Alumina Composite: The Present Generation of Load Bearing Ceramics in Orthopedics

ISO Standard

ISO 6474-1

ISO 13 356

ISO 6474-2

Material

Unit

Pure alumina

Zirconia

Alumina zirconia type X

Average bulk density

≥3.94 g/cm³

≥6.00 g/cm³

≥99 %

Chemical composition

wt %

Al₂O₃ ≥ 99.7, MgO ≤ 0.2, impurities ≤ 0.1

ZrO₂ + HfO₂ + Y₂O₃ ≥ 99.0, Y₂O₃ 4.5–6.0, HfO₂ ≤ 5, Al₂O₃ ≤ 0.5, Others ≤ 0.5

Al₂O₃ 60–90, ZrO₂ + HfO₂ 10–30, additives ≤10, impurities ≤ 0.2

Grain size

Mean value AI₂O₃

µm

≤2.5

≤1.5

Standard dev

≤40 %

≤25 %

Mean value ZrO₂

µm

≤0.4

≤0.6

Standard dev.

≤40 %

Strength

MPa

≥500

≥800

≥1000

Weilbull modulus (4 pt bending)

≥8

Young’s modulus

GPa

≥380

≥320

Fracture toughness

MPa √m

≥2.5

≥4.0

Hardness HV1

GPa

≥18

≥16.0

Wear resistance

Info

For comparison before and after accelerated aging

Cyclic fatigue limit

No failure at 200 MPa

No failure at 320 MPa

No failure at 400 MPa

Upper limit of monoclinic phase

≤20

Radioactivity zirconia raw material

Bq/kg

≤200

Accelerated aging

5 h authoclaving ≤ 25 % monocle phase

10 h autoclaving

Autoclave (0.2 MPa, 134° C)

Strength decrease not more than 20 %

Alumina Matrix Composites

Alumina-matrix composites represent the latest development in alumina technology, and are identified by some Authors as “the fourth generation alumina”. Alumina-matrix composites had been developed to obtain a ceramic in which the biocompatibility and stability of alumina would be joined to enhanced toughness and mechanical properties with respect to pure alumina [7]. Materials with different microstructures may be achieved in these systems, as exhaustively reported by Claussen [8]. Zirconia-Toughened Aluminas (ZTAs) were investigated in France and Italy [9–11], in view of applications in orthopedics without leading to clinical applications up to now. It was in the 2000s that an in situ reacted Alumina Matrix Composite (BIOLOX® delta) had been qualified for clinical applications [4].

BIOLOX® delta is obtained by the following chemical-physical reactions taking place within the lattice during sintering [12]:

The basic transformation equations are known as follows:

$2{\mathrm{Y}\mathrm{CrO}}_3\ \to {\mathrm{Y}}_2{\mathrm{O}}_3 + {\mathrm{Cr}}_2{\mathrm{O}}_3$

(14.1)

${\mathrm{SrZrO}}_3\ \to\ \mathrm{S}\mathrm{r}\mathrm{O} + {\mathrm{ZrO}}_2$

(14.2)

${\mathrm{Al}}_2{\mathrm{O}}_3 + {\mathrm{Cr}}_2{\mathrm{O}}_3\ \to\ {\mathrm{Al}}_2{\mathrm{O}}_3:\ \mathrm{C}\mathrm{r}$

(14.3)

$\mathrm{S}\mathrm{r}\mathrm{O} + 6{\mathrm{Al}}_2{\mathrm{O}}_3:\mathrm{C}\mathrm{r}\ \to\ {\mathrm{SrAl}}_{12\hbox{-} \mathrm{x}}{\mathrm{Cr}}_2{\mathrm{O}}_{19}$

(14.4)

${\mathrm{ZrO}}_2 + {\mathrm{Y}}_2{\mathrm{O}}_3\ \to\ {\mathrm{ZrO}}_2:\mathrm{Y}$

(14.5)

The sub-micron particles of yttria-stabilized tetragonal zirconia (Y-TZP) are forming about 17 vol% of the material while strontium aluminate platelets are accounting for about 1 vol%, the balance being fine grained alumina containing in solid solution chromium dioxide, forming the matrix of the ceramic. toughened by phase transformation in the Y-TZP grains and reinforced by the presence of platelets acting as a secondary, reinforcing structure in the composite.

Behavior of BIOLOX® Delta

Mechanical Properties

The bending strength and toughness values characteristic of BIOLOX® delta have been summarized in Table 14.1. These properties are based on the presence both of transformation toughening by zirconia and by the reinforcement due to the platelets within the alumina matrix.

Transformation Toughening by Zirconia

The presence of the Y-TZP phase is crucial for the reinforcement of BIOLOX® delta. These sub-micron particles evenly distributed within the microstructure are metastable in nature, and on their ability to transform their lattice from tetragonal to monoclinic relies their contribution to the mechanical properties of the composite. The tetragonal to monoclinic transformation in Y-TZP biomaterials had been already comprehensively described elsewhere [13]. In Alumina Matrix Composite when the constraint exerted onto the zirconia grains by the stiff alumina matrix is relieved, i.e. by a crack advancing in the material, the zirconia grains near the crack tip shift simultaneously and in an ordered way into monoclinic phase The 3–4 % volume expansion and the shear strain associated to tetragonal-to-monoclinic transformation of grains absorbs energy. This dissipates the energy of the advancing crack at the crack tip due to the T-M transformation. The crack needs to overcome the compressive stress field created within the matrix by the volume expansion of the grains to progress further. These energy dissipating mechanisms are the basis of the toughening contribution of the YTZP phase to BIOLOX® delta.

New Experimental Evidence for the Impact of Platelets

Incorporation of fibres, whiskers or platelet shaped components into a ceramic composite material is a well-known concept for toughness improvement. The toughening mechanism of such components is usually described based on crack deflection and crack bridging. Crack deflection requires strong reinforcing elements and a relatively weak interface. Furthermore, the volume share of the reinforcing phase should cover at least 10 % or more.

Analysis of crack extension on the surface of a BIOLOX® delta specimen was conducted. It was found that the platelets visible on the surface do not provide any crack deflection nor crack bridging. While this analysis does not examine platelets in the bulk material, on the surface the cracks preferably cross the platelet crystals. It is suggested from this analysis that crack shielding and bridging may not be the only key mechanism in this material.

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