26 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Effect of manufacturing technology of ball-and-socket joint made of Co28Cr6Mo alloy on its tribological properties

Adrian Mróz1*, Dariusz Garbiec1, Jarosław Jakubowicz2, Łukasz Łapaj3

1Metal Forming Institute, Poznan, Poland, 2Institute of Materials Science and Engineering, Poznan University of Technology, Poland, 3Department of General Orthopaedics, Musculoskeletal Oncology and Trauma Surgery, Poznan University of Medical Sciences, Poland,

*adrian.mroz@inop.poznan.pl

The article presents the results of friction and wear testing of ball-and-socket joints, made of ASTM F75 alloy powder, applying the technologies of selective laser melting and spark plasma sintering. The reference material constituted joints produced by machining from a ASTM F1537 LC rod. The tribological tests were carried out using the movement simulator of a spinal motion segment of our own design, in the environment of distilled water. The test results consisted of a comparison of the friction resistance values occurring in the friction couple and comparison of the wear indicators of the upper and lower components, which simulate the tribological system of an intervertebral disc endoprosthesis in the spinal lumbar segment. Analysis of the wear mechanism of the bearing surfaces was performed on the basis of microscopic observations and measurements of friction surface roughness. Regardless of the manu-facturing technology, the coefficient of friction varied in the range of 0.25 to 0.30, wherein a median value for all the tested material combinations was in the range of 0.27 to 0.29. The lowest resistance to tribological wear was demonstrated by the joints produced employing spark plasma sintering technology. The wear of the joints produced with this technology was about 20% higher in comparison to those joints formed by selective laser melting, and about 30% higher compared to the joints produced in a conventional manner from ASTM F1537 LC alloy. Observations performed using scanning electron microscopy showed that regardless of the production technology, the dominant type of wear was abrasive-adhesive wear. Unlike the joints produced in a conventional manner, the bearing surfaces of the components produced by means of selective laser melting and spark plasma sintering technologies, there is evidence of fatigue damage. The results call into question the possibility of using alternative technologies in the manufacturing process of endoprosthesis components exposed to tribological wear.

Key words: Co28Cr6Mo alloy, selective laser melting, spark plasma sintering, ball-and-socket joint, simulator testing, wear.

Inżynieria Materiałowa 1 (215) (2017) 26÷31DOI 10.15199/28.2017.1.5© Copyright SIGMA-NOT MATERIALS ENGINEERING

1. INTRODUCTION

The metal components of osteoarticular system endoprostheses usu-ally are produced from cobalt matrix alloys. The Co28Cr6Mo alloy owes its popularity in orthopedics to the fact that it has relatively the best tribological wear resistance compared to other currently used biomaterials [1]. However, we are still witnessing the emergence of new reports on periprosthetic tissue response to wear products generated by friction. This information encourages researchers to take measures to develop new materials, or methods to improve the properties of existing alloys. Despite the favourable properties of selective laser melting (SLM) and spark plasma sintering (SPS) in implant manufacturing or prototyping [2, 3], little is known about the tribological performance of implants with bearing surfaces fab-ricated using those technologies. That is why we decided to evalu-ate the tribological performance of ball-and-socket joints made of the Co28Cr6Mo F75 alloy using SLM and SPS in comparison with one machined from a forged ASTM F1537 LC alloy (subsequently ASTM F1537). The geometric properties of the working surfaces (friction) of the ball-and-socket joint elements have been selected in such a way as to reflect the tribological system bottom plate–insert (three-element intervertebral disc endoprostheses) or upper plate–bottom plate (two-element endoprostheses) [4÷6], providing an adequate range of motion relative to human anatomical planes [7].

This paper is a continuation of research, the results of which are presented in [8, 9]. In the context of [8], a comparison was made of the physical, mechanical and corrosion resistance properties of samples produced using SLM and SPS technologies, in respect to the reference material ASTM F1537. The previous publication [9], was devoted to preliminary verification of the resistance to tribo-logical wear of samples produced by means of SLM and SPS tech-nologies. The aim of this study is to analyze the tribological wear mechanism of bearing surfaces of the ball-and-socket joint elements

in kinematic conditions of motion and loads similar to those that oc-cur in a natural lumbar spinal motion segment. The tribological tests were carried out using an original movement simulator of a spinal motion segment designed by the authors.

2. MATERIALS AND METHODS

The choice of starting materials and technological parameters of the selective laser melting, spark plasma sintering, and machining pro-cesses was conducted in the same way as it was during the perfor-mance of the tests described in [8, 9]. The used equipment, starting materials, most important technological parameters and the selected material properties are summarized in Figure 1 and Table 1. The elements of a ball-and-socket joint with a radius of 10.5 mm (Fig. 2) were polished. The required starting value of the Ra roughness of the friction surfaces was less than 0.05 μm.

The friction and wear tests were carried out using a move-ment simulator of a spinal motion segment of our own design, an SBT-03.1 simulator. The tribosystem made of the test material was pressed from above with an alternating test force of a maximum instantaneous value of 1500 N. The tests were performed at 37°C in the medium of distilled water. Each tested friction pair underwent 1·106 test cycles with a motion frequency of 1.25 Hz. The upper component during a single cycle performed bending movement in the range of –5 to +8° relative to the loading force, whereas the lower component performed rotational movement relative to the axis of rotation in the range of –3 to +3°.

Immediately before and after completing the tests, the samples were washed twice in acetone using an ultrasonic bath (twice for 15 min), and then dried using a vacuum drying system (pressure less than 66 kPa) for 60 min. After 90 minutes from the end of the drying process, the sample was weighed with an analytical scale, R200D (Sartorius, Germany). Roughness measurements were

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carried out using a Hommel profilometer ETAMIC T8000 RC (Jenoptik AG, Germany). Analysis of the wear mechanism of the test materials was made on the basis of SEM observation of the bearing surfaces. Observations were carried out using a scanning electron microscope Inspect S (FEI, The Netherlands).

3. REULTS

The measuring system used in the simulator makes it possible to determine the coefficient of friction based on recording the fric-tion torque. Figure 3 shows a comparison of the minimum and maximum median values of the friction coefficient against the total range of friction coefficient values.

The obtained results indicate that regardless of the manufactur-ing technology, the friction resistance was similar. Analysis of all the partial values of the friction coefficient (audit every 50 000 cy-cles) do not differ significantly statistically on the adopted signifi-cance level p (α) <0.05.

Table 2 shows the mean weight loss values for the individual components as well as the mean wear values calculated on the basis of those weight loss values.

Fig. 1. Physical and mechanical properties of tested joints [8, 9]Rys. 1. Właściwości fizyczne i mechaniczne badanych przegubów [8, 9]

Table 1. Used equipment, starting materials and most important technological parameters [8, 9]Tabela 1. Zastosowane urządzenia, materiały wyjściowe i najważniejsze parametry technologiczne [8, 9]

SLM SPS ASTM F1537 LC

Starting mterial Alloy powderASTM F75, particle size: 20÷60 μm

ASTM F1537 LC rod, diameter: 40 mm

Equimpment SLM® 250 HL (SLM Solutions GmbH, Germany)

SPS HP D 25-3 (FCT Systeme GmbH, Germany)

HS 1000 milling machine (AVIA, Poland)

Technological parameters

laser beam power 175 Wwavelength 1070 nmspot diameter 200 µmlaser speed 685 mm/s

powder layer thickness 30 µmatmosphere O2 <0.3%

overlap 30%

sintering temp. 1000°Cheating rate 100°C/min

compaction pressure 50 MPa impulse duration 15 ms

impulse interval 3 mssintering time 5 minatmosphere vacuum

cutting speedroughing: 25 m/minfinishing: 28 m/min

feed rateroughing: fz = 0.05 mmfinishing: fz = 0.05 mm

a) b)

Fig. 2. Ball-and-socket joint elements: a) upper component – concave surface, b) lower component – convex surface Rys. 2. Elementy przegubu kulowego: a) komponent górny – powierzch-nia wklęsła, b) komponent dolny – powierzchnia wypukła

Fig. 3. Median friction coefficientRys. 3. Mediana współczynnika tarcia

28 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

Table 2. Mean weight loss, mean wear indicators and mean bearing surface roughness before and after test completion Tabela 2. Średnie zużycie masowe, średnie wskaźniki zużycia oraz średnia chropowatość powierzchni trących przed i po zakończeniu próby

Upper component Lower component

Weight loss mg

Wear indicator mm3/Nm

Roughness Ra, µm Weight loss mg

Wear indicator mm3/Nm

Roughness Ra, µm

before after before after

SLM 9.5 1.807·10–7 0.04±0.01 1.11±0.01 7.4 1.408·10–7 0.04±0.01 1.14±0.02

SPS 11.7 2.278·10–7 0.04±0.01 2.05±0.50 8.9 1.733·10–7 0.04±0.01 1.43±0,04

ASTM F1537 LC 7.9 1.492·10–7 0.03±0.01 1.09±0.04 6.5 1.228·10–7 0.04±0.01 1.07±0.04

Based on the obtained results, it can be concluded that the least resistance to tribological wear was characterized by the joints pro-duced with SPS technology. The wear of the joints produced us-ing this technology was about 20% higher in comparison to those formed by selective laser melting, and about 30% higher compared to the ASTM F1537 joints. Furthermore, regardless of the technol-ogy used, the wear of the lower component was 20÷30% less com-pared to the upper component.

Table 2 also presents the mean Ra values of the friction surfaces. The presented data show that before the start of the test, the mean values of this parameter ranged from 0.03±0.01 to 0.04±0.01 mm. As a result of the conducted the tests, the bearing surface roughness of the tested joints increased. The greater the Ra value, the greater the tribological wear — the Pearson linear correlation coefficient value was 0.87.

Exemplary results of SEM observation of the friction surface morphology after the tribological tests are shown in Figure 4. The observations performed using scanning electron microscopy showed that regardless of the production technology, the dominant type of wear was abrasive-adhesive wear. As a result of friction, the cyclic formation and destruction of adhesive connections between the cooperating surfaces occurred. In this way, wear products were generated, which while remaining in the friction couple, initiated the wear process (Fig. 4a÷c). In the case of joints produced by SLM technology, the process was most likely intensified by spallation of the material. The process of cracking was initiated in places where casting defects occurred, formed during crystallization of the alloy (Fig. 4a).

Moreover, on the friction surfaces of the SLM joints, the pres-ence of traces of fatigue wear, manifesting characteristic parallel discontinuities in the material, was revealed. (Fig. 4b). The tribo-logical wear resistance was the smallest for the joints produced by means of SPS technology. This is evidenced by the numerous and clear signs of wear. In Figure 3c, traces of plastic deformation, which were probably due to fatigue are visualized. On the bearing surfaces of all the tested joints, traces of pitting and delamination were found (Fig. 4b, d, e).

4. DISCUSSION

In paper [8] a comparison of the physical, mechanical and corro-sion resistance properties of the samples produced using SLM and SPS technologies in relation to ASTM F1537 LC was presented. The same phase composition, comparable corrosion resistance, comparable (for samples sintered by SPS) or greater (for sam-ples produced by SLM) hardness as compared to the reference material, compelled the authors to formulate the claim that SLM and SPS technologies can potentially be used in the production of endoprostheses. The previous publication [9] was devoted to preliminary verification of the wear resistance of tribological samples produced by means of SLM and SPS technologies . The obtained results were not satisfying. It turned out that the sam-ples prepared from the Co28Cr6Mo alloy using SLM and SPS technologies displayed lower tribological wear resistance com-pared to the reference material. It should be noted that the applied

research methodology has certain limitations: the tribosystem, motion kinematics and load conditions did not reflect the working conditions that exist in the human osteoarticular system.

Reducing the articulating speed, different type of motion and employing a lubricant influenced changes in friction regimes compared to [9]. This is evidenced by the results of the coefficient of friction calculations. Regardless of the technology used for producing the joint, the coefficient of friction varied in the range of 0.25 to 0.30, wherein the median value for all the tested mate-rial combinations was 0.27÷0.29 [9]. The proposed methodology allowed the authors to disclose many more wear mechanisms of the friction surfaces (Fig. 4). Besides traces of abrasive-adhesive wear, signs of wear fatigue (delamination, plastic deformation, pitting) were also observed. Similar phenomena was observed by the authors of [10]. They substantiate the occurrence of the fatigue wear mechanism, the occurrence of local stresses at the contact point of the friction couple elements. As a result of these stresses, chromium carbide and cobalt are deformed and torn, resulting in additional surface fatigue and through consequential denting in the bearing surfaces initiating wear. From this point of view, the coarse structure of the Co28Cr6Mo alloy after the SLM process seems to be less favourable compared to the reference material.

Chemical segregation and occurrence of casting defects due to alloy crystallization in the SLM process was most likely the cause of crack initiation of the material in this area.

Of all the tested material combinations, the joints formed by SPS technology have the lowest resistance to tribological wear. On the basis of SEM observations and roughness measurement results, it can be concluded that the interaction between the bear-ing surfaces and reinforced wear debris was the most intensive. It is known that the interaction between wear debris and the bearing surfaces depends on the shape of the wear products but also on the hardness of the friction surfaces [11]. The hardness of the bear-ing surface of joints produced using SPS technology was rela-tively the lowest and amounted to 385 HV0.5. A further drawback of the SPS joints was their porosity, comparatively the highest (over 3%).

We would like to highlight several limitations of this study for the reader. First, indeed the simulator works close under loads similar to those that occur in a natural L4–L5 motion segment, but enables the realization of motion only in two instead of three ana-tomical planes. Irrespective of these limitations, use of the spine motion simulator gives more valuable results. Second, during the tribological tests distilled water instead of a water solution of bo-vine serum with around a 20 g/L protein concentration around was applied. We had decided to use only distilled water in view of the more aggressive wear regime which produces a greater volume of wear debris of similar morphology and size to that produced un-der similar conditions in bovine serum [12]. Of course we should keep in mind that the tribological properties of CoCrMo alloys are very different in distilled water and solutions such as bovine or calf serum that mimic the in vivo conditions. In the appropriate biological solutions the wear and tribocorrosion are completely different to what they are in distilled water.

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5. CONCLUSIONS

The obtained results allow the authors to draw the following con-clusions: 1. The ball-and-socket joints made from the cobalt alloy

Co28Cr6Mo, using the techniques of selective laser melting and spark plasma sintering are characterized by inferior tribological properties in comparison with the reference material produced from ASTM F1537 LC.

2. Regardless of the production technology, the dominant type of wear of the friction surfaces was abrasive-adhesive wear. The

use of the spinal motion simulator enabled the disclosure of signs of fatigue wear (delamination, plastic deformation, pitting).The study showed poorer tribological wear resistance of the ball-

and-socket joints produced by means of SLM and SPS technologies compared to the reference material. The results call into question the possibility of using these technologies in the manufacturing pro-cess of endoprosthesis components exposed to tribological wear. However, it should be noted that no optimization of technological parameters of these processes was carried out. It is possible that a change in one of them will be beneficial to increase the tribologi-cal wear resistance of the Co28Cr6Mo alloy.

Upper component Lower component

SLM

a) b)

SPS

c) d)

ASTM F1537

LC

e) f)

Fig. 4. Morphology of bearing surfaces depending on applied technology: a), b) SLM, c), d) SPS, e), f) ASTM F1537 LC; BSE Rys. 4. Morfologia powierzchni trących w zależności od zastosowanej technologii: a), b) SLM, c), d) SPS, e), f) ASTM F1537 LC; kontrast BSE

30 INŻYNIERIA MATERIAŁOWA MATERIALS ENGINEERING ROK XXXVIII

REFERENCES

[1] Pourzal R., Catelas I., Theissmann R, Kaddick C., Fischer A.: Character-ization of wear particles generated from CoCrMo alloy under sliding wear conditions. Wear 271 9-10 (2011) 1658÷1666.

[2] Attar H., Calin M., Zhang L., Scudino S., Eckert J.: Manufacture by selec-tive laser melting and mechanical behavior of commercially pure titanium. Science and Engineering A 593 (21) (2014) 170÷177.

[3] Garbiec D., Rybak T., Heyduk F., Janczak M.: Nowoczesne urządzenie do iskrowego spiekania plazmowego proszków SPS HP D 25 w Instytucie Obróbki Plastycznej. Obróbka Plastyczna Metali 22 (2011) 221÷225.

[4] Guyer R., McAfee P., Hochschuler S., Blumenthal S., Fedder I. L, Ohn-meiss D., Cunningham B.: Prospective randomized study of the Charité artificial disc: data from two investigational centers. The Spine Journal 4 (6) (2004) 252÷259.

[5] Valdevit A., Errico T.: Design and evaluation of the FlexiCore metal-on-metal intervertebral disc prosthesis. The Spine Journal 4 (2004) 276÷288.

[6] Moghadas P., Mahomed A., Hukins D., Shepherd D.: Friction in metal-on-metal total disc arthroplasty: Effect of ball radius. Journal of Biomechan-ics 45 (2012) 504÷509.

[7] White A., Panjabi M.: Clinical biomechanics of the spine. Lippincott Wil-liams & Wilskins (1990).

[8] Mróz A., Jakubowicz J., Gierzyńska-Dolna M., Wiśniewski T., Wendland J.: Wpływ technologii wytwarzania wyrobów ze stopu Co28Cr6Mo na ich właściwości fizyczne, mechaniczne i odporność korozyjną. Inżynieria Materiałowa Materials Engineering 36 (1) (2015) 2÷8.

[9] Mróz A., Garbiec D., Jakubowicz J., Wielowiejska-Giertuga A., Łapaj Ł., Gierzyńska-Dolna M.: Effect of manufacturing technology on tribological properties of Co28Cr6Mo alloy. Inżynieria Materiałowa Materials Engi-neering 37 (5) (2016) 245÷251.

[10] Harper M., Dooris A., Pare P.: The fundamentals of biotribology and its application to spine arthroplasty. SAS Journal 3 (2009) 125÷132.

[11] Nosal S.: Tribologia. Wprowadzenie do zagadnień tarcia, zużywania i smarowania. Wydawnictwo Politechniki Poznańskiej, Poznań (2012).

[12] Posada O. M., Gilmour D., Tate R. J., Grant M. H.: CoCr wear particles generated from CoCr alloy metal-on-metal hip replacements, and cobalt ions stimulate apoptosis and expression of general toxicology-related genes in monocyte-like U937 cells. Toxicology and Applied Pharmacol-ogy 281 (2014) 125÷135.

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Wpływ technologii wytwarzania przegubu kulowego ze stopu Co28Cr6Mo na właściwości tribologiczneAdrian Mróz1*, Dariusz Garbiec1, Jarosław Jakubowicz2, Łukasz Łapaj3

1Instytut Obróbki Plastycznej w Poznaniu, 2Instytut Inżynierii Materiałowej, Politechnika Poznańska, 3Klinika Ortopedii Ogólnej, Onkologicznej i Traumatologii, Uniwersytet Medyczny w Poznaniu, *adrian.mroz@inop.poznan.pl

Inżynieria Materiałowa 1 (215) (2017) 26÷31DOI 10.15199/28.2017.1.5© Copyright SIGMA-NOT MATERIALS ENGINEERING

Słowa kluczowe: stop Co28Cr6Mo, selektywne przetapianie laserowe, iskrowe spiekanie plazmowe, przegub kulowy, testy symulatorowe, zużycie.

1. CEL PRACY

Celem pracy była analiza mechanizmu zużycia tribologicznego powierzchni trących elementów przegubu kulowego w warunkach kinematyki ruchu i obciążeń zbliżonych do tych, które występują w naturalnym segmencie ruchowym odcinka lędźwiowego kręgo-słupa. Uzyskane wyniki badań pozwoliły odpowiedzieć na pytanie, czy istnieje potencjalne uzasadnienie możliwości zastosowania technologii selektywnego przetapiania laserowego SLM (selecti-ve laser melting) i iskrowego spiekania plazmowego SPS (spark plasma sintering) w procesie wytwarzania elementów endoprotez współpracujących ślizgowo.

2. MATERIAŁ I METODYKA BADAŃ

W pracy przedstawiono wyniki prób tarciowo-zużyciowych prze-gubów kulowych, wytworzonych z proszku stopowego ASTM F75 z zastosowaniem technologii selektywnego przetapiania lasero-wego oraz iskrowego spiekania plazmowego. Materiał referencyj-ny stanowiły przeguby wytworzone obróbką skrawaniem z pręta ASTM F1537 LC. Próby tribologiczne przeprowadzono z zastoso-waniem symulatora ruchu segmentu ruchowego kręgosłupa własnej konstrukcji. Podczas realizacji prób tribologicznych maksymalna wartość siły obciążającej wynosiła 1500 N. Badania przeprowa-dzono w temperaturze 37°C w środowisku wody destylowanej. Każda badana para trąca pracowała 1·106 cykli z częstotliwością ruchu 1.25 Hz. Komponent górny podczas pojedynczego cyklu wy-konywał ruch zginający w zakresie od –5 do +8° względem siły obciążającej, natomiast komponent dolny ruch obrotowy względem osi obrotu w zakresie od –3 do +3°.

W celu określenia zużycia badanych materiałów próbki ważono za pomocą wagi laboratoryjnej R200D (Sartorius, Niemcy). Pomia-ry chropowatości przeprowadzono z wykorzystaniem profilometru Hommel Etamic T8000 RC (Jenoptik AG, Niemcy). Analizę me-chanizmu zużycia badanych materiałów wykonano na podstawie obserwacji SEM powierzchni trących. Obserwacje przeprowadzo-no z wykorzystaniem skaningowego mikroskopu elektronowego Inspect S (FEI, Holandia).

3. WYNIKI I ICH DYSKUSJA

Zastosowany w symulatorze system pomiarowy pozwala na wyzna-czenie wartości współczynnika tarcia na podstawie rejestrowanych wartości momentu siły tarcia. Na rysunku 2 porównano wartości minimalnych i maksymalnych median wartości współczynnika

tarcia na tle całkowitego zakresu wartości współczynnika tarcia. Otrzymane wyniki badań świadczą o tym, że niezależnie od za-stosowanej technologii wytwarzania opory tarcia w węźle tarcia był zbliżone.

W tabeli 3 zestawiono wartości średnie zużycia masowego dla poszczególnych komponentów oraz obliczonych na jego podstawie średnich wartości wskaźników zużycia. Najmniejszą odpornością na zużycie tribologiczne charakteryzowały się przeguby wytworzo-ne technologią SPS. Zużycie przegubów wytworzonych w tej tech-nologii było około 20% większe w porównaniu z przegubami wy-tworzonymi metodą selektywnego przetapiania laserowego i około 30% większe w porównaniu z przegubami ASTM F1537.

Tabela 3 przedstawia wartości średnie chropowatości Ra po-wierzchni trących. Przed rozpoczęciem prób średnie wartości tego parametru wynosiły od 0.03±0.01 do 0.04±0.01 μm. Wskutek prze-prowadzenia prób chropowatość powierzchni trących testowanych przegubów zwiększyła się. Zwiększenie wartości Ra było tym większe im większe było zużycie tribologiczne — wartość współ-czynnika korelacji liniowej Pearsona wynosiła 0,87.

Wyniki obserwacji SEM morfologii powierzchni trących po zakończeniu prób tribologicznych przedstawiono na rysunku 3. Wykazały one, że niezależnie od zastosowanej technologii wytwa-rzania dominującym rodzajem zużycia było zużycie ścierno-adhe-zyjne. Na powierzchniach trących przegubów SLM stwierdzono ślady zużycia zmęczeniowego, objawiającego się charakterystycz-nymi, równoległymi nieciągłościami materiału. Na powierzchniach trących przegubów wytworzonych w technologii SPS obserwowa-no ślady odkształcenia plastycznego, najprawdopodobniej spowo-dowane zmęczeniem materiału w strefie kontaktu.

4. PODSUMOWANIE

Przeguby kulowe wytworzone ze stopu kobaltu Co28Cr6Mo z za-stosowaniem technologii selektywnego przetapiania laserowego oraz iskrowego spiekania plazmowego charakteryzują się gorszymi właściwościami tribologicznymi w porównaniu z wytworzonymi z materiału referencyjnego ASTM F1537 LC.

Niezależnie od technologii wytwarzania dominującym rodza-jem zużycia powierzchni trących było zużycie ścierno-adhezyjne. Zastosowanie symulatora ruchu kręgosłupa ujawniło ślady zuży-cia o charakterze zmęczeniowym. Otrzymane wyniki stawiają pod znakiem zapytania celowość aplikacji tych technologii w procesie wytwarzania komponentów endoprotez narażonych na zużycie tri-bologiczne.