Laser structuring of hydrogenated DLC scaffolds: Raman spectroscopy and nanotribology

The speciality of hydrogenated diamond-like carbon (DLC) named a-C:H (amorphous carbon hydrogen coatings a-C:H according standard VDI 2840) is the inclusion of a certain amount of hydrogen atoms in holes of the sp3 and sp2 hybridized CC network. In addition, hydrogen is bonded to carbon directly. Hydrogen is set free or rearranged during exposure to non-thermal and thermal effects or when subjected to sliding. The first is true for laser irradiation. The used laser pulse is ultra-fast. For laser structuring in the present study laser pulse with a duration of 220 fs was applied. The laser beam had a diameter of 15 μm and possessed a wavelength of 1028 nm. The micro-pattern consisted of an array of dimples with a spatial distance of 60 μm. Graphitization of the hydrogenated DLC occurred due to laser fluences of 0.78 J cm−2, 1.71 J cm−2 or 2.86 J cm−2. As indicated by Raman spectroscopy (532 nm), with increasing fluences, the fraction of six-fold aromatic carbon rings increased, as well as their disorder. ID/IG ratio of the “as deposited” a-C:H was 0.48, in comparison, ID/IG ratio for the fs-laser treated dimples ranged between 1.04 and 1.54. In addition to the D and G peak, the Raman spectra of fs-laser generated dimples produced peaks at around 2600 cm−1 and 2900 cm−1, which could be attributed to the 2D band and the D + G peak of damaged graphene. In the present study, the peak at around 2600 cm−1 has the highest intensity, when dimples were generated with a fluence of 1.71 J cm−2. Coefficient of friction COF was measured with a nanotribometer at 34 % relative humidity and 23 °C. Using an Al2O3 ball as sliding counterpart, a COF of 0.059 ± 0.016 was measured for “as deposited” a-C:H. COF is considerably higher for fs-laser structured a-C:H, but still below 0.1 for fluences of 0.76 J cm−2 and 1.92 J cm−2. It increases up to 0.107 ± 0.02 for the highest fluence of 2.86 J cm−2. A possible reason for this can be, that the damaged graphene is curved and less smooth, which may lead to the “ripple-rug” mechanism during sliding. Precise designed COF due to the free up of graphite-like activated carbon are interesting for usage in biomedical and/or signal transmitting applications.