John Nunn

High resolution measurement of dynamic (nano) indentation impact energy: a step towards the determination of indentation fracture resistance

A Micro Materials (UK) NanoTest has been modified for fast data acquisition at up to 200 kHz to enable high-resolution (sub-pJ) impact energy measurement. Displacement versus time data were acquired at 20 kHz in 3-s bursts to capture the acceleration, impact and rebounding of the pendulum-mounted indenter. The data acquired were analysed to provide measurements of position, velocity, acceleration, impact distance, impact depth (elastic and plastic) potential and kinetic energy throughout the impact cycle and provided high-resolution information throughout the impact event itself, which lasted only ∼ 10 ms. The ability of the system to detect fracture events and measure energies associated with these events is investigated. The effect of impact energy and indenter geometry is demonstrated and it is shown that the system is able to detect damage events and measure the difference in the energy absorbed in an impact when damage occurs.

The feasibility of an infrared system for real-time visualization and mapping of ultrasound fields

In treatment planning for ultrasound therapy, it is desirable to know the 3D structure of the ultrasound field. However, mapping an ultrasound field in 3D is very slow, with even a single planar raster scan taking typically several hours. Additionally, hydrophones that are used for field mapping are expensive and can be damaged in some therapy fields. So there is value in rapid methods which enable visualization and mapping of the ultrasound field in about 1 min. In this note we explore the feasibility of mapping the intensity distribution by measuring the temperature distribution produced in a thin sheet of absorbing material. A 0.2 mm thick acetate sheet forms a window in the wall of a water tank containing the transducer. The window is oriented at 45 degrees to the beam axis, and the distance from the transducer to the window can be varied. The temperature distribution is measured with an infrared camera; thermal images of the inclined plane could be viewed in real time or images could be captured for later analysis and 3D field reconstruction. We conclude that infrared thermography can be used to gain qualitative information about ultrasound fields. Thermal images are easily visualized with good spatial and thermal resolutions (0.044 mm and 0.05 degrees C in our system). The focus and field structure such as side lobes can be identified in real time from the direct video output. 3D maps and image planes at arbitrary orientations to the beam axis can be obtained and reconstructed within a few minutes. In this note we are primarily interested in the technique for characterization of high intensity focused ultrasound (HIFU) fields, but other applications such as physiotherapy fields are also possible.

Performance of thermal barrier coatings in industrial gas turbine conditions

Abstract The effect of aerofoil geometry on the oxidative degradation mechanisms experienced by thermal barrier coatings (TBCs) used on industrial turbine blades has been investigated. Modified aerofoil-shaped samples (CMSX4 coated with high-velocity oxy-fuel sprayed AMDRY 995 and air plasma sprayed TBC) were oxidised at five temperatures in furnaces from 900 to 1000°C. Scanning electron microscopy and energy dispersive X-ray analysis were used to characterise details of the microstructural evolution of the thermally grown oxide and to monitor inter-diffusion between the bond coating and substrate. Additionally, a novel non-destructive examination technique (flash thermography) was used to detect and track the spread of cracks beneath the TBCs. Multiple samples cracking in identical locations suggested an effect of geometry in the failure of coatings. Furthermore, it was observed that coating curvature influenced spinel formation.

Application of thermography in the evaluation of early signs of failure of thermal barrier coating systems

Abstract For a number of years piezospectroscopy (Cr fluorescence) has been used to monitor the stress levels in the thermally grown oxide (TGO) that forms between the bondcoat and the thermal barrier coating (TBC) in TBC systems. The purpose of that work has been to observe early signs of failure and thus allow operators to schedule service intervals before failure of the TBC system occurred. This paper reports the use of thermography as an additional tool that can be used to assess the “health” of TBC systems. The technique consists of imaging the surface of the TBC coated specimen with a high spatial resolution infra-red camera while the specimen is heated, and monitoring the temperature of the outer surface of the TBC. Conductive heating through the substrate and radiative heating incident on the TBC have been studied. Early results are encouraging, revealing a clear correlation between thermograms obtained using the conductive and radiative forms of heating, some of the stress maps obtained using piezospectroscopy and direct metallographic evidence. Examples of electron beam physical vapour deposited (EB PVD) and air plasma sprayed (APS) TBC systems have been studied as they were progressively aged. Cracking and disbonding associated with the TGO and/or TBC have been observed in places where thermography showed differential heating.

Comparison of the Thermal Cycling Performance of Thermal Barrier Coatings under Isothermal and Heat Flux Conditions

Ceramic Thermal Barrier Coatings (TBCs) have been developed for advanced gas turbine engine components to improve the engine efficiency and reliability. The integrity and reliability of these coatings is of paramount importance. Accurate prediction of service lifetimes for these components relies upon many factors, and is not straightforward as knowledge of the service conditions and accurate input data for modelling are required. The main cause of failure of coatings is through debonding which develops as a consequence of thermally induced strains between the metallic bondcoat and the alumina TGO layers due to the differences in the thermal expansion coefficients of the individual layers. Thermal transients due to the power cycles of turbines will then cause these fractures to grow between the TGO and the bondcoat. When these fractures reach a critical size they can grow rapidly and cause the TBC to spall off. Thermal cycling of TBCs is used therefore to evaluate and rank TBC performance. Within the laboratory this is often conducted under isothermal conditions. Whilst this test method has performed adequately in the past it does not fully simulate service conditions. Work has been underway therefore to develop a more complex test method, which better simulates the service conditions experienced by the TBC. The approach here employs a gas torch to heat the operating face of the TBC whilst cooling the rear of the substrate with compressed air, thereby imparting a heat flux on the specimen. The specimen is then cycled by removing the gas torch and cooling with compressed air on the front and rear faces. Tests have been conducted on a TBC system consisting of an IN738 substrate with a CN334 bondcoat and EBPVD TBC. Thermal cycling tests have been performed under both isothermal and heat flux conditions. During the course of the tests the samples were examined non-destructively using a thermal camera to identify early indications of spallation. This paper reports on the performance of the flame rig equipment and the results from the exposures on the TBC system.