Bond-coating in plasma-sprayed calcium-phosphate coatings

The influence of bond–coating on the mechanical properties of plasma-spray coatings of hydroxyatite on Ti was investigated. Plasma-spray powder was produced from human teeth enamel and dentine. Before processing the main apatite coating, a very thin layer of Al₂O₃/TiO₂ was applied on super clean and roughened, by Al₂O₃ blasting, Ti surface as bond-coating. The experimental results showed that bond-coating caused significant increase of the mechanical properties of the coating layer: In the case of the enamel powder from 6.66 MPa of the simple coating to 9.71 MPa for the bond-coating and in the case of the dentine powder from 6.27 MPa to 7.84 MPa, respectively. Both tooth derived powders feature high thermal stability likely due to their relatively high content of fluorine. Therefore, F-rich apatites, such those investigated in this study, emerge themselves as superior candidate materials for calcium phosphate coatings of producing medical devices. The methods of apatite powder production and shaping optimization of powder particles are both key factors of a successful coating. The methods used in this study can be adopted as handy, inexpensive and reliable ways to produce high quality of powders for plasma spray purposes. An erratum to this article is available at .     

Characteristics of gas-tunnel plasma-sprayed coatings

The quality of Al₂O₃ coatings formed by gas-tunnel plasma-spraying apparatus has been evaluated with respect to hardness and porosity under various spraying conditions. The deposit characteristics of Al₂O₃ powder sprayed onto a substrate by this type of plasma spraying were also studied and the relation between the deposit characteristics and the spraying distance was investigated. Estimation of the deposit characteristics was found to be a very effective method for determining the proper spraying conditions under which a good quality coating can be obtained.     

Dissolution behavior of plasma-sprayed hydroxyapatite coatings

The long-term stability of plasma-sprayed hydroxyapatite coatings is influenced by the dissolution behavior of the coating in in vivo conditions. Plasma-spraying generates a mixture of phases and this study has focused on how the balance of phases affects the in vitro dissolution behavior of the coatings in double distilled-deionized water and in tris-buffer solutions. The pH changes in double distilled-deionized water were monitored, whilst the pH value was maintained at 7.25 for the tris-buffer solution at 37 °C with 5% CO₂atmosphere. The phosphate and calcium ions released were measured using UV-Visible Spectrophotometer and Atomic Absorption Spectroscopy respectively. Changes in crystal and surface topology were also studied. The results indicate that the dissolution behavior of the coatings depends on several factors. The rate of release of phosphate ions was found to increase significantly for the tris-buffer solution compared to the deionized water, indicating that the presence of electrolyte constituents affects the dissolution behavior of the coatings. The Ca/P ratio in the tris-buffer solution is approximately three. Increases in the level of crystallinity of the coatings significantly decreased the dissolution rate and hence, the amount of phosphate ions released. The higher the percentage of crystallinity, the higher the stability of the coating under in vitro conditions.     

Structural characterization of plasma-sprayed hydroxylapatite coatings

Plasma-sprayed hydroxylapatite coatings, widely used on metallic surgical implants to improve their adhesion to bone, are formed by rapid quenching of molten, or partly molten, particles which impact the substrate at high velocity. the performance of these coatings in the body depends upon their structure, which is not well understood. Coatings prepared under a range of spraying conditions have been studied by X-ray diffraction (XRD). differential thermal analysis (DTA), thermogravimetric analysis (TGA) and solid-state nuclear magnetic resonance spectroscopy (NMR). The results suggest that particles partly melt and lose combined water at lower plasma torch input powers forming a glass, by quenching of the liquid phase, and an OH-depleted hydroxylapatite residual crystalline phase. At higher power inputs an increasing amount of P₂O₅ is also lost and the coatings contain CaO and Ca₄P₂O₉. Heat treatment of coatings in air at 600°C results in crystallization of the glass phase and reaction with water vapour to form hydroxylapatite. The results show that XRD is relatively insensitive to some of the structural details of hydroxylapatite coatings which may be significant to their performance. NMR provides more structural information and is a significant tool for coating characterization.Sadly Professor McPherson died after completing this paper. He made a distinctive contribution to the fields of ceramics and thermal spraying and will be missed by the international community.     

Microstructural investigation of plasma-sprayed aluminum coatings

Although plasma-sprayed coatings have been used extensively in industry, limited studies have been carried out on the microstructural details. In this program scanning and transmission electron microscopy, combined with other techniques, were used to examine the grain morphology in plasma-flame-sprayed aluminum coatings.In plasma spraying, the initial particles which impinge onto the cold substrate solidify at rates comparable with those obtained by splat quenching. Since such particles are responsible for the bonding between the coating and the substrate, the first layer has received the central emphasis in this study. Because subsequent layers experience lower cooling rates, a non-uniform grain morphology is obtained which is substantially affected by the particle morphology.The topography of the surface on which the particles impinge is shown to play a decisive role with respect to the orientation of the grains within each particle. These morphological details are considered in the light of models proposed for splat quenching. Another important factor in grain morphology is the position of the particle within the coating. For aluminum, the layers adjacent to the substrate contain highly oriented elongated grains, with the longitudinal axis perpendicular to the substrate. However, because of heating by the flame itself, the particles located away from the substrate are composed of large, equiaxed and randomly oriented grains.Finally, the reaction of the molten droplets with the plasma effluent and the surrounding environment is considered. Oxidation and porosity result when aluminum is sprayed in air. The oxidation of the particles results in a thin layer of oxide which (i) hinders particle-particle interaction and thus results in the formation of interparticle pores and (ii) provides a path for crack propagation and failure, which can occur by delamination. The pore structure of the coating was investigated and correlated with fractography.     

Use of corrosion-resistant plasma-sprayed coatings in diesel engines

The aim of this investigation was to study corrosion and thermal shock resistance and thermal barriers to be used on heat-loaded parts in diesel engines.The coatings which were tested were well bonded to the base material. The corrosion rates of these coatings were somewhat different however. A nickel-chromium-aluminum-yttrium alloy proved to be the best alloy coating as far as corrosion resistance was concwrned. A nickel-chromium-aluminum coating with minor additions of oxides showed approximately the same corrosion rate as the pure alloy coating. Coatings of the same type of alloy but with an intermediate layer of chromium and a top coat of yttrium-stabilized zirconia showed no signs of corrosion or cracks even after 120 h of combustion rig testing.A detailed discussion of the results from tests of coated piston crowns and valves in ship engines in service is presented. The test periods were up to 30 000 h for piston crowns and up to 5000 h for valves.     

Thermal expansion study of plasma-sprayed oxide coatings

The thermal expansion characteristics of overlay coatings is of considerable importance when the coated parts are exposed to thermal cycling. In this study we investigated the thermal cycling behavior of oxide coatings plasma sprayed onto mild steel. Thermal expansion measurements were carried out on Al₂O₃TiO₂ and zirconia coatings under different conditions of coating-substrate adherence and on coatings that were substrate free. A study was initiated of the effects on the expansion coefficient of variations in composition, porosity, heating rate and thermal cycling. In addition, metallographic and acoustic emission methods were employed to examine the compatibility of these oxide coatings with steel substrates under varying conditions.     

In vitro testing of plasma-sprayed hydroxyapatite coatings

Hydroxyapatite-coated coupons were tested in various physiological media. Immersion in Ringer's solution showed that heat-treated coatings displayed a weight gain but the assprayed coatings underwent a weight loss. Dissolution of the coating was measured by weighing the specimen before and after ageing. Immersion of hydroxyapatite coatings showed the appearance of small spheres that were identified by X-ray diffraction as hydroxyapatite. Changes in coating morphology were detected and the coating degradation mechanisms are discussed. This paper thus looks at the morphology, composition, crystallinity and dissolution of hydroxyapatite coatings aged in Ringer's solution.     

Microstructure of zirconia-yttria plasma-sprayed thermal barrier coatings

The objective of this paper is to report on the characterization of the highly complex microstructure of zirconia coatings, which arise as a result of the plasma-spraying process. The fine structure has been observed to change through the thickness of the coating, behaviour which has been related to the cooling rate and crystallization of the deposited material. Microstructural features such as an amorphous bond coat/ceramic interfacial film and a grain-boundary glassy phase, which are believed to have a significant effect upon coating properties such as adhesion and compliance, have been shown to be present.     

Characterization of plasma-sprayed and whisker-reinforced alumina coatings

We have strengthened plasma-sprayed alumina coatings by incorporating SiC or Si₃N₄ whiskers. As a result, we found that the whisker-reinforced coatings were greatly improved in properties such as thermal shock resistance and adhesion. Major features of the plasma-sprayed Al₂O₃–5.0 wt % Si₃N₄, coating were investigated by means of scanning electron microscopy, secondary ion mass spectroscopy, X-ray diffraction, X-ray fluorescence spectroscopy and thermal radiation measurements.     

The acoustic emission testing of plasma-sprayed coatings

A study was made of the amplitude distributions of acoustic emission (AE) from several kinds of plasma-sprayed coatings during four-point bending. A method is described whereby the amplitude distribution at small increments of load can be readily obtained. It is shown that by means of a _χ² analysis of the AE pattern thus obtained it is possible to discern differences in spraying parameters or substrate preparations much more readily than from either b plots or event counting. It is suggested that under suitable circumstances the method could be applied as a means of non-destructive evaluation during proof testing.     

Mechanical testing of plasma-sprayed coatings of ceramics

Mechanical tests have been performed on two types of plasma-sprayed ceramic coatings: magnesium zirconate (ZM) on aluminium alloy and chromium oxide (CO) on cast iron. Tensile strength, shear strength, energy relaxation rate and crack velocity have been determined. Results obtained with double-torsion tests show good agreement with those of double-cantilever-beam tests. Fracture always occurred in the ceramic for the ZM coating and at the interface for the CO one. Finally, acoustic emission monitoring carried out during bending tests was used to point out different types of emission, and to correlate them with micrographic examinations in order to identify some damaging processes.[/p]     

Electrical resistivity of plasma-sprayed titanium diboride coatings

Plasma-sprayed TiB₂ coatings (50–600 m thick) on alumina substrates have been developed and characterized. X-ray diffraction studies, thermal analysis and oxygen analysis of the coatings show that there is appreciable oxidation of TiB₂ during the spray process. Partial oxidation of the boride during spraying strongly influences the electrical conductivity of the coatings, which is found to be 100–1000 times less than that of pure TiB₂. Although use of argon as shield gas during the spray process brings down the resisitivity substantially, partial oxidation of TiB₂ could not be totally avoided.     

Deuterium trapping measurements in aluminum and plasma-sprayed aluminum coatings

Bulk and plasma-sprayed aluminum samples were bombarded with 10 keV D₃⁺ to a standard fluence of 10¹⁸ atoms cm^-2 at implant temperatures of 100–400 °C. Gas re-emission, thermal desorption and D(³He,α)H measurements were conducted. The results indicate that the saturation deuterium trapping level was about 10¹⁶-10¹⁷ atoms cm^-2 and was primarily confined to about the first 0.2 μm depth. Trapping of about 10¹⁶ atoms cm^-2 was still observed after anneals to 530 °C.     

Chemical vapour deposition densification of plasma-sprayed oxide coatings

Chemical vapour deposition (CVD) was used to produce an additional protective layer on the surface of plasma-sprayed alumina coatings. This layer was deposited from an AlCl₃-CO₂-H₂ gas mixture at a temperature of 1000°C. The thickness of the layer varied from 10 to 20 μm and the grain size varied from about 3 to 30 μm. The CVD-produced alumina had the crystallographic structure of stable α-Al₂O₃. The protective quality of the composite coatings was verified with electrochemical measurements. These indicated that completely tight coatings could be obtained.     

Mathematical analysis of thermoelastic characteristics in plasma-sprayed thermal barrier coatings

The thermoelastic characteristics of plasma-sprayed thermal barrier coatings (TBCs) have been analyzed using mathematical modeling. Two types of TBC model, cylinder and circular disk which are commercial plasma-sprayed TBCs, subjecting to symmetric temperature distribution to the radial and longitudinal directions, respectively, were taken into consideration. Based on the thermoelastic theories, a second order ordinary differential equation was derived for the cylinder model and a pair of partial differential equations were set up for the circular disk model. The analytic solution was obtained from the ordinary differential equation, while a finite volume method was developed for numerical solutions to the pair of partial differential equations due to the complexity of governing equations. The thermoelastic characteristics of TBC models, such as temperature distributions, displacements, and stresses, were displayed according to the obtained solutions. The rate of heat conduction in the section of the top coat is relatively slow in comparison with the substrate, and no profound difference appears in the temperature distribution between two TBC models. The highest longitudinal tensile stress is expressed at the bond coat of both models, and the substrate is under the compressive stresses to the circumferential direction. While the cylinder expands to the positive longitudinal direction only, the expansion in the circular disk occurs to both the positive and negative longitudinal directions. Relatively large displacement and stresses exhibit in the cylinder as compared with the circular disk. In the circular disk, the stresses to the radial direction undulate at each section, and the displacement profile displays that the width of the circular disk is slightly narrowed. The results demonstrate that the mechanical and thermal properties of the top and bond coats are the crucial factors to be considered in controlling the thermoelastic characteristics of plasma-sprayed TBCs.     

On the properties of plasma-sprayed oxide and metal-oxide coatings

Alumina (with 3% TiO₂), zirconia (with 13% Y₂O₃) and powder mixtures of zirconia (with 13% Y₂O₃) and molybdenum (in the proportions 70%:30% and 30%:70%) were plasma sprayed under standard conditions onto several steel substrates. Intermediate Ni-Cr metal layers were used.The adhesive strength of the coatings under static and dynamic stress and the thermal shock resistance were tested as well as the wear and corrosion resistance, the porosity and other properties of these oxide and oxide-metal compound layers. In all the tests the number of samples tested was sufficient for reliable average results to be determined.Resistance against friction and against abrasion, sometimes in combination with corrosion, can be controlled by the use of pure oxide layers or, depending on the type of chemical reactions and mechanical behaviour, of oxide-molybdenum compounds. Some theoretical aspects of the behaviour of the coatings tested will be discussed.A knowledge of the mechanical, thermal and corrosion behaviour of the sprayed layers and the interfaces enables selections for practical use. The results reported are part of an industry-sponsored test program for industrial applications of plasma-sprayed coatings.