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Ti–N multilayer systems for compressor airfoil sand erosion protection
Surface & Coatings Technology 204 (2009) 1092–1096
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Ti–N multilayer systems for compressor airfoil sand erosion protection A. Feuerstein ⁎, A. Kleyman Praxair Surface Technologies, Inc., 1500 Polco Street, Indianapolis, Indiana 46222, United States
a r t i c l e
i n f o
Available online 2 October 2009 Keywords: Erosion coating TiN Cathodic arc Helicopter compressor blade erosion
a b s t r a c t Frequently aircraft, tank and helicopter gas turbine engines are operated in a desert environment where the gas turbine compressor rotor blades and vanes are exposed to erosive media such as sand and dust. Base metal erosion leads to increased fuel consumption, efficiency loss, and can cause damage to compressor and turbine hardware. Erosion resistant coatings such as Ti–N, Ti–C–N, Ti–Zr–N, Ti–Zr–C–N, Ti–Al–N, and Ti–Al– C–N applied by cathodic arc physical vapor deposition or other PVD processes can be used to prolong the life of compressor airfoils in a sand erosion environment. Praxair Surface Technologies, Inc. has developed unique multilayered Ti–N coating systems with optimized erosion resistance compared to conventional single layers. The multilayer structure can be tailored to the erosion media particle size distribution. The key features of two selected coating architectures are outlined. Selected erosion performance data with different erosion media are presented. And finally the aspects to ensure high quality mass production are addressed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Solid particle erosion mechanism and principles of coating design
Solid particle erosion of compressor blades and vanes of military and civil aircraft engines is becoming of increasing importance. Ti–N, Ti–C–N, Ti–Zr–N, Ti–Zr–C–N, Ti–Al–N, and Ti–Al–C–N coatings applied by cathodic arc physical vapor deposition are potential candidates to prolong the life of compressor airfoils in a sand erosion environment. Praxair Surface Technologies, Inc. has developed a multilayered coating system based on substoichiometric TiN/TiN1 − × (called “24k Type II™”) with much improved erosion resistance compared to conventional Ti–N coatings. This coating system has been tested to provide superior sand erosion protection in a variety of aircraft engines including civil engines. Because of the increased demand in erosion coatings for desert environment the Navy initiated a program to prolong the lifetime of T64 helicopter engine compressor blades and vanes by coating these with an erosion protection coating [1,2]. Praxair has established a production facility to cost effectively apply the 24k Type II™ multilayer coating and has become one of the main suppliers for aviation applications. Since then more than several hundred thousand blades and vanes have been coated. The key properties of the substoichiometric 24k Type II™ multilayer coating and critical to quality aspects of the production process are reviewed. Additionally the challenges of laboratory erosion testing, and the introduction of an enhanced coating system for large particle erosion protection will be discussed.
Excellent mechanical properties of single layer nitride coating such as high hardness and Young's modulus make it a very attractive material for the protection against the different types of wear. Solid particle erosion mechanisms of metals and brittle materials such as Ti–N and other nitride coatings have been discussed by Kleis and Chen [3,4]. It was demonstrated that the erosion rate of brittle coating compared to ductile coatings is lower at low impact angle but is higher at high impact angle. In principle erosion wear of a brittle coating starts with nucleation of brittle fracturing of the coating when according to the Rankine criterion the impact induced stress exceeds the tensile strength of the coating. The amplitude of the tensile stress is related to the particle impact energy whereas the tensile strength is an intrinsic coating property. Thus the erosion resistance can be linked to the stress in the coating surface. Bielawski and Beres [5] have calculated the stress distribution in a multilayer architecture with alternating high and low modulus layers and have shown that the surface coating stress of the coating and therefore the tendency to develop crack nucleation at the surface can be reduced. Multilayer coating structures with alternating brittle and ductile (metallic) coatings have been investigated by Holleck [6] and Bromark [7]. Brittle/ductile multilayer systems have also been applied successfully in commercial applications [8]. Further improvement of the erosion resistance by increasing the coating toughness can be accomplished by toughening the brittle component by creating nano-composites such as Ti–Si–C–N and other chemistries [9,10]. Holleck [6] describes the mechanisms of coating toughening as inhibition of grain growth, crack deflection, reduced stress concentration, and “Nano plasticity”. Swaminathan, Wei and Gandy [9] have demonstrated improved erosion resistance
⁎ Corresponding author. E-mail address: [email protected] (A. Feuerstein). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.053
A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
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of a nano-composite Ti–Si–C–N by a combination of high hardness and lower modulus in comparison to single phased TiN. 2. Substoichiometric TiN1 − × coating Substoichiometric TiN1 − × was identified as a promising candidate material for improved erosion resistance because of its higher hardness, toughness, and slightly reduced compressive stress compared to stoichiometric TiN [11]. It has been found that a smaller TiN crystallite size is beneficial for the coating toughness and can be achieved by lowering the nitrogen content in the coating. Previously published data from Sue [11] show that at a nitrogen content of approximately 40–45 at.% the individual material properties such as micro hardness, residual compressive stress, texture, and crystallite size achieve an optimum leading to a positive combined effect on the erosion resistance (Fig. 1). 2.1. Substoichiometric TiN/TiN1 − × “24k Type II™” multilayer coating for fine to medium size sand in low angle impact
Fig. 2. Coating architecture of a TiN/TiN1 − 24k Type II™” multilayer coating system for small and medium particle erosion protection.
3. Up-scaling to a production coating process The originally developed process used a single cathode concept with Praxair's own 100 mm diameter cathode design operating at arc current of 150–300 A. The process with one cathode had limitations with regard to substrate size and shape. For high volume production process the following challenges had to be addressed:
A monolithic coating concept is typically sufficient against smaller media. Larger particles which tend to shatter a brittle monolayer coating require enhancements by adding compliance layers which stop crack propagation. The multilayer concept also allows deposition of a significantly larger thickness without accumulating excessive compressive stress inherent to ion plated coating. The Praxair substoichiometric TiN/TiN1 − × “24k Type II™” multilayer coatings [12,13] was specifically designed for helicopter engine applications which exposes the compressor to medium sized filtered sand. In this application the dominant erosion mechanism is primarily low angle impact resulting in a reduction of the cord width of the blades and vanes [14]. The coating architecture is shown in Fig. 2. Substoichiometric TiN1 − × layers of approximately. 1 μm thickness alternate with fully stoichiometric TiN layers of less than 0.2 μm thickness. The multilayer coating consists of up to 60 layers. The interface layer to the substrate is a full stoichiometric TiN layer which is produced by a seamless transition from a Ti sputter cleaning step where nitrogen is added gradually. The total coating thickness is typically 15 to 25 μm. The top layer of the multilayer stack is a full stoichiometric layer. Fig. 3 shows the comparative 20° and 90°–50 μm alumina erosion performance of the multilayer concept versus a monolithic Ti–N coating, thermal spray coatings (such as Chromium Carbide and Tungsten Carbide), and Ti–6Al–4V substrate material. The multilayer coating outperforms the other coatings. All coatings were produced by Praxair.
A commercially available cathodic arc coater with multiple 100 mm cathodes was selected. An automated control procedure was developed to achieve the necessary long term adjustment of the nitrogen flow due to cathode wear. The coating process consists of the following main steps:
Fig. 1. Erosion resistance of TiN1–x coating for 90° and 20° 50 μm angular alumina impact as a function of nitrogen content. The optimum is at a nitrogen content of approximately 40 to 45at.%. Figure presents re-formatted data previously published from Sue [11].
Fig. 3. Comparative erosion resistance of TiN/TiN1 − × “24k TypeII™” multilayer coating versus a monolithic TiN coating, thermal spray Cr–C and W–C coatings and Ti-6Al-4V substrate material. All coatings produced by Praxair.
• Scaling from one cathode to a multiple cathode process • Development of a reliable holding and masking fixture • Automation of the process, especially when depositing the substoichiometric layers.
• • • • • • • •
Substrate preparation by surface polishing Aqueous cleaning (degreasing) Fixturing the blades and vanes Pumpdown and preheating of substrates to approximately 460 °C with radiant heaters Glow discharge cleaning Argon/Ti ion etch designed to prevent overheating of the blade tips (< 500 °C) Deposition of alternating TiN/TiN1 − × layers by automated control Controlled cool down.
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A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
The substrate coating geometry is designed to minimize selfshadowing and maximize the coating thickness on the platforms and fillet areas of the blades and vanes. The coating fixture needs to securely hold and mask the dovetail of the blades and vanes without scratching this highly stressed area. Single rotation demonstrated acceptable coating thickness uniformity and erosion resistance. The flexible concept allows processing full loads by mixing and matching parts with different size thus utilizing the coating equipment to 100%. Preemptive design of coating repair processes in case of aborted runs due to a possible machine malfunction was part of the qualification program. A rigid quality system with lot based destructive erosion and metallographic testing was put into place. One sample per lot is evaluated for thickness, microstructure, coating imperfections, and adhesion (scratch adherence). On IN718 substrate with a hardness HRC 40 the highly efficient ion etching process delivers scratch adherence > 30 N with a process performance index (Ppk) of > 1.5. A review of electronic run data such as gas flow, pressure, temperature, evaporator current and voltage, substrate bias voltage and current provides valuable SPC data. After demonstrating an acceptable process control the destructive sampling frequency was reduced significantly. A First Time Yield (FTY) of > 95% and a Final Yield (FY) of > 99% was achieved.
4. Erosion testing Development of next generation coatings, especially for improved large particle erosion resistance, requires a good understanding of the erosion process and the erosion coating properties. Choosing the appropriate test conditions presents a challenge to deliver accurate and meaningful results regarding the expected field performance. The American Society for Testing and Materials Standard G-76 uses 50 μm alumina and exposes only a small surface area to the erodent. Because of the resulting small weight loss the measuring accuracy has to be extremely high and the documentation of the results is complex. Randomly located coating imperfections may skew the results leading to significant variation in the erosion rate. 100 mm standoff and a nozzle geometry which creates an impact area of approximately 6 cm2 achieved reproducible results at impact angles of 20° and 90°. The standard erosion panel size is 5 × 2.5 cm. The blast pressure is adjusted such as to yield a particle velocity of approximately 90–100 m/s at the point of impact for 50 μm angular alumina. The identical blast pressure was also used for other media. This set up allows testing of engine hardware such as compressor airfoils in different sizes. Fig. 4 shows a compressor blade in test and the size of the eroded area after application of 2500 g of erodent (Coarse Arizona Road Dust).
Fig. 4. Room temperature erosion test setup with compressor blade at 100 mm standoff and impact zone on a coated and uncoated airfoil after 2500 g of erodent (Coarse Arizona road dust).
5. Erosion media and erosion mechanism For better understanding of the erosion mechanism three different erosion media were used. • 50 μm angular alumina because of its high hardness and sharp cutting edges is the ideal media for an accelerated test to simulate the long term effect for medium size particle impact • To simulate large particle erosion commercially available silica sand with rounded particles — a Quikrete product #1152 with a particle size distribution up to 700 μm was used. The median is 230 μm, 10% is below 87 μm, 10% is above 335 μm, and the maximum particle size is 700 μm. • For simulation of desert sand as it affects helicopter engines with filtered air inlet Coarse Arizona Road Dust with a particle size up to 200 μm was used. Fig. 5 shows the particle shape of these media and is helpful for the interpretation of the erosion results and the observed coating failure mechanism. The high hardness and sharp cutting edges of angular alumina gradually removes the coating. Fig. 6 shows the erosion of a (Ti,Al)N/TiN multilayer system coating consisting of 15 alternating (Ti, Al)N and TiN layers of equal thickness and a total coating thickness of 20 μm to better demonstrate this effect. When using large silica sand particles the erosion damage looks very different. The large particles shatter the coating and at first create isolated and then overlapping defects until full coating removal. Arizona Road Dust acts in between
Fig. 5. Solid particle erosion media as used for erosion testing — 50μm angular alumina (A), Silica sand (B), Coarse Arizona road dust (C).
A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
Fig. 6. Gradual coating removal by 50 μm angular alumina at 90° and 20° impact revealing the individual layers of a (TiAl)N/TiN multilayer coating.
the above extremes. Fig. 7 shows a typical weight loss curve and erosion pattern of 20° and 90° impact with Arizona Road Dust. 5.1. Leading edge erosion Coating loss on the leading edge of compressor blades can be another significant threat to compressor performance. Some compressor designs are very sensitive to degradation of the leading edge profile and subsequent disruption of airflow. Therefore it is critical to maintain the integrity of the leading edge at different incidence angles. Fig. 8 shows an example where the leading edge suffers significant erosion, but the coating on the pressure side of the airfoil remains intact. It is believed that coating spallation with subsequent erosion attack of the area where the coating was removed is a likely reason of this failure mode. A test methodology to study this erosion phenomenon in the progression was developed. 6. Coating imperfections Coating imperfections can lead to premature breakup of the coating, mainly caused by larger particles forming a point of attack for medium particle erosion. Coating defects such as macro-particles inherent to the unfiltered cathodic arc process and particulate contamination, which can produce kernel type defects, have to be minimized. In spite of the tendency to embed macro-particles into the
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Fig. 8. Leading edge coating erosion in laboratory testing. Substrate material is SST, TiN/ TiN 1 − x “24k Type II™” multilayer coating, leading edge erosion tested with Arizona Road Dust under 90 ° impact angle.
coating cathodic arc is still the preferred process because of its maturity and cost effectiveness. 6.1. Coating thickness Coating thickness and hardness of the underlying substrate are the key factors to improve the erosion protection especially against large particle erosion protection. Obviously a thinner coating is more prone to shattering by erodent. Fig. 9 shows the large particle erosion of a 24k Type II™ coating with a thickness of 25 μm and 50 μm. The 50 μm coating is a self- supporting layer which outlasts the thinner coating significantly. For medium particle erosion the thinner and lower cost coating may provide adequate protection. 6.2. Substoichiometric TiN/TiN1 − × 24k Type IV multilayer coating for large particle erosion applications In essence the combined information on the erosion rates and erosion mechanism for different media provides a valuable interpretation of the protective function and the prospective service life of the coating. Although the Praxair TiN/TiN1 − × 24 k Type II™ multilayer coating provides very good protection for fine and medium sized media, further improvement in the coating architecture had to be made for better protection against larger media. One option is the addition of ductile metallic compliance layers. Those provide an “onion peeling effect” which restricts the erosion damage by large particles to the upper layer of the coating without removing the complete coating. Fig. 10 shows the enhancement of the large particle erosion for this “Type IV” coating compared to the 24k Type II™ coating. The large particle erosion resistance is enhanced by roughly a factor of 5. Current development is also targeted towards new compositions such as e.g. Ti–Al–Si–N to combine the benefits of the nano-composite microstructure of Ti–Si–C– N [5] with the higher temperature capability of Ti–Al–N. 7. Summary and conclusion
Fig. 7. 90° and 20° impact of coarse Arizona road dust — isolated complete coating removal by large particle shattering the coating in 90° impact, basically no attack in 20° impact.
Substoichiometric TiN1 − × provides improved erosion resistance compared to TiN because of its increased hardness and toughness due to significantly smaller crystallites. Praxair has successfully applied a TiN/ TiN1 − × multilayer coating for the erosion protection of helicopter compressor blades and vanes against medium size sand. Using a coating composition based on Ti alone minimizes the production cost significantly compared to other coating systems on the market by avoiding the complexity of alternating layers with different chemistry requiring
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Fig. 9. Large particle erosion (silica sand) of 25 and 50 μm coating at 20° impact. The thicker coating is self supporting and has much larger life.
Acknowledgements The authors thank Scott Cain, Dave Sharp, Tony Hull, Roy Newman, James Knapp, and Michael Brennan from Praxair Surface Technologies, Inc., for their help and support.
References
Fig. 10. Weight loss curve of the “24k TiN/TiN1 − × Type IV” multilayer coating with additional compliance layers vs. “24k TiN/TiN1 − × Type II™” multilayer coating for large particle erosion (Quikrete sand).
multiple sets of cathodic arc sources. Erosion testing with different media such as angular alumina, large silica particles, and Arizona road dust provides a comprehensive picture of the erosion protection properties and allows optimizing the coating architecture for new applications such as large particle erosion. A modified coating architecture with additional metallic compliance layers (24k TiN/TiN1 − × Type IV) demonstrated an improvement of the large particle erosion resistance by a factor of five.
[1] M. Klein, G. Simpson, Proceedings of ASME Turbo Expo 2004, June 14–17, 2004, p. 1, GT2004–54336. [2] W. Tabakoff, G. Simpson, Proceedings of the 22nd Heat Treating Society and the 2nd International Surface Engineering Congress, Indianapolis, Indiana, USA, September 15-17, 2003, pp. 453–462. [3] I. Kleis, P. Kulu, Solid particle erosion, Springer-Verlag London Limited, 2008. [4] Q. Chen, Y. Li, Wear, vol. 255, 2003, p. 78. [5] M. Bielawski, W. Beres, J. Phys. Confer. Ser. 100 (2008) 082001, doi:10.1088/1742-6596/ 100/8/082001, p. 1–4. [6] H. Holleck, V. Schier, Surf. Coat. Technol. 76–77 (1995) 328. [7] M. Bromark, M. Larsson, P. Hedenqvist, S. Hogmark, Surf. Coat. Technol. 90 (3) (1997) 217. [8] T. Brendel, F. Heutling, W. Eichmann, M. Uecker, T. Uehlein, The Engine Yearbook, Aviation Industry Press, London, 2008. [9] V.P. Swaminathan, R. Wei, D. Gandy, Proc. ASME Turbo Expo 2008, GT2008, June 9–13, 2008, p. 1, GT2008–50713. [10] V.I. Gorokovsky, C. Bowman, P.E. Gannon, D. VanVorous, A.A. Voevodin, C. Muratore, Y.S. Kang, J.J. Hu, Wear 265 (5–6) (August 2008) 741. [11] J.A. Sue, Surf. Coat. Technol. 61 (1993) 115. [12] J.A Sue, H.H. Troue, US Patent 5, 185, 211, 1993. [13] J.A Sue, H.H. Troue, US Patent 5, 071, 693, 1991. [14] G. D'Alessio, D. Nagy, J. Can. Ceram. Soc. 63 (1) (1994) 59.
Contents lists available at ScienceDirect
Surface & Coatings Technology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / s u r f c o a t
Ti–N multilayer systems for compressor airfoil sand erosion protection A. Feuerstein ⁎, A. Kleyman Praxair Surface Technologies, Inc., 1500 Polco Street, Indianapolis, Indiana 46222, United States
a r t i c l e
i n f o
Available online 2 October 2009 Keywords: Erosion coating TiN Cathodic arc Helicopter compressor blade erosion
a b s t r a c t Frequently aircraft, tank and helicopter gas turbine engines are operated in a desert environment where the gas turbine compressor rotor blades and vanes are exposed to erosive media such as sand and dust. Base metal erosion leads to increased fuel consumption, efficiency loss, and can cause damage to compressor and turbine hardware. Erosion resistant coatings such as Ti–N, Ti–C–N, Ti–Zr–N, Ti–Zr–C–N, Ti–Al–N, and Ti–Al– C–N applied by cathodic arc physical vapor deposition or other PVD processes can be used to prolong the life of compressor airfoils in a sand erosion environment. Praxair Surface Technologies, Inc. has developed unique multilayered Ti–N coating systems with optimized erosion resistance compared to conventional single layers. The multilayer structure can be tailored to the erosion media particle size distribution. The key features of two selected coating architectures are outlined. Selected erosion performance data with different erosion media are presented. And finally the aspects to ensure high quality mass production are addressed. © 2009 Elsevier B.V. All rights reserved.
1. Introduction
1.1. Solid particle erosion mechanism and principles of coating design
Solid particle erosion of compressor blades and vanes of military and civil aircraft engines is becoming of increasing importance. Ti–N, Ti–C–N, Ti–Zr–N, Ti–Zr–C–N, Ti–Al–N, and Ti–Al–C–N coatings applied by cathodic arc physical vapor deposition are potential candidates to prolong the life of compressor airfoils in a sand erosion environment. Praxair Surface Technologies, Inc. has developed a multilayered coating system based on substoichiometric TiN/TiN1 − × (called “24k Type II™”) with much improved erosion resistance compared to conventional Ti–N coatings. This coating system has been tested to provide superior sand erosion protection in a variety of aircraft engines including civil engines. Because of the increased demand in erosion coatings for desert environment the Navy initiated a program to prolong the lifetime of T64 helicopter engine compressor blades and vanes by coating these with an erosion protection coating [1,2]. Praxair has established a production facility to cost effectively apply the 24k Type II™ multilayer coating and has become one of the main suppliers for aviation applications. Since then more than several hundred thousand blades and vanes have been coated. The key properties of the substoichiometric 24k Type II™ multilayer coating and critical to quality aspects of the production process are reviewed. Additionally the challenges of laboratory erosion testing, and the introduction of an enhanced coating system for large particle erosion protection will be discussed.
Excellent mechanical properties of single layer nitride coating such as high hardness and Young's modulus make it a very attractive material for the protection against the different types of wear. Solid particle erosion mechanisms of metals and brittle materials such as Ti–N and other nitride coatings have been discussed by Kleis and Chen [3,4]. It was demonstrated that the erosion rate of brittle coating compared to ductile coatings is lower at low impact angle but is higher at high impact angle. In principle erosion wear of a brittle coating starts with nucleation of brittle fracturing of the coating when according to the Rankine criterion the impact induced stress exceeds the tensile strength of the coating. The amplitude of the tensile stress is related to the particle impact energy whereas the tensile strength is an intrinsic coating property. Thus the erosion resistance can be linked to the stress in the coating surface. Bielawski and Beres [5] have calculated the stress distribution in a multilayer architecture with alternating high and low modulus layers and have shown that the surface coating stress of the coating and therefore the tendency to develop crack nucleation at the surface can be reduced. Multilayer coating structures with alternating brittle and ductile (metallic) coatings have been investigated by Holleck [6] and Bromark [7]. Brittle/ductile multilayer systems have also been applied successfully in commercial applications [8]. Further improvement of the erosion resistance by increasing the coating toughness can be accomplished by toughening the brittle component by creating nano-composites such as Ti–Si–C–N and other chemistries [9,10]. Holleck [6] describes the mechanisms of coating toughening as inhibition of grain growth, crack deflection, reduced stress concentration, and “Nano plasticity”. Swaminathan, Wei and Gandy [9] have demonstrated improved erosion resistance
⁎ Corresponding author. E-mail address: [email protected] (A. Feuerstein). 0257-8972/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2009.09.053
A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
1093
of a nano-composite Ti–Si–C–N by a combination of high hardness and lower modulus in comparison to single phased TiN. 2. Substoichiometric TiN1 − × coating Substoichiometric TiN1 − × was identified as a promising candidate material for improved erosion resistance because of its higher hardness, toughness, and slightly reduced compressive stress compared to stoichiometric TiN [11]. It has been found that a smaller TiN crystallite size is beneficial for the coating toughness and can be achieved by lowering the nitrogen content in the coating. Previously published data from Sue [11] show that at a nitrogen content of approximately 40–45 at.% the individual material properties such as micro hardness, residual compressive stress, texture, and crystallite size achieve an optimum leading to a positive combined effect on the erosion resistance (Fig. 1). 2.1. Substoichiometric TiN/TiN1 − × “24k Type II™” multilayer coating for fine to medium size sand in low angle impact
Fig. 2. Coating architecture of a TiN/TiN1 − 24k Type II™” multilayer coating system for small and medium particle erosion protection.
3. Up-scaling to a production coating process The originally developed process used a single cathode concept with Praxair's own 100 mm diameter cathode design operating at arc current of 150–300 A. The process with one cathode had limitations with regard to substrate size and shape. For high volume production process the following challenges had to be addressed:
A monolithic coating concept is typically sufficient against smaller media. Larger particles which tend to shatter a brittle monolayer coating require enhancements by adding compliance layers which stop crack propagation. The multilayer concept also allows deposition of a significantly larger thickness without accumulating excessive compressive stress inherent to ion plated coating. The Praxair substoichiometric TiN/TiN1 − × “24k Type II™” multilayer coatings [12,13] was specifically designed for helicopter engine applications which exposes the compressor to medium sized filtered sand. In this application the dominant erosion mechanism is primarily low angle impact resulting in a reduction of the cord width of the blades and vanes [14]. The coating architecture is shown in Fig. 2. Substoichiometric TiN1 − × layers of approximately. 1 μm thickness alternate with fully stoichiometric TiN layers of less than 0.2 μm thickness. The multilayer coating consists of up to 60 layers. The interface layer to the substrate is a full stoichiometric TiN layer which is produced by a seamless transition from a Ti sputter cleaning step where nitrogen is added gradually. The total coating thickness is typically 15 to 25 μm. The top layer of the multilayer stack is a full stoichiometric layer. Fig. 3 shows the comparative 20° and 90°–50 μm alumina erosion performance of the multilayer concept versus a monolithic Ti–N coating, thermal spray coatings (such as Chromium Carbide and Tungsten Carbide), and Ti–6Al–4V substrate material. The multilayer coating outperforms the other coatings. All coatings were produced by Praxair.
A commercially available cathodic arc coater with multiple 100 mm cathodes was selected. An automated control procedure was developed to achieve the necessary long term adjustment of the nitrogen flow due to cathode wear. The coating process consists of the following main steps:
Fig. 1. Erosion resistance of TiN1–x coating for 90° and 20° 50 μm angular alumina impact as a function of nitrogen content. The optimum is at a nitrogen content of approximately 40 to 45at.%. Figure presents re-formatted data previously published from Sue [11].
Fig. 3. Comparative erosion resistance of TiN/TiN1 − × “24k TypeII™” multilayer coating versus a monolithic TiN coating, thermal spray Cr–C and W–C coatings and Ti-6Al-4V substrate material. All coatings produced by Praxair.
• Scaling from one cathode to a multiple cathode process • Development of a reliable holding and masking fixture • Automation of the process, especially when depositing the substoichiometric layers.
• • • • • • • •
Substrate preparation by surface polishing Aqueous cleaning (degreasing) Fixturing the blades and vanes Pumpdown and preheating of substrates to approximately 460 °C with radiant heaters Glow discharge cleaning Argon/Ti ion etch designed to prevent overheating of the blade tips (< 500 °C) Deposition of alternating TiN/TiN1 − × layers by automated control Controlled cool down.
1094
A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
The substrate coating geometry is designed to minimize selfshadowing and maximize the coating thickness on the platforms and fillet areas of the blades and vanes. The coating fixture needs to securely hold and mask the dovetail of the blades and vanes without scratching this highly stressed area. Single rotation demonstrated acceptable coating thickness uniformity and erosion resistance. The flexible concept allows processing full loads by mixing and matching parts with different size thus utilizing the coating equipment to 100%. Preemptive design of coating repair processes in case of aborted runs due to a possible machine malfunction was part of the qualification program. A rigid quality system with lot based destructive erosion and metallographic testing was put into place. One sample per lot is evaluated for thickness, microstructure, coating imperfections, and adhesion (scratch adherence). On IN718 substrate with a hardness HRC 40 the highly efficient ion etching process delivers scratch adherence > 30 N with a process performance index (Ppk) of > 1.5. A review of electronic run data such as gas flow, pressure, temperature, evaporator current and voltage, substrate bias voltage and current provides valuable SPC data. After demonstrating an acceptable process control the destructive sampling frequency was reduced significantly. A First Time Yield (FTY) of > 95% and a Final Yield (FY) of > 99% was achieved.
4. Erosion testing Development of next generation coatings, especially for improved large particle erosion resistance, requires a good understanding of the erosion process and the erosion coating properties. Choosing the appropriate test conditions presents a challenge to deliver accurate and meaningful results regarding the expected field performance. The American Society for Testing and Materials Standard G-76 uses 50 μm alumina and exposes only a small surface area to the erodent. Because of the resulting small weight loss the measuring accuracy has to be extremely high and the documentation of the results is complex. Randomly located coating imperfections may skew the results leading to significant variation in the erosion rate. 100 mm standoff and a nozzle geometry which creates an impact area of approximately 6 cm2 achieved reproducible results at impact angles of 20° and 90°. The standard erosion panel size is 5 × 2.5 cm. The blast pressure is adjusted such as to yield a particle velocity of approximately 90–100 m/s at the point of impact for 50 μm angular alumina. The identical blast pressure was also used for other media. This set up allows testing of engine hardware such as compressor airfoils in different sizes. Fig. 4 shows a compressor blade in test and the size of the eroded area after application of 2500 g of erodent (Coarse Arizona Road Dust).
Fig. 4. Room temperature erosion test setup with compressor blade at 100 mm standoff and impact zone on a coated and uncoated airfoil after 2500 g of erodent (Coarse Arizona road dust).
5. Erosion media and erosion mechanism For better understanding of the erosion mechanism three different erosion media were used. • 50 μm angular alumina because of its high hardness and sharp cutting edges is the ideal media for an accelerated test to simulate the long term effect for medium size particle impact • To simulate large particle erosion commercially available silica sand with rounded particles — a Quikrete product #1152 with a particle size distribution up to 700 μm was used. The median is 230 μm, 10% is below 87 μm, 10% is above 335 μm, and the maximum particle size is 700 μm. • For simulation of desert sand as it affects helicopter engines with filtered air inlet Coarse Arizona Road Dust with a particle size up to 200 μm was used. Fig. 5 shows the particle shape of these media and is helpful for the interpretation of the erosion results and the observed coating failure mechanism. The high hardness and sharp cutting edges of angular alumina gradually removes the coating. Fig. 6 shows the erosion of a (Ti,Al)N/TiN multilayer system coating consisting of 15 alternating (Ti, Al)N and TiN layers of equal thickness and a total coating thickness of 20 μm to better demonstrate this effect. When using large silica sand particles the erosion damage looks very different. The large particles shatter the coating and at first create isolated and then overlapping defects until full coating removal. Arizona Road Dust acts in between
Fig. 5. Solid particle erosion media as used for erosion testing — 50μm angular alumina (A), Silica sand (B), Coarse Arizona road dust (C).
A. Feuerstein, A. Kleyman / Surface & Coatings Technology 204 (2009) 1092–1096
Fig. 6. Gradual coating removal by 50 μm angular alumina at 90° and 20° impact revealing the individual layers of a (TiAl)N/TiN multilayer coating.
the above extremes. Fig. 7 shows a typical weight loss curve and erosion pattern of 20° and 90° impact with Arizona Road Dust. 5.1. Leading edge erosion Coating loss on the leading edge of compressor blades can be another significant threat to compressor performance. Some compressor designs are very sensitive to degradation of the leading edge profile and subsequent disruption of airflow. Therefore it is critical to maintain the integrity of the leading edge at different incidence angles. Fig. 8 shows an example where the leading edge suffers significant erosion, but the coating on the pressure side of the airfoil remains intact. It is believed that coating spallation with subsequent erosion attack of the area where the coating was removed is a likely reason of this failure mode. A test methodology to study this erosion phenomenon in the progression was developed. 6. Coating imperfections Coating imperfections can lead to premature breakup of the coating, mainly caused by larger particles forming a point of attack for medium particle erosion. Coating defects such as macro-particles inherent to the unfiltered cathodic arc process and particulate contamination, which can produce kernel type defects, have to be minimized. In spite of the tendency to embed macro-particles into the
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Fig. 8. Leading edge coating erosion in laboratory testing. Substrate material is SST, TiN/ TiN 1 − x “24k Type II™” multilayer coating, leading edge erosion tested with Arizona Road Dust under 90 ° impact angle.
coating cathodic arc is still the preferred process because of its maturity and cost effectiveness. 6.1. Coating thickness Coating thickness and hardness of the underlying substrate are the key factors to improve the erosion protection especially against large particle erosion protection. Obviously a thinner coating is more prone to shattering by erodent. Fig. 9 shows the large particle erosion of a 24k Type II™ coating with a thickness of 25 μm and 50 μm. The 50 μm coating is a self- supporting layer which outlasts the thinner coating significantly. For medium particle erosion the thinner and lower cost coating may provide adequate protection. 6.2. Substoichiometric TiN/TiN1 − × 24k Type IV multilayer coating for large particle erosion applications In essence the combined information on the erosion rates and erosion mechanism for different media provides a valuable interpretation of the protective function and the prospective service life of the coating. Although the Praxair TiN/TiN1 − × 24 k Type II™ multilayer coating provides very good protection for fine and medium sized media, further improvement in the coating architecture had to be made for better protection against larger media. One option is the addition of ductile metallic compliance layers. Those provide an “onion peeling effect” which restricts the erosion damage by large particles to the upper layer of the coating without removing the complete coating. Fig. 10 shows the enhancement of the large particle erosion for this “Type IV” coating compared to the 24k Type II™ coating. The large particle erosion resistance is enhanced by roughly a factor of 5. Current development is also targeted towards new compositions such as e.g. Ti–Al–Si–N to combine the benefits of the nano-composite microstructure of Ti–Si–C– N [5] with the higher temperature capability of Ti–Al–N. 7. Summary and conclusion
Fig. 7. 90° and 20° impact of coarse Arizona road dust — isolated complete coating removal by large particle shattering the coating in 90° impact, basically no attack in 20° impact.
Substoichiometric TiN1 − × provides improved erosion resistance compared to TiN because of its increased hardness and toughness due to significantly smaller crystallites. Praxair has successfully applied a TiN/ TiN1 − × multilayer coating for the erosion protection of helicopter compressor blades and vanes against medium size sand. Using a coating composition based on Ti alone minimizes the production cost significantly compared to other coating systems on the market by avoiding the complexity of alternating layers with different chemistry requiring
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Fig. 9. Large particle erosion (silica sand) of 25 and 50 μm coating at 20° impact. The thicker coating is self supporting and has much larger life.
Acknowledgements The authors thank Scott Cain, Dave Sharp, Tony Hull, Roy Newman, James Knapp, and Michael Brennan from Praxair Surface Technologies, Inc., for their help and support.
References
Fig. 10. Weight loss curve of the “24k TiN/TiN1 − × Type IV” multilayer coating with additional compliance layers vs. “24k TiN/TiN1 − × Type II™” multilayer coating for large particle erosion (Quikrete sand).
multiple sets of cathodic arc sources. Erosion testing with different media such as angular alumina, large silica particles, and Arizona road dust provides a comprehensive picture of the erosion protection properties and allows optimizing the coating architecture for new applications such as large particle erosion. A modified coating architecture with additional metallic compliance layers (24k TiN/TiN1 − × Type IV) demonstrated an improvement of the large particle erosion resistance by a factor of five.
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