Applications of Energy-Assistance to the formation of novel surface coatings

In the area of manufacturing, surface coating of materials is a widely-used process representing a multi-billion pound per annum industry. the choice of a suitable coating allows the design engineer to choose a material for its optimum bulk properties but, at the same time, tailor the surface properties for a specific application. For example, components made of iron have good mechanical strength but, because the iron surface forms an oxide, a suitable coating (usually chromium-based) is used to protect the product. Over the last decade interest has surged in the potential use of more exotic coatings with controlled micro- and nano-structure. Professor John Colligon and Dr Vladimir Vishnyakov have developed techniques for production of such coatings

Ion sputter-deposition and in-air crystallisation of Cr2AlC films

Ternary alloys of composition close to Cr2AlC have been deposited by ion beam sputtering onto unheated and heated to 380 °C Si substrates. As-deposited films are amorphous. Annealing of the film in vacuum at 700 °C leads to crystallisation with 39.2 nm crystallites. Crystallisation also can be achieved by annealing in air but there is also partial oxidation of the film surface to the depth of approximately 120 nm, which represents an oxide layer less than 5% of the total film thickness. There is an increase of lattice size along the c-axis during crystallisation in air, which indicates a small incorporation of oxygen. Film structure and crystallisation have also been analysed by Raman spectroscopy. Changes in Raman spectra in Cr2AlC have been correlated with the film crystallisation and it was observed that MAX-phase related peaks become clearly defined for the crystallised film

Amorphous Boron containing silicon carbo-nitrides created by ion sputtering

Silicon carbo-nitride films with Boron were deposited onto Silicon, glass and SS304 Stainless Steel substrates using the ion beam assisted deposition (IBAD) method. The coating composition, rate of ion-assistance and substrate temperature were varied. Films were examined by X-Ray Diffraction, Scanning Electron microscopy, Energy Dispersive X-Ray analysis, Cathodoluminescence, Atomic Force Microscopy and Nano-indentation. The composition and chemical bonding variation was found to be dependent on deposition conditions. All coatings were amorphous, fully dense and showed high hardness up to 33 GPa. It is suggested that the low friction coefficient of about 0.3, measured against Al2O3 using the pin-on-disc method, may be the result of the presence of C nanoclusters which are formed under the low energy deposition conditions. Films deposited on Stainless Steel had an onset of rapid thermal oxidation at 1150 °C in air as determined by thermogravimetric analysis. The films have a Tauc bandgap between 2.2 and 2.8 eV and were also exceptionally high electrical resistive which may indicate the presence of localised state

Retention of microbial cells in substratum surface features of micrometer and sub-micrometer dimensions.

Surfaces were produced with defined topographical features and surface chemistry. Silicon wafers, and wafers with attached nucleopore filters and quantifoils were coated with titanium using ion beam sputtering technology. Irregularly spaced, but regularly featured surface pits, sizes 0.2 and 0.5 microm, and regularly spaced pits with regular features (1 and 2 microm) diameter were produced. The smallest surface feature that could be successfully produced using this system was of diameter 0.2 microm. Ra, the average absolute deviation of the roughness irregularities from the mean line over one sampling length, Rz, the difference in height between the average of the five highest peaks, and the five lowest valleys along the assessment length of the profile and surface area values increased with surface feature size, with Ra values of 0.04-0.217 microm. There was no significant difference between the contact angles observed for smooth titanium surfaces with 0.2 and 0.5 microm features. However, a significant difference in contact angle was observed between the 1 and 2 microm featured surfaces (p<0.005). Substrata were used in microbial retention assays, using a range of unrelated, differently sized microorganisms. Staphylococcus aureus (cells 0.5-1 microm diameter) were retained in the highest numbers. S. aureus was well retained in the 0.5 microm sized pits and began to accumulate within larger surface features. Rod shaped Pseudomonas aeruginosa (1 microm x 3 microm) were preferentially retained, often end on, within the 1 microm surface features. Some daughter cells of Candida albicans blastospores were retained in 2 microm pits. For S. aureus and P. aeruginosa, the greatest numbers of cells were retained in the largest (2 microm) surface features. The number of C. albicans was similar across all the surfaces. The use of defined surfaces in microbial retention assays may lead to a better understanding of the interaction occurring between cells and surface features

The production of surfaces of defined topography and chemistry for microbial retention studies, using ion beam sputtering technology

Microbial cells retained in surface defects on an inert substratum may pose problems in cross-contamination (food contact surfaces), and provide a focus for infection (medical devices). Although the type and degree of substratum surface roughness affects the retention of microorganisms on a surface, few studies have investigated this phenomenon using well-characterised substrata. A range of surfaces with features of regular and defined dimension on the micron and submicron scale has been produced. Various filters/grids of known pore size acted directly as templates, and a range of masks were placed upon smooth silicon wafers. Titanium was applied to the surfaces via magnetron thin film deposition, and the surfaces were visualised and characterised using atomic force microscopy, in preparation for microbial retention studies