Excellent biocompatibility of hydroxyapatite (HA) is the main reason of application plasma-sprayed coatings onto orthopedic prostheses. A careful optimization of spray parameters is necessary to avoid thermal decomposition of HA onto less biocompatible products such as e.g. tricalcium phosphate, tetracalcium phosphate, calcium oxide and amorphous calcium phosphates. The spray parameters influence considerably the decomposition and the present study is devoted to understand this influence using on an experimental way. The design of experiments (DOE) was made using two-level 2N plan of experiments (N=5). In total, 32 experiments of spraying were carried out by varying following operational parameters: (i) composition of plasma working gases; (ii) electric power input; (iii) art of spraying (into water or onto substrate); (iv) carrier gas flow rate and; (v) art of injection (external and internal). Plasma-sprayed coatings and powders were analyzed by Fourier Transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The X-ray diagrams enabled to find the content of crystal phases. The content was a first response function described by a polynomial regression equation. The morphology of obtained deposits was also characterized using Scanning Electron Microscope (SEM). Their porosity was estimated using image analysis of coatings cross section images.

Amorphous PEEK coating was prepared on Al substrate, employing flame spraying technology. The amorphous coating was subjected to annealing treatments under different conditions. Both differential scanning calorimetry (DSC) and wide angle X-ray diffraction (WAXD) analysis revealed that the isothermally treated coating exhibited semi-crystalline structure. Coexistence of double crystal entities in semi-crystalline PEEK coating was deduced from the results obtained by DSC and WAXD measurements. Annealing temperature and holding time under this temperature affect the morphology of the minor crystal entity which is metastable. The mechanical properties of the isothermally treated coating were investigated considering coating microhardness and friction and wear properties. The variations of the coating mechanical properties were correlated with the modifications in the coating structure induced by the different annealing conditions.

The goal of this paper is to evaluate high temperature ageing properties of a new high temperature titanium blade compatible abradable material DurabradeTM 2614. Coatings were tested in as-sprayed condition and after ageing at 550°C and 655°C for up to 8,030 hours. Coating properties such as coating hardness, erosion resistance, and cohesive strength were evaluated at regular time intervals. Abradability was tested in as-sprayed condition and after ageing. The results show that coating hardness, GE erosion resistance, and cohesive strength of the new material change most in the first 200 hours and SMC90 erosion resistance, and oxidation weight gain change most in the first 1,000 hours and then they stabilize at values that guarantee good seal performance. The good performance of the new seal after 8,030 hours ageing has been demonstrated by abradability and erosion testing.

In some applications like car armouring for personal safety, in bulk containers in pebbles working, in off-shore applications or in offroad-vehicles, impact of bodies with different sizes and high velocities has to be calculated in material design. A possibility to design under economical aspects is to use to low-cost material as substrate with a protective coating on its surface instead of using expensive bulk materials. One conceivable material system to be used as protective coating is WC-CoCr, which is usually applied by HVOF and shows high hardness combined with a good toughness of the matrix. These properties are very important for dissipation of the impact energy and for high fracture toughness. In the presented case study, WC-CoCr coatings with different carbide sizes were produced with the JP5000 system from TAFA. Projectiles with high velocities of more than 250 m/s were shot by an experimental shooting device on the coatings and for comparability on the uncoated substrate. Impact tests were done with balls and cylinders with varying impact angles between 10° and 90°. The powders and the coatings were characterized by means of microscopy and XRD. Furthermore, the hardness of the coatings was measured. After impact tests, the coatings were investigated with optical microscopy. In the case of ball impact, a significant dependency can be found between the impact behaviour and carbide size. Coatings with coarse carbides show the lowest impact depth. The dependency of the impact depth of coatings with one carbide size on their hardness level is relatively low. No dependencies of the carbide size or the hardness level can be found for impact with cylinders.

Water atomized cast iron powder of Fe-2.17C-9.93Si-3.75Al (in wt%) were deposited onto an aluminum alloy substrate by atmospheric DC plasma spraying to improve its tribological properties. Pre-annealing of the cast iron powder allows to precipitate considerable amounts of graphite structure in the powder. However, significant reduction in graphitized carbon in cast iron coatings is inevitable after plasma spraying in air atmosphere due to the in-flight burning and the dissolution into molten iron droplets. Hexagonal boron nitride (h-BN) powders which have excellent lubricating properties like graphite were incorporated to the cast iron powder as solid lubricant by sintering process (1300 °C) to obtain protective coatings with low friction coefficient. The performance of each coating was evaluated using ring-on-disk type wear tester under paraffin base oil condition in air atmosphere. Conventional cast iron liner which has flaky graphite embedded in pearlitic matrix was also tested in similar conditions in order to make a comparison. Sections of worn surfaces and debris were characterized and wear behaviour of plasma sprayed coatings are discussed.

The quality and operation performance of layer composites manufactured by thermal spraying is influenced by two different processes, the activation process of the surface prior to coating deposition and the coating process itself. The activation of the substrate surface is mainly performed by grit blasting operations. Surface activation by grit blasting is used in order to improve the bonding between substrate and coating, which is strongly related to the size and nature of the surface topography and roughness generated by the blasting process. Besides the roughening effect, grit blasting induces compressive residual stresses into the substrate surface which can be critical especially for thin walled components, e.g. piston rings, where the component shape is an important factor for the operation behavior and functionality. Another effect is an increase of hardness in the surface region related to the induced compressive stresses. A variety of blasting parameters can influence the surface characteristics, like nozzle diameter, grit medium and size, blasting pressure, distance and time. The influence of these parameters on the surface roughness, hardness, component deformation and residual stresses was investigated by tactile surface metrology, universal hardness and Almen tests as well as experimental residual stress analysis with the incremental hole drilling and milling method. All investigations were performed on rectangular steel strips. The results are discussed concerning quality control features for grit blasting processes in serial production.

Three adhesion measurement methods for thermal spray coatings, namely tensile adhesive strength (according to EN 582), interfacial indentation and in-plane tensile tests were investigated in terms of accuracy of the results and application potential for different coating / substrate conditions. Whereas the tensile adhesive strength test is widely used in industry, the other two methods are still under development in research laboratories and therefore only few experimental data on the accuracy of the methods and on the potential in an industrial context are available. For that reason, dissimilar coating-substrate combinations covering a wide range of types of thermal spray coating-substrate systems were tested using all these methods. Ceramic (Al 2 O 3 ) and metallic (NiCr 80-20) coatings were thermally sprayed by flame spraying with two different thickness on titanium alloy and steel substrates exhibiting each two distinct roughness levels. The distinguished coating properties include the coating toughness, shear strength, interfacial toughness, and adhesive strength. Thermally sprayed coatings do not only show an interfacial complexity, but also the integrity of the interface of substrate and coating has to be considered, as well as porosity, cracks and residual stresses. In this paper, each measurement method was found to be related to certain type of loading conditions and fracture mode. The results of the different methods are compared and the limits of applicability of the different methods are discussed.

Plasma-sprayed 7 wt.% Y 2 O 3 -ZrO 2 (YSZ) stand-alone coatings were subjected to 10, 50 and 100-hr heat treatments at 1200°C, followed by mechanical testing in compression at 25°C, 1050°C, and 1200°C. The mechanical tests performed on the samples included cyclic loading/ unloading, stress-relaxation, and creep. In cyclic compression tests at 25°C, it was observed that the YSZ coating that had been heat-treated for 50-hr at 1200°C demonstrated a higher modulus and more strain hysteresis as compared to an as-sprayed sample. As heat-treatment time increased, the YSZ stand-alone coating demonstrated less relaxation of the initial applied stress for stress-relaxation tests run at 1050°C and 1200°C. The steady-state creep rate was observed to decrease with increasing heat-treatment time prior to testing; as expected, the steady-state creep rate increases when the testing temperature was increased from 1050°C to 1200°C. Density (via Archimedes’) and phase analysis (via X-ray diffraction) were performed on YSZ coatings before (i.e. as-sprayed) and after heat-treating at 1200°C. Porosity was observed to decrease only slightly (~1%) after a 100-hr heat-treatment at 1200°C as compared to the as sprayed porosity. The high-yttria metastable tetragonal phase observed in the as-sprayed coatings was observed after a 100-hr heat-treatment at 1200°C. The same metastable tetragonal phase was also observed after a 100-hr heat-treatment at 1200°C followed by a 3-hr stress-relaxation test at 1200°C.

To enhance the efficiency of computational experiment covering all links of a technological chain ‘generation of plasma jet – injection of particles – formation of dust-laden plasma jet – coating deposition from drops of a melt’ it is very attractive to integrate the experimentally tested analytical solutions characterizing splats formation along with 2-D/3-D models describing dust-laden technological flow. The authors of the present paper carried out a computer simulation of a plasma spraying process from zirconia particles injection to a coating formation conducted by integrating particle-laden plasma flow, metal oxide splat formation and coating formation models with reference to the thermal barriers. The velocity and temperature of both plasma flow and zirconia particles under an applied RF electromagnetic field were clarified by using the first model. Radial distributions of particle impact location, velocity and temperature were obtained based on both an unsteady effect of a plasma flow and distributions of particle size and injection velocity. Secondly, splat thickness and diameter after zirconia droplets impact onto substrate were clarified by using the earlier experimentally probated analytical solution. Finally, the coating thickness distributions were evaluated by using the last model. Abstract only; no full-text paper available.

In the present study, TiO 2 and TiO 2 -Al composite coatings were prepared by HVOF spraying using reconstituted nanosized feedstock powders prepared via the spray drying technique. In the flame, the powders were injected by two methods: internal injection i.e. as in conventional HVOF process and external injection i.e. outside the torch nozzle. The microstructure of the coatings was characterized by scanning electron microscopy and X-ray diffraction. It was found that the amount of anatase in the coatings depends on the nature of the powder and also on the type of the injection method. The coatings were tested for their photocatalytic properties regarding the conversion rate of nitrogen oxides. Coatings elaborated by external injection presented a better photocatalytic activity than those obtained by the conventional HVOF process.

Thermal spray coatings are formed by successive molten droplets impinging onto a substrate. It is commonly admitted that the flattening behavior determines the interaction of splats with the substrate or previously deposited under layer and hence governs the overall quality of the sprayed deposit. Therefore a number of works have been devoted to explain the splashing mechanisms and the flattening behavior from different aspects, such as fluid dynamics, solidification behavior, surface wettability or surface adsorbates/condensates. It was thus shown that surface conditions such as surface roughness, contaminants, adsorption, etc. play a very important role in controlling the splat flattening behavior. In this study, the role of the substrate condition as well as that of the impinging particle parameters are emphasized in the case of a copper deposit formed on a titanium base alloy and on a stainless steel substrates. The plasma sprayed copper splats were examined for different surface conditions. It was thus confirmed that favorable surface conditions tend to suppress splashing and to promote the occurrence of disc-shaped splats while the substrate nature and particle parameters also affect the splat flattening behavior. Results also indicated that disc-shaped copper splats prepared on a cold substrate have a smaller flattening degree than those obtained on a preheated substrate.

The high-velocity oxy-fuel (HVOF) gun represented a major development towards forming dense coatings. But, compared to the electric arc-spray process, the HVOF process is more difficult to apply and in general more costly. Therefore, some process development efforts have aimed at exploiting the attributes of both the electric arc-spray technique combined with those of the HVOF technique. Specifically, this hybrid process utilizes the electric arc spray benefits of using wire stock and high deposition rates combined with the higher coating densities obtained using HVOF. This paper presents an in-depth, in-flight particle characterization of a hybrid spray gun. A DPV-2000 particle diagnostics sensor was used to measure particle velocity, temperature, size and distribution. The influence of feed material, electric arc-spray parameters, and HVOF parameters on the particle characteristics is presented. It is observed that the velocity of the particles in the hybrid mode are slightly lower than the velocity of particles by HVOF alone but significantly higher than typical electric arc-spray velocities. In addition, the particle temperature in the hybrid mode isn’t significantly different than those by HVOF only. The particles produced by the hybrid gun tend to be more uniform and smaller compared to traditional electric arc-spray coatings.

Al 2 O 3 and Cr 2 O 3 coatings were deposited by atmospheric plasma spraying. The tribological properties of coatings against copper alloy were evaluated with a block-on-ring configuration under dry friction conditions at room temperature. Microstructure of powders and coatings were observed using scanning electron microscopy (SEM) and optical microscope (OM). Some thermo-physical properties of coatings were measured. Results showed that the Cr 2 O 3 coating exhibits cracking failure in the sliding condition with a normal load of 500N and a sliding velocity of 0.84 m/s. This failure of the plasma sprayed Cr 2 O 3 coating is explained in terms of its low thermal conductivity.

Nanostructured and conventional WC/Co coatings were prepared by supersonic plasma spraying process respectively. The bonding strength, microhardness, tribological characteristics such as friction coefficient and wear resistance, microstructure and the phase ingredient were studied. The result of the XRD suggests that the coating contains low-carbon phase. TEM analysis reveals that WC phase with the dimension of about 100nm is dispersed in the nanostructured coating. Results of mechanical and tribological properties tests show that nanostructured coating has better performance in comparison with conventional one, with increase of about 40% in the bonding strength, 3% in the microhardness, 14% in the wear resistance, and a decrease of 0.06 in friction coefficient, respectively. After tribological properties tests, obvious plastic deformation was observed on the wear trace surface of the nanostructured coating, while microcrack existed on the surface of conventional coating. Superfine grain strengthening is the primary reason that enhances the performances of the coating.

The plasma spray process is unique because the coating properties depend indirectly on the processing parameters. Therefore, the in-service properties derive mostly from the coating architecture. The coating architecture is related to the intrinsic lamellae formation mechanism; which is related to the impinging particle characteristics that are linked to the power and feedstock injection. The coating architecture is also related to the extrinsic spray pattern formation mechanism; i.e., the kinematics and geometric spray parameters such as spray velocity, spray angle, and stand off distance. Understanding relationships between coating architecture and its properties requires consideration of the whole coating formation process. Ultimately, this permits selection of process parameters that demonstrate enhanced spray efficiencies and manufacturing capability. Included in this study are the pore network architecture and residual stress level because they play important roles in coating cohesion from which derive most of the in-service properties. Coating manufacturing mechanisms, from the spray pattern to an actual coating formed by several successive patterns, are investigated in this paper. The case of atmospheric plasma sprayed Al 2 O 3 -TiO 2 (13% by wt.) is considered. Data are statistically assessed by implementing Gaussian and Weibull analyses. The critical role of pores which develop within the spray pattern and between successive patterns is examined in detail. There is contamination between layers and the intrinsic roughness of the coating is altered during the spray process. In summary, within this work, it is intended to define the intrinsic and extrinsic operational variables that contribute to the coating architecture and, thereby, suggest technology that can be implemented to improve coating quality and deposition efficiency.

The impact and solidification of 4 mm molten aluminum alloy 380 droplets on a tool steel substrate was studied both analytically and experimentally. Temperature histories at different radial location on the substrate surface under impacting droplets were recorded using an array of thin film thermocouples with response times less than 1 µs. Photographs were taken of droplet impact onto the substrate. Initial substrate temperature was varied from room temperature to 300°C and average surface roughness from 0.5 to 5.0 µm. Estimates of thermal contact resistance were made by matching measured substrate temperatures with an analytical solution for surface temperature variation. A model of the true area of contact between molten metal and a rough surface was developed in order to predict how contact resistance changes with surface roughness and contact pressure. Impact of molten aluminum alloy droplets was simulated using a three-dimensional numerical. Using values of thermal contact resistance predicted by the model gave good agreement between computed and observed droplet shapes during impact.

Microscale wear mechanisms in thermally sprayed hard coatings were examined. A test procedure for examining microwear with various abrasives was developed. Different abrasivity of kaolin, precipitated calcium carbonate and titania was found to affect wear mechanisms. Fine-particle abrasion caused the surface to loose its gloss and smoothness. Coatings subjected to fine-particle abrasion were examined with optical gloss measurements and scanning electron microscopy. It was shown that in tungsten carbide based hard coatings the microscale wear was governed by preferential wear of the soft binder phase. However, also coating defects like pores or poorly adhered splats tended to provide nucleation points for microscale wear. It was found that coatings that performed well in dry-sand rubber wheel abrasion or wet abrasion tests did not necessarily have good microwear resistance. The results showed that different abrasives had effect on the wear phenomena and wear rate of hard coatings. Coatings also behaved differently and novel modifications in the composition affected the wear behaviour. In conclusion, the results provided deep understanding of the role of various abrasives in the wear phenomena of thermally sprayed hard coatings.

The characterization of the adhesive and cohesive strength of thermally sprayed coatings is often evaluated according to given standardized testing procedures. These tests require the preparation of normally large coupons which have to be fixed together using an appropriate adhesive. Additionally they need time for preparation (e.g. annealing/curing of the adhesive) and require test equipments normally not available at job shops for coating development. One of the largest limitations of these tests is the applicability only for non-porous coatings, and in some cases the limited strength of the adhesive. Within a European CRAFT research project on “standards, measurements and testing”, a new shear test method was developed to characterize the mode and value of failure of thermally sprayed layers in a more reliable and less limited manner. This new shear test does not need any adhesive and yields more intrinsic information on coating quality than conventional tensile tests.

A novel material has been used for plasma spraying by WSP. The material is composed of three main phases, namely corundum (aluminum oxide), baddeleyite (zirconium oxide), and glassy phase (silicon oxide). The material is a refractory and exhibits very high hardness, extremely high abrasion resistance, and chemical resistance. Conventionally, the material is fabricated by melt casting and machining. Cast tiles of the material were ground and sieved to obtain the right powder cut size for plasma spraying by water stabilized plasma torch (WSP). Both dense coatings and free standing parts were achieved with the new material, which sprays very well with WSP. Spraying parameters were varied and molten particles were monitored in flight by DPV 2000. The coatings exhibit very low porosity and high hardness. The as-sprayed material is mostly amorphous with some nanocrystalline grains of aluminum and zirconium oxide present. The phase composition of the as-sprayed material is thus different from that of the feedstock material, which is mostly crystalline with a small fraction of amorphous silica glass. The microstructure of the newly sprayed material was studied by electron microscopy (SEM, TEM) and is very complex. Upon annealing, the as-sprayed material crystallizes around 950ºC. This result and other thermal properties were measured by TMA and DTA. The ease of plasma spraying and the coating properties make this material a suitable candidate for many industrial applications.

Based on the HVO-AF (High Velocity Oxygen-Air Fuel) thermal spray system, the Low Temperature High Velocity Air Fuel Spray was realized by additional liquid feed stocks. In this paper, the microstructure and characteristics of composite coatings sprayed by this spray technology were analyzed. Composite powders were composed at three mass fractions, Fe, 5mass%Fe -polymer, 15mass%Fe-polymer. In the experiments, the coatings properties were tested. The results indicated that all the coatings microstructure is dense and low porosity; metal particles were dispersed with polymer in the coatings. There were little oxide phase in the coatings. The coatings were closely combined with substrate, the reflectance coefficient of 5mass%Fe-polymer composite coatings is better than others, the reflectance coefficient curve of the coatings is 2~6dB at 2~18GHz.