| The transition metal carbides(TMC)and nitrides(TMN)films,as hard protective films,have been widely used in aerospace,machining tools,geological exploration,mold industry and other industrial fields due to their high hardness,high melting point,wear resistance,corrosion resistance,excellent thermal and chemical stabilities.With the rapid development of manufacturing technologies and the increasing environmental complexity of material application,higher requirements are put forward for improving the comprehensive properties of the transition metal carbides and nitrides films,and it is necessary to further improve the hardness,toughness and wear resistance of the films.Generally,the increase in hardness of films will increase the brittleness and weaken the toughness.Hence,it is important to do some research on strengthening and toughening design of the improvement of hardness and toughness of the transition metal carbides and nitrides films.Furthermore,it is a very important method for reducing the coefficient of friction of the films by activating tribological reaction to form self-lubricating tribo-films during tribological process.Historically,doping or alloying was a very common approach for tailoring the comprehensive properties of the transition metal carbides and nitrides films,but the mechanism of doping of rare earth and noble metal has not yet been elucidated.In this paper,we designed and prepared the films by alloying Au and Ce into transition metal carbides and nitrides films with magnetron cosputtering equipment for the purpose of high hardness,high toughness,wear resistance and low friction.The microstructure,mechanical and tribological properties of these films were explored.The main conclusions of this paper are summarized as follows:1.Au was doped into TaC and TaN by magnetron cosputtering,and prepared the Ta-Au-C and Ta-Au-N films,respectively.The effects of Au content on the microstructure,mechanical properties and tribological properties of the films were investigated.(1)Firstly,for Ta-Au-C films,substitutional solid solution have been formed due to Au occupy the Ta sites in TaC sublattice.At R=0.03,Ta-Au-C film reaches the maximum hardness value of29.3 GPa and the best toughness at the same time,and this means that the strengthening-toughening has been achieved;moreover,the friction of coefficient was also reduced and the wear resistance was yet improved.Both the experimental and theoretical calculations showed that the introduction of a low content of Au can tailor the valence electron concentration(VEC)of the solid solution,and improve the toughness of the films.Moreover,Au as a weak carbon forming(WCF)metal,can accelerate the friction oxidation reaction of TaC and release of C to form a lubricated C+TaO_x transfer layer during tribological process.(2)For Ta-Au-N films,TaC/Au nanocomposite structure has been formed due to Au existence as precipitated Au nanoparticles;the addition of Au can inhibit the growth of TaN grains,which makes the grain refinement of the films.When film was doped with low content of Au(R=0.03),the film grain size was about 21 nm,achieving strengthening-toughening target with maximum hardness of 27.7 GPa and improved toughness,together with best wear resistance and lower friction coefficient during sliding in oil.During sliding in PAO10 oil for Ta-Au-N nanocomposite films,the amorphous carbon lubricating transfer layer can be formed via decomposing C from the olefins in the lubricating oil,achieving a low coefficient of friction in an oil medium environment.2.Nb-Ce-N films were fabricated by doping rare earth Ce atoms into NbN,and the mechanical,tribological properties and surface wettability of Nb-Ce-N films were evaluated.The microstructure of the Nb-Ce-N films were characterized,and we found that the substitution solid solution Nb-Ce-N was formed due to the occupation of partial Nb sites by Ce atoms.The increase of hardness was attributed to the lattice distortion caused by larger Ce atoms in Nb-Ce-N lattice,which limited the dislocation motion.The addition of Ce tailored the valence electron concentration(VEC)of the solid solution structure,thereby yielding the reduced shear resistance and enhanced toughness.At R=0.02,the strengthening-toughening of the films has been achieved,coupled with enhanced wear resistance and reduced friction coefficient.The Ce doping in the NbN film not only retains its mechanical properties,but also induces the surface wettability transformation from hydrophilicity to hydrophobicity via triggering self-oxidation to formation of hydrophobic cerium oxide groups on the surface that inhibits formation of hydrogen bonds from water molecules.It is a new way to control the hydrophobic surface based on the transition group metal nitride film,which can withstand some harsh complex environments.This design can not only improve the mechanical properties and wear resistance of the films,but also greatly enrich the application range of the films by tuning the wettability. |
PDF Full Text Request