【作者】 陈孝文；

【导师】 施太和；

【作者基本信息】 西南石油大学 ， 油气田材料与应用， 2018， 博士

【摘要】 钛合金具有比强度高、生物相容性好等优点,被广泛应用于航空、航天、化工和医疗等领域。钛合金钻杆本体主要选用Ti-6Al-4V(TC4)钛合金材料制造。与目前普遍使用的钢钻杆相比,钛合金钻杆具有柔性大、结构应力小、耐疲劳、耐腐蚀、质量轻等优点,在高曲率井眼的钻井应用中具有广阔的应用前景。但是,钛合金的硬度较低,耐磨性较差,在磨损过程中易发生咬合、粘着,与异种金属接触时易发生电偶腐蚀,使构件在使用过程中发生早期失效。研究发现微弧氧化处理后在钛合金表面形成的陶瓷层能有效改善其耐蚀和耐磨性能,但很少有人研究添加石墨烯对钛合金微弧氧化行为及膜层性能的影响,本文拟研究新型添加剂对钛合金的微弧氧化行为及膜层性能的影响规律及机理,以进一步提高微弧氧化膜层的性能。本文采用恒定电流模式在硅酸钠+六偏磷酸钠+丙三醇电解液体系中对钛合金钻杆常用的TC4钛合金进行微弧氧化处理,在最优化工艺的基础上制备了石墨烯改性、TiO2/(聚四氟乙烯(Polytetrafluoroethylene,PTFE)+石墨)复合和钨酸钠改性三种复合膜层,探索了添加剂含量对微弧氧化电压、厚度、粗糙度、硬度、形貌、相组成的影响,然后研究石墨烯改性、TiO2/(PTFE+石墨)复合和钨酸钠改性三种复合膜层的腐蚀行为、摩擦磨损行为及膜层的微区扫描开尔文探针(Scanning Kelvin Probe,SKP)电位分布特性。在此基础上,深入分析了钛合金微弧氧化复合膜层的成膜机理、改性机理、耐磨机理及耐腐蚀机理,探索提高钛合金钻杆材料表面微弧氧化膜性能的新途径。通过正交试验,得到了硅酸钠+六偏磷酸钠+丙三醇电解液体系中的最佳工艺参数,在基础电解液和最佳工艺参数下分别添加石墨烯和钨酸钠,溶液的电导率增大,膜层粗糙度增加。石墨烯和钨酸钠添加到基础电解液中改变了微弧氧化行为,降低了起弧电压,缩短了起弧时间,稳定电压增大。随着添加剂含量的增加,膜层厚度和硬度先增后减。EDS分析表明:碳和钨元素已经进入膜层中。石墨烯改性后膜层的相组成主要为金红石型和锐钛矿型TiO2,SiC,非晶态的SiO2和少量石墨烯。钨酸钠改性后的膜层比较平整,膜层的相组成主要为金红石型和锐钛矿型TiO2,WO3和非晶态的SiO2。采用两步复合处理的方法制备得到TiO2/(PTFE+石墨)复合膜层,表面较为平整,碳和氟元素进入到膜层中,膜层中的相组成除了锐钛矿型TiO2和金红石型TiO2外,还含有PTFE和石墨。PTFE+石墨主要分布在微弧氧化层的微孔内,并在原微弧氧化层表面覆盖一层较薄的PTFE+石墨复合膜层。研究结果表明,与经基础电解液处理后的微弧氧化层相比,石墨烯改性、TiO2/(PTFE+石墨)复合和钨酸钠改性三种复合膜层在3.5%的NaCl溶液中的自腐蚀电位升高,自腐蚀电流密度降低,腐蚀速率降低;与S135钻杆用钢在3.5%的NaCl溶液中偶合后,偶合电位升高,偶合电流密度降低,TC4、G0、G3、TiO2/(PTFE+石墨)、W3膜层的电偶腐蚀加速系数分别为185%、73%、16%、11%和19%;在甲硝基类型的泥浆中耐高温高压腐蚀性能有所增强。复合膜层的厚度增大,反应生成耐腐蚀的第二相(SiC和WO3),非晶相的存在和膜层致密性的提高都有助于复合膜层耐蚀性的提升。与基体相比,三种复合膜层的摩擦系数降低,磨痕深度变小,相对耐磨性显著提高,分别为12.2、18.7和10.4。第二相、少量吸附在膜层中的石墨烯和非晶相的存在均有助于提高膜层的耐磨性。在有盐水存在的环境中的摩擦系数进一步降低,相对耐磨性提高。TC4钛合金基体以黏着磨损为主,磨粒磨损为辅;复合膜层则以磨粒磨损为主,黏着磨损为辅。不同工艺处理后微弧氧化层的精细结构存在差异,膜层成分及物相也不一样。石墨烯改性的微弧氧化层表面微孔直径偏大。一部分石墨烯在微弧氧化过程中参与了化学反应生成了新的物质SiC,一部分仍以石墨烯的形式存在于膜层中。TiO2/(PTFE+石墨)复合膜层表面微孔处已被填充和覆盖,膜层表面较为平整,表面含碳和氟元素较高。钨酸钠改性复合膜层表面仍有大小不等的孔洞,反应后生成WO3,但与石墨烯改性的膜层相比,孔洞的深度明显减小。SKP电位测试结果表明,由于膜层结构的不完整性(存在孔洞),使得SKP电位分布不均匀,复合膜层的SKP电位波动比TC4钛合金基体的波动更大。从微区分布的平均电压值来看,G3膜层最大,TiO2/(PTFE+石墨)复合膜层表面SKP电位最小,但均比基体材料的SKP电位高。微弧氧化热力学条件分析表明:在一定温度下,锐钛矿型TiO2将向金红石型TiO2转变;当温度高于1314K时,WO42-就会向WO3转化。动力学条件分析表明,随着氧化时间的增加,膜层厚度增大,电解液的电导率越大,膜层厚度增加越快,最终膜层厚度也越大。钛合金微弧氧化过程由初始膜层的形成、电击穿和膜层增厚三个过程组成。初始膜层能否形成是发生微弧氧化的关键,电击穿过程受电解液种类及电导率等因素影响,膜层增厚是微弧氧化持续进行的结果。石墨烯和钨酸钠改性微弧氧化层以及PTFE+石墨复合膜层的改性机理分析结果表明,石墨烯和钨酸钠加入电解液后,改变了溶液的电导率,在微弧氧化过程中发生了化学反应,生成了硬度高、耐蚀性好的物质,同时,石墨烯比表面积大,吸附能力强,有少量石墨烯进入膜层;PTFE+石墨复合机理主要是利用PTFE乳液和石墨微粒填充微弧氧化层的微孔,并在原微弧氧化层表面覆盖薄薄的一层PTFE+石墨复合膜层,从而有利于提高膜层的耐磨性和耐腐蚀性能。研究成果为制备新型钛合金钻杆微弧氧化膜层提供了理论依据,实现了进一步提高微弧氧化膜层性能的目的,拓宽了钛合金的应用范围。更多还原

【Abstract】 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.更多还原