Study of antiwear adhesive coating under different erosion conditions

Wear, 162464

(1993) 569-573

569

Study of antiwear adhesive coating under different erosion conditions Xiang-Dong

Ma, Fu-Yan Lin and He-Sheng Shao

Betjing Graduate School, China University of Mining and Technology 100083 Beijing (China)

Abstract The characteristics

of antiwear adhesive coating in solid particle erosion and slurry erosion were investigated. The influence of the angle of impingement, the size of antiwear filler and the size of abrasive was studied. It was discovered that the erosion mechanisms of the coating under the two erosion conditions were similar. It was indicated that the filler (aluminium oxide) exhibited typical erosion behaviour of brittle material and the binder exhibited semiductile behaviour. The experiments also showed that the erosion resistance of the coating in slurry erosion was better than that in solid particle erosion.

1. Introduction Erosion is one of the important wear phenomena in modern industry. Many machines are destroyed by erosion, e.g. hydraulic turbines, pumps, steam turbines, coal gasification systems and even aircraft and satellites [l-5]. It was estimated by Eyre that erosion made up 8% of the total wear damage in industrial production [6]. The goal of studying wear is to find ways to solve wear problems and to retrieve the losses caused by wear. Applying an antiwear adhesive coating is one of the methods to prevent and repair worn materials and to prolong the service life of machines. Adhesive coatings have been developed for erosion wear conditions, eg. for spillway structures [7] and repairs to pumps [8]. In this study, solid particle erosion (SPE) and slurry erosion (SLE) of a coating were investigated over a range of angles of impingement, sizes of filler and sizes of abrasive. The erosion mechanisms of the coating are also discussed.

2. Experimental details 2.1. Apparatus SPE tests were conducted in the gas-blast-type rig depicted in Fig. 1. The velocity of the abrasive can be controlled by the air pressure. SLE tests were carried out in the slurry erosion device shown in Fig. 2. The velocity of the slurry impinging on the target can be controlled by the rotational velocity of the motor. 0043-1648/93/$6.00

Chambm Fig. 1. Schematic diagram of solid particle erosion apparatus.

Hcildcr'-' Samfle

Era sion --&amber

Fig. 2. Schematic diagram of slurry erosion apparatus.

2.2. Abrasive The abrasive used in the experiments was quartz sand with mean diameters of 130 and 230 pm. In SLE the test liquid was water and the content of abrasive in the slurry was 10 wt.%. 2.3. Specimens The specimens were cuboids of thickness 5 mm and rectangular surface area 25 X 10 mm2. The test materials are divided into two groups. Sample A is pure binder (epoxy resin) and sample B is antiwear adhesive coating consisting of binder and antiwear filler (aluminium 0 1993 - Elsevier Sequoia. AU rights reserved

570

X.-D. Ma et al. I Antiwear adhesive coating

oxide). The content of filler in sample B is 75 wt.%. The shear strength of the binder is 20 MPa and its density is 1.17 g cm- 3. The samples were cured at ambient temperature for about 12 h. The erosion rate (E) of a sample is defined as

4. Discussion

volume loss abrasive weight

E=

increases. If the filler grains are quite small and the abrasive grains quite large, E increases quickly (Fig. 4(a)). In SLE E varies very little with changing size of filler and size of abrasive (Fig. 4(b)).

2.4. Test conditions The tests were carried out at a temperature of 20&-2 “C and a velocity of 46 f 1 m s-l. A scanning electron microscope was used to photograph the eroded surfaces. The eroded specimens were plated with a very thin gold film for scanning electron microscopy (SEM) studies.

4.1. Solid particle erosion In Fig. 3(a) we find that the angle at peak E of sample A is about 35”. Hence the binder should be semiductile [9]. On examining the eroded surfaces with SEM, we find that there are plastically deformed lips and cracks on the surface of sample A as shown in Fig. 5. This indicates that the binder cannot suffer large-scale deformation. Thus the binder exhibits semiductile erosion behaviour. When the filler is added to the binder (forming sample B), the angle at peak E shifts to 60”. This

3. Results Erosion rate (E) vs. impingement angle curves are presented in Figs. 3(a) and 3(b) for erosion by quartz sand of size 230 pm. The size of filler in sample B is about 530 pm. It is seen that the erosion rates in SLE are lower than those in SPE and the angles at peak E in SLE are larger than those in SPE. Curves of E vs. size of filler and size of abrasive are shown in Figs. 4(a) and 4(b). In SPE E increases as the size of filler decreases and the size of abrasive

II

(4

30 15 GO -\rwle (de!Jrees)

is

Fig. 3. Erosion rate as a function of impinging angle in (a) SPE and (b) SLE. The size of abrasive is approximately 230 pm. The size of filler is about 530 pm for sample B.

N

g--_.

w5 I I

II

B/

I

100 200 300 400 500 600 Size of Filler (pm)

O”

1.100 200_.300 -loo 500- 6OU I

Size of Filler (pm) (4 Fig. 4. Erosion rate as a function of size of filler at impinging angle of 30” in (a) SPE and (b) SLE. The specimen is sample B. A, 230 pm abrasive; B, 130 pm abrasive.

(b) Fig. 5. Scanning electron micrographs of eroded surface of sample A (epoxy resin): (a) plastic deformation (original magnification, x 1067.5); (b) cracking (original magnification, X 1586).

X.-D. Ma et al. I Antiwearadhesivecoating

implies that sample B tends to be of brittle nature. The reason is that the filler is a brittle material. Figure 6 shows the eroded topographic feature of sample B for 30” angle and 130 lrn abrasive. It is seen that filler grains protrude on the surface, which demonstrates that the filler has better erosion resistance than the binder. In this case the filler grains act as an antiwear framework. From the experiments we have found that the erosion mechanisms of the filler are brittle fracture and breaking, with wear easily taking place at the edges and corners of the filler grains (Fig. 6). Therefore, if the shapes of the filler grains are needle or flake like, the grains are easily broken and worn out. If the shapes of the filler grains are approximately spherical, the grains are only broken with difficulty. Thus the shape of the filler is one of the factors influencing the erosion resistance of the coating material. Figure 4(a) shows that E is closely related to the size of filler and the size of abrasive. We find that if the size of filler is quite small, breaking of the filler may be severe as shown in Fig. 7. In this case the filler grains cannot act as an antiwear framework, so E increases quickly. From the above discussion we can see that E is closely related to the size of filler and the size of abrasive. We can calculate the force applied on the filler grains by physical and mathematical methods. In order to simplify the calculation, we assume that the shapes of the eroding particles and filler grains are spherical (radii R, and R2 respectively, as shown in Fig. 8). When a projectile with velocity Y impacts a filler grain at an angle fx, the maximum impact pressure q can be calculated from the stored elastic energy. If the kinetic energy is assumed to be completely converted into elastic energy during impact and a quasi-static approbation is used, q is given by

Fig. 6. Scanning electron micrograph of eroded surface of sample B in SPE: impinging angle, 30”; size of filler, 530 pm; size of abrasive, 130 pm (original magnification, X43.2).

571

Fig. 7. Scanning electron micrograph of eroded surface of sample B in SPE: impinging angle, 30”; size of filler, 130 Nrn; size of abrasive, 230 pm (original ma~ifi~tion; x189).

Fig. 8. Schematic

diagram of impact of two spheres.

(1) where h= *+ &

1--UZ2 &

which depends on Poisson’s ratio U, of the projectile and U, of the filler and on Young’s modulus E, of the projectile and Ez of the filler. In eqn. (1) p is the density of the projectile, Y is the impinging velocity and (r is the impin~ng angle. By calculation we find that the maximum tensile stress, the maximum shear stress and the maximum compressive stress applied on the filler grain are proportional to q. Thus q determines the stress state of the filler grain. Therefore the breaking condition of the filler can be expressed by q. We assume that there are two critical values, 4% and qp If qq2, q is large enough to produce cracks which can penetrate the filler grain. Then breaking of the filler is severe and the erosion

X.-D. Ma et al. J Antiwear adhesive coating

572

rate increases quickly as 4 increases. It is seen from eqn. (1) that q varies with changing R, and R,. If R, increases and R2 decreases, q and the erosion rate will increase. If q is beyond q2, the filler breaks severely and the erosion rate increases quickly (Figs. 4(a) and 7). The value of q can be determined by experiment. We found that at a velocity of 65 m s-l, an angle of 30” and RJR,= 1, q is equal to q2 [lo]. By calculation we obtain that q2 is about 3.0 GPa. In reality the shape of the abrasive or filler grains cannot be approximated as a sphere. In this case the maximum impact pressure q applied on a filler grain is given by q=[$h-‘m(i

+

k-(Vsin

fzr)2r

where m is the mass of the projectile and RI and R2 are the curvature radii of the projectile and filler grain respectively at the contact point. Thus there is no relationship between the sizes of abrasive and filler and the curvature radii. Generally, the smaller R, or R, is, the sharper the shape of the abrasive or filler grain will be. Equation (2) shows that q increases as m increases. Hence a large projectile may lead to more severe breaking of filler grains than a small one does. If m is constant, q varies with changing R, or R,. Since q increases as R, decreases, the sharper the abrasive is, the more wear will be produced. Since q also increases as R, decreases, when projectiles impact on the edges and corners of filler grains, the grains will be easily broken and worn out (as shown in Fig. 6). 4.2. Sluny erosion Figures 3(b) and 4(b) show that the erosion rates in SLE are much lower than in SPE. By examining the eroded surfaces, we find that the erosion mechanisms of samples A and B are very similar in SPE and SLE. Figure 9 shows the eroded topographic feature of sample B in SLE. It is seen that filler grains protrude very clearly. The only difference between the two erosion conditions is the medium which carries the abrasive eroding the target. The medium is air in SPE and water in SLE. We know that the viscosity of water is much higher than that of air. When slurry is incident on the surface of the target, it spreads over the surface and forms a layer of liquid (Fig. 10). Before the abrasive can impact the target it must penetrate the liquid layer; thus velocity of the abrasive will be reduced by the resistance of the liquid. However, on impacting the target surface at an angle (Y,the abrasive particles will rebound. The rebounded particles will exit the sample surface at a distribution of angles. Some of the re-

Fig. 9. Scanning electron micrograph of eroded surface of sample B in SLE: impinging angle, 30”; size of filler, 330 pm; size of abrasive, 230 pm (original magnification, x 121.5).

Fig. 10. Schematic target.

diagram of slurry spreading

over surface of

bounded particles might collide with incoming particles. Thus the average real velocity of the abrasive impacting the target surface is much smaller than the nominal velocity. This velocity loss effect also exists in SPE, but the inlluence of the effect is smaller than that in SLE. From eqn. (1) we know that if the velocity is reduced, the stress applied on the filler will be decreased and the breaking of the filler will be reduced. Thus at the same nominal velocity the erosion rates of materials in SPE are higher than those in SLE. Figure 4(b) shows that the erosion rates vary even with changing size of filler (from 70 to 600 pm in our tests). This illustrates that the maximum impact pressure q is very small (possibly lower than ql); thus the fillers break very little and have good erosion resistance in SLE. Because of the spreading of slurry over the target surface, the average real impinging angle is also smaller than the nominal angle. Thus the angle at peak E in SLE is larger than that in SPE. From the above discussion we have found that an antiwear adhesive coating is suitable for low velocity and small abrasive erosion working conditions (low impact energy), especially for some machines suffering slurry erosion, e.g. pumps and hydraulic turbines. An adhesive coating has been used on the fender of a coal water pump [8]. The fender was made of high chrome cast steel. The erosion rate of the fender without the coating was 7.8~ 10m3 mm h-l and the service life was about 1200 h. The erosion rate of the

X.-D. Ma et al. / Antiwear adhesive coating

coated fender was 2.2X 10e3 mm h-’ and the service life was prolonged to 2200 h. The thickness of the coating was 3 mm. These data show that the antiwear adhesive coating applies to the pump successfully and the service life with the coating is longer than that of the metal on its own.

573

erosion. (4) The average real impact angle of the abrasive in slurry erosion is lower than the nominal angle because of the slurry spreading over the surface of sample. (5) The coating is suitable for low velocity and small abrasive erosion conditions. References

5. Conclusions

(1) An antiwear adhesive coating is composed of a binder and an antiwear filler. The erosion characteristics of the coating are similar in solid particle erosion and slurry erosion. The erosion behaviour of the binder is of the semiductile type and the erosion mechanisms are plastic deformation and cracking. The erosion mechanisms of the filler are brittle fracture and breaking. (2) The size of filler and the size of abrasive are the factors which influence the erosion rates of the coating, especially in solid particle erosion. (3) The average real impact velocity of the abrasive is lower than the nominal velocity, especially in slurry

1 C. G. Duan, Silt Abmsive Erosion of Hydraulic Turbine, Qing Hua Press, Beijing, 1981, p. 1. 2 E. Reask, Wear, 13 (1968) 301. 3 T. Wakeman and W. Tabakoff, J. Aircraft, 16(12) (1979) 828. 4 J. Zahavi and G. F. Schmitt, Wear, 71 (1981) 179. 5 A. B. S. Willmott and A. Resente, Rot. ASiUE Int. Con& on Wear of Materials, ASME, New York, 1987, p. 753. 6 T. S. Eyre, in D. Scott (ed.), Treatise on Materials Science and Technology, Vol. 13, Academic Press, New York, 1979, p. 363. 7 P. V. Rao, Wear, ,122 (1988) 77. 8 X. D. Ma, F. Y. Lin and H. S. Shao, Lubr. Eng. (China), 4 (1991) 11. 9 G. P. Tilly, Wear, I4 (1969) 63. 10 X. D. Ma, F. Y. Lin and H. S. Shao, C-MRS Znt. 1990 Syrup., Vol. 5, Elsevier, Amsterdam, 1991, p. 561.