On the Mechanical Properties and Failure of Low-Pressure Cold-Sprayed Tungsten Carbide-Nickel Metal Matrix Composite Coatings

• Author / Creator

  Munday, Guriqbal S

• Tungsten carbide-Nickel (WC-Ni) metal matrix coatings (MMC) were fabricated using a low-pressure cold spray unit. The coatings were designed according to 50, 71, and 92 weight percentages of WC in the feedstock powder. These coatings were subsequently prepared for uni-axial quasi-static tensile testing to evaluate their mechanical properties, namely the tensile strength, the Young’s modulus, and the toughness. Further, the evolving strains during tensile testing were computed using the Digital Image Correlation (DIC) technique. The coatings were characterized for their microstructure, namely the total particle-matrix interfacial area, the mean free path between the particles, the average particle size, and the porosity of the coating by image analysis on the captured scanning electron microscope (SEM) micrographs. The results indicate that with an increase in the content of WC in the coatings, there was an increase in tensile strength, strain to failure, Young’s modulus, and the toughness of the coatings. This increase was attributed to the refined microstructural features that occurred with the decrease in the porosity of the coating that was caused by the greater consolidation of the Ni matrix at higher WC content in the metal matrix. Additionally, this improvement was linked to the increase in the particle-matrix interfacial area caused by the increase in carbide content, a decrease in the average WC particle size in the coating, and a reduction in the mean free path between the WC particles. Using this quantification of key microstructural features and mechanical properties, a combined microstructural feature’s effect on the tensile strength based on the individual functional relationships between the microstructure and mechanical properties was also explored. This revealed that there exist distinct tensile strength regimes based on the content of WC wt.% in the coatings. The coatings with content of WC beyond 30 wt.% reported a significant increase in their tensile strength. This was reiterated to the refined microstructure at higher content of WC in the coating and their ability to absorb higher mechanical energy to failure. Next, damage in the coatings was quantified by calculating the Poisson’s ratio and it was used to explain the tensile failure of the coatings. The coatings with WC content less than 15 wt.% were accompanied with higher damage than the coatings with WC content beyond 30 wt.%. Finally, a preliminary exercise is presented to model the tensile stress-strain behaviour of the tested coatings with the application of the cohesive zone between the metal matrix and the reinforcing phase. The modelling results indicated that the failure strength of the cohesive zone was approximately 215 MPa for 15 wt.% WC in the coating. The model also suggests that the tensile strength of the coating increases with an increase in particle-matrix interface strength. The mechanical properties and the microstructural data collected in this thesis is imperative to validate mechanism-based models to predict the stress-strain response and the performance of cold sprayed MMCs. Overall, besides quantifying the mechanical properties of the low-pressure cold-sprayed WC-Ni MMC coatings, this thesis study emphasizes the importance of tailoring the microstructure of cold spray MMC coatings to develop the next generation of coatings with improved mechanical properties and performance.

• Subjects / Keywords

  • Failure
  • Coatings
  • Mechanical Properties
  • Metal Matrix Composites

• Graduation date

  Fall 2019

• Type of Item

  Thesis

• Degree

  Master of Science

• DOI

  https://doi.org/10.7939/r3-e307-a265

• License

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