State of the Art of Laser Hardening and Cladding

In this paper an overview is given about laser surface modification processes, which are developed especially with the aim of hardness improvement for an enhanced fatigue and wear behaviour. The processes can be divided into such with and without filler material and in solid-state and melting processes. Actual work on shock hardening, transformation hardening, remelting, alloying and cladding is reviewed, where the main focus was on scientific work from the 21st century

Investigation of UV and IR Laser Processing of Single- Crystalline 4H:SiC and Characterisation of Laser Grown Graphene Derivatives

The formation of graphene (G) on the surface of silicon carbide (SiC) has gathered interest over recent years as a potential component in high power nano and microdevices. However, it is still in the early stages of research, therefore there are many challenges to overcome. Among the existing problems, the formation of good quality graphene/SiC is one of the most critical factors that determine the behaviour of this heterostructure. Here we report a full study of the formation of graphene and its derivative structures on SiC using different laser systems with different controlled irradiation conditions.Laser ablation experiments on polished 4H-SiC wafers using a 193 nm ArF laser over a fluence range of 0.3Jcm−2–5Jcm−2 are reported. An onset of material modification was measured at a laser fluence of 925 ± 80 mJcm−2, and a concomitant etch rate of ∼200 pm per pulse. Laser ablation sites have been analysed using optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), Raman microscopy and white light interferometry (WLI). Different surface modifications were observed. The influences of the laser fluence, number of pulses, and scanning velocity on the position of the microchannel are discussed. At a laser fluence in the region of 1.0 Jcm−2, the irradiated site removed material forming a uniform crater. At a higher laser fluence, in the region of 2.7 Jcm−2, nodule-like structures form on the base of the ablation crater. An increased fluence led to a smoother surface with higher etched depth and ripple formation. The dissociation of laser irradiated 4H-SiC was discussed. Graphene oxide (GO) and reduced graphene oxide (rGO) formed on the SiC surface by 193 nm laser- induced high-temperature thermal decomposition of the SiC substrate. The decomposition resulted in the presence of silicon (Si), especially on the edge of the irradiated site.Graphene formation on the 4H:SiC surface by high power CO2 laser. Two distinct ablation threshold energies of 4.3 mJ and 73 mJ were found. The etch rate was dependent on the applied pulse duration, laser power, the scanning velocity and the number of pulses. High temperature thermal decomposition of the SiC substrate was achieved with a CO2 laser over a power range of 1-30 W. The structure was different from the structure obtained from the UV laser irradiated samples. More rough surfaces were prepared with small islands of graphene, GO and rGO on SiC in addition to the ripples. Monolayer and Multilayer graphene was also achieved. The laser-induced surface decomposition of the SiC was controlled spatially. The processing was held at room temperature, and the operation carried out in either a vacuum chamber or at atmospheric pressure. A fast graphene growth rate was achieved. This method is achievable, scalable and compatible with semiconductors technology due to the onsite direct writing of graphitic structure formed by the laser. This method is cost-effective as it does not necessitate SiC pre- treatment, there is no need for a processing vacuum chamber, and it can be achieved on the nano/microsecond time scale.Analytical and Finite element simulations using COMSOLTM MetaphySiCs, 5.3 have been used to calculate laser-induced temperature rise of 4H-SiC as a function of laser fluence. The simulated temperature was always less the temperature anticipated analytically. The 193 nm laser fluence required to reach the melting points of silicon, silicon carbide, and carbon, have been calculated and correspond to ∼0.97, 1.95 and 2.6 Jcm−2, respectively. Extreme heating and cooling rates controlled the growing process of graphene and its derivatives. The CO2 laser-induced temperature rise was also estimated. The CO2 laser acted as a heat source for the SiC. High power was used to reach the high temperature needed to decompose the SiC. Pulse duration also played a significant role in controlling the temperature and the depth distribution inside the SiC.This work reports the graphene formation on the surface of SiC by laser-induced thermal decomposition for electrical characterisation. Current-voltage (I-V) measurements show a decrease of the electrical resistance per unit length by nine orders of magnitude. The lowest resistance per unit length was obtained using a laser fluence of ~1.5 Jcm-2, a pulse repetition frequency of 10 Hz and using a sample translation speed of 0.01 mms-1. Temperature simulations have been performed using the finite element method (FEM) to assist in understanding the dissociation mechanisms of SiC and hence optimise the experimental variables. 2D axis-symmetric FEM calculations predict a surface temperature of ~2500K at a laser fluence of 1.5 Jcm-2. Laser-irradiated 4H:SiC is an efficient and controllable method of producing highly reproducible electrically conducting tracks. An increase in the conductivity was observed when the graphitic structure was produced with the CO2 laser. However, the conductivity was less than the graphitic structure produced by the 193 nm laser. It is expected that the different graphene interfaces, including Ohmic contact and Schottky contact, was created

2D Microfluidic Devices for Pore-Scale Phenomena Investigation: A Review

Underground porous media are complex multiphase systems, where the behavior at the macro-scale is affected by physical phenomena occurring at the pore(micro)-scale. The understanding of pore-scale fluid flow, transport properties, and chemical reactions is fundamental to reducing the uncertainties associated with the dynamic behavior, volume capacity, and injection/withdrawal efficiency of reservoirs and groundwater systems. Lately, laboratory technologies were found to be growing along with new computational tools, for the analysis and characterization of porous media. In this context, a significant contribution is given by microfluidics, which provides synthetic tools, often referred to as micromodels or microfluidic devices, able to mimic porous media networks and offer direct visualization of fluid dynamics. This work aimed to provide a review of the design, materials, and fabrication techniques of 2D micromodels applied to the investigation of multiphase flow in underground porous media. The first part of the article describes the main aspects related to the geometrical characterization of the porous media that lead to the design of micromodels. Materials and fabrication processes to manufacture microfluidic devices are then described, and relevant applications in the field are presented. In conclusion, the strengths and limitations of this approach are discussed, and future perspectives are suggested

Experimental studies and simulation of laser ablation of high-density polyethylene films

This thesis lays the groundwork for a simulation model for the laser ablation of polymer materials. A thorough review of the laser ablation of various polymer materials has been provided. The current trends and challenges in utilizing laser ablation for micro/nano manufacturing and information essential to the choice of an appropriate laser source for a polymer material have been provided. Experimental studies on laser ablation-based drilling of micro-holes on high-density polyethylene films have been performed. The influence of an increasing number of pulses and laser power on the depth and area of the micro-holes has been analyzed. The experimental results were utilized to validate a quantitative area-depth approximation model that was formulated based on the gain factors and the laser intensity profile. Additionally, a finite element method-based model has been developed for predicting the surface temperature and depth profile evolution with time during laser ablation of polymer materials

Laser Assisted Mechanical Micromachining of Hard-to-Machine Materials

There is growing demand for micro and meso scale devices with applications in the field of optics, semiconductor and bio-medical fields. In response to this demand, mechanical micro-cutting (e.g. micro-milling) is emerging as a viable alternative to lithography based micromachining techniques. Mechanical micromachining methods are capable of generating three-dimensional free-form surfaces to sub-micron level precision and micron level accuracies in a wide range of materials including common engineering alloys. However, certain factors limit the types of workpiece materials that can be processed using mechanical micromachining methods. For difficult-to-machine materials such as tool and die steels, limited machine-tool system stiffness and low tool flexural strength are major impediments to the use of mechanical micromachining methods. This thesis presents the design, fabrication and analysis of a novel Laser-assisted Mechanical Micromachining (LAMM) process that has the potential to overcome these limitations. The basic concept involves creating localized thermal softening of the hard material by focusing a solid-state continuous wave laser beam of diameter ranging from 70-120 microns directly in front of a miniature (300 microns-1 mm wide) cutting tool. By suitably controlling the laser power, spot size and speed, it is possible to produce a sufficiently large decrease in flow stress of the work material and, consequently, the cutting forces. This in turn will reduce machine/tool deflection and chances of catastrophic tool failure. The reduced machine/tool deflection yields improved accuracy in the machined feature. In order to use this process effectively, adequate thermal softening needs to be produced while keeping the heat affected zone in the machined surface to a minimum. This has been accomplished in the thesis via a detailed process characterization, modeling of process mechanics and optimization of process variables.Ph.D.Committee Chair: Melkote, Shreyes; Committee Member: Vengazhiyil, Roshan; Committee Member: Graham, Samuel; Committee Member: Johnson, Steven; Committee Member: Liang, Steve

Metal Micro-forming

The miniaturization of industrial products is a global trend. Metal forming technology is not only suitable for mass production and excellent in productivity and cost reduction, but it is also a key processing method that is essential for products that utilize advantage of the mechanical and functional properties of metals. However, it is not easy to realize the processing even if the conventional metal forming technology is directly scaled down. This is because the characteristics of materials, processing methods, die and tools, etc., vary greatly with miniaturization. In metal micro forming technology, the size effect of major issues for micro forming have also been clarified academically. New processing methods for metal micro forming have also been developed by introducing new special processing techniques, and it is a new wave of innovation toward high precision, high degree of processing, and high flexibility. To date, several special issues and books have been published on micro-forming technology. This book contains 11 of the latest research results on metal micro forming technology. The editor believes that it will be very useful for understanding the state-of-the-art of metal micro forming technology and for understanding future trends