Radim Ctvrtlik

Nanocharacterization of the negative stiffness of ferroelectric materials

Phase changing materials such as ferroelectric materials could exhibit negative stiffness under certain thermomechanical environments. This negative stiffness is embodied by a deflection along the opposite direction of the applied load. So far negative stiffness materials were investigated with the specific morphology of embedded inclusions in stiff matrices then the resulting composite is studied to measure the behavior of each constituent indirectly. In this study, a modified nonisothermal nanoindentation method is developed to measure the negative stiffness of triglycine sulfate single crystal directly. This in-situ method is intended to first demonstrate the feasibility of detecting the negative stiffness via nanoindentation and nanocreep of a ferroelectric material at its Curie point and then to quantify the negative stiffness without the need for embedding the crystal within a stiffer matrix.

Pulsed laser welding of thin metals

Both types of industrial lasers, CO2 and Nd:YAG, are used for welding with wide range of maximal power values. Successful results of laser welding depend on many different factors: energy, diameter, mode structure, polarization and focus position of laser beam; thickness, accurate positioning and gap of welded parts, kind of shielding gas. With LASAG Nd:YAG pulsed laser system KLS 246-102 we made optimization of welding parameters for different metals 0.5 mm thick and some types of the weld geometry. Our experimental results were used for welding of the side joint of the steel motor covers. These precious parts are used in automobile racing models. Finite elements model is used to predict a heat affected zone.

Tribological Properties of Magnetron Sputtered Amorphous Silicon Carbide and Silicon Carbonitride Coatings

The tribological properties of magnetron sputtered amorphous silicon carbide (a-SiC) and silicon carbonitride coatings (a-SiCN) with thickness of 2.2 and 3.4 µm were investigated. Samples were additionally annealed at temperature of 700°C or 900°C in air. Progressive load scratch tests were performed on the annealed samples as well as on the as deposited ones. An acoustic emission signal was detected during all tests using the sample holder with embedded sensor of our own design. Results indicate no change in wear resistance of SiCN sample after high temperature exposure up to 900°C, unlike in the tests of SiC coatings. Detection of acoustic emission generated during the scratch test proved to be a significant improvement for the coating evaluation.

Mechanical properties of a-C, SiC and Ti-C: H films

Abstract a-C, a-SiC and Ti-C: H coatings were deposited on two steel substrates with different hardness by magnetron sputtering. Their mechanical properties were investigated by depth sensing indentation and dry sliding testing. Load – displacement curves were obtained using a NanoTest™ NT600 instrument equipped with a Berkovich indenter. The depth profiles of the mechanical properties (indentation hardness H, effective modulus Eeff, and also H/Eeff and H3/E2eff ratios) on both substrates are presented. The goal of this article is to compare the dependence of the real response of a coating/substrate system on the substrate. Regardless of substrate type the highest and the lowest values of hardness and modulus belong to a-C and Ti-C: H films, respectively.

Nanocrystalline diamond protects Zr cladding surface against oxygen and hydrogen uptake: Nuclear fuel durability enhancement

In this work, we demonstrate and describe an effective method of protecting zirconium fuel cladding against oxygen and hydrogen uptake at both accident and working temperatures in water-cooled nuclear reactor environments. Zr alloy samples were coated with nanocrystalline diamond (NCD) layers of different thicknesses, grown in a microwave plasma chemical vapor deposition apparatus. In addition to showing that such an NCD layer prevents the Zr alloy from directly interacting with water, we show that carbon released from the NCD film enters the underlying Zr material and changes its properties, such that uptake of oxygen and hydrogen is significantly decreased. After 100–170 days of exposure to hot water at 360 °C, the oxidation of the NCD-coated Zr plates was typically decreased by 40%. Protective NCD layers may prolong the lifetime of nuclear cladding and consequently enhance nuclear fuel burnup. NCD may also serve as a passive element for nuclear safety. NCD-coated ZIRLO claddings have been selected as a candidate for Accident Tolerant Fuel in commercially operated reactors in 2020.

High Temperature Nanoindentation Testing of Amorphous SiC and B4C Thin Films

Amorphous silicon carbide (a-SiC) and boron carbide (a-B4C) thin films were deposited using reactive magnetron sputtering of SiC and B4C target, respectively. Nanoindentation tests performed up to 450 °C in air were performed to explore and compare their hardness and elastic modulus.Hardness of a-B4C film decreases at smaller rate in comparison to a-SiC film up to 450 °C. Similarly, elastic modulus value of B4C is more stable with temperature than that of a-SiC.

Utilization of Acoustic Emission in Scratch Test Evaluation

The scratch test is a well-established instrumental method for assessment of the cohesive-adhesive parameters of thin films and coatings. Its evaluation is classically performed using the microscopic analysis of residual scratch and the indenter depth-change record. However, these analysis methods can be insufficient for detection of the very first film-to-substrate adhesion failures. To overcome this difficulty, an independent method of detection of acoustic emission signals can be employed. The detection system of acoustic emission, developed in our laboratory, utilizes a special holder and continuous recording during the whole scratch test. The piezoelectric sensor with 2 MHz sampling rate and sophisticated software allow a thorough post-process analysis of recorded acoustic emission signal. Failure events can be observed on microsecond scale and their frequency spectra can be evaluated.The demonstration of the acoustic emission probe detection capability is performed on the model layers. Comparison of the acoustic emission record to residual scratch image and indenter depth-change record shows a detection sensitivity of the method. Analysis of failure mode dynamics at the appropriate time scale is outlined.

Effect of Nitrogen Content on the Mechanical Properties of Amorphous SiCN Films

Amorphous silicon carbonitride (a-SiCxNy) thin films were deposited using reactive magnetron sputtering of SiC target in the mixture of Ar and N gasses. The films with nitrogen content from 0 - 40 at.% were sputtered at various N2/Ar flow ratios in the range of 0 - 0.48. The as deposited films were additionally annealed in argon at 700 °C and vacuum at 900 °C. Analysis of mechanical properties was performed using the regular nanoindentation and short duration nanoindentation creep test (600 s).Hardness of the a-SiCxNy films increases with the decrease of nitrogen content from approx. 19 GPa (a-Si30C30N40) to 22 GPa (a-SiC). Annealing of the films in inert atmosphere or vacuum leads to the increase of both the hardness and the elastic modulus. This increase is more pronounced for the SiC film than for the SiCN films. The nanoindentation creep test (600 s) showed that the rate of the steady-state creep growths with the increase of nitrogen content.

Wear of Human Enamel and Dentin

The wear of human tooth enamel and dentin has been studied and compared using a repetitive constant load scratch test. Depth sensing indentation with spherical tip was used for measurement of hardness and reduced modulus. An analysis of residual wear tracks was performed with scanning laser confocal microscopy. A procedure for evaluation of repetitive scratch test was proposed. Results showed that the microtribological behavior of enamel differs obviously from that of dentin.

Wear Behavior of Hard Dental Tissues and Restorative Materials

Human teeth are exposed to various chemical and mechanical factors. From mechanical point of view it includes attrition, abrasion or their combination. Teeth and dental restorative materials are subjected to normal and shear loads. Therefore the contact-based stresses during mastication and teeth wear are of considerable importance. In order to study wear behavior of enamel, dentine and two dental restorative composite materials scratch test at various contact conditions was employed. Hardness and elastic modulus were measured using nanoindentation with spherical and pyramidal indenters. Residual wear tracks were observed using laser scanning confocal microscopy.