Materials and Design 36 (2012) 511–521

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Materials and Design

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Damage assessment of titania filled zinc–aluminum alloy metal matrix composites

in erosive environment: A comparative study
Amar Patnaik a,⇑, T.G. Mamatha b, Sandhyarani Biswas c, Predeep Kumar a
a
Department of Mechanical Engineering, N.I.T., Hamirpur, Himachal Pradesh, India
b
Department of Manufacturing Technology, JSSATE, Noida, Uttar Pradesh, India
c
Department of Mechanical Engineering, N.I.T., Rourkela, Orissa, India

a r t i c l e i n f o a b s t r a c t

Article history: The present research work, describes the development of particulate filled alloy composites consisting of
Received 20 August 2011 ZA-27 alloy as a base material and titania particulate as reinforcing materials. The objective of the present
Accepted 22 November 2011 work is to study the erosion wear behavior of particulate filled alloy composites by Taguchi experimental
Available online 1 December 2011
design technique at four different impact velocities (25–79 m/s), erodent temperature (40–100 °C),
impingement angles (30–90°) and erodent size (250–550 lm) respectively. A finite element simulation
Keywords: model (ANSYS/AUTODYN) for damage assessment in erosion is developed and validated by a well
Non-ferrous metals and alloys
designed set of experiments. It is observed that the experimental results are in good agreement with
Wear
Mechanical
the computational results and the proposed simulation model is very useful for damage assessment.
However, a series of steady state erosion analysis is also conducted by varying the impingement angle
and impact velocity one at a time while keeping other factors remains constant before studying the
Taguchi experimental results. From this analysis it is observed that the peak erosion rate occur at about
60° impingement angle at constant impact velocity (66 m/s), erodent size (450 lm), stand-off-distance
(75 mm) and erodent temperature (60 °C) for all the composites. However, the steady state erosion rate
as a function of impact velocity for unfilled composite shows maximum erosion rate as compared to filled
composites at constant impingement angle (45°), erodent size (450 lm), stand-off-distance (75 mm) and
erodent temperature (60 °C) respectively. Finally, the damage mechanisms of eroded surfaces are inves-
tigated using scanning electron microscope (SEM).
Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction properties at high temperature (150–800 °C), ceramic or metal par-

ticulates can be reinforced. However, many researchers have
Owing to remarkable tribo-mechanical, physical and mechani- reported that Zn–Al alloy composites by inserting second phase
cal properties like enhanced strength, service temperatures, hard particles increased the mechanical characteristics and wear
improved wear resistance, high toughness, excellent fluidity, cast- properties particularly at higher speeds and loads. It has also been
ability, etc. Zinc–aluminum (ZA) alloys become commercially via- suggested by few of researchers that reinforcing of hard second
ble material for versatile engineering applications [1]. The phase particulates to the base matrix material is more useful re-
prominent types available in ZA-casting alloys are: ZA-8, ZA-12 lated to sliding wear, as wear requiring better thermal stability
and ZA-27. Among types, ZA-27 is classified as high strength alloy and load bearing capacity as compared to the unfilled Zn–Al alloys
with tensile strength substantially higher than that of ordinary cast [6–8]. Bobic et al. [9] studied the effect of Al2O3 particles on micro-
aluminum alloys. The cost reduction from 25–50% is observed structure and strength of ZA-27 based composites. The composite
when ZA alloys are used in place of aluminum and 40–75% when exhibits higher strength than the as-cast alloy and the strengthen-
replacing brass alloys respectively [2–4]. This alloy can be effec- ing effect of smaller size Al2O3 particles is more pronounced than
tively utilized for bearings and bushing applications, as a replace- the larger particles size. Sharma et al. [10] studied the effect of zir-
ment for bronze bearings because of its low cost and equivalent con particles on the mechanical properties and fractography of
or superior bearing performance [5]. Hence, this material shows ZA-27 alloy composites and observed that the fracture behavior
poor mechanical properties in high temperature environment con- of the composites was altered significantly by the reinforcement
ditions. In order to improve their physical and mechanical of the zircon particles and the reinforcing particles resulted in
the final fracture. Seah et al. [11] studied the effect of graphite par-
ticulate on wear characteristics of as-cast ZA-27 composites. The
⇑ Corresponding author. Tel.: +91 1972 204746; fax: +91 1972 22243.
wear test results revealed that the wear rate of the composites
E-mail addresses: amarp@nitham.ac.in, amar_mech@sify.com (A. Patnaik).

0261-3069/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.matdes.2011.11.054
512 A. Patnaik et al. / Materials and Design 36 (2012) 511–521

decreased with the increase in graphite filler content and the wear 2.3. Experimental design
rate increased with the increase in wear load and wear speed. Sim-
ilarly, Prasad and Das [12] studied the significance of the matrix Classical experimental approaches are complex and difficult to
microstructure on solid lubricant characteristics of graphite filled perform real experiments with entire accuracy. Therefore, in such
aluminum alloy composites and suggests that the matrix micro- methods a large number of experiments have to be carried out to
structure plays a major role in the solid lubrication characteristics get the valid and accurate results [14,15]. In the present research
of graphite filled aluminum alloys. Although a great deal of work work, Taguchi design of experiment has been used to get experi-
has been made on the advantage of reinforcement of hard second mental results accurately. With this, powerful tool/approach
phase particle to the base matrix material. However, erosion wear effective parameter design were obtained so that composite sam-
behavior of particulate filled Zn–Al alloy composites for erosive ples were effectively evaluated for their better erosion wear resis-
environment is hardly been found. tance. The Table 2, presents the operating conditions under which
To this end, the present research work is undertaken to study erosion tests were evaluated following Taguchi L16 (45) orthogo-
the erosion wear behavior of titania (TiO2) filled ZA-27 composites. nal array design (shown in Table 3). The every column of Table
A Taguchi design for the experiments has been used to evaluate 3, presents a test parameter whereas, each row present testing
erosive data in a controlled way. Hence, the individual effect of de- conditions i.e. combination of parameter levels. In full factorial
signed parameters on the wear behavior has been established sys- experiment design, it would require 45 = 1024 number of runs
tematically and compared with the proposed non-linear finite to study five factors each at four levels whereas, Taguchi’s facto-
element model (ANSYS/AUTODYN) results for the validation rial experiment approach reduces the same to 16 runs only,
purposes. thereby offering a great advantage in terms of experimental time
and cost.
2. Experimental details The plan of the experiment as per Taguchi L16 (45) orthogonal
array design are: the first column is assigned to impact velocity,
2.1. Preparation of the composites the second column to TiO2 content, the third column to erodent
temperature, the forth column to impingement angle and the
In the presented work, TiO2 particulate (size of 80 lm) filled (0, fifth column to erodent size at constant stand-off distance
4, 8 and 12 wt.%) ZA-27 alloy (having elemental composition (75 mm) for all the test runs to estimate the erosion rate. There
according to ASTM B669-82; Table 1) matrix composite material are several S/N ratios available depending on the type of perfor-
were fabricated following stir-casting technique. Initially, ZA-27 mance characteristics required. For the better erosion wear per-
alloy were super-heated above its melting point (>500 °C) in a formance smaller-the-better characteristic S/N ratio has been
graphite crucible using muffle furnace, thereafter particulates are adopted and computed as logarithmic transformation of loss
added and stirred via mechanical stainless-steel stirrer having function (Eq. (1)).
aluminite coating. The stirring were carried at 750 rpm almost S 1 
for 2–3 min. to achieve mechanical isotropy while mixing. Thereaf- Smaller-the-better characteristic : ¼ 10 log Ry2i Þ ð1Þ
N n
ter, molten-mixer was poured into permanent molds and gradually
whereas, ‘n’ is the number of experiments in the L16 orthogonal ar-
cools down. The prepared samples were then sized as per the spec-
ray design and yi is the measured values (i.e. erosion wear rate).
imen size and the surface is grounded/polished with fine emery pa-
per, thereafter examined for various characterization like
2.4. Materials modeling
mechanical, physical, erosion behavior and surface morphology.
The worn surface morphology is examined via SEM using ZEISS-
2.4.1. Johnson–Cook plasticity model
EVO 40 apparatus with EDAX attachment.
The Johnson–Cook (JC) plasticity visco-plastic model is a strain
rate dependent plasticity model. It is successfully used in various
2.2. Erosion test apparatus
problems where strain-rate changes repeatedly over a larger range.
Therefore, in the present work, Johnson–Cook model is used for the
The solid particle erosion experiments were carried out on ero-
prediction of erosion wear resistance of the isotropic material and
sion test rig following ASTM G-76 [13]. The test rig is capable of
is more suitable for the proposed material (TiO2 filled ZA-27 alloy
creating reproducible erosive environment in order to evaluate
composite) as they undergoes extensive plastic deformation. The
the erosion wear performance of the investigated composites. For
simulated results are in agreement with the experimental
this experimental work, dry silica sand of particle size (250, 350,
observations.
450 and 600 lm) are used as erodent. The erodent temperature
The Johnson–Cook and Grüneisen equation of state are used to
(40, 60, 80 and 100 °C) could be maintained via inbuilt heater.
model the particulate filled ZA-27 alloy composite [16,17]. The John-
The different impingement angle of erodent could be achieved by
son–Cook (JC) model is purely empirical and gives the following
holding the specimen at different angles with-respect-to the direc-
relation for the flow stress (ry):
tion of erodent flow via swivel and an adjustable sample clip. After
each experimental run, every sample is cleaned with cotton dipped
in acetone to remove any attached erodent particle with specimen
surface and then dried for few minutes. Thereafter weighted using
an electronic balance (Denver Instruments Germany, TB-2150) Table 2
with an accuracy of ±0.1 mg) and further weight loss and erosion Levels of the variables used in the experiment.
rate are calculated. Control factor Level Units
I II III IV
Table 1 Impact velocity 25 43 66 79 m/s
Chemical composition of Za-27 alloy in weight percent (ASTM B669-82). Alumina content 0 4 8 12 %
Erodent temperature 40 60 80 100 °C
Element Aluminum Magnesium Copper Zinc
Impingement angle 30 45 60 90 °
Percentage composition (wt.%) 25–28 0.01–0.02 2.0–2.5 Balance Erodent size 250 350 450 550 lm
 A. Patnaik et al. / Materials and Design 36 (2012) 511–521 513

Table 3
Taguchi L16 (45) orthogonal array design.

Sl No. Impact velocity (m/s) Filler content (%) Erodent temperature (°C) Impingement angle (°) Erodent size (lm)
1 1 1 1 1 1
2 1 2 2 2 2
3 1 3 3 3 3
4 1 4 4 4 4
5 2 1 2 3 4
6 2 2 1 4 3
7 2 3 4 1 2
8 2 4 3 2 1
9 3 1 3 4 2
10 3 2 4 3 1
11 3 3 1 2 4
12 3 4 2 1 3
13 4 1 4 2 3
14 4 2 3 1 4
15 4 3 2 4 1
16 4 4 1 3 2

 
ry ¼ ðA þ Bðep Þn Þð1 þ C  ln e_  Þð1  T  Þm ð2Þ shock velocity, Vp is the particle velocity and l ¼ qq0 ; 1 is the rela-
tive change of volume.
where A, B, C, n, and m are the material constants
ep ¼ effective plastic strain rate and 2.4.3. Finite Element (FE) model
In the present research work, a finite-element-model has been
e_ p developed that actually simulates the solid particle impacts on
e_  ¼ _ the target material via explicit dynamic code ANSYS/AUTODYN
e0
and simultaneously facilitates computation of the erosive wear
T  T room rate. The geometrical modeling was performed using Pro/Engineer
T  ¼ Homologous temperature ¼ ð3Þ software tool and then exported in ANSYS Design modeler in ‘IGES’
T melt  T room
format. Thereafter, it is meshed using meshing module by choosing
The strain energy at fracture is given by: explicit environment at ANSYS workbench platform. The explicit
hex meshing was done which consist of 113,760 nodes and
ef ¼ ½D1 þ D2 exp D3 r ½1 þ D4 ln e_  ½1 þ D5 T   ð4Þ
99,224 finite elements. The solid particles were modeled using
⁄
where Di = 1, . . .5 are the input parameters and r is the ratio of Smooth Particle Hydrodynamics (SPH) with the properties of silica
pressure divided by effective stress: sand. The material model applied SPH sand particles were modeled
p using compaction Equation-of-State (EOS), Granular strength, and
r ¼ ð5Þ Hydro (Pmin) failure material model. In this study, 50 spherical sand
reff
SPH particles were used to ensure the accuracy of the model. All of
Fracture occurs when the damage parameter D ¼ R Deef p reaches

the sand particles strike the target area at random locations. All the
the value of 1. bottom nodes of the target material were fixed. The rotation degree
The target material has been isolated by small elements for the of freedom has constrained for all the particles. ElTobgy et al. [18]
explicit dynamic finite element method and through the dynamic reported that simulating a single particle was insufficient and
finite element method, the RDepi of each element is calculated. therefore three or more particles were needed to simulate the
But in case of arbitrary element, say i during each time step, the
damage parameter Di = 1. Then the element i was marked as a fail-
ure and subsequently removed from the model.

2.4.2. Grüneisen (Shock) equation of state

The Mie-Grüneisen (Shock) equation of state is used to simulate
the shock wave effect at the time of erosion test run and produced
by high velocity solid particle erosion test for target materials. This
equation of state generally defines the pressure–volume-relation-
ship in one or two different ways, depending on the composite
material is compressed and expended. Therefore, in this work,
the Mie-Gruneisen equation of state with cubic shock velocity as
a function of particle impact velocity defines pressure for com-
pressed material as:
   
q0 C 2 l 1 þ 1  c20 l  2a l2
ph
2 3
i2 þ ðc0 þ alÞE ð6Þ
1  ðS1  1Þl  S2 llþ1  S3 ðllþ1Þ2

where C is velocity curve intercept (Vs–Vp curve); S1, S2, and S3 are
the coefficients of the slopes of the Vs–Vp curve, c0 is the initial
Gruneisen gamma, a is the first order volume-correction to c0, q Fig. 1. Variation of erosion rate with impingement angle for the TiO2 filled ZA-27
is the current density of the composites, q0 is the initial density composites (At constant impact velocity: 66 m/s, erodent size: 450 lm, stand-off
of the composite material, p is the hydrostatic pressure, Vs is the distance: 75 mm, erodent temperature: 60 °C).
514 A. Patnaik et al. / Materials and Design 36 (2012) 511–521

erosion process. In this analysis, there were 216 particles used and erodent temperature: 60 °C for TiO2 filled ZA-27 alloy composites.
the particles are grouped in 12 planes and each plane consists of 18 Tilley [20] studied for aluminum alloys and suggested that the
particles. According to Woytowitz and Richman [19], the distance peak rate is occurs around at 15°. Similarly for Ti-6AlAV the peak
between any two alternative particles centers in the same group rates occur in between 30° and 39° [21], while for super-alloys
is no less than 0.6r to avoid the damage interaction (where r is the peak erosion rates occur around 40°–45° under high velocity
the radius of the erodent particles). The erosion rate is defined as impact conditions [21,22]. In the present study, all the TiO2 filled
the ratio of cumulative mass loss of target materials and the and unfilled Zn–Al alloy composites shows maximum erosion rate
amount of erodent used during each experimental test. occurring at 60° impingement angle as shown in Fig. 1. Similar,
observations was also observed by Nicholls and Stephenson [23]
3. Results and discussion for the titanium alloy by using Monte Carlo modeling of erosion
processes and the maximum erosion rate at 60°. At 60° impinge-
3.1. Erosion wear characteristic of the composites ment angle, the erosion rate variation is marginal among the par-
ticulate filled composites and 8 wt.% TiO2 filled ZA-27 alloy
3.1.1. Influence of impingement angle on erosion wear rate composite which shows least erosion rate as compared with other
In the present study, the steady state erosion rate is estimated particulate filled composites (Fig. 1). Similar, observations are also
as a function of impingement angle under constant impact veloc- obtained in case of finite element simulated results for all the com-
ity: 66 m/s, erodent size: 450 lm, stand-off distance: 75 mm, posites as shown in Fig. 1. The difference between experimental

(a)

Highly stressed
regions

Elements failure
regions

(b)

Sliding particles
Embedded particle

Fig. 2ab. Von Mises equivalent stress distribution on ZA-27 with 4 wt.% TiO2 for 45° target material: (i) plastic deformation and element failure region and (ii) sliding
particles after impact.
 A. Patnaik et al. / Materials and Design 36 (2012) 511–521 515

(c)

Highly stressed region

Eroded
elements

(d)

Embedded
particles
Sliding particles

Fig. 2cd. Von Mises equivalent stress distribution on ZA-27 with 8 wt.%% TiO2 for 60° target material: (i) plastic deformation and element failure region and (ii) sliding
particles after impact.

results and finite element simulated results following same trends. yield stress, because this region the material absorbed total energy
Fig. 2 shows Von Mises stress distribution for particulate filled of the particle, but when multi particles are impacted, the elements
composites as a function of the impingement angle. Fig. 2ab show of highly stressed region removed and new surfaces are formed.
the maximum erosion rate at about 45° impingement angle for This may be due to the high erosion rate is predicted at 60°
4 wt.% TiO2 filled ZA-27 alloy composite and causes severe plastic impingement angle. There may be another reason that the im-
deformation within the target region. It is clearly visible from the pacted energy mostly converted it into internal energy (ElTobgy
above Fig. 2ab that at lower impingement angle the erodent parti- et al. [18]). However, the single particle erosion analysis was not
cles are rubbing the composite surface and embedded the compos- sufficient to get accurate erosion rate [18]. Therefore, ElTobgy
ite surface instead of bouncing back from the composite surface. et al. [18] subsequently increased the number of impacted particle
However, a change in impingement angle from 45° to 60° and filler from one to three or more to simulate the erosion process success-
content (8 wt.% TiO2) the particle rubbing action automatically re- fully. Similarly, for 12 wt.% TiO2 filled ZA-27 alloy composite has
duced and the erosion rate starts increasing as shown in Fig. 2cd. been studied with similar impingement angle (60) and the eroded
This may be due to when first few erodent particles impacted on surfaces are shown in Fig. 2ef. From Fig. 2ef shown that the internal
the composite, initiates highly stressed region generated up to a energy causes element failure in the composites and kinetic energy
516 A. Patnaik et al. / Materials and Design 36 (2012) 511–521

(e)

Highly
stressed
region

Elements failure
regions

(f)

Embedded
particle

Sliding particles

Fig. 2ef. Von Mises equivalent stress distribution on ZA-27 with 12 wt.% TiO2 for 60° target material: (i) plastic deformation and element failure region and (ii) sliding
particles after impact.

generated due to rebound/reback from the target material and the to the impact velocity are shown in Fig. 3. Many investigators have
flow of Von-Mises stresses changes with each cycle. Therefore, reported that the erosion wear is a strong function of the erodent
from this analysis it is clear that filler content and types of filler impact velocity hence a small decrease in the impact velocity
also play major role in determining the erosion rate of composites may result in large reduction in the wear [24–30]. It is seen from
for tribological applications. the figure that the erosion rate of the composites are lower than
that of the unfilled ZA-27 alloy with similar experimental condi-
3.1.2. Influence of impact velocity on erosion wear rate tions. This trend reduces the mean impact velocity of bigger parti-
The steady-state erosion rate are measured at four different cles and thus, reduction in wear increases with increase in fine
velocities (i.e. 25–79 m/s), which enables the determination of particles size in the base materials. The erosion rate of all the com-
one of the major control factor i.e. impact velocity dependence posites with in 25–43 m/s impact velocity show least effect but at
on the erosion rate. The experimental results of the erosion rate higher impact velocity the erosion rate increases monotonically
for the particulate filled ZA-27 alloy composite materials related with the increase in impact velocity (Fig. 3). Similar observation
 A. Patnaik et al. / Materials and Design 36 (2012) 511–521 517

flows. This could also be one of the reasons for wear reduction
due to reinforcement of fine particles in the matrix material.

3.2. Design of experiment based on Taguchi method

In Table 4, the eighth columns represents S/N ratio of the wear

rate which is in fact the average of two replications of composites.
The overall mean for the S/N ratio of the wear rate is found to be
84.70 db by using the software MINITAB 15. Analysis of the exper-
imental result leads to the conclusion that the factor combination
gives minimum erosion rate for TiO2 filled ZA-27 alloy composites
as shown in Fig. 4. This analysis establishes that erosion rate also
depends upon the types of filler, filler content and fabrication
techniques.

3.3. Surface morphology and EDAX characterization

Eroded surfaces of the unfilled ZA-27 alloy composites are

shown in Fig. 5. The surfaces are observed to be smooth with shal-
Fig. 3. Variation of erosion rate with impact velocity for the TiO2 filled ZA-27
composites (At constant impingement angle: 45°, erodent size: 450 lm, stand-off low wear grooves at low impact velocity (Fig. 5a) (See Table 4,
distance: 75 mm, erodent temperature: 60 °C). Experiment No. 1). Seizure of the specimens led to a considerably
increased extent of surface damage (Fig. 5b) (See Table 4, Experi-
ment No. 5). However, an increase in impact velocity from
was also reported by Bain and Bonnington [25] that the presence of 25–43 m/s for particulate filled composite, the wear surfaces of
fine particles helps in reducing the turbulence in solid–liquid the composite shows formation of grooves along with patches of

Table 4
Experimental design using L16 orthogonal array.

Sl No. Impact velocity (m/s) Filler content (%) Erodent temperature (°C) Impingement angle (°) Erodent size (lm) Erosion rate (gm/gm) S/N ratio (db)
1 25 0 40 30 250 0.0000128 97.851
2 25 4 60 45 350 0.0000014 117.056
3 25 8 80 60 450 0.0000021 113.534
4 25 12 100 90 550 0.0000175 95.117
5 43 0 60 60 550 0.0000725 82.798
6 43 4 40 90 450 0.0000730 82.736
7 43 8 100 30 350 0.0000693 83.186
8 43 12 80 45 250 0.0000939 80.550
9 61 0 80 90 350 0.0000051 105.870
10 61 4 100 60 250 0.0004174 67.590
11 61 8 40 45 550 0.0000216 93.319
12 61 12 60 30 450 0.0003307 69.611
13 79 0 100 45 450 0.0004502 66.932
14 79 4 80 30 550 0.0005347 65.437
15 79 8 60 90 250 0.0003572 68.942
16 79 12 40 60 350 0.0005825 64.695

Fig. 4. Effect of control factors on solid particle erosion rate.

518 A. Patnaik et al. / Materials and Design 36 (2012) 511–521

20µm
1.00KX 20µm
1.00KX

(a) (b)

30µm 10µm
1.00KX 1.00KX

(c) (d)

100µm 100µm
400X 400X

(e) (f)

100µm 100µm
400X 400X

(g) (h)
Fig. 5. SEM micrograph of TiO2 filled ZA-27 alloy composites under different operating conditions.

damaged regions as shown in Fig. 5c (See Table 4, Experiment No. 8 wt.% TiO2 filled composites under similar impact velocity
6). This may be due to the increase in erodent size and impinge- (43 m/s) with higher erodent temperature (100 °C) the micrograph
ment angle for 4 wt.% TiO2 filled ZA-27 alloy composite. For clearly shows sticking of debris particles on the wear surface and
 A. Patnaik et al. / Materials and Design 36 (2012) 511–521 519

causes severe surface damage as observed in Fig. 5d (See Table 4, potential sites for the nucleation and propagation of micro cracks
Experiment No. 7). However, on further increase in impact velocity [32–37]. Finally, the unfilled and particulate ZA-27 alloy compos-
(i.e. 61 m/s) and with lower magnification deeper wear grooves, ites are observed by an Energy Dispersive Spectrometer (EDAX)
micro-cracks and fragmented dispersed phase are observed in to assess the chemical compositions in the eroded samples. EDAX
Fig. 5e and f. Therefore, the extent of deformation increases with was performed to analyze the composition of the crystals formed
the increase in impact velocity and breaking of micro-constituents on the 0 wt.% to 12 wt.% TiO2 particulate in the matrix material
in the regions close to the wear surface as shown in Fig. 5g. Micro- as shown in Fig. 6a shows the amount of Zn and Al content present
cracking in the top layer of the subsurface region was also in the unfilled eroded metal matrix composite. But with the inclu-
observed and the flow of micro-constituents in the direction of sion of TiO2 in the base matrix material the amount TiO2 content
sliding in the nearest vicinity of wear surface was another feature available in different sets of composites are shown in Fig. 6b–d.
observed. Due to lower impingement angle the impacting particles This shows that from EDAX analysis one can predict the hard
mostly slides on the target material instead of grooving action (See TiO2 converts to Ti and rest oxides are formed when high temper-
Table 4, Experiment No. 14). Fig. 5h reveals microstructure of the ature erodent particles are impacted the composite surface. Also
composite beneath the worn surfaces. Regions in a process of being there may be chances during composite fabrication; the reinforced
separated from the bulk are evident in Fig. 5h at maximum impact TiO2 may be combined with the base matrix material (Fig. 6b–d).
velocity 79 m/s for 12 wt.% TiO2 filled composite. The dispersion of However, due to the gold coating used for the preparation of the
hard TiO2 particles also imparts thermal stability to the softer Zn– samples for SEM–EDAX analysis, the characteristic Zn peak at
Al alloy matrix. This allows more effective transfer of load between X-ray energy of 1.05 keV is overlapped by the strong gold signal
the matrix and dispersed phase [31]. Ultimately the hard phase in this energy region.
provides protection to the alloy matrix and acts as a load bearing
capacity to the alloy system [12]. In the case of composites, mi- 3.4. ANOVA analysis
cro-cracking tendency predominates at lower operating tempera-
tures, which deteriorates the wear properties of the material. On The results of ANOVA are used to analyze the influence of fac-
the contrary, thermal stability predominates at relatively higher tors such as impact velocity, TiO2 content, erodent temperature,
operating temperatures [30,31]. The hard second phase particles impingement angle and erodent size on the wear performance
like TiO2, when added to a metallic matrix, introduce micro-crack- characteristic. This analysis is carried out for a level of significance
ing tendency in the matrix material as shown in Fig. 5h. The dis- of 5% (i.e. the level of confidence 95%). Table 5 shows the results of
persed hard particulate and matrix interfacial regions act as ANOVA analysis. From Table 5 it is observed that the impact

Fig. 6. EDAX analysis of unfilled and particulate filled eroded composites.

520 A. Patnaik et al. / Materials and Design 36 (2012) 511–521

Table 5 (3) The eroded samples observed under SEM observation shows
ANOVA for erosion rate. formation of lips and their flow along the direction of the
Source DF Seq SS Adj SS Adj MS F P erodent. At medium impingement angle (say 60°), the mate-
Impact velocity (m/s) 3 2709.0 2709.0 2709.0 16.94 0.002 rial loss is governs mostly by cutting action. It also depicts
Filler content (%) 3 135.9 135.9 135.9 0.85 0.378 formation of coarser and larger lips.
Erodent temperature (°C) 3 31.7 31.7 31.7 0.20 0.666 (4) However, at a higher impingement angle (say 90°), the
Impingement angle (°) 3 83.6 83.6 83.6 0.52 0.486 material loss is dictated by micro-fracturing mechanism
Erodent size (lm) 3 5.9 5.9 5.9 0.04 0.852
Error 0 1598.7 1598.7 159.9
but TiO2 particulates provides the protection against erosive
Total 15 4564.9 wear in the composite.

DF: degree of freedom, ##Seq SS: sequential sum of squares, $Adj. SS: extra sum of


squares, $$Seq MS: sequential mean squares, F: F-test, ###P: percent contribution. References

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