No abstract

The structure and properties of tungsten carbide hardmetal with 8 wt.% Co produced by cold and hot isostatic pressing with the same starting powder mixture and conventional initial pressing of billets in metal dies were studied. The first batch of the samples was prepared by vacuum sintering of the pressed billets. The second batch was prepared by vacuum sintering followed by hot isostatic pressing of the billets at 5 MPa. The third batch was prepared by hot isostatic pressing at 200 MPa followed by vacuum sintering. The sintered samples had a density of 14.57 g/cm 3 in batch 1, 14.60 g/cm 3 in batch 2, and 14.63 g/cm 3 in batch 3. Microstructural analysis showed that cold isostatic pressing promoted more homogeneous and finer structure. According to the coercive force determination and structural analysis performed with scanning electron microscopy and X-ray diffraction, the average size of carbide grains was 1.315 m for the samples in batch 1, 1.396 μm in batch 2, and 1.062 m in batch 3. Determination of residual stresses indicated that they were compressive in both phases (WC and Co) of the batch 3 samples and tensile for the batch 1 and 2 samples. The average values of measured Rockwell hardness were 88, 87, and 90 HRA for the samples in batches 1, 2, and 3. Mechanical tests of the samples indicated that the bending strength and fracture toughness were 1820

The paper studies the kinetics of the diffusion redistribution of phases in the MoSi 2 -W system when tungsten samples with molybdenum silicide coatings are heated in air at 1500-1800°C. It is established that the (Mo x ,W y ) 5 Si 3 phase, which represents a molecular solid solution of lower molybdenum and tungsten silicides, forms in an exchange reaction between molybdenum and tungsten at the MoSi 2 -W 5 Si 3 interface. The MoSi 2 -W system is much more stable than the WSi 2 -W and MoSi 2 -Mo systems.It is of current importance to develop structural materials for components and mechanisms designed to operate in oxidizing and reducing media above 1500°C used in, for example, aerospace and electrothermal engineering. Refractory metals and alloys with protective silicide coatings are used as such materials. Protective coatings based on boron silicides have proved to be especially effective. The effectiveness of coatings depends on how stable the metal-coating system is. At high temperatures, the coating material actively interacts with the base metal to form intermediate compounds. In particular, the formation of lower silicide phases leads to substantial changes in the specific volume of interacting materials. The resulting stresses and worse oxidation resistance of lower silicides deteriorate the performance of the coatings. Therefore, to increase the stability of the system, the rate at which the coating interacts with the base needs to be decreased.The diffusion of silicon in silicides of refractory metals and the stability of silicide coatings at high temperatures was examined in [1][2][3][4][5][6]. It was established there that the diffusion mobility of silicon decreases in the following sequence: Mo 5 Si 3 , W 5 Si 3 , Nb 5 Si 3 , Ta 5 Si 3 . It was also pointed out that the diffusion mobility of silicon was especially low in mixed lower phases because of the exchange diffusion of metals when metal Me 1 was coated with a silicide of another metal Me 2 .It was established [2,3] in examining the WSi 2 -W system that excess silicon that was mainly concentrated in the near-surface layer of WSi 2 coating on tungsten greatly influenced the kinetics of phase redistribution in the system, thus increasing the stability and oxidation resistance of the coating. To increase the concentration of silicon in the near-surface layer, a complex coating was examined, which was produced by siliconizing a molybdenum layer preliminary gas-sprayed onto tungsten [3]. As compared with the WSi 2 -W system, the rate at which the higher