| Description |
Polycrystalline cubic boron nitride (PCBN) is one of the few materials suitable for friction stir welding (FSW) of hardened steels, which demands tool materials to possess high temperature strength, toughness, abrasion resistance, thermal and chemical stabilities. In FSW process at temperatures higher than 980°C PCBN consisting of AlB2, as one of the major reaction products, underwent a reverse peritectic reaction: AlB2^ A l ('i)+AlBj2 and released liquid Al, which was believed to cause inter-cBNgranular cracking in the PCBN tool during FSW of hardened steel. In the present research, PCBN starting with decreased Al additive and addition of fine cBN powder, was HP/HT-sintered at higher B:Al ratio and increased available BN surface area for fast in situ reaction with liquid Al in favor of forming AlB12 instead of AlB2. Titanium powder in HP/HT-sintering of PCBN under the same pressure and temperature conditions resulted in cBN, TiN, and TiB2 reaction products but could not achieve the densification comparable to the Al additives. However, using Ti-coated cBN particles with addition of the fine cBN powder led to PCBN with highly packed reaction products of cBN, TiN, and TiB2 and strong mechanical properties. Applying electron backscattered diffraction (EBSD) method, the PCBN with 10vol%Al additive was detected to consist of 0.5vol% and finely dispersed AlB2 in addition to AlB12 as the major borides in addition to cBN and AlN. Such PCBN was shown to possess high thermostability measured at 1000°C and measureable increases in flexural strength at temperatures up to 1100°C, while room temperature fracture toughness value was retained up to 1100°C as well. Combine x-ray diffraction (XRD) and EBSD results revealed the increase in high-temperature strength was resultant of high residual compressive-stress on cBN grains applied by thermal expansion of the AlNAlB12 grainboundary phase. The PCBN sintered from cBN with 10vo%Al powder, possessing improved thermostability and mechanical strength at temperatures up to 1100°C benefiting from significantly reduced AlB2 and increased cBN vol%, is suitable for high temperature applications. The PCBN sintered using the Ti-coated cBN particles showed significant decrease in strength and toughness measured in 900°C to 1100°C range, due to oxidation reactions of TiN and TiB2 at high temperatures. |
