Rudolf Podoba

Mechanical and Tribological Properties of HiPIMS and HiTUS W-C Based Coatings

Several series of W-C based coatings were deposited by HiPIMS and HiTUS deposition technologies with different deposition parameters (power, frequency and pulse length in HiPIMS and pressure, acetylene content and RF bias in HiTUS). To investigate their effect on the hardness and tribological properties. The hardness of HiTUS coatings was up to 36 GPa and CoFs of these coatings were around 0.5, whereas the HiPIMS coatings exhibited lower hardness (18 – 20 GPa) but also lower CoFs (0.18 - 0.2).

Mechanical properties of alumina porcelain during heating

The mechanical strength and Young’s modulus of green alumina porcelain (50 wt. % of kaolin, 25 wt. % of Al2O3, and 25 wt. % of feldspar) were measured during heating up to 900 °C and 1100 °C, respectively. To this end, we used the three point-bending method and modulated force thermomechanical analysis (mf-TMA). The loss liberation - of the physically bound water (20 – 250 °C) strengthens the sample and Young’s modulus increases its values significantly. The dehydroxylation that takes place in the range of 400 – 650 °C causes a slight decrease in Young’s modulus. On the other hand, the mechanical strength slightly increases in this temperature range, although it has a sudden drop at 420 °C. Beyond the dehydroxylation range, above 650 °C, both Young’s modulus and mechanical strength increase. Above 950 °C, a sharp increase of Young’s modulus is caused by the solid-state sintering and the new structure created by the high-temperature reactions in metakaolinite.

Some open problems in physics of disordered materials (and their possible solutions on the base of the barrier-cluster-heating model)

The paper briefly summarizes the basic problems in physics of disordered materials, namely in physics of non-crystalline semiconductors. Although there was much attention devoted to the research in this area, many questions about the nature of processes taking place in these substances still remain unanswered. This article familiarizes the readers with the barrier-cluster-heating model of disordered semiconductors and discusses the possibility of explanations of the mentioned phenomena on its base. This article offers the explanation of the mid-gap absorption and mid-gap photoluminescence on the base of the barrier-cluster-heating model.

The Firing Temperature of Romanesque Brick from Pác

Abstract A pavement brick taken from a Romanesque part of the church in Pác, in the Trnava County, Slovakia, was investigated by x-ray diffraction analysis (XRD) and thermal analyses as differential thermal analysis (DTA), thermogravimetry (TG) and thermodilatometry (TD). It was found that the brick contained dehydroxylated illitic clay, calcite and quartz. As revealed, dehydroxylation was completely finished and no redehydroxylation was observed. Partial decomposition of calcite was also found. The estimated firing temperature is between 600 °C and 700 °C.

Nanohardness of DC Magnetron Sputtered W – C Coatings as a Function of Composition and Residual Stresses

The effects of residual stresses in thin W-C based coatings were investigated with the aim to find their influence on nanohardness and indentation modulus. Ten samples of W-C based coatings were deposited on microslide glass substrates using DC magnetron sputtering at the identical deposition parameters. Their thickness was in the range from 500 to 600 nm. The residual stresses in the coatings varied from 1.5 GPa up to 4.4 GPa. Increase of residual stress caused linear increase of HIT from 16 to 19.5 GPa. This increase was only the result of the compressive stresses. EIT of the studied coatings was not sensitive to residual stresses and corresponded to 185 GPa ± 15 GPa.

Cracking in Brittle Coatings during Nanoindentation

The FIB/SEM investigations of the microstructure changes in the hard brittle W-C based coating deposited on softer steel substrate after nanoindentation tests revealed that a set of approximately equidistant circular cracks forms in the coating in a sink-in zone around the indent and single cracks appear under the indenter tip. Finite element modeling (FEM) indicated development and concentration of the highest principal tensile stresses in the sink-in zone and in the zone below the indenter, which are considered to be the reason for the experimentally observed cracking. The distance from the indenter tip to the first circular crack combined with the calibration curve obtained from the FEM of the location of tensile stress maxima in sink-in zone can be used as a simple method for the determination of the strength of the studied coatings.

Accelerated ageing in testing bricks used in the conservation of historic buildings

The effect of accelerated climate ageing on historical bricks in the laboratory is investigated in the paper. Differences in thermal properties are experimentally determined and studied before and after bricks exposure to climate ageing, which consists of 60 freeze–thaw cycles. For measuring thermal conductivity, diffusivity and specific heat, pulse method is used.