A. 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.^{Reference Erdemir, Eryilmaz and Kim53} DLC coatings with about 40 at.% of hydrogen are denoted as hydrogenated DLC coatings, whereas, those with less than 1 at.% are known as nonhydrogenated DLCs.^{Reference Erdemir54} It is possible to achieve ultralow friction coefficients with hydrogenated DLC coatings in the presence of inert gases and under vacuum,^{Reference Erdemir, Eryilmaz and Fenske55} whereas, no such behavior is associated with nonhydrogenated DLCs.^{Reference Andersson, Erck and Erdemir56} This improvement in friction performance can be attributed to the evolution of hydrogen due to strain and thermal effects experienced during sliding and formation of soft porous graphitic layer, independent of tribochemical interaction between DLC coatings and lubricant additives.^{Reference Komori and Umehara57} It has also been observed that DLC coatings passivated with hydrogen atoms offers low coefficient of friction compare with those passivated with environmental species such as oxygen or moisture.^{Reference Paulmier, Zaidi, Nery, Huu and Mathia58} On the other hand, relatively inferior friction performance of nonhydrogenated DLC coatings can be improved by either increasing the humidity level or by using a lubricant containing glycerol mono-oleate (GMO).^{Reference Erdemir, Eryilmaz and Fenske55,Reference Bouchet, Matta, Le-Mogne, Martin, Zhang, Goddard, Kano, Mabuchi and Ye59} Hardness of DLC coatings is also dependent on the atomic concentration of hydrogen in their structure and it is generally decreased with an increase in hydrogen content.^{Reference Komori and Umehara57,Reference Bhushan60,Reference Flege, Hatada, Ensinger and Baba61} Due to their high hardness, hydrogen-free DLC coatings (a-C and ta-C) are more wear-resistant compared with hydrogenated DLCs.^{Reference Hainsworth and Uhure62}

In an experimental investigation, Vengudusamy et al.^{Reference Vengudusamy, Green, Lamb and Spikes63} investigated the effect of hydrogen concentration on the tribological characteristics of self-mated a-C:H coatings by varying it from 17 to 25%. A decrease in friction levels between symmetrical a-C:H contact was observed with an increase in hydrogen content. The lowest value of friction was observed with 22 at.% of hydrogen (0.05), whereas, 18 at.% (0.07) gave the highest value irrespective of lubricant formulation. No particular relation of wear coefficient with concentration of hydrogen can be drawn from the experimental results given in the literature. However, a decrease in zinc dialkyldithiophosphate (ZDDP)-derived tribofilm concentration, comprised of zinc, phosphorus, and sulfur, was observed with DLC coatings rich in hydrogen. A similar behavior was also observed by Haque and his team.^{Reference Haque, Morina, Neville, Kapadia and Arrowsmith64} They found no evidence of ZDDP-derived tribofilm on a-C:H coatings containing 30 at.% of hydrogen when tested in the presence of ZDDP and molybdenum dithiocarbamate (MoDTC) containing lubricant.

In a similar study, de Barros'Bouchet et al.^{Reference de Barros'Bouchet, Martin, Le-Mogne and Vacher65} tested a-C:H coating containing 50 at.% hydrogen in its structure in combination with polyalphaolefin (PAO) additivated with MoDTC and ZDDP and compared the tribological parameters with a-C coating. They used American Iron and Steel Institute (AISI) 52100 steel cylinders as counterbody. Improved tribological performance was observed in the coating containing hydrogen in its structure. In case of a-C:H/steel contact, friction coefficients of 0.05 and a wear coefficient of 0.8 × 10⁻⁸ mm³/(N m) were recorded, whereas, a-C/steel tribopair gave values of 0.08 and 1.5 × 10⁻⁸ mm³/(N m), respectively. According to hard soft acids bases principle, hydrogenated DLC coatings are soft bases and preferably react with the soft acids such as Mo⁴⁺ resulting in the formation of MoS₂ which acts as solid lubricant. On the other hand, nonhydrogenated DLCs are hard bases and are attracted toward Mo⁶⁺ to form MoO₃ which is ineffective in enhancing the friction performance.

Komori and his team^{Reference Komori and Umehara57} investigated the effect of hydrogen content on the tribological performance of various DLC coatings sliding against steel counterbodies using ZDDP and MoDTC containing fully formulated oil. They observed that ratio of MoS₂/MoO₃ in additive-derived tribofilm generally increases with an increase in hydrogen content which in turn decreases the values of coefficient of friction. It was also witnessed that a-C:H coatings with very high surface roughness and hardness values also showed the formation of additive-derived tribofilms but with less durability eventually resulting in lower MoS₂/MoO₃ ratio. A similar behavior was also observed with hydrogen-free DLC coatings having high hardness and surface roughness values. The highest concentration of metallic-molybdenum and molybdenum carbide was detected with DLCs containing hydrogen content in the range of 25–40 at.%, whereas, no such species were found in case of DLCs with lower hydrogen content. It is interesting to note that highest value of metallic-molybdenum and molybdenum carbide was observed in a-C:H coating (33 at.% hydrogen) which was completely worn out during tribotesting. This effect can be attributed to decrease in hardness with an increase in hydrogen content which resulted in structural transformation from diamond to soft graphite phase. Due to this transformation, aggravated wear was observed in DLC coatings with higher concentration of hydrogen. The other factors which played a vital role in accelerated wearing of the DLC-coated surface and steel counterbody include the formation of transfer layer, tribochemical reaction with MoDTC, and the formation of hard compounds such as molybdenum carbide.

In another experimental investigation, Masuko et al.^{Reference Masuko, Ono, Aoki, Suzuki and Ito66} compared the tribological performance of self-mated a-C:H coatings containing 10 and 20 at.% of hydrogen with ta-C coating. MoDTC and other molybdenum-containing compounds were used as lubricant additives in PAO. An increase in wearing rates of hydrogenated DLC coatings was observed when MoDTC and other molybdenum-based additives were used as additives, whereas, no such behavior was observed with hydrogen-free DLC (ta-C). Contrary to that, PAO additivated with MoDTC maintained the coefficient of friction at a value of 0.055 for all three tribopairs (a-C:10H/steel, a-C:30H/steel and ta-C/steel). It was also observed that the additivation of MoDTC-containing lubricant with ZDDP can result in decreased wear of hydrogenated DLC coatings. Although MoO₃ was detected on all types of DLC coatings tested with MoDTC-additivated lubricants but no direct relation between amplified wear and MoO₃ was observed.

B. sp ³/sp ² content

Like other intrinsic factors, sp ³/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 sp ² or sp ³ configuration with a negligible amount of sp ¹ bonded carbon atoms.^{Reference Neuville and Matthews67,Reference Foong, Koh, Hsieh and Chua68} DLC coatings which have high percentage of sp ² bonded carbon atoms in their structure are more inclined toward graphite in terms of tribological characteristics and are relatively soft.^{Reference Grill17,Reference Sullivan, Friedmann and Hjort69} On the other hand, hard DLC coatings have high sp ³ content due to which their mechanical and tribological properties are closer to that of diamond.^{Reference Donnet, Fontaine, Grill and Le Mogne19,Reference Foong, Koh, Hsieh and Chua68,Reference Wang, Ma, Tang and Sun70} Suitability and service life of DLC coatings for various applications can be improved if somehow coatings with high hardness can be manufactured.^{Reference Neuville and Matthews67} Among the DLC coatings, the highest percentage of sp ³ content is found in ta-C (above 80%), whereas, a-C contains the lowest content of sp ³ bonded carbon atoms in its structure.^{Reference Al Mahmud, Kalam, Masjuki, Mobarak and Zulkifli71} Hydrogenated DLCs generally have higher sp ³/sp ² ratio.^{Reference Bhushan60} The sp ³/sp ² ratio of the coatings can be adjusted according to the application by varying the deposition parameters to impart either diamond or graphite-like characteristics.^{Reference Vetter72} One drawback associated with increasing the sp ³/sp ² ratio is that it amplifies the level of internal stresses during the deposition process. It makes the coatings more prone to delamination during the operation due to poor adhesion with the substrate and drastically affects the service life of the components.^{Reference Dearnley, Neville, Turner, Scheibe, Tietema, Tap, Stüber, Hovsepian, Layyous and Stenbom73,Reference Bewilogua, Cooper, Specht, Schröder, Wittorf and Grischke74}

In an experimental study, Vengudusamy and his team^{Reference Vengudusamy, Mufti, Lamb, Green and Spikes75} investigated the effect of sp ³ concentration on the tribological performance of self-mated ta-C coating in combination with API group III (A-III) base oil. Two different coatings with sp ³ concentration of 40 and 75% were tested at a temperature of 100 °C using identical tribotest conditions. It was observed that coatings with higher concentration of sp ³ content offered improved wear resistance compared to the one which is rich in sp ² content. Disc and ball wear coefficients recorded for ta-C coating containing 40% sp ³ content were 1.6 × 10⁻⁸ and 1.1 × 10⁻⁸ mm³/(N m), respectively, whereas, values of 1.4 × 10⁻⁸ and 0.5 × 10⁻⁸ mm³/(N m) were observed for ta-C coating having higher sp ³/sp ² ratio (75% sp ³ content). However, no significant change in the friction performance was realized. This improved wear resistance of ta-C coating with 75% of sp ³ content can be attributed to its higher hardness/elastic modulus ratio relative to the one having a sp ³ concentration of 40%.

C. 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. Moreover, lubricant additives have a threshold temperature under which they cannot interact with the sliding surfaces to form beneficial tribofilms and perform their intended role. Therefore, a lot of experimental investigations were carried out to understand the effect of temperature variations on the tribological behavior of various DLC coatings in combination with additivated and nonadditivated lubricants.

In an experimental study, Al Mahmud et al.^{Reference Al Mahmud, Varman, Kalam, Masjuki, Mobarak and Zulkifli76} investigated the effect of temperature on the tribological performance of ta-C/steel and a-C:H/steel tribopairs in combination with society of automotive engineers (SAE) 40 fully formulated lubricant. Three sets of tribotests were performed at 50, 100, and 150 °C with a constant applied load of 392 N. At higher test temperatures, an increase in I _D/I _G ratio of both tribopairs was verified by Raman spectroscopy which confirmed that phenomenon of graphitization took place as a result of sliding. Although, improvement in friction performance was observed at higher temperatures due to graphitization but it adversely affected the wear resistance of DLC coatings especially a-C:H which was completely delaminated during sliding at 150 °C. On the other hand, ta-C/steel contact was able to withstand the harsh tribotest conditions due to the formation of ZDDP-derived tribofilm on the interacting surfaces which not only prohibited the delamination of DLC coating but also prevented the wearing of the counterbody.

A similar behavior was observed by Kim and his team^{Reference Kim and Kim26} in multilayered W-DLC coating sliding against AISI 52100 steel balls in the presence of SAE 5W20 fully formulated lubricant rich in GMO. When temperature was increased from 27 to 160 °C in steps, improvement in friction performance of the contact was observed due to the increase in the extent of graphitization and enhanced tribochemical interaction between lubricant additives and interacting surfaces. Additive-derived tribochemical compounds were majorly found on steel surfaces, whereas, no such compounds were found on DLC-coated surfaces. Contrary to friction results, an increase in composite wear coefficient was observed with an increase in temperature. This deterioration in wear resistance can be attributed to the increase in the intensity of corrosive wear and decrease in durability of tribofilms at elevated temperatures.

In a similar study, Amanov and his team^{Reference Amanov, Watabe, Tsuboi and Sasaki77} analyzed silicon-doped DLC (Si-DLC) sliding against alumina balls in the presence of nonadditivated PAO to see the effect of temperature variation on friction and wear characteristics. When test temperature was increased from 25 to 100 °C, friction coefficient was also increased from 0.14 to 0.18 due to the formation of oxygen-rich film and accumulation of wear debris. Contrary to that, an exponential improvement in friction performance of Si-DLC was observed at 200 °C due to structural transformation from diamond to graphite phase and friction coefficient was reduced to its lowest value of 0.10. No significant change in the wear coefficient of Si-DLC coated disc was observed at lower test temperatures i.e., 25, 50, and 100 °C. It is well known that graphitization adversely affects the structural integrity of nondoped DLC coatings due to which they become soft and more vulnerable to excessive wearing. Although, occurrence of graphitization was witnessed at 200 °C but still a substantial improvement in wear characteristics of Si-DLC was observed and wear coefficient was reduced from 0.4 × 10⁻⁹ to 0.24 × 10⁻⁹ mm³/(N m). From these observations, it can be concluded that doped-DLC coatings such as Si-DLC behaves differently from nondoped DLC coatings as far as wear behavior at elevated temperature in combination with graphitization is concerned.

In another study, Abdullah Tasdemir and his team^{Reference Abdullah Tasdemir, Wakayama, Tokoroyama, Kousaka, Umehara, Mabuchi and Higuchi78} studied the effect of changing temperature, ranging from 25 to 110 °C, on wear characteristics of ta-C/steel contact using additivated and nonadditivated versions of PAO as lubricants. Overall, deterioration in wear properties of ta-C coatings was observed at high temperatures in case of nonadditivated PAO, whereas, additives proved to be helpful in overcoming the excessive wear by forming additive-derived tribofilms and surface passivation. No effect of temperature variation on the wear performance of ta-C coating was observed when PAO + ZDDP was used. Contrary to that, significant increase in wear coefficient was seen with nonadditivated PAO when temperature was increased beyond 50 °C and the highest value of 34.0 × 10⁻⁸ mm³/(N m) was observed at 110 °C. This effect can be attributed to different wear mechanisms involved at elevated temperatures such as delamination, brittle fracture, graphitization, and tribofilm formation compared to polishing wear which usually occurred at room temperature.^{Reference Ouyang and Sasaki79} An improvement in wear performance of ta-C/steel tribopair was experienced with GMO-additivated lubricants. Effectiveness of GMO in reducing friction between the contacts amplified with an increase in temperature in both PAO + GMO and PAO + GMO + ZDDP lubricants, whereas, opposite behavior was observed with PAO + ZDDP. In case of PAO, coefficient of friction was initially decreased when temperature increased from 25 to 50 °C but significant increase from 0.02 to 0.08 was observed when temperature was further elevated to 80 and 110 °C. A similar deterioration in wear characteristics and improvement in friction performance of titanium-doped DLC (Ti-DLC)/steel and a-C:H/a-C:H contacts at elevated temperature were also observed in the experimental studies carried out by Ouyang and Sasaki^{Reference Ouyang and Sasaki79} and Kalin et al.^{Reference Kalin, Roman and Vižintin80}

D. 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. It is mentioned in the literature that coefficient of friction is generally decreased with an increase in sliding speed and applied load.^{Reference Ouyang, Sasaki, Murakami, Zhou and Zhang30,Reference Yasuada44} A similar behavior also exists between wear coefficient and applied load, whereas, opposite is true for sliding speed.^{Reference Kim and Kim26,Reference Ouyang and Sasaki79}

Zaidi and his team^{Reference Zaidi, Le Huu, Robert, Bedri, Kadiri and Paulmier81} studied the effect of sliding speed and normal load on tribological characteristics of a-C:H coatings in vacuum under dry conditions. They found an indirect relation of sliding speed and normal load with coefficient of friction. At 4 N applied load, value of friction coefficient was 0.16, whereas, it decreased to 0.12 when load was increased to 8 N. Similarly, coefficient of friction was reduced from 0.1 to 0.085 when sliding speed of the contact was increased from 0.03 to 0.27 m/s. With an increase in normal load, a decrease in endurance life or abrasive wear resistance was also detected. This tribological behavior can be attributed to increase in contact temperature at high values of applied loads and sliding speeds due to which hydrogen is desorbed from the coating resulting in structural transformation from diamond to graphite phase.

In a similar study, Erdemir and his team^{Reference Erdemir, Bindal, Fenske, Zuiker and Wilbur82} used zirconia balls sliding against a-C:H coated disks at three different velocities i.e., 1, 2, and 6 m/s. Tribological experiments were performed in air without using any liquid lubricant between the interacting surfaces. The highest and lowest values of coefficient of friction were observed at 1 and 6 m/s, respectively. An improvement in friction performance with an increase in sliding speed can be attributed to the transformation of DLC coating from diamond to graphite phase. It was also observed that steady state coefficient of friction can be achieved much earlier at higher sliding speeds but it adversely affects the service life of DLC coatings.

In another study, Kim and Kim^{Reference Kim and Kim27} investigated the friction and wear characteristics of multilayered a-C:H coated disc sliding against AISI 52100 steel balls under dry conditions. To investigate the influence of applied load and sliding speed on tribological characteristics of a-C:H/steel contact, six different values of sliding speeds (0.0625, 0.125, 0.25, 0.5, 1, and 2 m/s) and three different levels of applied loads (6.1, 20.7, and 49.0 N) were selected. Tribological experiments were performed in the presence of ambient air with a relative humidity of 26% and temperature of 18 °C. Reduction in friction was observed with an increase in both sliding speed and applied load. Lowest value of friction coefficient (0.05) was observed at a sliding speed of 1 m/s and applied load of 49.0 N, whereas, the highest level of friction (0.18) between the interacting surfaces was observed at sliding speed and applied load of 0.06 m/s and 6.1 N, respectively. This enhanced friction performance of the contact can be attributed to surface graphitization and formation of transfer layer at extreme test conditions. Similar to friction results, lowest values of wear coefficients of a-C:H coated disc [5.0 × 10⁻⁸ mm³/(N m)] and AISI 52100 steel ball [0.2 × 10⁻⁸ mm³/(N m)] were observed at elevated values of sliding speed (2 m/s) and applied load (49.0 N), whereas, highest values [55.0 × 10⁻⁸ mm³/(N m) and 1.2 × 10⁻⁸ mm³/(N m), respectively] were observed at an applied load of 6.1 N and intermediate values of sliding speeds (0.125–0.25 m/s). Wear coefficients of both surfaces were initially increased and then decreased with an increase in sliding velocity. This effect can be attributed to the asperity breakages at higher sliding velocities which resulted in decreased number of asperity contacts and increase in the tribochemical products concentrations on the interacting surfaces.^{Reference Kim and Kim26}

Kano and his team^{Reference Kano, Yasuda, Okamoto, Mabuchi, Hamada, Ueno, Ye, Konishi, Takeshima and Martin83} studied the effect of sliding velocity on the friction characteristics of ta-C/steel and a-C:H/steel contacts using pin-on-disc configuration and reciprocating rig. SAE 5W30 and PAO additivated with GMO were used as lubricants and tribological results were compared with needle bearing in pure rolling contact. A decrease in coefficient of friction was observed with an increase in sliding speed from 0.1 to 1 m/s irrespective of contact type. However, a-C:H/steel contact showed high values of friction compared to ta-C/steel tribopair and needle bearings irrespective of oil formulation. A steady-state and ultralow coefficient of friction of 0.006 was achieved in case of ta-C/steel when tested in the presence of PAO + GMO at a sliding speed of 0.10 m/s however, no further reduction in friction was observed with increase in speed. This value of friction is equal to the value obtained with needle bearings in pure rolling contact. This shows that level of friction observed in case of rolling contact can also be obtained with sliding contact under boundary lubrication regime using ta-C coatings and PAO + GMO as a lubricant.

Ouyang and his team^{Reference Ouyang, Sasaki, Murakami, Zhou and Zhang30} studied the effect of varying normal load i.e., 2 and 5 N, on friction and wear performance of Ti-DLC coating sliding against steel and alumina balls under dry conditions and at room temperature. Ti-DLC surfaces showed aggravated wear and amplified friction when slid against steel compared with alumina. An increase in wear coefficient of Ti-DLC coated flat was observed in both Ti-DLC/steel and Ti-DLC/alumina tribopairs with an increase in applied load. When the load was increased from 2 to 5 N, wear coefficient of Ti-DLC coating was increased from 0.182 × 10⁻⁸ to 0.287 × 10⁻⁸ mm³/(N m) when alumina ball was used as counterbody. Similar deterioration in wear behavior was also observed in case of Ti-DLC/steel tribopair and wear rate of Ti-DLC coated surface was increased from 0.434 × 10⁻⁸ to 0.517 × 10⁻⁸ mm³/(N m). Comparatively, high value of friction was achieved with Ti-DLC/steel tribo-pair irrespective of the value of applied load. Although, a decrease in coefficient of friction was noted when load was increased from 2 to 5 N in both Ti-DLC/steel and Ti-DLC/alumina tribopairs, however, improvement in friction performance was more pronounced when steel was used as counterbody. When load was increased from 2 to 5 N, friction coefficients was decreased from 0.135 to 0.13 and 0.235 to 0.19 for Ti-DLC/alumina and Ti-DLC/steel contacts, respectively.

In a similar study, Gangopadhyay and his team^{Reference Gangopadhyay, Zdrodowski and Simko84} tribologically evaluated Si-DLC coating using cylinder-on-flat geometric configuration in combination with SAE 5W30 fully formulated oil at a temperature of 100 °C and sliding speed of 0.3 m/s. No additive-derived tribofilm was observed on the DLC-coated flat till the value of applied load was increased to 240 N. Beyond 240 N, additive-derived tribofilm started to build up and kept on thickening with the subsequent increase in the value of applied load. On further increasing the load, a decrease in the tribofilm thickness was observed. When the same tests were conducted on uncoated steel flat at an applied load of 80 N, calcium containing tribofilms containing identical concentrations were observed on the interacting surfaces. These results shows that there exists a threshold value of applied load at which additive-derived tribofilms can be formed on the sliding surfaces and that level is very much higher for DLC-coated surfaces as compared to ferrous-based materials. In addition to that, thicker tribofilms were observed on the steel surfaces as compared to Si-DLC coating.

E. 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.^{Reference Moon, Jensen, Hutchinson, Oh and Evans85} 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.^{Reference Bewilogua and Hofmann1,Reference Bubenzer, Dischler and Nyaiesh13} Interlayers also help in uniformly distributing the load across the substrate resulting in increased load carrying capacity of the coatings.^{Reference Kržan, Novotny-Farkas and Vižintin86} Fatigue and corrosion resistance can also be improved by depositing suitable interlayer before the deposition of top DLC coating.^{Reference Forsberg, Gustavsson, Renman, Hieke and Jacobson87} Some of the most commonly used interlayers include titanium diboride (TiB₂), titanium (Ti), titanium nitride (TiN), chromium (Cr), chromium carbide (CrC), chromium nitride (CrN), tungsten (W), and silicon nitride (Si₃N₄).^{Reference Bewilogua and Hofmann1,Reference Cruz, Rao, Rose, Lawson and Nicholls88}

In an experimental investigation, Kalin and his team^{Reference Kalin and Vižintin89} studied the effect of two different types of adhesion promoting interlayers on the tribological performance of a-C:H coating. TiN and silicon-based compound with thickness values of 0.5 and 1 µm, respectively, were used as interlayers. The adhesion strength of the coatings was measured using a scratch tester. The test results revealed that TiN proved to be more efficient in enhancing the bonding between the substrate and a-C:H coating. Since, only the top layer comes into contact during the sliding therefore, no effect of interlayer on the friction and wear performance of the contact was seen.

In another study, Forsberg and his team^{Reference Forsberg, Gustavsson, Renman, Hieke and Jacobson87} used two different types of interlayers in a-C:H coatings. In both coatings, chromium layer was used to increase the adhesion between the substrate and interlayers. In addition to that, CrN was also deposited in one of the coatings to increase its load bearing capacity, corrosion resistance and fatigue strength, whereas, W-DLC interlayer was deployed in the second coating for reducing the level of internal stresses induced during the deposition process. After tribological experiments, it was revealed that a-C:H coating which has CrN interlayer is less vulnerable to delamination and has better adhesion strength compared to the one having W-DLC interlayer.

Although silicon-based interlayers proved to be helpful in improving the adhesion between the substrate and DLC coatings in most of the applications, they are inappropriate for biomedical implants because such layers are vulnerable to corrosive attack by body fluid.^{Reference Hauert, Thorwarth, Müller, Stiefel, Falub, Thorwarth and Joyce90} Due to the imperfections and crevices in top layer of DLC coatings, body fluid can flow down to the underlying interlayers resulting in corrosion and decreased adhesion strength. Therefore, it is extremely important to test the corrosion stability and compatibility of all the interlayers in the structure of DLC coating before employing them for a particular industrial application otherwise there is an increased likelihood that coated parts will fail on a long-term exposure to harsh environmental and operating conditions.

F. Doping

It is an established fact that DLC coatings are chemically inert in nature compared to conventional engineering materials such as metals.^{Reference Grill, Patel and Meyerson91} 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. Compared to nondoped DLCs, metal-doped ones have higher surface energy and abundant active sites that facilitates in interactions with lubricant additives and environmental species. During the deposition of DLC coatings on the substrates, internal stresses are generally entrapped within the structure which aggravates with the increase in film thickness.^{Reference Gangopadhyay92} As a result, these coatings become more vulnerable to surface defects such as cracking and delamination resulting in reduced service life. Doping of DLC coatings with nonmetals is proved to be effective in mitigating the catastrophic effects of these stresses on the structural integrity and reliability of DLC coatings.^{Reference Robertson93,Reference Grill94} Moreover, the presence of nonmetals in the structure of DLC coatings can also alter their sp ³/sp ² ratio. It is mentioned in the literature that the incorporation of silicon in DLC coatings results in the increase of sp ³ content in their structure, whereas, opposite is true for fluorine.^{Reference Park, Choi, Park, Jang, Hong and Kwon95}

de Barros'Bouchet et al.^{Reference de Barros'Bouchet, Martin, Le-Mogne and Vacher65} compared the tribological performance of a-C, a-C:H, and Ti-DLC coatings using additivated and nonadditivated versions of PAO. Doping of a-C:H coating with titanium decreased its hardness from 1600 to 1000 HV and Young's modulus from 150 to 80 GPa. In case of DLC/steel contacts, the lowest value of steady-state coefficient of friction and wear rate were observed with Ti-DLC coating irrespective of the lubricant formulation. Almost similar tribological behavior was also observed with symmetrical DLC contacts. When PAO and PAO + MoDTC were used as lubricants separately, Ti-DLC/Ti-DLC tribopair demonstrated excellent tribological performance followed by a-C:H/a-C:H and steel/steel. With PAO + MoDTC + ZDDP, a-C:H/a-C:H demonstrated slightly better friction and wear behavior than Ti-DLC/Ti-DLC tribopair. This effect can be attributed to the enhanced tribochemical interaction of lubricant additives with richer-hydrogenated DLC such as a-C:H. Although some tribochemical interaction was also observed in case of Ti-DLC/Ti-DLC couple but not to an extent of a-C:H/a-C:H.

Effect of tungsten doping concentration (12 at.% W, 14 at.% W, 18 at.% W, and 21 at.% W) on the tribological performance of W-DLC in the presence of A-III base oil was studied by Vengudusamy and his team.^{Reference Vengudusamy, Mufti, Lamb, Green and Spikes75} DLC-coated balls and discs were used as interacting surfaces sliding against each other at a test temperature of 80 °C. Increase in the hardness of W-DLC was observed with an increase in tungsten concentration till 18 at.%. However, further increase in tungsten concentration to 21 at.% resulted in a decrease in hardness value from 1318 to 1190 HV. Wear coefficients of DLC-coated balls remained unchanged for different values of tungsten concentration. On the other hand, higher concentration of tungsten resulted in accelerated wear of DLC-coated discs. Lowest value of friction was observed with W-DLC coating containing 18 at.% of W, whereas, it remains constant at a level of 0.10 for rest of the concentrations.

In a similar study, Vengudusamy et al.^{Reference Vengudusamy, Green, Lamb and Spikes96} compared the tribological behavior of self-mated W-DLCs with those of a-C/a-C, a-C:H/a-C:H, ta-C/ta-C, ta-C:H/ta-C:H, and Si-DLC/Si-DLC tribopairs in combination with A-III and A-III + MoDTC lubricants. When nonadditivated A-III lubricant was used, W-DLC/W-DLC contact offered more friction compared to ta-C/ta-C and a-C/a-C contacts, whereas, rest of the tribopairs showed higher values of friction coefficients. Additivation of A-III with MoDTC improved the friction behavior of almost all tribopairs. Lowest values of friction coefficients were observed for W-DLC/W-DLC and ta-C/ta-C contacts. Ultralow friction coefficient observed in case of symmetrical W-DLC contact can be attributed to the formation of WS₂ along with MoS₂. Contrary to friction results, highest values of wear coefficients were observed for W-DLC/W-DLC when both A-III and A-III + MoDTC were used as lubricants separately. This deterioration in wear performance due to tungsten-doping of DLC coating can be attributed to its vulnerability to graphitize.

Abdullah Tasdemir and his team^{Reference Abdullah Tasdemir, Tokoroyama, Kousaka, Umehara and Mabuchi97} compared the friction and wear performance of Si-DLC and chromium-doped DLC (Cr-DLC) with nondoped DLCs. Both doped-DLCs showed reduced wear in both PAO and PAO + ZDDP lubricants compared with other DLCs. Opposing to that, doping of DLC coatings with silicon and chromium resulted in increased friction between the sliding surfaces irrespective of the lubricant formulation.

In another investigation, Kalin et al.^{Reference Kalin, Roman, Ožbolt and Vižintin98} compared the wear characteristics of conventional DLC coatings with Ti-DLC and W-DLC in the presence of formulated lubricants. The highest level of tribochemical interaction with ZDDP and consequently the lowest value of wear coefficient were observed in case of Ti-DLC. This improved wear resistance with titanium doping can be attributed to 25 times increase in P/S ratio due to the formation of ZDDP-derived tribochemical products on the interacting surfaces in large quantities (Fig. 5). Although, some tribochemical reaction was also observed between W-DLC and ZDDP, it did not effectively improve the wear resistance of interacting surfaces resulting in the highest value of wear coefficient. Graphitization also played a key role in accelerating the wear of W-DLC coated surfaces.

FIG. 5.

Structural formula of ZDDP.^{Reference Spikes99}

G. 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. Since, lubricant companies design the lubricants by keeping in mind that their products will only interact with the ferrous surfaces, therefore, a lot of researchers conducted research to investigate how most widely used conventional lubricant additives and fully formulated lubricants interact with various types of DLC coatings in symmetrical and asymmetrical contacts. Some of the lubricant additives which were most widely investigated in combination with various DLC coatings include MoDTC, ZDDP, GMO, and amine phosphates (AP).

In an experimental investigation, de Barros'Bouchet and his team^{Reference de Barros'Bouchet, Martin, Le-Mogne and Vacher65} investigated the effect of MoDTC on the tribological performance of Ti-DLC/Ti-DLC contact using PAO as a base oil. When PAO + MoDTC was used as a lubricant, tribochemical interaction took place between MoDTC and Ti-DLC coated surfaces resulting in lower coefficient of friction (0.06) compared to nonadditivated PAO (0.07) due to the formation of MoS₂. Although, MoDTC is primarily a friction modifier, it was able to further enhance the wear resistance of self-mated Ti-DLC contact when mixed with PAO and wear coefficient was decreased from 0.2 × 10⁻⁸ to 0.01 × 10⁻⁸ mm³/(N m). Similarly, an improvement in friction performance was also observed in case of W-DLC/W-DLC, Si-DLC/Si-DLC and symmetrical tungsten carbide-doped DLC WC-DLC/WC-DLC contacts when MoDTC was used as an additive in A-III base oil.^{Reference Vengudusamy, Green, Lamb and Spikes96}

Effectiveness of ZDDP in reducing the wear coefficient of doped-DLC coatings was studied by Kalin and his team.^{Reference Kalin, Vižintin, Barriga, Vercammen, Van Acker and Arnšek100} When paraffinic mineral oil (M) was used in nonadditivated form, accelerated wear of W-DLC/W-DLC and Ti-DLC/Ti-DLC contacts was noticed due to delamination and coating spallation. Additivation of ZDDP not only reduced the wear substantially but also helped in improving the service life of DLC coatings by overcoming the surface defects. In case of self-mated Si-DLC coating, an improvement in wear characteristics was realized as a result of ZDDP-derived tribochemical products due to which adhesive wear was replaced by polishing wear. A similar behavior was also observed by Abdullah Tasdemir et al.^{Reference Abdullah Tasdemir, Tokoroyama, Kousaka, Umehara and Mabuchi97} in Si-DLC/Si-DLC and Cr-DLC/Cr-DLC contacts when ZDDP was used as an additive in PAO due to the formation of ZDDP-derived pad-like tribofilm comprised of zinc, sulfur, and phosphorus.

A reduction in wear coefficient of ta-C/ta-C contact was observed by Abdullah Tasdemir and his team when PAO was additivated with GMO compared to nonadditivated PAO and it was reduced from 4.0 × 10⁻⁸ to 2 × 10⁻⁸ mm³/(N m).^{Reference Abdullah Tasdemir, Wakayama, Tokoroyama, Kousaka, Umehara, Mabuchi and Higuchi101} This improvement in wear performance can be explained by the interaction of hydroxyl group in the structure of GMO resulting in passivation of dangling carbon atoms. It is mentioned in the literature that passivation of carbon bonds decreases the surface energy of DLC coatings due to which their ability to react with environmental species such as oxygen and moisture also decreases.^{Reference Al Mahmud, Varman, Kalam, Masjuki, Mobarak and Zulkifli76} As a result, surface deterioration due to oxidation is minimized and service life of the contact increases. GMO also proved to be effective in facilitating the sliding of Cr-DLC coated surfaces and resulted in decreased friction coefficients when used as an additive in A-II base oil.^{Reference Topolovec-Miklozic, Lockwood and Spikes102} This enhancement in friction characteristics can be attributed to the adsorption of viscous GMO-derived tribofilm on the asperities and extrusions.

AP is also one of the conventional lubricant additives that are widely investigated in combination with DLC coatings. The primary role of AP is to minimize the composite wear of a contact by increasing the surface energy and preventing the material transfer between interacting surfaces. In an experimental investigation, Kalin and his team^{Reference Kalin and Vižintin103} analyzed the effectiveness of AP in enhancing the tribological properties of various nondoped and doped-DLC coatings in symmetrical and asymmetrical contacts using paraffinic mineral base oil. Detrimental effect of AP additive was observed on the friction performance of all the contacts resulting in 5–25% increase in coefficient of friction. Contrary to that, AP significantly improved the wear characteristics of W-DLC/steel, a-C:H/a-C:H, and W-DLC/W-DLC tribopairs by preventing coating spallation/delamination and replacing adhesive wear by polishing wear.

H. 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. Texturing emerged as a surface modification technique using which tribological performance of various mechanical components can be improved significantly.^{Reference Dumitru, Romano, Weber, Pimenov, Kononenko, Sentis, Hermann and Bruneau104–Reference Etsion107} Various techniques such as reactive ion etching, ion beam machining and lasers can be used to make grooves, dimples, patterns or channels of different orientations, sizes, and shapes on DLC-coated surface/substrate.^{Reference Etsion107,Reference Oksanen, Hakala, Tervakangas, Laakso, Kilpi, Ronkainen and Koskinen108} In Fig. 6, textured substrates with varying dimple diameter, dimple depth and dimple density are shown. These depressions not only act as hydrodynamic bearings to generate additional lift but also serve as reservoirs to continuously supply lubricant and trap wear debris to prevent third-body interaction. Moreover, texturing also obviate surface deterioration by abrasive wear and reduce contact area of mating surface.

FIG. 6.

Textured substrates with varying dimple diameter, dimple depth, and dimple density.

In an experimental investigation, Amanov and his team^{Reference Amanov, Watabe, Tsuboi and Sasaki77} compared the tribological performance of Si-DLC coating deposited on textured and untextured bearing steel substrates at various temperatures. In sliding tests, alumina balls were used as counterbodies and PAO as lubricant. An improvement in friction and wear characteristics of Si-DLC coating was observed when textured substrates were used instead of the untextured ones. At 200 °C, untextured Si-DLC coating gave a friction and wear coefficient of 0.1 and 0.024 × 10⁻⁸ mm³/(N m), respectively, whereas, coefficients values of 0.07 and 0.006 × 10⁻⁸ mm³/(N m) were observed in case of textured Si-DLC coating. This enhancement in tribological performance due to texturing can be attributed to enhanced mechanical properties, microstructural alterations, and phase transformation from diamond to nanocrystalline graphite.

The effect of dimple diameter, density, and depth on the tribological performance was studied by Shum and his team.^{Reference Shum, Zhou and Li109} They found that improvement in tribological performance of DLC coatings can only be achieved with dimples having optimum size and density. They observed a significant reduction in friction and wear coefficients of DLC coating with a dimple diameter of 100 µm, density of 10%, a depth of 10 µm, and a dimple diameter to contact diameter ratio of approximately 0.4. As mentioned earlier, this improvement can be attributed to the beneficial role of dimples in retaining the lubricant and supplying it to the contact in case of starved conditions so that metal-to-metal contact can be avoided. In addition, dimples also remove the wear debris from the interface and trap them inside to refrain sharp-edged hard metal particles from deteriorating the interacting surfaces.

In another study, Masuko et al.^{Reference Masuko, Kudo and Suzuki110} studied the effect of surface roughened substrates on the delamination strength and tribological performance of a-C:H coating using air, water, and n-decane as lubricants. Roughened substrates were made by wet blast surface finish method in combination with slurries of different hard particles such as alumina (6, 25, and 40 µm), glass (2–20 µm), and zirconia (63 µm) in angular and spherical shapes. Because of their amorphous nature, DLC coatings usually mimic the surface roughness of the substrate.^{Reference Bubenzer, Dischler and Nyaiesh13} A considerable improvement in the delamination strength and tribological properties of a-C:H coating was experienced using roughened substrates especially when spherical zirconia and glass particles were used in slurries. It was also observed that slight polishing of the substrate material before deposition of DLC coating can significantly improve adhesion strength and composite wear coefficient of a contact due to the removal of hard projections.

In another study, Abdullah Tasdemir et al.^{Reference Abdullah Tasdemir, Wakayama, Tokoroyama, Kousaka, Umehara, Mabuchi and Higuchi78} used germanium and AISI 52100 chrome steel discs sliding against ta-C coated pins to see the influence of counterbody materials on the friction and wear characteristics of DLC coatings. The tests were conducted at different temperatures using PAO as a lubricant base oil. In case of ta-C/germanium tribopair, low values of friction and wear coefficients were observed irrespective of test temperature. Contrary to that, aggravated wear of ta-C coating was observed at elevated temperatures (80 and 100 °C) due to delamination, which also resulted in higher friction coefficient, when steel disc was used as counterbody. Lower values of wear coefficients achieved with germanium can be attributed to extremely low solubility of carbon in germanium compared with steel.^{Reference Olesinski and Abbaschian111} In addition to that, only polishing wear was observed in case of germanium, whereas, thermally activated tribochemical wear was the predominant cause of delamination in ta-C/steel tribopair.

Ouyang and his team^{Reference Ouyang, Sasaki, Murakami, Zhou and Zhang30} used steel and alumina as counterbody materials to see the effect of counterbody material on the tribological performance of Ti-DLC coatings under dry conditions. DLC/alumina showed better friction and wear performance as compared to DLC/steel due to difference in hardness values of two counterbody materials under identical tribotest conditions. In a similar study, Ouyang and Sasaki^{Reference Ouyang and Sasaki79} compared the tribological characteristics of Ti-DLC using AISI 52100 chrome steel balls and CrN-coated pins as counterbodies. They found a significant dependence of friction and wear performance of DLC coatings on counterbody material irrespective of the test conditions. DLC/CrN tribopair showed better friction performance, whereas, DLC/steel couple proved to be more wear-resistant.