The friction behavior of the hot filament chemical vapor deposition(HFCVD) diamond film plays a critical role on its applications in mechanical fields and largely depends on the environment.Studies on the tribological properties of HFCVD diamond films coated on Co-cemented tungsten carbide(WC-Co) substrates are rarely reported in available literatures,especially in the water lubricating conditions.In this paper,conventional microcrystalline diamond(MCD) and fine-grained diamond(FGD) films are deposited on WC-Co substrates and their friction properties are evaluated on a reciprocating ball-on-plate tribometer,where they are brought to slide against ball-bearing steel and copper balls in dry and water lubricating conditions.Scanning electron microscopy(SEM),atomic force microscopy(AFM),surface profilometer and Raman spectroscopy are adopted to characterize as-deposited diamond films;SEM and energy dispersive X-ray(EDX) are used to investigate the worn region on the surfaces of both counterface balls and diamond films.The research results show that the friction coefficient of HFCVD diamond films always starts with a high initial value,and then gradually transits to a relative stable state.For a given counterface and a sliding condition,the FGD film presents lower stable friction coefficients by 0.02-0.03 than MCD film.The transferred materials adhered on sliding interface are supposed to have predominate effect on the friction behaviors of HFCVD diamond films.Furthermore,the effect of water lubricating on reducing friction coefficient is significant.For a given counterpart,the stable friction coefficients of MCD or FGD films reduce by about 0.07-0.08 while sliding in the water lubricating condition,relative to in dry sliding condition.This study is beneficial for widespread applications of HFCVD diamond coated mechanical components and adopting water lubricating system,replacing of oil lubricating,in a variety of mechanical processing fields to implement the green production process. The friction behavior of the critical role on its applications in mechanical fields and largely depends on the environment. Based on the tribological properties of HFCVD diamond films coated on Co-cemented tungsten carbide (WC -Co) substrates are rarely reported in available literatures, especially in the water lubricating conditions. In this paper, conventional microcrystalline diamond (MCD) and fine-grained diamond (FGD) films are deposited on WC-Co substrates and their friction properties are evaluated on a reciprocating ball-on-plate tribometer, where they are brought to slide against ball-bearing steel and copper balls in dry and water lubricating conditions. Scanning electron microscopy (SEM), atomic force microscopy (AFM), surface profilometer and Raman spectroscopy are adopted to characterize as-deposited diamond films; SEM and energy dispersive X-ray (EDX) are used to investigate the worn region on the surfaces of both count erface balls and diamond films. the research results show that the friction coefficient of HFCVD diamond films always starts with a high initial value, and then gradually transits to a relative stable state. For a given counterface and a sliding condition, the FGD film presents lower stable friction coefficients by 0.02-0.03 than MCD film. The transferred materials adhered on sliding interface are supposed to have predominate effect on the friction behaviors of HFCVD diamond films. Still more, the effect of water lubricating on reducing friction coefficient is significant. For a given counterpart, the stable friction coefficients of MCD or FGD films reduce by about 0.07-0.08 while sliding in the water lubrication condition, relative to in dry sliding condition. This study is beneficial for widespread applications of HFCVD diamond coated mechanical components and adopting water lubricating system , replacing of oil lubricating, in a variety of mechanical processing fields to implement the green production process.