Mark I. Jones

Protein adsorption and platelet attachment and activation, on TiN, TiC, and DLC coatings on titanium for cardiovascular applications

The hemocompatibility of a TiN/TiC/diamond-like carbon (DLC) multilayer structure, deposited on titanium substrates for use as coatings for a heart valve prosthesis, has been studied through the adsorption of blood proteins and the adhesion and attachment of blood platelets. All of the surfaces were characterized by stylus profilometry and water contact angles. The adsorption of albumin and fibrinogen to the surfaces was assessed using the Amido Black assay, whereas platelet attachment was studied by scanning electron microscopy and quantified using stereological techniques. The degree of platelet spreading on the surfaces was seen to correlate with differences in surface energy, indicated from contact angle measurements. The greatest spreading was seen on the more hydrophilic surfaces. When studying protein adsorption to the surfaces, no correlation could be determined between contact angle results and levels of adsorption, although the most hydrophilic surfaces did appear to promote greater amounts of fibrinogen adsorption. Thrombus formation was observed to some degree on all of the surfaces, with the exception of the DLC coating. This coating also promoted less spreading of platelets than the other surfaces. The good hemocompatibility of the DLC coating is attributed to its hydrophobicity and smooth surface, resulting in a higher ratio of albumin to fibrinogen than any of the other surfaces.

HAEMOCOMPATIBILITY OF DLC AND TIC-TIN INTERLAYERS ON TITANIUM

Abstract An interlayer approach was adopted to achieve adequate adhesion between a DLC coating deposited by plasma-assisted CVD and a titanium substrate as part of a study to coat a bi-leaflet heart valve prosthesis. The interlayers (TiC and TiN) were deposited by d.c. ion plating. TiN was also deposited by reactive r.f. sputtering. The substrate, interlayers and DLC were assessed for haemocompatibility and thrombogenicity using a dynamic blood method and interactions with rabbit blood platelets, respectively. The adhesion, activation and morphology of the platelets were determined by stereological techniques using SEM. All surfaces were assessed topographically by stylus profilometry and AFM. Surface energies were compared by liquid drop goniometry. The structure and crystallographic orientation of the TiN interlayers were determined using XRD. The coatings produced no haemolytic effect, except in the case of TiN deposited by reactive r.f. sputtering, which showed a significant effect after longer assessment periods. In contrast to the DLC coating, all of the interlayers showed a slight tendency towards thrombus formation during the later stages of incubation. Platelet spreading correlated with the surface energy of the coatings.

Effect of substrate preparation and deposition conditions on the preferred orientation of TiN coatings deposited by RF reactive sputtering

Titanium nitride (TiN) coatings have been deposited by RF reactive magnetron sputtering of a titanium target, in an atmosphere of argon and nitrogen. The coatings were deposited using different deposition conditions of gas composition, total pressure and indicated substrate temperature, on to titanium substrates that had received different preparation treatments. The structure of the coatings was examined as a function of deposition conditions by X-ray diffraction, and the crystallographic orientation was determined by use of a texture coefficient. The coatings on ground titanium substrates developed a strong (111) orientation from the earliest stages of growth, although the degree of orientation was dependent on deposition conditions. Although gas composition had no effect on film orientation, the intensity of the (111) orientation diminished with increasing pressure, as well as with increasing substrate temperature. On polished titanium substrates, the initial texture was (220), but this changed to (111) orientation with increasing coating thickness. The results are discussed in terms of thermodynamically stable orientations and the kinetics of coating development as a function of atomic matching of the coating and substrate.

A comparison of the abrasive wear behaviour of HVOF sprayed titanium carbide- and titanium boride-based cermet coatings

Cermet coatings are commonly employed to enhance the wear resistance of a wide range of engineering components. Thermal spray techniques, such as plasma spraying and high velocity oxy-fuel (HVOF) spraying are commonly utilised to deposit these coatings. This work outlines a route for cermet powder production based on the technology of self-propagating high-temperature synthesis (SHS) and subsequent HVOF spraying of coatings from these powders. Reactions of elemental mixtures of Fe, Cr, Ti and either C or B powders have resulted in powders consisting predominantly of an Fe(Cr) matrix with a dispersion of either TiC or TiB2 hard particles, ∼5m in size. Coatings ∼100m thick were deposited from these powders and dry sand–rubber wheel abrasive wear tests were performed on these coatings with both alumina and silica abrasives in the 500–600m size range. Both coatings exhibited wear rates superior to those of Ni(Cr)–Cr3C2 coatings deposited from commercially available blended powders with the TiB2-containing coating exhibiting lower wear rates than its TiC-based counterpart. Differences in the wear behaviour are rationalised in terms of the coating microstructures.

Reaction in Al–TiC metal matrix composites

Reactions in Al–10 wt.% TiC metal matrix composites have been investigated by heating samples between 600 and 900°C for 48 h and holding at 700°C for periods up to 240 h. X-ray diffraction, scanning electron microscopy and image analysis have been used to identify the composition, morphology and quantities of the reaction phases present. A maximum reaction rate was observed at 700°C and at this temperature the reaction products formed were large Al3Ti precipitates in the bulk of the matrix and Al4C3 blocks at the particle–matrix interface. At 900°C, TiC appeared to be stable in Al. The reaction kinetics for TiC followed a parabolic rate indicating a diffusion-controlled process. The rate of TiC dissolution is very much less than that for SiC in pure Al.

Wear properties of Y–α/β composite sialon ceramics

Abstract The tribological properties of yttrium containing α/β composite sialon ceramics have been studied under non-lubricated conditions by means of block-on-ring and ball-on-disk type experiments against a commercial silicon nitride material. The sialon ceramics were produced by hot pressing powder mixtures of Si3N4, AlN, Al2O3 and Y2O3, resulting in composite ceramics containing different amounts of the α/β phases. The effects of microstructural differences on the mechanical properties of the ceramics, and their wear characteristics under a range of testing conditions have been assessed. It was found that Vickers hardness decreased whilst both fracture toughness and bending strength increased with increasing amount of β phase in the composite. Under mild testing conditions, material removal was considered to occur by polishing of the surface, and in this case the high α-sialon composites exhibited the highest wear resistance, reflecting their higher hardness. Under severe testing conditions, the wear behaviour was characterised as microcracking caused by the higher Hertzian stress levels, and resulted in grain removal or “dropping” from the surface of the materials. Under these conditions, the elongated microstructure and higher fracture toughness of the low α-sialon composites hinder the crack propagation and result in better wear characteristics when compared to the fine equiaxed α-sialon materials.

Grain growth in microwave sintered Si3N4 ceramics sintered from different starting powders

Abstract Silicon nitride ceramics have been produced by microwave sintering at 28 GHz with Y 2 O 3 , Al 2 O 3 and MgO as sintering additives. The effect of initial β content of the Si 3 N 4 starting powder on the microstructural development has been assessed by scanning electron microscopy (SEM) and quantitative image analysis. Phase transformation behaviour was assessed by X-ray diffraction. Mechanical properties of the sintered bodies were determined through assessment of hardness and fracture toughness. It was found that the samples sintered from powders with lower initial β content developed larger grains than those from higher β content powders, due to fewer nucleation sites during the α→β transformation. However, attempts to develop a more bimodal microstructure by using a mixture of the two grades of powder, in an effort to increase both fracture toughness and fracture strength, were unsuccessful. In this case the microstructure was similar to that developed in the materials produced from higher β content powders. The mechanical properties of the sintered bodies were very similar, despite differences in microstructure. This was attributed to the strong bonding between the grains and grain boundary phase resulting in crack paths in all the materials that were predominantly transgranular, with little debonding or crack deflection. Under these circumstances the effect of larger grains is eliminated.

Complete desorption of interlayer hydrogen phosphate in Mg/Al-layered double hydroxides by means of anion exchange with 1-octanesulfonate

Hydrogen phosphate intercalated Mg/Al-layered double hydroxides (LDH-HPO4) were synthesized by a homogeneous precipitation method with urea and anion exchange. The intercalated hydrogen phosphate was removed by anion exchange with Na2CO3, NaOH–NaCl, and 1-octanesulfonate solutions. The phosphate removal behavior was characterized by X-ray diffraction (XRD), Fourier Transform-InfraRed (FT-IR) spectroscopy, and elemental analysis. Complete removal of interlayer hydrogen phosphate from LDH-HPO4 was realized using 1-octanesulfonate solution. Two reasons are proposed to explain complete hydrogen phosphate removal from the interlayer space of LDH-HPO4. Complete desorption and cyclability suggest this is a viable technique to use as an efficient adsorption/desorption system in industrial applications.

Influence of carbon fibre content on the processing and tribological properties of silicon nitride/carbon fibre composites

Abstract Si 3 N 4 /carbon fibre composites have been fabricated, and the effect of fibre content on the tribological properties was investigated under dry sliding conditions. The friction coefficient of the composites was around 30% of that of a monolithic Si 3 N 4 composite. A fibre content of 5 vol.% was sufficient to maintain a graphite interface during the sliding tests, such that the friction coefficient did not decrease further with increasing graphite fibre content above this level. The carbon fibre content was effective for maintaining a low friction coefficient throughout the duration of the experiment.

Anionic surfactant enhanced phosphate desorption from Mg/Al-layered double hydroxides by micelle formation

Desorption of interlayer hydrogen phosphate (HPO4) from hydrogen phosphate intercalated Mg/Al-layered double hydroxide (LDH-HPO4) by anion exchange with surfactant anions has been investigated under controlled conditions. Three types of surfactant, Dodecylbenzenesulphonate (DBS), Dodecylsulphate (DS) and 1-Octanesulphonate (OS), anions were used for intercalation experiments over a range of concentrations, and for all solutions, it was shown that the desorption of hydrogen phosphate is enhanced at concentrations close to the critical micelle concentration (CMC). Intercalation of the surfactant anions into LDH-HPO4 was confirmed by X-ray diffraction (XRD), Fourier Transform Infrared Spectroscopy (FT-IR) and Scanning electron microscopy (SEM). More than 90% removal of the hydrogen phosphate was achieved at CMC. Repeat adsorption tests to investigate recyclability showed that desorption with 0.005 M DBS improved subsequent phosphate re-adsorption, allowing around 90% of the original adsorption over three cycles. This is much higher than when desorption was conducted using either Na2CO3 or NaCl-NaOH solutions, even at much higher concentrations. This study suggests potential economic and environmental advantages in using these surfactants in improving the cycling performance of LDH materials as absorbents for clean-up of water systems.