Research On Microstructure,ablation And High-temperature Oxidation Behavior Of Ta Coating By Magnetron Sputtering

In the service process,the gun barrel must bear the erosion of high temperature,high pressure and high-speed gunpowder gas,which will lead to physical and chemical changes of the metal inside the gun barrel.That will also inevitably result in the degradation of ballistic performance,such as the decrease of initial velocity,the shortening of fireshot range and the decline of firing accuracy,affecting the service life and combat capacity of the gun seriously.Therefore,improving barrel performance and prolonging barrel service life are extremely important in the process of artillery research and development.Although the electroplated chromium coating can protect the barrel to some extent,its intrinsic microcrack defect becomes the channel for gunpowder residue to corrode the substrate preferentially,and then the Cr coating peels off rapidly.Moreover,the hexavalent chromium process seriously pollutes the environment,so it is imperative to study the substitute of electrodeposited Cr coating.In this paper,nanocrystalline Ta coating is deposited on 30Cr Ni3Mo V steel by magnetron sputtering in negative glow space.The thesis conducts the following research,and the corresponding conclusions are drawn as follows:（1）Ta coating is sputtered on steel substrate.The coating microstructure during growth is deeply studied by XRD,SEM and TEM.The key influencing factors ofβpreferential formation and depositing phase transformation behavior are analyzed.Studies shows that the sputtered Ta atoms rob the physico-chemical adsorbed oxygen on the steel surface.In the process of nucleation,Ta O_x bottom layer is produced by in-situ reactive sputtering.Based on the radiative thermal effect of negative glow space,the deposition behavior of Ta coating is a transformation course of Ta O_x→amorphous Ta→α+βmixed phase→αphase.（2）Compared with electroplated Cr coating,the simulated ablation process by laser pulse heating on nanocrystalline Ta coating is characterized by XRD,metallographic microscope and SEM,in order to analyze their service failure mechanism.The results show that the electroplated chromium coating is easy to produce transverse microcracks,which is caused by cyclic thermal stress.In addition,bulk-spalling behavior occurs when the transverse microcracks are connected with longitudinal cracks.The substrate at the coating/substrate interface is prone to oxidation,and deep heat affected zone happens at the substrate surface.Nanocrystalline Ta coating is basically intact during the laser pulse heating,and microcracks caused by thermal stress releasing occur after 100 pulses.An interconnection region appears at the Fe/Ta interface,which is caused by the sub-surface boiling effect of laser pulse scanning.The interfacial boiling phenomenon reduces the adhesion between nanocrystalline Ta coating and substrate,which will become an approaching problem of interface instability during service.（3）Compared with rolled Ta plate,the short-time oxidation behavior of nanocrystalline Ta coating is studied by XRD,SEM and TEM.The failure mechanism of nanocrystalline Ta coating is determined in comparison with rolled Ta plate,and the pulverization process and mechanism of Ta during the oxidation process are also found.The results show that for the rolled Ta plate,the expansion and disintegration of the most surface Ta oxide are completed in a few seconds,so that the internal matrix of Ta plate is quickly exposed to air and oxidized again at high temperature,resulting in cyclic disintegration and pulverization.Due to the characteristics of nanocrystalline Ta coating,its intrinsic pinholes provide a large number of channels for the rapid inward diffusion of oxygen,which makes it difficult for the oxygen to agglomerate quickly at the coating surface.Because the coating is not as dense as rolled Ta plate,the defects between columnar crystals provide a released zone for the volume expansion process during oxidation,which makes it feasible to form a comparatively dense layer of Ta oxide on the surface and close the channel of oxygen diffusion along the intergranular.Moreover,in the initial oxidation course,the oxidation of nanocrystalline Ta coating also experiences the transformation from amorphous state to crystalline state.The crystallization of Ta oxide can produce self-sustaining energy,which will urge the local temperature to exceed the melting point of Ta oxide,and then the outward bursting behavior of Ta oxide will occur.As to the collapsed Ta oxide,further volume shrinkage behavior happens after cooling and solidification,leading to becoming powdered individuals,and thus disintegrating rapidly.