Direct Metal Laser Sintering (DMLS) technique is one of the technologies which is generally used to built prototypes and tooling applications. DMLS uses powder bed fusion to bond particles together by laser energy. A new powder layer is spread on top of the previous layers and the process is repeated up to required shape of part can be produced. This review paper presents development, current status and challenges of the DMLS technique with emphasises on material processed by DMLS and is aimed to understand influence of density, microstructure, micro-hardness, tensile strength and wear behaviour of built-up parts. It also highlights the process through proofs based on classical results in terms of advantages and applications.

Due to the superposed thermal and mechanical stress profile, thermo-mechanically coupled forming processes require tools and machine components which meet high demands. High forming forces and process temperatures in the contact zone between the tool and the workpiece limit the life span of these tools. A promising approach for protecting such tools is a combination of thermally sprayed coatings and physical vapor deposited layers. This coating system combines the characteristics of the individual layers and leads to superior mechanical, tribological as well as thermal properties under the mentioned coupled stresses. In this study thermally sprayed alumina (Al 2 O 3 ) and yttria-stabilized zirconia (ZrO 2 ) coatings were produced by atmospheric plasma spraying. Therefor different coating porosities were adjusted in order to varied the effect of thermal insulation for the substrate made of AISI H11 (1.2343). After the coating process the surface roughness of the thermal barrier coatings (TBC) were reduced by polishing process in preparation for the PVD top layer. Subsequently, wear and heat resistant hard TiAlSiN and CrAlSiN coatings were deposited on top of the polished TBCs by using magnetron sputtering process. As a reference the PVD coatings were also applied on a nitrided steel samples. Titanium and chromium interlayers were applied by PVD technique in different coating thicknesses (50 – 150 µm) between PVD and thermally sprayed coatings. Afterwards the influence of these metallic interlayers on coating adhesion of PVD coatings were analyzed by performing scratch tests. Hardness and young’s modulus of PV coatings were investigated by means of nanoindentation. The morphology and topography of the coatings were analyzed by scanning electron microscopy, light microscopy and optical three-dimensional surface analyzer. EDX analyses and X-ray diffraction were used to determine the chemical composition of the PVD coatings. Finally the wear resistant of the PVD top layers were determined at different temperatures (20°C, 500°) by using a high temperature tribometer.

B 4 C-Ni powders ranging in content from 5-60 wt% Ni were fabricated by pressurized hydrogen reduction and deposited on mild carbon steel substrates by air plasma spraying. The microstructure, morphology, and phase composition of the powders and coatings were evaluated by means of SEM and XRD analysis. The influence of Ni content on coating microstructure, fretting wear resistance, hardness, and adhesive strength was investigated in detail. The results show that Ni affects fretting wear resistance, which was found to be highest in the coating with 40 wt% nickel. The B 4 C-40Ni coating also proved superior in terms hardness, porosity, and friction coefficient, although its adhesive strength was the lowest.

In this investigation, spherical Cr 7 C 3 -(Ni,Cr) 3 (Al,Cr) powder with different compositions was prepared by vacuum melting and gas atomization. During the solidification process, Cr 7 C 3 is in-situ synthesized and uniformly distributed in the Cr-doped Ni 3 Al phase. Coatings produced from the powders by HVOF spraying were characterized based on composition, microstructure, hardness, and tribological properties. The results show that the coatings compare well with commercial Cr 3 C 2 -NiCr coatings used on piston rings in heavy duty diesel engines. Optimization routes for further improvement are also discussed.

In this study, Vickers indentation testing is used to determine the fracture toughness of hard flame spray coatings produced from cored-wire feedstocks, including WC-10Co4Cr, 75CrCo-25NiCr, and WC-12Co. Average measured values are compared with fracture toughness values calculated from seven different equations found in the literature in order to validate the experimental results and to better understand the relationship between fracture toughness and coating hardness for each of the tested materials.

In this study, 1-1.5 μm chromium carbide particles are used to prepare CrC-NiCr and CrC-Ni-Cr powders by spray drying and vacuum sintering with the aim of optimizing the properties while minimizing the cost of HVOF-sprayed NiCr-CrC coatings. The powders are characterized by means of SEM and XRD analysis and the coatings are evaluated based on microstructure, microhardness, bonding strength, and erosion resistance.

This study assesses the abrasive wear resistance of Cr 3 C 2 -NiCr coatings produced by HVOF spraying. Abrasion tests were conducted in a three-body solid-particle rubber-wheel test rig using silica and alumina grits under different loads. The results indicate that abrasion performance is controlled by cohesion between splats, which can be further improved. The removal of carbide particles was the main wear mechanism and is controlled by the content and size of the Cr 3 C 2 particles. It is shown that the abrasive wear resistance of carbide-based cermet coatings is significantly higher than that of mild steel.

The anti-wear coatings in this study are developed for the use in forming tools, which need a good resistance against different wear mechanisms such as surface fatigue, adhesion and abrasion. NiCrBSi is suitable for this purpose because of its high resistance against different types of wear at ambient and high temperatures. Due to the low thermal and kinetic energy during the spraying process, the arc-sprayed NiCrBSi coating shows a high interlaminar boundary porosity as well as numerous interface cracks between the coating and the substrate, which can reduce the wear resistance and the adhesion to the substrate. In this work, arc-sprayed NiCrBSi coating were remelted by a treatment with an oxyacetylene flame, induction heating, furnace and laser. The treatment parameters were investigated and optimized. The modified microstructure was characterized using an optical microscope and a scanning electron microscope as wells as an EDS analysis. The hardness of the coatings was evaluated before and after remelting. To examine various wear mechanisms, pin-on-disc tests were conducted with different wear counterpart. The wear and coefficients of the as-sprayed and remelted coating were determined. In order to show the change of the wear resistance as well as the wear mechanisms, wear tracks were characterized with a scanning electron microscope.

HVOF sprayed coatings are widely used to improve the service life of machines components. Influence of spraying parameters and different kinds of materials on coatings properties have been investigated in many works. Our investigations concern properties of HVOF sprayed two tungsten carbide coatings after applying electrospark deposition process (ESD). After finishing microstructure of sprayed coatings were changed by applying tungsten carbide electrode. Changes of microstructure and properties of the sprayed and post ESD process coatings were analysed with a scanning microscope JOEL JSM-5400. The element distribution was analysed with a microprobe ISIS 300 Oxford Instruments whereas phase composition was analysed by diffractometer D8 Advance (BRUKER). The roughness of coatings was measured with Talysurf-4, whereas Matuzawa MMT-X3A was used to study changes of their hardness. Ball on disc tester and dry abrasive rubber wheel tester were applied to estimate coefficient of friction and wear resistance of sprayed coatings before and after EDM process.

Hardmetal coatings prepared by high-velocity oxy-fuel (HVOF) spraying represent an advanced solution for surface protection against wear. The work presented in this paper focussed on the comparison of the tribological behaviour of WC-Co, Cr 3 C 2 -NiCr and (Ti,Mo)(C,N)-NiCo hardmetal coatings under dry sliding wear conditions at room and elevated temperatures (at 500°C and 700°C). The friction properties of these HVOF-sprayed coatings were studied in detail. The progression of the coefficient of friction (CoF) was measured during the pin-on-disc tests at the different temperatures. The mechanisms of wear and the heat-related changes in surface roughness and microhardness of the coatings were studied.

The significant erosion of the boiler tube at high temperature becomes an important problem for the safe operation of circulating fluidized bed boiler. This paper investigated the erosion behavior of the HVOF sprayed Cr 3 C 2 -NiCr coating at high temperature comparing with the typical mild steel for boiler tube. Results showed that the erosion rate of the mild steel increased with the increase of temperature. The erosion rate of the mild steel at 800°C was 4 times that at 300°C at an erosion angle of 30°. However, the erosion rate of the HVOF sprayed Cr 3 C 2 -NiCr coating was not influenced by the temperature in the range of 300 to 800°C. It is found that the erosion resistance of HVOF sprayed Cr 3 C 2 -NiCr coating was more than 3 time higher than that of the mild steel at 700 to 800°C. In addition to the ploughing on the surface of the worn coating, the cracking along splats interfaces in the coating was clearly observed on the cross-sectional microstructure of the coating. The results indicate that the erosion performance of the HVOF sprayed Cr 3 C 2 -NiCr coating is controlled by the cohesion between splats in the coating and can be further enhanced by improving splat cohesion.

Three types of cobalt-based cermet coatings were prepared by high velocity oxy-fuel (HVOF) spraying using cobalt- clad TiC-50Co, SiC-50 Co and WC-18Co powders. The microstructure of three coatings was characterized using a scanning electron microscope (SEM). The adhesive strength of the coatings was tested according ASTM C633-79 standard. The hardness of three coatings was measured using a HV-5 Vickers hardness tester. The abrasive wear performance of the coatings was examined by a dry-sand rubber wheel tester according to ASTM G65-61 standard. The results show that the density, thermo physical properties and volume fractions of the solid carbide phases in the spray particle have a significant influence on the adhesive strength of the coatings. The hardness of WC-18Co coating is higher than that of TiC-50Co and SiC-50 coatings and is much lower than WC-17Co coating deposited with sintered-crushed powders. Moreover, the abrasive wear volume loss of the WC-18Co coating is about 60 times higher than that of the WC-12Co coating sprayed by sintered-crushed powder, and greatly lower than that of TiC-50Co and SiC-50 coatings. The wear mechanisms of three coatings are discussed.

HVOF-sprayed coatings (WC-17Co, WC-10Co-4Cr, Co-28Mo-17Cr-3Si) have been compared with various kinds of industrially manufactured hard chrome coatings (HCC), whose substrate preparation, deposition process, post deposition treatments greatly affect their characteristics. Microstructure, micromechanical properties, tribological behaviour and corrosion resistance (electrochemical polarization tests and Corrodkote test) have been studied. HVOF-sprayed cermets are harder but less tough than HCC, Co-28Mo-17Cr-3Si are less hard than HCC. Splats detachment causes a comparable or higher mass loss in three-body abrasion than HCC coatings. Forming a uniform surface film, cermet coatings definitely overcome HCC in two-body sliding, while Co-28Mo-17Cr-3Si has insufficient hardness to display good sliding wear resistance. HVOF coatings show no passivation in corrosive media but cermets posses more noble corrosion potentials than HCC, and undergo generalized corrosion in HNO 3 and HCl, with similar corrosion current densities (I C ). HCC passivate and resist well in HNO 3 0.1N, but undergo pitting corrosion in 0.1N HCl. Definitely different E C and I C are recorded for various HCC in HCl. HVOF-sprayed cermet coatings show lower I C in 0.1N HCl solution than several kinds of HCC. No visible damage occurs on HVOF-sprayed coatings after the Corrodkote test, while non de-hydrogenated HCC suffered pitting corrosion.

The delamination wear mechanism of the thermally sprayed coatings was studied by analyzing coatings structural feature and stress distribution on the warm surface, and the influencing factors on the delamination wear were discussed. And the delamination wear mode of coating was developed. The results show that, the thermally sprayed coatings have typical aspect of lamellar structure. There are oxide layers between splats, and there also exist porosity and micro-crack in the coatings. The coating surface was subjected to alternately tensile stress and compression stress caused by normal load and friction force during sliding. In a certain depth below the surface, there exists maximum shear stress. Therefore fatigue damage will take place at subsurface of the coating under alternate stress. The adhesion strength between splats of coating prepared by HVAS is by far lower than casting material because of lamellar structure. And the adhesion strength between splats is further weakened due to the defects (such as porosity and micro-crack) appearing mostly on the boundaries between thin oxide sheets and splats. When the fatigue damage accumulates to a certain value, micro-cracks initiate at the defects between splats. Then these micro-cracks grow, connect, and propagate along the defects between splats. Finally, these cracks shear to the coating surface leading to spallation of the splats, and thus wear debris is generated. By repeating the above process delamination of the coatings will occur. Reducing friction coefficient, increasing coatings hardness and adhesion strength between inter-splats are the basic methods to improve the wear resistance of thermally sprayed coatings. Abstract only; no full-text paper available.