Structure and tribological performance of diamond-like carbon based coatings for aerospace component processing

Copyright @ 2009 The Surface Science Society of JapanThis work examines diamond-like carbon (DLC) coatings deposited by plasma enhanced chemical vapour deposition (PECVD) as an environmentally friendly alternative to chromium plating in restoration of worn or damaged aircraft components. DLC coatings offer superior mechanical properties; however, high internal stresses and poor adhesion can prevent the deposition of thick films. This work examines a series of layered structures based on epoxy-resin interlayers with DLC applied as a surface film. Wear testing and examination with scanning electron microscopy and atomic force microscopy lead to the development of an optimum DLC/epoxy system with wear characteristics superior to those of chromium-plated steel. This new coating system has a great potential in restoring aircraft components in a more efficient and environmentally friendly manner.This work is funded via the Engineering and Physical Sciences Research Council (EPSRC)

Sparkling touchdown

Diamond-like carbon combines the properties of graphite and diamond to provide an inert, hard wearing, low-friction, thin-film barrier coating that can be deposited uniformly over a large area. Medical and electronic applications – including surgical implants and hard-disk heads - currently benefit from this versatile material, which has recently emerged into the limelight on the latest razor blades, and for high-performance manufacturing applications. Researchers at Brunel University and Hawker Pacific Aerospace, collaborating as part of an EPSRC sponsored Environmental Technology Engineering Doctorate (EngD) scheme, are investigating diamond-like carbon as a hard wearing, anti-corrosion coating on aircraft components, as an alternative to current techniques that rely on heavy metals

Argon plasma treatment techniques on steel and effects on diamond-like carbon structure and delamination

Copyright © 2011 Elsevier B.V. All rights reserved.We demonstrate alteration in diamond-like carbon (DLC) film structure, chemistry and adhesion on steel, related to variation in the argon plasma pretreatment stage of plasma enhanced chemical vapour deposition. We relate these changes to the alteration in substrate structure, crystallinity and chemistry due to application of an argon plasma process with negative self bias up to 600 V. Adhesion of the DLC film to the substrate was assessed by examination of the spallated fraction of the film following controlled deformation. Films with no pretreatment step immediately delaminated. At 300 V pretreatment, the spallated fraction is 8.2%, reducing to 1.2% at 450 V and 0.02% at 600V. For bias voltages below 450V the adhesion enhancement is explained by a reduction in carbon contamination on the substrate surface, from 59at.% with no treatment to 26at.% at 450V, concurrently with a decrease in the surface roughness, Rq, from 31.5nm to 18.9nm. With a pretreatment bias voltage of 600V a nanocrystalline, nanostructured surface is formed, related to removal of chromium and relaxation of stress; X-ray diffraction indicates this phase is incipient at 450V. In addition to improving film adhesion, the nanotexturing of the substrate prior to film deposition results in a DLC film that shows an increase in sp3/sp2 ratio from 1.2 to 1.5, a reduction in surface roughness from 31nm to 21nm, and DLC nodular asperities with reduced diameter and increased uniformity of size and arrangement. These findings are consistent with the substrate alterations due to the plasma pretreatment resulting in limitation of surface diffusion in the growth process. This suggests that in addition to deposition phase processes, the parameters of the pretreatment process need to be considered when designing diamond-like carbon coatings.This work is partially supported by the Technology Strategy Board, reference BD266E

Diamond-like carbon/epoxy low-friction coatings to replace electroplated chromium

A series of layered structures based on epoxy resins coated with diamond-like carbon (DLC) are examined as potential replacements for electroplated chromium in aerospace applications. Diamond-like carbon coatings can offer superior mechanical properties and tribological performance; however, in some applications high internal stresses and poor adhesion limit their practical use. A DLC/epoxy system is developed and studied utilising pin-on-disk testing, scanning electron microscopy and atomic force microscopy, resulting in an optimum system with characteristics superior to those of chromium-plated steel. This new coating system has a great potential in restoring worn or damaged aircraft components, without the health and environmental issues associated with chromium plating. The components can be rebuilt and improved over the original condition thus allowing an extension of service life and eliminating the need for costly replacements

The Effect of substrate geometry and surface orientation on the film structure of DLC deposited using PECVD

Potential applications of diamond-like carbon (DLC) coatings range from precision tools and biomedical implants to micro mechanical devices and engine components. Where uniform coatings are required on substrates with complex geometries, plasma enhanced chemical vapour deposition (PECVD) is often a preferred deposition method. As a non-line of sight process, the geometry of the substrate is often considered negligible. For this reason analysis of PECVD coatings, such as amorphous carbon, has mostly been concerned with reactor deposition variables, such as bias voltage, pressure and gas ratios. Samples are therefore usually prepared and positioned to minimise the influence of other variables. By depositing nominally similar DLC films on silicon samples positioned horizontally and vertically on the reactor cathode plate it was possible to examine the variations in the coating characteristics and mechanical properties that occur due to the geometry of the substrate being coated. Topographic measurements and analysis of bonding structures revealed significant heterogeneity in the coatings. Electron microscopy showed variation in surface structure as well as thickness disparities of up to 50% in the vertical sample. Atomic force microscopy showed roughness, Ra, varied from 0.37 nm to 15.4 nm between samples. Raman spectroscopy highlighted variations in the sp2/sp3 bonding ratios whilst micro wear tests demonstrated how these variations reduce the critical load performance. These effects are explained in terms of the deposition mechanisms involved and are related to variation in deposition species and geometrical field enhancements within the deposition chamber. Improved understanding of these local variations will aid in the optimization of coatings for complex substrate geometries

Antifriction aminopropyltriethoxysilane films on thermoplastic elastomer substrates using an APPJ system

The study of friction coefficients has long been of great importance in the automotive industry where some areas of the vehicle are subject to slippage. One example is the space between the window channels and the glass. The polymeric materials that are used in these areas, like thermoplastic elastomers (TPE), involve a high degree of friction. So, in order to decrease the friction coefficient of the TPE, companies are using such techniques as flocking. However their high energy consumption, irregular distribution of fibers and poor adhesion are drawbacks. In order to overcome these drawbacks, this work attempts to obtain a SiOx-based thin film over a TPE substrate using aminopropyltriethoxysilane (APTES) with similar or lower friction coefficients and the same durability. Since TPE is heat-sensitive, an atmospheric-pressure plasma jet system (APPJ) with a dielectric barrier discharge (DBD) was used in this study. The influence of the plasma power and number of passes was characterized by Profilometry, Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Attenuated Total Reflectance-Fourier Transform Infrared (ATR-FTIR) Spectroscopy, X-Ray Photoelectron Spectroscopy (XPS), Water Contact Angle (WCA) measurements and friction coefficient. The average surface temperature of the samples and the coating thickness seem to be the key variables in determining the friction behavior. Successful samples (those that have a lower friction coefficient than those of the current industrial solutions - flocked seals and polyamide tap) were coated at an average surface temperature of  1000 nm. Sample coated in six passes and the lowest power (350 W) proved to have the best friction performance. This sample has a friction coefficient that is 46% lower than that of the flocked seals. The results of this research permit one to conclude that a promising antifriction technology using APPJ with a DBD could be an alternative to the current industrial solutions. © 2016 Elsevier B.V