Properties Of Doped MoS₂ Solid Lubricating Composite Films By Magnetron Sputtering Method

Friction and wear exist widely in aerospace and automotive parts.solid lubricant is suitable for high and low temperature,high vacuum,strong radiation and other special conditions.As one of the most widely-used solid lubricant,MoS₂ was called “the king of solid lubrication”.Pure MoS₂ has low friction coefficient and good lubrication performance in vacuum.However,in the air,MoS₂ is easy to oxidize,leading to the rapid increase of the friction coefficient.Furthermore,the beating capacity and wear resistance of the pure MoS₂ is limited due to its low hardness.In this experiment,MoS₂-Cr、MoS₂-Ni、MoS₂-Zr、MoS₂-C and MoS₂-Ti-C composite film were deposited by ion source assisted magnetron sputtering method.Influences of doped element content on the structure and properties of the composites were investigated.The surface morphology and cross section structure of the films were characterized by Scanning Electron Microscope,the phase composition of the films were analyzed by X-ray Diffraction and Raman spectrum,The mechanical properties of the films were investigated by Micro Vickers tester and Rockwell-C hardness tester;The tribological properties were evaluated by ball-on-disk rotating friction and wear tester,The wear depth and wear track contour of the films were tested by 3D surface profiler.The characterization results showed that MoS₂ surface transfer to granular structure from wormlike structure after doped Cr,Ni,Zr.With the increase of the doping content,the densification of composite films increased,the film（100）,（110）diffraction peaks gradually weakened and disappeared,the broad amorphous diffraction characteristics appeared,which indicates that doped elements refined film’s grain size and film forming nanocrystalline or amorphous structure with the increase of doping amount.The density and orientation of MoS₂ films are more sensitive to the doping of Zr than Cr and Ni.The carbon doped could also improve the loose structure of MoS₂ film,but surface granular radius of film is larger than metal doped,and it can’t completely inhibit the growth of MoS₂（100）,（110）active oxide surfaces.The surface particle size of Ti and C Co-doped film is almost the same as metal doped.The mechanical properties of the film show that the hardness of pure MoS₂ film is about 60 HV,the hardness of MoS₂-Cr,MoS₂-Ni,MoS₂-Zr,MoS₂-Ti-C composite film is 300⁵00HV,and the hardness of MoS₂-C composite film is 120¹90HV.Doped elements increased the hardness of the MoS₂ film,the hardness of the MoS₂ film is improved greatly by doping metal elements because of the atomic radius of metal elements is much larger than the radius of Carbon atoms,.The adhesion strength of film and substrate is related to the doping element and its content.The film and substrate has a poor adhesion strength when doped content is too low,the indentation rating lower than HF4 grade;The combination of film and substrate is good when doped concentration is about 15 at.%,indentation grade is HF1;When doped elements is too high,the substrate is still combined good with the composite film which doped Ni,Zr,C and indentation rating higher than HF2 grade,but combined poor with composite film doped Cr and the indentation grade is HF6,which is mainly related with the plastic toughness of Ni,Zr,C is higher than Cr.The tribological properties of MoS₂ composite films in atmospheric environment show that the lubricating properties of the composite films doped different elements are different.The metal Cr is hard and brittle,and toughness is poor,friction coefficient of the MoS₂-Cr composite film with the high Cr content is larger;Ni is the iron affinity element,the friction coefficient of MoS₂-Ni composite films were larger because it is easy to adhesive in the friction process;While Zr is relatively good ductility and difficult to dissolve in the Fe,the friction coefficient of MoS₂-Zr film is low because it is hard to adhesive with friction part in the friction process;The friction coefficient of MoS₂-C composite film is larger because of the atomic radius of C is smaller than that of the metal and the active oxide surface of composite film has not been inhibited.The content of doped element has a great effect on lubricating properties of composite films.The optimum concentration of doped Cr,Ni,Zr is about 15 at.%,the friction coefficient of MoS₂-Cr,MoS₂-Ni,MoS₂-Zr are 0.05,0.17,0.09;the wear rate are 2.4 × 10-6mm3·N-1·m-1,1.2 × 10-6mm3·N-1·m-1,9.3 × 10-7mm3·N-1·m-1.The optimum concentration of doped C is 34.54 at.%,the friction coefficient is 0.15 and wear rate is 3.8×10-6mm3·N-1·m-1.The prepared MoS₂-Ti-C composite film has good lubricating performance,the friction coefficient is 0.07 and wear rate is 5.0×10-6mm3·N-1·m-1.The results of MoS₂ composite film’s tribology performance in high humidity（100%RH）show that the lubricating performance of MoS₂-metal（Cr,Ni,Zr）composite film be bad apparently in high humidity environment.However,MoS₂-C and MoS₂-Ti-C composite film has a better lubricating performance than MoS₂-metal composite film in high humidity.It is due to graphite can play a good lubricating performance in high humidity environment.The MoS₂-metal（Cr,Ni,Zr）composite films with 15 at.% doped content were heat treated by different temperatures.The results show that friction coefficient of MoS₂-Cr,MoS₂-Ni,MoS₂-Zr composite film unchanged when the temperature is 300℃;when temperature is 400℃,MoS₂-Cr composite film oxide seriously,Raman spectroscopy detected strong MoO₃ peaks and the friction coefficient increases sharply,the MoS₂-Ni and MoS₂-Zr composite films were only detected weak MoO₃ spectrum peaks and the friction coefficient increases slightly;when the temperature reaches 500℃,MoS₂-Cr,MoS₂-Ni,MoS₂-Zr composite film are basically out of the matrix and the friction coefficient increases sharply.