Tribology of Diamond-Like Carbon Films

Diamond-like carbon (DLC) is an amorphous carbon (a-C) or hydrogenated amorphous carbon (a-C:H) thin film material with a high fraction of sp³ carbon bonding. It is generally prepared by a deposition process which involves energeticions. The sp³ bonding is metastable compared to sp² bonding, unless it is stabilised by C-H bonds. The various types can be classified according to their fraction of sp³ bonding and hydrogen (H). DLC variants alloyed with other elements such as Si, metals or B, N and F are also found.

The availability of reliable characterisation tools for carbon films down to a few atomic layers' thickness is one of the most decisive factors for technology development and production. In particular, non-destructive techniques are preferred. This chapter reviews the use of x-ray reflectivity, surface acoustic waves, and Raman spectroscopy to characterise carbon films in terms of density, thickness, layering, elastic constants, roughness, structure, and chemical composition. Raman spectroscopy, in particular, allows to probe of most of the materials properties, even if indirectly. The use of atomic force microscopy (AFM) will be considered to assess the basic growth mechanism of carbon films. The measurement of thermal conductivity of carbon films will also be reviewed.

This chapter reviews the mechanical characteristics of diamond-like carbon (DLC) materials. It examines the motivations behind such studies, presents the experimental methods used to characterise them and discusses the main findings. DLC usually present wide-ranging mechanical properties, with the best specimen being tetrahedrally bonded in some cases approaching the performances of diamond. The chapter also discusses the effect of internal stress, doping and layering. However, we note a number of remaining issues, which concern mainly the nanoindentation protocols for ultra-thin layers, a significant aspect of the problem being the simultaneous effect of substrate deformations and tip blunting. To that end, a new analytical method was designed which extracts the intrinsic film's hardness H_F. Surveying a variety of DLC layers, we find that the H^F value tends to increase with thickness, at least up to 50 nm. This result is supported by complementary non-mechanical analysis. It is also observed elsewhere and is consistent with current growth models. Such independent correlation of the intrinsic characteristics of very thin DLC layers is a difficult task.

The origin of residual stresses in thin films and various models invoked to explain the development of such stresses are reviewed in the first part the chapter. Compressive intrinsic stresses develop in fully dense films such as diamond-like carbon (DLC) films produced at room temperature by energetic deposition techniques. Fundamental aspects related to residual stresses and adhesion of films, as well as the determination of the stress magnitude and adhesion strength of DLC films are presented in the second part. The effects of deposition parameters on the intrinsic stress level are analyzed in detail. The reduction of the stress level and improvement of film adhesion can be achieved via various approaches described in the last part of the chapter.

This chapter provides an overview of the fundamental processes that may govern friction and wear of diamond-like carbon (DLC) coatings. First, the general cases of traditional and carbon-based solid lubricants in tribology are considered, since they exhibit low friction and high wear resistance. The emphasis is then shifted to DLC films and the specific contributions of three main phenomena — abrasion, adhesion, and interfacial shearing — are considered in detail in order to account for the very interesting tribological behavior of these films. Corresponding critical parameters are identified and their roles are evaluated with respect to the known structures and properties of most DLC films. Finally, the specific examples of adhesive phenomena controlling friction and wear during tests in ultrahigh vacuum (UHV) are discussed. Adhesion appears to be necessary for tribofilm buildup and hence friction reduction; however, continued occurrence of strong adhesive phenomena is detrimental to the superlow friction behavior of these films. Overall, the exceptional tribological behavior of DLC films appears to be due to a unique combination of surface chemical, physical, and mechanical interactions at their sliding interfaces.

Diamond-like carbon (DLC) coatings have low friction and high wear resistance compared to bulk materials and to other wear-resistant coated surfaces. The surrounding environment, gas atmosphere, humidity and temperature, affect the friction and wear performance of DLC films dramatically. In dry and inert atmospheres, the highly hydrogenated DLC films typically exhibit low friction performance, but the hydrogen-free DLC films have high friction accompanied with increased wear. In humid environment, the friction coefficient of both types of DLC films is similar varying in the range 0.05–0.2 and the best wear resistance can be achieved with hydrogen-free ta-C films. At elevated temperatures, the advantageous tribological properties of hydrogenated DLC films may be disturbed due to effusion of hydrogen and graphitization of the film structure in rather low temperatures. The hydrogen-free ta-C films on the other hand can survive in higher temperatures, even though the friction coefficient reaches higher values.

Much of the literature on diamond-like carbon (DLC) tribological coatings emphasizes the processing—structure—tribological property interrelationships but often ignores or speculates on the mechanisms which control friction and wear. In reality, third-body processes, such as transfer films that form in the moving contact, are responsible for the long life of DLC coatings. In this chapter, the friction and wear behavior of amorphous diamond-like nanocomposite (DLN) coatings, Ti- and W-doped DLC, and hydrogenated DLC coatings in low speed, dry sliding contact have been investigated using a home-built in situ Raman tribometer. In situ optical microscopy identified how third-body processes controlled friction and wear behavior of these DLC coatings in reciprocating sliding against sapphire hemispheres in dry (~4% RH) and humid (~20–60% RH) air between contact stresses of 0.7 and 1.1 GPa. In situ visual observations monitored the health (e.g., formation, thickening, thinning, and loss) of transfer films at the sliding (buried) interface. For most of the coatings, interfacial sliding between the transfer film and underlying wear track was the dominant velocity accommodation mode (VAM) responsible for steady-state low friction coefficients between 0.03 and 0.2, with lower values obtained at high contact stress and lower RH percentage. Transfer film behavior with the W-doped DLC coating was also studied when lubrication changed from dry air to a perfluoropolyether (PFPE) lubricant.

Diamond-like carbon (DLC) films have emerged as a class of very important tribological materials in recent years mainly because of their outstanding properties, such as high mechanical strength and hardness, excellent chemical inertness, and exceptional friction and wear performance under both dry and lubricated sliding conditions. Persistent and systematic research efforts during the last decade have resulted in the development of a new breed of DLC films providing superlubricity and near-wearless sliding even under very harsh contact and environmental conditions. In fact, the dry sliding friction and wear coefficients (i.e., as low as 0.001 and 10⁻¹¹ mm³/N.m, respectively) of these films are among the lowest reported to date, and such unusual tribological properties may have huge positive impacts on efficiency, durability, and performance characteristics of a wide range of mechanical systems, including magnetic hard disks, sliding and/or rolling contact bearings, gears, mechanical seals, scratch-resistant glasses, invasive and implantable medical devices, microelectromechanical systems (MEMS) and many more. In this chapter, we attempt to provide an up-to-date overview of these novel DLC films that can provide superlubricity and discuss in detail those factors that control their very unique lubrication mechanisms. Specifically, we concentrate on the state of the art in our understanding of their superlow friction and wear mechanisms, and how these mechanisms may relate to their structural chemistry, mechanical properties, and test and environmental conditions. In particular, various intrinsic (film-specific) and extrinsic (or test condition-specific) factors that play major roles in friction and wear of DLC films are discussed in detail and correlated with their friction and wear mechanisms.

Wear-resistant coatings using hard hydrogen-free and tetrahedral bonded ta-DLC offer a unique combination of high hardness, low friction, and extremely low wear rates. The related coating deposition techniques of such wear-protective coatings are reviewed, including filtered vacuum arc and pulsed laser deposition (PLD), where process parameters and plasma characteristics are correlated to ta-DLC bonding and mechanical characteristics. Advancements in hybrid plasmadeposition methods and process-control arrangements allow production of hard ta-DLC with high adhesion and fracture resistance through the development of graded and multilayer coating architectures. Further enhancement came with nanocrystalline carbide/amorphous DLC compositions, where high toughness is achieved by using grain boundary sliding for strain accommodation without brittle fracture. Multiple crystalline/amorphous interfaces help to divert and split nanocracks, avoiding coating macro-cracking even when relatively softer substrates experience a high degree of deformations. Since ta-DLC is exceptionally good in ambient conditions, adaptive “chameleon” composites were developed to use this tribological benefit of ta-DLC, while introducing additional solid lubricant phases for other environments and/or protecting DLC material from degradation in extreme environments, including dry air or inert gas, high vacuum, and elevated temperatures in air. DLC is an important part of today's tribological coating applications, where a high wear resistance and the absence of lubricating fluids are critical, e.g., aerospace, dry machining, and MEMS. The coatings with hard DLC phases can provide friction reduction and wear-life extension without the need of lubrication systems, reducing mechanism complexity, costs, weight, and environmental impact.

During the past several decades, diamond-like carbon (DLC) and various carbon-based coatings were developed with a wide range of chemical, mechanical, and physical properties. Because of the difference in synthesis techniques and tribotesting conditions explored in these studies, many experimental results appeared to contradict one another. This chapter presents an overview of the effects of the environment and surface chemistry on friction and wear performance of several carbon-based coatings: silicon carbide, boron carbide, DLC, and amorphous carbon nitride. In addition, we present a model to explain time-dependent effects on the friction behavior of hydrogenated DLC coatings based on the combined effects of ambient gas adsorption and mechanical removal.

This chapter first describes the general features of triboemission and triboplasma generation as the basic knowledge to understand the phenomena in regard to diamond-like carbon (DLC) films. Next, the electrical properties and structure of DLC films are described, which are essential for an understanding of the triboemission phenomena in DLC films. This is followed by a description and discussion on triboemission on amorphous carbon (a-C), hydrogenated amorphous carbon (a-C:H), and nitrogenated amorphous carbon (a-C:N) films in connection with triboplasma generation, in addition to a presentation of the triboplasma distribution on tetrahedral hydrogen-free amorphous carbon (ta-C) film. In regard to a-C and a-C:H films, the characteristics of triboemission of electrons, ions, and photons, and plasma generation are also described and discussed in relation to the friction coefficient, hydrogen content, and the electric resistivity of the films under the circumstances of vacuum, ambient air, and oil lubrication. For a-C:N films, triboemission phenomena are described in comparison with those of a-C:H films. With respect to ta-C film, the chapter also presents the distribution of triboplasma generation with two-dimensional photons emitted from the vicinity of sliding contact

Doped or alloyed diamond-like carbon (DLC) coatings is an important category of DLC characterized by the incorporation of different elements in their struc-ture to achieve multifunctionality and improved properties in respect to pure DLC films. By controlling the nature, content and distribution of the dopants, tailored synthesis of doped-DLC with properties adapted to a desired value for specific applications can be obtained. Common dopants are light elements (B, Si, N, O or F), metals and combinations thereof to modify properties such as hardness, tribological properties, internal stress, adhesion, electrical conductivity or biocompatibility. The purpose of this chapter is to provide an overview of the different alloyed-DLC and more novel nanostructured coatings reported in the literature in relation with the property of interest. The tribological properties will be discussed in light of their chemical composition and microstructure trying to obtain general trends or correlation between them when possible due to the high number of parameters influencing their practical tribological response.

Carbon nitride (CN _x ) coatings exhibit a wide range of very attractive properties (such as low friction and wear, high hardness, good thermal and chemical stability, etc.) which make them very suitable for demending mechanical and tribological applications. During sliding against uncoated silicon nitride (Si₃N₄) and CN _x -coated Si₃N₄ in gaseous nitrogen atmosphere or in nitrogen gas flow, it provides superlow friction and wear. The beneficial effect of nitrogen on reducing friction and wear is much more pronounced if the portion of the runningin period is carried out in air or in oxygen before introducing nitrogen to the sliding interface. In this chapter, we will provide a comprehensive overview of the unique tribological properties of CN _x coatings in general and the effects of various gaseous environments on these properties in particular.

The tribological behaviour of several diamond-like carbon (DLC) coatings of the type a-C:H has been investigated under gross-slip fretting conditions and the effects of the counterbody material, ambient atmosphere, ambient temperature and liquid lubricating medium were assessed. DLC films have a significant friction- and wear-reducing effect under fretting conditions. Specifically, with their uses as thin protective films on tribological surfaces, very low friction coefficients in the range 0.01–0.1 with wear coefficients of about 1·10⁻⁷ mm³/Nm were realized. This is not only valid for dry sliding in ambient air, but also under lubricated sliding conditions in both the aqueous (deionized water) and organic media (liquid lubricant). The presence of sodium chloride in aqueous test media does not show any significant effect on altering the friction and wear characteristics of these films. However, when increased ambient temperatures are imposed on DLC as one of the sliding partners, the coating structure begins to degrade. Up to 100°C, their friction coefficients are not affected (in fact, noticeable decreases in friction are observed with increasing temperature). However, transformation processes to graphitic carbon seem to be induced immediately with further increases in temperature leading to increased wear coefficients and a significant reduction of the coating lifetime. This reduction of coating lifetime itself depends on the material of the counterbody.

Today diamond-like carbon (DLC) coatings are used in many applications such as in medical tools, machine tools, computer devices like hard disks and many more. In the near future, the tribological properties of the coatings will allow replacing some of the common lubricant additives like extreme pressure and anti-wear additives. This would have a beneficial impact on the environment and on fuel consumption.

Owing to their attractive properties, diamond-like carbon (DLC)coatings are becoming increasingly important in the field of machine components. The extreme hardness, high elastic modulus, excellent wear and corrosion resistance, high thermal and chemical stability, and the low-friction nature of these coatings open further possibilities in improving tribological performance and reliability of different components. Despite the low friction coefficients, normally observed for DLC-coated surfaces under dry sliding conditions, only few DLC-coated tribological components are likely to be operated completely without a lubricant. There are many reasons for that. First, tribological properties of unlubricated DLC coatings are very sensitive to the surrounding atmospheric conditions, notably the relative humidity. Further, it is not always possible or economically desirable to coat all surfaces in the system, while the lubricant also serves other functions, such as cooling, in mechanical systems. Thus, the majority of DLC-coated components will continue to be operated under lubricated conditions, mainly under boundary lubrication, and will initially use the same lubricants as originally developed for uncoated surfaces. Therefore, the knowledge about DLC coatings' tribological behavior under different lubrication conditions and influence of lubricants and their constituents is crucial, if the possibilities and benefits of the DLC coatings in mechanical systems are to be fully exploited.

Hard DLC coatings combine unique mechanical, chemical, and electrical properties. The possibility to produce them at very low temperatures, compared to those of conventional hard coatings, makes them an excellent choice for wear parts being coated without loss in hardness. The lifetime of cutting tools, not necessarily coated at low temperatures, can significantly be improved by using hard DLC coatings in machining. Sophisticated industrial coating technology capable of combining PVD-sputtering and PACVD can be provided for that. Steadily increasing demands for the performance of wear parts and cutting tools in connection with the versatility of these technologies will lead to an increasing market and further diversification.

In this chapter, diamond-like carbon (DLC) films for the manufacturing industry are discussed. Section 2 is dedicated to the production of a-C:H coatings by PACVD technology and the difficulty of the control of the plasma when it is generated by biasing the parts to be coated. This section also describes how process parameters can be interconnected and how they can influence the coating properties. Section 3 is related to some functional characterisations of DLC-coated parts that allow checking of the coatings properties and quality. The combination of coatings allows improving the performances of DLC. This can be achieved in vacuum equipment that can do both PVD and PACVD processes. It is particularly interesting to combine a hard chromium nitride layer deposited by reactive magnetron sputtering and a DLC top layer. This process can be used to increase the scratch resistance of DLC. Some tribology characterisations are also discussed in Section 4. These tests are very important for the optimisation of a DLC coating as they can, in some case, simulate the real working conditions.

Diamond-like carbon (DLC)-based coatings have found widespread applications during the last 10 years and further application possibilities are rapidly emerging. Over the years, the DLC coatings have been highly optimized and tailored to meet the increasingly stringent application requirements of numerous mechanical and/or tribological systems. In this chapter, a brief overview is given of the typical wear modes and mechanisms encountered in practical applications. Effective solutions currently available for the various wear modes are also provided. Finally, some of the industrially used deposition technologies and practical applications are presented.

Diamond-like carbon (DLC) has outstanding tribological properties and is, additionally, tolerated well by the body. Due to this advantageous combination of properties, research and development efforts have been made toward the use of DLC coatings in biomedical applications. It has been demonstrated that DLC coatings do not trigger any adverse effects on attached cells and that DLC can be considered to be biocompatible by in vivo and also many in vitro experiments. DLC surfaces also have an excellent haemocompatibility and DLC-coated cardiovascular implants such as artificial heart valves, blood pumps, and stents are already commercially available. The different studies presented demonstrate that DLC has the ability to reduce wear, more or less independently of the lubricant used, in load-bearing implants when sliding against metals or against DLC. However, it seems that when DLC slides against ultra high molecular weight polyethylene (UHMWPE) in the presence of body fluids, the good tribological properties that DLC shows in air could not be obtained. The in vitro experiments of DLC sliding against UHMWPE apparently showed different results, due to variations in experimental setups (ball-on-disk, hip or knee simulator, surface roughness) and especially the different liquids used as lubricants. In some medical applications such as guidewires, urinary tract catheters, and orthodontic archwires, the in vitro and in vivo experiments on DLC-coated parts showed an improved tribological performance. When implanting a DLC-coated material, it has to be considered that the reaction layer at the DLC/substrate interface has to have a high chemical durability under in vivo conditions to guarantee lifetime adhesion.

One of the best materials to use in applications that require very low wear and reduced friction is diamond, especially in the form of a diamond coating. Unfortunately, true diamond coatings can only be deposited at high temperatures and on selected substrates, and they require surface finishing. However, hard amorphous carbon — commonly known as diamond-like carbon (DLC) coating — has similar mechanical, thermal, and optical properties to those of diamond. It can also be deposited at a wide range of thicknesses using a variety of deposition processes on various substrates at or near room temperature. The coatings reproduce the topography of the substrate removing the need for finishing. The friction and wear properties of some DLC coatings make them very attractive for some tribological applications. The most significant current industrial application of DLC coatings is in magnetic storage devices.

The chapter deals with the multifaceted interrelations between the laser processing and the diamond-like carbon (DLC) films.

Fuel economy and reduction of harmful elements in lubricants are becoming important issues in the automotive industry. An approach to respond to these requirements is the potential use of low friction coatings in engine components exposed to boundary lubrication conditions. Diamond-like carbon (DLC) coatings present a wide range of tribological behavior, including friction coefficients in ultrahigh vacuum below 0.02. The engine oil environment which provides similar favorable air-free conditions might lead to such low friction levels.

In 1994, researchers at Linköping University discovered the fullerene-like allotrope of carbon nitride (FL-CN _x ) by using reactive magnetron sputtering in a nitrogen-containing atmosphere at rather low ion energy assistance. FL-CN _x is a predominantly sp₂-hybridized material with nitrogen structurally incorporated either substitutionally in a graphite sheet or in a pyridine-like manner, which initiates bending by formation of pentagons and cross-linking, respectively. The assumed nitrogen-induced cross-linkage between the sheets contributes considerably to the strength of FL-CN _x by preventing interplanar slip. This results in an extremely fracture tough, elastic, and compliant material, which deforms by reversible bond rotation and bond angle deflection rather than slip and bond breaking.