Elsevier

Applied Surface Science

Volume 255, Issue 6, 1 January 2009, Pages 3745-3751
Applied Surface Science

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

https://doi.org/10.1016/j.apsusc.2008.10.026Get rights and content

Abstract

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.

Introduction

Aluminium alloys show low weight, high specific strength and corrosion resistance in oxidizing environments, making them of interest for automotive and aerospace industry. Casting Al–Si alloys having excellent castability are suitable for production of large-series of complex-shape components, such as engine blocks, pistons, cylinder liners, cylinder heads, wheels, etc. In some applications, however, they suffer from insufficient wear resistance. To prolong the life time of components, several approaches have been adopted in industrial scale. They involve reinforcement with particles or fibers producing Al–Si matrix composites [1], increase of Si content in alloys [2], hard coatings. The hard PVD and electrodeposited chromium or nickel coatings provide sufficient improvement of hardness and wear resistance. However, problems often arise when complex components having internal surfaces or holes are coated. These problems are in part avoided when using electroless Ni–P coatings. The electroless Ni–P coatings provide significant improvement of wear and corrosion resistance and are not limited to the steel substrates. Various procedures have been developed to coat Al- or Mg-based substrates [3], [4]. It has been reported in many studies that the physical properties of these coatings vary considerably with their internal structure, content of phosphorus and ternary additives. Hardness of the as-deposited coatings generally lies between 500 and 800 HV and it depends on phosphorus concentration [5], [6]. Further improvement of hardness and wear resistance can be achieved by the incorporation of hard particles (boron carbide, silicon carbide, diamond, aluminium oxide, etc.) in the coatings and by the heat treatment [7], [8], [9], [10]. Generally, the annealing at 400 °C/1 h leads to the maximum hardening which is attributed to the decomposition of amorphous Ni–P phase, formation of crystalline Ni and precipitation of Ni3P. By applying higher annealing temperatures or longer periods, hardness of the Ni–P alloy progressively reduces due to the growth of Ni grains and phosphide particles. Though this process can be slowed to some degree by adding some ternary additives, such as tungsten [11], [12], [13], [14], [15], [16], higher annealing temperatures are generally not used to harden the Ni–P coatings. However, it should be noted that Al–Si castings may be exposed to temperatures above 400 °C. As an example, the solution annealing of age hardenable Al–Si-based alloys is commonly performed at around 500 °C for several hours. In addition, some thermally loaded components of engines made of aluminium alloys may be exposed to elevated temperatures of almost 400 °C for relatively long periods. Besides the crystallization of Ni, precipitation of phosphides and their growth, solid state reactions between Ni–P alloy and Al-based substrate may occur under these conditions. However, only little information is available on these processes, though reaction products may significantly modify properties of the surface zone. For this reason, in the present study our attention is devoted to the effect of heat treatment at temperatures up to 550 °C on the phase composition and mechanical characteristics of a surface zone of an Al–Si alloy with an electroless Ni–P coating. To investigate reactions between the coating and substrate, the annealing periods are prolonged up to 24 h.

Section snippets

Experiment

Commercial Al–12 wt.% Si alloy, see Table 1, was used as substrate for electroless deposition. The material was provided by an industrial supplier, remelted in a vacuum induction furnace and cast into cast-iron metal mould to prepare cylindrical ingots having a diameter of 20 mm and length of 200 mm. Disc-shaped samples of 10 mm in thickness were cut out directly from the ingots. Surface of samples was progressively ground with P60–P1200 SiC papers to obtain defined surface roughness of 3 μm. A

Structure and phase composition

Fig. 1 presents LM images of cross-sectioned samples both after deposition and after heat treatments. The as-deposited Ni–P coating (Fig. 1a) having a thickness of about 8 μm shows a relatively good adherence to the substrate and uniformity. Concentration of phosphorus in the coating is 17.4 at.%. Silicon particles from the substrate are in part incorporated in the coating, suggesting that the chemical pre-treatment has removed a thin surface layer of α(Al) phase. The coating heat-treated at 400 

Discussion

It is presented that the annealing at 350 °C already induces internal phase transformations in the electroless Ni–P coating, including crystallization of Ni and precipitation of Ni3P. These transformations result in the considerable hardening effect. At temperatures higher than 400 °C hardness of the Ni–P coating reduces due to the grain growth and precipitate coarsening, but solid state reactions between Al–Si substrate and coating become significant. Mechanism of these reactions includes three

Conclusions

Due to the heat treatment at high temperatures, various phase transformations take place in a surface zone of the Al–Si alloy coated with the electroless Ni–P layer. This sequence can be written as follows:

  • 1.

    Ni–P (amorphous)  Ni (crystalline) + Ni3P (crystalline);

  • 2.

    Ni + Al  Al3Ni;

  • 3.

    Al3Ni + Ni  Al3Ni2;

  • 4.

    Ni3P  Ni12P5 + Ni;

  • 5.

    Ni12P5  Ni2P + Ni.

The formation of Al–Ni intermetallics is observed at above 400 °C and is associated with the significant hardening of the surface zone, exceeding that obtained by the commonly

Acknowledgements

Authors wish to thank the Grant Agency of Czech Republic (project no. 104/08/1102), the Ministry of Education, Youth and Sports of Czech Republic (project no. MSM 6046137302) and the ICT Prague (project no. 106/08/0015) for their financial supports provided for this research.

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