ICMCTF2015 Session E2-3: Mechanical Properties and Adhesion

The recent drive to reduce the characteristic size of devices prompted by the evolution of nanotechnology renders the understanding of mechanical properties a pivotal issue. Recently, nanoindentation using depth-sensing technique has been utilized to characterize the hardness and surface properties of material at small scale. In this proposal, we have applied this technique to characterize physical phenomena and mechanical properties for a wide array of materials. T his study employs a series of experimental in-situ techniques and theoretical methods including the nanomechanics and continuum mechanics to investigate tribological behaviors and mechanical properties of nanomaterials.

Real-time buckling deformation of an individual carbon nanotube (CNT) was studied through in-situ TEM nanoindentation. These in-situ observationsreveal a significant shell-to-Euler phase transformation in the buckling response of CNTs. Objective evidences that the CNT possesses time-dependent characteristic were first suggested by combining in-situ TEM nanoindentation performed strain rate influences on the CNTs with molecular dynamics simulations. Moreover, CNTs reinforced metal matrix nanocomposites have been fabricated by a molecular level process, which involves aligned CNTs in by surface construction, depositing Cu ions upon CNT alignment and reduction, etc, to investigate the nano-mechanism for enhancement of martial strength.

Lateral junction growth (LJG) at nano-scale is fundamental for the atomic origins of macroscopic friction and wear. Our in-situ characterization reveals that the lateral junction growth during incipient sliding coincides with the occurrence of dislocation in the junction. This observation echoes our atomic simulations and suggests that plastic deformation is the central mechanism for LJG. Additionally, our results also reveal that the presence of an adsorbed layer on the asperity surface significantly delays the onset of LJG until the adsorbed layer is splayed out from the interface. Comparisons among molecular simulations, continuum approach and experimental findings are conducted to elucidate the underpinning of the tribological mechanisms and shed light on how to build durable systems to fulfill the compelling energy saving trend. Our studies uncover the underpinning of using soft pad for fine surface polishing and provides a theoretical foundation for the selection and fabrication of a polishing pad and the selection of operation parameters during polishing that cannot be achieved with empirical findings and phenomenological approaches.

The use of plastic substrates in different areas of application (optical lenses, consumer electronics, displays etc.) frequently leads to problems related to their compatibility with inorganic coatings. The deposition of durable inorganic layers on compliant substrates becomes difficult due to the large mismatch in the elastic and thermal properties of the substrate and the coating. In addition, daily use of such coated devices implies that they must withstand various external mechanical solicitations such as cleaning, interaction with hard particles, humidity and thermal cycling, radiation effects and others. By keeping in mind that modern optical coating systems involve complex multilayer stacks with individual layers as thin as several tens of nanometers, it is clearly necessary to develop films possessing controlled optical characteristics coupled with superior mechanical and tribological properties.

The application of hybrid organosilicone materials is a promising avenue to enhance the tribo-mechanical properties of low index optical materials. Indeed, by combining both inorganic (SiO_x) and organic (CH_x) molecular groups, we found that it is possible to achieve improved film characteristics such as higher elasticity than for standard SiO₂ evaporated films and increased resistance to crack formation and propagation.

In the present work, we systematically studied hybrid SiOCH coatings deposited onto transparent plastic substrates using Ion Beam Assisted Chemical Vapor Deposition (IBACVD) of the octamethylcyclotetrasiloxane (OMCTS) precursor. The flexibility of the IBACVD technique allows one to synthesize films with different hybridicity levels. Particular attention has been paid to the effect of the hybridicity on the tribo-mechanical properties of such films assessed by newly developed characterization techniques such as in situ scratching and in situ real-time nano-wear testing. We particularly address the effect of the chemical composition on the elastic rebound, mechanical stress and the environmental degradation mechanisms of the hybrid films.

Scratch resistance, while not a fundamental property of materials, is often a significant and common property of importance in many applications such as glass screens on mobile phones. In the broadest sense, scratch deformation occurs during any loading condition in which both normal and shear force components are applied. This combination occurs very frequently in both everyday life and during industrial machining.

Modern scratch testing systems incorporate a variety of sensors to quantify the deformation which occurs during the application of a controlled normal load and translation rate. The parameters measured by these sensors typically include initial, loaded, and residual penetration depths; normal and tangential loads; and acoustic emission intensity. Less commonly, due to the scarcity of the instrumentation, the scratch deformation is also observed in situ using optical microscopy to observe the contact underneath the translating indenter or electron microscopy to obliquely observe the surface near the indenter.

In this work, the advantages of oblique observation used in electron microscopy are combined with those of long working distance optical microscopy to achieve unique observational capabilities for opaque materials. A variety of materials were selected to demonstrate the advantages of the imaging capabilities. A ductile metal were chosen to highlight the ability to visualize the plastic pile-up around the scratch. Both a transparent, glass, and opaque, silicon, brittle solid were chosen to demonstrate the ability of the system to observe crack formation. Lastly, a brittle coating was selected to demonstrate the ability of the system to characterize the cracking and delamination of the coatings.

While shot peening is a widely used method to improve the mechanical properties of materials for highly stressed components, grit blasting is commonly used for cleaning surfaces. A combination of both methods as surface preparation before coating steel substrates leads to a distinct change in the adhesion and the tribological behavior of DLC coating (a-C:H).

In the case of shot peening, globular grains of WC/Co were applied which leads to high induced residual stress, a change in texture, roughness, micro hardness and surface energy. Additionally, the surface is contaminated heavily with residuals from the grains.

In the case of grit blasting, a slurry with a fixed ratio of water and sharp-edged grains of Al₂O₃ was used which results in surface smoothening and cleaning from WC/Co residuals. The characteristics of the above described effects is strongly dependent on the shot peening/grit blasting parameters like distance, angle, velocity (air pressure), grain size and form, grain hardness and peening time.

Depending on the surface topography and the degree of WC/Co residuals, the film adhesion could be improved distinctly. A better mechanical and chemical bonding as well as different failure mechanisms compared to polished surfaces could be identified. Additionally, the tribological tests showed that the friction coefficient can be decreased by choosing specific substrate preparation methods.

The objective of the talk is to present the benefit of the in-situ observation in case of transparent material. We have developed to analyse the scratching and the surface behaviour of polymeric surfaces a home made device which allows to record trough the sample the contact area, over a large range of velocity and temperature. Using spherical tips of different radiuses to avoid the effects of singularity of the tip, the mean contact stress versus the mean contact strain analysis will be presented. In case of coated surfaces, some cracking and delimitation mechanisms will be explained.

Regarding the FEM simulations, anti-scratch coated system is studied in order to define interfacial damage criteria in relation with the observed damage mode. To achieve this goal we plot shear stress according to normal stress for the nodes at the coating/substrate interface. We obtain many loops which represent how the nodes undergo stresses. Thus we can locate those which are under tensile stress and are likely to be the cause of the film damages.

Key issue of this study is the development of a microwave plasma assisted process in order to archive surface recrystallization of fine-grained cemented carbides with the objective to improve layer adhesion of CVD diamond. The effect of plasma parameters on the surface microstructure and this, in turn on the coating adhesion is evaluated. By controlling gas composition, the surface properties and its characteristics concerning hardness, roughness and surface energy could be influenced. Since Cobalt back diffusion has proved to be problematic, its quantification and impact on diamond quality and adhesion was analyzed. For the purpose of comparing interface and layer adhesion the common chemical etching pretreatment using Murakami solution and Caro’s acid was used and optimized. Microstructure investigations were done by SEM, EDS, XPS and Raman spectroscopy. Layer adhesion was tested by Rockwell indentation.

The results show that by recrystallizing the WC grains at the surface, contiguity and surface roughness increases. An enhanced nucleation density is archived by deposition of a thin layer of graphene or graphite on top of the surface. Addition of Oxygen during diamond deposition allows reducing the interaction between Cobalt and the growing Diamond coating. Taking all these improvements into account, the new approach improves layer adhesion compared to the wet chemical pretreatment and enhances tool life.