Adhesion and, in particular, the transfer of aluminium alloys to the bearing surface of a die are two of the main reasons for tool failure and the poor surface quality of products, especially at elevated temperatures. The present work was focused on the EN-AW6060 aluminium alloyʼs transfer initiation and the evolution to an AISI H13 hot-work tool steel, as well as CrN and TiAlN coatings in a cross-cylinder, single-pass, dry-sliding contact at room (20 °C) and elevated temperatures (300-500 °C). The contact was investigated in terms of the surface area and volume of the transferred aluminium alloy to the tool surface, the topography of the wear trace and the corresponding change in the coefficient of friction. The results show a strong dependence of the tribological properties of the investigated materials on the temperature and only a limited dependence on the sliding distance, especially for the TiAlN coating. At room temperature the lowest coefficient of friction and the smallest amount of material transfer were measured for the TiAlN coating. At higher temperatures both the CrN and TiAlN coatings showed similar friction values and amounts of transferred aluminium alloy, while the nitrided hot-work tool steel exhibited an inferior tribological performance.
COBISS.SI-ID: 14029595
High-performance polymers show a great potential to replace machined metal components in a wide variety of applications owing to their promising mechanical, thermal and tribological properties. However, further improvements in their properties are still needed. The aim of this study was to evaluate and compare the effect of the material type and the morphology of well-known solid lubricant nanoparticles on the tribological properties of poly-ether-ether-ketone (PEEK) composites. Different nanoparticles were added to the PEEK matrix: WS2 fullerene-like (WS2F), WS2 needle-like (WS2N), carbon nanotubes (CNT) and graphene nanopowder (GNP). The results of dry-sliding tribological tests show that the material and the morphology of the nanoparticles have a crucial effect on both the wear and the coefficient of friction, primarily by affecting their macroscopic hardness, as well as the thickness and the surface coverage of their transfer films. The WS2-based nanoparticles outperformed the carbon-based nanoparticles in terms of wear performance; compared to PEEK, WS2F improved the wear rate by 10 % and the WS2N by 60 %, while the carbon-based particles deteriorated the wear behaviour by 20 % (CNT) and as much as three times in the case of the GNP. The addition of WS2F nanoparticles and the CNT resulted in a lower coefficient of friction compared to the WS2N particles and the GNP.
COBISS.SI-ID: 13905691
The evolution of the nano-mechanical properties of tribofilms formed in steel/steel, steel/a-C:H and steel/Si-DLC contacts lubricated with two commercial oils containing different amounts of SAPS additives (E6 and E7 grade) and a mineral base oil containing ZDDP additive were examined in this investigation for two very different time periods. An atomic force microscope (AFM) was used in different modes to measure the topography, film thickness and stiffness, while the nano hardness was measured with a nano-indenter. In addition, ATR-FTIR microscope (spectroscopy) was used on selected samples to explain some of the tribofilmʼs mechanical modifications with chemical changes. The results have shown that the tribofilmʼs evolution and growth are very much surface and additive dependent, and are different for steel and DLC coatings.
COBISS.SI-ID: 14398491
In this paper, we show how the slide-to-roll ratio (SRR), the contact kinematics and the surface energy all have important effects on the elastohydrodynamic friction. As reported previously, diamond-like-carbon (DLC) contacts of the type DLC/DLC provide the lowest coefficient of friction, in particular those DLC materials with the lowest surface energies (three different DLC coatings were used in this study). A friction reduction of up to 48 %, compared to a steel/steel contact, was obtained. A surprising new finding from this investigation is that the friction in DLC contacts is significantly reduced for high SRRs, i.e. a difference of up to 27 % was measured in the DLC/DLC contacts when the SRR increased from 0.5 (rolling prevails) to 1.8 (sliding prevails). However, even more surprising is the effect of the SRR on the friction in mixed steel/DLC contacts, which is related to the contact design and its kinematics. We found that if the DLC is coated on the slower surface, a high SRR will not reduce the friction to any significant extent, typically about 5 %, and at most 13 %, compared to steel/steel contacts. However, a significant friction reduction is observed when applying a low-surface-energy DLC coating to the faster surface on average about 20 % and as high as 33 %. In this case (a high SRR and DLC on the faster surface), mixed steel/DLC surfaces can experience a very similar friction as that seen for DLC/DLC contacts. Qualitatively, the same behaviour was found for both positive and negative SRR values.
COBISS.SI-ID: 14197019
The properties of tribochemical films play an important, or even the key role with respect to friction in boundary lubrication. While their chemical behavior has already been widely studied, their mechanical properties are much less well understood. However, their nanoscale mechanical properties and behavior may reveal important information about a correlation with the macroscopic friction behavior. In this investigation, we looked at steel, a-C:H and Si-DLC in contact with steel, lubricated using two commercial oils containing different amount of SAPS additives (E6 and E7 grade) and a mineral base oil containing the ZDDP additive. The tribofilms were characterized using an atomic force microscope with seven different parameters, i.e., the topography (features morphology), tribofilm surface coverage, nano-roughness, adhesion, film thickness, lateral force, i.e., nanoscale friction, and the film stiffness through force modulation. The results confirmed the formation of tribofilms on all the selected coatings and showed that the film formation and its nanoscale properties are dependent on the coating and the additive. Two distinctive groups of parameters were identified: one closely related to the surface energy of the materials and the other clearly distinguishing the DLC coatings from the steel. The adhesion and tribofilm thickness were found to correlate directly with the macroscopic friction, while the other parameters have higher-order dependences.
COBISS.SI-ID: 14196763