Tension relief: Crack-resistant materials for aerospace

Excessive stress in a material can lead to fatigue, often rendering it unusable. To prevent this, researchers have investigated a particularly promising composite at MLZ’s Material Science Lab.

Dr Xingxing Zhang at work on the STRESS-SPEC diffractometer. © FRM II / TUM

Dr Xingxing Zhang at work on the STRESS-SPEC diffractometer. © FRM II / TUM

In both industry and research, materials must meet a wide range of requirements. Scientists are therefore constantly searching for materials that are stronger, more durable, and more resistant to corrosion – so that spacecraft can withstand high temperatures and cosmic radiation, or ships can withstand constant contact with salty water.

Reducing wear

The research team around Dr Michael Hofmann, instrument scientist at STRESS-SPEC at MLZ has investigated a specific composite material that can reduce wear. It consists of a metal alloy reinforced with tungsten carbide on the surface.

At the MLZ’s Materials Science Laboratory, Armin Kriele carried out the investigations using a scanning electron microscope (not in the picture). © Bernhard Ludewig; FRM II / TUM

At the MLZ’s Materials Science Laboratory, Armin Kriele carried out the investigations using a scanning electron microscope (not in the picture). © Bernhard Ludewig; FRM II / TUM

“Tungsten carbide is almost as hard as diamond, making the materials particularly wear-resistant,” says Michael Hofmann. The tungsten carbide particles were delivered on the metal alloy substrate surface to form a composite coating with the help of laser heating. This manufacturing process is called laser melt injection. “This is a very industry-oriented application,” explains Dr. Xingxing Zhang, “it concerns ship propellers, for example. If the coating breaks off, the material is damaged.” In other materials, tungsten carbide has already reduced wear by up to 80%.

Scanning electron microscope image of a sample cross-section. The tungsten carbide spheres (green) reinforce the surface of the copper bronze (brown). © Armin Kriele, FRM II / TUM

Scanning electron microscope image of a sample cross-section. The tungsten carbide spheres (green) reinforce the surface of the copper bronze (brown). © Armin Kriele, FRM II / TUM

Neutrons as the method of choice

The problem with the tested material currently lies in high internal stresses, so-called residual stresses of the material. These limit the performance and lifespan of the composite material. If ignored, they quickly lead to cracking, deformation and rapid fatigue. “We first tried X-raying the material,” explains Michael Hofmann. However, tungsten carbide proved too dense for this. “It allows hardly any radiation to pass through. Therefore, neutrons were essential.”

Dr Michael Hofmann is adjusting a sample on the STRESS-SPEC instrument. © Bernhard Ludewig

Dr Michael Hofmann is adjusting a sample on the STRESS-SPEC instrument. © Bernhard Ludewig

The researchers carried out the neutron measurements at the OPAL research reactor of the Australian Nuclear Science and Technology Organisation (ANSTO). They then used the scanning electron microscope at the Materials Science Lab at MLZ to resolve the material structure and distribution of the elements in detail. The scanning electron microscope is jointly operated by the Jülich Centre for Neutron Science and the Helmholtz-Zentrum hereon at the MLZ.

Heating leads to less stress

It turned out that heating the material could reduce internal stress. “In an industrial process, you don’t want to have to go to the research reactor every time – you want a simulation that delivers reliable results,” says Michael Hofmann, “with our data, it’s possible to set up and validate such a model.”