Elsevier

Surface and Coatings Technology

Volume 205, Issue 4, 15 November 2010, Pages 1119-1126
Surface and Coatings Technology

Fatigue behavior of a SAE 1045 steel coated with Colmonoy 88 alloy deposited by HVOF thermal spray

https://doi.org/10.1016/j.surfcoat.2010.01.011Get rights and content

Abstract

An investigation has been conducted in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a Ni-base alloy known commercially as Colmonoy 88, deposited by HVOF spray technique. Fatigue tests were conducted under axial conditions (R = 0.1), employing samples of the substrate material in the as-polished condition, after grit blasting with alumina particles and after grit blasting and coating with a deposit of about 250 μm thick. The fatigue tests were conducted at maximum stresses in the range of 380–533 MPa, depending on the condition of the material. A detailed fractographic analysis of some selected samples tested at different stresses was carried out, aimed mainly at determining the crack nucleation and propagation sequence. The results indicate that the deposition of such a coating leads to a fatigue strength debit of the substrate in the range of 10–20% and a similar debit in fatigue limit of ∼ 11–13%. It has been found that grit blasting is the process responsible for the fatigue strength debit observed in the coated samples. Fatigue cracks have been observed to initiate at the substrate–coating interface and at the free surface of the coating, mainly close to alumina particles embedded on the substrate and sharp notches produced during the process. The fractographic analysis of the fracture surface of the coated specimens points out the characteristic heterogeneous nature of the coating, particularly regarding some of its mechanical properties, such as fracture toughness.

Introduction

High velocity oxygen fuel (HVOF) spray technique is one of the leading technologies that have been proposed as a feasible alternative to the replacement of electrolytic hard chromium (EHC) plating in a number of engineering applications. EHC is widely employed for the improvement of surface properties of parts and components that operate under conditions that require corrosion and wear resistance. Particularly in the aircraft industry such parts include, among others, landing gear cylinders, axel journals and pins, hydraulic actuator rods, pistons and cylinders, shafts and bearing holders of gas turbine engines, gears, arrestor hooks, locators and different wear surfaces.

The success of EHC relies on the fact that it is cheap and simple; coatings can be obtained in a wide range of thickness, possess an excellent bond strength and are easily stripped from the substrate. However, in the past few years, significant efforts have been made to substitute EHC due mainly to the numerous environmental hazards involved in its production. Chromium plating baths contain chromic acid solutions, in which the chromium is in the hexavalent state, with hexavalent chromium (hex Cr) and several catalytic anions being known as carcinogenic substances. Also, hex Cr has a level of toxicity greater than arsenic or cadmium and is known to cause a wide variety of medical problems [1], [2], [3], [4], [5]. From the technical point of view, EHC has also some disadvantages which include an unreliable performance due to the variations found for different vendors meeting the same specifications, delamination in service, hydrogen embrittlement and decrease in the fatigue properties of the substrate [6], which is particularly important in critical structural applications.

HVOF constitutes a more friendly deposition process from the environmental point of view, which has already been applied successfully for the replacement of EHC by means of other coatings such as WC–17Co, WC–10Co–4Cr and Co–Mo–Cr alloys. Such coatings are characterized by possessing a high density and good adherence to the substrate, while their deposition could be carried out to reach a wide range of coatings thicknesses [7], [8], [9], [10]. More important, from their qualification as an acceptable replacement of EHC for structural applications, such coatings give rise to a fatigue strength debit of the substrate that is never greater than that produced by EHC plating.

It widely acknowledged that the substrate fatigue strength debit that arises from EHC plating can be attributed to the presence of high tensile residual stresses and microcracks density already contained within the coating [11]. However, in contrast to EHC plating, the substrate fatigue strength debit that arises from HVOF thermal spray is not only a consequence of the coating characteristics in terms of microstructure and residual stresses, but also of the grit blasting process applied to the substrate prior to coating deposition, aimed at improving the mechanical bonding of the coating to the substrate. This process is particularly important in the case of high strength substrate materials, such as structural steels.

Thus, given the great concern that arises from the effect of thermal spray coatings on the fatigue and corrosion-fatigue behavior of different substrates, in the past few years a number of research studies have been carried out [12], [13], [14], [15], [16], [17], [18]. Such investigations have mainly aimed at the evaluation of the fatigue properties of such substrate–coating systems and the quantification of the changes that are produced in the fatigue properties of the substrate as a consequence of the presence of the coating, as well as prior grit blasting. An example is the work conducted by Hernández et al. [12], regarding the deposition of a Colmonoy 88 alloy of ∼ 220 μm in thickness, deposited by HVOF thermal spray onto SAE Q&T 4340 steel.

This investigation concluded that such a steel substrate undergoes a substantial decrease in fatigue properties due to the alumina particles that remained embedded at the surface of the material after grit blasting. Such particles are able to act as stress concentrators and promote the initiation of fatigue cracks. However, these researchers also observed that the subsequent coating of the grit blasted substrate with the Colmonoy 88 alloy led to a further reduction in fatigue properties associated with the fracture and partial delamination of the coating from the substrate along the substrate–coating interface, and the reduction in the area of the load-carrying segments of the substrate–coating system during fatigue testing.

More recently, Puchi-Cabrera et al. [16] conducted a study of the fatigue behavior of a SAE 1045 steel both uncoated and also coated with a Colmonoy 88 alloy, but of about 410 μm thick, deposited by HVOF spray method. In this investigation, prior to deposition, the samples were also grit blasted with alumina grit of approximately 1 mm in equivalent diameter. As in the previous study, fatigue tests were conducted under rotating bending conditions (R =  1, where R represents the stress ratio) and the results indicated that the presence of the coating gave rise to a reduction in the fatigue life of the coated samples tested in air in comparison with the uncoated specimens. On the contrary, when the coated samples were tested in a NaCl solution at alternating stresses less than 350 MPa, these showed an increase in fatigue life in comparison with the polished uncoated samples.

The analysis of the fracture surfaces of the specimens tested in air revealed again that alumina particles present on the surface of the grit blasted samples acted as stress concentrators, inducing the initiation of fatigue cracks at the substrate–coating interface, which explained the reduction in fatigue life. However, under corrosive conditions and low alternating stresses, the presence of the coating provided an effective protection against corrosion-fatigue failures, giving rise to an improvement of the corrosion-fatigue performance of the coated system. On the contrary, at elevated alternating stresses, the coating was observed to delaminate from the substrate, leading to an impairment of the corrosion-fatigue behavior of the coated samples.

Thus, the present investigation has been conducted in order to study furthermore the fatigue behavior of the SAE 1045 steel substrate grit blasted with alumina particles prior to HVOF thermal spray coating with Colmonoy 88 alloy, but tested in air under axial conditions (R = 0.1), where a mean stress greater than zero is applied to the coated material, leading to more severe testing conditions than those reported previously for the same coated system.

Section snippets

Experimental alloy and sample preparation

The present investigation has been carried out employing samples of a SAE 1045 steel, with the following chemical composition (wt.%): 0.46 C, 0.75 Mn, 0.04 P, 0.05 S and Fe bal. This material is commonly employed in the manufacture of machine parts such as shafts, gears, screws, tools, bolts and different components that could undergo friction and wear during operation. The material was provided in the form of rectangular bars of 9 × 75 × 1000 mm3, which were subsequently machined in order to obtain

Experimental results

The evaluation of the static mechanical properties indicated that the uncoated substrate has a yield stress of ∼ 396 MPa and ultimate tensile of ∼ 814 MPa. These properties were not modified significantly due to grit blasting and coating, remaining in the range of ∼ 390–400 MPa and ∼ 810–840 MPa, respectively for both conditions. Also, the elongation of the materials was maintained in the range of ∼ 20%. It was clearly observed that during tensile testing of the coated specimens, the Colmonoy deposit

Discussion

As indicated in the previous section, the Colmonoy 88 deposit tends to delaminate from the substrate at maximum stresses above the yield stress of the steel substrate, which was found to be less than 400 MPa. In the present investigation, fatigue tests on the coated samples were conducted up to maximum stresses of 450 MPa, that is to say well into that plastic domain of the substrate. Under these conditions, it would be expected the occurrence of delamination of the coating to a significant

Conclusions

The deposition of a Colmonoy 88 alloy deposit of ∼ 250 μm in thickness, onto a SAE 1045 steel substrate by means of HVOF thermal spray, leads to a fatigue strength debit of the substrate in the range of 10–20% and a similar debit in fatigue limit of ∼ 11–13%, when the substrate–coating system is tested under axial conditions (R = 0.1) at maximum stresses of 410–450 MPa. The fatigue curves that were determined for the grit blasted and grit blasted and coated specimens do not exhibit a significant

Acknowledgements

The present investigation has been carried out with the financial support of the Scientific and Humanistic Development Council of the Universidad Central de Venezuela (CDCH-UCV), through the projects PI-08-7727-2009/1 and PG-08-7775-2009/1, as well as the National Fund for Science, Technology and Innovation (FONACIT) and the Centre National de la Recherche Scientifique (CNRS), France, through the project PI-2007000923.

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