The Mechanical And Anti-corrosion Behavior Of Inorganic Thin Films

The coating technology is very important and applied widely in industry due to the excellent performance of film, such as good resistance to wear, corrosion, heat, and excellent optical, thermal, magnetism and electrics properties. The advances made in thin film drive the expansion of photoelectronics, computer, microelectronics, sensors and information industry and provide high-tech materials and devices for mechanics, energy sources, traffic and national defence department. In this thesis, the film deposition technique, the mechanical and anti-corrosion behaviors of films are studied by analyzing the BTO, DLC and Fe-P films. Some interesting results are obtained. Thin BTO films were deposited by magnetron sputtering on Si and glass substrates. The thickness of the BTO layer was varied between 35 nm and 1 μm. The stress, dielectric permittivity, and ferroelectric hysteresis loop were measured after deposition and during heating. The combined effect of film thickness and stress on BTO ferroelectric behavior was studied. The thermal expansion and bilayer-modulus were calculated by two methods. It is found that the BTO films are under tensile stress at the Curie temperature. The tensile stress in BTO films increases with decreasing film thickness leading to a shift of the Curie point to a lower temperature, a decrease of remanent polarization, and an increase of the coercive field, especially obvious as the film thickness lower than 250 nm. The effect of tensile stress on the Curie temperature is dominant within a restricted range of stresses between 300 and 450 mega Pascals(MPa), where the Curie temperature decreases linearly by about 0.16℃/Mpa. A good correlation is found among the stress, dielectric and ferroelectric measurements with respect to the temperature of the Curie point, which is determined based on identifying changes in the slope of film stress versus temperature plot, the peak characteristic of dielectric permittivity and the shrinkage of hysteresis loop during heating. The ferroelectric hysteresis loop shows that the ferroelectric-paraelectric phase transition starts at the Curie temperature of the BTO films and continues within a range of temperatures until about the Curie temperature of the bulk BTO material. The ferroelectric behavior indicates the presence of various types of domains. The ferroelectric domains within the interior part of the BTO film have a dominant effect on its ferroelectric behavior, while the contribution of surface domains is relatively small. The biaxial modulus and expansion coefficient of BTO films were determined based on the change of stress in BTO films and of the volume of BTO unit cell due to phase transition or the stress slope of BTO films on Si and glass substrates versus temperature plot. The biaxial modulus of BTO film is little higher than that of bulk BTO materials, while the expansion coefficient of BTO films is little lower than that of bulk BTO materials. To improve the adhesion strength of film on substrate, a dual ion-beam bombardment and mixing (DIBM) technique was developed and employed to prepare diamond-like carbon (DLC) films on steel substrates. The mechanical performance and anti-corrosion behavior of DLC films were studied. The results show that DIBM technology not only increases the compound hardness of DLC film and steel substrate, but also leads to an energy shift corresponding to the maximum microhardness to lower ion energy. By implanting with medium energy Ar ion beam and introducing a mixed interface, DIBM technology improves the anti-wear performance and adhesion strength of DLC film, the critic load of DLC films prepared by DIBM + IBD increases by more than a factor of two than that by IBD. Also, the DLC films increase the stability and reduce corrosion rate of substrates in corrosive solution by functioning as a physical barrier and restraining from anodizing. In addition, the DLC films improve the steel substrate resistance to pitting corrosion remarkably. Thus, the DLC film provides a favorable protection for steel to be applied in corrosion environments. Also studied in the thesis was the effect of bath composition and process on the deposition rate and corrosion behavior of electroless Fe-P deposits by weight and electrochemical method. During the deposition process, the copper substrate was coupled with aluminum foil, which shifted the potential negatively and decreased the resistance for cathodic and anodic reaction, thus induced the Fe-P deposition. The reaction orders of bath composition and the activation energy were determined by plotting the logarithm of deposition rate versus the logarithm of concentration of bath and reciprocal of temperature, respectively. And the reaction orders of NaH2PO2,FeSO4, H+ and the activation energy are 0.34, 0.18, 0.13 and 4.425 Kcal/mol, respectively. The deposition rate determined by electrochemical method is higher than that by weight method due to the other reactions, such as hydrogen evolution. Compared with the corrosion rate in NaCl solution, the corrosion rate in acid solutionis large. In the polarization curve of Fe-P deposits in NaOH solution, there are several current peaks, suggesting the appearance of new reaction or the change of surface state. The cycle voltammetric behavior at different scan rate of the deposits in NaOH solution shows that the oxidation reaction is irreversible.