Surface & Coatings Technology 200 (2005) 1173 – 1177

www.elsevier.com/locate/surfcoat

Wear behavior of flame-sprayed Al2O3 –TiO2 coatings on

plain carbon steel substrates
I.M. Kusoglua,*, E. Celika,c, H. Cetinelb, I. Ozdemir a,d, O. Demirkurtc, K. Onela
a
Dokuz Eylul University, Faculty of Engineering, Department of Metallurgical and Materials Engineering, Bornova, Izmir 35100, Turkey
b
Celal Bayar University, Faculty of Engineering, Department of Mechanical Engineering, Manisa 45140, Turkey
c
KROMA, Inc., AOSB, 10036 Sk., No:8, Cigli, Izmir 35620, Turkey
d
Material Processing Lab, Toyota Technological Institute, 2-12-1, Hisakata Tempaku, Nagoya 468-8511, Japan
Available online 12 May 2005

Abstract

In this study, Al2O3 – TiO2 powders were sprayed using a flame-spray technique after a NiCrMo bond layer was deposited on plain carbon
steel substrate. The produced layers were characterized by X-ray diffraction (XRD), optical microscope, scanning electron microscope (SEM)
including energy-dispersive spectroscopy (EDS), surface roughness and microhardness tester. Friction and wear behaviour of the coatings
were also evaluated in the present study. The Al2O3 – TiO2 coatings were subjected to sliding wear against AISI 303 stainless steel counter
body under dry and acidic environments. A pin-on-plate type apparatus was used with normal loads in the range of 49 – 129 N. Wear
resistance of the coatings in acid environment is better than that in dry conditions.
D 2005 Elsevier B.V. All rights reserved.

Keywords: Al2O3 – TiO2; Flame spray; Friction coefficient; Wear; Corrosion

1. Introduction Commercially used alumina (corrundum) particles are in

stable a form, but after thermal spraying, they soften and
Increasing demand for engineering products to work in transform into the g form because of the lower nucleation
severe corrosive and tribological environments calls for apt energy [4]. This phase transformation is occasionally seen
surface design. Surface design is usually concerned with as a problem in flame spraying according to its lower
surface texture and surface chemistry to counter possible process temperature. In addition, fracture toughness is an
wear modes. While surface texture is achieved by mechan- important phenomena affecting tribological properties and
ical treatment, chemistry is usually controlled by surface this is allowed by the TiO2 additive to improve the wear
modification in the form of coating/deposition. Although resistance [5,6]. The present study focuses on wear
there are different techniques available for deposition of behaviour of flame-sprayed Al2O3 –TiO2/NiCrMo coatings
materials on suitable substrates, thermal spraying process is on plain carbon steel substrate used in mechanical parts for
being widely used to deposit thick coating for various its resistance to chemical and abrasive wear characteristics.
industrial applications [1]. Alumina and its modifications
with titania coatings are conventionally used hard coatings
for its resistance to chemical and abrasive wear character- 2. Experimental
istics [2]. Commonly used thermal spraying processes are
flame and plasma spraying for Al2O3 –TiO2 coatings [3]. Prior to the coating process, the plain carbon steel
substrates were cleaned, degreased and grit-blasted by using
* Corresponding author. Tel.: +90 232 3882880; fax: +90 232 3887864.
35-grit alumina abrasives. A flame-spray system with Metco
E-mail address: murat.kusoglu@deu.edu.tr (I.M. Kusoglu). 442 gun model was used to deposit NiCrMo and Al2O3 –
URL: http://www.deu.edu.tr (I.M. Kusoglu). TiO2 coatings on the substrate. Notably, NiCrMo coatings
0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2005.02.219
1174 I.M. Kusoglu et al. / Surface & Coatings Technology 200 (2005) 1173 – 1177

Table 1 Al2TiO5 and TiO2. Conversion of a-Al2O3 powders to

Flame-spraying parameters soften the g-Al2O3 phase frequently presented problems
Spray Parameters Values after thermal spray processes [7]. XRD patterns suggest that
NiCrMo Al2O3 – TiO2 a-Al2O3 particles were not completely transferred to soften
Powder feeding rate (lb/min) 4 4 g-Al2O3 phase after the flame-spray process. This is a good
Spraying distance (mm) 150 120 – 150 result for tribological behaviour where the hardness plays an
Oxygen pressure (bar) 0.4 0.4 important role in wear resistance.
Fuel (acetylene) pressure (bar) 0.7 0.7
Fig. 2 demonstrates surface morphology of Al2O3 – TiO2
Dry air pressure (bar) 3 3
Powder size (Am) 65 – 125 45 – 60 coating on NiCrMo/plain carbon steel substrate. Roughly
disc-shaped grains were observed from SEM studies. These
grains were formed by the molten droplets of coating
were used to improve the mechanical adhesion of the material, each grain corresponding to a flattened solidified
Al2O3 –TiO2 layer. Flame-spray parameters for NiCrMo and droplet. Flattening is caused by the striking of molten
Al2O3 –TiO2 coatings are listed in Table 1. droplets on the substrate. Residual stress by thermal shock
The crystal structure of the coating was analyzed using during the spray process resulted in transgranular micro-
Rigaku RTP 300 X-ray diffraction (XRD) system with cracks in the grain (see Fig. 2a). This is an extensive event
CoKa radiation (k = 1.79 Å). Surface morphology and cross for alumina titania coatings [8]. On the other hand, it has
section of the coating were examined using a scanning been reported that the fracture toughness of the ceramics can
electron microscope (SEM) (JEOL JSM 6060) with an be increased by crack deflections through a toughness
attached energy-dispersive spectroscopy (EDS) system. mechanism [9].
Thickness and porosity volume content of the coatings are Three layers such as Al2O3 – TiO2 top layer, NiCrMo
determined by an image analyzer (LUCIA 4.21). Addition- bond layer and plain carbon steel substrate is represented in
ally, surface roughness measurements were taken by using Fig. 2b. The dark gray structure is Al2O3 where light gray
Mitutoyo SJ-301 surface roughness measurement tester. The TiO2 structure is in corrugated form in the top layer. Image
coating microhardness as from surface to substrate was analysis indicated that the average thicknesses of Al2O3 –
measured by using a standard microhardness tester with TiO2 top and NiCrMo bond layers were determined to be 235
Vickers indenter.
Dry and acidic sliding tests were carried out using PLINT
TE88 multistation friction and wear test machine at room
(a)
temperature. Wear tests were performed in a pin-on-plate
configuration. The dimension of the coated samples used in
the test was 13 mm  13 mm  3 mm and a cylindrical
stainless steel counterpart was used with a diameter of 5
mm. The pin was fixed where plate stroke was 12 mm,
corresponding to a linear speed of 1 Hz. Several loads were
applied between 49 and 129 N in dry and acidic conditions. splat
After the wear test, the surfaces of worn coatings were
examined by SEM for each sample.

3. Results and discussion

(b)
The XRD pattern of the Al2O3 –TiO2 coated sample is
given in Fig. 1. The formed phases are a-Al2O3, g-Al2O3,

Fig. 1. XRD pattern of Al2O3 – TiO2 coating on NiCrMo deposited plain Fig. 2. Surface morphological and cross-sectional SEM micrographs of
carbon steel substrate by flame technique. Al2O3 – TiO2 coatings. The scale bars are (a) 10 Am and (b) 100 Am.
 I.M. Kusoglu et al. / Surface & Coatings Technology 200 (2005) 1173 – 1177 1175

Am and 50 Am and the amount of the porosity in the structure (a) Dry
was found to be around 16% after several measurements.

Wear Loss (micron)

400
The microhardness change from surface to center in the 49N
cross section is determined by several measurements. The 300
69N
microhardness value of the top layer was determined as
200 89N
1000 HV at applied load of 80 g, but there was a decrease
down to 765 HV according to the presence of porosities. The 100 129N
microhardness of the bond layer was around 250 HV but
0
unmolten droplets in spherical form in the structure changed 0 500 1000 1500 2000
the hardness value in different regions. Duration (Sec)
Microstructure, microhardness, surface roughness, tough-
ness, friction coefficient and environmental conditions are (b) Acidic
400
important parameters for wear resistance of thermally
sprayed oxide coatings. Assessments of the friction coef- 300

Wear Loss
49N

(micron)
ficient of coating against the stainless steel counterpart at
200 69N
several loads in the range of 49 –129 N under dry and acidic
89N
conditions are given in Fig. 3. As seen in this figure, there is 100
a very short period of an increase in friction coefficient 129N
0
related to a Frunning-in_ step. This situation is the same for 0 1000 2000
all applied loads and environments. A general consideration Duration (sec)
for the influence of the environment is the higher friction
coefficient observed at dry conditions than that at an acidic Fig. 4. Wear loss vs. duration curves of Al2O3 – TiO2/NiCrMo/steel sample
for 49 N, 69 N, 89 N and 129 N loads under (a) dry and (b) acidic
environment. This is an expected finding caused by wear
conditions.
debris. The surface roughness (R a) results also supports this
observation. The surface roughness decreased from 9.3 Am
down to 2.56 Am under dry conditions and 5.4 Am under where [10]. The mechanism is not the same for acidic
acidic environment under a 129 N load. A decrease in the conditions because the wear products were removed by acid
roughness is related to the increase in friction coefficient. attack. The friction coefficient dramatically increases during
The obtained results suggest that there is a decrease down to the Frunning in_ step followed by a stable value at 0.3 for the
0.3 for an applied load of 49 N, and 0.4 for other applied load of 49 N and 0.4 for the applied loads of 69 N and 89 N
loads at dry test condition in the stabilization step of friction to the end of the stabilization step for acidic environment.
coefficient. Wear products behave as a solid lubricant in wear Fig. 4 shows wear loss values during sliding time for all
mechanism for dry test condition that decrease the friction applied loads in dry and acidic environments. Wear loss
coefficient. The effect of solid lubricants is detailed else- increased by the increase in time as an expected result. The
loss in dry condition is higher than that in an acidic
(a) environment. This is a reason of wear debris effect as
Dry
mentioned above. Weight loss during sliding time is not
Friction Coefficient

1
49 N notable for the applied load of 49 N in both environments.
0,8
69 N No difference was obtained in weight loss between the
0,6
89 N
applied loads of 49 N and 69 N while a noteworthy increase
0,4
in weight loss is observed at 129 N for each environment.
0,2 129 N
The worn surfaces of the coating after dry and acidic
0
0 500 1000 1500 2000 wear tests were observed by SEM (see Figs. 5 and 6).
Duration (sec) The adhesion of the wear products was obtained for worn
(b) surfaces after dry test condition as shown in Fig. 5. Wear
Acidic products increase on the worn surface by the increase in
0,8
Friction Coefficient

the applied load. The wear debris was generated during

0,6 49 N
sliding of the coating against the counterpart. The pits
69 N
0,4 caused by wear is filled with wear products. The friction
89 N track on the coating is rough. The wear products were
0,2 129 N removed by acid attack from the worn surfaces as seen
0 from Fig. 6. Adhesion of fractured particles from the
0 500 1000 1500 2000 grains has been observed on the worn surface. As
Duration (sec) mentioned previously, transgranular microcracks occurred
Fig. 3. Friction coefficient vs. duration for curves of Al2O3 – TiO2/NiCrMo/ at the grains on the surface after flame-spray process.
steel sample for (a) 49 N, (b) 69 N, (c) 89 N and (d) 129 N loads. During sliding in acidic environment, these particles were
1176 I.M. Kusoglu et al. / Surface & Coatings Technology 200 (2005) 1173 – 1177

Fig. 5. SEM micrographs of worn surfaces under dry test condition at (a) 49 N, (b) 69 N, (c) 89 N and (d) 129 N.

fractured with increasing applied load and extended to observation increased with increasing applied load as seen
sliding directions. Also, extension in the grains on the from Fig. 6. This event can be another reason why wear
sliding direction is observed and the frequency of this loss is lower than dry test conditions. Because of the

Fig. 6. SEM micrographs of worn surfaces under acidic test condition at (a) 49 N, (b) 69 N, (c) 89 N and (d) 129 N.
 I.M. Kusoglu et al. / Surface & Coatings Technology 200 (2005) 1173 – 1177 1177

increase in larger boundary volume with adhesion of environment is better than that in dry conditions. No
fractured grains, more energy could be absorbed and notable wear loss was observed during wear test at 49 N
resulted in a higher wear resistance. for each environment but increased with increasing applied
loads. Friction coefficient was constant and nearly equiv-
alent after prolonged duration.
4. Conclusions

Al2O3 – TiO2/NiCrMo coatings were fabricated on plain References

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[6] E. Lugscheider, H. Jungklaus, P. Remer, J. Knuuttila, Proc. 14th Int.
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50 Am, respectively. The microhardness value of the (2001) 118.
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[10] M. Gell, E.H. Jordan, Y.H. Sohn, D. Goberman, L. Shawn, T.D. Xiao,
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N which was 9.3 Am at the beginning. Tribological results
indicated that wear resistance of the coatings in acid