Mark Koopman

An electrospun triphasic nanofibrous scaffold for bone tissue engineering

A nanofibrous triphasic scaffold was electrospun from a mixture of polycaprolactone (PCL), type-I collagen and hydroxyapatite nanoparticles (nano-HA) with a mixture dry weight ratio of 50/30/20, respectively. Scaffolds were characterized by evaluating fiber morphology and chemical composition, dispersion of HA particles and nanoindentation. Scanning electron microscopy revealed fibers with an average diameter of 180 +/- 50 nm, which coincides well with the collagen fiber bundle diameter characteristic of the native extracellular matrix of bone. The triphasic fibers, stained with calcein and imaged with confocal microscopy, show a uniform dispersion of apatite particles throughout their length with minor agglomeration. Scaffold fibers of triphasic (50/30/20), collagen/nano-HA (80/20), PCL/nano-HA (80/20), pure PCL and pure collagen were each pressure consolidated into non-porous pellets for evaluation by transmission electron microscopy and nanoindentation. While the majority of apatite particles are uniformly dispersed having an average size of 30 nm, agglomerated particles as large as a few microns are sparsely distributed. Nanoindentation of the pressure-consolidated scaffolds showed a range of Youngs modulus (0.50-3.9 GPa), with increasing average modulus in the order of (PCL < PCL/nano-HA < collagen < triphasic < collagen/nano-HA). The modulus data emphasize the importance of collagen and its interaction with other components in affecting mechanical properties of osteoconductive scaffolds.

Microstructure-based simulation of thermomechanical behavior of composite materials by object-oriented finite element analysis

Abstract While it is well recognized that microstructure controls the physical and mechanical properties of a material, the complexity of the microstructure often makes it difficult to simulate by analytical or numerical techniques. In this paper we present a relatively new approach to incorporate microstructures into finite element modeling using an object-oriented finite element technique. This technique combines microstructural data in the form of experimental or simulated microstructures, with fundamental material data (such as elastic modulus or coefficient of thermal expansion of the constituent phases) as a basis for understanding material behavior. The object-oriented technique is a radical departure from conventional finite element analysis, where a “unit-cell” model is used as the basis for predicting material behavior. Instead, the starting point of object-oriented finite element analysis is the actual microstructure of the material being investigated. In this paper, an introduction to the object-oriented finite element approach to microstructure-based modeling is provided with two examples: SiC particle-reinforced Al matrix composites and double-cemented WC particle-reinforced Co matrix composites. It will be shown that object-oriented finite element analysis is a unique tool that can be used to predict elastic and thermal constants of the composites, as well as salient effects of the microstructure on local stress state.

Mechanical properties of a hybrid cemented carbide composite

Microstructural effects on the mechanical properties of a hybrid metal matrix composite, double cemented (DC) carbide, have been investigated. DC carbide contains granules of WC/Co cemented carbide in a matrix of cobalt. Overall composite hardness increases with decreased granule cobalt content as well as with decreased intergranular matrix fraction of cobalt. High-stress abrasive wear resistance also increases with decreased granule cobalt content and matrix fraction. Fracture toughness of the composite increases with increased cobalt matrix fraction and to a lesser extent with increased granule cobalt content. Increased granule size increases both fracture toughness and wear resistance. DC carbide exhibits a superior combination of fracture toughness and high-stress wear resistance than conventional cemented carbide. The combination of toughness and wear resistance in the composite improves with increased granule hardness.

Axial fatigue behavior of binder-treated versus diffusion alloyed powder metallurgy steels

A comparative study has been conducted on the microstructure, tensile, and axial fatigue behavior of two Fe‐0.5Mo‐1.5Cu‐ 1.75Ni alloys, made by binder-treated and diffusion alloying processes. The mechanical properties will be explained in terms of the pore size and morphology, as well as the heterogeneous microstructures typical of ferrous powder metallurgy materials. Binder treatment can provide a variety of advantages in manufacturing, over diffusion alloyed powders, including faster and more consistent flow into the die cavity, increased green strength, and reduction of fine particle dusting. In addition to conventional porosity, smaller, ‘‘copper diffusion’’ pores were observed where copper particles had been prior to forming a liquid phase during sintering and diffusing into the Fe particles. The microstructure in both alloys was typical of P:M alloy steels, with a heterogeneous microstructure consisting of areas of ‘‘divorced pearlite,’’ martensite, and nickel-rich ferrite. The modulus and tensile strength of both types of alloys were equivalent. Yield strength in the binder-treated alloy was higher which coincided with somewhat lower ductility. The fatigue behavior in terms of stress versus cycles (S‐N curves) was almost identical for the two systems. Fractographic observations showed fracture to have initiated primarily at pore clusters in the surface region. Fracture surfaces after fatigue tests showed ductile fracture in the interparticle bridge regions, cleavage facets in pearlitic regions, and striations.

Nanoindentation Behavior of Nanolayered Metal-Ceramic Composites

Small-length scale multilayered structures are attractive materials due to their extremely high strength and flexibility, relative to conventional laminated composites. In this study, nanolayered laminated composites of Al and SiC were synthesized by DC/RF magnetron sputtering. The microstructure of the multilayered structures was characterized, and the mechanical properties measured by nanoindentation testing. The influence of layer thickness on Young’s modulus and hardness of individual and multilayers was quantified. An analytical model was used to subtract the contribution of the Si substrate, to extract the true modulus of the films.

Electrophoretic Deposition of Carbon Nanotubes on Metallic Surfaces

A method based on electrophoretic deposition has been developed to produce uniform deposits of multi-walled carbon nanotubes on stainless steel substrates. Aqueous suspensions were used under constant voltage conditions in the range of 5 to 50 V, with deposition times ranging from 0.5 to 10 minutes. The thickness of the coatings was controlled by variation of voltage and deposition time during EPD. Coatings of up to 100μm thickness were achieved, which exhibit homogeneous microstructure. The EPD technique is fast, cost-effective, and it can be applied to complex shapes. Possible applications of CNT coatings are in heat extraction devices or porous nanostructured coatings for tissue engineering scaffolds.

Thermal-shock behavior of a Nicalon-fiber-reinforced hybrid glass-ceramic composite

Abstract A Nicalon-fiber-reinforced hybrid composite with a matrix of barium magnesium aluminosilicate (BMAS) glass with silicon carbide whiskers was subjected to thermal shock from elevated to ambient temperatures. The combination of SiC whisker and BMAS glass resulted in a hybrid matrix with a lower thermal expansion than that of the fibers, inducing tensile stresses in the fiber upon thermal shock. This stress state resulted in microstructural damage in the form of fiber cracking and cracking along the fiber/matrix interface, as opposed to the conventional matrix cracking which is typically observed in ceramic-matrix composites. Significant damage in the composite was only observed after three thermal shock cycles. Flexural resonance measurements, used to evaluate thermal shock-induced changes in Youngs modulus, showed a reduction in modulus that correlated well with the onset of microstructural damage. Finally, fiber push-out tests, performed to evaluate changes in fiber/matrix interface strength after thermal cycling, indicated a slight decrease in interfacial strength, which was attributed to recession of the carbon-rich fiber surface during thermal shock.

Powder metallurgy of titanium – past, present, and future

ABSTRACT Powder metallurgy (PM) of titanium is a potentially cost-effective alternative to conventional wrought titanium. This article examines both traditional and emerging technologies, including the production of powder, and the sintering, microstructure, and mechanical properties of PM Ti. The production methods of powder are classified into two categories: (1) powder that is produced as the product of extractive metallurgy processes, and (2) powder that is made from Ti sponge, ingot, mill products, or scrap. A new hydrogen-assisted magnesium reduction (HAMR) process is also discussed. The mechanical properties of Ti-6Al-4V produced using various PM processes are analyzed based on their dependence on unique microstructural features, oxygen content, porosity, and grain size. In particular, the fatigue properties of PM Ti-6Al-4V are examined as functions of microstructure. A hydrogen-enabled approach for microstructural engineering that can be used to produce PM Ti with wrought-like microstructure and properties is also presented. Abbreviations: AM: additive manufacturing; ARC: Albany Research Center; BE: blended elemental; BUS: broken-up structure; CCGA: close-coupled gas atomisation; CHIP: CIP-sinter-HIP; CIP: cold isostatic pressing; CP-Ti: commercially pure Ti; DRTS: direct reduction of Ti-slag; CSIR: Council for Scientific and Industrial Research (South Africa); CSIRO: Commonwealth Scientific and Industrial Research Organization (Australia); EIGA: electrode induction gas atomisation; EMR: electronically mediated reduction; FFC: Fray, Farthing, and Chen; GA: gas atomisation; GIF: gaseous isostatic forging; GSD: granulation-sintering-deoxygenation; HAMR: hydrogen-assisted magnesium reduction; HDH: hydride–dehydride; HIP: hot isostatic pressing; HSPT: hydrogen sintering and phase transformation; MA: master alloy; MER: Materials & Electrochemical Research Corporation (US); MHR: metal hydride reduction; MIM: metal injection molding; OM: optical microscope; OS: Ono and Suzuki; PA: pre-alloyed; P/C: performance to cost ratio; PIF: pneumatic isostatic forging; PM: powder metallurgy; PREP: plasma rotating electrode process; PP: post-processing; PS: press and sinter; QIT: Quebec Iron & Titane, Inc. (Canada); SEM: scanning electron microscope; SPS: spark plasma sintering; SOM: solid oxide membrane; THP: thermohydrogen processing; TMP: thermomechanical processing; UFG: ultrafine grain; UGS: upgraded titanium slag; UTS: ultimate tensile strength; USTB: University of Science and Technology Beijing (China); VA: vacuum atomisation; VHP: vacuum hot pressing; WP: wrought process; YS: yield strength

Microstructure/hardness relationship in a dual composite

Double cemented (DC) carbide [1–3] is a novel dual composite composed of granules of conventional cemented carbide (WC + Co) dispersed in a metal matrix, typically cobalt (see Fig. 1). This concept of a “composite-within-a-composite” facilitates microstructural design to optimize properties for different applications. Properties of the overall composite are variable through control of granule and matrix properties, granule size and volume fraction, typically 70 to 90%. Granule properties are controlled by the grade of cemented carbide granules, i.e., Co content and WC particle size. Matrix properties are controlled by the matrix metal employed, modifiable through alloying or heat treatment. Studies to date have shown that this microstructural flexibility can achieve unusual combinations of properties, such as improved toughness and wear resistance compared to conventional cemented carbide, due to the larger metal matrix mean free path [2]. The particulate nature of the granules enables fairly conventional powder processing. DC carbide is currently being introduced in oil well drill bit inserts and should have use in many other applications requiring combined wear resistance and toughness. This material is also an ideal model for systematically investigating the microstructure/property relationships of a particulate reinforced dual composite, a design concept readily extendable to other metal and ceramic systems. Prior studies by others have investigated particulate dual composites containing lower volume fractions of cemented carbide granules in steel matrices [3–7] to improve wear and thermal shock resistance. Several

Determination of Elastic Constants in WC/Co Metal Matrix Composites by Resonant Ultrasound Spectroscopy (RUS) and Impulse Excitation

Elastic moduli were determined by impulse excitation and resonant ultrasound spectroscopy (RUS) techniques for composite systems of WC/Co. Two types of composites were used, including a novel microstructure termed double cemented carbides consisting of WC/Co granules in a Co matrix, as well as a conventional isotropic WC/Co. The results compared favorably with theoretical values predicted by the bounds of the Hashin-Shtrikman equations for quasi-isotropic materials. Various aspects of the RUS and impulse excitation techniques are discussed, particularly in regard to composite materials.