Stronger, tougher, harder: How scientists unravel the behaviour of materials
Scrrmp – the heavy doors close with a rich sound a bit like a kiss. The small experimental chamber, crammed full of instruments, can now no longer be entered. A concrete and lead wall protects the outside world from the X-rays inside Helmholtz-Zentrum Geesthacht's (HZG) Nanofocus Endstation at Beamline P03 of the Deutsches Elektronen-Synchrotron (DESY) in Hamburg.
Two successive hexapod positioners were used on the beamline. Six drive elements each move the platform with the sample into the beam. Photo: HZG/ R. Otzipka
While the deflected X-ray beam shines through the sample inside, physicists Dr Christina Krywka and Prof Jozef Keckes excitedly watch the monitors in the next room. Now the head of HZG’s X-ray Imaging with Synchrotron Radiation Department and the group leader of the Department of Materials Physics at Montanuniversität Leoben (Austria) can only hope that the sample has safely survived all of the preparations as there’s nothing they can do to change the sample now.
There! It shows the characteristic circle-shaped scattering pattern of a diffraction experiment. Relieved, the two look at each other: the sample is intact, so these data can be used and analysed. Now they can attack their materials science questions in a systematic way.
"We study the mechanical properties of zhe lyers using a nanoindenter." - Keckes. Photo: HZG/ R. Otzipka
The scientists are investigating novel coating processes for special tools, such as those used by industrial partners, including manufacturers of engines and turbines. These extremely hard tools are used to mill or cut individual components out of raw materials. Tungsten carbide is often used as the material for the tools themselves, and there is virtually no harder material. Even so, these tools wear down over time, and so each can be used to produce only a limited number of components. In order to extend the lifetime of these tools, they are treated with special coatings. The wafer-thin layers are approximately two microns (two one-millionths of a metre) thin and made of, for example, titanium nitride.
Jozef Keckes and his group study and develop these coatings. Why not just make the hard layers on the tools twice, or three times, as thick? Jozef Keckes explains: “Due to the deliberately introduced residual stress, the layers can't be any thicker, as they would spall off. Also, the layers are grown on the substrate by a process called sputtering. That means they are built up atom by atom in a plasma. That can take hours, sometimes even days.”
The Nanofocus terminal on the Beamline P03
The positioners push the sample into the tip of the indenter with a defined force. Photo: HZG/ R. Otzipka
Materials researcher Jozef Keckes has been coming with his group to Hamburg regularly since 2011 to carry out research into the coatings at the Nanofocus Endstation.
Often at his side is Christina Krywka who, on behalf of HZG, constructed and now maintains this equipment at Beamline P03.
Jozef Keckes explains: “We study the mechanical properties of the layers using a nanoindenter. Our question was, ‘how does the material respond on the nanoscale to external influences, such as high pressure?’”
An indenter is an extremely sharp, very small diamond tip, which, with a defined force, creates a tiny notch into the layer of the sample. For example, it is pressed with a force of 10 millinewtons, which corresponds to a weight of 1 gram.
Observe deformations in real time
Photo: HZG/ R. Otzipka
Christina Krywka and HZG Institute Director Prof Martin Müller had the idea to build a device which would make it possible to observe the deformation process in real time, fulfilling a long-held desire of Jozef Keckes. While the indenter presses into the layer, scattering patterns can be recorded at the beamline. The great advantage of such in situ (real-time) experiments is that they can be used to observe the elastic deformations, which only occur while the pressure is actually applied. These are no longer present after the tip is removed. Correspondingly, during ex situ measurements (i.e., after the indent is made), only the plastic deformation of the material remains.
The response is also measured for several samples under different, changing forces. This way, the immediate elastic response of the material, the residual stress, can be determined as a function of the force and penetration depth, as well as a function of the environmental conditions.
There is time to discuss upcoming experiments while the measurements are carried out and also later in the office. Photo: HZG/ R. Otzipka
“We want to understand how the material at this length scale deals with an external force. Until we developed our experiment, stress fields could not be recorded in situ. Here, modellers used to have to make assumptions, and the understanding of the model needed to be extremely precise. The smallest errors in the assumptions lead to large variations in the material,” Jozef Keckes explains.
The challenge for the real-time measurement was to design the geometry of the apparatus so that it would be compatible with the geometry of the X-ray beam and the instrumentation at the beamline. For this purpose, two connected hexapod positioners were used. These special devices align samples in all directions, both horizontally and vertically. The frame around the indenter, and the hexapod positioner, also had to be designed in such a way that they did not deform under the relatively high forces because this would distort the measurements.
The solution: one of the hexapods
Among other things, the measurements are compared with images taken under the electron microscope. Photo: HZG/ R. Otzipka
The solution: one of the hexapods uses a defined force to press the sample from underneath onto the fixed indenter, whose diamond tip is stationary in the X-ray beam. The other hexapod aligns the entire instrument with relation to the beam, and finally the precise scanningmotion is performed using a piezo positioner sitting in between the two hexapods.
A further difficulty was to make the system light enough that the delicate hexapods and all the other built-in positioners could withstand the load, as these extremely precise devices do not tolerate overloading. “We dived right into the electronics and customised the controllers of some of these positioners”, explained Krywka.
From 2011 to 2013, samples were first measured ex situ; the first measurements with the indenter in the beam began at the end of 2013. “We had to reinvent the technology for ourselves, and the sample preparation also gave us headaches. Then everything had to be formulated mathematically and, for example, the residual stress in the material predicted. We needed around two years to take all components into account and to be able to correctly analyse and then publish the data,” Jozef Keckes tells us.
"We are planning what you might call the Swiss army knife of in situ nanoindentation" - Krywka. Photo: HZG/ R. Otzipka
With success: now they can measure with the nanoindenter on the beamline in real time, using any loads or forces that cause the layers to fail. In their first findings, they were able to confirm in real experiments a number of modellers' assumptions.
So far, Keckes and Krywka were the first in the world to put this combination of X-ray nanodiffraction and nanoindentation into practice. Soon they will push their endeavours even further. Christina Krywka explains, “We are planning what you might call the Swiss army knife of in situ nanoindentation. We are currently developing a new nanoindenter which will be capable of covering the entire range of forces, from a few millinewtons to several newtons. When we achieve that, even more classes of materials could be studied.” In addition, the force sensor will include a digital interface to the beamline hardware.
This will ensure that Christina Krywka and Jozef Keckes will be sitting in front of the beamline monitors for many more days to come. The pair won't be letting go of their materials science questions.
Author: Heidrun Hillen (HZG)
Published in in2science #6 (June 2018)