Solution

Detecting Micro-Cracks in Fired Ceramics

IET internal friction measurement detects cooling-rate damage in porcelain stoneware and terracotta that Young's modulus alone misses.

ceramicsporcelainterracottainternal-frictiondampingmicro-cracksquality-controlfiring 2 min read

The Challenge

Ceramic manufacturers fire porcelain stoneware and terracotta at temperatures above 1100 °C, then cool them at rates dictated by production throughput. Fast cooling (200 °C/min by air ventilation) saves time but risks micro-crack formation. Quartz undergoes an allotropic transformation at 573 °C during cooling, and rapid passage through this transition generates internal stresses that crack the ceramic matrix. These micro-cracks weaken the product but are too small for visual inspection. Manufacturers need a way to verify that their firing and cooling cycles produce structurally sound parts.

The Solution

Researchers at the Ceramic Technical Center of Limoges used IET to measure both elastic moduli and internal friction of porcelain stoneware and terracotta specimens fired under different conditions. They compared fast-cooled samples (200 °C/min air ventilation) against controlled-cooled samples (50 °C/h), and varied firing temperatures by 25-50 °C within each material’s production range.

The internal friction measurement proved decisive. Young’s and shear moduli shifted by only 2.0-2.7% between fast and controlled cooling. Internal friction changed by a factor of 2.5. The damping signal picked up micro-crack damage that stiffness measurements alone would have flagged as within tolerance. Annealing fast-cooled samples at 700 °C partially healed the damage, confirming the micro-crack mechanism.

Results

IET internal friction distinguished cooling-rate damage with 2.5 times the sensitivity of elastic modulus. For terracotta, a 50 °C increase in firing temperature produced measurable property gains tied to reduced open porosity. The study demonstrated that a single IET measurement, capturing both modulus and damping in one tap, can verify heat treatment quality on the production floor. Ceramic manufacturers can use damping as an early warning for cooling-rate problems before mechanical failure occurs downstream.

Key takeaway: Internal friction measured by IET is 2.5 times more sensitive to cooling-rate damage than elastic modulus, making damping the critical parameter for verifying heat treatment quality in ceramic production.

Source

Evaluation of ceramic mechanical properties by Impulse Excitation Techniques: Effects of heating temperature and cooling rate

Antoine Coulon, Alexandre Filhol, Gérard Pillet

2021, Ceramics International (Elsevier)

The characterization of the ceramic materials integrity without damage has become a major issue for many industrial sectors. In this context, a research study has been initiated to assess the influence of the firing conditions on mechanical strength behaviors of Porcelain stoneware and Terracottas specimens. A focus is made on a non-destructive characterization method to evaluate the residual mechanical stresses, based on the Impulse Excitation Techniques. The influence of the cooling rate (≈200 °C/min by air ventilation vs. controlled cooling at 50 °C/h) and the heating temperature (1160 °C vs. 1185 °C for Porcelain stoneware; 1100 °C vs. 1150 °C for Terracotta) have been highlighted. The effect of samples annealing at 700 °C with a heating and cooling rate of 50 °C/h was also evaluated. A significant variation of the internal friction (2.5 times less important with a controlled cooling) and a small variation of 2.0% of the Young and shear moduli of porcelain stoneware samples caused by the cooling rate variation was shown. The negative effect of the fast-cooling was also discussed on terracotta samples by a small decrease (2.7%) of the Young and shear moduli. This decrease caused by a fast-cooling is led to micro-cracks formation induced by the quartz allotropic transformation during the cooling. In the case of terracotta, the 50 °C increase of the heating temperature induces a significant increase of mechanical strength properties linked to the open porosity decrease. The internal friction calculation from Impulse Excitation Techniques are able to efficiently evaluate the heat treatment quality of a porcelain stoneware ceramic material which allows the detection of mechanical performance decrease generated by micro-cracks in ceramic products.

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Frequently Asked Questions

How does IET detect micro-cracks caused by fast cooling in ceramics?

IET measures internal friction (damping), which is 2.5 times more sensitive to cooling-rate damage than Young's modulus. Fast cooling through the quartz alpha-beta transformation at 573 degrees Celsius creates micro-cracks that increase damping while barely shifting the elastic modulus. A controlled cooling rate of 50 degrees Celsius per hour versus rapid air ventilation at 200 degrees Celsius per minute produces measurably different internal friction values.

Can IET distinguish between different ceramic firing conditions?

Yes. For porcelain stoneware, IET detected a 2.0% variation in Young's and shear moduli between fast-cooled and controlled-cooled specimens, and a 2.5-fold difference in internal friction. For terracotta, a 50 degree Celsius increase in firing temperature produced measurable increases in mechanical properties linked to decreased open porosity. Annealing at 700 degrees Celsius partially recovered the properties lost to fast cooling.

Why is internal friction more revealing than elastic modulus for ceramic quality control?

Elastic modulus reflects bulk stiffness and changes only slightly with micro-crack formation. Internal friction measures energy dissipation at crack interfaces during vibration. Micro-cracks from quartz transformation during fast cooling create friction surfaces that damp vibrations without reducing overall stiffness. IET captures both measurements in a single tap, giving ceramic producers a more complete picture of heat treatment quality.

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