Effects of carbide size and Co content on the microstructure and mechanical properties of HVOF-sprayed WC-Co coatings

Twelve commercially available WC-Co powders with different average WC grain 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 process. Hardness, Young's modulus, and fracture toughness of the coatings were measured. While the hardness and Young's modulus decreased with increasing cobalt content from 1600 to 1100 Hv and from 400 to 300 GPa respectively, the fracture toughness remained in the range from 4 to 6 MPam^1/2. The coatings with 2 μm carbide showed lower hardness than those deposited from 0.2 and 6 μm carbide. These measured mechanical properties were discussed with the help of microstructures of the coatings investigated by scanning electron microscopy, X-ray diffraction and chemical analysis. Finally, the hardness of the binder phase in these coatings was estimated to range from 1000 to 1300 Hv by applying the mixture rule for composites to the experimental data, demonstrating that such hardening of the binder phase is a key factor affecting the mechanical properties of the coatings.     

Comparison of Isolated Indentation and Grid Indentation Methods for HVOF Sprayed Cermets

This paper compares the results of two approaches of instrumented indentation for characterization of mechanical properties of HVOF coatings. Three types of HVOF sprayed coatings (Cr₃C₂-NiCr, WC-Co, (Ti, Mo)(C,N)-NiCo) were investigated by the means of isolated nanoindentation and grid indentation methods. The results of the isolated indentation revealed hardness and elastic modulus of the individual phases in very good agreement with the corresponding bulk material. The grid indentation method, based on statistical evaluation of a large number of indentations, was influenced by the carbide-matrix interface, which gave rise to a third peak apart from the two peaks corresponding to the carbides and metallic matrix. As a consequence, the bimodal Gaussian fit was insufficient and a trimodal fit had to be used. The results extracted from low load grid nanoindentations were quite close to the results of isolated indentations whereas higher load grid nanoindentation revealed overall properties of the coating.     

Mechanical Properties and Microstructure of VPS and HVOF CoNiCrAlY Coatings

In this study, high velocity oxy-fuel (HVOF) and vacuum plasma spraying (VPS) coatings were sprayed using a Praxair (CO-210-24) CoNiCrAlY powder. Free-standing coatings underwent vacuum annealing at different temperatures for times of up to 840 h. Feedstock powder, and as-sprayed and annealed coatings, were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and x-ray diffraction (XRD). The hardness and Young’s modulus of the as-sprayed and the annealed HVOF and VPS coatings were measured, including the determination of Young’s moduli of the individual phases via nanoindentation and measurements of Young’s moduli of coatings at temperatures up to 500 °C. The Eshelby inclusion model was employed to investigate the effect of microstructure on the coatings’ mechanical properties. The sensitivity of the mechanical properties to microstructural details was confirmed. Young’s modulus was constant up to ~200 °C, and then decreased with increasing measurement temperature. The annealing process increased Young’s modulus because of a combination of decreased porosity and β volume fraction. Oxide stringers in the HVOF coating maintained its higher hardness than the VPS coating, even after annealing.     

Microstructure and properties of HVOF-sprayed chromium alloyed WC-Co and WC-Ni coatings

In this paper the influence of the type of binder metal (nickel or cobalt) and chromium as an additional alloying element on the microstructure, mechanical properties and wear resistance of high velocity oxy-fuel (HVOF)-sprayed WC-based hardmetal coatings was studied. Plain WC-Co and WC-Ni as well as five chromium alloyed compositions were sprayed with a liquid-fueled HVOF-spray process from commercial and experimental agglomerated and sintered feedstock powders. The coating characterization included optical microscopy and SEM of metallographically prepared cross-sections, hardness measurements, determination of the Young's modulus and phase composition by X-ray diffraction. Erosion and dry oscillating sliding wear were studied. The resistance to erosive wear was found to be improved when cobalt was used as binder metal. A dependence on the chromium content was not detected. For the oscillating sliding wear against a hardmetal counterbody there is no dependence of the wear rate on the type of binder metal or the amount of chromium.     

Mechanical properties of thermal barrier coatings after isothermal oxidation.: Depth sensing indentation analysis

Thermal barrier coatings (TBC) are extensively used to protect metallic components in applications where the operating conditions include aggressive environment at high temperatures. Isothermal oxidation degrades the performance of these coatings, so this work analyses the mechanical properties (Young's modulus, E, and hardness, H) of TBC and its evolution after thermal exposure in air. ZrO₂(Y₂O₃) top coat and NiCrAlY bond coating were air plasma sprayed onto an Inconel 600 Ni base alloy. The TBC were isothermally oxidized in air at 950 °C and 1050 °C for 72, 144 and 336 h. Depth sensing indentation tests were carried out on the ceramic coating to evaluate E and H in the as-sprayed materials and after isothermal oxidation. An approach based on multiple tests at different loads was used to determine size independent apparent E an H. These mechanical properties, measured perpendicular to the surface, clearly decreased after isothermal oxidation as a consequence of microcracking within the ceramic coating.     

燃料类型及喷涂参数对HVOF喷涂WC10Co4Cr涂层的组织及力学性能的影响

超音速火焰喷涂（HVOF）制备的WC基金属陶瓷涂层广泛应用于金属构件的磨损、腐蚀及空蚀防护。分别采用氢气燃料及煤油液体燃料HVOF喷涂设备分别在9种不同的工艺条件下制备了WC10Co4Cr涂层,研究了燃料类型对涂层的组织、残余应力及力学性能的影响规律。在两种燃料HVOF工艺各自优化的喷涂参数条件下,通过对基体曲率的原位监测对比测试了涂层中的平均残余应力;利用显微维氏硬度、压痕法（断裂韧性）和球盘摩擦磨损对比研究了涂层的力学性能。结果表明：液体燃料（LF）HVOF焰流中粒子的温度更低,速度更高。LF-HVOF喷涂的WC10Co4Cr涂层内的残余压应力更高且涂层致密度更高,而气体燃料（GF）HVOF喷涂的WC10Co4Cr涂层内为残余拉应力。LF-HVOF涂层（1280 HV_0.3, 7.3 MPa·m^0.5）比GF-HVOF涂层（1032 HV_0.3, 4.5 MPa·m^0.5）具有更高的硬度和断裂韧性,LF-HVOF涂层的耐磨性约为GF-HVOF涂层的1.7倍。     

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.     

Nanoindentation on the surface of thermally sprayed coatings

The ability to quantify surface mechanical properties is valuable for assessing the quality of thermal spray coatings. This is especially important for prostheses where loading is placed directly on the surface. Hydroxyapatite was classified to small (20-40 μm), medium (40-60 μm) and large (60-80 μm) particle sizes and thermal sprayed to produce a coating from spread solidified hydroxyapatite droplets. It was revealed for the first time, that nanoindentation can be successfully used to determine the hardness and elastic modulus on the surface of well spread solidified droplets at the hydroxyapatite coating surface. Comparison with indentation results from polished cross-section exhibited comparable values and statistical variations. The hardness was 5.8 ± 0.6, 5.4 ± 0.5 and 5.0 ± 0.6 GPa on coatings produced from small, medium and large sized powder. Similarly, the elastic modulus decreased from 121 ± 7, 118 ± 7 to 114 ± 7 GPa, respectively. Use of several indentation loads gave comparable results with sintered hydroxyapatite suggesting good inter-splat bonding within the coating. MicroRaman spectroscopy and X-ray diffraction confirmed a larger degree of dehydroxylation for the smaller particles also revealing a lower elastic modulus. This shows the influence of particle size and possibly dehydroxylation of hydroxyapatite on the mechanical properties of the coating surface.     

Effect of Thermal Treatment on High-Temperature Mechanical Properties Enhancement in LPPS, HVOF, and APS CoNiCrAlY Coatings

The mechanical properties of a MCrAlY coating significantly influence the initiation of cracks in the superalloy substrate under thermomechanical-fatigue conditions. Previous studies have developed a convenient method for evaluating the mechanical properties of sprayed coatings by lateral compression of a circular tube coating. This method does not need chucking, and manufacturing the free-standing coating is quite straightforward. In this study, the mechanical properties of the free-standing CoNiCrAlY coatings prepared using low-pressure plasma spraying (LPPS), high-velocity oxyfuel (HVOF) spraying, and atmospheric plasma spraying (APS) were systematically measured with the lateral compression method at room temperature through to 920 °C. The effect of postspray thermal treatments, in vacuum and in air, on the mechanical properties was investigated in the 400 to 1100 °C temperature range. It was found that high-temperature thermal treatment in air was effective in increasing the bending strength and Young’s modulus. It was especially effective on the APS coatings, which were produced using powders with average size 60 μm, and on HVOF coating, whose bending strengths increased by approximately three times. On the contrary, the enhancement in the LPPS and APS coatings produced with powders 21 μm in size was found to be approximately 1.6 times.     

Thermal barrier coating systems — analysis of nanoindentation curves

Mechanical properties (Young's modulus E, hardness H, degree of plasticity) of all three components of thermal barrier coatings systems, prepared by electron beam physical vapour deposition (EB PVD), have been investigated by nanoindentation. The power-law exponents n and m, describing the shapes of the loading and unloading nanoindentation curves, increase with peak load for the yttrium-stabilized zirconia top coat (TC), containing 4 mol% Y₂O₃, and the NiCoCrAlY bond coat (BC). The variations of m are correlated to the degree of plasticity. Decrease of the hardness with increasing peak load, generally known as indentation size effect (ISE), is observed only for the TC and the BC. The ISE in the TC is explained using a new empirical equation based on the concept of elastic recovery. The average Young's moduli of the Ni-based superalloy substrate, the BC and the TC are 189 ± 11 GPa, 166 ± 7 GPa, and 126 ± 25 GPa, respectively. The corresponding average hardness values are 3.3 ± 0.3 GPa, 5.5 ± 0.2 GPa, and 6.2 ± 1.7 GPa, respectively. The mechanical properties of the TC show complex behaviour upon annealing at 1000°°C in air, which can be explained by changes in the porosity and the residual stresses.     

Determination of Young's modulus and Poisson's ratio for coatings

Designing thin-film coatings to meet engineering needs requires the knowledge of accurate mechanical properties of the coatings. Young's modulus and Poisson's ratio are two basic mechanical properties of materials, which should be conveniently measured. This paper reports a direct and non-destructive method for the measurement of the Young's modulus and Poisson's ratio of a thin-film coating and its substrate based on the extended Hertz theory for the contact of coated bodies. The theory is used to analyze load-displacement data from a spherical indentation in the elastic range, in which the substrate effect is intrinsically modeled. The Young's modulus and Poisson's ratio are determined at the same time through minimizing the difference between the measured and specially defined modified Young's moduli. Two sets of validation experiments are also reported. This new method does not require any assumptions on pressure distribution and Poisson's ratio and can be easily incorporated into current indentation analysis systems.     

The Instrumented Indentation Study of HVOF-Sprayed Hardmetal Coatings

Elastic-plastic properties, namely, hardness and Young’s modulus, of four HVOF-sprayed hardmetal coatings were measured by instrumented indentation using Oliver-Pharr method Nanoindenter XP MTS with a continuous stiffness measurement (CSM) module. The results show that with sufficient number of CSM measurements, one can distinguish between indents made in the hard particles and indents made in the binder material. This can be accomplished by analyzing the plots of hardness and Young’s modulus versus load (or versus indentation depth). Further development of the dependence curves enables the load (or indentation depth) to be set to correspond to the point of transition from a single structure component to the composite material and to determine the properties of both. Comparison of results of CSM measurement with the results of single indentation measurement at a defined load reveals a new perspective on the origin of the indentation size effect in hardmetal coatings. The measurements show that the increase in both the hardness and Young’s modulus with decreasing load is caused mainly by the predominant influence of hard particles in the coatings.     

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.     

Indentation properties of plasma sprayed Al2O3–13% TiO2 nanocoatings

Young’s modulus and hardness were determined by depth sensing indentation in plasma sprayed Al₂O₃–13% TiO₂ nanocoatings. Results were compared to conventional coatings and the relevance of the nanostructure was analyzed. An indentation size effect was observed. Data provided by indentation tests at different maximum loads were used to estimate size-independent hardness and elastic modulus. Enhanced properties were observed in the nanostructured coating compared to the conventional one. Partially melted zones in the nanocoating, which act as reinforcements in the ceramic matrix composite, are likely responsible for the enhancement.     

The effect of an electric current on the nanoindentation behavior of tin

Electrical–thermal–mechanical interactions determine the reliability and performance of microelectromechanical devices and systems. Using the nanoindentation technique the effect of an electric current on the indentation deformation of Sn strips was studied for an indentation load in the range 50–200 μN. During the indentation an electric current density in the range 993.05–4087.89 A cm^?2 was passed through the Sn strips, which introduced electrical–thermal–mechanical interactions. The experimental results showed that the reduced contact modulus decreased with increasing electric current density. For an electric current density less than 4087.89 A cm^?2 the decrease in the reduced contact modulus with increasing electric current density was mainly controlled by Joule heating due to an electrothermal interaction. The electrothermal interaction caused surface softening of the Sn strips. A simple relation is proposed to describe the dependence of the reduced contact modulus on the electric current density. The indentation hardness decreased with increasing indentation load, showing a normal indentation size effect. Using the relationship between indentation hardness and indentation depth from strain gradient plasticity theory we curve fitted the experimental data and found that both the indentation hardness at the limit of infinite depth and the characteristic length were dependent on the electric current density. Finite element analysis was performed to analyze the indentation deformation of a two-dimensional tin strip under the simultaneous action of an electric current. The simulation results showed that the contact modulus of tin decreased linearly with the square of the electric current density, qualitatively in accordance with experimental observations for an electric current density ? 2803.7 A cm^?2.     

Chromium carbide–CNT nanocomposites with enhanced mechanical properties

Chromium carbide is widely used as a tribological coating material in high-temperature applications requiring high wear resistance and hardness. Herein, an attempt has been made to further enhance the mechanical and wear properties of chromium carbide coatings by reinforcing carbon nanotubes (CNTs) as a potential replacement of soft binder matrix using plasma spraying. The microstructures of the sprayed CNT-reinforced Cr₃C₂ coatings were characterized using transmission electron microscopy and scanning electron microscopy. The mechanical properties were assessed using micro-Vickers hardness, nanoindentation and wear measurements. CNT reinforcement improved the hardness of the coating by 40% and decreased the wear rate of the coating by almost 45–50%. Cr₃C₂ reinforced with 2 wt.% CNT had an elastic modulus 304.5 ± 29.2 GPa, hardness of 1175 ± 60 VH_0.300 and a coefficient of friction of 0.654. It was concluded that the CNT reinforcement increased the wear resistance by forming intersplat bridges while the improvement in the hardness was attributed to the deformation resistance of CNTs under indentation.     

Mechanical properties of nanocrystalline nickel films deposited by pulse plating

This paper reports the mechanical properties of Ni films fabricated by pulse electrodeposition. Transmission electron microscope revealed that the prepared films had an average grain size of 25 nm with a narrow size distribution and the absence of dislocations. Small grain size leads to an increasing hardness as high as 7.8 GPa while Young's moduli keep a constant bulk value of 215 GPa, resulting in an increasing ratio of hardness (H) to elastic modulus (E). Interestingly, the wear resistance was also improved significantly. Under a constant normal load of 500 μN, the penetration depths of indenter slightly increased from 25 nm to 30 nm and the coefficient of friction varied from 0.12 to 0.20, depending on sliding scans. Depth sensing instrumented indentation experiments performed at different loading rates on specimens revealed an increasing rate-sensitivity of hardness, which concerns with a significantly small activation volume for plastic flow.     

Nanocrystalline NiAl Coating Prepared by HVOF Thermal Spraying

Nanocrystalline NiAl intermetallic powder was prepared by mechanical alloying (MA) of Ni₅₀Al₅₀ powder mixture and then deposited on low carbon steel substrates by high velocity oxy fuel (HVOF) thermal spray technique using two sets of spraying parameters. X-ray diffraction (XRD), scanning electron microscopy (SEM), transition electron microscopy (TEM), differential scanning calorimetry (DSC), and hardness test were used to characterize the prepared powders and coatings. The MA of Ni₅₀Al₅₀ powder mixture led to the formation of NiAl intermetallic compound. The resulting powder particles were three dimensional in nature with irregular morphology and a crystallite size of ~10 nm. This powder was thermally sprayed by HVOF technique to produce coating. The deposited coating had a nanocrystalline structure with low oxide and porosity contents. The hardness of coatings was in the range of 5.40-6.08 GPa, which is higher than that obtained for NiAl coating deposited using conventional powders.     

Influence of the N2 partial pressure on the mechanical properties and tribological behavior of zirconium nitride deposited by reactive magnetron sputtering

Zirconium nitride was deposited by reactive unbalanced magnetron sputtering at different N₂ partial pressures, on an AISI 316L stainless steel substrate. The mechanical properties of the coatings were evaluated by means of nanoindentation tests employing a Berkovich indenter and loads which varied between 120-9000 µN. The sliding wear behavior of the substrate-coating systems was studied under a normal load of 2 N using a ball-on-disc tribometer, with an AISI 52100 ball (6 mm diameter) as counterpart. It has been found that N₂ partial pressure has a significant effect both on the hardness and corresponding Young's modulus of the coatings. As the N₂ partial pressure increases from 1 × 10^− 4 Torr to 10 × 10^− 4 Torr, the hardness and Young's modulus of the coatings decrease from 26 to 20 GPa and 360 to 280 GPa, respectively. The nanoindentation tests revealed the presence of a third oxide layer (10 nm thick, approximately) on the surface of the coating. Scanning electron microscopy (SEM) analysis performed on the worn triboelements indicated that both abrasive and adhesive wear mechanisms could take place in addition to the substrate plastic deformation. The deposition conditions and coating mechanical integrity determine the predominant wear mechanism.     

Mechanical characterization of nano-structured materials using nanoindentation

The principal strengths of the nanoindentation technique, which is used extensively to measure the mechanical properties of nano/micro materials, are easy sample preparation and simple experimental method. Hardness and Young's modulus are essential properties measured by nanoindentation; hardness corresponds to resistance to plastic deformation whereas Young's modulus is related to elastic deformation. Two key difficulties arise in association with nanoindentation on small volumes: measurement accuracy and material response. Here we discuss the indentation size effect (ISE) considering tip bluntness and variation in hardness of nano-multilayers with a bilayer period, representative research on measurement improvement, and material response at nanoscales.