PhD student (materials engineering): "Microstructure features controlling white etching cracks (WEC) in roller bearings"

Last application date
Feb 28, 2018 23:59
TW08 - Department of Electrical Energy, Metals, Mechanical Constructions and Systems
Limited duration
Master of Science degree in Metals science and technology
Occupancy rate
Vacancy Type
Research staff

Job description

Bearing failures form the majority of gearbox failures in wind turbines. The dominant failure mode is axial cracking associated with local microstructural alterations. Such cracks can cause failure at less than 30% of the classical rolling fatigue lifetime L10, and are referred to as white etching cracks (WECs) because the resulting altered microstructure appears white in an etched micrograph (white etching areas or WEA). Conditions leading to the formation of WEC in wind turbine bearings are highly debated and ultimately unknown. A list of potential influence factors includes but is not limited to: applied strain due to normal loading or impact loading associated with torque reversals, thermal heat from high friction or sliding during accelerations or decelerations linked to transients, or electrical discharge from stray currents stemming from the generator to other components. Moreover, embrittlement by externalities such as atomic hydrogen (originating from lubricant decomposition or water contamination) could compromise the steel structure.

In collaboration with Free University Brussels (VUB; project coordinator), SIRRIS and National Renewable Energy Laboratory (NREL; USA)), Ghent University (UGent) has originated a project that aims to provide the wind energy value chain players with an improved understanding, prediction and monitoring of white etching cracking (WEC) in wind turbine drivetrain bearings.

The project is backed by an impressive industrial advisory board covering near to the entire windmill value chain. Starting from the level of material microstructure and supported by numerical damage modelling and in-the-field measuring campaigns, material based Probability of Failure (MB-PoF) predictions are envisaged to lead to better operation (less failures) and improved maintenance strategies.

In total, four vacancies for PhD researchers are released for this project, each of which will have a duration of four years. Their close collaboration will be crucial to the success of the project. Two PhD researchers will be supervised by the project coordinator (VUB), focusing on monitoring based techniques for WEC prognosis. The other two PhD researchers will be affiliated to Ghent University. One researcher will develop a multi-physics based approach to model the hypothesized WEC initiation and growth mechanism, supervised by Prof. Stijn Hertelé () and Wim De Waele () at the Fracture Mechanics research group of Soete Laboratory, UGent.

Another PhD to which this vacancy is oriented will deal with the microstructural evaluation of WEC damage from a material science perspective, leading to a hypothesized microstructural based physical mechanism towards the initiation and growth of WEC. This work is supervised by Prof. Roumen Petrov () at the MST (Material Science & Technology) research group, UGent. The aim of the PhD work is via detailed microstructural characterization from macro- to nano-meter scale of white etching cracks (WEC) damage to hypothesize the physical metallurgical mechanisms that control nucleation an growth of WEC. The data will be used for development of a material based model that describes WEC initiation and propagation to failure. The data from this model could give directions for improving the microstructure of the bearing steels in order to increase their resistance to failure. This PhD project is foreseen to be in close collaboration and data exchange with the PhD project supervised by Prof Stijn Hertelé and Prof Wim De Waele. One of the goals of the PhD is to define the places where the damage appears and to associate this place with the microstructure and the specific loading conditions in this zone. Two main type of defects are subject of interest (i) WEC and (ii) butterfly wings. Microstructural characterization will be addressed to both through hardened and case hardened bearing steels. The qualitative and quantitative characterization of loaded in different condition bearings will be carried out via optical microscopy, scanning and transmission electron microscopy, x and -ray diffraction, electron backscatter diffraction (including transmission Kikuchi diffraction and automatic crystallographic orientation mapping in TEM), atom probe tomography. These studies aim to identify the zones of microstructure where the strains localize and rich critical values for damage (the weakest link) and to understand the operating damage mechanism on microstructural level. Based on these data a microstructural based model will be developed for the specific microstructures and loading conditions in the bearings. The data of the experiments and the model will be used to suggest directions forbearing steels with optimum microstructure with respect to damage resistance.

Profile of the candidate

  • you have a strong motivation for (and preferably a history of) conducting scientific research and working with complex questions;
  • your expertise resides in metallurgical engineering, but you additionally have experience and interest in material science , materials characterisation techniques and mechanical engineering.
  • you possess structured and creative problem-solving abilities;
  • you possess strong analytical and technical skills and take responsibility for the development of your work;
  • you can work independently as well as in team;
  • you have excellent English communication skills (written and presentation);
  • knowledge of damage and fracture mechanics is an advantage; experience with programming in Python or Matlab is advantage;
  • experience with experimental work especially SEM, TEM EBSD, XRD, FIB, is an advantage;
  • experience with work of bearing steels is an advantage;
  • knowledge of Dutch is a plus (but not compulsory).

How to apply

The application form (See - section Research) should be sent in pdf format to Prof Roumen Petrov (), including:

  • CV; 
  • an official record of transcripts (bachelor and master are required) and a copy of degree certificate;
  • a brief motivation letter, showing why the applicant wishes to engage in the scientific research;
  • List of publication (can be attached to the CV); 
  • two or more references.

For further information on the vacancy and/or how to apply, contact Prof. Roumen Petrov () and/or Prof. Stijn Hertelé ().