Material properties and tribological performance of rf-PECVD deposited DLC coatings

Diamond-like carbon (DLC) coatings were deposited on to silicon, glass and metal substrates, using an rf-plasma enhanced chemical vapour deposition (rf-PECVD) process. The resultant film properties were evaluated in respect of material and interfacial property control, based on bias voltage variation and the introduction of inert (He and Ar) and reactive (N2) diluting gases in a CH4 plasma. The analysis techniques used to assess the material properties of the films included AFM, EELS, RBS/ERDA, spectroscopic, electrical, stress, microhardness, and adhesion. These were correlated to the tribological performance of the coatings using wear measurements. The most important observation is that He dilution (>90%) promotes enhanced adhesion with respect to all substrate material studies. Coatings typically exhibit a microhardness of the order of 10-20 GPa in films 0.190%) promotes enhanced adhesion with respect to all substrate materials studied. Coatings typically exhibit a microhardness of the order of 10-20 GPa in films 0.1 < d < 2 μm thick, with associated electrical resistivity in the range 108 < ρ < 1012 Ω·cm, coefficient of friction <0.1 and surface RMS roughness as low as 2 A. The results are discussed with respect to surface pre-treatment, ion surface bombardment, interfacial reactivity and changes in plasma gas breakdown processes.

Dispersion rheology of carbon nanotubes in a polymer matrix

We report on rheological properties of a dispersion of multiwalled carbon nanotubes in a viscous polymer matrix. Particular attention is paid to the process of nanotubes mixing and dispersion, which we monitor by the rheological signature of the composite. The response of the composite as a function of the dispersion mixing time and conditions indicates that a critical mixing time t* needs to be exceeded to achieve satisfactory dispersion of aggregates, this time being a function of nanotube concentration and the mixing shear stress. At shorter times of shear mixing t< t*, we find a number of nonequilibrium features characteristic of colloidal glass and jamming of clusters. A thoroughly dispersed nanocomposite, at t> t*, has several universal rheological features; at nanotube concentration above a characteristic value nc ∼2-3 wt. % the effective elastic gel network is formed, while the low-concentration composite remains a viscous liquid. We use this rheological approach to determine the effects of aging and reaggregation. © 2006 The American Physical Society.

Electron microscopy of polymer-carbon nanotubes composites

Characterization of polymer nanocomposites by electron microscopy has been attempted since last decade. Main drives for this effort were analysis of dispersion and alignment of fillers in the matrix. Sample preparation, imaging modes and irradiation conditions became particularly challenging due to the small dimension of the fillers and also to the mechanical and conductive differences between filler and matrix. To date, no standardized dispersion and alignment process or characterization procedures exist in the trade. Review of current state of the art on characterization of polymer nanocomposites suggests that the most innovative electron and ion beam microscopy has not yet been deployed in this material system. Additionally, recently discovered functionalities of these composites, such as electro and photoactuation are amenable to the investigation of the atomistic phenomena by in situ transmission electron microscopy. The possibility of using innovative thinning techniques is presented. © 2010 Copyright SPIE - The International Society for Optical Engineering.

Tailoring the electrical properties of carbon nanotube-polymer composites

Advances in functionality and reliability of nanocomposite materials require careful formulation of processing methods to ultimately realize the desired properties. An extensive study of how the variation in fabrication process would affect the mechanism of conductivity and thus the final electrical properties of the carbon nanotube-polymer composite is presented. Some of the most widely implemented procedures are addressed, such as ultrasonication, melt shear mixing, and addition of surfactants. It is hoped that this study could provide a systematic guide to selecting and designing the downstream processing of carbon nanocomposites. Finally, this guide is used to demonstrate the fabrication and performance of a stretchable (pliable) conductor that can reversibly undergo uniaxial strain of over 100%, and other key applications are discussed. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Multi-colour switching of polymer stabilized chiral nematic liquid crystal devices

We have fabricated a series of polymer stabilized chiral nematic test cells for use as flexoelectro-optic devices. The devices fabricated were based on commercial chiral nematic mixtures which were polymer stabilized so as to enhance the uniformity and stability of the uniform lying helix texture in the cells. During fabrication and test procedures a series of unusual scattering states have been observed within the devices at different viewing angles. The observations made so far indicate that the properties of the scattering state lies somewhere between the focal conic texture and the Grandjean or planar texture and that the devices exhibit both a helical pitch selective reflection and scattering effect. What is even more dramatic is that the wavelength selectivity of the scattering effect can be tuned by an applied field. In addition, we show that it is possible to achieve good uniform lying helix textures from such devices. Moreover, we show that in certain cases the spontaneous alignment of the helix in the plane of the device opens up the possibility of a new mode of switching. Flexoelectric, Redshift, Coloured scattering, Liquid crystal, Polymer-stabilized liquid-crystal;.

Elastodynamic erosion of thermal barrier coatings

The finite element method is used to analyze the elastodynamic response of a columnar thermal barrier coating due to normal impact and oblique impact by an erosive particle. An assessment is made of the erosion by crack growth from preexisting flaws at the edge of each column: it is demonstrated that particle impacts can be sufficiently severe to give rise to columnar cracking. First, the transient stress state induced by the normal impact of a circular cylinder or a sphere is calculated in order to assess whether a 2D calculation adequately captures the more realistic 3D behavior. It is found that the transient stress states for the plane strain and axisymmetric models are similar. The sensitivity of response to particle diameter and to impact velocity is determined for both the cylinder and the sphere. Second, the transient stress state is explored for 2D oblique impact by a circular cylindrical particle and by an angular cylindrical particle. The sensitivity of transient tensile stress within the columns to particle shape (circular and angular), impact angle, impact location, orientation of the angular particle, and to the level of friction is explored in turn. The paper concludes with an evaluation of the effect of inclining the thermal barrier coating columns upon their erosion resistance. © 2011 The American Ceramic Society.