Thermal stability of a-C:H:SiOx thin films in hydrogen atmosphere
Introduction
In recent years, hydrogen has been expected to be used as an alternative to fossil fuels, and many hydrogen storage and transportation systems that use high-pressure hydrogen are being developed. However, it is well known that hydrogen has a negative impact on the mechanical properties of many metallic materials such as carbon steel, stainless steel, and aluminum alloys. Stainless steels with especially high mechanical strength have high hydrogen diffusivity, and tend to show hydrogen embrittlement [1]. Therefore the development of a technology for preventing degradation of materials by hydrogen is required.
Barrier coatings for preventing hydrogen from diffusing into the metals are studied by many researchers. For example, different ceramic coatings such as Al2O3, BN, TiC, and TiN have been reported as effective hydrogen barriers [[2], [3], [4], [5]]. They have a thickness of several micrometers and can decrease hydrogen permeability by several orders of magnitude. There are examples of application of diamond-like carbon (DLC) coatings as a H2 gas barrier [6,7]. It was shown that after deposition of a DLC coating with a thickness of about 3 μm on stainless steel sample, the hydrogen permeation rate was reduced to 1/1000 compared to that without a coating [7].
Diamond-like carbon films, representing a variety of amorphous hydrogenated or nonhydrogenated forms of carbon, may have unique properties such as high hardness and elastic modulus, low friction coefficient, good wear resistance and corrosion resistance depending on the ratio of sp2 and sp3 hybridized carbon [[8], [9], [10]]. Nevertheless, sometimes the use of DLC coatings is restricted due to high compressive residual stress (up to 13 GPa) [11], poor coating adhesion and poor thermal stability with graphitization starting at ~300–400 °C [12,13]. In general, the DLC film suffers from low thermal stability. The sp3 bond converted to sp2 at relatively lower temperature (300 °C) [14]. The graphitization process of DLC films reveals itself in an increase in the ratio of the D and G peaks of the Raman spectra [15]. But, this type of bond conversion could be prevented by incorporation of Si which forms a SiC bond, as Si does not form sp2 bonding, i.e. DLC film stability increases [16]. It has been proven that adding SiOx to a DLC film reduces some of its drawbacks, such as high internal compressive stress and poor adhesion [17]. The doping of DLC films is easily carried out in the process of plasma-chemical synthesis by introducing the doping component into the plasma. As such, components, various siloxanes, silazanes, silanes are used. Obtained amorphous material consists of two interpenetrating carbon and silicon networks, stabilized by hydrogen and oxygen, respectively. SiOx-containing DLC presents higher thermal stability and fracture toughness than undoped DLC films [18].
Effect of annealing in air on the structural and mechanical properties of undoped (a-C, a-C:H) DLC films has been studied by many researchers [[19], [20], [21]]. However, limited study has been reported on the structural changes after annealing of a-C:H:SiOx film in air [[22], [23], [24]]. Moreover, the effect of annealing in hydrogen on properties of a-C:H:SiOx film has not been reported yet.
The tribological characteristics and thermal stability of diamond-like carbon films containing SiOx were investigated in [22]. DLC-SiOx films were deposited using a plasma-based ion implantation method, after which they were annealed for 1 h in a vacuum, in an atmosphere of argon and air at 400, 600, and 750 °C. The structure of the DLC-SiOx films did not change after thermal annealing in vacuum at 600 °C, while it underwent changes after thermal annealing in argon and in air at 400 °C. Muller et al. [25] using Raman spectroscopy studied the structural changes in SiOx containing a-C:H films after annealing in an argon atmosphere. They revealed that transition temperature from the amorphous structure to the more graphitic-like structure increases by as much as 100 °C when SiO is incorporated into the a-C:H film. Venkatraman et al. [26] showed that the tribological properties of a-C:H:SiOx films remained unchanged up to 400 °C in air. The hardness of the films decreased by <15% after annealing in air at temperatures up to 400 °C.
In this paper, we have reported the effect of annealing temperature in hydrogen atmosphere on structural and mechanical properties of a-C:H:SiOx film deposited on stainless steel by plasma-assisted chemical vapor deposition (PACVD) method. The films were annealed at temperatures ranging from 300 to 700 °C, with 100 °C interval. The structural changes have been investigated using Raman spectroscopy, optical and atomic force microscopy (AFM). The stability of mechanical properties and structure as a function of temperature was used to evaluate the thermal stability of a-C:H:SiOx films. Such a study is useful for determining the maximum temperature that a-C:H:SiOx films can withstand and be effective in high-temperature protective applications.
Section snippets
Film preparation
The PACVD system used in the present study for film deposition described in detail in [27]. Stainless steel 12H18N10T (analog of AISI321), the chemical composition of which is presented in Table 1, was used as the substrate material. This steel is widely used as a structural material for atomic reactors.
The mechanically polished stainless steel substrates (2.5 × 2.5 × 0.4 cm3) were cleaned by conventional cleaning method in an ultrasonic bath. Cleaned substrates were loaded into the PACVD
Effect of annealing temperature on the morphology and mechanical characteristics of the a-C:H:SiOx films
To verify the uniformity of the films and adhesion, a simple optical control was applied. Typical images of the surface of samples with a-C:H:SiOx film after four hours of exposure to hydrogen at various temperatures are shown in Fig. 1. Image data suggests that heat treatment in a hydrogen atmosphere leads to significant changes in the surface of the samples only at 700 °C. In this case, there is a partial destruction and delamination of the film from the substrate. At lower temperatures,
Conclusion
Measurements of the surface morphology, mechanical properties, structure, and chemical composition of the a-C:H:SiOx films showed that during annealing in hydrogen, these parameters begin to change at a temperature of 200 °C more than during annealing in an air atmosphere. The effect of increase of bonded hydrogen content in the annealed to 600 °C films is evident from the increase of photoluminescence background in the samples. Raman spectroscopy indicates that there is an onset of structural
Acknowledgement
The work was carried out within Government task of Institute of High Current Electronics № 0366-2016-0010 and the framework of Tomsk Polytechnic University Competitiveness Enhancement Program. The authors are thankful to Tomsk Regional Center for Collective Use of the TSC SB RAS for the provided NanoTest 600 nanoindentator and AFM Solver P47 atomic force microscope.
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