  Polycarbonate (PC) and protective (Ti,Al)N coatings exhibit extremely
different material properties, specifically crystal structure, thermal
stability, elastic and plastic behavior as well as thermal expansion
coefficients. These differences present formidable challenges for the
deposition process development as low-temperature synthesis routes have to be
explored to avoid a thermal overload of the polymer substrate. Here, a
large-area sputtering process is developed to address the challenges by
systematically adjusting target peak power density and duty cycle. Adhering
(Ti,Al)N coatings with a critical residual tensile stress of 2.2 +/- 0.2 GPa
are obtained in the pulsed direct current magnetron sputtering range, whereas
depositions at higher target peak power densities, realized by high power
pulsed magnetron sputtering, lead to stress-induced adhesive and/or cohesive
failure. The stress-optimized (Ti,Al)N coatings deposited onto PC with a target
peak power density of 0.036 kW cm-2 and a duty cycle of 5.3% were investigated
by cross-cut test confirming adhesion. By investigating the bond formation at
the PC | (Ti,Al)N interface, mostly interfacial CNx bonds and a small fraction
of (C-O)-(Ti,Al) bonds are identified by X-ray photoelectron spectroscopy,
indicating reactions at the hydrocarbon and the carbonate groups during
deposition. Nanoindentation reveals an elastic modulus of 296 +/- 18 GPa for
the (Ti,Al)N coating, while a Ti-Al-O layer is formed during electrochemical
impedance spectroscopy in a borate buffer solution, indicating protective
passivation. This work demonstrates that the challenge posed by the extremely
different material properties at the interface of soft polymer substrates and
hard coatings can be addressed by systematical variation of the pulsing
parameters to reduce the residual film stress.
