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- Bio-oils (ester, PAG, glycols, bio-no-tox, low ash) (1)
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Two new substoichiometric titania (TiOx) coatings designated for cylinder liner application were deposited on specimen of grey cast iron GG20HCN with high carbon content by plasma spraying. First, a TinO2n-1 coating was prepared by atmospheric plasma spraying (APS) using a sintered and agglomerated Magnéli-type spray powder. Second a TiO1.95-x coating was deposited with a vacuum plasma spray (VPS) process using a commercial, fused and crushed TiO1.95 powder. The tribological behaviour of these coatings under lubricated conditions was compared with uncoated specimen of this grey cast iron. As counter bodies a widespread used APS-sprayed Mo-NiCrBSi piston ring coating (MKP81A®), an advanced HVOF-sprayed WC/Cr3C2-based (MKJet502®) ring coating as well as non-commercial prototype APS-sprayed TinO2n-1 and APS-sprayed (Ti,Mo)(C,N) + 23NiMo (TM23-1) coatings were tribotested.
The interaction of the pairs with prototype engine oils based on esters and polyglycols were studied under mixed/boundary lubrication using the BAM test method. Lubricants were factory fill engine oils, ester-containing lubricants with low-SAP (sulphurashphosphor) and/or bio-no-tox properties as well as polyglycole-based lubricants. The ester and polyglycole-based engine oils respond both to bio-no-tox criteria and are polymer-free. They follow different strategies to reduce zinc, phosphorus and sulphur to assure a low ash content.
Both TiOx coatings designated for cylinder liners meet or exceed the wear resistance of the grey cast iron with high carbon content when paired with APS-sprayed TinO2n-1 or Mo-NiCrBSi piston ring coatings. Overall, in nearly all pairs the wear rates of the APS TinO2n-1 coating were lower than those of the VPS TiO1.95-x coating.
In order to characterize the tribological behaviour under oil-off, dry-running conditions, additional tests were performed under unlubricated unidirectional sliding conditions at 22 and 400 °C for a sliding speed of 1 m/s against sintered polycrystalline Al2O3 as stationary specimen.
Gas turbine efficiency is significantly influenced by the pressure losses caused by the inevitable gap between the rotating turbine blades and the stator parts.
Because contact between rotor and stator cannot be completely avoided with a small gap, systems are designed to allow for some rubbing. Honeycombs or abradable MCrAlY stator coatings are normally used to minimize the wear of the rotor blades when rubbing occurs. But, more efficient solutions must be developed on modern engines, because both suffer temperature limitation and standard MCrAlY have usually poor abradability. For higher temperature and closer control of the gap, a thick thermal-barrier coating must be applied on the stator, but presently with no rub-tolerance.
To increase the abradability of the stator coating, its composition and structure can be modified, but such changes must be strictly limited with respect to the loss of erosion resistance.
The solution can then lie in the hardening of the rotating blade tip to permit the use of harder ceramic or metallic coatings on the stator with greater resistance to erosion. These systems typically consist, on the blade tip, of hard particles embedded in a softer metallic matrix.
In the present programme, it is therefore intended:
- to assess the present European solutions, standard or in development, for outer air seals and blade tips.
- to develop coated grits or new grits for the hardening of the blade tips, as well as to improve the matrix and the stator coating for long life systems.
- to optimize the most promising of these systems, that will be tested on development rigs and on engine.
The consortium includes several engine manufacturers, which are the potential users of the innovating solutions to be developed, companies having a large experience in the field of abradables and tip coatings
and non industrial partners involved in tribology and materials science.