František Lofaj

Creep Behavior of Improved High Temperature Silicon Nitride

New data are presented on the tensile creep behavior of silicon nitride sintered with Lu2O3. The data are compared with two earlier sets of data collected on the same material. The older sets gave results that are difficult to explain theoretically: a high value for the stress exponent, n=5.33, and no cavitation. The new set of data also gave no cavitation, but gave a stress exponent, n=1.81, that can be rationalized theoretically in terms of solution-precipitation creep of the silicon nitride grains. An analysis of variance showed that one of the earlier sets of data was statistically consistent with the newer set, whereas the other set of data was not. Combining the two sets of data that agreed statistically yields a consistent picture of creep with a low value of the stress exponent and no cavitation. The stress exponent for the combined set of data is n=1.87±0.48 (95 % confidence limits). The tensile creep mechanism of the silicon nitride containing Lu2O3, solution-precipitation, differs from those of other silicon nitrides, for which tensile creep has been attributed to cavitation. Enhancement of the creep resistance of the silicon nitride sintered with Lu2O3 may be a consequence of the fact that Lu2O3 produces a more deformation resistant amorphous phase at the two grain junctions, than do Y2O3 or Yb2O3. In parallel, reducing the amount of secondary phase below a critical limit, or increasing the viscosity of the two grain boundaries relative to three-grain junctions reduces the ability of the material to cavitate during creep, and forces the creep mechanism to change from cavitation to solution-precipitation.

Rheological Properties of the Rare-Earth Doped Glasses

RE-Si-Mg-O-N glasses with RE = Sc, Y, La, Yb, Sm, or Lu with two different nitrogen contents were investigated to understand the effects of rare-earth elements and nitrogen on microhardness, indentation modulus, fracture toughness and viscosity. The microhardness and indentation modulus of glasses containing lanthanide dopants increase approximately linearly with the increase of the cationic field strength (CFS) of the corresponding RE element, whereas fracture toughness is almost unaffected. Nitrogen content increase shifts the dependencies to higher values. The properties of the glasses with non-lanthanide dopants, Y and Sc, deviate from linear dependence. At elevated temperatures, variations of the softening temperature Tg from linearity were found in Yband Sc-containing glasses. Additional differential thermal analysis experiments on Yb-containing glass revealed the possibility of changes in glass structure at elevated temperatures. CFS concept seems to be insufficient to describe all the effects of rare-earth dopants on the properties of the oxynitride glasses.

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).

Nanohardness of Individual Phases in WC – Co Cemented Carbides

The nanohardness of WC – Co hardmetals has been investigated using instrumented indentation and Berkovich tip indenter. The nanohardness, HIT, and indentation modulus, EIT, of Co phase and individual WC grains and the influence of their crystalographic orientation have been studied. SEM, AFM and EBSD methods were used for the characterization of the microstructures and indents and for the identification of crystallographic orientation of WC grains, respectively. Strong indentation load-size effect and significant influence of the crystallographic orientation of WC crystals on HIT and EIT have been found. The nanohardness of Co binder was approximately 10 GPa and that of WC grains varied between 25 and 50 GPa, depending on the grain orientation and load. The nanohardness values of the basal and prismatic planes of individual WC grains at load of 10 mN were 40.4 ± 1.6 GPa and 32.8 ± 2.0 GPa, respectively.

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.

Nanohardness vs. Friction Behavior in Magnetron Sputtered and PECVD W-C Coatings

The nanoindentation and tribological measurements were performed on DC and RF magnetron sputtered W-C coatings to optimize the deposition conditions to obtain maximum nanohardness and to compare their properties and behavior with earlier studied PECVD W-C coatings. Despite number of similarities, some consequences of a tribo-chemical transfer film formation are inconsistent with the existing wear model for W-C coatings with controlled carbon content. Transfer films in the friction contact consist from iron- and tungsten oxides and carbon generated during carbide oxidation. The role of carbon in the transfer films leading to high coefficients of friction has to be principally different from the expected lubricative role of carbon in W-C coatings and needs further study.

Fracture of Thin Walled Translucent Polycrystalline Alumina (PCA) Tubes

Room temperature fracture behavior of short thin walled translucent coarse-grained polycrystalline alumina (PCA) tubes was investigated under compression loading in radial direction using conventional mechanical tests combined with the finite element modelling (FEM). FEM indicated strongly inhomogeneous stress distribution with the maximum tangential tensile stresses in the centre of the inner surface of a tube and opposite to the line of loading. Another zone of tangential tensile stresses form on the outer surface of the tube and oriented perpendicularly to the loading direction. However, these tensile stresses are lower compared to those under contact lines. Radial stresses were negligible compared to tangential stresses which were concluded to control tube fracture. Fracture occurs in two steps: firstly along the loading lines under the lines of contacts and secondly in the direction perpendicular to loading. It results in four regular pieces along the lines with the highest tangential tensile stresses. The results were analysed using Weibull statistics and standard fractography methods. The characteristic strength of the PCA tubes was 186 MPa and the Weibull parameter m‘corr = 16.9 ± 0.5. High m values and the absence of the obvious fracture origins were attributed to homogeneous microstructure and well developed PCA technology.

Thermal Shock Resistance of the RE-Oxide and Oxynitride Glasses

Thermal shock resistance of the RE-Si-Mg-O-N glasses (RE = La, Nd, Yb, Lu) with 0 and 20 eq.% of nitrogen was investigated by the indentation-quench method based on propagation of Vickers cracks. Crack growth was measured on the same sample for a test series of different quenching temperatures. Thermal shock resistance of the studied materials was determined as a temperature difference resulting in 10 % growth of the initial cracks (∆T10) and by the thermal shock parameter R calculated from the material properties. Although the comparison of ∆T10 and R values as a function of glass composition revealed some differences between these two approaches, also a common trend was observed. Thermal shock resistance increased with the fractional glass compactness resulting from RE type and N content increase.

Contact Strength of the RE-Oxynitride Glasses

Effects of the rare-earth additives (RE = Sc, Y, La, Nd, Gd, Sm, Yb and Lu) and nitrogen on contact strength of the oxide and oxynitride glasses were investigated at room temperature. The advantage of this relatively new technique is its experimental simplicity and using of miniature samples, however, a relatively complex stress distribution around the contact points reduces Weibull parameter in comparison with the conventional bending strength. Contact strength of the tested glasses is also strongly influenced by surface finish. The differences in the effects of REmodifiers in oxide and oxynitride glasses suggest that the contributions from RE and nitrogen may not be simply additive, as observed earlier for various thermo-mechanical properties. However, additional study is necessary to eliminate the effects of surface defects to support this assumption.