The need
X-ray Photoelectron Spectroscopy (XPS) is the most important method used to measure the surface chemistry and structure of materials and coatings in a wide range of industrial sectors. Better understanding of coating chemistry and thickness informs better design and manufacturing decisions.
For 50 years, UK-based Kratwww.kratos.com/os Analytical Ltd has led the world in XPS and the development of new instruments. 95% of their instruments are exported, where they are used by coatings and advanced materials companies to research new products and spot defects and contaminants that may compromise material performance.
Kratos has recently developed a new XPS instrument – a High Energy Ag L alpha X-ray source – which can measure chemistry to twice the depth of normal XPS. However, the standard calibration and measurement infrastructure is not designed for XPS at these X-ray energies, reducing confidence in the measurement results.
The new instrument could identify the presence of elements deeper in the sample but could not quantify them with enough certainty. Kratos approached NPL to explore methods to determine exactly how they could turn data from their new instrument into meaningful information.
The solution
NPL used fundamental physical parameters and theoretical models of how electrons at different energies would be emitted from materials to predict intensities in the new instrument. These were compared to experimental intensities from material samples of known composition to understand the efficiency of the instrument. This produced a set reference data to allow accurate calibration of the instrument and sensitivity factors for every element, enabling the composition of any sample to be accurately measured. These NPL procedures have now been incorporated into Kratos’ instrument software. The updated Kratos software was launched in September 2020 and instruments around the world are now using ‘Cant transmission functions’ and ‘Cant sensitivity factors’ named after David Cant from NPL.
The impact
Kratos have demonstrated that their new instrument generates quantitative data about the amount of each element in the surface of a sample. It provides customers with a new capability to measure chemical composition accurately and at greater depths. It also allows direct comparison with traditional XPS, giving users more information about the depth each element appears.
This is valuable for investigating surface chemistry of advanced materials and coatings, including pharmaceutical release systems, smart glass and touchscreens. It can also be used for analysing protective coatings for enhanced lifecycle of sensitive components such as wind turbines, automotive engines and energy storage devices. XPS is a key technique for quality control particularly for spotting defects, identifying contaminants and corrosion, and in patent protection.
Dr Chris Blomfield, Surface Analysis Business Manager at Kratos, says: “We have seen a marked uptake in the use of the High Energy Ag L alpha X-ray source as we have rolled out the software to our user base with new relative sensitivity factors, derived directly from our collaboration with NPL. A hitherto qualitative tool has been turned into a quantitative technique, enabling our users, both current and future, to get more value from their XPS system equipped with the higher energy Ag L-alpha source. What is quite remarkable is this development came about through an InnovateUK funded project with colleagues at NPL whom we have collaborated with informally for many years, the formality of the Innovate project produced real results."
This work was completed through InnovateUK funding.