Interfacial indentation test of FeB/Fe2B coatings

The present study uses interfacial indentation testing to estimate the adhesion of the FeB/Fe₂B coating formed on the surface of borided AISI 316 steel. This technique creates and propagates a crack along the FeB/Fe₂B interface and defines the apparent fracture toughness, which can then be related to the adhesion and mechanical support of the aforementioned interface. The boriding process was performed on the surface of AISI 316 steel by means of the powder-pack method at temperatures of 1123, 1173, 1223 and 1273 K with 2, 4, 6, 8 and 10 h for each temperature. The Young's modulus for each surface layer was obtained by Knoop microindentation at a constant indentation load. Vickers microindentation fracture technique was used to generate microcracks at the FeB/Fe₂B interface with varying indentation loads. The applied load, Young's modulus, hardness, and lateral crack lengths generated from the corners of the indentations, along with the depth of the FeB layer, were used to determine the apparent fracture toughness and adhesion of the FeB/Fe₂B interface. The apparent fracture toughness of the FeB/Fe₂B interface varied between 3.56 and 4.45 MPa . Finally, the intensity of residual stress at the FeB/Fe₂B interface was estimated as a function of the FeB layer thickness.     

The interfacial indentation test to determine adhesion and residual stresses in NiCr VPS coatings

The interfacial indentation test allows determining the interface toughness of a coating obtained by thermal spraying. During this test, a Vickers indentation is carried out on a cross-section of the sample. A crack is initiated and propagated along the interface. An analytical model allows defining an interface toughness representing the coating adhesion. The objective of this study is to compare this test with other tests (tensile adhesive test, shear test) and to specify its applicability. The residual stresses are also estimated by two different methods. Their influence on adherence is discussed in a third part. Those experiments were conducted on NiCr 80-20 VPS coatings with different thicknesses and roughnesses. In particular, it is shown that the interfacial indentation test is the most universal one and that compressive residual stresses improve coating adhesion.     

Tensile fracture behavior of thermal barrier coatings on superalloy

Tensile fracture behavior of thermal barrier coatings (TBCs) on superalloy was investigated in air at room temperature (RT), 650 °C and 850 °C. The bond coat NiCrAlY was fabricated by either high velocity oxygen fuel (HVOF) or air plasma spraying (APS), and the top coat 7%Y₂O₃-ZrO₂ was deposited by APS. Thus two kinds of the TBC system were formed. It was shown that the coating had little effect on tensile stress-strain curves of the substrate and similar tensile strength was obtained in two kinds of the TBC system. However, the cracking behavior in the two kinds of TBC system at RT was different, which was also different from that at 650 °C and 850 °C by scanning electron microscopy. The interface fracture toughness of the two kinds of TBC system was evaluated by the Suo-Hutchinson model and the stress distribution in the coating and substrate was analyzed by the shear lag model.     

Measurement of residual stress in thermal spray coatings by the incremental hole drilling method

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.     

Thermal cyclic behavior of air plasma sprayed thermal barrier coatings sprayed on stainless steel substrates

Thermal barrier coatings (TBCs) were deposited by an Air Plasma Spraying (APS) technique. The coating comprised of 93 wt.% ZrO₂ and 7 wt.% Y₂O₃ (YSZ); CoNiCrAlY bond coat; and AISI 316L stainless steels substrate. Thermal cyclic lives of the TBC were determined as a function of bond coat surface roughness, thickness of the coating and the final deposition temperature. Two types of thermal shock tests were performed over the specimens, firstly holding of specimens at 1020 °C for 5 min and then water quenching. The other test consisted of holding of specimens at the same temperature for 4 min and then forced air quenching. In both of the cases the samples were directly pushed into the furnace at 1020 °C. It was observed that the final deposition temperature has great impact over the thermal shock life. The results were more prominent in forced air quenching tests, where the lives of the TBCs were observed more than 500 cycles (at 10% spalling). It was noticed that with increase of TBC's thickness the thermal shock life of the specimens significantly decreased. Further, the bond coat surface roughness varied by employing intermediate grit blasting just after the bond coat spray. It was observed that with decrease in bond coat roughness, the thermal shock life decreased slightly. The results are discussed in terms of residual stresses, determined by hole drill method.     

Fracture toughness of thermal spray ceramic coatings determined by the indentation technique

The indentation technique for determining material toughness is applied to spinel and yttria-stabilized zirconia plasma-sprayed coatings in this investigation. Fracture toughness of the coatings ranged from 1.9 to 3.4 MPa√m for spinel and 2.0 to 3.3 MPa√m for yttria-stabilized zirconia. These results are in good agreement with those obtained by other experimenters for bulk materials.     

The adhesion strength and indentation toughness of plasma-sprayed yttria stabilized zirconia coatings

In this work, yttria stabilized zirconia (YSZ) coatings were deposited by atmospheric plasma spray (APS) technique under various powder feed rates and spray distances. The microstructure and phase analysis were studied by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Tensile adhesion test (TAT) and interfacial indentation test (IIT) methods were used to evaluate adhesion of coatings. The effect of spray parameters on the coatings adhesion as well as its toughness was investigated. The obtained results revealed that with increasing powder feed rate and spray distance, adhesion of the coatings reaches to a maximum and then reduces. Interfacial toughness values change in the same manner. Adhesion/toughness behaviors of the YSZ coatings were related to microstructural characteristics including the volume fraction and size of pores and unmelted particles.     

Fundamentals of adhesion of thermal spray coatings: Adhesion of single splats

Indentation experiments were performed inside a scanning electron microscope to measure adhesive strength of individual alumina splats on a steel substrate. The in situ nature of experimental evaluations made characterization of interfacial crack propagation possible by direct observation. The increase in the strain energy of brittle alumina splats originating from indentation deformation was correlated to the strain energy release rate through the characterization of interfacial crack propagation. An analytical model previously reported and evaluated in studies of the adhesive strength of thin films was employed. An average calculated strain energy release rate of 80 J m⁻² was found for single splats. This high value suggests that splat adhesion can make a significant contribution to the adhesion of thermal sprayed 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.     

Effects of annealing on the microstructure and the mechanical properties of EB-PVD thermal barrier coatings

The effects of thermal annealing at 1000 °C in air on the microstructure and the mechanical properties (Young's modulus and hardness) of thermal barrier coatings consisting of a 4 mol% Y₂O₃ partially stabilized ZrO₂ top coat and a NiCoCrAlY bond coat, deposited by electron beam physical vapour deposition on nickel-based superalloy IN 625, have been investigated using X-ray diffraction, Raman spectroscopy, scanning electron microscopy (SEM), image analysis and nanoindentation. During annealing, the ceramic top coat undergoes sintering and recrystallization. These processes lead to stress relaxation, an increase of the intra-columnar porosity and the number of large pores as measured by image analysis of SEM micrographs. An increase of the grain size of the γ-phase in the bond coat, accompanied by changes in the morphology of γ-grains with annealing time, is also observed. Correlations between these microstructural changes in the top coat and the bond coat and their mechanical properties are established and discussed.     

Microstructure and adhesion of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate

Thermally sprayed of 100Cr6 steel coatings are widely used to combat degradation of components and structures due to mechanical wear. In this paper, the microstructure and adhesion energy of 100Cr6 steel coatings thermally sprayed on a 35CrMo4 steel substrate are investigated. The microstructure characteristics of the deposits are studied using the combined techniques of X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) including energy-dispersive spectroscopy (EDS). The practical work of adhesion of flame-sprayed 100Cr6 on steel substrate is determined using a four-point delamination bending test. The influence of a molybdenum bond coat on the adhesion is also studied. Microstructure suggests that the coating is mainly constructed by splats of γ-phase (fcc) and FeO. Phase analysis also confirms that during spraying process, a stable α-phase (bcc) was transformed into a new γ-phase (fcc). The highest values of the fracture energy are obtained for the 35CrMo4 substrate/100Cr6 steel deposit type samples. On the contrary, when a molybdenum bond coat is introduced (composite system 35CrMo4 substrate/Mo bond coat/100Cr6 steel deposit), the fracture energy decreases in a ratio of approximately three. So, the presence of a Mo bond coat as a barrier between the coating and the substrate has a negative role on the adhesion.     

Modified tensile adhesion test for evaluation of interfacial toughness of HVOF sprayed coatings

Tensile adhesion test is widely used to evaluate the adhesion strength of coatings sprayed by High Velocity Oxy-Fuel (HVOF) technique. But there are two issues to be improved. Firstly, when the coatings have high adhesion strength, failure occurs in an adhesive layer, and secondary, the edge of a substrate is heavily deformed and rounded due to the high impact energy of sprayed particles. This deformation causes large scatter of adhesion test results. In this paper, a new technique to evaluate the interfacial fracture toughness has been proposed by introducing pre-crack at the interface of a conventional tensile adhesion test specimen. The asymptotic analytical formula was derived for interfacial toughness evaluation. Numerical analysis was also carried out for comparison. The difference between the numerical and the theoretical data was less than 5%. The developed procedure was applied for the SUS316 L steel coatings and the significant effects of the surface roughness and preheating temperature on adhesion strength were reconfirmed quantitatively.     

Evaluation of microhardness,fracture toughness and residual stress in a thermal barrier coating system: A modified Vickers indentation technique

The evolution of microhardness, fracture toughness and residual stress of an air plasma-sprayed thermal barrier coating system under thermal cycles was investigated by a modified Vickers indentation instrument coupled with three kinds of indentation models. The results show that fracture toughness on the top coating surface after thermal cycles changes from 0.64 to 3.67 MPa m^1/2, and the corresponding residual stress near the indented region varies from − 36.8 to − 243 MPa. For the interface region of coating and bond coat, fracture toughness in the coating close to interface ranges from 0.11 to 0.81 MPa m^1/2, and residual stress varies from − 5 to − 30 MPa, which are consistent with available data. For the lateral region of coating, fracture toughness and residual stress display strong gradient characteristics along the thickness direction due to the special layered structure.     

Development of electrically conductive plasma sprayed coatings on glass ceramic substrates

The production of functional coatings on glass or glass ceramic substrates is of outstanding interest in modern product development due to the specific thermophysical properties of glasses, like low or even negative CTE, low heat conductivity and high dimensional stability. Atmospheric plasma spraying (APS) is an adequate technology for the deposition of a wide variety of materials on glasses and opens new application fields for thermal spraying technology in engineering and consumer industries.Metals are the frequent solution to produce electrically conductive layers in thermal spraying operations. Concerning applications with glass ceramic as a substrate, an intermediate oxide ceramic coating is applied before depositing the metallic layer, so that the distribution of residual stresses in the composite caused during and after the deposition process due to the mismatch in the materials thermophysical properties is minimized. However, the electrical properties required for the developed coatings presented in this paper can be fulfilled using other spraying materials, like mixed phases of oxide ceramics and metal powders, or pure ceramic materials. In this way, mono-layer electrically conductive systems which ensure the required stability and adhesion of the coating can be developed, reducing as well production time and costs.In the proposed approach, the three systems, metal oxide layer-composites, ceramic-metal mixed layers and ceramic mono-layers as conductive coatings on glass ceramics were thermally sprayed with APS. The coatings were characterized in terms of residual stress distribution and electrical conductivity. The influence of the process parameters on the coating electrical and mechanical properties was analyzed using the design of experiments (DOE) methodology.     

Interfacial adhesion characterization of plasma coatings by V(z) inversion technique and comparison to interfacial indentation

This paper presents the development of V(z) inversion technique and its application to quantitative determination of interface adhesion by measuring interface tangential stiffness parameter K_T. The measurement is performed in two steps on an air plasma sprayed bronze-aluminum alloy coating (on a steel substrate) having different adhesion levels: the determination of the coating elastic constants and the determination of the interface tangential stiffness K_T are performed by optimizing the inverted angular-frequency reflectance function R(θ,f). The results obtained by the V(z) inversion are then compared to mechanical testing by interfacial indentation on the same sample. The same behavior (quality of interface adhesion) can be seen by both methods: the coating adhesion decreases as the interfacial stiffness parameter K_T decreases.     

Interface fracture toughness and fracture mechanisms of thermal barrier coatings investigated by indentation test and acoustic emission technique

Interface fracture toughness and fracture mechanisms of plasma-/sprayed thermal barrier coatings (TBCs) were investigated by interfacial indentation test (IIT) in combination with acoustic emission (AE) measurement. Critical load and AE energy were employed to calculate interface fracture toughness. The critical point at which crack appears at the interface was determined by the IIT. AE signals produced during total indentation test not only are used to investigate the interface cracking behavior by Fast Fourier Transform (FFT) and wavelet transforms but also supply the mechanical information. The result shows that the AE signals associated with coating plastic deformation during indentation are of a more continuous type with a lower characteristic frequency content (30-60 kHz), whereas the instantaneous relaxation associated with interface crack initiation produces burst type AE signals with a characteristic frequency in the range 70-200 kHz. The AE signals energy is concentrated on different scales for the coating plastic deformation, interface crack initiation and interface crack propagation. Interface fracture toughness calculated by AE energy was 1.19 MPam_1/2 close to 1.58 MPam_1/2 calculated by critical load. It indicates that the acoustic emission energy is suitable to reflect the interface fracture toughness.     

Characterization and residual stresses of WC-Co thermally sprayed coatings

The present investigation has been conducted in order to determine the residual stresses of an as-ground WC-12Co coating of two different thicknesses, by means of two different methods. Firstly, X-ray diffraction techniques, which allowed the determination of the surface residual stresses of the coating by means of the method called “sin²ψ” method. Secondly, an incremental hole drilling technique together with the integral method, which allowed the analysis of the non-uniform through-thickness residual stresses present in the coatings. It has been determined that the surface residual stresses are of a compressive nature, which could be due to the grinding that was applied to the coatings in order to achieve the desired thicknesses. On the contrary, the results of the incremental hole drilling tests indicated that the through-thickness residual stress distributions are not uniform and are characterized by the presence of tensile peak stresses, at depths in the range of ~ 50-125 μm. Such stresses were observed to decrease towards the coating-substrate interface where the compressive component of the stress state becomes greater than the tensile component. It has been found that the mean residual von Mises stress is higher in the thinner coating than in the thicker one, of approximately 180 and 107 MPa, respectively.     

Fracture toughness measurements of plasma-sprayed thermal barrier coatings using a modified four-point bending method

In this study, the adhesion strength of thermal barrier coatings 8YSZ (ZrO₂ + 8 wt.% Y₂O₃) deposited on NiCrAlY bond coats by atmospheric plasma spraying is investigated experimentally. A modified four-point bending specimen that can generate a single interface crack to facilitate the control of crack growth was adopted for testing. The fracture surfaces were examined using a scanning electron microscope. Images show that cracks are initiated along YSZ/NiCrAlY interfaces, then kink and grow uniformly within the YSZ layer. The load-displacement curves obtained indicate three distinct stages in crack initiation and stable crack growth. Based on a microstructural model, finite element analyses were performed to extract the bonding strength of the thermal barrier coatings. The fracture toughness of the plasma-sprayed 8YSZ coatings, in terms of critical strain energy release rate G_c, can be reliably obtained from an analytical solution or from a numerical simulation of the cracking process using compliance methods.     

Study of molten metal droplet adhesion to organic anti-spatter coatings

In this study, the protection mechanism of four typical anti-spatter products based on polymer coating is investigated. These coatings protect the weld nozzle against weld spatter in a similar way as water droplets on a hot plate. During the short contact of the molten metal droplet with the coating, part of the coating evaporates in the form of carbon monoxide, hydrogen, water and other volatile degradation products. During welding, these gases act as a buffer layer between the coating and the weld spatter and can significantly reduce the potential spatter/substrate adherence and spatter agglomeration.     

Design and manufacture of Nd-Fe-B thick coatings by the thermal spray process

Permanent magnetic coatings show potential in micro scale applications such as micro motors and micro generators. Nd-Fe-B magnetic coatings of average thickness 50 μm were produced via the flame spray method where the behavior of Nd-Fe-B particles in the thermal spray process was studied through single splat formation. There are generally five types of Nd-Fe-B splat morphology; which correspond to different solidification routes and varying degree of splashing. Microstructures of coating cross sections exhibit features such as cracks and porosity. The microstructural features were related to the physical properties and the brittle nature of the rare-earth alloy feedstock. Cross sections of the coatings also exhibited the presence of two distinct phases: Nd-rich and Fe-rich regions, which have been validated by EDS analysis. Metastable phase formation and decomposition followed by non-equilibrium solidification of the molten droplet prior to impact have been suggested to cause phase separation and were also identified in the formation of single splats. Hardness tests further confirmed the two distinct phases as Nd-rich and Fe-rich areas.