PacSurf2018 Session TF-TuE: Next-generation Protective Coatings and Tribological Applications

Water lubrication has the characteristic of low environmental burden. It is known that silicon carbide (SiC) shows low friction coefficient due to the formation of a tribofilm by chemical reaction at sliding interfaces under water lubrication. Thus, understanding of the chemical reaction mechanism is essential to improve friction characteristic for practical use and application. However, it is difficult to observe directly such a complicated phenomenon including friction and chemical reaction by experiments. Therefore, in this study, molecular dynamics (MD) simulation using reactive force field, which can simulate chemical reaction, was conducted to analyze the structure and formation mechanism of tribofilm in the friction process of amorphous SiC under water environment.

In the simulation, we used a model in which amorphous SiC ball and disk were rubbed in water (See supplementary document (SD) Fig.1). The SiC ball was slid on the SiC surface with nominal pressure of 0.5 GPa and sliding speed of 100 m/s.

During the friction, the surface wears as the SiC ball and the SiC disk come into contact with each other. Silica (SiO₂) particle and hydrocarbon were confirmed as wear debris. It was also observed that the SiO₂ particles dissolved in water and forms colloidal silica. Firstly, we investigated the changes in number of water molecules and Si-O-Si and C-H bonds during the friction (See SD Fig. 2, Fig.3). It was found that the number of water molecules decrease continuously while the number of Si-O-Si and C-H bonds increase correspondingly. This result indicates that hydrolysis reaction of SiC (SiC + H₂O → Si-OH + C-H) occurs at the sliding interface. Next, we investigated the change in the number of Si and C contained in the wear debris (See SD Fig. 4). We found that the number of Si in the debris was always larger than that of C in the debris. This result indicates that the Si atoms are easily dissolved from the SiC surface as SiO₂ particle, whereas the C atoms tend to remain on the SiC surface. The distribution of C, Si, H, and O atoms except H₂O molecules shows that the C and H atoms were concentrated on the surface of the SiC disk and SiC ball (See SD Fig. 5, Fig. 6). On the other hand, the Si and O atoms were dissolved in water, forming the colloidal silica. Meanwhile, the Si and O atoms also located between the SiC disk and SiC ball. This fact indicates that the colloidal silica prevent the contact of the surfaces, leading to low friction. Thus, we concluded that colloidal silica, which is produced by the hydrolysis reaction of SiC surface, lowers the friction of SiC in water lubrication.

Surface Engineering (SE) is the science and technology of improving the surface properties of materials for protection in demanding contact conditions and aggressive environments. SE also encompasses engineering new multi-functional surface properties, such as electrical, optical, thermal, chemical, and biochemical properties. It involves multiple or hybrid processes which include substrate modification and deposition of overlayers in complex architectures. These processes enhance adhesion and optimize composition or microstructure to enhance protective properties coupled with other functionality. The substrates may be of complex shapes, like metal-cutting tools and automotive or aerospace components, and range in size from micrometers, such as in MEMS or NEMS devices, to meters, such as in architectural glass. The applications are wide-ranging, and include, for example, control of friction, wear-resistance, corrosion-resistance, thermal-barrier coatings, decorative coatings, bioimplants, antimicrobial layers, web-coatings, and thin films with engineered electrical and optical responses. Areas of scientific interest range from first-principle atomistic studies of new materials, to scientific and technological advances in synthesis methods, structural and chemical characterization techniques, property measurements, and performance characterization of surface-engineered parts. I will highlight a few selected SE advances from the past three years which include: complex SE architectures for joint-replacement implants [1], duplex coating for superior wear resistance of Ti alloy compressor blades or landing gear components [2], fracture-resistant thin-film metallic glass [3] and non-stick syringe needles[4]. TiB_x thin films grown from compound TiB₂ targets by magnetron sputter deposition are typically highly over-stoichiometric due to differences in Ti and B preferential ejections angles and gas-phase scattering during transport. I will describe two methods to control of the B/Ti ratio in sputter deposited titanium diboride coatings, involving preferential ionization of sputter-ejected Ti atoms [5,6]. The ability to obtain stoichiometric TiB₂ films is a prerequisite to obtaining high-quality epitaxial transition metal diboride layers. Overall, Surface Engineering continues to be a vibrant interdisciplinary field in the area of interest of AVS.

₁ P Hovsepian et al, J. Mat. Sci: Mater Med (2016) 27:147

² D Goldbaum et al, Surf. Coat. Technol. 291 (2016) 264

³ CC Yu et al, APL MATERIALS 4, 116101 (2016)

⁴ JP Chu et al, Scientific Reports 6:31847

⁵ I Petrov et al, J. Vac. Sci. Technol. A 35 (2017) 050601

⁶ B Bakhit et al, J. Vac. Sci. Technol. A 36 (2017) 030604

MoDTC friction modifier for engine oil is well known to reduce friction under boundary lubrication where friction surfaces locally contact each other. MoDTC decomposes during friction and then, tribo-film of 2D-MoS₂ layers oriented at sliding interface is formed from the decomposition products of MoDTC, leading to low friction [1] . It is well known that the effect of MoS₂ largely depends on sliding condition. However, understanding of the dependency is not sufficient because of the difficulty of in situ observation at the sliding interface although sliding condition optimization is important to improve friction property. As one cause of this dependency, it is considered that the defect structure in MoS₂ tribo-film has some influence on friction. Thus, in order to investigate influence of defect structures in MoS­­₂ tribo-film generated from MoDTC at sliding interface on friction behavior, we performed molecular dynamics simulation by using reactive force field (ReaxFF), which takes into account the chemical reactions.

In the simulation, we prepared models in which some MoS₂ layers are sandwiched between two diamond-like carbon (DLC) substrates because DLC is a coating material used in engine cylinders. Here, we investigated the influence of the defects by comparing the friction behavior of the MoS₂ layers with and without defect structures. As the defect structures, we considered grain boundary structures in the MoS₂ layer and a distorted layer structure in which layers unoriented at the interface. In order to simulate friction process, one DLC substrate was fixed and the other was slid at 100 m/s with 3 GPa of a contact pressure.

The friction simulation the model without defect showed that the contact surface of DLC and MoS₂ was the sliding interface. The simulation the model with grain boundaries showed that the contact surface of MoS₂ and MoS₂ was the sliding interface since C-S bonds were formed between DLC and MoS₂ around grain boundaries. In addition, the MoS₂ layers with grain boundaries showed higher friction force because the grain boundaries in neighboring MoS₂ layers interact each other. The simulation the model with distorted layer structure showed that the MoS₂ edge adhered to DLC since C-S and Mo-C bonds were formed between DLC and MoS₂. The shear stress acting on the unstable MoS₂ edge accelerated the chemical bond formation. In summary, we found that interactions between MoS₂ grain boundaries and formation of chemical bonds between MoS₂ and DLC sliding surface around defect are significant frictional resistance.

[1] J. Graham et al., J. Tribol., 44 (2001) 4

In this study, a silver(Ag) coated copper(Cu) core-shell microparticles ink were fabricated and screen-printed on Polyimide(PI) substrates. It was sintered via flash light sintering technique. Flash light irradiation condition (i.e. pulse duration, irradiation energy) was optimized to obtain high conductivity and good oxidation resistance characteristics of Ag coated Cu core-shell pattern. To increase the packing density of the electrodes and its oxdiation resistance, Ag nanoparticles(NPs) were added to Cu/Ag core-shell microparticles (MP) ink. To determine optimal amount of the Ag NPs, the hybrid ink having different mass ratios of the Ag NPs and Cu/Ag core-shell MP were fabricated and tested. The sheet resistance of the hybrid ink was measured using the four-point probe method. To analyze the sintering behavior and degree of oxidation of hybrid pattern, a scanning electron microscope (SEM) was used. To demonstrate the mechanism of the sintering process on hybrid ink, multiphysics COMSOL simulation and temperature monitoting was conducted. As a result, hybrid pattern sintered with flash light showed excellent oxidation resistance (resistance increase rate in 300℃ for 5 hours: 54.34 %), and high electrical conductivity (9 μΩ·cm).

Acknowledgments

This work was supported by a National Research Foundation of Korea (NRF), funded by the Ministry of Education (2015R1D1A1A09058418, 2012R1A6A1029029, and 2013M2A2A9043280). This work was also supported by the Industrial Strategic technology development program (10076562, Development of fiber reinforced thermoplastic nano-composite via fiber bundle spreading for high quality resin impregnation process and its application to the underbody shield component for protecting battery pack of an electric-vehicle) funded by the Ministry of Trade, industry & Energy (MI, Korea).