Ján Balko

Highly wear-resistant and low-friction Si 3 N 4 composites by addition of graphene nanoplatelets approaching the 2D limit

Graphene nanoplatelets (GNPs) have emerged as one of the most promising filler materials for improving the tribological performance of ceramic composites due to their outstanding solid lubricant properties as well as mechanical and thermal stability. Yet, the addition of GNPs has so far enabled only a very limited improvement in the tribological properties of ceramics, particularly concerning the reduction of their friction coefficient. This is most likely due to the challenges of achieving a continuous lubricating and protecting tribo-film through a high GNP coverage of the exposed surfaces. Here we demonstrate that this can be achieved by efficiently increasing the exfoliation degree of GNPs down to the few-layer (FL) range. By employing FL-GNPs as filler material, the wear resistance of Si3N4 composites can be increased by more than twenty times, the friction coefficient reduced to nearly its half, while the other mechanical properties are also preserved or improved. Confocal Raman spectroscopy measurements revealed that at the origin of the spectacular improvement of the tribological properties is the formation of a continuous FL- GNP tribo-film, already at 5 wt% FL-GNP content.

Influence of Microstructure on Tribological Properties and Nanohardness of Silicon Carbide Ceramics

The influence of microstructural variations on the tribological properties and nanohardness of liquid phase sintered silicon carbide (LPS SiC) has been observed. In order to modify the microstructures samples were further heat treated at 1650°C and 1850°C for 5 hours to promote grain growth. The depth-sensing indentation tests of SiC materials were performed at several peak loads in the range 10-400 mN. The pin-on-flat dry sliding friction and wear experiments have been made on SiC ceramics in contact with Al2O3 ceramic ball at 10-50 N loads in an ambient environment. The nanohardness of samples with plate-like microstructure was about 34 GPa i.e. 3 GPa higher than nanohardness of SiC with fine globular microstructure. The SiC materials with coarser plate-like microstructure had similar COF (0.4-0.55) and better wear resistance (one order of magnitude at normal forces 10-20N) than SiC materials with fine globular microstructure.

Microstructure Modification of Tool Steel X38CrMoV5-1 by Fibre Laser

In the present work, we investigate the influence of laser radiation on the evolution of microstructure and wear resistance of X38CrMoV5-1 tool steel in hardened and tempered condition. The microstructure of the laser surface hardened zone consists of fine and coarse grained carbides (M23C6, M7C3, MC or M2C) which are dispersed within martensite matrix. On the other hand, the laser melted microstructure is characterized by martensite, retained austenite and fine carbides precipitated in the inter-dendritic zone. The laser surface affected zone microstructure exhibits the enhanced hardness in the range from 857 to 775 HV in comparison with quenched and tempered conditions (720-655 HV) and decreases from the surface to the soft zone with a typical hardness of 530 HV. The wear resistance of the laser treated samples is investigated by “ball on disc” method, which shows a significant improvement as compared to that in quenched and tempered condition.

Wear Behavior of Stoichiometric and Nonstoichiometric Zirconia

Zirconium oxide samples were sintered at two different temperatures (1250°C and 1450°C) using SPS method with maximum holding time 10 min. to obtain nonstoichiometric composition. Some of the samples were subsequently annealed for 6 hours at 1100°C to obtain stoichiometric composition. Wear rate and friction coefficient were obtained from tribological tests carried out on the UMT3 equipment (Bruker) using two different static partners (zirconia and alumina ball). Main aim of this work is to study the influence of composition and sintering temperature on the wear behavior, friction coefficient and basic mechanical properties.

Non-Destructive Surface Diagnostics of Tools

The aim of the study was to validate non-destructive diagnostics of surface of tools and metal sheets after steel grit blasting. We used replicas produced by fy Struers, known under commercial name RepliSet-F5. The replicas were extracted from surfaces of thirty specimens with PVD coatings (CrN, TiCN) and from flat specimens after steel grit blasting. Qualitative and quantitative parameters characteristic of surface morphology were determined by means of an optical profilometer, and confocal microscope was used for surface contactless metrology measurements. We observed a qualitative coincidence of morphological images of original surfaces and replicas. The values of the quantitative parameter Sa (Arithmetic mean height according to ISO 25178) of surfaces and replicas extracted from them differed at the second decimal place. Replicas document well the surface morphology and allow one to obtain information about the state of surface in difficult-to-access sites of the tools or constructions.

Wear and Mechanical Properties of Various Bone Cements – Influence of Saline Environment

Several bone cements were prepared in two ways of mixing (manual and vacuum bowl). Wear behavior, friction coefficient were studied by ball on disc method. Nano-hardness and Young`s modulus was studied by instrumented indentation. Obtained results were summarized by taking into account their way preparation, antibiotics content and testing conditions. There was found no significant time dependence of saline acting on Young`s modulus and nano-hardness values. Friction coefficient in saline was less than half in compare to dry sliding conditions

Instrumented Indentation of Composite Materials Prepared by Methods of Mechanochemistry

Four materials TixTa1-xCN-20%Co of two chemical composition (x=0.9 and 0.95) and two high energy milling methods have been prepared. Nanoardness and elastic modulus for microstructure as a whole and both phases (matrix and hardmetal grain) were obtained (Tab. 1). Instrumented indentation was cerried out on the nanoindentation equipment TTX NHT (CSM instruments). Single load mode was used. Maximum applied loads of 20 and 50 mN for individual phases and 300 mN and 400 mN for microstructure as a whole were used.

Local Mechanical Properties of Various Bone Cements

Various bone cements with different compositions were prepared in several model configurations which differed in volume (i.e. setting temperature profile) and mode of mixing (manual and vacuum bowl). Local mechanical properties (nanohardness, elasticity modulus) of all experimental states were measured by nanoindentation technique and compared. Role of the preparation route as well as influence of composition, particularly of presence of antibiotics, on the measured properties was investigated.