Residual stresses and adhesion of thermal spray

Residual stresses and adhesion of thermal spray coatings P. Araujo1, D. Chicot2, M. Staia3 and J. Lesage2* Residual stresses generated in coatings during thermal spraying could have very different inside intensity and distribution, depending on the materials and processing conditions. As it is recognised that residual stresses play a major role in the adhesion of coatings, it is necessary to evaluate precisely their influence. It is not possible to conduct these measurements directly, and no indication on how the stresses should be taken into account has been reported in the literature. Moreover, depending on the test used to evaluate adhesion, different volumes of the coating can participate in the delamination process. In order to take into account these observations, it is proposed to define two stress parameters related either to the stresses in the coating or to the stresses at the interfacial zone. In these conditions, it is possible to explain the variation in adhesion as a function of the coating thickness, i.e. to explain the maximum value obtained for the bonding strength deduced from tensile tests and the monotonic increase in adhesion toughness deduced from interfacial indentation tests. Keywords: Thermal spray coatings, Adhesion, Interfacial adhesion, Interface toughness, Bond strength, Residual stresses, Stress parameter, Tensile test

Introduction The interfacial indentation test is increasingly accepted as an alternative to other tests characterising the adhesive properties of thermally sprayed coatings.1–3 This test consists in measuring the length of the cracks generated at the coating–substrate interface produced by Vickers or Knoop indentations.4 A particular feature of the thermally sprayed coatings is that they contain residual stresses after spraying and cooling.5–10 The residual stress state in the coatings is very important, because it can play a significant role in the service performance of the mechanical parts as well as in the adhesive properties of the coatings.11–15 The goal of the present work is to obtain an insight in order to understand the role of residual stresses state, their intensity and their distribution on the coatings adhesion. The first part of the present paper aims to present some general aspects on the stresses generated in the thermally sprayed coatings as a consequence of processing conditions, the materials and the characterisation of the spraying process. Unfortunately, there are few systematic studies defining the role of stresses on the adhesive properties of coatings. Godoy et al.16 have only recently presented some useful results in an attempt to explain the generation of stresses as a function the 1

Universidade Tiradentes, UNIT, ITP, Campus II, Av. Murilo Dantas, 300, Farolaˆndia, 49032-490 Aracaju-SE, Brazil Laboratoire de Me´canique, CNRS UMR 8107, Universite´ de Lille, IUT A GMP, Rue de la Recherche, BP 179, 59 653 Villeneuve d’Ascq, France 3 CENMACOR, Universidad Central do Venezuela, Apt 49141, Caracas 1042-A, Venezuela 2

*Corresponding author, email [email protected]

! 2005 Institute of Materials, Minerals and Mining Published by Maney on behalf of the Institute Received 8 April 2004; accepted 25 July 2004 DOI 10.1179/174329405X30020

Origin and diversity of residual stresses in thermal spray coatings It is generally acknowledged that the residual stress state in a thermal sprayed coating has two main origins. During the first stage of deposition, individual molten particles heat the substrate and solidify. Complete contraction is not possible, owing to the presence of the substrate and/or the neighbouring particles. This phenomenon leads to residual stresses, called ‘quenching stresses’. The second stage of the spraying process is related to the cooling of the coating. The presence of the cooling stresses is due to both the mismatch between the thermal expansion coefficients and the temperature difference between the coating and the substrate. A schematic representation of the residual stress state is presented in Fig. 1.8 Depending on the spraying process, the distribution, intensity and sign of the residual stresses can be very different. Figure 2 gives a schematic representation of the stresses that could be obtained using different thermal spray processes, such as air plasma (APS), thermal wire arc (TWA) and high velocity oxygen fuel (HVOF). The processing conditions also have a significant influence in promoting very different residual stress

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thickness of the coating. Starting from these data and using the experimental results obtained for various substrate–coating systems, coupled with the hypotheses of linear fracture mechanics, a new relationship for the apparent interface toughness involving a term related to the residual stresses is presented.

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Stresses and adhesion of thermal spray coatings

1 Schematic representation of residual stress state in thermal sprayed coating8

a APS; b TWA; c HVOF 2 Schematic representation of residual stresses due to different spraying process of Ni–5Al deposit on steel substrate (from Sampat et al.7)

a low Td; b middle Td; c high Td 3 Schematic profile of residual stress as function of temperature of deposition of molybdenum (from Matejicek et al.14)

states, as shown in Fig. 3, where the deposition temperature Td is associated with cold or plasma spraying of molybdenum coatings and, in Fig. 4, the cooling conditions of ceramic coatings/Al alloys are given as examples. In order to ensure adhesion between the coating and the substrate, it is often necessary to use a bond coat.17 The presence of such a bond coat could severely affect the residual stress distribution, as shown in Fig. 5. Such diversity of situations shows that the interpretation of the effect of residual stresses such as fatigue or adhesion on the mechanical properties is a very complex problem. At present, there are no complete studies defining what is the most important parameter for interpreting their effect and, at the same time, showing the optimum spraying conditions for a given

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application. It is clear, however, that high compressive stresses at the surface promote better fatigue properties.18,19 Better adhesion between a coating and its substrate is also expected when the mean residual stresses in the region of the interface are as low as possible.20,21 To find some simple criterion representative of the residual stress state in the coated material is therefore an objective of great interest, because it could help in interpreting the results of mechanical tests and provide a tool for the optimisation of the coatings with regard to their use. The schematic representation of a typical residual stress state can be seen in Fig. 6, where Ds surf int is the difference between the stress in the coating near the interface s C int and the stress at the surface ssurf, and Ds S;C is the difference between the stress in the coating int

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6 Residual stresses coating studies 4 Effect of conditions of cooling of ceramic coatings/Al alloys on residual stress state calculated by finite element simulation (from Wenzelburger et al.6)


parameters

for

thermal

sprayed

should be involved in this process. These two situations are now examined.

Standard tensile test (ASTM standard C633-7922)

5 Example of influence of NiAl bond coat on residual stress state in molybdenum coating (from Laribi et al.9)

near the interface s C int and the stress in the substrate near the interface s Sint . S Both Ds surf int and Ds int should be significant parameters to be taken into account when interpreting the mechanical properties of thermal spray coatings such as fatigue or adhesion, respectively. Since traditional tests for the adhesion of the thermal spray coatings are referred to in the literature, i.e. tensile test or the interfacial indentation test, some considerations should be made in relation to the importance of the parameters just mentioned above. For the tensile test, the loading concerns the whole volume of the coating and, as a consequence, the two Ds parameters should be involved in the delamination process. Conversely, the interfacial test concerns only the interface surrounding material, and so only Ds S;C int

In a recent paper, Godoy et al.16 studied the effect of the thickness of NiCrAl coatings deposited on two different stainless steel substrates. By measuring the change in curvature caused by the deposition process, the authors were able to describe the residual stress state in each coating using the calculation methodology proposed by Clyne and Gill.23,24 Adhesive bond strength is deduced from the standard tensile test. Figure 7a presents some results obtained by Godoy et al.16 The bond strength exhibits a maximum value for a coating thickness of y400 mm. However, the authors were not able to explain this behaviour using only the level of residual stresses at different sites across the coating. As mentioned above, the residual stresses act in different ways at the surface and at the interface and, depending on the thickness of the coating, one or the other could be predominant. As a consequence, the Ds parameters introduced above could explain this behaviour. Figure 7b shows the increase in bond strength S;C surf surf when Ds S;C int .Ds int and a decrease when Ds int ,Ds int , respectively.

Interfacial indentation test The interfacial indentation test consists of measuring the length of a crack generated at the coating–substrate

a experimental results from Godoy et al.;16 b role of stress parameters 7 Adhesive bond strength of NiCrAl coatings

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9 Interfacial indentation toughness as function of coating thickness

1 logarithm of Vickers hardness indent half diagonal (d/ 2); 2 logarithm of interface crack length (a) 8 a Principle of interface indentation test; b schematic representation of results

and the subscripts i, S and C stand for interface, substrate and coating, respectively.

interface as a result of a Vickers indentation at the interface using different applied loads (Fig. 8). In bilogarithmic coordinates, the relation between the crack radius and the applied load is linear and allows a critical load under which no crack is generated at the interface to be defined. In a previous paper,4 the adhesion between the coating and the substrate was represented by the following equation ! "1=2 PC E : Kca~0 015 3=2 (1) H aC i

Materials and results In order to validate the assumptions made above, some interfacial indentations results obtained on several coating/substrate systems were reinterpreted. Samples were obtained from specimens of five metallic substrates coated with chromium carbide (75%), nickel–chromium (25%) alloy using the HVOF spraying process. Parallelepiped specimen (120615610 mm3) were first sand blasted in order to eliminate impurities and confer a mean roughness to the surface, varying between 6 and 10 mm, favourable to the mechanical anchorage of the molten particles. The thermal spraying process was performed on the two largest faces of the samples. The thickness of the coating varied between 200 and 600 mm. Grade 80–1200 SiC paper was used for prepolishing, which was followed by a diamond final polishing. Additional data obtained from the literature25 for Stellite/stainless steel system were also analysed. Since the intensity of residual stresses is known to depend on the coating thickness,8 at least two different coating thicknesses were tested for each substrate in

where Kca is the interfacial indentation toughness, PC is the critical load, and aC is the corresponding indentation diagonal measured in the plan of the interface. E is Young’s modulus, and H is the Vickers hardness of the material. # E $1=2 At the interface, the factor H was expressed by i the relation 1=2

1=2

ðE=H Þi ~

1=2

ðE=H ÞS

ðE=H ÞC z 1=2 1z(HS =HC ) 1z(HC =HS )1=2

(2)

Table 1 Mechanical characteristics of materials and calculated interface parameter Substrates

Coating

HS, GPa

ES, GPa

HC, GPa

EC, GPa

(E/H) i1=2

Low carbon steel Grey cast iron Globular graphite cast iron Austenitic steel Low alloyed steel Stainless steel25

Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Cr3C2–NiCr Stellite

1.30 2.14 2.56 1.99 3.68 1.65

210 105 105 210 210 203

9.07 9.07 9.07 9.07 9.07 3.73

125 125 125 125 125 175

10.24 5.93 5.47 8.18 6.06 9.40

Table 2 Coordinates of critical point and apparent interface toughness for each substrate and thickness

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Substrate

Low carbon steel to obtain

Grey cast iron

t, mm PC, N aC, mm Kca, MPa m1/2 Substrate t, mm PC, N aC, mm Kca, MPa m1/2

0.27 0.30 15.1 22.6 52 63 6.2 6.9 Austenitic steel 0.36 0.44 3.4 10.2 21 37 4.3 5.6

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0.58 0.27 5.8 1 27 11 3.7 2.2 Stainless steel25 0.22 0.29 2 7.7 20 40 3.2 4.3

0.56 6.2 26 3.8 0.38 12.2 50 4.9

0.49 16.7 58 5.3

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a from present experimental results; b from results of Godoy et al.16 10 Variation in Ds as function of coating thickness

coating thickness (Fig. 10a), where an appreciable decrease is observed. Figure 10b, obtained by plotting the variation in Ds with the thickness for the results presented by Godoy et al.,16 shows the same monotonic tendency. It is clear from these results that Ds S;C int should be considered the main stress parameter when studying adhesion by the interfacial indentation method.

order to evaluate their effect. Table 1 shows the data and measurements for the systems tested: Young’s modulus, Vickers hardness and the parameter (E/H) i1=2 which appears in the apparent interfacial toughness formulae. Five indentations were performed for each level of load in order to determine a reliable mean crack length. The additional data reported by Choulier25 are also included in the table. The use of logarithmic coordinates allows a straight line to be obtained for the crack length–load relationship. Applying the linear regression method to the experimental crack points (a, P) as well as to the apparent hardness points (d/2, P), it was possible to determine the coordinates of the critical point (Pc, ac) and to calculate the apparent interface toughness Kca, using equation (2). The corresponding values are given in Table 2. From these results, it is possible to represent the variation in interfacial toughness Kca with the thickness of the coatings, as shown in Fig. 9, where a monotonic increase is observed. As Kca is a function of a parameter related to residual stresses, as proposed previously,4 it was possible to calculate its value for each coating/substrate system. However, it was not possible to relate the Kca value in a simple way to a pertinent stress parameter. Because the Ds S;C int parameter proposed above as the main parameter related to the interface indentation test, it could be considered in expressing the toughness interface Kca as Kca~Kca0 zaDsS,C int

Conclusions Intensity and distribution of the residual stresses generated in coatings during thermal spraying can be very different. Because it is recognised that residual stresses play a major role in the adhesion of coatings, it is necessary to evaluate their influence precisely. Depending on the test used to evaluate adhesion, different volumes of coating can participate in the delamination process. As a result, there could be discrepancies between the apparent behaviour associated with adhesion tests such as the tensile test and the interfacial indentation test. This has motivated the preent attempt to characterise the residual stress state in terms of stress criteria in the coating Ds surf int or at the . Using these criteria, it was interfacial zone Ds S;C int possible to explain the variation in adhesion as a function of coating thickness, i.e. to explain the maximum value obtained for the bonding strength deduced from the tensile tests and the monotonic increase in adhesion toughness deduced from the interfacial indentation tests. And finally, it was proposed to include Ds S;C int parameter in the interfacial indentation toughness relationship in order to quantify the effect of residual stresses on adhesion.

(3)

then DKca~Kca{Kca0 ~aDsS,C int

(4)

References

where Kca0 is obtained by extrapolating the Kca values for an infinite thickness.4 a is calculated using the following formulae26–28 2 1=2 a~ pffiffiffi aC p

(5)

&pffiffiffi where aC represents the actual crack length and 2 p the coefficient associated with the geometry of the Vickers indent. Following these considerations, it is possible to represent the evolution of DKca as function of the