| Solid lubrication over a wide range of temperature is a challenge for decades and has yet to overcome. Most common lubricant performs only within a narrow temperature range, such as graphite, molybdenum disulfide are effective below 400℃in air, above which they will oxidize or decompose and lose their lubricious nature. Some oxides lubricate well above 600-800℃, but are abrasive at lower temperature. The combination of two or more than two lubricants is one of the good methods to realize lubrication over a wide temperature.In provious study, the self lubricating composite was usually prepared by adding solid lubricant in the metal matrix and lubricating coating was prepared by deposition technology. The frictional property was less affected if the adding amount of lubricant was too low. However, if the adding amount of lubricant was too high, the mechanical properties would decrease significantly. The wear life of lubricating coating was limited and its failure leaded to catastrophic incident.Nickel-based composites containing graphite, sliver, MoS2 and CeO2 were prepared by powder metallurgy (P/M) method. Ni-20Cr powders mixed with tungsten, aluminum, titanium, and different contents of graphite, molybdenum disulfides or silver were hot-pressed in a vuccum furnace. Laser surface texturing (LST) was performed on the Ni-base composite in order to modify its contact state and surface molybdenizing was carried out on the textured composite in order to improve its wear resistance. The molybdenizing and nitriding duplex-treatment was also performed on the textured nickel-base composite. The molybdenizd layer prepared on the solid lubricating composite can relize self lubrication over a wide temperature range and improve its surface mechanical properties. It is meaningful to solve the friction and lubricating problems of transport part and seal in the turbine engine and heavy vehicle, which work under an extreme environment.The mechanical properties and anti-oxidization properties of nickel-base composite, molybdenzing and duplex-treated layer were measured. Their tribological properties from room temperature to 700℃were tested on a pin-on-disk trimometer with Al2O3 as counterfaces. The effects of lubricant addition amount, temperature, load, sliding velocity and counter face materials, texturing and molybdenizing on the tribological properties were investigated. The microstructure and morphology were analyzed by X-Ray diffraction (XRD) and scanning electrical microscope (SEM) attached with EDS. The element distributions, micro-hardness of molybdenized layer were investigated. The worn surfaces were observed by optical microscope, SEM, EDS and X-ray photoelectron spectroscopy (XPS).Chromium sulfide and tungsten carbide were formed in the composite by adding molybdenum disulfide and graphite in the nickel-base alloy. The nickel-base composite with graphit and MoS2 consisted of hard phases (Mo2C, WC) and lubricant phases (CrxSx+1, graphite). The hardness and anti-bending strength of composites increase after adding MoS2. The high temperature oxidization resistance decreases with the increase of solid lubricant. The activity energy of oxidization increases with the increase of MoS2 adding amount.The addition of MoS2 is favored to the reduction of friction coefficient of composite at high temperature. The lowest friction coefficients (0.14～0.27) and wear rates (1.0～3.5×10-6mm3/(Nm)) from room temperature to 700℃are obtained due to synergetic lubricating action of graphite and molybdenum disulfide. The graphite plays the main role of lubrication at room temperature, while sulfides are responsible for low friction at high temperature.The friction coefficient at room temperature decreases after adding silver in the composite. Meanwhile, the wear rates at high temperature are reduced by adding cerium oxide. The wear rates are reduced by more than one order of magnitude after adding silver and CeO2. The SEM analysis of worn surface at high temperature shows that the rare earth compound is helpful for the formation of compact oxide film, which improves the wear resistance at high temperature. The nickel-base silver-containing composite consisted of lubricant phase such as sliver, Cr3S4, and CeS. The friction coefficients of silver and CeO2 containing Ni-base composite from room temperature to 700℃are in the range of 0.16～0.26.The molybdenized layer on nickel-base alloy is approximately 20-30μm in thickness. The content of molybdenum on the surface of molybdenized layer reaches 34% and the hardness and elastic modulus is improved by molybdenizing. The friction coefficient and wear rate of alloy decrease after molybdenizing. The diffusion of sliver and the forming of trioxide of molybdenum affected by temperature are responsible for the friction reduction at elevated temperatures. The molybdenizing and nitriding duplex-treated layer shows the similar tribological character with that of molybdenized layer. The compound MoN layer hinders the diffusion of silver.The laser surface textured dimples on nickel-base alloy are 150μm in diameter and 45-50 u m in depth and the distances between dimples are 500μm. The friction coefficient of alloys decreases by texturing and molybdnizing duplex-treatment. The wear rate of duplex-treated alloy is one order of magnitude lower than that of pristine alloy at elevated temperature due to the lubrication of trioxides of molybdenum. Furthermore, the micro-dimples can relieve abrasive wear by storing hard wear particles, which is also responsible for the reduction of wear rates. After texturing, the dimples on the MoN layer act as diffusion channels. The MoN layer on textured surface is hard enough to carry the load and protects the silver film.The parabolic parameter of dynamic oxidization is two orders of magnitude higher than that of static oxidization. the calculated wear rate of composite based on oxidizing wear model is lower than the experiment value while the calculated wear rate based on melt wear model is two orders of magnitude higher than experiment value. A high temperature melting wear model of oxide is developed for nickel-base composite and molybdenized layer. The oxide on high temperature frictional surface experienced formation, break or partly melt and compaction of compact layer.
|
PDF Full Text Request