Picking the right coating for wear applications.
When selecting a coating for wear resistance it is not simply a case of choosing the hardest coating and then relying on it to perform. The coating selected must match the application to which the coated part will be subjected and the part surface finished and toleranced to allow the coating to be effectively used.
Identify the Wear Mechanism First
The type of wear that is acting on a part can be broadly classified as either Abrasive wear, Adhesive wear, Fretting or Erosive wear, each causing different types of surface damage. As a general rule the abrasive and erosive wear resistance of a coating can be improved by increasing its hardness. Thus hard chrome (850-1000 HV) and thermal spray tungsten carbide (HV>1100) are recommended for severe abrasive and erosive wear applications. For mild adhesive wear and fretting corrosion, intermittent lubrication or the use of a low friction coefficient, electroless nickel (900-1000 HV) is recommended.
Check Coating Hardness Values
Hardness values are often useful for comparisons of wear resistance, however it is essential to note that there are many variations of surface treatments and hard materials, each with its own unique characteristics and thus it is essential to select the correct material for your specific wear problem. In general terms Hard chrome will average around 850-1000HV, electroless nickel in the heat-treated condition will average around 900-1000HV, whilst tungsten carbide thermal spray average around 1100HV or more.
Consider the Substrate Material
Some surface treatments are not suitable for certain base materials and require an intermediate layer to achieve adequate bonding. For example, a zinc-immersion coating is typically required before nickel plating of aluminium. Steel substrates are generally easier to coat but high-strength materials may require post-coating bake to relieve hydrogen embrittlement.
Factor In Dimensional Tolerances
There are varying amounts that can be added to a surface by a coating, and in many cases this can be very uneven. Where close dimensional tolerances are required, a uniform deposit such as that produced by electroless nickel is generally the better choice. Where a coating such as hard chrome is used, it is typically possible to true up the surface to the required tolerance after applying the coating, but this can be expensive and time consuming. If you want Surface Treatments, www.poeton.co.uk/surface-treatments is a good place to start.
Account for the Operating Environment
Temperature, corrosion by chemicals, lubrication etc. All aspects that affect wear in the long run. The nickel-phosphorus coating is sensitive to high temperatures. It degrades already at temperatures of about 350°C. In contrast to this, the wear resistance of thermal spray coatings is not affected by high temperatures. Moreover, in aggressive media such as acids or salty water the coating has to be corrosion resistant as well as wear resistant.
When considering surface treatment for wear resistance, all of the above points need to be considered as a checklist rather than afterthoughts.