Chemical contribution to friction behavior of sintered hexagonal boron nitride in water

Sliding characteristics of sintered hexagonal boron nitride (hBN) are investigated in water. The friction coefficient of sintered hBN/hBN decreases to 0.06 in distilled water, whereas it is 0.23–0.25 in dry air. As the result of tribochemical reactions, ammonia was formed during sliding in water, and its amount was determined quantitatively for tribochemical reactions and the wear mechanism investigation. The percentage of NH3/total worn nitrogen was observed to be as high as 20% in regions of low wear rate, while it was around 2% in almost all of the tested regions of hBN. The percentage chemical wear was proposed as an index to distinguish the dominant wear process: chemical or mechanical wear process. On the slid surface, X-ray diffraction (XRD) analysis revealed that (002) crystal orientation of hBN was dominant, which corresponds to the sliding plane of the hBN film transferred onto the steel surface. The sintered hBN with CaO additive (hBN–CaO) had a higher frictional force and a lower specific wear rate than sintered pure hBN (p-hBN). Surface analysis of hBN–CaO revealed that Ca(OH)2 condensed on the slid surface and a Ca(OH)2 film covered hBN to reduce ammonia formation as well as the chemical wear ratio of hBN–CaO. The minor component CaO additive was proven to take an important role in the characteristics of hBN in water.