Improvement in wear resistance of plasma sprayed yttria stabilized zirconia coating using nanostructured powder

Tribology International 37 (2004) 77–84 www.elsevier.com/locate/triboint

Improvement in wear resistance of plasma sprayed yttria stabilized zirconia coating using nanostructured powder J.F. Li a, H. Liao a,∗, X.Y. Wang a, B. Normand a, V. Ji b, C.X. Ding c, C. Coddet a a

c

LERMPS, Universite´ de Technologie de Belfort-Montbe´liard, 90 010 Belfort, France b Ecole Nationale Supe´rieure d’arts & me´tiers de Paris, 75013 Paris, France Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China

Received 17 April 2002; received in revised form 23 June 2003; accepted 10 July 2003

Abstract Plasma sprayed yttria stabilized zirconia coatings were prepared using nanostructured and conventional powders with optimized process parameters for the highest deposition efficiency, the smallest porosity and the highest microhardness. The tribological properties of these coatings against 100C6 steel were then tested with a ball-on-disc arrangement. Results showed that although the friction coefficients of the coatings sprayed using the nanostructured powder were slightly different from those of the coatings sprayed using the conventional powder, the former coatings were more wear resistant than the latter coatings. The wear mechanisms of all the coatings were explained in terms of adhesion-induced spallation and micro-fracturing of lamellae. The improvement in wear resistance of the coatings sprayed using the nanostructured powder could be mainly ascribed to the decrease of micrometersized defects such as pores and interlamellar and intralamellar cracks in the coatings.  2003 Elsevier Ltd. All rights reserved. Keywords: Friction and wear; Plasma spraying; Yttria stabilized zirconia; Nanostructured powder

1. Introduction Thermally sprayed coatings conventionally using 10– 100 µm sized starting powders have been widely practiced for many years to prepare metallic and ceramic coatings applied as wear-resistant surface layers [1]. Correspondingly, significant study has related friction and wear behaviour of thermal sprayed coatings to operating conditions such as applied load, sliding velocity, temperature and lubricated media [2–6]. This research has improved the understanding of the tribological properties of thermal sprayed coatings and consequently promoted the development and applications of these coatings. Over the past years, industrial trends have required more and more wear-resistant coatings. Using nanostructured powders as feedstocks for thermal spraying is

Corresponding author. Tel.: +33-3-84-58-32-42; fax: +33-3-8458-32-86. E-mail address: [email protected] (H. Liao). ∗

0301-679X/$ - see front matter  2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0301-679X(03)00138-5

expected to be an effective approach to deposit more wear-resistant coatings. This is because coatings with thinner lamellae and improved microstructures may be obtained when the nanometer-sized grains or particles are reconstituted into micrometer-sized agglomerates and used as the starting powders for thermal spraying [7–9]. In fact, Wang et al. [10] have confirmed that the abrasive wear resistance of the coatings produced using nanostructured Al2O3/TiO2 powder was greatly improved compared to coatings produced using conventional Al2O3/TiO2 powder. Because of a low density and high hardness, stiffness, strength and refractoriness, zirconia-based ceramics have been regarded as potential candidate materials for tribological applications [11]. Plasma sprayed yttria stabilized zirconia coatings have been used in engines and gas turbines as thermal barrier coatings. The coatings sprayed onto cylinder liners could enhance the thermal efficiency of internal combustion engines and meanwhile increase the service life of piston ring/cylinder liner pairs [12]. In previous work [13], the plasma spraying of nanostructured yttria stabilized zirconia powders was investigated

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and compared with that of the conventional powder. It was found that the nanostructured powder clearly improved the deposition efficiency, coating microstructure and microhardness. In the present work, the coatings, optimised with respect to deposition efficiency, porosity and microhardness, were sprayed onto medium carbon steel discs using both the nanostructured and conventional powders. Subsequently, the friction and wear behaviour of these coatings under sliding against 100C6 steel was studied.

2. Experimental procedure 2.1. Coating preparation All the coating samples were prepared using a SulzerMetco F4-MB (Sulzer-Metco, Switzerland) plasma gun mounted on an ABB IRB2400 robot (ABB, Sweden). They were sprayed onto medium carbon steel discs of 65 mm in diameter and 5 mm in thickness initially gristblasted by alumina abrasive. Prior to the spraying of the yttria stabilized zirconia coatings, an MCrAlY (SulzerMetco AMDRY 345) layer was applied as bond coating to enhance adhesion and reduce thermal expansion mismatch between the substrate and the ceramic coatings. The thickness of the bond coating was around 100 µm on average, and the top yttria stabilized zirconia coatings were in the range of 300 to 450µm. Table 1 lists the sample codes, starting powders and plasma spray process parameters of the yttria stabilized zirconia coatings and the MCrAlY bond coating. The nanostructured powder (7%Y2O3-ZrO2) was synthesized using the co-precipitation method and then reconstituted into micrometer-sized agglomerates by spray drying [13]. The conventional powder was the commercially available Sulzer-Metco 204NS-G powder (8%Y2O3-ZrO2). Fig. 1 illustrates the scanning electron microscopy (SEM) micrographs of the two kinds of yttria stabilized zirconia starting powders. The major difference between the two powders was the density. Because of many empty spaces, the density of the nanostructured powder was one half that of the conventional powder. Based on previous work [13], for the nanostruc-

Fig. 1. The SEM micrographs of (a) the reconstituted nanostructural powder and (b) the conventional Sulzer-Metco 204NS-G powder.

tured powder, the N1 coating had optimal deposition efficiency (74.1%), and the N2 coating had the optimal porosity and microhardness. For the conventional powder, the C2 coating had all the optimal deposition efficiency (about 52%), porosity and microhardness. The C1 coating was sprayed to compare with the other three coatings.

Table 1 The sample codes, starting powders and plasma spray process parameters Code

Starting powder

Current (A)

Ar flow rate (l/min)

H2 flow rate (l/min)

Spray distance (mm)

Powder feed rate (g/min)

N1 N2 C1 C2 Bond coating

Nanostructural powder Nanostructural powder Conventional powder Conventional powder Sulzer Metco AMDRY 345

620 620 620 620 600

30 30 30 30 55

8 13 8 13 8

90 90 90 90 90

18.4 18.4 46.0 46.0 40.0

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2.2. Coating characterization The microstructure, phase composition, porosity and microhardness of the yttria stabilized zirconia coatings were characterized. The preparation of the cross sections of the coatings was the same as described in the previous paper [13]. The microstructure of the coatings was observed with a Nikon optical microscope (OM, Nikon, Japan), and their porosity was measured using an image analysis method [13]. The microhardness measurement was operated by indenting the coating cross sections at a load of 2.942 N for 15 s using a Leitz RZD-DO Vickers hardness tester (Leitz, Germany). The phase composition of the coatings was determined by X-ray diffraction (XRD) employing a Simens D500 diffractometer (Simens, Germany) with Co Kα radiation, and the average grain size of the coatings was estimated basing on the XRD peak broadening according to the Scherrer formula [9]. The phase composition at worn surface was examined by a Seifert TS4 high resolution micro beam XRD equipment with Cr Kα radiation (l = 0.227 nm) within an irradiated surface 1 mm × 2 mm. 2.3. Friction and wear tests A friction and wear test was carried out using a ballon-disc arrangement on a CSEM tribometer (CSEM, Switzerland). The ball samples were 6 mm in diameter 100C6 steel balls. Prior to the friction and wear tests, the coatings were ground using SiC sandpaper and then polished using diamond slurries down to an average surface roughness of 1.6–2.2 µm.

Fig. 3. The XRD patterns of the two starting powders and the four coatings.

The experiments were performed under the following conditions: room temperature, air environment, applied load of 10 N and sliding velocity 1.0 m/s. During the test, the friction force samples were directly measured with a sensor and fed into a computer at a frequency of 12 values per min. The friction force data were simply divided by the applied loads to give the friction coefficients. The wear rates represent the worn volumes per unit of the applied load and of the sliding distance. The cross-sectional areas of the worn tracks were measured using a Taylor Hobson Surtronic 3P profilometer (Rank Taylor Hobson Ltd., UK), and then multiplied by the length (perimeter) of the worn tracks to give the total worn volumes. In addition, the worn surfaces and debris

Fig. 2. The OM micrographs of the cross sections of the four coatings: (a) N1, (b) N2, (c) C1 and (d) C2. From left-to-right, for each micrograph, the layers are stabilized zirconia, bond coat, and substrate

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of the frictional pairs were observed with a JEOL JSM5800LV scanning electron microscopy (SEM, JEOL, Japan) to investigate the wear mechanisms.

3. Results and discussion 3.1. Coating characterization Fig. 2 shows the microstructure of the four coatings as revealed by optical microscopy. It seems the nanostructured powder improves the microstructure of the coatings. Among the four coatings, the N2 coating is the densest and exhibits the least pores and interlamellar and intralamellar cracks, and the C1 coating is the most porous and possesses the most interlamellar and intralamellar cracks. Although the N1 and C2 coatings contain similar porosity, the former coating apparently has fewer interlamellar and intralamellar cracks than the latter. Fig. 3 shows the XRD patterns of the two starting powders and the four coatings. Besides the main nontransformable tetragonal phase, both powders contained some monoclinic phase. The four coatings predominately consisted of a non-transformable tetragonal phase, but very weak diffraction peaks of monoclinic phase could be detected from the N1 and C1 coatings. Table 2 presents the results of the porosity, microhardness, and average grain size of the coatings. The porosity of various coatings was in good agreement with their microstructure as shown in Fig. 4. Despite almost identical porosity, the N1 coating sprayed using the nanostructured powder had a higher microhardness than the C2 coating sprayed using the conventional powder. Although the average grain size measured based on XRD peak broadening gives only an estimate of the grain size [14], the apparent difference among the average grain size of N1, N2 and C2 coatings is obvious. The grains of the N1 coating should be the smallest overall, those of the C2 coating the largest, and those of the N2 coating between those of the N1 and C2 coatings.

Fig. 4. Typical evolutions of the friction coefficient versus the sliding distance.

3.2. Friction and wear Fig. 4 shows two curves representing the evolution of the friction coefficient as a function of the sliding distance. The friction coefficient data is somewhat variable. Therefore, the average friction coefficients, as shown in

Table 2 The porosity, microhardness and average grain size of the coatings Coating code Porositya

N1 N2 C1 C2

Hv0.3a

Average grain Wear rate size (nm) 10⫺5mm3N⫺1m⫺1

Mean (%)

Std. Dev. (%)

x-95 (%)

x+95 (%)

Mean (kg/mm2)

Std. Dev. (kg/mm2)

x-95 (kg/mm2)

x+95 (kg/mm2)

5.6 3.6 8.8 5.7

0.9 0.6 1.6 0.9

5.0 3.2 7.7 5.0

6.2 4.1 10.0 6.3

842 960 537 721

112 84 88 67

790 920 495 689

895 999 578 752

35.3 48.0 … 62.0

10 3.3 18.5 12.8

a For each coating, 10 images of 321×246 µm and 20 indentations under the indenter load of 300 grams randomly on the metallographic cross section were used to measure the porosity and microhardness, respectively.

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Fig. 6. Fig. 5. Comparison of the average friction coefficients of the four frictional pairs.

Fig. 5, were used to compare friction behaviour. Some differences in the friction coefficients can be observed between the coatings sprayed using the nanostructured powder and the coatings using the conventional powder. But still as shown in Fig. 5, a rather high error can be found with statistic calculation. That means there is very little difference in friction coefficient between these coatings. Fig. 6 presents the wear rates of the four coatings and the 100C6 steel balls mated to them. Among the four coatings, the coating N2 was the most wear resistant, followed by the coating N1, then the coating C2, and

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Comparison of the wear rates of the four frictional pairs.

the coating C1 was the least wear resistant. It should be emphasized that the wear rate of the coating N2 was less than one fourth of that of the coatings C1 and C2. In addition, Fig. 6 also revealed that the larger the wear rate of the coating, the larger the wear rate of the ball mated to the coating. 3.3. Worn surfaces and debris Fig. 7 shows the SEM micrographs of the worn surfaces of the four coatings. There were interlamellar interfaces, fracture fragments and relatively smooth patches on these worn surfaces. Some cracks also appeared on the worn surfaces and were clearer on the smooth

Fig. 7. The SEM micrographs of the worn surfaces of the four coatings: (a) N1, (b) N2, (c) C1 and (d) C2.

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patches. The individual area of the smooth patches on the worn surfaces of the N1 and N2 coatings were apparently larger than that on the worn surfaces of the C1 and C2 coatings. The element Fe was more prevalent on these smooth patches than on the other regions of the worn surfaces (Fig. 8). Thus it can be considered that these smooth patches resulted from the material transferred from the mated balls and then plastically deformed due to contact stress. From these worn features, it can be seen that the wear mechanisms of the four coatings were all adhesion-induced spallation and fracturing of lamellae. The wear of the 100C6 steel balls mated to the four coatings all showed plastic deformation and ploughing. Fig. 9 presents the SEM micrographs of the worn surfaces of the 100C6 steel balls against the N2 and C2 coatings. Besides the plastic deformation and ploughing, fragments of the coatings also appeared on the worn surface. Fig. 10 shows the SEM micrographs of the worn debris of the four frictional pairs. The debris consisted of granules smaller than 5 µm (most of them smaller than 1 µm) and flat particles larger than 10 µm. Therefore, the morphology of the debris further confirmed the wear mechanisms of the four coatings, the adhesioninduced spallation and fracturing of lamellae. The granular debris resulted from the lamellae fracturing, and the flat debris mainly came from the adhesion-induced spallation of the lamellae. 3.4. Discussion Except for the area of the relatively smooth patches, there was no substantial difference on the worn surfaces of the various coatings. Thus no obvious difference in the friction coefficients of the coatings can be found (Figs 5 and 7). The micrometer-sized defects such as the pores, the interlamellar and intralamellar cracks in the coatings might act as the cracking origins during sliding. The more micrometer-sized defects in the coating, the more easily the adhesion-induced spallation took place. The fracturing of the lamellae was probably related to the phase transformation of zirconia around the contact locations of the frictional pairs. According to the micro beam XRD analysis (Fig. 11) a little phase transform-

Fig. 8.

Fig. 9. The SEM micrographs of the worn surfaces of (a) the 100C6 steel ball against coating N2 and (b) the 100C6 steel ball against coating C2.

ation was detected in the worn surface: formation of monoclinic phase in both cases: in N1 and C1 coatings, but it is difficult to quantify it. Although the non-transformable tetragonal zirconia is rather stable, it can slowly decompose into the equilibrium tetragonal and cubic phases during high temperature (⬎1000 °C) exposures [15]. Such a high temperature might be reached around the contact locations of the frictional pairs under the friction and wear tests with the friction coefficients shown in Fig. 6 [16]. Consequently, microcracking resulting from the phase transformation

The X-ray energy disperse spectra of (a): point A and (b): point B on the worn surface of coating C2 (Fig. 7 (d)).

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Fig. 10.

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The SEM micrographs of the worn debris of the four frictional pairs: (a) ball/N1, (b) ball/N2, (c) ball/C1 and (d) ball/C2.

adhered to the worn surfaces of the N1 and N2 coatings were larger than those adhered to the worn surfaces of the coatings C1 and C2 (Fig. 7). Fewer pores, interlamellar and intralamellar cracks as well as smaller grain size would also improve the microhardness of the coatings [18]. Thus the wear rates of the coatings, as shown in Fig. 12, appeared to be linearly related to their microhardness.

4. Conclusion

Fig. 11. Spectra of micro beam XRD of as-received coatings and worn surfaces.

of the friction-induced tetragonal structure to monoclinic occurred around these contact locations and resulted in the fracturing of the lamellae [12]. The phase transformation and microcracking may be somewhat suppressed by finer grains in the materials [17]. Therefore, the improvement in wear resistance of the coatings sprayed using the nanostructured powder could be ascribed to both the decrease in the micrometer-sized defects and in the grain size. However, comparing the wear rate of the coating N1 with that of the N2 coating, it can be seen that the decrease in micrometer-sized defects played a more important role than the decrease in the grain size (Fig. 6 and Table 2). It was also the adhesion-induced spallation of the lamellae related to the micrometer-sized defects that led to the fact that the smooth patches

The tribological properties of plasma sprayed yttria stabilized zirconia coatings using nanostructured powder and optimized process parameters were tested under slid-

Fig. 12. ings.

The plot of wear rate versus microhardness of the four coat-

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ing against 100C6 steel and compared with those of the coatings sprayed using commercially available powder. The friction coefficients of the coatings sprayed using the nanostructured powder were almost the same as those of the coatings sprayed using the conventional powder. However, the former coatings were more wear resistant. The wear rate of the most wear resistant coating sprayed using the nanostructured powder was less than one fourth of that of the coatings sprayed using the conventional powders. The wear mechanisms of all the coatings were explained in terms of adhesion-induced spallation and fracturing of lamellae. The more the micrometer-sized defects in the coating, the more easily the adhesioninduced spallation took place. The fracturing of the lamellae was probably related to the phase transformation of zirconia around the contact locations of the frictional pairs. The improvement in wear resistance of the coatings sprayed using the nanostructured powder could be mainly ascribed to the decrease in micrometersized defects such as pores, interlamellar and intralamellar cracks in the coatings.

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Acknowledgements The authors would like to thank the Advanced Research Program of Franco-Chinese Cooperation (PRA MX02-03, C. CODDET & C. DING)

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