Abstract
Composite Ni-P-Al₂O₃ coatings were prepared on pre-treated Al and Al-Si casting alloy by electroless deposition in suspensions of 5-20g/l Al₂O₃ in a solution containing nickel lactate, nickel hypophosphite and lactic acid. In the as-deposited coatings, ceramic particles were homogeneously distributed in extremely fine-grained nanocrystalline Ni-P matrix. Adhesion of coatings to substrate was better in case of Al-Si alloy due to silicon present in its structure. Hardness, P content and grain size of the Ni-P matrix were 549-574 HV, 10.0 wt.% and 1.9 nm, respectively. These attributes did not depend either on Al₂O₃ volume fraction or on substrate chemical composition. The relatively low hardness value was discussed with respect to the inverse Hall-Petch effect. Due to composite coatings, abrasion resistance of Al-based alloys was significantly increased. The strongest effect was observed after deposition in a bath containing 5g/l Al₂O₃. Further additions of ceramic particles to the bath did not have so strong effect on abrasion resistance. Heat treatment of composite coatings at 400 °C/1 h induced decomposition of Ni-P phase to Ni and Ni₃P that led to significant hardening (almost 1000 HV) of the Ni-P matrix. The coatings were also heat treated at 500 °C/24 h that simulated solution annealing. This heat treatment led to an excessive softening of Ni-P matrix. In addition, solid state reaction between Al and Ni produced layers of hard Al₃Ni₂ and Al₃ Ni intermetallics.

Key words
Al-Si alloy, aluminium, Ni-P-Al₂O₃ coating, heat treatment, abrasion, hardness

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