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Nov 6, 2011 - of Al–SiC powder metallurgy composites during cold upsetting was attempted in the present work. Three levels of sintering temperature and ...
Int J Adv Manuf Technol (2012) 61:237–252 DOI 10.1007/s00170-011-3709-4

ORIGINAL ARTICLE

Effect of sintering temperature and time intervals on workability behaviour of Al–SiC matrix P/M composite J. Bensam Raj & P. Marimuthu & M. Prabhakar & V. Anandakrishnan

Received: 1 October 2011 / Accepted: 17 October 2011 / Published online: 6 November 2011 # Springer-Verlag London Limited 2011

Abstract The investigation on the effect of sintering temperature and time intervals on workability behaviour of Al–SiC powder metallurgy composites during cold upsetting was attempted in the present work. Three levels of sintering temperature and time have been considered to evaluate their effect on workability behaviour. The amount of SiC reinforcement content has been varied as 0%, 10% and 20%. The experimental results were analyzed for

workability under triaxial stress state condition as a function of the relative density. The Formability Stress Index (βσ), the Formability Strain Index (βε), stress ratio parameters namely σθ/σeff and σz/σm were obtained for all the cases. As a result, the exhibited tremendous variations in the various parameters for different sintering temperatures and time intervals were studied and reported. Keywords Aluminium metal matrix composites . Powder metallurgy . Workability . Plastic behaviour

J. Bensam Raj (*) Department of Mechanical Engineering, St. Anne’s College of Engineering and Technology Cuddalore District, Cuddalore, Tamil Nadu, India e-mail: [email protected] J. Bensam Raj Carmel Nager, Nagercoil, Kanyakumari District, 231, Carmel School opposite, Nagercoil, Tamil Nadu, India 629 004 P. Marimuthu Department of Mechanical Engineering, Syed Ammal Engineering College, Ramanathapuram, Tamil Nadu, India e-mail: [email protected] M. Prabhakar Department of Mechanical Engineering, Sudharsan Engineering College, Pudukkottai, Tamil Nadu, India e-mail: [email protected] V. Anandakrishnan Department of Production Engineering, National Institute of Technology, Trichirappalli, Tamil Nadu, India e-mail: [email protected]

Nomenclature F Force applied on the cylindrical preform for deformation h0 Initial height of the cylindrical preform hf Height of the barreled cylinder after deformation Initial diameter of the preform D0 Bulged diameter of the preform after deformation DB DTC Top contact diameter of the preform after deformation Bottom contact diameter of the preform after DBC deformation α Poisson’s ratio σz True stress in the axial direction σθ True stress in the hoop direction True stress in the radial direction σr True effective stress σeff σm True hydrostatic or mean stress True strain in the axial direction εz εθ True strain in the hoop direction True strain in the radial direction εr Formability Stress Index βσ ro Initial preform density of the preform

238

rf rth r RðorÞ r f th K n h0/D0

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Density of the preform after deformation Theoretical density of the fully dense material Relative density Strength coefficient value Strain-Hardening Index value Aspect ratio

Fig. 1 Photographs of hydraulic press and parts of compacting die tool

1 Introduction Powder forging is attractive due to its ability to produce the products with no wastage of material and its ability to fabricate high-quality, complex parts to close tolerances in an economical manner [1]. Powderforged parts are performing with high strength along with the absolutely uniform grained microstructure. Particles reinforced aluminium alloy matrix composite is one of the best materials to substitute the conventional structural alloys. The sintering is also difficult in some cases which is not giving satisfactory densifications [2] due to improper sintering temperature and time. Similarly, we can get better corrosion-resistant materials through these sintering variations [3]. Sintering is one of the important consolidation processes, which is very essential for cold compaction processes, due to very low strength of the green compact. Press-and-sinter fabrication technique is one of the oldest methods to produce good quality P/M products. In a powder metallurgical forging process, the sintering effect has been modelled by few of the authors to evaluate the effect of sintering in forging processes [4, 5]. Selecka et al. studied the behaviour of liquid-phase sintering effect on the specimen sintering at 1,200°C for 60 min in hydrogen for Ni-, Mo- and Cr-alloyed structural elements with boron. They identified the increase of specimen density and formation of a new microstructure type. Sintering of powder is the root cause for the establishment and growth of bonds between the particles of powder at their areas of contact and migration of the grain boundaries formed at the bonds. The spheroidization of the pores between the particles, and the elimination of small pores (and possibly the growth of larger pores) were also caused by the sintering effects. As the sintering temperature increases, porosity decreases and shrinkage increases [6]. Bonds form between the particles during sintering, and the number of particle bonds increase as the temperature increases. As generally agreed by the results presented in several previous works [7, 8] successful sintering of aluminium alloys can only be carried out through the formation of a liquid phase able to disrupt the extremely stable aluminium

oxide film always covering the aluminium particles. Such liquid phase must be able to penetrate the oxide film, through the discontinuities created during cold pressing. Thus, facilitating material transport and hence the development of adequate bonding by the formation of necks between particles. Khairaldien et al. have studied the sintering effect of Al–SiC composites fusion behaviour and weldability behaviour of the composite and found that the weldability behaviour is reducing, if the aluminium picks oxygen during the sintering and also he found that the increase in sintering temperature is reducing the weldability problems [9]. Workability is to which a material can be deformed in a specific metal working process without the initiation of cracks [10]. Abdel-Rahman et al. [11] presented the effect of relative density on the forming limit of P/M compacts during upsetting. They have proposed the criteria called Formability Stress Index (β) for describing the effect of the mean stress and the effective stress with the help of two theories, proposed by Kuhn-Downey and Whang-Kobayashi. Narayanasamy et al. [12] investigated some of the important criteria generally used for the prediction of workability. They have done tremendous experimental work on workability behaviour of aluminium [13, 14], Al–Al2O3 [15–20], Al–Fe

Fig. 2 The SEM photograph of as sintered Al-20% SiC at 605°C at 3 h

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239

F=0 F>0 D TC D0 h0

D8

hf

Dead Metal Zone D BC

Fig. 3 Schematic of upset test of preform (before and after deformation)

[21, 22], Fe [23], Fe–TiC [24] and Al–SiC [25] composites during cold upsetting. The previous literature are not dealing with the effect of sintering temperature and sintering time on workability under triaxial stress state condition and the workhardening behaviour on Al–SiC porous composite. The present investigation is an attempt to evaluate the effect of the sintering temperature and sintering time on particle size on the workability parameter under triaxial stress state condition and work-hardening effects in porous Al–SiC composite during upsetting at room temperature. The relationships between the various stress ratios namely σθ/σeff and σz/σm, and the relative density and its effect on the sintering effect are also attempted in this work.

2 Experimental details 2.1 Compacts preparation Atomized aluminium powder of −100 μm was used for analysis of its purity. The purity level of the powder was found to be 99.7% and insoluble impurities to be 0.3%. The flow rate, apparent density and particle size distribution of aluminium powder was studied. Al–10%SiC and Al–20% SiC powder mix were blended on a pot mill to obtain a homogeneous powder blend. The particle size of silicon

Fig. 4 a–b Photographs showing preform—before and after deformation test

carbide powder mixed was 50 μm. Green compacts of the powder blend was prepared on a 1.0 MN capacity hydraulic press using suitable punch and die assembly as shown in Fig. 1. The compact pressure was maintained for all composition of SiC composites to maintain the 92% relative density level. The green compact surfaces were coated with an indigenously developed ceramic mixture [21] and dried under room-temperature conditions for a period of 9 h. A second coating was applied at a direction of 90° to the direction of first coating and was allowed to dry for a further period of 9 h under the same conditions as stated above. 2.2 Sintering The ceramic-coated compacts were sintered in an electric muffle furnace in the temperature range of 600±10°C for a period of 120 min and allowed to be cooled to room temperature in the furnace itself. The sintered preforms of pure aluminium and aluminium–SiC composites prepared. The microstructure of sintered Al–SiC composites with sintering temperature of 605°C at 3 h is shown in Fig. 2. The ceramic coatings applied over specimen were removed by machining. The aspect ratio was maintained as 0.90 for all compositions of composites. 2.3 Deformation test Initial diameter (D0), initial height (h0) and the initial preform relative density (ρ0) of the specimen were measured and recorded. Each compact was subjected to the incremental compressive loads of 0.01 MN and the upsetting was carried between two flat, mirror finished open dies on a hydraulic press of 1.0 MN capacity. The deformation was carried out until the appearance of the first visible crack on the free surface. After each interval of loading, dimensional changes in the specimen such as height after deformation (hf), top contact diameter (DTC), bottom contact diameter (DBC), bulged diameter (DB) and density of the preform (ρf) were measured. The schematic diagram showing the various parameters measured before and after deformation is

240

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Fig. 5 a The variation of relative density (R) with respect to the true axial strain (εz) for the sintering temperature of 630°C at various time intervals. b The variation of relative density (R) with respect to the true axial strain (εz) for the sintering temperature of 605°C at various time intervals (c)

provided in Fig. 3. Using the Archimedes principle, the density of upset preforms was also determined after every loading interval. The deformation test is continued until the fracture occurs at outer surface of the specimen as shown in Fig. 4a, ab.

3 Theoretical investigations In a triaxial stress state condition pertains to actual stresses developed during cold upsetting of cylindrical preform with friction and barreling. Here, the axial stress is compressive

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241

Fig. 5 (continued)

a R sz sq

is is is is

Poisson’s ratio relative density the axial stress and the hoop stress

in nature, the hoop and the radial stresses are tensile in nature. In order to simplify the plasticity theory, the radial stress can be taken as equivalent to the hoop stress. Narayanasamy et al. [26] reported the state of stress in a triaxial stress condition is as follows: The relationships between stress and strain increment for an ideal plastic solid, where the elastic strains are negligible, are called flow rules or Levy–Mises equations. The relationship between the strain increment and stresses for triaxial stress state condition for a porous material is written as follows [26]: α, which is nothing but the ratio of the hoop strain increment to the axial strain increment. Say

In triaxial stress condition for the known values of Poisson’s ratio (α), relative density(R) and axial stress (σz), the hoop stress component(s q ) can be determined as explained elsewhere [27], shown as follows:

  A a¼ B

In the above Eq. 2, the relative density (R) plays a major role in determining the hoop stress component (s q ). The hydrostatic stress for axisymmetric upset forging condition is given by

Where   A ¼ ð2 þ R2 Þ s q  R2 ðs z þ 2s q Þ   B ¼ ð2 þ R2 Þ s z  R2 ðs z þ 2s q Þ

ð1Þ

 sq ¼

sm ¼

2 a þ R2 2  R2 þ 2 R2 a

ðs z þ s r þ s q Þ 3

 sz

ð2Þ

ð3Þ

Since s r ¼ s q for axisymmetric or cylindrical upsetting, the above equation can be written as: sm ¼

ðs z þ 2 s q Þ 3

ð4Þ

242

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Fig. 6 a The variation of Formability Stress Index (βσ) with respect to the relative density (R) for the sintering temperature of 630°C at various time intervals. b The variation of Formability Stress Index (βσ) with respect to the relative density (R) for the sintering temperature of 605°C at various time intervals. c The variation of Formability Stress Index (βσ) with respect to the relative density (R) for the sintering temperature of 560°C at various time intervals. d The variation of Formability Strain Index (βσ) with respect to the relative density (R) for the sintering temperature of 630°C at various time intervals. e The variation of Formability Strain Index (βσ) with respect to the relative density (R) for the sintering temperature of 605°C at various time intervals. f The variation of Formability Strain Index (βσ) with respect to the relative density (R) for the sintering temperature of 560°C at various time intervals

The effective stress can be determined from the following expression in terms of cylindrical coordinates for axisymmetric upset forging condition as explained [28]: 

 s 21 þ s 22 þ s 23  R2 ðs 1 s 2 þ s 2 s 3 þ s 3 s 1 Þ ¼ ð2R2  1Þ s 2eff

ð5Þ

The above Eq. 5 can be written in terms of cylindrical coordinates as follows:  2  s z þ s 2q þ s 2r  R2 ðs z s q þ s q s r þ s r s z Þ 2 ð6Þ s eff ¼ 2R2  1

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243

Fig. 6 (continued)

Since s q ¼ s r for axisymmetric or cylindrical upsetting, the Eq. 6 can be modified as: 

s eff ¼

 1=2 s 2z þ 2s 2q  R2 ðs 2q þ 2s z s q Þ 2R2  1

ð7Þ

In the above Eq. 7 for the known values of s z , s q and R, the effective stress s eff can be determined. The stress ratio parameters, namely (σθ/σeff) and (σz/σm) are calculated from the above equations.

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Fig. 6 (continued)

3.1 Formability stress parameter The Formability Stress Index parameter (βσ) can be determined for triaxial stress state condition. As an evidence of experimental investigation implying the importance of the spherical component of the stress state on

fracture, Vujovic and Shabaik [29] proposed a parameter called a Formability Stress Index ‘bs ’ given by,  bs ¼

3s m s eff

 ð8Þ

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245

where σm is the mean or hydrostatic stress component and σeff is the effective stress component calculated through Eqs. 4 and 8. This index determines the fracture limit as explained in the reference [26].

Since for the case of axisymmetric upsetting, "r ¼ "q , Eq. 10 can be written as follows:

3.2 The effective strain and mean strain under triaxial system "eff ¼

As described elsewhere [27], the effective strain rate for the case of axisymmetric upsetting, can be written as follows: 0 "eff

B ¼@

h i 11=2 3ð2 þ R2 Þ ð"1  "2 Þ2 þ ð"2  "3 Þ2 þ ð"3  "1 Þ2 C A ð"1 þ"2 þ"3 Þ2 2 þ ð1  R Þ 3

ð9Þ

The above equation can be written in terms of cylindrical coordinates as follows: 0 B "eff ¼ @

2 3ð2þR2 Þ

h

ð"z  "q Þ2 þ ð"q  "r Þ2 þ ð"r  "z Þ2

ð"z þ"q þ"r Þ2 ð1  R2 Þ þ 3

Table 1 The values of maximum Formability Stress Index (βσ) for various sintering temperature at various time intervals

Sintering temperature (°C) 630°C

C A

ð10Þ

In Eq. 11, "z , "q and R are known. Therefore, the effective strain "eff can be determined. The hydrostatic strain is given by,

"m ¼

ð"z þ "r þ "q Þ 3

ð12Þ

Sintering time (h)

SiC percentage

Maximum Formability Stress Index (βσ)

Maximum Formability Strain Index (βε)

4.5

Pure aluminium

0.446836

2.143575

10

0.443675 0.439983 0.45157

2.081776 2.014705 2.206692

20

0.448334 0.445154 0.461532

2.110817 2.088851 2.403806

Pure aluminium

0.457743 0.455177 0.442314

2.276057 2.248295 2.054056

0.438017 0.434278 0.448685 0.44397 0.440895 0.457744 0.452944

1.974429 1.936176 2.144436 2.074168 2.023689 2.292379 2.17684

0.449793 0.436033 0.432671 0.431117 0.443209 0.438334 0.436747 0.451693 0.448707 0.446709

2.122578 1.965738 1.898765 1.884128 2.008552 1.93582 1.910088 2.160308 2.096032 2.038317

3 1.5 4.5 3 1.5 4.5 3 1.5 4.5 3 1.5 4.5 3 560°C

ð11Þ

i 11=2

3 1.5 4.5

605°C

! 1=2  2  ð"z þ 2"q Þ2 2 2 2 þ 2"  4" " Þ þ 2" ð1  R q z q z 3 3ð2 þ R2 Þ

1.5 4.5 3 1.5 4.5 3 1.5 4.5 3 1.5

10

20

Pure aluminium

10

20

246

Int J Adv Manuf Technol (2012) 61:237–252

Since "r ¼ "q for axisymmetric or cylindrical upsetting, the above Eq. 12 can be written as: "m ¼

ð"z þ 2"q Þ 3

Fig. 7 a The variation of stress ratio parameter (σθ/σeff) with respect to the relative density (R) for the sintering temperature of 630°C at various time intervals. b The variation of stress ratio parameter (σθ/σeff) with respect to the relative density (R) for the sintering temperature of 605°C at various time intervals. c The variation of stress ratio parameter (σθ/σeff) with respect to the relative density (R) for the sintering temperature of 560°C at various time intervals

ð13Þ

3.3 Formability strain parameter Similar to the formability stress parameter as mentioned above, Narayanasamy et al. [30] proposed a strain-based

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247

Fig. 7 (continued)

formability parameter called a Formability Strain Parameter ‘b" ’ as given by,  b" ¼

3"m "eff

Table 2 The values of maximum stress ratio parameter (σθ/σeff) for various sintering temperature at various time intervals Sintering temperature (°C)



ð14Þ

630°C

Sintering time (h)

SiC percentage

Maximum stress ratio parameter (σθ/σeff)

4.5

Pure aluminium

0.797796

3

0.788721

1.5 4.5 3

4 Results and discussions

0.790981 20

3 4.5

0.814582 Pure aluminium

3

0.763671 10

3

0.780142 20

3 4.5

0.798855 Pure aluminium

3

0.75457 10

3

0.781568 0.769032

1.5 4.5

0.768719 0.758066

1.5 4.5

0.820237 0.806571

1.5 560°C

0.799676 0.788337

1.5 4.5

0.784612 0.772066

1.5 4.5

0.831726 0.818888

1.5 605°C

0.807952 0.796324

1.5 4.5

The workability of metals is one of the most important parameters that must be considered in the design of forming an operation. A preform with high relative density (pores of small size) is at a relatively greater stress, while a preform with low relative density (pores of larger size) is at a relatively smaller stress. The application of a compressive hydrostatic stress will close the pore and will increase the relative density. Similarly, the application of tensile hydrostatic stress will increase the size of the pores and reduces the relative density. Bulk forming is a process that reflects the complicated workability of materials. It is very important to guide the production practice according to the forming limits. Figure 5a–c have been plotted between the relative density and the true axial strain (εz) for aluminium containing different silicon carbide percentage and for three different sintering time at namely 1.5, 3 and 4.5 h with sintering temperature of 560°C,

0.778215 10

0.764665 20

0.803983

3

0.795433

1.5

0.788283

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Fig. 8 a The variation of stress ratio parameter (σz/σm) with respect to the relative density (R) for the sintering temperature of 630°C at various time intervals. b The variation of stress ratio parameter (σz/σm) with respect to the relative density (R) for the sintering temperature of 605°C at various time intervals. c The variation of stress ratio parameter (σz/σm) with respect to the relative density (R) for the sintering temperature of 560°C at various time intervals

605°C and 630°C. It is observed that the relative density increases with increasing amount of SiC added in the Al–SiC composite. For any given composition of composite, as the sintering time increases, the relative density increases. From these figures, it is understood that the relative density and axial strain are affected by

the amount of SiC content and sintering behaviour. The same behaviour has been observed when the sintering temperature increases. As SiC content and sintering temperature increases, the porosity level decreases and the relative density increases for the same compacting pressure. This may be one of the reasons for the

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249

Fig. 8 (continued)

increasing stresses for higher amount of SiC content. Furthermore, as the amount of SiC content in the composites increases, the SiC particulates impede the motion of dislocations and hence the stress required for further plastic deformation increases. Due to these reasons, all stresses increase with increasing amount of SiC. Figure 6a–c have been plotted between the Formability Stress Index (βσ) and the true axial strain (εz) as well as Fig. 6d–f have been plotted between the Formability Strain Index (βε) and the true axial strain (εz) for aluminium containing different SiC percentage and for three different sintering time at namely 1.5, 3 and 4.5 h with sintering temperature of 560°C, 605°C and 630°C. Table 1 provides the maximum value of the Formability Stress Index (βσ) and Formability Strain Index (βε) for various sintering time and temperature increment in the Al–SiC composites. It is observed that as the sintering temperature increases, the Formability Stress Index (βσ) also increases. The reason is that as the sintering temperature increases, the relative density also increases and porosity decreases. This may be the reason that the Formability Stress Index increases with increasing sintering temperature. It is further observed that as the sintering time increases, the Formability Stress Index also increases. The reason may be due to increase in relative density with increasing sintering time. Therefore, the Formability Stress Index (βσ) also increases with sintering time. The same behaviour has been observed irrespective of the sintering temperature in the

Table 3 The values of minimum stress ratio parameter (σz/σm) for various sintering temperature at various time intervals Sintering temperature (°C) 630°C

Sintering time (h) 4.5

SiC percentage Pure aluminium

3

3.248296 10

3

3.226568 20

3 4.5

3.185421 Pure aluminium

3

3.272989 10

3

3.244718 20

3 4.5

3.209374 Pure aluminium

3

3.288067 10

3

3.23821 3.259803

1.5 4.5

3.264785 3.281649

1.5 4.5

3.175044 3.196076

1.5 560°C

3.211837 3.23132

1.5 4.5

3.237971 3.257782

1.5 4.5

3.158441 3.17565

1.5 605°C

3.199173 3.214887

1.5 4.5

3.217727 3.231713

1.5 4.5

Minimum stress ratio parameter (σz/σm)

3.267183 20

3.201083

3

3.214443

1.5

3.224444

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Fig. 9 a The variation of fracture strain (εf) with respect to Formability Stress Index (βσ) value for the sintering temperature of 630°C at various time intervals. b The variation of Fracture Strain (εf) with respect to Formability Stress Index (βσ) value for the sintering temperature of 605°C at various time intervals. c The variation of Fracture Strain (εf) with respect to Formability Stress Index (βσ) value for the sintering temperature of 560°C at various time intervals

Al–SiC composite. Further it can be understood that the true mean stress (σm) also increases with increasing sintering temperature and sintering time to the true

effective stress (σeff). Therefore, the Formability Stress Index increases with increasing sintering temperature. As the sintering time increases, the Formability Stress

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251

Fig. 9 (continued)

Index also increases. The reason is due to the association of fine pores in higher sintering temperature. The same behaviour has been observed in the case of Formability Strain Index also which are plotted in Fig. 6d–f, when other stress ratio parameter (σθ/σeff) is plotted against the relative density as shown in Fig. 7a–c. The values of maximum stress ratio parameter (σθ/σeff) for various SiC content in composites are provided in Table 2. However, when the stress ratio parameter (σz/σm) is plotted against the relative density as in Fig. 8a–c, the opposite behaviour has been experienced. The reason is the true axial stress (σz) is compressive in nature. The values of minimum stress ratio parameter (σz/σm) for various sintering temperature are tabulated in Table 3. Figure. 9a–c has been plotted between the fracture strain (εf) and the Formability Stress Index (βσ) for the upsetting of Al–SiC composite preforms of various percentage of SiC and at various sintering temperature and time intervals under triaxial stress state condition. It is observed that as the SiC content increases, the fracture strain (εf) decreases. From these figures, it is further noted that for preforms with higher sintering temperature, the initiation of crack appears at a higher fracture strain. However, it exhibited higher Formability Stress Index (βσ) because of association with fine pores.

5 Conclusions The following conclusions can be drawn from the above results and discussions. &

& & &

& & &

As the sintering temperature increases the pore size becomes smaller. As the pore size becomes smaller, the Formability Stress Index value (βσ) increases. Twenty percent (20%) SiC added composites show higher Formability Stress Index value (βσ) compared to other composite because of better densification. The stress ratio parameter (σθ/σeff) is found to be higher for Al–SiC composites compared to pure aluminium because of better densification. The stress ratio parameter (σz/σm) decreases in the case of Al–SiC composites compared to pure aluminium because of fine pore size associated with high hydrostatic stress (σm). For preforms with higher percentage addition of SiC, the initiation of crack exhibits at lower fracture strain. Higher sintering temperature initiates cracks with higher fracture strain. Higher sintering time given composites initiate crack at higher fracture strain.

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