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.