Nano-impact testing to compare the fatigue performance of multilayered coatings

1. Introduction
Lead (Pb) based coatings which are known for their lubricious behaviour are increasingly unfavoured for use in large industrial IC engines as Pb constitutes a serious health and environmental hazards. Tin (Sn) based coatings which are also known for their superior tribological performance represent a Pb-free alternative. The authors have previously developed multilayer Sn based coatings which demonstrated significant improvement to monolayer Sn coating structure [1-2]. Nano-impact testing technique has been used in the past to study the fatigue resistance of hard coatings [3-4]. In the current research, this testing technique has been employed to evaluate the fatigue performance of soft Sn based multilayer coatings and benchmarked against the monolayer Sn coating structure.

2. Experimental details
Nano-impact testing on Sn based coatings using a Nanotest Vantage system (Micromaterials Ltd., Wrexham, UK) was carried out. Three different structures of Sn based coatings were studied. Electroplating was used to deposit a monolayer coating of a soft Sn matrix, and two multilayer coatings of 3-layers and 5-layers structures consisting of the soft Sn matrix with either one or two hard SnNi intermetallic sublayer (s), respectively. Different combinations of test parameters such as load and indenter tip radius were employed to obtain correct test conditions for capturing the effect of hard intermetallic sublayer (s) and at the same time avoiding the effects of surface roughness and substrate hardness. Low frequency cyclic loading pattern (one cycle every 4 secs) was used due to the soft nature of Sn coatings. The depth of penetration of the indenter was recorded in real time at each impact. Post-test surface analysis of impact sites was also performed using SEM and 3D surface characterisation software.

3. Results
Results showed superior performance of the multilayer Sn coatings compared to the monolayer coating. Figure 1 and 2 clearly show much deeper penetration and larger residual impact craters for the monolayer (1-layer Sn) coating. The findings suggest that hard intermetallic sublayer in multilayer structures results in hard-soft interfaces between sublayers which act as barrier and resist the penetration of impact loading, therefore making the multilayer coatings more fatigue resistant.