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DLC Coatings

  2. INTRODUCTION In the last few decades, diamond-like carbon (DLC) coatings have attracted much attention of industries and research groups from all over the world. This increase in demand can be attributed to outstanding physical, chemical, tribological, and optical properties of DLC coatings which make them suitable for a wide range of industrial applications especially those involving boundary lubricant regimes.
  3. The benefit associated with the usage of DLC coatings is that they can be deposited on literally any material ranging from metals to ceramics, nonmetals, and metalloids.
  4. INTRODUCTION DLC COATINGA DLC film can be defined as an amorphous film consisting of an irregular mixture of diamond atoms that form sp3 bonds (diamond structure) and diamond atoms that form sp2 bonds (Graphite structure) Unique mechanical, chemical, optical, and electrical properties. Quite hard, strong, and stiff. Most DLC films are electronically insulating and can be made optically transparent to visible and ultraviolet light. DLC films are chemically inert and impervious to acidic and saline media. They are amorphous.
  5. INTRODUCTION DLC films may also have large amounts of hydrogen in their amorphous structures. Hydrogen-free DLC films can also be deposited. Doping DLC films with dopant such as Cr, F, Ag to achieve better electrical and mechanical properties is also possible.
  6. TRIBOLOGY PROPERTY The mechanical and tribological properties depend on microstructures, . chemistry, hydrogen content, sp2/sp³ bonded carbon. Friction coefficients of the DLC films: 0.01 to 0.5. Relative humidity has the greatest effect on the friction of DLC films. Hydrogen-free DLC films: best in humid air. Hydrogenated DLC films: best in dry or inert conditions. At high temperatures, most undoped DLC films undergo permanent chemical and microstructural changes that degrade their friction and wear behavior (e.g., graphitization).
  7. TRIBOLOGICAL BEHAVIOUR Tribological behavior of DLC coatings is strongly dependent on various intrinsic and extrinsic parameters. To select an optimum DLC coating for a particular application, there is a need to evaluate how various types of coatings perform in operating conditions replicating real life scenarios.
  8. A lot of experimental studies have been carried out by the tribologists to investigate the effects of intrinsic and extrinsic factors on friction and wear characteristics of DLC coatings.
  9. EVOLUTION OF DLC COATING Diamond-like carbon (DLC) first appeared in a paper in 1971. DLC was discovered by accident during research on vapor-phase synthesis of diamond. In the 1950s, high-pressure synthesis of crystalline diamond was developed but it required special and expensive equipment. Therefore, a lot of research was conducted on vapor-phase synthesis for growing diamond crystals from hydrocarbon gas or carbon vapor (gaseous phase). During this process, Aisenberg et al. published a paper on an amorphous hard film mainly composed of carbon in 1971, which was later called DLC.
  10. After that, various DLC deposition processes and films were developed. Having superior characteristics as a lubricative material, such as a low coefficient of friction, high hardness, and chemical stability, DLC films have driven forward development in a unique way different from the vapor-phase synthesis of crystalline diamond. In particular, the low friction coefficient of DLC has been drawing attention due to the requirement to address environmental problems, and the importance of DLC has been increasing in reducing the fuel consumption of automobile engines by decreasing friction.
  11. After 1990, number of experimental investigations focusing on the exploration of mechanical and tribological properties of DLC coatings started to increase exponentially.1The primary objective of these studies was to evaluate the effectiveness of DLC coatings in improving the friction and wear characteristics of the interacting surfaces. Although DLC coatings were limited to optical applications till 1980s but as a result of systematic studies from 1990 to 2010, DLC coatings were started to be used as surface protective films in diverse engineering applications.
  12. CLASSIFICATION OF DLC COATINGS DLC coatings can be classified into three wide categories : DOPED DLCs NON-DOPED DLCs HYBRID DLCs
  13. NON D0PED DLCs In case of nondoped DLC coatings, carbon atoms are bonded with either carbon or hydrogen atoms, whereas, no other element is present in the structure. Based on the hydrogen concentration, nondoped DLC coatings can be further subdivided into hydrogenated [hydrogenated amorphous DLC (a-C:H) and hydrogenated tetrahedral DLC (ta-C:H)] and nonhydrogenated DLC coatings [hydrogen-free amorphous DLC (a-C) and tetrahedral DLC (ta-C)].
  14. DOPED DLCs Based on the type of doping element, dopedDLC coatings can be divided into metal-doped [titanium (Ti), tungsten (W), molybdenum (Mo), and chromium (Cr)] and nonmetal doped-DLC coatings [silicon (Si), fluorine (F), and nitrogen (N)].
  15. HYBRID DLCs In order to further enhance the load carrying capacity of DLC coatings and equal distribution of load, hybrid type known as multilayered DLC (ML-DLC) coating is used in industrial applications. ML-DLC coatings (a-C:H/W-DLC, a-C/W-DLC, VTiN, etc.) comprised of very thin alternate layers of both nondoped and doped DLC coatings stacked together to combine the advantages of both types in a single coating. Generally, tungsten-doped DLC (W-DLC) coating is used as a top layer but in some studies, a-C:H and a-C coatings are also used.
  16. INFLUENCE OF INTRINSIC/EXTRINSIC CONDITIONS The tribological performance of DLC coatings is not only dependent on its synergistic correlation with lubricant constituents but also on various conditions and operating parameters.
  17. INTRINSIC CONDITIONS Internal or intrinsic conditions can be defined as those that specify the inherent nature of DLC coatings and are independent of the external conditions. These features are generally defined during the deposition of DLC coatings on the substrates such as hardness, hydrogen content, nature of dopants, configuration of carbon atoms within the structure, carbon precursor, film thickness, and interlayers.
  18. EXTRINSIC CONDITIONS External or extrinsic conditions are those that are application specific and define the environment in which DLC coatings are supposed to operate. These parameters include temperature, lubrication regime, applied load, sliding speed, geometric configuration, lubricant formulation, counterbody material, and type of motion.
  19. HYDROGEN CONTENT DLC coatings are well known for their high hardness and excellent tribological properties. Hydrogen plays a vital role in defining the tribological and mechanical properties of DLC coatings.53 DLC coatings with about 40 at.% of hydrogen are denoted as hydrogenated DLC coatings, whereas, those with less than 1% are known as nonhydrogenated DLCs.
  20. sp3 /sp2 content Like other intrinsic factors, sp3 /sp 2 ratio of C–C bonds plays an important role in defining the tribological behavior of DLC coatings. Within the structure of DLCs, carbon atoms are bonded with each other either in sp2 or sp3 configuration with a negligible amount of sp1 bonded carbon atoms. DLC coatings which have high percentage of sp2 bonded carbon atoms in their structure are more inclined toward graphite in terms of tribological characteristics and are relatively soft. On the other hand, hard DLC coatings have high sp3 content due to which their mechanical and tribological properties are closer to that of diamond.
  21. Temperature Among the tribotest conditions, temperature is one of the most critical factors that can directly influence the tribological performance of a contact by changing viscosity of the lubricant and eventually the lubrication regime. In addition to that, different phenomena that can change the friction and wear characteristics of a tribosystem such as graphitization are also temperature dependent.
  22. Sliding speed and load Sliding speed and applied load are some of the application specific conditions that can significantly alter the tribological behavior and service life of DLC-coated components. The coefficient of friction is generally decreased with an increase in sliding speed and applied load.
  23. INTERLAYERS To improve the suitability of DLC coatings for various industrial applications and increase their service life, there is a need to promote their adhesion with various substrate materials and minimize the residual stresses entrapped in their structure during the deposition process. This can be achieved by depositing either ceramic or metallic adhesion promoting interlayers that can be bonded strongly with both substrate surface and top DLC layer as a result of a chemical reaction.
  24. DOPING It is an established fact that DLC coatings are chemically inert in nature compared to conventional engineering materials such as metals.91 To increase the thermochemical reactivity of DLC coatings with lubricant constituents and impart application specific properties, different metals and nonmetals are generally incorporated in their structure. By doing so, extraordinary tribological characteristics of DLC coatings such as high hardness, wear resistance, and chemical reactivity of metals can be combined in a single material
  25. Lubricant formulation ◦ Modern day industrial applications involve rigorous operating conditions such as high temperatures, extreme pressures and heavier loads. In these scenarios, either lubricant slips out of the contact or only a thin layer of lubricant resides between the interacting surfaces. To avoid metal-to-metal contact and enhance the tribological performance of the contact, lubricants generally contain specially designed additive packages. These additives are capable of interacting with the interacting surfaces and form tribologically beneficial tribofilms.
  26. SUBSTRATE AND COUNTERBODY MATERIALS There is a strong dependence of tribological characteristics of DLCs on physical and chemical properties of substrate and counterbody materials. A lot of improvements in adhesion strength between DLC coatings and substrates can be made by using surface alteration techniques such as texturing, hardening and roughening
  27. MANUFACTURING PROCESS  Process: Plasma Enhanced Chemical Vapor Deposition (PE-CVD) at room temperature. Other methods are magnetron sputtering method, ion beam process etc.  Source gas for manufacture. o Pure methane. o Mixture of methane and increasing hydrogen  Generally film thickness used is 1 um.  DLC films deposition range: subzero to 400°C..  Processes: plasma or ion beam- PVD and CVD.  Carbon source: hydrocarbon gas like CH4, C₂H₂-
  28. APPLICATIONS Medical devices and implants: Anti bacterial properties. Glass: anti-scratch. Moulds: to produce smooth surface and reduce wear. Engine and space parts: low friction . Cutting tools: Wear resistance and to increase lifetime. Oil & gas pipelines: super lubricity in pipes.
  29. The effects of intrinsic and extrinsic conditions on the tribological characteristics of various DLC coatings have been investigated, The main findings can be summarized as follows:  Ultralow friction coefficients can be achieved with hydrogenated DLC coatings, whereas, hydrogen-free DLCs are comparatively more wear-resistant due to their high hardness values. Interlayers such as chromium, titanium nitride and tungsten can increase the service life and reliability of DLC coatings by promoting the adhesion with the substrate and uniformly distributing the applied load.
  30.  Hardness and subsequent wear resistance of DLC coatings can be enhanced by tweaking the deposition parameters in such a way that they result in the coatings with higher sp3 /sp 2 ratio. By doing so, properties of DLC coatings can be made more inclined toward diamond. Friction coefficients of DLC coatings generally decrease with an increase in operating temperature especially beyond 100 °C due to the structural transformation from diamond to graphite phase. Lower levels of friction in symmetrical and asymmetrical DLC contacts can be achieved at heavier applied loads and increased sliding speeds. This can be attributed to the accelerated formation of harder and tougher tribofilm at extreme test conditions along with pressure induced graphitization of transfer layer.
  31. Surface modification techniques such as roughening and laser texturing can improve tribological performance of DLC coatings by efficiently lubricating the contact especially under starved conditions. Conventional lubricant additives such as ZDDP, MoDTC, GMO, and AP, which are primarily designed for interacting with ferrous surfaces, are also effective on DLC coatings. Tribochemical interactions between lubricant additives and DLC coatings can occur even in the absence of ferrous counterbodies and these interactions do not always necessarily result in enhanced tribological performance.