The selection of the proper coatings and treatments to be used for a wear resistant job is the most important step in doing the job successfully. Each wear coating has different characteristics of strength, shrinkage, hardness etc such that there is a material for every wear resistant category. There are two general material approaches to wear resistance. Firstly, wear resistant material can be used. Such a material resists wear by virtue of its composition or its properties. Second, a structural material can be used and its surface modified by treatments and coatings to provide any desired level of wear protection.
A wide variety of treatments and coatings have been developed using different coating techniques and methods of application. These coatings and treatments fall into three categories:
- Soft Coatings or Solid Film Lubricants. These coatings provide protection by preventing adhesion between the substrates. Since they have low shear strength they shear in preference to the substrate giving a low friction coefficient. The coating will also flow to better distribute the contact area. This promotes increased load and temperature capacity.
- Surface Treatments. Surface treatments modify the surface either to make it harder or to provide a more wear resistant alloy at the surface. Generally, these treatments increase the surface hardness and provide wear resistance by virtue of that increase. Nitriding is an example of this approach.
- Hard Surface Replacement Coatings (Hard Surfacing). Such coatings do not modify the existing surface but replace it with another surface. The wear behavior is only a function of the coating and not the base material. Examples of such coatings are chrome plate and weld overlays. Sprayed carbide and ceramics are also included in this category.
Hard Surfacing Category can easily be controlled by an engineering technique to achieve a maximum reliability, since a desired controlled surface can be derived mechanically. Actually, hard-surfacing is a process by which an alloy coating is welded, fused, or sprayed onto a critical area of a metal part. Although the name implies a wear resistant coating, hard-surfacing is undertaken to provide a variety of desired properties including wear resistance, corrosion resistance, oxidation resistance and strength. At this point, it is worth mentioning that, there are two kinds of hard-surfacing: Hard-facing and spray coatings. Hard-facing is applied using welding techniques which assures a metallurgical bond; spraying is accomplished by impinging the substrate with high velocity powder. Bonding in this case is usually mechanical. Spraying applies a coating which is up to 0.050 cm.
There are two general uses for hard-surfacing. It can be applied to repair worn surfaces, and it can be applied to obtain more desirable surface properties. Hard-surfacing competes with surface treatments and soft coatings and should be given equal consideration in the design of mechanical components. Irrespective of the range of coating properties, the advantages of hard-surfacing are the greater thickness available for maximum achievable reliability at minimum cost with technological inheritance technique.
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