Electrical insulators are devices used in electricity supply networks, and porcelain has long been used as an electrical insulator. The advantages of porcelain insulators include superior electrical properties, good mechanical properties (especially tensile strength), good creep resistance at room temperature and high corrosion resistance.¹

An insulator might reach the end of its working life for many reasons. Cracks in the body might be caused by a mechanical failure or a thermal mismatch between the ceramic part and the metal part. In addition, flashover might result from contamination on the glaze or weathering that leads to a small crack on the surface.² Thermal cycling causes material failure since it promotes the growth of micro-cracks on the surface. Thermal stress is generated by temperature differences between day and night, which may result in 80ºC differences on the glaze surface. The heat generated may also come from the passage of fault-current arcs in the insulator.³

Generally, the mechanical strength of the porcelain body can be improved by reducing the internal thermal mismatch between crystalline particles and glassy phases.⁴ An increase in mullite crystals can result in a change of the thermal expansion property of the body. The coefficient of thermal expansion is a thermal property related to chemical composition, firing temperature, particle size distribution of raw materials, particle packing of green and fired bodies, crystal structure, and the glassy phase of the body. Crazing of the glaze can be caused by a thermal mismatch between the body and glaze. Usually, the thermal expansion of the body should be higher than that of the glaze in order to generate compressive stress (instead of tension) in the glaze layer.

Many methods are available to determine the coefficient of thermal expansion, including calculation from the proportion and types of oxides in the body composition. However, this method is not accurate because many factors affect the calculated values. The most precise method is through measurement by a dilatometer.

Insulators are produced in many shapes, which are formed by different forming processes. For complicated shapes that cannot be formed by jiggering, slip casting is used. Other products, like a fused support with a simple shape, can be formed by pressing.

It is important to understand the physical, mechanical and thermal properties of porcelain bodies that are formed by slip casting, extruding and pressing. The effects of forming processes on fired properties can be explained by the phases and microstructures of the fired products. Understanding all of these relationships will result in increased accuracy when predicting and determining the thermal mismatch and life cycles of porcelain insulators.

Experimental Procedure

Enlarge this picture Table 1.

Porcelain insulator bodies from different forming processes were investigated to study the effect of the forming process on thermal expansion and sintering. The raw materials for the porcelain body included 24% feldspar, 7% kaolin, 30% ball clay, 10% washed ball clay, 3% Al₂O₃, and 26% silica. The body was prepared by milling in a ball mill to produce a body slip that controlled residue at 0.2-0.5% on 230 mesh. The slip was then filtered to make a cake prior to the selected preparation for each process.

The chemical composition of the raw materials used for the porcelain body and the overall chemical composition of the porcelain body are shown in Table 1. The parameters for forming and the dimensions of specimens corresponding to each forming process are shown in Table 2. The moisture content of the powder for pressing was 5-6%, while the extruding clay was about 18%. The water contents for the slip casting specimens were 40%, 50% and 60%.

Enlarge this picture Table 2.

The formed specimens were dried at 150ºC in an electric dryer and fired in an electric furnace at 1150, 1200 and 1250ºC with 20 minutes soaking time. The drying and firing shrinkages of the fired samples were measured, and the bulk density and water absorption of the specimens were measured according to Archimedes’ method. The mechanical strength of the fired bodies was measured by a three-point bending test. Crystal phases of the fired specimens were identified using a Bruker diffractometer model D8 advance (Ni-filtered Cu Ka radiation; l = 1.5406 Å). Diffractographs of the samples were recorded over the 2q range from 15° to 80° at a scanning rate of 1°/min.

The microstructure of the porcelain insulator body fired at 1250ºC was observed with a JEOL JSM-6480 LV scanning electron microscope (SEM) equipped with energy dispersive X-ray (EDS). The crystal morphology, amount of porosity and glassy phase in the specimens from different forming processes were also investigated.

The sintered specimens were ground with SiC paper (140, 240 and 400) followed by diamond paste (6, 3 and 1µm). All specimens were cleaned with ethanol in an ultrasonic bath after polishing. Before undergoing the SEM scanning, the samples were etched with HF solution (10%) for 15-20 seconds⁵ and finally gold sputtered.

The coefficients of thermal expansion  of the fired specimens were measured using a NETZSCH DIL 402 C dilatometer from room temperature to 1000ºC with a heating rate of 10ºC/min. The specimen size for measuring was made to a rectangular bar with dimension of 5 x 5 x 25 mm³, and the temperature range of the calculation was 20-500ºC.

Physical and Mechanical Properties

Enlarge this picture Figure 1. Comparison of the porcelain insulator bodies’ modulus of  rupture in different forming processes.

The total shrinkage of the porcelain insulator bodies from the three forming processes was clearly different. Pressed specimens showed lower shrinkage than the cast ones due to higher solid content, and the shrinkage decreased from 14.29 to 11.33% when increasing pressure from 50 to 150 bars due to better compaction. The high shrinkage of the cast specimens was obviously dependent on the solid content.

Not much difference existed in the % total shrinkage of the three types of forming at low firing temperatures (1150-1200ºC). As the temperature increased to 1250ºC, however, the difference became large. The % total shrinkage of the extruded specimen can be regarded in the same way as the pressed specimens due to the similar solid content. Moreover, it seemed that the shrinkage of the pressed specimens at 100 and 150 bars, as well as the extruded ones, attained their maxima at ~ 1200ºC.

At the firing temperature of 1250ºC, the water absorption of the specimens from most forming processes was nearly 0. The exception was the cast body (with 60% water content), at 1.05% water absorption. At this temperature, the porcelain body completely vitrified, especially the specimens that were formed by extrusion. This means that the apparent porosity of the extruded body was lower than others. However, water absorption data is not the only key to conclude the sinterability of the body; it must be compared with the bulk density of the fired specimens.

The bulk density of the fired specimens formed through pressing at 150 bars was the highest, followed by the extruded specimens and those pressed at 100 bars. The lowest bulk density was that of the cast specimen with 60% water content. The values were 2.43, 2.39, 2.37 and 2.31, respectively. The bulk density, which illustrates the densification of the specimens from different forming processes, indicated that the specimen pressed with high pressure was the most densified.

The high pressure may affect the closed pores. The final bulk density was still improved by a high green density. Thus, the benefits of high pressing pressure include a higher sintered density and more compact rigidity. It is surprising to find that the extrusion process effectively reduced porosity for both closed and open pores due to the homogeneity of the body. The reason is that the vacuum system of extrusion reduced air bubbles in the extruded clay. Therefore, the green density of the specimen was as high as the pressed specimen with 150 bar pressure.

Figure 1 shows that the bending strength of the porcelain body increased with increasing firing temperatures. The extruded specimens attained maximum bending strength of about 756 kg/cm² at a firing temperature as low as 1200ºC. The strength of the sintered bodies of 150 bar pressure was 739.3 kg/cm², almost equivalent to the extruded specimen. The cast body with 60% water content attained the lowest bending strength of 589.3 kg/cm², which was lower than the extruded and pressed specimens even at the high firing temperature of 1250ºC. Theoretically, maximum bending strength develops when the apparent porosity decreases to 0 (full vitrification).⁶

Thermal Properties

Enlarge this picture Figure 2. Comparison of the porcelain insulator bodies’ coefficient of  thermal expansion in different forming processes.

When pressure was increased, the thermal expansion coefficient of all specimens decreased with increasing firing temperatures, as shown in Figure 2. At 1250ºC, the coefficient of thermal expansion of the specimens pressed with 150 bar pressure was higher than that of the specimens pressed with 50 bar (by about 4%). The cast specimens with high solid content had a higher coefficient of thermal expansion than the specimen with low solid content because of the higher bulk density.

The dilatometric data show that the coefficient of thermal expansion of the pressed specimens of 150 bar was about 8.93 x 10^-6 ºC^-1, which was higher than the extruded and cast specimens at all firing temperatures. The reason is related to bulk density. Bodies that had the same chemical composition and firing temperatures but different forming processes exhibited different thermal expansions.

The specimen formed by a high bulk density process (high-pressure pressing) obtained higher thermal expansion than the low bulk density process after firing. The theoretical value of the coefficient of thermal expansion calculated from a proportion of oxides in the porcelain body from Table 1 is 8.99 x 10^-6 ºC^-1.^7,8,9 The measured thermal expansion of the specimen with low bulk density has lower thermal expansion than the calculated value.

On the other hand, the measured and calculated thermal expansion of the specimen with high bulk density did not show any significant difference. Therefore, the thermal expansion coefficient can be estimated from the bulk density of fired specimens. The forming process had a strong effect on the densification of the specimen and was related to the differences in thermal expansion as well.

The XRD patterns of the specimens showed that quartz, mullite and cristobalite were the main crystal phases. However, the intensity of the quartz peak at 26.5º of the pressed body was higher than the extruded and cast bodies. In order to examine the effect of forming force on crystal orientation and thermal expansion, the morphologies of the fired specimens were examined by cutting the specimen in the direction perpendicular to the forming force.

Figure 3. SEM images of sintered porcelain bodies at 1250ºC formed by different forming processes (500 x).

The low-magnification SEM micrographs in Figure 3 show the variations in the shape and number of porosities in the specimens. SEM examination revealed that the closed porosity developments were due to the glassy phase and trapped gases in pores.

The pore shapes of the specimens from the casting process were cylindrical pores, and the casting specimens included more pores than the other specimens. The specimens that were formed by extrusion had less of a tendency to form cylindrical pores than the casting specimens, while the pressed specimens had round-shaped pores and fewer pores than the extruded and cast specimens.

Pores are an inherent part of liquid phase sintering. Pores are presented in the compaction of green specimens as interparticle voids, and also can result from the viscosity of the glassy phase when firing temperatures are increased. The shape of the pores depends on the surface tension of the glassy phase. The particle shape, size of particle and pores, contact angle between particles, and amount of glassy phase affect the surface tension of the glassy phase.¹⁰

Enlarge this picture Figure 4. SEM images and EDS graphs of sintered porcelain bodies at 1250ºC formed by different forming processes (10,000 x).

Figure 4 shows the acicular mullite crystals and quartz grains in the fired porcelain body. From the XRD result, there was no evidence of different phase formation. However, there was clearly a difference in the morphology of mullite crystals formed in the specimens from different processes. As can be seen in Figure 4a, the morphology of crystallized mullite is an interlocked short needle that can be distinguished from the mullite crystals in Figure 4b and 4c.

The morphology of the mullite crystals in the specimens from extruding and pressing were interlocked with higher aspect ratios compared to the morphology of the mullite in the cast specimen. EDS analysis showed that the short needle mullite of the cast specimens was primary mullite, while the long needle mullite of the pressed specimens was secondary mullite.¹¹ In general, primary mullite tends to change to secondary mullite with increased firing temperature, soak time or change in composition. However, this research showed that the densification of the specimen has an effect on the morphology of mullite crystal.

Conclusions

From the above discussion, it is clear that forming processes affected the physical and thermal properties of the porcelain body. In addition, the microstructure and crystal phases of the fired specimens also differed when the forming processes of the body were changed.

The pressed specimens obtained higher bulk density and bending strength compared to the extruded and cast specimens. The coefficient of thermal expansion of the pressed specimens was also higher than those of the other processes at the same firing temperature.

As examined by SEM, the microstructure showed a spherical shape of closed pores in pressed specimens, while the cast specimens showed elongated porosity. The amount of porosity in the cast specimens was higher than pressed specimens, so it is clear that the cast specimens exhibited a lower coefficient of thermal expansion than the pressed specimens.

The coefficient of thermal expansion changed when the bulk density and microstructure of the specimens changed. The coefficient of thermal expansion does not depend only on the raw materials used in the composition. Forming processes and their parameters should be taken into account in order to understand the mechanical and thermal properties of fired products and better predict thermal mismatch problems and product life cycles.

For more information, e-mail sirithan.j@chula.ac.th.