Feeling the force and testing the temperature

Drs. Wilfred T. Tysoe & Nicholas D. Spencer | TLT Cutting Edge February 2021

Photoluminescent nanoparticles can be used as robust in situ monitors of both pressure and temperature. 

We have previously discussed how stress-sensitive fluorescent molecules can be used as in situ probes of the local viscosity in a fluid film¹ or the real area of a rough solid-solid contact during loading or sliding.² While such molecules provide the basis for powerful experimental methods in fundamental studies, they are relatively fragile. As a result, being able to measure the conditions in a sliding contact, especially under severe conditions, remains a challenging problem. Environmentally sensitive nanoparticles might provide a solution.

The Romans were aware that adding small amounts of a metal such as gold or silver could impart intense colors to glass, the most spectacular example being the Lycurgus Cup, which changes color depending on the direction from which it is viewed. It was discovered that the strong florescence was due to the presence of nanometer-size particles in the glass, perhaps one of the first examples of the use of nanotechnology.

More recently, it has been possible to tailor the architectures of nanoparticles to maximize their photoluminescence by fabricating core-shell structures in which a central nanoparticle is surrounded by one or more shells of different materials.

STLE member professor Philippe Vergne and his colleagues David Philippon and Nicolas Fillot, with engineers Lionel Lafarge and Nicolas Devaux and graduate student Tarek Seoudi, all from the University of Lyon, collaborated with Alexandre Mondelin from SKF Aerospace France to exploit such core-shell nanoparticles as an in situ sensor in elastohydrodynamic (EHL) contacts in a paper published in STLE-affiliated journal “Tribology Letters.”³ The 6-nm diameter particles consisted of a cadmium selenide core with concentric shells of cadmium and zinc sulfide that fluoresce strongly in the green. Such small particles have a tendency to agglomerate, and this was prevented by capping them with an organic layer, which also rendered them oil soluble.

The nanoparticles were dispersed at very low concentration in squalane. Because the fluorescent emission energy depends on the particle size, it varies linearly as a function of both pressure and temperature (see Figure 1). This, along with the nanoparticle size, homogeneity and robustness, means that they provide ideal in situ nanosensors.


Figure 1. Calibration of the emission energy of 0.125 mg/mL of CdSe/CdS/ZnS nanoparticles in squalane versus pressure at three temperatures in the 0–1.2 GPa range. Lines represent the bilinear regression. Reproduced by kind permission of Springer-Nature.³

To confirm this idea, the Lyon group used the core-shell nanoparticles as sensors in an EHL contact in an apparatus shown in Figure 2a using various ballon-flat contacts. A relatively common green argon-ion laser line was used to interrogate the contact, and the scattered light was analyzed using a spectrometer. The conditions in the fluid were measured using the calibration curve in Figure 1.

Isothermal experiments showed pressure distributions in the contact that agreed very well with theoretical predictions. However, as demonstrated in Figure 2b, the same nanoparticle sensors were also capable of quite accurately measuring the fluid temperature.


Figure 2. (a.) Experimental equipment for in situ experiments showing a cut-away diagram of the ball-on-disc apparatus. (b.) Comparison between measured and simulated average temperature profiles for: a glass-steel, b glass-Si₃N₄, c sapphire-steel and d sapphire-Si₃N₄ non-isothermal contacts operated at an ambient temperature of 313 K with an entrainment velocity of 5 m/s and SRR of 10%. Reproduced by kind permission of Springer-Nature.³

Of course, the conditions in these experiments were carefully controlled to only vary one of the parameters. However, either monitoring the nanoparticles at two excitation wavelengths or by using two types of nanoparticle should allow both the temperature and pressure to be monitored simultaneously.

Such environmentally sensitive nanoparticle probes will not only provide detailed information on the behavior of buried interfaces, but also seem to be sufficiently robust that they show potential to be used as condition monitors in practical applications.
FOR FURTHER READING
1. Tysoe, W.T. and Spencer, N.D. (2016), “Looking at lube in a new light,” TLT, 72 (4), p. 72. Available here.
2. Tysoe, W.T. and Spencer, N.D. (2018), “Lighting up the contacts,” TLT, 74 (4), p. 86. Available here.
3. Seoudi, T., Philippon, D., Fillot, N., Lafarge, L., Devaux, N., Mondelin, A. and Vergne, P. (2020), “CdSe-based quantum dots as in situ pressure and temperature non-intrusive sensors in elastohydrodynamic contacts,” Tribology Letters, 68: Article 73.
Eddy Tysoe is a distinguished professor of physical chemistry at the University of Wisconsin-Milwaukee. You can reach him at wtt@uwm.edu.

Nic Spencer is professor of surface science and technology at the ETH Zurich, Switzerland, and editor-in-chief of STLE-affiliated Tribology Letters journal. You can reach him at nspencer@ethz.ch.