The temperature of a substrate during thermal spraying may have a significant influence on the quality of coatings, especially for low-temperature materials such as polymers. The temperature must be high enough to thermally activate the interfacial bonding mechanisms but low enough to avoid degradation of the coating material or the substrate surface. It is thus necessary to understand and control the processes governing the temperature of the coating and substrate. However, thermal-spray deposition is a complex, dynamic process and the temperature distribution across the surface and through the thickness of the coating and substrate depends on many parameters. The scanning characteristics, the nature of the spray technique and the substrate dimensions are known qualitatively to influence the temperature profiles but the quantified inter-relationships are inadequately understood. It is difficult to characterize by experimental measurements the shifting patterns of temperature within the substrate because of the innumerable combinations of operating parameters. A computational model is therefore developed in this paper to simulate the temperature patterns during the deposition process. The influence of operating conditions, the spray technique and the dimensions of the substrate are taken into account in the computational model.

Plasma-sprayed coatings are formed by the impact and accumulation of feedstock particles with random sizes, temperatures and velocities on the surface of a substrate. The simulation results obtained for a single particle are not able to represent the behaviour of the feedstock in the process. A statistical analysis is required to describe this stochastic behaviour and in this research, the Monte-Carlo method was used to simulate the particles in the plasma jet. Parcels of groups of particles based on the particle size distribution were employed to provide a simplified and mathematically tractable representation of real feedstock powders. The distributions of particle temperature, velocity and trajectory were directly linked to the particle size distribution and the injection conditions occurring in practice. The statistical approach enables the prediction of the mean value and standard deviation of particle temperature, particle velocity and deposition impact position. The influence of particle-injection velocity, injection position and plasma parameters on the quality of the coatings was studied. The research shows how statistical techniques can be used to control and optimise the plasma-spray process.

Non-oxide ceramics, such as silicon nitride, have a unique combination of high strength, toughness, wear resistance, thermal and chemical stability. However, the use of these materials as thick protective coatings on engineering components has been severely restricted by their decomposition behavior. Silicon nitride, for instance, does not melt but decomposes at ~1900oC and so thermal spraying of pure silicon nitride powder is impracticable. A limited amount of research has been carried out on depositing silicon nitride in various metallic or ceramic matrix materials but none have produced adequate coating microstructures or coating properties. This paper concerns the design of oxide matrix systems for silicon nitride composite coatings. A quantitative model is developed for the viscous flow of two-phase feedstock particles on impact with the substrate and is applied to the deposition of silicon nitride – ceramic matrix coatings. A number of matrix systems are investigated including a series of yttria-alumina and yttria-alumina -silica compositions. The research shows that the oxide matrices successfully protect the silicon nitride from decomposition but that the matrix composition and particle loading have a critical influence on splat flow and coating quality.

The motion and heating of a single particle in a plasma jet is strongly influenced by the feedstock injection conditions and can be predicted using computational modelling as described in the literature. In practice, however, the size and initial velocity of a given particle are essentially random variables within the fairly wide limits of the injected feedstock population. This is compounded by the non-uniformity of the plasma jet. As a result, the motion and heating of particles take on a substantial random element. There are three independent variables that jointly affect the particle motion and heating in a given jet: the diameter, the radial coordinate and the azimuthal angle of a particle. Nevertheless, these parameters cannot be specified for every particle in the jet in a deterministic manner owing to the above complexities and so a simulation based on a single particle cannot provide a realistic prediction of the deposition process. In the present study, the random element present in practical spraying is simulated using a Monte-Carlo approach. The distributions of the motion and heating of a population of particles are simulated rather than those for a single particle. The statistical method presented in this paper gives more detailed information on the effects of processing parameters and the assessment of process quality. The results show that this is able to provide a more accurate means of simulating the thermal spraying process.

Ultra-high molecular weight polyethylene (UHMWPE) has remarkable properties in the bulk state and has substantial potential for use as a protective coating on metals. However, the molecular architecture responsible for these exceptional properties also causes difficulties in the formation of coatings by flame spraying. This paper studies two UHMWPE materials with molecular weights of 2 million and 6 million. The flow of splats for each UHMWPE and blends of selected polyethylenes were characterized and a model developed for the flow of these polymers with respect to polymer composition, viscosity and thermal spray parameters. The model was applied to the polyethylene system and the experimental results show that controlling the composition and the process parameters is essential for the deposition of high-quality coatings.

In this paper, a quantitative model of the viscous behavior of two-phase particles hitting a substrate is used to optimize a plasma spraying process for silicon-nitride composite layers. The model is derived from the observed behavior of Si 3 N 4 -YAS (Y 2 O 3 -Al 2 O 3 -SiO 2 ) layers and provides a basis for further study of ceramic-matrix composite layers. Paper includes a German-language abstract.

A computational model of the effect of the tail end of the flame on the temperature of polymer coatings during thermal spraying is presented. The low thermal conductivity of polymers results in a substantial build up of temperature at the surface of the coating and large temperature gradients are developed throughout its thickness. This is particularly problematic for polymer deposition owing to their low decomposition temperatures. The model quantifies the heat transfer from the impinging flame and the in-coming feedstock particles to the coating and the subsequent heat flow into the substrate and surroundings. The work shows that the heat input from the in-coming particles can be neglected in first-order computations. The scanning action of the flame across the substrate is simulated and the temperature profiles within the coating and substrate are calculated. The predictions are consistent with the experimental measurements. The model shows that overheating of polymer coatings can readily occur during combustion flame spraying and indicates remedial measures.

Jets produced in conventional atmospheric plasma spraying are inevitably heterogeneous due to the mechanics of a rapid gas flow entering a stagnant environment. The resulting high air content together with considerable velocity and temperature variations will adversely affect the quality of coatings. This paper applies a CFD code to simulate the effect of a solid shield and a gas shroud on the characteristics of a plasma jet. The computational results show that the geometry and gas flow rates of the shrouding systems have an important influence on the quality of the plasma flow. The work shows that shrouding can substantially reduce the entrained air content and the temperature and velocity gradients in the jet. The model is applied to predict the optimum shrouding conditions for plasma spraying.

Experimental measurements have been carried out with the aim of investigating the residual stresses generated during plasma spray deposition of glass composite coatings. The research shows that the behaviour of these materials is fundamentally different from metals and ceramics. The quench stress in the glass composites can be eliminated by plasma-scanning. This is attributed to their low glass transition temperatures, which enable the stresses to be completely relaxed. The work also shows that the addition of alumina as a second phase allows the expansion mismatch between the coating and the steel substrate to be controlled. Control of the second-phase volume-fraction enables the residual stress in the composite coatings to be reduced to zero. Real-time measurements on deflection and temperature show that the dimensions of the substrate, plasma operating conditions and scanning rate have substantial effects on the temperature profiles within the deposits. Keywords: glass composite coatings, thermal stress, plasma spraying.

A ball-milled mixture of glass and alumina powders has been plasma sprayed to produce alumina-glass composite coatings. The coatings have the unique advantage of a melted ceramic secondary phase parallel to the surface in an aligned platelet composite structure. The alumina raises the hardness from 300HV for pure glass coatings to 900HV for a 60wt% alumina-glass composite coating. The scratch resistance increases by a factor of three and the wear resistance by a factor of five. The glass wears by the formation and intersection of cracks. The alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reached a plateau at 40-50vol% alumina, which corresponds to the changeover from a glass to a ceramic matrix. Keywords: glass composite coatings, wear, thermal spraying

This paper investigates the degradation of polymers during thermal spraying with special reference to the changes in molar mass. A study is carried out on the degradation of polymethylmethacrylate (PMMA) during plasma spraying. The infrared spectroscopy showed that the coating did not lead to any significant chemical reactions. However, the gel permeation chromatography measurement revealed larger changes in the molar mass with a decrease of up to 70% in the average numerical molar mass of the PMMA. The results show that increasing the arc power resulted in a substantial increase in degradation. Because of the extremely low thermal conductivity of polymers, the magnitude of these effects becomes even worse. The results indicate that it is important to consider molar mass measurements along with spectroscopic analysis when characterizing thermally sprayed polymer coatings. Paper includes a German-language abstract.

Kinetic and heat transfer analysis have been undertaken in order to predict the decomposition of polymer feedstock particles during thermal spraying. Thermogravimetric measurements indicated that the decomposition of PMMA had an order of reaction of unity and an activation energy of 135 kJ mol -1 . The polymer decomposition temperature is shown to be a function of the particle residence time in the flame and is much higher than in conventional polymer processing. This has an important influence on process modelling, since the choice of decomposition temperature used in the heat transfer analysis has a major effect on the calculated temperature profiles. The work shows that realistic predictive data can only be obtained by using the dynamic decomposition temperature. Application of the model indicates that only the surface layers of the polymer feedstock particles undergo significant decomposition during plasma spraying and that the feedstock injection position is an important control parameter.

An investigation has been undertaken on the analysis of residual stress in glass coatings during plasma spraying. Theoretical analysis and in-situ experimental measurements show that the residual stresses in glass coatings are particularly sensitive to the heat input from the plasma flame, since this can raise the temperature to above the glass transition temperature. Control of the spraying parameters enables the quench stress of splats to be relaxed by the end of the spraying and the only significant remaining source of stress derives from the differential contraction between the coating and substrate during cooling. The analysis also shows that a stress transition occurs during cooling and that the sign of the final residual stress depends upon the expansion coefficient of the glass. The residual stresses are shown to govern the critical coating thickness for cracking and the coating adhesion.

The adhesion of plasma sprayed polyamide and PMMA coatings to steel depends markedly on the plasma arc power, the spraying distance and the substrate temperature. Each of these process parameters shows an optimum value with respect to adhesion. The underlying reason for this behaviour is the pronounced sensitivity of polymers to temperature. Heat transfer analysis and electron microscopy indicate that a critical amount of heat is required to be transferred from the flame to the feedstock particles in order to provide sufficient splat flow but avoid coating thermal degradation. Inadequate flow leads to interfacial voidage while degradation gives inferior bonding and porosity.

Experimental work has been undertaken to investigate the importance of the temperature of the substrate during deposition on the coating-adhesion of plasma sprayed borosilicate glass coatings. The work shows that the measured adhesion increases markedly with substrate temperature up to 400°C above which no further major increase takes place. Heat transfer and fluid mechanics calculations predict that the effect of substrate temperature is due to its influence through the cooling rate on the viscosity and flow of the molten glass particles as they impact on the substrate surface. The theoretical calculations also predict large temperature gradients through the thickness of the splats and glass coatings, and the consequent non-uniform thermal stress distributions are expected to contribute to the reduced splat retention rate and coating-adhesion at low substrate temperatures. The predictions were confirmed by an electron microscopy examination of the morphology of isolated splats, the deposits and the coating-substrate interface.

Plasma spray deposition of epoxies under normal conditions produces coatings with low wear resistance. The research shows that the difficulty in achieving satisfactory properties is a result of the rapid heat flow from the coating to the substrate, which suppresses the crosslinking reaction. The results indicate that the use of substrate preheating or ceramic undercoats enhances the wear resistance by promoting the curing reaction during spraying.

A heat transfer analysis has been undertaken to predict the influence of process parameters on the decomposition of in-flight particles and deposited layers during thermal spraying of polymer coatings. The theoretical analysis shows that polymers are unique in developing large temperature gradients, which accelerates the degradation of the surface of the particles and the coating layers. However, the analysis indicates that the degradation can be limited by the control of the plasma gas composition, the spraying distance and the torch traverse speed. The theoretical analysis has been confirmed by weight loss measurements, wear tests and microstructural observations of plasma sprayed PMMA coatings. The work shows the existence of a critical traverse speed below which satisfactory coatings cannot be produced.