Gradient Films Research Articles

Quantitative Analysis of Compositional Distribution in Biodegradable Poly(ethylene oxide)/Poly(3‐hydroxybutyrate) Blend Film with Compositional Gradient by FT‐IR Microspectroscopy

AbstractSummary: FT‐IR microspectroscopy was used to map the compositional distribution along the cross‐section on the gradient films of fully biodegradable poly(ethylene oxide) (PEO)/poly(3‐hydroxybutyrate) (PHB) blend system. First, a linear fitting line for a calibration, related the absorbance ratio of two peaks to the fraction of PEO in the blend, was established. During linear fitting, a new equation was deduced and the influence of the different crystallinities on the absorption peaks at the wavenumber 962 and 1 342 cm−1 for PEO, 980 and 1 380 cm−1 for PHB, have been taken into account. For the PEO/PHB blend system, it was found that the crystallinity has little effect on the absorbance ratios of A962/A1 380 and A1 342/A1 380. Based on the results from the linear fitting, which comes from the relation of the absorbance ratios of A962/A1 380 or A1 342/A1 380 and the combination of the weight fraction of PEO (WPEO/(1 − WPEO)), the compositional distributions on the cross‐section of three kinds of compositional gradient films of the PEO/PHB blend prepared by different technologies have been successfully estimated. Furthermore, the better method for preparing the PEO/PHB blend film with compositional gradient was found based on the result of the quantitative analysis of the compositional distribution.The compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.magnified imageThe compositional distribution along the cross‐section of the gradient films based on PEO/PHB binary system for the Type I gradient film.

Diamond/Carbide Nano-Composite Gradient Films: a Route to solve the Adhesion Issues of Diamond Films

ABSTRACTDue to their outstanding mechanical properties diamond films are ideal candidates for many cutting and machining applications. However, industrial applications of these films are limited due to poor adhesion. Two main reasons causing this poor adhesion, which are based on the extrinsic physical and chemical properties of diamond, can be identified: High mechanical stresses induced by a difference of the thermal expansion coefficient between the diamond film and the substrate as well as a catalytic effect in case of metallic substrates containing iron-, cobalt- and nickel that, in combination with a methane atmosphere during deposition, leads to soot formation. One option to overcome these difficulties is to provide an interfacial layer that acts as adhesion layer as well as barrier layer to prevent the catalytic effect of the substrate elements. Even though some successful examples exist, this approach usually requires a time consuming and expensive multi-step process.In this paper, the synthesis of nanocrystalline diamond/carbide composite films with a compositional gradient will be reported. Focusing on the example of diamond/ß-SiC the possibility to create a gradient layer ranging from ß-SiC to diamond in a controlled manner will be shown. The films are prepared by a Microwave Assisted Plasma Chemical Vapour Deposition process (MWCVD) using H2, CH4 and Tetramethylsilane (TMS) as reactive gases. The structure, grain sizes, and volume fractions of the components of these composite films, which consist of a mixture of diamond and carbide phase, can be controlled by adjusting the concentrations of the reactive gases in the gas mixture. This strategy, which handles all depositions in one process step, should allow for an improved diamond film adhesion on tools. The preparation and characterization of the composite films with special emphasize on their mechanical and tribological properties will be discussed and a short outlook on other diamond/carbide systems will be given.

High-throughput Studies on Photochemical Properties of Transition Metal-Doped SrTiO3 Epitaxial Thin Films

AbstractCombinatorial pulsed laser deposition approach is suitable to study complex effects such as composition, thickness and surface structure on thin film-based photocatalysis in high-throughput manner. We fabricated composition spreads and thickness gradient films of transition metal-doped SrTiO3(001) on various oxide single crystal substrates and evaluated their photocatalytic activity by the use of photoreduction of Ag+ in AgNO3 solution. We found anomalous film thickness and substrate effects on the Ag-photodeposition in the V-doped SrTiO3 epitaxial thin films grown on the Nb:SrTiO3(001) substrate.

Inverse and Reversible Switching Gradient Surfaces from Mixed Polyelectrolyte Brushes

We report on a thin polyelectrolyte film (mixed polyelectrolyte brush) with a gradual change of the composition (ratio between two different oppositely charged surface-grafted weak polyelectrolytes) across the sample. The gradient of surface composition creates a gradient in surface charge density and, consequently, a gradient of the wetting behavior. The gradient film is sensitive to a pH signal and can be reversibly switched via pH change.

Deposition of Ti/C compositional gradient film onto superplastic Ti‐alloy by magnetron sputtering

AbstractDeposition of Ti/C compositional gradient film onto superplastic titanium‐based alloy such as Ti6Al4V or Ti4.5Al3V2Fe2Mo was performed by magnetron sputtering, in order to improve not only the biocompatibility and the abrasion resistance of the alloy but also the adhesion between the deposited film and the alloy substrate while preserving the high hardness of such ceramic coatings. The Ti/C compositional gradient films were deposited by co‐sputtering of two sputter cathodes which had pure titanium and titanium carbide targets, respectively, and their compositional gradient was realized by varying continuously the electric power supplied to each sputter cathode. Under visual observation, the obtained film appeared to be uniform and adhesive. According to AES in‐depth profiles, the carbon (C) concentration in the film gradually decreased in depth direction from the surface toward the substrate, confirming that Ti/C compositional gradient film had formed on the alloy substrate. On the basis of XRD, it was found that titanium carbide (TiC) and α‐titanium phases were formed in the films. Furthermore, the Vickers hardness of the films reached over HV = 1800. Therefore not only the biocompatibility and the abrasion resistance of the alloy but also the adhesion of the deposited hard coatings were expected to be improved by this method. Copyright © 2004 John Wiley & Sons, Ltd.

Characterization of Combinatorial Polymer Blend Composition Gradients by FTIR Microspectroscopy.

A new FTIR technique was developed for characterizing thin polymer films used in combinatorial materials science. Fourier transform infrared microspectroscopy mapping technique was used to determine the composition of polymer blend gradients. Composition gradients were made from poly(L-lactic acid) (PLLA) and poly(D,L-lactic acid) (PDLLA) in the form of thin films (6 cm × 2 cm) deposited on IR reflective substrates. Three composition gradient films were prepared and characterized. The results demonstrate the reproducibility and feasibility of a new, high-throughput approach for preparing and characterizing polymer composition gradients. The combination of composition gradient film technology and automated nondestructive FTIR microspectroscopy makes it possible to rapidly and quantitatively characterize polymer composition gradients for use in combinatorial materials science.

The study of composition, structure, mechanical properties and platelet adhesion of Ti–O/TiN gradient films prepared by metal plasma immersion ion implantation and deposition

Titanium oxide and titanium nitrogen gradient films were prepared by three different processes using metal plasma immersion ion implantation and deposition (MPIII-D). The mechanical properties of the films synthesized on silicon wafers, Ti6A14V and low temperature isotropic carbon (LTIC) were evaluated by nano-indentation, pin-on-disc wear, and scratching test. The hardness of the film was measured to be 19.5 GPa. Investigation by XRD shows that the surface Ti–O layer possesses a rutile structure and analysis by X-ray photoelectron spectra (XPS) discloses that the surface composition of the synthesized TiN/Ti–O films is non-stoichiometric. The gradient characteristics of the films were corroborated by qualitative analysis of secondary ion mass spectroscopy (SIMS). The thickness of films was 510–940 nm. Platelet adhesion experiments adopted to estimate the blood compatibility of the films show that the adsorption and deformation of platelets on the synthesized TiN/Ti–O gradient films have been significantly suppressed compared to LTIC. Scanning electron microscopy (SEM) used to assess the wear and scratch tracks discloses that the films exhibit good wear resistance and high adhesion strength.

Influences of co-monomers and electrolyte acidity on morphological structure of copper-in-copolymer gradient film

In this paper, the influences of composition of copolymers and acidity of electrolyte in an electrochemical reactor on morphological structure of copper-in-polymer gradient composite film were investigated. For binary copolymers, poly(acrylonitrile-co-methyl acrylate) [P(AN-co-MA)] and poly(acrylonitrile-co-sodium allyl sulfonate) [P(AN-co-SAS)], the charged group SO in P(AN-co-SAS) improves the swelling of the copolymer phase and copper reduction to form gradient morphology; the carboxylic ester group in P(AN-co-MA) is not effective because of its poor hydrophilicity, but it is a cooperating component with P(AN-co-SAS) to avoid excess of counterion (i.e., Na+) in SCF, which might severely interrupt Cu2+ coexistence. The swelling of the polymer phase is helpful to decrease the energy of the transfer ions in SCF and to enhance copper deposition and gradient formation. The increase of surface energy because of cluster growth raises the surface energy level of deposited Cu0 clusters. The conteraction between these two energy factors allows the size of clusters to be 50–100 nm. The appropriate H+ concentration improves active Cu2+ reduction and thus deposited gradient copper phase in the copolymer matrix. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 373–380, 2004

Adhesion properties of functionally gradient diamond composite films on medical and tool alloys

We have investigated the adhesion properties of microcrystalline diamond thin films on Ti-Al-V alloy, Co-Cr-Mo alloy and steel. Microcrystalline diamond possesses high hardness, a low coefficient of friction, extreme chemical inertness and biocompatibility; these properties can enhance the performance of metal alloys used in medical implants and in machine tools. We have adopted three methods for improving the adhesion of microcrystalline diamond to commonly used metal alloys: (1) by alloying the substrate surface to minimize graphitization; (2) by employing appropriate buffer layers between the diamond film and the substrate; and (3) by creating functionally gradient diamond-(titanium carbide, tungsten carbide, titanium nitride and aluminum nitride) composites. We have demonstrated that functionally gradient discontinuous buffer layers of titanium carbide, titanium nitride, aluminum nitride and tungsten carbide are able to control stress and graphitization in microcrystalline diamond thin films. This work on buffer layers and functionally gradient coatings should allow the development of more adherent crystalline diamond films for medical and tribological applications.

On the behaviour of indentation fracture in TiAlSiN hard thin films

Structural properties of nanocomposite TiAlSiN thin films prepared as monolithic single layer, multilayers and gradient films have been characterized by X-ray diffraction, nano-indentation and atomic force microscopy. The films were deposited onto WC–Co substrates using cathodic arc-plasma. The Al content was varied within the range of Al/Ti=0.5–1.5 and the Si content did not exceed 3.5 at.%. The highest hardness (38–39 GPa) was obtained for samples with Al/Ti=1.0, associated with more effective combination of solid solution hardening and Hall–Petch effect arising from structure refinement by Al and Si addition. In contrast, the films with Al/Ti=1.5 showed the lowest hardness values (30–33 GPa) due to the formation of softer wurtzite-like hcp-AlN phase. Formation of layered films slightly improved hardness with respect to monolithic single layer films of comparable chemical composition. Meanwhile, gradient films (Al/Ti=1.0) showed hardness and modulus values very similar to those obtained for single layer films. Unlike mechanical properties, the formation and propagation of indentation induced cracks appeared more sensitive to film structures (columnar, equiaxed nanograins, …) as well as to growth morphology (monolithic, layered, gradient). The primary cracks appear for all films beneath the indenter at the contact site. The morphology of these cracks consists of a network of short, discontinuous, irregular cracks in columnar films, which gives rise to straight, well-organized repeated crack in fine grain structures. Such modification of crack patterns is attributed to the role of grain boundary sliding which is more pronounced in coarse columnar films than in nanograined materials.

A wear map of bearing steel lubricated by silver films

Wear map of bearing steel lubricated by silver film has been constructed to delineate the wear transition behavior with the change in operating conditions. Experiments were performed in dry sliding conditions using two types of ball-on-disk type test rigs under the contact pressure of 100–1000 MPa and the sliding speed of 20–1000 mm/s in ambient air. For the silver coating, an ultra thin IBAD silver bond layer was firstly deposited on AISI 52100 steel surfaces and then a relatively soft silver film was deposited by a thermal evaporation method onto the first layer. This functionally gradient film showed a great improvement in the life, mainly owing to the better bond strength. In order to build up a general framework on the tribological behavior of the functionally gradient silver films, all test data were plotted on a map whose axes are contact pressure and sliding speed. As a result, three main regimes were clearly identified: (i) elastic/plastic deformation of silver coating without failure, (ii) mild wear regime after initial failure of silver coating and (iii) severe wear regime. In the mild wear regime, the contact surfaces were covered with transfer layers of agglomerated wear particles. The transfer layer acted as a protective layer and resulted in low friction after initial failure of the coating. The formation of transfer layer was suppressed by several destructive actions, when the sliding speed was high. And, above a critical sliding speed, no transfer layer was able to form. In the discussion, an empirical model that could explain the existence of critical sliding speed was proposed.

Diamond-like carbon gradient film prepared by unbalanced magnetron sputtering and plasma immersion ion implantation hybrid technique

Diamond-like carbon (DLC) coatings have been used as solid lubricating coatings in vacuum technology because of their good physical and chemical properties. In this paper, the hybrid technique of unbalanced magnetron sputtering and plasma immersion ion implantation (PIII) was adapted to fabricate diamond-like carbon-based functionally gradient N/TiN/Ti(N,C)/DLC film on the 304 stainless steel substrate. Atomic force microscopy (AFM) was used to analyze the topography and surface roughness of the gradient film. The mechanical properties were evaluated by nano-indentation. The results showed that this DLC gradient film had good qualities as a solid lubricating coating.

Controlling synthesis of Ti–O/Ti–N gradient films by PIII

Titanium oxide and titanium nitrogen gradient films were synthesized using plasma-immersion ion implantation and deposition (PIIID). A intelligent control system was developed to control pushing of the metal cathode source and variation of the gas composition by programmable logical control (PLC); cathode pushing rules can be conveniently set, multi-channel gas composition control was achieved by D/A and mass flow control (MFC), and gas change has quick dynamic response velocity and a good degree of linearity. The results show that the whole system is stable and reliable, has strong anti-interference capacity and sets flexibly correlative technological parameters. The mechanical properties of the films synthesized on silicon wafer, titanium and low-temperature isotropic carbon (LTIC) were analyzed by microhardness tests, pin-on-disc wear experiments and scratch adhesion tests. X-Ray diffraction (XRD) and scanning electron microscopy (SEM) were used to evaluate the properties. Good mechanical properties were achieved by controlling the synthesis of the Ti–O/Ti–N gradient films discussed in this paper.

Deposition of epitaxial ternary transition metal carbide films

Thin epitaxial carbide films have been deposited in UHV by co-evaporation of Mo, Nb, Ti and V, with C 60 as carbon source. Two separate systems were studied, Ti 1− x V x C y on MgO(001) and Nb 1− x Mo x C y on MgO(111). We demonstrate the possibility to tune the cell parameter of an epitaxial ternary carbide film by control of the composition. Analysis with reciprocal space mapping show that deposition of Ti 0.34V 0.66C 0.81 at 500 °C yields a strain-free film with perfect match towards the MgO(001) substrate. Also, a good manual control of the individual fluxes allows the design of tailor-made compositional gradient structures. An epitaxial linear carbide gradient film going from TiC to VC was deposited at 500 °C. Furthermore, the low deposition temperature allows the deposition of metastable carbide structures. This was shown with epitaxial growth of a Nb 1− x Mo x C y film at 500 and 600 °C.

High‐throughput characterization of pattern formation in symmetric diblock copolymer films

AbstractSurface‐pattern formation in thin block copolymer films was investigated by utilizing a high‐throughput methodology to validate the combinatorial measurement approach and to demonstrate the value of the combinatorial method for scientific investigation. We constructed measurement libraries from images of subregions of block copolymer films having gradients in film thickness and a range of molecular mass,M.A single gradient film covers a wide range of film morphologies and contains information equivalent to a large number of measurements of films having a fixed thickness,h.Notably, the scale of the surface patterns is generally much larger than the molecular dimensions so that the interpretation of the patterns is more subtle than ordering in bulk block copolymer materials, and there is no predictive theory of this type of surface‐pattern formation. We observed a succession of surface patterns that repeat across the film with increasingh[extended smooth regions, regions containing circular islands, labyrinthine (“spinodal”) patterns, holes, and smooth regions again]. The extended smooth regions and the labyrinthine patterns appear to be novel features revealed by our combinatorial study, and these patterns occurred as bands ofhthat were quantized by integral multiples of the bulk lamellar period,Lo.The magnitude of the height gradient influenced the width of the bands, and the smooth regions occupied an increasing fraction of the film‐surface area with an increasing film gradient. The average size of the spinodal patterns, λ, was found to scale asλ ∼ Lorλ ∼ M−1.65and reached a limiting size at long annealing times. The hole and island features had a size comparable to λ, and their size likewise decreased with increasingM.The smooth regions were attributed to an increase in the surface‐chain density in the outer brushlike block copolymer layer with increasingh,and the scaling of λ withMwas interpreted in terms of the increasing surface elasticity withM.© 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2141–2158, 2001

C–SiC–Si gradient films formed on silicon by ion beam assisted deposition at room temperature

Ion beam assisted deposition (IBAD) of carbon under medium energy (35 keV) Ar ion bombardment onto a silicon target results in the formation of silicon carbide. The formation of a-C–SiC–Si gradient films is dependent on the ion/atom arrival ratio ( I/ A). All films are deposited at room temperature. The gradient layers formed are examined for their composition using RBS. Depending on the I/ A ratios, mixed interfaces of different widths, where silicon carbide exists, are formed. The SiC bonding in the mixed region is proven by XPS. The film formed on top of some samples contains amorphous carbon regions as shown by Raman spectroscopy. The surface of the layers grown is rather smooth with a roughness of several nanometres.

Bragg Reflections In Cholesteric Liquid Crystals: From Selectivity To Broadening And Reciprocally

Due to the existence of a macroscopic helicoidal structure, cholesteric liquid crystals exhibit many remarkable properties like selective light reflections. Bragg reflections occur when the helix pitch is of the order of the wavelength of the incident light propagating along the helix axis. The wavelength bandwidth is due to the medium birefringence and is typically limited to 50 nm, which is insufficient for some applications (full-colors displays, for example). The purpose of this paper is to show that, by favoring a pitch gradient in the helix during a novel experimental process, it is possible to get a cholesteric glassy material whose spectral characteristics evolve from a selective to a broad-band filter. Light reflection occurs on several hundreds of nanometers and several intermediary optical states are available during a thermal treatment. From a reciprocal viewpoint, the inverse transition, from a broad- to a narrow-band filter, is also demonstrated. Such a process is driven by one parameter: the annealing temperature. Then, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled. Different possible stages occuring during the thermal history of the sample are discussed. The very glassy state of these reflectors would give new opportunities for optical data storage.

From selective to wide-band light reflection: a simple thermal diffusion in a glassy cholesteric liquid crystal

: Due to their helicoidal structure, cholesteric liquid crystals exhibit remarkable optical properties. Selective light reflection occurs when the pitch (repeat distance) is of the order of the wavelength of incident light propagating along the helix axis. The wavelength bandwidth, due to the optical anisotropy, is typically limited to 50 nm which is insufficient for some applications (full-colors displays, for example). By introducing a pitch gradient in the helix during a novel two-step process in a cholesteric glass, we show that reflection may occur over a wavelength bandwidth greater than 300 nm. First, the reflection bandwidth is adjusted by thermal annealing. Then, the optical properties are permanently stored by quenching the viscous material to a glass at room temperature. The two steps, pitch gradient establishment and film hardening, are independently controlled. The present process exhibits some reversibility and properties intrinsic to the glassy state are gained: laser-writing high resolution full-color images on solid films for image recording or high-density optical data-storage are indeed conceivable.

Erosion by solid particles of W/W–N multilayer coatings obtained by PVD process

Abstract Sand-particle erosion remains a crucial problem in aeronautical engines, especially on compressor blades. These pieces are made from a titanium-based alloy, which is known to have a poor erosion resistance. To increase the endurance of these blades, it is planned to protect them with tungsten-based multilayer coatings obtained by PVD process. Coatings were first sputtered on flat samples, and their erosion resistance was evaluated using a horizontal projection device. The influence of several deposition parameters (Kr or Ar pressure, N2 flow) on erosion resistance was studied on two types of monolayer films: pure tungsten layers and layers reinforced with N atoms (called W–N). Based on these results, multilayer W/W–N coatings were then produced and tested. The observation of degradation mechanisms led us to study a new type of films, with a gradual change in N content. These gradient films showed an erosion resistance far higher than that of all multilayer coatings previously tested.

Latex Blends of Fluorinated and Fluorine-Free Acrylates: Emulsion Polymerization and Tapping Mode Atomic Force Microscopy of Film Formation

Latex based upon fluorinated acrylates such as perfluorooctylethyl methacrylate (FMA) and perfluorooctylethyl acrylate (FA) and fluorine-free acrylates such as n-butyl methacrylate (BMA) was prepared by means of emulsion polymerization in the presence of octadecyltrimethylammonium bromide (C18TAB) and 2,2‘-azobis(2-amidinopropane) dihydrochloride (V50) as water-soluble azo initiator. Reaction calorimetric studies revealed that acetone addition, preferably 15−25 wt %, was the key to achieve high monomer conversion and to produce stable latex with average particle sizes varying between 50 and 300 nm. Monomer conversion was increased, and latex particle size was reduced when 2 wt % n-butyl acrylate (BA) was added together with fluorine-containing monomers. The reaction enthalpy of FMA was determined to be 62 kJ/mol with respect to 75 kJ/mol for the more reactive FA. Stable poly(perfluorooctylethyl methacrylate)/poly(n-butyl methacrylate) (PFMA/PBMA) latex blends were formed. According to tapping mode atomic force microscopy, PBMA formed a dense film at 65 °C containing PFMA nanoparticles dispersed in the PBMA matrix. Annealing at 100 °C caused accumulation of PFMA at the surface and formation of a gradient film containing a surface layer with high fluorine content. Such latex blends are of interest in water-, oil-, and soil-repellent coatings.