Effects of bias voltage on coating structures and anticorrosion performances of PA-PVD Al coated NdFeB magnets

Al coated NdFeB magnets were prepared by plasma-assisted-physical-vapor-deposition(PA-PVD)method fo r enhancing the corro sion resistance.Mo rphologies and structures were characterized by an Xray diffractometer and a scanning electron microscope.Corrosion behaviors of the samples with different bias voltages were studied by electrochemical methods and neutral salt spray test,respectively.The bias voltage during the deposition process was optimized according to the coating structures and corrosion resistances.The density and flatness of Al coating increase with increasing the bias voltage,and at the same time the thickness decreases.The coating density and thickness turn to be stable when bias voltage is 1800 V.Also,the corrosion behaviors of PA-PVD Al coatings in different media,such as NaOH,HNO_(3) and NaCl solutions,were studied.The self-corrosion potentials(E_(corr)) of PA-PVD Al coatings almost keep constant in NaOH and HNO_(3) solutions with diffe rent concentrations.However,the self-corrosion current densities(J_(corr)) decrease with the increasing concentrations.E_(corr) shifts to negative potential and J_(corr)decreases gradually when increasing the concentration of NaCl solution.The corrosion mechanisms of Al coatings are discussed based on the corrosion behaviors in different media.

Corrosion resistance and mechanical properties of (Ho,Nd)FeB magnets

(Ho,Nd)FeB magnets with different Ho contents were prepared by Ho substitution for part of Nd during the casting process.Effects of Ho contents on the corrosion resistance and mechanical properties of(Ho,Nd)FeB magnets were analyzed by a highly accelerated aging tester,an electrochemical workstation,a microhardness tester,a bending tester,a scanning electron microscope and an X-ray diffractometer.Results show that the addition of Ho can change the main phase structure,optimize the distribution of rare-earth rich(RE-rich) phases in grain boundary,and improve the corrosion resistance and mechanical properties of NdFeB magnets.When the content of Ho increases from 0 to 21.0 wt%,the weight loss of magnets decreases from 2.672 to 0.933 mg/cm^(2),and the microhardness and bending strength increase from 528.74 HV and 374.92 MPa to 633.84 HV and 459.80 MPa,respectively.