Understanding the influence of graphene-based lubricant/coating during fretting wear of zircaloy

In order to study the relative effectiveness of oil-based and graphene-based lubrications towards suppressing the friction and wear damage of zircaloy-4 during low amplitude linear reciprocatory wear, fretting wear tests were performed with varying normal loads (5–20 N) under unlubricated, oil-lubricated (using Castrol TDN-81) and graphene-lubricated/coated (using ∼7 layers graphene film) conditions against stainless steel ball. While the oil-based lubricant has been found to be more effective towards lowering the co-efficient of friction at lower loads, the effectiveness of the graphene-based lubricant/coating increase significantly with an increase in load. In the contexts of suppressing the extent/severity of wear damage, the graphene-based lubricant/coating has been found to be significantly more effective than the oil-based lubricant. In fact, increase in effectiveness of the graphene-based lubricant/coating with load results in the wear volume remaining nearly invariant upon increase in load; thus, causing reduction in wear rate. This is in contrast to the increase in wear volume/rate with increase in load for the unlubricated and oil lubricated conditions; such that the wear volume and rate are lower for the graphene-lubricated/coated condition by more than a factor of 2 with respect to the oil-lubricated condition at 20 N. In the unlubricated condition severe sub-surface cracking induced material removal/delamination, presumably aided by incorporation of Fe and O due to tribochemical reaction, form the major mechanisms of wear damage of zircaloy-4. By contrast, all these get suppressed in the presence of the graphene-based lubricant/coating, even at 20 N; thus, leading to significant reduction in the overall wear damage.