Spark plazma sinterleme yöntemi ile üretilen molibden-molibden karbür esaslı yapıların mikroyapı ve tribolojik karakterizasyonu
Microstructural and tribologic characterization of molybdenum-molybdenum carbide structures produced by spark plasma sintering (SPS)
Yazar: CAN ÇEKLİ
Danışman: PROF. DR. GÜLTEKİN GÖLLER
İstanbul Teknik Üniversitesi / Fen Bilimleri Enstitüsü / Malzeme Bilimi ve Mühendisliği Ana Bilim Dalı / Malzeme Bilimi ve Mühendisliği Bilim Dalı
Derece: Doktora, 2019
Konu: Metalurji Mühendisliği = Metallurgical Engineering

Özet:
Refrakter metaller grubuna ait molibden (Mo), yüksek ergime sıcaklığı, yüksek ısıl ve elektriksel iletkenlik özellikleri sayesinde, havacılık, elektrik-elektronik endüstrileri ile yüksek sıcaklık uygulamalarında kullanılmaktadır. Yapılan doktora çalışmasında %99,97 saflıkta molibden tozunun spark plazma sinterleme yöntemi ile 40 MPa sabit basınç altında, 100 °C/dk ısıtma hızı ile vakum atmosferinde şekillendirilmesinde, sinterleme sıcaklığı ve sinterleme sürelerinin, yoğunlaşma davranışları, mekanik özellikler, mikroyapı ve aşınma üzerine etkileri incelenmiştir.
Bu çalışmada kullanılan başlangıç tozları, 50 mm çapında disk şeklinde numuneler elde etmek amacıyla, grafit kalıplara yerleştirilerek spark plazma sinterleme işlemine tabi tutulmuşlardır. Sinterleme sıcaklıkları 1650, 1700 ve 1725 °C olarak, sinterleme süreleri ise 180, 360 ve 540 saniye olarak belirlenmiştir. Sinterleme işlemi sırasında numunelerin sıcaklık ve zamana bağlı olarak lineer çekilmesi izlenmiş, çekilmenin 950 °C'de başlayıp 1400 °C'de tamamlandığı, sinterleme parametrelerinin çekilme eğrisi üzerinde önemli bir değişiklik oluşturmadığı belirlenmiştir.. 
Farklı sinterleme sıcaklığı ve sinterleme sürelerinde üretimi yapılan molibden numunelerin relatif yoğunluk ve mekanik özelliklere etkilerini belirlemek amacıyla, yoğunluk ve mikrosertlik testleri yapılmıştır. Molibden tozlarının yoğunlaşma davranışı incelendiğinde, sinterleme sıcaklığı ve sinterlenme süresinin artmasıyla relatif yoğunluk değerlerinin azaldığı, 1650 °C sinterleme sıcaklığında ve 180 saniyelik sinterleme süresinde en yüksek relatif yoğunluk değer olan %97,55'e ulaşıldığı belirlenmiştir.
Yüzey üzerinde yapılan mikrosertlik testlerinde, 15,2 GPa değerine ulaşılmış, kesitlerde ölçülen maksimum değer ise 2,17 GPa olmuştur. Sonuçlar, tüm numunelerin yüzeyinde molibden karbür yapısının oluşumunu gösterirken, kesitlerde elde edilen verilerin molibden ana metaline ait literatürdeki veriler ile uyumlu olduğunu göstermiştir. Molibden ana metalini temsil eden kesit sertlik sonuçları 2,04 ve 2,17 GPa arasında değişmektedir, hem sinterleme sıcaklığının hem de sinterleme süresinin ana malzemenin mikro sertliği üzerinde önemli bir etkiye sahip olmadığı tespit edilmiştir.
Ayrıca, 13,70 ile 15,24 GPa arasında değişen yüzey mikrosertlik değerlerinin, sinterleme sıcaklığı ve sinterleme süresi arttıkça azaldığını göstermiştir. Diğer taraftan sinterleme sıcaklığı ve sinterleme süresindeki artış, numunelerin yüzeyinde oluşan molibden karbür tabakasının kalınlığında bir artışa neden olmuştur. Molibden karbür tabakasının kalınlığındaki artışa rağmen, sertlik değerlerinde görülen azalma, mikroyapı bölümünde incelenen katmanın poroz yapısı ile açıklanmıştır.
Numunelerin mikroyapı incelemeleri mikrosertlik sonuçlarını desteklemiş, yüzeyde oluşan molibden karbür tabakasının en az 288 µm, en çok 589 µm kalınlığında olduğu tespit edilmiştir. Buna ilave olarak, molibden ve molibden karbür tabakaları arasında tipik ötektoid yapısının oluştuğu fakat disk şeklindeki numunelerin merkezlerinden kenarlara ilerledikçe oluşan sıcaklık kaybı nedeniyle, bu tabakanın kalınlığının azalarak yok olduğu görülmüştür.
1725 °C sinterleme sıcaklığında ve 540 saniyelik sinterleme süresinde üretilen numunenin mikroyapı incelemesinde, ana metal ile ilgili eşeksenli tane yapısı gözlemlenmiş, ortalama tane boyutunun 58,6±2 µm olduğu belirlenmiştir.
10 mm çaplı alumina top kullanılarak, 2 N normal yük altında gerçekleştirilen aşınma testleri sonucunda, 1700 ve 1725 °C'de sinterlenmiş numunelere ait sürtünme katsayılarının sinterleme süresindeki artışla arttığı, fakat 1650 °C'de sinterlenmiş numunelerde önemli bir değişiklik olmadığı belirlenmiştir. 4 N normal yük altında gerçekleştirilen çalışmaların deney sonuçlarından, bu numune grubu için uygun olmadığı anlaşılmıştır.  1700 °C sinterleme sıcaklığı ve 180 saniye sinterleme süresinde üretilen numunenin, 0,488 değeri ile tüm numuneler arasında en düşük sürtünme katsayısına sahip olduğu belirlenmiştir. 
Aşınma direncinin iyileştirilmesinin, daha düşük porozitenin elde edildiği, düşük sinterlenme süresinde üretilen numunelerde sağlandığı tespit edilmiştir. Kesite uygulanan aşınma deneyi ile, molibden metalinin aşınma özelliklerinin yüzeyde elde edilen molibden karbür tabakası ile zenginleştiği kanıtlanmıştır.

Abstract:
Refractory metals, including tantalum (Ta), tungsten (W), molybdenum (Mo), niobium (Nb) and rhenium (Re), have the highest melting points (>2468 °C; Mo: 2617 °C) among all metals except osmium and iridium. Aerospace industry and electronics industry are the main fields that these metals are chosen due to their excellent heat and wear resistance.
Pure Molybdenum (Mo) is one of the most important refractory metals because of its high melting point, low thermal expansion coefficient and good thermal conductivity (138 W/(m.K)). Due to these remarkable properties it is used primarily as alloying element in non ferrous metals and iron-steel production processes, also for a wide scale of engineering applications. Some examples of molybdenum applications for high temperature structural parts are rocket nozzles, heat radiation shields, heat sinks and turbine wheels.
In general, pure molybdenum or its alloys are manufactured by melting processes or by powder metallurgy techniques. Solid state, isothermal and spark plasma sintering (SPS) techniques are widely used powder metallurgy methods for manufacturing of molybdenum. Molybdenum can be manufactured at relatively lower temperatures for shorter holding periods by SPS technique when compared to conventional sintering techniques. In the SPS technique, a pulsed direct current passes through the graphite punch rods and dies simultaneously under uniaxial pressure. Rapid heating suppresses the grain growth and densification is accelerated at higher temperatures.
The densification behavior of molybdenum by different production techniques has been investigated in various studies. Sintering of pure molybdenum up to 1450°C, effect of interstitial impurities (O and C), effect of adding other metallic elements (Ni, Cu and Pd) were reported earlier. In addition, the effect of compaction pressure and sintering temperature on consolidation of molybdenum was also inspected. Consolidation of pure molybdenum powder by using SPS technique was also investigated by means of heating rate, sintering pressure and temperature. However the formation of carbon-rich layer during SPS process was not evaluated. Carbon diffusion from graphite dies to molybdenum sample during SPS process is inevitable and leads to formation of carbide layer on the surfaces. Formation, thickness, morphology and composition of the carbide layer could have an effect on wear, hardness, density and oxidation properties of the sintered sample and should be investigated by means of sintering parameters.
Molybdenum carbide exhibits superior wear resistance than molybdenum due to its low friction coefficient and high hardness, therefore in this study molydenum is produced as surface modified in order to endure abrasive conditions.
In accordance with this purpose, pure molybdenum powder (purity >99.97%, oxygen content >600 ppm) was sintered using Spark Plasma Sintering technique. Molybdenum starting powder particles were in spherical shapes and had sizes of 3-5 µm. However, microstructural investigations showed that these particles were agglomerated and formed larger spherical shaped particles between sizes of 40-120 µm.
Cylindrical graphite die with 95 mm outer diameter and 50 mm inner diameter was filled with 108.4 g of molybdenum powder without any additives, in order to have 4 mm thick disk shaped samples. Graphite sheets with a thickness of 0.5 mm were placed between the punches and the powder, also between the die and the molybdenum powder for easy removal and better conductivity. 
Sintering processes were carried out under constant pressure of 40 MPa at 100 °C/min heating rate under vacuum atmosphere. Sintering temperatures were 1650-1700-1725 °C and holding periods were 180-360-540 seconds. The uniaxial pressure was released during cooling for all of the samples. The effects of sintering temperature and holding period on relative density, microhardness, microstructure and wear properties of the specimens were investigated. Moreover, carbide formation on the surface due to carbon diffusion was also inspected. Prior to sampling for characterization process, final products were cleaned by sandblasting in order to remove residual graphite sheets on the surfaces. Various samples were taken from 4 different locations on each disk unit representing sintering parameters, in order to perform characterization studies and evaluate homogeneity of SPS process. Sampling was performed as follows; Sample C was cut from center, Samples D1 and D2 from 5 mm away from center and Sample E was cut from edge of each disk unit. As in the earlier studies, Sample C was investigated by means of densification, microhardness and microstructure. In addition to outcomes of Sample C; samples D1, D2 and E were investigated to determine the homogeneity of microstructure and carbon diffusion from graphite die to samples throughout the entire disk unit in detail.  Moreover, tribologic characterization was conducted on additional samples taken from disk units, by means of investigating wear properties of each parameters.
The linear shrinkage of the specimens depending on temperature and time were recorded during sintering process. Shrinkage was begun at 950°C and completed at 1400°C showing no significant change in curves, among all parameters observed. Densification behavior of molybdenum powders showed that, relative density values decreased with increasing sintering temperature and holding period, obtaining highest value of 97.55% at 1650 °C sintering temperature and 180 seconds of holding period.
Microhardness value of 15.2 GPa was reached on the surface while maximum value measured at cross section was 2.17 GPa, showing the formation of molybdenum carbide structure on the surface of all specimens. Cross sectional hardness investigations representing base material, varying between 2.04 and 2.17 GPa showed that both sintering temperature and holding period had no significant effect on microhardness of the base material. Average cross sectional hardness value was compatible with theoretical hardness of pure molybdenum (1.96 GPa) given in literature.
Moreover, surface microhardness values, varying between 13.70 and 15.24 GPa, showed that hardness values were decreased with increasing sintering temperature and holding period. On the other hand, increase in sintering temperature and holding period resulted an increase in the thickness of molybdenum carbide layer formed on the surface of the samples. Despite the increase in the thickness of molybdenum carbide layer, reduction in the hardness values could be explained by porous structure of this layer investigated in Microstructure section in detail.
Microstructural analysis supported microhardness results and showed minimum 288 µm and reached to maximum 589 µm thick molybdenum carbide layer on surface. Microstructural analysis performed on the samples revealed that, decreasing effect on microhardness values was caused by the increase in porosity together with thickness. In addition, typical eutectic microstructure was also observed between molybdenum and molybdenum carbide layers, however thickness of eutectic layer decreased and gradually disappeared from center to the edge of disc shaped specimens due to decrease in temperature. 
Additional test performed on sample, sintered at 1650 °C with 360 seconds holding time, clearly demonstrates both equaxial grain structure regarding to base metal and carbide layer formed on surface by the difference of fracture behavior. It is also observed that fracture type of base metal corresponds to intergranular fracture.
Equaxial grains related to the base metal were observed at the cross sectional center of sample, sintered at 1725 °C with 540 seconds holding time, was measured to have small and large equaixed molybdenum grains with an average grain size of 58.6±2 µm.
Reciprocating wear tests on the samples were conducted against alumina balls of 10 mm diameter with 2 N and 4 N normal load applied. Inconsistent friction coefficient values and peaks were obtained from wear tests performed under 4 N normal load and therefore were not appropriate for testing sample sets. , Friction coefficents of samples sintered at 1700 and 1725 °C increased with the increment in holding periods, while samples sintered at 1650 °C had no significant change with varying holding period. Sample sintered at 1700 °C with 180 seconds holding period, showed the lowest friction coefficient, ~0.5, among all samples.
Wear resistance of surfaces was improved by decreasing holding period where non-porous carbide layer and highest microhardness values were achieved. Cross sectional wear test analysis corresponding to molybdenum base metal proved that, formation of molybdenum carbide layer as a result of carbon diffusion, improved wear properties of the sample with lower friction coefficient and lower wear depth.