Fraunhofer ISC, Center for High Temperature Materials and Design HTL, Bayreuth
The characterization of ceramic and metallic materials at high temperatures is a research focus of Fraunhofer-Center HTL. For this purpose, the HTL has commercially available systems for thermal characterization and analysis as well as specially developed measuring furnaces for determining specific properties. With commercial systems for thermal analysis, weight loss, heat capacity, heat of reaction and gas emissions amongst others are measured in a controlled atmosphere (TG-DSC-MS).
With specially developed ThermoOptical Measuring devices (TOM), the industrial heat treatment process can be copied in the laboratory. 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, dimensional changes during sintering are measured with extremely good reproducibility, acoustic emission signals during debinding are detected with sensitive microphones. The characterization and analysis of high-temperature material properties can also be done with the TOM systems.
The TOM systems are 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:
a) Thermal properties
b) Mechanical properties
c) Thermomechanical properties
d) Chemical properties
e) 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. Besides, the measured volume must be representative of the material. For highly heterogeneous materials like coarse pottery, refractory materials or composite materials, some 10 to 100 cm³ are required. These requirements are met with TOM systems.
TOM Methods for Process Optimization
At Fraunhofer-Center HTL, eight self-developed TOM facilities are available - specifically designed for in-situ measurement D and optimization of thermal processes. The systems are used for the following heat treatment processes:
Moreover, the quality of forming processes can be checked with the TOM systems. Using a high-precision contour tracking technique, dimensional changes of the samples are measured in situ during the heat treatment. Besides, weight changes are measured. Microphones register the acoustic emission in the case of formation of cracks in the samples. By applying a uniaxial load with special pushrods, the creep behaviour of the sample is examined.
TOM Methods for High-Temperature Characterization
At Fraunhofer-Center HTL, five new TOM-systems have been 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 (c)
- Dynamic modulus of elasticity
- Thermal diffusivity and conductivity (a)
- Specific heat capacity and thermal expansion
- Wetting behaviour of melts
- Integral and spectral emissivity (a)
- Corrosion resistance against gases and dust particles
- Electrical impedance (e)
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 thermos cycling experiments can be performed with a well-controlled laser heating. The characterization and analysis of thermal material properties are also done without contact with a laser flash method specifically developed for large samples.
Fraunhofer-Center HTL also develops ThermoOptical Measuring systems (TOM) according to customer requirements. We provide 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 the high-temperature measurement of thermal properties such as thermal conductivity, thermal expansion, thermal shock resistance etc. The construction of the measuring systems is done in a certified centre for device development (CeDeD) of the Fraunhofer ISC.
TOM Systems:
- TOM_ac for controlled atmospheres, graphite heated up to 2200 °C:
> Dimensional changes, load tests and gravimetry - TOM_air for operation in the air up to 1750 °C:
> Dimensional changes, load tests, gravimetry and acoustic emission measurement - TOM_metal for measurement in hydrogen atmosphere or overpressure, graphite heated up to 1800 °C:
> Dimensional Measurements - TOM_gas for measurements in gas burner atmosphere up to 1500 °C:
> Gravimetry - TOM_II for measurements in gas burner atmosphere up to 1500 °C:
> Dimensional Measurements - TOM_pyr1) for debinding measurements in controlled gas flow up to 650 °C and 20 m/s:
> Gravimetry and acoustic emission measurement - TOM_wave for the measurement of thermomechanical properties up to 1750 °C:
> Thermal shock, thermal conductivity, modulus of elasticity, emissivity etc. - TOM_I for measurement of the thermal diffusivity of small samples up to 2000 °C:
> Thermal diffusivity, shrinkage - TOM_imp for measurements of the electric impedance in the air up to 1000 °C
- TOM_mech1) for measurements of mechanical properties up to 1800 °C:
> Strength, stiffness, elongation at break, fatigue, creep etc. - TOM_fiber for measurements of mechanical properties of fibres up to 1500 °C:
> Strength, stiffness, elongation at break, creep - TOM_chem for measurement of corrosion by particle and gas flows up to 1500 °C and 40 m/s:
> Gravimetry - TOM_dry for measurements of changes in weight during drying in a controlled atmosphere in a laminar gas flow
1) Test mode