Oxidation behavior of diamond-like carbon films

Oxidation behavior of diamond-like carbon films

Surface and Coatings Technology 120–121 (1999) 138–144 www.elsevier.nl/locate/surfcoat Oxidation behavior of diamond-like carbon films Da-Yung Wang a...

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Surface and Coatings Technology 120–121 (1999) 138–144 www.elsevier.nl/locate/surfcoat

Oxidation behavior of diamond-like carbon films Da-Yung Wang a, *, Chi-Lung Chang a, Wei-Yu Ho b a Institute of Materials Engineering, National Chung-Hsing University, Taichung, Taiwan, ROC b Surftech Corp., Taichung, Taiwan, ROC

Abstract Diamond-like carbon (DLC ) films were synthesized by a combined PVD and PECVD process to produce a multilayered structure consisting of a Ti interlayer, a graded transition layer, and the carbon film. The oxidation behavior of DLC films was investigated using thermogravimetric (TGA) and differential thermal analyses (DTA). The phase identification and microstructural examinations were conducted by XRD, Raman, and SEM/EDS. According to those results, DLC films disintegrated at 350°C, showing typical graphitic transformation and oxidation behavior. The weight loss as a result of oxidation of carbon followed a linear reaction rate of −2.3×10−4 g/min cm2. At 450°C, oxidation of the underlying TiN/TiC N interlayer occurred in addition x y to the oxidation of DLC. Weight gain caused by the formation of TiO was observed. The overall oxidation kinetics of DLC is 2 close to a parabolic behavior with k =5.48×10−5 mg2/cm4 h. Surface cracking of the film resulted from stress relief. The p microhardness of DLC films decreased with increasing annealing temperature due to the graphitization and oxidation of DLC. The transition temperature was confirmed using the results from the Raman analysis. © 1999 Elsevier Science S.A. All rights reserved. Keywords: DLC; DTA; Oxidation; TGA

1. Introduction DLC films possess advanced properties such as a high level of hardness [1], low friction coefficient (<0.15 in air) [2,3], high wear resistance, and chemical inertness. The amorphous DLC hard coatings, either hydrogenated (a-C:H ) or non- hydrogenated (a-C ), can be synthesized using ECR-CVD [4,5], d.c.- and r.f. PECVD [6–9], laser ablation [10,11], or magnetron sputtering [12–14]. DLC coatings suffer from environmental sensitivity and insufficient adhesion to steel substrates as a result of Fe–C interdiffusion. Various studies have attempted to enhance the film adhesion between a DLC and its substrate by applying an interlayer such as B or Ti to prevent the interdiffusion of carbon and cobalt at elevated temperatures [15–17]. Other developments have attempted to enhance the mechanical properties, by doping TiN, TiCN, and TiC layers in the DLC matrix [18]. Monaghan et al. designed a DLC film that was graded in both composition and microstructure [19], with significant improvements in film adhesion. The hydrogenated DLC film in this study was synthesized * Corresponding author. Tel.: +886-4-3381042; fax: +886-4-3367010. E-mail address: [email protected] (D.-Y. Wang)

by using unbalanced magnetron sputtering, in which hydrocarbon sources such as C H were decomposed 2 2 with the assistance of the ion flux of Ti. The use of pulsed plasma technology eliminated the substrate heating and target-arcing problems. In the UBM-based process, an interface layer of Ti and a transition layer of TiC N were incorporated to provide a supportive x y foundation for DLC and to solve the adhesion problem between DLC and steel substrates. In this study, the oxidation behavior of the compound DLC superhard coating was examined by thermal analyses. The oxidation kinetics and reaction mechanism were investigated as well.

2. Experimental DLC films were synthesized using an unbalanced magnetron sputtering system ( UDP-900) made by Teer Coating Ltd. Four targets of 99.98% pure Ti were used for both PVD deposition of the interface layer and PECVD deposition of the final DLC films. DLC coatings were deposited on M2 tool steel, Si wafer, and SS304 strips of dimensions 50×10×0.025 mm3, which was designed to fit into TGA/DTA crucibles in spiral

0257-8972/99/$ – see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S0 2 5 7- 8 9 7 2 ( 9 9 ) 0 0 35 0 - 3

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Fig. 2. TEM/SAD diffraction pattern of DLC.

Fig. 1. TEM cross-sectional micrograph showing the multi-layered structure of a DLC compound coating.

forms. Target poisoning was resolved using a d.c. power supply linked to a 2 kHz medium-frequency pulse generator. A pulsed d.c. power supply, with a variable frequency of 20–100 kHz, was applied to the substrates to control the substrate arcing and radical excitation during the final DLC formation. Before deposition, the chamber was evacuated to 1×10−3 Pa. The target-to-substrate distance was 13 cm. The substrate surfaces were sputtercleaned and heated to 250°C before deposition. During deposition, the argon pressure was 0.24 Pa with a pulsed biasing voltage of −70 V. The reactive gases of N and 2 C H were controlled by a closed-loop optical emission 2 2

monitor (OEM ). The as-coated DLC coatings were then subjected to thermal analysis in both a Cahn-2000 thermogravimetric ( TGA) system and an Ulvac TGD-7000HR combined TGA/DTA system. The oxidation of DLC films was conducted between room temperature and 800°C at a heating rate of 15°C/min in the atmosphere. The identification of DLC was performed by Raman spectroscopy. The morphologies of the oxidized DLC films were examined by cross-section TEM, selected area electron diffraction (SAD) and SEM/EDS.

3. Results and discussion 3.1. Deposition of diamond-like carbon films High-quality diamond-like carbon films were deposited on M2 tool steel. DLC was synthesized as a consequence of a PEVCD process within the UBM

Fig. 3. TGA/DTA analyses of DLC films.

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Fig. 4. XRD spectrum of DLC coating annealed at 400°C.

Fig. 5. TGA curves of isothermal oxidation of the DLC films at 600°C for 4 h.

sputtering system, where acetylene decomposition occurred with the assistance of UBM plasma. Atomic hydrogen helped in the formation of sp3 diamond bonds. The microhardness of DLC deposited in this study is 3618 HV . Metallic titanium and TiC N were (25g) x y co-deposited with DLC and were embedded in the DLC matrix, resulting in an improved film ductility. Excellent film adhesion was observed between the DLC coating and the steel substrate. A compound interface consisting of a Ti interlayer and a graded TiC N transition layer x y was deposited sequentially before the final deposition of DLC. The interdiffusion between the DLC and the steel

substrates was effectively prevented. The multilayered and graded film structures improved the severe mismatch in lattice and physical properties between DLC and M2. A medium-frequency magnetron drive and biasing power supplies were used to suppress target and substrate arcing interference and damage. Details of the microstructure of DLC compound coatings were revealed by cross-sectional TEM (Fig. 1), in which the deposition sequence of the Ti, TiN, TiC N , and DLC x y layers can be clearly observed. The interfaces between each layer are free from any noticeable voids, gaps, or other defects. The selected area diffraction (SAD)

D.-Y. Wang et al. / Surface and Coatings Technology 120–121 (1999) 138–144

pattern revealed the existence of diamond, graphite, and TiC microcrystallines within DLC, as shown in Fig. 2. 3.2. Thermal analyses of DLC DLC films were examined by TGA and DTA. The compound DLC film was deposited on thin strips of SS304 stainless steel to increase the detection sensitivity of the oxidation reactions. As shown in Fig. 3, DTA and TGA tests of DLC were conducted simultaneously at a heating rate of 15°C/min between room temperature and 800°C. According to those results, oxidation occurred at 350 and 450°C. The DTA curve revealed a typical endothermic reaction in this temperature range. The possible reaction mechanism includes the following reactions: C+O CO (1) 2 2 Ti+O TiO (2) 2 2 TiC+2O TiO +CO . (3) 2 2 2 Below 350°C, crystallization and graphitization of the amorphous DLC films occurred, as depicted in the DTA curve. Between 350 and 450°C, a significant weight loss resulted from the predominant oxidation of carbon, according to reaction (1). The reaction products CO 2 escaped with the vent gas out of the crucible. Oxidation of the embedded Ti and TiC N according to reactions x y (2) and (3) was also anticipated. The XRD analysis of the DLC films annealed at 400°C showed TiO peaks, 2 as shown in Fig. 4. At temperatures above 450°C, the overall reaction mechanism turned to the oxidation of

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the TiN/TiC N interlayer. Weight gains caused by the x y formation of TiO are clearly demonstrated in Fig. 3. 2 Fig. 5 depicts the isothermal TGA analysis of DLC films at 600°C for 4 h. Two distinctive stages of oxidation were observed. The first stage lasted about 12 min. In stage I, DLC films were quickly oxidized, according to reaction (1). The linear reaction mechanism was due to the interface-controlled evaporation behavior [12–14]. The oxidation rate constant was –2.3×10−4 g/min cm2 at 600°C. Most of the carbon contents of the compound DLC films were exhausted following stage I. In stage II, selective oxidation of the exposed TiN and TiC N x y occurred according to reactions (2) and (3). After 12 min of oxidation at 600°C, the oxidation mechanism changed to a protective growth of TiO scales. The 2 reaction followed a diffusion-controlled behavior [15]. As shown in Fig. 6, the parabolic oxidation rate constant was 5.48×10−5 mg2/cm4 h at 600°C. The degree of parabolic fit of the TGA curve was affected by reaction (1) and the thermal grooves of the oxidized DLC film. Fig. 7 presents SEM micrographs of the isothermally oxidized DLC films following 10, 20, and 240 min of exposure at 600°C. After 10 min (stage I oxidation), thermal grooves formed on DLC films as a result of graphitization and stress relief of the highly stressed DLC film, as shown in Fig. 7(a). The surfaces of DLC films were cleaned of any oxidation products. After 20 min, during the onset of stage II oxidation, evidence of scattered oxidation of Ti and TiC N was demonx y strated by precipitation of TiO on film surfaces. 2 Fig. 7(b) depicts the surface morphology of DLC films as well as the TiO precipitates. EDS analysis revealed 2

Fig. 6. Oxidation kinetics of DLC oxidized at 600°C.

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(a) 10 min.

(b) 20 min.

(c) 240 min. Fig. 7. Scanning electron micrographs of DLC films oxidized at 600°C.

that the oxide precipitates were TiO , as shown in Fig. 8. 2 After 240 min of exposure [Fig. 7(c)], the TiO scale 2 thickened and continued to grow. 3.3. Structural stability of DLC at high temperatures DLC coatings were oxidized at various temperatures to examine their structural stability. Graphitization and oxidation affect the structure of DLC. Fig. 9 depicts the change in hardness as a function of annealing temperatures. The microhardness of DLC dropped at 300°C and leveled with the microhardness of TiCN and TiN

at 400°C. This finding was in accordance with the twostage oxidation of DLC as revealed by the TGA study. DLC is a metastable form and can only exist up to a certain temperature level. The loss of DLC characteristics due to oxidation was examined by Raman spectroscopy, as shown in Fig. 10. Accordingly, significant graphitization and oxidation of DLC were confirmed by a decrease in the I /I ratio (intensity ratio of DD G and G-band of DLC ) occurring at temperatures above 300°C. At a higher temperature, the microhardness of DLC films resembled that of the underlying TiC N /TiN interlayer films. x y

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Fig. 8. EDS analysis of precipitates of DLC annealed at 600°C.

Fig. 9. Correlation between hardness and annealing temperature of DLC.

4. Conclusions In this study, a compound DLC film with a superior adhesion strength was deposited. The oxidation behaviors were analyzed by TGA and DTA thermal analyses. The results are summarized as follows: 1. DLC films consisting of a Ti/TiC N interlayer and x y a metal-doped amorphous carbon film were successfully deposited on M2 steels by unbalanced magnetron sputtering. The film adhesion was improved by applying medium-frequency pulsed power supplies. 2. The TGA/DTA analysis of DLC films revealed two distinctive oxidation temperatures at 350 and 450°C.

The stage I oxidation comprised the evaporation of oxidized carbon, where carbon was oxidized following a linear reaction rate of −2.3×10−4 g/ min cm2. At 450°C, oxidation of the underlying TiN/TiC N interlayer occurred in addition to the x y oxidation of DLC. Weight gain as a result of the formation of TiO appeared during stage II oxida2 tion, where the overall oxidation kinetics was close to a parabolic behavior with k =5.48× p 10−5 mg2/cm4 h. Surface cracking resulted from stress relief. 3. The microhardness of DLC films decreased with increasing annealing temperatures due to graphitiza-

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Fig. 10. Raman spectra of DLC films oxidized at different temperatures.

tion and oxidation. The hardness transition temperature was in accordance with the phase transition of DLC subjected to high-temperature annealing.

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