To achieve the aspired product quality via heat treatment, green bodies must feature adequate properties, too. The following aspects generally apply: The green bodies must be as homogeneous as possible on the macro-, meso- and micro-scale. Reasons for this are:
- Microscale: The individual particles must be distributed as uniformly as possible. Otherwise, preferential sintering of neighboring particles will take place and early onset of grain growth occurs.
- Mesoscale: At this magnitude, sintered components frequently exhibit structural defects such as inclusions, large pores or cracks, which determine the fracture behavior. Even a minor quantity of structural defects per cubic centimeter result in a degradation of the mechanical stability and the reliability.
- Macroscale: The density should be constant across the entire component. Otherwise sintering will cause irregular shrinkage and lead to distortion. Distortion is associated with a loss in quality and an increased effort and expense in post-processing.
The complexity of the development of powder metallurgical manufacturing processes can be reduced significantly by a qualified evaluation of the green body quality. Optimization steps in the areas of raw material selection, preparation and molding can be undertaken mostly independent from the optimization of the heat treatment and the finishing. The following methods are available for the evaluation of the green body quality at the Fraunhofer-Center HTL.
The particle size of most green bodies is in the size range of 0.1 – 100 µm. At this scale, scanning electron microscopy (SEM) is the most suitable method for structure imaging. For quantitative measurements of the degree of homogeneity, straight cuts through the structure are required. In green bodies, these are made using a special, almost artifact-free ion beam process, the so-called Cross Section Polishing (CSP). The green bodies are subsequently imaged at high contrast in the SEM. Using special software developed at the HTL, an automatic assessment of the homogeneity of the microstructure is undertaken by variance analyses.
Computer Tomography (CT) is used for the examination of structural defects in green bodies above a size of approx. 20 µm. It delivers a 3D image of the green body with a resolution of up to a few micrometers. Using special software developed at the HTL, variance analyses for the assessment of the structure homogeneity can be undertaken by CT as well. As an alternative, a method was developed at the HTL where structural defects can be detected in ceramic green bodies even under manufacturing conditions. In this method, the green bodies are debindered and then soaked with an immersion solution having the same refractive index as the ceramic. As a result the samples become transparent, and defects can be detected by a light microscope. The immersion method can be used with minimal effort for examining the green body quality, as soon as a suitable immersion solution has been found. However, it will require the use of laboratory fume hoods.
The defects detected in the green bodies can be exposed with special target preparation methods and then analyzed using energy-dispersive X-ray analysis (EDX) in the SEM. Based on the composition and/or type of error, it is possible, for example, to narrow down the origin of the contaminant.
In principle, many measurement methods are available for the analysis of the porosity and the pore size distribution or the density of green bodies:
- The green bodies can be cut into small samples and their density can be determined geometrically or by the Archimedean method.
- Using an x-ray method (radiographic or CT), the density of green bodies can be determined, too.
- The distortion due to sintering can be evaluated and used indirectly to determine the density distribution in green bodies.
The Archimedean or geometric measurement of the density of small samples, taken from the green body, requires only minor use of equipment but is demanding with respect to the work involved. The accuracy of the density measurement is limited to approx. ± 0.5 % porosity, which is frequently insufficient for near net shape sintering. X-ray methods require more sophisticated equipment, but important information concerning the distribution of binder and porosity in the component can be provided, too. However, X-ray methods are only of limited informational value on the edge areas of green bodies due to scattering effects, particularly where high density gradients are expected. The most precise methods for the detection of porosity gradients on the macroscale are in-situ measurements with respect to the distortion during sintering. Samples must be extracted from suitable locations of the green body for this purpose. Small components can also be examined as a single piece. With the ThermoOptical measurement systems (TOM) at the HTL center, the shrinkage of the samples during sintering is measured with great precision. The porosity in the green body is calculated from the shrinkage with an accuracy better than ±0.2 %. More precise information on the individual green body characterization methods can be found in ,.
 Raether, F.: Energy efficiency during production of ceramics, CFI Ceramic Forum International 10/2013, p. E27 ff.