Stress and crack monitoring during plasma spraying of TBC

Two types of process monitoring techniques are compared and discussed in this presentation. The first one is in-situ curvature monitoring, by which it was possible to evaluate the stress evolution during plasma spraying and separately identify the sources of stresses, i.e., the quenching stress and thermal stress as shown in Fig.1 (a). By changing the spraying parameters, it was possible to prepare specimens at largely different deposition temperatures, which resulted in significantly different levels of residual stresses. Also, it was found that the mechanical properties of the obtained YSZ coatings such as the elastic modulus are strongly dependent on the deposition temperature as shown in Fig.1 (b). Four-point bending test was conducted to these coatings, which clearly showed that the compressive residual stress effectively offset the applied tensile stress to initiate cracking in the YSZ coatings. Another method is based on acoustic emission (AE). Non-contacting laser AE sensors as shown in Fig.2 were used to detect cracking in YSZ coatings during spraying. Due to the intensive noise from the plasma spraying environment, extensive signal processing techniques have been developed to eliminate the noise in the frequency and time domains by using digital filtering and multi-threshold techniques. The obtained results so far indicate that the through thickness temperature gradient during spraying plays a major role in the formation of deep vertically segmentation cracks

Acquisition and Analysis of Continuous Acoustic Emission Waveform for Classification of Damage Sources in Ceramic Fiber Mat

Waveforms of acoustic emission (AE) events come close and sometimes overlap each other when AE activity is very high. Conventional AE measurement systems which handle discrete AE events are not suitable for this situation because miss-detection of AE event occurs frequently. A new AE measurement system named as Continuous Wave Memory (CWM) was developed to solve this problem by recording the AE waveforms continuously to hard disks for several hours throughout the testing time. This new system enabled multiple analysis of one waveform with different filtering parameters. Short time Fourier transform (STFT) gave the time–frequency–magnitude characteristic of continuous AE waveforms and useful information for evaluation of degradation of materials. In this study, the degradation of ceramic fiber mat during cyclic compression test and the effect of binder-addition were evaluated by this new system. STFT results clearly showed the classification of degradation of the mat; breakage of fibers was the main source in the early compression cycles and sporadic friction between fibers became the main source of AE in the later compression cycles. The effect of organic binder to prevent the degradation of the mat was also estimated. It was observed that the friction signal disappeared and the breakage signal weakened in the binder-added specimens. [doi:10.2320/matertrans.I-MRA2007850

Crack Monitoring during Plasma Spraying of Ceramic Coatings by Non-Contact Acoustic Emission Method

Atmospheric plasma spraying (APS) is an effective process to make ceramic coatings. However, several types of cracks are inherently present in the coatings and among them, delamination cracks within the coatings or at the interface with the underlying layer are believed to reduce their durability and reliability. In this study, non-contact laser acoustic emission (AE) method and original AE measurement system successfully detected the cracks during APS in real time, which was very difficult because of noise from the APS system. AE waveform was sampled and recorded continuously during the testing time. AE events due to delamination cracks were successfully detected from noisy waveform by this system. Influences of the scanning speed and the power of the plasma jet to the delamination were also confirmed. [doi:10.2320/matertrans.M2010004