t‐ZrO₂ toughened Al₂O₃ free‐standing films and as oxidation mitigating thin films on silicon nitride via colloidal processing of flame made nanopowders (NPs)

Abstract

is a widely studied structural ceramic with applications as grinding media, for cutting tools, high-temperature engine components, prosthetic implants, and substrates for power electronics due to its high melting point, excellent mechanical strength, hardness, and chemical stability as well as its widespread abundance. 1-17 However, applications using pure α-Al 2 O 3 are limited by its low resistance to crack propagation, that is, poor fracture toughness (4-5 MPa m 1/2) 18,19 that can lead to catastrophic failure. 1-3 To enhance Al 2 O 3 fracture toughness, a common practice involves incorporating additives, such as tetragonal zirconia (t-ZrO 2), to increase toughness while maintaining the hardness and chemical resistance of Al 2 O 3 , commonly referred to as zirconia toughened aluminas (ZTAs). 2-6,20-29 Room-temperature stabilized t-ZrO 2 will transform to monoclinic (m-ZrO 2) if subjected to mechanical stress. This transformation is accompanied by a 4% volume expansion creating a compressive stress field in the surrounding grains, which opposes crack propagation. 2-4,20-24 In this process, some fraction of the energy needed for crack propagation