RBS investigations of ion beam mixed hard coatings on tool steel

Silicon nitride hard coatings on tool steel produced by ion beam mixing following sputter or vapour deposition were investigated by RBS and Vickers microhardness measurements. Atomic mixing in the coating/steel interface region resulting in an improved adhesion and in a significant increase of the microhardness of the surface layers was found. Ion beam mixing with gas ions and high fluences leads to blistering of the implanted atoms in the silicon nitride layer.     

Evaluation of the continuous optical discharge for spectrochemical analysis

The continuous optical discharge (COD) has been studied as a spectrochemical excitation source for atomic emission spectroscopy. The COD was generated by focusing a 45-W cw-CO₂ laser beam in Xe gas at pressures between 1150 and 3200 torr. The high temperature ( 10 000 K) and electron density (~10 ¹⁷ cm ^?3) of the plasma should provide good excitation for elements difficult to excite by more conventional sources. Some characteristics of the plasma were examined as a function of laser power and gas pressure. The design of a gas cell for analytical measurements which increases plasma stability is presented. Linear calibration curves for O₂; and Cl₂ introduced into the plasma were obtained and detection limits established. Detection limits were also determined for solid materials laser ablated into the COD. Because the COD operates at pressures above atmospheric, gas samples are most easily introduced for analysis. To prevent contamination of optical components by analyte dissociation products, the COD should be operated as a plasmatron.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd?:?YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented.     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics.     

Saturation effects in the laser ablation of stainless steel in air at atmospheric pressure

A pulsed Nd : YAG laser was used to generate a plasma from stainless steel targets in air at atmospheric pressure. Laser focusing was found to be an important factor in the ablation process. The influence of focal conditions on spatial profiles of plasma, emission intensity and averaged ablation rate (AAR, μm pulse^–1) of stainless steel samples as a function of laser energy are discussed. At high energies and depending on laser beam focusing, ablation efficiency tends to decrease compared to that at lower energies. This effect can be due to plasma shielding and air breakdown. The averaged ablation rate was found to be dependent on the thickness of the sample. This effect results in shielding of the incoming laser beam and redeposition of removed material in the crater. By focusing the beam inside the material free expansion of plasma is allowed, resulting in more efficient erosion of the sample at larger energies. For comparative purposes, data on ablated mass per pulse are presented. Received: 25 January 1999 / Revised: 7 April 1999 / Accepted: 30 April 1999     

Laser ablation of polished and nanostructured titanium surfaces by nanosecond laser pulses

A comparison of the IR nanosecond laser ablation parameters for polished and nanostructured titanium samples has been performed. The titanium foil was mechanically polished and pres-structured by multiple 744-nm femtosecond laser pulses producing large surface spots covered by ripples with periods in range of 400–500 nm. In order to evaluate the influence of such nanoripples, the nanosecond laser ablation and laser plasma properties were compared for polished surface, surface with nanoripples parallel and orthogonal to the laser beam polarization. A substantial decrease of the nanosecond ablation threshold was observed for the nanostructured in contrast to polished surface was detected while no influence of the ripple orientation vs. beam polarization was revealed. The comparison of plasma spectra for the ablation cases demonstrated that intensity of basic atomic lines and plasma emission duration were 2–5 times larger for the polished sample while spectra evolution was faster for the nanostructured sample. Plasma temperature and electron density were slightly lower for nanostructured sample while laser beam polarization has no effect on plasma properties.     

Characterization of acoustic signals produced by ultraviolet laser ablation inductively coupled plasma atomic emission spectrometry

A simple device was designed to measure the acoustic signal accompanying laser ablation. The potential use of this signal for laser ablation-inductively coupled plasma atomic emission was examined. A frequency quadrupled pulsed Nd:YAG laser radiation was used for the ablation of glass, steel and ceramic samples. The relation between the acoustic signal, the laser energy, the analyte signal and the amount of ablated material was studied and evidence of the use of the acoustic signal for the exact focusing of the laser beam onto the sample surface was given. A more intense acoustic signal was observed for the exact focusing with a formation of larger ablation craters in glass and ceramics. Received: 25 June 1998 / Revised: 25 September 1998 / Accepted: 30 September 1998     

Laser surface treatment of high‐speed tool steel (AISI M2)

High‐speed tool steel (AISI M2) surface is pre‐prepared to form a thin carbon film containing 5% B₄C particles prior to laser treatment process. Morphological and metallurgical changes are examined in the treated layer using electron microscope, energy dispersive spectroscopy, and X‐ray diffraction. The microhardness and the residual stress formed at the treated surfaces are measured for samples with and without B₄C particles. It is found that the micro‐stresses formed in the neighborhood of B₄C particles at the treated surface contributed to the microhardness enhancement at the surface. This is associated with the mismatch of thermal expansion coefficients between B₄C particles and the base alloy. The nitride phases are formed at the treated surface, which also contribute to the microhardness increase at the surface. Copyright © 2012 John Wiley & Sons, Ltd.     

Laser surface treatment of high‐speed steel: presence of TiC particles at the surface

Laser treatment of a high‐speed steel surface is carried out and metallurgical and morphological changes in the laser‐treated layer are examined using SEM, EDS and XRD. A carbon film of 50 µm thickness and containing 5% TiC particles is formed at the workpiece surface prior to the laser treatment process. The carbon film formed at the surface enhances the absorption of laser irradiation and retains TiC particles at the workpiece surface. The residual stress formed at the laser‐treated surface is determined using the XRD technique while the indentation tests are carried out to measure microhardness and fracture toughness of the resulting surface. It is found that ε‐Fe₃N, and ε‐Fe₃ (N,C) compounds are formed at the laser‐treated surface, which are attributed to the presence of carbon film and high‐pressure nitrogen‐assisting gas. The fracture toughness of the laser‐treated surface reduces because of the increased hardness and dense layer formed at the surface vicinity. Copyright © 2011 John Wiley & Sons, Ltd.     

Study of heterogeneities in steels and determination of soluble and total aluminium and titanium concentration by laser ablation inductively coupled plasma mass spectrometry

A methodology for bulk analysis of Al and Ti and for determination of soluble and total Al and Ti concentration in steel samples by laser ablation inductively coupled plasma mass spectrometry was developed. The spatial distribution (both at surface and within the sample) of the insoluble fraction of Al and Ti was also qualitatively estimated. Certified reference materials (CRMs) SS-451 to 460 (carbon steel) and 064-1 (Nb/Ti interstitial free steel), from BAS, and JK 2D (carbon steel) and JK 37 (highly alloyed steel), from SIMR, were studied. It was demonstrated that the insoluble fraction of Al and Ti is heterogeneously distributed. A series of nine glass samples (fused beads) with fixed Fe content and different Al and Ti contents was prepared by melting appropriate amounts of Fe₂O₃, Al₂O₃ and TiO₂ with a lithium tetraborate–sodium carbonate mixture. Quantitative determinations were performed by using calibration graphs obtained from the synthetic fused beads, with ⁵⁷Fe as internal standard; line scan laser sampling mode was used, focusing the laser beam at the sample surface. The optimized laser operating parameters were: laser pulse energy of 1.5 mJ, pulse repetition rate of 5 Hz, scanning speed of 5 μm s⁻¹ and preablation time of 20 s. The concentrations obtained for bulk analysis of CRM samples corresponded with the certified values within the experimental uncertainty. An acceptable concordance between certified and found values was attained for the determination of soluble and total Al and Ti in CRM 064-1 sample.     

镧、铈、钕在722M24钢离子氮化中的作用

将纯稀土金属镧、铈、钕分别放入离子氮化炉中作为溅射源,对722M24钢进行离子氮化,用扫描电镜进行了氮化代表层物相的二次电子象观察,并用能谱仪、二次离子质谱仪、X射线衍射仪和辉光放电光谱仪对氮化表层进行了元素分析及物相结构分析,显微硬度计测量了加不同稀土氮化后沿氮化层的分布,稀土元素在不同程度上影响辉光发电特笥,稀土金属离子氮化时部分溅射并沉积在钢的表面,从而影响了氮化效果,不同稀土元素在离子氮化中的作用有所不同。     

Large area mapping of non-metallic inclusions in stainless steel by an automated system based on laser ablation

Large area compositional mapping (>6 mm²) using a fast and automated system based on laser-induced plasma spectrometry is presented. The second harmonic of a flat top Nd:YAG laser beam was used to generate a microline plasma on the sample surface. The emitted light from the microline plasma was imaged onto the entrance slit of an imaging spectrograph and was detected by an intensified charge-coupled device to generate a spatially and spectrally resolved data set. Individual LIPS images, each measuring roughly 2500×2500 μm with spatial resolution of 50 μm between adjacent craters and 4.8 μm along the microline are presented. These large area maps were acquired in less than 1 min. Steel samples containing MnS and TiN inclusions were chosen as the most adequate for this study. The results are presented for the characterization of inclusionary material in stainless steel products in terms of morphology, distribution and abundance.     

Open-path laser-induced plasma spectrometry for remote analytical measurements on solid surfaces

Open-path laser-induced plasma spectrometry has been studied for elemental analysis at a distance of 45 m from the target. The 230-mJ pulsed radiation of a Q-switched Nd:YAG laser at 1064 nm has been used to produce a plasma on the sample and light emission has been collected under an off-axis open-path scheme. Under such conditions, the main variables influencing the signal response such as beam focal conditions, laser incidence angle and laser penetration depth have been identified and diagnosed on the basis of spectral signal-to-noise ratio considerations. The incidence angle is critical beyond 60°. Crater morphology and ablation rates have been studied also. A semi-quantitative analysis of several stainless steel grades has been implemented using a pattern recognition algorithm, which allowed to discriminate successfully the samples on the basis of their variable content in alloying elements.     

Investigation of polarization effects for nanosecond laser-induced breakdown spectroscopy

The spectral and temporal polarization dependencies of nanosecond laser-induced plasmas are explored for analysis of gaseous and solid samples using various experimental configurations. Plasma emission measurements were resolved into vertical and horizontal polarization components, and the ratio of the two polarization-resolved measurements was calculated for each sample and configuration. For the solid target, measurements were recorded with the sample oriented both normal to the incident laser beam as well as at oblique angles of incidence. The results for the breakdown of a pure, nitrogen gaseous sample revealed no degree of polarization in either the continuum or atomic emission, with the ratios of the horizontally-to-vertically resolved plasma emission showing values equal to unity when resolved both temporally and spectrally. The analysis of both copper and steel solid samples also showed no polarization dependency in the spectral and temporal data when the laser was incidentally normal to the sample surface. For oblique angles of incidence, some polarization (< 10%) was observed within the first tens of nanoseconds of plasma lifetime. The polarization was manifested as a slight reduction in the horizontal component of plasma emission, but significantly, the observed polarization was found to be spectrally flat, with no difference observed between continuum and atomic emission features. The small polarization effect was found to diminish with plasma residence time, effectively vanishing by about 1 μs following breakdown. The transient polarization is hypothesized to arise from reflection effects (i.e. Fresnel reflectivity) between the plasma light and the solid target surface present with oblique angles of incidence for reflected light, with temporal effects due to the dynamic nature of the plasma development and plasma–surface interactions. Overall, no evidence was found to support any inherent anisotropy or polarization specific to the plasma continuum or the atomic emission for the transitions studied.     

Laser Based Optical Emission Studies of Zinc Oxide (ZnO) Plasma

We present the optical emission characteristics of the zinc oxide (ZnO) plasma produced by the first (1,064 nm) and second (532 nm) harmonics of a Q switched Nd: YAG laser. The target material was placed in front of laser beam in air (at atmospheric pressure).The experimentally observed line profiles of neutral zinc (Zn I) have been used to extract the electron temperature using the Boltzmann plot method, whereas, the electron number density has been determined from the Stark broadening. The electron temperature is calculated by varying distance from the target surface along the line of propagation of plasma plume and also by varying the laser irradiance. Beside we have studied the variation of number density as a function of laser irradiance as well as its variation with distance from the target surface. It is observed that electron temperature and electron number density increases as laser energy is increased.     

Micro- and nanosecond laser TiN coating/steel modification: Morphology studies

Morphology effects induced during interaction of μs- (Transversely Excited Atmospheric (TEA) CO₂ laser) or ns- (HF laser) pulses with titanium nitride (TiN) coating, deposited on austenitic stainless steel AISI 316, were studied. Experiments were carried out in regime of focused laser beam in air at atmospheric pressure. The used laser fluences were found to be sufficient for inducing intensive surface modifications of the target. The energy absorbed from the CO₂ as well as HF laser beam is mainly converted into thermal energy, causing different effects like ablation, appearance of hydrodynamic features, etc. Morphology characteristics obtained during ns-pulses irradiation (HF laser) were different to those initiated by μs-pulses (TEA CO₂ laser). The changes on the target surface in form of massive resolidifed droplets and crown-like structures were observed only for ns- (HF laser) pulses. It was found that these effects are a consequence of higher temperature and better coupling of the HF laser radiation with the target. Recent investigations of ps-Nd:YAG laser interaction with the same TiN coating showed that morphology picture is quite different including the reduction of thermal effect. The article is published in the original.     

Studies on laser defocusing effects on laser ablation inductively coupled plasma-atomic emission spectrometry using emission signals from a laser-induced plasma

The effect of laser defocusing on analytical performance of laser ablation inductively coupled plasma atomic emission spectrometry (LA-ICP-AES) was studied by varying laser focus conditions with respect to the surface of a low-alloy steel and a powdered sediment pellet. Laser-induced plasma (LIP) and LA-ICP-AES emission signals and LIP excitation temperatures (LIP T_ex) were determined and compared for different laser defocus conditions. LIP Fe and LA-ICP-AES Fe emission signals and LIP T_ex decreased when the laser was defocused for the low-alloy steel. On the other hand, when the sediment pellet was ablated, LIP T_ex decreased when the laser was defocused. However, LA-ICP-AES Fe emission signals increased at first, then decreased when the laser was defocused more. It was concluded that LIP T_ex and LIP and LA-ICP-AES Fe emission signals are dependent on laser shot conditions (focus–defocus), and are also dependent on sample type (texture, mineralogy, hardness, conductivity and heat capacity).     

The influence of laser-particle interaction in laser induced breakdown spectroscopy and laser ablation inductively coupled plasma spectrometry

Particles produced by previous laser shots may have significant influence on the analytical signal in laser-induced breakdown spectroscopy (LIBS) and laser ablation inductively coupled plasma (LA-ICP) spectrometry if they remain close to the position of laser sampling. The effects of these particles on the laser-induced breakdown event are demonstrated in several ways. LIBS-experiments were conducted in an ablation cell at atmospheric conditions in argon or air applying a dual-pulse arrangement with orthogonal pre-pulse, i.e., plasma breakdown in a gas generated by a focussed laser beam parallel and close to the sample surface followed by a delayed crossing laser pulse in orthogonal direction which actually ablates material from the sample and produces the LIBS plasma. The optical emission of the LIBS plasma as well as the absorption of the pre-pulse laser was measured. In the presence of particles in the focus of the pre-pulse laser, the plasma breakdown is affected and more energy of the pre-pulse laser is absorbed than without particles. As a result, the analyte line emission from the LIBS plasma of the second laser is enhanced. It is assumed that the enhancement is not only due to an increase of mass ablated by the second laser but also to better atomization and excitation conditions favored by a reduced gas density in the pre-pulse plasma. Higher laser pulse frequencies increase the probability of particle-laser interaction and, therefore, reduce the shot-to-shot line intensity variation as compared to lower particle loadings in the cell. Additional experiments using an aerosol chamber were performed to further quantify the laser absorption by the plasma in dependence on time both with and without the presence of particles. The overall implication of laser-particle interactions for LIBS and LA-ICP-MS/OES are discussed.     

Rapid in-situ analysis of liquid steel by laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) denotes a technique where a pulsed laser beam is used to ablate small amounts of the target material. The characteristic optical emission line intensities of the excited species in the laser-generated plasma allow a quantitative chemical analysis of the target material. LIBS is a fast, non-contact method allowing large working distances between the sample under investigation and the detection system. These properties make LIBS applicable to process control in metallurgy. We describe an apparatus designed for rapid in-situ analysis of solid and molten metals at variable distances of up to 1.5 m. A variable lens system allows compensation for varying positions of the liquid steel surface. The LIBS signal is guided by a fiber optic bundle of 12-m length to the spectrometer. Analysis of an element's concentration takes 7 s. Laboratory experiments using an induction furnace showed that the addition of admixtures to liquid steel results in rapid response of the system. Results including the in-situ monitoring of Cr, Cu, Mn and Ni within certain concentration ranges are presented (Cr: 0.11–13.8 wt.%; Cu: 0.044–0.54 wt.%; Mn: 1.38–2.5 wt.%; Ni: 0.049–5.92 wt.%).     

Chemical maps of patterned samples by microline‐imaging laser‐induced plasma spectrometry

The potential of a microline‐imaging laser‐induced plasma spectrometry (LIPS) system for surface and depth analysis of heterogeneous solid samples in air at atmospheric pressure has been demonstrated. A pulsed Nd : YAG laser beam operating at 532 nm, with a homogeneous energy distribution (flat top laser), was used to generate a microline plasma on the sample surface. Subsequent light from the microline plasma was resolved spectrally and spatially and detected with an imaging spectrograph and an intensified charged‐coupled device detector. A patterned metal sample was chosen as the most appropriate for this study. Three‐dimensional chemical maps of Ni and Cu from the edge connectors of a printed circuit board have been obtained. With this experimental configuration, the lateral resolution (limited by crater width) was 42 µm and the spatial resolution along the spectrometer slit was 17.4 µm. The results illustrate the capability of microline imaging for fast mapping of large‐area samples and for depth profiling purposes. Copyright © 2003 John Wiley & Sons, Ltd.