ZrO2 (Y2O3) Composites

JOURNAL OF RARE EARTHS V o l . 2 5 , Suppl., Jun. 2007, p . 5 3

Thermal Shock Resistance of A1203/Zr02( Y2O3) Composites Ma Weimin (4#58)’* , Wen Lei (PTJ %)’, Sun Xudong Cui Tong ( B k ) 3Qiu , Guanming (sF%tVJ)4 ( 1 , Department .f Materials Science and Engineering, Shnyang Institute of Chemical Technology , Shenyang 110142, China ; 2 . Shenyang National Laboratory for Materials Science , Institute of Metal Research , The Chinese Academy of Sciences , Shenyang I10016 , China ; 3 . Institute of Materials and Metallurgy , Northeastern University , Shenyang 110044, China ; 4 . Changchun University of Science and Technology, Changchun 130022, China) Abstract: ZrO2 containing 2% (mol fraction)

3% (mol fraction) Y 2 0 3 were added into A1203 matrix, compositing composites with 15% volume fraction of addictives mentioned above. The testing of property and analysis of SEM presented that, after vacuum sintering at 1550 “c , thermal shock resistance of two composites was superior to AI2O3ceramic. The experiment showed that the properties of A1203 composites was higher than A1203ceramic, and A1203/Zr02(3Y)was higher than A1203/ZrO2(2Y) in thermal shock resistance. Improvement of thermal shock resistance of composites was attributed to many toughness machanisms of ZrO2(Y203) . By calculation, the fracture energy of A1203, A1203/Zr02(2Y) and A1203/Zr02(3Y) was 38100.8 and 126.2 J ~ r n - respectively. ~, Cracks initiation resistance (R’) of A1203/2&(3Y) and A1203/ZrO2(2Y) was higher than A1203ceramic by 1.57 and 1.41 time, respectively, and cracks propagation resistance ( Rrr”)was higher than A1203 ceramic by 1.46 and 1 .38 time, respectively, which was corresponding to the results of residual strength. Y203 and

Key words : fracture energy ; transformation toughness ; A1203/Zr02 ( Y203) composites ; thermal shock resistance ; inorganic non-metallic material ; rare earths

CLC number: TG174.453; TG113.25

Document code: A

Alumina is one of the most widely used engineering ceramic material because of its beneficial properties, such as high wear resistance, chemical stability, high temperature strength and creep resistance. But ceramic materials have high brittleness, high Young’s modulus, negligible plastic yields and poor thermal conductivity. They are so sensitive to thermal transient and thermal fatigue that the strength of materials can be weakened and this situation may lead to catastrophic failure. Therefore, as a structural material of high temperature usage, its thermal shock resistance or thermal fatigue is one of the key factors determining the service life of such material. Obviously, there are two methods to improve the thermal shock resistance of ceramic materials : to decrease power-damage and to enhance thermal shockresistance, which are effective ways for ceramics. This paper shows a research on thermal shock resistance of vacuum-sintered A1203/ 15%zr02(2Y) and A1203/15%Zr02(3Y) composites, a calculation of cracks initiation resistance and cracks propagation resistance and an analysis of heat-shock

Article ID: 1002-0721(2007) -0053-05

stability. The results are corresponding to residual strength of thermal shock experiments.

1 Experimental 1.1 Preparation procedure The starting materials were A1203(99.99% ) powder of 0.2 p m , Z r02(2Y ) (99.9%) powder of 0.02 p m and ZrOz ( 3 Y ) powder of 0.02 pm . Al2O3/15% Zr02 ( 2 Y ) and A1203/15% Zr02 ( 3Y ) powders were fabricated by milling composite powders of

A1203,

ZrOz( 2Y) and ZrOz(3Y ) for 48 h , respectively. The mixed powders were dried, loaded with two-face model and then isostatically pressed at 200 MPa. The specimens were preheated at 600 “c in a box-furnace for 2 h and then sintered at 1550 “c for 2 h in a VSF vacuum sintering furnace with the vacuum degree lower than 1 x Pa. The dimension of specimens reached 30mm~5mm~5mm.

Received date : 2006 - 09 - 26 ; revised date : 2007 - 03 - 13 Foundation item: Project supported by the Natural Science Foundation of Liaoning Province (20032002) and the Key Program of Science and Technologies of Shenyang (1053090-2-05)

*

Biography: Ma Weimin ( 1956 - 1, Male, Doctor, Professor Corresponding author (E-mail : rnaweimin56@ 163. corn )

.

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1.2 Characterization The fracture toughness was examined by SENB technique and can be calculated according to the following equation”’ : 3PL KIc = Y ---A 2bW2 where Y is a geometrical constant, P is the fracture load ( k W ) , b and W are related to the width and thickness, respectively, and a is the depth of specimens center crack. The SENB technique was used to induce controlled cracks (depth of specimen center is 0.025 mm) on face of polished specimens. Thermal shock properties were measured by quenching method. Then specimens were annealed for 1 h in air at 1300 “c to maximize the amount of tetragonal ZrOz and to relieve surface compression stress by particle-induced transformation. The specimens were heated at 300, 500, 700, 900 and 1100 “c , respectively. Holding 20 min at each temperature then put into boiling water rapidly. All specimens were cyclically quenched 1 time and 5 times. Instron 4206 machine with loadspeed 0.05 mm * min- was used to measure specimens bending strength, fracture toughness and residual bending resistance of quenched specimens . Elastic modulus, Poisson’ s ratio, thermal expansion coefficient and heat conductivity of specimens were measured according to 1iteratu1-e‘~’. The fracture surface of specimens before and after thermal shock fracture were observed with a scanning electron Microscope ( Model EPM-810Q).

’

2 Results and Discussion 2.1 Thermal shock resistance The physical and mechanical properties of A1203 ceramic and the composites are shown in Table 1. The thermal shock properties of A1203 ceramic and the composites are shown in Fig. 1. As shown in Fig. 1 , the bending strengths of single phase Alz03, A12O3/ 15%ZrOz(2Y) and Al20,/15% Zr02 (3Y) composites is 361, 637 and 779 MPa, respectively. After introducing SENB method, the corresponding strengths is 189, 474 and 641 MPa, respectively. The strength of three materials decreases about 50%, 35. 8% and 27.5 % , respectively. It was known that, owning to the fracture toughening effects, sensitivity to cracking of A1203/15%Zr02(3Y) and Al2O3/l5%ZrO2(2Y) is lower than that of A1203ceramic. It matches with the results that the fracture toughness of A1203/15% ZrOz (3Y) (7.8 MPa milz ) and A1203/15%ZrOz (2Y) ( 6 . 7

MPa*m’/’) is higher than that of Alz03 ceramic ( 3 . 1 MPa*rn’”). The resistance of thermal shock of A1203/Zr02 composites is improved greatly compared with that of A1203ceramic. The strength of A1203 decreases quickly according to Fig. 1. The loss of strength is 30% at 300 “c, after that it changes slowly. The loss of strength is 85.2% at 900 “c . In the same condition, the strength of Al2O3/15%ZrOz ( 3Y ) composite decreases slowly after one time quenching from 300 to 900 “c . However, strength of the composite decreases smoothly below 700 “c and decreases significantly above 700 “c after five quenching cycle times. The loss of strength after one cycle and after five cycles obviously increases about 25 % and 37.3 % at 1100 “c , respectively. The strength of Al2O3/15%ZrO2 ( 2Y ) composite shows decrease slower than that of A1203/ 15%zr02(3Y) at the temperature below 900 T,but it rapidly decreases above 900 “c , which is 29% and 37.6% , respectively. Therefore, because of the ZrOz ( Y203) transformation toughness mechanism, the thermal shock resistance of composites is higher than that of A1203ceramic. The difference between A1203/15% z r O ~ ( 3 Y )composite and Al2o3/15% ZrOz(2Y) composite is the major effect of transformation toughness mechanism , expansion coefficient and heat conductivity. These differences shows that composites have the character of thermal shock crack propagation of static state e~pansion‘~], which is corresponding to the balance of the elastic strain energy and the absorption of fresh fracture surface energy. Fig. 1 illustrates that the loss of strength of composites after five cycles decreases significantly compares with the strength of composites after one cycle time. I

7001

0

200

400

600

800

1000

1200

Temperature difference/‘(: Fig. 1

Residual strength of A1203 ceramic and A1203/Zr02 composites after quenching at various temperature difference 0 A1203/ZrO~(3Y) Single q;ench; A A1203/Zr02( 3Y) Five quenches; A1203/Zr02(2Y Single quench; A1203/Zr02 (2Y) Five quenches; A1203Single quench

*

+

Ma W M et a1 .

Thermal Shock Resistance of A1203/Zr02 ( Y2O3 )

55

mal shock resistances of Al2O3/15%ZrO, ( 3 Y ) and Al2O3/15%ZrO2( 2Y) composites is much more excellent than that of A1203 ceramic. Moreover, thermal shock resistance of Al2O3/15% Zr02(3Y) composite is more excellent than that of A1,O3/15% ZrO, ( 2 Y ) composite.

/ /

2. 2

Temperature difference/”(: Strength loss factor of A1203 ceramic and A1203/Zr02 composites after quenching with various temperature differences 0 A1203/Zr0, (3Y ) Single quench ; A A1203/Zr02 ( 3Y ) Five quenches; A1203/Zr02( 2Y ) Single quench ; A120JZr02 (2Y) Five quenches ; Alz03 Single quench

*

+

Fig.2 shows that the loss factor of strength is related to different thermal shock temperatures, the loss factor increases with increasing temperature difference. The strength loss factors of A1203, Al&/15% ZrOz(3Y) and Al2O3/15%Zr0,(2Y) is about 85.2%, 18% and 1 2 . 4 % , respectively at 900 “c. After five cycles, the strength loss factor of these composites is 37.3% and 37.6% , respectively. It shows that ther-

Fig.4

Microstructure factures of fracture surface

Scanning electron macrographs (SEM) of fracture surface of A1203 ceramic are shown in Figs. 3 ( a ) and ( b ) . Fracture surface is smooth before quenching and it is an obvious phenomenon of single crack. Fracture surface is rough after quenching from 1100 “c . More cracks appear and macro-cracks are observed. Fracture surface is broken up a few parts during bending test. The fracture surfaces of A1203/15% ZrO, ( 3 Y ) and &o3/15% ZrOz ( 2Y ) composites are shown in Figs. 4 and 5. Fracture surface is smooth with small particles before quenching. The toughening phase is distinct. Under high thermal shock temperature, specimens do not break up into few parts. Because of several times of rupture, it is difficult to observe macrocracks. Therefore, these photographs present that thermal - shock resistance of composites is excellent to

SEM of fracture surfaces of A1203/15% ZrOz(3Y) composites before ( a ) and after ( b ) quenching from 1100 T

JOURNAL OF RARE EARTHS, Vol. 2 5 , Suppl. , Jun . 2007

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Fig. 5

SEM of fracture surfaces of Ai2O3/15% ZrOz(2Y) composites before ( a ) and after (b) quenching frpm 1100 T

coefficient of thermal expansion. Vf is the work of fracture, 2 y f is the fresh fracture surface energy. Therefore, for most materials, it is hard to enhance rupture thermal shock parameter R ' and breakage parameter R" at the same time. If the work of rupture is enhanced, rupture parameter R ' and breakage parameter R" should be enhanced simultaneously. Thermal shock parameter R ' and breakage parameter R" can be calculated according to Eqs. ( 2 ) and (4). Heat conduction ratio A and Poisson's ration u can be calculated according to the following equations :

ceramics. The surfaces of A1203/15%Zr02(3Y) and Al2O3/15% Zr02( 2 Y ) composites without appearing cracks after quenching at 300 "c compares with that of A1203 ceramic with appearing cracks. At 1100 "c , surface of composites can be shown cracks which are difficult to observe, the thermal cracks on A1203 ceramic surfaces begin to increase and propagate with increasing of thermal shock temperature difference . These results illustrate that the micro-cracks from transformation toughness can absorb stress-strain energy[*'. A1203

2.3 Calculation of thermal shock parameter

E,= 91) + E l 9 1 A = Az(1- 91) + A191

There are tn, theories on thermal shock resistance. Firstly, the point of view is based on elastic machanics . Secondly, on the view of fracture mechanics. The physical meaning of thermal shock parameter R ' and breakage parameter R" is crack-initiation resistance and crack-spread resistance, respectively. Therefore, the thermal shock parameter R ' and breakage parameter R" are given by:

R" =

EWf 0x1- Y)

Y

=

Y2(1-

91) + V l 9 ,

(7)

where suffix 1 and 2 represent toughness-increase phase and matrix phase, respectively, 9 is the volume ratio of toughness-increase phase, K is the volume shrinkage factor.

- 2EYf - &1- u )

KIcZ (4) a;( 1 - Y ) where of is the rupture stress, Y is the Poisson's ration, E is the modulus of elasticity, a is the linear So R " =

E2

where K2 =

3( 1 - 2 ~ 2 )

K1 =

EI 3( 1 - 2 ~ 1 )

The physical constants and mechanical properties of A1203ceramic, A1203/15%ZxQ (2Y) and Al2O3/15% Zr02( 3Y) composites are listed in Table 1.

Table 1 Physical and mechanical properties of A1203ceramic and composites A1203

Z d z ( 3y zroz (2Y) AIz03/Zr02(3Y) A1zO3/ZrOz(2Y1 * Experimental date

EIGPa 380.0 217 .O 148.0 355.6 345.2

A / ( J . ~ - * - ~ - ' . K - ~d) 1 0 - ~

29. OM) 3.300 2.100 25.145 24.965

8.237 9.900 10.100 8.412 8.371

(6)

and the coefficient of thermal expansion a is given by:

because of Klc =

Composites

(5)

v

of*/MPa

K c "/(MPa*m"2)

0.260 0.250 0.250

361

3.1

-

-

0.183 0.183

779 637

7.8 6.7

Ma W M et a1 . Thermal Shock Resistance of A1203/Zr02( Y 2 0 3)

The calculated values of R ' and R" are shown in Table 2. Crack initiation resistance and crack propagation resistance of Al2O3/15% ZrOz ( 3Y) and AlZO3/ 15% ZrOz(2Y) are both higher than that of Alz03 ceramic according to Table 2, respectively. Crack initiation resistances R ' of A12O3/15% ZrOz ( 3Y ) and A12O3/15% ZrOZ(2Y) composites is higher than that of Alz03ceramic by 2. 16 and 1 .82 times, respectively. Crack propagation resistances R" of two composites is higher than that of A1203 ceramic by 1 . 2 3 and 1 . 3 7 times, respectively. Obviously, thermal shock resistances of composites are excellent to ceramics. It matches with the result of experiment. At the same time, the result of calculation illustrates that enhancement of crack-spread resistance improves the thermal shock resistance of A1203/15% ZrOz( 3Y ) and A1203/ 15% ZrO2 (2Y ) composites . Generally, it is impractical that the change of strength and toughness could simultaneously increase R ' and R" . But it is possible to increase R ' and R" simultaneously if rupture work of materials increases ; AlZo3/15%zrOz(3Y) and Al2O3/15%ZrOz(2Y) composites are examples. Rupture work of material is replaced with released energy of crack propagation on place strain state A G[I3' :

where Klc is the fracture toughness, ER is modulus of elastivity and v is Poisson's ratio. Fracture work of Alz03ceramic, Al2O3/15% Zr02(3Y) and AlZo3/15% ZrOz(2Y) composites are 24, 162 and 1126 Jam-', respectively. The increase of thermal shock capability of materials can be realized by means of increasing rupture work of materials. Therefore, thermal shock resistances increased. It is beneficial to the improvement of thermal.

3 Conclusions 1. Both mechanical properties and thermal shock resistance of Al2O3/15% ZrOz ( 3 Y ) and Al2O3/15% ZrO2( 2Y) composites were higher than that of single phase A1203 ceramic. The calculations showed that crack initiation resistances ( R ' ) of A12O3/15% ZrOz (3Y) and AlZo3/15% Zr02(2Y) was higher than that

57

of A1203ceramic by 2.16 and 1.82 times, respectively, and their crack-spread resistances (R") was higher than that of A1203 ceramic by 1.23 and 1.37 times, respectively. 2. Toughness-increase phase transformation of ZrOz( 3Y) and ZrOz ( 2 Y ) changed the properties of A1203matrix. Both composites improved thermal shock fracture resistance and thermal shock damage resistance. Fracture energy of AlZ03, A1203/15% ZrO2 (3Y) and AlZo3/15%Zr02(2Y) was 24, 162 and 126 J * m - 2 , respectively. The resistance to thermal shock of A1203/15% Zroz(3Y) was superior to that of A1203 ceramic and A120J 15% ZrO2 ( 2Y ) composite . 3, The stress induction transformation toughing mechanism and micro-crack toughing mechanism took an important role in improving material's thermal shock resistance.

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