Influence of nitrogen doping on the thermal stability of hydrogenated amorphous diamond coating
Introduction
Because of their amorphous structure mixing sp2 and sp3 hybrid carbon atoms, diamond-like coatings (DLCs) present excellent physical and chemical properties and offer potential as advanced materials applied extensively in mechanical engineering, biomedicine, and aeronautical or aerospace fields [1,2]. However, the intrinsic high residual stress, weak adhesion strength, and nonideal thermal stability of DLCs are their key drawbacks in practical applications. The thermal stability of DLCs is mainly related to graphitization process at elevated temperature, in which sp3 C transforms to sp2 C due to bond breakage and atomic rearrangement [3], [4], [5], [6], [7]]. Graphitization not only affects the practical performance of DLCs [8,9], but also changes their size stability induced by volume expansion of the coating [10].
Nitrogen doping in DLCs is beneficial for leading to release of intrinsic residual stress, improving adhesion strength and wear resistance, and the surface modification of coatings [11], [12], [13], [14], [15], although it degrades the surface hardness when the nitrogen content is higher [16,17]. There are much argument about the effect of nitrogen doping on the thermal stability of DLCs. Carbon atoms preferentially bond with N atom to depress CH bonds in the coating [18], even form extremely strong CN valence bonds to improve the thermal stability of DLCs [19], [20], [21]. On the other hand, the doping nitrogen in DLCs can enhance sp2 C fraction, even promote the transformation of C = N to C = C bonds during annealing to intensify the graphite clustering and graphitization [22,23]. Thus, the so called “better thermal stability of DLCs containing nitrogen” is probably related to its high sp2 fraction in the as-deposited coatings.
Whether the DLC is hydrogenated or not, a few reports concern that sp3 fraction of as-deposited coating is reinforced by the doping N under low nitrogen content [24]. It is not surprising because the fraction of sp3 and sp2 of CC bonds and CN bonds is controlled by fabrication processing, especially the bonding environment and inputting ion energy [25,26]. But the mechanism for this sp3C “abnormally” rising with nitrogen addition is unknown up to now, and there is little study on the thermal stability of this kind of N-doped DLCs. Recently we also find that nitrogen doping can enhance the sp3 fraction under some specific processing condition. Thus, it is necessary to investigate the mechanism behind these test results.
In this study, the nitrogen content of a-C:H:N coatings was limited to the 0–3 at% range by controlling the nitrogen gas flow, and their structure, nanohardness, and residual stress evolution during annealing were investigated. The results presented should provide some ideas on fabricating a-C:H:N coatings with combined excellent properties of high hardness, low intrinsic residual stress, and high thermal stability.
Section snippets
Experimental procedure
The a-C:H:N coatings were deposited onto 316 L stainless steel and silicon wafer substrate coupons by means of a Hauzer Flexicoat 850 PECVD system. First, the polished specimen were cleaned in a ultrasonic ethanol bath and dried by nitrogen gas flow. Then the specimen were introduced into the vacuum chamber, which was then pumped to <5 × 10−3 Pa. The specimens were then argon ion etched by a hot filament ion source for 90 min, during which the current was 100 A, the Ar gas (99.999% pure) flow
Raman spectra characterization
The Raman spectra of DLCs typically exhibit two feature peaks, i.e., at wavenumbers of 1580 and 1350 cm−1 for the G and D peaks, respectively, which include much information about hybridization bond fraction, carbon clustering, disorder, and so on. The G peak comes from the stretching vibration of C sp2 of either the ring or chain, and the D peak reflects vibration induced by structure disorder, and it is mainly related to the aromatic ring in DLCs [[1], [2], [3], [4], [5],29,30]. Under certain
Conclusions
The a-C:H:N coatings with high sp3 fraction and 0–3 at.% nitrogen content were prepared by PECVD, in which the total sp3 includes Nsp3 and Csp3. In this study, the doping N enhances sp3 fraction till around 0.12 at.% N in the as-deposited coating. When more nitrogen is added, extra nitrogen dominantly forms C = N bonds in the coating. Thus a-C:H:N coatings with low nitrogen constitute a spacious structure with dense and distorted sp3 network frame, dense and small graphite clusters pinned by C
CRediT authorship contribution statement
Jihua Peng: Supervision, Methodology, Supervision, Conceptualization, Methodology, Writing - review & editing. Manzhong Yang: Investigation, Data curation, Writing - original draft, Writing - original draft, Data curation, Investigation, Validation. Jiwei Zeng: . Dongyi Su: . Jingwen Liao: Investigation. Man-lung Yick: .
Declaration of Competing Interest
None.
Acknowledgments
This work was supported by the Science and Technology Program of Guangzhou City (Grant Nos. 201902010018, 201807010091, and 2017010160670) and the Science and Technology Program of Guangdong Province (Grant No. 2015B090923006).
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