Effect of heat treatment on cobalt and nickel electroplatedsurfaces with Cr2O3 dispersions

K. Kumara,*, R. Chandramohana, D. Kalyanaramanb

aDepartment of Physics, Sree Sevugan Annamalai College, Devakottai 630302, IndiabDepartment of Chemistry, Sundarsan Engineering College, Pudukottai 622501, India

Received 27 April 2003; received in revised form 4 November 2003; accepted 12 December 2003

Dedicated to Late Prof. M. Kandasamy

Abstract

The growing interest in the electrochemically deposited composite coatings is owing to the flexibility of the deposition

process and due to the increasing demand for the highly wear resistant composite coatings. In the electrodeposited coatings a

second phase is incorporated in the coating matrix like fine tough particles to improve the surface properties. In this work, the

surface mechanical behavior of cobalt and nickel coatings with dispersion of chromium oxide in the matrix is reported. The

effect of heat treatment of these coatings on the surface mechanical properties is also studied. The surface microhardness, wear

loss due to rubbing abrasion are studied before and after heat treatment in the range of 100–600 8C and are reported for both the

systems. The composite containing 12 vol.% of chromium oxide (Cr2O3) in a Co matrix and heat treated at 500–600 8C were

found to exhibit high abrasive resistance. This is attributed to the formation of glass like layer found in the cobalt matrix. The

coatings are found to be highly adherent and uniform and are found to be suitable for high temperature applications.

# 2004 Elsevier B.V. All rights reserved.

PACS: 82.65Dp; 81.15Pq

Keywords: Electrodeposition; Wear index; Abrasion; Microhardness; Tribological systems; High temperature coatings; Dispersive coatings;

Wear resistive coatings

1. Introduction

Machine part surfaces, which are subjected to wear

and high temperature under stress are usually provided

with a coating [1]. This surface build up should have

resistance to wear at elevated temperatures. Nickel and

cobalt coatings are extensively used in engineering

applications to combat tough rubbing conditions [2–

4]. Electrodeposition is a low cost versatile method

used for coating over conducting materials [5]. Cobalt

matrix with dispersion of tough fine particles is very

much desirable for high temperature applications.

Cr2O3 particles were incorporated in nickel and cobalt

matrix to observe the surface behavior towards abra-

sive wear. The coatings were subjected to post heat

treatment and were tested for microhardness (HV) and

abrasive wear resistance. Ni-base depositions were

Applied Surface Science 227 (2004) 383–386

* Corresponding author. Present address: No. 15, Sivan Kovil

North St, Devakottai 630302, India. Tel.: þ91-4561-272251;

fax: þ91-4561-272251.

E-mail address: secltkumar@military.com (K. Kumar).

0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.apsusc.2003.12.016

made from a watts bath (chloride) and for Co-base

coating a sulphate bath was employed. The coating

specimens were subjected to heat treatment for 1 h

from 100 to 600 8C to observe the surface changes.

They were then treated for surface microhardness and

abrasive wear resistance using a precision built test

instrument Taber Abraser (model 174) as reported

earlier [6].

2. Experimental

The dispersion coatings were deposited onto mild

steel panels (15 cm � 15 cm) from suspensions com-

posed electrolytes containing 50–200 g/l of fine tough

Cr2O3 particles. The coating thickness was kept at

30 mm. Optimum deposition conditions like pH, cur-

rent density, temperature, concentration, etc., for good

coatings were arrived by performing many trials.

Abrasive weave tests were done using Taber Abraser.

This reveals abrasion resistance at all angles relative to

the wear or grain of the material. The wear rate was

established by a relative term typical of the testing

instrument called ‘Taber Wear Index’. This is the loss

in milligram per thousand cycles of rotation for a test

performed under specific set of conditions. For under-

stating the surface morphology of the coatings SEM

studies were carried out using scanning electron

microscope (JEOL JSM 35). Topography of rough

and smooth Co and Ni coated surfaces were also

studies using a topographic profilometer.

3. Results and discussion

A 12 vol.% inclusion of Cr2O3 in both the Co and Ni

matrix were achieved, which were then heat treated at

various temperature for 1 h. Fig. 1 shows the variation

of microhardness with temperature before and after

abrasion for Ni–Cr2O3 surfaces. With increase in

temperature, an increase in surface microhardness

was observed in the case of nickel deposition. The

values are found to be less for the abraded surfaces. At

200 8C the increase in hardness value in abraded Ni

surface was found to be higher than expected value.

Fig. 2 shows the variation of Taber Wear Index with

temperature for Ni–Cr2O3 surfaces. It could be seen

that the surface could be abraded well under 250 8C as

the resistance offered was less. Above 250 8C the wear

rate is reduced to a certain extent only.

Fig. 3 shows the variation in surface microhardness

before and after abrasion for Co–Cr2O3 surfaces. It is

observed that the rise in the hardness value with

Fig. 1. Variation of microhardness in Vickers scale with tempera-

ture before and after abrasion for Ni–Cr2O3.

Fig. 2. Variation of Taber Wear Index with temperature for Ni–

Cr2O3.

Fig. 3. Variation of microhardness in Vickers scale with tempera-

ture before and after abrasion for Co–Cr2O3.

384 K. Kumar et al. / Applied Surface Science 227 (2004) 383–386

temperature is more when compared to the nickel

deposit. Also no anomalous behavior around 200 8Cis observed for cobalt surfaces. Fig. 4 show the

variation of Taber Wear Index with temperature for

Co–Cr2O3 surfaces. It is observed that under 250 8Cthe surface could be abraded well as resistance offered

was less even with higher surface microhardness

compared to nickel based deposition. Above 250 8Cthe wear rate is reduced sharply. Also after 250 8Cheating a glass layer was formed on the Co–Cr2O3

surface which was not observed in the Ni–Cr2O3

surfaces. From 450 to 600 8C the surface was found

to be shining after abrasion. The glassy layer that is

observed could be because of an oxide formation [4].

This oxide built up on the surface offers higher hard-

ness and abrasive wear resistance. For cobalt deposits

the Taber Wear Index was found to reduce consider-

ably as there is very little wear loss. The surface build

up after heat treatment offers wear resistance and

withstands higher stress.

Fig. 5 shows the SEM micrograph of Co–Cr2O3

unabraded surface after heat treatment around 200 8C.

It could be seen that the coating is uniform with

dispersed Cr2O3 particles. The SEM micrograph of

Co-coated abraded surface is shown in Fig. 6. The

peaks and valleys are found to be present. This is due

to the abrasive wear found on the Co-coated surface.

These studies indicate that Co–Cr2O3 surfaces stand as

a good choice in the high temperature applications.

Topographic studies on the coated surfaces revealed

that abraded surfaces of Co and Ni are difficult to

follow than the smooth surface. The difference in the

real and traced topographies are greater in case of

abraded surfaces than the smooth ones.

4. Conclusion

Co–Cr2O3 and Ni–Cr2O3 coatings were performed

on to mild steel panels successfully. Effect of heat

treatment on these coating were studied. The surface

topography and morphology of the coatings were

also studied and reported. Heat treatment has very

high influence on surface properties of Co–Cr2O3

deposits compared to Ni–Cr2O3 deposits. Initially for

Ni-coatings the wear rate is lower compared to Co-

deposits. The Co-coatings with 12 vol.% dispersion

of Cr2O3 in the matrix heated up to 250 8C resulted in

higher wear due to abrasion and after 250 8C the

wear index was found to decrease sharply. This is

attributed to the glass like oxide surface layer of the

Fig. 4. Variation of Taber Wear Index with temperature for Co–

Cr2O3.

Fig. 5. A typical SEM micrograph of unabraded Co–Cr2O3

coatings after heat treatment around 200 8C.

Fig. 6. A typical SEM micrograph of Co-coated abraded surface.

K. Kumar et al. / Applied Surface Science 227 (2004) 383–386 385

cobalt coating which was not observed in the case of

nickel deposits. Hence, cobalt coatings with suitable

dispersion stand as a good choice in high temperature

applications. It is worth evaluating these coated

surfaces in different type of wear to understand

the wear mechanism of the glass like layer. Efforts

are underway to estimate the thickness of the glass

like layer by spectroscopic methods. Further studies

on the depth profile of the glassy layer and to

evaluate this surface against different contacting

surfaces are in progress.

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386 K. Kumar et al. / Applied Surface Science 227 (2004) 383–386