The tribological characteristics of electroless NiP coatings

Wear 249 (2001) 389–396

The tribological characteristics of electroless NiP coatings R. Taheri, I.N.A. Oguocha, S. Yannacopoulos∗ Department of Mechanical Engineering, 57 Campus Drive, University of Saskatchewan, Saskatoon, Sask., Canada S7N 5A9 Received 18 July 2000; received in revised form 31 January 2001; accepted 22 February 2001

Abstract The utilization of electroless NiP (EN) coating process in various industries such as printing, textile, oil and gas, food, electronics, automotive, aerospace, chemical, and mining has witnessed a tremendous increase during the last few decades. The endearing characteristics of EN coatings include excellent corrosion and wear resistance, superior mechanical and electrical properties, uniform coating thickness, and outstanding surface finish properties. In the present work, samples of 1018 substrate having various levels of surface roughness were coated with low, medium, and high phosphorus EN coatings. The different substrate surface morphologies were obtained using different machining parameters. The effects of EN coatings on the tribological and morphological properties of the substrate material were investigated using surface roughness and hardness measurements, wear and coefficient of friction tests, optical microscopy (OM), and scanning electron microscopy (SEM). It was found that the surface roughness characteristics of substrates can be enhanced and their coefficients of friction (COFs) significantly altered by EN coatings. The results further showed that, unlike conventional electroplated deposits, EN coatings do not subvert the substrate surface shape and profile even for large thickness coats. It was also found that the wear resistance of EN coatings is significantly affected by heat treatment. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Electroless NiP coating; Coefficient of friction; Substrate; Tribology

1. Introduction Electroless NiP (EN) coating is the autocatalytic deposition of a NiP alloy from an aqueous solution unto a substrate without the application of electric current. The electroless bath typically comprises an aqueous solution of metal ions, complexing agent(s), reducing agent(s), and stabilizer(s) operating in a specific metal ion concentration, temperature, and pH ranges. It provides a deposit that follows all the contours of the substrate, without building up at the edges and corners. A sharp edge receives the same thickness of deposit as does a blind hole. Therefore, EN plating differs significantly from the conventional electroplating process that depends on an external source of direct current (dc) to reduce nickel ions in the electrolyte to nickel metal on the substrate. EN coatings have a wide range of industrial applications including valves for fluid handling, medical equipment, and hydraulic cylinders owing to their excellent mechanical, physical, electrical, and corrosion and wear resistance properties. Other outstanding characteristics of EN coatings ∗ Corresponding author. Tel.: +1-306-966-5446. E-mail address: [email protected] (S. Yannacopoulos).

include the ability to be applied to a variety of substrate materials and the ability to plate uniformly on intricate part geometries. The properties and microstructures of EN coatings depend on the amount of phosphorus (P) alloyed in the deposit [1–6] and post-deposition heat treatment [3–8]. The structure of the as-plated EN coatings has been reported to be either crystalline, amorphous, or a mixture of both [1,6,9–12]. In general, low phosphorus (1–5% P) EN deposits are microcrystalline, medium phosphorus (6–9% P) coatings have mixed crystalline and amorphous microstructures, whereas high phosphorus (10–13% P) EN alloys are amorphous. High phosphorus amorphous EN coatings have outstanding corrosion resistance, but their hardness and wear resistance are lower than their low phosphorus counterparts [13–16]. Remarkable improvement in the wear resistance of EN coatings has been reported when hard solid particles are incorporated [2,5,17–19]. Studies comparing the hardness and wear characteristics of EN, hard chromium, and nitriding coatings have been documented [18–21]. In these studies, well-known wear test methods including Taber Abraser, Falex Pin and Vee Block, and Crossed Cylinder were used. It was shown that by increasing the hardness of EN coatings with heat treatment,

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dry abrasive wear resistance (as measured by the Taber Abraser test) may be improved to or beyond the level offered by hard chromium. Another important aspect related to hardness and wear is the coefficient of friction. In a recent study, Hadley and Tulsi [22] used the Crossed Cylinder wear test to study the metal-to-metal contact between EN coating, EN–PTFE composite, and mild steel. The least amount of wear occurred in the EN–PTFE composite and this was attributed to the excellent self-lubricating properties of PTFE. Substrate surface morphology and its metallurgical condition can affect the quality of EN coatings. Substrates with passive spots that will not initiate EN coating result in non-uniform deposition and cause porosity. The porosity of EN coatings has been found to depend not only on their surface roughness but also on their surface morphology [23–25], which can be affected by the pre-coating conditions of the substrate. The effects of substrate surface roughness and coating thickness on the properties of EN coatings were studied by Ernst et al. [26]. It was found that the roughness of the coatings increased with the substrate surface roughness. There is scanty information in the open literature regarding systematic studies of the coefficient of friction (static and kinetic) of EN coatings. Secondly, it is often assumed that EN coating does not subvert the substrate surface morphology when compared with conventional electroplating. The question that arises is do EN deposits follow exactly the substrate surface profile or do they just seal off

surface asperities (the troughs and peaks) like in electroplating? Certain aspects of the tribological properties of EN coatings were investigated in the present work. These include the coefficient of friction and the effects of EN coatings on substrate surface profile. The effect of heat treatment on the coefficient of friction of EN specimens was also considered. 2. Materials The substrate material used in this study was commercially available 1018 carbon steel. Cylindrical specimens used for surface measurements were machined from 13 mm diameter steel bars using different feed rates in order to obtain samples with different surface roughness. Samples used for studying the coefficient of friction of EN coatings were cut from flat steel bars into rectangular coupons measuring 50 mm × 38 mm × 6.4 mm. They were subsequently sandblasted with size 7 (medium) glass beads. 3. Experimental procedure 3.1. EN coating All the samples used in the present study were subjected to the following pre-treatment and plating procedure, which is adapted from the pre-cleaning method suggested by Riedel [8]:

Fig. 1. EN plating apparatus.

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Table 1 EN bath compositions and conditions Particulars

Bath solution P (wt.%) pH Coating rate (␮m/h) Temperature (◦ C)

Type of EN coating Low phosphorus

Medium phosphorus

High phosphorus

NI-429 (200 ml/l) 4.5 6.2 16.6 85

EN-435A (60 ml/l), EN-435B (150 ml/l) 7.5 5.0 9.0 88

NI-425A (60 ml/l), E-425B (180 ml/l) 11.3 4.8 9.4 87

• Ultrasonic cleaning in industrial cleaning solution at 60◦ C for 30 min. • Rinsing by immersion in water at room temperature (RT) for 2 min. • Cleaning in 20 vol.% H2 SO4 at RT for 30 s. • Rinsing by immersion in de-ionized water at RT for 2 min. • Cleaning in 5 vol.% H2 SO4 at RT for 30 s. • Rinsing by immersion in de-ionized water at RT for 2 min. • Electrocleaning in 40 g/l potassium carbonate solution at RT for 30 s. The current density applied was 0.5 A/cm2 . In order to provide a better scrubbing action on the surface of the specimens during electrocleaning, periodic starts and stops were introduced (5 s on and 1 s off). • Rinsing by immersion in de-ionized water at RT for 30 s. • EN coating using an in-house designed computercontrolled automated bath (see Fig. 1) containing a

commercial Enthone-OMITM EN bath solution. The bath compositions and conditions are shown in Table 1. EN deposits of various compositions were prepared by varying the pH of the bath. A linear deposition rate was found for each type of coating under the present experimental conditions. The rectangular samples were coated with approximately 20 ␮m thick EN deposits. The cylindrical specimens were coated with various thicknesses of high phosphorus EN deposit. • Rinsing by immersion in water at RT for 2 min. 3.2. Heat treatment Heat treatment for EN specimens used for friction tests and surface roughness measurement was carried out in a cylindrical tube furnace. The specimens were wrapped with

Fig. 2. Schematic diagram of the friction test apparatus.

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aluminum foil to prevent phosphorus depletion [4] and surface oxidation and were subsequently heat treated at 400 ± 5◦ C for 1 h. 3.3. Hardness measurement The effect of aging time on the hardness of EN deposits was studied by heat treating samples at 300◦ C for various lengths of time. Hardness measurements were carried out using a Buehler Vickers Microhardness Tester (Micromet II) with a diamond indenter under 100 g load. The reported values represent the average of at least five microhardness readings. 3.4. Surface roughness measurements The surface roughness of all samples before and after EN coating was measured by means of a Mitutoyo 211 surface roughness tester. The standard roughness estimation parameters, namely Ra and Rz , were used to describe the surface roughness. All the reported data represent the average of at least 10 surface roughness measurements.

two ticker marks, therefore, represents the distance traveled by the specimen in 1/60 s. With this, the velocity as well as the acceleration of the specimen can be calculated. The relationship between kinetic COF (m) and acceleration (a) is given as µ = tan θ −

a g cos θ

(1)

where m is the kinetic COF, a the acceleration of the specimen down the ramp, g the gravitational acceleration constant (9.81 m s−2 ), and θ is the ramp angle. The friction experiment was conducted on surfaces with different Ra values. The surfaces used were 240 grit emery paper (R a = 13.157 ␮m), 600 grit emery paper (R a = 4.928 ␮m), and polyethylene plate (R a = 0.254 ␮m). All the data reported represent the average of at least two friction measurements. In order to compare the dry friction behavior of the coated specimen and the wet friction behavior of the bare substrate, two lubricating oils, namely, 3-in-1TM multipurpose oil and Leco® silicone mold-release lubricant were applied on the specimen surface before testing. Also, the effect of heat treatment on the COF of EN coatings was investigated.

3.5. Scanning electron microscopy (SEM) The phosphorus content of the three EN coatings was determined by the X-ray energy dispersive spectrometry (EDS) method in a JEOL JSM-5900LV SEM.

4. Results and discussion

3.6. Optical microscopy

The effect of electroless NiP coating on the substrate Ra parameter is shown in Fig. 3. The initial substrate Ra values ranged from 0.05 to 0.65 ␮m. In general, the roughness of

Two of the surface roughness samples with the highest Ra values were chosen for optical microscopy. They were cut in the axial direction to expose the surface contours, mounted in a plastic mold, polished following standard metallurgical procedure, etched in 2% nital solution, and dried before being examined in a Nikon optical microscope.

4.1. Surface roughness

3.7. Friction tests The coefficient of friction (COF) of EN coatings was measured using a simple adjustable ramp apparatus equipped with a tape-ticker device (Fig. 2). The ramp apparatus consisted of a U-shaped plastic channel mounted on the inclined wooden base of the ramp. The use of the U-channel was to ensure that the samples travel in a straight line without wandering on the ramp or even falling off the ramp before reaching the desired destination. The ticker device was used to measure the acceleration of the specimens as they traveled down the ramp. The working principle of the ticker is very simple. An electric spark capable of indenting a heat-sensitive ticker tape is generated between its tips at a rate of 60 sparks per second. The tape is attached to the specimen. As the specimen slides down the ramp, the tape is pulled through the energized ticker device which, in turn, indents it with a series of burn marks. The distance between

Fig. 3. Relationship between substrate and EN coating Ra parameter for 1018 steel.

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Fig. 4. The effect of high-phosphorus EN coating thickness on the surface morphology of the substrate.

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Fig. 6. The effect of phosphorus content of as-plated EN coatings on the coefficient of friction of various surfaces.

Fig. 5. Optical micrographs of 20 ␮m thick high phosphorus coating: (a) R a = 2.46 ␮m; (b) R a = 5.97 ␮m.

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EN coatings increased with the initial substrate roughness. A plot of R a (S) = R a (EN) (where Ra (S) refers to the Ra of the substrate and Ra (EN) refers to the Ra of the EN coating) cuts the experimental plot at a single point, thereby indicating the existence of a transition or critical substrate roughness for EN coating process. The transition surface roughness is approximately 0.19 ␮m for the 1018 steel used in the present study. Therefore, it can be seen from Fig. 3 that on very smooth substrate surfaces, the Ra values increase, whereas on rough surfaces the application of EN coatings tend to decrease the Ra values. Similar results were obtained for the Rz parameter. The results obtained in this study are in fair agreement with those published by Ernst et al. [26]. The authors have studied the effect of EN coating on substrate surface roughness and found that a transition or critical substrate Ra exists. The effect of substrate surface roughness and coating thickness on the surface roughness of EN coatings was also studied for substrate Ra values of 0.33–2.49 ␮m. Fig. 4 shows the results of this investigation. A close examination of the effect of EN coating thickness on the Ra parameter of EN coatings in Fig. 4 reveals no dramatic result. At a given substrate Ra , coating thickness does not affect the resulting EN Ra parameter substantially. The R a (S) = R a (EN) plot which passes through points ‘A’ and ‘B’ is essentially coincident with the straight line fitted to the data in Fig. 4. This indicates that the substrate Ra parameter remains practically unchanged irrespective of the EN coating thickness. The results in Fig. 4 suggest that EN coating does not necessarily seal off the substrate asperities. Rather, it follows the surface morphology of the substrate material. This feature is depicted more clearly in Fig. 5(a) and (b), which show the optical micrographs of samples of the 1018 steel substrate (R a = 2.47 and 5.97 ␮m, respectively) coated with 20 ␮m thick high phosphorus EN deposit. It can be seen from both figures that EN coating follows the profile of the substrate surface waviness rather than filling it up as usually obtained in conventional electroplating.

4.2. Coefficient of friction measurements The kinetic COFs of EN coatings and bare substrate were determined under different conditions. Fig. 6 shows the effect of as-plated EN coatings on the COF of substrate surfaces. It can be seen that EN coatings generally reduce the kinetic COF of the bare substrate. As expected, the COF of EN coatings increases with increasing Ra parameter of the test surface. For example, the COF of as-plated high phosphorus EN coating is 0.764 for R a = 0.254 ␮m, whereas it is 0.828 for R a = 13.157 ␮m. A similar trend is obtained for other classes of EN coatings. The effect of heat treatment on the COF of EN coatings was also investigated. The results were similar to those of as-plated EN coatings. A summary of the results obtained from the friction experiment is shown in Table 2. Some other interesting results can be deduced from Fig. 6 and Table 2. At low ramp surface Ra , the COF of EN coatings tends to decrease with decreasing phosphorus content, whereas at high Ra , the COF seems to be independent of phosphorus content. Heat treatment is known to increase the hardness of EN coatings very substantially [3–8] but the results shown in Table 2 indicate that it does not affect their kinetic COF significantly. The COFs of as-plated and heat treated samples are practically identical. This unique combination of high hardness and low COF may partly explain the high wear resistance reported for EN coatings [18–21]. Fig. 7 shows a comparison of percentage reduction in COFs due to EN coatings and lubrication. Although lubrication inhibits the extent of wear of sliding surfaces in most cases, it does not necessarily reduce the COF of the substrate. This may be as a result of the lubricant creating an adhesive bonding between two sliding surfaces. This will result in an increase in the kinetic COF of the two sliding surfaces as depicted in Fig. 7. On the other hand, EN coatings can increase the wear resistance but reduce the COF at the same time. It can also be seen that the effectiveness of EN coating in reducing the COF of the substrate is more significant than that of lubrication. This suggests that coating

Table 2 Coefficients of friction obtained for EN coatings and bare substrate Material

As-plated high phosphorus EN coating Heat treated high phosphorus EN coating As-plated medium phosphorus EN coating Heat treated medium phosphorus EN coating As-plated low phosphorus EN coating Heat treated low phosphorus EN coating Bare substrate Bare substrate and oil (wet) Bare substrate and silicone (wet) a

Unless otherwise stated, value is for dry kinetic friction.

Coefficient of frictiona Ra = 13.157 ␮m

Ra = 4.928 ␮m

Ra = 0.254 ␮m

0.828 0.807 0.843 0.840 0.829 0.810 0.930 0.865 0.871

0.800 0.812 0.783 0.807 0.786 0.774 0.846 0.844 0.889

0.764 0.768 0.725 0.728 0.723 0.744 0.776 0.798 0.741

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Fig. 7. The effect of EN coatings on reduction of the coefficient of friction of various surfaces.

a steel substrate with a EN deposit may give better friction performance than lubrication. 4.3. Hardness Fig. 8 shows the effect of aging time at 300◦ C on the hardness of EN coatings. The figure exhibits the typical bell-shaped characteristics of an age-hardening material. As

expected, the hardness of the coatings increased with heat treatment. The hardness increased with aging time to a maximum and then decreased with further increase in aging time. The increase in hardness has been attributed to fine Ni crystallites and hard intermetallic Ni3 P particles precipitated during the crystallization of the amorphous phase [4,7–9,27–29]. The Ni3 P phase has a bct structure with a = 0.835 nm, and c = 0.439 nm [8]. However, Hur et al. [28] have reported that the crystallization process of amorphous NiP solution involved more than one intermediate phase. Fig. 8 also shows that low phosphorus coating reached its maximum hardness in a shorter aging time (∼6 h) than the high phosphorus coating (∼9 h). The degree of hardening depends on the amount of phosphorus, aging temperature, and aging time. Baudrand and Bengston [4] have reported that low phosphorus EN deposits require about 350◦ C to attain optimum hardness, whereas high phosphorus deposits require 375◦ C. The hardness results shown in Fig. 8 are consistent with their results.

5. Conclusions

Fig. 8. The effect of phosphorus content and aging time at 300◦ C on the hardness of EN deposits.

1. EN coating follows the substrate surface profile rather than just fill the spaces between surface asperities. This is in contrast to what one obtains in conventional electroplating techniques. 2. A transition substrate roughness exists for EN coating. However, at a given substrate Ra value, the Ra of EN coatings is independent of coating thickness.

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3. The Ra parameters of the substrate and the EN coating are practically identical. 4. EN coatings lower the kinetic coefficient of friction of their substrate, irrespective of their heat treatment condition. 5. The degree of hardening of EN deposits at a given temperature depends on phosphorus content and aging time. Acknowledgements The authors would like to thank the Potash Corporation of Saskatchewan (PCS) and the Natural Science and Engineering Research Council (NSERC) for their financial assistance. Also, we would like to thank Dr. M.C. Charturvedi and his staff at the Metallurgy Laboratories, University of Manitoba, for allowing us to access their facilities. References [1] M. Allen, J.B. VanderSande, Scripta Metall. 16 (1982) 1161–1164. [2] O. Berkh, S. Eskin, J. Zahavi, Metal Finish. 94 (1996) 35–40. [3] R. Parkinson, Nickel Development Institute Technical Series no. 10081, 1995, pp. 1–37. [4] D. Baudrand, J. Bengston, Metal Finish. 93 (1995) 55–57. [5] D. Barker, Trans. Instit. Metal Finish. 71 (1993) 121–125.

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