| As a kind of new hardness film, CrN coating has superior hardness, wear-resisting, chemical inertness with good corrosive resistance and high temperature oxidation resistances, therefore, it has been a wide industrial application in cutting tools, dies mold, decoration and so on as a protective coating. In this study, CrN coating was deposited by using Arc Ion Plating, and deposition technological parameters such as nitrogen partial pressure, bias voltage and pre-treatment process were systematically investigated, and its corrosion behaviors were tested in three typical industrial medium at room temperature. Finally the high temperature protective performances of CrN coatings which were deposited on two kinds of titanium alloys such as y-TiAl and O phase TiAlNb were also studied. The results are shown as following:For preparation of CrN coating by using arc ion plating, when N2 partial pressure was lower, the phase of the layer was Cr and Cr2N, and the surface morphologies of CrN coating was rough and had big melting droplet. With the increasing of content of N2 partial pressure, the intense of diffraction peak of Cr and Cr2N decreased whereas the intense of the phase CrN increased ultimately the atom ratio of N and Cr nearly approached 1:1. The macro-particle on the surface of coating decreased and the surface tended to become smooth. With N2 partial pressure continuously increasing, the macro-particle on the surface of coating increased. When no bias voltages were employed, the priority orientation was CrN (220). By varying substrate-bias, the melting droplet diameter and density on CrN coating surface decreased with the increasing of substrate-bias. The signal of CrN (200) became strong. A higher voltage applied to the substrate, macro-particles emerged again. Both polishing and peening could reduce the formation of large particles on coating surface, and polishing prompted (220) preferred orientation, otherwise peening prompted (200) preferred orientation. The best preparation process was on condition of 0.8 Pa nitrogen partial pressure,-300V bias voltages, peening pretreatment. The deposition speed was 10μm/h, and micro-hardness of CrN was 2400HV. The result of friction wear test showed the coefficient of friction of coating was about 0.6.Corrosion electrochemical behaviors of CrN coating in 3.5%NaCl,10%H2SO4,10%NaOH solution were studied by potentiodynamic polarization and electro-chemical impedance spectroscopy (EIS) at room temperature. The results showed corrosion potential of CrN coating in three kinds of solutions shifted to positive comparing with SS304, and potentiodynamic polarization curves showed that the CrN coating had a lower corrosion current density than that of SS304, which resulted in lower corrosion tendency. In the 3.5%NaCl solution, potentio -dynamic polarization curves of SS304 substrate appeared very slight passivation features, the CrN coating had obvious passivation phenomenon. In the 10%H2SO4 solution, both substrate and CrN coating showed apparent passivation features and appeared the second passivation features. In the 10%NaOH solution, polarization curve of CrN coating appeared active dissolved phenomenon, substrate still appeared passivation features, however, CrN presented a lower corrosion current density than that of SS304 excepting for its passive range of SS304 (52 mV-473 mV). Additionally, Electrochemistry impedance spectra (EIS) and Bode plots before and after potentiodynamic polarization tests of CrN coatings in three medium showed a single capacitive impedance arc, indicating that corrosion process was controlled by an electrochemical process. Charge transfer resistance Rt of CrN coating had a larger value than that of SS304 in three kinds of medium, indicating CrN coating had a lower corrosion rate. After potentiodynamic polarization tests, EIS tests were performed for evaluated re-passivation of the coating. The results showed that the coatings had superior re-passivation capability.Isothermal and cyclic oxidation experiments showed that CrN and CrN/Cr coatings could significantly decreased weight gain comparing with y-TiAl and O-TiAlNb at 800℃and 900℃. Whether in isothermal oxidation or in cyclic oxidation, the surface of two kinds of high temperature titanium alloy substrate all formed mixed non-protective oxidation scales, and it appeared visible cracks and spallings, especially severely in cyclic oxidation. However, when CrN and CrN/Cr coatings were coated on the substrates, the coating exhibited excellent oxidation resistance because a single, compact, protectiveness chrome oxide was formed on the surface of coating during oxidation process, which protected the coating from severe oxidation, therefore, the substrate alloys could be protected by the two coatings. When a 5μm pure Cr diffusion barrier coating was employed between CrN coating and substrates, inter-diffusion between CrN coating and Ti alloys substrates were effectively inhibited. |
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