TMA indentation viscometry

Here a probe is pressed into the surface of a flat, polished glass sample which is held isother-mally at the required temperature. Many geometries can be used for the probe tip, but spherical and cylindrical are the most suited to the TMA. Although the viscosity measurement 

The theoretical justification of viscosity measurement based on the surface penetration of the glass-forming supercooled liquid by a flat bottom cyhndrical indenter was considered by Ne-milov [34]. Both the elastic and viscous elements are involved but by considering the steady-state condition the elastic terms can be neglected. This simphfied model gives the viscosity as  

To verify the technique the viscosity-temperature curve of the NBS (National Bureau of Standards, USA) 711 glass was determined using a Nimonic 105 indenter. The TMA was set to ramp at 100°C/min to 40°C below the required temperature and then at 10°C/min to the set temperature. The force (F) applied to the sample was chosen by estimating the 

With k of Equation (10.6) adjusted to 8, the fit of the measured viscosity-temperature ciuve between 10 ° and 10 Pas had a mean error of 1.2%, for the particular experimental set-up. 

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Figure 10.16 Photograph of the experimental arrangement for thermomechanical analysis (TMA) indentation viscometry using a cylindrical indenter (32).

Figure 10.17 The viscosity-temperature data set measured using thermomechanical analysis (TMA) indentation viscometry of the National Bureau of Standards (NBS), USA, 711 glass is compared to that obtained using the Vogei-Fuicher-Tammann [9] model issued by the NBS [32].

Figure 10.18 Photograph of a sample of the fluoroaluminate glass 37AIF3-12BaF2-15CaF2-12MgF2-9SrF2-15Yp3 (molar %) that has undergone thermomechanical analysis (TMA) indentation viscometry with a cylindrical probe. The sample has devitrified during the indentation [35].

High viscosities, more than 10 Pa s,canbesatisfactorilymeasuredinthecommercialTMA and it is in this region that the glass-forming liquid is typically shaped. Table 10.2 summarises the way in which TMA has been used for beam bending, indentation and parallel-plate viscometers to cover the viscosity range 10 - 10 Pa s. The TMA indentation and parallel-plate viscometry are described in detail below (Section 10.5.5 and 10.5.6, respectively). 

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