High power diode lasers (HPDL) at the level of ? 6 kW are efficient cladding tools in heavy engineering applications where thick (up to 5 mm) wear and corrosion resistant coating layers are required. Large beam geometry makes possible the overlap of thick 20 mm wide cladding tracks side by side without coating defects. Compact size and closed cooling water circulation enable HPDL cladding process to take place also at a site of new or worn high-value machine parts, which have worn in operation or been damaged already during overseas transportation. Instead of moving parts of several tons’ weight, it would be perhaps more cost efficient to transport HPDL cladding unit.

In the present study, surface engineering related to simultaneously thermally sprayed and laser melted corrosion resistant coatings has been carried out. A 6 kW high power diode laser or a Nd:YAG laser and a flame spray gun were used to construct the laser assisted thermal spraying system. The main aims of the study were to find out the optimal processing parameters and materials to create dense corrosion resistant coatings on steels. The corrosion resistance of the manufactured coatings was measured and the microstructure characterized using optical and scanning electron microscopy.

Alumina and stabilized Zirconia were plasma sprayed in air using the water-stabilized plasma torch. In the case of Alumina two different stand-off distances were applied at spraying. Nd-YAG laser was then used for additional treatment of plasma sprayed coatings. The laser was maintained in a quasi-continual regime and defocused from the surface to increase the treated coating's area. Energy density was varied together with the laser scanning velocity to ensure variance in thermal history of the treated surfaces. Microhardness, surface roughness and slurry abrasion resistance (SAR) were measured before and after the laser treatment. Results vary in dependence on the laser treatment's parameters. When the treatment results in substantial changes of the structure (up to a complete re-melting of the surface), enhancement of all measured properties was proven. It is demonstrated that the extent of change of mechanical properties can be correlated with optical properties of coating materials at the laser wavelength. Microstructural aspects of the laser treatment are discussed as well, especially at the boundary between the laser-annealed layer and the basic coating's microstructure. It is pointed out that laser overheating due to use of an extremely high energy density can cause delaminating of the coatings.

Laser cladding is a surface treatment technology in which thick, dense and metallurgically adhered metallic layers are deposited on various structural steels with relatively low heat input, high accuracy and reproducibility. Laser cladding processes used in industrial cladding are largely based on the use of CO 2 or Nd:YAG lasers. High power diode lasers (HPDL) with rectangular beam spots are regarded as ideal laser sources for laser cladding processes, due to their compact size, high electrical to optical efficiency, easy operation, and low investment and running costs. In laser cladding of large surface areas, the affectivity of the laser cladding process becomes more important, i.e. high laser powers, wide laser beam spots, and high coating material feedrates are regarded as beneficial. In order to optimise the cladding process for such applications, special attention has to be put on devices used to deliver the coating power to the process. In the present work, various parameters in effective HPDL cladding are described and new approaches to optimised HPDL cladding process are described. The performance of a new HPDL cladding powder delivery nozzle will be presented and discussed. Abstract only; no full-text paper available.

The paper describes the some differences of laser coating (laser cladding or laser spraying) process in comparison to thermal spraying. Laser coating is a novel coating process, which produces coatings with high density, metallurgical bonding and low heat input to the substrate. Laser coating types and coating properties are reviewed and compared with thermally sprayed coatings. Typical application areas of laser coatings are presented.

In gas turbines and diesel engines there is a demand for Thick Thermal Barrier Coatings (TTBCs), because of the increased process combustion temperatures. Unfortunately the increased thickness of plasma sprayed TBCs normally leads to a reduced coating lifetime. So for that reason the coating structures have to be modified. When modifying the structure of TTBCs, the focus is normally set on elastic modulus reduction of the thick coating, in order to improve the coating strain tolerance. On the other hand, coating structural modification procedures, such as sealing treatments, can be performed when increased hot corrosion resistance or better mechanical properties are needed. In this paper we introduced several modified zirconia based TTBC structures and their specific microstructural properties. Coating surface sealing procedures such as phosphate sealing, laser-glazing and sol-gel impregnation were studied as potential methods in increasing the hot corrosion and erosion resistance of TTBCs. Some microstructural modifications were also made by introducing segmentation cracks into the coating structures by laserglazing and by using special spraying parameters. These last two methods were studied in order to increase the strain tolerance of TTBCs. The coating microstructures were characterized by optical microscopy, SEM, TEM, EDS analysis and X-ray diffraction. The effect of sealing procedures was studied on basic thermal and mechanical properties of the coatings. In the paper it was also presented some correlations between the coating properties and microstructures, and discussed about the advantages and drawbacks of each modification procedure.

Thermal spray processes are widely used to deposit high-chromium nickel-chromium coatings to improve high temperature oxidation and corrosion behaviour. However, in spite of the efforts made to improve the present spraying techniques, such as HVOF and plasma spraying, these coatings may still exhibit certain defects such as unmelted particles, oxide layers at splat boundaries, porosity and cracks, which are detrimental to corrosion performance in severe operation conditions. Due to low process temperature only mechanical bonding is obtained between the coating and substrate. Laser remelting of the sprayed coatings was studied in order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties. The coating material was high-chromium nickel-chromium alloy, which contains small amounts of molybdenum and boron (53.3%Cr- 42.5%Ni - 2.5%Mo - 0.5%B). The coatings were prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power fiber coupled continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of scanning speed and beam width (10 mm and 20 mm). Coating remelted with the highest traverse speed tended to suffer cracking during rapid solidification inherent to laser processing. However, choosing appropriate laser parameters, non-porous, crack-free coatings with minimal dilution between coating and substrate were produced. Laser remelting resulted in the formation of dense oxide layer on top of the coatings and full homogenization of the sprayed structure. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy (OPM, SEM). Dilution between coating and substrate was studied with EDS. The properties of the laser remelted coatings were directly compared with properties of as-sprayed HVOF coatings.

Zirconia based, 8Y 2 O 3 -ZrO 2 and 22MgO-ZrO 2 thick thermal barrier coatings (1000µm), were studied with different sealing methods for diesel engine applications. Aim of the sealing procedure was to improve hot corrosion resistance and mechanical properties of porous TBC coatings. The surface of the TTBCs was sealed with two different methods, phosphate based sealing treatment and laser glazing. The thickness of the densified top layer in all cases was 50-400µm. XRD analysis showed some minor phase changes and reaction products caused by phosphate based sealing treatment and some crystal orientation changes and phase changes in laser-glazed coatings. The porosity of the outer layer of the sealed coating decreased in all cases, which led to increased microhardness values. The hot corrosion resistance of TTBCs against 60Na 2 SO 4 - 40V 2 O 5 deposit was determined in isothermal exposure at 650°C for 200 h. Corrosion products and phase changes were studied with XRD after the test. Short-term engine test was performed for the reference coatings (8Y 2 O 3 - ZrO 2 and 22MgO-ZrO 2 ) and for the phophate sealed coatings. Engine tests were performed at the maximum load of the engine and it was aimed to evaluate the thermal cycling resistance of the sealed coatings. All the coatings passed the engine test, but some vertical cracks were detected in the phosphate sealed coatings.

Thermal spray processes are widely used to protect materials and components against wear, corrosion and oxidation. Despite the use of the latest developments of thermal spraying, such as HVOF and plasma spraying, these coatings may in certain operation conditions show inadequate performance, e.g. due to insufficient bond strength and/or mechanical properties and corrosion resistance inferior to those of corresponding bulk materials. The main cause for a low bond strength in thermal sprayed coatings is the low process temperature, which results only in mechanical bonding. Mechanical and corrosion properties typically inferior to wrought materials are caused by the chemical and structural inhomogeneity of the thermal sprayed coating material. In order to overcome the drawbacks of sprayed structures and to markedly improve the coating properties, laser remelting of sprayed coating was studied in the present work. The coating material was nickel based superalloy Inconel 625, which contains chromium and molybdenum as the main alloying agents. The coating was prepared by high-velocity oxy-fuel spraying onto mild steel substrates. High power continuous wave Nd-YAG laser equipped with large beam optics was used to remelt the HVOF sprayed coating using different levels of power and scanning speed. The coatings as-sprayed and after laser remelting were characterized by optical and electron microscopy. Laser remelting resulted in full homogenization of the sprayed structure. This strongly influenced positively the performance of the laser remelted coatings in adhesion, wet corrosion and high temperature oxidations test. The properties of the laser remelted coatings were compared directly with the properties of as-sprayed HVOF coatings, and with PTA overlay coatings and wrought Inconel 625.