JPH01188659A - Formation of thermally sprayed ceramic layer - Google Patents

Abstract

PURPOSE:To form a thermally sprayed ceramic layer capable of surely and sufficiently preventing peeling and falling by thermally spraying a ZrO2-type material containing Al2O3 and transforming Al2O3 in the thermally sprayed layer into alpha-Al2O3 by means of heating so as to expand the volume and cause cracking to the thermally sprayed layer. CONSTITUTION:A thermal spraying material in which 20-50wt.% of Al2O3 is mixed with ZrO2-type material is thermally sprayed on a base material or an undercoat layer, by which a thermally sprayed ceramic layer in which gamma-Al2O3 is dispersed in the ZrO2-type material is formed. Subsequently, by heating the surface of the above thermally sprayed layer up to 1200 deg.C or above, gamma-Al2O3 in the thermally sprayed layer is transformed into stable alpha-Al2O3 and, at this time, a volume expansion of about 3-9% is brought about. As the result, fine cracks are caused to the ZrO2-type material principally around the above gamma-Al2O3. By this method, the thermally sprayed ceramic layer in which peeling and falling are prevented and durability is improved to a greater extent than heterofore can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application This invention uses ceramics for heat insulation, heat shielding, etc. for parts used in high-temperature atmospheres, such as parts for automobile engines or parts for turbochargers. The present invention relates to a method of forming a sprayed layer.

Conventional technology Traditionally, parts such as automobile engine pistons that have parts that are heated to high temperatures, especially parts whose base material is aluminum alloy, have been made with ceramics that have low thermal conductivity and excellent heat resistance on the surface of the base material. Ceramic thermal sprayed members with improved heat insulation and heat resistance are being used by thermally spraying to form a ceramic solvent layer.

Such a conventional ceramic sprayed member will be described in more detail below using a piston for an automobile engine as an example.

In recent years, as pistons used in engines, pistons cast from aluminum alloy have been increasingly used, with a focus on weight reduction in order to reduce the inertia of reciprocating parts in the engine. However, since aluminum alloy is a material with high thermal conductivity, in engines using aluminum alloy pistons, the combustion heat generated by combustion of fuel in the combustion chamber is transferred to the outside of combustion via the piston, and the This tends to deteriorate the thermal efficiency of the engine, resulting in lower engine output and fuel efficiency. Therefore, in order to reduce the heat loss that is transferred to the combustion side through the aluminum ram alloy piston, a ceramic material with low thermal conductivity is sprayed on the top surface (piston head) of the piston, etc., and the ceramic material has improved heat insulation properties. Attempts have been made to apply thermal spray pistons (e.g. [CIJllIinS/TACOHA Advanced
AdiabatiCEngin, JR, Kamo e
tal, S^E Paper No. 840428, etc.).

However, in ceramic sprayed pistons in which a ceramic sprayed layer is formed using an aluminum alloy as a base material, there is a large difference between the coefficient of thermal expansion of the aluminum alloy that is the base material and the thermal expansion coefficient of the ceramic material. During repeated heating and cooling associated with engine operation, cracks occur at the interface due to the difference in thermal expansion between the surface of the aluminum alloy base material and the ceramic sprayed layer.
Eventually, the ceramic sprayed layer may peel off or fall off from the base material surface.

Therefore, recently, zirconia (Zr
02) It is becoming more common to select the material of the system. However, even if a ceramic sprayed member is coated with a sprayed layer made of a zirconia material, it is difficult to reliably prevent the solvent layer from peeling off or falling off.

On the other hand, as a conventional method to prevent the peeling of the ceramic sprayed layer due to the coefficient of thermal expansion between the aluminum alloy base material and the ceramic sprayed layer, the thermal expansion coefficient is set on the surface of the base material in advance between the base material and the ceramic sprayed layer. Metals that are intermediate between the above and have good adhesion to ceramics, such as Ni-Cr-A1 alloy, N1-Cr-AN-Y alloy, Ni-Go-Cr-
A method is known in which a base sprayed layer called a pond layer or an intermediate layer is formed by spraying a thin layer of Af-Y alloy, and a ceramic sprayed layer is sprayed on top of the base sprayed layer (for example, Even when the base sprayed layer was formed in this way, it was still not sufficient to prevent the ceramic sprayed layer from peeling or falling off due to the difference in thermal expansion.

Therefore, we introduced fine racks into the ceramic solvent layer,
The fine rack reduces thermal stress during use,
A method to prevent the peeling and falling off of the sprayed layer was published in Japanese Patent Application Laid-Open No. 5
No. 8-87273. In other words, in this proposed method, after the ceramic is sprayed, while the sprayed layer is still at a high temperature, an inert gas (651) is blown onto the sprayed layer to rapidly cool it, thereby creating fine particles within the sprayed layer. This is a method in which the Nari rack is made into a benshi, and the thermal stress is alleviated by the microscopic cracks. In addition, the applicant's patent application 1986-118
In d3 of 59'll, high-density energy such as a laser is applied to the surface of the ceramic sprayed layer after thermal spraying, as a method of introducing fine racks over the entire thickness of the ceramic sprayed layer with particularly large coating pieces to alleviate thermal stress. is irradiated to rapidly remelt and resolidify only the outermost layer of the sprayed layer, producing fine cracks in the outermost layer, and then subjecting the sprayed layer to a cooling and heating cycle to eliminate cracks throughout the entire thickness of the sprayed layer. Methods have been proposed to grow the species over a period of time.

Problems to be solved by the defense In the method of JP-A-58-87273 mentioned above, the thermal stress due to the difference in thermal expansion is alleviated to a considerable extent by the am-shaped rack introduced into the ceramic sprayed layer, and micro-cracks are introduced. Compared to the case where no coating is used, the ceramic sprayed layer is less likely to peel or fall off. However, in this method, cracks are generated by rapid cooling after thermal spraying, so it is not possible to control the position of the crack, and cracks also occur at the interface with the base material or the base layer, and the cracks at the interface On the contrary, the ceramic sprayed layer may easily peel off, so it has been difficult to reliably and sufficiently prevent the ceramic sprayed layer from peeling or falling off and improve its durability to a satisfactory level. . In addition, in the case of the above method, the quenching to introduce cracks is performed by spraying inert gas, so if the ceramic sprayed layer is thick, the quenching effect is insufficient and the cracks become coarse, making it difficult to alleviate thermal stress. There were also problems that made it less effective.

On the other hand, the method disclosed in Japanese Patent Application No. 1185'99/1985 has the advantage of being able to form relatively fine racks over the entire thickness of the ceramic sprayed layer even if it is thick; When growing the crack over its entire thickness, the above-mentioned JP-A-58-. 87273
Similar to the method in No. 1, it is not possible to control the growth position of cracks, and cracks occur at the interface with the base material and underlying layer, making it easy for the sprayed layer to peel off from that area.As a result, the results are as satisfactory as above. It was difficult to improve durability to this extent.

This invention was made against the background of the above-mentioned circumstances, and it is necessary to introduce cracks at the interface with the base material and base layer when introducing fine racks into the sprayed layer in order to alleviate the thermal stress generated in the ceramic solvent layer. This makes it possible to generate uniform and fine cracks only inside the sprayed layer, and also to control the degree of cracking, thereby preventing the peeling and falling off of the ceramic sprayed layer without causing the above-mentioned problems. It is an object of the present invention to provide a method for forming a ceramic sprayed layer that can reliably and sufficiently prevent the above-mentioned problems and has significantly improved durability compared to conventional methods.

Means to Solve the Problem The method of forming the ceramic sprayed layer in this defense uses a zirconia-based material mixed with 20 to 50% by weight of alumina as a solvent material, and after spraying this, the surface of the sprayed layer is heated. The alumina in the solvent layer was changed from γ-A1203 to α-7A20.
3, and the volumetric expansion of alumina caused by this transformation causes cracks in the solvent layer.

Function: If a thermal spray material consisting of a zirconia material mixed with 20 to 50% by weight of alumina is sprayed onto a base material or an underlying layer, a ceramic spray layer with alumina dispersed in the zirconia material will be formed. Ru.

Here, even if the alumina before thermal spraying is alumina in another phase other than α-Al2O3, the alumina in the thermal spray layer and the alumina cooled and solidified after iso-spraying are γ-Al203.
This is usually the case. Therefore, in the method of the present invention, the surface of the sprayed layer is heated again after the solvent is used to transform the alumina in the sprayed layer from γ-Al2O3 to stable α-8203. Note that the temperature at which γ-8203 transforms into stable α-Aβ203 is about 1200°C, so if the surface of the sprayed layer is heated to 1200°C or higher, the alumina in the sprayed layer will transform from γ to α. can be done.

When γ-Aβ203 metamorphoses into α-7A1203,
A volume expansion of about 3 to 9% occurs. Therefore, the sprayed layer is heated as described above.1. By subjecting the aluminum dispersed in the sprayed layer to γ→α transformation, the volume of the dispersed alumina expands, causing a fine rank to occur mainly in the zirconia-based material surrounding the alumina. The finely shaped rack thus produced functions to relieve thermal stress due to thermal expansion differences during use.

Here, since the fine racks 4 caused by the transformation and expansion of alumina are often generated near areas where alumina is present, there is less risk of cracks occurring at the interface between the base material and the base layer as in the case of the conventional method. .

In addition, by controlling the distribution of alumina in the melt layer, an appropriate number of alumina can be obtained for alleviating thermal stress.

A uniform and fine rack can be formed.

Note that if the amount of alumina contained in the thermal spray material is less than 20% by weight, the dispersion Φ of alumina in the ceramic solvent layer will also be reduced. The number of microscopic cracks generated is small, and therefore the effect of relieving thermal stress cannot be obtained sufficiently.

On the other hand, if the amount of alumina exceeds 50% by weight, the amount of alumina in the ceramic melt layer will be too large, and the number of cracks will be too large, making the solvent layer brittle, making it more likely that the sprayed layer will peel off or fall off. . Therefore, the mixing ratio of alumina necessary to achieve the purpose of this invention is 20 to 50% by weight.
is within the range of

Furthermore, the zirconia-based materials used in this invention include not only simple Z, rO2, but also stabilized zirconia and partially stabilized zirconia, such as ZrO2/Y2O3, ZrO2, etc.
Of course, it also includes O2, CaO1, ZrO2, MqO, etc.

Example 1 Example 11 ZrO2.8Y203 was used as a zirconia-based material, and 30% by weight of alumina was mixed therein to prepare a ceramic thermal spray material. On the other hand, as shown in Figure 1, JIS
The surface of a 50 x 50 x 10 IHI flat base material (base material) 1 made of AC8A alloy was shot blasted and preheated to 100 to 150°C with a plasma spray gun. An Aβ alloy was thermally sprayed as the base layer 2. On top of that, the above-mentioned ceramic spraying material was applied at a current of 500A, a voltage of 65V, and a powder supply of 160g/
A ceramic sprayed layer 3 in which alumina 32 is dispersed in a zirconia-based material 31 was formed by plasma spraying to a thickness of 0.5 minutes.

Thereafter, using the same plasma spray gun, the ceramic solvent layer 3 was rapidly heated from the surface thereof to a temperature of 1200° C. or higher. As a result, as shown in FIG. 1, fine racks 4 were formed in the zirconia-based material 31 around the alumina 32 dispersed in the ceramic sprayed layer 3.

In addition, in FIG. 1, J3 and reference numeral 5 are pores.

[Comparative Example 11] Only ZrO2.8Y20a was used as the ceramic spray material without mixing alumina, and the process up to the formation of the ceramic spray layer was carried out in the same manner as in Example 1.

[Comparative Example 21 A ceramic sprayed layer was formed in the same manner as in Example 1 using only ZrO2.8Y203 as a ceramic spraying material without mixing alumina, and the ceramic sprayed layer was further heated and
Cracks were introduced by rapid cooling treatment.

[Performance Evaluation Test 1] The following thermal cycle test was conducted on each of the thermal sprayed layers of Example 1 and Comparative Examples 1.2. That is, the central part of the ceramic sprayed layer was heated by an acetylene-M elementary burner.
After heating for 0 seconds, quenching in water maintained at 50°C and holding for 60 seconds, the heating-cooling cycle was repeated 5000 times.
The cycle was repeated. As a result, in the sprayed layer of Comparative Example 1,
The ceramic solvent layer peeled off after 1100 cycles, and the ceramic sprayed layer of Comparative Example 2 peeled off after 3000 cycles, but no abnormality was observed in the sprayed layer of Example 1 until the end of 5000 cycles. It was found to have excellent durability.

Here, when we investigated the structure of the sprayed layer before and after the test, we found that
In Comparative Example 1, it was found that during evaluation, large cracks were generated near the interface between the base layer 2 and the ceramic sprayed layer 3, leading to peeling of the sprayed layer. It is thought that such cracks at the interface are caused by a difference in thermal expansion between the base material 1 and the ceramic sprayed layer 3.

In addition, in Comparative Example 2, fine cracks were generated everywhere in the ceramic sprayed layer 3 before the test, and there were also parts where many cracks were generated near the interface between the base layer 2 and the ceramic sprayed layer 3. During the evaluation test, it was discovered that the cracks near the interface were connected and became large, leading to the peeling of the sprayed layer. On the other hand, in the sprayed layer of Example 1, alumina is uniformly and finely dispersed, and microcracks exist only around the alumina, so cracks do not concentrate near the interface even during the evaluation IalI test. Therefore, it is thought that excellent durability was obtained.

1 Example 21 Various combinations and cuts as shown in symbols A to G in Table 1
A ceramic spray material made of a mixture of r 02 8Y203 and alumina was prepared, a ceramic solvent layer was formed in the same manner as in Example 1, and the ceramic spray layer was applied to soil at 1200°Cg in the same manner as in Example 1. After rapid heating, a fine rack was introduced.

Table 1 [Performance Evaluation Test 1] A heating-cooling five-cycle test similar to the performance evaluation test 1 was conducted for each thermal sprayed layer of Example 2 formed of each of the ceramic thermal sprayed materials labeled A to G.

As a result, the solvent layer peeled off after 3,500 cycles when the thermal spray material with code A was used, and after 4,000 cycles and 3.5 cycles when the spray material with code F1G was used. OO
Peeling occurred in the sprayed layer after O cycles, but when a sprayed material within the specified range of symbols B-E was used, no abnormality occurred until the end of the test of 5000 cycles.

Here, when we investigated the structure status before and after evaluation for each sprayed layer, when the sprayed material with code A was used, the number of microcracks in the ceramic sprayed layer before spraying was small;
As a result, similar to the case of Comparative Example 1 in which no alumina was mixed, large cracks occurred near the interface between the ceramic sprayed layer and the base layer during the evaluation test, leading to peeling of the ceramic sprayed layer. . Furthermore, when thermal spraying materials with symbols F and G are used, the amount of alumina is too large, so the number of cracks in the initial ceramic sprayed layer is also extremely large, which makes the ceramic sprayed layer extremely brittle and causes the early formation of the thermal sprayed layer. It was confirmed that the peeling occurred. On the other hand, the mixing ratio of alumina is within the range of this excuse (20-50Φ suspension%)
When using thermal spray materials with codes B to F, the number of microcracks introduced into the ceramic solvent layer is appropriate, and early peeling does not occur even after 60 heating-cooling cycles. This was confirmed.

Effects of Defense According to the ceramic sprayed layer forming method of this defense, a ceramic sprayed layer in which alumina is dispersed in a zirconia-based material is created using a sprayed material in which 20 to 50% by weight of alumina is mixed with a zirconia-based material. In addition, the alumina in the ceramic sprayed layer is transformed from γ-alumina to α-alumina, and the volumetric expansion accompanying the transformation generates fine cracks near the periphery of the alumina for thermal stress relaxation. This method prevents the occurrence of cracks at the interface with the base material and underlayer, which would make it easier for the ceramic sprayed layer to peel off, which is the case with conventional methods.Moreover, by controlling the dispersion of alumina, thermal stress can be alleviated. It is possible to produce an optimal number of uniform and fine cracks, and therefore the ceramic sprayed layer formed by the method described in this defense can sufficiently relieve thermal stress under the usage conditions where thermal cycles are applied, so that it does not peel or fall off. can be reliably and sufficiently prevented, and its durability is dramatically improved compared to conventional methods.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal sectional view schematically showing an example of a ceramic sprayed layer formed according to this explanation. DESCRIPTION OF SYMBOLS 1... Base material, 2... Base layer, 3... Ceramic solvent layer, 31... Zirconia material, 32... Alumina, 4... Ram crack. Applicant Toyota Motor Corporation Agent Patent Attorney Hi 1) Takehisa (one idiot) Figure 1

Claims (1)

[Claims] The thermal spraying material is a mixture of 20 to 50% by weight of alumina in a zirconia material, and after this is thermally sprayed, the surface of the thermally sprayed layer is heated to transform the alumina in the thermally sprayed layer from γ-Al_2O_3 to α-Al_2O_3. A method for forming a ceramic sprayed layer characterized by causing cracks in the sprayed layer due to volumetric expansion of alumina due to transformation.