The influence of steel composition on the formation and effectiveness of anti-wear films in tribological contacts

The effectiveness of antiwear additives in laboratory tests is commonly evaluated using specimens made of AISI 52100 through-hardened bearing steel. However, many lubricated machine components are made of steels with significantly different material compositions, which raises an important practical question of whether the performance of antiwear additives with these other steel types is different from that established with AISI 52100. To help answer this question, this paper investigates the influence of steel composition on the formation and effectiveness of antiwear films. Four steels that are commonly used in tribological applications, namely AISI 52100 through-hardened bearing steel, 16MnCr5 case-carburised gear steel, M2 high speed steel and 440C stainless steel are tested in rolling-sliding, ball-on-disc contacts lubricated with three custom-made oils, one containing ZDDP and two containing different types of ashless antiwear additives. The relative effectiveness of their boundary films was assessed by measuring their thickness and associated wear and friction over 12 h of rubbing at two specimen roughness levels. For ZDDP it was found that the formation of antiwear film was not significantly influenced by steel composition or specimen surface roughness. A similar tribofilm thickness, final tribofilm roughness and friction was observed with all four steels. No measurable wear was observed. By contrast, for the ashless antiwear additives the thickness and effectiveness of their tribofilms was strongly influenced by steel composition, particularly at higher roughness levels. The exact trends in film thickness vs steel relationship depended on the specific chemistry of the ashless additive (ester-based or acid-based) but in general, relative to AISI 52100 steel, M2 steel promoted ashless tribofilm formation whilst 440C retarded ashless tribofilm formation. This behaviour is attributed to the presence of different alloying elements and the ability of the additives to extract metal cations from the rubbing surfaces to support the growth of a tribofilm. In all cases ZDDP films were thicker and rougher, and produced higher friction than those formed by the ashless additives. However, unlike ZDDP, ashless blends generally produced significant wear, particularly with 16MnCr5 and M2 steels. The results indicate that to ensure reliable performance of a given machine component, the chemistry of an ashless antiwear additive should be matched with the types of steel present in the lubricated machine

Tribochemistry of bearing steels:A new AFM method to study the material-tribofilm correlation

A new AFM method for relating microstructure to tribofilm formation is presented. Three aspects of this method distinguish it. Firstly, the new method maps the same region employing atomic force microscopy (AFM) in three sequential steps: preEDTA and postEDTA, before and after the tribofilm removal; and as-etched, after revealing the steel microstructure. Secondly, a new software was written for correcting the displacement between the sequential AFM topography images, and for analysing the data. Thirdly, the method allows to correlate the tribofilm with the steel microstructure, and relating it with protrusion of residual carbides and tribofilm roughness for regions as large as 50 μm×50 μm. This allows to performing statistical analyses to correlate tribofilm thickness with different microstructural constituents

The correlation between ZDDP tribofilm morphology and the microstructure of steel

The microstructure of most hard steels used in tribological applications is inhomogeneous at a micro-scale. This results in local variations in chemical composition and mechanical properties. On a similar scale, tribofilms formed by ZDDP and other anti-wear additives are commonly observed to exhibit a patch-like morphology. ZDDP tribofilms formed under controlled contact conditions on four different steel grades were carefully studied with a new AFM technique to analyse the relationship between the steel microstructure and the tribofilm morphology. Tribofilms were found to be thinner on residual carbides than on the martensitic matrix in all grades containing residual carbides. In most cases, the difference in tribofilm thickness is larger than the carbide protrusion

Grain boundary carbides as hydrogen diffusion barrier in a Fe-Ni alloy: A thermal desorption and modelling study

© 2018 A significant decrease in hydrogen absorption in the presence of grain boundary carbides compared to the carbide-free microstructure in the Ni-based HR6W alloy was measured by thermal desorption analysis (TDA). This novel observation is at odds with numerous existing reports – precipitate-rich microstructures generally absorb more hydrogen due to trapping effects. This discrepancy can only be explained by grain boundary diffusion which is known to be fast in Ni-based alloys. It is proposed that grain boundary diffusion is hindered by carbides, resulting in decreased hydrogen absorption. Further experimental evidence corroborates the hypothesis. In addition, a diffusion model was developed to quantify the experimental results, incorporating trapping, grain boundary diffusion and temperature effects. It was successfully applied to the reported TDA data as well as additional diffusion data from the literature. A parametric analysis showed that hydrogen absorption scales strongly with grain size and grain boundary diffusivity while grain boundary segregation energy has a much lower impact. The results of the study point at grain boundary precipitation as a possible means of hydrogen embrittlement mitigation in Ni alloys and austenitic stainless steels.EPSRC: EP/H022309/1 and EP/H500375/1 Royal Academy of Engineering for Research Fellowship fundin