| Transition metal nitrides(MN)have been ubiquitously used in the field of mechanical industry,aerospace,transportation,energy environment and electronic information due to their outstanding mechanical,electrical,thermal and optical properties.Especially as hard film materials,they had caused an extensive technical revolution in mechanical processing industry in the 1980s and now are still the preferred materials for coating reinforcement modification of most tools and components.Because almost all the MN have simple phase structure with large solution ranges,the strengthening mechanisms i.e.,correlations between the composition,structure and performance of MN films are always the research focuses.However,due to the limitations of preparation equipment,technology,detection means and other factors,the exploration of their strengthening mechanisms is always limited to meso scale,such as grain refinement,preferred orientation and residual stress,while little deeper and thorough understanding.It is one of the most important fundamental problems left over from decades of researches that how to explore strengthening mechanism of the MN films with wide solution ranges intrinsically and how to clarify the performance limits and achieve the limits through composition design.Therefore,further exploration on strengthening mechanism of the MN films with wide solution ranges is of great importance and necessity.This paper aims to explore and clarify the atomic-scale strengthening mechanism of the MN films with large solution ranges,and then design and prepare new single-layer superhard film material based on it.It will contribute to enrich the database and perfect basic theory of hard film materials.The autonomous design enhanced magnetic filtering pulsed bias arc ion plating(PBAIP)was utilized in this paper.Its arc source structure has been improved according to the physical characteristics of arc plasma by adding the two-stage magnetic coils to constrain the arc spot combustion and control plasma transmission,and then enhance the filtering capability of large particles.High-quality binary and ternary films,including TiNx,ZrNx,HfNx,(TixZr1-x)Ny and Tix(CyN1-y),were deposited on the Si(100)and high speed steel substrates after the careful design of macroscopic experiment parameters based on the influence of factors such as pulsed bias on deposition quality.And the correlations between compositions,structures and properties of these films were investigated systematically.In this paper,the influence of meco-scale factors was weakened as far as possible and the composition dependent hardness enhancement was attributed to different atomic-scale chemical bonding states and electronic structures.Furthermore,the single-layer superhard quaternary(Zr,Al)NO film with hardness up to 41.2 GPa was successfully designed and prepared based on the atomic-scale strengthening mechanism.Key points and results are listed as follows:1.High-quality binary single-phase MNx films,including TiNx(x=0.75-0.99),ZrNx(x=0.74?0.95)and HfNx(x=0.67?0.89),were successfully deposited on Si(100)substrates varying the nitrogen flow rate using the method of enhanced magnetic filtering PBAIP.And other deposition parameters such as deposition pressure,temperature,pulsed bias,deposition time,were precisely controlled to weaken the influence of conventional meso-scale factors as possible.Three groups of MNx films all exhibited stable Bl-NaCl single-phase structure and kept the preferred orientation unchanged within large composition ranges.The thicknesses,grain sizes and residual stresses of the same kind of MNx films with different nitrogen contents were also controlled effectively and remained stable.Both the hardness and elastic modulus of the MNX films were found highly depended on their composition and the maximums were obtained at the similar stoichiometry point near x=0.82:32.3 GPa and 433.6 GPa at TiN0.82,29.6 GPa and 392.1 GPa at ZrN0.784,29.0 GPa and 309.2 GPa at HfN0.82.The nitrogen content dependent mechanical enhancement of the three MNx films in this paper was attributed to the filling of covalent pda bondings and metallic dda bonding states by the valence electrons,and also the hardening effect of nitrogen vacancy in the MNx lattice,rather than the conventional meso-scale strengthening mechanisms,such as texture harding,grain refinement and residual stress reinforcement.The covalent pdd? bonding states formed by M electrons and N electrons can effectively resist shear deformation and will give a positive contribution to the hardness and elastic modulus,while the metallic dd? bonding states will give a negative contribution.And the electrical properties of ZrNx films are investigated by Hall measurements system as the assisted evidence of filling of the electron band by valence electron.The best performances of the three MN.films were obtained at the stoichiometry points where the valence electron concentrations(VEC)were 8.10,8.20 and 8.10 respectively,all near 8.15.2.High-quality ternary single phase(TixZr1-x)Ny(x=0.49?0.51,y=0.70?0·86)and Tiy(CyN1-y)(x=0.64?0.80,y=0.37?0.55)films were successfully deposited on Si(100)and high speed steel substrates by varying the nitrogen flow rate using the method of enhanced magnetic filtering PBAIP.The as-deposited(TixZr1-x)Ny and Tix(CyN1-y)films within large composition ranges all exhibited stable Bl-NaCl single-phase structure,and the thicknesses,grain sizes and residual stresses were also controlled effectively to keep stable.Mechanical properties of the two films both highly depended on their compositions and VECs.As the VEC increases from 7.50 to 8.30,hardness and elastic modulus of(TixZr1-x)Ny film first increase and then keep stable,reaching the maximum between 8.15 and 8.30:31.9 GPa and 351.2 GPa.Because VECs of all the Tx(Cy1-y)films are less than 8.20,the hardness and elastic modulus keep increasing from 26.6 GPa,310.0 GPa to 34.3 GPa,383.2 GPa as the VEC increases from 7.16 to 7.78.The performances of mechanical properties of the two films were fully compatible with the atomic-scale strengthening mechanism confirmed by the binary MNx films.3.The quaternary monolayer superhard(Zr,Al)NO film was successfully designed and prepared based on the atomic-scale strengthening mechanism by the enhanced magnetic filtering PBAIP.The stable phase structure,preferred orientation,residual stress and other conventional meso-scale properties were observed.And the hardness and elastic modulus of(Zr,Al)NO films first increase and then decrease with the increasing of Al content from 8.2 at.%to 21.9 at.%,reaching the top at 10.9 at.%:41.2 GPa,441.1 GPa.The(Zr,Al)NO film reached the super hard level at the stoichiometry point of(Zr0.80Al0.20)N0.59O0.26 where the VEC is 8.31,which is consistent well with the expected value of composition design. |
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