1. Introduction
Thermal barrier coatings (TBCs) are widely used for gas turbine and aero-engine parts, providing thermal protection for the parts to increase their operating temperature, thus improving the efficiency and performance of the engine [1,2,3]. TBCs can be prepared in a variety of ways, and the coatings prepared by different techniques differ in structure and properties [4,5]. At present, the common preparation techniques include atmospheric plasma spraying (APS), electron beam physical vapor deposition (EB-PVD), and plasma spraying-physical vapor deposition (PS-PVD). Solution precursor plasma spray (SPPS) directly uses solution precursors for spraying, breaking the limitation of powder particle size in traditional plasma spraying, and can prepare coatings composed of a large number of fine nano-particles, which has attracted a lot of attention in the recent years [6,7]. Compared with PS-PVD, SPPS has the advantages of high deposition efficiency and low cost.
As the engine operating temperature rises, the operating temperature of some parts exceeds the melting point of silicate sediments (mainly composed of CaO, MgO, Al2O3, and SiO2, referred to as CMAS) in the environment [8]. CMAS melts and adheres to the surface of TBCs, causing serious harm to them. Currently the most commonly used ceramic material for TBCs, 6–8 wt% yttrium oxide partially stabilized zirconia (6-8YSZ), is not resistant to CMAS corrosion [9,10,11]. Therefore, improving CMAS corrosion resistance of YSZ coatings has become a hot research topic in the field of thermal barrier coatings.
Researchers have proposed various protection methods to resist the CMAS corrosion. Lotus leaf has good hydrophobicity due to its surface structure consisting of micron convexes and nano waxy villis. Researchers have prepared a number of coatings with good hydrophobicity and self-cleaning properties mimicking this structure [12,13]. There are various methods for depositing superhydrophobic coatings on alloy surfaces, including hot dip method [14,15], chemical vapor deposition method [16,17], slurry method [18], etc. Moreover, this micro-nano double scale structure has been used on the surface of TBCs, which can also have the effect of resisting the adhesion and wetting of molten CMAS [19,20,21]. Therefore, weakening the wettability of coatings with molten CMAS by changing the surface structure is considered to be an effective method to resist CMAS corrosion [22].
At present, there are fewer studies on non-wetting type protection methods and the wettability of TBCs/CMAS systems. The key influencing factors and optimization methods for the wetting and infiltration performance are still unclear [23,24,25]. Yang’s research [26] shows that the smoother the surface of TBCs is, the larger the contact angle is with the molten CMAS, and consequently, the smaller the area of damage was on the TBCs. Zhang [4] compared the wettability of molten CMAS on the surface of coatings prepared by different techniques and showed that PS-PVD coatings have excellent resistance to CMAS adhesion and wettability due to the micro-nano double scale structure of the surface, and conventional APS coatings and EB-PVD coatings have weaker resistance to molten CMAS wettability. Thus, existing studies on non-wetting type protection methods have mostly been conducted on physical vapor deposition (PVD) coatings. It is regrettable that TBCs prepared by PVD techniques have a large number of vertical pores that provide favorable conditions for molten CMAS infiltration [27,28]. Zhang [29] investigated the CMAS corrosion resistance of gadolinium zirconate (GZ) coatings prepared by PVD and found that the GZ with higher Gd content promotes the generation of Gd-apatite, blocking the inter-columnar gap and the micro-cracks inside the column efficiently, which is helpful for the PVD application of the CMAS-resistance coating.
Whereas the SPPS technique allows easy and fast deposition of coatings with micro-nano double scale structures by the shadowing effect of precursor droplets without creating vertical cracks. Xu [30,31,32] prepared Yb2O3 coatings with a cauliflower-like micro-nano structure and good hydrophobic properties using the SPPS technique. Cai [33] similarly prepared rare earth oxides (REOs) coatings using the SPPS method and investigated their hydrophobic properties, and showed that the water contact angle of SPPS coatings was enhanced by 65% compared to smooth REOs surfaces. However, the above studies only focused on the hydrophobic properties of SPPS coatings, rather than the CMAS wetting properties. In addition, the highly tunable microstructure of SPPS coatings is well suited for the study of the wettability of TBCs/CMAS systems, but fewer scholars have applied the SPPS technique to this field. There is not even direct evidence that SPPS coatings have a significant advantage in resistance to molten CMAS wetting.
Therefore, this study attempts to apply the SPPS technique to the preparation of non-wetting CMAS corrosion resistant protective coatings and to demonstrate the potential of SPPS technique in this field. In this study, SPPS technique was used to prepare CMAS corrosion protective layers with micro-nano double scale structure on the surface of conventional APS coatings, as shown in Figure 1. First, this study investigates the influence of process parameters on the structure, analyzing how each parameter affects the shadowing effect and thus the structure. Furthermore, this study explores the effect of the size of the micro-nano double scale structures on the CMAS wetting performance and screens the important factors that determine the wetting and infiltration behavior of molten CMAS, aiming to enhance the CMAS corrosion resistance of TBCs in a more efficient and convenient way.
2. Materials and Methods
2.1. TBCs Preparation and Process Parameters
The coatings were prepared by atmospheric plasma spraying equipment (APS-2000, Beijing Aeronautical Manufacturing Technology Research Institute, Beijing, China). The IN-738 nickel-based superalloys with thicknesses of 3 mm and diameters of 25.4 mm were used as the substrates. IN-738 has a coefficient of thermal expansion similar to that of YSZ and is widely used as a substrate material for TBCs [34]. Before spraying, the substrates were grit-blasted and ultrasonically cleaned. The commercially available NiCrAlY powders (15–45 μm, Beijing Sunspraying New Material Co., Ltd., Beijing, China) and 8YSZ powders (15–45 μm, Beijing Sunspraying New Material Co., Ltd., Beijing, China) were used to deposit the APS bonding layer and the APS ceramic layer, respectively.
Argon was used as the main gas and hydrogen as the auxiliary gas in the coating preparation process. The APS bonding layer was prepared with a gun power of 30 kW (500 A/60 V), a gun travel speed of 500 mm/s, and a spraying distance of 100 mm. The APS ceramic layer was prepared with a gun power of 40 kW (650 A/62 V), a gun travel speed of 150 mm/s, and a spraying distance of 80 mm.
Before spraying SPPS layer, the APS layer was polished with sandpaper to reduce roughness and ultrasonically cleaned. The surface roughness of the APS layer is characterized by the Ra value. Zr(CH3COO)4 (99.99%, Anhui Zesheng Technology Co., Ltd., Anhui, China), ZrO(NO3)2-5H2O (99.5%, Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China), and Y(NO3)3-6H2O (99.5%, Shanghai Macklin Biochemical Technology Co., Ltd., Shanghai, China) were dissolved in a mixture of deionized water and anhydrous ethanol to prepare the desired solution precursors. Among them, Y(NO3)3-6H2O was the yttrium source, and Zr(CH3COO)4 and ZrO(NO3)2·5H2O were the zirconium sources.
For SPPS layer preparation, the power of the spraying gun was 32 kW (800 A/40 V), the gun travel speed was 400 mm/s, and the rest of the parameters are shown in Table 1. The choice of solute and the ratio of water to alcohol in the solvent will significantly affect the viscosity and surface tension of the precursor, which in turn will have a significant impact on the fragmentation of the precursor in the flame arc [6]. Spraying distance and APS layer roughness will directly affect the shadowing effect [32]. Therefore, these parameters are enumerated as important variables.
After spraying, the substrates were removed with aqua regia to obtain the free-standing coatings. The coatings were then vacuum-treated to remove the loosely bound oxygen from the surface.
2.2. Preparation of CMAS and CMAS Wetting Test
The oxide powders (99.99%, Sinopharm Group Chemical Reagent Shanghai Co., Ltd., Shanghai, China) were mixed uniformly in the ratio of 38CaO-5MgO-4Al2O3-50SiO2-Fe2O3-Na2O-K2O [35,36,37], placed at a temperature of 1500 °C for 8 h to allow a full reaction, and then quenched with water. In order to obtain the desired CMAS powders, the resulting glass was then crushed by ball milling and dried.
CMAS wetting test was carried out using the drop-weight method. In this step, the CMAS powders were uniaxially pressed into a cylinder with a thickness and diameter of 1 mm to make small cylindrical blocks and then placed on the free-standing coatings surfaces [11]. Finally, the samples were placed in a ceramic fiber furnace at 1300 °C and heat treated for various durations (180 s, 300 s, and 600 s).
2.3. Sample Characterization
The phase analysis of the coatings were examined by X ray diffraction (XRD, D/max, 2550VB/PC, RIGAKU, Tokyo, Japan) using filtered Cu Kα radiation at an accelerated voltage of 18 kV and a current of 450 mA. The microstructure of the coating surface and cross-section as well as the spreading and infiltration of CMAS were observed by scanning electron microscope (SEM, S-3400N, Hitachi Ltd., Tokyo, Japan) equipped with an energy dispersive spectrometer (EDS, EDAX Falcon, AMETEK, San Diego, CA, USA).
3. Results and Discussion
3.1. Phase Composition of the As-Sprayed Coatings
There are a series of complex physicochemical changes such as solvent volatilization, droplet splitting, precursor solute precipitation, high temperature pyrolysis, sintering, melting, and re-solidification during the SPPS layer preparation process. Modulating the parameters will affect these changes, which in turn will affect the phase composition and microstructure of the coatings [38,39]. The XRD pattern of the as-sprayed coatings is shown in Figure 2. The JCPDS reference patterns of tetragonal phase ZrO2 (No. 50-1089) and monoclinic phase ZrO2 (No. 37-1484) are also included in Figure 2. The sharp peaks matched the t-ZrO2 JCPDS reference pattern at all peak locations. All groups of coatings are composed of t-ZrO2 and almost no m-ZrO2 peaks can be observed. It proves that the precursors were completely pyrolyzed and well crystallized for all sets of process parameters.
3.2. Microstructure of the As-Sprayed Coatings
The microstructure of the as-sprayed coating is shown in Figure 3. It can be observed that the SPPS layer consists of nano-particles with diameters of 0.2–1.5 μm stacked on top of each other, which is looser compared to the conventional APS layer. This is because, in this study, the SPPS layer was sprayed with relatively low power and the small droplets solidified earlier during the flight. Group 1 and group 2 have more lath-shaped structures formed by unbroken large droplets with sizes below 10 μm. The viscosity of each precursor was measured using a pins viscometer. Groups 1 and 2 used Zr(CH3COO)4 as the zirconium source with a viscosity of 5.3 mPas and 4.9 mPas, respectively, and groups 3 to 6 used the same formulation with a viscosity of 3.3 mPas. It is inferred that the reason for the formation of lath-shaped structures in group 1 and group 2 is the high viscosity of the precursors, which leads to poor atomization of the droplets during the spraying process. But the size of the nano-particles did not change very significantly in each group of SPPS layers. It indicates that the size of the nano-particles is insensitive to the process parameters. To modulate the morphology of the nanostructures, an atomization device may be required.
In addition, “cauliflower”-like structures with diameters of 10–50 μm (referred to as cauliflower structures) appeared on the surfaces of groups 3 to 6. The reason for the formation of these structures is the shadowing effect during the flight of droplets [32,40,41]. While the plasma jet approaches the surface of the APS layer, the airflow is forced to turn, causing the airflow in this region to be parallel to the surface of the APS layer. Small droplets have less inertia and are susceptible to steering by parallel airflow, so a large number of small droplets hit the raised peaks of the APS layer, and these droplets agglomerate into cauliflower structures as the number of spraying passes increases. While large droplets with larger inertia can cross the parallel airflow and hit the APS layer surface vertically, the shadowing effect is weaker, so group 1 and group 2 do not form obvious cauliflower structures, but nano-particles still show a tendency to agglomerate and stack into small clusters of 3–12 μm in diameter, which can be regarded as the prototype of cauliflower structures.
Table 2 shows the dimensions of the cauliflower structures of groups 3 to 6. Group 3 has a closer spraying distance relative to group 5, and the cauliflower structures of group 3 are smaller in diameter and more densely distributed, with narrower gaps. The reason is that the closer spraying distance leads to a stronger parallel airflow on the coating surface and a stronger shadowing effect. Droplets can form stacks around some of the smaller peaks, which in turn leads to an increase in the number of cauliflower structures and a denser distribution.
Groups 4 to 6 have different surface roughness of the APS layers. It can be observed that the smoother the APS layer is, the smaller and denser the cauliflower structure is. A large number of high peaks exist on rough surfaces, which capture most of the droplets and are the first to form cauliflower structures. Then, as the cauliflowers grow, more and more droplets accumulate there, resulting in a concentrated stack of droplets that cannot be formed by the surrounding small peaks, resulting in a larger and dispersed surface morphology of the cauliflowers. On smooth surfaces, large peaks have been smoothed out and the remaining ones are more uniform in size, thus making it easier to obtain a more uniform structure when droplets are deposited on them.
In addition, cracks were observed in all groups of coatings. This is due to the fact that the closer spraying distance during the preparation of SPPS layer will bring more heat to the substrate and APS layer, leading to cracking of the denser APS layer and eventually together with the SPPS layer.
3.3. Wetting Performance of CMAS
3.3.1. Comparison between SPPS Coatings and APS Coatings
In order to contrast with the conventional APS coatings, group 7 and group 8 are specially set out as control groups, which are APS coatings without SPPS layer deposited on. Group 7 is the as-sprayed APS coating with a Ra value of about 5.81 μm, and group 8 is the polished APS coating with a Ra value of about 3.08 μm.
CMAS wetting tests were conducted on each group of coatings, and the variation curves of molten CMAS wetting diameter and contact angle with time are respectively shown in Figure 4 and Figure 5. The molten CMAS on the surface of the SPPS layers has a larger contact angle and smaller spreading diameter than the conventional APS coatings, and the CMAS spreading reaches a stable state more quickly. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% on average. In addition, CMAS is completely flat on the surface of APS coatings, and the steady-state contact angle is below 10°, while it is not completely flat on the surface of SPPS layers, and the steady-state contact angle is maintained at about 30°. The contact angle is one of the most sensitive parameters in material surface analysis and directly responds to the strength of the wettability [42]. The wetting diameter, on the other hand, directly reflects the size of the area subjected to molten CMAS corrosion, which is essentially the movement of the three-phase line. This result directly indicated that SPPS technique does play a significant role in improving the resistance of coatings to CMAS wetting.
Infiltration is a prerequisite for CMAS corrosion. EDS elemental analysis was performed on areas of the coating cross-section at different depths after 600 s heat treatment to obtain the Ca elemental content at each depth, and the results are shown in Figure 6. The overall Ca contents of the SPPS coatings are lower and a significant decrease in Ca content occurs in the depth of 0 to 100 μm. The APS coatings, on the other hand, show a significant weakening of the elemental Ca signal only at a depth of 100 to 200 μm. This demonstrates that the micro-nano double scale structure hinders CMAS infiltration by weakening the wettability properties.
The wettability of a liquid on a solid surface is mainly determined by the surface energy as well as the surface structure [43]. The surface energy of each group of samples is shown in Table 3. It can be seen that there is no significant difference in surface energy because the composition of each group of coatings is the same, and the stray bonds that may be generated by long exposure to air were removed by vacuum treatment. Therefore, the excellent resistance to CMAS wetting of SPPS layers is attributed to the micro-nano double scale structure.
In order to explain why SPPS coatings and APS coatings exhibit different wettability properties, the corrosion interfaces of both with molten CMAS was observed. Figure 7 shows the interface between CMAS and conventional APS coatings after different times of heat treatment. It can be observed that the molten CMAS and the APS coating always remained in complete contact, and the molten CMAS started to infiltrate into the coating at 180 s. Such a contact state is called Wenzel state [42]. Figure 8 shows the interface between CMAS and SPPS layers after different times of heat treatment. A good air layer was formed between the molten CMAS and the SPPS layer surface at 180 s, which greatly reduces the contact area between CMAS and the coating. It can be observed from the enlarged view in Figure 8a that the molten CMAS has not yet penetrated inside the SPPS layer at this time. Thus, the reason for the significant improvement of the non-wettability of the coatings after covering the SPPS layers is that the cauliflower structures and the nano-particles form the micro-nano double scale structures, which are similar to the surface of a lotus leaf and can effectively trap air and form an air layer between the coating surface and the molten CMAS. Although group 1 and group 2 do not form obvious cauliflower structures, the cluster structures on their surface and the nano-particles also form the micro-nano double scale structures. As a result, the contact surface is divided into two parts: molten CMAS/SPPS layer and molten CMAS/air. The contact state between the coating and the molten CMAS is Cassie state [44]. Air can be considered as an absolutely non-wetting material with a contact angle of 180°, so the Cassie state has better non-wetting properties compared to the Wenzel state [45].
3.3.2. Comparison between Different SPPS Layers
It can be seen from Figure 4 and Figure 6 that there is no significant difference in the overall wetting performance of different SPPS layers. The only observable difference is that group 1 and group 2 have a larger wetting diameter and a smaller contact angle at 300 s. This means that these two groups of coatings reach the steady state faster than the others. The reason for this phenomenon is that the raised cauliflower structures in other groups of coatings have a pinning effect on the movement of the three-phase line, preventing the molten CMAS from spreading outward [46].
The steady-state contact angles of all groups of SPPS layers are around 30°. Whether a typical cauliflower structure is formed and the dimension of the cauliflower structure have less influence on the steady-state CMAS wetting performance. The infiltration of CMAS shown in Figure 6 follows the same pattern. It is inferred that the nanostructure in the micro-nano dual-scale structure has a greater effect on the CMAS wetting performance. Air is trapped in nano-pores rather than in micron-sized grooves. However, the nano-particle sizes of the SPPS layers in each group are similar, resulting in a similar ability to trap air. From the other side, such results indicate that the preparation of CMAS corrosion protection layer using the SPPS technique is not demanding in terms of process parameters and surface topography.
3.3.3. Reliability of SPPS Layers
The sustainability of protection for TBCs is a very important indicator. However, it can be observed from Figure 8 that the lifetime of the SPPS layer in this study was not satisfactory. At 300 s, the molten CMAS infiltrated into the SPPS layer and filled the pores, and the air originally occupying the pores converged into air bubbles, and the SPPS layer surface started to be peeling off. Eventually, the SPPS layer was completely degraded at 600 s, broken and floated on the molten CMAS. This is in stark contrast to conventional APS coatings. After 600 s of corrosion, the APS coatings did not change significantly, and there were no small bubbles inside the molten CMAS.
Molten CMAS will preferentially attack and dissolve the boundaries between YSZ grains, and the fine-grained regions with high porosity are more susceptible to corrosion than the coarse-grained regions. Conventional YSZ particles will degrade irreversibly after 0.2 h of heat treatment in CMAS environment, and the boundaries of the particles will gradually break up [47,48,49]. It can be seen from the magnified images in Figure 8 that the nano-particles originally bonded to each other after CMAS corrosion were separated, and the fragmentation occurred first in the area of fine grains with weaker bonding, and the fragmentation was accompanied by some small bubbles. The main reason for the occurrence of SPPS layers degradation is the dissolution of the boundaries of YSZ particles in the SPPS layers [50].
The SPPS layers in this study consist of very fine particles with large specific surface area and more pores, the cohesion of the SPPS layers is weak, and the CMAS infiltration has a very large contact area to accelerate the dissolution of YSZ. The dissolution of YSZ leads to a further weakening of the interparticle bonding, which in turn leads to the fragmentation of the SPPS layers. In addition, the bubbles that converge inside the SPPS layers push the broken coatings into the CMAS as they grow and float upward.
The relatively low spraying power of the SPPS layer in this study is a double-edged sword. On the one hand, the droplets start to solidify earlier in the flight process, which makes the SPPS layers have a good nano-particle structure and enhances the CMAS wetting resistance. On the other hand, it also leads to a loose structure and weakened cohesion of the SPPS layers, which accelerates the infiltration of CMAS and the degradation of the SPPS layers. Therefore, further studies will focus more on the stability and reliability of the SPPS layers. In terms of material, the YSZ material will be doped to make it more inert to CMAS corrosion. Previous studies have found that Yb3+ exhibits a lower diffusion rate than that of Y3+ while interacting with CMAS [50]. Using Yb2O3-Y2O3 co-stabilized ZrO2 as a material can significantly improve the lifetime of SPPS layers corroded by CMAS. In terms of structure, increase the spraying power to strengthen the SPPS layer cohesion, and adjust the spraying distance to avoid cracking, aiming to obtain a long-lasting and reliable micro-nano double scale corrosion-resistant coating.
In addition, the CMAS concentration used in the CMAS wetting tests was much larger than the actual working conditions [51]. Under this condition, the SPPS layers with micro-nano double scale structures still effectively attenuated the wetting performance of molten CMAS on the surface within 600 s. The wetting diameter was reduced by 40% on average, the steady-state contact angle increased up to three times, and the infiltration rate was slowed down. It proves that the structure has good practicality and research value.
4. Conclusions
This study has used SPPS technique to prepare CMAS corrosion protection layer with micro-nano double scale structure on the surface of conventional APS coating, to investigate the effect of process parameters on the micro-nano double scale structure and the wetting and infiltration behavior of molten CMAS on the surface. The specific conclusions are as follows:
The morphology of micron structure in SPPS layers is significantly influenced by the process parameters. The precursor droplet fragmentation plays a key role in the formation of cauliflower structures. Closer spraying distance and smoother APS layer surface are favorable to the shadowing effect and produce finer cauliflower structures. Nanostructures, on the other hand, are relatively insensitive to the above process parameters.
Compared to conventional APS coating, the SPPS layer significantly enhances the CMAS wetting resistance of the coating and also plays a positive role in hindering CMAS infiltration. After covering the SPPS layer, the CMAS wetting diameter is reduced by about 40% and the steady-state contact angle increased up to three times. The reason is that the micro-nano double scale structures can effectively trap air and form an air layer between the coating surface and the molten CMAS.
There is no significant difference in the overall wetting performance of different SPPS layers. It is inferred that nano-particles play a more important role in the formation of the air layer, which in turn determines the steady-state wettability properties. The raised cauliflower structures have a pinning effect on the movement of the three-phase line, which can influence the time needed to reach the steady state.
The SPPS layer has a loose structure and weakened cohesion, and is prone to degradation and failure. Therefore, the focus of further research will be on the stability and reliability of the SPPS layer. Selecting materials with CMAS inertness and increasing spraying power are both feasible methods.
Conception and design of the experiments, Y.W., W.W. and H.F.; performance of the experiments, Y.W., Z.X. and Z.Y.; resources, C.Z.; analysis of data, Y.W., Z.Y. and T.Y.; article writing, Y.W. and W.W. All authors have read and agreed to the published version of the manuscript.
Not applicable.
Not applicable.
Not applicable.
The authors declare no conflict of interest.
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Enlarge this image.Figure 3. SEM images of sample surface and section: (a) group 1; (b) group 2; (c) group 3; (d) group 4; (e) group 5; (f) group 6.
Enlarge this image.Figure 7. Interface between conventional APS coatings and CMAS after different times of heat treatment: (a) 180 s; (b) 300 s; (c) 600 s.
Enlarge this image.Figure 8. Interface between SPPS layers and CMAS after different times of heat treatment: (a) 180 s; (b) 300 s; (c) 600 s.
Process parameters of SPPS layers.
| Group | Precursor Zirconium Source | Precursor Yttrium Source | Ethanol to Water Ratio | Precursor Concentration (mol/L) | Spraying Distance (mm) | Ra Value of APS Layer |
|---|---|---|---|---|---|---|
| 1 | Zr(CH3COO)4 | Y(NO3)3-6H2O | 1:1 | 0.75 | 30 | 2.87 |
| 2 | Zr(CH3COO)4 | Y(NO3)3-6H2O | 3:7 | 0.75 | 30 | 2.87 |
| 3 | ZrO(NO3)2-5H2O | Y(NO3)3-6H2O | 1:1 | 0.75 | 30 | 2.87 |
| 4 | ZrO(NO3)2-5H2O | Y(NO3)3-6H2O | 1:1 | 0.75 | 40 | 1.91 |
| 5 | ZrO(NO3)2-5H2O | Y(NO3)3-6H2O | 1:1 | 0.75 | 40 | 2.87 |
| 6 | ZrO(NO3)2-5H2O | Y(NO3)3-6H2O | 1:1 | 0.75 | 40 | 4.28 |
Dimensions of the cauliflower structures.
| Group | Cauliflower Diameter |
Gap Width |
|---|---|---|
| 3 | 24.03 ± 5.21 | 7.79 ± 2.20 |
| 4 | 22.25 ± 5.11 | 8.20 ± 2.31 |
| 5 | 31.80 ± 5.77 | 9.69 ± 2.65 |
| 6 | 34.07 ± 8.28 | 10.01 ± 2.87 |
Surface energy of each group of samples.
| Group | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 |
|---|---|---|---|---|---|---|---|---|
| Surface energy |
73.03 | 70.96 | 70.69 | 67.88 | 72.53 | 72.13 | 72.84 | 73.05 |
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