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The impact damage evolution of thermally sprayed WC-Ni coatings was studied under 1-8000 impacts at a fixed load of 10 kN order. With increasing impact number, the accumulative damages of coatings evolve by in-sequence occurrence of WC spallation, coating cracking, and mixed coating/interface cracking accompanied with crack length and width growing. To comprehensively evaluate the coated component performance by observed accumulative damages as the failure criteria, a critical impact number Nc with critical load Lc is proposed for the onsets of different cracking features. The impact resistance is quantified, by Nc(Lcc) for onset of the coating cracking, and Nc(Lcm) for the mixed coating/interface cracking. A subclassification is identified as Nc(Lcm1) and Nc(Lcm2), respectively, for sequential occurrence of the isolated interface cracking and the propagated interface cracking of the penetrated from the coating surface. As the coating thickness is increased from 50 to 200 μm, the impact resistance under a load of 11.1 kN is correspondingly improved with Nc(Lcc) prolonged from 3000 to 4500, and Nc(Lcm1) from 4500 to 5000. In particular, Nc(Lcm2) is 6000 for the 50 μm coating, but no Nc(Lcm2) failure for the 200 μm coating up to 8000 impacts. The impact stress field analysis reveals reduced amplitudes of cyclic surface tensile stress and interface shear stress for the thicker coating, together with the interface shear stress shifting from a tensile-compressive alternating mode to a monotonous tensile one. The impact resistance of coated components under repetitive impacts is interpreted on thermodynamics where the thicker coatings mitigate impact mechanical energy dissipation of plastic strain energy in both coating and steel substrate, and accommodate larger reversible elastic strain energy during the periodic loading-unloading of impact cycles. [ABSTRACT FROM AUTHOR]

The impact resistance of coating/substrate systems is still lacking comprehensive understanding, especially for the thermal spray of wear-resistant coatings for coated components in potential applications with heavy-load impact service condition. The impact responses of a thermal spray WC-Ni coating/steel substrate were studied systematically by varying the coating thicknesses from 50 to 300 μm under an impact load of 1–10 kN. Besides conventionally employed crater depth/volume for impact evaluation, a critical impact load is identified to quantify the impact resistance with cracking. Considering the observed cracking modes, two critical loads of Lcc and Lcm have been defined corresponding to the cohesive failure of cracking in coatings and the mixed cohesive–adhesive failure including interface cracking, respectively. Increasing the coating thickness up to 200 μm raised the critical load Lcm from 7.4 to 11.1 kN, followed by a drop to 7.4 kN of Lcc at 300 μm. To reveal the influence of coating thickness, a major surface integrity parameter is proposed as the coating deformation in proportion to the total value of coating/substrate system in correlation to the impact resistance. The dependence of impact resistance on coating thickness is interpreted through the surface integrity changes by the thermodynamic mechanism of effective energy input and dissipation into the coating/substrate system from the external impact load. [ABSTRACT FROM AUTHOR]

Oxidation or decomposition of thermally sprayed coatings during deposition may noticably impair the coating features and characteristics, i.e. the surface integrity of coated components, including coating porosity, phase, microhardness, elastic modulus and toughness, etc., having a profound influence on the final performance of components. Further degradations along with enhanced oxidation of the coatings were observed for an off-angle spraying scheme in industrial practice, where inconsistent results were previously reported for spray angle-dependent degradation of various thermal spray coatings, with the mechanism not being fully understood with the less controlled surface integrity. An improved off-angle HVOF thermal spray has been developed for cemented carbide coatings, using pretreated WC-Ni feedstock powders covered with a nanoscale capsulizing nickel layer. A comprehensive study on the formation of multiple surface integrity parameters and the correlative interactions between them is thus facilitated with controllable surface integrity generation using the capsulized powders. Decarburization is mainly attributed to the exposure of pristine WC grains causing enhanced porosity in inter-splats regions, and then microhardness depends mainly on porosity and partly on phase composition, while the elastic modulus depends on intra-splats cohesion and the indentation fracture toughness on inter-splats cohesion, respectively. Abrasive wear from mild to severe regime transition could be interpreted by the inter-splats and intra-splats cohesion correlation. It has been demonstrated by the improved off-angle thermal spray that a manufacturing process could be designed and optimized on the identified unique correlations between the processes, surface integrity and the final performance. [ABSTRACT FROM AUTHOR]