Ensuring Reliable Performance from Ceramic Components in Medical Applications

Author: Dr Phil Jackson

www.ceram.com

This work by Ceram is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License

Introduction

ISO 13356 “Implants for surgery – Ceramic materials based on yttria-stabilised tetragonal zirconia (Y-TZP)” is a standard1 created to ensure consistent performance of Yttria-Stabilised Zirconia (YSZ) ceramics in implants for surgery. The pass limits set can already meet or be exceeded using existing, mainstream processing routes. However, with the average age of humans increasing (resulting in over-65s making up a higher percentage of the population), there is a challenge to increase the lifetime of zirconia implants towards 30 years or more. This white paper highlights some of the testing involved in ISO 13356 and discusses how recent and on-going research into ceramic processing provides opportunities to meet the challenge.

Yttria-Stabilised Zirconia

Pure zirconia has a monoclinic crystal structure at room temperature. A denser tetragonal form exists at temperatures over 1000°C. Since the inversion between the two crystal phases is accompanied by a large volume change, pure zirconia has poor thermal shock resistance. The tetragonal crystal form has superior toughness and, by introducing yttria at ~5-7 wt%, a partially stabilised form of zirconia (containing a metastable tetragonal phase) exists at room temperature. By adding greater amounts of yttria, a fully stabilised form of zirconia can be produced: This is a cubic solid solution form that undergoes no phase transformation between room temperature and 2500°C and is valued for ion-conducting properties in applications such as oxygen sensors and solid oxide fuel cells. However, partially stabilised zirconia is prized for medical applications such as hip replacements since this form of zirconia has enhanced strength, toughness and hardness for longevity in service. It is partially stabilised zirconia for which ISO 13356 has been developed. It should be noted that the various preparation routes proposed for different zirconias2 have an impact on a number of properties (purity, crystallite size, surface area, etc.) that, as discussed later in this paper, can have a bearing on final sintered properties.

The Tests Involved

ISO 13356 demands that a range of properties are measured. These properties together with the appropriate analytical tests and standards covered are shown in the table below. The associated pass/fail criteria are recorded in ISO 13356 and not reproduced in this article.

Bulk density is measured using standard immersion techniques, as defined in standards including ISO 18754 and BS EN 623-2.

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By measuring chemical composition with XRF-type analysis and, in some cases, other techniques such as atomic absorption, information on impurities is gathered. Not only does this provide a cross-check on the radioactivity test (identification of elements present in zirconia that could contribute to radioactivity), it also highlights elements with the potential to act as fluxes and so generate unwanted glass phases. As reported earlier, different crystalline phases have different changes in volume with increased temperature which can lead to unwanted stresses developing during sintering - X-Ray Diffraction (XRD) allows the level of such phases and the desired tetragonal crystal phase to be monitored.

Property to be Measured Analytical Technique / Relevant StandardsBulk Density ISO 18754; BS EN 623-2Chemical Composition XRFGrain Size SEM / BS EN 623-3; ASTM E112% Monoclinic Zirconia Present XRD / ASTM F1873-98Strength 4-point bend / ISO 14704; ISO 7500-1;2004

Or bi-axial flexural test / ASTM C1499; ISO 7500-1; ISO 3611;

Cyclic Fatigue 4-point bend test with sample immersed in saline solution

Radioactivity Gamma Spectroscopy / ASTM G136-03Accelerated Ageing XRD , 4-point bend or bi-axial flexural test

performed after accelerated ageing using autoclave

The grain size calculation involves recording SEM images using a mean linear intercept method, in accordance with ASTM 112 or BS EN 623-3. These are made on samples that have been firstly polished and then subjected to thermal etching. The latter process involves heating the sample to ~50-100°C below the sintering point for zirconia. This heating initiates the grain growth process but only to a small extent; more importantly, it highlights the grain boundaries to make the calculations

demanded by ISO 13356 more reliable. Once the grain boundaries are clearly in evidence, a series of lines are drawn in random directions. Both the length of the line and the number of grain boundaries crossed are recorded. A mean boundary rate is then determined, with higher values denoting a smaller grain size that (in the absence of voids, cracks) is likely to be associated with increased strength.

The monoclinic content of the material is determined on polished material using XRD to interrogate the crystal structure and quantify the different polymorphs of zirconia.

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ISO 13356: Property summary and associated tests/standards

SEM (Scanning Electron Microscopy)

Direct measurements of sintered strength can either be made using a 4-point bend or bi-axial flexural strength test according to the defined standards. In the former case, great care must be taken to prepare rods of sintered zirconia that are dimensionally accurate and free of blemishes that could weaken the structure. Sintered ceramics are brittle, with strength dependent on the size distribution and orientation of flaws present. There is likely to be a spread of strengths when a number of supposedly identical samples are tested. By calculating the Weibull modulus (highly recommended) producers can gain an idea of how reliable the mean strength value calculated is. The Weibull modulus3,4 is represented by "m" in the following equation:

F = 1 - exp {-(σ - σ n)/ σ o))m}

where F = probability of failure; σ = Stress; σn = the minimum strength recorded and σo = the strength at which there is a 63.2% chance of failure.

The higher the Weibull Modulus (m), the more consistent the material performance will be. Or, put another way, the more reliable the recorded mean strength will be in indicating strength. Values for m of 10-15 are typical in sintered ceramics. This translates to a + or 10% variation in strength. When m reduces to 5, strength variations rise to + or - 30%.

The cyclic fatigue test involves applying/removing a load sinusoidally over 105 cycles as part of a 4-point bend test. By carrying out the test with the sample immersed in saline solution, data representative of the effects delivered to prostheses during normal every day movement by the patient is ensured. By recording the breaking strength following cyclic testing, any resulting loss of strength due to crack formation is revealed.

Radioactivity measurements are demanded by the standard since natural zirconia (baddeleyite) can be associated with significant amounts of natural radioactivity from uranium decay series; clearly the application demands that any zirconia used exhibits minimal radioactivity.

Accelerated ageing is designed to confirm that there is no catastrophic degradation of the zirconia implant over time. The ageing is conducted using a pressured autoclave and repeat measurement of either strength or monoclinic zirconia content by XRD is performed.

Variables Impacting on Performance Against the Standard

There are many process and material variables that the ceramic producer has to both consider and work with to ensure good performance against the limits imposed by ISO 13356.

Sub-micron and nano-sized YSZ powders both have an important role to play in enhancing the performance of sintered zirconia. By processing smaller size powders, there is great potential in increasing the number of grain boundaries present in the final sintered product and so enhancing both the aforementioned prized properties and level of confidence in consistently attaining them. In embracing nanotechnology, there are two potential problems however:

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1) Maximising the wt% loadings of nano-powder in water prior to subsequent processing. The greater the level of nano-powder present, the lower the contraction upon drying and sintering. Given the huge surface area associated with dispersed nano-powder, agglomeration and associated rises in suspension viscosity (to levels greater than those tolerated in subsequent processing) can quickly cause problems. Recent research6 however has given hope that high solids loadings can be achieved, even in nano-powder suspensions. Work carried out at Ceram as part of an FP6 European project ("SAPHIR") has also shown that, for nano-titania at least, correct selection of dispersant (type and wt% used) combined with (a) gradually adding the dry nano-powder to raise solids content (b) ageing and (c) use of ultrasonics, can all help to deliver 45wt%+ suspensions that can subsequently be spray-dried. There is no reason to suspect that this approach cannot be repeated for other nano-oxide systems. Indeed, Ceram is managing a TSB-funded project "NASTRAC" - Nano-structured Advanced Ceramics - that is looking to apply similar technology patented by Loughborough University in zirconia and barium titanate systems.

2) Retention of nano-structure during sintering. Sintering is needed to remove porosity and raise bulk density. However, elevated temperatures also encourage ion diffusion and grain growth. It has been shown6 that the use of novel temperature/time profiles (especially a sharp rise to the peak temperature followed by a temperature drop and then soak at the lower temperature) and/or microwave-assisted firing can retard grain growth. For YSZ, final grain sizes of ~65nm in a 99% dense component have been achieved from a starting powder size of ~16nm. Working with Carbolite, Ceram has various projects investigating the impact that microwave-assisted firing can have in manipulating sintered microstructures.

Novel Processing of Ceramics

Novel processing of ceramics also opens up possibilities in terms of controlling green state and sintered micro-structures. A high solids content suspension (whether 100% nano or a nano/micron mix to optimise particle packing) allows direct consolidation casting as a route for near-net shape fabrication from liquid suspensions. Direct consolidation covers a range of techniques8 in which chemical reactions (triggered by temperature, pH change etc.) convert a liquid, high solids de-flocculated suspension to a solid flocculated solid. The advantages of near net shape in terms of no or reduced machining in the green or sintered state are obvious. In achieving a high solids content slurry, particle size distribution has a key role to play. By maximising packing of particles, not only will contraction during drying and sintering be minimised (due to limited opportunities for particles to re-arrange), but suspension properties will be helped too. Good packing of particles in the suspended state minimises the amount of liquid medium trapped between particles and so maximizes the amount of liquid available to impart fluidity for suspension processing.

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Where pressing of a dry YSZ granulate is preferred as the shaping technique, Ceram has demonstrated that Freeze Dry Granulation of suspensions can deliver a more

homogeneous (although more porous) granulate compared with spray drying. Freeze Dry Granulation involves atomising a nano-powder suspension and immediately mixing it in liquid nitrogen whilst stirring. The frozen granulate are then transferred to a sublimation unit to deliver a dry granulate. Whilst attention has to be paid to the lower strength of the granulate (e.g. granulate breakdown during transport could be a problem) and higher compaction ratios, a freeze dry granulate is more likely to compact to a blemish-free pressed component. The aforementioned

work on maximising solids content in suspensions has a key role to play in allowing this granulation technology to be transferred into ceramics applications.

Conclusion

In conclusion, ISO 13356 provides a comprehensive suite of tests to assess zirconia component performance against the demands of implants. The standard needs to be considered carefully in conjunction with: a) current processing variables and b) novel materials/processes – both can impact heavily on the experimental results recorded and therefore on in-service performance of the implant.

References

1. ISO 13356:2008 Implants for surgery – Ceramic materials based on yttria-stabilised tetragonal zirconia (Y-TZP)

2. Web-site article “applications and preparations of zirconia and stabilized zirconia powders” at http://www.americanelements.com

3. “A Statistical Theory of the Strength of Materials”, W. Weibull, Ingeniörsvetenskapsakademiens Handlingar NR 151, 1939

4. Breviary Technical Ceramics, article on Weibull Modulus at http://www.keramverband.de/brevier_engl/5/3/3/5_3_3_4.htm

5. “Dense nano-structured zirconia by 2-stage conventional / hybrid micro-wave sintering”, J. Binner et al. J. Eur. Ceram. Soc. Vol 28 No.5 2008, pp973-977

6. “Processing of bulk nano-structured ceramics” J. Binner & B. Vaidhyanathan. J. Eur. Ceram. Soc. Vol 28 No.7 2008, pp1329-1339

7. NASTRAC Project, Ceram: http://www.ceram.com/ceramics/nastrac.html

8. “Novel Powder-Processing Methods for Advanced Ceramics”, W.M. Sigmund; N.S. Bell; L.Bergstrom. J. Am. Ceram. Soc Vol. 83 No.7, 2000 pp1329-1339

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Freeze dry granulation equipment

About Ceram

Ceram is an independent expert in innovation, sustainability and quality assurance of materials.

With a long history in the ceramics industry, Ceram has diversified into other materials and other markets including aerospace and defence, medical and healthcare, minerals, electronics and energy and environment.

Partnership is central to how we do business; we work with our clients to understand their needs so that we can help them overcome materials challenges, develop new products, processes and technologies and gain real, tangible results.Headquartered in Staffordshire, UK, Ceram has approved laboratories around the world.

About the Author

Dr Phil JacksonExpertise in: Powders; Medical DevicesBusiness Development Manager

Phil Jackson holds a BSc Hons in Chemistry and a PhD in Thermodynamics, both gained at Leicester University. Owing to over twenty years of experience, Phil’s areas of expertise lie in glass chemistry, novel shaping processes and in the practical control of powder suspensions.

Moving from a focus on glaze and ceramic powder processing for the traditional ceramic industry in the early part of his career, Phil transferred his expertise to advanced ceramic process issues, including control of nano-suspensions and precipitation processes for electronics applications.

More recently Phil has focused on applications in the medical devices sector and in particular to the development of implanted structures, surgical instrumentation and implant/anti-bacterial coatings.

Phil is also a Technology Translator for the Materials KTN, identifying powder needs in various industries and helping to connect companies and universities with complementary skills.

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