Validation of a High Performance Alumina Matrix Composite for Use in Total Joint Replacement

Fatigue

Resistance against long time static and alternating load

Ageing

Resistance against hydrothermal or other chemical attack

Wear

Durability under abrasive conditions

In this chapter, the life time limiting mechanisms and the relevance for the application as a surgical implant are discussed. It is shown how life time of the ceramic material BIOLOX®delta can be described and evaluated. The unique microstructure and reinforcing mechanisms of the material not only support the short term performance like fracture toughness and strength but also improve substantially the long term reliability.

Description of Biolox®Delta

BIOLOX®delta is an alumina based composite ceramic. 80 vol % of the matrix consist of fine grained high purity alumina which is very similar to the well known material BIOLOX®forte. As it is the case in any other composite material, the basic physical properties like stiffness, hardness, thermal conductivity etc. are mainly predetermined from the dominating phase. It was the basic idea for the development of the new material to preserve all the desirable properties of BIOLOX®forte which has millions of components in service but to increase its strength and toughness. These properties are rigorously improved by implementation of reinforcing elements. Figure 15.1 shows the microstructure of BIOLOX®delta.

Fig. 15.1

Microstructure of BIOLOX®delta

Two reinforcing components are integrated in BIOLOX®delta. 17 vol % of the matrix consist of tetragonal zirconia particles. The average grain size of the zirconia is around 0.2 μm. As a further reinforcing element, apprx. 3 vol % of the matrix are built by platelet shaped crystals of the ceramic composition strontiumaluminate. The platelets stretch to a maximum length of apprx. 3 μm with an aspect ratio of 5–10

Additionally to the reinforcing components, there are other elements doped to the material. The composite contains yttrium which is solved in the zirconia and which supports the stabilization of the tetragonal phase. The material contains Chromium, this element is known to be soluble in the alumina matrix, similar to the gemstone ruby which is an alumina single crystal with certain chromium content. The only effect of chromium in BIOLOX®delta is the eye-catching pink color of the final material

The reinforcing elements, in particular the zirconia, substantially increase fracture toughness and strength of the material [1, 2]. Fracture toughness (K_IC) is a measure for the ability of the material to withstand crack extension. Strength (s_c) is defined as the maximum stress within a structure that causes failure of the component. Consequently, when the fracture toughness of the alumina is increased also the strength is directly improved. This basic principle is the concept of the development of BIOLOX®delta. The microstructure is designed in order to provide a maximum of resistance against crack extension.

The benefit in crack resistance which is obtained from incorporating zirconia into an alumina matrix are well known in the science of high performance ceramics, as it is shown in Fig. 15.2.

Fig. 15.2

Reinforcing mechanism in BIOLOX®delta at crack initiation and propagation

The figure represents a realistic part of the microstructure. In the case of severe overloading crack initiation and crack extension will occur. High tensile stresses in the vicinity of the crack tip trigger the tetragonal – monoclinic phase transformation of the zirconia particles. The accompanied volume expansion leads to the formation of compressive stresses which are very efficient for blocking the crack extension.

As it is shown this reinforcing mechanism is fully activated within a region of a few micrometers. For the macroscopic performance of the material it is extremely important that immediately at the beginning of crack initiation also the reinforcing mechanisms are activated. Regarding Fig. 15.2 one should keep in mind that the average distance between the reinforcing zirconia particles is apprx. 0.2 μm, i.e. similar to the grain size. Thus, the reinforcement is activated immediately when any microcrack is initiated.

The reinforcing ability of zirconia particles is a consequence of the phase transformation, i.e. the spontaneous change from the tetragonal to the monoclinic phase. The phase transformation is accompanied by a volume change of 4 % of the zirconia particle, i.e. a linear expansion of 1.3 %. Spontaneous phase transformation is a well known principle in material science. For example, the properties of high performance steels also rely on spontaneous phase transformation from austenite to martensite.

It should be emphasized that the ability of phase transformation is the precondition for any benefit of the zirconia within the material. The composite is designed such that phase transformation occurs when it is needed, i.e. in the case of microcrack initiation. In contrast to pure zirconia (which draws its high strength from the same principle) the main source of the stability of the tetragonal phase is the embedding of the zirconia particles in the alumina matrix. In contrast, the stability of pure zirconia only relies on the chemical stabilisation (i.e. doping with yttria) and the grain size, which should not exceed a certain range. This is the most important distinction of the composite material BIOLOX®delta to pure zirconia. In particular, the mechanical stabilization of the stiff alumina matrix is not sensitive to any ageing effect.

Correlation of Material and Component Properties

Material properties are determined to ISO 6474-2 which is applicable for zirconia toughened alumina composite ceramics. In comparison to a pure alumina material, these composites are distinguished by a significantly improved strength and toughness. It should be noticed that the high strength limit in ISO 6474-2 of 1000 MPa is outperformed by BIOLOX delta by almost + 40 % ! In this chapter it is intended to discuss shortly how these materials data correlate to component properties, e.g. the strength of ball heads and inserts.

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