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Microstructure and properties of Cr2O3 coating deposited by plasma spraying and dry-ice blasting
Surface & Coatings Technology 225 (2013) 58–65
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and properties of Cr2O3 coating deposited by plasma spraying and dry-ice blasting Shujuan Dong a,⁎, Bo Song a,⁎, Bernard Hansz b, Hanlin Liao a, Christian Coddet a a b
LERMPS-UTBM, Site de Sévenans, 90010 Belfort Cedex, France HMRexpert, Impasse Bliss 25490 Fesches-le-Châtel, France
a r t i c l e
i n f o
Article history: Received 11 December 2012 Accepted in revised form 4 March 2013 Available online 21 March 2013 Keywords: Dry-ice blasting Cr2O3 Atmospheric plasma spray (APS) Porosity Wear resistance
a b s t r a c t Dry-ice blasting is applied in atmospheric plasma spray process with an aim to improve the properties of Cr2O3 coatings and save the cost. Microstructure, the tensile adhesive strength and the wear resistance of plasmasprayed Cr2O3 coatings without and with the treatment of dry-ice blasting were compared. The results indicate that dry-ice blasting has a significant effect on the porosity of Cr2O3 coating. After the treatment of dry-ice blasting, the porosity of Cr2O3 coating decreases from 6.6 ± 1.1% to 2.0 ± 0.1% and a noticeable improvement in the adhesion between the coating and 25CrMo4 substrate has been obtained from 13 ± 2 MPa to 46 ± 5 MPa. They could be attributed to the sublimation effect of dry-ice pellets on the evaporated Cr and the cleaning effect of dry-ice blasting on the coated substrate. The dry-ice blasted coating was more wear resistant than that deposited without dry-ice blasting. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ceramics have received much attention in industry applications, due to their high hardness, high chemical stability, high oxidationresistance at high temperatures, tribological properties, and so on. Among them, chromium oxide (Cr2O3) exhibits excellent wear and friction characteristics; therefore, this material was often used as coating material for thermal protection and wear resistance, such as piston engine rings [1,2]. Atmospheric plasma spray (APS) is one of the most frequently used thermal spraying methods to deposit Cr2O3 coatings, because this technique has relatively high deposition efficiency, low cost and high flexibility and the temperature is on the order of 10,000 K during this process [3]. However, the plasma-sprayed Cr2O3 coatings have a high porosity, which are considered to have some detrimental effects for the structure and properties of the deposits. The decrease of porosity, for example, could improve the service life of Cr2O3 coating applied to components such as piston ring in the automobiles. Therefore, the ability to control the coating porosity is essential for attaining the excellent protection. Apart from the porosity, the adhesion is also very important for determining the life of Cr2O3-coated parts. As a representative of ceramic coatings, the failure of the coating occurs easily from the interfaces between the coating and the substrate, because of the large difference of thermal expansion coefficient between ceramic coating and metallic substrate. The coatings must be properly bonded to the substrates in
⁎ Corresponding authors. Tel.: +33 03 84583164; fax: +33 03 84583286. E-mail addresses: [email protected] (S. Dong), [email protected] (B. Song). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.03.016
order to well protect the components in service. Adhesion depends on the surface preparation, the distribution of the residual stress in the coating, and so on. Continuous efforts are under way to improve the adhesion between the coating and the substrate, for example sandblasting the substrate prior to the deposition process. Especially for the Cr2O3 coating, an interlayer of Cr, NiCr or NiCrAl has been applied to the substrate prior to deposition [4–6]. It is therefore not surprising that adhesion becomes one of the most important research topics for Cr2O3 coatings. In addition, it can be recognized that the resistance to wear of the Cr2O3 coatings can be achieved only if the coating is properly bonded to the substrate [7–9]. Wear is closely related to the adhesion of the coating to the substrate because if the adhesion is poor, the coating will wear off quite rapidly in a reciprocating sliding tester and the deterioration of the substrate will be greatly accelerated [7]. Currently, the more popular method to improve the wear performance is the preparation of nanostructured coatings using nanostructured feedstock. The wear resistance improvement of the nanostructured coatings obtained from nanostructured powder could be ascribed to both the decrease of the defects size and the grains size [10–13]. The fine grains may somewhat suppress the phase transformation and microcracking. As reported by Luo et al. [14], the mixed microstructure of nanostructured coating composed of fully melted splats and partially melted particles also contributes to the higher fracture resistance, because the partially melted regions can provide a variety of cracks arrest and deflection mechanisms. However, the essential problem is that this deposition process requires the agglomerated powder, because it is difficult to introduce those nanopowders into the plasma jet due to their very poor flowability and low mass [10,13]. This will undoubtedly increase the processing cost.
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
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Fig. 1. (a) SEM observation and (b) size distribution of the used Cr2O3 powder.
In this work, dry-ice blasting is applied in APS process, with an aim to improve the properties of Cr2O3 coatings and to save the processing cost. It is a relatively new technique employed for pre-treating the coated substrate and cooling the sample during thermal spraying process. The purpose of this investigation is to identify the microstructure of the plasma-sprayed Cr2O3 coatings and to understand the influence of dry-ice blasting on the microstructure and their properties. 2. Experimental procedure 2.1. Materials A commercially available Cr2O3 powder (Sulzer-Metco Amdry 6410) was used as feedstock material. Fig. 1a illustrates the morphology of the Cr2O3 powders. Powder size distribution was measured with a laser particle size analyzer (Mastersixer 2000, Malvern Instruments, UK). It exhibits a distribution with D0.1 of 20.52 μm, D0.5 of 35.24 μm and D0.9 of 59.14 μm, as shown in Fig. 1b.
front of plate sample holder, the spraying torch moved along the rectangular lines with a speed of 300 mm/s. The plasma spray distances were fixed at 115 mm for all coatings. Dry-ice blasting was carried out using a mobile blasting device (ic4000 system, HMRexpert, France), which comprises a similar-Laval nozzle with a rectangular outlet dimension of 9 × 40 mm, a mass flow controller with a pneumatic motor, a storage tank, and a compressed air supplier. For the current work, mass flow rate of dry-ice pellets was 42 kg h−1 under a gas pressure of 0.6–0.8 MPa. The distance between the axis-exit of the dry-ice blasting nozzle and substrates is about 25 mm. A robot (ABB, Sweden) was employed to vertically move the spray torch and the dry-ice blasting nozzle with a line speed of 15 mm/s for a uniform and reproducible deposition of coatings. All the sandblasted substrates were pre-treated for four passes by dry-ice blasting; meanwhile, plasma flame was turned on but not providing the powder. This aims to avoid the condensation arising from the treatment of dry-ice blasting for long time. Afterwards, the sending powder system was turned on to deposit coatings. In other words, during the deposition process, dry-ice blasting was also employed.
2.2. Coating preparation All specimens were produced by atmospheric plasma spraying using a Sulzer-Metco F4 plasma gun. Argon was used as both plasmaoperating and powder carrier gas. Spraying parameters for these materials are displayed in Table 1. Disc-shaped and plate-shaped steel samples were used as the coated substrates. Among them, the disc-shaped 25CrMo4 steel substrates were used to measure the adhesive strength based on the tensile test. Some plate-shaped cast iron samples were also coated by Cr2O3 coatings in order to simulate the Cr2O3 protected heads. They were sandblasted prior to spraying, were fixed in a cylindrical holder (diameter of 160 mm) or a plate holder. The spraying torch was mounted on the flange of a robot and moved in front of cylindrical holder (D = 160 mm) which rotated with a speed of 150 rev/min. In
Table 1 Plasma spraying parameters used for the deposition of Cr2O3 coatings. Parameters/Unit
Cr2O3
Arc current/A Arc voltage/V Primary plasma gas/Ar/SLPM Secondary plasma gas/H2/SLPM Powder carrier gas/Ar/SLPM
630 68 32 12 3.4
2.3. Coating characterization The as-sprayed coatings were examined by an optical microscope (OM). The porosities of the as-sprayed coatings were estimated using an image analysis of the OM micrographs. More than five photos randomly observed in the polished cross section were averaged to evaluate the porosity. The cast iron samples were observed using a binocular. The tensile adhesive strength of Cr2O3 coating was measured according to American Society for Testing and Materials C633-01. For each experiment, five specimens were used. A commercially available adhesive (FM 1000) was used for the test. After the tensile test, the samples were examined using a binocular in order to determine whether there is coating residue on the substrate. A friction and wear test was carried out using a ball-on-disk CSEM tribometer (CSEM, Switzerland) in the air environment at room temperature. The counterparts were WC–Co balls with the diameter of 6 mm. The applied load and sliding velocity were 5 N and 10 cm/s. The friction force was measured with a linear variable differential transducer sensor and dynamically recorded into a computer at a frequency of 12 values per min. Before the friction and wear tests, the coatings were polished using SiC sandpaper and then polished using diamond slurries down to an average surface roughness of 1.6–2.2 μm. The friction force data were simply divided by the applied loads to give the friction coefficients. The wear rate was defined as the worn volume per unit of applied load and sliding distance. The cross-sectional areas of the
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b
a
Pores
c
d
Fig. 2. OM micrographs of Cr2O3 coatings plasma-sprayed (a)–(b) without and (c)–(d) with dry-ice blasting.
worn tracks were measured using a Taylor-Hobson Surtronic 3P profilometer (Rank Taylor Hobson Ltd., UK) after a 1000 m relative sliding and then multiplied by the length (perimeter) of the worn tracks to give the total worn volumes. In addition, the worn surfaces and debris were observed by means of scanning electron microscopy (SEM, JSW-5800LV, JOEL) and were analyzed by the associated energy dispersive spectrometer (EDS). 3. Results and discussion 3.1. Microstructure of the Cr2O3 coating Fig. 2 shows the cross-sectional microstructure of Cr2O3 coatings deposited by plasma spraying without and with dry-ice blasting. It is
a
surprising that there is a marked difference between the two structures. For the Cr2O3 coating deposited by plasma spraying without dry-ice blasting (Fig. 2a–b), it exhibits a great number of pores, as marked by arrow. Pores were apparent in the entire polished cross section of the coating. This structural feature is closely related to the spraying features during the APS process, in which a deposit is formed by the accumulation of the molten droplets. Inevitably, there exist different types of pores in the coating due to the incomplete filling and infiltration of the molten droplet, the rebound dissipation of a portion of semi-molten particles, the involvement of the gas, and so on. However, with the application of dry-ice blasting during the APS process, the Cr2O3 coating presents much denser structure (Fig. 2c–d). The statistic analysis of the porosity by the software of Image J yields a value of 2.0 ± 0.1% for the coating deposited with dry-ice blasting, while it
b
Fig. 3. Binocular observation of the samples (a) without and (b) with the treatment of dry-ice blasting.
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
yields 6.6 ± 1.1% for that deposited without dry-ice blasting. This result indicates that dry-ice blasting has a significant effect on the porosity of Cr2O3 coating. The reduction of the porosity, after the application of dry-ice blasting during the APS process, can be explained from two aspects. On one hand, the sublimation effect of dry-ice pellets on the evaporated Cr vapor plays an important role in the diminution of the porosity. This effect can be confirmed by the visual observation of the real samples. Fig. 3 displays typical photomicrographs of two samples with partial Cr2O3 coating deposited on the cast-iron substrate by plasma spraying without dry-ice blasting and with dry-ice blasting. It is clearly seen from the high-magnification images of the yellow frame that the majority of substrate is covered by a layer of the brown substance, while the substrate is relatively clean with the application of dry-ice
a
blasting during the deposition process. Further microscopic confirmation by SEM combined with EDS cartographic chemical analysis shows that the brown substance is the chromium particle, which formed during the APS process as presented in Fig. 4. For the deposition process of chromium oxide coatings, the chromium can easily be evaporated [15,16], because the temperature is on the order of 10,000 K during this process. The evaporated chromium, rapidly solidified, and then could be entrained like a non-molten particle in the coating. In fact, the entrained particles easily lead to pores between the lamellas and even fall off the coating because of the weak bonding with the surroundings. This is one reason of the high porosity in the plasma-sprayed Cr2O3 coating. However, with the application of dry-ice blasting during the APS process, the evaporated chromium would be significantly taken away by the expanding CO2 gas and the stream of the compressed air.
b
CKA
c
d
OKA
SiKA
e
f
CrKA
61
FeKA
Fig. 4. SEM micrograph of the brown substance in Fig. 3a and the corresponding EDS cartographic chemical analysis.
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S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
Fig. 5. Schematic diagram of the active processes by dry-ice blasting: (a) the cleaning effect of dry-ice blasting on the substrate, (b) the effect of expansion CO2 gas on the evaporated particles.
As described above, before plasma spraying, dry-ice pellets were blasted to treat the substrate and pre-coatings. Dry-ice pellets sublime upon the impact, and their state of aggregation changes directly from solid to gaseous, as illustrated in Fig. 5. This is known as the sublimation of carbon dioxide. The sublimation entails an instantaneous volume expansion by approx. 600–800 times [17,18], thus potentially taking away the evaporated chromium formed in the spraying process. Therefore, the pores which were led by those evaporated chromium would decrease. On the other hand, the decrease of the porosity after the application of dry-ice blasting can partially be explained by the cleaning effect of dry-ice blasting on the substrate and pre-coatings. This effect is based on three different effects—a thermal, a mechanical and an expansive effect, because this impacting process occurs under the condition of high velocity which can reach 100 m/s [19] and very low temperature. Before the molten or semi-molten droplets arrived on the substrate or pre-coatings, the micro- and nano-scale convex volume which adheres to the substrate and pre-coating had been
cleaned (Fig. 5). The pores would decrease because of the diminution of these convex volumes as the contaminants in the coating. 3.2. Adhesion of the Cr2O3 coating Fig. 6 presents the adhesion strength of plasma-sprayed Cr2O3 coatings without and with dry-ice blasting. As expected, the plasmasprayed Cr2O3 coating really has a very poor bonding with the substrate. Without the application of dry-ice blasting during the deposition process, the coating has a strength value of only 13 ± 2 MPa. It is of great interest to find that dry-ice blasting results in a remarkable improvement in the adhesion between the chromium oxide coatings and 25CrMo4 steel substrate. The adhesion increases to 46 ± 5 MPa after the application of dry-ice blasting during the APS process. The amount of increase in the adhesion is about 33 MPa, which is nearly three times of the initial adhesion value. This is closely related to the microstructure. Before the molten or semi-molten droplets arrived on the substrate, the micrometer- and nanometer-scale contaminant had been cleaned, which would improve the bonding between the substrate and the coating. In addition, the evaporated chromium had been taken away by the expanding inert gas; it thus avoids the deposition of the pre-solidified particles on the substrate, which is not beneficial to the bonding between the substrate and the coating. Fig. 7 displays the samples after the tensile test and the corresponding detachment characteristics. From the high-magnification images of the yellow frame, it can be distinguished that there isn’t coating still adhering on the substrate. They indicate that both the coatings were completely removed from the substrate when detachment occurred. 3.3. Friction behavior of the Cr2O3 coating
Fig. 6. Comparison of the tensile adhesive strength of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
The average friction coefficients and the wear rates of Cr2O3 coating, as shown in Fig. 8, were used to compare friction behavior. It can be revealed that there is very little difference in friction coefficient between the two coatings. With regards to the wear rate, it can be found that the Cr2O3 coating deposited with dry-ice blasting has the better wear resistance. After all, the wear rate is closely related to the
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
a
63
b
substrate
coating
substrate
coating
Fig. 7. Samples of the tensile test and the corresponding detachment characteristics: (a) without and (b) with dry-ice blasting.
adhesion of the coating to the substrate because if the adhesion is poor, the coating will fall off quite rapidly and the deterioration process will be greatly accelerated [7]. Fig. 9 shows the SEM micrographs of the worn surfaces of the two coatings. It can be distinguished from Fig. 9a that there are some fracture fragments and fine debris on the worn surface of the coating deposited without dry-ice blasting. While for the coating deposited with dry-ice blasting in Fig. 9b, the coating is in the initial period of shedding scales and there is almost no debris, although it was destroyed with the same sliding distance. Part of the coating was being delaminated owing to fatigue cracking and weak adhesion to the underlying surface. The next process is expected to be the removal of the delaminated film as wear debris. Some compact folds vertical to the sliding direction are also visible. The occurrence of the shedding scales indicates that the adhesion-induced spallation of the lamellae dominates for the tribological behavior of Cr2O3 coatings. In other words, the wear of Cr2O3 coatings appears to be strongly dependent on the adhesion and cohesion
strength of the coating as well as its fatigue strength. The micrometersized defects such as the pores in the coatings might act as the cracking origins during sliding. The more micrometer-sized defects in the coating, the more easily the adhesion-induced spallation took place, as reported in the literature [11]. Accordingly, the improvement in wear resistance of the coating sprayed using dry-ice blasting could be attributed to the decrease in the micrometer-sized defects. SEM examination of the groove bottom of the scratches in Figs. 10 and 11 revealed that with the repeated stressing, a friction fatigue occurs with many cracks, which are perpendicular to the lamellae. It is clear that nearly the whole surface of the groove bottom is full of micro cracks. Typical brittle fracture occurs during the sliding process. This result is determined by the high brittle character of chromium oxide. Combined with the EDS cartographic chemical analysis in Figs. 10 and 11, it can be considered that the worn surface contains the material transferred from the mated WC–Co balls, because that the element Co and W were prevalent on these worn surfaces. Fig. 12 presents the SEM micrographs of the worn surfaces of the WC–Co balls against the Cr2O3 coatings plasma-sprayed without and with dry-ice blasting, respectively. The wear surface of the WC–Co ball mated to the Cr2O3 coating plasma-sprayed without dry-ice blasting predominately presented plastic deformation and some ploughs (Fig. 12a), while almost no ploughs or obvious worn debris could be found at the surface of the WC–Co ball against the Cr2O3 coating deposited with dry-ice blasting (Fig. 12b). Few pores would also improve the microhardness of the coating from 1211 ± 105 to 1460 ± 108 HV0.3. Similar results were reported in our previous studies [20]. The increase of the microhardness is one reason for the improvement of the wear resistance, because the hardness is expected to be one factor in promoting good wear resistance in coatings, as reported in the literature [21]. 4. Conclusions
Fig. 8. Comparison of the average friction coefficient and the wear rate of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
In this study, the effect of dry-ice blasting on plasma-sprayed Cr2O3 coatings was investigated in terms of microstructure, the tensile
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S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
a
b
Sliding direction
Sliding direction
Fig. 9. SEM overview of the scratches of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
in the adhesion can be obtained using dry-ice blasting. The adhesive strength strongly depends on the microstructure. (3) The dry-ice blasted coating was more wear resistant than that deposited without dry-ice blasting. The wear mechanisms were explained in terms of adhesion-induced spallation, brittle fracture and material transfer. It is related to the microstructure, the adhesion and the microhardness.
adhesive strength and the wear resistance. The following conclusions could be drawn: (1) The Cr2O3 coating deposited by plasma spraying without dry-ice blasting exhibits a great number of pores. With the application of dry-ice blasting during the APS process, the Cr2O3 coating presents much denser structure. The porosity decreases from 6.6 ± 1.1% to 2.0 ± 0.1%. The sublimation effect of dry-ice pellets on the evaporated Cr vapor and the cleaning effect of dry-ice blasting on the coated substrate determine much difference in the microstructure. (2) The plasma-sprayed Cr2O3 coating really has a very poor bonding with the 25CrMo4 steel substrate, only having the adhesion strength of 13 ± 2 MPa. A remarkable increase of about 33 MPa
a
Acknowledgments The authors would like to thank C. Adam, J. Chauvelot and G. Schweitzer for their help in the APS and dry-ice blasting process. In addition, the authors gratefully acknowledge S. Lamy, O. Ribet and A. Lamraoui for their help in the SEM and EDS characterization.
b
CrKA b
CoKA
b
WLA
Fig. 10. (a) SEM (back scattered electrons) micrographs at high magnification and (b) cartographic analysis of the groove bottom corresponding to Fig. 9a.
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
a
65
b
CrKA b
CoKA
b
WLA
Fig. 11. (a) SEM (back scattered electrons) micrographs at high magnification and (b) cartographic analysis of the groove bottom corresponding to Fig. 9b.
Fig. 12. SEM micrographs of the worn surfaces of the WC–Co balls against Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
References [1] F. Rasteger, A.E. Craft, Surf. Coat. Technol. 61 (1993) 36. [2] J.E. Fernández, Y.L. Wang, R. Tucho, M.A. Martin-Luengo, R. Gancedo, A. Rincón, Tribol. Int. 29 (1996) 333. [3] Joseph R. Davis, Handbook of Thermal Spray Technology, Thermal Spray Society Training Committee, ASM International, 2004. [4] C.L. Li, H.X. Zhao, M. Matsumura, T. Takahashi, M. Asahara, H. Yamaguchi, Surf. Coat. Technol. 124 (2000) 53. [5] E. Sourty, J.L. Sullivan, M.D. Bijker, Tribol. Int. 36 (2003) 389. [6] In-Woong Lyo, Hyo-Sok Ahn, Dae-Soon Lim, Surf. Coat. Technol. 163–164 (2003) 413. [7] C.S. Richard, J. Lu, G. Beranger, F. Decomps, J. Therm. Spray Technol. 4 (1995) 342. [8] A.W. Batchelor, G.W. Stachowiak, J. Mater. Process. Technol. 48 (1995) 503. [9] C.S. Richard, J. Lu, G. Beranger, F. Decornps, J. Therm. Spray Technol. 4 (1995) 347.
[10] Y. Zeng, S.W. Lee, C.X. Ding, Mater. Lett. 57 (2002) 495. [11] J.F. Li, H.L. Liao, X.Y. Wang, B. Normand, V. Ji, C.X. Ding, C. Coddet, Tribol. Int. 37 (2004) 77. [12] J.H. Ouyang, S. Sasaki, Wear 249 (2001) 56. [13] A. Cellard, V. Garnier, G. Fantozzi, G. Baret, P. Fort, Ceram. Int. 35 (2009) 913. [14] H. Luo, D. Goberman, L. Shaw, M. Gell, Mater. Sci. Eng. A 346 (2003) 237. [15] Z. Zeng, S. Kuroda, H. Era, Surf. Coat. Technol. 204 (2009) 69. [16] M.D. Lima, R. Bonadimann, M.J. de Andrade, J.C. Toniolo, C.P. Bergmann, J. Eur. Ceram. Soc. 26 (2006) 1213. [17] G. Spur, E. Uhlmann, F. Elbing, Wear 233–235 (1999) 402. [18] E. Uhlmann, A. El Mernissi, Prod. Eng. Res. Dev. 2 (2008) 133. [19] S.J. Dong, B. Song, B. Hansz, H.L. Liao, C. Coddet, Mater. Res. Innov. 16 (2012) 61. [20] S.J. Dong, B. Song, B. Hansz, H.L. Liao, C. Coddet, Surf. Coat. Technol. 220 (2013) 199. [21] G.S.A.M. Theunissen, Tribol. Int. 31 (1998) 519.
Contents lists available at SciVerse ScienceDirect
Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat
Microstructure and properties of Cr2O3 coating deposited by plasma spraying and dry-ice blasting Shujuan Dong a,⁎, Bo Song a,⁎, Bernard Hansz b, Hanlin Liao a, Christian Coddet a a b
LERMPS-UTBM, Site de Sévenans, 90010 Belfort Cedex, France HMRexpert, Impasse Bliss 25490 Fesches-le-Châtel, France
a r t i c l e
i n f o
Article history: Received 11 December 2012 Accepted in revised form 4 March 2013 Available online 21 March 2013 Keywords: Dry-ice blasting Cr2O3 Atmospheric plasma spray (APS) Porosity Wear resistance
a b s t r a c t Dry-ice blasting is applied in atmospheric plasma spray process with an aim to improve the properties of Cr2O3 coatings and save the cost. Microstructure, the tensile adhesive strength and the wear resistance of plasmasprayed Cr2O3 coatings without and with the treatment of dry-ice blasting were compared. The results indicate that dry-ice blasting has a significant effect on the porosity of Cr2O3 coating. After the treatment of dry-ice blasting, the porosity of Cr2O3 coating decreases from 6.6 ± 1.1% to 2.0 ± 0.1% and a noticeable improvement in the adhesion between the coating and 25CrMo4 substrate has been obtained from 13 ± 2 MPa to 46 ± 5 MPa. They could be attributed to the sublimation effect of dry-ice pellets on the evaporated Cr and the cleaning effect of dry-ice blasting on the coated substrate. The dry-ice blasted coating was more wear resistant than that deposited without dry-ice blasting. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Ceramics have received much attention in industry applications, due to their high hardness, high chemical stability, high oxidationresistance at high temperatures, tribological properties, and so on. Among them, chromium oxide (Cr2O3) exhibits excellent wear and friction characteristics; therefore, this material was often used as coating material for thermal protection and wear resistance, such as piston engine rings [1,2]. Atmospheric plasma spray (APS) is one of the most frequently used thermal spraying methods to deposit Cr2O3 coatings, because this technique has relatively high deposition efficiency, low cost and high flexibility and the temperature is on the order of 10,000 K during this process [3]. However, the plasma-sprayed Cr2O3 coatings have a high porosity, which are considered to have some detrimental effects for the structure and properties of the deposits. The decrease of porosity, for example, could improve the service life of Cr2O3 coating applied to components such as piston ring in the automobiles. Therefore, the ability to control the coating porosity is essential for attaining the excellent protection. Apart from the porosity, the adhesion is also very important for determining the life of Cr2O3-coated parts. As a representative of ceramic coatings, the failure of the coating occurs easily from the interfaces between the coating and the substrate, because of the large difference of thermal expansion coefficient between ceramic coating and metallic substrate. The coatings must be properly bonded to the substrates in
⁎ Corresponding authors. Tel.: +33 03 84583164; fax: +33 03 84583286. E-mail addresses: [email protected] (S. Dong), [email protected] (B. Song). 0257-8972/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.surfcoat.2013.03.016
order to well protect the components in service. Adhesion depends on the surface preparation, the distribution of the residual stress in the coating, and so on. Continuous efforts are under way to improve the adhesion between the coating and the substrate, for example sandblasting the substrate prior to the deposition process. Especially for the Cr2O3 coating, an interlayer of Cr, NiCr or NiCrAl has been applied to the substrate prior to deposition [4–6]. It is therefore not surprising that adhesion becomes one of the most important research topics for Cr2O3 coatings. In addition, it can be recognized that the resistance to wear of the Cr2O3 coatings can be achieved only if the coating is properly bonded to the substrate [7–9]. Wear is closely related to the adhesion of the coating to the substrate because if the adhesion is poor, the coating will wear off quite rapidly in a reciprocating sliding tester and the deterioration of the substrate will be greatly accelerated [7]. Currently, the more popular method to improve the wear performance is the preparation of nanostructured coatings using nanostructured feedstock. The wear resistance improvement of the nanostructured coatings obtained from nanostructured powder could be ascribed to both the decrease of the defects size and the grains size [10–13]. The fine grains may somewhat suppress the phase transformation and microcracking. As reported by Luo et al. [14], the mixed microstructure of nanostructured coating composed of fully melted splats and partially melted particles also contributes to the higher fracture resistance, because the partially melted regions can provide a variety of cracks arrest and deflection mechanisms. However, the essential problem is that this deposition process requires the agglomerated powder, because it is difficult to introduce those nanopowders into the plasma jet due to their very poor flowability and low mass [10,13]. This will undoubtedly increase the processing cost.
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
59
Fig. 1. (a) SEM observation and (b) size distribution of the used Cr2O3 powder.
In this work, dry-ice blasting is applied in APS process, with an aim to improve the properties of Cr2O3 coatings and to save the processing cost. It is a relatively new technique employed for pre-treating the coated substrate and cooling the sample during thermal spraying process. The purpose of this investigation is to identify the microstructure of the plasma-sprayed Cr2O3 coatings and to understand the influence of dry-ice blasting on the microstructure and their properties. 2. Experimental procedure 2.1. Materials A commercially available Cr2O3 powder (Sulzer-Metco Amdry 6410) was used as feedstock material. Fig. 1a illustrates the morphology of the Cr2O3 powders. Powder size distribution was measured with a laser particle size analyzer (Mastersixer 2000, Malvern Instruments, UK). It exhibits a distribution with D0.1 of 20.52 μm, D0.5 of 35.24 μm and D0.9 of 59.14 μm, as shown in Fig. 1b.
front of plate sample holder, the spraying torch moved along the rectangular lines with a speed of 300 mm/s. The plasma spray distances were fixed at 115 mm for all coatings. Dry-ice blasting was carried out using a mobile blasting device (ic4000 system, HMRexpert, France), which comprises a similar-Laval nozzle with a rectangular outlet dimension of 9 × 40 mm, a mass flow controller with a pneumatic motor, a storage tank, and a compressed air supplier. For the current work, mass flow rate of dry-ice pellets was 42 kg h−1 under a gas pressure of 0.6–0.8 MPa. The distance between the axis-exit of the dry-ice blasting nozzle and substrates is about 25 mm. A robot (ABB, Sweden) was employed to vertically move the spray torch and the dry-ice blasting nozzle with a line speed of 15 mm/s for a uniform and reproducible deposition of coatings. All the sandblasted substrates were pre-treated for four passes by dry-ice blasting; meanwhile, plasma flame was turned on but not providing the powder. This aims to avoid the condensation arising from the treatment of dry-ice blasting for long time. Afterwards, the sending powder system was turned on to deposit coatings. In other words, during the deposition process, dry-ice blasting was also employed.
2.2. Coating preparation All specimens were produced by atmospheric plasma spraying using a Sulzer-Metco F4 plasma gun. Argon was used as both plasmaoperating and powder carrier gas. Spraying parameters for these materials are displayed in Table 1. Disc-shaped and plate-shaped steel samples were used as the coated substrates. Among them, the disc-shaped 25CrMo4 steel substrates were used to measure the adhesive strength based on the tensile test. Some plate-shaped cast iron samples were also coated by Cr2O3 coatings in order to simulate the Cr2O3 protected heads. They were sandblasted prior to spraying, were fixed in a cylindrical holder (diameter of 160 mm) or a plate holder. The spraying torch was mounted on the flange of a robot and moved in front of cylindrical holder (D = 160 mm) which rotated with a speed of 150 rev/min. In
Table 1 Plasma spraying parameters used for the deposition of Cr2O3 coatings. Parameters/Unit
Cr2O3
Arc current/A Arc voltage/V Primary plasma gas/Ar/SLPM Secondary plasma gas/H2/SLPM Powder carrier gas/Ar/SLPM
630 68 32 12 3.4
2.3. Coating characterization The as-sprayed coatings were examined by an optical microscope (OM). The porosities of the as-sprayed coatings were estimated using an image analysis of the OM micrographs. More than five photos randomly observed in the polished cross section were averaged to evaluate the porosity. The cast iron samples were observed using a binocular. The tensile adhesive strength of Cr2O3 coating was measured according to American Society for Testing and Materials C633-01. For each experiment, five specimens were used. A commercially available adhesive (FM 1000) was used for the test. After the tensile test, the samples were examined using a binocular in order to determine whether there is coating residue on the substrate. A friction and wear test was carried out using a ball-on-disk CSEM tribometer (CSEM, Switzerland) in the air environment at room temperature. The counterparts were WC–Co balls with the diameter of 6 mm. The applied load and sliding velocity were 5 N and 10 cm/s. The friction force was measured with a linear variable differential transducer sensor and dynamically recorded into a computer at a frequency of 12 values per min. Before the friction and wear tests, the coatings were polished using SiC sandpaper and then polished using diamond slurries down to an average surface roughness of 1.6–2.2 μm. The friction force data were simply divided by the applied loads to give the friction coefficients. The wear rate was defined as the worn volume per unit of applied load and sliding distance. The cross-sectional areas of the
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b
a
Pores
c
d
Fig. 2. OM micrographs of Cr2O3 coatings plasma-sprayed (a)–(b) without and (c)–(d) with dry-ice blasting.
worn tracks were measured using a Taylor-Hobson Surtronic 3P profilometer (Rank Taylor Hobson Ltd., UK) after a 1000 m relative sliding and then multiplied by the length (perimeter) of the worn tracks to give the total worn volumes. In addition, the worn surfaces and debris were observed by means of scanning electron microscopy (SEM, JSW-5800LV, JOEL) and were analyzed by the associated energy dispersive spectrometer (EDS). 3. Results and discussion 3.1. Microstructure of the Cr2O3 coating Fig. 2 shows the cross-sectional microstructure of Cr2O3 coatings deposited by plasma spraying without and with dry-ice blasting. It is
a
surprising that there is a marked difference between the two structures. For the Cr2O3 coating deposited by plasma spraying without dry-ice blasting (Fig. 2a–b), it exhibits a great number of pores, as marked by arrow. Pores were apparent in the entire polished cross section of the coating. This structural feature is closely related to the spraying features during the APS process, in which a deposit is formed by the accumulation of the molten droplets. Inevitably, there exist different types of pores in the coating due to the incomplete filling and infiltration of the molten droplet, the rebound dissipation of a portion of semi-molten particles, the involvement of the gas, and so on. However, with the application of dry-ice blasting during the APS process, the Cr2O3 coating presents much denser structure (Fig. 2c–d). The statistic analysis of the porosity by the software of Image J yields a value of 2.0 ± 0.1% for the coating deposited with dry-ice blasting, while it
b
Fig. 3. Binocular observation of the samples (a) without and (b) with the treatment of dry-ice blasting.
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yields 6.6 ± 1.1% for that deposited without dry-ice blasting. This result indicates that dry-ice blasting has a significant effect on the porosity of Cr2O3 coating. The reduction of the porosity, after the application of dry-ice blasting during the APS process, can be explained from two aspects. On one hand, the sublimation effect of dry-ice pellets on the evaporated Cr vapor plays an important role in the diminution of the porosity. This effect can be confirmed by the visual observation of the real samples. Fig. 3 displays typical photomicrographs of two samples with partial Cr2O3 coating deposited on the cast-iron substrate by plasma spraying without dry-ice blasting and with dry-ice blasting. It is clearly seen from the high-magnification images of the yellow frame that the majority of substrate is covered by a layer of the brown substance, while the substrate is relatively clean with the application of dry-ice
a
blasting during the deposition process. Further microscopic confirmation by SEM combined with EDS cartographic chemical analysis shows that the brown substance is the chromium particle, which formed during the APS process as presented in Fig. 4. For the deposition process of chromium oxide coatings, the chromium can easily be evaporated [15,16], because the temperature is on the order of 10,000 K during this process. The evaporated chromium, rapidly solidified, and then could be entrained like a non-molten particle in the coating. In fact, the entrained particles easily lead to pores between the lamellas and even fall off the coating because of the weak bonding with the surroundings. This is one reason of the high porosity in the plasma-sprayed Cr2O3 coating. However, with the application of dry-ice blasting during the APS process, the evaporated chromium would be significantly taken away by the expanding CO2 gas and the stream of the compressed air.
b
CKA
c
d
OKA
SiKA
e
f
CrKA
61
FeKA
Fig. 4. SEM micrograph of the brown substance in Fig. 3a and the corresponding EDS cartographic chemical analysis.
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Fig. 5. Schematic diagram of the active processes by dry-ice blasting: (a) the cleaning effect of dry-ice blasting on the substrate, (b) the effect of expansion CO2 gas on the evaporated particles.
As described above, before plasma spraying, dry-ice pellets were blasted to treat the substrate and pre-coatings. Dry-ice pellets sublime upon the impact, and their state of aggregation changes directly from solid to gaseous, as illustrated in Fig. 5. This is known as the sublimation of carbon dioxide. The sublimation entails an instantaneous volume expansion by approx. 600–800 times [17,18], thus potentially taking away the evaporated chromium formed in the spraying process. Therefore, the pores which were led by those evaporated chromium would decrease. On the other hand, the decrease of the porosity after the application of dry-ice blasting can partially be explained by the cleaning effect of dry-ice blasting on the substrate and pre-coatings. This effect is based on three different effects—a thermal, a mechanical and an expansive effect, because this impacting process occurs under the condition of high velocity which can reach 100 m/s [19] and very low temperature. Before the molten or semi-molten droplets arrived on the substrate or pre-coatings, the micro- and nano-scale convex volume which adheres to the substrate and pre-coating had been
cleaned (Fig. 5). The pores would decrease because of the diminution of these convex volumes as the contaminants in the coating. 3.2. Adhesion of the Cr2O3 coating Fig. 6 presents the adhesion strength of plasma-sprayed Cr2O3 coatings without and with dry-ice blasting. As expected, the plasmasprayed Cr2O3 coating really has a very poor bonding with the substrate. Without the application of dry-ice blasting during the deposition process, the coating has a strength value of only 13 ± 2 MPa. It is of great interest to find that dry-ice blasting results in a remarkable improvement in the adhesion between the chromium oxide coatings and 25CrMo4 steel substrate. The adhesion increases to 46 ± 5 MPa after the application of dry-ice blasting during the APS process. The amount of increase in the adhesion is about 33 MPa, which is nearly three times of the initial adhesion value. This is closely related to the microstructure. Before the molten or semi-molten droplets arrived on the substrate, the micrometer- and nanometer-scale contaminant had been cleaned, which would improve the bonding between the substrate and the coating. In addition, the evaporated chromium had been taken away by the expanding inert gas; it thus avoids the deposition of the pre-solidified particles on the substrate, which is not beneficial to the bonding between the substrate and the coating. Fig. 7 displays the samples after the tensile test and the corresponding detachment characteristics. From the high-magnification images of the yellow frame, it can be distinguished that there isn’t coating still adhering on the substrate. They indicate that both the coatings were completely removed from the substrate when detachment occurred. 3.3. Friction behavior of the Cr2O3 coating
Fig. 6. Comparison of the tensile adhesive strength of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
The average friction coefficients and the wear rates of Cr2O3 coating, as shown in Fig. 8, were used to compare friction behavior. It can be revealed that there is very little difference in friction coefficient between the two coatings. With regards to the wear rate, it can be found that the Cr2O3 coating deposited with dry-ice blasting has the better wear resistance. After all, the wear rate is closely related to the
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a
63
b
substrate
coating
substrate
coating
Fig. 7. Samples of the tensile test and the corresponding detachment characteristics: (a) without and (b) with dry-ice blasting.
adhesion of the coating to the substrate because if the adhesion is poor, the coating will fall off quite rapidly and the deterioration process will be greatly accelerated [7]. Fig. 9 shows the SEM micrographs of the worn surfaces of the two coatings. It can be distinguished from Fig. 9a that there are some fracture fragments and fine debris on the worn surface of the coating deposited without dry-ice blasting. While for the coating deposited with dry-ice blasting in Fig. 9b, the coating is in the initial period of shedding scales and there is almost no debris, although it was destroyed with the same sliding distance. Part of the coating was being delaminated owing to fatigue cracking and weak adhesion to the underlying surface. The next process is expected to be the removal of the delaminated film as wear debris. Some compact folds vertical to the sliding direction are also visible. The occurrence of the shedding scales indicates that the adhesion-induced spallation of the lamellae dominates for the tribological behavior of Cr2O3 coatings. In other words, the wear of Cr2O3 coatings appears to be strongly dependent on the adhesion and cohesion
strength of the coating as well as its fatigue strength. The micrometersized defects such as the pores in the coatings might act as the cracking origins during sliding. The more micrometer-sized defects in the coating, the more easily the adhesion-induced spallation took place, as reported in the literature [11]. Accordingly, the improvement in wear resistance of the coating sprayed using dry-ice blasting could be attributed to the decrease in the micrometer-sized defects. SEM examination of the groove bottom of the scratches in Figs. 10 and 11 revealed that with the repeated stressing, a friction fatigue occurs with many cracks, which are perpendicular to the lamellae. It is clear that nearly the whole surface of the groove bottom is full of micro cracks. Typical brittle fracture occurs during the sliding process. This result is determined by the high brittle character of chromium oxide. Combined with the EDS cartographic chemical analysis in Figs. 10 and 11, it can be considered that the worn surface contains the material transferred from the mated WC–Co balls, because that the element Co and W were prevalent on these worn surfaces. Fig. 12 presents the SEM micrographs of the worn surfaces of the WC–Co balls against the Cr2O3 coatings plasma-sprayed without and with dry-ice blasting, respectively. The wear surface of the WC–Co ball mated to the Cr2O3 coating plasma-sprayed without dry-ice blasting predominately presented plastic deformation and some ploughs (Fig. 12a), while almost no ploughs or obvious worn debris could be found at the surface of the WC–Co ball against the Cr2O3 coating deposited with dry-ice blasting (Fig. 12b). Few pores would also improve the microhardness of the coating from 1211 ± 105 to 1460 ± 108 HV0.3. Similar results were reported in our previous studies [20]. The increase of the microhardness is one reason for the improvement of the wear resistance, because the hardness is expected to be one factor in promoting good wear resistance in coatings, as reported in the literature [21]. 4. Conclusions
Fig. 8. Comparison of the average friction coefficient and the wear rate of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
In this study, the effect of dry-ice blasting on plasma-sprayed Cr2O3 coatings was investigated in terms of microstructure, the tensile
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a
b
Sliding direction
Sliding direction
Fig. 9. SEM overview of the scratches of Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
in the adhesion can be obtained using dry-ice blasting. The adhesive strength strongly depends on the microstructure. (3) The dry-ice blasted coating was more wear resistant than that deposited without dry-ice blasting. The wear mechanisms were explained in terms of adhesion-induced spallation, brittle fracture and material transfer. It is related to the microstructure, the adhesion and the microhardness.
adhesive strength and the wear resistance. The following conclusions could be drawn: (1) The Cr2O3 coating deposited by plasma spraying without dry-ice blasting exhibits a great number of pores. With the application of dry-ice blasting during the APS process, the Cr2O3 coating presents much denser structure. The porosity decreases from 6.6 ± 1.1% to 2.0 ± 0.1%. The sublimation effect of dry-ice pellets on the evaporated Cr vapor and the cleaning effect of dry-ice blasting on the coated substrate determine much difference in the microstructure. (2) The plasma-sprayed Cr2O3 coating really has a very poor bonding with the 25CrMo4 steel substrate, only having the adhesion strength of 13 ± 2 MPa. A remarkable increase of about 33 MPa
a
Acknowledgments The authors would like to thank C. Adam, J. Chauvelot and G. Schweitzer for their help in the APS and dry-ice blasting process. In addition, the authors gratefully acknowledge S. Lamy, O. Ribet and A. Lamraoui for their help in the SEM and EDS characterization.
b
CrKA b
CoKA
b
WLA
Fig. 10. (a) SEM (back scattered electrons) micrographs at high magnification and (b) cartographic analysis of the groove bottom corresponding to Fig. 9a.
S. Dong et al. / Surface & Coatings Technology 225 (2013) 58–65
a
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b
CrKA b
CoKA
b
WLA
Fig. 11. (a) SEM (back scattered electrons) micrographs at high magnification and (b) cartographic analysis of the groove bottom corresponding to Fig. 9b.
Fig. 12. SEM micrographs of the worn surfaces of the WC–Co balls against Cr2O3 coatings plasma-sprayed (a) without and (b) with dry-ice blasting.
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