ÖZET Anahtar Kelimeler : Şok İşlem, Plazma nitrürleme, Tİ-6A1-4V alaşımı, Difözyon katsayısı, Aktivasyon enerjisi, Kayma aşınması, sürtünme katsayısı, Bilye-disk aşınma sistemi Titanyum ve alaşımları mukavemet - ağırlık oram, kırılma tokluğu, korozyon direnci ve insan vücuduna uyumluluğu açısından mükemmel olmasına rağmen aşınma performansı pek tatmin edici değildir. Titanyum ve alaşımlarının nitrürlenmesi yorulma ve aşınma direncini artırarak onlara geniş bir uygulama alam sağlar. Plazma nitrürleme titanyum alaşımlarının yüzeyinin sertleştirilmesinde kullanılan en etkili metotlardan biridir. Bu işlem sayesinde titanyum ve alaşımlarının yüzeyinde yüksek sertlik, aşınma ve korozyon direncine sahip tabakalar ve difuzyon tabakası oluşmaktadır. Bu çalışmada, patlamalı şok işlemine maruz bırakılmış Tİ-6A1-4V alaşımı 700, 800 ve 900 °C'lerde 3, 6, 9 ve 12 saat sürelerle plazma ünitesinde nitrürlenmiştir. Nitrürlenen Tİ-6A1-4V alaşımının yüzey performansı, difuzyon kinetiği, aşınma ve sürtünme davranışları incelenmiştir. Plazma nitrürleme işlemi sonrası oluşan nitrür tabakalarının yüzey morfolojileri ve faz analizleri optik, taramalı elektron mikroskobu ve x-ışınları difraksiyon analizi yardımıyla, aşınma sonrası yüzey topografyası optik ve elektron mikroskobu yardımıyla incelenmiştir. Ayrıca elde edilen nitrür tabakalarındaki azotun difuzyon kinetiği analitik ve nümerik yolla belirlenmiştir. Metalografik incelemelerde, TiN (8) fazı, onu takiben Tİ2N (e) fazı ve daha aşağıda titanyum içerisinde azotun çözünmesinden dolayı azotça zengin bir katı çözelti tabakası oluştuğu, bu nitrürlü tabakaların yüzeye paralel olarak büyüdüğü ve düz bir morfolojiye sahip olduğu belirlenmiştir. Ayrıca bu bulgular XRD çalışması ile desteklenmiştir. XRD çalışmasında düşük sıcaklıklarda VN fazı da bulunurken, sıcaklığın artması ile bu faz kaybolmuştur. Alaşımın yüzey tabakalarında özellikle 900 °C'de 12 saat nitrürlenmiş alaşımda oldukça yüksek mikrosertlik değerleri (2546 HV) elde edilmiştir. TiN(8) ve Tİ2N(s) tabakalarındaki azotun aktivasyon enerjisi sırasıyla 18950(±2116) ve 27925(±1105) dir. Bu değerler literatürdeki değerler ile kıyaslandığında oldukça düşüktür. Bu da patlamalı şok işleminin oluşturduğu bir avantajdır. Bilye (WC)-disk sisteminde yapılan aşınma sonucunda, plazma nitrürleme sıcaklık ve süresinin artmasının aşınma direncini artırdığı belirlenmiştir. 900 °C'de 12 saat nitrürlenmiş alaşımın aşınma direnci nitrürlenmemiş alaşıma göre 200 kat artış göstermektedir. Nitrürlenmemiş Tİ-6A1-4V alaşımının sürtünme katsayısı 0,46-0,49 civarındayken, plazmada nitrürlenmiş alaşımların üzerindeki sert tabakanın sürtünme katsayısı 0,18-0,20 arasındadır. Plazma nitrürleme sonucu oluşan TiN alaşımın sürtünme katsayısını düşürmüştür.
INVESTIGATION OF WEAR BEHAVIOUR AND DIFFUSION KINETICS OF PLASMA NITRTOED Tİ-6A1-4V ALLOY Keywords : Shock treatment, Plasma nitriding, Tİ-6A1-4V alloy, Diffusion coefficient, Activation energy, Sliding wear, Friction coefficient, Ball on-disc system Surface diffusion treatments (i.e. nitriding, carburizing and boronizing ) are prominent choices for a wide range of tribological applications where the control of friction and wear is significant. These treatments can dramatically enhance the surface mechanical properties of metal components. Plasma nitriding, that is one of these surface treatments, is a ion treatment which surface harden metals. The process is used commercially and has a number of distinct advantages over gas nitriding. Plasma nitriding is carried out in a nitrogen-hydrogen gas mixture at 1 to 10 Ton- pressure. A glow discharge is operated between an anode and the sample to be hardened which is used as the cathode. Titanium and titanium alloys possess several excellent properties like a good corrosion resistance and high strength to weight ratio. Its light weight and ability to withstand extreme temperature make it suitable for aircraft applications. However, a disadvantages of titanium alloy is its high friction and poor wear resistance. This problem can be tackled by nitriding, the surface which results in a hard ceramic surface layer and as a consequence enhance both wear and friction properties. The mechanistic approaches to plasma nitriding behaviour of titanium alloys have been consistent in identifying the strong affinity between nitrogen and titanium and therefore the large driving force to form TiN. Titanium nitride is of great technological interest owing to its unique characteristic such as extreme hardness, high chemical and physical stability, good thermal and electrical conductivity and high melting point. There are several successful studies of TiN production on titanium or Tİ-6A1-4V substrates like reactive sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), plasma assisted chemical vapor deposition (PACVD), ion implantation, laser nitriding and plasma ion nitriding. Plasma ion nitriding of titanium and its alloys has been found to produce a thin surface layer of TiN (8) followed by Ti2N (e) layer and an interstitial nitrogen diffusion zone in the adjacent a-titanium matrix. Diffusional growth and multiphase diffusion of intermediate layers in binary diffusion couples were studied by many researchers. General solutions for the determination of diffusion coefficient in binary multiphase system are produced. The xviianalytical expressions for multiphase diffusion in binary metal - interstitial systems were developed with the following assumptions; 1. The interface advancement occurs due to the motion of interstitial atoms. The flux of the metal atoms can be neglected as a contributing factor to the layer growth. 2. Compositionally independent diffusion coefficients for nitrogen are used. 3. Thermodynamic equilibrium is assumed at the phase interface. The interface and surface compositions are assumed to be constant during nitriding and remains unchanged. Planar and stable interface are assumed. Wear has been defined as the removal of material from solid surfaces as a result of one contacting surface moving over another. Thus both friction and wear are simultaneously the results of the same tribological contact process that takes place between two moving surfaces. The removal of material from the surface is described by several wear mechanism. Typical of such mechanism are adhesive, abrasive, fatigue, delamination and chemical wear. Tribological properties of plasma nitrided titanium alloys are very impressive. Low coefficient of friction and low wear rates have been demonstrated for plasma nitrided Tİ-6A1-4V compared with untreated material. Fatigue performance of the nitrided material is highly dependent on prior substrate condition. Annealed Tİ-6A1-4V that was subsequently nitrided showed a marked reduction in endurance life compared with untreated Tİ-6A1-4V; this can be attributed to coarsening of the a grains and production of a continuous a matrix. In this study, Tİ-6A1-4V alloys were explosively shock treated by a parallel explosive welding arrangement. The plasma nitriding was carried out at 5 Torr nitrogen partial pressure with 99,99 % purity nitrogen gas for Tİ-6A1-4V alloy that explosively shock treated. The glow discharge was operated with a potential difference of 500 to 800 V to obtain the prescribed nitriding temperature. The nitriding temperatures were controlled by changing the power input. After 2 h of Ar sputtering, nitriding treatments were carried out at temperature of 700, 800 and 900 °C for 3, 6, 9 and 12 h durations. After nitriding, the samples were cut on cross-section and prepared using standard metallographic techniques. The thickness of plasma nitrided layers in the samples were measured by an optical and scanning electron microscopy (SEM). The surface nitrides were identified by using X-ray diffractometer. The interface velocity equations were derived from Fick's laws and a numerical method has been used to compute the diffusion coefficients of nitrogen in a binary multiphase Ti - TiN system. Wear tests was carried out using a ball-on-disc machine in ambient air of 38 % RH under unlubricated dry sliding conditions. WC ball was used as counterface material. The stationary ball was loaded by a dead weight put on the top of the ball. Tİ-6A1-4V discs plasma nitrided were driven by a d.c. motor. The tangential force originated from the normal force was measured by a load cell mounted on the loading arm. For this purpose a microprocessor controlled data acquisition system was used. For wear XVIIItests, track diameter, sliding speed and loads were selected as 0,006 m, 0,12 m/s and 2,5 N, 5 N, 7,5 N, 10 N, 12,5 N respectively. A thin film of TiN with a golden yellow colour formed at the surface of Tİ-6A1-4V alloy that was exposed to nitrogen plasma. This was confirmed by XRD analysis. XRD analysis showed that the nitrided layers consist of TiN(5) and Tİ2N(e) and a- Ti phases. Increasing the nitriding temperature and time resulted in the increase of the growth of TiN(8) and Tİ2N(s) phases and the decrease in the a- Ti phase. At the low temperature nitriding XRD analysis showed VN phase, but at 900 °C for 12 h this phase disappeared in the XRD pattern. Optical and electron (SEM) micrographs obtained from cross-section of the samples showed that the layers consist of TiN(8) phase followed by Tİ2N(s) phase subsequently a thick layer of a - Ti phase. To evaluate the effects of nitriding time and temperature on the extent of nitrogen, diffusion microhardness measurements were carried out on samples which were treated at 700, 800 and 900 °C for different times. Nitriding at high temperatures also resulted in high hardness values. Even at lower temperatures (700 °C) as values high as 1734 vickers hardness value was obtained. Nitriding at 900 °C for 12 hours produced maximum surface hardness values around 2546 vickers, which is at least a sixfold increase over the original microhardness of the unprocessed sample. The thickness TiN(ö) layer is seen to increase with increasing temperature of plasma nitriding and is parabolic with time. The s layer also shows parabolic growth with time in the all temperatures. Since the temperatures studied were the below the a~P transformation temperature, data for the thickness of the a- phase at 700 and 800 °C couldn't be measured. At those temperatures a- phase was the terminal phase. At 900 °C terminal phase was transformed to P-titanium. The diffusion analysis was firstly analysed at 900 °C. The interface velocity derived from Fick's laws for the each interface are as follows: 5/e expK-%=)2] f-n expK-^=)2] Bs24D~t = Jp,Bt24DJZjDgJ ZJDS1 2jDsJ T^JDJ 2JDJ exp[-(-^M2]._ exp[-(-ŞiM2] J a/p JxZea k*L_u>rf( kT) - erf(-^=)] JO*Z** k*> [rrf( *<* ) - erf( kas -)] expH-^)2]._ exp[-(__^=)2] %[DJ iJdj iJdj 2/^7 iJd^s XIXThese non-linear equations were solved using a numerical method by Matlap® mathematical program. Arrhenius equations for Ds and D8 n < v-n T in-»,-18950(+2116). Ds = 5,3(+2,73).10 sexp(-^'-) D.^.HffO^.lO-expC2792^1105') Ku for the temperature range 700 °C to 900 °C. The obtained values of activation energies in 8 (18950 cal/mole)and 8 (27925 cal/mole) phases are lower than that of values reported in literature. It is well known that explosive shock treatment increases dislocation densities 2 or 3 times when compared with cold worked metals. High dislocation density result in increase in atomic vacancies that accelerates the diffusion of nitrogen atoms to titanium network structure. Because of the high vacancy concentration the critical energy barrier is thought to decrease for nitrogen atom movement. Wear resistance of Tİ-6A1-4V alloy increases as the time and temperature of plasma nitriding increase. Under the 5 N load, wear resistance of Tİ-6A1-4V alloy, plasma nitrided at 700 °C for 12 hours, increases 15 % according to the Tİ-6A1-4V alloy without plasma nitrided. Wear resistance of Tİ-6A1-4V alloy, plasma nitrided at 800 and 900 °C for 12 h, increases 5 and 123 times respectively. Wear rate increases with the increase in applied load. At the all nitriding time of the samples nitrided at 700 and 800 °C, the nitrided layers completely worn out at the end of 500 m of sliding. Friction coefficient of Tİ-6A1-4V alloy against WC ball varied between 0,46 and 0,49 values. After nitriding treatment, coefficient of friction of Tİ-6A1-4V alloy nitrided, especially 900 °C for 9 and 12 h, varied between 0,18 and 0,2 values. In this study, the results of friction coefficients confirmed the wear behaviour of Ti- 6A1-4V alloy plasma nitrided. Electron microscopy (SEM) observations for unprocessed Tİ-6A1-4V alloy showed the plastic deformation areas, produced oxidized particles and large abrasive cavities. During the wear, metallic and black coloured powder were observed as wear production. This result was confirmed by XRD. Electron microscopy (SEM) observations of Tİ-6A1-4V nitrided at 700 °C and worn under the 2,5 and 12,5 N load showed wear particles in wear track and ploughing produced by effect of wear particles. Electron microscopy (SEM) observations of Tİ-6A1-4V alloy, nitrided at 900 °C for 12 h and worn under the 2,5 N load, showed that microcrack formed at the end of 500 m of sliding and plate particles removal initiated wear damage at the end of 1000 and 1500 m of sliding. XX
