Browsing by Author "Kovaci, Halim"

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Biomechanical Analysis of Spinal Implants with Different Rod Diameters Under Static and Fatigue Loads: An Experimental Study
(Walter de Gruyter GmbH, 2019) Kovaci, Halim; Yetim, Ali Fatih; Celik, Ayhan
Spinal implants are commonly used in the treatment of spinal disorders or injuries. However, the biomechanical analyses of them are rarely investigated in terms of both biomechanical and clinical perspectives. Therefore, the main purpose of this study is to investigate the effects of rod diameter on the biomechanical behavior of spinal implants and to make a comparison among them. For this purpose, three spinal implants composed of pedicle screws, setscrews and rods, which were manufactured from Ti6Al4V, with diameters of 5.5 mm, 6 mm and 6.35 mm were used and a bilateral vertebrectomy model was applied to spinal systems. Then, the obtained spinal systems were tested under static tension-compression and fatigue (dynamic compression) conditions. Also, failure analyses were performed to investigate the fatigue behavior of spinal implants. After static tension-compression and fatigue tests, it was found that the yield loads, stiffness values, load carrying capacities and fatigue performances of spinal implants enhanced with increasing spinal rod diameter. In comparison to spinal implants with 5.5 mm rods, the fatigue limits of implants showed 13% and 33% improvements in spinal implants having 6 mm and 6.35 mm rods, respectively. The highest static and fatigue test results were obtained from spinal implants having 6.35 mm rods among the tested implants. Also, it was observed that the increasing yield load and stiffness values caused an increase in the fatigue limits of spinal implants.
Designing and In Vitro Testing of a Novel Patient-Specific Total Knee Prosthesis Using the Probabilistic Approach
(Walter de Gruyter GmbH, 2022) Korkmaz, Ismail H.; Kaymaz, Irfan; Yildirim, Omer S.; Murat, Fahri; Kovaci, Halim
In order to prevent failure as well as ensure comfort, patient-specific modelling for prostheses has been gaining interest. However, deterministic analyses have been widely used in the design process without considering any variation/uncertainties related to the design parameters of such prostheses. Therefore, this study aims to compare the performance of patient-specific anatomic Total Knee Arthroplasty (TKA) with off-the-shelf TKA. In the patient-specific model, the femoral condyle curves were considered in the femoral component's inner and outer surface design. The tibial component was designed to completely cover the tibia cutting surface. In vitro experiments were conducted to compare these two models in terms of loosening of the components. A probabilistic approach based on the finite element method was also used to compute the probability of failure of both models. According to the deterministic analysis results, 103.10 and 21.67 MPa von Mises stress values were obtained for the femoral component and cement in the anatomical model, while these values were 175.86 and 25.76 MPa, respectively, for the conventional model. In order to predict loosening damage due to local osteolysis or stress shield, it was determined that the deformation values in the examined cement structures were 15% lower in the anatomical model. According to probabilistic analysis results, it was observed that the probability of encountering an extreme value for the anatomical model is far less than that of the conventional model. This indicates that the anatomical model is safer than the conventional model, considering the failure scenarios in this study.
Effect of Boronizing on the Structural, Mechanical and Tribological Properties of CoCrW Dental Alloy Produced by Selective Laser Melting
(Emerald Group Publishing Ltd, 2019) Uzun, Yakup; Kovaci, Halim; Yetim, Ali Fatih; Celik, Ayhan
Purpose This paper aims to investigate the effects of boriding on the structural, mechanical and tribological properties of CoCrW dental alloy manufactured by the method of selective laser melting. Design/methodology/approach In this study, CoCrW alloy samples that are used in dentistry were manufactured by the method of laser melting, and boriding treatment was made on the samples at 900 degrees C and 1,000 degrees C for 1, 4 and 8 h. The structural, mechanical and tribological effects of boriding on the samples were analyzed using X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, microhardness and an abrasion test device. Findings According to the results, the best outcomes in terms of abrasion strength and hardness were obtained in the sample that was subjected to boriding at 1,000 degrees C for 4 h. Originality/value This study produced CoCrW alloys, which are fundamental biomaterials that are used in dentistry, by a different production method called selective laser melting and improved their surface characteristics by boriding.
The Effects of Boriding on Metal-Ceramic Bond Strength of Co-Cr Alloy Fabricated by Selective Laser Melting
(Taylor & Francis Ltd, 2021) Uzun, Yakup; Yanikoglu, Nuran; Kovaci, Halim; Yetim, Ali Fatih; Celik, Ayhan
Boriding is a method used to increase the biocompatibility and strength of metallic materials. However, by this time, no study has been encountered in the literature regarding investigation of the effects of boriding on metal-ceramic bond strength of Co-Cr alloy fabricated by selective laser melting. Boriding of metallic surfaces may be an alternative to increase the roughness and adhesion strength of the metal surface. The aim of this study was to analyze the effects of boriding on the metal-ceramic bond strength of a Co-Cr alloy fabricated by selective laser melting. In this study, metal substructures of 0.1 mm, 0.2 mm and 0.3 mm thickness values were produced from CoCrW powders with the method of selective laser melting, borided at 900 degrees C for 1 h and coated with porcelain. After the procedures, the effects of the boriding process on metal-porcelain bonding were determined by in vitro fracture tests. The 0.1 mm 900 degrees C 1 h (521.22 +/- 56.37 N), (p < 0.001), 0.2 mm 900 degrees C 1 h (619.55 +/- 20.94 N), (p = 0.051) and 0.3 mm 900 degrees C 1 h (592.11 +/- 260.58 N) specimens showed the highest bond strength, significantly higher than that of the 0.1 mm Untreated (299.88 +/- 61.56 N), 0.2 mm Untreated (480.55 +/- 11.87 N) and 0.3 mm Untreated (520.00 +/- 165.52 N), (p = 0.29) specimens. According to the results, with the help of the boriding process applied on the surfaces of the metal substructure samples, the bonding resistance of the porcelain applied onto the metal substructures increased by approximately 74, 29 and 14% in comparison to the samples that were not borided respectively for the thicknesses of 0.1, 0.2 and 0.3 mm. Boriding of metallic surfaces increased the bond strength of the metal material with ceramics.
Influence of Plasma Nitriding Treatment on the Adhesion of DLC Films Deposited on AISI 4140 Steel by PVD Magnetron Sputtering
(Taylor & Francis Ltd, 2017) Kovaci, Halim; Baran, Ozlem; Bayrak, Ozgu; Yetim, Ali Fatih; Celik, Ayhan
Duplex surface treatments composed of diamond like carbon (DLC) coating followed by plasma nitriding have drawn attention for a while. In this study, AISI 4140 steel substrates were plasma nitrided at different treatment temperatures and times. Then, DLC films were deposited on both untreated and plasma nitrided samples using PVD magnetron sputtering. The effect of different plasma nitriding temperatures and times on the structural, mechanical and adhesion properties of DLC coatings was investigated by XRD, SEM, microhardness tester and scratch tester, respectively. It was found that surface hardness, intrinsic stresses, layer thickness values and phase distribution in modified layers and DLC coating were the main factors on adhesion properties of duplex coating system. The surface hardness and residual stress values of AISI 4140 steel substrates significantly increased with both DLC coating and duplex surface treatment (plasma nitriding+DLC coating). Increasing plasma nitriding temperature and time also increased the diffusion depth and the thickness of modified layers. Hard surface layers led to a significant improvement on load bearing capacity of the substrate material. However, it was also determined that the process parameters, which provided lower intrinsic stresses, improved the adhesion properties of the duplex coating system.
Investigation of the Usage Possibility of CuO and CuS Thin Films Produced by Successive Ionic Layer Adsorption and Reaction (SILAR) as Solid Lubricant
(Elsevier Science Sa, 2018) Kovaci, Halim; Akaltun, Yunus; Yetim, Ali Fatih; Uzun, Yakup; Celik, Ayhan
CuO and CuS films were coated on AISI 4140 steel samples by Successive Ionic Layer Adsorption and Reaction (SILAR) to investigate their usability as solid lubricant. Wear tests were performed using pin-on-disk tribo-tester under dry and lubricated conditions. The structural, morphological and morphological features of untreated, CuO and CuS coated samples were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and microhardness, scratch analysis. The surface examinations indicated that needle-like structures formed after coating processes and CuO and CuS films exhibited oleophobic behavior. CuO and CuS thin films reduced the coefficient of friction due to the low shear strength of bonding in transfer films. CuO and CuS films decreased the wear rates in comparison to untreated sample for both dry and lubricated conditions. Also, CuO thin films exhibited better wear resistance than CuS films in dry and lubricated wear tests. The overall results revealed that both CuO and CuS films produced by SILAR can be an alternative to conventional solid lubricants.
A Novel Method for Improving Plasma Nitriding Efficiency: Pre-Magnetization by DC Magnetic Field
(World Scientific Publ Co Pte Ltd, 2017) Kovaci, Halim; Yetim, Ali Fatih; Bozkurt, Yusuf Burak; Celik, Ayhan
In this study, a novel pre-magnetization process, which enables easy diffusion of nitrogen, was used to enhance plasma nitriding efficiency. Firstly, magnetic fields with intensities of 1500G and 2500G were applied to the untreated samples before nitriding. After the pre-magnetization, the untreated and pre-magnetized samples were plasma nitrided for 4 h in a gas mixture of 50% N-2-50% H-2 at 500 degrees C and 600 degrees C. The structural, mechanical and morphological properties of samples were examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), microhardness tester and surface tension meter. It was observed that pre-magnetization increased the surface energy of the samples. Therefore, both compound and diffusion layer thicknesses increased with pre-magnetization process before nitriding treatment. As modified layer thickness increased, higher surface hardness values were obtained.
Synthesis and Characterization of Wear and Corrosion Resistant Ni-Doped Al2O3 Nanocomposite Ceramic Coatings by Sol-Gel Method
(Elsevier Science SA, 2022) Yetim, Tuba; Turalioglu, Kerem; Taftali, Merve; Tekdir, Hilmi; Kovaci, Halim; Yetim, Ali Fatih
Cp-Ti is frequently used in biomedical applications because of its good corrosion and high biocompatible properties. However, its poor wear resistance limits its use in some applications. Therefore, this study aims to improve its wear and corrosion resistance by producing Ni-doped Al2O3 nanocomposite coatings on Cp-Ti. In order to achieve this goal, undoped Al2O3 and Ni-doped Al(2)O(3 )nanocomposite ceramic coatings were produced on Cp-Ti samples at two different rotational speeds by the spin coating process, which is a sol-gel method. Afterward, their structural and morphological features were examined by XRD, SEM, Raman, and 3D surface profilometer. Reciprocating wear tests and electrochemical investigations were performed to determine their wear and corrosion properties.
Tribological Behavior of DLC Films and Duplex Ceramic Coatings Under Different Sliding Conditions
(Elsevier Sci Ltd, 2018) Kovaci, Halim; Yetim, Ali Fatih; Baran, Ozlem; Celik, Ayhan
Diamond-like carbon (DLC) films are effectively utilized to improve the tribological properties of materials due to their low friction coefficient, good wear resistance and high hardness. In recent years, different pre-treatments such as plasma assisted diffusional processes have been used to increase long-term durability of these coatings. In the present work, DLC films were deposited on untreated and plasma nitrided AISI 4140 low-alloy steel samples using physical vapor deposition method (PVD). The effect of nitride layers on the structural and mechanical properties of DLC films was examined by XRD, SEM, microhardness tester and reciprocating tribotester. The wear tests were performed under dry, saltwater and lubricant environments. The microhardness results indicated that surface hardness considerably increased after duplex treatment and this caused to increase plastic deformation resistance of the material. The reciprocating test results showed that DLC films deposited on untreated and plasma nitrided samples reduced the coefficient of friction because of the formation of transfer film between sliding/mating surfaces. Also, wear rates improved significantly after duplex treatment in consequence of high plastic deformation resistance of duplex treated samples. The highest wear resistance was obtained in lubricated condition while the lowest wear resistance was for the samples tested in dry conditions.