Effect of beam angle on HAZ, recast and oxide layer characteristics in laser drilling of TBC nickel superalloys

Industrial applications of laser drilling include the production of cooling holes at acute angles in certain parts of the aero-engine components. These parts are often covered with ceramic thermal barrier coatings (TBC) to protect them from reaching excessive temperatures in hot engine environments. Acute angle TBC drilling brings three major simultaneous complications to the process. These are: (i) multi-layer drilling, (ii) non-symmetrical geometry and melt ejection, and (iii) increased depth of drilling. In a previous investigation by the authors, delamination of TBC was found as a main problem of angled drilling and mechanisms involved were studied. In the present study, implications of these difficulties on the hole quality is investigated through a comparative study of vertical and acute angle drilled holes. Characteristics of recast layer, heat-affected zone (HAZ), oxide layer and TBC delamination are investigated. Variation of these metallurgical characteristics with the depth of the hole is evaluated. Results for vertical and inclined holes are compared. The extent of HAZ, recast layer and oxide layer is seen to vary significantly with location and is found increasing with decreasing drilling angle to surface. Numerical simulation of pulsed laser heating of TBC Nimonic 263 was carried out for acute angle drilling with assist gas considerations. Results from the simulation suggested that the total heat transfer rate is higher on the leading edge side than the trailing edge of the heated region. Experimentally observed larger HAZ on leading edge side and larger recast layer on trailing edge side are explained by the analysis of heat flow characteristics obtained with the model.     

Investigation of delamination mechanisms during a laser drilling on a cobalt-base superalloy

Temperatures in the high pressure chamber of aircraft engines are continuously increasing to improve the engine efficiency. As a result, constitutive materials such as cobalt and nickel-base superalloys need to be thermally protected. The first protection is a ceramic thermal barrier coating (TBC) cast on all the hot gas-exposed structure. The second protection is provided by a cool air layer realized by the use of a thousand of drills on the parts where a cool air is flowing through. The laser drilling process is used to realize these holes at acute angles. It has been shown on coated single crystal nickel-base superalloy that the laser drilling process causes an interfacial cracking (also called delamination), detected by a cross section observation. The present work aims at characterizing interfacial cracking induced by laser drilling on coated cobalt-base super alloy. On the one hand, this work attempted to quantify the crack by several microscopic observations with regards to the most significant process parameters related as the angle beam. On the other hand, we studied the difference of the laser/ceramic and the laser/substrate interaction with real time observation by using a fast movie camera.     

A failure analysis of water jet drilling in composite laminates

Delamination is a major concern in the manufacturing processes of composite materials. It reduces not only the structural integrity of the laminate but also the long-term reliability of the assembly. Water jet drilling, in spite of its advantages of no tool wear and thermal damage, often creates delamination composite laminate at bottom. The current paper presents an analytical approach to study the delamination during drilling by water jet piercing. The analysis uses fracture mechanics with plate theory to describe the mechanism of delamination. This model predicts an optimal water jet pressure for no delamination as a function of hole depth and material parameters (opening-mode delamination fracture toughness and modulus of elasticity). Good agreement is achieved with data obtained from water jet drilling of graphite epoxy laminate. The predicted optimal water jet pressure can be applied in a control scheme for maximizing the productivity of water jet drilling of composite laminates.     

Investigation by 3D FE simulations of delamination crack initiation in TBC caused by alumina growth

In gas turbines, thermal barrier coatings (TBCs) applied by air plasma spraying are widely used to reduce the temperature in hot components. The TBC allows higher gas temperature and/or reduces the need for internal cooling in the hot components, thus increasing the efficiency of the gas turbine. Spallation is a common failure mechanism of TBC and occurs after a critical number of thermal cycles, when the alumina layer has grown to a critical thickness. The influence of the growing alumina layer and the top/bond-coat interface roughness in the TBC has been investigated. The primary goal was to identify failure mechanisms that can be incorporated into a life model of the TBC, and to increase the understanding of the delamination process in the TBC. A new formulation of alumina growth is proposed, in which the swelling strains caused by the volumetric increase during alumina growth depends on the stress state. The alumina growth model is used in 3D FE thermal cycling simulations of a TBC in which the thermal cycle time is long enough to characterize a typical cycle of a gas turbine. From the simulations, the growing alumina layer is observed to be one failure mechanism of the TBC. Without an alumina layer in the model, high delamination stress is observed at room temperature, above ridges of the top/bond-coat interface in the top coat. When the alumina is growing, the point of maximum delamination stress is moved towards the valleys. When the thickness of the alumina layer has grown to approximately 8–10 μm, positive delamination stress is found above the valleys in the top coat. The movement of the positive delamination stress region can explain why a delamination crack develops, which will cause spallation of the TBC during shutdown to room temperature.     

Effects of exit back-up on delamination in drilling composite materials using a saw drill and a core drill

Machining of composites has caught greater attention in manufacturing of structural parts in aerospace, automobile and sporting goods. Composite materials have advantageous features in strength and stiffness coupled with lightweight compared to the conventional metallic materials. Amongst all machining operations, drilling is the most commonly applied method for generating holes for riveting and fastening the structural assembly. Delamination is one of the serious concerns in drilling holes in composite materials at the bottom surface of the workpiece (drill exit). Quite a few references of the drilling of fiber-reinforced plastics report that the quality of cut is strongly dependent on drilling parameter as well as the drill geometry. Saw drills and core drills produce less delamination than twist drills by distributing the drilling thrust toward the hole periphery. Delamination can be effectively reduced or eliminated by slowing down the feed rate when approaching the exit and by using back-up plates to support and counteract the deflection of the composite laminate leading to exit side delaminations. The use of the back-up does reduce the delamination in practice, which its effects have not been well explained in analytical fashion. This paper predicts the effects of backup plate on delamination in drilling composite materials using saw drill and core drill. The critical drilling thrust force at the onset of delamination is calculated and compared with that without backup. The well known advantage of industrial use of backup can be understood fundamentally by the fact that the threshold thrust force at the onset of delamination is increased making the delamination less induced.     

Discrimination Between Over-Thickness and Delamination of Thermal Barrier Coatings by Apparent Thermal Effusivity Thermographic Technique

When the delamination extent of thermal barrier coatings (TBC) is over a sufficiently large area to assess the adhesion of coatings, Pulse thermography (PT) is an effective technique, but often the distinction between delamination and TBC over-thickness is very difficult to determine. In this work, a recently developed algorithm based on apparent thermal effusivity has been applied to automatically distinguish between delamination and over-thickness during the inspection of a serviced gas turbine TBC-coated vane.     

Feasibility study of the rotary ultrasonic elliptical machining of carbon fiber reinforced plastics (CFRP)

Carbon fiber reinforced plastics (CFRP) are used for various aircraft structural components because of their superior mechanical and physical properties such as high specific strength, high specific stiffness, etc. However, when CFRP are machined, rapid tool wear and delamination are troublesome. Therefore, cost effective and excellent quality machining of CFRP remains a challenge. In this paper, the rotary ultrasonic elliptical machining (RUEM) using core drill is proposed for drilling of holes on CFRP panels. This method combines advantages of core-drill and elliptical tool vibration towards achieving better quality, delamination free holes. The cutting force model and chip-removal phenomenon in ultrasonic elliptical vibration cutting are introduced and analyzed. The feasibility to machine CFRP for RUEM is verified experimentally. The results demonstrate that compared to conventional drilling (CD), the chip-removal rate has been improved, tool wear is reduced, precision and surface quality around holes is enhanced, delamination at hole exits has been prevented and significant reduction in cutting force has been achieved.     

An Effective Electrical Resonance-Based Method to Detect Delamination in Thermal Barrier Coating

This research proposes a simple yet highly sensitive method based on electrical resonance of an eddy-current probe to detect delamination of thermal barrier coating (TBC). This method can directly measure the mechanical characteristics of TBC compared to conventional ultrasonic testing and infrared thermography methods. The electrical resonance-based method can detect the delamination of TBC from the metallic bond coat by shifting the electrical impedance of eddy current testing (ECT) probe coupling with degraded TBC, and, due to this shift, the resonant frequencies near the peak impedance of ECT probe revealed high sensitivity to the delamination. In order to verify the performance of the proposed method, a simple experiment is performed with degraded TBC specimens by thermal cyclic exposure. Consequently, the delamination with growth of thermally grown oxide in a TBC system is experimentally identified. Additionally, the results are in good agreement with the results obtained from ultrasonic C-scanning.     

Delamination reduction in drilling composite materials by active backup force

Composites have been widely employed in various industries due to their outstanding mechanical properties and corrosion resistance. Drilling is an indispensible operation for building a load-carrying structure. Delamination, however, is among the serious concerns in drilling composite-based components in practice. This paper describes a novel method for the reduction of delamination during drilling of composites by active backup force. The applied backup force contributes to suppression of the growth of the delamination at drilling exit by 60–80%. The proposed novel drilling technique reveals the potential for fabrication of composite components at low cost and minor delamination with high feed rate.     

On the role of imperfections in the failure of a thermal barrier coating made by electron beam deposition

An impression test has been used to explore the remnant toughness and the delamination characteristics of thermal barrier coatings (TBCs) after extended thermal exposure. The delamination trajectory is found to change as the thermally grown oxide (TGO) thickens. At small thicknesses, delamination occurs predominantly within the TGO and TBC. With a thicker TGO, developed after 100 h exposure at 1100°C, the delamination extends predominantly along the TGO/bond coat interface, but with small oxide domains remaining embedded in the bond coat. The changes in the interface adhesion and the mechanics responsible for this transition are addressed, along with a discussion of the role of morphological imperfections in the TGO in failure nucleation. A method for determining the effective in-plane modulus of the TBC from the curvature of decohered TGO/TBC bilayers is also presented.     

Experimental and numerical life prediction of thermally cycled thermal barrier coatings

This article addresses the predominant degradation modes and life prediction of a plasma-sprayed thermal barrier coating (TBC). The studied TBC system consists of an air-plasma-sprayed bond coat and an air-plasma-sprayed, yttria partially stabilized zirconia top layer on a conventional Hastelloy X substrate. Thermal shock tests of as-sprayed TBC and pre-oxidized TBC specimens were conducted under different burner flame conditions at Volvo Aero Corporation (Trollhättan, Sweden). Finite element models were used to simulate the thermal shock tests. Transient temperature distributions and thermal mismatch stresses in different layers of the coatings during thermal cycling were calculated. The roughness of the interface between the ceramic top coat and the bond coat was modeled through an ideally sinusoidal wavy surface. Bond coat oxidation was simulated through adding an aluminum oxide layer between the ceramic top coat and the bond coat. The calculated stresses indicated that interfacial delamination cracks, initiated in the ceramic top coat at the peak of the asperity of the interface, together with surface cracking, are the main reasons for coating failure. A phenomenological life prediction model for the coating was proposed. This model is accurate within a factor of 3.     

Estimation of spallation life of thermal barrier coating of gas turbine blade by thermal fatigue test

Plasma-sprayed thermal barrier coatings (TBCs) are applied to protect the blades of a gas turbine system from high-temperature gas and to lower the surface temperature of the blades. The failure of TBC is directly connected to the failure of the blades because the spallation of a ceramic layer leads to the acceleration of local corrosion and oxidation at the location of failure. Therefore, the spallation life of TBC is very important in the evaluation of the reliability of a gas-turbine blade.In this study, thermal fatigue tests were performed at 1100 °C and 1151 °C. Then, c-scanning and bond strength tests were performed for TBC specimens that were thermally aged by thermal fatigue tests. From the results, an empirical equation based on the ratio of the delamination area and the thermal cycle number was presented and the spallation life of a TBC specimen could be roughly estimated using the relationship between the delaminated area and the number of cycles.     

High speed imaging of the flow during close-coupled gas atomisation: Effect of melt delivery nozzle geometry

We describe the application of a high-speed still imaging technique to the study of close-coupled gas atomisation. A pulsed Nd:YAG laser is used to obtain pairs of still images with an effective 6 ns exposure time, from which velocity maps of the flow of the atomised fluid can be reconstructed. We demonstrate directly that the melt spray cone consists of a jet precessing around the surface of a cone. Further, we demonstrate that the width of this jet is directly related to the geometry of the melt nozzle. By applying Particle Image Velocimetry techniques we are also able to map the flow field in both the primary and secondary atomisation zones, demonstrating an asymmetric recirculation eddy exists at the circumferential edge of the gas-melt interface in the primary atomisation zone.     

Finite element analysis of stress distribution in thermal barrier coatings

A numerical simulation of crack development within APS TBC systems is presented. The TGO thickening and creep deformation of all system constituents is modelled. Two dimensional periodic unit cell is used to examine the effect of interfacial asperity on stress distribution and subsequent delamination of APS TBC. A study of cyclic loading and of creep of the base material on the stress distribution close to the asperity at the TGO/BC interface is made, revealing a small in?uence influence of both on the stress state in the thermal barrier coating system subjected to temperature loading. Cohesive zone elements at the oxide/ceramic interface model the development of the interfacial micro-crack. The finite element analysis shows that the development of the interfacial crack allows for a micro-crack formation within APS TBC. Subsequent TGO growth results in a tensional zone within the oxide layer. Linking of the micro-cracks at the interface and within TBC through TGO could lead to a coating delamination in the unit cell.     

Ultrasonic Detection of Delamination and Material Characterization of Thermal Barrier Coatings

This article describes ultrasonic nondestructive evaluation (NDE) to detect the changes of material properties and provide early warning of delamination in thermal barrier coating (TBC) systems. NDE tests were performed on single-crystal René N5 superalloy coupons that were coated with a commercially available MCrAlY bond coat and an air plasma sprayed 7% yttria-stabilized zirconia (YSZ) top coat deposited by Air Plasma Spray method, as well as Haynes 230 superalloy coupons coated with MCrA1Y bond coat, and an electron beam physical vapor deposit of 7% YSZ top coat. The TBC coupons were subjected to either cyclic or isothermal exposure for various lengths of time at temperatures ranging from 900 to 1100?°C. The ultrasonic measurements performed on the coupons had provided an early warning of delamination along the top coat/TGO interface before exposure time, when delamination occurred. The material's property (Young??s modulus) of the top coat was estimated using the measured wave speeds. Finite element analysis (FEA) of the ultrasonic wave propagation was conducted on a simplified TBC system to verify experimental observations. The technique developed was also demonstrated on an as-manufactured turbine blade to estimate normalized top coat thickness measurements.     

Delamination during drilling in polyurethane foam composite sandwich structures

The objective of this paper is to study the influence of drilling velocity, feed rate, and flank length on the delamination of polyurethane foam sandwich structures. A Taguchi-based design of experiments was used to assess the importance of the drilling parameters, and scanning electron microscopy (SEM) was used to assess the damage from drilling. The drilling of sandwich structures results in significant damage caused by delamination and surface roughness around the drilled holes. The drilling process was evaluated based on a factor called the delamination factor, which is defined as the ratio of the maximum diameter of the damage zone, measured using SEM, to the standard hole diameter (drill diameter). Analysis of variance of the experimental results showed that cutting speed was the most significant parameter among the controllable parameters during drilling of sandwich specimens followed by flank length and feed rate. Finally, confirmation tests were performed to make a comparison between the experimental results and the correlation results. The damage mechanisms are explained using SEM.     

Damage mechanisms and lifetime behavior of plasma sprayed thermal barrier coating systems for gas turbines—Part I: Experiments

Detailed damage analyses of a plasma sprayed ZrO₂/8 wt.-% Y₂O₃-MCrAlY-CMSX-4 TBC system during isothermal and cyclic oxidation tests with different dwell times at high temperature have been performed. The resulting failure mode, i.e. the particular delamination crack path, is strongly dependent on the temperature cycle applied. Isothermal exposure promotes crack propagation within the TGO, whereas thermal cycling shifts the crack path towards the TBC. Thermal cycling with dwell time at high temperature leads to a mixed delamination crack path (partly within TBC and TGO). The respective correlation between TBC lifetimes and duration of high temperature dwell time per cycle (cycle frequency) is shown and discussed.     

Sequential laser and mechanical micro-drilling of Ni superalloy for aerospace application

Laser percussion drilling is inherently associated with poor geometry and thermal defects. While mechanical micro-drilling produces good quality holes, premature drill breakage often occurs and it is difficult to drill holes at acute angles. This paper presents the feasibility and basic characteristics of a new approach for micro-drilling In718 alloy sheets at an acute angle, using sequential laser and mechanical drilling. The results demonstrate that sequential laser-mechanical micro-drilling alleviates the defects associated with laser-drilled holes, reduces burr size and machining time and increases the tool life compared with mechanical drilling.     

Highly Segmented Thermal Barrier Coatings Deposited by Suspension Plasma Spray: Effects of Spray Process on Microstructure

Effects of the ceramic powder size used for suspension as well as several processing parameters in suspension plasma spraying of YSZ were investigated experimentally, aiming to fabricate highly segmented microstructures for thermal barrier coating (TBC) applications. Particle image velocimetry (PIV) was used to observe the atomization process and the velocity distribution of atomized droplets and ceramic particles travelling toward the substrates. The tested parameters included the secondary plasma gas (He versus H₂), suspension injection flow rate, and substrate surface roughness. Results indicated that a plasma jet with a relatively higher content of He or H₂ as the secondary plasma gas was critical to produce highly segmented YSZ TBCs with a crack density up to ~12 cracks/mm. The optimized suspension flow rate played an important role to realize coatings with a reduced porosity level and improved adhesion. An increased powder size and higher operation power level were beneficial for the formation of highly segmented coatings onto substrates with a wider range of surface roughness.     

Effect of deviation on delamination by saw drill

Various cutting techniques are available to drill holes, but drilling is the most common way in secondary machining of composite materials owing to the need for structure joining. Twist drills are widely used in the industry to produce holes rapidly and economically. Since the twist drill has a chisel edge, increasing the length of a chisel edge will result in an increase in the thrust force generated. Whereas, a saw drill has no chisel edge; it utilizes the peripheral distribution of the thrust force for drilling. As a result, the saw drill can achieve better a machining quality in drilling composite laminates than twist drill. The deviation of cutting edge that occurs in saw drill would result in an increase of thrust force during drilling, causing delamination damage when drilling composite materials in particular. A comprehensive model concerning delamination induced by the thrust force of a deviation saw drill during drilling composite materials has been established in the present study. For a deviation saw drill, the critical thrust force that triggers delamination increases with increasing β. A lower feed rate has to be used with an increasing deviation saw drill in order to prevent delamination damage. The results agree with real industrial experience. A guide for avoiding the drill deviation during drill regrinding or drill wear has been proved analytically by the proposed model, especially when the deviation ratio (β) affects the critical thrust force. This approach can be extended to examine similar deviation effects of various drills.