Advances in Materials Physics and Chemistry, 2012, 2, 68-70
doi:10.4236/ampc.2012.24B019 Published Online December 2012 (http://www.SciRP.org/journal/ampc)
Abrasive Wear Behavior of Different Thermal Spray
Coatings and Hard Chromium Electroplating
on A286 Super Alloy*
Macid Nurbaş, Elif Nazik Atabay Durul
1ESOGU, Chemical Eng. Dept. Eskisehir, Turkey
2Air Supply and Maintenance Center Command Eskisehir, Turkey
Received 2012
ABSTRACT
In cases of decorative and functional applications, chromium results in protection against wear and corrosion combined with chemi-
cal resistance and good lubricity. However, pressure to identify alternatives or to improve conventional chromium electroplating
mechanical characteristics has increased in recent years, related to the reduction in the fatigue strength of the base material and to
environmental requirements (1). In the present study plasma sprayed coatings (aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide,
chrome carbide) and electrolytic hard chrome coatings abrasive wear properties have been compared. The wear tests were conducted
with a Taber abraser, at room temperature.
Keywords: Thermal Spray; Abrasive Wear; Electrodeposited Hard Chrome; Hardness; SEM
1. Introduction
Chromium has been widely used in surface finishing of metals
because of the favorable properties it imparts to substrates and
because the processes used are relatively mature, well under-
stood, widely specified, and cost effective (2). This coating is
produced from a wet chemical bath containing hexavalent
chromium ions (Cr+6). In all environmental regulations, Cr+6 is
classified as a confirmed human carcinogen. Hard chromium
plating produces large volumes of chromium containing toxic
waste, air pollution and water contamination (3). Potential
process substitutions for hard chromium plating are electroless
nickel in certain applications, several nickel-tungsten composite
plating, and spray applications such as plasma spray coatings
(2). Plasma spraying is a process widely used in industry for
depositing protective and functional coatings for a large variety
of applications. Industrial sectors such as aerospace, automotive,
energy, mining, biomedical, etc. take advantage of the unique
properties of the sprayed coatings (4). The applications of
thermal spray coatings are extremely varied, but the largest
categories of use are to enhance the wear and/or corrosion re-
sistance of a surface (2). The deposition methods for the wear
protective coatings are atmospheric plasma spraying (APS) and
high velocity oxygen fuel (HVOF) flame spray processes. Both
of these methods have their own characteristics, e.g. different
spray particle velocities and flame temperatures. As a result, the
coatings have different microstructures and properties (5).
2. Experimental Coatings
As aforementioned, electroplated chrome and four kinds of
plasma spray were involved in the study. Their characteristics
are listed in Table 1. The A286 super alloy was used as sub-
strate materials for all five coatings. The thicknesses of coating
layers were controlled in the range of 100-150 µm. The sub-
strates were sand blasted prior to spraying using 36 grit alumina
sand. Sulzer Metco 9MB plasma gun and GH / 732 nozzles
were used. The spraying parameters were given in Table 2.
Table 1. The characteristics of the present coating materials.
Designation Composition (wt.%) Powder size (µm)
Aluminum oxideAl2O3 %3 TiO2 -45 +11
T400 Co-28Mo-8Cr-2Si -45 +15
Tungsten carbideWC %12Co -45 +15
Chrome carbide%75 Cr3C2 %20 Ni %5 chromium -45 +5
Table 2. Spray parameters.
Coating Materials Aluminum
oxide T400 Tungsten carbide
Chrome
carbide
Plasma gases Ar + H2 Ar + H2 Ar + H2 Ar + H2
Plasma gases flow
rates (scfh) 100 - 15120 – 12,5 110 – 12,590 - 10
Plasma gases
pressure (psig) 100 - 5090 - 50 100 - 50 100 - 50
Current (amper) 500 500 400 500
Spray distance (inch)4,5 4,5 4,5 4,5
Traverse speed %90 - 2%80 %95 %100
Powder feeder carrier
gas pressure (psig) 50 50 50 50
Powder feeder carrier
gas flow (scfh) 13,5 13 13,5 15
Feed rate (g/min) 50 45 30 50
Air jet - Paralle,
50 psi
Parallel,
60 psi
Parallel, 50
psi
*Acknowledgment, Thank you 1. Air Supply and Maintenance Center Command
Eskisehir, Turkey for the A286 super alloy materials supplied and analysis facili-
ties.

M. NURBAŞ ET AL. 69
Electroplated chrome coatings were produced in an industrial
plant, using the industrial deposition parameters listed in Table
3. The de-hydrogenation thermal treatment (2000C for 3 h) has
also been performed on the coating.
2.1. Characterization
Roughness was measured with Diavite DH-5, also hardness off
each coating were measured by the Vickers microhardness
tester (Wilson/Tucon) and given in Table 4.
The thicknesses of the coatings were determined by micro
hardness tester (Wilson/Tucon). Scanning electron microscopy
technique (SEM) was used to observe two different parts of the
test coupons which performed abrasive wear test (right) and
which didn’t (left).
2.2. Abrasive Wear Tests
For abrasive wear tests, samples were prepared from A286 with
4 mm thickness and 100 mm square, according to FED-STD-
141C. AMS 5525 (A286 plate form), electrolytic hard chrome
and aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide, chrome
carbide plasma spray coated test panels were subjected to abra-
sive wear test. The wear tests were conducted with a Taber
abraser, at room temperature, using a 1000 g load and CS-17
abrading wheel. The results were analyzed by wear index
(mg/1000 cycles) and total wear (mg/10000 cycles) data. Cy-
cles to mg/1000 weight loss is shown in Table 5 and Figure 1,
cycles to total mg weight loss is given in Table 6 and Figure 2.
3. Results and Comments
In this study, heat treated electrolytic hard chrome and alumi-
num oxide, Co-28Mo-8Cr-2Si, tungsten carbide and chrome
carbide plasma spray coated test coupons were characterized
and abrasive wear behaviors were evaluated.
Table 3. Hard chrome deposition parameters.
Bath composition CrO3 250 g/l; H2SO4 2.5 g/l;
no additives
Bath temperature (0C) 52-57
Voltage (V) 2.5-3
Approximate current density (A/dm2) ≈ 40
Bath stirring method Pneumatic stirring
Table 4. Coating roughness and Vickers microhardness of coatings.
Coating
Roughness;
Ra (µm)
(lt = 4.8;
lc = 0.8)*
Standard
deviation
Vickers
microhardness
(HV 0.1)
Standard
deviation
Electrolytic
hard chrome 144,2 16,87 662,27 4,56
Aluminum
oxide 237,09 22,36 695,34 167,2
Co-28Mo-8Cr-2Si 248 29,05 484,93 45,23
Tungsten carbide 284,05 23,1 932,98 314,6
Chrome carbide 205,05 14,63 772,92 79,45
Table 5. Abrasive wears weight loss (Cycles – weight loss, mg/1000).
mg/1000
Cycles
A286 K1 191 361 371 381
0 0 0 0 0 0 0
10000,026650,009580,0946 0,1405 0,229070,1655
20000,00910,006470,05117 0,072 0,073570,0677
30000,00540,005980,03313 0,0513 0,066270,0594
40000,017050,00570,0317 0,0332 0,059970,0236
50000,009950,00410,0281 0,0308 0,058270,0185
60000,004750,003250,05883 0,05155 0,062230,0485
70000,01670,00560,05597 0,0469 0,0543 0,02395
80000,006850,003550,0508 0,0455 0,0442 0,0124
90000,007150,004350,04853 0,0351 0,033630,0255
100000,013850,004450,04703 0,04295 0,023330,0217
Figure 1. Abrasive wear weight loss vs. number of cycles.
Table 6. Abrasive wears weight loss (Cycles – Total weight loss, mg).
Total mg
Cycles
A286 K1 191 361 371 381
0 0 0 0 0 0 0
10000,026650,009580,0946 0,1405 0,229070,1655
2000 0,035750,016050,14577 0,2125 0,30263 0,2332
30000,041150,022030,1789 0,2638 0,3689 0,2926
40000,05820,027730,2106 0,297 0,428870,3162
50000,068150,031830,2387 0,3278 0,487130,3347
60000,07290,03508 0,29753 0,37935 0,549370,3832
70000,08960,04068 0,3535 0,42625 0,603670,40715
8000 0,096450,04422 0,4043 0,47175 0,647870,41955
90000,10360,04858 0,45283 0,50685 0,68150,44505
100000,117450,053020,49987 0,5498 0,704830,46675

M. NURBAŞ ET AL.
70
weight loss are represented in Table 5, Table 6 and Figure 1,
Figure 2. The hard chromium plating on A286 substrate mate-
rial shows better wear resistance properties. Hard chromium
coating reduces the abrasion rate of the substrate material.An
initially higher wear weight loss for the plasma spray coatings
occurred, decreasing continuously and then nearly stabilized.
However, the stabilized abrasive wear rates were still higher
than the hard chrome coatings.
The initial peak which is typical for plasma spray coatings
(Figure 1) was due to the higher surface roughness. Table 4
figures out that the surface roughness values of all other coating
materials are higher than those of electrolytic hard chrome.
Plasma sprayed materials show rough surface properties, in-
volving many pores, oxides and inclusions.
Figure 2. Total weight loss vs. number of cycles: K : Electrolytic
hard chrome, 19 : Aluminum oxide, 36 : Co-28Mo-8Cr-2Si, 37 :
Tungsten carbide, 38 : Chrome carbide. It is possible to compare the tested coating materials with
electrolytic hard chrome coatings on the SEM micrographs
which were given in Figure 3. It can clearly be observed that
the electrolytic hard chrome coatings show dense and smooth
surface properties. On the other hand, the plasma spray coatings
have porous coating structure.
Experimental data from abrasive wear tests were conclusive,
indicating better results from the hard chrome coating. The abra-
sive wear resistance of plasma spray coatings and hard chro-
mium plating was evaluated and the results in terms of wear
Figure 3. SEM micrographs of electrolytic hard chrome and plasma spray coatings (for each coating, left side picture non-weared, right side
picture weared (From left to right electrolytic hard chrome, aluminum oxide, Co-28Mo-8Cr-2Si, tungsten carbide, chrome carbide) (250
X)).
In terms of hardness values, as it can be seen on Table 4, in
comparison with the electrolytic hard chrome coated test cou-
pons, similar or higher hardness values were reached by plasma
spray coated test coupons.
As it can be seen in Table 5, Table 6 and Figure 1, Figure 2,
aluminum oxide coatings show better abrasive wear resistance
among all plasma spray coupons. This is due to high oxide
content of the coating material. Coating of high oxide content is
usually harder and is more wear resistant [6].
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