Ni, Ni-Al and Ni-Cr powder particles were plasma sprayed onto the mirror polished metallic substrate surface and the effect of substrate temperature on the particle/substrate interface microstructure was investigated. Sprayed particles were fully melted, sphere in a shape and oxidized on their surfaces during spraying in an air atmosphere. The surface oxide layers were detected as Al or Cr rich thin layer, respectively by electron probe micro analysis (EPMA). After spraying of the particles onto the substrate, cross section microstructures at particle/substrate interfaces were investigated. As a result, almost no oxide layer was detected at the particle/substrate interface when the substrate was kept at room temperature. On the other hand, the oxide layer was apparently recognized at the interface when the substrate was kept at a certain elevated temperature, such as at 673K. The difference in the existence of the oxide layer at interface seems to relate the wettability of the substrate by the liquid particles. The transition temperature, Tt, for each powder material was measured. The meaning of elemental addition to the base metal was considered from the changing tendency in Tt value of each powder material. Through the investigation results obtained, dominating factor on the flattening of the thermal sprayed particles onto the flat substrate surface was estimated.

Effect of substrate temperature and ambient pressure of deposition chamber on contact condition between free falling droplet and substrate surface were investigated. Cu droplet and SUS304 substrate were used for the experiment. To estimate the contact condition, temperature history of molten droplet was measured at splat-substrate interface by thermocouples embedded in the substrate and high frequency data logging system. Quantitative porosity on splat bottom surface and degree of circularity of splat shape was evaluated. Splat adhesion strength experiment was conducted and dynamic wetting behavior of substrate by molten droplet was captured by ultra-high speed video. Heating substrate and vacuuming chamber pressure enhanced the heat transfer from splat to substrate, which can be attributed to the good contact at splat bottom surface. Adhesion strength of the splat to the substrate corresponded well with degree of circularity. Consequently, substrate temperature and ambient pressure of deposition chamber have an equivalent effect to contact condition at interface between droplet and substrate surface.

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.

Though a large literature exists on thermal spray coating, there have been few attempts to directly observe molten droplets as they land on a substrate, freeze, and coalesce into a solid layer. In this study, investigators photograph the impact of tin droplets landing on stainless steel surfaces at high speeds. They also measure the effect of substrate temperature and surface roughness on deposition efficiency, the fraction of coating material that adheres to the surface. The paper describes the apparatus used to create the tin droplets and record their impact with the target. It also provides a detailed review of the test results and discusses the practical implications of the study. Paper includes a German-language abstract.

The modeling work presented in this paper is intended to simulate a plasma jet impinging on a flat surface and its influence on substrate temperature during layer build-up. In the first step, a two-layer model is incorporated into a commercial CFD code to express the turbulent plasma jet and the viscous sublayer that forms on the surface of the impact plane, or in this case, the substrate. In a subsequent step, heat dissipation in the substrate is expressed with a 3D volume model based on finite volume approximation of thermal exchanges. This two-step approach allows surface temperatures on the substrate to be determined as the plasma-sprayed layer grows. The influence of different substrate materials is also considered in this investigation, particularly in regard to transient surface temperatures. Paper includes a German-language abstract.

Mechanical properties of thermal-sprayed ceramic coatings were investigated. Al 2 O 3 and Y 2 O 3 -stabilized ZrO 2 (YSZ) coatings were deposited on plate substrates. Stainless steel plates and aluminum plates, of different thermal expansion coefficients, were used as substrates. The coatings were prepared at two different thicknesses. During deposition of each sample, the history of substrate temperature was recorded. Four-point bending tests were carried out, while strains at the coating surface and the substrate surface were measured with strain gages. The apparent Young's modulus of the coating was determined using the composite beam theory. In addition, the rupture strain of the coating was measured by three-point bending test. The relationship between the results of these tests and the temperature of each substrate during deposition is discussed.

Residual stresses are inherent in thermal barrier coatings (TBC's) and can influence in-service performance and life of the coatings. Therefore, the effective design and processing of TBC's requires knowledge about residual stress generation and the effect of residual stresses on TEC life. Understanding residual stress generation and the effects on thermal barrier coating life are formidable tasks that have received little attention in the literature. This work addresses the first task. Specifically, the objectives of this work were to better understand how processing and post-processing residual stresses are generated in TBC's. The approach was to evaluate the effect of substrate temperature during processing and the effect of post-processing thermal cycling on the generation of coating residual stresses. Residual stress measurements were conducted using an experimental residual stress evaluation technique called the "Modified Layer Removal Method." Results showed residual stresses could be changed both by controlling the substrate temperature during processing and by thermal cycling after processing. Residual stresses in specimens with a higher substrate temperature during processing were found to be more compressive than residual stresses in specimens with a lower processing substrate temperature. Post-processing thermal cycling caused the residual stresses to become more compressive for specimens with both the higher and lower substrate processing temperatures. Residual stresses for one and ten post-processing thermal cycles were evaluated. For both substrate processing temperatures, the change in TBC compressive residual stresses for the first cycle was more than three times the total residual stress change that occurred in cycles two through ten. Interestingly, the increase in residual stresses in cycles two through ten for the higher substrate processing temperature was greater than that for the lower processing substrate temperature. In other words, based on results obtained here, compressive residual stresses generated during thermal cycling appear to depend on the existing processing residual stress. For these conditions, higher processing compressive residual stresses lead to higher post-processing changes in compressive stresses per thermal cycle.

Substrate temperature is nowadays recognized as a key parameter to optimise the coating quality in the thermal spraying process. Generally parts being processed are in motion and therefore non contact temperature measurement devices are appropriate. In contrast to thermocouples, optical pyrometers have several advantages. First, they are easy to install and second they do not bring any disturbance to the measured system. Meanwhile, several problems may arise with those devices which are not always considered as they should be and in particular the variation of material emissivity temperature, the effect of the reflection of the external radiation or the attenuation of the optical signal due to the variable transmissivity of the optical path. The aim of this work was to develop algorithms for correcting optical pyrometer temperature measurements during thermal spraying by taking into account emissivity variations and radiation reflexion on the components. Emissivity of some materials with respect to the specific spectral band of the pyrometer and the influence of reflected radiations were measured. Results are discussed in order to point out the influence of each parameter on the temperature value.

The current study has been focused on the final morphology of atmospheric plasma sprayed 8% yttria stabilized zirconia single splats. Single splats of two different sizes (-25 µm and +25/-45 µm) of ZrO 2 8Y 2 O 3 powder have been collected on polished stainless steel substrates at three different temperatures (Room, 300°C, and 600°C). The splat morphology and diameter, satellite particles, and splashing behavior were investigated using both scanning electron microscopy and image analysis software. The splat/substrate interface and splat curl up were studied from cross-sections prepared by focused ion beam milling. Results showed primarily pancake morphology and no evidence of delamination along the splat/substrate interface at 300oC substrate temperature and 100 mm spray distance. Overlapped splats showed evidence of melting (microwelding) at the splat boundaries. Splat thickness was measured to be less than 1 µm for all spray conditions. Roughness profiles of the surface of the deposited splats indicated microcracks had formed within the splats. Image analysis results exhibited a higher volume fraction of the splats relative to satellite particles at longer spray distance and higher substrate temperature. The average splat diameter increased as the substrate temperature increased.

When describing the cold spray process, one of the most widely used concepts is the critical velocity. Current models predicting critical velocities take the temperature of the sprayed particles explicitly into account but not the surface temperature (substrate or already deposited layers) on which the particle impact. This surface temperature is expected to play an important role since the deformation process leading to particle bonding and coating formation takes place both on the particle and the substrate side. The aim of this work is to investigate the effect of the substrate temperature on the coating formation process. Experiments were performed using aluminum, zinc and tin powders as coating materials. These materials have a rather large difference in critical velocities that gives the possibility to cover a broad range of deposition velocity to critical velocity ratio using commercial low pressure cold spray system. The sample surface was heated and the temperature was varied from room temperature to a high fraction of the melting point of the coating material for all three materials. The change in temperature of the substrate during the deposition process was measured by means of a high speed IR camera. The coating formation was investigated as a function of (1) the measured surface temperature of the substrate during deposition, (2) the gun transverse speed and (3) the particle velocity. Both single particle impact samples and thick coatings were produced and characterized. Both the particle-substrate and interparticle bondings were evaluated by SEM and confocal microscopy

A new thermal spray process is under development in order to produce thin coatings (thickness lower than 50 µm), with a fine microstructure (grain size smaller than 1 µm). It consists in injecting in a Direct Current (D.C.) plasma jet a suspension containing submicronic particles of the material to be deposited. To study the interaction between the plasma jet and the suspension, a system based on a pendulum allows the collection of particles at different distances from the injection point. In this paper the effect of substrate temperature upon the formation of micrometric zirconia splats was studied, glass and stainless steel were used as substrate materials.

A numerical model is presented for the computation of heat transfers during the APS thermal spray process. This model includes the contributions of both the impinging plasma jet and that of the particle flux on the substrate heating. The contribution of the impinging plasma jet is taken into account using a computational fluid dynamic model describing the impact of the plasma jet on the substrate. For this part of the work, a two-layer extension to the Chen-Kim k-s model was used allowing the description of both the turbulent plasma jet and that of the flow in the viscous sub-layer formed on the substrate surface. The contribution of the sprayed particles is taken into account considering their distribution in the spray jet. Since this is an important parameter that could affect the model accuracy, measurements of the deposit thickness profiles were first performed using the non-destructive acoustic microscopy method and the corresponding particle flux distribution was then deduced. Heat transfers inside the substrate were then computed using a three dimensional in-house code based on a finite volume approach. In the case studied, the results show that the contribution of the sprayed particles forming the coating is much more focalized than that of the plasma flow itself whereas the substrate nature has a strong influence on the thermal flux dissipation (not presented in the following). These elements are expected to provide useful information concerning the coating adhesion mechanisms and the formation of residual stresses during the coating elaboration.

This paper aims at improving the adhesive strength of SS 316L coating by substrate preheating (400, 600 and 700°C). The relationships between the adhesive strength of coating/substrate interface and the substrate preheating temperature are discussed. It was found that stronger adhesion is able to occur despite the presence of a thick oxide film on the substrate surface. The preheated substrate surface undergoes a stronger plastic deformation that disrupts the oxide films for obtaining an intimate contact between particle and substrate material. In addition, the oxide films on the substrate surface can prevent the generation of material jet of the substrate. The effects of substrate preheating on the microstructure and hardness were also investigated.

In this work, the influence of the substrate temperature on the deposition efficiency and on the coating properties and residual stress was investigated. Pure Al coatings were deposited on Al 6061 alloy substrates using CGT Kinetics 3000 deposition system. The substrate temperature was ranged between 20°C (room temperature) and 375 °C and was kept nearly constant during the deposition while all the other deposition parameters were unchanged. The deposited coatings were quenched in water (within one minute from the deposition) and then characterized. The residual stress was determined by Almen gage method (Ref 1, 2, 3), Modified Layer Removal Method (Ref 4, 5, 6), and XRD (Ref 7) in order to identify both the mean coating stress and the stress profile through the coating thickness from the surface to the coating- substrate interface. The residual stress results obtained by these three methods were compared and discussed. The coating morphology and porosity were investigated using optical and scanning electron microscopy.

This study investigates the influence of substrate temperature on the adhesion strength of cold spray coatings. Copper, aluminum, and iron powders were deposited on preheated substrates similar in composition with the respective powder. Adhesion strength was determined by shear adhesion testing. The results show that substrate preheating improves adhesion strength for specific combinations of coating and substrate materials, likely due to the relief of thermal stress and the formation of an oxide layer on the substrate surface.

The focus of this study is to deposit metallic coatings onto ceramic substrates for application in power electronics. One possibility to achieve the required surface activation is to heat the substrate during spraying. An increased substrate temperature significantly influences bond strength and coating properties. This is investigated for cold-gas sprayed copper and aluminum on thermally sprayed Al 2 O 3 layers. The study examines the adhesion of single-impacting Cu particles as well as coating microstructures and mechanical and electrical coating properties. It is found that increasing the substrate temperature as well as increasing the surface roughness enhances the adhesion strength of single particles. Coatings sprayed on heated substrates adhere very well and show low compressive stresses. Their hardness is reduced significantly, while their electrical conductivity is optimized to values of over 90 % IACS (IACS: International annealed Copper standard, 100 % IACS equals 58 MS/m).

Wollastonite coatings were deposited on Ti6Al4V substrate at the substrate temperatures of room temperature, 200 °C, 400 °C and 600 °C by the atmosphere plasma spraying, respectively. The effect of substrate temperature on the microstructure, phase composition, mechanical properties and dissolution behavior of wollastonite coatings were investigated. The microstructure and phase composition of coatings were examined by SEM and XRD. The hardness and elastic modulus were obtained by the Knoop indentation tests. In addition, the dissolution behavior of coatings was evaluated by immersion in SBF solution. The results indicate that a slight decrement of porosity and an obvious increment of crystallinity were found with the substrate temperature. The hardness and elastic modulus of coatings increased with the substrate temperature up to 400 °C firstly, and then a decrement was observed with the temperature further increasing to 600 °C. The dissolution rate of coatings characterized by the pH changes and the released Ca, Si and P concentration in the SBF decrease with the substrate temperature, which is related to the porosity and crystallinity of coatings. It is revealed that through increasing the substrate temperature during plasma spraying is a feasible method to improve the mechanical properties and to decrease the dissolution of wollastonite coatings.

Aiming at clarifying the individual splat formation mechanism in thermal spray process, commercially available metallic powders were thermally sprayed onto AISI304 substrate surface. The splats changed from a distorted shape with splash to a disk-shaped splat in flattening after collision onto substrate surface, through substrate preheating and/or reducing the ambient pressure. Accordingly, both substrate temperature and ambient pressure have an equivalent effect on the shape transition. The observation on the bottom surface morphology of single splat indicated that the ring-shaped initial solidification might play an important role during splat formation process. As a simulation of the real thermal spray process, free falling experiment has been conducted. The thermal history of the free falling metal droplet onto AISI304 substrate indicated that the flattening pattern is decided so quicky just after collision onto solid surface, which is enough earlier to the finalization of the flattening.

It is essential to understand and clarify what causes adhesive strength between the thermally sprayed deposit and the substrate. It is known that the adhesive bond strength is strongly related to the roughness of the substrate surface, however, we are yet to know why and how the roughness affects the strength. Although the roughness works as a socalled anchor or interlocking effect, we cannot show how the effect quantitatively relates to the strength. Before we use the word “roughness”, we need to define roughness as a strict meaning in connection with the adhesive strength to avoid any ambiguity in expressing the roughness itself. Seeking the true roughness to relate to the bond strength, we have introduced a newly developed roughness indicator correlated closely with the adhesive strength. This indicator is derived by theory on the basis of the assumption that the bond strength is primarily caused by the mechanical friction between the deposit and the substrate. It was supposed that chemical and physical adhesions were secondary effects except that the molten particle, the substrate temperature or both of them are high enough to form these bonds by thermal interaction. The increase in contact area between the splat and the substrate could raise the friction force between them. The higher substrate temperature could increase the contact area when the molten particle interacts with the substrate, because the molten particle could wet better on the high temperature substrate. The substrate temperature influence on the adhesive strength was also investigated.

Experimental studies involving aluminum particles sprayed onto polished AISI304L substrates using the Valuarc 200 wire arc previously showed that there exists a transition temperature from splash to disk splats. Increasing the substrate temperature above the transition temperature was seen to increase the number of disc splats, thus producing coatings of improved properties. XPS test results have shown that increasing the substrate temperature also results in increased oxygen content on the surface of the substrate. Experiments also show that prolonged heating of a substrate at a particular (fixed) temperature further promotes oxidation of the substrate surface, thus increasing the surface roughness (Ra). Samples generated on substrates that were held at or above 350°C (above Tt) for prolonged periods of time (over 20 minutes) were seen to promote splashing. This is in contrast to the previous findings that showed substrate temperatures above Tt further promoted disc type splats and improved adhesion between the splat and substrate. Samples generated in this study consistently showed that splashing can be seen at temperature well above the transition temperature, if the substrate has been heated for too long a duration. The cause of splashing is believed to be related to increased surface roughness resulting from prolonged oxidation of the substrate surface. Abstract only; no full-text paper available.