Hardness SiAlONs

Mandal H, Hoffmann MJ (2000) Hard and Tough a-SiALON Ceramics. In Hampshire S, Pomeroy MJ (eds) Nitrides and Oxynitrides. Trans Tech Publications Ltd, Switzerland, p 219... 

With increasing zvalue the density of P-SiAlON decreases linearly, similarly lowering Young s modulus, strength, thermal conductivity, hardness and fracture toughness.30 On the other hand, for low zvalues (z< 1) the hardness and fracture toughness increase and when z > 1 they decrease. It has been also depicted that glass-free microstructures could be obtained in P-SiAlON polycrystals with substitutional level z> 2.31... 

The a-SiA10N has twice the unit-cell parameter in the c-direction compared to P-SiAlON, and the doubling of the Burgers vector for c[0001] dislocations means that dislocation movement is more difficult and the hardness is enhanced. 

Fig. 6.9). Hardness values increased gradually from 16 GPa to about 19 GPa, shifting from the P-SiAlON-rich region to the a-SiAlON-rich one (Fig. 6.10). However, the gradient layer was longer in this sample, which may be due to easier diffusion of species in a larger amount of liquid phase in B3. 

Both XRD and SEM studies support the results of hardness measurement, as illustrated in Fig. 6.13. Hardness values decreased gradually from 19 to 15 GPa, over a distance from a-SiAION to P-SiAlON rich sides for each functionally graded SiAION ceramic. 

Ekstrom, T., (1992a), Preparation and properties of alpha-sialon ceramics , J. Hard. Mater., 3, 109-118. 

The bulk densities of all the materials were determined using Archimedes method (AS 1774.5, 1979). The Vickers indentation technique was used to measure the hardness in each case. The applied load in the Vickers hardness tests was 10 kg for silicon nitrides and sialons. However, using the same load produced severe lateral cracking in silicon carbides around indents, which prevented the accurate measurement of the diagonals of indents. Therefore the load was reduced to 0.3 kg for silicon carbide samples. 

Better understanding of the microstructural formation of the oc -SiAlON materials in the future will be necessary for the improvement of the mechanical properties, especially fracture toughness and strength, and the wider application of this group of materials, having higher hardnesses than the P-Si3N4 and P-SiAlON materials (see Fig. 16). 

This chapter is focused on CS and SPS of high-purity p-SiAlON, which includes the reaction mechanism of CS the physical properties of Vickers hardness and thermal properties and the corrosion resistance properties in different atmospheres. 

The Vickers hardness of polished CS-SPSed P-SiAlON compacts was measured using a Vickers microhardness tester with a diamond indenter of regular pyramid with an opposite angle of 136°. The experiments were performed under the loads between 0.981 N (0.1kg) and 19.614N (2.0kg) at room temperature. The dwell time for each load was 20 s. An average of at least five readings at different locations of the specimen surfaces was taken for each specimen. The Vickers hardness GPa) was calculated according to Equation 4.4... 

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