Introduction
Protective coating can increase the life of piping, vessels, and several other equipment and structures, as the coating protects the substrate against corrosion and wear mechanisms [1, 2, 3]. Damage mechanisms such as fatigue, high temperature oxidation [3], wear [4] pose a risk to the equipment surface quality. Thus, surface modification techniques are implemented to provide coatings on the surface of the base material [5].
Fatigue failure is important in components and structures, because when the crack that nucleates and propagates reaches its critical size, it can lead the component to a catastrophic rupture.[6 - 8]. The roughness of the surface that is subjected to positive stresses in the fatigue loading is an important parameter, since fatigue nucleation is dependent on the stress raisers attributed to the surface roughness. In addition, residual stresses have been considered a critical factor for the fatigue behavior [9]. The fatigue performance is improved by surface compressive stresses which hinder crack nucleation, as these compressive stresses will be an additive to external stresses for determining the effective stress acting on the component [10]. One of the great benefits of coatings is to promote compressive residual stresses on the surface [6]. Some parameters such as torch operating parameters, feedstock characteristics, deposition rates and differences in the coating and substrate’s thermal expansion coefficient have all demonstrated great influence in coating stresses [9]. Furthermore, coating’s microstructures with higher toughness lead to delayed crack nucleation [6].
Metal matrix composites and dispersed carbide microparticles are common in the surface coating in industrial practice. Currently, the deposition of nickel-chromium (NiCr) alloy has been the subject of increased attention. The NiCr alloy as a metal matrix gives toughness to the layer, as well as corrosion and oxidation resistance. Chromium carbides particles (Cr3C2) are well known and widely used for wear protection purposes [11], as these particles can increase the mechanical strength of the layer and act against erosive and abrasive wear [2]. The Cr3C2 has high strength properties, high coating adhesion, low density and good chemical stability [11].
Nanostructured coatings can undergo potentialities in their physical and mechanical properties, such as mechanical strength, hardness, ductility, specific heat, diffusivity [12]. Coatings obtained from milled powders down to the nanometer level, whose structure contains nanometric crystallites, provide coating with greater adhesion to the substrate, significantly lower porosity and higher surface quality, lower roughness, in addition to the increased hardness and wear resistance [11]. The high energy milling (HEM) is a technique where powder mixtures of different metals are ground together to obtain a homogeneous alloy [13]. This process causes intense plastic strain of the powders, leading the particles to hardening, fracture and subsequent cold welding in successive stages [14]. The structure of the particles is continuously refined due to the energy transferred by the continuous impact of the grinding bodies. The structure undergoes intense multiplication of dislocations, which leads to the reduction of its long-range order, promoting particles fracture up to the nanometer scale, although they remain as crystallites. The HEM allows nanostructured materials to be synthesized easily in a short time [13]. Moreover, the main objectives of the HEM for the synthesis of Cr3C2-25NiCr coating powder are: i) to reduce the granulometry of the composite; ii) to ensure that the carbide particles will be surrounded by the precursor constituent of the NiCr binder phase; iii) to create new active surfaces and structural defects in the composite [15]. Reducing the particle size of the powder causes an increase in the grain boundary area of the layer splats, which increases the hardness of the layer due to the restriction of dislocations [15].
The coating method may also be an important factor for the resulting properties of the layer. The high velocity oxygen fuel (HVOF) is a thermal spraying technique, widely used for coating’s manufacture [8, 15 – 20]. The hypersonic flame at speeds of up to 3000 m/s releases large energy that is converted into heat and pressure. The particles reach the substrate forming a dense and hard coating, with extremely low porosity and high adhesion strength, retaining nanometric crystallites after deposition [21], resulting in dense, hard [22], fatigue strength [8] and well bonded coatings [9, 22]. Nanostructured material sprayed by HVOF makes the coating more resistant to crack propagation [12]. Although conventional thermal spraying causes particles to melt, high temperature and high-speed spraying cause plastic strain of the powder upon impact with the substrate, forming mechanically bonded splats that cool at a rate of up to 1x106 K/s [4]. The accumulation of large number of splats gives rise to the coating building. The rapid solidification of the Cr3C2-25NiCr particles can leads to the formation of an amorphous / nanocrystalline nickel supersaturated in chromium binder phase, besides retaining the Cr3C2 and NiCr phases [2 - 5, 13 - 15].
The strong mechanical bond between splat and substrate impedes the relative sliding of splat [8, 11, 15, 23]. Thus, fatigue cracks arising from the coating surface would cause increased stresses at the interface with the substrate, favoring the crack propagation into the substrate. Since the concentrated stress is greater than the applied stress, it reduces the fatigue limit [23].
The size of sprayed powders intimately effects the physical and mechanical properties of the coating. Structure formed by nanometer crystallites of the coating and high volumetric fraction of atoms in the layer’s grain boundaries can improve mechanical properties [13]. Coatings obtained from nanometric powders have been researched more recently, and their use enhances coating performance gains [1, 10, 22, 24]. Studies show that coatings increase wear resistance over the substrate [10, 18, 25 – 26]. However, concern about the fatigue performance of coatings is important in the case of equipment operating under cyclical stresses. Therefore, it is important to evaluate the fatigue strength of the coating obtained from nanometric powder and combinations of this with micrometric powder to determine whether there is advantage in expending high energy milling effort on this fatigue strength property.
The present study evaluates the fatigue performance of the coatings given by particles of Cr3C2 and NiCr, both in nanometric scale, micrometric scale and a combination of 50 % of both, sprayed by the HVOF technique on ASTM A516 steel substrate.
Materials and Methods
The commercially manufactured micrometric Cr3C2-25NiCr powder (PRAXAIR Surface Technologies) containing 75 wt % of Cr3C2, 20% of Ni and 5% of Cr was used in the present study [27].
Pulverisette 6 planetary grinding mill equipped with steel-coated grinding jar and 5 mm zirconium oxide spheres of grinding bodies allowed to evaluate the HEM method, to give the nanometric powder. The mass ratio of the spheres and raw material were 5:1. The milling speed was 800 rpm. 20 ml of ethyl alcohol and 3% of alumina was added as a milling medium, to increase the grinding efficiency, inhibiting the particle’s growth, controlling the grain size and avoiding the agglomeration of nanometric particles [28]. The powder was ground up for 12 hours. Them, it was dried in an oven at 50°C for 20 minutes and finally sieved on 325 Mesh, according to a recommended protocol [15].
Laser diffraction granulometer with detection range of 0.04 to 2500 μm (CILAS - 1180) provided the particle size analysis. Samples with 10 mg of powder were dispersed in water and it was subjected to ultrasonic agitation for 60 seconds for deagglomeration prior to the analysis.
X-ray diffraction (XRD) analysis enabled the phases characterization, using the Philips X-Ray Analytical Equipment X’Pert-MPD System, console PW3040/00 and PW3373/00, anode CuKα. Parameters used were 40 kV and 40 mA, 10 mm window, 1º slot, angle between 50 and 75º. The diffractograms were treated and analyzed through the software WinFit 1.2, employing the Single - Line method to calculate the mean diameter of the crystallites [13]. Scanning electron microscope (SEM JOEL Carry Scope JSM-5700) and energy dispersive spectroscopy (EDS) techniques were applied for the powders and coating analysis.
Three different layers were established: 1) coating obtained from the nanometric powder (Nano); 2) coating obtained from micrometric powder (Micro) and 3) coating containing 50 wt % of both powder’s granulometry (NM). The specimens of ASTM A516 steel (75 x 15 x 5 mm) were machined and alumina blasting was carried out over one face of each specimen to enhance the coating anchoring. The coating equipment used was the TAFA JP 5000 HP/HVOF System 5120, PRAXAIR and TAFA GUN 5220 pistol. To reduce the analysis variables, it was attempted to evaluate the coatings by fixing factors such as thickness and porosity and parameters for HVOF spraying are optimized and listed in Table 1.
Table 1. HVOF spraying parameters of the Cr3C2-25NiCr coating.