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TECHNICALNOTE• Industrial Ceramics •

THE OPTICAL FLEXIMETER

TO STUDY

DEFORMATIONS ON

CERAMICS

M. Paganelli, D. SighinolfiExpert System Solutions, Modena, Italy

Generally, the deformations of ceramic materials during

and after firing may have a complex origin. If the

products are made up of a single material, such

deformations are mainly due to pyroplastic phenomena.

In the case of glazed materials, two further factors must

be considered: the state of tension established between

glaze and body, and their differences in behaviour during

sintering.

1. New techniques, old problems

During firing, a glaze applied on a ceramic body undergoessome transformation which can be simply listed as: losing ofthe forming water of the clay components, glass transitionand softening. In correspondence of this point, the glass partof the glaze start to melt, giving as a result a continuous liquidlayer. During cooling, the glaze viscosity increases until theglaze becomes rigid and it starts to contract simultaneouslywith the ceramic body.Delayed crazing is due to the volume increase of the body,because of absorption of humidity after firing. The body, infact, can show a slow but inexorable tendency to react withthe water present as humidity in the air, which changespermanently its properties. This dimensional increasing ofthe ceramic bodies occurs in a period of time which canvary from some days to some years.In the past, this phenomena was not understood and it wasindeed the biggest problem of porous ceramic bodies. Duringthe last 50 years it was deeply investigated and it is nowcompletely understood. The solution looks easy at first sight:minimise the moisture expansion and increase thecompression of the glaze on the body. This approach indeed

gave excellent results for many years, but now a day theproblem is showing up again in different circumstances.Fast firing is now the dominating technology and the timeavailable to reach a optimal stabilisation of the body isreduced to few minutes. The size of the tiles is growingquickly: now the market is asking for wall tiles up to onemeter in size and above. Thickness is down to the minimum,to save in cost and transportation. The value of the productlays in the richness of the surface and the thickness of theglazes is rising. Geometrical perfection is a must: tiles aremechanically squared to give a marble-like look to the wall.The big problem now is planarity. The old trick of increasingthe state of compression of the glaze onto the body to avoiddelayed crazing was working well on the old thick and smalltiles. Now the same level of compression on the very samebody and glaze but with a much larger size, has a nasty sideeffect: bending. To reduce the bending there is only onesolution: reduce to the minimum the amount ofcompression. A second deflect could appear in case of toohigh compressive tension in glaze: peeling. In some casesthe tendency of the glaze to peel off the body appear in thesharp corner of three-dimensional products, such askitchenware.The problem, now, is to know how much it is the minimumamount of compression. Since all glazes, like glasses, arecharacterized by a very low resistance to tractions stress, alittle value of traction may causes their rupture. If the bodyexpands, even if in a small percentage, the glaze may easilycrack, unless it is in a state of compression with respect to thebody. In this case, a little expansion of the body reduces thestate of compression, without generating a dangeroustraction. This tendency to react with water in the course oftime cannot be completely eliminated: all glazed bodiesshow this problem. The only type of body that can beconsidered stable during the time is the completely sinteredbody, characterized by a lack in porosities through whichthe humidity would penetrate into the body.A body formulation and a firing cycle intended to avoiddelayed crazing do not exist. The problem, however, can besuccessfully tackled by studying the state of tension betweenglaze and body, by means of thermal expansion and bendingtests.The state of tension between glaze and body dependsessentially on two factors: the relation between theircoefficients of thermal expansion and their coupling tempe-rature.Generally, a common error consists of performing a simplecomparison between coefficients of thermal expansion inorder to prevent the defect. However, the fact that the glazecoefficient of thermal expansion (CTE) at 400°C is lower

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than the body CTE at the same temperature cannot assurethat the glaze will be in a state of compression with respect tothe body after firing. Also a glaze with a CTE lower than thebody one can give rise to delayed crazing, as it will be shownlater. In fact, another important parameter must beconsidered: it is the coupling temperature between glazeand body. Until now, the instrument used to measure thecoupling temperature was the Steger tensiometer and theanalysis was quite problematical. The test was performedwith very long (30 cm) specimen which was not placedcompletely inside the kiln, with consequent lack inhomogeneity in sample temperature.

2. A novel optical technique

Now, thanks to MISURA® FLEX-ODLT (shown in Figure 1),the new instrument developed by Expert System Solutions,

it is possible to measure the state of tension of ceramic glazedbodies.With this instrument (which combines together OpticalFleximeter and Optical Dilatometer), it is possible to performboth thermal expansion measurements and Steger analysis,which allows to identify the coupling temperature.The same instrument can be used to study the problemrelated to the opposite situation, i.e. when there is a too highcompressive state of the glaze with respect to the body.This instrument works with no contact with the samplestested: thus, their behaviour is never modified by themeasuring system. The specimens can be cut directly froman industrial tile; then there are placed into the kiln, whichis equipped with an automatic temperature controller andcan reach a heating rate of 30 °C per minute and a maximumtemperature of 1600 °C. All the experimental part of thiswork was performed by using this optical instrument.

Figure 1. The combined instrument MISURA® FLEX-ODLT of Expert System Solutions

Figure 2. Thermal expansion curves of body and glaze

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TECHNICAL NOTE

3. Delayed crazing

Starting from a glazed product, we want to study the state oftension between glaze and ceramic body.The first experimental step consists of two thermal expansiontests, performed on fired glaze and body (see Figure 2). Twobeams of lights illuminate both the ends of a specimen 50mm long placed horizontally into the furnace and two digitalcameras capture the images of the last two hundreds micronsof each tip. The specimen, completely free to expand orcontract, is measured by the image that it projects on animage sensor. With a wavelength of 478 nanometres, aresolution of 0,5 microns can be obtained.The second experimental phase consists of a flexion analysisperformed on a fired glazed body (Figure 3). Two holdingrods, 70 mm spaced, support an 85 mm long sample. Acamera frames the centre of the sample, which movesdownward or upward during the test. The test is performedheating the sample up to a temperature high enough to cau-se the glaze softening and following the cooling below toabout 300 °C.In Figure 3, the flexion curve of a glazed tile is expressed as afunction of the temperature.At the beginning of the cooling phase, the glaze is liquid andfollows the body contraction without developing tensions.At a certain moment, a rapid variation of inclination occurs:this mean the glaze has become rigid enough to build uptensions, causing the flexion of the sample. Incorrespondence of the coupling temperature, the glaze

softens during heating (absorbing tensions) and solidifiesduring cooling (building up tensions). In this case the couplingtemperature is 720 °C.In order to obtain the final result, it’s necessary to translatethe glaze thermal expansion curve so that it coincides withthe body thermal expansion curve in correspondence ofthe coupling temperature. The result of the translation isreported in Figure 4.The two curves, after the translation, do not coincideanymore at the origin (room temperature). This difference isindeed the of traction or compression which has establishedbetween glaze and body immediately after firing. From thismoment, the body start to react with the air humidity ,increasing its volume. This is a hydrothermal phenomenon,which is favoured by the presence of humidity and hightemperatures. This process of rehydration, in the case of tilesalready placed on the floor, may require some years, butcan be also artificially accelerate by increasing the watervapour pressure and the temperature. Performing anautoclave test at high temperature and pressure, the processcan be completed in few hours. The measurement of thepercentage of expansion due to the adsorption of watershould be compared with the measurement of thepercentage of compression of the glaze layer with respect tothe body.If the value of expansion after the autoclave test is lower thanthe level of compression established between glaze and body,then the product will not be at risk of delayed crazing.The following example (Figure 5) shows the thermal

Figure 3. Flexion curve of a fired glazed tile

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Figure 4. Thermal expansion and flexion curves overlapped to identify the state of tension of the glaze with respect to the bodyafter firing

Figure 5. Although the glaze CTE is lower with respect to the body CTE, the glaze at room temperature will be in a state of traction

expansion and flexion curves of a product which is stronglyat risk of crazing. A simple comparison between coefficientsof thermal expansion is misleading: the glaze CTE at 400 °C

is 6.38·10-6 °C-1, while the body at the same temperature is6.58·10-6 °C-1. Although the glaze CTE is lower than the bodyCTE, the glaze is in a state of traction with respect to the body

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TECHNICAL NOTE

during the whole cooling phase. This is due to the fact thatthe coupling temperature is considerably higher than theglass transition temperature. The glaze is in a state of tractionwhich respect to the glaze that can provoke its rupture, evenimmediately after firing.Considering the various experimental results it is easy tounderstand that it very difficult to solve the various problemsof deformations without the support of complete laboratorydata. The combined instrument MISURA® FLEX-ODLT wasdesigned to meet this requirement.

4. Pyroplasticity

Another parameter of great technological importance ispyroplasticity, in other words the tendency of the body todeform within a given temperature range. This parameteraffects the productivity of a production plant because itdetermines the relationship between the thickness and therate of deformation of the material as a function of firingtemperature. It is now possible to monitor changes inpyroplasticity during the thermal treatment by measuringthe bending of a body sample suspended between twosupports 70 mm apart. This measurement is particularlyimportant for bodies that have to be completely sinteredbecause during the final stage of firing they develop anabundant vitreous phase with a sufficiently low viscosity tocause rapid deformation of the material. The stress sufferedby the sample inside the instrument is certainly much greater

than that suffered by the material inside the roller kiln. As thematerial moves over the rollers, the supporting points shiftcontinuously across the entire width of the piece, where asin the laboratory instrument the material is suspended ontwo static supports. However, unfired ceramic materials maybehave in very different ways when subjected to the thermalfiring treatment. In some cases the highest bending rate occurswhen fusion of the feldspars commences; then at highertemperatures the tendency to bend falls as the vitreous phasedissolves other mineral components of the body, thusbecoming more viscous. The addition of vitroceramicmaterials to bodies can reduce bending, indicating anincrease in modulus of elasticity caused by the transitionfrom the vitreous phase to the crystalline phase. Figure 6illustrates the behaviour of two samples of body for glazedporcelain tile which have undergone a firing cycle with 10minutes dwell at 1220 °C. The black curve is the one inwhich the temperature rises as a function of time, while thered and green curves relate to the bending of two bodysamples. In order to be able to display the progression of thethermal treatment, we need to show on the graph the dataregarding bending and temperature as a function of time,which is therefore shown on the x-axis.The differences in behaviour are very clear, sample 2 bendstwice than sample 1, situation that can cause seriousproduction problems, adversely affecting the productivityand quality yield of an entire factory. Of great importance isthe measurement of the bending rate (first derivative of the

Figure 6. Pyroplasticity test on two porcelain stoneware samples

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bending curve) during the maximum temperature dwell:while the sample 1 is decreasing it, sample 2 does not stopbending but exhibit an increase in bending rate. Themeasurements for monitoring the behaviour of the sampleduring the thermal treatment are all made using the newoptical measuring systems, which allow for very precisemeasurements without any contact with the material underexamination. Most of the properties that are displayed athigh temperature would be substantially modified by ameasuring system in contact with the material, especially inthe sintering stage. Pressure of any kind applied to the samplealters its behaviour. Using optical systems it is now possible toperform surface tension measurements even on molten glazeand to monitor the behaviour of material as it undergoessuccessive stages of wetting and drying and its wet surfacebecomes completely plastic.

References

1. A.N. Smith, - Trans. Brit. Ceram. Soc. 54 300 (1955).2. James E. Young e Wayne E. Brownell, “Moisture Expansion

of Clay Products” - J. Am. Ceram. Soc. 42 (12) 571-81(1959).

3. I.C. Hope, J. Gabriel e I.C. McDowall, “Moisture Movementand Readsorption Phenomena in Dried Clay Articles” -J. Am. Ceram. Soc. 43(11) 553-60 (1960).

4. A.G. Verduch, “Expansiòn por humedad de los productosceràmicos” - Bol. Soc. Esp. Ceràm. 4 (3) 259-84 (1965).

5. Ravaglioli, C. Fiori e G. Vecchi, “Espansione di piastrelleceramiche per imbizione di acqua e vapore. Studio Stati-stico”. - La Ceramica 17-26 (luglio-agosto 1976).

6. R. Bowman, “Importanza della cinetica dell’espansionein vapore - The importance of the kinetics of MoistureExpansion” - Ceramica Acta 5 (4,5) 37-60 (1993).