Study on the Properties of the Electroless Nickel Phosphorus Coating on the Surfaces of the Lotus-type Porous Copper and the Titanium Alloy Tubes

Author:Cui Chuan Yu

Supervisor:xiong tian ying du zuo


Degree Year:2019





Due to its high thermal conductivity,oriented pore structure and smooth pore walls,the lotus-type porous copper will likely be a promising candidate in the fields of fuel filtration,heat sink and energy absorbing materials.However,the poor corrosion and wear resistance of pure copper limits its wider application in the above fields.In order to mitigate these problems,the electroless Ni-P coating and pack cement techniques were applied on the lotus-type porous copper to improve its corrosion and wear resistance and compression behavior.Electroless Ni-P coatings were successfully plated on the inner and the outer walls of the lotus-type porous copper By studying the corrosion behavior of the coatings on the inner wall and the outer wall of the porous copper,it was found that although the narrow space in the pore limits the diffusion of the reactants in the plating solution into the pores,rendering the inner wall coating thinner than its outer wall counterpart,but,on the contrary,it has better corrosion resistance.This is because the columnar growth mechanism of electroless Ni-P produces a competing growth behavior on the pore walls.This competitive growth can force the hydrogen generated during the plating process out of the inter-columnar spaces,thereby obtaining a denser coating and reducing defects on the surface of the coating.In addition to the corrosion behavior,we also studied the improvement of the compression behavior of the coating on the porous copper.It has been found that the coating plated on the pore walls can significantly improve the compressive yield strength and compression energy absorption of the lotus-type porous copper.The improvement in mechanical properties can be attributed to the internal stress between the coating and the pore walls,the dislocation slip barrier during the deformation of the pore walls,and the energy release of the coating crack.The new interface between the coating and the walls prevents dislocations from leaving the copper matrix and causes dislocations to form dislocation pileups and stacking faults at the interface,which further hinder the movement of dislocations toward the interface,increasing the stress required for plastic deformation,This plays a more important role in the improvement of mechanical properties.In addition to electroless Ni-P coatings,the possibility of embedding aluminized coatings on the inner walls of the lotus-type porous copper was also explored.Studies have shown that aluminized coatings can be successfully obtained on the inner and outer walls of the porous copper.The thickness of the aluminized layer is largely determined by the pore size:the larger the pore size,the thicker the thickness of the aluminized layer.As the pore size decreases,the thickness of the aluminized layer decreases drastically.When the pore diameter is less than 300 the aluminized layer on inner wall of the pore is difficult to form.After pack cementation treatment,the surface of the porous copper is increased by nearly three times,and the wear mechanism of these samples was changed from the original adhesive wear to the abrasive wear,which greatly reduces the wear rate.Similar to the porous copper,the titanium alloy Ti6A14V has excellent mechanical properties and high specific strength,which is very suitable for the barrel material of the current military equipment.However,the poor wear resistance of titanium alloy limits its use in the ballistic environment.Electroless Ni-P as an excellent wear-resistant coating is very suitable to solve this problem.However,titanium alloys are not easy to plate,because the oxide layer on the surface will passivate the electroless plating reaction.In order to activate the titanium alloy,strong mixed acids are needed to activate the titanium surface,but the strong acids are harmful to the operator and the environment,which greatly increases the production difficulty.In this thesis,we have successfully prepared the coating on the titanium alloy substrate by an in-situ activation method,and the diffusion zone formed by heat treatment increases the adhesion and thermal shock resistance of the coating to the substrate.Moreover,we also studied the wear resistance of the coating after heat treatment in an air environment of 400℃.The result showed that the heat treatment temperature has a great influence on the wear resistance of the electroless Ni-P coating.The coatings heated at 600℃ for 2 h has the highest hardness and the lowest wear rate;the hardness of the coating decreased when the heat treatment temperature was elevated to 700℃,but the wear rate and the wear mechanism remain somewhat unchanged;the 800℃ sample has the lowest hardness and the highest wear rate,and the worn surfaces exhibit typical adhesive wear.Combining the results regarding the wear resistance,the adhesion strength and the thermal shock resistance,the 700℃ 2 h vacuum heat treatment was deemed the optimal heat treatment process for the electroless Ni-P coating on titanium alloy substrate.