An investigation on the microstructure and oxidation behavior of laser remelted air plasma sprayed thermal barrier coatings

NiCrAlY bond-coat was coated on Inconel 718 substrate by air plasma spraying (APS) followed by APS ZrO₂-8 wt.%Y₂O₃ as top-coat. Using CO₂ laser of different energy densities, ceramic top-coat surface was remelted. Laser remelting with high energy density (4 J/mm²) produced a dense microstructure over the whole thickness of top-coat, while low energy density (0.67 J/mm²) laser remelting produced a ~ 50 μm thick dense layer on the top-coat surface. It was found that the volume fraction of monoclinic phase decreased from 9% in as-sprayed coating to 4% and 3% after laser remelting with high and low energy density respectively. After isothermal oxidation at 1200 °C for 200 h, the thickness of oxide layer (TGO) in the sample produced by low energy density laser remelting was ~ 5.6 μm, which was thinner than that of oxide layer in as-sprayed (~ 7.6 μm) and high energy density laser remelted (~ 7.5 μm) samples. A uniform and continuous oxide layer was found to develop on the bond-coat surface after low energy density laser remelting. Thicker oxide layer containing Cr₂O₃, NiO and spinel oxides was observed in both as-sprayed and high energy density laser remelted coatings. After cyclic oxidation at 1200 °C for 240 h, the weight gain per unit area of as-sprayed coating was similar to that of high energy density laser remelted coating while a significantly smaller weight gain was found in low energy density laser remelted coating.     

Efficient first-order resonant near-infrared quantum cutting in β-NaYF4:Ho,Yb

We demonstrated an efficient two-photon near-infrared (NIR) quantum cutting (QC) in Ho³⁺-Yb³⁺ co-doped hexagonal β-NaYF₄, which could efficiently convert an incident high-energy photon in the wavelength region of 300-550 nm into two NIR photons. Underlying mechanism for the two-photon NIR-QC process is analyzed in terms of static and dynamic photoemission and monitored excitation spectra. It is found that NIR-QC can occur through two possible energy transfer (ET) approaches: (i) the excited Ho³⁺:⁵F₃ state may simultaneously excite two Yb³⁺ neighbors via a cooperative ET process, and (ii) the NIR-QC can be feasibly induced by a first Ho³⁺(⁵S₂,⁵F₄) + Yb³⁺(²F_7/2) → Ho³⁺(⁵I₆) + Yb³⁺(²F_5/2) resonant ET process and a sequential ⁵I₆ → ⁵I₈ transition of Ho³⁺. This novel NaYF₄:Ho³⁺,Yb³⁺ NIR-QC phosphor, may explore a new approach to maximize the performance of solar cells.     

Effect of laser irradiation on failure mechanism of TiCp reinforced titanium composite coating produced by laser cladding

Laser cladding is an effective technique to coat a metallic substrate with a layer of a different nature. It has been widely reported that the most important combined parameters controlling the quality of the coating are the specific energy (E) and the powder density (Ψ). In the present work, clad deposits of Ti6Al4V + 60 wt.% TiC were prepared on a Ti6Al4V substrate using an optimum combination of E_c = 24 J/mm² and ψ_c = 3 mg/mm². These experiments were performed using a laser power of 400 and 600 W, in order to study the effect of laser power on the properties of the clad. The microstructure, phase composition and nanohardness of the coatings were investigated by optical microscopy, scanning electron microscopy and X-ray diffraction. During laser processing, TiC can be partially converted to TiC_X (X = 0.5) due mainly to the TiC dissolution into the laser-generated melting pool and subsequent precipitation during cooling. It was observed that the lower laser power limit reduces primary TiC dissolution but it also promotes secondary carbide alignment at the interface. On the other hand, the damage mechanism induced by high laser power is dominated by primary TiC particle cracking by the high stress concentration at the particle–matrix interface followed by ductile failure of the matrix. It is also remarkable that irradiance affects the TiC/TiC_x ratio despite E_c and ψ_c are fixed and it determines hardness distribution inside the coating.     

High-energy heavy ion beam annealing effect on ion beam synthesis of silicon carbide

Silicon carbide (SiC) is a superior material potentially replacing conventional silicon for high-power and high-frequency microelectronic applications. Ion beam synthesis (IBS) is a novel technique to produce large-area, high-quality and ready-to-use SiC crystals. The technique uses high-fluence carbon ion implantation in silicon wafers at elevated temperatures, followed by high-energy heavy ion beam annealing. This work focuses on studying effects from the ion beam annealing on crystallization of SiC from implanted carbon and matrix silicon. In the ion beam annealing experiments, heavy ion beams of iodine and xenon, the neighbors in the periodic table, with different energies to different fluences, I ions at 10, 20, and 30 MeV with 1-5 × 10¹² ions/cm², while Xe ions at 4 MeV with 5 × 10¹³ and 1 × 10¹⁴ ions/cm², bombarded C-ion in implanted Si at elevated temperatures. X-ray diffraction, Raman scattering, infrared spectroscopy were used to characterize the formation of SiC. Non-Rutherford backscattering and Rutherford backscattering spectrometry were used to analyze changes in the carbon depth profiles. The results from this study were compared with those previously reported in similar studies. The comparison showed that ion beam annealing could indeed induce crystallization of SiC, mainly depending on the single ion energy but not on the deposited areal density of the ion beam energy (the product of the ion energy and the fluence). The results demonstrate from an aspect that the electronic stopping plays the key role in the annealing.     

Synthesis and electrochemical properties of porous LiV3O8 as cathode materials for lithium-ion batteries

In this paper, we report on the synthesis of porous LiV₃O₈ by using a tartaric acid-assisted sol-gel process and their enhanced electrochemical properties for reversible lithium storage. The crystal structure, morphology and pore texture of the as-synthesized samples are characterized by means of XRD, SEM, TEM/HRTEM and N₂ adsorption/desorption measurements. The results show that the tartaric acid plays a pore-making function and the calcination temperature is an important influential factor to the pore texture. In particular, the porous LiV₃O₈ calcined at 300 °C (LiV₃O₈-300) exhibits hierarchical porous structure with high surface area of 152.4 m² g⁻¹. The electrochemical performance of the as-prepared porous LiV₃O₈ as cathode materials for lithium ion batteries is investigated by galvanostatic charge-discharge cycling and electrochemical impedance spectroscopy. The porous LiV₃O₈-300 displays a maximum discharge capacity of 320 mAh g⁻¹ and remains 96.3% of its initial discharge capacity after 50 charge/discharge cycles at the current density of 40 mA g⁻¹ due to the enhanced charge transfer kinetics with a low apparent activity energy of 35.2 kJ mol⁻¹, suggesting its promising application as the cathode material of Li-ion batteries.     

Effects of Ce concentration, beam voltage and current on the cathodoluminescence intensity of SiO2:Pr-Ce nanophosphor

SiO₂:Pr³⁺-Ce³⁺ phosphor powders were successfully prepared using a sol-gel process. The concentration of Pr³⁺ was fixed at 0.2 mol% while that of Ce³⁺ was varied in the range of 0.2-2 mol%. High resolution transmission electron microscopy (HRTEM) clearly showed nanoclusters of Pr and Ce present in the amorphous SiO₂ matrix, field emission scanning electron microscopy (FE-SEM) indicated that SiO₂ clustered nanoparticles from 20 to 120 nm were obtained. Si-O-Si asymmetric stretching was measured with Fourier transform-IR (FT-IR) spectroscopy and it was also realized that this band increased with incorporation of the activator ions into the SiO₂ matrix. The broad blue emission from the Ce³⁺ ions attributed to the 5d¹-4f¹ transition was observed from the SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺ phosphor. This emission was slightly enhanced compared to that of the singly doped SiO₂:1 mol%Ce³⁺ phosphor. Further investigations were conducted where the CL intensity was measured at different beam voltages and currents from 1 to 5 kV and 8.5 to 30 μA, respectively, in order to study their effects on the CL intensity of SiO₂:0.2 mol% Pr³⁺-1 mol% Ce³⁺. The electron-beam dissociated the SiO₂ and as a result an oxygen-deficient surface dead or non-luminescent layer of SiO_x, where x < 2 on the surface, was formed.     

Embedded phase change material microinclusions for thermal control of surfaces

The performance and lifetime of sensors, microelectronic devices, mini air vehicle components and other systems can benefit from maintenance of a constant temperature profile or a local temperature that does not exceed a pre-selected, critical value during short-term transient loading events. In this work, the utility of surfaces featuring phase change materials (PCMs) encapsulated within micro-reservoirs was evaluated as a passive thermal management system for mitigation of transient temperature spikes. Patterned silicon substrates with 40 μm diameter, 10 μm deep trenches were prepared by reactive ion etching in an Ar/CF₄ plasma. The features were filled with an organic phase change material possessing a high latent heat and known to undergo melting at a target operating temperature of approximately 60 °C. An infrared microscope was used to produce temporally and spatially resolved temperature maps of the surface during heating. When a constant heat flux of approximately 200 W m^− 2 was used for uniform heating of the sample area from 50 to 75 °C the PCM encapsulated materials demonstrated an isothermal plateau period lasting 5–8 s near the PCM melting point. A similar plateau was also observed during cooling below the PCM melting point. From the isothermal time during heating the areal thermal energy storage density was estimated to be approximately 800 J m^− 2, close to that predicted by the calculated volume of PCM contained within the sample. The effects of PCM inclusions on surface temperature gradients during pulsed laser heating were also investigated. An infrared (980 nm) laser at a fixed power and repetition rate was focused into a 1 mm diameter beam on surfaces with and without PCM encapsulation for rapid heating (analogous to heating from resistive contacts in microelectronic circuits) of different duration from 0.5 to 5.0 s. The surface temperature stayed below the desired temperature limit for 100 mW laser pulses lasting up to 2 s in duration. Transient temperature profiles of Si/PCM surfaces showed that micro-scale volumes of embedded phase change material stored sufficient quantities of heat to stabilize and otherwise control surface temperatures for potentially useful periods of time in selected applications.     

Effect of high current pulsed electron beam treatment on surface microstructure and wear and corrosion resistance of an AZ91HP magnesium alloy

This paper reports an analysis of the microstructure and property modifications on a commercial Mg alloy AZ91HP treated by using High Current Pulsed Electron Beam (HCPEB). It is shown that, a thin layer consisting of nanograined MgO formed on the top surface after HCPEB (electron energy ∼ 30 keV, pulse duration 1 μs, energy density ∼ 3 J/cm²) treatment, below which is a melted layer with depth of about 8-10 μm. The heat affected zone (HAZ) underneath the melted layer and stress wave affected zone contains many stress induced deformation marks. Meantime, a nearly complete dissolution of the intermetallic phase Mg₁₇Al₁₂ in the melted layer is observed, leading to the formation of a super-saturated solid solution on the surface. This is due to the solute trapping effect occurred during the fast solidification process. As a result, the wear and corrosion resistance of the alloy were significantly improved as shown by sliding friction, wear and immersion tests.     

Modification of hard alloy by the action of high power ion beams

Phase composition, sizes of coherent scattering regions, hardness and friction coefficient of WC-TiC-Co hard alloy and “(Ti,Cr)N coating-on-alloy” system treated by high power ion beams (HPIB) are studied. HPIB treatment was carried out with power densities from 4 to 16 J/cm². The effect of HPIB pretreatment (4 J/cm²) before deposition procedure was studied. HPIB action with energy densities of 8-16 J/cm² on hard alloy results in partial fusion of tungsten and titanium carbides. The hardness of surface layer increases by two times and wear resistance improves. HPIB action on “coating-on-alloy” system leads to coarsening of (Ti,Cr)N crystallites, partial decomposition of (Ti,Cr)N and precipitation of chromium phase. Preliminary HPIB treatment (4 J/cm²) followed by HPIB impact (16 J/cm²) provides best effect on wear-resistance of (Ti,Cr)N solid solution on hard alloy.     

AC corrosion - Part 1: Effects on overpotentials of anodic and cathodic processes

AC-induced corrosion is a controversial subject and many aspects of it need to be clarified, first and foremost, the mechanism and relationship between AC density and corrosion rate. This paper (Part 1) presents and discusses the effects of AC interference on kinetics parameters; the effects on corrosion rate and corrosion mechanism will be discussed in Part 2. Polarisation curves were obtained in different solutions (soil-simulating solution, 35 g L⁻¹ NaCl, 1 M FeSO₄, 1 M CuSO₄ and 1 M ZnSO₄) on different metallic materials (carbon steel, galvanised steel, zinc and copper) in the presence of AC interference (30-1000 A/m²).     

Pyrometry in laser surface treatment

The pyrometers specially developed for laser machining were applied to analyse temperature fields in laser cladding. It is shown that 2D temperature mapping is useful to optimise the cladding parameters: zone of powder injection in relation to laser beam, temperature gradients and their evolution versus cladding parameters. Multi-wavelength pyrometer was applied to restore the value of true temperature that is useful to control melting/solidification when complex powder blends are used and when it is necessary to minimise thermal decomposition of certain compounds. The developed multi-wavelength pyrometer and the applied notch filters are appropriate instruments to measure the evolution of surface temperature produced by Nd:YAG laser pulses of millisecond duration. The variations of several characteristics of the thermal cycle, such as the maximum peak temperature, T_max, the instant when melting starts, t_m, the melt lifetime, τ_lt, the duration of the solidification stage, τ_s, with various energy inputs (in the range 10-33 J) and pulse durations (10-20 ms), have been determined for rectangular laser pulses. By appropriate modification of the laser pulse shape (keeping the same energy input and pulse duration), it is possible to realise rather different temperature profiles to vary the melt lifetime and the instant when melting starts. In order to minimise surface temperature variation, to minimise thermal decomposition of certain melt compounds and to increase the melt lifetime, it is necessary to apply higher energy density flux at the beginning of the laser pulse. To obtain a higher peak of the surface temperature, for the given energy input and pulse duration, it is necessary to apply higher energy density flux at the pulse end. This will minimise the melt lifetime as well. In general, it is possible to impose the instant when melt cooling starts and thus to realise an intensive melt cooling during laser irradiation. The above results correspond to the action of laser pulses in the millisecond range with relatively low energy density flux (4 · 10⁸-10⁹ W m^− 2) on metallic materials, whose thickness is larger than the heat affected zone (i.e. semi-infinite body from the heat transfer point of view).     

On time-of-flight ion energy deposition into a metal target by high-intensity pulsed ion beam generated in bipolar-pulse mode

The energy deposition of high-intensity pulsed ion beam (HIPIB) into a titanium target was studied in TEMP-6 apparatus of bipolar-pulse mode using a self-magnetic field magnetically insulated ion diode (MID), where anode plasma was pre-generated by a first negative voltage and then mixed carbon ions and proton beam was extracted during the positive stage of the bipolar pulse. According with the time-of-flight (TOF) of ions, C⁺ arriving at the target 14 cm downstream from the MID was delayed by 55 ns relative to H⁺ at a peak accelerating voltage of 250 kV and the ion energy spectrum varied greatly, starting with a Gaussian profile at exit of MID and arriving with a multi-energy complex distribution. The TOF ion energy deposition of HIPIB showed that the energy deposition proceeded firstly in a deeper depth delivered by H⁺ and then moved towards a top layer dominated by C⁺. It is found that, the contribution of H⁺ to the energy deposition is negligible at the beam composition of 70%C⁺ and 30%H⁺. As a result, the gradient of energy deposition profile in target is negative by C⁺ deposition through the whole pulse. This unique feature of HIPIB energy deposition can lead to different thermal and dynamic effects as compared to previous studies of H⁺-abundant HIPIB, electron or laser beam, especially limiting subsurface heating that is concerned as a major cause of droplet ejection and surface cratering and waviness formation.     

Selective Laser Melting of in-situ TiC/Ti5Si3 composites with novel reinforcement architecture and elevated performance

A novel Selective Laser Melting (SLM) process was applied to prepare bulk-form TiC/Ti₅Si₃ in-situ composites starting from Ti/SiC powder system. The influence of the applied laser energy density on densification, microstructure, and mechanical performance of SLM-processed composite parts was studied. It showed that the uniformly dispersed TiC reinforcing phase having a unique network distribution and a submicron-scale dendritic morphology was formed as a laser energy density of 0.4 kJ/m was properly settled. The 96.9% dense SLM-processed TiC/Ti₅Si₃ composites had a high microhardness of 980.3HV_0.2, showing more than a 3-fold increase upon that of the unreinforced Ti part. The dry sliding wear tests revealed that the TiC/Ti₅Si₃ composites possessed a considerably low friction coefficient of 0.2 and a reduced wear rate of 1.42 × 10^− 4 mm³/Nm. The scanning electron microscope (SEM) characterization of the worn surface morphology indicated that the high wear resistance was due to the formation of adherent and strain-hardened tribolayer. The densification rate, microhardness, and wear performance generally decreased at a higher laser energy density of 0.8 kJ/m, due to the formation of thermal cracks and the significant coarsening of TiC dendritic reinforcing phase.     

Classification of laser shock forming within the field of high speed forming processes

TEA-CO₂-laser induced shock waves are used to form metal foils, such as aluminum or copper. The process utilizes an initiated plasma shock wave on the target surface, which leads to forming of the sheet. Several pulses can be applied at one point in order to achieve a high forming degree without increasing the energy density beyond the ablation limit. During the process, typical pressure peaks up to 15 MPa can be achieved. In order to classify the process in the framework of high speed forming processes, the strain rates as well as the temporal varying deformation velocity due to different materials have been identified on the basis of a bending process and a theoretical model for deformation velocity is proposed.     

Model based optimization criteria for the generation of deep compressive residual stress fields in high elastic limit metallic alloys by ns-laser shock processing

Laser Shock Processing (LSP) is based on the application of a high intensity pulsed Laser beam (I > 1 GW/cm²; τ < 50 ns) on a metallic target forcing a sudden vaporization of its surface into a high temperature and density plasma that immediately develops inducing a shock wave propagating into the material.The main acknowledged advantages of LSP consist on its capability of inducing a relatively deep compression residual stresses field into metallic alloy pieces allowing an improved mechanical behavior, explicitly, the life improvement of the treated specimens against wear, crack growth and stress corrosion cracking. Due to these specific advantages, Laser Shock Processing is considered as a competitive alternative technology to classical treatments for improving fatigue, corrosion cracking and wear resistance of metallic materials, and is being developed as a practical process amenable to production technology.In this paper, a model based systematization of process optimization criteria and a practical assessment on the real possibilities of the technique is presented along with practical results at laboratory scale on the application of LSP to characteristic high elastic limit metallic alloys, showing the induced residual stresses fields and the corresponding results on mechanical properties improvement induced by the treatment. The homogeneity of the residual stress fields distribution following the laser treatment spatial density will be specially analyzed.     

Photoluminescence and energy transfer of Ce and Tb doped oxyfluoride aluminosilicate glasses

The transmission and photoluminescence (PL) properties of Ce³⁺ or Tb³⁺ doped and Tb³⁺/Ce³⁺ codoped oxyfluoride aluminosilicate glasses were reported. The X-ray diffraction (XRD) and differential scanning calorimetry (DSC) were applied to confirm the structure and thermal stability of samples. PL spectra revealed a bright and broad violet-blue emission derived from Ce³⁺ [5d (²D) → ²F_5/2,7/2] and an intense sharp green emission (543 nm) derived from Tb³⁺ (⁵D₄ → ⁷F₅) in the Ce³⁺ and Tb³⁺ doped glasses, respectively. Concentration quenching is not observed even the mole ratio of Tb³⁺ is up to 8% in Tb³⁺ doped glass. This indicates that the as-made host glass provides a good distribution of Tb³⁺ activators in glass matrix. For Tb³⁺/Ce³⁺ codoped glasses, a strong green emission corresponding to Tb³⁺ (⁵D₄ → ⁷F₅) and an energy transfer phenomenon from Ce³⁺ to Tb³⁺ were observed upon excitation with an UV wavelength (289 nm). It was also observed that their PL intensity depends on the concentration of Ce³⁺ when the concentration of Tb³⁺ is fixed. The mechanism involved in the energy transfer between Ce³⁺ and Tb³⁺ was explained with an energy level diagram.     

Temperature changes of copper nanoparticle ink during flash light sintering

The copper nanoparticle ink was coated on polyimide substrates using a doctor blade method. The films thus formed were then sintered by flash light irradiation at room temperature under ambient conditions. The flash light energy was varied from 2 J/cm² to 12 J/cm². To measure the temperature change, a non-inverting amplifier circuit with an op-amp and a type-K thermocouple was devised. The sheet resistance change was simultaneously monitored using a Wheatstone bridge circuit. An analytical temperature calculation was conducted, considering the heat transfer phenomena during the flash light irradiation. As the results, the temperature of the copper nanoparticle films was reached to (318 °C) in 10 ms at the flash light irradiation energy higher than 12 J/cm² and they were melted and fully sintered. The analytical solutions of the temperature profile of copper nanoparticles film and polyimide substrate (maximum temperature of copper nanoparticles film and polyimide substrates are 279 °C and 140 °C, respectively) in which the latent heats for phase changes of the copper nanoparticles and the binder (PVP) were concerned, agrees well with the experimentally measured temperature profiles of them (maximum temperature of copper nanoparticles film and polyimide substrates are 318 °C and 135 °C, respectively). The analytical calculation method proposed, could be used to design the flash light sintering variables applicable to various low-temperature flexible substrates.     

Ductile and brittle material removal mechanisms in natural nacre—A model for novel implant materials

Nacre is a composite material found in the inner layer of sea shells. It consists of soft organic and hard inorganic components arranged in a complex hierarchical structure. Due to this arrangement, nacre exhibits outstanding mechanical properties (elastic modulus: 64.70 ± 3.50 GPa, hardness: 4.41 ± 0.45 GPa, density: 2.6 g/cm³). Therefore, nacreous implant materials have a high potential in many fields. In medical science, these materials might be used for bone replacement. This article provides an insight into the material removal mechanisms occurring in the scratching of natural nacre. Scratch tests are a simplification of the grinding process and are used to investigate the influence of single input parameters on the material removal. Different scratch tool geometries and varying processing parameters are applied, so that the material removal efficiency can be evaluated by analyzing the process forces. Additionally, the scratch geometry is examined by using a scanning electron microscope (SEM) and optical profilometer images as well as photomicrographs. The results of these examinations provide knowledge on the machinability of nacre and also on the machinability of new nacreous materials.     

Analysis of the influence of tool type, coatings, and machinability on the thermal fields in orthogonal machining of AISI 4140 steels

Micro-thermal imaging was used to determine the amount of heat flowing into the tool, chip and workpiece during orthogonal cutting at speeds up to 400 m min⁻¹. Two AISI 4140 steels with different machinability ratings and three types of tools were compared: (i) uncoated with 0° rake angle, (ii) coated with −6° rake angle and (iii) coated with chip breaker. A control volume approach was used to estimate the energy partition from thermal images and energy outflow was compared to direct measurement of the cutting power. This provides a new physical tool for examining machinability, tool wear and subsurface damage.     

Corrosion resistance of Al ion implanted AZ31 magnesium alloy at elevated temperature

The Al ion implantation into AZ31 magnesium alloy was carried out in a MEVVA 80-10 ion implantation system at an ion energy of 40-50 keV with an ion implantation dose ranging from 2 × 10¹⁶ to 1 × 10¹⁷ ions/cm² at an elevated temperature of 300 °C induced by an ion current density of 26 μA/cm². The concentration-depth profile of implanted Al in AZ31 alloy measured by Rutherford backscattering spectrometry (RBS) is a Gaussian-type-like distribution in a depth up to about 1200 nm with the maximum Al concentration of about 8 at.%. The X-ray diffraction (XRD) analysis revealed the formation of α-Mg(Al) phase, intermetallic β-Mg₁₇Al₁₂, and MgO phase on the Al ion implanted samples. The potentiodynamic anodic polarization curves of the Al ion implanted samples in the 0.01 mol/l NaCl solution with a pH value of 12 showed increases of the corrosion potential and the pitting breakdown potential, and a decrease of the passive current density, respectively. The Al ion implanted samples with 6 × 10¹⁶ ions/cm² achieved the high pitting breakdown potential to about − 480 mV (SCE). In the 0.08 mol/l NaCl solution with pH = 12, the Al ion implanted samples with 1 × 10¹⁷ ions/cm² showed an increased pitting breakdown potential to about − 1290 mV (SCE), from around − 1540 mV (SCE) of unimplanted samples. It is indicated that different corrosion mechanisms are responsible for improvement in corrosion resistance of the AZ31 magnesium alloy in the NaCl solutions with the varied concentrations.