The structural and mechanical properties, in terms of surface roughness, hardness and wear resistance, of WC/12wt%Co coatings are investigated as a function of powder properties, such as powder size and WC grain size, as well as high velocity oxygen-fuel (HVOF) spray conditions. It has been found that the WC/12wt%Co coatings with relatively smoother surface, about 2 µm in average surface roughness (Ra), retaining high wear resistance can be formed by optimizing the powder and spray parameters. Further, we have found that powder size can control the surface roughness over a wide range; from about 2 to 7 µm (about Δ5 µm) in Ra. On the other hand, Ra changed only about Δ1 µm or more when changing spray conditions, such as barrel length and spray distance.

The adhesive strength between substrate and sprayed coatings seems to be mainly due to the mechanical interlocking effect. The physical bonding of spray particles to the substrate is the second factor. In this paper, the bonding mechanisms of sprayed coatings were studied on basis of experiments with the substrate at different surface roughness. The substrate surface roughness was quantitatively evaluated using a surface roughness tester. The tensile adhesion strength test method was used to evaluate the adhesive properties. The relationship between surface roughness and adhesive strength was investigated. The adhesive strength of plasma sprayed coating decreased with an increase in substrate surface roughness, as the size of the grit particles for roughening increased. On the contrary, the adhesive strength of arc sprayed coating increased.

This paper evaluates different techniques for measuring bond coat surface roughness, explores the influence of different thermal spray processes on the surface roughness of bond coats, and correlates bond coat surface roughness with spray parameters. The results of an evaluation of various techniques for measuring the surface roughness of the adhesive layer including the influence of the thermal spraying process, the powder size, the travel speed, and the coating thickness on the roughness of the thermal barrier coatings (TBCs) adhesive layers are presented. Light microscopy, a needle-tracking profile stylus, and non-contact interferometry with white light are used to characterize the adhesive layer surfaces. The results show that the measurement technique used had significant effects on some of the measured surface roughness values. Paper includes a German-language abstract.

To understand performance of thermal barrier coatings (TBCs) in various industrial applications of Siemens medium size gas turbines, effects of three types of thermal exposures i.e., high temperature isothermal exposure, thermal cycle fatigue (TCF) test, and burner rig test (BRT) on adhesion strength of an air plasma sprayed (APS) TBC have been studied and reported in this paper. It has been seen that the TBC adhesion strength is influenced by the type of thermal exposures differently. Together with a microscopic examination on TBC microstructures and fractography, a correlation between failure mechanisms and types of thermal exposures is discussed. In addition to the impact of various engine operation conditions on behavior of TBC, impacts of TBC surface roughness on turbine performance have also been evaluated. Surface profile and surface roughness on as-sprayed and polished TBC and cast metal (uncoated) have been measured and two different polishing methods have been compared. As a result, a requirement of TBC surface roughness and a preferable polishing method are suggested.

This work deals with the flattening of alumina molten particles, called droplets, on stainless steel substrates either smooth or blasted and preheated at different temperatures. The blasted surface roughness has been limited to Ra= 1.4 µm to image the flattening droplet. Besides flattening and splat cooling, the wettability of melted millimeter-sized alumina drops on the same substrates was measured. The transition temperature, Tt, has been shown to be different between smooth and rough surfaces. For a smooth surface, Tt, is 170°C, and at 200°C 100% of disk shaped splats are obtained. For the rough surface, Tt is close to 300°C with porous splats, becoming almost dense at 450°C. Close to alumina melting temperature, wettability does not vary with the substrate preoxidation, which may not be the case when the temperature is much over the melting one as in plasma spray conditions.

Though the adhesion between substrate and sprayed coating seems to be mainly due to the anchoring effect, quantitative examinations have not been sufficient. In this paper, the surface condition of a roughened substrate is quantitatively evaluated using a surface roughness tester, and the adhesive relationship is clarified. Substrate used was S45C(JIS standard) carbon steel, and the roughening methods were blasting and endless belt polishing with alumina grit, in various roughening conditions. A pure copper coating was formed using the powder flame spraying method on roughened substrate. The drawing adhesion strength test method was used to evaluate the adhesive properties. The adhesive strength is reduced, though the surface roughness of the substrate increases, as the size of the grid particles for roughening increase. The alumina grits remained on the substrate after roughening, and they were removed by ultrasonic cleaning. Though there is no relation between the adhesive strength and the ratio of the increase in the surface area of substrate due to the roughening treatment, it is recognized that the adhesive strength also increases when the roughness gradient of the substrate increases.

In order to utilize the blasting process for preparing surfaces of precision products, the factors affecting the work piece distortion and the degree of the distortion were investigated. The long and flat shaped specimens of the different materials and the thickness were blasted, and the phenomena induced by the blasting were investigated. In the result, almost the linear relationship between the surface roughness and the maximum deflection existed. In addition, the equivalent load was applied to the simple model of the distortion, and the correlative expression among the maximum deflection, the specimen's size, and the equivalent load was deduced. The results made it possible to select and design beforehand the products, and to optimize the blasting condition.

Thermal spraying produces coatings with relatively rough surfaces compared to other deposition methods. In this work, NdFeB coatings were deposited on stainless steel by plasma spraying at various standoff distances. Some of the coatings were also annealed. Surface roughness profiles of as-sprayed and heat-treated coatings were measured by contact profilometry and analyzed via statistical methods. The effect of standoff distance and annealing on roughness is discussed along with the significance of measurement direction and evaluation length.

Twin wire arc sprayed aluminum coatings are used extensively in semiconductor processing equipment. In certain applications, such as metal reclamation, rough coating textures are desirable as the added surface area facilitates the collection of metals and lowers manufacturing costs. There are many factors that affect the surface roughness of twin wire arc sprayed coatings. This study investigates coating conditions that produce rough textures and the influence of surface roughness on bond strength. Traditional theory of arc spraying has been that to obtain a rough coating, particle velocity should be reduced by decreasing the atomizing air pressure of the system. Decreasing air pressure, however, may have a negative impact on bond strength, the extent of which is assessed in this study.

Cold Spray is a material deposition process where the effects of substrate roughness on cratering phenomenon are often observed. In order to understand and explain crater formation on cold sprayed coatings, the laser surface texturing technique is used. This innovative process allows to control the substrate surface roughness and to create a controlled topography. In this study, five hole sizes from 20 to 100 μm diameters with an angle of 45° were drilled to obtain different working craters. Subsequent, build up of the coating was investigated. Aluminum powder and nitrogen were used for this study. The main gas temperature and pressure were respectively 500°C and 3MPa. The morphology and the microstructure of aluminum coatings were characterized by optical microscopy and scanning electron microscopy. Surface improperly filled crater affects bond strength. The objective is to determine the effect of surface morphology on craterisation weakening the bond strength. The erosion velocity creates locally a hydrodynamic penetration leading to strong erosion.

Thermal spray layers are formed on rough surfaces; however, the flattening process on rough surfaces has not yet been clarified. A mathematical flattening model which takes into account the roughness of the substrate or previously coated layers is proposed in this paper. As a result of surface roughness, the flattening degree and the flattening time decrease with increasing surface roughness in this model. In addition, the characterization of surface roughness is introduced for the flattening model. Several calculated cases of the flattening model are shown.

Different types of tungsten carbide materials (fused tungsten carbide, nickel clad fused tungsten carbide, macrocrystalline WC and sintered and crushed WC/Co) are used for laser dispersing of construction steel surfaces. Surface roughness analyses and metallographic evaluation of cross sections concerning efficiency of carbide embedding as well as crack formation tendency are carried out. Generally, all types of tested carbides permit production of rough surfaces with metallurgical bonding to the metallic matrix, but only use of nickel clad fused tungsten carbide permits to prevent crack formation. The effectiveness of silicon and silicon carbide for production of durable rough surfaces on aluminium alloys is investigated. Both silicon and silicon carbide qualify for production of rough surfaces by laser dispersing. While silicon carbide particles show higher hardness, use of silicon does not include danger of embrittlement due to formation of aluminium carbide.

Thermal barrier coating systems are used to enhance the temperature resistance of hot section components in gas turbines. The coatings protect the underlying nickel based components and consist of the bond coat (BC) the thermal barrier coating (TBC) and a thermally grown oxide (TGO) between the BC and TBC. The coating systems fail in service at or near the TBC/TGO interface. To study the failure mechanisms a simplified coating system is introduced which consists of a MCrAlY bond-coat material as the substrate, a TGO, and a yttria-stabilised zirconia TBC as a topcoat. The TBC is applied by atmospheric plasma spraying on top of specimens with defined roughness profiles, manufactured by a micromachining process. The main advantage of micro-machining is a defined interfacial roughness between the TBC and the BC in contrast to sandblasted specimens. Furthermore, a FEM simulation of the coating system was developed which approximates the interface by sinusoidal functions. This simplified model system and additional FEM calculations show the influence of varying the interfacial roughness between the BC and the TBC.

Surface treatments and coatings are widely used to protect components from wear and corrosion. Of all available methods, thermal spraying is arguably the most versatile with regard to coating material and morphology. Surface roughness and porosity can be adjusted in a wide range depending on the requirements. However, as-sprayed coating surfaces inevitably exhibit a certain roughness necessitating post-treatment if a smooth surface is required. The surface roughness of thermal spray coatings is usually determined by the used powder fraction and the particles’ melting degree. Using wires as feedstock material allows for a certain influence on the particle size distribution by adjusting process parameters. In this study, the influence of nozzle geometry and atomizing gas pressure on coating quality, surface roughness and cost-efficient post-treatments of wire-arc sprayed Fe-based alloys with a wide hardness-range is investigated. To allow for easy transfer to real components, the sample geometry is based on real world examples of coatings for new components and repair of worn parts. Using adapted process parameters and air-flow, the surface roughness could be decreased to allow for a less time-consuming post-treatment by grinding. Especially in repair coatings for large area applications requiring a smooth surface finish, significant runtime and cost reductions are feasible.

The objective of this study was to investigate the effects of thermal spray process selection and corresponding bondcoat surface roughness on thermal barrier coating (TBC) performance. TBC's consisting of a 300 µm (12 mil) thick air plasma sprayed (APS) top coating of ZrO 2 -8 Wt.% Y 2 O 3 and CoNiCrAlY bondcoats deposited by three different thermal spray processes were produced and their surface roughness characterized. The bondcoats were deposited using low pressure plasma spray (LPPS), shrouded air plasma spray (SPS) and high velocity oxy-fuel (HVOF) combustion spray. Bondcoat surface profiles were measured by profilometric and interferometric techniques and surface roughness values calculated. TBC performance was evaluated by adhesive bond strength testing, thermal shock and thermal cycling testing, and microstructural analysis. Results showed that the bondcoat deposition process used and corresponding surface roughness had significant effects on the adhesive strength, thermal shock and thermal cycling lifetime, and failure mechanisms.

In state-of-the-art manufacturing of sliding bearings, brass components are soldered to respective parts, which is costly and energy-intensive. Furthermore, up to now most bearings still contain lead, which by EU regulations for new part has to be omitted due to associated health risks. Cold spraying can be employed as additive manufacturing technique and opens the perspective to deposit the requested bearings in desired leadfree layout where needed. Aside cohesion and tribological behaviour, sufficient adhesion of the coating is essential for applications. The present study aims to systematically elucidate the influence of surface roughness on adhesion. The surface roughness was adjusted by varying the grit blasting material, grit size, blast pressure, blast distance and substrate material with the aim to study influences by impact conditions, surface topography on particle deformation and bonding in cold spraying. The results show that the adhesion strength reaches a maximum for a certain roughness. The ideal surface roughness to ensure good adhesion of cold-sprayed coatings apparently depends on specific impact conditions related to the powder material strength but also on the substrate material strength and particle size distribution. By systematic tuning of blasting conditions, coating adhesion can be increased by about a factor of two, thus meeting the requirements for new lead-free bearings.

In the field of additive manufacturing, the demand for Extreme High-Speed Laser Material Deposition (EHLA) is increasing due to its unique process characteristics, economic efficiency as well as its great resource efficiency. The process is currently mostly used for surface functionalization through coating, by means of corrosion and wear protection. Thereby, almost all materials can be processed and nearly all material combinations can be created. The layers produced are dense and metallurgical bonded, and furthermore the surface roughness produced is low, so that only 20-100 μm has to be removed to produce a finished surface. However, it can also be used for the generation of 3D geometries. The greatest cost factor in the production is the coating material. With increasing requirements, for example in wear protection, cost-intensive special alloys or materials must be used. An opportunity to increase the areas of application in the field of wear resistance as well as increasing material efficiency is offered by combining EHLA with the innovative post-processing methods of hammering, solid as well as smooth rolling. Using these processes, the surface roughness can be reduced to a value of Rz 1-3 μm on the one hand and the surface hardness can be increased on the other hand. The hammering and solid rolling processes differ in their depth of impact. In the case of hammering, the impact depth can be a few millimeters and in the case of solid rolling only a few tenths of a millimeter. So far, the influence of hammering or solid rolling of additive manufactured volumes or surfaces has not been investigated. In the context of this study, the influence of hammering and solid rolling on a volume produced with EHLA is investigated. For this purpose, an EHLA produced volume of IN718 is built up and the influence of hammering as well as solid rolling on the surface roughness and hardness is analyzed.

A design of experiments approach was used to characterize process-microstructure relationships in the twin wire arc process using zinc feed stock. Specifically, the effect of arc current, primary atomizing gas pressure, secondary atomizing gas pressure, and standoff distance on deposition efficiency, spray pattern shape, atomization behavior, coating porosity, and coating surface roughness were investigated. All work was conducted using a Praxair 8835 torch spraying 02Z zinc feed stock. It was found that primary and secondary atomizing gas pressure significantly affected spray pattern shape, atomization behavior, coating porosity and coating surface roughness. Arc current significantly affected spray pattern shape. Standoff distance significantly affected deposition efficiency, spray pattern shape, and surface roughness. Discussion will focus on using the relationships identified through this experiment to tune the wire arc process.

The aim of this study was to investigate the microstructure (including surface roughness, microstructure and microhardness) of cold sprayed copper coatings on 2017 Al alloy, before and after annealing at 350°C for 1h. It seemed that keeping constant the powder flow rate and increasing the gas pressure from 2.0 to 2.5 MPa, the surface roughness of the coating tended to decrease, while the thickness of the coating tended to increase. Also, it seemed that keeping constant the pressure and increasing the powder flow rate from 22 to 130 g/min, the surface roughness and the thickness of the coating tended to increase. The microstructural study of the coatings after etching revealed particle interfaces and in some cases grain boundaries. Twins were observed in some of the coatings. Negligible porosity, absence of cracks and good adhesion of the coatings to the substrate were observed. The microhardness of the coatings varied between 95 HV 0.3 and 150 HV 0.3 for the different employed cold sprayed conditions. After coatings’ annealing the microhardness of all the coatings (cold sprayed and HVOF sprayed) decreased. The aforementioned results were compared with those of HVOF coating.

Surface preparation is very important for reliable adhesive bonding of cold sprayed coatings to the substrate. In this work, the grit blasting of low-carbon A516 steel substrates with Al2O3 particles was studied and the roughness parameters Ra and Rt of the grit blasted surfaces were then measured. The influence of alumina grit size on the roughening of the A516 steel substrate, and the resulting effect on the roughness of the Cu coating – steel interface were studied. The results showed that variations of the grit blast size had significantly affected the resultant surface roughness of the substrate. The adhesive strength of the formed copper coatings on A516 steel substrates depends on the surface roughness and hardness of the base material. The adhesive strength about 110-200MPa was achieved. The specific features of the Cu coating-A516 steel interface topography were examined and discussed.