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Duplex surface treatments for al components


After having discussed and analysed the tribological system in which the coated component works, the complete process chain in deposition (cleaning, pre-treatments, adapting different combinations of layers) was investigated and the surface was analysed at different stages by different tools GDOS, SEM, TEM). Most important for both the deposition and application was to analyse the interface base material/coating (oxide/carbon contamination), the composition and the grain growth of the deposited layer. In the second part of the project the best coatings were tested (model tribological tests) and applied in service life or near service life. The results show for deposition of duplex layers on automotive parts that low temperature deposition by PVD has been reached on high strength Al-alloys for automotive components and mould application. But the deposition temperature always remains a challenge, also for duplex coatings. The load bearing capacity of the Al-base materials (casted or wrought) can be very different and has to be adapted together with roughness and surface topography to the duplex coating according the tribological demands in the relevant application.
WC/Co as a base coat of 5 µm on Aluminium - deposited by PVD at partner Daimler/EADS has been found to grow nearly amorphous independently on the angle of incident during deposition as most of the reactively sputtered nitrides and carbides are unlikely to show. This novelty makes the WC/Co hard coating attractive for depositing on 3D components and tools in general and in particular on Aluminium base material. The top layer on the WC/Co is a co-sputtered layer of MoS2 and WC/Co; the later is mixed in with an amount of about 5% and increases the interlaminar shear strength. This reaches the release of lubricant from the top layer during run in and a smoothened surface at the end.

In order to coat the industrial components (liners and real injection moulds) Genta has modified the industrial "Arc-Cathodic" PVD equipments with the optimisation of the Plasma etching phase in a large chamber (1 m x 1 m). In particular Genta has modified its large industrial PVD equipment to improve the ion bombardment of Al alloys during all the heating cycle. To obtain this improved bombardment was designed and manufactured a new power supply system with a better and more precise control of the plasma formation, and was designed a new procedure with this different power supply.
With this modified PVD equipment Genta has been performed the deposition of innovative PVD coatings on Al real parts delivered from EADS, Renault and CRF

High precision components with tight tolerances and with surfaces of high quality are worth to be coated by the expensive thin film plasma technology as applied in this project. Automotive components with insufficient liquid lubrication are considered to overcome wear and friction by duplex coatings on Aluminium components. Duplex coatings (low wear coating + low friction coating) are very interesting for mechanical applications. The results show interesting friction reduction comparatively to conventional surfaces and a good wear behaviour.

Thin coatings on tools for Plastic industry have become more important, but no Plasma assisted coating on moulds made of Aluminium exists. The surfaces of high precision in moulds have to be polished, wear-resistant and non-adhesive to the injected plastic material. The tribology behaviour of plastic material to the coating of the mould is very specific and was tested and evaluated in series of tests at partner CR Fiat. Tests were set up to reproduce working conditions of a real mould: glass reinforced plastic material was used as counterpart in friction tests that were performed in heated condition. Thin WC/Co coatings on Aluminium show good results in terms of coating adhesion and friction coefficient; moreover the deposition temperature was low enough to preserve the strength of the chosen Aluminium alloy.

Shedding of vehicle weight in the car, connected to environment stresses, is one of the priority axes in automotive industry and at Renault. Even if some parts in the engine (cylinder head...) and in the body (front bonnet...) are ever made in aluminium, the development of new aluminium applications need to increase friction properties.
One solution to solve this problem is to coat the surface of the aluminium part with duplex coatings in order to use an anti-wear (hard coating) and a low friction (soft coating) effect. WC/Co + MoS2 is a good candidate to mechanical applications and had presented a very interesting behaviour concerning adherence and reduction of friction and wear.

The second original objective was to find condition to implement new Plasma Diffusion Techniques. A new plasma immersion Ion implantation reactor was developed by Partner JRC and characterised. The process was used for plasma diffusion treatment and deposition of well adherent and dense material layers on Aluminium. Due to the strong limitations in deposition temperature, the diffusion of implanted or deposited material like nitrogen in Aluminium is restricted. After the midterm assessment it was agreed to focus on adherent coatings of Silicon and diamond like carbon on Aluminium. The potential of this deposition technique is high for corrosion protective layers on Aluminium. The deposition of thin silicon layers on aluminium shows an increase in corrosion resistance. The implantation of further corrosion inhibitors - to replace Cr6+ - on aluminium surfaces is likely by this technique.
Objectives and content:

In mechanical industry, there is a need for reducing the mass of components, in order to decrease inertia of moving parts and increase the system efficiency. Better design of components has reached its limits, and substitution of steel by lightweight alloys is an alternative. Aluminium alloys are candidates of choice because they can be shaped easily and at relatively low cost.

However, the use of Al alloys is limited because of:
- the poor friction properties due to the alumina scale on the surface;
- their low load carrying capacity.

Surface properties of A1 can be improved by Surface modification techniques similar to those applied to steel:
- Plasma Diffusion Treatment (nitriding) and Plasma Immersion (PI) in order to improve the load bearing capabilities of the Al surface;
- Coatings deposition to improve the functional properties (e.g. friction).

However, the specific properties of Al alloys surface request the following R&D tasks:
- Cleaning and etching of the Al surface;
- Development of Plasma Diffusion Treatments and Plasma Immersion;
- Adaptation of wear and corrosion resistant coatings by PVD and PACVD;
- Development of duplex coating processes combining l), 2) and 3);
- Evaluation of the mechanical and tribological properties of the modified surfaces produced.

The first objective of the present project is to define surface modification treatments which will improve the performance of aluminium components in mechanical industries, particularly polymer forming dies and automotive sector. Applications considered as injection dies for plastics, engine components (pistons, sliding bearings, separating rings of pumps), accessories (door hinges), and specific target products (space frames, panels, fasteners) have been selected as test components having good potential for transfer to industrial production in a later stage.

The second objective is to find conditions for the implementation of the Aluminium Plasma Diffusion Treatment in the plants which are presently used for nitriding steel. Conditions for a rapid application of the processes to the complete mechanical industry will be met, if the two objectives are achieved.

The industrial sectors which might benefit from the project are:
- the transportation sector with automotive, aerospace and train applications. In the case of cars, a weight reduction of 200kg by 2005 is expected, leading to an average fuel saving of 1.2l/lOOkm per vehicle;
- in food processing industry (mixers, packaging machines etc);
- In textile industry (shuttles, cylinders, wire guides);
- In household products: small mechanical components and tools. In all cases, a longer lifetime of components and better system efficiency are expected. The objectives of the project follow the European Consortium for Automotive Research (EUCAR) directives. BE97-4615

Funding Scheme

CSC - Cost-sharing contracts


DaimlerChrysler AG

81663 München

Participants (5)

Strada Torino 50
10043 Orbassano
Via Enrico Fermi 1
21020 Ispra
GIE Renault Recherche et Innovation
860,Quai De Stalingrad
93109 Boulogne Billancourt
1-3,Via E. Fermi 1-3
10040 Caselette
Österreichisches Forschungszentrum Seibersdorf GmbH

2444 Seibersdorf