| Titanium alloy is widely used in aviation,aerospace,chemical industry,medical and other fields,due to its high specific strength and good biocompatibility etc.The titanium alloy drill pipe is mainly made of Ti-6Al-4V(TC4)titanium alloy material.Compared with steel drill pipe which is currently used,titanium alloy drill pipe has a promising application prospect in drilling high curvature wellbore as a result of the large flexibility,small structural stress,fatigue resistance,corrosion resistance and lightweight.However.titanium alloy has lower hardness and poorer wear resistance,and is prone to seizure and adhere during wear.When it is contacted with dissimilar metals,galvanic corrosion is likely to occur,resulting in early failure of the components in use.It has been suggested that a ceramic coating formed on the surface of titanium alloy by micro-arc oxidation(MAO)treatment can effectively improve its corrosion and wear resistance,while few literature has reported the effect of graphene on the micro-arc oxidation behavior and coating properties of titanium alloy.In this thesis,the influence and mechanism of new additives for micro-arc oxidation coating are studied to further improve the performance of micro-arc oxidation coatings.In this work,TC4 titanium alloy,usually used for titanium alloy drill pipe,was micro-arc-oxidized in sodium silicate,sodium hexametaphosphate and glycerol electrolytic liquid under constant current mode.Three types of composite coatings modified by graphene,TiO2/(PTFE+graphite)and sodium tungsten were prepared based on the optimized process.The effect of additive content on micro-arc oxidation voltage,thickness,roughness.hardness,morphology and phase composition was investigated.And the corrosion resistance,wear resistance and the distribution characteristics of the Scanning Kelvin Probe(SKP)potential in the micro-area of the membrane scans of three different composite coatings were investigated.Furthermore,the mechanism of formation,modification,wear resistance and corrosion resistance of micro-arc oxidation coating was investigated in order to explore a new method to improve the surface performance of micro-arc oxide coating of titanium alloy drill pipe material.The optimum process parameters in the sodium silicate,sodium hexametaphosphate and glycerol composite electrolyte system were obtained by orthogonal experimental design.When graphene and sodium tungstate were added into the base electrolyte under the optimum process parameters,the conductivity of the solution and the roughness of the coating increased.Besides,graphene and sodium tungstate affected the micro-arc oxidation behavior,namely lowered the arcing voltage,shortened the arcing time,and increased the stability voltage.As the additive content increased,the coating thickness and hardness increased first and then decreased.EDS analysis indicated that the carbon and tungsten elements were involved in the coating.Rutile TiO2,anatase TiO2,and SiC were the main phases of the coating after graphene modified,and amorphous SiO2 and a small amount of graphene were also contained.The coating after sodium tungstate modified is relatively smooth,and the phase composition of the coating is mainly rutile TiO2,anatase TiO2.WO3 and amorphous SiO2.The TiO2/(PTFE+graphite)composite coating generated by two-step composite treatment also has a smooth surface.Besides rutile and anatase TiO,,PTFE and graphite can be also found in coating.PTFE and graphite are mainly distributed in the micropores of micro-arc oxidation coating,and a thin coating composed with PTFE and graphite was generated on the surface of the original micro-arc oxidation coating.The results show that compared with the micro-arc oxidation coating using the base electrolyte,the self-corrosion potential in a 3.5%NaCl solution of the three composite coatings modified by graphene,composite TiO2/(PTFE+graphite)and sodium tungstate increases,the self-corrosion current density and corrosion rate decreases.After coupling with S135 drill pipe steel in a 3.5%NaCl solution,the coupling potential increases and the density of coupling current decreases.The acceleration coefficients of galvanic corrosion of TC4,G0.G3,TiO2/(PTFE+graphite)and W3 coatings were 185%,73%.16%,11%and 19%,respectively.The high temperature and high pressure corrosion resistance is enhanced in the methyl nitro type mud.The improved corrosion resistance can be attributed to the increased thickness of the composite coating,second phases(SiC and WO3),amorphous phase and the improvement of the denseness of the coating.Compared with the matrix,the friction coefficients of the three composite coatings and the depth of the wear scars are reduced,and the relative wear resistance is significantly improved,which are 12.2,18.7 and 10.4,respectively.The presence of a few graphene,second phase and amorphous phase contributes to the improvement of the wear resistance of the coating.The coefficient of friction is further reduced and the relative wear resistance is improved in the presence of brine.The wear of TC4 titanium alloy matrix is mainly adhesive wear with some abrasive wear,while the composite coating shows an opposite situation.The fine structure of the micro-arc oxidation coatings with different processes is different,and the composition and phase of the coatings are also different.The diameter of micropore on surface of graphene modified coating is lager.A part of graphene participated chemical reaction during micro-arc oxidation and formed SiC,and some still existed in the coating in its original form.The surface of the TiO2/(PTFE+graphite)composite coating has been filled and covered,and the surface of the coating is relatively smooth,containing more carbon and fluorine elements.There are still holes of different sizes on the surface of the tungsten modified composite coating,and WO3 is formed after the reaction,but the depth of the holes is significantly reduced compared with the graphene modified coating.SKP potential testing results show that the SKP potential distribution is not uniform,and the SKP potential fluctuation of the composite coating is larger than that of the TC4 titanium alloy matrix due to the incompleteness of the coating structure(the presence of holes).According to the distribution of average voltage in micro-area,the graphene modified coating is the largest,while the surface of the TiO2/(PTFE+graphite)composite coating has the smallest SKP potential.It should be noted that both of them are higher than the SKP potential of the matrix material.The analysis of thermodynamics of micro-arc oxidation shows that at a certain temperature,anatase will transform to rutile TiO2.When the temperature is higher than 1314K,WO42-will be converted to WO3.The kinetic analysis shows that as the oxidation time increases,the thickness of coating increases.The greater the conductivity of the electrolyte,the faster the thickness of the coating grows,and the greater the thickness of the final coating.The micro-arc oxidation process of titanium alloy consists of three processes:initial coating formation,electrical breakdown and coating thickening.Whether the initial coating can be formed is critical to micro-arc oxidation.The electric breakdown process is affected by the factors such as electrolyte type and conductivity.Thickening of the coating is resulted from a continual micro-arc oxidation.The analysis of the modification mechanism of graphene and sodium tungstate modified micro-arc oxidation coating and PTFE+graphite composite coating show that the addition of graphene and sodium tungstate to the electrolyte changes the conductivity of the solution during micro-arc oxidation.When chemical reaction occurs,and the substance with a higher hardness and better corrosion resistance is formed.In addition,a few graphene enter into coating due to its strong adsorption capacity.The mechanism of PTFE+graphite composite is mainly the filling of the PTFE emulsion and graphite particles into the micropores on micro-arc oxidation coating,covering the surface of the original micro-arc oxidation coating with a thin coating of PTFE+graphite composite coating,which is beneficial to improve the wear resistance and corrosion resistance of the coating.The research results provide a theoretical fundation for the preparation of a novel micro-arc oxidation coating on titanium alloy drill pipe,and achieve the purpose of further improving the performance of micro-arc oxidation coating,broadening the application range of titanium alloy. |
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