Effect of deposition rate on the microstructure of electron beam evaporated nanocrystalline palladium thin films

Highlights

• Fraction of twinned grains and twin boundary density increase with deposition rate.

• Clear increase of dislocation density was observed for the highest deposition rate.

• A moderate increase of the mean grain size with increase of deposition rate is found.

• For the highest deposition rate, the twin boundaries lose their coherency.

• Fraction of high angle grain boundary (55–65˚) increases with deposition rate.

Abstract

The influence of the deposition rate on the formation of growth twins in nanocrystalline Pd films deposited by electron beam evaporation is investigated using transmission electron microscopy. Statistical measurements prove that twin boundary (TB) density and volume fraction of grains containing twins increase with increasing deposition rate. A clear increase of the dislocation density was observed for the highest deposition rate of 5 Å/s, caused by the increase of the internal stress building up during deposition. Based on crystallographic orientation indexation using transmission electron microscopy, it can be concluded that a {111} crystallographic texture increases with increasing deposition rate even though the {101} crystallographic texture remains dominant. Most of the TBs are fully coherent without any residual dislocations. However, for the highest deposition rate (5 Å/s), the coherency of the TBs decreases significantly as a result of the interaction of lattice dislocations emitted during deposition with the growth TBs. The analysis of the grain boundary character of different Pd films shows that an increasing fraction of high angle grain boundaries with misorientation angles around 55–65° leads to a higher potential for twin formation.

Introduction

Palladium (Pd) is a promising candidate material for H₂ purification and sensing [1]. For these applications the metal surface area plays an important role regarding the optimization of the interaction between Pd and H₂. In order to obtain a higher surface to volume ratio, special attention has been paid to nanostructured forms of Pd such as nanoparticles, nanowires, nanotubes, nanocrystals, and thin films. Nano-wires have a high surface to volume ratio but suffer from poor mechanical stability [2]. Conversely, thin nanocrystalline (nc) Pd thin films deposited on a flat substrate offer improved mechanical stability, but relatively low surface to volume ratio [1], [3]. Moreover, it has been reported in the literature that nc thin metallic films often suffer from a lack of ductility which limits their use in applications in which the ability of the materials to deform, stretch, or permanently change shape without cracking must be controlled [4], [5], [6]. The low ductility is attributed to the high strength and to the lack of strain hardening due to the absence of forest hardening mechanisms especially when the grain size is in the submicron range [7].

Recently, highly textured and twinned nc metallic bulk samples and layers have been produced using pulsed electro deposition [8], [9], [10] and sputter deposition [11], [12], [13], [14], [15], [16]. All these studies confirm that the introduction of nanoscale twins considerably improves the strength/ductility balance. Typically, in nc metals with high-angle grain boundaries (GBs) the increased strength is accompanied by a loss of ductility, thermal stability, and electrical conductivity. However, nanotwinned metals such as copper exhibit very high tensile strengths, with good ductility, thermal stability, and electrical conductivity at room temperature [8], [17].

In order to amplify the formation of beneficial growth twins, the efforts for the synthesis were intentionally focused on metals with low and moderate stacking fault energy (SFE), such as Cu [9], [10] and stainless steels [12], [13], [14], [15]. However, although the technological needs for other nc twinned metallic films with higher SFEs are clear, for instance, in the case of Pd membranes to be used for hydrogen technologies, such materials have not as yet benefited much from similar efforts.

Recently, it has been reported that the strength/ductility balance of Pd thin films deposited by electron beam (e-beam) evaporation with an average grain size of 30 nm can be significant above 3–5% and sometimes up to 12% [7]. The origin of this behavior was extensively investigated using conventional and advanced transmission electron microscopy (TEM) and was partly attributed to the presence of the nanotwins which act as barriers to dislocation motion as well as sources for dislocation storage and multiplication via specific twin boundary (TB)/dislocation reactions, providing an isotropic hardening contribution to the strain-hardening capacity [17]. The results were also used by Colla et al. [19] for the development of a semi-analytical model based on homogenization theory in order to predict the strength and the evolution of the strain hardening capacity of the twinned Pd films. Back stress effects also partly related to the presence of twins constitute another origin for the good ductility of the Pd films. The high strain hardening capacity of the twinned Pd films was obtained with only 20% to 30% of the grains containing nanotwins, revealing a huge potential to raise even more the strength/ductility balance of the Pd films [19]. Therefore, a series of experiments to determine the driving forces as well as the fundamental mechanisms controlling the formation of growth twins in nc Pd films has been conducted. The final goal is the optimization of the deposition parameters to produce Pd films with a well-controlled population of nanoscale twins by analogy with recent experimental results obtained on Cu [10], [17]. Very recently, Wang et al. [20] have demonstrated the absence of nanoscale growth twins in sputter deposited {111} textured nc Pd films. On the contrary, an absence of clear crystallographic texture was observed in electron-beam-evaporated films accompanied by the presence of nanoscale growth twins in these films, even though Pd has a high SFE [21]. Based on careful High Resolution TEM (HRTEM) analysis of the TBs, the formation mechanism of the twins was attributed to the splitting and the subsequent migration of a GB segment instead of a twinning mechanism involving the nucleation and glide of Shockley partial dislocations (SPDs) [22], [23], [24], [25], [26]. It was concluded that the large misorientation between the grains in the non-textured films constitutes the driving force for the activation of the twin growth mechanism. However, several key questions were left open regarding the influence of other parameters such as the effect of the deposition rate and of the internal stress. Also, the experimental methods used by Wang et al. [18] to analyse the texture-dependent twin formation did not include the investigation of the character and the type of the GBs susceptible to act as preferential sites for the nucleation of the twins.

Recently, Castrup et al. [27] studied the structural properties and residual stresses of 500 nm thick magnetron sputtered nc Pd and PdAu films on flexible polymeric substrates, while changing the pressure of the sputtering gas (Ar) in the range of 0.3 to 2 Pa, which results in deposition rates in a relatively narrow range from 2.1 to 1.2 Å/s, respectively. X-ray diffraction (XRD) measurements showed that the intensity of the (111) texture as well as the grain size decreases while the (220) texture becomes more dominant with increasing Ar pressure. Besides, no significant change of microstructure was observed when compared to deposition on hard substrates. In the present work, the microstructure changes of e-beam evaporated Pd thin films including grain size, texture changes, and GB analysis at nanoscale are investigated while changing deposition rates in a relatively wide range from 0.3 to 5 Å/s. The density of TBs was measured using conventional Bright Field (BF) and Dark Field (DF) TEM while HRTEM was used to measure the dislocation densities and the interaction of the dislocations with growth TBs based on the analysis of Fast Fourier Transforms (FFTs) and Geometric Phase Analysis (GPA) maps. Furthermore, Automated Crystallographic Orientation Indexation in a TEM (ACOM-TEM) has been used to characterize the evolution of the grain size distribution, the crystallographic texture, and the character of the GBs. These results provide insightful information to guide the generation of microstructures with enhanced strength/ductility balance in high SFE nc metallic thin films.