Fundamentals

SCS has developed smart coatings for multiple applications

The wide range of technology offered by SCS is based on the fundamental concept of using luminescent ceramic materials for sensing purposes. The design and manufacture of the coating is tailored to the requirements of both the sensing and substrate application. As such several different physical properties can be measured and, importantly, the coating can survive for as long as the underlying material.

What is observed?

The sensor coating technology is based on phosphor materials. Essentially, in our applications, phosphorescence is a process whereby light is used to excite the material and as a result light of a different wavelength (colour) is emitted. The emitted light comes from atoms within the material and can be analysed to give important information about the state of the material itself such as temperature, pressure or crystallinity. This interrogation can all happen remotely by detecting the light through cameras and glass fibres and without wiring electronics to the component.
Note: These types of materials are already an essential part of our daily life, since they are used in TV displays, energy-efficient lighting and LEDs. Hence, the materials are a commodity rather than exotic.

How does luminescence or phosphorescence work?

The optically active components within the phosphor are either rare earth or transition metal elements. When excited with UV-light, these dopants absorb the energy (directly or indirectly), which promotes their electrons to higher energetic states. These higher energetic states are unstable, and the electrons eventually fall back to their stable ground state and, in doing so,  emit visible light which is called luminescence. The time it takes for the luminescence to cease depends on various factors, but can have fundamentally different meanings. Phosphorescence light shows slow emission rates, while fluorescence shows very fast emission times.

The Technologies

Thermal History

When manufactured in a certain way, the coating material has the capability of changing irreversibly when exposed to high temperatures. After cooling, the state changes in the coatings remain and can be interrogated at room temperature. The physical or chemical changes are reflected in the luminescence properties and can thus be used as a measure of the thermal history. The material then acts as a tool that remembers the thermal conditions it has been exposed to. The measurement device can then interpret the luminescence changes and provide temperature information.

Further details of the thermal history technology are available in the following scientific publication . . .

Figure 1 Thermal history sensing

Figure 2 Aging Intensity Ratio

Ageing

Phase changes occur in some coating materials when exposed to high temperatures for extended periods of time. These phase changes cause subtle changes to the luminescence characteristics of the material which can be calibrated and quantitatively related to the percentage of the new phase and, hence, its ageing characteristics. Furthermore, the effect of ageing does not affect the temperature measurement capability which is outlined above, enabling both detection methods to be conducted using the same coating.

Figure 3 Rainbow Coating for erosion detection

Erosion

The emission properties of the phosphorescence are primarily dependent on the dopant used. As such, different dopants in the same host emit light at different wavelengths (colours). Therefore, if different dopants are applied in layers in a coating, as the coating is eroded the different colour layers are revealed. The structure of the coating is known so a quantitative measurement of erosion can be made simply by imaging the luminescence across the surface.

Further details on the assessment of erosion by photoluminescence are available in the following scientific publication . . .

Figure 4 Corrosion detection

Corrosion

During corrosion the chemical composition of the coating material changes when certain materials attack the coating. For example, a vanadium attack, typical during hot corrosion, causes a significant change to the emission spectrum which can be detected using optical equipment.

Online temperature measurement – when temperature transients are important

The coatings can also be produced so that there is no thermal history effect!  In this case, the luminescence in the coatings changes reversibly with temperature so that measurements are recorded while the material is at temperature. This technique, termed phosphor thermometry, has been known for many years however Southside Thermal Sciences (the sister company to Sensor Coating Systems) has pioneered its practical application using patented robust coatings and applied the technology successfully in the hot section of a Rolls-Royce Viper 201 engine. Please see the Southside Thermal Sciences website for further details . . .