R&D

COATING

Metal-carbide- (TC, TiC or CrC) or Diamond-like-carbon- (DLC) coatings are used when there is a possibility of wear, friction or the risk of galling, all of which must be minimized. Such layers are necessary in the last resort, if no satisfactory performance can be achieved through conventional surface hardening methods in combination with extreme loading conditions. These coatings comply with their high wear resistance over a wide temperature range, its good adhesion to the substrate, the high corrosion resistance and the low coefficient of friction of many requirements.

The main disadvantage of these coatings is that they can only be applied in very thin layers and that they tend to spalling because of their usually large production-related residual compressive stresses. This spalling risk is enhanced in addition by the fact that the highest macroscopic shear stresses are located, due to the thin layers, below the surface in much softer base material. A hardened base material cannot be used in the most cases for reasons of poor fatigue behaviour. These problems can be overcome if there is a relatively hard intermediate layer between the coating and base material.

BHDT‘s experiences

The big challenge is, that coatings perform in a way that there is maximum adhesion between the individual layers. Furthermore, it has to be ensured that the stress peaks from the microscopic frictional contact are located within the coating, while those resulting from the macroscopic contact are located within the intermediate layer, so that there is no plastic deformation in the base material. The material of the intermediate layer should have a high strength so as not to plasticize itself and a large modulus of elasticity so as not to transmit much strain to the base material, which would lead therein for plasticizing.

BHDT GmbH already has years of experience in terms of hard-facing using wear resistant layers, for example on valve stems and seats. To be able to offer valves for high temperatures, with minimal wear and low frictional losses under corrosive media influence, BHDT GmbH is working on a continuous development of the wear layers used. For example, a coating system is currently used for deep-sea ball valve tests. This coating consists of a ductile base material on which a particle-hardenable nickel-based super-alloy layer is coated with a thin layer of tungsten carbide.

BHDT Gmbh provides tailor-made sealing concepts for special applications. Currently a self-energized gasket for a deep-water application is developed, which is equally suitable for large internal and external pressure.

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The left figure shows a valve stem made of super duplex steel, which has a hardened intermediate layer using Kolsterising® and is additionally coated with a tungsten carbide layer. The right figure shows a micrograph through this coating.