Formation of compositional gradient in Al/SiC FGMs fabricated under huge centrifugal forces using solid-particle and mixed-powder methods

Formations of graded distribution of SiC ceramic particles within the hollow cylindrical shaped Al/SiC functionally graded materials (FGMs) fabricated by centrifugal solid-particle method (CSPM) and centrifugal mixed-powder method (CMPM) under huge centrifugal force are experimentally and theoretically investigated. The movement of SiC ceramic particles in viscous liquid under centrifugal force is explained based on Stoke's law. The effect of compositional gradient of particles on viscosity is taken into account. Also, the effect of temperature distribution on viscosity and density are considered. A computer code to simulate the formation of compositional gradient in the Al/SiC FGMs fabricated by CSPM and CMPM is developed. From the results, it is found that the volume fraction of SiC ceramic particles can be graded from the inner to the outer surface of hollow cylindrical shaped Al/SiC FGMs by CSPM. Meanwhile by CMPM, the SiC ceramic particles can be dispersed on the outer surface of hollow cylindrical shaped Al/SiC FGMs. The graded distribution in Al/SiC FGMs under huge centrifugal force is found to be significantly affected by the mold temperature but less affected by the temperature of molten Al and casting atmosphere.     

Electrical conductivity and electromagnetic shielding properties of Ti3SiC2/SiC functionally graded materials prepared by positioning impregnation

Ti₃SiC₂/SiC functionally graded materials (FGMs) were prepared by hot-pressing/positioning impregnation. Positioning impregnation is a novel local impregnation technique targeted at layers exhibiting poor sintering behaviour, resulting in uniform densification across the different layers and minimizing the deviation from the designed composition distribution. With the increased Ti₃SiC₂ content in the layer, the resistivity decreased significantly firstly and then decreased slowly and finally tended to be a constant. The resistivity of the layers with SiC content of 80–100 vol.% decreased significantly via positioning impregnation and further decreased by introducing γ-Al₂O₃. However, introducing γ-Al₂O₃ facilitated the decomposition of Ti₃SiC₂ into TiC. The FGMs had a total shielding effectiveness over X band of 43–52 dB and is suitable for potential applications in excellent electromagnetic shielding materials. The dominating shielding mechanism of the FGMs is reflection, and the FGMs had high intrinsic absorption capabilities.     

Experimental analysis on deformation and damage behavior of Al6061/SiC functionally graded plates under low-velocity impact

Due to their promising features provided by ceramic and metal constituents in a single volume, Functionally Graded Materials (FGMs) have received great attention for impact applications. Most of the available studies on the low-velocity impact behavior of FGMs have been carried out by analytical or numerical methods. This study addresses an experimental analysis on the low-velocity impact response of Al6061/SiC FGM plates. The influence of the material composition of the FGM plate (from metal-rich to ceramic-rich) on the energy absorption mechanisms as well as on the deformation and damage behavior was investigated. The ceramic-rich FGM plate exhibits a quasi-brittle response that includes a combination of elastoplastic indentation and brittle failures with increasing impact energy, while the metal-rich and linear FGM plates show elastoplastic behavior. Plastic deformation is the primary energy absorption mechanism for the metal-rich and linear FGM plates, whereas plastic deformation, brittle failures (radial cracks and conoidal crack/fracture), delamination, and pore collapse are effective on the energy absorption of the ceramic-rich FGM plate.     

Characterization of magnesium-hydroxyapatite functionally graded composites prepared by rapid microwave sintering technique

The use of the microwave technique for the sintering of materials has been noticed due to the reduction in processing time, the achievement of uniform microstructure, and high effectiveness. In this study, magnesium/hydroxyapatite (Mg/HA) functionally graded materials (FGMs) in the form of a 2-layer, 3-layer, and 4-layer samples with different contents of HA (0, 5, 10, and 15 wt%) and also single-layer pure Mg samples were successfully fabricated by powder metallurgy and rapid microwave sintering technique. For this purpose, the prepared powders were first mixed and then poured into a die in a layer-by-layer manner at a predetermined arrangement. The stacked powders were compacted under a pressure of 300 MPa and sintered via microwave technique at 530 °C for 12 min under an argon atmosphere. The samples were investigated by the Archimedes test, optical microscope (OM), scanning electron microscopy (SEM), mechanical and corrosion tests. Achieving high relative densities in a short time for pure Mg (99.5%) and its multi-layer composites suggested the high effectiveness of this technique in processing these materials. In addition to the uniform distribution of HA particles in the Mg matrix, the microstructural studies confirmed the proper inter-layer bonding. The mechanical properties of the samples were evaluated by the microhardness indentation method and the compression test. The results showed that the microhardness and compressive strength of the layered samples is much higher than pure Mg. The highest microhardness (63 ± 5 Hv) and compressive strength (148 ± 10 MPa) were observed in the 3-layer sample. The corrosion behavior of the samples was also examined in simulated body fluid (SBF) at 37 °C through immersion tests which indicated superior corrosion resistance of layered samples compared to pure Mg. The 3-layer sample exhibited the best corrosion behavior.     

Fabrication of a material with composition gradient for metal/ceramic assembly

This study presents a method and an apparatus for fabricating graded materials with powders. The application is about making some transition interlayer components when assembling metal to ceramic by brazing for example. The chosen applicative couple is inco600 with zirconia. The densification technique is hot isostatic pressing. After calibration of the equipment designed to make some bi-constituent powder gradients, a cylindrical component with a symmetrical axial gradient has been manufactured. Phase volume fraction composition determined through image analysis and micro-hardness study has demonstrated that the obtained composition and properties vary continuously along the gradient direction. The fabricated material also exhibits some cracks probably due to low sinterability of zirconia used (50 μm balls). Other tests should be done with a finer zirconia powder. This technique should then be employed to fabricate ceramic metal assemblies through the manufacture of intermediated graded parts between the metal and ceramic components.     

聚氯乙烯共混改性研究进展(续)

介绍了近期国内外在聚氯乙烯（PVC）的共混改性领域内的研究和应用进展及其发展方向。     

Understanding centrifugal casting in the manufacture of functionally graded materials

Centrifugal casting is a shaping technique intended for ceramic tubular structure manufacture, where the particle size and density are exploited to produce asymmetrical membranes and functionally graded materials. However, some well-established insights about particle segregation are debatable and remain unclear. For instance, small particles do not necessarily stay in the inner region, and the bigger particles do not accumulate in the outer. Herein several manufacturing parameters were studied through Taguchi’s robust design, using discrete element method-based simulations to generate the data. Alumina powder was used as model material and water as the liquid phase, to assess the green cast formation time, the cast thickness, the roundness, the changes in relative density, and particle size distribution along the cast’s cross-section. The mean particle diameter and the rotation speed are the most influential parameters for the casting time. The volume fraction of solids on the precursor slurry is decisive regarding the cast structural properties, and particle segregation is negligible, except for size differences above one order of magnitude. When a fraction of denser nickel powder was added, density segregation was also observed, but the size differences can overshadow its effect. In addition, alumina and nickel particles were cast in a lab-scale centrifuge and experimentally validated the segregation of particles. The centrifugal casting method was successfully applied for producing the Al₂O₃-Ni composites with a gradient distribution of the Ni phase.     

The adhesive contact problem between a graded coated half-space and a cylindrical indenter by using a Maugis model

In this study, the adhesive contact problem for a functionally graded coating with exponentially varying modulus is investigated. The MD model is used to describe the adhesion effect between a rigid cylindrical indenter and a functionally graded coated half-space. The method of Fourier integral transform is applied to convert the controlling equation of the two-dimensional adhesive contact problem to the singular integral equations with Cauchy kernel. The numerical calculation is conducted to achieve the effects of the adhesion parameter and the stiffness ratio on the pull-out force, indentation, contact stresses and adhesion region. The results provide a way to improve the performance of coating on contact phenomena.     

Extrusion-based additive manufacturing of functionally graded ceramics

The Ceramic On-Demand Extrusion (CODE) process has been recently proposed for additive manufacturing of dense, strong ceramic components via extrusion with uniform layered drying. This study focuses on enabling CODE to fabricate functionally graded ceramics. A controlled volumetric flowrate for each ceramic paste was used to achieve a gradient between alumina and zirconia. A dynamic mixer was built to mix constituent ceramic pastes homogeneously. Functionally graded alumina/zirconia samples were printed, sintered, and tested to examine the capability of CODE in fabricating functionally graded components. The desired and actual material compositions were compared using energy dispersive spectroscopy. Dimensions of sintered samples were evaluated to study the deformation of functionally graded components during drying and sintering. Vickers hardness was also measured at different locations, corresponding to different material compositions. Finally, a case study was conducted to demonstrate the capability of the proposed method to build functionally graded ceramics with complex geometries.     

Cobalt doped glass for the fabrication of percolated glass–alumina functionally graded materials

The present research was focused on the development of a new glass to produce glass–alumina FGMs. The glass formulation, belonging to the CaO–ZrO₂–SiO₂ system, was doped with cobalt, by adding a small molar percentage (about 0.1 mol%) of CoO, in order to obtain a blue glass, which could be useful to appreciate the final compositional gradient. The glass was accurately characterized, evaluating its thermal behaviour, its mechanical properties, and its attitude to crystallize during a thermal treatment. Subsequently, the glass was used to produce glass–alumina FGMs via percolation and the so obtained specimens were analysed in order to evaluate the effect of the glass infiltration. The possible development of new crystal phases, in particular, was tested via micro X-ray diffraction and the elastic properties gradient associated with the compositional gradient was measured via depth-sensing Vickers microindentation.     

One-step Sinter-HIP method for preparation of functionally graded cemented carbide with ultrafine grains

Ultrafine crystalline functionally graded cemented carbides (FGCCs) with a surface zone enriched in binder phase were prepared by a one-step Sinter-HIP method. The influence of sintering pressure and cubic carbide composition on the formation of gradient layer was examined. The results show that the ultrafine FGCC with surface zone enriched in binder phase can be formed by the one-step Sinter-HIP method. The process of the gradient layer formation is accelerated under higher sintering pressure; the gradient layer thickness increases with the sintering pressure increasing. The gradient layer thickness is controlled by diffusion distance of cubic carbide formers, such as Ti, Ta and Nb. The addition of (Ta,Nb)C leads to decrease the thickness of gradient layer.     

Formation of cylindrical phase structure in PMMA/HBP polymer blend films

The evolution of the phase separation was investigated for poly(methyl methacrylate)(PMMA)/hyperbranched poly(ester-amide)(HBP) blend films on glass substrate by means of phase contrast microscopy. The films with different component ratios show different phase separation processes and phase morphologies. At a film thickness of about several hundreds nanometers, a cylindrical dispersed phase was observed in the films with lower HBP content. The effects of the composition and sample thickness on the formation of the special morphology were also studied. It is found that the interaction between the substrate and HBP and the thickness of blend film are essential factors for the formation of the phase morphology and the appearance of the special cylindrical morphology depends on the component ratio and the film thickness. There is a critical film thickness, above which the special morphology could be observed. The critical thickness varies as the HBP weight percent changes. Our research provides a possible strategic way to obtain polymer films with special structure which are important for an increasing number of applications in wide fields.     

Functionally graded adhesive joints by graded mixing of nanoparticles

The main objective of the present study was to use a functionally modified adhesive in order to have an adhesive with properties that vary gradually along the overlap, allowing a uniform stress distribution along the overlap. This allows for a stronger and more efficient adhesive joint. It is possible to work with much smaller areas, reducing considerably the weight of the structure and obtaining more reliable joints. The adhesive stiffness would vary along the overlap, being maximum in the middle and minimum at the ends of the overlap. The processes tested in this work were dielectric heating using a domestic microwave oven and conventional oven heating. Different amounts of carbon black were used and functionally dispersed along the bondline length in order to obtain a functionally graded joint. The functionally graded joints were found to have a higher joint strength compared to the cases where the carbon black was dispersed uniformly along the overlap or where the adhesive was used without carbon black. An analytical model was used to assist with the prediction and the assessment of the possible effectiveness of a graded joint concept.     

External normal pressure prevents preferential wetting of PS/PMMA blend thin films

Substrate wetting of the component(s) of thin polymer blend films strongly dictates their phase evolution during thermal annealing. In the case of wetting by one component being preferential than the other, a continuous wetting layer at the substrate will form. Here, we report that the preferential wetting of PMMA within a PS/PMMA thin film can be prevented under normal pressure. Moreover, the external pressure drives the PMMA wetting layer at the substrate (or a PMMA cushion layer intentionally placed between the blend film and the superstrate) into the isolated PMMA domains within the blend film. This results in a film morphology normally observed on neutral surfaces, revealing that normal pressure can potentially be used to effectively control the blend film morphology by preventing the hydrodynamic wetting.     

Stress reduction mechanisms during photopolymerization of functionally graded polymer nanocomposite coatings

From the experimental analysis of the photocuring process in terms of reaction kinetics as well as modulus and shrinkage build-up, the residual stresses arising during the photopolymerization of functionally graded composite coatings based on an acrylate matrix and Fe₃O₄@SiO₂ core@shell nanoparticles are evaluated through a Finite Element Modeling approach. Owing to the monotonous variation of volume fraction of the constituent phases that influences the local conversion of the polymeric matrix, these coatings are able to decrease the residual stresses at the coating/substrate interface by as much as ≈25% compared to those encountered in composites with homogeneous compositions, and by as much as ≈40% compared to those arising in the pure polymer. The influence of substrate stiffness, nanoparticle stiffness and conversion degree of the polymer matrix was also analyzed, providing further information for the optimization of the stress reduction mechanism in graded nanocomposite coatings.     

Microstructure and properties of Nb2O5/Mg gradient coating on AZ31 magnesium alloy by magnetron sputtering

A Nb₂O₅/Mg gradient coating was synthesized on AZ31 magnesium alloy through the magnetron sputtering technique. The microstructure and properties of the coating were investigated by SEM, AFM, EDS and XPS, scratch tester, nanoindenter, friction tester, and electrochemical workstation, with a Nb₂O₅ monolayer coating as a control. The results show that all the as-disposed films have an amorphous columnar structure, and can improve the mechanical, anti-wear and anti-corrosion properties of AZ31 magnesium alloy. The Nb₂O₅/Mg gradient coating shows a gradual change in chemical composition through its depth, decreasing the residual stress of the coating/substrate system, thus reducing the risk of film cracking, and increasing compactness of the coating. Compared with a Nb₂O₅ single-layer film of the same thickness, the gradient coating exhibits increased adhesion, H/E and H³/E² increased by about 16 times, 7.8% and 100% respectively, and a wear rate reduced by more than an order of magnitude. In addition, the gradient coating has better corrosion resistance, having a two orders of magnitude lower current density and one order of magnitude higher polarization resistance. This study provides a workable strategy for improving the performance of ceramic coatings on magnesium alloy, for medical applications.     

Depth profiling of a functionally graded alumina/calcium-hexaluminate composite using grazing incidence synchrotron-radiation diffraction

Grazing incidence synchrotron radiation diffraction (GISRD) has been successfully used for near-surface depth profiling of phase composition, texture and residual strains in a functionally-graded alumina/calcium-hexaluminate (CA₆) composite prepared by infiltration process. Depth profiling of near surface information both in the nanometre and micrometre ranges have been done by using angles of incidence below and above the critical angle (_c) for total external reflection. The penetration depth increased to several hundred angstroms as approached _c. Above _c there was a rapid increase in penetration depth to a thousand angstroms or more. As the penetration depth increased the intensity of CA₆ peaks relative to those of alumina became less intense, indicating a distinct gradation in the phase abundance. The distribution of CA₆ grains at the near-surface was highly textured and showed a distinct depth-dependent gradation in texture. The presence of graded residual strains in the composite due to thermal expansion mismatch between the phases has been computed and verified from the display of line shifts. The unique but powerful capability of GISRD as a complementary tool for depth profiling the near-surface information of graded materials has been demonstrated in this work.     

Fabrication process of smooth functionally graded materials through a real-time inline control of the component ratio

Functionally graded materials (FGMs) play an essential role in tissue engineering because of their satisfactory histocompatibility and excellent mechanical performance. While traditional manufacturing methods allow production of simple FGMs, precise control of composition and customized property at transition between the dissimilar materials is still a challenge. Here, an extrusion-based functionally graded additive manufacturing (FGAM) platform was developed to generate smooth graded parts by thrusting out monolithic cylindrical filaments with high viscosity. Furthermore, the rheological properties, hydrodynamic behavior, and mixed homogeneity of the non-Newtonian fluids were studied. Therefore, the appropriate solid contents, alternative energy-efficient mixers, and optimized printing parameters were proved to be beneficial for an outstanding deposition effect of the suspension. Ultimately, an object with smooth gradient was successfully manufactured. The validity of this strategy was verified via optical microscopy combined with an image processing method to gauge homogeneity and a scanning electron microscope to investigate graded composition and microstructure.     

Direct additive manufacturing of melt growth Al2O3-ZrO2 functionally graded ceramics by laser directed energy deposition

Functionally graded ceramics (FGC), which combine properties of different ceramics in one part, usually have better comprehensive function and structural efficiency. In this study, four different gradient transition Al₂O₃-ZrO₂ FGC samples were prepared by laser directed energy deposition (LDED) method. The results show that there is an obvious interface in direct transition sample. The transition section bears tensile stress caused by difference of thermophysical properties of materials, resulting in significant longitudinal cracks. Element transition in interface region shows a step sharp transition. The direct transition sample shows intergranular fracture and the bonding strength is very low. Gradient transition mode can effectively suppress cracks, and avoid the step transition of microstructure and elements. Elements, microhardness of 25, 20 wt% FGC samples realized a nearly linear smooth transition. The interface fracture of FGC samples changed to transgranular fracture, bonding strength was significantly improved, and the maximum flexural strength reached 160.19 MPa.