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Magic angle spinning nuclear magnetic resonance (MAS-NMR) is complementary to other methods of solid-state structural characterisation and has an advantage over phase identification by X-ray diffraction (XRD) regarding its ability to distinguish between different atomic environments. This is o f particular interest for complex structures, such as silicon carbide, where the XRD data may be confused by the overlapping of diffraction peaks shared by different polytypes. MAS-NMR can assist in distinguishing between the different polytypic structures by revealing the local atomic co-ordination of the nuclei. The technique is described in more detail in reference [140].

A Bruker MSL 360 spectrometer was used to perform MAS-NMR (using the pulsed Fourier Transform method) on ceramic specimens at ambient temperature. The specimens were crushed into a fine powder and compacted into a cylindrical, alumina 'spinner' which was then placed inside the coil of the Bruker NMR probe. The coil was electrically tuned to the resonant frequency of 29Si nuclei, being 71.5 MHz under the conditions used. To reduce spectral broadening from anisotropic atomic interactions and to allow Fine resolution within the spectral dispersion obtained, the sample was spun at 3-4 kHz inside the coil which was orientated at the magic angle (54.7°) to the

magnetic field (8.45 T). Short (1 ps, so as to irradiate all spectral frequencies

uniformly), intense pulses of an r.f. magnetic field were applied to the sample by the coil at three minute intervals (the relaxation delay time) in order to produce a resultant Field at 30° to the static magnetic Field. (Ideally, the resultant field would have been perpendicular to the static magnetic Field, but a shorter relaxation time is required when using a 30° resultant Field, making possible the collection o f a larger number of measurements in a given time period.)

The data was collected during relaxation of the nuclei inbetween these pulses and the 'data acquisition' cycle was repeated about 300 times for each experiment. The data was then Fourier transformed to produce chemical shift spectra which were expressed in parts per million (ppm) with reference to the chemical shift of the

a single resonance peak at the high frequency end of the 29Si spectrum (most other chemical shifts are on the low frequency side of this peak and so have negative values), all the Si sites are crystallographically indistinguishable; with four methyl (CH3) groups being symmetrically arranged around a central Si atom. The ceramic phases were identified by comparison with published chemical shift data and the spectra obtained were consistently accurate within 0.5 ppm.

3.7 DENSITY MEASUREMENT

Measurements of specimen density were made with a digital balance (Precisa 125A), using Archimedes’ principle. The specimens were weighed (i) in air and (ii)

immersed in a fluid (water). Any evidence of encapsulant materials or other

contamination adhering to the surface layers was removed by surface grinding, followed by ultrasonic cleaning in acetone. After being weighed in air, the specimens were placed in a weighing pan that was permanently suspended in water inside an enclosed compartment below the main balance. Evidence for open porosity in the specimens was indicated by a steady rise in weight whilst they were immersed in water.

The measured densities were compared with theoretical density values calculated (Appendix B) using the rule of mixtures and assuming complete reaction of the sintering additives with no residual phases, according to

2Si3N4 + Y203 + 3 S i02 -> Si3N4 + Y2Si207 + 2Si2NzO (3.2)

The composition of the post-sintered matrix phase was expected to be 88.20 wt% Si3N4,

7.66 wt% Y2Si207 and 4.14 wt% Si2N20 , having a theoretical density of 3.235 g/cm3.

3.8 INDENTATION EXPERIMENTS

An indentation technique was used measure the microhardness of fabricated ceramics and also to provide an indication of their fracture toughness, taking advantage of the small sample volume that this method requires. A Vickers diamond indenter

(pyramidal with apex angles of 136°) was used with a high enough load to generate cracking around the indentation and additional experiments were performed using a Knoop indenter (with apex angles of 172.5° and 130°). The ceramic surfaces were

polished to a good optical finish (with 1 pm diamond paste) in preparation for the

experiments. These were carried out using an Instron 1122 universal testing instrument (5 kN capacity) with a 0-50 kg loadcell (calibrated using a 1 kg weight) and the experimental arrangement is shown in figure 3.7.

Figure 3.7: Experimental arrangement for indentation

To perform the indentation, the crosshead was lowered towards the specimen, at a speed

of 8 pm/sec and as the diamond indenter contacted the specimen surface, load was

applied until a preset value (2 kg) was measured by the loadcell. The load was then maintained for a dwell time of 15 seconds to form each indentation and the variation of load over time was monitored using a pen chart recorder. The specimens to be indented were supported in a bakelite mould which was positioned on a sliding X-Y table and the indentations were made at regular intervals (to facilitate their location in a microscope). Measurements were made on a series of indentations either using an Olympus (BH-2) optical microscope (with maximum magnification of x500) or using the SEM, for

which (in both cases) the scale markers were calibrated against a copper graticule of 1 0

pm spacing. The hardness values were calculated using the relationship:

For the Vickers indenter, the indentation area is taken as being the (pyramidal) area of the surface in contact with the indenter, given by

A = d2 / 2 sin(136°/2) (3.4)

where d = diagonal length of the indentation (in pm). The equation used to obtain Vickers microhardness values (in GPa) is,

Hv = 1854.4 x ( P / d 2). (3.5)

For the Knoop indenter, the projected indentation area (in the plane of the surface) is used in calculating the hardness:

HK = 14230 x ( P / d , 2) (3.6)

where d, = the length of the long indentation diagonal (in pm).

The indentation fracture toughness was determined from measurements of the lengths of radial cracks emanating from the comers of the Vickers impressions (see figure 3.8), using the equation derived by Evans and Charles [2, 141]

KIc = 0.16 Hv a 1/2 ( c /a ) ^ (3.7)

where K|C = the fracture toughness (MPam1/2), a = the half-diagonal length (ptm) and c = the surface crack length (pm).

Figure 3.8 (right):

A typical surface impression made by a Vickers diamond indenter.