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The results of the density and porosity determinations on the sintered samples are reported in this section.

4.3.1 Sources and estimation of experimental error.

Three estimates of experimental error in the density determinations were made, using similar methods to those described for error estimation in the shrinkage results (section 4.2.1).

i.e. (a) Estimate of measuring error.

(b) Estimate of (post doping) experimental error (c) Estimate of total experimental error.

(a) The error in the measurement of density and porosity was thought to be due to the limitations in accuracy of

the balance (a Mettler AE 100 electronic balance reading to four decimal places [O.OOOlg]), the presence of

trapped air bubbles in or on the specimens, and the (non) penetration of water into the pore structure of the test specimens. The removal of water from internal pores during the specimen surface drying operation was also a possible source of error.

To attempt to estimate the errors present in the

measurement (and to give more reliable results) the wet density measurements were repeated four times and the mean and standard deviation calculated. These values are shown in the tabulated results.

The measuring error was estimated from these as the 95% confidence limits for each of the measurements, with an overall average value for all specimen types being

calculated as described for the shrinkage results.

Table 4.3.1 (a) (appendix 1) shows the calculations used

and results obtained for the estimation of measuring error by this method.

The 95% confidence limits, representing the range of

range of possible error (for 95% confidence) being

approximately plus or minus 0.02 g per cm3.

(b) The errors in the fabrication and sintering results were thought to arise from density variations between the green pressed specimens, and from temperature

inhomogeneities in the heat treatment furnace. These errors were estimated from measurements on a series of duplicate pellets made from 3 of the doped powders and produced under the same experimental conditions.

As with the equivalent error estimate in the shrinkage calculations, the upper confidence limit of the mean density difference was used as the estimation of the

error in this part of the experiment. A value of plus or minus 0.05 g/cm3 was calculated for the experimental

error by this method. The calculations used to generate the estimate of experimental error, and the results obtained by this method are shown in table 4.3.1(b)

(appendix 1).

(c) The error present in the whole experiment (including the effects of the doping process) was estimated from density measurements on pellets produced from separate

(repeat) batches of powders of two of the compositions (0.25% and 0.75 wt. % doped alumina).

As with the equivalent error estimate in the shrinkage measurements, the 95% upper confidence limit in the mean difference between the density of pellets of the two powder batches (of each composition) under the same

experimental conditions was used as the estimate of total

experimental error. The estimated experimental error by this method gave a value of plus or minus 0.05g/cm3.

Table 4.3.1(c) (appendix 1) shows the data and calculations used to generate this estimate of

experimental error. The total experimental error is shown in the graphs as error bars representing the 95%

4.3.2 Undoped powders.

The density and porosity results for the undoped TZP samples are shown in table 4.3.2 (appendix 1), with the effect of sintering temperature on the sintered density of the undoped specimens illustrated in figure 4.3.2(a). The graph shows the behaviour of the unmodified TZ3Y material, as supplied by the manufacturer, and the blank

treated material which had undergone the doping process, but with no dopant additions made.

The results show that no difference could be determined in the densification behaviour of the two materials by this method.

The majority of the densification took place at sintering temperatures below 1350°C, with only a slight increase in density possibly occurring (less than possible

experimental error) for sintering temperatures in excess of this.

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Effect of sintering temperature on the density of undoped TZP. 6.2 5.B 95% c o n f id e n c e l i m i t 5.4 5.0 4.6 4.2 3.8 3.4 1100 1200 1300 1400 1500 1600 1700 1800

Sintering Temperature (°C) .

4.3.3 Alumina doped powders.

The density results for the alumina doped samples are shown in table 4.3.3 (appendix 1), with the effect of sintering temperature on the densification behaviour

shown in figure 4.3.3(a).

The results show that the majority of the densification in these materials took place at sintering temperatures up to 1250°C/ with a significant decrease in density in the majority of the specimens for sintering temperatures in excess of 1650°C.

The effect of alumina addition on the densification for different sintering temperatures is shown in more detail in figures 4.3.3(b)- (d).

The effect of alumina additions appears to give rise to four types of behaviour depending upon the sintering temperature used.

The results show a significant increase in density with

alumina addition for sintering temperatures below 1350°C, the majority of this occurring for alumina additions of 0.25% or less.

For sintering temperatures of 1350-1450°C, the effects of

alumina addition are less significant, and less clearly defined, being within the range of possible experimental

error. However, there appears to be a slight increase in density with alumina addition up to 0.25-0.5 mass %.

For sintering temperatures of 1550-1650°C, there appears to be a small decrease in density for increased alumina addition, this becoming more pronounced at the higher sintering temperature.

For sintering temperatures in excess of 1650°C, the effect of increased alumina additions becomes

increasingly significant on the density, with a large decrease in density apparent for alumina additions in excess of 0.5 mass %.

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Effect of sintering temperature on the densification behaviour of alumina doped TZP.

95% c o n f id e n c e l i m i t s 5.5 5.3 1800 1700 1600 1500 1400 1200 1100

S i n t e r i n g T e m p e r a t u r e (°C) .