Surface Modification Of WC-Ni Cemented Carbide Irradiated By High-intensity Pulsed Charged Particle Beams

Replacement of Co by Ni is found to improve the corrosion reistance, heat stability and anti-radiation of WC based cemented carbide, but its mechanical properties and wear resistance are lower than that of WC-Co system cemented carbide. In order to improve the wear resistance and lifetime of the WC-Ni system cemented carbide and its components under the extreme condition, the experimental investigation of the WC-13Ni cemented carbide irradiated by high-intensity pulsed charged particle beams are performed in TEMP-6type high-intensity pulsed ion beam (HIPIB) apparatus and compact high-intensity pulsed electron beam (HIPEB) apparatus, respectively. The modification mechanism of nickel cemented tungsten carbide irradiated by high-intensity pulsed charged particle beams are illustrated in terms of the change on surface morphology, phase structure, surface roughness, surface microhardness, and tribological performance of modified surface, respectively.The WC-13Ni cemented carbides were irradiated by HIPIB at energy densities of1-6J/cm2and shot number of1～10with a pulse width of70ns. The surface remelting and selective ablation of nickel binder phase resulted in the formation of a "hill-valley" surface morphology with smoothed and densified in microscale on the irradiated surfaces. The surface roughness Ra increased with increasing energy density and shot number, and Ra for irradiated sample at6J/cm with10shots significantly increased from0.13μm for original sample to1.62μm. It is found that the phase transformation from hexagonal a-WC to cubic β-WC1-x underwent in the irradiated surface layer, and the amount of β-WC1-x phase increaed with increasing energy density and shot number. Surface hardening in different degrees is observed on the HIPIB irradiated samples compared with the original samples. Surface microhardness of the irradiated samples at a fixed shot number presented a similar hardening tendency with increasing the energy densities, i.e."increase-decrease-increase", and surface microhardness of irradiated sample at6J/cm2with10shots significantly increased from11.84GPa for original sample to16.60GPa. The HIPIB irradiation led to the "long range hardening" on the surface layer of WC-13Ni samples, and the depth of hardened layer increased with increasing energy density and shot number, where the depth of hardened layer reached to160μm for the irradiated sample at6J/cm2with10shots. The block-on-ring wear test demonstrated that the friction coefficient and specific wear rate of HIPIB irradiated samples obviously decreased. The HIPIB irradiated samples at6J/cm2with10shots exhibited the superior tribological performance, the friction coefficient decreased from0.80for the original sample to0.48. Correspondingly, the specific wear rate decreased from1.2×10-6mm3/Nm to3.6×10-7mm3/Nm, which is approximately reduced by70%compared with that of the original sample.The WC-13Ni cemented carbides were irradiated by HIPEB at energy densities of3～34J/cm2and shot number of1-10with a pulse width of180ns. The surface remelting, ablation and ejection, as followed by fast re-solidification resulted in a typical ablated-ejected surface topography with some craters and blow holes formed on the irradiated surfaces. Similar to the HIPIB irradiation, the higher energy density and multiple shots,Ra increased more remarkably, and Ra significantly increased from0.21μm for original sample to1.26μm for irradiated sample at34J/cm2with10shots. It is found that the phase transformation from hexagonal a-WC to cubic β-WC1-x and hexagonal a-W2C underwent in the irradiated surface layer, and the amount of β-WC1-x, phase increased with increasing energy density and shot number. Moreover, the graphite C phase can also be observed on the surface of irradiated sample at the higher energy density of34J/cm2. Surface hardening and softening in different degrees was observed on the HIPEB irradiated samples compared with the original samples. The surface microhardness of irradiated samples reach to a maximal value of13.70GPa at3J/cm2with1shot, and reach to a minimal value of10.49GPa at34J/cm2with5shots. The "long range hardening" phenomenon is observed on all the HIPEB irradiated WC-13Ni samples with surface hardening and softening, and the depth of the hardened layer increased with increasing energy density and shot number, where the depth of hardened layer reached to380μm for the irradiated sample at34J/cm2with10shots. The block-on-ring wear test demonstrated that the friction coefficient and specific wear rate of HIPEB irradiated samples obviously decreased. The HIPEB irradiated samples at34J/cm2with10shots exhibited the superior tribological performance, the friction coefficient decreased from0.80for the original sample to0.54. Correspondingly, the specific wear rate decreased from1.2X10-6mm3/Nm to3.8×10-7mm3/Nm, which is approximately reduced by68%compared with that of the original sample.The maximum energy of HIPIB irradiation deposited in the several hundreds nm range of the surface layer of WC-13Ni samples, and led to more stronger surface melting and ablation to form a "hill-valley" surface morphology. However, the maximum energy of HIPEB irradiation deposited in the subsurface layer of WC-13Ni samples, about several μm, and led to the subsurface layer melting and melt eruption to form the craters. High-intensity pulsed charged particle beams irradiation with rapid heating and cooling led to carbon loss of WC in short pulse and rapid quenching, and contributed to the formation of metastable β-WC1-x phase. Moreover, HIPEB irradiation have longer pulse action time and larger energy deposited range than HIPIB irradiation led to slower heating and cooling rate and longer high-temperature reaction, and contributed to much more carbon loss of WC and decomposition of some β-WC1-x phases to form metastable a-W2C phase and graphite C phase. Surface hardening of the irradiated samples is resulted from the decreasing of the nickel binder phase content and densification in the surface layer, and the formation of a-W2C phase on the surface layer of HIPEB irradiated samples also increased hardening effect. The formation of metastable β-WC1-x phase only reduced the surface hardening degree of HIPIB irradiated samples, but the reason of surface softening of HIPIB irradiated samples is the formation of β-WC1-x phase and some defects, such as cracks and blow holes and so on. The reason of the "long range hardening" on the surface layer of the irradiated samples is the formation of crystal defects, such as dislocation, vacancy and so on, which is resulted from the impact action of the stress wave by high intensity pulsed charged particle beams irradiation. Hardening effect depends on the width and intensity of stress wave. HIPEB irradiation have deeper hardening layer than HIPIB irradiation because it have longer width of stress wave, but an more significant hardening effect is achieved for HIPIB irradiated samples at lower energy density. The wear mechanism of original WC-Ni cemented carbide during the wear process is abrasive wear with the main characteristics of plastic deformation of Ni binder phase and fall-out of WC grains, and wear resistance depends on the hardness of cemented carbide. The irradiated samples by both high-intensity pulsed charged particle beams have similar wear mechanism, and the improved wear resistance of irradiated samples is not only related to surface hardness, but also associated with the features of modified structure. Irradiation led to surface healing of original defects of micro-pores between WC grains and binder phase by surface remelting. At the initial stage of sliding wear process, the melted layer was gradually worn out and some regions smeared flack-shape debris, and the main mechanisms of the irradiated samples are micro-abrasion and adhesive wear. In the steady-state stage of sliding wear, the wear mechanism of the irradiated samples is similar to that of original sample, but the micro-abrasion of Ni binder phase and subsequent pull-out of WC grains can be effectively restricted in the irradiated samples during the sliding wear process, as result of bonding force between WC grains and Ni binder phase, and strengthening of binder phase itself by the hardened effect in the long range from shock waves of beams irradiation.