Hertzian Indentation Behavior of Electroless Ni-P-Ti Composite Coatings
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DUCTION
ELECTROLESS Ni-P coatings have been utilized in many industries such as electronic, automotive, and oil and gas.[1–4] Ni-P coatings have been employed in the electronic industry due to their magnetic properties[1] as well as employed in the automotive and oil and gas industries due to their high hardness as well as outstanding corrosion resistance.[2,3] Although used across these industries, Ni-P coatings exhibit low toughness and therefore are constrained to applications which do not require high dent and crack resistance. To improve the properties of Ni-P coatings to make them more applicable in the current and new industries, Ni-P composite coatings have been increasingly developed in recent years. These composite coatings can enhance the hardness, wear resistance, or corrosion resistance compared to Ni-P coatings. For example, the addition of SiO2 has been reported to improve the hardness and corrosion resistance,[5] the addition of W has a lower friction coefficient and higher wear resistance,[6,7] and better cavitation erosion resistance was found with the addition of SiC.[8] Although these additions show improvements, little attention has been given to enhancing the toughness of the Ni-P coatings. One way to
ZHI LI and ZOHEIR FARHAT are with the Department of Mechanical Engineering, Dalhousie University, Halifax, NS B3J 2X4, Canada. Contact email: [email protected] Manuscript submitted December 16, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
toughen the Ni-P coatings is to incorporate superelastic NiTi particles into the Ni-P matrix.[9] Superelastic NiTi alloy has high toughness, which can withstand a large amount of deformation without permanent damage due to the martensitic transformation.[10] Nevertheless, the high price of superelastic NiTi powder limits the applications of Ni-P-NiTi coatings. Therefore, to reduce the cost, in this study, Ni-P-NiTi composite coatings were prepared by electroless co-depositing Ni-P and Ti particles and annealing the composite coatings. In addition, after annealing, an improved adhesion between Ti particles and Ni matrix is expected. To determine the toughness of a coating, a tensile test conducted on a standalone coating is a very attractive method. Zhou et al. performed single edge notch tensile test on standalone Ni-P coatings to measure the fracture toughness.[11] They found that the average fracture toughness of the as-deposited Ni-P coatings is 7.1 MPa m1/2, which is close to the result (7.4 MPa m1/2) obtained by other researchers using Vickers indentation method.[12] Other mechanical properties, such as Young’s modulus and tensile strength can also be determined through tensile testing a standalone coating with no effect from the substrate. However, tensile tests performed on the Ni-P or Ni-P composite standalone coatings have rarely been reported due to the challenges of preparing standalone coatings as well as the difficulties of performing tensile tests on brittle coatings. Besides tensile testing, an indentation test is another simple but effective method t
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