Structural evolution of electroless Ni–P coating on Al–12 wt.% Si alloy during heat treatment at high temperatures

The work is concerned with the high-temperature heat treatment of an Al–12 wt.% Si alloy coated by an electroless Ni–P layer. The electroless deposition took place on a pre-treated substrate in a bath containing nickel hypophosphite, nickel lactate and lactic acid. Resulting Ni–P deposit showed a thickness of about 8 μm. The coated samples were heat-treated at 200–550 °C/1–24 h. LM, SEM, EDS and XRD were used to investigate phase transformations. Adherence to the substrate was estimated from the scratch test and microhardness of the heat-treated layers was also measured. It is found that various phase transformations occur, as both temperature and annealing time increase. These include (1) amorphous Ni–P → Ni + Ni3P, (2) Al + Ni → Al3Ni, (3) Ni3P → Ni12P5 + Ni, (4) Ni12P5 → Ni2P + Ni, and (5) Al3Ni + Ni → Al3Ni2. The formation of intermetallic phases, particularly Al3Ni2, leads to significant surface hardening, however, too thick layers of intermetallics reduce the adherence to the substrate. Based on the growth kinetics of the intermetallic phases, diffusion coefficients of Ni in Al3Ni and Al3Ni2 at 450–550 °C are estimated as follows: D(Al3Ni, 450 °C) ≈ 6 × 10−12 cm2 s−1, D(Al3Ni, 550 °C) ≈ 4 × 10−11 cm2 s−1, D(Al3Ni2, 450 °C) ≈ 1 × 10−12 cm2 s−1 and D(Al3Ni2, 550 °C) ≈ 1 × 10−11 cm2 s−1. Mechanisms of phase transformations are discussed in relation to the elemental diffusion.