| The cermet,either ceramic matrix composites or metal metrix composites which comprise both metal and ceramics,behaves in interesting ways because of its tailorable physical and chemical properties,e.g.thermal expansion coefficient and mechanical properties between metal and ceramics.Thus,it can better serve in many particular working conditions,e.g.it offers a perfect solution in components such as resistors and vacuum tubes,which demand both heat resistance and electric conductivity,or machine tools,which demand both wear resistance and toughness.Nanomaterials have been received great attentions of engineerers and scientists and have a wide variety of proven and potential applications due to their extraordinary properties beyond ordinary materilas.In this work,the microstructural manipulation of nanocrystalline cermet coatings of Ni-based metal-nitride composites and their microstrure dependence of high temperature oxidation and cavitation erosion behavior were investigated in order to further understand the nature of the nano-cermet materials and to guide the development and applications of advanced cermet materials.Two nanocrystalline cermet coatings,Ni-based NiCrAlYSiHfN/AlN multilayer and NiCrAlTiN cermet coatings,were designed and prepared by means of reactive sputtering.The investigation was focused on high temperature oxidation mechanism of the multilayer coating,the influences of parameters in magnetron sputtering and heat treatment on the microstructures and cavitation errosion behavior of the NiCrAlTiN coatings.Three conclusions are drawn according to the experimantal results:1.The NiCrAlYSiHfN/AlN multilayer coating quickly transformed into ?-(Ni,Cr)Al/AlN alternate structure,and an exclusive ?-Al2O3 scale was formed on the surface with a parabolic constant equivalent to that of ?-NiAl when the coating was oxidized at 1100?.The ?-Al2O3 scale alternated coarse-grained and fine-grained layer by mechanism of layer by layer oxidation of the ?-(Ni,Cr)Al and AlN layers.During the oxidation process,the continuity of AlN layer was gradually destroyed,and a small amount of y'-Ni3Al and AlN particles were formed in ? phase layer.2.At the initial stage of oxidation,the transformation from ?-Al2O3 to ?-Al2O3 on the surface of the multilayer coating was promoted significantly for the higher nitrogen content incorporated.The presence of AlN particles changes the growth mode of the Al2O3 scale.In the oxidation process,nitrogen atoms may migrate into the ?-Al203 to produce more oxygen vacancies,thus promoting the phase transformation of ???phase.3.The nanocrystalline NiCrAlTiN cermet coating was composed of metallic?/?'matrix and dispersive Ti2N/TiN nanoparticles and AlN submicron-particles after vacuum annealing at 1000? for 3 h.The density and size of the nitrides was dependent on the nitrogen content.The cavitation erosion resistance of the NiCrAlTiN coating was superior to austenitic stainless steel 304L,arc ion plating TiN hard coating and plasma spraying WC-12Co cermet coating.However,nitride precipitation in the NiCrAlTiN coating in this study exhibited negative effects on the cavitation resistance,even worse for the coatings containing more N.It probably resulted from the weakened strength of the ?/?' matrix due to the precipitation of nitrides and the weakened interface strength due to coarse nitride precipitates.4.Further study reveals that the cavitation erosion resistance of the NiCrAlTi-xN coatings can be manipinated in wide range by controlling the vacuum annealing temperature and time.The NiCrAlTi-3N coating annealed at 800 ? for 3 hours exhibited the best cavitation erosion resistance among the studied coatings,even better than that of its counterpart of NiCrAlTi coating and equiatomic NiTi alloy.This coating demonstrates that simultaneous strengthening the strength and toughness by nanocrystallizing gains and second-phase nanoparticle is possible in practice if the grain sizes and the matrix/particle interfaces are under well controled.In this NiCrAlTi-3N coating,the average grain sizes are several nanometers while the interfaces between the precipitated ?'-Ti2N nanoparticles and the matrix are semi-coherent.As a result,its hardness,toughness and elastic recovery are greatly enhanced.During incubation period,cavitation leads to formation of an outmost?10 nm thick layer with lattice distortiona while the nitride/metal interfaces keep semi-coherent and the matrix contains lots of stacking faults and mechanical twinning.The matrix/nitride interfaces look stronger than the majority of the cavitation inpacts,so they can constain the plastic deformation of matrix and thus effectively dissipate cavitation energy. |
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