Study On Preparation Technology Of Powder Metallurgy Molybdenum Rod And Its Surface Silicification Modification

Molybdenum owning good high temperature strength,and the compactness of molybdenum tube prepared by thermoplastic processing is excellent.It can satisfy the use of tungsten rhenium thermocouple wire in complex atmosphere.With the development of modern industry,higher requirements are put forward to the service life and the testing atmosphere of the thermocouple.The molybdenum protection tube used in temperature measurement requires high density and temperature strength,high conductivity and corrosion resistance.At present,most sintered molybdenum tubes are used in the market,however,this kind of molybdenum tubes with low density and inadequate strength,which limited the protection to thermocouples.Therefore,the preparation of molybdenum rods with high density and excellent mechanical properties,and then drilling pipe has become a common processing for molybdenum protection pipe with small specification.In addition,the high temperature antioxidant capacity of molybdenum is poor,which limits the application of molybdenum as antioxidant thermocouple protection tube.What's more,the molybdenum has been widely used in the field of electronic device,heater element,glass fiber processing industry,aerospace industry and other high temperature fields,study and improve the antioxidant properties of molybdenum is of great importance.So,this paper relying on Chongqing materials research institute and its national functional instrument materials research institute,based on the traditional powder metallurgy production line,excavated and improved the preparation and processing parameters of small-sized molybdenum rods,prepared and modified the surface anti-oxidation coating of molybdenum matrix.In this paper,by excavating and improving the traditional powder metallurgy process,the pressing process parameters of molybdenum powder were determined,the cold isostatic pressing pressing process was developed,and the technological parameters of vertical sintering and rotary forging of molybdenum rod were optimized,so as to improve the density and mechanical properties of molybdenum rod.In addition,the packing cementation silicone coating were prepared on the molybdenum surface by in-situ chemical vapor deposition?CVD?process,and the effects of infiltration components and process parameters to the the thickness,structure and phase composition of the coating were studied,and the anti-oxidation and anti-acid-alkali corrosion properties of siliconizing coating were evaluated.On this basis,the titanium-modified molybdenum silicide coating was prepared,the influence of deposition process on the coating was studied,and the oxidation resistance and acid and alkali corrosion resistance of the coating were also evaluated.The research contents are as follows:?1?Molybdenum powder with average length diameter D[1,0]of 4.87?m and median diameter D50 of 31.639?m,pretreat in hydrogen atmosphere and then screen using 60 mesh.The obtained molybdenum powder has wider particle size and better fluidity,which is conducive to pressing formation.The relationship between pressing pressure and density of molybdenum powder conforms to"double logarithmic equation".Its hardening index m value is between 2.737 and 3.043,the value of elastic modulus M is about 0.20⁰.27×10⁸Pa.The double logarithmic pressing equation of this particle size composition of molybdenum powder is:?2?The relationship between the actual pressing pressure and the compacting density of molybdenum powder screened 60 mesh conforms to the theoretical pressure-density curve.After holding pressure for more than 5min,the density of pressing blank remains constant.In the actual production process,the pressing pressure of 200MPa and the holding time of 5min can be selected,and the pressing density can reach 6.91g/cm³.For compacting molybdenum with the weight of 400g,vertical sintering current of 2600A and holding time of 15min can obtain high-quality sintered molybdenum billet with a density of 9.56g/cm³,and the grain size at the edge of sintered billet is smaller than that at the core,and the vickers hardness at the edge is larger than that at the core.When the rotary forging temperature is 1450?,the roller compression ratio is 20%,and the total compression rate is 60%,the molybdenum density can reach 10.17g.cm^-3,and its micro-hardness distribution of cross section is more uniform.?3?In the packing cementation silicon experiment of molybdenum samples,when the mass fraction ratio is calculated as Si:NH₄F:Al₂O₃=40:10:50 and deposited at 1000? for 5 hours,the coating thickness can reach 53.3?m.The coating surface presents an ellipsoidal packing arrangement.the coating is closely bound to the substrate and no gap or crack existed,and its bonding strength is high.The outer layer of the coating is single MoSi₂ phase,and the transition phase between coating and substrate is Mo₅Si₃.The thickness of coating increases with the rising of deposition temperature,meanwhile the thickness of transition layer also increases obviously.In the process of packing cementation,the appropriate thickness of the transition layer can be obtained by controlling the deposition temperature,in order to increase the thermal shock resistance of coating.With the extend of holding time,the increase of coating thickness present parabolic change,and the thickness of transition layer changed slightly,meanwhile the phase composition and structure of the coating did not change.The kinetic law of coating thickness and holding time satisfied H=18.5·???+6.95.?4?The service life of a single silicone-permeated coating prepared at 1000?under high and low temperature cyclic oxidation at 1200?is up to 140h,which is higher than the antioxidant life of the coating prepared at 900? and 1100?under the same infiltration time.With the extension of oxidation time,the MoSi₂ layer becomes thinner and the Mo₅Si₃ layer becomes thicker.In addition,during the cycling oxidation of the coating at high and low temperature,the cracks in coating formed by thermal stress accelerate the failure of coating.In the corrosive medium of dilute sulfuric acid,the surface of coating tends to form a stable passivation film composed of Si-Mo-O,thus reducing the corrosion rate of the coating in dilute sulfuric acid solution.The coating has no obvious passivation platform in NaOH solution,that is because passivation film reacted with NaOH solution,resulting in partial dissolution and discontinuity of passivation film.?5?In the coating modified by Ti and Si co-deposition process,when the mass fraction ratio is calculated as Ti:Si:NH₄F:Al₂O₃=20:20:10:50 and deposited at 1000?for 5 hours,the outermost layer of coating is distributed discontinuous?Mo,Ti?Si₂phase,and the second outer layer is MoSi₂ phase.With the decrease of Ti content in the buried material,no effective deposition of Ti occurred on coating surface,and only deposition of Si occurred to form a single siliconizing coating,indicating that the co-deposition process could not achieve effective deposition of Ti.The coating was prepared by two-step deposition process contains first titanizing and then siliconizing.When the titanizing temperature was 1000?,the coating thickness was larger than 900? and denser than 1100?.AlMoTi₂ and AlMo₃Ti_x are included in the coating after titanizing.The introduction of aluminum is caused by the reaction of fluoride containing Ti with dispersant Al₂O₃ to form Al fluoride,which is then deposited on the Mo substrate.The coating modified by titanizing was then treated by siliconizing.the deposition efficiency of coating increased more obviously,when the deposition temperature is greater than 1000?.The coating growth process is still controlled by the elemental solid diffusion.The modified silicide coating with uniform thickness of?Mo,Ti?Si₂ ternary compound on the outer layer,MoSi₂ layer on the second layer and Mo₅Si₃ transition layer between the second layer and the matrix.With the increase of siliconizing temperature,the thickness of?Mo,Ti?Si₂ layer did not change significantly,and the thickness of MoSi₂ layer increased obviously.When the siliconizing temperature reached at 1100?,the thickness of Mo₅Si₃ transition layer increased significantly.?Mo,Ti?Si₂ layer contains a trace of Al₃Mo precipitated phase.The existence of element Al is caused by the introduction of trace Al in the process of titanizing.With the extend of siliconizing holding time,the coating thickness showed a parabolic change,the thickness of?Mo,Ti?Si₂ layer change slightly,and the thickness of MoSi₂ layer increased to a certain extent,while the thickness of Mo₅Si₃transition layer increased slowly.The results show that the effect of prolonging siliconizing holding time on coating growth is less than that of temperature increasing.The kinetic law of coating thickness and holding time satisfied H=16.56·???+6.90.The growth coefficient of Ti modified silicide coatings is slightly lower than that of single siliconizing coatings,indicating that in Ti modified silicide coatings,the modified layer?Mo,Ti?Si₂ slows down the diffusion rate of silicon to some extent.?6?The coating samples titanizing at 1000?for 5h,and then siliconizing at1100? for 5h,carried out high and low temperature cyclic oxidation at 1200?.The coating did not show obvious weight loss after continuous cyclic oxidation of 180h,which is because the Si-Ti-O dense oxide layer can effectively improve the antioxidant performance of the silicide coating and increase the antioxidant life of coating.In the corrosive medium of dilute sulfuric acid,the surface of Ti modified silicide coating is easy to form stable passivation film composed of Si,Mo,Ti,Al oxide,thus reducing the corrosion rate of coating in dilute sulfuric acid solution.The coating has no obvious passivation platform in NaOH solution,that is because passivation film reacted with NaOH solution,resulting in partial dissolution and discontinuity of passivation film.What's more,the dissolution degree of above passivation film is greater than that in single siliconizing coating.The corrosion resistance of Ti modified silicide coating in dilute sulfuric acid solution is similar to that of siliconizing coating and superior to that of molybdenum matrix.The corrosion resistance in NaOH solution was lower than that of siliconizing coating and higher than that of molybdenum substrate.