Fatigue crack growth in self-standing plasma sprayed tungsten and molybdenum beams with artificially introduced notches subjected to pure bending was studied. Beams width, thickness and length was 4 mm, 3 mm and 32 mm respectively. Fatigue crack length was measured using the differential compliance method and fatigue crack growth rate was established as a function of stress intensity factor. Unusual crack opening under compressive loading part of the cycle was detected. Fractographic analysis revealed the respective crack formation mechanisms. At low crack propagation rates, the fatigue crack growth takes place by intergranular splat fracture and splat decohesion for Mo coating. In W coating, intergranular splat fracture and void interconnection formed the fatigue crack. Frequently, the crack deflected from the notch plane being attracted to stress concentrators formed by porosity. At higher values of the stress intensity factor, the splat intergranular cracking become more common and the crack propagated more perpendicularly to the specimen surface.

Many applications of thermally sprayed coatings call for increased fatigue resistance of coated parts. Despite the intensive research in this area, the influence of coating on fatigue is still not completely understood. In this paper, the spatiotemporal localization of crack initiation and the dynamics of crack propagation are studied. The resonance bending fatigue test is employed to test flat specimens with both sides coated. Hastelloy-X substrates coated with classical TBC YSZ/NiCoCrAlY composites were tested. The strain distribution on the coating surface is evaluated by the digital image correlation method (DIC) through the whole duration of the fatigue test. Localization of crack initiation sites and the mode of crack propagation in the coated specimen are related to the observed resonance frequency. The individual phases of specimen degradation, i.e. the changes of material properties, crack initiation, and crack propagation are identified. The tested coatings strongly influenced the first two phases, the influence on the crack propagation was less significant.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate was studied for Inconel 738LC alloy built by the Dynamic Metal Deposition technique (3DMD, a high-speed Directed Energy Deposition technique). The measured quasi-static properties, i.e. stress-strain and fracture toughness showed only subtle anisotropy, with no more than 10% differences found for different orientations. The fatigue crack growth rate was influenced by the specimen orientation more significantly (30% for fatigue crack growth threshold, up to 90% for Paris exponent and coefficient). This pilot study attributes the anisotropy of fatigue crack growth properties to material texture and the columnar grain geometry resulting from directional solidification. The obtained testing results indicate that 3DMD technology can produce materials with good mechanical and fracture properties even from materials considered as non-weldable such as In 738LC. The study provides a solid experimental base for further investigation of the fatigue crack growth mechanism relation to the material texture in 3DMD In 738LC.

Diamond-reinforced composites prepared by cold spray are emerging materials simultaneously featuring outstanding thermal conductivity and wear resistance. Their mechanical and fatigue properties relevant to perspective engineering applications were investigated using miniature bending specimens. Cold sprayed specimens with two different mass concentrations of diamond 20% and 50% in two metallic matrices (Al – lighter than diamond, Cu – heavier than diamond) were compared with the respective pure metal deposits. These pure metal coatings showed rather limited ductility. The diamond addition slightly improved ductility and fracture toughness of the Cu-based composites, having a small effect also on the fatigue crack growth resistance. In case of the Al composites, the ductility as well as fatigue crack growth resistance and fracture toughness have improved significantly. The static and fatigue failure mechanisms were fractographically analyzed and related to the microstructure of the coatings, observing that particle decohesion is the primary failure mechanism for both static and fatigue fracture.

Fatigue properties of the Al 2 O3 plasma-sprayed SUS316L stainless steel rod specimens coated on different spraying conditions have been studied in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of surgical implant application. Fatigue tests were conducted in push-pull loading at the stress ratio of R = -1, and frequency of 2 Hz. Microstructure related with fatigue damage was examined by SEM and TEM. The fatigue strength of Al 2 O 3 plasma-sprayed metals significantly depended on spraying conditions: the effects of spraying on fatigue strength decreased with increasing the applied stress amplitude. As-blasted specimens were higher in fatigue strength than Al2O3 plasma-sprayed specimens. It was found that the plasma spraying had significant effects on fatigue crack growth behavior in the early stage of crack propagation. Fatigue cracks preferentially originated from dents that had been caused on the substrata metal surface subjected to grit-blasting. These results are discussed with both the compressive residual stresses due to the grit blasting which was carried out prior to plasma spraying and the corrosion-resistance of the alumina deposit.

Thick deposits were produced from pure Al powder of three different sieve sizes using cold spraying at the same process parameters. The in-plane mechanical and fracture properties of the deposits were investigated using bending of small specimens in four specimen orientations. It was shown that increasing the Al particle size by approximately 50% and 100% leads to small, but statistically significant differences of yield strength. Further, the increase in the powder particle size led to higher fracture toughness K IC but lower fatigue crack growth threshold ΔK thr . This can be attributed to two different fracture mechanisms in the cold sprayed deposits. A trans-particular fracture in the near-threshold fatigue regime is controlled by the microstructure and work hardening of the particles. At higher cyclic loads and in quasi-static regime, the particle decohesion and the resulting crack path determine the fracture behavior instead. However, the observed effect of particle size was rather small, much smaller than the effect of spray process parameters observed in the previous research.

Anisotropy of stress-strain behavior, fracture toughness, and fatigue crack growth rate of Ti6Al4V deposited by cold spray using nitrogen was studied. For that, flat deposits were tested with stress acting in the in-plane directions and tubular deposits were tested in the out-of-plane stress directions. In all tests, unified small-size specimens were used. It was shown that for the in-plane stress, the deposits can be considered isotropic, whereas the out-of-plane stress led to significantly lower values of the measured properties. The obtained results were related to fractography and microstructural analysis. While a combination of trans-particle and inter-particle fracture determined the fatigue properties in the near-threshold regime, at higher loads, inter-particle fracture was dominant. It was also shown that the different particle-to-stress orientations influenced the resulting fatigue and static properties.

Mechanical and fatigue properties of cold sprayed (CS) Cu 20 Sn bell metal were tested in order to assess the potential applicability of the technology to repair impact areas of church bells. The CS bell metal was compared to its traditional cast counterparts, a fine-grained Cu 22 Sn bell metal seen in small bells, and a coarse-grained Cu 20 Sn seen in large bells. Similar to other CS metals, it was shown that both the strength as well as the fatigue crack growth rates at low loading are similar to the cast materials. The fracture toughness of the CS material was comparable with the finegrained Cu 22 Sn bell metal, while both were significantly lower than the coarse-grained Cu 20 Sn bell metal. The impact damage rate of the CS material determined by a periodic impact test was significantly higher than the (finegrained) cast material. Both materials showed a stabilized, very slow damage rate after the relatively fast initial crater formation. The results presented in this paper identify CS as a feasible restoration technology for church bells, and the introduced methodology presents a characterization method for quantitative description of bell metal impact damage.

In situ values of Young's modulus and Poisson's ratio for thermal spray coatings are needed to evaluate properties and characteristics of thermal spray coatings such as residual stresses, in-service stresses, bond strength, fracture toughness, and fatigue crack growth rates. It is important to have methods documented in detail so that people can follow the document and use the methods. Such a document requires more pages than are allowed in conference proceeding and journal papers. Thus, Recommended Practices and Standards describing these methods are needed. Currently, there is not a recommended practice or standard for evaluating Young's modulus and Poisson's ratio for thermal spray coatings. The ASM International Thermal Spray Society has recognized this need and formed a committee on Recommended Practices for Thermal Spray Coatings. This paper describes one of the recommended practices being written by the Mechanical Properties Evaluation Subcommittee of the Recommended Practices Committee. The specimen is a coated substrate in the form of a cantilever beam. The method is easy to use and inexpensive. The equipment needed is a vise or clamping fixture, strain gages, a strain indicator, a micrometer, a ruler, a hanger, and a set of weights. The specimen is easy to machine and spray. The loading is easy to apply and remains constant during readings. The method can be used to evaluate Young's modulus and Poisson's ratio in tension or compression. A description of the method, a verification, and a sensitivity analysis was done and published in Reference [1]. Some of the details of implementing the method and the data sheet are presented here.

Tungsten heavy alloy (WHA) of W-Ni composition was deposited from a blend of standard thermal spray powders using a radio frequency inductively coupled plasma torch in a protective atmosphere. The coating contained a fully developed WHA structure, i.e., spherical W particles embedded in a Ni-rich matrix. Bending tensile strength R m , bending yield strength R p,0.2 , and elastic modulus were measured and compared with W-Ni-Co references fabricated by sintered and quenched (SQ) and forged and annealed (FA) powder metallurgy (PM) processes. The fatigue and fracture properties of the plasma spray deposits are comparable with those of the SQ-PM reference material, but inferior to those of the FA-PM reference. The results of various property tests are presented and analyzed in the paper.

The fracture toughness of pure Al, Cu, Ni, and Ti deposited by cold spraying was investigated to gain a better understanding of the damage process and quantify material performance. Rectangular specimens of self-standing deposits with fatigue pre-cracks were tested in three-point bending. KIC values were obtained from J-R curves and stress-strain curves were plotted. The cold-sprayed deposits exhibited significantly lower fracture toughness than the same wrought materials, and fractographic analysis revealed either ductile or cleavage intergranular fracture as the major failure mode.

Rotating bending fatigue tests have been conducted at room temperature in laboratory air using specimens of medium carbon steel (S45C), low alloy steel (SCM435) and titanium alloy (Ti-6AI-4V) with HVOF sprayed coating of a cermet (WC-12%Co) and S45C with WFS sprayed coating of a 13Cr steel (SUS420J2). Plane bending fatigue tests were also conducted at stress ratios, R, of -1, -0.5 and 0 for S45C with WC-12%Co coating. The fatigue strength and fracture mechanisms were studied. The fatigue strength evaluated by nominal stress was strongly influenced by substrate materials, R and the thickness of sprayed coatings. Detailed observation of crack initiation on the coating surface and fracture surface revealed that a crack was initiated in the coating and then cracks were initiated in the substrate due to the stress concentration of the crack in the coating. The fatigue strength of the sprayed materials was dominated by that of the sprayed coating. Therefore, the fatigue strength could be evaluated uniquely in terms of the true stress on the coating surface. The influence of compressive residual stress of the sprayed coatings on fatigue strength was discussed based on the fatigue mechanisms at different stress ratios.

The SUS316L stainless steel rod specimen coated with plasma-sprayed Al 2 O 3 deposits has been fatigued in a physiological saline solution (0.9 % NaCl solution) to evaluate the potential of its application to prosthetic implant materials. Push-pull loading fatigue tests were conducted at the stress ratio of R = -1, and at the frequency of 2 Hz. Pure titanium powder was selected for undercoat. Fatigue damage was examined on longitudinal section of the specimen and fracture surface by optical and electron microscopy from the microstructural viewpoints. The plasma spraying of Al 2 O 3 powder has significantly improved fatigue properties of the substrate metal in the longer range of fatigue lives, compared with the results of the non-coated steel specimen. It was found from electrochemical experiments that titanium for undercoat metal has acted as sacrificial anode to protect the substrate metal from corrosive attack and under lower stress amplitudes the plasma sprayed Al 2 O 3 coating kept the solution out at an early stage of fatigue lives. Fatigue cracks preferentially originated from flaws, which had been caused on the substrate metal surface through grit blasting, and extended into the bulk of substrate metal. Fatigue cracks appear not to develop into plasma-sprayed deposits while the deposits could accommodate themselves to the crack opening displacement at the surface of substrate metal. It was understood that the plasma sprayed coating has enhanced fatigue properties in the solution both by keeping the solution out during the early stage of fatigue lives and by electrochemical effects of the undercoat metal when the topcoat was cracked in macroscopic scale.

Thermal barrier coatings (TBC) in general fail by delamination of the ceramic partially stabilised Zirconia (PSZ) top coat (TC) from the underlying metallic bond coat (BC). The process is initiated by crack initiation and growth either in the TC or in the thermally grown oxide (TGO) that will form at the interface between top and bond coat. The aim of the present paper is to describe the degradation due to crack growth in such a way that data can be used for FEM modelling work. Flat rectangular test coupons have been subjected to thermal cyclic fatigue (TCF) in air with a temperature range from 100°C to 1100°C. Identical samples were removed from the TCF furnace at different times of thermal cycling in order to achieve material with different degree of damage. After mounting, cutting and sectioning the specimen were investigated by light optical microscopy (LOM) and scanning electron microscopy (SEM) together with an energy dispersive spectrometer (EDS). Image analysis of LOM micrographs was used for measurement of crack distribution and degree of TC damage. A method for crack growth measurement based on the degree of TC / TGO damage has been developed. Furthermore, a measure of TBC damage as a function of elapsed fatigue cycles was introduced. The TBC material shows a mixed black and white fracture surface after TCF cycling. Delamination crack growth data are presented. Delaminated TC/BC interface surface as a function of fatigue cycles follows an S-curve behaviour.

Methodology to evaluate the adhesive strength between the MCrAlY alloy coating film and the Ni-base superalloy substrate was studied and proposed. By employing the double cantilever beam specimens which were taken from the CoNiCrAlY alloy coated Ni-base superalloy, the fatigue crack propagation tests along the interface were carried out. Through the work particular attention was given to the threshold level to the fatigue crack propagation along the interface as a measure to represent the adhesive strength, based on fracture mechanics approach. The effects of temperature, the surface finishing of the substrate and the long term thermal aging on the adhesive strength were also investigated.

The effect of high frequency pulse detonation (HFPD) and HVOF thermally sprayed WC-Co coatings on the high cycle fatigue (HCF) behavior of 2024-T4 aluminum was investigated. The fatigue life distributions of specimens in the polished and coated conditions are presented as a function of the probability of failure. The monotonic and cyclic deformation behaviors of the as-received and as-coated specimens were investigated. The conclusions show that, (i) the HFPD sprayed specimens exhibited slightly higher fatigue lives compared to the uncoated specimens, (ii) the HVOF sprayed specimens exhibited significantly higher fatigue lives compared to the uncoated specimens, and (iii) the as-coated specimen was cyclically stable.

To determine the effect of bond coat oxidation on the coating life, thermal shock testing were performed, using three different thermal cycles. The failure mode and crack paths were investigated in scanning electron microscope. A finite element model was developed to simulate the thermal shock tests. First, transient temperature fields during the thermal cycling were calculated. Second, stresses and strains evolving in the coatings due to thermal expansion mismatches and temperature gradients during the cycling were computed. The stress concentration at the interface due to the roughness of the bond coat was accounted for by using an ideal sinusoidal interface in the model. By adding an oxide layer with and without residual stresses to the model, the influence of the bond coat oxidation was determined. Both the experimental and numerical results revealed that the TBC failed by crack initiating in the ceramic top coat very close to the grown oxide layer at the interface followed by coating fatigue failure. Numerical simulation indicated that bond coat oxidation led to stress concentration at the peak of the asperity of the interface proceeding crack growth. It also showed that bond coat inelasticity and ceramic creep might further enhance the crack growth. There was little effect on coating behavior due to the residual stresses in the oxide layer.

This paper investigates the effect of HVOF thermally sprayed WC-Co coatings on the cyclic deformation of the aluminum alloy 2024-T4 and the steel SAE 12L14. Strain gages are glued on the specimen surface to measure the strain response after certain intervals of fatigue cycles. The strain is also measured during the fatigue test. A calibrated beam is used to apply the bending moment and the stress calculated using the flexural formula. The stress-strain curves for uncycled and cycled specimens are determined. The dynamic strains for the coated and uncoated specimens are also monitored during the fatigue test. Paper includes a German-language abstract.

Environmental problems of hard chrome plating are raising its cost and shrinking availability. HVOF and Detonation spray technologies for application of tungsten carbide and chrome carbide based coatings have proved to be cleaner and more effective than chrome plating. In this paper, the results of fatigue tests for WC-17Co coating deposited onto AISI 4340 steel by HVOF are compared to those for hard chrome plating. The fatigue life distributions as a function of the probability of failure for the coated AISI 4340 steel specimens showed that the HVOF coated specimens exhibited extraordinarily higher fatigue lives compared to the uncoated specimens whereas the hard-chrome-plated imparted fatigue strength degradation to the AISI 4340 steel.

Nanostructured and conventional titania (TiO 2 ) powders were thermally sprayed using APS and HVOF processes. The fatigue and mechanical properties of the coatings investigated. Coatings were characterized using SEM to investigate the microstructural features and Vickers indentation to determine the hardness. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess crack nucleation and to study the mechanisms of deformations. The fatigue strength of coatings deposited onto low-carbon steel (AISI 1018) showed that the nanostructured titania coated specimens exhibited significantly higher fatigue strength compared to the conventional titania