Effect of substrate on microstructure and mechanical properties of sol–gel prepared (K, Na)NbO3 thin films

Lead-free ferroelectric (K, Na)NbO3 (KNN) thin films (∼200 nm thickness) were prepared using a modified sol–gel method by mixing K and Na acetates with the Nb–tartarate complex, deposited by spin-coating method on Pt/Al2O3 and Pt/SiO2/Si substrates and sintered at 650 °C. Pure perovskite phase of K0.65Na0.35NbO3 in film on silicon were revealed, while film on alumina contained also small amount of secondary pyrochlore Na2Nb8O21 phase. Homogenous microstructure of film on Si substrate was smoother with the lower roughness (∼7.4 nm) and contained spherical (∼50 nm) particles. The mechanical properties of films were characterized by nanoindentation. The modulus and hardness of KNN films were calculated from their composite values of film/substrate systems using discontinuous and modified Bhattacharya model, respectively. The KNN film modulus was higher on alumina substrate (91 GPa) in comparison with silicon substrate (71 GPa) and values of film hardness were the same (4.5 GPa) on both substrates.