Laser irradiation effects on the surface,structural and mechanical properties of Al–Cu alloy 2024

Laser irradiation effects on surface, structural and mechanical properties of Al–Cu–Mg alloy (Al–Cu alloy 2024) have been investigated. The specimens were irradiated for various fluences ranging from 3.8 to 5.5 J/cm² using an Excimer (KrF) laser (248 nm, 18 ns, 30 Hz) under vacuum environment. The surface and structural modifications of the irradiated targets have been investigated by scanning electron microscope (SEM) and X-ray diffractometer (XRD), respectively. SEM analysis reveals the formation of micro-sized craters along the growth of periodic surface structures (ripples) at their peripheries. The size of the craters initially increases and then decreases by increasing the laser fluence. XRD analysis shows an anomalous trend in the peak intensity and crystallite size of the specimen irradiated for various fluences. A universal tensile testing machine and Vickers microhardness tester were employed in order to investigate the mechanical properties of the irradiated targets. The changes in yield strength, ultimate tensile strength and microhardness were found to be anomalous with increasing laser fluences. The changes in the surface and structural properties of Al–Cu alloy 2024 after laser irradiation have been associated with the changes in mechanical properties.     

Surface micro/nanostructuring of titanium under stationary and non-stationary femtosecond laser irradiation

In this paper the surface topography of titanium samples irradiated by femtosecond laser pulses is described. When the fluence is about 0.5 J/cm² periodic ripples with a period of about 700 nm are formed. For fluences between 0.5 and 2 J/cm², a microcolumnar surface texture develops in the center of the irradiated spots and ripples are formed in the periphery of the spots. When experiments are performed with a non-stationary sample, the microcolumns exhibit ripples similar to those observed when the radiation fluence is about 0.5 J/cm² and in the outer regions of the irradiated areas for fluences between 0.5 and 2 J/cm². Since the energy distribution in the transverse cross-section of the laser beam is Gaussian, we conclude that the ripples form when the microcolumns are subjected to fluences near the melting threshold of the material at the trailing edge of the moving laser beam.     

Silicon substrate temperature effects on surface roughness induced by ultrafast laser processing

We report the effect of substrate temperature (T_sub) in the range 300-900 K on the surface roughness of silicon wafer resulted from femtosecond laser ablation. The surface roughness observed at the laser fluences less then 0.3 J/cm² increases with increasing T_sub. However, the surface roughness decreases with increasing T_sub for the laser fluences between 0.5 and 1.0 J/cm². If the laser fluence is higher than 2.0 J/cm², the surface roughness is independent of T_sub. The effect of T_sub on the surface roughness can be understood in terms of the temperature dependence of optical absorption coefficient of silicon substrate, which eventually alters a mechanism underlying the fs-laser-material ablation process between optical penetration and thermal diffusion processes.     

Picosecond laser ablation of nickel-based superalloy C263

Picosecond laser (10.4 ps, 1064 nm) ablation of the nickel-based superalloy C263 is investigated at different pulse repetition rates (5, 10, 20, and 50 kHz). The two ablation regimes corresponding to ablation dominated by the optical penetration depth at low fluences and of the electron thermal diffusion length at high fluences are clearly identified from the change of the surface morphology of single pulse ablated craters (dimples) with fluence. The two corresponding thresholds were measured as F _th(D1)¹=0.68±0.02 J/cm² and F _th(D2)¹=2.64±0.27 J/cm² from data of the crater diameters D _1,2 versus peak fluence. The surface morphology of macroscopic areas processed with a scanning laser beam at different fluences is characterised by ripples at low fluences. As the fluence increases, randomly distributed areas among the ripples are formed which appear featureless due to melting and joining of the ripples while at high fluences the whole irradiated surface becomes grainy due to melting, splashing of the melt and subsequent resolidification. The throughput of ablation becomes maximal when machining at high pulse repetition rates and with a relatively low fluence, while at the same time the surface roughness is kept low.     

Swift heavy ion provoked structural, optical and electrical properties in SnO2 thin films

SnO₂ thin films grown on glass substrates at 300 ^°C by reactive thermal evaporation and annealed at 600 ^°C were irradiated by 120 MeV Ag⁹⁺ ions. Though irradiation is known to induce lattice disorder and suppression of crystallinity, we observe grain growth at a certain fluence of irradiation. X-ray diffraction (XRD) revealed the crystalline nature of the films. The particle size estimated by Scherrer’s formula for the irradiated films was in the range 10–25 nm. The crystallite size increases with increase in fluence up to 1×10¹² ions?cm^?2, whereas after that the size starts decreasing. Atomic force microscope (AFM) results showed the surface modification of nanostructures for films irradiated with fluences of 1×10¹¹ ions?cm^?2 to 1×10¹³ ions?cm^?2. The UV–visible spectrum showed the band gap of the irradiated films in the range of 3.56 eV–3.95 eV. The resistivity decreases with fluence up to 5×10¹² ions?cm^?2 and starts increasing after that. Rutherford Backscattering (RBS) reveals the composition of the films and sputtering of ions due to irradiation at higher fluence.     

Modifications induced by swift heavy ions in rapidly solidified Sn–8.5Sb–5.5Cu alloy

Alloys of composition Sn–8.5Sb–5.5Cu (in atomic percent) were rapidly solidified by a melt-spinning technique. The samples were irradiated at room temperature with GeV uranium ions of fluences between 9×10⁸ and 9×10¹¹ ions cm^?2. X-ray diffraction analysis revealed the formation of a new-phase Cu₁₁Sb₃ as well as a reduction in the axial ratio (c/a) of the matrix (β -Sn) indicating the regular re-arrangement of atoms. Scanning force microscopy showed no surface topographic changes with the ion fluence. The mechanical properties (Young's modulus and hardness) of the irradiated alloys were studied as a function of ion fluence. The radiation-annealing process is discussed in terms of the evolution of both resistivity and hardness as a function of ion fluence.     

Synthesis of chromium silicide with laser pulses

Thin chromium films, 60 nm thick, were deposited onto single-crystal silicon wafers. The samples were irradiated with 30 ns single pulses from a Nd: glass laser at fluences ranging from 0.4 to 2.25 J/cm². Rutherford backscattering spectrometry, transmission electron microscopy and electron diffraction measurements evidence the formation of CrSi₂ layers at the Cr/Si interface. The silicide thickness depends on the laser fluence.     

Changes induced in structure and optical properties of PM-355 nuclear track detector due to laser irradiation

PM-355 is a class of polymeric solid-state nuclear track detectors which has a lot of applications in several radiation detection fields. Samples from sheets of PM-355 have been exposed to infrared (IR) laser fluences ranging from 1 to 12.8?J/cm². The effect of IR laser radiation on the structural properties of PM-355 has been investigated using X-ray diffraction and Fourier transform infrared spectroscopy. The results indicate that the samples exhibit chain scission under the effect of laser irradiation up to 4.2?J/cm², thus producing free radicals that led to the formation of new bonds started and continued until 12.8?J/cm². This reduces the ordering structure, giving the PM-355 polymer more resilience. In addition, the laser irradiation at the fluence range 4.2–12.8?J/cm² led to a more compact structure of PM-355, which resulted in an improvement in its isotropic nature with an increase in Vickers hardness and refractive index. Further, the color changes due to laser irradiation were computed using the transmission data in the wavelength range of 370–780?nm. It is found that the color intensity, which is the color difference between the irradiated samples and the non-irradiated one, increases with increasing the laser fluence, largely depending on the proportions of the blue color component.     

The generation,detection and measurement of laser-induced carbon plasma ions and their implantation effects on brass substrate

The generation, detection and measurement of laser-induced carbon plasma ions and their implantation effects on brass substrate have been investigated. Thomson parabola technique was employed to measure the energy and flux of carbon ions. The magnetic field of strength 80?mT was applied on the graphite plasma plume to provide an appropriate trajectory to the generated ions. The energy of carbon ions is 678?KeV for laser fluence of 5.1?J/cm² which was kept constant for all exposures. The flux of ions varies from 32?×?10¹¹ to 72?×?10¹⁴?ions/cm² for varying numbers of laser pulses from 3000 to 12,000. In order to explore the ion irradiation effects on brass, four brass substrates were irradiated by carbon ions of different flux. Scanning electron microscope (SEM) and X-ray diffractometer (XRD) are used to analyze the surface morphology and crystallographic structure of ion-implanted brass, respectively. SEM analysis reveals the formation and growth of nano-/micro-sized cavities, pores and pits for the various ion flux for varying numbers of laser pulses from 3000 to 12,000. By increasing ion flux by increasing the number of pulses up to 9000 shots, the dendritic structures initiate to grow along with cavities and pores. At the maximum ion flux for 12,000 shots, the unequiaxed dendritic structures become distinct and the distance between the dendrites is decreased, whereas cavities, pores and pits are completely finished. The XRD analysis reveals that a new phase of ZnC (0012) is formed in the brass substrate after ion implantation. Universal tensile testing machine and Vickers microhardness tester are used to explore the yield stress, ultimate tensile strength and microhardness of ion-implanted brass substrate. The mechanical properties monotonically increase by increasing the ion flux. Variations in mechanical properties are correlated with surface and structural modifications of brass.     

Surface topography of ultrashort laser-irradiated CaF2

A single-crystal CaF₂ (111) was irradiated with single and multiple laser (Ti:sapphire, 800 nm, 25 fs) shots at fluences ranging from 0.25 to 1.5 J cm^?2. In this fluence regime, a single laser pulse usually leads to typical bump-like features ranging from 200 nm to 1.5 μm in diameter and 10–50 nm in height. These bumps are related to compressive stresses due to a pressure build-up induced by fast laser heating and their subsequent relaxation. When CaF₂ is irradiated with successive (in our case 20) shots at a laser fluence of 1.5 J cm^?2, nanocavities at the top of the microbumps are observed. The formation of these nanocavities is regarded as an explosion and is attributed to the explosive expansion generated by shock waves due to laser-induced plasma after the nonlinear absorption of the laser energy by the material. Such kinds of surface structures at the nanometre scale could be attractive for nanolithography.     

Argon and krypton ion-induced changes in permalloy thin films

The effects of 75-keV Ar and 100-keV Kr ion irradiations of 72-nm thin DC-sputtered permalloy (Ni₈₁Fe₁₉) films on Si(100) wafers were studied at fluences of up to 10¹⁶ ions/cm². The changes of the structural and magnetic properties were measured via X-ray diffraction, Rutherford backscattering spectroscopy, and magneto-optical Kerr effect. The irradiations increase the lattice constant and improve the crystallinity of the samples. They induce also strong changes of the magnetic polarisation and the coercive field for increasing ion fluence. The hysteresis loops suggest that, with increasing ion fluence, the reversal of the magnetisation changes gradually from rotation-dominated in the as-deposited films to domain-wall-motion dominated at the highest ion fluences. The results are compared with those obtained for Ni-, Cr-and Xe-ion irradiated permalloy films.     

Analyses of surface coloration on TiO2 film irradiated with excimer laser

TiO₂ film of around 850 nm in thickness was deposited on a soda-lime glass by PVD sputtering and irradiated using one pulse of krypton-fluorine (KrF) excimer laser (wavelength of 248 nm and pulse duration of 25 ns) with varying fluence. The color of the irradiated area became darker with increasing laser fluence. Irradiated surfaces were characterized using optical microscopy, scanning electron microscopy, Raman spectroscopy and atomic force microscopy. Surface undergoes thermal annealing at low laser fluence of 400 and 590 mJ/cm². Microcracks at medium laser fluence of 1000 mJ/cm² are attributed to surface melting and solidification. Hydrodynamic ablation is proposed to explain the formation of micropores and networks at higher laser fluence of 1100 and 1200 mJ/cm². The darkening effect is explained in terms of trapping of light in the surface defects formed rather than anatase to rutile phase transformation as reported by others. Controlled darkening of TiO₂ film might be used for adjustable filters.     

High-vacuum time-resolved laser-induced incandescence of flame-generated soot

We have measured time-resolved laser-induced incandescence of flame-generated soot under high-vacuum conditions (4.1×10⁻⁶ mbar) at an excitation wavelength of 532 nm with laser fluences spanning 0.06–0.5 J/cm². We generated soot in an ethylene/air diffusion flame, introduced it into the vacuum system with an aerodynamic lens, heated it using a pulsed laser with a spatially homogeneous and temporally smooth laser profile, and recorded LII temporal profiles at 685 nm. At low laser fluences LII signal decay rates are slow, and LII signals persist beyond the residence time of the soot particles in the detection region. At these fluences, the temporal maximum of the LII signal increases nearly linearly with increasing laser fluence until reaching a plateau at ∼0.18 J/cm². At higher fluences, the LII signal maximum is independent of laser fluence within experimental uncertainty. At these fluences, the LII signal decays rapidly during the laser pulse. The fluence dependence of the vacuum LII signal is qualitatively similar to that observed under similar laser conditions in an atmospheric flame but requires higher fluences (by ∼0.03 J/cm²) for initiation. These data demonstrate the feasibility of recording vacuum LII temporal profiles of flame-generated soot under well-characterized conditions for model validation.     

The effects of fast-neutron irradiation on tensile strengths of carbon fibers

Two types of commercially available carbon fibers (high tensile strength, HTS, and high modulus, HMS) were irradiated in the Ground Test Reactor in environments of air and of liquid nitrogen (LN₂). The tensile strength of HTS fibers irradiated in air increased sharply above a fast-neutron fluence of 6 × 10¹⁷ n/cm² (E> 1 MeV) and was 17 per cent greater than the strength of unirradiated control fibers at a fluence of 8.5 × 10¹⁷ n/cm², but then the strengrh began to decrease for additional neutron exposure in air and fell 25 per cent below the control strength at the highest fluence of 4.5 × 10¹⁸ n/cm². However, when irradiated in LN₂ where surface oxidation did not take place, the room-temperature strength of HTS fibers continued to increase beyond 8.5 × 10¹⁷ n/cm² and became almost 30 per cent greater than the control strength for a fluence of 3 × 10¹⁸ n/cm². The tensile strength of HMS fibers irradiated in air increased slowly but steadily with neutron exposure and was only 4 per cent greater than the control strength at the highest fluence of 4.5 × 10¹⁸ n/cm²; the room-temperature strength of the HMS fibers decreased by 13 per cent when irradiated to a fluence of 3 × 10¹⁸ n/cm² in LN₂.     

Simulation of high fluence fast neutron damage with heavy ion bombardment

316 stainless steel has been irradiated with 5 MeV Cu ions to a fluence of 2 × 10¹⁶ ions/cm² at 500°C. Transmission electron microscopy of this sample reveals that 6 × 10¹⁵ voids/cm² of average diameter equal to 180 Å were produced. A method for correlating the fluence of ions with equivalent neutron fluences is described. This method predicts that the Cu bombardment in this study should produce a microstructure similar to that found in steel irradiated with 2–5 × 11²² neutrons/cm². A comparison of the ion produced voids with those found after previous neutron irradiation experiments confirms this prediction.     

Investigation of radiation resistance of fullerenes under irradiation with fast neutrons

Fullerenes C₆₀ and C₇₀ synthesized by the electric arc method and fractionated (purity grades of 99.99 and 99.90 wt %, respectively) were irradiated in a solid phase in the WWR-M reactor (Konstantinov Petersburg Nuclear Physics Institute, National Research Centre “Kurchatov Institute,” Gatchina, Russia) with the aim of determining the survivability in the range of fast neutron fluences Φ = 4 × 10¹⁵?3 × 10¹⁷ n/cm². The irradiated samples were dissolved in carbon disulfide, and intact fullerenes were extracted. With an increase in the fluence, their weight fraction in the samples S(Φ), a measure of radiation resistance of molecules, decreased, to a first approximation, exponentially: S(Φ) = exp(?Φ/Φ _D ). The estimated characteristic fluences were Φ _D = 2.4 × 10¹⁷ and 4.0 × 10¹⁷ n/cm² for C₆₀ and C₇₀, respectively.     

Raman spectroscopy of apatite irradiated with swift heavy ions with and without simultaneous exertion of high pressure

Durango apatite was irradiated with energetic U ions of 2.64 GeV and Kr ions of 2.1 GeV, with and without simultaneous exposure to a pressure of 10.5 GPa. Analysis by confocal Raman spectroscopy gives evidence of vibrational changes being marginal for fluences below 5×10¹¹ ions/cm² but becoming dominant when increasing the fluence to 8×10¹² ions/cm². Samples irradiated with U ions experience severe strain resulting in crystal cracking and finally breakage at high fluences. These radiation effects are directly linked to the formation of amorphous tracks and the fraction of amorphized material increasing with fluence. Raman spectroscopy of pressurized irradiated samples shows small shifts of the band positions with decreasing pressure but without a significant change of the Grüneisen parameter. Compared to irradiations at ambient conditions, the Raman spectra of apatite irradiated at 10.5 GPa exhibit fewer modifications, suggesting a higher radiation stability of the lattice by the pressure applied. PACS 61.80.Jh; 62.50.+p; 07.35.+k     

Effect of proton irradiation on the physical properties of PC/PBT blends

Samples from polycarbonate/poly (butylene terephthalate) (PC/PBT) blends film have been irradiated using different fluences (1?×?10¹⁵– 5?×?10¹⁷ H⁺/cm²) of 1?MeV protons at the University of Surrey Ion Beam Center, UK. The structural modi?cations in the proton irradiated samples have been studied as a function of fluence using different characterization techniques such as X-ray diffraction and UV spectroscopy. The results indicate that the proton irradiation reduces the optical energy gap that could be attributed to the increase in structural disorder of the irradiated samples due to crosslinking. Furthermore, the color intensity ΔE, which is the color difference between the non-irradiated sample and those irradiated with different proton fluences, increased with increasing the proton fluence up to 5?×?10¹⁷ H^+/cm², convoyed by an increase in the red and yellow color components. In addition, the resultant effect of proton irradiation on the thermal properties of the PC/PBT samples has been investigated using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). It is found that the PC/PBT decomposes in one weight loss stage. Also, the variation of transition temperatures with proton fluence has been determined using DSC. The PC/PBT thermograms were characterized by the appearance of two endothermic peaks due to the glass transition and melting temperatures. The melting temperature of the polymer, T_m, was investigated to probe the crystalline domains of the polymer, since the proton irradiation destroys the crystalline structure so reducing the melting temperature.     

Photoluminescence and optical properties of He ion bombarded ultra-high molecular weight polyethylene

Ion bombardment is a suitable tool to improve the physical and chemical properties of polymer surface. In this study UHMWPE samples were bombarded with 130 keV He ions to the fluences ranging from 1 × 10¹² to 1 × 10¹⁶ cm⁻². The untreated and ion beam modified samples were investigated by photoluminescence, and ultraviolet-visible (UV-vis) spectroscopy. Remarkable decrease in integrated luminescence intensity with increasing ion fluences was observed. The reduction in PL intensity with increase of ion fluence might be attributed to degradation of polymer surface and formation of defects. The effect of ion fluence on the optical properties of the bombarded surfaces was characterized. The values of the optical band gap E_g, and activation energy E_a were determined from the optical absorption. The width of the tail of the localized states in the band gap (E_a) was evaluated using the Urbach edge method. With increasing ion fluences a decrease in both the energy gap and the activation energy were observed. Increase in the numbers of carbon atoms (N) in a formed cluster with increasing the He ion fluence was observed.     

AC electrical properties of proton irradiated EVA films

MeV ions passing through polymeric films modify their electrical, optical and thermal properties and these changes are related to changes in the chemical structure of the polymers. Ethylene vinyl acetate (EVA) films were irradiated with 3 MeV proton beam at different fluences of 10¹³, 10¹⁴ and 10¹⁵ ions/cm². AC electrical properties of pristine and irradiated samples were studied in the frequency range 100 Hz to 100 kHz by means of an LCR meter. There is an exponential increase in conductivity with log frequency and conductivity increases as fluence increases. The dielectric loss/constant is observed to change with the fluence. FTIR spectra reveals significant change in intensities of functional groups at a fluence of 10¹⁵ ions/cm² due to scissioning of polymer chains.