On Shock Processing And Strengthening By Irradiation Using High-intensity Pulsed Ion Beam

High-intensity pulsed ion beam (HIPIB) irradiation into electron beam physical vapor deposited (EB-PVD) thermal barrier coating (TBC) and WC-Co hard alloy is carried out in TEMP-6 type HIPIB apparatus, respectively. Using the shock processing and strengthening by HIPIB irradiation, a new surface sealing technology is developed for EB-PVD TBC, and WC-Co hard alloy is strengthened with forming a wear-resistant surface layer. The modification mechanisms in oxidation resistance of EB-PVD TBC and wear resistance of WC-Co hard alloy are also illustrated in terms of changes in composition, structure and properties of the surface layer, respectively.For the HIPIB irradiated EB-PVD ZrO₂-7wt.%Y₂O₃ TBC at the ion current densities of 100-300 A/cm² with the shot number of 1-10, no phase transformation is detected and the TBC surface layer is still composed of tetragonal ZrO₂ phase. Under 1-shot irradiation, selective ablation of micro-protrusions and grain crevices on TBC surface occurred due to three-dimensional heat flow from incoming ion beam and the adjacent zone. At 100 A/cm², the remelting of irradiated TBC surface remains in local regions, so a disconnected bridging layer is produced on the gaps between columnar grains. With increasing the ion current density to 200 A/cm², a continuous bridging layer of about 1μm in thickness is introduced by filling of remelted ZrO₂ into the gaps under the shock precessing effect of HIPIB irradiation. While the micro-protrusions and grain crevices can be more intensely heated and then the preferential melting is expected as compared to the flat region at 300 A/cm², so severe ablation on the irradiated surface causes deterioration of sealing of the crevices. Under multi-shot conditions, the remelting and ablation expand to a larger local region including more micro-protrusions and grain crevices. The continuous filling of remelted ZrO₂ into the crevices forms a nearly 3-μm-thick bridging layer under the shock processing of HIPIB irradiation, meanwhile, microcracks are induced at the bridging region due to thermal stress concentration on TBC surface.Cyclic oxidation tests at 1050℃demonstrate that oxidation resistance of the EB-PVD TBC further increases under HIPIB sealing. The weight gain of the sealed TBC at 200 A/cm² with 1 shot is decreased by about 60 % from 0.8 mg/cm² of the as-deposited one. With presence of the thin bridging layer as effective diffusion barrier, the thickness of thermally grown oxide (TGO) formed in the irradiated TBC at 200 A/cm² with 1 shot is only about 1μm, which is about 35% of that for the as-deposited TBC. Formation of the thinnest TGO layer in the sealed TBC at 200 A/cm² with 1 shot among the TBC is in good agreement with the lowest weight gain. Severe localized oxidation where the oxides extended into the bond coat, as occurred in the as-deposited TBC, hardly took place at the bond coat/top coat interface in the sealed TBC at 200 A/cm² with 1 shot after thermal cycling. And the peak content of oxygen in the TGO layer decreased to about 30 wt.%, compared with that of about 40 wt.% for the as-deposited one. The thin bridging layer provided atmosphere shielding for the TBC along with matching strain tolerance of the columnar top coat during thermal cycling. While the several-μm-thick bridging layer formed under multi-shot condition is not compliant with the strain tolerance of the top coat. So the improving effect of bridging layer is limited since surface cracking during thermal cycling.WC-10Co hard alloy was modified by HIPIB with an ion current density of 300 A/cm² and shot number up to 10. Co binder is preferentially ablated due to lower melting/boiling points compared with those of WC, and Co₃W₉C₄ carbide precipitates in the surface layer resulting from the high heating and cooling rate during HIPIB irradiation. As increasing the shot number to 10, the selective remelting and ablation of Co binder expands to a large scale, and many protrusions of about ten urn in size emerge on the irradiated surface with forming a net structure. The irradiated hard alloy showed a roughening surface with increasing the shot number due to the emergence of the protrusions. The surface roughness R_a significantly increased to 1.72μm as compared with that of the original one, being 0.29μm. HIPIB irradiation induced hardening both in near-surface layer and at the depth of about 100μm away from the hard alloy surface. The surface microhardness is about 20.2 GPa of the irradiated hard alloy with 10 shots which increased about 15 % relative to that of the original one, and the hardening layer extended down to about 120μm away from the irradiated surface.Significant improvement in wear resistance is achieved for HIPIB-irradiated WC-10Co hard alloy using a block-on-cylinder type wear test machine at dry condition without lubricant under a normal load of 400 N and a sliding speed of 0.42 m/s. The specific wear rate of the irradiated hard alloy with 10 shots is about 1.92×10^-7 mm³/N·m during 30-min sliding, as decreased nearly 65% compared with that of the original one. In general, wear of WC-Co hard alloy is believed to result from adhesion, ploughing and deformation of the Co binder of surface layer. Wear proceeds by removal of the Co binder between the WC grains by a combination of plastic deformation and microabrasion. When sufficient binder was removed, the WC grains and the WC grain clusters were detached from the hard alloy surface. HIPIB irradiation induces segregation of dispersive Co₃W₉C₄ carbide in the surface layer resulting in hardening of the Co binder. Deformation of the hardening Co binder was restrained due to improved pressing resistance against mating materials, so the removal of Co binder decreases during sliding. Meanwhile, the surface layer gets fine crystal grain with dispersive and homogeneous distribution of WC grains and Co binder from rapid melting and resolidification during HIPIB irradiation. Minimizing the mean free path in Co binder by decreasing the WC grain size leads to an increase in resistance to plastic deformation in the binder. HIPIB localizes high energy deposition to the near surface, including shock waves that propagate deeply into the hard alloy. The shock waves produced defects that significantly increase hardness over depth up to 100μm. The hardening of the surface layer improves wear resistance of HIPIB-irradiated hayrd alloy.