Fatigue behavior of a SAE 1045 steel coated with Colmonoy 88 alloy deposited by HVOF thermal spray

An investigation has been conducted in order to study the fatigue behavior of a SAE 1045 steel substrate coated with a Ni-base alloy known commercially as Colmonoy 88, deposited by HVOF spray technique. Fatigue tests were conducted under axial conditions (R = 0.1), employing samples of the substrate material in the as-polished condition, after grit blasting with alumina particles and after grit blasting and coating with a deposit of about 250 μm thick. The fatigue tests were conducted at maximum stresses in the range of 380-533 MPa, depending on the condition of the material. A detailed fractographic analysis of some selected samples tested at different stresses was carried out, aimed mainly at determining the crack nucleation and propagation sequence. The results indicate that the deposition of such a coating leads to a fatigue strength debit of the substrate in the range of 10-20% and a similar debit in fatigue limit of ∼ 11-13%. It has been found that grit blasting is the process responsible for the fatigue strength debit observed in the coated samples. Fatigue cracks have been observed to initiate at the substrate-coating interface and at the free surface of the coating, mainly close to alumina particles embedded on the substrate and sharp notches produced during the process. The fractographic analysis of the fracture surface of the coated specimens points out the characteristic heterogeneous nature of the coating, particularly regarding some of its mechanical properties, such as fracture toughness.     

Effect of spray distance on the corrosion-fatigue behavior of a medium-carbon steel coated with a Colmonoy 88 alloy deposited by HVOF thermal spray

The present work has been conducted in order to determine the influence of the spray distance, on the corrosion-fatigue behavior of a SAE 1045 steel substrate coated with a Ni base coating deposited by high velocity oxygen fuel (HVOF) thermal spray. The spray distances employed in the present investigation were of 380, 425 and 470 mm. The mechanical properties of the coated systems were evaluated by means of tensile and corrosion-fatigue tests conducted with cylindrical samples. Corrosion-fatigue tests were carried out under rotating bending conditions (R = − 1), at a frequency of 50 Hz and maximum alternating stresses in the range of 250-420 MPa, employing a 3 wt.% NaCl solution. The results indicate that varying the spray distance in the range of 380-470 mm has apparently no significant influence on the corrosion-fatigue behavior of the coated systems. However, the presence of the Ni base coating gives rise to a significant increase in the corrosion-fatigue life of the coated substrate, in comparison with the uncoated steel. Such an increase varies between ∼ 90 and 440% in the interval of maximum alternating stresses investigated in the present work. Also, the coated systems exhibited a better corrosion-fatigue performance in comparison with the same steel substrate coated with an electrolytic hard chromium (EHC) deposit.     

Effect of spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by HVOF thermal spraying

The present work has been conducted in order to determine systematically the influence of the spraying distance on the microstructure and mechanical properties of a Colmonoy 88 alloy deposited by means of HVOF thermal spray onto a SAE 1045 steel substrate. The spray distance varied between 380-470 mm and the evaluation of the deposits characteristics and properties was carried out both on their surface and on cross section. Both hardness and elastic modulus of the coatings were determined according to the model of Oliver and Pharr. The yield strength of the coatings was also estimated following the methodology developed by Zeng and Chiu for the analysis of the loading and unloading curves obtained from nanoindentation experiments, as well as from classical static spherical indentation tests. The microstructural analysis indicated a significant increase in the unmelted particles volume fraction and the development of interlamellar microcracks as the spraying distance increases, leading to a decrease in the elastic modulus of the coatings. Both hardness and elastic modulus showed an anisotropic behavior and were found to be higher on the cross section of the coating than on the deposition plane. A satisfactory comparison between the predicted and experimental values of the coatings yield strength was observed for all the conditions investigated.     

Fretting wear of HVOF Ni-Cr based alloy deposited on SAE 1045 steel

The present study has been carried out in order to investigate the fretting wear performance of a Ni-Cr based alloy, containing B and C, deposited onto a SAE 1045 steel substrate by HVOF thermal spray. Tests were conducted on both the uncoated and coated substrate, under unlubricated dry conditions, at different applied normal loads, cycles and amplitudes in order to analyze the influence of these parameters on the wear behavior. It has been determined that the coated substrate exhibits a very good tribological performance in comparison to the uncoated substrate. The decrease of the wear volume of the coated substrate is of ~ 95-97% of that determined for the uncoated substrate. The increase in the fretting wear resistance provided by the coating has been attributed to the presence of a large amount of dispersed Ni and Cr carbide and/or borides in the Ni-Co matrix. Examination of the fretted surfaces by SEM indicates that the uncoated substrate undergoes an abrasive wear mechanism. On the contrary, it has been observed that the wear mechanism of the coating-substrate system depends on the magnitude of the applied load. At loads of 30 N, a polishing wear mechanism has been determined, whereas between 40 and 50 N, the initiation and propagation of micro-cracks takes place through the coating. The numerical integration of the wear scar depth profile curves employed in the present work allows an estimation of the wear volume which is in good agreement with that determined experimentally by means of 3D profilometry. It has been determined that, at a constant wear amplitude the wear volume increases with the applied normal load and that at under constant load conditions, the wear volume decreases as the wear test amplitude also decreases, becoming insignificant when it is less than ~ 100 µm.     

Fatigue and corrosion fatigue behavior of an AA6063-T6 aluminum alloy coated with a WC-10Co-4Cr alloy deposited by HVOF thermal spraying

The present investigation has been conducted in order to study the fatigue and corrosion fatigue behavior of an AA6063-T6 aluminum alloy substrate coated with a WC-10Co-4Cr deposited by HVOF thermal spraying. It has been determined that the deposition of such a coating on the aluminum substrate gives rise to significant gains in fatigue life in comparison with the uncoated substrate, when testing is carried out both in air and in a 3 wt.% NaCl solution. It has been shown that during testing in air, the fatigue gain ranges between ~ 540 and 4300%, depending on the maximum alternating stress applied to the material. Larger fatigue gains are associated with low alternating stresses. Also, when fatigue testing is conducted in the NaCl solution, the gain in fatigue resistance varies between ~ 620 and 1460%. Fatigue cracks have been observed to initiate at the coating surface and then grow towards the substrate after propagating through the entire coating thickness. Crack growth along the coating has been observed to occur mainly along the regions formed by the agglomeration of W and W-Co-Cr-rich particles, flanking the tougher Co-Cr-rich areas. Although in the present work residual stresses were not measured, it is believed that the gain in fatigue life of the coating-substrate system is due to the presence of compressive residual stresses within the coating which hinder fatigue crack propagation. The deposition of the coating does not give rise to significant changes in the static mechanical properties and hardness of the aluminum alloy substrate. It has been observed that the WC-10Co-4Cr coating displays a significant indentation size effect and has a mean hardness of ~ 9.4 GPa.     

Fatigue behavior of a 4140 steel coated with a NiMoAl deposit applied by HVOF thermal spray

The fatigue behavior of a quenched and tempered AISI 4140 steel has been investigated in three different conditions: as-polished, as-grit blasted with Al₂O₃ particles and as-coated, after grit blasting, with a deposit of Ni–Al–Mo alloy (Metco 447) of approximately 300 μm in thickness, applied by HVOF thermal spraying. It has been determined that after grit blasting with particles of 20 mesh (83 μm) at a pressure of 345 kPa, a significant decrease in the fatigue properties of the material takes place. It has also been observed that such particles, are retained at the substrate surface during blasting and become stress concentrators that enhance the nucleation of fatigue cracks. The latter give rise to a decrease in the fatigue strength of the blasted material. Further coating of the grit blasted specimens with a deposit of Metco 447 of approximately 300 μm thick, applied by HVOF thermal spraying, leads to a further reduction in the fatigue strength of the material. Under these conditions, the fatigue cracks are also nucleated at the alumina particles retained after blasting. It is believed that such a further decrease is mainly associated with two different causes. Firstly, the extensive fracture and delamination of the coating from the substrate which has been observed from the microscopic analysis. Secondly, the possible existence of tensile residual stresses in the substrate, in the vicinity of the substrate–deposit interface, which would assist in the propagation of the fatigue cracks nucleated at the alumina particles. The fatigue properties of the steel substrate in the three different conditions investigated, has been described in terms of the simple parametric relationship earlier proposed by Basquin.     

Microstructural and mechanical characterization of Ni-base thermal spray coatings deposited by HVOF

The present work has been conducted in order to determine the microstructural features, hardness and elastic modulus of two different Ni-base coatings deposited by means of HVOF thermal spray, onto a SAE 1045 plain carbon steel substrate. The morphology and chemical composition of the phases that are present in the coatings were characterized by means of SEM, EDS and XRD techniques. Image analysis was used for the evaluation of the coatings porosity. Both conventional and instrumented indentation tests were also carried out on the surface and cross section of the coatings, in order to evaluate the effect of coating microstructure on hardness and elastic modulus. Conventional indentation tests were conducted using a Knoop indenter and a maximum load of 9.8 N. Instrumented indentation tests, in which the indenter depth and applied load were recorded continuously, were carried out employing a Vickers indenter and maximum loads of 0.49, 0.98, 1.96, 4.9 and 9.8 N. Instrumented nanoindentation tests (in a continuous stiffness measurement mode) were also conducted employing a Berkovich indenter with a maximum load of 9.8 N. The elastic modulus was computed by means of the Oliver and Pharr method and compared with the values determined by means of the method earlier advanced by Marshall et al. The results obtained indicate that the elastic modulus values determined on the cross section of the coatings are higher than those obtained on the surface, clearly indicating the anisotropy of the structure. Also, the values found employing a Berkovich indenter are very similar to those derived by means of the Vickers indenter. In addition, the these values are in agreement with those determined by taking into consideration the elastic recovery of the short Knoop diagonal after removal of the load.     

Evaluation of WC–17Co and WC–10Co–4Cr thermal spray coatings by HVOF on the fatigue and corrosion strength of AISI 4340 steel

It is known that chromium electroplating is related to the reduction in the fatigue strength of base metal. However, chromium results in protection against wear and corrosion combined with chemical resistance and good lubricity. Environmental requirements are an important point to be considered in the search for possible alternatives to hard chrome plating. Aircraft landing gear manufactures are considering WC thermal spray coating applied by the high-velocity oxygen-fuel (HVOF) process an alternative candidate, which shows performance at least comparable to results, obtained for hard chrome plating. The aim of this study is to compare the influence of WC–17Co and WC–10Co–4Cr coatings applied by HVOF process and hard chromium electroplating on the fatigue strength of AISI 4340 steel, with and without shot peening. S–N curves were obtained in axial fatigue test for base material, chromium plated and tungsten carbide coated specimens. Tungsten carbide thermal spray coating results in higher fatigue strength when compared to hard chromium electroplated. Shot peening prior to thermal spraying showed to be an excellent alternative to increase fatigue strength of AISI 4340 steel. Experimental data showed higher axial fatigue and corrosion resistance in salt fog exposure for samples WC–10Co–4Cr HVOF coated when compared with WC–17Co. Fracture surface analysis by scanning electron microscopy (SEM) indicated the existence of a uniform coverage of nearly all substrates.     

Fatigue response of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B with varied surface quality and thermal exposure history

Fatigue specimens with four types of designed surface (EDM plane-sited, EDM notched, shot peened, electropolished) were assessed under three exposure conditions (no exposure, block exposure, individual exposure-oxidation at 700 °C for 10000 h) to quantify the effects of surface roughness, stress concentration, oxidation and inner microstructural embrittlement on fatigue behaviour of a grain refined TiAl alloy Ti-44Al-5Nb-1W-1B. With the yield strength of 568 MPa, fatigue is found to occur under a loading condition of σ_max<σ_0.1. Local plastic deformation is difficult to occur. The alloy becomes sensitive to surface damages but not to V-notch because the small surface area sampling the highest stress significantly reduces the EDM impact. Electropolishing rather than shot peening is found to be more effective in improving fatigue strength for the high strength alloy. When subjected to block exposure, both annealing effect (beneficial) and microstructural embrittlement (detrimental) occurred on all the surfaces, and the latter was dominant in governing fatigue behaviour except for EDM surfaces. After individual exposure-oxidation, fatigue performance deteriorated significantly for the shot peened and moderately for the electropolished but not for EDM surfaces. The mechanism for specific fatigue behaviour is discussed individually based upon whether or not the beneficial effects outweigh the detrimental effects.