Research On Microstructure And Properties Of TiB₂ Super-hard Thin Films Deposited By RF Magnetron Sputtering

In recent years, nanocrystalline TiB2 thin films have an increasing need in the fields of materials science to be devoted to wear resistant coatings. The main synthesis technique is the low pressure magnetron sputtering system because it can improve the coating properties through the control of ion bombardment during growth. The ion bombardment is a strongly non-equilibrium process which controls the growth of the film by the energy delivered to the growing film by condensing and bombarding particles. Energetic particle bombardment modifies the substrate surface or influences the nucleation and growth of the depositing film material. The use of energetic particles (ions and atoms) has become increasingly important in magnetron sputtering deposition techniques.The cross-section morphology, crystallographic structure, preferred orientation, chemical composition and B/Ti ratio of the films obtained were characterized by field emission scanning electron microscopy (FESEM), the grazing incidence X-ray diffraction (GIXRD), synchrotron radiation and Auger electron spectroscopy (AES), respectively. And the mechanical properties of the thin films have been examined also.It was found that the (001) crystal orientation is characteristic of the films irrespective of the deposition process in our work. Using the nanoindentation testing, the hardness and ratio H3/E*2 values of the films are 50 GPa and 0.65 GPa under—50V bias. With increasing the negative bias voltage, the grain size decreases, the hardness and resistance to plastic deformation (H3/E*2) decrease as well. For further characterization of the mechanical properties of the coatings, we conducted extensive investigation on their sliding friction and wear behavior at room temperature without lubrication. It was found that the coatings demonstrate superior wear resistance, which can be indicated by the close adherence of the coatings to the substrate even after 60 min severe sliding friction.The experimental parameters were selected in light of Thornton's model in order that the deposited films were located in zone T. Therefore, the homologous temperature was pointed to guarantee that all the deposits finish with the ratio of Ts/Tm between 0.1 and 0.2. Fracture cross sections in SEM images show that the growth rate of films varies from 0.09 to 0.06 nm/sec, which is depending on the negative bias applied to the substrates. The SEM analysis shows that the substrate temperature has a strong effect on Zone T structure. It was found that the TiB2 thin films were overstoichiometric at lower substrate temperature and the diffusion of Ti and B atoms on the substrate surface was greatly improved at 350℃. At this temperature, the value of B/Ti ratio is approximate to the normal stoichiometric proportion of TiB2. Moreover, a new dense structure, named "equiaxed" grain structure was observed by FESEM at this substrate temperature. Combined with FESEM and AES analysis, it was suggested that the "equiaxed" grain structure was located in Zone 2 at the normalized substrate temperature as low as 0.18.The structural feature of TiB2 thin films deposited by magnetron sputtering is the preferred orientation, its close-packed (001) planes tend to grow parallel to the substrate surface. Therefore, the TiB2 thin films usually exhibit a strong out-of-plane (fiber) texture. This fiber texture can be explained by the evolutionary-selection model from van der Drift. The grazing incidence X-ray diffraction (GIXRD) technique is widely used in the crystallographic structure determination during thin film examination. However, during the GIXRD examination, the direction of the diffraction vector for the textured crystals is not the growth direction of grains. We understand that the crystal orientation of the textured grains could be regarded as random in the growth plane because the orientation distribution of the textured grains are characterized by the axial rotational symmetry, that is rotated around the sample surface normal. However, the van der Drift's model originated from the typical case (or axial rotation) of texture characterization in thin films and then was used to examine the actual preferred-orientation tendency in the thin film. Moreover, the actual textured grains are preferentially oriented along, not only the normal axis direction (or typical case) but also the deviations from the axis direction. Hence those preferred-orientation planes with their diffraction vectors deviation from the normal can be observed by the grazing-incidence x-ray diffraction method. At the same time, their diffraction intensity will change when GIXRD is performed at different incidence directions because they are no longer randomly oriented. To keep this in mind, we designed the three different directions relative to the substrates during the GIXRD study. According to the GIXRD results in the three different directions, we used Knuyt's model for texture evolution in PVD thin film growth, to explore in detail and explained the problems of the preferred orientation during the GIXRD interpretation.To study the lateral structure of the thin film including lattice constant, grain size and in-plane direction of the information, we use the synchrotron radiation to test the out-of-plane (fiber) texture by the ordinary XRD and Grazing incidence in-plane X-ray diffraction (GIIXRD) mode, respectively. Synchrotron radiation allows us to focus on the effects of ion bombardment on the preferred orientation of grains, combined with knuyt's model, the microscopic characteristics of grains in the TiB2 thin film with preferred orientation could be presented through a very vividly depicture.