Optical techniques for non-destructive detection of flaws in ceramic components

• Main Author: Matysiak, Mateusz
• Other Authors: Shephard, Jonathan ; Hand, Duncan
• Published: Heriot-Watt University 2012
• Subjects: 666
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.681939

This thesis primarily concerns development of a non-destructive inspection method for 3mol% Yttria-Stabilised Zirconia Polycrystal (3Y-TZP) ceramics used for dental applications and a scoping study on applying the technique to other ceramic materials applied in thermal barrier coatings and other fields. Zirconia ceramics are materials of great interest for various engineering applications, primarily due to their stiffness, hardness and wear resistance. These factors in combination with the complex manufacturing processes may reduce the material strength and durability due to induced cracking. Knowledge of the extent of this cracking must be obtained and often, if each part is unique as in biomedicine, the assessment must be carried out for every part non-destructively so the part can be subsequently used. Only a few techniques are known for inspection of Zirconia ceramics, however these techniques are not able to detect flaws in thick (above 500 μm) parts. The main limitation for optical inspection of 3Y-TZP is the highly scattering nature of the material due to its multicrystalline grain structure (grains size of 500 nm) which, particularly in the visible region, reduces imaging capabilities. However, a transmission window in the mid-infrared (between 3 and 8 μm) exists opening up the potential for inspection at these wavelengths. Mid-Infrared Transmission Imaging (MIR-TI) and Confocal Mid-Infrared Transmission Imaging (CMIR-TI) techniques were developed for inspection of 3Y-TZP parts which allow for detecting sub mm scale cracks. The measured imaging resolution for the MIR-TI is 42 ± 5 μm, whereas for the CMIR-TI it is below 38.5 ± 5 μm. The maximum sample thickness inspected with the MIR-TI and CMIR-TI is 6 mm and 3.5 mm respectively, considerably more than currently available inspection methods. The MIRTI technique provides fast inspection of the part due to the large field of view (11 by 7 mm), however the high cost and limited imaging resolution make this technique less attractive. The CMIR-TI technique on the other hand is more cost effective due to reduced cost of the infrared sensor and it provides an enhanced imaging capabilities. The promising results achieved with the MIR-TI and CMIR-TI techniques led to the development of reflection equivalents (Camera-MIRI and Confocal-MIRI) for ceramic coating measurements, however further in-depth experiments to determine and quantify the capabilities of both techniques are required.