Yüksek Safiyette Molibden Tozunun Spark Plazma Sinterleme Yöntemi İle Şekillendirme Şartlarının Belirlenmesi Ve Karakterizasyonu

Abstract

Yüksek ergime sıcaklığı, yüksek ısıl ve elektriksel iletkenlik özelliklerine sahip olan refrakter metaller arasında bulunan molibden, elektrik-elektronik endüstrisinde, havacılık endüstrisinde ve yüksek sıcaklık yapı elemanlarında yaygın kullanıma sahip bir metaldir. Yapılan bu çalışmada, yüksek saflıktaki (%99-99,97) molibden tozunun spark plazma sinterleme yöntemi ile şekillendirilmesinde; sinterleme sıcaklığı, basınç ve süre parametrelerinin, sinterleme davranışları, mekanik özellikler ve mikroyapı üzerine etkisi incelenmiştir. Bu çalışmada; kullanılan molibden tozları, 50 mm çapına sahip grafit kalıplara yerleştirilmek suretiyle spark plazma sinterleme işlemine sokulmuştur. Sinterleme işlemleri, 1550, 1600, 1650, 1675, 1700 ve 1750°C sıcaklıklarında, vakum ortamında, 30-40-60 MPa basınç altında, 4-8 dk. sinterleme süresi ile spark plazma sinterleme sistemi kullanılarak gerçekleştirilmiştir. Sinterleme parametrelerinden, sinterleme sıcaklığı ve basınç değişimlerinin üretimi gerçekleştirilen molibden numunelerin relatif yoğunluk ve mekanik özelliklerine etkisinin incelenmesi amacıyla, yoğunluk ve sertlik değerleri ölçülmüştür. Relatif yoğunluk ölçümleri, Arşimet prensibi kullanılarak yoğunluk ölçüm cihazı ile ölçülmüştür. Üretimde kullanılan yüksek saflıktaki molibden tozunun faz analizi X-ışınları difraktometresi (XRD) ile yapılmıştır. Farklı sinterleme sıcaklığı ve basıncına sahip numunelerin sertlikleri, mikrosertlik ölçüm cihazında; 1,96 N (200 gf) yük uygulanarak ölçülmüştür. Spark plazma sinterleme yöntemi ile gerçekleştirilen üretimlerde, sinterleme sıcaklığının artışının üretilen numunelerin relatif yoğunluklarını arttırdığı gözlenmiştir. 1550°C’de yapılan üretimlerde relatif yoğunluklar yaklaşık %95 olarak ölçülmüşken, sıcaklığın 1650°C olduğu üretimlerde %97’nin üzerinde relatif yoğunluklar elde edilmiştir. 1750°C’de yapılan üretimde ise %98 relatif yoğunluğa ulaşılmıştır. Sabit sinterleme sıcaklığı ve sabit sinterleme süresi koşulları altında, artan sinterleme basıncının da numunelerin relatif yoğunluklarını arttırdığı gözlenmiştir. 1650°C ve 4 dk. bekleme süresi altında yapılan üretimlerde; 40 MPa basınç altında üretilen numunenin relatif yoğunluğu %97,29 iken, 60 MPa basınç altında üretilen numunenin relatif yoğunluğu %98,20 olarak hesaplanmıştır. Sinterleme sıcaklığı, sinterleme basıncı ve sinterleme süresi parametrelerinden; diğer ikisi sabit kalmak koşulu ile, artan sinterleme sıcaklığının sertlik değerlerini bir miktar düşürdüğü tespit edilmiştir. Sinterleme basıncı ve sinterleme süresi parametrelerinin ise sertlik üzerine belirgin bir etkisi gözlenmemiştir. Sinterleme koşulları 1550°C - 30 MPa - 4 dk. olan numune sertliği 2,87 GPa iken, koşulları 1550°C - 40 MPa - 4 dk. olan numune sertliği 2,76 GPa olarak ölçülmüştür. Sinterleme koşulları 1650°C - 40 MPa - 4 dk. olan numune sertliği 2,53 GPa iken, koşulları 1650°C - 60 MPa - 4 dk. olan numune sertliği 2,71 GPa olarak ölçülmüştür. Artan sinterleme süresinin ise sertlik değerlerinin bir miktar yükselttiği görülmüştür. Sinterleme koşulları 1650°C - 40 MPa - 4 dk. olan numune sertliği 2,53 GPa iken, koşulları 1650°C - 40 MPa - 8 dk. olan numune sertliği 2,59 GPa olarak ölçülmüştür. Artan sinterleme sıcaklığının ise sertlik değerlerini bir miktar düşürdüğü, 1650°C-1700°C arasında yükselttiği gözlenmiştir. 1550°C’de 40 MPa basınç altında sinterlenen numunenin sertliği 2,76 GPa iken aynı basınç altında 1600°C’de sinterlenen numunenin sertliği 2,70 GPa’a, aynı basınç altında 1650°C’de sinterlenen numunenin sertliği ise 2,53 GPa’a düşmüştür. 1700°C’de sinterlenen numunenin sertliği 2,62 GPa olarak ölçülmüştür. 1750°C’de sinterlenen numunenin yüzey sertliği 6,57, kesit sertliği ise 2,43 GPa olarak ölçülmüştür. Yüzey ve kesit arasındaki bu sertlik farkının nedeni için ince film XRD analizi yapılmış ve yüzeyde bir miktar Mo2C oluşumunun bu sertlik artışına neden olduğu görülmüştür. 1750°C, 40 MPa ve 4 dk. koşullarında sinterlenen bu numuneye yapılan ince film XRD analizi göstermiştir ki; SPS, kısa süreli bir proses olmasına rağmen molibden tozunun grafit kalıp ve levha ile teması sonucu yüzeyden içeri doğru karbon difüzyonu bu oluşuma neden olmuştur. Numunenin kesitinden alınan mikroyapı görüntüleri ışığında, Mo2C oluşumlarının yüzeyden 350 mm içeri difüze olduğu görülmüştür. Her 50 mm aralıkta yapılan EDS analizleri ışığında, 200 mm sonrasında karbon yüzdesinin azalma eğilimi gösterdiği tespit edilmiştir.

Refractory metals are the metals that having the highest melting points and lowest vapor pressures among all metals, except the two of the platinum-group metals, osmium and iridium. These refractory metals include tantalum (Ta), tungsten (W), molybdenum (Mo), niobium (Nb) and rhenium (Re). Refractory metals have excellent wear and heat resistance. These metals at one time have a limited use in applications like heating elements, lamp filaments, electrical contacts and electron tube grids, however they have since found widespread application like rocket nozzles, honeycomb structures, inertial guidance systems, leading edges and nose caps for hypersonic flight vehicles in aerospace industry; capacitors, transducers, electron tube parts, heaters, cathodes, X-ray targets, electrical contacts, filaments in electronics industry etc. Molybdenum, which is one of these refractory metals, is primarily using as an alloying element in iron-steel and non-ferrous metals’ production processes. Moreover, molybdenum is a very widely-used refractory metal in different applications like cathodes, cathode supports for radar devices, magnetron end hats, current leads in the electrical and electronic industries. Also it is used in missile industry for high-temperature structural parts such as rocket nozzles, heat-radiation shields, heat sinks, turbine wheels, leading edges of control surfaces and reentry cones. Besides high melting point, high wear and heat resistance properties, molybdenum also has high electrical and thermal conductivity and shows low thermal expansion. Electrical conductivity value is only lower than tungsten among the refractory metals. (W = 1.89 x 107, Mo = 1.87 x 107) Also this electrical conductivity value of molybdenum is higher than metals like cobalt (Co), zinc (Zn), nickel (Ni). Temperature constant (K-1) value of molybdenum is higher than some refractory metals as W and Ta, and also some widely used metals as aluminum (Al), copper (Cu) and zinc, and also precious metals as silver (Ag), platinum (Pt) and gold (Au). Work function, which is defined as “the minimum energy needed to remove an electron from a solid to a point immediately outside the solid surface”, of the molybdenum is lower than metals like Ni, Re, Cr, W, Ta. Because of that, molybdenum becomes an alternative metallic material at electrical and electronic industry. There are two types of production process of molybdenum. One of them is melting, and the other one is powder metallurgy. Production from melting is generally holding by vacuum arc furnaces with the VAR (vacuum arc remelting) process and this production technique is an unpreferable method when comparing with the powder metallurgy. More than 95% of the total world production of molybdenum and molybdenum alloys high in molybdenum is produced by the powder metallurgy. With powder metallurgy, fine-grained microstructure and higher mechanical properties are gained against the melting method. Molybdenum production by powder metallurgy performed by different types of production techniques like conventional sintering, hot isostatic pressing, die pressing and spark plasma sintering. Spark plasma sintering is a pressure-assisted sintering method that the sample is heated by a pulsed direct current which passes through graphite punch, die and powder in this technique. This sintering method provides some advantages like low sintering temperature, fast heating rate, ease of operation and accurate control of sintering energy. By these advantages, sintering process complete in a few minutes and grain growth is inhibited. In this study, bulk molybdenum metallic materials produced by spark plasma sintering method to determine the parameter changes of sintering temperature, sintering pressure and sintering (holding) time on the effect of the sintering behaviour and mechanical properties were investigated. High purity (99-99,97%) molybdenum powder as starting material. The phase analysis of the molybdenum powder was performed with X-ray diffraction device with the range of 2θ = 20-80°. Sintering processes are carried out at sintering temperatures 1550, 1600, 1650, 1675, 1700 and 1750°C, sintering pressure 30-40-60 MPa and sintering (holding) time 4-8 minutes. All these processes are fulfilled in vacuum atmosphere. The characterization stage involves the following analyses of molybdenum metallic materials: densification and sintering behaviour (shrinkage rate, displacement rate), relative density, phase analysis, microstructure characterization and mechanical properties characterization. The densification behaviour of molybdenum materials showed that, while sintering pressure and sintering time parameters are constant, sintering temperature does not significantly effect the beginning of the shrinkage temperature. While sintering temperature and sintering time parameteres are constant, sintering pressure effects the beginning of the shrinkage temperature. Increasing sintering pressure decreases the beginning of the shrinkage. The large slopes between the beginning of the shrinkage and finishing of the shrinkage indicates that the consolidation occurs mainly during fast heating up of spark plasma sintering. Densification of molybdenum samples depend on greatly the acting temperature, generated by resistance heating in the powder. It was determined that, the relative density of the molybdenum metallic materials are increased with both the sintering pressure, sintering temperature and sintering time, while two of the other parameters are constant. A relative density of 98% and higher can be reached in three conditions. One of them is 98.02% at 1750°C sintering temperature, with 40 MPa external pressure and 4 min. of holding time. The other one is 98.20% at 1650°C sintering temperature, with 60 MPa external pressure and 4 min. of holding time. The highest relative density value is revealed 98.25% at the following conditions: 1675°C sintering temperature, 60 MPa sintering pressure and 4 min. of sintering time. It is revealed that the microhardness values of molybdenum metallic materials are higher when comparing with the previous studies. Increasing sintering pressure is not significantly effect on hardness values of the samples. It is calculated that hardness of sample which is sintered at 1550°C, 30 MPa, 4 min. sintering conditions, is 2.87 GPa. Besides, the sample which is sintered at the same sintering temperature and sintering time with 40 MPa sintering pressure, hardness value is calculated as 2.76 GPa. Hardness of sample which is sintered at 1650°C, 40 MPa, 4 min. sintering conditions, is 2.59 GPa. Besides, the sample which is sintered at the same sintering temperature and sintering time with 60 MPa sintering pressure, hardness value is calculated as 2.71 GPa. Hardness values of the samples decreases with the increasing sintering temperature. It is calculated that hardness of sample which is sintered at 1550°C, 40 MPa, 4 min. sintering conditions, is 2.76 GPa. Besides, the sample which is sintered at the same sintering pressure and sintering time at 1600°C, hardness value is calculated as 2.70 GPa. Also, when the sintering pressure and time parameters remain constant, at 1650°C, hardness value of the sample calculated as 2.53 GPa. At 1700°C, with the same sintering pressure and sintering time, hardness value of the sample calculated as 2.62 GPa. At 1750°C sintering temperature, hardness value of the sample calculated 6.57 GPa from surface and 2.43 from cross section. On the other hand, hardness values of the samples slightly increases with the increasing sintering (holding) time. It is calculated that hardness of sample which is sintered at 1650°C, 40 MPa, 4 min. sintering conditions, is 2.53 GPa. Besides, the sample which is sintered at the same sintering pressure and sintering temperature with 8 min. sintering time, hardness value is calculated as 2.59 GPa. For the investigation of this hardness difference, thin film XRD analysis was carried out. It is revealed that; when thin film XRD diagram of the sample that sintered at 1750°C investigated with the database patterns, there are molybdenum carbide (Mo2C) peaks obtained from three different Mo2C pattern. It is seen that the peaks that are not related with molybdenum’s characteristic peaks comes from the Mo2C phases at the sample surface. It is revealed that the differences at the fractured surface micrographs and hardness of cross-section and surface comes from this formation of Mo2C in sample surface. Despite the fact that, spark plasma sintering (SPS) is a short-time process, contact of molybdenum powder with graphite die and sheet causes to diffusion of carbon inside the samples. For the detection of the thickness of Mo2C structure through inside the surface, a SEM analysis was carried out from the cross-section of the sample that sintered at 1750°C sintering temperature. It is seen that, Mo2C morphology is seen through nearly 350 mm of inside the main structure from the surface. In conclusion, high purity molybdenum powder are successfully consolidated by spark plasma sintering technique with the sintering temperatures 1550 – 1750°C, sintering pressures 30, 40 and 60 MPa, 240 to 480 min. of holding time. The relative densities of the metallic molybdenum samples shows increase with the increasing sintering temperature, external pressure and holding time at maximum temperature. The highest relative density value obtained as 98.25%. In comparison with the values of the literature, these densities obtained exceed those by conventional, isothermal sintering and hot-isostatic pressing.