Barry Twomey

Co-blasting of titanium surfaces with an abrasive and hydroxyapatite to produce bioactive coatings: Substrate and coating characterisation

The aim of this work is to assess the influence of two blast media on the deposition of hydroxyapatite onto a titanium substrate using a novel ambient temperature coating technique named CoBlast. CoBlast was developed to address the problems with high temperature coating techniques. The blasting media used in this study were Al2O3 and a sintered apatite powder. The prepared and coated surfaces were compared to plasma sprayed hydroxyapatite on the same substrates using the same hydroxyapatite feedstock powder. X-ray diffraction analysis revealed the coating crystallinity was the same as the original hydroxyapatite feedstock powder for the CoBlast samples while evidence of amorphous hydroxyapatite phases and β-TCP was observed in the plasma sprayed samples. The blast media type significantly influences the adhesive strength of the coating, surface roughness of both the substrate and coating and the microstructure of the substrate. The coating adhesion increased for the CoBlasted samples from 50 MPa to 60 MPa for sintered apatite powder and alumina, respectively, while plasma spray samples were significantly lower (5 MPa) when tested using a modified pull-test. In conclusion, the choice of blast medium is shown to be a key parameter in the CoBlast process. This study indicates that sintered apatite powder is the most suitable candidate for use as a blast medium in the coating of medical devices.

Generation of Active Species in a Large Atmospheric-Pressure Plasma Jet

Low-temperature atmospheric-pressure plasma jets (APPJs) are being increasingly used in surface activation, cleaning, wound care, and sterilization applications. The development of successful applications using these systems depends on the ability to tailor the active species generated in the plasma jets to match the treatment requirements. This paper presents an investigation of the effect of sinusoidal drive frequency (20-140 kHz), on a helium discharge formed using an APPJ. The discharge was formed in a large-orifice 16-mm-diameter quartz tube with a treatment area of ≈ 2cm2 at the nozzle exit. Optical, polychromic emission, and thermographic imaging data were correlated with electrical measurements. These measurements indicated that the coupling efficiency was frequency dependent. As a result of differences in coupling efficiency, variations in active species (N2, N2+, O, and NO) present in the discharge were observed. The concentration of active species was also dependent on the distance from the powered electrodes. As well as altering the concentration of active species in the discharge, changes in frequency resulted in higher discharge temperatures (25 °C at 20 kHz to 40 °C at 80 kHz). The temperature was measured on the quartz tube, and steady state was reached after 120 s. This paper presents a detailed analysis of the frequency/distance dependence on the active species in the discharge. This dependence makes it possible to control the active species present at the plasma jet orifice by tailoring the frequency and tube length.

Comparing Deposition Properties in an Atmospheric Pressure Plasma System Operating in Uniform and Nonuniform Modes

A large-scale atmospheric pressure plasma has been generated in helium, and the time-resolved optical and electrical properties have been shown to produce a homogeneous dielectric barrier discharge. Introducing tetraethyl orthosilicate as a liquid aerosol into this plasma produced clear, uniform, and smooth plasma polymerized coatings. Optical imaging studies have shown that adding 1% oxygen to the gas mixture induced a switch from a homogeneous plasma to a filamentary or microdischarge mode of operation, and this has been shown to dramatically alter the morphology of the deposited coatings. Surface analysis reveals significant particulate inclusions in coatings deposited from the filamentary mode of operation.

Blast Coating of Superelastic NiTi Wire with PTFE to Enhance Wear Properties

This work investigates the deposition of polytetrafluoroethylene (PTFE) onto a superelastic NiTi wire using an ambient temperature-coating technique known as CoBlast. The process utilises a stream of abrasive (Al2O3) and a coating medium (PTFE) sprayed simultaneously at the surface of the substrate. Superelastic NiTi wire is used in guidewire applications, and PTFE coatings are commonly applied to reduce damage to vessel walls during insertion and removal, and to aid in accurate positioning by minimising the force required to advance, retract or rotate the wire. The CoBlast coated wires were compared to wire treated with PTFE only. The coated samples were examined using variety of techniques: X-ray diffraction (XRD), microscopy, surface roughness, wear testing and flexural tests. The CoBlast coated samples had an adherent coating with a significant resistance to wear compared to the samples coated with PTFE only. The XRD revealed that the process gave rise to a stress-induced martensite phase in the NiTi which may enhance mechanical properties. The study indicates that the CoBlast process can be used to deposit thin adherent coatings of PTFE onto the surface of superelastic NiTi.

Plasma power can slash small run sintering times

A group of Irish researchers have demonstrated that rapid discharge sintering can dramatically reduce sintering times for processing small numbers of green compacts. But while mainstream sintering technologies have the upper hand for the time being, commercial prospects for plasma are being assessed…

High-Temperature Solar Reflector Coating for the Solar Orbiter

SolarWhite is a diffusely reflective (white) inorganic thermal-control surface tailored for use in high-temperature environments, including European Space Agency’s Solar Orbiter mission. The material is applied via a liquid spray process to substrates including, but not limited to, titanium and aluminum alloys and requires a multistage thermal curing cycle. This coating is required to exhibit long-term thermal/thermo-optical stability under extreme ultraviolet, vacuum-ultraviolet, proton, and electron radiation while possessing the electrical conductivity necessary to prevent surface arcing or internal electrostatic discharge in service, with specific requirements for the Solar Orbiter mission. This paper presents the results of the early ultraviolet/vacuum-ultraviolet-stability testing, carried out in the second Synergistic Temperature-Accelerated Radiation facility space environment simulator at the European Space Technology Centre; the outgassing characteristics of the surface have previously been exam...

Comparison between microwave and microwave plasma sintering of nickel powders

The objective of this study is to investigate the use of microwave plasma treatments as a processing technology for the sintering of metal powders. The volumetric heating process achieved with microwaves is considerably more efficient compared with resistance heating. The sintering study was carried out on 20 mm diameter by 2 mm thick compacted discs of nickel powder, with mean particle size of 1 µm. The discs were fired in a 5 cm diameter microwave plasma ball, under a hydrogen atmosphere at a pressure of 2 kPa. There was an increase in fired pellet transverse rupture strength (TRS) with plasma treatment duration. The mechanical properties of the sintered nickel discs were compared based on TRS, Rockwell hardness tests and density measurements. The morphology of the sintered discs was compared using microscopy and SEM. Comparison disc sintering studies were carried out using both a non plasma microwave and tube furnace firing. Using the microwave plasma sintering process full sintered disc strength of ≈1000 N (based on 3-point bend tests) was achieved after a 10 minute treatment time. In contrast the sintering time in the tube furnace treatment involved total processing time of up to 6 hours. The non plasma microwave system involved intermediate treatment periods of 2 hours. The degree of sintering between the individual nickel powder particles can be precisely controlled by the duration of the treatment time in the plasma.

Evaluation of real-time non-invasive diagnostic tools for the monitoring of a pilot scale atmospheric pressure plasma system

A set of real-time non-invasive multivariate analysis tools were evaluated using LabVIEW software for the process monitoring of an atmospheric pressure plasma system. During system operation, it was observed that the optical and electrical properties are subject to a deterministic jitter effect caused by momentary changes in the discharge characteristics. This temporal jitter in the voltage, current and frequency of the applied power to an atmospheric pressure plasma was monitored to assess the plasma processing conditions. Electrical diagnostic tools were used to determine the transition of the plasma from the primary glow mode to the secondary glow mode and were correlated with photo diode (PD) analysis of the plasma. Intensified charge-coupled device (ICCD) imaging of the plasma was also used to distinguish between glow and Townsend discharge properties at low applied powers (400 W). The electrical observations were recorded in real time, plotted on a principal component analysis (PCA) loading plot and analysed using non-parametric cluster analysis (NPCA). It was observed from the plotted electrical parameters that data clusters were formed which relate to both the geometry of the atmospheric plasma chambers and the mode of plasma operation. The development of these tools facilitates real-time analysis of this reel-to-reel atmospheric pressure plasma processing system.

Novel rapid discharge sintering technique for damage-free diamond metal matrix composites

Abstract The performance of diamond/nickel metal matrix composites (MMCs) produced using a novel microwave plasma technique, rapid discharge sintering (RDS), and conventional tube furnace sintering in argon is compared. The MMCs were sintered at temperatures between 850 and 1050°C in both cases. The RDS treatments were carried out at 20 mbar in plasmas containing hydrogen or hydrogen–nitrogen gases. The addition of nitrogen gas to the hydrogen plasma facilitated a substantial increase in composite firing temperatures. A significant reduction in sintering times, to 10 min from several hours, was achieved using the RDS technique. A further advantage of the RDS treatments was the absence of any diamond graphitisation (as detected by X-ray diffraction), which was reflected in higher sintered densities and flexural strength for RDS than for furnace sintering.

Comparison of thermal and microwaveassisted plasma sintering of nickel–diamond composites

There is considerable interest in processing technologies which can lead to more energy efficient sintering of metal powders. Microwave sintering has been shown to reduce energy usage as volumetric heating is more efficient than resistance heating. Plasma sintering meanwhile delivers heat via uniform excitation of the processing gas. The use of a rapid, novel microwave-assisted plasma sintering (MaPS) technology has been evaluated for processing nickel-diamond composites. Discs fired in a low pressure microwave plasma under a hydrogen atmosphere were compared with discs sintered in a conventional tube furnace. MaPS is very rapid, with full disc strength being achieved within 10 min, compared with 8 h for furnace treatment. MaPS produced similar or superior mechanical properties to furnace sintering but with sintering cycle time reduced by up to 95%.