Traditional sintered ceramic bodies like stoneware or porcelain undergo a viscous flow sintering process. The primary driving force of the process is surface tension, leading to decreased overall surface area by the aggregation of separate, discrete glassy phases into a single cohesive body. The speed of this processed is moderated by the viscosity of the glassy phase itself.¹

Industrial process optimization has led to fast firing as the dominant sintering technology. In these rapid processing settings, the time available to reach optimal stabilization of the body is reduced to a few minutes. Firing to maximum densification minimizes porosity, improving the mechanical strength of the tile.

A major concern in the design of fast sintering bodies is the determination of the best top firing temperature. This is the temperature at which the material is able to achieve full densification, without bloating, with the minimum time requirement.¹ An excessively high firing temperature leads to bloating, which degrades mechanical properties and produces deformations caused by the growth of entrained gas bubbles within the body.

Optimizing the firing process and predicting sintering behavior are not recent challenges; they have been fundamental objectives of sintering studies for many decades. The continuing emphasis on short manufacturing times, however, requires that successful ceramic engineers pursue techniques that effectively capture these fast processes and low force deformations.

Most of the sintering models available in literature propose idealized particle geometries,³ are limited to solid-state sintering of a single component³ and analyze systems in which diffusion is the dominant mechanism for material transport.^4,5 In traditional ceramics, the behavior of real batches is often too complex to be understood a prioribased on the thermo-physical parameters of the single components. In this field, experimental methods are the most effective route to study sintering and optimize industrial processes.

Experimental Methods

Conventional thermal analysis techniques such as differential thermal analysis (DTA), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA) are not able to provide complete information about the sintering onset temperature or the sintering rate. The sintering process is best described by measurements of the dimensional change, a direct representation of the densification process.

The measurement and observation of a sample while undergoing a sintering process was first demonstrated by Harkort and Paetsch in 1960, when they published a work based on investigations of feldspars by means of heating microscopy. The heating microscope was actually used as an optical dilatometer to study the dimensional change of powder compacts during sintering.

In the early 1990s, heating microscopy was applied intensively to study sintering in several material systems.⁶ Optical measurement techniques are ideally suited to optimize fast-firing sintering. Free from the heat sinks or temperature-profile-specific calibrations of some pushrod systems, optical dilatometry allows for experiments that mimic fast-firing processes and capture the large dimensional changes and very low forces that are present.

The heating microscope technique was enhanced significantly by the development of the double-beam optical dilatometer (see Figure 1), designed and patented by Expert System Solutions (now part of TA Instruments) in 2001. Two independent optical systems focus on either end of the specimen, optimizing precision and sensitivity compared to the single microscope methodology. The instrument is based on the double-beam concept. The first optical path is used to measure the top of the specimen: a stepper motor is used to continuously maintain proper alignment. The second optical path is focused on the sample holder and acts as a reference beam to correct the mechanical drift of the sample holder.

Optimizing the Sintering Process

The objective of the proposed experimental procedure is to optimize fast-firing processes by determining the optimal top firing temperature that provides full densification without swelling with the minimum processing time. The specimens are prepared with a laboratory press equipped with a steel die, which yields a 15 x 5 x 5 mm sample. The powder can be prepared according to any standard laboratory procedure, or it can be taken directly from the production line.

Maximum care must be taken in the preparation of the specimen, since the measurement is largely affected by the powder preparation, humidity and specific pressure. Samples are typically pressed with a specific pressure of 400 or 500 kg/cm², according to the normal industrial practice, but they can be cut from an industrially prepared ware. The sintering tests shown in this work are performed on materials wet milled and dry pressed with 5% humidity at 400 kg/cm².

Because the specimen will undergo a very fast heating cycle, they must be completely dry, or they could explode inside the dilatometer kiln, just as in the industrial firing kiln.¹ Results were obtained for a sample of technical porcelain using an optical dilatometer.* Sintering behavior was studied through three different testing phases.

Determining the Temperature of Maximum Sintering Speed

This preliminary test is performed with a heating rate of 80°C/min up to 1400°C. The dimensional change is represented in black in Figure 2. The expected densification is observed, followed by complete bloating due to over-firing.

The temperature of maximum sintering speed is identified by plotting the first derivative of the sintering curve dimension change with respect to temperature. The peak of the derivative curve indicates the maximum sintering speed temperature. In this case, this temperature is 123°C, which is accompanied by a sintering speed of 0.08%/s.

Determining the Temperature of Optimal Densification

This second experiment is used to verify the temperature that allows rapid and complete densification while preventing bloating. The heating cycle is designed based on the results of the previous determination of the maximum sintering speed temperature (in the present case, 1230°C). A constant heating rate of 80°C/min is applied, followed by isothermal segments of 2 min at successive temperature steps. The steps start at 1190°C and end at 1260°C, roughly centered around the previously determined temperature of maximum sintering speed. The temperature profile is shown as the red curve in Figure 3.

This test reveals the temperature at which the material reaches the highest level of densification without bloating. The highest suitable temperature is desired to minimize firing time, while needing to be low enough to avoid bloating, which occurs at high temperatures. For each temperature, the dimensional change is observed through the isothermal step. The highest temperature that shows densification (contraction) without any expansion is the optimal temperature. In the present case, this temperature is found at 1220°C, which is 10°C below the temperature of maximum sintering speed.

Confirmation of the Optimal Firing Cycle

Previous experiments have identified the temperature for optimal densification. To confirm the suitability of these conditions, the complete firing profile is created within the optical dilatometer. The sample is heated at a rate of 80°C/min to 1220°C, followed by an isothermal hold for 10 min. The desired behavior is maximum, stable sintering without swelling throughout the isothermal segment.

In the present case, the sample shows a maximum shrinkage of 7.3% and is completely sintered after 5 min of the isothermal segment. There is no tendency to bloat even after its complete sintering, confirming the suitability of the firing conditions. This experiment allows confirmation that an ideal firing cycle for this technical porcelain consists of a heating rate of 80°C/min to 1220°C, followed by an isothermal dwell of up to 10 min, or as little as 5 min.

The determination of the optimum firing cycle, however, is not the sole demand of a correct industrial practice.

Pyroplasticity

Pyroplasticity, the gravity-driven viscous deformation of a ceramic material during firing, affects the productivity of a manufacturing plant. It determines the relationship between the thickness and the rate of deformation of the material as a function of firing temperature.

This pyroplastic deformation can be quantified by measuring the bending of a body sample during heating. The exact stresses experienced by a specimen vary with specimen dimensions and support spacing. The stress on the sample inside the analytical instrument is typically greater than the stress present in the industrial roller kiln; as the material moves over the rollers, the supporting points shift continuously across the entire width of the piece, whereas in the laboratory instrument the material is suspended on two static supports. However, qualitatively the results are fundamental to understanding the behavior of the glassy phases during sintering and to optimize the firing cycle.

This measurement is particularly important for those bodies that have to be completely sintered. During the final stage of firing, they develop an abundant vitreous phase with a sufficiently low viscosity to cause rapid deformation of the material. The primary deformation mechanism is viscous flow, therefore having a time dependence for the bending deformation. The first derivative of the bending curve represents the deformation speed and is a function of the material viscosity within the limit of small deformations. In fact, during the final stage of firing, an abundant vitreous phase is developed with a sufficiently low viscosity to cause rapid deformation of the material. However, ceramic materials can show other behaviors during firing; in some cases, the highest bending rate occurs when fusion of the feldspars commences, not at the maximum temperature. At higher temperatures, the tendency to bend may decrease; the vitreous phase dissolves other mineral components of the body, thus becoming more viscous.⁷

Pyroplasticity can be measured directly with an optical fleximeter. The sample is suspended between two support rods 70 mm apart, while a camera frames the center of the sample, which moves during heat treatment (see Figure 5, p. 31). Pyroplastic viscous flow produces a downward movement at the sample midpoint. Glazed and other multi-component systems can also exhibit upward motion caused by the mismatch of thermal expansion and resulting internal stresses within the specimen.

In Figure 6, the two possible behaviors, or pyroplasticity, are demonstrated. Two samples of technical porcelain were subjected to a heating cycle consisting of a heating rate of 30°C/min, an isothermal dwell of 5 min at 1220°C and a second isothermal dwell of 5 min at 1230°C (red curve).

Technical Porcelain 1 (solid curve) shows a final bending of more than 6 mm, and the maximum deformation rate occurs at the maximum temperature (1230°C). Technical Porcelain 2 (dashed curve) shows a final bending of less than 3 mm and the maximum deformation rate occurs at the beginning of the first isothermal segment at 1220°C.

The deformation curve for Porcelain 1 reaches a maximum deformation of 6,401 µm at 1230°C, reaching a speed of deformation equal to 19.17 x 10^-3 µm/s, while Porcelain 2, at 1230°C, reaches a maximum deformation of 2,815 µm and a speed of deformation equal to 7.4 x 10^-3 µm/s.

Surprisingly, Porcelain 2 reduces the speed of deformation while the temperature increases. This is a clear sign that the glassy phases are changing their compositions, becoming more viscous. The addition of vitroceramic materials to bodies, for example, can reduce bending, indicating an increase in modulus of elasticity caused by the transition from the vitreous phase to the crystalline phase.

This example shows how pyroplasticity and deformation speed may affect the productivity of a manufacturing plant. Since it is not possible to determine ab initio the flexion behavior of a material, the analysis performed with the optical fleximeter is of fundamental importance to characterize the material behavior during heat treatment.

Conclusion

The optical, non-contact measuring approach is extremely effective for the optimization of industrial processes, making it possible to achieve reliable measurements of the thermal behavior in terms of sintering speed (measured as shrinkage), bloating, pyroplastic deformation speed and bending.  

References

1.         Paganelli, M., “Using the Optical Dilatometer to Determine Sintering Behavior,” American Ceramic Society Bulletin, Vol. 81, No.11.

2.         Li, Da., Chen, Shaou, Shao, Weiquan, Ge, Xiaohui, Zhang, Yongcheng, and Zhang, Sasha, “Application of Master Sintering Curve Theory to Predict and Control the Sintering of Nanocrystalline TiO₂ Ceramic,” Key Engineering Materials,Vols. 368-372 (2008) pp. 1588-1590.

3.         Johnson, John L., and German, Randall M., “Theoretical Modeling of Densification During Activated Solid-State Sintering,” Metallurgical and Materials Transactions, Volume 27A, February 1996.

4.         Park, Seong Jin, Chung, Suk Hwan, Blaine, Debby, Suri, Pavan, and German, Randall M., “Master Sintering Curve Construction Software and its Applications,” Advances in Powder Metallurgy and Particulate Materials, Part 1, Metal Powder Industries Federation, Princeton, N.J., pp. 13-24, 2004.

5.         Blaine, Deborah C., Park, Seong Jin, German, Randall M., LaSalle, Jerry, and Nandi, Hill, “Verifying the Master Sintering Curve on an Industrial Furnace,” Advances in Powder Metallurgy and Particulate Materials, Proceedings of the 2005 International Conference on Powder Metallurgy and Particulate Materials, PowderMet 2005, 2005:13-19.

6.         Boccaccini, A.R., Hamann, B., “Review In Situ High-Temperature Optical Microscopy,” Journal of Material Science, 34 (1999) 5419-5436.

 7.         Paganelli, M., “The Properties of Ceramic Material During the Firing Process,” Ceramic World Review, n. 55/2004.