Effect of substrate roughness on the fatigue behavior of a SAE 1045 steel coated with a WC–10Co–4Cr cermet, deposited by HVOF thermal spray

The present investigation has been carried out in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a WC–10Co–4Cr cermet, of approximately 200 μm thick, deposited by HVOF thermal spraying. Particular emphasis has been paid to the influence of the substrate roughness prior to HVOF deposition, as a mean of improving the mechanical bonding of the coating, on the fatigue life of the coated system. Fatigue tests were conducted under rotating bending conditions (R = −1), employing samples with different surface roughness, including as-polished, fine-grinding with abrasive paper grit 400 and coated and grit blasted with alumina particles of two different sizes (<1 and <3 mm, respectively) and coated. For comparative purposes, tests were also carried out on as-grit blasted specimens employing alumina particles of 3 mm in size. Fatigue tests were carried out at maximum alternating stresses in the range of 358–588 MPa, depending on the condition of the material, in order that the number of cycles to fracture varied in the range of 105 to 106. Selected samples tested at different applied stresses were analyzed after fracture by SEM techniques, which allowed the determination of the crack nucleation and propagation sequence. The results indicate that the presence of the cermet coating gives rise to a delay in the initiation of fatigue cracks at deep notches and alumina particles embedded at the steel substrate after grit blasting, leading to an insignificant fatigue strength debit of the coated specimens, in comparison with the as-polished ones. Fine-grinding, on the other hand, impairs the mechanical bonding of the coating, giving rise to its delamination from the substrate at elevated maximum alternating stresses.

Research highlights▶ Surface roughness of the steel substrate is of utmost importance prior to HVOF deposition. ▶ Although a fine-ground surface does not introduce deep notches and particles, which are stress concentrators for the nucleation of fatigue cracks, it impairs the mechanical bonding of the coating to the substrate. ▶ Early delamination of the coating from the base steel is expected to occur at relatively high maximum alternating stresses. ▶ Grit blasting with alumina particles improves mechanical bonding, but introduces flaws that enhance the initiation of fatigue cracks. ▶ Grit blasting with alumina particles improves mechanical bonding, but introduces flaws that enhance the initiation of fatigue cracks. ▶ HVOF deposition of a WC–10Co–4Cr cermet onto the grit blasted steel substrate, delays significantly fatigue crack initiation at such flaws, allowing a similar behavior of the coated substrate in comparison with the as-polished substrate. ▶ The beneficial effect of the WC–10Co–4Cr cermet coating is due to its compressive residual stress state. ▶ Grit blasting with alumina particles of sizes <1 or <3 mm does not bring about significant changes in the substrate roughness, as determined by the Ra parameter. ▶ The materials with the above surface conditions exhibit a similar fatigue behavior after coating. ▶ The WC–10Co–4Cr cermet constitutes a suitable coating material for parts and components that operate under severe tribological and corrosion conditions. ▶ The high hardness, intrinsic compressive residual stresses and corrosion resistance of the WC–10Co–4Cr cermet enable its performance in harsh environments without compromising the fatigue strength of substrate.