Surface Modification Of WC-Ni Hard Alloy Coatings Irradiated By High-intensity Pulsed Ion Beam Of Carbon And Protons

The experimental investigation of WC-Ni hard alloy coatings irradiated by high-intensity pulsed ion beam (HIPIB) of carbon and protons are performed in TEMP-6 and TEMP-4M apparatus, respectively. The hard alloy coatings are prepared on the surface of 17-4PH stainless steel substrate by high velocity oxygen fuel (HVOF) spraying. In order to illustrate the modification mechanism of nickel cemented tungsten carbide coatings irradiated by HIPIB, the surface morphology, surface roughness, phase composition and microhardness of WC-Ni hard alloy coatings before and after HIPIB irradiation were studied.The WC-10Ni and WC-12Ni hard alloy coatings were treated by HIPIB with ion current density of 200 A/cm2 at a pulse duration of 70 ns up to 10 shots in TEMP-6 apparatus. With increasing shot number, the surface remelting and selective ablation of nickel binder phase resulted in the smoothing and densification in micro scale on the irradiated surfaces though a hilly topography formed with some craters and micro cracks leading to a higher surface roughness, where both of these hard alloy coatings’surface roughness Ra significantly increased from 0.637 μm and 0.019 μm for original sample to 2.443 μm and 0.150 μm at 200 A/cm2 with 10 shots. Meanwhile the thickness of surface remelting layer on the irradiated coatings was enhanced to 3-4μm. It is also found that the phase transformation from hexagonal WC to cubic β-WC1-x underwent in the irradiated surface layer, and the amount of WC phase transformation became more and more with increasing shot number. With increasing shot number, the surface microhardness of WC-lONi and WC-12Ni hard alloy coatings presented different hardening tendency:"increase-decrease" and "increase-unchanged-increase", and the surface microhardness of WC-10Ni samples increased from 10.4 GPa for original sample to 15.4 GPa with 5 shots, while that of WC-12M samples increased from 10.5 GPa for original sample to 13.3 GPa with 10 shots. The HIPIB irradiation caused a significant long-range hardening behavior beyond the range of ion beams, where the hardened depth of WC-10Ni and WC-12Ni were up to 140 μm and 90 μm with 10 shots respectively.In addition, the WC-12Ni hard alloy coatings were irradiated by HIPIB at current density of 200 A/cm2 and shot number of 1～10 with a pulse width of 100 ns in TEMP-4M apparatus. Multiple irradiation by HIPIB resulted in the smoothing and densification in micro scale and the formation of some craters and micro cracks on the irradiated surfaces, thus leading to the surface roughness Ra showing an "increase-decrease" tendency, i.e., Ra increased from 0.021 um for original sample to 0.164 μm with 1 shot remarkably, and then decreased to 0.118 μm with 5 shots and 0.111 μm with 10 shots gradually. The phase transformation from hexagonal WC to cubic β-WC1-x in the irradiated surface layer were also observed, which is similar to be irradiated by HIPIB on TEMP-6 apparatus. The thermal-dynamical effect of HBPIB irradiation significantly increased the surface microhardness from 10.5 GPa for original sample to 14.1 GPa with 10 shots by producing the surface remelting top layer on hard alloy coating, and simultaneously caused a phenomenon of multi-maximum of microhardness in the underlayer beneath the heat affected zone.