The nanoindentation technique has been applied to thermal-sprayed metal, cermet and ceramic deposits. The hardness and elastic modulus were determined from the load-displacement curves. Each test was implemented by varying the penetration depth (100, 200, 300 and 400 nm) in the same test location and at least 20 tests were performed. The results were compared to those from microindentation tests. The nanoindentation test, essentially, measured the submicrometer scale properties of thermal spray deposits, which can be considered as "near-intrinsic" properties of the coatings. Thus, these measurements exclude most of the microstructural factors that influence the "macroscale" properties. The nanoindentation test exhibits significantly greater hardness and elastic modulus values than the microindentation test.

The effect of WC-Co coating on the high cycle fatigue (HCF) behavior of SAE 12L14 steel and 2024-T4 aluminum was investigated. The fatigue tests were performed at room temperature and 370°C. The fatigue life distributions of specimens in the polished, grit blasted, peened, and coated conditions are presented as a function of the probability of failure. HVOF sprayed WC-Co coating has influenced the fatigue life of aluminum and steel. Factors contributing to this influence, which include grit blasting, elastic modulus, and residual stress, are discussed. A three-dimensional finite-element model (FEM) of the coated specimen was used to calculate the stress distribution across the coating and the substrate. The results of the analytical model are in good agreement with fatigue lives observed experimentally.

Tungsten caibide (WC) thermal spray coatings are being used for wear protection on selected components of aircraft. Tungsten carbide coatings are being used on aircraft flap tracks and fan and compressor blade mid-span dampers. However, a larger use of tungsten carbide coatings is being considered for other commercial aircraft applications where it would be used as a replacement for chrome plating. For instance, WC coatings are currently being tested on aircraft landing gear parts. One factor that affects the suitability of WC coatings for these applications is the fatigue life of the coated part. Coatings, whether chrome plating or thermal spray coating, can reduce the fatigue life of the part compared to an uncoated part. This study compares the fatigue life of uncoated 6061 aluminum specimens to the fatigue life of WC thermal sprayed coated 6061 aluminum specimens. The relation between the residual stress level in the coating and the fatigue life of the specimens is also investigated. Fatigue tests were run on cantilever flat beam specimens that were coated on one side. Specimens were cycled in bending so that the coatings experienced tensile fatigue stresses. Residual stress levels for each type of coating were determined using the Modified Layer Removal Method on specimens processed along with the cantilever flat beam specimens. Test results show that the fatigue life of the WC coated specimens is directly related to the level of compressive residual stress in the coating.

Two types of WC-12wt%Co powders, each manufactured by a different process, were thermally sprayed on a medium carbon steel by HVOF, and repeated load tests (rolling contact fatigue test and high cycles fatigue test) were carried out. The surface damages for the two types of coatings were investigated. It has been clear that the coating damages depend on the types of powders. It has been found that in rolling contact fatigue, there are the coatings, in which damage is characterized by delamination, and by a mixture of delamination and cracks. And it has also been found that in high cycles fatigue, there are the coatings, in which damage is characterized by net-like fatigue cracks, and by linear fatigue cracks.

The use of MoSi 2 as a high temperature oxidation resistant structural material is hindered by its poor elevated temperature creep resistance. The addition of second phase Si 3 N 4 holds promise for improving the creep properties of MoSi 2 without decreasing oxidation resistance. The high temperature impression creep behavior of atmospheric plasma sprayed (APS) and hot pressed (HP) MoSi 2 /Si 3 N 4 composites was investigated. Values for steady state creep rates, creep activation energies, and creep stress exponents were measured. Grain boundary sliding and splat sliding were found to be the dominant creep mechanisms for the APS samples while grain boundary sliding and plastic deformation were found to be the dominant creep mechanisms for the HP samples.

A technique has been developed to characterize the elastic modulus of zirconium oxide - 8 % yttrium oxide plasma sprayed deposits. A commercial hardness indenter has been modified to record load - displacement as a spherical ball is elastically loaded onto the surface of the material to be measured. The resulting data are used to calculate the elastic modulus. Since the loads used are in the elastic region, the technique is, in theory, nondestructive. Relatively small areas of the material, approximately 50 μm in diameter, are sampled by the indenter, allowing local mapping of elastic modulus variations throughout the deposit. Using this technique, elastic modulus variations have been measured through the thickness of the deposit. Also, different moduli were measured in the cross-section and through the thickness and these differences are correlated with the microstructure. Finally, significant increases in elastic modulus have been found in samples annealed for 2.5 h at 1100°C. These changes have been correlated with small angle neutron scattering measurements of void surface area.

The role of surface roughness in coating adhesion mechanism is studying for detonation spraying. Roughness was produced by conventional grit blasting, D-gun blasting and was formed as a result of spraying of high-adhesive thin layer of detonation coating. Cermet and alloy powders were sprayed by detonation gun Ob. The coating bonding strength measurements show the WC+25Co adhesion to be above 200 MPa independently of a substrate surface preparation. Contrary, NiCrSiB coatings are very sensitive to surface conditions their adhesion varies from 180 MPa to zero. As-sprayed alloy particles fail in adherence because of insufficient energy to fuse substrate material at a flat surface. Only developed (wide scale) roughness may be fused partially by these particles for their bonding to the substrate. Otherwise, high heated cermet particles do not need special surface preparation (except cleaning) for fusion of substrate material to provide high bonding with it. The wide scale and ball shape roughness, which is similar to the self-reproduced coating roughness, provides the best conditions for the coating bonding and it is recommended as the purpose of surface treatment before thermal spray coating.

The attractive bioactive properties of HA are significantly reduced upon plasma spraying because of the phase transformation that accompanied the deposition process. One major factor that influence the extent to which the transformation occur appears to be the morphology and physical states of the HA raw powders. This paper reports the study on the influence of powder morphology and property on the fracture behaviour and tensile adhesive strength of plasma sprayed HA coatings. Three types of powders were used in the study; calcined HA (CHA), spray dried HA (SDHA) and flame spheroidised HA (SHA). The particle size range of 53 - 75 μm was employed for all 3 types of powders to effect an accurate comparison of the powders. Results show that the cohesive bond strength of the SHA coating was the highest because of the denser microstructure created by well-formed lamella splats. A correspondingly lower bond strength was recorded with less coherent coatings generated by agglomerated CHA and SDHA powders.

The preheating of metallic substrates before powder deposition in air plasma spraying improves the adhesion of oxide coatings, provided heating is performed with an optimal procedure to avoid a too high oxidation state of the surface. It means that the temperature level and heating time have to be monitored carefully. In these conditions a thin layer (<100 nm) of oxides is formed on the substrate surface, the resulting contact of the molten droplets impinging the hot substrate is excellent (R th <10 -7 m 2 .K/W) and the adhesion properties of coatings are enhanced. This paper is devoted to the study of the metallic surface. The substrate heating and thus oxidation are obtained with a d.c. Ar-H 2 plasma jet flowing in air of which the stand off distance is maintained at 100 mm. The parameters investigated are macroscopic surface temperature and heating time. The characterization of the oxide layers is achieved by Mossbauer spectroscopy, near grazing X-ray diffraction, near UV-Visible-near IR spectroscopy and specular reflection infrared spectroscopy. At the end of this paper an attempt will be made to correlate these characterizations to the splats microstructure and coatings adhesion.

WC-Co-Cr powders with different WC particle size have been sprayed by the HVOF process. At constant spraying conditions the powders give coatings of different quality. The deposition efficiency during spraying of powders containing large WC particles was found to be low compared to powders with finer WC grains. In addition the amounts of porosity and cracks were different. The coatings have been characterised by different methods. Erosion and erosion-corrosion tests showed that the WC particle size also influence the wear resistance of the coatings. Small WC particle size was found to be beneficial. Chemical composition of the matrix was also found to be decisive for the coating properties. An increase of the chromium content improved the erosion-corrosion resistance.

Mechanical properties of WC-Co D-Gun coatings produced from various powders were determined by using a four-point bend test equipped with a special device for strain measurements. The MOR(Modulus of Rupture), elastic modulus, fracture strain and toughness were measured from stress-strain curves using the four-point bend tests. The fracture strength values were increased in the order of clad, sintered, agglomerated(no densification), blended and cast & crushed powder coatings. The Co content blended with WC-Co cermet powders significantly increased fracture strain values and decreased elastic modulus values. It was found that the type of powder more significantly influenced the mechanical properties of D-Gun coatings than the composition of powders. The toughness of thermal spray coatings was increased by using WC-Co powders (high MOR) blended with self-fluxing alloy powders (high fracture strain).

Different mechanisms of development of the substrate-coating adhesion during thermal spraying are considered. One of the most important is mechanical interlocking formed chiefly due to roughness of the substrate surface, high pressures developed upon the droplet impact and solidification of the lower part of the splat. Possible deformation of the substrate surface and rebounding of the impinging droplets are considered. Thermal mechanisms involving partial or complete melting and subsequent solidification in the substrate interfacial region is shown to be effective in creation of the adhesive bonds. The role of the diffusion processes and the influence of the splat morphology on adhesion is discussed. Mechanisms of splashing of droplets impinging onto the substrate surface during thermal spraying and their influence on the coating-substrate adhesion are considered. Roughness of the substrate surface is shown to be critically important to obtain good adhesion. Transition temperature is shown to exist which determines the splat morphology on the smooth surface of the substrate. With a "cold" substrate when its initial temperature is less than the transition temperature the splashing occurs. When the substrate initial temperature exceeds the transition one a regular splat is formed. The theoretical results agree well with the observed behaviour of the thermal spray coatings and in the case of the thermal mechanisms of adhesion these results are in a good agreement with the experimental data.

A Round Robin study involving 19 coating suppliers and independent laboratories was conducted on tensile testing of thermally sprayed coatings to determine the accuracy and consistency of the tensile data among the participating labs and within a lab. One coating system (NiCrAl) and two adhesive types (film vs. liquid epoxy) were used. The results showed the average tensile strengths for the coating system using the liquid epoxy systems (EC2086/EC2214) were consistently higher than the average values which resulted using the film epoxy system (FMIOOO). However, less scatter in the results was observed when the FMIOOO film epoxy was used.

In wire arc spraying, atomizing gas velocity and particle velocity are important factors influencing coating quality. A nozzle with secondary gas injection has been developed to increase the gas velocity and to improve coating quality. In this study, wire arc spraying of stainless steel on aluminum substrates has been investigated with the objective of establishing correlations between atomizing gas velocities, particle velocities, particle sizes and coating bond strength. Cold gas velocity is measured with a Pitot tube. Particle velocities are determined from high speed images of particle streaks taken with a Kodak high speed vision system and evaluated using image analysis. Bond strength is measured with pull-off tensile test. Secondary gas atomization clearly leads to improved adhesion due to additional metallurgical bonding between the coating and the substrate achieved through higher particle temperatures at the moment of impact.

Future development of thermal spray processes and new composite materials raises an important problem concerning the transition from qualitative to quantitative methods of coatings evaluation. It is well known that thermal spray coating deposition in most cases is accompanied by the formation of temporal and residual stresses through the coating thickness. For proper evaluation of formed stressed state it is extremely important to know the real value of elastic characteristics in different layers of the coating. This problem has become more complicated taking into consideration the variety of materials, different spray parameters, number of coating layers and extreme service conditions. These values can be obtained only from experimentation. Elastic characteristics (EC) could be used in many calculations, such as durability, stiffness, fatigue, vibration and others. This paper describes new methods of experimental determination of elastic characteristics presumed as variable throughout the coating thickness. Influence of coating composition, particle size of initial powders, spray parameters, post-treatment and other factors on elastic modulusses were studied. Obtained experimental data for different materials supplement existing data and can be used for evaluation of residual stresses and other purposes.

Low cycle fatigue tests were performed at room temperature (RT) and at 673 K for l%Cr-0.5%Mo steel comparing the specimens coated with chromium carbide by gas spraying and the ordinary uncoated specimens, and the mechanism of fatigue crack formation was investigated. Following observations and conclusions were made: (1) When sprayed with ceramic, the fatigue life suffers reduction at either temperature, but at 673 K, the degradation was so much smaller than that at RT that the fatigue life was actually, though slightly, longer than that at RT. (2) The cracks are initiated in the ceramic layer very early in the whole fatigue life, the crack initiation lifetime becoming the longer, the smaller the strain range. (3) The fatigue failure process can be viewed as comprising following steps: first, early initiation of fatigue crack at the surface of the ceramic coating, rapid propagation through it to the substrate metal, and initiation of crack in the metal, the initial rate of propagation of such a crack being a number of times (perhaps as much as one full order of magnitude) faster than that in uncoated steel.

The mechanical properties (i.e., tensile and shear properties) of vacuum plasma spray copper deposits presenting different porosity levels were determined. The tensile properties of the copper plasma spray deposit appeared to strongly depend on the porosity level, but whatever was the porosity level of the sample, the fracture always exhibited a ductile character, with the failure cups expanding preferentially on the pores and secondarily on the grain boundaries of the annealed microstructures. The tests clearly shown that the material presented an isotropic ductile-plastic hardening behavior typical of copper based materials, but this behavior being significantly influenced by the porosity level of the deposit.

Mechanical properties of water stabilized plasma spray-formed Al 2 O 3 -13wt%TiO 2 have been investigated by using indentation techniques and a four-point bend test. Young's modulus was determined by four point bend, nano- and Knoop indentation tests. Modulus of rupture was measured by the four-point bend test. The anisotropic behavior as well as the reduced Young's modulus and strength of thermal spray deposits are extensively investigated and can be related to the unique microstructure; i.e., the spheroidal shape of pores, cracks and splats as well as inter-lamellar boundaries. The heat treatment substantially enhanced the mechanical properties and the microstructure becomes more homogeneous and isotropic; that is, the deposits lose their unique nature and become more similar to bulk materials.