1. No category

Surface and Coatings Technology 133᎐134 Ž2000. 572᎐582
Influence of a commercial electroless Ni᎐P deposit on the
fatigue properties of a notched and unnotched SAE 4140
steel
A. Pertuz a , J.A. Berrıos
´ b, E.S. Puchi Cabrerab,U
a
School of Mechanical Engineering, Faculty of Engineering, Uni¨ ersidad Central de Venezuela, Apartado Postal 47885,
Los Chaguaramos, Caracas 1045, Venezuela
b
School of Metallurgical Engineering and Materials Science, Faculty of Engineering, Uni¨ ersidad Central de Venezuela,
Apartado Postal 47885, Los Chaguaramos, Caracas 1045, Venezuela
Abstract
The effect of a commercial electroless Ni᎐P deposit on the fatigue properties of an SAE 4140 steel in the quenched and
tempered condition, has been investigated when the substrate is in a notched and an unnotched condition. The application of
such a coating to the substrate gives rise to a significant reduction of the fatigue life in comparison with the uncoated samples.
The coated specimens in the as-deposited condition showed a reduction of approximately 88%, whereas in the notched uncoated
and coated conditions it was of approximately 94᎐95%. The decrease in fatigue properties in the unnotched coated samples is
comparable to that reported for the notched uncoated specimens. It has been observed that the dominant crack responsible for
the fracture process is nucleated at the root of the notch, regardless of the presence of the deposit. A rough estimate of the
fracture toughness of the material has been determined from the results of the experiments conducted with the notched uncoated
specimens, together with the values of the critical crack length at different alternating stresses. The material constants involved in
the Paris relationship, for the description of the fatigue crack growth rate as a function of the stress intensity factor at the crack
tip, were also determined from the results obtained with the notched uncoated samples. 䊚 2000 Elsevier Science B.V. All rights
reserved.
Keywords: Electroless nickel; Fatigue; 4140 steel
1. Introduction
Electroless Ni᎐P ŽEN. deposits have been reported
to diminish severely the fatigue properties of highstrength steels w1᎐4x. Among the most recent studies
regarding this subject, Wu et al. w1x reported a reduction in the fatigue limit of a 30CrMo steel Ž0.30 C, 1.09
Cr and 0.24 Mo., in the quenched and tempered condition, of approximately 39% for the plated substrate and
U
Corresponding author. Tel.: q58-2-6628-927; fax: q58-2-7539017.
E-mail address: [email protected] ŽE.S. Puchi Cabrera..
a reduction of 20% when the substrate was previously
shot peened before plating. Also, Zhang et al. w2x,
working on the same substrate, reported that plating it
with an EN deposit reduced the fatigue limit of the
material in comparison with the unplated substrate,
although such a decrease was observed to be less
marked if the substrate was previously shot peened. In
both investigations, the low fatigue strength of the
coating was found to be responsible for the decrease in
the fatigue limit of the plated steel and that for this
type of composite material, the fatigue properties depend primarily on the fatigue resistance of the coating
itself. More recently, Garces
´ et al. w3x reported that
plating a quenched and tempered AISI 4340 steel with
0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 8 9 9 - 9
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
an EN deposit leads to a significant reduction of the
fatigue life of the material that can reach up to 92%.
The microscopic observation of the fracture surfaces of
the samples conducted in this investigation indicated
that the fatigue process was initiated at the surface of
the deposit and, subsequently, transferred to the substrate, with the assistance of the metallic bonding established at the deposit᎐substrate interface. More important, this study have concluded that the EN deposit
actually acted as a surface crack source or surface
notch which decreased the fatigue life of the coated
material by reducing the crack nucleation stage.
Thus, the present investigation has been conducted
with two different purposes. Firstly, to corroborate the
hypothesis put forward above, by comparing the fatigue
curves that are obtained in a quenched and tempered
SAE 4140 steel, both in a notched and an unnotched
condition, uncoated and coated as-deposited with a
commercial EN plating of 20 ␮m thickness and a P
content ranging approximately between 12 and 13 wt.%
deposited industrially. Secondly, to test a method for
obtaining an approximate estimate of the fracture
toughness of the substrate material as well as the
material parameters involved in the Paris relationship
for the description of the fatigue crack growth rate as a
function of the stress intensity factor.
2. Experimental techniques
The present investigation has been carried out with
samples of a SAE 4140 steel of the following composition Žwt.%.: 0.39 C, 0.75 Mn, 0.24 Si, 0.24 Cu, 0.95 Cr,
0.17 Mo and 0.15 Ni. This material is widely used in the
automotive industry, for the manufacture of connecting
rods, crankshafts, knuckles, rear axle and trailer axle
shafts. Also, in the aircraft industry it is employed for
making shapes and tubing, and in the oil industry for
the production of bits, core drills, reamer bodies, drill
collars, tool joints, piston rods and pump parts. The
material was provided as bars of approximately 14 mm
diameter and 6 m length. Such bars were cut to pieces
of approximately 120 mm length for machining tensile
specimens and of 90 mm length for machining the
fatigue samples. Both type of specimens had a gage
diameter of 6.35 mm, gage length of 12.7 mm, fillet
radius of 25.4 mm and shoulder diameter of 12.7 mm,
according to the ASTM standard E-606. The alloy was
already provided in the quenched and tempered condition. The specimens were subsequently ground with
successive SiC papers grit 80᎐1200 and polished mechanically to a mirror-like finish. Sixty fatigue samples
were notched by spark machining employing a copper
electrode, applying a potential of 45᎐50 V for 2 min.
The stress concentrators thus produced were of a hemispherical shape with a radius of 0.5 mm. In order to
573
maintain this geometry, the electrode was remachined
every three notches.
The samples to be plated were degreased in a 5%
HCl solution at 348᎐353 K for 7 min, rinsed in distilled
water, rinsed in a sodium bicarbonate Ž100 grl. solution and rinsed again in water. The EN deposition was
conducted industrially at Tecnologıas
´ Aplicadas C. A.
ŽSan Diego de los Altos, Venezuela., employing a bath
composed of 30 grl of nickel sulfate, 30 grl of sodium
hypophosphite, 35 grl of malic acid, 1.5 ppm of lead
sulfate, 10 grl of succinic acid and a stabilizer. During
deposition the pH was maintained at approximately
4.6᎐4.8, at a mean temperature of approximately 363
K. The deposition rate was of approximately 12 ␮mrh.
The deposit applied had a phosphorous content of the
order of 12᎐13 wt.% and a thickness of approximately
20 ␮m, which was corroborated by means of the ball
cratering technique ŽCalotest, CSEM. and image analysis ŽLECO 500.. The chemical analysis of the plating in
the as-deposited condition was determined by means of
SEM techniques ŽHitachi S-2400. with EDS facilities.
The observations were conducted at a constant potential of 20 kV.
Tensile tests were carried out on a computer-controlled servohydraulic machine ŽInstron 8502, USA. at
a cross head speed of 3 mmrmin. At least three
samples were employed for characterizing the monotonic mechanical properties of both the coated and
uncoated substrate. All the fatigue tests were carried
out under rotating bending conditions ŽFatigue Dynamics, RBF-200, USA. at a frequency of 50 Hz and
alternating stresses of 474, 510, 545 and 580 MPa,
which corresponded to approximately 50, 54, 58 and
62% of the yield stress of the unplated substrate. A
total of 24 samples were employed for evaluating the
fatigue properties of the material under the four different conditions investigated. The number of fatigue
samples employed to determine the fatigue life curves
fulfills the ASTM standard 739 Ž12᎐24 samples. for
reliability data required in S᎐N testing. Thus, the testing procedure employed in the present investigation
allowed a replication of more than 80%. The meaningful comparison of the fatigue life of the materials
under different conditions was possible by machining
and polishing all the specimens in order to have similar
mirror-like polished surfaces before testing.
SEM techniques were employed for the examination
of the fracture surfaces of the samples, especially regarding three important aspects of the present investigation: Ža. the role of the EN deposit in the nucleation
of cracks in the unnotched coated specimens; Žb. verification of the stress concentrator dimensions; and Žc.
estimation of the critical crack size, at each stress level,
at which the final fracture of the fatigue samples occurred.
574
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
3. Results and discussion
3.1. Characteristics of the deposit
Fig. 1 shows a detailed view of the substrate ᎐deposit
interface between the EN coating and the base steel of
one of the samples after fracture. In this picture, some
secondary cracks can also be observed along the interface, which possibly indicates a poor adhesion between
both materials. Also, it is clearly observed that the
fracture features generated during the fatigue process
are shared between the deposit and the substrate.
Some fatigue striations are noticed within the plating.
The thickness of the coating determined from the
photomicrograph, of approximately 20 ␮m, agrees with
the measurements conducted by means of the ball
cratering technique. Fig. 2 illustrates one of the EDS
analyses conducted on the deposit, which allowed to
determine a P content in the range of approximately
12᎐13 wt.%. Such results, according to Parker and
Shah w4x, would indicate the existence of a compressive
residual stress pattern within the coating. The results
reported by Wu et al. w1x and Zhang et al. w2x would
also corroborate this assumption.
3.2. Mechanical properties
The influence of the EN deposit employed in the
present study on the monotonic mechanical properties
of the composite coating᎐substrate material was evaluated by conducting a number of tensile tests with
samples both in the uncoated and coated conditions.
For the substrate material, the yield stress was found to
be approximately 817 " 6 MPa, whereas the ultimate
tensile strength ŽUTS. was found to be approximately
Fig. 1. SEM view of the substrate ŽS. ᎐deposit ŽD. interface between
the EN coating and the base steel after fracture. Secondary cracks
ŽSC. can also be observed along the interface. It is clearly noticeable
that the fracture features generated during the fatigue process are
shared between the deposit and the substrate. Some fatigue striations
ŽFS. are seen within the plating.
941 " 2 MPa. In the as-deposited condition the material had a yield stress of 819 " 2 MPa and a UTS of
939 " 2 MPa, which indicates that the deposit plated
onto the substrate does not give rise to any change
either in yield stress or in the UTS. Such a result is not
surprising given the small thickness of the coating in
relation to the diameter of the sample. As reported by
Garces
´ et al. w4x, during testing of the coated samples,
the deposits were observed to detach severely from the
substrate, indicating a poor bonding at the
substrate ᎐deposit interface and the brittle nature of
the coating.
As far as fatigue testing is concerned, the evaluation
of the monotonic mechanical properties of the material
allowed to determine a stress amplitude range of
Fig. 2. Typical EDS spectrum for the EN deposits involved in the present work.
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
Table 1
Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the
substrate samples
Table 3
Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the
coated specimens
Stress ŽMPa.
Mean
S.D.
575
Stress ŽMPa.
474
510
545
580
474
510
545
580
1 112 300
737 100
596 100
544 800
778 200
4 697 500
1 411 000
1 480 948
678 600
628 600
412 700
920 000
184 500
200 500
504 150
265 077
98 700
219 000
251 200
154 300
115 400
171 100
168 280
53 680
80 200
109 300
87 000
44 600
107 000
76 500
84 100
21 607
46 500
103 900
74 300
45 900
215 900
123 100
101 600
58 332
92 200
32 800
61 400
61 200
35 800
90 800
62 367
23 384
84 600
64 200
43 400
40 300
68 800
27 900
54 867
19 305
44 100
36 100
37 700
55 600
21 400
32 100
37 833
10 495
474᎐580 MPa to conduct the fatigue tests of both
coated and uncoated materials. Such stress interval
corresponded to a fraction of approximately 0.50᎐0.62
of the yield strength. Tables 1᎐4 present the data of
number of cycles prior to fracture Ž Nf . in terms of the
alternating stress applied to the material Ž S . for all the
conditions investigated. Fig. 3 illustrates the results
obtained from which it can be seen that at least six
tests were conducted at each alternating stress. It has
already been mentioned that this amount of samples
allowed the fulfillment of the reliability conditions prescribed in the ASTM standard E-739. Several important aspects must be discussed in relation to Fig. 3.
Firstly, the linear relationship between the alternating stress and the number of cycles to failure in a
double logarithmic scale for all the conditions analyzed
indicates the validity of the simple parametric expression of the type earlier proposed by Basquin w5x for the
description of this kind of data:
Mean
S.D.
where A and m are constants that depend on both
material properties and testing conditions; A represents the fatigue strength coefficient of the material
and m the fatigue exponent. Table 5 summarizes the
values of the parameters A and m for the four sets of
data represented in Fig. 3. Such parameters would be
of upmost importance for the design of structural components and parts that could fail by high cycle fatigue
under service. This is particularly relevant for those
parts made of high strength steels that are coated with
EN deposits either as a mean of restoring their dimensions after severe wear in service or to improve both
their corrosion or abrasive wear resistance before going
into service.
Also, it can be observed from Fig. 3 that plating an
EN deposit of these characteristics onto the 4140 substrate significantly decreases the fatigue life of the
material in relation to the uncoated steel, in spite of
the fact that the deposit is under compressive residual
stresses. At an alternating stress of 580 MPa the reduction in fatigue life reaches 68% whereas at 474 MPa it
reaches 88%. These results, are consistent with those
obtained by Wu and co-workers w1x and also by Zhang
et al. w2x regarding the decrease in the fatigue limit of
the 30CrMo steel, up to 52%, when it is plated with an
EN deposit. Similarly, the present results agree with
those reported by Garces
´ et al. w3x, for an AISI 4340
steel coated with an EN deposit, in the as-deposited
condition, for which the decrease in fatigue life varied
between 49 and 78% when the fatigue tests were
conducted at alternating stresses in the range of
663᎐590 MPa.
Table 2
Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the
uncoated notched specimens
Table 4
Mean number of cycles to failure Ž Nf . vs. stress amplitude Ž S . for the
notched coated specimens
S s ANfm
Ž MPa .
Ž1.
Stress ŽMPa.
Mean
S.D.
Stress ŽMPa.
474
510
545
580
474
510
545
580
92 100
50 300
61 900
71 700
54 100
47 100
62 287
15 367
44 600
57 200
33 800
29 000
38 700
58 800
43 683
11 182
67 900
43 000
28 300
31 500
33 500
18 800
37 167
15 492
28 400
19 500
40 700
30 500
15 600
17 500
25 367
8778
48 200
41 700
37 900
72 800
86 100
33 100
53 300
19 416
32 000
21 800
24 700
54 300
21 600
53 100
34 583
13 952
19 800
26 100
45 200
23 200
14 800
21 700
25 133
9612
20 800
16 800
19 300
15 900
39 800
23 700
22 717
8058
Mean
S.D.
576
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
Fig. 3. Mean number of cycles prior to fracture Ž Nf . as function of
the alternating stress applied to the material Ž S . for the uncoated,
uncoated notched, coated as-deposited unnotched and coated notched
specimens.
As expected, the presence of a hemispherical notch
in the uncoated samples also gives rise to a significant
reduction in the fatigue life of the substrate material
since the time required for the nucleation of the crack
is virtually suppressed. The decrease in fatigue life
varies between 69 and 94% in the alternating stress
range employed in this investigation. It is interesting to
observe that at 580 MPa the reduction in fatigue life
induced by the stress concentrator is comparable to
that found for the coated samples in the as-deposited
condition. At low alternating stresses Ž474 MPa., the
notch gives rise to a reduction in the fatigue life slightly
greater Ž6%. than that produced by the EN deposit.
These observations lead to the conclusion that the EN
deposit acts as a surface notch or stress concentrator
either because its lower mechanical properties enhance
the nucleation of cracks that are subsequently transferred to the substrate or due to the presence of cracks
within the deposit that were nucleated during its synthesis, which reach the critical size for propagation
after few cycles of loading.
If the notched samples are also coated with the EN
deposit, the fatigue life of the substrate is decreased
Table 5
Parameters involved in the Basquin relationship for the conditions
tested
Condition
A ŽMPa.
m
Substrate
Coated as-deposited
Uncoated notched
Coated notched
1119.5
1336.2
1850.9
1560.2
0.060
0.085
0.120
0.105
further, particularly at elevated alternating stresses
where such a reduction can reach up to 78%. At low
alternating stresses the decrease in fatigue life reaches
approximately 95%, which is marginally superior than
that induced by the stress concentrator alone, but
within the experimental scatter of the results. Under
low and elevated alternating stresses, the notch is the
site initiation of the dominant crack that gives rise to
the final fracture of the sample and therefore, as discussed later, the fracture surfaces of the notched specimens, both uncoated and coated, display a single crack
site initiation and a flat surface.
The above results could be interpreted more clearly
in terms of the estimated number of cycles required for
the nucleation of a fatigue crack of 0.5 mm, which at
each alternating stress would be given by the difference
between the number of cycles to fracture of either the
uncoated or coated substrate and that corresponding to
the notched uncoated specimens, which represents the
number of cycles required for the propagation of such
a crack. The relevant data, based on the mean of the
number of cycles, is reported in Table 6 and 7 and
shown graphically in Fig. 4a,b for both the uncoated
and coated substrate, respectively. As it can be observed
from Fig. 4a, the estimated number of cycles for the
nucleation of a crack of this size in the uncoated
substrate is very similar to that required for fracture,
which means that for the uncoated material, most of
the fatigue life is spent in the nucleation of the crack
rather than propagating it. According to Table 6, at an
alternating stress of 580 MPa, approximately 70% of
the fatigue life is consumed in the nucleation of the
crack, whereas at 474 MPa it takes approximately 96%.
On the contrary, for the coated material, as shown in
Fig. 4b and Table 7, most of the fatigue life is consumed in the propagation of the fatigue crack. At an
alternating stress of 580 MPa, only 40% of the fatigue
life is expended in the nucleation of the crack, whereas
at 474 MPa such a process requires approximately
47.5%. Thus, these results show clearly that by plating
the substrate material with the EN deposit, the nucleation of fatigue cracks is accelerated significantly and,
therefore, that the coating acts effectively as a notch.
Table 6
Mean crack length at fracture as a function of the alternating stress
and mean number of cycles to fracture for the uncoated notched
samples
Stress
ŽMPa.
Mean number
of cycles of
propagation Ž Np .
Mean critical
crack length
Žmm.
KIc
ŽMPa
m1r 2 .
474
510
545
580
62 867
43 683
37 167
25 367
3.69
3.49
3.32
3.15
62.0
61.9
62.0
61.9
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
577
Table 7
Mean number of cycles to fracture for the coated notched samples
Stress, ŽMPa.
Mean of total
number of cycles
Mean number of
cycles of propagation
Mean number of
cycles of nucleation
474
510
545
580
101600
62367
54867
37833
53300
34583
25133
22717
48300
27784
29734
15116
3.3. Fracture toughness of the substrate
⌬ Ks f Ž a. ⌬ S'␲ a
Ž2.
which, at fracture can be simply expressed as:
The fracture toughness of the substrate material can
be estimated from the classical expression that relates
the stress intensity factor, at the crack tip, ⌬ K, with
stress applied, ⌬ S, and the crack length, a:
K Ic s f Ž a c . Smax ␲ a c
'
In the above equations, f Ž a. represents a geometrical factor that depends on the applied loads and
geometry of the body and crack. However, due to the
complexities of the problems, exact solutions for edge
cracks in rods under bending are not available w6x. For
example, according to Toribio and co-workers w7x, for a
hemispherical notch in a solid bar subjected to bending:
f Ž a. s 0.821y 0.486
Fig. 4. Ža. Estimated number of cycles for the nucleation and propagation of a crack of 0.5 mm in the uncoated substrate. Žb. Estimated
number of cycles for the nucleation and propagation of a crack of 0.5
mm in the coated substrate.
Ž3.
ž da / q 2.003 ž da /
2
Ž4.
where d represents the diameter of the bar. Si w6x has
also proposed a solution for determining f Ž a. by combining selected solutions for curved and straight fronted
cracks previously published. In this work, f Ž a. is given
in a table as a function of the ratio ard. Particularly,
for 0 F ardF 0.6, it is reported that 0.74F f Ž a. F 1.5.
Thus, by measuring experimentally the critical crack
length from the fracture surfaces of the samples it is
possible to estimate the K Ic of the material. The SEM
observations conducted on the fracture surfaces of the
uncoated and coated notched samples, tested at different alternating stresses, allowed to measure the
mean critical crack length as a function of the stress
applied and the mean number of cycles to fracture, as
shown in Table 6. Such data, together with Eqs. Ž3. and
Ž4., yielded an estimation of the fracture toughness of
the substrate of approximately 62 MPa m1r2 . Such a
result agrees reasonably well with the value reported by
Le May and Shaw w8x for this material of 66 MPa m1r2 ,
after tempering at 673 K. In order to determine the
fracture toughness of the material by means of any of
the standard test methods developed for this purpose
w9x, it is necessary to specify the specimen dimensions
on the basis of an approximate value for K Ic , in order
to fulfill the condition that such dimensions must be
sufficiently large in comparison with the plastic zone
dimensions at the crack tip. Therefore, it is concluded
that the present experimental approach could be em-
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
578
ployed for determining satisfactorily a reasonable estimate of K Ic .
3.4. Crack propagation rate
The early work conducted by Paris and Erdogan w10x
showed that the rate of crack propagation is related to
the stress intensity factor by means of a simple parametric equation of the form:
da
p
sCŽ⌬ K .
dN
Ž5.
where C and p represent material properties. Forman
and co-workers w11x, in an attempt to improve the
correlation between the crack growth rate and ⌬ K,
proposed an alternative relationship of the form:
r
da
BŽ⌬ K .
s
dN
Ž 1 y R1 . K Ic y ⌬ K
Ž6.
where, again, B and r represent material parameters,
R1 the load ratio and K Ic the fracture toughness of the
material. The test method for conducting fatigue crack
growth measurements is fully described in the ASTM E
647-88a standard and basically it consists in growing
the crack by cyclic loading and monitoring K min , K max
and the crack length throughout the test. The test
fixtures and specimens design are similar to those employed for fracture toughness testing. However, a rough
estimate of the materials constants involved in Eqs. Ž5.
and Ž6. can be obtained from the fatigue experiments
conducted with the uncoated notched and unnotched
samples. As it has been shown before, due to the stress
concentrator in these specimens, the number of cycles
to fracture is approximately equal to the number of
cycles for the propagation of the crack. Thus, by considering that under rotating bending conditions the
propagation of the crack occurs due to the action of
the maximum tensile stress at the crack tip, and therefore ⌬ Kf f Ž a. Smax Ž ␲a.1r2 , Eq. Ž5. can be combined
with Eq. Ž4. and integrated directly to give:
Np s
1
C
ac
da
0
p
Ž f Ž a. '␲ a .
Ha
Syp
Ž7.
where Np represents the number of cycles during the
propagation of the crack, a0 the initial crack length Ž0.5
mm., ac the critical crack length at fracture and S the
maximum alternating stress applied to the specimen
during testing. The data presented in Table 6 can be
employed to determine the value of the constants C
and p by means of non-linear regression analysis.
Thus, by defining the error sum of squares as:
Fig. 5. Ža. Typical fracture surface of an uncoated notched sample
tested at 474 MPa. The radial lines ŽRL. indicate that the crack
started to propagate from the notch root ŽN.. Žb. Detailed view of the
previous picture at the notch. Some of the radial lines actually
correspond to secondary cracks ŽSC. that were also nucleated at this
site.
N
␸s
Ý
is1
½
1
Np i y
C
2
ac i
Ha
0
da
Ž f Ž a. '␲ a .
p
Syp
i
5
Ž8.
and solving the system of equations derived from the
condition that:
⭸␸
s0
⭸C
and
⭸␸
s0
⭸p
Ž9.
In Eq. Ž8. N represents the number of experimental
data available. Thus, it has been determined that for
the substrate material C s 3.15= 10y1 1 mmrcycle and
ps 4.22. In the above equations, ac is input in mm and
S in MPa. If the solution for f Ž a. proposed by Si w6x is
employed instead of that of Toribio and co-workers w7x,
it is obtained that C s 1.56= 10y1 0 mmrcycle and
p s 4.24. Such values are very close to those reported
by Lemaitre and Chaboche w12x for similar materials.
3.5. Fracture surfaces of the samples
Several specimens tested at different alternating
stresses were examined after failure by SEM in order
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
Fig. 6. Ža. General fracture surface of a coated sample tested at 474
MPa showing the origin ŽO. of the crack. Žb. Closer analysis of the
deposit illustrating the presence of fatigue striations ŽFS. within it.
to verify the size of the notch machined, to measure
the crack length at fracture and also to study more
closely the sites of crack initiation, particularly in the
coated specimens. For example, Fig. 5a illustrates a
photomicrograph of a typical fracture surface of an
uncoated notched sample tested at 474 MPa. As expected, the radial lines indicate that the crack started
to propagate from the notch root. The photomicrograph in Fig. 5b, which represents a more detailed view
of the previous picture at the notch, indicates that
some of the radial lines actually correspond to secondary cracks that were also nucleated at this site.
The photomicrograph of Fig. 6a illustrates the general fracture surface of a coated sample tested at 474
MPa in which the origin of the crack is also well
defined. A close analysis of the deposit, Fig. 6b, allowed
to determine fatigue striations within it, which confirm
our view in the sense that the EN deposit is able to
develop fatigue cracks that subsequently are transferred to the substrate. Fig. 7a depicts the general
fracture surface of a notched and coated sample, tested
at 474 MPa. As for the uncoated specimens, it is
observed that the fracture process occurs as a consequence of the propagation of the crack nucleated at
the notch, giving rise to a flat fracture surface. Fig. 7b
illustrates a detailed view of the notch root from which
579
Fig. 7. Ža. General fracture surface of a sample coated and notched,
tested at 474 MPa showing that the fracture process occurs as a
consequence of the propagation of the crack nucleated at the notch
ŽN., giving rise to a flat fracture surface. Žb. Detailed view of the
notch root showing the nucleation of several secondary cracks ŽSC..
several secondary cracks were also observed to originate. On the other hand, Fig. 8 illustrates a detailed
view of Fig. 1, particularly of the fatigue striations that
were observed within the EN deposit. These observations agree with the results previously reported by
Zhang et al. w2x who were able to find fatigue markings
within the EN deposits plated onto similar high strength
steels as substrates.
Fig. 8. Detailed view of Fig. 1, showing fatigue striations ŽFS. within
the EN deposit ŽD..
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A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
Finally, Fig. 9a shows the general fracture surface
typically observed in the notched and coated samples
tested at 580 MPa. At elevated alternating stresses it
can be seen that the morphology of the fracture surface is quite similar to that observed at low stresses. It
can be clearly noticed that the main crack was nucleated at the notch root.
Fig. 9b illustrates a closer look of the notch root,
indicating a number of fracture steps which characterize the transcrystalline propagation of the fatigue crack.
Fig. 9. Ža. General fracture surface of a notched and coated sample tested at 580 MPa. The main crack was nucleated at the notch ŽN. root. Žb.
Closer look of the notch root indicating a number of fracture steps ŽFST. which characterize the transcrystalline propagation of the fatigue crack.
At the inner surface of the notch, a number of nodules ŽND. are observed to be present. Žc. Magnified view of zone A in Ža., illustrating the
partial detachment of the deposit from the substrate through secondary cracking ŽSC.. Fatigue striations ŽFS. within the coating and fatigue
marks ŽFM. on the substrate are also clearly visible. Žd. Magnified view of zone B in Ža., showing fatigue marks on the substrate. Most of the
substrate ᎐deposit interface is observed to remain relatively free of secondary cracks.
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
At the inner surface of the notch, a number of nodules
formed during the synthesis of the deposit are observed
to be present, which give rise to an ‘orange-peeling’
type of surface finish all over the surface of the coated
specimens. Hardness indentations conducted on such
nodules allowed the determination of their solid nature. The zone on the left of the notch surface, designated as A, has been magnified in Fig. 9c. Here, the
partial detachment of the deposit from the substrate
through secondary cracking can be clearly observed, as
well as fatigue striations within the coating and fatigue
marks on the substrate. The zone on the top of the
notch surface, designated as B, has also been magnified
in Fig. 9d where fatigue marks are clearly visible on the
substrate. In this area, most of the substrate ᎐deposit
interface is observed to remain relatively free of secondary cracks.
Thus, the present results corroborate those previously discussed regarding the fatigue tests conducted
on coated and uncoated samples in the sense that EN
deposits are bound to undergo fatigue failure before
the substrate and to allow the transference of the
fatigue cracks to it, giving rise to a reduction in its
fatigue strength, particularly if the substrate has better
mechanical properties than the deposit, as in the present case. Such a detrimental effect is almost comparable to the presence of a notch or stress concentrator
from which the fatigue cracks nucleate and propagate.
However, in the case of the coated materials the situation could be even worse than in the uncoated notched
samples since it has been observed that at elevated
stresses the presence of the deposit could accelerate
the nucleation of the cracks that lead to the final
fracture of the specimen.
4. Conclusions
A number of fatigue experiments conducted on samples of a quenched and tempered SAE 4140 steel
showed that plating this material with an EN deposit
led to a significant reduction in the fatigue life. It has
been determined that if the coating is in the as-deposited condition, at a stress amplitude of 580 MPa,
such a reduction can reach up to approximately 68%,
whereas at 474 MPa it could reach up to approximately
88%. The experiments conducted on the uncoated
notched samples revealed that the presence of a stress
concentrator of 0.5 mm in length, gave rise to a reduction in fatigue life that varied between approximately
69 and 94% depending on the alternating stress applied
to the material. This decrease in fatigue properties is
comparable to that reported for the coated specimens,
which suggest that the EN deposit effectively acts as a
surface fatigue crack source, causing a significant decrease in the fatigue properties of the substrate mate-
581
rial. If the notched samples are further coated, then at
elevated alternating stresses the reduction in fatigue
life is increased up to approximately 78%, whereas at
low stresses it remains at a similar value to that
observed for the notched and uncoated specimens. The
analysis of the fracture surfaces of the coated and
uncoated notched samples tested, revealed that at low
alternating stresses, the dominant crack responsible for
the fracture process is nucleated at the root of the
notch, regardless of the presence of the deposit. Such
study also revealed clear evidence of fatigue striations
within the deposit and the continuity of certain fracture
features between coating and substrate. Therefore, it
has been concluded that the decrease in fatigue life in
the as-deposited samples occurs as a result of the
passage of fatigue cracks from the coating to the substrate. Extensive secondary cracking along the
coating᎐substrate interface has been revealed after
fatigue testing, which indicates that the adhesion of the
EN deposit is somewhat poor and therefore the interface is not able to sustain the stresses applied to the
material. The experiments conducted with the notched
and uncoated specimens, together with the experimental measurement of the critical crack length at different
alternating stresses, allowed to conduct a rough estimation of the fracture toughness of the substrate material
which agrees satisfactorily with the values reported in
the literature for this important property. Also, based
on the information provided by the results obtained
from the notched samples, a non-linear regression
method has been presented by means of which it has
been possible to estimate satisfactorily the parameters
that enter in the Paris relationship for the description
of the crack growth rate of the substrate, in terms of
the stress intensity factor at the crack tip. The two
methods advanced here, which can be easily implemented, can provide both useful and reliable information about the fracture properties of the material being
tested before conducting the more elaborated procedures already standardized.
Acknowledgements
This investigation has been conducted with the financial support of the Venezuelan National Council
for Scientific and Technological Research ŽCONICIT.
through the project LAB-97000644 and the Scientific
and Humanistic Development Council of the Central
University of Venezuela ŽCDCH-UCV. through the
project 08-17-4595-2000. The authors would like to
acknowledge the assistance of Mr E. Batoni and B.
Lozada in the conduction of the experimental work.
J.A. Berrıos
´ is deeply grateful to the School of Mechanical Engineering, Faculty of Engineering and Architecture of the University of El Salvador.
582
A. Pertuz et al. r Surface and Coatings Technology 133᎐134 (2000) 572᎐582
References
w1x Y.Y. Wu, Y.Z. Zhang, M. Yao, Plating Surf. Finish. Ž1995.
83᎐85.
w2x Y.Z. Zhang, Y.Y. Wu, M. Yao, J. Mater. Sci. Lett. 15 Ž1996.
1364᎐1366.
w3x Y. Garces,
J. Berrıos,
´ H. Sanchez,
´
´ A. Pertuz, J. Chitty, H.
Hintermann, E.S. Puchi, Thin Solid Films 355r356 Ž1999.
487᎐493.
w4x K. Parker, H. Shah, Plating 58 Ž1971. 230.
w5x O.H. Basquin, Proc. ASTM 10 Ž2. Ž1910. 625.
w6x E. Si, Eng. Fract. Mech. 37 Ž4. Ž1990. 805.
w7x J. Toribio, V. Sanchez,
M. Astiz, J. Campos, J. Eng. Fract.
´
Mech. 39 Ž1991. 357.
w8x I. Le May, J.D. Shaw, in: J. Backlund, A.F. Blom, C.J. Beevers
ŽEds.., Fatigue Thresholds: Fundamentals and Engineering Applications, v噛ol. II, Engineering Materials Advisory Services
Ltd, The Chameleon Press Ltd, London, UK, 1982, p. 603.
w9x E 399-83, Standard test method for plane᎐strain fracture
toughness of metallic materials, American Society for Testing
and Materials, Philadelphia, 1983.
w10x P.C. Paris, F. Erdogan, J. Basic Eng. Trans. ASME Ser. D 85
Ž4. Ž1963. 528.
w11x R.G. Forman, V.E. Kearny, R.M. Engle, J. Basic Eng. Trans.
ASME 89 Ž1967. 459.
w12x J. Leimatre, J.L. Chaboche, Mechanics of Solid Materials,
Cambridge University Press, New York, 1994, p. 484.