K. P. Shaha

Nanoscale deformation mechanism of TiC/a-C nanocomposite thin films

This paper concentrates on the deformation behavior of amorphous diamondlike carbon composite materials. Combined nanoindentation and ex situ cross-sectional transmission electron microscopy investigations are carried out on TiC/a-C nanocomposite films, with and without multilayered structures deposited by pulse dc magnetron sputtering. It is shown that by controlling the distribution of nanocrystallites forming nanoscale multilayers, the system can be used as a “microstructural ruler” that is able to distinguish various deformation patterns, which can be hardly detected otherwise in a homogeneous structure. It is shown that rearrangement of nanocrystallites and displacement of a-C matrix occur at length scales from tens of nanometer down to 1 nm. At submicrometer scale homogeneous nucleation of multiple shear bands has been observed within the nanocomposites. The multilayered structure in the TiC/a-C nanocomposite film contributes to an enhanced toughness.

Tunable self-organization of nanocomposite multilayers

In this letter we report the controlled growth and microstructural evolution of self-assembled nanocomposite multilayers that are induced by surface ion-impingement. The nanoscale structures together with chemical composition, especially at the growing front, have been investigated with high-resolution transmission electron microscopy. Concurrent ion impingement of growing films produces an amorphous capping layer 3 nm in thickness where spatially modulated phase separation is initiated. It is shown that the modulation of multilayers as controlled by the self-organization of nanocrystallites below the capping layer, can be tuned through the entire film.

On the dynamic roughening transition in nanocomposite film growth

Surface roughness and dynamic growth behavior of TiC/a-C nanocomposite films deposited by nonreactive pulsed-dc (p-dc) magnetron sputtering were studied using atomic force microscopy, cross-sectional scanning, and transmission electron microscopy. From detailed analyses of surface morphology and growth conditions, it is concluded that a transition in growth mechanisms occurs, i.e., a mechanism dominated by geometric shadowing at a p-dc frequency of 100 kHz evolving to a surface diffusion mechanism driven by impact-induced atomistic downhill flow process by Ar+ ions at a p-dc frequency of 350 kHz. It is shown that rapid smoothening of initially rough surfaces with rms roughness from ∼6 to <1 nm can be effectively achieved with p-dc sputtering at 350 kHz pulse frequency, leading to a transition from a strong columnar to a columnar-free microstructure.

On the evolution of film roughness during magnetron sputtering deposition

The effect of long-range screening on the surface morphology of thin films grown with pulsed-dc (p-dc) magnetron sputtering is studied. The surface evolution is described by a stochastic diffusion equation that includes the nonlocal shadowing effects in three spatial dimensions. The diffusional relaxation and the angular distribution of the incident particle flux strongly influence the transition to the shadowing growth regime. In the magnetron sputtering deposition the shadowing effect is essential because of the configuration of the magnetron system (finite size of sputtered targets, rotating sample holder, etc.). A realistic angular distribution of depositing particles is constructed by taking into account the cylindrical magnetron geometry. Simulation results are compared with the experimental data of surface roughness evolution during 100 and 350 kHz p-dc deposition, respectively.

Surface roughness evolution of nanocomposite thin films

An analysis of dynamic roughening and smoothening mechanisms of thin films grown with pulsed-dc magnetron sputtering is presented. The roughness evolution has been described by a linear stochastic equation, which contains the second- and fourth-order gradient terms. Dynamic smoothening of the growing interface is explained by ballistic effects resulting from impingements of ions to the growing thin film. These ballistic effects are sensitive to the flux and energy of impinging ions. The predictions of the model are compared with experimental data, and it is concluded that the thin film roughness can be further controlled by adjusting waveform, frequency, and width of dc pulses.

On the nature of the coefficient of friction of diamond-like carbon films deposited on rubber

In this paper, the nature of the coefficient of friction (CoF) of diamond-like carbon (DLC)-protected rubbers is studied. The relative importance of the viscoelastic and adhesive contributions to the overall friction is evaluated experimentally by modifying the contact load and the adhesive strength between the surface and the counterpart. The results indicate that the increase of CoF during the tribotests under non-lubricated conditions is caused by the increase of the adhesive contribution to friction motivated by the growth of the contact area during the test. In the case of oil lubricating condition, the adhesive force is minimized and the CoF is observed to decrease during the tribotest. This is caused by the reduction of the viscoelastic contribution due to the variation of the shape of the contact area. The role of the microstructure of the DLC film on the efficiency of the oil lubrication is also discussed.

Nanoscale deformation in TiC/a-C multilayered nanocomposite coatings

Nanoscale deformation in TiC∕a-C nanocomposite coatings is revealed by combined nanoindentation and high resolution cross-sectional transmission electron microscopy. A process characterized by rearrangement of TiC nanocrystallites and the displacement of a-C matrix is found to dominate the deformation mechanism at length scales ranging from tens of nanometers down to 1nm and contributes to toughening of the nanocomposite coatings by delocalizing local plastic deformations.

Dynamic smoothing of nanocomposite films

In contrast to the commonly observed dynamic roughening in film growth we have observed dynamic smoothing in the growth of diamondlike-carbon nanocomposite (TiC/a-C) films up to 1.5 μm thickness. Analytical and numerical simulations, based on the Edwards–Wilkinson model and the Mullins model, visualize the effect of the diffusivity parameters and the noise strength on the interface evolution of dynamic smoothing. The prediction is in a good agreement with the measured roughness evolution. High-resolution transmission electron microscopy shows that the formation of an amorphous front layer 2 nm thick excludes possible influence of nanocrystallites on the dynamic growth behavior of the nanocomposite film.

Pulsed DC sputtered DLC based nanocomposite films: controlling growth dynamics, microstructure and frictional properties

Abstract Surface smoothness of diamond-like carbon based thin films becomes a crucial property for developing nearly frictionless protective coatings. Surface roughness and the dynamic growth behaviour of TiC/a-C nanocomposite films, deposited by non-reactive pulsed DC (p-DC) sputtering of graphite targets, were studied using atomic force microscopy and cross-sectional scanning electron microscopy. Upon increasing the intensity of concurrent ion impingement by raising the frequency of p-DC sputtering, a transition from dynamic roughening to dynamic smoothing was revealed, leading to a transition from a strong columnar to a columnar free microstructure. It was shown that smooth films can be deposited on initial rough surfaces at a higher pulse frequency (350 kHz). Also dense, tough, ultrasmooth and ultralow friction TiC/a-C:H nanocomposite films were successfully deposited on industrial polished steel substrates by reactive p-DC sputtering of Ti targets at 350 kHz frequency in an argon/acetylene atmosphere.

Dynamic smoothening and tribological properties of pulsed-DC sputtered DLC based nanocomposite films

Interface roughness and dynamic growth behavior of TiC/a-C nanocomposite films deposited by pulsed-DC magnetron sputtering were studied using atomic force microscopy and scanning electron microscopy. Upon increasing the intensity of concurrent ion impingement by raising the frequency of pulsed-DC sputtering, a transition from dynamic roughening to dynamic smoothening is revealed in the growth behavior of TiC/a-C nanocomposite films. Analyses of surface morphology and growth conditions imply that there is a transition of dominating growth mechanism from geometric shadowing to surface diffusion driven by impact-induced atomistic downhill flow process due to enhanced impingement of Ar+ ions, which occurs upon the change of pulse frequency from 100 kHz to 350 kHz. Also ultra-smooth TiC/a-C:H films were successfully grown on initial rough steel substrates (Sa similar to 6 nm) by pulsed-DC sputtering at 200 and 350 kHz frequency. These nanocomposite films exhibit superb toughness, vicar resistance and ultralow friction.