Tungsten carbide-cobalt coatings are extensively used to protect surfaces from wear in many types of applications, such as compressor piston rods, pump plungers, shaft sleeves on centrifugal pumps and fans, and midspans of compressor blades in gas turbines. The wear behavior in any application is strongly influenced by the basic physical and mechanical properties of such coatings. Fracture toughness as a mechanical property indicates the resistance to fracture in the presence of a sharp crack, and thus provides a measure of the intrinsic strength of the cemented carbides coatings. In this study, Vickers indentation tests have been used to quantify the in-plane fracture behavior of various WC-based coatings deposited by the High Velocity Oxy-Fuel (HVOF) spray process. The indentation cracks are analyzed in terms of standardized relations that utilize radial-median crack geometries. It is shown that the fracture properties of HVOF WC-Co coatings are anisotropic, and depend strongly on the microstructure and composition of the coatings. The crack propagation is determined by the porosity, binder mean free path, and the shape, size, and distribution of the reinforcing carbide particles. The erosion resistances of the coatings have also been discussed as a function of the fracture properties and mechanisms. It is shown, in this study, that the Vickers indentation method is a useful and convenient technique for determining the in-plane fracture toughness of HVOF sprayed WC-based coatings.

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.

Adhesive strength of the plasma-sprayed thermal barrier coating (TBC) is one of the most important parameters which influence the reliability during service. In the past, numerous test methods were reported to measure the coating adhesion. However, most of them require careful and time consuming preparation. Consequently, limited information could be obtained to establish the relationship between the processing conditions and the adhesive property. To produce more measurements using a simpler procedure, the interfacial indentation test and the modified tensile adhesive test are examined. In this paper, the interfacial fracture toughness of the plasma-sprayed ZrO 2 coatings, deposited on Al substrates, were evaluated by these two tests. In order to study the effects of the powder injection, samples were sprayed with various carrier gas flow rates. The test results show a certain correlation between the melting index and the interfacial fracture toughness. In addition, variations between the results obtained from the two different methods are discussed.

In this study, WC-12Co powders with multimodal-sized WC particles were prepared by ball milling and deposited onto stainless steel substrates by cold spraying. Microhardness and fracture toughness were measured on cross-sections by the indentation approach. Coatings produced from powders with small carbide particles, resulting from long milling times, were found to exhibit high microhardness and high fracture toughness. Deposits sprayed with high porosity powders produced by cold compaction had low microhardness, but high fracture toughness.

In this study, Vickers indentation testing is used to determine the fracture toughness of hard flame spray coatings produced from cored-wire feedstocks, including WC-10Co4Cr, 75CrCo-25NiCr, and WC-12Co. Average measured values are compared with fracture toughness values calculated from seven different equations found in the literature in order to validate the experimental results and to better understand the relationship between fracture toughness and coating hardness for each of the tested materials.

In this work, one new technique named as supersonic suspension plasma spraying (SSPS) is applied to deposit quasi-columnar scandia-yttria co-doped zirconia (ScYSZ) and yttria-stabilized zirconia (YSZ) thermal barrier coatings (TBCs). The phase composition, microstructural evolution, fracture toughness and failure behavior of both TBCs before and after thermal cycling tests at 1300 °C were systematically studied. It was found that both as-sprayed TBCs were fully non-transformable tetragonal (t’) phase. After the thermal cycling test, tetragonal (t) phase and cubic (c) phase formed for the SSPS-YSZ TBC, while single t’ phase retained for the SSPS-ScYSZ coating. The fracture toughness of the ScYSZ coating was comparable or superior to that of the YSZ coating. As for the thermal cycling behavior, the lifetime of the ScYSZ coating was better than that of the YSZ coating, which confirmed that ScYSZ was a promising alternative material for YSZ.

The FeAl intermetallic compound coatings were sprayed by low pressure plasma spraying (LPPS), air plasma spraying (APS) and high velocity oxy-fuel spraying (HVOF). The influence of three kinds of thermal spraying processes on the microstructure, microhardness, elastic modulus and fracture toughness of coatings were investigated. The results showed that the APS and HVOF coatings exhibited similar microhardness, about 540HV 0.3 , which is much lower than that of LPPS coatings, 860HV 0.3 . The elastic modulus measured for APS, HVOF and LPPS coatings using Knoop indentation were 96, 84, 176GPa, respectively. The APS coatings were also observed to have lower elastic modulus values in the in-plane direction than those in the perpendicular direction, as a result of microcracks scattered within the coatings. In fracture toughness tests, the LPPS coatings revealed the lowest fracture toughness, as compared with other two spraying processes, indicating low porosity and crack levels are related to low fracture toughness. From these results, it appeared that potential improvements to certain mechanical properties can be achieved using low pressure plasma spraying process.

In this work, the influence of microstructure, hardness, elastic modulus, and fracture toughness on the tribological behavior of flame-sprayed Al 2 O 3 -TiO 2 and WC-NiFeCr coatings is investigated. Fracture toughness was found to be the most influential property, with higher toughness corresponding to greater wear resistance. This relationship, however, is verified only up to the point where the applied stresses cause detachment of particles from the surface. Hardness emerged as a less dominant factor than fracture toughness, elastic modulus, and porosity due to the fact that ceramic coatings, despite their high hardness, have low relative wear resistance.

Cavitation erosion (CE) damage, which occurs in the main parts (made of high chromium cast steel) of hydroelectric power generation machine, is one of the serious problems. It is expected that life time of those parts would be prolonged if the suitable CE-resistant coating is applied on the surface of the cast steel. In this study, WC-cermet coatings (WC-CoCr and WC-Cr 3 C 2 -Ni), which were fabricated by high-velocity oxygen-fuel (HVOF) thermal spraying process, was interested in protecting CE attack to the cast steel. To clarify CE property of the WC-cermet coatings, the ultrasonic vibration tests were conducted, and the amount of volume loss characterized as CE damage was measured. The microstructure and the fracture toughness, which was evaluated by the indentation test method, of the coatings were related with the CE damage. As the results obtained in this study, the fragment which was spalled from the surface after CE test was almost flake-like shape, and its size was from 2µm to 50µm. SEM observation indicated that this fragment included both WC particle and metal binder, which means that WC particle and metal binder was still strongly bonded together. It was also confirmed that the amount of volume loss could relate directly with the fracture toughness KIC rather than Vickers hardness. It was considered that CE damage was progressed into the depth by throwing out the fragment originated from micro crack initiation. Thus, it was required that the CE resistance of the developed coatings could be labelled through the fracture toughness.

A Vickers indentation method has been applied to determine the interfacial fracture toughness of modern multilayer thermal barrier coatings. The delamination behavior of four types of coating systems will be discussed and compared with results based on modified four-point bending (4PB) tests. The investigated multi-layer coating system consists of a CoNiCrAlY-bond coat applied via low-pressure plasma spray (LPPS) on a nickel-based superalloy and an atmospheric-plasma sprayed (APS) top layer of type gadolinium zirconate (GZO) and yttria-stabilized zirconia (YSZ). A conventional YSZ mono-layer system is used for reference. The effects of GZO and YSZ microstructure were investigated using top coats with low and high porosities for both (multi- and single-layer) coating systems. Isothermal oxidation tests at 1100 °C up to 500 hours were performed to study the interaction between thermal aging and fracture behavior. Investigations of microstructure and sintering behavior show a significant influence of the annealing conditions on fracture toughness. It has been observed, that with increasing annealing time, the stiffness and thus the crack driving force of the GZO layer is increased due to sintering effects and healing of submicron defects. The lower stiffness and higher defect density of GZO seem to be the main reason for the reduced fracture toughness of the YSZ / GZO interface compared to the YSZ / CoNiCrAlY interface. As a result, the delamination of the top coat is observed to shift from the top coat / bond coat interface into the top coat double-layer.

Twelve commercially available WC-Co powders with different average carbide 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 Characterization of the coatings showed that the average carbide sizes and carbide volume contents in the coatings were lower than those of feedstock powders. Hardness and fracture toughness of the coatings were investigated using indentation techniques. Young’s modulus was measured by an ultrasonic technique. The hardness and Young’s modulus decreased with increasing cobalt content, while fracture toughness slightly increased. The effect of carbide size on the hardness showed no specific trend. These behaviors were discussed with the help of microstructure observations of the coatings by scanning electron microscopy, X-ray diffraction and chemical analysis. Using an improved HVOF process with a gas shroud could result in less decomposition of the powder and higher fracture toughness.

Fracture toughness and phase stability are crucial properties of thermal barrier coatings (TBC) during highly loaded thermomechanical operations in gas turbines. While several alternative TBC materials have exhibited excellent thermal resistance, their potential applicability has been limited due to poor endurance to cyclic stresses. The addition of TiO 2 to the non-transformable tetragonal t´-YSZ has been found to effectively enhance the fracture toughness and phase stability of YSZ at high temperature exposures. Thermal cycling tests in a burner rig were conducted on TBCs prepared from atmospheric plasma sprayed titania-doped YSZ to verify this phenomena. Exposure temperature was 1400°C at the surface and thermal gradient across the sample was provided by simultaneous back-cooling treatment. Cycling tests reveal that the slight increase in the tetragonality of the deposited coatings with increasing amount of dopant did not cause a significant effect to the lifetime of the TBCs. Moreover, increasing amount of Ti-substitution did not influence the fracture toughness of the bulk YSZ.

Micro-structural design had attracted increasing interests in modern developments of hard coatings. The ability of cold spray process to retain the feedstock microstructure into coating makes it possible to design coating microstructure through feedstocks for development of different coating properties. In this study, a multi-size modal WC-12Co powder containing nano-sized WC particles was designed to deposit WC-Co deposition with multi-sized WC. Multimodal WC-12Co powders were prepared with ball-milling of a commercial WC-12Co powder, cold-compacting, sintering in hydrogen atmosphere and crushing. WC particle size in the powder exhibits a distribution with two peaks in tens of nanometers and several micrometers. The multimodal WC-12Co deposition was prepared by cold spraying using helium as driving gas. The multimodal size of WC particles in the powders was retained into the deposit. The micro-hardness and fracture toughness of the multimodal structured WC-12Co deposit was compared with bulk WC-12Co. It was found that the multimodal deposition exhibits a comparable hardness to nano-sized WC-12Co and a high fracture toughness compared with micro-sized WC-12Co. The simultaneous strengthening and toughening of WC-12Co can be realized through the bimodal microstructure design of WC-Co.

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.

Gadolinium zirconate (Gd 2 Zr 2 O 7 , GZ) as one of the promising thermal barrier coating materials for high-temperature application in gas turbine was toughened by nanostructured 3mol% yttria partially-stabilized zirconia (3YSZ) incorporation. The fracture toughness of the composite of 90mol%GZ-10mol% 3YSZ (GZ-YSZ) was increased by about 60% relative to the monolithic GZ. Both the GZ and GZ-YSZ composite coatings were deposited by atmospheric plasma spraying on Ni-base superalloys and then thermal-shock tested under the same conditions. The thermal-shock resistance of GZ-YSZ composite coating was improved significantly, which is believed to be mainly attributed to the enhancement of fracture toughness by the addition of nanostructured 3YSZ. In addition, the failure mechanisms of the thermal-shock tested GZ and GZ-YSZ composite coatings were also discussed.

The mechanical deformation and microstructure formation at the interface of a high velocity oxygen fuel (HVOF) sprayed WC-Co splat on a carbon steel (0.45%C) have been studied by the focused ion beam (FIB) cross-sectioning and examined by scanning electron microscopy (SEM). Several types of WC-12wt.%Co powders with different WC particle sizes, 0.2 ~ 7.0µm were used for spraying to investigate the size effect of these WC particles onto the splat morphology and microstructure around the interface. The interface fracture toughness of the coatings made of these powders was evaluated by the pre-notched four-point bending test and the correlation between the splat microstructure and the toughness variation was investigated. The interface fracture toughness of these WC-12%Co coating / carbon steel was evaluated as about 600~1700J/m 2 . It was revealed that the WC particle size in the original powders has large effects onto the interface adhesion.

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.

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.

The aim of this study is to investigate the effect of multimodal powders and annealing on cold sprayed deposits. In the experiments, a multimodal WC-Co powder containing micro- and nano-sized particles was prepared by ball milling and deposited on stainless steel substrates. The hardness of the resulting layer was comparable to that of the nanostructured WC-Co bulk and its fracture toughness reached a level close to that of the microstructured bulk. It is thus shown that with multimodal particle distribution cold-sprayed WC-Co can be simultaneously strengthened and toughened. Investigators also show that interface cohesion between WC and cobalt phases can be improved by annealing, which leads to further toughening as well.

HVOF thermal spraying has been developed to deposit dense ceramic coatings with improved protective properties for various applications. Even though HVOF coatings are much denser as compared to ordinary plasma sprayed coatings, the coating properties are inferior as compared to bulk ceramics because of pores and microcracks, which influence adversely the coating properties, i.e. toughness, hardness and wear resistance. One strategy to improve the properties of the coatings is to decrease the grain size of the ceramic phase and to add toughening elements to the microstructure. Nanocrystalline coatings have been found to offer better thermal shock resistance, lower thermal conductivity and better wear resistance than their conventional counterparts. In this paper we describe the development of nanocrystalline ceramic composite coatings, where the grain size of ceramic has been decreased and a few percents of alloying element has been added in order to toughen the coating. Dense nanostructured alumina coatings alloyed with Ni and ZrO 2 nanosized particles were manufactured by HVOF spraying by using HV2000 spray gun. Mechanical properties, especially elastic modulus and relative fracture toughness were studied. Used techniques were instrumented nanoindentation and KIC evaluation. As a result coatings with nearly 100% improvements in relative fracture toughness were produced for nanoreinforced alumina composite coating. Results are compared with the microstructure, hardness and abrasive wear resistance of the coatings.