الفهرس Only 14 pages are availabe for public view
 |  | from | 166 |  |  |
| | | from | 166 | | |
Abstract
High-performance alloys and hard facing application equipment are used by diverse industries as mining oil and gas, metalworking control, medical, automotive, aerospace, power generation and chemical and food processing as either manufactured products or within the manufacturing processes. The principal reasons for nitriding or carbonitriding could be used to achieve high surface hardness, high wear resistance, long fatigue life, high corrosion resistance and to obtain a surface that is resistant to the softening effect of heat at temperatures up the nitriding/carbonitriding temperature.
In the last years, the commercial production of titanium and its alloys has increased steadily. Titanium and its alloys are widely used as engineering materials and also for biological applications. They are suitable for high stress conditions and do not present toxic behavior to biological environments. However, these materials show weak wear resistance and high friction coefficients, ascribed to the hexagonal close packed structure with a low c/a ratio of the α -Ti phase. As a consequence of this structure, titanium shows prismatic and pyramidal slip systems, which cause relatively low shear strength and high friction coefficient.
The present work was planned with the aim of enhancing the mechanical properties of Ti and Ti–6Al–4V alloy surfaces using both inductively coupled radio frequency plasma (ICRFP) and CO2 laser techniques. Nitrogen and/or carbon are the effective elements to improve the tribological properties of Ti and Ti–6Al–4V alloy by forming TiN, Ti2N, TiNC and Ti(N) super-hard phases. We have achieved a hard titanium surface with high efficiency process by ICRFP and CO2 laser techniques. The properties and the characteristics of the processed samples are evaluated using Vickers microhardness test (HV), nanoindentation technique, optical and scanning electron microscopy (SEM), crystal structure and pole figures (XRD), wear resistance and friction coefficient measurements (oscillating ball-on-disk type tribometer wear tester without lubrication), and surface roughness measurements by Dektak3 ST profilemeter .
This work includes four groups of samples. The first one reports on the effect of the input rf plasma processing power in the range of 350-650 W on the microstructure and the mechanical properties of plasma carbonitrided Ti. The plasma-processing time was fixed for 20 min and a gas mixture of 15% C2H2 and 85% N2, from the total gas pressure, was used. The measured surface hardness values of the compound layer shows a maximum value of 2050 HV0.1 for the sample that was treated at a plasma power input of 550 W. The thickness of the carbonitrided layer continuously increases as the plasma power increases. Moreover, the highest carbonitriding rate of 3.52 (μm2/s) was observed when the input plasma power was adjusted at 600 W. This high carbonitriding rate of the treated titanium samples is ascribed to the high concentration of active carbon and nitrogen species in the plasma atmosphere and the formed microcracks in the near surface of the sample during the plasma processing.
In the second group of the samples, experiments were carried out to treat a surface of pure titanium by ICRFP. Increasing the efficiency of carbonitriding process with decreasing the plasma treatment time to minutes instead of many hours is our main goals. The effects of different plasma-processing times from 5 to 35 min in step of 5 min on the microstructure and the mechanical properties of the plasma carbonitrided Ti were examined. The plasma power input was fixed at 550 W and a gas mixture of 15% C2H2 and 85% N2, from the total gas pressure, was adjusted. The surface microhardness and the thickness of the compound layer of the carbonitrided Ti increase as the plasma-processing time increases. The surface energy, yield strength and Young’s modulus for the carbonitrided titanium were calculated from the Vickers microhardness data. An anomalously high carbonitriding rate has been achieved and a maximum value of 3.03 (µm2/s) for the sample that was treated for 25 min was obtained. Moreover, high value of microhardness, 2288 (HV0.1) for the sample treated at plasma-processing time of 35 min was observed.
In the third group of the samples, experiments were carried out to treat a surface of Ti–6Al–4V alloy by ICRFP. The effects of plasma-processing time in the range of 5–35 minutes on the microstructure and the mechanical properties of the plasma nitrided Ti–6Al–4V samples were studied. The plasma power input was adjusted at 450 W and pure N2 gas was introduced to establish a treatment pressure of 8-8.4x10-2 mbar. The results show that, the surface microhardness increases as the plasma-processing time increases to reach 2000 HV0.1 at the plasma-processing time of 35 min. Moreover, the surface energy increases from 32.134 for the untreated sample to 107.7 N/mm for the sample that was treated at 35 min. A high nitriding rate of 2.81μm2/sec at plasma- processing time of 25 min was achieved.
In these three groups, the formation of the hard phases TiN, Ti2N, TiNC and Ti(N) in the titanium surfaces are found to be the reason for these attractive results. Moreover, we have adapted the formed microcracks mechanism reported by El-Hossary (2002), together with the diffusion mechanism. At a certain temperature (depending on the treated material) and at a high nitrogen concentration and high plasma energy, the chance of the surface microcracks and a high penetration of the nitrogen species occurring are high. The nitrogen diffuses into the native material through the sample surface and the wall of the formed microcracks, in which the interface between treated and native titanium sample is large enough for a fast nitriding process due to the concentration gradient. The diffusion of the nitrogen in titanium sample might be due to interstitial or vacancy mechanism depending on the sample temperature.
In the fourth group of the samples, experiments were carried out to treat a surface of pure titanium using continuous wave CO2 laser technique. Set of Ti samples were processed by 45 laser overlapping tracks using different mixtures of nitrogen to argon. The first one was in 40% N2 and 60% Ar, the second one was in 60% N2 and 40% Ar, and the third one was in 80% N2 and 20 % Ar. The laser nitriding was done using 3.1 kW laser power, 3.5 mm spot diameter, 9 mm/s traverse speed, 2.75 mm track overlap, and a constant gas flow of 100 L/min. It was found that the nitrogen content in the gas atmosphere has a massive effect on the microstructure and the mechanical properties of the laser nitrided samples. Thick TiN coatings can be obtained and that the formation of TiN is increasing with the nitrogen gas fraction in the processing atmosphere. The hardness and the wear resistance are drastically improved by this treatment. High values of microhardness, good wear resistance, thick nitrided layers, and low values of the strain have been found especially for the sample that was treated in 80% N2. The high abundance of the cubic TiN phase with high (200) texture in this sample has found to be the reason of these attractive mechanical properties. Thus, laser nitriding is an effective way to enhance the microstructure and the mechanical properties of the titanium samples.