Turgut Gulmez

Corrosion fatigue behaviour of ion nitrided AISI 4140 steel

Abstract Machine components suffer from corrosion degradation of fatigue characteristics and improvement can be attained by the application of a nitriding treatment, particularly to low alloy steels. In the present study, the effect of ion nitriding on corrosion fatigue performance of AISI 4140 steel has been investigated by conducting a series of rotary bending corrosion fatigue tests at 95 Hz, in 3% NaCl aqueous solution. Hourglass shaped, 4 mm diameter fatigue specimens were ion nitrided at 748 K for 1, 3, 8 and 16 h prior to the tests. It was observed that distinct fatigue limit behaviour of ion nitrided steel in air completely disappeared in corrosive environment besides severe degradation in fatigue characteristics. An improvement reaching to 60% in corrosion fatigue strength can be attained by successive ion nitriding practice based on a fatigue life of 10 7 cycles. An attempt was made to establish an empirical relationship between corrosion fatigue strength and relative case depth, which considers the size of the ion nitrided specimen. It was also determined that a power relationship holds between corrosion fatigue strength and fatigue life of ion nitrided steel. The presence of white layer has resulted in additional improvement in corrosion fatigue resistance, and it was observed that corrosion fatigue cracks were initiated dominantly under the white layer by pit formation mechanism.

Study on the Electroless Ni‐B Nano‐Composite Coatings

Coating of materials is one of the known surface modification technique in recent years and electroless method is one of the growing areas of both scientific and commercial region. An optimal formulation for electroless Ni‐B coating is developed. After doing successful Ni‐B coating, nano‐sized diamond particles are incorporated into the coating to improve hardness and wear resistance properties of the deposits. To disperse diamonds to the bath, five different suspensions are prepared and also five different electroless composite Ni‐B coatings are obtained. The rates and microhardness of coatings after in as‐plated and heat‐treated conditions are measured. The better composite coatings are chosen to study. SEM examination and then wear test (ball on plate) is done to the nano‐composite coatings, electroless Ni‐B coating and uncoated plain carbon steel substrates. Microhardness of the nano‐composite coatings greatly increased because of the presence of the nano particles. The hardest electroless composite c...

The effect of zirconia thickness on the biaxial flexural strength of zirconiaceramic bilayered discs

The aim of this study was to assess the effect of zirconia core thickness on the biaxial flexural strength values of zirconia-porcelain bilayered discs. A total of 60 discs with 0.3, 0.4, and 0.5 mm thickness were obtained from a fully sintered zirconia block. A 1.5-mm thick layer of veneer porcelain was fired on the zirconia specimens and biaxial flexural strength tests were performed on the bilayered discs. In each group, the loading surface was the veneer porcelain in half of the specimens (core in tension) and the zirconia core surface in the other half (core in compression). The zirconia core thickness had no effect on the biaxial flexural strength of zirconiaporcelain bilayered discs when the core was in tension (p>0.05). Whereas, when the core was in compression, an increase in the zirconia core thickness resulted in an increase in the biaxial flexural strength (p<0.05).

Effect of stacking sequence on tensile, flexural and thermomechanical properties of hybrid flax/glass and jute/glass thermoset composites

This paper investigates tensile, flexural, and dynamic mechanical properties of natural and hybrid thermoset composite laminates made from flax/glass and jute/glass fibres. Hybrid laminates with various stacking sequences were manufactured by vacuum infusion method. Weight and cost of composites decreased using cheaper and lightweight natural fibres (flax and jute). Results showed that composite laminates made from natural fibres had higher specific strength values when the results were normalised to same glass fibre volume fraction, although they had lower tensile and flexural strength than that of glass composites without normalization. Composite elastic properties were predicted using classical lamination theory through rule of mixture and Halpin–Tsai models, and compared with experimental values. Changing the stacking sequence did not affect the tensile strength and modulus of composites significantly, whereas there were notable differences on flexural strength of composites when the outer layers contained glass fibres. Dynamic mechanical analyses showed similar results as flexural test, while natural fibre and some of hybrid composites had higher damping characteristics than glass-reinforced composites.

Investigations on Tensile Properties of Waste Fillers Reinforced Composites

In this study, low density polyethylene (LDPE) matrix composites were manufactured with the ratios of 20%, 30% and 40%wt hazelnut and peanut reinforcements as the natural fibers. Composite plates were manufactured by using a single screw extruder. Various tests and measurements are performed to obtain mechanical properties such as density, tensile strength (TS), Youngs modulus (YM), failure strain (FS), impact strength (IS) etc. as well as the effect of maleic anhydride grafted resin as an additive with different ratios was investigated and the optimum composite content was obtained. This preliminary work showed that hazelnut and peanut fillers could be utilized with proper additives in order to produce the composite materials with good physical and mechanical properties.

Effect of Different Surface Applications and Adhesive Systems on Bond Strength of Porcelain Repair Material

The purpose of this study was to evaluate the effect of different surface treatments and adhesive systems on bond strength of composite resin to porcelain when there is need to repair the restoration.Materials and methods: Rectangular discs (10mm x 12mm) with a height of 2mm were prepared from leucite-reinforced feldspathic ceramic CAD/CAM blocks (Vitablocs), embedded in acrylic resin, and bur cut porcelain surfaces were created. The specimens were divided into three groups (n=20) according to the surface treatments: Er:YAG laser at 2W for 20 seconds, air abrasion with 50µm Al2O3 at 2.5 bar for 15 seconds, and the last group left untreated as control. After silane (Monobond S) application on ten specimens from each group, composite rods (Aelite Flo LV) with 4.25mm diameter and 4mm height were bonded to ceramic surfaces with a self-etch adhesive system (All-Bond SE). The remaining ten specimens from each group were etched with 36% orthophosphoric acid (Blue Etch Dental Etching Gel), and after silane and bonding agent (One-Step Plus) application composite rods with the mentioned dimensions were bonded. All groups were stored in distilled water at 37°C for one day before mechanical testing. Shear testing of all groups were performed on a universal testing machine with a load cell of 50kg and at a crosshead speed of 1.5mm/min. Results were statistically analyzed using Mann Whitney U and Kruskall Wallis tests.Results: The bond strength values of the self-etch adhesive system in the lased and control groups were statistically higher in comparison to the other adhesive system (p<0.01, p<0.05, respectively). The highest bond strength values were obtained in the Er:YAG lased and self-etch adhesive applied group.Conclusions: When there is need to repair of a porcelain restoration, a self-etch adhesive application after Er:YAG laser treatment could be the method of choice.

Mechanical Properties of CAD/CAM Manufactured and Prefabricated Endodontic Posts

The aim of this study was to evaluate the stiffness, elastic limit and strength of CAD/CAM manufactured metal and zirconia posts in comparison to prefabricated zirconia and glass-fiber posts. Materials and methods: Four types of posts were investigated in this study (n=40). Prefabricated posts made of zirconia (Cosmopost) and glass-fiber reinforced composite (FRC Postec Plus), and Cr-Co (Magnum Lucens) and zirconia (Zirkonzahn) posts manufactured with a CAD/CAM system were used. Artificial root canals with similar diameters to those of the posts were drilled on a stainless steel metal block and the posts were cemented with a self-etch/self-adhesive dual cure resin cement (Totalcem). The posts were loaded in a universal testing device at an inclination of 135 degrees at a crosshead speed of 1mm/min. The force-deflection curves were recorded, and stiffness, elastic limit and strength of the posts were determined. Data were analyzed statistically using Mann Whitney U and Kruskall Wallis tests.Results: The difference between the strength, elastic limit and also stiffness values of the groups was statistically highly significant (p<0.01). CAD/CAM Cr-Co posts showed the highest values of strength, elastic limit and stiffness amongst all the posts investigated. CAD/CAM zirconia posts had the lowest values of strength, and FRC had the lowest values of elastic limit and stiffness. Cosmopost had higher values of elastic limit and stiffness compared to CAD/CAM zirconia posts.Conclusions: If an endodontic post restoration is required, functional load and aesthetics should be considered in addition to mechanical properties of the material selected.