The full force of microscopy

The development of a portable scanning force microscope

Scanning force microscope (SFM) is a new portable version of the atomic force microscope (AFM) for onsite inspection of industrial components and for nano- or micro-level early-stage damage detection. 

Traditionally used in medical research, AFMs are a high-resolution imaging and measurement tool used to visualise and analyse surfaces at the nanoscale level. 

Unlike electron microscopes, AFMs can operate in ambient conditions (air, liquid or vacuum) and do not require preparation – like metal coatings of non-metallic samples when studied under a scanning electron microscope (SEM). 

A sharp tip is scanned across a sample surface, measuring the interactions between the tip and the sample to generate a 3D image. The sharp tip is attached to a cantilever, which acts as a spring. The tip is raster scanned across the sample surface. As the tip encounters the surface, it experiences forces like van der Waals or electrostatic interactions that cause the cantilever to deflect. 

A laser beam is focused on the cantilever, and its reflection is detected by a photodiode detector. The cantilever deflection is measured by the change in the reflected laser beam position and this data is used to construct a 3D image of the sample surface.

The traditional version of AFM used in the laboratory needs a cut-out, mounted, polished and etched specimen, similar to that required to study under an optical microscope. 

However, for in situ plant inspection, we require a device that can be directly mounted on an industrial component to study the detailed microstructural damage at a polished and etched spot on the component, avoiding damage to the component when cutting out a sample. 

ETD Consulting commissioned a manufacturer to develop a portable version that can be taken directly to a plant and inspect, non-destructively, for early-stage creep or fatigue damage, which may appear in steels such as ASME P91 or P92 at nano-levels to start with.

Portable solution 

ETD and its partners have now developed this portable version of the microscope for onsite use in industry, civil and aviation structures – a first-ever development for industry use. 

Development began about 15 years ago during work on early-stage, creep cavitation, damage detection in ASME P91 martensitic steel structures for high-temperature power and process plants. 

Established non-destructive examination (NDE) techniques couldn’t detect early-stage creep damage in high-strength P91 steel used for high-pressure, high-temperature piping, valves and pressure vessels. 

Even powerful NDE techniques such as phased array ultrasonic testing couldn’t detect such damage until 70-80% of its life. By this time, it would be too late and could result in a critical high-value component’s catastrophic failure before the next plant shut-down for inspection. 

Powerful onsite inspection for early-stage creep cavitation damage is therefore a necessity. Scanning force microscopy solves this problem and produces nanoscale resolution images comparable to SEM. 

It uses the same preparation steps as traditional replicas of high-temperature plant components i.e. polishing to 1µm-level and etching to reveal the microstructure – another technique for non-destructive onsite inspection.  

However, replicas are usually studied under an optical microscope with a resolution of only a few micrometres, but SFM can resolve finer features in 3D. It can also be used to study precipitates in metallic alloys.  

The SFM head is mounted on a mobile frame that sits on a fixed frame, attached by magnets or straps to the component under investigation. 

Once the SFM is positioned on the polished and etched area of a component, a laptop controls its movement to scan the area under investigation, which is usually about 30mm² in size. 

The image of the scanned area is then displayed on the laptop and shows the creep cavities, other damage, or even the size and shape of the precipitates. 

The associated software is used to calculate the cavity depth and volume, as well as the surface area. Any of these parameters can then be plotted against the life consumed and a relationship developed to calculate the component life from the cavity volume, for example (see below for the images produced). 

Such relationships have been established by ETD for some of the high-temperature steels that are used to determine a component’s safe remaining life. As an example, in the graph above, the life fraction of an ASME P91 steel was plotted against a creep test interrupted specimens’ cavity volume. 

These specimens were tested at 600°C and 625°C, and were interrupted at various life fractions. The cavity volume was measured and plotted against the life fraction. 

The portable SFM has now been developed to a stage where it can be used onsite in an industrial plant for early-stage creep cavitation, fatigue or corrosion-damage detection in any material. 

In addition to defect assessment of the material microstructure, SFM can assess conductivity, surface electric potentials, magnetic domains and friction properties of the material or component being tested – using, for example, magnetic, ceramic or other tip types. 

As well as onsite industrial use, SFM can be used in the laboratory, just like an SEM, for studying polished and etched specimens, or even replicas. It also has great potential for analysing aerospace and nuclear plant structures. 

Gamma precipitates in a gas  turbine blade 

The efficiency of a steam turbine depends on the reliability of its blades. 

These aerofoil-shaped components are mounted in the rim of a turbine disc to extract energy from the high temperature and high pressure of the combustor. 

To survive the harsh environment, the gas turbine blades are usually made from superalloys and are designed for material stress. However, time, pressure and temperature all take their toll.

Preliminary work has been carried out by ETD Consulting on the non-destructive examination (NDE) of gas turbine blades and their life assessment, saving large costs. 

Several parts of the blade require efficient NDE. The cold root of a turbine blade is fixed to the turbine rotor and experiences lower temperatures (600-650°C) compared with the rest of the blade (~1,300-1,400°C). 

The leading edge is the hottest part of a turbine blade that first encounters the hot gas, and the trailing edge is where the exiting gas meets the blade. 

The original gamma prime particles in a gas turbine blade have a cuboid shape but during operation become spherical and coarsen. This happens over time, with increased temperature and pressure. 

The micrographs below are of the unaffected cold root of a blade fixing and the leading edge microstructures. They show a change from cuboid to spherical of the γ’ precipitates.

This shape change and the increasing particle size can be related to a decrease in the safe remaining life of these blades.