Abradable material for a turbo machine

Abstract: A turbo machine including a movable member and a stationary member which has a coating layer disposed adjacent to the movable member, formed by flame spray coating and including at least one selected from the group consisting of hexagonal system boron nitride and ceric oxide. Since the coating layer is easily machined by the movable member, the coating layer comes to have a surface generated by machining the movable member and the clearance between the movable member and the stationary member is made zero substantially (0) even at a high temperature. Thus, the efficiency of the turbo machine has improved. In addition, since the coating layer has a high thermal shock resistance, the durability of the apparatus has improved also.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to non-positive displacement apparatus, such as a turbocharger, a gas turbine and the like, which comprises a movale member and a stationary member disposed adjacent to each other for relative displacement at a high temperature. In particular, the present invention relates to non-positive displacement apparatus which enables to make a clearance between the movale member and the stationary member substantially zero (0) during the operation thereof. Description of the Prior Art A conventional non-positive displacement apparatus will be hereinafter described with reference to an automotive turbocharger illustrated in Figure 24. This turbocharger has a turborotor 100 and an impeller 200 as a movable member, and a turbohousing 101 and a compressor housing 201 as a stationary member. In the operation of the turbocharger, the turborotor 100 is rotated by the energy of the exhaust gas of an engine (not shown), a shaft 300 is then rotated, and the impeller 200 is rotated by the rotation of the shaft 300. Whereby air is supercharged into the engine. In this way, the turborotor 100 and the turbohousing 101 as well as the impeller 200 and the compressor housing 201 are disposed adjacent to each other and displaced relatively at a high temperature during the operation of the turbocharger. Here, it has been known that the efficiency of the turbocharger can be improved by making a clearance C100 between the turborotor 100 and the turbohousing 101 and a clearance C200 between the impeller 200 and the compressor housing 201 as small as possible. In the case that the clearances C100 and C200 are reduced, however, there is a possibility of damaging the turborotor 100 and the impeller 200 since the eccentricity and so on occurred during the manufacture of the shaft 300 results in the contact or collision of the turborotor 100 with the turbohousing 101 and the contact or collision of the impeller 200 with the compressor housing 201. Accordingly, in the conventional turbocharger, it is necessary to set the clearance C100 between the turborotor 100 and the turbohousing 101 at approximately 0.6 to 0.8 mm and to set the clearance C200 between the impeller 200 and the compressor 201 at approximately 0.3 to 0.5 mm. The conventional turbocharger thus has insufficient efficiency. Therefore, it has been desired to develop a technology which can improve the efficiency of the conventional non-positive displacement apparatus by making the clearance between the movable member and the stationary member as small as possible and which can avoid the damage to the movable member. A technology has been disclosed so far in which a coating layer composed of a mixture of soft metal and resin or graphite is formed on the compressor housing 201 by flame spray coating. In the conventional technology, the formed coating layer is easily machined off by the contact of the impeller 200 with the compressor housing 201 resulting from the eccentricity and so on of the shaft 300. Whereby it is possible to make the clearance C200 between the impeller 200 and the machined compressor housing 201 substantially zero (0). Here, the impeller 200 is not damaged by the operation. The technologies disclosed in Japanese Unexamined Patent Publication (KOKAI) No. 18085/1974 and United States Patent No. 4,405,284 are included in the category of the technology which utilizes the machinability of the coating layer in order to make the clearance between the movable member and the stationary member zero (0) substantially. These publications disclose technologies of an Ni-graphite coating and an NiCrFeAl-BN coating. Further, United States Patent No. 4,269,903 discloses an invention relating to a ceramic seal. The publication discloses a technology for coating a porous stabilized zirconium oxide layer having a porosity of 20 to 33 %. This technology is basically identical with the above-mentioned technology. According to this technology, it is also possible to make the clearance between the movable member and the stationary member zero (0) substantially by utilizing the machinability of the porous stabilized zirconium oxide layer. However, even when the relatively displacing apparatus is manufactured by the above-mentioned technologies, there arises the following problems in the relatively displacing apparatus manufactured. Namely, the technology disclosed in the United States Patent No. 4,269,903 utilizes the zirconium oxide, which is resistible to a thermal shock, as the coating layer in view of the high temperature application. The zirconium oxide is made porous in order to secure the machinability of the coating layer. However, since the coating layer having the porosity of 20 to 33 % is formed by flame spray coating the zirconium oxide only and since the zirconium oxide having a high hardness of Hv 1000 or more is contained therein according to the technology, the movable member, i.e., a mating member of the coating layer, is likely to be worn by the coating layer. Further, when a coating layer having a porosity of 33 % or more, for instance a coating layer having a porosity of 40 % is formed in order to improve the machinability, the thermal shock resistance of the coating layer deteriorates and the coating layer comes off or falls off accordingly. Further, since the coating layer is metallic in the technologies disclosed in the Japanese Unexamined Patent Publication (KOKAI) No. 18085/1974 and the United States Patent No. 4,405,284, it is impossible to endure a severe application condition, for instance the application condition of an aircraft engine or a gas turbine engine, i.e., a high temperature of approximately 1000°C at maximum for a long period of time. The coating layer is eventually oxidized and corroded, and it should be repaired accordingly. In addition to the above-mentioned technologies, Japanese Examined Patent Publication (KOKOKU) No. 690/1975 discloses an invention relating to a gas turbine engine. The publication does not disclose the technology utilizing the machinability of the coating layer, but discloses a technology for avoiding the damage to a turbine blade in which a turbine casing is molded and sintered with a soft ceramic material being softer than a material for forming the turbine blade. However, since the force for binding the ceramic materials is weak, the gas turbine engine manufactuered by the technology lacks the durability. Moreover, Japanese Unexamined Patent Publication No. 168926/1987 discloses a technology for optimizing the clearance between the turborotor 100 and the turbohousing 101 or the clearance between the impeller 200 and the compressor housing 201 in which the inner surface of the turbohousing 101 of the compressor housing 201 is coated with a composite material. However, the publication does not disclose the quality of the coating layer material at all. US-A-3,084,064 discloses an abradable coating material of aluminum-boron nitride or nickel-chromium-boron nitride applied by flame spray coating. EP-A-0,166,097 discloses a flame spray material which may be formed of ceric oxide. US-A-3,879,831 discloses an abradable coating material which contains ceric oxide. SUMMARY OF THE INVENTION The present invention has been developed in view of the problems of the above-mentioned technologies. It is therefore an object of the present invention to provide a non-positive displacement apparatus having a coating layer of a favourable machinability even in a high temperature application. The invention provides a relatively displacing apparatus in accordance with Claim 1. A non-positive displacement apparatus according to the present invention may be a turbocharger or a gas turbine for an automobile or an aircraft. The non-positive displacement apparatus comprises the movable member and the stationary member which are disposed adjacent to each other and which displace relatively at a high temperature. For instance, if a turbocharger is the non-positive displacement apparatus, the impeller and the turborotor correspond to the movable member, and the compressor housing and the turbohousing correspond to the stationary member. Likewise, if a gas turbine is the non-positive displacement apparatus, the turbine blade corresponds to the movable member, and the turbine casing corresponds to the stationary member. In addition, the relative displacement between the movable member and the stationary member may be either rotary displacement or linear displacement. The stationary member has the coating layer disposed adjacent to the movable member. The coating layer is formed of an abradable material including hexagonal system boron nitride and ceric oxide, and it is formed by flame spray coating. The abradable material may include the hexagonal system boron nitride by 5 to 45 % by volume and the ceric oxide by 10 % or more by volume. As for the boron nitride, the hexagonal system boron nitride, not cubic system boron nitride, is employed in order to effect the advantages of the present invention. This is because the cubic system boron nitride is hard and the hexagonal system boron nitride is soft. It is preferred to employ the abradable material including the hexagonal system boron nitride (hereinafter simply referred to as "BN") by 5 to 45 % by volume in order to effect the advantages of the present invention. When the BN is included therein by less than 5 % by volume, the machinability of the coating layer is not improved sufficiently. When the BN is included therein by more than 45 % by volume, the machinability of the coating layer is improved excessively and the thermal shock resistance deteriorates, thereby causing the coating layer more likely to come off or fall off. Moreover, the average particle size of the BN is preferred to fall in a range of 5 to 50 µm in view of practicability. As for the oxide, the following ceramic powder may be employed: zirconium oxide (ZrO₂) powder, yttrium oxide (Y₂O₃) powder, aluminum oxide (Al₂O₃) powder and the like. Moreover, the average particle size of the oxide is preferred to fall in a range of 10 to 100 µm in view of practicability. As aforementioned, the abradable material may include at least the ceric oxide by 10 % or more by volume. When the ceric oxide is included therein by less than 10 % by volume, the machinability of the coating layer is not improved sufficiently. The more the ceric oxide is included therein by volume, the more the machinability is improved. The reason will be hereinafter described why the ceric oxide is extremely appropriate for the abradable material for adjusting the clearance in the non-positive displacement apparatus. Table 1 sets forth major oxides and their Mohs scales and thermal expansion coefficients. It is apparent from Table 1 that the ceric oxide is softer than most of the other oxides and has a thermal expansion coefficient substantially equal to that of metal. The latter property is favorable one for a component part used at a high temperature of 800 to 1000 °C. However, among the major oxides, calcium oxide (CaO), barium oxide (BaO) and strontium oxide (SrO) have favorable Mohs scales, but they are not appropriate oxides to be included in the abradable material because they react with moisture content in atmosphere to generate hydroxides. Moreover, the average particle size of the ceric oxide is preferred to fall in a range of 10 to 100 µm in view of practicability. Further, the following powder may be included in the abradable material together with the ceric oxide and BN powder: oxide powder such as aluminum oxide (Al₂O₃) powder, zirconium oxide (ZrO₂) powder and yttrium oxide (Y₂O₃) powder, graphite powder, mica powder and the like. The BN powder, the graphite powder and the mica powder work as an auxiliary powder for improving the machinability of the coating layer. For instance, by including the BN powder in the coating layer, the machinability is further improved by the laminated structure of the BN. Here, the average particles size of the oxide powder is preferred to fall in a range of 10 to 100 µm, and the average particle size of the BN powder, the graphite powder, the mica powder and the like falls in a range of 5 to 50 µm in view of practicability. As for the flame spray coating, plasma jet flame spray coating, gas flame spray coating and the like may be employed. The coating layer has the generated surface. The generated surface is machined and generated by the movable member during the operation of the non-positive displacement apparatus. A non-positive displacement apparatus according to the present invention comprises a stationary member having the coating layer disposed adjacent to a movable member, and the coating layer includes the BN and the ceric oxide. The coating layer has a structure, which results from the property of the BN, as schematically illustrated in Figure 17. In the structure of the coating layer, the BN particles 52 are present on the boundaries of the oxide particles 51 in a laminated structure, and the pores 53 are also present on the boundaries of the oxide particles 51 and the BN particles 52. Here, the stationary member is designated at 61 in Figure 17. As schematically illustrated in Figure 18, the inventors of the present invention consider that there are four mechanisms in which the coating layer having the above-mentioned structure is machined by the movable member 62 displacing relatively with respect to the stationary member 61. (a) Shear fracture of the oxide particles 51 (designated at "a-1") (b) Falling off of the oxide particles 51 disposed on the boundaries of the pores, 53 (designated at "a-2") (c) Falling off of the oxide particles 52 disposed on the boundaries of the BN particles 52 (designated at "a-3") (d) Shear fracture of the BN particles 52 (designated at "a-4") On the other hand, as schematically illustrated in Figure 19, the inventors of the present invention consider that there are two mechanisms in which the conventional coating layer is machined by the movable member 62. The conventional coating layer includes the oxide only and has a structure as schematically illustrated in Figure 19. (a) Shear fracture of the oxide particles 51 (designated at "b-1") (b) Falling off of the oxide particles 51 disposed on the boundaries of the pores 53 (designated at "b-2") Among the above-mentioned mechanisms, since the oxide particles 51 are extremely hard, for instance, since the zirconium oxide has a hardness of Hv 1000 or more, it is believed that a large force is required to cause the shear fracture of the oxide particles 52 ("a-1" and "b-1"). On the contrary, since the BN particles 52 are soft, namely they have a hardness of approximately Hv 3, and since they are thus softer than the oxide particles 51, it is believed that a small force is required to cause the shear fracture of the BN particles 52 ("a-4"). In addition, since the oxide particles 51 are bound by a small force exerted by the BN particles 52 having a low wettability, it is also believed that a small force is needed to complete the falling off of the oxide particles 51 ("a-3") disposed on the boundaries of the BN powder particles 52. Finally, since the pores 53 are present on the boundaries of the oxide particles 51 and since the oxide particles 51 are adhered by a weak adhesion force, it is believed that an intermediate force is required to cause the falling off of the oxide particles 51 ("a-2" and "b-2") disposed on the boundaries of the pores 53. Therefore, according to a non-positive displacement apparatus of the present invention, no large force is required to machine the coating layer and consequently it is possible to carry out the machining by the small forces or the intermediate force. Moreover, the coating layer including the ceric oxide as well as BN powder as an auxiliary powder for improving the machinabilitiy the BN powder, also has the structure as schematically illustrated in Figure 17. In the structure of the coating layer, the auxiliary powder particles 52 are present on the boundaries of the oxide particles 51, i.e., the ceric oxide particles, in a laminated structure, and the pores 53 are also present on the boundaries of the oxide particles 51 and the auxiliary powder particles 52. The inventors of the present invention believes that the above-mentioned four mechanisms are also applicable to the machining of the coating layer having such a structure. Namely, since the pores 53 are present on the boundaries of the oxide particles 51 and since the oxide particles 51 are adhered by a weak adhesion force, it is believed that an intermediate force is required to cause the falling off of the oxide particles 51 ("a-2") disposed on the boundaries of the pores 53. Further, since the oxide particles 51 are bound by a small force exerted by the auxiliary powder particles 52 having a low wettability, it is also believed that a small force is needed to complete the falling off of the oxide particles 51 ("a-3") disposed on the boundaries of the auxiliary powder particles 52. Accordingly, the machinability of the coating layer is mostly governed by the forces required for causing the shear fracture of the oxide particles 51 ("a-1") and the shear fracture of the auxiliary powder particles 52 ("a-4"). In the case that the other conditions are identical, the coating layer becomes to be easily machinable when the forces required for causing the shear fractures ("a-1" and "a-4") are small. Here, the force required for causing the shear fracture of the oxide particles 51 is believed to be in proportion to the hardness of the oxide particles 51 themselves. Therefore, since the soft ceric oxide particles are included as the oxide particles 51 in the coating layer, it is believed that a small force is only needed to machine the coating layer. In a non-positive displacement apparatus of the present invention, the coating layer is thus machined easily without causing the damage to the movable member, thereby generating the generated layer. Since the clearance between the movable member and the stationary member is made substantially zero (0) by the generated surface, the gas leakage and the like has been prevented from happening and the efficiency of the apparatus is improved. Additionally, in an apparatus of the present invention, the porosity of the coating layer is not increased, nor the coating layer is formed of a metallic abradable material. As a result, the coating layer does not come off, fall off or corrode at a high temperature in an apparatus of the present invention. Hence, an apparatus of the present invention comprises a movable member being less likely to be damaged and a stationary member having the coating layer being less likely to come off, fall off or corrode even when it is applied to a high temperature for a long period of time, and accordingly the clearance between the movable member and the stationary member can be made substantially zero (0). The apparatus thus has an excellent efficiency and a long life. BRIEF DESCRIPTION OF THE DRAWINGS A more complete appreciation of the present invention and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein: Figure 1 is a partial cross sectional view of a turbocharger of a preferred embodiment according to the present invention; Figure 2 is an enlarged partial cross sectional view of the turbocharger thereof; Figure 3 is another enlarged partial cross sectional view of the turbocharger thereof; Figure 4 is a still another enlarged partial cross sectional view of the turbocharger thereof; Figure 5 is a perspective view illustrating how a machinability test is carried out on a test piece having a coating layer formed in accordance with the' present invention; Figure 6 is a scatter diagram illustrating relationships between wear amounts and machined depths of fourth and fifth coating layers as well as comparative examples 15 through 20; Figure 7 is a column chart illustrating hardness of the fourth and fifth coating layers as well as comparative examples 15 through 20; Figure 8 is a microphotograph showing a particulate structure of a test piece having a coating layer formed in accordance with comparative example 15; Figure 9 is a microphotograph showing a particulate structure of a test piece having a coating layer formed in accordance with comparative example 17; Figure 10 is a microphotograph showing a particulate structure of a test piece having a coating layer formed in accordance with the fourth coating layer; Figure 11 is a microphotograph showing a particulate structure of a test piece having a coating layer formed in accordance with comparative example 20; Figure 12 is a scatter diagram illustrating relationships among machined depths, wear amounts and addition amounts of the BN of the fourth and fifth coating layers; Figure 13 is a column chart illustrating thermal shock resistance or number of endured thermal cycles exhibited by fourth and fifth coating layers; Figure 14 is a scatter diagram illustrating relationships among machined depths, wear amounts and porosities of the fourth and fifth coating layers as well as another comparative example; Figure 15 is a scatter diagram illustrating relationships between number of endured thermal cycles and porosities exhibited by the fourth and fifth coating layers as well as a still another comparative example; Figure 16 is a line chart illustrating relationships between weight variations and testing times exhibited by the fourth and fifth coating layers as well as a still another comparative examples; Figure 17 is a schematic cross sectional view for illustrating how a coating layer of the present invention works; Figure 18 is another schematic cross sectional view for illustrating how the coating layer thereof works; Figure 19 is a schematic cross sectional view for illustrating how a conventional coating layer works; Figure 20 is a scatter diagram illustrating relationships between wear amounts and machined depths of a ninth coating layer; tenth and eleventh coatings, which are embodiments of the present invention; twelfth and thirteenth coating layers as well as comparative examples 23 through 30; Figure 21 is a column chart illustrating hardnesses of the ninth through thirteenth coating layers as well as comparative examples 23 through 30; Figure 22 is a scatter diagram illustrating relationships among machined depths, wear amounts and addition amounts of the ceric oxide of the thirteenth coating layer; Figure 23 is a column chart illustrating thermal shock resistance or number of endured thermal cycles exhibited by the twelfth and thirteenth coating layers as well as comparative examples 24 and 27; and Figure 24 is a cross sectional view of a conventional turbocharger. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Having generally described the present invention, a further understanding can be obtained by reference to certain specific preferred embodiments which are provided herein for purposes of illustration only and are not intended to be limiting unless otherwise specified. The preferred embodiments are embodied in a turbocharger. They will be hereinafter described together with further coating layers, not in accordance with the present invention as well as further comparative examples manufactured in comparison therewith. First Coating Layer. As it is apparent from Figure 1 illustrating the partial cross sectional view of the turbocharger, the turbocharger is basically identical with the above-mentioned conventional turbocharger (see Figure 24). The turbocharger comprises a turbohousing 81 having an inside diameter of 55 mm and a turborotor 82 connected to a shaft 80. In the turbocharger, the turbohousing 81 has a coating layer 85 at a position "P" disposed adjacent to the turborotor 82. A procedure for manufacturing the turbocharger will be hereinafter described. (a) In the turbocharger, as illustrated in Figure 2, there was a clearance C0 of approximately 0.8 mm between the turbohousing 81 and the turborotor 82 before forming the coating layer 85. (b) A shot blasting treatment was carried out on the portion "P" of the turbohousing 81 adjacent to the turborotor 82 with a calcined aluminum oxide powder having an average particle size of 1200 to 1400 µm. (c) An alloy layer 84 was formed as a substrate layer on the shot-blasted portion "P" in a thickness of t1, for instance 0.08 to 0.1 mm, by plasma jet flame spray coating with an NiCrAl (94(80Ni-20Cr)-6Al) alloy in advance. The alloy layer 84 was for improving the adhesion. (d) A ZrO₂·8Y₂O₃ powder ("K90" produced by Showa Denko Co., Ltd.) having an average particle size of 10 to 74 µm and a hexagonal system BN powder ("UHP-EX" produced by Showa Denko Co., Ltd.) having an average particle size of 35 to 45 µm were prepared. Then, 60 % by volume of the ZrO₂·8Y₂O₃ powder and 40 % by volume of the BN powder were compounded to make an abradable material. (e) The abradable material was coated on the portion "P," which had been coated with the alloy layer 84, in a thickness of t2, for instance approximately 1.0 mm, by plasma jet flame spray coating. The coating layer 85 was thus formed. (f) After forming the coating layer 85, the coating layer 85 was machined by a numerical control machine tool so that there was a clearance Cl of 0.05 mm between the turbohousing 81 and the turborotor 82. In this way, the turbocharger of the first coating layer was manufactured. Second Coating Layer The turbocharger of a second coating layer was identical with that of the first coating layer except that an abradable material was employed which included 65 % by volume of an aluminum oxide (Al₂O₃) powder and 35 % by volume of a BN powder. Here, the aluminum oxide powder ("101B" produced by Metco Co., Ltd.) had the average particle size of 35 to 74 µm, and the BN powder was identical with the one employed in the first coating layer. Third Coating Layer The turbocharger of a third coating layer was identical with that of the first coating layer except that an abradable material was employed which included 60 % by volume of an aluminum oxide (Al₂O₃) powder and 40 % by volume of a BN powder. Here, the aluminum oxide powder was identical with the one employed in the second coating layer, and the BN powder was identical with the one employed in the first coating layer. Comparative Example 11 The turbocharger of comparative example 11 was basically identical with that of the first coating layer except that the alloy layer 84 and the coating layer 85 were not formed on the portion "P" of the turbohousing 81. Comparative Example 12 The turbocharger of comparative example 12 was identical with that of the first coating layer except that an abradable material was employed which included a ZrO₂·20Y₂O₃ powder. Here, the ZrO₂·20Y₂O₃ powder was produced by Showa Denko Co., Ltd., and had an average particle size of 10 to 44 µm. The coating layer had a porosity of 28 %. Comparative Example 13 The turbocharger of comparative example 13 was identical with that of the first coating layer except that an abradable material was employed which included 75 % by weight of nickel (Ni) powder and 25 % by weight of a graphite powder. Here, the nickel powder had an average particle size of 10 to 74 µm, and the graphite powder had an average particle size of 10 to 30 µm and was produced by Perkin Elmer Co., Ltd. Comparative Example 14 The turbocharger of comparative example 14 was identical with that of the first coating layer except that an abradable material was employed which included 94.5 % by weight of an NiCrFeAl alloy powder and 5.5 % by weight of a BN powder. Here, the NiCrFeAl alloy had the following composition: Cr by 14 %, Fe by 8.0 %, Al by 3.5 %, BN by 5.5 % and the balance of Ni, and had an average particle size of 45 to 120 µm. The BN powder was identical with the one employed in the first coating layer. Product Evaluation In the turbochargers of the first and second coating layers, even in the case that the shaft 80 had an eccentricity and that the turborotor 82 was brought into contact with or collided with the turbohousing 81 during the operation, the coating layer 85 of the turbohousing 81 was easily machined by the turborotor 82, and a generated surface 851 was thereby generated on the coating layer 85 as illustrated in Figure 4. Accordingly, the turbochargers could prevent the turborotor 82 from being damaged. Namely, the turbochargers of the first and second coating layer as well as comparative example 11 were subjected to a product performance test for evaluating a total efficiency (%) under the condition of the rotor speed of a hundred thousand rpm. The results of the product performance test are set forth in Table 2. As set forth in Table 2, the turbochargers of the first and second coating layers had the total efficiency improved by 5 to 6 % with respect to that of comparative example 11. The improvement is believed to result from the fact that the generated surface 851 made the clearance between the turbohousing 81 and the turborotor 82 substantially zero (0) as illustrated in Figure 4 and accordingly the gas leakage could be prevented to a minimum degree. Further, the turbochargers of the first and third coating layers as well as comparative examples 12, 13 and 14 were subjected to a durability test including a noise test during a 300 hour-operation, a coating layer checking test after the operation and a turborotor checking test after the operation. Additionally, weight reductions (in grams) of the turborotors 82 were measured after the operation. The results of the tests are set forth in Table 3. In accordance with the results of the tests, the turbochargers were rated in a "Total Evaluation" column of Table 3 with either a "Good" sign identifying an excellent turbocharger and a "Bad" sign identifying an inferior turbocharger. As set forth in Table 3, the turbochargers of the first and third coating layers did not show any faulty operation. On the other hand, the turbochargers of comparative examples 12, 13 and 14 generated chattering when the turborotors 82 were brought into contact with the turbohousings 81 in the operation because the coating layers were of inferior machinability. As for the state of the coating layer after the operation, the coating layers of the turbochargers of comparative examples 12 and 13 were not machined, but they were come off when the turborotors 82 were brought into contact with the turbohousings 81 in the operation. Moreover, the turbochargers of comparative examples 13 and 14 had corroded coating layers after the operation. Finally, the turbochargers of comparative examples 12, 13 and 14 had deformed turborotors 82 after the operation, and the turborotors 82 thereof exhibited the weight reductions due to wear. It is thus apparent that the turbochargers of the first, second and third coating layers had the coating layers 85 of favorable machinability. Accordingly, the turborotors 82 could be prevented from being damaged, and the efficiency of the turbochargers could be improved. Fourth and Fifth Coating Layers (1) Machinability Test Abradable materials were prepared whose compositions are set forth in Table 4. Then, test pieces of the fourth and fifth coating layers as well as comparative examples 15 through 20 were prepared with the abradable materials and subjected to a test for comparing their machinabilities. The test pieces were prepared in the following manner: First, the NiCrAl alloy were coated as a substrate layer in a thickness of 0.1 mm on a flat plate (30 mm x 30 mm x 5 mm) made of S45 (carbon steel as per JIS) by plasma jet flame spray coating. Then, the abradable materials were coated on the substrate layer in a thickness of 1 mm by plasma jet flame spray coating. Here, the ZrO₂·8Y₂O₃ powder, the Al₂O₃ powder, the BN powder and the graphite powder were identical with those employed by the above-mentioned first and second coating layers and comparative example 13. The mica powder had an average particle diameter of 35 to 45 µm, and was produced by Showa Denko Co., Ltd. In addition, all of the additives other than the ZrO₂ and the Al₂O₃ had a laminated structure, and they themselves had a property of easily disintegrating. The additives were added in order to make the machinability of the coating layers favorable. The fourth and fifth coating layers and comparative examples 15 through 20 were subjected to a machinability test for evaluating the machinabilities of their coating layers. The machinability test were carried out as illustrated in Figure 5. Namely, a ring 90 ( 10 mm) made of Inconel (Trade Mark), i.e., an identical material for making the turborotor 82, was rotated on the test pieces under the following conditions in the direction of the arrow in Figure 5: Load "W" : a surface load of 150 gf/mm² Speed : 1000 rpm Testing Time : 1 minute A ring-shaped groove was machined by the ring 90 in the coating layer of the test piece, and the depth was measured and taken as machined depth (in mm). Also, the wear amount of the ring 90 was measured in milligrams and taken as mating part attack tendency. The results of the machinablity test are illustrated in Figure 6. It is understood from Figure 6 that the coating layers including the BN powder and formed on the test pieces of the fourth and fifth coating layers were machined easily and that they were favorable. On the contrary, the coating layers formed on the test pieces of comparative examples 15 through 20, namely the coating layers including the graphite and mica and the coating layers formed of the oxides only, exhibited the machined depth of substantially zero (0) and heavy ring wear amounts, and accordingly they were not favorable. In addition, Figure 7 illustrates the results of a hardness measurement on the hardnesses of the coating layers formed on the test pieces in accordance with the fourth and fifth coating layers and comparative examples 15 through 20. The hardness measurement was carried out to obtain the Vickers hardness under a load of 5 kgf. It is apparent that the coating layers of the fourth and fifth coating layers had extremely low Vickers hardnesses compared with those of the coating layers of comparative examples 15 through 20. Moreover, a microphotograph (x 400) of the coating layer formed on the test piece in accordance with comparative example 15 is shown as Figure 8, and was taken by a scanning electron microscope (hereinafter referred to as "SEM"). Likewise, microphotographs (x 400, taken by the SEM) of the coating layers formed on the test piece in accordance with comparative example 17, the fourth coating layer and comparative exmaple 20 are shown as Figure 9, 10 and 11, respectively. It is understood that the coating layers formed on the test pieces in accordance with the fourth and fifth coating layers had structures in which the BN was present, thereby giving low hardnesses and favorable machinabilities. Namely, the coating layers including the BN were easily machined. This is believed to result from the fact that the BN had low wettability with the oxides therearound because it was not oxide, and that the force for binding the particles were weak because the BN was so soft that it had a Mohs scale of 1 to 2. On the other hand, it is apparent from Figure 9 that substantially no graphite was present in the coating layer formed on the test piece in accordance with comparative example 17. This is because the graphite was burned up during the plasma jet flame coating. As for the coating layer formed on the test piece in accordance with comparative example 19 or 20, it seems from Figure 11 that the coating layer was formed firmly. This is believed to result from the fact that the mica included therein had good wettability because the mica mainly consisted of SiO₂ and Al₂O₃ which were oxides similar to the aluminum oxide compounded together with the mica in the abradable material, and that the mica itself was harder than the BN. Therefore, it seems that the coating layer including the mica and formed on the test piece in accordance with comparative example 19 or 20 was harder than those of the test pieces of the other comparative examples, and that it had bad machinability. (2) Evaluation on Optimum BN Addition Amount An optimum BN addition amount was evaluated in view of the machinability and the thermal shock resistance. (2)-(a) Machinability Test for Evaluating BN Addition Amount Influence on Machinability The coating layers of the fourth and fifth coating layers were formed on the test pieces in the same manner as aforementioned, however the addition amounts of the ZrO₂·8Y₂O₃ and the Al₂O₃ were varied in a range of 50 to 90 % by volume and the addition amount of the BN was varied in a range of 10 to 50 % by volume. The test pieces thus prepared were subjected to the machinability test described in section (1) above, and their machined depths (mm) and ring wear amounts (mg) were measured similarly. The results of the test are illustrated in Figure 12. The following is understood from Figure 12: In the coating layer including the ZrO₂·8Y₂O₃ and also in the coating layer including the Al₂O₃, the more the BN addition amount increases, the more the machinability becomes favorable. Accordingly, it is preferable to add the BN more in consideration of the machinability only. (2)-(b) Thermal Shock Test for Evaluating BN Addition Amount Influence on Thermal Shock Resistance The thermal shock resistance of the coating layer including the BN was evaluated by a thermal shock test, i.e., a thermal cycle test. At first, the coating layers of the fourth and fifth coating layers were formed on the test pieces in the same manner as aforementioned, however the addition amounts of the ZrO₂·8Y₂O₃ and the Al₂O₃ were varied in a range of 50 to 100 % by volume and the addition amount of the BN was varied in a range of 0 to 50 % by volume. The test pieces thus prepared were subjected to a thermal cycle test. One cycle of the test consisted of a first step of heating the test pieces with an oxygen-acetylene burner to approximately 1000 °C in 32 seconds and a second step of quenching the test pieces by immersing them into water. The cycle was carried out repeatedly until part of or whole of the coating layers came off or fell off. The thermal shock resistance of the test pieces were rated by the number of the repeated cycles until the coming off or falling off. Here, checking was done every 50 cycles, and the cycle was repeated up to a maximum of 2000 times. The results of the test are illustrated in Figure 13. As can be seen from Figure 13, the thermal shock resistance began to improve starting at the BN addition amount of 5 % by volume, marked the maximum improvement at the BN addition of 25 % by volume, and deteriorated starting at the BN addition amount of 45 % by volume. In addition, the coating layers come off at less than 200 cycles when the BN addition amount was 50 % by volume. According to the results of the machinability test in section (2)-(a) and the thermal shock test in section (2)-(b) as illustrated in Figures 12 and 13, it is verified that an effective coating layer in respect of the machinability and the thermal shock resistance can be formed when the BN addition amount falls in a range of 5 to 45 % by volume. Further, it is believed that the ZrO₂·8Y₂O₃ is a more preferable oxide to be included in the coating layer than the Al₂O₃ is. Additional Evaluation No. 1 The coating layers including the BN formed in accordance with the fourth and fifth coating layers were compared with the porous coating layer formed in accordance with the United States Patent No. 4,269,903 with regard to their machinabilities and thermal shock resistances. The fourth and fifth coating layers were formed on the test pieces in the same manner as aforementioned, however the addition amounts of the ZrO₂·8Y₂O₃ and the Al₂O₃ were varied in a range of 50 to 90 % by volume and the addition amount of the BN was varied in a range of 10 to 50 % by volume. Further, a coating layer was formed on test pieces in accordance with the United States Patent No. 4,269,903, and the coating layer included porous ZrO₂·20Y₂O₃ and had a porosity of 20 to 33 %. The test pieces thus prepared were subjected to the machinability test described in section (1) above, and their machined depths (mm) and ring wear amounts (mg) were measured similarly. The results of the test are illustrated in Figure 14. In Figure 14, the machined depths and the ring wear amounts are plotted with respect to the same horizontal axis specifying the porosity of the coating layers of the test pieces in percentage. As can be seen from Figure 14, the coating layers formed in accordance with the fourth and fifth coating layers exhibited more favorable machinabilities than the coating layers formed in accordance with the United States Patent No. 4,269,903 did at identical porosities. This is believed to result from the fact that the BN particles having the softness and disposed in the laminated structure were present on the boundaries of the oxides such as zirconium oxide and the like. Further, the test pieces were subjected to the thermal shock test described in section (2)-(b) above, and their thermal shock resistances were evaluated similarly. The results of the test are illustrated in Figure 15. As can be seen from Figure 15, the coating layers including the BN exhibited better thermal shock resistances than the coating layers free from the BN in a porosity range of 20 to 33 %. Although the coating layers free from the BN exhibited increased machinability in a porosity range of over 40 % according to Figure 14, the thermal shock resistance deteriorated so sharply that the coating layers free from the BN were hardly applicable to a practical use as can be seen from Figure 15. Accordingly, the coating layers including the BN formed were superior to the conventional porous coating layers in respect of the machinability and the thermal shock resistance. Additional Evaluation No. 2 The coating layers including the BN formed in accordance with the fourth and fifth coating layers were compared with the metallic coating layer mainly composed of nickel and disclosed in the Japanese Unexamined Patent Publication No. 18085/1974 and the United States Patent No. 4,405,284 with regard to their corrosion resistances. The coating layers of the fourth and fifth coating layers were formed on the test pieces in the same manner as aforementioned, however the addition amounts of the ZrO₂·8Y₂O₃ and the Al₂O₃ were set at 75 % by volume, and the addition amount of the BN was set at 25 % by volume. Further, a coating layer including nickel and graphite was formed on the test pieces. Furthertnore, a coating layer including NiCrFeAl alloy and the BN was formed on the test pieces. Namely, the coating layer including nickel and graphite was formed of the abradable material according to comparative example 13, and the coating layer including NiCrFeAl alloy and the BN was formed of the abradable material according to comparative example 14. The test pieces thus prepared were subjected to a corrosion resistance test. The corrosion resistance test was carried out in the following procedure: First, the test pieces were put in an oven whose temperature is held at 1000 °C. Then, the test pieces weighed from time to time to check their weight variations in mg, thereby evaluating their degrees of the oxidation. The results of the test are illustrated in Figure 16. As can be seen from Figure 16, the test pieces having the coating layers including the oxides and the BN showed no weight variation even at a high temperature of 1000 °C, and no failure resulting from the oxidation and the like occurred therein. On the other hand, the test pieces having the conventional nickel-base coating layers gained their weights as time passed because the oxidation developed therein. Accordingly, the coating layers including the BN formed were much better than the conventional nickel-base coating layers in respect of the corrosion resistance. Sixth Coating Layer A turbocharger of a sixth coating layer was manufactured in the seme manner as the turbocharger of the first coating layer except that a ceric oxide (CeO₂) powder was employed for preparing an abradable material in the step (d) of the procedure for manufacturing the turbocharger. The ceric oxide powder had an average particle size of 10 to 74 µm and was produced by Showa Denko Co., Ltd. The turbocharger of the sixth coating layer thus manufactured has a coating layer 85 having a porosity of 23 %. Seventh Coating Layer The turbocharger of a seventh coating layer was identical with that of the first coating layer except that an abradable material was employed which included 75 % by volume of an ceric oxide powder and 25 % by volume of a ZrO₂·20Y₂O₃ powder. Here, the ZrO₂·20Y₂O₃ powder (produced by Showa Denko Co., Ltd.) had an average particle size of 10 to 44 µm, and the ceric oxide powder was identical with the one employed in the sixth coating layer. The turbocharger of the seventh coating layer thus manufactured had a coating layer 85 having a porosity of 21 %. Eighth Coating Layer The turbocharger of an eighth coating layer was identical with that of the first coating layer except that an abradable material was employed which included 30 % by volume of a ceric oxide powder and 70 % by volume of a ZrO₂·20Y₂O₃ powder. Here, the ZrO₂·20Y₂O₃ powder was identical with the one employed in the seventh coating layer, and the ceric oxide is the one employed in the sixth coating layer. The turbocharger of the eighth coating layer thus manufactured had a coating layer 85 having a porosity of 19 %. Comparative Example 21 The turbocharger of comparative example 21 was identical with that of the first coating layer except that an abradable material was employed which included 100 % by volume of ZrO₂·20Y₂O₃ powder. Here, the ZrO₂·20Y₂O₃ powder was identical with the one employed in the seventh coating layer. The turbocharger of the comparative example 21 thus manufactured has a coating layer having a porosity of 30 %. Comparative Example 22 The turbocharger of comparative example 22 was identical with that of the first coating layer except that an abradable material was employed which included 100 % by volume of an Al₂O₃ powder. Here, the Al₂O₃ powder was produced by Metco Co., Ltd., and had an average particle size of 35 to 74 µm. The turbocharger of the comparative example 22 thus manufactured had a coating layer having a porosity of 28 %. Product Evaluation The turbochargers of the sixth, seventh and eighth coating layers as well as comparative example 21 and 22 were subjected to the above-mentioned 300-hour durability test simulating an actual operation for evaluating the noise during the 300 hour-operation, the coating layer state after the operation and the turborotor state after the operation. The weight reductions (in grams) of the turborotors 82 were also measured after the operation. The results of the tests are set forth in Table 5. Likewise, in accordance with the results of the test, the turbochargers were rated in a "Total Evaluation" column of Table 5 with either a "Good" sign identifying an excellent turbocharger and a "Bad" sign identifying an inferior turbocharger. As set forth in Table 5, the turbochargers of the sixth and seventh coating layers did not show any faulty operation such as chattering, coming off or falling off of the coating layer 85, damaged turborotor 82, because the coating layer 85 of the turbohousing 81 was easily machined by the turborotor 82 and the generated surface 851 is thereby generated on the coating layer 85 as illustrated in Figure 4 even in the case that the turborotor 82 was brought into contact with or collided with the turbohousing 81 during the operation. Additionally, the turbocharger of the eighth coating layer showed any particular problem other than that it generated a bit of low chattering. On the other hand, the turbochargers of comparative examples 21 and 22 generated loud chattering when the turborotors 82 were brought into contact with the turbohousings 81 in the operation because the coating layers were of inferior machinability. As for the state of the coating layer after the operation, the coating layer of the turbocharger of comparative example 21 was not machined, but it was come off when the turborotor 82 was brought into contact with the turbohousing 81 in the operation. Moreover, the turbocharger of comparative example 21 had the deformed blades of the turborotor 82, and showed a weight reduction due to wear after the operation. Finally, in the turbocharger of comparative example 22, one third of the coating layer come off because of the poor thermal shock resistance at 950 °C, i.e., the maximum temperature to which the turbochargers were applied. Moreover, in the turbocharger of comparative example 22, the blades of the turborotor 82 were deformed and cracked. The deformed and cracked blades are believed to result from the poor machinability of the coating layer and the collisions of the fragments of the come-off coating layer. The turbocharger of comparative example 22 accordingly showed the largest weight reduction or wear amount of 12 grams. The turbochargers of the sixth and seventh coating layers were also subjected to the product performance test for evaluating the total efficiency (%) under the condition of the rotor speed of a hundred thousand rpm. The turbochargers of the sixth and seventh coating layers had the total efficiency improved by 5.3 and 5.1 % respectively with respect to the turbocharger without the coating layer which had the total efficiehcy of 51 %. It is believed that the generated surface 851 made the clearance between the turbohousing 81 and the turborotor 82 zero (0) substantially as illustrated in Figure 4 and accordingly the gas leakage could be suppressed to a minimum degree, thereby making the improvement possible. Therefore, in the turbochargers of the sixth, seventh and eighth coating layers having the coating layer 85 which included the ceric oxide, the turborotors 82 could be prevented from being damaged, and the efficiency of the turbochargers could be improved. Ninth through Thirteenth Coating Layers The present invention will be hereinafter described with reference to the results of the above-mentioned tests for evaluating the test pieces having coating layers formed in accordance with a ninth through thirteenth coating layers. (1) Machinability Test Abradable materials were prepared whose compositions are set forth in Table 6. Then, coating layers were formed of the abradable materials of the ninth through thirteenth coating layers as well as comparative examples 23 through 30 in the same manner as described in section (1) of Fourth and Fifth Coating Layers. Here, the BN powder ("UHP-EX" produced by Showa Denko Co., Ltd.) had an average particle size of 35 to 45 µm, and the CeO₂ powder, the Al₂O₃ powder and the ZrO₂·20Y₂O₃ powder were identical with those employed by the sixth through seventh coating layers or the comparative examples 21 and 22. The test pieces thus prepared were subjected to the machinability test for comparing their machinabilities described in section (1) of Fourth and Fifth Coating layers. The results of the machinability test are illustrated in Figure 20. It is understood from Figure 20 that the coating layers including the ceric oxide and formed in accordance with the ninth through thirteenth coating layers were machined easily and that they were favorable. It is especially apparent when the characteristics of the coating layers of the ninth through thirteenth coating layers are compared with those of the coating layer including ZrO₂·20Y₂O₃ of a large porosity and formed in accordance with the comparative example 24 and the coating layer formed in accordance with the comparative example 27 which has been employed in an aircraft conventionally. Further, when the coating layers of the comparative examples 23 and 24 are compared with the coating layers of the comparative examples 25 and 26, the machinabilites of the ZrO₂·20Y₂ O₃ -base coating layers were improved by adding the BN. However, since these coating layers had slightly strong mating part attacking tendencies or slightly large ring wear amounts, they were a bit inadequate for an abradable material for adjusting the clearance in a non-positive displacement apparatus. On the other hand, it was found that the CeO₂ -base coating layers of the ninth through thirteenth coating layers could have weak mating part attacking tendencies and excellent machinabilities even when the auxiliary powder such as the BN and the like was not added therein. (2) Vickers Hardness Measurement The abradable materials set forth in Table 6 were coated on the test pieces by plasma jet flame spray coating in the same manner as described in section (1) of Fourth and Fifth Coating layers. Then, the Vickers hardnesses of the coating layers were measured. Figure 21 illustrates the results of the measurement. It is assumed from Figure 21 that the coating layers of the ninth through thirteenth coating layers were much softer and much more machinable than the coating layers of the comparative examples 23 to 30. In addition, when comparing the hardness of the coating layer of the ninth coating layer with those of the coating layers of the tenth and eleventh coating layers, which are in accordance with the present invention, it was found that the machinability was further improved by adding the BN. (3) Evaluation on Optimum Ceric Oxide Addition Amount Since the coating layer formed by flame spray coating the ceric oxide had a favorable machinability, an optimum ceric oxide addition amount was evaluated in view of the machinability and the thermal shock resistance. (3)-(a) Machinability Test for Evaluating Ceric Oxide Addition Amount Influence on Machinability The coating layers of the thirteenth coating layer were formed on the test pieces in the same manner as aforementioned, however the addition amounts of the ceric oxide and the ZrO₂·20Y₂O₃ were vaired variously in a range of 0 to 100 % by volume. Here, the ZrO₂·20Y₂O₃ and the ceric oxide were identical with the above-mentioned.ones. The test pieces thus prepared were subjected to the machinability test described in section (1) of Fourth and Fifth Coating layers and their machined depths (mm) and ring wear amounts (mg) were measured similarly. The results of the test are illustrated in Figure 22. It is understood from Figure 22 that the coating layer came to have more favorable machinability as the ceric oxide was added more in the abradable material. When the ceric oxide was added by 10 % by volume, the machinability, i.e., the machined depth of the coating layer, deteriorated substantially as low as that of the coating layer of the comparative example 24 (See Figure 20.). The coating layer of the comparative example 24 was formed in accordance with the United States Patent No. 4,269,903. Further, when the ceric oxide was added by less than 10 % by volume, the machinability of the coating layer decreased sharply. Consequently, it is necessary to include the ceric oxide by 10 % by volume or more in order to obtain a coating layer of a favorable machinability. (3)-(b) Thermal Shock Test for Evaluating Ceric Oxide Addition Amount Influence on Thermal Shock Resistance The thermal shock resistance of the coating layer including the ceric oxide was evaluated by the thermal shock test described in section (2)-(b) of Fourth and Fifth Coating layers. At first, the coating layers of the twelfth and thirteenth coating layers were formed on the test pieces in the same manner as aforementioned, however, in the coating layers of this thirteenth coating layer, ZrO₂·8Y₂O₃ was used instead of the ZrO₂·20Y₂O₃, and the addition amount of the ceric oxide with respect to the ZrO₂·8Y₂O₃ or the Al₂O₃ was varied variously in a range of 0 to 100 % by volume. Further, comparative test pieces were prepared on which the abradable materials of the comparative examples 24 and 27 were coated by plasma jet flame spray coating. Also in this test, the alloy including the NiCrAl alloy was coated by plasma jet flame spray coating in a thickness of 1 mm before flame coating the abradable materials. Here, the ZrO₂·8Y₂O₃ powder ("K90" produced by Showa Denko Co., Ltd.) had an average particle size of 10 to 74 µm, and the Al₂O₃ and the ceric oxide were identical with the above-mentioned ones. The test pieces thus prepared were subjected to the thermal cycle test. ' The results of the test are illustrated in Figure 23. As can be seen from Figure 23, the thermal shock resistances of the coating layers were substantially equivalent to those of the comparative examples 24 and 27. However, when the ceric oxide was included by 10 % by volume or more, the thermal shock resistances of the coating layers improved to equivalent to or more than those of the coating layers of the comparative examples 24 and 27. Additionally, it was found that the ZrO₂. 8Y₂O₃ was superior to the Al₂O₃ as the oxide to be included in the coating layer in respect of the thermal shock resistance. According to the results of the machinability test in section (3)-(a) and the thermal shock test in section (3)-(b) as illustrated in Figures 22 and 23, it is verified that an effective coating layer in respect of the machillability and the thermal shock resistance can be formed when the ceric oxide addition amount falls in a range of 10 % by volume or more. Therefore, in consideration of the results of the tests described in sections (1), (2) and (3) above, it can be said that the abradable material including the ceric oxide by 10 % by volume or more is an optimum flame spray coating material for adjusting the clearance in the non-positive displacement apparatus. Having now fully described the present invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the scope of the invention as set forth in the accompanying claims.

Claims: A non-positive displacement apparatus comprising: a movable member; and a stationary member; said movable member and said stationary member disposed adjacent to each other and displacing relatively at a high temperature; said stationary member having a coating layer disposed adjacent to said movable member, said coating layer being formed by flame spray coating, and including hexagonal system boron nitride and ceric oxide, and having a surface generated by machining with said movable member. A non-positive displacement apparatus according to Claim 1, wherein said coating layer includes said boron nitride by 5 to 45% by volume and ceric oxide by 10% by volume or more. A non-displacement apparatus according to Claim 1 or Claim 2, wherein said boron nitride has an average particle size of 5 to 50 µm. A non-displacement apparatus according to any one of Claims 1 - 3, wherein said ceric oxide has an average particle size 10 to 100 µm. A non-displacement apparatus according to any one of Claims 1 - 4, wherein said coating layer further includes at least one oxide selected from the group consisting of zirconium oxide, yttrium oxide and aluminum oxide. A non-positive displacement apparatus according to Claim 5, wherein said oxide has an average particle size 10 to 100 µm.

Inventors: MIYAMOTO NORITAKA, TOMODA TAKASHI, MIYAMOTO, NORITAKA, TOMODA, TAKASHI
Applicants: TOYOTA MOTOR CO LTD, TOYOTA JIDOSHA KABUSHIKI KAISHA
Application year: 1990
Main classification: F01D11
IPC codes: F01D11