Powder Metallurgy and Metal Ceramics, Vol. 53, Nos. 1-2, May, 2014 (Russian Original Vol. 53, Nos. 1-2, Jan.-Feb., 2014)
EFFECT OF Mo2C ON THE MICROSTRUCTURE AND MECHANICAL PROPERTIES OF (Ti, W)C–Ni CERMETS 1,3 Changan Tian1 Chengliang Han,
UDC 621.762 (Ti, W)C–Ni cermets with different contents of Mo2C were produced by the spark plasma sintering (SPS) method. The grain size (GS), composition of ceramic phases, and mechanical properties of the sintered cermets were investigated. The amount of Mo2C had a significant influence on the microstructure and mechanical properties of as-prepared cermets. GS and fracture toughness (KIc) were decreased as a result of increasing the amount of Mo2C. By increasing the amount of Mo2C, the transverse rupture strength (TRS) and hardness (HRA) were enhanced. However, above 10 wt.%, the TRS was reduced. The conventional black cores observed by field-emission scanning electron microscopy (FE–SEM) in backscattered electron imaging (BSE) in (Ti, W)C–Ni cermets will be partially turned into some white cores which contain higher Mo, except for Ti and W elements, when content of Mo2C reaches ~15 wt.%. Batch mechanical tests indicate that cermets with some white cores have refined microstructure and higher hardness, but relatively lower transverse rupture strength (TRS) and fracture toughness (KIc) at room temperature. Keywords: (Ti, W)C–Ni cermets, microstructure, mechanical properties, spark plasma sintering (SPS).
INTRODUCTION TiC-based cermets are promising cutting materials due to unusual combinations of physical and chemical properties such as high hardness, high melting point, good electrical and thermal conductivities, and high resistance to thermal shock and wear [1, 2]. Except for TiC and Co/Ni powders, some secondary carbides such as WC, Mo2C, and NbC are usually chosen to improve microstructure and mechanical properties of the sintered cermets [3–5]. Many researchers found that the introduction of WC to Ti(C, N)-based cermets could improve the wettability between the ceramic and metallic phases, which lead to highly compact sintered samples with excellent mechanical properties [6–8]. Similarly, molybdenum (Mo) or molybdenum carbide (Mo2C) is also introduced in the system of TiC or Ti(C, N) cermets. Lasalvia and Yan Li [9, 10] pointed out that the presence of Mo or Mo2C greatly enhanced the density of sintered compacts, decreased particle growth rate, and increased the fracture toughness in Ti(C, N)-based cermets. Previous studies showed that Mo, Mo2C, and WC could react with TiC or Ti(C, N) to form (Ti, Mo, W)C or (Ti, Mo, W)(C,N) solid solution on the surface of TiC or Ti(C, N) grains, which substantially enhanced the interface bonding between TiC or Ti(C, N) and Ni [11–13]. However, there are arguments
1Department 2 Institute 3 To
of Chemical and Material Engineering, Hefei University, China.
of Solid State Physics, Chinese Academy of Sciences, Hefei, China.
whom correspondence should be addressed; e-mail:
[email protected].
Published in Poroshkovaya Metallurgiya, Vol. 53, No. 1–2 (495), pp. 73–80, 2014. Original article submitted June 6, 2013. 1068-1302/14/0102-0057 2014 Springer Science+Business Media New York
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about initial state of powders (Mo or Mo2C) and their contents. Shi [14] regarded that less than 5 wt.% Mo2C is advantageous to microstructures and mechanical properties. Others [15, 16] insisted that the optimal amounts of Mo are in the range of 10–20 wt.%. In addition, previous works mainly concentrated on the effect of Mo or Mo2C on growth of rim phases in the system of Ti(C, N)–Ni/Co. The effect of Mo2C on the microstructure and mechanical properties of (Ti, W)C–Ni cermets has not been well known. The mechanical properties such as transverse rupture strength, fracture toughness, and hardness were measured and correlated to the microstructural evolution.
EXPERIMENTAL PROCEDURES Specimen Preparation. TiC–12WC–xMo2C–24Ni (in wt.%, x = 0, 12, 15) cermets were prepared for this study using TiC (~2.54 m), WC (~3.65 m), Mo2C (~1.52 m), and Ni (~4.36 m) powders. In typical synthesis, firstly, the above powders were weighed and mixed by attrition milling with WC–Co balls in ethanol for 24 h. Secondly, the dried powders mixtures were compacted into a rectangular green samples under a uniaxial pressure of 60 MPa. Finally, the sintered samples were obtained by spark plasma sintering (SPS) from the green samples at 1442C for 1 h. Experimental Methods. In general, the values of mechanical properties have statistical characteristic. The following values of mechanical properties were obtained from the method of the mean. Six sintered samples with the same composition were tested. The values of transverse rupture strength (TRS) and fracture toughness (KIc) were figured out by measuring the critical rupture loading on an MTS801-23 universal machine using a constant 3PL 3PL strain rate mode. The formulas for TRS and KIc were TRS and K Ic Y c , respectively, where Y is a 2 2bh 2bh2 2 3 4 c c c c geometrical factor (Y = 1.93–3.07 +14.53 – 25.11 + + 25.8 ); P is the critical rupture loading; c h h h h is the size of the pre-made crack; L, h, and b are the length, height and width of tested samples, respectively. The hardness (HRA) was measured on the common Rockwell hardometer. The microstructure and fractured surfaces of the as-fractured cermets were observed by field-emission scanning electron microscopy (FE-SEM, FEI, sirion). The grain size(GS) of sintered cermets was obtained by image analysis (Analysis software).
RESULTS AND DISCUSSION XRD Analysis. Figure 1 shows the XRD patterns of the as-prepared products produced by SPS at 1442C for 1 h. All of the diffraction peaks can be readily indexed from the standard powder diffraction file of the cubic phase TiC (JCPDS 02-1179) and Ni (JCPDS 65-0380), which are also marked with solid squares and solid circles, respectively, in Fig. 1. These data clearly show that all sintered products are TiC–Ni composites.
Fig. 1. XRD patterns of as-prepared TiC-based cermets with various contents of Mo2C, wt.%: 0 (a), 12 (b), and 15 (c) 58
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Fig. 2. Microstructures of as-prepared TiC-based cermets with various contents of Mo2C, wt.%: 0 (a), 12 (b), and 15 (c)
Microstructural and Mechanical Features. Figure 2 shows the FE-SEM microstructure of as-obtained TiC– Ni cermets with various contents of Mo2C. It can be found that the addition of Mo2C to TiC–Ni cermets had a significant effect on the microstructure. Firstly, the grain size obviously reduced with increasing Mo2C. Secondly, in backscattered electron imaging (BSE), all ceramic grain phases exhibited a core-rim structure. Ahn and Kang [17] have indicated that the black core is made of nearly pure TiC ceramic phase and gray rim mainly consists of (Ti, W, Mo. . .)C solid solution formed through the dissolved Mo2C, WC, and TiC at the liquid-phase sintering stage and co-precipitated in subsequent cooling process. However, some white cores have been observed in the assintered TiC–Ni cermets with ~15 wt.% Mo2C. Lindahl et al. [18] reported that the increasing content of Mo in cermets would cause the volume fraction of rim to increase. Here, we also found that the amount of Mo2C (~15 wt.%) not only resulted in decreasing of the volume fraction of rims, but also changed the color of cores. In order to well understand the formation of white cores, the energy spectrum analyses (EDX) of cores were conducted and results were shown in Fig. 3. It was found that the core contained more Mo and W elements compared with the core without Mo2C addition. The core without Mo2C addition is almost pure TiC and the core with proper content of Mo2C is (Ti, W, Mo)C. It was shown that the black cores would be changed into white cores when the content of Mo exceeded W in TiC–Ni cermets (seen from Fig. 3b and c). The formation of white cores with higher content of Mo can be explained by assuming that Mo2C increases the solubility of TiC and WC in the binder. Namely, at the liquid sintering stage, TiC and WC raw materials were almost completely dissolved in the metal nickel phase when the content of Mo2C reached ~15 wt.%. Then, the Morich (Ti, W, Mo)C solid solution would be formed by co-precipitation during cooling. If the content of Mo2C was equal to WC, Ti, W, and Mo would form a black W-rich (Ti, W, Mo)C solid solution (seen from Fig. 3b). The decrease in grain size with increasing content of Mo is explained by Yan Li et al. [10]. They considered that the solution of TiC grains could be hindered by the formation of a Mo-rich rim, which is less soluble in the binder than pure TiC. As a result, proper content of Mo could effectively refine the ceramic grains. However, when the content of Mo2C exceeds 12 wt.%, the grain sizes are diminished remarkably (seen from Fig. 3c). 59
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Fig. 3. EDX of cores in as-prepared TiC-based with various contents of Mo2C, wt.%: 0 (a), 12 (b), and 15 (c) It can be explained that high content of Mo2C will lead to the solution–recrystalization progress instead of solution–coprecipitation one. It was the severe partition of the Mo2C in the carbide that induced the refined microstructure. The values of grain size (GS), transverse rupture strength (TRS), fracture toughness (KIc), and hardness (HRA) versus the content of Mo2C are shown in Fig. 4. It is found that values of GS and KIc decreased linearly with increasing of Mo2C. It is well known to us all that refining of materials will help to improve the TRS and KIc according to the Hall–Petch formula. But for our experimental results, the highest value of TRS was not from the cermets with the least grain size (see Fig. 4a). Cutard et al. [13, 19] have demonstrated that in the case of Ni-binder cermets, the distribution of Mo both in the ceramic and metal binder is a key to interpret the mechanical behaviors. The reason of relatively lower values of TRS and KIc of as-obtained cermets with the highest content of Mo2C can be ascribed to the difference of Mo 60
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Fig. 4. Grain size and mechanical properties of as-prepared TiC-based cermets with various content of Mo2C distribution in ceramic cores. Furthermore, the high amount of Mo2C will result from the diminishing of the binder mean free path due to the partition of ceramic grains in the binder. As binder mean free path and grain size decrease, the hardness increases, but the fracture toughness decreases. It can be noted that composition of ceramic
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Fig. 5. Fracture micrographs of as-prepared TiC-based cermets with various contents of Mo2C, wt.%: 0 (a), 12 (b), and 15 (c) 61
cores and amount of the binder phase have vital roles in improving mechanical properties except the ultrafine microstructures in TiC–Ni cermets. Figure 5 shows the fractured surfaces of the tested cermets with various content of Mo2C at room temperature. It can be found that the main fractural feature of as-obtained cermets without Mo2C is the cleavages of the (Ti,W)C ceramic grains (see black arrows in Fig. 5a) and the binder phases can also be observed to fail by interfacial decohesion from the (Ti,W)C grains. The intergranular and transgranular fractured modes were observed in cermets with 12 wt.% Mo2C. Furthermore, some microdds and nests were found in the matrix with high content of Mo2C samples (see white arrows in Fig. 5c). It means that the ceramic grains fail dominantly by transgranular fractured modes. The above three microstructural fractured features correspond to the mechanical properties of asprepared (Ti, W)C–Ni cermets with different contents of Mo2C.
CONCLUSIONS (Ti, W)C–Ni cermets with various contents of Mo2C were produced by SPS technique. The microstructure with the highest content of Mo2C exhibited a white core surrounded by a gray rim structure. The white core contained higher Mo and Ti, and W elements which could be regarded as (Ti, W, Mo)C solid solutions. The cermets with some white cores had refined microstructure and higher hardness, but relatively lower strength and toughness at room temperature. The cermets with higher hardness were promising to be used as tool materials which had excellent resistant performance for continuously cutting carbon steel at high cutting speed.
ACKNOWLEDGEMENTS The authors acknowledge with thanks the support of this research by the Natural Science Foundation of Education Department of Anhui Province (No. KJ2012B148), the Natural Science Foundation of Hefei University (No. 13RC09), and the National Natural Science Foundation of China (No. 51102073).
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