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

Advanced Scanning Probe Microscopy for Materials Research

Scanning probe microscopy (SPM) encompasses a set of advanced techniques for mapping the structure and properties of the surfaces of materials from the atomic to micro scales. The most widely used SPM technique is atomic force microscopy (AFM), in which forces exerted between the tip of a needle probe and the sample surface can be measured with extremely high precision. By recording these forces as the tip rasters across the surface, an image of the sample surface topography is obtained. Beyond the surface topography, several SPM techniques can provide quantitative information about the properties of a material’s surface. These include scanning Kelvin probe force microscopy (KPFM) for surface potential measurements, scanning capacitance microscopy (SCM) for surface capacitance mapping, conductive and tunneling AFM (C-AFM and TUNA) for imaging the electrical conductance of a surface, as well as several techniques for imaging the mechanical properties of a surface. These advanced SPM techniques provide tools for direct structure-property correlations in materials at the nanoscale and are powerful capabilities for materials research, especially when co-located with other surface analytical techniques. Each of these advanced SPM techniques is available for materials research in the Boise State University Surface Science Laboratory

The Influence of Heat Treatment on Corrosion Behavior of Martensitic Stainless Steel UNS 42670

Ceaseless demand for lighter, faster, and more efficient aircraft has been one of the greatest driving forces behind bearing steel innovations. Recent studies demonstrate that corrosion is one of the leading causes of bearing failure in both military and commercial aircraft. High-performing bearing steels are available but are not being used in US military applications due to high cost and security issues when steels are produced outside of the continental United States. One approach to address this issue is to engineer steels that are cost-efficient and heat treated for corrosion resistance, long wear life, etc. This dissertation presents information on the effects of heat treatment on bearing steels, specifically UNS 42670 (Pyrowear 675, or simply P675). P675 is a martensitic stainless steel (MSS) engineered for use in the aerospace industry. Through proprietary heat treatments, P675 can be transformed from a mediocre performing steel to one which can withstand fatigue more than all other steels in its class, while maintaining acceptable corrosion resistance. Here we demonstrate the effects of heat treatments on the new generation of bearing steels to inform and aid steel developers in designing cost-efficient steels that can provide superior corrosion resistance while maintaining required tribological performance. Samples studied were heat treated using three different methods; High temperature tempering (HTT), Low temperature tempering (LTT), and Carbo-Nitriding (CN). This study was initiated to test the following hypotheses: Electrochemical techniques (i.e. anodic polarization (AP), electrochemical impedance spectroscopy (EIS)) will yield faster and more accurate results than conventional corrosion testing methods for screening bearing steels for corrosion behavior. HTT samples will have the lowest corrosion resistance due to a larger depletion of chromium from the matrix experienced at the highest tempering temperature which will lead to the highest microgalvanic couple between the carbides and matrix. CN will have the highest corrosion resistance from the steels tested due to the addition of nitrogen and encouraged passivation at the oxide/metal interface. The objective of this dissertation is to understand and explain the implications of heat treatments on the newest and upcoming generation of MSS. A combination of accelerated corrosion testing, modeling, and nanoscale surface analysis was used to determine corrosion mechanism and provide recommendations. Key results from this study include the following: Corrosion performance of P675 is highly dependent on heat treatment where CN outperforms all three heat treatments for corrosion testing, while HTT has the lowest corrosion resistance. EIS data was fitted to an equivalent circuit and a mechanism of corrosion attack was proposed for each of the bearing steels studied where HTT experienced general corrosion attack while LTT and CN pitting corrosion. SKPFM Volta potential difference (VPD) measurements in an inert environment showed HTT as the thermodynamically most favorable to experience microgalvanic corrosion between the chromium-rich precipitated carbides and the surrounding martensitic matrix, with a measured carbide-matrix VPD of 200 mV, while LTT (150 mV) and CN (90 mV) were less. Corrosion propagation was also monitored in real time via in situ AFM and revealed that HTT underwent the most rapid spread of corrosion attack across the sample, while LTT and CN were less affected and showed much more localized, intergranular attack and adjacent to carbides. Bulk electrochemical testing results agreed with in situ AFM results, with LTT and CN showing distinct passive regions as compared to HTT, confirming the nanoscale differences in corrosion behavior observed between the steel heat treatments investigated. Corrosion rate measurements alone are not adequate to be a predicting factor of bearing performance. The mechanism of corrosion initiation and propagation must be investigated to properly design new bearing steels. Based on this work, HTT would be recommended over the other two tempering procedures for use in aerospace bearings where corrosion is not a primary concern. However, when the bearing assembly is prone to corrosion attack, CN is recommended for bearing use due to its high resistance to both corrosion onset and propagation. In conclusion, this study will allow the United States Armed Forces a new tool (electrochemistry coupled with surface analysis via SPM) to screen candidate bearing steels for gas turbine engine applications and will give steel developers insight into the effects of heat treatment on the corrosion performance of MSS (i.e P675). This work is a quintessential application of the materials engineering triangle; By varying the heat treatment (processing) of the steel, the microstructure (structure) of the surface of the steels were changed, thus altering the corrosion behavior (properties) and affecting the overall performance

Accelerated Testing to Investigate Corrosion Mechanisms of Carburized and Carbonitrided Martensitic Stainless Steel for Aerospace Bearings in Harsh Environments

Carburizable martensitic stainless steels (MSSs) are attractive candidates for bearings due to their high corrosion resistance, high hardness, and high temperature performance. Wear performance in tribocorrosion applications is strongly influenced by the surrounding environment. Electrochemical testing was used to evaluate three different surface treatments on AMS 5930 steel developed for advanced gas turbine engine bearing applications: low temperature (LTT), high temperature (HTT), and carbonitrided (CN). HTT had a higher corrosion rate that increased with time, whereas LTT and CN had lower corrosion rates that were stable over time. Accelerated testing revealed that surface treatment significantly influenced how corrosion propagated: HTT was more uniform; conversely, LTT and CN showed localized attack. Degradation mechanisms developed from electrochemical methods provide rapid insight into long-term wear behavior

Microgalvanic Corrosion Behavior of Cu-Ag Active Braze Alloys Investigated with SKPFM

The nature of microgalvanic couple driven corrosion of brazed joints was investigated. 316L stainless steel samples were joined using Cu-Ag-Ti and Cu-Ag-In-Ti braze alloys. Phase and elemental composition across each braze and parent metal interface was characterized and scanning Kelvin probe force microscopy (SKPFM) was used to map the Volta potential differences. Co-localization of SKPFM with Energy Dispersive Spectroscopy (EDS) measurements enabled spatially resolved correlation of potential differences with composition and subsequent galvanic corrosion behavior. Following exposure to the aggressive solution, corrosion damage morphology was characterized to determine the mode of attack and likely initiation areas. When exposed to 0.6 M NaCl, corrosion occurred at the braze-316L interface preceded by preferential dissolution of the Cu-rich phase within the braze alloy. Braze corrosion was driven by galvanic couples between the braze alloys and stainless steel as well as between different phases within the braze microstructure. Microgalvanic corrosion between phases of the braze alloys was investigated via SKPFM to determine how corrosion of the brazed joints developed

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