Y. Bao

The influence of process parameters on the degradation of thermally sprayed polymer coatings

Heat transfer theory has been used to predict the influence of process parameters on the decomposition of in-flight particles and deposited layers during the thermal spraying of polymer coatings. The methodology is applicable to all polymers and the generic family of thermal spray processes, including plasma, combustion flame, and high-velocity oxy-fuel spraying. The work shows that polymers are unique amongst engineering materials in developing large temperature gradients, which accelerate the degradation of the surface of the particles and the coating layers. However, the analysis also indicates that the degradation can be limited by the control of the 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. The critical traverse speed is a characteristic parameter for all polymers, and its value is dependent upon the degradation behaviour of the particular material.

Abrasive wear resistance of plasma-sprayed glass-composite coatings

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 plateletlike-composite structure. The alumina raises the hardness from 300 HV for pure glass coatings to 900 HV for a 60 wt.% alumina-glass composite coating. The scratch resistance increases by a factor of 3, and the wear resistance increases by a factor of 5. The glass wears by the formation and intersection of cracks, while the alumina wears by fine abrasion and supports most of the sliding load. The wear resistance reaches a maximum at 40 to 50 vol.% alumina, above which there is little further improvement. This critical alumina content corresponds to the changeover from a glass to a ceramic matrix.

The effect of feedstock particle size on the heat transfer rates and properties of thermally sprayed polymer coatings

SummaryA heat transfer analysis has been undertaken to predict the temperature profiles of in-flight polymer particles in a plasma jet. It shows that the development of large temperature gradients below the surface of particles during plasma spraying is a distinctive characteristic of polymers and is primarily due to their low rate of internal heat conduction relative to that of metals and ceramics. The large temperature gradients together with the low decomposition temperatures result in a kinetic difficulty in achieving complete melting of in-flight polymer particles despite their low melting temperatures. The theoretical analysis indicates that a suitable balance between maximizing the degree of melting and minimizing degradation can be achieved by control of the feedstock particle size. The experimental results are consistent with the theoretical predictions and show that size control provides an appreciable improvement in the density, tensile properties and wear resistance of polyamide coatings.

Process model of plasma enamelling

A computational model has been developed to simulate the deposition of enamel on steel substrates by the use of plasma spraying. The model predicts the temperature profiles of the feedstock particles during their flight in the gas jet and the concurrent heating of the substrate. A process window is predicted for enamel deposition in terms of plasma gas composition and feedstock particle size. The model also predicts that the plasma jet produces a thermal shock at the surface of the coating and a high temperature gradient through its thickness during the scanning action. A series of experimental trials confirmed that plasma spraying could successfully produce dense coatings on steel. An inherent advantage of the process is that the enamel feedstock powder is fused separately in the plasma while the substrate remains at a low temperature. This enables enamelling to be carried out in a single stage operation without the need for a furnace, which offers the potential of widening the applicability of enamel coatings.

Influence of secondary phase on residual stress of plasma sprayed glass coatings

Abstract The behaviour of glass coatings during thermal spray deposition is fundamentally different from that of metal and ceramic coatings because quench stresses can berelaxed during spraying. However, the expansion coefficient mismatch between the coating and the steel substrate remains an important source of residual stress and this paper investigates the effect of incorporating an alumina second phase into the glass to reduce the mismatch. Real time measurements on the deflection of coated specimens during spraying show that the residual stress can be reduced to zero by controlling the second phase content. The results also show that substantial temperature variations may be developed across large substrates during spraying as a consequence of the scanning action. These variations can affect the residual stress owing to the thermally activated nature of the stress relief and the marked sensitivity of glass properties to temperature. This research indicates that the pattern of residual stress developed in glass coatings is complex and will depend upon the substrate dimensions and the process operating conditions.

The mechanism of crack formation in plasma sprayed enamel

Experimental work has shown that enamel frits can be plasma sprayed but there is a tendency for cracks to form in the deposits. This investigation indicates that cracking is due to the development of residual stresses during deposition. Theoretical analysis and in-situ experimental measurements show that the residual stresses in enamel 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 spraying parameters enables the quench stress of splats to be relaxed by the end of the spraying operation 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 enamel. The residual stresses are shown to govern the critical coating thickness for cracking and to affect the coating adhesion.

Plasma sprayed PMMA coatings on steel

SummaryPMMA coatings have been applied to steel by plasma spraying. The work shows that the polymer decomposition temperature is a function of the particle residence time in the plasma and is much higher than in conventional polymer processing. The analysis indicates that realistic prediction of the temperature profiles of in-flight polymer particles can only be obtained by using this dynamic decomposition temperature. The process model predicts that only the surface layers of the polymer feedstock particles undergo significant decomposition under optimized spraying conditions.

Sintering behaviour of glass-silicon nitride composites for thermal spray precursor powders

Abstract Silicon nitride composite powders have been prepared in which each individual powder particle contains submicron silicon nitride particles completely encapsulated in a glass matrix. This necessitated raising the sintering temperature as the silicon nitride content increased in order to reduce the viscosity of the glass and enhance the Darcian flow. However, these higher sintering temperatures caused porosity due to thermal degradation of the glass, and as a consequence, the density of the composites decreased monotonically with increasing silicon nitride content. The hardness results revealed two conflicting effects of increasing the silicon nitride content and as a result, an optimum silicon nitride content of 30 vol.-% giving a hardness of over 680 HV was found. The silicon nitride was also found to have a greater effect on scratch resistance than hardness due to its greater ability for inhibiting crack development than shear banding.

Processing and properties of plasma sprayed silicon nitride-glass composite coatings

Abstract Abstract Silicon nitride decomposes before it can melt, and so thermal spraying of pure silicon nitride powder is impracticable. To address this difficulty, feedstock powder for plasma spray deposition has been developed in which each particle is a composite of silicon nitride in a low temperature borosilicate glass matrix. The research showed that the silicon nitride did not decompose in the plasma because the low thermal conductivity of the glass matrix ensured a low heat transfer rate and the particle temperature remaining below the decomposition temperature. The coating density initially increased with plasma arc power because of increasing splat flow but then declined at high power levels owing to decomposition of the glass matrix. The silicon nitride dispersion substantially reduced the splat flow, particularly near the maximum packing fraction, but also had the beneficial effect of restricting crack propagation, resulting in an optimum content for wear resistance of 30 vol.-% silicon nitride.