This study investigates the effects of plasma spray parameters on the microstructure and porosity of yttria-stabilized zirconia coatings. In the experiments, the torch was held in place perpendicular to the substrate to produce cone or stalagmite shaped deposits. Stand-off distance (80, 90, 120 mm) and spray time (15, 30, 60 s) were varied to assess their effect on microstructure-property relationships as well as particle temperature, velocity, and impact angle. The shapes of the deposits were recorded using a CMM and surface topography and roughness were measured with a 3D optical profiler.

A primary goal in modern orthopaedics is increasing the rate and long-lasting anchoring of implants in the human body. Hydroxyapatite, having a chemical structure and chemistry that is identical to bone, significantly enhances the on growth and subsequent ingrowth of natural bone material. Nevertheless some aspects of the performance of hydroxyapatite coatings have not been investigated sufficiently; for example the in vivo, long-term behaviour of the material, including the time-dependent dissolution accompanied by changes of mechanical properties have been poorly documented. The current study creates an idealized, virtual microstructural coating model that examines the time-dependent behaviour and properties of hydroxyapatite coatings. The analyses examine time vs. dissolution dependencies that reflect in vivo behaviour.

Nanostructured and conventional Alumina–13wt.% Titania powders were thermally sprayed using air plasma spray(APS) process. Scanning electron microscopy (SEM) was used to examine the morphology of the agglomerated powders and the cross section of the alumina-titania coatings. The microstructure and phase composition of the coatings were characterized by X-ray diffraction (XRD), and scanning electron microscopy (SEM).The fatigue and mechanical properties of the coatings were investigated. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess the mechanisms of deformations. The experimental data indicated that the nanostructured coated samples exhibited higher stiffness, hardness, and fatigue strength compared to the conventional coated samples.

Parametric drifts and fluctuations occur during plasma spraying. These drifts and fluctuations originate primarily from electrode wear and intrinsic plasma jet instabilities. One challenge is to control the manufacturing process by identifying the parameter interdependencies, correlations and individual effects on the in-flight particle characteristics. Such control is needed through methods that (i) consider the interdependencies that influence process variability and that also (ii) quantify the processing parameter-process response relationships. Artificial intelligence is proposed for thermal spray applications. The specific case of predicting plasma power parameters to manufacture grey alumina (Al 2 O 3 -TiO 2 , 13% by wt.) coatings was considered and the influence of the plasma spray process on the in-flight particle characteristics was investigated.

Stresses developed within a thermal barrier coating (TBC) due to the mismatch in thermal expansion of different coating components can cause coating failure. Nanostructured materials have an increased volume fraction of grain boundaries and this microstructural attribute may allow coatings to relieve the strain in the coating structure thereby improving the effectiveness and the lifetime of the TBC. Multi – layered TBCs were prepared using two techniques; atmospheric pressure plasma spray (APS) using a commercial system, and reduced pressure plasma spray using the Triple Torch Plasma Reactor (TTPR). The coatings were deposited on mullite and on NiCrAlY-coated steel substrates, and consisted of an inter – layer of nano-phase partially stabilized zirconia (n – PSZ) and a layer of conventional partially yttria stabilized zirconia coating (c – YSZ) as the top thermal barrier coat. The coatings were heat treated at various temperatures and the microstructural changes analyzed using scanning electron microscopy (SEM) images. Mechanical properties of the coating were studied using four point bend testing to better understand the effect of the n-PSZ inter-layer on the strain relief mechanisms that may be operative within the TBC.

Polymeric coatings manufactured by thermal spray processes exhibit variable mechanical and adhesion properties that depend on their exact processing schedules. One important advantage of these coatings is that they can be readily repaired by re-spraying any delaminated or otherwise defective regions. In some instances the repaired region exhibits better mechanical attributes than the original coating. In this study the repairability of several classes of polymeric and polymer-ceramic composite coatings were investigated with a focus on the interfacial adhesion properties. The coatings include those of monolayer and bilayer ethylene methacrylic acid (EMAA), and CaCO 3 -EMAA composites. The coating thickness did not influence the interfacial adhesive strength between the coating and substrate; while a higher preheat temperature produced a greater interfacial cohesion for the monolayer coating on a metal substrate. The substrate preheat temperature played a dominant role concerning the peel strength of the coating. Greater peel strengths were achieved between polymers, at least twofold greater than that between the polymer and the steel substrate when the pre-heat temperature was greater than the melting point of the polymer. The peel strength of the composite coating decreased with filler content; both on the steel substrate and on a previously sprayed polymer coating. On the basis of these observations, the adhesion mechanism between polymers was explained with a model that relied on the formation of welding points.

Multilayer thermal barrier coatings are being investigated for high temperature applications by employing a strain accommodating interlayer. Plasma sprayed coatings of nanostructured feedstock have shown promise in this direction. Layers of nanostructured yttria stabilized zirconia (nano–YSZ) and conventional YSZ were deposited on mullite substrates using the triple torch plasma reactor (TTPR), and on NiCrAlY coated steel substrates using the Praxair SG-100 plasma torch. The coatings were heat treated and the microstructure evaluated. Heat treating the samples lead to the formation of larger pores with a significant proportion of partially molten particles. The porosity evolved from the partial sintering of the nano-agglomerates. Porosity change during the sintering process was measured and the microstructure observed using electron microscopy. The nanostructured coatings were compared to conventional YSZ coatings.

Phase composition control is a prime concern for plasma sprayed hydroxyapatite [Ca10(PO4)6(OH)2; i.e., HA] coatings due to the complexity of both HA structure and plasma spray process. The present study aims to better understand the phase formation mechanism in the HA coating through compositional, structural and microstructural studies of HA coatings obtained from various spraying processes. A process model was established by considering both a single HA splat formation and coating buildup processes. Three HA recrystallization mechanisms were proposed on the basis of the temperature-time experiences of particles, their cooling rates, and the heat and hydroxyl accumulation during coating formation. The model explained very well the experimental results. It was concluded that crystallinity alone was not capable of reflecting the coating composition due to the existence of various portions of crystalline HA; i.e., unmelted, recrystallized and dehydroxylated HA, as well as the gradient compositional structure from the coating interface to the surface. Some newly formed nanocrystalline regions were also revealed in the coating microstructure.

The plasma spray process is unique because the coating properties depend indirectly on the processing parameters. Therefore, the in-service properties derive mostly from the coating architecture. The coating architecture is related to the intrinsic lamellae formation mechanism; which is related to the impinging particle characteristics that are linked to the power and feedstock injection. The coating architecture is also related to the extrinsic spray pattern formation mechanism; i.e., the kinematics and geometric spray parameters such as spray velocity, spray angle, and stand off distance. Understanding relationships between coating architecture and its properties requires consideration of the whole coating formation process. Ultimately, this permits selection of process parameters that demonstrate enhanced spray efficiencies and manufacturing capability. Included in this study are the pore network architecture and residual stress level because they play important roles in coating cohesion from which derive most of the in-service properties. Coating manufacturing mechanisms, from the spray pattern to an actual coating formed by several successive patterns, are investigated in this paper. The case of atmospheric plasma sprayed Al 2 O 3 -TiO 2 (13% by wt.) is considered. Data are statistically assessed by implementing Gaussian and Weibull analyses. The critical role of pores which develop within the spray pattern and between successive patterns is examined in detail. There is contamination between layers and the intrinsic roughness of the coating is altered during the spray process. In summary, within this work, it is intended to define the intrinsic and extrinsic operational variables that contribute to the coating architecture and, thereby, suggest technology that can be implemented to improve coating quality and deposition efficiency.

Nanostructured and conventional titania (TiO 2 ) powders were thermally sprayed using APS and HVOF processes. The fatigue and mechanical properties of the coatings investigated. Coatings were characterized using SEM to investigate the microstructural features and Vickers indentation to determine the hardness. SEM analyses were also carried out on the fracture surfaces of fatigue-tested samples to assess crack nucleation and to study the mechanisms of deformations. The fatigue strength of coatings deposited onto low-carbon steel (AISI 1018) showed that the nanostructured titania coated specimens exhibited significantly higher fatigue strength compared to the conventional titania

Applying an environmental barrier coating (EBC) and a thermal barrier coating (TBC) on the next generation gas turbine structural materials such as silicon carbide matrix composites will lead to large stresses due to thermal expansion mismatch; thereby limiting the coating's effectiveness and lifetime. Nanostructured materials possess a large volume fraction of grain boundaries and are conjectured to partially relieve the strain in the coating structure. A Triple Torch Plasma Reactor (TTPR) was used to spray multi-layered TBCs consisting of a mullite EBC deposited either on a silicon carbide or a mullite substrate, a nano-phase partially stabilized zirconia coating (n- PSZ), and a yttria stabilized zirconia coating (YSZ) as the TBC. The nanostructure of the n-PSZ could be maintained during the deposition process. The coatings were heat treated at 1300°C and the change in microstructure and mechanical properties were analyzed using scanning electron microscopy (SEM), micro-indentation and scratch testing applied to the coating cross section. While a change in the microstructure was observed, in particular grain growth, the hardness and elastic modulus appeared to be little affected by the heat treatment giving a preliminary validation of the multilayer concept.

Nanostructured materials and nanocomposites have the potential to have a dramatic impact on technological progress in the 21st century. Of great interest, is the outstanding deformation behavior of nanostructured materials and nanocomposites. In this study, the fatigue and mechanical properties of HVOF sprayed nanostructured and conventional WC-Co coatings were investigated. High velocity oxy-fuel process was used to spray the WC-12Co from a feedstock in which the WC phase was mainly in the micron size range (conventional) or contains a significant fraction of nanosized grains (multimodal).The fatigue strength of coatings deposited onto Aluminum alloy showed that the nanostructured WC-Co coated specimens exhibited higher fatigue strength compared to the conventional WC-Co coatings. Microhardness test was performed. Relationships between the microstructures and mechanical properties were discussed.

Environmental problems of hard chrome plating are raising its cost and shrinking availability. HVOF and Detonation spray technologies for application of tungsten carbide and chrome carbide based coatings have proved to be cleaner and more effective than chrome plating. In this paper, the results of fatigue tests for WC-17Co coating deposited onto AISI 4340 steel by HVOF are compared to those for hard chrome plating. The fatigue life distributions as a function of the probability of failure for the coated AISI 4340 steel specimens showed that the HVOF coated specimens exhibited extraordinarily higher fatigue lives compared to the uncoated specimens whereas the hard-chrome-plated imparted fatigue strength degradation to the AISI 4340 steel.

The chemical and physical properties of thermally sprayed coatings depend in part on surface characteristics, such as texture and roughness, that can be difficult to quantify. This paper demonstrates the use of fractal analysis for quantifying surface roughness based on images obtained from cross-sectioned samples. An isolated profile of an Al 2 O 3 -TiO 2 coating on an aluminum substrate is used as the test case. The first part of the paper covers the basic concept and the procedures used. The second part presents practical examples and precautions that must be observed to obtain accurate and reproducible results. Paper includes a German-language abstract.

Hydroxyapatite/polymer composite coatings of different volume ratios were produced using a Plastic Flame Spray (PFS) system. The intent of this processing is to obtain a coating with an optimal combination of biological and mechanical properties of these two materials for skeletal implants. The composite coatings were produced with a mechanical blend of EMMA and hydroxyapatite powder from a fluidized bed powder feeder. Characterization was conducted by scanning electron microscopy on the surface morphology, polished cross-sections and fracture surface morphology of the coatings. The bioactivity of the coatings was evaluated with a calcium ion meter, and the stress-strain behavior was investigated by tensile testing. The biological and mechanical properties were found to be related to the volume and the distribution of the hydroxyapatite in the polymer matrix.

The cold spray process was used to prepare nanostructured WC-Co coatings. The coating microstructural characteristics and phase composition were analyzed via optical microscopy, SEM and XRD. The morphology and microstructure of the nanostructured WC-Co powder were also analyzed by SEM and XRD. A 10µm thick coating was achieved. The results show that there is no degradation of the WC-Co powder during the cold spray process and well-bonded and phase-pure WC coating can be produced by the cold spray process.

Yttria partially stabilized zirconia (YSZ) was atmosphere plasma sprayed on to mild steel substrates. The spray parameters were varied to determine their effects on the elastic modulus of the coating. The parameters were (i) continuous spray vs. paused spray, (ii) bond coat vs. no bond coat, and (iii) cooled vs. not-cooled. The elastic modulus was measured using laser ultrasonics and Knoop Indentation. Using indentation, the continuous/ paused spray exhibited the greatest effect with the paused spray samples having a lower elastic modulus value regardless of the condition of the other parameters. The other parameters did not reveal any statistically significant effect. The laser ultrasonics measurements showed that cooling and no-cooling had a greater effect on elastic modulus, with the other parameters having little effect. Laser ultrasonics detected parameters whose influence can be detected near the surface (in this work the cooling and no-cooling), but did not detect those parameters that influence the properties throughout the coating. Indentation detected the parameters that influence the properties throughout the coating, in this work continuous and paused spraying.

This work examines nanocrystalline thermal spray coatings prepared under several power and stand off distances. The coating material was nanocrystalline alumina + titania and zirconia (ATZ) powder mixture deposited using an air plasma spray process. The experimental design employs a full factorial method totaling four samples. The mechanical properties of four coatings were compared via scratch and hardness testing. The scratching process was monitored for transverse loads during the test and the variation in load monitored. The profiles of scratches were analyzed using white light interference profilometry and studied further via SEM. The results indicate that the microstructure reflected features observed in the scratch test. It was also shown that the scratch process and morphology depend on the normal load and spray process.

The technology of thermal spray will reach its centenary anniversary in the first decade of the 21 st century. It is appropriate to summarize the early literature of the 1900's and present, for posterity, several of the key publications upon which this vital technology rests. This article is broken up into three sections; (i) a summary of early US and UK patent literature is presented in tabular format along with a brief commentary; (ii) several articles (prior to 1915) and books (prior to 1970) from the early literature are mentioned; and (iii) three Appendices reproduce those patents which are indicated as "Master Patents" for thermal spray and which spear-headed its art and science. This article is intended to guide the interested reader towards primary sources of literature upon which present thermal spray science and engineering is based.

Calcined spray-dried hydroxyapatite (Ca10(PO4)(OH)6; i.e., HA) powders were atmospherically plasma sprayed (APS) using various process parameters. The resulting phases within the coating surface and the interface between the coating and the substrate were analyzed using X-ray diffraction (XRD) methods. This XRD revealed the presence of both amorphous (i.e., amorphous calcium phosphate: ACP) and crystalline phases. The crystalline phases included both HA and some impurity phases from the decomposition of HA, such as tricalcium phosphate (α-TCP and β-TCP), tetracalcium phosphate (TTCP) and calcium oxide (CaO). The crystallinity of HA decreased with increasing spray power and stand-off distance (SOD). The percentage of all impurity phases increased with the spray power. The percentage of both TCP and TTCP decreased with the SOD while the CaO percentage increased. In addition, the percentage of ACP and CaO were higher in the interface than at the surface of the coating while the percentage of TCP and TTCP exhibited the opposite effect.