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[en] As an element, carbon is rather unique and offers a range of rare opportunities for the design and fabrication of zero-, one-, two-, and three-dimensional nanostructured novel materials and coatings such as fullerenes, nanotubes, thin films, and free-standing nano-to-macroscale structures. Among these, carbon-based two-dimensional thin films (such as diamond and diamond-like carbon (DLC)) have attracted an overwhelming interest in recent years, mainly because of their exceptional physical, chemical, mechanical, electrical, and tribological properties. In particular, certain DLC films were found to provide extremely low friction and wear coefficients to sliding metallic and ceramic surfaces. Since the early 1990s, carbon has been used at Argonne National Laboratory to synthesize a class of novel DLC films that now provide friction and wear coefficients as low as 0.001 and 10^-11-10^-10 mm³ N^-1 m^-1, respectively, when tested in inert or vacuum test environments. Over the years, we have optimized these films and applied them successfully to all kinds of metallic and ceramic substrates and evaluated their friction and wear properties under a wide range of sliding conditions. In this paper, we will provide details of our recent work on the deposition, characterization, and tribological applications of near-frictionless carbon films on glass and ceramic substrates. We will also provide chemical and structural information about these films and describe the fundamental tribological mechanisms that control their unusual friction and wear behaviour

[en] In this study, we introduce a new diamondlike carbon (DLC) film providing a friction coefficient of 0.001 and wear rates of 10^-9-10^-10 mm³/N m in inert-gas environments (e.g., dry nitrogen and argon). The film was grown on steel and sapphire substrates in a plasma enhanced chemical vapor deposition system that uses a hydrogen-rich plasma. Employing a combination of surface and structure analytical techniques, we explored the structural chemistry of the resultant DLC films and correlated these findings with the friction and wear mechanisms of the films. The results of tribological tests under a 10 N load (creating initial peak Hertz pressures of 1 and 2.2 GPa on steel and sapphire test pairs, respectively) and at 0.2 to 0.5 m/s sliding velocities indicated that a close correlation exists between the friction and wear coefficients of DLC films and the source gas chemistry. Specifically, films grown in source gases with higher hydrogen-to-carbon ratios had the lowest friction coefficients and the highest wear resistance. The lowest friction coefficient (0.001) was achieved with a film on sapphire substrates produced in a gas discharge plasma consisting of 25% methane and 75% hydrogen. (c) 2000 American Vacuum Society

[en] In this paper, we present the results of a systematic study directed toward submicrometric scale triboactivity of a range of hydrogenated diamond-like-carbon (H : DLC) films derived from source gases with different hydrogen-to-carbon ratios. The H : DLC films were deposited on Si substrates in a plasma enhanced chemical vapour deposition system. Specifically, we produced three kinds of H : DLC films, using pure acetylene, pure methane and 25% methane + 75% hydrogen as the precursor source gases. Samples were subjected to wettability and depth sensing ultramicroindentation tests, and micro-to-nanoscale friction and wear studies using a nanotribometer and an atomic force microscope. The results of our study revealed a very close correlation between the wettability and the tribo-mechanical response of the H : DLC films at micro-to-nanoscales and their hydrogen-to-carbon ratio, i.e. lower hydrogen-to-carbon ratio leads to higher hardness (H) and lower water contact angles. Moreover, our results indicated a strong correlation between the hardness of the films and the threshold for severe wear damage. This threshold can be expressed by the ratio between the average Hertzian contact stress and the hardness which, in this study, is close to unity.

[en] Hydrogen-free diamond-like carbon (DLC) films (both amorphous (a-C) and tetrahedral amorphous carbon (ta-C)) suffer high friction and severe wear losses when tested in inert and/or high vacuum environments. However, they provide anomalous superlow friction and wear coefficients in the presence of hydrogen gas, water vapour and alcohol molecules in the test environment. In this paper, we used such films in a systematic study to further confirm that hydrogen indeed plays an important role in their friction and wear behaviours. To study the effect of hydrogen, we conducted sliding tests in a hydrogen-containing test chamber and analysed the chemistry of their sliding contact surfaces using a time-of-flight secondary ion mass spectrometer. Clearly, the sliding contact regions of the carbon films became very rich in hydrogen after tribological tests in the hydrogen-containing chamber. In an attempt to understand the fundamental tribochemical mechanisms involved, we performed additional tests on these DLC films using a highly instrumented tribometer that permitted us the visualization of triboplasmas generating at or in the vicinity of the sliding surfaces. In this test system, we confirmed the formation of a triboplasma inside the contact area of the DLC films as evidenced by the characteristic UV radiation. Based on these observations, we believe that the formation of such triboplasmas within the contact zones of these DLC films may have triggered unique tribochemical reactions between hydrogen and carbon atoms on their sliding surfaces and thus resulted in very low friction and wear during tests in hydrogen-containing environments.

[en] New results about the wear resistance of CoB-Co₂B coatings under dry sliding conditions were estimated in this work. The cobalt boride coatings were developed at the surface of the ASTM F1537 alloy by means of the powder-pack boriding process using two experimental conditions: 1223 K with 6 h of exposure and 1273 K with 10 h of exposure. Before the sliding wear tests, Vickers depth-sensing microindentation tests were conducted on the cross section of the cobalt boride coatings to estimate the distribution of hardness, Young’s modulus, and residual stresses. Otherwise, the sliding wear tests were performed on both boriding conditions and on the untreated material, using a ball-on-flat configuration comprised of an alumina ball as a counterpart with applied loads between 5 and 20 N. The wear rates of the borided ASTM F1537 alloy were ranged between 4.02 and 8.91 × 10⁻⁶ mm³ N⁻¹ m⁻¹ compared with the values of the untreated material (13.90 and 15.78 × 10⁻⁶ mm³ N⁻¹ m⁻¹) for the overall set of experimental conditions; nevertheless, the influence of boriding conditions (1273 K with 10 h of exposure) tended to increase the CoB coating thickness, developing a more brittle layer that decreased the sliding wear resistance at the surface of the borided ASTM F1537 alloy. Finally, the presence of failure mechanisms on the surface of the wear tracks was analyzed for both borided ASTM F1537 alloy and untreated material.