Investigation On The Strengthening-toughening,Oxidation-and Corrosion-resistance Properties For N-rich Zr₃N₄-based Films

Transition metal nitrides（TMNs）films have been widely utilized in aerospace,automotive industry,mechanical processing and other fields as surface protective coating,due to their high hardness,excellent corrosion-and oxidation-resistance properties.The harsh and complex service environments put forward higher requirements for the application of materials performance,and thus the design of materials in accordance with a need and precise control of service performance is the inevitable development tendency of advanced materials.The hardness and fracture toughness of the protective films determine its service life,while the thermal stability,oxidation-and corrosion-resistance are also vital to broaden application scenarios and improve durability.In order to adapt the increasingly harsh service environment,higher requirements are put forward for the toughness,oxidation-and corrosion-resistance of the protective films,and it is urgent to seek new nanostructured materials with excellent comprehensive properties.The TMNs with Na Cl-structure have been extensively studied,in general,alloying and building nanomultilayer structures are effective strategies to improve their mechanical properties,oxidation-and corrosion-resistance.Recent years,N-rich TMNs have attracted widespread attentions due to their peculiar electronic structure and excellent properties,which make them regard as a new type of protective films.As a typical N-rich TMNs,Zr₃N₄ possesses complicated crystal structure and high coordination number,high oxidation state(Zr⁺⁴)and low charge transfer ability,which render it as a potential hard protective material with high environmental tolerance,such as high temperature,corrosive environments.Learning form the research progress in rocksalt-structure TMNs,it is expected that alloying and constructing nanomultilayered architecture can also be utilized to regulate the microstructure and properties of N-rich Zr₃N₄films,however,the related research is still in its infancy.Based on the above analysis,we adopt N-rich Zr₃N₄ as the research object,and employ the routes of alloying and constructing nanomultilayered architecture to regulate the microstructure of Zr₃N₄ films using magnetron sputtering technology,further observing their effects on strengthening-toughening,oxidation-and corrosion-resistance properties of Zr₃N₄-based films.Firstly,the influence of Hf alloying on the microstructure,strengthening-toughening behavior,oxidation-and corrosion-resistance properties of Zr₃（Hf）_xN₄ films are explored.Subsequently,high-crystallinity Ti Al N and low-crystallinity Ta N are respectively employed as templated layers to construct Ti Al N/Zr₃N₄ and Ta N/Zr₃N₄ nanomultilayered films for improving their mechanical properties.Specifically,the Zr₃N₄-nanolayer-thickness(l _Zr3N4)dependent strengthening-toughening and fracture behavior in Ti Al N/Zr₃N₄ nanomultilayers and strengthening-toughening behavior,oxidation-and corrosion-resistance properties in Ta N/Zr₃N₄ nanomultilayer are investigated.The main conclusions of this thesis are summarized as follows:1.By adjusting the Hf target current during co-sputtering process,the effect of Hf content on the microstructure,strengthening-toughening,corrosion behaviors and thermal stability of Zr₃（Hf）_xN₄ films have been investigated.The Hf content in the Zr₃（Hf）_xN₄ film gradually raised from 15.7 at.%to 29.8 at.%with increasing Hf current from 0.2 A to 0.5 A,mainly forming solid-solution structure.At low Hf content（15.7at.%Hf）,Hf atoms are dissolved into o-Zr₃N₄ to form solid-solution o-Zr₃（Hf）₃N₄,resulting in grain refinement and formation of ultra-fine nanocrystalline o-Zr₃（Hf）₃N₄films;this ultra-fine nanocrystalline solid-solution structure yields enhanced hardness（17.9 GPa）relative to Zr₃N₄monolayer（15.8 GPa）and best fracture toughness suggested by invisible radial cracks.Further increment in the Hf content（≥20.2 at.%Hf）results in phase transition from o-Zr₃（Hf）₃N₄at 15.7 at.%Hf to c-Hf（Zr）N solid-solution that is same as Hf N monolayer.In which,the crystallinity of c-Hf（Zr）N films is significantly improved,and the hardness increase to 20.5-21.6 GPa,higher than the hardness of c-Hf N film（18.9 GPa）.Although the fracture toughness of c-Hf（Zr）N films is deteriorated as compared with o-Zr₃（Hf）₃N₄film,it is still better than o-Zr₃N₄ and c-Hf N monolayers.In the thermal stability test,it is found that the alloyed Zr₃（Hf）_xN₄films exhibit excellent phase-structure and mechanical stability at 800°C.In addition,the formation of solid-solution structure can obviously improve the corrosion-resistance ability of Zr₃（Hf）_xN₄ films,especially,formation of c-Hf（Zr）N at high Hf content provides both high corrosion potential and low corrosion current(E_corr=-0.1574V,I_corr=0.007μA/cm²,29.8 at.%Hf),showing superior corrosion-resistance properties.2.The Ti Al N/Zr₃N₄ nanomultilayer films are successfully fabricated by alternatively inserting Zr₃N₄ nanolayers onto Ti Al N nanolayers,and then the l _Zr3N4-dependent microstructure,super-hard effect and fracture behavior are investigated.When l _Zr3N4=1.1 nm,Zr₃N₄ nanolayers are forced to grow into metastable c-Zr₃N₄pseudocrystal structure under the template effect of c-Ti Al N sublayers and form good coherent epitaxial relationship with c-Ti Al N sublayers.This,in return,result in the abnormal hardness enhancement but with superior fracture toughness.However,at thicker l _Zr3N4（4.2 nm）,the coherent growth between Zr₃N₄ and Ti Al N nanolayers are gradually destroyed because of the phase transition of Zr₃N₄ nanolayers from the pseudocrystal c-Zr₃N₄ to bulk-energy-stabilized o-Zr₃N₄,thus yielding a drop in both hardness and fracture toughness.The indenter impression of the Ti Al N/Zr₃N₄nanomultilayer with l _Zr3N4=4.2 nm is determined by FIB to observe the deformation behavior.Even after significant plastic deformation at the indentation apex,the plastic deformation is primarily accommodated by appearance shear steps and bending of nanolayers;At the same time,partial film delamination and appearance of large amount of nanoscale longitudinal and lateral cracks are beneficial to effectively release high stress,avoiding presence of macro-cracks,finally providing both enhanced hardness and toughness in the Ti Al N/Zr₃N₄ nanomultilayers.3.In addition,the Ta N/Zr₃N₄ nanomultilayer films with different l _Zr3N4 are prepared,then the l _Zr3N4-dependent microstructure,mechanical properties,oxidation-and corrosion-resistance are investigated in detail.As l _Zr3N4 is small（2 nm）,adjacent Zr₃N₄ and Ta N nanolayers can form local c-Ta N/c-Zr₃N₄ coherent interface structure,which provide higher hardness and fracture toughness;Whereas at thicker l _Zr3N4（10nm）,Zr₃N₄ nanolayers gradually change to the bulk-energy-stabilized o-Zr₃N₄ structure,worsening the local c-Ta N/c-Zr₃N₄ coherent growth,resulting in a drop in both hardness and fracture toughness.The oxidation-resistance test identify that Ta N/Zr₃N₄nanomultilayers with relative thicker l _Zr3N4 possess more superior oxidation resistance at 600℃.When l_Zr3N4=10 nm,there is no sign of oxidation on the surface and cross-section.The reason for the improvement of oxidation-resistance is formation of Zr O₂passivation layer on the surface at high temperature,blocking the diffusion of oxygen into the interior.In addition,the corrosion-resistance ability of Ta N/Zr₃N₄nanomultilayer films are significantly improved compare to constituent Ta N and Zr₃N₄monolayers,achieving best corrosion-resistance ability at l _Zr3N4=2 nm.The introduction of high-density interlaminar interfaces can block the continuous growth of columnar crystals,reduce the internal defects,improve the density of the films,smooth the film surface to reduce the corrosion active sites and transform the vertical penetration of corrosive ions into lateral diffusion,which decline the diffusion rate and improve the corrosion resistance.