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Contenu archivé le 2024-04-15

NEW THERMAL BARRIER CERAMIC COATINGS ON ALUMINIUM ALLOYS

Objectif

The aim of this proje ct is the manufacturing of plasma sprayed Thermal Barrier Coatings on aluminium alloy components such as pistons and cylinder heads to be tested in a turbocharged diesel engine. The new aspect is that the coatings are manufactured inside an argon filled chamber where two or four special nozzles, situated on both sides of the torch, deliver, when required, a definite amount of liquid argon droplet fog on top of individual ceramic layers being deposited in order to master coatings deposition temperature.
In the laboratory, practically stress free thermal barrier ceramic coatings (TBC) up to 3 mm thick were obtained. These had a very homogeneous microstructure which was segmentedinside the individual lamellae. The coating was truly strain tolerant, showing outstanding resistance to thermal cycling. This can be explained by the correlation between deposition temperature and microstructure.

It was further discovered by computer simulations that TBC increases piston surface temperatures and decreases the temperature of inner parts. Increasing the thickness by more than 2 mm does not result in higher insulating capacities.

A new cooling system was investigated for the manufacture of TBCs on aluminium alloys, called atmosphere temperature control (ATC). This procedure maintains a low and constant coating temperature while spraying TBCs. The plasma torch, operating in an argon-filled chamber, is equipped with nozzles delivering a defined amount of argon droplets on top of individual ceramic layers being sprayed. Liquid argon can also be replaced by liquid carbon dioxide.

The work was structured in four parts:
set up of optimized cooling conditions and plasma spray parameters, according to an iterative optimization procedure;
laboratory characterization of coated samples, including measurement of residual stresses, thermal shock tests, adhesion tests, and metallographic investigations on cross sections;
computer simulation of the thermal field of a piston;
manufacturing of TBCs on the combustion chamber of components of a turbocharged diesel engine under optimized plasma spray and cooling conditions.

Engine bench tests showed that the ceramic coating will become damaged after severe stop/go cycles producing thermal shock. No temperature or time details were submitted.

In cylinder heads, a comparatively thin layer of TBC prevents any fire damage. The selected system consists of an aluminium-silicon (35%) bonding layer and a zirconia-yttria ceramic top coat .
THE INNOVATIVE IDEAS IN THIS PROJECT CONCERN:

- PLASMA SPRAY TECHNOLOGY
- CHEMISTRY OF BONDING LAYERS
- SPECIAL TREATMENTS AGAINST HOT CORROSION.

CONCERNING PLASMA TECHNOLOGY THE NEW ASPECT IS THAT THE COATINGS WILL BE MANUFACTURED IN A PURPOSELY DEVISED CHAMBER OPERATING UNDER CONTROLLED TEMPERATURE AND ATMOSPHERE CONDITIONS. INDIVIDUAL CERAMIC LAYERS WILL BE COOLED DURING THEIR DEPOSITION BY MEANS OF A PATENTED CRYOGENIC SYSTEM IN ORDER TO AVOID THERMAL GRADIENT STRESSES AND THIS WILL MAKE IT IMPOSSIBLE TO GROW THICK COATINGS.

CONCERNING BONDING LAYERS, THEIR CHEMISTRY WILL BE MODIFIED TO MATCH DIFFERENCES IN THERMAL EXPANSION COEFFICIENTS.

POST-TREATMENTS WILL ALSO BE INVESTIGATED IN AN EFFORT TO IMPROVE HOT CORROSION RESISTANCE WITHOUT IMPAIRING THERMAL BARRIER PROPERTIES.

THE NEW COATINGS ARE TO BE PRIMARLY USED FOR PISTONS AND CYLINDER HEADS THUS CONTRIBUTING TO THE THERMAL INSULATION OF AN AUTOMATIVE HIGH SPEED TURBO-CHARGED DIESEL ENGINE WITH WELL KNOWN BENEFITS SUCH AS HIGHER THERMAL YIELDS AND THE POSSIBILITY OF AVOIDING THE COOLING SYSTEM.

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Coordinateur

Centro Sviluppo Materiali SpA
Contribution de l’UE
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Adresse
Via del Castel Romano 100/102
00129 Roma
Italie

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