Materials Science Forum Vols. 534-536

Abstract: The effect of WC or NbC addition on various properties of Ti(C0.7N0.3)-Ni cermets was investigated. The microstructure of Ti(C0.7N0.3)-xWC-20Ni showed a typical core/rim structure, irrespective of the WC content, whereas the structure of Ti(C0.7N0.3)-xNbC-20Ni was different and was dependent on the NbC content. The hardness (HV) and the fracture toughness (KIC) had a tendency to increase marginally, while the coercive force (HC) and the magnetic saturation (4πσ) decreased gradually with an increase in WC or NbC content in the systems studied. In addition, increasing WC content in Ti(C0.7N0.3)-xWC-20Ni system, decarburization was retarded, while denitrification was accelerated.

Abstract: The present work is a study on the argon gas pressure effects of Sinter/HIP sintering on microstructure and strength of different grades of TiC-NiMo cermets. Titanium carbide in the composition of different grades of TiC-NiMo cermets was ranged from 40 to 80 wt.% and the ratio of nickel to molybdenum in the initial powder composition was 1:1, 2:1 and 4:1 respectively. On the sintered alloys, the main strength characteristic, transverse rupture strength (TRS) and erosion wear resistance were measured. Furthermore, the microstructure parameters of some alloys were measured and the pressure effect on pore elimination was evaluated. All the results were compared with common, vacuum sintered alloys. The TRS values of TiC-NiMo cermets could be considerably improved by using Sinter/HIP technique, for high-carbide fraction alloys and for alloys sintered at elevated temperatures. The results provide new possible application fields for Sinter/HIP-ed TiC cermet materials to areas where, in addition to wear performance, higher strength properties are demanded.

Abstract: The effect of TiC content on oxidation behavior of the sintered WC-TiC-TaC alloys with 2 mass% TaC and different TiC amount of 3-45 mass% was investigated through oxidation tests in air at 973K in order to clarify their oxidation mechanism at high temperature. Based on the results of mass change, SEM observation, elemental map analysis and composition analysis of the samples before and after the oxidation test, it was revealed that with increasing TiC content in the alloys, mass changes from oxidation and thickness of the oxidation scale decreased. Thus, it is considered that the main component of the scales changed from WO3 to TiO2 gradually with increasing TiC content in the alloys, and oxygen diffusing through the oxidation scale to the alloys was inhibited more and more.

Abstract: Nano-sized WC particles in WC/Co composite powders were synthesized by mechanochemical method. The raw powders(WO3, Co3O4, VC, Cr3C2 and graphite) were mixed by planetary milling with ethyl alcohol for 30 hours. The compositions were WC-10 and -20 wt% Co after reduction and carburization. The powders of VC and Cr3C2 were added as the inhibitors to inhibit the formation of nano-sized WC particles. The direct reduction and carburization of the oxides with carbon were carried at 900°C for 1 to 3 hours under flowing Ar gas. The resultant powders were compacted and sintered at 1300~1360°C for 0.5 hour. The degree of the direct reduction and carburization of the mixed powders increased with the reaction time. The mean size of WC particles in WC/Co composite powders was about 16 nm, and the mean size of WC/Co composite powders was about 240 nm. In WC-10 and -20 wt% Co, the sintered compacts shrank by 16% and 24%, respectively. The mean sizes of WC in WC/Co composite powders were about 50 and 30 nm.

Abstract: In the present paper, the formation of high hardness and large strength structure of WCCo composite material containing particles of diamond were investigated and optimal operating parameters were defined. Tungsten carbide - cobalt - diamond composite was produced by the method of high voltage electrical discharge together with applying mechanical pressure to powder compact. It was found that the density and hardness of composite material reach its maximum values at certain magnitudes of applied pressure and high voltage electrical discharge parameters. We found that there is [a maximum level -Comment: Don’t you mean a maximum level] for the discharge voltage and applied pressure beyond which the powder WC-Co-diamond composite material disintegrates like an exploding wire. Near this level, the cobalt particles are in a fused condition and are redistributed in the compact volume due to magnetic pressure of discharge current pulse. [The distribution of magnetic pressure is defined by the distribution of a current density in the powder compact. The magnetic pressure is more homogeneous in powder compact volume when the skin effect is strong.-Comment: Are these two statements of fact or conclusions made based on experimental observations - Answer: The first statement is fact and the second statement is conclusion made based on experimental observations.]

Abstract: Direct reduction and carburization process was thought one of the best methods to make nano-sized WC powder. The oxide powders were mixed with graphite powder by ball milling in the compositions of WC-5,-10wt%Co. The mean size of mixed powders was about 250 nm after milling. The mixture was heated at the temperatures of 600~800°C for 5 hours in Ar. The reaction time of the reduction and carburization was decreased as heating temperatures and cobalt content increased. The mean size of WC/Co composite powders was about 260 nm after the reactions. And the mean size of WC grains in WC/Co composite powder was about 38 nm after the reaction at 800°C for 5 hours.

Abstract: Twinned WC grains are sometimes observed in WC powder and sintered WC-Co alloys. The present investigation has studied the formation of twinned WC grains during carburization of an Eta phase. Eta grains were carburized at 700-1450°C for 1 min to 9 h. Twinned WC grains formed during the carburization. Crystallographic characterization of the formed twins were made using SEM and TEM. The formation of twins was found to be affected by the carbon activity during carburization. The twins formed under high carbon activities while no twins formed under low carbon activities. Two kinds of twins with different orientations were observed. The present experimental observation suggests that the twins formed via 2-dimensional nucleation and layer-bylayer growth on small WC clusters under high supersaturation and high driving force for the growth of WC grains.

Abstract: To investigate sliding friction and wear behaviors of WC–Co/WC–Co pairs containing different WC grain sizes, the ball-on-disc test in air were carried out for WC-Co containing WC grain sizes of 0.5 μm (F.G.) and 1.5 μm (C.G.). The results show that the wear volume of F.G. pin for F.G. pin/C.G. disc is larger than that of F.G. pin for F.G. pin/F.G. disc due to higher friction coefficient, and the surfaces after wear test are richer in oxygen compared to those before test. Furthermore, the wear debris, which is composed of nona-scale grains, after the wear test are remarkably richer in oxygen than to those before test. The diffraction pattern reveals sharp ring

Abstract: To improve the mechanical properties of WC-Co cemented carbides, a dual composite was studied. The compositions of granule and matrix were nano-sized WC-6 wt% Co(granule) and normal sized WC-20 wt% Co(matrix), respectively. The granules were made by spray-drying method and sintered at 1380°C for 10 minutes in vacuum. After sieving, the granules were grouped to 50, 100 and 150 ㎛ and mixed with WC and Co powders as the volume fractions of granule to matrix were 50 to 50, 40 to 60 and 30 to 70. These compacts were sintered at 1380°C for 10 minutes in vacuum. The microstructure, transverse rupture strength and wear resistance were investigated. The relative densities of the dual composites were about 98% after sintering. The mean size of the WC grains in the dual composite was about 300~400 nm. As the volume fraction of the matrix in the dual composite increased, the transverse rupture strength increased and hardness decreased. The wear resistance of the dual composite increased with decreasing matrix volume fraction and increasing granule size.

Abstract: The behavior of hardmetals under cyclic loads is investigated. Unnotched specimens were employed to obtain practical information regarding fatigue in hardmetals. All the tested hardmetals exhibit an increase in the number of cycles until failure with a decrease in the maximum stress, i.e., the hardmetals exhibit a high fatigue sensitivity. The fatigue strength increases with the cobalt content. Although distinct fatigue limits, as observed in metals, cannot be observed, the calculated fatigue limit stress at 107 cycles is found to be approximately 70% of the flexural strength, and the stress value exhibits a linear relationship with the flexural stress.