Coatings for wear, corrosion, and oxidation protection
Coatings are routinely used to add corrosion, oxidation, and wear protection to materials. We have extensive experience and publication history using established coatings as well as developing new coatings for metallic, and graphitic carbon-carbon materials. The table below lists some of this experience. However, the application of a coating has both an economic direct cost and a cost associated with changing underlying material behavior. We can work with your organization to develop and manage a characterization plan to implement protective coatings which accounts for these broader influences.
An example of this is how various diffusion based coatings such as nitriding or carburizing can give rise to compressive residual stress. From a fatigue standpoint, this is generally good. The residual stress can be measured using x-ray diffraction and it can be observed in rotary bending fatigue testing, where the fracture origins move to the sub surface region.
Another more concerning effect occurs when anodizing aluminum and magnesium alloys. The coating which provides corrosion protection degrades the fatigue life of the underlying material. In the case of AA2219 aluminum plate, we were first to publish a report showing a 40 % reduction in fatigue life after anodization and establishing a mechanism. The effect was most pronounced in large cross section plate where ingot casting artifacts (Al2Cu) were not sufficiently broken down during wrought processing. These structures were immediately dissolved out of the aluminum surface leading to a fatigue fracture origin. We have experience developing high voltage spark anodization methods to mitigate this effect.
Wear coatings too influence material behavior. The coating thickness and adhesion need to be considered in light of maximum shear stress depth so that coating spalling does not occur. Returning to anodization, a hard anodize coating intended primarily for wear protection on aluminum can cause close to 60 % fatigue strength loss.
Coating system experience base
Ledgend: X: primary use; x: secondary use; O: not intended
|Anodize||Aluminum, magnesium||X||x||X||Fatigue strength loss|
|Hard anodize||Aluminum||x||X||x||Fatigue strength loss, loss of particle erosion resistance|
|Spark anodize||Aluminum, Magnesium||X||X||X||Can minimize fatigue strength loss|
|Titanium nitride/carbonitride (PVD)||Steel, stainless steel||X||Residual stress can cause flaking. Adhesion to substrate|
|Carburize||Steel||X||Compressive residual stress, deformation|
|Nitride||Stainless steel||O||X||O||Loss of corrosion resistance, compressive residual stress, minimal deformation|
|Boride||Nickel, cobalt alloys, steel||X||Brittle, very high temperature process.|
|Kolsterising||Stainless steel||X||Sub critical temp. carburize, minimal deformation|
|Hard chrome||Steel, stainless steel, nickel, cobalt alloys||x||X||X||Environmental hazard (hexavalent chrome)|
|Chrome replacement: Co-W, etc.||Steel, stainless steel, nickel, cobalt alloys||X||X||X||Superior wear vs chrome (dry sliding)|
|Electroless nickel||Most alloys||X||X||X||Pitting, adhesion|
|Thermal spray (D-Gun, HVOF, etc.)||Most alloys||x||X||X||Adhesion|
|Phosphate glass||Carbon-Carbon, graphite||O||X|