Khurram S. Munir

Deterioration of the Strong sp2 Carbon Network in Carbon Nanotubes during the Mechanical Dispersion Processing—A Review

ABSTRACT The unique mechanical, thermal, and electrical properties of carbon nanotubes (CNTs) make them an ideal reinforcement for the metal matrix composites (MMCs). The successful incorporation of CNTs as reinforcement in MMCs can result in the development of lightweight and high-strength structures which can eventually result in weight savings for the automobile and aerospace industries. In the last two decades extensive research has been carried out to improve the dispersion of CNTs in metal and polymer matrices. Challenges remain to effectively disperse CNTs within the matrix materials with minimal damage during the composite processing stages. The ultra-high Youngs modulus and other superior mechanical and thermal properties of CNTs have been attributed to the strong sp2 carbon-carbon (C-C) bonds present in their structures. In order to fully utilize the unique properties of CNTs as reinforcement, damages to CNTs in the form of damaging these sp2C-C bonds have to be minimized. A variety of processing techniques have been developed to fabricate CNTs reinforced MMCs but mechanical alloying (MA) via powder metallurgy (PM) is most widely used process to develop the nano-composites. The role of processing variables during PM and their effects on the structural integrity of CNTs have been reviewed in this work. Governing principles to predict the mechanical properties of CNTs with incorporating the key process variables are deduced. With the help of these governing equations, critical study of the processes parameters and their effects on the structural integrity of CNTs, it is possible to optimize the processing methodologies of CNTs reinforced MMCs and get the maximum benefit from the unique properties of CNTs. It is assumed that better dispersion of CNTs in the metal matrices, retaining the structural integrity of CNTs and optimization of process parameters would result in better mechanical and tribological properties of CNTs reinforced MMCs.

Calcium Phosphate-Based Composite Coating by Micro-Arc Oxidation (MAO) for Biomedical Application: A Review

ABSTRACT Calcium phosphate (Ca-P) based composites have attracted great attention in the scientific community over the last decade for the development of biomedical applications. Among such Ca-P-based structures, carbonate apatite (CA) and hydroxyapatite (HA) materials have received much attention in the clinical and biomedical fields, mainly because of their unique biological characteristics. These characteristics can promote the biocompatibility of implant materials and osseointegration between the implant and host bone. Various studies have been carried out on the fabrication of Ca-P coatings on orthopedic and dental implants using the micro-arc oxidation (MAO) process; however, there has not been a comprehensive review of the control of MAO parameters to achieve an optimal coating structure. This article presents a critical analysis of the synthesis techniques that have been adopted for the fabrication of Ca-P-based coatings on both commercially pure titanium (CP-Ti) and biomedical grade Ti alloys. Moreover, this work elucidates the influence of MAO processing parameters such as electrolyte concentration, pH value, voltage, and time on the crystal structure and surface morphology of Ca-P coatings. It is shown that the surface thickness, crystal structure, and surface morphology of Ca-P coatings directly influence their biocompatibility.

Extraordinary high strength Ti-Zr-Ta alloys through nanoscaled, dual-cubic spinodal reinforcement

While titanium alloys represent the current state-of-the-art for orthopedic biomaterials, concerns still remain over their modulus. Circumventing this via increased porosity requires high elastic admissible strains, yet also limits traditional thermomechanical strengthening techniques. To this end, a novel β-type Ti-Zr-Ta alloy system, comprised of Ti-45Zr-10Ta, Ti-40Zr-14Ta, Ti-35Zr-18Ta and Ti-30Zr-22Ta, was designed and characterized mechanically and microstructurally. As-cast, this system displayed extremely high yield strengths and elastic admissible strains, up to 1.4GPa and potentially 1.48%, respectively. This strength was attributed to a nanoscaled, cuboidal structure of semi-coherent, dual body-centered cubic (BCC) phases, arising from the thermodynamics of interaction between Ta and Zr; this morphology occurring with dual BCC-phases is heretofore unreported in Ti-based alloys. Further, cell proliferation investigated by MTS assay suggests this was achieved without sacrificing biocompatibility, with no significant difference to either empty-well or commercially-pure Ti controls noted. STATEMENT OF SIGNIFICANCE The current research details microstructural, mechanical, and biological investigations into four novel biomedical alloys in a hitherto uninvestigated region of the Ti-Zr-Ta alloy system; Ti-45Zr-10Ta, Ti-40Zr-14Ta, Ti-35Zr-18Ta and Ti-30Zr-22Ta. We find that the investigated alloys display 0.2% yield strengths of up to 1.40GPa and elastic admissible strains of up to 1.48%, along with biological properties comparable to that seen in the conventional metallic biomaterial ASTM Grade-2 CP-Ti, achieved in the complete absence of traditional thermomechanical processing techniques. This is attributed to the presence of a dual-BCC cuboidal nanostructure, achieved via spinodal decomposition; while similar structures have been reported in e.g. Ni-based superalloys, we believe this is the first such structure investigated in a Ti-based material. As such, this work is felt to be of great interest in aiding the design and manufacture of highly-biocompatible, porous, metallic biomaterials for orthopedic application.

Chemical and structural analyses of the graphene nanosheet/alumina ceramic interfacial region in rapidly consolidated ceramic nanocomposites:

Graphene nanosheets (GNS) reinforced Al2O3 nanocomposites were prepared by a rapid sintering route. The microhardness and fracture toughness values of the resulting nanocomposites simultaneously increased due to efficient graphene nanosheet incorporation and chemical interaction with the Al2O3 matrix grains. The properties enhancement is attributed to uniformly dispersed graphene nanosheet in the consolidated structure promoted by high surface roughness and ability of graphene nanosheet to decorate Al2O3 nanoparticles, strong GNS/Al2O3 chemical interaction during colloidal mixing and pullout/crack bridging toughening mechanisms during mechanical testing. The GNS/Al2O3 interaction during different processing stages was thoroughly examined by thermal and structural investigation of the interfacial area. We report formation of an intermediate aluminum oxycarbide phase via a confined carbothermal reduction reaction at the GNS/Al2O3 interface. The graphene nanosheet surface roughness improves GNS/Al2O3 mechanical attachment and chemical compatibility. The Al2O3/GNS interface phase facilitates efficient load transfer, thus delaying failure through impediment of crack propagation. The resulting nanocomposites, therefore, offer superior toughness.

Titanium-niobium pentoxide composites for biomedical applications

The strength of titanium scaffolds with the introduction of high porosity decreases dramatically and may become inadequate for load bearing in biomedical applications. To simultaneously meet the requirements of biocompatibility, low elastic modulus and appropriate strength for orthopedic implant materials, it is highly desirable to develop new biocompatible titanium based materials with enhanced strength. In this study, we developed a niobium pentoxide (Nb2O5) reinforced titanium composite via powder metallurgy for biomedical applications. The strength of the Nb2O5 reinforced titanium composites (Ti-Nb2O5) is significantly higher than that of pure titanium. Cell culture results revealed that the Ti-Nb2O5 composite exhibits excellent biocompatibility and cell adhesion. Human osteoblast-like cells grew and spread healthily on the surface of the Ti-Nb2O5 composite. Our study demonstrated that Nb2O5 reinforced titanium composite is a promising implant material by virtue of its high mechanical strength and excellent biocompatibility.

Investigation of tip sonication effects on structural quality of graphene nanoplatelets (GNPs) for superior solvent dispersion

The exceptional properties of graphene and its structural uniqueness can improve the performance of nanocomposites if it can attain the uniform dispersion. Tip sonication assisted graphene solvent dispersion has been emerged as an efficient approach but it can cause significant degradation of graphene structure. This study aimed to evaluate the parametric influence of tip sonication on the characteristics of sp2 carbon structure in graphene nanoplatelets by varying the sonication time and respective energy at three different amplitudes (60%, 80% and 100%). The study is essential to identify appropriate parameters so as to achieve high-quality and defect-free graphene with a highly desirable aspect ratio after solvent dispersion for composite reinforcement. Quantitative approach via Raman spectroscopy is used to find the defect ratio and lateral size of graphene evolved under the effect of tip sonication parameters. Results imply that the defect ratio is steady and increases continually with GNPs, along with the transformation to the nano-crystalline stage I up to 60 min sonication at all amplitudes. Exfoliation was clearly observed at all amplitudes together with sheet re-stacking due to considerable size reduction of sheets with large quantity. Finally, considerable GNPs fragmentation occurred during sonication with increased amplitude and time as confirmed by the reduction of sp2 domain (La) and flake size. This also validates the formation of edge-type defect in graphene. Convincingly, lower amplitude and time (up to 60 min) produce better results for a low defect content and larger particle size as quantified by Raman analysis.

Effects of solution treatment and aging on the microstructure, mechanical properties, and corrosion resistance of a β type Ti–Ta–Hf–Zr alloy

Titanium and some of its alloys have become increasingly important for biomedical materials due to their high specific strength, good corrosion resistance, and excellent biocompatibility compared to the biomedical stainless steels and cobalt–chromium based alloys. In this study, a β type TTHZ alloy (Ti–40Ta–22Hf–11.7Zr) was prepared with the cold-crucible levitation technique. The corrosion behavior and the effects of solution treatment (ST) and aging on the microstructures and mechanical properties of the TTHZ alloy were investigated using electrochemical analysis, XPS (X-ray photoelectron spectroscopy), OM (optical microscopy), XRD (X-ray diffractometry), TEM (transmission electron microscopy) and compressive testing. The results indicate that the as-cast alloy exhibited a β + ωath microstructure, which transformed into a single β phase after ST at 900 °C for 1 h. The β phase further transformed into β + α′′, β + α′′ + α, and β + α + ωiso after aging for 15 min, 1.5 h, 12 h and 24 h, respectively. The different phases of the TTHZ alloy showed significantly different mechanical properties and corrosion behavior. The solution-treated TTHZ alloy exhibited a compressive yield strength of approximately 1018 MPa and an excellent compressive strain as no fracturing was observed; and the compression tests were stopped at a compressive strain of ∼70%. The TTHZ alloy after solution treatment plus aging exhibited an increase in the compressive yield strength with a decreased compressive strain. The solution-treated TTHZ alloy exhibited a single β phase with the highest corrosion resistance, compared to the as-cast and solution-treated alloy, followed by aging samples. The open-circuit potential (OCP) analysis indicates that the corrosion resistance of the as-cast TTHZ alloy was superior to those of both CP-Ti and Ti6Al4V.

Metallic scaffolds manufactured by selective laser melting for biomedical applications

Abstract Additive manufacturing (AM) has emerged as a promising technique for fabricating customized tissue engineering scaffolds and implants. The scaffolds not only act as temporary support structures for bone ingrowth and tissue regeneration but also provide temporary biological functions. The complex defect sites in human knee, dental, hip, skull, and craniofacial require a precise design of scaffold to match and fit in those defect sites. These scaffolds are designed to stay in the human body as implants for longer periods. This requires matching of mechanical properties and biocompatibility of these scaffolds with surrounding hard tissues. Extensive research has been carried out to provide patients with productive and cost-effective solutions by fabricating complex and near net shape metallic scaffolds using the potential of laser-assisted AM techniques, ie, selective laser sintering (SLS) and selective laser melting (SLM). This chapter focuses on the development of metallic scaffolds and implants using such laser-assisted AM techniques. For this reason, discussion on SLS is limited to fundamentals, as this particular AM technique is not useful for fabricating metallic scaffolds. However, the discovery of SLS laid the foundation for the development of SLM by incorporating high-energy lasers, which are useful for developing near net shape and complicated metallic scaffolds with minimum defects in their microstructures. SLM has been explored exclusively for fabricating tissue engineering scaffolds and implants from a wide range of metals. This chapter elucidates the key processing parameters of SLM and their effects on the functionality of fabricated scaffolds. The progress, limitations, challenges, and potential of SLM in the manufacturing of metallic scaffolds and implants are also explained. An overview of the research gaps and future research directions is provided.