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Ultra ne amorphous strontium borate was prepared using an ethanol supercritical uid drying (ESFD) technique and characterized using transmission electron microscopy (TEM), X-ray diffractometry (XRD), small angle X-ray scattering (SAXS) and nitrogen physical adsorption. Its tribological properties, as an antiwear additive of lubricating oil, were evaluated with four-ball and block-on-ring tribotester. The wear scar was characterized using scanning electron microscopy (SEM). Results indicated that strontium borate prepared in this way possessed a porous structure with a pore size below 20 nm. The wear resistance and load-carrying capacity of lubricating oil were improved by the strontium borate and the friction coef cient was also slightly decreased. The improvement of lubricating oil tribological properties resulted from the deposition of strontium borate and its tribochemical reaction product containing boron.