Reduction of soil resistance through the use of a composite coating.

Generally, soil seriously resists the motion of soil-engaging components of terrain machines and tillage equipment because soil has such strong adhesion. In order to develop a method to treat this problem, a polysiloxane/T8 composite coating was used in this study. The wettability of steel 45, steel T8, polysiloxane coating, and polysiloxane/T8 composite coating was measured using a contact angle testing instrument (JCC-1, made in China) at 20[degrees]C in air. The abrasive wear resistance of both steel 45 and polysiloxane/T8 composite coating was tested on a turnplate abrasive wear testing machine. The resistance of soil to bulldozing plates made of the aforementioned materials was measured on bulldozing resistance testing equipment. The experimental results showed that the polysiloxane/T8 composite coating had excellent hydrophobicity, and that the contact angle of water on its surface reached up to 92[degrees], which is much larger than that of steel 45 and steel T8. The relative volume wear loss of the polysiloxane/T8 composite coating was 65% and its bulldozing resistance was reduced by 15.2%-22.3% compared to bare steel 45.

Keywords: Surface analysis, scanning electron microscope, composite coatings, spray application, adhesion, mechanical properties, abrasive wear, wettability, and steel

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Soil adhesion is a natural phenomenon that is harmful to terrain machines and tillage equipment that have soil as their work medium, such as automobiles, tractors, engineering machines, spades, hoes, and plows. Soil adhesion increases motion resistance and energy consumption, quickens the damage of the soil-engaging components, and lowers work quality. For this reason, many methods have been studied to treat this problem. (1) The most promising method is the surface modification of soil-engaging components. The results of previous investigations (1-3) have indicated that when the contact angle of water on materials is smaller, the adhesion of soil to materials is more difficult and the soil resistance is smaller.

Because soil animals' skins possess a strong hydrophobic character, soil animals can freely move in soil and it does not adhere to their skin. (3) Metals, metal oxides, and inorganic materials are all high surface energy materials and possess hydrophilic character.

Organic materials, however, are all low surface energy materials and have an excellent hydrophobic nature. (4) Under the same conditions, polymers perform much better at reducing the adhesion of soil, (1-3) and show a lot worse abrasive wear resistance when compared with metals. (5) Wettability, soil adhesion, and friction-wear of [Al.sub.2][O.sub.3]/PTFE(+PPS) composites were reported in reference (6). Th. Uelzen et al. (7) investigated the wettability of the smooth and rough surfaces of three different materials: tetrafluoroethylene (TFE), gold, and hexamethyldisiloxane (HMDSO). They indicated that the wettability of a smooth hydrophilic surface, is improved by roughening it, whereas the opposite effect is observed with a smooth hydrophobic surface, i.e., by roughening the surface, the contact angle will increase. B. Elkin et al. (8) also reported similar results about the wettability of plasma-polyfluorocarbon layers.

The aim of this study is focused on a composite coating, polysiloxane/T8, which can effectively reduce soil resistance and has a certain wear resistance. For this, surface modification was carried out on metal material steel 45, (9) a medium carbon steel with 0.42%-0.50% carbon. The surface of steel 45 was coated with steel T8, (9) a high carbon steel with 0.75%-0.84% carbon, by flame spraying. The T8-treated steel 45 was then dipped in polysiloxane. The wettability, abrasive wear resistance, and performance of reducing soil resistance were examined for the polysiloxane T8 composite coating.

EXPERIMENTAL

Preparation of the Polysiloxane/T8 Composite Coating

First, the surface of steel 45 substrates was rinsed with acetone and roughened by spraying corundum; second, the steel T8 coating was applied on the roughened substrates by flame spraying (the pressures of oxygen and acetylene were 294-588 kPa and 49-98 kPa, respectively); then, the steel T8 coating was dipped with polysiloxane at room temperature for one to four hours.

Abrasive Wear Test

The sample size utilized in abrasive wear testing was 60 mm X 30 mm X 6 mm. Materials tested were bare steel 45 (quenched and tempered) and steel 45 with polysiloxane/T8 composite coating. Abrasive wear experiments were carried out on a turnplate abrasive wear testing machine (JMM, made in China), as shown in Figure 1. The abrasive was composed of 450-900 [micro]m quartz sands of 94 wt%, bentonite of 3 wt%, and water of 3 wt%. The buried depth of the specimen was 40 mm (see Figure 1), and the angle between the normal of wear surface and the line from wear surface to the circle's center was 35[degrees] [+ or -] 10'. Three tested material specimens (samples 1-3 in Figure 1) and one comparative material specimen (sample 4 in Figure 1) were fixed on the clamping apparatus at the same time. During the wear process the specimens did not move, and the turntable rotated at 49 r/min, causing the abrasive to move. This resulted in a relative movement between specimen and abrasive at a speed of 123.6 m/min and a wear of specimen located at the wear position. Every specimen from sample 1 to sample 4 was located at wear position and worn in turn via the intermission rotation of the clamping apparatus. At the wear position, the wear distance of each sample in each wear turn was 803.4 m ("wear distance" is the distance of the relative movement between specimen located at wear position and abrasive). Total wear distance of each specimen was 16068 m. The abrasive wear resistance of samples was determined by a relative volume wear loss ([[omega].sub.v]). Tests were repeated nine times to obtain one average value as the abrasive wear resistance of this material:

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

[[omega].sub.v] = [V.sub.c]/[V.sub.t] X 100% (1)

where [V.sub.c] is the volume wear loss of the comparative sample (steel 45), [V.sub.t] is the volume wear loss of tested samples, and [[omega].sub.v] is the relative volume wear loss of the tested sample, i.e., the percentage of the volume wear loss of the comparative sample to that of the tested sample. If the volume wear loss of the tested sample is larger than that of the comparative sample, namely, [V.sub.t] > [V.sub.c], then the relative volume wear loss of the tested sample, [V.sub.c]/[V.sub.t] X 100%, is smaller than 100%, which indicates that the wear resistance of the tested sample is worse than that of the comparative sample. On the contrary, if [V.sub.t] < [V.sub.c], then [V.sub.c]/[V.sub.t] X 100% is larger than 100%, which indicates that the wear resistance of the tested sample is better than that of the comparative sample.

Measurement of Contact Angles

The contact angles of distilled water on the surfaces of steel 45, steel T8, polysiloxane coating, and polysiloxane/T8 composite coating were measured by contact angle testing instrument JCC-1 at 20[degrees]C in air.

Bulldozing Resistance Test of Polysiloxane/T8 Composite Coating

The soil used in this experiment was yellow clay, whose particle size distribution consists of 0.1-0.05 mm of 33 wt%, 0.05-0.01 mm of 37 wt%, 0.01-0.005 mm of 11.2 wt%, 0.005-0.002 mm of 8 wt%, and < 0.002 mm of 10.80 wt%. Plastic limit ([w.sub.p]) was the moisture content of 18.83%. Liquid limit ([w.sub.1]) was the moisture content of 35.27%. The moisture content of soil was 29.4% and 33.0%, respectively.

The size of the bulldozing plate was 250 mm X 130 mm X 10 mm. Two kinds of bulldozing plates were used: one made of steel 45 and another made of steel 45 and coated with polysiloxane/T8 composite coating. The plowing velocities of the bulldozing plates were 0.02 and 0.04 m/sec, the plowing depth was 20 mm, and the plowing angle was 45[degrees]. The measurement tests were carried out using the self-made bulldozing resistance testing equipment, as shown in Figure 2. The bulldozing plates were fixed on a sensor and did not move, the soil trough translated to move back and forth via a gear mechanism to produce the relative movement between the bulldozing plates and the soil. The plowing velocities of the bulldozing plates were adjusted through the transmission-gear box and the plowing distance was controlled by a limit switch. The bulldozing resistance, i.e., the force which soil exerts on the surface of bulldozing plates to resist forward movement, was measured on a YD-15 sensor and strain gauge, and the output was recorded on a magnetic tape recorder and treated by a CF-920 dynamic signal analytical instrument.

RESULTS AND DISCUSSION

Wear Resistance

The experimental results of abrasive wear showed that the volume wear loss of coated steel 45 was 63% that of uncoated steel 45; i.e., [[omega].sub.v] of the polysiloxane/T8 composite coating is 63%. This means that the wear resistance of polysiloxane/T8 composite coating is lower than that of uncoated steel 45. The reason is that the wear resistance of the composite coating depends on the T8 matrix which appears as a net-like structure, as shown in Figure 3a. From the photomicrogragh (Figure 3a), the scratch trace can only be found on the T8 matrix, but from the photomicrogragh of Figure 3b, which is an amplification of the steel T8 matrix in 3a, the furrow and polygonal pits can be found on the T8 matrix. In front of the polygonal pits there exists a stack of materials, and some of the polygonal pits and some of the furrows intersect each other. This suggests that plowing on a microscopic basis on the surface of the steel is the predominating wear mechanism of the T8 matrix. For quenched and low-temperature tempered steel 45, its worn morphology is similar to that of T8 matrix, as shown in Figure 3c. Its prevailing wear mechanism is also microscopic plowing. Figures 3a and 3d show that homogeneous holes exist in the flame-sprayed T8 coating and the holes are full of polysiloxane. Figure 3d also shows that polysiloxane transfers onto the T8 matrix of the composite coating's worn surface, due to the friction action of the abrasive. Therefore, the polysiloxane distribution after wear is more extensive than before wear. This is beneficial for increasing the hydrophobicity of the composite coating and reducing the adhesion and resistance of soil.

Wettability

The wettability of materials is expressed in contact angles of liquids on their surfaces. When the contact angle of water on the surface of a material is smaller than 90[degrees] the material is called hydrophilic; the smaller the contact angle, the stronger the hydrophilicity. When the contact angle for water on the surface of a material is larger than 90[degrees]C the material is called hydrophobic, and the larger the contact angle, the stronger the hydrophobicity. The experimental results showed that the contact angles of water on steel 45, steel T8, polysiloxane coating, and polysiloxane/T8 composite coating were 68[degrees], 70[degrees], 101[degrees], and 92[degrees], respectively. Polysiloxane/T8 composite coating possesses hydrophobicity. The contact angle of liquids on a composite surface composed of two different materials can be calculated using the Cassie equation (10):

[FIGURE 3 OMITTED]

cos[[theta].sub.c] = [f.sub.a]cos[[theta].sub.a] + [f.sub.b]cos[[theta].sub.b] (2)

where [[theta].sub.c] is the contact angle of a liquid on a composite surface, [f.sub.a] is the surface fraction of material A on a composite surface and [f.sub.b] for material B; correspondingly, [[theta].sub.a] is the contact angle on material A and [[theta].sub.b] for material B.

Based on equation (2), if the surface fraction of the material with larger contact angle increases the contact angle of a composite surface increases. The friction-wear process makes polysiloxane transfer onto the T8 matrix of the composite surface, so that the surface fraction of polysiloxane becomes larger after wear than before wear. Therefore, the hydrophobicity of the composite surface was further enhanced. The experimental results showed that the contact angle of water on the composite coating after wear was up to 97[degrees]. Therefore, if this composite coating is used as the surface coating for soil-engaging components, when soil-engaging components work, the friction of soil to composite coating will lead to the transfer of polysiloxane, as previously mentioned. This is more beneficial for reducing the adhesion and resistance of soil.

Performance of Reducing Soil Resistance

The bulldozing resistance experiments were carried out using self-made testing equipment. The results and their standard deviations are listed in Table 1. The results of the statistical tests (F-test and t-test at the 95% confidence level) indicated that there is not a significant difference between the precisions in the sets of data but that there exists a significant difference between the two means being compared. Table 1 shows that the bulldozing resistance of the plate with the polysiloxane/T8 composite coating is much smaller than that of the uncoated steel 45 plate. The moisture content of soil and the plowing velocity have a strong influence on the bulldozing resistance. When the moisture content of soil is kept constant, the reducing rate of bulldozing resistance increases with increasing plowing velocity; the greater the moisture content of the soil, the greater the effect. When plowing velocity does not change the reducing rate of bulldozing resistance increases with increasing moisture content of a soil; the greater the plowing velocity, the greater the effect. Thus, plowing velocity and soil moisture content have an effect on the reducing rate of bulldozing resistance. The bulldozing resistance and reducing rate of bulldozing resistance increase with the increase of plowing velocity and soil moisture content.

[FIGURE 4 OMITTED]

The reason the composite coating effectively reduces soil resistance is that it effectively reduces soil adhesion. The typical states of soil adhesion on the surface of bulldozing plates are shown in Figure 4. In the bulldozing resistance experiment, when the bulldozing plates moved backward, the soil adhering to the surface of steel 45 bulldozing plates did not fall off, as shown in Figures 4a and 4b. However, for the bulldozing plates treated with polysiloxane/T8 composite coating, when the bulldozing plates moved backward, the adhering soil fell off from the surface of the bulldozing plates and only a little soil remained on the plate surface, as shown in Figures 4c and 4d. Hence, the polysiloxane/T8 composite coating effectively reduces soil resistance.

CONCLUSIONS

(1) When the quenched and tempered steel 45 was used as comparative material, the relative volume wear loss of the polysiloxane/T8 composite coating was 63%. The wear resistance of the polysiloxane/T8 composite coating depends on the porous T8 matrix. The predominant wear mechanism on a microscopic basis on the surface of the steel is plowing.

(2) The contact angle of water is larger on the surface of the polysiloxane/T8 composite coating than on the surface of steel T8, which indicates that the polysiloxane/T8 composite coating is more hydrophobic than steel T8.

(3) Under the experimental conditions, the polysiloxane/T8 composite coating effectively reduces soil resistance. When the moisture content of soil and the plowing velocity of bulldozing plates are increased, the reduction extent in bulldozing resistance increases; the maximum reduction is 22.3% as compared to that of uncoated steel 45.

References

(1) Ren, L.Q., Chen, D.X., and Chen B.C., "Review about Studies on Soil Adhesion," Transactions of the Chinese Society of Agricultural Engineering, 1, No. 1, 6 (1990).

(2) Jia, X., "Theoretical Analysis of the Adhesion Force of Soil to Solid Materials," Biosystems Engineering, 489, No. 4, 87 (2004).

(3) Ren, L.Q., Chen, D.X., and Hu, J.G., "Analysis about Principles of Soil Animals Reducing Soil Adhesion," Transactions of the Chinese Society of Agricultural Engineering, 15, No. 1, 6 (1990).

(4) Wu, S., Polymer Interface and Adhesion, Marcel Dekker, Inc., New York, 1982.

(5) Vaziri, M., Spurr, R.T., and Stopp, F.H., "An Investigation of the Wear of Polymeric Materials," Wear, 329, 122 (1988).

(6) Lu, X.C., Wen, S.Z., Tong, J., Chen, Y.T., and Ren, L.Q., "Wettability, Soil Adhesion, Abrasion and Friction Wear of PTFE (+PPS) + [Al.sub.2][O.sub.3] Composites," Wear, 48, No. 1, 193 (1996).

(7) Uelzen, Th. and Muller, J., "Wettability Enhancement by Rough Surfaces Generated by Thin Film Technology," Thin Solid Film, 311, No. 1-2, 434 (2003).

(8) Elkin, B., Mayer, J., Schindler, B., and Vohrer, U., "Wettability, Chemical and Morphological Data of Hydrophobic Layers by Plasma Polymerization on Smooth Substrates," Surf. Coat. Technol., 836, No. 9, 116-119 (1999).

(9) Zeng, Z.M., Practical Technical Manual for Engineering Materials, China Machine Press, Beijing, 2002.

(10) Cassie, A.B., "Contact Angles," Faraday Society Discussion, 11, 3 (1948).

Xian Jia** -- University of Science and Technology Beijing*

Xiaomei Ling -- Peking University ([dagger])

* Department of Science and Technology, Beijing 100083, China.

([dagger]) Department of Analytical Chemistry, School of Pharmaceutical Science, Beijing 100083, China. Email: [e-mail].

** Author to whom correspondence should be addressed. Email: [e-mail].

Table 1 -- Bulldozing Resistance and Its Reduction ([bar.x] [+ or -] s,
n = 7)

Materials                                Steel 45

Soil moisture content, %        29.4             29.4
Plowing velocity, [ms.sup.-1]    0.02             0.04
Bulldozing resistance, N       828 [+ or -] 14  890 [+ or -] 16
Reduction in resistance, %

Materials                                Steel 45

Soil moisture content, %         33.0              33.0
Plowing velocity, [ms.sup.-1]     0.02              0.04
Bulldozing resistance, N       1040 [+ or -] 18  1145 [+ or -] 17
Reduction in resistance, %

Materials                           Composite Coating

Soil moisture content, %        29.4             29.4
Plowing velocity, [ms.sup.-1]    0.02             0.04
Bulldozing resistance, N       702 [+ or -] 17  747 [+ or -] 16
Reduction in resistance, %      15.22            16.07

Materials                           Composite Coating

Soil moisture content, %        33.0             33.0
Plowing velocity, [ms.sup.-1]    0.02             0.04
Bulldozing resistance, N       851 [+ or -] 17  890 [+ or -] 15
Reduction in resistance, %      18.17            22.27

Note: [bar.x] -- average value; s -- standard deviation.