Papers by Author: Yao Gen Shen

Abstract: Nanocomposite coating films have been increasingly used in industrial applications because of their unique mechanical and physical properties. Residual stresses generated during the growth of sputter-deposited thin films due to a strain mismatch between the film and the substrate may lead to significant failure problems. Large residual stresses may generate buckling, delamination and film fracture. Although buckles with cracks in thin films have been experimentally observed, their origins are still not well understood.

Abstract: CrN/CNx nano-scale multilayered films were deposited on Si (100) substrate by closed-field unbalanced magnetron sputtering. Designed experimental parameters enabled an evaluation of the effects of negative substrate bias voltage (Vb), and bi-layer thickness λ (by changing substrate rotation rate) during deposition on the structural and mechanical properties of multilayer films. These multilayers were characterized and analyzed by transmission electron microscope (TEM), X-ray diffraction (XRD), atomic force microscopy (AFM), and nanoindentation measurements. In all cases, the CNx layers were amorphous and independent of Vb, while the microstructures of the CrN layers were dependent primarily on Vb. The CrN layers showed a mixed structure phase consisting of CrN, Cr2N, and Cr at Vb = -(40-120) V. At higher Vb values (-140 V or above), the Cr2N phase was dominant along with low CrN phase content. AFM measurements revealed that the root-mean-square (rms) surface roughness of the CrN/CNx film was 2 nm at Vb= -200 V whereas the rms values were about 9.5-3.3 nm for lower Vb values of -(40-180 V). By nanoindentation measurements, a maximum hardness of about 36 GPa was observed at Vb= -140 V. The improved mechanical properties of the films are correlated to the phase formation during deposition.

Abstract: Nano-structured TiN/TiBN multilayer thin films were deposited onto unheated Si(100) substrates by reactive unbalanced dc-magnetron sputtering in an Ar-N2 gas mixture at a pulsed-bias voltage of –60 V. The effects of the bilayer thickness (Λ = 1.8-7.7 nm) on microstructures and mechanical properties have been analyzed using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), and microindentation measurements. Microstructure studies revealed that the TiN layers were fcc B1-NaCl structure comprising of (111)- and (200)-oriented grains depending on Λ, while the TiBN layers were amorphous. Significant relationships were found between hardness (H) and Λ. A maximum hardness of ~30 GPa was observed in a multilayer film with
 = 1.8 nm. The possible hardness enhancement mechanism was also discussed.

Abstract: Based on nanoindentation techniques, the evaluation of hardness of two nanostructured thin films, AlN and Ti-Al-N, is discussed. In the case of AlN films, the indentation size effect of hardness can be modeled using the concept of geometrically necessary dislocations, whereas in the case of Ti-Al-N films, the measured hardness increases exponentially as the indentation depth decreases. The results show that, as thin films approach superhard, dislocation-based plastic deformation is gradually replaced by grain-boundary mediated deformation.

Abstract: A combination of high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy are used to establish that Ti-B-N films with different boron concentrations prepared by reactive unbalanced magnetron sputtering exhibit a two-phase nanocomposite microstructure, showing nanocrystalline Ti(N, B) grains embedded in amorphous (TiB2, BN) matrices. Using Monte Carlo simulations and based on a simple model employing a kinetic grain growth theory, we also investigate the effects of the amorphous TiB2-BN phase on the microstructure evolution and grain growth in nanocrystalline-Ti(N, B). Our study demonstrates that the formation of such an amorphous phase at the grain boundary could hinder the growth of Ti(N, B) grains and the mean grain size shows an exponential decay with boron concentration, in good agreement with our experimental observations.

Abstract: The pattern formation during delamination and buckling in sputter-deposited tungsten thin films under large compressive stresses was investigated. The films were analyzed in situ by a cantilever beam technique, and ex situ by atomic force microscopy (AFM) and focused ion beam. Depending on the magnitude of compressive strain in thin films, different types of buckling patterns were observed. For stresses above a critical value, there was a regime of steady growth in which the incipient blister evolves into a regular sinusoidal-like propagation. At higher strains, the sinusoidallike wrinkles were developed with constant widths and wavelengths. Some of the wrinkles bifurcated to form branches. With further increase in stress the complicated buckling patches were formed with many irregular lobes. These types of pattern formation have been supported by elastic energy calculations.

Abstract: Thin films of molybdenum nitride (MoNx with 0≤x≤0.35) were deposited on Si(100) at room temperature using reactive DC magnetron sputtering. The residual stress of films was measured as a function of sputtering pressure, nitrogen incorporation, and annealing temperature by wafer curvature-based technique. It was found that the stress of the films was strongly related to their microstructure, which depended mainly on the incorporation of nitrogen in the films. The film stresses without nitrogen addition strongly depended on the argon pressure and changed from highly compressive to highly tensile in a relatively narrow pressure range of 0.8-1.6 Pa. For pressures exceeding ~5.3 Pa, the stress in the film was nearly zero. Cross-sectional transmission electron microscopy indicated that the compressively stressed films contained a dense microstructure without any columns, while the films having tensile stress had a very columnar microstructure. High sputtering-gas pressure conditions yielded dendritic-like film growth, resulting in complete relaxation of the residual tensile stresses. It was also found that the asdeposited film was poorly ordered in structure. When the film was heated at ~775 K, crystallization occurred and the stress of the film drastically changed from –0.75 to 1.65 GPa. The stress development mechanism may be due to volumetric shrinkage of the film during crystallization.