Effect Of Coating On The Mechanical Properties Of Bulk Metallic Glass

Bulk metallic glass possesses many distinctive mechanical properties andits deformation mechanism is different from traditional materials due to itslong-range disorder and short-range order structure. And so the amorphous alloyhas obtained the extensive concern of the scientific community since it wasinvented. Its high strength and hardness, excellent bend ductility, excelllentcorrosion quality, outstanding oxidation resistance and wear resistance made itbecome the focus of many scientists. However, these desirable properties arealways accompanied with their inability to undergoing homogeneous plasticdeformation due to the absence of dislocation-mediated crystallographic slip.According to the present research, bulk metallic glass has a particular mode ofdeformation, that is deforming through shear banding. Because there is nobarrier during the propagation of shear bands, metallic glass become quite brittleand fail catastrophically. Therefore, low plasticity even none plasticity becomesan fatal flaw during bulk metallic glasses’ engineering application, which shieldsits other excellent properties. Two kind of coating were plated on the bulk metallic glass matrix toimprove its plasticity in this study. One was electroplating a Ni layer on theamorphous alloy, and detected the effect of coating on its compressive andbending properties under room temperature; Another was plating a Ni-P alloy onthe bulk metallic glass through electroless plating, and detected the effect ofcoating on its tensile and compressive properties under room temperature. Theresults proved that:(1) The bending strength and fracture deflection is increasingly improved withthe coating thickness of Ni-layer. That is to say, after coating, the plasticity hasbeen improved, and a thicker coating correspond to a better plasticity. Normally,Three point bending test is a kind of tension and compression symmetry test,neutrosphere is always located in the middle of the specimen height. Aftercoating, the stress state has been changed with the Ni film, which resulted in theneutral layer moving to the compressive side.(2) For compressive tests, the corresponding plastic deformation of a3hours-coating sample is9%, which improves7%than the uncoated sample.However, the compressive plasticity is reduced by a6hours-coating with athicker (100μm) film.That is to say, for compression samples, suitable coatingthickness can improve its plasticity, but the plating thickness is not as thick aspossible.(3) Because Ni is contained in the Zr52.5Cu17.9Ni14.6Al10Ti5(Vit105), and Ni haveautocatalytic effect in the process of electroless plating, chemical deposition method can be chose to electroless Ni-P alloy on bulk metallic glass. Researchindicated that, the P content of the coating is decreased with the increasing of thepH value, and with the improving of PH value, coating structure has a trend ofchanging from amorphous to crystalline.(4) After coating Ni-P alloy, as the combination of coating and substrate wasvery well, the compression plasticity at room temperature of bulk metallic glasswas obviously improved, especially when the PH is5.7, the plastic strainreached4.5%, and due to the strength of Ni-P is higher than the Ni, the drop ofthe strength is not very obvious.(5) As the combination of coating and substrate was very well, we deduced thatthe Ni-P coating could improve the tensile plasticity of amorphous alloy. Butdue to the strength of the coating is lower than substrate, the plasticity of thebulk metallic glass has not been improved.