Soutenance publique de thèse de doctorat en Sciences physiques - Essam Sadik TALAAT SERAG

Tailored coatings for enhanced long-term tribological performance of low speed journal bearings in liquid lead bismuth eutectic

Jury

• Prof. Robert SPORKEN (Université de Namur), Président
• Prof. Stéphane LUCAS (Université de Namur), secrétaire
• Prof. Paul SCHUURMANS (University of Hasselt)
  
• Dr Emile HAYE (Université de Namur)
• Prof. Thomas PARDOEN (Université catholique de Louvain)

Abstract

 

The objectives of this research are to develop optimized coatings aimed at enhancing the tribological performance of journal bearings operating in Lead Bismuth Eutectic (LBE), and to gain a comprehensive understanding of the corrosion and wear mechanisms of these coated bearings in LBE.

The research followed a systematic approach to achieve its objectives. Initially, an extensive literature review was conducted to survey recent advancements and innovations in the field of mitigating LBE corrosion, and mechanical wear. Various solutions have been introduced, with some addressing only LBE corrosion, while others offer effective protection against both corrosion and wear. Among these solutions, coatings emerge as a promising and effective means of protecting structural materials and mechanical components immersed in LBE. Based on the insights gained, two promising candidate coatings have been identified for further investigation: TiAlN and carbon-based coatings.

The results revealed that surface roughness, hardness and residual stresses of the TiAlN coatings increased with higher bias voltage and/or lower working pressure. Additionally, the material and thickness of the adhesion layers were selected based on the acquired results. TiAlN coatings demonstrated the best adhesion to stainless steel substrates without requiring a TiAl intermediate layer. On the other hand, carbon-based coatings showed the best adhesion when a Cr+CrN double layer was deposited on the substrate, with a deposition duration of 2 minutes for each sublayer.

Following the optimization process, a series of LBE corrosion tests were conducted. Exposure TiAlN coatings to LBE at 410 °C for 500 h led to the creation of an oxide bilayer comprising TiO₂ (outer sublayer) and (Ti, Al)O_x depleted of Ti (inner sublayer). The CrxC1-x coatings (x = 0.2, 0.5, and 0.8), immersed in high oxygen concentration LBE at 360°C for 1000 h, showed different behaviour depending on the Cr content of the coatings. The coating with the lowest Cr content (Cr_0.2C_0.8) got completely removed, leaving the substrate exposed to LBE. In contrast, the other two coatings with higher Cr content developed a protective Cr₂O₃ layer, acting as a diffusion barrier for oxygen atoms.

The next phase involved applying the optimized coatings to the surfaces of journal bearing shafts. The test results indicated that the uncoated shaft performed the worst, failing immediately due to adhesive and abrasive wear mechanisms. The PA-CVD carbon-coated shaft failed shortly after the test began. This failure was attributed to the tribofilm wear mechanism.

The chrome-plated and TiAlN coated shafts showed varying performance depending on the mating material. Multi-layer-coated shafts, with a bottom layer of TiAlN, an intermediate layer of both TiAlN and carbon, and a top layer of pure sputtered carbon, survived the tests thanks to the lowest average torque compared to all the other tests.

It is crucial to recognize that wear is a system property rather than a material property, making it a complex phenomenon influenced by various factors. Consequently, there is no universal solution that fits all situations.

 

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