JPH04333556A - Method for thermal-spraying chromia and sprayed deposit - Google Patents

Abstract

PURPOSE:To prevent the generation of voids damaging the denseness of sprayed deposit and the precipitation of metallic chromium reducing its sliding wear resistance. CONSTITUTION:A plasma flame 73 obtd. by forming an arc between a main torch 51 and a subtorch 52, forming the turning flow of a plasma gas 83 contg. oxygen around the arc 67 in the main torch and executing heating by an arc 67 is fed with chromia 70 as granular substance, and melting is executed. Then, the molten drops 71 are ejected on a base metal 75 to form sprayed deposit 74 of chromia.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention is based on Chromia (Chromia), which has high hardness, excellent sliding wear resistance and chemical resistance, and exhibits a beautiful black color when mirror-finished, and is used for surface treatment of rolls, plungers, mechanical seals, etc. The present invention relates to a thermal spraying method of Cr2 03 ) and a thermal spray coating.

0002

[Prior Art] Since Chromiya's thermal spray coating is often used as a sliding member, its required characteristics are (
1) In order to ensure smooth sliding, the surface roughness should be Ra0.
．． Capable of mirror finish of 0.3μm or less. For this purpose, it is important to obtain a thermal sprayed coating that is dense without voids and has high interparticle bonding strength. (2) In order to minimize wear due to sliding, the composition of Chromia must not change due to thermal spraying, and in particular, precipitation of metallic chromium due to reduction of Chromia must be suppressed. In conventional plasma spraying, as shown in FIG.
into the plasma flame 2 which supplies high-temperature fine molten droplets 27 and ejects them from the outlet 28 of the plasma torch 21.
4 and accelerate the molten droplets 27 to collide with a base material 29 disposed ahead of the plasma flame 24 to form a thermally sprayed coating 30 of Chromiya on the surface of the base material.

At this time, the plasma flame 24 attracts surrounding air 31 in the space from the outlet of the plasma torch 21 to the base material, and the plasma flame 24 expands into the shape shown in the figure, and the molten droplets therein are expanded. The thermal history of 27 varies widely depending on the path, and the molten droplet
The speed at which the sprayed coating 30 hits the base material 29 decreases, impeding the uniformity of the sprayed coating 30 formed on the surface of the base material 29, and reducing its density. In order to improve this problem, as shown in FIG. 6, the length from the tip 32 of the cathode 22 of the plasma torch 21 to the anode point 33 of the anode 23 is made longer than that in FIG. An annular gas passage 35 is provided outside and concentrically with the plasma gas passage 34 around the cathode 22, and a tangential passage is formed in the annular wall between the two gas passages 34, 35 to form a plasma gas inlet. Plasma gas 39 introduced from 38 is swirled in the plasma gas passage 34 around the cathode 22 and flows toward the outlet 28, and this gas is passed through a relatively long arc 20 that is generated between the cathode tip 32 and the anode point 33. It is heated sufficiently to form an elongated plasma flame 24, and the molten droplets 2 contained therein are
There is a plasma spray device that improves the focusing and stability of the 7 beam.

[0005]

[Problems to be Solved by the Invention] However, if the length L6 of the plasma flame 24 is made sufficiently longer than L5 in this way, when the molten droplets of the thermal spray material collide with the base material 29, the plasma flame that transports them It also collides with the base material, heats the base material 29 with the plasma flame, and the base material 29
There is a risk of damaging the material. In addition, there is an arc anode spot (anode spot) on the anode 23 of the torch shown in FIGS. 5 and 6, and the anode spot is in contact with the plasma gas. The parts that are marked are subject to wear and tear, making long-term operation impossible. To prevent this, argon, helium, nitrogen, or the like is usually used as a plasma gas. However, when thermal spraying is performed by a plasma spraying apparatus using such plasma gas, there is a problem in that Chromia is reduced and a large amount of metallic chromium is deposited in the thermal sprayed coating, resulting in a significant loss of sliding wear resistance.

[0006] The present invention is a plasma sprayed Chromia coating that can prevent the formation of voids that impair the density and the precipitation of metallic chromium that lowers the sliding wear resistance, which are the functional problems of the conventional plasma sprayed Chromia coating. The object of the present invention is to provide a thermal spraying method and a thermal spray coating of Chromia obtained by the thermal spraying method.

[0007]

[Means for Solving the Problems] The present invention melts chromia powder by feeding it into a plasma flame obtained by heating plasma gas with an arc, and sprays the molten droplets onto the base material. In Chromiya's plasma spraying method for forming a thermal spray coating on a surface, the arc is formed between the main torch and the sub-torch, and a swirling flow of oxygen-containing plasma gas is formed around the arc within the main torch. Chromiya's thermal spraying method is characterized by:
In the Chromiya plasma spraying method, powdered Chromiya is fed into a plasma flame obtained by heating plasma gas with an arc, melted, and the molten droplets are sprayed onto the base material to form a sprayed coating on the surface of the base material. , the arc is formed between the main torch and the sub-torch, and a swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch, and plasma is generated just before the base material in the plasma flame. Chromiya's thermal spraying method, which is characterized in that:

Furthermore, an arc is formed between the main torch and the sub-torch, a swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch, and the resulting plasma flame is heated by the arc. This is a thermal spray coating of Chromia, characterized in that it is formed by feeding and melting granular Chromia and spraying the molten droplets onto the base material, or by applying an arc between the main torch and the sub-torch. A swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch, and granular Chromia is fed into the plasma flame obtained by heating by the arc and melted. , a thermal spray coating of Chromiya characterized in that it is formed by separating the plasma from the plasma immediately before the base material in the plasma flame and then spraying the molten droplets onto the base material,
It is.

[0009]

[Operation] A swirling flow of plasma gas is formed around the arc formed between the main torch and the sub-torch, so the arc is focused by the so-called pinch effect, and even if the plasma flame is not in a laminar flow state, it is squeezed and expanded. , resulting in a high-speed plasma flame. Therefore, the molten droplets of Chromia collide with the base material at high temperature and high speed, resulting in a dense thermal sprayed Chromia coating. If the plasma is separated just before the base material in the plasma flame, only the molten droplets will collide with the base material, resulting in a higher quality Chromia sprayed coating. Furthermore, since the plasma gas contains oxygen, the plasma flame is in an oxygen atmosphere. Therefore, the reduction of Chromia is prevented and the precipitation of metallic chromium is prevented.

0010

DESCRIPTION OF THE PREFERRED EMBODIMENTS Examples of the present invention will be explained with reference to the accompanying drawings. In the composite torch-type plasma spraying apparatus P shown in FIG. 1, an insulator 77 is provided with a main gas inlet 55 of the same diameter and concentrically on the axis of the main cathode 53, a main mantle 54 having a discharge port, and a main plasma gas inlet. The main torch 51 is constituted by the insulator 79 provided with the insulating material 82 and the main second mantle 81 having the constricted opening. As shown in FIG. 2, the protective gas 56 or the main plasma gas 83 is first fed into the gas annular chamber 98 from the main gas inlet 55 or the main plasma gas inlet 82, and then flows through one swirling flow forming hole 99 or It passes through a plurality of equally spaced swirling flow forming holes 99 and is fed as shown by an arrow 101 so as to swirl around the inner wall of the insulator 77 or the insulator 79 .

[0011] Next, the sub-torch starting electrode 59 arranged to intersect with the central axis of the main torch 51 is arranged concentrically with an insulator 78, a sub-first mantle 60 having a discharge port, and a sub-first mantle 60 having an ejection port.
It is attached by an insulator 80 and an auxiliary second mantle 86, and is further attached by an insulator 77 of the main torch 51 or an insulator 7.
Insulator 78 having swirling gas forming means 97 similar to 9
The auxiliary gas 62 is fed through the auxiliary gas inlet 61 provided in the insulator 80, and the auxiliary second gas 88 is fed through the auxiliary second gas inlet 87 provided in the insulator 80. . Here, the main power source 57 has its negative terminal connected to the main cathode 53, and its positive terminal connected to the main jacket 54 and the main second jacket 81 via switch means 58 and 84, respectively. constitutes the main torch 51 as a whole. The positive terminal of the sub power source 63 is connected to the positive terminal of the main power source 57 and the sub first jacket 60 of the sub torch 52, and the sub power source 63
The negative terminal of No. 3 is connected to the sub-torch activation electrode 59 via the switch means 64, and these form the sub-torch 52 as a whole. A pair of sub-torches 52 are provided. Note that a gas containing oxygen, such as oxygen or air, is used as the main plasma gas.

Activation of each torch shown in FIG. 1 is performed in the following order. That is, by closing the switch 58 and using the main power source 57, a main starting arc 65 is first formed between the main cathode 53 and the discharge port of the main mantle 54, whereby the protective gas 56 is heated and the gas flows from the tip of the main mantle 54. A conductive plasma is emitted. At this time, when the switch means 84 is closed and then the switch means 58 is opened, the main starting arc 65
The arc emitted from the tip of the main cathode 53 at the same time as the main cathode 53 is extinguished forms a main second mantle starting arc 85, which heats the protective gas 56 and the main plasma gas 83 to form a conductive plasma 100. is emitted to the outside of the main torch 51. Next, the switch means 64 is closed, and the auxiliary power supply 63
When a sub-starting arc 66 is formed between the sub-first mantle 60 and the sub-torch starting electrode 59, the sub-gas 62 is heated by this arc, and a conductive plasma 68 passes through the constriction opening to the sub-torch 52. released to the outside. When these processes are completed, the main torch 51 and the sub-torch 52 are installed so that their central axes intersect, so
Conductive plasma 100, 68 emitted from each
At this stage, when the switches 84 and 64 are opened, a steady hairpin arc 67 is formed by the main power source 57 from the tip of the main cathode 53 toward the outer surface of the constriction opening of the sub-mantle 60. By adjusting the amount of gas fed into the torch 51 and the amount of gas fed into the sub-torch 52, plasma can be created that is approximately concentric with the central axis of the main torch 51, as shown in FIG. A frame 73 is formed. At this time, gas is supplied so as to form a strong swirling flow around the arc columns of the torches 51 and 52, so the arc columns are maintained at the axial center position of the torch and a concentric swirling annular gas sheath is formed. and vice torch 5
The heat load exerted on the inner walls of the constriction openings of the first and second outer mantles 54 and 60 and the first and second second mantles 81 and 86 is uniformly reduced, and the arc current can be increased. As a result, the so-called pinch effect is promoted, and the arc is further focused, enabling high-output, high-temperature, high-speed thermal spraying.

The powdery Chromia 70 is fed toward the plasma flame 73 from the material feed pipe 69 in FIG.
The molten droplets 71 are immediately heated to a high temperature by the plasma flame 73 and melted into molten droplets 71, which proceed toward the base material 75 without spreading much while being entrained by the plasma flame 73. Plasma flame 73 containing this molten droplet 71
is the plasma decomposition means 72 provided immediately before the base material 75
By this, only the plasma is separated.

As this plasma separation means, for example, a two-fluid nozzle 90 of an atomizer that supplies water and air to the plasma flame 73 immediately in front of the base material is used. When the atomizer 90 sprays water, the plasma flame 73 is cut and formed into a conical shape. This conical plasma flame 73 acts as a tunnel-like jacket that minimizes cooling of the molten droplets 71;
The excess heat load on the base material 75 and the thermal spray coating 74 can be removed by the heat of vaporization of the water sprayed by the atomizer, and the adverse effects of the heat load can be prevented. Further, with the explosive expansion of the water at this time, the gas is rapidly injected along the plasma gas, accelerating the molten droplets 71, and as a result, forming a dense sprayed film.

The molten droplets 71 separated from the plasma in this manner immediately collide with the base material 75 to form a sprayed coating 74. At this time, by providing a means for supplying gas to form a strong swirling flow around the arc column, the arc column is maintained at the axial center position of the torch, and a concentric swirling annular gas sheath is formed, unlike the conventional laminar flow. In a turbulent region that could not be achieved with a plasma spraying device that forms a plasma flame, the plasma flame 73 is narrowed to a high density,
Thermal spraying can be performed in an elongated and stable state, and since Chromia melts well and is sprayed onto the base material 75 at high speed, a high-quality thermal spray coating 74 of Chromia can be obtained with high efficiency. Furthermore, since a gas containing oxygen, such as oxygen or air, is used as the main plasma gas, the plasma flame is in an oxygen atmosphere. Therefore, since the reduction of Chromia is prevented, the precipitation of metallic chromium in the sprayed coating can be prevented.

[0016] Embodiments of the present invention are not limited to the above, and for example, in addition to the above-mentioned air or oxygen, a mixed gas thereof or a small amount of an inert gas such as argon, helium, or nitrogen may be used as the plasma gas. It is also possible to use a gas with added . Further, in this example, a composite torch type plasma spraying device was used as the plasma spraying device, but the present invention is not limited to this, and other plasma spraying devices can be used as long as they can use a gas containing oxygen as the plasma gas. Of course, a plasma spraying device may also be used. Moreover, the present invention
Not only Chromiya's thermal spray coating, but also other oxide ceramics, such as material coatings that are extremely resistant to reducing atmospheres and can exhibit unique high performance in oxidizing atmospheres, such as ferrite, alumina, titania, and zirconia. It can also be used for thermal spray coatings such as Further, when feeding the powdery chromia into the plasma flame, a gas is used, but a liquid such as water may be used instead of the gas.

Next, an experimental example of the present invention will be explained. (A) Sample preparation conditions Conditions for producing Chromiya (Cr2O3) film

[Table 1
] Shown in . The composite torch type plasma spraying apparatus of the above embodiment was used as the plasma spraying apparatus. Furthermore, as the plasma gas, air (AIR) was used in case A, and oxygen (O2) was used in case B. X-ray diffraction on a flat sample,
We measured wear loss, etc. (B) X-ray diffraction measurement results FIG. 3 shows the X-ray diffraction measurement results of the Cr2O3 material powder (FIG. 3C) and the sprayed coating using oxygen as the plasma gas (FIG. 3B). In this figure, ○ (white circle) is Cr2O3
, ● (black circle) is Cr, and the vertical axis is Intensity (
intensity), and the horizontal axis shows 2θ (deg.), respectively. Generally, when a Cr2O3 film is produced by plasma spraying, a portion of the film is reduced and metal Cr is precipitated, causing problems such as spoiling the appearance after surface finishing and loss of electrical insulation. However, in the film using oxygen as the plasma gas (FIG. 3B), the crystal structure is almost the same as that of the material powder. Furthermore, when observing the cross section of a Cr2O3 sprayed coating with a microscope, the precipitated Cr may be observed as a band-like layer with a metallic luster, but when oxygen is used as the plasma gas (Fig. 3B), the metallic Cr Almost no precipitation was observed. On the other hand, in the film using air as the plasma gas (FIG. 3A), the peak of metal Cr was slightly larger than when oxygen was used. This is not a problem in practice, but it is thought to be due to the low partial pressure of oxygen in the plasma gas, and being able to use oxygen as a plasma gas is quite effective for spraying oxides that are easily reduced. I think that the.

(C) Wear loss (wear resistance) measurement results Cr
The results of the abrasion test for the 2O3 film are shown in Figure 4. In this figure, the vertical axis is wear loss (mg), the horizontal axis is the number of reciprocating frictions (DS), ○ (white circle) is the main plasma gas of oxygen, ● (
The black circles) indicate the main plasma gas of air. The torch output is the same for both levels (95Kw
, 99Kw), the difference in wear loss was due to
As inferred from the above-mentioned X-ray diffraction results, this is thought to be due to the difference in the main plasma gas. (D) Pickers hardness test result output Cross section of a 99Kw (O2) sample. As a result of hardness measurement, the variation range was 1408 to 1666,
The average value Hv was 1537, the variation width was relatively small, and the value was close to the highest level as a value obtained by atmospheric pressure plasma spraying. (E) Surface finish In order to ensure high wear resistance, the Cr2O3 film is required to have a mirror finish with a surface roughness of 0.5 μmRmax or less. Low-pressure plasma spraying is currently said to be the best method to obtain such high-quality coatings, but it has low productivity because it is processed in a sealed vacuum container, and it is not suitable for large parts. There are problems such as not being able to do so. Therefore, it would be of great significance if a Cr2O3 film with excellent surface finish could be obtained by atmospheric pressure plasma spraying, which is a relatively easy process. When a sample with an output of 99Kw, which showed high wear resistance in this test, was polished and finished, it reached Rmax = 0.4μm, and the quality of the coating produced by the composite torch type plasma spraying device was comparable to that of the vacuum sprayed coating. It was shown that

[0019]

[Effects of the Invention] Since the present invention is constructed as described above, the arc is focused by the swirling flow of plasma gas, and even if the plasma flame is not in a laminar flow state, it is narrowed and elongated, resulting in a high-speed plasma flame. Therefore, since the molten droplets of Chromia can be caused to collide with the base material at high temperatures and high speeds, a dense thermally sprayed Chromia coating can be obtained. Furthermore, since an arc is formed between the main torch and the sub-torch, continuous operation is possible even with oxygen-containing plasma gas, unlike the conventional single torch. Therefore, since thermal spraying can be performed using a plasma gas containing oxygen, the plasma flame becomes an oxidizing atmosphere. Therefore, since the reduction of Chromia is prevented and the precipitation of metallic chromium is prevented, a high-quality thermal sprayed Chromia coating can be obtained. Furthermore, if the plasma in the plasma flame is separated and only the molten droplets of Chromia collide with the base material, a sprayed coating of Chromia of even better quality can be obtained.

[Brief explanation of drawings]

FIG. 1 is a longitudinal sectional view showing an embodiment of the present invention.

FIG. 2 is a sectional view taken along line II-II in FIG. 1;

FIG. 3 is a diagram showing the results of X-ray diffraction of a sprayed coating.

FIG. 4 is a diagram showing abrasion test results for thermal spray coatings.

FIG. 5 is a longitudinal sectional view showing a conventional example.

FIG. 6 is a longitudinal sectional view showing another conventional example.

[Explanation of symbols]

51 Main torch 52 Deputy torch 56 Protective gas 62a Secondary gas 62b Secondary gas 68 Plasma 70 Kuromiya 71 Melted droplets 72 Plasma separation means 74 Thermal spray coating 75 Base material 83 Main plasma gas 88a　Sub-secondary gas 88b Sub-secondary gas 97a Swirl flow gas forming means 97b Swirl flow gas forming means 99 Swirling flow forming hole

Claims (5)

[Claims] [Claim 1] Chromia powder is fed into a plasma flame obtained by heating plasma gas with an arc, melted, and the molten droplets are sprayed onto a base material to form a sprayed coating on the surface of the base material. In the plasma spraying method of
Chromiya thermal spraying method, characterized in that the arc is formed between a main torch and a sub-torch, and a swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch. [Claim 2] Chromia powder is fed into a plasma flame obtained by heating plasma gas with an arc, melted, and the molten droplets are sprayed onto the base material to form a sprayed coating on the surface of the base material. In the plasma spraying method of
The arc is formed between the main torch and the sub-torch, and a swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch, and the plasma is separated just before the base material in the plasma flame. Chromiya's thermal spraying method is characterized by 3. The method for thermal spraying Chromia according to claim 2, wherein the plasma is separated by spraying from an atomizer toward a plasma flame. 4. A plasma flame obtained by forming an arc between a main torch and a sub-torch, forming a swirling flow of oxygen-containing plasma gas around the arc in the main torch, and heating by the arc. A thermal spray coating of Chromiya, characterized in that it is formed by feeding and melting granular Chromiya, and spraying the molten droplets onto the base material. [Claim 5]
An arc is formed between the main torch and the sub-torch, and a swirling flow of oxygen-containing plasma gas is formed around the arc in the main torch. A thermal spray coating of Chromiya, characterized in that it is formed by feeding and melting Chromiya, separating the plasma just before the base material in the plasma flame, and then spraying the molten droplets onto the base material.