The characterization of materials at high temperatures is one of the research focuses of the Fraunhofer-Center HTL. For this purpose, the Fraunhofer-Center HTL has commercially available systems for thermal analysis as well as specially developed measuring furnaces for determining specific properties. With commercial systems for thermal characterization and analysis, the Fraunhofer-Center HTL measures weight loss, heat capacity, heat of reaction and gas emissions amongst others in a controlled atmosphere (TG-DSC-MS).
The industrial heat treatment process can be reconstructed in the laboratory with specially developed ThermoOptical measuring devices (TOM). The TOM systems can simulate all relevant industrial furnace atmospheres: gas burner atmosphere, air, inert gases, forming gas, hydrogen, vacuum, pressure, etc.
They are equipped with detectors which measure the material changes in situ during the heat treatment with high accuracy. For example, the dimensional changes during sintering are measured with extremely good reproducibility, acoustic emission signals during debinding are measured with sensitive microphones. The characterization and analysis of high temperature properties of materials can also be done with the TOMsystems.
The TOM systems can be used for the in situ characterization and analysis of very different material properties in the temperature range from room temperature up to about 2000 °C.
- Thermal properties
- Mechanical properties
- Thermomechanical properties
- Chemical properties
- Electrical and optical properties
Exact measurements of high temperature properties require a precisely defined local temperature and an atmosphere which is adapted to the sample material, such as inert gas atmosphere for oxidation-sensitive samples for instance. In addition, the volume of the sample must be representative of the material. For highly heterogeneous materials (coarse pottery, refractory materials, composite materials) some 10 to 100 cm³ are required. These requirements are met with the TOM systems.
TOM methods for process optimization
At Fraunhofer-Center HTL, seven TOM facilities are available specifically designed for in-situ measurement and optimization of high temperature processes. The systems are used for the following heat treatment processes:
Moreover, the quality of the forming can be reviewed with the TOM systems. Using a high-precision shadow imaging technique, the dimensional changes of the samples can be measured in situ during the heat treatment. In addition, changes in weight can be measured in situ. Microphones register the acoustic emission in the case of formation of cracks in the sample. By applying uniaxial load with special push rods, the creep behavior of the sample can be examined.
TOM methods for high-temperature characterization
At Fraunhofer-Center HTL, five new TOM-systems are developed that can measure material properties in a controlled atmosphere. The following high-temperature properties are determined:
- Strength, stiffness and elongation at break
- Vibration resistance
- Softening under load, creep
- Thermal shock and thermocycling resistance
- Dynamic modulus of elasticity
- Thermal diffusivity and -conductivity
- Specific heat and thermal expansion
- Wetting behavior of melts
- Integral and spectral Emissivity
- Corrosion resistance against gases and dustparticles
- Electrical impedance
For the mechanical characterization, load cells with maximum forces between 3 N and 5 kN are used. The fatigue tests are designed for frequencies up to 300 Hz. Thermal shock and thermocycling experiments can be performed with a well-controlled laser heating. The characterization and analysis of thermal material properties are done without contact with a laser flash method specifically developed for large samples.
The Fraunhofer-Center HTL develops ThermoOptical measuring systems (TOM) according to customer requirements. The Fraunhofer-Center HTL also provides measuring furnaces for the desired maximum temperature, size and atmosphere. In addition to the optical in situ dimension measurement, mechanical properties such as modulus of elasticity, creep resistance, wedge splitting resistance or viscosity are available. Furthermore, special systems are developed for high temperature measurement of thermal properties (thermal conductivity, thermal expansion, thermal shock resistance, etc.). The construction of the measuring systems is done in a certified center for device development (CeDeD) of the Fraunhofer ISC.