Pal Molian

Femtosecond laser-induced periodic structure writing on diamond crystals and microclusters

Three-dimensional (3D), periodic nanowriting on diamond clusters is reported in this letter. Concentric circular rings were observed on diamond microclusters, nucleated near the periphery of a laser-irradiated region, when chemical-vapor deposited diamond was processed in air, with laser pulses of 380 fs duration and at a wavelength of 248 nm. Periodic ripples also have been observed on single-crystal and polycrystalline diamond surfaces. Further, it is experimentally shown that the periodicity of these corrugated two-dimensional and 3D structures is shorter than that of the laser wavelength used (248 nm for the excimer fs laser and 825 nm for the Ti–sapphire fs laser).

Coulomb explosion-induced formation of highly oriented nanoparticles on thin films of 3C–SiC by the femtosecond pulsed laser

We report the formation of highly oriented, uniform, and spherical nanoparticles of 3C–SiC as a result of Coulomb explosion during the interaction of near-infrared ultrafast laser pulses with 3C–SiC thin films grown on Si substrate. Experiments were performed at laser fluences well below the single shot, thermal modification threshold.

A novel laser technique for oxidation-resistant coating of carbon–carbon composite

Abstract High-temperature protection of carbon–carbon (C/C) composites against oxidation by ambient air is a serious problem with no single, universal solution. In this paper, we present the results of a novel laser-induced chemical decomposition (LICD) process used to apply two types of oxidation protection system (OPS) coatings on C/C composites. Silicon carbide and iridium were selected as the candidate materials for OPS based on their chemical compatibility with carbon and oxidation protection capabilities, and deposited on the C/C substrates by LICD of Si 3 N 4 and IrCl 3 . High-temperature oxidation tests of the coatings in air environment at 650°C showed a mass loss improvement of about 10% for single layer SiC and 50% for single layer iridium.

Nanocrystalline TiC powder alloying and glazing of H13 steel using a CO2 laser for improved life of die-casting dies

Abstract Premature failures of die-casting dies used in the metal casting industry occur because of the damage caused by thermal fatigue, erosion, stress corrosion, and soldering on the die surfaces. In this work, the effects of two laser surface-treatment methods for the prevention of die failures were investigated. A 1500-W CO 2 laser with round and line beam-shapes was employed to glaze H-13 steel substrate or alloy the substrate with TiC of various particle sizes (30 and 2 μm, and 300 nm). Laser parameters for the glazing and surface alloying processes were optimized, the criteria being a specified surface finish and integrity. The corrosion and erosion properties of laser-treated samples in aggressive casting conditions were evaluated by testing them in molten aluminum alloy A390. Laser-glazed and -alloyed specimens with μm-sized particles exhibited hardness 30–100% higher than that of heat-treated H-13 steel substrates. However, the hardness of specimens that were laser-alloyed with 300-nm particles was lower, approximately 25% of that of the substrate. The anomalous effects of nanocrystalline powder alloying could not be explained satisfactorily by the microstructural evidence obtained by the use of optical and scanning electron microscopy, and X-ray diffraction. However, it is hypothesized that some titanium dissolves in steel, promoting the formation of ferrite in preference to austenite at high temperatures, thereby decreasing the hardness. Laser glazing and alloying improved the resistance of H-13 steels to both corrosion and erosion, but a marked improvement occurred in the specimens alloyed with nanocrystalline powders. The beneficial effects of nanocrystalline alloying are attributed to smooth, crack-free, and tough surface layers, as well as to uniform and homogeneous microstructures. Laser surface processing of nanocrystalline materials is potentially important in the casting industry for improving die life and reducing downtime.

Pulsed laser deposition of AlMgB14 on carbide inserts for metal cutting

Abstract Nanocrystalline AlMgB 14 containing 0–30 mol% additives are a family of new superhard materials with hardness comparable to that of TiB 2 on the lower end and to that of cubic BN on the higher end. Compared with diamond and cubic BN, AlMgB 14 is an equilibrium material with excellent electrical conductivity, high chemical stability, and lower density. The projected cost of manufacture of the boride is 10% of the cost of diamond and cubic BN. AlMgB 14 materials appear to be congruently melting/evaporating, which would allow them to be processed with techniques such as pulsed laser deposition (PLD). In this work, the feasibility of PLD for synthesizing thin films of baseline AlMgB 14 (0% additive) is demonstrated and compared with TiB 2 . A 248-nm, 23-ns KrF excimer laser was used to prepare baseline boride thin films on cemented carbide (ANSI C-5 and C-2) tool inserts. The films were dark blue, continuous and fairly uniform with few particulates. An impact fracture test showed that adhesion of the films to the substrate was excellent. The deposition rate was 0.08 nm per pulse at an energy density of 7 J/cm 2 . Nanoindentation hardness tests revealed that the films exhibited hardness 60% higher than the carbide substrate. Lathe turning tests with cold-drawn 1045 steel bars indicated that C-5 tools coated with 0.5 μm baseline AlMgB 14 have an average flank wear reduction of 12% compared to uncoated C-5 tools. Further machining tests on C-2 tools showed that the tools coated with baseline boride have much better flank (23% reduction) and nose wear resistance (26% reduction) compared with TiB 2 coated tools. In addition, multilayer composite coating of AlMgB 14 and TiB 2 outperformed single layer boride coating in minimizing the tool wear. This pioneering work sets the stage and serves as a catalyst for rapid and innovative advances in the development of new boride materials for numerous tool and hard coating applications, including bulk cutting tools, hard and erosion-resistant coatings, wear-resistant electrical switch contacts, and conductive thin films for MEMS.

Weldability of aluminium-lithium alloy 2090 using laser welding

Lithium-containing aluminium alloys are of considerable current interest in the aerospace and aircraft industries because lithium additions to aluminium improve the modulus and decrease the density compared to conventional aluminium alloys. Many such alloys are under develøpment for aircraft applications, which usually involves mechanical fastening. While aluminium-lithium alloys are fusion weldable with gas metal arc, gas tungsten arc and electron beam processes, they suffer from problems of weld porosity, heat-tearing cracking, poor penetration and low joint efficiency. In this paper, the weldability of aluminium-lithium alloys is briefly reviewed. The weldability of commercial aluminium-lithium alloy 2090 in the peak-aged condition was studied using laser welding. The quality of the welds was evaluated through mechanical tests (hardness and tensile tests) and microscopical observations. Mechanical property data and microscopical observations of the welds on prior surface-prepared (milled) material revealed a low degree of the weld surface degradation and an absence of porosity. This coupled with the attractive joint efficiencies suggest the superiority of the laser welding to conventional arc welding of this alloy. The performance of laser-welded butt joints is rationalized.

Micro- and Sub-Micromachining of Type IIa Single Crystal Diamond Using a Ti:Sapphire Femtosecond Laser

A 200-fs pulsed Ti:Sapphire laser was used to micromachine Type IIa single crystal diamond. The effects of pulse energy and exposure time were investigated. Both blind and through holes were generated by trepanning and percussion modes. Trenches were produced by the direct-writing mode. Scanning electron and atomic force microscopy analysis revealed that the holes are in the range 0.65-100 μm and are free from taper. In addition, there was little recast layer around the holes. The damage threshold was approximately 4 J/cm 2 , which is smaller than those obtained from other lasers. A two-temperature model was used to establish the electron temperatures and to predict the ablation depth per pulse. It is evident from this work that femtosecond lasers are capable of producing micron and sub-micron structures with very high precision.

Ultrashort pulsed laser ablation of diamond

Laser processing of diamond and chemical-vapor-deposited diamond thin films are important in the microelectronics and cutting tool industries because the manufacture of diamond films with low surface roughness and complex shapes has proven to be difficult. In this paper we present a review of current laser polishing and ablation processes followed by a discussion of ultrashort pulsed processing of diamond. Compared with the use of longer pulsed lasers, the use of 248-nm, 500-fs duration pulses at extreme intensity offered multiple advantages, including a lack of lateral thermal damage and significant improvements in the structural purity of the ablated surface. Experimental data, including Raman spectra and scanning electron micrographs, were presented to demonstrate the superior capabilities of this new class of lasers for diamond processing.

Femtosecond pulsed laser micromachining of single crystalline 3C–SiC structures based on a laser-induced defect-activation process

A femtosecond pulsed Ti:sapphire laser with a pulse width of 120 fs, a wavelength of 800 nm and a repetition rate of 1 kHz was employed for direct write patterning of single crystalline 3C–SiC thin films deposited on Si substrates. The ablation mechanism of SiC was investigated as a function of pulse energy. At high pulse energies (>1 µJ), ablation occurred via thermally dominated processes such as melting, boiling and vaporizing of single crystalline SiC. At low pulse energies, the ablation mechanism involved a defect-activation process that included the accumulation of defects, formation of nano-particles and vaporization of crystal boundaries, which contributed to well-defined and debris-free patterns in 3C–SiC thin films. The interactions between femtosecond laser pulses and the intrinsic lattice defects in epitaxially grown 3C–SiC films led to the generation of nano-particles. Micromechanical structures such as micromotor rotors and lateral resonators were patterned into 3C–SiC films using the defect-activation ablation mechanism.

Ultra-short pulsed laser ablation of highly oriented pyrolytic graphite

Abstract Highly oriented pyrolytic graphite (HOPG) targets were irradiated by use of an ultra-short pulsed (pico to femtosecond) Ti:Sapphire laser operating at 825-nm wavelength. The morphology and quality of laser ablated surfaces were characterized by stylus profilometry, scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy. The results were also compared with those obtained in nanosecond pulsed excimer laser ablation. The ablation rates expressed in depth per pulse were substantially higher for ultra-short pulsed lasers, and were in close agreement with the theoretical model predictions of laser–solid interactions in the short-pulse regime. Post-analysis of laser irradiated regions revealed a reduction in thermal effects and a decrease in the formation of diamond-like carbon as the pulse width was shortened. This work demonstrates the clean and precise machining capabilities of ultra-short pulsed lasers for HOPG that could be applied in the areas of thin-film deposition, nanotube synthesis, and dust-free machining.