Focused Ion Beam Cross-sectioning Research Articles

Surface degradation mechanisms of cemented carbide drill buttons in iron ore rock drilling

The wear behavior of cemented carbide rock drill buttons is influenced by many factors, which include the composition and microstructure of the cemented carbide material, the nature of the rock material, and the conditions of the rock drilling operation. Depending on the type of rock and on the drilling procedure used, the cemented carbide is exposed to substantially differing mechanical and thermal conditions. In the present study, the surface degradation and wear mechanisms of cemented carbide drill buttons exposed to iron ore rock drilling have been characterized based on a combination of high resolution scanning electron microscopy (SEM), focused ion beam cross-sectioning (FIB), energy-dispersive X-ray spectroscopy (EDS) and electron back scatter diffraction (EBSD).The results show a significant difference in surface degradation and wear between the front and peripheral buttons of the drill bits. While the front buttons display a relatively smooth worn surface with shallow surface craters the peripheral buttons display a reptile skin pattern, i.e. plateaus, 200–300µm in diameter, separated by valleys, typically 40–50µm wide and 15–30µm deep, The reptile skin pattern is obtained in regions where the peripheral buttons are in sliding contact against the drill hole walls and exposed to high surface temperatures caused by the frictional heating. The results indicate that the reptile skin pattern is related to friction induced thermal stresses rather than mechanical contact stresses, i.e. the reptile skin pattern is formed due to thermal fatigue, rather than mechanical fatigue, caused by the cyclic frictional heating generated at the cemented carbide button/iron ore interface.

Measurements of the Phase and Stress Evolution during Initial Lithiation of Sn Electrodes

We have measured the phase kinetics and stress evolution during potentiostatic lithiation of Sn thin film layers to form the first lithiated phase (Li2Sn5). Real-time measurements of the wafer curvature are used to determine the stress evolution during lithiation; the corresponding layer structure is determined from the measured charge and confirmed with cross-sectional focused-ion beam (FIB) images and X-ray diffraction (XRD). Prior to new phase formation, a 20-hour conditioning step is used to form a solid-electrolyte interphase (SEI) layer. The Stress in the SEI and Sn layers are determined by stress measurements carried on different thicknesses of Sn film. When the potential is lowered to initiate Li2Sn5 formation, both the electrochemical current and stress exhibit a complex transient behavior as Li2Sn5 phase nucleates and grows. The stress evolution in this regime is attributed to rate-dependent plasticity in the Sn layer. At longer times, as the new phase propagates into Sn, a steady-state is reached that enables us to determine the stress in the Li2Sn5 layer. The phase evolution in the steady-state regime is analyzed with a kinetic model that enables us to estimate the Li diffusivity in Li2Sn5 and the reaction rate coefficient controlling the phase transformation.

Root Cause Investigation of Lead-Free Solder Joint Interfacial Failures After Multiple Reflows

Solder joint interconnects in three-dimensional (3D) packages with package stacking configurations typically must undergo multiple reflow cycles during the assembly process. In this work, interfacial open joint failures between the bulk solder and the intermetallic compound (IMC) layer were found in Sn–Ag–Cu (SAC) solder joints connecting a small package to a large package after multiple reflow reliability tests. Systematic progressive 3D x-ray computed tomography experiments were performed on both incoming and assembled parts to reveal the initiation and evolution of the open failures in the same solder joints before and after the reliability tests. Characterization studies, including focused ion beam cross-sections, scanning electron microscopy, and energy-dispersive x-ray spectroscopy, were conducted to determine the correlation between IMC phase transformation and failure initiation in the solder joints. A comprehensive failure mechanism, along with solution paths for the solder joint interfacial failures after multiple reflow cycles, is discussed in detail.

Structural and Quantitative Investigation of Perovskite Pore Filling in Mesoporous Metal Oxides

In recent years, hybrid organic–inorganic perovskite light absorbers have attracted much attention in the field of solar cells due to their optoelectronic characteristics that enable high power conversion efficiencies. Perovskite-based solar cells’ efficiency has increased dramatically from 3.8% to more than 20% in just a few years, making them a promising low-cost alternative for photovoltaic applications. The deposition of perovskite into a mesoporous metal oxide is an influential factor affecting solar cell performance. Full coverage and pore filling into the porous metal oxide are important issues in the fabrication of highly-efficient mesoporous perovskite solar cells. In this work, we carry out a structural and quantitative investigation of CH3NH3PbI3 pore filling deposited via sequential two-step deposition into two different mesoporous metal oxides—TiO2 and Al2O3. We avoid using a hole conductor in the perovskite solar cells studied in this work to eliminate undesirable end results. Filling oxide pores with perovskite was characterized by Energy Dispersive X-ray Spectroscopy (EDS) in Transmission Electron Microscopy (TEM) on cross-sectional focused ion beam (FIB) lamellae. Complete pore filling of CH3NH3PbI3 perovskite into the metal oxide pores was observed down to X-depth, showing the presence of Pb and I inside the pores. The observations reported in this work are particularly important for mesoporous Al2O3 perovskite solar cells, as pore filling is essential for the operation of this solar cell structure. This work presents structural and quantitative proof of complete pore filling into mesoporous perovskite-based solar cells, substantiating their high power conversion efficiency.

Development of yttrium-containing self-passivating tungsten alloys for future fusion power plants

Tungsten is a prime material candidate for the first wall of a future fusion reactor. In the case of a loss-of-coolant accident (LOCA) wall temperatures of about 1450 K could be reached lasting about 30–60 days due to nuclear decay heat. In the worst case scenario combining LOCA with air ingress, the formation and release of highly volatile and radioactive tungsten trioxide (WO3) into the environment can occur. Smart self-passivating tungsten alloys preventing the formation of WO3 can be a way to mitigate this release.In this contribution we present the studies of a new yttrium-containing W-Cr-Y alloys. The extent up to which yttrium acts as an active element improving the adherence and stability of the protective Cr2O3 layer formed during oxidation is assessed. The approach is similar to the one taken for high-temperature steels where active elements stabilize the oxide layers at a substantially reduced thickness by changing the oxygen diffusion and improving the adherence of the protective oxide layer by e.g. avoiding of pores. Further, simulations on mobilized material for the case of a LOCA are developed. In addition, the loss of alloying elements during normal operation of a reactor is estimated. This is done by modelling a thermally activated diffusion, using a diffusion coefficient which is extrapolated from experimental data at higher values.The oxidation behaviour of magnetron sputtered and therefore alloyed at the atomic level W-Cr-Y alloys is tested in a thermo-gravimetric facility. The isothermal oxidations are performed in a gas mixture, containing 20 kPa oxygen and 80 kPa argon under ambient pressure at temperatures of 1273 K and 1473 K, respectively. Experiments with W-Cr-Y show a parabolic oxidation rate of kp=3·10−6mg2cm−4s−1 which is more than five orders of magnitude lower than that of pure tungsten at 1273 K. Investigations using X-ray diffraction analysis and focused ion beam cross-sections in combination with scanning electron microscopy and energy dispersive X-ray spectroscopy are conducted. A protective Cr2O3 layer is detected on the surface with a thickness between 100 and 300 nm.

Impact of diamond seeding on the microstructural properties and thermal stability of GaN-on-diamond wafers for high-power electronic devices

The impact of seeding of the diamond growth on the microstructural properties of GaN-on-diamond wafers was studied using in situ focused ion beam cross-sectioning and scanning electron microscopy imaging. Microstructural studies revealed that the seeding conditions are a critical parameter to obtain an optimal material, allowing the manufacture of GaN-on-diamond wafers with no microscopic defects and with structural stability under thermal annealing at 825°C. The use of the right seeding conditions also results in homogeneous thermal properties across four inch GaN-on-diamond wafers, which is of critical importance for their use for ultra-high power microwave electronic devices.

Nanowire formation is preceded by nanotube growth in templated electrodeposition of cobalt hybrid nanostructures

Cobalt fluted nanowires, novel nanostructures with a diameter of 200 nm consisting of a solid nanowire base and a thin, nanotubular flute shape, were grown in track-etched polycarbonate membranes via templated electrodeposition. The structures were characterized electrochemically via cyclic voltammetry, chronoamperometry, and charge stripping, and structurally via scanning electron microscopy, transmission electron microscopy, and focused ion beam cross-sectioning. Electrochemical and structural analysis reveals details of their deposition kinetics, structure, and morphology, and indicate possible mechanisms for their formation and control. These unique structures provide inspiration for an array of possible applications in electronics, photonics, and other fields.

Deep-groove nickel gratings for solar thermal absorbers

This paper presents measured and modelled optical absorptance and reflectance for deep-groove nickel nano-gratings in the 450–950 nm wavelength range. The structures have been fabricated using focused ion beam etching and characterised using Fourier spectroscopy and the field distributions on the gratings have been studied using finite difference time domain modelling. Realistic grating structures have been modelled based on focused ion beam cross sections and these results are in good agreement between measured and modelled results. The roles of surface plasmon polaritons and slot modes are highlighted in the strong broadband absorbance that can be achieved with these structures.

Effect of surface films on tribologically induced metallurgical transformations of steel in oil lubricated contacts

This study was initiated with the aim to assess the influence of boundary films on the tribologically induced metallurgical transformations and deformations of metals in lubricated contacts and their effect on wear. For this, the tribological behavior of a carbon steel contact was studied in two commercially available oils expected to form boundary films of quite different chemistry and structure. Surface analysis by AES and XPS revealed that under the investigated conditions the first oil led to the formation of an iron–zinc oxide surface film while the second to a calcium–carbon–oxygen rich film. The results show that, without influencing the coefficient of friction, the nature of the formed films significantly affected the metallurgical transformations (characterized by electron microscopy of focused ion beam cross sections) occurring in the near surface region of the metal and the corresponding wear response. The effect of boundary films on wear was attributed to their capability to influence the plastic flow of the nano-grained structures generated in the studied tribological contacts.

Coupling Surface Imaging, FIB, and Spectroscopies to Understand Silicon Anodes Lithiation and Ageing Mechanisms

To follow growing energy demand of new devices, batteries energy density needs to be improved. Silicon-based anode is a serious option since it offers a specific capacity almost ten times higher than carbonaceous materials. Even so silicon anodes suffer from a drastic capacity fading making it unusable after few cycles. A fine understanding of the lithiation mechanism of silicon could participate to the elaboration of more robust architectures. In this work1, composite silicon electrodes have been electrochemically cycled vs Li. After a transfer from a glove box to a ToF-SIMS by using an air-tight vessel, chemical mapping of the inside of silicon grains can be done by achieving an in-situFIB cut. The first lithiation has been extensively studied. By stopping lithiation at different stages it is possible to follow the evolution of the core-shell structure (ie LiSi alloy shell and pure Si core) as shown in fig.1. This method also permits to observe the Si particles, considering their position within the depth of the electrode. To complete this study, cross analysis with Auger spectrometer, SEM and TEM have been performed, allowing to quantify Li presence in the LixSi alloy and better understand the pure Si core morphology. At the end, a complete mechanism for silicon particles is proposed. In a following step, Si electrodes have been observed after 3, 5 and 10 cycles (limited capacity of 1000 mAh.g-1) in both lithiated and delithiated states. After three cycles the core-shell structure is still visible in lithiated state. After 10 cycles the presence of Si particles containing lithium at the end of the charge are observed and associated to particle getting out of the conductive grid and trapping lithium. This phenomena are detrimental to battery lifespan and participate in the failure of limited cycling strategies. (1) Bordes, A.; De Vito, E.; Haon, C.; Boulineau, A.; Montani, A.; Marcus, P., Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by Time-of-Flight Secondary Ion Mass Spectrometer Imaging Performed on an in Situ Focused Ion Beam Cross Section. Chem. Mater. 2016. Figure 1

Coating Ni particles to preserve their conductivity during sintering in oxidizing atmospheres

In this study, a new Ni electrode was introduced which was able to preserve its electrical conductivity during sintering in oxidizing atmosphere. The Ni electrode was manufactured from Ni nanoparticles, which were coated by Li salts, namely Li2CO3, LiOH, and LiF, with an innovative coating method. The coating process is schematically shown in the graphical abstract. After the confirmation of a successful coating on Ni particles by SEM (Scanning Electron Microcopy) imaging, the Ni particles with Li2CO3 coating demonstrated the greatest preservation in conductivity among the other salts. Thus they were selected for further investigation; the effect of Li2CO3 percentage, sintering program, and pO2 on electrode resistance were systematically studied. The results suggest that the coating can reduce the resistance by five orders of magnitude at oxidizing atmosphere (e.g. pO2=2×10−4atm). The reduced resistances were as low as 1Ω, suitable to be used as an electrode. SEM and FIB (Focused Ion Beam) cross section imaging were used to document the Ni oxidation, the sintering process of Ni particles, and the Li2CO3 decomposition. Finally, two mechanisms were speculated to be the reasons of the conductivity conservation and the importance of this invention in the reducing of oxygen vacancies in electroceramic devices was discussed.

Friction stir processing of A-286 stainless steel: Microstructural evolution during wear

The effect of friction stir processing (FSP) on mechanical and wear behavior was investigated for A-286 stainless steel, an Fe–Ni–Cr based austenitic, precipitation hardened alloy. The alloy was characterized in different processed conditions, namely as rolled (AR)+aged and FSP+aged. High frequency reciprocating sliding wear behavior and wear mechanisms were investigated at room temperature. The Vickers microhardness and wear rates were measured and compared for each processing condition. It was determined that along with increasing microhardness in the stir zone, FSP resulted in improved wear resistance. Specifically, the wear rate in the stir zone was reduced from 1×10−6 to 6×10−7mm3/Nm due to FSP. Mechanistic studies were conducted to determine the effect of FSP on the microstructural evolution during wear. Scanning electron microscopy revealed increased coarse abrasion with the AR+aged alloy as compared to much finer-scale microabrasion with the FSP+aged alloy. This resulted in smaller and less abrasive wear debris, and hence lower wear rate. Furthermore, cross-sectional focused ion beam microscopy studies inside the stir zone of the FSP+aged alloy determined that increased microhardness was due to FSP-induced microscopic grain refinement resulting in Hall–Petch strengthening, and the corresponding wear rate decrease was due to even finer wear-induced grain refinement. With both effects combined, the level of surface fatigue wear was suppressed resulting in reduction of the wear rate. In contrast, the absence of FSP-induced grain refinement in the AR+aged alloy resulted in decreasing hardness and increasing wear rate. Overall, FSP of A-286 stainless steel alloy resulted in a lower wear rate suggesting it is a viable surface engineering technique to target and mitigate site-specific wear.

Multiscale Investigation of Silicon Anode Li Insertion Mechanisms by Time-of-Flight Secondary Ion Mass Spectrometer Imaging Performed on an In Situ Focused Ion Beam Cross Section

Considering its specific capacity, silicon is one of the most promising materials to replace graphite in lithium ion batteries anodes. However, its rapid capacity fading prevents its use in current batteries. Understanding lithiation and degradation mechanisms of silicon is important for improving its cyclability. In this work a novel approach is developed by using a focused ion beam implemented in the analysis chamber of a state-of-the-art time of flight secondary ion mass spectrometer. Detailed mapping of elements distribution, including lithium, inside a silicon particle or in the entire depth of the electrode, can thus be performed. During the first lithiation, a core–shell mechanism is observed and its evolution upon electrochemical cycling was examined. This mechanism is observed for all particles in the electrode, independently of their position. Cross analysis with Auger spectroscopy allowed Li concentration in the entire shell to be quantified. Fast lithiation paths getting through the pure silic...

Kinetics of carbide formation in the molybdenum–tungsten coatings used in the ITER-like Wall

The kinetics of tungsten carbide formation was investigated for tungsten coatings on carbon fibre composite with a molybdenum interlayer as they are used in the ITER-like Wall in JET. The coatings were produced by combined magnetron sputtering and ion implantation. The investigation was performed by preparing focused ion beam cross sections from samples after heat treatment in argon atmosphere. Baking of the samples was done at temperatures of 1100 °C, 1200 °C, and 1350 °C for hold times between 30 min and 20 h. It was found that the data can be well described by a diffusional random walk with a thermally activated diffusion process. The activation energy was determined to be (3.34 ± 0.11) eV. Predictions for the isothermal lifetime of this coating system were computed from this information.

Influence of Indium Addition on Whisker Mitigation in Electroplated Tin Coatings on Copper Substrates

Among many factors that influence whisker nucleation and growth in electroplated tin, it is now well established that small additions of Pb leads to whisker mitigation. To date, a good non-toxic elemental alternative to Pb that would mitigate whiskers remains elusive. In this work, a 50–100 nm In electroplated layer was incorporated into a 1-μm-thick electroplated Sn on a pure Cu substrate. In order to permit diffusion of In into Sn, heat treatments (HTs) between 125°C and 160°C were performed. The diffusion profile of In was altered by varying the dwell times of the HT and by utilizing two variants of In layer deposition, namely, (1) electroplating In at the top of the Sn plating, and (2) by sandwiching the In plating between two Sn layers, each approximately 500 nm thick. Appropriate control samples of pure Sn were utilized to permit valid data on the influence of In on whisker mitigation. Indium additions reduced whisker growth by at least two orders of magnitude following the 160°C treatment, independent of the location of the In layer. X-ray microanalysis of a focused ion beam cross section of the sandwich plating confirmed that In had indeed diffused into the Sn through the 160°C HT and was a likely reason for the mitigation of Sn whiskers.

Femtosecond laser surface ablation of polymethyl-methacrylate with position control through z-scan

Spatial resolution of laser micromachining of polymers can be improved with the use of femtosecond laser pulses. Due to the short interaction time, thermal effects are significantly reduced. Additionally, the non-linear character of the interaction of ultrashort laser pulses with transparent materials allows the modification inside their bulk also. However, this creates the challenge to accurately focus the laser beam in the surface when only surface ablation is required. Thus, this work presents a study of the laser ablation of a transparent polymer at different pulse energies and focusing positions controlled through z-scan transmittance measurements. Experiments were performed using an Yb:KYW laser with 450 fs pulses and 1027 nm wavelength. Morphological analysis of the polymer surface after irradiation was performed using scanning electron microscopy. Similar ablation craters were found for a range of sample positions around the beam waist. However, focused ion beam cross-sections of the craters unveil significant inner modifications under most of the focusing conditions leading to surface ablation. Hence, surface ablation without damaging the bulk material only occurs at critical positions where the beam waist is located slightly outside the sample. In situ monitoring of the sample position can be made through transmittance measurements.

Reproducing automotive engine scuffing using a lubricated reciprocating contact

The frequency and severity of scuffing in automotive engines has the potential to increase due to new low-viscosity lubricants for fuel efficiency and increased cylinder power output. This work is to understand the fundamental causes and events resulting in piston ring and liner scuffing. A TE-77 high frequency reciprocating tribometer was used with a synthetic PAO base oil (4cSt) to reciprocate a 52100 G5 barrel against a ground pearlitic Grade 250 grey cast iron.Samples were run-in at 50N and 10Hz prior to a temperature ramp to 150°C followed by a discreet load ramp to 1kN (0.49GPa). The tests were terminated when a sharp increase in the average friction force was observed indicating that scuffing had occurred. 3D optical profilometry showed that the scuffed cast iron surface consisted of smeared platelets and craters of 35μm depth. SEM and EDX analyses suggested adhesive transfer of cast iron material to the counter-surface was occurring by failure along lamellar graphite interfaces. Tests were repeated using instantaneous high-speed friction data and indicated that micro-scuffing initiated at a load of 620N. Focused ion beam cross-sections of the mildly scuffed surface confirmed the mechanism of sub-surface crack initiation occurring along lamellar graphite boundaries.

Obtaining accurate cross-section images of supported polymeric and inorganic membrane structures

Detailed and accurate information about the morphology of supported membranes structures is important for the analysis of their performance and fabrication. In this work it is shown that Focused Ion Beam (FIB) nano-machining can be used to provide such information for polymeric and inorganic membrane layers and composite structures containing those layers. Single-sided cuts are used for scanning electron microscopy (SEM) observations; ~100nm thin slabs obtained with two-sided cuts are used for transmission electron microscopy (TEM) imaging. Details of the FIB milling are provided in addition to examples of cross-sections of polymer, hybrid and inorganic structures. It is shown that the quality of cross-section information, obtained with FIB, is superior to that obtained with transmission electron microscopy of micro-tome sections. In addition it is shown how Energy Dispersive X-ray Spectroscopy (EDS) mapping of FIB cross-sections provides information about the relation between chemical composition and membrane structure that cannot be obtained otherwise.

Depth profiling of the modification induced by high-flux deuterium plasma in tungsten and tungsten–tantalum alloys

The present work reports the results of an experimental study of the depth distribution and fluence dependence of deuterium plasma-induced material modification of tungsten and tungsten–tantalum alloys. Plasma-induced damage was created by exposure to high-flux deuterium plasma in the plasma generator Pilot-PSI, followed by the degassing and subsequent decoration of created defects with deuterium by another plasma exposure. The depth distribution of deuterium from the decorating exposure reflects the distribution of plasma-induced defects. Depth profiling of this decorating deuterium, was performed by nuclear reaction analysis. It was found that plasma-induced material modification, which manifested itself as an increase of the deuterium concentration in the samples pre-exposed with high-flux plasma in comparison to the samples without such pre-exposure extends down to more than 5 µm from the surface. This increase features a tendency to saturation with increasing fluence of the damaging high-flux plasma. Over the entire probing range, with the exception of the narrow surface region and the deep region beyond 5 µm, the deuterium content is lower in pre-exposed W–Ta than in similarly pre-exposed W. Sub-surface features formed as a result of high-flux plasma exposure were studied with the help of focused ion beam cross-sectioning. W was found to contain plasma-induced cavities down to much larger depth than W–Ta.

Deuterium retention in tungsten films after different heat treatments

Tungsten films deposited by magnetron sputtering on polycrystalline tungsten substrates were used as a model system to study the influence of the film microstructure on deuterium retention behavior. Different microstructures were produced by annealing the films up to recrystallization temperature and the corresponding structural changes were investigated by scanning electron microscopy combined with focused ion beam (FIB) cross sectioning. The influence of the induced structural changes on D retention was investigated by both nuclear reaction analysis and temperature-programmed desorption. D concentration in the investigated W films is higher than in polycrystalline bulk tungsten by a factor of 3. D retention in the films decreases as a function of annealing temperature. After annealing at 2000K, FIB cross-section images reveal that cavities appeared at the grain boundaries within the film and at the initial interface between the W film and W substrate. This new microstructure strongly affects the D depth profile and leads to the increase of D retention. Although a further increase of the holding time at 2000K or an increase of the annealing temperature to 2150K lead to the reduction of the retained D amount, the D concentration in the recrystallized W films cannot be reduced to a level as low as that of bulk W recrystallized at 2000K for 30min.