The submitted manuscript ha_ beeh I authored by a contractor of the U. S.I Governmentunder contract No. W.31-I 109-ENG-38. Accordingly, the U. S.] Government retains a nonexclusive,] royalty-free license to publish or I reproducethe published form of thisl contribution,or allowothers to doso, for[ U. S. Governmentpurposes. I DISCLAIMER This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

This report was prepared as an account of work sponsored by an agency of the United States Government. Neither the United States Government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.

It was shown recently that nanocrystalline diamond films can be grown using fullerenes as precursors in an argon microwave plasma without the addition of hydrogen or oxygen. Extensive fragmentation of C60 in the microwave discharge leads to a copious production of the carbon dimer molecule, C2, as evidenced by intense Swan-band emission. Here we have investigated hydrogen–methane–argon plasmas and found that high argon fractions (≳50%) lead to intense C2 emission, indicating significant production of C2 in the plasma. In situ measurements of the substrate reflectivity were used to determine the growth rate. A correlation between the C2 emission intensity and growth rate was observed. These results prompted us to propose a scheme for diamond film growth on the (100)–(2×1): H reconstructed diamond surface with C2 as the growth species. Each surface carbon atom (bonded twice to carbons in the bulk, once to a surface carbon, creating a ‘‘dimer’’ and then forming a five-membered ring) is terminated with hydrogen. With C2 as the growth species, no hydrogen abstraction reactions are required because its very high energy of adsorption (815 kJ/mol C) allows the C2 molecule to insert directly into the dimer bonds. A second C2 can form across the adjoining trough. The added carbons then dimerize, forming a new (2×1) surface on a new layer with dimer rows orthogonal to the original rows.

Nanocrystalline diamond films have been deposited using a microwave plasma consisting of argon, 2-10% hydrogen and a carbon precursor such as C6o or C&. It was found that it is possible to grow the diamond phase with both carbon precursors, although the hydrogen concentration in the plasma was 1-2 orders of magnitude lower than normally required in the absence of the argon. Auger electron spectroscopy, x-ray diffraction measurements and transmission electron microscopy indicate the films are predominantly composed of diamond. Surface roughness, as determined by atomic force microscopy and scanning electron microscopy indicate the nanocrystalline films grown in low hydrogen content plasmas grow exceptionally smooth (30-50 nm) to thicknesses of 10 pm. The smooth nanocrystalline films result in low friction coefficients (p=0.04-0.06) and low average wear rates as determined by pin-on-disk measurements.

DISCLAIMERThis reportwas preparedas an accountof worksponsoredby an agencyof the United States Government. Neitherthe UnitedStates Government nor any agencythereof,norany of their employees,makesany warranty, expressor implied,or assumesany legal liabilityor responsibility for the accuracy, completeness, or usefulnessof any information, apparatus, product,or processdisclosed,or represents that its use wouldnot infringeprivatelyownedrights.Reference hereinto any specificcommercialproduct,process, or serviceby tradename,trademark, .anufacturer, or otherwisedoes not necessarilyconstituteor imply its endorsement, recom-.,endation,or favoringby the United States Government or any agencythereof.The views and opinionsof authors expressedherein do not necessar'_ L_aL¢or renect those of the United States Governmentor anyagencythereof. ¢',? i-_i_THiBi,iTiOi_Oi: _HhSDOCUMENTIS UNt.IMITI_ ABSTRACT In this study, we investigated the friction and wear performance of ion-beam-deposited diamondlike-carbon (DLC) films (1.5 pm thick) on AISI 440C steel substrates. Furthermore, we ran a series of long-duration wear tests under 5, 10, and 20 N load to assess the load-bearing capacity and durability limits of these films under each load. Tests were performed on a ball-ondisk machine in open air at room temperature =22+1°C, and humidity, =30_+5%.For the test conditions explored, we found that (1) the steady-state friction coefficients of pairs without a DLC film were in the range of 0.7 to 0.9 and the average wear rates of 440C balls (9.55 mm diameter) sliding against uncoated 440C disks were on the order of 10-5mm3/N.m, depending on contact load; (2) DLC films reduced the steady-state friction coefficients of test pairs by factors of 6 to 8, and the wear rates of pins by factors of 500 to 2000; (3) The wear of disks coated with a DLC film was virtually unmeasurable while the wear of uncoated disks was quite substantial, (4) these DLC films were able to endure the range of loads, 5 to 20 N, without any delamination and to last over a million cycles before wearing out. During long-duration wear tests, the friction coefficients were initially on the order of 0.15, but decreased to some low values of 0.05 to 0.07 after sliding for 15 to 25 km, depending on the load, and remained low until wearing out. This low-friction regime was correlated with the formation of a carbon-rich transfer film on the wear scar of 440C balls. Microlaser-Raman spectroscopy and scanning-electron microscopy were used to examine the structure and chemistry of worn surfaces and to elucidate the wear-and friction-reducing mechanisms of the DLC film.