Excellent tribological behavior of hexadecylphosphonic acid films formed on titanium alloy

Titanium alloy(Ti6Al4V)surfaces are generally modified to achieve some specific surface properties to satisfy requirements of clinical medicine.In our work,hexadecylphosphonic acid(HDPA)films were successfully formed on Ti6Al4V and subsequently confirmed by X-ray photoelectron spectroscopy(XPS)and atomic force microscopy(AFM)measurements.The tribological properties of the HDPA-modified Ti6Al4V were investigated using a ball-on-disk tribometer with a linear reciprocating movement.Experimental results indicate that the HDPA-modified Ti6Al4V can maintain a low friction coefficient(approximately 0.06)for 4 h when sliding against polytetrafluoroethylene(PTFE)balls under a load of 0.8 N in comparison to bare Ti6Al4V(approximately 0.2);the friction coefficient of the HDPA-modified Ti6Al4V shows a 70%decline.In addition,the wear rate of PTFE balls sliding against bare Ti6Al4V was almost twenty times that of PTFE balls sliding against the HDPA-modified Ti6Al4V.Moreover,results of tribological experiments for different speeds(from 3 to 24 mm/s)and loads(from 0.8 to 3.2 N)proved that the HDPA-modified Ti6Al4V was not sensitive to both velocity and load.The friction coefficients were still low and stable even under a high load of 3.2 N or at a high speed of 24 mm/s.This indicates that this soft modification is an optional method of improving tribological properties of Ti6Al4V.