C. N. David, 1 M. A. Athanasiou, 1 K. G. Anthymidis, 1 and P. K. Gotsis 1 Journal of ASTM International, Vol. 5, No. 6 Paper ID JAI101571 Available online at www.astm.org Impact Fatigue Failure Investigation of HVOF Coatings ABSTRACT: Dynamic impact-wear and coating fatigue at cyclic loading conditions demonstrates a very demanding failure mode, which occurs in a number of mechanical applications and it becomes very critical when the application concerns aggressive working environments. The coating impact testing is a novel experimental technique developed to investigate the fatigue behavior of coating-substrate compounds, which was not possible with the common testing methods previously available. The objective of this study is to investigate the influence of the impact load on the fatigue strength of thermal spray high velocity oxy-fuel HVOF coatings. Furthermore, the overall aim of the current research is to prove the reliability of the impact testing method to assess the coating lifetime against fatigue, to interpret the coating failure modes, and thereby to explore its capability, whether this nonstandard test can be used in industrial scale as a reliable technique in the development and optimization of fatigue resistant coatings. Based on the above method the current research provides experimental results concerning the coating fracture in terms of cohesive and adhesive failure modes. The fatigue strength of the tested coatings is determined in terms of fatigue-like diagrams by means of scanning electron and white light interference microscopy, as well as by electron dispersive x-ray analysis EDX at discrete loads and number of loading cycles. From the conducted experiments, it was shown that the optimum HVOF coating against fatigue is the WC-CoCr. KEYWORDS: HVOF coatings, impact fatigue, cohesive-adhesive coating failure Introduction The modern power generation steam turbines are being designed to have higher efficiencies and to meet the stringent environmental regulations, ensuring plant reliability, availability, and maintainability without compromising cost. High efficiencies can be achieved at higher temperatures. Therefore, the operating temperature is expected to rise from 550 C to 650 C and from the material perspective to implement turbine components protected by spallation and oxidation resistant coatings. To guarantee the reliability of coated steam turbines components, used in power plants, the lifetime assessment of the coatings and their failure prediction become very important. Microhardness, scratch adhesion and pin-on-disk sliding tests are commonly used for rapid evaluation of the mechanical properties of coatings 1. However, they do not model the dynamic cyclic impact fatigue. The impact test method has been introduced as a convenient experimental technique to assess the fatigue strength of coatings being exposed in successive impact loads 2 5. According to this method a coated specimen is exposed to a cyclic impact load. The superficially developed Hertzian pressure induces a complex stress field within the coating, as well as in the interfacial zone between the layer and the substrate. Both these stress states are responsible for distinct failure modes such as the cohesive or adhesive one. The exposure of the layered compound against impulsive stresses creates the real conditions for the appearance of the coating fatigue phenomena based upon structural transformation, cracking generation, and cracking growth, which are responsible for the gradual microchipping and the degradation of the coating. Experimental Procedure The objective of this experimental study was to investigate the influence of the impact stress state on the performance and fatigue strength of thermal spray HVOF coatings. Furthermore, the overall aim of the current research was to prove the reliability of the impact test, as a new testing method, to assess the coating lifetime against high cycle fatigue, to interpret the failure modes of coatings, and thereby to Manuscript received November 14, 2007; accepted for publication May 14, 2008; published online June 2008. 1 Mechanical Engineering Department, Technical University of Serres, 62124Serres, Greece, e-mail: david@teiser.gr Copyright 2008 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.
2 JOURNAL OF ASTM INTERNATIONAL TABLE 1 Specific properties of the investigated coating steel layered compounds. Coating/substrate WC-CoCr/ P91 Steel Ni20% Cr/ P91 Steel CrC25% Ni/ P91 Steel Hardness HV 520/270 340/271 851/257 Thickness m 100 100 130 Deposition method HVOF HVOF HVOF examine its capability, whether this nonstandard test can be used as a useful method for the development and optimization of fatigue resistant coatings working under impact Hertzian loading. Thermal spray coatings have been applied in many different industries to provide wear and corrosion protection. It is common knowledge today that high velocity oxy-fuel HVOF deposition systems are capable of producing coatings with high density, higher hardness, and superior bond strength compared to other thermal spray methods. This paper is concerned with HVOF thermal spray coatings, deposited on high-strength Cr-steel P91. In Table 1 specific properties of the investigated coating steel layered compounds are shown. The coating impact test was used as the most convenient experimental method to assess the fatigue strength of the examined coating systems under cyclic impact loading. Figure 1 shows the impact test rig where the experiments have been conducted 6,7. The stress strain problem related to the impact test is the Hertzian contact, which is developed between the spherical indentor cemented carbide ball consists of 88 % WC and 12 % Co with a diameter of 5 mm and the examined layered space Fig. 2 a. To minimize the friction in the contact area during the impact lubricant film is used. The resulted impact craters were analyzed using both scanning electron microscopy and 3D-optical profilometry Fig. 2 b. A window of the impact force signal in time domain acquired during a typical experiment is shown in Fig. 2 c. The impact frequency amounts to 50 Hz and was the same for all conducted experiments. In order to determine experimentally the coating fatigue life curve Fig. 2 d a number of experiments with varied impact force amplitude and a number of cycles have to be carried out 2,4. In the impact craters resulting from such experiments three different zones inside and around the impact cavity can be identified 5. A central zone in the mid of the impact cavity, where the coating is strained with compressive stresses and gradual cohesive degradation takes place. The intermediate zone inside the piled up rim formed around the impact cavity, where tensile and shear stresses are building up and both cohesive and adhesive coating failure arises. Finally, the outer zone of the impact cavity, where macrocracks might propagate and coating adhesive failure occurs depending on the coating brittleness. Gradual intrinsic coherence coating release and coating microchipping leading to progressive degradation of the film is characterized as cohesive failure mode. Abrupt coating fracture and suddenly exposure of the substrate material, due to poor coating adhesion with the substrate, designates the adhesive failure mode. Both coating failure modes and their extent were assessed by SEM observations, EDX analysis, as well as by white light interference microscopy 3D-optical profilometry. The appearance of the substrate material due to coating removal in a local area of the impact crater was selected as failure criterion. The failure was identified by EDX analysis inside the crater in terms of iron detection belonging to the substrate. The proposed testing method has been proved to be repeatable regarding the coating fatigue strength identification, due to the fact that for each of the examined samples, a couple of experiments have been carried out in order to determine the fatigue curve. From these experiments we observe that in all fatigue diagrams the resulted fatigue curves have the typical shape of fatigue life curves. FIG. 1 Impact test rig.
DAVID ET AL. ON IMPACT FATIGUE FAILURE OF HVOF COATINGS 3 FIG. 2 (a) Working principle of the impact testing, (b) scanned impact craters, (c) impact force signal, (d) coating fatigue life curve. During the indentation of the impact ball in the crater, friction and microslip mainly exist at the intermediate area of the impact cavity and superficial abrasive wear also takes place there. However, from the experimental analysis it is observed that coating failure in adhesive mode arises in this zone. Although friction is present here, the result is that the induced shear and tensile stresses cause the coating fatigue. Furthermore, coating failure occurs in the center of the impact cavity, where the friction due microslip is negligible and in the crater vicinity where no contact exists, as well. Hence, it can be concluded that the major coating failure mechanism is the coating fatigue and the friction does not significantly affect the coating failure in this test. Discussion of Experimental Results The main failure of the examined coating-substrate compounds occurred in the central zone of the impact crater with gradual coating degradation. However, depending on the impact load amplitude the tensile and shear stresses in the intermediate zone of the impact crater take high values. In this case microcracks propagate and sheet-like debris of the coating layer flakes can occur. Figure 3 a illustrates the cross section of the examined WC-CoCr thermal spray coating deposited on P91 steel substrate. The microhardness value 520 HV indicates the relatively high wear resistance of this coating. Furthermore, the observed homogeneous coating microstructure, in relation with its composition, is responsible for the high coating toughness. In Fig. 3 b the impact crater is depicted by SEM and white light interferometry images. The EDX analysis, inside the crater, indicates the coating cohesive failure initiation in local regions, where Fe belonging to the steel substrate is detected. The fatigue strength evaluation of this coating, after the conclusion of the necessary experiments to find out the coating fatigue curve, is outlined in Fig. 3 c. From the results it is concluded that the coating sustains the dynamic impact load to high cycles, presenting also enhanced fatigue limit 200 N. The overall coating behavior can be attributed to the enhanced fracture toughness of this coating and the perfect adhesion with the substrate. However, superficial abrasive wear as reported in other works 8 has been observed. Particular attention was paid also to the coating adhesive failure mode, which occurs in the coatingsubstrate interfacial zone. This kind of failure has been observed by the investigation of the Ni20% Cr coating compound Fig. 4 a 9. High tensile and shear stresses developed in the intermediate vicinity of the impact crater, due to the plastic deformation of the substrate, have initiated the development of a large
4 JOURNAL OF ASTM INTERNATIONAL FIG. 3 (a) Microhardness measurement of the WC-CoCr/ P91 steel layered compound, (b) SEM and white light interferometry image of the impact crater and EDX analysis indicating the WC-CoCr coating failure (cohesive failure mode), (c) WC-CoCr fatigue life curve.
DAVID ET AL. ON IMPACT FATIGUE FAILURE OF HVOF COATINGS 5 FIG. 4 (a) Microhardness measurement of the Ni20% Cr/ P91 steel layered compound, (b) SEM image of the impact crater with coating microcracks and EDX analysis indicating the Ni20% Cr coating failure (adhesive failure mode), (c) Ni20% Cr fatigue life curve.
6 JOURNAL OF ASTM INTERNATIONAL FIG. 5 (a) Microhardness measurement of the CrC25% Ni/ P91 steel layered compound, (b) SEM image of impact crater with total coating removal after 5 10 5 impact cycles at 600 N impact force and magnified SEM image with coating microcracks inside the crater at lower impact force 550 N, (c) CrC25%Ni fatigue life curve.
DAVID ET AL. ON IMPACT FATIGUE FAILURE OF HVOF COATINGS 7 FIG. 6 Comparison of the impact fatigue curves of the examined HVOF thermal spray coatings. number of microcracks and caused thereby the coating abruption and the substrate exposure Fig. 4 b. Microcracks arise inside the coating layer and propagate perpendicular to its surface, when the coating is not tough enough to accommodate the stresses, induced by the ball indenter, and to follow the flexure and deformation of the substrate. When the above damage mechanism is present, and in the case where the bonding of the coating with the substrate is faulty, adhesive failure takes place in the form of sheet-like debris. Figure 4 c outlines the fatigue life curve of this coating determined by impact testing. Figure 5 shows a typical example of adhesive fatigue failure as it has been observed at the brittle CrC25%Ni coating, which has considerably higher hardness Fig. 5 a in comparison to the previous one, Ni20% Cr. In the magnified view of the failure area inside the impact cavity, microcracks are visible after 5 10 5 impacts at 550 N indicating the coating failure initiation Fig. 5 b right part. At the same impact cycles with increased impact load 600 N, the total removal of the coating and the exposure of the substrate is observed. An overview of the fatigue strength of this coating is given in Fig. 5 c. From the above experiments we conclude that the adhesive failure abrupt coating removal like flakes from the substrate appears in low cycle fatigue at high impact force amplitude. Instead of that cohesive failure occurs in high cycle fatigue at low impact force gradual coating degradation. Figure 6 outlines the fatigue strength performance of the three examined thermal spray coatings by means of the experimental TABLE 2 Fatigue strength evaluation of the investigated coating steel layered compounds. Failure Mode Bonding with Substrate Fatigue Strength Fatigue Limit Failure Mechanism Coatings Experiments Behavior WC-CO 24 cohesive ductile perfect high 200 N Micro chipping, gradual degradation Ni20% Cr 19 adhesive ductile poor small 100 N Large macro cracks in the interface and coating removal in flakes CrC25% Ni 19 adhesive brittle good medium 100 N Coating micro cracks and intrinsic coherence release
8 JOURNAL OF ASTM INTERNATIONAL determined coating fatigue curves. Apparently, the WC-CoCr coating demonstrates higher fatigue strength against cyclic impact loading in comparison to the other two HVOF coatings. An overview of the fatigue strength of the examined coatings and their failure modes is presented in Table 2. Conclusions The work presented here explores the impact testing method in understanding the failure mechanisms of HVOF thermal spray coatings and provides a feedback approach for optimizing the design of surface engineered components being used in cycle power plants steam turbine components. More specifically the paper reports the results of a novel experimental approach adopted to investigate the performance of HVOF coating systems and to deliver a reliable testing method for coating development. The current impact testing investigations revealed the good fatigue strength of HVOF thermal spray coatings. Acknowledgments The authors are greatly indebted to the Research Committee of the Technical University of Serres, Greece, for financing this research project. References 1 Loeffler, F., Methods to Investigate Mechanical Properties of Coatings, Thin Solid Films, Vol. 339, 1999, pp. 181 186. 2 Bouzakis, K., Vidakis, N., and David, K., The Concept of an Advanced Impact Tester Supported by Evaluation Software for the Fatigue Strength Characterization of Hard Layered Media, Thin Solid Films, Vol. 351, 1999, pp. 1 8. 3 Bouzakis, K., David, K., Siganos, A., Leyendecker, T., and Erkens, G., Investigation of the Fatigue Failure Progress of PVD Elastoplastic Coatings with Various Roughness During the Impact Testing, International Conference on Metallurgical Coatings and Thin Films, San Diego, CA, 30 April 2001, p. 116. 4 Knotek, O., Bosserhoff, B., Schrey, A., Leyendecker, T., Lemmer, O., and Esser, S., A New Technique for Testing the Impact Load of Thin Films: The Coating Impact Test, Surf. Coat. Technol., Vol. 54-55, 1992, pp. 102 107. 5 Bantle, R. and Matthews, A., Investigation Into the Impact Wear Behavior of Ceramic Coatings, Surf. Coat. Technol., Vol. 74-75, No. Part 2, 1995, pp. 857 868. 6 David, C., Anthymidis, K., and Tsipas, D., A Comparative Study of the Fatigue Resistance of Aluminide Coatings on P91 Steel Substrate Under Cyclic Impact Loading, Particle and Continuum Aspects of Mesomechanics, ISTE Ltd., 2007, pp. 721 728. 7 David, C., Anthymidis, K., Agrianidis, P., and Tsipas, D., Determination of the Fatigue Resistance of HVOF Thermal Spray WC-CoCr Coatings by Means of Impact Testing, J. Test. Eval., Vol. 35, No. 6, 2007, pp. 630 634. 8 Ahmed, R., Contact Fatigue Failure Modes of HVOF Coatings, Wear, Vol. 253, 2002, pp. 473 487. 9 Padilla, K., et al., Fatigue Behavior of a 4140 Steel Coated with a NiMoAl Deposit Applied by HVOF Thermal Spray, Surf. Coat. Technol., Vol. 150, 2002, pp. 151 162.