How Chrome Plating Handles High-Load Sliding Wear
Why is Chrome plating, a hard wear surface coating, so widely specified for parts that are subject to high load sliding wear, and how does it perform?
Hardness and Low Friction Working Together
Hard chrome thicknesses typically have a hardness of 850-1000 HV, and this makes it very effective in preventing adhesive wear. The natural lubrication of the Chromium, and low friction, of around 0.15-0.2 against steel, also helps to prevent metal pickup and wear.
The Role of Chromium’s Natural Lubricity
The Chromium in hard chrome plating forms a thin stable oxide film on the surface of the part which acts as a lubricant. This surface film reduces direct metal to metal contact allowing the sliding surfaces to ‘slide’ rather than ‘grip’ and thus preventing adhesive wear by reducing the risk of metal pickup. Even with moderate loads and without the formation of full hydrodynamic lubrication the surface is able to perform well.
How Thickness Affects Load-Bearing Capacity
Thick coatings are generally better at distributing load than thin ones. However, very thick chrome can introduce tensile stress in the substrate and also suffer from micro-cracking. In flexing or rotating parts, this can have serious effects on fatigue life. If you are looking for Chrome Plating, see //www.poeton.co.uk/advanced-treatments/apticote-100-hard-chrome-plating/.
Surface Finish After Grinding Matters
For Sliding it’s also the surface finish after grinding of the plated part which affects the friction behaviour. An average surface roughness (Ra) value of 0.1 to 0.4 µm is typical for Sliding applications. Higher values of Ra result in asperities of the surface coming into contact under load which in turn result in high friction. Lower values of Ra result in the lubricant being ‘locked’ into the fine surface features which again results in high friction.
Where Hard Chrome Loses Its Advantage
In applications where abrasive particles are to enter into the surfaces of contact, chrome will offer little advantage over other wear resisting coatings due to its relatively low resistance to abrasion by hard grit. Additionally, in high impact applications, chrome’s inherent brittleness makes it prone to cracking and spalling under repeated shock loading. In such cases, a tougher thermal coating or wear resistant ceramic coating will generally be more suitable.
The key is to match the coating to the wear mechanism(s) and so hard chrome is well suited to most hard sliding wear applications.