The Influence Of Nickel On The Structure And Properties Of Boride Layer

The boriding layer has many advantages,such as high temperature strength,good wear resistance,high temperature oxidation resistance and corrosion resistance.However,the borided layer,with high brittleness and low impact fatigue resistance,may bring some negative effects for the further application.The boriding layer can also form looses and holes in the surface easily which decrease hardness and wear resistance.Therefore,improving toughness and reducing looses of borided layer is a main objective for researchers.In this thesis,nickel was added into boriding agent with B₄C under protective atmosphere to study the influence of nickel on the structure and properties of boride layer.The results show as follows:Using NiB as a donor with the protection of N₂ atmosphere,under the holding temperature920℃,holding time 6 h,a single phase Fe₂B boronizing layer with thickness to 62μm was obtained.The hardness of the layer was up to 1538 HV,boride layer was pyknotic and smooth without obvious cracks,and there were little holes and loose in the outer surface of boride layer.K_ICC value of 3.2 MPa·m^1/2 was tested in the most hardness position of boriding layer.Diffusion after electroplated nickel coating with thickness of 14.5μm,then used the best boriding process parameters to obtain a electroplated nickel-boriding layer,which is mainly composed of Ni₂B and（Fe,Ni）₂B,Fe₂B phase andγ-（Fe,Ni）phase.Due to the kirkendall effect and complex process,there were holes and loose in the outer surface of boride layer.Holes and loose in the outer surface of boride layer was reduced with nickel-boronizing layer after heat treatment.Maximum hardness in nickel-boriding layer appeared in the single Fe₂B phase layer,hardness valued to 1513 HV,hardness of the surface improved to 1046 HV,nickel-boriding layer has a more stable hardness gradient.K_ICC value improved from 4.9 MPa·m^1/2 to5.6 MPa·m^1/2 after heat treatment.Ni powder can regulate boron potential and improve suface quality of boriding layer,moreover,proper content of Ni powder can increase thickness of layer.Adding pure Ni powder to the boriding agent,a good suface quality and pyknotic boriding layer was obtained with thickness of 120μm under temperature 920℃,holding time 5 h.Maximum hardness valued to1580 HV in single Fe₂B phase,hardness of the surface valued to 1200 HV.K_ICC value of 4.4MPa·m^1/2 was tested in the 50μm distance from the surface.When 1.5 wt%nickel powder was added into boriding agent,the single Fe₂B phase can be obtained under 1 wt%to 10 wt%B₄C content.