Elaboration Of Nano-composite Coatings Using Sputtering Processes With Application To The Substitution Of Electrolytic Cr Coating

Chromium is widely used in many applications;however,there is increasing pressure on electroplated chrome industry.This research focuses on the synthesis and characterization of CrN based multilayer coatings for the replacement of electrolyplated chrome.The studied materials include the chromium nitrides coatings,Cr-Zr-N ternary multiple phase and multilayer coatings and Si added CrN/ZrN multilayer coatings.Because of the diversity of the applications of the electroplated chrome,in this thesis,the objective is to demonstrate the controllable coating structures and properties based on the magnetron sputtering technology so that the coating performance can be optimized to satisfy the replacement of Cr in industry scale.The depositions are accomplished by a conventional magnetron sputtering system. Coating structures are characterized by various techniques such as glow discharge optical spectrometer(GDOS),X-ray diffraction(XRD),scanning electron microscope(SEM), transmission electron microscope(TEM).The mechanical,tribological and corrosion properties of the deposited coatings are evaluated using nanoindentation,scratch test, Pin-on-disk and dynamic polarization techniques respectively.The investigations of chromium nitrides with an increased nitrogen flow rate show that their phase structures evolute from a single bcc-Cr phase to Cr+Cr₂N(or Cr(N)), stoichiometric Cr₂N,the mixed Cr₂N + CrN and stoichiometric CrN.The hysteresis curve and target voltage versus nitrogen flow rate curve are used to predict the chromium nitride depositions.The stoichiometric Cr₂N,CrN and the nanocrystalline Cr(N) solid solution with a <200> preferred orientation demonstrate the higher hardness of nearly 25 GPa,which in turn result that they performed better to wear resistance.The abilities of corrosion resistance of the chromium nitride coatings in a 3.5%NaCl solution are a-CrN＞Cr＞Cr₂N＞CrN＞AISI 304. Optical emission spectroscopy(OES) analysis for various RF bias reveals that the increased substrate bias result in the decrease of the nitrogen content in the coatings;thus the evolution of structures from stoichiometric to N deficiency CrN.The optimized RF bias is-60 V for CrN coating deposition.In Cr-Zr-N ternary system,Zr modified CrN coatings with various Zr contents and nanoscale multilayer CrN/ZrN coatings with the bilayer thickness ranging from 11.7 to 66.7 nm are prepared.The Zr modified CrN coatings form a nanocrystalline solid solution structure.The maximum hardness value of these coatings is approximately 24 GPa.These coatings show excellent abrasive wear resistance.The additions of Zr into CrN improve their chemical inertness.For CrN/ZrN multilayers,it is found that they consist of nanocrystalline CrN and ZrN individual layers.The multilayer coatings present a constant hardness of 29 GPa in spite of the variations of their bilayer thicknesses.Their friction coefficients and wear rates increase with decreasing the bilayer thickness.Good chemical inert and very low corrosion current densities are obtained in the corrosion tests.A further effort to add Si into ZrN layer in CrN/ZrN multilayer is carried out.The CrN/ZrSiN multilayers with a bilayer thickness from 11 nm to 153 nm form the nanocrystalline/amorphous period structure.They present almost a constant hardness of 30 GPa and elastic modulus of 350 GPa.In addition,the multilayer structure shows a good combination of high hardness from ZrSiN and excellent toughness from ZrN.The additions of Si into ZrN individual layer have been proved to be an efficient way to inhibit pitting corrosion.