Functional Gradient Structure Research Articles

Mechanical and microstructural profiling of additively manufactured cobalt–nickel functional gradient structure

In this study, a functional gradient material (FGM) structure composed of a nickel alloy (IN718), and a cobalt alloy (CoCrMo) was additively manufactured using a co-axial powder-fed laser-directed energy deposition (L-DED) system. The high manufacturing flexibility of L-DED enabled the seamless deposition of IN718-CoCrMo FGM structure through interfacial bonding driven by thermal gradient and cool-down mechanisms. Microscopic analysis confirmed the absence of cracks or delamination in the CoCrMo-IN718 interfacial bonding region. The SEM-EBSD microstructural analysis and micro-hardness testing were performed on the as-printed CoCrMo-IN718 FGM primarily to correlate the hardness properties-microstructural grain phase morphology between the parent alloys and their interfacial bonding region. It was observed that the average hardness of the CoCrMo-IN718 interface zone (322.83 HV1) fell between IN718 (268.67 HV1) and CoCrMo (359.75 HV1) regions. The EBSD analysis reports indicated that along the build direction of CoCrMo-IN718 FGM, the preferential grain orientation aligned in [100] with grain structure transitioning from equiaxed → columnar → elongated columnar dendrites → equiaxed grain, predominantly comprised of face-centered cubic (FCC)-gamma (γ) phase crystal structures. Relatively, coarser grain structures were observed in the interface bonding region, attributed to the analogous crystallography space groups, and prolonged localized thermal gradients.

Optimization of the Stress-Strain State of Functional Gradient Structures with Stress Raisers

Optimization of the Stress-Strain State of Functional Gradient Structures with Stress Raisers

Passive and active oxidation of Al in the Al–Al2O3 composite refractory at high temperatures in air

The active and passive oxidation of Al in Al–Al2O3 composites was investigated in air at high temperatures. As fresh air impinged on the surface of the Al–Al2O3 composite sample, a thin Al2O3 layer was preferentially formed via passive oxidation of Al, which consumed most of the oxygen and lowered the local Po2 inside the sample. At low Po2, active oxidation of Al occurred and predominated in the interior transition layer, which further lowered the local Po2 to extremely low values within the non-oxide stability field. The local equilibrium PAlxOy generated by the active oxidation of Al reached a micro-positive pressure state and served as a gas barrier, hindering the inward diffusion of O2. The Po2 inside the composite was lowered by Al in the outer layers via a self-gettering process. Subsequently, Al4O4C and Al2OC-AlN solid solution were formed by both the direct reaction of Al(l) and the indirect reaction of AlxOy(g). After sintering in air, the Al–Al2O3 composite exhibited a functional gradient structure with a thin oxide layer and an interior non-oxide-reinforced corundum. A reaction mechanism model was established.

Transparent Bamboo with Multiple Light Transmittance levels, Fiber Textures, and Colors Based on the Gradient Structure of Bamboo

Transparent bamboo (TB) is a new type of biomass and light transmitting material with potential to serve as a substitute for transparent wood. However, the potential of the bamboo structure in preparing transparent composites is underexplored. Inspired by the functional gradient structure of bamboo, we used a simple and efficient two‐step strategy to prepare TB with different transmittance levels, fiber textures, and colors. We evaluated the performance of single‐ and multilayer TB with various fiber volume contents prepared using different lamination methods. We found that the optical and mechanical properties of TB differed with the fiber volume content and the lamination method used. TB composed of veneers with low fiber volume showed the highest light transmittance (44.6%), whereas TB composed of veneers with high fiber volume had the highest tensile strength (45.7 MPa). In terms of water resistance, cross lamination inhibited expansion in width (water absorption rate, 16.37–21.26%) and thickness (thickness swelling rate, 10.22–21.74%). The lamination structure and fiber volume content in the preparation of TB should be selected according to the specific application scenarios. The graded utilization of bamboo veneer with a fiber gradient can effectively realize the rational allocation of bamboo resources and promote sustainable development.This article is protected by copyright. All rights reserved.

Understanding of trabecular-cortical transition zone: Numerical and experimental assessment of multi-morphology scaffolds

Applications of additive manufacturing (AM) in tissue engineering develop rapidly. AM offers layer-by-layer creation of complex objects, developed to restore functionality of, or replace, damaged tissues. Porous 3D-printed functional gradient structures are of particular interest: their special architecture makes it possible to simulate the heterogeneity of the replaced tissue and, by continuously changing the mechanical properties, to avoid the concentration of stresses that can be caused by abrupt geometric changes. Such structures also allow combinations of different types of unit cells and a smooth transition between them, making design of personalised scaffolds with optimal parameters for the replacement of damaged host tissue at the interface between tissues possible. This paper presents the results of development of scaffold structures with gradients of porosity and multi-morphology using unit cells based on triply periodic minimal surfaces (TPMS). The mechanical behaviour of additively manufactured scaffold prototypes made of polylactide acid (PLA) was studied under compressive loading. Strain fields on their surface were captured using the Vic-3d Micro-DIC digital image correlation system and compared with those obtained with detailed numerical simulations, employing elastic-plastic properties of PLA, obtained in experiments. The effect of gradient parameters and unit-cell morphology on the stress distribution in scaffolds was analysed. A smooth gradient transition between cells with different morphologies was found to reduce the probability of structural failure under intense compressive loading. A good agreement between numerical results and experimental data was achieved, which justifies application of the developed approach to design of personalised bone scaffolds.

Composition Gradient Cellulose–Aerogel Nanocomposites Regulating Thermal Insulation

Over the last few decades, functional gradient structures have evolved through natural biological systems, while gradient structures lead to tailored mechanical and physical performance due to the gradual structural change. Herein, composition gradient cellulose and aerogel nanocomposites that regulate their thermal insulation and mechanical performance are reported. The as‐prepared gradient composite shows a thermal conductivity of 32.2 mW m−1 K−1 and flexural modulus of 660 MPa while exhibiting superhydrophobicity and superior reusability. The unique orientation‐dependent thermal insulation and mechanical strength arise from the composition gradients formed by the silica aerogel distribution during the cellulose‐fiber‐percolated network formation, which opens a pathway toward green building thermal insulation materials.

Design theory and anti-ballistic effect simulation of dual phase hybrid functionally graded ceramic composite armor

Functional gradient structure design is one way to improve the anti-ballistic performance of ceramic composite armors. In this paper, the design theory of dual phase hybrid functionally graded ceramics is established. Through the finite element simulation of the anti-ballistic behaviour, three structural effects of dual phase hybrid functionally graded ceramics composite armor are studied: monolithic ceramics (MC), unidirectional functionally graded ceramics (UFGC) and bidirectional functionally graded ceramics (BFGC). The ceramics structure of composite armor is optimized. The simulation results of the optimized structure of unidirectional and bidirectional functionally graded ceramics are better than that of monolithic ceramics, and the weight is lighter. This research can provide theoretical support for the design of functionally graded ceramic composite armor.

Mechanical properties of Al2O3 and Al2O3/Al interpenetrated functional gradient structures by 3D printing and melt infiltration

In this paper, Al2O3 functional gradient structures (FGS) were prepared by digital light processing-based 3D printing, and Al2O3/Al interpenetrated FGS was subsequently obtained by melt infiltrating. The microstructure of Al2O3FGS and Al2O3/Al interpenetrated FGS was studied. And the mechanical properties of Al2O3 FGS and Al2O3/Al interpenetrated FGS under quasi-static compression loading were investigated in detail. Compressive strength, Young’s modulus, and energy absorption of Al2O3 FGS and Al2O3/Al interpenetrated FGS were experimental tested, compared, and discussed. The failure behavior of Al2O3 FGS and Al2O3/Al interpenetrated FGS was analyzed by experiments and FEA. This study provides a novel approach for the fabrication of ceramic FGS and ceramic/metal interpenetrated FGS with high mechanical performance.

Construction relationship between a functionally graded structure of bamboo and its strength and toughness: Underlying mechanisms

The special functional gradient structure of bamboo determines its excellent properties of strength and toughness, which make it an excellent candidate for biomimicking purposes in advanced material design. However, the construction relationship and underlying mechanism of the functionally graded structure in the direction of the bamboo wall and its excellent strength and toughness have not been elucidated. Accordingly, this study first conducts a quantitative analysis of the relationship between the directional gradient structure of the bamboo wall layer and its chemical composition and physical and mechanical properties. To this end, in situ scanning electron microscopy (in situ SEM) was used to investigate the mechanical behavior and failure mode of the bamboo wall layer under the effect of external loads, and to investigate the structure–activity relationship between the gradient structure of the bamboo wall layer and its strength and toughness, as well as the potential mechanism of action. From the perspective of structural mechanics, the mechanical model of the bamboo wall direction under the applied load was analyzed, and the mechanism by which the bamboo wall gradient structure improves the strength and toughness of bamboo was further elucidated. The results showed that the uniform gradient distribution of the vascular bundles and their interfaces in the direction of the bamboo wall determines the gradient distribution of its chemical components and physical and mechanical properties. Together, the uneven deformation of parenchyma cells and fibers, multi-path propagation of cracks, fiber bridging, pull-out and fracture constitute the mechanism by which the bamboo gradient structure contributes to its strength and toughness. This study provides a theoretical basis and innovative ideas for the design of biomimetic advanced materials based on the bamboo wall gradient structure by revealing the potential close relationship between the excellent mechanical properties of bamboo of high strength and toughness and its functional gradient structure.

Mechanical and degradation properties of triply periodic minimal surface (TPMS) hydroxyapatite & akermanite scaffolds with functional gradient structure

High porosity and high mechanical strength are both important for bone scaffolds, but they are always contradictory, which can be tackled by reasonable structural design. In this paper, we proposed a series of functional gradient lattice structures with the local porosity increasing continuously from the inside out. Based on the triply periodic minimal surface (TPMS), four types of gradient scaffolds were designed with the same porosity (65%) and fabricated by digital light processing (DLP) technology. Finite element (FE) analysis and compressive tests were conducted to investigate the mechanical property, and degradation test was conducted to investigate the biological property. Results showed that gradient structures can effectively enhance the overall compressive strength at the same porosity level. Scaffolds with exponential structure have the highest compressive strength and elastic modulus, reaching 4.45 ± 0.52 MPa and 0.580± 0.034 GPa respectively, almost twice that of uniform structure. Gradient structure has a trivial effect on degradation performance. The degradation rates of all scaffolds can reach more than 5% after immersing for one day. Surface morphology of distinct parts of a scaffold are almost the same due to the highly connected internal structure. These results confirmed that gradient structure can improve mechanical property while maintaining the degradation rate, which have great advantages in bone scaffold engineering.

Bamboo-Inspired Renewable, High-Strength, Vibration-Damping Composites for Structural Applications

Renewable structural materials derived from natural biological materials can potentially replace non-renewable synthetic materials for civil engineering and transportation applications. Here, inspired by the functional gradient structure of bamboo, we propose a simple and efficient two-step preparation strategy to convert natural bamboo into a lightweight, high-strength, damping, and sound-insulating structural engineering material called densified bundle-laminated veneer lumber (DBLVL). DBLVL was prepared by finely grading and thinning three layers of bamboo slices and removing the lignin and hemicelluloses from the surface of bamboo bundles. This was followed by penetrating the phenol formaldehyde resin and then solidifying and densifying it in situ. DBLVL had superior mechanical properties due to its bamboo-like gradient laminated structure, three-dimensional network distribution of adhesive under high temperature and high pressure, in situ curing, and solid interfacial bonding. It had a specific tensile strength of 297 MPa cm3 g–1, which was superior to those of many other construction materials. The tensile strength of DBLVL reached 363 MPa, and the bending strength reached 219 MPa, which were 97.28 and 92.11% higher than those of natural bamboo, respectively. The impact toughness reached 15.4 J/cm2. DBLVL also showed excellent damping and vibration reduction (the first three damping ratios were 2.35, 1.81, and 2.40%) and dimensional stability (the thickness expansion rate after 24 h of water absorption was 4.43%). Because of its excellent mechanical properties and hygrothermal stability, DBLVL is expected to replace non-renewable synthetic materials as a green and sustainable structural material for engineering applications.

Mechanical and Degradation Properties of Triply Periodic Minimal Surface (Tpms) Hydroxyapatite & Akermanite Scaffolds with Functional Gradient Structure

Mechanical and Degradation Properties of Triply Periodic Minimal Surface (Tpms) Hydroxyapatite & Akermanite Scaffolds with Functional Gradient Structure

Fabrication of alumina ceramics with functional gradient structures by digital light processing 3D printing technology

Alumina ceramics with different unit numbers and gradient modes were prepared by digital light processing (DLP) 3D printing technology. The side length of each functional gradient structure was 10 mm, the porosity ratio was controlled to 70%, and the number of units were (1 × 1 × 1 unit) and (2 × 2 × 2 unit) respectively. The different gradient modes were named FCC, GFCC-1, GFCC-2 and GFCC-3. SEM, XRD, and other characterization methods proved that these gradient structures of alumina ceramics had only α-Al2O3 phase and good surface morphology. The mechanical properties and energy absorption properties of alumina ceramics with different functional gradient structures were studied by compression test. The results show that the gradient structure with 1 × 1 × 1 unit has better mechanical properties and energy absorption properties when the number of units is different. When the number of units is the same, GFCC-2 and GFCC-3 gradient structures have better compressive performance and energy absorption potential than FCC structures. The GFCC-2 gradient structure with 1 × 1 × 1 unit has a maximum compressive strength of 19.62 MPa and a maximum energy absorption value of 2.72 × 105 J/m3. The good performance of such functional gradient structures can provide new ideas for the design of lightweight and compressive energy absorption structures in the future.

The Suppression Effect of Functional Gradient Design on Concentration Polarization of Multiple Cations Molten Salt Electrolyte for Thermal Batteries

The energy density of traditional Li−Si/FeS2 thermal batteries, which multiple cations are contained in molten salt electrolyte, is limited by the deficiency of Li+ in cathode when a high discharge current is applied. To address this issue, LiCl−KCl is taken as an example to build a functional gradient structure through Li−Si/FeS2 thermal batteries. By levering the Li+ concentration in cathode and preventing the formation of high concentration polarization resistivity as well as molten salt precipitation, the energy density of Li−Si/FeS2 single cell with a discharge current of 500 mA cm−2, increase from 157.0 Wh kg−1 to 218.4 Wh kg−1 at 450 °C (with an increase ratio of 39.1%), compared with the original one. Similarly, a 25.6% raise is observed at 525 °C. Moreover, the effects of LiCl-rich electrolyte and the functional gradient structure on the electrochemical performance of Li−Si/FeS2 single cells are discussed and classified.

Hierarchical Structural Engineering of Ultrahigh-Molecular-Weight Polyethylene.

Nature has inspired the design of next-generation lightweight architectured structural materials, for example, nacre-bearing extreme impact and paw-pad absorbing energy. Here, a bioinspired functional gradient structure, consisting of an impact-resistant hard layer and an energy-absorbing ductile layer, is applied to additively manufacture ultrahigh-molecular-weight polyethylene (UHMWPE). Its crystalline graded and directionally solidified structure enables superior impact resistance. In addition, we demonstrate nonequilibrium processing, ultrahigh strain rate pulsed laser shock wave peening, which could trigger surface hardening for enhanced crystallinity and polymer phase transformation. Moreover, we demonstrate the paw-pad-inspired soft- and hard-fiber-reinforced composite structure to absorb the impact energy. The bioinspired design and nonequilibrium processing of graded UHMWPE shed light on lightweight engineering polymer materials for impact-resistant and threat-protection applications.

RETRACTED ARTICLE: Ablation Behavior of SiC/ZrB2 Multilayer Coating Prepared by Plasma Spray Method

In this study, a SiC/ZrB2 multilayer coating with a functional gradient structure was prepared on graphite to increase the ablation resistance. The SiC coating as the inner layer was applied by the reactive melt infiltration method. Then, a ZrB2 outer layer coating was applied by argon gas shrouded plasma spray. The effect of the shielding gas flow rate on the amount of oxides and quality of the ZrB2 coating was studied. To evaluate the ablation resistance of the coatings, the specimens were exposed perpendicular to an oxy-propane flame for 60 second with a temperature of 2473 K and heat flux of 3000 kW/m2. The single-layer SiC coating showed 24.24 pct mass loss. The results indicated that applying the ZrB2 coating significantly improved the ablation resistance and reduced the mass ablation rate. In the plasma spray process, by applying the argon shrouding with a flow rate of 150 L/min, the oxidation of ZrB2 phase decreased from 41.6 to 4.8 pct. In addition, the weight loss and mass ablation rate decreased from 12.79 and 1.857 × 10−3 g/cm2/s to 0.12 pct and 0.0393 × 10−3 g/cm2/s, respectively. The improvement of the ablation resistance of the coating prepared by argon gas shrouded plasma spray could be attributed to the lower amount of oxides and pores in the as-sprayed coating and better cohesion of splats.

3D printing of multiple metallic materials via modified selective laser melting

Selective laser melting (SLM) is a powder bed layer-by-layer fusion technique mainly applied for additive manufacturing of 3D metallic components of complex geometry. However, the technology is currently limited to printing a single material across each layer. In many applications such as the manufacture of certain aero engine components, conformably cooled dies, medical implants and functional gradient structures, printing of multiple materials are desirable. This paper reports an investigation into the 3D printing of multiple metallic materials including 316L stainless steel, In718 nickel alloy and Cu10Sn copper alloy within a single build-up process using a specially designed multiple material SLM system combining powder-bed with point by point powder dispensing and selective material removal, for the first time. Material delivery system design, multiple material interactions, and component characteristics are described and the associated mechanisms are discussed.

Production of centimeter-scale gradient patterns by graded elastomeric tip array.

Large-area patterned surfaces with chemical and/or morphological gradients have significant applications in biology, chemistry, and materials science. In this work, we developed a unique lithographic strategy to fabricate 2D and 3D gradient patterns with gradually varying feature size or height over centimeter-scale areas by utilizing a large-area polydimethylsiloxane (PDMS) tip array with programmable tip apex as a conformal photomask in near-field photolithography. Meanwhile, a new strategy was developed to create the PDMS tip array with graded apex size, which was employed to fabricate gradient patterns with the lateral feature sizes changing from sub-100 nm to several microns on one single substrate over macroscopic (square centimeter) areas. Furthermore, 3D gradient patterns with spatially varying feature height were enabled by employing gradient exposure dose. The formation of gradient feature size was ascribed either to gradient contact areas between tips and substrates or to exposure dose gradient. This lithography strategy combines the advantages of a wide range of feature sizes, simplicity, high-throughput, low-cost and diversified feature shapes, making it a facile and flexible approach to manufacture various functional gradient structures.

Functional gradient effects explain the low transverse shear modulus in spruce – Full-field strain data and a micromechanics model

An important failure mechanism in glulam beams is cracking caused by out-of-plane transverse loads. It has been demonstrated that the low transverse shear modulus G RT in spruce contributes to large transverse strain inhomogeneities due to the annual ring structure in combination with shear coupling effects. In the present study, improved understanding of annual ring effects is achieved by the development of a micromechanical model. It relates the functional density gradient in spruce annual rings to shear modulus G RT . The geometrical basis is a hexagonal cell model, and in shear it is demonstrated to deform primarily by cell wall bending. Full-field strain measurements by digital speckle photography (DSP) show very strong correlation with predicted shear strains at the annual ring scale. Predictions are obtained by implementation of the micromechanics model in a finite element (FE) model developed for the single cube apparatus shear specimen. The low G RT of spruce is due to the strong dependence of G RT on relative density ρ / ρ s ( G RT ∝ ( ρ / ρ s ) 3 ) . This is particularly important in spruce. Even though average density is typically quite high, the functional gradient structure includes local densities as low as 200 kg / m 3 .

Characterization of functional gradient structures in duplex stainless steel castings

Titanium carbide particle reinforced metal-matrix composite coatings, formed on duplex (austenitic/ferritic) stainless steel castings, were studied. These coatings, developed for wear applications, develop a functional gradient structure through a self-propagating high-temperature synthesis (SHS) of titanium carbide (TiC). This SHS-reaction was ignited in reactive inserts by the heat of liquid steel. The reactive inserts were compressed from mixtures of titanium, carbon and molybdenum powders together with iron or duplex stainless steel powders. The coating micro- structure depends on the reactive insert composition. Molybdenum reduced the TiC particle diameter, and reducing the binder content reduced the areal fraction of TiC.