| Spline in aeroengine power transmission system is one of the important parts that directly affect the safety and service life of aeroengine.However,spline employed in aeroengine experience adverse working environment,and spline tooth subjects wear,fatigue,fracture and fretting,which makes spline failure.Therefore,improving the performance of spline,especially internal spline with a high aspect ratio,is critical to develop the advanced aerogene.To enhance the performance of friction-reduction and wear resistance of aviation internal spline,this paper prepares Si-DLC films on the inner surface of internal spline by plasma enhanced chemical vapor deposition(PECVD),utilizing plasma generated by hollow cathode discharge(HCD).The hollow cathode discharge characteristics of tube and internal spline with an aspect ratio of 10 and an internal diameter of 30mm are studied,respectively.A two-dimensional fluid model is used to study effects of spline tooth on spatiotemporal distribution of plasma.The problem of non-uniform films prepared on complex geometric shapes,for instance,spline tooth is noticed,and the effects of discharge parameters on microstructure and properties of Si-DLC films on spline tooth are analyzed,revealing key factors of the film radial uniformity.In view of the common problem of poor bonding strength of hard thin films on soft substrates,the bonding strength of HCD-PECVD deposited Si-DLC films on various substrates are studied,and failure modes of films on substrates with various hardness are discussed.The effects of Si interlayer with different thickness on Si-DLC film’s bonding strength are analyzed.Severe fretting makes aviation spline failure,and fretting performance of Si-DLC films on aviation spline material is tested.The fretting failure mechanism of hard Si-DLC films on soft substrates is proposed.Finally,solid lubricated Si-DLC films are prepared on internal spline of accessories gearbox,and verified by spline tester.An HCD-PECVD equipment was designed and set up according to the requirements of the inner-surface film deposition for tubular workpiece.The DLC film has been deposited by this equipment using a condition of low gas pressure with high voltage pulse,to realize an adjustable voltage pulse within a range of 0～-2000V.Si-DLC films are deposited on the inner surface of slender tube successfully via HCD-PECVD equipment.The amplitude of breakdown voltage decreases with increasing pressure within 8～13Pa under argon atmosphere.Enlarged cathode area due to spline teeth emits more secondary electrons during the discharge,which leads to a higher discharge current than slender tube,the current of the former is 51.5%and 60%higher than the same-sized tube with a voltage pulse of-1100V under argon and mixed gas atmosphere,respectively.According to the results of fluid model,the negative glow regions fail to enter the concave area formed by spline teeth,namely tooth root,and the cathode sheath is non-uniform along the teeth,where the thickness of sheath on the tooth root is twice as it is on the tooth top.The peak density of argon ions and electrons is located at the center of discharge structure,and the peak value of hollow cathode with spline teeth is higher than hollow cathode without spline teeth.The cathode sheath on the surface of tooth is initially modeled on the tooth after the pulse ignition discharge.As the sheath expands and the discharge enters the steady state,the sheath is no longer conductive to the uniform deposition of the film on tooth structure.The increase of the amplitude of pulse voltage and the deposition pressure,or the decrease of pulse frequency,may increase HCD plasma density,result in a loose columnar structure of Si-DLC films on spline tooth.The overlap of sheath on the surface of spline tooth thickens the film at the tooth top rather than the film at the tooth middle and tooth root.By reducing the amplitude of the pulse voltage or increasing the HCD plasma density,the reduction of the sheath thickness can uniform the film thickness along radial direction,where the non-uniformity value drops below 0.4.Higher density of HCD plasma results in more graphite-like structure in Si-DLC films on internal spline,and the hardness and elastic modulus of films decline.Si-DLC films deposited by small amplitude voltage and low pressure show great anti-friction performance.The bonding strength of the film on substrate with high hardness and good plastic deformation resistance is better.Si-DLC films show different failure modes on substrates with different hardness.The films completely peel off inside the scratch on substrate with high hardness under heavy load,but experience brittle fracture inside and at both sides of the scratch due to yielding of soft substrates under relative low load.The increase of Si interlayer thickness could tune the bonding strength of Si-DLC films,which increases first and then decreases.The Si-DLC films can significantly reduce the friction coefficient of aviation spline material during fretting,and keep its reliability and friction-reduction performance after fretting cycles of 3×10~4.After the DLC film preparation of fretting pairs,the friction-reduction performance is even better.The fretting failure of the hard Si-DLC films on soft substrates shows the gradual loss of Si-DLC films at fretting marks,and the film debris is extruded into layers,which is accompanied by oxidative wear,and the Si-DLC films around fretting marks is partially peeled off from the substrate due to the stress introduced by fretting.The static friction test shows the maximum friction force of Si-DLC deposited aviation internal spline of accessories gearbox can be reduced by more than 40%.The size of Si-DLC deposited internal spline still meets the design standard after the service life test of 587.78h,which meets the usage requirements and realizes mass production.There is no crack observed on membrane disc after Si-DLC deposition due to excessive friction force during flight verification. |
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