Ab Initio Investigation on Cu/Cr Codoped Amorphous Carbon Nanocomposite Films with Giant Residual Stress Reduction

The influence of hydrogen concentration in amorphous carbon films on mechanical properties and fluorine penetration: a density functional theory and ab initio molecular dynamics study

Amorphous carbon (a-C) films have attracted significant attention due to their reliable structures and superior mechanical, chemical and electronic properties, making them a strong candidate as an etch hard mask material for the fabrication of future integrated semiconductor devices. Density functional theory (DFT) calculations and ab initio molecular dynamics (AIMD) simulations were performed to investigate the energetics, structure, and mechanical properties of the a-C films with an increasing sp3 content by adjusting the atomic density or hydrogen content. A drastic increase in the bulk modulus is observed by increasing the atomic density of the a-C films, which suggests that it would be difficult for the films hardened by high atomic density to relieve the stress of the individual layers within the overall stack in integrated semiconductor devices. However, the addition of hydrogen into the a-C films has little effect on increasing the bulk modulus even though the sp3 content increases. For the F blocking nature, the change in the sp3 content by both atomic density and H concentration makes the diffusion barrier against the F atom even higher and suppresses the F diffusion, indicating that the F atom would follow the diffusion path passing through the sp2 carbon and not the sp3 carbon due to the significantly high barrier. For the material design of a-C films with adequate doped characteristics, our results can provide a new straightforward strategy to tailor the a-C films with excellent mechanical and other novel physical and chemical properties.

In vitro and in vivo biocompatibility and bio-tribological properties of the calcium/amorphous-C composite films for bone tissue engineering application

Carbon-and diamond-like-carbon coated Ti alloys hold great promise for tissue engineering applications. Unfortunately, their strong intrinsic stress leads to the adhesion failure of the films. Herein, a series of a-C films with different Ca content were prepared on Ti₆Al₄V via co-sputtering deposition technology. Homogeneous spherical Ca nanoclusters, with an inner diameter of 2-6 nm, were formed in an amorphous carbon matrix. The addition of Ca induced indistinctive variation in either phase composition or topography. However, the introduction of Ca not only improved the mechanical properties of a-C film but also significantly strengthened its adhesion to osteoblasts. The bio-tribological properties of Ca/a-C films were also assessed using a tribometer in FBS solution. The Ca/a-C films exhibited a low friction coefficient of 0.083 and a low wear rate of 1.02-1.24×10^-6 mm³/Nm. The low coefficient of friction (COF) of the Ca/a-C films indicates their superior mechanical properties, making them the promising target of nanocomposite films used in bio-tribological applications. Well-stretched cells and the developed actin filaments were distinctly observed on the Ca/a-C films in the osteoblast cell adhesion experiments. In addition, the Ca/a-C films promoted cell proliferation and showed high cell viability. After being implanted for 4 weeks, the Ca/a-C implant material still adhered well to the muscle tissue, without inducing hyperergic or inflammatory reactions. Collectively, our results suggest that the Ca/a-C film is an ideal mounting material for bone tissue engineering.

First principles investigation on energetics, structure, and mechanical properties of amorphous carbon films doped with B, N, and Cl

Amorphous carbon (a-C) films have received significant attention due to their reliable structures and superior mechanical, chemical and electronic properties, making them a strong candidate as a hard mask material. We investigated the energetics, structure, and electronic and mechanical properties of the B, N, and Cl doped a-C films based on density functional theory (DFT) calculation. Our DFT calculated results clearly show that introducing B and N atoms into a-C films makes the bulk modulus slightly reduced as a function of the concentration increases. Interestingly, it is noted that introducing Cl atom into a-C films makes the bulk modulus is drastically reduced, which suggests that the films softened by Cl doping would relieve residual stress of the individual layers within the overall stacks in integrated semiconductor devices. These requirements become more important and increasingly more challenging to meet as the device integrity grows. In the perspective of F blocking nature, B doping into a-C films pulls in and captures the F atom due to the strong bonding nature of B‒F bond than C-F bond. Unlike the B doping, for the N doped a-C film, F atom has extremely large diffusion barrier of 4.92 eV. This large diffusion barrier is attributed to the electrostatically repulsive force between both atoms. The Cl doped a-C film shows consistently the similar results with the N doped a-C film because both N and Cl atoms have large electro-negativity, which causes F atom to push out. If one notes the optimized designing with the suitable doped characteristics, our results could provide a new straightforward strategy to tailor the a-C films with excellent mechanical and other novel physical and chemical properties.

Transformation of amorphous carbon to graphene on low-index Ni surfaces during rapid thermal processing: a reactive molecular dynamics study

The transformation of amorphous carbon to graphene on different Ni surfaces during rapid thermal processing was explored using reactive molecular dynamics simulation. Due to the difference in activation energy, Ni surfaces affected the diffusion behavior of C into Ni and thus modulated the remnant number of C atoms, dominating the formation and quality of graphene, which accorded with the developed empirical equation.

Impact of Film Thickness on Defects and the Graphitization of Nanothin Carbon Coatings Used for Metallic Bipolar Plates in Proton Exchange Membrane Fuel Cells

Metallic bipolar plates (BPPs) are considered promising alternatives to traditional graphite BPPs used in proton exchange membrane fuel cells (PEMFCs). Major auto companies, such as Toyota, GM, Ford, and BMW, are focusing on the development of metallic BPPs. Amorphous carbon (a-C) coating are widely known to be effective at enhancing the performance of metallic BPPs. However, a-C coatings prepared by sputtering are mostly micrometers thick, which can render mass production difficult due to their low deposition rates. In this study, we investigate effects of thickness on the formation of defects and the graphitization of nanothin a-C layers deposited by magnetron sputtering from scanning electron microscope (SEM) and transmission electron microscope (TEM) observations, internal stress measurements, X-ray diffractometer (XRD) data, Raman spectra, and X-ray photoelectron spectroscopy (XPS). Furthermore, corrosion and interfacial contact resistance (ICR) test results show that an approximately 69 nm a-C layer, with a deposition time of only 15 min, can meet ex situ technical targets of US Department of Energy. As the thickness of a-C layers increases, vacancy-like defects become more pronounced, which is accompanied by stress relaxation. Furthermore, the larger the graphite-like clusters, the more sp²-hybridization carbon atoms found in loose a-C films. The good properties of nanothin a-C layers are attributed to their limited defects and proper graphitization.

Ab Initio Study of Interfacial Structure Transformation of Amorphous Carbon Catalyzed by Ti, Cr, and W Transition Layers

Amorphous carbon (a-C) films composited with transition layers exhibit the desirable improvement of adhesion strength between films and substrate, but the further understanding on the interfacial structure transformation of a-C structure induced by transition layers is still lacked. In this paper, using ab initio calculations, we comparatively studied the interfacial structure between Ti, Cr, or W transition layers and a-C film from the atomic scale, and demonstrated that the addition of Ti, Cr, or W catalyzed the graphitic transformation of a-C structure at different levels, which provided the theoretical guidance for designing a multilayer nanocomposite film for renewed application.

Highly Corrosion Resistant and Sandwich-like Si3N4/Cr-CrNx/Si3N4 Coatings Used for Solar Selective Absorbing Applications

Highly corrosion resistant, layer-by-layer nanostructured Si₃N₄/Cr-CrN_x/Si₃N₄ coatings were deposited on aluminum substrate by DC/RF magnetron sputtering. Corrosion resistance experiments were performed in 0.5, 1.0, 3.0, and 5.0 wt % NaCl salt spray at 35 °C for 168 h. Properties of the coatings were comprehensively investigated in terms of optical property, surface morphology, microstructure, elemental valence state, element distribution, and potentiodynamic polarization. UV-vis-near-IR spectrophotometer and FTIR measurements show that the change process in optical properties of Si₃N₄/Cr-CrN_x/Si₃N₄/Al coatings can be divided into three stages: a rapid active degradation stage, a steady passivation stage, and a transpassivation degradation stage. With the increase in the concentration of NaCl salt spray, solar absorptance and thermal emittance experienced a slight degradation. SEM images reveal that there is an increase in surface defects, such as microcracks and holes and -cracks. XRD and TEM measurements indicate that the phase structure changed partially and the content of CrO_x and Al₂O₃ has increased. Auger electron spectroscopy shows that the elements of Cr, N, and O have undergone a minor diffusion. Electrochemical polarization curves show that the as-deposited Si₃N₄/Cr-CrN_x/Si₃N₄/Al coatings have excellent corrosion resistance of 3633.858 kΩ, while after corroding in 5.0 wt % NaCl salt spray for 168 h the corrosion resistance dropped to 13.759 kΩ. However, these coatings still have an outstanding performance of high solar absorptance of 0.924 and low thermal emittance of 0.090 after corroding in 3.0 wt % NaCl salt spray for 120 h. Thus, the Si₃N₄/Cr-CrN_x/Si₃N₄/Al coating is a good choice for solar absorber coatings applied in the high-saline environment.