Study On Preparation And Tribological Behavior Of Electroless Ni-B-W Coating On Titanium Alloy

TC4 titanium alloys were broadly applied in many areas because of their low-density,good corrosion resistance,wide temperature range adaptability,and superior specific strength.Nevertheless,wider applications were limited by their poor hardness and wear resistance.This research aimed to improve the hardness and wear resistance of TC4 by preparing Ni-B-W multiple alloy coatings on its surface through electroless plating,extending the service life and broadening the application scenarios of titanium alloys.The morphology,composition,microstructure,mechanical and tribological properties of the coatings were characterized and tested by scanning electron microscopy,energy dispersive spectroscopy,optical microscopy,Vickers hardness tester,nanoindentation/scratch tester,friction and wear testing machine,white light scanning profilometry,and X-ray diffractometer.The conclusions are listed as followed:（1）The co-deposition of W element reduced the content of B element in the coatings,ranging from 6.8 wt.%to 8.53 wt.%,all of which belonged to rich-boron coatings,meanwhile,the reaction between tungstate(WO₄^2-)ions and tetrahydroborate（BH₄^-）ions also reduced the content of Ni element in the coatings.（2）The hardness,fracture toughness and bonding strength with the substrate of the coatings were improved owing to the presence of W element.The existence of hard metal element W hindered the formation and growth of microcracks within the coating under external forces,resulting in a shift in the direction of crack propagation,releasing the stress that drove the cracks to develop along the axial and radial directions,thereby boosting the properties of alloy films.（3）When the concentration of sodium tungstate was 20 g/L,the wear volume of the coating was the smallest,reducing about 91.2%and 84.3%compared to TC4 and Ni-B coatings,respectively.Mixed wear form of abrasive wear,adhesive wear and oxidation wear in the coatings was found in both Ni-B and Ni-B-W coatings.The excellent mechanical properties enabled coatings to better resist deformation and abrasive particles indentation during friction and wear movements,exhibiting better tribological properties.Compared to Ni-B coatings,Ni-B-W could better adapt to more severe friction and wear environments,like greater loads,higher speeds,and longer periods of time.（4）Annealing crystallized the Ni-B and Ni-B-W amorphous coatings,the mechanical properties and wear resistance of the coatings were further enhanced after annealing at 350℃.Because after annealing treatment,hard phases such as Ni,Ni₂B,Ni₃B and solid solution of Ni-W were formed in the coatings.The mechanical and tribological properties of coatings with crystalline structure were increasingly enhanced by grain boundary strengthening.However,as the annealing temperature increased to 500℃and 650℃,the grains grew to coarser and the structural characteristics changed,which made the hardness of the coatings smaller.Although the fracture toughness was better,the wear of the coatings was significantly increased,indicating that the hardness played a dominant role in impacting the wear resistance of the coatings.After annealing at different temperatures,the performance of Ni-B-W coating was always better than that of Ni-B coating,which may be related to Ni-W solid solution strengthening.