Effect of nitrogen in the properties of diamond-like carbon (DLC) coating on Ti₆Al₄V substrate

Abstract

The great interest in the study of diamond-like carbon films (a-C:H) is justified by its mechanical and tribological properties. However, the high internal stress of the film results in its difficult adhesion to the metallic substrate, which can be solved by nitrogen incorporation in the a-C:H film, allowing a formed film of lower internal stress. The objective of this work is to evaluate the influence of flow (20, 30 and 40sccm), CH 4 /Ar ratio (90/10 and 70/30) and voltage (400, 500, 600 and 700 V) in the a-C:H formation. For the best condition of the film, we studied the effect of nitrogen incorporation in the hardness and wear resistance of the a-C:H(N), modifying the nitrogen percentage in the treatment at 10% to 60% N 2 . The treatments were carried out in the Ti 6 Al 4 V substrate by DC-PECVD for two hours. For good adhesion of the films on the substrate, a silicon interlayer must be produced. The increase in the voltage above 600 V increases the I D /I G and film thickness, causing its delamination, and the gas ratio did not influence the a-C:H characteristics. The a-C:H film deposited with 30 sccm, 90/10 and 500 V was characterized as a-C:H (hard), with properties such as the hardness of 17 GPa, 30% H, 39% sp 3 and I D /I G ratio of 0.58. Since nitrogen reduced the deposition rate, the total gas flow for the production of a-C:H(N) was performed with 40 sccm. The Raman spectra of a-C:H(N) films showed changes in D band intensity and displacement in relation to the nitrogen-free film spectrum, evidencing the incorporation of nitrogen in the film. The XPS analysis showed the linear increase of the nitrogen incorporation in the a-C:H(N) film with the increase of the amount of the N 2 gas in the treatment, which caused, in general, a decrease in the amount of C-C sp 3 bonding, increasing the adhesion of the film in the substrate and not necessarily the low wear resistance of the formed film.According to Robertson [4] the DLC film that presents hydrogen concentrations between 30 and 50% receives the nomenclature of hydrogenated amorphous carbon (a-C:H), and presents lower hardness and internal stress than ta-C, facilitating the adhesion process of the film in the metallic substrates. This occurs due to sp 2 bonds formed in detriment of sp 3 bonds.The DLC films have been used in a variety of applications due to their notable properties, as protective coatings against wear and corrosion, in optical windows (optical filters), data storage hard drives, high performance automotive parts, coatings for biomedical products and electromechanical microdevices [9][10][11][12].However, as reported by Chen et al [13] and confirmed by Cemin et al [11], the high intrinsic compressive stress of the film, the high variation of the elastic coefficient, the difference in thermal expansion and the low chemical affinity between DLC and substrate difficult the adhesion process of the DLC film mainly when deposited on metallic surfaces. Thus, the use of techniques as oxide layer removal and the use of film interlayer previously to ...

Enhancement of sp3 C Fraction in Diamond-like Carbon Coatings by Cryogenic Treatment

“…The raw and fitted peaks of C 1s spectra of different specimens are shown in Figure 3a-f. The standard core level binding energies (BE) of sp 2 C=C, sp 3 C-C, and C-O bonds are 284.4 ± 0.2, 285.2 ± 0.2 and 286.5 ± 0.3 eV, respectively [6,12,19]. During fitting, the fitted BE deviated slightly from the above ideal value probably due to the collective effect of the chemical environment, defect density, together with residual stress [17].…”

“…The variation of hardness with DCT temperature agree with the above change of I D /I G and sp 2 /sp 3 bond ratios. Because specimens with or without DCT kept almost the same residual stress, and the nano-indention depth was less than 1/8 thickness of the DLC coatings, thus, the hardness improvement of EM-140 surely was responsible for sp 3 C fraction enhancement [2,12,20]. One hand, the mechanical and micro-structural change of EM 35 alloy substrate induced by DCT [34,35,37] provided strong support for the coating; another hand, the sp 2 /sp 3 C ratio of the coating affected the interface strength between DLC and the interlayer.…”

“…At present physical and plasmaenhanced chemical vacuum vapor depositing are still the main methods to prepare DLCs in which the energic C + impinges the top surface and implants into the subsurface to create a thermal spike site with high intrinsic stress and high local temperature, thus, stabilize the sp 3 C hybrid [11]. Thus, the sp 3 C fraction can be controlled by varying the depositing parameters, such as bias voltage, working gas, doping species, and even the multiple layer structure [12][13][14][15][16][17]. The sp 2 C/sp 3 C bonding ratio plays a crucial role in determining the performance of DLCs [18][19][20].…”

Study of the Effect of Nitrogen on Corrosion, Wear resistance and Adhesion Properties of DLC Films Deposited on AISI 321H

“…Thereafter, the deposition of a silicon-based layer was carried out, using hexamethyldisiloxane (HMDSO) as a precursor, with 70% HMDSO and 30% Ar at a total gas pressure of 0.06 torr for 15 min. Then, the deposition of the DLC or DLC(N) films was carried out with the parameters based on the study carried out by Almeida et al 19 . The substitution of argon for nitrogen in the gas mixture for the DLC(N) films followed the studies carried out by Franceschini and Freire 25 and Franceschini, Freire and Silva 16 .…”

“…Several works report the effect of the DLC doping on their property modification using different elements [12][13][14][15][16][17][18] , such as nitrogen that added to DLC films can considerably improve its adhesion to metallic substrates 19 , with a great influence on the effectiveness of corrosion protection and wear resistance of the treated material 20,21 . Almeida et al 21 observed that the introduction of intermediate amounts of N 2 (30%) on the DLC film by PECVD reduces the fraction of sp 3 bonds *e-mail: solano.larissa@gmail.com keeping the mechanical characteristics of the DLC film, and improving the wear and adhesion of the film to the metallic substrate.…”