Thermal stability of diamondlike carbon films

Abstract

IR and UV-VIS optical spectroscopy have been used to study the transformations of the properties of diamondlike carbon films following isothermal anneals from T a ϭ300 to 650°C. Several processes due to the annealing have been observed: ͑1͒ the increase of the absorption due to vibrations of unsaturated bonds of C͑sp 2 ͒ atoms at ϳ1600 cm Ϫ1 for T a у350°C, ͑2͒ the decrease of the absorption due to C͑sp 3 ͒-H bonds at T a у350°C, and ͑3͒ the reduction of the optical energy gap. Analysis of the kinetics has shown that the dehydrogenation of the alloys and the formation of unsaturated bonds may proceed independently. The reduction of the energy gap is related to the formation of C͑sp 2 ͒ atoms with unsaturated bonding which occurs mostly in hydrogen-free regions. Intensive graphitization of the films occurs above T a ϭ650°C. Transformations of C-H bonds are proposed to occur via fast rearrangement in stressed regions leading to formation of new C͑sp 2 ͒-H bonds and formation of methane molecules as the most important product of the anneals inside the polymeric highly hydrogenated regions in the alloys. It appears that both bond removal and reordering have taken place as a result of annealing.

Thermal stability in oxidative and protective environments of a-C:H cap layer on a functional gradient coating

“…Concomitantly to thermal decomposition of a-C:H films into gaseous products, such as H 2 and hydrocarbon C x H y , in particular CH 4 and C 2 H 6 [12][13][14] , the conversion of sp 3 to sp 2 bonds and the consequent loss of diamond-like properties have been observed. Typically for a-C:H coatings, the graphitization onset temperature ranges from 250 to 500 ºC depending on the deposition and growth process [3][4][5][6][7][8][9][10][11] . Therefore, the study of the irreversible changes on the a-C:H coatings structure by annealing is an important issue for practical coatings applications.…”

“…As the temperature is increased, the structure of DLC films, which is claimed to be a metastable form of carbon with mixture of sp 3 /sp 2 bonds [1,2] , collapses into a more sp 2 -bonded network by a sp 3 diamond-like domains conversion. This instability, denominated as graphitization process, is extremely noticeable in the one of the major DLC family, the H-containing form of amorphous carbon (a-C:H) [3][4][5][6][7][8][9][10][11] . It is well known that hydrogen stabilizes the diamond structure by maintaining the sp 3 hybridization configuration.…”

Continuum theory of carbon phases

“…Then, using Eqs. (2), (20), (22), and the data of [4], we obtain that A n ð0 K; 80 GPaÞ ¼ 6DG G=D ð0 K; 80 GPaÞ % 441: kJ=mol. Using a linear approximation for the pressure dependence of the coefficient A n we can estimate @A n =@P % 1:79 cm 3 =mol.…”

“…First, these materials are always produced in the form of thin films, wires, or small particles that is, nanostructures. Second, although theoretical calculations and numerical simulations of such materials produced energies of formation significantly higher than those of graphite and diamond at the same temperatures [13,[21][22][23], these materials possess certain degree of thermodynamic stability. The MD/MC cluster simulation methods are an excellent tool of materials study when equilibrium properties of phases-graphite, diamond, liquid, and amorphous-are considered [5][6][7][8][9][10][11][12][13].…”

Characterization of temperature-induced changes in amorphous hydrogenated carbon thin films

“…The basic observations can be summarized as follows: The optical gap E 04 was found to decrease for both hard and soft a-C:H [26][27][28]. Fourier transform infrared spectroscopy (FTIR) indicated a transformation of sp 3 carbon into sp 2 carbon and a loss of hydrogen for both hard and soft films [5,27,28], in accord with an aromatization (sometimes also referred to as 'graphitization') observed in Raman spectroscopy by an increase of the D peak intensity and a shift of the G peak position to higher wave numbers [6]. The loss of hydrogen was also confirmed by ion beam analysis [6,29].…”