Scanning Probe Microscopy for Nanoscale Characterization of Electrical and Magnetic Properties

Atomic force microscopy (AFM) is a nanoscale scanning probe microscopy (SPM) characterization technique useful for obtaining topographical maps of surfaces and their associated nanomechanical properties. Complementary SPM modes such as Kelvin probe force microscopy (KPFM) and magnetic force microscopy (MFM) can simultaneously elucidate the electrical and magnetic properties of materials with nanoscale resolution, thereby expanding AFM’s utility. KPFM measures the Volta potential difference between a conductive AFM probe and the sample surface, which can be related back to the work function of the material and correlated with co-localized elemental mapping via energy dispersive spectroscopy (EDS). This can be useful for understanding and predicting initiation and propagation of galvanic corrosion in metal alloys. MFM employs a magnetized AFM probe tip to detect magnetic interactions between the sample and the tip, thereby mapping out the magnetic structure of the sample surface. Here we present KPFM studies of case-hardened stainless steels engineered for bearing applications in high performance jet engines destined for operation in corrosive marine environments. MFM studies of Ni-Mn-Ga, a magnetic shape memory alloy, connect experimental data with computational modeling to understand the growth of twins in response to bending. Together, these studies highlight the widespread applicability of AFM, KPFM, MFM, and other SPM techniques for illuminating nanoscale structure-property relationships in material systems

Electrochemically Induced Amorphous-to-Rock-Salt Phase Transformation in Niobium Oxide Electrode for Li-Ion Batteries

Intercalation-type metal oxides are promising negative electrode materials for safe rechargeable lithium-ion batteries due to the reduced risk of Li plating at low voltages. Nevertheless, their lower energy and power density along with cycling instability remain bottlenecks for their implementation, especially for fast-charging applications. Here, we report a nanostructured rock-salt Nb2O5 electrode formed through an amorphous-to-crystalline transformation during repeated electrochemical cycling with Li+. This electrode can reversibly cycle three lithiums per Nb2O5, corresponding to a capacity of 269 mAh g−1 at 20 mA g−1, and retains a capacity of 191 mAh g−1 at a high rate of 1 A g−1. It exhibits superb cycling stability with a capacity of 225 mAh g−1 at 200 mA g−1 for 400 cycles, and a Coulombic efficiency of 99.93%. We attribute the enhanced performance to the cubic rock-salt framework, which promotes low-energy migration paths. Our work suggests that inducing crystallization of amorphous nanomaterials through electrochemical cycling is a promising avenue for creating unconventional high-performance metal oxide electrode materials

Impact of Grain Orientation and Phase on Volta Potential Differences in an Additively Manufactured Titanium Alloy

This work introduces a method for co-localized multi-modal imaging of sub-μm features in an additively manufactured (AM) titanium alloy. Ti-6Al-4V parts manufactured by electron beam melting powder bed fusion were subjected to hot isostatic pressing to seal internal porosity and machined to remove contour–hatch interfaces. Electron microscopy and atomic force microscopy-based techniques (electron backscatter diffraction and scanning Kelvin probe force microscopy) were used to measure and categorize the effects of crystallographic texture, misorientation, and phase content on the relative differences in the Volta potential of α-Ti and β-Ti phases. Given the tunability of additive manufacturing processes, recommendations for texture and phase control are discussed. In particular, our findings indicate that the potential for micro-galvanic corrosion initiation can be regulated in AM Ti-6Al-4V parts by minimizing both the total area of {111} prior-β grains and the number of contact points between {111} β grains and α laths that originate from {001} prior-β grains

Corrosion Initiation and Propagation on Carburized Martensitic Stainless Steel Surfaces Studied via Advanced Scanning Probe Microscopy

Historically, high carbon steels have been used in mechanical applications because their high surface hardness contributes to excellent wear performance. However, in aggressive environments, current bearing steels exhibit insufficient corrosion resistance. Martensitic stainless steels are attractive for bearing applications due to their high corrosion resistance and ability to be surface hardened via carburizing heat treatments. Here three different carburizing heat treatments were applied to UNS S42670: a high-temperature temper (HTT), a low-temperature temper (LTT), and carbo-nitriding (CN). Magnetic force microscopy showed differences in magnetic domains between the matrix and carbides, while scanning Kelvin probe force microscopy (SKPFM) revealed a 90–200 mV Volta potential difference between the two phases. Corrosion progression was monitored on the nanoscale via SKPFM and in situ atomic force microscopy (AFM), revealing different corrosion modes among heat treatments that predicted bulk corrosion behavior in electrochemical testing. HTT outperforms LTT and CN in wear testing and thus is recommended for non-corrosive aerospace applications, whereas CN is recommended for corrosion-prone applications as it exhibits exceptional corrosion resistance. The results reported here support the use of scanning probe microscopy for predicting bulk corrosion behavior by measuring nanoscale surface differences in properties between carbides and the surrounding matrix