Christopher C. Berndt

A review of the application of anodization for the fabrication of nanotubes on metal implant surfaces.

Metal implants are the best choice for the long-term replacement of hard tissue, such as hip and knee joints, because of their excellent mechanical properties. Titanium and its alloys, due to their self-organized oxide layer, which protects the surface from corrosion and prevents ion release, are widely accepted as biocompatible metal implants. Surface modification is essential for the promotion of the osseointegration of these biomaterials. Nanotubes fabricated on the surface of metal implants by anodization are receiving ever-increasing attention for surface modification. This paper provides an overview of the employment of anodization for nanotubes fabricated on the surface of titanium, titanium alloys and titanium alloying metals such as niobium, tantalum and zirconium metal implants. This work explains anodic oxidation and the manner by which nanotubes form on the surface of the metals. It then assesses this topical research to indicate how changes in anodizing conditions influence nanotube characteristics such as tube diameters and nanotube-layer thickness.

Transition metal-substituted cobalt ferrite nanoparticles for biomedical applications

Transition metals of copper, zinc, chromium and nickel were substituted into cobalt ferrite nanoparticles via a sol-gel route using citric acid as a chelating agent. The microstructure and elemental composition were characterized using scanning electron microscopy combined with energy-dispersive X-ray spectroscopy. Phase analysis of transition metal-substituted cobalt ferrite nanoparticles was performed via X-ray diffraction. Surface wettability was measured using the water contact angle technique. The surface roughness of all nanoparticles was measured using profilometry. Moreover, thermogravimetric analysis and differential scanning calorimetry were performed to determine the temperature at which the decomposition and oxidation of the chelating agents took place. Results indicated that the substitution of transition metals influences strongly the microstructure, crystal structure and antibacterial property of the cobalt ferrite nanoparticles.

Cell response of anodized nanotubes on titanium and titanium alloys.

Titanium and titanium alloy implants that have been demonstrated to be more biocompatible than other metallic implant materials, such as Co-Cr alloys and stainless steels, must also be accepted by bone cells, bonding with and growing on them to prevent loosening. Highly ordered nanoporous arrays of titanium dioxide that form on titanium surface by anodic oxidation are receiving increasing research interest due to their effectiveness in promoting osseointegration. The response of bone cells to implant materials depends on the topography, physicochemistry, mechanics, and electronics of the implant surface and this influences cell behavior, such as adhesion, proliferation, shape, migration, survival, and differentiation; for example the existing anions on the surface of a titanium implant make it negative and this affects the interaction with negative fibronectin (FN). Although optimal nanosize of reproducible titania nanotubes has not been reported due to different protocols used in studies, cell response was more sensitive to titania nanotubes with nanometer diameter and interspace. By annealing, amorphous TiO2 nanotubes change to a crystalline form and become more hydrophilic, resulting in an encouraging effect on cell behavior. The crystalline size and thickness of the bone-like apatite that forms on the titania nanotubes after implantation are also affected by the diameter and shape. This review describes how changes in nanotube morphologies, such as the tube diameter, the thickness of the nanotube layer, and the crystalline structure, influence the response of cells.

The 2016 Thermal Spray Roadmap

Considerable progress has been made over the last decades in thermal spray technologies, practices and applications. However, like other technologies, they have to continuously evolve to meet new problems and market requirements. This article aims to identify the current challenges limiting the evolution of these technologies and to propose research directions and priorities to meet these challenges. It was prepared on the basis of a collection of short articles written by experts in thermal spray who were asked to present a snapshot of the current state of their specific field, give their views on current challenges faced by the field and provide some guidance as to the R&D required to meet these challenges. The article is divided in three sections that deal with the emerging thermal spray processes, coating properties and function, and biomedical, electronic, aerospace and energy generation applications.

Selection of the Implant and Coating Materials for Optimized Performance by Means of Nanoindentation

Mechanical compatibility between a coating and a substrate is important for the longevity of implant materials. While previous studies have utilized the entire coating for analysis of mechanical compatibility of the surface, this study focuses on the nanoindentation of a uniformly thermally sprayed splat. Hydroxyapatite was thermally sprayed to create a homogeneous deposit density, as confirmed by microRaman spectroscopy, of amorphous calcium phosphate. Substrates were commercially pure Ti, Ti-6Al-4V, Co-Cr alloy and stainless steel. Nanoindentation revealed that splats deposited on the different metals have similar hardness and elastic modulus values of 4.2 ± 0.2 GPa and 80 ± 3 GPa, respectively. The mechanical properties were affected by the substrate type more than residual stresses, which were found to be low. It is recommended that amorphous calcium phosphate is annealed to relieve the quenching stress or that appropriate temperature histories are chosen to relax the stress created in cooling the coating assembly.

Development of surface nano-crystallization in alloys by Surface Mechanical Attrition Treatment (SMAT)

Nanometer-sized grain structures that exhibit a large number of grain boundaries on the surface of a bulk material demonstrate excellent properties relative to their coarse-grained (CG) equivalents. Surface modification using surface mechanical attrition treatment (SMAT) is an option that cab be used to tailor the corrosion, tribological, mechanical, and chemical reaction properties of a surface. SMAT is an effective route to create the nanostructured surface layer. The SMAT process has unique advantages compared with the other coating and deposition techniques for surface nanocrystallization. For example, SMAT does not alter the chemical composition of the nanocrystalline surface layer in the matrix. In addition, SMAT has been demonstrated to activate the material surface layer by surface modification and enhance the atomic diffusivity. This article presents a review of the advantages offered by the SMAT technique for the creation of high performance surface layers. The influence of the created nanocrystalline layer on mechanical, physical, and chemical properties is assessed. Developments and the current status of the surface nanolayer that are formed are evaluated from a physical approach. Finally, prospects for the future development of grain refinement on the surface of a material matrix and potential applications are presented.

Nanocomposite coatings: thermal spray processing, microstructure and performance

Abstract The processing of nanomaterials and nanocomposites has advanced since the 1990s. The growth and opportunities afforded by this technological domain is evident through the trends of research and development (R&D) funding, Science Citation Index (SCI) publications, and patent applications presented in this paper. This article reviews the current state for the development of thermal sprayed nanocomposite coatings. The types of nanocomposite thermal spray feedstock materials that are available commercially, as well as those currently in the development phase, are critically assessed. The thermal spray approaches to deposit nanocomposite coatings are discussed, including the conventional plasma spray and high velocity oxygen fuel (HVOF) processes and the more recently developed cold spray, suspension thermal spray (STS), and solution precursor thermal spray (SPTS) processes. These processes are assessed in relation to their deposition mechanisms and the specific nanocomposite materials associated with each technique. The unique microstructure of the coatings deposited by each method is highlighted in relation to process and compositional control. The exceptional attributes of nanocomposite coatings, such as mechanical strength and toughness, wear resistance, thermophysical, and electrical properties, are also presented together with specific applications.

Effect of Power and Stand-Off Distance on Plasma Sprayed Hydroxyapatite Coatings

Plasma sprayed hydroxyapatite coatings were deposited onto mild steel substrates under several power and stand-off distance conditions. The microstructure of the coatings was observed and imaged using scanning electron microscopy (SEM). The crystal structure and crystallinity of the coatings were analyzed using X-ray diffraction (XRD). The Ca/P ratio of the coating was measured using energy dispersive X-ray spectroscopy (EDX). The deposition efficiency of the coatings was measured by weighing the substrate before and after coating. The porosity, surface roughness, and microhardness of the deposited coatings were characterized by using ImageJ, surface profilometer, and Vickers microhardness tester, respectively. The results indicated that microhardness and deposition efficiency increased with increasing power and stand-off distance. Porosity, surface roughness, and crystallinity decreased as the Ca/P ratio steadily decreased.

Improving the Generalization Ability of an Artificial Neural Network in Predicting In-Flight Particle Characteristics of an Atmospheric Plasma Spray Process

This paper presents the application of the artificial neural network into an atmospheric plasma spray process for predicting the in-flight particle characteristics, which have significant influence on the in-service coating properties. One of the major problems for such function-approximating neural network is over-fitting, which reduces the generalization capability of a trained network and its ability to work with sufficient accuracy under a new environment. Two methods are used to analyze the improvement in the network’s generalization ability: (i) cross-validation and early stopping, and (ii) Bayesian regularization. Simulations are performed both on the original and expanded database with different training conditions to obtain the variations in performance of the trained networks under various environments. The study further illustrates the design and optimization procedures and analyzes the predicted values, with respect to the experimental ones, to evaluate the performance and generalization ability of the network. The simulation results show that the performance of the trained networks with regularization is improved over that with cross-validation and early stopping and, furthermore, the generalization capability of the networks is improved; thus preventing any phenomenon associated with over-fitting.

Sputtered Hydroxyapatite Nanocoatings on Novel Titanium Alloys for Biomedical Applications

Titanium and titanium alloys have been extensively studied for many applications in the area of bone tissue engineering. It was believed that the excellent properties of titanium al‐ loys, e.g. lightweight, excellent corrosion resistance, high mechanical strength and low elas‐ tic modulus compared to other metallic biomaterials such as stainless steels and Cr-Co alloys, would provide enhanced stability for load-bearing implants. However, they usually lack sufficient osseointegration for implant longevity, and their biocompatibility is also an important concern in these applications due to the potential adverse reactions of metallic ions with the surrounding tissues once these metallic ions are released from the implant sur‐ faces. One approach for consideration to improve the healing process is the application of a hydroxyapatite nanocoating onto the surface of biomedical devices and implants. Hydrox‐ yapatite, with its excellent biocompatibility, and similar chemistry and structure to the min‐ eral component of bone, provides a bioactive surface for direct bone formation and apposition with adjacent hard tissues. The deposition of a SiO2 interlayer between the im‐ plant surface and the hydroxyapatite nanocoating is necessary to further improve the bio‐ compatibility of metal implants, as SiO2 has its own excellent compatibility with living tissues, and high chemical inertness, which lead to enhanced osteointegrative and functional properties of the system as a whole.