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Polycarbonate (PC) and protective (Ti,Al)N coatings exhibit extremelydifferent material properties, specifically crystal structure, thermalstability, elastic and plastic behavior as well as thermal expansioncoefficients. These differences present formidable challenges for thedeposition process development as low-temperature synthesis routes have to beexplored to avoid a thermal overload of the polymer substrate. Here, alarge-area sputtering process is developed to address the challenges bysystematically adjusting target peak power density and duty cycle. Adhering(Ti,Al)N coatings with a critical residual tensile stress of 2.2 +/- 0.2 GPaare obtained in the pulsed direct current magnetron sputtering range, whereasdepositions at higher target peak power densities, realized by high powerpulsed magnetron sputtering, lead to stress-induced adhesive and/or cohesivefailure. The stress-optimized (Ti,Al)N coatings deposited onto PC with a targetpeak power density of 0.036 kW cm-2 and a duty cycle of 5.3% were investigatedby cross-cut test confirming adhesion. By investigating the bond formation atthe PC | (Ti,Al)N interface, mostly interfacial CNx bonds and a small fractionof (C-O)-(Ti,Al) bonds are identified by X-ray photoelectron spectroscopy,indicating reactions at the hydrocarbon and the carbonate groups duringdeposition. Nanoindentation reveals an elastic modulus of 296 +/- 18 GPa forthe (Ti,Al)N coating, while a Ti-Al-O layer is formed during electrochemicalimpedance spectroscopy in a borate buffer solution, indicating protectivepassivation. This work demonstrates that the challenge posed by the extremelydifferent material properties at the interface of soft polymer substrates andhard coatings can be addressed by systematical variation of the pulsingparameters to reduce the residual film stress.

Large-area deposition of protective (Ti,Al)N coatings onto polycarbonate