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Title: Studies of nanostructured Ti/TiB₂and TiN/TiBN multilayer films
Other Titles: Na mi jie gou Ti/TiB₂ji TiN/TiBN duo ceng bo mo de yan jiu
納米結構 Ti/TiB₂及 TiN/TiBN 多層薄膜的研究
Authors: Chu, Kenneth (朱冠霖)
Department: Department of Manufacturing Engineering and Engineering Management
Degree: Doctor of Philosophy
Issue Date: 2007
Publisher: City University of Hong Kong
Subjects: Thin films.
Nanostructured materials.
Notes: xvi, 150 leaves : ill. 30 cm.
Thesis (Ph.D.)--City University of Hong Kong, 2007.
Includes bibliographical references (leaves 126-132)
CityU Call Number: TA418.9.T45 C58 2007
Type: thesis
Abstract: Nanostructured multilayers represent a new class of engineering materials that are made up of alternating nanometer scale layers of two different materials. A range of technologically interesting properties including increased hardness and yield strength achieved in these materials at the nanometer scale make them attractive for a wide range of applications, such as high-strength/high-conductivity materials, load-bearing components, x-ray optics, thin film magnetic heads, recording media, wear-resistance coatings, cutting tools, and micro-electro-mechanical systems. Significant research is being conducted worldwide on the synthesis and characterization of the structural, mechanical, electrical, magnetic and optical properties of these multilayered materials. In this study, nanostructured Ti/TiB2 and TiN/TiBN multilayers were deposited onto Si (100) wafers and hardened AISI M42 tool steels at room temperature by unbalanced dc-magnetron sputtering. The effects of bilayer thickness (Λ) on structural, mechanical and tribological properties of the multilayers were studied. The first part of the thesis focuses on the structural, mechanical and tribological properties of Ti/TiB2 multilayers. X-ray photoelectron spectroscopy (XPS) studies revealed that the presence of Ti and TiB2 bonds was found in Ti and TiB2 layers, respectively. The Λ values of the films obtained by low-angle X-ray diffraction (XRD) were varied from 1.1 to 9.8 nm. By XRD θ~2θ scans and high-resolution transmission electron microscopy (HRTEM) measurements, the growth of (002) oriented nanocrystalline (nc)-Ti layers was interrupted by amorphous (a)-TiB2 layers when Λ decreased from 9.8 to 1.1 nm. The root-mean-square (rms) surface roughness measured by atomic force microscope (AFM) showed that it increased with Λ. By scaling analysis, it was also found that the evolution of surface roughness obeyed scaling law and could be explained by linear diffusion process with the roughness exponent of α ~ 0.90. The hardness of the multilayers was closely related to the Λ, exhibiting the great enhancement compared to the values from the individual single layer. The maximum hardness of ~33 GPa was obtained in the film with Λ = 1.9 nm. The hardening behavior is mainly attributed to the Hall-Petch effect. The residual stress was also found to be dependent on Λ. For the tribological properties, the multilayer with Λ = 1.9 nm showed the best cohesive and adhesive strength, which was evidenced in terms of the critical load values of LC1 (~26 N) and LC2 (~62 N), respectively. Moreover, by dynamic impact testing this multilayer could endure impact cycles up to 4 × 105 without adhesive failure. In wear tests, it was found that the frictional coefficient during nano-scratch tests under single-pass and constant-load conditions decreased with Λ and increased with normal load due to the ploughing effect. The enhanced hardness in the multilayers with small Λ values also improved the wear resistance and lowered the frictional coefficients in ball-on-disc wear tests. In the second part of this thesis, the structural, mechanical and tribological properties of TiN/TiBN multilayers with Λ = 1.4 to 9.7 nm are studied. XPS measurements showed that the TiN layer mainly consisted of TiN bond while TiBN layer consisted of BN, TiB2 and TiN bonds. Microstructure studies revealed that the TiN layers were fcc B1-NaCl structure comprising of (111) oriented grains depending on Λ. With decrease of Λ, the preferred orientation of TiN gradually transformed from (200) to (111), while the TiBN layers were amorphous. The rms surface roughness was found to be increased with Λ. By scaling analysis, the roughness exponent α obtained ~0.60 which indicated surface diffusion dominated the smoothing mechanism with lateral growth. A maximum hardness of ~30 GPa was observed in a multilayer with Λ = 1.8 nm. This film also showed the best cohesive and adhesive strength in terms of critical load values of LC1 (~37 N), LC2 (>80 N) and can endure dynamic impact test up to 4 × 10 cycles without adhesive failure. Similarly, the nano-scratch tests showed that the friction coefficients decreased with Λ and increased with normal load. The wear performance studied by ball-on-disc wear tests showed much higher wear resistance compared with Ti/TiB52 multilayers. The high wear resistance in these multilayers with small Λ is mainly due to the high hardness and the presence of hexagonal (h)-BN, detected by Fourier transform infrared spectroscopy (FTIR), which acted as a lubricant and protective layer.
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