Characterization of the Near-Eutectic Al₂O₃-40 Wt.% ZrO₂ Composite Coating Fabricated by Atmospheric Plasma Spray. Part II: Microstructure and Mechanical Properties of Nano Composite Coating

To cite this Article: Bai, Yu , Yu, Fang Li , Lee, Soo Wohn , Chen, Huang and Yang, Jian Feng (2011) ’Characterization of the Near-Eutectic Al ₂O ₃–40 Wt.% ZrO ₂ Composite Coating Fabricated by Atmospheric Plasma Spray. Part II: Microstructure and Mechanical Properties of Nano Composite Coating’, Materials and Manufacturing Processes, 1, doi: 10.1080/10426914.2011.551951, First posted on: 10 May 2011 (iFirst)

Abstract

Keywords: Atmospheric plasma spray; Coating; Cohesion; Feedstock; Near-eutectic

INTRODUCTION

 

With the development of nano-science and nano-technology, the interest in the preparation of nano structured coatings is growing, since they have greatly improved mechanical properties in engineering. Recently, plasma sprayed ceramic coatings using nano sized powders have become more and more attractive [1-3]. It was found that, compared with the traditional coating, the plasma sprayed coatings using nano sized powders showed better mechanical properties, such as high thermal shock resistance [4]. However, individual nano sized powders can not be carried in a moving gas stream and deposited on a substrate due to its low mass and bad flowability in the spraying process, so it is necessary to reconstitute nano sized powders into micro sized granules. There are usually two ways for preparing micro sized granules for plasma spraying. One is to use spray drying process, which is usually very expensive in terms of the powders cost. Another is to fuse and crush the agglomerated particles, which is less expensive compared with the former process [5]. Noticing that although the microstructures and properties of plasma sprayed nano structured alumina or zirconia coatings have been extensively dealt with in many reports, only limited researches have been published on nano structured Al₂O₃-ZrO₂ composite coatings, and in particular, few reports are currently available on the preparation of nano-structured Al₂O₃-ZrO₂ composite coatings prepared by sol-gel method and plasma spraying process [1, 6-9].

In part I [10], agglomerated nano sized Al₂O₃-40 wt.% ZrO₂ composite powders used as feedstock for plasma spraying were directly synthesized by sol-gel method and post-heating process. The effects of heating temperature and holding time on the phase composition and grain size of powders were investigated. In this paper, atmospheric plasma spray (APS) was applied to deposit the near-eutectic Al₂O₃-40 wt.% ZrO₂ composite coating, which was expected to be used as wear-resistant or thermal barrier coating. The phase composition, microstructure and mechanical property of the resulting coatings were investigated.

EXPERIMENTAL

 

Preparation of Coating

 

As described in Part I, when the dried gel was calcinated below 1100°C with holding time lower than 8 h, the grain size of obtained powders was less than 100 nm. So, in this part, the powders prepared at 1100°C for 4 h were used as feedstock for plasma spraying. The sprayed coatings were deposited directly on the 304 stainless-steel substrate (50  20  2 mm). The substrate was degreased ultrasonically in acetone and blasted with alumina grit before spraying. The plasma spraying was carried out with Metco A-2000 atmospheric plasma spraying equipment with F4-MB plasma gun (Sulzer Metco AG, Switzerland). The feedstock was fed with Twin-System (Plasma-Technick AG, Switzerland) for plasma spraying. The external powder port injector (outside the nozzle torch) was perpendicular to the torch axis. Argon and hydrogen were the primary and secondary plasma gas during the process of spraying. During the spraying process, the substrate was cooled by compressed air (about 0.6 MPa) and the temperature of substrate was held about 180°C, monitored by a NiCr/NiSi thermocouple attached to the back of substrate. The scheme of temperature measuring and air cooling system is presented in Fig. 1 and the plasma spray parameters are listed in Table 1.  FIGURE 1. —Scheme of temperature measuring and air cooling system during plasma spraying.

TABLE 1.—Plasma-spray parameters for Al₂O₃-ZrO₂ coating.

Parameters	Value
Gun nozzle diameter (mm)	8
Powder port injector inlet diameter (mm)	1.8
Current (A)	660
Voltage (V)	74
Primary argon gas flow rate (slpm)	41
Secondary hydrogen gas flow rate (slpm)	14
Carrier argon gas flow rate (slpm)	4.5
Spray distance (mm)	110
Gun traverse speed (mm/s)	800
Distance between adjacent torch passes (mm)	3
Total number of torch cycles	25
Substrate preheat temperature (°C)	100

Microstructural and Morphological Analysis of the Coating

 

The coating was characterized by X-ray diffraction (XRD) using Cu K radiation (Rigaku X-RAY DIFFRACTOMETER, Japan). Microstructure of as-sprayed coating was observed by scanning electron microscope (SEM, VEGAII XMU, Tescan, Czech Republic) with energy dispersive X-ray spectroscopy (EDX, INCA-AE350, Oxford Instruments, UK) analysis and high resolution transmission electron microscopy (HRTEM, JSM-6700 F, JEOL, Japan). After substrate was removed from the as-sprayed coating by dissolution in hydrochloric acid, plan-view specimen for TEM analysis was prepared by mechanical grinding, polishing and dimpling, followed by Ar-ion milling using a Gatan 691 precision ion polishing system (Gatan Inc. USA). The HRTEM was equipped with an X-twin objective lens with coefficient of spherical aberration C_s = 1.0 mm (point to point resolution of 0.23 nm) and operated at 200 kV accelerating voltage, meanwhile, selected area electron diffraction pattern (SAEDP) was used to confirm the different phase exactly.

The porosity of coating was calculated through digital image analysis (IA), 15 fields of view at magnification of 1000 on cross sections were used for the porosity measurement. The set-up and requirements of measurement were described in more detail in previous articles [11].

Properties of As-Sprayed Coating

 

Micro hardness of coating was measured by micro hardness tester (MICROMET 3 MICRO HARDNESS TESTER, Buehler Ltd. USA). The polished cross section of coating was measured 35 times at different areas with a load of 300 g pressure and holding time of 10 s, while maintaining a distance between consecutive indentations at least five times larger than the diagonal length of indentation.

The cohesion inside the as-sprayed coating was evaluated by scratch testing method using Revetest Scratch Tester (CSM Instruments, Peseux, Switzerland). The morphologies of scratch tracks were observed using an optical microscope (Olympus OLS 1100, Olympus, Japan). The scratch tests were performed on the top surface and cross section of as-sprayed coating. Surfaces with Ra < 0.5 &micro;m for the top surface and cross section were obtained by standard metallographic procedures such as sectioning, cleaning, sample mounting, grinding and polishing. The Rockwell N-203 diamond stylus was 200 &micro;m in radius. Load was continuously increased from 0 to 100 N at a speed of 100 N/min. The sliding speeds were 20 mm/min and 4 mm/min, respectively. The crack formation was detected by acoustic emission (AE) attached on the scratch tester. The scratch testing was repeated for five times at different locations on the top surface and cross section of as-sprayed coating. It should be noted that, during the test carried out on polished cross section of as-sprayed coating, the indenter moved perpendicular to the sample surface and the scratch direction was from the substrate into the coating. Furthermore, in order to compare the cohesion of present coating to that of other counterparts, a series of comparative scratch tests under the same conditions were also carried out. Plasma sprayed Al₂O₃ and Al₂O₃-40 wt.% ZrO₂ coatings using commercial 100-150 nm Al₂O₃ powders (Sumitomo Chemical CO. Ltd. Japan) and 40-70 nm ZrO₂ powders (Tosoh Corporation, Japan) as feedstock were chosen as comparative samples. Before plasma spraying (plasma spray parameters are shown in Table 1), the above powders were spray dried to form spherical feedstock with particle size about 10-45 &micro;m. Spray-drying was performed using a laboratory scale spray dryer (SD 1000, Eyela, Japan) with a pneumatic nozzle (0.2 mm diameter) under the following set of conditions: powder content in slurry (water), 40 wt.%; PVA (Junsel chemical CO., Ltd. Japan) content in slurry, 2 wt.%; feeding rate of slurry, 12 ml/min; inlet temperature, 160&deg;C; atomization air pressure, 130 KPa; drying air flow rate, 0.70 m³/min.

RESULTS AND DISCUSSION

 

Phase Composition of Feedstock and As-Sprayed Coating

 

Figure 2 presents the X-ray diffraction (XRD) patterns of the powders and as-sprayed coating. As shown in Fig. 1, the powders were composed of &theta;-Al₂O₃, -Al₂O₃, m-ZrO₂ and t-ZrO₂. However, XRD pattern of the as-sprayed coating contained only peaks of t-ZrO₂, &gamma;-Al₂O₃ and -Al₂O₃. The presence of metastable t-ZrO₂ was attributed to the fact that the transformation from t-ZrO₂ to m-ZrO₂ was prohibited under the high cooling velocity of feedstock (>10⁶ K/s [12]). Moreover, it is well established that the preferential formation of &gamma;-Al₂O₃ is attributed to the high cooling rate of the molten particles during plasma spraying and easy nucleation of &gamma;-Al₂O₃ from the melt superior to -Al₂O₃ thanks to lower interfacial energy between crystal and liquid [13-14].  FIGURE 2. &mdash;X-ray diffraction (XRD) patterns of the powders and as-sprayed coating, (a) powders; (b) as-sprayed coating.

Structure of As-Sprayed Coating

 

Figure 3 shows the SEM micrograph of polished cross-sections and surface morphology of as-sprayed coating. As seen from Fig. 3a, the thickness of coating was about 450 &micro;m and micro sized pores were distributed homogenously in the as-sprayed coating. However, no macro cracks were observed at the interface between the top coating and underlying substrate, indicating that the bonding between them was good. The typical disc-shaped splats of plasma-sprayed ceramic coatings were obvious and many pores can be observed in Fig. 3b. The above results can be attributed to the fact that, during the plasma spraying process, the powders were injected into the plasma jet, where they should be melted (droplet) and accelerated against the substrate to form splats. Meanwhile, due to the poor wetting/adhesion among splats, a large number of pores were kept in the structure of coating. Besides, according to the result obtained from digital image analysis (IA), the porosity of as-sprayed coating was about 7%, which was less than the other reports [15, 16]. The result demonstrated that the powders prepared by sol-gel method were suitable for plasma spraying and the as-sprayed coating had a denser structure compared with other coatings with the same composition.  FIGURE 3. &mdash;SEM micrograph for polished cross-section and surface morphology of as-sprayed coating, (a) the polished cross-section; (b) surface morphology.

The coating was removed from the stainless-steel substrate and used for micro-structural studies by high resolution transmission electron microscopy (HRTEM). Figure 4 shows the TEM micrographs of the original powders produced by sol-gel method and as-sprayed coating. As shown in Fig. 4a and Fig. 4b, the grain size of original powders was less than 50 nm and the as-sprayed coating was composed of regular shaped grains with size 20-50 nm and the boundaries of grains were very clear as seen from Fig. 4c. The result can be attributed to the short dwell time the powders experienced in the plasma jet, which helped to preserve the nano structure of starting powders obtained from sol-gel method. Besides, plasma-sprayed composite powders cooled rapidly, which also let to retain nano structure in the final coating. However, as shown in Fig. 4d, it was found that amorphous phase (marked as F) formed among some nano sized grains (marked as A, B and C), since the eutectic composition had the lower melting temperature (1866 &plusmn; 7&deg;C [17]) and greater potential for obtaining amorphous structures under very high cooling rate during the plasma spraying process. Besides, the chemical composition within the amorphous phase, confirmed by EDX, was found to be Al/Zr = 78:22 on an atomic basis, which was nearly equal to the normal chemical composition (Al/Zr = 79:21) of original feedstock, indicating the as-sprayed coating was homogenous in chemical composition. It is well known that one of the biggest challenges in plasma spraying nano composite coating is to retain the preexisting nanostructure of the powders. If nano sized powders are fully melted during spraying, then the traditional behavior of plasma sprayed particles, such as, solidification, nucleation and growth will take place. Such processes will be able to destroy the original nano structured features of the powders. But in the present work, this phenomenon was overcome by using the composite powders obtained from sol-gel method and nanostructure was well preserved because unmelted nano-particles were retained in the as-sprayed coating.  FIGURE 4. &mdash;TEM micrographs of the original powders produced by sol-gel method and as-sprayed coating, (a) original nanometric particles produced by sol-gel method; (b) bright-field image of nano polycrystalline; (c) grain boundary and inside single nano grain; (d) amorphous structure (arrow) among nano grains (marked by A, B and C).

Meanwhile, as shown in Fig. 2b, the XRD pattern of as-sprayed coating showed the presence of t-ZrO₂, &gamma;-Al₂O₃ and -Al₂O₃. This result indicated that the nanostructured crystalline grains were composed of separate alumina and zirconia phases. Furthermore, some degree of melting of starting powders during plasma spray was indicated by the formation of flattened disk-like splats as shown in Fig. 3b. However, even if the original powders were partially melted during spraying, the simultaneous formation of alumina and zirconia grains could prevent overgrowth of both phases. Once a growing alumina crystal and a growing zirconia crystal came into contact, each one stopped the growth of the other. This may be another reason why the coating exhibited the nanometric crystal size.

Properties of As-Sprayed Coating

 

The micro hardness of plasma-sprayed coating is often used to predict the wear behavior. However, for a given ceramic coating, the distribution of pores and cracks differs from sample to sample. And the micro hardness of ceramics varies unpredictably even if identical specimens are tested under identical loading conditions. The Weibull distribution has been used successfully to describe a wide range of problems including the mechanical properties of brittle materials [18]. The Weibull function, in the two-parameter form, is given as   where F(x) is the cumulative density function of probability, x is the micro hardness data, x_o is the scale factor, which gives 63.2% of the cumulative density and m is the Weibull modulus. The Weibull modulus reflects the data scatter within the distribution. The Weibull plot is the most common and easiest way to obtain the Weibull modulus. A Weibull plot can be drawn by rearranging Equation 1 and taking natural logarithms twice. Thus x_o and m can be determined by fitting the following equation  

Figure 5 shows the Weibull plot of micro hardness (HV0.3) of as-sprayed coating. The average micro hardness (HV0.3) measured on the polished cross-section of as-sprayed coating reached to about 726 kg/mm ² and the slope m equaled to 16 determined by linear fitting of data points. Although this value of HV0.3 was a little lower than the previous work of others, the Weibull modulus was much higher indicating a narrower micro hardness distribution [ 15]. The above results demonstrated the flaws, such as micro pores and cracks were homogenously distributed in the structure of as-sprayed coating, which was expected to have excellent mechanical properties.   FIGURE 5. &mdash;Weibull plot of micro hardness (HV0.3) of as-sprayed coating.

 

Figure 6 shows the scratch testing curve of the polished top surface of as-sprayed coating, which consisted of four parts: normal force, frictional force, friction coefficient and the intensity of acoustic emission (AE). During the test, the process of crack formation was detected by AE attached to the scratch tester. It should be noted that, scratch testing is always used to assess the interfacial adhesion between the thin coating and substrate [19]. However, for a thicker coating, this result only reflects the cohesion (refers to bonding strength of splats) inside the coating. As shown in Fig. 6, the occurrence of the highest peak of friction coefficient or intensity of AE at the scratch length about 5.7 mm indicated a large-area detachment of splats from the surface of as-sprayed coating, which was also observed by optical microscopy in the test system. Corresponding to the highest peak of friction coefficient or intensity of AE, critical normal force and frictional force were 67 N and 44 N respectively. Figure 7 shows the SEM micrograph for surface morphology of as-sprayed coating after scratch testing. The scratch groove and detachment of splats from the surface of coating were clearly observed in Fig. 7. Some investigators described the failure process during scratch testing, which can be summarized as follows: under the interaction of normal force and frictional force, wedge crack formed ahead of the moving indenter and forward motion of indenter opened up an interfacial crack. Finally, crack propagated until large-area splats peeled off [20]. Besides, Xie and Hawthorne [21, 22] also interpreted the failure mechanism of coating during scratch testing, as indicating that when scratching was performed on a flat surface, the predominant mechanism of material removal was found to be the micro-fracture driven by the inelastic strain and the micro-fracture always took place preferentially at splat boundaries and pre-existing cracks. From this point of view, it would be very useful to strengthen splat boundaries and reduce the number the pre-existing cracks during the plasma spray process for improving the cohesion of as-sprayed coating.  FIGURE 6. &mdash;The scratch testing curve of the polished top surface of as-sprayed coating.  FIGURE 7. &mdash;SEM micrograph for surface morphology of as-sprayed coating after scratch testing, showing the scratch groove and detachment of splats from the surface of coating.

The cohesion of splats was also characterized by the test carried out on polished cross section of as-sprayed coating. Figure 8 shows the SEM micrograph for the cross section of as-sprayed coating after scratch testing. The typical scratch groove and the cone-shaped spalled region due to the detachment of splats were clearly presented in Fig. 8. The main failure mechanism of coating during test was associated with large cracks propagated along the scratch direction, which resulted in the detachment of splats [23]. Furthermore, the scratch testing curve is shown in Fig. 9. The occurrence of highest peak in AE indicated detachment of splats on the cross section of the as-sprayed coating. Corresponding to the highest peak of friction coefficient or intensity of AE, critical normal force and frictional force were 69 N and 41 N respectively, which was well in agreement with the result obtained from test on the top surface of as-sprayed coating. Table 2 shows the results of critical normal force (L_c) in the comparative tests. As shown in Table 2, the L_c of the present coating using sol-gel-derived powders was much higher than that of other reference coatings, which indicated the cohesion of present coating was excellent and the sol-gel-derived Al₂O₃-ZrO₂ powders can be used as feedstock for fabricating high-performance Al₂O₃-40 wt.% ZrO₂ composite coating. Meanwhile, the next steps will consist of a comparative analysis of microstructures in the above reference coatings, which are expected to be responsible for the difference in cohesion.  FIGURE 8. &mdash;SEM micrograph for cross section of as-sprayed coating after scratch testing, the scratch direction was from the substrate to the coating.  FIGURE 9. &mdash;The scratch testing curve of the polished cross section of as-sprayed coating.

TABLE 2.&mdash;The critical normal forces (L_c) in comparative tests.

Samples	Lca	Lcb
a Lc, Critical normal force obtained from top surface scratching test.
b Lc, Critical normal force obtained from cross section scratching test.
Al2O3-40 wt.% ZrO2 coating*, using spray-dried agglomerated powders.
Al2O3-40 wt.% ZrO2 coating+, present coating (using powders synthesized by sol-gel method).
Al2O3 coating	49	53
Al2O3-40 wt.% ZrO2 coating*	54	59
Al2O3-40 wt.% ZrO2 coating+	67	69

CONCLUSION

 

In this paper, near-eutectic Al₂O₃-40 wt.% ZrO₂ coating composed of nano polycrystalline structure with grain size of 20-50 nm and amorphous phase was deposited by atmospheric plasma spray (APS) using nano sized powders prepared by sol-gel method as feedstock. The as-sprayed coating consisting of -Al₂O₃, &gamma;-Al₂O₃ and metastable t-ZrO₂ was obtained with lower porosity about 7% and average micro hardness of 726 kg/mm² with higher Weibull modulus 16. Compared with other as-sprayed coating with the same chemical composition, the micro pores and cracks were more homogenously distributed in the present as-sprayed coating confirmed by higher Weibull modulus. Furthermore, the results from a comparative study in scratching test indicated the present coating exhibited a higher cohesion compared with its counterparts and the sol-gel-derived Al₂O₃-ZrO₂ powders can be used as feedstock for fabricating high-performance Al₂O₃-40 wt.% ZrO₂ composite coating.

ACKNOWLEDGMENT

 

This work was supported by the National Natural Science Foundation of China (No. 50772086, 50821140308), and the High-Tech R & D Program of China (863, No. 2007AA03Z558), and by Research Fund for the Doctoral Program of Higher Education under contract NO. 20060698008.

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List of Figures

  FIGURE 1. &mdash;Scheme of temperature measuring and air cooling system during plasma spraying.

  FIGURE 2. &mdash;X-ray diffraction (XRD) patterns of the powders and as-sprayed coating, (a) powders; (b) as-sprayed coating.

  FIGURE 3. &mdash;SEM micrograph for polished cross-section and surface morphology of as-sprayed coating, (a) the polished cross-section; (b) surface morphology.

  FIGURE 4. &mdash;TEM micrographs of the original powders produced by sol-gel method and as-sprayed coating, (a) original nanometric particles produced by sol-gel method; (b) bright-field image of nano polycrystalline; (c) grain boundary and inside single nano grain; (d) amorphous structure (arrow) among nano grains (marked by A, B and C).

  FIGURE 5. &mdash;Weibull plot of micro hardness (HV0.3) of as-sprayed coating.

  FIGURE 6. &mdash;The scratch testing curve of the polished top surface of as-sprayed coating.

  FIGURE 7. &mdash;SEM micrograph for surface morphology of as-sprayed coating after scratch testing, showing the scratch groove and detachment of splats from the surface of coating.

  FIGURE 8. &mdash;SEM micrograph for cross section of as-sprayed coating after scratch testing, the scratch direction was from the substrate to the coating.

  FIGURE 9. &mdash;The scratch testing curve of the polished cross section of as-sprayed coating.

List of Tables

TABLE 1.&mdash;Plasma-spray parameters for Al₂O₃-ZrO₂ coating.

Parameters	Value
Gun nozzle diameter (mm)	8
Powder port injector inlet diameter (mm)	1.8
Current (A)	660
Voltage (V)	74
Primary argon gas flow rate (slpm)	41
Secondary hydrogen gas flow rate (slpm)	14
Carrier argon gas flow rate (slpm)	4.5
Spray distance (mm)	110
Gun traverse speed (mm/s)	800
Distance between adjacent torch passes (mm)	3
Total number of torch cycles	25
Substrate preheat temperature (°C)	100

TABLE 2.&mdash;The critical normal forces (L_c) in comparative tests.

Samples	Lca	Lcb
a Lc, Critical normal force obtained from top surface scratching test.
b Lc, Critical normal force obtained from cross section scratching test.
Al2O3-40 wt.% ZrO2 coating*, using spray-dried agglomerated powders.
Al2O3-40 wt.% ZrO2 coating+, present coating (using powders synthesized by sol-gel method).
Al2O3 coating	49	53
Al2O3-40 wt.% ZrO2 coating*	54	59
Al2O3-40 wt.% ZrO2 coating+	67	69