Microstructural Characteristics And Influence On The Mechanical Properties Of Superhard Nano-Scale Thin Films

Superhard coatings having a hardness exceeding 40GPa have been successfully used for protection of materials and particularly to enhance the life of cutting tools since the 1970s. Crystalline diamond has been regarded the most hard materials for many years. But Liu and Cohen theoretically predicted in 1989 the existence of a new superhard compound , carbon nitride (CsN^). This compound has been of great interest to the field of materials science and engineering.TiN/CNx multiplayer composite superhard coatings has been prepared using a closed-field unbalanced magnetron sputtering(CFUBMS) by the group of Fan in Wuhan University. They had used the X-ray diffraction(XRD) and transmission electron diffraction to prove whether the carbon nitride is crystal. But the microstructural characterisation and mechanical properties of this kind of superhard coatings has not been researched before.In the present work, the composition, microstructural characteristics, morphology of fracture surface and mechanical properties of TiN/CNx superhard thin films have been researched systematically. On the base of above anlaysis, the mechanism of growth of this coatings and the influence of all major parameters on the mechanical properties are discussed, which will be beneficial for the preparation of good coatings.The dissertation consists of nine Chapters. From Chapter One to Chapter Three briefly introduce superhard thin films, carbon nitride thin film and TiN/CNx thin film, mainly about preparations, present research and application of superhard thin films. Chapter Four gives the purpose and major work of the project. Chapter Five is the materials and experimental procedures.From Chapter Six to Chapter Eight, the microstructural characteristics, composition and mechanical properties of TiN/CNx superhard thin films have been studied systematically by using scanning electron microscopy(SEM), X-rayenergy dispersive spectrometer(EDS) and X-ray diffraction (XRD). Some conclusion can be drawn. The mechanism of growth of this coatings is put forward: when the temprature of substrates and the negative bias is high enough, the growth of the coatings is layer by layer, so the coatings is thin and integrity and a high hardness but a poor adhesion; on the contrary, the growth of the coatings is nanocrystalline- amorphism, so the coatings is not a lot thin and has much defects and a low hardness but a good adhesion. The pressure of nitrogen of all coatings is not enough to form TiN. Ti2N was formed in TiN/CNx thin films. And the measurable preliminary ion bombardment is beneficial for adhesion .In chapter 9, the modulation period, A,where the hardness reaches the maximum, has been discussed. A lot of research data about multilayed nanometer scaled superhard films analyzed, a conclusion can be drawn that this period is affected by the lattice constant and the preferred orientation, and theymeet a formula quantitatively: d: average crystal plane space; the mismatch;...