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Microstructure and tribological properties of nanostructured and conventional plasma sprayed alumina–titania coatings
Surface & Coatings Technology 268 (2015) 190–197
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Microstructure and tribological properties of nanostructured and conventional plasma sprayed alumina–titania coatings Wojciech Żórawski a,⁎, Anna Góral b, Otakar Bokuvka c, Lidia Lityńska-Dobrzyńska b, Katarzyna Berent b a b c
Kielce University of Technology, Poland Institute of Metallurgy and Materials Science, Cracow, Poland University of Zilina, Slovakia
a r t i c l e
i n f o
Available online 16 September 2014 Keywords: APS Alumina–titania Nanostructure COF Abrasion
a b s t r a c t The nanostructured and two conventional alumina–titania coatings were deposited by atmospheric plasma spray. The presented studies show that nanoparticles are the predominant component of nanostructured Al2O3–13TiO2 powder grains. The microstructure of coating consisted of two distinct regions: fully melted and unmelted or partially melted nanostructured areas, which were comprised of components of a starting powder. Microhardness and modulus of these coatings were significantly lower than the values obtained for nanostructured coating. It was found that the plasma-sprayed nanostructured Al2O3–13TiO2 coating possessed better tribological properties than those of conventional alumina–titania coatings. The lower coefficient of friction and wear of the nanostructured Al2O3–13TiO2 coating is attributed to the bimodal microstructure and the enhanced mechanical properties in comparison to conventional alumina–titania coatings. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured materials are of particular scientific interest because of their physical and mechanical properties, which are superior to those of conventional materials. Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD) or sol–gel techniques are the most common processes which are used to produce nanostructured coatings involving atomic or molecular layer deposition. An advantage of these processes is that the coatings possess very good properties. These processes have significant limitations in regard to applied coatings and substrate materials. Additional problems include high vacuum (PVD), high temperature (CVD), additional post-treatment (CVD, sol–gel), and the necessity of work piece positioning for deposition in line of sight [1,2]. Additional inconveniencies consist of small deposition rate, film thicknesses, and high cost of manufacturing [3–5]. All above disadvantages can be avoided by application of nanocrystalline coatings through thermal spraying processes. Moreover, some investigations reveal that the nanostructured coatings possess better properties than the same coatings sprayed with conventional feedstock [6–12]. Friction coefficient of the HVOF sprayed nanostructured WC–Co coating was 20% lower in comparison with the conventional WC–Co coating [13]. It was reported that VPS and HVOF-sprayed nanostructured TiO2 coatings revealed the highest abrasion resistance and crack propagation resistance than those exhibited by the best conventional coatings [14].
⁎ Corresponding author. E-mail address: [email protected] (W. Żórawski).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.014 0257-8972/© 2014 Elsevier B.V. All rights reserved.
Nanostructured plasma sprayed Al2O3–13TiO2 coatings were in the area of interest over the last decade because they showed improved properties over conventional coatings [10,17–38]. Spraying of such coatings using standard plasma systems is possible thanks to applications of nanostructured powders with the mean diameter of 30 μm which are prepared from nanocrystalline grains. Parameters of plasma spraying process should be carefully controlled because of high enthalpy of plasma stream which influences the degree of powder melting. This causes changes in the nanostructure of powder grain. Consequently sprayed coatings consist of partially and fully melted regions which create bi-modal microstructure. Better wear characteristics of nanostructured alumina–titania coatings in comparison to conventional coatings were shown for the first time by Wang et al. [29]. Abrasive wear resistance of the nanostructured Al2O3–13TiO2 powder with the addition of CeO2 and ZrO2 was about four times higher than that of sprayed with a corresponding Metco 130 powder. These results were confirmed by Shaw et al. [30]. Moreover, the strong dependency of coating density and hardness on the ratio of electrical power to the primary argon gas flow rate was also demonstrated. This function known as critical plasma spray parameters (CPSP) was the base of investigations carried out by Jordan et al. [17] and Gell et al. [31] using coatings sprayed with optimized parameters. Analysis of the test results show superior mechanical properties as indentation crack resistance, abrasive wear resistance, adhesion strength, and spallation resistance. These enhancement properties were attributed to unique bi-modal structure of coatings that have a retained nanostructure originated from nanostructured powders. Goberman et al. [18] analyzed the influence of CPSP on volume percent
W. Żórawski et al. / Surface & Coatings Technology 268 (2015) 190–197 Table 1 Plasma spraying parameters. Parameter
Value
Current, A Voltage, V Plasma gas pressure, MPa Spraying distance, mm Powder feeding rate, g/min
600 60 0.7 100 45
fraction of γ-Al2O3 phase in bi-modal and conventional coatings. The content of this phase grew with increasing CPSP for nanostructured powders. In the case of the conventional powder (Metco 130) the share of γ-Al2O3 phase was unchanged. As CPSP increased over 390, the participation of this phase in both coatings decreased which caused an increase in the α-Al2O3 content. Toughening mechanisms in nanostructured Al2O3–13TiO2 coatings produced at optimized CPSP were investigated by Luo et al. [20]. Such coatings with 15–20% of the partially melted particulate regions, possessed approximately 100% improvement in the crack growth resistance. These regions allow to deflect and trap boundary cracks. Bolledu et al. [32] investigated the influence of CPSP on volume fraction of α-Al2O3 and γ-Al2O3 phases in nanostructured coatings with the application of nitrogen and argon as primary plasma gases. The effect of CPSP on coating properties in the case of nitrogen was negligible but the application of argon caused significant changes in coating characteristics. The optimum plasma spray parameters for the deposition of nanostructured Al2O3–13TiO2 coatings can be achieved by application design of experiment. Yusoff et al. [38] applied two-level factorial design to obtain optimum properties (wear resistance, surface roughness and microhardness) by changing primary gas pressure, carrier gas pressure and powder feed rate. Sanchez et al. [22] confirmed better tribological properties and higher microhardness of nanostructured coatings over conventional coatings. It was found that changed spraying parameters did not possess significant influence on coating properties. Also investigations carried out by Zhang et al. [33] revealed negligible influence of plasma stream power (30 and 45 kW) on the wear of nanostructured coatings. Additionally, Liu et al. [34] found that the applied spraying system (plasma or HVOF) had a significant influence on the coating abrasive wear resistance but the microstructure of powder did not. Tian et al. [35] showed that nanostructured coatings can reach two times higher fretting wear resistance than conventional coatings in tests carried out on polished samples against steel balls. The enhancement fretting wear resistance of nanostructured coatings was attributed to the nanosized grains, reduced lamellar structures, and amorphous phases. Rico et al. [36] investigated wear rates of nanostructured and conventional coatings by means of a pin on disk tribological tester at a wide range of loads. In all cases wear rates of the nanostructured material were lower but friction coefficient for both materials was similar.
a)
b)
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The main wear mechanism was attributed to the brittle propagation of cracks. In the low load regime the cracks tend to propagate along the splat boundaries, but at high contact loads the cracks pass through the splats. Wang et al. [37] studied the thermal shock behavior of two nanostructured Al2O3–13TiO2 and corresponding conventional coatings (Metco 130). Nanocoatings exhibited a much higher thermal shock resistance at temperatures of 650 °C and 850 °C. Moreover, the crack propagation mode was quite different for nanostructured and conventional coatings. Rico et al. [23] studied the effect of temperature on tribological properties of both conventional and nanostructured Al2O3–13TiO2 coatings at 500 °C. Wear resistance of nanostructured coatings was superior to that of conventional coatings under all experimental conditions. It was observed that at high temperatures plastic deformation appears as the main wear mechanism. In the present study, the nanostructured Al2O3–13TiO2 coating and two kinds of conventional Al2O3–13TiO2 and Al2O3–3TiO2 coatings were plasma sprayed. Although plasma-sprayed nanostructured Al2O3–13TiO2 coatings have been extensively studied in recent years there is still a need for a detailed analysis of structure and composition of nanostructured powder grains and coating. The presented studies provide this analysis and in addition offer an analysis of the topography of sprayed coatings, which is significant for the determination of mechanical properties of coatings. Moreover the presented results add to tribological studies by means of providing new information on the coefficient of friction and abrasive wear resistance of coatings.
2. Experiment Three types of alumina–titania powder feedstock were applied in this study: nanostructured powder Al2O3–13TiO2 (Infralloy Nanox S2613S), (N), and two standard powders Al2O3–13TiO2 (Amdry 6228), (S), and Al2O3–3TiO2 (FST C-336.31), (S). Plasma spraying was performed by means of a Plancer PN-120 plasma spraying system with a Thermal Miller 1264 powder feeder. Plasma spraying parameters are listed in Table 1. For the metallographic sections and abrasive examination, the coatings were deposited on flat low-carbon steel samples with dimensions of 30 mm × 30 mm × 3 mm, whereas for the ball on disk test the coatings were deposited on disk-shaped samples in low-carbon steel with dimensions of ϕ30mm × 6 mm. Before plasma spraying, all coupons were degreased and grit blasted with electrocorundum EB-12 at a pressure of 0.5 MPa. After spraying the thickness of the coatings was 0.35 ÷ 0.5 mm. The morphologies of the powders and the coatings were analyzed using the following microscopes: SEM JSM-5400 with an ISIS 300 Oxford (EDS) microprobe, SEM FEI Nova™ NanoSEM 200 and TEM Philips CM20 with EDAX EDX. Phase composition was studied using a
c)
Fig. 1. Morphology of powder grain: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
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Fig. 2. Morphology of the Al2O3–13TiO2 (N) powder grain surface.
Bruker D8 Advance diffractometer with Co-Kα radiation of wavelength λ = 1.78897 Å. Friction coefficient for plasma sprayed coatings at unlubricated conditions was determined by means of a T-05 ball-on-disk type tribotester. The bearing steel 100Cr6 balls of 6 mm in diameter were used as a counter body. The sprayed coatings on the disk samples were subsequently ground and polished for 1 h. The action of the friction force in the function of time was registered and controlled using a computer. The parameters of the tribotester were as follows: radius — 19 mm, load — 5 N, linear velocity v = 0.1 m/s, and total distance — 2000 m. The abrasive wear resistance of all alumina–titania coatings was evaluated by applying a dry sand rubber wheel tester (T07). The alumina powder of 300 μm was applied as an abrasive medium in this test. Investigations were repeated three times for each coating. The micromechanical testing of coatings was carried out by using a nanoindentation technique (MCT-CSM) with a Vickers indenter (the Olivier and Pharr methodology). Ten readings were taken for each coating. The topography of coatings “as sprayed” and after polishing was analyzed by means of a Talysurf CCI-Lite non-contact 3D profiler [15].
α -Al2O3
TiO2
α -Al2O3
CeO2
ZrO2 500 nm Fig. 3. TEM micrograph of Al2O3, TiO2, ZrO2 and CeO2 nanoparticles.
Fig. 4. Linear analysis of Al2O3–13TiO2 (N) powder grain cross-section.
3. Results and discussion 3.1. Characterization of alumina–titania powders Three types of alumina–titania commercial feedstock were used in the study: nanostructured Al2O3–13TiO2 powder (Nanox™ S2613S Inframat Advanced Materials), (N) and two conventional Al2O3–13TiO2 (Amdry 6228), (S) and Al2O3–3TiO2 (FST C-336.31), (S) powders. Fig. 1a shows the morphology of the nanostructured alumina-powder. Most grains have a regular spherical shape, only some of them are fractured. Imaging of the grain surface at high magnification SEM shows that each grain of powder consists of nanograins which create cauliflower-like high porous agglomerates (Fig. 2). The slight necking connecting nanograins are a result of the manufacturing process which allows to obtain grains in size range of −50 + 10 μm [16,17]. To further analyzed the morphology of the nanostructured powder, TEM analysis of a single agglomerated powder was performed and is presented in Fig. 3. Besides Al2O3 and TiO2 nanograins, the addition of ZrO2 and CeO2 nanoparticles is present. This image confirms that a single grain of powder is made up of nano-sized particles ranging from tenth to 500 nm in size. Additional components were added to lower the sintering temperature and to improve the densification process [18], however they were not reported in some works [16,17,19,27]. Linear analysis of grain crosssection (Fig. 4) revealed that additional oxides exist not only in the range of nanometers; significantly bigger particles of ZrO2 and CeO2 in the range of several micrometers are also components of the nanostructured powder. Nanostructured Al2O3–13TiO2 powder consists of wt.%: Al2O3 — 75.7%, TiO2 — 11.3%, ZrO2 — 8%, and CeO2 – 5%. Such composition of the nanostructured powder was reported by Bandyopadhyay et al. [21]. Fig. 1a and b shows the morphology of the particles of the conventional alumina–titania powders. The almost irregular angular shape of the particles is a characteristic of powders produced by fusing and crushing. These conventional powders are mixtures of alumina (dark
W. Żórawski et al. / Surface & Coatings Technology 268 (2015) 190–197
a)
b)
193
c)
Fig. 5. Morphology of coating surface: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
grains) with different contents of titania (light grains) with a grain size of −45 + 15 μm for Al2O3–13TiO2 and −53 + 10 μm for Al2O3–3TiO2. 3.2. Characterization of alumina–titania coatings The surface morphology of the plasma-sprayed coatings is shown in Fig. 5. The splats of the nanostructured powder seem to be homogenous (Fig. 5a), however nanograins of incomplete melted powder grains are visible at higher magnification (Fig. 6b). Differences in the quantity of light phase splats represents titania on the surface of conventional powders (Fig. 5b and c) and is the result of the share of titania in the mixture with alumina, 13 and 3% respectively. These splats were segregated in an appropriate proportion and are also well seen at the cross-section of both conventional coatings (Fig. 6b and c). Such conventional microstructure was reported by Sanchez et al. [22]. Dark areas represent pores or oxides which are an inherent feature of plasma sprayed coatings. Unlike the typical lamellar microstructure of the conventional coating where grains of powder are completely melted, the microstructure of the nanostructured coating consists of regions with different degrees of melting (Fig. 6a). Such bimodal microstructure containing melted and unmelted particles is typical for coatings sprayed with reconstituted nanostructured feedstock and was previously reported by other researches [21–25]. The formation of such microstructure is a result of lower heat transfer within the agglomerated particles in comparison to the conventional milled feedstock. Additionally, the degree of melting is influenced by grain size and its trajectory in the plasma stream [17,26,27]. On impact, the cooling process starts from the surface of grain and outer part of nanostructured grain which solidifies while the center preserves original nanocrystal structure is preserved (Fig. 7a). In the case of incomplete melting of particle, the nanosized grains are retained in the melted matrix (Fig. 7b). Further TEM investigations revealed that these nanosized grains are Al2O3 nanocrystals which are surrounded by the mixture of melted TiO2, ZrO2 and CeO2 nanograins (Fig. 8a). Such matrix composition may be due to the higher melting point of alumina (2072 °C) in comparison to titania (1843 °C).
a)
b)
Titania is melted entirely in the plasma stream and unmelted alumina nanograins can get embedded in the melted titania [23]. This description does not explain the presence in the matrix of additional oxides with significantly higher melting temperature (zirconia — 2715 °C, ceria — 2400 °C) and dimensions (Fig. 4). TEM observations of fully melted area show the typical lamellar microstructure with the presence of γ-Al2O3 and are easily recognizable by a gray contrast amorphous phase (Fig. 8b). Table 2 presents the chemical composition specified on the base of EDS analysis of the fully melted nanostructured coating (Fig. 9). Visible decrease of aluminum amount is present in the amorphous phase (points 2 and 6). The changes of titanium content are slight across the coating in the range from 2.8 to 4.2 at.%. Uniform distribution of Zr and Ce allows to assume that these elements were dissolved in the γ-Al2O3 phase. The XRD patterns of all alumina–titania powders are presented in Fig. 10a. The main constituents of all powders are α-Al2O3 and titania in the form of rutile. In the case of the nanostructured powder, additional peaks confirm the presence of earlier described additives in the form of ZrO2 and CeO2. In comparison to conventional powders nanostructured feedstock presents broader peaks. Nanoparticles have much less atoms hence the lattice sum is not able to converge to a diffraction line but broadens out. The XRD pattern of all sprayed coatings (Fig. 10b) revealed an additional phase of γ-Al2O3. The presence of this phase in all plasma sprayed coatings is due to the melting of the alumina–titania powders and the fact that at a higher cooling rate γ-Al2O3 commonly nucleates in the preference to α-Al2O3 during the rapid solidification of liquid droplets. The reason for this phenomena is lower energy for nucleation from the liquid for the γ-phase than for the α-phase [40]. Peaks corresponding to ZrO2 and CeO2 were not detected in the XRD pattern of the nanostructured coating surface. The force–depth penetration curves obtained from nanoindentation are shown in Fig. 11. The nanostructured coating was much harder because it exhibited a lower penetration and higher stiffness (the highest slope in the unload branch at a maximum load) than the conventional coatings. Young's modulus for the nanostructured coating
c)
Fig. 6. Microstructure of coating: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
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a)
b)
Fig. 7. Morphology of the Al2O3–13TiO2 (N) coating: a) fracture and b) surface.
was 164 HV0.1 ± 19 GPa and 123 HV0.1 ± 14 GPa and 102 HV0.1 ± 15 GPa for both conventional coatings, Al2O3–13TiO2 and Al2O3–3TiO2 respectively. The nanostructured plasma sprayed Al2O3–13TiO2 coating possesses a higher microhardness (1192 ± 180 HV0.1) than the conventional coatings (980 ± 186 HV0.1 and 683 ± 99 HV0.1). The presented results showed a significantly better microhardness and Young's modulus of the nanostructured coating. The main reason for this is the distinctive bi-modal microstructure obtained by spraying reconstituted nanostructured feedstock. It consists of fully melted and splat-quenched regions (γ-Al2O3 supersaturated with Ti4+ and amorphous phase) and partially melted particulate where the initial microstructure of the nanostructured agglomerates is retained. The partially melted region consists of α-Al2O3 particles embedded in γ-Al2O3 supersaturated with Ti4 + with the presence of Zr and Ce. The bi-modal distribution of microstructure and grain size led to the superior mechanical properties. Optimum properties are provided by the presence of 15–20% of partially melted regions [20]. These nano-scale and submicron-scale microstructures in the coatings form a skeleton-like
structure. The nano-scale and submicron-scale skeleton structures play a very important role in the behavior of bi-modal coatings against external impact [29]. On the other hand, the microstructure of the conventional coating consists primarily of fully melted splats (γ-Al2O2). Moreover, weak splat boundaries in such coating typically contain many small voids and have a significantly lowered fracture toughness. Soft partially melted regions in hard nanostructured ceramic coatings improve ductility and fracture toughness, and consequently, the resistance to crack initiation and propagation, which is important in wear resistance. The crack arrest phenomenon is due to crack trapping within the partially melted regions and by crack deflection at the interface between partially melted and fully melted regions. These results indicate that the partially melted particulate region and the interface between partially melted and fully melted regions have a better crack arresting potential than the fully melted splat microstructure. These reduced crack propagation capabilities are a result of crack deflection, crack trapping and crack stopping at the interface [20,38]. Moreover, Sebastiani et al. [39] suggests that the presence in microstructure of
a)
b)
Fig. 8. TEM micrograph of the Al2O3–13TiO2 (N) coating: a) partially melted area and b) fully melted area.
W. Żórawski et al. / Surface & Coatings Technology 268 (2015) 190–197 Table 2 Results of EDS analysis of the fully melted Al2O3–13TiO2 (N) coating. No.
1 2 3 4 5 6 7 8
195
a)
Chemical composition, at.% Al
O
Ti
Zr
Ce
50.0 46.6 49.4 50.5 48.2 45.2 45.8 49.6
44.5 47.2 45.6 45.4 44.7 48.3 47.0 46.8
3.5 3.7 3.4 2.9 3.6 3.5 4.2 2.8
1.0 1.3 0.6 0.4 1.7 1.4 1.8 0.4
1.0 1.2 1.0 0.8 1.8 1.6 1.2 0.4
glassy splats, which are free of micro-cracks and subjected to near zero tensile quenching stress, can play an important additional role in toughening Al2O3\TiO2\ZrO2\CeO2 plasma sprayed coatings.
b)
3.3. Topography of the coatings Results of the topography of all plasma as-sprayed coating surfaces are presented in Table 3. Surfaces were scanned and measured to estimate coating influence on the wear process. The highest arithmetic mean of the surface height Sa of the as-sprayed nanostructured coating is slightly higher than for both conventional coatings, however the ranges of grain size distribution of all powders were very close. This result can be attributed to the earlier described lower heat transfer within the reconstituted powder causing enhanced coating roughness. The similar dependency was observed for the mean squared surface height, Sq. Negative value of asymmetry of the surface, Ssk, indicated that the both surfaces Al2O3–13TiO2 (N) and Al2O3–3TiO2 (S) were flattened and occurring peaks were rounded. Kurtosis Sku is responsible for steep irregularities and defects. The value of kurtosis over 3 indicated that the distribution of profile ordinate corresponds to a higher concentration around the mean value. Parameters: the maximum peak Sp, the maximum valley Sv and the maximum height Sz are nearly two times lower in the case of the nanostructured coating surface. It results from the participation of smaller spherical grains of the nanostructured powder in the spraying process in comparison to angular grains of conventional powders. The same range of topography measurements were carried out on coating surfaces after polishing before the investigation of coefficient of friction. Results of measurements are reported in Table 4. All plasma sprayed alumina–titania coatings were polished to obtain a surface with mirror reflectivity. To avoid influence of treatment on surface topography the same conditions of polishing were applied to
Fig. 10. XRD patterns of: a) Al2O3–13TiO2 (N), Al2O3–13TiO2 (S) and Al2O3–3TiO2 (S) powders, and b) Al2O3–13TiO2 (N), Al2O3–13TiO2 (S) and Al2O3–3TiO2 (S) coatings.
nanostructured and conventional coatings. In the case of the polished nanostructured coating, parameter Sa = 0.05263 μm is nearly twofold lower than Sa for both conventional coatings. As a consequence, the roughness of the nanostructured coating is significantly lower. 3.4. Measurements of friction coefficient and abrasive wear resistance Results of the carried out tests showed that the friction coefficient for the nanostructured coating was 0.51 and for conventional coatings was 0.62 and 0.58 respectively, after a 2000 m distance. Lower friction coefficient of the nanostructured coating is probably due to its fundamentally different microstructure in comparison to the typical lamellar
120
Al2O3-13TiO2 N Al2O3-13TiO2 S A2O3-3TiO2 S
100
Load, mN
80 60 40 20 0 0
500
1000
Displacement, nm Fig. 9. TEM micrograph of the Al2O3–13TiO2 (N) fully melted area with the distribution of elements along the marked line.
Fig. 11. Load–displacement nanoindentation curves on alumina–titania plasma sprayed coatings.
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200
Table 3 As-sprayed surface topography parameters according to ISO 25178. Al2O3–13TiO2 (N)
Al2O3–13TiO2 (S)
Al2O3–3TiO2 (S)
6.63564 8.8498 −0.0426 4.8029 35.3753 39.0988 74.4741
6.3652 8.1312 0.2221 3.8954 82.1862 88.7426 170.9288
5.7312 7.0842 −0.0460 3.0116 76.9685 65.7015 142.6700
Al2O3-13TiO2 S
160
Al2O3-3TiO2 S
140
Δ mg
Parameter Sa Sq Ssk Sku Sp Sv Sz
Al2O3-13TiO2 N
180
120 100 80
microstructure of conventional coatings. The nanostructured microstructure containing agglomerates with different degrees of melting limited the propagation of any crack generated in the sprayed coating [25]. Additionally, such microstructure revealed a significantly higher mechanical properties than both conventional coatings. In the conditions of dry sliding test, probably the presence of unmelted nanoparticles on the surface collaborating with steel 100Cr6 balls can positively influence the friction process in which small contact areas cause high pressures. Also the important factor is the roughness of the nanostructured coating surface which is two-fold lower than the roughness of both conventional coatings [28]. Fig. 12. presents the results of all alumina–titania coating abrasive wear tests (dry sand rubber wheel). The carried out measurements showed that the nanostructured coating has the best wear resistance after 20 min of testing. The worst wear resistance was demonstrated by the conventional Al2O3–3TiO2 coating which was nearly three times lower in comparison to the nanostructured coating. Wear resistance of the conventional Al2O3–13TiO2 coating was nearly 30% lower than the best coating. The roughness of all as-sprayed coatings was nearly the same, so the microstructure and mechanical properties of bimodal nanostructured coatings appear to be the essential factors which influenced the significantly higher abrasive wear resistance.
4. Conclusions The novelty and significance of the presented studies consist of: 1. Detailed analysis of reconstituted nanostructured feedstock demonstrated that nanocrystal Al2O3 grains, which were a constituent of the sprayed powder, were present in the nanostructured Al2O3– 13TiO2 coating. Besides Al2O3 and TiO2 nanograins, the ZrO2 and CeO2 nanoparticles were also present. Nanograins with the dimension of several tens of nanometers created cauliflower-like highly porous agglomerates. Significantly bigger particles of ZrO2 and CeO2 in the range of several micrometers are also components of the nanostructured powder. 2. Detailed analysis of the morphology of sprayed and polished nanostructured surface of the coating. Measurements of topography of the nanostructured Al2O3–13TiO2 coating after polishing reveals that parameters for nanostructured coatings are nearly two-fold lower in comparison to conventional alumina–titania coatings.
60 40 20 0 0
Parameter
Al2O3–13TiO2 (N)
Al2O3–13TiO2 (S)
Al2O3–3TiO2 (S)
Sa Sq Ssk Sku Sp Sv Sz
0.05263 0.0780 −2.051 9.957 0.3846 0.4914 0.8760
0.09513 0.1361 −1.714 8.814 0.6440 0.8516 1.496
0.09338 0.1261 −1.11 5.553 0.4536 0.6175 1.071
10
15
20
t, min Fig. 12. Abrasive wear of plasma-sprayed alumina–titania coatings.
3. The nanostructured Al2O3–13TiO2 coating exhibits a better tribological properties than the conventional Al2O3–TiO2 coating. The coefficient of friction of the polished nanostructured Al2O3–13TiO2 coating and rate of abrasive wear was lower than that for the conventional plasma sprayed coatings. 4. The hardness and the modulus of the nanostructured Al2O3–13TiO2 coating was distinctly higher than those of both alumina–titania coatings. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Table 4 After polishing surface topography parameters according to ISO 25178.
5
[23] [24] [25] [26] [27] [28] [29] [30] [31]
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Contents lists available at ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and tribological properties of nanostructured and conventional plasma sprayed alumina–titania coatings Wojciech Żórawski a,⁎, Anna Góral b, Otakar Bokuvka c, Lidia Lityńska-Dobrzyńska b, Katarzyna Berent b a b c
Kielce University of Technology, Poland Institute of Metallurgy and Materials Science, Cracow, Poland University of Zilina, Slovakia
a r t i c l e
i n f o
Available online 16 September 2014 Keywords: APS Alumina–titania Nanostructure COF Abrasion
a b s t r a c t The nanostructured and two conventional alumina–titania coatings were deposited by atmospheric plasma spray. The presented studies show that nanoparticles are the predominant component of nanostructured Al2O3–13TiO2 powder grains. The microstructure of coating consisted of two distinct regions: fully melted and unmelted or partially melted nanostructured areas, which were comprised of components of a starting powder. Microhardness and modulus of these coatings were significantly lower than the values obtained for nanostructured coating. It was found that the plasma-sprayed nanostructured Al2O3–13TiO2 coating possessed better tribological properties than those of conventional alumina–titania coatings. The lower coefficient of friction and wear of the nanostructured Al2O3–13TiO2 coating is attributed to the bimodal microstructure and the enhanced mechanical properties in comparison to conventional alumina–titania coatings. © 2014 Elsevier B.V. All rights reserved.
1. Introduction Nanostructured materials are of particular scientific interest because of their physical and mechanical properties, which are superior to those of conventional materials. Physical Vapor Deposition (PVD), Chemical Vapor Deposition (CVD) or sol–gel techniques are the most common processes which are used to produce nanostructured coatings involving atomic or molecular layer deposition. An advantage of these processes is that the coatings possess very good properties. These processes have significant limitations in regard to applied coatings and substrate materials. Additional problems include high vacuum (PVD), high temperature (CVD), additional post-treatment (CVD, sol–gel), and the necessity of work piece positioning for deposition in line of sight [1,2]. Additional inconveniencies consist of small deposition rate, film thicknesses, and high cost of manufacturing [3–5]. All above disadvantages can be avoided by application of nanocrystalline coatings through thermal spraying processes. Moreover, some investigations reveal that the nanostructured coatings possess better properties than the same coatings sprayed with conventional feedstock [6–12]. Friction coefficient of the HVOF sprayed nanostructured WC–Co coating was 20% lower in comparison with the conventional WC–Co coating [13]. It was reported that VPS and HVOF-sprayed nanostructured TiO2 coatings revealed the highest abrasion resistance and crack propagation resistance than those exhibited by the best conventional coatings [14].
⁎ Corresponding author. E-mail address: [email protected] (W. Żórawski).
http://dx.doi.org/10.1016/j.surfcoat.2014.09.014 0257-8972/© 2014 Elsevier B.V. All rights reserved.
Nanostructured plasma sprayed Al2O3–13TiO2 coatings were in the area of interest over the last decade because they showed improved properties over conventional coatings [10,17–38]. Spraying of such coatings using standard plasma systems is possible thanks to applications of nanostructured powders with the mean diameter of 30 μm which are prepared from nanocrystalline grains. Parameters of plasma spraying process should be carefully controlled because of high enthalpy of plasma stream which influences the degree of powder melting. This causes changes in the nanostructure of powder grain. Consequently sprayed coatings consist of partially and fully melted regions which create bi-modal microstructure. Better wear characteristics of nanostructured alumina–titania coatings in comparison to conventional coatings were shown for the first time by Wang et al. [29]. Abrasive wear resistance of the nanostructured Al2O3–13TiO2 powder with the addition of CeO2 and ZrO2 was about four times higher than that of sprayed with a corresponding Metco 130 powder. These results were confirmed by Shaw et al. [30]. Moreover, the strong dependency of coating density and hardness on the ratio of electrical power to the primary argon gas flow rate was also demonstrated. This function known as critical plasma spray parameters (CPSP) was the base of investigations carried out by Jordan et al. [17] and Gell et al. [31] using coatings sprayed with optimized parameters. Analysis of the test results show superior mechanical properties as indentation crack resistance, abrasive wear resistance, adhesion strength, and spallation resistance. These enhancement properties were attributed to unique bi-modal structure of coatings that have a retained nanostructure originated from nanostructured powders. Goberman et al. [18] analyzed the influence of CPSP on volume percent
W. Żórawski et al. / Surface & Coatings Technology 268 (2015) 190–197 Table 1 Plasma spraying parameters. Parameter
Value
Current, A Voltage, V Plasma gas pressure, MPa Spraying distance, mm Powder feeding rate, g/min
600 60 0.7 100 45
fraction of γ-Al2O3 phase in bi-modal and conventional coatings. The content of this phase grew with increasing CPSP for nanostructured powders. In the case of the conventional powder (Metco 130) the share of γ-Al2O3 phase was unchanged. As CPSP increased over 390, the participation of this phase in both coatings decreased which caused an increase in the α-Al2O3 content. Toughening mechanisms in nanostructured Al2O3–13TiO2 coatings produced at optimized CPSP were investigated by Luo et al. [20]. Such coatings with 15–20% of the partially melted particulate regions, possessed approximately 100% improvement in the crack growth resistance. These regions allow to deflect and trap boundary cracks. Bolledu et al. [32] investigated the influence of CPSP on volume fraction of α-Al2O3 and γ-Al2O3 phases in nanostructured coatings with the application of nitrogen and argon as primary plasma gases. The effect of CPSP on coating properties in the case of nitrogen was negligible but the application of argon caused significant changes in coating characteristics. The optimum plasma spray parameters for the deposition of nanostructured Al2O3–13TiO2 coatings can be achieved by application design of experiment. Yusoff et al. [38] applied two-level factorial design to obtain optimum properties (wear resistance, surface roughness and microhardness) by changing primary gas pressure, carrier gas pressure and powder feed rate. Sanchez et al. [22] confirmed better tribological properties and higher microhardness of nanostructured coatings over conventional coatings. It was found that changed spraying parameters did not possess significant influence on coating properties. Also investigations carried out by Zhang et al. [33] revealed negligible influence of plasma stream power (30 and 45 kW) on the wear of nanostructured coatings. Additionally, Liu et al. [34] found that the applied spraying system (plasma or HVOF) had a significant influence on the coating abrasive wear resistance but the microstructure of powder did not. Tian et al. [35] showed that nanostructured coatings can reach two times higher fretting wear resistance than conventional coatings in tests carried out on polished samples against steel balls. The enhancement fretting wear resistance of nanostructured coatings was attributed to the nanosized grains, reduced lamellar structures, and amorphous phases. Rico et al. [36] investigated wear rates of nanostructured and conventional coatings by means of a pin on disk tribological tester at a wide range of loads. In all cases wear rates of the nanostructured material were lower but friction coefficient for both materials was similar.
a)
b)
191
The main wear mechanism was attributed to the brittle propagation of cracks. In the low load regime the cracks tend to propagate along the splat boundaries, but at high contact loads the cracks pass through the splats. Wang et al. [37] studied the thermal shock behavior of two nanostructured Al2O3–13TiO2 and corresponding conventional coatings (Metco 130). Nanocoatings exhibited a much higher thermal shock resistance at temperatures of 650 °C and 850 °C. Moreover, the crack propagation mode was quite different for nanostructured and conventional coatings. Rico et al. [23] studied the effect of temperature on tribological properties of both conventional and nanostructured Al2O3–13TiO2 coatings at 500 °C. Wear resistance of nanostructured coatings was superior to that of conventional coatings under all experimental conditions. It was observed that at high temperatures plastic deformation appears as the main wear mechanism. In the present study, the nanostructured Al2O3–13TiO2 coating and two kinds of conventional Al2O3–13TiO2 and Al2O3–3TiO2 coatings were plasma sprayed. Although plasma-sprayed nanostructured Al2O3–13TiO2 coatings have been extensively studied in recent years there is still a need for a detailed analysis of structure and composition of nanostructured powder grains and coating. The presented studies provide this analysis and in addition offer an analysis of the topography of sprayed coatings, which is significant for the determination of mechanical properties of coatings. Moreover the presented results add to tribological studies by means of providing new information on the coefficient of friction and abrasive wear resistance of coatings.
2. Experiment Three types of alumina–titania powder feedstock were applied in this study: nanostructured powder Al2O3–13TiO2 (Infralloy Nanox S2613S), (N), and two standard powders Al2O3–13TiO2 (Amdry 6228), (S), and Al2O3–3TiO2 (FST C-336.31), (S). Plasma spraying was performed by means of a Plancer PN-120 plasma spraying system with a Thermal Miller 1264 powder feeder. Plasma spraying parameters are listed in Table 1. For the metallographic sections and abrasive examination, the coatings were deposited on flat low-carbon steel samples with dimensions of 30 mm × 30 mm × 3 mm, whereas for the ball on disk test the coatings were deposited on disk-shaped samples in low-carbon steel with dimensions of ϕ30mm × 6 mm. Before plasma spraying, all coupons were degreased and grit blasted with electrocorundum EB-12 at a pressure of 0.5 MPa. After spraying the thickness of the coatings was 0.35 ÷ 0.5 mm. The morphologies of the powders and the coatings were analyzed using the following microscopes: SEM JSM-5400 with an ISIS 300 Oxford (EDS) microprobe, SEM FEI Nova™ NanoSEM 200 and TEM Philips CM20 with EDAX EDX. Phase composition was studied using a
c)
Fig. 1. Morphology of powder grain: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
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Fig. 2. Morphology of the Al2O3–13TiO2 (N) powder grain surface.
Bruker D8 Advance diffractometer with Co-Kα radiation of wavelength λ = 1.78897 Å. Friction coefficient for plasma sprayed coatings at unlubricated conditions was determined by means of a T-05 ball-on-disk type tribotester. The bearing steel 100Cr6 balls of 6 mm in diameter were used as a counter body. The sprayed coatings on the disk samples were subsequently ground and polished for 1 h. The action of the friction force in the function of time was registered and controlled using a computer. The parameters of the tribotester were as follows: radius — 19 mm, load — 5 N, linear velocity v = 0.1 m/s, and total distance — 2000 m. The abrasive wear resistance of all alumina–titania coatings was evaluated by applying a dry sand rubber wheel tester (T07). The alumina powder of 300 μm was applied as an abrasive medium in this test. Investigations were repeated three times for each coating. The micromechanical testing of coatings was carried out by using a nanoindentation technique (MCT-CSM) with a Vickers indenter (the Olivier and Pharr methodology). Ten readings were taken for each coating. The topography of coatings “as sprayed” and after polishing was analyzed by means of a Talysurf CCI-Lite non-contact 3D profiler [15].
α -Al2O3
TiO2
α -Al2O3
CeO2
ZrO2 500 nm Fig. 3. TEM micrograph of Al2O3, TiO2, ZrO2 and CeO2 nanoparticles.
Fig. 4. Linear analysis of Al2O3–13TiO2 (N) powder grain cross-section.
3. Results and discussion 3.1. Characterization of alumina–titania powders Three types of alumina–titania commercial feedstock were used in the study: nanostructured Al2O3–13TiO2 powder (Nanox™ S2613S Inframat Advanced Materials), (N) and two conventional Al2O3–13TiO2 (Amdry 6228), (S) and Al2O3–3TiO2 (FST C-336.31), (S) powders. Fig. 1a shows the morphology of the nanostructured alumina-powder. Most grains have a regular spherical shape, only some of them are fractured. Imaging of the grain surface at high magnification SEM shows that each grain of powder consists of nanograins which create cauliflower-like high porous agglomerates (Fig. 2). The slight necking connecting nanograins are a result of the manufacturing process which allows to obtain grains in size range of −50 + 10 μm [16,17]. To further analyzed the morphology of the nanostructured powder, TEM analysis of a single agglomerated powder was performed and is presented in Fig. 3. Besides Al2O3 and TiO2 nanograins, the addition of ZrO2 and CeO2 nanoparticles is present. This image confirms that a single grain of powder is made up of nano-sized particles ranging from tenth to 500 nm in size. Additional components were added to lower the sintering temperature and to improve the densification process [18], however they were not reported in some works [16,17,19,27]. Linear analysis of grain crosssection (Fig. 4) revealed that additional oxides exist not only in the range of nanometers; significantly bigger particles of ZrO2 and CeO2 in the range of several micrometers are also components of the nanostructured powder. Nanostructured Al2O3–13TiO2 powder consists of wt.%: Al2O3 — 75.7%, TiO2 — 11.3%, ZrO2 — 8%, and CeO2 – 5%. Such composition of the nanostructured powder was reported by Bandyopadhyay et al. [21]. Fig. 1a and b shows the morphology of the particles of the conventional alumina–titania powders. The almost irregular angular shape of the particles is a characteristic of powders produced by fusing and crushing. These conventional powders are mixtures of alumina (dark
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a)
b)
193
c)
Fig. 5. Morphology of coating surface: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
grains) with different contents of titania (light grains) with a grain size of −45 + 15 μm for Al2O3–13TiO2 and −53 + 10 μm for Al2O3–3TiO2. 3.2. Characterization of alumina–titania coatings The surface morphology of the plasma-sprayed coatings is shown in Fig. 5. The splats of the nanostructured powder seem to be homogenous (Fig. 5a), however nanograins of incomplete melted powder grains are visible at higher magnification (Fig. 6b). Differences in the quantity of light phase splats represents titania on the surface of conventional powders (Fig. 5b and c) and is the result of the share of titania in the mixture with alumina, 13 and 3% respectively. These splats were segregated in an appropriate proportion and are also well seen at the cross-section of both conventional coatings (Fig. 6b and c). Such conventional microstructure was reported by Sanchez et al. [22]. Dark areas represent pores or oxides which are an inherent feature of plasma sprayed coatings. Unlike the typical lamellar microstructure of the conventional coating where grains of powder are completely melted, the microstructure of the nanostructured coating consists of regions with different degrees of melting (Fig. 6a). Such bimodal microstructure containing melted and unmelted particles is typical for coatings sprayed with reconstituted nanostructured feedstock and was previously reported by other researches [21–25]. The formation of such microstructure is a result of lower heat transfer within the agglomerated particles in comparison to the conventional milled feedstock. Additionally, the degree of melting is influenced by grain size and its trajectory in the plasma stream [17,26,27]. On impact, the cooling process starts from the surface of grain and outer part of nanostructured grain which solidifies while the center preserves original nanocrystal structure is preserved (Fig. 7a). In the case of incomplete melting of particle, the nanosized grains are retained in the melted matrix (Fig. 7b). Further TEM investigations revealed that these nanosized grains are Al2O3 nanocrystals which are surrounded by the mixture of melted TiO2, ZrO2 and CeO2 nanograins (Fig. 8a). Such matrix composition may be due to the higher melting point of alumina (2072 °C) in comparison to titania (1843 °C).
a)
b)
Titania is melted entirely in the plasma stream and unmelted alumina nanograins can get embedded in the melted titania [23]. This description does not explain the presence in the matrix of additional oxides with significantly higher melting temperature (zirconia — 2715 °C, ceria — 2400 °C) and dimensions (Fig. 4). TEM observations of fully melted area show the typical lamellar microstructure with the presence of γ-Al2O3 and are easily recognizable by a gray contrast amorphous phase (Fig. 8b). Table 2 presents the chemical composition specified on the base of EDS analysis of the fully melted nanostructured coating (Fig. 9). Visible decrease of aluminum amount is present in the amorphous phase (points 2 and 6). The changes of titanium content are slight across the coating in the range from 2.8 to 4.2 at.%. Uniform distribution of Zr and Ce allows to assume that these elements were dissolved in the γ-Al2O3 phase. The XRD patterns of all alumina–titania powders are presented in Fig. 10a. The main constituents of all powders are α-Al2O3 and titania in the form of rutile. In the case of the nanostructured powder, additional peaks confirm the presence of earlier described additives in the form of ZrO2 and CeO2. In comparison to conventional powders nanostructured feedstock presents broader peaks. Nanoparticles have much less atoms hence the lattice sum is not able to converge to a diffraction line but broadens out. The XRD pattern of all sprayed coatings (Fig. 10b) revealed an additional phase of γ-Al2O3. The presence of this phase in all plasma sprayed coatings is due to the melting of the alumina–titania powders and the fact that at a higher cooling rate γ-Al2O3 commonly nucleates in the preference to α-Al2O3 during the rapid solidification of liquid droplets. The reason for this phenomena is lower energy for nucleation from the liquid for the γ-phase than for the α-phase [40]. Peaks corresponding to ZrO2 and CeO2 were not detected in the XRD pattern of the nanostructured coating surface. The force–depth penetration curves obtained from nanoindentation are shown in Fig. 11. The nanostructured coating was much harder because it exhibited a lower penetration and higher stiffness (the highest slope in the unload branch at a maximum load) than the conventional coatings. Young's modulus for the nanostructured coating
c)
Fig. 6. Microstructure of coating: a) Al2O3–13TiO2 (N), b) Al2O3–13TiO2 (S), and c) Al2O3–3TiO2 (S).
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a)
b)
Fig. 7. Morphology of the Al2O3–13TiO2 (N) coating: a) fracture and b) surface.
was 164 HV0.1 ± 19 GPa and 123 HV0.1 ± 14 GPa and 102 HV0.1 ± 15 GPa for both conventional coatings, Al2O3–13TiO2 and Al2O3–3TiO2 respectively. The nanostructured plasma sprayed Al2O3–13TiO2 coating possesses a higher microhardness (1192 ± 180 HV0.1) than the conventional coatings (980 ± 186 HV0.1 and 683 ± 99 HV0.1). The presented results showed a significantly better microhardness and Young's modulus of the nanostructured coating. The main reason for this is the distinctive bi-modal microstructure obtained by spraying reconstituted nanostructured feedstock. It consists of fully melted and splat-quenched regions (γ-Al2O3 supersaturated with Ti4+ and amorphous phase) and partially melted particulate where the initial microstructure of the nanostructured agglomerates is retained. The partially melted region consists of α-Al2O3 particles embedded in γ-Al2O3 supersaturated with Ti4 + with the presence of Zr and Ce. The bi-modal distribution of microstructure and grain size led to the superior mechanical properties. Optimum properties are provided by the presence of 15–20% of partially melted regions [20]. These nano-scale and submicron-scale microstructures in the coatings form a skeleton-like
structure. The nano-scale and submicron-scale skeleton structures play a very important role in the behavior of bi-modal coatings against external impact [29]. On the other hand, the microstructure of the conventional coating consists primarily of fully melted splats (γ-Al2O2). Moreover, weak splat boundaries in such coating typically contain many small voids and have a significantly lowered fracture toughness. Soft partially melted regions in hard nanostructured ceramic coatings improve ductility and fracture toughness, and consequently, the resistance to crack initiation and propagation, which is important in wear resistance. The crack arrest phenomenon is due to crack trapping within the partially melted regions and by crack deflection at the interface between partially melted and fully melted regions. These results indicate that the partially melted particulate region and the interface between partially melted and fully melted regions have a better crack arresting potential than the fully melted splat microstructure. These reduced crack propagation capabilities are a result of crack deflection, crack trapping and crack stopping at the interface [20,38]. Moreover, Sebastiani et al. [39] suggests that the presence in microstructure of
a)
b)
Fig. 8. TEM micrograph of the Al2O3–13TiO2 (N) coating: a) partially melted area and b) fully melted area.
W. Żórawski et al. / Surface & Coatings Technology 268 (2015) 190–197 Table 2 Results of EDS analysis of the fully melted Al2O3–13TiO2 (N) coating. No.
1 2 3 4 5 6 7 8
195
a)
Chemical composition, at.% Al
O
Ti
Zr
Ce
50.0 46.6 49.4 50.5 48.2 45.2 45.8 49.6
44.5 47.2 45.6 45.4 44.7 48.3 47.0 46.8
3.5 3.7 3.4 2.9 3.6 3.5 4.2 2.8
1.0 1.3 0.6 0.4 1.7 1.4 1.8 0.4
1.0 1.2 1.0 0.8 1.8 1.6 1.2 0.4
glassy splats, which are free of micro-cracks and subjected to near zero tensile quenching stress, can play an important additional role in toughening Al2O3\TiO2\ZrO2\CeO2 plasma sprayed coatings.
b)
3.3. Topography of the coatings Results of the topography of all plasma as-sprayed coating surfaces are presented in Table 3. Surfaces were scanned and measured to estimate coating influence on the wear process. The highest arithmetic mean of the surface height Sa of the as-sprayed nanostructured coating is slightly higher than for both conventional coatings, however the ranges of grain size distribution of all powders were very close. This result can be attributed to the earlier described lower heat transfer within the reconstituted powder causing enhanced coating roughness. The similar dependency was observed for the mean squared surface height, Sq. Negative value of asymmetry of the surface, Ssk, indicated that the both surfaces Al2O3–13TiO2 (N) and Al2O3–3TiO2 (S) were flattened and occurring peaks were rounded. Kurtosis Sku is responsible for steep irregularities and defects. The value of kurtosis over 3 indicated that the distribution of profile ordinate corresponds to a higher concentration around the mean value. Parameters: the maximum peak Sp, the maximum valley Sv and the maximum height Sz are nearly two times lower in the case of the nanostructured coating surface. It results from the participation of smaller spherical grains of the nanostructured powder in the spraying process in comparison to angular grains of conventional powders. The same range of topography measurements were carried out on coating surfaces after polishing before the investigation of coefficient of friction. Results of measurements are reported in Table 4. All plasma sprayed alumina–titania coatings were polished to obtain a surface with mirror reflectivity. To avoid influence of treatment on surface topography the same conditions of polishing were applied to
Fig. 10. XRD patterns of: a) Al2O3–13TiO2 (N), Al2O3–13TiO2 (S) and Al2O3–3TiO2 (S) powders, and b) Al2O3–13TiO2 (N), Al2O3–13TiO2 (S) and Al2O3–3TiO2 (S) coatings.
nanostructured and conventional coatings. In the case of the polished nanostructured coating, parameter Sa = 0.05263 μm is nearly twofold lower than Sa for both conventional coatings. As a consequence, the roughness of the nanostructured coating is significantly lower. 3.4. Measurements of friction coefficient and abrasive wear resistance Results of the carried out tests showed that the friction coefficient for the nanostructured coating was 0.51 and for conventional coatings was 0.62 and 0.58 respectively, after a 2000 m distance. Lower friction coefficient of the nanostructured coating is probably due to its fundamentally different microstructure in comparison to the typical lamellar
120
Al2O3-13TiO2 N Al2O3-13TiO2 S A2O3-3TiO2 S
100
Load, mN
80 60 40 20 0 0
500
1000
Displacement, nm Fig. 9. TEM micrograph of the Al2O3–13TiO2 (N) fully melted area with the distribution of elements along the marked line.
Fig. 11. Load–displacement nanoindentation curves on alumina–titania plasma sprayed coatings.
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200
Table 3 As-sprayed surface topography parameters according to ISO 25178. Al2O3–13TiO2 (N)
Al2O3–13TiO2 (S)
Al2O3–3TiO2 (S)
6.63564 8.8498 −0.0426 4.8029 35.3753 39.0988 74.4741
6.3652 8.1312 0.2221 3.8954 82.1862 88.7426 170.9288
5.7312 7.0842 −0.0460 3.0116 76.9685 65.7015 142.6700
Al2O3-13TiO2 S
160
Al2O3-3TiO2 S
140
Δ mg
Parameter Sa Sq Ssk Sku Sp Sv Sz
Al2O3-13TiO2 N
180
120 100 80
microstructure of conventional coatings. The nanostructured microstructure containing agglomerates with different degrees of melting limited the propagation of any crack generated in the sprayed coating [25]. Additionally, such microstructure revealed a significantly higher mechanical properties than both conventional coatings. In the conditions of dry sliding test, probably the presence of unmelted nanoparticles on the surface collaborating with steel 100Cr6 balls can positively influence the friction process in which small contact areas cause high pressures. Also the important factor is the roughness of the nanostructured coating surface which is two-fold lower than the roughness of both conventional coatings [28]. Fig. 12. presents the results of all alumina–titania coating abrasive wear tests (dry sand rubber wheel). The carried out measurements showed that the nanostructured coating has the best wear resistance after 20 min of testing. The worst wear resistance was demonstrated by the conventional Al2O3–3TiO2 coating which was nearly three times lower in comparison to the nanostructured coating. Wear resistance of the conventional Al2O3–13TiO2 coating was nearly 30% lower than the best coating. The roughness of all as-sprayed coatings was nearly the same, so the microstructure and mechanical properties of bimodal nanostructured coatings appear to be the essential factors which influenced the significantly higher abrasive wear resistance.
4. Conclusions The novelty and significance of the presented studies consist of: 1. Detailed analysis of reconstituted nanostructured feedstock demonstrated that nanocrystal Al2O3 grains, which were a constituent of the sprayed powder, were present in the nanostructured Al2O3– 13TiO2 coating. Besides Al2O3 and TiO2 nanograins, the ZrO2 and CeO2 nanoparticles were also present. Nanograins with the dimension of several tens of nanometers created cauliflower-like highly porous agglomerates. Significantly bigger particles of ZrO2 and CeO2 in the range of several micrometers are also components of the nanostructured powder. 2. Detailed analysis of the morphology of sprayed and polished nanostructured surface of the coating. Measurements of topography of the nanostructured Al2O3–13TiO2 coating after polishing reveals that parameters for nanostructured coatings are nearly two-fold lower in comparison to conventional alumina–titania coatings.
60 40 20 0 0
Parameter
Al2O3–13TiO2 (N)
Al2O3–13TiO2 (S)
Al2O3–3TiO2 (S)
Sa Sq Ssk Sku Sp Sv Sz
0.05263 0.0780 −2.051 9.957 0.3846 0.4914 0.8760
0.09513 0.1361 −1.714 8.814 0.6440 0.8516 1.496
0.09338 0.1261 −1.11 5.553 0.4536 0.6175 1.071
10
15
20
t, min Fig. 12. Abrasive wear of plasma-sprayed alumina–titania coatings.
3. The nanostructured Al2O3–13TiO2 coating exhibits a better tribological properties than the conventional Al2O3–TiO2 coating. The coefficient of friction of the polished nanostructured Al2O3–13TiO2 coating and rate of abrasive wear was lower than that for the conventional plasma sprayed coatings. 4. The hardness and the modulus of the nanostructured Al2O3–13TiO2 coating was distinctly higher than those of both alumina–titania coatings. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
Table 4 After polishing surface topography parameters according to ISO 25178.
5
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