Humidity effect on the critical number of friction cycles for wear particle generation in carbon nitride coatings

Wear particle generation in carbon nitride coatings by a spherical diamond counterpart in repeated sliding contacts has been studied with an emphasis on the effect of relative humidity varying from 0.4 to 80%. An environmental scanning electron microscope (E-SEM), in which a pin-on-disk type apparatus was installed, has in situ provided direct evidence that when and where the wear particle generation do occur. The in situ examination of non-conductive carbon nitride coatings are therefore available free from surface charging with controllable relative humidity and a sliding speed of 50 mm/s varying normal load from 10 to 250 mN. Based on the in situ examination, the shape transition maps for generated wear particles have been obtained for carbon nitride coatings in various relative humidity. The results show that the critical number of friction cycles ( N c) for detecting a cluster of wear particles is observed to be generally increasing with an increase in relative humidity. It also appears that the increase in relative humidity results in a decrease in peak-to-valley (P-V) values of contact tracks during the first friction cycle, which is essentially induced by a combined plastic deformation behavior of carbon nitride coated silicon surface. This further implies that the effective surface property such as hardness has been possibly changed by introducing water vapor into the contact interface between the carbon nitride coating and the spherical diamond counterpart, if assuming the contact pressure has a constant value.