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Erosion resistance of coatings for metal protection at elevated temperatures
SURfACE
&COARNIiS
ELSEVIER
Surfaceand Coatings Technology 86-87 (1996)88-93
HCHNDLDGY
Erosion resistance of coatings for metal protection at elevated temperatures Vesselin Shanov *, Widen Tabakoff Departmentof Aerospace Engineering andEngineering Mechanics, University ofCincinnati, Cincinnati, OH 45221, USA
Abstract An experimental study was conducted to investigate the behavior of coated metal substrates exposed to erosion by chromite particles. Chemical vapor deposition technique (CVD) was used to apply titanium carbide (TiC) on nickel based alloy INCa 718 and on AISI 410 (stainless steel 410). Another group of specimens made of the same substrate materials was subjected to surface treatment by nit riding in glow discharge plasma (ion nitriding). The erosive wear of the samples was investigated experimentally by exposing them to particle laden flow at velocities from 180 to 305 m s-\ temperatures from ambient to 538°C and impingement angles from 20 to 90° in a specially designed erosion wind tunnel. The erosion results show the effect of the velocity, temperature, and the impingement angle on the erosion rate. The cumulative effects of the impacting particles mass on the weight loss and on the erosion rate of the protective coatings were also investigated. The coating erosion rate variation with the impingement angle shows brittle erosion patterns for the TiC coating and ductile patterns for the ion nitrided substrates. The CVD coated INCa 718 revealed longer lifetime than that of coated stainless steel 410 in sequential tests. Both substrate materials coated with TiC showed one order magnitude less erosion rates compared to some commercial coatings on INCa 718. The ion nitrided samples did not reveal any improved erosion resistance compared to the uncoated metal substrates. In addition, the eroded surfaces were examined by scanning electron microscopy (SEM). This study indicates that the tested CVD ceramic coating provides very good erosion resistance for stainless steel 410 and INCa 718 when exposed to elevated temperatures.
of
Keywords: Erosion rate; High temperature; CVD coating; Ion-nitriding
1. Introduction Machine parts, turbines, and engines operating under particulate flow are exposed to erosion and performance deterioration. Erosion studies of protective coatings applied on stainless steel or superalloy-based substrates by plasma spraying, sputtering, detonation gun spraying, and electro-spark detonation are well described in many articles [1-5]. The erosion resistance of ceramic coatings is strongly dependent on the coating process and on the substrate material [1,2]. Our previous works demonstrated the excellent protection that CVD coatings provided for cemented tungsten carbide, for ceramic substrates, and for super alloys in particulate flow environment [6-8]. In this study, the particulate erosion test results obtained on INCO 718 and on AISI 410 (stainless steel 410), both coated with CVD titanium carbide, are described. The nickel-based alloy INCa 718 is frequently used as
* Corresponding author. 0257-8972/96/$15.00 © 1996 Elsevier Science SA Allrights reserved PI! 80257-8972(96)02965-9
substrate material for different commercial coatings, protecting steam turbine components [5,9]. For comparison, the erosion behavior of ion nitrided specimens made of the same substrate materials, was also evaluated. Implantation of nitrogen into the surface of the metal substrates is expected to increase their hardness and the wear resistance [10l
2. Experimental details 2.1. Experimental set-up
A hot wall CVD reactor was used for deposition of the TiC coating [7]. The reactor operated at a temperature of 1000°C and total pressure of 40 Torr (5333 Pa) [6,7]. AISI 410 (stainless steel 410) and INCO 718 were used as substrates on which a 10 urn thick TiC coating was grown at a deposition rate of about 1 um h -1. Nitriding in glow discharge plasma (ion nitriding) was performed in ION-20 equipment made by EFTOM-
V. Shanov, W. Tabakoff[Sutface andCoatings Technology 86-87 (1996) 88-93
ION, Sofia. The metal substrates served as a cathode and the nitrogen generated plasma enabled formation of the nitrided layers. After nitriding for 7 hours at a temperature of 520°C and a total pressure of 10Torr (1333 Pa), surface diffusion layers about 100 um thick were obtained. The microhardness of the nitrided INCa 718 varied within the layer from 740 to 860 HV 100 gf and that of the nitrided stainless steel 410 was in the range of 400-630 HV 100 gf. The high temperature erosion test facility was designed to provide erosion data in the range of operating temperatures experienced in compressors and turbines [11]. In addition to the high temperatures, the facility properly simulates all the erosion parameters which were determined to be important from an aerodynamic point of view.
2.2. Test conditions and materials The test parameters were varied in the present study for two coatings applied on two metal substrates. The compositions of the metal substrates used in this program were as follows: INCa 718: 52.5 Ni, 0.04 C, 0.8 Mn, 18.5 Fe, 0.008 S, 0.18 Si, 0.15 Cu, 19 Cr, 0.5 AI, 0.9 Ti, 3.05 Mo, 5.13 Cb+Ta. AISI 410 (stainless steel 410): 0.15 C, 1.0 Mn, 0.03 S, 12.0 Cr, 1.0 Si, 0.04 P, rest Fe. Since the solid particles in the steam turbines are mainly boiler scales, chromite powder (75 um) has been used in the present investigation. It is usually considered to be a solid solution of various spinels. Chromite used for the erosion tests was reported in our previous publications [1,5,9]. Flat rectangular specimens were used in the presented test program. The samples were 26 mm long, 3 mm thick and 13 mm wide. The impact velocity changed from 180 to 305 m S-l at temperatures from ambient to 538°C. Test data were accumulated by setting the particle impingement angle at 20,30,50,70 and 90°. The erosion tests were conducted to obtain two types of erosion data, namely the erosion rates and the cumulative erosion mass loss for both coated substrates. The erosion rate is defined as the ratio between the change of the sample mass and the mass of the impacting particles. The erosion rate tests were carried out in one cycle using a certain amount of particles impacting the sample surface. The cumulative erosion mass loss tests, on the other hand, were conducted in multiple cycles.
89
surface morphology of the same coating on other substrate materials was described previously [6,7]. The ion nitrided samples did not reveal any specific morphology compared to the uncoated substrates.
3.1. Impact angle effecton the erosion rate The effect of the impingement angle on the erosion rate of the tested coatings is presented in Fig. 1 and Fig. 2. Inspection of Fig. 1 shows that the TiC coating erosion rate increases with the impingement angle and reaches a maximum at 90° for both metal substrates. For this coating we assume brittle type of wear at particulate flow. Similar behavior was reported previously for the same coating on nickel based super alloy MAR 246 [8]. The TiC coating reveals better protection on stainless steel 410 than on INCa 718, when it was eroded by a particle mass of 20 g chromite powder at 0.12 . - - - - - - - - - - - - - - - , ,...,0.10
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Fig. 1. Titanium carbide coating erosion rate variation with impingement angle: T=538°C, Vp=305 m s-1, 20 g chromite mass, 7.00 , - - - - - - - . , . . - - - - - - - - - - . , 6.00
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3. Erosion test results and discussion
The TiC coating on both INCa 718 and stainless steel 410 prepared by a CVD process exhibits a fine grained structure which was observed by SEM. Similar
Fig. 2, Erosion rate variation of ion nitrided metal substrates with impingement angle: T=538°C, Vp =305 m s-1, 20 g chromite mass.
90
V. Shanov , JY. TahakoffiSurface and Coatings Technology 86-87 ( /996) 88 -93
an impingement angle below 90° (Fig. 1). During the bombardment of the specimen, the erosion for the TiC coating is probably made predominantly by chipping. Observation of the eroded surface of coated INCO 718 (Fig. 3) shows that the coating does not have any cracks and plastic deformation, Similar surface structure reveals also the coated stainless steel 410 after erosion. The coating morphology is not substantially affected by the angle of attack and exhibits the initial grained structure of CVD titanium carbide. As a result of this, the coating erosion rate variation with the impingement angle is small, especially for coated INCa 718. This behavior, we believe, is related to the fine grained structure of the coating and its good adhesion to the substrate. Similar conclusions were made by Levy et al. concerning the low erosion wear of CVD silicon carbide coating [12]. TiC coating provides excellent erosion protection to INCa 718 and stainless steel 410. For comparison, commercial coatings like SDG-2207 and LC-IH on INCa 718 revealed about one order of magnitude higher erosion rate than that of CVD titanium carbide coating on the same substrate material at erosion parameters identical to those in Fig. 1. The erosion data for the two reference coatings are presented in our previous works concerning the erosion behavior of coated INCO 718 and stainless steel 410 exposed to chromite particle flow [5,9]. Variation of the erosion rate with the impact angle for the ion nitrided substrates used in this study is shown in Fig. 2. The erosion rates for both specimens pass through a maximum at 30° impact angle. These patterns indicate the ductile nature of the surface nitrided layers. The chromite particles, striking the specimen, "cut" metal chips along the surface and cause plastic deformation. This is shown in Fig. 4 for the case of nitrided INCa 718 at the maximum erosion impingement angle of 30°. The rugged morphology observed in Fig. 4 is close to that of nitrided stainless steel 410 after erosion.
Fig. 3. Scanning electron micrograph of TiC coating on INCa 718 after erosion at 90' impingement angle: T=538'C, Vp=305 m S- l, 20 g chromite mass.
Fig. 4. Scanning electron micrograph of ion nitrided INca 718 after erosion at 30° impingement angle: T = 538°C, V p = 305 m s - I , 20 g chrornite mass.
The material removal mechanism is probably based on flaking and ploughing [13]. Similar ductile behavior showed uncoated INCa 718, stainless steel 410 and Waspaloy, exposed to particulate flow environment [2,5,9,13]. The ion nitrided specimens did not reveal any impro ved erosion resistance compared to the uncoated metal substrates, although the surface hardness increased by the nit riding process. It seems that the glow discharge plasma has affected the substrate in a negative way. Similar to the ion implantation process, the forced diffusion into the substrate could create structure defect layers . A detailed study is required which should involve surface healing after the ion nitriding by a proper thermal treatment. 3.2. Particle mass effect on the erosion rate
The cumulative mass erosion test results for the TiC coating on INCO 718 and stainless steel 410 substrates at 90° erosion angle of the chromite abrasive are shown in Fig.5 and Fig. 6. The nonlinear behavior of mass erosion with the particle mass (Fig. 5) indicates that there is a continuous change in the erosion rates. This is displayed in Fig. 6, which presents the erosion rate variation with the mass of particles. The erosion rate of the TiC coating on stainless steel 410 initially increases slowly. After a particle dose of 25 g it increases rapidly and reaches the uncoated substrate erosion rate of about 1.3 mg g-l. The latest indicates that the coating has been penetrated. In the sequential tests, the CVD titanium carbide on INCO 718 revealed a longer life time than that of the same coating on stainless steel 410. This can be observed in Fig. 6, where the erosion rate of the coated INCO 718 remains below the erosion rate of the coated stainless steel 410. The substrate erosion rate of the uncoated INCO 718 is around 2.8 mg gland it was not reached after particle dose of 145 g. The surface morphology of coated stainless steel 410 after erosion,
V. Shanov, W TabakofffSurface and Coatings Technology 86-87 ( 1996) 88-93
91
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Fig. 5. Titanium carbide coating weight loss variation with particle mass at 90° impingement angle: T=538°C, V p = 305 m s-\ chromite particles.
Fig. 7. Scanning electron micrograph of TiC coating on stainless steel 410 after erosion carried out in multiple cycles at 90° impingement angle: T=538°C, V p=305 m s", total amount of 40g chromite particles. 750.0 , . . . - - - - - - - - - - - - - - - ; = ; - - ,
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Fig. 6. Variation of the titanium carbide coating erosion rate with particle mass at 90° impingement angle: T= 538°C, V p=305 m s-\ chromite particles.
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Fig. 8. Weight loss variation of ion nitrided metal substrates with particle mass at 90° impingement angle: T=538°C, Vp =305 m s-\ chromite particles. 4.8 , . . . - - - - - - - - - - - - - ,
carried out in multiple cycles, is displayed in Fig. 7. This Figure shows that the coating chips off probably because of generated cracks, which propagate along the surface. This causes a shorter lifetime of the coating on stainless steel 410 in sequential erosion tests compared to INCa 718. The incremental mass erosion test results for ion nitrided INCa 718 and stainless steel 410 are presented in Fig. 8 and Fig. 9. The high weight losses of both specimens during the sequential runs (Fig. 8) results from their high erosion rates which can be observed in Fig. 9. Fig. 9 shows that the erosion rate of the nitrided INCa 718 fluctuates around the value of the uncoated substrate. A nonlinear concentration distribution of the diffused nitrogen ions into the substrate surface could result in that variation. The erosion rate of the ion nitrided stainless steel 410 decreases with increasing the
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Fig. 9. Erosion rate variation of ion nitrided metal substrates with particle mass at 90° impingement angle: T=538°C, V p=305 m S-l, chromite particles.
v: Shanov,
92
W. Tabakoff[Surface and Coatings Technology 86-87 ( 1996) 88-93
particle mass and reveals a trend to reach the substrate erosion rate (Fig. 9). This behavior confirmsthe assumption about a damaged surface layer caused by the glow discharge plasma nitriding. Improvement is expected after an additional thermal treatment of the nitrided metal specimens.
3.3. Temperature andparticle velocity effecton the erosion rate Additional experiments have been performed to study the effect of the temperature and of the particle velocity on the erosion rate of CVD titanium carbide on INCa 718 and stainless steel 410. The tests were conducted at an impact angle of 90°, corresponding to the maximum erosion rate. Variation of the coating erosion rate with the temperature are presented in Fig. 10. The erosion rate of both coated substrates decreases significantly with increasing the temperature from ambient to 538°C. This effect is weaker for the coated stainless steel 410. As is shown in Fig. 10, the TiC coating gives much better protection to the metal substrates at elevated temperatures. Similar behavior was reported for CVD titanium carbide on ceramic based substrates [6, 7J. These results demonstrate that the tested coating is a promising high temperature material. The rise in temperature causes an increase of the energy requirement for erosion of the coated samples. The substrate materials do not contribute this improvement, because their erosion resistance is reduced at elevated temperatures [9,14,15]. Factors related to the temperature dependence of the material's physical properties, such as strength and hardness of the coating and of the composite coated body, are of significant importance for interpretation of the obtained results [13-15]. Further investigation at the coating/substrate interface, including
composition, oxidation and fracture mechanic issues, is required. The effect of the particle impact velocity on the erosion rate of titanium carbide coated INCa 718 and stainless steel 410 is presented in Fig. 11. The experimental data show that the particle velocity has a significant influence on the erosion rate of both coated substrates. The logarithmic plots exhibit linear variation of the erosion rate with velocity. The velocity exponents n obtained from the power low curve fitting are 3.06 for INCa 718 and 2.27 for coated stainless steel. These values are close to those obtained from the same coating on cemented tungsten carbide substrate [6J. The values of the velocity exponent n occur within the approximate range from 2 to 4, which is considered to be for brittle materials [16]. 4. Summary andconclusions CVD of titanium carbide and ion nitriding techniques were used for erosion protection of nickel-based alloy INCa 718 and of AISI 410 (stainless steel 410). It was found that the ion nitriding treatment in a glow discharge plasma does not improve the wear performance of the studied substrate materials. The CVD titanium carbide coating on both metals behaves as a brittle material and its erosion resistance increases significantly at elevated temperatures. It was established that the erosion rate of the TiC coated metals is proportional to the particle velocity to the power n. The CVD coated INCa 718 had a longer lifetime than coated stainless steel 410 in sequential tests. This study demonstrated that the CVD titanium carbide coating provides excellent erosion protection for INCa 718 and stainless steel 410 when subjected to impact by chromite particles at elevated temperatures.
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Fig. 11. Variation of the titanium carbide coating erosion rate with velocity at 90 impingement angle: T = 538°C, 20 g chromite mass. Q
V. Shanov, W: Tabakoff/Surface and Coatings Technology 86-87 ( 1996) 88-93
Acknowledgement The support of the FUlbright Foreign Scholarship Board to Dr. V. Shanov in the pursuit of this work is gratefully acknowledged.
References [lJ [ 2J [3] [4]
J. Qu reshi and W. Tabakoff, Surf Coat. Technol., 36 (1988) 433. W. Tabakoff, Surf. Coat. Techno/., 39/40 (1989) 97. P.J. Burnet and ns Rickerby, J. Mater. Sci., 23 (1988) 2429. B. Jonsson, L. Akre, S. Johansson and S. Hogmark, Thin Solid Films, 137 (1986) 65. [5] P. Walsh and W. Tabakoff Ado. Steam Turbine Technol. Power Generation, 10 (1990) 1.
93
[6J V. Shanov, W. Tabakotf and M. Metwally, Surf. Coat. Technol.; 54/55 (1992)25. [7J V. Shanov, W. Tabakotf and A. Hamed, Surf Coat. Techno!., 68/69 (1994) 92. [8] W. Tabakotrand V. Shanov, Surf. Coat. Technol.; 76/77 (1995)75. [9] W. Tabakoff, M. Metwally and A. Hamed, J. Eng. Gas Turbine Power, 117 (1) (1995) 146. [10] M.G. Hocking, V. Vasantasree and P.S. Sidky, Metallic and Ceramic Coatings: Production, High Temperature Properties and A pplications, LongmanScientific and Technical, UK, 1989, p. 349. [11] W. Tabakotr and T. Wakeman, ASME Special Publication, 664 (1979) 123. [12] A. Levy, D. Boone, A. Davis and E. Scholz,in J.E. Field and N.S. Corney (eds.), Proceedings of 6th lilt. Conf onErosionby Liquid and Solid Impact, Cavendish Laboratory, Cambridge, 1983,46. [13J S. Chinnadurai, and S. Bahadur, Wear, 186/187 (1995) 299. [14] N. Gat and W. Tabakoff, J. Test. El>aluation, 8 (4) (1980) 177. [15J T. Wakeman and W. Tabakoff, J. Aircraft, 16 (12) (1978) 828. [16] JAC. Humphrey, Int. J. Heat Fluid Flow, 11 (1990) 3.
&COARNIiS
ELSEVIER
Surfaceand Coatings Technology 86-87 (1996)88-93
HCHNDLDGY
Erosion resistance of coatings for metal protection at elevated temperatures Vesselin Shanov *, Widen Tabakoff Departmentof Aerospace Engineering andEngineering Mechanics, University ofCincinnati, Cincinnati, OH 45221, USA
Abstract An experimental study was conducted to investigate the behavior of coated metal substrates exposed to erosion by chromite particles. Chemical vapor deposition technique (CVD) was used to apply titanium carbide (TiC) on nickel based alloy INCa 718 and on AISI 410 (stainless steel 410). Another group of specimens made of the same substrate materials was subjected to surface treatment by nit riding in glow discharge plasma (ion nitriding). The erosive wear of the samples was investigated experimentally by exposing them to particle laden flow at velocities from 180 to 305 m s-\ temperatures from ambient to 538°C and impingement angles from 20 to 90° in a specially designed erosion wind tunnel. The erosion results show the effect of the velocity, temperature, and the impingement angle on the erosion rate. The cumulative effects of the impacting particles mass on the weight loss and on the erosion rate of the protective coatings were also investigated. The coating erosion rate variation with the impingement angle shows brittle erosion patterns for the TiC coating and ductile patterns for the ion nitrided substrates. The CVD coated INCa 718 revealed longer lifetime than that of coated stainless steel 410 in sequential tests. Both substrate materials coated with TiC showed one order magnitude less erosion rates compared to some commercial coatings on INCa 718. The ion nitrided samples did not reveal any improved erosion resistance compared to the uncoated metal substrates. In addition, the eroded surfaces were examined by scanning electron microscopy (SEM). This study indicates that the tested CVD ceramic coating provides very good erosion resistance for stainless steel 410 and INCa 718 when exposed to elevated temperatures.
of
Keywords: Erosion rate; High temperature; CVD coating; Ion-nitriding
1. Introduction Machine parts, turbines, and engines operating under particulate flow are exposed to erosion and performance deterioration. Erosion studies of protective coatings applied on stainless steel or superalloy-based substrates by plasma spraying, sputtering, detonation gun spraying, and electro-spark detonation are well described in many articles [1-5]. The erosion resistance of ceramic coatings is strongly dependent on the coating process and on the substrate material [1,2]. Our previous works demonstrated the excellent protection that CVD coatings provided for cemented tungsten carbide, for ceramic substrates, and for super alloys in particulate flow environment [6-8]. In this study, the particulate erosion test results obtained on INCO 718 and on AISI 410 (stainless steel 410), both coated with CVD titanium carbide, are described. The nickel-based alloy INCa 718 is frequently used as
* Corresponding author. 0257-8972/96/$15.00 © 1996 Elsevier Science SA Allrights reserved PI! 80257-8972(96)02965-9
substrate material for different commercial coatings, protecting steam turbine components [5,9]. For comparison, the erosion behavior of ion nitrided specimens made of the same substrate materials, was also evaluated. Implantation of nitrogen into the surface of the metal substrates is expected to increase their hardness and the wear resistance [10l
2. Experimental details 2.1. Experimental set-up
A hot wall CVD reactor was used for deposition of the TiC coating [7]. The reactor operated at a temperature of 1000°C and total pressure of 40 Torr (5333 Pa) [6,7]. AISI 410 (stainless steel 410) and INCO 718 were used as substrates on which a 10 urn thick TiC coating was grown at a deposition rate of about 1 um h -1. Nitriding in glow discharge plasma (ion nitriding) was performed in ION-20 equipment made by EFTOM-
V. Shanov, W. Tabakoff[Sutface andCoatings Technology 86-87 (1996) 88-93
ION, Sofia. The metal substrates served as a cathode and the nitrogen generated plasma enabled formation of the nitrided layers. After nitriding for 7 hours at a temperature of 520°C and a total pressure of 10Torr (1333 Pa), surface diffusion layers about 100 um thick were obtained. The microhardness of the nitrided INCa 718 varied within the layer from 740 to 860 HV 100 gf and that of the nitrided stainless steel 410 was in the range of 400-630 HV 100 gf. The high temperature erosion test facility was designed to provide erosion data in the range of operating temperatures experienced in compressors and turbines [11]. In addition to the high temperatures, the facility properly simulates all the erosion parameters which were determined to be important from an aerodynamic point of view.
2.2. Test conditions and materials The test parameters were varied in the present study for two coatings applied on two metal substrates. The compositions of the metal substrates used in this program were as follows: INCa 718: 52.5 Ni, 0.04 C, 0.8 Mn, 18.5 Fe, 0.008 S, 0.18 Si, 0.15 Cu, 19 Cr, 0.5 AI, 0.9 Ti, 3.05 Mo, 5.13 Cb+Ta. AISI 410 (stainless steel 410): 0.15 C, 1.0 Mn, 0.03 S, 12.0 Cr, 1.0 Si, 0.04 P, rest Fe. Since the solid particles in the steam turbines are mainly boiler scales, chromite powder (75 um) has been used in the present investigation. It is usually considered to be a solid solution of various spinels. Chromite used for the erosion tests was reported in our previous publications [1,5,9]. Flat rectangular specimens were used in the presented test program. The samples were 26 mm long, 3 mm thick and 13 mm wide. The impact velocity changed from 180 to 305 m S-l at temperatures from ambient to 538°C. Test data were accumulated by setting the particle impingement angle at 20,30,50,70 and 90°. The erosion tests were conducted to obtain two types of erosion data, namely the erosion rates and the cumulative erosion mass loss for both coated substrates. The erosion rate is defined as the ratio between the change of the sample mass and the mass of the impacting particles. The erosion rate tests were carried out in one cycle using a certain amount of particles impacting the sample surface. The cumulative erosion mass loss tests, on the other hand, were conducted in multiple cycles.
89
surface morphology of the same coating on other substrate materials was described previously [6,7]. The ion nitrided samples did not reveal any specific morphology compared to the uncoated substrates.
3.1. Impact angle effecton the erosion rate The effect of the impingement angle on the erosion rate of the tested coatings is presented in Fig. 1 and Fig. 2. Inspection of Fig. 1 shows that the TiC coating erosion rate increases with the impingement angle and reaches a maximum at 90° for both metal substrates. For this coating we assume brittle type of wear at particulate flow. Similar behavior was reported previously for the same coating on nickel based super alloy MAR 246 [8]. The TiC coating reveals better protection on stainless steel 410 than on INCa 718, when it was eroded by a particle mass of 20 g chromite powder at 0.12 . - - - - - - - - - - - - - - - , ,...,0.10
'"
<,
'"
$0.08 i.I.J
!;( 0.06
a::
z a
~ 0.04
ffi
0.02
Fig. 1. Titanium carbide coating erosion rate variation with impingement angle: T=538°C, Vp=305 m s-1, 20 g chromite mass, 7.00 , - - - - - - - . , . . - - - - - - - - - - . , 6.00
Dl <,
15 .00 4,00 ~ a:: z 3.00
i.I.J
a
Vi
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3. Erosion test results and discussion
The TiC coating on both INCa 718 and stainless steel 410 prepared by a CVD process exhibits a fine grained structure which was observed by SEM. Similar
Fig. 2, Erosion rate variation of ion nitrided metal substrates with impingement angle: T=538°C, Vp =305 m s-1, 20 g chromite mass.
90
V. Shanov , JY. TahakoffiSurface and Coatings Technology 86-87 ( /996) 88 -93
an impingement angle below 90° (Fig. 1). During the bombardment of the specimen, the erosion for the TiC coating is probably made predominantly by chipping. Observation of the eroded surface of coated INCO 718 (Fig. 3) shows that the coating does not have any cracks and plastic deformation, Similar surface structure reveals also the coated stainless steel 410 after erosion. The coating morphology is not substantially affected by the angle of attack and exhibits the initial grained structure of CVD titanium carbide. As a result of this, the coating erosion rate variation with the impingement angle is small, especially for coated INCa 718. This behavior, we believe, is related to the fine grained structure of the coating and its good adhesion to the substrate. Similar conclusions were made by Levy et al. concerning the low erosion wear of CVD silicon carbide coating [12]. TiC coating provides excellent erosion protection to INCa 718 and stainless steel 410. For comparison, commercial coatings like SDG-2207 and LC-IH on INCa 718 revealed about one order of magnitude higher erosion rate than that of CVD titanium carbide coating on the same substrate material at erosion parameters identical to those in Fig. 1. The erosion data for the two reference coatings are presented in our previous works concerning the erosion behavior of coated INCO 718 and stainless steel 410 exposed to chromite particle flow [5,9]. Variation of the erosion rate with the impact angle for the ion nitrided substrates used in this study is shown in Fig. 2. The erosion rates for both specimens pass through a maximum at 30° impact angle. These patterns indicate the ductile nature of the surface nitrided layers. The chromite particles, striking the specimen, "cut" metal chips along the surface and cause plastic deformation. This is shown in Fig. 4 for the case of nitrided INCa 718 at the maximum erosion impingement angle of 30°. The rugged morphology observed in Fig. 4 is close to that of nitrided stainless steel 410 after erosion.
Fig. 3. Scanning electron micrograph of TiC coating on INCa 718 after erosion at 90' impingement angle: T=538'C, Vp=305 m S- l, 20 g chromite mass.
Fig. 4. Scanning electron micrograph of ion nitrided INca 718 after erosion at 30° impingement angle: T = 538°C, V p = 305 m s - I , 20 g chrornite mass.
The material removal mechanism is probably based on flaking and ploughing [13]. Similar ductile behavior showed uncoated INCa 718, stainless steel 410 and Waspaloy, exposed to particulate flow environment [2,5,9,13]. The ion nitrided specimens did not reveal any impro ved erosion resistance compared to the uncoated metal substrates, although the surface hardness increased by the nit riding process. It seems that the glow discharge plasma has affected the substrate in a negative way. Similar to the ion implantation process, the forced diffusion into the substrate could create structure defect layers . A detailed study is required which should involve surface healing after the ion nitriding by a proper thermal treatment. 3.2. Particle mass effect on the erosion rate
The cumulative mass erosion test results for the TiC coating on INCO 718 and stainless steel 410 substrates at 90° erosion angle of the chromite abrasive are shown in Fig.5 and Fig. 6. The nonlinear behavior of mass erosion with the particle mass (Fig. 5) indicates that there is a continuous change in the erosion rates. This is displayed in Fig. 6, which presents the erosion rate variation with the mass of particles. The erosion rate of the TiC coating on stainless steel 410 initially increases slowly. After a particle dose of 25 g it increases rapidly and reaches the uncoated substrate erosion rate of about 1.3 mg g-l. The latest indicates that the coating has been penetrated. In the sequential tests, the CVD titanium carbide on INCO 718 revealed a longer life time than that of the same coating on stainless steel 410. This can be observed in Fig. 6, where the erosion rate of the coated INCO 718 remains below the erosion rate of the coated stainless steel 410. The substrate erosion rate of the uncoated INCO 718 is around 2.8 mg gland it was not reached after particle dose of 145 g. The surface morphology of coated stainless steel 410 after erosion,
V. Shanov, W TabakofffSurface and Coatings Technology 86-87 ( 1996) 88-93
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Fig. 7. Scanning electron micrograph of TiC coating on stainless steel 410 after erosion carried out in multiple cycles at 90° impingement angle: T=538°C, V p=305 m s", total amount of 40g chromite particles. 750.0 , . . . - - - - - - - - - - - - - - - ; = ; - - ,
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Fig. 8. Weight loss variation of ion nitrided metal substrates with particle mass at 90° impingement angle: T=538°C, Vp =305 m s-\ chromite particles. 4.8 , . . . - - - - - - - - - - - - - ,
carried out in multiple cycles, is displayed in Fig. 7. This Figure shows that the coating chips off probably because of generated cracks, which propagate along the surface. This causes a shorter lifetime of the coating on stainless steel 410 in sequential erosion tests compared to INCa 718. The incremental mass erosion test results for ion nitrided INCa 718 and stainless steel 410 are presented in Fig. 8 and Fig. 9. The high weight losses of both specimens during the sequential runs (Fig. 8) results from their high erosion rates which can be observed in Fig. 9. Fig. 9 shows that the erosion rate of the nitrided INCa 718 fluctuates around the value of the uncoated substrate. A nonlinear concentration distribution of the diffused nitrogen ions into the substrate surface could result in that variation. The erosion rate of the ion nitrided stainless steel 410 decreases with increasing the
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v: Shanov,
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W. Tabakoff[Surface and Coatings Technology 86-87 ( 1996) 88-93
particle mass and reveals a trend to reach the substrate erosion rate (Fig. 9). This behavior confirmsthe assumption about a damaged surface layer caused by the glow discharge plasma nitriding. Improvement is expected after an additional thermal treatment of the nitrided metal specimens.
3.3. Temperature andparticle velocity effecton the erosion rate Additional experiments have been performed to study the effect of the temperature and of the particle velocity on the erosion rate of CVD titanium carbide on INCa 718 and stainless steel 410. The tests were conducted at an impact angle of 90°, corresponding to the maximum erosion rate. Variation of the coating erosion rate with the temperature are presented in Fig. 10. The erosion rate of both coated substrates decreases significantly with increasing the temperature from ambient to 538°C. This effect is weaker for the coated stainless steel 410. As is shown in Fig. 10, the TiC coating gives much better protection to the metal substrates at elevated temperatures. Similar behavior was reported for CVD titanium carbide on ceramic based substrates [6, 7J. These results demonstrate that the tested coating is a promising high temperature material. The rise in temperature causes an increase of the energy requirement for erosion of the coated samples. The substrate materials do not contribute this improvement, because their erosion resistance is reduced at elevated temperatures [9,14,15]. Factors related to the temperature dependence of the material's physical properties, such as strength and hardness of the coating and of the composite coated body, are of significant importance for interpretation of the obtained results [13-15]. Further investigation at the coating/substrate interface, including
composition, oxidation and fracture mechanic issues, is required. The effect of the particle impact velocity on the erosion rate of titanium carbide coated INCa 718 and stainless steel 410 is presented in Fig. 11. The experimental data show that the particle velocity has a significant influence on the erosion rate of both coated substrates. The logarithmic plots exhibit linear variation of the erosion rate with velocity. The velocity exponents n obtained from the power low curve fitting are 3.06 for INCa 718 and 2.27 for coated stainless steel. These values are close to those obtained from the same coating on cemented tungsten carbide substrate [6J. The values of the velocity exponent n occur within the approximate range from 2 to 4, which is considered to be for brittle materials [16]. 4. Summary andconclusions CVD of titanium carbide and ion nitriding techniques were used for erosion protection of nickel-based alloy INCa 718 and of AISI 410 (stainless steel 410). It was found that the ion nitriding treatment in a glow discharge plasma does not improve the wear performance of the studied substrate materials. The CVD titanium carbide coating on both metals behaves as a brittle material and its erosion resistance increases significantly at elevated temperatures. It was established that the erosion rate of the TiC coated metals is proportional to the particle velocity to the power n. The CVD coated INCa 718 had a longer lifetime than coated stainless steel 410 in sequential tests. This study demonstrated that the CVD titanium carbide coating provides excellent erosion protection for INCa 718 and stainless steel 410 when subjected to impact by chromite particles at elevated temperatures.
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V. Shanov, W: Tabakoff/Surface and Coatings Technology 86-87 ( 1996) 88-93
Acknowledgement The support of the FUlbright Foreign Scholarship Board to Dr. V. Shanov in the pursuit of this work is gratefully acknowledged.
References [lJ [ 2J [3] [4]
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[6J V. Shanov, W. Tabakotf and M. Metwally, Surf. Coat. Technol.; 54/55 (1992)25. [7J V. Shanov, W. Tabakotf and A. Hamed, Surf Coat. Techno!., 68/69 (1994) 92. [8] W. Tabakotrand V. Shanov, Surf. Coat. Technol.; 76/77 (1995)75. [9] W. Tabakoff, M. Metwally and A. Hamed, J. Eng. Gas Turbine Power, 117 (1) (1995) 146. [10] M.G. Hocking, V. Vasantasree and P.S. Sidky, Metallic and Ceramic Coatings: Production, High Temperature Properties and A pplications, LongmanScientific and Technical, UK, 1989, p. 349. [11] W. Tabakotr and T. Wakeman, ASME Special Publication, 664 (1979) 123. [12] A. Levy, D. Boone, A. Davis and E. Scholz,in J.E. Field and N.S. Corney (eds.), Proceedings of 6th lilt. Conf onErosionby Liquid and Solid Impact, Cavendish Laboratory, Cambridge, 1983,46. [13J S. Chinnadurai, and S. Bahadur, Wear, 186/187 (1995) 299. [14] N. Gat and W. Tabakoff, J. Test. El>aluation, 8 (4) (1980) 177. [15J T. Wakeman and W. Tabakoff, J. Aircraft, 16 (12) (1978) 828. [16] JAC. Humphrey, Int. J. Heat Fluid Flow, 11 (1990) 3.