Paulo José Modenesi

Bio: Paulo José Modenesi is an academic researcher from Universidade Federal de Minas Gerais. The author has contributed to research in topics: Welding & Gas tungsten arc welding. The author has an hindex of 15, co-authored 64 publications receiving 909 citations.

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

Abstract: 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.

Toughness evaluation of HVOF WC–Co coatings using non-linear regression analysis

Abstract: The models for predicting indentation toughness of materials are usually functions of both indenter and induced crack (i.e. Palmqvist or Half - Penny ) geometries. Palmqvist cracks have already been identified in sintered WC–Co cermets. In the present work, the indentation toughness of high velocity oxy-fuel thermally sprayed WC–12%Co coatings has been evaluated using a standard metallographic procedure. Two crack regimes were found to occur in such coatings, depending on the indentation load: at low indentation loads, Palmqvist cracks appeared whilst Half - Penny cracks were detected at high indentation loads. A statistical methodology was developed in order to determine the indentation toughness of WC–12%Co coatings, which were heat-treated at 1173 and 1463 K, from induced crack sizes measured at different loads. The non-linear regression analysis statistically confirmed the existence of two regimes for crack propagation and two distinct toughness values were recorded for the same material under different load conditions. The present results indicate that adequate post-heat treatments can lead to an increase in coating toughness.

American Society for Testing and Materials

Abstract: The American Society for Testing and Materials (ASTM) is an independent organization devoted to the development of standards.

Using finite element modeling to examine the flow processes in quasi-constrained high-pressure torsion

Abstract: Finite element modeling was used to examine the flow processes in high-pressure torsion (HPT) when using quasi-constrained conditions where disks are contained within depressions on the inner surfaces of the upper and lower anvils. Separate simulations were performed using applied pressures from 0.5 to 2.0 GPa, rotations up to 1.5 turns and friction coefficients from 0 to 1.0 outside of the depressions. The simulations demonstrate the distribution of effective strain within the depressions is comparable to the prediction by ideal torsion, and the applied pressure and the friction coefficient outside the depressions play only a minor role in the distribution of effective strain. The mean stresses during processing vary linearly with the distance from the center of the disk such that there are higher compressive stresses in the disk centers and lower stresses at the edges. The torque required for rotation of the anvil is strongly dependent upon the friction coefficient between the sample and the anvil outside the depressions.

Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC–Co coatings

Abstract: Twelve commercially available WC–Co powders with different average WC grain sizes (0.2, 2, and 6 μm) and cobalt contents (8, 12, 17 and 25 wt.%) were sprayed on carbon steel substrates using High Velocity Oxy-Fuel (HVOF) spraying process. Hardness, Young's modulus, and fracture toughness of the coatings were measured. While the hardness and Young's modulus decreased with increasing cobalt content from 1600 to 1100 Hv and from 400 to 300 GPa respectively, the fracture toughness remained in the range from 4 to 6 MPam 1/2 . The coatings with 2 μm carbide showed lower hardness than those deposited from 0.2 and 6 μm carbide. These measured mechanical properties were discussed with the help of microstructures of the coatings investigated by scanning electron microscopy, X-ray diffraction and chemical analysis. Finally, the hardness of the binder phase in these coatings was estimated to range from 1000 to 1300 Hv by applying the mixture rule for composites to the experimental data, demonstrating that such hardening of the binder phase is a key factor affecting the mechanical properties of the coatings.