High Temperature Characterization

Heat management of high-temperature processes is becoming increasingly important regarding increased product requirements and more energy-efficient manufacturing processes. Today, it is mostly carried out with Finite-Elements (FE) methods. For heat management, it is expected that all relevant material data is available for the selected high-temperature materials. These are, in particular, the thermal material properties, including thermal conductivity, heat capacity, temperature conductivity and emissivity. To estimate the service life of high-temperature materials, mechanical material properties such as high-temperature stability, material fatigue, elasticity and creep behavior are also relevant. Thermomechanical properties such as thermal shock and thermal shock resistance as well as chemical properties such as corrosion resistance are important as well.

While the room temperature characteristics of many materials are known or can be determined relatively easily, hardly any FE-compatible data is available for higher temperatures. The determination of defined and representative measuring conditions, such as temperature homogeneity and furnace atmosphere, is difficult in determining the high-temperature characteristic values.  There often are undesirable interactions between the measuring equipment, the measuring furnace and the sample material. Many high-temperature materials are characterized by a coarse microstructure. In order to obtain representative measurement data, large measuring volumes of 10 to 100 cm³ are necessary. Conventional thermal analysis methods are not designed for this kind of sample dimensions. Practical procedures with large measuring volumes, on the other hand, frequently provide parameters which are suitable for a comparison of different materials which have been measured under the same conditions. However, they are not suitable for the derivation of physically defined material properties: Example of thermal shock tests according to DIN EN 993-11.

In order to circumvent such obstacles and limitations, the Fraunhofer-Center HTL has been developing special ThermoOptical measurement systems (TOM) for more than two decades and is qualifying these devices for high-temperature measurements. The HTL is certified according to ISO 9001: 2015. Sample volumes of 10 to 100 cm³ are preferably used n the TOM devices. The measuring furnaces are designed for homogeneous temperatures and controlled atmospheres. The high-temperature measurements are preferably carried out in a non-contact manner in order to minimize interactions with the sample. The interpretation of the high-temperature measurements and the evaluation of the measured data are often carried out at the HTL using FE methods or specially developed algorithms to improve the meaningfulness and accuracy of the methods. Three recently developed TOM systems - TOM_wave, TOM_air and TOM_chem - represent the spectrum of measurement possibilities at the HTL:

ThermoOptical Measurement system TOM_wave

TOM_wave is equipped with a gas-tight water-cooled measuring furnace with a maximum temperature of 1750 °C and can be operated with different atmospheres. TOM_wave is equipped with a gas-tight water-cooled measuring furnace with a maximum temperature of 1750 °C and can be operated with different atmospheres. The samples used are disk-shaped with a diameter of up to 35 mm and a thickness of up to 20 mm. With a defined furnace temperature, the samples can be heated with powerful CO₂ lasers on one or both sides. One-sided sample heating is used for a specially developed laser flash method for determining the temperature conductivity. The CO₂ laser radiation is absorbed directly at the sample surface, unlike the radiation of conventionally used lasers. The heating of the material is recorded on the back of the sample with a long-wave pyrometer. In this way, any contact with the sample material and also a coating of the samples can be avoided. The heat distribution after the laser radiation is adapted to the measured data using inverse FE simulations. In this way, sample volumes unusually large for laser flash measurements can be taken into account. An automatic sample changer ensures efficient generation of representative high-temperature measurement data. The FE simulation also allows a determination of the heat capacity from the heating and cooling curves after laser radiation. One-sided laser heating can also be used to excite the self-oscillations of the samples. These are measured acoustically, so that the modulus of elasticity and the damping behavior can be determined. Two-sided heating of the samples is used to determine the thermal shock and thermal cycling behavior. The samples can be heated by the laser in a controlled manner from the set oven temperature by more than 1000 K within seconds. Any damage to the samples is recorded acoustically. The spectral emissivity of the samples is measured with an IR spectrometer in the wavelength range from 0.9 μm to 28 μm.

ThermoOptical measurement system TOM_air

TOM_air is equipped with a chamber furnace with a maximum temperature of 1750 °C and is operated in air. Similar TOM systems, TOM_ac or TOM_metal or TOM_two have gas-tight furnaces with maximum temperatures of 2200 °C or 1800 °C and can be operated with inert and reducing atmospheres or with a gas burner atmosphere - TOM_metal aditionally with 100% hydrogen or 30 bar overpressure. The sample geometry is variable in these TOM systems. The special feature of the TOM systems is their optical beam path, with which the sample contours can be recorded in the shading process and can be measured exactly. Changes in dimensions that can occur at high temperatures can be measured without contact. This allows studies on the expansion, shrinkage, deformation and wetting behavior at high temperatures. The algorithms used ensure a high reproducibility and resolution of up to 0.1% and 0.1 μm, respectively. TOM_air is equipped with a weight sensor which can be used to register weight changes of samples up to a weight of 200 g with a resolution of 0.1 mg. In addition, TOM_air has microphones, which can also be used to record very weak sound emissions of the samples, which can be caused by cracks during degassing. Finally, in TOM_air, uniaxial forces can be transferred to the sample up to a maximum force of 4 kN. In this way, creep processes can be investigated in particular. Unlike conventional measuring methods, the high-temperature optical measurement of the creep allows a detection of the axial and radial deformations. Therefore, the uniaxial viscosity as well as the viscous Poisson's ratio can be calculated from the measured data. Both sizes are required to correctly calculate the deformation behavior of components at high temperatures using FE simulation.

ThermoOptical measurement system TOM_chem

TOM_chem is equipped with a heater system that achieves maximum temperatures of 1500 °C in a controlled atmosphere. The flow velocity of the gases can be increased up to 40 m/s. The gas stream can be loaded with vapors and particles to simulate corrosive atmospheres in different applications. Its own furnace serves as steam injector, through which the steam generating media are driven at a constant speed. The particle injector functions via a fluidized bed process, with which particles with a size of up to 100 μm and a rate of up to 10 g / Nm³ are induced into the gas stream. The weight changes of samples weighing up to approx. 50 g are measured in TOM_chem using a magnetic suspension balance in situ. Corrosion in a gas burner atmosphere can also be measured in situ on large components with a weight of up to 15 kg. In addition, the released materials are examined for structural analysis by conventional methods in order to elucidate the corrosion mechanisms.

The new TOM systems are currently being tested for different applications. For this purpose, many already established TOM systems or commercially available and/or already standardized measuring methods are available at the HTL. For example, temperature conductivities on small samples up to temperatures of 2000 °C can be measured with an older TOM system. Commercial systems for measuring the temperature conductivity up to 1100 °C by Laserflash method (Netzsch LFA 457) and Hot Plate/Disk method (Hot Disk® TPS 500) are also used at the HTL. Thermal analyzes are carried out with a commercially available DTA/TG/MS system (differential thermal analysis coupled with thermogravimetry and mass spectrometry, Netzsch STA 449 C with QMS 403 C Aeolus) and a DSC system (differential heat flow calorimetry, Netzsch DSC 204 F1). Thermal expansion coefficients can be determined using a conventional push-pull dilatometer (Netzsch DIL 402). A DMTA system is available for mechanical material testing at temperatures up to 1500 °C (Dynamic-Mechanical-Thermal Analysis, Gabo Explexor 4000 N). The system is used to measure mechanical stabilities such as 3- or 4-point bending stability, tensile stability, static and dynamic modulus as well as high-cycle fatigue properties. Thermal shock tests are also carried out according to DIN EN 993-11 and are evaluated by measuring the ultrasonic speed or by computer tomography. Printing softening tests based on DIN EN ISO 1893 are carried out with the TOMMI or TOM_ac systems. The HTL provides the necessary measuring methods to cover all relevant high-temperature properties in a controlled atmosphere.