PBF-LB/M, also Selective Laser Melting (SLM): Laser powder-bed fusion for metallic materials.
The Institute of Materials is a member of the SLM Cluster within the Faculty of Mechanical Engineering. The facilities and equipment were acquired exclusively for research and teaching purposes. Pure contract manufacturing without a research component is not possible. More information here.
System: SLM 125 HL (Nikon SLM Solutions)
400 W laser
125 mm x 125 mm x 125 mm build volume
Optional: Reduced build volume for small components
The Institute of Materials is equipped with a state-of-the-art coating facility. Coatings can be applied here using both plasma spraying and high-velocity oxygen fuel spraying (HVOF). Thanks to powder feed lines that can be controlled independently of one another, the coating composition can be adjusted even during the process, making graded coatings possible.
For safety reasons, the coatings are sprayed in a closed spray booth, which protects the operating personnel from noise exposure and the very intense UV radiation of the plasma jet. An industrial robot serves as the torch manipulator, and the system also features a rotating sample table, allowing even complex geometries to be coated.
Atmospheric Plasma Spraying (APS)
In atmospheric plasma spraying (APS), spray particles are applied to the surface to be coated using a plasma jet. A plasma is a hot gas in which, due to the high temperature, neutral particles dissociate and ionize. Thus, unlike in a gas, a plasma also contains charged particles such as electrons and ions. To generate a plasma, a high-frequency ignition creates an arc between the cathode and anode in a so-called plasma torch. With an appropriately selected gas supply, a plasma jet with high thermal energy forms, which flows out of the nozzle of the plasma torch at high speed in a focused stream. The temperatures in the hottest part of the plasma cone reach approximately 30,000 K.
The powder to be sprayed is introduced into the plasma jet via an injector. Depending on the process, argon or nitrogen is used as the carrier gas (to transport the spray powder to the torch with the necessary kinetic energy). After the powder is introduced, heat and momentum are transferred to the powder particles, causing them to melt and accelerate. Depending on the selected parameters, the powder particles strike the substrate at a certain velocity and temperature.
HVOF coating of a test segment for rocket engine tests.
In high-velocity oxygen-fuel (HVOF) spraying, the powder is injected into a kerosene flame. This process achieves flame temperatures of up to 3000 K and velocities of up to 2000 m/s. One advantage of this process is the short residence time of the powder particles in the flame, resulting in significantly reduced oxidation compared to other processes. In addition, the high particle velocity ensures a comparatively dense coating structure. The coatings produced in this way typically have an oxide content and porosity of less than 2% each.
Materials for Thermal Spraying
In general, any ceramics or metals that are available in powder form, can be melted or fused at the process temperatures (see above), and remain stable even at high temperatures are suitable.
Analytics at the Institute for Materials Science showing optical microscopes and hardness testers.
The metallography laboratory at the Institute of Materials Science. Right: Hot-pressing machines, center: Automatic grinding and polishing machines, left: Manual polishing machines.
Sample chamber of the laser test bench. Image Source: T. Fiedler (2018), Wärmedämmschichten für Raketentriebwerke, NFL Forschungsbericht 2018-01.
The laser test bench uses a 3.3 kW diode laser with wavelengths of 980 nm and 1030 nm. The laser beam is guided into the test chamber via an optical fiber and expanded divergently by a focusing lens. A ratio pyrometer with measurement wavelengths of 750 µm and 1100 µm is coupled into the laser beam path, allowing the surface temperature of the sample to be measured in situ. Depending on the distance between the laser optics and the sample, a laser spot diameter of approximately 10 mm to 30 mm is possible. This results in a maximum heat flux density of approximately 30 MW/m².
The laser power can either be set to a constant value or controlled based on a preset target temperature for the sample surface. The measurement range - and thus the range of possible surface temperatures for controlled operation - is 500 °C to 1500 °C.
After each laser test, the sample can be automatically immersed in liquids such as water to induce thermal shock during cooling.
All materials capable of sufficiently absorbing the laser radiation used - such as metals - are suitable for the tests. Some ceramic materials, such as zirconia thermal insulation layers, must be blackened prior to the test.
One advantage of the ratio pyrometer is that the measurement is independent of the surface’s emissivity. However, this applies only if the emissivity is the same for both measurement wavelengths. Otherwise, it must either be measured, or a correction factor must be determined in advance for the measured surface temperature.
