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Wear-Resistant Peripheral Coatings
PVD Coatings

PVD technology (Physical Vapour Deposition) is a method of depositing hard coatings reactively from the vapour phase onto the surface of piston rings. This involves vaporizing and ionizing a metal by means of an arc or an inert gas ion bombardment. The metal ions are accelerated to the surface of the component, discharged and deposited. In the reactive process the metal atoms react with the reactive gas atoms, forming nitrides, carbides, oxides, etc. This causes thin and dense coatings to form true to contour on the surface of the component.

Owing to the ceramic character of these coatings, PVD coated piston rings are exceptionally scuff resistant and extremely wear resistant. The PVD coatings preferred for piston rings are based on the CrN system (Fig. 28). Alternative material systems like DLC (Diamond-Like Carbon) are being increasingly used with a view to reducing friction. DLC coated piston rings in gasoline engine applications demonstrate a significantly improved early life behaviour and reduced cylinder wear especially in combination with Al cylinder surfaces.

Given the ceramic structure of PVD coatings they are characterized by high residual stresses, placing restrictions on the usable coating thickness and the ring design (O.D. edge). However, it is anticipated that further improvement of the coating systems (multi-layer, multi-component, graduated hard coatings) compared to the CrN systems mostly in service today will enable more widespread use in the future.


Fig. 28: Cross Sectional Metallographic Specimen of a PVD CrN Coated Piston Ring

 

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