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

Abstract

Three types of hydrogenated amorphous carbon (a-C:H) coatings were synthesized on stainless steel substrates by a Plasma Assisted CVD process, containing hydrogen contents in the range from 25 to 29 at.%. The effect of annealing up to 600 ºC in two different environments on both the structure and the mechanical properties of the coatings was investigated by means of Differential Scanning Calorimetry/Thermogravimetry (DCS/TG), Raman Spectroscopy and Depth Sensing Indentation. The results indicate that the structural modifications occurred in the coatings in both protective and oxidative atmospheres up to 400ºC were due to a complex atomic rearrangement involving the dehydrogenation reaction. A small weight loss, detected by isothermal TG analysis confirmed the H 2 effusion. This dense effect proceeds without a change of hardness which was maintained in the diamond-like regime. The annealing in non-oxidative ambiance at temperatures above 500ºC causes both gaseous products effusion and sp 3 to sp 2 transformation. Raman parameters and hardness values were, under these conditions, similar to those known for a typical graphite-like regime. While the onset temperature of the graphitization process was found to be almost independent of the H content range investigated, the situation was completely different inrelation to the oxidation reaction. The highest oxidation resistance was found for coatings with the lowest H content.

Annealing Effects on Structure and Optical Properties of Diamond-Like Carbon Films Containing Silver

“…According to [ 9 ], it means decreased sp 3 /sp 2 carbon bond ratio (graphitization of the amorphous carbon matrix). This fact is in good accordance with the data reported by other studies on annealing of undoped DLC films [ 23 , 24 ]. In [ 23 ], annealing of undoped hydrogenated DLC films in oxygen-containing ambient resulted in mass loss, decrease of thickness, and sp 3 /sp 2 carbon bond ratio decrease of the hydrogenated DLC film.…”

Origin of temperature-induced low friction of sputtered Si-containing amorphous carbon coatings

“…graphitization) [17][18][19]. This explains the decreased performance of carbon films at elevated temperatures, where oxidation and decomposition make their applications difficult [15,20,21]. To enhance the thermal stability of DLC materials, silicon was reported to be a promising candidate, because it promotes stabilization of the disordered carbon structure by forming strong, thermally stable SiAC sp 3 interlinks [22].…”

Hydrogen stability in hydrogenated amorphous carbon films with polymer-like and diamond-like structure

“…For this purpose, many different characterization techniques such as Raman spectroscopy, infrared absorption, differential scanning calorimetry/thermogravimetry, x-ray photoelectron spectroscopy, Auger electron spectroscopy, high resolution transmission electron microscopy, elastic recoil detection analysis (ERDA), and thermal desorption have been employed. 11,[15][16][17][18][19] Walters et al 19 performed a neutron diffraction and infrared absorption study on the structure of a-C:H films ($35 at. % H) and they evidenced a significant structural reorganization up to about 300 C. Overall, the initial roomtemperature amorphous network (a mixture of single bonds and olefinic double bonds) became progressively aromatic, then graphitic as hydrogen is desorbed up to 1000 C. 19 Recently, Peter et al 11 performed a comparative analysis of the thermal induced gas evolution from a-C:H films deposited from different hydrocarbons using thermal desorption spectroscopy.…”