Structural evolution of electroless Ni–P coating on Al–12 wt.% Si alloy during heat treatment at high temperatures
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.
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.
References (26)
- et al.
Appl. Surf. Sci.
(2008) - et al.
Appl. Surf. Sci.
(2008) - et al.
Surf. Coat. Technol.
(2000) - et al.
Surf. Coat. Technol.
(2003) - et al.
Scripta Mater.
(1998) - et al.
Surf. Coat. Technol.
(2001) - et al.
Surf. Coat. Technol.
(2006) - et al.
Thin Solid Films
(2002) - et al.
Wear
(2008) - et al.
Thin Solid Films
(2004)
Thin Solid Films
Surf. Coat. Technol.
Appl. Surf. Sci.
Cited by (50)
Picosecond laser remelting of electrodeposited Ni–P coating: Parameters optimization and electrochemical corrosion behavior
2023, Surface and Coatings TechnologyEffect of laser irradiation on high-temperature crystallization behavior, oxidation resistance, and corrosion performance evaluation of electrodeposited amorphous Ni-P coatings
2022, Journal of Non-Crystalline SolidsCitation Excerpt :Especially, the crystallization rate of LECD-formed coating is slower than that of ECD-formed coating. When crystallized, nanocrystalline Ni precipitates first, followed by the precipitation of some metastable phases, such as Ni12P5 and Ni5P2 [24–26]. The precipitated metastable phase process is related to the P element [27].
Microstructure evolution and corrosion resistance of Ni–Cu–P amorphous coating during crystallization process
2019, Applied Surface ScienceHardness evaluation from a bilayer coating system of Ni-P deposited on carbon steel plates by multicycle indentation tests
2018, Surface and Coatings TechnologyCitation Excerpt :According to the load-curve from the diffusion layer between the Cr-N layer and Al substrate, it appears the pop-ins in the loading part of the curve and it would be interesting to study the nature of this interlayer and its contribution with the composite hardness of the multilayer system. The electroless nickel-based coatings are extensively used to improve the surface properties of a base material, such as electrochemical behavior and hardness [8], and it is also utilized as a good catalytic electrode material in hydrogen evolution reaction, although it has a low electrocatalytic performance [9,10]. This type of coating is used above all in electroless engineering technologies owing to its low cost and malleability [11–14].
A protocol for fast electroless Ni-P on Al alloy at medium-low temperature accelerated by hierarchically structured Cu immersion layer
2017, Surface and Coatings TechnologyEffect of bath agitation on surface properties and corrosion behaviour of ENi-P coatings along with annealing temperature
2016, Engineering Science and Technology, an International Journal