Effect of bond coat modulus on the durability of silicone duplex coatings

Scratch tests were used to determine the effect of the bond coat modulus on the durability of silicone duplex coatings under continuous loading and low speed sliding contact. Two sets of duplex coatings were tested. Each had the same top coat but different bond coats, with layers ranging in thickness from 65 to 485 μm. Depth sensing indentation tests were performed to determine the modulus, elastic and irrecoverable energy of each layer. The top coat layer had an indentation modulus of 2.7 ± 0.2 MPa with a percent irrecoverable energy of 11%. One bond coat had an indentation modulus of 3.9 ± 0.5 MPa with a percent irrecoverable energy of 9%; the other bond coat had an indentation modulus of 155 ± 12 MPa with a percent irrecoverable energy of 54%. Scratch tests were performed with a 1.6 mm radius steel ball sliding at 5 mm/min during loading at 97 N/min. Coating failure was detected by a sudden drop in the tangential force at a critical value Tc. For both sets of coatings Tc increased linearly with both bond and top coat thickness. The thickness dependence of the bond coat with the higher indentation modulus was about 2.2 times more than for the lower modulus bond coat. The dependence of Tc on the modulus ratio of bond coat to top coat has been calculated for a sphere loaded into a duplex layer; Tc was found to increase monotonically with increasing modulus ratio. It has also been demonstrated that there should be an optimal value for the bond coat's indentation modulus that provides the most tear resistance. A tearing energy criterion was also used to qualitatively explain the bond coat's contribution to toughening the duplex coating, in that bond coats that dissipate more energy during deformation will require more energy for tearing.