Combustion engine components with dynamic thermal insulation coating and method of making and using such a coating

A component for an engine is provided. The component includes a thermal barrier coating applied to a body portion formed of metal, such as steel or another ferrous or iron-based material. According to one embodiment, a bond layer of a metal is applied to the body portion, followed by a mixed layer of metal and ceramic with a gradient structure, and then optionally a top layer of metal. The thermal barrier coating can also include a ceramic layer between the mixed layer and top layer, or as the outermost layer. The ceramic includes at least one of ceria, ceria stabilized zirconia, yttria, yttria stabilized zirconia, calcia stabilized zirconia, magnesia stabilized zirconia, and zirconia stabilized by another oxide. The thermal barrier coating can be applied by thermal spray. The thermal barrier coating preferably has a thickness less than 200 microns and a surface roughness Ra of not greater than 3 microns.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to engine combustion components for internal combustion engines, and methods of manufacturing the same.

2. Related Art

Modern heavy duty diesel engines are being pushed towards increased efficiency under emissions and fuel economy legislation. To achieve greater efficiency, the engines must run hotter and at higher peak pressures. Thermal losses through the combustion chamber can be problematic under these increased demands. For example, typically about 4% to 6% of available fuel energy is lost as heat through the piston into the cooling system. One way to improve engine efficiency is to extract energy from hot combustion gases by turbo-compounding. For example, about 4% to 5% of fuel energy can be extracted from the hot exhaust gases by turbo-compounding.

another approach to improving engine efficiency is to insulate the crown of the piston in order to reduce the heat otherwise lost to the cooling system. Insulating layers of ceramic are one approach to insulating the piston. It is known to apply a metal layer to the body portion of the piston followed by application of a ceramic layer. However, ceramic is inherently porous and the combustion gases can pass through the ceramic layer and oxidize the metal layer causing a failure at the ceramic/metal layer interface and eventual spalling and failure of the ceramic layer. There is also a mismatch in the thermal expansion coefficients of the ceramic and metal layer, further adding to the potential delamination and spalling of the ceramic layer over time.

another example is a thermally sprayed coating formed of yttria stabilized zirconia. This material, when used alone, can suffer destabilization through thermal effects and chemical attack in diesel combustion engines. It has also been found that thick ceramic coatings, such as those greater than 500 microns, for example 1 mm, are prone to cracking and failure.

Although more than 40 years of thermal coating development for pistons is documented in literature, there is no known product that is both successful and cost effective to date. It has also been found that typical aerospace coatings used for jet turbines are not suitable for engine pistons because of raw material and deposition costs associated with the highly cyclical nature of the thermal stresses imposed.

Another approach to piston protection specific to aluminum pistons is to convert the surface of the aluminum crown to aluminum oxide via plasma oxidation and then the pores of the conversion layer are sealed with polysilazane. The conversion zone is very thin (50-70 microns) and is understood to be a high insulation and dissipation material that quickly heats and cools so it cycles with the heat of combustion. This relatively thin conversion approach for aluminum pistons has no application for use with steel or other iron-based pistons.

SUMMARY

One aspect of the invention provides a component for exposure to a combustion chamber of an internal combustion engine and/or exhaust gas generated by the internal combustion engine. The engine component comprises a body portion formed of metal, and an improved thermal barrier coating applied to the body portion. According to one embodiment, the thermal barrier coating includes a bond layer formed of metal disposed on the body portion, a mixed layer disposed on the bond layer, and a top layer disposed on the mixed layer. The mixed layer is formed of a mixture of ceramic and metal, and the top layer is formed of metal and fills pores of the ceramic of the mixed layer.

According to another embodiment, the thermal barrier coating includes a bond layer formed of metal disposed on the body portion and a mixed layer disposed on the bond layer. The mixed layer includes a mixture of ceramic and metal, and the thermal barrier coating has a thickness of not greater than 700 microns.

According to yet another embodiment, the thermal barrier coating includes a bond layer formed of metal disposed on the body portion and a mixed layer disposed on the bond layer. The mixed layer includes a mixture of ceramic and metal. In this embodiment, a ceramic layer is formed entirely of a ceramic material is disposed on the mixed layer. The ceramic layer presents an outermost exposed surface of the thermal barrier coating and has a surface roughness Ra of not greater than 3 microns, and the thermal barrier coating has a total thickness of not greater than 200 microns.

Another aspect of the invention provides a method of manufacturing a component for exposure to a combustion chamber of an internal combustion engine and/or exhaust gas generated by the internal combustion engine. The method includes applying a thermal barrier coating to a body portion formed of metal. According to one embodiment, the step of applying the thermal barrier coating includes applying a bond layer formed of metal to the body portion, applying a mixed layer formed of a mixture of ceramic and metal to the bond layer, and applying a top layer formed of metal to the mixed layer, the top layer filling pores of the ceramic of the mixed layer.

According to another embodiment, the step of applying the thermal barrier coating includes applying a bond layer formed of metal to the body portion, and applying a mixed layer formed of a mixture of ceramic and metal to the bond layer. The thermal barrier coating has a total thickness of not greater than 700 microns.

According to yet another embodiment, the step of applying the thermal barrier coating includes applying a bond layer formed of metal to the body portion, applying a mixed layer formed of a mixture of ceramic and metal to the bond layer, and applying a ceramic layer formed entirely of a ceramic material to the mixed layer. The ceramic layer presents an outermost exposed surface of the thermal barrier coating and has a surface roughness Ra of not greater than 3 microns. The thermal barrier coating has a total thickness of not greater than 200 microns.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

One aspect of the invention provides an engine component for use in an internal combustion engine20, such as a heavy duty diesel engine or alternatively a gasoline engine, with a thermal barrier coating22applied to the engine component. The thermal barrier coating22reduces heat loss and thus improves engine efficiency. The thermal barrier coating22is also more cost effective and stable, as well as less susceptible to chemical attacks, compared to other coatings used to insulate engine components.

Various different components of the internal combustion engine can be coated with the thermal barrier coating22. As shown inFIG. 1, the thermal barrier coating22can be applied to one or more components exposed to the combustion chamber24, including a cylinder liner28, cylinder head30, fuel injector32, valve seat34, valve face36, valve back37, seal ring54, exhaust port surface56, and firedeck62. Typically, the thermal barrier coating22is only applied to a portion of the component20exposed to the combustion chamber24. For example, an entire surface of the component20exposed to the combustion chamber24could be coated. Alternatively, only a portion of the surface of the component exposed to the combustion chamber24is coated. The thermal barrier coating22could also be applied to select locations of the surface exposed to the combustion chamber24, depending on the conditions of the combustion chamber24and location of the surface relative to other components.

In the example embodiment ofFIG. 1, the thermal barrier coating22is only applied to a portion of an inner diameter surface38of the cylinder liner28located opposite a top land44of the piston26when the piston26is located at top dead center, and the thermal barrier coating22is not located at any other location along the inner diameter surface38, and is not located at any contact surfaces of the cylinder liner28. However, according to another embodiment, the thermal barrier coating22is applied to other surfaces of the cylinder liner28.FIG. 2is an enlarged view of the portion of the cylinder liner28including the thermal barrier coating22. In this embodiment, the inner diameter surface38includes a groove40machined therein. The groove40extends along a portion of the length of the cylinder liner28from a top edge of the inner diameter surface38, and the thermal barrier coating22is disposed in the groove40. Also in this example, the length1of the groove40and the thermal barrier coating22is 5 mm to 10 mm. In other words, the thermal barrier coating22extends 5 mm to 10 mm along the length of the cylinder liner28. In the example embodiment ofFIG. 1, the thermal barrier coating22is also applied to the valve face36.FIG. 3is an enlarged view of the valve face36including the thermal barrier coating22. However, the thermal barrier coating22could be applied to another portion or surface of a valve guide or valve, such as a shaft or valve back37between the valve seat face36and stem. The thermal barrier coating22can be applied to the valve back37for heat management.

The thermal barrier coating22could also be applied to the seal ring54on a cylinder opening of a head gasket, as shown inFIG. 4; exhaust port surfaces56in a head of the engine, as shown inFIG. 5; the firedeck62of the cylinder head30, as shown inFIG. 6; and selective regions on side faces or running surfaces of a piston, such as a top land64of the piston26, as shown inFIG. 7.

The thermal barrier coating22could also be applied to other components of the internal combustion engine20, or components associated with the internal combustion engine20, for example other components of a valve train, post-combustion chamber, exhaust manifold, and turbocharger. The thermal barrier coating22is typically applied to components of a diesel engine directly exposed to hot gasses of the combustion chamber24or exhaust gas, and thus high temperatures and pressures, while the engine20is running. A body portion42of the component is formed of a metal material, preferably a ferrous material, such as steel or another iron-based material. The steel used to form the body portion26can be an AISI 4140 grade or a microalloy 38MnSiVS5, for example. The steel used to form the body portion26preferably does not include phosphate, and if any phosphate is present on the surface of the body portion26, then that phosphate is removed prior to applying the thermal barrier coating22.

The thermal barrier coating22is applied to one or more components of the internal combustion engine20or exposed to exhaust gas generated by the internal combustion engine20, to maintain heat in the combustion chamber24or in exhaust gas, and thus increase efficiency of the engine20. The thermal barrier coating22is oftentimes disposed in specific locations, depending on patterns from heat map measurements, in order to modify hot and cold regions of the component. The thermal barrier coating22is designed for exposure to the harsh conditions of the combustion chamber24. For example, the thermal barrier coating22can be applied to components of the diesel engine20subject to large and oscillating thermal cycles. Such components experience extreme cold start temperatures and can reach in excess of 700° C. when in contact with combustion gases. There is also temperature cycling from each combustion event of approximately 15 to 20 times a second or more. In addition, pressure swings up to 250 to 300 bar are seen with each combustion cycle. The thermal barrier coating22is oftentimes disposed in a location aligned with and/or adjacent to the location of the fuel injector, fuel plumes, or patterns from heat map measurements in order to modify hot and cold regions along the body portion.

The thermal barrier coating22is designed for exposure to the harsh conditions of the combustion chamber. For example, the thermal barrier coating22can be applied to the component20for use in a diesel engine which is subject to large and oscillating thermal cycles. This type of component20experiences extreme cold start temperatures and reaches up to 760° C. when in contact with combustion gases. There is also temperature cycling from each combustion event of approximately 15 to 20 times a second or more. In addition, pressure swings up to 250 to 300 bar are seen with each combustion cycle.

According to an exemplary embodiment shown inFIG. 8, the thermal barrier coating22includes a mixed layer50, a top layer51, a bond layer52, and a ceramic layer60. The initial bond layer52is applied directly to the metal surface of the component20, followed by the mixed layer50, then the ceramic layer60, and then the top layer51.FIG. 9shows another embodiment including the bond layer52, the mixed layer50, and the ceramic layer60.FIG. 10shows another exemplary embodiment including the bond layer52, the mixed layer50, and the ceramic layer60.FIG. 11shows another embodiment including the bond layer52and the mixed layer50in the as-applied condition.FIG. 12is a flow chart illustrating various possible embodiments of the thermal barrier coating22.

The bond layer52is formed of metal and achieves good adhesion to the metal body portion26. The bond layer52also presents a thin but robust bond surface on which to apply the remainder of the thermal barrier coating22. The material used to form the bond layer52may be the same material, or similar to, or different from the material used to form the body portion26, for example a ferrous material, such as steel or another ferrous or iron-based material. The material of the bond layer52is compatible with the ferrous or other material used to form the body portion26. The material of the bond layer52could also be formed of chromium, nickel, and/or cobalt. The bond layer52could also be formed a chromium alloy, nickel alloy, and/or cobalt alloy. The body layer52could also be a high performance superalloy, such as a nickel-based superalloy or cobalt based superalloy. For example, the metal bond layer52could include or consist of at least one of alloy selected from the group consisting of CoNiCrAlY, NiCrAlY, NiCr, NiAl, NiCrAl, NiAlMo, and NiTi. According one preferred embodiment, the metal bond layer52is formed of NiCrAlY or NiCrAl.

The thermal barrier coating22typically includes the metal bond layer52in an amount of 5 percent by volume (% by vol.) to 33% by vol. %, more preferably 10% by vol. to 33% by vol., most preferably 20% by vol. to 33% by vol., based on the total volume of the thermal barrier coating22. The metal bond layer52is provided in the form of particles having a particle size of −140 mesh 105 μm), preferably −170 mesh 90 μm), more preferably −200 mesh 74 μm), and most preferably −400 mesh (<37 μm). The thickness limit of the metal bond layer52is dictated by the particle size of the material forming the metal bond layer52. A low thickness is oftentimes preferred to reduce the risk of delamination of the thermal barrier coating22. The thickness of the bond layer52may be between 20 to 100 microns, but preferably is between 20 and 50 microns.

Prior to application of the bond layer52, the metal surface of the body portion26is appropriately cleaned, such as by grit blasting, and the bond layer52is then deposited on to the bare surface of the body portion26by plasma spray, high velocity oxy-fuel (HVOF), and/or wire arc. It is noted that the surface to be coated with the barrier coating22is preferably bare steel and is free, for example, of a phosphate coating.

Applied to the bond layer52is a composite or mixed layer50of ceramic and metal material. The metal material in the mixed layer50may the same, similar, or different from the candidate materials identified above for the bond layer52. In other words, the composition of the metallic material selected for the bond layer52may be the same, similar, or different from that used in the mixed layer50of the barrier coating22.

The ceramic material of the mixed layer50is typically at least one oxide, for example ceria, ceria stabilized zirconia, yttria, yttria stabilized zirconia, calcia stabilized zirconia, magnesia stabilized zirconia, zirconia stabilized by another oxide, and/or a mixture thereof. The ceramic material has a low thermal conductivity, such as less than 1 W/m·K. When ceria is used in the ceramic material, the thermal barrier coating22is more stable under the high temperatures, pressures, and other harsh conditions of a diesel engine. The composition of the ceramic material including ceria also makes the thermal barrier coating22less susceptible to chemical attack than other ceramic coatings, which can suffer destabilization when used alone through thermal effects and chemical attack in diesel combustion engines. Ceria and ceria stabilized zirconia are much more stable under such thermal and chemical conditions. Ceria has a thermal expansion coefficient which is similar to the steel which can be used to form the body portion26. The thermal expansion coefficient of ceria at room temperature ranges from10E-6to11E-6, and the thermal expansion coefficient of steel at room temperature ranges from11E-6to14E-6. The similar thermal expansion coefficients help to avoid thermal mismatches that produce stress cracks.

In one embodiment, the ceramic material is present in an amount of70percent by volume (% by vol.) to 95% by vol., based on the total volume of the thermal barrier coating22. In one embodiment, the ceramic material used to form the thermal barrier coating22includes ceria in an amount of 90 to 100 weight percent (wt. %), based on the total weight of the ceramic material. In another example embodiment, the ceramic material includes ceria stabilized zirconia in an amount of 90 to 100 wt. %, based on the total weight of the ceramic material. The ceria stabilized zirconia preferably includes ceria in an amount of 20 to 25 wt. %, based on the total weight of the ceria stabilized zirconia. In another example embodiment, the ceramic material includes yttria or yttria stabilized zirconia in an amount of 90 to 100 wt. %, based on the total weight of the ceramic material. In yet another example embodiment, the ceramic material includes ceria stabilized zirconia and yttria stabilized zirconia in a total amount of 90 to 100 wt. %, based on the total weight of the ceramic material. In another example embodiment, the ceramic material includes magnesia stabilized zirconia, calcia stabilized zirconia, and/or zirconia stabilized by another oxide in an amount of 90 to 100 wt. %, based on the total weight of the ceramic material. In other words, any of the oxides can be used alone or in combination in an amount of 90 to 100 wt. %, based on the total weight of the ceramic material. In cases where the ceramic material does not consist entirely of the ceria, ceria stabilized zirconia, yttria, yttria stabilized zirconia, magnesia stabilized zirconia, calcia stabilized zirconia, and/or zirconia stabilized by another oxide, the remaining portion of the ceramic material typically consists of other oxides and compounds such as aluminum oxide, titanium oxide, chromium oxide, silicon oxide, manganese or cobalt compounds, silicon nitride, and/or or functional materials such as pigments or catalysts. For example, according to one embodiment, a catalyst is added to the thermal barrier coating22to modify combustion. A color compound can also be added to the thermal barrier coating22. According to one example embodiment, thermal barrier coating22is a tan color, but could be other colors, such as blue or red.

The material selection and proportions of the mixed layer50can be controlled to achieve a good bond with the body portion26and to tune the desired thermal characteristics of the thermal barrier coating22. The metal material mixed in with the ceramic material also serves to protect the ceramic material (which is naturally porous) from thermal and corrosive attack from the hot combustion gases that can otherwise infiltrate and compromise the integrity of the mixed layer50, subjecting it to delamination from the body portion26. According to a preferred embodiment, the mixed layer50is a 50:50 mix by weight of NiCrAlY or NiCrAl metal combined with ceria stabilized zirconia (20 wt. % ceria, 80 wt. % zirconia). Having a higher concentration of ceramic increases the insulating effect of the thermal barrier coating22which protects the body portion26, but too high of concentration can cause the body portion26to retain the heat at the surface instead of cycling with the thermal transients of the combustion chamber to which it may be is exposed. By increasing the metal content, the pores of the ceramic material are filled and protected against attack and also the thermal barrier coating22becomes more thermally dynamic and its temperature at the combustion chamber surface is able to swing or cycle more closely with that of the combustion chamber environment to which it is directly exposed. The thickness/thinness of the mixed layer50can also play a role in the thermal properties of the thermal barrier coating22, with thicker coatings being more insulating and thinner coatings being more dynamic in their thermal properties. According to an example embodiment, the thickness of the mixed layer 50 is 200 microns or less, or 100 microns or less, and preferably 20 to 50 microns.

According to one embodiment, the ratio of ceramic to metal material in the mixed layer50is a 50:50 mix by weight. More or less ceramic in the mix will increase and decrease, respectively, the thermal insulation and retention properties of the thermal barrier coating22. The skilled artisan will understand that the ratio together with the thickness can be adjusted to tune the mixed layer50to achieve the desired thermal properties. For example, in the present case it is desired that the thermal barrier coating22sufficiently insulate the metal body portion26from thermal and oxidative damage from exposure to the environment of the combustion chamber of an internal combustion engine, and in particular a diesel engine. On the other hand, the thermal barrier coating22for the present case also is tuned to be sufficiently dynamic in its thermal properties to enable the thermal barrier coating22to cycle in sync with the transient temperature swings of the combustion cycle. In addition, these competing properties are to be achieved in the thermal barrier coating22that is sufficiently robust to withstand the corrosive attack of the hot combustion gases, and this is satisfied in large part by mixing the metal and ceramic in the mixed layer50so that the pores of the ceramic are infiltrated by the metal and the hot corrosive gases cannot penetrate the ceramic to the degree it could without the metal present which may otherwise lead to failure of the ceramic. This does not require the pores of the ceramic to be 100% filled, but rather sufficient metal to block the access of the hot gases through the surface and deep into the ceramic of the mixed layer50. If one were to section the mixed layer50of a 50:50 ceramic/metal mixed layer50, one would expect to see 20% or more of the pores of the ceramic material to contain the metal material and very few open passages extending from the surface to the base of the thermal barrier layer22. An increase in the proportion of metal to ceramic would increase the proportion of metal seen in cross section and thus an increase in porosity fill.

According to an alternative embodiment, the mixed layer50of ceramic and metal and could be applied as a gradient structure whereby there would be a higher concentration of metal compared to ceramic close to the metallic bond layer52, and progressing outward with increasing concentrations of ceramic until reaching the outer surface where the mixed layer50may be essentially all ceramic. For example, the gradient structure can be formed by gradually or steadily transitioning from 100% of the metal to 100% ceramic material. Alternatively, on the outer surface of the mixed layer50, both metal and ceramic material could be present. The transition function of the gradient structure can be linear, exponential, parabolic, Gaussian, binomial, or could follow another equation relating composition average to position. The gradient structure of the mixed layer50helps to mitigate stress build up through thermal mismatches and reduces the tendency to form a continuous weak oxide boundary layer at the interface of the ceramic and the metal material. The gradient structure may be more compatible in some applications for the transition from steel or another metal to ceramic and may yield a more robust thermal barrier coating22if required for a given application. Similar dynamic temperature profiles as described above are expected from the mixed layer50with the gradient structure.

An outermost surface of the mixed layer50with the gradient structure could be polished to reveal both ceramic and metal and finished following application to achieve desired roughness. For example, a surface roughness of the mixed layer50with the gradient structure after spraying may have a surface roughness of Ra 10-15 microns, but can be polished to a surface roughness less than Ra 15 microns, such as 3 microns or less, and more preferably 1 micron or less.

As indicated above, an uppermost portion and/or uppermost surface of the mixed layer50is typically formed entirely of ceramic, but may contain both metal and ceramic. Also, the additional ceramic layer60formed entirely of a ceramic material can be located on top of the mixed layer50, as shown inFIGS. 13, 9, and 10. The ceramic layer60could be the outermost layer and thus present the outermost exposed surface of the thermal barrier coating22, or could be located below the metal top layer51. This optional ceramic layer60can have a thickness of 20 to 80 microns. The ceramic material used to form the ceramic layer60can be the same or different from the ceramic of the mixed layer50.

According to one embodiment, the thermal barrier coating22includes the bond layer52, the mixed layer50, the ceramic layer60disposed on the mixed layer50, and the top layer51formed of metal disposed on the ceramic layer60. The top layer51is smoothed to a surface roughness Ra of not greater than 3 microns, or not greater than 1 micron, or less. The top layer51can be abraded until some of the ceramic layer60is exposed or protrudes through the top layer51, as shown inFIG. 8. Alternatively, the top layer51can be smoothed to provide a continuous outermost surface so that none of the ceramic layer60is exposed through the top layer51.

According to another example embodiment, the thermal barrier coating22includes the bond layer52, the mixed layer50, and the ceramic layer60formed entirely of a ceramic material disposed on the mixed layer50, wherein the ceramic layer60is an outermost exposed layer of the thermal barrier coating22, as shown inFIGS. 9 and 10. In this case, the ceramic layer60is processed to a thickness of not greater than 200 microns, preferably not greater than 100 microns, and most preferably 20-80 microns. The ceramic layer60is also processed or smoothed to a surface roughness Ra of not greater than 5 microns, not greater than 3 microns, or less. InFIG. 9, the ceramic layer60is smoothed to various degrees along the surface, so that the thickness of the ceramic layer60is greater in some portions than others, or the ceramic layer60could be completed eliminated in some areas. The surface roughness and thickness of the ceramic layer60can be adjusted depending on how much the ceramic layer60is smoothed or processed. InFIG. 10, the ceramic layer60is smoothed to a more uniform thickness.

According to another example embodiment, the thermal barrier coating22includes the bond layer52, the mixed layer50, so that the mixed layer50is the outermost layer of the thermal barrier coating22, as shown inFIG. 11. InFIG. 11, the mixed layer50is shown in the as-sprayed condition, before being processed or smoothed. However, the mixed layer50could be smoothed or processed to achieve the desired thickness and surface roughness. Also, the metal top layer51could be applied directly on the mixed layer50.

When the thermal barrier coating22includes the top layer51, it is typically the very outermost layer. The top layer51is formed of metal and is applied over the mixed ceramic/metal layer50and/or the ceramic layer60to fill the pores and seal off the surface of the ceramic. The top layer51is then typically polished to achieve the desired roughness. The top layer51is typically formed of 100 wt. % metal, based on the total weight of the top layer51. The top layer51can be the same or similar material as the bond layer52or it can be different. For example, the material used to form the top layer51could be a ferrous material, such as steel or another iron-based material. The material of the top layer51may also be chromium, nickel, and/or cobalt. The top layer51could also comprise a chromium alloy, nickel alloy, and/or cobalt alloy. The top layer51could also be a high performance superalloy, such as a nickel-based superalloy or cobalt based superalloy. For example, the metal top layer51could include or consist of at least one of alloy selected from the group consisting of CoNiCrAlY, NiCrAlY, NiCr, NiAl, NiCrAl, NiAlMo, and NiTi. According to preferred embodiments, the metal top layer51is formed of NiCrAlY or NiCrAl, chromium, and/or chromium alloy. The top layer51is typically deposited on the mixed layer50by plasma, HVOF and/or wire arc spray. This top layer51can serve as a protective layer to the ceramic material.

As indicated above, the top layer51is optionally polished to a degree where some of the peaks of the underlying ceramic material are revealed through the metal top layer51. Depending on the amount of abrading and the initial thickness of the top layer51, there can be areas of the top layer51where peaks of the underlying ceramic material show through or the ceramic peaks can show through uniformly across all of the top layer51. The top layer51may be abraded smooth to a surface roughness Ra of 3 microns or less, or even 1 micron or less. The Ra of 3 micron or less finish provides a very smooth and highly polished surface, which can benefit the flow and guidance of a fuel plume during the combustion cycle, and further resists carbon buildup. The thickness of the top layer51typically ranges from 10 to 100 microns, depending on how much material is removed during the smoothing process, and whether it is desirable to have peaks of the ceramic material exposed and showing through. According to one embodiment, no mixed layer50or ceramic layer60is exposed under the top layer51, so that the top layer51provides a smooth continuous exposed surface. According to another embodiment, some of the mixed layer50or some of the ceramic layer60is exposed through the top layer51.

The resulting outermost final surface can consist of the top layer51, or some of the underlying ceramic material may be revealed through the abrading operation such that a mix of ceramic and metal is present at the final outermost surface. In the latter case for this embodiment, the final surface would have a majority of the metallic material with peaks or specks of the ceramic dispersed and appearing in the otherwise continuous top layer51, and especially where there may have been more abrading than in other areas of the final surface. Visually, one would see a largely metallic final surface with specks of the ceramic dispersed either evenly throughout or more heavily in some regions than others. This can give the surface a mottled appearance with specks of the ceramic appearing in the otherwise continuous top layer51of metal.

It is to be understood that the various layers as-applied are not perfectly smooth and are typical of what one skilled in the art would expect when applying coating materials by plasma spray. Roughness can affect combustion by trapping fuel in cavities on the surface of the thermal barrier coating22. It is typically desirable to avoid coated surfaces rougher than the examples described herein. Immediately after plasma spraying, the thermal barrier coating22preferably has a surface roughness Ra of less than 15 μm, and a surface roughness Rz of not greater than 110 μm. However, the thermal barrier coating22can be smoothed. The same is true if HVOF or wire arc processes are used for the deposition. The material is applied in splats and builds to develop a layering effect due to overlapping of adjacent deposits, but it is not applied smooth nor necessarily uniform. It would be typical to have a series of peaks and valleys (as seen on the micro scale) and an intermixing of materials as a subsequently applied material may come to rest in a valley of a previously applied material, and a peak of prior material may project through a layer of a subsequently applied material. The intermix effect is enhanced when subsequent abrading operations are performed to smooth the surface, wherein some of the overlying material is stripped away and some of the underlying material (especially peaks) are revealed at the abraded surface.

The total thickness of the thermal barrier layer22may range from 50 to 350 or 700 microns, but preferably 200 microns or less or 150 microns or less or even less than 100 microns. For example, the overall coating (bond layer52, mixed layer50, and top layer51) may have a thickness of 250 microns or less, with the bond layer52having a thickness of 20 to 50 microns, the mixed layer50have a thickness of 20 to 50 microns, and the top layer51having a thickness of 50 to 100 microns. If the ceramic layer is present between the mixed layer50and the top layer51, the ceramic layer can have a thickness of 20 to 100 microns. As stated above, according to one embodiment, the thermal barrier coating22includes only the bond layer52and the mixed layer50with a total thickness of 700 microns or less.

Typically, 5% to 25% of the entire thickness of the thermal barrier coating22is formed of the bond layer52, and about 30% to 90% of the thermal barrier coating22could be made up of the mixed layer50. If the ceramic layer is present, about 5 to 50% of the thickness could be made up of the ceramic layer.

As described above, the thermal barrier coating22of the example embodiment includes a smooth surface with pores filled by the top layer51and thus is able to give similar fuel swirl characteristics as a non-coated surface. The thermal barrier coating22is not expected to absorb fuel or lubricant since the pores are filled.

The horizontal splat pattern of the top coat51is not expected to admit hot combustion gases because of the closed network of splats from the plasm spray. The thin ceramic-based mixed layer50insulates the body portion26but follows the transient temperature of the combustion, and the top layer51protects against hot oxidation due to the metal chemistry. The metal body portion26is thus protected from thermal and oxidative damage, while producing efficiency benefits.

When the thermal barrier coating22includes the bond layer52and the mixed layer50, but not the top layer51of metal, the total thickness of the thermal barrier coating22of this embodiment is up to 700 microns, preferably not greater than 400 microns, such as 50 to 400 microns, and more preferably not greater than 200 microns, or not greater than 150 microns. This two-layer structure is typically plasma sprayed onto the surface of the body portion26. Complex geometries of the body portion26can be coated, such as surfaces with wavy or curved features.

According to one embodiment, the bond layer52of the thermal barrier coating22is applied to the body portion26after grit blasting the surface. There is preferably no phosphate coating or other material applied to the surface of the body portion26prior to applying the bond layer52. Preferably, the bond layer52is applied by a plasma spray, to an average thickness of 50 to 100 microns, but may be applied using one of the other methods discussed herein. The material of the bond layer52of this embodiment may be the same as those described above with regard to the first example embodiment. Typically, the bond layer52is formed of chromium, nickel, cobalt, or an alloy thereof, or a nickel based superalloy or cobalt based superalloy. Preferably, the bond layer52is formed of NiCrAlY or NiCrAl.

The mixed layer50may be applied directly on the bond layer52, typically by plasma spraying. There are no sharp interfaces in the thermal barrier coating22, and thus thermal stress concentration is avoided. The mixed layer50of this embodiment can include the same ceramic materials and metal materials discussed above with regard to the first example embodiment. For example, the metal can be the same material used to form the bond layer52, such as chromium, nickel, cobalt, alloy thereof, nickel based superalloy, or cobalt based superalloy. The ceramic can be at least one oxide, for example ceria, ceria stabilized zirconia, yttria, yttria stabilized zirconia, calcia stabilized zirconia, magnesia stabilized zirconia, zirconia stabilized by another oxide, and/or a mixture thereof. The composition of the mixed layer50can be varied to tune the thermal properties. The mixed layer50can vary from 10 wt. % to 90 wt. % ceramic material, based on the total weight of the mixed layer50, and the remainder is formed of the metal material, such as one of the metal materials used to form the bond layer52described above. In this embodiment, the mixed layer50could be applied as the gradient structure discussed above. Typically, the uppermost portion of the mixed layer50is formed entirely of the ceramic material. Optionally, the ceramic layer could be applied to the mixed layer50, as discussed above.

The mixed layer50can have a thickness of 50 to 350 microns, such that the total thickness is less than 700 microns, for example between 100 to 450 microns, with a preferred total thickness of about 200 microns or less. No other coatings of metal or ceramic are applied on top of the mixed layer50in this embodiment, such that the thermal barrier layer22is a two-layer structure. The sprayed roughness of the mixed layer50is about Ra 10-15 microns, but the outermost surface of the mixed layer50can be abraded as described above to smooth the surface to have an Ra of 3 microns or less if desired.

A preferred example composition of the mixed layer50is a 50:50 mix by volume of NiCrAlY or NiCrAl combined with ceria stabilized zirconia (20 wt. % ceria, 80 wt. % zirconia). The bond layer52is also preferably the NiCrAlY or NiCrAl superalloy. Also, a preferred total thickness of the thermal barrier layer20is about 200 microns, with the bond layer52having a thickness of 50 to 100 microns, and the remaining length is the mixed layer50.

The thermal barrier coating22provides numerous advantages, including good thermal protection of the metal body portion26. The thermal barrier coating22has a low thermal conductivity to reduce heat flow through the thermal barrier coating22. Typically, the thermal conductivity of the thermal barrier coating22having a thickness of less than 1 mm is less than 1.00 W/m·K, preferably less than 0.5 W/m·K, and most preferably not greater than 0.23 W/m·K. The specific heat capacity of the thermal barrier coating22depends on the specific composition used, but typically ranges from 480 J/kg·K to 610 J/kg·K at temperatures between 40 and 700° C. The low thermal conductivity of the thermal barrier coating22is achieved by the porosity of the ceramic material50. Due to the composition and low thermal conductivity of the thermal barrier coating22, the thickness of the thermal barrier coating22can be reduced relative to comparative coatings, which reduces the risk of cracks or spalling, while achieving the same level of insulation relative to comparative coatings of greater thickness. It is noted that the advantageous low thermal conductivity of the thermal barrier coating22is not expected. When the ceramic material50of the thermal barrier coating22includes ceria stabilized zirconia, the thermal conductivity is especially low.

Various evaluations and tests have been conducted to evaluate the characteristics and performance of the thermal barrier coating22. For example, thermal imaging was used as a rapid (<1s) way to estimate the speed of cooling of the thermal barrier coating22on the metal body portion26. The thermal barrier coating22has also demonstrated to be very capable of cycling with the temperature of the combustion cycle. One way the dynamic cycling capability of the thermal barrier coating22was evaluated was to measure the rate at which the coated surface of the body portion26cooled (thermal decay) when exposed to a heating/cooling cycle.

Tests of the thermal barrier coating22were performed on a metal sample according to an example embodiment, wherein the metal sample was formed of AISI 4140 with a bond layer52formed of NiCrAlY, a mixed layer50formed of 50:50 by weight of mixed NiCrAlY and ceria stabilized zirconia, and a ceramic material51formed of 100% ceria stabilized zirconia as the final exposed layer. Competitive coatings on aluminum substrates were tested for comparative purposes. Total coating thicknesses between 70 microns and 390 microns were tested. In addition, tests were done on an AISI 4140 sample with a two layer thermal barrier coating22containing a NiCrAlY bond layer52with a mixed layer50formed of 50:50 by weight layer of NiCrAlY and ceria stabilized zirconia, such that the total coating thickness was not more than 200 microns.

One approach was to expose the coated surface of the sample to a heat source, remove the heat source and monitor the temperature drop at the surface as a function of time. The heat source may be a lamp flash, and thermal imaging with a FLIR camera may be used to measure the change in temperature values as a function of time after the lamp is cycled off In this case, the lamp flashes then frames are recorded at 60 Hz while cooling.

The test included evaluating the average thermal decay time of the thermal barrier coating22on the metal sample, and the results are shown inFIG. 13. This assessment of thermal decay included determining how fast the coated surface dropped to half of its starting temperature. Using the same lamp flash cycling and sample, the coated surface was heated to about 100° C. and the lamp cycled off. Using thermal imaging, the temperature of the coated surface averaged over a line from the outer diameter of the sample to a center axis of the sample was measured.FIG. 13compares the time taken by variants of thermal barrier coatings to drop to half after the lamp flashes and delivers thermal energy to the coated surface.

The above temperature cycling profiles of the coated sample demonstrate that the average thermal decay time of the coated body portion26can be tuned to be close to that of the average decay time of the combustion gases that are seen during a combustion cycle in an internal combustion engine. The thermal barrier coating22thus protects the metal body portion26against corrosive and thermal damage while providing a very thermally dynamic surface that is able to swing with the rapid temperature rise and fall of combustion.

Another advantage when the thermal barrier coating22includes the gradient structure is that the bond strength of the thermal barrier coating22is increased due to the gradient structure50and the composition of the metal used to form the body portion26.

The bond strength of the thermal barrier coating22having a thickness of 0.38 mm is typically at least 2000 psi when tested according to ASTM C633.

The thermal barrier coating22with mixed layer50can be compared to a comparative coating having a two layer structure, which is typically less successful than the thermal barrier coating22with the mixed layer50. The comparative coating includes a metal bond layer applied to a metal substrate followed by a ceramic layer with discrete interfaces through the coating. In this case, combustion gases can pass through the porous ceramic layer and can begin to oxidize the bond layer at the ceramic/bond layer interface. The oxidation causes a weak boundary layer to form, which harms the performance of the coating.

It has been found that the reduction in heat flow of a metal sample coated with the thermal barrier coating22is at least 50%, relative to the same sample without the thermal barrier coating22. By reducing heat flow through the metal body portion26, more heat can retained in the exhaust gas produced by the engine, which leads to improved engine efficiency and performance.

The thermal barrier coating22of the present invention has been found to adhere well to the body portion26. However, for additional mechanical anchoring, the surfaces of the body portion26to which the thermal barrier coating22is applied is typically free of any edge or feature having a radius of less than 0.1 mm. In other words, the surfaces of the body portion26to which the thermal barrier coating22is preferably free of any sharp edges or corners.

According to one example embodiment, the body portion26can include a broken edge or chamfer machined along an outer surface of the body portion26. The chamfer allows the thermal barrier coating22to creep over the edge of the surface and radially lock to the body portion26. Alternatively, at least one pocket, recess, or round edge could be machined along the surface and/or edges of the body portion26. These features help to avoid stress concentrations in the thermal sprayed coating22and avoid sharp corners or edges that could cause coating failure. The machined pockets or recesses also mechanically lock the thermal barrier coating22in place, again reducing the probability of delamination failure.

Typically, the thermal barrier coating22is only applied to a portion of the component exposed to the combustion chamber. For example, an entire surface of the component exposed to the combustion chamber could be coated. Alternatively, only a portion of the surface of the component exposed to the combustion chamber is coated. The thermal barrier coating22could also be applied to select locations of the surface exposed to the combustion chamber, depending on the conditions of the combustion chamber and location of the surface relative to other components. In an example embodiment, the thermal barrier coating22is only applied to a portion of the inner diameter surface of the cylinder liner28located opposite the top land44of the piston26when the piston26is located at top dead center, and the thermal barrier coating22is not located at any other location along the inner diameter surface, and is not located at any contact surfaces of the cylinder liner28.

Another aspect of the invention provides a method of manufacturing the coated component for use in the internal combustion engine, for example a diesel engine. The body portion26, which is typically formed of steel or another ferrous or iron-based material, can be manufactured according to various different methods, such as forging or casting. The method can also include welding sections of the component together. As discussed above, the body portion26can comprise various different designs. Prior to applying the thermal barrier coating22to the body portion26, any phosphate or other material located on the surface to which the thermal barrier coating22is applied must be removed.

The method next includes applying the thermal barrier coating22to the body portion26. The thermal barrier coating22can be applied to the entire surface of the body portion26, or only a portion of the surface. The ceramic material50and metal bond material52are provided in the form of particles or powders. The particles can be hollow spheres, spray dried, spray dried and sintered, sol-gel, fused, and/or crushed. In the example embodiment, the method includes applying the metal bond material52and the ceramic material50by a thermal or kinetic method. According to one embodiment, a thermal spray technique, such as plasma spraying, flame spraying, or wire arc spraying, is used to form the thermal barrier coating22. High velocity oxy-fuel (HVOF) spraying is a preferred example of a kinetic method that gives a denser coating. Other methods of applying the thermal barrier coating22to the body portion26can also be used. For example, the thermal barrier coating22could be applied by a vacuum method, such as physical vapor deposition or chemical vapor deposition. According to one embodiment, HVOF is used to apply a dense layer of the metal bond material52to the body portion26, and a thermal spray technique, such as plasma spray, is used to apply the mixed layer50. Also, the mixed layer50can be applied by changing feed rates of twin powder feeders while the plasma sprayed coating is being applied.

The example method begins by spraying the metal used to form the bond layer52in an amount of 100 wt. % and the ceramic used to form the mixed layer50in an amount of 0 wt. %, based on the total weight of the materials being sprayed. Once the bond layer52is formed, the method includes spraying a mixture of the ceramic and metal to form the mixed layer50. To form the gradient structure, throughout the spraying process, an increasing amount of ceramic material can be added to the composition, while the amount of metal bond material is reduced. Thus, the composition of the thermal barrier coating22gradually changes from 100% metal bond material52at the body portion26to 100% ceramic material50at an outermost surface, which may or may not be an exposed surface. Multiple powder feeders are typically used to apply the thermal barrier coating22, and their feed rates are adjusted to achieve the desired structure. When the mixed layer50includes the gradient structure, the gradient structure is achieved during the thermal spray process. To form the thermal barrier coating22of the first example embodiment, the method includes applying the top layer51on the mixed layer50, typically depositing by plasma, HVOF and/or wire arc spray.

The thermal barrier coating22can be applied to the entire body portion26, or a portion thereof. Non-coated regions of the body portion26can be masked during the step of applying the thermal barrier coating22. The mask can be a re-usable and removal material applied adjacent the region being coated. Masking can also be used to introduce graphics in the thermal barrier coating22. In addition, after the thermal barrier coating22is applied, the coating edges are blended, and sharp corners or edges are reduced to avoid high stress regions.

The thermal barrier coating22has a thickness t extending from the body portion26to the exposed surface58, as shown inFIG. 8. According to example embodiments, the thermal barrier coating22is applied to a total thickness t of not greater than 1.0 mm, and preferably not greater than 200 microns. The thickness t can be uniform along the entire surface of the body portion26, but typically the thickness t varies along the surface. In certain regions along the body portion26, for example where a shadow from a plasma gun is located, the thickness t of the thermal barrier coating22can be lower. In other regions, for example regions which are in line with and/or adjacent to fuel injectors, the thickness t of the thermal barrier coating22is increased. For example, the method can include aligning the body portion26in a specific location relative to the fuel plumes by fixing the body portion26to prevent rotation, using a scanning gun in a line, and varying the speed of the spray or other technique used to apply the thermal barrier coating22to adjust the thickness t of the thermal barrier coating22over different regions of the body portion26.

In addition, more than one layer of the thermal barrier coating22having the same or different compositions, could be applied to the body portion26. Furthermore, coatings having other compositions could be applied to the body portion26in addition to the thermal barrier coating22.

Prior to applying the thermal barrier coating22, the surface of the body portion26is washed in solvent to remove contamination. Next, the method typically includes removing any edge or feature having a radius of less than 0.1 mm. The method can also include forming the broken edges or chamfer56, or another feature that aids in mechanical locking of the thermal barrier coating22to the body portion26and reduce stress risers, in the body portion26. These features can be formed by machining, for example by turning, milling or any other appropriate means. The method can also include grit blasting surfaces of the body portion26prior to applying the thermal barrier coating22to improve adhesion of the thermal barrier coating22.

After the thermal barrier coating22is applied to the body portion26, the coated component can be abraded to remove asperities and achieve a smooth surface. The method can also include forming a marking on the surface of the thermal barrier coating22for the purposes of identification of the coated component when the component is used in the market. The step of forming the marking typically involves re-melting the thermal barrier coating22with a laser. According to other embodiments, an additional layer of graphite, thermal paint, or polymer is applied over the thermal barrier coating22. If the polymer coating is used, the polymer burns off during use of the component in the engine. The method can include additional assembly steps, such as washing and drying, adding rust preventative and also packaging. Any post-treatment of the coated component must be compatible with the thermal barrier coating22.

The resultant overall thermal barrier coating22presents a thermal barrier for the ferrous component when exposed to combustion gases and the cycle of an internal combustion engine, and is able to readily cycle with the temperature of the intake and combustion gases better than a thicker ceramic coating. The metal top layer51seals the remainder of the coating22against attack from the corrosive fuel environment that can sometimes penetrate and compromise thermal barrier coatings. The application technique of the top layer51(e.g., plasma spray) is believed to be particularly effective at shielding the top layer51and mixed layer50against attack from the hot corrosive environment. The applied metal top layer51has a close network of horizontally spreading splats of the metal material that resists absorption of fuel since they do not present vertical boundaries of the metal top layer51that would be present if for example the top layer51were applied by electrodeposition and that are more prone to absorption and attack by the combustion gasses and fuel. The smoothness of the abraded top layer51presents a surface that is comparable to an uncoated component and allows the component to perform in fuel plume management to the level of an uncoated component and much better than a ceramic coated component alone.

Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the following claims. In particular, all features of all claims and of all embodiments can be combined with each other, as long as they do not contradict each other.