Impact test

1. Impact Testing a Capable Method to Investigate the Fatigue Resistance
P. Agrianidis, Professor of Mechanical Engineering Department, Technical University of Serres (TEI),
Terma Magnisias St., 62124 Serres, Greece, Email: pagri@teiser.gr
K.G. Anthymidis, Head of Materials Department in Applied Research Center of Serres, Terma
Magnesias St. 62124 Serres, Greece, Email: kanth@teiser.gr
C. David, Professor of Mechanical Engineering Department, Technical University of Serres (TEI),
Terma Magnisias St., 62124 Serres, Greece, Email: david@teiser.gr
D.N. Tsipas, Professor of Mechanical Engineering Department,
Aristotle University of Thessaloniki, 54006 Thessaloniki, Greece, Email: tsipas@eng.auth.gr
ABSTRACT
The impact testing is an efficient experimental procedure that enables the assessment of the fatigue strength of mono- and
multilayer coatings, which was not possible with the common testing methods previously available. This paper presents a
novel impact testing procedure capable to assess the fatigue failure resistance of HVOF thermal spray coatings working under
cyclic loading conditions. From the experimental results it was concluded that the WC-CoCr coating deposited on P91 steel
substrate showed superior fatigue strength in comparison to that of CrC-NiCr and Ni20Cr coatings deposited on the same
substrate.
Keywords: impact testing, coatings, fatigue, adhesion
1. INTRODUCTION
The modern power generation steam turbines are being designed to have high efficiency and to meet the stringent
environmental regulations, ensuring plant reliability, availability and maintainability, without increased cost. High power
generation efficiency can be achieved at elevated temperatures. Therefore, the operating temperature of steam turbines 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-disc 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. Recently, the impact test method has been introduced as a convenient experimental technique to
evaluate the fatigue strength of coatings being exposed in alternate impact loads [2-5]. According to this testing method a hard
carbide ball repetitively impacts with a frequency of 50 Hz on the specimen surface and cyclically loads the coated specimen.
The superficially developed Hertzian pressure induces a complex stress field within the coating, as well as, in the interfacial
zone. Both these stress states are responsible for distinct failure modes, such as a cohesive or adhesive one. The exposure of
the layered compounds against impulsive stresses creates the real conditions for the appearance of 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.

2. 2. EXPERIMENTAL PROCEDURE
In the frame of the current work the investigated coatings were characterized using the testing device, which is shown in
Figure 1. The system consists of three main parts:
• The main test device – mechanical unit (center)
• The power supply unit (right)
• The evaluation and controlling unit (left)
Figure1: Impact tester system
The concept of the working principle of the tester was based on the on-line control and in real time monitoring of its operating
parameters. The mechanical unit, where the impact test is conducted, is located in an appropriate stationary bench stand.
This experimental set up is simple, user friendly and allows the determination of the fatigue behavior of a wide range of single
and multielement coatings. The working principle of the impact tester is based on the alternate Laplace magnetic forces
produced by the electromagnetic field, which is induced within the mechanical unit. The stress strain problem related to the
impact test is the Herzian contact, which develops between the spherical indentor (carbide ball) and the examined layered
space. Gradual intrinsic coherence release and coating microchipping, or abrupt coating fracture and consequent exposure of
the substrate material designate the coating failure. In all impact craters resulted from the experiments three different zones
inside the impact cavity were identified (see fig. 2). A central zone in the mid of the impact cavity, where the coating is strained
with compressive stresses and a gradual cohesive degradation takes place. The indermediate zone inside the piled up rim
formed around the impact cavity, where tensile and shear stresses are building up and both cohesive and adhesive
delamination arises. Finally, the peripheral zone of the impact cavity, where macrocracks might propagate and coating failure
occurs. The coating failure mode and its extent were assessed by SEM observations and EDX analysis. The contact load
leading to coating fatigue fracture was recorded in diagrams (endurance strength curves) versus the number of impacts. In the
present paper the fatigue strength of HVOF thermal spray coatings was investigated in such a system.
High velocity oxy-fuel (HVOF) thermal spraying has been used in industry during the last twenty-five years. This process has
been widely used for the production of high quality carbide cermet coatings, due to it’s moderate process temperature and
higher particle velocities which result in avoiding reduction of carbides to brittle carbides and oxy – carbides and leading to
better coating properties like higher bond strength and density [7,8,9]. Consequently, this results in higher quality, more wear
resistant coatings, with higher levels of retained reinforced material and less porosity [10].

3. Imprint
Impact
direction
Carbide
ball
Substrate
Central Zone
Outer Zone
Middle Zone
Outer Zone
Central Zone
Coating
Figure 2: Impact crater with the developed coating failure
3. RESULTS AND DISCUSSION
The main failure of the examined coating-substrate compounds occurred in the central zone of the impact crater with coating
delamination. However, when the tensile and shear stresses in the outer zone of the impact crater are to high sheet-like debris
of the coating layer due to poor adhesion can occur. In case of relatively tough coating microstructure with high wear
resistance as the WC-CoCr thermal spray coating is, the coating layer sustains the cyclic impacts without any sign of cohesive
delamination failure. Instead of that, only superficial abrasive wear and spalling failure, as reported in other works [6], has
been observed (Fig 3). This behaviour can be attributed to the higher fracture toughness of this coating.
Figure 3: WC-CoCr coating failure initiation (cohesive failure mode) and microhardness measurement of the layered
compound

4. Particular attention was paid to the extent of the delamination and fracture of the investigated coatings at the substrate-deposit
interface during the cyclic loading. For the examined Ni20Cr coating in combination with the relatively plastic strained
substrate, the high tensile stresses in the immediate vicinity of the impact crater, due to the plastic deformation of the
substrate, caused the development of a large number of microcracks and thereby the fracture of the coating leading to the
exposure of substrate (Fig.4).
Microcracks and
adhesive failure
EDX analysis of substrate
Microcracks and
adhesive failure Number of impacts: 6x 105
Impact force: 275 N
EDX analysis of substrate
Figure 4: Ni20Cr coating adhesive failure and microhardness measurements
In general, micro- and macrocracks arise inside the coating layer and propagate perpendicular to its surface when the coating
is not tough or ductile enough to accommodate the stresses induced by the ball indenter and to follow the flexure and
deformation of the substrate. Figure 5 shows a typical example of this kind of fatigue failure as it has been noticed by the brittle
CrC-NiCr coating. The right lower part of the figure illustrates a magnified view of the failure area with the building up of
microcracks after 500000 impacts. As the experiment proceeds, after 600000 impacts at the same impact load, we can
observe the total removal of the coating and the exposure of the substrate (upper left figure part).
20 µm
500 µm
Number of impacts: 5x105
Impact force: 550 N
Number of impacts: 6x105
Impact force: 550 N
Figure 5: CrC-NiCr coating failure and microhardness measurements

5. Figure 6 gives an overview of the endurance performance of the examined thermal spray coatings by means of the
experimental determined fatigue curves. Apparently, the WC-CoCr coating revealed in comparison to the other two coatings
the higher fatigue strength against cycle impacting.
0,2 0,4 0,6 0,8 1,0 1,2 1,4 1,6
100
200
300
400
500
600
700
Impactforce(N)
Number of impacts X 10
6
WC-CoCr
CrC-NiCr
Ni20Cr
Figure 6: Comparison of the impact fatigue strength of HVOF thermal sprayed coatings
4. CONCLUSIONS
The work presented here shows a step forward in understanding the failure mechanisms of HVOF thermal spray coatings and
provides a feedback approach for optimizing the design of surface engineering components being used in cycle power plants
(steam turbine components). More specifically the paper reports the results of a novel experimental approach adapted to
investigate the endurance performance of coating systems with refer to their mechanical properties and to deliver a semi-
empirical design approach. The current impact testing investigations revealed the good fatigue strength of HVOF thermal
spray coatings in case where the layer compound is ductile and simultaneously hard enough in order to be wear resistant.
5. ACKNOWLEDGMENT
We express our gratitude to the EU for financing this work through the project SUPERCOAT, Contract No: ENK5-CT-2002-
00608.
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