Sliding wear behavior of remelted Al 2O3-TiO 2 plasma sprayed coatings [600403]

Sliding wear behavior of remelted Al 2O3-TiO 2 plasma sprayed coatings
on titanium
UTU Ion -Dragos1,a, MARGINEAN Gabriela2,b, HULKA Iosif1,c and
SERBAN Viorel -Aurel1,d
1University Politehnica Timisoara, Piata Victoriei no. 2, Timisoara, Romania, Department of
Materials Engineering and Fabricat ion
2 Westphalian University of Applied Sciences, D -45897, Gelsenkirchen, Germany
[anonimizat] , bgabriela.marginean@w -hs.de , [anonimizat] , [anonimizat]
Keywords: Al2O3-TiO 2 coatings, APS spraying, sliding wear behavior
Microstructure and wear properties of th e Al 2O3-13.wt% TiO 2 thermally sprayed coatings before
and after remelting were investigated in this study. The coatings were deposited on a pure titanium
substrate using the atmospheric plasma spraying (APS) process. The as -sprayed coatings were
electron b eam (EB) modified in order to improve their compactness and bonding strength.
The effect of EB remelting on the microstructure, phase constituents and wear properties was
investigated using scanning electron microscopy (SEM), X -Ray diffraction technique a nd hardness
measurements. The sliding wear behavior was tested using a pin on disk method.
The results showed that the remelting process had a positive effect removing the lamellar defect of
the as -sprayed coating and improving the compactness, hardness a nd wear behavior.

Introduction
It is well known that coatings deposition is one of the viable solution s in order to improve the
surface characteristics in terms of fretting and tribological properties of titanium and titanium
alloys . Plasma sprayed ceramic coatings such as Al2O3-TiO 2 coatings have attracted much attention
in this field due to the wide appli cation to different components for enhancing resistances to wear ,
corrosion, oxidation, erosion and high temperature [1,2].
However, the plasma sprayed coatings possess lamellar structures with a high density of defects like
porosity and crack s which reduce the hardness, wear and corrosion properties affecting the coatings
performances [1,3]. Another important problem by plasma spraying is the inadequate bonding
strength between the coating and substrate which can cause the spallation and peeling off of the
deposited material under high bending stress or heavy load [4].
Application of post treatment s by high energy heat sources (laser, electron beam, welding methods)
and triggering of the remelting effect of plasma sprayed coatings seems to be a very interesting
solution for removing the above presented drawbacks [4, 5] .
In the current study, Al 2O3-TiO 2 type coatings were deposi ted by APS thermal spraying method
onto the surface of titanium substrate and further electron beam (EB) irradiated . The aim of this
work is to investigate the microstructure and sliding wear properties of Al 2O3-TiO 2 ceramic coatings
before and after EB re melting process .

2. Materials and experimental procedures
Commercial titanium was used as substrate material, which was cut in round specimens of 60 mm
diameter. The Al 2O3-13.wt% TiO 2 powder (−35 + 5 μm) was deposited by atmospheric plasma

spraying (APS) at the University of Tampere Finland. Before depositions the surface was sand
blasted and cleaned with acetone. The coating thickness was about 260 microns.
The morphology and the microstructure of the ceramic powder a nd of the APS sprayed coating has
been characterized by scanning electron microscopy (SEM: Philips XL -30 scanning electron
microscope equipped with EDAX analyzer).
The samples were remelted with an EBW700/6 –60 CNC electron beam apparatus from firma PTR
Praezisionestechnik Gmbh . The input power was 3 kW. The electron beam wa s focused onto the
specimen, the irradiated surface was 100 mm2. The coating s were remelted keeping constant the
voltage respectively the current intensity for different maintaining time .
Because of the low thermal conductivity and conductance of the Al 2O3-13.wt% TiO 2 ceramics and
in order to assure a full EB penetration depth , the coatings in as sprayed state were thinned by
grounding until a thickness of about 10 0 µm.
The phase composi tion of coatings before and after EB remelting has been investigated by X -ray
diffraction analysis using a Cu Kα radiation (Philips X’Pert Diffractometer) .
The sliding wear resistance was determined using the pin -on disk testing method by calculating the
variation of the wear track depth with applied load. The equipment used was a m icro pin on disk
tribometer Tr -20 Micro from DUCOM -Material Characterization System . The operation conditions
were: normal load 20 N, the relative velocity between the ball (WC) and surface v=20 cm/s, and the
testing distance was up to 2000 m (the trajectory was a circle with a radius of 15 mm).
The hardness of the coatings was determined using a ZHV µ micro Vickers hardness tester from
Zwick/Roell Company applying a load of 300 gf (HV0.3).

3. Results and discussion
3.1 Coatings morphology
Figure 1 presents the SEM micrograph s (at different magnifications) of the APS sprayed coating. It
can be noticed that the coating exhibit s an irregular structure of lamellar splats and a certain degree
of porosity and voids [6]. The presence of the oxide incl usions at the interface coating /substrate can
affect the adherence between the both materials.

Fig. 1 SEM micrograph of the APS -sprayed coatings
After the electron beam remelting treatment the structure of the material was refined and
compacted. The oxides were removed , the lamellar defect was eliminated and a diffusion zone at
the interface of th e coating -substrate was formed improving the bounding between them. Moreover,
the diffusion of the titanium inside the ceramic coating can be clearly seen along the interface (see
figure 2) .

Fig. 2 SEM microg raphs of the EB remelted coating s
3.2. XRD diffraction measurements
The XRD pattern of the APS sprayed coating is shown in F igure 3 and reveals the identified phases
like: η-Al2O3 and α-Al2O3 and TiO 2 [7].
Figure 4 shows the pattern of the EB remelted coating. The metastable η-Al2O3 phase in the
remelted region transferred into stable phase of α -Al2O3 because of the remelting and
recrystallization of the as -sprayed coating [8]. Beside α -Al2O3 and TiO 2 a new phase, tistarite Ti 2O3,
was formed. Tistarite is a Ti -rich phase (where Ti is totally in the 3+ oxidation state with occupied
3d orbitals, exhibiting a corundum structure ) which was formed because of the EB remelting
process.

Fig. 3 XRD diffraction pattern of the APS sprayed Al 2O3-TiO 2 coating

Titanium diffusion

Fig. 4 XRD diffraction pattern of the EB remelted Al 2O3-TiO 2 coating

3.3. Hardness and sliding wear behavior of the coatings
The microhardness values HV 0.3 (Table 1) were determined along the cross section of the coatings
and represent the average of performed indentations. As it can be seen the EB remelting had a
positive effect regarding the mechanical properties of material . The hardness has increased fro m 850
to 1470 HV 0.3. The lower values (1410 HV 0.3) of the remelted coating were recorded at the interface
right above the substrate in the Ti -rich diffusion zone.

Table 1 Hardness values of the Al2O3-TiO 2 coating s
Material Al2O3-TiO 2 – as sprayed Al2O3-TiO 2 – EB remelted
HV 0.3 850± 30 1470±60

These results influenced also the sliding wear behavior of the coatings. As mentioned in the chapter
2 the testing distance was established for both coatings to a value of 2000 m. In case of the APS
method the coating was destroyed and exfoliated after a testing distance of about 250 m [7].
In case of the EB remelted coating the sliding resistance was clearly higher than that in case of the
as-sprayed coatings. The evolution of the friction coefficient for the tested coatings is shown in
Figure 4 and 5 and the values of the wear rates are presented in Table 2.

Figure 4. Evolution of the friction coefficient for the as -sprayed Al2O3-TiO 2 coating

Figure 5. Evolution of the friction coefficient for the EB remelted Al2O3-TiO 2 coating

Table 2 Val ues of the sliding wear parameters
Al2O3-TiO 2 Worn section track, µm2 Testing distance d, mm Wear rate, mm3/N/m
as- sprayed 96 84 5.6 2000 0.000 1931
EB-remelted 113 236.8 250 0.00302

Conclusions
The obtained results performed in this study reveal clearly that the EB surface treatment had a
positive effect regarding the microstructure , hardness and sliding wear properties.
A more compact and homogenous structure was observed, the defects like the lamellar effect ,
internal oxidation and porosity were removed and the coating adherence to the substrate was
improved by means of interdiffusion reactions .
The heat developed during the EB remelting process provided recrystallization phenomena wh ich
transferred the metastable η -Al2O3 phase into stable α-Al2O3 oxide.
As a consequence of this the coating hardness was improved (from 850 to 1470 HV 0.3) and EB
remel ted coatings having the highest sliding wear resistance.

Acknowledgments
This work was supported by a grant of the Ministry of National Education, CNCS – UEFISCDI,
project number PN II -ID-PCE-2012 -4-0104.

References
[1]. W. Tian, Y. Wang, T. Zhang, Y. Yang, Sliding wear and electrochemical corrosion
behaviour of plasma sprayed nanocomposite Al 2O3-13%TiO 2 coatings, Materials Chemistry
and Physics 118 (2009), 37 -45;
[2]. E.P. Song, J. Ahn, S. Lee, N.J. Kim, Microstructure and wear resistance of nanostructured
Al2O3-8%TiO 2 coatings, plasma -sprayed with nanopowders, Surface & Coatings
Technology 201 (2006), 1309 -1315;
[3]. Y. Wang, C.G. Li, W. Tian, Y. Yang, Laser surface remelting of plasma sprayed
nanostructured Al 2O3-13%TiO 2 coatings on titanium alloy, Applied Surface Sci ence 255
(2009) 8603 -8610.
[4]. J.A. Haynes, E.D. Rigney, M.K. Ferber, W.D. Porter, Surface & Coatings Technology, 86 –
87 (1992), 102 -2018;
[5]. A. Dudek, J. Iwaszko, Structural and phase investigations of oxide coatings of TiO 2 and
Al2O3-13wt.%TiO 2 after remelting, Archives of Materials Science and Engineering,
International Scientific Journal, Vol. 33, Issue 1 (2008), 39 -44;

[6]. C. Li, Y. Wang, l. Guo, J. He, Z. Pan, l. Wang, Laser remelting of plasma -sprayed and
nanostructured Al2O3-13wt.%TiO 2 coatings on titanium alloy, Journal of Alloys and
Compunds 506 (2010) , 356 -363;
[7]. I.D. Utu, G. Marginean, I. Hulka, V.A. Serban, D. Cristea, Properties of the thermally
sprayed Al 2O3–TiO 2 coatings deposited on titanium substrate , Int. Journal of Refractor y
Metals and Hard Materials 51 (2015) , 118–123;
[8]. Y. Gao, C. Wang, M. Yao, H. Liu, The resistance to wear and corrosion of laser -cladding
Al2O3 ceramic coating on Mg alloy Applied Surface Science 253 (2007), 5306 -5311.