Native Oxide Thickness Research Articles

Self-formation of compositionally complex surface oxides on high entropy alloys observed by accelerated atom probe tomography: a route to sustainable catalysts.

Sustainable catalysts rely on abundant elements which are prone to oxidation. A route to non-noble electrocatalysts is opened by directing the formation of unavoidable surface oxides towards creating a few atomic layers of an active and stable electrocatalyst, which is in direct contact with its metallic, conducting support. This is enabled by combining possibilities of compositionally complex solid solutions with accelerated atomic-scale surface characterization. Surface composition changes from the as-synthesized state to states after exposure to the oxygen evolution reaction (OER) are investigated using a Cantor-alloy-catalyst-coated tip array for atom probe tomography (APT): The film on top of the tip forms a nanoreactor which enables acquisition of intrinsic properties. The as-deposited film has an around 3 nm thick native oxide; short and prolonged OER exposures result in an oxygen-influenced surface layer with lower oxidation depth and altered metal composition. This shows that as-synthesized complex compositions can be used to obtain active and stable surface oxides under electrochemical load, while their surface evolution is observed by accelerated APT.

Comparative Analysis of Surface Characterization Techniques for Atomic Layer Etching

The growing demand for precise atomic level control in device fabrication has led to the rising popularity of Atomic Layer Etching (ALE).1-3 As the name implies, the process facilitates layer-by-layer material removal in an atomic scale, making low surface damage one of the key features of this process. The low-power ion bombardment and the self-limited nature of different process steps help to reduce the damage induced by ALE. However, the damage is not easy to characterize. Atomic Force Microscopy (AFM) is one of the methods that is widely used for surface characterization. However, roughness measurements of the surface using AFM may be misleading as it primarily reflects the top surface, which is typically composed of native oxide rather than the actual Si surface.In this work, we discuss how we used X-Ray reflectometry (XRR) together with ellipsometry to measure the damage induced to the Si device layer of a Silicon on Insulator (SOI) wafer.The experiments were done on an SOI wafer with a 55 nm thick device layer and 145 nm box layer. In order to remove native oxide, all samples were treated with Buffered oxide etch (BOE) 10:1 right before the etch process. The cyclic process was performed in a commercial Inductively Coupled Plasma Reactive Ion Etcher (ICP-RIE) Takachi™ tool from Plasma-Therm LLC, USA. The process gases were Chlorine to activate the Si surface and Ar to remove the activated surface. There was no plasma during activation and purging, it was only ignited during the removal step. XRR, AFM, and ellipsometry characterizations were used to monitor the damage induced by the process. These methods were used to measure the surface roughness before and after the process. In the ellipsometry model, a damage layer is incorporated to evaluate the Si surface that has undergone plasma bombardment resulting in its transformation into amorphous Si. This layer is typically referred to as the surface damage layer. The XRR can measure the roughness on Si layer right beneath the native oxide which again demonstrates the surface damage. On the other hand, AFM measures the roughness at the surface of native oxide.Our results show that while in XRR and ellipsometry, the roughness change follows the same trend, the AFM measurement is not. Results have revealed that in the case of a non-ALE process, both XRR and ellipsometry techniques exhibit a substantial level of surface damage, whereas AFM indicates lower damage. This provides evidence that AFM alone may not be a reliable approach for assessing surface damage. In addition, both XRR and ellipsometry confirm a thick native oxide on top which is very much correlated to the damage. Typically, the thickness of the native oxide is in the ranges between 15-25 Å, and our results support that the thickness of the measured native oxide layer increases with the level of surface damage.

Electron Microscopy Investigation of Porous Transport Layer Coatings

The flow of reactants and biproducts to and from the anode in polymer electrolyte membrane (PEM) water electrolyzers is dictated by the properties of the porous transport layer (PTL). The PTL is made of titanium (Ti) fibers or sintered particles to provide stable thermal and electronic conduction pathways at the high overpotentials and acid conditions experienced at the anode. To prevent the formation of a thick native oxide on the Ti PTL surface, precious metal coatings are required. These coatings enable and maintain a reduced interfacial resistance between the PTL and catalyst layer. Understanding the impact of surface treatments and coatings on the morphology and composition of this interface is an important step to minimizing interfacial resistance, degradation and production costs. In this work, ion and electron beam techniques, including focused ion beam (FIB), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and scanning transmission electron microscopy (STEM), are used to investigate PTL coatings at the nanoscale.A series of fiber-type PTLs with a range of platinum (Pt) coating thicknesses were synthesized using physical vapor deposition. The uniformity of the coatings was analyzed using SEM-EDS in both plan-view and cross-section. Sections of individual fibers were then removed using FIB lift-out technique and thinned to less than 50 nanometers for further analysis by STEM. The thickness and uniformity of the Pt coating was quantified from STEM images for each Pt loading and correlated with interfacial resistance measurements. Changes to the thickness and composition of the coatings and oxide layers after single cell tests will also be reported alongside additional insights from correlative characterization approaches like atomic force microscopy, X-ray photoelectron spectroscopy, and time-of-flight secondary ion mass spectrometry. Overall, these findings will help steer the development of PTL treatments and coatings for highly durable and efficient PEM water electrolyzers towards meeting the U.S. Department of Energy’s Hydrogen Shot to reduce the cost of clean hydrogen production to $1 per kg by 2031.Funding for this work was provided by U.S. Department of Energy Office of Energy Efficiency and Renewable Energy, Hydrogen and Fuel Cell Technologies Office through the H2NEW Consortium. Electron microscopy research conducted as part of a user project at the Center for Nanophase Materials Sciences (CNMS), which is a US Department of Energy, Office of Science User Facility at Oak Ridge National Laboratory.

Ferroelectric Al1−xBxN thin films integrated on Si

Ferroelectric Al0.93B0.07N thin films are prepared (100) Si substrates. The necessary c-axis out-of-plane orientation to observe macroscopic ferroelectric switching was achieved by implementing an initial Ar/N2 plasma treatment, followed by a thin layer of AlN to initiate the desired texture and a 150 nm W layer. The plasma treatment facilitates crystallinity enhancement of the AlN template layer, allowing for subsequent growth of highly oriented W and Al0.93B0.07N layers. The W layer exhibits random in-plane orientation and exclusive (110) out-of-plane orientation with a rocking curve width of 1.4°. When grown on these W surfaces, 175 nm thick Al0.93B0.07N films exhibit random in-plane orientation and exclusive (001) texture with rocking curve full-width-half-max values of 1.6° and RMS roughness values less than 1 nm. Polarization hysteresis measurements show robust hysteresis with coercive field values of 5.4 MV/cm and remanent polarization values of 136 μC/cm2. XPS depth profile analysis suggests that the plasma treatment converts the existing native oxide to a nitrogen rich oxynitride with approximate composition Si3O0.5N3.67. Cross-sectional TEM reveals that the oxynitride interlayer is amorphous and ∼3.4 nm thick, more than double the native oxide thickness measured by multiwavelength ellipsometry, implying that (oxy)nitride growth continues after conversion of the native oxide. This new family of ferroelectric wurtzites is interesting from an integration perspective given their chemical compatibility with mainstream semiconductors. Developing synthesis routes that promote needed texture while preserving compatible processing windows is an important step toward practical integration.

Density control of GaN nanowires at the wafer scale using self-assembled SiN x patches on sputtered TiN(111)

The self-assembly of heteroepitaxial GaN nanowires using either molecular beam epitaxy (MBE) or metal-organic vapor phase epitaxy (MOVPE) mostly results in wafer-scale ensembles with ultrahigh (>10 μm−2) or ultralow (<1 μm−2) densities, respectively. A simple means to tune the density of well-developed nanowire ensembles between these two extremes is generally lacking. Here, we examine the self-assembly of SiN x patches on TiN(111) substrates which are eventually acting as seeds for the growth of GaN nanowires. We first found that if prepared by reactive sputtering, the TiN surface is characterized by {100} facets for which the GaN incubation time is extremely long. Fast GaN nucleation is only obtained after deposition of a sub-monolayer of SiN x atoms prior to the GaN growth. By varying the amount of pre-deposited SiN x , the GaN nanowire density could be tuned by three orders of magnitude with excellent uniformity over the entire wafer, bridging the density regimes conventionally attainable by direct self-assembly with MBE or MOVPE. The analysis of the nanowire morphology agrees with a nucleation of the GaN nanowires on nanometric SiN x patches. The photoluminescence analysis of single freestanding GaN nanowires reveals a band edge luminescence dominated by excitonic transitions that are broad and blue shifted compared to bulk GaN, an effect that is related to the small nanowire diameter and to the presence of a thick native oxide. The approach developed here can be principally used for tuning the density of most III–V semiconductors nucleus grown on inert surfaces like 2D materials.

Surface composition, structure and corrosion properties of a ZrTiNb alloy: Effect of impact treatment energy

A comparative study of the microstructure, surface chemical and phase composition, microtopography, surface roughness, microhardness, and corrosion properties of Zr31Ti18Nb alloy in as-prepared state and after ultrasonic impact treatment under two loading schemes has been carried out by using XRD, XPS, SEM/EDX and TEM/SAED. The surface of as-prepared alloy was considerably enriched with Zr as compared to the nominal composition, covered by a thick native oxide and comprised carbide and nitride precipitates. The processing of the alloy for 2 min in (i) a multi-pin impacting mode combined with a reciprocating movement of the sample and (ii) single-pin normal impacting mode caused deformation-enhanced uptake of oxygen promoting further oxidation of the alloy components. Besides, the impact treatment brought about the accumulation of structural defects, texturing, generation of compressive residual stresses, a decrease in surface roughness, an increase in microhardness, and a significant increase in the corrosion resistance of the alloy in a 0.9% NaCl solution. The highest effect of surface modification of the alloy was obtained after treatment in the single-pin impacting mode. This was related to a larger energy density and power density deposited in the impact area during processing in this mode as evaluated from measurements of pins’ energetics.

Hydrogen plasma enhanced oxide removal on GaSb planar and nanowire surfaces

Due to its high hole-mobility, GaSb is a highly promising candidate for high-speed p-channels in electronic devices. However, GaSb exhibits a comparably thick native oxide causing detrimental interface defects, which has been proven difficult to remove. Here we present full oxide removal from GaSb surfaces using effective hydrogen plasma cleaning, studied in-situ by synchrotron-based X-ray photoelectron spectroscopy under ultrahigh vacuum (UHV). GaSb nanowires turn out to be cleaned faster and more efficiently than planar substrates. Since the UHV conditions are not scalable for industrial sample processing, H-plasma cleaning is furthermore used as pre-treatment prior to atomic layer deposition (ALD) of a protective high-k layer to demonstrate the use of the cleaning step in a more realistic fabrication situation. We observe a cleaning effect of the H-plasma even in the ALD environment, but we also find residual Ga- and Sb-oxides at the GaSb-high-k interface, which we attribute to re-oxidation of the cleaned surface. Our results indicate that an improved control of the ALD reactor vacuum environment could realize oxide- and defect-free interfaces in GaSb-based electronics.

Modulating the Reactivity of Liquid Ga Nanoparticle Inks by Modifying Their Surface Chemistry.

Micro- and nanosized particles of liquid metals, particularly Ga-based alloys, are attracting increasing attention for applications in several fields. The surface functionalization of Ga-based nanoparticles (NPs) with organic ligands renders easily processable inks. However, little is known about the interaction of these molecules with the native oxide skin, which regulates many properties of liquid metal NPs. Here, we investigate the impact of selected capping ligands on the native oxide thickness of Ga NPs and on their chemical reactivity, choosing the galvanic replacement reaction (GRR) as one example. We demonstrate that amines and carboxylic acids promote thicker oxide shells while thiols and phosphines hinder the oxide growth. Upon pondering thermodynamics and kinetics factors, we conclude the affinity of the anchoring group toward the metal core being the major driver in determining the oxide thickness. We go on to prove that thicker shells foster the formation of Cu-Ga nanodimers following the reaction of the Ga NPs with a copper-amine complex. In contrast, thinner oxides lead to formation of isolated Cu NPs. This study reveals the importance of the choice of ligand when studying Ga-based metal NPs for different applications since both their surface chemistry and reactivity are largely affected by this decision.

Thickness and composition of native oxides and near-surface regions of Ni superalloys

The surface chemistry and thickness of the native oxide, hydroxide, and modified sub-surface layer of three Ni superalloys (alloy 59, 625, and 718) were determined by synchrotron X-ray Photoelectron Spectroscopy (XPS) and X-ray Reflectivity (XRR). Taking advantage of the synchrotron radiation techniques, a procedure for normalizing the photoelectron intensity was employed, which allowed for accurate quantitative analysis revealing a total oxide thickness for all samples of 12–13 Å, a hydroxide layer of 2–3 Å, and a thickness of the sub-surface alloy layer of 20–35 Å. The thickness results were compared to structural atomic models suggesting that the oxide thickness corresponds to four planes of metal cations in the oxide matrix. The XPS data revealed that the native oxides were enriched in Cr3+, Mo(4,5,6)+, and Nb5+, while no Ni oxide was detected. The hydroxide layer mainly contained Ni2+ and Cr3+ hydroxide. The sub-surface layer was enriched in Ni and depleted in Cr, Fe, Mo, and Nb. The obtained oxide composition can be explained using thermodynamics, and it was found that the oxide composition correlates with the enthalpy of oxide formation for the metal elements in the alloys. Finally, the advantages of synchrotron radiation for composition and thickness determination are discussed.

Role of oxide at interface between organic layer and silicon substrate in hybrid solar cells

Degradation in PEDOT:PSS-silicon hybrid heterojunction solar cells is centered at the organic–silicon interface characterized by an s-shaped JV curve. The effects of interfacial silicon oxide and the degradation of PEDOT:PSS films on solar cell performance were simulated and fit to experimental results and found to be good predictors of JV performance and the development of s-shapes. Transmission line measurement (TLM) studies showed the Ag-PEDOT:PSS interface remains ohmic over 5 weeks and has a contact resistivity < 0.1ohm cm2 over 10 days. X-ray photoelectron spectroscopy (XPS) showed that the interfacial silicon oxide develops rapidly after fabrication with high amounts of suboxide defects that grows and chemically saturates to the native oxide thickness (1.5 nm) and composition as the devices age. Hard X-ray photoelectron spectroscopy (HAXPES) was used to investigate the buried organic–silicon interface and showed that doping in the PEDOT:PSS backbone decreases and the PEDOT:PSS-silicon band alignment does not change measurably as the devices age.

Effect of native oxide on the crystal orientation contrast in SEM micrographs obtained at hundreds, tens and units of eV

This paper aims to elucidate the effect of native air-formed oxide on the crystallographic contrast between differently oriented copper grains in scanning electron microscope images obtained at energies from 0 eV up to 1 keV. The contrast between the Cu grains is strongly affected by the presence of native oxide. The crystallographic orientation contrast between the grains without covering the native oxide layer is relatively weak at hundreds of eV, negligible at tens of eV, and dramatically increases at energies below 10 eV. At extremely low landing energies, say below ~ 1 eV, the surface potential differences caused by work function variations between the differently oriented Cu grains affect the primary electrons, which enables us to obtain the micrographs with high crystallographic contrast. This contrast becomes surprisingly visible even if the grains are covered by a several nm thick native oxide layer. The presence of the native air-formed oxide layer on the Cu surface is inconsiderable for the contrast formation at energies close to the mirror conditions (< 1 eV). The surface potential differences originating in the substrate can affect the incident electrons through the native oxide film situated on the Cu surface. Scanning low-energy electron microscopy is a powerful tool for mapping local work function differences with a spatial resolution slightly better than 30 nm due to high sensitivity to local electrical potentials.

Synthesis of model Al-Al2O3 multilayer systems with monolayer oxide thickness control by circumventing native oxidation

A hybrid thin film deposition system was built by combining physical vapor deposition (PVD) with atomic layer deposition (ALD) without breaking high vacuum, which was achieved by a chamber-in-chamber design. The vacuum is better than 1 × 10−5 Pa during the entire sample transfer process between the ALD and PVD chambers. The Al-Al2O3 multilayers synthesized by the PVD-ALD integrated system consist of ALD Al2O3 sublayers between pure PVD Al sublayers. The Al2O3 sublayers, with incremental thickness of 1 nm from 1 to 10 nm, effectively interrupted the grain growth of the ~250 nm thick Al sublayers. Native surface oxide formation on the PVD deposited pure Al sublayers was circumvented by keeping the substrate in high vacuum between processing steps. The pure Al sublayers are constituted of equiaxed grains with no epitaxial or texture relationship with the neighboring layers. The Al grain boundaries are parallel to the film growth direction and extend the height of each Al sublayer, to form a brick-and-mortar type microstructure. In this system, ALD allows precise control of the thickness of each ultrathin Al2O3 layer, to thicknesses below the native surface oxide thickness of pure Al, and avoids further natural oxidation through the low-oxygen environment of the PVD-ALD system.

Native oxide layer effect on polarization cancellation for mirrors over the visible to near-infrared region

Abstract The presence of native oxide layers on aluminum mirrors can be a nuisance for precision optical design. As the native oxide thickness varies from mirror to mirror, its effect cannot be completely canceled even in the conventional crossed fold mirror geometry. We show how this effect arises and how it can be mitigated, and provide an experimental demonstration in which the residual linear retardance and linear diattenuation are reduced to &lt;0.14° and &lt;0.001, respectively, over the visible and near-infra-red spectral range.

Investigation of the mechanism of transport across the poly/monocrystalline silicon interface in polysilicon-emitter bipolar transistors based on variations in the interface treatment process

Electrical transport across the poly/monocrystalline silicon (poly-Si/c-Si) interface has been investigated by introducing variations in the interface treatment process affecting the thickness of the native oxide layer formed on the silicon surface. The I-V relationship characterizing the resistance associated with this interface is extracted using an improved open-collector method, which eliminates the inherent nonlinearity arising from the reverse active characteristics of the bipolar transistor. The nonohmic behavior of the interface at low electric fields is demonstrated to be consistent with direct tunneling through a rectangular barrier presented by an ultrathin interfacial oxide layer. An approximate tunnel junction model is proposed that fits the tunneling current in the low bias regime, predicting the experimentally observed increase in the low-voltage leakage current associated with ultrathin oxides. The thickness of the interfacial oxide extracted by fitting the measured data to the proposed rectangular-barrier tunnel junction model is in good agreement with reported experimental values. Finally, a relation between experimentally introduced variations in the interface treatment process and the magnitude of the resistance associated with the interface is identified, which can be quantitatively explained based on the direct tunnel junction model in terms of differences in the native oxide thickness.

Beyond conventional secondary electron imaging using spectromicroscopy and its applications in dopant profiling

Despite decades of research, the technique of measuring dopant concentration by the scanning electron microscope (SEM) has so far been limited to probing idealized test pn junction specimens, where only step dopant concentrations have been quantified. This article presents experimental results to demonstrate that the key to making this method reliable is to use a high signal-to-noise secondary electron (SE) energy spectrometer attachment and how quantifying small changes in spectral signal shape overcomes longstanding problems, such as making dependable measurements in the presence of a silicon native oxide layer. Dopant profiling of a solar cell precursor device were also found to be in good agreement with other conventional dopant profiling methods. In addition, new possibilities have been discovered, such as in situ measurement of native oxide thickness and the mapping of space-charge density. These results point towards SE energy spectroscopy having the potential to become a new analytical technique for SEMs.

In-Situ X-Ray Observations of Sn Electrodeposition into Anodized Alumina

Aluminum is used widely for various products and application because of its characteristics of high strength, low density and good corrosion resistance. When aluminum is exposed to air a native oxide forms on its surface and protects it from corrosion[1,2]. The native oxide can by an electrochemical method called anodization be grown thicker to give a better corrosion protection of the aluminum[3]. By using acidic electrolytes for the anodization the grown oxide will become porous, these pores can become ordered under certain anodization conditions[4]. The grown Nano-Porous Anodic Aluminum Oxide (NP-AAO) can be used as a template for growing nano wires and can be colored by deposition of metals into the pores. We have followed in-situ the electrochemical deposition of Sn into the NP-AAO by Grazing Transmission Small Angle X-Ray Scattering (GTSAXS), X-Ray Fluorescence (XRF) and X-Ray Absorption Near Edge Structure (XANES). A two-step anodization method was used to create the ordered NP-AAO, this is achieved by anodizing the sample in acidic electrolytes in two anodizing steps with a chemical etch between the steps. The second anodization step was followed in-situ with GTSAXS[5]. To achieve a better deposition into the pores of the oxide, either a barrier layer thinning or a pore widening step was used after the second anodization. These methods will decrease the barrier layer at the bottom of the pores by a controlled down stepping of the anodization potential and make the pores wider trough a chemical etch. Both methods were followed continuously by GTSAXS. The electrochemical deposition of Sn was achieved using an alternating voltage and an electrolyte containing tin ions. We followed the deposition continuously with GTSAXS, while XRF and XANES was measured at intervals during the deposition. Our GTSAXS measurements shows an increase in intensity during deposition, indicating deposition into the pores of the aluminum oxide. From our XRF measurements an increase of deposited Sn could be detected as the XRF signal at the sample increased over time. We used XANES for determining the chemical state of the deposited Sn. Ex-situ FIB-SEM measurements shows the pores filled with Sn, confirming the in-situ measurements. The sample also had a change in color confirming the Sn presence inside the NP-AAO. References L.P.H. Jeurgens, W.G. Sloof, F.D. Tichelaar, C.G. Borsboom, E.J. Mittemeijer, Determination of thickness and composition of aluminium-oxide overlayers on aluminium substrates, Appl. Surf. Sci. 144, 11 (1999)J. Evertsson et al, The thickness of native oxides on aluminum alloys and single crystals, Appl.Surf. Sci. 349, 826 (2015)F. Bertram et al, In situ anodization of aluminum surfaces studied by x-ray reflectivity and electrochemical impedance spectroscopy, J. Appl. Phys. 116, 034902, (2014)W. Lee. and S.-J. Park, Porous Anodic Aluminum Oxide: Anodization and Templated Synthesis of Functional Nanostructures, Chem. Rev. 114, 7487 (2014)Nikolay A. Vinogradov, Gary S. Harlow, Francesco Carlà, Jonas Evertsson, Lisa Rullik, Weronica Linpé, Roberto Felici, and Edvin Lundgren, Observation of Pore Growth and Self-Organization in Anodic Alumina by Time-Resolved X-ray Scattering, ACS Applied Nano Materials, Article ASAP

Phosphonate monolayers on InAsSb and GaSb surfaces for mid-IR plasmonics

Stable functionalization of semiconductor surfaces is a prerequisite for all-semiconductor mid-IR biophotonics. This work demonstrates the adsorption of phosphonic acids on oxygen plasma activated GaSb and InAsSb surfaces. X-ray photoelectron spectroscopy shows that oxygen plasma treatment, used to activate the investigated III–V surfaces, increases the surface oxide layer beyond its native oxide thickness. Phosphonates with different terminal groups, either ethylene glycol or fluorinate carbon terminated groups, allow to modify the hydrophobicity of the surfaces and to protect the surfaces by an anti-fouling cover layer. Infrared spectroscopy indicates partial deprotonation of the phosphonic acid and thus phosphonate bonding to the surfaces. Adsorption of phosphonates on an all-semiconductor mid-IR plasmonic grating structure is detected by a shift and by a shape modulation of the plasmonic resonance peak. Compared with molecule adsorption on flat mirror-like layers a ten-fold signal enhancement is found. The adsorbed molecules are stable upon baking at 120° C, ultrasonic cleaning with organic solvents and storage for several weeks at ambient conditions. These results show that stable functionalization of InAsSb and GaSb surfaces by phosphonate monolayers is possible. All-semiconductor enhanced plasmonic sensing in the mid-IR was demonstrated.

Stencil lithography of superconducting contacts on MBE-grown topological insulator thin films

Topological insulator (Bi0.06Sb0.94)2Te3 thin films grown by molecular beam epitaxy have been capped in-situ with a 2nm Al film to conserve the pristine topological surface states. Subsequently, a shadow mask - structured by means of focus ion beam - was in-situ placed underneath the sample to deposit a thick layer of Al on well-defined microscopically small areas. The 2nm thin Al layer fully oxidizes after exposure to air and in this way protects the TI surface from degradation. The thick Al layer remains metallic underneath a 3–4nm thick native oxide layer and therefore serves as (super-) conducting contacts. Superconductor-Topological Insulator-Superconductor junctions with lateral dimensions in the nm range have then been fabricated via an alternative stencil lithography technique. Despite the in-situ deposition, transport measurements and transmission electron microscope analysis indicate a low transparency, due to an intermixed region at the interface between topological insulator thin film and metallic Al.

Size Dependence of Nanoscale Wear of Silicon Carbide.

Nanoscale, single-asperity wear of single-crystal silicon carbide (sc-SiC) and nanocrystalline silicon carbide (nc-SiC) is investigated using single-crystal diamond nanoindenter tips and nanocrystalline diamond atomic force microscopy (AFM) tips under dry conditions, and the wear behavior is compared to that of single-crystal silicon with both thin and thick native oxide layers. We discovered a transition in the relative wear resistance of the SiC samples compared to that of Si as a function of contact size. With larger nanoindenter tips (tip radius ≈ 370 nm), the wear resistances of both sc-SiC and nc-SiC are higher than that of Si. This result is expected from the Archard's equation because SiC is harder than Si. However, with the smaller AFM tips (tip radius ≈ 20 nm), the wear resistances of sc-SiC and nc-SiC are lower than that of Si, despite the fact that the contact pressures are comparable to those applied with the nanoindenter tips, and the plastic zones are well-developed in both sets of wear experiments. We attribute the decrease in the relative wear resistance of SiC compared to that of Si to a transition from a wear regime dominated by the materials' resistance to plastic deformation (i.e., hardness) to a regime dominated by the materials' resistance to interfacial shear. This conclusion is supported by our AFM studies of wearless friction, which reveal that the interfacial shear strength of SiC is higher than that of Si. The contributions of surface roughness and surface chemistry to differences in interfacial shear strength are also discussed.

Recyclability assessment of Al 7075 chips produced by cold comminution and consolidation using spark plasma sintering

A solid-state recycling route for processing aluminium alloy 7075 chips is proposed. This process involves pulverizing the chips using an industrial scale comminution line to produce a broad particle size distribution of granulated products and powders. Upon sieving, the −140 mesh fraction of the pulverised material was consolidated by spark plasma sintering into fully dense compacts, i.e. >99%. The properties of the pulverised powders and compacts were compared to commercially available aluminium alloy 7075 powder used in industrial press-sinter applications. The comminution line produced powder particles which possessed a 7·9 ± 0·6-nm thick native oxide layer with adhering exogenous contaminants, and aluminium oxide particles dispersed throughout the powder. The presence of the oxide layer and exogenous contaminants was found to be deleterious to the powder consolidation and mechanical properties. A flexural strength and strain and hardness of 94·7 ± 7·2 MPa, 2·1 ± 0·3% and 69·3 ± 5·2 HV were obtained, respectively.