State ot the Art in Ceramic Manufacturing R. Lenk

State ot the Art in Ceramic Manufacturing

R. Lenk

Introduktion

Ceramics are among the oldest materials in the service of mankind. First of all the ceramic materials which were used were predominately natural, silicates in origin, i.e. mixtures of clay, feldspars and quartz. In the twentieth century the range of available materials was enlarged by the addition of synthetics, as for example oxides, carbides, titanates and nitrides. These materials also known as technical ceramics exhibit a series of exceptional properties such as high hardness, high strength, good thermal and chemical stability and various functional properties. Well known for example is the piezo electric effect which makes possible the conversion of mechanical pressure or sound into electrical signals and vice versa. Today by means of multiple and yet unique combinations of mechanical, electrical, optical and chemical properties modern ceramic materials open up completely new application possibilities. Evidence of this point are many innovative examples taken from energy technology, from mechanical, construction or automotive engineering, as for example high temperature fuel cells, cutting tools and brakes for Formula 1 racing cars.

The chemical composition and structure determine the properties of high perform-ance ceramic materials. The spectrum of structural characteristics extends from open porosity (e.g. for filtration applications or as carriers for catalysts) to porosity-free and 100% dense (e.g. for sealing rings In water pumps or bath mixers). For high performance ceramics not only the particle size of the original powder, but also the grain size of the microstructure of the material are often in the region of one micron in size or finer. If the region is in nanometers, then the properties such as hardness and strength become especially good. If the grains are smaller than the wavelength of light, then aluminum oxide ceramic can even become transparent. In many cases the original powder is dosed with additives in order to modify the material properties or the formation of the material structure during the sintering process.

Ceramics are produced using powder technology. The material is created by means of material transport processes which occur during sintering and which initiate within the powder package at high temperature, provided the particle size is small (and the associated sintering activity is high) and provided the inter- particle separation is small. The resulting material properties are determined by the fineness and homogeneity of the post-sintering microstructure. For this reason very pure and very fine powder is used. This raw material powder is in the form of undefined agglomerates and first of all it must be prepared. To create a material of the desired quality various powders and sintering additives must be mixed without contamination until they are homogeneous. In the powder pressing process defined granules with optimized processing properties ore used, so that homogeneous densification con be achieved both in pressing and in sintering.

Damage to the material as a result of processing either in the green state or post-

sintering must be reduced to a minimum. All production processes must be held

within tight tolerances and must be extensively controlled. There are many

possibilities for quality assurance in the characterization of both the process and the nnaterial itself.

In this way by means of the design of the structure and the optimization of the tech-nology high performance materials have been developed, whose properties with re-gard to reliability and longevity need fear no comparison. Take, for example, ceramic roller bearings, which have a clearly longer working life than the conventional steel equivalents, which reduce both friction and weight, which have good corrosion resistance and high temperature stability and which are very suitable for lubricated and dry use. So it is not surprising that today nine out of ten Formula 1 racing car teams - Including that of the world champion of the past 6 years - rely on ceramic roller bearing technology and that 90% of all microprocessors and D-RAMs memory modules worldwide are produced with the help of high performance ceramic roller bearing technology. A Formula 1 front wheel bearing, which lasts about 2,000 km in a race, lasts about 200,000 km in a production vehicle such as a Porsche 911 and at the some time gives a weight reduction of 85%. In all three main engines of the US Space Shuttle silicon nitride cylindrical rollers were employed in one of the fastest rotating roller bearings in the world. (The speed of rotation was 3.5 million mm x min \ The bearing was lubricated with liquid hydrogen at a temperature of - 253 °C). These bearings achieved a &d times higher service life than the equivalent steel bearings and were suitable for use in 12 shuttle missions, rather than in a single one, as was the case for steel bearings [1].

Project Description

Ceramic Implants for hip joint replacement belong together with ceramic high per-formance roller bearings and many other products to the group of already established and reliable high technology products made from high performance ceramics. CeramTec AG is known as the leading manufacturer woridwide of ceramic ball heads and cup inserts under the trade name BIOLOX®.

The failure rate of BIOLOX-forfe ball heads in the last few years was between 0.004 and 0.02% depending on ball head diameter. The failure rate of BIOLOX- forfe (Cer-alock) cup inserts is even lower [2]. The identified failure rate has continued to fall from the time of the first introduction of alumina as a bioinert ceramic material for use in hip joint replacements (1975). The reason for this reduction was the improvement of the material (the use of high purity powder, the development of finer grain structure) together with the improvement of the technology (hot isostatic pressing, alternative marking processes). With the development of BIOLOX-de/fa, a fine grained mixed oxide ceramic, the strength of implant ceramics has been once again significantly increased.

In order to evaluate more comprehensively possible risks in the manufacturing

proc-ess, CeramTec AG initiated a project with the Fraunhofer Institutes IKTS in

Dresden and IPT in Aachen. The objective of this collaboration was to determine

and specify possible risks in the manufacture of hip joint implants made from

BIOLOX-forfe and BIOLOX-delta. By means of the perspective of an independent

party with applicable technical expertise it is intended that CeramTec's existing

internal and external evaluations may be enlarged upon, so that the remaining

residual risks can be identified and so that appropriate risk reduction actions can

be initiated.

Method

The appraisal of the Fraunhofer Institute was made taking into account the descrip-tion of the manufacturing processes as specified in the in-house manufacturing standards for "Powder Preparation", "Green Part Production",

"Material Development / Sintering", "Hard Machining / Measurement of Dimensions" and "Quality Management / Production Logistics". Not only were the risks evaluated with respect to their technical relevance, but also further risks were token into consideration. The analysis of ceramic implant failure statistics showed that the cause of existing failure cases can in principle lie both within production and elsewhere. Even if the analysis of the existing cases reveals that no material or production defect was the likely cause of failure, it may under certain circumstances be required to verify that the product was manufactured without defect and that complete production and quality assurance records were available. The risks associated with this verification process were also evaluated.

The assessment evaluated the special features which occurred in the manufacture of ball heads and cup inserts. Over the entire manufacturing process, especially with reference to the formation of the material itself, special attention was paid to the manufacturing of BIOLOX-de/fa, because for implants of this quality the long term experience with this material was less than with BIOLOX-forfe. The assessment of the design and product development processes for individual components and for combinations of components did not however form a part of this assessment.

Results and Discussion

Expert teams were formed consisting of scientists from the Fraunhofer Institute (eleven in all) plus employees of CeromTec AG having the necessary responsibility. The five tasks were performed in parallel and in various phases and were evaluated regularly and in such a way that all the special topic areas were encompassed. First in the "Process Analysis" phase the actual state of the manufacturing processes in the relevant areas of production at CeromTec were inspected and documented. In the second phase, the "Process Description", the sequences of manufacturing steps were investigated for possible deficiencies which would be relevant to the later use of the components in the human body.

In the "Risk Analysis" phase possible residual risks from identified sources of defects within the manufacturing processes were defined, and in doing so a special importance was assigned to "Risks which could lead to Implant Failure". The residual risks were compared both within the individual special topic areas and also were compared with identified risks associated with other topic areas. Finally in order to obtain a complete description or portfolio of the risks each residual risk was assigned on individual weighting factor and they were all viewed visually [3].

In this way all determinable risks could be evaluated using a single method with

re-spect to their probability of occurrence and to the significance of the resulting

defect in a worst case situation, so that a comprehensive, special topic area

encompassing evaluation of their potential could be obtained. Next the risks with

high and middle potential were evaluated, having assigned to them individual

weighting factors which took into account the actual likelihood of the risk causing

damage and the likelihood of the resulting defects being detected. Finally in this

w a y a n evaluation of the remaining residual risks was m a d e possible. The risks with middle risk value w e r e e v a l u a t e d with respect to their i m p a c t one u p o n another.

In this w a y b o t h risks w h i c h h a v e a relatively high a c t i v e influence value a n d also those w h i c h h a v e a strong mutual effect w e r e identified.

All identified risks in the m a n u f a c t u r i n g process are only residual risks, whose prob-ability of having an effect upon patients in the form of a ceramic implant failure, be-cause of the high quality standard and the 100% testing, can be considered as very small. As a result of the combination of the manufacturing standard, the procedures, the work instructions and the testing which is done the manufacturing processes are in their entirety very well controlled, which is also apparent from the high level of product quality which is achieved. The Fraunhofer Institute scientists, who took part in this project, based on their extensive experience can confirm that in the manufacture of hip joint ball heads and cups made from BIOLOX-forfe and BIOLOX-de//a there is a high level of quality awareness.

Potential causes of implant failure in vivo, in addition to the manufacturing processes (material, design and manufacture), are the further use of the implants including their approval for use in combination with other products. The risks associated with this use are higher than all others. While those risks which have been identified in the manufacturing process are considered to have an exceptionally low effect on patient safety, as confirmed statistically by the retrospective data and analysis of the existing ceramic Implant failures for BIOLOX-forfe products, it is however theoretically possible that as a result of sudden, undiscovered manufacturing non-conformities both single defects and also, depending on the actual lot size, in unfavorable circumstances multiple defects could arise. In practice, however, the latter possibility can be eliminated, as the analysis of the measures to assure quality before, during and immediately after product manufacture has confirmed; from the material ageing process the residual risk is unknown but of a low magnitude. The risks, however, associated with the use of the product are in certain circumstances high, but as a rule lead to single failures and are randomly occurring single events, so that the risk of multiple failures can be excluded. The likelihood that the rate of ceramic failures rises significantly above the 1:10,000 level is low. The possibility that multiple failures may occur, which result from defective manufacturing processes, can be practically eliminated.

The procedures and methods which are used in the development,

manufacture and testing of ceramic implants precisely define the status of the

technology. The costs associated with the control of the manufacturing and

quality assurance processes are exceptionally high, but because of the product

safety requirements they are necessary. Compared with any other high

technology products made from high performance ceramics, ceramic ball

heads and cup Inserts for use in hip joint replacement demonstrate best of all that

competent and responsibility-conscious processing using currently available

materials and technologies makes possible the safe application of high

performance ceramics.

Reference

1. Infornnation from CEROBEAR GmbH, Herzogenrath, Germany, In: AdvanCer Newsletter 1/04,2004.

2. Gorino J. et. ol. Ceramic Component Fracture: Trends and Recommendations with Modern Components Based on Improved Reporting Methods, 2005 American Academy of Orthopedic Surgeons annual meeting, Scientific Exhibit number 008.

3. Klugl R: Produktionsrisiken beurteilen, QZ (Qualitat und Sicherheit), 49 (2004), 2, 30.