Nanostructured thin films for prosthetic dentistry applications Introduction

Romanian Biotechnological Letters Copyright © 2010 University of Bucharest

Vol. 15, No.3, 2010, Supplement Printed in Romania. All rights reserved ORIGINAL PAPER

Nanostructured thin films for prosthetic dentistry applications Received for publication, April 6, 2010 Accepted, May 22, 2010 MADĂLINA PRODAN1, IULIANA STANESCU1, V. CIUPINA1, DOINA GHEORGHIU2, COMSA STANCA2, V. EUGENIU3, G. PRODAN1 1 “Ovidius” University of Constanţa, Constanta, 900527, Romania 2 The National Institute of Research and Development in Mechatronics and Measurement Technique, Bucharest, 021591, Romania 3 Metav Research-Development Bucharest, 020011, Romania [email protected]

Abstract Nanostructured carbon thin film has a large application in different working area. These work will present nanostructured carbon thin films obtained by TVA (Thermionic Vacuum Arc) method, with nanometric size (thickness < 100nm). The most important feature of these films is hardness given by sp2-sp3 ratio of films composition. Titan alloy are used frequently in biomedical application due to highly biocompatibility. The resulted prosthetic from these coated alloy, with high hardness carbon films, lead on substantial improvement quality, from mechanical point of view with about 100% related to other protection methods. TVA method allows a morphological smoothly DLC (diamond like carbon) type carbon films to obtain. The resulted DLC is thicker and has diamond properties, like: high hardness (3000-5000kg/mm2), small friction coefficient (0,1-0,2), chemical inactive, abrasion hardness (1,6x10,9mm3/mN) and also electrical properties. Obtained films were studied by electron microscopy techniques (TEM, SEM, HRTEM, EELS, SAED and PED). PED (Precession of Electron Diffraction) method is recently developed and substantially improves the crystalline structure studies by means of electron diffraction patterns acquisition very close to kinematical condition. Also, the mechanical resistance was tested and compared for coated and uncoated alloys. The geometry of used sample is cylindrical with 100 mm length and 3 mm diameter. Dynamic tensile and bend tests were performed using method described by the SR EN 10002-1:2002 standard. Dynamic tensile test show increasing of sample length from 4 mm (uncoated sample) to approximately 6 mm for coated sample, and the maximum stress and bending momentum values gives that carbon coated samples are more elastic.

Keywords: prosthetic dentistry, TVA, DLC, SAED

Introduction For applications in medical devices, nanostructured materials have unique properties to interact with cells, proteins and human DNA. These materials are defined as materials containing structural elements (i.e. clusters, crystallites or molecules) with dimensions range from 1 to 100 nm. Recent advances in medical applications are the result of the development of two complementary phenomena. First, is a natural evolution from micro to nano scale as processing and characterization techniques become available. Second, the nanostructured materials allow specific protein interactions, DNA, viruses and other biological structures at the nano scale (HAN & al, [1]. Specific interactions between them and nanostructured materials can generate biological features that are not possible when using micro materials (PIELES & al, [2]. Field of nanostructured ceramic materials for medical applications are rapidly evolving. As biomedical applications, pharmaceuticals and cosmetics are major target market for suppliers of nanoparticles and nanotechnologies, in this stage we propose a description of different types of nanoparticles, including nanoparticles of ceramics, metals and alloys in the 109

Nanostructured thin films for prosthetic dentistry applications

manufacture of implantable medical devices etc. applicable in the manufacture of implantable medical devices and mechanical properties characterization. Experiments have shown that materials with less than one micron grain, under certain conditions, mechanical properties significantly improved compared with the same largegrained materials. However, the basic structure of materials, physical processes and manufacturing conditions can have a significant impact on the performance of nanostructured materials for special applications.

Materials and methods The pure titanium material was used to sample preparation. Titanium is used in manufacturing dental implants, because is a biocompatible material, non-toxic and not cause allergic reactions (HOHENHOFF & al [3]. They also must have strength, resistance to high flow, low density and elastic modulus. To study the mechanical properties of nanostructured materials deposition compared with those without deposits were made three specimens of pure titanium. Samples were obtained from pure titanium rolled bars turned to cut the length of 100mm. Two of them to a structure made of carbon-look diamond DLC (Diamond like Carbon) using the precursor on the inner surface (MUSA & al [4, 5]. By controlling pressure, precursor type and polarization of the grid, the method prevents the film from breaking contact with the substrate surface. DLC (Diamond like Carbon) is the diamond type carbon material structure having a high proportion of sp3 bonds in comparison with sp2 bonds. Depending on the method this report can be varied. DLC is deposited in the form of thin films and shows many properties of diamond such as high hardness (3000-5000kg/mm2), low friction coefficient (0.1-0.2), chemical inertia, resistance wear (1.6 x10.9mm3/mN) and electrical properties. Bending and tensile test were performed to study mechanical behavior for each samples, pure titanium with and without deposition of carbon films. Bending test was done in the elastic bending range of material to reduce damage of the samples, so the same samples were used to tensile test. Bending test procedure was performed according to methods described in “STAS 1660:1980 Metals testing, Bending cast iron”, and “EN ISO 178:2003 Plastics, Determination of bending properties” [COMSA & al, [6]. Characterization of deposited films in terms of morphology and structure was done by electron microscopy (TEM and SEM). Carbon film structure was analyzed using electron diffraction, by interpreting the radial distribution function of electrons scattered by the carbon atoms. The film thickness was estimated form scanning electron microscopy images on a video cross-medium geometry. Special peculiarities of the film were shown using transmission electron microscopy high resolution. Amorphous solids, liquids and gases are largely present in the current research. Among these materials we can remember liquid crystals, clean amorphous carbon layers or doped with impurities, such as boron, nitrogen and hydrogen. Local characteristics of ordering for about (2-5) lattice constant resulting in further material properties are a subject of study for X-ray and electron diffraction. Debye relationship came to radiation intensity as reflected by such materials, assuming a radial distribution which is the density of atoms or electrons as function of distance from an atom or electron, taken as origin. Intensity value is found by Debye as: 4πr 2 ρ (r ) = 4πr 2 ρ 0 +

2r

π

∞

∫ Si(S )sin rSdS 0

where fm and fn are the scattering factors of atoms m and n are the NMR distance and S = (4πsinq) / λ. Above equation are useful to establish the correct atomic configuration by comparing the experimental intensities with those calculated for different models. 110

Romanian Biotechnological Letters, Vol. 15, No. 3, Supplement (2010)

MADĂLINA PRODAN, IULIANA STANESCU, V. CIUPINA, DOINA GHEORGHIU, COMSA STANCA, V. EUGENIU, G. PRODAN

Using Fourier transform Debye relationship becomes:

I = ∑∑ f m f n m

n

sin Srmn Srmn

Where ρ(r) is the density of atoms at distance r from the atom taken as origin, ρ0 is the average density of atoms in the sample, and i(S) = (I / N) f2-1.

Results and discussion TEM examination reveals a thin film (Figure 1) of the film, penetrable by electrons accelerated at 100kV. Thickness was estimated from SEM images (Figure 2) obtained on film cross-section support relatively small film thickness ~ 16 nm films allow investigation under kinematical approximations.

Fig.1 TEM image of DLC thin film and detail for marked area

Fig. 2. SEM images of cross-section DLC-support sample

The results achieved using precession electron diffraction are close to those obtained using conventional diffraction. Electron diffraction (Figure 3) shows that the film has a strong amorphous character, with two rings / bands situated at 0.206 and 0.111 nm and can be ascribe to the diamond structure. Profile analysis was made by means CRISP2 (ELD module) application. We obtain RDF (Radial Distribution Function) and using an algorithm implemented in Maxima, calculations were made to extract PDF. Romanian Biotechnological Letters, Vol. 15, No. 3, Supplement (2010)

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Fig. 3. Electron diffraction for DLC film and RDF function extracted by means of CRISP2 ELD module.

The results are shown in the figure 4, RDF fit with a linear function, and after deducting the linear contribution, PDF is obtained in figure 5. The PDF exhibit two peaks for small scattering angle. The first peak at 1.1Å can be associated with tetrahedral bonds (sp3) and the second peak at 2Å can be associated with sp2 bonding film (theoretical value is approximately 1.7 A) (OANCEA STANESCU & al [7]. A comparative study was carried out between TEM and image simulation of an amorphous film composed of carbon atoms bound 50% sp2 and 50% sp3. Carbon properties are a direct consequence of the arrangement of electrons around the nucleus. There are six electrons in a carbon atom, evenly distributed between the orbital 1s, 2s, and 2p. Because the earliest steps can support up to six 2p electrons, the carbon can achieve up to four links, however, valence electrons involved in chemical bonds, so the earliest steps occupy the earliest steps 2s and 2p. Such solid phase carbon can exist in three allotropic forms: graphite, diamond and fullerenes. Diamond has a crystalline structure where each sp3 hybridized carbon atom form a link with four other atoms in a tetrahedron arrangement (ROBERTSON & al, [8].

Fig.4. RDF function

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Fig. 5. PDF function

This lattice gives hardness of crystalline diamond (being the hardest substance known) and excellent thermal conductivity (approximately five times greater than that of copper).

Fig.6. Carbon film and Fourier space representation of the area marked on the image

Sp3 hybridized bonds justify electrically insulating property and optical transparency. Graphite sheet consists of sp2 hybridized atoms arranged in a hexagonal network. Bonds geometry gives graphite chemical characteristics: flexible, slippery, opaque, and electrically conductive. Compared with the diamond, each carbon atom in a graphite sheet is connected with only three atoms, the electrons can move freely between the p-type orbital allowing electrical conductivity (ROBERTSON & al, [9].

Fig. 7. Amorphous carbon thin film in QSTEM application


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The model used for simulation was generated using gbmaker form package of simulation application QSTEM.

Fig.8. Simulated diffraction figure

Tensile and bending tests were conducted in accordance with the method described in SR EN 10002-1:2002. The figures below are presented tensile test for both types of samples uncoated (Figure 9) and DLC coated (Figure 10), and charts that results from the tests.

Fig.9. Experimental assembly tensile test for uncoated sample (A. clamping device, breaking the sample B, C rod after break) and tensile test D. diagram uncovered cylindrical sample

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Fig.10. Experimental assembly tensile test for DLC coated sample tensile test sample (sample B A broken rod after fracture) and C. The tensile test diagram of DLC film coated cylindrical sample

Values of maximum pressure force and bending moments show that carbon coated specimen is more elastic. This elasticity and softening of the material is probably due to fact that deposition process determines heating of material surface. Comparing the values of bending strength is observed that specimens with nanostructured carbon deposit layer have a higher resistance.

Conclusions DLC films appear to have great potential for their use of medical applications. These thin films containing many bond sp3, and it proved to have biological properties, adhesion and high wear resistance. They find application in many medical devices such as cardiac stents, prostheses for joints, biosensors and other implantable medical devices. Samples coated with a nanostructured layer have bending strength and elasticity higher than the uncovered sample. This behavior of the material is close to the bone and dental implants are better in case of biocompatible materials with nanostructured coatings. Same elastic behavior is observed in tensile testing for specimens with nanostructured carbon film deposition. Such elongation of these specimens and constriction coefficient is higher than uncovered pure titanium sample. Different results of percentage elongation after fracture specimens can be attributed to the method covered by carbon layer deposition that is dividing in two steps: first depositing a layer of 20nm, stopping the process and return the sample to 180 °, and then continued with another layer of 20nm. The process induces uneven thickness and elastic properties determine the default uneven sample. Also observed that tensile strength is higher in coated samples compared with uncoated sample.


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Acknowledgments This work is the result of research projects MATSTOM 71-070/2007 and NANCARB 71-121/2007.

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