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A small, round, compact mystery

Yellowish in colour, round in shape, this tiny little thing can, at least in theory, be transformed into something that is harder than diamond.

With a lot of hard work, a group of researchers has succeeded in producing coating targets out of carbon nitride powder (CN powder). These can be used in the deposition of CN based thin films.  The material was completely new for the researchers, exotic and mysterious.

The researchers managed to produce test samples out of pure CN powder and powders in which CN was combined with another component such as Teflon or boron compounds. The end products made from the CN and Teflon possess both the low friction properties of Teflon and the durability of carbon nitride. The combination of CN and boron compounds can be made into exceptionally hard coatings, which provide improved resistance against wear.

The researchers noted that the end result was affected both by what was done before and during the consolidation process.

The study used a SPS (Spark Plasma Sintering) device, which applies intense electrical current during the consolidation process.

‘Careful mixing and drying of the powder prior to its consolidation into a solid form was crucially important to the end result. Ball milling turned out to be the best method for preparing both the pure CN and mixed compounds for consolidation, the latter of which had to be carefully mixed to ensure a homogeneous mixture,’ explains doctoral candidate Juho Lotta, who is working on his dissertation under the supervision of Professor Simo-Pekka Hannula in his research group in the Department of Materials Science and Engineering.

Drying the mixture also proved important, as the CN powder is prone to absorbing moisture. When the researchers were examining the consolidation process, they observed that of all the process parameters temperature had the greatest impact on the end result  - too high a temperature resulted in chemical decomposition of the material, while too low a temperature resulted in poor densification. The end result was also influenced by the pressure and time used in the consolidation process.

Sensitive to water

After the test sample was compressed, the researchers were able to dry polish it to create a smooth  surface. The effect of water on the test sample was catastrophic, so efforts were made to avoid exposure to water. The test samples were characterised using various methods, including scanning electron microscopy, in order to study densification and microstructure.

‘The project was a challenge, primarily due to the difficulties we had in producing  good quality test samples. This was also the first time that we'd worked with CN, which is completely different to the metallic and ceramic powders we’d been studying prior to this. The project taught us a lot about the new materials and the SPS apparatus used in the work.  We will definitely be able to make use of the knowledge we gained when studying other materials,’ says Erkin Cura, another doctoral student, who was also working on the project. 

‘It was a really good feeling to finally succeed in producing good quality samples. It happened unexpectedly once we managed to optimise certain factors in the pre-treatments and processing of the powder,’ Juho Lotta recalls, and also noting that the project was both interesting and successful. 

In the same way that carbon is fragile in its graphitic state and diamond hard, CN is not hard in its graphitic state. Instead, its structure is only finalised during the coating process.

The PM Products with New Material and Processing Solutions research project was part of the Demanding Applications (DEMAPP) programme, which was a FIMECC (Finnish Metals and Engineering Competence Cluster) research programme in 2009-2014. The project partners included VTT - Technical Research Centre of Finland, Metso Group, Wärtsilä Corporation, and Carbodeon Ltd.  The CN powder used in the research was synthesised by Carbodeon. The Department of Materials Science and Engineering was represented in the DEMAPP research programme by the research groups of Professors Simo-Pekka Hannula, Jari Koskinen, and Pekka Taskinen.

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