Characterization of a-CSi:H films prepared by PECVD in terms of adhesion
Introduction
Film adhesion together with its mechanical properties is one of the most important material properties for industrial and scientific applications. Thin films prepared for example by PVD (physical vapour deposition) or CVD (chemical vapour deposition) with controlled adhesion are essential in many applications such as anti-scratch, barrier, transparent, wear-resistant, bioactive and anti-reflective coatings or in unconventional optics for surface modified materials to achieve additional functional properties [[1], [2], [3], [4], [5], [6], [7]]. The greatest potential of this research for industrial applications is in functionalized surfaces, protective coatings and mainly in the glass-fibre reinforced polymer composites without sharp interfaces, mimicking natural and biological systems [8].
Advanced materials, such as composites, are based on the principle of mutual reinforcement of their components. This means that transfer of stress takes place from the matrix to the reinforcement (particulates or fibres) when the material is under stress. To ensure efficient transfer of stress, a strong adhesion among all components is necessary. In the case of glass fibres, it is therefore beneficial to modify them with a thin layer (or a gradient multilayer), resulting in compatibility and bonding between the fibres and the polyester matrix which allows to increase adhesion, hence improving the properties of the composite [[8], [9], [10]]. This can be achieved uniquely by means of films prepared by plasma-enhanced chemical vapour deposition (PECVD) using a tetravinylsilane precursor [[11], [12], [13]].
The value of the load at which film adhesion failure is detected from the nanoscratch test is known as the critical normal load (Lc) and is used as a semi-quantitative measure of film adhesion to the substrate [[14], [15], [16]]. However, this value usually provides sufficient information for comparative purposes [16,17]. The Lc also corresponds to the first significant fluctuation of the lateral force component in the loading curves. This marked fluctuation in the lateral force corresponds to the point of adhesion failure, which is usually also verified by either Optical Microscopy (OM), Scanning Electron Microscopy (SEM) [18] or Atomic Force Microscopy (AFM) [19]. The choice of these microscopic methods depends on the dimensions of the scratches, as well as on the thicknesses and the character of films. It is also possible to employ the measurement of acoustic emissions during the scratch test to detect Lc [15].
Since the Lc value is influenced by many extrinsic and intrinsic parameters [14,20], further proposed determination of the work of adhesion seems to be more appropriate criterion used to characterize film adhesion to a substrate. One of the first models used in determination of the work of adhesion was the energy balance approach; this assumes that the film under compressive stress in the front of the indenter releases its accumulated elastic energy by cracking, spallation of the film and delamination from the substrate at Lc. The stress responsible for adhesion failure can then be related to the work of adhesion Wadh = σappl(x)2·t/2E, where σappl(x) is the applied stress at the leading edge of the indenter, t is the film thickness and E is the Young's modulus of the film [[21], [22], [23]]. However, this model does not adequately describe the stresses in materials that exhibit plastic deformation [24]. Burnett and Rickerby identified three contributing factors responsible for the film detachment from the substrate during the nanoscratch test, namely ploughing, internal stress and adhesive friction. The friction force F is then a linear sum of these three contributions and is related to the Wadh [25]. Based on later studies by Bull et al.; F is equal to Lc·μc, where μc is friction coefficient at Lc. Finally, work of adhesion can be expressed from equationwhere dc is the critical scratch width and ν is Poisson's ratio [24].
Section snippets
Materials
Tetravinylsilane Si(CH=CH2)4 (TVS, purity 97%, Sigma-Aldrich) was used as an organosilicon precursor for the preparation of the thin films. These were deposited on planar double-side polished p-type silicon wafer (100) with native silicon oxide layer, which is known to be amorphous (10 × 10 × 0.6 mm3, boron doped, ON Semiconductor CZ). Argon gas (99.999%, Linde Gas) was used for pre-treatment substrate to clean surface from adsorbed gasses and reach reproducible adhesion of thin films. Argon
Critical normal load – evaluation and results
Fig. 1 shows a specific relationship between normal and lateral forces during the scratch tests as well as the superimposed AFM image to the same scale. This figure clearly shows the correlation between the point of adhesion failure and the critical normal load Lc, obtained graphically as indicated by the red dashed lines. In this specific case the loading curve was obtained from a-CSi:H thin film prepared from TVS using 2 W. The graph of the lateral versus normal force (Fig. 1) shows a sudden
Conclusions
Studies of a-CSi:H films deposited using tetravinylsilane as a precursor employing PECVD with power source operated in two modes – pulsed plasma and continuous wave were undertaken. The individual investigated properties of films prepared using these two different power modes corresponded very well to each other and showed similar trends. The critical load as a measure of adhesion by nanoscratch test was determined and correlated with the AFM images; Lc values rise from 1.7 up to 4.6 mN at
Author's contribution
T. Plichta performed all measurements of mechanical and tribological properties together with AFM, evaluated results and provided their analysis and discussion, wrote and edited the article; M. Branecky deposited thin films and characterized their thickness by spectroscopic ellipsometry; V. Cech provided the idea for this study, revised and edited the article.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the Czech Science Foundation, Grant No. 16-09161S and the Specific Research of Brno University of Technology, Grant No. FCH-S-19-5834.
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2023, Surface and Coatings TechnologyCitation Excerpt :Deposition is controlled by many parameters which may affect the properties of prepared film, thus tailored films of variable chemical or physical properties can be obtained [1–3]. Amorphous hydrogenated silicon carbide (a-CSi:H) thin films generally reach good adhesion to the substrate [4], friction coefficient lower than 0.15 [4–7] and relatively low values of internal stress, commonly below −1 GPa, where the minus sign indicates the compression internal stress [5,8,9]. As these films can be applied in many industrial sectors, it is essential to know the most important mechanical properties, such as modulus of elasticity, in order to prepare composites without sharp interfaces (glass fiber-reinforced composites) [10] or hardness for surface and wear protective treatments or adhesion studies [11,12].
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