WO2020090103A1 - Thermal spray coating - Google Patents

Abstract

This coating is formed on a cylinder bore inner circumferential surface of an aluminum or aluminum alloy cylinder block, and is a coating of an iron alloy having iron (Fe) as the primary component thereof and including 8mass% or more of chromium (Cr) and 12% or less by Ni equivalent of austenite phase-stabilizing elements. The present invention can also provide a coating wherein the percentage of austenite phase present within the crystal structure of the coating is more than 0% and less than or equal to 20%, and the coating demonstrates superior adhesion to the cylinder bore, in spite of demonstrating superior corrosion resistance by having a high concentration of chromium.

Description

Thermal spray coating

The present invention relates to a thermal spray coating, and more specifically to a coating formed on the inner peripheral surface of a cylinder bore of a cylinder block made of aluminum or aluminum alloy.

A cast iron liner is provided on the inner surface of the cylinder bore of a cylinder block of an internal combustion engine made of aluminum or aluminum alloy to improve functions such as strength, wear resistance, and slidability.

However, since the cast iron liner requires a certain amount of wall thickness due to the method of manufacturing the cylinder block using the cast iron liner, the weight of the entire cylinder block increases, and in addition, voids easily occur at the joint surface with the cylinder block, resulting in thermal conductivity. Is easy to decrease.

Therefore, instead of a cast iron liner, a spray coating is formed on the inner peripheral surface of the cylinder bore to reduce the weight of the cylinder block.

Patent Document 1 describes a spray wire used for spraying the inner surface of a cylinder bore.
When chromium (Cr) is contained in the sprayed wire in order to prevent corrosion of the sprayed coating caused by low-quality fuel having a high sulfur content, the bonding force between the sprayed droplets forming the sprayed coating decreases. It is disclosed that although the peeling resistance of the thermal spray coating is lowered, it can be solved by using a predetermined composition containing manganese (Mn).

Japanese Patent Laid-Open No. 2012-41617

However, in recent years, from the viewpoint of improving fuel economy, increasing the air-fuel ratio, returning exhaust gas to the cylinder again and burning it, etc. are being carried out.

The operating temperature of such an internal combustion engine is low because the amount of fuel supplied to the cylinder is small and the amount of heat generated is small. Therefore, the exhaust condensed water generated in the cylinder is hard to evaporate and easily stays in the cylinder.

Nitrogen oxides and sulfur oxides in the exhaust gas are dissolved in the condensed water and stay in the cylinder, so the corrosive environment in the cylinder has become more severe than before, and when corrosion occurs in the cylinder, the piston Combined with the sliding load of No. 2, especially near the top dead center, the wear becomes large, and it becomes the starting point of peeling. Therefore, the sprayed coating having a small film thickness is required to have further improved corrosion resistance.

However, if the coating has a high chromium concentration, the coefficient of thermal expansion becomes extremely large, and the difference in thermal contraction between the aluminum alloy cylinder block and the thermal spray coating after thermal spraying becomes large, and the thermal spray coating is more than the shrinkage of the cylinder block. Greatly contracts, and the entire sprayed coating peels from the cylinder block.

In other words, the thermal spray coating melts the thermal spray wire at a temperature higher than the melting point and sprays fine thermal spray droplets onto the inner peripheral surface of the cylinder bore, whereby the thermal spray droplets adhere to the inner peripheral surface of the cylinder bore and heat the cylinder block. Are deprived of the heat, cool, and solidify.

At this time, the sprayed droplets adhere to the cylinder block and give heat to the cylinder block, and the temperature drops by 1200 K or more before the temperature becomes the same as that of the cylinder block, causing a large heat shrinkage.

On the other hand, since the melting point of the aluminum alloy that constitutes the cylinder block is approximately 450 ° C to 660 ° C, the temperature of the cylinder block during thermal spraying does not exceed the above temperature, and the temperature rise due to thermal spraying is slight.

However, if the thermal spray coating largely shrinks by the time the temperature of the thermal spray coating becomes the same as the temperature of the cylinder block, the diameter of the thermal spray coating attached to the inner surface of the cylinder bore will be smaller than the inner diameter of the cylinder bore, and the plasticity of the thermal spray coating will decrease. Deformation causes peeling beyond the range where stress can be relaxed.

Since the peeling of the sprayed coating occurs at the interface between the sprayed coating and the cylinder block, it cannot be prevented by improving the bonding force between the sprayed droplets containing chromium, and it is difficult to form the sprayed coating itself.

The present invention has been made in view of the above problems of the conventional technique, and an object thereof is to provide a sprayed coating having excellent adhesion (peel strength) with a cylinder bore while improving corrosion resistance. Especially.

The present inventor, as a result of repeated intensive studies to achieve the above object, the iron-based alloy containing a high concentration of chromium, the thermal contraction rate is different depending on the metal structure, the austenite phase abundance ratio in the crystal structure of the thermal spray coating. It has been found that the above object can be achieved by setting the amount to be a predetermined amount or less, and the present invention has been completed.

That is, the thermal spray coating of the present invention is a coating formed on the inner peripheral surface of the cylinder bore of a cylinder block made of aluminum or aluminum alloy.
An iron-based alloy containing iron (Fe) as a main component, chromium (Cr) in an amount of 8 mass% or more, and an austenite phase stabilizing element in an amount of Ni equivalent to 12% or less, and an austenite phase existence ratio in the crystal structure are included. Is more than 0 and 20% or less.

According to the present invention, the existence ratio of the austenite phase in the crystal structure is set to 20% or less, so that it is possible to provide a coating film having a high chromium concentration and excellent corrosion resistance, but excellent adhesion to the cylinder bore. it can.

It is a graph which shows the relationship between the film thickness of a film, and peeling strength.

The coating film of the present invention will be described in detail.
The coating is a coating formed on the inner peripheral surface of the cylinder bore of a cylinder block made of aluminum or an aluminum alloy (hereinafter, may be simply referred to as “aluminum alloy”), and contains iron (Fe) as a main component and chromium. It is a coating of an iron-based alloy excellent in corrosion resistance, containing (Cr) 8 mass% or more and an austenite phase stabilizing element in Ni equivalent of 12% or less.
In the crystal structure of the coating, the austenite phase abundance ratio is more than 0 and 20% or less.

The austenite phase has a face-centered cubic lattice structure, and the filling rate of atoms is higher than that of the ferrite phase or martensite phase of the body-centered cubic lattice structure. That is, since there are few gaps between atoms, the volume change due to heat (vibration of atoms) is large.

The coating of the present invention has an austenite phase content of 20% or less in its crystal structure, has a large amount of ferrite phase and martensite phase, and has a small coefficient of thermal expansion. Thermal contraction due to the decrease in temperature is small.

After the thermal spray coating and the cylinder block reach the same temperature, since the thermal expansion coefficient of the aluminum alloy forming the cylinder block is larger than the thermal expansion coefficient of the coating, the cylinder block presses the thermal spray coating from the surroundings. Close together.

The austenite phase abundance ratio of the coating can be measured by conducting a structural analysis of the metal structure by electron diffraction.

The content of the austenite phase stabilizing element in the above film is Ni equivalent represented by Ni + 30 × C + 0.5 × Mn is 12 mass% or less. If the Ni equivalent exceeds 12% by mass, the entire coating may become an austenite phase depending on the metal composition of the coating, and the peeling resistance may be reduced.

The crystal structure of the coating can be adjusted by adjusting the cooling rate according to the metal composition of the iron-based alloy and the amount of thermal spraying.

Austenite phase stabilizing elements such as nickel (Ni), carbon (C), and manganese (Mn) are elements that increase the austenite phase existence ratio.

Further, a ferrite phase stabilizing element such as chromium (Cr) molybdenum (Mo) silicon (Si) niobium (Nb) is an element that forms a ferrite phase or a martensite phase and reduces the austenite phase existence ratio.

However, since the martensite phase is formed by the transformation of the austenite phase, unavoidably contains a retained austenite phase regardless of the metal composition, but the martensite phase transformation is sufficient to reduce the residual austenite phase. Thus, the abundance ratio of the austenite phase can be reduced.

The coating film contains chromium (Cr) in an amount of 8% by mass or more and forms a self-reproducible passivation film, and thus has excellent corrosion resistance. Since the corrosion resistance improves as the amount of chromium increases, the content of chromium preferably exceeds 11.5 mass%.
Since molybdenum (Mo) also exhibits corrosion resistance, molybdenum may be replaced by reducing the content of chromium.

The coating film preferably has an austenite phase existence ratio of 1% or more.
From the viewpoint of peel resistance due to the coefficient of thermal expansion, the smaller the austenite phase, the more preferable, but in the martensite phase and the ferrite phase, the strength of the coating increases due to the presence of many martensite phases, and the peel resistance And wear resistance are improved.

Both ferrite phase and martensite phase have body-centered cubic lattice structure, and it is not easy to distinguish them. Then, as described above, since the austenite phase inevitably remains in the martensite phase while the ferrite phase increases, the austenite phase does not increase, so that the martensite phase may exist. Was indirectly indicated by the existence ratio of the austenite phase being 1% or more.

The upper limit of the chromium content of the coating film is preferably 26% by mass or less, and more preferably 16% by mass or less.
When chromium is contained in an amount of more than 26% by mass, the corrosion resistance is improved, but the thermal expansion coefficient is increased and the coating film may be easily peeled off. Further, the martensite phase is not formed and the strength of the coating film is reduced, resulting in peeling. May be easier to do.

The content of the ferrite phase stabilizing element in the coating film is preferably such that the Cr equivalent represented by Cr + Mo + 1.5 × Si + 0.5 × Nb is 20 mass% or less.
When the Cr equivalent is 20% by mass or less, the martensite phase is easily formed, the strength of the coating film is improved, and the peel resistance is improved.

The coating preferably has a cross-sectional oxide ratio of 4 area% or less.
Since the thermal spray coating is formed by melting the thermal spray wire at a high temperature, it is easily oxidized.If chromium in the coating becomes an oxide and the amount of metallic chromium decreases, corrosion resistance decreases, so it is necessary to increase the content of chromium. , The coefficient of thermal expansion increases.
When the proportion of the oxide contained in the coating film is 4 area% or less, both corrosion resistance and peeling resistance can be compatible.

The proportion of oxide in the film can be measured by identifying the oxide based on the difference in iris from the optical microscope image of the film cross section, and binarizing and quantitating the cross-sectional image.

The proportion of oxide in the coating can be adjusted by the spray atmosphere.
Specifically, the ratio of oxides can be reduced by spraying a non-oxidizing gas such as nitrogen as a shield gas while using a nitrogen gas as a carrier gas.

The film thickness of the coating film is preferably 100 μm or more and 400 μm or less.
If the thickness of the coating is less than 100 μm, it is difficult to form irregularities of sufficient height on the inner circumference of the cylinder bore to enhance peeling resistance, and if it exceeds 400 μm, heat is retained during thermal spraying, and the coating strength decreases and resistance to The releasability may decrease. Further, the iron-based alloy forming the coating has a smaller thermal conductivity than the cylinder block made of the aluminum alloy, so that the cooling efficiency decreases as the coating becomes thicker.

In the present invention, unevenness can be provided on the inner peripheral surface of the cylinder bore to improve the peeling resistance of the coating. When the unevenness is provided, the film thickness of the coating means the thickness from the bottom of the unevenness.

The relationship between the film thickness of the coating and the peel strength is shown in FIG.
FIG. 1 is a graph showing the peel strength when the film thickness is increased, where the peel strength of a film having a film thickness of 200 μm is 1.
It can be seen from FIG. 1 that the peel strength decreases with an increase in the film thickness, and when it exceeds 400 μm, the peel strength is 10% or more lower than that of a 200 μm coating film, and sufficient peel resistance cannot be obtained.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following Examples.

Grooving was performed on the inner surface of the cylinder bore of the ADC12 alloy gasoline engine cylinder block so that irregularities with a height of approximately 85 μm were created.

Using a sprayed wire having the composition shown in Table 1, a coating film having a thickness of 200 μm was formed by an arc spraying method.
The composition of the sprayed wire was determined by dissolving the sprayed wire in nitric acid and conducting IPC analysis (Inductively Coupled Plasma).
Further, the coating film after thermal spraying was scraped off, and the same IPC analysis was performed to confirm that the composition was the same as the wire composition shown in Table 2.

For the thermal spraying, the cylinder block was preheated to 120 ° C., a nozzle was inserted into the cylinder bore, and nitrogen gas was sprayed at 1200 L / min for spraying sprayed droplets, and nitrogen gas was sprayed at 500 L / min as a shield gas. It was run in the atmosphere.
Only in Example 5, air was used for the spraying of the sprayed droplets, and the spraying was performed without flowing the shield gas.

<Evaluation>
The coating film was evaluated by the following methods. The evaluation results are shown in Table 2.

(Whether or not a film can be formed)
The presence or absence of peeling after cooling to room temperature was confirmed.

○: No peeling of thermal spray material ×: Complete removal of thermal spray material from cylinder lock

(Peeling resistance)
The cylinder block on which the coating was formed was cut into 20 mm squares, a jig was attached to the surface of the coating with an adhesive, and a tensile test was performed. The peeling resistance was evaluated by the stress (MPa) until the coating peeled.

(Presence ratio of austenite phase)
The cross section of the coating film was observed by SEM, two visual fields of 50 × 200 μm were arbitrarily extracted, phase analysis was performed by electron diffraction, and the austenite phase existence ratio (area%) was measured.

(Oxide ratio)
The cross section of the coating film was surface-analyzed by an electron probe micro analyzer (EPMA) to identify an oxide. Next, the cross section of the coating film was magnified 20 times and the oxide ratio (area%) was calculated by an optical microscope. Based on the difference in the luminosity of the oxide specified by the electron microprobe analyzer, the cross-sectional image was binarized from the optical microscope image to calculate the oxide ratio (area%) in the image.

(Corrosion resistance)
The coating film was cut into □ 20 mm, immersed in a 1% aqueous nitric acid solution at room temperature for 1 hour, and the corrosion resistance was evaluated by the weight loss (mg).

　表２中、―　は、未測定である。

In Table 2, − is unmeasured.

It can be seen that the coatings of Examples 1 to 5 containing 8 mass% or more of chromium and having an austenite phase existence ratio of 20% or less are excellent in peel strength and corrosion resistance.
In addition, in Example 5, the corrosion resistance was lower than that of the other Examples because the oxide content was less than 4% by mass.
Further, in Comparative Example 3 containing no chromium, peeling of the coating did not occur, but the corrosion resistance was extremely low.

Claims (6)

A coating formed on the inner peripheral surface of the cylinder bore of a cylinder block made of aluminum or an aluminum alloy,
It consists of an iron-based alloy containing iron (Fe) as a main component, chromium (Cr) of 8 mass% or more, and an austenite phase stabilizing element of 12% or less by Ni equivalent, and the austenite phase existence ratio in the crystal structure is 0. Over 20% or less. The coating according to claim 1, wherein the abundance ratio of the austenite phase in the crystal structure of the coating is 1% or more. The coating according to claim 1 or 2, wherein the content of chromium (Cr) is more than 11.5 mass% and 26 mass% or less. The coating according to any one of claims 1 to 3, wherein the oxide contained in the coating has a ratio of 4 area% or less. The coating film according to any one of claims 1 to 4, which contains a ferrite phase stabilizing element in an amount of 20% or less in terms of Cr equivalent. The coating film according to any one of claims 1 to 5, wherein the film thickness is 100 μm or more and 400 μm or less.

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