporization and phase explosion, which correspond to gentle and strong ablations, are correlated mainly with the applied laser power. From measured data, two ablation thresholds are calculated using a piece-wise linear fitting. Moreover, surface recast layer associated with strong ablation can be eliminated by an appropriate selection of laser processing parameters. The Hirschegg model is applied to analyze the evolution of hole depth as a function of laser processing parameters for the two wavelengths. For the 532-nm wavelength laser ablation, the shapes and morphology of micro-holes evolve differently with low power from high power. In addition, the related reasons inducing multiple featured hole shapes are discussed.

High repetition rate can produce high laser ablation efficiency. However, there is the issue of shortened pulses interval in the case of high repetition rate. Moreover, both the ejecting time of the ablated materials and the cooling time of the substrate material have been decreased, due to the high repetition rate. These all make some significant influences on micro-hole drilling using ultra-short pulse laser. In this paper, the effect of the repetition rate, on the dimension and morphology of micro- shallow and deep holes, has been systematically studied in metals ablated by picosecond ultra-short pulse laser. The relationship between the repetition rate and micro-hole dimensions, such as diameter and depth, is described. Most importantly, the effect of the repetition rate on micro-hole surface and side-wall morphology is presented for the first time. The possible influencing mechanisms are discussed.

Investigations on the morphological features of holes and grooves ablated on the surface of stainless steel using the picosecond dual-wavelength laser system with different powers combinations are presented based on the scarce researches on morphology of dual-wavelength laser ablation. The experimental results show the profiles of holes ablated by the visible beam appear V-shaped while those for the near-infrared have large openings and display U-shaped, which are independent of the ablation mechanism of ultrafast laser. For the dual-wavelength beam (a combination of visible beam and near-infrared), the holes resemble sunflower-like structures and have smoother ring patterns on the bottom. In general, the holes ablated by the dual-wavelength beam appear to have much flatter bottoms, linearly sloped side-walls and spinodal structures between the bottoms of the holes and the side-walls. Furthermore, through judiciously combining the powers of the dual-wavelength beam, high-quality grooves could be obtained with a flat worm-like structure at the bottom surface and less resolidified melt ejection edges. This study provides insight into optimizing ultrafast laser micromachining in order to obtain desired morphology.

Stainless steel was ablated by a 10 ps Q-switched Nd:VAN pulsed laser (Austria) in air under different operating conditions to explore high-efficiency ultrafast laser micromachining. The relationship between the ablation characteristics and laser parameters (number of pulses, fluence, wavelength and repetition rate) was analyzed through scanning electron microscopy and laser scanning confocal microscopy. The saturation pulses are approximately 2000–2500 in our experiments at varying other parameters attribute to the multi-pulse damage threshold of material. A critical fluence is found corresponding to the deepest hole and the optimized ablation efficiency which is related to the formation of dense plasma at the surface of the material. The short wavelength has significant advantages for the holes processing with high aspect ratios because of the lower optical absorption coefficient and the ablation threshold. Moreover, the maximum ablation depth with the least time could be achieved at the critical repeti...