Nanoscale Coatings Research Articles

Interrelationship between atomic species, bias voltage, texture and microstructure of nano-scale multilayers

A matrix of binary and ternary nitrides containing lighter elements (Al, Ti, V and Cr) with atomic mass <52 and heavier elements (Nb and W) with atomic mass >89 has been formulated. These have been grown as nano-scale multilayer coatings (bi-layer thickness approx. 3.0 nm) on stainless steel substrates using an industrial size multiple-target ABS coater. When lighter elements are incorporated into the multilayer at a lower bias voltage ( U B =−75 V) pronounced {111} or {110}, textures develop which are determined by the dominating species present. A {111} or {110} texture develops when TiAlN or VN and or CrN dominates the matrix, respectively. In contrast when a heavier element is incorporated a {100} texture is observed. Additionally, there is a strong indication that in the case when heavy elements (>89) are involved in the growth process, which evolves by continuous re-nucleation. Conversely, when only light elements (<52) are involved then the coating evolves by competitive growth. This observation is limited only for the lower bias voltage range of U B −75 to −120 V. However, as the bias voltage is increased (up to U B =−150 V) the texture becomes increasingly sharp and in all cases a {111} texture develops. A lower residual compressive stress (typically −1.8 GPa) is observed when one of the bi-layers is dominated by a heavier element. The stress increases (up to −6.8 GPa) in these coatings when the bias voltage is increased to U B =−150 V which is always systematically lower than in coatings containing only lighter elements which are typically up to −11.7 GPa at the same bias voltage. In parallel this results in an increase in plastic hardness (80 GPa) and in the sliding wear coefficient by an order of magnitude regardless of the type of lattice growth observed.

Processing and Development of Nano-Scale HA coatings for Biomedical Application

ABSTRACTFunctionally graded Hydroxyapatite coating with graded Crystallinity across the thickness of the film has been processed and tested as a more effective orthopedic/ dental implant coating. The present study aims to increase the service-life of an orthopedic/ dental implant by creating materials that form a strong, long lasting, bond with the Ti substrate as well as juxtaposed bone. The health relatedness of the new material is to increase bonding between an implant and juxtaposed bone so that a patient who has received joint or dental replacement surgery may quickly return to a normal active lifestyle. Cross-sectional transmission electron microscopy analysis displayed that the films have a graded crystal structure with the crystalline layer near the substrate and the amorphous layer at the top surface. Compositional analysis was performed using SEM-EDX at the top surface as well as STEM-EDX at the cross section of the film. The average calcium to phosphorous ratio at the surface is 1.46, obtained by SEM-EDX. The Ca/P ratios in the crystalline and amorphous layers of the film are 1.6 to 1.7, close to the ratio of 1.67 for HA.

The effect of (Ti+Al):V ratio on the structure and oxidation behaviour of TiAlN/VN nano-scale multilayer coatings

Nano-scaled multilayered TiAlN/VN coatings have been grown on stainless steel and M2 high speed steel substrates at U B=−85 V in an industrial, four target, Hauzer HTC 1000 coater using combined cathodic steered arc etching/unbalanced magnetron sputtering. X-ray diffraction (XRD) has been used to investigate the effects of process parameters (Target Power) on texture evolution (using texture parameter T*), development of residual stress (sin 2 ψ method) and nano-scale multilayer period. The composition of the coating was determined using energy dispersive X-ray analysis. The thermal behaviour of the coatings in air was studied using thermo-gravimetric analysis, XRD and scanning electron microscopy. The bi-layer period varied between 2.8 and 3.1 nm and in all cases a {1 1 0} texture developed with a maximum value T*=4.9. The residual stress varied between −5.2 and −7.4 GPa. The onset of rapid oxidation occurred between 628 and 645 °C depending on the (Ti+Al):V ratio. After oxidation in air at 550 °C AlVO 4, TiO 2 and V 2O 5 phases were identified by XRD with the AlVO 4, TiO 2 being the major phases. The formation of AlVO 4 appears to disrupt the formation of Al 2O 3 which imparts oxidation resistance to TiAlN based coatings. Increasing the temperature to 600 and 640 °C led to a dramatic increase in the formation of V 2O 5 which was highly oriented (0 0 1) with a plate-like morphology. At 640 °C there was no evidence of the coating on XRD. Increasing the temperature to 670 °C led to further formation of AlVO 4 and a dramatic reduction in V 2O 5.

Combined steered cathodic arc/unbalanced magnetron grown C/Cr nanoscale multilayer coatings for tribological applications

C/Cr nano-scale multilayer coatings have been produced by the combined steered cathodic arc/unbalanced magnetron sputtering technique. The coating was deposited by non-reactive unbalanced magnetron sputtering from three graphite targets and one chromium target at deposition temperatures of 250, 350 and 450 °C. In scratch adhesion tests C/Cr coatings deposited on M2 HSS showed values exceeding critical load of 70 N. The XRD and TEM analysis revealed the amorphous nature of the coatings. Higher deposition temperatures led to an increase in crystallinity. Raman spectroscopy also showed that the amount of the carbon disorder depends on the deposition temperature, increasing from 68 to 86% for deposition temperatures of 250 and 450 °C, respectively. In pin-on-disc tests, C/Cr coatings exhibit a low coefficient of friction of between 0.1 and 0.2, when sliding against 100Cr6 steel ball. These low values were retained for tests in air, de-ionised water and engine oil. Remarkably low sliding wear coefficients of 2×10 −17 m 3/Nm for the coating and 1×10 −19 m 3/Nm for the counterpart were measured after 22.6 km sliding distance. However, the increased crystallinity of the coatings produced at higher deposition temperature leads to an increase in the friction coefficient. The C/Cr coatings, used as an overcoat on TiAlCrYN coated cemented carbide end mills, led to reduction of the rake and the flank wear rates by factor of 7 when machining extremely abrasive Ni based alloys.

Nanoskalige Schutzschichten für hochbeanspruchte Werkzeuge und Bauteile

Abstract Nanoscale coatings for tribological applications can be subdivided into nanostructured multilayer films, nanomodulated superlattice films, nanocrystalline metastable films, nanostabilized single and multilayer films and nanograded films. Aside from materials selection and deposition characteristics, the interface areas, grain size, single layer thickness, surface and interface energy, texture and the epitaxial stress and strain are principal factors determining constitution, properties and performance of these coatings. The functional and structural design of the films can result in tailored multifunctional coatings to be applied as protective coatings under severe and complex loading for tools and components. New metastable nanocrystalline films can be deposited by vapor quenching. A thermodynamic and kinetic modelling results in new film materials, tailored with respect to constitution, properties and performance. Various nanoscaled film composites and new concepts for protective coatings, elab...

Interfacial properties and protein resistance of nano-scale polysaccharidecoatings

For many applications, it is essential to be able to control the interface betweendevices and the biological environment by nanoscale control of the composition ofthe surface chemistry and the surface topography. Application of molecularthickness coatings of biologically active macromolecules provides predictableinterfacial control over interactions with biological media. The covalent surfaceimmobilization of polysaccharides, proteins and synthetic oligopeptides can beachieved via nanometres thick, interfacial bonding layers deposited by gas plasmamethods, and the multi-step coating schemes are verified by XPS analyses.Interactions between biomolecular coatings and biological fluids are studied byMALDI mass spectrometry and ELISA assays. Using a colloid-modified AFM tip,quantitative measurement of interfacial forces is achieved. Comparison withtheoretical predictions allows elucidation of the key interfacial forces that operatebetween surfaces and approaching bio-macromolecules. In this way, it is possibleto unravel the fundamental information required for the guided design andoptimization of biologically active nanoscale coatings that confer predictableproperties to synthetic carriers used for the fabrication of bio-diagnostics andbiomedical devices. By studying the relationships between interfacial forcesand the adsorption of proteins, we have established the key propertiesthat make specific polysaccharide coatings resistant to the adsorption ofproteins, which is applicable to biomaterial, biosensor and biochip research.

Biaxial testing of nanoscale films on compliant substrates: Fatigue and fracture

Two problems of technological importance for microelectromechanical systems (MEMS) and microelectronics industry are addressed: fatigue of thin films and nanoscale film cracking. A device is described that can (1) conduct biaxial fatigue tests on thin films and (2) be utilized to study fracture patterns in nanoscale coatings under biaxial stress state. Thin-film specimens, in the form of circular membranes, are exposed to cyclic pressures between two fixed pressure limits. Corresponding pressure and specimen deflection are measured. Experimental results, including hysteresis loops spanning deflections of 15–50 μm are presented for 4.6-μm-thick polyimide films. Furthermore, the evolution of crack patterns in a 150-nm-thick Al film deposited on a polyimide substrate is studied. Critical mode I stress intensity factor for Al is extracted from experimental results.

High surface area polymer coatings for SAW-based chemical sensor applications

High surface-to-volume ratio coatings consisting of nano-scale polymer particles were applied to surface acoustic wave (SAW) transducers. The resulting sensors were tested upon exposure to analyte vapors and were compared with sensors developed from bulk films of the same polymers. The relative importance of surface adsorption and bulk absorption was investigated and it is shown that high surface area coatings can be used to improve the sensitivity and response time particularly in the case of polymers exhibiting low vapor permeability. Furthermore, because the mean particle diameter is small compared to the spacing of the SAW interdigital electrodes, nano-scale particulate coatings appear acoustically uniform and are therefore, compatible with SAW technology. The high surface-to-volume ratio coatings were deposited using a spray-on technique known as rapid expansion of supercritical solutions (RESS) and both glassy and viscoelastic polymers were studied.

Nanoscale Coating of Silicon and Manganese on Ferrimagnetic Yttrium Iron Garnets

A uniform microstructure of Gd‐substituted yttrium iron garnet (Y2Gd1Fe5O12) was obtained with additives of Si and Mn using a sol‐gel coating process. Tetraethyl orthosilicate (TEOS) and manganese acetate were used as Si and Mn sources for the sol‐gel coating procedure, respectively. TEM/EDS analysis confirmed the presence of a relatively uniform nanoscale coating of Si and Mn around each particle. High density and uniform microstructure were obtained after sintering at 1400°C in O2 atmosphere. The density and grain size of the specimens depended on Mn content. The results were compared with materials fabricated without additives incorporated by the sol‐gel coating procedure.

On the simultaneous determination of the optical constants and of the thickness of thin absorbing films

Ti- and Si-doping effects on morphology, structure, optical and photo-response of α-Fe2O3 nanoscale coatings from atmospheric-pressure chemical vapour deposition (APCVD) have been studied. Si- and Ti-doping led to larger clusters with finer grains and smaller clusters with larger grains, respectively. Photocurrent performance was increased remarkably by doping, especially Si. Excellent agreement was found for band gaps and optical properties compared to hybrid-density functional theory. Substitutional replacement of Fe by Si shrinks the volume more than Ti-doping; it is conjectured that this affects hopping probability of localised charge-carriers more and leads to enhanced photocurrent activity for Si-doping, supported by experiment.