Homogeneous microstructure during the size up of FAST/SPS sample: Use case with UHTC composite

Towards large sample production?

It is now well-known that Spark Plasma Sintering (SPS) is a versatile process used to produce new advanced materials, such as new metallic alloys (HEA, MAX Phase, superalloys), ceramics and composites while reaching higher densification rates than conventional processes. However, the production of large homogeneous refractory samples such as UHTC (Ultra High Temperature Ceramic) requires a high sintering temperature between 1800 and 2200°C. This could result in several drawbacks that can discourage the FAST/SPS user:

• Expense of time and feedstock. The trial-and-error approach, mold breakage and long cycle times associated with the development of a new material can be quite expensive.
• Microstructure inhomogeneity. The temperature strongly affects densification and grain size, and thus, the physical properties of the final part.

How to optimize and reach large sample production?

To reduce the development time of new materials and produce large samples, one economic strategy is to optimize the process and sintering parameters using small samples (15 – 50 mm of diameter). Once the conditions of production are set, the use of new numerical tools helps to size up to large samples with minimum effort. However, many think that this size up necessarily increases the thermal gradient and thus, leads to inhomogeneity of the microstructure inside the sample. However, you can control and reduce thermal gradient by optimizing the design of the mold specifically for your application. Numerical mold design is a new tool now available to the FAST/SPS user to limit thermal gradients inside large parts.

Case study: production of a UHTC sample

Here we report a case study to produce a new UHTC composite (130 mm diameter and 20 mm thickness). Optimization of the mechanical properties and composition (ratio between the two phases) was carried out with a sample of 36 mm diameter and 5 mm thickness. This screening led to a target composition, in a very short time. Afterwards two size-up operations were performed to sinter 60 and 130mm samples. Numerical modeling was used to set sintering parameters to keep the same sample average temperature. Mold design optimization was also useful to reduce the thermal gradient into the sample from 57°C (3%) for the 36mm sample to 14°C (<1%) for the 130mm one.

The SEM images show a very close microstructure between the edge and the center of all samples and between the different diameters. Grain size measurements highlight the homogeneity of the microstructure as well as its stability during the size up from 36 to 130 mm with a density of 99%.

The optimization of the mold dimensions as well as the sintering conditions allowed to limit the thermal gradient during the size up process. Thus, the same microstructure is achieved for all sample sizes.

Arnaud Fregeac, PhD Ph. D Development by SPS of ceramic coatings for turbomachine applications

Antoine Van der Laan , Ph.D in Modeling of complex shapes by SPS?