The present investigation has been carried out in order to propose a new model for the determination of the surface hardness of thin films, employing data easily obtained from standard ...micro-indentation tests. The model is based on the consideration that the substrate has a predominant behavior at the highest indentation loads, whereas at low indentation loads the predominant behavior is that of the film. Such a description is achieved by combining a series and parallel additive laws associated to each behavior, respectively. This approach has been applied for the computation of the hardness of a number of films, including Al2O3, Cr, diamond like carbon, Ti, TiC, TiCN, TiN, TiNx and ZrNx, deposited onto different substrates. It is shown that the results obtained are in good agreement with the predictions provided by other models reported in the literature.
The present study has been conducted in order to formulate a rational description of the flow stress of structural steels, when these are deformed in tension under hot-working conditions. The ...simplified formulation that has been developed only involves the analysis of the positive work-hardening region of the stress–strain curve and is based on the concepts behind the Mechanical Threshold Stress Model. Accordingly, it is assumed that the flow stress of the material arises from three different contributions: athermal hardening and solid solution, which do not evolve in the course of plastic deformation and work-hardening, the only evolutive component of the flow stress. The evolution of work-hardening is introduced in the formulation by means of the phenomenological work-hardening law earlier proposed by Sah and co-workers, expressed in differential form. The temperature and strain rate dependence of the contributions of solid solution hardening to the flow stress, as well as that of the saturation stress, within the work-hardening law, are described by means of the kinetic model earlier advanced by Kocks. The formulations derived are able to reproduce satisfactorily both the flow stress and work-hardening rate of the material employing a relatively small number of material parameters and therefore, these could be a valuable tool for the analysis of hot-working processes.
Rotating bending fatigue experiments, both in air and in a 3% NaCl solution, have been carried out in order to study the fatigue behavior of a 7075-T6 aluminum alloy coated with a WC–10Co–4Cr cermet ...by high velocity oxygen fuel (HVOF) thermal spray, without any grit blasting prior to the coating deposition. The results indicate that the presence of the coating gives rise to a significant increase in the fatigue strength of the substrate and therefore, that from the fatigue behavior point of view this coating could be a feasible replacement of electrolytic hard chromium plating in aircraft applications. Such an increase in fatigue and corrosion-fatigue strength is believed to be associated with the intrinsic microstructural characteristics, corrosion resistance and mechanical properties of the coating, its compressive residual stresses and the possible compressive residual stress field induced in the substrate during coating deposition. The fractographic analysis of the specimens indicates that the final fatigue fracture could be due either to the joint action of a large number of cracks, which propagate from the substrate–coating interface, or to a single dominant fatigue crack, depending on the maximum alternating stress applied to the coated system. It is shown that the lack of grit blasting prior to HVOF deposition does not seem to compromise the functionality of the coated system, while it avoids the introduction of further sharp stress concentrators for the nucleation of fatigue cracks.
The present investigation has been conducted in order to evaluate the change in fatigue properties of a 316L stainless steel substrate when this material is coated with a hydrogenated, amorphous ...carbon (a-C:H) solid lubricant of approximately 2
μm in thickness, which has been developed in recent years and is known commercially as Dymon-iC™, a film deposited by means of physical vapor deposition (PVD) closed field unbalanced magnetron sputter ion platting (CFUBMSIP). The fatigue behavior of the uncoated and coated alloy has been evaluated under rotating-bending conditions at maximum alternating stresses in the range of 430–520
MPa, depending if testing was conducted in air or in a 3
wt.% NaCl solution. The results indicate that the presence of the coating provides a significant increase in fatigue properties, particularly under corrosive conditions, an outcome that is believed to be closely associated with the amorphous structure of the film, besides its elevated mechanical strength, a possible existence of a compressive residual stress state and good adhesion to the substrate. The fractographic analysis conducted both on the fracture surfaces of the tested specimens and along sections normal such surfaces clearly shows that fatigue cracks are nucleated on the surface of the coating and that propagate into the substrate once have crossed over the entire coating thickness.
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.
The experimental measurement of residual stresses originating within thick coatings deposited by thermal spray processes onto solid substrates plays a fundamental role in the preliminary stages of ...coating design and process parameters optimization. The main objective of the present investigation was to determine the residual stresses by means of the incremental hole drilling method in order to perform the measurement of the stress field through the thickness of two different HVOF Nickel-based coatings. The holes through the coatings were carried out by means of a high velocity drilling machine (Restan). A finite element calculation procedure was used to identify the calibration coefficients necessary to evaluate the stress field. The Integral method was used for the analysis of non-uniform through-thickness stresses. The results for both coatings indicate that the nature of the residual stresses is tensile and their values are between 150–300 MPa.
In this work, the total and plastic hardness of a variety of materials were determined from the work-of-indentation. We show that by taking into account the influence of the tip defect, both the ...total and plastic hardness have the same values and that the hardness-load dependence is considerably reduced, thus explaining that part of the indentation size effect can be related to an incorrect estimation of the tip defect. To consider the influence of the indenter tip defect, we propose a calculation of the impression volume involving the length of the truncated indenter tip. For the residual impression volume, we have demonstrated that it can be simply calculated from the total volume using a proportionality constant equal to the residual to maximum depth ratio. Moreover, the hardness derived from work-of-indentation is found to be equal to the Meyer hardness. Finally, the ratios of Meyer hardness to the reduced modulus and elastic work to total work of indentation are found to be very close to the proportionality factor (π tanα) of Stilwell and Tabor.
•The hardness of different materials was calculated from the work-of-indentation.•The indenter tip defect length is introduced into the hardness computation.•No indentation size effect is observed when the tip defect is taken into account.•Hardness numbers were compared to that calculated from the work-of-indentation.•Hardness linked to the work of indentation agrees with Meyer and Vickers hardness.
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.
▶ 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
R
a 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.
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 10
5 to 10
6. 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.
Mechanical properties of an Al91-Mn6-Nd3 nanostructured alloy CHICOT, D; VODA, M; DECOOPMAN, X ...
Materials science & engineering. A, Structural materials : properties, microstructure and processing,
08/2011, Letnik:
528, Številka:
22-23
Journal Article
Recenzirano
A methodology for obtaining a nanocrystalline Al91-Mn6-Nd3 alloy and characterizing its mechanical properties is presented. The fabrication procedure, based on the melt spinning method, allowed the ...preparation of thin ribbons of a arrow right 430 mu m in thickness. The hardness and elastic modulus have been determined by classical and instrumented indentation tests considering the influence of the ribbon thickness on the indentation measurements. The Martens hardness has been calculated by means of instrumented indentation tests, for which the fixing support of the sample has no influence on the indentation measurement. The results have been subsequently compared with those obtained from classical Vickers hardness measurements, for which some composite hardness models have been applied in order to separate the contribution of the fixing support to the hardness measurement. It has been concluded that both methodologies lead to similar values for the mechanical properties of the alloy, i.e. a arrow right 43.4GPa for the macrohardness and a arrow right 43.8MPam1/2 for the hardness length-scale factor of the material. The elastic modulus of the material, resulting from the analysis of the unloading indentation curves, has been found to be a arrow right 4131GPa.
