Showing papers by "Liangchi Zhang published in 2018"

A review on metallic porous materials: pore formation, mechanical properties, and their applications

Abstract: This paper presents a comprehensive review on the mechanism of pore formation, mechanical properties, and applications of metallic porous materials. The different manufacturing techniques of metallic porous materials using various pore-forming agents (e.g., sodium chloride, polymethyl methacrylate, magnesium, and cenosphere) are highlighted in the first part of this review. Subsequently, the pore formation mechanism and pore morphology in final products as well as corresponding pore-forming agent removal techniques (e.g., sintering-dissolution process, thermally stimulated decomposition, thermally melted elimination, and embedding cenosphere technique) are specifically discussed. Then, some major influential factors on the mechanism of pore formation, including pore size, shape, distribution, and porosity, are analyzed in detail. Meanwhile, the primary mechanical properties such as compressive strength, elastic modulus, fatigue properties, and flexural strength of metallic porous materials depending on pore morphology and porosity are explored in detail. Furthermore, their applications in structural and functional aspects according to their pore morphology and mechanical properties are emphatically summarized. Finally, this review article highlights some important factors for advanced wear-resistant tool and biomedical implant applications of porous metallic materials.

Effective Mechanical Properties and Thickness Determination of Boron Nitride Nanosheets Using Molecular Dynamics Simulation

Abstract: Research in boron nitride nanosheets (BNNS) has evoked significant interest in the field of nano-electronics, nanoelectromechanical (NEMS) devices, and nanocomposites due to its excellent physical and chemical properties. Despite this, there has been no reliable data on the effective mechanical properties of BNNS, with the literature reporting a wide scatter of strength data for the same material. To address this challenge, this article presents a comprehensive analysis on the effect of vital factors which can result in variations of the effective mechanical properties of BNNS. Additionally, the article also presents the computation of the correct wall thickness of BNNS from elastic theory equations, which is an important descriptor for any research to determine the mechanical properties of BNNS. It was predicted that the correct thickness of BNNS should be 0.106 nm and the effective Young’s modulus to be 2.75 TPa. It is anticipated that the findings from this study could provide valuable insights on the true mechanical properties of BNNS that could assist in the design and development of efficient BN-based NEMS devices, nanosensors, and nanocomposites.

Effect of micropores on the microstructure and mechanical properties of porous Cu-Sn-Ti composites

Abstract: The porous Cu-Sn-Ti composites were fabricated by using a sintering-dissolution process with carbamide particles as space holders. The macroporosity of the composites was consistently controlled. The micropores among the pore-walls and wall intersections were quantitatively analyzed by using 2D micrographs and 3D X-ray tomographic reconstruction technique. The size, shape and distribution of the micropores and their effect on the mechanical properties of the composites (compressive strength, Young's modulus, and flexural strength) were investigated in relation to the composite formation mechanism and micropores evolution. It was found that the amount of graphite particles has a significant impact on the micropores formation and further on the mechanical properties of the bulk material. The rapid increase in the number of micropores as well as the gradual decrease in the compressive and flexural strengths were found to be caused by the increase of the graphite particle concentration. The mechanical performance could be improved by optimizing the alloy composition content and arranging the distribution of carbamide particles.

A micromechanics analysis of the material removal mechanisms in the cutting of ceramic particle reinforced metal matrix composites

Abstract: In previous investigations on the cutting of ceramic particle reinforced metal matrix composites using the finite element (FE) method, the particles are usually considered to be rigid. This is inco...

Characterisation of high thermal conductivity thin-film substrate systems and their interface thermal resistance

Abstract: This paper characterises the high thermal conductivity thin-film substrate systems and the interface thermal resistances between the films. First, three-omega (3ω) method was proposed and verified, obtaining a reliable thermal conductivity measurement at shallow thermal-penetration depths. The method was then applied to a thin-film/substrate system to identify the individual thermal conductivities. The interface thermal resistance between the thin films was then successfully characterised with the aid of theoretical modelling and experimental measurements. As an example of the method application in the IC industry, the AlN/Si systems were investigated. The study identified that the thermal conductivity of the 2 μm-thick AlN film in an AlN/Si system is 172.1 W/mK and the AlN/Si interface thermal resistance is 1.796 × 10− 9 m2 K/W.

A simple approach for analysing the surface texture transfer in cold rolling of metal strips

Abstract: This paper presents a simple approach for analysing the surface texture transfer in cold rolling of metal strips. The approach made use of the advantages of the slab method and accommodated the surface roughness effect of a rigid work roll. A numerically generated rough surface, whose heights generally follow a Gaussian distribution and distribute transversely, was used in the calculation. The transient distribution of contact stresses and instant texture transfer were then predicted. The interface contact pressure and friction stresses predicted by the established method were verified by the finite element method under the same rolling conditions. It was found that the new approach is efficient and cost-effective. The application of the approach revealed that due to the surface texture of the work roll, the interface stress in the rolling bite can be discontinuous, and that a higher roughness transfer ratio can be expected when reduction ratio increases.

Critical loading conditions of amorphization, phase transformation, and dilation cracking in 6H-silicon carbide

Abstract: Amorphization, phase transformation, and dilation cracking are 3 major deformation/failure mechanisms of monocrystalline 6H-SiC. This paper studies their critical formation conditions and mechanisms under hydrostatic pressure and uniaxial compression and tension with the aid of large-scale molecular dynamics simulations. It was found that under hydrostatic pressure the major deformation mechanism is amorphization, that under uniaxial compression the major mechanism turns to phase transformation at low temperature and amorphization at high temperature, and that under uniaxial tension the dominating mechanism becomes dilation cracking. Increasing the temperature reduces the thresholds significantly and brings about a heterogeneous deformation mode. The study further concluded that these deformation mechanisms and their thresholds can be predicted theoretically.

Deformation of Ti-Based Bulk Metallic Glass Under a Cutting Tip

Abstract: This study aims to investigate the fundamental deformation mechanism of Ti-based bulk metallic glass under mechanical loading by a cutting tip. The cutting tip interaction is resolved into two separate actions, namely nanoindentation by using a Berkovich diamond indenter and contact sliding under a pin-on-disk configuration. The deformation details in the specimens due to the action of the corresponding mechanical loading were analyzed by SEM and TEM. It was found that under nanoindentation, the plastic deformation is evidenced by a series of pop-ins in load–displacement graphs. The temperature rise at the contact interface during sliding must have been above the glass transition temperature and the onset temperature of crystallization, leading to the formation of discrete nanocrystalline particles in the immediate subsurface and hence bringing about an increase in hardness of the metallic glass in the wear tracks.

Investigation into the room temperature creep-deformation of potassium dihydrogen phosphate crystals using nanoindentation

Abstract: It has been a tremendous challenge to manufacture damage-free and smooth surfaces of potassium dihydrogen phosphate (KDP) crystals to meet the requirements of high-energy laser systems. The intrinsic issue is whether a KDP crystal can be plastically deformed so that the material can be removed in a ductile mode during the machining of KDP. This study investigates the room temperature creep-deformation of KDP crystals with the aid of nanoindentation. A stress analysis was carried out to identify the creep mechanism. The results showed that KDP crystals could be plastically deformed at the nano-scale. Dislocation motion is responsible for creep-deformation. Both creep rate and creep depth decrease with decrease in peak force and loading rate. Dislocation nucleation and propagation bring about pop-ins in the load-displacement curves during nanoindentation.

Thermal aging effects on microstructure, elastic property and damping characteristic of a eutectic Sn–3.5Ag solder

Abstract: The present work describes the microstructural changes and their impacts on the electrical resistivity, elastic modulus and damping property of a eutectic Sn–3.5wt% Ag solder material when exposed to high temperature. A detail microstructural study was conducted through the scanning electron microscopy with energy-dispersive spectroscopy analysis and electron backscattered diffraction technique. In as-cast eutectic Sn–Ag solder alloy, sub-micrometer size Ag3Sn intermetallic compound (IMC) particles and bamboo-like dendritic structure with a dimension of length 20–30 µm and width 3–5 µm formed during solidification. However, after thermal aging treatment at 150 °C for 60 days, the fine Ag3Sn IMC particles and β-Sn grain appeared with coarse microstructure with the formation of twinning having the twin axis and 60° rotation. As a result, microstructure and Sn-crystal orientation of Sn–Ag solder greatly impact on its overall properties and turned inferior. From material properties evaluation, it was confirmed that the electrical resistivity, elastic and shear moduli values were significantly reduced with aging time. Consequently, the values of damping capacity improved due to the reduction of moduli.

Mechanisms of the Complex Thermo-Mechanical Behavior of Polymer Glass Across a Wide Range of Temperature Variations.

Abstract: This paper aims to explore the mechanisms of the complex thermo-mechanical behavior of polymer glass across a wide range of temperature variations. To this end, the free vibration frequency spectrum of simply supported poly(methyl methacrylate) (PMMA) beams was thoroughly investigated with the aid of the impulse excitation technique. It was found that the amplitude ratio of the multiple peaks in the frequency spectrum is a strongly dependent on temperature, and that the peaks correspond to the multiple vibrational modes of the molecular network of PMMA. At a low temperature, the vibration is dominated by the overall microstructure of PMMA. With increasing the temperature, however, the contribution of the sub-microstructures is retarded by β relaxation. Above 80 °C, the vibration is fully dominated by the microstructure after relaxation. The relaxation time at the transition temperature is of the same order of the vibration period, confirming the contribution of β relaxation. These findings provide a precise method for establishing reliable physical-based constitutive models of polymer glass.

Metallic glass hardening after thermoplastic forming

Abstract: Thermoplastic forming is an important manufacturing technique for the shaping of metallic glass (MG) components. However, property changes of MGs after such forming are unclear. This paper systematically studied the effect of thermoplastic forming on the structure and mechanical properties of a Zr-based MG. It was found that the MG after forming is of higher hardness; yet this effect increases with the forming temperature rise. Localized shear deformation as commonly observed around a Vickers indentation mark in MGs was not found around the indentation mark of the thermoplastically formed MG. The study concluded that the primary mechanism of such property change of the MG is due to the free volume annihilation induced by the structural relaxation during the thermoplastic forming.

Effects of sliding speed and lubrication on the tribological behaviour of stainless steel

Abstract: Due to their specific properties, the rolling of stainless steel can experience adhesion-induced ‘heat scratch’. This paper investigates experimentally the effects of sliding speed and lubrication on the tribological behaviour of stainless steel with the aid of the pin-on-disc technique. The disc was made from the commonly used roll material, i.e. 5Cr in production, and the pins were of the 443 stainless steel, the typical stainless steel that experiences heat scratch problems. To explore the lubricant effects, the surface topography of the pins was compared with those obtained under dry contact sliding. The sliding-induced debris was also analyzed. It was found that there is a threshold of sliding speed beyond which the adhesion of stainless steel occurs and sliding process can become unstable. A larger contact pressure tends to lower down such speed threshold. Lubricant and additives can considerably reduce the interface friction, wear rate and surface damage of the stainless steel.