| With the continuous development of two-engine technology,higher requirements are put forward for the performance of engine compressor blades and ground gas turbine blades.Therefore,the protection of related components not only needs to have good mechanical properties(such as high strength and toughness,low internal stress)to meet the effect of erosion resistance,but also need to have a certain high temperature resistance.It is an effective method to prepare protective coating on precision components in engineering field by surface coating technology.There have been reports on the application of physical vapor deposition(PVD)technology to the preparation of erosion-resistant coatings in aero-engine compressor blades.However,the domestic research in this field started late and has not yet been systematized,especially lack of theoretical guidance on coating erosion failure behavior and mechanism,and it cannot satisfy the high-temperature environment work requirements of related components.Based on the above problems,Cr-based anti-erosion coatings were designed and prepared on the surface of titanium alloy by pulsed electromagnetic and strong permanent magnetic field composite arc ion plating technology in this paper.The influence of the compound magnetic field on the large particles of the coating was systematically studied,and the influence of the microstructure of the coating(monolayer/multilayer)on the main properties of residual stress,erosion resistance and other major properties,the fracture failure mechanism of the coating(monolayer/multilayer)was further studied,the evolution law of the multilayer coating under high temperature environment was obtained.The main research results are as follows:(1)By adjusting the electromagnetic field parameters in the composite magnetic field,the motion range and motion velocity of the arc spot are obtained,and the internal connection between the electromagnetic field parameters and the structural properties of the coating was established.It was found that the number of large particles on the surface of the CrAlN coating is the smallest(about 6.09%)and the roughness is the smallest(Ra 0.136 ?m)at medium-intensity voltage(25 V)and high electromagnetic frequency(16.7 Hz).On this condition,the hardness of the coating(2072.34 Hv),bonding force(41.5 N),friction and wear(the friction coefficient is about 0.35,the wear rate is 2.77×10-6 mm3·N-1·m-1)and the erosion resistance(The erosion rate at 30° is about 0.17 ?m/g,and at 90° it is about 1 ?m/g),the above properties of the coating were all the best.The erosion fracture morphology of the CrAlN coating is analyzed,and it is found that the coating is a typical brittle fracture mechanism,and the size and number of micro-particles in the coating have a great negative effect on the performance.(2)The CrAl/CrAlN micro-nano multilayer structure coating with alternating metal soft layer/nitride hard layer system was designed and prepared.Each cycle in the coating consists of a layered CrAl layer(25 nm thick),a fine columnar CrAl layer(25 nm thick)with a column width of 3 nm,and a thick columnar CrAlN layer(150 nm thick)with a column width of 20 nm Compared with monolayer CrN and CrAlN coatings,it is found that the multilayer coating has higher bonding strength(46.2 N),fracture toughness(8.7 MPa·m1/2),minimum residual stress(-0.932 GPa)and average erosion rate(0.916 ?m/g),which shows a good erosion resistance.However,with the increase of deposition period(film thickness),the surface particles of the CrAl/CrAlN multilayer coating increase,and the roughness and residual stress also increase.While the fragmentation occurs when the thickness of the coating increases to about 8 ?m,and it is difficult to prepare the coating with thickness above 8 ?m.(3)Based on the design concept of strength,toughness and internal stress control,the Cr/CrN/Cr/CrAlN multilayer coating with a cycle of 200 nm and a total thickness of 8 ?m was designed and prepared,taking the wear resistance and protection requirements of the coating under the action of multi-angle of attack solid particles as the basic requirements.The effect of different modulation ratios of Cr/CrN and Cr/CrAlN on the microstructure,mechanical properties and erosion resistance of the coating is revealed.It is found that the Cr/CrN/Cr/CrAlN multilayer coatings have high hardness(above 3000 HV)and good film-base bonding strength.When the modulation ratio is 1:2,the bonding force can reach up to 54.6 N,and the indentation level can reach HF1.Compared with the titanium alloy substrate,the erosion resistance of the coating is increased by 8 times and 5 times under the conditions of 30°and 90°,respectively.Using the method of finite element simulation to study the internal reason of the coating fracture due to the erosion of sand and gravel,the main reason is the high tensile stress concentration at the lower surface of the CrAlN layer.At the same time,it is found that the coatings with modulation ratios of 1:2 and 1:3 have better performance of ability to absorb stress.The crack propagation morphology and the structural changes of the fracture micro-area in the coating were analyzed,and it was found that the fracture mechanism of the Cr/CrN/Cr/CrAlN multilayer coating was mainly brittle fracture.In addition,based on the influence of different interfaces between the multilayer structure on the crack tip,the propagation/termination mechanism of the crack in the coating is analyzed.The cracks propagated easily from the hard layer(nitride layer)into the soft layer(metal layer),a large amount of energy was consumed in the soft layer and the cracks ended at the next soft and hard interface.It shows that the metal intermediate layer provides a good ability to inhibit crack propagation and coordinate deformation,thereby improving the brittle fracture resistance of the coating.Finally,a multilayer coating with a modulation ratio of LCr/CrN:LCr/CrAlN of 1:2 has the best erosion resistance.(4)The coating with the thickness of 12 ?m was prepared using a Cr/CrN/Cr/CrAlN multilayer coating process with a modulation ratio of 1:2,and thermal cycle fatigue tests were carried out at 300?,500?,700? and 900? respectively.The experiment explored the internal relationship between the evolution of the micro structure of the coating and its mechanical and erosion properties in a high-temperature environment.With the increase of thermal cycling temperature,the surface appears discoloration and wrinkles after thermal cycling at 900?,and peeling occurs at the edge area at the same time.Analyzing the phase structure and microscopic morphology changes,it is found that the nitride phase disappears and the Cr2O3 and Al2O3 phases appear at this time,it is revealed that the main reasons for the coating cracking failure are the thermal expansion mismatch stress and the Cr-rich oxide generated in the high temperature environment growth stress.The hardness and bonding force of the coating after thermal cycling decrease with the increase of the thermal cycling temperature,and the erosion resistance becomes worse.Under the conditions of 30° and 90°,the erosion resistance of the coatings with a thermal cycle temperature of 700? is about 6 times and 4 times higher than that of the titanium alloy matrix,indicating that good erosion protection performance can be maintained at this time,but the temperature reaches 900?,the coating has completely failed and has no protective effect.The friction and wear performance of the coatings at room temperature,300?,500? and 700? were studied.It was found that the friction coefficient decreased with the increase of temperature,and the wear rate increased with the increase of temperature.This is due to the oxidation of the coating surface under high temperature conditions,Cr2O3 can play a similar role as a "lubricant".However,as the temperature increased,the coating softened,and the oxidation wear of "pitting" occurred in the abrasion zone.Finally,it can be determined that the effective use temperature of the 12 ?m grade Cr/CrN/Cr/CrAlN coating prepared in this experiment is less than 700?. |
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