Wear Resistant
Wear Resistant Coatings
Acree has developed wear resistant coatings that offer superior protection against abrasive, adhesive and surface fatigue wear for gears, bearings, bushings and other high-wear surfaces. We offer a wide variety of coatings that provide improved surface hardness and toughness. A number of material approaches are used, such as titanium nitride and carbide. Typical hardness values for the TiCN system range from 2270 – 3150 HV, depending upon the exact composition. For reference, heat treated tool steel possesses a hardness of about 850 HV. While improvements are obtained with single layer coatings, multilayer coatings add additional features, such as combining low surface friction and high hardness simultaneously. Multilayer and nanoscale engineered structures provide improved performance for wear characteristics. Examples include nanocomposite materials and super lattice multilayer films. These structures can also be graded to improve adhesive strength and reduce stress.
In nanocomposite films, nanocrystals of a given material are present in an amorphous phase or material. Typical examples include TiN nanocrystals in an amorphous Si3N4 background or TiC in a matrix of DLC. These coatings possess high hardness values and excellent wear resistance. The DLC or amorphous matrix provides for low coefficient of friction (typically 0.1) while the dispersion of nanocrystals achieves the required hardness. At the same time, brittleness is suppressed by the ductility of the background phase and the small grain size aiding in resistance to fracture or fatigue.
To provide some background, wear mechanisms can be broken into three general categories: abrasive; adhesive; and surface fatigue wear. In gear and bearing assemblies during oil-out and l<1 operation (where lambda is the ratio of lubricating layer thickness/surface roughness) asperities on the two surfaces come into contact with each other and wear occurs either through adhesive or abrasive action. In adhesive wear, the asperities tend to weld to each other and then get pulled away from the surface, leading to pitting. In abrasive wear, hard foreign particles in the gear/bearing system cut into the surface, producing pitting and/or erosion. The Hertzian stresses in the area of contact between components are constantly cycled since the parts contact or pass over each other again and again. This load cycling can generate cracks in the surface that progress inward leading to failure via surface fatigue. This crack growth can result in the dislodgment of small pieces of material called spallation. If the components are also subject to small oscillatory vibrations material will be removed leading to fretting failure of the components. The potential for component failure in these scenarios is mitigated through the appropriate choice of surface coatings to provide appropriate toughness and resistance to wear. Acree’s coatings provide excellent protection against wear.