Graded Transition Layer Research Articles

Microstructure and thermal stability of a structurally graded tungsten and reduced activation ferritic/martensitic steel joint

The leading design for plasma-facing high heat flux components in proposed fusion reactors involves joining plasma-facing tungsten tiles to underlying reduced activation ferritic-martensitic (RAFM) steel structures. Due to significant differences in the physical properties between W and steel, an effective method for joining them while preserving the mechanical and microstructural properties under service thermo-mechanical load is yet to be demonstrated. A transitional multilayer consisting of three layers (VCrTi, VCrAl, and FeCrAl) between W and steel is designed with the help of thermodynamic simulation and diffusion kinetics in an attempt to form solid solution bonding without forming brittle intermetallic phases. The solid solution bonding is desirable to mitigate the drastically different thermal expansion between W and steels. The multilayer structure was fabricated using spark plasma sintering (SPS). The microstructure of the bonded transition layers was analyzed after long-term annealing at 620 °C up to 1000h, through scanning electron microscope, energy dispersive spectroscopy and electron backscattering diffraction. Interdiffusion kinetics between layers and reliability of mobility database were also analyzed. This work provides a good understanding of thermal stability as well as the efficacy of multilayer functionally graded transition layer for joining W to ferritic/martensitic steel.

Microstructure and shear fracture behavior of Mo/AlN/Mo symmetrical compositionally graded materials

The metallization of AlN ceramic is a critical prerequisite for applying AlN ceramics to electronic component packaging. In this study, a concept of functionally graded materials (FGMs) was used for the metallization of AlN ceramic. Symmetrical Mo/AlN/Mo FGMs with different composition exponent p (p=1, p<1, and p>1) were designed and fabricated by hot-press sintering. The influence of the p-value on the shear performance and fracture path of the Mo/AlN/Mo FGMs was investigated. Compared to specimens with p=1, the specimens with p<1 and p>1 show the distribution of Mo-rich and AlN-rich in the graded transition layer, respectively. The shear strength of the specimens reaches a maximum value of 48.0 MPa as p < 1 and 22.5 MPa higher than the minimum value as p > 1. Besides, three fracture paths were found for the Mo/AlN/Mo FGMs with different values of p, including the cracks propagate obliquely along the axis in the graded transition layer, the cracks penetrate through the AlN ceramic and the graded transition layer, and the cracks only propagate horizontally in a single graded layer. The crack initiation of Mo/AlN/Mo FGMs depends on the strength of each graded layer. Moreover, the directions of fracture paths are closely related to the strength of the ceramic interlayer and the composition exponent p.

Additive manufacturing of tantalum-zirconium alloy coating for corrosion and wear application by laser directed energy deposition on Ti6Al4V

In this work, a preplaced TaZr alloy powder layer (Ta:Zr = 7:3 in wt%) is coated on a Ti6Al4V substrate by laser-based directed energy deposition. Crack-free, smooth TaZr alloy coating is acquired, and the content of elements (Ta, Zr, Ti, Al, V) distributes in a gradient. In this coating, the bcc α-Ta grains with a rectangular shape and a size of 10–20 nm surrounded by the TiZr phase, the Al3Zr4 phase mainly gathers in grain boundary. The corrosion resistance of the coating and substrate is tested via electrochemical measurement and the coating exhibits a better resistance in 0.5 mol/L H2SO4 solution. The average microhardness of the coating is around 600 HV, which is ~1.7 times than that of the substrate. The wear test results show the mass loss of the coating is approximately 60 times lower than the substrate at room temperature. It is believed that this coating could shed light on not only the protection of Ti-based alloy, but also the Fe-based and Ni-based components with the functionally graded transition layers.

Challenges in multiphysics modeling of dual-band HgCdTe infrared detectors.

We present three-dimensional simulations of HgCdTe-based focal plane arrays (FPAs) with two-color and dual-band sequential infrared pixels having realistic truncated-pyramid shape, taking into account also the presence of compositionally graded transition layers. After a validation against the spectral responsivity of two-color, mid-wavelength infrared detectors from the literature, the method is employed for a simulation campaign on dual-band, mid-, and long-wavelength infrared FPAs illuminated by a Gaussian beam. Simulation results underscore the importance of a full-wave approach to the electromagnetic problem, since multiple internal reflections due to metallizations and slanted sidewalls produce non-negligible features in the quantum efficiency spectra, especially in the long-wavelength band. Evaluations of the optical and diffusive contribution to inter-pixel crosstalk indicate the effectiveness of deep trenches to prevent diffusive crosstalk in both wavebands. In its present form, the detector seems to be subject to significant optical crosstalk in the long-wavelength infrared band, which could be addressed through pixel shape optimization.

Dissimilar metals deposition by directed energy based on powder-fed laser additive manufacturing

In this research, powder-fed laser additive manufacturing based on directed energy deposition (DED) technology is utilized for 3D printing/fabrication of dissimilar alloy walls. Three-dimensional fabrication of metallic layers (A410-L stainless steel, A316-L stainless steel, and zirconium) with different crystallographic structures such as body-centered cubic (BCC), face-centered cubic (FCC), and hexagonal close-packed (HCP) are studied. Solidification cracking of the HCP metal structure inhibits the manufacturing of a sound dissimilar wall during direct deposition of zirconium on stainless steel. Different techniques are employed to eliminate cracking during the deposition of such dissimilar walls. First, a functionally graded transition layer is employed to gradually transition from a BCC or FCC layer to a zirconium layer. However, several longitudinal and horizontal cracks are observed in the functionally graded structure, deteriorating the overall integrity of the wall. Subsequently, inter-layers are applied to reduce the metallurgical differences between the BCC, FCC, and HCP metal structures. These inter-layers are designed to suppress the formation of brittle intermetallic compounds and decrease the level of thermal stress leading to cracking. Several metallic powders including nickel, titanium, vanadium, and copper are tested and analyzed as inter-layer materials during layer by layer fabrication of the dissimilar wall. Cross-sectional examinations are performed for each case, in order to study the feasibility of fabricating a sound dissimilar wall without deleterious phases. Embrittlement and presence of nickel-rich intermetallics within the Zr matrix restricts the successful fabrication of a dissimilar alloy wall through the formation of vertical cold cracks. Samples with titanium and vanadium inter-layers demonstrate horizontal hot solidification cracking at the stainless steel interface, which is attributed to the large solidification temperature range. Nonetheless, best results are attained with the copper inter-layer due to its high compatibility with both of the stainless steel and Zr metals.

Performance of Full Compositional W/Cu Functionally Gradient Materials Under Quasi-Steady-State Heat Loads

To investigate the performance of graded structure materials under steady-state heat load, six-layered W/Cu functionally gradient materials with full compositional distributions (from 0% to 100%) fabricated by resistance sintering under ultrahigh-pressure method were tested under the electron beam material testing scenario using the fusion relevant quasi-steady-state heat load in the range of 6.5–9.8 MW/m2 and compared to the W/Cu samples with sharp joint. The specimens were loaded for about 5 s, and then indirectly cooled for 30 s. It is identified that the graded transition layers can really ameliorate the thermal mismatch, and thus prolong the fatigue lifetime. These results were examined by the thermal-stress analysis using finite element method. Moreover, the inserting of graded interlayers may suppress the formation of center vertical cracks of W layer. However, the copper melt exudation within gradient layers occurred under extreme high heat loads of 9.8 MW/m2, which should be avoided by essential thermal analysis and proper structure design.

Interface behavior of a thin-film bonded to a graded layer coated elastic half-plane

The interface behavior of a thin-film adhering to an elastic half-plane with an arbitrarily graded transition layer is analyzed by a linearly unified model when the interface between the thin-film and the graded layer is subjected to a mismatch strain. A series of continuous but piecewise linear functions are adopted to describe different graded variation laws of the transition layer's modulus. The fundamental solution of a graded-layer coated half-plane under a pair of normal and tangential concentrated forces is obtained first, and then the mismatch problem of a thin-film bonded to an elastic half-plane with an arbitrarily graded transition layer is further investigated. Solving the obtained integral governing equation of Cauchy singularity leads to the interfacial shear stress, the normal stress in the film, as well as the stress intensity factor near the bonded edge. It is found that the interfacial mechanics of a thin-film bonded to a graded layer is significantly affected by varying gradient law, stiffness of the film, ratio of shear modulus and length scale of the graded transition layer. The results should be helpful for the design of systems with thin-films and functional graded materials and could guide engineers to choose proper graded materials for particular applications.

Two-dimensional contact mechanics problems involving inhomogeneously elastic solids split into three distinct layers

This paper investigates the frictionless two-dimensional contact problem of an inhomogeneously elastic material under a rigid punch. The inhomogeneous solid is deemed to comprise three distinct regions which represent a homogeneously elastic coating and substrate joined together by a functionally graded transition layer (interlayer) whose shear modulus depends exponentially on the vertical coordinate.We propose closed form solutions for the horizontal and vertical displacements of the solid which are analytic if the contact pressure is known exactly. These solutions are further used to derive a fast and efficient iterative algorithm from which the contact footprint resulting from the rigid stamp problem may be computed. A selection of numerical results are then presented using this method and it is found that our model compares well with those of other authors in the two particular limiting cases considered here. We then investigate the effects of material inhomogeneity and coating thickness on the cylindrical stamp problem and it is found that the maximum principal stress is highly dependent on the thickness and mechanical properties of the layer. In particular, it is found that the maximum principal stress that occurs in a material with a hard coating may be reduced by increasing the thickness of the transition layer whilst lower stresses are achieved in materials with soft coatings by decreasing interlayer thickness.

Effects of Heat Treatment on the Creep Crack Growth Behavior in Al/Al-4wt%Cu Functionally Graded Material

A conventional hot-pressing method was used to produce Al/Al-4wt%Cu functionally graded material (FGM). Heat treatment, which included solid solution treatment (T4) and aging treatment (T6), was carried out on hot-pressed (F) specimens. The creep crack growth tests were performed under constant loading of the creep-testing machines. The distribution of copper composition was investigated by line analysis via electron probe microanalysis. Fracture morphology and creep crack paths were studied by scanning electron microscopy. During heat treatment, the thickness of the graded transition layer increased due to copper composition redistribution. Creep crack growth retardation was found when crack propagated from the graded transition region to the Al-4wt%Cu layer. Greater improvement in creep crack growth resistance was achieved by the T4 and T6 states of Al/Al-4wt%Cu FGM. For T4 and T6 state specimens, the micro-cracks and crack kinking in the transition region were observed, which prevented creep crack growth.

Growth and characterization of integrated nano- and microcrystalline dual layer composite diamond coatings on WC–Co substrates

In this work, integrated composite diamond (ICD) coatings have been achieved with top layer nanocrystallinity, low friction coefficient and enhanced integrity. ICD coatings were deposited on chemically treated tungsten carbide (WC–Co) substrates using hot filament chemical vapour deposition technique. Nanocrystalline diamond (NCD) layer was deposited over microcrystalline diamond (MCD) layer with a coating architecture of NCD/transition layer/MCD/WC–Co. Graded transition layer thickness of ~1μm was realized by controlling the process parameters such as methane concentration and chamber pressure in order to integrate the MCD and NCD layers. Integrity of the coatings was examined by the cross-sectional studies. Structural and microstructural characteristics of ICD coatings were compared with those of MCD coatings. The measured average nanohardness of ICD coating was ~96GPa. A low and stable friction coefficient of ~0.06 was observed for ICD coatings against silicon nitride (Si3N4). ICD coatings were anticipated to exploit the advantages of both NCD and MCD coatings and these coatings can be promising candidates for various mechanical applications.

Structure of SiC coatings from polycarbosilane on graphite for fuel element matrix of high temperature gas-cooled reactor

The structures of the SiC coatings fabricated from polycarbosilane (PCS) on graphite for fuel element matrix of high temperature gas-cooled reactor were studied. It was shown that uniform dense SiC coatings could be obtained by this method when the PCS concentration of impregnation solution was in the range of 10–30 wt.%. The graded transition layers at the interface between coatings and graphite substrates were obtained for the content of SiC in pores transformed from PCS became smaller with the increase of depth due to the gradually changing impregnation resistance of PCS/xylene in the pores. The SiC coatings became denser and thicker with increase of PCS concentration. The largest thickness of SiC coatings by one impregnation was 1–2 μm. The SiC coatings mainly contained β-SiC, but free carbon, which was detected by Raman spectrum, still existed in the coatings.

Mixed Mode Loading of an Interface Crack Between Graded Creeping Solids

Interface cracks are seldom subjected to pure Mode I conditions. Stationary cracks between two distinct, bonded elastic-creep materials subjected to remotely applied mixed mode loading are simulated using the finite element method (FEM). Plane strain conditions are assumed. In most cases a functionally graded transition layer is included between two homogeneous, yet distinctive, materials. Examples of such systems include bonded structures or repaired components subjected to elevated temperatures. Numerical solutions based on the asymptotic fields of the homogeneous and heterogeneous Arcan-type specimens are presented. Creep ductility-based damage models are used to predict the initial crack propagation trajectory. Incorporation of functionally graded transition layer regions was determined to affect the evolution of time-dependent stress components in the vicinity of the crack tip. The magnitude and direction of crack tip propagation can then be optimized with respect to interface properties.

Numerical simulation of time-dependent fracture of graded bimaterial metallic interfaces

Stationary cracks along and near interfaces between two time-dependent materials are simulated using the finite element method (FEM) to examine crack tip fields and candidate driving force parameters for crack growth. Plane strain conditions are assumed. In some cases, a thin transition layer is included between the two materials. This transition layer features a functionally graded blend of properties of the two materials. An example of such a system is that of weld metal (WM) and base metal (BM) in a weldment, with the transition layer corresponding to the heat-affected zone (HAZ). Numerical solutions for the stress and strain fields of homogeneous and heterogeneous Compact Tension (C(T)-type) specimens are presented. The equivalent domain integral technique is employed to compute the J-integral for elastic-plastic cases as well as the C(t)-integral and transition times for creep behavior. Results from parametric studies are curve-fit in terms of transition layer thickness and crack position, inelastic property mismatches, and other independent model parameters. Results indicate that the incorporation of functionally graded transition layer regions leads to less concentrated stress and strain components along interfaces ahead of the crack tip. It is also shown that the computed fracture parameters are influenced by the transition layer properties.

Mechanical properties of polytypoidally joined Si 3N 4–Al 2O 3

Crack-free joining of alumina and silicon nitride has been achieved by a unique approach introducing sialon polytypoids as a functionally graded materials (FGM) bonding layer. The polytypoid compositions are identified in the phase diagram of the Si 3N 4–Al 2O 3 system. The thermal stresses of this FGM junction were analyzed using a finite element analysis program taking into account both coefficient of thermal expansion and modulus variations. From this analysis, the result showed a dramatic decrease in radial, axial and hoop stresses as the FGM changes from three layers to 20 graded layers. Scaling was considered, showing that the graded transition layer should constitute about 75% or more of the total sample thickness to reach a minimal residual stress. Oriented Vickers indentation testing was used to qualitatively characterize the strengths of the joint and the various interfaces. The indentation cracks were minimally or not deflected at the sialon layers, implying strong interfaces. Finally, flexural testing was conducted at room temperature and at high temperature. The average strength at room temperature was found to be 581 MPa and the average strength at high temperature (1200°C) was found to be 262 MPa. Scanning electron microscope observation of fracture surfaces at a different loading rates indicated that the strength loss at higher temperatures was consistent with a softening of glassy materials present at grain junctions.

Wear-resistant multilayered diamond-like carbon coating prepared by pulse biased arc ion plating

Diamond-like carbon coatings have been deposited by a pulse biased arc ion plating. In order to improve their adherence to metal substrate, two systems of graded transition layers, namely Ti/TiN/TiC and Ti/TiCN/TiC, have been applied. The structure and composition of the diamond-like carbon/transition composite coatings were studied by scanning electron microscopy, Raman spectroscopy and energy dispersive X-ray spectroscopy. The total thickness of the coatings was within a range of 1.0–2.0 μm. Such multi-layer coatings showed excellent properties including high hardness, low friction coefficient and long wear-resistant lifetime. Diamond like carbon coatings as well as their wear tracks developed by sliding steel balls have been investigated by scanning electron microscopy. The results of the analysis, particularly that of the tribological study showed that the wear resistance and film-to-substrate adherence of diamond like carbon coatings with stainless steel surface were dramatically improved by using a graded transition layer and pulse biased arc ion plating.

Oxidation behavior of diamond-like carbon films

Abstract Diamond-like carbon (DLC) films were synthesized by a combined PVD and PECVD process to produce a multilayered structure consisting of a Ti interlayer, a graded transition layer, and the carbon film. The oxidation behavior of DLC films was investigated using thermogravimetric (TGA) and differential thermal analyses (DTA). The phase identification and microstructural examinations were conducted by XRD, Raman, and SEM/EDS. According to those results, DLC films disintegrated at 350°C, showing typical graphitic transformation and oxidation behavior. The weight loss as a result of oxidation of carbon followed a linear reaction rate of −2.3×10−4 g/min cm2. At 450°C, oxidation of the underlying TiN/TiCxNy interlayer occurred in addition to the oxidation of DLC. Weight gain caused by the formation of TiO2 was observed. The overall oxidation kinetics of DLC is close to a parabolic behavior with kp=5.48×10−5 mg2/cm4 h. Surface cracking of the film resulted from stress relief. The microhardness of DLC films decreased with increasing annealing temperature due to the graphitization and oxidation of DLC. The transition temperature was confirmed using the results from the Raman analysis.

Characterization of sputter deposited tungsten films for x-ray multilayers

The microstructural and optical properties of tungsten thin films prepared by dc magnetron sputtering were investigated as a function of the argon pressure. The films were characterized by x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), x-ray diffraction (XRD), grazing x-ray reflectometry (GXR), and spectroscopic ellipsometry (SE). From the analysis of the XPS data, the films were found to consist of a base tungsten layer, a graded oxide transition layer, and a surface oxide layer. Also, the thickness of the oxide layers were determined. XRD indicated that the films were either amorphous or composed of crystallites with grain size &amp;lt;100 Å. STM and GXR have revealed that the films grown at low pressures were the smoothest. Finally, SE measurements allowed the determination of the dielectric function of a compact tungsten layer. Further SE analysis of the films indicated that with increasing argon pressure, the films developed into a more voided columnar structure.

Organometallic vapor phase epitaxial growth and characterization of InAsBi and InAsSbBi

InAs1−xBix with x≤0.026 and InAs1−x−ySbyBix with x≤0.017 and y≤0.096 have been successfully grown on InAs (100) oriented substrates by atmospheric pressure organometallic vapor phase epitaxy using the precursors trimethylindium, trimethylbismuth, trimethylantimony, and arsine. Good surface morphologies for both InAsBi and InAsSbBi epitaxial layers were obtained at a growth temperature of 400 °C. A key growth parameter is the V/III ratio. Only a very narrow range near 4 (considering the incomplete pyrolysis of AsH3) yields smooth InAsBi epilayers. Typical growth rates were 0.02 μm/min. X-ray diffractometer scans show clearly resolved Kα1 and Kα2 peaks for the layer of InAs0.889Sb0.096Bi0.015 grown on an InAs substrate with a graded transition layer to accommodate the lattice parameter difference. The half widths of the peaks are comparable to those of the substrate. For the first time, photoluminescence (PL) at 10 K from these Bi-containing alloys has been measured. The PL peak energy is seen to decrease with increasing Bi concentration at a rate of 55 meV/at. % Bi. InAsSbBi is a potential material for infrared detectors operating in the wavelength range from 8 to 12 μm.

Vacuum chemical epitaxial growth of III–V materials for multi-band-gap solar cells

The growth of GaAs, GaAsP and GaASSb epitaxial layers made by vacuum chemical epitaxy (VCE) for monolithic two-color terrestrial solar cells is described. The utilization of materials and safety aspects are also discussed. We show the changes made in our VCE reactor to improve film quality and safety. Data are presented on the use of trimethylgallium as a p-type dopant source in solar cells with 90% quantum yields. We discuss the measurement of minority carrier diffusion lengths of over 2 μm in carbon-doped cells using a new technique which we have developed and called sequentially etched quantum yield. The characterization of defects using Schottky barrier electron-beam-induced currents of GaAsP grown on graded transition layers on GaAs substrates is also discussed.

Near-term higher efficiencies with mechanically stacked two-color solar batteries

Two-band-gap (or two-color) solar batteries have the potential for much higher efficiencies than can be achieved with single-junction devices. While two-terminal monolithic stacks with two active junctions and a series interconnect junction grown sequentially onto a single substrate are conceptually very elegant, they require the development of three new components in a complex multi-element III–V alloy system. Although such devices have been demonstrated, each of the three novel components (the two cells and the interconnect) must be optimized so that it will perform near its theoretical limit in order for the two-color device to outperform the more developed single-junction devices. Tandem mechanically stacked two-color solar batteries offer a shorter path to commercialization, primarily because one of the two active cells can be an already-developed Si or GaAs cell and because the interconnect problem can be easily solved with conventional wire-bond technology. Thus, the problem of developing three new components for the monolithic-device option is replaced by the problem of developing one new component for the mechanical-stack option. We previously observed that, if the Si cell is chosen as the well-developed cell, a GaAs 0.7P 0.3 cell on a GaP substrate would be a logical choice for the novel component in a solar battery. In studying this particular option, we recently noted that the four terminals available in a mechanical-stack battery can be used in a series-parallel interconnect scheme, allowing voltage matching at the module level. This interconnect scheme makes the module energy-conversion efficiency quite insensitive to variations in the solar spectrum and opens up a broader range of materials that are usable in mechanical stacks. In particular, it may be desirable to choose a GaAs cell with a band gap of 1.42 eV as the well-developed cell and a GaSb cell with a band gap of 0.72 eV as the novel cell in a two-color solar battery. The advantage of this option is that the high-band-gap GaAs cell currently holds the world record for energy-conversion efficiency, and the GaSb cell would simply boost this record. Moreover, GaSb is advantageous in comparison to GaAsP because it is a binary compound rather than an alloy and would not require compositionally graded transition layers between the active junction and the substrate. Both of these considerations should greatly improve the chances for higher performance. GaSb photodiodes with high quantum efficiencies have already been described in the literature, and available data allow the projection of stack efficiencies over 30% (AM 1.5) with this option.