When components slide against one another, the friction arising can wear the part through galling and overheating. Wear is the undesired removal of material from a component through tribosystems such as abrasion, adhesion, cavitation, erosion, fatigue and stress cracking, fretting, and impact. In conjunction with a corrosive environment, tribocorrosion will occur. This synergistic effect can cause very rapid loss of material, component failure, and machinery seizure. Surface technology can effectively reduce the friction coefficient, prevent wear, and improve lubrication.
Abrasion can result in very rapid wear. It is classified into two primary mechanisms: In two-body abrasion, a hard, rough surface scratches, cuts, or spalls a softer surface. In three-body abrasion, a hard third body damages one or both of the sliding surfaces. This is usually due to grit or dirt getting in between the sliding interface. This wear can be minimalized by hard coatings that are strongly adhered onto the substrate and feature diffusion layers.
When surfaces slide relative to one another, there is a tendency for one material to transfer onto the counterface. In its most severe form, adhesive wear results in galling and possibly machinery seizure through cold welding.
Adhesion can be combated with coatings that provide dissimilar materials and harder surfaces. We offer a variety of surface solutions using coating and diffusion techniques that match specific applications. For highly loaded components such as gears, plasma nitriding is the best solution.
Cavitation wear occurs on surfaces exposed to fluids in which entrained bubbles collapse at or near the surface. The collapse releases a jet of fluid that impacts the surface, causing a severe hammering effect.
Cavitation is best combated through component redesign that either stops bubble formation or inhibits bubble collapse near the affected surfaces. Coatings can provide temporary relief prior to the redesign, or can be used in applications where redesign is not feasible.
Intense cavitation is combated through tough materials that strongly work-harden, such as cobalt-based alloys. These are applied using processes that lead to metallurgical bonding, such as welding and fusing. For mild to weak cavitation, High Velocity Oxygen Fuel (HVOF) coatings and carbonitriding can be used to reduce wear.
Erosion is caused by gad or liquid particles striking the surface of a component. The severity of wear strongly depends upon the velocity and hardness of the particles, as well as the angle of impact. It is crucial to consider all these conditions when designing a surface solution.
For high-angle attack, select a somewhat compliant coating or a very thick coating applied by welding or thermal spray. For low-angle attack, very hard coatings are preferred.
Fatigue and stress cracking
Changing mechanical stresses cause fatigue and stress. Mechanical cycling of components or surfaces can result in local crack initiation and growth. This can cause catastrophic component failure or localized pitting, as often seen on gear faces.
Generally, this is best solved by mechanical redesign. However, diffusion treatments such as nitriding are very effective in increasing fatigue resistance by introducing compressive stresses. Thermal spray processes, especially the High Velocity Oxygen Fuel (HVOF) method, introduce compressive stresses within the coating that benefit components exposed to high-stress cycling.
Fretting is damage caused by low amplitude vibration and/or small oscillations between two surfaces contacting each other. This damage often goes unnoticed for a long time. Over long periods, particularly at high temperatures or in corrosive environments, this friction can lead to fatigue cracking and loss of component functionality.
Sulzer offers coating solutions that provide excellent anti-fretting properties. For example, thermal sprayed copper-nickel-indium prevents fretting on the roots of gas turbine blades.
Impact is the sudden striking of one object against another with high force. Repetitive impact often causes substrate materials to weaken and crack.
Impact-resistant coating applications require materials that are hard, thick, and especially tough. Such materials should then be applied using a system that metallurgically bonds the coating to the substrate, such as Plasma Transformed Arc (PTA) overlaying or fused thermal spraying.