Erosion of protective coatings

Surface and Coatings Technology, 43/44 (1990) 875—887

875

EROSION OF PROTECTIVE COATINGS ZHENG RONG SHUI and BU QIAN WANG Xian Institute of Highways, Xian, Shaanxi (People’s Republic of China) ALAN V. LEVY Lawrence Berkeley Laboratory, University of California, Berkeley, CA 94720 (U.S.A.)

Abstract The room temperature erosion behavior of four different coatings on 1018 mild steel, namely clad WC + NiCrB, detonation gun Cr3C2, sputtered Cr3 C2 and electroless Ni + P, were investigated. The erosion tests were carried out under conditions that were representative of those which could occur in coal particle, energy production systems, with the exception of the test temperature. It was determined that the clad WC + NiCrB coating had a low erosion rate compared with most other materials tested, and that the hardness of the coating did not relate directly to the erosion resistance. For the hard coatings tested at low particle velocity, the erosion mechanism appeared to be cracking and chipping. As the particle velocity increased, more plastic deformation of the clad alloy was observed.

1. Introduction It is known that the density of protective coatings on substrate alloys has a direct relationship to their erosion resistance [1]. The purpose of this investigation was to determine the difference in erosion behavior between dense coatings applied either by brazing a previously melted layer on the surface of the mild steel to be protected or by building up the coating by an application process that melts individual coating particles and propels them to the surface to be protected. It is also known that the test variables, i.e. particle shape, size, velocity and impact angle, have a significant effect on the erosion behavior of materials. Since different end uses for protective coatings have different sets of operating conditions, the evaluation of the erosion behavior of coatings must be carried out over a relatively wide range of test conditions. In an earlier report [2] we investigated the erosion behavior of a group of 12 coatings on types 403 and 422 martensitic stainless steel eroded with 74 j.tm mixed oxide particles at T = 538 °C, c~= 30° and 90°, and a high velocity, V = 150 m s’, for application on steam turbine blades. It was determined that the Cr3 C2 (detonation gun) and WC + NiCrB (clad) coatings 0257-8972/90/$3.50

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876

had the lowest erosion rates and the electroless Ni + P and sputtered Cr3 C2 + Triballoy 800 coatings had the highest erosion rates. In a subsequent study [3] the erosion behavior of 403SS and two types of coatings on it, Cr3 C2 (detonation gun) and WC + NiCrB (clad), was investigated under milder conditions more representative of those in coal conversion and utilization systems. They were eroded by three different erodent particles, namely 130 ~tm angular SiC, 130 ~.tmrounded agglomerated Al2 03 and 74 I.Lm mixed oxides, at room temperature and 500 °C, ~ = 30°and 90°,and at half the previous velocity, V = 70 m s~. It was found that the erosion behavior of the steel and coatings was markedly dependent on the shape and size of the erodent particles as well a~on the velocity. Different erosion behavior occurred for the same material or coating upon impact by the different particles, and the ranking of the materials tested for erosion resistance varied with the different erodent particles. In this work the behaviors of four of the coatings tested earlier on stainless steel were evaluated on 1018 carbon steel substrate under even milder conditions to determine their relative behavior for use in coal utilization systems. In ref. 3 it was found that when eroding coatings with 130 jim angular SiC at ~ = 30°, V = 70 m s~ and both room temperature and 500°C, the WC + NiCrB (clad) coating had the lowest erosion rate. The current tests used lower particle velocities down to V = 15 m s~ and a somewhat more erosive erodent (more angular and larger size) with the emphasis on the WC + NiCrB (clad) coating. Marked differences in behavior and ranking among the coatings occurred in the different velocity regimes, as reported in this and the previous two papers. Thus the spectrum of erosion behavior of protective coatings has been determined over an order-of-magnitude difference in particle velocity, along with the controlled variation of other important variables.

2. Test conditions Materials The materials that were tested are listed in Table 1. 1018 carbon steel was used as the coating substrate. Detonation gun Cr3 C2 and clad WC + NiCrB were selected as the best-performing coatings from the previous work [2] and electroless Ni + P and sputtered Cr3 C2 were used as the worst-performing coatings from the earlier tests. The specimen dimensions were 5 cm x 1.8 cm x 0.5 cm and the test surfaces of the coatings were polished to 600 grit prior to testing. 2.1.

2.2. Test description The tests were carried out in the Lawrence Berkeley Laboratory room temperature erosion tester [4]. Angular SiC erodent particles with an average size of 200 j.Lm were used. Tests were performed at room temperature at impingement angles of 30°and 90°in an air stream with impact velocities of

877 TABLE 1 Coating parameters Coatings

Application techniques

Density 3) (g cm

Surface hardness (HV500)

Thickness as-applied (pm)

WC + NiCrB Ni + P (Ni—MDC) Cr 3 C2 + triballoy 800 Cr3 C3

Clad Electroless Ni

11.65 7.85

1049 617

330 90

8.64 6.40

615 1000

30 180

Sputtered Detonation gun

aCoating substrate: 1018 steel.

1 particle loading. The particle velocity was 15, 30 and 70 m s~ at a 5 g min calculated using a program described in ref. 5. The speciments of clad WC + NiCrB coating were eroded incrementally in small gram increments until a steady state erosion rate was reached. The specimens of the other three coatings were eroded at three particle loadings, 100 g (at V = 15 m s1), 80 g (at V = 30 m s’) and 50 g (at V = 70 m which resulted in steady state material loss conditions. After the tests the specimens were cleaned with an air jet and in alcohol in an ultrasonic cleaner. Erosion rates were measured by weight loss using a balance that could be read to 0.1 mg ±0.3 mg.

3. Results and discussion 3.1. Erosion rate analysis Figure 1 shows the incremental weight loss curves for the clad WC + NiCrB coating from the V = 15 and 30 m s’ tests. Figure 2 shows the curves for the same coating from the V = 70 m s’ tests. It can be seen that the clad WC + NiCrB coating had higher steady state erosion rates at higher particle impact velocities. At the lowest particle velocity there was relatively little difference in the erosion rates at a = 30° and 90° impact angles. As the particle velocity increased to 30 m s’ and then to 70 ms1, the difference in erosion rates at a = 30°and 90°increased. As the velocity increased, more material was eroded at a = 90° than at a = 30°, which is generally the pattern of behavior of a brittle material. This behavior differs from that reported in ref. 6 for a test of the same coating carried out at 180 m s’ and eroded by fly ash, where the impingement angle was reported not to have resulted in any change in the erosion rate. It is known [7] that materials that consist of both brittle and ductile constituents can behave in either a ductile or a brittle manner, as indicated by different parameters. Thus, as a function of impact angle a, the coatings behaved like brittle materials. However, the peak in the curves near the beginning of the tests is typical of ductile metal erosion [8].

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Fewer particles were required to reach the steady state erosion condition as the particle velocity increased. At particle velocities of 15, 30 and 70 m s~,the steady state condition was reached at total weights of particles of 100, 60 and 30 g respectively, indicating that erosion tests can be relatively short in duration [9]. Table 2 shows the erosion rate of the four coatings eroded by 200 jim angular SiC under steady state conditions. The erosion rates are normalized for the different densities of the coatings and are presented as volume of material or coating removed per gram of erodent. For comparison, the test results of these four coatings on 403SS from refs. 2 and 3 are also listed in Table 2. The test conditions for each test are noted in the table. The absence of an effect of hardness on erosion behavior can be observed in Tables 1 and 2. The clad WC + NiCrB coating had the same hardness as the detonation gun Cr3 C2 coating. However, the former had the lowest erosion rate and the latter had the highest erosion rate among the four coatings tested. The two soft coatings, Ni + P and sputtered Cr3 C2, had lower erosion rates than the harder detonation gun Cr3 C2 coating and higher rates than the clad WC + NiCrB hard coating.

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TABLE 2 Erosion rate at steady state condition 3 g’ x 106)

Coating

Erosion rate (cm V=15ms’

WC + NiCrB Ni + P (Ni—MDC) Cr 3 C2 + triballoy 800 Cr3 C3—Detonation gun Test conditions: T

=

V=30ms~

V=70ms’

2=300

2=900

2=300

2=900

6=300

1.5 4.1

2.1 2.9

5.4 13.0

8.5 10.2

45 82

71 75

3.5 5.2

2.7 8.2

10.8 17.0

8.9 24.0

78 95

65 130

8=900

room temperature; 200 pm angular shaped SiC; coating substrate: 1018 steel.

880

From Table 2 it can be seen that the particle velocity had an effect on the erosion of the coatings tested. The erosion rates of both hard coatings in Table 2 using 200 jim SiC particles compared with their rates using smaller particles indicate the effect on the resultant erosion rates of trade-offs between angularity and size of the erodent particles and their impact velocity. The more angular and larger size SiC particles used in the current tests reported in Table 2 compared with the less angular and smaller size mixed oxide particles used in the earlier tests more than offset the higher impact velocity of the mixed oxide erodent in the earlier tests and caused higher material losses. At the same particle velocity of 70 ms1 at room temperature, the erosion rates in this work seen in Table 2 are almost twice those from the test using smaller, less angular particles in the earlier work [3]. Therefore the shape, composition and size of the erodent particles as well as the test conditions of particle velocity, impact angle and test temperature had an important effect on the erosion rate of the coatings tested. Also, different relative erosion behaviors occurred when the same coating was impacted by different erodent particles. For instance, the soft coating Ni + P had a lower erosion rate than the detonation gun Cr 3 C2 coating at room temperature in the current test (Table 2). In contrast, this soft coating had the highest erosion rate [2] in the earlier tests [3] at 538 °C. The erosion of the clad WC + NiCrB coating can be compared with other hard materials tested under the same conditions [10] in the bar chart shown in Fig. 3. All erosion rates are normalized for the differing coating densities. The clad WC + NiCrB coating had one of the lowest erosion rates, comparable with those of Kennemetal’s WC—Co high density materials K701 and K703 and AMS4777 NiCrB coating. It is thought that the few voids that are in the clad WC + NiCrB, as shown later, may have caused the higher erosion rates compared with K701, K703 or AMS4777. The absence of an effect of hardness on erosion behavior can also be observed in the bar chart. Thus the low-side 1049 VHN hardness of the clad WC + NiCrB coating is not a deterrent to its good erosion behavior. 3.2. Metallographic analysis Under steady state erosion conditions at a = 90°impact, as seen in Fig. 4, the eroding surface of the clad WC + NiCrB coating had no grinding marks remaining and each of the surface conditions from the three different particle velocity tests were different in appearance, having a more gross texture with increasing particle velocity. There were cracks in the coatings at all three test velocities. There was a high tungsten content on the tested surface, although specific WC particles are difficult to identify, indicating that some of the coating remained after the test. There is evidence of plastic flow of the binder material, particularly on the surface of the specimen tested at V =70 m s~. The surfaces of the clad WC + NiCrB coating specimens at the three impact velocities and a = 30°impact angle are shown in Fig. 5. The appear-

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ances of the surfaces are similar to their a = 90°counterparts, while the rates of erosion for the V = 30 and 70 m s~ tests are considerably lower (see Table 2). The lower energy-dispersive X-ray (EDX) peaks in Fig. 5(d) are attributed to a shorter counting period. The distribution of WC particles in the NiCrB matrix is shown in the cross-section micrograph of Fig. 6(a). The slight roughening of the eroded surfaces compared with the untested ground surface can be seen. There is no strong relationship between the eroded surface contour and the location of WC particles or ductile matrix alloy at the surface. The voids shown in the three surface views of Fig. 6 were typical occurrences on the eroded surface. They appear to have had their edges eroded to some degree, but there is no indication that the presence of the voids significantly enhanced the overall erosion rate, as occurs on plasmasprayed coatings [2, 10]. The morphologies of the other three coatings tested, before and after eroding at a particle velocity of 70 m s~ and impact angles of 30°and 90°,are shown in Figs. 7—9. It can be seen from Fig. 7 that the surface of detonation gun Cr3 C2 coating specimens showed the brittle nature of the hard material.

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= 25 °C;t = 30, 50, 75 mm.)

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Fig. 5. Eroded surface of the clad WC + NiCrB coating at a = 30°impact in air. (a) V = 15 ms~, loading 150 g; (b) V = 30 ms’, loading 150 g; (c) V = 70 ms’, loading 60 g; (d) composition of surface (a = 30°, V = 30 ms’, loading 150 g). (Erosion conditions: 200 pm SiC; T = 25°C; t = 30, 75 mm).

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Fig. 6. Erosion surfaces of the clad WC + NiCrB coating in air. (a) Cross-section of the distribution of WC particles in NiCrB matrix (uneroded); (b) a = 30°, V = 30 ms’, loading 150 g; (c) =90°, V=30ms’, loading 150g; (d) a=30°, V=7Oms~, loading 7.5g; (e) a =90°, V = 70 ms~, loading 60g. (Erosion conditions: 200pm SiC; T = 25°C; t = 4, 30, 75mm.)

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3 C2 (detonation gun) coating in air. (a) Uneroded surface; (b) eroded surface, a = 70°; (c) eroded surface, a = 90°;(d) composition of surface, a = 90°.(Erosion conditions: V = 70 m s~ 200 pm SiC; T = 25 °C;t = 10 mm detonation gun.)

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3 C2 (triballoy 800) coating in air. (a) Uneroded surface; (b) eroded surface, a = 300; (c) eroded surface, a = 90°;(d) cross-section of surface, a = 90°. (Erosion conditions: nozzle coater; V = 70m s’; 200pm SiC; T= 25°C; t = 10 mm; loading 50g.)

Cracks and brittle fractures were observed, many small pieces of coating were chipped off and many small voids and pits formed. Some small gouges and striations can also be seen at the surface of the specimen eroded at a = 30°.The EDX peak analysis of the specimen eroded at a = 90°indicated that some coating remained at the end of the test, in spite of a severely damaged surface and the highest material loss for this coating at a = 90° (see Table 2). From Figs. 8 and 9 it can be seen that the surfaces of eroded electroless Ni + P and sputtered Cr3 C2 coating specimens show their ductile material nature, consisting of platelets and shallow craters as well as large gouges and striations. The craters and gouges are larger for the a = 30° tests than for the a = 90° tests where the erosion rates were lower (see Table 2).

887

4. Conclusions (1) The clad WC + NiCrB coating has a low erosion rate compared with most other materials tested. (2) Hardness does not directly relate to erosion resistance. (3) For the hard coatings tested at low particle velocity, the erosion mechanism appeared to be cracking and chipping. As the particle velocity increased, more plastic deformation of the clad alloy was observed. (4) The softer coatings tested showed a typical ductile material erosion behavior. (5) The voids in the clad coating do not appear to have a significant effect on the erosion behavior.

Acknowledgment This research was sponsored by the Department of Energy under DOE/ FEAA 10 10 0, Advanced Research and Technical Development, Fossil Energy Materials Program, Work Breakdown Structure Element LBL-3, under Contract No. DE-AC035F00089.

References 1 A. G. Davis, D. H. Boone and A. V. Levy, Erosion of ceramic thermal barrier coatings, Wear, 110(2) (1986) 101—116. 2 J. Qureshi, A. V. Levy and B. Q. Wang, Characterization of coating processes and coatings for steam turbine blades, Proc. mt. Conf. on Metallurgical Coatings, San Diego, CA, April 1986, J. Vac. Sci. Technol. A, 4 (6) (1986) 2638—2647. 3 A. V. Levy and B. Q. Wang, Erosion of hard material coating systems, Wear, 121 (3) (1988) 325—346. 4 A. V. Levy, The solid particle erosion behavior of steel as a function of microstructure, Wear, 68 (3) (1981) 269—287. 5 D. M. Kleist, One dimensional, two phase particulate flow, M.S. Thesis, Report LBL-6967, Lawrence Berkeley Laboratory, University of California, 1977. 6 M. R. Dustoor and D. E. Shewell, New method of applying wear resistant coatings, Metal Progress, Nov. (1983) 1—6. 7 A. V. Levy and G. Hickey, Erosion of corrosion-resistant surface treatments on alloy steels, Wear, 108(1) (1986) 61—80. 8 A. V. Levy, M. Aghazadeh and G. Hickey, The effect of test variables on the platelet mechanism of erosion, Wear, 108 (1) (1986) 23—42. 9 A. V. Levy, The platelet mechanism of erosion of ductile metals, Wear, 108 (1) (1986) 1—22. 10 A. V. Levy, T. Bakker, E. Scholz and M. Azahabeh, Erosion of hard metal coatings, Proc. Symp. on High-Temperature Protective Coatings, Atlanta, GA, March 1983, The Metallurgical Society of AIME, New York, pp. 339-358.