Abstract
In this study binary and ternary boride-carbide 3000°C-super-Kelvin fused composites are tungsten-cemented fused boride(s)-carbide(s) with solidus temperatures being higher than 3000°C. Conventional cermets, sintered WC, TiC and TaC cemented mainly with Co or Ni, were patented by Schröter in 1923. Although these composites possess high hardness of carbides and high toughness of Co and Ni, they have a serious defect of low relative density and, consequently, have no predicted superior mechanical and other related properties due to (liquid phase) sintering. In order to improve high melting point of composites, this study uses tungsten as a binder to cement transition metal carbide(s) and boride(s) and adopts melting process to provide 100% relative density. The effect of presence of merely carbide(s) and presence of both carbide(s) and boride(s) on mechanical properties and microstructure is thus investigated. Choice of TiB2 and ZrB2 is due to their relative inexpensive cost and high melting points, while for carbides are TiC, ZrC, NbC, TaC, WC, HfC and VC by considering their high hardness and high melting points. By means of various combinations as well as compositions of carbide(s) and boride(s), this study investigates hardness of composites at both ambient and elevated temperatures up to 1100oC, and toughness, wear, corrosion, and machinability of composites at ambient temperature. Super Kelvin fused composites in this study are typically a solidified structure of dendrite-interdendrite type. At ambient temperature these composites possess hardness of 1200 HV to 2300 HV and KIC toughness ranging from 6 MPa m1/2 to 13 MPa m1/2. In one carbide-one boride (TiB2 or ZrB2) composites, the hardness of the composites of ZrC or HfC has significantly lower value than that of the other carbides, while their toughness obviously has higher value than that of the other carbides. In ZrB2 composites of various carbides, the hardness of the composites with ZrB2 is higher than that of the background composites without ZrB2. In the pin-on-belt abrasive wear test with 6 kg loading on the counterpart Al2O3 belt, the wear resistance reaches 95.6 m/mm3, as compared with the value of a commercial hardmetal WC-Co of 120 m/mm3. Another conclusion of this test is that the wear resistance is closely related to the hardness of the composites as well as to their toughness. The hardness of the composites at 1100oC still has value ranging from 800 HV to 1300 HV. These composites have the following two merits over commercial WC-Co; i.e., (1) their lowering temperature coefficient in hardness is lower than that of commercial WC-Co; (2) their elevated temperature hardness is obviously higher than that of commercial WC-Co. With high hardness at elevated temperature, their performance in heavy duty turning machining is thus extraordinarily better than that of commercial WC-Co. Their anticorrosion behavior in 3.5 wt% NaCl artificial sea water linear potentiodynamic polarization test shows a better result than that of commercial WC-Co. In conclusion, these high temperature fused cermets are suitable for high-temperature environments that require both high hardness and high toughness, such as super high speed machining tools and hard facing applications in repairing rolling rolls.