The Ti-6Al-4V alloy is widely used in aerospace applications for its excellent mechanical properties, however, it presents low wear resistance. It is often coated with a cermet using high-velocity oxy-fuel (HVOF) spraying to improve its wear performance. The Cr3C2-NiCr cermet becomes particularly interesting since it is non-carcinogenic, compared to traditional cermet coatings containing tungsten-cobalt compounds. While the improvement in wear resistance of Ti-6Al-4V with this coating has been demonstrated, its impact on the fatigue performance of the alloy remains to be studied. This is precisely the aim of this study, which focuses on the fatigue life of a Cr3C2-25NiCr-coated Ti-6Al-4V alloy. Among the various influencing factors, surface preparation represents a significant source of crack initiation, particularly in the case of sandblasted surfaces. Indeed, the inclusion of fragmented alumina particles can produce stress concentration zones. Thus, laser texturing, which is a method involving the creation of anchoring points through controlled ablation, can be considered today as a less harmful surface preparation technique. The results obtained from cyclic tensile fatigue tests with a stress ratio of 0.1 for these two surface preparation methods are presented in this paper.

Recent development of plasma spraying with liquid feedstocks enabled deposition of novel thermal barrier coatings (TBCs) with top-coats incorporating various desirable features, such as columnar microstructure or ultrafine porosity. Moreover, different materials may be relatively easily combined, e.g., by alternating feedstocks in the feed line or using feedstock mixtures. Coatings with gradient change of chemistry/microstructure towards the free-surface can be also prepared by gradual change of the feedstock composition, which may be potentially beneficial for example to mitigate stresses at the macroscopic interfaces between TBC layers (typically bond-coat/top-coat or within layered top-coat). In this study, three experimental TBCs with gradient top-coats were successfully deposited by hybrid water/argon-stabilized WSP-H plasma torch, i.e., one version of coating with 8 wt.% yttria stabilized zirconia transitioning into gadolinium zirconate Gd 2 Zr 2 O 7 (YSZ→GZO) and two versions of Al 2 O 3 transitioning into YSZ (Al 2 O 3 →YSZ). Thermal cycling fatigue (TCF) test with peak temperature of 1100 °C showed outstanding thermal shock resistance of the YSZ→GZO coating and mediocre to poor resistance of both Al 2 O 3 →YSZ coatings.

As a critical technology, thermal barrier coatings (TBC) have been used in both aero engines and industrial gas turbines for a few decades, however, the most commonly used MCrAlY bond coats which control air plasma sprayed (APS) TBC lifetime are still deposited by the powders developed in 1980s. This motivates a reconsideration of development of MCrAlY at a fundamental level to understand why the huge efforts in the past three decades has so little impact on industrial application of MCrAlY alloys. Detailed examination of crack trajectories of thermally cycled samples and statistic image analyses of fracture surface of APS TBCs confirmed that APS TBCs predominately fails in top coat. Cracks initiate and propagate along splat boundaries next to interface area. TBC lifetime can be increased by either increasing top coat fracture strength (strain tolerance) or reducing the tensile stress in top coat or both. By focusing on the reduction of tensile stress in top coats, three new bond coat alloys have been designed and developed, and the significant progress in TBC lifetime have been achieved by using new alloys. Extremely high thermal cycle lifetime is attributed to the unique properties of new alloys, such as remarkably lower coefficient of thermal expansion (CTE) and weight fraction of β phase, absence of mixed / spinel oxides, and TGO self repair ability, which cannot be achieved by the existed MCrAlY alloys.

This study presents the results of thermal cycling experiments on thermal barrier coatings deposited using hybrid water/argon-stabilized plasma (WSP-H) torches. Topcoats produced from YSZ suspensions and powders were successfully prepared and evaluated by thermal fatigue testing. Quad-layer coatings with topcoats consisting of yttria stabilized zirconia, gadolinium zirconate, and yttrium aluminum garnet were also prepared and tested at high temperatures and thermal gradients. The results obtained show the potential of WSP-H technology for applications where protection of large components or deposition of thick coatings are required.

In this work, process models combining statistics, materials science, and mechanics were developed to optimize and qualify HVOF sprayed coatings as if for aerospace application. In an attempt to correlate process variables with typical coating properties, a preliminary microstructure model was developed using statistical data obtained from 34 design-of-experiments runs. A second model, based on fatigue testing, was also developed. The fatigue model was subsequently used to guide the selection of variables for a final model that is shown to aid in both process qualification and the optimization of process-operating parameters.

The aim of this study is to evaluate the thermal lifetime properties of yttria-stabilized zirconia (YSZ) coatings with a columnar microstructure. YSZ suspensions were sprayed under different conditions in order to obtain a sample lot with columnar microstructures varying from well-separated to closely spaced. Thermo-cyclic fatigue (TCF) tests were performed at 1100 °C and the results are presented. Coatings with well-separated columns reached 2150 cycles prior to failure compared to 1300 cycles in the case of coatings with compact columns. The apparent lower TCF resistance is attributed to a loss of thermal compliance inducing the development of sharp intercolumnar cracks. Failures seem to be linked to debonding at the TGO-substrate interface. The bond coat and substrate surface roughness also play a role in such failures and their impact is discussed.

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

The effects of bond coat species on the fracture behavior and lifetime performance were investigated in thermal barrier coating (TBC) systems through thermally graded mechanical fatigue (TGMF) tests. Two types of process, air-plasma spray (APS) and high-velocity oxygen fuel (HVOF), were employed to prepare the bond coats of about 300 μm thickness, and then the top coat of about 600 μm thickness was coated on the both bond coats by APS process. The TGMF tests with two tensile loads of 100 and 150 MPa were performed until 900 cycles at a surface temperature of 850 and 1100 °C for a dwell time of 10 min, and then the sample was cooled at the room temperature for 10 min at each cycle. When the tensile load applied in TGMF tests was 100 MPa at 850 °C, the TBC with APS bond coat showed delamination phenomena at the interface between the top and bond coats and small cracks on the surface after about 250 cycles, while the TBC with HVOF bond coat showed a long crack at interface between the top and bond coats without delamination phenomena until 900 cycles. As the tensile load in TGMF tests was increased to 150 MPa at 850 °C, delamination and/or cracks were created at the relatively low cycles, after about 130 and about 279 cycles for the TBCs of the APS and HVOF bond coats, respectively. When the tensile load applied in TGMF tests was 100 MPa and the temperature increase to 1100 °C, the TBC with APS and HVOD bond coats were delaminated after about 65 and about 110 cycles, respectively. These evidences indicate that the TBC with HVOF bond coat is more efficient in improving lifetime performance than that with APS bond coat in the thermal and mechanical environments.

This study assesses the strength and adherence of VPS titanium coatings on ultrahigh molecular weight polyethylene (UHMWPE) substrates. Four-point bend tests show the existence of a critical tensile strain of 1% corresponding to the onset of cracking. For strains up to 6%, crack density increases with no observed debonding. Fatigue tests over 106 cycles reveal that the coating remains uncracked at a strain of 1% and stays in a stable cracked state without debonding as strain is increased to approximately 6%. A laser shock test developed specifically for titanium-polymer interfaces revealed the existence of a debonding threshold corresponding to the adhesion strength. The results serve as a guide for the design of orthopedic implants on which VPS titanium coatings are used and, more generally, open the way for systematic measurement of adhesion between metallic coatings and polymer substrates.

This study investigates the development of fatigue failure in steel specimens coated by various spraying methods with and without grit blasting. Commercial titanium powder was deposited on structural steel substrates by low-pressure and portable cold spray as well as plasma and warm spray. Coating samples were subjected to strain-controlled cyclic bending, while monitoring resonant frequency as a measure of accumulated damage. A change in frequency of 4 Hz was chosen as the test-stop with the corresponding cycle count serving as the main indicator of fatigue life. Test results are presented in the paper along with explanations of fatigue mechanisms and process-related factors.

To understand performance of thermal barrier coatings (TBCs) in various industrial applications of Siemens medium size gas turbines, effects of three types of thermal exposures i.e., high temperature isothermal exposure, thermal cycle fatigue (TCF) test, and burner rig test (BRT) on adhesion strength of an air plasma sprayed (APS) TBC have been studied and reported in this paper. It has been seen that the TBC adhesion strength is influenced by the type of thermal exposures differently. Together with a microscopic examination on TBC microstructures and fractography, a correlation between failure mechanisms and types of thermal exposures is discussed. In addition to the impact of various engine operation conditions on behavior of TBC, impacts of TBC surface roughness on turbine performance have also been evaluated. Surface profile and surface roughness on as-sprayed and polished TBC and cast metal (uncoated) have been measured and two different polishing methods have been compared. As a result, a requirement of TBC surface roughness and a preferable polishing method are suggested.

The thermal cycle test of Al 2 O 3 sprayed 1%Cr-0.5%Mo steel and 18%Cr-8%Ni steel was performed, and the failure type of thermally sprayed coating on both steels was compared. The results obtained are follows. (1)The thermal cycle fatigue life of thermal sprayed 1%Cr- 0.5%Mo steel was shorter than that of thermal sprayed 18%Cr-8%Ni steel. (2)In the case of thermally sprayed 18%Cr-8%Ni steel that is difficult to oxidize at high temperature, during thermal cycle, a tensile stress occurs in a sprayed coating and a compressive stress occurs in a substrate. Thus the thermal strain occurs at the interface of sprayed coating and substrate. The sprayed coating delaminates when the thermal cycles reach the limit value. (3)In the case of thermal sprayed 1%Cr-0.5%Mo steel that is easy to oxidize at high temperature, during thermally cycle, dense oxide film forms at the interface between the sprayed coating and substrate, and adhesive strength in the interface becomes low. Thus the sprayed coating delaminates under the lower thermal strain.

Thermal barrier coatings have got considerable importance for the improvement of gas turbine efficiency. These materials are applied on the surface of gas turbine blades and vanes and are based on a layer of low-oxidation material (mainly MCrAlY alloys, where M stay of Co, Ni or a combination of both) and a ceramic top layer that acts as proper thermal barrier (normally Yttria Partially Stabilized Zirconia). Coating removal is an important aspect in the production of these blades and vanes. “Decoating” or “stripping” is needed during the production of new components as well as for the reconditioning of existing ones. The present paper is dedicated to a new removal method of the ceramic Zirconia layer, based on dry ice blasting. This method will not impact on the roughness and morphology of the bond coat surface, making it suitable for re-coating with TBC, without any further operation before TBC recoating. This possibility has an important impact on the stripping costs and time, avoiding all the operations related to the bond coat. The paper presents the process tests to get the process set up and the characterization of the surfaces comparing the stripped ones with the “original ones” coated by LPPS on new components, ready to be TBC coated. Optical and SEM microscopy, 3D profilometry have been used for characterization. Finally a Thermal Cycling Fatigue test has been carried out in order to validate the procedure of stripping and re-coating.

Plasma sprayed coatings, thanks to their specific structure, are considered for applications with thermal shocks. Alumina, Alumina-Titania as well as less common spraying materials like mullite, forsterite and ilmenite were selected for testing. Carbon steel coupons were used as substrate. Flame-sprayed bond coats of Ni-Cr and Ni-Al under plasma sprayed ceramic coating were applied for comparison in selected cases. Two different regimes were used to simulate high and low temperature thermal cycling. Before the test and after given number of cycles coating's adhesion was tested by a standard tensile test (EN 582). After the test also the microstructure of samples was observed to evaluate the damage mechanism. It has been proved that Titania addition to Alumina improves the adhesion of coatings under thermal cycling, especially for the coatings without any bond coat. All materials had higher adhesion after each given number of cycles with bond coat than without. Cracking occurred dominantly on the boundary with the base in the case of pure Alumina and mullite while a cohesive cleavage is preferred in ilmenite and combination of both types is typical for other tested materials.

Industry faces the continual challenges of operating in less space and accelerating line speed in order to increase productivity, improve quality, and reduce cost. These challenges are particularly evident in the film and papermaking markets. To maintain competitive products in the market place, it is essential to incorporate rollers that are; lightweight, high stiffness, and compact. To satisfy these requirements, rolls made of carbon fiber reinforced plastics (hereinafter referred to CFRP) have proven effective and have come into wide use in many industrial fields. CFRP rolls are the structural materials that were developed mainly to improve the Young's modulus of rolls. Guiding and transfer rollers frequently require surface features such as non-adhesion and wear resistance. The typical surface treatments to CFRP currently employed are; painting, plating, and rubber lining or the like, but these treatments have their limits in the manufacturing process and effectiveness. Consequently, we have developed the technology to provide rolls with the desired surface by spraying various metals, ceramics, or cermets, directly on CFRP substrate. These sprayed rolls have been intensely monitored in actual operations to insure high speed capability and longevity. In this presentation, we will talk about the rotary bending fatigue test of CFRP rolls on which the carbide cermet was plasma-sprayed and the quality stability of the roll with the two-layer composites that was confirmed based on the test results.

Cyclic, mechanical fatigue testing of coated materials has increased with the wider use of thermal spray coatings on components experiencing fatigue loading. Fatigue testing of coated specimens presents all of the difficulties associated with fatigue testing of uncoated specimens and several difficulties that do not arise when testing uncoated specimens. A summary of fatigue test methods and test specimen geometries for both coated and uncoated specimens is presented. Issues of specimen standardization, geometry, substrate preparation, and post-spray surface finishing are discussed. Several specimen configurations are described.

Fatigue and delamination resistance of overlay coatings is critical to their performance in tribological applications involving Hertzian loading. This study addresses the influence of coating thickness and contact stress fields on the fatigue/delamination resistance of thermal spray (WC-12%Co) coatings, deposited by a JP-5000 system. These coatings were deposited in three different thicknesses on the surface of 440-C steel substrate cones. Fatigue tests were conducted using a modified four-ball machine under various tribological conditions of contact stress and configuration. Results are discussed in terms of Hertzian contact stress fields, coating thickness and Scanning Electron Microscope (SEM) observations to comprehend the performance and ascertain the fatigue failure modes of coated rolling elements. These results indicate that by appropriate control of coating thickness and tribological conditions, it is possible to achieve a fatigue life in excess of 70 million stress cycles without failure. Further studies in this field can thus trigger an area of new novel applications of thermal spray coatings.

One candidate alternative to chrome plating and hard anodizing is a tungsten carbide (WC) coating applied by the High Velocity Oxy-Fuel (HVOF) process. HVOF WC coatings are currently being evaluated in many service life tests, including fatigue. The purpose of this paper is to compare the fatigue life of HVOF WC coated specimens with the fatigue life of hard anodized and bare aluminum specimens. This work examines WC thermal spray coatings as candidates for replacement of hard chrome plating and hard anodizing in aircraft and helicopter applications such as landing gear. In fatigue testing, the results showed an expected fatigue deficit for hard anodizing as compared to bare aluminum. Paper includes a German-language abstract.

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

This article presents one of the most advanced technologies for the application of powder coatings, the detonation spray method. The devices for measuring the temperature and the velocities of the powder particles are presented and the results obtained are shown. The article deals with results from wear tests as well as from fatigue strength tests. The tests are carried out on the coatings of the Ni-Cr-B-Si type applied to C45 steel. In addition, X-ray examinations, linear distribution of the chemical elements, microhardness measurements and metallographic examinations are carried out. The coating properties of the detonation-sprayed layers are compared with the same coatings, which were applied by means of plasma or flame spraying. Paper includes a German-language abstract.