As thermal spraying contractors in the business of applying a wide range of coatings to meet engineering surfacing requirements, Plasma Coatings Ltd. receives regular enquiries for abrasion resistant coatings.
Abrasion resistant coatings applied by thermal spraying fall broadly into two main categories namely, abrasion caused by impact from abrasive particles and abrasion caused by close contact sliding abrasive wear.
Wear caused by dry abrasive particle impingement or wet slurries is best combated using abrasion resistant coatings based on composites such as tungsten carbide/cobalt, tungsten carbide/cobalt/chrome, chrome carbide/nichrome and a group of nickel based abrasion resistant coatings known as ‘self-fluxing alloys’.
These coatings, applied by plasma, HVOF and LVOF processes exhibit high hardnesses and high internal strength by virtue of ‘binders’ in the coatings which help prevent grain loosening and breakup of the coatings when exposed to particulate impact. The most durable abrasion resistant coatings are those of the ‘self-fluxing alloy’ family which are thermally sprayed and subsequently heat treat ‘fused’ either by torch or vacuum furnace to metallurgically bond the coating to the substrate. During the fusing process phases of nickel borides, silicides and chrome carbide are formed which impart high bond strength, extreme toughness and high hot hardness to the coatings making them ideal abrasion resistant coatings for extreme abrasive wear environments at normal or elevated temperatures up to 900 deg. C.
For resisting abrasive sliding wear such as close contact seal applications on shafts and pump sleeves, abrasion resistant coatings based on hardfacing ceramic coatings are in widespread use. Although they do not possess the high internal cohesive strength of the abrasion resistant coatings described above, ceramic coatings such as plasma sprayed chrome oxide exhibit the highest hardness of any thermally sprayed coating and this combined with chemical inertness make it ideal, when appropriately sealed, for use in highly corrosive water and chemical environments.
The chrome oxide coating compositions now used incorporate silicon dioxide to act as a toughening agent which reduces the brittle nature of the coating and prevents grain loosening of the coating in service.
