Nanoscratching deformation and fracture toughness of electroless Ni–P coatings

In the present investigation electroless Ni–P coatings were prepared. Structural characterizations indicated that the as-deposited coating had an amorphous structure with a P content of 23 at.%. The deformation behavior of an electrolessly amorphous Ni–P coating was investigated by using the Vickers indentation and the Tribo-indenter instrumented nano-indentation technique. The hardness of the Ni–P coating is remarkably improved after proper heat-treatment and the hardness is as high as 12.7 GPa for the coating annealed at 400 °C for 1 h. However, the cracks were observed during the indentation of the Ni–P coatings annealed at 400 °C and 500 °C for 1 h. The corresponding fracture toughness was evaluated as 2.58 MPa m0.5 and 1.33 MPa m0.5, respectively. Nanoscratching tests indicated that the wear resistance of the Ni–P coatings was improved significantly with an increasing ratio of hardness (H) to elastic modulus (E). It was observed that the friction coefficient increased from 0.083 ± 0.006 for the Ni–P coating annealed at 300 °C up to 1.337 ± 0.009 for the IF steel substrate, while the H/E simultaneously decreased from 0.084 (10.7/128) to 0.009 (1.85/200). The study revealed that the electrolessly amorphous Ni–P coating had offered better corrosion resistance than the Ni–P coatings after heat-treatment. An annealing temperature of 300 °C is preferentially suggested for the trade-off between the wear resistance property and anti-corrosion property of the Ni–P coating.