Additives and their role in running-in (P8)
The process of running-in has been studied for many years but, right now few people have focused its interests on the role of lubricant with fresh surfaces: it is known that some additives are important to have a faster and successful running-in but which reactions are the most important? Is it only matter of mechanical contacts? Let’s find out!!
The purpose, once the chemical process acting in the running-in will be understood, is to improve this process: avoiding from the beginning the running in and obtain directly a so called “steady state” period.
1. Master Degree in Chemical Sciences with specialization in Industrial and Applied Chemistry at the University of Cagliari – Sardinia (Italy).
Tribochemistry process in non ferrous material
Analytical Spectroscopy, Surface Analysis, Lubrication, Tribology.
SKF Engineering Center
For more information contact Elio;
Supervisor: Dr. Pascal Ehret
When two fresh unworn solid surfaces are loaded for the first time and moved relatively to one other, one can observe changes in friction, temperature, roughness and wear. These temporary changes were sometimes accepted as the normal course of operation; however, it has been found that improving the initial performance (low friction, roughness and wear rate) of components such as gears, bearings, and cams is advantageous for the life of components. This process has been generally called running-in.
The project, focused on the role of oil additives during running-in period. This research studied the tribochemistry process and the aspects of reactions between lubricant additives and different materials. The main idea behind the project has been to fill the gap present in the literature about the connection between mechanical studies to understand the running-in process and the chemical investigation of the additives/surfaces interactions during running-in, in rolling/sliding environment.
First of all a definition of running-in was required. The problem with the definition of running-in is that, since it is really broad subject, different papers can give different definitions. A general definition says that running-in is a “net result of simultaneous transitional process occurring within the interface”.
The running-in in literature has been widely studied, mostly from a mechanical point of view. The interactions with additives and nowadays with different surface materials ask for a deeper understanding of this process in order to find the way to have a short and optimum running-in, leading then to a steady state with reduced surface distress and preventing premature failures of the materials in contact.
The meaning of an optimum running-in is to have low wear rate, low friction and initial absence of surface fatigue.
Beside the study of running-in, this project had brought a novelty regarding the type of rig used to perform the tribological tests: in fact a Micro-Pitting rig has been used for the first time in this topic. This rig has the possibility to run tests with a higher number of cycles in a shorter time than MiniTraction machine. Moreover the MPR reproduces a contact between a roller and a raceway, similar to the one in a real rolling element bearing. For these reasons, the MPR has been widely used in the present work.
The project focused on observing friction, wear and roughness modifications of the surface in presence of base oil, additives and with the surface of the samples coated with different type of coatings. These coatings were made of two types, one harder that steel called Diamond-like-Carbon (DLC) and one softer than the steel called Iron Oxide.
The findings of this work can be resumed in several points:
- A mechanical process which act during running-in has been described and analysed; this process is an equilibrium between two different processes which are very well known in the literature: plastic deformation due to flattening of asperities and mild wear; following these processes, it has given the possibility to follow the surface modifications during running-in which can lead to an optimum running-in;
- The presence of additive, change the running-in process; when ZDDP is present, this lead to formation of a thick tribolayer which avoid further wear and surface modifications with the time; this tribolayer is important because it reduces the running-in period to a very short time. The presence of a friction modifier additive (such as MoDTC) changes the mechanical processes acting on the surface during running-in time; although this time is shorter than the one observed without additives, it lasts longer than the ZDDP ones.
- The presence of different coatings suggested that the best performance (which means low friction and wear and short running-in time) are obtained by coating the rougher surface with the Black oxide coating; these process was not only giving a good performance in terms of running-in but it was also creating a thick tribolayer which then was protecting the surface against further wear.