Preparation And Investigation Of Coatings With Self-lubricating And Wear-resistant Properties Used At Elevated Temperatures In Turbo-engines

Nowadays,brush seal system has been employed on the high-pressure compressor and low-pressure turbine in advance turbo-engine.The major part of the brush seal system is the brush bristle which has a direct frictional contact with the rotor shaft surface in the combined combustion gas and high engine speed circumstances.Therefore,in order to reduce the wear rates of both rotor shaft and brush bristles,thermally sprayed coatings with self-lubricating and wear-resistant properties are always deposited on the surface of rotor shaft.Because of the harsh service environment,this coating would not only reduce the friction and wear,but also satisfy the performance requirements of high temperature oxidation resistance,hot corrosion resistance and thermal cycling resistance.In this study,the Ag,Ag-Mo,CoNiCrAlY-Cr₂O₃ and NiCoCrAlY-Cr₂O₃ powders were prepared by chemical reduction,electroless plating,spray granulation and solid state alloying technologies.Then the CoNiCrAlY-Cr203,NiCoCrAlY-Cr203,NiCoCrAlY-Cr₂O₃-Ag and NiCoCrAlY-Cr₂O₃-AgMo composite coatings were fabricated by atmospheric plasma spray?APS?.Microstructure and properties of these coatings were systematically investigated,and the main conclusions are summarized as follows:?1?The CoNiCrAlY-Cr₂O₃ and NiCoCrAlY-Cr₂O₃ composite coatings fabricated by atmospheric plasma-spray technology have dense microstructure.Tribological behavior indicates that the friction coefficient of these two coatings decreased as temperature increasing from room temperature to 800?.The CoNiCrAlY-Cr₂O₃ coating has a lower friction coefficient than that of NiCoCrAIY-Cr₂O₃ coating below 600 ?,while exhibits a similar value above 600 ?.The wear rates are all at the order of 10-5mm³/Nm.The CoNiCrAlY-Cr₂O₃ shows a better wear resistance below 400 ? while a little worse than NiCoCrAlY-Cr₂O₃ coating at elevated temperatures.The oxidation tests at 800? indicate that NiCoCrAlY-Cr₂O₃ coating shows a better resistance to oxidation as compared to CoNiCrAlY-Cr₂O₃ coating and the oxidation kinetics curves of the two coatings can be divided into three stages:severely slopeing line,parabolic and slightly slopeing line.The spinel oxides formed on the surfaces and the Cr₂O₃ deposited in coatings protect the coatings from being severe oxidized.Hot corrosion tests suggest that the melting point as well as the stability of Co·Co₃S₂ are higher than that of Ni·Ni₃S₂,which means the CoNiCrAlY-Cr₂O₃ coating has a better hot corrosion resistance.?2?The Ag film deposited on the hard substrate shows an extreme low friction coefficient of about 0.2 at room temperature.The plasma sprayed NiCoCrAlY-Cr₂O₃,NiCoCrAlY-Cr₂O₃-1OAg and NiCoCrAlY-Cr₂O₃-30Ag composite coatings have dense microstructure with a similar porosity about 2.5%.It was found that the NiCoCrAlY-Cr₂O₃ coating shows a microhardness of about 650HV0.2,while it decreases as the addition of Ag.However,the tensile strength of the NiCoCrAlY-Cr₂O₃-30Ag coating is 53MPa,which is 17MPa more than that of the NiCoCrAlY-Cr₂O₃ coating.The tribological tests indicate that the addition of Ag effectively reduces the coating's friction and wear from room temperature to 800?,and reaches the lowest value of 0.3 at 600?.The friction and wear are significantly increased due to the addition of 30%Ag.At low temperatures,Ag would be smeared on the sliding surface forming a lubricating film,while at high temperatures,the wear mechanism of the coatings containing Ag is dominated by sever adhesive wear and plastic deformation due to the diffusion and excessive softening of Ag.The thermal shock resistance of coating is improved by the addition of Ag,while the NiCoCrAlY-Cr₂O₃ coating shows a better oxidation resistance at 800 ?.?3?NiCoCrAlY-Cr₂O₃-10AgMo and NiCoCrAlY-Cr₂O₃-15AgMo coatings were fabricated by plasma spray technologies.It was found that both coatings have a similar microhardness about 600HV0.2.Tensile strength of these two coatings is 42±2MPa and 45±5MPa respectively.The coatings with Ag-Mo as the composite solid lubricants exhibit excellent self-lubricating properties over a wide temperature range,and reaches a low value of about 0.3 at 800?.The wear rates are all at an order of 10-5mm³/Nm in the entire testing temperature ranges.Above 600 ?,synergistic solid lubrication is provided by the formed NiMoO₄ and Ag₂MoO₄.A higher normal load applied on the friction process would promote the formation of the Ag₂MoO₄ lubricating film,and contribute to the compaction of the debris into a glaze layer.?4?A Ag-Mo composite was prepared by cold-pressed and sintering method.Analysis of the wear mechanisms of this composite at various temperature suggest that,Ag forms a continuous lubricating film on the friction surface at low temperatures while the formed series silver molybdates with lower melting point provide both solid and liquid lubrication above 500?.A model of solid reaction processes in the Ag-Mo-O₂ system was established due to analysis of detailed reaction mechanism.As a high temperature lubricant,Ag₂MoO₄ shows a stable performance in the air at 1000? with no obvious decomposition and evaporation.As Ag₂MoO₄ can completely attach to the surface of NiCoCrAlY-Cr₂O₃ coating after melting,it can be viewed Ag₂MoO₄ as a stable solid lubricant in the service environment of high temperature and high speed.?5?The simulations by finite element analysis indicate that the thermal stress distribution of the coating and substrate would vary with the different elastic modulus and coefficients of thermal expansion.Due to the large stress gradient causing by the increasing elastic modulus and mismatch of thermal expansion coefficients,the seal layer is prone to crack and peel off from the adhesive layer during the thermal cyclic process.The addition of Ag could effectively reduce the coating's elastic modulus of 34%and elevate the coefficient of thermal expansion to better match the substrate,and consequently improve the thermal shock resistance.