degradation resistance of 3y-tzp ceramics sintered using spark plasma sintering

Degradation resistance of 3Y-TZP ceramics sintered using spark plasma sintering

R Chintapalli 1, F G Marro 1, J A Valle1, H Yan2, 3, M J Reece2, 3 and M Anglada1 1Center for Structural Integrity and Reliability of Materials, Department of Materials Science and Metallurgical Engineering, Universitat Politècnica de Catalunya, Av. Diagonal 647, 08028 Barcelona, Spain 2School of Engineering and Materials Science, Queen Mary College, University of London, Mile End Road, London E1 4NS, UK 3Nanoforce Technology Ltd, Mile End Rd, London E1 4NS E-mail: [email protected] Abstract. Commercially available tetragonal zirconia powder doped with 3 mol% of yttria has been sintered using spark plasma sintering (SPS) and has been investigated for its resistance to hydrothermal degradation. Samples were sintered at 1100, 1150, 1175 and 1600 0C at constant pressure of 100 MPa and soaking for 5 minutes, and the grain sizes obtained were 65, 90, 120 and 800 nm, respectively. Samples sintered conventionally with a grain size of 300 nm were also compared with samples sintered using SPS. Finely polished samples were subjected to artificial degradation at 131 0C for 60 hours in vapour in auto clave under a pressure of 2 bars. The XRD studies show no phase transformation in samples with low density and small grain size (<200 nm), but significant phase transformation is seen in dense samples with larger grain size (>300 nm). Results are discussed in terms of present theories of hydrothermal degradation.

1. Introduction Stabilised tetragonal zirconia polycrystals doped with 3% molar of yttria (3Y-TZP) are used in implants because of their excellent mechanical properties and bio-compatibility. They have an interesting feature in the form of transformation toughening, that is, the increase in fracture toughness induced by the increase in stress in front of cracks. This stress concentration triggers locally the tetragonal-monoclinic phase transformation accompanied by an increase in volume in a small region surrounded by material undergoing only elastic deformation [1].

An important drawback for 3Y-TZP ceramics is the phenomenon of low temperature degradation (LTD) in aqueous environments at relatively low temperatures. This effect was first observed by [2] at temperatures close to 250 ºC in air. Degradation activates surface transformation from tetragonal to monoclinic phase and rapidly deteriorates surface mechanical properties. This phenomenon is widely documented in the literature [3-7]. At lower temperatures, that is, between 250 ºC and room temperature, ageing also occurs but transformation proceeds at much slower rate. In fact, during many years it has been thought that this effect was negligible at the human body temperature. This is also the reason for assessing the in vitro resistance to LTD of medical grade zirconia at higher temperatures in an autoclave [8]. Much work has also been done to reduce LTD by various methods [9]: refining the grain size, surface coatings and surface modification with cationic or anionic dopants in order to increase the number of vacancies.

5th International EEIGM/AMASE/FORGEMAT Conference on Advanced Materials Research IOP PublishingIOP Conf. Series: Materials Science and Engineering 5 (2009) 012014 doi:10.1088/1757-899X/5/1/012014

c© 2009 IOP Publishing Ltd 1

The effect of decreasing the particle size in free powder has been previously studied by [10] and by [6]. It was found in both cases that degradation was stopped if the particle size was reduced to the nanometric scale. However, in conventionally sintered bodies the grain size is usually equal or larger than 0.3 µm, so that the studies of LTD have been concentrated on this range of grain sizes [11-13]. Degradation is faster in sintered bodies than in free particles. However, there are very few studies of the behaviour of sintered bodies of 3Y-TZP of nanometric grain size under hydrothermal degradation. Here, we have produced sintered bodies of 3Y-TZP by spark plasma sintering (SPS) with a nanometric grain size and we have analysed their resistance to LTD in specimens sintered under different conditions. 2. Materials and Experimental The commercially available yttria stabilised zirconia powder (Tosoh Co, Japan) with 3% molar of yttria was chosen for this study. The mean crystallite size of the powder is ~29 nm. Samples were sintered using spark plasma sintering (SPS FCT HP D25I, FCT system GmBh) at temperatures of 1100, 1150, 1175 and 1600 0C for a soak time of 5 minutes at a pressure of 100 MPa. For comparison, 3Y-TZP specimens were also conventionally compacted and sintered at 1450 0C for 2 hours in the absence of pressure. At this point, it is necessary to mention that these samples will be referred as “AS” further in the paper and they are studied only for comparison, as the main focus is given to the study of SPS specimens.

The samples were ground using a silicon carbide grinding disc and well polished with 30 µm diamond paste and finally polished with 0.3 µm colloidal silica. The surface roughness of the samples was measured in the range of Ra 0.02-0.05 µm using laser confocal microscopy (Olympus, LEXT OLS 3100). Vickers indentations (20 kg load) were performed for estimating the facture toughness of the samples. Density of all samples was measured using Archimedes principle. Table 1 shows the detailed description of samples with listed properties. Thermally etched SPS samples were observed in the atomic force microscope (Veeco Instruments Inc). A group of samples were subjected to hydrothermal ageing using an autoclave at 131 0C for 60 hours and in contact with water vapour under a pressure of 2 bars. All the samples were analysed by X-ray diffraction with a Bruker AXS D8 diffractometer and using Ni- filtered Cu-Kα radiation. The samples will be referred with the reference code further in the article, as shown in the table.1 3. Results Properties of all materials are listed in table 1. Hardness (HV1) values were measured with 1kg Vickers indentations. Contact hardness and elastic modulus were obtained by nanoindentations with a Berkovich tip up to 2 µm depth. The fracture toughness was calculated from the equation proposed by [14]. Because of the small starting particle size with good sintering activity, moderate to fully dense materials were obtained using SPS technique. A fully dense material was also obtained with conventional sintering, and its density was higher than by SPS. Figure 1.a shows the relative densities obtained at various soaking temperatures. It is clear from the curve that higher densities can be achieved with higher sintering temperatures.


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Table.1 Conditions and measured properties of the different specimens considered.

All samples were thermally etched at 100 ºC below sintering temperature for 1 hour in order to

observe their microstructures by AFM. Figure 2 shows the microstructures of all SPS samples. Grain size was measured using a line intercept method by counting at least 250 grains. The mean grain size of SPS-795 is 800 nm and from the microstructure it can be observed that the material is dense, no large pores were present and the grains appear with sharp edges. In SPS-808 the mean grain size is 120 nm and grains appear more like rounded shape, also no open pores appear in the microstructure suggesting a dense material. In SPS-802 and SPS-804, the mean grain sizes were 90 and 65 nm, respectively, and the grains have a round shape. In these two materials large pores can be seen resulting in low density; this is because of poor sintering conditions, the soak time was probably too short. Figure 1.b shows the relationship between mean grain size and sintering temperature. It can be appreciated that grain size increases with sintering temperature. When the soaking temperature is between 1050 and 1200 ºC the grain size is nanometric (< 200 nm), while at above 1200 ºC the grain is in the submicron range ( > 200 nm). This suggests that high soaking temperatures will accommodate grain growth, at least in SPS process [15].

Figure 1: Properties of SPS samples with respect to sintering temperature a) after relative density and b) after mean grain size

Reference SPS-804 SPS-802 SPS-808 SPS-795 AS

Sintering Temp. (oC) 1100 1150 1175 1600 1450

Density(g/cm3) 5.4 5.7 6 6,05 6.1

Mean Grain Size (nm) 65 90 120 800 300

Vickers hardness HV1 (GPa) 6.2-8 10 - 11.5 13-14.5 10-12.5 13-14

Contact hardness (GPa) 6.5 12.5 17.129 17.105 17

Elastic modulus (GPa) 122 200 240 245 240

Fracture toughness MPa m 5 - 5.2 5 - 5.2 7 - 7.8 5 - 5.4 5 - 5.7

93.1%

89.8%

98.8%98.1%

88

90

92

94

96

98

100

1050 1150 1250 1350 1450 1550 1650

Sintering temperature / 0C

Rel

ativ

e d

ensi

ty %

a)

0

100

200

300

400

500

600

700

800

900

1050 1150 1250 1350 1450 1550 1650

Sintering temperature / 0C

Mea

n G

rain

siz

e (n

m) b)


3

Figure 2: Microstructures of thermally etched SPS sintered samples observed in atomic force microscope (tapping mode): i) SPS-804; ii) SPS-802; iii) SPS-808; iv) SPS-795

Figure 3.a shows the X-ray diffraction patterns of all samples after sintering and fine polishing. The

material is purely tetragonal and no monoclinic phase is found. Tetragonal phase was detected at 2θ values of 30.2° as a single peak and a double peak was observed at 34.5° and 35.2°. After 60 hours of ageing in autoclave at 131 0C all samples were analysed by X-ray diffraction again to quantify the phase change. Monoclinic weight fraction (Xm) was calculated by the method of [16] and monoclinic volume fraction (Vm) by [17] using the following equations:

)101()111()111(

)111()111(

tmm

mmm III

IIX

+++

= (1)

m

mm 0.311X1

1.311XV

+= (2)

Figure 3: X-ray diffraction patterns of all samples: a) after sintering and fine polishing; b) after 60 hours autoclave ageing. (t-tetragonal, m-monoclinic)

SPS 802

SPS 804

SPS 808

SPS 795

t ASmt

m

0

2000

4000

6000

8000

10000

12000

24 26 28 30 32 34 36

2θ

Inte

nsi

ty

b)

SPS 802

SPS 804

SPS 808

SPS 795

t

t

AS

0

5000

10000

15000

20000

25000

30000

35000

40000

45000

24 26 28 30 32 34 36

2θ

Inte

nsi

ty

a)

iv)

ii)

iii )

i)


4

71.4%65.4%

No monoclinic volume fraction found error <4%

0

10

20

30

40

50

60

70

80

65 90 120 300 800

Mean grain size(nm)

Mo

no

clin

ic v

olu

me

frac

tio

n %

X-ray diffraction patterns obtained after ageing are shown in figure 3.b. The tetragonal phase

remains stable after hydrothermal ageing, as no monoclinic phase is observed in samples SPS-802, 804, 808. But in samples SPS-795 and AS the tetragonal phase is destabilised and transformed to monoclinic. From figure 3.b, monoclinic peaks can be observed at 2θ values of 28.1° and 31.4°. Figure 4 shows the amount of phase transformation (t-m) in relation to the grain size. The monoclinic volume fraction was not detected for grain sizes 65, 90 and 120 nm. On the other hand, large amounts of monoclinic volume fractions, 65.4 and 71.4 %, were detected in specimens of grain sizes of 300 and 800 nm, respectively.

Figure 4: Monoclinic volume fraction in relation to the grain size corresponding to the degraded specimens.

4. Discussion It is well known that soak time has a large influence on the density [15, 18]. Porous materials (relative density between 61-90 % of theoretical density) have been sintered when the temperature is below 1050 ºC and the soak time is limited to 15 minutes, but dense materials are achieved only when the soak time is above 60 minutes [15]. Under the present conditions, it is necessary to increase the soaking temperature above 1100 ºC in order to achieve dense materials. The densification mechanisms were proposed by [15] for SPS. When the effective compaction stress and temperature are low, a pure diffusion mechanism is responsible for densification. At intermediate stress and temperature, densification proceeds by grain boundary sliding accommodated by an in-series mechanism controlled reaction step. At high temperatures and stresses, densification proceeds by a dislocation-climb controlled mechanism.

Under the conditions of hydrothermal degradation studied here, it is clear that LTD does not occur for specimens with grain sizes equal or smaller than 120 nm. These smaller grain size specimens have low density because of the short time and low soaking temperature used during sintering; but, in spite of that, they are immune to LTD under the conditions used here. In conventional 3Y-TZP with grain size of about 0.35 µm, it is well known that porosity has a large effect in decreasing resistance to LTD. However, this is not the case in SPS low density specimens with nanometric grain size, although the porosity is connected to the surface, as can be seen in figure 2. Resistance to LTD in sintered bodies with nanometric grain size has been hardly studied in 3Y-TZP. In loose powder it has been shown that nanometric 3Y-TZP powder is resistant to LTD [10]. Here, we also measured the resistance to degradation of the starting 3Y-TZP powder used to produce the sintered bodies (average particle size 70 nm) and no degradation was detected after 30 hours at 131 ºC.

Eichler et al [19] have recently studied hydrothermal degradation of 3Y-TZP with grain sizes in the range of 110–480 nm and its effect on fracture strength by means of fractography and Raman microscopy. Hydrothermal degradation was simulated by applying a pressure of 16 bars to the specimens at 200 ºC for 6 h in an autoclave. They analysed the monoclinic concentration on the fracture surface in terms of the distance to the tensile surface of broken flexure specimens, and observed that significant amounts of monoclinic phase were formed by degradation only in grain sizes


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larger than 210 nm. Although their degradation tests used shorter times and higher temperature and vapour pressure, their results are similar to ours in specimens sintered for SPS. However, it should be indicated that some very small amount of monoclinic was also found by these authors in sintered bodies with 110 nm grain size.

The mechanism of low temperature degradation is still at debate. However it is accepted that OH-

ions are formed at the surface by reaction of H2O with oxygen:

H2Oad + O´ surf → 2(OH) surf (3)

H2Oad is water adsorbed on the surface, O´´surf is an oxygen of the crystal at the surface, (OH) ´surf is the OH- ion that is formed on the surface, which can react with an oxygen vacancy (V● ●

O ) according to the following reaction:

(OH) surf + V● ●

O → (OH) ●O + SxO, surf (4)

where (OH) ●O is an oxygen vacancy site filled by OH- and Sx

O surf, is an oxygen vacant site at the surface. At least at low temperature, the penetration of OH- should be easier along grain boundaries. Similarly in this case:

(OH) gb + V● ●

O → (OH) ●O + SxO, gb (5)

where (OH) gb and Sx

O, gb are, respectively, OH- ion at the grain boundaries and vacant grain-boundary site for oxygen. In equations (4) and (5), oxygen vacancies are annihilated and according to [6] their concentration near the surface could be low enough to destabilise the tetragonal phase, resulting in t–m transformation in the surface layer. When adequate amount of the phase transformation takes place, both micro- and macrocracks can be produced in the transformed surface layers due to the volume expansion associated with the phase transformation. These cracks open up new surfaces to react with water species, leading to a further spontaneous transformation. In addition grain boundaries and cracks are believed to be the main paths for diffusion of OH- ions, due to this phenomenon degradation proceeds further along grain boundaries and cracks.

However, in the nanometric 3Y-TZP specimens (SPS- 802, 804, 808) there is a large density of grain boundaries and pores, and in spite of that, no degradation was detected in these specimens after aging. By contrast, large monoclinic contents were measured in denser specimens, but with grain size larger than 300 nm. In order to account for this, it is important to take into account that the change in free energy for the transformation t-m in a sintered body can be written as [6]:

−∆G = ∆GChem + ∆GResidual stresses + ∆GTransformation+∆GSurface + ∆GTwinning (6)

Where ∆G is the free energy change, which is due to: chemical free energy, residual stresses, transformational strain, surface-free energy and surface energy of the twin planes respectively. The surface term, ∆GSurface, leads to a dependence of the activation barrier for t-m transformation on the particle size. Therefore, the activation barrier to form a critical nucleus is decreased by an increase in particle size. This effect would qualitatively explain the dependence of degradation on grain size. On the other hand, this effect is very well known in transformation toughening of 3Y-TZP since increasing the grain size makes the material more transformable and increases the effectiveness of the toughening mechanism.

However, according to the work of [20] the two elastic contributions, ∆GResidual stresses and ∆GTransformation have a dominating influence over both twinning and surface energy terms. ∆GTransformation is related to the change in free energy related to the volume increase that accompanies transformation and it makes it more difficult, since it induces compressive stresses on the nucleus. On the other hand, the term ∆GResidual stresses is associated to the residual stresses present in the manufactured body and to those that may be created by the diffusion of OH- inside the material [6]. Apparently, the water radicals lead to a change in lattice parameters which shows basically a lattice contraction of a tetragonal unit cell. This contraction due to proton penetration leads to a greater energy difference


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∆GResidual stresses between t- and m-phase which has a destabilising effect because it leads to a smaller activation barrier for transformation.

The resistance to LTD of free nanometric tetragonal particles is the result that the nucleus for transformation is too large since only the first term of equation (6) changes during transformation. The dependence of grain size can be explained in terms of the critical nucleus size necessary for t-m transformation [20] but this nucleus is affected by different factors. In fact the results showed here are in agreement with the size of the critical nucleus (200 nm) calculated by [21]. A smaller grain size implies a higher activation energy barrier for transformation.

The stability of a grain depends on internal stresses, which themselves depend on the level of anisotropy of the thermal expansion. For 2Y-TZP the shear stresses are larger than in 3Y-TZP because of higher thermal expansion anisotropy [20]. It is also found that larger grains contain high stresses in the area close to the grain edge compared to smaller grains [22]. Starting from the same level, the stresses decrease more rapidly for smaller grain sizes. Additionally, from figure 2 it can be observed that the grains appear with round edges in samples SPS 802, 804, 808. The stress level in grains with round edges is lower than perfectly sharp-edged grains [22]. Therefore, in our materials both the grain size and shape were the main contributors for maintaining a stable tetragonal phase during ageing. 5. Conclusions The main conclusions of the present work are summarised as follows: i). 3Y-TZP sintered by spark plasma sintering with grain sizes less than 300 nm were highly

resistant to hydrothermal degradation, in spite of very low density. ii). Fully dense 3Y-TZP sintered either by SPS at high temperature or by conventional isostatic cold

pressing and sintering with grain sizes larger than 350 nm had low resistance to degradation under the same conditions.

iii). High activation barrier due to smaller grain size and less residual stress levels are considered the main causes for the high resistance to LTD.

Acknowledgements RC is grateful to Spanish Ministerio de ciencia e innovación for providing the financial grant under the project MAT 2008-03398, and all authors want to thank Alvaro Mestra and Emilio Jimenez, for their help in experiments. References

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[20] Schmauder S and Schubert H 1986 J. Am. Ceram. Soc. 69(7) 534-540 [21] Matsui M, Masuda M, Soma T and Oda I 1988 In Advances in Ceramics Science and

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degradation resistance of 3y-tzp ceramics sintered using spark plasma sintering

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