Plasma Nitrading

J.J. Castings has the largest sub-contract Plasma Nitriding facilities in Great Britain

Plasma Nitriding works by placing the component to be treated in a vacuum furnace so that it is electrically insulated from the furnace.

The furnace is evacuated and the treatment gas is introduced. A DC voltage is applied between the component and the furnace wall which ionises the treatment gas (the component being the cathode and the furnace wall anode). The positive ions of the treatment gas are accelerated towards the negatively charged component. On hitting the surface of the component the ions transfer their kinetic energy in the form of heat, thus heating up the component.

The vacuum employed is relatively low, normally between 2mB and 8mB, and so the DC voltage does not drop in a linear way. Almost the entire applied voltage drops directly in front of the cathode (component), producing the typical luminous purple glow seam in the cathode fall region around the component outline. The glow seam follows every contour of the component so that all surfaces receive uniform ion bombardment and therefore uniform surface hardness and case depth.

Plasma Nitriding is successfully carried out at a lower temperature and at a greater rate than gas nitriding. Plasma Nitriding at low temperature enables components to be treated without the loss of base hardness and without distortion – provided that the components are in a stress-free condition before they are nitrided. Plasma Nitriding produces a micro structure which is considerably different from that produced by gas nitriding.

The outermost layer is extremely thin, typically 2-10 microns. It is mono-phased and contains either γ (Gamma) or ε (Epsilon) ionitrides. It is unaffected by alcoholic nitric acid etch and remains white – hence it is referred to as the white layer. The mechanical properties of the surface ie. wear resistance, ductility and resistance to rolling friction are much improved by this thin and homogenous white layer.

Beneath the white layer is the diffusion zone, the hardness and depth of which depends on the material being treated, the treatment temperatures and the treatment time. The treatment gas ions penetrate the component surface where they impact, thus through grain as well as grain boundary diffusion occurs.  This produces small and evenly distributed nitride precipitates within the nitrogen diffusion zone. As the nitrogen precipitates in this zone are smaller and more evenly dispersed than those produced by gas nitriding, the ductility and tortional properties are considerably improved.