Limited load-carrying capacity and impact loading resistance greatly restrict the use of hard coatings in forming applications, making substrate hardness and resistance crack initiation and propagation very important. Therefore, the aim of this research work was to investigate the effect of substrate fracture toughness and hardness on the load carrying capacity and impact wear resistance of coated tool steel, coated by monolayer (TiAlN), multilayer (AlTiN/TiN) and nano-composite ((Ti,Si)N) PVD coatings. By using different combinations and parameters of vacuum heat treatment and deep cryogenic treatment effect of the substrate fracture toughness and hardness on the load-carrying properties was determined under progressively loading dry sliding conditions, while ball-on-plate impact fatigue test was employed to investigate impact wear resistance. Results clearly show, that substrate hardness is the most important property influencing load-carrying capacity and impact wear resistance of the coated surface. However, with increased hardness and brittleness of the coating increase in fracture toughness although on the expense of the reduced hardness becomes beneficial.
COBISS.SI-ID: 28797479
Fatigue damage is a parameter which plays an important role in lifetime and reliability predictions of randomly loaded structures. In this paper, four different forms of the Miner linear accumulation damage rule are examined. They are used to compare observed experimental damage and the results of numerical simulations for both uniaxial and multiaxial fatigue. First, critical fatigue damage values for all forms of the Miner accumulation damage rule are calculated for uniaxial cases. Based on the results presented in this paper, it has been established that for high strength materials the critical damage value is around 0.3. These results are then applied to the multiaxial cases. The multiaxiality is taken into account using either the signed von Mises or critical plane method. Finally, the fatigue fracture surface has been calculated and compared to the experimental pattern of fatigue fracture. The results show that the fatigue fracture surface determined by the numerical simulation is comparable to the fatigue fracture observed experimentally.
COBISS.SI-ID: 14219547
The aim of this work was to show that with the use of the surface-roughness parameters S[sub]sk and S[sub]ku we can predict tribological behaviour of contact surfaces and use these parameters to plan surface texturing. This paper presents a continuation of our research on virtual texturing and experimental work on surface textures in the form of channels. For this investigation steel samples were laser surface textured in the shape of dimples with different spacings between the dimples and different dimple depths. The experimental results confirmed that the parameters S[sub]sk and S[sub]ku can be used to design the surface texturing, where a higher value of S[sub]ku and more negative S[sub]sk lead to lower friction.
COBISS.SI-ID: 14605851
Laser-induced periodic surface structures (LIPSS) are produced on cold work tool steel by irradiation with a low number of picosecond laser pulses. As expected, the ripples, with a period of about 90% of the laser wavelength, are oriented perpendicular to the laser polarization. Subsequent irradiation with the polarization rotated by 45° or 90° results in a corresponding rotation of the ripples. This is visible already with the first pulse and becomes almost complete - erasing the previous orientation - after as few as three pulses. The phenomenon is not only observed for single-spot irradiation but also for writing long coherent traces. The experimental results strongly defy the role of surface plasmon-polaritons as the predominant key to LIPSS formation.
COBISS.SI-ID: 14711835
To improve tribological properties of components, different kinds of textures are added to their surfaces. Although the added imprints or craters are only few tens of micrometers wide and a few micrometers deep, they form an initial crack from which a fatigue crack may propagate. In this article, a numerical approach for calculating the fatigue life of a body with present micro-imprints with standard finite-element software will be presented. The objective is to show how the imprint shape and its manufacturing process influences the fatigue life. The considered specimens are round bodies that were made from Boehler K890 microclean cold-working steel. Three shapes of the imprints are studied: the imprints produced by a Vickers pyramid and a Rockwell cone and a half-spherical crater void produced by a laser device. The flat-surface round body served as a reference. The imprint shape and its manufacturing process is also considered. For this reason, a simulation of impression of Vickers and Rockwell indenters is simulated first, which is followed by a numerical fatigue-life prediction. In this manner, the residual stresses from the imprinting process are considered during the fatigue-life prediction. For the laser-induced craters and the reference body, only the fatigue life is numerically predicted. Because the material inhomogeneities are much smaller than the geometrical ones, the calculated fatigue lives for different texturing represent the fatigue-life reduction caused by the applied textures. To validate the presented numerical approach, the calculated fatigue lives are compared to the experimentally determined.
COBISS.SI-ID: 1292714