Characterization of Films with Thickness Less than 10 nm by Sensitivity-Enhanced Atomic Force Acoustic Microscopy

We present a method for characterizing ultrathin films using sensitivity-enhanced atomic force acoustic microscopy, where a concentrated-mass cantilever having a flat tip was used as a sensitive oscillator. Evaluation was aimed at 6-nm-thick and 10-nm-thick diamond-like carbon (DLC) films deposited, using different methods, on a hard disk for the effective Young's modulus defined as E/(1 - ν2), where E is the Young's modulus, and ν is the Poisson's ratio. The resonant frequency of the cantilever was affected not only by the film's elasticity but also by the substrate even at an indentation depth of about 0.6 nm. The substrate effect was removed by employing a theoretical formula on the indentation of a layered half-space, together with a hard disk without DLC coating. The moduli of the 6-nm-thick and 10-nm-thick DLC films were 392 and 345 GPa, respectively. The error analysis showed the standard deviation less than 5% in the moduli

Rißausbreitung in Hartmetallen unter monotoner und zyklischer Belastung

  &nbsp

Simulation der Disruptions-Belastung von Erste-Wand-Hochtemperturmaterialien

The behaviour of carbon materials under thermal load in fusion reactors has been simulated by laser-pulse irradiation in a scanning electron mocroscope (SEM). In this way material damage, such as thermal shock crack formation and propagation, and erosion behaviour, can be studied in situ in the SEM with high lateral resolution. The dependence of damage initiation and propagation on the laser-beam parameters (pulse number, energy, spot size, spot duration and energy density), is of special interest. The damage behaviour is strongly determined by special materials structures. Because of its fibre reinforcement, the investigated CFC composite materials proved to be more stable to erosion and crack formation than homogeneous finegrained graphites. High-temperature damage may be diminished by the use of carbon materials with creep-resistant components

Untersuchung der laserinduzierten Rißbildung an Hochtemperatur-Kohlenstoffwerkstoffen im LAREM

Essential damage of carbon-fiber reinforced carbon materials is caused by short high temperature loading conditions. They may be modeled by irradiation with intense laser pulses. For direct observation of the damaging the irradiation is carried out inside a scanning electron microscope. Erosion and crack formation is described in dependence on irradiation conditions. The crack pattern is quantitatively analyzed

In situ laser irradiation of WC-Co hardmetals inside an SEM. Part 1. General features of surface modification.

Samples of WC-6% Co hardmetals were irradiated with a pulsed CO2 laser inside a scanning electron microscope in order to study the mechanisms and development of surface modification in microscopic detail. The photon emission of the irradiated surface was measured simultaneously. Samples with surfaces prepared in different ways were irradiated in a multiple pulse regime. The laser power density was somewhat lower than that which would lead to a melting of the polished surface. It is shown that under such irradiation conditions the decisive step in surface modification is the successive solution of the carbide in the lower melting binder phase. Above a critical number of laser pulses, in the order of a thousand, this leads to the onset of large-scale melting, pointing to a marked rise of absorbency. These alterations of surface state are well reflected in the photon emission measurements. The influence of the original binder distribution within the surface layer, changed by grinding or p olishing, is investigated

In situ laser irradiation of WC-Co hard metals inside an SEM. Part 2. Behaviour of the cobalt binder.

Cemented carbides with higher binder contents were irradiated with a CO2-pulse laser within a SEM to investigate the structural development in more detail. The decisive medium is the melting binder which dissolves the refractory carbide phase. In this way a superficial layer of an oversaturated solid solution with highly dispersed secondary carbides is produced. At higher pulse numbers a periodic structure is engraved in the surface by local evaporation as result of a self-amplifying resonance phenomenon

Thermoschockrißbildung durch laserinduzierte Hochtemperaturrelaxation

Crack formation by thermal shocks is usually discussed on the base of thermal-elastic misfits and corresponding reversible stress fields notwithstanding the fact that dangerous tensile stresses in this case are only produced by thermal quenching. The cracking by temperature peaks in high temperature applications is caused by relaxation processes on the high temperature level leading to permanent tensile stresses after cooling. The underlying processes are theoretically investigated for the case of carbon materials irradiated by short laser pulses. Temperature and stress fields and the corresponding stress intensities are calculated. The crack propagation is discussed in the framework of linear fracture mechanics