Nanoindentation as an alternative to mechanical abrasion for assessing wear of polymeric automotive coatings

Mechanical abrasion followed by transmission delta-haze measurement is a standard means to assess wear in polymeric silicone automotive hard coatings. However, the drawbacks of this abrasion-transmission (A-T) technique (drift, variability, sample size, and test time) make an alternative measurement method desirable. Literature reports have shown that the ratio of hardness to modulus can successfully predict wear performance in ceramic and metallic nanocomposite coatings. This work studied measures from both nanoindentation and nanoscratch testing to determine which could be a viable alternative to the historical A-T test for a specific polymeric coating system. Both nanoindentation measures of hardness (H) and the ratio of hardness to modulus (H/Er) showed high repeatability compared with the other measures evaluated in this study and compared with the historical test. Of these two measures, the ratio H/Er with an exponential fit showed the strongest correlation with A-T delta-haze measurements. Key formulation and process factors affecting abrasion resistance in automotive coatings were analyzed in a designed experiment with historical A-T delta-haze and nanoindentation H/Er as responses. Analysis showed significant benefits to the use of the H/Er measure of abrasion resistance in modeling coating performance.