Thermal spray deposits are built layer by layer following several passes of the spray gun in front of the part to be coated. Between each pass, dusts from the surrounding atmosphere and condensed vapors are deposited and hence blur the surface onto which the following layer will be formed; these phenomena weaken the deposit cohesion and lead in the same time to the formation of pores and others stacking defaults. To circumvent such a problem and to improve the deposit overall quality, in situ laser ablation was performed during the coating manufacturing. The ablation stage occurred between each pass. This paper presents at first the experimental device and the major considered processing parameters. Among those, energy density is one of the most critical. It then investigates the structural modifications and the induced improvements obtained for grey alumina coatings considering principally the deposit cohesion and the porosity level.

Thermal spraying technique has been widely applied for the production of ceramic protection layer on metals that used in various hostile environments. However, all sprayed coatings have defects such as connected pores and unmelted particles, which deteriorate coating properties. To improve the properties of sprayed coatings, a lot of approaches have been undertaken such as laser irradiation, seal sintering with liquid alloys and sol-gel infiltration technique. Lasers are promising technological tools due to its speedy treatment and simplicity of process control. Moreover, laser treatment technology enables not only the post-treatment but also the pre and simultaneous treatment by combining with spraying process. Generally, wide beams of as uniform as possible are preferred for use in laser surface treatment to obtain a uniform depth of melting, alloying or cladding and to cover a large area by partially overlapping of tracks. However, it is not easy to produce a uniformly treated coating by conventional laser treatment method as desired. To obtain a near-uniform beam intensity for practical laser irradiation, a kaleidoscope was installed in a conventional YAG laser. In this research, laser beam properties of YAG laser equipped with a kaleidoscope and its effect on surface modification of plasma sprayed zirconia coatings and WC-Co system coatings prepared by HVOF spraying was investigated.

This paper deals with a comprehensive evaluation of the laser glazing or re-melting route as a possible means of specifically enhancing the performance of thermal sprayed WC-Co coatings. In the present study, a high-power continuous-wave 9kW CO 2 laser was utilized for laser treatment of plasma sprayed as well as detonation sprayed WC-Co coatings. The influence of the two most important laser-related variables, namely laser power and scan speed, on the properties of the laser-treated layers was investigated. Both mere surface densification by melting a thin top layer of the coating as well as melting of the entire portion of the coated layer were targeted during laser treatment. In each case, the laser treated coatings were fully characterized by optical microscopy, scanning electron microscopy, and microhardness measurements. In addition, the influence of laser processing on the elemental distribution, phase constitution and extent of defects in the treated layers was investigated. The tribological performance of the laser-glazed coatings was also evaluated and compared against the performance of their as-sprayed counterparts. The study has revealed significant differences between the response of plasma and detonation sprayed WC-Co layers when subjected to laser treatment. The potential of plasma-sprayed coatings to match the performance of the inherently superior detonation sprayed coatings by adopting laser glazing as a post-processing step has also been assessed.

This article describes a method of preparing bioactive and porous titanium coatings on titanium-based substrate using vacuum plasma spraying and chemical treatment in alkali solution. The porous titanium coating was fabricated in two-layer structure. The bond strength, average porosity and roughness (Ra) of the porous titanium coating are 55MPa, 30% and 21µm, respectively. The chemical treatment of as-sprayed titanium coatings was carried out in 5.0M NaOH solutions at 40 °C for 24h. The surface morphology and structure of the porous titanium coating before and after chemical treatment were examined by SEM and laser Raman spectroscopy. The treated titanium coating was immersed into SBF to evaluate its bioactivity by examining apatite formation on its surfaces. It was observed by SEM and TF-XRD that apatite was formed on the surface of the treated titanium coating after immersion in SBF. The spherical aggregates gradually grew large by consuming calcium and phosphate ions in SBF and covered the surface with increase in soaking time. Incorporation of CO 3 2- ions into apatite crystal lattice was revealed by FT-IR. The results obtained indicated that net-like sodium titanate formed due to NaOH attack was responsible for apatite nucleation and growth. It is concluded that vacuum plasma spraying and subsequent chemical treatment is an effective way to produce bioactive and porous titanium coatings.

Hard surfacing layers of WC-Co/Ni-based self-fusing alloy (SFA), Ni-based SFA and Cr 3 C 2 -NiCr alloy were formed using powder and an electron beam. When the layers were examined using the Vickers hardness test, a sand erosion test and an immersion corrosion test, they were found to display high erosion and corrosion resistance. The WC-Co/Ni-based SFA, Ni-based SFA, and Cr 3 C 2 -NiCr alloy layers displayed high hardness of 1400HV, 780HV and 900HV, respectively.

Microstructural study of plasma sprayed chromia coatings sealed with aluminum phosphate, was carried out for determining strengthening mechanisms of the sealant. Characterization was accomplished by X-ray diffractometry, scanning electron microscopy, and analytical transmission electron microscopy. The main phase in the coating is the eskolaite type α-Cr 2 O 3 . The overall structure of the coating is lamellar with columnar grains parallel to the lamella thickness. Amorphous aluminum phosphate sealant has penetrated into the coating filling the structural defects such as cracks, gaps and pores between the lamellas. The average composition of the sealant in the coating is 25 at% aluminum and 75 at% phosphorus giving the molar ratio P/Al of 3, that corresponds to metaphosphates Al(PO 3 ) 3 . The aluminum phosphate sealing in the chromium oxide coatings is based on adhesive binding due to the attractive forces between the condensed phosphates and the coating. There were no indications about chemical binding due to reactions between the sealant and the coating in the sealing treatment for chromia coatings.

Thick alumina coatings produced by Air Plasma Spraying have an interconnected porosity, thus the use of these coatings in oxidizing or corrosive environment is not suitable. In this paper, a study is developed in order to limit this problem on metallic substrates. It consists in using two successive techniques: APS and PECVD. Two parameters have been shown to be important: the roughness and the preheating temperature. Two types of duplex (PECVD coating as top coat or as bond coat) have been achieved on two substrates (TA6V and stainless steel 316L). The optimization of each process has shown that the substrate has to be grit blasted and preheated (360°C for PECVD and 250°C for APS). This study has revealed that a good (36 ± 5 MPa) APS coating adhesion was obtained on smooth TA6V substrates (due probably to a chemical reaction between TiO 2 and alumina) while for stainless steel substrates, the Ra has to be at least 2µm to achieve 66 ± 5. When observing the first APS splats sprayed on the PECVD alumina smooth layer, they exhibited a specific appearance: low flattening degree (about 2 against 5 on metallic substrates) with most of the alumina in the splat rim or some sort of lace morphology. However, as a whole, the adhesion of the APS coating on the PECVD one was excellent: 60 + 4. An electrochemical method has shown that the PECVD layer on APS coating has reduced drastically its open porosity.

The sealing effect of various sealants applied to sprayed coatings in sodium chloride aqueous solution was investigated with the galvanostatic technique in which the same quantity of electricity is fed to each test material coated with sealant film cut crossly. The amount of dissolved sprayed coating sealed with sealant can be obtained as the difference between the dissolved material volume at the crosscut part and that in the solution. From the result it was found that the higher the sealing effect of sealant the more aluminum is dissolved from the sprayed coating at the crosscut part, which is considered to mean in case of the sealant poor in sealability, sprayed coating is dissolved even where it is coated with sealant and electrolytic current is dispersed. This proves that the galvanostatic technique is effective in evaluating the sealing effect of sealants.