Preparation And Properties Of NiCrCoAlY-TiB₂ Nano-powders And Its Composite Coating Applied To HVOF

High Velocity Air Fuel spraying?AC-HVAF?is a kind of spraying method for preparing coatings with excellent bonding strength and high density.Spray feeding plays the most important role in the spraying process and has been attracted much attention.Due to its ultra-high hardness?micro hardness 34GPa?,high melting point?2980??,excellent resistance to high temperature oxidation?1000?,air?and other characteristics,TiB₂ shows excellent wear resistance,corrosion resistance and high temperature performance.M-TiB₂ cerament composites coating is considered to be one of the best materials to replace the traditional coatings working on conditions of high temperature?450?¹000??and great wear resistance.The NiCrCoAlY alloy has good properties at high temperature and can be used as the bonding phase of TiB₂ ceramic powder.It can increase the bonding strength and reduce the stress caused by different thermal expansion coefficient.Compared with micro-structure coatings,nano-structure coatings have many excellent properties,such as higher hardness,high bonding strength and good wear resistance.In this thesis,the preparation of TiB₂-NiCrCoAlY composite powders used as upersonic flame spraying structure feed were studied systematically.Properties of composite powders and coatings were also characterized.The nano-sized TiB₂-NiCrCoAlY composite powders were synthesized by high energy ball milling.After wet milled at 320r/min for 20 h and dry milled at 220r/min for 5h By optimizing the ball milling parameters,NiCrCo AlY-TiB₂ MMCs nanoscale powders with good fluidity and high cohesive were obtained,in which nanometer titanium boride particles dispersing in binder phases.The particles obtained are ranging from 5?m to 50?m and most of the powders are of spherical morphology.Compared with the initial mixed raw materials powders,repose angle and HR value of milled powders are reduced from 38.4° to 32.9° and from 1.925 to 1.248,respectively.It is indicated that the flow ability of feed powders has been improved obviously.For further improving density and fluidity of thermal spraying feed,the ball milled powders were reconstructed by plasma spheroidization technology.After being spheroidized,the HR value of ball milled powders is increased to 1.13,the compressibility is decreased to 11%,the angle of repose is reduced to 28.4°.Nanometer titanium boride particles remain dispersing in binder phases.This result shows that physical properties of ball milled powders,such as flow ability and density have been greatly improved.The particle size of spheroidized powders is mainly ranging from 5 to 50?m and suitable for supersonic flame spraying.Nanostructured NiCrCoAlY-TiB₂ composite coating is successfully prepared by HVAF technique using spheroidized powders on a Cr12 MoV substrate.The coatings are of compact and uniform structures,as well as good physical and mechanical properties.Micro-hardness,fracture toughness,porosity and the bonding strength of the nanostructured coating are 991.425kgf·mm-2,4.12MPa·m1/2,0.965% and 75.43 MP,respectively.