The crack propagation characteristics obtained by observing the microstructure of bent coatings at different stages of applied strain were conducted. At the same time, acoustic emission (AE) features of released events were recorded. Generated cracks were found to emit weak acoustic emission events at the initiation (creation) of the crack at the coating surface, and then become strong releasing a uniform acoustic emission energy per event, when propagating transversally in the coating. However, cracks propagating at the interface with the substrate could generate both weak and strong events. It was also found that increasing the thickness of the coating leads to a decrease in the number of events and an increase of their released acoustic emission energy.

Plasma Facing Materials (PFMs) suffer from very high heat load including quasi-stationary high heat load during normal operation and transient events with extremely high heat load during normal plasma operation and off-normal events. In this paper, W/Cu functional gradient coating was applied on CuCrZr substrate (250mm × 120mm × 30mm) with compositionally gradient W/Cu as bond coat (0.4-0.6 mm) and 1.5 mm thickness W coating as top coat via VPS for continuous deposition duration of 5 h. VPS-W/CuCrZr mokeup with built-in cooling channel was prepared for evaluating the transient vertical displacement and plasma disruption events applied by high energy electron beam. The formation of cracks and surface melting of VPS W/Cu mokeup were investigated under the two transient high heat loads (HHL). The coatings were able to absorb about 2 MJ/m2 in HHL without significant damage.

Four-point bend test using acoustic emission (AE) was used to compare coating properties under mechanical solicitation, mainly the toughness and the spalling behaviour. Coatings are made from the same material; the 2005NS (WC-17Co), sprayed with an HVOF gun at various spray parameters. Coatings deposited on thin rectangular substrates were tested with the coating bent in tension and in compression. AE features like the event number, energy per event and cumulative energy were used to assess the damage in the coatings. The results are analyzed in relation with the coating microstructure.

Microscopic fracture mechanisms of thermal spray coatings under bending stress are investigated. Samples of thermally sprayed coatings were made using three distances. The sprayed powder was pure molybdenum. Vertical microcracks occur in lamellae and subsequently, these cracks join together and form vertical macrocracks in the samples sprayed with a short spraying distance. On the other hand, horizontal microcracks occur at the lamellae interfaces, and these cracks link together in the samples sprayed with a long spraying distance. These tendencies can be explained in terms of the hardness of the lamella and the bonding strength between each lamella. It is clarified that the bonding strength between each lamella corresponds to the applied strain at the point of rapid increase of the acoustic emission (AE) event. The amplitude and rate of AE beyond the point of rapid increase are high in the coatings which formed macrocracks. It is concluded that the coating which has high resistance to crack formation has a high point of AE increase, low AE amplitude and low AE increasing rate.

This research examined the use of 4-point bend testing, with in-situ acoustic emission analysis, for the characterization of the deformation of WC-Co coatings sprayed by the high-velocity oxy-fuel method. Coatings were deposited from WC- 17wt.%Co powders using a Praxair JP5000 system. Two sets of gun operating parameters were employed to produce coatings with distinctly different structures. In the present studies the cracking behaviour of the coatings was investigated through analysis of the acoustic emission (AE) data. In particular, AE data recorded has shown clear differences between the two coating types, in terms of the critical strain level and amount of energy released during cracking. The critical strain levels for the different coatings were 0.32% and 0.6% respectively. Analysis of the coatings by scanning electron microscopy and X-ray diffraction has allowed correlations to be made between the principle microstructural features of the coatings and the mechanical behaviour under bending. The AE responses for these cermet coatings were highly reproducible.

The aim of this work is to study the adhesion of WC-Co coatings using acoustic emission testing. The coatings were produced by HVOF spraying then subjected to four-point bending while in situ acoustic emission measurements were made. The test specimens were then examined under a microscope, revealing regularly spaced transverse cracks on the coating surface as well as interfacial cracking. An analysis of the acoustic emission data revealed two types of acoustic events based on absolute energy and amplitude. This appears to be linked to the cracks that were observed forming the basis for an interfacial cracking model.

In this paper, plasma sprayed thermal barrier coatings (TBCs) with and without bond coat are stressed to various stress levels under four point bending with in situ acoustic emission (AE) to monitor any cracking activities. Micro- and macro-cracks occurring during the tests are investigated to better understand the failure mechanisms of TBCs. The results show that limited AE activities were detected in the first four stress-relief cycles, while plastic deformation and the greatest AE activity were observed when the applied load exceeded the yield point. In addition, they show that a TBC system that contained an adhesive layer had less AE activity (cracking events) than the TBC without an adhesive layer. In addition, the samples tested at a main speed of 5 micrometer/s resulted in a higher AE activity than the samples tested at 10 micrometer/s. With increasing plastic deformation, macro cracks and surface cracks also occurred. Paper includes a German-language abstract.

Preface for the International Thermal Spray Conference 2002. Includes a publishing history for proceedings from previous thermal spray conference events.

Preface for the National Thermal Spray Conference 1996. Includes a publishing history for proceedings from previous thermal spray conference events.

Preface for the United Thermal Spray Conference 1997. Includes a publishing history for proceedings from previous thermal spray conference events.

The Electromagnetic Powder Deposition (EPD) process converts pulsed electrical energy into kinetic and thermal energy to accelerate and heat powder material to conditions suitable for bonding. A high pressure plasma armature is electromagnetically accelerated using a railgun. A supersonic pressure wave is created when the armature accelerates through and "snowplows" the ambient gas ahead of it. The gas column is heated, compressed, and accelerated to the entrainment section of the gun, where some of the thermal and kinetic energy is transferred to an injected stream of powder material. The acceleration burst is repeated rapidly to supply the required deposition rate and to achieve steady thermal conditions. Development of a starter plasma which is reliable at ambient pressure was a major programmatic task. Generation of a low pressure linear arc required to form a planar armature during the pulsed event was investigated. Several geometries (point-to-point breakdown, rail-to-rail breakdown, and confined glow discharge) were explored using different voltage sources (dc, 60 Hz ac, 150 MHz rf, pulsed). Satisfactory operation of the confined glow discharge approach at atmospheric pressure was achieved using rf excitation. Results of testing under the various scenarios are presented and critiqued.

A summary of the efforts involved in organizing the 1998 thermal spray conference and proceedings. Also includes a publishing history for proceedings from previous events.

Preface for the International Thermal Spray Conference 2000. Includes a publishing history for proceedings from previous thermal spray conference events.

Preface for the International Thermal Spray Conference 2003. Also includes a list of the editorial committee members and a publishing history for proceedings from previous thermal spray conference events.

Most of the early applications of thermal spray coatings were focused towards providing a remedy to excessive wear degradation. However, as the introduction of such coatings into wider industrial sections increases there is also exposure to other potential degradation processes - aqueous corrosion is one such process. The complex microstructures in cermet coatings have been shown to translate to complex modes of corrosion attack. In this paper an electrochemical test methodology to probe the local/micro aspects of corrosion initiation and propagation will be described. A new electrochemical cell has been devised in which the corrosion can be followed `live` and in `real time`. The surface is subjected to in-situ imaging by Atomic Force Microscopy (AFM). This paper reports some of the exciting findings which have enabled the sequence of corrosion events in cermet coatings to be defined. The study has revealed that a precursor to coating degradation is the attack of the Co-base matrix. This sets up an environment which can catalyse WC dissolution. The practical and fundamental importance of the results will be discussed.

The adhesion of splats formed by impact of molten metal droplets was studied experimentally. Tin droplets (550 µm diameter) were produced using a drop-on-demand generator. To achieve high impact velocities the stainless steel coupons used as substrates were mounted on the rim of a rotating flywheel and heated using cartridge heaters. To hit a falling droplet with the substrate and photograph its impact, a timing circuit was used to synchronize three events with the position of the substrate: ejection of a droplet, triggering of the camera and a flash to provide illumination. The impact velocity was varied from 10 – 40 m/s whereas the substrate average roughness (2.0 µm) and the droplet diameter (~550 µm) were kept constant. We measured the adhesion strength of splats by a simple pull test. A wire was attached to the upper surface of each splat using epoxy and the force required to separate the splat from the substrate was recorded. A significant increase in adhesion strength was observed as the impact velocity was increased. Coatings were produced by depositing many droplets sequentially. Substrate temperature and impact velocitiy were the main parameters varied. SEM images of cross-sections through coatings showed that increasing impact velocity and substrate temperature produced better adhesion between the coating and substrate.

Somewhat unconventional plasma sprayed TBC systems were produced and evaluated by interrupted or cyclic furnace oxidation life testing. Approximately 250 µm thick 8YSZ coatings were directly sprayed onto grit blasted surfaces of PWA 1484, without a bond coat, in order to take advantage of the excellent oxidation resistance of this superalloy. For nominal sulfur contents of 1 ppmw, total coating separation took place at relatively short times (200 hr at 1100oC). Reductions in the sulfur content, by melt desulfurization commercially (0.3 ppmw) or by hydrogen annealing in the lab (0.01 ppmw), improved scale adhesion and extended life appreciably, by factors of 5-10. However, edge-initiated failure persisted, producing massive delamination as one sheet of coating. To subvert this mechanism, samples surfaces of melt desulfurized PWA 1484 were EDM’ed with a grid of grooves or ribs (~250 µm wide and high), resulting in a segmented TBC surface macrostructure. Now failure only occurred as independent single segments events. For grooved samples, 1100 C segment life was extended to ~1000 hr for 5 mm wide segments, with no failure observed out to 2000 hr for segments ≤ 2.5 mm wide. Ribbed samples were even more durable, and segments ≤ 6 mm remained intact for 2000 hr. Larger segments failed by buckling at times inversely related to the segment width and decreased by oxidative effects at higher temperatures. This critical buckling size was consistent with that predicted for elastic buckling of a TBC plate subject to thermal expansion mismatch stresses. Thus, low sulfur substrates demonstrate appreciable coating lives without a bond coat, while rib segmenting extends life considerably.

Preface for the International Thermal Spray Conference 2001. Includes a publishing history for proceedings from previous thermal spray conference events.

The acoustic emission (AE) technique, when used with the four-point bend test, is potentially an effective method for characterization of the ductility and fracture behaviour of certain thermally sprayed coatings. Monitoring of AE during such tests on brittle coatings reveals that the rate of the occurrence of events with increasing load changes gradually over the duration of the test. In this work, finite element modelling (FEM) of a brittle coating on an elastic substrate in four-point bending is used to analyze the fracture behaviour of the coating and predict its AE behaviour. The results from the FE analysis show good agreement with experimental data from four-point bending of thermally sprayed WC-Co coatings.

In this study, a new laser based technique was evaluated for the characterization of plasma-sprayed oxide coatings. It uses the contactless laser generation and detection of ultrasonic waves in the bi-layered systems. For this purpose, a nanosecond pulsed Nd:YAG laser (λ : 1064 nm, τ =14 ns) was used for irradiating the ceramic coating, whilst the longitudinal displacements of the rear surface of the metallic substrate were detected at the epicenter using a laser heterodyne interferometer. The acoustic signal recorded at the rear surface of the substrate was found to be characteristic of the different events taking place within the irradiated system. In this way, the longitudinal wave velocity, the porosity, as well as the Young's modulus of the coatings can be easily determined, whilst the coating/ substrate adhesion strength can be calculated, taking into account both the thermal, as well as the acoustic effects of the laser radiation. The proposed technique was applied to alumina coatings deposited onto stainless steel coupons by Atmospheric Plasma Spraying and the results were found to be in accordance with those obtained by the techniques commonly used for testing thermal spray coatings (interfacial indentation test, porosity measurement, etc.).