Investigation On The Microstructure,Mechanical Properties And Tribological Behavior For TaC-based Films

The group IVB-VIB transition metal carbide films（TMC）have been widely utilized in aerospace,automotive industries,machining-tool industries,geological drilling as surface protective films due to their excellent physical and chemical properties,such as high hardness,high melting point,low friction coefficient,wear resistance,corrosion resistance and oxidation resistance.Recently,as a research in highlight in the field of films,TMC films have attracted numerous attentions.In these applications,the mechanical and tribological properties of the films are of utmost importance,as they determine the performances and service lifetime in practical applications.Although ceramic films have high hardness,the intrinsic brittleness of ceramic films often limits their cutting and tribological applications due to spontaneous failure.Hence,it is indispensable to design and fabricate of superhard and high toughness films.Meanwhile,improving hardness and toughness can enhance the wear resistance of the film to prolong service lifetime.Adding a third elements into TMC films and building multilayer structure are common and effective methods to improve the properties of films,which provides an approach to achieve the best blend of the superhard,high toughness,low friction coefficient and wear resistance properties.Tantalum carbide（TaC）films exhibit excellent physical and chemical properties suggesting a potential use in the extreme environments.Therefore,in this thesis,TaC was taken as the research object,and the properties of TaC films were improved by adding a third elements and building multilayer structure.Firstly,the influence of C content on the microstructure and properties for Ta C films was investigated.Then,the N or Si atoms were incorporated in TaC films to form TaC-X（X=N,Si）,and the effect of N or Si content on the microstructure,mechanical and tribological properties was explored.In addition,by building multilayer structure,the TaC/SiC nanomultilayer films were deposited,and the dependence of microstructure,mechanical properties and tribological behavior on SiC layer thicknesses was investigated.The main conclusions of this thesis are summarized as follows:1.By changing the CH₄ gas flow rate,the effect of C content on the microstructure,hardness and tribological behavior of TaC films had been investigated.The C content of the films gradually increased from 30.3 to 74.3 at.%,along with a phase transition from hexagonal-Ta₂C through cubic-TaC to nc-TaC/a-C nanocomposite structure,while at the highest C content,the film only consisted of amorphous phase.The hardness of TaC films first increased,and then decreased after reaching the maximum.At 54.4 at.%C,the film had a small amount of a-C and achieved the highest hardness,while the film had a high friction coefficient and wear rate.With further increasing the a-C content in the film,the hardness of film rapidly decreased,but the film had a low friction coefficient and wear rate.Hence,in nc-TaC/a-C system,the tribological properties were strongly dependent on the a-C content in the film.Although more a-C in the film can improve the tribological properties,it will deteriorate the mechanical properties.2.In order to fabricate film with low friction coefficient and high hardness,the TaC_xN_y films were deposited by incorporating N atoms into TaC film.The influence of N content on microstructure and properties were primarily investigated.With increasing of N content,all N atoms occupied the C vacancies in TaC lattice to form NaCl-type Ta（C,N）solid solution.The hardness and toughness of films first increased,and then decreased after reaching the maximum at 17 at.%N.The hardness of the film was mainly controlled by the valence electron concentration（VEC）,and the VEC can be taiored by N content in the film.At 17 at.%N,the film achieved the optimal VEC,resulting in the maximum hardness.The wear resistance was strongly dependent on hardness and fracture toughness of films.High hardness and fracture toughness can resist for abrasive wear and plastic deformation,respectively.The film with 17 at.%N reached the lowest wear rate caused by high hardness and fracture toughness.The friction coefficient decreased as the increased of N content,due to the introduction of N reducing the oxidation temperature of the film and opening the tribochemistry reaction.The tribochemistry reaction was identified as TaC_xN_y+O₂→TaO_x+C+N₂/NO_x,resulting in forming tribofilms comprised of TaO_x and graphitic clusters and reducing the friction coefficient.3.Moreover,the Si atoms also were incorporated in TaC film to deposite Ta-Si-C films.The effect of Si content on the microstructure,hardness,fracture toughness and tribological behavior were investigated.At low Si content（≤5.6 at.%）,the C vacancies in the TaC lattice were occupied by Si to form Ta（C,Si）solid solutions.With increasing of Si content（13.7 at.%²0.8 at.%）,the C sublattice vacancies in the TaC lattice were gradually filled and then resulted in forming of amorphous phase（a-C,C-Si）to form Ta（C,Si）/（a-C,C-Si）nanocomposite films.At the highest Si content（30.8 at.%）,the film only consisted of amorphous phase.The effect of Si content on the hardness was similar to N.At 5.6 at.%Si,the film exhibited the maximum in hardness and fracture toughness,which can be ascribed to the formation of a solid solution.With further increasing the Si content,the amorphous phase gradually appeared,leading to a decrease in hardness and fracture toughness.The wear rate was dependent on hardness and fracture toughness of films.Thus,the film obtained the lowest wear rate at 5.6 at.%Si,which was attributed to the highest hardness and fracture toughness.With increasing of Si content,the friction coefficient decreased from 0.29 to 0.19,which can be ascribed to the a-C content in the film and the formation of lubricious oxide on the surface of wear tracks.4.In addition,by building multilayer structure,a series of TaC/SiC nanomultilayer films with different SiC layer thicknesses(t_{Si C})were prepared and the t_SiC-dependent structure,hardness,fracture toughness and tribological behavior were explored.At low t_SiC（<1.2 nm）,the SiC crystallized and grew coherently with TaC layers under the template effect of TaC layers.Maximum hardness and toughness of46.06 GPa and 4.21 MPa m^1/2 were achieved at t_SiC=0.8 nm with good coherent interface.With further increasing of t_SiC,SiC layers partially transformed to an amorphous structure and gradually lost their coherent interface,leading to a rapid drop in hardness and fracture toughness.At t _SiC=0.8 nm,the film achieved the lowest wear rate,because the SiC crystallized and grew coherently with TaC layers,achieving the highest hardness and fracture toughness.Meanwhile,the friction coefficient in multilayers was reduced when compared to the individual monolayers,which was attributed to the formation of lubricious Ta silicate（TaSiO_x）on the surface of wear tracks during sliding.