Electroless Ni-P Coatings: Preparation and Evaluation of Fracture Toughness and Scratch Hardness
Ni-P chemical coatings have been used to prevent wear, corrosion and as an alternative for hard chromium, since the latter's deposition processing is very harmful to the human health and the environment. In the present paper, Ni-P coatings with 8 and 10% P were deposited in steel AISI 1020 and thermally treated. Ni-1wt%P coatings with incorporation of hard particles of Al2O3 were also investigated. The microstructure and phase relationships were analyzed and correlated with the fracture toughness and scratch hardness of the coatings.The results show that the fracture toughness of the coating was smaller when thermally treated at 400°C for 1 hour and the scratch hardness reached a peak in this temperature. The relation of chemical composition and microstructure with mechanical properties of Ni-P coatings is presented. The phosphorus contents, the crystallization, and the incorporation of hard particles in the coatings change the values of toughness fracture and scratch hardness.
The use of coatings in surface engineering continues to increase in recent decades. Ni-P coatings, commercially available, combine high wear resistance and adhesion to the substrate. These kinds of coatings present a uniform thickness and can be used instead of hard chromium, decreasing health and environment problems. The wide use of these coatings stems from new prospects for improving the tribological properties of the coatings by thermal treatment and by the incorporation of hard particles such as alumina . The process consists of electroless chemical reduction of Ni+2 to Ni0 and P+1 to P0 and simultaneous deposition on desired substrates. In the present study the effects of thermal treatment and the alumina hard particles incorporation were evaluated and related to fracture toughness and scratch hardness of Ni-P coatings.
The coatings were deposited on steel AISI 1020. Before the coatings, the substrate was polished in SiC of 100 and 180 mesh. The substrates were cleaned through the use of absolute alcohol and etching with HCl, 30 wt%, before the coatings to be applied . The process was performed in a nickel sulphate and sodium hypophosphite solution at 90°C and the hydrogen ions concentration at the solution (pH) was monitored according to the phosphorus content in the coatings. Figure 1 shows the influence of pH on the percentage of phosphorus in the coatings .
Branco et al. in 2006  noted that for a pH = 4 the coatings present 10 wt%P content, for a pH = 5 the phosphorus content is 8 wt%, and for a pH = 8 the phosphorus content is 1wt%P. As the pH of the bath has a tendency to lower to it becomes necessary drip an alkaline solution with ammonium hydroxide to 50% by volume. The higher the deposition time is, the increased the thickness of the coating. The Ni-8 wt%P, Ni-10 wt%P, and Ni-1wt%P/Al2O3 coatings were prepared in three chemical baths as shown in Table 1.
After the coatings the samples of Ni-P were thermally treated in furnace at 300°C, 400°C, and 600°C for 1 hour and cooled in the air.
The Ni-P/Al2O3 coatings were performed by incorporation of alumina with 3 μm size particles. The use of calgon surfactant is necessary in the chemical baths to stabilize the dispersion and improve the deposition of Ni-P/Al2O3 coatings.
The chemical composition of the coatings was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). The thickness of the coating was determined by an optical microscope coupled with a graduated scale. The identification and the evaluation of the present phases in the coat were performed by X-ray diffraction (XRD). The microstructure of the sample and the present precipitates was examined by a scanning electron microscope (SEM). The coated samples were polished in SiC at 240, 400, 600, 800, and 1500 mesh, and the last step was 1 μm diamond. The chemical etching was carried out by 20 seconds in aqueous solution with 50% nitric acid and 30% acetic acid .
The fracture toughness () for the Ni-P coated samples was measured by indentation test. A load “” of 100 N with a Vickers indenter was applied for 20 seconds in order to cause cracks in the samples. The value of semi-diagonals of impression (“”) and the length of cracks (“”), Figure 2, was measured by optical microscope.
The fracture toughness of coating, in MP a.m1/2, was determined by Equation (1), proposed by . whereis the value of fracture toughness (MPa·m1/2);is the value of applied load indentation (N); is the semidiagonals of indentation (m);is the length of cracks in the direction of diagonal (m).
The scratch test was used to evaluate the hardness of the coatings. The tests were performed with a Rockwell C indenter, with a diamond cone and opening angle of 120° at rate of 100 N/min and speed of 10 mm/min. The scratches measured roughly 7 mm and were analyzed by optical microscope.
The scratch hardness of coatings was determined using (2) : where is the scratch hardness of the material (MPa); is the normal force in scratch test (N); is the width of scratch (mm) relative to this normal force.
3. Results and Discussion
3.1. The Thickness and Chemical Composition of Ni-P and Ni-P/Al2O3 Coatings
The thickness of the coating measured roughly 10 μm. Table 2 shows the chemical analyses of Ni-P and Ni-P/Al2O3 coatings. The phosphorus content in Ni-8% P and Ni-10% P coatings is the same, as estimated by the chemical baths with their pH = 5 and pH = 4, respectively. Note that Al2O3 content in the coatings is roughly 31%.
3.2. Analyses of X-Ray Diffraction
The diffractograms of Ni-8%P coatings, thermally treated, and Ni-1%P/Al2O3 are shown in Figure 3. The profile of Ni-8%P coatings, as deposited, is characteristic of amorphous materials. There is a peak at 2θ = 44.5° for Ni-β phase, which has a face-centered cubic (FCC) structure. The α-alumina phase, present in Ni-1%P/Al2O3 coatings, has a rhombohedral structure at 2θ = 35.1°, 37.1°, and 45.3°. Li et al. in 2004  also found similar results in their studies
. The phase that corresponds to the nickel phosphides appears when the coatings of Ni-P are treated at 300°C for 1 hour. At this temperature, the amorphous Ni crystallizes to FCC. The structure of the nickel phosphide is tetragonal . The intensity of the peaks of Ni-P coatings increases with annealing temperature because of the crystallization of phosphides. At high temperatures, 400°C and 600°C the peaks of Ni, Ni3P and Al2O3 have high intensity. Gao et al. in 2005  also found similar results in their works.
The microstructures of the coatings are shown in Figure 3. The β phase consists of a solid solution of nickel and phosphorus that can hold up to 4.5%, by weight, of phosphorus. The γ phase is metastable where the phosphides precipitates appear between 11 and 15%P . The nickel phosphides are a hard phase. Figure 4(a) shows a sample of Ni-10% P coating as, deposited and Figure 4(b) shows the Ni-P coatings with incorporation of Al2O3. This phase is harder than the nickel phosphides .
3.4. Toughness Fracture of Coatings
When the phosphorus content increases, from 8 to 10%, this increases the nickel phosphides content and the propagation of Palmqvist type cracks (Figure 5). The increase of phosphides precipitates embedded in nickel matrix decreases the fracture toughness of the coatings. The propagation of cracks has a higher probability of finding a hard phase of nickel phosphides. The toughness fracture of coatings with 8%P is greater than coatings with 10%P in same conditions. For the coatings with alumina hard particles, such as the case of Ni-1%P/Al2O3 coatings, this probability increases.
The fracture occurs when the hard phase is subjected to stress fields . The influence of temperature on the fracture toughness of the Ni-P coatings can be evaluated in the graphs shown in Figure 6. There were no cracks in Ni-P and Ni-P/A2O3 coatings as deposited.
Note that there is a decrease in fracture toughness when the coatings were treated until 400°C for 1 hour. The fracture toughness of Ni-P coatings has a lower value, 1.4 MPa·m1/2 (Ni-10%P), at 400°C. The highest toughness values occur at 600°C, about 2.3 MPa·m1/2 (Ni-8%P) and 1.9 MPa·m1/2 (Ni-10%P). The values of fracture toughness are high for these amorphous coatings, that is, as deposited. Bozzini et al. 2001  obtained a value of 7.5 MPa·m1/2 for the fracture toughness in Ni-9% P coatings. Increasing the fracture toughness is related to the coalescing of particles Ni3P, where the ductile matrix Ni isolates the hard particles of Ni3P, preventing the spread of breakage particle to particle.
3.5. Results of Scratch Hardness
The scratch hardness was determined by (2) where b is the width of the scratch. The values are shown in the graph in Figure 7. On the Mohs scale, used by mineralogists, diamonds have a hardness of 10 which corresponds to 61 GPa . The scratch hardness of Ni-P/A2O3 coatings as deposited, 18 GPa, is greater than the hardness of Ni-8%P and Ni-10%P in same conditions, 15 GPa and 17 GPa, respectively.
At temperatures below 400°C, the hardness increases due to phase transformation of amorphous Ni-P to cermets Ni3P embedded in matrix of crystalline Ni . For the temperature at 400°C, the scratch hardness of coatings reaches greater values. The larger value is roughly 21 GPa for Ni-10%P coating. At temperatures above 400°C, the size of the particles of phosphides grows and reduces the hardness.
Figure 8 presents an image of the scratch after delaminating of the substrate, obtained in the scratch test for Ni-8%P coatings treated at 600°C for 1 hour.
The evolution phase of coatings thermally treated was observed from the results of X-ray diffraction that show the presence of Ni-β phase and nickel phosphides that crystallize from the amorphous phase. These peaks of crystallization increase with annealing temperature.
Ni-1%P/Al2O3 composite coatings plated from stable dispersions in EN baths exhibit excellent particle distribution even at particle incorporation levels approaching 30% by weight. A large increase in particle concentration in the growing layer during plating was observed for surfactants systems plated from baths with particle concentration of 5 g/L.
The fracture toughness and scratch hardness vary depending upon the annealing treatment. This fact is due mainly to the crystallization and precipitation of the phases present and the coalescing of phosphides. The values of fracture toughness decrease until the thermal treatment of 400°C and increase after this temperature. The values of fracture toughness by Vickers indentation of Ni-P and Ni-1%P/Al2O3 coatings, as deposited, were not found. These values are very large because there was no crack by indentation. The value of scratch hardness of Ni-P coatings increases until the thermal treatment temperature of 400°C and decreases after this temperature. The incorporation of Al2O3 particles on the Ni-P coatings, as deposited, increases the scratch hardness of these coatings. This process is the hardening by incorporation of hard particles. The Ni-P/Al2O3 coatings, as deposited, are harder than the Ni-8%P and Ni-10%P coatings.
The results of the present study reveal a relation of chemical composition, microstructure with mechanical properties of Ni-P coatings. Thus, the phosphorus contents, the crystallization, and the incorporation of hard particles in the coatings change the values of toughness fracture and scratch hardness.
The authors acknowledge the financial support from the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) and Fundação Centro Tecnológico de Minas Gerais (CETEC).
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