Preparation And Properties Of TiB₂-TiC-Co/NiAl Coatings On Magnesium Alloy

Magnesium alloys have low density and high specific strength.However,their poor wear resistance and corrosion resistance seriously restrict their application and development in the fields of automobile,mechanical engineering,electronics,and aerospace.Coatings or surface treatment techniques can effectively retard the wear and corrosion of magnesium alloys.TiB₂-TiC composite ceramics are good coating materials because of their high hardness,good thermal stability,excellent wear and corrosion resistance.In this study,two kinds of composite coatings of TiB₂-TiC-Co and TiB₂-TiC-NiAl were prepared on the surface of AZ91D magnesium alloy by atmospheric plasma spraying?APS?to improve the wear resistance and corrosion resistance of magnesium alloy.It was found that the addition of NaCl as a diluent in Co-Ti-B₄C system produced TiB₂-TiC-Co composite with a high ceramic content by self-propagating high-temperature synthesis?SHS?,and reduced the grain size of the resultant ceramic.The SHS product powder slurry was spray-dried to prepare dense,nearly spherical agglomerated powder,and the agglomerated powder was sprayed by APS to fabricate TiB₂-TiC-Co cermet coating on the surface of magnesium alloy.The electroless Ni-P and NiCrAlY layers were used as the interlayer to improve the bonding strength between the cermet coating and the substrate,and the value was as high as 21.4 MPa.The fracture failure of the coating occurred inside the interlayer.The microhardness of TiB₂-TiC-Co coating exceeds 1000 HV,which significantly improves the wear resistance of the substrate.Compared with the coarse-grained cermet coating,the fine-grained one has a more uniform microstructure and better wear resistance.The wear mechanism of cermet coatings is mainly abrasive wear,hard phase peeling and micro-cutting.The Ni-Ti-B₄C system SHS-reacted TiB₂-TiC-Ni cermet powder and Ni-Al mixture were spray-dried to prepare agglomerated powder.TiB₂-TiC reinforced NiAl intermetallic coating was deposited on the surface of magnesium alloy coated with Ni-P interlayer via reactive plasma spraying?RPS?.The bonding strength,hardness,corrosion resistance and friction and wear behavior of the coating were studied.The influence of ceramic content on the microstructure and properties of TiB₂-TiC-NiAl composite coating was revealed.The RPS coating has a compact structure with phase composition of NiAl,TiB2-TiC and the by-product Ni3Al.Meanwhile,the coating contains a small amount of unreacted Ni and Al,and its content decreases sharply with the increase of ceramic content in the spray powder.The ceramic particles in the RPS coating melt well,which is mainly attributed to the reaction of Ni-Al during the spraying process,releasing a large amount of heat.A few of incompletely melted particles form pores in the coating due to insufficient spreading.With the increase of TiB₂-TiC ceramic content,the microhardness and wear resistance of RPS coatings show an upward trend;however,the number of pores and cracks in the coating increases relatively,resulting in a reduction in bonding strength and corrosion resistance.The wear mechanism of the TiB₂-TiC-NiAl composite coating is abrasive wear and slight brittle flaking.