Picosecond time scale imaging of mechanical contacts

By means of an ultrafast opto-acoustic technique we study the nanoindentation of thin chromium films on sapphire substrates using a ceramic ball bearing. Acoustic pulses at 40 GHz returning from the film–indenter interface allow the film indentation profiles to be probed to sub-nanometer resolution over contact areas 25 lm in radius. The deformation of the films during loading is hereby revealed. Furthermore, thermal wave imaging of the contact at megahertz frequencies is simultaneously achieved

Diffraction Model of Thermoreflectance Data

Diffraction based mathematical model is developed to address the issue of spatial resolution in thermoreflectance imaging at the scale of 1 and 10 μm. Thermoreflectance imaging provided non-contact temperature measurement at micro and nano scale but the spatial resolution is limited by diffraction. By virtue of this work mathematical model is developed for the analysis of thermoreflectance data. In the development of model both the diffraction occurring at sample and substrate is combined to calculate intensity of thermoreflectance signal. This model takes into account the effective optical distance, sample width, wavelength, signal phase shift and reflectance intensity. Model shows qualitative and quantitative agreement with experimental data for the two wavelengths under investigation, 470 nm and 535 nm

Thermal conductivity and flash temperature

The thermal conductivity is a key property in determining the friction-induced temperature rise on the surface of sliding components. In this study, a Frequency Domain Thermoreflectance (FDTR) method is used to measure the thermal conductivity of a range of tribological materials (AISI 52100 bearing steel, silicon nitride, sapphire, tungsten carbide and zirconia). The FDTR technique is validated by comparing measurements of pure germanium and silicon with well-known values, showing discrepancies of less than 3%. For most of the tribological materials studied, the thermal conductivity values measured are reasonably consistent with values found in the literature. However the measured thermal conductivity of AISI 52100 steel (21 W/mK) is less than half the value cited in the literature (46 W/mK). Further bulk thermal conductivity measurements show that this discrepancy arises from a reduction in thermal conductivity of AISI 52100 due to through-hardening. The thermal conductivity value generally cited and used in the literature represents that of soft, annealed alloy, but through-hardened AISI 52100, which is generally employed in rolling bearings and for lubricant testing, appears to have a much lower thermal conductivity. This difference has a large effect on estimates of flash temperature and example calculations show that it increases the resulting surface temperatures by 30 to 50%. The revised value of thermal conductivity of bearing steel also has implications concerning heat transfer in transmissions

FINITE ELEMENT AND IMAGING APPROACHES TO ANALYZE MULTISCALE ELECTROTHERMAL PHENOMENA

Electrothermal effects are crucial in the design and optimization of electronic devices. Thermoreflectance (TR) imaging enables transient thermal characterization at submicron to centimeter scales. Typically, finite element methods (FEM) are used to calculate the temperature profile in devices and ICs with complex geometry. By comparing theory and experiment, important material parameters and device characteristics are extracted. In this work we combine TR and FEM with image blurring/reconstruction techniques to improve electrothermal characterization of micron and nanoscale devices