Subsurface Modifications Research Articles

Investigation on material removal rate, surface and subsurface characteristics in wire electro discharge machining of Ti50Ni50-xCux shape memory alloy

TiNiCu shape memory alloys have superior properties as compared with NITINOL due to their greater ductility, reduced hysteresis temperature range, and quick actuation response. The present article investigates the surface and subsurface modifications occurring due to wire electro discharge machining of Ti50Ni50-xCux shape memory alloy. The machining experiments were performed considering the pulse on time, pulse off time, and servo voltage as the process parameters. The influence of these parameters was studied on the material removal rate, surface roughness, recast layer thickness, microhardness, and phase changes in the machined surface. Longer pulse on time causes greater discharge energy, hence leading to higher material removal rate, surface roughness, and recast layer thickness. The machined surface hardness increased up to 900 Hv, which is about 59% increase with respect to the base material for longer pulse on time due to the recast layer thickness and the formation of oxides. A phase change on the machined surface was observed to cause the shape recoverability of the alloy. The microstructure, composition through EDAX, and the phase changes of the machined surface are also discussed in the article.

Corrosion Resistance of Friction Stir Processed AZ91D Magnesium Alloy under a Salt Fog Environment

Magnesium alloys are widely used in applications where weight reduction is of primary importance. MgAZ91D is an Mg-Al-Zn alloy and its application in the automotive sector is limited by its poor corrosion resistance. Recent advances in solid state processing techniques have made it easier to modify the mechanical and corrosion characteristics of various alloys. Friction stir processing (FSP) is such a solid-state process for surface and sub-surface modification, which increases the microstructural densification, thereby producing fine and equiaxed grains. Through this work, an attempt was made to analyse the effect of friction stir processing on the corrosion resistance of the alloy in an enclosed salt spray chamber. Micro-analysis tools like FESEM and EDS are used to supplement our results. It is seen that, FSP significantly contributes to the increase in the corrosion resistance by homogenising the distribution of α and β phases and hence making the use of the alloy more practical in moisture rich environments.

Crystal structure of laser-induced subsurface modifications in Si

Laser-induced subsurface modification of dielectric materials is a well-known technology. Applications include the production of optical components and selective etching. In addition to dielectric materials, the subsurface modification technology can be applied to silicon, by employing near to mid-infrared radiation. An application of subsurface modifications in silicon is laser-induced subsurface separation, which is a method to separate wafers into individual dies. Other applications for which proofs of concept exist are the formation of waveguides and resistivity tuning. However, limited knowledge is available about the crystal structure of subsurface modifications in silicon. In this work, we investigate the geometry and crystal structure of laser-induced subsurface modifications in monocrystalline silicon wafers. In addition to the generation of lattice defects, we found that transformations to amorphous silicon and Si-iii/Si-xii occur as a result of the laser irradiation.

Surface and Sub Surface Evaluation in Coated-Wire Electrical Discharge Machining (WEDM) of INCONEL® Alloy 718

Wire Electrical Discharge Machining (WEDM) is one of the most versatile and useful technological processes for cutting complex shapes made of conductive materials such as those typical of aerospace applications. With the aim to optimize process parameters, this paper studies the surface and subsurface modifications of INCONEL® alloy 718 (UNS N07718) machined by WEDM using a Zinc coated brass wire. Machining was performed under roughing and finishing conditions by setting different values of discharge energy and wire feed rate. Surface roughness of the cut surfaces was measured as well as the micro-hardness profile on polished sections. WED-machined surfaces and their cross sections were also observed by Scanning Electron Microscope (SEM) and analyzed by Energy Dispersive X-ray Spectroscopy (EDS) to evaluate possible variations of the surface chemical composition. In this investigation, results are discussed as a function of the feed rate and of the single pulse discharge energy, which is determined by duration, peak current and discharge voltage of the discharge pulse. The research demonstrates that the required surface roughness can be achieved by properly setting the feed rate and the single pulse discharge energy on the WEDM machine. Experimental results also show that under the re-melted layer thickness no significant white layer formation or thermal modification occur, indicating that the chosen set of operating parameters minimizes secondary and unwanted chemical reactions.

Cryogenic cooling effect on surface and subsurface microstructural modifications in burnishing of Co–Cr–Mo biomaterial

The aim of the present work is to investigate the effect of a severe plastic deformation (SPD) process, cryogenic burnishing, on the surface integrity modifications of a Co–Cr–Mo alloy due to the burnishing-induced surface integrity properties. A set of experiments was conducted to investigate the influence of different burnishing parameters on distribution of grain size, phase structure and hardness of the processed material. The results from this work show that the proper selection of burnishing conditions can significant improve the surface integrity of the Co–Cr–Mo alloy due to refined microstructure, high hardness, and favorable phase structure on the surface layer, which could potentially lead to advanced wear performance of such material.

Finite element modeling of microstructural changes in dry and cryogenic cutting of Ti6Al4V alloy

This paper presents a customized FE model for describing the microstructural changes during dry and cryogenic cutting of Ti6Al4V. It addresses the importance to modify the material behavior taking into account the microstructural changes and the cooling/lubrication effects during the cutting process. With this aim, a user subroutine is implemented in the FE code to describe the surface and subsurface modifications taking place during the cutting process and to implement them in order to properly modify the material flow stress. Thus, the material flow stress is continuously updated during the simulation according to the new microstructure characteristics. The proposed FE model is calibrated and validated by comparison with experimental results.

Subsurface modifications in powder metallurgy aluminium alloy composites reinforced with intermetallic MoSi2 particles under dry sliding wear

The effect of dry sliding wear on the subsurface of six AA6061/MoSi2/15p composites processed by powder metallurgy with varying mixing methods (wet blending, rotating cube and ball milling) and reinforcement sizes was investigated. Three regions could be distinguished: the tribolayer or mechanically mixed layer (MML), the elasto-plastically deformed layer, where deflection of material occurs, and the unaffected bulk. The MML was not uniform along the surface and no relation could be found between the size/shape of MML areas and processing variables or applied loads. In the second region, particles close to the MML were found to be fragmented only in the composites processed using low energy. These composites were the only ones to show hardening, together with the similarly processed unreinforced alloy. It can be concluded that, although the subsurface of these composites are noticeably affected by dry wearing through formation of a mechanically mixed layer and material deflection, the ball milled composites do not suffer subsurface hardening and their reinforcing particles do not break during the test. It is also evident from the present research that no simple correlation exists between the size/shape of the tribolayer and deflection parameters and processing variables or applied loads.

Two-temperature model for pulsed-laser-induced subsurface modifications in Si

We investigated the laser-material interaction during the production of laser-induced subsurface modifications in silicon with a numerical model. Such modifications are of interest for subsurface wafer dicing. To predict the shape of these modifications, a two-temperature model and an optical model were combined. We compared the model results with experimental data obtained by focusing laser pulses in the bulk of silicon wafers using a microscope objective. This comparison revealed a good agreement between the simulations and the experimental results. A parameter study was performed to investigate the effect of the laser wavelength, pulse duration and pulse energy on the formation of subsurface modifications. We found that both single- and multi-photon absorption may be used to produce subsurface modifications in silicon. © 2013 Springer-Verlag Berlin Heidelberg.

Erosion–corrosion behaviour of lean duplex stainless steels in 3.5% NaCl solution

The interaction between corrosion and erosion has often been a subject of debate among scientists and engineers. The total material degradation when corrosion and erosion interact is often more than the individual components acting separately. Much effort has been directed towards the study of erosion–corrosion behaviour of carbon steels, standard austenitic and duplex stainless steels, highly alloyed austenitic and super duplex stainless steels. However, the subject of erosion–corrosion of lean duplex stainless steels is still rarely reported. This present work studies the erosion–corrosion behaviour of lean duplex stainless steels UNS S32304 and UNS S32101 in an aerated 3.5% NaCl environment. The erosion–corrosion behaviour of these alloys is compared with an austenitic stainless steel, UNS S30403 and a duplex stainless steel, UNS S32205. Sub-surface microstructure modifications induced by work-hardening as a result of the sand impingement were analysed by X-ray Diffraction (XRD) and Scanning Electron Microscopy (SEM). Results show that lean duplex stainless steels, UNS S32101 and UNS S32304, have higher resistance to pure erosion than UNS S30403 and UNS S32205; better erosion–corrosion resistance than UNS S30403 austenitic stainless steel; and equivalent erosion–corrosion resistance to UNS S32205 duplex stainless steel. Also, erosion with 500mg/l sand at 15m/s in de-aerated tap water was able to cause subsurface deformation, phase transformation and an increase in hardness on the stainless steels.

Influence of grit blasting treatment using steel slag balls on the subsurface microhardness, surface characteristics and chemical composition of medical grade 316L stainless steel

Slag is a residue from the steel making processes which is at present still considered as an industrial waste due to its low utility. In this paper, the spherical steel slag balls obtained from the slag atomization process are evaluated for use in grit blasting treatment of medical grade 316L stainless steel. The modification in subsurface microhardness, surface characteristics (morphology, roughness, mass loss) and chemical composition of the stainless steel after the grit blasting treatment with these particles is examined. The blasting treatment was carried out for 5–20min using steel slag balls with a size of 1–2mm in diameter and 0.7MPa compressed air flow in normal direction toward the surface of the specimen. The result shows the increasing subsurface microhardness, surface irregularity and roughness of the stainless steel by this treatment. Surface material removal takes place as well during the blasting treatment as indicated by the mass loss of the specimen. The mechanisms of the subsurface microhardness modification as well as those for the surface roughness and mass loss evolution during the grit blasting treatment are elucidated in this paper. The blasting treatment with the steel slag balls also introduces some bioactive elements such as Ca, Si and Mg on the specimen surface. In conclusion, the grit blasting treatment using the steel slag balls has potential for improving mechanical properties and bioactivity of stainless steel based biomedical implants.

Complex frictional analysis of self-lubricant W-S-C/Cr coating

Transition metal dichalcogenides belong to one of the most developed classes of materials for solid lubrication. However, one of the main drawbacks of most of the self-lubricating coatings is their low load-bearing capacity, particularly in terrestrial atmospheres. In our previous work, alloying thin films based on tungsten disulfide with non-metallic interstitial elements, such as carbon or nitrogen, has been studied in order to improve tribological performance in different environments. Excellent results were reached with the deposited coatings hardness, in some cases, more than one order of magnitude higher than single W-S films. In this work, W-S-C films were deposited with increasing Cr contents by co-sputtering chromium and composite WS2-C and targets. Two films were prepared with approx. 7 and 13 at.% of Cr. Alloying with chromium led to dense films with amorphous microstructure; the hardness and adhesion was improved. Sliding tests were carried out in dry and humid air using a pin-on-disc tribometer with 100Cr6 steel balls as a counterpart. To analyse the sliding process, the surfaces in the contact were investigated by X-ray photoelectron spectroscopy (bonding), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Raman spectroscopy. Surface and sub-surface structural modification of the coating and composition of the transferred tribolayer are discussed in detail. High friction in humid air was attributed to the absence of a well-ordered WS2 sliding interface. On the other hand, the existence of such an interface explained the very low friction observed in dry air.

Microsized subsurface modification of mono-crystalline silicon via non-linear absorption

We introduce a novel method of optically inducing microsized subsurface structures using non-linear absorption of near infrared light in mono-crystalline silicon. We discuss the physical processes such as multi-photon absorption and self focussing in the material. The results presented in this paper demonstrate a new method of subsurface modifications in silicon and may open up novel avenues for optical devices embedded in silicon and optical process for the separation of wafers from their ingots.

Improvement of performance of lubricating materials with additives of clayey minerals

The paper presents the results of comparative tests of serpentine and kaolin minerals using the criteria of wear, scuffing, and friction resistance properties. It is established that, irrespective of the silicate stock type, the structure of the appearing protective layer and the mechanism of subsurface chemical modification are quite similar. The possibility is established that kaolin can serve as an equivalent substitute for serpentine.

Localized subsurface modification of materials using micro-low-energy multiple ion beamlets

Generation of focused multiple ion beamlets from an intense microwave plasma source is investigated for the creation of localized subsurface modification of materials. Unlike conventional single element focused ion beam (FIB) systems, the plasma source is capable of providing ion beams of multiple elements. Two types of plasma electrodes (PE) are employed, one with a honeycomb structure with notched apertures and another with a 5×5 array of through apertures, both attached to the plasma source and are capable of generating focused ion beamlets (50 - 100 μm diameter) in a patterned manner. Measurements of ion saturation current near the PE indicate that the plasma is uniform over an area of ∼ 7 cm2, which is further confirmed by uniformity in extracted beam current through the apertures. The ion beams are applied to investigate change in electrical sheet resistance Rs of metallic thin films in a controlled manner by varying the ionic species and beam energy. Results indicate a remarkable increase in Rs with beam energy (∼ 50 % at 1 keV for Ar ions), and with ionic species (∼ 90% for Krypton ions at 0.6 keV), when 80 nm thick copper films are irradiated by ∼2 cm diameter ion beams. Ion induced surface roughness is considered as the main mechanism for this change as confirmed by atomic force microscopy (AFM) measurements. Predictions for micro-beamlet induced change in Rs are discussed. The experimental results are verified using TRIM and AXCEL-INP simulations.

The impact of microstructural alterations at spray coated cylinder running surfaces of diesel engines – Findings from motor and laboratory benchmark tests

Future motor engine design and technology demand new materials that meet the requirements in terms of friction and wear properties. In order to reduce weight of the engine block and further more reduce the emissions thermal spray coatings are options to manufacture wear resistant and low-friction cylinder running surfaces. A motor cylinder with a thermally sprayed mild steel coating after a road test is prepared for transmission electron microscopy (TEM) investigations in the positions top dead center (TDC) and bottom dead center (BDC). In the dead centers of engines mixed/boundary lubrication conditions prevail. Within this study a new nanocrystalline thermal spray coating is investigated using two different laboratory tests where mixed/boundary lubrication regime is presumed. Conventional mild steel coatings and the new high alloyed Fe-base coatings are tested against two different piston rings in a reciprocating sliding wear test and an oscillating sliding wear test, known as SRV © test. TEM studies of the surface near microstructure demonstrate the difference in the microstructural alterations and wear mechanisms. Findings from the motor cylinder in surface and subsurface modifications are considered when discussing the results from laboratory tests in terms of initial grain size, influence of lattice defects and chemical influences due to oil additives.

Self-adaptive low friction coatings based on transition metal dichalcogenides

This paper deals with three fundamentally different concepts of self-lubricant coatings based on the transition metal dichalcogenides (TMD) and deposited by magnetron sputtering. The first two designs could be considered as traditional: the TMD films doped by carbon or nitrogen. The main attention is aimed at qualitative description of surface and sub-surface modification of the films as the result of the sliding process. Based on a thorough analysis of the worn surfaces, two features emerge: self-adaptability, where originally randomly oriented TMD phase becomes well oriented (i.e. with basal planes parallel to the surface), and limited role of carbon or nitrogen in the contact. The films doped with carbon are the best solution for humid environment, while those doped with nitrogen are ideal for use in dry environment or vacuum. The last concept deals with the combination of a hard TiN matrix deposited by reactive magnetron sputtering with fullerene-like TMD nanoclusters, which are directly injected into the growing TiN film. Despite the embryonic stage of the process development limiting in many ways the final properties of the film, the mechanical properties seem to be promising. The fullerene-like structure of TMD is well maintained and the composite hardness is relatively high compared to the competitive self-lubricant coating designs. On the other hand, the tribological tests did not show any effect of the embedded TMD nanoclusters in the sliding process.

Platinum Monolayer Electrocatalysts for the Oxygen Reduction Reaction: Improvements Induced by Surface and Subsurface Modifications of Cores

This paper demonstrates that the ORR activity of PtML electrocatalysts can be further improved by the modification of surface and subsurface of the core materials. The removal of surface low‐coordination sites, generation (via addition or segregation) of an interlayer between PtML and the core, or the introduction of a second metal component to the subsurface layer of the core can further improve the ORR activity and/or stability of PtML electrocatalysts. These modifications generate the alternation of the interactions between the substrate and the PtML, involving the changes on both electronic (ligand) and geometric (strain) properties of the substrates. The improvements resulted from the application of these approaches provide a new perspective to designing of the new generation PtML electrocatalysts.

Modifications on the bovid bone assemblage from Dunefield Midden, South Africa: stage one of a multivariate taphonomic analysis

A three-stage taphonomic analysis was performed on the small, small-medium and large bovid bone assemblages from Dunefield Midden, an open-air Later Stone Age site on South Africa's west coast. The results of the first stage of the analysis, which focuses on surface and subsurface bone modifications, are presented here. A forthcoming paper will detail the subsequent two stages, in which skeletal element abundance, longbone breakage patterns and overall taphonomic variability are investigated. The Dunefield Midden bovid bone assemblages are amenable to such an indepth study since they were subjected to a comprehensive refitting operation, with particular emphasis on longbone reconstruction. This enabled the systematic incorporation into the analysis of shaft fragments, a crucial yet frequently ignored component of mammalian zooarchaeological assemblages. Including shafts not only improves estimates of skeletal element abundance and thus our understanding of how prehistoric foragers acquired, transported and distributed animal carcasses, but also yields better assessments of the techniques employed for carcass butchery and bone processing. This paper explores the latter two aspects of subsistence at Dunefield Midden by integrating quantitative and configurational data on bovid bone modifications. It shows that the assemblage was principally shaped from within-bone nutrient acquisition by humans. Variation in bone modification patterning is suggested to stem largely from size-related differences in butchery effort and utility-related differences in skeletal element treatment.

Influence of Mechanical and Thermal Material Properties on Laser-Produced Crater-Morphology and their Study by Focused Ion Beam & Scanning Electron Microscope Imaging

Ablation of material can be obtained by high intensity laser beams and resulting in crater formation in samples. Crater morphology is due to concurring processes: subsurface vaporization, heterogeneous or/and homogeneous boiling, subsurface heating, spherical shock wave propagation, and the volume of craters is expected to be inversely proportional to the bulk modulus of target material. FIB-SEM dual beam system can be used to investigate the role of mechanical and thermal material properties in the laser produced crater morphology. In particular, FIB-SEM images of craters in conductive samples show a higher deposition around craters than in insulators, due to the different thermal conductivity of materials. Due to subsurface modifications on samples, both circular and noncircular craters may be formed. Therefore the analysis of the profile of these structures can be used to back up at dissimilar laser ablation processes

Writing Parameters for 3D Refractive Index Structures in Polymethyl Methacrylate Using Femtosecond Radiation at 400 nm

Sub-surface modification by a femtosecond laser, producing pulses of 100fs duration at 400nm wavelength for fabrication of optical components is investigated, within sub-millimetre thick sheets of undoped polymethyl methacrylate (PMMA). For effective structuring, the influence of laser writing parameters and focusing conditions were studied to optimise the pulse energy, pulses spacing and focusing depth in the material. Permanent sub-surface refractive index structures with a minimum feature size (similar to 3 mu m) at a depth of similar to 500 mu m demonstrated the feasibility of writing flexible 3-D microstructures within a thin PMMA substrate. This investigation of focusing depth location in PMMA bulk material enabled photonic structures at the desired spatial resolution to be written within the core of single mode polymer optical fibre under controlled laser writing conditions. A long period fibre grating (LPG) consisting of a 2.8 mu m wide refractive index structure with a periodicity of 250 mu m and containing a series of attenuation bands in visible range, demonstrated a photonic structure written into a single mode polymer optical fibre.