The effect of surface adsorbates on splat formation during thermal spraying is examined by controlling substrate adsorption.Splats are formed on a polished flat stainless steel substrate surface by plasma spraying. The adsorption state of the substrate iscontrolled with different organic substances of different boiling points and different preheating temperatures. The droplet materialsused are aluminum, nickel, and Al O 2 3. Three kinds of organic substances used are xylene, glycol and glycerol, which are brushedon the surface of the substrate before spraying. It is revealed that when preheating temperature of the substrate is lower than theboiling point of the organic adsorbates on the substrate surface, the splashing occurs during droplet flattening which results in theformation of the splats with an irregular morphology. However, when the preheating temperature exceeds approximately 50 8Cover the boiling point of the organic adsorbates and evaporates all organic substance adsorbed on the surface, the regular disktype splats are formed which corresponds to no occurrence of the splashing during splatting. It is clearly shown that the existenceof the evaporable adsorbates on the substrate surface influences significantly the splatting and consequently the splat morphology.On the basis of the result, the evaporated-gas-induced splashing model is proposed to explain the occurrence of splashing duringsplatting of a spray molten droplet and the formation of the reduced splat. The effect of preheating temperature of a substrtaesurface on water adsorption and desorption characteristics in ambient atmosphere is disscussed from the point of view to create aclean surface and regular disc splat.

The deposition process of WC-Co and Cr3C2-NiCr cermet coatings through a high velocity oxyfuel (HVOF)spray process was examined using powders of different carbide particle sizes that ranged from 1.5 mm to over15 mm. The effect of carbide particle size on the thickness of flattened particles was investigated. The carbide size inthe coating was compared to that in the original feedstock. Results showed that most of the carbide particlesremained in a solid state after passing through the flame, and the deposition of the cermet coating involves lateralflattening of a solid-liquid two phase droplet rather than a single liquid droplet, as in plasma spraying of metallicand ceramic materials under optimised conditions. It was found that the size of carbide particles in a sprayed coatingdepends greatly on the corresponding carbide size in the starting powder. The large solid carbide particles in a twophase droplet tend to rebound easily during HVOF spraying when the droplet impacts on a substrate surface, whilesmall carbide particles in the droplet may follow the flattening of the liquid binder phase and be easily retained in thedeposit. Using a powder consisting of small carbide particles leads to substantial retention of carbide particles in thecoating and consequently an improved wear performance. A model is proposed to explain the effect of carbideparticle size on the splat formation by a solid-liquid two phase droplet.

Cr3C2–25% NiCr coatings were deposited by high velocity oxy-fuel (HVOF) spraying process using two commercial powders. The microstructure of the deposited coating was characterized by scanning electron microscopy. The carbon contents in both the deposited coatings and the collected powders were characterized by chemical analysis to clarify the main mechanism controlling the carbon loss during deposition of Cr3C2–NiCr coating by HVOF spraying. The results revealed that the carbon loss in the collected powders was much lower than that in the coatings. A model involved in a solid–liquid two-phase particle deposition behavior and rebound-off of large carbide particles during splatting was proposed to explain the effect of droplet conditions including carbide particle size on the carbon loss during deposition of Cr3C2–NiCr. It was suggested that the rebound-off of larger carbide particles when the two-phase droplet impacts on the surface is main mechanism responsible for overall high carbon loss during HVOF spraying of Cr3C2–NiCr.

The electrochemical behaviour of plasma-sprayed SS41 steels with The Al2O3-TiO2 system. Cr2O3 and ZrO2 stabilized by Y2O3 ceramics was studied using a potenliostat by measuring Iheir natural potential und anodic polarization curves in 3.5%, NaCI aqueous solution Fhe natulal patential of ceramic-sprayed SS41 steels is tile sa me as tha t olthe stlbstrat. TIle anodie polarization bchaviour of ceramic-sprayed SS4 slecls is also dominaIed by Ibe subslratc through connected porosity. However, the polarizatian current density of ceramic sprayed SS41 steel is lower than that of the substrate because tile current densdy is conlrolled greally by Ihe anlotlllt ptrosity il1 the coating. The connected pt~rosity of ceramic coatings was L}blained quantitatively The polarlzation currenl density ratio between the ceramic-sprayed specinlcn and lhe substratc at Ihe carrcsponding polarization potentkd, The slrong rekltion between the ctmnected portlsily and the porosity of cross-sections of the ceramic coatings, shown using micrographs, was recognized By measuring electrochemically the connected porosity ill coalings obtained under various spraying conditions, it would bc possible to select the oplirnum spraying condibons Io spray coatings which have less porosity.

Thermally sprayed deposits have layered stracture composed of individual splats. The individual splats have quenchin microstruture of quasi-stabl prferred fine grains. Howerer, this fine-grained microstructure of the deposlts is usually not reflected by improved performance of the deposits bvecause a layered structure with two-dimensional vnids occurs between lamellar interfaces. The microstructure of the thermal spray deposits with the emphasis on the layer structural parameters is reviewed. Conventionally. one of the most common quantitative parameters uased to characerize the mierostracruter of the thermally sprayed deposits is the porosity, measured by different methods. However, it is illustrated that the relationships between propterties and porosity for bulk porous materials processed by conventional processes cannot be applled to thermally sprayed deposits owing to the two-dimenslonal char-acteristics of vodis. The total porosity in the depesists is not meaningfol from the viewpoint of prediction off the deposit properties. An idealfzed stracaral model and related parameterx, imstedad of porosity, are proposed to characterize quantitatively the microstructure of the thermally sprayed deosit. the retationships between the properties and the stturchural parameters are preseted for the plasma-sparyed ceramid deposits based on the proposed microstructure model. the properties include the Young's modulus. fracture toughmess, erosion resistance, and thermal conductiveity of the plasma sprayed ceramic deposis. The correlations of theocetical relationships with reported expertimental data are disenssed. An agreeement of theoretical with observed values suggests that the lamellar structure of the deposit with limited interace bonding is the dominant factor controling the performance of the deposit.

Five representative types of WC Co powders were selected to clarify the dependence of thestructure of sprayed coatings on the structure of powders themselves. The WC Co coatingswere sprayed with the Jet-Kote process and plasma spraying as well. The structure ofWC Co coatings was primarily characterized by X-ray diffraction, The X-ray diffractionpatterns of the sprayed coatings were illustrated compared with those of powders, whichaims at a better understanding of the structure of thermally sprayed WC Co coatings, Theselected coating was also analysed by differential scanning calorimetry (DSC). Thedecarburizing process, and the effects of powder structure and spray conditions on thecrystal structure of sprayed WC Co coatings are discussed in detail.

A WC-Co coaling was sprayed by the high-velocity oxy fuel process using a feedstock of tungs(en carbide clad wilh cobalt. The struccture of the sprayed coating was characterized by x-ray diffractiun (XRD), dif-ferential scanning caloriulptry (DSC), and differential thermal analysis (DTAL It was found that an amorphous phase of Co-W-C lernary alloy observed as a large, broad peak in the XRD paltern can he formed in tile as-sprayed WC-Co coating. The DSC, DTA. and XRD analyses revealed that Ihe amor-phous phase cryslallized al a lemperature of around 873K to metallic cobalt. Co6W6C, and tuugslen witb appreciable precipilation of free carbon. The heat treatment of as-sprayed WC-Co coating at a high tem-perature of 1173K suggests that annealing at a temperature higher than about i104 K will promote Ihe reaction of lungsten and cobalt with carbon to form the complex carbide Co6W6C.