X-ray diffraction measurement of residual stress in WC-Co thermally sprayed coatings onto metal substrates

Investigation of the residual stresses and microstructural properties associated with HVOF thermal spray coating of WC-17 wt% Co of same thickness on three substrates with coefficients of thermal expansion different to that of WC. The residual stresses were measured by X-ray diffraction sin²ψ techniques using CoKα radiation. The results indicated residual stresses that have different natures for the as-sprayed coatings despite using the same powder as feedstock. The magnitudes of the stresses in the as-sprayed condition are low.     

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.     

Methods and application of residual stress analysis on thermally sprayed coatings and layer composites

Coating and layer composite manufacturing most commonly involves high temperature gradients and intensive heat transfer between the different composite materials. This can be noticed not only for thermal spraying, but also for other coating techniques. The combination of temperature gradients and materials with different thermophysical properties leads to the formation of thermal stresses in the composite, which are superimposed by stress generating effects during coating solidification, phase transformation or recrystallization. The final state of residual stresses affects the structural and functional properties of the coating as well as the component reliability during operation. Therefore, residual stress analysis is an important tool for the optimization of coatings and layer composite manufacturing processes in order to ensure stability of the processes, adhesion and compatibility of the coating, and finally, the reliability of the components in various technical systems.The most common residual stress measurement techniques are described and compared, with the focus on the incremental hole drilling and milling method. The advantages and disadvantages of the methods are discussed with respect to their application on industrial machine parts. The typical application fields for the different methods are given with respect to the specific measurement principles. The incremental hole drilling method is presented in more detail with application examples that illustrate the suitability of this method for the optimization of thermal spraying processes in industrial layer composite manufacturing by managing the heat and mass transfer in a most appropriate way.     

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.     

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.     

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.     

A comparative study of tribological behavior of plasma and D-gun sprayed coatings under different wear modes

In recent years, thermal sprayed protective coatings have gained widespread acceptance for a variety of industrial applications. A vast majority of these applications involve the use of thermal sprayed coatings to combat wear. While plasma spraying is the most versatile variant of all the thermal spray processes, the detonation gun (D-gun) coatings have been a novelty until recently because of their proprietary nature. The present study is aimed at comparing the tribological behavior of coatings deposited using the two above techniques by focusing on some popular coating materials that are widely adopted for wear resistant applications, namely, WC-12% Co, A1₂O₃, and Cr₃C₂-MCr. To enable a comprehensive comparison of the above indicated thermal spray techniques as well as coating materials, the deposited coatings were extensively characterized employing microstructural evaluation, microhardness measurements, and XRD analysis for phase constitution. The behavior of these coatings under different wear modes was also evaluated by determining their tribological performance when subjected to solid particle erosion tests, rubber wheel sand abrasion tests, and pin-on-disk sliding wear tests. The results from the above tests are discussed here. It is evident that the D-gun sprayed coatings consistently exhibit denser microstructures and higher hardness values than their plasma sprayed counterparts. The D-gun coatings are also found to unfailingly exhibit superior tribological performance superior to the corresponding plasma sprayed coatings in all wear tests. Among all the coating materials studied, D-gun sprayed WC-12%Co, in general, yields the best performance under different modes of wear, whereas plasma sprayed Al₂O₃ shows least wear resistance to every wear mode.     

Residual stress evaluations of thermally sprayed coatings by a modified Almen method

Based on the Almen method,the authors have modified this test method so that the results of the simple testcan be correlated to the absolute values of residual stress measured fy conventional methods such as X-ray diffraction ormechanical techniques.The technique utilizes changes in the strain of standardized test substrates, which is measured byBHP-700℃ temperature-compensated reststance strain gauges, as an indication of the level of residual stress in thesprayed coatings. Based on the concept that the sprayed coating on the surface of substrate with residual stresses acts aforce (N) and bending momeat (M) on the test piece,the calculating equation of residual stress in the coatings hasbeen proposed. The results of the test method showed that the technique is potentially reliable and accurate, althoughdeficiencies and many controversies are still exist.     

Microstructure and stresses in HVOF sprayed iron aluminide coatings

The microstructure and state of stress present in Fe₃Al coatings produced by high velocity oxygen fuel (HVOF) thermal spraying in air at varying particle velocities were characterized using metallography, curvature measurements, x-ray analysis, and microhardness measurements. Sound coatings were produced for all conditions. The microstructures of coatings prepared at higher velocities showed fewer unmelted particles and a greater extent of deformation. Residual stresses in the coatings were compressive and varied from nearly zero at the lowest velocity to approximately −450 MPa at the highest velocity. X-ray line broadening analyses revealed a corresponding increase in the extent of cold work present in the coating, which was also reflected in increased microhardness. Values of mean coefficient of thermal expansion obtained for assprayed coatings using x-ray analysis were significantly lower than those for powder and bulk alloy.     

Determination of residual stress distribution from in situ curvature measurements for thermally sprayed WC/Co coatings

An in situ monitoring of curvature of the specimen during spraying using a high speed video system was implemented to determine stresses in thermally sprayed WC/Co coatings. Influences of different spray-ing techniques (atmospheric plasma spraying and high-velocity oxygen fuel) and cooling levels were con-sidered using a mathematical model. Results show that temperature history of a part is of paramount importance in stress generation and distribution.     

超音速等离子喷涂FeCrBSi涂层组织和残余应力分析

采用超音速等离子喷涂技术制备了FeCrBSi涂层,利用扫描电子显微镜(SEM)、X射线衍射仪(XRD)和纳米压痕仪等研究了涂层的微观组织和力学性能。采用X射线应力仪对不同厚度及不同温度退火后涂层的表面残余应力进行测试。结果表明涂层表面的残余应力为拉应力,且随着涂层厚度的增加而增加;对试样进行退火处理可以有效地缓和涂层表面的残余应力,随着温度的升高涂层表面的残余应力不断降低,到260℃左右变为压应力;压应力值随着退火温度的升高而变大,但当温度升高到大约400℃以上时,保持在80 MPa左右。     

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.     

Influence of dopant on the behavior under thermal cycling of two plasma- sprayed zirconia coatings Part 2: residual stresses

The evolution of coating morphology and surface residual stresses was followed for three different pow-ders: zirconia stabilized with 8 wt% yttria (YSZ), 9.9 wt% dysprosia (DSZ), and 9.8 wt% ytterbia (YbSZ). The YSZ reference powder was fused and crushed (-45 +22 μm), and the other two were agglom-erated and sintered (-90 +10 μm). According to the size distributions and manufacturing process, the plasma-sprayed YSZ particles were fully molten, resulting in dense coatings with good contact between the splats; the DSZ and, especially, the YbSZ particles were partially molten. In general, the surface residual stresses were slightly compressive before thermal cycling. The YSZ and DSZ coatings were insensitive to aging (600 h in air at room temperature), as shown by the surface stress evolution, which was not the case for YbSZ coatings. Six hundred furnace thermal cycles from 1100 °C to room temperature indicated excellent behavior of YSZ and DSZ coatings, with almost no variation of sur-face residual stresses, compared to a high dispersion for YbSZ coatings with the development of macrocracks parallel and perpendicular to the substrate within the coating. Part 1 of this article was printed in the Journal of Thermal Spray Tech-nology, Vol 5 (No. 4), 1996, p 431-438.     

Characterization of thermal sprayed nanostructured WC-Co coatings derived from nanocrystalline WC-18wt.%Co powders

Nanostructured WC-Co coatings were synthesized using high velocity oxygen fuel (HVOF) thermal spray. The nanocrystalline feedstock powder with a nominal composition of WC-18 wt.%Co was prepared using the novel integrated mechanical and thermal activation (IMTA) process. The effects of HVOF thermal spray conditions and powder characteristics on the microstructure and mechanical properties of the as-sprayed WC-Co coatings were studied. It was found that the ratio of oxygen-to-hydrogen flow rate (ROHFR) and the starting powder microstructures had strong effects on decarburization of the nano-coatings. Decarburization was significantly suppressed at low ROHFR and with the presence of free carbon in the powder. The level of porosity in the coatings was correlated with the powder microstructure and spray process conditions. The coating sprayed at ROHFR=0.5 exhibited the highest microhardness value (HV_300g=1077), which is comparable to that of conventional coarse-grained coatings.     

Non-destructive evaluation of plasma sprayed functionally graded thermal barrier coatings

Acoustic emission (AE) as a non-destructive evaluation technique has recently been used in a number of studies to investigate the performance and failure behavior of plasma sprayed thermal barrier coatings. The mechanism of coating failure is complex, especially when considering the composite nature of the coating. In the present paper, the thermal shock tests with in situ acoustic emission are used to study the cracking behavior of plasma sprayed functionally graded thermal barrier coatings. Each thermal cycle consists of 8 min heating in the furnace at 1000°C and 8 min cooling from 1000°C to the room temperature by a compressed air jet. The AE signals are recorded during the quench stage. Three, four and five layer functionally graded coatings have been tested. The results show that the five layer functionally graded coatings appear to have the best thermal shock resistance in the specimens tested, because of the gradual changes in material properties. Higher AE energy counts and cumulative counts recorded by the tests are associated with the macro-crack initiation and growth. On the other hand, micro cracking and phase transformation only give rise to lower AE signals.     

Adhesion improvements of Thermal Barrier Coatings with HVOF thermally sprayed bond coats

Thermal barrier coatings (TBC) are an effective engineering solution for the improvement of in service performance of gas turbines and diesel engine components. The quality and further performance of TBC, likewise all thermally sprayed coatings or any other kind of coating, is strongly dependent on the adhesion between the coating and the substrate as well as the adhesion (or cohesion) between the metallic bond coat and the ceramic top coat layer. The debonding of the ceramic layer or of the bond coat layer will lead to the collapse of the overall thermal barrier system. Though several possible problems can occur in coating application as residual stresses, local or net defects (like pores and cracks), one could say that a satisfactory adhesion is the first and intrinsic need for a good coating. The coating adhesion is also dependent on the pair substrate-coating materials, substrate cleaning and blasting, coating application process, coating application parameters and environmental conditions. In this work, the general characteristics and adhesion properties of thermal barrier coatings (TBCs) having bond coats applied using High Velocity Oxygen Fuel (HVOF) thermal spraying and plasma sprayed ceramic top coats are studied. By using HVOF technique to apply the bond coats, high adherence and high corrosion resistance are expected. Furthermore, due to the characteristics of the spraying process, compressive stresses should be induced to the substrate. The compressive stresses are opposed to the tensile stresses that are typical of coatings applied by plasma spraying and eventually cause delamination of the coating in operational conditions. The evaluation of properties includes the studies of morphology, microstructure, microhardness and adhesive/cohesive resistance. From the obtained results it can be said that the main failure location is in the bond coat/ceramic interface corresponding to the lowest adhesion values.     

Deposition effects of WC particle size on cold sprayed WC-Co coatings

The WC particle size and its influence on the deposition of Co-based cermets are examined. Micron and nanostructured powders with similar Co content were employed. Varying the WC particle size influenced significantly the deposition efficiency of the coating process. Micrometer-structured WC-Co feedstocks did not permit coating build up when processed under comparable or elevated thermal spray parameters used for the nanostructured WC-Co feedstocks. In addition, micrometer-structured WC-Co coatings exhibited a conjoint erosion and deposition effect on the surface. Fine WC particles (< 1 μm) were observed near the substrate interface and larger WC particles (1-2 μm) in the vicinity of the coating surface. These observations indicate the existence of a critical WC particle size for deposition by the cold spray method and that the size criteria arises due to the formation and cohesion mechanisms within the coating layer.Nanostructured test specimens displayed (i) a dense microstructure with little presence of porosity, (ii) a crack free interface between the coating and substrate that indicated good adhesion, and (iii) no observable phase changes. The XRD patterns of each powder and their respective coatings did not have observable peak differences but the diffraction peak broadening of coatings indicated that there was grain refinement during the coating process. Furthermore, all nanostructured as-sprayed WC-Co coatings exhibited Vickers hardness values above HV1000. The nanostructured WC-Co coatings demonstrated adhesive strengths that exceeded the limits of the glue (60 MPa).     

Towards engineering isotropic behaviour of mechanical properties in thermally sprayed ceramic coatings

It is widely recognized by the scientific community that thermal spray coatings exhibit anisotropic behaviour of mechanical properties, e.g., the elastic modulus values of the coating in-plane (i.e., parallel to the substrate surface) or through-thickness (i.e., perpendicular to the substrate surface) will tend to be significantly different due to their anisotropic microstructures. This work shows that thermally sprayed ceramic coatings may exhibit isotropic mechanical behaviour similar to that of bulk materials even when exhibiting the typical anisotropic coating microstructure. Elastic modulus values on the in-plane and through-thickness directions were measured via Knoop indention and laser-ultrasonic techniques on a coating produced via flame spray (FS) using a nanostructured titania (TiO₂) powder. No significant differences were found between the coating directions. In addition, four major cracks with similar lengths were observed originating near or at the corners of Vickers indentation impressions on the coating cross-section (i.e., a typical characteristic of bulk ceramics), instead of two major cracks propagating parallel to the substrate surface, which is normally the case for these types of coatings. It was observed by scanning electron microscopy (SEM) that coatings tended to exhibit an isotropic behaviour when the average length of microcracks within the coating structure oriented perpendicular to the substrate surface was about twice that of the microcracks aligned parallel to the substrate surface. Modelling, based on scalar crack densities of horizontal and vertical cracks, was also used to estimate when thermal spray coatings tend to exhibit isotropic behaviour.     

Residual stresses as a factor in the selection of tungsten carbide coatings for a jet engine application

Tungsten carbide thermal spray coatings are important to the aerospace industry for the mitigation of midspan damper wear on jet engine fan and compressor blades. However, in some cases the coating can fail due to spallation and cracking, and in other situations the fatigue life of a fan or compressor blade is reduced when a coating is applied. Coating failures can result in decreased engine performance and costly maintenance time. A comprehensive experimental research program was conducted to evaluate coating crack resistance in bending, low-cycle fatigue properties of the coating and substrate, coating performance in jet engine tests, and microstructures for a wide range of coating compositions and application processes. Coating residual stress distributions also were evaluated. Eleven coatings were ranked according to their performance relative to the other coatings in each evaluation category. Results from the bend and low-cycle fatigue evaluations were compared to the experimentally evaluated residual stresses. Comparisons of rankings indicate a strong correlation between performance and the residual stresses in the coatings. Results from the program were used to select a suitable coating system for final in-service use based on two important criteria: (1) the coating must not fail while in service, and (2) the coating must not induce crack propagation into the substrate of the midspan damper.     

Sintering characteristics of plasma sprayed zirconia coatings containing different stabilisers

Zirconia powders with different types of stabiliser (Y₂O₃, Dy₂O₃ and Yb₂O₃) have been air plasma sprayed onto metallic substrates. The coatings were detached and dimensional changes during heat treatment were measured by dilatometry. Ytterbia-stabilised specimens exhibited the highest rates of shrinkage, in both in-plane and through-thickness directions. However, it was noted that these specimens had higher initial porosity levels, and a finer microstructure, than coatings containing the other stabilisers. In-plane stiffness and through-thickness thermal conductivity were also measured after different heat treatments. These increased at greater rates for specimens with higher porosity levels (ie the Yb-stabilised coatings). Changes in pore architecture during heat treatments were also studied. Fine scale porosity is rapidly reduced during heat treatment. This correlates with enhanced inter-splat bonding and healing of intra-splat microcracks. In general, the sintering behaviour, and consequent changes in microstructure and properties, appear to be more sensitive to the pore architecture than to stabiliser type. This is correlated with theoretical expectations that it is grain boundary and surface diffusion which will dominate the sintering behaviour, rather than lattice diffusion, and these are more likely to be affected by pore structure, and possibly by the presence of certain types of impurity, than by stabiliser content. It is also noted that thermal cycling appears to retard sintering, at least in terms of the rate of shrinkage. This effect, which could be of practical significance, is briefly discussed.