Thermal barrier coatings (TBCs) are widely utilized in gas turbine engines for power generation. In recent years, the application of TBCs in automotive has been introduced to improve engine efficiency. Low thermal conductivity and high durability are desired coating properties for both gas turbine engines and automotive. Also, suspension plasma spraying (SPS) permits a columnar microstructure that combines both properties. However, it can be challenging to deposit a uniform columnar microstructure on a complex geometry, such as a gas turbine component or piston head, and achieve similar coating characteristics on all surfaces. This work’s objective was to investigate the influence of spray angle on the microstructure and lifetime of TBCs produced by SPS. For this purpose, SPS TBCs were deposited on specimens using different spray angles. The microstructures of the coatings were analyzed by image analysis for thickness, porosity, and column density. Thermal and optical properties were evaluated on each TBC. Lifetime tests, specifically designed for the two applications, were performed on all investigated TBCs. The lifetime results were analyzed with respect to the TBC microstructure and thermal and optical properties. This investigation showed that there is a limit to the spray angle that achieves the best compromise between TBC microstructure, thermal properties, optical properties, and lifetime. © 2022, The Author(s).

Internal combustion engines have a key role in the social and economic advancement of modern society but also a significant contribution to greenhouse gas emissions. For these engines, to preserve their role, a higher efficiency, that dramatically reduces the environmental impact, is necessary. To achieve increased engine efficiency, a technical solution is to lower the heat losses in the combustion chamber. Among them, the heat losses to the pistons are the preferential route, due to their extensive impact on fuel consumption. In this paper, porous thermal barrier coatings with large pores were applied to the pistons of diesel engines to improve engine efficiency. Atmospheric Plasma Spray (APS) process and porosity former TBC feedstock were employed to obtain high porosity coatings with large pores. Scanning Electron Microscopy (SEM) was utilized to investigate the microstructure of the coating in coupons and pistons. The optical properties of the coatings were explored with two methods: the spectral normal hemispherical reflectivity at room temperature (SNHRRT) and spectral normal emissivity at high temperature (SNEHT). The coatings’ behavior under thermal cyclic conditions was assessed by Flame Rig Test. Microstructure analysis was also performed before and after the test to identify the failure mechanisms. The engine efficiency was evaluated by measuring the Indicated Specific Fuel Consumption (ISFC) in a single-cylinder engine test. The results showed that porous coating with large pores combined with a higher emissivity can withstand the engine environment well and have the potential to provide enhancements in engine efficiency.

The upward trend in internal combustion engine efficiency is driven by the demands for emission reduction and fossil fuel depletion. No present alternative fuel can produce energy comparable to that produced by conventional oil; this necessitates the reasonable, efficient usage of oil. For several decades, thermal barrier coatings (TBCs) have been studied as an answer for increasing the thermal efficiency of gas turbine engines. However, TBCs have not been extensively evaluated for application to internal combustion engines owing to their emissions, costs, and demanding working conditions. Much effort is needed to simultaneously address this problem and expand the applicability of thermal barrier coatings.The objective of this study is to investigate the relationships between the spraying parameters, microstructural variations, thermal properties, and performance of TBCs applied to diesel engines. The further objective is to harness this knowledgeto fabricate coatings that result in high automotive engine efficiencies. Different feedstock materials were used with various spraying methods and classified into separate TBC types. The first TBC type had a lamellar bond coat deposited by atmospheric plasma spray (APS) and lamellar yttria-stabilized zirconia (YSZ) APS top coat. The second TBC type was derived from the first TBC type and had a lamellar APS bond coat, and the top coat was deposited by APS using a feedstock along with a porosity former, resulting in high-porosity top coats. The third TBC type had a dense bond coat deposited with high velocity air fuel (HVAF) and a columnar top coat deposited by suspension plasma spray (SPS) using a feedstock of YSZ or gadolinium zirconate (GZO). The SPS technique can generate a variety of microstructures, and the TBCs containing these microstructures were tested in an internal combustion engine for the first time. The fourth TBC type had a dense bond coat deposited HVAF, a columnar top coat produced with SPS and an additional top layer, which functioned as the sealing layer.

For the thermophysical property investigation of all coating types, experimental and modeling techniques—laser flash analysis (LFA) and object-oriented finite element (OOF) analysis, respectively—were employed. To evaluate the optical properties of the coatings, two methods were adopted, namely, spectral normal hemispherical reflectivity at room temperature (SNHRRT) and spectral normal emissivity at high temperature (SNEHT). The functional performance of the coatings was evaluated on the basis of the TBC behavior under cyclic thermal loads; thermal cyclic furnace test, flame rig test, thermal swing test, and a single cylinder engine experiment were conducted. The coatings were characterized by scanning electron microscopy (SEM) before and after the functional performance test. The coating performance was correlated to the microstructural, thermophysical, and optical properties of the coatings.

The results of this study infer that the TBC type significantly influences the thermal properties and thermal cyclic performance, which can be correlated with the porosity levels and the pore types. The complex substrate geometry of the piston resulted in inherent variations in spray angle and spray distance, leading to different coating microstructures and porosities owing to the changes in the particle trajectory and in-flight characteristics. Further, the single-cylinder engine evaluation demonstrated that the high-emissivity second TBC type or the third TBC type with a porous microstructure and a low thermal effusivity resulted in a high engine efficiency.

En ökad effektivitet i förbränningen hos dieselmotorer krävs för att kunna uppfylla de allt högre kraven på minskad miljöpåverkan och ett minskat användande av fossila bränslen. Inget alternativt bränsle finns idag som kan producera energi som är jämförbar med den som produceras av konventionell olja, vilket förutsätter en effektiv användning av olja. Under flera decennier har termiska barriärbeläggningar (TBC) varit en lösning för att öka den termiska effektiviteten hos gasturbinmotorer. Beläggningarna har dock inte utvärderats i tillräcklig omfattning för dieselmotorer. Forskning är av detta skäl nödvändig för att skapa förutsättningar för att denna typ av beläggningar kommersiellt skall kunna tillämpas i dieselmotorer.

Syftet med detta arbete är att utforska sambandet mellan processparametrar, beläggningsmikrostruktur, termiska egenskaper och prestanda hos termiska barriärbeläggningar för dieselmotorer. Målet med arbetet är att skapa beläggningar som resulterar i hög förbränningsverkningsgrad. Olika beläggningsmaterial utvärderades och kategoriserades med olika spruttekniker. Det första materialet som utvärderades var yttriumoxidstabiliserad zirkoniumdioxid (YSZ) som sprutades med atmosfärisk plasmassprutning (APS). Denna beläggning användes som referensprov. Det andra materialet var en vidareutveckling av den första beläggningen där porositet i beläggningen skapades med hjälp av ett tillsatsmaterial, vilket resulterade i beläggningar med hög porositet. Den tredje typen var en så kallad kolumnär mikrosstrukturbeläggning som skapades med suspensionsplasmasprutning (SPS) och tillsatsmaterialet YSZ. SPS tekniken valdes även för gadoliniumstabiliserad zirkoniumdioxid. Anledningen till att tekniken SPS utvärderades var att denna teknik i gasturbinapplikationer visat sig kapabel till att producera ett stort antal olika typer av mikrostrukturer. Den fjärde typen av beläggning som utvärderades var ett SPS producerat beläggningssystem där ytterligare ett så kallat toppskikt som tätskikt sprutades.

Prestanda och egenskaper utvärderades för samtliga skapade beläggningar genom såväl experimentella tekniker som modellering där laser flash analysis (LFA) och finita elementanalys är exempel. För att utvärdera beläggningarnas optiska egenskaper användes två metoder, spektral normal hemisfärisk reflektivitet vid rumstemperatur (SNHRRT) och spektral normal emissivitet vid hög temperatur (SNEHT). Beläggningarnas livslängd utvärderades indirekt med hjälp av termisk utmattning i ugn, termisk utmattning med förbränningslåga och med ett motorexperiment. Beläggningarnas mikrostruktur analyserades genom svepelektronmikroskopi (SEM) före och efter prestandautvärderingarna. Beläggningsprestanda korrelerades därefter till beläggningarnas mikrostruktur, termiska och optiska egenskaper samt de funktionella egenskaperna.

Resultaten av studien visar att valet av TBC-typ signifikant påverkar de termiska egenskaperna och de termiska utmattningsegenskaperna, och att dessa skillnader kan korreleras med porositetsnivåerna och portyperna i beläggningarnas mikrostruktur. Motorkolvens komplexa substratgeometri orsakade variationer i sprutvinkel och sprutavstånd, vilket ledde till olika beläggningsmikrostrukturer på kolvens olika ytor. Motorutvärderingen visade att den andra TBC-typen med hög emissivitet och den tredje TBC-typen med en porös mikrostruktur och en låg termisk effusivitet resulterade i hög motoreffektivitet.

The efficiency of internal combustion engines is gaining increased interest due to the impact of fuel consumption on greenhouse gas emissions and the goals of countries to minimize emissions. Thermal barrier coatings (TBCs) have shown great potential in improving the efficiency of internal combustion engines. The TBCs, applied on the surface of the piston, apart from thermal isolation, should also follow the surface temperature variations in the combustion chamber, reducing the energy loss and not affecting volumetric efficiency, and thus accomplish a raise in fuel efficiency. This characteristic of the TBC can be associated with the thermal properties, but the best performance test for TBCs is the single cylinder engine test. The single cylinder engine test is an expensive and time demanding procedure, making it not easily accessible. The purpose of this work was to develop a thermal swing test method to evaluate the applicability of TBCs in the combustion chamber of an internal combustion engine. This was carried out by measuring the temperature variation on the surface of the coating (thermal swing response) exposed to heat pulses from a high velocity air fuel (HVAF) spray torch. The TBCs were tested as sprayed (AS) and after grinding them to reduce roughness (RR) in order to ensure similar thickness and roughness along the different TBCs. Characterization of the coating microstructure was carried by scanning electron microscopy (SEM) together with image analysis techniques, and the thermal properties were measured by laser flash analysis (LFA). By correlating the thermal swing response with the microstructure and thermal properties of the coatings, it was determined that the coatings with large open pores exhibited the highest thermal swing response, which was as high as 200 degrees C.

Thermal barrier coatings (TBCs) are considered a promising solution for improving the efficiency of internal combustion engines. Among the thermal spray processes, the relatively newly developed suspension plasma spray (SPS) is an attractive candidate due to its unique microstructural features that have already demonstrated increased performance in gas turbine applications. To achieve these features, thermal spray conditions play an essential role. In specific uses, such as piston of diesel engines, parameters as spray angle and spray distance pose challenges to keep them constant during the whole spray process due to the complex geometry of the piston. To understand the effect of the spray distance and spray angle, a comprehensive investigation of the produced thermal spray microstructure on the piston geometry was conducted. Flat and complex geometry surfaces were coated using the same plasma parameters while the spray angle and distance were changed. Characterization was performed using scanning electron microscopy (SEM) combined with the image analysis technique to perceive the variation of the thickness and microstructures features such as pores, cracks, column density, and column orientation. The results showed that the changes in spray angles and spray distances due to the complex shape of the substrate have a significant influence on the microstructure and thermal properties (thermal conductivity and thermal effusivity) of the coatings. The thermal conductivity and thermal effusivity were calculated by modeling for the different regions of the piston and measured by laser flash analysis combined with modeling for the flat-surfaced coupon. It was shown that the modeling approach is an effective tool to predict the thermal properties and thus to understand the influence of the parameters on the coating properties. Connecting the observations of the work on the microstructural and thermal properties, the complex geometry’s influence on the produced coatings could be diminished by tailoring the process and generating the most desirable TBC for the internal combustion engines in future applications.

The upward trend in internal combustion engine efficiency is likely driven by the depletion of fossil fuels. Since no replacement in sight can deliver energy comparable to the conventional oil, there is a need to use it more rationally and effectively. Thermal barrier coatings have been seen for a long time as a solutionto increase the thermal efficiency of gas turbine engines but suffer from the lackof strong applicability in internal combustion engines. This is due to the different restrictions when comparing the environment on the gas turbines and in internal combustion engines. To overcome this problem and, at the same time, expand the application field of thermal barrier coatings, more efforts need to be devoted.In this work, different top coat materials using various deposition techniques were evaluated and categorized in three different thermal barrier coating (TBC) architectures. The first was the lamellar yttria-stabilized zirconia (YSZ) top coat deposited by atmospheric plasma spray (APS), used as a reference sample. The second architecture was a columnar suspension plasma spray (SPS) TBC with YSZ and gadolinium zirconate (GZO) top coat. The SPS process can produce avariety of microstructures, and they were, for the first time, tested in an internal combustion engine. The third architecture was an SPS top coat, with an additional layer on the top, called a sealing layer of either metallic or ceramic material, both never investigated in a diesel engine application earlier.

For the thermophysical properties investigation, a combination of laser flashanalysis (LFA) and modeling with object-oriented finite element (OOF) was employed to understand the properties in all the applications. The performance of the coatings was evaluated in two different ways, by thermal cyclic tests, basedon the TBCs behavior under cyclic thermal loads and by single-cylinder engine experiment. The characterization of the coatings was done by scanning electron microscope (SEM) before and after the thermal cyclic tests.The performance properties were correlated with coatings microstructure and thermophysical properties. It was shown that a columnar TBC produced by SPS had a superior engine efficiency in the single cylinder engine experiment.

The increased demands for higher efficiency and environmentally friendly diesel engines have led to a continuous search for new coating processing routes and new ceramic materials that can provide the required properties when applied on engine components such as pistons and exhaust manifolds. Although successful in gas turbine applications, thermal barrier coatings (TBCs) produced by suspension plasma spraying (SPS) processes have not been employed so far in the automotive industry. This work aims to achieve a better understanding of the role of thermal conductivity and thermal effusivity on the durability of SPS TBCs applied to pistons of diesel engines. Three different coating architectures were considered for this study. The first architecture was yttria-stabilized zirconia (YSZ) lamellar top coat deposited by APS (Atmospheric Plasma Spray) and used as a reference sample in this study. The second architecture was a columnar SPS top coat of either YSZ or gadolinium zirconate (GZO) while the third architecture was an SPS columnar top coat, "sealed" with a dense sealing layer deposited on the top coat. Two types of sealing layers were used, a metallic (M) or a ceramic thermal spray layer (C). Laser Flash Analysis (LFA) was used to determine the thermal conductivity and thermal effusivity of the coatings. Two different thermal cyclic tests were used to test the TBCs behavior under cyclic thermal loads. Microstructure analysis before and after the thermal cyclic tests were performed using SEM in different microstructures and materials. The thermal cyclic test results were correlated with coatings microstructure and thermophysical properties. It was observed that the columnar coatings produced by SPS had an enhanced service life in the thermal cyclic tests as compared to the APS coatings.

The objective of this investigation was to improve the thermal properties of plasma sprayed thermal barrier coatings (TBC) for internal combustion engines. There is a need for further reduction of thermal conductivity and volumetric heat capacity and the negative effects on heat loss and combustion phasing of surface roughness and permeable porosity, typical for plasma sprayed coatings, should be minimized. Four measures for improvement of TBC properties were evaluated: I) modification of the coating’s microstructure by using a novel suspension plasma spraying method, ii) application of gadolinium-zirconate, a novel ceramic material with low thermal conductivity, iii) polishing of the coating to achieve low surface roughness, and iv) sealing of the porous coating surface with a polysilazane. Six coating variants with different combinations of the selected measures were applied on the piston crown and evaluated in a single cylinder light duty diesel engine. The experimental data was modeled with multiple linear regression to obtain confidence intervals for the measurement results and to correct the data for variations of surface roughness, combustion phasing and compression ratio for the different pistons. The main tool for evaluation of the coating properties was cylinder pressure analysis, providing the apparent rate of heat release, indicated efficiency, wall heat loss, and exhaust loss. The new TBC microstructure from suspension plasma spraying in combination with the use of gadolinium-zirconate showed promising results with respect to indicated efficiency and heat loss reduction. © 2019 SAE International and © 2019 SAE Naples Section. All rights reserved.

Demands for improved fuel efficiency and reduced CO2 emissions of diesel engines have been the driving force for car industry in the past decades. One way to achieve this would be by using thermal spraying to apply a thermal insulation layer on parts of the engine’s combustion chamber. A candidate thermal spray process to give coatings with appropriate properties is suspension plasma spray (SPS). SPS, which uses a liquid feedstock for the deposition of finely structured columnar ceramic coatings, was investigated in this work for application in light-duty diesel engines. In this work, different spray processes and materials were explored to achieve coatings with optimized microstructure on the head of aluminum pistons used in diesel engine cars. The functional properties of the coatings were evaluated in single-cylinder engine experiments. The influence of thermo-physical properties of the coatings on their functional properties has been discussed. The influence of different spray processes on coating formation on the complex piston head profiles has been also discussed. The results show that SPS can be a promising technique for producing coatings on parts of the combustion chamber, which can possibly lead to higher engine efficiency in light-duty diesel engines.