Cohesive and adhesive properties of nanocrystalline Ti thin films on polyimide substrates

Abstract

In this work, cohesive and adhesive properties of Ti thin films on polyimide substrates were studied systematically as a function of grain size by in situ optical microscopy, resistance measurements and synchrotron X-ray diffraction under uniaxial tensile testing. The characteristic of cohesive and adhesive behavior can be well reflected in the stress-strain curves determined by synchrotron X-ray diffraction. Generally, the yield strength decreases while the cracking strain increases with increasing grain size, due to the constraint effect of grain size on dislocation activity. Different from the yield strength, the fracture strength is almost unchanged with grain size. The fracture toughness is found to increase as the grain size increases, which is attributed to the enhanced plasticity during fracture process. Additionally, the interfacial adhesion energy is determined to be 3.8 J m⁻². The buckling strain increases with increasing grain size as a result of enhanced plastic dissipation. The formation of patches, instead of buckles, in Ti film with smaller grain size is ascribed to the low deformation capability of the film.

Introduction

Titanium (Ti) thin films have arose much more attention for their extensive applications in microelectromechanical systems (MEMS), medical implants, and intelligent materials due to high mechanical strength, excellent thermal stability, good corrosion resistance in extreme conditions and intrinsic biocompatibility [1], [2], [3], [4]. Many works have been focused on the investigation of chemical and structural characteristics [1], [5], [6], phase transformation [7], [8], and hardness/strength [9] of Ti thin films, and also their dependence on film thickness [10], temperature [9], and deposition conditions [6]. Generally, pure Ti and most its alloys form an hexagonal close-packed (hcp) structure ($\alpha$-Ti) under the low temperature condition, and it is transformed into a body-centered cubic (bcc) structure ($\beta$-Ti) under the high temperature condition [11]. Additionally, a phase transformation from face-centered cubic (fcc) to hcp structure with increasing film thickness ($h$) has been observed in polycrystalline Ti thin films [7].

Recently, with the emergence and rapid development of flexible electronics, composing of metallic thin films on flexible substrates, complex service environment brings forth the increasingly stringent requirements for materials. The capabilities to be bent, folded, stretched, compressed, twisted and even deformed to arbitrary shapes without failure are needed for the effective utilization of metallic film/compliant substrate systems. Consequently, much effort has been devoted to cohesive and adhesive failure/properties and their film thickness dependence [12], [13], [14], [15], [16], [17], [18]. Lu et al. [14] and Niu et al. [19] performed uniaxial tensile tests on polyimide-supported Cu films and reported an monotonically enhanced cracking strain (${\epsilon }_C$) with increasing film thicknesses. Frank et al. [20], [21] investigated the cohesive and adhesive failure of thin tantalum films by an uniaxial tensile testing combined with in situ optical microscopy and synchrotron X-ray diffraction and developed a two dimensional shear lag model to describe the biaxial stress field within a film fragment. The cracking strain was found to decrease as the film thickness increases [21]. Similar results were also reported in polyethylene terephthalate (PET) substrates-supported Cr films [22]. Schlich et al. [12], [23] revealed two different film thickness dependence of the buckling strain (${\epsilon }_B$) and proposed two delamination mechanisms to describe them. For fracture toughness measurements on thin films, several methods such as indentation-based techniques [24], [25], bulge tests [26], [27] and tensile tests [20], [28] have been used. However, only the tensile tests will be suitable to determine the fracture toughness ($K_{IC}$) for films adherent to flexible substrates. Based on tensile tests, some energy-based models [20], [29], [30] have been developed.

Actually, the aforementioned works mainly focused on the Ti films adherent to the rigid substrates, such as Si wafer. Yet, to date, the cohesive and adhesive properties of Ti films on compliant substrates were less studied, although such systems are also widely employed as adhesion layers [16], [31] in flexible electronics, as bolometers in infrared sensors [32], and as electrical connectors in thermal insulating structures [33]. Therefore, systematic investigation of the mechanical properties of Ti films on compliant substrates needs to be developed urgently. Additionally, it is well known that the grain size usually increases with film thickness for metallic films [14], [34], [35]. Thus, the film thickness dependence of the cohesive and adhesive properties in previous works is usually a result of the superposition influence of film thickness and grain size. The individual effect of grain size on the cohesive and adhesive properties is still unclear and the underlying mechanism is also poorly understood. In this work, we overcome this gap by investigating the cohesive and adhesive properties of Ti thin films with the same thickness but different grain sizes ($d$) ranging from 20 nm to 80 nm. Uniaxial tensile testing combined with synchrotron X-ray diffraction, optical microscopy, and resistance measurements are performed to determine the cracking strain, yield strength, fracture strength, fracture toughness, buckling strain, and adhesion energy. Also, these properties will be discussed elaborately in light of the grain size-dependent deformation.