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.     

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.     

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.     

Interfacial failure analysis of functionally graded adhesively bonded double supported tee joint of laminated FRP composite plates

This paper deals with three-dimensional non-linear finite element analyses to assess the structural behavior of adhesively-bonded double supported tee joint of laminated FRP composites having embedded interfacial failures. The onset of interfacial failures is predicted by using Tsai–Wu coupled stress failure criterion with pre-determined stress values. The concept of fracture mechanics principle is utilized to study the sustainability of the tee joint having interfacial failures pre-existed at the critical locations. Individual modes of the strain energy release rates (SERR) G_I, G_II and G_III, are considered as the damage growth parameters and, are evaluated using the Modified crack closure integral (MCCI) technique based on the concept of linear elastic fracture mechanics (LEFM). Based on the stress analyses, it has been observed that the interfacial failures in tee joint structure trigger at the interface of base plate and adhesive layer from both ends of base plate. Depending on the SERR magnitudes, it has been noticed that the interfacial failure propagates under mixed mode condition. Therefore total SERR (G_T) is considered as the governing parameter for damage propagation. Furthermore, efforts have been made to retard damage propagation rate by employing functionally graded adhesive (FGA) instead of monolithic adhesive material. Series of numerical simulations have been performed for varied interfacial failure length in functionally graded adhesively bonded double supported tee joint structure in order to achieve the significant effect of FGA with various modulus ratios on SERR. Material gradation of adhesive indicates significant SERR reduction at the incipient stage of failure which necessitates the use of functionally graded adhesive for the tee joint and prolong the service life of the structure.     

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.     

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.     

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.     

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.     

Estimation of the compositional gradient in a PVC/PMMA graded blend prepared by the dissolution-diffusion method

We reported a new preparation method in the previous paper, by which an excellent graded polymer blend was simply prepared. In this report, we propose a new model for the dissolution-diffusion process. The model was derived by obeying Fick's second law for diffusion and by assuming that the evaporation of the solvent in PMMA solution during the diffusion could be neglected. We then proved that this model was applicable to the graded structure of a PVC/PMMA graded blend. Further, it was confirmed that the dissolution rate of PVC into PMMA solution remained constant during preparation. Thus, our model was found to be reliable. However, while the apparent diffusion coefficients (D_ABs) of PVC in 0.183 and 0.274 ml/cm² of the initial solution volumes were equal to each other, they were larger than the D_AB in 0.091 ml/cm² of the initial solution volume. Thus, the effects of PVC concentration dependency of D_AB and changing of PMMA concentration in the solution on D_AB were discussed, because all of the D_ABs should be equal to each other. Then, our model was modified as follows. We considered that PVC finished diffusing below a PMMA concentration of 0.18 g/ml, because the diffusion coefficient of PVC immediately increased at a higher PMMA concentration. Then, the solution layer was shrunk to about 1/6, in order to form a blend film. Thus, it was found that the D_AB in each of the initial solution volumes was 4.2 ± 0.25 × 10⁻⁸ cm²/s. Further, the D_ABs estimated by diffusion thickness method, were from 1.5 to 3.5 times larger than those estimated by our model. Therefore, it was proved that our modified model was applicable to the experimental data.     

Design and analysis of functionally graded adhesively bonded double supported tee joint of laminated FRP composite plates under varied loading

The present research deals with three-dimensional nonlinear finite element analyses for a functionally graded adhesively bonded tee joint made of laminated fiber reinforced polymeric composites when the tee joint is subjected to different types of loadings. The out-of-plane stress components have been evaluated along the interfacial surfaces of bond line of the tee joint. Using the stress values, the failure indices are computed by using Tsai–Wu coupled stress failure criterion in order to predict the location of onset of failures within the interfacial surfaces. Accordingly, critical location is identified based on the magnitude of failure indices for varied load conditions. It has been observed that tee joint under bending load is vulnerable for early failure compared with that when the joint is subjected to tensile and compressive loading. The location of failure is found to be different in tee joint under bending load compared with tensile and compressive loadings. Further, efforts have been made to reduce out-of-plane stress concentration by implementing functionally graded adhesive (FGA) with appropriate smooth and continuous gradation function profile. Further, effects of material gradation function profile with varied modulus ratios on out-of-plane stresses and failure indices are observed along the different interfacial surfaces. Series of numerical simulation result significant reduction in peak values failure index. Based on the present research findings, the FGA is recommended for higher and efficient joint strength. Results also exhibit delayed failure onset and improved structural integrity in the tee joint structure with the use of FGA material.     

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.     

A study on the transition between neighbouring drum segregated bands and its application to functionally graded material production

The composition transitions between two neighbouring segregated bands in a rotating drum were studied using binary glass beads of 6 different size ratios. The concentration gradient of the coarser particles in the transition zone increased dramatically when the particle size ratio increased from 1.68 to 2.01 and then gradually decreased as the particle size ratio increased from 2.01 to 3.37. The concentration gradient profile could be explained by the differences of the dynamic angles of repose between the two components of the binary mixture. A novel, convenient and particulate-level composition controlled FGM processing method is proposed based on the transition zone structure between two neighbouring segregated bands. A Cu/Al₂O₃/Cu functionally graded material was successfully produced based on the proposed method using binary mixture of Cu and Al₂O₃ powders.     

Mechanical behaviour and pore morphology of functionally graded alumina preforms and their composites

Functionally-graded ceramic composites were produced using a hot pour-and-set method via freeze casting of alumina slurries with solid loading between 40% and 20%, with gelatine as a binder. The slurry and additives were tailored for controlling the microstructure and mechanical properties, such as pore morphology, preform density and compressive strength. Varying the gelatine concentration between 2.5% and 9%, transformed the pore morphology from lamelllar to honeycomb and into closed cell. At 3% concentration, increasing the solid loading from 10% to 30% yielded higher compressive strength from 48 MPa to 317 MPa. The resultant compressive behaviour closely matched to Gibson-Ashby closed cell predictive model. Alumina/epoxy composite mechanical performance plateau as the solid loading increased; the 20% solid loading composite produced the best performance. The compressive strengths of the alumina/epoxy and alumina/aluminium composites were on average 300% and 1110% higher than their respective preform counterparts, across a solid loading range of 10–20%.     

Fatigue behavior and damage analysis of PIP C/SiC composite

In this work, we study the fatigue behavior of a C/SiC composite produced by several cycles of polymer infiltration and pyrolysis (PIP). Fatigue tests were performed with maximum stresses corresponding to 60–90% of the tensile strength of the composite. During the fatigue tests, acoustic emission (AE) monitoring was performed and the measured AE energy was utilized to quantify the damage and distinguish possible damage mechanisms. Most of the fatigue damage in the form of matrix cracking, interface damage and fiber breakage occurs in the first cycle. As loading cycles proceeded, damage in form of matrix crack re-opening and interfacial friction constantly accumulates. Nevertheless, all samples survived the run-out of 1,000,000 cycles. After the fatigue tests, an increase of the tensile strength is observed. This phenomenon is associated with the relief of process-induced internal thermal stresses and the weakening of the fiber-matrix interface. In general, the studied material shows very high relative fatigue limit of 90% of its tensile strength.     

In-vitro electrochemical and bioactivity evaluation of SS316L reinforced hydroxyapatite functionally graded materials fabricated for biomedical implants

Three sets of FGMs of stainless steel 316L (SS) reinforced with micro-, nano- and mixed (1:1 mass ratio) hydroxyapatite (HA) were fabricated by powder metallurgy route. The concentration of the HA was varied from 0 to 20 wt% with the increment of 5 wt% in all sets to strengthen the discrete layers of FGMs. The sintered densities along the discrete layers of FGMs continually decreased as a function of HA content which enhanced the bioactivity of the FGMs towards the end with high content of HA. All FGMs experienced an excellent corrosion response in the 0.9% NaCl solution with high passivity. Heavy diffusion of chromium (Cr) form SS to HA made the matrix Cr deficient. The chromium depletion regions around the interface of SS and HA caused an active corrosion behavior of FGMs in 0.1 M HCl solution. FGMs with micro-sized HA demonstrated the better corrosion properties than that of other FGMs. After being immersed in the simulated body fluid (SBF) solution for 7 days, the apatite layer formed on the surface of FGMs with micro-sized HA had a mature spherical morphology and leaf like shape for the other two FGMs. The apatite morphology and gained weight results proved the highest bioactivity for micro-sized HA reinforced FGMs.     

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 TiCp reinforced Al2O3-ZrO2 eutectic functionally graded ceramics by laser directed energy deposition

Laser directed energy deposition (LDED) provides an ideal manufacturing technique to produce ceramic matrix composites, which are endows with superior and customizable mechanical properties by the reinforcement of gradient distribution of rigid particles. In this paper, we for the first time manufactured TiCp reinforced Al₂O₃-ZrO₂ eutectic functionally graded ceramics with two different transition modes using LDED. The results show that the gradient transition realizes gradually increase of TiCp particles in Al₂O₃-ZrO₂ eutectic matrix. With the increase of TiCp content, the morphology of Al₂O₃-ZrO₂ eutectic matrix changes from lamellar or rod-shaped to irregular shape. Meanwhile, LDED realizes controllable fabrication of properties in different positions of gradient materials, and the wear resistance of TiCp rich regions has increased by 43.4% compared with pure Al₂O₃-ZrO₂ region.     

Experimental additive manufacturing of green body of SiC/Graphite functionally graded materials by stereolithography

Ceramic stereolithography (CSL)-additive manufacturing (AM) technology is used to create a functionally graded ceramic (FGC) green body made of silicon carbide (SiC) and graphite. For the SiC/graphite FGC, the mixing parameters of ceramics powders and ultraviolet (UV) curing resin are improved, and correlations of the resultant slurry curing depth with integrated light intensity are discovered. Therefore, the SiC/graphite FGC-produced green body has no flaws, pores, or cracks on its surfaces. According to the association between cure depth and integrated light density for each slurry's composition, several interfacial collapses discovered in a cracked cross-section might be decreased.     

Centrifugally cast functionally graded materials: Fabrication and challenges for probable automotive cylinder liner application

In response to stringent environmental rules and rising public awareness, internal combustion (IC) engines have undergone fast improvement to reduce friction and wear in recent decades. Liner is a sacrificial engine component that protects and provides a smooth reciprocation surface to the engine block. High hardness aluminum alloys are used to produce cylinder liners. When reinforced with suitable ceramic particles, the strength, hardness, stiffness, thermal stability, and wear resistance of these alloys are improved. Here, the underlying challenges in liners are discussed, along with various solutions. The detailed fabrication process of centrifugally cast functionally graded composite materials (FGMs) for prospective use as liners is emphasized. Various parameters and their effect on mechanical and tribological properties are discussed in depth, and a comparison is made with existing Aluminum liners. A general framework for optimal material selection, parameter selection, and processing procedure is proposed to develop the FGM liner. In addition, challenges, research opportunities, and possibilities for the development of this field are presented.     

Effects of different production techniques on glass–alumina functionally graded materials

Glass–alumina functionally graded materials were obtained using two different methods: percolation, which was representative of natural transport based processes, and plasma spraying, which was representative of constructive processes. The specimens produced in this way were investigated to evaluate the effect of production techniques on the final microstructure and gradient, which, in turn, govern the properties and performances of the graded systems. Moreover, post-production heat treatments were performed in order to improve the reliability of the materials examined.