Fabrication And Properties Of Erosion Resistance Gradient Coating For Outer Cylinder Of MWD Instrument

With the continuous increase of oil drilling depth,the geological structure is growing increasingly more complex.The measurement while drilling(MWD)technology plays a significantly important role in drilling operations.However,due to the extremely harsh environment conditions during the drilling process,the drilling fluid will seriously damage the outer cylinder of the MWD instrument,which can lead to problems such as failure of MWD pulse signal transmission,shorten the service life and frequent replacement of the outer cylinder of the MWD instrument,which seriously affects the drilling equipment safety and production efficiency.This work proposed a new idea of developing and applying functionally graded material coating on the copper alloy(QBe2)outer cylinder of MWD instrument.NiCrAlY/Al₂O₃-20%TiO₂ gradient coating is prepared by plasma spraying technology.First,the microstructure and the interface bonding quality of the gradient coating are evaluated through bonding strength test and the thermal shock cycle test;Second,Study the wear properties(change the test load and the speed of the friction pair)and the erosion properties(change the erosion angle,erosion velocity and sand content);Finally,the gradient coating for the outer cylinder of the MWD instrument was evaluated based on the results of the field application.The gradient coating structure designed in this paper from the substrate surface to the ceramic coating is:QBe2 alloy substrate,NiCrAlY bond coating,gradient transition region(75%NiCrAlY+25%AT20?50%NiCrAlY+50%AT20?25%NiCrAlY+75%AT20),AT20 ceramic coating.The spraying power gradually increases with increasing AT20 powder content to ensure that the powder particles are fully melted.The phase of the AT20ceramic coating are mainly composed of?-Al₂O₃,?-Al₂O₃,Rutile-TiO₂ and Al₂TiO₅.The surface of the NiCrAlY/AT20 gradient coating has a typical lamellar structure.The gradient coating has a smooth transition of composition?uniform thickness and good interface.(1)The microhardness of the NiCrAlY/AT20 gradient coating gradually increases from QBe2 substrate to the AT20 ceramic coating,and the microhardness of the AT20 ceramic coating is 875 HV_0.05;A mechanical bond is formed between the gradient coating and the QBe2 substrate,and the bond strength is 34.8 MPa;After thermal shock tests at 400?,the thermal shock failure of gradient coating occurred after 467 cycles.Gradient transition structure alleviates the problem of easy spalling caused by the mismatch of TEC between the QBe2 alloy and AT20 ceramic coating.The gradient coating has excellent thermal stability with the QBe2 substrate and good interfacial bonding quality,thus ensuring the reliability and durability of the application of the outer cylinder of the MWD instrument.(2)Under dry friction conditions,the friction coefficient of NiCrAlY/AT20 gradient coating is 0.511?0.581,which is lower than the friction coefficient of QBe2 alloy substrate 0.736?0.768,and the fluctuation is smaller,showing better wear resistance.The AT20 ceramic coating has a small amount of wear entent and shallow abrasion marks.The wear mechanism of NiCrAlY/AT20 gradient coating mainly has abrasive wear,fatigue wear and adhesive wear.(3)When erosion at a small angle,the erosion of the gradient coating are shown as:cutting wear and particle spalling;When erosion at a large angle,the erosion of gradient coatings are shown as:particle spallation,brittle fracture and plastic failure;The erosion of AT20 ceramic coating are shown as:small particles peeling off and microcrack propagation;The erosion of the NiCrAlY/AT20 gradient transition coating are shown as:particle spallation,brittle fracture and plastic failure,and with the increase of NiCrAlY content,the more obvious the plastic failure of the gradient coating.The NiCrAlY/Al₂O₃-20%TiO₂ gradient coating has been successfully applied to the outer cylinder of the MWD instrument.It has good erosion resistance and improves the comprehensive performance of the surface of the outer cylinder of the MWD instrument without affecting its size and magnetic properties,and the service life has been increased from 512 hours to 1400hours.And the gradient coating has still not completely fallen off,and the outer barrel of the MWD instrument can continue to be used,which has good economic benefits.