Laser Glazing Process Research Articles

Improvement of CMAS resistance of laser glazed and nano-modified YSZ thermal barrier coatings

The present investigation used atmospheric plasma spray (APS) as a method for the production of thermal barrier coatings composed of Yttria-stabilized zirconia (YSZ). A CO2 laser was used to remove surface cracks from the YSZ coatings' surface. Several parameter variations were applied to generate laser-glazed tracks on the coating surface, resulting in an impure surface microstructure with excellent surface properties. Scanning electron microscopy was utilized to analyse the tracks' microstructure. The optimal laser parameters were determined to be 77.7 W laser power, 215 mm laser distance, and, 120 mm s−1 scanning speed based on microstructural characteristics. Once the laser parameters had been optimized, a whole coating surface was laser glazed. In order to ascertain the impact of laser glazing, stability analysis, fracture toughness, and hardness evaluations were conducted on both unglazed and laser-glazed coatings. The growth of a network of cracks was seen on the surface. The surface roughness exhibited a reduction of about tenfold in comparison to the unglazed YSZ. The presence of the monoclinic ZrO2 phase within the coating structure was successfully eradicated. The laser glazing process resulted in enhancements to both the fracture toughness and surface hardness of the coating. Despite the fact that the gaps and imperfections on the coating surfaces were repaired during laser glazing, some cracked structures nonetheless appeared. Nano YSZ was deposited on laser-glazed surfaces by the electrophoretic method in order to eliminate cracks from laser-glazed surfaces. The experimental results obtained from conducting calcium-magnesium-alumina-silicate (CMAS) and hot corrosion tests on laser-glazed coatings indicate that the use of nano YSZ filler leads to a significant reduction in salt penetration during both glassy melt and hot corrosion processes.

Finite element analysis on temperature field and stress distribution of thermal barrier coatings by laser modification and CMAS corrosion

Laser glazing to produce a modified layer on the surface of thermal barrier coatings (TBCs) is a promising method to alleviate calcium-magnesium-alumina-silicate (CMAS) attacks to coatings. In this study, finite element analysis is carried out to investigate the temperature field and stress distribution of TBCs after the laser glazing process and the modified TBCs after CMAS corrosion. Results revealed that along the direction of laser scanning, the principal stress was in a tensile state, which increased along the direction of the laser spot movement; while perpendicular to the laser scanning direction, the maximum principal stress appeared at the interface between the glazed layer and the unmodified coating, where could be a potential danger zone for crack initiation and coating spallation. For the modified TBCs with CMAS attack, in regions that are located in the range of 0–6 mm along the radial direction, the radial stress and maximum principal stress were high (∼1094 MPa), and the coating edge had complex shear stress state; as a result, these areas were easy to crack.

Microstructural characterization of modified plasma spray LZ/YSZ thermal barrier coating by laser glazing

One of the high potential material, lanthanum zirconate (LZ) is deposited on superalloy 625 due to its excellent phase stability, low thermal conductivity as well as the high melting point. However, microstructural defects were detected due to lamellar structure behaviour exhibited from the plasma-sprayed coating techniques. This paper investigated the effect of surface modification on LZ thermal barrier coating (TBC) by using laser glazing method. The double ceramic layer (DCL) TBCs are composed of NiCoCrAlYTa bond coating formed via the HVOF process while 8YSZ ceramic top coat and La2Zr2O7 as the third layer were produced by utilizing the APS technique. Nd:YAG pulsed laser has been used to modify the surface layer of plasma-sprayed LZ coatings. The microstructural morphologies of both as-sprayed and laser glazed samples were investigated using Scanning Electron Microscopy (SEM). The phases of the coatings were analysed with X-ray Diffractometry (XRD). The results showed that the laser glazing process reduced surface roughness attributed to the sealing of porosities and other structural defects of the coating. Finally, the impacts of laser energy input and laser distance on the characterization of the glazed layer are discussed.

Microstructural study and hot corrosion behavior of bimodal thermal barrier coatings under laser heat treatment

In this study, nanostructured YSZ powders were deposited on the Hastalloy X Superalloy substrate coated with a metallic bond coat by plasma spraying to produce a nanostructured thermal barrier coating with bimodal microstructure. After that, the coated samples were heat-treated using a Nd:YAG laser. Then, the microstructures of the conventional and nanostructured TBCs before and after the laser glazing process were examined using a scanning electron microscope (SEM). The coating phases were studied by X-ray diffractometry (XRD). The high-temperature corrosion behavior of the nanostructured plasma sprayed coating in the presence of Vanadium pentoxide and Sodium sulfate molten salt was compared with that of the conventional coatings before and after laser treatment at 1050 °C. The hot corrosion results showed that the coatings had a similar degradation mechanism based on which the corrosive molten salt reacted with the stabilizer of YSZ, producing hot corrosion products such as YVO4. It led to an unwanted phase transformation from tetragonal (t) to monoclinic (m) Zirconia and the final degradation of the TBC system. However, reducing molten salt penetration, decreasing surface roughness and, reduction of the specific surface area are three important mechanisms that improved hot corrosion resistance, finally extending the lifetime of the glazed samples. The results also showed that the nanostructured TBC had higher hot corrosion resistance in comparison with other samples.

The effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings

The aim of the present research was to investigate the effect of laser surface treatment on the thermal shock behavior of plasma sprayed Al2O3/YSZ multilayer thermal barrier coatings. In this regard, specimens of Inconel 718 were sprayed with Ni–22Cr–10Al–Y bond coat and Al2O3/YSZ top coat using an atmospheric plasma spraying approach. Then, top surfaces of specimens were under exposed to Nd-YAG laser radiation and thereby were glazed. Thermal stabilities of both coatings were evaluated by rising the environment temperature up to 1000 °C, keeping at this temperature for 15 min, and also rapid cooling of specimens in cold water. Using FESEM with EDS and XRD, the microstructure and morphology observations, elemental investigations and also phase identification were carried out. Findings displayed that both sprayed and glazed coatings were damaged as a spallation of top coats. Furthermore, by checking the life-time of coatings, the number of thermal cycles led to 20% destruction of top surfaces of coatings, increased from 232 cycles in sprayed coating to 325 cycles in laser glazed Al2O3/YSZ multilayer coating which displayed 40% improvement of thermal shock resistance after laser glazing process. This enhancement in resistance to thermal shock was due to strain adaptability improvement via a network of segmented cracks on the top surface produced by laser glazing process.

Evaluation of hot corrosion behavior of plasma sprayed and laser glazed YSZ–Al2O3 thermal barrier composite

The evaluation of hot corrosion behavior of sprayed and laser glazed YSZ + Al2O3 composite coatings was conducted in the presence of 55% V2O5 + 45% Na2SO4 molten salt was conducted.Inconel 718 coupons were sprayed with a Ni–22Cr–10Al–1Y bond-coat and then with an YSZ–Al2O3 top-coat as composite coatings. The sprayed coatings were glazed using a Nd:YAG laser. The results displayed that both coatings illustrated similar mechanisms of hot corrosion. However, the reaction between corrosive molten salt with Y2O3 and production of YVO4, leaching Y2O3 from YSZ, led to a destructive phase transformation of tetragonal-ZrO2 phase (t-ZrO2) to monoclinic-ZrO2 phase (m-ZrO2) and production of YVO4, accompanied by 3–5% volume expansion during cooling which could lead to cracking and degradation of TBCs. Findings revealed that by comparing the destabilization fractions of coatings, the content of m-ZrO2 fraction decreased from 80% in sprayed coating to 47% in laser glazed YSZ + Al2O3 coating, which displayed double improvement of hot corrosion resistance in laser glazing process. Reduction of the reaction between molten salt and zirconia stabilizer was the result of a surface roughness decrease and a specific reactive surface gained by laser glazing process.

Influence of preheating processes on the microstructure of laser glazed YSZ coatings

Laser glazing is considered to be a promising surface sealing technique for thermal barrier coating. The dense top layer with reduced surface roughness and the segment cracks perpendicular to the surface are considered to be suitable for improving the thermal cycling and hot corrosion resistance of these kind of coatings. In present study, yttria stabilized zirconia ceramic coatings were manufactured by atmospheric plasma spraying and then subjected to a Nd-YAG pulsed laser source. During the laser glazing process, coatings were preheated to 600°C and 800°C in order to obtain different microstructure of the laser glazed coatings. The surface morphologies and cross-sections of the coatings were examined by scanning electron microscopy and microhardness measurements of coatings were carried out. The results indicate that preheating process induces a reduction of the grain size of laser glazed coatings in conjunction with an increasing of microhardness and toughness. In addition, preheating also decreases the substrate-coating interface tensile stress which leads to a reduction of crack surface density.

The influence of laser treatment on thermal shock resistance of plasma-sprayed nanostructured yttria stabilized zirconia thermal barrier coatings

Abstract The main goal of this paper was to evaluate the effects of laser glazing on the microstructure and thermal shock resistance of nanostructured thermal barrier coatings (TBCs). To this end, nanostructured yttria stabilized zirconia (YSZ) top coat and NiCrAlY bond coat were deposited on Inconel 738LC substrate by air plasma spraying (APS). The Nd:YAG pulsed laser was used for laser treatment of top coat surface. The thermal shock behavior of plasma-sprayed and laser-glazed coatings was investigated by quenching the samples in cold water from 1000 °C. The microstructure and phase composition of the coatings were characterized by scanning electron microscopy (SEM) and X-ray diffractometry (XRD). Energy dispersive spectroscopy (EDS) was used to analyze the interface diffusion behavior of the bond coat elements. The results of SEM revealed that the laser glazing process reduced the surface roughness, eliminated the porosity of the surface and produced network cracks perpendicular to the surface. XRD results also indicated that both as-sprayed and laser glazed coatings consisted of non-transformable (T′) phase. Thermal shock test results showed that the lifetimes of the plasma-sprayed TBCs were almost doubled by laser glazing. Continuous network of segmented cracks perpendicular to the surface produced by laser glazing improved the strain accommodation and recognized it as the main enhancement mechanism for TBC life extension.

Thermal Simulation of Laser Surface Modification of H13 Die Steel

In this study, the effects of laser parameters on the properties of glazed die steel were investigated. A Rofin DC-015 diffusion-cooled CO2 slab laser with 10.6 µm wavelength was used. Die steel sample surfaces were prepared with a 3 µm roughness and chemically treated to improve CO2 laser wavelength absorbance. One set of processing parameters were processed through the thermal simulation program and correlated with physical results determined from actual test samples. Set processing parameters were 1138 W peak power, 2900 Hz PRF, 24% duty cycle and 261 mm/s traverse speed. Scanning Electron Microscopy (SEM) micrographs and micro-hardness properties of the affected surface were measured. An analytical mathematical model of the heat field generated in the laser glazing process was used to predict the nominal temperature distribution in the surface and dimensions of melt pool. A thermal model using point source surface energy inputs was used to predict the thermal profiles in the die steel. This allowed estimation of the depths of microstructurally altered regions. For higher energy absorbance, the depth of the glazed surface increased from 20 μm to 40 μm. At high nominal heating to cooling rate ratios, high micro-hardness values were recorded.

Thermal stability of laser treated die material for semi-solid metal forming

This paper presents laser surface modification work performed to improve the lifetime of die materials. Die material AISI H13, with typical hardness in the range of 42 to 48 HRC, offers high wear and corrosion resistance. However the cyclic high temperature conditions along with exposure to high viscosity molten metal in semi-solid forming cause the die to wear and crack with resultant shortened die lifetime. In this study, the thermal stability of die material at elevated temperature was investigated through micro-hardness testing and a metallographic study. AISI H13 samples were laser glazed using CO2 continuous wave mode laser with 10.6 µm wavelength. Samples were attached to a specially designed rotating chuck to enable it to be rotated at speeds up to 1500 rpm and allow flat surface glazing to take place. The micro- hardness was measured for as-glazed samples and annealed samples which were held at temperatures ranging from 550oC to 800oC with 50oC intervals. The metallographic study conducted examined the formation of three zones at different depths which were the glazed zone, the heat affected zone and the substrate. As a result of rapid heating and cooling from the laser glazing process, a metallic glass layer was developed which exhibited an average micro-hardness of 900 HV when exposed to 3.34E+10 W/m2 laser irradiance within a range of 0.0011 to 0.0018 s exposure time. Crystallization in glazed zone increased as the annealing temperature increased. As the annealing temperature reached above approximately 600oC, the micro-hardness decreased to approximately 600 HV (equivalent to approx. 54 HRC) due to local crystallization. These findings show potential direct application of glazed dies for non-ferrous semi-solid forming and the requirement for thermal barrier protection for application at higher temperatures.

Microtopographic inspection of laser-glazed thermal barrier coatings

Thermal barrier coatings (TBCs) are extremely useful in gas turbine engines. The quality of these coatings can be improved by subjecting their surface to laser glazing. A reduction in surface roughness, eliminating open porosity on the surface, and the generation of a controlled segmented crack network, can be achieved. TBCs consisting of atmospheric plasma sprayed (APS) ZrO 2 -8%wtY 2 O 3 are subjected to a CO 2 continuous wave laser glazing process to seal their surface porosity, generating an external dense layer. Different amounts of irradiation are applied to the specimens. The microtopograpic inspection of the samples reveal a significant decrease of the surface roughness after laser treatment. However, a network of surface cracks, dependent on the laser scanning speed and track overlapping, are noticed. The cracks have a tendency to be oriented in two perpendicular directions, one in the direction of the laser beam travel direction, the other perpendicular to it. Cracks parallel to the beam scanning direction are found to be at the overlapping zone, in the edge of the subsequent track.

Continued Developments in Easily Carburizable High Speed Steel Alloys

Abstract A technical update on how bearing fatigue life in high speed steels (HSS) is enhanced by modifying the carbide distributions and morphologies is presented. A brief review of the laser glazing process as applied to bearings (U.S. Patents 5,861,067 and 5.879,480) will be presented. For wrought M50 steel, this processing resulted in an increase in bearing L10 % fatigue life of approximately 10×. The microstructural enhancements created by this processing will be illustrated. How the knowledge gained from the laser glazing process led to the development of the easily carburizable high speed steel alloys (U.S. Patent 6.702,981) will be discussed. Developments made for these alloys since the last bearing steel symposium in 2001 will be presented. The most notable achievement in the development process is a 20× increase in bearing L10 % fatigue life for the easily carburizable version of M50 as compared to standard M50 HSS. Other data pertinent to these alloys will be presented.

Refining carbide size distributions in M1 high speed steel by processing and alloying

The distributions of carbides in M1 high speed steel (HSS) manufactured by four different types of processing will be considered. For conventionally processed ingot steel, the characteristic microstructural features in ingots, intermediate size products and smaller bar product will be discussed. The carbide size distributions in steel bars made by gas atomized powders will be shown. The refinement of the carbides in M1 by using a patented laser glazing process will be illustrated. The last topic is the development of a new patent-pending family of low carbon high speed alloys that can easily be carburized by conventional processing. Various beneficial effects that these alloys provide will be discussed.

Microstructural changes in laser hardened Zr-2.5Nb alloy

Laser surface treatment of metals is a process in which heating or melting of the surface of the metal by laser irradiation is followed by a rapid cooling via self-quenching. This process can produce metallurgical structures with improved wear and corrosion resistance. For zirconium alloys, laser melting (glazing) has been used to modify the microstructure of the surface layers of Zircaloy-2 and -4 tubing, and Zr-2.5Nb pressure tube alloy (1-4) without affecting the bulk properties. A supersaturated [alpha][prime]-martensitic structure was produced by the laser-glazing process. In addition, laser glazing produced a rough surface finish, which was attributed to rippling effects during the rapid solidification. In the present work the authors have investigated laser surface heating because such a treatment is expected to produce a similar microstructure as laser glazing, but with a better surface finish. Laser irradiation could raise the specimen's surface temperature into the [beta]-phase region and the subsequent rapid cooling should be sufficient to achieve the martensitic phase transformation, without requiring surface melting. The microstructure produced by laser surface heating was characterized by transmission electron microscopy. The depth profiles of the laser surface heated layers were obtained using microindentation. This paper reports the results obtained in this investigationmore » of microstructural changes produced in the surface layers of Zr-2.5Nb alloy by laser surface heating.« less

Thermal cyclic response of laser-glazed plasma-sprayed ZrO 2-19.5wt.%Y 2O 3/Ni-22Cr-10Al-1Y thermal barrier coatings

A pulsed CO 2 laser with a cyclindrical focal lens has been employed to glaze the surface layer of plasma-sprayed ZrO 2-19.5wt.%Y 2O 3/MCrAlY thermal barrier coatings (TBCs). The effects of laser glazing on the thermal cyclic response of these coatings have been evaluated. Results of this research showed that laser glazing enhanced the thermal cyclic life of plasma-sprayed TBCs by a factor of about 2–6 depending on the heating cycle length and the definition of cyclic life. The effect on the life improvement was more pronounced for specimens with a short heating cycle length. The thermal cyclic life of laser-glazed TBCs increased with increasing input energy, i.e. increased with increasing melting depth, while the overlap had no significant influence on the thermal cyclic life. The laser-glazing process did not have an evident effect on reducing the oxidation rate of the bond coat. Segmented cracks produced by laser glazing were beneficial for accommodating the strain which resulted either from the thermal cycle or from oxidation of the bond coat. Improving the strain tolerance was the major enhancement mechanism for TBC life extension. The failure of laser-glazed TBCs was similar to that of plasma-sprayed TBCs, which was initiated and propagated mainly within the top coat near the top coat-bond coat interface just above the higher asperities. The laser parameters and heating cycle length had no influence on the fracture morphologies. After cyclic tests no phase transformation was found in either the plasma-sprayed or laser-glazed top coats.

Laser-beam and photon-assisted processed materials and their microstructures

Laser processing is a relatively new technique for modifying the near-surface region of materials without altering the in-bulk characteristics. A single laser can perform several functions by manipulating processing conditions such as laser power, beam diameter, and traverse speed. Lasers have shown attractive applications, such as cutting, welding, glazing, alloying, and cladding. A laser glazing process has demonstrated an improvement in the microstructure of vacuum plasma-coated copper-based alloys containing cavities, unmelted particles, and segregation. Laser glazing has also been shown to restore the degraded microstructure of components and make them equivalent to, or better than, the original wrought alloy. The laser cladding concept was used to develop nickel-based alloys for high-temperature applications that exhibited higher thermal stability than the nickel-based Rene-95 alloy. Rapid melting and quenching occurred during the laser glazing, alloying, and cladding processes resulting in a fine-grained microstructure, metastable phases and extended solid solubility of alloying additions in the matrix.

Characterization of laser glazed plasma sprayed yttria stabilized zirconia coatings

A CO 2 continuous wave laser with defocused beam was employed to glaze the surface layer of plasma sprayed 20 wt.% yttria stabilized zirconia/MCrAlY coatings on 420 stainless steel substrates. An evaluation was made of the applicability of the thermal conduction model proposed by Greenwald. Results of experiments in this study showed that the laser glazing process was capable of producing shiny surfaces of low roughness on the plasma sprayed zirconia coatings. The coupling efficiency of the laser beam with the plasma sprayed zirconia coatings was about 65% at the travel speed of 4500 mm min −1. Laser glazing increased the microhardness from about 550 Hv for the as-sprayed layer to about 1550 Hv for the as-glazed layer. There was a heat affected zone (HAZ) under the resolidified layer, and the glazed layer revealed exclusively cubic phase with columnar and columnar-dendritic structures which grew perpendicular to the surface with {100} preferred orientation.

Pulsed laser glazing

An analysis of the parameters of the pulsed laser glazing process is presented and the effect of the shape of the pulse on the heat transfer efficiency is pointed out. The efficiency of the process is discussed in terms of the temporal and spatial profile of the pulse. Shorter pulses are more efficient but may lead to vaporization. Bulk samples are surface melted while plasma-sprayed coatings are either shallow densified or fully densified and cladded to the substrate material. The influence of the process parameters on the nature of coatings and substrates is discussed. Ways to improve the laser beam coupling efficiency and uniformity are indicated. Results obtained with an optical integrator which consists basically of a gold-coated hemisphere are shown.