Mechanical Robustness of Atomic-Layer- and Molecular-Layer- Deposited Coatings on Polymer Substrates.

摘要

The mechanical robustness of atomic layer deposited alumina and recently developed molecular layer deposited aluminum alkoxide ( alucone ) films, as well as laminated composite films composed of both materials, was characterized using mechanical tensile tests along with a recently developed fluorescent tag to visualize channel cracks in the transparent films. All coatings were deposited on polyethylene naphthalate substrates and demonstrated a similar evolution of damage morphology according to applied strain, including channel crack initiation, crack propagation at the critical strain, crack densification up to saturation, and transverse crack formation associated with buckling and delamination. From measurements of crack density versus applied tensile strain coupled with a fracture mechanics model, the mode I fracture toughness of alumina and alucone films was determined to be K(IC)=1.89 plus or minus 0.10 and 0.17 plus or minus 0.02 MPa m(0.5), respectively. From measurements of the saturated crack density, the critical interfacial shear stress was estimated to be Tau(c)=39.5 plus or minus 8.3 and 66.6 plus or minus 6.1 MPa, respectively. The toughness of nanometer-scale alumina was comparable to that of alumina thin films grown using other techniques, whereas alucone was quite brittle. The use of alucone as a spacer layer between alumina films was not found to increase the critical strain at fracture for the composite films. This performance is attributed to the low toughness of alucone. The experimental results were supported by companion simulations using fracture mechanics formalism for multilayer films. To aid future development, the modeling method was used to study the increase in the toughness and elastic modulus of the spacer layer required to render improved critical strain at fracture.