Microstructure Performance And Reactive Synthesizing Mechanism Of Ni₃Al-MoS₂ Self-lubricating Coating

In high-tech fields such as aerospace,aviation and nuclear power,some important moving parts need to operate under high vacuum,high load,strong radiation and other conditions.Under such extreme conditions,only solid lubricants can meet the lubrication requirements.MoS₂ is a kind of lamellar material with good lubricity,which is especially suitable for vacuum environment.However,the wear-resistant life of pure MoS₂ coating is short because of its small thickness and poor adhesion.Self-lubricating coatings with good friction and wear resistance can be obtained by combining MoS₂ with hard materials.Ni₃Al and NiAl intermetallic compounds,which have high melting point and hardness,can be produced by self-propagating reaction of nickel and aluminum.Utilizing the reaction between Ni and Al,and overcoming the burning loss of MoS₂ during preparation through certain technical means,it is hopeful that Ni₃Al-MoS₂ composite coatings with good lubricity can be rapidly prepared on the surface of steel matrix.In this paper,the reaction process and mechanism of molybdenum disulfide with nickel,aluminum and Ni₃Al were studied by DSC/TG thermal analysis and high temperature in situ XRD.The results showed that molybdenum disulfide reacted with nickel to form sulfide Ni₃S₂ and nickel-molybdenum solid solution when the heating temperature exceeded 630?.With the increase of heating temperature,sulfides with high sulfur content,such as NiS and Ni₇S₆,appeared in the products.When the amount exceeded the solid solubility in nickel,molybdenum reacted with nickel to form Ni₄Mo.Molybdenum disulfide reacted with aluminum when the heating temperature exceeded 561?.The reaction process can be divided into two parts:aluminum replaces sulphur in molybdenum disulfide to form Al₂S₃ and Mo;and then aluminium reacts with molybdenum to form aluminium-molybdenum compounds.With the increase of temperature,the Al-Mo compounds in products gradually changed from Al-rich phase,such as Al₁₂Mo and Al₅Mo,into Mo-rich phase,such as Al₈Mo₃ and AlMo₃.The intermetallic compound Ni₃Al has a stable thermodynamic state and does not react with molybdenum disulfide even when heated to 1200?.The temperature distributions in Ni/Al self-propagating reaction under different conditions were studied by finite element simulation.When only Ni/Al compact is on the surface of steel substrate,the maximum temperature of NiAl layer during combustion could reach 1406.1?.According to the study of the reaction process between molybdenum disulfide and metals,the temperature would cause serious burning loss of MoS₂.In order to prevent burning loss,MoS₂ is pre-placed in Ni₃Al as lubricating layer.Combustion reaction of Ni/Al compacts spreaded between lubricating layer and steel matrix.At this condition,the maximum temperature of the joint between lubricating layer and Ni/Al compact was between 1026 and 1430?.The maximum temperature of the node at the thickness of 0.0380mm in the lubricating layer could reach 1200?.The lubricating phase MoS₂ in other parts of lubricating layer could be well preserved during the bonding process.When the mixed powders of nickel,aluminium and molybdenum disulfide were hot pressed at 530?,the reactants Ni and Al transformed into Ni₃Al by solid phase diffusion reaction.The lubricating phase MoS₂ was completely preserved in Ni₃Al.Ni₃Al/MoS₂ lubricating layer and NiAl layer can be well connected by self-propagating reaction of Ni/Al compacts.After self-propagating bonding,a small amount of MoS₂ near the interface burns,and the thickness of the burned area is about40?m.Other MoS₂ is completely stored in the lubricating layer.In self-propagating bonding,copper-zinc filler metal was added to the interface between NiAl layer and steel matrix to improve the bonding.The phase composition at the interface includes:copper-zinc alloy,NiAl,unreacted nickel particles and the Ni₃Al outer layer.With the increase of bonding pressure and preheating temperature,the density of the microstructure at the interface can be increased,and then the bonding strength is enhanced.When the connection pressure is 40 MPa and the preheating temperature is 100?,the adhesion strength of the coating reaches the maximum value,which is 64.7 MPa.When the preheating temperature exceeds100?,the excessive volatilization of zinc in the filler metal leads to the formation of pore in the interface,which leads to the decrease of the bonding strength.In the tribological testing,the friction coefficient of Ni₃Al-MoS₂ layer first keeped a high value of 0.4.With the development of wear,the lubricating phase MoS₂was squeezed on the friction surface and a lubricating layer was formed.Therefore,the friction coefficient was gradually reduced and maintained at about 0.2.The wear rate of the lubricating layer was 1.04×10^-5 mm³N^-1m^-1 with load 3N and sliding rate0.1m/s after 15 minutes of tribological testing.