Structure and properties of nickel-phosphorus coatings in relation to annealing temperature and time

Abstract

1. With diffusion annealing, the structure of nickel-phosphorus coatings depends on their interaction with the steel substrate. A mechanism was proposed for the interaction of the two-phase nickel-phosphorus coating with the steel substrate, consisting of decomposition of nickel phosphides in the transition layer with diffusion of iron from the substrate into the coating, and formation of phosphides in the surface layers still not “diluted” with iron. 2. With an increase of annealing temperature and time the front of the phosphides is forced back to the surface layers of the coating, with simultaneous growth of the phosphorus-free layer. The reduction in the thickness of the phosphorus-containing layer ceases with formation of the single-phase Ni3P layer. 3. The thicknesses of the phosphorus-free and phosphorus-containing layers during annealing depend not only on the annealing parameters but also the original thickness of the coating. With decreasing thickness of the original coating the formation of the single-phase layer of nickel phosphide is completed in a shorter time. 4. The microhardness of the phosphorus-containing layer depends on the annealing parameters, the thickness of the original coating, and the development of precipitation hardening processes in it, softening due to coalescence, and subsequent hardening as the result of enrichment with phosphorus up until the formation of the single-phase layer. 5. At low temperatures and short annealing times the maximum microhardness of the phosphorus layer is reached due to precipitation hardening, while at high temperatures and long annealing times it is due to the increase in the phosphorus concentration. With decreasing thickness of the original coating the annealing time required to attain the maximum hardness as the result of enrichment with phosphorus is shorter.