Deposition time is kept constant for each and every specimen so that the average coating thickness remains approximately the same. The surface roughness of electroless Ni-P coating is found to be in the range 0.62-0.83 μm. After the deposition is over, samples are washed with distilled water. The coated samples are annealed in a box furnace for 1 hour at 400 °C.

Tribo-corrosion test in alkaline environment

The coated specimens serve as test specimens which are held perpendicular against a rotating alumina disk (∅ 100 mm × 8 mm thickness). As the hardness of the counter disk (alumina) is 1680 HV₁, which is higher than the hardness of Ni-P coated specimen (average microhardness value is 880 HV₁), the counter disk material undergoes negligible wear compared to the coated specimen. Normal load is applied by placing dead weights on loading pan. The depth of wear is measured by a LVDT (Syscon) which has a measurement range of ±2 mm with accuracy of 0.1 ± 1% μm. The frictional force is measured by a frictional force sensor (IPA) that uses a beam type load cell (range 0 to 100 N) with accuracy of 0.1 ±2% N. The experiments are carried out at sliding speed ranging from 0.084-0.126 m/s and load in the range of 10-50 N for 10 min duration considering the small coating thickness. Wear rate is evaluated by the following expression: wear rate = mass loss / (density × sliding distance). The mass loss due to sliding test is evaluated by weighing the pins before and after each test on a digital balance with accuracy up to 0.01 mg. The density of the EN coating is considered as 8.0 g/cc.

Coating characterization

The coating is characterised by its surface morphology, composition and phase structure. In the present study, surface morphology of the coating is observed by SEM (JEOL, JSM-6360) to analyze the microstructure of the deposited coatings before and after heat treatment at 400 °C. SEM is also done after tribological testing to see the wear track patterns in order to predict the wear mechanism. EDX analysis (Inca, Oxford) is done in conjunction with SEM to study the composition of the EN coatings in terms of the weight percentages of nickel and phosphorous. The crystalline phase structure is studied with the help of X-ray diffraction (XRD) analysis (Rigaku, Ultima III) so that the different precipitated phases both pre and post heat treatment can be identified.

Results and discussion

Wear behavior

It is observed that the wear rate increases with decreasing the sliding speed and gradually increases with the increase in the applied load up to 40 N beyond which the wear rate decreases with increasing in the applied load.

The wear rate of EN coating gradually increases with increase in load for both alkaline medium and dry condition but the same decreases after 40 N load in alkaline medium. Also it is observed from the plot that wear rate is higher in NaOH solution compared to dry condition. This is because alkaline environment produces corrosive effects at the sliding interface. In the present study in alkaline medium, the wear rate increases gradually with increasing with applied load but it decreases after applied load of 40 N at all sliding velocities. However, for dry condition, the wear rate shows a continuous increasing trend. This may partly be explained by the fact that the corrosion resistance in the case of a metal or an alloy depends on the presence of reaction layers at its surface. These layers may sometimes be only a few atoms thick, resulting from an interaction between the material and the surrounding environment. Such layers can be formed as oxides, solid precipitates, adsorbed layers, or passive surface films. Some of them like dense oxide layers, precipitates, or passive films play a protective role by isolating the underlying metal from a direct contact with a surrounding corrosive environment. Now, continuous breakdown is believed to occur due to the mechanical rubbing. But at a load of 40 N the surface layer exposed to the alkaline environment may be repassivating to form a stronger layer which may be contributing to lower wear rate.

Friction behavior

It is noticed that the friction coefficient decreases with increase in the applied load and then remained at nearly steady level at higher applied load. Also, it is found that the friction coefficient is in general lower at higher sliding speed.

The friction coefficient does not change with the sliding duration after the initial transient running-in time. It is clear that after two minutes, no significant change in friction coefficient is detected. Also the steady-state value of the friction coefficient is nearly the same for different sliding speeds.

The friction coefficient of EN coating decreases with increase in the load for both alkaline medium and dry condition but the friction coefficient in NaOH solution is lower than at the dry condition. This may be due to lubrication and cooling effect in alkaline environment.

In NaOH solution, the friction coefficient does not change significantly with sliding duration after the initial transient running-in time. It is clear that after two minutes, no remarkable change of friction coefficient is detected in NaOH solution. But the friction coefficient in dry condition gradually increases with increase in test duration. Also, the friction coefficient in dry condition is higher than in NaOH solution.

Wear mechanism

It is clear from the figure that the load is taken by all of nodules. The presence of longitudinal grooves along the sliding direction with high degree of plasticity can be clearly observed. This is indicative of the occurrence of micro-cutting and micro-ploughing effect and characterized as ductile failure. Almost no pits or prows are observed on the worn surface. Hence it can be concluded that the abrasive wear is the predominant wear mechanism.

Coating surface morphology and compositional study

Microstructure study

The micrographs of these surfaces exhibit many globular particles on the surface of the substrate. The surface is optically smooth and of low porosity. No obvious surface damage is found. The surface of the EN coatings appears to be dense. Also, by careful observation, it can be noted that the Ni-P nodules are quite deflated and flat in as-deposited condition but grow in size with heat treatment giving rise to coarse grained structure.

Compositional study

From EDX analysis, the average weight percentage of nickel and phosphorous is around 91.5% and 8.5%, respectively. With heat treatment and change in other process parameters, no significant change in nickel and phosphorous content is observed.

Phase structure study

From the figures, it is evident that in as-deposited condition the phase is mostly amorphous but turns crystalline with heat treatment. The major peaks of Ni₂P, Ni₃P and Ni₅P₂ are found in heat-treated specimen. Hence, the Ni-P coating changes with temperature to produce various nickel phosphide compounds (as observed in XRD plots) which may be impeding the movement of dislocations thereby contributing to the hardness of the coating.

Conclusions

Tribo-corrosion behavior of electroless Ni-P coatings in alkaline environment sliding against alumina is studied. The wear rate of Ni-P coating decreases slightly with sliding speed and increases with applied load up to 40 N. The wear rate gradually increases with increase in load for dry condition but decreases after 40 N in alkaline medium. Wear rate is lower in dry condition compared to NaOH solution because the alkaline medium has corrosive effects. The friction coefficient of electroless Ni-P coating decreases with increase in load under alkaline medium at applied loads of 10 N-40 N. The friction coefficient is also lower at higher sliding speeds. The friction coefficient decreases with increase in load for both NaOH solution and dry condition. However, friction coefficient in NaOH solution is lower than at dry condition because alkaline medium provides lubrication and cooling effect. Also, friction coefficient in dry condition gradually increases with increase in test duration. It is observed that the surface of the coatings is smooth, of low porosity and dense, with average nickel and phosphorous content of around 91.5% and 8.5%, respectively. The coatings are amorphous in as-deposited condition but after heat treatment, the coating turns to crystalline with different nickel phosphide compounds deposits.

References