TRIBOLOGICAL BEHAVIOR OF A PLASMA-SPRAYED Al2O3-TiO2-Cr2O3 COATING
TRIBOLOŠKO PONAŠANJE S PLAZMO NAPRŠENEGA Al2O3-TiO2-Cr2O3 NANOSA
Yeçim Sert, Nil Toplan
Sakarya University, Department of Metallurgical and Materials Engineering, Esentepe Campus, 54187 Sakarya, Turkey
toplan@sakarya.edu.tr
Prejem rokopisa — received: 2012-08-31; sprejem za objavo - accepted for publication: 2012-10-08
Alumina-titania, titania, chromia and chromia-titania coatings, deposited on aluminium substrates with atmospheric, plasma-spray, coating techniques (APSCTs), were tested on a low-frequency reciprocating-sliding tribometer. The surface wear of the coatings was investigated with SEM and optical microscopy. The denser Cr2O3 coatings showed a higher wear resistance than the more porous Al2O3-TiO2 and TiO2 coatings. An increase in the titania content diminishes the coating hardness and the wear resistance.
Keywords: Cr2O3-TiO2, Al2O3-TiO2 coatings, textile parts, plasma-spray coating, wear
Nanosi Al2O3-TiO2, TiO2, Cr2O3, Cr2O3-TiO2 z atmosferskim plazemskim nanašanjem z brizganjem (APSCT) na podlago iz aluminija so bili preizkušeni na nizkofrekvenčnem protismernem drsnem tribometru. Obrabljena površina nanosa je bila preiskovana s SEM in svetlobno mikroskopijo. Gostejši nanos Cr2O3 je pokazal večjo odpornost proti obrabi kot bolj porozna nanosa Al2O3-TiO2 in TiO2. Povečanje vsebnosti TiO2 zmanjšuje trdoto nanosa in zmanjša odpornost proti obrabi.
Ključne besede: nanosi Cr2O3-TiO2, Al2O3-TiO2, kosi tkanine, plazemski nanos z brizganjem, obraba
1 INTRODUCTION
It is well known that aluminum (Al) alloys have been considered to be some of the most useful and versatile materials because of their metallurgical characteristics, such as high strength-to-weight ratio, and high thermal conductivity. They are also easy to shape and relatively inexpensive. However, the low hardness results in poor tribological characteristics and prevents their wide use, especially in the situations where a hard surface is needed. To improve the wear resistance, many techniques, such as metal-matrix composites, plasma spraying, thermal spraying and hard anodizing have been explored1.
The APSCT is an economical and effective method applied to various machine parts to improve the component performance in wear, corrosion, thermal barrier, and electric insulation. Plasma-sprayed Al2O3-TiO2 has been widely used as a wear-resistant coating in textile, machinery and printing industries2-5. &2O3 has a wide range of applications such as green pigments, coating materials for thermal protection and wear resistance as
well as refractory applications due to the high melting temperature (about 2435 0C)3,6. The present paper deals with the wear resistance of the plasma-sprayed alumina-titania, titania, chromia and chromia-titania coatings that increased the service life of the shutters (Al-based) used in the textile industry.
2 EXPERIMENTAL PROCEDURE
The commercial feedstock powders in the mass fractions 13 % TiO2-Al2O3 (Metco 130), 40 % TiO2-Al2O3 (Metco 131VF), 100 % TiO2 (Metco 102) and 100 % &2O3 (Metco 106) were supplied by SULZER METCO Powder Technology. AW3, TiO2 and Cr2O3 powders were premixed to form five different compositions (Table 1) and these were prepared on an aluminium alloy (AA1050). The mixtures were ball-milled for 2 h by using ZrO2 balls and distilled water as the milling media to provide homogenous mixtures. After drying the powders were screened and sieved to
Table 1: Coating powders (w/%) Tabela 1: Prahovi za nanašanje (w/%)
Composition	Al2O3	TiO2	Cr2O3
1	87	13	—
2	60	40	—
3	—	100	—
4	—	50	50
5	—	—	100
Table 2: Plasma-spray parameters Tabela 2: Parametri nabrizgavanja s plazmo
Coating parameters (for the 1—5 coded compositions)	
Primary-gas flow rate (Ar, L/min)	80
Secondary-gas flow rate (H2, L/min)	15
Carrier-gas flow rate (Ar, L/min)	40
Spray distance (mm)	100
Current (A) Voltage (V)	500 60—70
achieve the correct particle-size distribution needed for plasma spraying. Prior to the deposition process, aluminium substrates were grit blasted with AI2O3 particles and this was followed by ultrasonic cleaning in acetone for 15 min. A 40 kW plasma-spraying system (METCO 3MB) was utilized to produce the coatings using the parameters summarized in Table 2.
The surface roughness was measured with a surface-roughness tester (Perthometer M4P) and the average roughness Ra, defined as the arithmetic mean of the departures of the profile from the mean line, was used to quantify the coating-surface roughness. A scanning electron microscope (SEM) (JEOL JSM-6060LV) equipped with an energy dispersive X-ray spectrometer (EDS) was used to examine the microstructures and chemical compositions of the coatings. An X-ray diffraction analysis (XRD) was carried out on RIGAKU DMAX 2200 to determine the phases of the coating(s). The microhardness values of the specimens were taken from the cross-sections of the polished samples at the load of 200 g and after a loading time of 15 s using LEICA VMHT MOT microhardness equipment. The wear tests were performed using a low-frequency reci-procating-sliding tribometer, connected to a computer monitoring the dynamic coefficient of friction (in both sliding directions), relative humidity and temperature. The tests were performed by applying a load of 5 N to a single-crystal Al2O3 (sapphire) ball with a diameter of 6 mm. The wear specimens had the dimensions of 3 cm x 3 cm x 3 mm, the shear rate was 0.15 m/s and the sliding distance was 150 m. The values of the coefficient of friction were calculated from the normal load and the friction force was obtained from a digital oscilloscope. After the wear tests, the morphology of each wear scar was observed with SEM.
3 RESULTS AND DISCUSSION
Table 3 summarizes five different compositions of the coating-test results. While the TiO2 coatings had the highest surface-roughness values, the values for the Cr2O3 coatings were found to be the lowest. Having a lower as-sprayed surface roughness is very important for technological applications because it reduces the number of post-deposition mechanical treatments necessary7 8. As a function of the substrate-surface roughness, the values of porosity and coating roughness increased, while the increase in the substrate-surface roughness grow up. The hardness values were also relatively reduced. The highest hardness, the lowest porosity and the lowest coating roughness were obtained at the value of the substrate roughness of 2.346 pm. The hardness values decrease with the increasing amounts of TiO2 in the Al2O3-TiO2 coatings. In the Cr2O3-TiO2-based coatings, the hardness values increase with the amount of Cr2O3. An increase in the porosity amount will result in a decrease in the hardness of the coating. The lowest
coefficient of friction (p) was achieved in the 100 % Cr2O3 coatings. Hardness has a strong influence on wear resistance. The higher the hardness, the better is the wear resistance. It is well known that an addition of TiO2 to an Al2O3 coating is to reduce the melting temperature of the oxide, thereby producing less porous and better performance coatings than the pure Al2O3 coatings. The melting temperature decreases due to the fact that TiO2 has a lower melting temperature (1854 °C) than ALO3 (2040 °C) and due to its ability to form a liquid solution with AFO3. It is also noted that the trend displayed by the coating densities is consistent with that exhibited by the degree of melting, i.e., a high degree of melting (e.g., Cr2O3, 2435 °C) results in high density. Increasing the microhardness leads to the improvements in the wear resistance of the coatings. The grain size also has an effect on the wear resistance. The nanocoating has a higher wear resistance than the commercial coating although its hardness is lower than that of the commercial coating. A related study on the abrasive wear has revealed that nanocoatings could have a two-to-four-fold increase in the wear resistance in comparison with the commercial coatings2,7.
Table 3: Surface roughness, hardness, density and friction coefficient of the coatings
Tabela 3: Hrapavost povrsine, trdota, gostota in koeficient trenja nanosa
Composition	1	2	3	4	5
Ra/|im	3.553	3.437	4.443	3.011	2.346
Hardness (HV)	1028	899	812	1010	1724
Relative density (%)	87.70	88.45	86.00	90.12	92.51
Average friction coefficient	0.142	0.190	0.223	0.144	0.074
The XRD analysis of the starting powders showed that the chromia powder consisted of an eskolaite phase (&2O3) and the alumina-titania powder of a-Al2O3 and anatase. It was also clear from XRD that the chromia coating consists of eskolaite, the chromia-titania coating consists of eskolaite and Ti2Cr2O7 and the alumina-titania coating consists mainly of y-A^O3 with some a-Al2O3, Al2TiO5, a glassy phase and a small amount of rutile. A very low amount of crystalline TiO2 indicates that it mostly dissolves in the molten Al2O3 2,8.
The main wear mechanisms of plasma-sprayed ceramic coatings were reported to be a plastic deformation, crack formation and spalling due to fatigue, brittle fracture and material transfer. In the reciprocated dry sliding, wear debris was considerably involved in the wear process in the steady state. The worn surfaces of the &2O3 and TiO2 coatings were observed with SEM at different magnitudes (Figure 1). In the SEM images the wear scar of the TiO2 coating was much larger than that of the &2O3 coating. The &2O3 coating is the hardest and the most anisotropic among the plasma-sprayed ceramics due to its low interlamellar cohesion; the Al2O3-TiO2 and TiO2 coatings are the most isotropic but
also less hard and less tough due to the formation of an alumina-titania glassy phase which favors intersplat adhesion but turns out to be quite brittle8. The smooth film, formed due to a large plastic deformation of the adhered wear debris, strongly influenced the friction and wear behavior. For the plasma-sprayed Cr2O3 coatings, similar wear mechanisms were reported under dry sliding conditions and the role of the wear-protective film formed by a plastic deformation of the adhered and compacted debris particles was discussed9. The abrasive wear mechanism of the coatings does not only depend on the coating hardness and density, but also on the particle size, the type of the powder used, the coating microstructure, as well as on the microstructural change during a wear testing. The average coarser powder particle size causes an appearance of a significant number of unsmelted particles.
4 CONCLUSIONS
Alumina-titania, titania, chromia and chromia-titania coatings were deposited with APSCT to increase the
wear behavior of the aluminium-based shutters. While the friction coefficient and the coating-surface roughness increased with an increase in the titania content, the coating density, hardness and wear resistance decreased.
5	REFERENCES
*K. Huang, X. Lin, C. Xie, T. M. Yue, J. Mater. Sci. Technol., 23 (2007) 2, 201-206 2M. Wang, L. L. Shaw, Surface and Coatings Technology, 202 (2007) 1, 34—4
3	V. P. Singh, A. Sil, R. Jayaganthan, Wear, 272 (2011), 149-158
4	S. Tao, Z. Yin, X. Zhou, C. Ding, Tribology International, 43 (2010),
69-75
5S. T. Aruna, N. Balaji, J. Shedthi, V. K. W. Grips, Surface & Coatings Technology, 208 (2012), 92-100
6	A. Cellard, V. Garnier, G. Fantozzi, G. Baret, P. Fort, Ceramics International, 35 (2009), 913-916
7	L. L. Shaw, D. Goberman, R. Ren, M. Gell, S. Jiang, Y. Wang, T. D. Xiao, P. R. Strutt, Surface and Coatings Technology, 130 (2000) 1, 1-8
8	G. Bolelli, V. Cannillo, L. Lusvarghi, T. Manfredini, Wear, 261 (2006), 1298-1315
9H. S. Ahn, O. K. Kwon, Wear, 225-229 (1999), 814-824