First-Principles Investigation On Thermal Stability And Oxidation Resistance Of Cr-Al-N Hard Coating Of Cutting Tool

As a high-quality surface modification technology,the cutting tool coating plays a vital role in enhancing the overall performance of cutting tools.TiAlN coating has been widely used as one of hard and wear resistant coating on the surface of cutting tools.However,its thermal stability and oxidation resistance have not yet met the requriments of modern high-speed and dry cutting technologies.CrAlN coating is considered as one of the most promising alternatives for TiAlN due to its high hardness,high wear-resistance as well as excellent high-temperature oxidation resistance.However,some drawbacks of CrAlN coating limit its practical applications as follows.1)With increasing Al solubility,CrAlN coating will happen to structural transfermation from face centered cubic?fcc?to hexagonal close packed?hcp?structure,which will lead to the weakened mechanical properties of CrAlN coating.2)The CrAlN coating will happen to decompose for the decreased t hermal stability upon the high temperature over 900?.3)The high-temperature oxidation resistance of CrAlN coating still remains to improve in order to meet the requirements of modern high-speed and dry cutting temperature.In order to improve the thermal stability and oxidation resistance of CrAlN coating,the structural stability,thermal decomposition mechanism and their alloying effects of CrAlN coatings are investigated systematically using first-principles calculations based on density functional theory in this thesis.The detailed conclusions are given as follows:?1?For CrAlN coating,its crystal cell with fcc structure gradually contracted with increasing Al solid solubility,while the magnitude of lattice distortion became large firstly and then became small.The magnitude of lattice distortion was the largest when the Al concentration x was 0.5⁰.7,which provided the driving force for the precipitation of hexagonal close-packed?hcp?AlN compound.The phase structure stability of fcc-Cr_1-xAl_xN was gradually weakened with increasing Al solid solubility,and they preferred to decompose according to the paths of fcc-Cr_1-xAl_xN??fcc-CrN?+?hcp-AlN???hcp-Cr2N?+N2+?hcp-AlN?.The weakened phase structure stability of fcc-Cr_1-xAl_xN with Al solid solution should be attributed to the weakened covalentbond strength between Cr and N in Cr_1-xAl_xN crystal cell with increasing Al solid solubility.?2?Based on the understanding about the phase structure stability and thermal decomposition mechanism of CrAlN coating,the influences and rules of the fourth element M?M = V,Ti,Nb,Ta,Mo,Zr,W?doping on its microstructure,phase structure stability and thermal decomposition properties are further studied.The results show that the elements M are inclined to replace Cr atoms in CrAlN.After M doping,the lattice of formed coating undergoes the expansion with different magnititute.The doping of V,Nb,Ta or Mo is beneficial to increasing the phase stability of CrAlN coating,while the doping of Zr,W is quite different.The caculated decomposition enthalpies according to the thermal decomposition path of fcc-CrMAlN ? fcc-CrMN + hcp-AlN show that the doping of V,Nb,Ta,Mo or W will help improve the thermal stability of CrAlN coating.?3?In order to investigate the oxidation mechanism at high temperature of CrAlN coating so as to improve its oxidation resistance,the micro-reaction behaviors and mechanisms of CrN,CrAlN and CrMAlN coatings with O2.are further investigated systematically.The results show that O2 decomposes spontaneously in CrN,CrN or CrAlN coating surface,but the O-O distances are different.For CrN coating,O-O distance reaches its maximum after O2 decomposition,indicating CrN coating is susceptible to oxidation.For CrAlN coating,the O-O distance shortens as compared with that in CrN coating,revealing Al doping will help improve the oxidation resistance of CrN coating;After the fourth element doping,the O-O distance of decomposed O2 on CrMAlN surface is short than that in CrN and CrAlN coatings.Noticably,the O-O distance is gradually decreased in the order of V,Ti,Nb,Ta,Mo,W and Zr-doped systems,suggesting that the fourth element doping can further improve the oxidation resistance of CrAlN coating.