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Title: Tribological properties and related microstructure of Ti-B-N thin films
Other Titles: Tai peng dan bo mo de mo ca xing neng ji xiang guan wei jie gou
Authors: Sit, Ping (薛萍)
Department: Dept. of Physics and Materials Science
Degree: Master of Philosophy
Issue Date: 2004
Publisher: City University of Hong Kong
Subjects: Thin films
Thin films -- Mechanical properties
Notes: CityU Call Number: TA418.9.T45 S57 2004
Includes bibliographical references (leaves 110-117)
Thesis (M.Phil.)--City University of Hong Kong, 2004
xviii, 119 leaves : ill. ; 30 cm.
Type: Thesis
Abstract: Thin films of Ti-B-N were deposited on Si (100) at room temperature by reactive unbalanced dc magnetron sputtering in an Ar-N2 gas mixture at room temperature. The effects of B and N contents on microstructure and tribological properties of Ti-B-N films have been analyzed by using optical microscopy (OM), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, auger electron spectroscopy (AES), atomic force microscopy (AFM), micro-indentation measurements and pin-on-disc tester (POD). Microstructure studies revealed that the Ti-B-N usually showed nanocomposite structure consisting of nanocrystalline (nc-) TiN and amorphous (a-) TiB2 or hexagonal BN phase. The fraction of amorphous phase (either TiB2 or BN phase) increased with increasing B or N content in Ti-B-N thin films, which affected not only surface smoothening, but also the hardness of the films. In the case of B addition, the films showed two- or three-phase nanocomposite structure. At B content below about 10 at.%, the films consisted of mainly TiN bonding with a small amount of TiB and BN bonds. As the B content increased, TiB gradually transformed to TiB2 and the films consisted of nanocrystalline (nc-) TiN and an amorphous (a-) TiB2. A maximum hardness of ~44 GPa was observed in a film with B content of 19 at.% corresponding to the mixture microstructure of nc-TiN/a-TiB2 possessing about 20 mol% TiB2. When B content reached ~42 at.% or above, an amorphous-like nanocomposite of nc-TiN/a-TiB2/a-BN was formed. The surface roughness value decreased gradually with increasing B content. Nitrogen also had a great effect on microstructure of Ti-B-N thin films. The films deposited without N addition exhibited to be a single hexagonal Ti(B) solid solution comprising TiB2 and Ti bonds. After adding about 28 at.% N, TiN bonding was formed accompanying with formation of a small amount of TiB and BN bonds. With further increasing N content, the fraction of BN bonding increased, while TiB2 bonding decreased. Finally, a nanocomposite nc-TiN/a-TiB2/a-BN was formed. Low microhardness of about 20 GPa was obtained in Ti(B) solid solution film, and a maximum microhardness value of about 50 GPa in the nc-TiN/a-TiB2 nanocomposite with a minimum BN bonding. Formation of amorphous h-BN phase would greatly decrease the hardness of the thin films. With further increasing N content a decrease of microhardness took place due to formation of h-BN phase, and a hardness value of about 35 GPa was obtained at high N content. With increasing N content, the surface roughness value gradually increased. It was found that the B or N content strongly affected not only the hardness value but also the tribological properties by changing the phase configuration, microstructure and surface roughness. For the tribological properties of the changing B content in Ti-B-N films, both friction coefficient and specific wear rate decreased with increasing B content in coatings which slid against AISI440C steel ball. This behavior was accompanied by the transformation of wear modes from abrasive wear (ploughing) to powder formation, then to plastic deformation. The wear transition was attributed to the increasing amount of amorphous phase and the building up of tribo-film. When the B content reached a critical value (~43 at.%), the wear rate slightly increased due to formation of hexagonal BN phase in Ti-B-N thin films. For the effect of wear parameter, the friction coefficient increased if applied loading increased from 2N to 10N, however the specific wear rate decreased with an increase of deformed wear debris. Increasing sliding velocities yielded similar results to increasing load, which might be due to the strain rate effect. Ti-B-N coatings with different N contents slid against AISI440C steel ball and WC-6wt%Co ball showed that the wear resistance of film depended on the amount of tribo-film participating in the sliding process. In the case of AISI440C steel, adhesive wear was pre-dominant and the wear rate increased sharply to a maximum when N content reached ~38 at. %, which corresponded to the least amount of deformed wear debris. This might be related to the change of film microstructure and phase configuration. If WC-6wt%Co ball was used, increasing ploughing component and less deformation wear debris were observed in 38 at. % N content, which was responsible for the rise of wear rate. Despite the different wear modes, friction coefficients in both cases mainly depended on the formation of tribo-film and the amount of the h-BN phase. During sliding, the tribo-film acted as a protective layer between the sliding surfaces, causing a drop in wear rate. Moreover, chemical analysis revealed that oxidation of films occurred during sliding tests, and formed mainly Ti oxide of various types, TiO, TiO2 and Ti2O3, together with minor amounts of FeO in debris, which contributed to an appreciable effect on the increasing wear resistance. The above results revealed that the tribological properties of Ti-B-N coating were mainly affected by phase configuration and microstructure of coatings. e.g. the surface roughness, coating hardness, phase mole fraction (such as a-TiB2, a- hexagonal BN and nc-TiN). It was found that the h-BN phase could improve the wear resistance and reduce the friction after formation of the tribo-film between the contact interfaces. This result is beneficial in industrial application, but it is not possible to apply on machines which require a very clean environment because of the formation of tribo-film during operation.
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