Abstract
Ti-B-C-N super-hard coatings are of increasing interest as wear resistant coating system for bearings, blades of steam turbine, cutting tools and forming tools, due to their excellent combination of physical, chemical, and mechanical properties, and so on. In this thesis, the developing progress of monolayer and nanocomposite coatings in Ti-B-C-N is reviewed, and then several common deposition methods of coatings are simply introduced.Besides, detailed description is made on the principle of radio-frequency magnetron sputtering as well as reactive sputtering.
TiB2 coatings were prepared by radio -frequency magnetron sputtering technique on the substrates of steel and silicon. The cross-section morphology, microstructure,compositions and chemical states of the coatings obtained were characterized by field emission scanning electron microscopy, glancing incidence X-ray diffraction and X-ray photoelectron spectroscopy, respectively. It was found that the TiB2 coatings show a smooth surface and dense nanostructure with a preferential crystalline orientation of [001],the origin for which is discussed from the thermodynamic view. The hardness and residual stresses of the films were also examined. It was found that TiB2 coatings exhibit considerably high hardness while showing a significantly low compressive residual stress.
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 120min severe sliding friction. However, the properties of TiB2 coatings will be degraded by theformation of titanium and boron oxides during the deposition process.
The effect of the sputtering bias voltage on microstructure and properties of TiB2 coatings was also investigated. The results show that the sputtering bias voltage shows no remarkable effect on the microstructure of the TiB2 coatings but significantly enhances their adhesion to the substrate, up to the maximum value of 16N under –100V, and markedly improves the wear resistance of the coatings, which can be indicated by the drastic decrease in friction coefficient from 0.52 under absence of bias voltage, down to 0.14 under the application of –150V bias voltage. Moreover, it was found that the mechanical properties of the coatings can be further enhanced by moderately increasing their thickness, whose hardness determined by Nano-indentation Test can reach 46GPa with the thickness of 1µm, indicating achievement of high-quality coatings.What’s more, another kind of coatings with composition of Ti-B-N was prepared by reactive magnetron sputtering technique on steel. Comparative study was undertaken on the structure, morphology, hardness and wear resistance of the TiB2 , Ti-B-N coatings which deposited without bias voltage. It was found that a very smooth surface of Ti-B-N coatings can be obtained by introduction of 10℅ nitrogen and the structure of the coatings translates correspondingly from columnar nanostructure into amorphous structure.
But the hardness of Ti-B-N coatings decreased. Sliding friction and wear resistance measurements revealed that the friction coefficient of the coatings formed under the atmosphere of N2 is moderately decreased, but the wear resistance doesn’t obtain any improvement. Actually, we observe the shelling off of the coatings after the test.We also deposited Ti-B-N coatings with the optimized preparing process variables. It was found that with the –100V bias voltage and decreased of N2 flux, the structure of the coatings transformed from amorphous to crystalline structure, the wear resistance and adhesion of coatings are excellent which can be indicated by the value of adhesion and the friction coefficient. The effect of the flux of N2 on the properties of Ti-B-N coatings is minor when the flux is not more than 4sccm.
Key words: radio -frequency magnetron sputtering; TiB2 coating; Ti-B-N coating;microstructure; adhesion; hardness;
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