Amorphous Carbon Nitride Coatings Research Articles

Stable and super‐low friction of amorphous carbon nitride coatings in nitrogen gas by using two‐step ball‐on‐disk friction test

AbstractEffect of running‐in process on friction behaviour of carbon nitride (CNx) coating in N2 gas stream was investigated with a newly introduced two‐step ball‐on‐disk friction test, where the rubbed Si3N4 ball in the pre‐sliding (step 1) was replaced by a new CNx‐coated Si3N4 ball in the subsequent sliding stage under N2 gas (step 2). The two‐step friction test is clarified to be a simple but effective technique for obtaining contact material combination of self‐mated CNx coatings and for achieving stable and low frictions of CNx coatings. Friction coefficients of CNx/CNx in N2 gas stream decrease greatly from 0.07 without pre‐sliding to less than 0.025 in two‐step friction tests. The minimum friction coefficient of 0.004 was obtained by introducing 500 cycles of pre‐sliding in ambient air. These stable and low frictions are attributed to the generation of self‐mated CNx coatings and the formation of a lubricious layer on the disk surface. Copyright © 2014 John Wiley & Sons, Ltd.

Carbon tribo-layer for super-low friction of amorphous carbon nitride coatings in inert gas environments

Super-low friction of amorphous carbon nitride (CNx) coating in inert gas environment has generally been attributed to the formation of a uniform and homogeneous carbon-rich transfer film (denoted as tribo-film) on the mating ball surface in the ball-on-disk tribo-system. However, the role of tribo-film in terms of its appearance (visibility and amount), composition and structure in the low friction mechanisms of CNx coatings is still not clearly understood. In this study, the key parameters of tribo-film for achieving low friction coefficients of CNx coatings in inert gas environments are systematically investigated by classifying the wear scars on the ball surfaces. The wear scars are classified into three modes according to the amount of tribo-film, such as no, partial, and almost full, accumulated inside of the contact area on the ball surface. As a result, it is confirmed that the amount as well as composition and structure of tribo-film on the wear scar of the ball surface can control the friction behavior of CNx coatings. Most importantly, a thin amorphous carbon layer (denoted as carbon tribo-layer) with a thickness of approximately 10nm has been found as sub-surface of the wear scar on the ball surface where tribo-film is not observed using naked eye as well as confocal microscopy. It is strongly argued that the formation of this carbon tribo-layer on the wear scar of the ball surface is enough for achieving stable and low friction coefficients (less than 0.05) of CNx coatings in inert gas environments. On the other hand, the tribo-film is not necessary and should be avoided from both viewpoints of low and stable friction.

Preparation and tribological characterization of amorphous carbon nitride coatings in a RF PECVD–DC PVD hybrid coating process

Amorphous carbon nitride (CNx) coatings were prepared on Si (100) substrates using a unique hybrid coating process with the combination of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and DC magnetron co-sputtering of graphite target at different N2/Ar flow ratios. The structural properties (i.e. deposition rate, composition and bonding structure, residual stress, hardness, and surface morphology) of those CNx coatings were greatly affected by the N2/Ar flow ratio. However, the friction behaviors of the CNx coatings in air and N2 gas stream showed less dependency on the N2/Ar flow ratio. The friction coefficients of CNx coatings sliding against AISI 52100 pins in air and N2 gas stream were in the ranges of 0.17–0.19 and 0.33–0.42, respectively. The analyses of the worn surfaces from optical microscopy and Raman spectroscopy suggested that the friction behaviors of the CNx coatings are mainly controlled by the tribo-films on the pin surfaces other than the composition and bonding structure of the coatings. Specifically, the moderate friction coefficients (0.17–0.19) in air are attributed to the formation of a polymer-like carbon tribo-film, whereas the high friction coefficients (0.33–0.42) in N2 gas stream are due to the directly sliding between CNx coating and steel pin surface.

Influence of normal load and sliding speed on the tribological property of amorphous carbon nitride coatings sliding against Si 3N 4 balls in water

Amorphous carbon nitride (a–CN x ) coatings were deposited on Si 3N 4 disks by an ion beam assisted deposition system. The composition, structure and hardness of the a–CN x coatings were characterized by Auger electronic spectroscopy, Raman spectroscopy and nano-indentation tester, respectively. The influences of normal load and sliding speed on the friction coefficients and the specific wear rates for the a–CN x/Si 3N 4 tribo-pairs were investigated and analyzed synthetically by ball-on-disk tribometer. The worn surfaces were observed by optical microscope. The results showed that the a–CN x coatings contained 12 at.% nitrogen, and their structure was a mixture of sp 2and sp 3 bonds. The a–CN x coatings’ nanohardness was 29 GPa. The influence of sliding speed on the friction coefficients and the specific wear rate of the CN x coatings was more obvious than that of normal load. The friction coefficients and the specific wear rate of the CN x coatings decreased as the sliding speed increased. At a sliding speed higher than 0.1 m/s, the friction coefficients were less than 0.04. The specific wear rates of the a–CN x coatings were higher than those of Si 3N 4 balls at a sliding speed below 0.1 m/s, while the specific wear rates of the a–CN x coatings and the Si 3N 4 balls all fluctuated around a lower level of 10 − 8 mm 3/Nm as the sliding speed increased beyond 0.2 m/s. To describe the wear behavior of a–CN x coatings sliding against Si 3N 4 balls in water with normal loads of 3–15 N and sliding speeds of 0.05–0.5 m/s, the wear-mechanism map for the a–CN x /Si 3N 4 tribo-pairs in water was developed.

Friction and wear property of a-CN x coatings sliding against Si 3N 4 balls in water

Amorphous carbon nitride coatings (a-CN x ) were deposited on Si(1 0 0) wafers and Si 3N 4 disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CN x coatings’ hardness was measured by nano-indentation and the friction and wear behavior of a-CN x coating sliding against a Si 3N 4 ball in water was investigated. The results indicated that the a-CN x coatings contained 12 at.% nitrogen and the major chemical bonding was sp 2 C N and sp 3 C–N. The nano-hardness of the a-CN x coatings was 29 GPa. At a sliding velocity of 0.16 m/s and after running-in, the mean steady-state friction coefficient varied around 0.02 when the normal load was lower than 3.5 N, and then decreased abruptly from 0.018 to 0.007 at 5 N. For self-mated Si 3N 4, the specific wear rate of a Si 3N 4 ball was a little higher than that of a Si 3N 4 disk, while for a-CN x /Si 3N 4, the specific wear rate of a Si 3N 4 ball was slightly smaller than that of a-CN x coating. Furthermore, the specific wear rate of Si 3N 4 ball sliding against a-CN x coating was reduced by a factor up to 35 in comparison to that against Si 3N 4 in water. This indicated that the wear mechanism of a-CN x coating/Si 3N 4 ball was the formation of a carbonaceous transfer film on the a-CN x coatings via a tribochemical reaction between a-CN x coatings and water induced by friction, while that of self-mated Si 3N 4 ceramics was the formation of silica gel on the contact zone via the reaction of silicon nitride and water.

Friction and wear property of a-CN x coatings sliding against ceramic and steel balls in water

The amorphous carbon nitride coatings (a-CN x ) were deposited on Si 3N 4 disks using ion beam assisted deposition (IBAD), and their composition and chemical bonding were determined by X-ray photoelectron spectroscopy (XPS). The a-CN x coatings' hardness was measured by nano-indentation and the friction and wear property of the a-CN x coatings sliding against Si 3N 4, SiC, Al 2O 3, SUS440C and SUJ2 balls in water were investigated by using ball-on-disk tribo-meter. The worn surfaces were observed using optical microscopy and analyzed by XPS. The results of XPS analysis showed that the a-CN x coatings contained 12 at.% nitrogen and the major chemical bonding was sp 2 C = N and sp 3C–N. The nano-hardness of the a-CN x coatings was 29 GPa, higher than those of balls. Among five kinds of tribo-systems, the lowest friction coefficient was obtained in the range of 0.01 to 0.02 for the tribo-systems with SiC and Si 3N 4 balls, the largest wear rate of the a-CN x coating of 1.77 × 10 − 7 mm 3/Nm was obtained as sliding against Al 2O 3 ball, while the smallest wear rate of a-CN x coating of 1.44 × 10 − 8 mm 3/Nm was gotten as sliding against Si 3N 4 ball. However, SUJ2 ball showed the highest wear rate of 7.0 × 10 − 7 mm 3/Nm, whereas Al 2O 3 ball exhibited the lowest wear rate of ball of 3.55 × 10 − 9 mm 3/Nm. The XPS analysis on the worn surface for the a-CN x coatings displayed that the nitrogen concentration decreased and the sp 2-bonding-rich structure was formed after sliding tests in water.

Wear-mechanism map of amorphous carbon nitride coatings sliding against silicon carbide balls in water

To understand the wear mechanism of amorphous carbon nitride coatings (a-CN x ) sliding against SiC ball in water, the influence of tribological variables such as normal load ( W) and sliding speed ( V) on the friction coefficients ( μ) and the specific wear rates ( w s) was studied. The worn surfaces on disks and balls were observed by optical microscope (OM). The results indicate that the influence of the sliding speeds on the friction coefficients and the specific wear rates was more obvious than that of the normal loads. The friction coefficients and the specific wear rates decreased with an increase in the sliding speed, whereas with an increase in the normal loads, the friction coefficients and the specific wear rates fluctuated slightly. The specific wear rate of the a-CN x coatings was ten times larger than that of the SiC balls. When the sliding velocity was larger than 0.1 m/s, the friction coefficient of the a-CN x /SiC ball tribo-pair in water lubrication was smaller than 0.05 and the specific wear rates of the a-CN x coatings and the SiC balls all were at a lowest level of 10 − 8 and 10 − 9 mm 3/Nm. Furthermore, the specific wear rates of a-CN x coatings and SiC balls increased linearly with friction coefficient ( μ), while decreased linearly with the friction heat per unit time ( μWV). To describe the friction and wear property of the a-CN x coatings sliding against SiC balls in water at the normal loads of 3∼15 N and the sliding speeds of 0.05∼0.5 m/s, the wear-mechanism map for the a-CN x /SiC tribo-pair in water was developed.

Friction and wear properties of CNx/SiC in water lubrication

Amorphous carbon nitride coatings (a-CN x ) were deposited on SiC disk by ion beam assisted deposition (IBAD). The tribological behavior of a-CN x coating sliding against SiC ball in water was investigated and compared with that of SiC/SiC system at room temperature. The influences of testing conditions on friction coefficient and specific wear rate of both kinds of tribopairs were studied. The worn surfaces on disks were observed by scanning electron microscope (SEM). The results indicate that the running-in period of a-CN x /SiC was shorter than that of SiC/SiC system in water. At a sliding velocity of 120 mm/s, the mean steady-state friction coefficients of SiC/SiC (0.096) was higher than that of a-CN x /SiC (0.05), while at 160 mm/s, lower friction coefficient (0.01) was obtained for SiC/SiC in water. With an increment of normal load, the mean steady-state friction coefficients after running-in first decreased, reaching a minimum value, and then increased. For self-mated SiC, the specific wear rate of SiC ball was a little higher than that of SiC disk, while for a-CN x /SiC, the specific wear rate of SiC ball were 10 times smaller than that of a-CN x coating. Furthermore, the specific wear rate of SiC ball sliding against a-CN x coating was reduced by a factor up to 100~1000 in comparison to that against SiC in water. The wear mechanism of SiC/SiC system in water is related to micro-fracture of ceramic and instability of tribochemical reaction layer. Conversely, wear mechanism for a-CN x /SiC is related to formation and transfer of easy-shear friction layer.

Structural, mechanical and tribological behavior of fullerene-like and amorphous carbon nitride coatings

The mechanical and tribological properties of fullerene-like (FL) and amorphous carbon nitride (CN x) coatings synthesized by reactive unbalanced magnetron sputtering were analyzed and compared to carbon coatings deposited without N 2. Elastic recoil detection analysis (ERDA) was used to determine the amount of incorporated nitrogen while X-ray photoelectron spectroscopy (XPS) was used to study the local bonding environment. Nano-indentation revealed a large spread in hardness, elastic modulus and elastic recovery. The tribological performance of the coatings was tested with a reciprocating sliding tribometer that allowed in situ visualization of the sliding contact. Tests were performed with a sapphire hemisphere sliding at 4 mm/s in dry and ambient humidity air. Friction coefficients of the FL–CN x coatings in both humidities were slightly higher than those of diamond-like carbon coatings (DLC); the values were between 0.1 and 0.25, dropping as the bias voltage increased. FL–CN x coatings had higher wear resistance than amorphous CN x, DLC and graphite coatings in sliding contact although the hardnesses of the coatings were comparable. Wear rates of the FL–CN x coatings in ambient air were lower than in dry air and orders of magnitude lower than the other carbon coatings. Furthermore, the most wear-resistant high-temperature FL–CN x coating had the highest hardness, but the shortest wear life of the three high-temperature FL–CN x coatings. In situ visualization indicated films transferred from the coating to the sapphire hemisphere; the transfer film isolated the hemisphere from the coating, thereby controlling both the friction coefficient and the wear behavior. Effects of deposition conditions, structure and hardness on wear and friction behavior are discussed.

The effects of texture height and thickness of amorphous carbon nitride coating of a hard disk slider on the friction and wear of the slider against a disk

In order to minimize the stiction force caused by contact of the extremely smooth surfaces of head sliders and disks in hard disk drives, texture is usually applied on the disk surface. For future contact/near-contact recording, the stiction-induced high friction between slider and disk will become a problem. Texture on the slider/disk interface will still be an expected method to reduce friction. Recently, it was suggested to texture the slider surface. A protective coating is usually required on the textured slider surface to reduce wear of the texture. The results showed that texture on the slider surface was effective in reducing the friction between head sliders and disks. On the other hand, the texture and coating on the slider surface increase the spacing between the read/write element and the magnetic layer of the disk. The necessary and effective texture height and coating thickness are still not clear. In the present research, island-type textures with different heights (3–18 mn) were formed on slider surfaces by ion-beam etching. Amorphous carbon nitride (a-CN x ) coatings of different thicknesses (0–50 nm) were coated on the textured slider surfaces as a protective overcoat. The friction and wear properties of these sliders were evaluated by constant-speed drag tests against hard disks coated with diamond-like carbon (DLC). The results show that 2 nm texture on a slider surface is sufficient for low (0.3–0.5) and stable friction of the slider against the disk in a drag test, and coatings thicker than 5 nm show similar wear resistances of the texture on slider surfaces.

Recent advances in the synthesis and properties of amorphous and crystalline carbon nitride†

As a result of the prediction by Liu and Cohen that crystalline β‐C3N4 has mechanical properties similar to diamond, many research groups have attempted to synthesize carbon nitride using various deposition techniques. Most groups succeeded in producing amorphous carbon nitride coatings. These coatings, though softer than diamond, display excellent tribological properties and are now used as protective overcoats for computer hard drive systems. The lack of success to make crystalline coatings suggests that β‐C3N4 is metastable. Using TiN(111) and ZrN(111) as seeding layers (which have similar lattice symmetry and commensurate unit cell dimensions as β‐C3N4(001)), we were able to synthesize fully crystalline carbon nitride composite coatings with hardness in the 50 GPa range, which is in the low end of diamond films.

SYNTHESIS AND MECHANICAL PROPERTIES OF CRYSTALLINE CARBON NITRIDE SUPERHARD COATINGS

Crystalline β- C 3 N 4 was predicted to have mechanical properties similar to those of diamond. Until recently, only amorphous carbon nitride coatings were made in spite of numerous attempts. This lack of success in making crystalline coatings suggests that β- C 3 N 4 is metastable. Using TiN as seeding layers and under proper processing conditions, we were able to synthesize fully crystalline carbon nitride composite coatings with hardness in the 45–55 GPa range, which is at the low end of diamond. Suggestions for further improvements and implementation with other deposition techniques will be made.

Tribological Behavior of Amorphous Carbon Nitride Overcoats for Magnetic Thin-Film Rigid Disks

Abstract Amorphous carbon nitride coatings of thickness of 5 and 30 nm were deposited onto 65 and 95 mm magnetic thin-film rigid disks surfaces using single-cathode and dual-cathode magnetron sputtering systems containing nitrogen-argon plasmas. Under optimum deposition conditions, amorphous carbon nitride coatings can be synthesized on ultrasmooth thin-film disks with no significant pinholes at thickness down to 5 nm, with hardness 22–28 GPa (compared to 7–12 GPa for amorphous carbon), and r.m.s. roughness as low as 0.25 nm. These amorphous carbon nitride coatings were shown to have better contact-start-stop performance and three-to-four times better pin-on-disk contact durability compared with amorphous carbon overcoats under identical testing conditions. Amorphous carbon nitride appears to be a promising candidate overcoat material for replacing amorphous carbon in the next-generation magnetic thin-film rigid disk systems.

Sputtered amorphous carbon nitride films

The recent announcement of the synthesis of C3N4 has increased interest in this unique material. Carbon nitride may have several useful applications as wear and corrosion resistant coatings, electrical insulators, and optical coatings. We have produced amorphous carbon nitride coatings containing up to 40% nitrogen using planar magnetron RF sputtering with and without an ion beam in a nitrogen atmosphere. Both wavelength dispersive x-ray spectrometry (WDX) and x-ray photoelectron spectroscopy (XPS) indicate this composition. Coatings up to 2 μm thick were produced on alumina, silicon, SiO2, and glass substrates using a graphite target. Films with transparency greater than 95% in the visible wavelengths and harder than silicon have been produced. The properties of these films are correlated with composition, fabrication, conditions, and subsequent heat treatments. A scanning tunneling microscope (STM) and transmission electron microscopy (TEM) were used to characterize the morphology of the films. XPS studies confirm the stability of a carbon nitrogen phase up to 600 °C. Compositional variations were determined with secondary ion mass spectrometry (SIMS) depth profiling, and the Raman spectra are compared with those of carbon and carbon nitride films prepared by other methods.

Composition, structure and tribological properties of amorphous carbon nitride coatings

Carbon nitride thin films were synthesized using d.c. unbalanced magnetron sputtering of a high-purity graphite target in a nitrogen-containing plasma onto various substrates held at ambient temperatures. The N/C ratio was found to vary from zero to 0.8 depending on deposition conditions. There was evidence of multiple bonding states for carbon and nitrogen (sp, sp 2 and sp 3), and the bulk of the film was found to be amorphous. Nanoindentation studies showed that under appropriate substrate bias conditions the carbon nitride coating hardness can be enhanced to levels well above that of conventional amorphous hard carbon coatings. CN x coatings replicate the substrate topography, giving rise to surface roughness equal to or better than the original substrates. Lubricated tribotesting showed the relationship between wear performance and deposition conditions. Under pure sliding in air, the CN x coating provided a 30-fold reduction in wear rate for M2 steels (compared with uncoated M2) under our testing conditions.