Coating fracture toughness determined by Vickers indentation: an important parameter in cavitation erosion resistance of WC–Co thermally sprayed coatings

The deficient performance of thermally sprayed coatings in cavitation erosion tests is often attributed to the nature of their lamellar microstructure. Poor coating/substrate adhesion, low toughness and tensile residual stresses, which are introduced during the deposition process, can also adversely affect their cavitation resistance. In order to improve the cavitation performance of such coatings, it is also important to control some of the coating properties such as elastic modulus (E) and hardness (H). By reducing E and increasing H (i.e. to ensure a higher H/E ratio), a better tribological performance is usually achieved. The erosive environment also plays a decisive role in determining the final chemical composition of the coating. Thermally sprayed WC–Co coatings are well known because of their high hardness and, in such cemented carbides, the corrosion resistance is frequently affected by the susceptibility of the cobalt binder to chemical attack. In this work, an attempt to improve the cavitation resistance of WC–Co coatings was made by either modifying coating composition (and therefore modifying some coating properties such as hardness, elastic modulus and toughness) or by carrying out a ‘melt’ post-deposition treatment in order to disrupt the intrinsic lamellar microstructure of the coating. Four different coatings were deposited onto an AISI 1020 steel substrate: (i) WC–12%Co; (ii) as-sprayed (AS) 50%(WC–12%Co)+50%(NiCr); (iii) post-melted (PM) 50%(WC–12%Co)+50%(NiCr) and (iv) a duplex system comprising a WC–12%Co top layer and a NiCrAl interlayer. The ‘PM’ coating produced from the pre-alloyed powder 50%(WC–12%Co)+50%(NiCr) displayed a higher elastic modulus (measured by Knoop indentation) and a lower hardness (and thus a lower H/E ratio) than the WC–12%Co. Also, the fracture toughness of the latter (measured by Vickers indentation tests) was increased from 1.6±0.9 to 32±12 MPa m1/2. The worst performance in cavitation erosion tests was achieved by the WC–12%Co coating, which showed the highest mass loss throughout the test. Conversely, the ‘PM’ 50%(WC–12%Co)+50%(NiCr) coating exhibited the best cavitation resistance and a correlation between coating toughness and cavitation resistance could be established.